Thermal oxidative breakdown of plasticized polyvinylchloride

K . S . MINSKER e$ al.

2338

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

7.

8, 9. 10. 11. 12. 13.

G. F. EHLERS, K. R. FISH and W. R. POWELL, J. Polymer Sci. 7: 2955, 1969 I. N. SHORT and H. W. HILL, J. Chem. Technol., 481, 1972 R. M. BLACK, C. F. ZIST and R. J. WELLS, J. Appl. Chem. 17: 269, 1967 N. S. I. CHRISTOPHER, I . I . COTTER, G. I. KNIGT and W. W. WRIGHT, ft. AppL Polymer Sci. 12: 863, 1976 R. T. HAWKINS, Macromolecules 9: 189, 1976 V. A. SERGEYEV, V. K. SHITIKOV, V. I. NEDEL'KIN, A. S. KOGAN, A. S. TKACHENKO, V. 1. MISYUREV, V. V. YAKOBSON and V. S. GLEBYCHEV, Aut. Cert. 583141, 1977; Bull. izobr., No. 45, 1977 V. B. GAVALYAN, V. I. NEDEL'KIN, V. I. ZHURAVLEVA, S. S. A. PAVLOVA a n d V. A. SERGEYEV, Vysokomol. soyed. B2O: 768, 1978 (Not translated in Polymer Sei. U.S.S.R.) V. M. LAKTIONOV and I. V. ZHURAVLEVA, Vysokomol. soyed. A17: 2813, 197~ (Translated in Polymer Sci. U.S.S.R. 17: 12, 3232, 1975) N. R. LERNER, J. Polymer Sei. 12: 2477, 1974 M. C. R. SYMONS, J. Chem. See,, Perkin Trans. 11: 1618, 1974 R. Kh. FREIDLINA, L I. KANDOR and R. G. GASANOV, Uspekhi khimii 47: 508, 1978 G. F. EHLERS, K. R. FISH and W. R. POWELL, Polymer Sci. 7, A-I: 2931, 1969 V. A. SERGEYEV, V. K. SHITIKOV and V. I. NEDEL'KIN, Vysokomol. soyed. BI9: 396, 1977 (Not translated in Polymer Sci. U.S.S.R.)

Polymer ScienceU.S.S.R.Vol. 22, ~o. 9, pp. 2388-2344,1980 Printed in Poland

0032-3950/80[092338-07507.50/O ©1081 Pergamonl~ess Ltd~

THERMAL OXIDATIVE BREAKDOWN OF PLASTICIZED

POL~VINYLCHLORIDE* m

K . S.

MI~sx~R, M.

I. ABI)ULLI~, V. G. KALASl~IKOV, V. P . BIRYUKOV, G. ]Vl. TROSM~'¢ a n d A. V. S~m'NIKov

Bashkir State University named after the 40th Anniversary of October All-Union Scientific Research Institute of Film Materials and Artificial Leather.

(Received 11 September 1979) The addition to PVC of ester plasticizers considerably changes the rate o f breakdown processes--elimination of HC1, formation of statistical isolated ~)C~C< bonds aud the formation of poly-eonjugated systems of double bonds in t h e macromolecules. The ratio of rates of these processes depends on the oxidative stability of the plasticizer its concentration and partial oxygen pressure in the reaction zone. Products of oxidation of esters intensify the formation of pelycne systems, whereaa * Vysokomol. soyed. A22: No. 9, 2131-2136, 1980.

Plasticized polyvinylchloride

2339;

statistical dehydrochlorinati0n of PVC is accelerated by free radicals formed during oxidation of plastieizers. On the other hand, plasticizers inhibit the formation of polyenes as a result of the solvation of polymer macromolecules.As a result ester pla~cizers have a complex effect on the dehydroehlorination of PVC. IT spite of the extensive application in practice of PVC plasticized b y diesters, there are only a few papers [1, 2], which deal with the effect of these compounds. on thermal decomposition of the polymer and hardly any kinetic investigationa have been carried out into this problem, although the high sensitivity of breakdown of PVC in the presence of various chemical agents in the polymer [3] is well known. A study was therefore made of kinetic regularities of thermal oxidative (in oxygen and air) breakdown of PVC (elimination of HCI, formation of internal ~ C = C ~ , formation of poly-conjugated systems of double bonds in macromoleeules), plasticized by esters--dioctylphthalate (DOP), dioetyladipate (DOA). and dioctylsebacate (DOS). S-70 PVC was used (GOST 14332-69). Characteristics of plasticizers are as. follows: Ester Density at 20°, g/cma Acid number, mg KOH/g Flash point, °C Colour according to the iodometric scale

~)OP

DOA

DOS

0.982 0.09 206

0-913 0-13 215

0.926 0.07 192

1.0

1.0

1.0

Methods of experiments have been described in fair detail previously [4, 5]. IX)P, DOA, DOS esters markedly reduce the rate of thermal decomposition o f PVC during breakdown in inert medium (nitrogen), plasticizers having a practically identical effect. A different kinetic pattern is observed during thermal exposure of the plasticized substance in oxygen, or air. Breakdown of PVC is not inhibited in this case on adding an ester to the polymer, on the contrary, the gross rate of elimination of HC1 VHClbegins to increase rapidly w~'th an increase in" plasticizer content in the composition; the higher the partial pressure o f oxygen in the reaction zone (Fig. 1), the higher the rate of this increase and the kinetic of dehydroctflorination of PVC become auto-catalytic (Fig. 2). I t should be noted that HC1 liberation is only accelerated by the action of a p]asticizer with concentrations of this plasticizer higher than the critical value of [SEP]ar.* Another special feature of thermal oxidative breakdown, compared w i t h thermal breakdown is the fact that plasticizers have a varying effect on kinetics of dehydrochiorination of the polymer; this is particularly apparent considering that the critical concentration of the plasticizer has varying values for t h e diesters studied, values of [SEP]or depending on partial oxygen pressure in the reae-* SEP--DOP,

DOA

or DOS.

2340

K.S.

MrNSKFm et ed.

tion zone. Critical concentrations of ester plasticizers observed in thermal oxidative dehydrochlorination of PVC plasticizer at 175 ° are as follows: Ester Medium

DOP air

DOA air

DOS air

DOP oxygen

DOA oxygen

DOS oxygen

[SEP]er,g/g PVC

0-20

0"10

0"05-0.06

0"15

0"06-0"07

0'01-0"02

These features observed in the effect of ester plasticizers on thermal oxidative breakdown of PVC are, evidently, due to the fact that the replacement of an inert medium for an atmosphere containing oxygen initiates various oxidative reactions with the participation of plasticizer molecules, whereby as judged from breakdown kinetics of plasticized substances, DOP, DOS, D0A are involved ~o yarying degrees in these processes.

mrno/e/molePVC / 60'a .'/! °

,4 HCI,

D 4

32

18

I

~0

SO

50[CEp]

I

~0

I

I

3O

.1o,g/s pvc

I

5O

FIG. 1. Dependence of the a m o u n t of hydrogen chloride separated in 60 rain on the concentration of ester plasticizer during the degradation of plasticized PVG (175 °) in oxygen (a) and air (b) by the action of heat: 1, 4 - - D O S, 2, 5 - - D O A , 3, 6 - - D O P .

In fact, a study of initiaiion kinetics (using di-tert-butyl peroxide) of oxidation of DOP, D0A and DOS shows that diesters interact at a varying rate with oxygen--ratios of rate constants k~/v/~ characterizing the ability of compounds to undergo oxidation [6] vary considerably. Ester T° b= - ~ × 103

DOP

DOA

DOS

DOP

125

125

125

135

1-31

2.68

3.38

1.61

DOA •

DOS

DOP

]3OA

DOS

135

135

145

145

145

3.70

4.90

1.99

4-74

7-66

Products formed during oxidation of esters are, apparently, the cause of intensive decomposition of PVC during thermal oxidation of the plasticized

Plasticized polyvinylchlorido

2341

polymer, the rate of formation in the system of breakdown products of plasticizers varying on using DOP, D 0 A and DOS because of their different resistance to oxidation. As a consequence the rate of elimination of HC1 varies differently on adding to the polymer identical amounts of the diesters (Fig. 2). This conclusion means t h a t the rate of decomposition of PVC in the presence ofplasticizers previously subjected to thermal oxidizing exposure is higher than on using initial esters because of the high content of breakdown products active in HCL.mmole/rno/ePVC 24

IG

Y

8

3

J I

20

]

I

SO ~'me,m/n

Fio. 2. Winotics of gross elimination of HC1 in thermal-oxidizing breakdown of PVC (175°; O2), containing DOS (1, 4), DOA (2, 5), DOP (3, 6): 1-3--0.3; 4-6-0.1 g/g PVC. respect of degradation. In fact, the Table shows t h a t VHCl increases with an increase in the duration of preliminary heat-treatment of the plasticizer (it was also noted in former papers [7, 8] .that the decomposition of ester plasticizers, which is accompanied by an increase in acidity, reduces the duration of heat-stability of PVC compositions). However, a comparison of these results with kinetic data of thermal oxidative degradatlon of plasticized substances shows t h a t the addition to PVC of a plasticizer previously subjected to oxidation accelerates the breakdown of polymer molecules less markedly t h a n could be expected (Fig. 2, curve 2 and Table). These results suggest t h a t the formation in the system of products from oxidative decomposition of plasticizers, e.g. acids, is not the chief and only cause of intensification of the elimination of HC1 during thermal oxidative exposure in mixtures

2342

K . S . ~V~INSKERet al.

with DOP, DOS, DOA. Another cause is probably the formation in the plasticized substance during oxidative breakdown of free radicals as a result of the interaction of ester molecules with oxygen. The radicals formed attack polymer macromolecules, which increases the rate of VHCl. EFFECT

OF PRELI~/III~ARY O X I D A T I O N OF E S T E R P L A S T I C I Z E R S METERS

Plasticizer (0.3 g/g PVC) DOA

O F T H E R M A L D E G R A D A T I O I ~ OF P V C

Duration of heat -treatment, hr

VHClX 106

I

[

ON K I N E T I C P A R A -

(175°, N~) "

ve X 10 7

VpX 107

mole HC1/mole PVC" sec

1.40 1-70

0.80 0.80 0.81 0.79 0.78

7.22 5.41 10.19 11-31 16.20

0.65 0.86 0-98 1-30

0-80 0.79 0.77 0-78

5.70 7-81 9.03 12.22

0.62 1.10 1.21

DOS

(175 °, ~R)

,

A study of the effect of ester plasticizers on the statistical elimination of HCI taking place at a rate of vc and the formation of systems Of conjugated ) C = C ( bonds in polymer macrochains at a rate of vp enabled further information to be obtained concerning the mechanism of thermal oxidative breakdown of plasticized PVC. I t appeared t h a t esters change the rates of these reactions over the entire concentration range examined (Fig. 3). The continuous increase of rate ve with the increase of diester concentration in the plasticized substance (Fig. 3, curves 1 - 3 ) is further evidence supporting the validity of the conclusion about the active role of free radicals formed during oxidation of the plasticizer in reactions of breakdown (formation of statistical bonds )C----C() of PVC as it is known [3] t h a t the formation of radicals in PVC (for example, as a result of the decomposition of peroxide compounds in the polymer) causes a sudden acceleration in statistical dehydrochlorination of polymer macromolecules. As far as the formation of polyconjugated systems is concerned, this reaction shows a more complex dependence on diester content in the polymer (Fig. 3, curves 4-6), which is due to the dual nature of their effect on vp: on the one hand, being decomposed b y the action of heat and oxygen, esters form products t h a t intensify the formation of polyenes (confirmed by the fact t h a t according t o results given, the addition to PVC of a plasticizer previously oxidized accelerates the breakdown of the polymer merely as a result of increasing rate vp) and on the other, plasticizers markedly inhibit this process as a result of the solvation o f polymer macromolecules [4].

Plasticized polyvinylchloride

2343

The opposite effect of the plasticizer on rates Vp and vc produces a maximum in the ratio Vc/Vpwith plasticizer concentration (Fig. 3). A s t u d y of the effect of DOP, DOS and DOA on rates of vo and Vp enables a deeper understanding to be achieved of the degradation of PVC in the presence of plasticizers and explain in particular w h y the process of gross-dehydrochlorination of PVC is independent in a number of cases of the presence in the composi~ion of plasticizers if their content is lower than a critical value (Fig. 1). This is AC x I0 q; Z~p~10 ~ rno/e/mo/e PVC

×

5

b

Z~c/A p

1.s 7 [

I

I0

h

J

30

h.

I

.,,. O.ff

50

ECEpJ .

I

I

10 g/s pvc

~

~ "~1 ~

30

50

l~c. 3. Dependence of the number of internal bonds formed in 60 min (175°, air) =C Ac(1-3) and the amount of HC1 liberated from PVC by the formation in macromolecules of polyconjugated sequences of double bonds Ap (4-6) (a) and the ratio of these pal~zneters (b) on DOS (1, 4, 8), DOA (2, 5, 9) and I)OP (3, 6, 7) content in the polymer. caused b y the fact t k a t although the addition of diesters to PVC is accompanied b y an increase in the rate of statistical separation of HCI, in some cases (depende n t on partial 02 pressure and the t y p e of ester), this increase is offset b y a reduction in the rate of formation of polyconjugated systems and since gross-dehydrochlorination of PVC includes both these reactions, vile1 may, as a result, be independent of the concentration of the plasticizer. However, exceeding a critical concentration disrupts compensation as a result of the rapid formation in the plasticized substance of products of oxidation of diesters, catalysing the formation of polyenes, therefore on increasing this concentration, the rate of gross-dehydrochlorination of PVC increases. These results are confirmed b y the fact that the ve/Vp ratio changes continuously in t h e entire concentration range of plasticized substances studied, including concentrations lower than the critical value (Fig. 3, ~urves 7-9).

2344

K . S . MI~Sm~R et al.

Therefore, t h e a d d i t i o n t o PVC o f esters n o r m a l l y causes a n intensification of t h e r m a l o x i d a t i v e b r e a k d o w n of p o l y m e r maeromolecules, which differs m a r k e d l y f r o m regularities o b s e r v e d during t h e r m a l decomposition of plasticized PVC. H o w e v e r , according t o t h e n a t u r e a n d p r o p o r t i o n of plasticizer a n d conditions o f the process, the gross r a t e o f d e g r a d a t i o n o f PVC m a y be i n d e p e n d e n t o f the existence in t h e p o l y m e r composition of a plasticizer or m a y even be slowed d o w n (e.g. a t low p a r t i a l pressures of oxygen, etc.) since plasticizers m a y n o t o n l y ~ccelerate (as a result o f decomposition t o form active products), b u t also inhibit the f o r m a t i o n o f p o l y - c o n j u g a t e d systems. These results p r o v e t h a t conclusions concerning the effect o f certain ester plasticizers on t h e b r e a k d o w n o f PVC should be d r a w n v e r y carefully in view of t h e fact t h a t kinetics o f decomposition o f the plasticized p o l y m e r d e p e n d to a m a r k e d e x t e n t on conditions o f breakdown. Translated by E. SE~EI~E REFERENCES

1. T. B. SH$I~HMINA and V. A. VOSKRESENSKII, Izv. vuzov, seriya khimiya i khimich: tekhnologiya 16" 447, 1973; 18. 1456, 1975 2. I. WYPYCH, Angew. Makromolek. Chemie 48: 1, 1975 3. ]~. S. MINSKER and G. T. FEDOSEYEVA, Destruktsiya i stabilizatsiya polivinilkhlorid~ (Breakdown and Stabilization of PVC). Izd. "Khimiya", 1979 4. K. S. MINSKER, M. L ABDULLIN, S. A. ARZHAKOV, V. I. MANUSHIN, A. V. MAMYKIN and V. A. KRAIKIN, Vysokomol. soyed. B21: 194, 1979 (Not translated in Polymer Sci. U.S.S.R.) 5. ]L S. MINSKER, V. V. LISITSKII, Z. VYMAZAL, M. KOLINSKI, Ya. KALAL, Ye. N. SHVAREV, I. B. KOTLYAR, I. I. GORBACHEVSKAYA and I. G. SAMOILOVA~ Plast. massy, No. 1, 19, 1976 6. N. M. EMANUEL', Ye. T. DENISOV and Z. K. MAIZUS, Tsepnyye reaktsii okisleniya uglevodorodov v zhiclkoi raze (Liquid Phase Chain Reactions of Oxidation of Hydrocarbons) Izd. "Nauka", 1965 7. Ye. N. ZIL'BERMAN and V. B. SALTANOVA, Plast. massy, No. 9, 10, 1968 8. R. S. BARSHTEIN, Plast. massy, 1~o. 12, 13, 1968