Correlations between the chemical structure and the thermal stability of polyvinylchloride. A review

Polymer Science U.S.S.R. Vol. 28, No. B, pp. 585-552, 1 9 8 1 Printed in Poland

00$2~950/81/080585-18507.50[0 O 1982 Pergamon Prew Ltd.

CORRELATIONS DETWEEN ~ CHEMICAL STRUCTURE AND ~ THERMAL STABILITY OF POLYYINYLCffLORIDE. A REVIEW* K. S.

M.I~SKER, V. V. LISITSKII and G. YP.. ZArgov

Chemical Physics Institute, U.S.S.R. Academy of Sciences State University, Bashkiria (Received 1 tfebruary 1980)

The main ideas about the new theory of PVC decomposition are described; they tackle such key questions as the chemical structure and the contents of anomalous groupings in the PVC, their effects on the thermal stability of the products, and the kinetics of HC1 elimination. In contrast with the generally accepted fl-allyl chloride activation of the PVC decomposition was developed the concept of oxygen-containing groups having the structure NC(O)--CH-~CH--CHC1 ~ playing a decisive part. Real PVC macromolecules have been shown to contain l0 -~ mole per base-mole of PVC .of the latter, which are the cause of the low thermal stability. The purity of the monomer, the presence of oxygen in the reaction zone, the polymerization temperature, and other factors 'have been established to have a significant influence on the content of the ~C(O)--CH-----CH--CHCI~ groups in PVC. PVC h ad been synthesized already in the 19-th century but appeared as an industrial product only 60 years later (in the 1930's). The polymer demanded a specific approach to processing and the solution of a number of complicated tasks connected with prolonged use umder outdoor conditions of the articles made from i t ; this w a s th en an unsurmountable obstacle. The PVC is at present one of the polymers produced in large tonnage, i.e. 5-7 million tonnes per annum. I t forms the basis for about 4000 types of products and articles used for a v a r i e t y of purposes, and the areas in which PVC is used widens each year. The processing difficulties and these of the production of materials with the required durability are however still with us. One of the main problems encountered with PVC is its low thermal stability. J u d g i n ~ from the kinetics of the thermal degradation of some low mol.wt, compounds modelling PVC structure, namely ~CH~--CHC1--CH~--CHC1 ~ (Table 1), the polymer ought to be stable (decompose above 500°K), but the thermal degradation (TD) te m per a t ur e of industrial PVC types is still below 400°K [4, 5]; on t h e basis of the experimental T D results got with the low mol.wt. Cl-containing * Vysokomol. soycd. A25: No. 3, 483-497, 1981. 535

.536

K.S.

MrssKm~ et a/.

model compounds (Table 1) the reason appears to be the existence of structural defects in the PVC macromolecules. One of the more important characteristics of PVC on which its specific stability depends is the position of the Cl-atom in the macromoleeule and the type of adjacent groups differing from the normal sequences with 1,2-additions of the vinyl chloride chain units ~ CH2--CHC1--CH2--CHCI ~. One regards the PVC macromolecule as containing vicinal (in 1,2-position) Cl-atoms [6, 7] formed as the result of macro-radical recombination, also branches of varying length [8-14], amongst them also Cl-atoms present on the tertiary C-atom [10, 11]; the latter are the result of chain-transfer on the polymer, of an isomcrization of the macrodicals or of an alternate monomer "head to head" addition with simultaneous isomerization. The internal [12-21] and terminal [21-25] unsaturated / ~ = C ( bonds form as a result of appropriate admixtures present, the partial HC1 elimination during the production and storage of the polymer, a chain transfer on the monomer, and a main-chain fracture by disproportionation. There are also various oxygen-containing groups present, including hydroperoxides [26, 27], hydroxyls [28] and carbonyls [29-31], which are PVCoxidationll products. I T A B L E 1. T H E T H E R M A L D E G R A D A T I O N OF SOME LOW MOL.WT. C1-CONTAININO COMPOUNDS I1ff Tlh~ L I Q U I D P H A S E

Compound

1,4,7-triehloroheptane 1,4,9 - t r i c h l o r o n o n a n e 2,4.diehloropentadecane 8-chlorohexadecane 4-chlorodqdec-2-ene 4-chlorodec-2-ene 4-chlorohex-2-ene 7 - c h l o r o n o n a - 3,5-diene 6-chloroocta- 2,4-diene

Decompo- ] s i t i o n tern- 1 Ea, perature, kcal/mole oK 503-533 493-523 517-565 509-557 430-453 438-469 433-463 343-369 360-386

22"7 31.5 16"3 36"0 22.7 22"2 13"6 19:4 17'9

k a t 448K, • log A

S~C - I

6"5 × 6.0 × 1.7 × 1.0 × 6.8 × 5.0 × 5.1 × 3.4 × 2.6 ×

4.88 9.14 2.18 10.55 6.90 6.53 3.34 7.99 7.14

10 -7 10 -7 10 -6 10 -7 10 -5 10 -~ 1 0 -4 I 0 -~

10 -3

Lit. ref.

[1] [1] [2] [2] [2] [2] [3] [2] [23

The vicinal groups content of PVC has not been reliably established; even when present they do not greatly reduce the stability [32]. Furthermore, mild and strictly controlled PVC chlorination, i.e. the formation of 1,2-dichloro:ethylene chain units in the maeromolecules, greatly improves thermal stability [15, 16, 33]. The number of branches present will depend on the production method and conditions; they number 0-40/1000 monomer units [10, 21, 26, 34-37] and only 2-2.5°/o of these probably contain a Cl-atom on the tert.C-atom [35], which is generally considered in evaluations of the PVC stability. However, the IR- and NMR-spectra of PVC do not show any absorption lines typical of the ~C--Clgroupe [37, 38]. /

Chemical structure and thermal ~ability of PVC

537

Experiments have shown that any PVC samples synthesized by radical polymerization methods [27, 30, 31] contain up to 6 × 10 -8 mole/base-mole of unsaturated ~ = C ~ bonds (Table 2), of which the main quantity are terminal groups. T h e presence of i n t e r n a l / ~ = C ( bonds ~0 is always smaller by one magnitude or so than that of unsaturated terminal groups (10 -~ mole/base-mole PVCI. PVC does not contain any polyene sequences. The PVC macromolecules can be expected to contain 3 types of terminal: /~C=C( groups and their appearance is due to disproportionation reactions of the macro-radicals ~CHz--¢HC1 + ¢HCI--CH2'~ ---,~CH~--CH~CI + CHCI=CH~, but also to main-chain termination by chain-transfer to the monomer ~CH~--CH2C1 + CH2----CC1.--.

(I[

~CH2--CHCI -~ CH2=CHCI --. I--' ~CH~--CH2C1 + CHCI=CH~

(II[)

I

CHz--CHC/2.+ CH~=CH~

(IX:)

By using a kinetic ozonolysis method on PVC and the respective low mol.wt. model compounds [39, 40], and on the basis of the O3 reaction constants with the end groups, with 0ct- -ene, hex-a-ene, vinyl chloride, polychloroprene, a 2,3-dichloroprop-l-ene mixture with 1,3-diehloroprop-l-ene (Table 3) one can conclude that the PVC macromolecules are unlikely to contain end groups of type ~ CCl= CH2 and/or ~ C H = CHCI, and that there are only CH2 = CH-- CHC1 ~ groups present. Accepted until quite recently was the inverse dependence of the mol.wt. (as / ~ ' ~ C ( end groups) on the dehydrochlorination rate of the PVC [22-25], but there are now quite reliable experimental results available (see Table 2) which indicate that the ~/C=C~/ end groups do not affect the thermal stability of the polymer. Thus there remain 2 structural defects of the macromolecules which could reduce the specific stability of PVC, namely the O- and internal )C-~C-containing groups. The O-containing groups, although regarded in many cases as likely structural defects [26-29, 41-45], are not being'considered in the decomposition scheme o f PVC despite the fact that the final product is in contact with atmospheric oxygen a n d subjected to oxidation during recovery and drying. Some firm ideas about the low specific stability of PVC existing at present blame it on the fl-allyl chloride activation of the dehydrochlorination of the polymer. This is quite understandable as the fl-allyl chloride group always forms.

538

K.S.

Mms~

e$ a/.

~o~s

o o ~

T ~ L ~ . 2. Trm EFFECT OF THE ~ f f i ~

o~

~

.

s

~

~

o~ P V C (448°K, 1 0 - ' Pa)

×'°', mo o a -ino o O-containing chloride I unsaturated groups groups

polyene seq uences

0 0 0 0 1.0

1.7 1"0 0.7 1.2 0.3

33 34 35

1"2

0.8 1.0 0.1

0 0 0

35 39 47 47 50 53 33

0 0 0 0 0 0 0

1.2

40

1.0

48 53 60

1.9 2.9 3.8

22 27 28 28 30

1-2 0.2 0.9 0-2 2.0 0.8

0 0 0

VHm × 10 e mole/ basemole PVC. "see

end groups

20"3 26-0 27'3 26"8 28"7

Remarks

X 10 -s

I

1.40 ! 0.80 0.60 0.95 0.25

132 150 235 108 108

32.2 33.0 33-7

0.65 0.85 0.16

140 100 120

0 0 0 0 0 0 0

33.8 37"8 46"8 46.1 49"8 51"0 32-2

1 "00

i 0.95 0.22 0.85 0.22 1.52 0.65

105 100 83 106 290 61 140

6"8

32.2

0.65

140

3"9 7"9 :4"0

32.2 32-2 32.2

0.65 0.65 0-65

140 140 140

Polymerization in the absence of 01 Polymerization in the absence O~

TD 20 min ,, ,, ,,

40 rain 60 rain 80 m i n

during macromolecular decomposition: ~CHCI--CH2.CHCI--CII2-~

-, ~ C H = C H - - C H C I - - C H z ~ + HCI

(V)

These groups can also form either during the vinyl chloride copolymerization with any admixture present (such as acetylene and its derivatives) "~CH2--(~HCI + C H ~ C H -* "~CH2--CHC1--CH=CH ~ C H ~ - - C H C I - - C H = C H - - C H 2 - - C HCI,

CH~CHCI (vt)

o r a s a r e s u l t o f t h e c h a i n - t r a n s f e r o n t h e p o l Y m e r w h i c h is f o l l o w e d b y a n HC1

Chemical structure and thermal stability of PVC

5N

CONSTANTS OF T H E OZOI~E R E A O T I O N W , ' J ~ ~ N ~ C = <

BOND C O N T A I N I N G

TABI~ 3. Tm~ ~ T Z

eOMPOU~-DS (293°K)

Compound

k' x 10' liquid phase Solid phase (CC14), kg/mole1./mole.

Group

•I~C-1

PVC Low tool. wt. PVC fraction Degraded PVC Vinyl chloride Mixture of dichloropentenes Polyehloroprene Chloropentene Allyl chloride Hex - 1-ene Oct - 1-ene

CH~:CH

~

CH~=CH ~ ~ CI~I,--( CH= CH) n---CHC1~ CH2=CHC1 CHCI:CH---CH2C1 and CH~=CC1---CHsC1 ~ CH2---CHC1-----CH--CHs ~ ~ CHC1--CH----CH ~ CH~:

CH---CTIICI

CH~:CH ~ CH,=CH ~

1.8

•sec-I

1.1 1.3 0.5

0.12" 0.4 0.42 4.0 0.85* 7.6* 10

* Result cited by WilllsJaason [62].

elimination f r o m t h e Cl-tert.C-atom b r a n c h e s R

R

I t ~CH~--CCI--CH~-- CHCI ~ -+ ~CH2--C'CH--CHCI ~ ~-HCI

(VI1)

The decomposition "results got with the low mol.wt, compounds modelling t h e fl-allyl chloride group clearly confirmed the possibility of such an activation (Table 1). The rate constant of the HC1 elimination increased (by several powers) when compared with that of the dehydrochlorination of the compounds modelling the normal structure of PVC. The mathematical calculations also confirmed it [18, 46, 47]. I n m a n y cases t h e r e was also a linear d e p e n d e n c e b e t w e e n t h e internal )C=< bonds cofltent of the macromolecule a n d t h e r a t e o f the PVC d e h y d r o c h l o r i n a t i o n (Fig. 1) [16, 20, 21, 30]. T h e PVC d e h y d r o c h l o r i n a t i o n is a complex process consisting of two parallelconsecutive reactions, n a m e l y a) the r a n d o m HC1 elimination a n d the p r o d u c t i o n o f single /\ C = C \ / b o n d s (statistical PVC dchydroehlorination): ~CHa--CHCI--CH~--CHCI~

kc ~ ~CH2--CH---CH--CHCIN -]- HCI,

(VI ID

a n d b) an increase o f the s y s t e m of polyconjugations due to a n a c t i v a t i o n o f t h e

HCl elimination by an adjacent

bond (allyl chloride activation/

*'-'CH,--CH~---Ctt--CItCI--CH,--CHCI,~ 2:tCl ~CH,--(CH-~CIt)s--CltGI,~

(IX}

~40

K. S. M i ~ s m

eta/.

The rate vc of the statistical PVC dehydrochlorination cletermined as the change of the internal ) C = ~ bonds content ~ in time (vc=a~/d0 was found to be constant (Table 4). Value vc is therefore a fundamental characteristic of PVC which shows that all the ~CHs--CHC1 ~ units present in the macromolecu!e participate equally in the random HC1 elimination process.

Q. 1"5

I

io0

t

j

~E ~

~:0.,

*'

,~ 60

4

&3

~20 I

I

1

1

,

2

240

120

Yo, mole~base-mole PVC

Time, ,seo

FIa. 2

FIG. 1

x.~

j

1. The dehydroehlorination rate vacz as a function of the internal /'U=tX,, bonds content 70 of the PVC (448°K, 10-= Pa); ~ × 10-4 values: 1--<10, 2--i0-11, 3=-11-13, 4-->13, 5--see text. :FIG. 2. The kinetic curves of the PVC dehydroehlorination: 1-3--without, 4-6--with consideration of the decay of polyene sequences; 70× 10' mole/base-mole PVC: 1,4--1.90, 2,5--1.40, 3,6--0.70; k¢× 104, sec-1:4--4,3, 5--6.2, 6--9.6 (448°K, 10-=Pa). The crosses are calculated values, the points experimental ones. The rate at which the system of polyconjugated /~3=C-bonds forms, when determined as the difference between t h ~ vHCl and vc reaction rates, i.e.

Vp=V~Cl--Ve

(1)

during the decomposition of suspension, bulk or emulsion PVC samples possessing differing mol.wt. (50,000-300,000)and internal ) C = C / ~ bond contents (~0=(0.2-2.5)×10-4mole/base-mole PVC) will be different. However, the rate constant of the reaction (kp) will be constant in practice for all the PVC samples (with the exception of freshly prepared samples and those synthesized in the absence of oxygen) when referred to T0 (kp=vp]~o) (Table 4). The constant/¢p varied in time during consideration of fl-allyl chloride activation (~t). This was one of the experimental results which disagreed with the theory of HC1 elimination from PVC activated b y the fl-allyl chlorides structure. There also exist a number of (well-proven)experimental facts Which disagree with the fl-allyl chloride activation theory, as well as about the correlation between the rate o f the dehydrochlorination and the fl:allyl :chloride groups content [30, 48-50].

541

Chemical structure and thermal stability of PVC

X

i!

r~

6 6 ~ 6 ~ o 6 6 ~ 6 ~ o 6

6666o6~66

~ 6 ~ 0 0 6 6 6 6 6 ~ 6 6 6

ooooooooo

0

~T xg 4

~

~ 6 6 6 6 ~ 6 6 ~ 6 6 6 5 ~ 6 6

666666666'

?

I~ o

o

I l l l l

B

r.5 6 6

fJ42

K . S. M I ~ T m

et ol.

The comparisons of kc and kp for PVC (Table 4) with the decomposition constants (reduced to 448°K) got for the respective low mol.wt, compounds in the liquid phase (Table 1) gave satisfactory agreement of the rate constant for the statistical HC1 elimination (kc= 0.8 × 10 -7 sec -1, 448°K) with its low mol.wt, analogues modelled by sequences of ~ CH2--CHC1 ~ u n i t s (10-7-10 -e sec -1) [1, 2, 4]. The rate constant Vpreferred to yo,(kp=0"75 × 10 -3 sec -1, 448°K) is two or more magnitudes above those calculated for the HC1 elimination by the model compounds containing a Cl-atom in fl-position to the isolated ~ C = C ~ bond (10-~-10-' sec -1, 488°K), but agrees well with the rate constant for the decomposition of low mol.wt, compounds containing conjugated double bonds, i.e. ~ CH~-- (CH----CH)~-CHC1 ~ (10 -~ sec -~, 448°K) [2-4] and ~ CH2(CH---- CH)a-- CHC1 ~ [51]. The original PVC samples do not however contain conjugated/~C----C~ bonds in their macromolecules [30, 31, 48, 52] (Table 2); according to experiment these appear only during the heat-treatment of PVC (Tables 2, 3). Assuming t h a t the dehydrochlorination of PVC is a combination of reactions (VIII) and (IX) d[HCi] d¢

a~=v~-kg~, dt

(2)

one gets for the initial stages where the-decay of the kinetic chains is negligible

VHCl=Vc~Vp=kcao-{-.kp(~o+-~)

(3)

There ought to be a spontaneous acceleration of the dehydrochlorination in this case (Fig. 2). In a correctly carried out ~xperiment there is actually a constancy of the HC1 elimination as a function of time up to substantial °/o conversions [14, 19, 23] and this was shown especially clearly by Arthur and Blouin [53]. Compensation for the internal/~C---=C~ bond accumulation in the macromolecules due to a limitation of the growth of polyene sequences (crosslinking, intra- and inter-molecular cyclization [54-57]) can result in an, agreement with the experimental results (linear dependence between HC1 concentration and time; Fig. 2), although t h e calculated ks values will greatly differ for various PVC samples, being a function of the initial interI~al ~ C : - C ~ bond concentration 70. For example, the /:g-values will be 9.6× 10 -4, 6.2× 10 -4 and 4=3× 10 -4 sec -1 for the samples having 70 of 0.7 × 10-', 1.4 × 10 -4 and 2.0 × 10, 4 mole/base-mole respectively. Finally, there is not always a strict linearity of VHCl----f(y0) in the case of some PVC samples (Fig. 1). This normally happens where the PVC is synthesized under specific conditions in which no 02 is present, or where the polymer is immediately removed from the reaction zone and dried in the absence of 03. The rate of the HCI liberation is t h e n always lower t h a n expected on the basis of the found To (Fig. 1,

Chemical structure and thermal stability of PVC points 5). When the same samples ~ e r e stor¢~ in oxygen there was a change o f the:HC1 liberation rate during thermal degradation. As soon as a steady ~ c l was reached (after several months; 298°K), the strictly equdvalent ?o (Fig. 1) did not cause any further change in v~cl provided the number of internal ~ = C ( bonds remained the same. The same effect was also observed under mild oxidation conditions of the PVC [30, 48]. A natural assumption was the existence, to a lesser extent, of two t y ~ s o f structural defects in PVC containing internal /C--C\\-/ bonds, namely ~ CH2-- (CH ~ CH)2-- CHC1 ~ and ~ C (0)-- CH = CH-- CHC1 having the structure of conjugated dienes (essential to this is numerical agreement of the k~, of about l0 -2 see -1, for PVC and the low mol.wt, models of the Cl-eontaining dienes; Table 1). By assuming the PVC not to contain any ~ C ~ conjugations [30, 31, 48, 51], and by also considering the part played by the oxidative processes in the vHCl increase (at constant ~0), one would think t h a t the required structure would from during participation of the O-atoms. Such a structure could be produced during the synthesis and storage of PVC, e.g. during the oxidation of t h e fl-hydrogen atoms by atmospheric oxygen of the separate \/C----C\ / bonds [58] 0 II

OOH O:

['--+ ~ C--CH=CH--CHCI~

I

•~CH~-- CH=CH--CHCI~ -+ .-~CH--CH=CH--CHCI,~ --

OH I

--+ ~ CH--CH----CH--CHCI~ (X) Strict proof of the presence of O-containing unsaturated groups by any direct, accurate methods is known to be difficult because of their low concentrations (10 -4 mole/base-mole PVC). Nevertheless, there a r e fairly weak 1600 and 1675 cm -x lines present in the IR-spectra recorded from fairly thick (0.4-1 mm) films which belong to t h e / ~ H = C H - - and the conjugated /~C-~O-group bond valence oscillations [59]. The hydrolysis of the polymer chains made it possible to identify the O-containing unsaturated groups present in the PVC macromolecules, The • ~ C (0) -- C H = C H - - CHC1 ~ type of group is known to hydrolyze easily in the presence of alkali or oxygen [60] which will result in macromolecular fracture a t the ~ C = C ~ bonds (like on ozonolysis [30, 31]); simultaneously the IR-spectra (taken on 50p thick films) will show the 1715 cm -1 IR-spectral line (valence oscillations of the ~ C = O terminal groups [59]) 0 tl ...CH=CH--C--CH2N

H mo / K0H > N C

~- C H s - - C - - C H l ~

0

0

(Xl}

-544

K . S. M ~ s ~ B

~ og.

Important is that the fl-allyl chloride groups ~ CH~--CH~-CH--CHC1 ~ are resistant to hydrolysis. By combining the alkaline hydrolysis with ozonolysis one can ,easily get a quantitative content of the O-containing and fl-allyl chloride groups present in the PVC (as indicated by the intrinsic viscosity [0] decrease [30, 31]). For example, there was a steady increase of the separate internal/NC----C~/bonds, determined from the mol.wt, change during ozonolysis when the PVC was s u b jeered to thermal dehydrochlorination in a vacuum (10 -3 Pa, 448°K), and the number of macro-chain fractures remained the same during hydrolysis (Fig. 3). T h e rate of the PVC decomposition remained constant a t the same time. The ~cheme equations ( 2 ) a n d (3) for the dehydrochlorination kinetics is therefore unlikely. ~=~ 1"5

;o;ooo lO

o_ 30 Time, rnin

i

I

I

1

I

0"5 1"5 2.5 ~'o , rno/e/ba,~e-rno/e PVC

FIG. 4 3 FIO. 3. Changes of the internal ~C=C~ bonds content during-the dehydroehlorination of PVC (448°K, 10-~ Pa): /--oxidative dissociation (H~O~) of ozonized samples, 2--alkaline (KOH) hydrolysis. FIG. 4. The rate of polyconjugated systems formation vp as a function of the internal content of ~ C (O) -- CH-----CH-- CHC1~ groups in the PVC maeromolecules (448°K, 10-I Pa). FIO.

A systematic study of a large number of industrial and laboratory PVC samples (Table 4) showed unequivocally that all the polymers contained a noticeable number of internal ~ C = C ~ bonds which were in O-containing groups in the majority of cases (no fl-allyl chloride groups). A content reduction of the O-containin~ unsaturated groups usually resulted in a reduction of the overall rate of PVC dehydrochlorination, i.e. in a thermal stability increase strictly in accordance with the linear dependence shown in Fig. 4. The presence of fi-allyl chloride groups in the macromolecules does n o t affect the changes of the rate o f PVC dehydrochlorination which indicates their relative stability (when compared w i t h t h e O-containing groups). There is therefore a strict correlation of the vHcl with t h a t of the formation of the polyc0njugated sequences vp and the T0-values calculated from the mol.wt, changes as a result of alkaline hydrolysis (Fig. 4). Only then will the experimental vp----f(~o ) function be valid for any polymer and any PVC synthesis variations b y the suspension, bulk or emulsion methods. T h e linearity of t h e vp----f(~o) extrapolates to the origin, which is one more evi~lence for the stability of the /\ C ~ C \ / end groups.

Chemical structure .and thermal stability of PVC

545

Independent proof for t h e presence of 0-containing unsaturated groups w a s the PVC reaction with organic phosphites by a nucleophilic addition reaction with proton donors, involving attack by the organic phosphite on the fl-C atom of the conjugated system present in the O-containing groups, followed by the protonization of the bipolar ion and the formation of a stable keto-phosphonate [49, 61]: H+

~C--CH:CH~ -~ P(OR)s ---,~C:CH II

!

O

O

• ---C-- CH2--CH~ ~ R+

\

Ir

CH.-~ O

[

(XII)

O=P(OR)2

P(OR)a The results of the PVC ozonolysis after even a mild heat treatment (353°K, 0.5-1 hr) in the presence of the phosphite indicated that there were no unsaturated /~C---~C( bonds present in the main chain (there was no [t/] depression); the organic phosphites did not react with the p-allyl chioride groups (at least not in these conditions), which was proved by competitive reactions with triallyl-alkyl phosphites and aryl phosphites in equimolar mixtures with methylvinylketone (the model for the O-containing unsaturated group) or 4-chloropent-2-ene (model for the p-allyl chloride group) at 353°K. The phosphites (the triaryl phosphites were less reactive in this reaction) reacted easily and exhaustively with the methylvinylketone, while the 4-chloropent-2-ene was unchanged except for a slight dehydrochlorination [49]. The main product of distilling the reaction mixture (64-70%) was dibutyl-3oxo-butyl phosphonate CH3--C--CH~--CH2 whose structure was confirmed

It

/

O

O=P(0C~Hp)

2

by IR-spectroscopy (1280 (P----O) and 1780 cm -1 (C----O)), and by elemental analysis. The gross rate of the PVC dehydrochlorination (as well as the 'low stability of the polymer) are thus determined by the initial content of internal 0-containing groups in the macromolecules ?0 of type ~C (O)--CH----CH--CHCI ~ which in practice is the only unstable group present in PVC. The PVC dehydrochlorination is generally a complex process which incorporates to a lesser extent the following parallel-consecutive reactions [30, 48]. 1. A statistical HC1 elimination (random) from. the normal PVC chain units and the formation of p-allyl chloride groups (kc=0.8 × 10-7 sec -2, 448°K): ~CHI--CHC1--CHz-~CHCIN --, ~CH2--CH=CH--CHCI--- + HCI

(Xnl)

2. An HC1 elimination and the formation o f a polyconjugated system of / ~ C = ~ double bonds initiated by internal ~ C ( O ) - - C H = C H - - C H C I ~ groups

546

K . S . MrssY~a e~ a/.

kp=0.75 × 10-~ see-1, 448°K)

kp -,~C(O)--CH=CH--CHCI--CH2--CHC!~ ' ~ ~C(O)--CH=CH--CH----CH--CHCI~q- HCI 3. A slow HCI elimination and the formation of polyconjugated )C----~ bonds which is activatedlby theinternM fl-allyl chloride groups ~ CH2--CH=CH---CHC1 ~ (kin= 10-5-10 -a see"1, 448°K [2-4]: ~CH2--CH=CH-eCHCI--CH~--CHCI~

......>- C H ~ - - C H = C H - - C H = C H - - C H C I ~

-}-HCI

(XV) 4. An HC1 elimination with the formation of polyconjugated systems oi /xC----C~/ bonds activated by the internal conjugated /~'C=C~ bonds /¢p2 =10 -2 see-1, 448°K) [2, 4]:

~Pz

,--CH,--(CH=CH)9--CHCI--CH~-~ ~

-~CH2--(CH=CH)s--- -}-HCI

(xvz)

In accordance with this scheme one can present the PVC dehydrochlorination in the shape of: PCV /¢_~cA1 ~

7kg inactive product A2,,~kp2 active product

(XVII)

in which k~ -- termination constant of the polyeonjugated ~ = C ~ system propagation. The changes of the individual \/ C ---- C/\ bonds concentration A1 can be represented by dial], , .... ~-~ = ~ea0--r~L.~lj ,

(4)

~c"C~o [All= ~ [1--exp(--]gpz$)]~-[A1]o'exp(--kp t),

(5)

from which one gets

in which a0--HC1 content of o~ginM PVC, [A1]o--content of internal fl-allyl chloride groups of original PVC. The constants ]co and kp~ ought to be the same in practice and the concentration changes of the conjugated )C----C~ bonds during the dehydrochlorination will be

d[.~,] ~. =kp,[A~]--ks[A,]

(6)

Chemical structure and. thermal arability of PVC

547

By inserting [A~] from eqn. (5) i~to~ (6) one sets d[A,] =k~ao[1--exp(--k~t)]+~p,[Ado exp (--/~,t)--~[A,] dt

and

[~/=

~a0--km[A1]

kp,-~

• exp(-kp,t)+(~0-

kca 0

(7)

kcao--km[A1]o kr,--kg

~ea o

- - • [(exp (kS)] + - kg kg

(8)

The rate of the PVC dehydrochlorination is described by: d[HCl]

dt

'

=k~ao+k~,[A,]+~p,[A,]

(9)

The insertion of [All and [A2] from eqns. (5) and (8) and the appropriate rearrangements will give

[HC1]=( 2k~°4-% a ~ t- kea°-km[Ar]° × kg

: [1-exp(-~p,t)]+ ~ o -

keao--kp,[A1]o' ~. . . . . . . ~ )L,--expt~-~j

(lo)

As the majority of the PVC samples does not contain any internal fl-allyl chloride groups, i.e. [A1]o=0, [HC1]-----( 2kea0-{- ~ ) •

kp. /

x [1-exp(-k~,t)]+ ~ o .

t----~[m\k~_kp,kea°(kg+kp'--km~×] kc" ao

k.

kcao

~T--k, [1-exp(-kJ)]

(n)

Equation (11) will become [HC1]=keaot+kp,~ot

(12)

in the initial stages of the hydrochlorination when k~t<
K. S. M I ~ s ~ a

"548

~ oZ.

this plays an important part in the changes of the use properties of the polymer. The branching and ultimately the formation of a network result in an increase of the mol.wt, and of the MWD, a partial solubility loss at the start, later s complete loss, and in changes of the whole complex of physico-meehanieal properties.

3'5 3"t,,,J;~q.3 Iogl/~ 200

",ooi .ca

~

10"8

2;/00 15o

6o

50

~ 5

20

L

60

120

,.2;

I

0.4 _

180~ ZL/O 300

Time, rain

~

l ]o ~ 10~

Fzo. 5

2.

-

•

3

mole/base-mole

Fie. 6

Fro. 5. The gel accumulation during the thermal degradation of PVC samples with 70× 10-4 mole/base-mole PVC : 1 - 3 - - 1.8,' d-- 1.4, 5 - - 1.0; temp., °K: 1 - - 468, 2 - - 458, 3 - 5 - - 448. Fie. 6. The start of gelling in various PVC samples as a function of their ~C (O)---CH=CH--CHCI-groups content ?0: /--during decomposition, 2--the logarithmic transformation of ] (4480K, 10 -s Pa). The formation of the gel-fraction during the decomposition of various PVC samples is preceded by an induction period which varies in length (Fig. 5). The moment at which gelling starts (~8) at constant temperature is connected with the content of internal O-containing groups of type ~ C ( O ) - - C H = C H - - C H C I ~ (rig. 6). The found dependence is clear experimental proof for the prevailing opinion [19, 56] that the macromoleeular crosslinking takes place as a result of the reactions of the polyene sequences present in the various macromolecules; this occurs most probably regardless of their length. As the content of the polyene sequences frorming at the start of the thermal degradation o f PVC depends on 70, the equation describing the concentration of lateral bonds C will be the following

dc/dt = ken [sequences]== kerl [70]=,

(13)

in which kerl -- rate constant of the crosslinking reaction, ~ -- concentration of polyene sequences. If one disregards the utilization of the latter, the content of the crosslinkages present at the moment of gelling will be Ccr=~cr] [70] - = Tg

(14)

However, the critical content of crosslinkages essential to gelling of a real polymer is known to be given by the Flory condition Ccr= ]/pc. Bearing in mind

Chemical structure and thermal stability of PVC

64~

that ~7/,1Ms=1.86 when ~]~w/~lfn=2 we get for the e~r in 1 mole of P V C Cer=

1.86 o

X 62"5

(15}

We therefore get on the basis of eqns. (13) and (15) the equation for the keri calculation: 1.86 x 62.5 kcrl-- - (16). tt 2M~ [~'0] Tg --0

The order of the reaction, ~, determined from the logarithmic transformation of vg-----f(?0)equals 2. Equation (16) was used to calculate the following parameters: k e r l = 6 " 8 ± l ' l (base-mole PVC/mole). sec -1 (448°K), Ecrl-----12+- 2 kcal/mole, and log Acrl=6"3 +_0"5 (438-468°K). The kinetic parameters of the macromolecular crosslinking in the PVC during thermal degradation in a vacuum are practically identical with those known from the Diels-Alder reaction (Eo=10-20 kcal/mole, log A=5-7); this indicates that the crosslinking takes place by the reaction scheme of a diene synthesis 0 II ~C CH=CH~ \CH=Ctl ~crt ~

CH--CH

~C "/n

0

CH--CH

"%CH--CH ~

CH=CH~ / CH--CH / \ CH CH--CH=CH,~ (XV). / \ / ~CH=CH CH=CH ~C

...->

\

\ CH-~

Very significant is that only 1 of 8-15 PVC macromolecules possesses an O-containing unsaturated grouping which determines all the thermal properties of the PVC. Important from the practical aspect is that the internal unsaturated groups (T0) are produced in the PVC macromoleculcs throughout the polymerization process, but especially at great intensity at the start and in later stages ( > 8 0 wt.%) of monomer conversion (Table 5). This demands a very thorough purification of the vinyl chloride, a limitation of the critical conversion and as complete an absence of oxygen as possible. The mol.wt, does not affect the thermal stability of the PVC and the known inverse dependence between vHClandll~v [20] is 'chiefly connected with a production of a relatively large T0-value at higher process temperatures, and of low _~rv-values (Table 5). A specifica!ly high stability of the PVC is obtainable by t h e synthesis o f products in which the content of the internal ) C = < bonds is reduced to a

T ~

5. Txrm EFFECTS

OF TECHNOLOGICAL AND OOMPOUNDII~O FACTORS

~][~.'~IrAT. S T A B I L I T Y (448°K, 10 -I Pa)

BONDS CONTENT AND ~ ' ~

[~>C=C~-_IX 10',

mole/base-mole

PVC

I

7o

34 28 27 28 30 38 41

1-5 1.2 1.1 0.9 1.3 2.3 3"3

22 25

0.8 1.2

26 26 34 26 48

0.7 0.8 1.4 1.6 2.0

I

v~cl

OF P V C

X 104~

I mole/base-mole

~ v × 10 -8

end I PVC'sec oups E ~ e c t of conversion (process a t 326 K) 1.4 70.0 32-5 1.1 82.0 26.2 1.0 83-0 25.9 1.0 27.1 86"0 88.0 28.7 0"96 1.2 87.5 35.7 1.5 86.0 37.7 Effect of oxygen (process a t 326 K) 137.5 21.5 0.7 137.5 • 23.8 1.0 Effect of temperature 235.0 25.3 0.6 140.0 25.2 0.7 32.6 1.2 94-5 24.4 1.3 80.0 61.5 46.0 1.5

10" 25* 40* 60* 80* 90* 98* Prior evacuation Oi present 313t 326 t 335 t 337, 345 t

* Conversion. wt.%. t Temperature, °K.

minimum. The upper limit of the HC1 liberated will be characterized by the rate o f the statistical HC1 elimination from the ~CH~--CHCI~ units present in the PVC maeromolecules (VHCl~=0"8>(10-7 sec-1 at 448°K). Translated by K. A. AIz.,E~

REFERENCES

1. 2. 3. 4. 5. 6. ft. 8. 9.

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551

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Polymer Science U.S.S.R. Vol. 25, No. 3, lap. 552-569, 1981

Printed in Poland

0052-8950/81]050552-18507.50/~ © 1982 Pergamon Press Ltd.

POLYVINYLCHLORIDE STABILIZATION METHODS. A REVIEW* K . S. ~-INSKER, S. V. KOLESOV a n d G. YE. Z ~ K O V Chemical Physics Institute, TJ.S.S.R. Academy of Sciences 40 Years of October State University, Bashkiria

(Received 10 April 1980) The m a i n results of PVC stabilization studies axe discussed. They relate t o modern ideas about the reasons for the thermal stability of PVC, on the c o m p l e x i t y of its dehydroehlorination and on the decomposition kinetics. The main source of t h e instability of the poly~aer is thought to be the presence of internal, unsaturated O-containing groups of the t y p e ~ C (O) -- CH-= C H - - CHC1 ~ . Typical processes leadin~ * Vysokomol. soyed. A28: No. 3, .498-5.12,1981.