Manufacture and Processing of PVC

MANUFACTURE AND PROCESSING OF PVC

MANUFACTURE AND PROCESSING OF PVC Edited by R.H.BURGESS Senior Research Chemist, PVC Research, Imperial Chemical Industries Ltd, Plastics Division, Bessemer Road, Welwyn Garden City, Herts., UK

ELSEVIER APPLIED SCIENCE PUBLISHERS LONDON and NEW YORK

ELSEVIER APPLIED SCIENCE PUBLISHERS LTD Crown House, Linton Road, Barking, Essex IG11 8JU, England This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” British Library Cataloguing in Publication Data Manufacture and processing of pvc. 1. Polyvinyl chloride I. Burgess, R.H. 668.4'237 TP118.V48 ISBN 0-203-49056-8 Master e-book ISBN

ISBN 0-203-79880-5 (Adobe eReader Format) ISBN 0-85334-972-X (Print Edition) WITH 28 TABLES AND 100 ILLUSTRATIONS © APPLIED SCIENCE PUBLISHERS LTD 1982 Reprinted 1986 Special regulations for readers in the USA This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside the USA, should be referred to the publisher. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher.

FOREWORD

In my long period of intimate involvement with the PVC industry I have seen the business expand from its small beginnings to the world-wide business of today. Much important research work was carried out in the early days laying the foundations for the sophisticated control of particle type both of the suspension and mass types of PVC and in emulsion polymerisation which is such a feature of the business today. Perhaps in no other large scale plastic are the physical forms in which the polymer powder can be produced (as opposed to the detailed chemical structure of the molecule) of such importance; and it is this feature which makes the interaction between what happens in the polymerisation vessel and the subsequent use of the polymer so crucial. The industry faced its most serious challenge in the early 1970s with the discovery of a link between exposure to vinyl chloride and a rare form of liver cancer. Its vigorous response to that challenge is a model for the way in which these problems should be tackled, and its successful outcome has ensured the continued growth of the business. The pace of change of the manufacturing process, in the understanding of the processes involved in converting raw PVC to the final article and in the processing equipment itself, remains high with new markets still being developed. All these factors are discussed in this book which has been written by five ICI colleagues who have worked with me for many years. I believe it represents a very fair and up-to-date view of the state of the art. A.W.BARNES Formerly ICI Plastics PVC Director and Chairman of CIA and European CEFIC Committees on VCM Toxicity

PREFACE

Polyvinyl chloride (PVC) has been produced commercially for 50 years but most of the expansion has taken place since the end of the 1939–45 War. From its early beginning in Germany the market has grown to the present 12 million tonnes per year worth £5 billion a year in turnover. It is manufactured by over 70 major companies in more than 30 countries. In turn each of these 70 manufacturers may have up to 100 customers who fabricate articles from PVC many of whom are themselves part of large organisations. The growth of this large market and the involvement of many major companies has stimulated much development work over the years on the manufacturing process, on the product produced and on its subsequent fabrication. For example, a typical manufacturer may have up to 100 or more technically qualified people working on various ways of improving the production and subsequent processing of PVC. Such a large effort has led to important advances in the technologies employed in the processes used to produce PVC and in the ways in which it is subsequently converted to the final article. ICI have been manufacturing PVC in the UK for approximately 40 years and throughout most of this period have developed their technology in close collaboration with Solvay, the leading continental European producer. Outside Europe, through subsidiary/associate companies and licensing arrangements, the ICI/ Solvay technology is also being operated in the USA, Argentina, Brazil, Australia, South Africa and Turkey. Consequently within the ICI organisation there is knowledge of the PVC industry world-wide. Five experts from ICI with experience ranging from 12 to 30 years in the business have contributed 10 chapters to this book covering the essential features of the major polymerisation processes for the production of PVC, including random and graft copolymers and blends of PVC with special resin and rubber additives. Particular attention is given to the so-called stripping step in the process, i.e. the effective removal of unreacted vinyl chloride—today a mandatory requirement. The important features of the subsequent processing of PVC resin to the final articles of commerce with which we are all familiar are also covered and the importance of the morphology of the PVC resin particle in this subsequent processing discussed. We are particularly grateful to Rhone-Poulenc, who contributed to the chapter on the bulk process for making PVC as this is not within ICI’s direct experience. Thus, this book contains in one volume the essential features of the technology of the PVC industry, including the most up-to-date thinking on the different established manufacturing processes and the subsequent fabrication of PVC into finished articles. The authors are grateful to ICI for permission to publish the information in the book and to J.H.Wilson who read the manuscript and made many useful suggestions which have been incorporated into the text. R.H.BURGESS

LIST OF CONTRIBUTORS

M.W.ALLSOPP, A.P.I., M.Sc. Senior Research Chemist, PVC Research, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. R.H.BURGESS, B.Sc., Ph.D. Senior Research Chemist, PVC Research, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. D.E.M.EVANS, B.Sc., Ph.D. Senior Research Chemist, PVC Research, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. V.G.LOVELOCK Consultant, 31 Westly Wood, Welwyn Garden City, Herts, UK. Formerly Senior Research Chemist, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK. D.A.TESTER, B.Sc., Ph.D. PVC Development Group Leader, PVC Technical Service, Imperial Chemical Industries Limited, Plastics Division, Bessemer Road, Welwyn Garden City, Herts AL7 1HD, UK.

CONTENTS

Foreword

iv

Preface

v

List of Contributors

vi

Introduction

xi

SUSPENSION POLYMERISATION OF VINYL CHLORIDE R.H.Burgess

1

1.1

Market for PVC and Principal Manufacturing Processes

1

1.2

Kinetics of Vinyl Chloride Polymerisation

2

1.3

Outline of Suspension Polymerisation Process

6

1.4

The Polymerisation Process

7

1.5

Cost of Manufacturing Suspension PVC

17

1.6

Use of Large Autoclaves

19

Acknowledgements

27

References

27

BULK PROCESSES FOR THE MANUFACTURE OF PVC M.W.Allsopp

28

2.1

Introduction

28

2.2

Development of Bulk Polymerisation of Vinyl Chloride

29

2.3

Evolution of St Gobain to Rhone-Poulenc

30

2.4

History of the Rhone-Poulenc Bulk Process for PVC

31

2.5

Rhone-Poulenc Two-stage Bulk Polymerisation Process

32

2.6

Control of Properties

36

2.7

Process Control

37

2.8

Degassing and Powder Handling

37

2.9

Morphology

38

Chapter 1

Chapter 2

viii

2.10

Copolymers

39

2.11

Cost Comparison of Suspension and Bulk Processes

40

2.12

Latest Development in the Two-stage Process: Vertical Autoclaves

40

2.13

Polymer Quality Status—Bulk Versus Suspension

42

2.14

Gas Phase Polymerisation

43

Acknowledgements

43

References

44

THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS D.E.M.Evans

45

3.1

Introduction

45

3.2

Types of Polymer Produced Using the Emulsion Process

46

3.3

Applications of Paste/Emulsion Polymers

46

3.4

Production of Paste/Emulsion Polymers

47

References

58

VINYL CHLORIDE COPOLYMERS AND PVC BLENDS R.H.Burgess

59

4.1

Introduction

59

4.2

Theory of Copolymerisation

60

4.3

Vinyl Acetate Copolymers

61

4.4

Olefin Copolymers

64

4.5

Other Copolymers

66

4.6

PVC Blends—General

67

4.7

Rubber Blends

67

4.8

Polymer Blends

70

4.9

Filler Blends

71

Acknowledgements

71

References

71

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC R.H.Burgess

72

5.1

Vinyl Chloride Toxicity

72

5.2

Protection of PVC Plant Operators

73

Chapter 3

Chapter 4

Chapter 5

ix

5.3

Vinyl Chloride Analysis

75

5.4

Removal of Residual VCM

77

Acknowledgements

85

References

85

ISOLATION PROCESSES FOR PVC V.G.Lovelock

87

6.1

Introduction

87

6.2

Suspension Polymerisation

87

6.3

Emulsion/Microsuspension Polymerisation

97

6.4

Bulk Polymerisation

107

6.5

PVC Dust—Possible Hazards

107

References

108

MORPHOLOGY OF PVC M.W.Allsopp

109

7.1

Introduction

109

7.2

Nomenclature

110

7.3

Classification of PVC Morphology

111

7.4

Mechanism of Suspension Polymerisation

116

7.5

Overall Morphology of Suspension PVC and Polymer Properties

125

7.6

Morphology of Rhone-Poulenc Bulk Polymer

128

7.7

Morphology of Gas Phase Polymer

128

7.8

Summary

130

Acknowledgements

132

References

132

MECHANISM OF GELATION OF RIGID PVC M.W.Allsopp

135

8.1

Introduction

135

8.2

Pre-mixing with Additives

135

8.3

Effect of Processing Parameters

137

8.4

Extruder Sampling Techniques

141

8.5

Twin-Screw Extruder Sampling

143

Chapter 6

Chapter 7

Chapter 8

x

8.6

Single-Screw Extruder Sampling

147

8.7

Other Extruders

150

8.8

Banbury High Shear Internal Mixer

150

8.9

Brabender Plasticorder

157

8.10

Two-roll mill

158

8.11

Summary

158

Acknowledgements

160

References

160

THE PROCESSING OF RIGID PVC D.A.Tester

161

9.1

Introduction

161

9.2

Feedstock Preparation

162

9.3

Extrusion

164

9.4

The Influence of Processing on Properties

165

9.5

The Influence of Formulation Ingredients on Processing Behaviour

170

9.6

Orientation

181

References

181

THE PROCESSING OF PLASTICISED PVC D.A.Tester

183

10.1

Introduction

183

10.2

Feedstock Preparation

183

10.3

Polymer/Plasticiser Interaction

185

10.4

Particle Morphology and Melt Flow

185

10.5

The Effect of Formulation Ingredients

188

10.6

The Processing of PVC Pastes

193

References

200

Index

201

Chapter 9

Chapter 10

INTRODUCTION

Polyvinyl chloride (PVC) has been produced commercially for 50 years. It was first produced in Germany in the early 1930s but its extensive use did not start until the 1939–45 War when mixtures with certain organic liquids (plasticisers) producing a flexible material found wide application as a rubber substitute, particularly in those countries denied access to natural rubber supplies. This early commercial success stimulated the development of cheap processes for the production of the monomer, vinyl chloride (VCM), initially based on the reaction of hydrochloric acid with acetylene, both materials cheap to produce and readily available. More recently cheaper processes involving the reaction of chlorine with ethylene (oxychlorination) have been developed. Many companies, interested in the electrolysis of brine to give chlorine and caustic soda, saw this new polymer as an important user of chlorine so maintaining the allimportant balance between sales of chlorine and caustic soda. It was soon realised that PVC/plasticiser mixtures could be used for a wide range of applications such as cable covering, raincoats, fabric coating, etc., where flexibility, toughness and, in some cases, transparency were required, and that these articles could be made on equipment already available for processing rubber. In many cases the product, used, for example, in cable covering, showed real advantages over that based on the materials used hitherto. The-combination of comparative cheapness, the link with the already important chlor-alkali business and the excellent properties of flexible products based on PVC persuaded a large number of major chemical companies to become interested in PVC. Their interest and that of their customers who fabricated the final articles, coupled with the excellent properties of the material, have seen the world market for PVC itself increase to its present 12 million tonnes per year (worth £5 billion a year in turnover). The market for PVC articles in total is worth several times this figure when machinery sales, stabilisers, lubricants, other additives, plasticisers, etc., are taken into account. This large growth has seen many changes to the polymerisation process, the fabrication techniques used, the types of articles produced and their properties. The first production process for PVC was based on emulsion polymerisation of VCM to produce a latex from which the PVC was separated by spray drying. This process was expensive since the product contained significant quantities of relatively high cost emulsifier and all water had to be removed by evaporating it with heat. Most manufacturers turned their attention to the suspension polymerisation process in which droplets of vinyl chloride were polymerised to produce coarser particles rather like grains of sand. This process is potentially cheaper because it uses much lower concentrations of additives and because much of the associated water can be removed by filtration. Commercially attractive suspension polymerisation processes were soon developed but, at least initially, the product was much more difficult to process into the final article than that based on the emulsion process, both in the flexible and rigid (i.e. unplasticised) applications, such as packaging film and foil, developing at that time. It was some time

xii

before the necessary control of particle structure was achieved to enable suspension PVC to be used for flexible and rigid applications but once this was achieved the cheaper suspension PVC largely superseded emulsion PVC in a wide range of PVC applications. The present suspension polymerisation process, including the addition polymerisation kinetics used in virtually all commercially important types of polymerisation, is described in Chapter 1. The rapid growth of the rigid PVC market in the last 30 years for such articles as film, sheet, pipe, rainwater goods, conduit and other profiles has been achieved in spite of the thermal instability of PVC at about the processing temperature needed to fabricate the PVC powder into the final rigid article. This problem has prompted the development of complex stabiliser systems, the production of intrinsically more stable PVC and detailed changes to the PVC grain structure to ensure rapid and uniform gelation of the PVC powder to give a tough final article. The morphology of PVC grains (the key property of PVC as sold) and the gelation mechanism are described in Chapters 7 and 8, respectively. PVC, when fabricated without plasticiser, produces a very tough, rigid article which is transparent if desired. However, the difficulty in processing the material normally means that only a narrow range of polymer molecular weights can be used, which limits both the toughness of the final product and the complexity of the article which can be produced. Toughness can be improved by blending rubbers with the PVC and the product melt flow increased by copolymerising vinyl chloride with other monomers, notably vinyl acetate. PVC blends and VCM copolymers are now used extensively for applications such as bottles, window profiles, calendered and extruded rigid film and foil, gramophone records, etc. These special polymers are normally based on the suspension polymerisation process and are discussed in Chapter 4. While these factors favour the growth of PVC manufacture using the suspension polymerisation process, such that 80 % of the total PVC made is produced by this process, the growth of markets such as PVC coated wallpaper, continuous vinyl flooring, etc., coupled with traditional applications such as gloves, bottle caps, fabric coating, etc., which are produced from PVC pastes (i.e. fine PVC particles suspended in plasticiser) has stimulated renewed interest in emulsion polymerisation processes. In order to obtain the required properties in the paste very sophisticated control of the polymerisation process and drying is required. About 10% of the present PVC made is produced by emulsion processes. The polymerisation processes used to make both emulsion and paste PVC are described in Chapter 3. The drying and milling processes which are such an important part of emulsion/paste polymer technology are described in Chapter 6 along with the centrifuging and drying steps used for suspension PVC. In the early 1960s a number of companies investigated the production of PVC in the absence of water. A major problem was ensuring that the correct PVC grain structure was achieved but one of these companies, Rhone-Poulenc, succeeded in developing a process in which a PVC seed particle was used to produce the desired final PVC grain morphology. This process is potentially cheaper than suspension PVC because of the absence of the drying step. However, the extra cost associated with the seed step and the comparative inflexibility of the process in terms of product properties have limited its penetration to less than 10% of the total PVC made. This process is described in Chapter 2. The discovery that the PVC monomer, VCM, represented a serious health hazard to man has posed many problems for the PVC industry. VCM is a gas at ambient temperatures and is usually handled as a liquid under pressure so that there is a high probability of some escape of the monomer into the environment of a VCM or PVC plant unless especial efforts are made to control that escape. The kinetics of VCM addition polymerisation are such that it is uneconomic to convert all the VCM to PVC so that PVC as made is always contaminated with VCM. In recent years processes have been developed to remove this VCM from the PVC very efficiently. The toxicity of VCM and the measures taken to prevent its escape from the PVC plant are described in Chapter 5.

xiii

Historically, PVC was first used mixed with plasticisers to make flexible articles. The market for these products has widened considerably since those early days to include flexible film, pipe, sheet, footwear and flooring applications, such that plasticised PVC articles still consume just over half the PVC produced. Although the basic steps of mixing PVC powder with plasticiser and other additives and fabricating the final article by the application of heat have been established for many years there has been constant development in the understanding of these steps and in fabrication techniques. Chapter 10 describes the processing of plasticised PVC from theoretical aspects such as polymer/plasticiser interaction and melt rheology. It also describes the processes used to make the final article with special emphasis on their production from PVC pastes. Over more recent years the growth of the rigid PVC market has been most rapid, presenting many problems to the processor because of the latent instability of PVC to heat. In addition to the important developments in blending, copolymerisation, PVC grain morphology and product heat stability already mentioned there has been much development in the machinery used to process rigid PVC and in the formulations of stabilisers and lubricants used to assist processing. Chapter 8 describes recent work on the mechanism ‘of gelation of rigid PVC while Chapter 9 deals more generally with the topic of making rigid PVC articles.

Chapter 1 SUSPENSION POLYMERISATION OF VINYL CHLORIDE R.H.BURGESS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

1.1 MARKET FOR PVC AND PRINCIPAL MANUFACTURING PROCESSES PVC has been produced commercially for over 40 years and world-wide sales have expanded greatly over that period with continued growth in the market forecast. The applications for which PVC is currently used span such a wide field that healthy growth is virtually assured. World sales of PVC are shown in Table 1.1, indicating the four-fold increase in sales over the past 15 years. TABLE 1.1 World Consumption of PVC (106 t per annum) Year Total world sales

1965 3.0

1970 6.0

1975 8.1

1980 12.0

1985 15 (estimate)

There are three major methods of manufacturing PVC; namely suspension, emulsion and bulk (mass) polymerisation. Limited quantities of PVC are made by solution polymerisation for speciality applications. Table 1.2 shows the proportion of PVC made by the three principal processes. These figures show the pre-eminence of the suspension polymerisation process in terms of the tonnage produced. The emulsion process, which leads to products with unique properties, also has a healthy growth record. The mass process, the most recent of these processes to become commercial, has grown rapidly over the last 20 years, but less rapidly in the TABLE 1.2 World Nameplate Capacity for the Manufacture of PVC by the Suspension, Emulsion and Mass Processes (103 t per annum) Year 1960 1965 1970 1975 1980

Suspensiona

Emulsiona

Mass

1430 360 12 2900 660 140 6200 1 160 340 10000 1450 1000 13200 1640 1200 a Approximate figures only since these capacities are, to some extent, interchangeable.

Total 1800 3700 7700 12500 16000

2

R.H.BURGESS

past five years than formerly. The products made by the mass process are similar to those made by suspension polymerisation and are used for the same market. Each of these major processes is described in detail in the following chapters. 1.2 KINETICS OF VINYL CHLORIDE POLYMERISATION Before proceeding to describe the suspension polymerisation process it is appropriate to discuss the kinetics of the vinyl chloride (VCM) polymerisation which are common to all the commercial processes. All are based on a free radical induced addition polymerisation described by the following six equations:1–6 (1.1) (1.2) (1.3) (1.4) (1.5) (1.6) where I are free radicals formed by decomposition of the initiator, R is a growing polymer free radical, M is a VCM monomer molecule and P is the final polymer molecule. The initiation step (eqn. 1.1) is commonly a first order decomposition of a labile molecule (peroxide or azo compound) but activation of this decomposition is commonly practised in emulsion polymerisation and is technically feasible in suspension polymerisation. Equations 1.2 and 1.3 represent the chain propagation step normally with similar rate constants, i.e. k2=k3. In some cases, for example azo initiators with a very short half-life for decomposition, the radical produced is so stable that initiation of the polymerisation is very inefficient (k270%), molecules produced by termination in the VCM phase at low conversion have little effect on the final product K-value. Similarly, K-value can be shown to vary by no more than 5K-units over the initiator concentration range 0.01–1% on VCM and to be virtually immeasurable over the commercially used range 0.01–0.1%. Consequently eqn. 1.11 is a good measure of the molecular weight achieved. This is further confirmed by the good agreement between the Kuchanov and Bort prediction and the experimental data of Freeman and Manning6 shown together with the ISO K-value temperature relationship in Fig. 1.4. It is possible to reduce the molecular weight at a given temperature by the use of chain transfer agents. Typical transfer agents which can be used are trichloroethylene, thiolesters and isobutyraldehyde but high concentrations are necessary (say 0.5−1% on VCM) to have any significant effect on polymer molecular weight. This is not surprising in view of the chain transfer properties of VCM itself and the fact that VCM is present, inevitably, in such high concentration. It is possible to increase the molecular weight by the addition of minor amounts of cross-linking agents7 (di- and tri-functional monomers such as divinylbenzene, glycol dimethacrylate, diallyl maleate and diallyl phthalate). Again, the technique is not widely used because of consequent changes in the product molecular weight distribution.

6

R.H.BURGESS

FIG. 1.4. The effect of temperature on molecular weight and K-value.

1.3 OUTLINE OF SUSPENSION POLYMERISATION PROCESS VCM is a gas at ambient temperature (BP—13°C)8 but is handled as a liquid under pressure. It forms explosive mixtures with air (explosive limits 3·6−26%v/v) and is toxic (see Chapter 5); consequently extreme care has to be taken in the design of the storage and transport equipment. VCM as produced is relatively stable and shows little tendency to polymerise. However, contamination with oxygen can give rise to vinyl chloride polyperoxide which decomposes and initiates polymerisation of the VCM. Consequently VCM manufacturers take steps to avoid contamination with oxygen, or if this is unavoidable, small quantities of stabiliser, usually phenol or phenol derivatives, can be added which prevent both peroxidation of the monomer and inhibit polymerisation. A very small quantity (2–10ppm) is normally sufficient to stabilise contaminated VCM against pre-polymerisation and is sometimes used to prevent polymerisation under long-term storage conditions. This stabiliser is not normally removed before

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

7

FIG. 1.5. Suspension polymerisation process. (Reproduced from Burgess, Developments in PVC Production and Processing—1, Whelan and Craft, Eds., Applied Science Publishers Ltd, 1977.)

the VCM is used in the polymerisation process. VCM is transported by pipeline from bulk storage (in spheres of up to 2000t capacity) or by road or rail tanker into stock tanks on the PVC plant. Since it is not economic to convert all the VCM charged to the polymerisation vessel to a quantity of VCM is always recovered from these vessels. This recovered VCM can be returned to the VCM plant for redistillation and blending with virgin VCM, or can be used in subsequent polymerisations as produced. The latter practice is normally followed, the recovered VCM being used blended with virgin VCM roughly in the proportions in which the recovered VCM is produced. In the suspension polymerisation process VCM is dispersed as droplets in water in a suitably designed pressure vessel and polymerised using free radical initiators until 80–90% of the VCM is converted to PVC. The residual VCM is then removed from the suspension of PVC in water by a process termed ‘stripping’, the stripped slurry is dewatered using a centrifuge, dried and then stored as required. This outline of the process is shown diagrammatically in Fig. 1.5. The stripping and polymer isolation processes are described in more detail in later chapters of this book and this chapter concentrates on the polymerisation process only. 1.4 THE POLYMERISATION PROCESS In a typical suspension polymerisation a known quantity of water, normally demineralised water or water of a known high quality, is charged to a pressure vessel (autoclave) and other polymerisation ingredients such as initiator, buffers and protective colloid are added. The autoclave is then sealed and evacuated. VCM is added either by metering, using a turbine meter, or by weight from a suitable weigh vessel. A typical recipe is shown in Table 1.3. TABLE 1.3 Typical Suspension Polymerisation Recipe for a 10m3 Autoclave Weight (kg) Water

5000

8

R.H.BURGESS

Weight (kg) Protective colloid Buffer Initiator Vinyl chloride

3.5 0.7 1.5 3500

Polymerise at 60°C for up to 6 h

The autoclave is heated up to the polymerisation temperature using a mixture of steam and water in the autoclave jacket. Once the autoclave reaches the set temperature, polymerisation starts and heat is evolved (heat of polymerisation of VCM (liquid) to PVC is—1540kJ/kg).9 This heat is removed by cooling water in the autoclave jacket. The autoclave pressure will be steady at the autogenous pressure of VCM for the polymerisation temperature ranging from 6·3 bar at 40°C to 15 bar at 80°C8 until ca 75% conversion of VCM to PVC, when there is no free VCM phase remaining. The pressure then begins to fall as the equilibrium vapour pressure of the PVC/VCM phase falls as its PVC content rises. At a given pressure below the autogenous pressure the polymerisation is terminated, usually by venting off the excess VCM. Since the polymerisation rate falls at high conversion (see Figs. 1.1 to 1.3) it is normally not economic to take the polymerisation beyond 90% and lower conversions are desirable for polymer quality reasons. Venting off the excess VCM reduces the polymerisation rate markedly and effectively stops the polymerisation. The remaining VCM is then stripped from the slurry either in the autoclave or in a separate stripping process (see Chapter 5). After discharge of the batch the autoclave is cleaned to remove any polymer remaining in the autoclave either as loose particles or adhering to the walls of the autoclave. The autoclave is then ready for the next polymerisation cycle. A typical autoclave cycle as operated in the 1950s and early 1960s is shown in Table 1.4. TABLE 1.4 Typical Suspension Polymerisation Cycle for a 10m3 Autoclave (1950–1960) Time (min) Charge aqueous phase Evacuate autoclave Charge VCM Heat up to 60°C Hold at 60°C Remove unreacted VCM Discharge autoclave Clean autoclave Total cycle:

30 15 15 60 360 120 30 60 — 690

Consequently such an autoclave is capable of producing about 3t of PVC every 690 min, corresponding to over 2000t per year assuming continuous operation and 90% equipment availability. A plant capable of producing, say, 20000t per year would thus consist of 10 such autoclaves or 20 for 40000t per year although it should be pointed out that normal practice today would be to use a smaller number of larger autoclaves with a shorter cycle (see Section 1.5).

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

9

1.4.1 Role of Water in the Process Water is present to act as a continuous phase so enabling the VCM to be broken up into discrete droplets, to act as a heat transfer medium and as a carrier for the protective colloid. The quantity of water used is determined to some extent by each of these activities but principally by the nature of the VCM and PVC particles, normally called granules or grains, produced during the process. Initially the VCM phase is broken up into 30–150 µ m droplets by the comminuting action of the agitation and prevented from coalescing by the action of the protective colloid. A minimum quantity of water is required to fill all the spaces between the spherical VCM particles and to provide some ‘free’ water to ensure a low viscosity mixture. Water:monomer ratios of 1:1 are perfectly adequate to achieve this aim. However, at the end of the polymerisation it is equally necessary for the PVC grains to be held apart by the water. The bulk density of PVC made varies widely depending on the market for which it is intended, but will normally be in the range 0·4−0·8 kg/litre. This in turn requires water: monomer ratios up to 1·75:1 to ensure a low viscosity mixture is present. Typically a water:monomer ratio of about 1·5:1 is used as shown in Table 1.3 (actually 1·43 in this example). The second major purpose of the water is to act both as a heat sink and as a heat transfer medium. Since the specific heat of VCM and PVC are approximately 0–25 cal/g and that of water is 1 cal/g, the heat capacity of the water at the typical 1·5:1 water: monomer ratio is six times that of the organic phase. This has a considerable moderating effect on any changes in temperature caused by either heat evolved by, or heat lost from, the system. However, the major purpose of the water is to act as a heat transfer medium. The total heat transfer (HT) from a polymerising system to the autoclave jacket is given by the relationship (1.12) where HI is the film coefficient on the inside of the autoclave, HM is the thermal conductivity of the autoclave wall itself and HJ is the jacket side film coefficient. Since HT has to be as high as possible for maximum autoclave output and HM and HJ are fixed and relatively high, it is vital that HI is as large as possible. HI for a well agitated slurry is at least 10 times that for a well agitated powder. The final role of the water is to carry the protective colloid. Most protective colloids, as described in the next section, are water soluble polymers and clearly can only act in an aqueous system. 1.4.2 Role of the Protective Colloid During the suspension polymerisation of VCM the VCM droplets are converted gradually from a non-sticky liquid (VCM), through a mixed droplet system consisting of a PVC/VCM phase with some free VCM, to PVC grains containing some VCM. This process has been described by a number of authors10–12 and can be shown schematically by Fig. 1.6, due to Sanderson. In the intermediate stages of this polymerisation the particles are viscous and sticky and tend to agglomerate. With most agitation systems this agglomeration process would occur to an uncontrolled extent such that large lumps of PVC would result. In the limit these lumps could not be agitated and heat transfer would be so poor that temperature control of the polymerisation would be lost. The presence of the protective colloid avoids this problem. Water soluble polymers are known which are capable of fully protecting the original droplets leading to PVC granules of low porosity and high packing density.

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FIG. 1.6. VCM suspension polymerisation—precipitation of PVC within droplets. (Reproduced from Sanderson, British Polmer Journal, 12, 186 (1980).)

PVC is used for a wide variety of applications but there are two essential properties which for almost all applications it must possess. One is that the PVC grains must be capable of absorbing stabilisers, lubricants and in many cases large quantities of plasticiser during the blending steps which take place before the PVC is fabricated into the final article. The second is that the PVC must be capable of being transformed into the final article as rapidly as possible without degradation. The final point is especially critical since PVC is essentially a heat labile material processed, particularly for rigid applications, close to its decomposition temperature so that a very short heat history is necessary. Both of these requirements call for PVC grains which are more-or-less porous. The protective colloid thus has the dual responsibility of both stabilising the polymerising droplets against too much agglomeration and the production of porous grains. Figure 1.7 shows typical PVC granules of both dense and porous structure made under the same conditions except for a change of protective colloid.12 As has already been indicated, most protective colloids are water soluble polymers, the characteristic of which is an affinity for both the water and the organic phase. They tend to gravitate to the interface between the two phases stabilising the droplets by lowering the interfacial tension between the two phases. It is possible to demonstrate this simply by mixing a VCM simulant, say ethylene dichloride, with water in the presence of 0.1% of a water soluble polymer such as partially hydrolysed poly(vinyl acetate) and agitating the mixture. If agitation is stopped the ethylene dichloride droplets are stable for many hours. In the absence of the protective colloid they will coalesce almost instantaneously. In addition to special grades of poly(vinyl acetate), cellulose derivatives such as methyl cellulose, sodium carboxyethyl cellulose, hydroxypropyl methyl cellulose, etc., are also widely used as protective colloids. Undoubtedly other water soluble colloids such as polyvinylpyrollidone, ethylene/maleic anhydride copolymers, polyacrylates and the naturally occurring materials such as gelatin and gum arabic can be used. In practice gelatin was used for much early suspension PVC manufacture, but, as the demands of the market

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

11

FIG. 1.7. PVC suspension polymer—appearance under transmitted light. (Reproduced from Sanderson, British Polymer Journal, 12, 186 (1980).)

place for more reproducible and sophisticated grain structures have developed, there has been a complete change to the use of man-made colloids especially tailored to the demands of the process. It seems that in some cases the protective colloid acts by stabilising the VCM droplets against coalescence in the early stages of the polymerisation and controlling the aggregation step shown in Fig. 1.6, so enabling a fine grain (50µ m) of controlled internal structure to be achieved. As polymerisation proceeds a certain amount of coalescence occurs, so producing the desired final grain size of about 100–150 µ m and the desired grain porosity. In other cases the protective colloid stabilises the original VCM droplet throughout polymerisation but produces the desired grain porosity by controlling the contraction of the grain (there is a density change VCM PVC of 0.85 1.4 kg/litre) and allowing the precipitating PVC primary particles to form in a specially suitable way. In recent years increasing attention has been paid to the use of secondary granulating agents such as short chain surface active agents mixed with the primary substituted cellulosic or polyvinyl alcohol (PVA) protective colloid. Their role is almost certainly to control the aggregation step shown schematically in Fig. 1.6.13 1.4.3 Role of the Agitator In the earlier sections it has been mentioned that the agitator has the functions of producing the necessary droplet size, maintaining a suspension of these droplets and ultimately of PVC grains, and ensuring good heat transfer from the polymerising mass to the autoclave walls. The ability of the agitator to produce a particular droplet size is relatively easily predicted from classical fluid mechanics.14 Most breakdown of the VCM into droplets occurs in the area of turbulence near the tip edges and trailing edges of the impellor. As would be expected, the droplet size decreases as the agitator speed increases. Consequently much higher speeds are required to produce 50 µ m droplets, as required for some processes, than are required for the coarser 150 µ m grains.

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Most of the protective colloids used for stabilising VCM droplets are very efficient so that droplet coalescence very rarely occurs, certainly at the beginning of the polymerisation. However, as the polymerisation proceeds the nature of the droplets changes as PVC is precipitated inside them and as they contract because of the density change VCM PVC. Basically the droplets become less stable, particularly at the intermediate stages of polymerisation. Also the system becomes much more viscous as the final PVC grains with their porous structure are formed. The requirements for the agitator system to prevent or at least control coalescence and to ensure good and uniform circulation are now much more onerous. This has called for the development of quite sophisticated agitation systems especially for the more modern, large autoclave plants. In the early days of PVC manufacture it was normal to agitate the vessel with a single simple 45° or 90° paddle on the end of a shaft entering from the top of the small (up to 10m3) vessel. Under these conditions the VCM passes through the area of high turbulence close to the impellor sufficiently frequently to ensure that the correct droplet size and distribution occur and to ensure adequate circulation of the whole batch. In these circumstances various protuberances inside the autoclave such as thermocouple pockets and the stirrer shaft itself undoubtedly helped the circulation by acting to some extent as baffles. As the autoclave size increased, the length of a shaft from the top of the vessel to the impellor became very long and the power required (perhaps l00hp for a 50m3 vessel) so large that the shaft and bearing became very expensive. For some years it has been common practice for new large autoclaves to be agitated by means of an impellor close to the bottom of the vessel driven by a short bottom entry shaft. At the same time the problem of ensuring adequate circulation within the vessel by a single impellor becomes greater as the autoclave volume increases. This problem can be overcome by using a second or third impellor above the first or by placing baffles in the autoclave. Most manufacturers use baffles of some sort and these greatly improve circulation by increasing the areas of moderate turbulence within the system. Most of the baffles take the form of a shape (cylinder, plate, etc.) on the inside of the vessel wall, usually in the top part of the autoclave. 1.4.4 Role of the Buffer It is impossible to polymerise VCM without the formation of some HC1. Additionally some initiators produce acidic products as they decompose. These processes are not under perfect control and the properties of most water soluble polymers can vary quite considerably with changing pH with consequent loss of control of the type of PVC granules produced. Thus it is quite normal to add inorganic salts to VCM suspension polymerisation in order to alter or control the pH of the system. Most additives are designed to raise the autoclave pH somewhat. Strictly speaking, not all of the additives used are buffers but merely bases. Sodium and magnesium hydroxides, carbonates and bicarbonates, phosphates and acetates have all been mentioned in the patent literature. A further virtue of the resulting higher pH of the slurry is that it is less likely to attack the plant equipment used to isolate the PVC from the slurry. In practice almost all the plant equipment from the autoclaves to the driers is constructed of stainless steel or is glass lined or epoxy coated. If coating is to be used, glass is the normal coating material where the environment is at its most active, such as in the autoclaves, while the cheaper epoxy coatings are used in less aggressive situations such as in the silos used to store the PVC. The coated substrate will normally be mild steel. Mild steel is, of course, attacked by aqueous systems but even stainless steel is attacked by PVC slurries in certain circumstances. This attack, called stress corrosion cracking, can lead to failure of the steel and is faster the higher the temperature and chloride ion

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

13

FIG. 1.8. A typical suspension polymerisation reaction.

concentration and the lower the pH. Since little can be done to prevent chloride ion formation, and high temperatures are also inevitable, some measure of pH control is desirable. 1.4.5 Role of the Initiator The choice of initiator for use in VCM suspension polymerisations is vitally important since it significantly affects the cost of the process. As described in Section 1.2 on the kinetics of VCM polymerisation (see Figs. 1.2 and 1.3), the reaction accelerates rapidly at a constant rate of initiation, because the locus of polymerisation moves from VCM to a PVC/VCM phase in which the kinetic termination step is markedly lower, as the VCM conversion to PVC proceeds. The reaction VCM PVC is exothermic giving out 1540 kJ/kg VCM polymerised, and it is necessary to remove this reaction heat at its rate of generation if the polymerisation temperature is to be maintained constant as is normally desired to produce the PVC K-value required. Removal of this heat is a problem in the commercial scale production of PVC because of the desire to minimise the reaction time and so make the maximum use of the autoclave. Since in most suspension polymerisation processes the heat of polymerisation is removed by cooling water circulating round the jacket of the autoclave, there is clearly a maximum rate of heat removal controlled by the heat transfer coefficient between the autoclave contents and the jacket and the temperature difference between the batch and the jacket. As the heat transfer coefficient between the contents and the jacket stays sensibly constant throughout a polymerisation, the rate of polymerisation is given by the temperature difference between the contents and the jacket. Typical polymerisation profiles are shown in Fig. 1.8. This shows the way in which the jacket temperature is used to increase the batch from ambient temperature to the

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polymerisation temperature (in this case 60°C) and the role of the jacket water in removing the heat of polymerisation, the rate of that polymerisation being given roughly by the temperature difference between the contents and the jacket. This temperature difference increases with time and mirrors the reaction rate increase shown in the 40% curve of Fig. 1.3. The polymerisation kinetics (initiator used, etc.) are arranged so that the heat evolution at the maximum polymerisation rate can be controlled just using the minimum cooling water temperature (in this case 15°C). Subsequently the polymerisation rate falls as the free VCM concentration falls below the 75/25 PVC/ VCM equilibrium concentration. Of course, the batch pressure which up to that point is the vapour pressure of VCM at 60°C, falls also. This typical reaction profile as discussed earlier is achieved by using a free radical initiator which decomposes to give free radicals. Typical initiators which can be used are azo compounds such as azodiisobutyronitrile and azobis(2,4-dimethylvaleronitrile), peresters such as t-butyl perpivalate and t-butyl perneodecanoate, acid peresters such as lauroyl and benzoyl peroxide, peroxydicarbonates such as dibutyl, di(2-ethylhexyl), dicetyl, and di(t-butylcyclohexyl) peroxydicarbonates and special initiators such as acetylcyclohexylsulphonyl peroxide. Whereas the azo compounds decompose by the elimination of nitrogen the peroxides are decomposed by splitting the O—O bond in the peroxide. All the initiators mentioned above are characterised by decomposing according to first order kinetics, see eqn. (1.1), with a rate of reaction expressed by the equation (1.13) That is, the rate of initiation is proportional to the initiator concentration and this, of course, falls with time as the initiator decomposes. The rate of decomposition is given by the rate constant k1 which is a characteristic of each particular initiator. This is conveniently expressed as the initiator half-life, i.e. the time taken at a particular temperature for the initiator concentration to fall to half its original value. Table 1.5 shows the half-lives of the initiators mentioned over a range of temperatures used in VCM suspension polymerisations. As can be seen from the data in the Table, most of these initiators are characterised by having half-lives of about 2 h at one temperature in the range 50–70°C which is that commonly used for suspension PVC manufacture. Thus in a reaction time of 6h given in Table 1.4 as being typical for a 10m3 autoclave, the initiator may be decomposing at a rate such that its final concentration is only 10–15% of that initially added with the VCM charge. This change of initiation rate with conversion has a powerful effect on the rate of polymerisation, greatly moderating the increase in that rate which would otherwise result. This has the result of reducing the instantaneous polymerisation rate at any one time, as shown in Fig. 1.3, so facilitating heat removal. The overall effect is to make possible a TABLE 1.5 Half-lives of Initiators used in VCM Suspension Polymerisation15 Initiator Azodi-isobutyronitrile Azobis(2,4-dimethylvaleronitrile) t-Butyl perpivalate Dibutyl peroxydicarbonate Di(2-ethylhexyl) peroxydicarbonate Di(t-butylcyclohexyl) peroxydicarbonate Lauroyl peroxide

Half-life (min) 50°C

60°C

70°C

4200 420 1300 300 250 250 3000

1080 120 360 70 60 60 800

300 35 100 18 15 15 200

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

Initiator Benzoyl peroxide Acetyl cyclohexylsulphonyl peroxide

15

Half-life (min) 50°C

60°C

70°C

— 80

3000 18

800 4

higher average polymerisation rate so making better use of the cooling capacity of the autoclave and increasing autoclave productivity. An examination of the data in Table 1.5 also shows that if a half-life of, say, 2–3 h is desirable to maximise reactor productivity, a particular initiator may only be especially suitable for use over a relatively minor range of polymerisation temperatures. Thus for example azodiisobutyronitrile and lauroyl peroxide might be used at 70°C, t-butyl perpivalate and azobis(2,4-dimethylvaleronitrile) at 60°C and the peroxydicarbonates or acetylcyclohexylsulphonyl peroxide at 50°C. The use of mixtures of any of these initiators may be especially advantageous. For example a mixture of an initiator which decomposes rapidly with one which decomposes slowly at the polymerisation temperature should increase the polymerisation rate at the beginning where the rate is normally slow (see Fig. 1.2) while maintaining an adequate rate of initiation at the end of the polymerisation. Without the latter feature it is quite possible for the VCM polymerisation to become very slow at high conversions as shown in the bottom curve of Fig. 1.3. Half-life is not the only basis for the choice of initiator. Some initiators are less efficient at starting a VCM polymerisation than others. For example, while the peroxides in general have a high radical efficiency the azo compounds are less effective. This is basically because the free radicals produced when a peroxide decomposes are very energetic and are sufficiently active to overcome the energy barrier needed to open the VCM double bond. By comparison azo compounds decompose to give much less energetic radicals and their efficiency in starting a VCM polymerisation is correspondingly less. This problem is of no real consequence for the longer half-life azo initiators such as azodi-isobutyronitrile but azo compounds such as 2,2 -azobis(4-methoxy-2,4-dimethylvaleronitrile), which has a half-life at 50°C of 40min,16 produces such stable radicals that it is a very ineffective initiator for VCM polymerisation. Such an initiator can, of course, be employed but very high concentrations which would probably be uneconomic have to be used. Another major factor in the choice of initiator is its stability and storage properties. Not surprisingly, the more active the initiator, i.e. the faster it will decompose when exposed to heat, the more unstable it tends to be. For example, dibutyl peroxydicarbonate has to be stored at lower than ambient temperature while lauroyl peroxide can be stored at temperatures up to 30°C. However, there are exceptions to this general rule which are related, once more, to the energetics of the radicals produced and to the physical form of the initiator. For example, as is well known, benzoyl peroxide has to be stored wet but lauroyl peroxide is stable dry at room temperature. The liquid peroxydicarbonates have auto-ignition temperatures (i.e. temperatures above which they self-ignite) below ambient temperature while the solid peroxydicarbonates have high autoignition temperatures. Dissolving the solid peroxydicarbonates in a solvent greatly reduces their autoignition temperatures15 so that they are then comparable with the liquid materials. Clearly if one initiator, for example lauroyl peroxide, can be stored at ambient temperature and presents no explosion hazard, there is a powerful incentive to choose it rather than another which requires special cooled storage conditions with safety devices designed to guard against any breakdown of the cooling system. Also initiators which self-ignite at ambient temperature have to be handled especially carefully to avoid any spillage. Not only are these disadvantages for the PVC manufacturer but the initiator manufacturer will also have increased costs because of the safety problems which have to be overcome.

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The initiator used in a PVC process tends in many cases to have a small effect on other features of the process. For example, the more water soluble initiators such as acetylcyclohexylsulphonyl peroxide tend to induce the formation of more fouling, i.e. deposition of PVC on the autoclave surface. Others may have an effect on the colour and heat stability of the PVC made and on the porosity of the PVC granules produced. A PVC manufacturer thus has to weigh many facts before choosing the initiator he will use. At the present time the most popular initiators are undoubtedly the peroxydicarbonates which are gradually replacing lauroyl peroxide, at least for the lower temperature polymerisations. t-Butyl perpivalate and azobis-2,4 (dimethylvaleronitrile) are used to some extent but this use is declining as is that of acetylcyclohexylsulphonyl peroxide. As is apparent from the earlier discussion a control over the initiation rate is very desirable to optimise autoclave productivity. There are a number of patents covering the activation of peroxides in a VCM polymerisation. For example, Rhone-Poulenc17 describe a process in which decomposition of lauroyl peroxide is induced with a mixture of ascorbic acid and a transition-metal salt. It is possible to activate the decomposition of benzoyl peroxide with aromatic amines. These and similar techniques are of considerable interest but they suffer from the disadvantage that the activation reaction only produces one free radical from each peroxide molecule instead of the normal two, and this significantly increases initiator cost. 1.4.6 Effect of the Evacuation Step It is customary to evacuate the autoclave before charging VCM in order to remove as much as possible of the oxygen otherwise present. This is a powerful inhibitor of the VCM polymerisation since it copolymerises very readily with VCM but only slowly propagates the chain reaction. mCH2=CHCl+nO2 → —(CH2—CHCl)m(O2)n— (1.14) This vinyl chloride polyperoxide is itself labile and breaks down either before or during the VCM homopolymerisation to give HC1 and various unsaturated or carbonyl containing PVC molecules, together with small quantities of carbon monoxide and formaldehyde. The low pH produced by this reaction is a major cause of corrosion attack on the PVC plant equipment since it can be responsible for stress corrosion cracking of stainless steel, direct attack on mild steel and, when concentrated by evaporation during the drying operation, direct attack on ducting, etc. The by-product PVC is also undesirable since unsaturated PVC or PVC containing carbonyl groups is known to be of poorer colour and lower thermal stability than the homopolymer itself. Carbon monoxide will copolymerise with VCM to give an unstable (to light and heat) copolymer. Since the large evacuation pumps used to remove air from PVC reactors are usually incapable of producing a perfect vacuum, manufacturers often reduce the oxygen further either by multiple evacuation steps with the back addition of nitrogen between evacuations or evacuate the autoclave when it contains warm water, so using the steam generated to sparge the air out of the autoclave. 1.4.7 Vinyl Chloride Quality Until around 1960 most VCM was produced from acetylene and hydrogen chloride in a single step addition process HC≡CH+HCl→CH2=CHCl (1.15)

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

17

but there was also some production of VCM by direct chlorination of ethylene to ethylene dichloride (EDC) followed by dehydrochlorination. CH2Cl—CH2Cl→CH2=CHCl+HCl (1.16) In some plants both processes were used in tandem, the HC1 from the cracking of EDC being combined with acetylene. Since 1965 virtually all major new VCM plants have been based on a combination of direct chlorination and oxychlorination of ethylene, the so-called balanced process. In the first step EDC is produced by the direct chlorination of ethylene. CH2=CH2+Cl2→CH2Cl—CH2Cl (1.17) In the second step the purified EDC is pyrolysed to give VCM and HC1. CH2Cl—CH2Cl→CH2=CHCl+HCl (1.18) In the third step this HC1, mixed with air or oxygen, is reacted with ethylene to form further amounts of EDC. CH2=CH2+2HCl+0.5 O2→CH2Cl—CH2Cl+H2O (1.19) The chief advantage of this process over those operated earlier is that it enables ethylene and chlorine plus air (or oxygen) to be used as the basic raw materials, and ethylene is cheaper than acetylene. There are also no major side products. However, the full economies of this process are only realised when it is operated on a very large scale (> 200ktpa) and at high occupacity. The process involving hydrochlorination of acetylene alone or coupled with EDC cracking involves lower capital investment and can be operated on a smaller scale. It tends, therefore, still to be used in some countries at an early stage in the development of their PVC industry. The acetylene route has also been favoured in South Africa, where the economy is based on coal. VCM as made by any of these processes is extremely pure and will normally be sold at greater than 99. 8% purity. The nature of the remaining impurities depends on the process used to make the VCM, the quality of the raw materials used in that process and the purification regime. They will consist principally of other chlorinated hydrocarbons and water. Neither is a major concern in suspension polymerisation although chlorinated hydrocarbons may act as chain transfer agents and contaminate either the PVC, if they remain in the polymer after VCM removal, or increase losses in the VCM recovery plant if they are removed during stripping. The major problem with water is that it promotes corrosion of the plant equipment which is itself serious but the resultant iron in the VCM may have more serious effects on the polymerisation especially for redox catalysed emulsion polymerisations. Other impurities are normally present at the ppm concentration level only. Nonetheless these small concentrations are thought to have a significant effect on the morphology of PVC grains produced by suspension polymerisation. The significance of each impurity will depend on the details of the suspension process and especially on the protective colloid used. 1.5 COST OF MANUFACTURING SUSPENSION PVC At the time of writing a ratio of monomer cost to PVC selling price of 60–65% is fairly normal. Assuming this ratio, the cost elements of producing PVC can be summarised very approximately as in Table 1.6. It is apparent from this table that the VCM cost is by far the biggest cost element, but that after that the next most important is associated with the

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TABLE 1.6 Approximate Cost of Manufacturing Suspension PVC Percentage of selling price Monomer cost assuming 98 % efficiency Variable services+ainitiator+other chemicals Total variable cost Amortisation of capital Research and development Selling costs and all other overheads Total cost Profit element to reward capital investment

a

64 6 —— 70 7.5 2.5 10 —— 90 10 —— 100

Steam, electricity, water, etc.

amortisation of, and reward for, the capital employed. There is a powerful incentive, therefore, to reduce the capital cost of building PVC plants. It is possible to divide a suspension PVC plant into sections, namely VCM storage and recovery, polymerisation, VCM removal, slurry storage and drying, isolation and storage, and laboratories and offices. Ignoring isolation and storage, the cost of which can vary very considerably depending on whether bulk storage is required or not, the polymerisation sections can account for up to half the total cost of the plant. Consequently there is a strong case to reduce costs in this area. In principle there are three ways in which this can be accomplished. (1) By reducing the cost of the individual items used. However, this cost-cutting approach may not be compatible with the ever-improving PVC quality standards and the pressure to increase the safety standards under which the PVC process operates. For example the discovery that VCM is a human carcinogen has greatly increased the cost of the equipment needed on a PVC plant to reduce both deliberate and adventitious VCM leaks to a minimum, to ventilate plants more effectively and to monitor VCM concentrations. (2) By increasing the amount of PVC made in a particular autoclave. An examination of Table 1.4 shows that a 10m3 autoclave operating under those conditions will produce 2000t per year but that there are a number of ways in which that productivity can be increased. In the cycle described (Table 1.4) over half the time is spent polymerising the VCM. The correct choice of initiator, as described in Section 1.4.5, coupled with adequate polymerisation heat removal, can reduce that time significantly with 300 min or less being achievable. The second largest item in the cycle is the VCM removal step. The provision of a separate vessel for VCM removal will eliminate this item and the cycle time saving may more than offset the extra cost. Over the past ten years there has been much activity in the PVC industry aimed at preventing PVC deposits in the autoclave (build-up) which caused numerous problems and required interbatch autoclave cleaning. This will be discussed in more detail in Section 1.6.5 but most companies now have chemical systems for controlling build-up formation such that very greatly reduced cleaning is required. When VCM is converted to PVC there is a marked volume change (density of VCM is 0.85 g/ml—density of PVC is l.4g/ml) so that there is a large volume contraction in the autoclave contents. Advantage can be taken of this fact by increasing the initial VCM charge at the expense of the initial water

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

19

and injecting water during the polymerisation as the volume contracts. In this way increased quantities of PVC can be produced per batch. Taking all these ideas into account it is theoretically possible to increase the VCM charge from the 3500 kg in Table 1.3 to, say, 4000 kg and to reduce the overall cycle to 480 min instead of the 690 min in Table 1.4. This increases the autoclave output to 305t of PVC every 8 h corresponding to over 3400t per year at 90% autoclave availability. Even with this increase in productivity the costs remain very high for this size of autoclave. A l00000tpa plant would still require 30×10m3 autoclaves, the expensive associated equipment having to be replicated on each autoclave. Consequently most PVC manufacturers have today moved to the use of a smaller number of larger autoclaves. (3) Increasing autoclave size. Making the assumption that the VCM charge possible and, hence, the amount of PVC produced, increases pro rata with the size of the autoclave, the use of a large autoclave, say 100m3, is very attractive. Clearly the capital cost of such an autoclave must be significantly greater than that for a 10m3 autoclave but it is not 10 times as much. The autoclave section building, the agitator and its drive, the cooling water circulating pump, the monomer and water charge pumps, etc., will be more expensive but again not by a ×10 factor. Other items such as instruments may be no more expensive than those used on the 10m3 scale but there is a need to increase instrumentation with increasing autoclave size to ensure satisfactory safety standards when handling such large quantities of VCM. As a rough approximation it is reasonable to assume that capital costs will be roughly in proportion to the square root of the autoclave size, e.g. a 100m3 autoclave polymerisation section to give the same total autoclave volume would cost 10/ 10, i.e. 0.3 times as much to build as one based on 10m3 autoclaves. However, there are a number of problems associated with the use of large autoclaves which either must be solved before saleable product is produced or have to be taken into account when designing a plant based on large autoclaves. These have been discussed by Terwiesch.18 1.6 USE OF LARGE AUTOCLAVES 1.6.1 Size of Plant When designing a new PVC plant the first concern is to decide how much PVC production capacity is required. Since transport costs may be a relatively high proportion of the selling price of PVC it is normal for a plant to be built to service primarily only its local market area (say one country of Western Europe or a part of the USA). Since a number of companies may be competing for this same total market, the amount available to a particular firm may be limited and this acts as a constraint on the size of any new plant to be built. Even in the highly developed markets of Europe, USA and Japan, the capacity of a new plant is at present unlikely to be much greater than 100000tpa although the capacity may be considerably increased by subsequent extensions (for example to as much as 300000 tpa). In many of the developing countries a plant to produce as much as 100000 tpa would be too large, so plants with a capacity of 35000 tpa or less have been built in recent years in the developing countries. Accepting the data in section 1.5 which suggest that one 10m3 autoclave can produce 3400tpa of PVC, a plant containing 10–11 autoclaves would be required for 35000tpa and 30 for 100000tpa. Alternatively, assuming for the moment that a 100 m3 autoclave will produce 10 times as much PVC as one of 10m3 (and this is not true—see below), a 35000tpa plant would need one 100m3 autoclave and a plant of 100000tpa three 100m3 autoclaves.

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There are, however, a number of other factors which have an important bearing on the choice of autoclave; one is the peak service usage, a second is its effect on downstream equipment and a third is autoclave downtime. Since the PVC suspension polymerisation process is a batch process involving services to charge the autoclave, to heat the autoclave, to cool the autoclave and to discharge it, these services must be adequate to perform these tasks. For example, in the limit, the steam service required to heat a single 100m3 autoclave might be 10 times that required for ten 10m3 autoclaves all sequenced so that they were heated up in turn. A similar argument can be applied to other services. Although on a chemical complex with many other plants sharing facilities such as steam raising, cooling water, etc., this effect can be reduced, it is most unlikely that a single autoclave plant can be justified economically. The effect on downstream activities can also be very large. For example, a 100m3 autoclave will require slurry storage facilities of at least 100 m3 capacity if the autoclave is not to be under-used while awaiting slurry storage room. A 10 m3 autoclave based plant could use a storage tank of, say, 30m3 and this would permit batch blending which might be desirable on product consistency grounds. Finally a plant containing very few autoclaves would be much more seriously affected by a breakdown of any one of those autoclaves than a plant based on a multiplicity of small autoclaves. Taking all these effects into account it is rarely economic to build a PVC plant containing fewer than four autoclaves except as an addition to an existing plant. 1.6.2 Effect of Size on Heat Removal As the size of the autoclave increases, its surface area to volume ratio falls unless the size increase is achieved by simple lengthening of the vessel. This latter is not possible, except to a limited extent, on agitation grounds and most autoclave size increases are achieved by increasing volume at a constant length: diameter ratio. Assuming a constant length: diameter ratio of 2 it is possible to calculate the way in which the surface area changes with volume (see Table 1.7), assuming that the vessel is cylindrical in shape and that only the cylindrical section, and not the ends, is useful surface for cooling. TABLE 1.7 Effect of Autoclave Size on Heat Transfer Surface Area Autoclave volume (m3)

Length (m)

Diameter (m)

Surface area (m2)

Surface area: volume (m-1)

1 3 10 30 100 300

1·72 2·48 3·71 5·34 7·98 11·52

0·86 1·24 1·85 2·67 3·99 5·76

4·65 9·66 21·56 44·79 100 208

4·65 3·22 2·16 1·49 1·00 0·695

The Table shows that the available surface area for cooling as a function of the possible VCM charge (autoclave volume) falls by about a factor of 10 for the autoclave size range 1 to 300 m3 or more realistically by over a factor of 2 for the range 10 to 100m3. However, there is an additional factor in that the actual thickness of the autoclave wall must increase as the diameter of the vessel increases if the fabricated vessels are to have the same pressure rating. As already

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

21

discussed the maximum temperature used for VCM suspension polymerisation is about 80°C, corresponding to an autogenous VCM pressure of 15 bar. After allowing suitable safety margins so that the autoclave pressure can be relieved without rupturing the vessel itself, a typical suspension polymerisation autoclave might be designed to 25 bar working pressure. Such a vessel would require a vessel wall thickness of from 10 to 40mm if built of steel. Typically a cylindrical vessel of 2 m diameter would require a wall thickness of 16mm while one of 4m diameter would require 32mm. Given that the available heat removal capacity is a function of the heat transfer through the walls of the vessel, this can have a significant effect (see eqn. 1.12). A doubling of the wall thickness could reduce the overall heat transfer coefficient by up to 20% (assuming HI, HJ and HM are of similar magnitude at the original wall thickness). Consequently, increasing the autoclave size from 10 to 100m3 may reduce the overall ability of the jacket to remove heat, as a proportion of the heat generated (i.e. the VCM charge) by almost a factor of 3. If no steps are taken to improve this aspect of the process and if the smaller autoclave were already operating on a heat transfer limitation, then the polymerisation time in the 100m3 autoclave would have to increase markedly by up to a factor of 3. The effect on the idealised autoclave process described in Section 1.5 would be to increase the reaction time from 300 to, say, 800 min and the overall cycle to, say, 1000 min giving a theoretical autoclave output, assuming a 40t charge and 90% utilisation, of 21 000 tpa. This is markedly lower at 210 t/m3/year than the 340 t/m3/year theoretically achievable with 10m3 autoclaves. In practice, of course, this theoretical output is very rarely achievable so that a real comparison for a 100 000 tpa plant might be that it would require 6×100m3 autoclaves compared with say 30×10m3 autoclaves. Accepting that 3×100m3 autoclaves would cost only 10/10=0.3 times as much as 30×10m3 autoclaves, then 6×100m3 autoclaves at 0–6 times the cost of 30×10m3 autoclaves would represent a significant saving which would be greater if the heat transfer limitation were reduced. 1.6.3 Reduction of Heat Transfer Problem The heat transfer problem can be reduced either by reducing the peak heat removal rate or by increasing the ability of the autoclave to remove that heat. 1.6.3.1 Reduction of Peak Heat Removal Rate As has already been discussed earlier it is important in a VCM suspension polymerisation to maintain an adequate control of the polymerisation temperature and this means that it is necessary to be able to remove polymerisation heat at the peak rate at which it is generated. In earlier sections the tendency of the polymerisation rate to accelerate with conversion has been described and the correct choice of initiator to balance this effect has been stressed. Unfortunately there are no really satisfactory initiators available to exploit the high cooling capacity of the small autoclaves when polymerising at the low temperatures (50–58° C) needed to make PVC for plasticised applications so that rather longer polymerisation times than necessary, on heat transfer grounds, are common. With 0the poorer heat transfer of the larger autoclaves, especially for low temperature polymerisations, this problem is less severe because at the longer reaction times used initiators already available produce a more square reaction profile. Consequently the effect on reaction time of the poorer heat transfer availability is less severe than would otherwise occur.

22

R.H.BURGESS

1.6.3.2 Increase of Heat Transfer Coefficient The overall heat transfer coefficient HT is given by eqn. 1.12. HI, the autoclave side film coefficient, can be increased somewhat by careful control of agitation to increase turbulence and reduce stagnant boundary layers at the surface and by reducing fouling on the autoclave wall with PVC (see Section 1.6.5). Similarly HJ, the jacket side film coefficient, can be increased slightly by the use of special nozzles or baffles designed to keep the jacket contents in contact with the wall moving as much as possible. However, it is HM, the heat transfer coefficient across the jacket, which has received the most attention. In early days many PVC autoclaves were glass lined and although glass lining techniques have improved so that the effect of the low thermal conductivity of glass has been reduced, almost all large PVC autoclaves have stainless steel, which has a much higher thermal conductivity, on the autoclave side of the jacket. However, the thermal conductivity of stainless steel is itself significantly less than that of mild steel so that most large autoclaves are now constructed of mild steel with a thin cladding of stainless steel in contact with the actual polymerising batch. 1.6.3.3 Use of Low Temperature Coolants Most suspension PVC plants use site cooling water to cool the autoclave contents. Clearly the temperature of that water, which is normally and most cheaply obtained using a cooling tower, will depend on ambient conditions with much higher temperatures resulting on hot, humid days than on cool, dry days. Commonly 15°C may be achievable much of the time in temperate climates but in hotter climates the minimum cooling water temperature achievable may be as high as 30°C. Since the autoclave cooling is determined directly by the temperature difference between the autoclave contents and the jacket these changes in cooling water temperature can have a profound effect on the maximum rate of heat removal, especially for the lower temperature polymerisations. A simple and widely practised means of overcoming this problem is the use of specially chilled water. In this process some or all of the autoclave coolant is reduced in temperature by refrigeration. If the coolant used is water then temperatures as low as 4°C are possible although somewhat higher temperatures (say 5– 10°C) may be more economic. The use of 10°C chilled water on a plant where 30°C cooling water can only be guaranteed by the use of a cooling tower can double the heat removal rate for a 50°C polymerisation temperature and hence increase autoclave productivity by a very considerable amount (50–100%). Lower temperatures than 4°C are possible by changing the coolant from water to water/methanol or to other materials such as ethylene glycol. As far as is known to the author, systems employing cooling below 4°C are not used for PVC manufacture, presumably because suitable initiators are not available to exploit this large extra cooling capacity which is, inevitably, very expensive to operate. Indeed, the use of chilled water itself is expensive since refrigeration equipment is expensive to install, maintain and run unless extremely cheap electricity happens to be available. 1.6.3.4 Use of Condensers These objections to the high cost of chilled water have prevented many manufacturers from exercising this option and increasing attention has been paid in recent years to the use of condensers to remove extra heat. Since VCM is a liquefied gas under polymerisation conditions, its vaporisation and subsequent

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

23

reliquefaction in a condenser can be used to remove polymerisation heat. The latent heat of evaporation of VCM varies with temperature ranging from 336, through 310 to 279 kJ/kg at 20, 50 and 80°C respectively. This compares with the heat of polymerisation of 1540kJ/kg at 75°C, showing that it is necessary to condense all the VCM about five times during the course of the polymerisation to remove all the polymerisation heat by this route. It can be shown that a condenser with a surface area of about 1000m2 is capable of condensing 40t per hour of VCM using a temperature difference across the cooling surface of 30° C, i.e. sufficient to remove the polymerisation heat from a 100m3 autoclave containing 40t of VCM assuming a uniform polymerisation rate and a 5 h reaction. Such a condenser if of the conventional shell and tube construction is quite modest in size and potentially cheap. However, the use of condensers poses certain other problems such as the maintenance of a uniform product remembering that condensed VCM will contain non-typical concentrations of initiator and protective colloid, the avoidance of fouling of the condenser (which is difficult to clean)19 and the diminishing amount of VCM available for condensation especially after 75 % conversion when there is no free VCM phase. In spite of these problems a number of PVC manufacturers are known to use condensers. 1.6.3.5 External Cooling Loop The removal of heat by pumping polymerising slurry out of the reactor, through an external cooler and back into the reactor has been proposed.20 Such a process is clearly capable of significant heat removal but formidable problems such as fouling of the pipe and heat exchanger and, once more, an adequate control of PVC grain properties have to be solved. Despite the reported use of external cooling loops in PVC production it is not thought that this process is much used commercially at this time. 1.6.4 Agitation The VCM suspension polymerisation process is widely practised because its use of protective colloid dissolved in the aqueous phase permits the production of the types of final granule which are required for the wide range of applications for which PVC is used. While it is relatively easy to produce the desired product type from a particular size of autoclave, much greater expertise is required to match the granule structure produced in one autoclave size with that made in another. The basic reason for this is that the production of the final grain is controlled as much by the agitation as it is by the protective colloid and the agitation inevitably changes on changing autoclave size geometry. This problem has been discussed in Section 1.4.3. 1.6.5 Autoclave Fouling PVC is insoluble in VCM so that any PVC deposited on the autoclave surfaces during a polymerisation is unlikely to be removed from the autoclave subsequently. Moreover, since PVC is always swollen with VCM when free VCM is present any PVC deposits (termed build-up) on the autoclave surface are likely to grow rapidly in subsequent polymerisations. The formation of this build-up is a major and long-standing problem with PVC production since it will reduce the heat transfer capability of the autoclave jacket and will lead to quality problems with the PVC produced if any small quantity finds its way into the final product. When

24

R.H.BURGESS

large quantities have been formed the build-up may alter the agitation, prevent good temperature control and block the autoclave discharge valve. These problems are so severe that for many years it was the practice of suspension PVC manufacturers to clean the autoclave of build-up between every batch. This was formerly a manual operation involving a man entering the autoclave and scraping the build-up off with paint scrapers or the like. About 15 years ago alternative methods of removing the build-up either with jets of high pressure water (100–300 bars)18 or with solvents21 were developed and started to come into increasing service. The discovering of a link between VCM exposure and acroosteolysis, a degeneration of finger bone tissue, in the mid 1960s and, more particularly, angiosarcoma, a rare form of liver cancer, in 1974, rendered it unacceptable to expose operators to the relatively high VCM concentrations in air produced by releasing VCM dissolved in the autoclave build-up unless they were wearing suitable protective clothing and breathing apparatus. However, most companies from that time onwards adopted high pressure water or solvent cleaning and entry of the autoclaves became a rare occurrence, and then only under strictly controlled conditions. In practice high pressure water cleaning is the more popular because of the high cost of solvent systems, especially the cost of separation of solvent from the dissolved PVC. 1.6.5.1 High Pressure Water Cleaning The use of high pressure water (e.g. 300 bars) involves passing water through a constriction so that a jet of water with a strong cutting action is produced. While hand-held devices are available their usefulness is strictly limited since the need is to avoid operator exposure to the inside of the autoclave. Consequently, rotating heads have been developed which are lowered into the autoclave and automatically cover a large area of the surface usually by rotating in both the vertical and horizontal planes. These cleaning heads are a complex piece of equipment operating under very onerous conditions, and in the early days were subject to quite frequent breakdown. However, much development work has been carried out by PVC and equipment manufacturers both on the high pressure water pumps and the rotary heads to improve reliability in service. These rotating heads were formerly spaced centrally in the autoclave and their efficiency as autoclave cleaners depended on the very high pressures of water used at the nozzles. The input energy of the water falls very rapidly with increasing distance from the nozzle and there is a tendency for the jet to break up on passing through the air. This is potentially very serious for large autoclaves since a 100m3 autoclave has double the diameter of a 10m3 vessel (Table 1.7). Increasing the water pressure to compensate for increased autoclave diameter would present routine operation problems with the pump and serious erosion at the nozzles. Since the jet impact energy falls with distance from the nozzle principally because of the resistance of the gas through which it is passing and the droplet break-up which occurs, high pressure water cleaning in a vacuum or partial vacuum has been proposed.22 However, most attention has been given to equipment development designed to place the nozzles nearer the autoclave walls. 1.6.5.2 Chemical Build-up Suppression The major event of the last 10 years in the field of build-up control has been the development of chemical systems designed to prevent build-up formation. Hundreds of patents have been filled on these systems and it is very difficult to generalise on the methods chosen but a consideration of the possible mechanisms of build-up formation gives some clue to the reasons why these systems are effective. The VCM suspension

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

25

polymerisation process is intended to produce grains of PVC using an initiator which is dissolved in the VCM or PVC/VCM phase. Provided the droplets of VCM or the PVC/VCM particles formed are sufficiently stable and the initiator is only in the organic phase then no build-up on the autoclave should be formed. In Section 1.4.2 the possible mechanism of grain formation was discussed and the probability of controlled agglomeration of the original droplets was mentioned. This inevitably implies that the droplets are not very stable and since this step is needed for the production of PVC with the desired grain structure little can be done to change this situation. In fact the use of large quantities of protective colloid can be shown largely to eliminate build-up formation at the expense of producing very fine glassy grains of little commercial value. Another and more effective method of reducing build-up formation is by eliminating areas of high shear in the autoclave, so that the somewhat unstable polymerising particles are not shattered by the high shear. Indeed it is well known that build-up forms on all discontinuities inside the autoclave, e.g. brackets, impellor, etc., and the elimination of these, or at least streamlining, is known to reduce buildup formation.23 The initiators used in VCM suspension polymerisations are monomer soluble and all tend to be mainly dissolved in the VCM. However, there is normally an excess of water present in the system and many of the initiators are soluble at least to a small extent in the water. Since VCM is also soluble in water to a limited extent there is a finite possibility of PVC formation in the aqueous phase. Such PVC is not stabilised by the protective colloid and may well be attracted to the autoclave wall where it forms build-up. It is true as a generalisation that the more water soluble the initiator the more build-up is formed. Initiators such as acetylcyclohexylsulphonyl peroxide and t-butyl perpivalate are known to be particularly poor in this respect while highly organic molecules such as lauroyl peroxide and dicetyl peroxydicarbonate are good. Other initiators such as the azo compounds produce little build-up possibly because they are incapable of activated decomposition on the surface induced by iron (Fe3+). Certainly it is known that build-up is rich in iron and mild steel or other corrodable steels are especially bad for build-up. It will be clear from this that although the choice of initiator, the protective colloid system and details of the autoclave design and agitation are very important in reducing build-up, further action is usually necessary to eliminate build-up completely. The largest class of chemical build-up suppressant systems described in the literature is based on chemicals which are inhibitors for VCM polymerisation which are themselves located on the autoclave wall. Clearly if these materials are complete inhibitors for the polymerisation and are located only on the surfaces to which build-up normally adheres a complete build-up suppressant system has been developed. In practice neither of these criteria is easy to achieve but many systems have been developed aimed at this ideal. A wide range of phenolic and nitrogen containing inhibitors has been patented25–27 which either have an affinity for metal surfaces, for example Nigrosine dyestuffs,25 or can be physically located on the autoclave wall, for example phenol formaldehyde resins.26 Taking these two examples as typical it is possible to identify some of the problems encountered in the development of build-up suppressant systems. Nigrosine dyestuffs are highly coloured and very powerful inhibitors for VCM polymerisation. Many of them are indeed very good build-up suppressants but there is a tendency for some of the Nigrosine to end in the bulk phase producing discoloration of the PVC and some retardation of the polymerisation. Much effort has been expended trying to ‘fix’ the Nigrosine on the autoclave wall using, for example, resin carriers.25 The use of phenol/formaldehyde resins located on the autoclave wall may involve a cross-linking baking step in their application since there is a tendency for all organic materials to be swollen with VCM during the polymerisation which can lead to breakdown of the layer on the surface. Other patents are concerned with suppressing reactions/interactions between the autoclave wall and the PVC. Oxidising agents,28 reducing agents,29 and iron chelators such as EDTA,30 have all been patented.

26

R.H.BURGESS

It is clear that most major PVC manufacturers have now developed build-up suppressant systems. It seems that almost all are different in detail, possibly because of differences in the equipment used to make PVC but more probably because the suppressant system must be compatible with the rest of the PVC process, i.e. it must have at most only a minimum effect on the quality of the PVC made. The development of these build-up suppressant systems has enabled PVC suspension polymerisation without interbatch opening to be a possibility. At the end of a typical suspension polymerisation after discharge of the unstripped PVC slurry, the autoclave is full of VCM. This must be removed by evacuation and purging before the autoclave is opened for high pressure water cleaning. These operations take some considerable time and inevitably some loss of VCM to the environment results. Furthermore after clearing the autoclave it is necessary to remove residual air before the next batch can be charged. Both these steps can be avoided if there is no necessity for interbatch cleaning although as a consequence of this it is necessary to be able to charge the recipe ingredients automatically. While it is generally simple to dissolve many of the polymerisation ingredients, protective colloid, buffer, etc., in water, some ingredients, notably the initiator, are not soluble in water and it is undesirable to dissolve them in VCM for safety reasons. Many of the more active initiators are already supplied as solutions in phthalate plasticisers or mineral oils but many have to be stored and handled at lower than ambient temperatures if dangerous decompositions are to be avoided. Some solid initiators could be dissolved in solvent but the presence of solvent in the final PVC or the liquid effluent from the plant is undesirable and dissolving some of the ‘stable’ active initiators such as the solid peroxydicarbonates actually reduces their stability so that they behave like their liquid lower molecular weight analogues. Consequently dispersions of these solid initiators in water have been developed31 for use on automated plants operating with so-called closed lid technology.31 1.6.6 Instrumentation and Safety Consideration Over the past 10 years the availability of sophisticated computerised equipment has revolutionised the possibilities for controlling a PVC suspension polymerisation plant. Prior to that time the instrumentation on a PVC plant consisted of accurate but limited batch controllers which, for example, would control one function such as the autoclave temperature with the remainder of the operations being carried out manually by the plant operators. While a batch process such as the VCM suspension polymerisation is much more complicated to control than a continuous one, the use of fewer and larger autoclaves has made the overall task significantly less. The very large quantities of VCM in process and the toxicity of VCM have increased the need to ensure the safety of the process. Finally the high cost of process labour has provided an economic case for eliminating the more mundane tasks. These factors have combined so that the more recent PVC plants18 are almost always computer controlled and many old plants have been converted to at least semi-automatic operation with the minimum of operator involvement. These installations range in complexity from a replacement of the batch controllers by a computer through to the most complex systems involving both master and slave computer. Perhaps the most common is a dual computer system where the first computer is used to control the charging of the autoclave, the polymerisation and its discharge together with the cleaning operation necessary before the next batch can be charged. Usually such a process will involve extensive safety checking to ensure, for example, that water and VCM are charged to one autoclave at a time through a metering system which is checked to ensure that it is operating correctly. The autoclave temperature and pressure might be monitored carefully and automatic systems to prevent abnormal polymerisations automatically triggered. Other operations such as stripping and drying might also be controlled by the computer. The second computer would back up the

SUSPENSION POLYMERISATION OF VINYL CHLORIDE

27

first in the event of a breakdown but might also be monitoring upstream and downstream activities so that a decision on discharging or charging an autoclave could be taken automatically. ACKNOWLEDGEMENTS The author is especially grateful to G.C. Maitland of Imperial College and to J.H.Wilson, P.D.Roberts and A.K.Sanderson of ICI Plastics Division for their help in preparing this chapter. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

TALAMINI, G., J. Polym. Sci., Part A2, 4, 535 (1966). CROSATO-ARNALDI, A., GASPARINI, P. and TALAMINI, G., Makromolek. Chemie, 117, 140(1968). ABDEL-ALIM, A.H. and HAMIELEC, A.E., J. Appl. Polym. Sci., 16, 783 (1972). UGELSTAD, J., LEVRICK, H., GARDINOVACKI, B. and SUND, E., Pure Appl. Chem, 26, 121 (1971). KUCHANOV, S.J. and BORT, D.N., Polym. Sci. USSR, 15, 2712 (1974). FREEMAN, M. and MANNING, P.P., J. Polym. Sci., A2, 2017 (1964). MONSANTO, British Patent 954983. DANA, L.T., BURDICK, J.N. and JENKINS, A.C., J. Amer. Chem. Soc., 49,2801 (1927). JOSHI, R.M., Ind. J. Chem., 2, 125 (1964). UEDA, T., TAKCUCHI, K. and KATO, M., J. Polym. Sci., Polymer Chem. Edn., 10, 2841 (1972). DAVIDSON, J.A. and WITENHAFER, D.E., J. Polym. Sci., Polymer Phys. Edn., 18, 51 (1980). SANDERSON, A.K., Brit. Polym. J., 12, 186 (1980). ZICHY, E.L., J. Macromol. Sci.—Chem., A11(7), 1205 (1977). UHL, V.W. and GRAY, J.B. (Eds.), Mixing Theory and Practice, Volume 2, Academic Press, London (1967). Data provided by Akzo Chemie. Data of Wako Pure Chemicals Industries Ltd. RHONE-PROGIL, British Patent 1435425. TERWIESCH, B., Hydrocarbon Processing, November 1976, 117. CONOCO, British Patent 1410128. ANIC, British Patent 2001659. IAMMARTINO, N.R., Chemical Engineering, 24 November 1975, 25. ICI, British Patent 1484866. GEORGIA PACIFIC, British Patent 1508818. HULS, German Patent Publication 2000397. SHIN ETSU, British Patents 1536160 and 1291145. ICI, British Patent 1439334; MITSUI TOATSU, Japanese Patent Publication 54 107991; GOODRICH, British Patent 1523041 GOODRICH, British Patent 1491115; UNIVERSAL PVC RESINS, US Patent 3778423. SHIN ETSU, British Patent 1373286; HULS, British Patent 1502335. WACKER, French Patent 2185678. Dow, Dutch Publication 68 10726. VERHELST, W.F., OOSTERWIJK, H.M.J. and VAN der BEND, D. Th., Kunstoffe, 70, 224(1980).

Chapter 2 BULK PROCESSES FOR THE MANUFACTURE OF PVC M.W.ALLSOPP Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

2.1 INTRODUCTION Vinyl chloride monomer (VCM) can be polymerised by any of the following techniques: suspension, bulk (mass or gas phase), emulsion and solution/diluent. The largest tonnage of PVC and its copolymers is made by the suspension polymerisation route which produces a coarse powder of 100–150 µ m mean size (see Chapter 1). A considerable quantity of PVC is made by the emulsion process, whose product is a latex containing particles below 1–2 µ m in size which can be used directly or, as is more usual, spray dried to produce a friable powder suitable for general purpose extrusion or for mixing with plasticisers to form pastes used in spreading and coating applications (see Chapter 3). A number of attempts have been made to produce PVC by the well-known bulk polymerisation route, which is used extensively for other monomers, e.g. styrene and methyl methacrylate, but the only process to reach commercial status is that developed by Pechiney St Gobain (PSG)—now Rhone-Poulenc Industries Ltd, this process being the subject of this chapter. A large tonnage of PVC is now made by this process, the majority by a number of PVC manufacturers who are licensees of the PSG process: see Table 2.1. Small quantities of vinyl acetate copolymers are produced by solution copolymerisation for coating applications but this is a rather specialised field involving only one major company, Union Carbide, and is not within the scope of this book. A few years ago much interest was created by the development of a bulk ‘gas phase’ polymerisation process based on a fluidised bed reactor but the process has not yet reached commercial status. This process is considered in more detail later in this chapter. All the above processes are operated in a batchwise form only, except for TABLE 2.1 Rhone-Poulenc Licensees of ‘Mass’ PVC Process Licensee

Country

Capacity (ktpa)

Approx. date of original licence

Wacker Wacker Wacker Toa Gosei Rio Rodanoa Dzerghinsk

Germany Germany Germany Japan Spain USSR

Small single-stage process plant Small two-stage process plant 60 ktpa two-stage process plant Small single-stage process plant 80 90

1956 1966 1971/72 1957 1966 1966

BULK PROCESSES FOR THE MANUFACTURE OF PVC

Licensee

Country

Calico Mills India BP Chemicals UK Hoechst Germany Huls Germany SCR Italy Polymeros Mexico Goodyear USA Goodrich USA Hooker USA Vinylplastika Yugoslavia Ohis Yugoslavia Certain Teed USA Diamond Shamrock Canada Rhone-Poulenc France Total mass capacity in World

Capacity (ktpa)

20 90 30 30 30 50 75 20 80 20 40 90–100 90–100 250 ca 106 a Rio Rodano is a Rhone-Poulenc affiliate.

29

Approx. date of original licence 1967 1967/68 1968 1968 1968 1968 1968 1968 1967/68 1969 1971 1972/73 1976 —

emulsion polymerisation, where a continuous process has also been in operation in Germany for many years (Chapter 3), and the solution polymerisation process. The different approaches to suspension and emulsion polymerisation are dealt with in Chapters 1 and 3 respectively whilst in this chapter bulk polymerisation techniques are described. 2.2 DEVELOPMENT OF BULK POLYMERISATION OF VINYL CHLORIDE Bulk polymerisation is defined as a single-phase process involving just monomer and the initiating species. Two modes of bulk polymerisation are possible, viz. homogeneous or heterogeneous. In the former case where the polymer is soluble in the monomer, e.g. polystyrene and poly(methyl methacrylate), bulk processes in which the monomer is converted to a very high viscosity polymer/monomer system result in a solid structureless mass of interspersed polymer chains. Both batch and continuous processes are well known1 and are characterised, in the case of polystyrene, by a fall in reaction rate as the conversion and viscosity of the system increases. Hence, the maximum rate of heat removal is required when the viscosity of the system is at its lowest. In contrast, in the heterogeneous mode where the polymer precipitates from the monomer at low conversion, e.g. poly(vinyl chloride) and polyacrylonitrile, the operation of a bulk process can present quite different problems. With PVC the polymer precipitates immediately as 0.1 µ m primary particles and the number of them is fixed at an early stage. They grow with conversion and then aggregate, so the viscosity of the system increases rapidly. Eventually a phase change results as the unreacted monomer is absorbed by the powdery polymer. For many years the problems of control of grain size and removing the heat of polymerisation at high viscosity retarded the development of a commercial bulk process for PVC. Many companies evaluated their own bulk polymerisation processes since the concept, compared to suspension, is simple especially as a continuous process for PVC. A number of attempts were made to overcome the problems but these met

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with limited success and eventually most PVC manufacturers dropped the bulk process in favour of increased development of their existing suspension or emulsion systems. It was left to St Gobain to pursue the bulk polymerisation route but the early days were fraught with problems of particle size control, buildup and overheating. Three success criteria were apparent to those working in this field. Initially, control the primary particle agglomeration step to give a uniform product of the correct size at low conversion and prevent further agglomeration as conversion rises by uniform agitation. Secondly, remove the heat of polymerisation uniformly from the powdery phase so as to control reaction rate, molecular weight and polymer properties. Thirdly, prevent encrustation of the reactor walls. The breakthrough came after many years’ development in 1960 when the above criteria were largely satisfied by the introduction of the two-stage process by the company, then operating as Pechiney-St Gobain. 2.3 EVOLUTION OF ST GOBAIN TO RHONE-POULENC As some readers may be confused over the proliferation of company names which are associated with apparently the same process, a short company history of bulk process now follows. Also, the term ‘mass’ polymerisation is used to describe the bulk process particularly in France. St Gobain has a very long history in the chemical industry, one of its principal antecedents going back to the time of Louis XIV in the 17th century when it was principally involved in glass manufacture. A famous name in the history of the company is Gay-Lussac. By the early part of the 20th century the company had become known by the name Manufactures des Glaces et Produits Chimiques de St Gobain, Chauney et Cirey SA. In the post-Second World War period expansion of the company was rapid and links were established with other French companies as well as with companies of different national identities. The early development of the PVC bulk process was achieved by P.C. de St Gobain. In 1960 St Gobain and another large French chemical company, Pechiney, also with PVC interests, decided to pool their activities and a-new 50/50 company called Produits Chimiques Pechiney-St Gobain was formed. Pechiney and St Gobain both had a whole network of relationships with other French and nonFrench companies, one of these being Rhone-Poulenc. As the development of the two-stage PVC bulk process was achieved and exploited at about this time it came to be identified with Pechiney St Gobain, or PSG. In the late 1960s a series of mergers began to take place within the network of companies related to Pechiney and to St Gobain and in 1969 it was announced that Rhone-Poulenc was taking a controlling (51%) interest in PSG. This led to some problems for other companies in the network of relationships and as a result further realignments began to follow. In 1971 there was a merging of Rhone-Poulenc’s two largest subsidiaries, viz. PSG and Progil, and the new company became known as Rhone-Progil. From then on the PVC bulk process became attributed to Rhone-Progil rather than to PSG, probably because Pechiney became increasingly closely related to another grouping, Produits Chimiques Ugine Kuhlmann (PCUK), which was also producing PVC but by another process and in competition with Rhone-Progil. A formal regrouping of these companies took place in 1974 with PCUK splitting off completely. In January 1975 there was a complete merger of the subsidiary Rhone-Progil with the parent Usines Chimiques Rhone-Poulenc, and a new company called Rhone-Poulenc Industries was formed. As from January 1975 the bulk PVC process was the property of Rhone-Poulenc.

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2.4 HISTORY OF THE RHONE-POULENC BULK PROCESS FOR PVC 2.4.1 Early Developments—the Single-Stage Process Work began in France in 1938 in the laboratories of Produits Chimiques St Gobain and by 1939 St Gobain had developed their first commercial process for the bulk polymerisation of VCM. At this point in time it consisted of a 0–8m3 vertical autoclave equipped with two agitators, a turbine type to mix the liquid phase and an anchor type to mix the precipitating solid phase. The product made in this way was reported to be of good quality but wall encrustation as well as problems of scale-up and of heat removal caused this system later to be abandoned, when the rapid expansion of the market demand brought the need for increased capacity. In 1956 a modified process was introduced by St Gobain. This involved the use of a 12m3 horizontal autoclave revolving about its own axis. Agitation was further assisted by the inclusion in the autoclave of a number of stainless steel balls weighted with lead. This process became known as the L51 process and it permitted a very marked increase in production capacity: without this development the mass process could not have remained competitive. A principal defect of the process, however, was that the speed of rotation of the autoclave had to be kept within strict limits and the conditions of agitation could not be varied, so the variations in grain structure were strictly limited. 2.4.2 Development of the Two-Stage Process From 1958 the chief research objective of St Gobain was to develop a process that permitted better control of particle morphology. This was achieved when it was shown to be possible to carry out the polymerisation in two steps: by the production of a seed polymer in stage 1, when conversion would be taken to 7–12%, and then transference of the seed in its VCM medium to a second autoclave to which more VCM was added and where conversion would be taken to 70–85%. For the first stage seed polymerisation a vertical stainless steel (SS) autoclave with a turbine type stirrer with flat blades giving strong agitation was found best for the right kind of nucleation. For the second stage polymerisation a horizontal SS autoclave of 12m3 capacity rotating as before on its axis and with steel balls to promote agitation was used initially. Later this was changed to a stationary autoclave with a slowly rotating (9 rpm) ribbon blender type of agitator. The new process was at first known simply as ‘modified L51’ but was renamed M60 when the shape of the agitator blades in the horizontal autoclave was changed and the shaft hollowed out to permit improved cooling. Such a stationary horizontal autoclave has been in commercial operation at St Fons since 1962. In 1966 the process was renamed M66 after being further modified by the addition of reflux condensers to both stage 1 and stage 2 autoclaves. The improved heat removal permitted shorter reaction times and therefore improved productivity. The next step in the development of the process was to use one pre-polymeriser, typically an 8m3 autoclave, to feed three horizontal autoclaves of 16m3 capacity and this combination formed a stream of ca 15ktpa capacity which could be duplicated to give a plant of ca 30ktpa capacity. Next there was a further increase in the number and size of the horizontal autoclaves that could be fed by the pre-polymeriser, which was itself increased in size. The preferred new combination comprised 1×17m3 pre-polymeriser and 5×30m3 horizontal autoclaves.

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FIG. 2.1. General layout of the Rhone-Poulenc bulk polymerisation plant (top) and a detailed representation of the prepolymeriser and the second stage horizontal autoclave (bottom). CY=cyclone. (Reproduced in part from Marks, Developments in PVC Technology, Henson and Whelan, Eds., Applied Science Publishers Ltd, 1973.)

Further changes were also made in the agitator and cooling arrangements. Such a plant was receded N7 and its capacity was ca 60ktpa per stream. In the mid 1970s a further increase in the size of the stage 1 and stage 2 autoclaves was made, the preferred combination being 1×25m3 pre-polymeriser and 5×50m3 horizontal autoclaves. This new combination was renamed N8 and the capacity per stream was raised to 100ktpa. All further discussion in this chapter relates to the N8 process involving one pre-polymeriser feeding five horizontal reactors unless otherwise indicated. 2.5 RHONE-POULENC TWO-STAGE BULK POLYMERISATION PROCESS There are two distinct phases in the process; firstly the formation of the grains in a liquid phase and secondly, the growth of the grains in an essentially solid powdery phase. Since the agitation requirements in the two phases are fundamentally different, each step must be carried out in two distinct reactors. Figure 2.1 shows the overall layout and the two reactors in detail.2,3 The importance of agitation in the first stage with respect to polymer properties is best understood by first considering the mechanism of particle formation in a VCM polymerisation.

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2.5.1 Mechanism of Particle Formation Vinyl chloride is polymerised in bulk by a free radical mechanism using an acyl peroxide or peroxydicarbonate as the initiating species. Since vinyl chloride boils at-13.4°C the polymerisation is carried out under pressure in stainless steel reactors at temperatures of 40–70°C and pressures of 5–12 bar (75–175psi). The vinyl chloride containing the initiator is rapidly brought to reaction temperature and polymerisation begins. After initiation the growing polymer chain rapidly becomes insoluble in the monomer and precipitates.4 Several polymer chains aggregate together to form the first identifiable species —the micro-domain or basic particle about 200 A in diameter. (See Chapter 7 for a full description of nomenclature.) These micro-domains contain a maximum of about 50 polymer chains and therefore cannot be representative of a single polymerisation site. They are negatively charged and are stable for a short period of time.5 After a brief growth stage the micro-domains coagulate to form primary particles, which contain approximately 103 micro-domains and are 0.1 to 0.2 µ m in diameter. This stage is attained before 2% conversion is reached; after completion of the aggregation step free microdomains are no longer observed and no new primary particles are formed. Therefore, all further growth takes place in, or on, the surface of the existing primary particles and two mechanisms are possible. If the monomer contains steric stabilisers further growth may continue to give monodisperse primary particles up to 1–2 µ m in diameter. In the absence of colloid stabilisers, where the system is electrostatically stabilised, the zeta potential gradually decreases with increase in particle size until the primaries reach a critical size at which van der Waals attractive forces exceed the electrostatic repulsive forces between the particles and coagulation into 1–2 µ m agglomerates results. Although the structure of each agglomerate is essentially close packed for minimum free energy, the inter-agglomerate structure depends very much on the level of agitation at the time of coalescence. Up to this point the mechanism of polymerisation of both bulk and suspension processes is the same but from this point onwards the objectives are different. In the suspension process (Chapter 1) agitation is used to control not only the coagulation of the primary particles but also the size, rate and degree of coalescence of the monomer droplets and hence the size and shape of the final grains. In the bulk process the rate and uniformity of the primary coagulation step alone defines the total morphology of the grains at this stage. In order to achieve a uniform grain this step is completed as rapidly and uniformly as possible by the use of very vigorous agitation in the first stage reactor—the pre-polymeriser (pre-po). This results in formation of the agglomerates at an earlier stage in the bulk process than in suspension. However, once formed the growth mechanisms converge and the agglomerates grow in size with conversion as a result of the growth of the component primary particles. The growth of the primaries from 0.2 µ m at low conversion to approximately 0.6–0.8 µ m results in the agglomerate dimensions increasing from 1–2 µ m when first formed up to 2–10 µ m (average 5µ m) at high conversion. The actual mechanism of growth with conversion is considered in more detail in Chapter 7. 2.5.2 The Influence of the First Stage Reactor (Pre-po) A flat blade turbine agitator is used in conjunction with baffles to prevent vortex formation. The diameter of the final grain depends on the tip speed used (Fig. 2.2).2 As well as grain size and size distribution it is very important to control the level of primary particle and aggregate cohesion since this parameter not only affects the final processing properties of the polymer but also determines the ability of the material to survive the severity of transfer to, and a long period of agitation

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FIG. 2.2. Variation of grain size of bulk polymer as a function of pre-polymeriser agitation speed. (Reproduced from Marks, Developments in PVC Technology, Henson and Whelan, Eds., Applied Science Publishers Ltd, 1973.)

(and possibly attrition) in, the second stage reactor. This is achieved by control of the polymerisation temperature in the pre-po. As higher reaction temperatures are used the degree of cohesion between primary aggregates increases. Therefore, to ensure aggregate cohesion, the first stage is always polymerised above 62°C. This has little effect on the molecular weight of the final polymer, since, on average, only 5% of the total polymer is formed in the pre-po. The temperature of pre-polymerisation can be selected at will independently of the molecular weight of the final PVC produced. However, the use of temperatures above 62°C in the pre-po will limit the ultimate porosity that can be achieved since the openness of the aggregate network depends to a considerable extent on pre-po temperature (Fig. 2.3).3 The morphology created in the initial 2% of conversion in suspension or bulk has a major influence on the overall morphology of the grain. As the spacial arrangement of the aggregates in the grain is fixed early on in the pre-po the morphology created in this step is important even though it only contributes less than 5% of the total conversion. In fact, the degree of conversion in the pre-po plays only a minor role in the final porosity, which mainly depends on the final conversion because growth continues in the second stage reactor. Therefore, if porosity needs to be increased the pre-po temperature can be reduced, but not below 62°C. In the pre-po sufficient initiator just to complete the reaction to 8% conversion is used so that the seed can be stored for some time if necessary. Reaction time is less than half an hour. It is likely that for seed produced at the lower end of the 62–75°C range a highly active initiator, e.g. acetylcyclohexylsulphonyl peroxide, is used whereas di-isopropyl peroxydicarbonate is used at the higher end of the temperature range. The latter initiator is probably used in the second stage reactor.

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FIG. 2.3. Cold plasticiser absorption versus pre-polymerisation temperature, K-value 65; conversion 80%. (Reproduced from Chatelain, British Polymer Journal, 5, 457, 1973.)

Cleaning of the pre-po is generally unnecessary since the conversion is very low and even if some material is held up from one batch to the next the slight increase in density and cohesion is unlikely to produce dispersion faults (‘fish eyes’) in the final product. The low viscosity (0.2cP at—14°C, 0.193cP at 25°C) system is easy to agitate initially but the viscosity increases rapidly with conversion so that above 15% the power requirement would prevent normal turbine operation. Conversion is normally 8% at which point grain cohesion is sufficient to allow transfer to the second stage reactor with a practical limit of 12%. The heat of polymerisation is removed by a combination of jacket cooling and reflux condenser. Experiments have shown that there is no need to submit all the monomer to the pre-polymerisation step. Generally, only half the monomer required for the polymerisation is charged to the pre-po. 2.5.3 The influence of the Second Stage Reactor The seed is transferred by gravity along with additional monomer, and the necessary initiator for the second step is injected in solution in plasticiser. Oxygen is removed from the reactor by vaporising a part of the monomer charge. To ensure that the pre-po remains free of polymer some of the additional monomer required can be used to flush the pre-po contents to the second stage reactor. In the second stage the grain becomes stronger due to fusion of the primary particle agglomerates which grow in size with subsequent infilling of the pores between them. Experiments have shown that the porosity is dependent on the temperature of polymerisation and on the degree of conversion in the second step (Fig. 2.4).3 If a high porosity level is required final conversion must be decreased, and/or a lower polymerisation temperature used. The physical nature of the material undergoes several changes with conversion. Initially a suspension of grains in VCM results, but this rapidly changes in appearance to that of a damp powder with no free monomer on the outside of the grain at about 20% conversion. As the reaction continues the liquid monomer is absorbed by the PVC grains leaving a ‘dry’ powdery material after a conversion of about 40%. Correct agitation of the powder is important at this stage to ensure that the monomer returned from the condenser is

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FIG. 2.4. Cold plasticiser absorption versus conversion at two different second stage temperatures. Pre-polymerisation temperature 70°C. (Reproduced from Chatelain, British Polymer Journal, 5, 457, 1973.)

well distributed. This is performed by a complex three-bladed helical agitator with incurved blades to prevent jerks when penetrating the polymer powder (Fig. 2.1). These rotate with minimum wall clearance at 6–7 rpm for a 50m3 autoclave. The conversion continues to rise until the predetermined end point is reached, when the reaction is terminated by removal of unreacted VCM. Progress of the polymerisation is followed by measuring the heat generated during polymerisation and is used to indicate the moment for transfer from the pre-polymeriser to the second stage reactor and for stopping the polymerisation. The normal level of conversion reached is 80% and reaction time in the second stage is 3–5 h depending on temperature. Use of a high level of conversion reduces porosity and gives rise to a highly integrated grain structure. 2.6 CONTROL OF PROPERTIES For a certain grade of polymer, molecular weight (and K-value) and hence second stage polymerisation temperature are fixed, since K-value depends solely on temperature due to the dominance of chain transfer, as in suspension polymerisation (see Chapter 1). Grain size is controlled by the speed of agitation during the first stage (Fig. 2.2). Temperature during the first stage affects porosity (Fig. 2.3), and the cohesion of the grains. The latter limits the temperature to 62°C and above, which has a slight influence on the level of porosity that can be attained. Conversion in the first stage defines the quantity of seed available and ensures sufficient cohesion for transfer to the horizontal reactor. In the second stage an increase of conversion lowers porosity and increases bulk density. The temperature of polymerisation in the second stage controls the molecular weight or K-value (see Chapter 1) of the polymer as well as its porosity (see Fig. 2.4). In practice, the porosity expected at a pre-po temperature of 70°C and a conversion of 80% is known (Fig. 2.5).3 If the porosity is too low, the pre-po temperature has to be lowered but a reduction below 62°C would give rise to problems of grain attrition during transfer and second stage reaction. If the theoretical temperature needs to be below 62°C to attain a higher porosity, a temperature of 62°C is used but conversion in the second stage is reduced. The second stage reactor temperature cannot be changed because it defines the K-value of the final polymer.

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FIG. 2.5. Cold plasticiser versus K-value. Pre-polymerisation temperature 70°C, conversion 80%. (Reproduced from Chatelain, British Polymer Journal, 5, 457, 1973.)

2.7 PROCESS CONTROL Rather than attempting to control the process temperature itself, pressure is monitored as it can be measured more accurately, particularly in the powdery stage, and its relationship with temperature is linear as long as conversion is not too high. Towards the end of the polymerisation when most of the vinyl chloride is used up the control system increases the temperature of the reactor in an attempt to maintain a constant pressure. The polymerisation of VCM is strongly exothermic. As the monomer is constantly in equilibrium with its vapour, it is easy to remove the heat of reaction by vaporisation and recondensation on the cool walls of the reactor or in a condenser so the reaction is carried out under reflux, which is possible as long as the degree of conversion remains below 88%. Therefore, if a grain becomes too hot the pressure in the immediate zone surrounding it becomes lower than that at saturation and some of the adsorbed monomer evaporates until equilibrium is again established. The condensation of the evaporated monomer can take place on any chilled surface and the normal cooling plan as the reaction accelerates is to bring progressively into operation firstly a cooled agitator, secondly the jacket, and finally the condenser(s). One condenser is used on reactors up to 30m3 capacity while two are used on the 50m3 scale. As in other processes, build-up can form on all surfaces if the correct precautions are not taken. In the bulk process build-up levels depend on monomer purity, initiator type and wall temperature; as long as the latter is kept low the build-up level is small. The pre-po does not require regular cleaning but the second stage reactor does. High pressure water cleaning is regularly used and operator entry is no longer required on a routine basis. 2.8 DEGASSING AND POWDER HANDLING Degassing takes place in the reactor and may be completed elsewhere. This step represents a significant proportion of the cycle which is a reflection of the problem of heating the polymer sufficiently rapidly at the end of the polymerisation in the absence of a good heat transfer medium. As the condenser removes the

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FIG. 2.6. Optical morphology of typical suspension and bulk polymer: (a) suspension; (b) bulk.

majority of the exothermic heat of reaction, conversely, the other forms of heat exchange can only provide a limited degree of heating. Prior to the industry’s awareness of the VCM cancer problem, the bulk process resins were usually fairly high in residual VCM ex-factory, although because of the porous nature of the grains the VCM level fell fairly rapidly when the resins were packed and stored in paper sacks. Bulk handling of resin did present increased problems and longer degassing times were required for resins handled in this way. More recently the problem was largely overcome by injecting a small quantity of water (steam or liquid) or nitrogen at the degassing stage to act as carrier6 and mass polymers ex-factory are probably now little different from well stripped suspension resins in terms of residual VCM. After completion of the polymerisation in the horizontal autoclave the unreacted VCM is recovered and dry PVC conveyed to a resin receiver. From there the resin is screened to remove small amounts of oversize material. The main bulk of the resin (95%) goes to finished product storage. The particle size of this grade A material is slightly narrower than for an equivalent suspension resin. The oversize material is ground and rescreened enabling some of it to be recovered as grade A product. 2.9 MORPHOLOGY Although the subject is dealt with in depth in Chapter 7, it is worth acquainting the reader at this stage with aspects of bulk polymer morphology compared to suspension which make the former instantly recognisable. If a sample of polymer is immersed in plasticiser, allowed to come to equilibrium and then viewed by transmitted light, the optical morphology is seen (Fig. 2.6). Whereas suspension polymers appear rounded and slightly different from different manufacturers, mass polymer is always typified by ‘squaresided’ grains and a background of fines, whatever the source. Scanning electron microscopy also shows large differences in the appearance of the grains (Fig. 2.7).

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FIG. 2.7. Scanning electron micrographs of typical suspension and bulk PVC grains: (a) suspension; (b) bulk.

2.10 COPOLYMERS Mass polymerisation is adaptable for the preparation of copolymers of vinyl chloride which are insoluble in their monomers but the reaction temperature must be lower than the softening point of the copolymer formed.

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The copolymers assume the characteristics of homopolymers made by bulk polymerisation in that the structure of the seed is maintained. Hence, it is possible to obtain copolymers with vinyl acetate having a suitable porosity. As for all copolymerisation, it is necessary to take into account the characteristics of the monomers used, in particular their rate of polymerisation. The continuous introduction of the co-monomer which polymerises faster is a simple operation, since the bead during polymerisation is not protected by any barrier. The conditions of the reaction must obviously be adapted to the monomers present. In particular the speed of agitation in the pre-polymeriser must be adjusted since the viscosity of the medium is often different from that of vinyl chloride. It is necessary in copolymerisation to maintain a constant temperature rather than a constant vapour pressure, since vapour pressure varies during the reaction with the change in monomer concentration. 2.11 COST COMPARISON OF SUSPENSION AND BULK PROCESSES The chief advantage of the bulk process over the suspension process is the absence of water and therefore the saving of energy costs in drying PVC wet cake. Comparing two modern plants of ca l00ktpa capacity, the saving in energy costs is 1–2% average selling price of general purpose PVC. In terms of capital costs there is little to choose between the two processes. Superficially the saving of capital on the installation of centrifuges and driers for handling PVC slurry would appear to be significant, but the fairly elaborate system of PVC powder handling/conveying, classification and grinding required for bulk polymers tends to offset this saving. A detailed appraisal of the costs of suspension and bulk processes is given in reports issued by the Stanford Research Institute which are available to subscribers. 2.12 LATEST DEVELOPMENT IN THE TWO-STAGE PROCESS: VERTICAL AUTOCLAVES Since July 1978, vertical 50m3 autoclaves have been in operation at St Fons.7 Before reaching such a size, smaller autoclaves had been operated for 15 years and the agitation system has been progressively improved. Today, a single screw agitator enters from the top while a scraper agitator is introduced from the bottom. This system allows a fast discharge and an easy cleaning of the autoclave (see Fig. 2.8). Much research work has been carried out on this development. This allowed construction of a successful 8m3 autoclave pilot plant which was then scaled up to a 30m3 unit. The results proved satisfactory. Meanwhile improvements of the agitation have been performed which avoid the non-stirred areas along the shaft areas where crusts occur. The advantages of this screw design can be summed up: suppression of dead areas and deposits on the screw, improved mixing of the material, improved heat exchange via the screw. A new production line has been erected at St Fons: it includes one prepolymeriser (30m3) and five vertical 50m3 autoclaves and is shown in Fig. 2.9.7 The vertical position of these autoclaves, as well as the free space left between the screw and the walls, bring several advantages.

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FIG. 2.8. General layout of Rhone-Poulenc 50m3 vertical reactor: (1) reactor shell and jacket; (2) upper screw agitator; (3) lower anchor agitator; (4) reflux condenser; (5) degassing filter; (6) maximum PVC resin level; (7) discharge valve; (8) manhole; (9) packing seal.

(1) The fact that the screw does not scrape the walls allows space for a thermo-probe giving the actual temperature of the powder, thus enabling control of the degassing without excessive heating and so maintaining satisfactory product colour. Moreover, the temperature at the bottom of the autoclave is also measured, giving interesting information on the progress of the polymerisation. (2) The vertical position of the autoclaves allows a very short discharge time. The flow-speed of the powder may reach at least 1 t/min. (3) The combination of these features (vertical position and free space between screw and walls) allows efficient cleaning of the reactor and agitator. For this purpose, two high pressure water cleaning heads are let down by two holes diametrically facing each other. These heads, fitted with two rotating nozzles, go down along the autoclave and thus remove the powder left at the end of the discharge. After each operation the autoclave is cleaned and the whole powder is thus removed. At the same time, the condenser is flushed by a separate line. The use of such a reactor, besides giving a reduction of investment costs due to simpler engineering and better productivity, allows for an appreciable improvement in the quality of resins. A higher efficiency in the cleaning of reactors eliminates totally the remaining PVC particles, which by overpolymerisation would lead to glassy beads difficult to degas and to gel. The improved control of temperature during the whole reaction, including the final degassing phase, permits the production of resins with a very low amount of residual monomer and good colour which are essential for most rigid and flexible applications.

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FIG. 2.9. General layout of Rhone-Poulenc 50m3 vertical reactor plant. (Reproduced from Elastomers, March 1980, p. 26.)

2.13 POLYMER QUALITY STATUS—BULK VERSUS SUSPENSION Bulk density of resins is a function of the degree of aggregate cohesion of primary particles and the particle size distribution. Generally the combination of properties achieved in a polymer from the mass process can be closely matched in the suspension process by recipe modification and vice versa. The best polymers from each process are similar in performance. Mass polymers hold about 10% of the total market and are used in all the principal applications as are suspension polymers. For example, both mass and suspension polymers are used for rigid pipe production. However there are detailed differences between the two which mean that the one or the other type tends to be preferred for certain segments of the market. For example, mass polymers are preferred for rigid injection mouldings whereas suspension polymers are preferred for large sections of the extrusion blow moulding business. A point to note is that the two types of polymer are not compatible, mixtures leading to powder flow problems. Hence fabricators at any one time will use only mass or only suspension polymers and avoid mixing the two types in storage silos. The bulk process is terminated at a lower conversion than most suspension polymerisations, which will obviously give advantages in laboratory tests involving rapid ad/absorption of large quantities of plasticiser. It is often claimed that the pericellular skin surrounding suspension polymer grains (Chapter 7) retards plasticiser absorption relative to bulk but the skin contains many holes, particularly at the bottom of reentrant pores and many grains are covered by an incomplete membrane. Although the skin is porous to plasticiser it may retard the wetting out process slightly but this seems to have little effect in practice. If the conversion of suspension polymer is reduced to that of the mass polymer, plasticiser ad/absorption behaviour is very similar.

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2.14 GAS PHASE POLYMERISATION In the late 1960s and early 1970s several companies simultaneously studied a variation of the bulk process which came to be known as gas phase or partial pressure polymerisation. A low conversion, highly porous ‘seed’ polymer is produced using suspension or conventional bulk polymerisation. This seed is added to the gas phase reactor, a fluid bed unit in a loop system; a solution of initiator in VCM is injected into the centre of the bed and the whole fluidised with hot VCM vapour at a pressure about 1 bar below saturation. Some of the fluidising vapour is absorbed by the PVC seed so the pressure in the reactor starts to fall and more monomer is injected, whereupon it immediately vaporises, re-establishing the operating pressure. Once polymerisation has started, reaction temperature is maintained by injecting cold liquid monomer into the fluid bed and the heat of polymerisation is removed as latent heat of vaporisation. In order to maintain the correct operating partial pressure, monomer is continually condensed in another part of the loop reactor system. The kinetics of a suspension or bulk polymerisation (Chapter 1) are typified by a gradually accelerating rate with the maximum occurring just after pressure drop, after which the rate falls due to monomer starvation. By operating at about 1 bar below saturation the gas phase process works at the optimum kinetic point and very high rates of polymerisation are possible. The progress of the reaction can be followed very closely by measuring actual monomer consumption, which is not possible in other processes, and the batch terminated at an exact point when the seed has grown in weight by a factor of 8–15—the growth factor. Reaction rate is linear throughout the process and the same temperature/K-value relationship that applies to suspension or bulk is followed. Since the process is dry it is an ideal vehicle for the use of organometallic catalysts with the possibility of producing more stereoregular PVC. The fact that the process does not appear to have reached commercial operation is probably related to two factors. In the first place, plant capital costs are not significantly lower than those of an equivalent tonnage suspension process, and secondly the properties of the gas phase polymer are such that it cannot be handled in the kind of processing equipment used widely by the PVC fabricating industry. The morphology of PVC undergoes considerable changes with conversion (Chapter 7). Since the gas phase process operates under partial pressure conditions the growth mechanism is typified by that operating after pressure drop in the suspension process. In the latter we see a densification of the aggregate structure even though only 10–20% of the polymer is produced under these conditions. It is not surprising then that gas phase polymers consist of rather dense grains of low porosity since the whole process is carried out in this post-pressure drop region; see Chapter 7. Also, the material tends to be somewhat coarser than that currently available from the bulk and suspension processes since the seed doubles in size for a growth factor of 8. The gelation behaviour (see Chapter 8) of gas phase polymer tends to be inferior to suspension and bulk but the polymer could be processed to give, say, good quality pipe on a twin-screw extruder. However, the screw length required for adequate gelation is longer than that found on conventional machines and until longer barrelled machines are available the gas phase polymer will represent a processing problem. ACKNOWLEDGEMENTS The author is indebted to a colleague, Dr J.H.Wilson, who provided commercial data on the Rhone-Poulenc process, and to J.C.Thomas of Rhone-Poulenc Industries Limited who kindly agreed to check the text for technical accuracy, since ICI do not operate a bulk polymerisation process themselves.

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REFERENCES 1. 2. 3. 4. 5. 6. 7.

RINGSDORF, H. Encyclopaedia of Polymer Science and Technology, Volume 2, Interscience, New York, pp. 642–66. MARKS, G.C. Developments in PVC Technology, J.H.L.Henson and A. Whelan (Eds.). Applied Science, London, 1973, pp. 17–40. CHATELAIN, J. Br. Polym. J., 5, 457–65 (1973). COTMAN, J.D., GONZALES, M.F. and CLAVER, G.C. J. Polym. Sci., 5, 1137–64 (1967). WILSON, J.C. and ZICHY, E.L. Polymer, 20, 264–5 (1979). RHONE-POULENC. British Patent 1506981. ANON., Elastomerics, March 1980, p. 26.

Chapter 3 THE MANUFACTURE OF PVC PASTE AND EMULSION POLYMERS D.E.M.EVANS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

3.1 INTRODUCTION The first PVC produced commercially was manufactured using an emulsion polymerisation process. Work on PVC was pioneered in Germany in the early 1930s and rapid progress was made in this period, spurred on by the German requirement for natural rubber substitutes. Thus the successful development of an emulsion polymerisation process to produce ‘Buna’ rubber had a significant influence on the way in which the development of PVC proceeded and following pilot plant production in the early thirties, full scale production was achieved in 1937 by I.G. Farbenindustrie.1 Work on suspension polymerisation also began early in Germany outside I.G. Farbenindustrie, Wacker Chemie commencing semi-technical production in 1935. Commercial scale production by Wacker of suspension homo- and co-polymer had begun at Burghausen before the outbreak of the Second World War and by war end output had reached about 7 ktpa. There was a hiatus period then for a year or so but during 1946 Wacker brought their Burghausen plant back into production and production continues there to this day. In contrast to Germany, in the UK there was little incentive in the 1930s to search for synthetic substitutes. Nevertheless in 1938, ICI became interested in PVC as part of a general programme of development work on plastics. Initially to avoid patent difficulties, attention was concentrated on a suspension technique for producing PVC similar to that already in use for ‘Diakon’* (poly(methyl methacrylate)) but an emulsion process was also developed. The outbreak of the Second World War rendered the patent situation irrelevant and a pilot scale plant was commissioned at Runcorn in late 1940 using the emulsion process. A small commercial scale plant (ca 500tpa) was started up at Runcorn in 1942. Also in 1942, Malaya was lost to the Japanese and the UK supply of natural rubber was thus cut off. The need for producing high quality material for cable insulation therefore became more urgent and although resins made by the emulsion process could be used, the impurities in the resin, an inherent feature of the process (see later), impaired the electrical properties. Hence product from a suspension process became more attractive for electrical applications. Based on earlier work, ICI were able to develop a suspension process quickly and by 1943 commercial production had been introduced on the Runcorn plant. Nevertheless the emulsion route was still favoured by ICI at that time and a new larger capacity (5ktpa)

* ICI Trade Name.

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plant designed to make emulsion resins was built and came on stream in 1944—(this was ‘Corvic’ 1 plant, now shut down), allowing the smaller Runcorn plant to specialise in suspension resin production. A similar development of the suspension process took place in the USA during the war and for a period imports of suspension PVC for use in electrical applications came into Britain under the Lend Lease Agreement. After the war the suspension process was developed rapidly in the UK and the USA whereas in Germany in the early post-war period more attention continued to be paid to the emulsion process. However, in the 1950s in Germany, as elsewhere in the Western World, the emphasis switched to the suspension process. Today the emulsion process accounts for only 15–20% of the total PVC production (30– 35% in Germany still but declining) and represents the speciality sector of the market. An excellent account of the historical development of PVC manufactured by the emulsion polymerisation process is contained in books by Kaufman.2,3 3.2 TYPES OF POLYMER PRODUCED USING THE EMULSION PROCESS Two main types of polymer are produced using the emulsion polymerisation process: paste and so-called ‘emulsion’ polymers. Paste polymers and may then be spread on to substrates, deposited on formers produce stable suspensions (‘plastisols’) when mixed with plasticisers by dipping, rotationally cast, etc., prior to fusion in an oven. Stringent control of the particle size of the latex at the polymerisation stage is necessary in order to control the rheological properties essential for subsequent conversion into finished articles (see later). Emulsion polymers, on the other hand, do not form pastes and are used for calendering and the production of thin complex sections by extrusion or in the manufacture of car battery separators using a sintering process. Although both types of polymer can originate from a common polymerisation process, apart from latex particle size control at this stage, subsequent processing (drying, milling, etc.—see Chapter 6) determines the final properties and hence the applications. 3.3 APPLICATIONS OF PASTE/EMULSION POLYMERS Paste polymers, when stirred with plasticisers, form stable suspensions (pastes, plastisols) and it is in this form that they are used to produce finished articles. Fillers, diluents, stabilisers, etc., can all be mixed into the paste at this stage. The major applications are in spread coatings, i.e. the paste is spread at room temperature in a thin layer on to a suitable substrate and is subsequently cured in an oven for a short period of time (1–5 min) to give a coherent film. A reverse roll coater is normally used to spread the film. Typical examples of finished products are vinyl coated wallcoverings, artificial leathercloth, foamed flooring, etc. In the latter case, a chemical blowing agent is mixed into the paste which produces gas at the oven curing temperature and thus produces a foam layer. Various layers may be built up prior to the final curing by heating the spread paste to an intermediate temperature (e.g. using a heated roller) which allows the plasticiser to be absorbed into the paste polymer particles (grains) to produce a coating which is dry but with little strength. All the layers built up may then be cured together in the final oven which causes fusion of the grains to a homogeneous state. Other applications include rotational casting, e.g. in the production of toys. Here the plastisol is poured into a mould which is then rotated in an oven. The finished article may be removed by opening the mould. Dipping processes are also employed, e.g. for the production of gloves, where a former

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is dipped into a plastisol bath. After heating, the glove is then peeled off. A more detailed account of such applications is given in Chapter 10. Emulsion polymers are used mainly in the production of thin complex profiles by extrusion. They have an advantage over suspension polymers in this application due to their rapid gelation rates. Battery separators (for car batteries) are manufactured by spreading the polymer powder (grains) on to a band which then passes through an oven to produce sintered sheet. Using the correct grade of polymer and optimum process conditions, sheets of the correct porosity can be produced for this application. 3.4 PRODUCTION OF PASTE/EMULSION POLYMERS 3.4.1 Properties Required of Dried Product Since plastisols are used in a variety of applications by many different processes, different properties are required both at room temperature and in the fused state. The most important of these at room temperature are the plastisol viscosity, and viscosity/shear rate relationships. By considering the various applications it can be seen that vastly different viscosities may be required and also the shear rates encountered are quite different, e.g. for dipping compared with high speed spread coating. Whilst the viscosity properties can be controlled to a certain extent by using different plasticisers and different additives, a significant contribution arises from the polymer itself and different grades of product are manufactured and recommended for different applications. One of the most convenient ways of controlling the viscosity is to alter the size distribution of the particles in the final plastisol. Since we are dealing with a dried product from a latex (from the emulsion polymerisation), which after drying consists of agglomerates of the particles formed at the polymerisation stage, the degree to which these agglomerates break down in the paste is significant in the final particle size distribution. Thus the final distribution is a mixture of primary particles (distributions containing particles < 1.5 µ m formed during polymerisation) and secondary particles (grains—mean particle size 30–60 µ m formed during drying and subsequently reduced to 5–20 µ m (mean) by milling). The ease and degree to which the secondary particles break down in the paste is determined by the drying conditions used. Nevertheless, the primary particle size distribution plays a significant role in determining the paste viscosity. Thus, for unimodal size distribution, the larger the size, the lower the viscosity, but such a paste would become very dilatant as the shear rate was increased. In order to reduce the viscosity and approach Newtonian behaviour, it is necessary to broaden the distribution, but the number of distributions is limited in practice by the degree of control available at the polymerisation stage. Figure 3.1 shows the effect of latex particle size distribution on the viscosity behaviour of the plastisol. The requirements for emulsion polymers differ from those for paste polymers. For extrusion applications, the emphasis is on easy gelation, good powder flow properties and high packing density. In general this normally means producing a latex of small particle size but a coarse powder after drying. For battery separator applications the flow properties of the powder have to be tailored to the process being used to produce the separator. In addition, good sintering properties are required to give good mechanical strength to the separator together with a low pore size and optimum total porosity. Clearly the particle size and distribution of both primary and secondary particles is important.

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FIG. 3.1. Effect of particle size distribution on plastisol viscosity. Recipe: 50phr DOP. (Reproduced from Rangnes and Palmgren, Journal of Polymer Science, Part C, 33, 181, 1971.)

3.4.2 General Description of the Process A schematic diagram of the process is shown in Fig. 3.2. The manufacture of paste/emulsion PVC polymers may be divided into six main parts: (a) emulsion polymerisation or (b) microsuspension polymerisation; (c) VCM removal; (d) latex storage; (e) drying; (f) finishing. 3.4.2.1 Batch Process Emulsion Polymerisation

The principal ingredients in an emulsion polymerisation recipe are VCM, water, initiator and emulsifier. The polymerisation takes place in an autoclave capable of withstanding the vapour pressure of VCM at the temperature of polymerisation. This is usually 40–60°C, corresponding to pressures of 6.4–10.0 bar. The polymerisation temperature is chosen depending on the molecular weight required for a particular grade. A major step in the polymerisation of PVC is chain transfer of radicals with VCM. As the rate of this reaction increases more rapidly with temperature than the chain propagation reaction, this means that the polymerisation temperature controls the molecular weight and other variables, e.g. initiator concentration, have little effect. Further control can be obtained by deliberately adding chain transfer agents. For paste polymers, low molecular weights are necessary when low fusion temperatures are required whereas high molecular weights give increased wear resistance in fused layers. The contents of the autoclave are agitated and heat is supplied or removed via a jacket through which a mixture of water and steam are pumped. The polymerisation reaction is strongly exothermic, and the normal requirement is for cooling. As the size of the autoclave is increased, the cooling requirement presents a problem as a less favourable surface area:volume ratio results. These problems have been overcome by designing vessels with thinner walls, using chilled water, and installing an external condenser

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FIG.3.2. Diagram of emulsion mircosuspension polymersation process.(Reproduced from Burgess, Developments in PVC Proucation and Processing—1, Whelan and Craft, Eds., Applied Science Publisher Ltd, 1977.)

to condense the VCM vapour.4 The size of the autoclave is usually governed by economics but modern plants currently use vessels in the 30–80 m3 range, although a 200m3 vessel is used by Huls.4 Larger autoclaves

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FIG. 3.3. Effect of emulsifier concentration and type on latex particle size. (Reproduced from Rangnes and Palmgren, Journal of Polymer Science, Part C, 33, 191, 1971.)

also present mechanical problems with respect to agitation. The conventional top entry stirrer shaft would become long and very thick. This is usually overcome by employing a bottom entry stirrer fixed in the base of the autoclave and driven by a relatively thin shaft. These effects are discussed in more detail for suspension polymerisation in Chapter 1 (section 1.6). For emulsion polymerisation, the agitation conditions and system can be critical and a balance has to be achieved between good mixing/heat removal and the mechanical stability of the latex, otherwise excessive coagulation will occur. The initiator used in emulsion polymerisation is soluble in water and is normally potassium persulphate or ammonium persulphate. Initiation takes place in the aqueous’phase by sulphate ion free radicals. The rate may be increased significantly by the addition of a reducing agent. Such systems were developed by ICI and independently in other industrial laboratories in 1940 and the work was subsequently published after the war. Bacon5 called such a system a ‘reduction activation’ system but the term ‘redox catalysis’ is now commonly used.6 Three-component systems, involving the use of a metal salt also, give further benefits7 and typical systems are ammonium persulphate/sodium bisulphite/copper sulphate or hydrogen peroxide/ ascorbic acid/ferrous sulphate. Using such systems, polymerisation may be carried out at lower temperature or better control of reaction rates may be achieved at the higher temperatures (say 50°C), particularly if some of the compounds are injected continuously or as required.8 Typical quantities (based on monomer) for persulphate redox systems are 0.01–0.3% persulphate, 0.001–1% reducing agent and 0.05–10ppm metal ion. One of the most important components of the emulsion polymerisation recipe is the emulsifier. The quantity used has a major effect on the final latex particle size achieved, by determining the number of particles initiated. The nature of the emulsifier can also affect the number of particles initiated, particularly at low concentrations.9,10 This is illustrated in Fig. 3.3. However, the number of particles formed is strongly influenced by the concentration of free emulsifier in the aqueous phase and accurate metering during reaction provides an effective method of controlling particle size and allows the minimum amount of emulsifier to be used.11 Since the latex formed is spray dried, the emulsifier remains on the dried particles and can influence, therefore, the properties of the dried polymer. Typical properties influenced are heat stability, colour,

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FIG. 3.4. Electron micrograph of PVC latex primary particles.

transparency, quality of chemical foam, etc.12,13 Typical emulsifiers used are sodium or ammonium salts of alcohol sulphates, alkyl sulphonates, alkyl aryl sulphonates, sulphosuccinates, and fatty acids. Although the emulsifier has a strong influence on the number of particles initiated, and therefore on the final latex particle size distribution, the use of preformed seed latex in the polymerisation recipe is a useful technique for controlling particle size distribution, particularly if large particles are required. In this way, by suitable choice of size and quantity of seed latex, growth to a predicted size distribution can be achieved with minimum or controlled initiation of new particles. It has been shown that the absence of free emulsifier in the aqueous phase is not the only requirement for the suppression of fresh initiation of particles and the surface area of the seed latex employed plays a major role.14,15 The normal requirement for emulsion polymer is to produce primary particles at the polymerisation stage of 1.0 µ m are desirable. A typical particle size distribution of a PVC latex is shown in Fig. 3.4. The quantity and type of emulsifier are governed mainly by a balance of cost and the properties required for the finished product. Another consideration is the mechanical stability of the latex both during polymerisation and for the subsequent handling operations. As mentioned above, the type of agitation used will play a role, together with the amount of polymer adhering to the walls of the vessel. These considerations clearly set the lower limit for emulsifier concentration, whereas, although high concentrations will reduce build-up, the upper limit is governed by the product properties, both by its effect on primary particle size and by the residual amount on the dried product after spray drying. The normal range is approximately 1–3 % on PVC. The control of pH during polymerisation is also important. For persulphate-initiated polymerisations, faster reactions are achieved at alkaline pH whereas if hydrogen peroxide is used, acid conditions are often more favourable.16 Conventional buffer systems can be used and in some cases, e.g. sodium carbonate or

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sodium bicarbonate, these can also act as heat prestabilisers. It has been claimed also that pH during spray drying can affect finished product properties.17 The quantity of VCM converted to PVC per batch is normally in the range 85–95 %. It is not generally economic to carry out the polymerisation to higher conversion as the rate of reaction becomes slow at the end of the reaction cycle. At approximately 70% conversion, the pressure in the autoclave starts to fall as liquid monomer is used up. The remaining gaseous monomer is usually vented to a gas-holder when the pressure falls to 3–4 bar. The solids content is governed by the ratio of monomer:water used and also the monomer converted. It is normally in the range 40–45 % solids and is restricted by the particle size distribution, which affects the latex viscosity and the control of unwanted secondary initiation. Micro suspension Polymerisation

In general, the ingredients and polymerisation vessel used in microsuspension polymerisations are similar to those used for the emulsion reactors. The principal differences are that a monomer-soluble initiator is used and some or all of the monomer is emulsified to small droplets using a mechanical homogeniser prior to the start of the polymerisation. A typical procedure, therefore, is to homogenise a mixture of VCM, water, emulsifier, and monomersoluble initiator and pump this mixture into an autoclave. The mixture is then heated, with agitation, to the desired temperature which allows polymerisation to take place to yield a stable emulsion with a particle size distribution of 0.1–1.5 µ m. Homogenisation of the mixture before polymerisation is carried out by the application of high shearing forces. Suitable equipment is a high speed pump, colloid mill or high speed stirrer. For a number of years, the preferred initiator was of the long chain diacyl peroxide type, normally lauroyl peroxide. However, this gave slow reactions with unfavourable kinetics, e.g. fast rate at the end of the polymerisation and slow at the start. The use of the more active peroxydicarbonate initiators gave undesirable quantities of particles 5) to overcome the problem of small particle formation and autoclave build-up.18 The long chain compounds were of the peroxide, ether, hydrocarbon, or carbonyl compound type. In this way significantly improved reaction kinetics were obtained. A further advance was the discovery that it was not necessary to homogenise all the VCM or water,19 thus reducing capital and energy costs associated with the homogenisation step. The scope for modifying the particle size distribution is more limited than for the emulsion process. A continuous distribution is obtained and the main opportunity for changing this is at the homogenisation stage. Nevertheless, the distributions obtained are suitable for making paste polymers giving medium-low viscosities and the products are useful for general applications. Recently, a process has been developed to alter the particle size distribution and to obtain a higher proportion of large particles which is analogous to the seeded emulsion process described in the previous section.ntf20 In this process a ‘seed’ latex is made by the microsuspension process described above but using a large excess of initiator, typically lauroyl peroxide. In the second stage, a portion of this seed is used together with VCM, water, and emulsifier. The polymerisation is activated using a redox system consisting of copper sulphate/ascorbic acid. As the copper sulphate/ascorbic acid forms a complex which is soluble in the monomer phase, the excess lauroyl peroxide contained in the seed particle is activated. In this way the seed latex can be grown to large particles without the problem of secondary initiation. A further development is to introduce an emulsion latex with the microsuspension seed latex to modify the particle size distribution further. In this case, very little

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polymerisation takes place on the emulsion seed and by suitable choice of size and quantity it is claimed that distributions which allow high solids contents (up to 55%) and low paste viscosities can be obtained.21 3.4.2.2 Continuous Emulsion Polymerisation Process The continuous emulsion process for the production of PVC was first developed by I.G.Farbenindustrie in Germany in the 1930s and has already been referred to in the Introduction (Section 3.1). Such a process is still operated in Germany by several companies which formerly constituted I.G.Farbenindustrie, disbanded after the Second World War. These companies have continued to develop the continuous emulsion process and some new plants have been built in recent years. However, outside Germany there is a preference for the batch emulsion polymerisation process and most of the new capacity has been of this type. Although good quality paste polymers can be produced using the continuous process, it does not possess the versatility of the batch process. Build-up formation can present problems and often higher emulsifier contents are necessary compared with the batch process (2–4% normally). Also residence time control can present problems. Continuous stirred tank reactors are generally preferred, usually with a height:diameter ratio of at least 3 and having a capacity of at least 5m3.22 VCM and other ingredients (similar to those used for the batch process) may be fed in separately or mixed/pre-emulsified separately shortly before introduction to the reactor. The portion of the reactor near the inlet is well agitated whereas the rest of the reactor is usually mildly agitated to maintain a homogeneous mixture together with adequate heat transfer. The reactants may be fed in at the top, although a recent patent22 claims that there is less build-up and a reduction in unwanted larger particles (‘pebble’) if the reactants are fed in at the bottom and the product removed from the side of the reactor at a level 30–90 % of the total internal height of the reactor above the feed inlet. If the outlet is too low then the product will contain excess unreacted material, whereas if it is too high, pressure regulation problems are encountered. The heat from the reactor is removed by a cooling jacket and, additionally, cold ‘fingers’ or condensers are used. High monomer conversion is desirable and this is measured either by computing the heat produced from the inlet/outlet temperatures and jacket flow rates23 or by taking samples at the outlet of the reactor and measuring the foam density, which is related to the free monomer remaining.24 The conversion is controlled by varying the monomer feed rate/reaction rate. In order to produce a consistent product, it is necessary to maintain a constant rate of reaction and conversion. Fluctuations in reaction rate are unavoidable in practice and it is desirable to have a highly reactive initiator system. Redox systems are normally suitable and some have been specially developed for the continuous process, e.g. hydrogen peroxide/ascorbic acid at a particular pH and in the absence of heavy metal salts.23 The process beyond the reactor is similar to the batch process although a continuous monomer stripping process is of course necessary. In the process as it is today operated conversions are usually > 90 % and the solids content approximately 45%, although in the early days much lower solids were normal. 3.4.3 VCM Removal In theory, because of the small size of the PVC latex particles produced by emulsion polymerisation, the removal of VCM from such polymers should proceed at a high rate. This in fact is achieved during the spray drying step and the VCM concentration in the finished product is often very low. However, this is not a

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suitable process for VCM removal since it is difficult to recover the monomer removed and thus there is an environmental problem. In practice therefore the VCM is removed at the end of the polymerisation stage, either in the autoclave or after transfer to a separate stripper vessel. The most efficient method of removing VCM at the autoclave stage is to boil the latex by applying sufficient heat and vacuum and to remove the VCM by first condensing the steam. The major problems encountered in practice are excessive foam formation due to the presence of anionic emulsifiers, and coagulation which in its mildest form produces excessive build-up formation in the vessel. Careful control of venting rates, temperature, agitation, etc., is therefore required and low levels of VCM in latex can be achieved but the cycle time can become unattractive with certain types of latex. Stripping external to the autoclave can be achieved by continuously spraying latex into an evacuated chamber against a countercurrent of steam.25,26 Foaming and build-up problems are reduced considerably using the latter technique. Removal of VCM from PVC latex is discussed in more detail in Chapter 5 (Section 5.4.3). 3.4.4 Drying of Latex In order to isolate the product from the latex it is necessary at some stage to remove the water. Although coagulation techniques have been investigated these are not in general use and can usually be applied only to certain types of latex. Also the structure of the particles (grains) formed by removing the water is important in determining the final properties of the plastisol. For this reason a spray drier is normally employed to isolate the solid product. As this process is expensive on energy costs, relatively small changes in the amount of water to be evaporated can lead to significant reduction in drying costs. Thus, it is desirable to produce latices with high solids contents at the polymerisation stage but the limitations of this technique have already been mentioned. Concentration of the latex after the autoclave stage is sometimes employed, e.g. thin film evaporation or ultrafiltration,27 but the choice of technique has to be carefully assessed if there are to be worthwhile savings. In general approximately 45–60% of the latex is water and this is finally removed by the drier. Using a spray drier a product containing 300 µ m, is removed by sieving and, in some instances, the particle size distribution can be modified by using an air classifier system. In this way the coarse particles can be removed either as reject or for further treatment with a mill. Alternatively, all the powder

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FIG. 3.7. Internal structure of secondary PVC particles (grains) showing aggregates of primary particles.

after sieving may be subjected to a milling (grinding) process to remove the large hard particles. This has the advantage of breaking the relatively soft agglomerates also and thus improves the ease with which the polymer will form a plastisol. However, other properties, in particular plastisol viscosity, are significantly affected depending on the conditions and type of equipment used. In general plastisol viscosities are increased using polymer which has been milled, particularly at high shear rates. The equipment generally used is of the attrition mill type, e.g. hammer mills. It is essential to be able to control the degree of milling and, to produce polymer for critical applications, some form of classification is required. This is often built into the equipment although a separate unit can be used if necessary. 3.4.6 Powder Handling The final product emerging from the drier or mills may be stored either in bags or in silos. There are no serious handling problems with emulsion polymers and both bag and silo storage are used. For paste polymers, however, problems do arise since the powder has poor flow properties and compacts readily, making bulk handling difficult. Special equipment is therefore necessary to handle these polymers in bulk both at the manufacturing site and the off-loading point. A recent patent39 claims to have overcome these problems by co-spray drying latex under special conditions with a fine suspension polymer, normally used as an extender resin in plastisol formulations to reduce viscosity (see Chapter 10). However, such polymers would have limited applications as not all formulations can accept extender resins, e.g. thin layers for paper coating.

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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.

I.G.FARBENINDUSTRIE, German Patent 900019. KAUFMAN, M., The History of Polyvinylchloride, Maclaren & Sons, London, (1969), p. 62. KAUFMAN, M., The First Century of Plastics, The Plastics Institute, London (1963), p. 75. TERWIESCH, B., Hydrocarbon Processing, November 1976, 117. BACON, R.G. R., Trans. Faraday Soc., 42, 169 (1946). BACON, R.G. R., Quart. Rev., 9, 287 (1955). MORGAN, L.B., Trans. Faraday Soc., 42, 169 (1946). WACKER-CHEMIE, German Patent 2006966. RANGNES, P. and PALMGREN, O., J. Polym. Sci., Part C, 33, 181–92 (1971). DIAMOND SHAMROCK, US Patent 3981837. CHEMISCHE WERKE HULS, German Patent 1964029. CHEMISCHE WERKE HULS, Belgian Patent 857235. CHEMISCHE WERKE HULS, Belgian Patent 830223. GATTA, G., VIANOLLO, G., BENETTA, G., La Chemica e l’Industrie, 51(11), 1234–9 (1969). MONTECATINI EDISON, British Patent 1120410. DYNAMIT NOBEL AG, German Patent 1720524. CHEMISCHE WERKE HULS, German Patent 2531780. ICI, British Patent 978875. ICI, British Patent 1458367. RHONE-PROGIL, British Patent 1435425. RHONE-POULENC, British Patent 1503247. HOECHST, German Patent 2625149. CHEMISCHE WERKE HULS, British Patent 1411875. HOECHST, German Patent 2521862. ICI, British Patent 1553829. HOECHST, US Patent 4158092. HOECHST, German Patent 2440634. SHTARKMAN, B.P., MUKHIMA, I.A. and SHARIKOVA, L.L., Koll. Zhur., 31(4), 611(1969). SHTARKMAN, B.P., MUKHIMA, I.A., VISHNEVSKAYA, I.N., Akad. Nauk. SSSR, Sb Statei, 124–7(1966). WIESEBACH, H. et al.. Plaste u. Kaut., 21(8), 576–8 (1974). MASTERS, K., Spray Drying Handbook, George Godwin Ltd, London, 1979. HERTE, P., Chemischen Technik, 4(7), 327–30 (1952). CHEMISCHE WERKE HULS, US Patent 3805869. CHEMISCHE WERKE HULS, US Patent 3883494. CHEMISCHE WERKE HULS, Belgian Patent 820496. A/S NIRO ATOMISERS, Belgian Patent 867667. CHEMISCHE WERKE HULS, Belgian Patent 869496. A/S NIRO ATOMISER, Belgian Patent 854317. STAUFFER, British Patent 1410202.

Chapter 4 VINYL CHLORIDE COPOLYMERS AND PVC BLENDS R.H.BURGESS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

4.1 INTRODUCTION Historically, PVC gained wide commercial acceptance because it could be mixed with organic liquids (plasticisers) to give a tough flexible material not unlike rubber in its properties and the way in which it could be processed. The large market thus developed soon produced reductions in the cost of the VCM monomer and the manufacturing process so that it was natural for PVC manufacturers and users to consider its use for unplasticised applications. PVC in its unplasticised state when correctly fabricated is capable of producing tough rigid articles of excellent surface appearance and transparency if desired. However, it is thermally labile and will decompose quite rapidly at or near the temperatures needed to melt the PVC sufficiently to produce the finished articles. Hence, before the full commercial potential of PVC could be realised, much progress had to be made improving the basic heat stability of the PVC, developing improved heat stabilisers and process formulations, and developing equipment capable of fabricating the required finished products. As a result of this successful development work, it is now possible to produce rigid, i.e. unplasticised, articles of very considerable commercial importance (pipes, profiles, etc.) which are in widespread use. Even so, their performance is limited necessarily by the basic properties of PVC. Some of these basic properties are listed in Table 4.1 for the narrow range of molecular weight (expressed as Kvalue) used for rigid applications. Attempts to widen this property range by molecular weight change lead either to an unprocessable material (at K-values above 70) or a brittle material without the most desired property of PVC, its toughness (at K-values below 50). These basic properties of rigid PVC can be extended quite considerably TABLE 4.1 Properties of Rigid PVC K-value 5kg Vicat softening point (°C) PVC minimum torquea in Brabender plasticorder (g/m)

Failure stressb (MN/m2)

56 60 64 68 72

12–5 13–3 14–0 14–8 15–6

79 80–5 82 83 84

720 970 1200 1420 1640

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K-value 5kg Vicat softening point (°C) PVC minimum torquea in Brabender plasticorder (g/m)

Failure stressb (MN/m2)

a

At equal gelation time. Fatigue resistance under cyclic on/off load 0–5 Hz. Fatigue stress at 106 cycles. b

either by copolymerisation or by blending with other polymers. Both these areas have been extensively studied and many products of commercial importance developed. 4.2 THEORY OF COPOLYMERISATION The copolymerisation of any two monomers is a complex process but the basic theory has been understood for some considerable time.1,2 Two concepts can be used, both of which recognise that copolymerisation in addition polymerisation is an event which must take account of the properties both of the monomer which is about to add to the chain and the nature of the group on the end of the chain. If one considers two monomers M1 and M2 then there will be two polymer radicals P1 and P2 containing M1 or M2 active ends respectively and four reactions possible with rate constants k11, etc. P1+M1→P2 k11 P1+M2→P2 k12 P2+M1→P1 k21 P2+M2→P2 k22 The first theory1 then defines reactivity ratios r1=k11/k12 and If this nomenclature is used it is possible to show that the copolymer produced (composition ratio M1/M2) will be related to the monomer mixture present ([M1] and [M2] being the respective concentrations of the two monomers) by the relationship M1/M2=[M1](r1[M1]+[M2])/[M2](r2[M2]+[M1]) (4.1) It is clear from this that only when r1[M1]+[M2]=r2[M2]+[M1] will the polymer composition be the same as the monomer mixture composition and only in the very special case of r1=r2=1 will this apply for all concentrations of M1 and M2. In virtually all practical situations these conditions are not met and the copolymer composition is not the same as that of the monomer mixture. Moreover a consequence of this is that the monomer mixture concentration changes continuously as one monomer is incorporated into the copolymer more rapidly than the other, which in its turn, changes the composition of the next copolymer produced. Thus copolymers of mixed comonomers content are produced unless special steps are taken to maintain a constant monomers composition. The usefulness of the reactivity ratio approach has been extended2 by postulating that the specific reactivity of a monomer is given by a resonance effect Q and the polar character of the radical adduct by e. Then for example : k12=P1Q2exp{−e1e2} (4.2) The reactivity ratios r1 and r2 would be given by the expressions r1=Q1exp{−e1(e1−e2)}/Q2 (4.3) r2=Q2 exp {−e2(e2−e1)}/Q1 (4.4) In principle the determination of the values of Q and e for a particular monomer in copolymerisation with any monomer can be used to predict its copolymerisation behaviour with any other monomer.

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Typical values for r1 and r2 and for Q and e for the more important VCM comonomers are given in Table 4.2. It can be seen that r1, r2, Q and e vary widely from monomer to monomer. For the easiest copolymerisation r1 and r2 should be as similar to one another as possible or the Q and e values of the two monomers to be polymerised (in this case VCM and one other) should be as similar as possible. Inspection of Table 4.2 shows this is rarely true. TABLE 4.2 Copolymerisation Parameters for Commonly Used Vinyl Chloride Comonomers assuming VCM is M1 Reactivity ratio Vinyl chloride Vinyl acetate Methyl methacrylate Vinyl isobutyl ether Vinylidene chloride Methyl acrylate Maleic anhydride Ethylene Propylene Styrene Butadiene

r1

r2

Q

e

__ 2 0·1 2 0·3 0·08 0·3 2·85 5·66 0·077 0·037

__ 0·25 10 0·02 3 9·0 0·008 0·055 0·0066 357 97

0·044 0·02 0·74 0·023 0·22 0·42 0·23 0·015 0·002 1 2·40

0·20 −0.59 0·40 −1.77 0·36 0·60 2·25 −0.20 −0.78 −0.8 −1.05

References 3 and 4 except where indicated.

4.3 VINYL ACETATE COPOLYMERS By far the most important copolymers commercially are those based on copolymerisation with vinyl acetate (VAM). These have been used in relatively large quantities for about 25 years, the VAM copolymerised markedly reducing the melt viscosity of the resultant polymer at the expense of softening point, and to a lesser extent heat stability and product toughness. Two main market areas have emerged, one requiring a product with the maximum melt flow for applications such as gramophone records and vinyl flooring, and the other a material of good melt flow, high melt elasticity and adequate mechanical properties mainly for packaging applications. The former market is met by a product of very low molecular weight (K-value 45– 50) with about 15% of copolymerised vinyl acetate, and the latter by a product of higher molecular weight (K-value 60) and a lower vinyl acetate content ( 10%). Typical properties of these products are listed in Table 4.3. The data in Table 4.2 show that when VCM and VAM are polymerised together there is a tendency for VCM to polymerise more rapidly than VAM. The reaction rate of either a VCM or VAM terminated polymerisation radical is more rapid with VCM than it is with VAM (r1 > 1,

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TABLE 4.3 Properties of Commercially Available Vinyl Acetate Copolymers K-value

VAM content (%)

TG (ºC)

Flow pressure at 140° Ductility factora at 23° C (psi) C (mm)

57 62 47 60

0 0 14·5 9·5

79 80 70 73

3220 3345 1070 2210

a

2·4 4·6 1·5 3·1

Ductility factor obtained by dividing the stress field intensity factor (KIc) by the yield stress y and squaring the result. Description of ductility factor to be published soon. Note that at low molecular weight (K-value) copolymers tend to have a higher ductility factor than homopolymers.

r2 < 1). The copolymer produced will consequently contain rather less VAM than is present in the monomer mixture. The two rates r1 and r2 are much more similar to one another, however, than are the rates for many other monomer mixtures (see Table 4.2) so that simple copolymerisation, in which both monomers are added at the beginning of the polymerisation, is possible and is commonly performed. Under these conditions a copolymer mixture with the range of compositions shown in Fig. 4.1 (which is discussed later) is produced. Both emulsion and suspension polymerisations are possible but for most applications the cheaper suspension process produces the type of product required and this method is consequently most used. The actual process is very similar to that described in Chapter 1 for suspension homopolymerisation with the same initiators, protective colloids and water:monomer ratios in common use. VAM is not a chain transfer agent and the preponderance of VCM used in the commercial processes means that the product molecular weight is still controlled primarily by chain transfer to VCM; consequently reaction temperatures are used as described in Chapter 1. Although the measured K-value for vinyl acetate copolymers is lower than for homopolymers of the same molecular weight, the K-value of the 15% vinyl acetate content copolymers required is still very low by homopolymer standards and rather higher polymerisation temperatures of up to 80°C are used. This may present a problem with autoclave temperature control so that additional chain transfer agents such as isobutyraldehyde or trichloroethylene have been used to reduce that temperature. The general reservations about impurities in the subsequent polymer (much trichloroethylene remains in the product after drying) have reduced the use of chain transfer agents in recent years. Fortunately the vapour pressure of a mixture of VAM and VCM is less than that of VCM alone, presumably because VCM is soluble in VAM to some extent, so that the maximum pressure of the polymerising mixture is little higher than the maximum generated in homopolymerisation in spite of the higher temperature used. Because of the high polymerisation temperatures, initiators such as lauroyl peroxide, benzoyl peroxide or azodi-isobutyronitrile are the most suitable for the 15% vinyl acetate copolymers (see Table 1.5 in Chapter 1). The more modest temperatures used for the K 60 ca 10% vinyl acetate copolymers require the normal initiators for the medium K-value range of homopolymers. The grain structure of PVC homopolymers is extremely important because it is a major factor in their subsequent processing. As discussed in Chapters 8, 9 and 10 this is related to the way in which the grains accept plasticiser, stabilisers, lubricants, etc., and the way in which the grains break down and fuse during processing. Vinyl acetate copolymers are rarely used for plasticised applications and because of their lower melt temperature and melt viscosity are much easier to process than homopolymers. Consequently there is not the need for the porous, easily broken down structure which is so important for PVC homopolymers. In practice the market is most interested in extruder output, where reasonably high packing density polymers are desirable and in the case of record manufacture very high packing density grades with dense spherical

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FIG. 4.1. Changes of composition during vinyl chloride/vinyl acetate copolymerisation.

grains are normally sold. This type of grain is very non-porous and processes such as those required to remove residual VCM are much more difficult to carry out satisfactorily than for homopolymers (see Chapter 5). This problem is even more acute if one considers the removal of the much less volatile residual VAM. Figure 4.1 shows the change of composition of polymer being produced throughout a VCM/VAM copolymerisation using the reactivity ratios given in Table 4.1 and a computer simulation technique (similar techniques using Q and e values give similar results8). The Figure shows that using an original VCM/VAM mixture containing 15% VAM by weight a copolymer with an average VAM content of 12% is produced after 90% of the original monomers charged have been converted to polymer. Polymer molecules are produced with a VAM content ranging from almost 8% VAM at the beginning to almost 24% at 90% conversion. At 90% conversion an unreacted monomer mixture containing 40% of VAM remains. This monomer mix is more soluble in the copolymer than VCM is in the homopolymer, so it exerts a lower vapour pressure. Removal of both VCM and VAM is difficult, especially so for the VAM which is both more soluble in the copolymer and being a liquid (BP 73°C) much more difficult to vaporise. It was common for up to 1–5% residual VAM to remain in the polymer after drying but the improved stripping techniques described in Chapter 5 and designed to remove VCM almost completely have greatly reduced this figure; however, 0–1% VAM is still quite common in the final polymer. In principle the homogeneity of the copolymer molecules produced can be improved by proper control of the monomer composition. For example, Fig. 4.1 shows that a 15% vinyl acetate copolymer requires a 73/27 VCM/VAM monomer mix. By maintaining such a composition all the copolymer made would have the desired copolymerised monomer mix. This can be achieved by constantly adding VCM to the VCM/VAM

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polymerising mixture at a rate just sufficient to balance the extra VCM being copolymerised. This can be done either by measuring the extent of polymerisation, perhaps by sampling a few runs and then assuming a similar polymerisation rate in future batches, by measurement of the heat of polymerisation evolved or by an indirect method such as measuring the pressure generated when operating below the saturated vapour pressure of VCM. All these methods have been used, especially for applications where homogeneous compositions, for example for solution coating applications, are required. In practice the range of composition produced in a simple copolymerisation presents no problem for applications other than solution coating and is often used perhaps with one or more late additions of VCM to improve copolymer homogeneity somewhat. 4.4 OLEFIN COPOLYMERS Olefin copolymers have been extensively studied and their reactivity with VCM falls markedly with increasing olefin molecular weight. In fact some higher olefins are retarders for VCM polymerisations. Both ethylene and propylene can be polymerised relatively easily and have the property of reducing the copolymer melt viscosity in much the same manner as vinyl acetate with the advantage that the product mechanical properties and heat stability are not impaired as in the case of vinyl acetate. 4.4.1 Ethylene Table 4.2 shows that ethylene copolymerises with VCM much less readily than VAM. A comparison of the data for VAM in Fig. 4.1 with those shown for ethylene in Fig. 4.2 shows the magnitude of these differences. For example a 13–5% ethylene content monomer mix is required to produce a 5% ethylene copolymer, compared with 27% VAM monomer mix for a 15% VAM copolymer. Moreover, in a single copolymerisation in which both monomers are added at the beginning, a very large change of composition of the copolymer produced throughout the conversion occurs with the ethylene content steadily increasing. Figure 4.2 shows that a 7% ethylene content copolymer is produced at 90% conversion from a 10% ethylene monomer mix with copolymer varying from 3–5 to 22% ethylene content and with residual gas containing 60% ethylene. This increase in ethylene content produces a large change in the pressure of the monomer mixture. For example,9 in a polymerisation carried out at 50°C producing a 4% ethylene copolymer, the pressure before ethylene addition was 8 bar, it was 22 bar after ethylene was added and this peaked to 31 bar during the polymerisation. These high pressures in ethylene copolymerisations are inevitable but the increase in pressure with conversion for a simple copolymerisation and the heterogeneity of the copolymer composition produced are additional reasons compared with VAM copolymerisation for feeding VCM continuously. The very high pressures necessarily require thick wall autoclaves which reduce heat transfer to the cooling jacket so that very long polymerisations and uneconomic recipes have to be used. At the end of the polymerisation a mixture of VCM and ethylene has to be removed from the copolymer; the resultant recovered monomer gas mixture is very difficult to separate into the constituent monomers by conventional compression and reliquefaction but can be used for further copolymerisations as a gas mixture.

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FIG. 4.2. Changes of composition during vinyl chloride/ethylene copolymerisation.

4.4.2 Propylene Mixtures of VCM and propylene produce a much lower pressure than ethylene mixtures at polymerisation temperatures so that thinner wall autoclaves can be used. For example,9 in a polymerisation carried out at 50°C producing a 2–5% propylene copolymer the pressure of the polymerisation never rose above 10 bar. However propylene is less reactive with VCM than ethylene (see Table 4.2) so that higher propylene content mixes have to be used to produce the required propylene content copolymer. This is exemplified in Fig. 4.3 which shows that a 7.5% propylene copolymer requires a monomer mixture containing 34% propylene, i.e. almost 5 times as much, compared with less than 2 times for VAM and 2–3 times for ethylene. In the example shown a 10% propylene content mixture would produce a 4–8% average propylene content copolymer at 90% conversion with polymer ranging in composition from less than 2% to 21% with a residual gas mixture containing over 70% propylene. Because of its low reactivity, propylene slows down the rate of polymerisation, but even more serious is the fact that propylene is a significant chain transfer agent so that lower polymerisation temperatures than usual have to be used to produce the product of required molecular weight. As a consequence only very slow polymerisation rates can be achieved even with high initiator concentrations. As for ethylene copolymers, a mixture of unreacted monomers very rich in the comonomer is obtained which is difficult to separate and the mixture is normally used in the production of further VCM/propylene copolymer. Comparative testing of the melt flow characteristics of vinyl acetate, ethylene and propylene copolymers has shown that a similar reduction in melt viscosity is produced for the same molar concentration of the three comonomers. For example, the 15% VAM, 7.5% propylene or 5% ethylene copolymers described

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FIG. 4.3. Changes of composition during vinyl chloride/propylene copolymerisation.

above would have similar melt viscosity characteristics if they have the same K-value but the difficulties in making the olefin copolymers with such high olefin content mean that comparisons have been made only at lower comonomer content (7–2% VAM, 3–5% propylene and 2–5 % ethylene). Of these three polymers both the ethylene and propylene copolymers show improved polymer heat stability, the ethylene polymer improved notched impact strength and the VAM copolymer the best deep draw characteristics.9 Both ethylene and propylene copolymers were marketed for some years by Union Carbide and Air Reduction Co. Ltd respectively, but had very little impact on the market, presumably because of the marginal improvement in product properties and their high cost compared with vinyl acetate copolymers. Nearly all have now been withdrawn from sale. 4.5 OTHER COPOLYMERS Copolymers of VCM with various acrylate and methacrylate esters can be made although all these common monomers tend to react more rapidly than VCM (see Table 4.2) so that lower acrylate content monomer mixes are required to make a given copolymer and the acrylate or methacrylate have to be added to the polymerising mixture if homogeneous copolymers are required. These copolymers are only of slight commercial interest although some are offered in latex form for coating applications and a copolymer resin of VCM and 2-ethylhexyl acrylate is sold because its superior processing eases the development of the optimum mechanical properties in the final product. Copolymers of VCM and methyl methacrylate are thermally unstable, yielding methyl chloride on heating.

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A very wide range of other comonomers have been studied, nearly always with the aim of reducing the product melt viscosity. Vinyl ethers and vinylidene chloride have been used and produce polymers with significantly lower melt viscosity. However, vinyl ether copolymers are expensive because of the high cost of the comonomer, and vinylidene chloride copolymers are more unstable to heat than vinyl acetate copolymers. Some comonomers have been studied in the hope of producing especial effects. VCM and carbon monoxide can be copolymerised together and the resultant copolymer is very unstable, especially to light. Copolymers with substituted N-phenylmaleimides produce copolymers with an increased softening point but these are thermally unstable. Methacrylic acid copolymers have been produced in the hope of matching the ‘ionomer’ properties of olefin copolymers.10 Homogeneous VAM copolymers are produced by solution polymerisation and the addition of a third monomer is of some commercial interest for solution coating applications (e.g. Vinylite resins from Union Carbide). These terpolymers containing, perhaps, 1% of a third monomer are used to improve, for example, metal adhesion. Copolymers of VCM, VAM and ethylene are of increasing interest in latex form for paint applications. Many potential comonomers, i.e. butadiene, styrene, -methylstyrene, higher olefins, etc., react relatively rapidly with polymerising VCM but the radical produced does not propagate the polymerisation chain so that such monomers are retarders for the polymerisation. In practice these comonomers are of some commercial importance, either because their presence as impurities in VCM has to be carefully controlled to avoid reaction time variability (e.g. butadiene) or because they are used as retarders to control potential runaway reactions (e.g. -methylstyrene). 4.6 PVC BLENDS—GENERAL PVC when processed is almost always used mixed with other materials to improve its heat stability or its processibility or to modify its properties. The formulation of PVC compositions to enable it to be processed is examined in detail by Matthews11 and discussed in Chapters 8–10 of this book. This section discusses the production of blends, with additives other than plasticisers, made at the polymerisation stage which are designed to significantly change the properties of the final composition. Basically there are three types of blends: (1) Those based on rubbers designed to increase product impact strength. (2) Those based on fillers designed to increase product rigidity. (3) Those based on compatible polymers where product properties intermediate between those of the blended polymers result. 4.7 RUBBER BLENDS 4.7.1 Butadiene Based Rubbers A large number of butadiene based rubbers are available for blending with PVC. These are either based on a butadiene/styrene or polybutadiene rubber grafted with poly(methyl methacrylate) or styrene/acrylonitrile

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and these products are then blended, usually at the 5–15% concentration with suspension PVC and the usual other additives. Fabricated articles made from these blends, when examined optically, are seen to consist of a continuous PVC phase in which are suspended discrete rubbery particles. The product consequently retains most of the properties (rigidity, softening point) of rigid PVC with enhanced impact strength. By correct choice of the composition and particle size of the rubber phase it is possible to produce transparent blends with these materials. They are extensively used for applications such as bottles where high impact strength and transparency are required. While these blends are now produced exclusively by blending specially produced grafted rubbers as powders with PVC, much work was carried out some years ago on the production of intimate blends at the autoclave stage. The most successful of these techniques was to coagulate a suitable rubber latex in the presence of suspension PVC and then dry the resultant mixed material using a conventional suspension PVC drier.12 This technique, when applied to butadiene based rubber latices, offers no great technical or economic advantage over simple powder blending. Moreover the expertise in producing the optimum product properties resides almost entirely in the production of the grafted rubber so that a few companies only now produce the speciality rubbers required. 4.7.2 Chlorinated Polythene Unsaturated rubber blends such as those based on butadiene rubbers suffer from the disadvantage that they degrade quite rapidly when exposed to sunlight. Consequently, they are quite unsuitable for the type of outdoor applications where rigid PVC is widely used. A number of saturated rubbery materials have been examined for these applications. Of these, chlorinated polythene (CPE) initially gained wide acceptance for such applications as PVC window-frames, house siding, etc. These blends are based on physical blends of a suspension PVC with 8–10% CPE. The degree of chlorination is important in order to ensure the correct balance between compatibility (to avoid cheesy composition), and incompatibility (to maintain PVC type rigidity and a higher impact strength). While optimum product properties are produced from physical blends when CPE containing 35% combined chlorine is used, rather different results are obtained if the CPE is incorporated into the VCM before polymerisation. In general the mechanical properties of physical blends deteriorate the lower the chlorine content of the CPE is below 30%, whereas ‘graft’ blends with excellent mechanical properties can be produced from CPE containing as little as 24% chlorine.13 Again, no great technical or economic advantage has been demonstrated for such ‘graft’ blends. The market is dominated by the product sold under the trade name ‘Hostalit Z’, which is believed to be produced by physical blending of PVC and CPE. 4.7.3 Ethylene/Vinyl Acetate Rubbers These ideas have been developed further with ethylene/vinyl acetate (EVA) rubbers where once more a wide range of ethylene: vinyl acetate ratio rubbers are available. The optimum product properties have been obtained by polymerising VCM in the presence of a roughly 50:50 ethylene:vinyl acetate rubber.14 Such a rubber is quite incompatible with PVC and a physical blend has poor mechanical properties. Two manufacturing processes have been proposed. In one14 the EVA is dissolved in VCM before polymerisation, which is carried out using the usual suspension polymerisation procedures, to give a 5–15% EVA content blend which is then used to fabricate the final article or is blended with more suspension PVC

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to produce a lower EVA content blend, usually about 6% EVA. In the other,15 finely broken down EVA rubber is suspension polymerised in the presence of VCM below its saturated vapour pressure, i.e. so that there is no free VCM phase. In this procedure the EVA particles grow by swelling with VCM and subsequent polymerisation. Both these methods can be used to make much higher EVA content grafts (50%) which can then be used for dilution with 5–10 times as much PVC homopolymer to produce the final composition. Blends of EVA and PVC give compositions with similar mechanical properties to those based on CPE and being saturated rubbers are suitable for use outdoors. They are somewhat easier to process than the CPE blends and in recent years have captured the major share of the European market for such applications as window frames, etc. 4.7.4 Ethylene/Propylene Rubbers It is possible to make graft blends of ethylene/propylene rubber (EPR) with PVC by polymerising VCM in the presence of EPR by the partial pressure method. These blends show no advantage over the established EVA blends. 4.7.5 Acrylate Rubbers Increasing attention is being paid to the use of blends of PVC with acrylate rubbers. Blends of PVC with poly(butyl acrylate) (PBA) or PBA grafted with methyl methacrylate can be produced either by coagulating the PBA latex in the presence of suspension PVC and codrying, by polymerising VCM in the presence of PBA rubber as described in Sections 4.7.2, 4.7.3 and 4.7.4 for other saturated rubbers, or by blending PVC with commercially available graft PBA powders (e.g. 8–12% Paraloid KM323B). The Vinidur SZ range of polymers marketed by BASF are in the form of ‘graft’ blends of PBA and PVC and can be used at lower concentrations of PBA than Paraloid KM323B to give the same product mechanical properties. Fabricated articles based on blends of PVC with these PBA materials have mechanical properties similar to those obtained with EVA and CPE blends and the customer chooses which blend to use on minor differences such as surface gloss and die relaxation in the particular application. All the PVC/saturated rubber blends described here, when containing the 5–10% rubber normally required for enhanced impact properties, are opaque or strongly translucent. This is of no concern for woodsubstitute application such as window-frames but makes the blends of no interest for clear applications. However, blends containing a lower concentration of PBA are sufficiently transparent for some clear applications. Clear rigid foil for packaging applications can be produced either from emulsion PVC or suspension PVC. However, the latter is difficult to process by calendering to give the highest transparency because of sticking to the calender bowl if the high bowl temperatures necessary for high transparency are used. This problem can be overcome either by incorporating some methyl methacrylate grafted PBA powder into the formulations, e.g. Paraloid K175, by coagulating PBA latex in the presence of suspension PVC, or by polymerising VCM in the presence of some PBA.16

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4.8 POLYMER BLENDS 4.8.1 Processing Aids Quite a wide range of polymers which are compatible with PVC act as processing aids when mixed at 1– 10% concentration in rigid PVC formulations. They apparently act by increasing the melt elasticity of the formulations and the molecular weight of the added polymer is very important (see Section 9.5.4). The most widely used additive polymer is poly(methyl methacrylate) (PMMA) or copolymers of methyl methacrylate and ethyl acrylate containing at least 90% MMA. These polymers are usually added as fine powders to suspension PVC at the mixing stage but they can be added to the polymerisation process either before polymerisation of the VCM or by coagulating a PMMA latex in the presence of suspension PVC. Styrene/ acrylonitrile polymers are also used as processing aids. 4.8.2 Poly(methyl methacrylate) Poly(methyl methacrylate) (PMMA) is compatible with PVC in all proportions. Consequently it is possible to produce any blend which is desired and the properties of these blends are intermediate between those of the constituent homopolymers. These blends can be produced by simple mixing of the respective powders followed by normal processing.17 They can also be produced by polymerising the second monomer in the presence of the first polymer. For example successive suspension polymerisations are possible in which VCM is polymerised to give the normal porous PVC granules, the VCM remaining is removed and MMA is added together, if desired, with extra initiator and chain transfer agent. The mixture is heated and an intimate mixture of PMMA and PVC is produced.18 Most compositions of commercial interest concentrated on 60–88 % PVC in the blend and have the expected properties of a cheaper, more flame resistant acrylic. Unfortunately the compositions share some of the less desirable features of their base polymers, i.e. the material is difficult to process and more expensive than PVC, and their commercial sale has now substantially ceased. These blends are potentially transparent although not all the products marketed were. Blends of PVC with PMMA have a slightly higher softening point than PVC itself. If the methyl methacrylate polymer used has a higher softening point than PMMA itself, for example by using a suitable MMA copolymer, a higher softening point blend results. 4.8.3 Chlorinated PVC The softening point of PVC can be increased by chlorination of the product either in a solvent or as a suspension. Normally one grade of chlorinated PVC (CPVC) containing 63% chlorine is produced with a softening point of 120°C compared with 80°C for PVC itself. This grade is difficult to process and expensive so that if a lower softening point is acceptable blends of this product and PVC will be used in the proportions necessary to give the desired softening point. Products of this type have not been very successful commercially but are used for speciality markets such as hot water systems in mobile homes in the USA and for fibre manufacture in France.

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4.9 FILLER BLENDS In principle the rigidity of a PVC composition can be increased by the addition of fillers such as talc, chalk, mica, glass, etc., to the mixture. These blends are of little commercial importance largely because of poor processing behaviour and the poor bonding between the filler and the PVC which gives poor long term properties. Various forms of finely divided calcium carbonate, surface treated with stearate to improve dispersion, are available and are extensively used at low concentration to improve toughness. It is possible to polymerise VCM in the presence of various fillers but no commercially important product has arisen even though this process should improve the PVC filler bond. ACKNOWLEDGEMENTS Thanks are due especially to R.W.Gould for his help in the preparation of the sections on blends. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

MAYO, F.R. and LEWIS, F.M., J. Amer. Chem. Soc., 66,1594 (1944); ALFREY. T. and GOLDFINGER, G., J. Chem. Phys., 12, 205 (1944). ALFREY, T. and PRICE, C.C., J. Polym. Sci., 2, 101 (1947). BRANDRUP, J. and IMMERGUT, E.H., Polymer Handbook, Interscience, New York (1966). FLEISCHER, G. and KELLER, F., Plaste u. Kaut., 20, 10 (1973). FLEISCHER, G. and KELLER, F., Plaste u. Kaut., 20, 7 (1973). SIELAFF, G. and HUMMEL, D.O., Makromol Chemie, 175, 1561 (1974). NASS, L.I., Encyclopedia of PVC, Marcel Dekker, New York (1976). HIBBERT, P.M., personal communication. DICKSON, J.T., unpublished work. EISENBERG, A. and King, M., Ion-Containing Polymers—Physical Properties and Structure, Academic Press, London (1977). MATTHEWS, G., Vinyl and Allied Polymers, Volume 2, Iliffe, London (1972). ICI, British Patent 1219352. MONSANTO, British Patent 1029634. BAYER, British Patents 1021324 and 1027710. CHEMISCHE WERKE BUNA, East German Patents 134232 and 135083. DYNAMIT NOBEL, German Patent 1595515; SOLVAY, British Patent 1530854. MONTECATINI, British Patent 1 043058. ICI, British Patent 1015334.

Chapter 5 THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC R.H.BURGESS Senior Research Chemist, ICI Ltd, Welwyn Garden City, UK

5.1 VINYL CHLORIDE TOXICITY Vinyl chloride (VCM) is a gas at ambient temperature with a boiling point of−13°C. It is usually handled in commercial PVC manufacture as a liquid at pressures ranging from 2 to 15 bar. Since the PVC manufacturing process, as has been seen in Chapters 1–3 of this book, is a batchwise one, large quantities of VCM are handled in very complex pipework and pressure vessels. There are necessarily in these processes many opportunities for leaks of VCM from the pressure envelope to the surrounding atmosphere. Moreover, as has already been described (Chapter 1, section 1.2) the kinetics of VCM polymerisation are such that it is uneconomic to polymerise the VCM completely to PVC and substantial quantities of VCM (10–20% of original charge) have to be removed subsequent to the polymerisation step. Prior to the early 1960s VCM was known as a gas which presented a severe explosion risk (explosive limits in air 3.6−26% v/v), with anaesthetic properties but with a not-unpleasant ethereal smell apparent at concentrations of > 2000 ppm v/v in air. Plants were designed to avoid the build-up of concentrations of VCM in air so that explosive concentrations and concentrations capable of rendering an exposed person unconscious were avoided. Basically this involved attention to the pressure envelope to minimise major leaks and concentrations in the atmosphere of plants of 100–1000 ppm v/v VCM in air were common. The assumed low toxicity of VCM at that time can be evidenced by proposals to use it as an anaesthetic for medical use, and as an aerosol propellant. While its use as an anaesthetic did not develop, it was used as an aerosol propellant up to the early 1970s in some countries, especially in the USA. In the mid 1960s the occurrence of a bone deficiency called acroosteolysis (AOL) in a very small number of PVC plant operators was noticed. This disease occurred especially in plant operators engaged in cleaning PVC reactors and usually took the form of a degeneration of bone in the tips of the fingers of the operators. After removal of the operator from autoclave cleaning, bone recovery usually took place and no other symptoms were observed. This discovery stimulated the development of methods of cleaning autoclaves alternative to the manual scraping then generally used, and further research into the toxicity of VCM. The various steps taken to develop non-manual autoclave cleaning have been described fully in Chapter 1, Section 1.6.5, and took the form of cleaning the autoclaves either with high pressure water or with solvents. In more recent years the development of chemical systems of preventing autoclave fouling has further reduced the need for manual cleaning of autoclaves. These changes have been sufficient to overcome the problem of AOL and indeed where these changes have been implemented no case of AOL has been reported.

THE TOXICITY OF VINYL CHLORIDE AND ITS REMOVAL FROM PVC

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In the late 1960s Professor Viola of Solvay1 tried to reproduce AOL in rats by exposing them to 30000ppm of VCM; he failed to produce AOL but found that the rats developed tumours. In the early 1970s work by Maltoni,2 sponsored by ICI, Solvay, Montedison and Rhone-Poulenc, showed that VCM over a wide range of doses induced tumours in rats, especially a particularly rare form of tumour, angiosarcoma of the liver (ASL). Quite clearly, VCM was a proven animal carcinogen. The VCM/PVC industry was alerted to do epidemiology studies to discover whether there were any comparable effects in humans. In early 1974 the discovery of cases of ASL among workers on the B.F.Goodrich PVC plant at Louisville, Kentucky,3 alerted the world to the fact that VCM appeared to be a human carcinogen. Epidemiological studies of workers associated with the manufacture and polymerisation of VCM revealed further cases of ASL world-wide, in the USA and elsewhere.4 The majority of the cases occurred among workers in polymerisation rather than VCM monomer plants, and again especially among autoclave cleaners. These findings confirmed the link between ASL and heavy exposure to VCM. Since then further animal experimentation has been carried out.5 As is the case with most forms of cancer, ASL has a long latency period (20y average), so that cases are still occurring, usually among operators exposed to VCM in some PVC plant in the 1940s, 1950s and 1960s. A total of 84 cases of ASL, world-wide, have so far been confirmed up to mid-1980. Although further cases can be expected there is reason to hope that the steps taken in the late 1960s to prevent AOL among autoclave cleaners will reduce the total number of ASL cases. Following the discovery of the link between ASL and VCM exposure very considerable efforts were made to reduce the exposure to VCM of: (a) VCM and PVC plant operators, (b) PVC fabrication personnel, (c) the public and other plant operators around the VCM and PVC plants, and (d) the public at large. This has involved: (a) large changes in operating procedures, especially for PVC plants, (b) marked developments in the techniques used for the measurement of VCM concentration, and (c) the development of new processes for the removal of unreacted VCM from PVC immediately after polymerisation. 5.2 PROTECTION OF PVC PLANT OPERATORS In the section 5.1 the strong correlation between the cases of ASL and workers on PVC manufacturing plants has been noted. Accordingly in 1974 effort was directed at reducing the exposure to VCM of such plant operators. A gradual tightening of standards from a threshold limit value (TLV) of 500 ppm to 200 ppm in the 10 years prior to 1974 had reduced the number of leaks on a PVC plant, such that the previous industry-wide average of, perhaps, 100–1000 ppm worker exposure to VCM had fallen to the 20–200 ppm region. Further reductions were rapidly achieved by changing operating procedures to avoid ingress into the plant atmosphere and to remove such VCM as did escape. The elimination of normal autoclave cleaning either by using automatic cleaning or by the avoidance of the necessity to clean autoclaves has been referred to earlier, but these changes were rapidly completed after the ASL link was uncovered. Interbatch opening

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of the autoclave is a major source of VCM emission into the plant. Changes of procedure designed either to degas the autoclave contents before opening or to ensure that air from the plant is always drawn into the autoclave when it is open have markedly reduced this source of VCM emission into the plant atmosphere. Some manufacturers have developed processes without interbatch opening which, of course, eliminate this source altogether. Other routine sources of VCM emission have been tackled by attempting to eliminate or reduce markedly the amount of the emission, but where this has not proved feasible controlled venting of the emission through tall stack pipes has greatly reduced operator exposure. VCM discharged in this way is quickly diluted very considerably and rapidly destroyed by the action of air and sunlight. A further source of VCM in the plant air is leaks from the many flanges and valves in the pipework and autoclave used to contain the VCM under pressure. Much attention has been paid to eliminating such leaks by regular inspection for VCM emissions, by improved maintenance and in many cases by replacement with more suitable equipment. Further reductions have been achieved by improved ventilation of the plant either by using forced ventilation (up to 10 air changes per hour) or by opening up the plant to the outside by removing parts of the plant walls. The latter step, of course, depends both on the detailed design of the plant and on the ambient conditions. Finally, residual VCM left in the polymer and, in the suspension polymerisation process, in the waste water, is a further source of VCM. The solution to this problem lies in the more efficient VCM removal by stripping after polymerisation. This is dealt with in detail later in the chapter (Section 5.4), but, where a significant source of residual VCM remains, emissions into the plant are reduced by local ventilation coupled with venting of contaminated air at high level. These changes enabled the PVC manufacturing industry to reduce operator exposure to VCM very considerably and meet the requirements of regulatory bodies in the various countries. For example, the British ‘Code of Practice’, arrived at after agreements between industry, workers’ representatives and Government, called for a maximum time-weighted average (TWA) of l0ppm v/v in the workplace, no excursions beyond 30ppm, and continuing efforts to produce as low concentrations as possible. This was superseded in December 1979 by the EEC Directive 78/610 which requires an annual average of