Thermal degradation studies of poly(vinyl chloride) and ethylene-vinyl chloride copolymer

Journal

ELSEVIER

Thermal

of Analytical and Applied 33 (1995) 243-252

JOURNALOI ANALYTICALand APPLIED PYROLYSIS

Pyrolysis

degradation studies of poly(viny1 chloride) ethylene-vinyl chloride copolymer S. Mayeda *, N. Tanimoto,

and

H. Niwa, M. Nagata

Poiwrwr Research Laboratory, TOSOH Corporation. Kasumi I-X, Yokkaichi 5/O. Japcm Received

6 July

1994: accepted

13 September

1994

Abstract The thermal degradation behavior of suspension-polymerized poly( vinyl chloride) (PVC) and ethylene-vinyl chloride copolymer (E-VC) in the early stages was compared by continuous measurement of the extent of dehydrochlorination. The behavior of E-VC was different from that of PVC; the rate of dehydrochlorination of E -VC decreased as a function of heating time whereas that of PVC was constant. It was presumed that the dehydrochlorination reaction was terminated at the ethylene units in the polymer backbone. At a very early degradation stage, however, the rate of dehydrochlorination of E-VC was greater than that of PVC. This degradation behavior arose from structural defects in the polymer, i.e. the content of tertiary chlorine increased according to the increase in ethylene content in E-VC during suspension polymerization. Keywords:

chloride);

Dehydrochlorination; Ethylene--vinyl chloride copolymer; Pyrolysis; Tertiary chlorine; Thermal degradation

Polyene;

Poly( vinyl

I. Introduction Poly( vinyl chloride) (PVC) is one of the most important large production volume plastics. Ethylene-vinyl chloride copolymer (E-VC) has also been investigated for improvements in the properties of PVC. It is well known that these polymers are easily degraded and colored by heating through dehydrochlorination and formation of a polyene structure. The thermal stability of these polymers is actually lower than expected from the regular structure, and a number of investigations [ I- 131

* Corresponding

author

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1995 ~ Elsevier

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Science B.V. All rights reserved

244

S. Mayeda et ul. 1 J. Anal. Appl. Pyrolysis 33 (1995) 243-252

have been carried out on the thermal degradation of PVC to clarify this problem. It was found that a decrease in thermal stability is caused by structural defects such as unsaturated groups and structures with tertiary chlorine atoms. Dehydrochlorination from the structural defects is the primary reaction during the thermal degradation of these polymers. Therefore, the measurement of evolved hydrogen chloride is the most suitable method for evaluation of the early stages of degradation. Although much research [ 14-251 has been done on the dehydrochlorination of PVC, a detailed investigation has not been reported on the very early stages of thermal degradation which involve structural defects in the polymer backbone. Further, all investigations on E-VC [26-301 are related to the thermal degradation of partially reduced PVC. As yet, the thermal degradation behavior of suspensionpolymerized E-VC has not been reported, but should be investigated because commercial E-VC is produced by suspension polymerization. In this study, we focused our attention on the early stages of the thermal degradation of suspension-polymerized PVC and E-VC, and will reveal the differences in degradation behavior of these polymers with respect to structural defects.

2. Experimental 2.1. Samples PVC homopolymer and three E-VC materials, which had different ethylene contents, were used in our investigation. Their average molecular weights and compositions are summarized in Table 1. The polymers were prepared by suspension polymerization at 56°C in a 50 1 reactor, using poly(viny1 alcohol) as the suspending agent. They were then sieved (loo- 150 mesh) to counteract the influence of heat conduction of the powder sample during thermal degradation. 2.2. Measurement

of dehydrochlorination

The apparatus for the measurement of evolved hydrogen chloride is shown in Fig. 1. It consisted of a GC oven (Hitachi 663-50), a pump (TOSOH CCPD), and a conductivity meter (TOSOH CM-8000). The volume of the glass reaction vessel

Table 1 PVC and E-VC

samples

investigated

Sample

iz;I ( x 10-4)

h;i, ( x 10-4)

I 2 3 4

13.8 13.5 13.7 15.3

6.1 5.2 6.4 6.7

Ethylene (mol”!,) 2.3 2.6 2.1 2.3

0 2.7 6.3 17.9

in copolymer

“‘2 +-

-

4 copper

L

tube

GC

OVEN

Fig. I. Apparatus

for the measurement

of evolved hydrogen

chloride.

was about 10 cm3. The nitrogen gas was heated in a copper tube (O.D. 3.12 mm. I.D. 1.5 mm, 5.2 m) before being introduced into the reaction vessel, and the gas flow rate was 500 ml/min. Before the degradation started, the reaction vessel was heated to 18O”C, and a 20 mg sample was then placed in the reaction vessel to be heated at 180°C. Hydrogen chloride evolved from the polymer was carried away by heated nitrogen gas and absorbed in water. The conductivity of this solution was measured continuously. 2.3.

U V/tChle

.spectroscopJ

The polyene sequence distributions of the degraded samples were determined with a Hitachi 220A UV/visible spectrophotometer. The samples for UV/visible measurement were degraded for 0.5, I .O and 2.0 h using the same apparatus (Fig. 1). After the degradation, the reaction vessel with degraded sample was detached and put in the ice water bath to quench the reaction immediately. The absorbance spectra of degraded samples were obtained in tetrahydrofuran solutions (0.5 wt’%,). which were carefully prepared under nitrogen before analysis. 2.4. NMR meusurements The structural defects of the samples were determined by JEOL GSX400 FTNMR (400 MHz). The concentrations of internal double bonds and allylic chlorine end groups were determined by ‘H NMR measurements. The samples were measured at 80-C as 4.0 wt% solutions in C,D,/1,2,4-trichlorobenzene ( 1:3). The number of scans accumulated was 1000. The pulse interval was 3.6 s and the pulse angle was 45”. The concentration of tertiary chlorines was determined by ‘%I NMR as the sum of ethyl and butyl branches after reductive dechlorination with (n-Bu),SnH [ 14.3 I].

246

S. Mayeda

et al. 1 J. Anal. Appl.

Pyrolysis

33 (1995) 243-252

The reduced samples were measured at 130°C as 10 wt% solutions in C,D,/1,2,4trichlorobenzene ( 1:3). The number of scans accumulated was 20 000. The pulse interval was 15 s and the pulse angle was 90”.

3. Results and discussion The dehydrochlorination behavior of the samples is shown in Fig. 2. The amount of hydrogen chloride evolved from PVC homopolymer has almost a linear relationship with heating time. This means that the rate of dehydrochlorination of PVC is constant. Such a result is in good agreement with other reports [ 16- 191.It is accepted that the basic processes in the thermal degradation of pure PVC are (1) relatively slow initiation, (2) fast allyl-activated (zipper-like) propagation of the dehydrochlorination and formation of polyenes, and (3) termination. The mechanisms of these reactions are still controversial. However, most investigators agree that the dehydrochlorination reaction is mainly initiated by structural defects, i.e. chloroallyl groups, end groups, head-to-head structures and tertiary chlorines [ 1 - 141. After the initiation of dehydrochlorination at structural defects in PVC, new structural defects (internal double bonds) were formed in the PVC, as shown in Scheme 1. Compared with the thermal degradation results of PVC homopolymer, the rate of dehydrochlorination of E-VC decreases as a function of heating time and as the content of ethylene in the E-VC increases. After the appropriate degradation, the UV/visible spectra of degraded samples were measured as shown in Fig. 3. The obtained UV/visible spectra of the degraded samples were a mixture of the 6000

5000

4000

3000

2000

1000

0

0

20

40

60

80

100

120

Heating Time (min) Fig. 2. Degree of dehydrochlorination the polymers (mol%): -, 0 (PVC; (sample 4).

of PVC and E-VC at 180°C in nitrogen. Content sample 1); P-m, 2.7 (sample 2); ---, 6.3 (sample

of ethylene 3); -.-,

in 17.9

S. Mayeda et al. 1J. And. CH$HCI &CHp

-C

i’

A&.

Pyrolwis

33 (1995) 243

247

252

-

-CH2-CH AI

-CH2

-CH

A,

-CH,

-CH

iI

structural defect

-CH,

-CH

LI

-

iI

-HCI

CH,CHCI *CH,

-C

=CH

-CH

-CH,

-CH

-CH,

-CH

-CH,

-CH

-

-CH,

-CH

-CH2

-CH

-

structural defect -HCI

CH,CHCI *CH2

-C

=CH

-CH

=CH

-CH

/

LI

Ll

A

structural defect Scheme

I

1.0

0

200

300

400

500

600

700

800

Wavelength Fig. 3. UV/visible polymers (mol%): 4).

spectra

of degraded

, 0 (PVC; sample

samples at 18O‘C in nitrogen for I h. Content of ethylene m the I); ~~. 2.7 (sample 2); ---. 6.3 (sample 3); ~. . 17.9 (sample

248

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et al. / J. Anal. Appl. Pyrolysis

33 (1995) 243-252

absorbed peaks from polyenes of various lengths. The absorbance in the long wavelength region around 300-600 nm decreases and that in the short wavelength region around 200-300 nm increases with an increase in the content of ethylene in the samples. This means that the amount of longer chain polyenes decreased and the amount of shorter chain polyenes increased at high ethylene content. In order to determine the distribution of polyene sequences, peak separation was carried out, and the average polyene length was calculated. It was found that the average polyene length of E-VC was shorter than that of PVC, as shown in Table 2. The dehydrochlorination reaction of E-VC is shown in Scheme 2. This reaction started Table 2 Average polyene

length

of degraded

samples

Heating

Sample

1 2 3 4

0.5

1.0

2.0

4.98 4.61 3.75 3.15

4.13 4.70 3.72 3.19

4.53 4.09 3.53 3.15

CH&HCl *CH2

-C

time (h)

-

-CHp-CH

-CH2

-CHP-CH2

-CH

-CH2

-CH

-

I Cl

I Cl t

structural

ethylene unit

defect -HCI

CH&HCI -

CHP -C

=CH

-CH

-CHp

-CHp-CHp

-CH

-CHp

--CH I Cl

rhhh

-CHp

-CH

-

I Cl t structural

defect

ethylene unit -HCI

CH&HCI -

CH? -C

=CH

-CH

=CH

-CH2-CHP

-CH I Cl

t ethylene unit Scheme 2.

I Cl

S. Mayeda et al.

/J. Anal. Appl. Pyrolysis 33 (1995) 243-252

749

at a structural defect in E-VC in the same way as in PVC, but no other double bonds were formed at the ethylene units in the polymer backbone. In other words, the dehydrochlorination reaction terminated at the ethylene units in the polymer backbone. Fig. 4 shows the degree of dehydrochlorination in the very early stages of degradation (up to 30 min). Increasing the content of ethylene in the sample tended and cause the starting point of to increase the rate of dehydrochlorination, dehydrochlorination to appear earlier. It was mentioned above that the dehydrochlorination is mainly initiated at structural defects in the PVC and E-VC. Therefore, the dehydrochlorination behavior in this degradation region is related to the amount and type of structural defects in the polymer. The NMR spectra of PVC and reduced PVC are shown in Fig. 5. The concentrations of internal double bonds and allylic chlorine end groups were determined by ‘H NMR ( Fig. 5(a)). The content of tertiary chlorine was determined as the sum of ethyl (E-a; 34.1 ppm) and butyl (B/L-r; 34.5 ppm) branches in 13C NMR spectra (Fig. 5(b)). The quantities of structural defects in PVC and E-VC are shown in Table 3. A head-to-head structure was not detected in all samples. The contents of the structural defects in the samples were almost the same, except for tertiary chlorine which originated from branch structures. The branch structure in E-VC increased as the content of ethylene increased. Ethylene radicals were formed during the suspension polymerization of E--VC. Because the reactivity of an ethylene radical is greater than that of a viny1 chloride radical, more intra- and intermolecular radical transfer reactions

1200T 1000 c h ul

\ 0 2 : s G I

800

600 400

0 0

5

10

15 Heating

Fig. 4. Degree of dehydrochlorination Content of ethylene in the polymers (sample 3); . 17.9 (sample 4).

Time

20

25

30

(min)

of PVC and E-VC at the very early stages at ISO’C in nitrogen. 0 (PVC; sample I); -, 2.7 (sample 2); ---. 6.3 (mol’%): --.

S. Mayeda et al. 1 J. Anal. Appl. Pyrolysis 33 (1995) 243-252

250

End group of allylic chlorine -CH2-CH=CH-CH&zI

0a CBC I

I

-CHC I-C&C b

I 1

-CHz-C&C

>CH-C&C

I

Internal double bond -CHl=

1

cy-

I

ai3 M

a. H-br

I

H-8

L

2 1 ‘..Mz-Cth-M,

P2 P sad-53-W

PM.

u

B/T-k

Fig. 5. ‘H and ‘% NMR spectra of PVC (sample 1). (a) ‘H NMR; (b) 13C NMR; this spectrum was obtained after reductive dechlorination of PVC (sample 1) with (n-Bu),SnH.

S. Mayeda et al. /J.

Table 3 Quantitative

determination

Sample

of structural

Anal. Appl. Pyrolysis 33 (1995) 243-252

defects

in PVC and E--VC

‘H NMR

“C

/I oooc

/ I oooc

NMR

tertiary

I 2 3 4

251

End group of allylic chlorine

Internal double bond

0.2 0.2 0.2 0.3

0.7 0.5 0.5 0.6

chlorine

0.4 0.8

I I 18

occurred in the copolymerization of E-VC than in the case of PVC polymerization. As a result of these reactions, branch structures were formed. This is the reason for the lower thermal stability of E-VC in the very early stages of degradation.

4. Conclusion The dehydrochlorination behavior of suspension-polymerized PVC and E-VC was investigated. Although the rate of dehydrochlorination of PVC was constant, that of E-VC decreased as a function of heating time. The average polyene length of degraded E-VC was shorter than that of PVC. This tendency was enhanced by increasing the content of ethylene in the polymer. This means that the dehydrochlorination reaction terminated at the ethylene units in the polymer backbone. In the very early stages of degradation ( < 30 min), the rate of dehydrochlorination of E-VC was greater than that of PVC. This behavior is related to the content of tertiary chlorines in the polymer backbone.

References [I] D. Braun, B. Bohringer, B. TvPn, T. Kelen and F. Tiid(is, Eur. Polym. J., 22 (1986) I. [2] B. IvBn, J.P. Kennedy, T. Kelen, F. Tiidiis, T.T. Nagy and B. TuresBnyi. J. Polym. Sci., Polym. Chem. Ed.. 21 (1983) 2177. [3] D. Braun, A. Michel and D. Sonderhof, Eur. Polym. J., I7 (1981) 49. [4] T. Hjertberg and E.M. S(irvik, Polymer, 24 (1983) 673. 685. [5] V.P. Gupta and L.E. St. Pierre, J. Polym. Sci.. Part A, 8 (1970) 37. [h] B. IvAn. J.P. Kennedy, 1. Kende, T. Kelene and F. Tiidos. J. Macromol. Sci., Chem., Al6 ( I98 I ) 1473. [7] A.R. Berens, Polym. Prepr., I4 (1973) 671. [8] D. Braun and M. Thallmaier, J. Polym. Sci., Part C, I6 (1967) 235. [9] L. Valko, 1. Tvarbska and P. Kovarik, Eur. Polym. J.. I I (1975) 41 I [IO] T. Hjertberg and E. Siirvik, J. Macromol. Sci., Chem.. Al7 (1982) 983. [I I] B. Baum and L.H. Wartman, J. Polym. Sci.. 28 (1958) 537.

252

S. Mayedu et al. / J. Anal. Appl. Pyrolysis 33 (1995) 243-252

[I23 C.J.M. Heuvel and A.J.M. Weber. Makromol. Chem., 184 (1983) 2261. [ 131 V.P. Gupta and L.E. St. Pierre, J. Polym. Sci., Polym. Chem., 11 ( 19733 1841. [ 141 M. Rogestedt and T. Hjertberg, Macromolecules, 25 ( 1992) 6332. [ 151J.H.L. Heuson and F.J. Hybart, J. Appl. Poiy. Sci., 16 ( 1972) 1653. [ 161 G. Vancso, T.T. Nagy, B. Turcsinyi, T. Kelen and F. Tiidos, Makromol. Chem., Rapid. Commun., 3 (1982) 527. [ 171 K.B. Abbls and E.M. Sorvik, J. Appl. Polym. Sci., 17 (1973) 3567. [18] T.T. Nagy, T. Kelen, B. Tursanri and F. TiidSis, Polym. Bull., 2 (1980) 77. [19] E. Martinsson, T. Hjertberg and E. Siirvik, Macromolecules, 21 ( 1988) 136. [20] A. Guyot and M. Bert, Polym. Prepr., l2(2) (1971) 303. [2l] K.B. AbbPs and E.M. Siirvik, J. Appl. Polym. Sci., I7 (1973) 3577. [22] Reference deleted. [23] S.R. Leijenaar, C.J.M. van den Heuvel and W.G. Huysmans, Makromol. Chem., I86 (1985) 1549. 1241 3. Stepek, Z. Vymazal and b. Dolezel, Mod. Plast., 40 (1963) 146. [25] K.B. Abbas and E.M. Siirvik, J. Appl. Polym. Sci., 19 f1975) 2491. [26] M.K. Naqvi, Polym. Deg. Stab.. 17 (1987) 341. 1271 M.K. Naqvi, Polym. Deg. Stab., 22 (1988) 1. [28] N. Pourahinady and P.I. Bak, J. Macromoi. Sci., Pure Appl. Chem., A29(11) (1992) 959. [29] D. Braun, W. Mao, B. Biihringer and R.W. Garbella, Angew. Macromol. Chem., 141 (1986) 113. [30] B. Turcsanyi and F. Tudos, Int. J. Polym. Mater., 13 (1990) 147. [3l] W.H. Starnes, Jr., Pure Appl. Chem., 57(7) (1985) 1001.