Temperature, C

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1 3 xygen-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 (PM) and acetylated polyoxymethylene (PMAc) 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 Temperature, C 400 Figure 3.1 TGA curves for polyoxymethylene (PM), acetylated PM (PMAc), and Delrin in air: ( ) PM; (o) PMAc, and (Δ) Delrin 5000X. Reproduced with permission from S. Igarashi, I. Mita and H. Kambe, Bulletin of the Chemical Society Japan, 1964, 37, 8, , Chemical Society of Japan [1] In this case, PM and PMAc showed a large weight loss between 150 C and 51

2 Thermo-oxidative Degradation of Polymers 200 C, and the temperature for complete decomposition was higher for Delrin than for PM. These results showed that, although PM 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 PM 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 PM 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 PM 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. 3.0 Weight loss, %/ C Temperature, C 300 Figure 3.2 Thermal decomposition of PM 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, , Chemical Society of Japan [1] 52

3 xygen-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 PM 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 PM in the absence of oxygen: H ~ H + ~ (A) (3.1) A + products (3.2) ~ ~ H + ~ ~ (B) (3.3) B = (C) + ~ (3.4) From this mechanism, Equations , 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 PM in a vacuum. In the presence of oxygen, the following scheme was given [2]: ~ ~ + 2 ~ ~ (D) H (3.5) D H + ~ ~ (E) (3.6) E ~ C H + ~ (3.7) 53

4 Thermo-oxidative Degradation of Polymers This mechanism, Equations , 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 xides (PP) 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 PP 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 PP [3, 4]. The complexity of the chemical structure of heterocyclic polymers, including PP 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 Polycarbonate Carbon monoxide (C), carbon dioxide (C 2 ), 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 (C, C 2 ) have established [3] that C forms exclusively via the oxidation of groups, whereas C 2 arises from the decomposition of ester groups and the oxidation of groups. 54

5 xygen-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, C and C 2, 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 C 2 elimination occurs according to the scheme: C C 2 + (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: C C H H 2 H + + C 3 + H C H (3.9) C C H RH CR + + H C H (3.10) 55

6 Thermo-oxidative Degradation of Polymers The high-temperature decomposition of PC at C results in the elimination of C 2, C, 4 and H 2, as well as in the formation of terminal phenol groups. This is explained by the following radical process: C C H + + C C H + + C 2 + C H (3.11) 2 H (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 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

7 xygen-containing Polymers Polyethylene Terephthalate (PETP) n heating PETP in atmospheres of air, oxygen and water vapour, the concentration of 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 group and one H 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 C 2 as a result of decarboxylation. Vinyl ester groups decompose to produce radicals: ~ CC 6 C = ~ ~ CC 6 C + = (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: ~ CC 6 C + ~ C 6 C ~ ~ = + ~ C 6 C ~ CC 6 + ~ C 6 C = + ~ C 6 C ~ (3.14) (3.15) n decomposition of the alkyl radical, terminal benzyl and acetaldehyde radicals are formed: ~ C 6 C CC 6 ~ ~ C 6 C + = CC 6 ~ (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: ~ C 6 C ~ C 6 C + (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

8 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 H 2, C 2, C, 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 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 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: C 6 H 5 H[CC CC 6 C]H C 6 H 5 (I) (3.18) 58

9 xygen-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: C; C; C; and 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 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 ( C); and a mixture of phthalic acid, phthalic anhydride, and a liquid containing low molecular weight esters of propylene glycol ( 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. n 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: C 6 H C 6 H 5 (3.19) 59

10 Thermo-oxidative Degradation of Polymers Table 3.1 Kinetic parameters for thermal degradation of styrenated polyester (Laminac 4116) Stage of reaction Temperature range ( C) rder of reaction In air Energy of activation (kcal/mole) First-order frequency factor (per s) , In argon 1, Reproduced with permission from D.A. Anderson and E.S. Freeman, Journal of Applied Polymer Science, 1959, 1, 2, , 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: C 6 H 5 C 6 H 5 C + H (3.20) C 6 H 5 C + H C 6 H 5 C + H (III) (3.21) 60

11 xygen-containing Polymers C 6 H 5 C C 6 H 5 C (II) + H (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: H CC 6 C CC C C 6 H 5 (C) 2 + C (IV) (3.23) The hydroxy ester may then be formed by reaction with hydrogen: C + H H C (3.24) Decarboxylation of the polyester should occur readily and account for the formation of propylene and carbon dioxide: C C 2C 2 + = (3.25) 61

12 Thermo-oxidative Degradation of Polymers In the fourth degradation stage ( 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 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: ~ ~ (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 C > > > 2 [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

13 xygen-containing Polymers decrease as the temperature rises, leaving carbon as a residue. The data suggest that cleavage polymer chains occur at the bond: C 6 Hˉ + H 2 H H + H H (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 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 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

14 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 uchi and Honda [23]. Within the boiling range of 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 uchi 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 uchi and Honda. From initial oxidation reactions, Equations 3.28 and 3.29: H H H H C [] Source (3.28) 64

15 xygen-containing Polymers H H [] Source H (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. ccurrence 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

16 Thermo-oxidative Degradation of Polymers (I) H C H H C + H H C H H H + C H H H H + C 2 (II) H H H + H H H + 66

17 xygen-containing Polymers (III) 4 H + + H H 2 C H or higher homologe + 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

18 Thermo-oxidative Degradation of Polymers H C H [] + H C + C (-C 2 ) H + From Route l H H (1) -C (2) Ring formation C Figure 3.4 Reactions proposed for char formation. Source: Author s own files 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

19 xygen-containing Polymers Weight loss, % T ( C) min = Temperature, C Resin C Resin B Resin A DER 331 Resin Figure 3.5 Thermogravimetric curves with boron trifluoride monoethylamine (BF 3 MEA) as curing agent. Reproduced with permission from J.A. Bremner, Industrial Engineering Chemistry Product Research and Development, 1964, 3, 1, , ACS [24] Br Br H 2 C C Br Br (V) (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 (BF 3 MEA), 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

20 Thermo-oxidative Degradation of Polymers Br Br H H 2 C C CL Br Br (VI) (3.31) 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

21 xygen-containing Polymers oxidation. wavenumbers (cm -1 ) Absorbance Units c b a 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, , 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

22 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) 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

23 xygen-containing Polymers C C Percent residue C 275 C 20 a Time (Hours) 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, , Elsevier [25] Ethylene xide-propylene xide 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

24 Thermo-oxidative Degradation of Polymers on the PP 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 (PE) propylene oxide (P) ethylene oxide (E) 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. PP was more sensitive to thermo-oxidation than PE due to the more stable tertiary radical that can be formed on the PP chains. Gallet and co-workers [27] showed that this was also true for poloxamer materials by identifying products of PP cleavage early in the degradation process. Although C homolysis while submitting PP 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 scissions. The mechanism started from the same tertiary alkoxy radical as displayed in Mechanism I (see next) followed by C homolysis and depropagation. 74

25 xygen-containing Polymers (a) Relative abundance Time (min) 17 (b) Relative abundance Time (min) (c) Relative abundance Time (min) 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, , Elsevier [27] 75

26 Thermo-oxidative Degradation of Polymers Crown ethers of E and P 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 H H + PH H 2 + P (3.32) C H 2 H -H 2 PH Acetate Formate + Methyl ether P (3.33) Mechanism II H H + PH H 2 + P (3.34) Formate C H H -H 2 PH + P Acetate Ethyl ether (3.35) 76

27 xygen-containing Polymers 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

28 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. PH Concentration (µg/g) Time of ven Ageing at 180 C(h) Figure 3.9 Hydroperoxide concentration (inserts are values of [PH] 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, , 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