Wood Technology: Chemical Aspects (Acs Symposium Series ; Number 43))

Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.fw001 Wood Technology: Chemical Aspects In Wood Technology:...

Author: Irving S. Goldstein

157 downloads 866 Views 14MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.fw001

Wood Technology: Chemical Aspects

In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.



W o o d Technology: Chemical Aspects Irving S. Goldstein,

EDITOR


North Carolina State University

A symposium sponsored by the Cellulose, Paper and Textile Division at the 172nd Meeting of the American Chemical Society, San Francisco, Calif., Aug. 31-Sept. 2, 1976.

43

ACS SYMPOSIUM SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON,

D.

C.

1977



Library of Congress CIP Data W o o d technology, chemical aspects ( A C S symposium series; 4 3 ) Includes bibliographies and index. 1. Wood-using industries—Congresses. 2. W o o d — Chemistry—Congresses. I. Goldstein, Irving S., 1 9 2 1 . II. American Chemical Society. Cellulose, Paper, and Textile Division. III. Series: American Chemical Society. A C S symposium series; 4 3 . TS801.W66 I S B N 0-8412-0373-3

674'.8

77-2368 A C S M C 8 4 3 1-372

Copyright © 1977 American Chemical Society All Rights Reserved. N o part of this book may be reproduced or transmitted i n any form or by any means—graphic, electronic, including photocopying, recording, taping, or information storage and retrieval systems—without written permission from the American Chemical Society.

PRINTED IN THE UNITED STATES OF AMERICA


ACS Symposium Series Robert F. G o u l d , Editor


Advisory Board Donald

G.

Crosby

J e r e m i a h P. F r e e m a n E. D e s m o n d Robert John

A.

L.

Goddard Hofstader

Margrave

Nina I. McClelland John B. P f e i f f e r J o s e p h V. R o d r i c k s Alan

C.

Sartorelli

Raymond

B.

Roy

Whistler

L.

Seymour

Aaron Wold



FOREWORD The A C S

SYMPOSIUM SERIES

was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the IN

SERIES

CHEMISTRY SERIES

parallels that of the continuing

ADVANCES

except that in order to save time the

papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form.

As a further

means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book.

Papers published in the A C S

SYMPOSIUM SERIES

are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.


PREFACE

h e r o l e of c h e m i s t r y i n the forest p r o d u c t s i n d u s t r i e s has a l w a y s b e e n A

s e c o n d a r y to m e c h a n i c a l influences.

M o s t c h e m i c a l effort has b e e n

e x p e n d e d i n t h e p u l p a n d p a p e r sector to the v i r t u a l n e g l e c t of s o l i d w o o d a n d b o a r d products even though the latter comprise 6 0 %

of the

volume. Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.pr001

A s p o i n t e d o u t i n a recent N a t i o n a l R e s e a r c h C o u n c i l r e p o r t ( " R e n e w a b l e Resources f o r I n d u s t r i a l M a t e r i a l s , " N a t i o n a l A c a d e m y of S c i ences,

W a s h i n g t o n , D . C . , 1976, 267 p p . ) , r e n e w a b l e resources

are a

great a n d u n d e r u s e d n a t i o n a l resource w h o s e s u b s t i t u t a b i l i t y f o r d w i n d l i n g c o a l a n d p e t r o l e u m resources

can reduce

U . S . dependence

on

f o r e i g n energy a n d m a t e r i a l s sources. W o o d is o u r m o s t a b u n d a n t r e n e w able

resource

a n d has s u b s t a n t i a l advantages,

particularly from

the

s t a n d p o i n t of e n e r g y r e q u i r e m e n t s , over a l t e r n a t i v e m a t e r i a l s . I t w a s i n this context

that the s y m p o s i u m p r e s e n t e d h e r e

was

p l a n n e d . T h e i n t e n t is to s u m m a r i z e the c h e m i c a l aspects of s o l i d w o o d t e c h n o l o g y , p r o v i d e this b a c k g r o u n d i n a u s e f u l reference v o l u m e , a n d h o p e f u l l y to s t i m u l a t e a d d i t i o n a l c h e m i c a l a c t i v i t y i n this area. M o s t of the p a p e r s p r e s e n t e d are r e v i e w s , b u t a f e w c u r r e n t r e s e a r c h reports h a v e b e e n i n c l u d e d to i n d i c a t e the p i o n e e r i n g w o r k b e i n g done. T h e b o o k opens w i t h a p a p e r o n the s t r u c t u r e a n d c o m p o s i t i o n of w o o d t o define the m a t e r i a l u n d e r d i s c u s s i o n a n d t h e n considers m o l d s , p e r m e a b i l i t y , w o o d p r e s e r v a t i o n , t h e r m a l d e t e r i o r a t i o n a n d fire r e t a r d ance, d i m e n s i o n a l s t a b i l i t y , a d h e s i o n , r e c o n s t i t u t e d w o o d b o a r d s s u c h as fiberboard

a n d particleboard, p l y w o o d , laminated beams, w o o d

wood-polymer

composites,

and wood

softening a n d forming.

p a p e r treats the c o m m o n t h e m e of w a s t e w a t e r m a n a g e m e n t .

finishes, A

final

O n l y one

of the papers p r e s e n t e d at the m e e t i n g is n o t i n c l u d e d i n this v o l u m e , a n d its subject of c o n v e n t i o n a l w o o d p r e s e r v a t i o n m e t h o d s is a d e q u a t e l y t r e a t e d i n d e t a i l e l s e w h e r e (e.g., N i c h o l a s , D . D . , Ed., " W o o d D e t e r i o r a t i o n a n d Its P r e v e n t i o n b y P r e s e r v a t i v e T r e a t m e n t s , " 2 vols., S y r a c u s e U n i v e r s i t y Press, 1 9 7 3 ) . A s is i n e v i t a b l e w i t h so m a n y topics a n d so m a n y authors, there is great d i v e r s i t y i n style, d e p t h , a n d q u a n t i t a t i v e or q u a l i t a t i v e d e g r e e of treatment.

T h e r e m i g h t b e m o r e t h a n a r e a d e r w o u l d w a n t to k n o w ix


about a particular subject and not enough about another. However, I hope that in toto this collection of papers by knowledgeable individuals will to some extent meet the symposium objectives. Raleigh, N.C.

IRVING S. GOLDSTEIN

Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.pr001

January 3, 1977

χ


1 W o o d : Structure and Chemical Composition R. J. THOMAS

Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.ch001

Department of Wood and Paper Science, North Carolina State University, Raleigh, N.C. 27607

W i t h i n living t r e e s , wood is produced to perform the r o l e s of support, conduction and storage. The support r o l e enables the t r e e stem to remain e r e c t d e s p i t e the heights to which a t r e e grows. Because of these h e i g h t s , wood a l s o must perform the r o l e of conduct i o n , that is, transport water from the ground to the upper parts o f the t r e e . F i n a l l y , food i s stored i n certain parts of the wood until required by the living t r e e . The wood cells which perform the r o l e o f conduction and/or support make up 60 to 90 percent of the wood volume. W i t h i n the living t r e e these c e l l s are dead, that is, the cytoplasm i s absent 1eaving a hollow cell with rigid walls. The only living cells w i t h i n the wood portion of a t r e e are the f o o d - s t o r i n g cells. The c l o s e r e l a t i o n s h i p be tween "form and f u n c t i o n " simplifies the study o f wood anatomy i f the r o l e o f the cells i n the living t r e e is kept c o n s t a n t l y in view. Gross Anatomical

Features

The e n d v i e w o f a l o g ( F i g u r e 1) e x p o s e s t h e wood and b a r k p o r t i o n o f a t r e e t r u n k . Each y e a r a g r o w i ng c e n t e r l o c a t e d b e tween t h e wood and b a r k i n s e r t s a new 1 a y e r o f wood a d j a c e n t t o t h e e x i s t i ng wood. I η a d d i t i o n , new b a r k i s depos i t e d n e x t t o t h e p r e - e x i s t i n g b a r k . Wood o c c u p i e s t h e l a r g e s t volume o f a t r e e stem b e c a u s e more wood c e l l s a r e p r o d u c e d t h a n b a r k c e l l s and a l s o b e c a u s e t h e wood e e l 1 s a r e r e t a i ned and t h u s a c c u m u l a t e w h i l e t h e o u t e r m o s t b a r k e e l 1 s a r e contînually d i s c a r d e d . The c e n t r a l wood p o r t i o n o f t h e l o g d e p i c t e d i n F i g u r e 1 i s c o n s i d e r a b l y d a r k e r i n c o l o r than t h e p a r t a d j a c e n t t o t h e bark. The d a r k - c o l o r e d wood i s termed heartwood and t h e 1 i g h t - c o l o r e d wood i s termed sapwood. The d i s c o l o r a t i o n i s due t o t h e p r o d u c t i o n and s e c r e t i o n o f s u b s t a n c e s w h i c h a r e a b y - p r o d u c t o f t h e death o f food-storage c e l l s . As new wood, t h a t i s sapwood, i s formed t o t h e o u t s i d e o f t h e t r e e s t e m , add i t i o n a l i n t e r i o r s a p wood a d j a c e n t t o t h e h e a r t w o o d zone i s c o n v e r t e d t o h e a r t w o o d . Some t r e e s do n o t f o r m d i s c o l o r e d h e a r t w o o d upon t h e d e a t h o f

1



2

WOOD

TECHNOLOGY :

CHEMICAL

Figure 1. End view of a log showing both the wood and bark. Note the small total volume occupied by the bark. The central, dark portion of the wood is heartwood, and the outer ring of light colored wood is sapwood.

Figure 2. End view of a softwood showing growth rings. Each growth ring consists of a light and dark area. The light portion is called springwood or earlywood, and the dark area is termed summerwood or latewood. 2 5 χ


ASPECTS


1.

THOMAS

Wood:

Structure

and Chemical

Composition

3

f o o d - s t o r a g e c e l l s , t h u s r e c o g n i t i o n o f h e a r t w o o d based on c o l o r i s not always p o s s i b l e . I t s h o u l d be a p p a r e n t t h a t t h e v a s t maj o r i t y o f c e l l s w h i c h c o n s t i t u t e t h e wood p o r t i o n o f a l i v i n g t r e e a r e dead* The sapwood zone c o n t a i n s t h e o n l y l i v i n g c e l l s f o u n d i n mature wood and t h e y c o n s t i t u t e , d e p e n d i n g upon s p e c i e s , f r o m 10 t o kO p e r c e n t o f t h e sapwood volume. As h e a r t w o o d i s formed t h e s e l i v i n g , f o o d - s t o r a g e c e l l s d i e , t h u s t h e h e a r t w o o d c o n t a i n s o n l y dead c e l l s . T r e e s a r e c l a s s i f i e d i n t o two m a j o r g r o u p s termed s o f t w o o d s (gymnosperms) and hardwoods ( a n g i o s p e r m s ) . The b o t a n i c a l b a s i s f o r c l a s s i f i c a t i o n i s w h e t h e r o r n o t t h e t r e e seed i s naked as i n s o f t w o o d s o r c o v e r e d a s i n hardwoods. A more f a m i l i a r c l a s s i f i c a t i o n , w h i c h w i t h some e x c e p t i o n s i s v a l i d , i s based on t h e r e t e n t i o n o f l e a v e s by s o f t w o o d s o r t h e s h e d d i n g o f l e a v e s by h a r d woods. Thus t h e s o f t w o o d s a r e o f t e n r e f e r r e d t o as e v e r g r e e n t r e e s and hardwood as d e c i d u o u s t r e e s . The m a j o r d i f f e r e n c e w i t h r e g a r d t o wood anatomy i s t h e p r e s e n c e o f v e s s e l s i n hardwoods. V e s s e l s a r e s t r u c t u r e s composed o f c e l l s c r e a t e d e x c l u s i v e l y f o r t h e c o n d u c t i o n o f w a t e r . S o f t w o o d s l a c k v e s s e l s b u t have c e l l s termed l o n g i t u d i n a l t r a c h e i d s w h i c h p e r f o r m a d u a l r o l e o f c o n d u c t i o n and s u p p o r t . The terms s o f t w o o d and hardwood a r e n o t t o be t a k e n a s a measure o f h a r d n e s s s i n c e some hardwoods a r e s o f t e r t h a n many softwoods. F o r t h e c o m m e r c i a l l y i m p o r t a n t d o m e s t i c woods, t h e a v e r a g e s p e c i f i c g r a v i t y f o r s o f t w o o d s i s .41 w i t h a range o f .29 t o .60. Hardwoods a v e r a g e .50 and v a r y f r o m .32 t o .81. Growth r i n g s , o r a n n u a l i n c r e m e n t s , as seen on t h e end o f a s o f t w o o d l o g a r e d e p i c t e d i n F i g u r e 2. Growth r i n g s a r e d e t e c t a b l e due t o d i f f e r e n c e s i n t h e wood p r o d u c e d e a r l y and l a t e i n t h e g r o w i n g s e a s o n . The wood p r o d u c e d e a r l y , c a l l e d e a r l y w o o d o r s p r i n g w o o d , i s c o n s i d e r a b l y l i g h t e r i n c o l o r t h a n t h e wood termed l a t e w o o d o r summerwood w h i c h i s p r o d u c e d l a t e i n t h e g r o w i n g s e a s o n . The c o l o r d i f f e r e n c e i s due p r i m a r i l y t o t h e d i f f e r e n t k i n d s o f c e l l s produced e i t h e r e a r l y o r l a t e i n t h e growing season. F i g u r e 3 d e p i c t s t h r e e g r o w t h r i n g s and p a r t o f a f o u r t h . Note t h a t a t t h i s low m a g n i f i c a t i o n y o u c a n d e t e c t t h e i n d i v i d u a l c e l l s w h i c h c o n s t i t u t e t h e s p r i n g w o o d a r e a ; h o w e v e r , t h e summerwood zone a p p e a r s m e r e l y as a d a r k z o n e . A t a h i g h e r m a g n i f i c a t i o n , a s shown i n F i g u r e 4, i n d i v i d u a l c e l l s o f b o t h s p r i n g w o o d and summerwood a r e e v i d e n t . The s p r i n g w o o d c e l l s have a l a r g e c r o s s - s e c t i o n a l a r e a w i t h t h i n w a l l s and a l a r g e , open c e n t e r . The l a r g e , open c e n t e r , c a l l e d t h e lumen, p e r m i t s e f f i c i e n t c o n d u c t i o n o f w a t e r . The summerwood c e l l s have a s m a l l e r c r o s s s e c t i o n a l a r e a , t h i c k e r w a l l s and a s m a l l e r lumen t h a n s p r i n g w o o d cells. O b v i o u s l y , t h i s type o f c e l l provides s u b s t a n t i a l support f o r t h e t r e e stem b u t i s n o t as e f f i c i e n t as s p r i n g w o o d c e l l s i n conduction. In some s o f t w o o d s p e c i e s t h e g r a d a t i o n between s p r i n g w o o d and summerwood i s g r a d u a l and f e w e r summerwood c e l l s a r e p r o d u c e d . When t h i s o c c u r s , i t i s o f t e n d i f f i c u l t t o d i s t i n g u i s h g r o w t h i n c r e m e n t s a s t h i s t y p e o f wood p o s s e s s e s a more



4

WOOD

TECHNOLOGY:

CHEMICAL

Figure 3. Softwood block showing three complete and part of two other growth rings in the cross-sectional plane (X). Individual springwood cells can be detected, whereas the smaller summerwood cells cannot be seen as individual cells. Also note the absence of vessels and the uniformity of the wood. Two longitudinal surfaces (R—radial; Τ— tangential) are illustrated. Food-storing cells can be easily detected on the radial surface (arrow). 47 X (Courtesy ofN.C. Brown Center for Ultrastructural Stud ies, S.U.N.Y. College of Environmental Science and Forestry)

Figure 4. Cross-sectional view of springwood and sum merwood cells. The springwood cells are larger and char acterized by thin cell walls and a large central opening. The smaller cells with thick cell walls and a small central opening are summerwood cells. The narrow ribbon-like rows of cells traversing both the springwood and sum merwood zones (arrow) are called rays. They consist of transversely oriented food-storage cells. 135χ


ASPECTS


1.

THOMAS

Wood:

Structure

and Chemical

Composition

5

uniform s t r u c t u r e . As i n d i c a t e d e a r l i e r , hardwoods a r e c h a r a c t e r i z e d by t h e presence o f vessels o r pores. Vessels a r e c e l l s which occupy a l a r g e c r o s s - s e c t i o n a l a r e a and c a n i n most s p e c i e s be d e t e c t e d w i t h t h e u n a i d e d e y e . The wood o f hardwood t r e e s i s c l a s s i f i e d as eî t h e r r i n g - p o r o u s o r d i f f u s e - p o r o u s depend i ng upon t h e a r rangement o f t h e v e s s e l s. I η a t y p i c a l r i n g - p o r o u s wood, as i l l u s t r a t e d i n F i g u r e 5, t h e v e s s e l s formed i n t h e s p r i ngwood a r e c o n s i d e r a b l y l a r g e r t h a n t h o s e formed i n t h e summerwood. F i g u r e 6 r e v e a l s a d î f f u s e - p o r o u s wood i n w h i c h t h e v e s s e l s a r e e s s e n t i a l l y t h e same s i z e t h r o u g h o u t t h e g r o w t h r i n g . As a r e s u l t , i n many d i f f u s e - p o r o u s woods i t i s d i f f i c u l t t o d i s t i n g u i s h g r o w t h rings. A c o m p a r i s o n o f F i g u r e s 3» 5 and 6 w i l l r e v e a l t h e m a j o r d i f f e r e n c e between hardwood and s o f t w o o d anatomy. Note t h e l a c k o f v e s s e l s and t h e r e l a t i v e l y un i f o r m a p p e a r a n c e o f t h e s o f t w o o d i n F i g u r e 3, compared t o t h e hardwoods d e p i c t e d i n F i g u r e s 5 and 6. Wood r a y s a r e f o u n d i n a l 1 s p e c i e s o f wood and cons i s t o f ribbon-1ike aggregations o f food-storing c e l l s extending i n the t r a n s v e r s e d i r e c t i o n from the bark toward t h e c e n t e r o f t h e t r e e . In t h e c r o s s - s e c t i o n a l v i e w , r a y s t a k e t h e f o r m o f 1 i n e s o f v a r y i n g w i d t h r u n n i n g a t r i g h t a n g l e s t o t h e g r o w t h r i n g s ( F i g u r e s 3» 4, 5 and 6 ) . S o f t w o o d Anatomy The anatomy o f s o f t w o o d s w i l l be d e s c r i b e d f i r s t b e c a u s e i t i s l e s s c o m p l e x t h a n hardwoods. The two mai η e e l 1 t y p e s wh i c h c o n s t i t u t e s o f t w o o d s a r e t r a c h e i d s , w h i e h c o n d u c t and s u p p o r t , and parenchyma w h i c h s t o r e f o o d . T h e s e two e e l 1 t y p e s c a n be f u r t h e r c l a s s i f i e d as t o t h e i r o r i e n t a t i o n , t h a t i s l o n g i tud i n a l or transverse. C e l l s o r i e n t e d i n t h e l o n g i tud înal d i r e c t i o n have t h e l o n g a x i s o f t h e e e l 1 o r i e n t e d p a r a 1 l e i t o t h e v e r t îcal a x i s o f t h e t r e e t r u n k w h e r e a s t r a n s v e r s e l y o r i e n t e d e e l 1 s have t h e i r long a x i s a t r i g h t a n g l e s t o t h e v e r t i c a l a x i s o f t h e t r e e stem. A g r e a t l y s împli f i e d model o f s o f t w o o d anatomy c a n be made by g l u i ng t o g e t h e r a g r o u p o f s o d a s t r a w s a l o n g t h e i r l e n g t h and d î s p e r s i ng t h r o u g h o u t t h i s g r o u p m a t c h e s , l a i d end t o end w i t h t h e l o n g a x i s o f t h e matches o r i e n t e d a t r i g h t a n g l e s t o t h e l o n g axi s o f t h e soda s t r a w s . The s o d a s t r a w s r e p r e s e n t t h e l o n g i t u d i n a l l y o r i e n t e d t r a c h e i d s w h i c h o c c u p y a b o u t 30% o f t h e vo1ume w h e r e a s t h e matches r e p r e s e n t t r a n s v e r s e l y o r i e n t e d parenchyma e e l Is o c c u p y i n g a b o u t 10% o f t h e vo1ume. The t r a n s v e r s e l y o r i e n t e d parenchyma a r e t h e e e l 1 s w h i c h c o n s t i t u t e wood r a y s . As t h e matches i n t h e model a r e cons i d e r a b l y s h o r t e r t h a n t h e soda s t r a w s , s o parenchyma e e l 1 s a r e c o n s i d e r a b l y s h o r t e r t h a n l o n g i tudinal tracheids. S i n c e l o n g i t u d i n a l t r a c h e i d s c o n s t i t u t e a b o u t 30% o f t h e vo1ume and a r e t h e r e f o r e l a r g e l y r e s p o n s i b l e f o r t h e r e s u l t i n g phys i c a l and chemi c a l p r o p e r t i e s o f s o f t w o o d s , a d e t a i l e d



6

WOOD

TECHNOLOGY:

CHEMICAL

Figure 5. Cross-sectional and longitudinal surfaces of a ringporous hardwood. In the crosssectional view (X) the largest diameter cells are springwood vessels whereas the smaller cells with obvious openings are sum merwood vessels. Smaller di ameter, thick-walled fibers con stitute most of the remaining volume. Transversely oriented food-storing cells can be seen on the radial surface (arrow). 40χ (Courtesy of N. C. Brown Cen ter for Ultrastructural Studies, S.U.N J. College of Environ mental Science and Forestry)

Figure 6. Cross-sectional and longitudinal views of a diffuseporous wood. Note in cross sec tion that the vessels, the large diameter cells, are essentially the same size throughout the growth ring. Both longitudinal views reveal the vessels (V) are formed as the result of indi vidual vessel cells stacked one on top of the other in the longi tudinal direction. The majority of the remaining cells are small diameter fibers. 70 χ (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. Col lege of Environmental Science and Forestry)


ASPECTS


1.

THOMAS

Wood:

Structure

and Chemical

Composition

7

d e s c r i p t i o n o f t h e i r anatomy i s m e r i t e d . F i g u r e 3» s h o w i n g a smal1 b l o c k o f p i ne magni f i e d 50 t i m e s , c l e a r l y î11ustrates t h a t s p r i n g w o o d and summerwood 1 o n g i t u d i n a l t r a c h e i d s make up t h e l a r g e s t vo1ume o f t h e wood. Note t h a t i n add i t i o n t o t h e c r o s s s e c t i o n a l v i e w , two l o n g i t u d i n a l v i e w s a r e e x p o s e d . Transversely o r i e n t e d parenchyma e e l I s , w h i c h make up t h e r a y s , c a n a l s o be s e e n i n t h e t r a n s v e r s e and l o n g i t u d i n a l v i e w s . I η the crosss e c t i o n a l v i e w , t h r e e c o m p l e t e and two p a r t i a l g r o w t h r i n g s a r e depicted. The l o n g i t u d i n a l t r a c h e i d i s a b o u t 100 t i m e s l o n g e r t h a n w i d e . D e p e n d i n g on s p e c i e s , most d o m e s t i c s o f t w o o d s have l o n g i t u d i n a l t r a c h e i d s r a n g i n g f r o m 3 t o 5 mm i n l e n g t h . Redwood has t h e l o n g e s t t r a c h e i d s , up t o 7.3 mm, and c e d a r t h e s h o r t e s t , a b o u t 1.18 mm i n l e n g t h . The w i d t h o f l o n g i t u d i n a l t r a c h e i d s f o r d o m e s t i c s p e c i e s r a n g e s f r o m 20 ym f o r c e d a r up t o 80 ym f o r redwood. S i n c e t h e l o n g i tud i n a l t r a c h e i d i s a l o n g , t h i n, c y 1 i n d r i c a l , t u b e - 1 i ke e e l 1, i t s a p p e a r a n c e depends upon how i t i s ν i ewed. For example, i f t h e c e l l i s c u t a t r i g h t a n g l e s t o i t s long a x i s , t h e c r o s s - s e c t i o n a l v i e w i s e x p o s e d on t h e c u t s u r f a c e . The c r o s s - s e c t i o n a l v i e w e x p o s e d i n F i g u r e 3 r e v e a l s î η t h e sprî ngwood a s q u a r e o r p o l y g o n a l s h a p e predomi natîng wh i l e i n t h e summerwood a more r e c t a n g u l a r shape i s a p p a r e n t . I η the longi tudi nal , r a d i a l p l a n e v i e w , t h e ends o f sprî ngwood t r a c h e i d s a r e rounded, w h i l e i n t a n g e n t i a l view they a r e p o i n t e d ( F i g u r e 7 ) . The ends o f summerwood t r a c h e i d s a r e p o i n t e d i n b o t h r a d i a l and t a n g e n t i a l views (Figure 7 ) . 11 s h o u l d be o b v i o u s t h a t t h e s t r u c t u r e o f t h e l o n g i t u d i n a l t r a c h e i d i s w e l l s u i ted t o perform t h e dual r o l e s o f conduct ion and s u p p o r t . S i nee w a t e r i s t r a n s l o c a t e d up t h e t r e e v i a t h e t r a c h e i d s , the o r i e n t a t ion o f the long a x i s o f the t r a c h e i d para i l e i t o t h e v e r t i c a l s t e m p e r m i t s a l o n g e r passageway p r i o r t o i n t e r r u p t i o n by a c e l l w a l l . The r i g i d c e l l w a l l s , o f v a r y i n g t h i c k n e s s , p r o v i d e adequate support. Of t h e s e v e r a l t y p e s o f m a r k i ngs f o u n d on t h e l o n g i t u d i n a l tracheîd w a l l ( F i g u r e s 7 and 8 ) , t h e most o b v i o u s a r e t h e c i r c u l a r dome-1i ke s t r u c t u r e s c a l l e d b o r d e r e d p i t s . Also depicted are c l u s t e r s o f egg shaped p i t s whi ch î n t e r c o n n e c t l o n g i t u d i n a l t r a c h e i d s t o t r a n s v e r s e l y o r i e n t e d parenchyma c e l l s . The s t r u c t u r e o f t h e b o r d e r e d p i t f a c i l i t a t e s l i q u i d f l o w between c e l l s . An o p e n i ng termed t h e p i t a p e r t u r e i s l o c a t e d i n t h e c e n t e r o f t h e dome-1ike s t r u c t u r e w h i c h i s c a l l e d t h e p i t b o r d e r ( F i g u r e 9 ) . Removal o f t h e b o r d e r r e v e a l s t h e p i t membrane w h i c h r e s e m b l e s a w h e e l w i t h a hub and s p o k e s ( F i g u r e 1 0 ) . The p o r t i o n o f t h e membrane s i m i l a r t o t h e p a r t o f t h e wheel w i t h spokes i s c a l l e d t h e margo. N o t e t h e many o p e n i n g s i n t h e margo t h r o u g h w h i c h l i q u i d can f l o w . T h e c e n t r a l p o r t i o n , r e s e m b l i ng t h e hub o f a w h e e l , has no d e t e c t a b l e open i n g s and i s termed t h e t o r u s . S i nee e a c h b o r d e r e d p i t wi t h i η a c e l l u s u a l l y has a c o m p l i m e n t a r y p i t i n t h e c o n t i g u o u s e e l 1, 1 i q u i d c a n f l o w f r o m t h e lumen t h r o u g h t h e p i t


8

WOOD

TECHNOLOGY:

CHEMICAL


Wood Science

Figure 7. Isolated springwood (earlywood) and summerwood (latewood) longi tudinal tracheids. Note resemblance of tracheids to long cylindrical tubes. Tracheid lengths in this figure are considerably re duced as tracheids are normally about 100 times longer than wide. Note the rounded end of the springwood tracheid in the radial view and the pointed end in the tangential view. Summerwood tracheid ends tend to be pointed in both views, a. Bordered pits to adjacent longitudinal tracheids; b. and c. pits to adjacent ray cells. (Drawing from: Howard, E. T., Manwiller, E. G., Wood Science (1969) 2,77-86)

Figure 8. View of internal cell walls of springwood longitudinal tracheids. The circular dome-like structures are bor dered pits which permit liquid flow between contiguous longitudinal tra cheids. The smaller egg-shaped pits in clusters lead to adjacent transversely oriented ray cells. 400χ (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environ mental Science and Forestry)


ASPECTS


1.

THOMAS

Wood:

Structure and Chemical

Composition

9

Figure 9. Innermost wall layer as seen from the inside of a springwood longi tudinal tracheid showing two bordered pits. Note the circular pit apertures and the stringlike microfibrils which are ori ented at approximately 90° to the long axis of the cell. Arrow indicates longi tudinal axis of the cell. 3,000χ

Figure 10. View of a bordered pit membrane with the dome-shaped pit border removed. The dark central portion is the torus. The stringlike microfibrils radiating from the torus constitute the margo portion of the pit membrane. Water flows freely from cell to cell through the openings between the margo microfibrils. 3,000X



10

WOOD

TECHNOLOGY:

CHEMICAL

Figure 11. Springwood longitudinal tra cheids showing bordered pits in crosssectional views. Note also the longitudinal surface views of bordered pits inside of the tracheids. In addition, narrow, elongated, transversely oriented food-storage cells which constitute rays are visible. 400X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)

Figure 12. Cross-sectional view of a bordered pit-pair. A: pit aperture; B: pit border; T: torus. Note the thick, nonperforated torus. Most of the thin margo was destroyed during specimen preparation, and only portions of it re main (arrow). Liquid flow occurs through the pit aper ture, around the torus through the margo, and out the other pit aperture into the adjacent cell. 5,000χ


ASPECTS


1.

THOMAS

Wood:

Structure

and Chemical

Composition

11

a p e r t u r e o f one c e l l , t h r o u g h t h e margo p o r t i o n o f t h e membrane and o u t t h e p i t a p e r t u r e i n t o t h e lumen o f t h e a d j a c e n t c e l l . C r o s s - s e c t i o n a l views o f bordered p i t - p a i r s i n t e r c o n n e c t i n g t r a c h e i d s a r e i l l u s t r a t e d i n F i g u r e 11. A v i e w o f a s i n g l e b o r d e r e d pi t - p a i r i n F i g u r e 12 r e v e a l s an e x t r e m e l y t h i η and d i s r u p t e d margo and a r e l a t i v e l y t h i c k t o r u s . B e c a u s e o f t h e t h i n and p o r ous n a t u r e o f t h e margo r e g i o n , i t i s o f t e n d i s r u p t e d when p r e p a r i n g specimens f o r m i c r o s c o p i c e x a m i n a t i o n . Not i c e i n F i g u r e 12 t h a t t h e t o r u s i s i n t h e c e n t r a l pos i t i o n whîle i n F i g u r e 13 i t has moved t o one s i d e and e f f e e t i v e l y s e a l e d one o f t h e a p e r t u r e s . I η t h i s cond î t i on t h e p i t i s i n t h e aspirated state. C o m p a r i s o n o f an a s p i r a t e d p i t - p a i r ( F i g u r e s 13 and 1k) w i t h a n o n - a s p i r a t e d p i t - p a i r ( F i g u r e s 11 and 12) c l e a r l y i 1 1 u s t r a t e s t h e d i s p l a c e m e n t o f t h e p i t membrane a g a i n s t t h e p i t b o r d e r and t h e e x t r e m e l y t i g h t s e a l w h i c h e x i s t s between t h e t o r us and p i t b o r d e r . A s p i r a t i o n o f t h e p i t membrane o c c u r s a s w a t e r i s removed f r o m t h e c e l l . In t h e l i v i n g t r e e , a s p i r a t i o n p r e v e n t s , i n t h e e v e n t a t r e e i s wounded, a i r e m b o l i s m s f r o m o c c u r r i n g t h r o u g h o u t t h e t r e e and i n t e r r u p t i n g a l 1 w a t e r c o n d u c t i o n . P i t membrane a s p i r a t i o n , w h i c h o c c u r s d u r i ng t h e d r y i ng o f wood, r e d u c e s wood permeab î1i t y as 1 i q u i d f l o w between e e l 1 s i s p r e vented. Thus any p r o c e s s i ng o f wood î n v o l v i ng t h e pénétrât i o n o f l i q u i d s a f t e r d r y i n g i s more d i f f i c u l t . The o t h e r m a j o r e e l 1 w a l l s t r u c t u r e found on l o n g i t u d i n a l t r a c h e i d s i s termed a r a y c r o s s i ng and i s î11ustrated i n F i g u r e s 7 and 8. Ray c r o s s î ngs c o n s i s t o f p i t s w h i c h î n t e r c o n n e c t l o n g i tud î n a l t r a c h e i d s t o r a y parenchyma. Due t o t h e d i v e r s e s t r u c t u r e o f r a y c r o s s i n g p i t s they a r e extremely u s e f u l i n t h e ident i f i c a t i o n o f wood and wood f i b e r s . However, s i nee i d e n t i f i c a t i o n i s beyond t h e s c o p e o f t h i s r e v i e w , a descrî pt i o n o f t h e d i f f e r e n t t y p e s o f p i t s found i n r a y c r o s s i n g s i s n o t i n c l u d e d . F i g u r e s 8 and 15 r e v e a l r a y c r o s s i ng p i t s as s e e n f r o m t h e i n s i d e o f l o n g i t u d i n a l t r a c h e i d s . The c o n s i d e r a b l y h i g h e r magnif i c a t i o n i n F i g u r e 15 shows a s o l i d p i t membrane. O p e n i n g s i n t h e p i t membrane w o u l d e x p o s e t h e c y t o p l a s m t o t h e h o s t île e n v i ronment o f t h e l o n g i t u d i n a l t r a c h e i d lumen and r e s u l t i n t h e d e a t h o f t h e parenchyma e e l 1. T h u s , t h e membranes a r e s o l i d and do n o t p r o v i d e a passageway f o r f r e e l i q u i d f l o w . Hardwood Anatomy O b v i o u s l y , s o f t w o o d anatomy i s relatî v e l y s i mp1e as o n l y two t y p e s o f e e l 1 s, l o n g i t u d i n a l t r a c h e i d s and r a y parenchyma, c o n s t i t u t e t h e b u l k o f t h e wood. Hardwoods have a more c o m p l e x a n a tomy a s more k i n d s o f e e l 1 s a r e p r e s e n t . The r o l e s o f c o n d u c t i o n and s u p p o r t a r e c a r r i e d o u t by d i f f e r e n t c e l l s and i n add i t i o n t o t h e t r a n s v e r s e r a y parenchyma, f o o d - s t o r a g e e e l Is o r i e n t e d i n t h e l o n g i tud i nal d i r e c t i o n a r e p r e s e n t . Parenchyma o r i e n t e d l o n g i t u d i n a l l y a r e c a l l e d l o n g i t u d i n a l o r a x i a l parenchyma. Vessel segments p e r f o r m t h e c o n d u c t i o n r o l e , and f i b e r s t h e s u p p o r t r o l e .



12

WOOD

TECHNOLOGY:

CHEMICAL

Figure 13. Cross-sectional view of an aspirated bordered pitpair. The pit membrane has moved to the border and sealed a pit aperture with the torus. In this condition, liquid flow no longer occurs between contiguous cells. 5,000χ

Figure 14. Surface view of an aspirated bordered pit mem brane. The imprint of the pit aperture through the torus is the result of an extremely tight seal. 6,200X


ASPECTS


1.

THOMAS

Wood:

Structure

and Chemical

Composition

13

T h u s , most hardwood s p e c i e s c o n t a i n f o u r t y p e s o f c e l l s , v e s s e l s e g m e n t s , f i b e r s , t r a n s v e r s e and a x i a l parenchyma, w h e r e a s most s o f t w o o d s p e c i e s p o s s e s s two t y p e s ; l o n g i t u d i n a l t r a c h e i d s and t r a n s v e r s e parenchyma. V e s s e l s a r e s t r u c t u r e s u n i q u e l y des i g n e d t o c a r r y o u t t h e conduct ion r o l e . A v e s s e l cons i s t s o f i n d i v i d u a l v e s s e l e e l 1 s s t a c k e d one on t o p o f t h e o t h e r i n t h e v e r t i c a l d i r e c t i o n o f t h e s t e m ( F i g u r e 6 ) . V e s s e l l e n g t h s up t o t h r e e m e t e r s have been n o t e d . The v e s s e l c e l l s o r s e g m e n t s , w h i c h c o n s t i t u t e a v e s s e l , d i f f e r w i d e l y i n t h e i r s i z e and s h a p e . The l e n g t h o f v e s s e l s e g ments v a r i e s f r o m 0.18 mm t o 1.3 mm and f r o m 20 ym t o 330 ym i n width. The s h o r t e s t and w i d e s t v e s s e l segments a r e f o u n d i n t h e s p r î ngwood o f r i n g - p o r o u s woods where o f t e n t h e w i d t h o f t h e v e s s e l segment i s g r e a t e r t h a n t h e l e n g t h . However, i n d i f f u s e p o r o u s woods t h e v e s s e l segments a r e u s u a l l y 8 t o 10 t i m e s l o n g e r than they a r e wide. Note i n F i g u r e s 5 and 6 t h a t v e s s e l segments i n c r o s s - s e c t i o n a l v i e w have a more o r l e s s rounded shape. Iη t h i s v i e w t h e y a r e o f t e n ca1 l e d p o r e s whi ch i s a t e r m o f c o n v e n i e n c e t o d e s c r i b e t h e c r o s s s e c t i o n o f a v e s s e l . As s e e n l o n g i t u d i n a l l y i n F i g u r e 16, t h e y range f r o m drum-shaped t o b a r r e l shaped t o o b l o n g , 1î n e a r - s h a p e d e e l 1 s. Two o b v i o u s s t r u c t u r a 1 f e a t u r e s o f v e s s e l segments a r e p e r forât i o n p l a t e s and p i t s . P e r f o r a t i o n p l a t e s a r e d i stî net openi n g s f o u n d a t b o t h ends o f t h e v e s s e l segment w h i c h l e a d t o a d j a cent v e s s e l segments. In some s p e c i e s a number o f o p e n i n g s a r ranged p a r a l l e l t o e a c h o t h e r f o r m t h e p e r f o r a t i o n p l a t e . These a r e termed s c a l a r i f o r m perforât i o n p l a t e s . A s i n g l e o p e n i ng i s c a l led a s impie p e r f o r a t i o n p l a t e . B o t h t y p e s o f perforât i o n p l a t e s a r e î11ustrated i n F i g u r e 16. The c r e a t i o n o f o p e n i ngs between v e s s e l segments p r o v i d e s an e l o n g a t e d t u b e l i k e s t r u c t u r e of considerable length h i g h l y s u i t e d f o r the l o n g i t u d i n a l transl o c a t i o n o f water. When v e s s e l s e n d , t h e y r a r e l y do s o i n i s o l a t i o n b u t r a t h e r among a g r o u p o f v e s s e l s. T r a n s l o c a t i o n c o n t i nues i n t o t h e a d j a cent v e s s e l s v i a the î n t e r v e s s e l pi t s . These pi t s d i f f e r from s o f t w o o d b o r d e r e d p i t s i n t h a t t h e y l a c k a t o r u s and o p e n i n g s l a r g e enough t o be r e a d i l y d e t e c t e d w i t h an e l e c t r o n m i c r o s c o p e . F i g u r e 17 d e p i c t s a t y p i ca1 i n t e r v e s s e l p i t membrane. D i f f e r e n t a r r a n g e m e n t s o f i n t e r v e s s e l p i t s c a n be d e t e c t e d and a r e u s e f u l i n t h e i d e n t i f i c a t i o n o f hardwood s p e c i e s . The l o n g i tud i n a l e e l 1 t y p e s r e s p o n s i b l e f o r t h e s u p p o r t r o l e i n hardwoods a r e f i b e r s . F i b e r s a r e t h i c k - w a l l e d , e l o n g a t e d e e l Is w i t h c l o s e d pointed ends. I t s h o u l d be n o t e d t h a t f r e q u e n t l y t h e term " f i b e r s i s used l o o s e l y f o r a l 1 t y p e s o f wood e e l 1 s. However, s p e c i f i c a l l y i t r e f e r s o n l y t o t h o s e e e l 1 t y p e s f o u n d i n hardwoods w h i c h meet t h e above d e f i n i t i o n . F i b e r s range i n l e n g t h f r o m 0.7 mm t o 3 mm w i t h an a v e r a g e s i i g h t l y l e s s t h a n 2 mm f o r domestic species. i n d i a m e t e r , an a v e r a g e o f l e s s t h a n 20 ym can be e x p e c t e d . The p e r c e n t a g e o f t h e volume o f wood o c c u p i e d by f i b e r s v a r i e s cons i d e r a b l y . I η sweetgum, f i b e r s c o n s t i t u t e o n l y 1 1



14

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

Figure 15. View from the inside of a lon gitudinal tracheid showing pits connecting a longitudinal tracheid to a ray cell. Note the lack of openings within the pit mem brane. 2,500χ

Figure 16. Types of vessel segments found in hardwoods. A and B: springwood vessels from a ring-porous wood. Note the short length compared with the diameter. C and D: typical vessel elements from dif fuse-porous woods with simple perforation plates at each end. E: typical diffuse-por ous vessel element with scalariform per foration plates at each end. 1 4 0 χ


Ε


1.

THOMAS

Wood:

Structure

and Chemical

Composition

15

26% w h e r e a s în h i c k o r y up t o 67% o f t h e volume i s made up o f fibers. In most c a s e s t h e h i g h e r t h e p e r c e n t a g e volume o f f i b e r s present the higher the s p e c i f i c g r a v i t y . E x c e p t i o n s , when n o t e d , a r e u s u a l l y due t o t h e f i b e r s b e i n g r e l a t i v e l y t h i n - w a l l e d . F i g u r e s 6 and 18 d e p i c t a wood i n w h i c h f i b e r s o c c u p y more t h a n 50% o f t h e vo1ume. A l s o i 1 1 u s t r a t e d a r e t h e o b v i o u s d î f f e r e n c e s i n t h e relatî ve s i z e , shape and w a l l t h i c k n e s s between fî b e r s and v e s s e l s. Cons i d e r a b l e v a r i a t i o n i n t h e amount o f t r a n s v e r s e and l o n g i tudî n a l parenchyma e x i s t s among hardwood s p e c i e s . For example, basswood has a p p r o x i m a t e l y t h e same as s o f t w o o d s , t h a t i s a b o u t 10%, w h i l e some oak s p e c i e s a p p r o a c h k0% parenchyma. As i n s o f t wood s, t h e parenchyma a r e u s u a l l y b r i c k - s h a p e d e e l 1 s a l though some v a r i a t i o n s o f t h i s shape o c c u r s . The r a y s , composed o f t r a n s v e r s e p a r e n c h y m a , r a n g e f r o m one t o thî r t y - p l u s e e l 1 s w i d e . The r a y i 1 1 u s t r a t e d i n F i g u r e 18 i s s e v e n e e l 1 s w i d e . Thus t h e h i g h e r parenchyma volume i s due t o w i d e r r a y s and t h e a d d i t i o n a l p r e s e n c e o f a x i a l parenchyma w h i c h i s r a t h e r r a r e i n s o f t w o o d species. Based on t h e wood a n a t o m i c a l d e s c r i p t i o n s p r e s e n t e d , i t i s o b v i o u s t h a t hardwoods and s o f t w o o d s d i f f e r cons i d e r a b l y f r o m e a c h o t h e r . F o r e x a m p l e , v e s s e l s a r e p r e s e n t i n hardwoods and absent i n softwoods. I η hardwoods more e e l 1 t y p e s , s h o r t e r e e l 1s, more parenchyma and a more v a r i a b l e a r r a n g e m e n t o f e e l 1 t y p e s o c c u r . The r e l a t i ve u n i f o r m i t y o f s o f t w o o d anatomy i s t h e r e s u l t of t h e preponderance o f a s i n g l e type c e l l , the l o n g i t u d i n a l tracheid. C e l l Wall

Structure

A l t h o u g h some v a r i a b i 1 i t y e x i s t s , t h e i n t e r n a l e e l 1 w a l l s t r u c t u r e d e s c r i b e d below r e p r e s e n t s t h e t y p i c a l s t r u c t u r e f o u n d i n most woody p l a n t c e l l s . A t t h e t i m e t h e c e l l i s p r o d u c e d by c e l l d i v i s i o n , i t cons i s t s o f a p r i m a r y w a l l wh i c h i s c a p a b l e o f e n l a r g i ng b o t h l o n g i t u d i n a l l y and t r a n s v e r s e l y . A f t e r the eel 1 r e a c h e s f u l 1 s i z e , a s e c o n d a r y w a l l i s depos î t e d î n t e r n a l t o t h e p r i m a r y w a l l add i ng t h i c k n e s s and r i g i d i t y t o t h e e e l 1 w a l l . F i g u r e 19 i 1 1 u s t r a t e s t h e e e l 1 w a l I s o f two m a t u r e , c o n t i g u o u s eel 1 s from a softwood spec i e s . Note t h e t h r e e d i s t i n e t 1 a y e r s w h i c h make up t h e c e l l w a l l . A d j a c e n t t o and on e a c h s i d e o f t h e middle l a m e l l a , a primary w a l l from both c e l l s i s p r e s e n t . However, t h i s w a l 1 i s t o o t h î η t o be eas i l y o b s e r v e d . T h e r e f o r e , the t e r m compound m i d d l e 1amel1 a, w h i c h r e f e r s t o t h e m i d d l e l a m e l l a and t h e two p r i mary wa11 s, î s o f t e n ut i 1 i z e d . A d j a c e n t t o the compound m i d d l e l a m e l 1 a and e a s i l y d e t e c t e d i s t h e f i r s t 1aye r o f t h e s e c o n d a r y w a l 1 termed t h e S^ l a y e r . The n e x t and c e n t r a l l a y e r , w h i c h i s t h e l a r g e s t , i s c a l l e d S^. The înnermost l a y e r a d j a c e n t t o t h e lumen i s termed The t o t a l e e l 1 wa11 t h i c k n e s s i s l a r g e l y c o n t r o l l e d by t h e t h i c k n e s s o f t h e $ l a y e r . T h a t i s, t h i c k - w a l l e d e e l 1 s r e s u l t 9



16

WOOD

TECHNOLOGY:

CHEMICAL

Figure 17. Fit membrane from an intervessel bordered pit. Note the absence of a torus and detectable openings in the mem brane. 2,400χ

Figure 18. Hardwood specimen showing a vessel cell, fibers, and ray cells. Note the retotive dif ferences in size and shape of the cells. The tangential view shows a ray which is up to seven cells wide. (Courtesy of N. C. Brown Center for Ultrastructural Stud ies, S.U.N.Y. College of Environ mental Science and Forestry)


ASPECTS

1.

THOMAS

Wood:

Structure

and Chemical

Composition

17

from a s u b s t a n t i a l i n c r e a s e î n the t h i c k n e s s o f the l a y e r and 1 i t t l e o r no i n c r e a s e i n t h e S. and S. 1 a y e r s . F o r e x a m p l e , n o t e i n F i g u r e 2 0 , w h i c h d e p i c t s a d j a c e n t v e s s e l and f i b e r c e l l w a l l s , t h e t h i n S 1 a y e r î η t h e t h i η v e s s e l w a l 1 and t h e t h î c k S£ i n t h e th î c k f i b e r w a l 1 . Not i ce a l s o t h a t t h e S^ a n d S- o f b o t h e e l 1 s a r e e s s e n t i a l l y t h e same s i z e . The a v e r a g e r e l a t i v e s i z e o f t h e v a r i o u s e e l 1 w a l 1 l a y e r s i s i n d î c a t e d i n T a b l e I . The t h i n n e s t i s o f c o u r s e t h e p r i m a r y w a l 1 w i t h t h e S~, S^ and in order of i n c r e a s i ng t h î c k n e s s .


Table I T h i c k n e s s o f V a r i o u s C e l l Wal1 L a y e r s and M i c r o f i b r i 1 A n g l e Wi t h î η t h e L a y e r s

Wal1

Layer

P.W. S S*

Relative Thickness (%)

Average Angle o f MicrofibriIs

>1

Random

10-22 70-90 2-8

50-70° 10-20° 60-90°

D e t e c t i o n o f t h e v a r i o u s e e l 1 w a l 1 l a y e r s i s due t o t h e p r e s e n c e o f m i c r o f i b r i 1 s wh i c h a r e o r i e n t e d a t d i f f e r e n t a n g l e s wi t h i η e a c h l a y e r . A m i c r o f i b r i 1 i s t h e bas i c n a t u r a l l y o c c u r r i ng un i t wh î c h c a n be eas i l y s e e n wi t h an e l e c t r o n m i c r o s c o p e . Note i n F i g u r e 21 t h e s t r i n g - l i k e a p p e a r a n c e o f t h e m i c r o f i b r i I s . In s i z e , m i c r o f i b r i I s range f r o m 1 0 0 t o 3 0 0 A i n d i a m e t e r , whereas t h e i r l e n g t h has n o t been d e t e r m i n e d . Wi t h i η e a c h o f t h e e e l 1 wal1 l a y e r s , the m i c r o f i b r i 1 s a r e o r i e n t e d at di f f e r e n t angles t o t h e l o n g a x i s o f t h e e e l 1. T a b l e I i n d i c a t e s t h e a v e r a g e m i c r o f i b r i 1 o r i e n t a t i o n wi t h i η t h e v a r i o u s e e l 1 w a l 1 l a y e r s . Figure 22 p r e s e n t s an i d e a l i z e d d r a w i n g o f t h e microfibrî1 o r i e n t a t i o n wi t h i η t h e v a r i o u s e e l 1 w a l 1 l a y e r s . Q

Chemical

Compos i t i o n o f C e l l Wal1

Chemically the eel 1 wal1 i s rather heterogeneous, c o n s i s t i n g p r i m a r i l y o f t h r e e po1yme r î c materîals: e e l 1 u l o s e , h e m i c e l l u l o s e and 1 i g n i n. T h e s e m a t e r îals a r e composed o f l a r g e m o l e c u l e s and c o n s t i t u t e f r o m 9 5 t o 98% o f t h e c e l l w a l l . The r e m a i n i n g 2 - 5 % a r e l o w e r m o l e c u l a r w e i g h t compounds c a l l e d e x t r a c t i v e s . The amount o f e a c h component, e s p e c i a l l y t h e h e m i c e l l u l o s e s , 1 i g n i n and e x t r a c t i v e s , v a r i es c o n s î d e r a b l y between hardwoods and s o f t woods ( T a b l e I I ) . O t h e r f a c t o r s s u c h as s p e c i e s , l o c a t i o n o f e e l 1 s wi t h i n t h e t r e e and g r o w t h e n v i ronment a l s o î n f 1 u e n c e t h e f i n a l chemi c a l c o m p o s i t i o n .



18

WOOD

TECHNOLOGY:

CHEMICAL

Figure 19. Cell walls in cross-sectional view from contiguous springwood longitudinal tracheids depicting wall layering. C: compound middle lamella. 1: Sj layer; 2: S layer; and 3: S layer. Note the S layers are the largest. 16,000 X 2

s

2

Figure 20. Cell walls of a vessel (V) and adjacent fiber (F) in cross-sectional view. Note the very large S layer in the thickwalled fiber and the small S layer in the thin-walled vessel. 13β00χ B

2


ASPECTS


1.

THOMAS

Wood:

Structure and Chemical

Composition

Figure 21. Cellulose microfibrils in the primary wall and Sj portion of the secondary wall from a longitudinal tracheid. Note the loosely packed and randomly arranged microfibrils in the primary wall (P). The S layer (S) consists of tightly packed, parallel microfibrils. 12,000X


19

20

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

Table ! I Average

% Chemical C o m p o s i t i o n o f Softwoods Softwoods


Cel1ulose Hemicel1ulose Lignî η Extractî ves

42 27 28 3

± ± ± ±

2 2 3 2

and Hardwoods Hardwoods 45 30 20 5

± ± ± ±

2 5 4 3

C e l 1 u l o s e i s a 1 i n e a r polymer o f anhydro-D-glucopyranose uni t s 1i nked by $-(l->4) g l y c o s id i c bonds. The number o f g l u c o s e res i dues v a r i es f r o m 7,000 t o 10,000. The c e l l u i o s e m o l e c u l e s are 1i nked l a t e r a l l y by h y d r o g e n bonds i n t o 1 i n e a r b u n d l e s . The e x t r e m e l y l a r g e number o f h y d r o g e n bonds r e s u l t s i n a s t r o n g l a t e r a l a s s o c i a t i o n o f t h e 1i n e a r c e l l u i o s e m o l e c u l e s . T h i s s t r o n g a s s o c i a t i o n and a l m o s t p e r f e c t alîgnment o f t h e c e l 1 u l o s e molec u l e s g i v e s r i s e t o c r y s t a l 1i ni t y . X - r a y measurements show t h a t the c r y s t a l 1i ne r e g i o n s a r e i n t e r r u p t e d e v e r y 600 a n g s t r o m s wi t h n o n - c r y s t a l 1i ne (amorphous) r e g i o n s . Whether t h i s i s due t o mi nor i m p e r f e c t i o n s i n t h e c r y s t a l 1i ne l a t t i c e o r a r e a l s t r u c t u r a l e n t i t y i s not c o m p l e t e l y c l e a r . The most w i d e l y h e l d c o n c e p t i s the 1atter i n which the c e l 1 u l o s e molecules a r e h i g h l y o r i e n t e d ( c r y s t a 1 1 i n e ) f o r a d i s t a n c e o f about 600 A, t h e n p a s s t h r o u g h an a r e a o f poor o r i e n t a t i o n (amorphous) and r e - e n t e r a c r y s t a l 1i ne r e g i o n . The p a t t e r n r e p e a t s t h r o u g h o u t t h e l e n g t h o f t h e c e l 1 u 1 ose m o l e c u l e . E l e c t r o n microscopy studies revealed t h r e a d l i k e s t r u c t u r e s ca1 l e d microfî b r i 1 s ( F i g u r e 2 1 ) . Mi c r o f i b r i1 w i d t h s , depend î ng upon m a t e r i a l s o u r c e and methods u s e d , v a r y f r o m 100 t o 300 angs t r o m s and a r e a b o u t h a l f as t h i c k as t h e y a r e w i d e . Microfibri1 l e n g t h has n o t been d e t e r m i n e d n o r has t h e înternal s t r u c t u r e o f the microfibrî1 been c l e a r l y e s t a b l i s h e d . Some învestigators bel i e v e t h a t t h e m i c r o f i b r i 1 cons i s t s o f a c r y s t a l 1i ne c o r e o f c e l 1 u l o s e s u r r o u n d e d by an amorphous r e g i o n c o n t a i ni ng hemi c e l 1 u loses. Depend i ng upon t h e î n v e s t i g a t o r ^ e s t i m a t e s o f t h e c r y s t a l 1 i ne c o r e s i ze r a n g e s f r o m 30 by 50 A t o 40 by 100 Â. F u r t h e r v a r i a t i o n i n t h e s i z e o f t h e microfî b r i1 i s due t o t h e amount o f hemi c e l 1 u l o s e s s u r r o u n d i ng t h e c r y s t a l 1î ne c o r e as w e l l as a g g r e g a t i o n s o f s îngle m i c r o f i b r i 1 s t o f o r m m i c r o f i b r i 1 s wi t h 1 a r g e r d iameters. S t u d i e s wi t h v e r y h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p e s c o u p l e d wi t h n e g a t i ve s t a î ni ng t e c h n i q u e s has l e d t o t h e v i e w t h a t a 35 Â w i d e c e l 1 u l o s e f i b r i l c a l l e d t h e e l e m e n t a r y f i b r i l i s the bas î c s t r u c t u r e . One p r o p o s a l f r o m t h i s work v i e w s t h e m i c r o f i b r i l as cons i s t i ng o f f o u r e l e m e n t a r y f i b r i l s wi t h h e m i c e l 1 u l o s e s , l i g n i n and w a t e r a r o u n d t h e o u t s i d e as w e l l as between t h e elementary f i b r i I s . More r e c e n t w o r k has i n d i c a t e d s o - c a l l e d s u b - e l e m e n t a r y f i b r i I s i n t h e 10 A r a n g e . O b v i o u s l y t h e phys i c a l s t r u c t u r e o f c e l 1 u l o s e i n t h e woody pi a n t c e l 1 w a l 1 i s f a r f r o m



1.

THOMAS

Wood:

Structure

and

Chemical

Composition

21

certaî n. H e m i c e l 1 u l o s e s , 1i ke c e l 1 u l o s e , a r e p o l y m e r s o f a n h y d r o s u g a r uni t s . They d i f f e r f r o m c e l 1 u l o s e î n t h a t a g i ven h e m i c e l 1 u l o s e m o l e c u l e may c o n t a i n s e v e r a l d i f f e r e n t s u g a r un i t s . In add i t i o n , h e m i c e l 1 u l o s e s a r e u s u a l 1 y b r a n c h e d m o l e c u l e s containîng o n l y 150 t o 200 s u g a r u n i t s . H e m i c e l l u l o s e s found î n wood a r e p o l y m e r s o f D - g l u c o s e , D - g a l a c t o s e , D - x y l o s e , D-mannose, L - a r a b i nose and 4-0methy1-D-glucuronic a c i d . The h e m i c e l 1 u l o s e s a l o n g wi t h 1 i g n i n s u r r o u n d t h e c r y s t a l 1i ne c e l 1 u l o s e . L i g n i n s a r e t h e m a j o r n o n - c a r b o h y d r a t e component o f wood. They a r e v e r y c o m p l e x , c r o s s 1i n k e d , t h r e e d imens i o n a l p o l y m e r s formed f r o m p h e n o l i c un i t s . The number o f b u i I d i ng un i t s v a r i e s cons i d e r a b l y f r o m a few up t o s e v e r a l h u n d r e d s . The a roma t î c nat u r e o f t h e p h e n o l i c u n i t s r e n d e r s 1i g n i η h y d r o p h o b i c and t h e t h r e e d imens i o n a l n e t w o r k p r o v i d e s r i g i d î t y and o p t îmum r e s i s t a n c e to compressive f o r c e s . The s t r u c t u r a l make-up o f 1 i g n i n f r o m hardwoods d i f f e r s f r o m s o f t w o o d 1 i g n i n î η t h a t t h e bas i c bu i I d i ng u n i t i s s y r i n g y l w h e r e a s gua i a c y 1 i s t h e p r imary bu îId i ng un i t f o r softwoods. B e c a u s e t h e gua i a c y 1 u n i t has a g r e a t e r number o f p o t e n t i a l r e a c t i ve s i t e s , a h i gher d e g r e e o f c r o s s l i n k i n g e x i s t s . F u r t h e r m o r e 1 i g n i η formed pr i marîly f r o m gua i a c y 1 h a s , on t h e a v e r a g e , a h i g h e r m o l e c u l a r we î g h t . T y p i c a l l y , s o f t w o o d 1i g n i η has a b o u t t e n t i m e s more g u a i a c y l t h a n s y r i n g y l un i t s w h e r e a s i n hardwoods t h e r a t i o i s u s u a l l y one t o one. Thus hardwood 1 i g n i n s a r e more eas i l y d e g r a d e d t h a n s o f t w o o d 1i gn i n s . The d i s t r i but i o n o f t h e c h e m i c a l c o n s t i t u e n t s a c r o s s t h e c e l 1 wal1 i s not un i f o r m . Wi t h r e g a r d t o c e l 1 u l o s e , t h e p r i m a r y wal1 c o n t a î ns a b o u t 10% i n c r e a s i ng t o more t h a n 50% i n t h e laye r and d e c r e a s i ng s i i g h t l y i n t h e S 1 a y e r . The 1 i g n i n c o n t e n t o f t h e m i d d l e 1ame11 a and p r i m a r y wal1 i s on t h e o r d e r o f 70%. The S c o n t a i n s a b o u t 22% and t h e S a p p r o x i m a t e l y 15%. I t should be n o t e d t h a t due t o t h e v e r y 1arge volume o f t h e S l a y e r , over h a l f o f t h e t o t a l 1 i g n i n i s found i n t h î s l a y e r a l t h o u g h t h e c o n centrât i o n i s l o w e r t h a n i n t h e compound m i d d l e 1 a m e l i a . A l s o , b e c a u s e t h e compound m i d d l e l a m e l l a i s so t h î η o n l y 10% o f t h e t o t a l 1i g n i n i s p r e s e n t d e s p i t e t h e v e r y h i g h c o n c e n t r a t i o n . The h e m i c e l 1 u l o s e f r a c t i o n t e n d s t o v a r y a b o u t t h e same as t h e c e l 1 u l o s e a c r o s s the c e l 1 wal1. I η add i t i o n t o t h e m a j o r c e l 1 wal1 components o f c e l 1 u l o s e , h e m i c e l 1 u l o s e and 1 i g n i n, wood c o n t a i n s v a r y i ng amounts o f sub s t a n c e s termed extractî v e s . The t e r m e x t r a c t i ves i n c l u d e s a w i d e range o f chemi c a l t y p e s and a v e r y 1arge number o f i nd i v i d u a l compounds. Some o f t h e m a j o r chemi c a l t y p e s a r e 1) T e r p e n e s and r e l a t e d compounds, 2) F a t t y a c i d s , 3) A r o m a t i c compounds and 4) Volâtile o i l s . S p e c i e s d i f f e r w i d e l y i n t h e t y p e and amount o f extractî ves p r e s e n t . A l s o t h e r e i s cons i d e r a b l e v a r i a t i o n i n t h e d i s t r i b u t i o n o f e x t r a c t îves t h r o u g h o u t t h e wood o f i n d i v i d u a l trees. A l t h o u g h some e x t r a c t i v e s a r e f o u n d i n sapwood, t h e h e a r t wood u s u a l l y c o n t a i ns t h e l a r g e s t amount. A l t h o u g h t h e e x a c t c a u s e o f h e a r t w o o d f o r m a t i o n i s not known,



22

WOOD TECHNOLOGY: CHEMICAL ASPECTS

the r e s u l t i ng changes have been w e l l d o c u m e n t e d .The parenchyma eel 1 s change thei r metabolic a c t i v i ty and tend to produce e x t r a c t i v e s from stored c a r b o h y d r a t e s . The parenchyma cel1 s d i e and the e x t r a c t ives d i ffuse i n t o adjacent cel1 s . Thus, the heartwood conta i ns no 1i v i ng cel1 s and in many species is d î s c o l o r e d as a r e s u l t o f the e x t r a c t i ve c o n t e n t . I η c e r t a i η softwoods res in canal s are another source o f e x t r a c t i v e s . These s t r u c t u r e s con ta i η cons i d e r a b l e quanti t i e s o f res in in the 1 î v i ng t r e e . Despi te the r e l a t i v e l y low percentage content o f e x t r a c t î ves (Table I I ) , they very often i nf1uence wood propert ies and thus play a r o l e i n u t i 1 i z a t i o n . Advantages accrue from the presence of c o l o r e d and vol at i l e e x t r a c t ives which provide e s t h e t i c values. Some of the phenolic compounds provide r e s i s t a n c e to i n s e c t and fungal a t t a c k . Other e x t r a c t i ves provî de useful products. From ta 11 o i 1 , products such as turpent i ne, ros in and f a t t y a c i d s are p r o d u c e d .I η add i t i o n , tanni ns, camphor, gum a r a b i c , n a t u r a l rubber and f1avonoids are some of the many products from e x t r a c t i ves. E x t r a c t i ves somet imes prevent or i nhi bi t the ut î1î zat ion o f w o o d . For example, woods contai ni ng phenolic type e x t r a c t i ves cannot be pulped v i a the s u l f î te p r o c e s s . The s o - c a l l e d " p i t c h t r o u b l e s " in the p u l ρ and paper î n d u s t r y r e s u l t from the tendency of the res i η type e x t r a c t i ves to coagulate and adhere to metal and f i b r o u s s u r f a c e s . A l s o the presence of e x t r a c t i v e s r e s u l t in a h i gher consumpt ion of pulpi ng chemical s and i n lower p u l ρ yields. Despi te the numerous products d e r i ved from e x t r a c t i v e s , much of the bas i c chemi s t r y o f the numerous compounds found in many species i s s t i l l v i r t u a l l y u n k n o w n .Future work in th i s area should provî de many new products. Literature Cited Wood Anatomy 1. 2. 3.

I senberg, I. H. "The S t r u c t u r e of Wood i n The Chemistry o f Wood" e d i t e d by B. L. Browning. I n t e r s c i e n c e P u b l i s h e r s , John Wiley and Sons. New York, New York. 1963. Jane, F. W. "The S t r u c t u r e o f Wood" 2nd Edition. Adam and Charles Black. London, England. 1970. Panshin, A . G. and Carl d e Z e e u w ."Textbook o f Wood Technology" 3rd Edition. McGraw-Hill. New York, New York. 1970.

Wood Chemistry 1.

2.

Browning, B. L. Editor. "The Chemistry o f Wood" I n t e r science P u b l i s h e r s , John Wiley and Sons. New York, New York. 1963. Hanna, R. B. and W. A . Côté, Jr. The Sub-elementary Fibril of P l a n t Cell Wall Cellulose. C y t o b i o l o g i e (1974) 10(1):102.


1. 3.

4.

Wood:

Structure

and Chemical

Composition

23

Hillis, Ε. W. "Wood Extractives and Their Significance to t h e P u l p and P a p e r I n d u s t r y " A c a d e m i c Press. New Y o r k , New York. 1962. P r e s t o n , R. D. "The Physical B i o l o g y o f Plant Cell W a l l s " Chapman and Hall. London. 1974. W i s e , L. E. and E. C. Jahn. "Wood C h e m i s t r y Volume I " ACS Monogram Series. Reinhold Publishing Corporation. New Y o r k , New Y o r k . 1 9 5 2 .


5.

THOMAS


2 Prevention of Stain and M o u l d in Lumber and Board Products A . J. C S E R J E S I


Western Forest Products Laboratory, Vancouver, British Columbia, V 6 T 1X2, Canada

Wood, like a n y o t h e r natural organic material is perishable, and may s e r v e a s a nutrient source for different microorganisms. Some wood-inhabiting m i c r o o r g a n i s m s (decay f u n g i ) can utilize as nutrients t h o s e wood c o m p o n e n t s w h i c h f o r m t h e structural elem e n t s o f wood (cellulose, 1ignin), while others (sapstain fungi and m o u l d s ) consume t h o s e wood c o m p o n e n t s w h i c h r e p r e s e n t t h e cell contents of the tree (starch, other carbohydrates, etc.) and thus no strength loss o f wood o c c u r s . To p r e v e n t g r o w t h o f wood-inhabiting f u n g i it is c u s t o m a r y to treat wood w i t h c h e m i c a l s w h i c h a r e toxic t o fungi. To protect wood in service f r o m deterioration by decay fungi, the chemicals, called wood preservatives, a r e applied in m o s t c a s e s by p r e s s u r e t r e a t m e n t b e c a u s e deep penetration into t h e wood i s essential. Wood p r o d u c t s i n t e n d e d f o r u s e where t h e d e c a y h a z a r d is l o w a r e unlikely to require p r e s s u r e treatment but i n many instances may require superficial fungicidal treatment to prevent fungal discoloration. H i s t o r y of

S a p s t a i n and Mould P r e v e n t i o n

W o o d - i n h a b i t i n g f u n g i n e e d w a t e r , a i r , n u t r i e n t s and a s u i t a b l e temperature for growth. Sapwood i n f r e s h l y c u t l u m b e r has t h e p r o p e r b a l a n c e o f a i r and w a t e r , c o n t a i n s n u t r i e n t s , and t h e r e f o r e i n warm w e a t h e r i t i s r e a d i l y a t t a c k e d b y f u n g i . Abs e n c e o f a n y one o f t h e s e f o u r r e q u i r e m e n t s s t o p s t h e g r o w t h o f t h e s e f u n g i , however t h e f u n g i a r e n o t n e c e s s a r i l y k i l l e d . Among t h e s e f o u r r e q u i r e m e n t s t h e m o i s t u r e c o n t e n t ( M . C . ) o f t h e wood i s t h e e a s i e s t t o c o n t r o l , a n d i f i t i s b e l o w a b o u t 20% f u n g a l growth i s stopped. I n t h e e a r l y d a y s a i r - d r y i n g was t h e m e t h o d u s e d t o r e d u c e t h e M . C . o f wood. H o w e v e r , a i r - d r y i n g i n humid and r a i n y w e a t h e r was r e l a t i v e l y s l o w , o f t e n t a k i n g a s l o n g a s two o r t h r e e m o n t h s t o d r y lumber t o l e s s t h a n 20% M . C . During t h i s period fungi w e r e a b l e t o grow o n ( m o u l d s ) and i n ( s t a i n i n g f u n g i ) t h e l u m b e r . T h i s p e r i o d however, i s too short f o r decay f u n g i to cause an

24



2.

csERjEsi

Prevention

of

Stain and

25

Mould

a p p r e c i a b l e amount of decay. P l a n i n g a i r - d r i e d lumber l a r g e l y removes surface d i s c o l o r a t i o n caused by moulds, but d i s c o l o r a t i o n i n the wood caused by s t a i n i n g f u n g i cannot be removed. K i l n - d r y i n g does not depend on the weather and, i n a d d i t i o n , due t o the h i g h temperature used, the wood i s e s s e n t i a l l y s t e r i l i z e d . However, i f k i l n d r y i n g i s delayed mould and s a p s t a i n f u n g i may d i s c o l o r the wood. For the p r e v e n t i o n of s a p s t a i n development d u r i n g a i r - d r y i n g , sawmills s t a r t e d to use f u n g i c i d e s as e a r l y as the end of the l a s t century. The f i r s t attempt t o prevent s a p s t a i n i n lumber was i n 1888, according to Bryant ( 4 ) . At the beginning of t h i s century, the use of sodium carbonate and bicarbonate became widespread i n s a w m i l l s , and remained the s a p s t a i n and mould preventi v e u n t i l about 1930 ( 4 ) . The s t u d i e s c a r r i e d out by the Southern Forest Experiment S t a t i o n and the Forest Products Laboratory i n the U.S.A. from 1928 to e a r l y 1930's (31) r e s u l t e d i n a change over from the use of sodium carbonate to the use of the sodium s a l t s of c h l o r i n a t e d phenols and organo-mercurie compounds. Since s a p s t a i n and mould p r e v e n t i v e s based on the t o x i c i t y of i n d i v i d u a l compounds were r e p o r t e d t o f a i l o c c a s i o n a l l y , the use of mixtures of m e r c u r i a l s and the sodium s a l t s of c h l o r i n a t e d phenols was suggested (39,40). Since then the sodium s a l t s of c h l o r i n a t e d phenols, sodium pentachlorophenate and sodium t e t r a c h l o r o p h e n a t e , buffered i n most formulations w i t h borax, w i t h or without organo-mercurials, has dominated the s a p s t a i n and mould p r e v e n t i v e market. In B r i t i s h Columbia the lumber i n d u s t r y v o l u n t a r i l y stopped u s i n g f o r m u l a t i o n s c o n t a i n i n g mercury i n the l a t e I 9 6 0 s . However s a p s t a i n and mould p r e v e n t i v e s c o n t a i n i n g m e r c u r i a l s are s t i l l i n use i n the U.S.A. The o r i g i n a l requirement f o r a s a p s t a i n and mould p r e v e n t i v e treatment was to prevent f u n g a l growth on lumber f o r the p e r i o d of a i r - d r y i n g , or f o r the storage p e r i o d between sawing and k i l n d r y i n g . Thus the chemicals were r e q u i r e d t o be e f f e c t i v e f o r about a two month p e r i o d . T r a d i t i o n a l l y lumber from the P a c i f i c Northwest i s shipped unseasoned, by ocean t r a n s p o r t . Shipping charges are based on volume not on weight, and n e i t h e r a i r nor k i l n - d r y i n g would be j u s t i f i e d economically. A f t e r the Second World War, i n order to reduce handling c o s t s , packaging of lumber was introduced. Although i t was recommended that the importer break up the packages on a r r i v a l and p i l e the lumber f o r d r y i n g , i n p r a c t i c e i t became customary t o break up the packages only when the lumber was a c t u a l l y used. This p e r i o d i s g e n e r a l l y l e s s than one year, but i s sometimes as much as two years (26). During storage of packaged green lumber, c o n d i t i o n s remain favourable f o r fungal growth and s i n c e the p i e c e s are i n c l o s e contact i n the packages, f u n g i , i n c l u d i n g decay f u n g i , may spread from one p i e c e to another i f f u n g i c i d a l treatment was inadequate. 1

%



26

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

T h i s extended time requirement f o r e f f e c t i v e c o n t r o l of f u n g a l growth by s a p s t a i n and mould p r e v e n t i v e s put more s t r i n g e n t requirements on the f u n g i c i d e s as w e l l as on the a p p l i c a t i o n methods. C h l o r i n a t e d phenols a r e e f f e c t i v e f u n g i c i d e s f o r t h i s purpose, as proven by use (17,26), however they may cause s k i n i r r i t a t i o n ( 4 ) , and an i n c r e a s e of the c o n c e n t r a t i o n i n the t r e a t i n g s o l u t i o n without the proper p r e c a u t i o n s may cause other problems i n h a n d l i n g f r e s h l y - t r e a t e d lumber (17,35). Searching f o r s a f e r and more e f f e c t i v e f u n g i c i d e s t o r e p l a c e c h l o r i n a t e d phenols f o r t h e p r e v e n t i o n o f s a p s t a i n and mould, t h e r e f o r e i s Important f o r both the chemical companies manufacturing s a p s t a i n and mould p r e v e n t i v e s , as w e l l as f o r the lumber i n d u s t r y u s i n g them. An e x t e n s i v e study i n Germany i n the e a r l y I 9 6 0 s (28,29) was c a r r i e d out by t e s t i n g about 300 chemicals f o r t h e i r t o x i c i t y to f u n g i and s t u d y i n g t h e i r p h y s i c a l and chemical p r o p e r t i e s . None of the t e s t e d chemicals were found t o be more e f f e c t i v e than c h l o r i n a t e d phenols. In New Zealand, Butcher found t h a t c a p t a f o l i s a s u i t a b l e replacement f o r c h l o r i n a t e d phenols (5_>6>7) and i t has been w i d e l y accepted f o r use i n t h a t country. In North America, Chapman Chemical Co. i s promoting d i f f e r e n t metal complexes of 8-hydroxyquinoline ( 9 ) , but the acceptance o f these f o r m u l a t i o n s i s slow. The Eastern Forest Products Laboratory, Ottawa, found t h a t ammoniacal z i n c oxide i s an i n h i b i t o r t o f u n g i i n pine lumber (16,33). Sanford Products Corp. i s o f f e r i n g 2^hiocyanomethy3(thio) b e n z o t h i a z o l e f o r p r e v e n t i o n of s a p s t a i n and mould i n Canada and U.S.A., and t r i b u t y l t i n o x i d e emulsion i s marketed i n Japan. Experiments were c a r r i e d out i n Europe t o use benomyl, d i ( g u a n i d i n o - o c t y l ) amine and p-chlorophenyl-3-iodo-propargyl formal t o be used i n s a p s t a i n and mould p r e v e n t i v e f o r m u l a t i o n s . L a b o r a t o r i e s , i n c l u d i n g the Western Forest Products Laboratory, are a l s o t e s t i n g new chemicals. However s a p s t a i n and mould p r e v e n t i v e f o r m u l a t i o n s c o n t a i n i n g c h l o r i n a t e d phenols as t h e main f u n g i c i d a l components, s t i l l dominate the market. 1

Sapstain and Mould P r e v e n t i v e Treatment i n P r a c t i c e An e f f e c t i v e s a p s t a i n and mould p r e v e n t i v e , does not n e c e s s a r i l y make a s u c c e s s f u l treatment. For a s u c c e s s f u l treatment the f o l l o w i n g requirements a r e a l s o necessary: 1. Lumber f r e e from s a p s t a i n , mould and a c t i v e decay; 2. Not more than 24 hours delay of the treatment a f t e r sawing ; 3. The s u r f a c e of lumber should be u n i f o r m l y covered w i t h the t r e a t i n g s o l u t i o n ( a p p l i c a t i o n method); 4. Proper c o n c e n t r a t i o n of the f u n g i c i d e i n the t r e a t i n g solution;


2.

csERjEsi

Prevention

of

Stain and

Mould

27

At l e a s t a temporary p r o t e c t i o n of lumber from the weather a f t e r treatment. P o i n t s 1 and 4 need no f u r t h e r comment. The requirement of 2 i s a l s o obvious, c o n s i d e r i n g that t h i s treatment i s r e s t r i c t e d o n l y t o a t h i n surface l a y e r of the wood and a delay of the t r e a t ment a l l o w s s t a i n i n g f u n g i to penetrate beyond the t r e a t e d l a y e r , where t h e i r growth i s u n i n h i b i t e d . The t h i r d requirement r e f e r s t o the a p p l i c a t i o n method which i s used to apply the s a p s t a i n and mould p r e v e n t i v e s . According to the author's experience, the use of a good a p p l i c a t i o n t e c h nique i s as important as, or perhaps more important than, the e f f e c t i v e n e s s of the f u n g i c i d e i n the f o r m u l a t i o n . The type of a p p l i c a t i o n methods used a r e : Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.ch002

5.

1

i ) Dipping* (a) hand d i p p i n g ; (b) d i p p i n g on the s o r t i n g c h a i n ; and (c) b u l k d i p p i n g (of packaged lumber). (2) Spraying. The author's preference f o r s a p s t a i n and mould p r e v e n t i v e treatment i s d i p p i n g . Experience, i n a n a l y z i n g commercially t r e a t e d lumber, i n d i c a t e s that the q u a l i t y of treatment by d i p p i n g i s s u p e r i o r t o spraying (12). Hand d i p p i n g i s used today only f o r experimental purposes. Dipping on s o r t i n g c h a i n i s one of the e a r l i e s t automated a p p l i c a t i o n methods (31). The advantage of t h i s l a t t e r i s , that i t r e q u i r e s a r e l a t i v e l y s m a l l volume of t r e a t i n g s o l u t i o n , which i s easy to handle. Although i t i s a good t r e a t i n g method, because of some problems i n handling wet lumber on the s o r t i n g c h a i n , i t had not been used i n B.C. f o r many years. Recently however i t was reintroduced and w i t h the development of automatic s o r t i n g equipment, t h i s t r e a t i n g method probably w i l l become more popular i n the f u t u r e . Bulk d i p p i n g of packaged lumber was introduced i n B.C. i n the I960's, p r i m a r i l y f o r the treatment of rough sawn lumber. A n a l y z i n g commercially and e x p e r i m e n t a l l y t r e a t e d lumber f o r c h l o r i n a t e d phenols a f t e r bulk d i p p i n g showed that t h i s t r e a t ment r e s u l t e d i n r e t e n t i o n comparable to t h a t of hand-dipped rough-sawn lumber. I t was a l s o observed that w i t h surfaced lumber, although each p iece i n the package r e c e i v e d some treatment, the c o n c e n t r a t i o n s of the f u n g i c i d e were much lower than i n hand dipped lumber, p o s s i b l y due to the r e s t r i c t e d space between p i e c e s (25). Bulk d i p p i n g i s done e i t h e r by d r i v i n g a c a r r i e r w i t h the lumber i n t o the " d i p tank", or d i p p i n g on a p l a t f o r m . The l a t t e r method u s u a l l y u t i l i z i n g automatic equipment, which lowers the lumber placed on the p l a t f o r m i n t o the s o l u t i o n , keeps i t down f o r a short time, and b r i n g s i t up. In some s m a l l s a w m i l l s , the package of lumber i s held under the f o r k s of a f o r k l i f t , which push them under the s o l u t i o n i n an above ground " d i p tank". A major advantage of b u l k d i p p i n g i s that the lumber i s not handled manually a f t e r treatment. A disadvantage i s the l a r g e


28

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

volume of t r e a t i n g s o l u t i o n needed to f i l l the " d i p tank". Spraying i s used mainly f o r the treatment of surfaced lumber and thus the equipment i s u s u a l l y i n s t a l l e d behind a high speed planer. The main problem w i t h t h i s method i s that the n o z z l e s of the spray system o f t e n become t e m p o r a r i l y plugged. Even a very short stoppage of spray, c o n s i d e r i n g the h i g h speed of the lumber (180-350 m/min), may r e s u l t i n l a r g e surface areas of the p i e c e s remaining untreated. These untreated areas are r e s p o n s i b l e f o r f a i l u r e i n the p r o t e c t i o n of lumber t r e a t e d by t h i s method.


Retention of Sapstain and Mould P r e v e n t i v e s on Lumber According to V e r r e l l (39) the e f f e c t i v e n e s s of the s a p s t a i n and mould p r e v e n t i v e treatment i s c o r r e l a t e d t o the r e t e n t i o n of f u n g i c i d e s on the lumber. T h i s r e t e n t i o n i s a f f e c t e d by some p h y s i c a l and p o s s i b l y chemical p r o p e r t i e s of wood i n a d d i t i o n to the a p p l i c a t i o n of f u n g i c i d e , as w e l l as by the method of i t s application. In l a b o r a t o r y experiments by the author i t was found that the moisture content of the wood has l i t t l e e f f e c t on the r e t e n t i o n of the t r e a t i n g s o l u t i o n on the lumber f o l l o w i n g a 15 second d i p (14). In another l a b o r a t o r y study (11) i t was found that although a l t e r n a t e w e t t i n g and d r y i n g caused complete l o s s of pentachlorophenol from wood, n e i t h e r l e a c h i n g nor d r y i n g alone caused s i g n i f i c a n t l o s s of pentachlorophenol a f t e r a 1 to 2 day f i x a t i o n p e r i o d i n the wet c o n d i t i o n . This suggests that lumber needs o n l y temporary p r o t e c t i o n from weather, although complete l o s s may occur from the surfaces of lumber i f exposed to weather due to repeated w e t t i n g and d r y i n g (12). Using the r e s u l t s of a l a r g e s c a l e f i e l d t e s t (26), a number of c o n c l u s i o n s about s a p s t a i n and mould p r e v e n t i v e treatments can be made. In t h i s experiment more than 10,000 p i e c e s of 2" χ 4" and 8 f t long lumber were t r e a t e d by hand d i p p i n g i n t o two s a p s t a i n and mould p r e v e n t i v e s , each used at four c o n c e n t r a t i o n s . Both contained sodium t et rac hlorophenat e as the main f u n g i c i d e . A l l p i e c e s were inspected f o r growth of s a p s t a i n and mould a f t e r d i f f e r e n t storage p e r i o d s , and more than 3000 p i e c e s were ana l y z e d f o r t e t r a c h l o r o p h e n o l . The c o n c e n t r a t i o n s of t e t r a c h l o r o phenol were ealeulated2on weight per surface area b a s i s (13) and were reported as mg/cm . Considering the l a r g e number of observations the f o l l o w i n g c o n c l u s i o n s were w e l l documented : 1. The same p r o t e c t i o n was observed against s a p s t a i n and mould f u n g i w i t h the same r e t e n t i o n (mg/cm ) r e g a r d l e s s of whether the lumber was rough sawn or surfaced (15). 2. Rough-sawn lumber r e t a i n e d an average of 2.5 to 3 times as much t e t r a c h l o r o p h e n o l as surfaced lumber d i d , when they were t r e a t e d i n the same s o l u t i o n . T h i s means t h a t surfaced lumber should be t r e a t e d i n a s o l u t i o n which i s



2.

csERjEsi

Prevention

of Stain and

Mould

29

three times more concentrated than t h a t used f o r roughsawn lumber f o r e q u i v a l e n t treatment (15). 3. Large v a r i a t i o n s i n the r e t e n t i o n s (the maximum being 10 χ higher than the minimum) were found i n the lumber t r e a t e d i n the same s o l u t i o n (26). T h i s v a r i a t i o n was about the same on both, surfaced and rough-sawn lumber (hand-dipped I). 4. On the premise that the p r o t e c t i o n depended o n l y on the r e t e n t i o n o f t e t r a c h l o r o p h e n o l , we c a l c u l a t e d t h a t f o r c l o s e 2 t o 100% p r o t e c t i o n , a minimum r e t e n t i o n o f 0.05 mg/cm i s necessary f o r 2 a 2 year p e r i o d and a minimum r e t e n t i o n of 0.04 mg/cm t e t r a c h l o r o p h e n o l f o r a one year p e r i o d (26). There i s no standard f o r s a p s t a i n and mould p r e v e n t i v e treatment, but f o r export lumber most c o n t r a c t s s p e c i f y t h a t t h e lumber should be " e f f e c t i v e l y t r e a t e d " . Savory and Cockroft i n 1961 (30) came t o the c o n c l u s i o n : " i t i s doubtful whether a c e r t i f i c a t e o f treatment employing the words ' e f f e c t i v e l y t r e a t e d ' has any r e a l meaning. I t i s suggested t h a t c e r t i f i c a t e s o f treatment would be more v a l u a b l e i f they s t a t e d the t r e a t i n g chemical and the c o n c e n t r a t i o n a t which i t was a p p l i e d . " T h i s suggestion i s not yet accepted. But f o l l o w i n g up t h e i r suggestion based on the r e s u l t s o f our experiment we c a l c u l a t e d t h a t 13 randomly taken samples would be r e q u i r e d t o estimate w i t h reasonable c e r t a i n t y whether the r e t e n t i o n was w i t h i n the d e s i r e d t a r g e t amount (15). Another f a c t o r which may reduce the e f f e c t i v e n e s s o f a s a p s t a i n and mould p r e v e n t i v e treatment i s the t o l e r a n c e and t h e a d a p t a t i o n o f f u n g i t o f u n g i c i d e s (10,39). However the t o l e r a n c e of f u n g i i s considered when the e f f e c t i v e c o n c e n t r a t i o n o f a f u n g i c i d e i s determined. Experimental

Methods

The experiment which made i t p o s s i b l e t o draw s e v e r a l con c l u s i o n s on s a p s t a i n and mould p r e v e n t i v e treatment was a l a r g e s c a l e t e s t that needed about three years t o complete. The q u a n t i t a t i v e c o n c l u s i o n s j u s t i f i e d such e x t e n s i v e exper imentat i o n . However, f o r screening new f u n g i c i d e s , such a l a r g e experiment would be too expensive and time consuming. F i e l d t e s t s u s i n g commercially produced lumber w i t h much s h o r t e r storage p e r i o d s (4 months) g i v e a very good i n d i c a t i o n o f the u s e f u l n e s s o f a f u n g i c i d e f o r s a p s t a i n and mould p r e v e n t i o n (7,25,31,40). Ex periments were a l s o c a r r i e d out w i t h s m a l l e r samples, up t o 3 f t long and s h o r t e r storage p e r i o d s (2-3 months). The r e s u l t s of these t e s t s a l s o gave a good i n d i c a t i o n o f the e f f e c t i v e n e s s of f u n g i c i d e s (5,31^,40). As an a l t e r n a t i v e t o these t e s t s , where untreated p i e c e s were used a s c o n t r o l s , p i e c e s h a l f dipped i n t o the t r e a t i n g s o l u t i o n were t e s t e d a g a i n s t the untreated other h a l v e s , which served as c o n t r o l s (Chapman Chem. Co.).



30

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

For p r e l i m i n a r y t e s t s of f u n g i c i d e s , d i f f e r e n t l a b o r a t o r y methods were developed. Several methods were described u s i n g a r t i f i c i a l media t o t e s t the t o x i c i t y of chemicals t o m i c r o organisms. One of the f i r s t t e s t s of t h i s k i n d was to use malt agar mixed w i t h the chemicals (19). The r e p r o d u c i b i l i t y of these t e s t s i s much b e t t e r than those which use wood as the medium (21). However, the r e s u l t s obtained w i t h a r t i f i c i a l media r a r e l y c o r r e l a t e w i t h those which are obtained u s i n g wood, and t h e r e f o r e a r t i f i c i a l media are best used f o r s p e c i a l experiments (e.g. a d a p t a t i o n of f u n g i to f u n g i c i d e s (10). Most of the l a b o r a t o r y r e s e a r c h w i t h f u n g i c i d e s intended to be used f o r wood p r o t e c t i o n has been c a r r i e d out w i t h a wood medium. Since no standard e x i s t s to t e s t s a p s t a i n and mould p r e v e n t i v e s , almost as many methods e x i s t as experiments c a r r i e d out (3,12,13,19,20,23,24,30,38). D i s c o l o r a t i o n of Non-Fungal O r i g i n on Lumber D i s c o l o r a t i o n , other than s a p s t a i n and mould (or decay) a l s o occurs on lumber. Some of t h i s d i s c o l o r a t i o n o r i g i n a t e s from the t r e e s and i s r e s t r i c t e d mainly to the heartwood I t may be caused by chemical r e a c t i o n s which do not continue a f t e r the t r e e s are harvested (1,31,34) or i t may be caused by f u n g i i n the l i v i n g t r e e , which may or may not continue to develop a f t e r h a r v e s t i n g (31,34). The o n l y p r o t e c t i o n against d i s c o l o r a t i o n which i s caused by f u n g i t h a t continue a f t e r h a r v e s t i n g , i s k i l n d r y i n g of the lumber ( 1 ) . A non-fungal d i s c o l o r a t i o n developing i n sapwood of lumber of s e v e r a l species i s due t o the o x i d a t i o n of c e r t a i n wood ext r a c t i v e s (2,22_,31) . C o l o u r l e s s carbohydrates or phenolic wood e x t r a c t i v e s , e i t h e r present i n sound wood or produced by m i c r o organisms e.g. b a c t e r i a i n ponded l o g s (18,37), migrate to the s u r f a c e , mainly onto the c r o s s - c u t ends of lumber. On the surface these e x t r a c t i v e s are converted to t a n n i n - l i k e , coloured m a t e r i a l s by o x i d a t i o n , i n most cases c a t a l y z e d by enzymes (2,22). This d i s c o l o r a t i o n i s mostly r e s t r i c t e d to the surfaces of sapwood, and more pronounced on the ends of pines (22,36) western hemlock (2,18) and a few other wood s p e c i e s . S i m i l a r d i s c o l o r a t i o n occurs d u r i n g k i l n - d r y i n g i n pine lumber, and i s caused by a s i m i l a r process. But i n s t e a d of enzymatic o x i d a t i o n , the high temperature a c c e l e r a t e s the formation of the coloured compounds. This d i s c o l o r a t i o n i s u s u a l l y not r e s t r i c t e d to the surfaces of the lumber (39). To c o n t r o l t h i s d i s c o l o r a t i o n i n p i n e s , buffered sodium a z i d e (36,37), and more r e c e n t l y the l e s s hazardous sodium f l u o r i d e have been used ( 8 ) . Recently ammoniacal z i n c oxide (32) and sodium carbonate or bicarbonate (41) have been suggested to prevent t h i s discoloration. The brown s t a i n on western hemlock sapwood i s not c o n t r o l l e d by sodium a z i d e (18). A number of chemicals (reducing agents and a c i d s ) prevent the c o l o u r formation i n l a b o r a t o r y experiments but f


2 . csERjEsi Prevention of Stain and Mould 31 i n p r a c t i c e they o n l y delay i t . The i n e f f e c t i v e n e s s of these chemicals i n p r a c t i c e i s probably due to the l o w e r i n g of the c o n c e n t r a t i o n of the chemicals below t h e i r e f f e c t i v e concen t r a t i o n s by r a i n and by the continuous movement of the e x t r a c t i v e s to the surface d u r i n g d r y i n g ( 2 ) . Since t h i s d i s c o l o r a t i o n i s r e s t r i c t e d to the surface, no p r e v e n t i v e measures are taken a g a i n s t i t s formation. This type of d i s c o l o r a t i o n i s l e s s important i n B r i t i s h Columbia than the d i s c o l o r a t i o n caused by f u n g i . Surface d i s c o l o r a t i o n can be r e a d i l y removed by s u r f a c i n g of lumber, and u s u a l l y obj e c t i o n i s r a i s e d o n l y when i t ' s observed on an unfam i l i a r wood species as was the case w i t h western hemlock.


Literature 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. Sci. 15. 16. 17. 18. 19. 20. 21. 22.

Cited

Ananthamarayana, S . , IPIRI J. (1975) 5(1) 20-25. B a r t o n , G.M. and Gardner, J.A.F., Can. Dep. F o r . Pub. 1147 (1966). Boocock, D., B.W.P.A. News Sheet No. 30 (1963) 4 pp. B r y a n t , R.C., "Lumber: its manufacture and distribution", 2nd ed. W i l e y , New York. (1938) pp. 213-216. Butcher, J.Α., Mat. U . Org. (1973) 8(1) 51-70. Butcher, J.A., N . Z . Wood I n d u s t r i e s (1974) 20(10) 9-11. Butcher, J.A. and D r y s d a l e , J., F o r . P r o d . J. (1974) 24(11) 28-30. Cech, M.Y., F o r . P r o d . J. (1966) 16(11) 22-27. Anon., B . C . Lumberman- (1974) 58(10) 60-61. C s e r j e s i , A.J., Can. J. M i c r o b i o l (1967) 13 1243-1249. C s e r j e s i , A.J. and R o f f , J.W., F o r . P r o d . J. (1964) 14(8) 373-376. C s e r j e s i , A.J. and R o f f , J.W., B . C . Lumberman (1966) 50(6) 64-66. C s e r j e s i , A.J. and R o f f , J.W., Mater. Res. Stand. (1970) 10(3) 18-20. C s e r j e s i , A.J., Quon. K . K . and Kozak, A., J. I n s t . Wood (1967) 4(1) 27-33. C s e r j e s i , A.J., R o f f , J.W. and Warren, W . G . , Mater. Res. Stand. (1971) 11(8) 29-32. D e s a i , R.L. and S h i e l d s , J.K., Can. F o r . Ind. (1975) 94(4) 43. Eades, H.W., "Sap stain and mould prevention on British Columbia softwoods." Can. Dep. Northern A f f . Nat. R e s . , F o r . Branch, Bull. 116 (1956) 39 pp. Evans, R.S. and H a l v o r s o n , H.N., F o r . P r o d . J. (1962) 12(8) 367-373. H a t f i e l d , I., A.W.P.A. P r o c . (1931) pp. 305-315. H a t f i e l d , I., Phytopathology (1950) 40(7) 653-663. H o r s f a l l , J.G., B o t . Rev. (1945) 11(7) 357-397. Millett, M.A., J. F o r . P r o d . Res. Soc. (1952) 6(12) 232-236.


32


23. 24. 25. 26.

27. 28.


29. 30. 31. 32. 33. 34. 35.

36. 37. 38. 39. 40. Bull 41.

Momoh, Z.O. and O l u y i d e , A.O., Dep. F o r . R e s . , N i g e r i a , Tech. Note No. 38 (1967) 10 pp. Richardson, B.A., Papper och Tra (1972) 10 613-624. R o f f , J.W. and C s e r j e s i , A.J., B.C. Lumberman (1965) 49(5) 90-98. R o f f , J.W., C s e r j e s i , A.J. and Swann, G.W., " P r e v e n t i o n of s a p s t a i n and mould in packaged lumber." Can. Dep. Environment, Can. For. Serv. Publ. 1325 (1974) 43 pp. Sandermann, W. and Casten, R., Holz als Roh-u. Werkstoff (1961) 19(1) 20-21. Sandermann, W., Casten, R. and Thode, Η., Holzforchung (1963) 17(4) 97-105. Sandermann, W., Thode, H . and Casten, R . , Holzforchung (1965) 19(2) 43-47. Savory, J.G. and C o c k c r o f t , R., Timber Trade J. (1961) 239(4440) 67-68. S c h e f f e r , T.C. and L i n d g r e n , R.M., U.S.D.A. Tech. Bull. No. 714, (1940) 124 pp. S h i e l d s , J.K., D e s a i , R.L. and C l a r k e , M . R . , F o r . P r o d . J. (1973) 23(10) 28-30. S h i e l d s , J.K., D e s a i , R.L. and C l a r k e , M.R., F o r . P r o d . J. (1974) 24(2) 54-57. S i e g l e , Η., Can. J. B o t . (1967) 45(2) 147-154. Smith, R.S., "Responsibilities and risks i n v o l v e d in the use of wood-protecting c h e m i c a l s . " Can. Dep. Nat. H e a l t h Welfare, Occup. H e a l t h Rev. (1970) 21(3-4) 1-6. S t u t z , R.E., F o r . P r o d . J. (1959) 9(12) 459-463 S t u t z , R.E., Koch, P. and Oldham, M.L., F o r . P r o d . J. (1961) 11(6) 258-260. Unligil, H.H., F o r . Prod . J. (1976) 26(1) 32-33. Verrall, A.F., B o t . Review (1945) 11(7) 398-415. Verrall, A.F. and Mook, P.V., "Research on chemical c o n t r o l of f u n g i in green lumber, 1940-51." U.S.D.A. Tech. No. 1046 (1951) 60 pp. Hulme, M.A., F o r . P r o d . J. (1975) 25(8) 38-41.


3 Chemical Methods of Improving the Permeability of Wood D A R R E L D. N I C H O L A S


Institute of Wood Research, Michigan Technological University, Houghton, Mich. 49931

In the past, it has been w e l l documented that the relative permeability of wood has a s i g n i ficant influence on the effectiveness of p r e s e r v a t i v e treatments. Without adequate p e n e t r a t i o n , even the best preservat i v e s will not provide s u f f i c i e n t protection and the wood will fail prematurely. Consequently, any d e v e l opments i n methods which increase the treatability of wood which are difficult to t r e a t could have a s i g n i f icant impact on the wood preserving i n d u s t r y . The o b j e c t i v e of t h i s paper i s to review methods of improving the permeability of wood. Both mechanical and chemical methods are possibilities, but t h i s d i s cussion will be l i m i t e d to the latter. S t r u c t u r a l and Chemical Factors i n Wood Which Affect Flow A complete review of the s t r u c t u r e of wood i s presented i n another paper i n t h i s symposium. Hence, t h i s d i s c u s s i o n will be l i m i t e d to a review of the p r i n c i p a l s t r u c t u r a l and chemical factors which could p o s s i b l y influence the flow of l i q u i d s i n wood. Structural Factors. Wood i s e s s e n t i a l l y a c l o s e d c e l l u l a r s y s t e m , and t h e c e l l w a l l s a r e c h a r a c t e r i z e d by t h e p r e s e n c e o f numerous p i t p a i r s w h i c h s e r v e as flow paths for l i q u i d s i n l i v i n g c e l l s . A f t e r the c e l l s d i e and t h e y a r e t r a n s f o r m e d i n t o h e a r t w o o d , t h e p i t s u n d e r g o a s p i r â t i o n and become o c c l u d e d w i t h wood extract ives. T h i s r e s u l t s i n a reduc t i o n i n the e f f e c t i v e pore s i z e which i n turn r e s t r i c t s flow of m a t e r i als . N e v e r t h e l e s s , e v i d e n c e s u g g e s t s t h a t t h e maj o r f l o w p a t h from c e l l to c e l l ( i n e i t h e r r a y s or t r a c h e i d s ) i s t h r o u g h t h e p i t membrane s i n c e t h i s i s t h e path of l e a s t r e s i s t a n c e . C o n s e q u e n t l y , an u n d e r s t a n d -

33


34

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS


i n g o f t h e s t r u c t u r e and c h e m i c a l c o m p o s i t i o n o f t h e p i t membrane i s p a r a m o u n t i n o u r a t t e m p t t o i m p r o v e t h e p e r m e a b i l i t y o f wood. Pit Structure. B e c a u s e t h e p i t membrane a p p e a r s t o be t h e c o n t r o l l i n g f a c t o r i n f l o w t h r o u g h wood, an e x a m i n a t i o n o f i t s s t r u c t u r e w o u l d be p e r t i n e n t . S i m p l e p i t p a i r s h a v i n g c o n t i n u o u s p i t membranes ( 1 ) are found between parenchyma c e l l s . T h e r e f o r e , the f l o w t h r o u g h t h i s t y p e o f c e l l must p a s s t h r o u g h t h e p i t membrane. C o n v e r s e l y , most s o f t w o o d t r a c h e i d s have b o r d e r e d p i t p a i r s which have a d i f f e r e n t i a t e d membrane c o m p o s e d o f a n e t w o r k - l i k e o p e n margo and a thickened c e n t r a l port i o n c a l l e d the t o r u s ( 1 ) . As l o n g as t h e b o r d e r e d p i t p a i r i s i n t h e u n a s p i r a t e d s t a t e , f l o w can o c c u r r e l a t i v e l y e a s i l y t h r o u g h the porous margo. However, the p i t p a i r s of heartwood a r e f r e q u e n t l y c l o s e d t o f l o w due t o a c o m b i n a t i o n o f a s p i r a t i o n and o c c l u s i o n by e x t r a c t i v e s o r l i g n i n - l i k e substances (2, 3). I n s a p w o o d t h e p i t s may o r may n o t be a s p i r a t e d , b u t do n o t c o n t a i n o c c l u s i o n s so f l o w i s g e n e r a l l y l e s s r e s t r i c t e d than i n the heartwood. I n an a s p i r a t e d p i t , the flow could occur e i t h e r through the t h i c k e n e d t o r u s , o r p o s s i b l y b e t w e e n t h e t o r u s and t h e overhanging border. B a s e d on e v i d e n c e a c c u m u l a t e d t o d a t e , i t i s not p o s s i b l e to determine which of these paths i s the major pathway of f l o w t h r o u g h a s p i r a t e d p i t p a i r s , b u t b o t h a r e p r o b a b l y f u n c t i o n a l and v a r y i n i m p o r t a n c e w i t h i n and among s p e c i e s . E l e c t r o n m i c r o s c o p i c s t u d i e s ( 4 , 5, 6, 7) r e v e a l t h a t t h e b o r d e r e d p i t membrane i s f i b r i l l a r i n n a t u r e . U n t i l r e c e n t l y , t h e margo was c o n s i d e r e d t o be an o p e n network of m i c r o f i b r i l s i n the green s t a t e . However, t h e w o r k by S a c h s and K i n n e y ( 8 ) i n d i c a t e s t h a t t h e margo i s e s s e n t i a l l y a c o n t i n u o u s membrane i n t h e g r e e n s t a t e but becomes q u i t e p o r o u s as a r e s u l t of d r y i n g stresses during seasoning. As l o n g a s wood i s d r i e d p r i o r to treatment, which i s n o r m a l l y the case, t h i s p o i n t i s s u p e r f l u o u s w i t h r e s p e c t to p e r m e a b i l i t y . A l t h o u g h t h e r e a r e no v i s i b l e o p e n i n g s i n t h e t o r us , e v i d e n c e s u g g e s t s t h a t o p e n i n g s do e x i s t ( 9 ) . I t i s e n v i s i o n e d t h a t f l o w through the t o r u s would occur through t o r t u o u s paths between randomly o r i e n t e d m i c r o f i b r i l s s i m i l a r to the openings between the f i b e r s i n f i l t e r paper. Extending t h i s analogy, the s t r u c t u r e and c o m p o s i t i o n o f t h e f i l t e r p a p e r d e t e r m i n e s t h e r a t e of f l o w . The f i n e r and more c o m p a c t t h e e l e m e n t s o f t h e p a p e r a r e , t h e s l o w e r t h e f l o w and t h e more s u s c e p t i b l e i t i s t o p l u g g i n g by p a r t i c u l a t e m a t t e r . This s i t u a t i o n would a l s o a p p l y t o t h e s t r u c t u r e of the mem-


3.

NICHOLAS

Methods

of Improving

Permeability

35


brane t o r u s . Hence, the r e l a t i v e p a c k i n g d e n s i t y of t h e m i c r o f i b r i l s and t h e amount and t y p e o f e n c r u s t a n c e p r e s e n t w o u l d be e x p e c t e d t o h a v e a s i g n i f i c a n t influe n c e on t h e f l o w o f f l u i d s . C h e m i c a l F a c t o r s . B a s e d on t h e s t r u c t u r e and f l o w p a t h s i n w o o d , t h e p i t membrane a p p e a r s t o be t h e comp o n e n t w h i c h s h o u l d be c h e m i c a l l y a l t e r ed i n o r d e r t o i n c r e a s e the p e r m e a b i l i t y . In order to accomplish t h i s , k n o w l e d g e o f t h e c h e m i c a l c o m p o s i t i o n o f t h e p i t membrane i s d e s i r a b l e . A number o f i n v e s t i g a t o r s h a v e s t u d i e d t h e chemi c a l compos i t i o n o f t h e p i t membrane ( 1 0 , 1 1 , 1 2 , 1 3 , 14) us i n g comb i n a t i o n s o f m i c r o s c o p y , s p e c i f i c enzymes, h i s t o c h e m i c a l m e t h o d s and UV microspectrophotometry. From t h i s w o r k , i t a p p e a r s t h a t t h e p i t membrane u n d e r goes c h e m i c a l m o d i f i c a t i o n d u r i n g heartwood f o r m a t i o n . I t ha s b e e n p o s t u l a t ed t h a t t h e m e c h a n i s m f o r t h i s t r a n s f o r m a t i o n i s t h a t the parenchyma c e l l s produce compound s w h i c h m i g r a t e t o t h e p i t membrane and s e r v e a s p r e c u r s o r s f o r t h e f o r m a t i o n o f p o l y p h e n o l i c compounds ( 1 0 ) . P e r o x i d a s e , w h i c h i s f r e q u e n t l y p r e s e n t i n sapwood p i t s , p r o b a b l y s e r v e s a s a c a t a l y s t f o r these r e a c t i o n s . I n s a p w o o d , t h e p i t membranes a p p e a r t o be p r i n c i p a l l y c o m p o s e d o f c e l l u l o s e and p e c t i n ( 1 3 ) . Howe v e r , i n a number o f g e n e r a , t h e sapwood p i t membranes a l s o c o n t a i n p o l y p h e n o l s i n some c a s e s , b u t t h e d i s t r i b u t i o n i s not u n i f o r m even w i t h i n spec i e s . I n h e a r t w o o d , a l l p i t membranes o f t h e g e n e r a s t u d i e d a p p e a r t o c o n t a i n p o l y p h e n o l s i n add i t i o n t o the o t h e r major c h e m i c a l components. The p r e s e n c e o f l i g n i n i n t h e p i t membrane h a s n o t b e e n p o s i t i v e l y established. However, the f a c t t h a t a monomeric C6-C3 compound has b e e n f o u n d i n t h e c a p i l l a r y l i q u i d o f sapwood t r a c h e i d s , s t r o n g l y s u g g e s t s t h a t l i g n i f i c a t i o n may o c c u r i n t h e p i t m e m b r a n e s . I n any e v e n t , i t appears t h a t the polyphenols present i n the h e a r t wood u n d e r g o p o l y m e r i z a t i o n r e a c t i o n s t o h i g h e r m o l e c u l a r w e i g h t compound s w h i c h r e s i s t e x t r a c t i o n by n e u t r a l s o l v e n t s. Henc e, t h e y e x h i b i t p r o p e r t i e s s i m i l a r to t h a t of l i g n i n . One o f t h e s i g n i f i c a n t f i n d i n g s by B a u c h , e t . a l . ( 1 3 ) i s t h e f a c t t h a t t h e p r e s e n c e o f l i g n i n - l i k e compound s i s n o t u n i f o r m w i t h i n s a m p l e s f r o m a g i v e n s p e cies. T h e r e a l s o a p p e a r s t o be a c o n s i d e r a b l e v a r i a t i o n i n the c h e m i c a l c o m p o s i t i o n of the p i t s w i t h i n a single tracheid. This suggests that treatment w i t h c h e m i c a l s t h a t d e g r a d e l i g n i n may n o t be n e c e s s a r y i n a l l c a s e s , s i n e e m o d i f i c a t i o n o f a few p i t membranes


36

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

p e r t r a c h e i d s h o u l d be s u f f i c i e n t t o s i g n i f i c a n t l y i m p r o v e t h e p e r m e a b i l i t y o f wood. I t s h o u l d be p o i n t e d o u t t h a t most o f t h e r e s e a r c h on c h e m i c a l c o m p o s i t i o n o f t h e p i t membrane h a s b e e n l i m i t e d to the bordered p i t s . The c h e m i c a l composition o f t h e s i m p l e p i t membranes i n t h e r a y p a r e n c h y m a c e l l s may o r may n o t be t h e same. However, s i n c e t h e p a r e n chyma c e l l s p r o d u c e t h e p r e c u r s o r s f o r t h e f o r m a t i o n of p o l y p h e n o l i c compounds, i t i s a n t i c i p a t e d t h a t t h e membrane o c c l u s i o n s w o u l d be s i m i l a r .


Methods of I n c r e a s i n g

the Permeability

o f Wood

T h e r e a r e a number o f p o s s i b l e c h e m i c a l m e t h o d s w h i c h c o u l d be e m p l o y e d t o i n c r e a s e t h e p e r m e a b i l i t y o f wood. These a r e : a) c h e m i c a l t r e a t m e n t s , b) m o d i f i c a t i o n o f t r e a t i n g s o l u t i o n p r o p e r t i e s , and c ) b i o l o g i c a l treatments. The p o t e n t i a l o f e a c h o f t h e s e m e t h o d s w i l l now be e x a m i n e d i n d e t a i l . Chemical Treatments. Over t h e y e a r s , a c o n s i d e r a b l e amount o f r e s e a r c h h a s b e e n c o n d u c t e d on t h e p o s s i b i l i t y o f us i n g v a r i o u s c h e m i c a l p r e t r e a t m e n t s t o i m p r o v e t h e p e r m e a b i l i t y o f wood. The b a s i c p r i n c i p l e behind such treatments i s e i t h e r t o e x t r a c t extraneous m a t e r i a l f r o m t h e p i t membrane o r d e g r a d e t h e p i t membrane i n order to enlarge the openings. P r e - e x t r a c t i o n o f Wood. Since the major f a c t o r c a u s i n g a r e d u c t i o n i n t h e p e r m e a b i l i t y o f wood d u r i n g h e a r t w o o d f o r m a t i o n i s o c c l u s i o n o f t h e p i t membranes w i t h e x t r a n e o u s m a t e r i a l , one w o u l d ant i c i p a t e t h a t p r e - e x t r a c t i o n o f t h e wood w i t h a s u i t a b l e s o l v e n t w o u l d be a m e t h o d o f i n c r e a s i n g t h e p e r m e a b i l i t y o f wood. T h i s c o n t e n t i o n h a s b e e n v e r i f i e d b y a number o f s t u d i e s ( 1 5 , 1 6 , 1 7 , 1 8 , 1 9 , 2 0 , 3, 2 1 ) . H o w e v e r , i t a p p e a r s d o u b t f u l t h a t s u c h t r e a t m e n t s w o u l d be comm e r c i a l l y f e a s i b l e since the solvents are expensive and e x c e s s i v e t i m e i s r e q u i r e d f o r t h e a d d i t i o n a l s t e p in the t r e a t i n g process. C h e m i c a l D e g r a d a t i o n o f t h e P i t Membranes. Chemi c a l mod i f i c a t i o n o f t h e p i t membranes i s a p o s s i b l e m e t h o d o f i n c r e a s i n g t h e p e r m e a b i l i t y o f wood a s l o n g as i t c a n be a c c o m p l i s h e d w i t h o u t i n c u r r i n g e x c e s s i v e strength loss. I n t h i s r e g a r d , sodium c h l o r i t e , p u l p i n g l i q u o r s , a c i d s and b a s e s h a v e b e e n u s e d t o i n c r e a s e t h e p e r m e a b i l i t y o f wood ( 1 9 , 2 2 , 3, 2 3 ) . U n f o r t u n a t e l y , e x c e s s i v e s t r e n g t h l o s s o f t h e wood r e s u l t e d from these treatments. N e v e r t h e l e s s , i t may be p o s s i b l e



3.

NICHOLAS

Methods

of Improving

Permeability

37

t o a c h i e v e t h e d e s i r e d r e s u l t s by s e l e c t i n g t h e p r o p e r c h e m i c a l s so t h i s a p p r o a c h s h o u l d n o t be a b a n d o n e d . F o r e x a m p l e , t h e w o r k by Emery and S c h r o e d e r ( 2 4 ) i n d i c a t e s t h a t wood c a n be c h e m i c a l l y o x i d i z e d w i t h a n i r o n c a t a l y z e d r e a c t i o n under a c i d i c c o n d i t i o n s . It i s c o n c e i v a b l e that t h i s type of treatment could deg r a d e t h e p i t membrane a n d i n c r e a s e t h e p e r m e a b i l i t y . F u r t h e r m o r e , T s c h e r n i t z ( 2 5 ) h a s shown t h a t t r e a t m e n t o f R o c k y M o u n t a i n D o u g l a s f i r sapwood w i t h h o t ammonium o x a l a t e improved the t r e a t a b i l i t y of t h i s m a t e r i a l . I n t h i s l a t t e r c a s e , t h e ammonium o x a l a t e p r o b a b l y s o l u b i l i z e d t h e p e c t i n s i n t h e p i t membrane. A n o t h e r c h e m i c a l method o f i n c r e a s i n g t h e p e r m e a b i l i t y o f sapwood i s t o s t e a m t h e g r e e n wood. This t e c h n i q u e i s f r e q u e n t l y used i n p r o c e s s i n g s o u t h e r n p i n e where i t a i d s i n t h e d r y i n g s t e p as w e l l as i n creasing the permeability. I t h a s b e e n shown t h a t t h e p r o b a b l e mechanism i n v o l v e d i n t h e change i n p e r m e a b i l i t y i s ac i d h y d r o l y s i s w h i c h r e d u c e s t h e e f f e c t i v e n e s s o f p i t aspirât i o n s i g n i f i c a n t l y ( 2 6 ) . A t t e m p t s h a v e b e e n made t o i n c r e a s e t h e p e r m e a b i l i t y of heartwood but without success. Furthermore, s t e a m i n g h a s n o t b e e n e f f e c t i v e on s p e c i e s o t h e r t h a n t h e s o u t h e r n p i n e s (27) . Mod i f i c a t i o n o f L i q u i d P r o p e r t i e s . The r e l a t i v e p e r m e a b i l i t y o f wood v a r i e s w i t h t h e t y p e a n d c o n d i t i o n of t h e i m p r e g n a t i n g l i q u i d . C o n s e q u e n t l y , by s e l e c t i n g the a p p r o p r i a t e parameters, i t i s p o s s i b l e to a l t e r the a p p a r e n t p e r m e a b i l i t y o f wood. T h e s e f a c t o r s w i l l now be d i s c u s s e d i n d e t a i l . Type o f L i q u i d . B o t h p e t r o l e u m h y d r o c a r b o n s and w a t e r a r e u s e d a s c a r r i e r s f o r wood p r e s e r v a t i v e s . A number o f s t u d i e s h a v e c l e a r l y shown t h a t p e t r o l e u m h y d r o c a r b o n s p e n e t r a t e wood much more r a p i d l y t h a n w a t e r ( 2 8 , 2 9 , 2 0 , 2 7 ) . The r e a s o n f o r t h i s d i f f e r e n c e i s n o t ent i r e l y c l e a r , b u t i t has been p r o p o s e d t h a t h y d r o g e n b o n d i n g a b i l i t y o f t h e l i q u i d i s t h e maj o r f a c t o r t h a t i n f l u e n c e s i t s a b i l i t y t o p e n e t r a t e wood (28, 29, 2 7 ) . Water has t h e a b i l i t y t o form s t r o n g hydrogen bonds between m o l e c u l e s w h i c h r e s u l t s i n a s t r u c t u r e d medium. I n a d d i t i o n , water has t h e a b i l i t y t o f o r m h y d r o g e n b o n d s w i t h t h e h y d r o x y l g r o u p s i n wood. H e n c e , t h e s e two f a c t o r s c o u l d p r o d u c e a f r i c t i o n a l d r a g a s w a t e r moves t h r o u g h wood a n d e f f e c t i v e l y r e duce the flow rate. S i n c e i t has never been proven c o n e l u s i v e l y t h a t hydrogen bonding a b i l i t y i s t h e reason f o r d i f f e r e n c e s i n p e n e t r a b i l i t y o f l i q u i d s , o t h e r p o s s i b i l i t i e s should



38

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

n o t be p r e m a t u r e l y r u l e d o u t . For example, the a b i l i t y o f a l i q u i d t o f o r m b u b b l e s may be s i g n i f i c a n t s i n c e i t i s known t h a t t h e p r e s e n c e o f b u b b l e s r e d u c e s t h e f l o w r a t e b e c a u s e c a p i l l a r y p r e s s u r e m u s t be o v e r c o m e d u r i n g imprégnât i o n ( 3 0 ) . I n t h i s r e g a r d , t h e r e i s a f u n d a m e n t a l d i f f e r e n c e b e t w e e n w a t e r and o r g a n i c l i q u i d s s i n e e i t h a s b e e n shown t h a t s t a b i l i z e d g a s m i c r o n u c l e i e x i s t s i n the former but not i n the l a t t e r ( 3 1 ) . Hence, water would have a tendency t o form b u b b l e s whereas o r g a n i c l i q u i d s would n o t . Another p o s s i b l e e x p l a n a t i o n o f t h e d i f f e r e n c e b e t w e e n t h e two l i q u i d s h a s b e e n a d v a n c e d by B a i l e y and P r e s t o n ( 3 2 ) . T h e y contend t h a t the d i f f e r e n c e i s a t t r i b u t a b l e to the dep o s i t i o n of h y d r o p h o b i c mater i a l i n the pores which ef f e c t i v e l y i n c r e a s e s the c o n t a c t a n g l e f o r w a t e r . As a r e s u l t , more p r e s s u r e i s r e q u i r e d t o i m p r e g n a t e wood i n a c c o r d a n c e w i t h J u r i n s equat i o n ( 3 0 ) . The a b o v e a r e o n l y 3 p o s s i b l e f a c t o r s w h i c h c o u l d i n f l u e n c e t h e p e n e t r a b i l i t y o f l i q u i d s i n t o wood. O t h e r p o s s i b i l i t i e s a r e : a) s u r f a c e t e n s i o n , b) m o l e c u l a r s i z e , c ) c h e m i c a l ac t i v i t y , d) s o l v e n c y , and e) a b i l i t y t o s w e l l wood. Some o f t h e s e f a c t o r s may be o p e r a t i v e i n t r e a t m e n t s w i t h p r o p y l e n e o x i d e w h i c h have b e e n c a r r i e d o u t by R o w e l l ( 3 3 ) . I n t h i s s t u d y , he u s e d a m i x t u r e o f 9 5 % p r o p y l e n e o x i d e and 5% t r i e t h y l amine ( v / v ) . He was a b l e t o c o m p l e t e l y t r e a t s o u t h e r n p i n e and r e d p i n e h e a r t w o o d , b o t h o f w h i c h w e r e c l a s s i f i e d as b e i n g r e f r a c t o r y . T h i s w o r k c l e a r l y shows t h a t w i t h t h e p r o p e r t r e a t i n g m e d i u m , h e a r t w o o d c a n be f u l l y penetrated. I n summary, i t c a n be c o n c l u d e d t h a t t h e t r e a t i n g l i q u i d c h a r a c t e r i s t i c s h a v e a s i g n i f i c a n t i n f l u e n c e on the treatment r e s u l t s . Consequently, a b e t t e r unders t a n d i n g of the f a c t o r s which a f f e c t p e n e t r a b i l i t y could l e a d to improved t r e a t m e n t methods. Liquid Contamination. As t r e a t i n g s o l u t i o n s a r e c o n t i n u o u s l y r e u s e d , t h e y become c o n t a m i n a t e d w i t h p a r t i c u l a t e m a t t e r f r o m c h e m i c a l r e a c t i o n s and e x t r a n e o u s sources. I t h a s b e e n shown t h a t t h i s p a r t i c u l a t e m a t t e r c a n s i g n i f i c a n t l y r e d u c e t h e pénétrât i o n o f p r e s e r v a t i v e s o l u t i o n s i n t o wood. C o n s e q u e n t l y , i t appears t h a t methods f o r c o n t i n u o u s l y r e m o v i n g p a r t i c u l a t e matt e r c o u l d r e s u l t i n i m p r o v e d pénétrât i o n o f p r e s e r v a tive solut ions. Chemical A d d i t i v e s . P r e v i o u s r e s e a r c h h a s shown t h a t t h e a d d i t i o n o f c h e m i c a l s t o w a t e r h a s an e f f e c t on i t s a b i l i t y t o p e n e t r a t e wood ( 7 ) . I n t h i s r e g a r d , s t a n d a r d p r e s e r v a t i v e c h e m i c a l s and wood e x t r a c t i v e s



3.

NICHOLAS

Methods

of

Improving

Permeability

39

t e n d t o r e d u c e t h e f l o w o f l i q u i d s i n wood. Hence, p r o p e r s e l e c t i o n o f c h e m i c a l s and t h e r e m o v a l o f e x t r a c t i v e s could i n s u r e that the treatment i s maximized. The f a c t t h a t t h e r e i s a s i g n i f i c a n t d i f f e r e n c e i n t h e p e n e t r a b i l i t y o f p o l a r and n o n - p o l a r l i q u i d s s u g g e s t s t h a t altérât i o n s o f t r e a t i n g s o l u t i o n s w i t h add i t i v e s may be a m e t h o d o f i m p r o v i n g t r e a t m e n t . In t h i s r e g a r d , B u c k m a n , e t . a l . ( 2 8 ) and H a r t m a n n - D i c k ( 2 0 ) h a v e shown t h a t t h e f l o w o f w a t e r i n wood c a n be s i g n i f i c a n t l y i n c r e a s e d by t h e add i t i o n o f t h e z i n c chloride. The r e a s o n f o r t h i s i m p r o v e m e n t i s n o t k n o w n , b u t i t s u g g e s t s t h a t t h e r e may be some p o t e n t i a l i n t h i s a r e a and o t h e r add i t i v e s s h o u l d be i n v e s t i gated . B i o l o g i c a l Treatments. The p o s s i b i l i t y o f u s i n g b i o l o g i c a l treatments to i n c r e a s e the p e r m e a b i l i t y of wood has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n by r e s e a r c h e r s . I n g e n e r a l , t h i s t y p e o f t r e a t m e n t c a n be separated i n t o 3 c a t e g o r i e s ; n a m e l y , a) t r e a t m e n t w i t h f u n g i , b) t r e a t m e n t w i t h b a c t e r i a , and c ) enzyme t r e a t m e n t s . Treatment w i t h Fungi. The u s e o f f u n g i t o i n c r e a s e t h e p e r m e a b i l i t y o f wood was i n i t i a t e d by L i n d g r e n and H a r v e y ( 3 4 ) and B l e w ( 3 5 ) . In these s t u d i e s , T r i c h o d e r m a m o l d was u s e d t o i n c r e a s e t h e p e r m e a b i l i t y of s o u t h e r n p i n e sapwood. F o l l o w i n g t h i s work, o t h e r s t u d i e s w e r e c o n d u c t ed w i t h D o u g l a s f i r ( 3 6 , 3 7 ) , j a c k ρine ( 3 8 ) , and s p r u e e and a s p e n (3 9 ) . A l l of t h e s e s t u d i e s i n v o l v e d the use of T r i c h o d e r m a mold. H o w e v e r , i n a l a t e r s t u d y by E r i c k s o n and Depreitas ( 4 0 ) , G l i o c l a d i u m f u s a r i u m , and C h a e t o m i u m w e r e u s e d along with Trichoderma. Improved p e n e t r a t i o n , h i g h e r p r e s e r v a t i v e r e t e n t i o n , and more u n i f o r m t r e a t m e n t s w e r e t h e g e n e r a l r u l e i n the above stud i e s . Fur t h e r m o r e , the r e s u l t s w e r e s i m i l a r among t h e s p e c i e s o f f u n g i t e s t e d . The m e c h a n i s m f o r i n c r e a s i n g p e r m e a b i l i t y i s i l l u s t r a t e d i n F i g u r e 1. T h i s shows a f u n g a l h y p h a e g r o w i n g i n s i d e a c e l l l u m e n and a b r a n c h h a s f o r m e d a t t h e p i t a p e r a t u r e so t h a t i t c a n p e n e t r a t e i n t o t h e next c e l l . I f t h i s o c c u r s f r e q u e n t l y enough, i t would e f f e c t i v e l y i n c r e a s e the p e r m e a b i l i t y . T h e r e a r e two maj o r l i m i t a t i o n s i n t h e processes d e s c r i b e d above. F i r s t of a l l , t h e f a c t t h a t t h e s e f u n g i a r e e f f e c t i v e o n l y i n t h e sapwood, w h i c h i n most instances i s r e a d i l y t r e a t a b l e , reduces i t s usefulness. S e c o n d l y , i t has b e e n shown by J o h n s o n and Gj o v i k ( 4 1 ) t h a t e x t r a n e o u s b a c t e r i a , r a t h e r t h a n f u n g i , may a c t u a l l y be r e s p o n s i b l e f o r t h e p i t membrane d e g r a d a t i o n .



40

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

C o n s e q u e n t l y , i t does not appear t h a t t h i s p a r t i c u l a r a p p r o a c h w i l l p r o v e t o be p r a c t i c a l i n c o m m e r c i a l a p plications . S i n c e i t h a s b e e n shown t h a t t h e d e g r a d a t i o n of t h e p i t membranes i n wood i s t h e most l o g i c a l m e t h o d o f i n c r e a s i n g p e r m e a b i l i t y , f u n g i must be c a p a b l e o f a t t a c k i n g t h i s s t r u c t u r e i n o r d e r t o be s u c c e s s f u l . As i s shown i n F i g u r e 1, t h e f u n g a l h y p h a e a r e a b l e t o l o c a t e and p e n e t r a t e t h e p i t membrane so t h i s i s a v i a b l e method. Because the heartwood p i t s c o n t a i n p o l y p h e n o l i c s u b s t a n c e s , t h e f u n g i must d e g r a d e t h e s e compounds. I n t h i s r e g a r d , t h e w o r k by K i r k (42) i s significant. K i r k has f o u n d t h a t t h e f u n g u s P h a n e r o chaete chrysosporium r e a d i l y degrades l i g n i n . Hence, i t m i g h t be p o s s i b l e t o u s e t h i s o r a s i m i l a r f u n g u s t o i m p r o v e t h e p e r m e a b i l i t y o f wood. S i n c e i t may be d e s i r a b l e t o d e g r a d e t h e c e l l u l o s e as w e l l as p o l y p h e n o l i c c o m p o u n d s , i t m i g h t be n e c e s s a r y t o u s e a m i x e d c u l t u r e ( 4 3 ) . T h a t s u c h a f u n g u s e x i s t s was v e r i f i e d by G r e a v e s and B a r n i c l e ( 4 4 ) when t h e y d i s c o v e r e d t h a t a f u n g u s was r e s p o n s i b l e f o r t h e i n c r e a s e d p e r m e a b i l i t y o f k a r r i h e a r t w o o d by s e l e c t i v e a t t a c k on t h e p i t membrane and wood r a y s . I f t h e r i g h t f u n g i c o u l d be f o u n d , t h e n i t may be p o s s i b l e t o p r e - t r e a t wood w i t h a s p o r e s u s p e n s i o n w h i c h w o u l d be a l l o w e d t o r e a c t s u f f i c i e n t l y t o o p e n up t h e s t r u c t u r e b e f o r e t r e a t m e n t . T h i s , of course, assumes t h a t the f u n g i w i l l s u f f i c i e n t l y d e g r a d e the p i t membranes b e f o r e s i g n i f i c a n t l y d a m a g i n g t h e c e l l w a l l s t r u c t u r e and c a u s e e x c e s s i v e s t r e n g t h l o s s . This may be p o s s i b l e b e c a u s e o f t h e a c c e s s i b i l i t y o f t h e p i t membranes. F u r t h e r m o r e , t h e i n c u b a t i o n p e r i o d m u s t be r e l a t i v e l y s h o r t i n o r d e r f o r s u c h a p r o c e s s t o be feasible. K i r k (42) i n d i c a t e s t h a t t h e s e r e a c t i o n s p r o c e e d r a p i d l y u n d e r t h e p r o p e r c o n d i t i o n s so t h i s may n o t be a s e r i o u s p r o b l e m . I n o r d e r t o make i t f e a s i b l e t o t r e a t h e a r t w o o d t o s a t i s f a c t o r y d e p t h s , i t may be n e c e s s a r y t o i n c i s e w i t h an o p e n p a t t e r n p r i o r t o a p p l y i n g t h i s s p o r e suspension. S i n c e t h i s c o u l d be d o n e i n a s i n g l e o p e r a t i o n , i t w o u l d be e c o n o m i c a l l y a t t r a c t i v e . Such a s y s t e m might p r o v i d e a means o f a c h i e v i n g a u n i f o r m p r e s e r v a t i v e d i s t r i b u t i o n , which i s c u r r e n t l y not p o s s i b l e w i t h i n c i s i n g a l o n e w i t h o u t i n c u r r i n g e x c e s s i v e damage t o t h e wood. Treatment w i t h B a c t e r i a . S e v e r a l y e a r s ago, i t was d i s c o v e r e d t h a t t h e p e r m e a b i l i t y o f wood was i n c r e a s e d by b a c t e r i a l a t t a c k when l o g s w e r e s o a k e d i n water f o r a p e r i o d of time (45). Following this,



3.

NICHOLAS

Methods

of

Improving

Permeability

41

add i t i o n a l s t u d i e s w e r e c o n d u c t e d t o d e t e r m i n e w h i c h b a c t e r i a w e r e i n v o l v e d , t h e i r mode o f a c t i o n and o p t i mum o p e r a t i n g c o n d i t i o n s ( 1 1 , 12, 46, 4 7 , 4 8 , 4 5 , 49, 5 0 , 5 1 , 5 2 , 5 3 , 54, 5 5 , 56, 57, 5 8 , 5 9 , 60, 6 1 ) . I n p i n e , i t was f o u n d t h a t B a c i l l u s p o l y m y x a was the major s p e c i e s i n v o l v e d (45, 55), whereas, i n s p r u c e t h e m a j o r s p e c i e s w e r e B a c i l l u s s u b t i l l s and Flavobacterium pectinovorum (49). In another study ( 5 3 ) , C l o s t r i d ium o m e l i a n s k i i was i d e n t i f i e d as t h e species at t a c k i n g softwoods. I n a l l s t u d i e s , i t was f o u n d t h a t t h e b a c t e r i a l a t t a c k on t h e p i t membranes was t h e r e a s o n f o r i n c r e a s e d p e r m e a b i l i t y o f t h e wood. F u r t h e r m o r e , i t was shown by F o g a r t y and Ward ( 4 9 ) t h a t b a c t e r i a d e g r a d e d t h e p i t membranes by e x c r e t i n g t h e enzymes a m y l a s e x y l a n a s e , and p e c t i n a s e . A t y p i c a l sapwood p i t membrane t h a t h a s b e e n a t t a c k e d by b a c t e r i a i s shown i n F i g u r e 2. As c a n be s e e n , t h e t o r u s i s s e v e r e l y d e g r a d e d and h a s w e l l d e f i n e d o p e n ings. The i m p o r t a n t t h i n g t o remember i n m o s t o f t h e s e e x p e r i m e n t s i s t h a t the b a c t e r i a have been used to imp r o v e the p e r m e a b i l i t y of sapwood. For s p e c i e s l i k e S i t k a s p r u c e w h i c h h a s sapwood t h a t i s i m p e r m e a b l e t o c r e o s o t e , t h i s t y p e o f t r e a t m e n t may p r o v e t o be commercially feasible. However, i n o r d e r to f u l l y exp l o i t t h i s method f o r i n c r e a s i n g p e r m e a b i l i t y , i t w i l l be n e c e s s a r y t o f i n d b a c t e r i a w h i c h h a v e t h e a b i l i t y to d e g r a d e h e a r t w o o d p i t membranes. I n d e e d , t h i s may be p o s s i b l e s i n c e G r e a v e s (50) h a s shown t h a t some b a c t e r i a can i n c r e a s e the p e r m e a b i l i t y of heartwood. In a d d i t i o n t o t h i s , i t w o u l d be d e s i r a b l e t o a c c e l e r a t e t h e r e a c t i o n as much a s p o s s i b l e i n o r d e r t o make i t compatible with a commercial operation. P o s s i b l e s t r e n g t h l o s s i s i m p o r t a n t w h e n e v e r an a t t e m p t i s made t o i n c r e a s e t h e p e r m e a b i l i t y o f wood. V a r i a b l e e f f e c t s h a v e b e e n r e p o r t e d by r e s e a r c h e r s . F o r e x a m p l e , B a u c h , e t . a l . ( 1 1 , 12) and U n l i g i l (58) observed a s i g n i f i c a n t r e d u c t i o n i n the impact s t r e n g t h o f wood t h a t had b e e n p o n d e d t o i m p r o v e p e r m e a b i l i t y . On t h e o t h e r h a n d , D u n l e a v y and F o g a r t y ( 4 8 ) r e p o r t e d no s i g n i f i c a n t s t r e n g t h l o s s i n s p r u c e p o l e s t h a t w e r e exposed to b a c t e r i a l a t t a c k . The r e a s o n f o r t h i s d i f f e r e n c e i n r e s u l t s may be due t o t h e b a c t e r i a l s p e c i e s involved. G r e a v e s ( 5 1 ) s t u d i e d t h i s i n d e t a i l and f o u n d t h a t some b a c t e r i a w e r e a b l e t o i n c r e a s e t h e p e r m e a b i l i t y o f wood w i t h o u t a f f e c t i n g t h e s t r e n g t h . Conv e r s e l y , o t h e r b a c t e r i a were a b l e to i n c r e a s e the p e r m e a b i l i t y b u t a t t h e same t i m e t h e y d e c r e a s e d t h e strength signif icantly. T h i s d i f f e r e n c e may a l s o be due t o t h e p r e s e n c e o f o t h e r m i c r o o r g a n i s m s s i n e e one



WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

Figure 1. A fungal hyphae growing inside the lumen of a tracheid. Note how the hyphae is branched to penetrate the pit membrane. (Courtesy of I. B. Sachs)

Figure 2.

A sapwood pit membrane that has been degraded by bacteria. (Courtesy of I. B. Sachs)



3.

NICHOLAS

Figure 3.

Methods

of Improving

Permeability

43

A Douglas-fir sapwood pit membrane which has been degraded by pectinase. (Courtesy of I. B. Sachs)

s t u d y ( 5 9 ) r e v e a l e d t.hat t h e i n c r e a s e d p e r m e a b i l i t y was due t o f u n g i r a t h e r t h a n b a c t e r i a . Enzymes. S i n c e b o t h b a c t e r i a and f u n g i u t i l i z e e n z y m e s t o d e g r a d e t h e p i t membrane i n w o o d , i t i s n o t s u r p r i s i n g t h a t t r e a t m e n t w i t h i s o l a t e d enzymes p r o duces s i m i l a r e f f e c t s . T h i s was shown t o be t h e c a s e by N i c h o l a s a n d Thomas ( 2 6 ) u s i n g c e l l u l a s e , h e m i c e l l u l a s e and p e c t i n a s e . S i m i l a r r e s u l t s were sub s e q u e n t l y o b t a i n e d by o t h e r r e s e a r c h e r s ( 6 2 , 63, 64, 65) . I n a r e c e n t s t u d y by Tshernitζ ( 2 5 ) , i t was v e r i f i e d t h a t e n z y m e s c o u l d be u s e d t o i n c r e a s e t h e p e r m e a b i l i t y o f Rocky M o u n t a i n Douglas f i r . By p r e t r e a t i n g t h e wood w i t h pec t i n a s e, a c o m p l e t e l y uniform t r e a t m e n t o f t h e sapwood z o n e was p o s s i b l e w i t h c r e o sote. This i s i n contrast to e r r a t i c treatment nor m a l l y o b t a i n e d when t h i s m a t e r i a l i s t r e a t e d . A t y p i c a l p i t membrane w h i c h h a s b e e n d e g r a d e d b y p e c t i n a s e i s shown i n F i g u r e 3. W i t h r e g a r d t o t h e e f f e c t o f e n z y m e s on t h e s t r e n g t h o f t r e a t e d wood, t h e work by Meyer (65) i n d i cated that i t i s p o s s i b l e to s i g n i f i c a n t l y increase t h e p e r m e a b i 1 i t y o f wood w i t h o u t a n y a p p r e c i a b l e


44

WOOD TECHNOLOGY:CHEMICAL ASPECTS

s t r e n g t h l o s s . Hence, i t does not appear t h a t t h i s would be a problem w i t h the use of enzymes. To d a t e , i t has not been p o s s i b l e to degrade heartwood p i t membranes w i t h i s o l a t e d enzymes. Since c o - f a c t o r s are p r o b a b l y r e q u i r e d ( 4 2 ) , i t appears t h a t use of a whole o r g a n i s m r a t h e r than i s o l a t e d enzymes may be r e q u i r e d i f the p e r m e a b i l i t y of heartwood i s to be i n c r e a s e d .

LITERATURE CITED


1. 2. 3. 4. 5. 6.

Panshin, A. J., DeZeeuw, C., Brown, H. P., "Text book of Wood Technology," Vol. 1, McGraw-Hill Book Company, Inc., New York, Ν. Y., 1964. Cote, W. A., J. Polymer Sci. C., (1963), (2):231-42. Krahmer, R. L., Cote, W. A., Tappi, (1963), 46 (1):42-49. Erickson, H. D., Wood Science, (1970), 2(3):149-158. Cote, W. A., Forest Prod. J., (1958), 8(10):296301. Cote, W. A., Krahmer, R. L., Tappi, (1962), 45(2):119-22.

7. 8.

Nicholas, D. D., Forest Prod. J., (1972), 22(5):31. Sachs, I. Β., Kinney, R. E., Wood Science, (1974), 6(3):200-205. 9. Sachs, I. Β., Personal communication, (1975). 10. Bauch, J., Berndt, H., Wood Science and Technol ogy, (1973), 7(1):6-19. 11. Bauch, J. Adolf, P., Liese W., Holz als Roh-und Werkstoff, (1973), 31(3):115-120. 12. Bauch, J., Liese, W., Berndt, H., Holzforschung, (1970), 24(6):199-205. 13. Bauch, J., Schweers, W., Berndt, H., Holzforschung, (1974), 28(3):86-91. 14. Nicholas, D. D., Thomas, R. J., Forest Prod. J., (1968), 18(1):57-59). 15. Benvenuti, R. R., Unpublished Masters thesis, North Carolina State Univ., Raleigh, N. C. (1963). 16. Buro, Α., Buro, Ε. A., Holz als Roh-und Werkstoff, (1959), 17(12):461-74. 17. Charuk, Ε. V., Razumova, A. F., Holztechnologie, (1974), 15(1):3-6. 18. Comstock, G. L., Forest Prod. J., (1965), 15(10): 441-49. 19. Erdtman, H., Svensk Papperstidning, (1958), 61625-32. 20. Hartmann-Dick, V., Forstwiss. CBI., (1955), 74(5/6):163-83. 21. Scarth, G. W., Spier, J. D., Royal Soc. of Can. Proc. and Trans., (1929), 23:281-88.


3. NICHOLAS 22. 23. 24. 25. 26. 27. Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.ch003

D. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.

Methods of Improving Permeability

J o h n s t o n , H. W., Maass, O., Can. J. Res., ( 1 9 3 0 ) , 3:140-73. Y o s h i m o t o , T., H a y a s h i , S., K i s h i m a , T., Wood Research, (1970), (52):90-105. Emery, J. A., S c h r o e d e r , Η. A., Wood Sci. and Technology, (1974), 8(2):123-137. T s c h e r n i t z , J. L., F o r e s t P r o d . J., ( 1 9 7 3 ) , 23(3):30-38. N i c h o l a s , D. D., Thomas, R. J., Proc. Am. Wood– Preservers' Assoc., (1968), 64:1-7. N i c h o l a s , D. D., S i o u , J. F., "Wood Deterioration and Its P r e v e n t i o n by Preservative Treatments," D. N i c h o l a s , Ed., Vol. II, S y r a c u s e University P r e s s , S y r a c u s e , Ν. Y., 1973. Buckman, S. J., S c h m i t z , H., G o r t n e r , R. Α., Chem., (1935), 39:103-19. E r i c k s o n , H. D., S c h m i t z , Η., G o r t n e r , R. A., M i n n e s o t a Ag. E x p . S t a . T e c h . Bull. No. 2 2 , ( 1 9 3 7 ) . S i a u , J. F., "Flow in Wood," S y r a c u s e U n i v . P r e s s , S y r a c u s e , Ν. Υ., 1971. Hayward, A. T. J., Am. Scientist, (1971), 59:434443. B a i l e y , P. J., P r e s t o n , R. D., H o l z f o r s c h u n g , (1969), 23(4):113-120. R o w e l l , R. Μ., P e r s o n a l c o m m u n i c a t i o n , ( 1 9 7 5 ) . L i n d g r e n , R. A., H a r v e y , G. M., F o r e s t P r o d . J., (1952), 2(5):250-56. B l e w , J. O., Jr., F o r e s t P r o d . J., ( 1 9 5 2 ) , 2(3): 85-86. Graham, R. D., F o r e s t P r o d . J., ( 1 9 5 4 ) , 4 ( 4 ) : 1 6 4 166. L i n d g r e n , R. Μ., W r i g h t , Ε., F o r e s t P r o d . J., (1954), 4(4):162-164. Panek, Ε., F o r e s t P r o d . J., ( 1 9 5 7 ) , 7 ( 4 ) : 1 2 4 - 1 2 7 . S c h u l z , G., F o r e s t P r o d . J., ( 1 9 5 6 ) , 6 ( 2 ) : 7 7 - 8 0 . E r i c k s o n , H. D., Defreitas, A. R., F o r e s t P r o d . J., (1971), 21(4):53-58. J o h n s o n , B. R., G j o v i k , L. R., P r o c . Amer. WoodPreservers' A s s o c . , (1970), 66:234-240. K i r k , Τ. Κ., P e r s o n a l c o m m u n i c a t i o n , ( 1 9 7 5 ) . S k i n n e r , K. J., Chem. and E n g r . News, ( 1 9 7 5 ) , 53(Aug. 1 8 ) : 2 2 - 4 1 . G r e a v e s , Η., B a r n a c l e , J. Ε., F o r e s t P r o d . J., (1970), 20(8):47-51. E l l w o o d , Ε. L., E c k l u n d , Β. A., F o r e s t P r o d . J., (1959), 9(9):283-92. D e G r o o t , R. C., S c h e l d , H. W., F o r e s t P r o d . J., (1973), 23(4):43-46. D u n l e a v y , J. A., M c Q u i r e , A. J., J. I n s t . Wood Sci., (1970), 5(2):20-28.


45

46 48. 49. 50. 51. 52.


53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65.


Dunleavy, J. Α., Fogarty, W. M., Proc. British Wood Preserving A s s o c . , (1971), 1-28. Fogarty, W. M., Ward, O. P., Wood Sci. and T e c h . , (1973), 7(4):261-270. Greaves, Η . , Holzforschung, (1970), 24 (1):6-14. Greaves, Η., Wood Sci. and T e c h . , (1971), 5(1): 6-16. Greaves, Η., Levy, J. F., "Proc. of the F i r s t I n t e r n a t i o n a l B i o d e t e r i o r a t i o n Symposium," p. 429441, E l s e v i e r P u b l i s h i n g Company, Ltd., Amsterdam, 1968. Karnop, G., M a t e r i a l and Organism, (1972), 7(3):189-203. Knuth, D. T., Forest Prod. J., (1964), 12(9):437442. Knuth, D. T., Unpublished Ph.D. d i s s e r t a t i o n , Univ. of Wisconsin, Madison. Univ. M i c r o f i l m , Ann Arbor, Michigan, (1964). S u o l a h t i , O., Wallen, Α., Holz als Roh-und Werks t o f f , (1958), 16:8-20. Unligil, Η. Η., J. I n s t . Wood Sci., (1971), 5(6):30-35. Unligil, Η. Η., Forest Prod. J., (1972), 22(9): 92-100. Unligil, Η. Η., Krzyzewski, J., Newfoundland bi monthly Research Notes, (1972), 28 (2/3):11-12. Ward, O. P., Fogarty, W. Μ., J. of the Inst. of Wood Sci., (1973), 6(2):8-12. Banks, W. Β., D e a r l i n g , Τ. Β., Mater. Organismen, (1973), 8(1):39-49. A d o l f , F. P., Holzforschung, (1975), 29(5):181-186. Imamura, Y., Harada, Η., Saiki, Η., Wood Sci. and Tech., (1974), 8(4):243-254. J u t t e , S. Μ., Levy, J. F., Acta Bot. Neerl., (1971), 20(5):453-466. Meyer, R. W., Wood Sci., (1974), 6(3):220-230.


4 Nonconventional W o o d Preservation Methods


ROGER M. ROWELL Forest Products Laboratory,P.O.Box 5130, Madison, Wis. 53705

A most effective way to extend the Nation's timber supply i s to use wood so that its service life i s increased. The service life of wood i n hazardous use conditions can be increased severalfold by the proper use of wood preservatives. I t i s e s t i mated that the preservative treatment of railway ties results i n an annual savings of 2.4 billion board feet of lumber and that, if utility poles were not treated, an additional 20 m i l l i o n mature trees of pole-stock quality softwoods would be needed each year simply as replacements for those destroyed by decay and termites. Some 275 m i l l i o n cubic feet of wood are treated annually for protection against decay fungi, insects, or marine borers. Nevertheless, losses to these agents are still large and may amount to between $1 and $2 billion in the United States annually. These losses may be attributed to either inadequate or no preservative treatment. Although conventional preservatives are generally effective, they are coming under increasing attack because of their t o x i c i t y , so information is urgently needed on newer, safer, more environmentally acceptable, effective preservatives. A l l of the commercial wood preservatives presently used i n the United States are effective i n preventing attack by microorganisms because of their toxic nature. Most of these preservatives are c l a s s i f i e d as broad spectrum preservatives, that i s , effective against several different types of l i v i n g systems. Because of the toxic hazards and environmental concerns and because prevention of wood decay i s needed i f we are to extend our timber resources by increasing i t s service l i f e , we have i n v e s t i gated alternative methods of wood preservation not based on t o x i c i t y for their effectiveness. This paper i s not meant to present finished data as much as i t i s to present ideas, concepts and research approaches i n the area of new methods for wood preservation. For this discussion, a nonconventional preservation method w i l l be defined simply as a concept or process not presently i n use but with future potential.



48

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

C o n t r o l l i n g micro-organisms by means other than p o i s o n i n g them can be i n v e s t i g a t e d by c o n s i d e r i n g the b a s i c needs o f the organism. I n order f o r wood-destroying micro-organisms to t h r i v e , they r e q u i r e : (1) oxygen, (2) water, (3) food, i n c l u d i n g a l l e s s e n t i a l t r a c e compounds, and (4) a f a v o r a b l e environment. To e l i m i n a t e any one of these w i l l e f f e c t i v e l y c o n t r o l the growth of the organism. I t i s very d i f f i c u l t to r e s t r i c t the oxygen from microorganisms so c o n t r o l measures based on t h i s approach would proba b l y be f r u i t l e s s . Wood below the f i b e r s a t u r a t i o n p o i n t does not decay. Therefore, by r e s t r i c t i n g the amount of water i n the wood c e l l w a l l below the f i b e r s a t u r a t i o n p o i n t , the microorganisms w i l l not t h r i v e . R e s t r i c t i n g or e l i m i n a t i n g an e s s e n t i a l component i n the micro-organism food chain such as m e t a l s , v i t a m i n s , e t c . , would cause the organisms t o look elsewhere f o r nourishment. M o d i f y i n g the wood so the organism d i d not r e c o g n i z e i t as food would a l s o p r o t e c t i t from a t t a c k . I n h i b i t i n g the enzyme systems such as the c e l l u l a s e s unique t o those organisms capable of b r e a k i n g wood down would a l s o p r o t e c t the wood without the treatment being harmful t o humans. The f i n a l c o n s i d e r a t i o n , a f a v o r a b l e environment, would c a p i t a l i z e on c r e a t i n g a h o s t i l e environment f o r the organism. For h i g h e r organisms that destroy wood, such as rodents, deer, b i r d s , e t c . , r e p e l l e n t s , which are not t o x i c to the organisms, but because of t h e i r s m e l l , t a s t e , or t e x t u r e , cause the pros p e c t i v e d i n e r to leave the t r e a t e d m a t e r i a l alone. Changing the pH of the wood o r m a i n t a i n i n g a temperature above or below that r e q u i r e d f o r organisms to t h r i v e w i l l e f f e c t i v e l y c o n t r o l t h e i r growth. The problem w i t h t h i s approach i s that some of the c o n d i t i o n s which are not f a v o r a b l e f o r the organisms a r e a l s o not f a v o r a b l e to m a i n t a i n the d e s i r a b l e p r o p e r t i e s of the wood. For example, a t a low pH, the wood components undergo h y d r o l y s i s causing severe s t r e n g t h l o s s e s . In d i s c u s s i n g concepts f o r p r e v e n t i n g a t t a c k by organisms not based on t o x i c i t y , the mechanism of e f f e c t i v e n e s s q u i t e l i k e l y i s not based on any s i n g l e mechanism, but a combination of s e v e r a l f a c t o r s . For example, i n the d i s c u s s i o n to f o l l o w on chemical m o d i f i c a t i o n of wood, the mechanism f o r the p r o t e c t i v e a c t i o n may be due t o : (a) b l o c k i n g conformational s i t e s r e q u i r e d f o r the h i g h l y s p e c i f i c enzyme-substrate r e a c t i o n s to take p l a c e , (b) p l u g g i n g h o l e s i n the l i g n i n - h e m i c e l l u l o s e s h i e l d p r o t e c t i n g the c e l l u l o s e , (c) s t a b i l i z i n g l a b i l e polymer u n i t s which may be the p o i n t of the fungus f i r s t a t t a c k , (d) removal of s o l u b l e chemicals i n the wood which a r e r e q u i r e d by the micro-organisms to s t a r t or s u s t a i n the a t t a c k , (e) changing the wood-water r e l a t i o n s h i p as to be i n i m i c a b l e t o m i c r o b i a l l i f e , and (f) combinations of these or other p o s s i b i l i t i e s . 1


4.

ROWELL

Research

Nonconventional

Wood

Preservation

49

Methods

Approaches

A. I r r a d i a t i o n . Ponderosa p i n e , red and white oak, sweetgum and D o u g l a s - f i r have been t r e a t e d w i t h h i g h l y p e n e t r a t i n g gamma r a d i a t i o n emitted by C o b a l t i n an attempt t o a l t e r the polymer s t r u c t u r e of the wood. A f t e r i r r a d i a t i o n of the wood, the decay r e s i s t a n c e was determined u s i n g : (a) the s o i l - b l o c k t e s t employing the fungus P o r i a m o n t i c o l a ( P o r i a p l a c e n t a ) (Madison 698) (1) o r agar-block t e s t s using L e n z i t e s trabea (Gloeophyllum trabeum) (Madison 617) ( 2 ) . R a d i a t i o n l e v e l s from 1 0 - 1 0 reps showed no change i n decay r e s i s t a n c e i n the i r r a d i a t e d wood over n o n i r r a d i a t e d c o n t r o l b l o c k s . I t has been found by s e v e r a l workers (3-5) that the primary e f f e c t s o f high-energy r a d i a t i o n on wood polymers are d e p o l y m e r i z a t i o n , d e c r y s t a l l i z a t i o n , and degradation. I t would be expected that the e f f e c t s o f i r r a d i a t i o n would cause a decrease i n the decay r e s i s t a n c e o f wood r a t h e r than an i n c r e a s e . B. Thiamine D e s t r u c t i o n . F a r r e r (6) showed that one o f the e s s e n t i a l m e t a b o l i t e s f o r f u n g a l growth, thiamine, was destroyed i n 2 hours a t 100°C a t pH 7. At the same temperature but a t pH 8, d e s t r u c t i o n was complete i n 1 hour and a t pH 9 i n 15 minutes. These r e s u l t s encouraged B a e c h l e r , et_ a l . (7_,8) t o t r e a t wood w i t h e i t h e r ammonia or sodium hydroxide to presumably destroy the thiamine, thus p r o t e c t i n g wood by removing an e s s e n t i a l t r a c e compound so long as o u t s i d e sources of thiamine were excluded. D o u g l a s - f i r , b i r c h , southern p i n e , and sweetgum b l o c k s were t r e a t e d w i t h 1% aqueous ammonia o r sodium hydroxide f o r v a r i o u s times, temperatures, and pressures ( 9 ) . These samples were subm i t t e d to s o i l - b l o c k t e s t s w i t h two brown-rot f u n g i P o r i a monticola (Madison 698) and L e n t i n u s l e p i d e u s (Madison 534) and two w h i t e - r o t f u n g i Polyporus v e r s i c o l o r ( C o r i o l u s v e r s i c o l o r ) (Madison 697) and _P. anceps (F 784-5) as w e l l as o u t s i d e exposure t e s t s (10). I n the s o i l - b l o c k t e s t s , the t r e a t e d wood was r e s i s t a n t to the two brown r o t t e r s , but was not r e s i s t a n t to the two white r o t t e r s . I n the outdoor stake t e s t s , the average l i f e time was 3.5 years w h i l e untreated c o n t r o l s had an average l i f e time o f 3.6 y e a r s . The outdoor t e s t s show that there i s no i n crease i n r o t r e s i s t a n c e by t h i s treatment. C. Heat Treatments. S e v e r a l woods have been heated under wet and dry h e a t i n g c o n d i t i o n s to determine the e f f e c t heat has on the decay r e s i s t a n c e of these woods. Alaska-cedar, A t l a n t i c white-cedar, b a l d cypress, D o u g l a s - f i r , mahogany, redwood, white oak, S i t k a spruce, and western redcedar were heated under dry c o n d i t i o n s o r wet c o n d i t i o n s a t temperatures of 80-180°C f o r v a r y i n g lengths o f time. Boyce (11) found that dry heat a t 100°C or steam heat a t 120°C f o r 20 minutes had no e f f e c t on the decay r e s i s t a n c e . S i m i l a r r e s u l t s were observed by Scheffer and E s l y n (12) i n s o i l - b l o c k t e s t s w i t h L e n z i t e s trabea f o r the heated softwoods and Polyporus v e r s i c o l o r f o r the heated hardwoods. Thus, heat treatments do not i n c r e a s e the decay r e s i s t a n c e of the 6 0


z


7


50

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

h e a t - t r e a t e d wood. I n some cases, a s l i g h t l o s s i n decay r e s i s t a n c e was observed. D. P l a s t i c Composites. Many d i f f e r e n t woods have been t r e a t e d w i t h o r g a n i c monomers and the monomers c a t a l y t i c a l l y polymerized w i t h i n the wood s t r u c t u r e . The s u b j e c t of t r e a t i n g wood t o form p l a s t i c composites i s covered by Dr. John Meyer i n another s e c t i o n of t h i s p u b l i c a t i o n . For the most p a r t , these composites have been prepared and s t u d i e d f o r t h e i r use i n dimen s i o n a l - s t a b i l i z e d products ( f o r example, see 13-17). Southern p i n e , D o u g l a s - f i r , and y e l l o w p o p l a r stakes were impregnated w i t h p h e n o l i c r e s i n and cured (impreg) o r impregnated w i t h p h e n o l i c r e s i n , compressed, and cured (compreg). Separate samples were t r e a t e d w i t h urea-formaldehyde and cured. These samples were p l a c e d i n the ground and t h e i r average l i f e t i m e determined. The r e s u l t s a r e shown i n Table I ( 1 8 ) . Table I . Average L i f e t i m e of Resin-•Impregnated Wood i n Ground Contact Treatment

Retention Lb/ft

3

Average

Life

ΧΕ

—

Ι.8-2.7

Impreg-phenolic r e s i n

5

6.8-11.7

Impreg-phenolic r e s i n

10

12.4-19.5

Compreg-phenol r e s i n

10

19.5

6

9.1

Control

Urea-formaldehyde

Ε. R e p e l l e n t s . I t i s q u e s t i o n a b l e whether a r e p e l l e n t would have any e f f e c t on micro-organisms, but they have been s t u d i e d f o r a p p l i c a t i o n f o r p r o t e c t i n g wood a g a i n s t higher l i f e forms. The amount of damage to wooden s t r u c t u r e s each year by animals i s c o n s i d e r a b l e . The b a s i c approach here i s to r e p e l the p r o s p e c t i v e d i n e r , not to k i l l i t . Various r e p e l l e n t s have been t e s t e d f o r d i f f e r ent animals (19). S o l u t i o n s or s l u r r i e s of these compounds have been p a i n t e d on wooden s t r u c t u r e s . Since no bonding to the wood components takes p l a c e , these r e p e l l e n t s are leached, v o l a t i l i z e d , broken down, and weathered out of the wood. A new approach i n t h i s area i s to encapsulate the r e p e l l e n t i n a r e s i s t a n t or slow r e l e a s e s h e l l . This way there i s very l i t t l e , i f any, r e p e l l e n c y u n t i l the animal comes i n t o contact w i t h the wood. T h i s contact causes the s h e l l to be broken and r e l e a s e s the r e p e l l e n t . The slow r e l e a s e type would slowly break



4.

ROWELL

Nonconventional

Wood Preservation

Methods

51

down and r e l e a s e the r e p e l l e n t over a p e r i o d of time depending on the s t a b i l i t y or weathering c h a r a c t e r i s t i c s of the capsule. The encapsulated chemicals could be added to a d i s p e r s a n t or p a i n t and a p p l i e d to the wood s u r f a c e . I f the capsules could be made small enough, deep p e n e t r a t i o n by pressure impregnation might be possible. F. Bound Toxins. Another approach to more environmentally acceptable p r e s e r v a t i v e s i s to c h e m i c a l l y bond a t o x i c compound onto a wood component so that i t cannot be leached out. The compound, once r e a c t e d , would have to r e t a i n i t s t o x i c p r o p e r t i e s . Compounds now used as wood p r e s e r v a t i v e s are t o x i c to the organism because they are ingested by the organism. I f the t o x i c compound were bound to the wood, they may be t o x i c to the organism only when ingested. Because of t h i s , the approach of perman e n t l y bound t o x i n s may not be a f r u i t f u l research area. I t i s a l s o p o s s i b l e to r e a c t a c i d c h l o r i d e s (20) or anhyd r i d e - c o n t a i n i n g compounds so as to form e s t e r bonds w i t h hydroxy1 groups on one of the wood components. E s t e r bonds could s l o w l y hydrolyze and r e l e a s e the bound t o x i n . In t h i s case, the r e l e a s e of the p r e s e r v a t i v e s would be a f u n c t i o n of the r a t e of h y d r o l y s i s and not d i r e c t l y r e l a t e d to weathering e f f e c t s ( f o r example, water s o l u b i l i t y , vapor pressure, UV degradation, e t c . ) . C o n t r o l l e d r e l e a s e f u n g i c i d e s based e i t h e r on slow h y d r o l y s i s or capsule e r o s i o n could g r e a t l y decrease the q u a n t i t y of preservat i v e needed to adequately p r o t e c t a wooden s t r u c t u r e , s i n c e l e a c h i n g could be c o n t r o l l e d . G. M e t a b o l i c D i f f e r e n c e . Micro-organisms a t t a c k wood by s e c r e t i n g enzymes i n t o the immediate s t r u c t u r e which i n t u r n break down the wood components i n t o s m a l l , s o l u b l e u n i t s that become n u t r i e n t s f o r the organism. The main d e s t r u c t i v e enzyme system the wood-rotters c o n t a i n i s a c l a s s of p r o t e i n s known as c e l l u l a s e s . These enzymes break down *~he polymeric c e l l u l o s e , the strong backbone of wood, i n t o d i g e s t i b l e u n i t s . Humans do not possess t h i s enzyme system; consequently, we cannot degrade cellulose-containing materials. C a p i t a l i z i n g on t h i s metabolic d i f f e r e n c e between higher forms of l i f e and micro-organisms i s the b a s i s f o r t h i s research approach to wood p r o t e c t i o n . Compounds are a v a i l a b l e which i n h i b i t the c e l l u l a s e enzyme systems ; however, t h e i r s p e c i f i c i t y has not been determined. MandeIs and Reese (21,22) found that the e x t r a c t s from the immature f r u i t of persimmon or the e x t r a c t from leaves of bayberry were very e f f e c t i v e i n h i b i t o r s of the c e l l u l a s e system. At c o n c e n t r a t i o n l e v e l s of .00005 and .00018%, r e s p e c t i v e l y , these two e x t r a c t s i n h i b i t e d the c e l l u l a s e enzymes i s o l a t e d from Trichoderma v i r i d e . I t i s not known what the a c t i v e component(s) are i n these two e x t r a c t s . New m a t e r i a l s need to be i n v e s t i g a t e d as p o s s i b l e s p e c i f i c i n h i b i t o r s to the c e l l u l a s e enzymes. This research approach w i l l r e q u i r e the screening of chemicals a g a i n s t pure enzyme s o l u t i o n s of known a c t i v i t y . S p e c i f i c i t y must be determined using a



52

w o o d t e c h n o l o g y : c h e m i c a l aspects

v a r i e t y of human-type enzymes such as t r a n s f e r a s e s , phosphoryl a s e s , dehydrogenases, e t c . U l t i m a t e success w i l l depend on f i n d i n g compounds which are only i n h i b i t o r y to the c e l l u l a s e enzymes. A more b a s i c approach to t h i s area would be to study the c e l l u l a s e enzymes themselves. I f the a c t i v e s i t e s and t r u e nature of t h i s p r o t e i n were known, s e l e c t i v e i n h i b i t i o n could be d e t e r mined . H. Chemical M o d i f i c a t i o n . The chemical m o d i f i c a t i o n of wood i n v o l v e s a chemical r e a c t i o n between some r e a c t i v e p a r t of a wood component and a simple s i n g l e chemical reagent, w i t h or w i t h out c a t a l y s t , to form a covalent bond between the two. The wood component may be c e l l u l o s e , h e m i c e l l u l o s e , or l i g n i n . The o b j e c t i v e of the r e a c t i o n i s to render the wood decay r e s i s t a n t . The mechanism of the e f f e c t i v e n e s s i s not known, but some p o s s i b l e explanations were given e a r l i e r . By f a r the most abundant r e a c t i v e chemical s i t e s i n wood are the hydroxyl groups on c e l l u l o s e , h e m i c e l l u l o s e , and l i g n i n . The types of covalent chemical bonds of the carbon-oxygen-carbon type that are of major importance are e t h e r s , a c e t a l s , and e s t e r s . The ether bond i s s t a b l e to bases, but l a b i l e to a c i d s , and the e s t e r bond i s l a b i l e to both a c i d s and bases. The t r e a t e d wood must s t i l l possess the d e s i r a b l e p r o p e r t i e s of untreated wood; the s t r e n g t h must remain h i g h , l i t t l e or no c o l o r change (unless a c o l o r change i s d e s i r a b l e ) , good e l e c t r i c a l i n s u l a t o r , not dangerous to handle, g l u a b l e , p a i n t a b l e , e t c . For t h i s reason, the chemicals to be considered f o r the m o d i f i c a t i o n of wood must be capable of r e a c t i n g w i t h wood h y d r o x y l groups under n e u t r a l or m i l d l y a l k a l i n e v-onditions at temperatures below 120°C. The chemical system should be simple and capab l e of s w e l l i n g the wood s t r u c t u r e to f a c i l i t a t e p e n e t r a t i o n . The complete reagent molecules should r e a c t q u i c k l y w i t h the wood components y i e l d i n g s t a b l e chemical bonds that w i l l r e s i s t weathering (23,24). These chemicals, once r e a c t e d , are e f f e c t i v e i n preventing a t t a c k by micro-organisms, but they are not t o x i c to the decay organisms. The important f a c t o r i n preventing a t t a c k i s to a t t a i n a treatment l e v e l which i n h i b i t s the growth of the organisms. A recent review on t h i s subject (23) shows that r e a c t i o n w i t h a c e t i c anhydride, dimethyl s u l f a t e , a c r y l o n i t r i l e , butylène oxide, phenyl isocyanate, and β-propiolactone a l l give good r o t r e s i s t ance at 17-25 weight percent gain (WPG). The exception to t h i s i s formaldehyde where a 2-5 WPG gives decay r e s i s t a n c e . I n t h i s case, there may be c r o s s l i n k i n g of l a r g e r wood u n i t s which gives i t d i f f e r e n t p r o p e r t i e s (25). Figure 1 shows that the decay r e s i s t a n c e of a c e t y l a t e d wood i s d i r e c t l y p r o p o r t i o n a l to the WPG (26). The degree of dimen s i o n a l s t a b i l i t y i s a l s o p r o p o r t i o n a l to the WPG so the e x c l u s i o n of c e l l w a l l or b i o l o g i c a l water may be a very important f a c t o r i n the decay r e s i s t a n c e mechanism.


4.

ROWELL

Nonconventional

Wood Preservation

Methods

53

The average s e r v i c e l i f e of a c e t y l a t e d y e l l o w b i r c h and cyanoethylated southern pine stakes i n ground contact i s shown i n Table I I ( 2 7 ) . Table I I . Average L i f e t i m e of Chemically Wood i n Ground Contact Treatment

--

Control Acetylation Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.ch004

L e v e l (WPG)

19.2

—

Control

Modified

Average L i f e

(Years)

2.7 17.5 3.6

Cyanoethylation

11

3.9

Cyanoethylation

15

5.3

I n p r e l i m i n a r y t e s t s , a l k y l e n e o x i d e - t r e a t e d southern pine (28) was found to be r e s i s t a n t to t e r m i t e a t t a c k and a t t a c k from the marine b o r e r s , Teredo (shipworm) and Limnoria. I . B a s i c Mechanism of A t t a c k . The u l t i m a t e s o l u t i o n f o r p r e v e n t i n g a t t a c k by micro-organisms w i l l come once we know how an organism breaks wood down. How does the organism know wood i s something to eat? What does i t recognize f i r s t t o s t a r t the a t t a c k ? What enzymes are v i t a l i n the i n i t i a l and sustained attack? I s there a s p e c i f i c weak l i n k i n those important enzymes that can be used to develop s e l e c t i v e i n h i b i t o r s ? I t i s a l s o p o s s i b l e that enzymatic r e a c t i o n s are not the only degrading r e a c t i o n s i n the d e t e r i o r a t i o n of wood by organisms. F i g u r e 2 shows i n the l e f t hand graph that only a 10-15% weight l o s s occurs i n the f i r s t 2 weeks of a t t a c k by brown-rot f u n g i . The graph on the r i g h t , however, shows that w i t h only a 10% weight l o s s , there i s a drop i n the average degree of p o l y m e r i z a t i o n of the h o l o c e l l u l o s e from 1,600 to 400 (29). This represents a f o u r - f o l d decrease i n DP, which a f f e c t s the strength of the wood w i t h very l i t t l e t o t a l weight l o s s . These r e s u l t s would i n d i c a t e t h a t , a t l e a s t i n the i n i t i a l a t t a c k , h y d r o l y t i c chemical r e a c t i o n s p l a y an important p a r t . I t has been suggested that hydrogen peroxide and i r o n a r e the cause of t h i s r a p i d depolymerization (30). I t i s p o s s i b l e that the i n i t i a l a t t a c k by micro-organisms i s not enzymatic but hydrol y t i c and o x i d a t i v e i n nature. I f t h i s i s t r u e , then a preservat i v e system could be based on a n t i - o x i d a n t p r o p e r t i e s of the chemical. I f the i n i t i a l a t t a c k can be stopped, then the t o t a l a t t a c k has been stopped. I t i s a l s o p o s s i b l e t h a t the i n i t i a l and sustained a t t a c k s are caused by a combination of chemical


54



40

r

WEIGHT PERCENT GAIN Figure 1.

Figure 2.

Decay resistance of acetylated wood

Action of brown-rot fungi on pine


4.

ROWELL

Ν unconventional

Wood

Preservation

Methods

55

(hydrolytic, oxidative, etc.) and enzymatic reactions.


Conclusions The purpose of this paper was to plant seeds for thought in the area of new methods of wood preservation, which are not based on broad spectrum toxicity for their effectiveness. The present concern for our environment has created the opportunity for re search in this area to find alternative wood preservatives which are effective in preventing attack by organisms and, at the same time, not harmful to the environment or man. Chemical modification of wood does result in a treatment which is nontoxic, effective, and nonleachable. The high chemi cal treatment level required for effectiveness, however, results in a rather expensive treatment. Dimensional stability is also obtained at these high (17-25 WPG) substitution levels so, for those products where both rot resistance and dimensional stability are important, the present state of the technology is close to a viable industrial process. Toxic chemicals which are permanently bound to the wood com ponents may be an environmentally acceptable preservation method. The actual effectiveness of a bound toxicant, however, s t i l l needs to be investigated. The encapsulation of preservatives is another interesting area for research. Procedures for encapsulation, capsule properties, and capsule size are important factors to be deter mined. Slow release fungicides by means of hydrolyzable link ages is also an interesting possibility. Basic knowledge of the nature of the attack of micro organisms on wood, the enzymes involved which are unique to micro-organisms, the chemical reaction which takes place in the i n i t i a l and sustained attack, and an investigation of specific inhibitors for these reactions is the most promising long-range approach. Literature Cited 1. Scheffer, T. C., Forest Prod. J . (1963), 13(5): 208. 2. Kenaga, D. L., and Cowling, Ε. Β., Forest Prod. J . (1959), 9(3): 112-116. 3. Lawton, E. J., Bellamy, W. D., Hungate, R. Ε., Bryant, M.P., and Hall, Ε., Science (1951), 113: 380-382. 4. Saeman, J . F., Millett, M. A., and Lawton, E. J., Ind. Eng. Chem. (1952), 44: 2848-2852. 5. Mater, J., Forest Prod. J . (1957), 7(6): 208-209. 6. Farrer, K.T.H., J . Proc. Austral. Chem. Inst. (1941), 8: 113. 7. Baechler, R., Forest Prod. J . (1959), 9(5): 166-171. 8. Gjovik, L. R., and Baechler, R. Η., Forest Prod. J . (1968), 18(1): 25-27. 9. Highley, T. L . , Phytopathology (1970), 60(11): 1660-1661.


56 10. 11. 12. 13. 14. 15.


16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

WOOD TECHNOLOGY: CHEMICAL ASPECTS Comparison of wood preservatives i n stake tests, USDA Forest Service Research Note FPL-02 (1975), 52. Boyce, J. S., Jr., J. of Forestry (1950), 48(1): 10. Scheffer, T. C . , and Eslyn, W. Ε., Forest Prod. J. (1961), 11(10): 485-490. Stamm, A. J., Forest Prod. J. (1959), 9(3): 107-110. Stamm, A. J., and V a l lier, Α. Ε., Forest Prod. J. (1954), 4(5): 305-312. Meyer, J. A., and Loos, W. Ε., Forest Prod. J. (1969), 19(12): 32-38. Loos, W. E., Wood Sci. & Tech. (1968), 2(4): 308-312. Choong, E. T., and Barnes, Η. Μ., Forest Prod. J. (1969), 19(6): 55-60. Ref. 10, 22: 13. Hampel, C. A., and Hawley, G. G., "The Encyclopedia of Chemistry," Van Nostrand Reinhold Co., 3rd ed., p. 968, 1973. A l l a n , G. G., Chopra, C. S., Neogi, Α. Ν., and Wilkins, R. M. Tappi (1971), 54(8): 1293-1294. Mandels, Μ., and Reese, E. T., Ann. Rev. of Phytopathology (1965), 3: 85-102. Mandels, Μ., and Reese, E. T., "Enzymic Hydrolysis of C e l l u lose and Related Materials," The MacMillan Co., New York, e d . , Ε. T. Reese, pp. 115-157, 1963. Rowell, R. M., Proc. Amer. Wood-Preservers Assoc. (1975), 71: 41-51. Rowell, R. M., Amer. Chem. Soc. Symp. Ser. No. 10 (1975), pp. 116-124. Stamm, A. J., and Baechler, R. Η., Forest Prod. J. (1960), 10(1): 22-26. Goldstein, I. S., Jeroski, E. G., Lund, Α. Ε., Nielson, J.F., and Weaver, J. W., Forest Prod. J. (1961), 11(8): 363-370. Ref. 10, 52: 26. Rowell, R. M., and Gutzmer, D. I., Wood Sci. (1975), 7(3): 240-246. Cowling, Ε. Β., "Comparative Biochemistry of the Decay of Sweetgum Sapwood by White-Rot and Brown-Rot Fungi," USDA Forest Service Tech. Bull. No. 1258, p. 50, 1961. Koenings, J. W., Wood & Fiber (1974), 6(1).


5 Thermal Deterioration of W o o d


FRED SHAFIZADEH and PETER P. S. CHIN Wood Chemistry Laboratory, University of Montana, Missoula, Mont. 59801

When wood is heated at elevated temperatures, it w i l l show a permanent loss of strength resulting from chemical changes in its components. The thermal decomposition can start at temperatures below 100°C if wood is heated for an extended period of time. Figure 1 shows that wood heated at 120° loses 10% of i t s strength in about one month, but it takes only one week to obtain the same loss of strength if it i s heated at 140° (1). Heating at higher temperatures gives v o l a t i l e decomposition products and a charred residue. The pyrolytic reactions and products control the combustion process and relate to the problems of c e l l u l o s i c f i r e s , chemical conversion of c e l l u l o s i c wastes and u t i l i z a t i o n of wood residues as an alternative energy source. In our laboratory, the pyrolytic reactions of wood and its major components have been investigated by a variety of analytical methods. Thermal analysis of cottonwood and i t s major components (2), as shown in Figures 2 and 3, indicates that the thermal behavior of wood reflects the sum of the thermal responses of i t s three major components, c e l l u l o s e , hemicellulose (xylan) and l i g n i n . A l l these substrates are i n i t i a l l y dried on heating at 50-100°. The hemicellulose component is the least stable and decomposes at 225-325°. Cellulose decomposes at higher temperatures within the narrower range of 325-375°. Lignin, however, decomposes gradually within the temperature range of 250-500°. The c e l l wall polysaccharides provide most of the v o l a t i l e pyrolysis products, while l i g n i n predominantly forms a charred residue. Since the thermal reactions of the wood components are highly complex and additive, they have been studied i n d i v i d u a l l y . Among the three major components, the pyrolysis reactions of cellulose have been most extensively studied. At temperatures above 300°, rapid cleavage of the glycosidic bond takes place,

57



WOOD

TECHNOLOGY:

1 hr.

CHEMICAL

.

Journal of the Forest Products Research Society

Figure 1. Effect of time and tempera ture on the modulus of elasticity of wood: (A) 10%,

(B) 2 0 % , and (C) 40%

Ί

m1n

'

fro

loss of modulus of elasticity (I),

50

150

250

îfe

ïfc—

im>. vo

350

450

Temperature (°C) Figure 2.

Differential thermal analysis of wood and its components


ASPECTS

Thermal

AND CHIN

Deterioration


SHAFizADEH

1 ^ fc I 110

130

I 10

I 15 I

150

I 20 I

170

I 25 I

190

1 30

min.

1 210

230

°C

R e t e n t i o n time (min) Temperature (degrees) Figure 4. Chromatogram of cellulose pyrolysis tar after reduction with NaBH : a, unknown; b, l,6-anhydro~β-Ό-glucopyranosè; c, lfi-anhydro-fi-O-glucofuranose; d, 3-deoxyhexitols; e, Ό-glucitol; f, oligosac charide derivatives i



60

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

producing 1,6-anhydro-g-D-glucopyranose (levoglucosan) and other t a r r y p y r o l y s i s products t h a t have been analyzed by chromatographic methods (see Figure 4) and are l i s t e d i n Table I ( 3 ) . This cleavage o f g l y c o s i d i c groups proceeds through a t r a n s g l y c o s y l a t i o n mechanism w i t h the p a r t i c i p a t i o n o f one of the f r e e hydroxyl groups, producing mainly 1,6-anhydro-6 -D-glucopyranose, which i s more s t a b l e than other anhydro sugars. The t a r f r a c t i o n a l s o contains randomly l i n k e d o l i g o - and p o l y s a c c h a r i d e s , p r o duced by secondary t r a n s g l y c o s y l a t i o n and condensation r e a c t i o n s . P y r o l y s i s of c e l l u l o s e under vacuum gives a high y i e l d of the v o l a t i l e p r o d u c t s , p a r t i c u l a r l y l e v o g l u c o s a n . At atmospheric p r e s s u r e , however, the y i e l d of levoglucosan drops s h a r p l y , due to f u r t h e r decomposition, which increases the y i e l d o f char. The t r a n s g l y c o s y l a t i o n r e a c t i o n s are preceded and accompanied by dehydration and e l i m i n a t i o n r e a c t i o n s t h a t produce wat e r and other dehydration products such as furan d e r i v a t i v e s and 1,6-anhydro-3,4-dideoxy-&-D-gjycero-hex-3-enopyranos-2-ulose (levoglucosenone) . The a d d i t i o n o f a c i d i c a d d i t i v e s , such as phosp h o r i c a c i d , diammonium phosphate, diphenyl phosphate and z i n c c h l o r i d e , can s i g n i f i c a n t l y c a t a l y z e the l a t t e r r e a c t i o n s . This i s i l l u s t r a t e d i n Figure 5 and Table I I . Figure 5 shows the enhanced y i e l d s o f levoglucosenone and 2-furaldehyde due to the a d d i t i o n of diphenyl phosphate, a strong Arrhenius a c i d , and Table II shows the e f f e c t o f H 3 P O 4 on promoting the production of levoglucosenone from various _Q-glucose-containing m a t e r i a l s i n c l u d i n g pure c e l l u l o s e , s t a r c h and wastepapers. The formation of levoglucosenone from the p y r o l y t i c dehydration o f c e l l u l o s e has r e c e n t l y been reported by d i f f e r e n t l a b o r a t o r i e s ( 4 - 6 ) . This compound was b e l i e v e d to be produced through the formation o f levoglucosan as shown i n Scheme 1, however, t h i s i s the s u b j e c t of some controversy ( 6 , 7 ) . In summary, the thermal degradation of c e l l u l o s e i n the temperature range of 300-350° can be dep i c t e d as shown i n Scheme 2. At higher temperatures, the i n t e r m e d i a t e s , i n c l u d i n g l e v o glucosan and the condensation products f u r t h e r p y r o l y z e to g i v e various products by f i s s i o n o f the carbohydrate u n i t s and r e a r rangement of the intermediate p r o d u c t s . Table I I I shows the p r o ducts obtained from the p y r o l y s i s of c e l l u l o s e and t r e a t e d c e l l u lose a t 600° ( 8 ) . The s i g n i f i c a n t increase i n the y i e l d s o f wat e r and char and decrease i n the y i e l d o f t a r i n the a c i d t r e a t e d c e l l u l o s e v e r i f i e s the p r e v i o u s l y mentioned promotion of dehydrat i o n and c h a r r i n g reactions by a c i d i c a d d i t i v e s . The p y r o l y s i s r e a c t i o n s i n v o l v e d i n h e m i c e l l u l o s e , i . e . , x y l a n , are s i m i l a r t o those i n v o l v e d i n c e l l u l o s e p y r o l y s i s . Table IV shows the p y r o l y s i s products formed from x y l a n at 300° (9). The p y r o l y s i s of x y l a n y i e l d s about 16% of t a r which cont a i n s 17% of a mixture of o l i g o s a c c h a r i d e s . Upon a c i d h y d r o l y s i s , they g i v e an approximately 54% y i e l d o f J - x y l o s e . Structural a n a l y s i s of the polymers shows t h a t they are branched-chain


SHAFizADEH

AND CHIN

Thermal

Deterioration


15

Scheme 1.

Pyrolysis of cellulose (A) to levoglucosan (B) and levoglucosenone (C )



62

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

polymers, i n d i c a t i n g t h a t they are d e r i v e d from random condensat i o n of x y l o s y l u n i t s which are formed by cleavage of the g l y c o s i d e groups s i m i l a r to t h a t occuring i n c e l l u l o s e p y r o l y s i s . The a d d i t i o n of a Lewis a c i d , i . e . , Z n C ^ s i g n i f i c a n t l y decreases the production of t a r and enhances the production of char due to the enhanced dehydration r e a c t i o n s . A t higher temperatures the g l y c o s y l u n i t s and the random condensation products are f u r t h e r degraded to a v a r i e t y o f v o l a t i l e p r o d u c t s , as shown i n Table V ( £ ) . Comparison o f t h i s t a b l e with the high temperature p y r o l y s i s products l i s t e d f o r c e l l u l o s e i n Table I I I shows t h a t the products of both f r a c t i o n s are b a s i c a l l y s i m i l a r . The s i g n i f i cant increase i n the y i e l d s o f 2 - f u r a l d e h y d e , water and char and decrease i n the y i e l d o f t a r by the a d d i t i o n of Z n C K v e r i f i e s the enhanced, dehydration and i s s i m i l a r t o observed e f f e c t s i n cellulose pyrolysis. Compared w i t h the p y r o l y s i s o f p o l y s a c c h a r i d e s , the p y r o l y s i s of 1 i g n i n i s r e l a t i v e l y unexplored. While the thermal r e a c t i o n s of l i g n i n occur over a wide temperature range of 2 5 0 - 5 0 0 ° , the decomposition i s most r a p i d between 3 1 0 - 4 2 0 ° , as i n d i c a t e d by the y i e l d o f gas and d i s t i l l a t e produced. Due to the complex s t r u c t u r e of l i g n i n , the mechanism of i t s thermal degradation i s not w e l l understood. Table VI l i s t s the major p y r o l y t i c products from l i g n i n ( 1 0 ) . The most abundant product i s c h a r , a h i g h l y condensed carbonaceous r e s i d u e , obtained i n about 55% y i e l d . The second f r a c t i o n of the p y r o l y t i c products i s an aqueous d i s t i l l a t e , produced i n about 20% y i e l d . I t contains mainly water and some methanol, acetone and a c e t i c a c i d . The y i e l d o f methanol f o r hardwood l i g n i n i s about 2%, twice as much as f o r softwood l i g n i n because i t c o n t a i n s a s y r i n g y l r a t h e r than guaiacy1 s t r u c t u r e . The y i e l d of a c e t i c a c i d from hardwood l i g n i n i s a l s o s i g n i f i c a n t l y higher than from softwood 1 i g n i n ; prèsumably i t o r i g i nates from the propanoid s i d e c h a i n s . The t h i r d f r a c t i o n of the p y r o l y t i c products i s t a r , which i s produced i n about 15% y i e l d . I t i s a mixture of p h e n o l i c compounds c l o s e l y r e l a t e d to phenolguaiacol and 2,6-dimethoxy-phenol, w i t h s u b s t i t u e n t s a t the p o s i t i o n para t o the hydroxyl group. The l a s t p y r o l y t i c f r a c t i o n i n cludes v o l a t i l e products such as CO, C H 4 , C 0 and ethane, p r o duced i n about 12% y i e l d . The 1 i s t o f p y r o l y s i s products of cottonwood shown i n Table VII (11) r e f 1 e c t s the summation of the p y r o l y s i s products of i t s three major components. The higher y i e l d s of acetone, p r o p e n a l , methanol, a c e t i c a c i d , C 0 , water and char from cottonwood, as compared to those obtainea from c e l l u l o s e and x y l a n , are l i k e l y a t t r i b u t e d to l i g n i n p y r o l y s i s . Other res u l t s are s i m i l a r to those obtained from the p y r o l y s i s o f c e l l - w a l l p o l y s a c c h a r i d e s . This f u r t h e r v e r i f i e s t h a t there i s no s i g n i f i c a n t i n t e r a c t i o n among the three major components during the thermal degradation of wood. The p y r o l y s i s products o f wood can be broadly grouped i n t o three c a t e g o r i e s as shown i n Scheme 3 , i . e . , the combustible 2

2


Thermal

Deterioration


SHAFIZAJDEH A N D C H I N



64

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

v o l a t i l e s , t a r and char. Upon f u r t h e r p y r o l y s i s , t a r i s f i n a l l y converted to v o l a t i l e s and c h a r . In the presence o f oxygen, the combustible v o l a t i l e s lead to f l a m i n g combustion w h i l e char r e acts by glowing combustion. The r a t e of combustible v o l a t i l e formation can be measured as a f u n c t i o n o f temperature by thermal e v o l u t i o n a n a l y s i s (TEA). This i n s t r u m e n t a t i o n u t i l i z e s a temperature programmed furnace combined w i t h a flame i o n i z a t i o n d e t e c t o r which responds i n a p r e d i c t a b l e manner to the evolved gases. The TEA of cottonwood i s shown i n Figure 6. I t shows the e v o l u t i o n o f combustible v o l a t i l e s i n two o v e r l a p p i n g stages due to the decomposition o f h e m i c e l l u l o s e and c e l l u l o s e f r a c t i o n s , r e s p e c t i v e l y . A previous study i n our l a b o r a t o r y revealed a good c o r r e l a t i o n between c a l o r i f i c values of the p y r o l y s i s products and t h e i r carbon c o n t e n t s , as shown i n Figure 7 (12). The TEA data can thus be used f o r c a l c u l a t i n g the heat content o f the v o l a t i l e s . An example i s given i n F i g u r e 8 (13), which shows the o r i g i n a l TEA data on the s c a l e on the l e f t and the converted heat values on the r i g h t f o r Douglas f i r needles before and a f t e r a sequence of e x t r a c t i o n s . P a r t Β of t h i s f i g u r e i s the o r i g i n a l cumulative d a t a , and Part A i s the data c a l c u l a t e d f o r d i f f e r e n t temperature i n t e r v a l s . This data i s important i n terms of p r e d i c t i n g the f l a m m a b i l i t y of the samples, s i n c e i t r e f l e c t s the f r a c t i o n of the t o t a l heat content a c t u a l l y made a v a i l a b l e through gas phase com b u s t i o n to propagate the f i r e . The c h a r r i n g o f c e l l w a l l polysaccharides i n v o l v e s a s e r i e s o f r e a c t i o n s i n c l u d i n g d e h y d r a t i o n , condensation and c a r b o n i z a t i o n . The dehydration and the e f f e c t of a c i d i c a d d i t i v e s i n p r o moting dehydration have been d i s c u s s e d . The dehydration products f u r t h e r undergo condensation r e a c t i o n s , e s p e c i a l l y i n the p r e sence o f Z n C l . The unique e f f e c t of ZnClo on promoting conden s a t i o n r e a c t i o n s i s i l l u s t r a t e d i n F i g u r e 9 {S) which shows that the a d d i t i o n of Z n C l s i g n i f i c a n t l y reduces the evaporation of levoglucosenone by promoting the condensation o f t h i s dehy d r a t i o n product to n o n v o l a t i l e m a t e r i a l s that are charred on f u r t h e r h e a t i n g . The c a r b o n i z a t i o n r e a c t i o n s i n v o l v e f u r t h e r e l i m i n a t i o n o f the s u b s t i t u e n t s , production o f s t a b l e f r e e r a d i c a l s and formation of new carbon bonds on f u r t h e r h e a t i n g . A study of the temperature dependence of f r e e r a d i c a l form a t i o n o f wood and i t s three major components by ESR i s shown i n Figures 10-13. This data i n d i c a t e s that up t o 3 5 0 ° , the f r e e r a d i c a l s formed from heating of wood are mainly from c e l l w a l l polysaccharides. L i g n i n , at t h i s temperature range, generates very small concentrations of f r e e r a d i c a l s . The a d d i t i o n of a c i d i c a d d i t i v e s lowers the decomposition temperature o f wood and i t s components, i n c l u d i n g 1 i g n i n . However, they promote f r e e r a d i c a l formation o n l y i n the c e l l wall p o l y s a c c h a r i d e s , not i n l i g n i n . An exception i s Z n C l which produces a s l i g h t increase i n f r e e r a d i c a l formation i n f i g n i η at temperatures above 3 0 0 ° . 2

9

2

2


SHAFizADEH

AND

Thermal

CHIN

Deterioration

65


5.

î

Figure 7.

I

I

30

40

I

1

50 60 P e r c e n t Carbon

I

70

Heat of combustion at 4O0°C vs. percent carbon: V • char, Ο vohtiles

fuels,


WOOD TECHNOLOGY:

CHEMICAL

ASPECTS


ËS BEFORE REMOVAL OF EXTRACTIVES Π AFTER REMOVAL OF ETHER EXTRACTIVES Μ AFTER REMOVAL OF TOTAL EXTRACTIVES

100

200

300

400

500

TEMPERATURE (*C) • BEFORE REMOVAL OF EXTRACTIVES • AFTER REMOVAL OF ETHER EXTRACTIVES • AFTER REMOVAL OF TOTAL EXTRACTIVES

3500

2500

8 ο 1500

S 500

300

500

TEMPERATURE ( C) e

Figure 8. Evolution of carbon and heat from Douglas-fir foliage (A) in tem perature intervals and (B) cumulative, based on dry weight of the unextracted sample


In Wood Technology: Chemical Aspects; Goldstein, I.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977. (degrees)

Figure 9. Thermal analysis of levoglucosenone samples: (a) neat, (b) -\-5% zinc chloride, (c) + 5 % diammonium phosphate, and (d) + 5 % diphenyl phosphate (D.t.a., differential thermal analysis; T.g., thermogravimetry; D.t.g., derivative of thermogravimetry)

Temperature


CHEMICAL

ASPECTS


WOOD TECHNOLOGY:

Jo

i


55

.BP


Figure 11.

The rate of free radical formation (—A—)

(«Ο

and weight loss ( — Δ — )

Temp,

on heating xylan and treated xylan


CD

pi


Figure 12.


(«Ο

and weight loss ( — Δ — )

Temp.

on heating of lignin and treated lignin



Figure 13.


(«ς)

and weight loss (--A--)

Temp,

on heating of wood and treated wood



72

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

This i n d i c a t e s t h a t the thermal decomposition o f 1 i g n i n at temperatures below 350° i s h e t e r o l y t i c , most 1 i k e l y o c c u r r i n g by the cleavage o f s i d e c h a i n s . This i s true i n neat 1 i g n i n as w e l l as i n the presence o f a c i d c a t a l y s t s . However, the r a t e of f r e e r a d i c a l formation of l i g n i n samples increases s h a r p l y at 3 5 0 ° , i n d i c a t i n g the homolytic f i s s i o n o f l i g n i n predominates at h i g h er temperatures. The ESR data from c e l l w a l l polysaccharides shows a twostage f r e e r a d i c a l f o r m a t i o n ; a low temperature f r e e r a d i c a l formation stage which corresponds to the i n i t i a l decomposition of the carbohydrate polymers, and a high temperature f r e e r a d i c a l formation stage which corresponds t o the f i n a l c h a r r i n g r e a c t i o n s between 300 and 3 5 0 ° . This two-stage f r e e r a d i c a l forma t i o n phenomena i s e s p e c i a l l y c l e a r f o r the Z n C ^ t r e a t e d x y l a n sample. This i s due to the low decomposition temperature f o r t h i s sample, which produces a c l e a r separation between these two s t a g e s . These phenomena i n d i c a t e t h a t during the i n i t i a l decomposition o f carbohydrate polymers, the h e t e r o l y t i c r e a c t i o n s , such as t r a n s g l y c o s y 1 a t i o n , are accompanied by homolytic react i o n s . The low temperature f r e e r a d i c a l formation i s probably a s s o c i a t e d with the dehydration and e l i m i n a t i o n r e a c t i o n s and the condensation of unsaturated p r o d u c t s . The increased r a t e of high temperature f r e e r a d i c a l formation i n c e l l w a l l polysacchar i d e s i s accompanied by a small weight l o s s , i n d i c a t i n g t h a t t h i s f r e e r a d i c a l formation i s caused by c r a c k i n g of the bonds i n the char substrate r a t h e r than by cleavage of the s u b s t i t u e n t s . The f r e e r a d i c a l formation i n wood i s roughly the summation of t h a t f o r i t s three major components. Tables V I I I and IX are a b r i e f summary o f r e s u l t s from work r e l a t e d to the use o f c e l l u l o s i c f u e l s as an energy source, both i n terms of propagation o f f i r e and as a renewable a l t e r n a t i v e energy source. Table V I I I shows the heats of combustion o f the f u e l and i t s p y r o l y s i s p r o d u c t s . Table IX shows the d i s t r i b u t i o n of the heat content i n the v o l a t i l e and char f r a c t i o n s . The energy released i n the gas phase i s much higher f o r c e l l u l o s e than f o r l i g n i n , although the heats o f combustion o f l i g n i n and i t s gaseous p y r o l y s i s products are much higher than those o f c e l l u l o s e . Consequently, softwood, although i t s heat of combustion i s over 500 cal/g more than that o f hardwood, produces very l i t t l e more heat i n the gas phase. This i s because the higher o r i g i n a l heat content o f softwood i s due t o i t s higher l i g n i n content. These data a l s o c l e a r l y p o i n t out the value o f these f u e l s as an energy source, e i t h e r a f t e r c a r b o n i z a t i o n or i n t h e i r o r i g inal state.


5.

SHAFizADEH

and

chin

Thermal

Deterioration

73

Appendix

TABLE I.

ANALYSIS OF THE PYROLYSIS PRODUCTS OF CELLULOSE AT 300° UNDER NITROGEN


Condition

Atm. pressure

1.5 Mm Hg, 5% S b C l

1.5 Mm Hg

3

Char

34.2%

17.8%

25.8%

Tar

19.1

55.8

32.5

levoglucosan

3.57

28.1

6.68

1,6-anhydro-e-Ilglucofuranose

0.38

5.6

0.91

trace

trace

2.68

^-glucose hydro!yzable material s

TABLE I I .

6.08

11.8

20.9

YIELDS OF LEVOGLUCOSENONE FROM THE PYROLYSIS OF DIFFERENT MATERIALS AT 3 5 0 ° *

Material

Neat

(55)

5% H P0 - t r e a t e d 3

4

Cellulose

1.2

11.1

Starch

0.3

9.0

Newsprint (with ink)

Tb

9.1

K r a f t shopping bags

Τ

10.2

a.

Determined by p y r o l y z i n g 5 mg samples and d i r e c t l y the v o l a t i l e s by GLC.

b.

Τ = t r a c e amount.

(%)

analyzing


74

WOOD

TABLE I I I .

PYROLYSIS PRODUCTS OF CELLULOSE AND TREATED CELLULOSE AT 600°

Product


TECHNOLOGY: CHEMICAL ASPECTS

Neat

Acetaldehyde

1.5

Furan

+5% H P 0 3

4

+5% ( N H ) H P 0 4

2

4

+5% ZnCl 2

0.9

0.4

1.0

0.7

0.7

0.5

3.2

Propenal

0.8

0.4

0.2

Τ

Methanol

1.1

0.7

0.9

0.5

Τ

0.5

0.5

2.1

2.0

2.0

1.6

1.2

2.8

0.2

Τ

0.4

Acetic acid

1.0

1.0

0.9

0.8

2-Furaldehyde

1.3

1.3

1.3

2.1

5-Methyl-2furaldehyde

0.5

1.1

1.0

0.3

Carbon dioxide

6

5

6

3

11

21

26

23

5

24

35

31

66

41

26

31

2-Methylfuran 2,3-Butanedione l-Hydroxy-2- . propanone I J

Glyoxal

Water Char Balance

(tar)

a

Percentage, y i e l d based on the weight of the sample; Τ = trace amounts.


SHAFizADEH

AND


TABLE IV.

CHIN

Thermal

Deterioration

PYROLYSIS PRODUCTS OF XYLAN AND TREATED XYLAN AT 300°

Product

Neat

Liquid condensate

30.6

+10X Z n C l

a

45.3

7.9

7.5

Char

31.1

42.2

Tar

15.7

3.2

Carbon dioxide

High mol. wt. component

(17)*

D-xylose from hydrolysis

(54)

2

C

Percentage, y i e l d based on the weight of the sample. ^Based on the weight of the tar 'Based on the weight of oligosaccharides.


76

WOOD TECHNOLOGY:

TABLE V.

Neat

Xylan +10% Z n C l

2

0-Acetylxylan +10% ZnCl Neat

0.1

1.0

1.9

Τ

2.0

2.2

3.5

0.3

Τ

1.4

Τ

1.3

1.0

1.0

1.0

Τ

Τ

Τ

Τ

1-Hydroxy-2propanone

0.4

Τ

0.5

Τ

3-Hydroxy-2butanone

0.6

τ

0.6

Τ

Acetic acid

1.5

τ

10.3

9.3

2-Furaldehyde

4.5

2.2

5.0

Carbon d i o x i d e

8

7

8

6

Water

7

21

14

15

10

26

10

23

64

32

49

35

Acetaldehyde

2.4

Furan Acetone

^

Propionaldehyde Methanol 2,3-Butanedione

Char Balance

ASPECTS

PYROLYSIS PRODUCTS OF XYLAN AND TREATED XYLAN AT 500*

Product


CHEMICAL

(tar)

a

10.4

Percentage, y i e l d based on the weight of the sample; Τ = trace amounts.


2


12 20 15 55

water, methanol, acetone, acetic a d d

phenolic compounds

carbonaceous residue

Liquid

Tar

Char

Yield

carbon monoxide, méthane» carbon d i o x i d e , ethane

Products

PYROLYSIS PRODUCTS OF LIGNIN AT 450-550°

Volatile

Fraction

TABLE V I .


(Ï)

WOOD TECHNOLOGY:

78


TABLE V I I .

CHEMICAL

PYROLYSIS PRODUCTS OF WOOD AND TREATED WOOD AT 600° +52

ZnCl

Product

Neat

Acetaldehyde

2.3

Furan

1.6

7.9

1.5

0.9

Propenal

3.2

0.9

Methanol

2.1

2.7

Acetone

%

Propionaldehyde

J

a

4.4

2>

2-Methylfuran

b

2,3-Butanedione

2.0

1.0

1-Hydroxy-2-propanone

2.1

Τ

Glyoxal

2.2

Τ

Acetic acid

6.7

5.4

2-Furaldehyde

1.1

5.2

Formic acid

0.9

0.5

5-Methyl-2-furaldehyde

0.7

0.9

2-Furfuryl alcohol

0.5

Τ

Carbon dioxide

12

6

Water

18

18

Char

15

24

28

22

Balance

ASPECTS

(tar)

Percentage, y i e l d based on the weight of the sample; Τ = trace amounts. ^Not c l e a r l y i d e n t i f i a b l e for wood.


2

SHAFiZADEH

AND

CHIN

Thermal

Deterioration


5.


79


Douglas F i r

Populus ssp.

Softwood

Hardwood

heated at 400° for 10 min.

Klason

Lignin

a

-4375

F i l t e r paper

Cellulose

-1546

-1987

-1050

Type

Source

a

Char ( c a l / g fuel)

-3072

-3169

-1995

-3093

a

Gas ( c a l / g fuel)

-4618

-5156

-6370

-4143

Total (cal/g)

DISTRIBUTION OF THE HEAT OF COMBUSTION OF WOOD AND ITS COMPONENTS

Fuel

TABLE I X .


5. SHAFIZADEH AND CHIN

81

Thermal Deterioration

Literature Cited 1.

Seborg, R.M., Tarkow, Η., and Stamm, A.J., J. Forest Prod. Res.

2.

16, 3.

Soc.,

(1953),

3,

59.

Shafizadeh, F. and McGinnis, G.D., Carbohyd. Res.,

(1971),

273-277.

Shafizadeh, F. and Fu. Y.L., Carbohyd. Res.,

(1973),

29,

113-122.

4.

Halpern, Y., Riffer, R., and Broido, A., J. Org. Chem.,

5.

Shafizadeh, F. and Chin,


(1973),

38,

204-209.

P.P.S.,

Carbohyd. Res.,

(1976),

46,

149-154. 6.

7.

Fung, D.P.C., Wood Science, ( 1 9 7 6 ) , 9, 5 5 - 5 7 . Broido, A., Evett, Μ., and Hodges, C.C., Carbohyd. ( 1 9 7 5 ) , 44,

Res.,

267-274.

8.

Chin, P.P.S., Ph.D. Dissertation, University of Montana

9.

Shafizadeh, F., McGinnis, G.D., and Philpot, C.W., Carbohyd.

(1973). Res.,

10. 11.

(1972),

25,

23-33.

A l l a n , G.G. and M a t t i l a , T., Lignins, Sarkanen, K.V. and Ludwig, C.H., Eds., Wiley-Interscience Publishers, New York, 1 9 7 1 , p. 5 7 5 . Philpot, C.W., Ph.D. Dissertation, University of Montana (1970).

12. 13.

Susott, R.A., DeGroot, W.F., and Shafizadeh, F. J. Fire and Flammability, ( 1 9 7 5 ) , 6 , 3 1 1 - 3 2 5 . Shafizadeh, F., Chin, P.P.S., and DeGroot, W.F., Forest Science, in press.


6 Effect of Fire-Retardant Treatments on Performance Properties of W o o d


CARLTON A. HOLMES Forest Products Laboratory, Forest Service, U.S. Dept. of Agriculture, P.O. Box 5160, Madison, Wis. 53705

The one m i l l i o n f i r e s i n buildings in the United States account for about two-thirds of the 12,000 people who die each year in f i r e s . The property loss i n building f i r e s i s about 85 percent of the t o t a l annual $3 billion property loss i n f i r e s (1). Building contents are often a primary source of f i r e and are usually responsible for fire-related deaths before struct u r a l members become involved. Nevertheless, wood and wood-base products, extensively used both as structural members and as interior f i n i s h i n housing and buildings, can be contributors to fire destruction. To reduce the contribution of wood to fire losses, much research through the years has gone into development of fire– retardant treatments for wood. A t o t a l of 21.3 m i l l i o n pounds of fire-retardant chemicals were reported used i n 1974 to treat 5.7 m i l l i o n cubic feet of wood products (2). The amount of wood treated was about one tenth of 1 percent of the t o t a l domestic production of lumber and plywood and has increased ninefold i n 20 years. How does our research stand i n rendering wood f i r e retardant? What i s the effect of fire-retardant treatments on the f i r e performance properties of wood and on the physical and mechanical properties of wood that are important to i t s u t i l i t y ? Discussion w i l l be limited to f i r e retardancy obtained by pressure impregnation, which i s currently the most effective method. F i r e retardant coatings, wood-plastic combinations, and chemical modi f i c a t i o n s of wood w i l l not be considered. Fire-Retardant Chemicals Past research on f i r e retardants, including those for wood, from about 1900 to 1968 i s reviewed i n John W. Lyons' comprehensive reference book, "The Chemistry and Uses of F i r e Retardants" (3). A more recent review by Goldstein (4) gives additional information on fire-retardant chemicals and treatment systems for wood and also discusses some of the topics of this 82



6.

HOLMES

Fire-Retardant

Treatment

83

present paper more thoroughly. These two r e f e r e n c e s , together w i t h the o l d e r review by Browne (5) , a r e recommended t o t h e reader as b a s i c reviews on s e l e c t i o n and chemistry of f i r e r e t a r d a n t s f o r wood. The chemistry of s y n e r g i s t i c e f f e c t s between chemicals i n a f i r e - r e t a r d a n t system i s presented by Lyons ( 3 ) ; i t was a l s o d i s cussed more r e c e n t l y by Junej a (6,7) and recommended by him as an area of needed i n v e s t i g a t i o n . F i r e - r e t a r d a n t chemicals used by the commercial wood-treating i n d u s t r y a r e l i m i t e d almost e x c l u s i v e l y to mono- and diammonium phosphate, ammonium s u l f a t e , borax, b o r i c a c i d , and z i n c c h l o r i d e (4 . I t i s b e l i e v e d t h a t some use i s a l s o made of the l i q u i d ammonium polyphosphates ( 9 ) . Some a d d i t i v e s such as sodium dichrornate as a c o r r o s i o n i n h i b i t o r a r e a l s o used. Aqueous f i r e r e t a r d a n t treatment s o l u t i o n s a r e u s u a l l y formulated from two o r more of these chemicals to o b t a i n the d e s i r e d p r o p e r t i e s and c o s t advantages. For l e a c h - r e s i s t a n t type treatments, the l i t e r a t u r e shows t h a t some or a l l o f the f o l l o w i n g a r e used : urea, melamine, dicyandiamide, phosphoric a c i d , and formaldehyde (10-12). E f f e c t of F i r e - R e t a r d a n t Treatment on F i r e Performance P r o p e r t i e s What a r e t h e f i r e performance p r o p e r t i e s of untreated wood and how a r e these p r o p e r t i e s a l t e r e d by f i r e - r e t a r d a n t treatments? Ignition Wood, l i k e a l l o r g a n i c m a t e r i a l s , c h e m i c a l l y decomposes— p y r o l y z e s — w h e n subjected t o h i g h temperatures, and produces char and p y r o l y s a t e vapors or gases. When these gases escape to and from the wood s u r f a c e and a r e mixed w i t h a i r , they may i g n i t e , w i t h o r without a p i l o t flame, depending on temperature. I g n i t i o n — t h e i n i t i a t i o n of c o m b u s t i o n — i s evidenced by glowing on the wood s u r f a c e or by presence of flames above the s u r f a c e . The temperature of i g n i t i o n i s i n f l u e n c e d by many f a c t o r s r e l a t e d t o the wood under thermal exposure and the c o n d i t i o n s of i t s environment (5,13,14). F a c t o r s i n c l u d e s p e c i e s , d e n s i t y , moisture content, t h i c k n e s s and s u r f a c e a r e a , s u r f a c e absorpt i v i t y , p y r o l y s i s c h a r a c t e r i s t i c s , thermoconductivity, s p e c i f i c heat, and e x t r a c t i v e s content. Environmental c o n d i t i o n s a f f e c t i n g temperature of i g n i t i o n i n c l u d e d u r a t i o n and u n i f o r m i t y of exposure, heating r a t e , oxygen supply, a i r c i r c u l a t i o n and v e n t i l a t i o n , degree of confinement or space geometry surrounding the exposed wood element or member, temperature and c h a r a c t e r i s t i c s of an adjacent or c o n t a c t i n g m a t e r i a l , and amount of r a d i a n t energy present. Reviews c o v e r i n g i g n i t i o n of c e l l u l o s i c s o l i d s by Kanury ( 1 3 ) , B e a l l and E i c k n e r (15), Browne Ç5), and Matson e t a l . (14) r e p o r t a wide range o f i g n i t i o n temperatures obtained on wood, and dependence on r a d i a n t or c o n v e c t i v e nature of heat. For r a d i a n t


84

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

heating of c e l l u l o s i c s o l i d s , Kanury (13) r e p o r t s spontaneous t r a n s i e n t i g n i t i o n a t a c r i t i c a l temperature of 600°C w i t h p i l o t e d t r a n s i e n t i g n i t i o n a t 300°C to 410°C. P e r s i s t e n t f l a m i n g i g n i t i o n i s r e p o r t e d a t a temperature g r e a t e r than about 320°C. With c o n v e c t i v e h e a t i n g of wood under l a b o r a t o r y c o n d i t i o n s , spontaneous i g n i t i o n i s r e p o r t e d as low as 270°C and as high as 470°C (5,14,15). Spontaneous i g n i t i o n of wood c h a r c o a l , which has e x c e l l e n t a b s o r p t i o n of oxygen and r a d i a n t heat, occurs between 150°C and 250°C ( 5 ) . I n one experiment on i g n i t i o n , oven-dried s t i c k s of n i n e d i f f e r e n t species were i g n i t e d by p i l o t flame i n 14.3 t o 40 minutes when held a t 180°C, i n 4 to 9.5 minutes when held a t 250°C and i n 0.3 to 0.5 minutes when held a t 430°C (16). Many f i e l d r e p o r t s c o l l e c t e d by U n d e r w r i t e r s L a b o r a t o r i e s , Inc. (UL) (14) show i g n i t i o n o c c u r r i n g a t or near 212°F (100°C) on wood next to steam pipes or other hot m a t e r i a l s . Laboratory experiments have not been a b l e to c o n f i r m these low i g n i t i o n tempe r a t u r e s (5,14,16,17). To p r o v i d e a margin of s a f e t y , Underw r i t e r s ' L a b o r a t o r i e s , Inc. suggests that wood not be exposed f o r long p e r i o d s of time a t temperatures g r e a t e r than 90 F (32°C) above room temperature or 170°F (77°C). The N a t i o n a l F i r e P r o t e c t i o n A s s o c i a t i o n handbook (18) g i v e s 200°C as the i g n i t i o n temperature of wood most commonly quoted, but g i v e s 66°C as the h i g h e s t temperature to which wood can be c o n t i n u a l l y exposed w i t h out r i s k of i g n i t i o n . McGuire (17) of the N a t i o n a l Research C o u n c i l of Canada suggests that 100°C would be a s a t i s f a c t o r y c h o i c e of an upper l i m i t i n g temperature f o r wood exposure. U s u a l l y the f i r e - r e t a r d a n t treatment of wood s l i g h t l y i n c r e a s e s the temperature a t which i g n i t i o n w i l l take p l a c e . There i s evidence, however, that wood t r e a t e d w i t h some chemical r e t a r d a n t s a t low r e t e n t i o n l e v e l s w i l l i g n i t e (flame) or s t a r t glowing combustion a t s l i g h t l y lower temperatures or i r r a d i a n c e l e v e l s than does untreated wood (19,20), though sustained comb u s t i o n i s u s u a l l y prevented or hindered.


1

Q

Thermal Degradation An e x t e n s i v e review of the l i t e r a t u r e to 1958 on thermal degradation of wood i s g i v e n by Browne ( 5 ) . B e a l l and E i c k n e r (15) and G o l d s t e i n (4) add a d d i t i o n a l review i n f o r m a t i o n on t h i s complex s u b j e c t . S h a f i z a d e h s (21) review of the p y r o l y s i s and combustion chemistry of c e l l u l o s e g i v e s a b a s i s f o r understanding these processes i n wood and the e f f e c t of f i r e - r e t a r d a n t treatment on these processes. Browne (5) d e s c r i b e d the p y r o l y s i s r e a c t i o n s and events which occur i n each of four temperature zones or ranges when s o l i d wood of a p p r e c i a b l e t h i c k n e s s i s exposed to heat i n absence of a i r . Zone A i s below 200°C; Zone B, 200° to 280°C; Zone C, 280° t o 500°C; and Zone D, above 500°C. These zones may be present s i m u l taneously. When wood i s heated i n a i r , events o c c u r r i n g i n these 1



6.

HOLMES

Fire-Retardant

Treatment

85

temperature zones i n c l u d e o x i d a t i o n r e a c t i o n s and, a f t e r i g n i t i o n , combustion of the p y r o l y s i s and o x i d a t i o n products. G o l d s t e i n (4) more simply d i v i d e d the thermal degradation processes i n t o those o c c u r r i n g a t low temperatures, below 200°C, and those a t h i g h temperatures, above 200°C. Decomposition of wood exposed to temperatures below 200°C i s slow but measurable (22,23). For example, the average l o s s i n weight of 11 s p e c i e s of wood was 2.7 percent i n 1 year a t 93°C and 21.4 percent i n 102 hours a t 167°C (22). Sound wood w i l l not g e n e r a l l y i g n i t e below 200°C s i n c e products evolved a r e mostly carbon d i o x i d e and water vapor. High temperature degradation processes above 200°C i n c l u d e r a p i d p y r o l y s i s of the wood components, combustion o f flammable gases and t a r s , glowing of the char r e s i d u e , and e v o l u t i o n of unburned gases, vapors, and smoke. The most w i d e l y accepted theory of the mechanism of f i r e r e t a r d a n t chemicals i n reducing flaming combustion of wood i s that the chemicals a l t e r the p y r o l y s i s r e a c t i o n s w i t h formation of l e s s flammable gases and t a r s and more char and water (4,5,8,21,24-29). Some f i r e r e t a r d a n t s s t a r t and end the chemical decomposition a t lower temperatures. Heat of combustion of the v o l a t i l e s i s r e duced. Shafizadeh (21) suggests that a primary f u n c t i o n of f i r e r e t a r d a n t s i s to promote dehydration and c h a r r i n g of c e l l u l o s e . The normal degradation of c e l l u l o s e to the flammable t a r , levoglucosan, i s reduced and the c h a r r i n g of t h i s compound i s promoted. Shafizadeh and coworkers used thermogravimetric (TG) and thermal e v o l u t i o n a n a l y s i s (TEA) data, to conf irm two d i f f e r e n t mechanisms i n v o l v e d i n flameproofing c e l l u l o s i c m a t e r i a l s : 1) d i r e c t i n g the p y r o l y s i s r e a c t i o n s to produce char, water, and carbon d i o x i d e i n p l a c e of flammable v o l a t i l e s , and 2) p r e v e n t i n g the f l a m i n g combustion of these v o l a t i l e s ( 2 7 ) . F i r e Penetration The property of a wood m a t e r i a l or assembly t o r e s i s t the p e n e t r a t i o n of f i r e or to continue to perform a g i v e n s t r u c t u r a l f u n c t i o n , or both, i s commonly termed f i r e r e s i s t a n c e . The measure of elapsed time that a m a t e r i a l or assembly w i l l e x h i b i t f i r e r e s i s t a n c e under the s p e c i f i e d c o n d i t i o n s of t e s t and performance i s c a l l e d f i r e endurance. Large furnaces a r e used t o measure f i r e endurance of w a l l s , f l o o r s , r o o f s , doors, columns, and beams under the standard ASTM E119 (30) time-temperature exposure c o n d i t i o n s . Wood has e x c e l l e n t n a t u r a l r e s i s t a n c e to f i r e p e n e t r a t i o n due to i t s low thermal c o n d u c t i v i t y and to the c h a r a c t e r i s t i c of forming an i n s u l a t i n g l a y e r of c h a r c o a l w h i l e burning. The wood beneath the char s t i l l r e t a i n s most of i t s o r i g i n a l s t r e n g t h properties. In wood c h a r r i n g s t u d i e s by S c h a f f e r a t the F o r e s t Products Laboratory (FPL) (31), 3 - i n c h - t h i c k p i e c e s of wood were v e r t i c a l l y


86

WOOD

TECHNOLOGY: C H E M I C A L ASPECTS

exposed to f i r e on one s u r f a c e . Rate of char development a t three constant f i r e exposure temperatures, 1,000°F (538°C) 1,500°F (816 C), and 1,700°F (927°C), was described by an equation w i t h an Arrhenius temperature-dependent r a t e constant. When specimens were exposed to the uniformly i n c r e a s i n g f i r e temperatures of ASTM E119 ( e a r l i e r l i n e a r p o r t i o n of timetemperature curve) (30), the r a t e of char development was constant, a f t e r the more r a p i d l y developed f i r s t 1/4 i n c h o f char. Under the standard ASTM f i r e exposure, temperatures 1/4 i n c h from the specimen s u r f a c e reached 1,400°F (760°C) a t 15 minutes, 1,700°F (927 C) a t 1 hour, and 1,850°F (1,010°C) a t 2 hours (31). When wood i s exposed to these c o n d i t i o n s , the f i r s t v i s u a l e f f e c t of thermal degradation (Figure 1) i s i n d i c a t e d by browning of the wood a t about 350°F to 400°F (175°C to 200 C). The temperature which c h a r a c t e r i z e d the base of the char l a y e r was 550°F (288°C). A f t e r the f i r s t 1/4 i n c h of char development, the r a t e that t h i s char l a y e r moved i n t o the s o l i d wood—the r a t e o f f i r e penet r a t i o n — w a s about 38 m i l l i m e t e r s per hour (1-1/2 i n / h r ) . Schaffer (31) found some d i f f e r e n c e s i n char development r a t e i n the three species s t u d i e d , D o u g l a s - f i r , southern pine, and white oak. Charring r a t e decreased w i t h i n c r e a s e i n dry s p e c i f i c g r a v i t y and w i t h i n c r e a s e i n moisture content. He a l s o found that growth-ring o r i e n t a t i o n p a r a l l e l to the exposed f a c e r e s u l t e d i n higher c h a r r i n g r a t e s than when o r i e n t a t i o n was perpendicular to the exposed f a c e . I n s t u d i e s a t the J o i n t F i r e Research Organi z a t i o n i n Great B r i t a i n (32) on r a t e of burning, increased p e r m e a b i l i t y along the g r a i n was found to i n c r e a s e r a t e of char. S c h a f f e r (33) found that impregnations of southern pine w i t h c e r t a i n f i r e - r e t a r d a n t and other chemicals d i d not s i g n i f i c a n t l y change the r a t e of c h a r r i n g . B o r i c a c i d , borax, ammonium s u l f a t e , monosodium phosphate, potassium carbonate, and sodium hydroxide v a r i o u s l y reduced the r a t e of c h a r r i n g a f t e r 20 minutes of f i r e exposure by about 20 percent over untreated wood. Only p o l y ethylene g l y c o l 1,000 reduced the r a t e of c h a r r i n g over the e n t i r e period of f i r e exposure by about 25 percent over untreated wood. T e t r a k i s (hydroxymethyl) phosphonium c h l o r i d e with urea, d i c y andiamide w i t h phosphoric a c i d , monoammonium phosphate, z i n c c h l o r i d e , and sodium c h l o r i d e had no e f f e c t on c h a r r i n g r a t e . Commercial f i r e-r etardant treatments g e n e r a l l y do not add s i g n i f i c a n t l y to the f i r e endurance of assemblies. I t i s o f t e n more advantageous from the cost standpoint, e i t h e r to use t h i c k e r wood members or to s e l e c t species w i t h lower c h a r r i n g r a t e s , than to add the cost of the f i r e - r e t a r d a n t treatment. In some a s semblies, however, i t has been found worthwhile to use some f i r e r e t a r d a n t - t r e a t e d components i n order to g a i n the extra time which w i l l b r i n g the f i r e endurance time up to the g o a l d e s i r e d . For example, t r e a t e d wood studs i n w a l l s and treated r a i l s , s t i l e s , and cross bands i n s o l i d wood doors have been used. e


e

e


6.

Fire-Retardant

HOLMES

Treatment

87


Flame Spread In the ASTM E84 25-foot t u n n e l furnace t e s t (34) f o r measuring flame spread of b u i l d i n g m a t e r i a l s , an i g n i t i n g p i l o t flame i s a p p l i e d to the underside of a h o r i z o n t a l l y mounted specimen. The flame heats the combustible m a t e r i a l to p y r o l y s i s , and the flammable gases g i v e n o f f a r e i g n i t e d by the p i l o t flame. I f the p y r o l y s i s - c o m b u s t i o n process becomes exothermic, the flaming on the specimen becomes s e l f - p r o p a g a t i n g . A flame-spread c l a s s i f i c a t i o n or r a t i n g number i s c a l c u l a t e d from the timed i s t a n c e progress of the flame along the l e n g t h of the specimen surface. The flame-spread number i s d e r i v e d r e l a t i v e to red oak (with an a r b i t r a r y flame-spread r a t i n g of 100) and to asbestos-cement board (rated zero)· N a t u r a l wood products ( 1 - i n c h lumber) u s u a l l y have flame-spread r a t i n g s of 100 to 150 i n the t e s t furnace of U n d e r w r i t e r s L a b o r a t o r i e s , Inc. (35). Some exceptions are poplar (170-185), western hemlock (60-75), redwood (70), and n o r t h e r n spruce (65). Wood w e l l t r e a t e d w i t h c u r r e n t commercial f i r e - r e t a r d a n t impregnation treatments w i l l have flame-spread r a t i n g s of 25 or l e s s . Many t r e a t e d wood products have obtained a s p e c i a l marking or d e s i g n a t i o n "FR-S" from UL (36) f o r having a flame-spread, f u e l c o n t r i b u t e d , and smoke-developed c l a s s i f i c a t i o n of not over 25 and no evidence of s i g n i f i c a n t p r o g r e s s i v e combustion i n an extended 30-minute ASTM E84 (34) t e s t procedure. The f u e l - c o n t r i b u t e d and smoke-developed c l a s s i f i c a t i o n s are a l s o c a l c u l a t e d r e l a t i v e to performance of red oak and asbestos-cement board. Eickner and S c h a f f e r (10) found t h a t monoammonium phosphate ( F i g u r e 2) was the most e f f e c t i v e of d i f f e r e n t f i r e - r e t a r d a n t chemicals i n reducing the flame-spread index of D o u g l a s - f i r plywood. They used the 8-foot t u n n e l furnace of ASTM E286 (37). The untreated plywood had a flame-spread index of 115. This was reduced to about 55 at a chemical r e t e n t i o n of 2 pounds per c u b i c f o o t , to 35 at 3 pounds, 20 a t 4 pounds, and to about 15 a t r e t e n t i o n s of 4.5 pounds and h i g h e r . Zinc c h l o r i d e was next i n e f f e c t i v e n e s s but r e q u i r e d higher r e t e n t i o n l e v e l s to reduce the flame-spread index v a l u e s e q u i v a l e n t to monoammonium phosphate. I t r e q u i r e d 5.5 pounds of z i n c c h l o r i d e to reduce flame spread to 35, and 7 pounds to reduce flame spread to 25. Ammonium s u l f a t e and borates were as e f f e c t i v e as z i n c c h l o r i d e a t r e t e n t i o n s of about 4.5 pounds per c u b i c f o o t and lower but not as e f f e c t i v e a t higher r e t e n t i o n l e v e l s . B o r i c a c i d had some e f f e c t i v e n e s s i n reducing flame spread. I t was e q u i v a l e n t to z i n c c h l o r i d e , ammonium s u l f a t e , and the borates at a r e t e n t i o n of about 2 pounds per c u b i c f o o t , but much l e s s e f f e c t i v e a t h i g h r e t e n t i o n l e v e l s . A r e t e n t i o n of 6 pounds per c u b i c f o o t reduced the flame-spread index of the plywood to o n l y 60. In many l a b o r a t o r i e s , flame-spread t e s t s of d i f f e r e n t types have c o n s i s t e n t l y shown t h a t the c u r r e n t acceptable treatments w i l l 1



WOOD TECHNOLOGY:

C H E M I C A L ASPECTS

Figure 1. Fire penetration into wood and formation of char hyer under the fire exposure conditions of ASTM E119 120

I

I

!

—

100

~

I

1

1

Nam

^80

60

-

\ _

20

BORATFX

**J^Vq>po

4

Ο

1 1 1 2 DRY CHEMICAL

1 3 RETENTION

1 4 (POUNDS

1 1 5 6 PER CUBIC FOOT)

7

Figure 2. Relationship of flame spread to level of chemical reten tion in 3/8-in. Douglas-fir plywood evaluated by the 8-ft tunnel furnace method


6.

HOLMES

Fire-Retardant

89

Treatment

p r e v e n t f l a m i n g c o m b u s t i o n o f t h e wood a n d p r e v e n t s p r e a d o f f i r e over the s u r f a c e . Wood, p r o p e r l y t r e a t e d , w i l l b e s e l f e x t i n g u i s h i n g o f b o t h f l a m i n g and g l o w i n g o n c e t h e p r i m a r y s o u r c e o f h e a t a n d f i r e i s removed o r e x h a u s t e d .


Glowing Glowing i s the v i s u a l evidence of combustion of the carbon i n t h e c h a r l a y e r o f t h e b u r n i n g wood. I f flaming of the released combustible gases has ceased, the glowing of the char i s u s u a l l y termed a f t e r g l o w . Of t h e s e v e r a l p o s s i b l e o x i d a t i o n r e a c t i o n s i n g l o w i n g c o m b u s t i o n , b o t h Browne (5) and L y o n s (3) i n t h e i r r e v i e w s show one p o s s i b i l i t y t o be a t w o - s t a g e r e a c t i o n : (1)

C + %0

(2)

CO + % 0

CO + 2 6 . 4 3 k i l o c a l o r i e s p e r m o l e

2

2

·+ C 0

2

+ 67.96 k i l o c a l o r i e s p e r mole

The f i r s t r e a c t i o n o c c u r s o n t h e s u r f a c e o f t h e char and t h e second i s a g a s - p h a s e r e a c t i o n . Wood t h a t h a s b e e n e f f e c t i v e l y t r e a t e d s h o u l d n o t e x h i b i t a n y afterglowing. Reviews C3,5) c o v e r i n g t h e s u b j e c t o f g l o w i n g p o i n t out t h a t t h e mechanism i n v o l v e d i n glow r e t a r d a n c e i s n o t c l e a r . B o t h p h y s i c a l and c h e m i c a l t h e o r i e s have been s u g g e s t e d . Physical methods i n c l u d e t h e e x c l u s i o n o f oxygen from t h e carbonaceous char by f o r m a t i o n o f c o a t i n g s o f t h e f i r e r e t a r d a n t d u r i n g t h e comb u s t i o n p r o c e s s o r by a c o o l i n g e f f e c t due to t h e f i r e r e t a r d a n t . The c h e m i c a l t h e o r y w i t h t h e most s u p p o r t i n g e v i d e n c e i n d i c a t e s t h a t e f f e c t i v e g l o w r e t a r d a n c e i n c r e a s e s t h e r a t i o o f CO t o C 0 . 2

I f t h e r e a c t i o n c a n b e d i r e c t e d m o s t l y t o t h e m o n o x i d e , s t e p (1) a b o v e , t h e h e a t l i b e r a t e d i s o n l y 28 p e r c e n t o f t h a t g i v e n o f f when the r e a c t i o n continues to the d i o x i d e . Thus g l o w i n g may b e e l i m i n a t e d b y a n i n s u f f i c i e n t amount o f h e a t t o c o n t i n u e combustion. E f f e c t i v e g l o w r e t a r d a n t s f o r wood a r e t h e ammonium p h o s p h a t e s , ammonium b o r a t e s , b o r i c a c i d , p h o s p h o r i c a c i d , a n d compounds t h a t y i e l d p h o s p h o r i c a c i d d u r i n g p y r o l y s i s 0 3 , 5 ) . Some c h e m i c a l s t h a t a r e r e p o r t e d t o s t i m u l a t e g l o w i n g a r e chromâtes, m o l y b d a t e s , h a l i d e s o f chromium, manganese, c o b a l t and c o p p e r , a n d f e r r i c and s t a n n i c o x i d e s ( 5 , 1 0 ) . Chemicals found to be i n e f f e c t i v e i n r e t a r d i n g a f t e r g l o w i n a l i m i t e d s t u d y were ammonium s u l f a t e and s o d i u m b o r a t e s (10). Combustion Products T h e c o m b u s t i o n p r o d u c t s o f b u r n i n g wood—smoke a n d g a s e s — a r e becoming o f i n c r e a s e d Importance. Code and b u i l d i n g o f f i c i a l s , b u i l d e r s , p r o d u c e r s o f b u i l d i n g m a t e r i a l s and f u r n i s h i n g s , and a l l



90

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

engaged i n f i r e r e s e a r c h are being d i r e c t e d by p u b l i c i n t e r e s t and s c r u t i n y toward a g r e a t e r concern f o r the r e a l hazard to l i f e s a f e t y of b u i l d i n g m a t e r i a l s i n a f i r e s i t u a t i o n . A study conducted by the N a t i o n a l F i r e P r o t e c t i o n A s s o c i a t i o n (18) of 311 f a t a l f i r e s i n one- and two-family d w e l l i n g s , i n c l u d i n g mobile homes and r e c r e a t i o n a l v e h i c l e s , r e v e a l e d that 73.6 percent of the deaths were caused by products of combustion r e s u l t i n g i n a s p h y x i a t i o n or a n o x i a . I n a study of f i r e f a t a l i t i e s that occurred i n the s t a t e of Maryland, a Johns Hopkins U n i v e r s i t y group (38) found that carbon monoxide was not o n l y the predominant f a c t o r i n h i n d e r i n g escape from the f i r e scene but was a l s o the primary agent i n the cause of death i n 50 percent of the 129 cases. I t was a l s o a major c o n t r i b u t o r to death i n another 30 percent of the cases i n combination w i t h heart d i s e a s e , a l c o h o l i n the b l o o d , and burns. V i s i b i l i t y i n a burning b u i l d i n g i s extremely important. Smoke can obscure v i s i o n and e x i t s , thereby r e t a r d i n g escape and r e s u l t i n g i n p a n i c . I t a l s o h i n d e r s the work of f i r e f i g h t e r s . The p a r t i c u l a r f r a c t i o n of smoke, e x c l u s i v e of any combustion gases, a c t s as an i r r i t a n t to the r e s p i r a t o r y system and may a l s o r e s u l t i n hypoxia and c o l l a p s e (39). Smoke from untreated wood.—For r e s e a r c h purposes, t h e r e a r e s e v e r a l methods used f o r measuring smoke developed by burning b u i l d i n g m a t e r i a l s (40). These t e s t s g e n e r a l l y measure the v i s i b l e smoke products. One method of smoke d e t e r m i n a t i o n being used f o r b u i l d i n g code purposes i s the 25-foot t u n n e l furnace method of ASTM E84 which y i e l d s a smoke-developed r a t i n g r e l a t i v e to red oak. Because the t e s t i s conducted under a s t r o n g flame, the r e s u l t s are not always i n d i c a t i v e of performance i n a b u i l d i n g f i r e where m a t e r i a l s may have some high-temperature exposure without the presence of flames. During the l a s t decade, the N a t i o n a l Bureau of Standards (NBS) and other l a b o r a t o r i e s worked to develop a meaningful t e s t method f o r measuring the smoke development p o t e n t i a l of burning wood and other b u i l d i n g m a t e r i a l s . The method developed a t NBS i s now e x t e n s i v e l y used (41,42). This method t h e r m a l l y exposes a s m a l l sample i n a closed chamber and s u p p l i e s a s p e c i f i c o p t i c a l d e n s i t y based on l i g h t t r a n s m i s s i o n , l i g h t path l e n g t h , burning a r e a , and volume of e n c l o s u r e . I t i s intended to r e l a t e to l i g h t o b s c u r a t i o n and the hindrance i n f i n d i n g e x i t s . This method has been accepted as a standard by the N a t i o n a l F i r e P r o t e c t i o n A s s o c i a t i o n (43) and i s expected to be accepted by others and more w i d e l y used f o r r a t i n g b u i l d i n g m a t e r i a l s f o r r e g u l a t o r y purposes. The chemical makeup of the combustion products, i n c l u d i n g a e r o s o l s and p a r t i c u l a t e s , w i l l change w i t h burning c o n d i t i o n s and the complex processes r e s u l t i n complex mixtures of products (28). More smoke i s produced under nonflaming combustion than under flaming combustion. The complexity of the smoke i s i n d i c a t e d by the f a c t that over 200 compounds have been found i n the d e s t r u c t i v e d i s t i l l a t i o n of wood by Goos (45).



6.

HOLMES

Fire-Retardant

Treatment

91

In complete combustion, the products from burning wood a r e carbon d i o x i d e , water, and ash. Other gases and vapors that may be present due to incomplete combustion i n c l u d e carbon monoxide, methane, formic a c i d , a c e t i c a c i d , g l y o x a l , and saturated and unsaturated hydrocarbons (46) . The a e r o s o l s can a l s o c o n t a i n v a r i o u s l i q u i d s such as levoglucosan and complex mixtures. The s o l i d s can c o n s i s t of unburned carbon p a r t i c l e s and high-molecularweight t a r s . There i s no standardized t e s t method f o r determining the combustion products g i v e n o f f from wood or other m a t e r i a l s dur ing a r e a l f i r e s i t u a t i o n . The gases and products obtained and t h e i r estimated hazard to l i f e w i l l depend on the experimental cond i t i o n s of any t e s t method s e l e c t e d . Most s t u d i e s on the t o x i c i t y of combustion products show that the dominant hazardous gas from burning wood i s carbon monoxide f o l l o w e d by carbon d i o x i d e and the r e s u l t i n g oxygen d e p l e t i o n (46-50). Considerable r e s e a r c h i s underway by v a r i o u s i n s t i t u t i o n s and agencies and by i n d u s t r y on the p h y s i o l o g i c a l and t o x i c o l o g i c a l e f f e c t s of smoke and gaseous products. Of p a r t i c u l a r note are the e x t e n s i v e r e s e a r c h programs at the U n i v e r s i t y of Utah under the d i r e c t i o n on I . N. Einhorn (44,48), at Johns Hopkins Unive r s i t y under R. M. F r i s t r o m (38,50), and a t the N a t i o n a l Bureau of Standards under M. M. B i r k y ( 5 1 ) . Smoke from t r e a t e d w o o d . — F i r e - r e t a r d a n t - t r e a t e d wood a l s o produces smoke and gaseous combustion products when burned. Many commercial f i r e - r e t a r d a n t - t r e a t e d wood products showed g r e a t l y reduced smoke development when t e s t e d i n the 25-foot tunnel furnace used f o r r a t i n g purposes by UL (36). This t e s t , however, does i n v o l v e flaming exposure, r e g a r d l e s s of the f l a m m a b i l i t y of the specimen. I n a recent study a t the FPL (52), r e s u l t s of t e s t s w i t h the NBS smoke chamber show that plywood t r e a t e d w i t h s p e c i f i c f i r e - r e t a r d a n t chemicals may g i v e o f f more or l e s s smoke than untreated wood depending on the chemicals employed and t h e c o n d i t i o n s of burning. Eickner and Schaffer (10) examined the e f f e c t s of v a r i o u s i n d i v i d u a l f i r e - r e t a r d a n t chemicals on f i r e performance of D o u g l a s - f i r plywood ( F i g u r e 3 ) . Using the 8-foot tunnel furnace t e s t method (37), they found that monoammonium phosphate and z i n c c h l o r i d e g r e a t l y increased the smoke d e n s i t y index v a l u e s f o r the plywood when treatment l e v e l s were above 2.0 pounds per c u b i c f o o t . B o r i c a c i d , a t r e t e n t i o n s above 5 pounds per cubic f o o t , a l s o increased smoke development. Sodium borates and sodium dichrornate c o n s i d e r a b l y reduced smoke development. A t low r e t e n t i o n l e v e l s of about 1 to 3-1/2 pounds per cubic f o o t , ammonium s u l f a t e was a l s o found e f f e c t i v e i n reducing smoke. I n the 8-foot furnace, e f f e c t i v e f i r e r e t a r d a n t s produced a lowflaming combustion and t h i s c o n d i t i o n g e n e r a l l y r e s u l t e d i n more smoke development than i n the flaming combustion of untreated wood.




WOOD TECHNOLOGY:

DRY

CHEMICAL

RETENTION

(POUNDS

PER CUBIC FOOT)

Figure 3. Relationship of smoke density to level of chemical retention in 3/8-in. Douglas-fir plywood evaluated by the 8-ft tunnel furnace method



6.

HOLMES

Fire-Retardant

Treatment

93

Satonaka and I t o (53) obtained reduced smoke from f i r and oak t r e a t e d w i t h e i t h e r ammonium s u l f a t e or diammonium phosphate, or w i t h the commercially used f o r m u l a t i o n s p y r e s o t e or m i n a l i t h . (Pyresote c o n s i s t s of z i n c c h l o r i d e 35 p e r c e n t , ammonium s u l f a t e 35 percent, b o r i c a c i d 25 p e r c e n t , and sodium dichromate 5 percent. M i n a l i t h c o n s i s t s of ammonium s u l f a t e 60 percent, diammonium phosphate 10 p e r c e n t , b o r i c a c i d 20 p e r c e n t , and borax 10 percent.) They a l s o obtained r e d u c t i o n s i n carbon monoxide and carbon d i o x i d e l e v e l s compared to the untreated wood w i t h each of the four treatments at the two p y r o l y s i s temperatures employed, 400°C and 700°C. The p o s s i b i l i t y of t o x i c gas f o r m a t i o n can o c c a s i o n a l l y be p r e d i c t e d from the chemical composition of f i r e - r e t a r d a n t f o r m u l a t i o n s . Chemicals c o n t a i n i n g c h l o r i n e may produce c h l o r i n e gas, hydrogen c h l o r i d e , or other c h l o r i n a t e d products. Ammonia gas may a l s o be a noxious gas from ammonia-containing compounds. The trend i n r e c e n t years has been toward increased i n v e s t i g a t i o n and use of o r g a n i c compounds as f i r e r e t a r d a n t s f o r wood. The thermal decomposition products from wood t r e a t e d w i t h these compounds i s not c l e a r l y understood, p a r t i c u l a r l y i n regard to t h e i r t o x i c o l o g i c a l and p h y s i o l o g i c a l e f f e c t s . I n f o r m a t i o n on r e s e a r c h i n t h i s area i s l a c k i n g . Heat C o n t r i b u t i o n At some time a f t e r the i n i t i a l exposure of wood to heat and flames i n a f i r e s i t u a t i o n , the burning process becomes exothermic and heat i s c o n t r i b u t e d to the surroundings. The t o t a l heat of combustion of wood v a r i e s w i t h species and i s a f f e c t e d by r e s i n content. I t v a r i e s from about 7,000 to 9,000 Btu per pound, but not a l l of t h i s p o t e n t i a l heat i s r e l e a s e d during a f i r e . The degree to which the t o t a l a v a i l a b l e heat i s r e l e a s e d depends on the type of f i r e exposure and the completeness of combustion. During the i n i t i a l stages of a f i r e , f i r e - r e t a r d a n t - t r e a t e d wood c o n t r i b u t e s l e s s heat than does untreated wood, e s p e c i a l l y from the flammable v o l a t i l e s (8,26). This means that the spread of f i r e to nearby combustibles i s slow. The f i r e tends to be confined to the primary source. I n the ASTM E84 t e s t f o r b u i l d i n g m a t e r i a l s , t r e a t e d specimens produce about 75 percent l e s s heat than untreated red oak. I n a t o t a l combustion t e s t , however, such as the N a t i o n a l Bureau of Standards " p o t e n t i a l heat method (54), both t r e a t e d and untreated wood r e l e a s e about the same t o t a l heat. Heat r e l e a s e r a t e i s another r e l e v a n t measure of the c o m b u s t i b i l i t y of a m a t e r i a l along w i t h ease of i g n i t i o n and flame spread. Smith (55) p o i n t s out that the r e l e a s e r a t e d a t a , obtained under d i f f e r e n t t e s t exposures, w i l l be u s e f u l i n p r e d i c t i n g the performance i n a c t u a l f i r e s under d i f f e r e n t f u e l l o a d i n g . Release r a t e data can thus be u s e d — a l o n g w i t h other 11



94

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

f i r e performance c h a r a c t e r i s t i c s — f o r s p e c i f y i n g m a t e r i a l s and products i n a p a r t i c u l a r l o c a t i o n i n an occupancy w i t h a given f u e l load r a t i n g . The r a t e of heat r e l e a s e during the i n i t i a l stages of f i r e exposure i s c o n s i d e r a b l y l e s s , however, f o r t r e a t e d wood than f o r untreated wood. Brenden (56), using the FPL r a t e of heat r e l e a s e method, obtained a maximum heat r e l e a s e r a t e of 611 Btu per square f o o t per minute f o r untreated 3/4-inch D o u g l a s - f i r plywood, w i t h an average r e l e a s e r a t e of 308 B t u per square f o o t per minute f o r the f i r s t 10 minutes. F i r e - r e t a r d a n t - t r e a t e d D o u g l a s - f i r plywood, w i t h 3.6 pounds per cubic f o o t d r y chemical and a r e ported flame spread of 25 to 28, had a maximum heat r e l e a s e r a t e of 132 B t u per square f o o t per minute a t 42 minutes and an average r a t e of 16 Btu per square f o o t per minute f o r the f i r s t 10 minutes. Treatment-Related P r o p e r t i e s of Fire-Retardant-Treated Wood Strength Gerhards (57) reviewed the r e s u l t s of 12 separate s t u d i e s on s t r e n g t h p r o p e r t i e s of f i r e - r e t a r d a n t - t r e a t e d wood conducted a t the FPL and other l a b o r a t o r i e s . He concluded that modulus of rupture (MOR) i s c o n s i s t e n t l y lower and modulus of e l a s t i c i t y (MOE) and work to maximum load a r e g e n e r a l l y lower f o r f i r e r e t a r d a n t - t r e a t e d wood than f o r untreated wood i f f i r e - r e t a r d a n t treatment i s followed by k i l n d r y i n g . The e f f e c t may be l e s s or n e g l i g i b l e i f the f i r e - r e t a r d a n t - t r e a t e d wood i s a i r d r i e d i n s t e a d of k i l n d r i e d . The most s i g n i f i c a n t l o s s was i n work to maximum l o a d , a measure of shock r e s i s t a n c e or brashness, which averaged 34 percent r e d u c t i o n . The l o s s e s from t r e a t i n g and k i l n drying f o r s m a l l c l e a r specimens averaged about 13 percent f o r MOR and 5 percent f o r MOE. Losses i n s t r u c t u r a l s i z e s were about 14 percent f o r MOR and 1 percent f o r MOE (57). Losses due to h i g h temperature k i l n d r y i n g , above 65°C may be c o n s i d e r a b l y greater ( 5 8 ) . The N a t i o n a l Forest Products A s s o c i a t i o n recommends that t h e a l l o w a b l e s t r e s s e s f o r f i r e - r e t a r d a n t - t r e a t e d wood f o r design purposes be reduced by 10 percent as compared to untreated wood; the a l l o w a b l e loads f o r f a s t e n e r s a r e a l s o reduced 10 percent ( 5 9 ) . The 10 percent r e d u c t i o n i n design s t r e s s e s was confirmed prov i d i n g the s w e l l i n g of the wood r e s u l t i n g from treatment i s taken i n t o account (57,60). Treated wood i s s l i g h t l y more hygroscopic than untreated, t h e r e f o r e the d e n s i t y of e q u i v a l e n t cross s e c t i o n s of the t r e a t e d t e s t samples was s l i g h t l y lower. B r a z i e r and Laidlaw (58) a t the P r i n c e s Risborough Laboratory have w r i t t e n t h a t , u n t i l more research i s done i n t h i s area, i t i s wise t o assume a l o s s i n bending s t r e n g t h of 15 to 20 percent f o r t r e a t e d wood d r i e d a t 65°C.



6.

HOLMES

Fire-Retardant

Treatment

95

In a d d i t i o n t o s t r e n g t h l o s s due to k i l n d r y i n g a t h i g h temperatures (above 65°C), p r o g r e s s i v e l o s s of s t r e n g t h i n t r e a t e d wood members can be caused by a c i d i c degradation of the wood by some treatment chemicals (58). There i s evidence t h a t phosphate and s u l f a t e s a l t s may be broken down t o a c i d i c r e s i d u e s w i t h i n the wood. The degradation and r e s u l t a n t l o s s i n s t r e n g t h may continue at a slower r a t e under use a t normal temperatures. The f i r e - r e t a r d a n t treatment of l a r g e s t r u c t u r a l members f o r a p p l i c a t i o n s where s t r e n g t h i s a predominant f a c t o r i s u s u a l l y not recommended. The adverse e f f e c t s of treatment chemicals on s t r e n g t h and other p r o p e r t i e s such as h y g r o s c o p i c i t y outweigh any b e n e f i t obtained by the treatment. Large wood members have good f i r e r e s i s t a n c e and i f treatment i s r e q u i r e d f o r reducing flame spread, i t would be b e t t e r to use f i r e - r e t a r d a n t c o a t i n g s or other protection. Hygroscopicity Wood t h a t has not been t r e a t e d w i l l absorb moisture from the surrounding a i r u n t i l i t s moisture content reaches an e q u i l i b r i u m c o n d i t i o n . The h y g r o s c o p i c i t y of wood t r e a t e d w i t h i n o r g a n i c f i r e r e t a r d a n t chemicals i s u s u a l l y g r e a t e r than that of the untreated wood and i s dependent on s i z e and species of wood, temperature, r e l a t i v e humidity, and type and amount of chemicals used (8,60). The i n c r e a s e i n e q u i l i b r i u m moisture content i s n e g l i g i b l e a t 27°C and 30 to 50 percent r e l a t i v e humidity. A 2 to 8 percent i n c r e a s e i n moisture content occurs i n the t r e a t e d wood a t 27°C and 65 percent r e l a t i v e humidity, and a t 80 percent r e l a t i v e humidity the moisture content may i n c r e a s e 5 to 15 percent and cause exudation of the chemical s o l u t i o n from the wood. In an unpublished study a t the U.S. F o r e s t Products Labo r a t o r y , the moisture content of wood t r e a t e d w i t h two commercial f o r m u l a t i o n s reached 48 to 58 percent (based on ovendry weight of t r e a t e d sample) i n 4 weeks exposure a t 27°C and 90 percent r e l a t i v e humidity. Continuous exposure of wood t r e a t e d w i t h waters o l u b l e s a l t s to c o n d i t i o n s above 80 percent r e l a t i v e humidity can r e s u l t i n l o s s of chemicals and i n adverse e f f e c t s on dimensional s t a b i l i t y and p a i n t c o a t i n g s . C o r r o s i o n of some metals i n contact w i t h the wood w i l l a l s o occur. Zinc c h l o r i d e w i l l add c o n s i d e r a b l y t o the e q u i l i b r i u m moisture content of wood i n the range of 30 to 80 percent r e l a t i v e humidity ( 8 ) . Ammonium s u l f a t e w i l l add a t r e l a t i v e h u m i d i t i e s exceeding 65 percent, and borax and b o r i c a c i d w i l l a t t r a c t water at lower h u m i d i t i e s . Phosphate s a l t s a f f e c t h y g r o s c o p i c i t y mostly when r e l a t i v e humidity exceeds 80 percent ( 5 8 ) . Most commercial treatment f o r m u l a t i o n s a r e developed f o r use under c o n d i t i o n s not g r e a t e r than 80 percent r e l a t i v e humidity. An e x t e r i o r type, l e a c h - r e s i s t a n t treatment t h a t i s not hygroscopic i s a v a i l a b l e (61). 1


96

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

Gluing G e n e r a l l y , the bonding o b t a i n a b l e w i t h f i r e - r e t a r d a n t - t r e a t e d wood i s s a t i s f a c t o r y f o r d e c o r a t i v e purposes. Treated wood members can be bonded i n t o s t r u c t u r a l assemblies w i t h s p e c i a l l y formulated adhesives under optimum bonding c o n d i t i o n s ( 8 ) . However, the q u a l i t y of bonds i s not u s u a l l y equal to that o b t a i n a b l e w i t h untreated wood, p a r t i c u l a r l y i n e v a l u a t i o n a f t e r exposure t o c y c l i c wetting and d r y i n g ( 6 2 ) .


Corrosivity Current treatment s o l u t i o n s c o n t a i n i n g c o r r o s i v e i n o r g a n i c s a l t s u s u a l l y a l s o c o n t a i n c o r r o s i o n i n h i b i t o r s such as sodium dichromate o r ammonium thiocyanate or a r e formulated t o a more n e u t r a l pH (60). However, s o l u b l e - s a l t - t r e a t e d wood i n contact w i t h metals should not be exposed t o high r e l a t i v e h u m i d i t i e s f o r prolonged p e r i o d s . The treatment chemicals can a t t a c k and det e r i o r a t e metal f a s t e n e r s . The c o r r o s i o n products i n t u r n d e t e r i o r a t e the wood. For example, under humid c o n d i t i o n s , ammonium s u l f a t e w i l l a t t a c k the z i n c and i r o n of galvanized punched-steel n a i l p l a t e s used i n t r u s s e s (58). A l k a l i n e and a c i d i c areas a r e developed i n the wood next to the attacked metal f a s t e n e r , and cause degradation of the wood (58,63). Paintability P a i n t a b i l i t y i s g e n e r a l l y not a problem under d r y normal c o n d i t i o n s . Unusually h i g h r e l a t i v e humidity c o n d i t i o n s can a f f e c t adhesion of the p a i n t f i l m or cause chemical c r y s t a l blooming on the p a i n t surface due to the increased moisture content of the wood. N a t u r a l or c l e a r f i n i s h e s a r e g e n e r a l l y not used f o r t r e a t e d wood because the chemicals may cause darkening or i r r e g u l a r s t a i n i n g . Machining The a b r a s i v e e f f e c t of treatments w i t h i n o r g a n i c s a l t c r y s t a l s can reduce t o o l l i f e . Where machining i s necessary, t h i s can be minimized by u s i n g t o o l s of abra s i v e-r es i s tant a l l o y s . Durability F i r e - r e t a r d a n t - t r e a t e d wood i s durable and s t a b l e under normal exposure c o n d i t i o n s . Treatments using i n o r g a n i c waters o l u b l e s a l t s , however, a r e not recommended f o r e x t e r i o r exposures to r a i n and weathering unless the treatment can be adequately protected by w a t e r - r e p e l l e n t c o a t i n g . E x t e r i o r - t y p e treatments i n which the chemicals a r e " f i x e d " i n the wood i n some manner a r e l e a c h r e s i s t a n t and nonhygroscopic.


6.

HOLMES

Fire-Retardant

Treatment

97


Current Research The l a t e s t " D i r e c t o r y of F i r e Research i n the United States 1971 to 1973," by the N a t i o n a l Research C o u n c i l (64), shows that only a few of the l i s t e d f e d e r a l , u n i v e r s i t y , p r i v a t e , and i n d u s t r i a l l a b o r a t o r i e s a r e doing research i n v o l v i n g f i r e - r e t a r d a n t impregnation treatments f o r wood. Published r e s e a r c h i n d i c a t e s that the current e f f o r t i s i n the development of l e a c h - r e s i s t a n t types of f i r e - r e t a r d a n t treatments f o r both e x t e r i o r and i n t e r i o r uses. Major emphasis i s on r e d u c t i o n of flame spread as determined by ASTM E84 (34), and r e d u c t i o n of flaming and f i r e p e n e t r a t i o n as determined by ASTM E108 (65). Development i s a l s o d i r e c t e d toward enhanced p r o p e r t i e s of the t r e a t e d wood i n nonh y g r o s c o p i c i t y , g l u a b i l i t y , p a i n t a b i l i t y , s t r e n g t h , and preserva t i o n against biodégradation. Some a t t e n t i o n — b u t not e n o u g h — i s being g i v e n to r e d u c t i o n of smoke and noxious gases. The c u r r e n t r e s e a r c h emphasis on the t o x i c o l o g i c a l and p h y s i o l o g i c a l e f f e c t s from the combustion products of n a t u r a l and s y n t h e t i c polymers i s expected to e v e n t u a l l y i n c l u d e f i r e - r e t a r d a n t - t r e a t e d wood. FPL Research At the U.S. Forest Products Laboratory, many of the r e s e a r c h programs i n v o l v e f i r e - r e t a r d a n t - t r e a t e d wood. This has i n c l u d e d extensive b a s i c study of p y r o l y s i s and combustion r e a c t i o n s of wood and i t s components and the e f f e c t s of chemical a d d i t i v e s on these r e a c t i o n s (15,24-26,28,29,66). A cooperative study (9) w i t h the D i v i s i o n of Chemical Development of the Tennessee V a l l e y A u t h o r i t y , showed the e f f e c t i v e n e s s of l i q u i d ammonium p o l y phosphate f e r t i l i z e r s as f i r e r e t a r d a n t s f o r wood. The commercial use of these products, made from e l e c t r i c furnace superphosphoric a c i d , has been shown t o be economically f e a s i b l e . Work has been completed by Schaffer (33) on the r a t e of f i r e p e n e t r a t i o n i n wood t r e a t e d w i t h d i f f e r e n t types of chemicals. Some r e s u l t s of t h i s study a r e reported elsewhere i n t h i s paper. Studies a r e c u r r e n t l y being conducted on smoke development and heat r e l e a s e r a t e from t r e a t e d and untreated wood and wood products (52,56). An e v a l u a t i o n of the a v a i l a b l e treatment systems f o r wood s h i n g l e s and shakes was completed using a r t i f i c i a l weathering (11). A f u r t h e r development from t h i s work was a new ASTM Standard Method D2898 (67,68) f o r t e s t i n g d u r a b i l i t y of f i r e - r e t a r d a n t treatment of wood. Other I n s t i t u t i o n s Using f u l l - s c a l e f i r e t e s t f a c i l i t i e s of the I l l i n o i s I n s t i t u t e of Technology-Research I n s t i t u t e ( I I T R I ) , C h r i s t i a n and Waterman (69) studied f i r e and smoke behavior of i n t e r i o r f i n i s h m a t e r i a l s i n c l u d i n g f i r e - r e t a r d a n t - t r e a t e d wood products. The authors found that the m a t e r i a l s performed according to a


98

WOOD TECHNOLOGY:

CHEMICAL

" r e l a t i v e hazard" p o s i t i o n , but that the t u n n e l t e s t flame-spread number does not q u i t e p l a c e them i n the proper o r d e r . They s t a t e that "attempts to d i s t i n g u i s h between hazards of m a t e r i a l s whose tunnel t e s t flame-spread numbers d i f f e r by 25 or l e s s do not seem j u s t i f i e d . " These same IITRI r e s e a r c h e r s (70) a l s o found t h a t i n some s i t u a t i o n s a s i g n i f i c a n t amount of m a t e r i a l w i t h a flame spread of 90 can be s a f e l y used on w a l l s of c o r r i d o r s wider than 6 f e e t when the c e i l i n g m a t e r i a l has a r a t i n g of 0 to 25. E f f e c t i v e f i r e - r e t a r d a n t treatments f o r wood f o r e x t e r i o r uses under c o n d i t i o n s of l e a c h i n g and weathering have been needed f o r many y e a r s . For wood s h i n g l e or shake r o o f i n g , a commercial treatment system has been developed (61) i n the United S t a t e s t h a t meets acceptance requirements of U n d e r w r i t e r s L a b o r a t o r i e s , Inc. Lumber and plywood are a l s o a v a i l a b l e w i t h t h i s e x t e r i o r - t y p e treatment. The success of t h i s treatment system i n d i c a t e d a breakthrough i n the development of a commercially s u c c e s s f u l system whereby f i r e - r e t a r d a n t chemicals are pressure impregnated i n t o the wood and f i x e d or converted to a l e a c h - r e s i s t a n t s t a t e without s e r i o u s impairment of the d e s i r a b l e n a t u r a l wood p r o p e r t i e s . This d e v e l opment has s t i m u l a t e d research w i t h l e a c h - r e s i s t a n t type treatments. Chemicals employed u s u a l l y i n v o l v e o r g a n i c phosphates and compounds t h a t can r e a c t w i t h phosphorous-containing chemicals or w i t h the wood c e l l u l o s e s t r u c t u r e to g i v e permanence of treatment. The E a s t e r n F o r e s t Products Laboratory (12,71) a t Ottawa, Ontario, has been a c t i v e i n development of l e a c h - r e s i s t a n t t r e a t ments u s i n g melamine or urea w i t h dicyandiamide, formaldehyde, and phosphoric a c i d . Decay r e s i s t a n c e i s a l s o shown f o r a ureabased treatment (72). One stystem has met the requirements f o r C l a s s C wood r o o f i n g under ASTM El08 by Underwriters' Labora t o r i e s of Canada (12,73). This treatment, or one s i m i l a r , i s expected to be introduced i n t o the United States w i t h i n the year as an approved e x t e r i o r - t y p e l e a c h - r e s i s t a n t treatment. McCarthy and coworkers (74) a t the A u s t r a l i a n F o r e s t Products Laboratory reported t h a t a pressure treatment f o r p i n e posts w i t h zinc-copper-chromium-arsenic-phosphorus p r e s e r v a t i v e produced a l e a c h - r e s i s t a n t treatment having both f i r e retardancy and p r e s e r v a t i o n a g a i n s t decay. This treatment system i s r e p o r t e d to have commercial a p p l i c a t i o n i n A u s t r a l i a . B a s i c r e s e a r c h on the chemistry of c e l l u l o s i c f i r e s i s being s t u d i e d by Shafizadeh at the U n i v e r s i t y of Montana (75,76). Working w i t h model compounds, he has shown how thermal r e a c t i o n s a f f e c t the cleavage of the g l y c o s i d i c bond w i t h breakdown of the sugar u n i t s through a t r a n s g l y c o s y l a t i o n mechanism which event u a l l y r e s u l t s i n formation of combustible t a r and v o l a t i l e p y r o l y s i s products. I n t e r f e r e n c e of the t r a n s g l y c o s y l a t i o n process by a c i d i c a d d i t i o n s , amino groups, and phosphate and halogen d e r i v a t i v e s has been demonstrated to r e t a r d combustion by producing more water and char. 1


ASPECTS



6.

HOLMES

Fire-Retardant

Treatment

99

One area of c o n t i n u i n g research a t the Stanford Research I n s t i t u t e (SRI) i s concerned w i t h the e f f e c t s of flame r e t a r d a n t s on thermal degradation of c e l l u l o s e (77,78). The r e s u l t s of a recent study (78) f o r t r e a t i n g wood showed t h a t e x i s t i n g wood r o o f s can be g i v e n a s e l f - h e l p f i r e - r e t a r d a n t treatment equiva l e n t t o a C l a s s C (65) r a t i n g f o r a 5-year p e r i o d . To o b t a i n adequate depth of p e n e t r a t i o n , the treatment i s e f f e c t i v e o n l y on weathered s h i n g l e or shake r o o f s a t l e a s t 5 years o l d . The t r e a t ment c o n s i s t s of a spray a p p l i c a t i o n of a 20 percent aqueous s o l u t i o n of diammonium phosphate, f o l l o w e d by a 20 percent aqueous s o l u t i o n of magnesium s u l f a t e to form the w a t e r - i n s o l u b l e magnesium ammonium phosphate. Of p a r t i c u l a r i n t e r e s t i s the a p p l i c a t i o n of the P a r k e r L i p ska model f o r s e l e c t i n g f i r e r e t a r d a n t s (77). This model of p y r o l y s i s processes p r e d i c t s the e f f i c i e n c y of candidate chemical f i r e r e t a r d a n t s based on increased char y i e l d and e l i m i n a t i o n of f l a m i n g . E f f i c i e n t r e t a r d a n t s w i l l be those t h a t have h i g h oxygen content per molecule and c o n t a i n phosphorus or boron to prevent a f t e r g l o w . I n a d d i t i o n , the s t u d i e s a t SRI have shown thaj: the o p t i m a l add-on weight of a chemical r e t a r d a n t i s about 10 mole per gram of c e l l u l o s e . Areas of Needed Research From the v i e w p o i n t of l i f e s a f e t y , the most urgent area of f i r e-r etardant r e s e a r c h i s the development of treatments f o r wood t h a t w i l l reduce not o n l y flame spread but a l s o smoke and noxious gases. The treatments should not add or c r e a t e new noxious gases. B a s i c r e s e a r c h on the combustion of wood being conducted i n many l a b o r a t o r i e s should be s t u d i e d and c a r e f u l l y gleaned f o r c l u e s on treatment chemicals or other means to a l t e r the c e l l u l o s e and l i g n i n s t r u c t u r e t o reduce smoke and harmful gases. Because comb u s t i o n products have been shown to be the primary cause of death i n f i r e s , r e s e a r c h on the r e d u c t i o n of smoke and gases should take precedence over r e d u c t i o n of flame spread. Another area of necessary r e s e a r c h i s development of treatments t h a t w i l l i n c r e a s e r e s i s t a n c e of wood to f i r e pene t r a t i o n . The work done by Schaffer (31,33) and others i n t h i s f i e l d should be c a r r i e d f u r t h e r . The slow r a t e of f i r e penet r a t i o n i n t h i c k wood members i s one of the b a s i c a s s e t s of wood and has been accepted and u t i l i z e d f o r many years i n heavy timber c o n s t r u c t i o n . But t h i n wood members and p a n e l i n g have a cons i d e r a b l y higher f i r e p e n e t r a t i o n r a t e than t h i c k wood members under severe f i r e c o n d i t i o n s . A f i r e - r e t a r d a n t system t h a t w i l l g i v e slower f i r e p e n e t r a t i o n means more a v a i l a b l e s a f e t y time f o r f i r e f i g h t i n g personnel and f o r evacuation of occupants from a burning b u i l d i n g . Continuing b a s i c r e s e a r c h i s needed i n the p y r o l y s i s , combustion, and f i r e chemistry of wood l e a d i n g toward the



100

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

s e l e c t i o n of f i r e - r e t a r d a n t chemicals and t h e i r more e f f i c i e n t a p p l i c a t i o n to wood. The h i g h l o a d i n g r e q u i r e d (2 to 6 pounds of dry chemical per cubic f o o t of wood) f o r chemicals i n present use puts a severe l i m i t a t i o n on cost of usable treatments. A higher cost t r e a t ment could be t o l e r a t e d i f i t proved more e f f i c i e n t . A l a r g e p a r t of t h e c o s t of t r e a t e d wood to the consumer i s the f u l l - c e l l pressure process r e q u i r e d by present-day f o r m u l a t i o n s . A l e s s c o s t l y method of g e t t i n g the chemical i n t o the wood i s needed. We need not l i m i t the choice of chemical candidates o n l y to those that can be used i n a w a t e r - t r e a t i n g s o l u t i o n . A p p l i c a t i o n w i t h hydrocarbon s o l v e n t s or l i q u i f i e d gases w i t h subsequent recovery of the c a r r i e r may prove p r a c t i c a b l e . Further research should be d i r e c t e d toward the development of t e s t methods to p r o p e r l y evaluate f i r e - r e t a r d a n t - t r e a t e d wood. Current methods have been c r i t i c i z e d (79) f o r not g i v i n g a t r u e hazard e v a l u a t i o n of m a t e r i a l s on t h e i r p o t e n t i a l performance i n a r e a l f i r e . The l i m i t a t i o n s of s m a l l - s c a l e t e s t methods should be understood as adequate only f o r products research and d e v e l opment . Even the 25-foot r a t i n g furnace of ASTM E84 (34) has been c r i t i c i z e d regarding i t s c o r r e l a t i o n w i t h f u l l - s c a l e f i r e s and the meaning of the numbers i t produces f o r flame spread and smoke d e n s i t y (40,69,80). There i s a trend toward more f u l l - s c a l e f i r e t e s t i n g w i t h the o b j e c t i v e of r e l a t i n g the r e s u l t s of s m a l l e r s c a l e t e s t s i n c l u d i n g the 25-foot furnace to performance i n r e a l f i r e s . F u l l - s c a l e t e s t s a r e too expensive, of course, to prove out b u i l d i n g products on a r o u t i n e b a s i s . Perhaps the c o r n e r - w a l l t e s t (80-82) i s adequately r e a l i s t i c and could be used i n conj u n c t i o n w i t h s m a l l - s c a l e t e s t s f o r determining product performance and f i r e hazard. As new c r i t e r i a a r e developed f o r d e f i n i n g c o m b u s t i b i l i t y , a method i s needed to r e a l i s t i c a l l y i n d i c a t e heat r e l e a s e r a t e of wood products exposed to b u i l d i n g f i r e s i n s t e a d of dependence on " t o t a l heat v a l u e s . " Continuing research must y i e l d i n f o r m a t i o n on t h e treatmentr e l a t e d p r o p e r t i e s of f i r e - r e t a r d a n t - t r e a t e d wood and methods f o r t h e i r improvement. The p r o p e r t i e s of the c o n v e n t i o n a l s a l t t r e a t ments which need improvement e s p e c i a l l y a r e h y g r o s c o p i c i t y , s t r e n g t h p r o p e r t i e s , g l u i n g , and f i n i s h i n g .


6.

HOLMES

Fire-Retardant Treatment

101

Literature Cited


1.

National Commission on Fire Prevention and Control "America burning: The report of the national commission on fire prevention and c o n t r o l . " 53 p. Washington, DC. 1973. 2. American Wood Preservers' Association "Wood preservation s t a t i s t i c s 1974." Compiled by Ernst & Ernst, In Am. Wood Preserv. Assoc. Proc. 71:225-263. AWPA, Washington, DC. 1975. 3. Lyons, J. W. "The chemistry and uses of f i r e retardants." WileyInterscience Div., John Wiley and Sons, New York. 1970. 4. Goldstein, I. S. Degradation and protection of wood from thermal attack. In "Wood Deterioration and Its Prevention by Preservative Treatment. Vol 1. Degradation and Protection of Wood," p. 307-339. D. D. Nicholas, ed. Syracuse Univ. Press. Syracuse, N.Y. 1973. 5. Browne, F. L. U.S.D.A. For. Prod. Lab. Rep. No. 2136, Madison, Wis. 1958. 6. Juneja, S. C. Synergism and fire retardance of wood and other c e l l u l o s i c materials. In "Advances in Fire Retardants, Part 2." V. M. Bhatnagar, ed. Technomic, Westport, Conn. 1973, 7. Juneja, S. C. Wood Sci. 7(3):201-208. 1975. 8. Eickner, H. W. J. Mater. 1(3):625-644. 1966. 9. Eickner, H. W., J. M. Stinson, and J. E. Jordan Am. Wood Preserv. Assoc. Proc. 65:260-271. 1969. 10. Eickner, H. W., and E. L. Schaffer Fire Tech. 3(2):90-104. 1967. 11. Holmes, C. A. Evaluation of fire-retardant treatments for wood shingles, Ch. 2. In "Advances i n F i r e Retardants, Part 1." V. M. Bhatnagar, ed. Technomic, Westport, Conn. 1971. 12. Juneja, S. C., and L. R. Richardson For. Prod. J. 24(5):19-23. 1974. 13. Kanury, A. M. Fire Res. Abstr. and Rev. 14(1):24-52. 1972. 14. Matson, A. F., R. E. Dufour, and J. F. Breen Bull, of Res. No. 51, p. 269-295. Underwriters' Lab. Inc., Northbrook, Ill. 1959. 15. B e a l l , F. C., and H. W. Eickner U.S.D.A. For. Serv. Res. Pap. FPL 130, For. Prod. Lab., Madison, Wis. 1970. 16. U.S. Forest Products Laboratory. U.S.D.A. For. Prod. Lab. Rep. No. 1464 ( r e v . ) , Madison, Wis. 1958.


102



17.

McGuire, J. H. F i r e Technol. 5(3):237-241. 1969. 18. National Fire Protection Association "Fire protection handbook." 13th ed. pp. 1-7, 5-7. N a t l . F i r e Prot. Assoc. Boston, Mass. 1969. 19. Broido, A. Chem. Tech. 3(1):14-17. 1973. 20. Prince, R. E. "Tests on the inflammability of untreated wood and of wood treated with fire-retarding compounds". In Publ. Proc. of Annu. Meet. N a t l . Fire Prot. Assoc. Boston, Mass. 1915. 21. Shafizadeh, F. Pryolysis and combustion of c e l l u l o s i c materials. In "Advances i n Carbohydrate Chemistry." V o l . 23, pp. 419-474. M. L . Wolfrom, and R. S. Tipson, eds. Academic press, New York. 1968. 22. MacLean, J. D. Am. Wood Preserv. Assoc. Proc. 47:155-168. 1951. 23. Stamm, A. J . Ind. and Eng. Chem. 48(3):413-417. 1956. 24. B e a l l , F. C. Wood Sci. 5(2):02-108. 1972. 25. Brenden, J. J. U.S.D.A. For. Serv. Res. Pap. FPL 80, For. Prod. Lab., Madison, Wis. 1967. 26. Browne, F. L., and J. J. Brenden U.S. For. Serv. Res. Pap. FPL 19, For. Prod. Lab., Madison, Wis. 1964. 27. Shafizadeh, F . , P. Chin, and W. DeGroot J. F i r e & Flammability/Fire Retardant Chem. 2:195-203. Aug. 1975. 28. Tang, W. K. U.S. For. Serv. Res. Pap. FPL 71, For. Prod. Lab., Madison, Wis. 1967. 29. Tang, W. Κ., and H. W. Eickner U.S. For. Serv. Res. Pap. FPL 82, For. Prod. Lab., Madison, Wis. 1968. 30. American Society for Testing and Materials ASTM Design. E119-73. Philadelphia, Pa. 1973. 31. Schaffer, E. L . U.S. For. Serv. Res. Pap. FPL 69, For. Prod. Lab., Madison, Wis. 1967. 32. Great B r i t a i n Ministry of Technology and Fire Offices' Committee, Joint F i r e Research Organization "Fire research 1964." Rep. of the F i r e Res. Board with the Rep. of the D i r . of F i r e Res. p. 12. London. 1965. 33. Schaffer, E. L . J. Fire & Flammability, Fire Retardant Chem. Suppl. 1:96-109, A p r i l 1974.


6.

HOLMES

Fire-Retardant

Treatment


34.

103

American S o c i e t y f o r T e s t i n g and M a t e r i a l s ASTM Desig. E84-75. P h i l a d e l p h i a , Pa. 1975. 35. U n d e r w r i t e r s ' L a b o r a t o r i e s , Inc. " W o o d — f i r e hazard classification, card data s e r v i c e . " Serial No. UL527. U n d e r w r i t e r s ' L a b o r a t o r i e s , I n c . , Northbrook, Ill. 1971. 36. U n d e r w r i t e r s ' L a b o r a t o r i e s , I n c . " B u i l d i n g m a t e r i a l s d i r e c t o r y P a r t I." B u i l d . Mater. List. Underwriters' L a b o r a t o r i e s , I n c . , Northbrook, Ill. Jan. 1976. 37. American S o c i e t y f o r T e s t i n g and M a t e r i a l s ASTM Design. E286-69 (reapproved 1975). P h i l a d e l p h i a , Pa. 1969. 38. H a l p i n , Β. Μ., E. P. Radford, R. F i s h e r , and Y. Caplan Fire J . 69(3):11-13, and 98-99. 1975. 39. Thomas, D. M. Fire Command 38(4):23-27. 1971. 40. Yuill, C. H., e t . al. Task Group of Subcomm. IV of ASTM Comm. E-5 on Fire Tests of Mater, and C o n s t r . , Mater. Res. and Stand., MTRSA 11(4):16-23, 42. 1971. 41. Brenden, J . J . For. Prod. J. 21(12):22-28. 1971. 42. Lee, T. G. "The smoke d e n s i t y chamber method f o r e v a l u a t i n g the potential smoke g e n e r a t i o n of b u i l d i n g m a t e r i a l s , " U.S. Dep. Commer., Natl. Bur. Stand., Tech. Note No. 757. 1973. 43. N a t i o n a l f i r e P r o t e c t i o n A s s o c i a t i o n N a t l . Fire P r o t . Assoc. NFPA No. 258. Boston, Mass. 1976. 44. Einhorn, I. N., D. A. Chatfield, J . H. Futrell, R. W. M i c k e l s o n , K. J . Voorhees, F. D. Hileman, and P. W. Ryan "Methodology f o r the a n a l y s i s of combustion p r o d u c t s . " UTEC 75-073, FRC/UU49. Univ. of Utah, S a l t Lake C i t y , Utah. 1975. 45. Goos, A. W. The thermal decomposition of wood, Ch. 20. In "Wood Chemistry." L.E. Wise and E. C. Jahn, eds. 2nd ed. R e i n h o l d , New York. 1952. 46. Wagner, J. P. Fire Res. A b s t r . and Rev. 1 4 ( 1 ) : 1 - 2 3 . 1972. 47. B i r k y , M. M. Polymer P r e p r . 14(2):1011-1015. 1973. 48. Einhorn, I. Ν., M. M. B i r k y , M. L. Grunnet, S. C. Packham, J. Η. Petajan, and J . D. Seader "The p h y s i o l o g i c a l and toxicological aspects of smoke produced during the combustion of polymeric m a t e r i a l s . " Annu. r e p . 1973-1974. UTEC-MSE 74-060, FRC/UU26. Univ. of Utah, S a l t Lake C i t y , Utah. 1974.


104 49. J. 50. 51.

52.


53. J. 54. 55. 56. 57. 58.

59.

60.

61. 62. 63. 64.

WOOD TECHNOLOGY: CHEMICAL ASPECTS O'mara, M. M. F i r e & Flammability 5(1):34-53. 1974. Robison, Μ. Μ., P. E. Wagner, R. M. Fristrom, and A. G. Schulz F i r e Technol. 8(4):278-290. 1972. Birky, M. M. "Review of smoke and toxic gas hazards i n fire environment." Int. Symp. F i r e Safety of Combust. Mater., Univ. of Edinburgh, Scotland. Oct., 1975. Brenden, J. J. U.S.D.A. For. Serv. Res. Pap. FPL 249, For. Prod. Lab., Madison, Wis. 1975. Satonaka, S., and K. Ito Jap. Wood Res. Soc. 21(11):611-617. 1975. Loftus, J. J., D. Gross, and A. F. Robertson "Potential heat of materials i n building fires." U.S. Dep. Commer., N a t l . Bur. Stand., Tech. News Bull. 1962. Smith, Ε. E. F i r e Technol. 12(1):49-54. 1976. Brenden, J. J. U.S.D.A. For. Serv. Res. Pap. FPL 230, For. Prod. Lab., Madison, Wis. 1975. Gerhards, C. C. U.S.D.A. For. Serv. Res. Pap. FPL 145, For. Prod. Lab., Madison, Wis. 1970. Brazier, J. D., and R. A. Laidlaw "The implications of using inorganic salt flame-retardant treatments with timber." BRE Information IS 13/74. In News of Timber Res. Princes Risborough Lab., Aylesbury, Buckinghamshire. Dec. 1974. National Forest Products Association "National design specifications for stress-grade lumber and its fastenings." N a t l . For. Prod. Assoc., Washington, DC. 1973. U.S. Forest Products Laboratory. "Wood handbook: Wood as an engineering material." pp. 15-10, 15-11. U.S.D.A. Agric. Handb. No. 72, (rev)., 1974. Shunk, Β. H. For. Prod. J. 22(2):12-15. 1972. Schaeffer, R. E. U.S.D. A. For. Serv. Res. Note FPL-0160, For. Prod. Lab., Madison, Wis. 1968. Baker, A. J. U.S.D.A. For. Serv. Res. Pap. FPL 229, For. Prod. Lab., Madison, Wis. 1974. National Research Council "Directory of f i r e research i n the United States 19711973." Comm. on F i r e Res., Div. Engr., N a t l . Res. Coun., N a t l . Acad. Sci., Washington, DC. 1975.


6.

HOLMES

Fire-Retardant Treatment


65. ASTM 66.

105

American Society for Testing and Materials Desig. E108-75. Philadelphia, Pa. 1975. Browne, F. L., and W. H. Tang U.S. For. Serv. Res. Pap. FPL 6, For. Prod. Lab., Madison, Wis. 1963. 67. American Society for Testing and Materials ASTM Desig. D2898-72. Philadelphia, Pa. 1972. 68. Holmes, C. A. U.S.D.A. For. Serv. Res. Pap. FPL 194, For. Prod. Lab. Madison, Wis. 1973. 69. Christian, W. J., and T. E. Waterman F i r e . Tech. 6(3):165-178, 188. 1970. 70. Christian, W. J., and T. E. Waterman F i r e J. 65(4):25-32. 1971. 71. King, F. W., and S. C. Juneja For. Prod. J. 24(2):18-23. 1974. 72. Juneja, S. C., and J. K.Shields For. Prod. J. 23(5):47-49. 1973. 73. Fung, D.P.C., Ε. E. Doyle, and S. C. Juneja Inf. Rep. OP-X-68, Dep. of the Environ., Canadian For. Serv., Eastern For. Prod. Lab., Ottawa, Ont. 1973. 74. McCarthy, D. F., W. G. Seaman, E.W.B. DaCosta, and L.D. Bezemer J. Inst. Wood Sci. 6(1):24-31. 1972. 75. Shafizadeh, F. Appl. Polymer Symp. 28, Proc. of the Eighth Cellulosic Conf., Part I , pp. 153-174. T. E. Timell, ed. John Wiley and Sons, N.Y. 1975. 76. Shafizadeh, F. J. of Appl. Polymer S c i . 12(1):139-152. 1976. 77. Amaro, A. J., and A. E. Lipska "Development and evaluation of p r a c t i c a l self-help fire retardants. Annual Report." SRI Proj. No. PYU-8150. Stanford Res. I n s t . , Menlo Park, C a l i f . 1973. 78. Lipska, Α. Ε., and A. J. Amaro "Development and evaluation of p r a c t i c a l and self-help fire retardants. F i n a l Report." SRI Proj. No. PYU-8150. U.S. Dep. Commer., NTIS No. AD-A014 492. Stanford Res. I n s t . , Menlo Park, C a l i f . 1975. 79. Yuill, C. H. Am. Soc. Test. Mater. Stand. News, STDNA 1(6) :26-28, 47. 1973. 80. Williamson, R. Β., and F. M. Baron J. Fire & Flammability 4(2):99-105. 1973. 81. Underwriters' Laboratories, Inc. "Flammability studies of c e l l u l a r p l a s t i c s and other building materials used i n i n t e r i o r finishes." Subj. 723. Underwriters' Laboratories, Northbrook, Ill. 1975.


106

U.S. Forest Products Laboratory. U.S.D.A. For. Serv. Res. Note FPL-0167. For. Prod. Lab., Madison, Wis. 1967.


82.



7 Properties of W o o d during Carbonization under Fire Conditions


F. C. BEALL Faculty of Forestry and Landscape Architecture, University of Toronto, 203 College St., Toronto, M5S 1A1 Canada

Most of the knowledge concerning the behavior of wood under high temperature conditions is limited to such externally-measured quantities as mass loss or gas generation. Two general approaches have been used to predict the pattern of wood deterioration during f i r e exposure. The more elegant approach is through the d i f f e r ential form of the unidirectional heat-flow equation. Thermal d i f f u s i v i t y , which is the equation "constant", has a very complex behavior with temperature, precluding a formal solution to the equation. However, numerical solutions, such as can be developed from finite-element techniques, permit a means of evaluating this "constant" for small temperature increments (1). No serious effort has been made in past research to fully evaluate thermal d i f f u s i vity of wood as i t changes with temperature. Such information is elementary for designing methods of f i r e retardation, which in the past, have been almost exclusively trial and error. It is also evident that the physical changes in wood during fire exposure should be examined in an effort to modify thermal d i f f u s i v i t y . A second approach involves the evaluation or creation of empirical equations based on experimental data. The most widely-used method relates the rate of charring to the change of wood density (2). However, just as the density factor in thermal d i f f u s i v i t y is r e l a t i v e l y undefined, so is the density change from wood to char. Fragmentary information has been published on the densi ty change (3, 4), but no systematic study has been done. The purpose of this study was to c l a r i f y the change of density with species, heating rate, and temperature under oxygen-defîcient conditions. The major constraint was an arbitrary specimen size (10-mm cube), based on the observation that thicknesses equal to or greater than about 6 mm (approximately the half-thickness of the cubes) produce consistent charring rates (5). The effect of thickness on density changes is currently being studied. Experimental Procedure Sample Preparation.

The six wood species (Table I) were

107



0.71

0.55

0.45

0.39

0.37

0.25

0.25

0.24

0.22

0.29

Hard Maple

Southern pi ne

Douglas f i r

Basswood

Redwood

0.23

0.24

0.26

1.17 1.25

0.367 0.262

0. 184 0. 180

Wood Handbook (£).

1.33

1.16

0.327

0.323

0. 188

1.38

1.44

0. 176

0.361

0. 176

0.46 0.35

0.326

0. 175

0.58

Subscript c = char; ο = ovendry c o n t r o l value

0.80

0.31

Whi te oak

0

5

Q

p

ο

1 .69

0.52

0.65

0.58

1 .58 1 .41

0.64

0.61

2 .06 1 .54

0.57

1 .87

0. 07

0. 16

0. 12

0. 12

0. 15

0. 16

Table I. Mass, d e n s i t y , and shrinkage r e l a t i o n s h i p s a t 600°C among wood species heated a t 1°C/min. a m c Δν/ν ^ ΔΤ/AR Po , c m ΔΤ/Τ AL/L (g/cm3) (g/cnv ) Species char moi sture char moisture ο


7.

BEALL

Carbonization

under Fire

Conditions

109


s e l e c t e d f o r a w i d e range o f densîty, s t r u c t u r e , and s h r i n k a g e (from moisture l o s s ) b e h a v i o r . A l 1 s p e c i m e n s were s e l e c t e d f r o m f l a t - s a w n , k i I n - d r i e d b o a r d s w h i c h were f r e e f r o m v i s i b l e d e f e c t s . S e v e r a l l e n g t h s o f n o m i n a l 10 by 10 mm s t o c k f r o m e a c h s p e c i e s wi t h good rad i a l and t a n g e n t i a l f a c e s provî ded s u f f i c i e n t specimen m a t e r i a l , e i t h e r end-matched o r c o n t a i n i n g v i r t u a l l y t h e same g r o w t h r i n g s . A f t e r t h e s t o c k was c r o s s c u t , f i n a l d i m e n s i o n s o f t h e 10 mm cubes were o b t a i n e d by sand i n g . A l 1 s p e c i m e n s were o v e n d r i e d and s t o r e d o v e r d e s i c c a n t u n t i 1 m e a s u r e d . Sample Measurements. Mass was d e t e r m i n e d u s i n g an a n a l y t i c a l b a l a n c e o f 10"i>g s e n s i t î v i t y . P r i o r r e s e a r c h on c a r b o n i zed wood has shown t h a t î t s spongy a n d / o r fragîle n a t u r e c o u l d c a u s e e r r o r s i n m e c h a n i c a l measurement o f d i m e n s i o n s . T h e r e f o r e , dime n s i o n s were o b t a i n e d u s i n g a camera mounted on an i n c i d e n t l i g h t m i c r o s c o p e t o p h o t o g r a p h s p e c i m e n s wi t h a c a l i b r a t e d g r i d ( m i c r o m e t e r e y e p i e c e ) r e s t i n g on e a c h o f t h e f a c e s . The d e v e l o p e d n e g a t i ves were mounted i n s 1i des and p r o j e c t e d t o d î r e c t i y measure e a c h f a c e . Two w i d t h s were d e t e r m i ned a t un î f o r m s p a c i n g i n e a c h a x i s, f o r a t o t a l o f h measurements p e r f a c e , o r 2k p e r c u b e . The p a i r e d measurements were l a t e r a v e r a g e d . A f t e r c a r b o n i z a t i o n , t h e cube f a c e s were p h o t o g r a p h e d i n an îdentical s e q u e n c e t o p e r m i t s h r i nkage a n a l y s î s o f i n d i v i d u a l f a c e s . Specimen e x p o s u r e t o atmospherîc c o n d i t i o n s was m i n i m i z e d . Sample Runs. A b l o c k d i a g r a m o f t h e s y s t e m i s shown i n F i g u r e 1. T h r e e s p e c i m e n s were p l a c e d i n a n i c k e l b o a t and posi t i o n e d i n the Vycor f u r n a c e tube. The s y s t e m was i n i t i a l l y f l u s h e d wi t h n i t r o g e n a t 0.8 Jtt/min t o remove o x y g e n , and r e d u c e d t o 0.2 £t/min d u r i ng t h e r u n . H e a t i n g r a t e (1, 10, 50°C/min) was preset on t h e t e m p e r a t u r e programmer whi ch m a i n t a i n e d a 1 i n e a r r a t e u s i n g a p l a t i n u m r e s i stance s e n s i n g element d i r e c t l y below the f u r n a c e t u b e and h a v i n g a p r o p o r t i o n a t i n g o u t p u t v o l t a g e t o the f u r n a c e . The sample t e m p e r a t u r e was moni t o r e d wi t h a CR/AL t h e r m o c o u p l e a d j a c e n t t o t h e f a c e o f one s a m p l e . T h e r m o c o u p l e EMF was f e d t h r o u g h an e l e c t r o n i c r e f e r e n c e j u n c t i o n t o a s t r i p c h a r t r e c o r d e r wi t h a c a l i b r a t e d s p a n . When t h e d e s i red f i n a l t e m p e r a t u r e was r e a c h e d , t h e h e a t i n g was s t o p p e d and t h e s p l i t f u r n a c e e l e m e n t opened t o expedî t e c o o l i n g . R e s u l t s and D i s c u s s i o n A typî c a l mass l o s s c u r v e f o r t h e t h r e e h e a t i n g r a t e s i s shown i n F i g u r e 2. The f r a c t i o n a l r e s i d u a l ( c h a r ) mass a t 600°C i s g i v e n f o r e a c h s p e c i e s i n T a b l e I. Considerable data are a v a i l a b l e f r o m t h e 1 i t e r a t u r e , p a r t i c u l a r l y from thermogravimetry s t u d i e s , on t h e înf1uence o f s p e c i m e n and h e a t i n g p a r a m e t e r s on mass l o s s c h a r a c t e r i s t i c s .


110

WOOD TECHNOLOGY:

-T/C

CHEMICAL

"S—®

iPROGh

R


Figure 1. Block diagram of heating system. G = gas; F = flowmeter; T/C = thermocouple; REF = reference junction; REC = recorder. Dashed line indicates boundary of furnace.

100

300

m

TEMPERATURE (0

_ 600

Figure 2. Mass loss of redwood at three heating rates to end temperatures of 250°, 300°, 350°, 400°, and 600°C

m

TBTOWUPOC)

6 0 0

Figure 3. Longitudinal, radial, and tangential shrinkage of redwood heated at 1 °C/min


ASPECTS


7.

BEALL

Carbonization

under Fire

Conditions

111

General Shrinkage Behavior. Shrinkage i n the major axes i s shown i n F i g u r e 3 f o r a t y p i c a l s p e c i e s h e a t e d a t l°C/min. C e r t a i n f e a t u r e s were common f o r a l l s p e c i e s : s i m i l a r s h r i n k a g e r a t e s i n a l l a x e s s t a r t i n g a t a b o u t 350°C, g r e a t e r t a n g e n t i a l t h a n r a d i a l s h r i n k a g e a t a l l t e m p e r a t u r e s , a l a g i n o n s e t and l o w e s t v a l u e f o r l o n g i t u d i n a l s h r i n k a g e , a n d p r a c t i c a l l y i dentî c a l l o n g i tud î n a l s h r i nkage b e h a v i o r and v a l u e s f o r a l 1 s p e c i e s ( T a b l e I ) . The T/R s h r i n k a g e r a t i o s f r o m c a r b o n i z a t i o n show a similarî t y t o t h o s e r e p o r t e d f o r moi s t u r e - l o s s s h r i nkage ( 6 ) , h o w e v e r , measurements must be made on c o n t r o l s b e f o r e t h e relatîonshî ρ can be establî s h e d . I η g e n e r a l , t h e r e i s a t e n d e n c y toward i s o t r o p i sm i n t r a n s v e r s e s h r i n k a g e ( T , R ) , p a r t i c u l a r l y f o r basswood a n d s o u t h e r n p i ne. Redwood, b e c a u s e o f î t s r e l a t i v e l y low t a n g e n t i a l s h r i n k a g e , behaves more i s o t r o p i c a l l y t h a n t h e o t h e r s . The a n a l o g y between c h a r and moi s t u r e i s l e s s c l e a r f o r volumetrî c s h r i n k a g e , a l t h o u g h a t r e n d i s o b v i o u s . As wi t h t h e T/R a n a l o g y , measurements must be made o n c o n t r o l s t o c l a r i f y any r e l a t i o n s h i p . D e n s i t y Changes. The v a r i a t i o n o f d e n s i t y wi t h t e m p e r a t u r e was t h e m a j o r relatîonshî ρ s o u g h t i n t h i s s t u d y . From t h e c h a r dens i t y v a l u e s o b t a i ned a t 600 °C, i t was p o s s i b l e t o e s t a b l i sh t h e fο 11ow î n g r e g r e s s i o n e q u a t i o n s : (1)

p

c

= -0.078 + 0.79 P

(2)

p

c

(3)

P

c

( t = 1 °C/min)

r

2

= 0.98

= -0.049 + 0.71 Po

(t = 10 °C/min)

r

2

= 0.99

= -0.006 + 0.56 p

(f = 50°C/min)

r

2

= 0.97

0

0

A l 1 o f t h e s e r e l a t i o n s h i p s a p p l y t o c o n d i t i o n s o f an o x y g e n d e f i c i e n t a t m o s p h e r e and r a p i d e s c a p e o f v o l a t i l e s . The more comp1ex dependence o f dens i t y o n b o t h t e m p e r a t u r e and heat i ng r a t e i s shown i n F i g u r e k. Hi gher r a t e s o f h e a t i n g d e l a y t h e d e n s i t y change u n t i 1 a b o u t 400 ΐ , where t h e c u r v e s c r o s s and show a d i r e c t dependence between d e n s i t y change and h e a t i n g r a t e . L o n g i tud i n a l S h r i n k a g e . Despite the possi ble analogies between c a r b o n i z a t i o n and moi s t u r e l o s s f o r t r a n s v e r s e s h r i n k a g e o f wood, l o n g i t u d i n a l s h r i n k a g e a t 600°C d i d n o t v a r y s i g n i fîc a n t l y among s p e c i e s . The mean v a l u e o f l o n g i t u d i n a l s h r i n k a g e f o r t h e s i x s p e c i e s was 18.0% wi t h a s t a n d a r d d e v i a t i o n o f 0.5%. C a l c u l a t i o n s by Bacon and Tang (7) show t h a t t h e r e d u c t i o n i n l e n g t h o f c e l 1 i b i o s e , i f i t we re t r a n s f o r m e d i n t o graphî t e , w o u l d be 17.3%. T h i s c l o s e agreement s u p p o r t s t h e c o n c e p t o f i n s i t u c e l 1 u l o s e l o s i n g oxygen and f o r m i n g a g r a p h i t i c - t y p e s t r u c t u r e d u r i n g c o n t r a c t i o n o f t h e c h a i n s . Addi t i o n a l l y , t h e t h r e e h e a t i n g r a t e s p r o d u c e d d î f f e r e n t s h r i nkage p a t h s , b u t t h e same ( s t a t i s t i c a l l y ) e n d p o i n t a t 600°C ( F i g u r e 5). A f u r t h e r d i f f e r e n c e i s o b v i o u s between l o n g i t u d i n a l and t r a n s v e r s e s h r i n k a g e when t h e s e a r e p l o t t e d a g a i n s t mass l o s s ( F i g u r e 6) i n s t e a d o f t e m p e r a t u r e ( F i g u r e 3 ) . The l a g i n l o n g i t u d i n a 1



112

WOOD TECHNOLOGY:

CHEMICAL

Figure 4. Density of southern pine as affected by heating rate and temperature. Dashed line is the probable path between points at 400° and 600°C.

IT)

Figure 5.

I o

ifl) T M M P E "(0

600

Effect of heating rate on longitudi nal shrinkage of redwood

I

20

I

I

m ω Y\SS LOSS (%)

L

at)

f

Figure 6. The rehtionship of shrinkage in the major axes to mass loss of southern pine


ASPECTS


7.

BEALL

Carbonization

100| 0

under

, 20

Fire

, «

113

Conditions

ι 50

ι 80

ι 100

TOTAL mss LOSS ω

Figure 7.

Effect of heating rate on loss of oxygen from Douglas fir

nXYPEN mss

(%)

Figure 8. Effect of heating rate on the rela tionship between longitudinal shrinkage and loss of oxygen



114

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

s h r i n k a g e i s much more p r o n o u n c e d , p a r t i c u l a r l y a t 300°C, w h i c h s t r o n g l y s u g g e s t s t h a t t h e m a j o r i n i t i a l mass l o s s ( t o a b o u t 30%) o c c u r s from v o l a t i l i z a t i o n o f c a r b o h y d r a t e s i d e g r o u p s . Shrinkage f r o m 1 i g n i n o r a n y l o s s o f backbone oxygen from t h e p o l y s a c c h a r i d e s s h o u l d c a u s e a component o f l o n g i t u d i n a l s h r i n k a g e . The r e l a t i o n s h i p s were s t u d i e d f u r t h e r by u l t i m a t e a n a l y s i s f o r o x y g e n and h y d r o g e n . Fî g u r e 7 shows t h e p e r c e n t a g e o f oxygen l o s s a s a f u n c t i o n o f t o t a l mass l o s s a t t h e t h r e e r a t e s o f h e a t ing. The c u r v e s c l e a r l y show a much g r e a t e r l o s s o f oxygen a t h i g h e r h e a t i n g r a t e s d u r i n g t h e i n i t i a l mass l o s s . By c o m b i n i n g Fi g u r e s 6 and 7 i n t o Fî g u r e 8, t h e r e l a t i o n s h i p s a p p e a r much c l e a r e r . A t l°C/min, backbone oxygen i s a p p a r e n t l y l o s t a t a s u f f i c i e n t l y s low enough r a t e t o permi t new c a r b o n - t o - c a r b o n v a l e n c e bonds t o f o r m . H i g h e r h e a t i n g r a t e s p r e f e r e n t i a l l y remove s i d e g r o u p ( h y d r o x y l ) oxygen a n d / o r c a u s e a l a g i n C-C bondi ng a t t h e n e w l y - c r e a t e d backbone s î t e s . However, t h e l o n g i t u d i n a l s h r i n k a g e a t 600°C i s r e a s o n a b l y c o n s t a n t f o r a l 1 s p e c i e s and h e a t i n g r a t e s .

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Knudson, R.M. and A . P . Schniewind. For. Prod. J. (1975) 25 (2):23-32. Schaffer, E . L . U.S. For. Serv. Res. Note FPL-0145. 1966. B e a l l , F.C. P.R. Blankenhorn, and G.R. Moore. Wood Science (1975) 6 (3):212-219. McGinnes, E . A . , S.A. Kandeel, and P.S. Szopa. Wood and Fiber (1971) 3 (2):77-83. A k i t a , Κ. Report Fire Res. Inst. Japan 9 (1,2). 1959. Anon. "Wood Handbook - wood as an engineering material". Govt. Printing Office. Washington. 1974. Bacon, R. and M.M. Tang. Carbon (1964) 2:221-225.

This research was funded by National Research Council Canada and the Canadian Forestry Service. Portions of the study were in cooperation with the Universi ty of Missouri.


8 Dimensional Changes of W o o d and Their Control A L F R E D J. STAMM


School of Forest Resources, Department of Wood and Paper Science, North Carolina State University, P.O. Box 5516, Raleigh, N.C. 27607

Wood, l i k e all other plant materials, i s l a i d down from aqueous solution. The c e l l u l o s e , hemicellulose, and l i g n i n polymers formed are no longer soluble in water, but water still dissolves in them to form s o l i d solutions on the polar hydroxyl groups. Water i s held within the c e l l wall structure by hydrogen bonding (1, 2). Sorption i s of the polymolecular sigmoid type (1, pg 146). Each adsorption s i t e , consisting primarily of hydroxyl groups, can take up 5 to 7 molecules of water (1, pg 162) as shown from adsorption isotherms using the equation of Brunauer, Emmett, and Teller (3). The energy of adsorption decreases rapidly as molecules beyond monomolecular are taken up (1, pg. 208). The final molecule adsorbed on any particular s i t e is taken up by a force just exceeding that required to open up the structure to accommodate it. The take up of water or other 1iquids within the c e l l walls of wood involve the take up of a molecule at a time and i t s movement from one adsorption s i t e to another (molecular jump phenomenon) under a concentration gradient. This i s d i s t i n c t from flow of bulk liquids into the coarse c a p i l l a r y structure under a c a p i l l a r y force or pressure gradient. Fundamentals of S h r i n k i n g and S w e l l i n g Adsorbed water v i r t u a l l y adds i t s volume to t h a t o f the dry c e l l w a l l s o f wood, causing s w e l l i n g (1_, Chapter 13). This i s the case because o f the f a c t that the dry c e l l w a l l s are v i r t u a l l y f r e e o f voids t h a t could f i l l w i t h water without s w e l l i n g o c c u r r i n g . The v o l u m e t r i c s w e l l i n g o f wood substance, with a s p e c i f i c volume of 0,685 (]_, Chapter 3) and a f i b e r s a t u r a t i o n p o i n t o f 30%, i g n o r i n g any a d s o r p t i o n compression, i s 43.7%. I f wood swelled l i k e a s t r e s s s t r a i n l e s s g e l , the voids i n wood would increase i n volume the same amount as the wood substance (1_, Chapter 13), F o r t u n a t e l y , natural v o i d s , such as the lumen o f f i b e r s and the v e s s e l s of hardwood change only s l i g h t l y i n volume w i t h changes i n volume o f the wood substance because of the i n t e r n a l r e s t r a i n i n g a c t i o n by f i b r i l wrappings. 115


WOOD


116

TECHNOLOGY:

CHEMICAL

ASPECTS

External dimensional s h r i n k i n g and s w e l l i n g of wood i s roughly p r o p o r t i o n a l to the s p e c i f i c g r a v i t y of the wood (]_, Chapter 13). S w e l l i n g of wood can be forced to be almost e n t i r e l y i n t e r n a l by a p p l y i n g strong external r e s t r a i n t (]_, pg. 233). Wood i s a n i s o t r o p i c , t h a t i s i t swel1 s and s h r i n k s d i f f e r e n t l y i n the three s t r u c t u r a l d i r e c t i o n s . S h r i n k i n g i n the f i b e r d i r e c t i o n i s u s u a l l y o n l y 0.1 to 0.3%, v a r y i n g w i t h the slope of the f i b r i l wrappings of the S2 l a y e r of the c e l l w a l l s and with the slope of the g r a i n (4, 2, pg 100). The t a n g e n t i a l shrinkage i s u s u a l l y 1,5 to 275 times t h a t i n the r a d i a l d i r e c t i o n . Tangential shrinkage of commercial woods grown i n the United States ranges from 7 to 11% and r a d i a l shrinkage from 3 to 7% (5). Greater t a n g e n t i a l than r a d i a l s h r i n k i n g has been explained on the basis of ray c e l l r e s t r a i n t ; g r e a t e r t a n g e n t i a l shrinkage of the denser latewood than of the earlywood f o r c i n g increased earlywood shrinkage i n the t a n g e n t i a l d i r e c t i o n ; g r e a t e r slope of f i b r i l s on r a d i a l faces of f i b e r s than on the t a n g e n t i a l faces due to c o n c e n t r a t i o n of p i t s on the r a d i a l f a c e s ; and g r e a t e r amounts of middle l a m e l l a i n the r a d i a l w a l l s than i n the t a n g e n t i a l w a l l s (2, pg, 106-118), A l l of these e f f e c t s may be o p e r a t i v e to various e x t e n t s . F u r t h e r , the r a t i o of t a n g e n t i a l to r a d i a l s w e l l i n g increases w i t h an increase i n moisture content above about 20% (6, 7J, S w e l l i n g i n aqueous s o l u t i o n s beyond the s w e l l i n g i n water i s almost e n t i r e l y i n the t a n g e n t i a l d i r e c t i o n (]_, pg. 251 ) . Dimensional S t a b i l i z a t i o n There are f i v e known methods by which the s h r i n k i n g and s w e l l i n g of wood can be m a t e r i a l l y reduced i n r a t e or i n f i n a l magnitude. They a r e : 1. a p p l y i n g mechanical r e s t r a i n t by c r o s s - l a m i n a t i n g , 2. a p p l y i n g external or i n t e r n a l water r e s i s t a n t c o a t i n g s , 3. reducing the h y g r o s c o p i c i t y of the wood components, 4. c h e m i c a l l y c r o s s - l i n k i n g the s t r u c t u r a l com ponents of the wood, 5. b u l k i n g the c e l l w a l l s of wood w i t h chemicals. C r o s s - l a m i n a t i n g . Wood because of i t s a n i s o t r o p i c n a t u r e , swelIs t h i r t y to one hundred times as much t r a n s v e r s e l y as l o n g i t u d i n a l l y ( χ , Chapter 13, 2 ) , When veneer i s made up i n t o plywood, the l a t e r a l external s w e l l i n g of each p l y i s mechanic c a l l y r e s t r a i n e d from being i t s normal amount due to the much s m a l l e r l o n g i t u d i n a l s w e l l i n g of the adjacent p l i e s . S w e l l i n g of plywood i n the two sheet d i r e c t i o n s i s only s l i g h t l y g r e a t e r than the l o n g i t u d i n a l s w e l l i n g of the unassembled p l i e s , The mechanical r e s t r a i n t reduces the h y g r o s c o p i c i t y by several p e r cent (8) but cannot alone account f o r the l a r g e r e d u c t i o n i n external s w e l l i n g . The c h i e f e f f e c t s are a r e l i e f of the s t r e s s e s by an increased s w e l l i n g i n the thickness d i r e c t i o n of the sheets and an i n t e r n a l s w e l l i n g i n t o the lumen of the f i b e r s


8.

S T A M M

Dimensional

Changes

and Their

Control

117

Cl» pg 2331.


This simple method f o r o b t a i n i n g dimensional s t a b i l i t y of plywood, i n the important sheet d i r e c t i o n s , has the shortcoming that i t promotes face c h e c k i n g . Plywood i s known to face check as a r e s u l t o f the r e s t r a i n i n g s t r e s s e s set up under a l t e r n a t e s w e l l i n g and s h r i n k i n g c o n s i d e r a b l y more than i n normal wood or i n p a r a l l e l laminates (9_). I t w i l l be shown l a t e r t h a t t h i s face checking can be g r e a t l y reduced by s u b j e c t i n g the face p l y s of plywood to a f i b e r b u l k i n g treatment before assembly. External C o a t i n g s . A p p l y i n g water r e s i s t a n t coatings or f i n i s h e s to wood w i l l a p p r e c i a b l y reduce the r a t e o f a d s o r p t i o n of l i q u i d water or a d s o r p t i o n of water vapor and thus reduce the r a t e of s w e l l i n g and f a c e c h e c k i n g , but has o n l y a minor e f f e c t upon e q u i l i b r i u r n s w e l l i n g . The e f f e c t i v e n e s s of coatings v a r i e s w i t h the nature of the c o a t i n g and the exposure c o n d i t i o n s . U n f o r t u n a t e l y a l l known coatings that adhere to wood are somewhat permeable to water. A p p l y i n g aluminum f o i l to a l l surfaces of small wood p a n e l s , w i t h curved edges and c o r n e r s , between coats of v a r n i s h or o i l base p a i n t s gave moisture excluding e f f i c i e n c i e s of 99% (weight gain of the uncoated con t r o l minus the weight gain of the coated specimen d i v i d e d by that of the c o n t r o l when exposed to a r e l a t i v e humidity of 97% f o r one week) (10, 11), Aluminum powder dispersed i n v a r n i s h or o i l base p a i n t gave values ranging from 90 to 95%, Two coats of pigmented o i l base p a i n t over a primer gave values ranging from 60 to 90%. These measurements were made p r i o r to the advent o f water bomb emulsion p a i n t s so they were not i n c l u d e d i n the s t u d y . They undoubtedly would have given s t i l l lower v a l u e s . Two coats of v a r n i s h e s , enamels, or c e l l u l o s e n i t r a t e laquers gave values ranging from 50 to 85%. F i v e coats of l i n s e e d o i l f o l l o w e d by two coats o f wax gave values o f o n l y about 8%. The moisture e x c l u d i n g e f f i c i e n c y of coatings decreases r a p i d l y w i t h t i m e , r e l a t i v e humidity c y c l i n g , and weathering exposure. When c y c l i c or weathering t e s t s are extended f o r periods of a year or more moisture e x c l u s i o n i s p r a c t i c a l l y eliminated. I n t e r n a l C o a t i n g s . Impregnating wood w i t h w a t e r - r e s i s t i n g m a t e r i a l s d i s s o l v e d i n a v o l a t i l e s o l v e n t has the advantage o f not being weathered away or degraded by u l t r a v i o l e t l i g h t . Experience has shown, however, t h a t l e s s p e r f e c t coatings are obtained i n t h i s way. I n t e r n a l c o a t i n g w i t h water r e p e l l e n t s (natural resins,waxes or d r y i n g o i l s d i s s o l v e d i n v o l a t i l e hydrocarbon s o l v e n t s c o n t a i n i n g a t o x i c agent such as penta chlorophenol) are used to some extent to g i v e temporary p r o t e c t i o n to mi Πwork, e s p e c i a l l y a g a i n s t a d s o r p t i o n of l i q u i d


118

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS


water (12, 13). They are u s u a l l y a p p l i e d to dry mi 11work by a simple three minute d i p technique. P e n e t r a t i o n i s c h i e f l y conf i n e d to end p e n e t r a t i o n . This s u p e r f i c i a l treatment imparts some decay r e s i s t a n c e to wood and reduces face checking and g r a i n r a i s i n g , b u t has l i t t l e or no e f f e c t on a l t e r n a t e seasonal s h r i n k i n g and s w e l l i n g . Reduction i n H y g r o s c o p i c i t y . Obviously any treatment or chemical change i n wood that reduces i t s a f f i n i t y f o r water w i l l reduce i t s tendency to s w e l l . Replacing p o l a r hydroxyl groups with l e s s p o l a r groups should accomplish t h i s . An i d e a l case would be to replace a l l hydroxyl groups accessable to water by hydrogen. Unfortunately a l l known hydrogénation procedures break down both c e l l u l o s e and 1 i g n i n (14, Chapter 17), Wood can, however, be a c e t y l a t e d without chemical break down of the s t r u c t u r e . This would be expected to reduce the h y g r o s c o p i c i t y and s w e l l i n g and s h r i n k i n g to about h a l f of normal. I t a c t u a l l y caused a g r e a t e r r e d u c t i o n due to b u l k i n g of the f i b e r s . A c e t y l a t i o n w i l l hence be considered under b u l k i n g . The only p r e s e n t l y known dimension s t a b i l i z i n g method f o r wood t h a t r e s u l t s from a l o s s i n h y g r o s c o p i c i t y alone i s heat s t a b i l i z a t i o n . Heat S t a b i l i z a t i o n , When wood i s heated, p r e f e r a b l y i n the absence of oxygen, under temperature-time c o n d i t i o n s that cause some l o s s of water of c o n s t i t u t i o n and other minor breakdown p r o d u c t s , s w e l l i n g and s h r i n k i n g are a p p r e c i a b l y reduced (1_, pg. 304). Figure 1 i s a p l o t of the logarithm of heating time against heating temperature f o r three d i f f e r e n t softwood species having d i f f e r e n t thicknesses t h a t were heated beneath the s u r face of a low f u s i o n Woods metal (15) to minimize o x i d a t i o n and cause r a p i d heat t r a n s f e r . L i n e a r p l o t s r e s u l t f o r the three d i f f e r e n t reductions i n s h r i n k i n g and s w e l l i n g . Reductions of 40% are obtained by heating at 315°C f o r one minute, 255°C f o r one hour, 210°C f o r one day, 180°C f o r one week, 160°C f o r one month, and 120°C f o r one y e a r . U n f o r t u n a t e l y , t h i s simple means of o b t a i n i n g dimensional s t a b i l i t y of wood i s accompanied by r e l a t i v e l y l a r g e strength l o s s e s , e s p e c i a l l y toughness, and abrasion r e s i s t a n c e . Abrasives a c t u a l l y gouge out e n t i r e f i b e r s r a t h e r than abrading away parts of f i b e r s . Table I gives data f o r the e f f e c t of heating wood f o r 10 minutes a t three d i f f e r e n t temperatures upon f o u r d i f f e r e n t s t r e n g t h p r o p e r t i e s (15). Heat s t a b i l i z a t i o n imparts c o n s i d e r a b l e decay r e s i s t a n c e to wood. Heating to a t t a i n a dimensional s t a b i l i z a t i o n of 40% gave a n e g l i g i b l e w e i g h t l o s s due to decay when subjected to block c u l t u r e t e s t s w i t h Trametes s e r i a l i s f o r two months (17). The corresponding weight l o s s of the unheated c o n t r o l s was 28.4%. Heat s t a b i l i z a t i o n was at f i r s t b e l i e v e d to be due to the formation of ether l i n k a g e s between adjacent c e l l u l o s e chains as a r e s u l t of s p l i t t i n g out of water between two hydroxyl groups (18). I t was l a t e r shown t h a t heat s t a b i l i z e d wood swel1 s to a



•^Heated i n a i r

^ F o r e s t Products Lab. toughness t e s t

(16)

40 92.

40.0

21,0

17.0

8,0

280

25

80.

20.0

12,5

5,0

3,0

245

10

40.

4,6

5,0

2,0

0,5

210

°C

Reduction i n s w e l l i n g and shrinking %

Toughness l o s s 1/ %

Abrasion resistance l o s s 2/ %

Hardness loss %

Modulus o f rupture l o s s %

Weight Loss %

Weight and s t r e n g t h l o s s e s accompanying heat s t a b i l i z a t i o n of dry softwoods heated beneath the surface of molten Wood's metal f o r ten minutes a t three d i f f e r e n t temperatures ( j j )

Temp,

Table I .



120

WOOD

TECHNOLOGY:

CHEMICAL

ASPECTS

g r e a t e r extent than unheated wood i n concentrated sodium hydro xide s o l u t i o n s and i n p y r i d i n e (19.). As n e i t h e r of these chemicals break ether bonds,another e x p l a n a t i o n f o r heat s t a b i l i z a t i o n was sought. H e m i c e l l u l o s e , the most hygroscopic component of wood, i s a l s o the most subject to thermal degradation (20) to f u r f u r a l and various sugar break-down products which polymerize under heat to water i n s o l u b l e polymers, thus reducing the hygroscopi c i t y . These polymers are presumably s o l u b l e or at l e a s t swell i n concentrated sodium hydroxide s o l u t i o n or p y r i d i n e thus accounting f o r the increased s w e l l i n g i n these media. This a l s o accounts f o r the extremely low abrasion r e s i s t a n c e of heat s t a b i l i z e d wood. In normal wood the f i b e r s are at l e a s t p a r t i a l l y held together by h e m i c e l l u l o s e chains that pass through the middle l a m e l l a (1 pg. 319). I f these chains are severed by heat, complete f i b e r s can be separated by a b r a s i o n . Hardboards, made from steam hydrolyzed wood chips,when heat tempered or s t a b i l i z e d lose l i t t l e i f any strength as the hemicelluloses are removed i n the h y d r o l y s i s step and they are no longer needed f o r bonding. Any a p p l i e d use of the simple heat s t a b i l i z a t i o n technique to wood w i l l be l i m i t e d by the l a r g e l o s s i n abrasion r e s i s t a n c e and toughness. Cross L i n k i n g . T i e i n g together o f the s t r u c t u r a l u n i t s of wood w i t h s t a b l e molecular c r o s s - l i n k s should g r e a t l y reduce i t s tendency to swel1. This i s i l l u s t r a t e d by the f a c t that i n c o r p o r a t i n g only small amounts of d i v i n y l benzene i n the v i n y l benzene used 1n making p o l y s t y r e n e , converts the polymer from a benzene s o l u b l e t o a benzene i n s o l u b l e m a t e r i a l (21), with s i n g l e c r o s s - l i n k s per several thousand carbon atoms i n each polymer chain. Formaldehyde has long been known to act as a c r o s s - l i n k i n g agent f o r c e l l u l o s e (22) and i s used as a crease r e s i s t a n t t r e a t ment f o r cotton f a b r i c s (23). Cotton f a b r i c s are soaked i n a formal i n s o l u t i o n c o n t a i n i n g a low concentration of a m i l d l y a c i d i c s a l t , f o l l o w e d by d r y i n g . Tarkow and Stamm (24) a p p l y i n g the treatment to wood, showed that dimensional s t a b i l i z a t i o n does not occur u n t i l the wood i s almost dry and then only when the a c i d i t y was q u i t e h i g h . I t thus seemed d e s i r a b l e to t r e a t the wood with formaldehyde i n the vapor phase over heated p a r a formaldehyde (25). A p p r e c i a b l e permanent dimensional s t a b i l i z a t i o n occurred only i n the presence of strong mineral a c i d s such as h y d r o c h l o r i c or n i t r i c a c i d , Permanent weight gains of 4 to 5% were accompanied by dimensional s t a b i l i z a t i o n s of up to 70%, expressed as a n t i s h r i n k e f f i c i e n c i e s , A S Ε A.à.t.

-1

- s

"

( S

S

S

t

}

t_

χ

1

0

0

where S i s the shrinkage of the c o n t r o l and S. that of the t r e a t e d specimen. Optimum ASΕ values were obtained when the



8.

S T A M M

Dimensional

Changes

and Their

Control

121

wood had a moisture content of 5 to 10% at the time of treatment (24), When formic or a c e t i c a c i d s were used as c a t a l y s t s a n t i s h r i n k e f f i c i e n c i e s of l e s s than 10% r e s u l t e d . U n f o r t u n a t e l y c r o s s - l i n k i n g f o r high dimensional s t a b i l i t y of wood r e q u i r e s a c a t a l y s t pH of 1.0 or l e s s , i n c o n t r a s t to the much lower a c i d i t y that i s adequate f o r o b t a i n i n g crease r e s i s t a n c e i n cotton f a b r i c s . Table II shows the d r a s t i c e f f e c t of the r e a c t i o n on the two most adversely a f f e c t e d s t r e n g t h p r o p e r t i e s o f wood. These losses are l a r g e l y due to a c i d h y d r o l y s i s of the h e m i c e l l u i oses and c e l l u l o s e , as they occur when wood i s t r e a t e d w i t h the c a t a l y s t s without formaldehyde present. Paper can be c r o s s - l i n k ed w i t h formaldehyde to g i v e good dimensional s t a b i l i t y w i t h l e s s a c i d i c c a t a l y s t s , and c o n s i d e r a b l y s m a l l e r permanent weight increase (26, 27, 28). The formaldehyde r e a c t i o n with wood i s undoubtedly one of c r o s s - l i n k i n g as i t i s accomplished w i t h a much s m a l l e r weight increase than i n the case of the b u l k i n g treatments and r e a c t i o n s , to be considered i n the f o l l o w i n g s e c t i o n . F u r t h e r , dimensional s t a b i l i z a t i o n i s a t t a i n e d by reducing s w e l l i n g r a t h e r than by a r e d u c t i o n i n s h r i n k a g e , which i s the case f o r b u l k i n g treatments. Formaldehyde reacted wood, u n l i k e heat s t a b i l i z e d wood, s w e l l s only s i i g h t l y i n concentrated sodium hydroxide s o l u t i o n s and i n p y r i d i n e , which would be expected i f the r e a c t i o n i n v o l v e d c r o s s l i n k i n g (24). Other aldehydes than formaldehyde have been t e s t e d as to t h e i r c r o s s - l i n k i n g a b i l i t y (24). None gave as good dimensional s t a b i l i t y as formaldehyde or proved as permanent,and a l l r e q u i r ed the high concentrations o f e m b r i t t l i n g a c i d s to c a t a l y z e the r e a c t i o n . C h l o r a l r e q u i r e d no a d d i t i o n of a c i d but i t developed i t s own embrîttling a c i d i t y on h e a t i n g , Other types of c r o s s l i n k i n g agents have been sought t h a t do not r e q u i r e the high a c i d i t y needed to a t t a i n high dimensional s t a b i l i t y by the formaldehyde r e a c t i o n . Although these e f f o r t s have not as y e t met w i t h success they should be continued because of the s m a l l e r amount of short c r o s s - l i n k i n g reactant needed compared to bulking reactions. Bulking Treatment with Water S o l u b l e Non-Reacting Chemicals When chemicals are e i t h e r deposited i n or c h e m i c a l l y reacted with the c e l l w a l l s of wood so as to increase the volume of the dry c e l l w a l l s , the external v o l u m e t r i c shrinkage of the wood i s m a t e r i a l l y decreased as a r e s u l t of b u l k i n g o f the f i b e r s , This p r i n c i p l e was f i r s t observed when t h i n cross s e c t i o n a l wafers of softwoods were swollen i n concentrated s a l t s o l u t i o n s f o l l o w e d by d r y i n g to e q u i l i b r i u m with v a r i o u s decreasing r e l a t i v e vapor pressures at which the t a n g e n t i a l and r a d i a l dimensions of the wafers were measured, Figure 2 i s a p l o t of the external c r o s s s e c t i o n a l shrinkage a g a i n s t the r e l a t i v e vapor pressure over saturated s o l u t i o n s of the f o l l o w i n g s a l t s and t h e i r f r a c t i o n -


122


Table


Π

C r i t i c a l s t r e n g t h , l o s s e s c a u s e d by f o r m a l d e h y d e c r o s s 1 i n k i ng o f s o f t w o o d s t o v a r i o u s permanent w e i g h t ga i n s and a n t i s h r i n k e f f i c i e n c i e s , A.S.E. Toughness l o s s 1/

Abrasion Resistance

10

27

60

0.55

25

45

80

2.20

50

70

91

4.20

70

84

95

Weight i n c r e a s e o f d r y wood

A.S.E.

0.10

—^Forest

P r o d u c t s Lab. Toughness T e s t ( 1 6 )

120

140

160

180 200 220 240 260 Heating Temperature (°C)

280

300

320

Industrial and Engineering Chemistry

Figure 1. Logarithm of heating time vs. temperature required to give three different reductions in swelling and shrinking when the heating was done beneath the surface of a molten metal to exclude oxygen (15). 0,1/16-in. thick Sitka spruce veneer; · , 1/2-in. thick cross sections of western white pine; O , 3/8-in. flat sawn western white pine; O , 15/16-in. thick eastern pine boards. Numbers on plot indicate antishrink efficiency (A.S.E.) in percent.


loss


8.

S T A M M

Dimensional

Changes

and Their

Control

123

a l r e d u c t i o n i n vapor pressure : barium c h l o r i d e , 0.916; sodium c h l o r i d e , 0.758; manganese c h l o r i d e , 0.543 ; magnesium c h l o r i d e , 0.331; and l i t h i u m c h l o r i d e , 0.117 at 25°C (29). The p l o t shows that no shrinkage occurs u n t i l the r e l a t i v e vapor pressure f a l l s below t h a t i n e q u i l i b r i u m w i t h a s a t u r a t e d . s o l u t i o n of the s a l t i n the wood. The shrinkage to the f i n a l oven dry c o n d i t i o n was i n each case reduced by the volume of s a l t f i n a l l y a t t a i n e d w i t h i n the c e l l w a l l s . Figure 3 i s a p l o t of the shrinkage versus the moisture content, g i v i n g v i r t u a l l y p a r a l l e l s t r a i g h t l i n e s . This i n d i c a t e s that s h r i n k a g e , i n a l l c a s e s , i s the same f u n c t i o n of the volume of water l o s t below the f i b e r s a t u r a t i o n p o i n t . Water thus v i r t u a l l y adds i t s volume to that of the c e l l w a l l s , f u r t h e r i n d i c a t i n g that the extent of voids i n the dry c e l l w a l l s must be v i r t u a l l y n e g l i g i b l e . Shrinkage due to b u l k i n g i s reduced merely because there i s l e s s moisture to be l o s t . Reducing the r e l a t i v e vapor pressure at which shrinkage begins has no advantage i n a t t a i n i n g dimension c o n t r o l , as the wood i n e q u i l i b r i u m w i t h higher r e l a t i v e vapor pressure values i s always damp. I t i s , however, advantageous i n so c a l l e d s a l t seasoning by reducing d r y i n g s t r e s s e s (30). The i d e a l b u l k i n g agent f o r wood would be a n o n - c o r r o s i v e n o n - v o l a t i l e s o l i d , approaching i n f i n i t e s o l u b i l i t y i n water, that does not m a t e r i a l l y reduce the vapor pressure of water. These c o n d i t i o n s are more n e a r l y approached with sugars than with s a l t s , as shown i n Figure 4 (31). Treatment of wood w i t h aqueous sugar s o l u t i o n s c o n t a i n i n g a t o x i c agent was commercially p r a c t i c ed i n England f o r a short p e r i o d (32). The c h i e f shortcoming was that the wood became damp at r e l a t i v e h u m i d i t i e s above 80% and that adhesion of wood f i n i s h e s was reduced. Polyethylene g l y c o l s proved to be c o n s i d e r a b l y b e t t e r b u l k ing agents than sugars, as shown i n Figure 5 (33). S i t k a spruce cross s e c t i o n s saturated w i t h 25% s o l u t i o n s of polyethylene g l y c o l s w i t h molecular weights of 1000 and l e s s gave almost complete replacement of the s o l u t i o n by the polymer on slow d r y i n g . Thus, the wood approaches having an a n t i s h r i n k e f f i c i e n c y of 100%. This can occur o n l y as the s o l u b i l i t y of the polymer i n water approaches 100%. The higher molecular weight polyethylene g l y c o l s are l e s s e f f e c t i v e b u l k i n g agents because of t h e i r l e s s e r s o l u b i l i t y i n water and the f i n d i n g t h a t f r a c t i o n a t e d polyethylene g l y c o l s w i t h molecular weights exceeding about 3500 cannot, because of t h e i r b u l k , penetrate the c e l l w a l l s of wood (34). The f a c t that presumably higher molecular weight polymer entered the c e l l w a l l s of wood (see Figure 5) can be explained on the basis that depolymerization occurred during b o i l i n g to put them i n t o s o l u t i o n and that the commercial polymers had an a p p r e c i a b l e spread i n molecular w e i g h t s . Figure 5 shows that a s l i g h t swel1 ing occurs during the i n i t i a l stages of d r y i n g i n the case of the low molecular weight polymers. This i s due to the f a c t t h a t s w e l l i n g i n aqueous s o l u t i o n s of hygroscopic chemicals increases



WOOD TECHNOLOGY:


Relative Vapor Pressure Journal of the American Chemical Society

Figure 2. External volumetric shrinkage vs. relative vapor pressure for thin Sitka spruce cross sections containing originally different quarter-saturated salt solutions (29)

Moisture Content (%) Journal of the American Chemical Society

Figure 3. External volumetric shrinkage vs. moisture constant for thin Sitka spruce cross sections containing originally different quarter-saturated salt soltuions (29)


S T A M M

Dimensional


0.1

0.2

Changes

0.3

and Their

Control

0.4 0.5 0.6 0.7 Relative Vapor Pressure

0.8

0.9 1.0

Industrial and Engineering Chemistry

Figure 4. External volumetric shrinkage vs. relative vapor pressure for thin white pine cross sections presoaked in different concentrations ' sucrose or invert sugar (31). O, water only; C, 6.25% sucrose; 12.5% sucrose; Q 25.0% sucrose; Φ, 80.0% sucrose; 3 , 12.5 invert sugar; Δ , 25.0% invert sugar; A , 50.0% invert sugar.

0

0.1

0.2

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Relative Vapor Pressure (%) Forest Products Journal

Figure 5. External volumetric shrinkage vs. rela tive vapor pressure for thin Sitka spruce cross sec tions presoaked in 25% by weight aqueous solutions of glycerine and polyethylene glycol having the average molecular weights given in parenthesis in the legend (33). • , water only; Φ, glycerine; ±, polyethylene glycol (200 and 400); Δ , polyethylene glycol (600); Q, polyethylene glycol (1,000); Φ, polyethylene glycol (1,540); O, polyethylene glycol (4,000); Μ, polyethylene glycol (6,000).



126

WOOD TECHNOLOGY:


w i t h an increase i n c o n c e n t r a t i o n of the s o l u t e up to about a 50% s o l u t i o n (1, pg, 249), Polyethylene g l y c o l treatment i s best a p p l i e d to green wood. The s i m p l e s t technique i s to merely soak the green wood i n a 30% by weight aqueous s o l u t i o n of polyethylene g l y c o l - 1000. The time of soaking v a r i e s w i t h the p e r m e a b i l i t y of the species and the amount of end g r a i n exposed as d i f f u s i o n i n the f i b e r d i r e c t i o n i s about ten to f i f t e e n times as f a s t as i n the t r a n s verse d i r e c t i o n s . Figure 6 i s a photograph of two adjacent c r o s s - s e c t i o n s of an o r i g i n a l l y green l o b l o l l y pine t r e e 3 cm t h i c k . One was soaked i n a 30% s o l u t i o n of polyethylene g l y c o l 1000 f o r one day f o l l o w e d by a i r drying of both specimens. The c o n t r o l developed a l a r g e wedge shaped check extending from the p i t h to the bark due to s t r e s s e s developed because the t a n g e n t i a l shrinkage was about twice the r a d i a l shrinkage. The t r e a t e d specimen shrank so l i t t l e that i t developed a minimum of damaging s t r e s s e s . This was accomplished with only a 16% take up of the polymer (35), An a l t e r n a t e method f o r t r e a t i n g green wood i s to apply several l i b e r a l coats of molten polyethylene g l y c o l - 1 0 0 0 a day apart to a l l surfaces,and s t o r i n g the specimens i n sealed p o l y ethylene bags between coatings to a v o i d d r y i n g . This should be repeated f o r a week to a month, depending on the p e r m e a b i l i t y and s i z e of the specimens, f o l l o w e d by a i r d r y i n g . Checking of the face p l i e s of plywood r e s u l t i n g from r e l a t i v e humidity c y c l i n g can be v i r t u a l l y e l i m i n a t e d by p r e t r e a t ment of the face p l i e s w i t h polyethylene g l y c o l - 1 0 0 0 so as to a t t a i n approximately a 25% dry weight i n c r e a s e . The t r e a t e d plys can be assembled w i t h untreated core p l i e s using any type of g l u e . To i n s u r e a good bond the face p l i e s should be oven d r i e d j u s t p r i o r to assembly to reduce the surface moisture content (35). Drying j u s t p r i o r to the a p p l i c a t i o n of f i n i s h e s i s a l s o d e s i r a b l e . A surface treatment of l o b l o l l y pine house s i d i n g w i t h polyethylene g l y c o l improved the weathering p r o p e r t i e s of a p p l i e d a l k y d emulsion p a i n t s and t h a t of two-can c l e a r polyurethane f i n i s h e s (36, 37), Wood t r e a t e d w i t h polyethylene g l y c o l has c o n s i d e r a b l e decay r e s i s t a n c e under non l e a c h i n g c o n d i t i o n s i n s p i t e of i t ' s non t o x i c i t y (17). This i s probably due to the f a c t t h a t there i s i n s u f f i c i e n t water present w i t h i n the c e l l w a l l s to support decay. The strength p r o p e r t i e s of polyethylene g l y c o l t r e a t e d wood are v i r t u a l l y those of the swollen wood. This i s not s u r p r i s i n g as the polymer tends to maintain green wood dimensions. U n l i k e heat s t a b i l i z e d and formaldehyde c r o s s - l i n k e d wood and wood bulked by r e s i n forming polymers w i t h i n the c e l l w a l l s (to be considered l a t e r ) , the toughness of the wood i s not adversely a f f e c t e d by polyethylene g l y c o l treatment (35), Green t r e e cross s e c t i o n s , w i t h bark i n t a c t , are being t r e a t ed, on a l i m i t e d s c a l e , w i t h polyethylene g l y c o l f o r t a b l e and stand tops and d e c o r a t i v e plaques to prevent checking (38), Green,



8.

S T A M M

Dimensional

Changes and Their

Control

127

Figure 6. Adjacent originally green loblolly pine tree cross sections (27-cm diameter and 3 cm thick). Left, soaked in a 30% by weight solution of polyethylene glycol (1000) for 24 hours; right, soaked in water for 24 hours, both followed by air drying. The treated specimen, on the left, developed no periferal radial checks. The control, on the right, developed a large periferal radial check extending almost to the pith. The treated specimen took up on the average 16% of the polyethylene on a dry weight basis (35).



128

WOOD TECHNOLOGY:


roughed out d e c o r a t i v e c a r v i n g blanks are a l s o being t r e a t e d by d i f f u s i o n w i t h polyethylene g l y c o l , The green t r e a t e d wood carves more e a s i l y than dry wood. Treatment i s more complete where there i s a maximum of end g r a i n , j u s t the parts of the c a r v i n g t h a t need treatment most to prevent checking on f i n a l d r y i n g . A r t i s t s can set t h e i r own pace i n c a r v i n g , as between c a r v i n g sessions the c a r v i n g should be immersed i n a 25 to 30% s o l u t i o n of polyethylene g l y c o l or stored i n a polyethylene bag f o l l o w i n g a p p l i c a t i o n o f the molten polymer to a l l s u r f a c e s , Gunstocks of e x o t i c woods are being commercially t r e a t e d w i t h polyethylene g l y c o l to g i v e them dimensional s t a b i l i t y and to avoid face checking (39), Merely d i p p i n g t h i n fancy c r o t c h face veneer i n a s o l u t i o n of polyethylene g l y c o l gives s u f f i c i e n t take up of the polymer to p l a s t i c i z e the sheets so t h a t they dry f l a t , thus a v o i d i n g breaking and checking when assembled with core p l i e s . Wood a r t i f a c t s , recovered i n the water logged c o n d i t i o n , are t r e a t e d w i t h polyethylene g l y c o l to prevent s e r i o u s break down of the s t r u c t u r e on d r y i n g , A notable example i s the treatment of the Swedish wooden b a t t l e s h i p Vasa, which was sunk i n the harbor of Stockholm i n 1628, and recovered i n 1961 (40), The most remarkable recovery case i s t h a t of a pine log hermetc a l l y sealed i n a bog i n a g l a c i a l moraine i n Northern Wisconsin, R a d i o a c t i v e d a t i n g technique showed t h a t the log was buried f o r a p e r i o d of 31,000 y e a r s . A i r d r y i n g of a s e c t i o n of the l o g r e s u l t e d i n s e r i o u s break down of the specimen to a p i l e o f chips as a r e s u l t of drying s t r e s s e s . Other s e c t i o n s of the log were soaked i n i n c r e a s i n g concentrations of polyethylene g l y c o l - 1 0 0 0 from 10 to 30% f o r several weeks. The specimens remained perf e c t l y sound upon a i r d r y i n g , w i t h no a d d i t i o n a l c h e c k i n g . The s i i g h t shrinkage t h a t d i d occur was presumably s u f f i c i e n t l y great to a l l o w hydrogen bonds to r e p l a c e broken covalent bonds. Recent experiments to determine the dimension s t a b i l i z i n g e f f i c i e n c y of water s o l u b l e f i r e retardent chemicals (41) showed ammonium sulfamate to be s u p e r i o r to phosphate s a l t s , g i v i n g a n t i s h r i n k e f f i c i e n c i e s of 51 to 66% compared to polyethylene g l y c o l - 1 0 0 0 values of 63 to 77%, Sodium s i l i c a t e , because of i t s a l k a l i n i t y , caused c o l l a p s e of the wood that r e s u l t e d i n negative a n t i s h r i n k e f f i c i e n c i e s . S t r o n g l y a l k a l i n e systems should hence be a v o i d e d . Bulking Treatment w i t h Water I n s o l u b l e Chemicals. The c h i e f shortcomings of dimensional s t a b i l i z a t i o n of wood w i t h p o l y ethylene g l y c o l are t h a t i t can be leached from the wood and that the wood f e e l s damp when held f o r prolonged periods of time a t r e l a t i v e h u m i d i t i e s of 80% and above. I t thus appears d e s i r a b l e to deposit water i n s o l u b l e m a t e r i a l s w i t h i n the c e l l w a l l s of wood. This can be done by a replacement process w i t h waxes (42). Water i n green wood i s replaced by C e l l o s o l v e (ethylene g l y c o l monoethyl e t h e r ) by soaking the wood i n t h i s



8.

S T A M M

Dimensional

Changes

and Their

Control

129

c h e m i c a l , f o l l o w e d by s l o w l y d i s t i l l i n g o f f the water which has a lower b o i l i n g p o i n t than the C e l l o s o l v e , No shrinkage occurs during t h i s f i r s t stage of replacement, i f c a r r i e d out s l o w l y . The specimens are then immersed i n a molten wax o r natural r e s i n and the C e l l o s o l v e s l o w l y d i s t i l l e d o f f . This step i n v a r i a b l y i n v o l v e s some shrinkage. A n t i s h r i n k e f f i c i e n c i e s of 80% a r e , however, o b t a i n a b l e i n t h i s way w i t h mixtures of beeswax and r o s i n . This treatment appears s u i t a b l e f o r the p r e s e r v a t i o n of wood a r t i f a c t s . Christensen (43) has t r e a t e d wood a r t i f a c t s by r e p l a c i n g the water w i t h t e r t i a r y butanol and t h i s with p o l y e t h y lene g l y c o l - 4 0 0 0 . A simpler approach f o r d e p o s i t i n g water i n s o l u b l e chemicals w i t h i n the c e l l w a l l s of wood i s to impregnate the wood w i t h s o l v e n t s o l u b l e r e s i n forming chemicals c o n t a i n i n g a c a t a l y s t that penetrate the c e l l w a l l s f o l l o w e d by evaporation of the s o l v e n t and then heating to polymerize the r e s i n . This has been accomplished w i t h the f o l l o w i n g water s o l u b l e r e s i n forming systems: phenol, r e s o r c i n o l , melamine and urea-formaldehydes, p h e n o l - f u r f u r a l , f u r f u r y l - a n i l i n e and f u r f u r y l a l c o h o l (44), The most successful of these has been phenol-formaldehyde (45), I t i s cheaper than r e s o r c i n o l and melamine-formaldehydes and gives higher dimensional s t a b i l i t y and i s more weather r e s i s t a n t than urea-formaldehyde (46). F u r t h e r , l e s s chemical i s l o s t on drying and p o l y m e r i z i n g than i n the case of f u r f u r a l - a n i l i n e and f u r f u r y l a l c o h o l when s l i g h t l y prepolymerized but s t i l l water s o l u b l e " A " stage phenol-forma1dehyde s l i g h t l y a l k a l i n e r e s i n i s used. A number of s u i t a b l e " A " stage r e s i n s are commercially a v a i l a b l e (47). T h e i r aqueous s o l i d r e s i n contents range from 33% to 70%, pH from 6.9 to 8,7 and r e l a t i v e v i s c o s i t i e s i n 33% s o l u t i o n s from 3.5 to 4 . 7 . Wood t r e a t e d w i t h these r e s i n s i s c a l l e d Impreg. D i f f i c u l t y was encountered i n adequately d i s t r i b u t i n g " A " stage r e s i n s i n s i z a b l e pieces of s o l i d wood. L i m i t e d amounts of Impreg have been made by impregnating e a s i l y t r e a t e d s o l i d woods such as ponderosa pine and basswood. Most of the Impreg p r e s e n t l y made i s laminated from t r e a t e d veneer. P r e d r i e d fancy face veneer, 1/32 inch or l e s s i s t h i c k n e s s , can be adequately t r e a t e d merely by soaking i n a 30 to 60% s o l i d content " A " stage r e s i n f o r a few minutes up to an hour or two depending upon the thickness and the amount of cross g r a i n . Cross g r a i n accentuates c a p i l l a r y absorption which i s f o l l o w e d by d i f f u s i o n i n t o the c e l l w a l l s . The r a t e of d i f f u s i o n i n t o the c e l l wall v a r i e s i n v e r s e l y with the square of the t h i c k n e s s . S t r a i g h t g r a i n veneer, 1/16 inch or more i n t h i c k n e s s , r e q u i r e s excessive soaking time f o r the take up of 25 to 30% of r e s i n forming c h e m i c a l , Veneer having a low to medium s p e c i f i c g r a v i t y , i n thickness up to 1/8 inch and moisture contents of 20 to 30%, can be r e a d i l y t r e a t e d using compression r o l l equipment (48). The veneer i s passed between compression r o l l s beneath the surface of the s o l u t i o n where i t i s compressed to about h a l f of i t s o r i g i n a l t h i c k n e s s . On



130

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

emerging from between the r o l l s , the veneer tends to recover i t s o r i g i n a l thickness and i n doing so sucks i n the t r e a t i n g s o l u t i o n . The c h i e f method f o r t r e a t i n g a i r dry t h i c k e r veneer i s by pressure impregnation i n a t r e a t i n g c y l i n d e r . The usual p r o cedure i s to immerse one sheet of veneer at a time i n a tank f i 1 1 e d with the t r e a t i n g s o l u t i o n to i n s u r e w e t t i n g of the faces of each p l y , making c l o s e p i 1ing p o s s i b l e without f e a r of form ing dry pockets. The sheets of veneer are then held down i n the s o l u t i o n with metal w e i g h t s . The height of the t r e a t i n g s o l u t i o n i s adjusted so that f o l l o w i n g impregnation the top sheet i s s t i l l submerged. The tank i s then r o l l e d i n t o the t r e a t i n g c y l i n d e r and 20 to 200 p s i of a i r pressure i s a p p l i e d f o r ten minutes to s i x hours, depending on the wood s p e c i e s , whether sapwood or heartwood and the thickness of the veneer. Heartwood of basswood or cottonwood veneer 1/16 inch t h i c k w i l l take up i t s own weight of 30% s o l i d s content s o l u t i o n i n 15 minutes at 30 to 40 p s i . B i r c h heartwood veneer 1/16 i n c h t h i c k w i l l r e q u i r e a pressure of 75 p s i f o r two to s i x hours to a t t a i n the same take up (45). The t r e a t e d veneer should then be c l o s e p i l e d f o r one to two days, with a water proof cover over i t , to a l l o w f o r e q u a l i z a t i o n of the r e s i n content by d i f f u s i o n . The veneer can then be d r i e d and the r e s i n polymerized i n a continuous veneer d r i e r or i n a dry k i l n . Real f a s t i n i t i a l drying should be avoided to prevent excessive m i g r a t i o n of the as y e t uncured r e s i n to the s u r f a c e s . The t r e a t e d veneer i s then laminated i n t o panels of any d e s i r e d t h i c k n e s s i n a hot press using phenolic glue (45). The dimensional s t a b i l i t y of Impreg made i n the aforegoing way increases w i t h an increase i n the r e s i n content of the veneer up to about 70% a n t i s h r i n k e f f i c i e n c y a t a r e s i n content of 30 to 35%. This ASΕ value i s l e s s than that o b t a i n a b l e w i t h polyethylene g l y c o l because of l o s s of water and subsequent c o n t r a c t i o n of the r e s i n forming chemicals w i t h i n the c e l l w a l l s as p o l y m e r i z a t i o n o c c u r s . Face checking of plywood and p a r a l l e l l a m i n a t e s , w i t h phenolic r e s i n t r e a t e d f a c e s , i s p r a c t i c a l l y e l i m i n a t e d on indoor exposure. Under o u t - o f - d o o r s weathering c o n d i t i o n s face check ing and e r o s i o n are m a t e r i a l l y reduced ( 9 ) . Phenolic r e s i n treatment imparts c o n s i d e r a b l e decay r e s i s t ance to wood as do other dimension s t a b i l i z a t i o n treatments (17). The treatment increases the e l e c t r i c a l r e s i s t a n c e m a t e r i a l l y (49). I t a l s o gives wood c o n s i d e r a b l e a c i d r e s i s t a n c e (45) and heat r e s i s t a n c e (50). Treated specimens have been subjected to c y c l i c heating t o T 0 5 ° C f o l l o w e d by c o o l i n g more than 50 times without v i s u a l harm, whereas untreated c o n t r o l s charred and d i s i n t e g r a t e d badly a f t e r a few heating c y c l e s . Phenolic r e s i n treatment, however, does not impart true f i r e r e s i s t a n c e to wood, but i t does improve the i n t e g r i t y of the c h a r , thus c u t t i n g down on f i r e spread (45). Phenolic r e s i n treatment causes a s l i g h t l o s s i n t e n s i l e strength p r o p e r t i e s of wood and a c o n s i d e r a b l e increase i n



8.

S T A M M

Dimensional

Changes

and Their

Control

131

compressive p r o p e r t i e s and hardness. F l e x u r a l p r o p e r t i e s are increased s l i g h t l y . Shear p a r a l l e l to the g r a i n i s decreased. Toughness i s , however, reduced to about o n e - t h i r d of normal (51, 1, pg. 131). Impreg i s used f o r automobile d i e models of a l l of the body surfaces (50). P a r a l l e l laminates of phenolic r e s i n t r e a t ed cati/o veneer are hot pressed to one i n c h t h i c k panels and these are glued together to the d e s i r e d thickness with c o l d s e t t i n g g l u e , f o l l o w e d by c a r v i n g . Impreg i s a l s o used f o r various s h e l l molding d i e s (50) where i t s e x c e l l e n t heat r e s i s t a n c e i s u t i l i z e d . The mofd i s imbedded i n sand c o n t a i n i n g a heat s e t t i n g r e s i n , heat c u r e d , c o o l e d , and the mold removed. The Impreg mold can be reused up to 50 times. Compreg i s s i m i l a r to Impreg except t h a t the t r e a t e d veneer p r i o r to heat c u r i n g i s a p p r e c i a b l y compressed. P h e n o l i c r e s i n , s t i l l i n the " A " s t a g e , i s an e x c e l l e n t p l a s t i c i z e r f o r wood. Pressures of 1000 p s i or l e s s a t 275 to 300°F are s u f f i c i e n t to compress most species to dry volume s p e c i f i c g r a v i t i e s of 1.2 to 1.35, thus approaching the s p e c i f i c g r a v i t y ( l , 4 6 ) o f the wood substance (52). Drying of t r e a t e d veneer without cure of the r e s i n can be accomplished by k i l n d r y i n g f o r f i v e to e i g h t hours at 140 to 150°F, (52). Compressed p a r a l l e l laminates can be made from d r i e d but uncured veneer c o n t a i n i n g a t l e a s t 30% of r e s i n forming s o l i d s without the use of a l a m i n a t i n g glue when compressed to a s p e c i f i c g r a v i t y of at l e a s t 1,3 at about 300°F as s u f f i c i e n t r e s i n exudes from the p l i e s to form a good bond. When the p l i e s are c r o s s e d , c o n t a i n l e s s than 30% of r e s i n forming chemicals, and are compressed to l e s s than a s p e c i f i c g r a v i t y of 1 . 3 , a d d i t i o n a l hot press phenolic bonding r e s i n must be used. I t i s important to predry the t r e a t e d p l i e s to a moisture content of 2 to 4% p r i o r to a p p l i c a t i o n of a waterborne l a m i n a t i n g glue as t h i s tends to introduce excessive moisture i n the panel which i s trapped on compression, I t i s f u r t h e r d e s i r a b l e to again dry the glue spread veneers to t h i s low moisture content before assembly. F a i l u r e to do t h i s may r e s u l t i n checking of the panels as they s l o w l y dry to t h i s reduced e q u i l i b r i u m moisture content. Treated p l i e s f o r t u n a t e l y respond to compression under heat and pressure more r a p i d l y than they cure even at 280°F, This makes p o s s i b l e molding of Compreg by a so c a l l e d expansion molding technique, S i n g l e sheets of dry uncured r e s i n t r e a t e d veneer w i t h a surface coat of bonding r e s i n are r a p i d l y p r e heated to 220 to 240°F and then compressed i n a f a s t operating c o l d press a t about 1500 p s i , The p l i e s respond r a p i d l y to compression. The contained r e s i n does not set i n a thermosetting sen se but sets i n a t h e r m o p l a s t i c sense as the veneer i s c o o l e d . The sheets of veneer can be kept i n t h i s compressed c o n d i t i o n f o r weeks at room temperature and low r e l a t i v e humid i t y without springback. They are cut to tempi ate s i z e s layed up i n proper sequence i n a s p l i t mold to completely f i l l the



132

WOOD TECHNOLOGY:

CHEMICAL

ASPECTS

mold. The mold i s f i r m l y locked i n a c l o s e d p o s i t i o n and then heated to about 270°F. The r e s i n loses i t s t h e r m o p l a s t i c s e t . The p l i e s tend to l o s e t h e i r compression and exert a pressure on the mold approaching that at which they were compressed. As heating continues the r e s i n sets i n a thermosetting sense (53). Compreg s w e l l s i n the t h i c k n e s s d i r e c t i o n two to three times as much as Impreg on the basis o f i t s compressed dimensions but the s w e l l i n g i s extremely slow and the panels do not recover from compression as do untreated compressed wood panels (52). I t has a golden to dark brown c o l o r , depending on the s p e c i e s . I t has a natural lustrous f i n i s h t h a t can be r e s t o r e d by merely sanding and b u f f i n g when cut or s c r a t c h e d . I t can be r e a d i l y cut or turned using metal working t o o l s operated at reduced speeds. Compreg can be glued to Compreg or normal wood w i t h both hot press phenolic and room temperature s e t t i n g r e s o r c i n o l glues (52). Compreg i s h i g h l y r e s i s t a n t to decay and a t t a c k by t e r m i t e s and marine borers (52). I t s e l e c t r i c a l and a c i d r e s i s t a n c e s are a l s o r e a l h i g h . The strength p r o p e r t i e s of Compreg are i n general increased over those of the wood from which i t was made about i n p r o p o r t i o n to the increase i n s p e c i f i c g r a v i t y except f o r the hardness which i s increased by ten to twenty f o l d (54) and the toughness which i s reduced to 0.75 of t h a t f o r the o r i g i n a l wood (51). The toughness i s improved i f the Compreg i s made w i t h a s p i r i t s o l u b l e p h e n o l i c r e s i n r a t h e r than an " A " stage water s o l u b l e (55), but the dimensional s t a b i l i t y i s not so good. Compreg was used during World War II l a r g e l y f o r the roots of wooden a i r p l a n e p r o p e l l e r s , f o r s h i p screw bearings and experimental a i r c r a f t landing surfaces of a i r c r a f t c a r r i e r s . More recent uses have been f o r forming d i e s and j i g s , weaving s h u t t l e s , k n i f e handles, g l a s s door p u l l s and r a i l r o a d t r a c k connectors where e l e c t r i c a l r e s i s t a n c e i s needed f o r automatic s i g n a l i n g systems. r

e

s

i

n

F u r f u r y l Alcohol Resin has been s u c c e s s f u l l y formed i n the c e l l w a l l s of wood to g i v e the wood high dimensional s t a b i l i t y (ASE values o f 65 to 75%) and high a l k a l i as w e l l as a c i d r e s i s t a n c e (56). Anhydrous f u r f u r y l a l c o h o l swel1 s dry wood very s l o w l y . Only about 5% of water present e i t h e r i n the f u r f u r y l alcohol or the wood makes i t a good s w e l l i n g a g e n t f o r wood. The r e a c t i o n r e q u i r e s an a c i d c a t a l y s t (57). The use o f strong mineral a c i d s should be avoided as the p o l y m e r i z a t i o n may p r o ceed even at room temperature w i t h e x p l o s i v e v i o l e n c e . The s h e l f l i f e of f u r f u r y l a l c o h o l w i t h c a t a l y s t present (90% f u r f u r y l a l c o h o l 5% water and 5% c a t a l y s t ) was tested by determining the time a t room temperature beyond which the v i s c o s i t y of the s o l u t i o n increased s i g n i f i c a n t l y . Of a s e r i e s o f a c i d s a l t s and d i - a n d t r i - b a s i c organic a c i d s t e s t e d o n l y z i n c c h l o r i d e and c i t r i c and m a l i c a c i d s gave s h e l f l i v e s over one month. These systems on heating at 100°C f o r 24 hours gave r e s i n y i e l d s o f


8.

Dimensional

S T A M M

Changes

and Their

Control

133


72 to 75%.

F u r f u r y l a l c o h o l r e s i n t r e a t e d wood v a r i e s i n c o l o r from dark brown to black depending on the r e s i n content. A t high r e s i n c o n t e n t s , a high degree o f p o l i s h i s a t t a i n e d by sanding and b u f f i n g . Hardness and crushing s t r e n g t h p e r p e n d i c u l a r to the g r a i n are increased m a t e r i a l l y . Toughness, as i n the case of p h e n o l i c r e s i n t r e a t e d wood, i s decreased ( i n terms of the Charpy impact t e s t from 70 to 30 inch - l b . } ( 5 6 ) . Relative Forest Products Lab toughness values obtained by the author ranged from 0 . 3 to 0 . 6 7 using d i f f e r e n t a c i d c a t a l y s t s and varying concentrations. D r a s t i c a l k a l i r e s i s t a n c e t e s t s c o n s i s t i n g of heating wood specimens i n b o i l i n g 10% NaOH f o r 16 days reduced the crushing strength a t the e l a s t i c 1 i m i t f o r untreated southern y e l l o w pine from 620 to 80 p s i and f o r the wood c o n t a i n i n g 71% f u r f u r y l a l c o h o l r e s i n from 2650 to 890 p s i ( 5 6 ) . V i n y l Resin Treatment. Considerable i n t e r e s t has developed i n recent years i n p o l y m e r i z i n g v a r i o u s v i n y l r e s i n s i n c e l l u losic materials. Most of the v i n y l monomers, w i t h the exception of a c r y l o n i t r i l e , swell wood o n l y s l i g h t l y , ( 5 8 , 59) and hence would not be expected to be good dimension s t a b i l i z i n g b u l k i n g agents except when a non-aqueous f i b e r p e n e t r a t i n g s o l v e n t i s used to a i d i n the f i b e r p e n e t r a t i o n . Normally the 1 i q u i d monomers are impregnated i n t o s o l i d wood and polymerized e i t h e r by gamma ray i r r a d i a t i o n which generate f r e e r a d i c a l s t h a t a c t as e x c i t â t ion s i t e s i n the system (60) ,by f r e e r a d i c a l s generated by thermal break down of a peroxide c a t a l y s t such as b e n z o y l peroxide ( 6 1 ) , or by Vazo , a DuPont c a t a l y s t t h a t breaks down on heating to two f r e e r a d i c a l s and a n i t r o g e n molecule ( 6 2 ) . D i s t r i b u t i o n of monomer was found to be good o n l y at high l o a d i n g which r e s u l t e d i n the polymer being mostly i n the homopolymer form i n the v o i d s t r u c t u r e ( 6 3 , 6 4 , 6 5 , 6 £ , 6 7 , 6 8 ) . These modified woods are being made to a T i m i t e d extent l a r g e l y to take advantage of the improved mechanical p r o p e r t i e s , e s p e c i a l l y hardness and a b r a s i o n r e s i s t a n c e ( 6 8 ) . Good dimens i o n a l s t a b i l i t y , 60-70% ASE, i s obtained only w i t h a c r y l o n i t r i l e and i t s combination w i t h other v i n y l monomers and then o n l y at high loadings ( 6 9 ) . V i n y l r e s i n t r e a t e d wood, at high loadings has a n a t u r a l lustrous appearance as does Compreg. I t s advantages over Compreg f o r f l o o r i n g are i t s g r e a t e r toughness, abrasion r e s i s t a n c e and undarkened c o l o r . Because of the much higher r e s i n content i t should be p o t e n t i a l l y c o n s i d e r a b l y more expensive than Compreg. The step of impregnating w i t h v i n y l monomers could be g r e a t l y s i m p l i f i e d and made more uniform i f veneer was t r e a t e d as i n the case of Impreg and Compreg. In t h i s case a low v o l a t i l i t y monomer, such as t r i butyl styrene (70) d i s s o l v e d i n a v o l a t i l e wood s w e l l i n g s o l v e n t such as methyl a l c o h o l should be the


134

WOOD TECHNOLOGY:



imprégnant to avoid d e p l e t i o n of r e s i n at the surfaces of the p l i e s a f t e r evaporating o f f the s o l v e n t and thus i n s u r i n g dimensional s t a b i l i t y . Curing of the r e s i n w i t h benzoyl peroxide or Vazo and heat could be c a r r i e d out i n a press simultaneously w i t h assembly o f the p l i e s . Chemical Reactants. The b u l k i n g agents f o r wood thus f a r considered depend merely upon d e p o s i t i o n of chemicals w i t h i n the c e l l w a l l s . Forming of r e s i n s w i t h i n the c e l l w a l l s may or may not i n v o l v e some chemical r e a c t i o n w i t h the wood. Even i f the r e s i n cannot be leached from the wood w i t h r e s i n solvents there i s no assurance t h a t i t i s c h e m i c a l l y attached a t the wood. I f polymers are formed w i t h i n the c e l l w a l l s with molecular weights exceeding about 3500 they may be merely mechanically entrapped as homopolymers (34). There i s , however, one group of b u l k i n g agents t h a t d e f i n i t e l y r e a c t with the a v a i l a b l e hydroxyl groups w i t h i n the c e l l w a l l s of wood. A c e t y l a t i o n has proved to be the most successful of these r e a c t i o n s . A c e t y l a t i o n of c e l l u l o s e to the t r i a c e t a t e has been c a r r i e d out without breaking down of the s t r u c t u r e w i t h a c e t i c anhydride c o n t a i n i n g p y r i d i n e to help open up the c e l l wall s t r u c t u r e and to act as a c a t a l y s t (71). This l e d Stamm and Tarkow (72) to t e s t the l i q u i d phase r e a c t i o n on wood. High dimensional s t a b i l i z a t i o n without break down of the s t r u c t u r e was o b t a i n e d , but excessive amounts of chemical were used. They hence devised a vapor phase method at atmospheric pressure that proved s u i t a b l e f o r t r e a t i n g veneer up to thicknesses of 1/8 i n c h . A c e t i c anhydride p y r i d i n e vapors generated by heating an 80-20% mixture of the l i q u i d s were c i r c u l a t e d around sheets of veneer suspended i n a box 1ined w i t h sheet s t a i n l e s s s t e e l , Hardwood veneer, 1/16 inch t h i c k , r e q u i r e d about a 6 hour exposure a t 90°C to o b t a i n an a c e t y l content of 18 to 20% and an ASE of 70%. Softwood veneer r e q u i r e d an a c e t y l content of 25% to o b t a i n the same ASE value and an exposure time of 10 to 12 hours. Clermont and Bender (73) showed that dimethyl formamide can be s u b s t i t u t e d f o r p y r i d i n e as the s w e l l i n g agent and c a t a l y s t f o r a c e t y l a t i o n of wood. G o l d s t e i n et a l . (74) showed that a c e t y l a t i o n of wood can be c a r r i e d out i n the l i q u i d phase with a c e t i c anhydride without the a d d i t i o n of a c a t a l y s t . Only one a c e t y l group of the anhydride molecule r e a c t s w i t h the wood, the other forming a c e t i c a c i d . Following surface r e a c t i o n on the wood the a c e t i c a c i d formed presumably helps open up the s t r u c t u r e and promote f u r t h e r r e a c t i o n . These i n v e s t i g a t o r s a l s o devised a means of a v o i d i n g the use of excessive amounts of a c e t i c anhydride by d i s s o l v i n g j u s t the needed amount i n an aromatic or c h l o r i n a t e d hydrocarbon, impregnating s o l i d wood w i t h t h i s s o l u t i o n under pressure of about 150 p s i i n a t r e a t e d c y l i n d e r , heating w h i l e s t i l l under pressure to 100 to 130°C f o r 8 to 16 hours to promote the r e a c t i o n f o l l o w e d



8.

S T A M M

Dimensional

Changes

and Their

Control

135

by d r a i n i n g the c y l i n d e r and evacuation to remove any excess of a c e t i c a n h y d r i d e , the s o l v e n t and formed a c e t i c a c i d . The d r a i n ed reactants were found to be reusable f o r several subsequent impregnations. The l i q u i d phase r e a c t i o n w i t h a c e t i c anhydride alone and a l s o when d i l u t e d to 25% w i t h xylene a t 125°C gave ASE values f o r twelve species of wood ranging from 70 to 80% ( 7 4 ) . This process was c a r r i e d out on p i l o t p l a n t s c a l e f o r severa l years by the Koppers C o . , P i t t s b u r g h , Pa, I t was never converted to a l a r g e s c a l e process f o r economic reasons. B a i r d , (75) showed that the vapor phase r e a c t i o n can a l s o be c a r r i e d out without a c a t a l y s t to a t t a i n a c e t y l contents of 20% i n 2 hours a t 130°C w i t h white pine cross s e c t i o n s . The a d d i t i o n of 15% of dimethylformamide gave an a c e t y l content of 25% under the same c o n d i t i o n s . The .presence of c a t a l y s t was found h e l p f u l o n l y i n a t t a i n i n g the higher l e v e l s of a c e t y l a t i o n . A c e t y l a t e d wood i s h i g h l y s t a b l e . Ten c y c l e s of r e l a t i v e humidity change between 30 and 90% at 27°C over a o e r i o d o f f o u r months gave no l o s s i n a n t i - s h r i n k e f f i c i e n c y (ASE) ( 7 6 ) , Soaking i n a 9% aqueous s u l f u r i c a c i d s o l u t i o n f o r 18 hours a t 25°C had no e f f e c t on the subsequent ASE. When the temperature was increased to 40°C the ASE dropped o n l y from 75 to 65%. Exposure of a c e t y l a t e d b i r c h panels i n the warm s a l t y water of the G u l f of Mexico f o r a year showed no a t t a c k by Teredo and no l o s s i n ASE whereas the untreated c o n t r o l s were baaly a t t a c k e d . A c e t y l ated b i r c h stakes i n s e r t e d i n t e r m i t e i n f e c t e d s o i l showed no sign of a t t a c k i n 5 y e a r s . A c e t y l ated S i t k a spruce w i t h an ASE of 70% when exposed to L e n z i t e s trabea i n a 3 month s o i l - b l o c k c u l t u r e t e s t showed a n e g l i g a b l e l o s s i n weight compared to 47% f o r the c o n t r o l s (17). S i m i l a r r e s u l t s were obtained by G o l d s t e i n et a l . , 174), using s i x d i f f e r e n t c u l t u r e s . Douglas f i r plywood with a c e t y l a t e d f a c e s , when exposed to the weather on a t e s t fence f o r two years without a surface f i n i s h developed only a s i i g h t roughening and checking whereas the c o n t r o l s weathered and checked badly (76)* The weathering of e x t e r i o r p a i n t s on panels w i t h acetylatecT faces were c o n s i d e r ably b e t t e r than on the c o n t r o l s . Presurface a c e t y l a t i o n a l s o seemed to improve the weathering p r o p e r t i e s of painted wood (36, 37). A c e t y l a t i o n i n general causes a s l i g h t bleaching o f the wood. I t causes l i t t l e change i n the s p e c i f i c g r a v i t y of wood, the weight increase being v i r t u a l l y o f f s e t by the b u l k i n g . Acetylat i o n causes v i r t u a l l y no change or a small increase i n most of the s t r e n g t h p r o p e r t i e s o f wood (72, 74, 75, 76) i n c l u d i n g toughness which i s adversely a f f e c t e d by a l l r e s i n forming b u l k i n g treatments. Other Reactants. Vapor phase r e a c t i o n s of isosyanates with wood have been s t u d i e d as a means of o b t a i n i n g dimensional s t a b i l i t y (Z5_)« Isocyanates are poor s w e l l i n g agents f o r wood. It was thus necessary to use an accompanying s w e l l i n g agent such as


WOOD TECHNOLOGY:


136

CHEMICAL

ASPECTS

dimethyl formamide to open up the s t r u c t u r e , The most s u i t a b l e i s o c y a n a t e , b u t y l , gave ASE values up to 78% f o r a weight increase of 49% when heated f o r two hours at 130°C. Toughness and abrasion r e s i s t a n c e were, however, reduced to 72 and 75% of the values f o r the untreated c o n t r o l s . Another b u l k i n g r e a c t i o n of i n t e r e s t i s w i t h ethylene oxide w i t h t r i m e t h y l ami ne present to open up the s t r u c t u r e and serve as a c a t a l y s t (77). Small wood specimens were evacuated a t 95°C in an a u t o c l a v e . Trimethyl ami ne at 65°C was admitted to a pressure of 1 p s i a b s o l u t e . Ethylene oxide was then introduced i n t o the system under a pressure of 50 p s i and held u n t i l the d e s i r e d extent of weight increase of 20 to 30% due to r e a c t i o n was a t t a i n e d , to give ASE values up to 65%. Recently Rowell and Gutzmer (78) have shown t h a t good dimens i o n a l s t a b i l i t y can be imparted to wood by r e a c t i o n s w i t h other a l k y l e n e oxides namely propylene and butylène oxides and e p i c h l o r o h y d r i n c a t a l yzed w i t h t r i ethyl ami ne. A l l of these chemicals are l i q u i d s at room temperature so that complicated gas handling equipment i s not needed. Optimum ASE values of 66 to 68% f o r Southern y e l l o w pine reacted w i t h propylene oxide were obtained when the add on weight ranged from 28 to 34%. Higher add on values e v i d e n t l y r e s u l t e d i n rupture of the f i b e r w i t h an a p p r e c i a b l e increase i n s w e l l i n g . The optimum ASE values were obtained when the wood was impregnated under a pressure of 150 psi with 95 parts of propylene oxide and f i v e parts of the t r i ethyl ami ne and heated f o r one hour at 110 to 120°C. E p i c h l o r o hydrin gave s i m i l a r ASE values w i t h a s i i g h t l y broader range of weight i n c r e a s e s . E p i c h l o r o h y d r i n treatment gave e x c e l l e n t decay r e s i s t a n c e as shown by block c u l t u r e t e s t s . Conclusions The most e f f e c t i v e dimension s t a b i l i z i n g treatments f o r wood thus f a r devised that introduce a minimum of accompanying d e t r e mental p r o p e r t i e s are a l l of the b u l k i n g t y p e s . The best a l l around treatment i s a c e t y l a t i o n . I t has the l e a s t e f f e c t on the appearance and s p e c i f i c g r a v i t y of wood. I t i s the o n l y treatment other than w i t h polyethylene g l y c o l t h a t does not reduce the toughness of wood. I t gives ASE values as high as 75% with weight increases of o n l y 18 to 20% f o r hardwoods and 26 to 28% f o r softwoods. I t i s h i g h l y s t a b l e and gives the optimum r e s i s t ance to organisms. The r e a c t i o n can be c a r r i e d out simply i n the vapor phase on veneer up to 1/8 inch t h i c k o r i n the l i q u i d phase on s o l i d wood when the r e a c t a n t i s d i s s o l v e d i n a hydrocarbon s o l v e n t . Other b u l k i n g treatments have t h e i r s p e c i a l a p p l i c a t i o n s . Phenolic r e s i n treatment, the f i r s t to be developed, gives high permanent dimensional s t a b i l i t y , decay, heat, a c i d , and e l e c t r i cal resistance. When compressed p r i o r to s e t t i n g of the r e s i n , i t gives the hardest t r e a t e d wood known, hardness increases up to


8.

S T A M M

Dimensional

Changes

and Their

Control

137


20 f o l d . There i s , however, a l o s s i n toughness. Furfuryl a l c o h o l r e s i n treatment imparts a l k a l i as w e l l as a c i d r e s i s t a n c e to wood, making i t s u i t a b l e f o r chemical processing equipment. The c h i e f improved property o f v i n y l r e s i n t r e a t e d wood i s i t s high abrasion r e s i s t a n c e making i t s u i t a b l e f o r f l o o r s u r f a c e s . Polyethylene g l y c o l treatment i s s u i t a b l e f o r the treatment of green wood, e s p e c i a l l y water swollen a r t i f a c t s as i t m a t e r i a l l y reduces the shrinkage t h a t occurs on d r y i n g , and the accompany ing degrade. The treatment i s a l s o h i g h l y useful i n c a r v i n g green wood and avoiding degrade on d r y i n g . The newest treatment w i t h a l k y l e n e oxides shows promise of being developed i n t o a commercial p r o c e s s .

Literature Cited (1)

Ν.

Stamm, A. J. "Wood and Cellulose Science" Ronald Press Co., New York (1964). (2) Skaar, C. "Water in Wood" Syracuse Univ. Press., Syracuse, Y. (1972), (3) Brunauer, S., Emett, P. H. and T e l l e r , Ε., J. Am. Chem. Soc. (1938) 60, 309, (4) Koehler, A. "Longitudinal Shrinkage of Wood", U. S. Dept. Agr. For. Prod. Lab. Report 1093 (1946). (5) Markwardt, L. J. and Wilson, T. R. C. "Strength and Related Properties of Woods Grown in the U. S.", U. S. Dept. Agr, Tech. B u l l . 479. (1935). (6) Keylwerth, R., Holz Roh Werkstoff, (1962) 20 (7) 252-259. (7) Hittmeier, M. E. Wood Sci. and Tech. (1967) 1 (2),109-121. (8) Barkas, W. W. "A Discussion of the Swelling Stresses and Sorption Hysteresis of P l a s t i c Gels", Great B r i t . Dept. Sci. Ind. Research, Forest Products Special Report No. 6 (1947). (9) Lloyd, R. A. and Stamm, A. J., For. Prod. J. (1958) 8 (8), 230-234. (10) Hunt, G. M. "Effectiveness of Moisture-Excluding Coatings on Wood", U. S. Dept. Agr. Circular No. 128 (1930). (11) Browne, F. L., Ind. Eng. Chem. (1933) 25, 835-842. (12) Browne, F. L., Architectural Record, (1949), Mar: 131-133. (13) Browne, F. L. and Downs, L. E. "A Survey of the Properties of Commercial Water Repellants and Related Products" U. S. For. Prod. Lab. Mimeo R1495. (1945). (14) Stamm, A. J. and Harris, Ε. Ε., "Chemical Processing of Wood", Chem. Pub. Co., Ν. Y. (1953). (15) Stamm, A. J., Burr, Η. Κ., and Kline, A. A., Ind. Eng. Chem. (1946) 38:630-637. (16) Forest Products Lab. "Toughness Testing Machine", U. S. For. Prod. Lab. Report 1308 (1956). (17) Stamm, A. J. and Baechler, R. Η., For. Prod. J. (1960) 10 (l):22-26. (18) Stamm, A. J. and Hansen, L. A., Ind. Eng. Chem. (1937) 29: 931-938.


138 (19) (20) (21) (22) (23) (24)


(25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36)

(37)

(38) (39) (40) (41) (42) (43) (44) (45) (46)


Seborg, R. Μ., Tarkow, Η., and Stamm, A. J., J . For. Prod. Research Soc. (1953) 3 (3):59-67. Stamm, A. J. Ind. Eng. Chem. (1956) 48 413-417. Staudinger, Η., Trans. Faraday Soc. (1936) 32:323-335. Eschalier, X., French Patent No. 374, 724 additions 8422 (1906); 9904 (1908); 9905 (1908); 10760 (1909). Gruntfest, I. J. and Gagliardi, D. D. Textile Research J. (1948) 18 643-649. Tarkow, Η., and Stamm, A. J., J. For. Prod. Research Soc. (1953) 3:33-37. Walker, J. F. "Formaldehyde", Reinhold Pub. Corp. New York, (1944). Cohen, W. E., Stamm, A. J. and Fahey, D. J., TAPPI (1959), 42, 934-940. Stamm, A. J., TAPPI, (1959) 42:44-50. Stamm, A. J., TAPPI, (1959) 42:39-44. Stamm, A. J., J. Am. Chem. Soc., (1934), 56:1195-1204, Loughboruogh, W. Κ., Southern Lumberman, (1939), Dec. pg. 137. Stamm, A. J., Ind. Eng. Chem. (1937), 29:833-836. Batson, Β. Α., Chem. Trade J. (1939) 105 (8) (2724):93, 98. Stamm, A. J., For. Prod. J. (1956) 6 (5):201-204. Tarkow, Η., Feist, W. C. and Southerland, C. F . , For. Prod. J. (1966) 16 (10):61-65. Stamm, A. J., For. Prod. J. (1959) 9 (10):375-381. Campbell, G. G. "An Investigation of Improving the Durabili ty of Exterior Finishes on Wood", M. S. Thesis, Dept. Wood and Paper S c i . North Carolina State Univ, Raleigh, N. C. (1966). Campbell, G. G. "The Effect of Weathering on the Adhesion of Selected Exterior Coatings to Wood", PhD Thesis, Dept. Wood and Paper Sci., North Carolina State Univ. Raleigh, N. C. (1970). M i t c h e l l , H. L. and Iverson, E. S., For. Prod. J. (1961) 11 (1), 6-7. M i t c h e l l , H. L. and Wahlgren, Η. E . , For. Prod. J. (1959) 9 (12), 437-441. Franzen, A. National Geographic Mag. (1962) 121 (1) 42-57. Stamm, A. J., Wood S c i . and Tech. (1974) 8:300-306. Stamm, A. J. and Hansen, L. Α., Ind. Eng. Chem. (1935) 27: 148-152. Christensen, Β. Β . , "The Conservation of Waterlogged Wood in the National Museum of Denmark", National Museum of Denmark Copenhagen (1970). Stamm, A. J., and Seborg, R. M . , Ind. Eng. Chem. (1936) 28: 1164-1170. Stamm, A. J. and Seborg, R. M . , Ind. Eng. Chem. (1939) 31: 897-902. M i l l e t t , M. A. and Stamm, A. J., Modern Plastics (1946) 24, 150-153.


8. STAMM (47) (48) (49) (50) (51)


(52) (53) (54) (55) (56) (57) (58) (59) (60) (61) (62) (63) (64)

(65) (66) (67) (68)

(69)

Dimensional Changes and Their Control

139

Burr, Η. Κ., and Stamm, A. J., "Comparison of Commercial Water-Soluble Phenol-Formaldehyde Resinoids for Wood Impregnation", U. S. For. Prod. Lab.Mimeo 1384 (1943). Stamm, A. J., "Wood Impregnation", U. S. Patent No. 2350135. (1944). Weatherwax, R. C . , and Stamm, A. J., Elect. Eng. Trans. (1945) 64:833-839. Seborg, R. M. and Vallier, A. E., J. For. Prod. Research Soc. (1954), 4 (5):305-312. Erickson, E. C. O. "Mechanical Properties of Laminated Modified Wood", U. S. For. Prod. Lab. Mimeo No. 1639 Revised. (1958). Stamm, A. J. and Seborg, R. M. Trans. Am. Inst. Chem. Eng. (1941) 37:385-397. Stamm, A. J. and Turner, H. D. "Method of Molding", U. S. Patent No. 2391489 (1954). Weatherwax, R. C . , Erickson, E. C. O., and Stamm, A. J., "Modulus of Hardness Test," Am. Soc. Testing Materials, Bull No. 153 (1948). Findley, W. H . , Werley, W. J., and Kacatieff, C. D . , Trans. Am. Soc. Mech. Eng. (1946) 68, 317-325. Goldstein, I. S., For. Prod. J. (1955) 5 (4) 265=267. Goldstein, I. S., and Dreher, W. A., Ind. Eng. Chem. (1960) 52, 57-58. Siau, J. F . , Wood S c i . (1969) 1 (4):250-253. Loos, W. Ε., and Robinson, G. L., For. Prod. J. (1968) 18 (9); 109-112. Chapiro, A., and Stannett, V. T . , International J. Applied Radiation and Isotopes (1960) 8, 164-167. Meyer, J. Α., For. Prod. J. (1965) 15 (9):362-364. DuPont Co. "DuPont Vazo 64 Vinyl Polymerization Catalyst" "Product Information" (1974). Kenaga, D. L., Fennessey, J . P. and Stannett, V. T . , For. Prod. J. (1962), 12 (4), 161-168. Kent, J. Α., Winston, A., and Boyle, W. R., "Preparation of Wood - P l a s t i c Combinations using Gamma Radiation to Induce Polymerization", U. S. Atomic Energy Commission Report O. R. O. - 600 and 612 (1962). Loos, W. Ε., Walters, R. E. and Kent, J. A., For. Prod. J. (1967) 17 (5): 40-49. Ramlingham, Κ. V., Werezak, G. N. and Hodgins, J. W., J. Polymer S c i . (1963) Part C Polymer Symposium No. 2: 153-167. Siau, J. F . , Meyer, J. A. and Skaar, C . , For. Prod. J. (1965) 15 (10):426-434. Ellwood, E . , Gilmore, R., Merrill, J . A. and Poole, W. Κ., "An Investigation of Certain Physical and Mechanical Pro perties of Wood-Plastic Combinations", U. S. Atomic Energy Commission Report ORO-638 (RTI-2513-T13) (1969). Ellwood, Ε., Gilmore, R., and Stamm, A. J. Wood Sci. (1972) 4 (3) 137-141.



140


(70) Kenaga, D. L., Wood and Fiber (1970) 2 (1), 40-51. (71) Hess, K., Ber., (1928) 61, 1460. (72) Stamm, A. J., and Tarkow, Η., J . Phys. and Colloid Chem. (1947) 31: 493-505. (73) Clermont, L. P. and Bender, Ε., For. Prod. J . (1957) 7 (5), 167-170. (74) Goldstein, I. S., Jeroski, F. Β., Lund, Α. Ε., Nielson, J . F., and Weaver, J . W., For. Prod. J . (1961) 11 (8), 363-370. (75) Baird, B. R., Wood and Fiber (1969) 1 (1) 54-63. (76) Tarkow, Η., and Stamm, A. J . and Erickson, E. C. O. "Acetylated Wood", U. S. Dept. Agr. For. Prod. Lab. Mimeo No. 1593 (1955). (77) McMillin, C. W., For. Prod. J . (1963) 13 (2), 56-61. (78) Rowell, R. Μ., and Gutzmer, D. I., Wood Sci. (1975) 7 240-246.


9 Dimensional Stabilization of W o o d with Furfuryl Alcohol Resin ALFRED J. STAMM


School of Forest Resources, Department of Wood and Paper Science, North Carolina State University, P.O. Box 5516, Raleigh, N.C. 27607

A number of methods have been developed i n the l a s t f o r t y years f o r the dimensional s t a b i l i z a t i o n o f wood ( 1 2 ) , Un f o r t u n a t e l y none of these have proved to be economically success f u l on a l a r g e s c a l e . One of the methods, i n v o l v i n g the t r e a t ment w i t h f u r f u r y l a l c o h o l (3) was shown by G o l d s t e i n (4, 5) to impart both alkali as w e l l as a c i d r e s i s t a n c e to wood, g i v i n g i t a distinctive black c o l o r . This treatment i s of renewed i n t e r e s t at the present time because f u r f u r y l a l c o h o l i s made from the renewable r e s o u r c e , corn cobs, and can a l s o be made from the h y d r o l i z a t e of hardwood waste ( 6 ) . G o l d s t e i n and Dreher ( 7 ) , have shown t h a t five parts of the c a t a l y s t z i n c c h l o r i d e or organic di-or tri-basic acids, such as citric acid, gave s o l u t i o n s i n 5 p a r t s of water added to 90 p a r t s of f u r f u r y l a l c o h o l t h a t remain s t a b l e f o r a month or more a t room temperature without a significant amount of p o l y m e r i z a t i o n , whereas they polymerize to g i v e high y ields of r e s i n when heated f o r a day at 100°C, This makes p o s s i b l e the impregnation o f wood with t h i s l i q u i d i n conventional t r e a t i n g c y l i n d e r s , d r a i n i n g o f f the excess 1 i q u i d f o r r e u s e , and then heat c u r i n g the r e s i n w i t h i n the wood. In t h i s way an a n t i s h r i n k e f f i c i e n c y (1 - % swel1ing of t r e a t e d wood χ 100) of 63 was obtained at % s w e l l i n g of untreated wood 48% r e s i n content and 70 at 120% r e s i n content w i t h Idaho pine cross s e c t i o n s (4, _7), The toughness of southern pine s t i c k s (0.5 by 0.5 by 4 i n c h span) as shown by the Charpy impact t e s t at 68% r e s i n content was, however, reduced from 69 to 27 f o o t pounds ( r elative toughness 0 , 3 9 ) . I t thus appeared d e s i r a b l e to determine if the use of l e s s c a t a l y s t would improve the toughness and if the long tie up of an oven or dr y k i l n f o r the cure of the resin could be a v o i d e d . p

Experimental Douglas f i r , l o b l o l l y p i n e , and Engelmann spruce specimens 4.5 by 4.5 cm i n the r a d i a l and t a n g e n t i a l d i r e c t i o n s were cut i n t o a s e r i e s of end matched cross s e c t i o n s 3 mm t h i c k f o r 141



142

WOOD TECHNOLOGY;

CHEMICAL

ASPECTS

treatment. Yellow poplar s t i c k s 1 by 1 cm i n the r a d i a l and t a n g e n t i a l d i r e c t i o n s by 40 cm long were a l s o t r e a t e d . A l l s p e c i mens were s t r e s s r e l i e v e d by s w e l l i n g i n water f o l l o w e d by a i r and then oven d r y i n g . P a r t of the s t i c k s were cut i n t o f o u r end matched s t i c k s 9.5 cm long f o r treatment and toughness t e s t s . Three d i f f e r e n t c a t a l y s t s were used. F i v e p a r t s by weight of z i n c c h l o r i d e or of c i t r i c a c i d were d i s s o l v e d i n 5 parts o f water and added to 90 parts by weight o f f u r f u r y l a l c o h o l , g i v i n g 5% c a t a l y s t c o n c e n t r a t i o n s . Zinc c h l o r i d e was a l s o used i n one f i f t h and one twenty f i f t h of the former c o n c e n t r a t i o n . Formic a c i d was used i n 5% by weight concentrations w i t h no water p r e s e n t . The presence of the small amounts of water i n the case of the two s o l i d c a t a l y s t s a i d i n t h e i r s o l u t i o n and cause almost as r a p i d s w e l l i n g of wood as i n water a l o n e , whereas the s w e l l i n g i n w a t e r - f r e e f u r f u r y l a l c o h o l i s extremely slow. Formic a c i d a c c e l e r a t e s the r a t e of s w e l l i n g of wood i n f u r f u r y l a l c o h o l , s i m i l a r to the e f f e c t of water. The h i g h l y permeable l o b l o l l y pine and Engelmann spruce cross s e c t i o n s were t r e a t e d by merely immersing the a i r dry specimens i n the t r e a t i n g s o l u t i o n s f o r 5 seconds. The much denser Douglas f i r heartwood cross s e c t i o n s were t r e a t e d by p u l l i n g a vacuum f o r 30 seconds over the s o l u t i o n immersed specimens. The 9.5 cm long y e l l o w poplar s t i c k s were t r e a t e d by immersing them f o r 10 minutes under vacuum i n the t r e a t i n g s o l u t i o n s . The 40 cm long y e l l o w poplar s t i c k s were t r e a t e d i n g l a s s tubes by p u l l i n g a vacuum of 0.1 mm of mercury on the oven dry specimens, running i n the t r e a t ing s o l u t i o n under vacuum and holding f o r one minute (8), The t r e a t e d specimens were wrapped i n aluminum f o i l and held at room temperature f o r one day to a l l o w f o r e q u i l i z a t i o n of the s o l u t i o n through the s t r u c t u r e by c a p i l a r i t y and d i f f u s i o n . The specimens were then weighed and the r a d i a l and t a n g e n t i a l dimensions determined. They were again wrapped i n aluminum f o i l and heat cured at 120°C f o r e i t h e r 18 or 6 hours. The specimens were weighed and measured, oven d r i e d f o r 2 hours to remove unpolymerized v o l a t i l e s and again weighed and measured. The specimens were then immersed i n d i s t i l l e d water f o r a t l e a s t two days, measured, a i r d r i e d f o l l o w e d by oven d r y i n g , weighing and measuri n g . Specimens that were w e l l cured l o s t l i t t l e weight and dimensions between the heat cured c o n d i t i o n and the f i r s t and second oven d r y i n g . A n t i - S h r i n k and P o l y m e r i z a t i o n E f f i c i e n c i e s A n t i s h r i n k e f f i c i e n c i e s (ASE) f o r the t r e a t e d specimens were c a l c u l a t e d from the changes i n cross s e c t i o n s between the o r i g i n a l untreated water swollen and oven dry c o n d i t i o n s and the t r e a t e d water swollen and second oven dry c o n d i t i o n . Figure 1 i s a p l o t of the ASE f o r the three species of cross sections and the y e l l o w poplar s t i c k s versus the r e s i n c o n t e n t . The ASE values increase approximately 1 i n e a r l y w i t h an increase i n r e s i n content as r e s i n



9.

Stabilization

S T A M M

with Furfuryl

Alcohol

Resin

143

i s formed w i t h i n the c e l l w a l l s to bulk the f i b e r . Above about 40% r e s i n content by weight, r e s i n i s deposited w i t h i n the natural v o i d s t r u c t u r e with l i t t l e i n c r e a s e i n ASE. This p o i n t corresponds to an optimum bulked volume of 32.5% as the s p e c i f i c g r a v i t y of c a s t f u r f u r y l r e s i n was found to be 1.23 by the sus pension method i n an aqueous z i n c c h l o r i d e s o l u t i o n (1 pg 55). This value i s o n l y s l i g h t l y g r e a t e r than the f i b e r s a t u r a t i o n p o i n t or optimum b u l k i n g of untreated wood by water, 30%. The f u r f u r y l a l c o h o l - c a t a l y s t s o l u t i o n s swel1ed the wood 6 to 8% beyond the s w e l l i n g i n water. The b u l k i n g by the s o l u t i o n s was thus 31.6 to 32.4%. This i n d i c a t e s a r e a l high e f f i c i e n c y of d i f f u s i o n of r e s i n forming chemicals i n t o the c e l l w a l l s of wood under the c o n d i t i o n s used. The e f f i c i e n c y w i t h which r e s i n i s formed from the weight of r e s i n forming m a t e r i a l taken up w i t h i n the wood was c a l c u l a t e d on the basis of one mole of water being l o s t f o r each mole of f u r f u r y l a l c o h o l polymerized (0.815) and a l l c a t a l y s t and water l o s t as vapor or from l e a c h i n g subsequent to the f i n a l water soak. The e f f i c i e n c y of p o l y m e r i z a t i o n i s then the f i n a l r e s i n content of the wood i n weight percent, d i v i d e d by the o r i g i n a l s o l u t i o n content of the wood before p o l y m e r i z a t i o n times one minus the i n i t i a l f r a c t i o n a l moisture content of the wood times the f r a c t ion of the weight of the t r e a t i n g s o l u t i o n t h a t was f u r f u r y l alcohol times 0.815. These e f f i c i e n c i e s together with the a n t i s h r i n k e f f i c i e n c i e s are given i n Table I f o r the matched y e l l o w poplar s t i c k s 9.5 cm long and i n Table II f o r the Douglas f i r and Engelman spruce cross s e c t i o n s . Under c o n d i t i o n s where p o l y m e r i z a t i o n was v i r t u a l l y complete (18 h r . cure) the e f f i c i e n c i e s were 90% or b e t t e r using both z i n c c h l o r i d e and c i t r i c a c i d catalysts. When formic a c i d was used as the c a t a l y s t , the e f f i c i e n c y was s i g n i f i c a n t l y lower. When the c u r i n g time was reduced to 6 hours high e f f i c i e n c y of p o l y m e r i z a t i o n r e s u l t e d only at z i n c c h l o r i d e concentrations of 1% and 5%, Mechanical

Properties

S t a t i c bending t e s t s were made on nine t r e a t e d y e l l o w pop l a r s t i c k s (40 χ 1 χ 1 cm) (three each w i t h 2.5% z i n c c h l o r i d e , 2.5% c i t r i c a c i d and 5% formic a c i d c a t a l y s t cured f o r 18 h r . a t 120°C), and f o u r c o n t r o l s t i c k s . Loading was i n the t a n g e n t i a l d i r e c t i o n . The r e s i n contents ranged from 40 to 72%, ASE values ranged from 72 to 77% (av. 74%), The average r e l a t i v e s t r e s s to p r o p o r t i o n a l l i m i t was 1 . 3 , the average r e l a t i v e modulus of rupture was 0.89 and the average r e l a t i v e modulus of e l a s t i c i t y was 2 . 0 . The increases i n the s t r e s s to p r o p o r t i o n a l l i m i t and the modulus of e l a s t i c i t y are due to s t i f f e n i n g of the f i b e r s . The l o s s i n modulus of rupture r e s u l t e d from a c i d embrîttlement of the f i b e r s . There was no s i g n i f i c a n t d i f f e r e n c e due to c u r i n g w i t h the d i f f e r e n t c a t a l y s t s except i n the case of the r e l a t i v e modulus of rupture which averaged 0,99 f o r the specimens



9

6

6

6

6

1% c i t r i c a c i d

1% z i n c c h l o r i d e

0.2% z i n c c h l o r i d e



71.2

74.4

73.8

91.8

102,3

91,5

Av. Solution Content Wt,%

6

6

6

18

18

18

hr.

Cure Time

53.7

49.7

20,7

55.9

67.1

51,2

Av. Resin Content Wt.%

53.2

57.6

57.2

59.5

74.3

66.5

Ut.%

Theoretical Resin Content

t

97,3

86.3

36.2

90,7

90.3

77,1

%

Effici

68.8

54,6

27.7

73.4

73,6

70.6

10

ASE

.33

.58

.74

.57

.55

0.78

Av. Relative Toughness-

U, S. Forest Products Lab, toughness t e s t (9) 3 i n c h span top w e i g h t . 30° a n g l e , p u l l i n t a n g e n t i a l direction.

9

Spec,

No.

s

E f f i c i e n c y of F u r f u r y l Alcohol Resin Treatment of End Matched Yellow Poplar S t i c k s (9,5 χ 1,0 χ l 0 cm) Cured at 120°C i n Aluminum F o i l The A n t i s h r i n k E f f i c i e n c y (ASE) and the R e l a t i v e Toughness

5% formic a c i d

Catalyst

Table I .



4

4

4

4

1% c i t r i c a c i d


0.2% z i n c c h l o r i d e


141.0

143.0

124.1

103.9

113,3

55.3

6

6

18

18

18

98.2

66.7

88.1

67.5

62.7

Engelmann Spruce

18

43.3

104.2

90.3

75.7

82.5

56.6

45.8

64.0

97.3

89.5

76.3

97.5

94.5

80.5

%

Efficiency

97.9

4

5% formic a c i d

77.8

18

59.4

wt, %

Theoretical Resin Content

5% z i n c c h l o r i d e 4 149.0 6 106.0 108.5 y O r i g i n a l moisture content of wood before treatment, 6% 2/ Loos Abrader (10)

4


62.9

47.7

wt. %

Av. Resin Content

95.5

4

λ% c i t r i c a c i d

81.5 18

hr.

wt. %

y

Cure Time

Av. Solution Content

102,8

8

Spec.

No.

Douglas F i r

66.7

72.0

61.7

73.6

72.5

69.2

74.1

77.2

73.0

%

ASE

.73

.80

.83

.63

.63

.66

.63

.64

0.71

y

Relative Abrasion Resistance

E f f i c i e n c y of F u r f u r y l Alcohol Resin Treatment of Douglas f i r and Engelmann Spruce cross Sections Cured at 120°C i n Aluminum F o i l , the A n t i s h r i n k E f f i c i e n c y (ASE) and the R e l a t i v e Abrasion R e s i s t a n c e .

5% formic a c i d

Catalyst

Table Γ Ι .


146

WOOD TECHNOLOGY:

CHEMICAL

|l Δ

ω ω

40


/

0

t

0

ο

•

/

Δ

-OH

OH

OH

HO^^

OH

^^OH

v

OH

^ Ί 3 Η


OH OH CH ÇH

OH

2 2

Ç-CH J^CH ^^CH J^) 2

2

OH

CX OH

2

OH

OH

Figure 1. Resorcinol—phenol—formaldehyde resin oligomers

HO

OH

HO

OH

OH

CH -r*S-CH 2

2

+ Ϊ CH

OH

2

I

^

OH

OH

I

HO-fS

I

OH

^OH

ψΟ

OH

CH -£>OH 2

HO

OH

HO

OH

CH -£-CH 2

HQ cHa

2

0 H

w

OH

OH

OH

HO

H HO-f^l OH +H 0

2

CH

Wood Technology: Chemical Aspects (Acs Symposium Series ; Number 43))

Good Laboratory Practices: Chemical Aspects (Acs Symposium Series)

Wood technlogy.. Chemical aspects

Chemical Sensors and Microinstrumentation (Acs Symposium Series)

Hemicelluloses: Science and Technology (Acs Symposium Series)

Isotopes and chemical principles: A symposium (ACS symposium series ; 11)

Carbohydrate Antigens (Acs Symposium Series)

Environmental Catalysis (Acs Symposium Series)

Fermentation Biotechnology (Acs Symposium Series)

Polyethers (ACS symposium series ; 6)

Microreactor Technology and Process Intensification (Acs Symposium Series)

Light-Activated Pesticides (ACS Symposium Series 339)

Ultraclean Transportation Fuels (Acs Symposium Series)

Food and Packaging Interactions (Acs Symposium Series)

Dietary Supplements (ACS Symposium Series 987)

Light-Activated Pesticides (ACS Symposium Series 339)

Zeolite Synthesis (ACS Symposium Series 398)

Smart Coatings II (ACS Symposium Series 1002)

Drug Metabolism Concepts (ACS symposium series ; 44)

Clinical Chemistry (ACS symposium series ; 36)

Metal-DNA Chemistry (Acs Symposium Series)

Molybdenum Enzymes, Cofactors, and Model Systems (Acs Symposium Series)

Introduction to Microlithography: Theory, Materials, and Processing (Acs Symposium Series)

Bioregulators: Chemistry and Uses (ACS Symposium Series, Volume 257)

Modeling NMR Chemical Shifts: Gaining Insights into Structure and Environment (ACS Symposium Series)

Analytical Chemistry of Bacillus thuringiensis (Acs Symposium Series)

Expression Systems and Processes for rDNA Products (Acs Symposium Series)

Protein Folding: In Vivo and In Vitro (Acs Symposium Series)

Chemistry of Wine Flavor (ACS Symposium Series, No. 714)

Advances in Teaching Physical Chemistry (Acs Symposium Series)

Wood Technology: Chemical Aspects (Acs Symposium Series ; Number 43))

Good Laboratory Practices: Chemical Aspects (Acs Symposium Series)

Wood technlogy.. Chemical aspects

Chemical Sensors and Microinstrumentation (Acs Symposium Series)

Hemicelluloses: Science and Technology (Acs Symposium Series)

Isotopes and chemical principles: A symposium (ACS symposium series ; 11)

Carbohydrate Antigens (Acs Symposium Series)

Environmental Catalysis (Acs Symposium Series)

Fermentation Biotechnology (Acs Symposium Series)

Polyethers (ACS symposium series ; 6)

Microreactor Technology and Process Intensification (Acs Symposium Series)

Light-Activated Pesticides (ACS Symposium Series 339)

Ultraclean Transportation Fuels (Acs Symposium Series)

Food and Packaging Interactions (Acs Symposium Series)

Dietary Supplements (ACS Symposium Series 987)

Light-Activated Pesticides (ACS Symposium Series 339)

Zeolite Synthesis (ACS Symposium Series 398)

Smart Coatings II (ACS Symposium Series 1002)

Drug Metabolism Concepts (ACS symposium series ; 44)

Clinical Chemistry (ACS symposium series ; 36)

Metal-DNA Chemistry (Acs Symposium Series)

Molybdenum Enzymes, Cofactors, and Model Systems (Acs Symposium Series)

Introduction to Microlithography: Theory, Materials, and Processing (Acs Symposium Series)

Bioregulators: Chemistry and Uses (ACS Symposium Series, Volume 257)

Modeling NMR Chemical Shifts: Gaining Insights into Structure and Environment (ACS Symposium Series)

Analytical Chemistry of Bacillus thuringiensis (Acs Symposium Series)

Expression Systems and Processes for rDNA Products (Acs Symposium Series)

Protein Folding: In Vivo and In Vitro (Acs Symposium Series)

Chemistry of Wine Flavor (ACS Symposium Series, No. 714)

Advances in Teaching Physical Chemistry (Acs Symposium Series)

Recommend Documents