Thermo-oxidative Degradation of Polymers

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Thermo-oxidative Degradation of Polymers

T.R. Crompton

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-184735-471-6 (hardback) 978-184735-472-3 (softback) 978-184735-473-0 (ebook)

Typeset by Argil Services Printed and bound by Lightning Source Inc.

P

reface

The oxidative and thermal degradation of polymers has very important implications with respect to their suitability for particular end-user applications, particularly in relation to their physical properties and the lifetime over which the manufactured article retains these properties after which they become unsuitable for purpose. By the correct selection of polymer type, the method of manufacture and addition of chemicals that slow down the polymer degradation, manufactured articles can be produced that have suitable processing and end-user physical properties such as tensile strength and avoidance of embrittlement. Particular applications that come to mind are critical applications of polymers to the manufacture of electronic components and the design of polymeric alloys for aircraft fuselage assembly which retain their properties for many years over which the alloy must retain the physical properties at high or low operating temperatures. A particular area of growing importance is the recycling of manufactured components for re-use. Here, the polymer must retain its thermo-oxidative resistance properties over several re-mouldings A wide and increasing range of types of polymers are being used in these critical applications, and an understanding of thermo-oxidative processes will be very important in developing new applications in the future. The object of this book is to bring together available information on the thermooxidative resistance of polymers to change during processing and end-use life. A further object is to review our present understanding of the chemical changes of the polymer that accompany degradation, and to discuss analytical methods by which these changes can be ascertained. The principal techniques used in thermo-oxidative studies are based on thermal analysis methods such as thermogravimetric analysis and differential scanning calorimetry, and on methods based on polymer pyrolysis followed by gas chromatography and mass spectrometry and/or infrared spectroscopy of the volatiles produced. Other techniques

iii

Thermo-oxidative Degradation of Polymers that have found much less application include nuclear magnetic spectroscopy, electron spin resonance spectroscopy, and methods based on chemiluminescence and positron annihilation lifetime mass spectrometry. Details of the more important techniques are discussed in Chapter 1. Chapters 2 to 6 are an organised review of recent work. Various classes of polymers, based for convenience on their elemental composition, i.e., carbon/hydrogen and oxygen, halogen, nitrogen or silicon types of polymers, are discussed. It is hoped that the book will be of interest to those involved in the investigation of polymer stability and studies of the mechanics of polymer degradation, to polymer manufacturers, and the users of polymers to manufacture articles. The book will also be of interest to those involved in the manufacture of stabiliser oxidation resistance for use in polymer manufacture, mechanical engineers, and designers of polymer products. Students engaged in these disciplines will have much to learn about the stability of polymers in critical applications.

Roy Crompton Beaumaris, UK July 2010

iv

C

ontents

1

Methodology of Thermo-oxidative Degradation of Polymers....................................................................................1 1.1

TGA....................................................................................1 1.1.1 Methods Involving Maximisation of Rate................7 1.1.2 Method of Multiple Heating Rates [13–15]...........10

2

1.2

Differential Scanning Calorimetry.....................................14

1.3

Evolved Gas Analysis (EGA)..............................................16

1.4

Pyrolysis-based Techniques................................................16

Carbon-Hydrogen-Type Polymers...............................................21 2.1

Polyethylene......................................................................21 2.1.1 Mechanism of Oxidative Degradation . .................21 2.1.2 Thermogravimetric Analysis (TGA).......................24 2.1.3 DSC.......................................................................28 2.1.4 Infrared (IR) Spectroscopy.....................................28 2.1.5 Pyrolysis–Gas Chromatography–Mass Spectrometry (Py–GC–MS)..........................................................30

2.2

Polypropylene....................................................................33 2.2.1 Mechanism of Oxidative Degradation . .................33 2.2.2 IR Spectroscopy.....................................................35 2.2.3 Differential Scanning Calorimetry..........................37 2.2.3.1 Differential Scanning Calorimetry with Chemiluminescence ................................39

v


2.2.4 DTA.......................................................................42 2.2.5 TGA.......................................................................42 2.2.6 Py–GC . .................................................................42 2.2.7 Positron Annihilation Lifetime Spectroscopy..........44

3

2.3

Rubbers.............................................................................44

2.4

Polystyrene (PS) and Poly(α-methyl styrene)......................45

Oxygen-Containing Polymers......................................................51 3.1

Polyoxymethylene..............................................................51

3.2

Polyphenylene Oxides (PPO).............................................54

3.3

Polyesters...........................................................................54 3.3.1 Polycarbonate........................................................54 3.3.2 Polyethylene Terephthalate (PETP).........................57 3.3.3 Polymethacrylates..................................................58 3.3.4 Styrenated Polyesters..............................................58 3.3.5 Phenol-formaldehyde (PF) Resins...........................62 3.3.6 Epoxy Resins..........................................................68 3.3.6.1 Thermogravimetric Analysis ...................70 3.3.6.2 Differential Scanning Calorimetry...........72 3.3.7 Ethylene Oxide-Propylene Oxide Copolymers........73 3.3.8 Ethylene Vinyl Acetate (EVA) Copolymers.............77 3.3.9 Phenolic Resins......................................................83

4

5

vi

Halogen-Containing Polymers.....................................................87 4.1

Polyvinyl Chloride (PVC)..................................................87

4.2

Chlorinated Natural Rubber..............................................89

4.3

Polytetrafluoroethylene (PTFE)..........................................89

Nitrogen-Containing Polymers....................................................97

Contents

6

5.1

Rigid Polyurethanes...........................................................97

5.2

Polyacrylonitrile..............................................................103

5.3

Polyimides (PI).................................................................105

5.4

Polyamides......................................................................112

5.5

Polycaprolactam..............................................................119

Silicon-Containing Polymers......................................................123

Abbreviations......................................................................................127 Index .................................................................................................131

vii


viii

1

Methodology of Thermo-oxidative Degradation of Polymers

The techniques most commonly used in thermo-oxidative studies on polymers are mainly based on thermal analysis methods such as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and on pyrolysis–gas chromatographic studies (particularly if they are linked to complimentary techniques such as mass spectrometry or infrared spectroscopy). Other techniques that have been used to a much lesser extent include chemiluminescence analysis, nuclear magnetic resonance (NMR) spectroscopy, electron spin resonance, and positron annihilation lifetime mass spectrometry.

1.1 TGA Various workers were involved in the development of the thermogravimetric technique [1–4]. In 1958, Freeman and Carroll [5] developed a widely used method for the determination of reaction kinetics using the thermobalance. During the past few years, much progress has been made in developing more suitable methods for the determination of kinetic parameters, as discussed below. The TGA technique involves continuous weighing of a polymer as it is subjected to a temperature programme. This technique can provide quantitative information about the kinetics of the thermal decomposition of polymeric materials from which the thermal stability can be evaluated. It is used to study the influence of factors such as effect of crystallinity, molecular weight, orientation, tacticity, substitution of hydrogen atoms, grafting, copolymerisation, and addition of stabilisers on polymer degradation. Figure 1.1 shows TGA decomposition profiles for polytetrafluoroethylene (PTFE) and fibre glass-reinforced Nylon.

1


Weight (Wt%)

100.0

(a)

50.0

T1 T2 Onset Base 0.0 300

340

521.43 ºC 623.29 ºC 606.67 ºC 98.549% weight 380

420

460 500 540 Temperature (ºC)

580

620

660

700

Weight (Wt%)

100.0

(b)

50.0 T1 T2 delta γ γ1 γ2 0.0

20

86

355.74 ºC 638.92 ºC 81.386% weight 97.683% weight 16.296% weight 152

218

284 350 416 Temperature (ºC)

482

548

614

Figure 1.1 TGA decomposition profiles for (a) PTFE and (b) fibreglass-reinforced Nylon. Reproduced with permission from E.S. Freeman and B. Carroll, Journal of Physical Chemistry, 1958, 62, 4, 394. ©1958, American Chemical Society [5] 2

Methodology of Thermo-oxidative Degradation of Polymers The lifetime (or ‘shelf-life’) of a polymer can be estimated from kinetic data. Ozawa [6] observed that the activation energy of a thermal event could be determined from a series of thermogravimetric runs carried out at different heating rates. As the heating rate increased, the thermogravimetric changes occurred at higher temperatures. A linear correlation was obtained by plotting the logarithm of the heating rate or scan speed against the reciprocal of the absolute temperature at the same conversion or weight-loss percentage. The slope was directly proportional to the activation energy and known constants. To minimise errors in calculation, approximations were used to calculate the exponential integral. It was assumed that the initial thermogravimetric decomposition curve (2–20% conversion) obeyed first-order kinetics. Rate constants and pre-exponential factors could then be calculated and used to examine relationships between temperature and conversion levels. The thermogravimetric decomposition kinetics can be used to calculate the: 1. Lifetime of a sample at selected temperatures 2. Temperature that will give a selected lifetime 3. Lifetimes at all temperatures at a known percentage conversion Thermograms (percentage weight versus temperature) can be prepared for specimens of polymer obtained at four heating rates (2.5, 5, 10 and 20 °C/min) in a dynamic air atmosphere. From these data, the rate of decomposition of the polymer, the activation energy, and the relationship between the rate constant or half-life and temperature can be calculated. The basic equation for the determination of kinetic parameters for a one-step polymer degradation process is as follows:

dW/dt = Ae– E/RT W^

(1.1)

where: W = Dimensionless mass of a polymer sample subjected to degradation Ae = Pre-exponential (frequency) factor E = Activation energy of the reaction n = Effective reaction order Various methods have been described for determining these parameters.

3

Thermo-oxidative Degradation of Polymers Thus, the method of Freeman and Carroll [5] for a decomposition of the type:

A (solid) → B (solid) + C (gas) may be written:

Rτ = −dW / dT = ( A / RH )e − B / RT W n

(1.2)

Where W is the weight of active material remaining for a particular reaction, and RH is the rate of heating. If Equation 1.1 is applied at two different temperatures and the resulting expressions subtracted from one another (RH = constant), the following is obtained:

Δ log Rτ   or  = nΔ log W − (E / 2.303R )Δ (1/ T ) Δ log Rt 

(1.3)

Where Rt = RH(Rr). From Equation 1.3 it can be seen that values of E and n may be calculated from a single TGA trace. Thus, Δ log Rt should be linear with Δ log W when Δ (1/T) is held constant. The slope of the resulting linear curve will give a value for n whereas the intercept will give a value for E. Figure 1.2 shows a primary thermogram for a sample of polyethylene. In Figure 1.3, the curves obtained using data from Figure 1.2 are shown. Values of Rt and W at equally spaced intervals of 1/T may thus be obtained. From such values, Equation 1.3 may be plotted. Above 35% reaction, it was found that n was essentially unity and that E had a value of 67 ± 5 kcal/mole.

4


Weight loss, mg

0

50

100

300

350

400

450

487

Temperature (ºC)

Figure 1.2 Thermogravimetric curve for the degradation of polyethylene at 1 mm Hg pressure. The sample weight was 100 mg and the heating rate was 5 °C/ min. Reproduced with permission from E.S. Freeman and B. Carroll, Journal of Physical Chemistry, 1958, 62, 4, 394. ©1958, American Chemical Society [5]

In many polymer pyrolyses, the TGA trace follows a relatively simple sigmoidal path. Thus, the sample weight decreases slowly as the reaction begins, then decreases rapidly over a comparatively narrow temperature range, and finally levels off as the reactant becomes spent. The shape of the trace depends primarily upon the kinetic parameters involved, i.e., upon reaction order (n), frequency factor (A), and activation energy (E). The values of these parameters can be of major importance in the elucidation of mechanisms involved in polymer degradation [7, 8] and in the estimation of thermal stability [9].

5


100

8

6

4

50

Weight of reactant (Wr), mg

Reaction rate (dW/dt), mg/min

10

2

0 1.35

1.40

1.45 1.50 103/T(°K) 1

1.55

0

Figure 1.3 Graph of (Δ) the first derivative of the thermogravimetric curve (dW/dt) and (O) the weight reactant (Wr) as a function of reciprocal absolute temperature for the degradation of polyethylene in a vacuum. Reproduced with permission from E.S. Freeman and B. Carroll, Journal of Physical Chemistry, 1958, 62, 4, 394. ©1958, American Chemical Society [5]

Thermogravimetric traces may be more complex than those described above. Thus, if a material degrades by a multistep mechanism which involves rate-controlling steps of similar order, and if the activation energies of the rate-controlling steps are of a similar magnitude, a relatively simple trace may be obtained which provides an overall activation energy for the sample degradation. However, if the values of E of the ratecontrolling steps differ sufficiently, the TGA trace may involve two or more sigmoids, and if the reaction orders for the various rate-controlling steps have values greater than zero, two or more inflection points may be observed. The separate sigmoidal traces may be individually analysed for values of E, n and A by methods similar to those employed for TGA traces which possess one sigmoid. However, if values of E for the rate-controlling steps are not sufficiently different, then one reaction may 6

Methodology of Thermo-oxidative Degradation of Polymers overlap another and analyses of the TGA traces may be difficult (if not impossible). TGA studies give values of overall kinetic parameters which may shed little light on the mechanism involved in any particular pyrolysis. It is therefore often necessary to complement TGA studies with differential thermal analysis and with chromatographic, infrared, and mass spectrographic methods. There are several advantages [10, 11] for using TGA methods rather than isothermal methods in the determination of kinetic parameters: • Considerably less data are required. The temperature dependence of the volatilisation rate may be determined over various temperature ranges from the results of a single experiment, whereas several separate experiments are required for each temperature range if isothermal methods are employed. • The continuous recording of weight loss versus temperature ensures that no features of the pyrolysis kinetics are overlooked. • A single sample is used for the entire TGA trace, thereby avoiding a possible source of variation in the estimation of kinetic parameters. • In the isothermal method, a sample may undergo a premature reaction, and this may make the subsequent kinetic data difficult (if not impossible) to analyse appropriately. Determination of the kinetic parameters of polymer degradation reactions under isothermal conditions yields more precise and accurate results, but this is a labourintensive method, needing much time and many samples. In this connection, dynamic thermogravimetry has been widely employed in recent years for the analysis of polymers and of polymer composites. Despite several substantial disadvantages (lack of reproducibility; difficulty of obtaining a control temperature and rate of heating; sensitivity to the presence of low-molecular-mass admixtures and to the thermal prehistory of the sample; overlapping of certain stages of the process), the method makes it possible not only to obtain the quantitative characteristics of the decomposition process (initial and final temperatures of the process; degree of decomposition as a function of temperature) but also to describe this process with reasonable accuracy in the form of kinetic equations, the parameters of which are calculated from experimental data.

1.1.1 Methods Involving Maximisation of Rate A method for evaluating kinetic parameters that emphasises the position of the inflection point on the primary thermogram has been reported by Reich and co-

7

Thermo-oxidative Degradation of Polymers workers [9] and Fuoss and co-workers [12]. If Equation 1.4 is differentiated with respect to T and the result set equal to zero, after rearranging [12] (Equation 1.5) we obtain:

n = (E / R )(WM / RM TM 2 )

RT = −dW

dt

= (A RH )e

−E

RT

(1.4)

Wn

(1.5)

A = Pre-exponential factor n = Effective order of reaction E = Energy of activation (kcal/mole) R = Raolt Gas constant RH = Rate of heating RM = Slope at inflection point on primary thermogram T = Temperature degrees absolute TM = Temperature at inflection point on the primary thermogram W = Weight active material remaining for a particular reaction - inflection point of primary thermogram WM = weight of active material at inflection point on primary thermogram Differentiation of the above equation with regard to T and the result set to zero, after rearrangement we obtain:

8

(1.6)

Methodology of Thermo-oxidative Degradation of Polymers If Equation 1.2 is converted into a logarithmic expression and Equation 1.4 is substituted into this expression we obtain:

(1.7)

In a plot of log RT versus WM/(TM)2(RM) from Equation 1.7, the slope of the resulting linear relationship RM will give the value of E activation energy, and the intercept will give the value of the pre-exponential factor A (Figure 1.4).

0

–1

–2 –5.5

–5.0 –4.5 [(WM /TM2RM)log W–1/2.303T] × 104

–4.0

Figure 1.4 Log Rt versus (WM/TM2 RM) log W– 1/2.303T × 104 for Teflon at heating rate of 6 °C per min. Source: Author’s own files

9


1.1.2 Method of Multiple Heating Rates [13–15] If the value of the constant heating rate is changed between runs with other conditions being identical, different TGA curves will be obtained. From Equation 1.8 it can readily be shown that:

ln Rt = ln A − E / RT + n ln W

(1.8)

If W is held constant, a plot of ln Rt versus 1/T, employing data from the different TGA curves, should give a linear relationship whose slope will give the value of E and whose intercept the value of A. It may be advisable to carry out a series of such plots at various (constant) values of W (Figure 1.5). In this manner, mean values of E and A may be obtained over a range of conversions. The series of curves obtained can indicate the conversion at which pyrolysis kinetics begin to vary.

3.5 W = 45

W = 50

3.0

W = 60 W = 40

Ln Rt

W = 65

2.5 2.0

W = 55 W = 70

1.5 1.0 1.5

1.6

1.4

1.5

1.6 1.5 10 /T

1.6 1.5

1.6

3

Figure 1.5 Ln Rt versus 1/T for m-phenylenediamine-cured halogenated epoxide at the indicated values of W. Reproduced with permission from D.W. Levi, L. Reich and H.T. Lee, Polymer Engineering and Science, 1965, 5, 3, 135. ©1965, Elsevier [7] 10

Methodology of Thermo-oxidative Degradation of Polymers To evaluate n, we may employ the following expression which applies at ln Rt = 0:

E / RT0 = ln A + n ln W

(1.9)

In this case, a plot of E/RTo versus ln W should give a linear relationship whose slope will be n. Studies have been conducted involving graphical methods based on variable heating rates for a single thermogram [16]. Graphical methods for determining reaction order n have been described by Reich and Levi [17]. Moyer and Lehr [18] have also described a method for evaluating the activation energy E. Results obtained from a static test on a phenolic resin in air were compared with thermobalance results (Table 1.1). To establish a criterion for evaluating resin decomposition, the temperatures at which 10% decomposition [10% decomposition temperature (DT)] and 50% decomposition (50% DT) had occurred were noted. Temperatures were also recorded at which maximum rates of decomposition occurred. From Table 1.1, it can be seen that, based upon resin types 1, 2, 3, 4 and 7, the thermal stability of the resins decreases with increasing molecular weight of the meta-substituted phenol, i.e., stability decreases in the order phenol > m-cresol > m-isopropylphenol > cardanol > m-tert-butylphenol. The anomalous position of the m-tert-butylphenol indicates that branching of the side chain has a significant effect, particularly if branching occurs from the α-alkyl carbon atom which is attached to the phenolic nucleus. Another approach to the measurement of the isothermal life of a polymer is the use of the equation:

log ti = − E / 2.3RT + constant

(1.10)

where ti is the isothermal ageing time, and T is the absolute TGA temperature corresponding to the equivalent ageing time, ti. When based upon isothermal experiments and TGA curves for Teflon in a nitrogen atmosphere, a value of E of 67 kcal/mole was obtained.

11


Table 1.1 Test results for evaluating the thermal stability of different resins Thermobalance results Static test

Weight loss curve maxima

Type Resin

Loss at 10% 300 °C DT after 1 (°C) h (%)

50% DT (°C)

Temperature Value (°C) (mg/min)

1

7.0

830

415

≥0.7

530

≥1.4

Phenol formaldehyde

430

2

m-Cresol formaldehyde

9.5

395

650

430

≥3.0

3

m-Isopropylphenol formaldehyde

12.7

360

610

450

≥2.4

4

m-tert-Butylphenol formaldehyde

26.4

335

490

355

≥3.2

5

p-tert-Octylphenol formaldehyde

7.15*

360

490

470

≥3.5

6

p-Dodecylphenol formaldehyde

19.4*

345

460

440

≥5.4

7

Cardanol formaldehyde 19.6

340

485

445

≥5.4

8

p-Octadecylphenol formaldehyde

22.05* 340

480

470

≥4.5

9

Phenol benzaldehyde

11.4

360

740

Broad peak at about 550 °C (1.2 mg/min)

10

Phenol furfural

14.2

320

640

Broad peak at about 410 °C (1.4 mg/min)

11

m-Isopropylphenol furfural

35.0*

290

520

300

≥1.6

410

≥1.7

12

m-tert-Butylphenol furfural

27.0*

270

440

330

≥2.6

13

p-tert-Octylphenol furfural

36.0*

315

490

390

≥2.0

12

485


14

p-Dodecylphenol furfural

28.75* 350

510

475 (Broad)

≥2.9

15

Cardanol furfural

22.2

340

485

445

≥5.0

16

Phenol formaldehyde (stearic acid modified)

13.0

375

710

455

≥2.0

17

Phenol formaldehyde (oleic acid modified)

13.1

390

665

455

≥1.8

18

Phenol formaldehyde (linseed fatty acids modified)

11.9

380

655

455

≥2.4

* = Resins in which the skin effect was observed or in which the caking effect occurred. Reproduced with permission from P.H. Moyer and M.H. Lehr, Journal of Polymer Science, Part B: Polymer Physics Edition, 1965, 3, 1, 231. ©1965, John Wiley and Sons [18]

An expression similar to Equation 1.10 is based on the isothermal rate expression:

dW

∫W

n

= kti

(1.11)

From this, it is possible to obtain for any value of n:

log (ti / T 2 ) = − E / 2.3RT + log

AR (RH ) Ek

(1.12)

This equation has been plotted for Teflon degradation. From Equation 1.12, it can be seen that, from a single TGA trace and from a single isothermal degradation experiment, the value of E may be readily obtained irrespective of reaction order. Equation 1.12 has been plotted for Teflon degradation in a vacuum using an isothermal temperature of 494 °C. From the slope of the linear 13

Thermo-oxidative Degradation of Polymers relationship obtained, a value of E of 74 kcal/mole was obtained. Once the value of E has been obtained, the isothermal stability of the material in question may be quantitatively estimated for other temperatures (other than 494 °C). Thus, plots for other temperatures should involve a series of parallel lines whose vertical separation, Δ may be ascertained from the expression:

Δ = (E / 2.3R )(1/ Ti1 − 1/ Ti )

(1.13)

where Ti´ and Ti are the particular isothermal temperatures involved. Thizon and co-workers [19] studied the effect of low concentrations of oxygen and water in nitrogen on the rate of thermal degradation of ethylene–propylene copolymer. Using TGA, they obtained estimates of the rate and change of polymer weight and time (dm/dt). Hence they also obtained the activation energy at a range of polymer temperatures (240–350 °C), oxygen contents in the head space gas (0–90 ppm) and water contents in the space gas (0–600 ppm):

dm  E  = K exp  dt  RT 

(1.14)

It is known that oxygen reacts with hydrocarbon polymers even at moderate temperatures [20]. The resulting peroxides or hydroperoxides which are thermally unstable induce chain degradation through reactions involving the RO2 radical. Thus, oxygen contamination of the head space gas in a thermogravimetric experiment is likely. The oxygen effect is severe, particularly at low temperatures. The degradation rate can be multiplied by a factor as large as six for an oxygen concentration of 90 ppm at 240 °C, and it is multiplied by 1.2 at 280 °C and by 2 at 240 °C for a concentration of 20 ppm. The water content of the carrier gas is not as critical (at least for ethylene– propylene copolymers) and does not appreciably affect thermal degradation.

1.2 Differential Scanning Calorimetry DSC measures the amount of energy adsorbed or released by a sample as it is heated, cooled, or held at a constant temperature. DSC is therefore useful in the studying energy

14

Methodology of Thermo-oxidative Degradation of Polymers changes accompanying the thermal oxidative degradation of polymers. The theory of the technique has been discussed by McNaughton and Mortimer [21]. DSC measures the temperature and the heat flow associated with chemical changes or transitions in material as a function of time and temperature, i.e., heat-flow-temperature plots. Such measurements provide qualitative and quantitative information about physical or chemical changes that involve exothermic or endothermic processes or changes in heat capacity. Such measurements can provide information on thermal stability and oxidative stability, as well as reaction kinetics or polymer curing reactions. An alternative technique is pressure differential scanning calorimetry (PDSC). This technique measures heat flow and temperatures of transitions as a function of temperature, time, and pressure (elevated pressure or vacuum). The ability to vary pressure from 1.3 Pa to 6.8 MPa makes PDSC ideal for the determination of the stability of oxidative polymers and for other pressure-sensitive reactions. High pressures of oxygen cause polymers to oxidise much faster than they would at atmospheric pressure. The oxidation process is observed as an exothermic peak. Thus, the oxidative stability of a polymer can be measured as the time to the onset of oxidation at a predetermined temperature and pressure. DuPont supply a dual sample pressure differential calorimetry cell. A DSC heating experiment in air on an impact-modified polypropylene sample shows the melting of two components followed by oxidative decomposition at 237.7 °C. When tested by TGA in nitrogen, the same material shows thermal degradation starting near 300 °C. However, an expanded view of the top 5% weight loss showed a component (0.66%) of the sample was lost between 126 °C and 184 °C at ambient pressure. The identity of the component was unknown, but the mere fact that the weight loss occurred makes oxidative stability results suspicious if they are generated at >125 °C and at ambient pressure. The advantages of PDSC over DSC and oven ageing for determination of oxidative stability have been identified, as listed next: • At an oxygen pressure of 4.1 MPa and a specimen size of 4–5 mg, the maximum oxidation rate is maintained throughout the reaction, and results are not dependent upon oxygen concentration or diffusion rate. • Temperature and pressure can be precisely controlled with commercial thermal analysis equipment. This eliminates errors caused by temperature variations commonly encountered in oven testing.

15

Thermo-oxidative Degradation of Polymers • PDSC oxidation exotherms are sharper and better defined than the usual differential scanning calorimetric tests at atmospheric pressure, making extrapolation of the onset of autoxidation easier and more precise. • The test temperature may be varied to permit analysis of samples in the molten, semi-molten, or non-molten state, or to permit changing the duration of the test. Conversely, studies may be conducted at relatively low temperatures if a longer test period can be tolerated. • Because high oxygen pressures accelerate autoxidation, the test can be carried out at sufficiently low temperatures to minimise volatilisation of stabilisers.

1.3 Evolved Gas Analysis (EGA) Even though in many instances measures of heat changes, phase changes, pressure changes and weight changes are very meaningful, they are limited in the information they can provide. TGA, for example, measures the weight of a sample as a function of temperature and time as the temperature is varied. This gives valuable quantitative information but tells us nothing about the chemistry of the process. To bridge this gap, EGA has developed in which monitoring the by-products of reactions associated with heat is possible. In EGA, the sample is heated at a controlled rate under controlled conditions and the weight changes monitored (i.e., TGA). Reaction products are simultaneously led into a suitable instrument for identification and, in some cases, quantification. Many variants of this approach have been developed based on three methods for thermally breaking down samples: pyrolysis, linear-programmed thermal degradation (i.e., without recording weight change), and the thermogravimetric approach (i.e., continuously recording of sample weight). Schole and co-workers [22] reported on polymer characterisation using an oxidative degradation method. In this method, the oxidation products of the polymers are produced in a short pre-column maintained at 100–600 °C just ahead of the separation column in a gas chromatograph. The oxidation products are swept on to the separation column and detected in the normal manner.

1.4 Pyrolysis-based Techniques Pyrolysis is the breaking of large, complex molecules into smaller fragments by the application of heat. If the heat energy applied to a molecule is greater than the energy of specific bonds in that molecule, these bonds will dissociate in a predictable, 16

Methodology of Thermo-oxidative Degradation of Polymers reproducible way. The smaller molecules generated in this bond-breaking process can be identified by several analytical tools, including gas chromatography and mass spectrometry. Once identified, they help in understanding the structure of the original macromolecule, as well as the effect of oxidation and heat on the original structure of the polymer. The time has long since passed when one could rely on gas chromatographic data alone to identify unknown compounds in gas chromatograms. The sheer number of compounds that could be present would invalidate reliance on the use of such techniques. The practice nowadays is to link a mass spectrometer (or NMR or Fourier-transform infrared spectrometers) to the outlet of the gas chromotograph so that a mass spectrum is obtained for each chromatographic peak as it emerges from the separation column. If the peak contains a simple substance, then computerised library searching facilities attached to the mass spectrometer will rapidly identify the substance. If the emerging peak contains several substances, then the mass spectrum will provide information on the substances present. Alternatively, different gas chromatographic columns can be tried that will resolve the mixture of substances in question. A good pyrolysis instrument must be able to heat a sample reproducibly to a preset temperature at a known rate for a specific amount of time. Inability to control any of these variables will result in a pyrogram that cannot be reproduced. Linear-programmed thermal degradation mass spectrometry has been used to monitor the thermal degradation of polymers and their oxidation products. Whereas pyrolysis–mass spectrometry and pyrolysis–gas chromatography involve a single mass spectrum or gas chromatogram obtained after rapid heating of the sample to a fixed temperature, linear-programmed thermal degradation mass spectrometry is based on a collection of sequential mass spectra during programmed heating of the sample. This enables the detection of subtle differences in polymer structure caused by heat and/or oxygen which would not be apparent with data from pyrolysis–mass spectrometry or pyrolysis–gas chromatography alone.

References 1.

C. Duval, Inorganic Thermogravimetric Analysis, 1st Edition, Elsevier, New York, NY, USA, 1953.

2.

A.E. Newkirk and E.L. Simons, Report No 63-RL-3498C, General Electric, 1963.

17

Thermo-oxidative Degradation of Polymers 3.

H. Saito, Bulletin of the Imperial Academy of Japan, 1926, 2, 58.

4.

D.W. van Krevelen, C. van Heerden and F.J. Huntjens, Fuel, 1951, 30, 253.

5.

E.S. Freeman and B. Carroll, Journal of Physical Chemistry, 1958, 62, 4, 394.

6.

T. Ozawa, Bulletin of the Chemical Society of Japan, 1965, 38, 11, 1881.

7.

D.W. Levi, L. Reich and H.T. Lee, Polymer Engineering and Science, 1965, 5, 3, 135.

8.

H.L. Friedman, Office of Technical Services PB Report No.145, US Department of Commerce, Washington, DC, USA, 1959, p.182.

9.

L. Reich and D.W. Levi, Die Makromolekulare Chemie, 1963, 66, 102.

10. A.W. Coats and J.P. Redfern, Analyst, 1963, 88, 1053, 906. 11. D.A. Smith, Rubber Chemistry and Technology, 1964, 37, 4, 937. 12. R.M. Fuoss, I.O. Salyer and H.S. Wilson, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1964, 2, 7, 3147. 13. H.L. Friedman, Journal of Polymer Science, Part C: Polymer Symposia, 1964, 6, 183. 14. H.C. Anderson, Polymer Preprints, 1963, 4, 2, 655. 15. H.C. Anderson, Journal of Polymer Science, Part B: Polymer Physics Edition, 1964, 2, 115. 16. L. Reich, H.T. Lee and D.W. Levi, Journal of Applied Polymer Science, 1965, 9, 1, 351. 17. L. Reich and D.W. Levi, Journal of Polymer Science, Part B: Polymer Physics Edition, 1964, 2, 1109. 18. P.H. Moyer and M.H. Lehr, Journal of Polymer Science, Part B: Polymer Physics Edition, 1965, 3, 1, 231. 19. M. Thizon, C. Eon, P. Valentin and G. Guiochon, Analytical Chemistry, 1976, 48, 13, 1861. 20. P.M. Norling and A. Taboesky in Thermal Stability of Polymers, Volume 1, Ed., R.T. Conley, Marcel Dekker, New York, NY, USA, 1970, Chapter 5.

18

Methodology of Thermo-oxidative Degradation of Polymers 21. J.L. McNaughton and C.T. Mortimer, Differential Scanning Calorimetry, Perkin Elmer Corporation, Norwalk, CT, USA, 1975. 22. R.G. Scholz, J. Bednarczyk and T. Yamauchi, Analytical Chemistry, 1966, 38, 2, 331.

19


20

2

Carbon-Hydrogen-Type Polymers

2.1 Polyethylene 2.1.1 Mechanism of Oxidative Degradation The thermal stability of polyethylene (PE) decreases sharply in the presence of oxygen. This effect is clearly observed at elevated temperatures, which promote rapid development of the oxidative processes that essentially decrease the mechanical properties of PE. Thus, low-pressure PE (0.93 g/cm3) completely loses its mechanical strength after exposure at 100 °C for 48 hours in air; the impact viscosity of such a material is only 7% of its initial value [1]. The oxidation process is complex not only from the chemical, but also from the physicochemical viewpoint. This is due to the uneven course of the process in the bulk of the PE. It has been shown that the amorphous zones of PE, being more accessible to oxygen molecules than the densely packed crystalline regions, are oxidised first. The amorphous zones of PE act as a buffer which protects the crystalline regions from attack during thermal degradation. Thus, the increase in crystallinity results in an increase in sensitivity of the mechanical properties of ethylene to oxidation (though its oxidation rate is lower) [2]. Most polymers (including polyalkenes) are oxidised at high temperatures by autoacceleration. Autoxidation is characterised by an induction period during which a noticeable transformation of the polymer is not observed. After the induction period, the oxidation rate increases sharply and may reach large values for a short time. Thus, for high-pressure PE powder, the induction period at 140 °C is 5 hours (Figure 2.1) [3]. During the induction period, the temperature of the polymer gradually rises, and on its termination the temperature rises sharply. In the same period, the rate of oxygen uptake by the polymer increases greatly.

21


T (°C)

[O2](dm3 kg)

200

200

180

150

160

100

140

50

0

7.2 14.4 21.6

0

7.2 14.4 21.6 t (103 s)

Figure 2.1 Kinetics of changes in the temperature and oxygen consumption by a polyethylene sample during thermal oxidation (thermostatted at 140 ºC). Source: Author’s own files

Studies on the mechanism of polyalkene oxidation have shown that hydroperoxides are its primary products, which then decompose to form other oxygen-containing compounds. The oxidation of polyalkenes is described by a radical-chain scheme as follows. Initiation may occur with the formation of radicals R or RO2·:

R R

R

R

R

O2

RO2

(2.1)

In the case of the developed oxidation process, initiation occurs owing to the decomposition of hydroperoxides with the formation of radicals by monomolecular or bimolecular reactions [4]:

22

ROOH

RO

OH

2ROOH

RO

RO2

(2.2)

H2O

(2.3)

Carbon-Hydrogen-Type Polymers The radicals formed attack the C–H groups of polyalkenes to produce hydroperoxides:

RO2

RH

ROOH

R

(2.4)

which, on decomposition, initiate new kinetic oxidation chains. Hydroperoxides, being thermally unstable primary products of the chain oxidation of polyalkenes, in addition to decomposing into radicals, may also decompose via a molecular mechanism to form a stable oxygen-containing polymer and low-molecular-mass compounds. Termination of the kinetic chain of oxidative degradation of polyalkenes is attributable to the reactions:

2R R

R R RO2

2RO2

(2.5)

ROOR ROOR

(2.6)

O2

(2.7)

For ideal PE molecules (e.g., polymethylene, see Chapter 3), the susceptibility to attack by oxygen is the same for all methylene groups, whereas double bonds are particularly vulnerable to oxygen via addition:

∼ CH = CH − CH2 ∼ + O2

∼ CH − CH − CH2 ∼ O2

(2.8)

However, the predominant oxidation of methylene groups in the β-position to double bonds may occur [2]:

∼ CH = CHCH2 ∼ + O2

∼ CH = CH − CH ∼ + HO2

(2.9)

The biradical formed via Equation 2.8 may be completely or partially deactivated:

23


∼ CH − CH − CH2 ∼ + ∼ CH = CH − CH2 ∼ OO ∼ CH − CH − CH2 ∼ + ∼ CH = CH − CH ∼ OOH

(2.10)

(2.11)

∼ CH − CH − CH2 ∼ + ∼ CH = CH − CH2 ∼ OHO ∼ CH − CH2 − CH2 ∼ + ∼ CH = CH − CH ∼ OOH

The numerous schemes suggested for this type of hydroperoxide decomposition are not always well substantiated. The chemical structure of polyalkene, as well as the process conditions (presence of admixtures, additives, temperature) play a decisive part in realising one or other mechanisms of hydroperoxide decomposition. Reactions with the participation of hydroperoxide groups and radicals cause cleavage of the molecular chain, i.e., polymer degradation. Additionally, structural changes associated with the branching, crosslinking and cyclisation of macromolecules take place on autoxidation of polyalkenes. However, these processes are typical only of polymers with double bonds, whereas saturated macrochains (particularly those containing side-groups) are subject to degradation.

2.1.2 Thermogravimetric Analysis (TGA) The TGA curves for two samples of non-purified PE and purified PE in air are shown in Figure 2.3. These curves are different from those obtained in a nitrogen atmosphere (compare with Figure 2.2) started at about 200 °C whereas that for non-purified PE started at about 250 °C. Also, from Figure 2.3, it appeared that the oxidative degradation occurred in three stages: the first, up to about 350 °C; the second, from 350 °C to about 390 °C; and the third, from 390 °C to 455 °C. If the curve for the purified sample is compared with that for the non-purified, the weight loss for the first stage for the non-purified material was smaller than that for the purified material. This result can be attributed to the presence of antioxidant in the latter sample. Also,

24

Carbon-Hydrogen-Type Polymers in the lower temperature range, the weight loss of the PE is attributed to degradation at the branching point in the polymer molecule. Thus, it would be expected that the less-branched non-purified PE (in Figure 2.3) would exhibit a smaller weight loss in this temperature range. Isothermal studies which involve oxygen absorption techniques tend to support this explanation. Willbourn [5] found that the oxidation rate of polymethylenes is dependent upon the degree of branching and upon the amorphous content. As the degree of branching increases, so does the oxidation rate. The differences in the rate of weight losses (Figure 2.3) at other stages may also be due (at least in part) to changes in PE morphology during oxidation [6].

4.0

Weight loss, %/ºC

3.0

2.0

1.0

0 350

400 Temperature, ºC

450

Figure 2.2 Thermogravimetric analysis curves for polyethylenes in a nitrogen atmosphere ( ) purified polyethylene and ( ) unpurified polyethylene. Source: Author’s own files 25


2.0

Weight loss, %/ºC

1.5

1.0

0.5

0

200

250

300 350 400 Temperature, ºC

450

Figure 2.3 Thermogravimetric analysis curves for polyethylenes in air: ( ) unpurified polyethylene, ( ) purified polyethylene. Source: Author’s own files

26

Polyethylene film Rate: 10.0 ºC/min

DSC METTLER 21-Jun-84 Fisons Instruments, Crawley

Peak temperature = 129.9 ºC

50

100 Polypopylene film Rate: 10.0 ºC/min

Onset = 231.0 ºC

150

200

File: 00002.001 Identi: 0.0

exo>

A2, PP 6.880 mg

ºC

DSC 04-Sep-85 Fisons Instruments, Crawley

Oxidation stability

Peak temperature = 164.1 ºC 50

100

Onset = 238.2 ºC 150

200

ºC

Figure 2.4 Differential scanning calorimetry (DSC) curve obtained during oxidation breakdown of polyethylene and polypropylene. Source: Author’s own files 27


10

mW

File: 00001.001 Identi: 5.0

exo>

20

mW

PE 2.360 mg


2.1.3 DSC Ding and co-workers [7] found that the commonly used oxidative stability measurement of oxidative induction time (OIT) at high temperatures, typically employing molten polymers, grossly overestimates stability at lower temperatures in the solid state. The continuity and suitability of OIT data at lower temperatures and under widely different experimental parameters were examined for high-density polyethylenes (HDPE) and polypropylene (PP) of various formulations. Upon raising the oxygen pressure from about 0.02 MPa to 4 MPa, for example, an approximately 20-fold acceleration was observed. For a hindered phenol-stabilised PP, a normal 1/OIT versus the square root of oxygen pressure was observed, whereas a hindered amine-stabilised PP showed marked departure from this behaviour at lower pressures. For HDPE, a marked loss in OIT was observed when airflow rate through the DSC cell was reduced, along with a reduction in apparent activation energy. These effects were interpreted from experimental parameters (if possible) with polymer degradation mechanisms. A sample is tested in flowing air or oxygen to examine oxidative breakdown. Oxidation is highly exothermic, so the onset of oxidation is clearly visible on a DSC trace. The time to the extrapolated onset of the exothermic reaction under isothermal conditions is called the ‘induction period’. Dynamic determination of oxidation stability (e.g., heating the sample at 10 °C/min) is much quicker than the isothermal procedure, and permits simultaneous measurement of the glass transition temperature or melting temperature. This type of analysis is shown in Figure 2.4 by the comparative performance between films of PE and PP. As with the isothermal induction period, the resulting onset temperature is a measure of polymer stability. Woo and co-workers [8] also applied DSC to the determination of the OIT of PE stabilised with a hindered phenol antioxidant and a phosphite synergist.

2.1.4 Infrared (IR) Spectroscopy Extensive IR studies have been carried out on oxidised PE [9–15]. IR absorption spectroscopy has been widely used to determine the oxidation products and the rate of formation of these products during the thermal or photo-oxidation of PE [11, 16, 17]. Acids, ketones, and aldehydes (the end products reported from these oxidations) have similar spectra in the region 5.50–6.00 µm. It is only in this carbonyl-stretching region that the products have suitable absorptivity to give quantitative data. The absorption band of the acid (5.84 µm), ketone (5.81 µm), and aldehyde (5.77 µm) groups present in oxidised PE are so overlapped that they only give a broad band on relatively low-resolution spectrometers. Interpretation of these data based on the increase in the total number of carbonyl groups rather than on a single chemical

28

Carbon-Hydrogen-Type Polymers moiety could lead to incorrect conclusions because of the large differences in the absorptivity of the various oxidation products. Acid absorptivity has been reported to be 2.4 times greater than that of ketones and 3.1 times greater than that of aldehydes. Rugg and co-workers [10], using a grating spectrometer for increased resolution, demonstrated that the carbonyl groups formed by heat oxidation are mainly ketonic, whereas in highly photo-oxidised PE, the amounts of aldehyde, ketone, and acid are approximately equal. This procedure is adequate for qualitative, but not suitable for accurate quantitative, data. An IR study of oxidative crystallisation of PE was made from examination of the 5.28 µm ‘crystallinity’ band and 7.67 µm ‘amorphous’ band and carbonyl absorption at 5.83 µm [18]. Miller and co-workers [19] used Fourier-transform infrared spectroscopy (FTIR) to study the effect of irradiation on PE. The number of aldehydic carbonyl and vinyl groups decreased and the number of ketonic carbonyl and trans-vinylene double bonds increased on irradiation. IR reflection was used in studies of the oxidation of PE at a copper surface in the presence and absence of the inhibitor N,N-diphenyl-oxamide [20]. Cooper and Prober [15] used alcoholic sodium hydroxide to convert the acid groups to sodium carboxylate (6.40 µm) to analyse PE oxidised with a corona discharge in the presence of oxygen and ozone. This procedure requires five days, and has been found to extract the low-molecular weight acids from the film. Heacock [21] described a method for the determination of carboxyl groups in oxidised polyolefins without interference by carbonyl groups. This procedure is based upon the relative reactivities of the various carbonyl groups present in oxidised PE film to sulfur tetrafluoride gas:

~ CO ~ + SF4 = SOF2 + ~ CF2 ~

(2.12)

The quantity of the carboxyl groups in the film is then measured as a function of the absorption at 5.45 µm. IR spectroscopic study of the thermal oxidation of PE under different temperature regimens established [22] that mainly ketone and aldehyde groups are formed in the oxidised PE. Ester and anhydride groups are not present in the polymer. Absorption bands typical of hydroperoxide groups are found in the spectra of oxidised PE [22, 23]. When the PE melt is heated in air for 40 min, the content of these groups increase and then becomes constant. This result is probably due to establishment of

29

Thermo-oxidative Degradation of Polymers a dynamic equilibrium between the formation and decomposition of hydroperoxide groups.

2.1.5 Pyrolysis–Gas Chromatography–Mass Spectrometry (Py–GC–MS) Westphal and co-workers [24] subjected low-density polyethylene (LDPE) to photooxidation and thermo-oxidation in a weatherometer oven at 60 °C and 100 °C. Parallel FTIR spectroscopy was used to monitor functional groups resulting from the degradation studies and the data obtained compared with data obtained by size exclusion chromatography, TGA and Py–GC–MS. Pyrolysis–gas chromatography (Py–GC) of LDPE gives details of its microstructure, and is a versatile tool to show the extent of degradation. Two LDPE were investigated: one was LDPE with a masterbatch consisting of LDPE, corn starch (7.7%), and prooxidant (SBS-1 manganese stearate) (LDPE-MB); the other was 30 µm films of pure LDPE with pro-oxidant (LDPE-PO). TGA showed that the LDPE samples all degraded around 480 °C. For the LDPE stabilised with corn starch and pro-oxidant (LDPE-MB) polymer a second shoulder appeared around 318–327 °C. The mechanism for photo-oxidation and thermooxidation starts with the formation of free radicals on the main chain, with subsequent formation of peroxy radicals. Later, intra- and inter-molecular reactions lead to the formation of hydroperoxides. Figure 2.5 shows the FTIR spectra of thermo-oxidised LDPE-MB and LDPE-PO. Ketone (5.84 µm), aldehyde (5.76 µm) and ester groups (5.63 µm) are found in the carbonyl region. Thermo-oxidised samples at 100 °C contain the largest number of carboxylic groups. Figure 2.6 shows the pyrograms of degraded LDPE samples; a series of triplets from C6 to C30 was identified. The peak maxima lies at C10, C14 and C18 for all samples, except thermo-oxidised LDPE. The highest peak for these samples instead lies at C9 and all the other peaks have very low intensities. A change in peak maxima from C10 to C9 occurs depending on the degree of oxidation of the LDPE. Tsuchiya and Sumi proposed that the major hydrogen-abstraction reaction is due to an intramolecular cyclisation [25]. Following an initial radical formation at C1, successive intramolecularhydrogen abstraction along the chain results in the formation of new radicals at C5, C9 and C13.

30

Carbon-Hydrogen-Type Polymers 100 90 LDPE MB

80 70

1 2 3

60 50 40 30 20 10

1900

1800

1700

1600

1500

1400

0 1300

wavelength (cm 1) 100 LDPE PO 1 2 3

90 80 70 60 50 40 30 20 10

1900

1800

1700

1600

1500

1400

0 1300

wavelength (cm 1)

Figure 2.5 Transmission FTIR spectra of thermo-oxidised LDPE-MB and LDPEPO: (1) Virgin; (2) 60 °C; (3) 100 °C. Reproduced with permission from C. Westphal and C. Perrot, S. Karlsson, Polymer Degradation and Stability, 2001, 73, 2, 281. ©2001, Elsevier [24]

31


(a)

LDPE-Starch 100

LDPE MB 100

C4

C10

C14 C14

0

0

5

10

C18

C11

0

15 20 Retention time (minutes)

5

10


LDPE-PO 100 C10

C14

0 (b)

5

C18

10 15 20 Retention time (minutes)

LDPE MB 100

C4

LDPE-MB Photo-oxidised C10

C14 C18 C14

0

5

10

C11


0

5


20

Figure 2.6 (a) Pyrograms of LDPE-MB, LDPE-PO and LDPE-starch thermooxidised at 100 °C; (b) pyrograms of photo-oxidised and thermo-oxidised (100 °C) LDPE-MB. Reproduced with permission from C. Westphal and C. Perrot, S. Karlsson, Polymer Degradation and Stability, 2001, 73, 2, 281. ©2001, Elsevier [24]

32


2.2 Polypropylene 2.2.1 Mechanism of Oxidative Degradation The oxidation of PP in the presence of oxygen occurs more rapidly than that of PE. On oxidation, PE becomes fragile, and brief heating of PP film at 100 °C leads to its complete degradation [1]. Polyisoprene behaves in the same way. The presence of weak carbon–carbon bonds at the tertiary and quaternary carbon atoms promotes oxidation of these polymers to yield hydroperoxide groups [26]:

∼ CH2 − CH(CH3) ∼

OOH

O2

∼ CH2 − C(CH3) ∼

(2.13)

as well as the oxidative decomposition by Equations 2.13–2.25. A trimolecular reaction of PP with oxygen leading to primary initiation of the kinetic chains of oxidation has been proposed [27]. Here the influence of oxygen is shown not only on the decomposition process but also on the crosslinking of the PP macromolecules. The oxidation of PP is described by a radical chain scheme as follows. Initiation may occur with the formation of radicals R· or RO2·:

R R

R +R

R + O2

RO2

(2.14)

but, in the case of the developed oxidation process, initiation occurs owing to the decomposition of hydroperoxides with the formation of radicals by monomolecular or bimolecular reactions (Equation 2.14):

ROOH

RO

OH

2ROOH

RO

RO2

(2.15)

H2O

(2.16)

The radicals formed attack the C–H groups of polyalkenes to produce hydroperoxides:

RO2 + RH

ROOH

R

(2.17)

33

Thermo-oxidative Degradation of Polymers which, on decomposing, initiate new kinetic oxidation chains. Hydroperoxides, being thermally unstable primary products of the chain oxidation of polyalkenes, in addition to decomposing into radicals, may also decompose via a molecular mechanism to form the stable oxygen-containing polymer and low-molecular-mass compounds. The termination of the kinetic chain of oxidative degradation of polyalkenes is attributable to the reactions:

2R R

R R RO2

2RO2

(2.18)

ROOR

(2.19)

ROOR O2

(2.20)

For ideal PE molecules (e.g., polymethylene) the susceptibility to attack by oxygen is the same for all methylene groups, whereas double bonds are particularly vulnerable to oxygen via addition:

∼ CH = CH − CH2 ∼ + O2

∼ CH − CH − CH2 ∼ O2

(2.21)

However, the predominant oxidation of methylene groups in the β-position to double bonds may occur [2]:

∼ CH = CHCH2 ∼ + O2

∼ CH = CH − CH ∼ + HO2

(2.22)

The biradical formed via Equation 2.20 may be completely or partially deactivated:

∼ CH − CH − CH2 ∼ + ∼ CH = CH − CH2 ∼ OO ∼ CH − CH − CH2 ∼ + ∼ CH = CH − CH ∼ OOH

34

(2.23)


∼ CH − CH − CH2 ∼ + ∼ CH = CH − CH2 ∼ OHO ∼ CH − CH2 − CH2 ∼ + ∼ CH = CH − CH ∼ OOH

(2.24)

The numerous schemes suggested for this type of hydroperoxide decomposition are not always well substantiated. The chemical structure of the polyalkene, as well as the process conditions (presence of admixtures, additives, temperature) play a decisive part in realising one or other mechanism of hydroperoxides decomposition. Reactions with participation of hydroperoxide groups and radicals cause cleavage of the molecular chain, i.e., polymer degradation. Additionally, structural changes associated with branching, crosslinking and cyclisation of macromolecules take place on autoxidation of polyalkenes. However, these processes are typical only of polymers with double bonds, whereas saturated macrochains (particularly containing side-groups) are subject to degradation.

2.2.2 IR Spectroscopy The appearance in the spectra of irradiated (gamma rays from 60Co) PP specimens, after storage in air, of a strong band in the region of 5.85 µm (corresponding to carbonyl groups) can be explained by the reaction of oxygen with the long-lived allyl radical, with the formation of peroxide radicals which form carbonyl groups by decomposition [28]:

CH3 ∼ CH2

C OO

CH3 CH

C∼

(2.25)

The intensity of the 5.85 µm band of PP (and consequently the degree of oxidation) increases sharply with time of storage of specimens in air. Irradiated amorphous specimens oxidise to a considerably lesser extent than isotactic polypropylene (iPP) specimens. The degree of oxidation of specimens at -196 °C increases more rapidly than if the specimens are irradiated at 25 °C. All these facts indicate that the lifetime

35

Thermo-oxidative Degradation of Polymers of the allyl radicals is longer in crystalline PP, and that the concentration of these radicals is higher in specimens irradiated at low temperature. The free radicals are destroyed only after heat treatment of the specimens in an inert atmosphere at 150 °C. After this heat treatment, the intensity of the 5.85 µm band ceases to increase on storage, i.e., no further oxidation occurs. Adams and Goodrich [29] compared, on a qualitative basis, the non-volatile oxidation products obtained by photo- and thermo-oxidation of PP. They used IR spectroscopy and chemical reactions. The major functional group obtained by photodecomposition is followed by vinyl alkene, then acid. In comparison, thermally oxidised PP contains relatively more aldehyde, ketone, and γ-lactone, and much less ester and vinyl alkene. Photo-degraded PE contains mostly vinyl alkene followed by carboxylic acid. Gel permeation chromatography determined the decrease in PP molecular weights with exposure time. Adams and Goodrich [29] determined that, in photochemical oxidation, there is one functional group formed per chain scission; in thermal oxidation there are two groups formed per scission. Adams and Goodrich [29] made the comments given next regarding the IR spectrum of oxidised PP. Hydroxyl region - The hydroxyl absorption in the IR spectrum of PP has a broad band centred at 2.90 µm (associated alcohols) with a definite shoulder at 2.77 µm (non-associated alcohols). At a similar extent of degradation, thermally oxidised polyolefins show hydroxyl bands of approximately half the absorbance values of the photo-oxidised polyolefins. Thus, thermal oxidation produces about half as many hydroxyl groups as photo-oxidation in polyolefins. A portion of PP hydroxyl absorption could be due to hydroperoxides. If so, then an exposed sheet, with the volatiles removed, heated in a nitrogen atmosphere for two days at 140 °C, should show a decrease in the hydroxyl IR band and an increase in the carbonyl band due to the decomposition of hydroperoxides. The IR spectrum of the photo-degraded PP sheet subjected to the thermal treatment showed a 20% decrease in the hydroxyl band. However, the broad carbonyl band at 5.75 µm did not increase but showed a 5% decrease. However, the small γ-lactone (5.75 µm) and vinyl alkene (6.08 µm) bands showed a slight increase. Thus, these results are due not to hydroperoxide decomposition but to some carboxylic acids converting to γ-lactones and some terminal alcohols dehydrating to vinyl alkenes at high temperature. Hydroperoxides are undoubtedly an intermediate in the photo-oxidation process, but they decompose too rapidly under ultraviolet light to build up any significant concentration. Carbonyl region - The PP carbonyl band after exposure for 335 hours is broad, with few discernible features except for the vinyl alkene band at 6.08 µm. The broadness of the carbonyl band indicates a large variety of functional groups, and makes accurate

36

Carbon-Hydrogen-Type Polymers quantitative analysis difficult. The large vinyl alkene band at 6.08 µm stands out clearly and distinct carboxylic acid (5.83 µm) and γ-lactone (5.58 µm) spikes can be readily identified. The carbonyl band for PP decreases after the volatile products are removed by the vacuum oven. Isopropanol extraction removes about 40% of the PP carbonyl. The carbonyl band is then narrow, and appears to centre at the ester absorption at 5.75 µm. Treatment with base converts lactones, esters, and acids to carboxylates (6.33 µm), leaving only a small band at 5.81 µm, which is due to aldehyde and ketone. Upon re-acidification of the PP, some of the original esters at 5.75 µm do not reform but instead become carboxylic acids and γ-lactones. Curiously, the vinyl alkene band becomes less intense with each step and broader shifting down to 6.10–6.25 µm. The vinyl groups may be isomerised into internal alkenes or become conjugated during the various treatments, although no such change occurs with the PP vinyl alkene or with the process-degraded PP vinyl alkene. Wood and Statton [30] developed a new technique to study the molecular mechanics of oriented PP during creep and stress relaxation based on use of the stress-sensitive 10.25 µm band and the orientationsensitive 11.12 µm band. The far-IR spectrum of iPP was obtained from 400 cm–1 to 10 cm–1, and several band assignments made [31]. The isotacticity of PP has been measured from IR spectra and Py–GC after calibration from standard mixtures of iPP and atactic polypropylene. The IR spectrum of oxidised PP indicated small amounts of OOH groups plus larger concentrations of stable cyclic peroxides or epoxides in the PP chain [31].

2.2.3 Differential Scanning Calorimetry A sample is tested in flowing air or oxygen to examine oxidative breakdown. Oxidation is highly exothermic, so the onset of oxidation is clearly visible on a DSC trace. The time to the extrapolated onset of the exothermic reaction under isothermal condition is called the ‘induction period’. Dynamic determination of oxidation stability (heating the sample at 10 °C/min, for example) is much quicker than the isothermal procedure, and permits simultaneous measurement of the glass transition temperature or melting temperature. This type of analysis is shown in Figure 2.4 by the comparative performance between films of PE and PP. As with the isothermal induction period, the resulting onset temperature is a measure of polymer stability. An alternative technique is pressure differential scanning calorimetry (PDSC). This technique measures heat flow and temperatures of transitions as a function of

37

Thermo-oxidative Degradation of Polymers temperature, time, and pressure (elevated pressure or vacuum). The ability to vary pressure from 1.3 Pa to 6.8 MPa makes PDSC ideal for the determination of the oxidative stability of polymers and for other pressure-sensitive reactions. High pressures of oxygen cause polymers to oxidise much faster than they would at atmospheric pressure. The oxidation process is observed as an exothermic peak. Thus, the oxidative stability of a polymer can be measured as the time to the onset of oxidation at a predetermined temperature and pressure. Antioxidants are common additives for polymers. Oven-ageing and thermal analysis techniques such as DSC and TGA have been used with varying degrees of success to measure the concentration and effectiveness of antioxidants. In most cases, the failure of these tests to correlate with actual end-use performance is due to the volatility of the antioxidants and other components at test temperatures. Even at a relatively modest test temperature (150 °C) the volatility of stabilisers varies greatly, e.g., butylated hydroxytoluene antioxidant loses 30% of its weight in 15 minutes whereas Irganox 1010 loses only 5% in the same time. If an isothermal test were carried out under these conditions, results would be greatly affected by the volatility of the antioxidant, and not accurately reflect enduse performance. PDSC eliminates the volatility problem in two ways, high pressure: (i) decreases the volatility by increasing the boiling point, and (ii) increases the concentration of the reacting gas, oxygen. This allows lower test temperatures or provides significantly shorter test times at equivalent temperatures. A DSC heating experiment in air on an impact-modified PP sample shows the melting of two components followed by oxidative decomposition at 237.7 °C. When tested by TGA in nitrogen, the same material shows thermal degradation starting near 300 °C. However, an expanded view of the top 5% weight loss shows a component (0.66%) of the sample is lost between 126 °C and 184 °C at ambient pressure. The identity of the component is unknown, but the mere fact that the weight loss occurs makes oxidative stability results suspicious if they are generated at >126 °C and at ambient pressure. DSC has been used in conjunction with differential thermal analysis (DTA) to investigate the kinetics of the oxidation of iPP [32]. DSC and oxygen-uptake experiments have been used to measure the oxidative stability of gamma-irradiated ethylene–propylene elastomers [33]. The oxidative irradiation environment generated peroxy radicals that were involved in the air-degraded

38

Carbon-Hydrogen-Type Polymers samples. The specific heat capacity dependences on temperature determined for the two methods of irradiation were dissimilar. Woo and co-workers [34] carried out estimation of polyolefin durability by oxidative stability testing using DSC. Ding and co-workers [7] found that the commonly used oxidative stability measurement of OIT at high temperatures, typically employing molten polymers, grossly overestimates stability at lower temperatures in the solid state. The continuity and suitability of OIT data at lower temperatures and under widely different experimental parameters were examined for HDPE and PP of various formulations. Upon raising the oxygen pressure from about 0.02 MPa to 4 MPa, for example, an approximately 20-fold acceleration was observed. For a hindered phenol-stabilised PP, a normal 1/OIT versus the square root of oxygen pressure was observed, whereas a hindered amine-stabilised PP showed marked departure from this behaviour at lower pressures. For HDPE, a marked loss in OIT was observed when airflow rate through the DSC cell was reduced, along with a reduction in apparent activation energy. These effects were interpreted from experimental parameters (if possible) with polymer degradation mechanisms.

2.2.3.1 Differential Scanning Calorimetry with Chemiluminescence Camacho and Karlsson [35] investigated the thermal and thermo-oxidative stability of recycled PP, HDPE and a 20:80 PP/HDPE blend. These samples were subject to extrusion cycles. The oxidation induction time of all three samples decreased with the number of extrusion cycles, as did the temperature of oxidation. Chemiluminescence runs showed two peaks: one sharp peak (corresponding to PP) and the other bimodal in shape (corresponding to PE). This type of information cannot be obtained by DSC or TGA. Figure 2.7 shows the results of these experiments compared with the ones measured by DSC. The plot shows good correlation between these techniques regarding the onset time of the oxidation phenomenon. DSC and chemiluminescence produced reliable and rapid OIT results if the experiments were conducted at temperatures well above the melting point. Thus, the effectiveness of the residual stabilising package can be determined identically by either of these techniques.

39


150

OITCL (min)

PE x1 100

PE

50 PP x0

Blend PP

Blend x0

0 0

50

100

150

OITDSC (min)

Figure 2.7 OIT obtained by DSC versus OIT obtained from chemiluminescence measurements. Reproduced with permission from W. Camacho and S. Karlsson, Polymer Degradation and Stability, 2002, 78, 2, 385. ©2002, Elsevier [35]

Figure 2.8 shows the chemiluminescence curves of PP, HDPE and their blend after the first extrusion step. The PP exhibits a sharp peak at the maximum on chemiluminescence intensity, whereas the HDPE curve shows a broad bimodal behaviour [36]. In the chemiluminescence curves of the blend, all these features were observed. This may be a strong indication of the existence of a two-phase system in the molten state, which was also pointed out by Braun and co-workers [37], although this was based on peroxide treatment of PP/PE blend melts. It appears that PP oxidises first and the oxidation sites created during this process accelerate, to some extent, the oxidation of PE phase. The overlap between the PP and PE traces in the blend can be interpreted as the interface of these two phases where the PE starts oxidising. In addition, the shape of the curves confirms that the oxidation mechanisms of these resins are different, and that this difference remains during the oxidation of the blend in the molten state.

40


Lumininescence intensity (au)

100000 90000

PP Blend

80000 70000 60000 50000 40000

PE

30000 20000 10000 0

0

5000 Time (s)

10000

Figure 2.8 Chemiluminescence curves of PP, PE and PP/PE blend after the second extrusion; au = arbitrary units. Reproduced with permission from W. Camacho and S. Karlsson, Polymer Degradation and Stability, 2002, 78, 2, 385. ©2002, Elsevier [35]

Fearon and co-workers [38] investigated the oxidative stability of multi-extruded PP using simultaneous DSC and physico-mechanical testing such as impact strength, the melt flow index (MFI) and the yellowness index. Determination of the residual oxidative stability of a polymeric formulation during its multi-pass extrusion is of immediate relevance to polymer recycling. These workers sought correlation between physico-mechanical tests and DSC– chemiluminescence-based techniques using MFI and DSC–chemiluminescence with PP stabilised with different antioxidant systems. Their results suggest a strong correlation between results obtained by the two types of test. There is, however, no correlation between the yellowness index and DSC–chemiluminescence.

41


2.2.4 DTA Duswalt [39] and Steiner and Koppelmann [40] applied DTA in measurements of the thermo-oxidative stability of PP. Steiner and Koppelmann [40] studied the thermooxidative stability of iPP by long-term DTA over a temperature range above and below the crystallite melting range, and applied it to samples of PP containing various levels of Irganox 1330 antioxidant. Oxidation maximum times were measured in the temperature range 90–210 °C up to 2000 hours, and long-term tensile tests carried out at 100–120 °C. This proved to be an excellent method of assessing thermooxidative stability. Figure 2.9 shows results of a long-term tensile test at 120 °C versus failure times obtained by long-term dynamic thermal analysis for iPP with different Irganox 1330 concentrations between 0 and 0.03 w/w%. The beginning of the abrupt decrease in tensile strength is coincident with the oxidation maximum time.

2.2.5 TGA This technique has very limited applications in the measurement of the oxidative stability of PP and its copolymers [41–44]. Wampler and Levy [45] and Chien and Kiang [44] coupled TGA with gas chromatography (GC)–mass spectrometry (MS) to identify volatile oxidation products produced upon heating PP in air.

2.2.6 Py–GC Chien and Kiang [44] carried out oxidative pyrolysis of PP at temperatures between 240 °C and 289 °C. The products were separated by GC and identified online by an interface GC peak-identification system. The major products were CO2, H2O, acetaldehyde, acetone, butanal, formaldehyde, methanol and other ketones and aldehydes. These identifications were confirmed by MS. Most of the products can be accounted for by well-known reactions of alkoxyl and peroxyl radicals; the major products are derived from the secondary alkoxy and peroxy species. Oxygen starvation is manifested in diffusion-limited products of olefins and dienes, and the increase in the formation of CO2 and H2O in an atmosphere of pure oxygen. The first-order rate constant at 240 °C is 2.4 × 10–3/s, with an overall activation energy of approximately 16 kcal/mol (67 kJ/mol). If one assumes that the oxidative pyrolysis shares the same reaction pathways as autoxidation at lower temperatures, then the observed rate constants and activation energy may be calculated from kinetic

42

Carbon-Hydrogen-Type Polymers parameters for autoxidation of PP carried out at temperatures between 11 °C and 140 °C. Good agreement was obtained, implying a similarity of oxidative degradation of the polymers spanning a wide temperature range.

Polypropylene

10 N mm2

5 2 1

Tensile Strangth O

Film 5 mm 100 μm

0.1

120 °C without 0.01 % 0.03 % Antioxidant (IRGANOX 1330)

ILDTA: 0.01

1

2

Id 5

10

1w 100

1m 103

1a h

104

Time t

Figure 2.9 Results of a long-term tensile test at 120 °C for isotactic polypropylene with different antioxidant concentrations, and comparison with isothermal long term DTA results; ILDTA = isothermal long-term differential thermal analysis. Source: Author’s own files

43


2.2.7 Positron Annihilation Lifetime Spectroscopy Positron annihilation lifetime spectroscopy has found limited application in study of the outdoor oxidative degradation of PP [46].

2.3 Rubbers The oxidation of diene rubbers proceeds under the influence of atmospheric oxygen even at room temperature, and results in the hardening and fragility of the surface layer. In its initial stages, the oxidative degradation of natural rubber is characterised by softening of the material and the appearance of stickiness; rubber elasticity then decreases and it cracks [2]. Non-vulcanised synthetic polyisoprene is oxidised extensively even at room temperature [47]. The oxidation of rubber proceeds at its double bonds and at the single bond α to the tertiary carbon atom. Dan and co-workers [47] in their investigation of the structure and stability of chlorinated natural rubbers (CNR) applied high-resolution Py-GC-MS coupled with FTIR and thermal analysis techniques. From the IR analysis, the following results were concluded. The main structures of gel and sol are similar, and there are much greater numbers of carbonyl groups and tertiary C–Cl bonds in the gel. The carbonyl group is an active group, and the activity of tertiary C–Cl is higher than those of primary and secondary C–Cl. The carbonyl group and tertiary C–Cl are the factors resulting in the poorer stability of CNR from latex polymerisation than that of CNR from solution polymerisation. Coupling of high-resolution pyrolysis with GC and MS showed that the gel present in this rubber is formed by crosslinking of linear chlorinated natural rubber molecules. DTA and TGA of the sol and gel were carried out in air and nitrogen. The decomposition reaction active energies of the sol and gel are presented in Table 2.1. The activation energies of the sol in nitrogen and air are higher than those of gel. This further indicates that the thermal and thermo-oxidative stability of the sol is better than that of the gel. The carbonyl group tertiary C–Cl group were thought to be the factors resulting in the poor stability of CNR latex than that of CNR from solution.

44


Table 2.1 Activation energy of sol and gel in thermal decomposition and thermo-oxidative decomposition Sample

Sol

Gel

N2

Air

N2

Air

E/ (kJ/mol)

110.46

94.22

92.42

84.54

R

0.9981

0.9964

0.9952

0.9940

P

0.0000

0.0001

0.0001

0.0002

Reproduced with permission from Y. Dan, L. Sidong, Z. Jieping and J. Demin, Journal of the China Synthetic Rubber Industry, 2003, 26, 47. ©2003, China Synthetic Rubber Industry [47]

2.4 Polystyrene (PS) and Poly(α-methyl styrene) In the presence of oxygen, PS and poly(α-methyl styrene) decompose at a higher rate than under vacuum or in an inert medium [2, 48]. When heated in air for 200 hours at 100 °C, PS films display brittleness without changing in colour or solubility; under identical conditions but at 125 °C, the polymer becomes yellow. The thermal degradation processes are accompanied by the production of peroxide and hydroperoxide groups at the sites of cleavage of C–C bonds and the abstraction of hydrogen atoms from the tertiary carbon atom [48]. On decomposing by a chain mechanism, the peroxide and hydroperoxide groups initiate subsequent degradation of the polymer. The thermal oxidative stability of poly(α-methyl styrene) is substantially lower than that of PS as a result of the weak bonds at the quarternary carbon atom which readily decompose under the attack of oxygen at elevated temperatures. Grassier and Weir [49] described an apparatus for the measurement of the uptake of small amounts of oxygen by PS with a high degree of precision. Grassie and Weir [50] also investigated the application of ultraviolet and IR spectroscopy for the assessment of PS films after vacuum photolysis in the presence of 235.7 nm radiation. During irradiation, there is a general increase in the 240 nm and 290–300 nm regions. Absorption in the 240 nm region is characteristic of compounds having a C=C in conjunction with a benzene ring. Styrene, for example, has an absorption band at 244 nm.

45

Thermo-oxidative Degradation of Polymers Schole and co-workers [51] have applied an oxidative degradation technique to the study of PS. In this technique the PS sample is mixed with a support in a pre-column which is mounted at the inlet to a GC column. Shaw and Marshall [52] have carried out an infrared spectroscopic examination of emulsifier-free PS which had been oxidised during polymerisation. Evidence was found for the presence of surface carboxyl groups bound to the polymer chains, presumably formed by oxidation during polymerisation. The banc at 5.86 µm was assigned in part to the carbonyl stretching mode of dimeric carboxylic acid, formed by oxidation, in the PS chains. Absorption at 5.65 µm, which was very weak, was tentatively attributed to the carbonyl stretching mode of the monomeric form of this acid. The structure of the acid end group was not established but the results obtained suggested that it was possible a phenylacetic acid residue or a residue of standard (unoxidised) and of oxidised emulsion polymerised PS in the region 12.5-25.0 µm. Scholz and co-workers [51] applied an oxidative degradation technique to the study of PS. In this technique, the PS sample is mixed with a support in a pre-column which is mounted at the inlet to a GC column. Shaw and Marshall [52] carried out IR spectroscopic examination of emulsifier-free PS which had been oxidised during polymerisation. Evidence was found for the presence of surface carboxyl groups bound to the polymer chains, presumably formed by oxidation during polymerisation. The band at 5.86 µm was assigned in part to the carbonyl-stretching mode of dimeric carboxylic acid, formed by oxidation, in the PS chains. Very weak absorption at 5.65 µm was tentatively attributed to the carbonyl-stretching mode of the monomeric form of this acid. The structure of the acid end group was not established, but the results obtained suggested that it was possibly a phenylacetic acid residue or a residue of standard (non-oxidised) and of oxidised emulsion polymerised PS in the region 12.5–25.0 µm.

References 1.

J. Voigt, Stabilization of Plastics Against Light and Heat, Monographs on Chemistry, Physics and Technology Plastics No.2, Springer-Verlag, Berlin, Germany, 1966.

2.

J. Voigt, Stabilization of Plastics Against Light and Heat, Monograph on Chemistry, Physics and Technology of Plastics No.10, Springer-Verlag, Berlin, Germany, 1966.

3.

B. Baum, Journal of Applied Polymer Science, 1959, 2, 6, 281.

46

Carbon-Hydrogen-Type Polymers 4.

K.S. Minsker, S.V. Kolesov and G.E. Zackov, Ageing and Stabilization of Vinyl Chloride-based Polymers, Nauka, Moscow, Russia, 1982. [in Russian]

5.

A.H. Willbourn, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1959, 34, 127, 569.

6.

L.P. Luongo, Journal of Polymer Science, Part B: Polymer Physics Edition, 1962, 1, 141.

7.

S. Ding, M.K.T. Ling, A.R. Khare, C.L. Sandford and L. Woo, Journal of Applied Medical Polymers, 2000, 4, 1, 28.

8.

L. Woo, C.L. Sandford and S.Y. Ding, Polymer Preprints, 2001, 42, 1, 394.

9.

L.H. Cross, R.B. Richards and H.A. Willis, Discussions of the Faraday Society, London, UK, 1950, 9, 235.

10. F.M. Rugg, J.J. Smith and R.C. Bacon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1954, 13, 72, 535. 11. A.W. Pross and R.M. Black, Journal of the Society of the Chemical Industry, 1950, 69, 115. 12. H.C. Beachell and S.P. Nemphos, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1956, 21, 97, 113. 13. H.C. Beachell and G.W. Tarbet, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1960, 45, 146, 451. 14. J.P. Luongo, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1960, 42, 139, 139. 15. G.D. Cooper and M. Prober, Journal of Polymer Science, 1960, 44, 144, 397. 16. G. Kimerle, Combustion Technology, 1974, 1, 4. 17. L. Kveder and G. Ungar, Nafta Zagreb, 1973, 24, 85. 18. D.L. Tabb, J.J. Sevcik and J.L. Koenig, Journal of Polymer Science, Part A: Polymer Physics Edition, 1975, 13, 4, 815. 19. P.J. Miller, J.F. Jackson and R.S. Porter, Journal of Polymer Science, Part B: Polymer Physics Edition, 1973, 11, 10, 2001.

47

Thermo-oxidative Degradation of Polymers 20. M.G. Chan and D.L. Allara, Polymer Engineering and Science, 1974, 14, 1, 12. 21. J.F. Heacock, Journal of Applied Polymer Science, 1963, 7, 6, 2319. 22. G. Kostov and E.I. Bakalova, Neftekhimya, 1985, 19, 14. 23. S.L. Madorsky, Thermal Degradation of Organic Polymers, Polymer Reviews Volume 7, Wiley Interscience, New York, NY, USA, 1964. 24. C. Westphal, C. Perrot and S. Karlsson, Polymer Degradation and Stability, 2001, 73, 2, 281. 25. Y. Tsuchiya and K. Sumi, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1968, 6, 2, 415. 26. V.V. Korshak, Chemical Structure of Temperature Characteristics of Polymers, Nauka, Moscow, Russia, 1977. [in Russian] 27. G.E. Zaikov, M. Iring, F. Tudes and Z. Fodor, Vysokomolekulyarnye Soedineniya Seriya A, 1986, 28, 842. 28. H. Kveder and G. Ungar, Neftekhimya, 1973, 24, 86. 29. J.H. Adams and J.E. Goodrich, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1970, 8, 5, 1269. 30. R.P. Wool and W.O. Statton, Journal of Polymer Science, Part B: Polymer Physics Edition, 1974, 12, 8, 1575. 31. M. Goldstein, M.E. Seeley, H.A. Willis and V.J.I. Zichy, Polymer, 1973, 14, 11, 530. 32. A.D. Daiwalt, Thermochimica Acta, 1974, 57, 8. 33. T. Zaharescu, V. Meltzer and R. Vilcu, Polymer Degradation and Stability, 1999, 64, 1, 101. 34. L. Woo, C.L. Sandford and S.Y. Ding, Polymer Preprints, 2001, 421, 394. 35. W. Camacho and S. Karlsson, Polymer Degradation and Stability, 2002, 78, 2, 385. 36. R. Broska and J. Rychlý, Polymer Degradation and Stability, 2001, 72, 2, 271.

48

Carbon-Hydrogen-Type Polymers 37. D. Braun, S. Richter, G.P. Hellmann and M. Ratzsch, Journal of Applied Polymer Science, 1998, 68, 12, 2019. 38. P.K. Fearon, N. Marshall, N.C. Billingham and S.W. Bigger, Journal of Applied Polymer Science, 2001, 79, 4, 733. 39. A.A. Duswalt, Thermochimica Acta, 1974, 8, 1/2, 57. 40. G. Steiner and J. Koppelmann, Polymer Degradation and Stability, 1987, 19, 4, 307. 41. T. Kashiwagi, E. Grulke, J. Hilding, K. Groth, R. Harris, K. Butler, J. Shields, S. Kharchenko and J. Douglas, Polymer, 2004, 45, 12, 4227. 42. S-B. Kwak and J-D. Nam, Polymer Engineering and Science, 2002, 42, 8, 1674. 43. M. Thizon, C. Eon, P. Valentin and G. Guiochon, Analytical Chemistry, 1976, 48, 13, 1861. 44. J.C.W. Chien and F.J.Y. Kiang, Makromolekulare Chemie, 1980, 181, 47. 45. T.P. Wampler and E.J. Levy, Journal of Analytical and Applied Pyrolysis, 1985, 8, 153. 46. L. Brambilla, G. Consolati, R. Gallo, F. Quasso and F. Severini, Polymer, 2003, 44, 4, 1041. 47. Y. Dan, L. Sidong, Z. Jieping and J. Demin, Journal of the China Synthetic Rubber Industry, 2003, 26, 47. 48. V.A. Radtsig, Y.N. Tolparov and E. Firsov, Vysokomolekulyarnye Soedineniya Seriya B, 1986, 28, 111. 49. N. Grassie and N.A. Weir, Journal of Applied Polymer Science, 1965, 9, 3, 963. 50. N. Grassie and N.A. Weir, Journal of Applied Polymer Science, 1965, 9, 3, 975. 51. R.G. Scholz, J. Bednarczyk and T. Yamauchi, Analytical Chemistry, 1966, 38, 2, 331. 52. J.N. Shaw and M.C. Marshall, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1968, 6, 3, 449. 49


50

3

Oxygen-Containing Polymers

3.1 Polyoxymethylene Igarashi and co-workers [1] studied the thermo-oxidative decomposition of polyoxymethylene in air by thermogravimetric analysis (TGA) and infrared (IR) spectroscopy. Polyoxymethylene (POM) and acetylated polyoxymethylene (POMAc) were included in this study. In the presence of air, TGA-derived curves showed two peaks for each material (Figure 3.1).

Weight loss, %/°C

2.5 2.0 1.5 1.0 0.5 0

100

200 300 Temperature, °C

400

Figure 3.1 TGA curves for polyoxymethylene (POM), acetylated POM (POMAc), and Delrin in air: (◊) POM; (o) POMAc, and (Δ) Delrin 5000X. Reproduced with permission from S. Igarashi, I. Mita and H. Kambe, Bulletin of the Chemical Society Japan, 1964, 37, 8, 1160. ©1964, Chemical Society of Japan [1]

In this case, POM and POMAc showed a large weight loss between 150 °C and

51

Thermo-oxidative Degradation of Polymers 200 °C, and the temperature for complete decomposition was higher for Delrin than for POM. These results showed that, although POM is stabilised by the introduction of an acetyl end group (in a nitrogen atmosphere), this stabilisation has a minor effect on oxidative degradation. Thus, it would appear that, in air, chain scission occurs at random points along the chain; because the decomposition of POM in the 200 °C region also occurs in nitrogen, its oxidative decomposition may consist of a combination of chain depolymerisation and random degradation [2]. The effect of oxygen concentration upon the TGA derived curves for POM is shown in Figure 3.2. The weight losses of the first stage in the 200 °C region increased with increasing oxygen concentration. TGA measurements were conducted at very low concentrations of oxygen (in vacuum at 0.01 mm Hg). Even under these conditions, degradation occurs in two stages. However, the fraction of the weight loss in the first stage for POM is larger than that obtained in nitrogen. This was attributed to the greater rate of volatilisation of the degradation products at low pressure, and indicated that the second stage of weight loss may correspond to the decomposition of stable fragments formed during the first stage of degradation.

Weight loss, %/°C

3.0

2.0

1.0

0 100

200 Temperature, °C

300

Figure 3.2 Thermal decomposition of POM in various concentrations of oxygen: (Δ) 0.24 mol%; (◊) 5.6 mol%, and (o) 21.0 mol%. Reproduced with permission from S. Igarashi, I. Mita and H. Kambe, Bulletin of the Chemical Society Japan, 1964, 37, 8, 1160. ©1964, Chemical Society of Japan [1]

52

Oxygen-Containing Polymers From IR absorption spectroscopy studies, Igarashi and co-workers [1] found that the carbonyl content increased as weight loss increased in the decomposition of POM in air, nitrogen, and in a vacuum. The oxygen present in a vacuum should have little effect on carbonyl formation, so it was assumed that this formation is due to the interchange of an ether linkage into a carbonyl group. The following mechanism was proposed [1] for the thermal degradation of POM in the absence of oxygen: HO

A ~ CH2

B

CH2

O

CH2

HO• + •CH2

O~

O

(A)

CH2

O~

CH2O + products

O

O = CH (C)

CH2

(3.1)

(3.2) H• + ~ CH2

O~

O (B)

CH

O~

(3.3)

O + ~ CH2

(3.4)

From this mechanism, Equations 3.1-3.4, the decomposition of a stable fragment, C, with carbonyl groups at the chain end may occur during the second stage of the thermal degradation of POM in a vacuum. In the presence of oxygen, the following scheme was given [2]: ~ CH2

O

CH2

O ~ + O2

~ CH2

O

CH OOH

D

OH• + ~ CH2

O

CH

~ CH2

O

C O

(3.5)

O ~ (E)

O• E

O ~ (D)

(3.6)

H + •O ~

(3.7)

53

Thermo-oxidative Degradation of Polymers This mechanism, Equations 3.5-3.7, can readily account for the fact that the quantity of carbonyl groups formed in the decomposition in air is greater than that in nitrogen.

3.2 Polyphenylene Oxides (PPO) The thermo-oxidative degradation of non-substituted oligophenylene oxides of metaand para structures reveals a sharp rise in viscosity of these compounds. It is believed that initiation occurs through the addition of oxygen to oligophenylene oxides in the middle or at the ends of macromolecules, with the formation and subsequent decomposition of hydroperoxides. The characteristics of the thermo-oxidative degradation of PPO are as follows: oxygen enhances the decomposition of the bridging groups at considerably lower temperatures, whereas the aromatic segments of the chain are virtually unchanged. The radical-chain oxidation of aromatic segments of the chain predominates at the second, high-temperature stage of the thermal degradation of PPO [3, 4]. The complexity of the chemical structure of heterocyclic polymers, including PPO which are very strong, thermally stable polymers, makes the study of their thermal and thermo-oxidative degradation difficult. The schemes suggested for their thermal degradation are, in many cases, only hypothetical, but available experimental data make it possible to delineate the major factors determining the thermal stability of these polymers [3–5].

3.3 Polyesters 3.3.1 Polycarbonate Carbon monoxide (CO), carbon dioxide (CO2), acetaldehyde, formaldehyde, methanol and water are found in the thermo-oxidation products of polycarbonates (PC). Their formation is explained by the isomerisation and decomposition of peroxide and hydroperoxide radicals, with aldehyde and hydroxyl groups being accumulated in the solid polymer residue. Studies on the mechanism of formation of the main gaseous products of the thermal degradation of PC (CO, CO2) have established [3] that CO forms exclusively via the oxidation of CH3 groups, whereas CO2 arises from the decomposition of ester groups and the oxidation of CH3 groups. 54

Oxygen-Containing Polymers Increasing the temperature to >400 °C may lead not only to the decomposition of carbonate groups, but also to the degradation of isopropylene groups. In this case, CO and CO2, together with methane, ethane, ethylene, propylene and considerable quantities of ethylphenol, isopropenyl phenol, isopropyl phenol and cresol, are found among the gaseous products [3, 6]. Decarboxylation is the main pyrolysis reaction of PC at temperatures >450 °C. It is thought that CO2 elimination occurs according to the scheme: O C

CO2 +

O

O

O

(3.8)

Large quantities of bisphenol-A eliminated during the decomposition of PC at about 427 °C may be formed in the thermal degradation of the polymer chain or be due to hydrolysis (alcoholysis) by phenolic compounds of the terminal carbonate group:

O OCO

CH3 C

OH

H 2O

CH3

OH + CH3

+ CO3 + HO

C

OH

CH3

(3.9) O OCO

CH3 C

O OH

ROH

OCOR +

CH3 CH3 + HO

C

OH

CH3

(3.10)

55

Thermo-oxidative Degradation of Polymers The high-temperature decomposition of PC at 700–1100 °C results in the elimination of CO2, CO, CH4 and H2, as well as in the formation of terminal phenol groups. This is explained by the following radical process:

CH3

O OCO

C

OH

O• +

CH3 CH3 • +C

O

C

O• +

OH

CH3

O CH3 + CO2 + •

C

OH

CH3

(3.11)

2

O•

O

OH

O

(3.12)

The IR spectroscopic and nuclear magnetic resonance analysis of the solid residue from the pyrolysis of PC at 500 °C indicates [3] that not only the ester-group content, but also the methyl-group content decreases, whereas that of the phenyl group increases. The activation energy of the thermal degradation of PC under vacuum calculated by kinetic curves for the elimination of volatile products at 300–400 °C is 117 kJ/mol [3, 6]. Very similar values (107 kJ/mol) of the activation energy are found for the thermal degradation of PC as investigated by the TGA over the same temperature range. This may be associated with the strong contribution of thermal degradation during the thermal oxidation of PC.

56


3.3.2 Polyethylene Terephthalate (PETP) On heating PETP in atmospheres of air, oxygen and water vapour, the concentration of COOH groups increases considerably. It has been hypothesised [7] that PETP decomposition under oxidation conditions proceeds mainly through ester bonds via their hydrolysis, with water formed from the decomposition of hydroperoxides. Such a scission of the polymer chain leads to the appearance of one COOH group and one OH group from one ester group. In addition, carboxyl groups may be formed from oxidation of terminal ethylene glycol groups, both those initially present and those appearing during the cleavage of ester bonds. The formation of new end-carboxyl groups leads to the additional production of CO2 as a result of decarboxylation. Vinyl ester groups decompose to produce radicals: ~ COC6H4COOCH = CH2 ~

•

•

~ COC6H4CO + O = CHCH2

(3.13)

After abstracting in a secondary hydrogen atom, radicals initiate the reaction via a chain mechanism. In this case a benzaldehyde end-group and acetaldehyde are produced: ~ COC6H4CO + ~ C6H4COOCH2CH2 ~

~ COC6H4CHO + ~ C6H4COOCHCH2

O = CHCH2 + ~ C6H4COOCH2CH2 ~

O = CHCH3 + ~ C6H4COOCHCH2 ~

(3.14) (3.15)

On decomposition of the alkyl radical, terminal benzyl and acetaldehyde radicals are formed: ~ C6H4COOCHCH2OOCC6H4 ~

~ C6H4CO + O = CHCH2OOCC6H4 ~

(3.16)

The formation of formaldehyde is evidently associated with cleavage of the C–C bond in the glycol link and subsequent decomposition of the radicals: ~ C6H4COOCH2

~ C6H4CO + CH2O

(3.17)

The compositions of the products of the thermal and thermo-oxidative degradation of PETP are the same, so the mechanism of initiation of these processes is identical. In this case, addition of oxygen to the products of the radicals promotes the development of degenerate branching. 57

Thermo-oxidative Degradation of Polymers All investigations on the mechanism of the thermo-oxidative degradation of PETP presuppose [3] that this process has a radical-chain character that proceeds by the formation and decomposition of peroxides and hydroperoxides. Simultaneously with the oxidation of aliphatic links, resulting in the formation of H2O, CO2, CO, and aldehydes, and in the appearance of new carboxyl and phenyl groups, there are also changes in the aromatic links of the chain associated with the formation of biphenyl structures and crosslinking of the polymer.

3.3.3 Polymethacrylates Electron spin resonance spectroscopy has been applied in oxygen stability studies on polymethacrylic acid [8, 9]. Kaezmarek and co-workers [10] investigated the course of photo-oxidative degradation of polyacrylic acid, polymethacrylic acid, and polyvinyl pyrrolidone using IR ultraviolet (UV) spectroscopy.

3.3.4 Styrenated Polyesters Anderson and Freeman [11] studied the thermal properties of a styrenated polyester synthesised by condensation involving a glycol and two dicarboxylic acids, one of which was unsaturated. A crosslinking reaction of the styrene (used as a solvent and copolymer) was effected by the use of free-radical initiators. TGA, dynamic thermal analysis, IR and mass spectrometric techniques were used to study the thermal degradation of this polymer in air and in argon. Based upon IR analysis, the unit basic structure of the polyester was taken to be as in Equation 3.18: CH CH2 C6H5

CH2

CH

H[OOCCHCHCOOCHCH2OOCC6H4COO]H C6H5

CH CH2 CH2 CH (I)

58

(3.18)

Oxygen-Containing Polymers Curves were derived using TGA for the thermal degradation of the styrenated polyester in air and in argon. For both types of atmospheres, degradation started at about 200 °C. However, in argon, the reaction was complete at about 450 °C whereas, in the case of air, there was another reaction stage from about 450 °C to 550 °C. In air, four reaction stages appeared: 200–260 °C; 260–360 °C; 360–450 °C; and 450–550 °C. There also appeared to be extensive overlap between the reactions involved in the second and third stages. In argon, degradation occurred in two stages with extensive overlap. The first stage occurred between 200 °C and 365 °C and the second between 365 °C and 450 °C. The thermal degradation of a styrenated polyester measured in air and in argon was compared by using a difference plot of the differential thermal analysis curves obtained (the curve in argon was subtracted from that in air). Two regions of exothermicity were apparent, from 150 °C to 290 °C, and from 470 °C to 550 °C. Also, degradation in air over the temperature range 290–413 °C was more endothermal than decomposition in argon. Upon heating the polyester in air from room temperature to about 500 °C, the following non-condensable compounds were obtained: carbon dioxide, hydrogen, methane, and propylene. The condensable compounds consisted of benzaldehyde and unsaturated hydroxy esters (between 200 °C and 300 °C); phthalic anhydride and an oily liquid containing hydroxy esters (300–400 °C); and a mixture of phthalic acid, phthalic anhydride, and a liquid containing low molecular weight esters of propylene glycol (400–500 °C). Degradation kinetics of the styrenated polyester in air were determined by the method of Freeman and Carroll [12]. Three linear relationships were obtained. These represented the initial stage of the reaction, the combination of the second and third stages, and the fourth (and last) stage of the reaction. The corresponding values of E, n, and A are listed in Table 3.1. The linear relationship obtained represented the reaction of the styrenated polyester in argon the combined first and second stages of the reaction. Values of E, n, and A in this atmosphere are also given in Table 3.1. On the basis of the various results obtained previously, Anderson and Freeman [11] postulated several schemes. Thus, for the degradation behaviour in air, it was postulated that the initial exothermal reaction stage (Equation 3.19) involved the formation of an unstable hydroperoxide intermediate, for example: CH C6H5

CH + O2

CH C6H5COOH

CH2

CH2

CH

CH

(3.19)

59


Table 3.1 Kinetic parameters for thermal degradation of styrenated polyester (Laminac 4116) Stage of reaction

Temperature range (°C)

Order of reaction

Energy of activation (kcal/mole)

First-order frequency factor (per s)

In air 1

200–260

0.4

19

1.3 × 105

2, 3

260–450

1.2

35

2.7 × 109

4

450–550

1.0

79

4.2 × 1019

20

4.8 × 103

In argon 1, 2

200–450

1.0

Reproduced with permission from D.A. Anderson and E.S. Freeman, Journal of Applied Polymer Science, 1959, 1, 2, 192. ©1959, John Wiley and Sons [11]

Rearrangement (Equations 3.20 and 3.21) and subsequent cleavage (Equation 3.22) could lead to products III and II, respectively, for example:

CH

CH

COOH

C6H5

C6H5CO• + •OH

CH2

CH2

CH

CH

CH C6H5C

CH2

60

(3.20)

O

C6H5CO• + •OH

CH

CH2 (III)

CH

+

CH OH

(3.21)

Oxygen-Containing Polymers O

O

C6H5C

C6H 5C

CH2

+ H

CH2 CH

(II)

CH

(3.22)

An E value of 19 kcal/mole for the initial degradation phase in air falls in the range of values reported for hydroperoxide formation. The second and third reaction stages in air were endothermal, indicating bond rupture. Values of E, n, and A for these stages were of the order expected for the rupture of chemical bonds [13, 14]. Based upon the gaseous products obtained from these stages, the following free-radical mechanism was suggested (Equations 3.23 to 3.25). Cleavage occurs at the carboxyl oxygen to form phthalic anhydride: CH3

OH

CH3

OOCC6H4COOCH2CHOOCCHCHCOOCHCH2OOC

CH3

C6H5(CO)2O + OCH2CHOOC (IV)

(3.23)

The hydroxy ester may then be formed by reaction with hydrogen: CH3

CH3

•OCH2CHOOC

+H

HOCH2CHOOC

(3.24)

Decarboxylation of the polyester should occur readily and account for the formation of propylene and carbon dioxide: CH3 COOCH2CHOOC

2CO2 + CH2 = CHCH3

(3.25)

61

Thermo-oxidative Degradation of Polymers In the fourth degradation stage (450–550 °C), the exothermal trend and the high value of E of 79 kcal/mole indicates that oxidation of carbon (formed in the third reaction stage) occurs. This is supported by reported values of E for the reaction between carbon and oxygen (80 kcal/mole) [15]. Furthermore, a residue of carbon is found after degradation in argon is complete.

3.3.5 Phenol-formaldehyde (PF) Resins In the presence of oxygen, the degradation of PF resins becomes enhanced owing to the formation of hydroperoxide and peroxide groups at the expense of oxidation, with methylene groups being attacked first. It has been established [16] that polyacenaphthylene: ~ CH

CH2 ~

(3.26)

which is produced on polyermisation of the monomer (125 °C) begins to decompose at temperatures >297 °C to form the initial monomer. Polyarylenequinones produced by the action of p-benzoquinone on different bisdinitrogenated aromatic diamines are stable in an inert atmosphere up to 697 °C and in air to 350 °C [17]. The copolymer of anthracene and styrene which at 305 °C loses only 5.5% of its initial mass after 4 hours is also a thermally resistant polymer. The data described previously demonstrate that the introduction of benzene rings into the vinyl polymer chain increases its thermal stability. The nature of the bridging groups which connect the aryl groups in polymers also plays a part in their thermal stability. The thermal stability of polymers containing various bridging groups between the aromatic rings decreases in the order CO > CH2 > O > O2 [18]. TGA of solid PF resins shows that the ratio of the initial components influences the thermal stability of the polymer produced. During the thermal decomposition, up to 50% of the initial mass is released as volatiles with a diverse composition. These include, at 357 °C, up to 11% of propanol, 6.7% acetone, 4% propylene and 3% butanols. In the involatile residue, the concentration of hydrogen and oxygen gradually

62

Oxygen-Containing Polymers decrease as the temperature rises, leaving carbon as a residue. The data suggest that cleavage polymer chains occur at the bond:

CH2

C6H4

OHˉ + H2O

H

H + HCHO OH

(3.27)

Heron [19] carried out a comprehensive study of the pyrolysis of a PF resin in air and an inert atmosphere. He used TGA, isothermal, and gas chromatographic techniques. As indicated by Jeffreys [20], Heron also found that, based upon TGA curves, that there were two stages in the oxidation of PF in air. However, the values of the maximum rates of weight loss found by the two authors were different. This is not too surprising because many factors can affect the types of TGA curves obtained (e.g., particle size), which indicated that there was diffusion control in the oxidative process. In this connection, Conley and Bieron [21] indicated, on the basis of isothermal studies of a PF condensate by IR spectroscopy, that sample thickness affects oxidation results and that therefore the oxidation is a surface phenomenon. In view of the variable nature of the TGA curves obtained and because of the large extent of the overlap between the two peaks obtained, Heron [19] used isothermal techniques to obtain values of the kinetic parameters. He obtained a value of E of about 15 kcal/mole for the oxidative degradation process observed over the temperature range of the first peak (about 300–380 °C). Conley and Bieron [21] found, on the basis of carbonyl formation determined by IR techniques, that the partially cured PF resin gave a value of E of 19.5 kcal/mole, whereas the fully cured PF resin gave a value of E of 15.6 ± 3.9 kcal/mole over a temperature range 140–220 °C. The latter value corresponds with the value for the first peak observed by Heron [19], so it appears that the initial oxidative degradation weight loss may be associated with the formation of carbonyl compounds. Conley and Bieron [21] proposed a scheme in which the PF resin is oxidised to the hydroperoxide at the methylene bridge, and that this peroxide then decomposes to form the corresponding hydroxy derivative and/or ketone derivative. These derivatives can then oxidise further to form quinone and acid fragments, as indicated by IR spectra [22]. At the higher reaction temperatures which involved the second peak (>400 °C), Heron [19] found that the oxidation became so strongly exothermic that parts of the powdered resin glowed red, and therefore the true sample temperature was difficult (if not impossible) to ascertain. At these temperatures, the maximum rate or weight loss was found to be less than for the first peak, suggesting that this was due to oxidative crosslinking.

63

Thermo-oxidative Degradation of Polymers Using gas chromatography, Heron [19] found that the major constituents in the oxidative degradation products are qualitatively similar to those formed by degradation in an inert atmosphere (argon). Thus, it was indicated that thermal and oxidative degradation processes occurred simultaneously (diffusion effects). Among the many degradation products identified are benzene, toluene, mesitylene, m-xylene, phenol, o- and p-cresols, methylphenols, and water. Heron attributed the formation of some of these products (e.g., water) to mechanisms proposed by Ouchi and Honda [23]. Within the boiling range of 50–250 °C of the decomposition products, it was also found that phenol is the main product from pyrolysis in air, which suggests that methylene links are the main points of oxidation, as also indicated by IR spectroscopy [21]. However, Conley and co-workers [22] also carried out high-temperature degradation studies of PF resin in air. They proposed a mechanism for the formation of water which is in contrast to that proposed by Ouchi and Honda [23]. Thus, water formation is attributed to condensation of methylol groups, which is consistent with the observed loss of such groups and the lack of detectable diphenylether linkages which should form according to Ouchi and Honda. From initial oxidation reactions, Equations 3.28 and 3.29:

OH

OH

OH

CH2

O

OH

C [O] Source

64

(3.28)

Oxygen-Containing Polymers OH

OH CH2

OH

COOH [O] Source

(3.29)

it was possible to extend the degradation reaction sequence to account for all of the observable products. Figure 3.3 summarises the oxidative degradation processes in a generalised form. Schemes for the formation of phenol, cresol, methane, and other methyl-substituted species (benzene, toluene, carbon dioxide) can be seen in this figure. The formation of char, which was previously indicated as occurring during the second stage of the oxidative reaction (second peak of TGA curves), occurred according to the scheme in Figure 3.4. At temperatures >450 °C, the decomposition to form char and carbon monoxide by means of ring scission was found to be rapid. Carbon monoxide did not form at lower temperatures. Char formation could be substantiated by the presence of a graphite-like line in the X-ray pattern of the PF resin residues, and was apparently formed by the decomposition of the oxidised resin through a quinone-type intermediate. Occurrence of char formation and of carbon monoxide occurred simultaneously. In contrast to the work of Heron [19], Conley [22] found that it was not necessary to invoke thermal non-oxidative degradation mechanisms, and that the post-cured PF resin showed no change in its IR spectrum after prolonged heating to 450 °C in a vacuum.

65


OH

O

OH

O

OH

C

(I)

OH

C• +

•

CH2 OH OH

O C

O CH

OH OH • +C

O

CH2

CH2 OH

OH CH2

OH + CO2

OH

(II)

OH

OH

OH CH2•

CH2

+

•

CH2 OH

OH CH3

66

•

+ CH3

OH

Oxygen-Containing Polymers (III)

CH3

CH•

CH4 +

OH

+ •OH

CH2 H2C•

•

O CH

CH2

OH

CH3 or higher homologe

COOH

+ CH2

Figure 3.3 Typical reactions proposed for resin decomposition at elevated temperature. Route I: oxidative degradation processes; Route II: fragmentation reactions; and Route III: formation of benzenoid species. Source: Author’s own files

67


OH

O

O

OH

OH O

C [O]

COOH

+

O C

+C

O

(-CO2)

CH• OH

+ OH

From Route l

OH (1) -CO (2) Ring formation C• O

Figure 3.4 Reactions proposed for char formation. Source: Author’s own files

3.3.6 Epoxy Resins Bremner [24] reported on various factors that affect the heat stability of brominated epoxy resins in the presence of air using TGA and isothermal methods. Figure 3.5 shows TGA curves for cured resins based on the structure (Equation 3.30).

68

Oxygen-Containing Polymers 80

Weight loss, %

70

Resin C Resin B Resin A DER 331 Resin

∆T (°C) = 3.7 min

60 50 40 30 20 10 0

0

100

200 300 400 Temperature, °C

500

600

Figure 3.5 Thermogravimetric curves with boron trifluoride monoethylamine (BF3MEA) as curing agent. Reproduced with permission from J.A. Bremner, Industrial Engineering Chemistry Product Research and Development, 1964, 3, 1, 55. ©1964, ACS [24]

Br O H2C

CH

CH2

O

CH3

O

C Br

CH3 (V)

Br OCH2

CH

CH2

Br

(3.30)

These resins were designated as A, B, and C, and their halogen content and epoxy equivalent increased from A to C. In Figures 3.5 it can be seen that, for the same curing agent (BF3MEA), as the halogen content increased, the thermal stability decreased. This effect may be due to the presence of chlorinated structures, such as structure VI (Equation 3.31):

69

Thermo-oxidative Degradation of Polymers Br O H2C

CH

CH2

O

CH3

Br

C Br

CH3 (VI)

OH O

CH2

CH

CH2

CL

Br

(3.31)

3.3.6 Thermogravimetric Analysis Dyakonov and co-workers [25] used programmed TGA and IR spectroscopy in their studies of the thermal and oxidative stability of some amine-cured epoxy resin systems based on the glycidyl ether of bisphenol-A and aromatic primary amines. They studied changes in network epoxy resin model systems brought about by exposure to elevated temperatures in the presence and absence of oxygen. Dyakonov and co-workers [25] elucidated some of the processes which result in the thermolysis of phenoxy resin and network epoxy resins under an inert atmosphere. The ultimate goal behind the elucidation of processes of thermal degradation under accelerated conditions is to be able to extrapolate the findings (determined for short times at high temperatures) to predict material lifetimes at lower temperatures and longer times. Unfortunately, this extrapolation is complicated by the influence of atmospheric oxygen on the degradation process. To determine or predict a user-defined lifetime for these materials as a structure–property correlation, the influence of oxygen on the degradation process must be determined. To this end, thin films of phenoxy resin were aged isothermally on salt plates, and IR spectra recorded in an attempt to elucidate the structural changes which accompany the thermo-oxidative process (Figure 3.6). Changes in the IR spectrum of network model epoxy resin systems after thermo-oxidation were similar to those observed in the spectrum of phenoxy resin. The transmission IR spectrum of the neat resin is shown in Figure 3.6a. Absorbances due to saturated (6.84 µm) C–H bending at 1462 cm–1 (6.84 µm) and the phenyl ether absorbance at 1042 cm–1 (9.60 µm) are noted for future reference. The IR spectrum of resin which had been aged in air for 5 days at 200 °C is reproduced in Figure 3.6b. A small reduction in the relative intensity of the 1042 cm–1 (9.60 µm) phenyl ether absorbance band is observed after this treatment regimen, along with a small build-up of absorbance at 1724 cm–1 (5.80 µm) due to creation of carbonyls. Carbonyls are not produced in phenoxy resin that has been degraded under an inert atmosphere. As such, the carbonyl centre may be identified as a true product of

70

Oxygen-Containing Polymers oxidation. wavenumbers (cm-1)

Absorbance Units

3800

2200 1400 800 1462 1042 1724

c 1724 b 1462

1042

a 3800

2200 1400

800

Figure 3.6 Infrared spectra of phenoxy resin degraded as a thin film onto a salt plate. (a) Unmodified resin; (b) resin which had been aged in air for 5 days at 200 °C, and (c) resin which had been aged in air for 1 hour at 300 °C. Absorbances which appear and grow or which diminish in intensity through the ageing process are annotated in the figure. Reproduced with permission from T. Dyakonov, P.J. Mann, Y. Chen and W.T.K Stevenson, Polymer Degradation and Stability, 1996, 54, 1, 67. ©1996, Elsevier [25]

The IR spectrum of resin which had been aged in air for 1 hour at 300 °C is reproduced in Figure 3.6c. Changes in the IR spectrum accompanying long-term oxidation at 200 °C are also observed after oxidation at 300 °C. However, the extent of oxidation is more pronounced after 1 hour at 300 °C. A large carbonyl band appears at 1724 cm–1 (5.80 µm), indicating that similar carbonyl products of oxidation are formed at the higher temperature/shorter time regimen, but at a higher yield. The phenyl ether

71

Thermo-oxidative Degradation of Polymers absorbance at 1042 cm–1 is almost completely removed after 1 hour of oxidation at 300 °C, as is the saturated C–H bending mode at 1462 cm–1 (6.84 µm). The reduction in intensity of the phenyl ether absorbance is more pronounced after degradation for 1 hour in air at 300 °C than after degradation of the resin for 1 hour under nitrogen at the same temperature (spectrum not shown). The single strong absorption in the fingerprint region of the spectrum (p-disubstituted aromatic) remains unaltered except for a small wavenumber shift. This indicates that oxidation for 1 hour at 300 °C does not involve quantitative reaction of the central isopropylidine carbons within the bisphenol-A structural motif, the inference being that the 1,3-di-phenoxy isopropanol chain extender forms the primary initial locus for oxidative degradation of the resin. If so, the oxidation reaction, as measured by the production of carbonyl products, may be coupled to the accelerated loss of phenyl ether content in the resin. Figure 3.7 shows programmed TGA curves for epoxy resin systems under an inert atmosphere and in air aged at 200 °C to 300 °C. At 200 °C, weight loss is minimal over a time interval of 200 hours. At ≥250 °C, weight loss is rapid over this time interval. Most striking is the very rapid initial weight loss at the higher temperatures. Resin number 4 is, at best, a ‘200 °C resin’. As a result of this work it was concluded that pheonxy resin, and a simpler model system containing the bisphenol-A structural motif, is more stable thermally and thermo-oxidatively than all the bisphenol-A-based epoxy resins studied; the inference being that the 1,3 di-phenoxy isopropanol chain extender is more stable than the di-(3-phenoxy, 2-hydroxy) tertiary aromatic/aliphatic amine extender/crosslink. Weight loss in phenoxy resin in the absence of oxygen was related quantitatively to scission at the bisphenol-A phenyl ether. The same reaction was observed in all the epoxy resin systems, as was another resulting in the production of a small amount of carbonyl-containing product. In air, degradation of phenoxy resin is accompanied by the production of carbonyl residues. Enhanced carbonyl content was also observed in all the epoxy residues degraded in air (presumably through the addition of oxygen to the resin).

3.3.6.2 Differential Scanning Calorimetry Park and Seo [26] applied differential scanning calorimetry (DSC) in a study of the thermo-oxidative degradation and mechanical properties of a range of epoxy resins based on the diglycidyl ethers of bisphenol-A produced using the cationic thermal catalyst N-benzylpyrazinium hexafluoroantimonate. It was found that the internal structure of the epoxy resins was stabilised and post-cured with increasing elapsed heating time, resulting in improved thermal thermo-oxidative and mechanical properties.

72


100

200 °C

250 °C

Percent residue

80

60 275 °C 40

300 °C

20

a 0

100 200 Time (Hours)

300

Figure 3.7 Isothermal weight loss curves produced by thermo-oxidation in a forced air oven. Stoichiometric thermoset resin number 4 from diglycidyl ether of bisphenol A and meta-phenylene diamine aged at four temperatures between 200 °C and 300 °C. Reproduced with permission from T. Dyakonov, P.J. Mann, Y. Chen and W.T.K Stevenson, Polymer Degradation and Stability, 1996, 54, 1, 67. ©1996, Elsevier [25]

3.3.7 Ethylene Oxide-Propylene Oxide Copolymers Gallet and co-workers [27, 28] studied the oxidative thermal degradation of poloxamer 407, a poly(ethylene oxide-propylene oxide-ethylene oxide) triblock copolymer, at 50 °C and 80 °C in air by solid-phase microextraction/gas chromatography–mass spectrometry (SPME/GC–MS). At 80 °C, it was found that degradation was initiated

73

Thermo-oxidative Degradation of Polymers on the PPO block of the copolymer by three mechanisms involving hydroperoxyl formation and depropagation; 1,2-propanediol, 1-acetate; 1,2-propanediol, 2-formate; and 1,2-propanediol, 1-acetate, 2-formate and 2-propanone, 1-hydroxy were the first degradation products produced. Random chain scissions and a sharp decrease in the molecular weight of the material followed the initiation period. Formic acid and acetic acid formed upon degradation participated in esterification reactions leading to the formation of the formate and acetate forms of 1,2-propanediol and ethanediol. Though degradation at 50 °C was much slower, the oxidative mechanisms leading to low molecular weight formats and acetates were the same as those observed at 80 °C. Figure 3.8 shows SPME/GC–MS chromatograms of polyoxamer 407 polyethylene oxide (PEO)–propylene oxide (PO)–ethylene oxide (EO) triblock copolymer from the virgin copolymer and after 4 days at 80 °C and 12 days at 80 °C. Butylated hydroxy toluene (BHT) was the main volatile product extracted (peak 19) from the matrix of the virgin polymer. BHT was almost completely consumed and the polymer degradation began. PPO was more sensitive to thermo-oxidation than PEO due to the more stable tertiary radical that can be formed on the PPO chains. Gallet and co-workers [27] showed that this was also true for poloxamer materials by identifying products of PPO cleavage early in the degradation process. Although C–O homolysis while submitting PPO to continuous oxygen flow at 125 °C has been observed, degradation proceeded mostly through C–C scissions under milder conditions, such as those used in this study (oxygen-starved and at relatively low temperature). These mechanisms, which involve hydroperoxide formation followed by C–C homolysis, are displayed in Scheme 1. Peak 1 (2-propanone, 1-hydroxy) was the only degradation product resulting from C–O scissions. The mechanism started from the same tertiary alkoxy radical as displayed in Mechanism I (see next) followed by C–O homolysis and depropagation.

74

Oxygen-Containing Polymers (a)

Relative abundance 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

(b)

2 19 5

6

7

8

9


(c)

17

10

11

12

13

14

Time (min)

2

15

16

17

18

19

20

21

17 19

6 3

11

10

11

1 5

6

7

8

9

14 13 15 12

20

18 13

14

Time (min)

15

16

17

18

19

20

21


20 18 6 11 14 13

3

12

2 5 7 10

1

4 8 9 5

6

7

8

9

10

11

19

15 16 17 12

13

14

Time (min)

15

16

17

18

19

20

21

Figure 3.8 Gas chromatography – mass spectrometry chromatograms of (a) virgin P407AABHT, (b) P407AABHT thermoxidised for 4 days at 80 °C, and (c) P407AABHT thermoxidised 12 days at 80 °C. P407 = commercial poloxamer, P407AA = P407 mixed with acetic acid, P407AABHT = P407 mixed with AA and butylated hydroxy toluene. Reproduced with permission from G. Gallet, B. Erlandsson, A-C. Albertsson and S. Karlsson, Polymer Degradation and Stability, 2002, 77, 1, 55. ©2002, Elsevier [27] 75

Thermo-oxidative Degradation of Polymers Crown ethers of EO and PO were suspected to be present as well as methyl and ethyl ethers from Mechanism I (Equations 3.32 and 3.33) and II (Equations 3.34 and 3.35): Mechanism I O

O

O

O

O

OOH HO• + PH

O

O

O

O• H 2O + P •

C• O H2

O

O

O•

(3.32)

O

O HOO H2O

O

Acetate

•

PH

O

O

CH3

+ P•

Methyl ether

Formate

(3.33)

Mechanism II OOH O

O•

O

O

O

HO• + PH

O

O

H 2O + P •

O• O

O

O

O

O

Formate

O C• H

(3.34)

O

HOO• H2O O O Acetate

76

PH

O O

O

Ethyl ether

+ P•

(3.35)


3.3.8 Ethylene Vinyl Acetate (EVA) Copolymers Allen and co-workers [29] degraded EVA polymer films containing 17% and 28% vinyl acetate for various times in a hot air oven at 180 °C, and then examined the products by TGA, Fourier-transform infrared (FTIR), spectroscopy, luminescence analysis, and carried out measurements of the yellowness index and hydroperoxide content. Thermal analysis indicates the initial loss of acetic acid followed by oxidation and breakdown of the main chain. The degradation rate is greater in an oxygen atmosphere, as is the formation of coloured products. FTIR spectroscopic analysis of the oxidised EVA shows evidence for de-acetylation followed by the concurrent formation of hydroxyl/hydroperoxide species, ketone groups, α, β-unsaturated carbonyl groups, conjugated dienes, lactones and various substituted vinyl types. Hydroperoxide evolution follows typical auto-oxidation kinetics to form ketonic species. In severely oxidised EVA, evidence is given for the subsequent formation of anhydride groups. The initial fluorescence excitation and emission spectra of EVA are not unlike that reported for polyolefins, confirming the presence of low levels of unsaturated carbonyl species. There are, however, significant differences in a long-wavelength component in the fluorescence emission, indicating the presence of other active chromophores. These long wavelength-emitting components grow in intensity and shift to longer wavelengths with ageing time. However, unlike studies on polyvinyl chloride (PVC), these emission spectra are limited due to the vinyl polyconjugation lengths and tend to be consistent with the formation of specific degraded units, possibly polyunsaturated carbonyl species of a limited length confined to the EVA blocks. During EVA oxidation, the original unsaturated carbonyl species remain as distinct emitting chromophores. This suggests that the growth and decay of these chromophores is virtually constant, indicating that they could be an integral part of the EVA polymer responsible for inducing degradation. Degradation is limited to the vinyl acetate moieties where hydroperoxides can lead to the formation of polyconjugated carbonyl groups. EVA degradation is therefore different from that of PVC degradation where in the latter case poly-conjugated vinyl groups are evident through conjugated absorption bands in the UV spectrum. In the case of degraded EVA, no such bands are observed. Also, degraded coloured EVA is not bleached by treatment with bromine, maleic anhydride or peracetic acid. Primary phenolic antioxidants exhibit variable activity in inhibiting the yellowing of EVA whereas combinations of these with phosphites generally display powerful synergism. The two EVA copolymers with 17% and 28% of vinyl acetate blocks were oven-aged at 180 °C and samples analysed at different stages for hydroperoxide concentration. Figure 3.9 illustrates that the hydroperoxides form and decay in a typical autooxidative fashion [28] as found previously for polyolefins. Hydroperoxide growth

77

Thermo-oxidative Degradation of Polymers is initially rapidly followed by a gradual decay where, after 2 hours, the rate of destruction of hydroperoxides exceeds the rate of formation. In terms of the percentage EVA there appears to be little difference in the curves apart from a small enhancement in hydroperoxide concentration for the 28% w/w vinyl acetate. At this stage, the polymer begins to discolour and carbonyl growth sets in.

POOH Concentration (µg/g)

3500 3308

3000

2901

2500

2295 1962

2000 1500

1798 1372

1000 500 0

1

204 64

1 1 Time of Oven Ageing at 180 °C(h)

1

Figure 3.9 Hydroperoxide concentration (inserts are values of [POOH] versus oven ageing time (h) for (○) 28% and () 17% EVA copolymers. Reproduced with permission from N.S. Allen, M. Edge, M. Rodriguez, C.M. Liauw and E. Fontan, Polymer Degradation and Stability, 2000, 68, 3, 363. ©2000, Elsevier [29]

Comparative evaluations of degradation were undertaken using the technique of attenuated total reflection with a ZnSe crystal. EVA samples were degraded to obtain discolorations extending from yellow, brown through to black. Full-range FTIR spectra

78

Oxygen-Containing Polymers for these degraded samples are compared in Figure 3.10. The main functional groups appearing are compiled in Table 3.2. Significant changes in functional groups were evident in the carbonyl region. These show an enhancement in lactone formation at 1770 cm–1 (5.05 µm) associated with a back-biting process in the vinyl acetate moieties by the acetate groups forming methane (Scheme 1 Equation 3.36). Ketone formation is also evident by a shift in the carbonyl group to the 1715 cm–1 (5.83 µm):

R

R2

R

1

2

H

O

O O

+ CH4

R1 O

CH3

(3.36)

band at 1175 cm–1 (Figure 3.10). These groups may be formed as shown in Scheme 2 (Equation 3.37) through the evolution of acetaldehyde:

R1

R2

R1

R2

H O

+ H

O

H O

CH3

O

CH3

(3.37)

Alternatively, hydroperoxides (β to the acetate group) can break down to give ketone groups and water (Scheme 3, Equation 3.38). These changes are primary steps in the yellow and brown coloured samples. Vinyl groups are also evident absorbing at 995 cm–1, 960 cm–1 and 842 cm–1 (10.05 µm, 10.42 µm and 11.88 µm) and are associated with mono, di and tri-alkyl substitution patterns. A broad peak at 1600 cm–1 (6.25 µm) due to diene formation also begins and forms a predominant sharp peak in the black sample. The ketone bands shift downwards to 1708 cm–1 (5.85 µm) and in conjunction with the diene band is consistent with the formation of α, β-unsaturated carbonyl products. These may be formed through the breakdown of hydroperoxides as shown in Scheme 4 (Equation 3.39) to form acetic acid. Here the α-alkyl radical site rearranges to give a double bond:

79

Thermo-oxidative Degradation of Polymers O2H R

R2

1

H

-OH•

R1

R2

H +

O

O O CH3

O O

H

-H2C

•

CH3

O R

R2

1

H

O O CH3

(3.38)

H

O2H R1

R2 H

O -OH•

O

H

O

R1 R3

+

H

O

O CH3

CH3

O R

1

R3 H [O] O R4

80

R3

[DEACETYLATION]

[O]

O R1 R5 H

(3.39)


Table 3.2 FTIR absorption bands undergoing change in ethylene vinyl acetate Wavenumber (cm–1)

Group

Comment

3700-3100

Hydroxyl

Alcohols, peroxides and hydroperoxides

3000-2800

Methylene, methyl

C-H stretch

3430

-OOH

O-H stretch

1845

Anhydride

C=O stretch

1770

Lactone

C=O stretch

1740

Aliphatic ester

C=O stretch

1715/1175

Ketone

C=-O stretch

1708

Ketone

C=O stretch

1600

Conjugated diene

C=C stretch

1160

Aliphatic ester

C-O-C stretch

995

R-CH=CH2

=CH stretch

960

R-CH=CH-R

CH out-of-plane bending

842

R1R2C=CHR3

CH out-of-plane bending

Reproduced with permission from N.S. Allen, M. Edge, M. Rodriguez, C.M. Liauw and E. Fontan, Polymer Degradation and Stability, 2000, 68, 3, 363. ©2000, Elsevier [29]

The data presented by Allen and co-workers [30] indicate that the main degradation routes for EVA involve the initial loss of acetic acid followed by oxidation and breakdown of the main chain. The degradation rate is greater in an oxygen atmosphere, as is the formation of coloured products. Hydroperoxidation is an important route in the oxidation of EVA copolymers, giving rise to ketone and unsaturated ketone groups. Their rate of formation and decay follow typical auto-oxidation kinetics. Using FTIR spectroscopy analysis of the oxidised EVA shows evidence for deacetylation followed by the concurrent groups, α, β-unsaturated carbonyl groups, conjugated dienes, lactones and various substituted vinyl types. The α, β-unsaturated carbonyl groups present initially in the polymer give rise to deacetylation. In severely oxidised EVA, 81

Thermo-oxidative Degradation of Polymers evidence is given for the presence of anhydride groups formed via the decomposition of ketonic groups.

0.26 0.24

EVA virgin **ATR** 2nd crystal V EVA yellow at 100 °C **ATR** 2nd crystal Y EVA brown at 150 °C **ATR** 2nd crystal BR EVA black at 200 °C **ATR** 2nd crystal BL

0.22 0.20

BROWN EVA

ABSORBANCE

0.18 0.16

BL

0.14 0.12 0.10 0.08

BLACK EVA

0.06

V

0.04

BL Y BR V

0.02 4000

3500

BL 3000

Y 2500 2000 WAVENUMBERS (cm 1)

BR 1500

VIRGIN EVA 1000

Figure 3.10 FTIR spectra using attenuated total reflectance spectroscopy (ATR) in the range 4000–700 cm–1 of EVA copolymer degraded to various coloured states from colourless (virgin), yellow to brown through to black. Reproduced with permission from N.S. Allen, M. Edge, M. Rodriguez, C.M. Liauw and E. Fontan, Polymer Degradation and Stability, 2000, 68, 3, 363. ©2000, Elsevier [29]

The initial fluorescence excitation and emission spectra of EVA are not unlike that reported for polyolefins, confirming the presence of low levels of α, β-unsaturated carbonyl species. There are however, significant differences in a long-wavelength component in the fluorescence emission, indicating the presence of other active chromophores. These long wavelength-emitting components grow in intensity and shift to longer wavelengths with ageing time. However, like studies on PVC, the emission spectra for degraded EVA are limited in conjugation length and tend to be

82

Oxygen-Containing Polymers consistent with the formation of specific degraded units, possibly polyunsaturated carbonyl species of a limited length confined to the EVA blocks. During oxidation of EVA, the original α, β-unsaturated carbonyl species remain as distinct emitting chromophores. This suggests that the growth and decay of these chromophores is virtually constant, indicating that they could be an integral part of the polymer responsible for inducing degradation. Degradation is limited to the vinyl acetate moieties where hydroperoxides can lead to the formation of polyconjugated carbonyl groups. This differentiates EVA from that of PVC where, in the latter case, polyconjugated vinyl groups are evident through conjugated absorption bands in the UV spectrum. In the case of degraded EVA, no such bands are observed. This is confirmed by the observation that degraded coloured EVA is not bleached by treatment with bromine, maleic anhydride or peracetic acid. In terms of stabilisation against discolouration, primary phenolic antioxidants exhibit some inhibition activity whereas combinations with phosphites can display powerful synergism. This is consistent with the involvement of a free radical oxidation process in deacetylation through ketone/hydroperoxide initiation.

3.3.9 Phenolic Resins Lyim and co-workers [31] studied the effect of PETP on the thermo-oxidative stability of Novolac-type phenolic resins using TGA, DSC and FTIR. It was found that the incorporation of PETP in blends improves stability. Degradation of polymer is avoided up to 370 °C.

References 1.

S. Igarashi, I. Mita and H. Kambe, Bulletin of the Chemical Society Japan, 1964, 37, 8, 1160.

2.

W. Kern and H. Cherdron, Die Makromolekulare Chemie, 1960, 40, 1, 101.

3.

B.M. Kavarskaya, A.B. Blyumenfeld and S.I. Levantovskaya, Thermal Stability of Heterochain Polymers, Khimya, Moscow, Russia, 1977. [in Russian]

4.

S.P. Pavlova, I.V. Zhuravleva and Y.F. Tolchinsky, Thermal Analysis of Organic and High Molecular Weight Compounds, Khimya, Moscow, Russia, 1983. [in Russian]

83


V.V. Korshak, Chemical Structure and Temperature Characteristics of Polymers, Khimiya, Moscow, Russia, 1970. [in Russian]

6.

G. Schnell, Chemistry and Physics of Polycarbonates, Khimiya, Moscow, Russia, 1967. [in Russian]

7.

I.I. Levantavskaya, O.A. Klapovskaya, N.V. Andrianova and B.M. Kovarskaya, Plastische Massy, 1971, p.44. [in Russian]

8.

Y. Hajimoto, N. Tamura and S. Okamoto, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1965, 3, 1, 255.

9.

Y. Sakai and M. Iwasaki, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1969, 7, 7, 1749.

10. H. Kaczmarek, A. Kaminska, M. Swiatek and J.F. Rabek, Angewandte Makromolekulare Chemie, 1998, 261-262, 1, 109. 11. D.A. Anderson and E.S. Freeman, Journal of Applied Polymer Science, 1959, 1, 2, 192. 12. E.S. Freeman and B. Carroll, Journal of Physical Chemistry, 1958, 62, 4, 394. 13. I. Marshall and A. Todd, Transactions of the Faraday Society, 1953, 49, 67. 14. S. Straus and L.A. Wall, Journal of Research of the National Bureau of Standards, 1958, 60, 39. 15. G. Blyholder and H. Eyring, Journal of Physical Chemistry, 1957, 61, 5, 682. 16. V.V. Korshak, Thermostable Polymers, Nauka, Moscow, Russia, 1969. [in Russian] 17. M.T. Bryk, Zhurnal Fizicheskoi Khimii, 1982, 14, 64. [in Russian] 18. V.V. Korshak, Chemical Structure and Chemical Characteristics of Polymers, Nauka, Moscow, Russia, 1970. [in Russian] 19. G.F. Heron, Private Communication. 20. K.D. Jeffreys, British Plastics, 1963, 36, 188. 21. R.T. Conley and J.F. Bieron, Journal of Applied Polymer Science, 1963, 7, 1, 171.

84

Oxygen-Containing Polymers 22. R.T. Conley, Journal of Applied Polymer Science, 1965, 9, 3, 1117. 23. K. Ouchi and H. Honda, Fuel, 1959, 38, 429. 24. J.A. Bremner, Industrial Engineering Chemistry Product Research and Development, 1964, 3, 55. 25. T. Dyakonov, P.J. Mann, Y. Chen and W.T.K. Stevenson, Polymer Degradation and Stability, 1996, 54, 1, 67. 26. S.J. Park and M.K. Seo, Macromolecular Materials and Engineering, 2003, 288, 11, 894. 27. G. Gallet, B. Erlandsson, A-C. Albertsson and S. Karlsson, Polymer Degradation and Stability, 2002, 77, 1, 55. 28. G. Gallet, S. Carroccio, P. Rizzarelli and S. Karlsson, Polymer, 2002, 43, 4, 1081. 29. N.S. Allen, M. Edge, M. Rodriguez, C.M. Liauw and E. Fontan, Polymer Degradation and Stability, 2000, 68, 3, 363. 30. N.S. Allen and M. Edge, Fundamentals of Polymer Degradation and Stabilization, Elsevier Applied Science Publishers, London, UK, 1992. 31. T.B. Lyim, S. Özgümüş and M. Orbay, Polymer - Plastics Technology and Engineering, 2003, 42, 1, 17.

85


86

4

Halogen-Containing Polymers

4.1 Polyvinyl Chloride (PVC) Gladyshev and co-workers [1] pointed out that, during the thermo-oxidative degradation of PVC, highly conjugated systems are produced and these can be conveniently followed by electron paramagnetic resonance (EPR) and ultraviolet (UV) spectroscopy. It is believed that, under vacuum, the polyconjugated chains grow at the expense of PVC dehydrochlorination, with the concentration of polyene radicals determined by EPR remaining unchanged. In the presence of oxygen the polyenyl radicals are destroyed, so their concentration (and consequently that of the polyene molecules in the PVC sample) may be determined by reaction with oxygen. It has been established that the introduction of oxygen decreases the intensity of the 291 nm and 330 nm bands in the UV spectrum of PVC, whereas the bands at wavelengths >400 nm are unchanged. Presumably, the 291 nm and 330 nm bands may be attributed to polyene radicals produced during the thermo-oxidative degradation of PVC via the cleavage of long polyconjugated chains. Heating PVC films in atmospheres of argon and air result in different behaviour in their UV spectra. On prolonged heating and with increasing temperature, the intensity of the UV spectral maximum increases and a hypsochromic shift is observed. The rate of formation of the conjugated system during the thermal degradation of PVC is higher than that during thermal oxidation. This may be caused by cleavage of the long conjugated chains due to oxygen. Additionally, the presence of oxygen inhibits the formation of polyconjugated systems. Various workers [1–7] have discussed the application of electron spin resonance (ESR) and EPR to the examination of paramagnetic centres in PVC. As a rule, the presence of oxygen increases the concentration of free radicals produced during polymer degradation; it increases the initiation rate of degradation due to random scission or increases the quantity of gel in the polymer residue due to interchain crosslinking. In several cases the degradation of chlorine-containing polymers leads to the formation of noticeable quantities of polyconjugated systems which are 87

Thermo-oxidative Degradation of Polymers characterised by the coloration of samples and the formation of paramagnetic centres [1]. The latter are formed via the local cleavage of π-bonds in the long conjugated chain and the production of excited triplet states. Ouchi [1] examined PVC which had been heated in air or a vacuum to temperatures up to 400 °C by ESR. The polymer was examined by means of a spectrometer with a modified microwave circuit. The ESR signals are first observed at an early stage of degradation and increase with heating time. In the case of heat treatment in a vacuum, the ESR stops increasing when dehydrochlorination is nearly finished whereas, in the case of treatment in air, the signal continues to increase after dehydrochlorination is finished. The effects of oxygen and the saturation characteristics of the ESR are quite different for the chars heat-treated at 400 °C. Below 400 °C, the signal shows strong saturation in a vacuum but not in air. Oxygen readily affects the signal, shortening the spin-lattice relaxation time. For the char treated at >400 °C, the signal does not saturate as strongly and is quenched by oxygen. PVC is known to show two-step carbonisation that is readily distinguishable by its thermogram or by its abrupt change to a tarry state at around 400 °C. In the first stage, up to 400 °C hydrochlorination is the chief reaction, and dehydrogenation does not occur unless by oxidation. In this study, PVC was heat-treated in most cases at 3F. Figure 5.6 shows IGA curves for PMR-II-50 at the temperatures listed. Although the accelerated weight loss observed proceeding from 475 °C to 500 °C is expected, the subsequent deceleration observed in rising to 550 °C and again to 575 °C is surprising. Indeed, at the highest isothermal temperature (575 °C), there is an initial period of time during which the sample maintains weight.

109


Table 5.3 Polyimide stability using isothermal ageing weight loss studies Method

3F (C)

PMR-II-50 (A)

VCAP-75 (B)

6F (D)

Coats/Redfern Ea at 480-575 oC (kJ/mol)

109.1 ± 3.27

124.07 ± 4.45

136.23 ± 3.88

140.21 ± 5.74

Ing/Mar Ea at 480575 oC (kJ/mol)

83.67 ± 16.02

172.56 ± 1.67

186.01 ± 5.33

162.44 ± 9.25

Hor/Metz Ea 3-90% conversion (kJ/mol)

122.73 ± 2.39

136.11 ± 3.60

129.68 ± 11.88

161.90 ± 4.83

Isothermal Ea (kJ/mol)

142.28

139.10

150.08

170.70

IWL weight loss aged at 370 oC for 400 h (%)

71.01 [22]

28.65 [23] ± 2.070

26.27 [23] ± 1.30

551.3 ± 0.4

Td (oC)

500.4 ± 2.2

545.0 ± 3.0

551.3 ± 0.4

560.4 ± 1.6

Td: Decomposition temperature Source: Author’s own files

Weight remaining %

575 °C 80 60

550 °C

475 °C

40 20

500 °C 0

100

200 300 Time, (min)

400

500

Figure 5.6 IGA curves for PMR-II-50 (see Figure 5.5). Reproduced with permission from M.J. Turk, A.S. Ansari, W.B. Alston, G.S. Gahn, A.A. Frimer and D.A. Scheiman, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1999, 37, 21, 3943. ©1999, Wiley [15] 110

Nitrogen-Containing Polymers This temporary weight gain is observed only for PMR-II-50 because it contains the greatest amount of oxidisable aliphatics compared with the other three PI in this study. The other PI (including 3F) displayed normal IGA weight loss (i.e., greater weight loss as IGA maximum temperature was increased). However, 3F at the highest IGA temperature (575 °C) starts to undergo a two-step decomposition (seen via TGA) that complicates analysis using IGA or TGA. Thus, only the linear portion of each curve, which occurred at the higher weight-loss range, could be used to determine Ea by the Arrhenius equation. Figure 5.7 summarises the two probable degradation pathways that could account for the five evolved products observed (CO2, CO, ArNCO, CHF3 and HF). The two pathways (CO2, CO and ArNCO versus CHF3 and HF) appear to occur independently. In one, thermal cleavage of the C–N imide linkage produces phenyl C–O radicals and amide radicals, both of which can undergo direct loss of CO. Any non-reacted anhydride end group can undergo direct loss of CO and CO2, although possibly CO or the phenyl CO radical can oxidise to account for some additional CO2 evolution. Other possible direct sources of CO2 are iso-imide and/or uncyclised imides (i.e., amic acids).

O

O

CF3

CF3

O

X

C O

N O

CF3

O

O

C

O

N

H OH

N

N

O

O X=

O

or CF3

H2O or CO2 O

X

CF3 C

C

O C

O

N O

O

Carbon Monoxide and Carbon Dioxide

Trifluoomethane and some Hydrogen Fluoride

O C N O

a = Carbon Monoxide b + c = p Phenylene Dilsocyanate b + d = Phenyl lsocvanate

Figure 5.7 Probable initial decomposition events and identified evolved degradation products. Source: Author’s own files

111

Thermo-oxidative Degradation of Polymers At the adjoining imide group, a similar thermal cleavage can generate more CO. More importantly, however, it can also generate the low levels of ArNCO, presumably as p-phenylene diisocyanate (rather than cleavage at the aromatic nitrogen bond that would lead to only phenyl isocyanate instead). The second pathway is the loss of CHF3 and F species from the 3F and 6F connecting linkages. This process appears to be independent of the evolution of CO2, CO and ArNCO. The CHF3 evolution data clearly show that the 3F linkage generates CHF3 at about three-times the rate and amount of the 6F linkage. The amount of HF from either linkage was minimal but real, and was not analysed quantitatively. Turk and co-workers [15] concluded that to best reproduce the ranking of long-term IWL studies, the Coats/Redfern dynamic TGA kinetic treatment should be used. In this study, Td via TGA studies correlates well with long-term IWL results which, together with the Coats/Redfern kinetic treatment, are an accurate means of ranking the thermo-oxidative stability of PI. In this study, the thermo-oxidative ranking was 5F PI > VCAP-75 > PMR-II-50 > 3F PI. This ranking reflects only the relative order of thermal stability and thermo-oxidative performance. The data suggest that PI degradation presumably occurs via two thermal pathways. In one pathway, the imide C–N bond cleaves homolytically, generating an amide radical and a phenyl CO radical to produce CO and possibly undergo further oxidation in air to CO2. Simultaneously, a second imide C–N bond cleavage on the other side of the phenyl ring would create another amide radical that would evolve a phenyl isocyanate (presumably p-phenylene diisocyanate). In the second pathway, loss of CHF3 and F species from 3F and 6F linkages occurs with 3F linkage degradation, producing almost three-times the CHF3 than the 6F linkage. The poorer thermo-oxidative of 3F may be attributed to the greater number of degradation sites generated on the 3F polymer chain.

5.4 Polyamides Differential scanning calorimetry (DSC) and TGA have been used to evaluate the oxidative and thermal stability of polyamide 6,6 [24, 25]. Eriksson and co-workers [24] evaluated thermal stability in an oxidative environment by isothermal and DSC. They found a decrease in the oxidative stability of the polymer as a result of repeated injection moulding. The effect of the isothermal temperature on the exothermic behaviour of Sample 1 is shown in Figure 5.8. During a run, the initial breakdown of stabilisation protection and oxidation of the polyamide resin is accompanied by an exothermal reaction

112

Nitrogen-Containing Polymers causing a deflection of the heat-flow curve. This exothermic effect is the signature of severe degradation of the polymer. As the isothermal temperature is raised, the peak height is increased and the peak width decreases. From these isothermal DSC scans, the oxidative induction time (OIT) was determined. The deviation from the baseline when oxidation starts was used as a criterion for induction time. This is defined as the intersection of the extrapolated baseline and the tangent at a point on the heatflow curve with the steepest portion. As expected, the induction time decreased with increase in temperature. Usually, the induction period follows an Arrhenius-type dependence on temperature, i.e., plots of induction time against reciprocal temperature being linear. The OIT was determined to be 21.4 minutes and 12.1 minutes at 260 °C and 265 °C, respectively. An increase of 5 °C corresponds to about a decrease of 50% in induction time. At 270 °C, it was not possible to detect an OIT. The shortest induction periods which can be measured are limited to a few minutes because of the time taken to establish thermal equilibrium. A typical DSC scan of heat-stabilised polyamide 6,6 is shown in Figure 5.9. The endotherm at approximately 260 °C is due to the crystalline melting of the polyamide. The onset temperature of oxidation, where the exothermic reaction causes a shift in the baseline which was observed by extrapolation, is approximately 305 °C. The oxidation peak temperature was determined to be about 330 °C.

Heat flow (w/g)

0.15

270 °C

0.125 0.1

265 °C

0.075 260 °C

0.05 0.025

0

10

20 30 40 Time (min)

50

60

Figure 5.8 Heat flow of Sample 1 (not reprocessed) at isothermal temperatures of 260 °C, 265 °C and 270 °C. Reproduced with permission from P-A. Eriksson, A-C. Albertsson, K. Eriksson and J-A.E. Månson, Journal of Thermal Analysis and Calorimetry, 1998, 53, 1, 19. ©1998, Springer [24]

113


Heat flow (w/g)

0.5

OPT

0 TOX

-0.5 -1 225

250

275 300 325 Temperature °C

350

Figure 5.9 Typical dynamic DSC scan of heat-stabilised polyamide 6,6. Reproduced with permission from P-A. Eriksson, A-C. Albertsson, K. Eriksson and J-A.E. Månson, Journal of Thermal Analysis and Calorimetry, 1998, 53, 1, 19. ©1998, Springerlink [24]

DSC was used for samples 1–4 (Table 5.4) at an isothermal temperature of 260 °C. The OIT was determined to be 21.4, 17.8, 14.1 and 8.6 minutes for Samples 1 to 4, respectively. Reprocessed samples exhibit lower OIT values compared with that of the virgin reference samples. The presence of glass fibres in the polyamide matrix seems to decrease the thermo-oxidative stability.

Table 5.4 Properties of samples examined Sample designation

Grade

Fibreglass-reinforced

Reprocessed

1

Technyl A218

No

No

2

Technyl A218

No

Yes

3

Technyl A218 V30

Yes

No

4

Technyl A218 V30

Yes

Yes

Reproduced with permission from P-A. Eriksson, A-C. Albertsson, K. Eriksson and J-A.E. Månson, Journal of Thermal Analysis and Calorimetry, 1998, 53, 1, 19. ©1998, Springer [24]

114

Nitrogen-Containing Polymers DSC was applied to Samples 1–4 (Table 5.4) at a heating rate of 10 °C/min. The onset temperature of oxidation was measured to be 306.3 °C, 298.3 °C, 289.8 °C and 283.0 °C for Samples 1 to 4. The oxidation peak temperature was determined to be 333.3 °C, 329.3 °C, 324.1 °C and 318.7 °C for the same samples. The order of oxidative stability between the samples determined by DSC measurements is the same as that based on the induction period during isothermal measurements. Repeated injection moulding and the presence of glass fibres both have a negative influence on the temperature of oxidation. Darie and co-workers [25] studied the thermal and thermo-oxidative behaviour of isotactic polypropylene/polyamide-6 (PA6) and tertiary isotactic polypropylene, PA6ethylene-propylene-diene blends by DSC and TGA. Polyamides and polyolefins are two important classes of commercial polymers. The former are frequently blended with lower-modulus polyolefins to prepare polymer alloys of enhanced characteristics. Usually, in a blend, polyolefins offer melt strength, flexibility, lubricity, impact strength, electrical resistance, low dielectric constant and dielectric loss, water resistance and low price. Polyamides offer melt fluidity, rigidity, strength, high-temperature performance and solvent resistance. The processing characteristics and DSC measurements showed the incompatibility of the binary isotactic polypropylene (iPP)/PA6 blends (especially for the 50/50 ratio of mixing). Functionalised iPP changed the melting behaviour of components of the blends, mainly for PA6, decreasing melting heat and crystallinity degree. This could be explained by the reactions of carboxyl or anhydride groups from functionalised iPP with the end-amino group from the polyamide leading to increase of the amorphous phase. The shape of the thermogravimetric and dynamic thermogravimetric (DTG) curves changes with the blend composition (Figure 5.10). Two distinct peaks or clear shoulders in DTG curves reveal the incompatibility of binary iPP/PA6 blends whereas larger peaks appear for ternary iPP/PA6/ethylene propylene diene terpolymer (EPDM) blends. Changes in the shape of thermogravimetric and DTG curves for the blends containing functionalised iPP indicated an increased degree of compatibilisation, and reactions between functional groups during processing and/or heating at high temperatures were in accordance with processing behaviour and DSC results. Maleic anhydride (MA)-, bismaleimide (BMI)-, and acrylic acid (AA)-functionalised iPP were shown to act as compatibilising agents for ternary iPP/PA6/EPDM blends. The compatibilising effect follows the order iPP-MA≈ iPP-BMI>iPP-AA for an optimum amount of 3 wt%.

115

Thermo-oxidative Degradation of Polymers Another technique that has been used to study the thermo-oxidative behaviour of polyamide 6,6 is headspace solid-phase micro-extraction combined with gas chromatography–mass spectroscopy (GC–MS) [26]. This technique was used to reveal the correlation between the degradation product pattern and mechanical properties of the polymer. The identified degradation products were categorised into four groups: cyclic imides, pyridine derivatives, chain fragments, and cyclopentanone derivatives. Five cyclic imides (2,6-piperidinedione (glutarimide), azepane-2,7-dione, 1-propyl2,5-pyrolidinedione, 1-butyl-2,5-pyrrolidinedione, and 1-pentyl-2,5-pyrrolidinedione) were detected after oxidation. The amounts of cyclic imides clearly increased as a function of the thermo-oxidation time, and larger amounts of cyclic imides were formed during the thermo-oxidation of recycled polyamide than during the thermooxidation of virgin polyamide 6,6. In particular, 1-pentyl-2,5-pyrrolidinedione showed a large increase in abundance. At the beginning of the oxidation, it was found only at trace levels but, after 1200 hours, its relative peak area had increased drastically, making it the most abundant degradation product. The increase was prominent for virgin and recycled materials, but substantially more 1-pentyl-2,5-pyrrolidinedione was formed from the recycled materials than from the virgin material. The amount of 1-pentyl-2,5-pyrrolidinedione that formed during the oxidation corresponded almost linearly to the number of recycling steps of the respective samples. Lower amounts of glutarimide, 1-propyl-2,5-pyrrolidinedione and 1-butyl-2,5-pyrrolidinedione were detected, and they showed similar behaviour toward oxidation and recycling as 1-pentyl-2,5-pyrrolidinedione; that is, their amount increased during thermooxidation and repeated processing. Approximately the same amount of azepane-2,7dione was detected in virgin and recycled materials, and the amount remained almost constant throughout the oxidation. This suggests that azepane-2,7-dione is formed through a different mechanism than the other succinimides within this group. It can be formed by ring closure of the adipamide produced as the polyamide 6,6 backbone is cleaved by oxidation of the N-vicinal methylene group of two neighbouring hexamethylenediamine units or as a result of oxidation of caprolactam. There was a good correlation between the recorded degradation product pattern and the changes in mechanical properties during the thermo-oxidation. Figure 5.11 shows the tensile strength and the amount of the most abundant thermo-oxidation product, 1-pentyl-2,5-pyrrolidinedione, as a function of the thermal oxidation time for a virgin polymer and a material recycled once. Head-space-solid-phase microextraction (SPME)/GC–MS and tensile testing could be used to differentiate between the samples recycled one, twice or thrice. SPME/GC–MS showed that the amounts of degradation products increased with the number of extrusions. FTIR and DSC measurements did not provide sufficiently clear and evident data to differentiate between samples recycled for different times.

116

Nitrogen-Containing Polymers (a) 100

(b) 100

409

40 20 400 300 Temperature °C

60 40 20

455 200

w, %

-dW/dT, au

w, %

335

60

-dW/dT, au

80

80

0

455

0

500

(c) 100

517 200

300 400 Temperature °C

500

460

-dW/dT, au

w, %

80 60 388

40 20 0

495 200

300 400 Temperature °C

500

Figure 5.10 Experimental thermogravimetric and DTG curves (––) and stimulated individual ( ) and total (---) DTG curves for thermo-oxidative decomposition of (a) iPP, (b) PA6, and (c) EPDM. au = arbitrary units Reproduced with permission from R.N. Darie, M. Brebu, C. Vasile and K. Kozlowski, Polymer Degradation and Stability, 2003, 80, 3, 551. ©2003, Elsevier [25]

117

400

80

300

60

200

40

100

20 0

25

100

500 1200 100 Ageing Time (hours) (a)

80

0

400 300

60

200

40

100

20 0

Relative Abundance (%)

100

Relative Abundance (%)

Tensile Strength (MPa)

Tensile Strength (MPa)


25

100 500 1200 Ageing Time (hours) (b)

0

Figure 5.11 Loss of tensile strength versus the formation of 1-pentyl-2,5pyrrolidinedione during 1200 h of thermo-oxidation of (a) virgin, and (b) once recycled polyamide 6,6 at 100 °C. Reproduced with permission from M. Gröning and M. Hakkarainen, Journal of Applied Polymer Science, 2002, 86, 13, 3396. ©2002, Wiley [26]

Imaging chemiluminescence has also been used to determine oxidation profiles of polyamide 6,6 [26-28].

118

Nitrogen-Containing Polymers

5.5 Polycaprolactam Heating polycaprolactam (PCL) at comparatively low temperatures (about 100 °C) in air causes the formation of peroxides which act as branching agents, leading to thermal oxidation via a degenerate branched chain mechanism. Peroxide compounds are not found in PCL at higher temperatures. Mass spectroscopic studies on the thermal degradation products of aromatic polyamides have established that oxygen is completely consumed in forming CO2 and H2O. Oxygen is not an initiator for the decomposition of aromatic polyamides but only participates in secondary oxidation reactions of the decomposition products [29]. Oxygen promotes the amido-iminol rearrangement and the iminol groups formed readily engage in intermolecular condensation to produce crosslinked polymer, releasing water that hydrolyses the amide bonds. Thus, the amide bond is the weakest link in aliphatic and aromatic polyamides of different chemical structure. In thermal and thermo-oxidative degradation, this bond is subjected to decomposition via homolytic and heterolytic mechanisms leading to the decomposition of polymer chains, their crosslinking and to isomerisation.

References 1.

J.K. Backus, W.C. Darr, P.G. Gemeinhardt and J.H. Saunders, Journal of Cellular Plastics, 1965, 1, 1, 178.

2.

A.F. McKay and G.R. Vasasour, Canadian Journal of Chemistry, 1958, 31, 7, 688.

3.

T.M. Laakso and D.D. Reynolds, Journal of the American Chemical Society, 1957, 79, 21, 5717.

4.

H.C. Beachell and C.P. Ngoc Son, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1964, 2, 11, 4773.

5.

S.L. Madorsky and S. Straus, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1959, 36, 130, 183.

6.

V.A. Andreva, Y.N. Tolparov and E.I. Firsov, Vysokomolekulyarnye Soedineniya Seriya B, 1986, 28, 111.

7.

S.L. Madorsky, Thermal Degradation of Organic Polymers, Polymer Reviews, Volume 7, Wiley Inter-Science, New York, NY, USA, 1964.

119


L. Costa, C. Camino, A. Guyot, M. Bert, G. Clouet and J. Brossas, Polymer Degradation and Stability, 1956, 14, 1, 85.

9.

M.I. Bessonove, M.M. Koton, V.V. Kudryavtsev and L.A. Laius, Polyimides – A Class of Thermostable Polymers, Nauka, Leningrad, Russia, 1983. [in Russian]

10. P.N. Gribikova, V.V. Rade, S.Y. Vygodskii, S.V. Virogradova and V.V. Korshak, Vysokomolekulyarnye Soedineniya Seriya A, 1970, 12, 220. 11. B.M. Kovarskaya, A.B. Blyumenfeld and S.I. Levontovskay, Thermal Stability of Heterochain Polymers, Khimiya, Moscow, Russia, 1977. [in Russian] 12. A.B. Blyumenfeld, B.M. Kovarskaya, V.A. Popov and A.I. Puzeev, Plasticheskie Massy, 1975, p.75. 13. L.A. Laius, E.N. Dergacheva, T.I. Zhukova and M.I. Bossonov, Vysokomolekulyarnye Soedineniya Seriya B, 1986, 28, 39. 14. V.V. Korshak, Chemical Structure and Temperature Characteristics of Polymers, Nauka, Moscow, Russia, 1970. [in Russian] 15. M.J. Turk, A.S. Ansari, W.B. Alston, G.S. Gahn, A. Frimer and D.A. Scheiman, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1999, 37, 21, 3943. 16. K.C. Chuang, R.D. Vannucci, I. Ansari, L.L. Cerny and D.A. Scheiman, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1994, 32, 7, 1341. 17. C. Pineche, D. Zaldívar, M. Pazos, S. Páz, A. Bulay and J. San Román, Journal of Applied Polymer Science, 1993, 50, 3, 486. 18. P.S. Bhandare and K. Krishnan, Mikrochimica Acta, 1997, 14, Supplement, 729. 19. M.L. Mittleman, D. Johnson and C.A. Wilkie, Trends in Polymer Science, 1994, 2, 391. 20. D.J. Johnson, D.A.C. Compton, R.S. Cass and P.L. Canale, Thermochimica Acta, 1993, 230, 293. 21. M.L. Mittleman and C.A. Wilkie, Private Communication.

120

Nitrogen-Containing Polymers 22. W.B. Alston in the Proceedings of the USAF Sponsored High Temperature Workshop XIV, 1994, pJ1-J20. 23. K.J. Laidler, Chemical Kinetics, McGraw-Hill, New York, NY, USA, 1950, p.76. 24. P-A. Eriksson, A-C. Albertsson, K. Eriksson and J-A. Månson, Journal of Thermal Analysis and Calorimetry, 1998, 53, 1, 19. 25. R.N. Darie, M. Brebu, C. Vasile and K. Kozlowski, Polymer Degradation and Stability, 2003, 80, 3, 551. 26. M. Gröning and M. Hakkarainen, Journal of Applied Polymer Science, 2002, 86, 13, 3396. 27. G. Ahlblad, D. Forsström, B. Stenberg, B. Terselius, T. Reitberger and L.G. Svensson, Polymer Degradation and Stability, 1997, 55, 3, 287. 28. D. Forsström, T. Reitberger and B. Terselius, Polymer Degradation and Stability, 2000, 67, 2, 255. 29. V.V. Rode, P.N. Gribkova, Y.S. Vygodsky, S.Vv. Vinogradova and V.V. Korshak, Izvestiya Akademii Nauk SSR, Seriya Khimicheskaya, 1969, p.85.

121


122

6

Silicon-Containing Polymers

The thermal stability of polyorganosiloxanes containing aromatic groups is higher than that of their aliphatic analogues, increasing considerably the introduction of phenylene groups into the backbone. The nature of the side-group also has a considerable effect on the thermal stability of polyorganosiloxanes: polymers containing a CH3 group are the most thermally stable, and those with the p-dimethylaminophenyl group being thermally stable. Crosslinked and ladder organosilicon polymers are characterised by high thermal stability [1]. Liquid polydimethylsiloxanes are stable up to 175 °C, whereas liquid polyphenylmethylsiloxanes are stable up to 300 °C. Polydimethylsiloxane (PDMS) decomposes under vacuum at 400 °C to form the cyclic oligomers hexamethylycyclotrisiloxane and oxamethylcyclotetrasiloxane. This process occurs at lower temperatures in the presence of terminal Si–OH groups or additives of electrophilic or nucleophilic character. The terminal OH groups interact with the Si–O bond by the scheme: Si(CH3)2 ~O

Si(CH3)2

O

O

HO

Si(CH3)2

(CH3)2Si

~O

Si(CH3)2

OH + [Si(CH3)2O]n

where n = 3.4

O

(6.1)

During the thermal degradation of polysiloxanes, the weaker Si–C bond is cleaved at random and then the radicals formed undergo several transformations: ~Si(CH3)2

O

Si(CH3)2

~Si(CH3)2 + O = Si(CH3)2

O~

~Si(CH3)2

O

Si(CH3)2

O~ + CH3

O~

(6.2)

123

Thermo-oxidative Degradation of Polymers CH2 O~

CH3 + ~Si(CH3)2

CH4 + ~Si(CH3)2

O

Si

O~

CH3 CH2 ~Si(CH3)2

O + Si

O~

CH3 O

Si(CH3)2

(6.3)

Si(CH3)2

O

O Si(CH3)2

(CH3)2Si O

O

Si(CH3)2 + [Si(CH3)2O]3

(6.4)

Thus it follows that a radical-chain process is the main mechanism for the thermal degradation of polyorganosiloxanes at comparatively low temperatures. Thermal degradation of organosilicon polymers of cyclolinear structure is more complex. Based on results from mass spectroscopy, nuclear magnetic resonance, and gas–liquid chromatography of the products of thermal degradation, it was possible to suggest an appropriate mechanism [2]. The data obtained suggest that all degradation products (cyclic and bicyclic organosiloxanes) are formed by the mechanism suggested for PDMS, according to which the decomposition begins, as shown above, with the formation of an intermediate four-centre complex followed by rearrangement of the siloxane sites of two linear segments of the polymer chain. Besides cyclosiloxanes, products such as formaldehyde, formic acid, CH3OH, H2O, CO, CO2, CH4 and solid residue are formed during pyrolysis of polyorganosiloxanes [2]. The degree of acceleration observed during the decomposition of polyorganosiloxanes in the presence of oxygen is associated with the oxidation of side-groups, leading to the formation of Si–OH groups, which are active in the degradation. The following main schemes are suggested for the thermal oxidation of organic groups at the silicon atom of polyorganosiloxanes [2, 3]:

124

Silicon-Containing Polymers

2 ≡ Si − CH3+ O2 ≡ SiCH2+ O2

• 2 ≡ Si − CH3+ H2O2 • ≡ Si − CH2OO

• ≡ Si − O + H3C − Si ≡

• ≡ Si − OOCH2

≡ Si − O• + CH2O CO + H2

• ≡ Si − OH + ≡Si − CH2

(6.5)

• ≡ Si − CH3+ O2 ≡ Si − CH2OOH • ≡ Si + OH ≡ Si − OH

≡ Si − CH3+ O2

(6.6)

• • ≡ Si − CH2 + HO2 • ≡ Si − CH2OO

≡ Si − CH2+ O2 • ≡ Si − CH2OO + H3CSi≡ ≡ Si − CH2OOH • ≡ Si − CH2O

• • ≡ Si + CH2O + OH

• ≡ SiCH2OOH + ≡Si − CH2

• • ≡ Si − CH2O + OH

• ≡ Si + CH2O

(6.7)

References 1.

M.P. Kharitonov and V.V. Ostrovsky, Thermal and Thermooxidative Degradation of Polyoxysiloxanes, Nauka, Leningrad, Russia, 1982. [in Russian]

2.

S.P. Pavlova, I.V. Zhurevleva and Y.I. Tolchinski, Thermal Analysis of Organics and High Molecular Weight Compounds, Khimiya, Moscow, Russia, 1983. [in Russian]

3.

V.S. Osipechik, M.S. Akutim, E.B. Lebedereva, V.V. Dudin and V.G. Frolop, Plasticheskie Massy, 1973, p.13.

125


126

A

bbreviations

AA

Acrylic acid

ATR

Attenuated reflectance spectroscopy

au

Arbitrary units

BF3MEA

Boron trifluoride monoethylamine

BHT

Butylated hydroxy toluene

BMI

Bismaleimide

CNR

Chlorinated natural rubber(s)

CO

Carbon monoxide

CO2

Carbon dioxide

DSC

Differential scanning calorimetry

DT

Decomposition temperature

DTA

Differential thermal analysis

DTG

Dynamic thermogravimetric

Ea

Activation energy

EGA

Evolved gas analysis

EO

Ethylene oxide

EPDM

Ethylene propylene diene terpolymer

EPR

Electron paramagnetic resonance

127

Thermo-oxidative Degradation of Polymers ESR

Electron spin resonance

EVA

Ethylene vinyl acetate

FTIR

Fourier-transform infrared

GC

Gas chromatography

GC-MS

Gas chromatography-mass spectroscopy

HDPE

Polyethylene(s)

HR-Py-GC-MS High-resolution pyrolysis–gas chromatography–mass spectroscopy IGA Isothermal thermogravimetric analysis ILDTA

Iso-thermal long-term differential thermal analysis

iPP

Isotactic polypropylene(s)

IR

Infrared

IWL

Ignition weight loss

LDPE

Low-density polyethylene(s)

LDPE-MB

Low-density polyethylene - masterbatch

LDPE-PO

Low-density polyethylene - pro-oxidant

MA

Maleic anhydride

MFI

Melt flow index

MS

Mass spectrometry

NMR

Nuclear magnetic resonance

OIT

Oxidative induction time

PA6

Polyamide-6

128

Abbreviations PC

Polycarbonate(s)

PCL

Polycaprolactam

PDMS

Polydimethylsiloxane(s)

PDSC

Pressure differential scanning calorimetry

PE

Polyethylene

PEO

Polyethylene oxide

PETP

Polyethylene terephthalate

PF

Phenol-formaldehyde

PI

Polyimide(s)

PO

Propylene oxide

POM

Polyoxymethylene

POMAc

Acetylated polyoxymethylene

PP

Polypropylene(s)

PPO

Polyphenylene oxide(s)

PS

Polystyrene

PTFE

Polytetrafluoroethylene

PVC

Polyvinyl chloride

Py-GC

Pyrolysis–gas chromatography

Py-GC-MS

Pyrolysis–gas chromatography–mass spectrometry

SPME

Space-solid-phase microextraction

SPME/GC-MS Solid-phase microextraction/gas chromatography–mass- spectrometry

129

Thermo-oxidative Degradation of Polymers Td

Temperature of onset of thermal decomposition

TGA

Thermogravimetric analysis

UV

Ultraviolet

130

I

ndex

A Activation energy 108-109, 111 Ageing 108 time 82 time, isothermal 11 Antioxidants, phenolic 83 Aromatic groups 123 Arrhenius equation 111 Autoxidation 21, 24

B Back-biting 79 Bisphenol-A based epoxy resins 72 Bisphenol-A phenyl ether 72 Bisphenol-A structural motif 72 Bond-breaking 17 Butylated hydroxyl toluene 74-75

C Caking effect 13 Carbonisation 88 Carbonyl band 36, 37 Char formation 65, 68 Chemiluminescence analysis 1 Chemiluminescence, imaging 118 Chlorinated natural rubber 44, 89 Chlorinated polyester urethane 97, 101 Chromatography-mass spectroscopy 116 Coats/Redfern kinetic treatment 108, 112 Copolymerisation 1 Curing agent 69 Cyclopentanone derivatives 116 Cyclosiloxanes 124

131


D Deacetylation 80, 83 Decarboxylation 55, 57, 61 Decomposition, two-step 111 Defluorination 89 Degradation, thermal 14, 21, 90, 104, 106 Degradation, thermo-oxidative 57, 72, 89, 102, 105-106, 119 Dehydrochlorination 88-89 Depolymerisation 102 chain 52 Differential scanning calorimetry 1, 14, 16, 27, 37, 72, 112 with chemiluminescence 15, 28, 37-41, 83, 12, 114-116 Differential thermal analysis 38, 97 Dynamic thermal analysis 42, 44, 58, 89, 102 Dynamic thermogravimetry 115, 117

E Electron paramagnetic resonance 1, 58, 87, 103, 104 Electron spin resonance 88-89 Epoxy resins 68, 70, 72 amine-cured 70 Ethylene oxide-propylene oxide copolymers 73 Ethylene-propylene copolymer 14 gamma-irradiated 38 Ethylene vinyl acetate copolymers 77, 81-83 Evolved gas analysis 16

F Flame retardant polyether urethane 97, 101 Fourier-transform infrared 30- 31, 44, 77-78, 81-83, 89, 108, 116 spectrometers 17, 108 Fragmentation reactions 67 Free radical mechanism 61

G Gas chromatography 44, 64 peak-identification system 42 Gas-liquid chromatography 124

H Hexamethylcyclotrisiloxane 123 High-density polyethylene 28, 39-40

132

Index High-resolution pyrolysis-gas chromatography-mass spectroscopy 89 Hydrochlorination 88 Hydrolysis 106 Hydroperoxidation 81 Hydroxyl region 36

I Ignition weight loss 108-109, 112 Imides, cyclic 116 Impact strength 41 Initiators, free radical 58 Injection moulding 112, 115 Isothermal test 38 Isothermal gravimetric analysis 108-109, 111 Isothermal gravimetric analysis – Fourier transform infrared spectroscopy 109

L Low-density polyethylene 30 Luminescence analysis 77

M Mass spectroscopy 44, 124 Mass spectroscopy, linear-programmed thermal degradation 17 Melt flow index 41

N Natural rubber 44, 89 Nuclear magnetic resonance spectroscopy 1, 17, 56, 124 Nylon, fibre glass-reinforced 1, 2

O Oligomers, cyclic 123 Organophosphorus flame 97 Oven-ageing 38 Oxamethylcyclotetrasiloxane 123 Oxidation, degree of 35 Oxidation, free radical 83 Oxidation, long-term 71 Oxidation, thermal 91-92 Oxidative induction time 28, 39-40, 113-114

133


P Phenol-formaldehyde resins 62-65, 70-72, 83 Phenylene groups 123 Phenolic resins, novolac-type 83 Photolysis, vacuum 45 Photo-oxidation 30, 36 Photo-oxidative degradation 58 Physico-mechanical testing 41 Poloxamer 73 Poly(α-methyl styrene) 45 Polyacrylonitrile 103-104 Polyalkenes 23-24, 34, 112, 115 oxidation of 22 Polyarylenequinones 62 Polybenzimidazoles 106-107 Polybenzoxazoles 107 Polycaprolactam 119 Polycarbonate 55-56 Polydimethylsiloxane 123-124 Polyester 54, 61 styrenated 58-59 Polyether urethane 97, 99-101 Polyethylene 21, 25, 27, 29, 33, 37, 41 purified 25-26 unpurified 25-26 Polyethylene terephthalate 57-58 Polyimides 105-106, 108, 111 Polyisoprene 33 Polymer degradation 5, 7, 24, 35, 87 one-step 3 Polymer density 93 Polymer, isocyanate 97 Polymer, lifetime of 3 Polymers, halogen-containing 87 Polymers, oxygen-containing 23, 51 Polymers, silicon-containing 123 Polymerisation 46, 62 Polymethacrylates 58 Polyolefins 36, 77, 82, 115 Polyorganosiloxanes 123-124 aliphatic analogues 123

134

Index Polyoxamer 74 Polyoxymethylene 51-53 Polyphenylene oxides 54 Polypropylene 27-28, 33, 37, 39, 40, 42-44 crystalline 36 Polypropylene/polyethylene 41 Polysiloxanes 123 Polystyrene 45-46 emulsifier-free 46 Polytetrafluoroethylene 1-2, 89-91 Polyvinyl chloride 83, 87-88 Positron annihilation lifetime spectroscopy 1, 44 Pressure differential scanning calorimetry 15, 37-38 Pyridine derivatives 116 Pyrolysis 16, 55-56, 63-64, 91, 105 instrumentation 17 Pyrolysis-gas chromatography 17, 30, 37, 42 Pyrolysis-gas chromatography-mass spectrometry 30, 44 Pyrolysis-mass spectrometry 17 Pyrrolidinedione 118

R Radical-chain mechanism 107, 124 oxidation of 54 Resins, cured 68

S Size exclusion chromatography 30 Skin effect 13 Solid-phase microextraction/gas chromatography-mass spectrometry 73, 116 Spectroscopy, infrared 28, 35-36, 44-45, 51, 56, 58, 63-64, 70, 100 Spectroscopy, ultraviolet 87 Spin-lattice relaxation time 88

T Tacticity 1 Teflon 92-94 degradation of 13 Tensile testing 116 long-term 43 Thermal analysis, isothermal long-term differential 43 Thermogravimetric analysis, weight-loss treatment 109

135

Thermo-oxidative Degradation of Polymers Thermogravimetric analysis – Fourier transform infrared spectroscopy 109 Thermal analysis 38, 44, 77, 89 Thermal decomposition 106, 108 temperature 108 Thermograms 3 Thermogravimetric analysis 1, 2, 5, 6, 11, 13, 16, 24, 30, 38, 42, 44, 51-52, 56, 58-59, 62-63, 65, 68, 70, 72, 83, 89, 91-93, 97, 102, 108-109, 112, 115 Thermo-oxidation 30, 116, 118

U Urethane foams, rigid 97, 100 Urethane polymers 102

V Volatilisation 89

Y Yellowness index 41

Z Zinc selenium crystal 78

136

Thermo-oxidative Degradation of Polymers

Stabilization and Degradation of Polymers

Microbial Degradation of Xenobiotics

Degradation 1

Degradation Rites

Lysosomal Pathways of Protein Degradation

Environmental Degradation of Industrial Composites

Polymers

Degradation 2

The Degradation of American History

Thermal Degradation of Polymeric Materials

Handbook of environmental degradation of materials

Extracellular Matrix Degradation

Handbook of environmental degradation of materials

Properties of Polymers

Physical Properties of Polymers

Electrical Properties of Polymers

Molecular Imprinting of Polymers

Encyclopedic Dictionary of Polymers

Electrical Properties of Polymers

Microhardness of polymers

Adhesion of Polymers

Electrical Properties of Polymers

Polyfunctional Stabilizers of Polymers

Electrical Properties of Polymers

Handbook of Conducting Polymers

Adhesion of Polymers

Handbook of Environmental Degradation of Materials

Extracellular Matrix Degradation

Polymer Degradation and Performance

Electrical Properties of Polymers

Thermo-oxidative Degradation of Polymers

Stabilization and Degradation of Polymers

Microbial Degradation of Xenobiotics

Degradation 1

Degradation Rites

Lysosomal Pathways of Protein Degradation

Environmental Degradation of Industrial Composites

Polymers

Degradation 2

The Degradation of American History

Thermal Degradation of Polymeric Materials

Handbook of environmental degradation of materials

Extracellular Matrix Degradation

Handbook of environmental degradation of materials

Properties of Polymers

Physical Properties of Polymers

Electrical Properties of Polymers

Molecular Imprinting of Polymers

Encyclopedic Dictionary of Polymers

Electrical Properties of Polymers

Microhardness of polymers

Adhesion of Polymers

Electrical Properties of Polymers

Polyfunctional Stabilizers of Polymers

Electrical Properties of Polymers

Handbook of Conducting Polymers

Adhesion of Polymers

Handbook of Environmental Degradation of Materials

Extracellular Matrix Degradation

Polymer Degradation and Performance

Electrical Properties of Polymers

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