Thermal degradation of polyepichlorhydrin elastomers

Thermal Degradation of Polyepichlorhydrin Elastomers" John Day and Walter W. Wright

The thermal degradation of polyepichlorhydrin and of the copolymer of epichlorhydrin with ethylene oxide, both in the uncompounded and compounded form, has been studied in air and inert atmosphere by thermogravimetry. A detailed examination has also been made of the evolution of hydrogen chloride from the homopolymer as a function of temperature and compounding formulation, specific attention being given to the efficiency of the acid acceptor. It is concluded that with suitable formulationohydrogen chloride evolution should not be a problem at temperatures at least up to 175 C.

1. INTRODUCTION The homopolymer of epichlorhydrin and its 1 : 1 copolymer 1

with ethylene oxide are elastomers with a good range of physical properties. The homopolymer has excellent resistance to ozone and weathering and good resistance to oils. Its gas permeability is outstandingly low. The copolymer is much more resilient than the homopolymer and is more suitable for low temperature applications. Major uses of these polymers are as control system hoses, tubing and diaphragms. The maximum service temperature is quoted as being 1 50°C, and there would be advantages in raising this temperature limitation. The structures of the polymers are such that hydrogen chloride (HCl) can be eliminated at elevated temperatures with consequent possible corrosion of certain metals. The thermal degradation of the homo- an( co-polymer in uncompounded and compounded form has, therefore, been studied in air or nitrogen by thermogravimetry. A detailed examination has been made of the evolution of HC1 from the homopolymer as a function of temperature and compounding formulation, specific attention being given to the efficiency of the acid acceptor.

2. EXPERIMENTAL 2.1 Materials Polyepichlorhydrin and a copolymer of epichlorhydrin and ethylene oxide were commercial products - Herchlor H and C made by Hercules Inc.

Polyepichlorhy drin

Analysis C H C1 Theoretical (%) 38.9 5.4 384 Found (%) 39.6 5.5 36.5

1 : 1 Copolymer of epichlorhydrin and ethylene oxide

fc~-~;~-cH2-cH2-o

Analysis C H C1 Theoretical (%) 440 6.6 26.0 Found (h) 45.5 6.7 22.3

Materials Dept., Royal Aircraft Establishment, Farnborough, Hants. *Presented in part of the First European Symposium on Thermal

Manuscript received 30 November 1976, accepted 14 December 1976. Analysis, Salford, UK, September 1976.

A standard formulation and cure were used for the compounded products as follows: 100 parts (by weight) of gumstock, 5Q parts of reinforcing filler (GPF black), 5 parts of acid acceptor (eg Pb304), 1.5 parts of curing agent (2- mercaptolmidazoline), 1 part of antioxidant (nickeldibutyl- dithiocarbamate) and 1 part of release agent (zinc stearate). Press-cure 45 min at 17OoC. Based on the analytical figures found for the gumstocks, the chlorine contents of the compounded homopolymer and copolymer would be expected to be 23.0% and 14.1% respectively. The actual amounts determined were 22.8% and 14.7%. Experiments were first carried out varying only the acid acceptor. The compounds used were Pb304, CaO, Ca(OH)2. LiOH, Mg(OH)2, CaC03, K2 C03, Liz C03, MgC03, all at a concentration of 5 parts by weight. These were first milled and only the powder which passed through a 300 mesh sieve was used in the formulation. The compounds were chosen on the basis of their efficiency as HF acceptors in the degradation of hydrofluoro elastomers,'I2 and as HC 1 acceptors in the degradation of p~lyvinylchloride.~ Other papers 475 refer to the stabilising effect of polyhy- droxy aromatic acids (specifically azelaic acid) on epichlorhydrin rubbers. Accordingly a second series of experiments was done using a modified formulation containing 7.5 parts of acid acceptor (Pb304, Ca(OH)2, CaC03, Liz C 0 3 , MgC03) and 4 parts of either azelaic or gallic acid. One or two more specialised formulations were also tested; these are referred to later in the text.

2.2 Techniques Weight loss measurements were made on samples weighmg a few milligrams using the Du Pont 950 thermogravimetric analyser. Experiments were carried out in a flow of air or nitrogen (40 cm3/min) at a rate of temperature rise of 2"C/min, or isothermally.

Fig.1 Apparatus for measuring HC1 evolution

The evolution of HC1 was monitored by a pH method. The apparatus is shown diagrammatically in Fig. 1. Samples

66 THE BRITISH POLYMER JOURNAL, MARCH 1977

(approximately lg in weight) in thin sheet form were treated isothermally at temperatures between 175 and 225°C in a stream of dry C02-free air (50 cm3/min). The gas stream was passed into a 0.1N potassium nitrate solution made up freshly with deionised water before the start of an experiment, so that the ionic strength of the liquid was relatively unchanged during the measurements. The solution was maintained at a temperature of 30°C and the change in pH monitored with a glass electrode and a sul- phate reference electrode combination. The -outputs of the electrodes were fed via a pH meter to an X-Y recorder and hence continuous plots were obtained of pH versus time at constant temperatures. Knowing the quantity of sample taken and its initial chlorine content, the plots could readily be transposed to graphs of yield of HC1 as a function of time temperature. The yields were expressed in terms of moles per gram of gumstock in the formulation to enable true comparisons to be readily made. First experiments were made with a chloride ion specific electrode instead of a glass electrode, but it proved to be too insensitive at low Cl'ion concentrations. With certain chosen formulations the tensile strength, tensile modulus at 100% elongation and the elongation at break were measured as a function of time of ageing at 150°C.

1.5 IY

cn 0 -I

1 -0

3. RESULTS AND DISCUSSION

3.1 Weight loss experiments The dynamic weight loss curves for the uncompounded homo- and co-polymer were superimposable in both nitrogen and air and there were only marginal differences between the curves in the two atmospheres. The curves for the compounded samples were also almost identical to those of the uncompounded analogues up to 30-50 per cent weight loss. Loss in weight was first detected at above 200°C and the rate increased rapidly at temperatures greater than 250°C.

.

2.0

1.5 CL m 0 -I

1 .o

- \ ; I d Homopolymer - air H

- H - a i r

H - compounded - Np

-

0.5 I I I 1 I 1.75 1.85 1.95 2.05

Ill

Copolymer C 2 'o r

C and C compounded - air C compounded - N 2

C-N,

-

0.5 ' I I I I 1.75 1.85 1.95 2.05

IIT Fig.2 Arrhenius plots of rates of weight loss

Isothermal experiments in the temperature range 2 10- 300°C did reveal slight differences in stability and these are best illustrated by Arrhenius plots of the data (Fig. 2). Considering the homopolymer first, the samples were somewhat more stable in nitrogen than in air and the gumstock was more stable in both atmospheres than the compounded material. The slopes of the lines were almost identical. The behaviour of the copolymer was very similar except that in air, for both gumstock and compounded forms, the thermal stability at the lower temperatures used was less than that of the other samples. This must be re- lated to the ethylene oxide component of the copolymer. These features are reflected in the values of the overall activation energies for thermal breakdown derived from the Arrhenius plots (Table 1).

Table 1. Overall Activation Energies for Thermal Breakdown -

Polymer sample Activation energy (kJ/mole)

Nitrogen Air Homopolymer 143 139 Homopolymer-compounded 139 134 Copolymer 143 92 Copol ymer-compounded 143 92

3.2 Evolution of HC1 The results for rising temperature (2"clmin) experiments are shown in Figs. 3 and 4 for homo- and co-polymer

7 Compounded h0mQpoi"mel - N I

,' / ,'j ' /

,' ,' 0 - 1

290 100 110 1 2 0 330 110 350 360 370 300 190 1ernpe,a,u,e * C

Fig.3 for homopolymer samples

Yield of HC1 as a function of rising temperature (2"C/min)

C0pol"mer - 1 5 1 -__---- _/--

T.m~.lll"r. .C

Fig.4 for copolymer samples

Yield of HC1 as a function of rising temperature (2'Clmin)

THE BRITISH POLYMER JOURNAL, MARCH 1977 67

6oo r 800

Y) 0 - 300 0,

2l 200 E - u I - 100 0

H compounded I

0 1 LO 160 180 200 2 2 0

Temperature "C HC1 evolution as a function of temperatures Fig.5

samples, respectively. In all cases HC1 evolution commenc- ed at temperatures greater than 290°C and proceeded at a rapid rate in the temperature range 310-330°C. The final HC1 yield at 400°C amounted to 30-35% of that available for practically all the samples. There was relatively little difference (- 10°C) between all the curves for the homopolymer samples in air, or nitrogen (Fig. 3). The same was true for the copolymer samples (Fig. 4) with the exception of the uncompounded sample, which appeared to be

700 -

600 -

500 -

n LOO -

l i m e hours

Figd Comparison of efficiencies of oxide acceptors at -225OC.

6 0 0 I

G O O t

Fig.8 -225°C.

Comparison of efficiencies of carbonate acceptors at

lime hours

Fig.9 -225°C.

Comparison of efficiencies of Pb3O4 formulations at

30 -

E 20 i

I0

PbiO' + a z e l a c a r i d I

0 I0 20 3 0 LO 50 60 70 80 90 100 110

rime hours

Fig.10 Comparison of efficiencies of HC1 acceptors at 175°C.

Fig.7 -225°C.

Comparison of efficiencies of hydroxide acceptors at Fig. 11 compound based formulations at -225OC.

Comparison of efficiencies of lithium and magnesium


rather more stable in nitrogen. The compounded materials containing 5% of F'b304 as an HC1 acceptor would have been expected to show superior behaviour. Isothermal experiments were carried out for all the samples in air and the results are illustrated in Fig. 5 plotting the total HC1 evolved after 6 h at temperature as a function of temperature. In this case HC1 evolution was detected at temperatures greater than 160°C and became rapid at temperatures above 200°C. The compounded samples containing Pb304 were superior to the uncompounded polymers at the lower temperatures studied, but this was no longer the case at > 200°C. Arrhenius plots of rates of HCl evolution versus temperature were curved and hence it was not possible to derive overall activation energies for the process. Further experiments were restricted to study of compounded samples of the homopolymer in air, varying the acid acceptor in an attempt to improve the efficiency of HCI absorption, Most comparisons were made at - 225°C. Figs. 6 ,7 and 8 show the results for oxide, hydroxide and carbonate compounds, respectively, all at the 5% level. The control sample was fully compounded with the exception of the acid acceptor and was cured as the other specimens. It is evident from the curves that there was relatively little difference in the efficiencies of the compounds studied as HC1 absorbers. Some of the compounds, in fact, gave results worse than those of the control and even the best compound only increased the time to attain a specific HCl yield by a factor of 2 over the control. This is in complete contrast to the behaviour of these compounds as HF acceptors during the degradation of hydrofluoroelasto- m e r ~ , ~ where the relative efficiencies at 225OC differ by a factor of more than 100 and the superiority over a control may be as much as 400-fold. Figs. 9-12 give data for various formulations with and without azelaic acid. The most interesting result was obtained with azelaic acid alone (Fig. 9); the initial yields of HC1 were the highest observed for any of the formulations studied, but the rate of evolution decreased markedly with time temperature and after a period of some hours the total yield was the lowest observed. It has been suggested that azelaic acid acts as a stabiliser in epichlorhydrin elastomers by linking up broken chains. The results detailed in this paper on its effest of HCl evolution show that this cannot be the whole story. Fig. 9 also includes the curves for a number of formulations containing Pb304. The addition of azelaic acid, gallic acid and azelaic acid plus a polymeric antioxidant all gave increased inducation periods compar- ed with Pb304 alone, but after 6-7 h at 225°C all the rates of HCI evolution were very similar. The differences were more marked at lower temperatures. Fig. 10 shows the results for Pb304 and Pb304 t azelaic acid formulations at 175°C. With the latter there was effectively no evolution of HCl during 160 ha t this temperature. Fig. 1 1 gives the data for lithium and magnesium compound based formulations and Fig. 12 for calcium compound based formulations. In all cases an initial reduction in rate of evolution of HCl was observed when the inorganic compound was used in combination with azelaic acid. Mixtures of azelaic acid with CaCO,, Ca(OV2 and Li2CO3 gave better results than a Pb3 04/azelaic acid mixture. Fig. 13 shows the results for the calcium compounds in a hydrofluoro elastomer system. Their much greater efficiency as HF acceptors in that system is obvious from the lugher temperature employed, the greater difference from the control and the longer time periods and lower yields of HF involved.

,

800

'00

600

Fig.12 formulations at -225°C.

Comparison of efficiencies of calcium compound based

I ' /

0 5 I 0 15 2 5 ID

------ r i m e h o u r s

Fig.13 Comparison of efficiencies of HF acceptors at 275°C.

I I , T i m e - d a y s a t 150.C

A P b g O ' l a z e l a i c ac td D L i ~ C O ~ I a z e l a i c a c l d

8 C a I O H l 2 l a z e l a i c a c i d

C C a C O j l a z e l a l c acdd F A z e l a l c acid

E P b j O '

Fig.14 Effect of ageing at 150°C upon tensile strength.


Table 2. Effect of Ageing at 15OoC upon Mechanical Properties

Formulation

Time at 150°C Tensile Tensile Modulus Elongation Hardness

(days) (MN/m2) (MN/m2) (%) (BS) Strength (M 100) at Break

Pb304/Azelaic acid 0 1 3 7

1 3 7

1 3 7

1 3 7

Ca(0H l2 /Azelaic acid 0

CaC03/Azelaic acid 0

Li2C03/Azelaic acid 0

Pb3 0 4

Azelaic acid

14.9 3.9 16.6 6.0 14.6 6.8 13.7 8.3

13.8 6.3 14.6 7.0 11.0 5.1 5.9 3.5

10.8 4.4 10.3 4.5 7.1 3.9 3.9 3.1

12.1 7.1 13.0 6.0 9.9 5.0 6.9 4.0

19.0 6.2 17.8 8.5 17.3 10.8 6.2 13.9

10.7 4.2 9.9 3.0 5.2 3.0 1.9 1.9

319 248 188 156

280 222 228 234

41 7 272 238 218

214 216 208 21 0

250 180 154 188

528 324 280 222

73 74 80 79

79 76 74 71

78 73 74 78

81 76 75 72

77 78 80 77

76 70 72 78

220 I

200 -

180 -

\

\ \ \ \ \

\

6 0 -

; F L O

0 1 2 3 L 5 6 7

I ‘\ c

120 - 0

\

6 0 -

; F L O

0 1 2 3 L 5 6 7 Time - d a y s at 150’C

A P b 3 0 , l a z e l a i c ac id D L 1 2 C O 3 l a z e l a i c ac id

B Ca10H12 l a z e l a i c a c i d E Pb,OL

C C a C 0 3 / a z e l a i c acid F Arelaic acid

3.3 Effect of ageing at 150°C upon mechanical properties Sheets were fabricated using the most efficient formulations listed above ie CaC03, Ca(OH)2 and LizC03 all with azelaic acid. For comparison purposes sheets were also

“ O d D

I I I I I , , 0 1 2 3 L 5 6 7

T i m e - d a y s at 150’C

A Pb301/ a z e l a i c a c i d D Li2C03 / a z e l a i c ac id

B C a ( O H I 2 / azela ic acid E Pb301

C C a C O 3 / a z e l a i c acid F Azela ic ac id

Fig.15 Effect of ageing at 150°C upon tensile modulus. Fig.16 Effect of ageing at 150°C upon elongation at break.


made with Pb304 plus azelaic acid, Pb304 alone and azelaic acid alone. The tensile strength, modulus at 100% elogation, elongation at break and BS hardness were measured. The sheets were aged in an air circulating oven at 150°C and the above properties redetermined after 1 , 3 and 7 days at this temperature. All tests were carried out at room temperature and the results quoted are the mean of five determinations. The actual data are given in Table 2 and plots of percentage retention of the initial property level versus time temperature are shown in Figs. 14-17. Some of the values for tensile strength and elongation at break were lower than those normally quoted (tensile strength 14-21 MN/m2 and elongation at break 320-350%). This is because the acid acceptor also affects the cure of the system and the final cross-link density of the elastomer. The tensile strength of the samples decreased rapidly with time at 150°C (Fig. 14) with the exception of the formulation containing Pb304 and azelaic acid, where after an initial increase in strength there was a relatively slow diminution so that after 7 days only 8% of the initial strength had been lost. The reason for this becomes plain when the changes in modulus curves are examined (Fig. 15). Once again the formulation containing Pb304 and azelaic acid was excep- tional in that the modulus increased steadily throughout the time of ageing indicating that further cross-linking was taking place. The sample containing only Pb304 followed an almost identical curve up to 3 days' ageing, but then the modulus decreased very rapidly with further time at temperature. All the other formulations showed a slow de- crease in modulus with ageing time. The elongation at break curves (Fig. 16) are of the form that would be expected from the above data. It is interesting that the Li2C03/ azelaic acid formulation showed very little change in elogation at break throughout the course of the experiment. There was relatively little change in hardness of any of the focmulations with time at temperature (Fig. 17).

3.4 Summary of results

(a) On the basis of the weight loss results the calculated time for a 50 per cent weight loss from the compounded (5% Pb304) homopolymer in air would be 770h at 175°C and 6,250h at 150°C. The weight loss corresponding to 7 days at 150°C would be just over 1 per cent. (b) With certain formulations ie azelaic acid in combination with Pb304, CaC03, Ca(OH)2 or Li2C03 there would be no detectable evolution of HC1 in 7 days at 175°C. Since the rate of evolution of HC1 does not obey an Arrhenius relationship .it' is not possible to extrapolate the results obtained to low temperatures.

(c) the elastomer and hence the level of mechanical properties and also the retention of these mechanical properties with time at 150°C. In 7 days at this temperature all the systems studied showed considerable changes in properties - either scission of chains or further corss-linking was taking place. (d) It is difficult to envisage the precise mechanistic role of the azelaic acid (HOOC(CH2),COOH). In conjunction

The different formulations affect both the cure of

I I

0 I 2 3 L 5 6 I 8 Time-days at 15O'C

A Pb3 OL lare la ic acid D L i 2 C 0 3 larelaic acid

B Ca 10H12/arelatc acid E P b i O i C CaC0,lazelaic acid F AzcIaIc acid

Fig.17 Effect of ageing at 150°C upon hardness.

with Pb304 it had a profound effect on the cure and the subsequent retention of mechanical properties and in its own right it has a very marked influence on the evolution of HCl.

4. CONCLUSIONS There is relatively little difference in the thermal stabilities (based on weight loss) of polyepichlorhydrin and a 1 : 1 copolymer of epichlorhydrin and ethylene oxide in air, or nitrogen or when fully compounded as an elastomer. HC1 evolution becomes rapid at temperatures above 2OO0C for both uncompounded and compounded materials, but with suitable formulation the rate is almost negligible at temperatures up to 175°C. Mechanical properties are, however, changing fairly rapidly at 150°C and protracted use at this temperature, or above, will require stabilisation against the modes of breakdown, which are taking place. The behaviour of the polyepichlorhydrins where the HC 1 is eliminate from a side chain is completely different from that of the hydrofluoro elastomes, where HF is eliminated from the main chain and the build up of conjugated sequences is possible.

5. ACKNOWLEDGEMENT The thermogravimetry was done by Mr P. Morgan, a sandwich student, during an industrial period at the RAE.

References 1 Knight, G. J . ; Wright, W. W.;BrPoljm J 1973. 5, 396 2 Wright, W. W.; Br Polym J 1974,6 ,141 3 O'Mara, M. M.;JPolymSci, 1971,Al ,9 , 1387 4 Adank, G.; Goshom, T. R.;Angew Makromol Chem 1971,

16/17, 103 5 Yamada, M.; Arai S.; & Masuda, Y.; Nippon Gomu Kvokaishi,

1973,46,404 6 Brown, J. H.;ProgressRubb. Technol. 1975, 31 7 Kay, E.; Thomas, D. K.; & Wright, W. W.; Paper F7 presented

at the International Rubber Conference, Brighton, 1972

Copyright 0 Controller HMSO, London 1976


Thermal degradation of polyepichlorhydrin elastomers

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