Kinetics of the thermal oxidation of polydimethylphenylene oxide

• 'o~marScienceU.S.S.R.Vol. 24, ~'o. 4, pp. 902-908, 1 9 8 2 ~mted in Poland

0082-~5018~7507,5010 O 1988P e ~ n Prem Ltd.

KINETICS OF TffE THERMAL OXIDATION OF P O L Y D I M E T H Y L P ~ L E N E OXIDE* I. A. S~R~KOVA and ¥ ~ . A. SHLYAP~IKOV Chemical Physics Institute, U.S.S.R. Academy of Sciellces

(Received 19 November 1980) The high-temperature oxidation of poly-2,6-dimethyl-l,4-phenylene oxide has been investigated in the temperature r~nge 230-270 ° under oxygen pressures up to 2 × 104 Pa. The oxidation takes place in ~ccordauoe with a n autocatalytic law, which is accounted for b y the accumulation of readily oxidizable aldehyde groups. The m a x i m u m rate of oxidation is a linear function of the 02 pressure. Oxidation is accompanied by erosslinking of the macromoleoules, the MW of the soluble fraction decreasing in advanced stages of the process. Low molecular weight r~iioais participate in the reaction, as is evident from the complicated dependence of the rate of O2 absorption on the thickness of the sample.

A numer of difficulties confront authors investigating mechanisms of high temperature oxidation processes of polymers, The volatility of low molecular weight compounds that may be used as models is such that scarcely any study o f their liquid-phase oxidation appears to have been made. Low molecular weight substances and new functional groups ~ccumulating in polymers during oxidation are not, generally speaking, primary products, but are the result of a number of intermediate stages that elude observation. On the other hand, the presence of a variety of functional groups in the majority of heat-stable polymers greatly impedes spectrum analysis of similar groups formed in the course of oxidation. The high temperature (220-400 °) oxidation of linear PE was investigated in [1]. The results showed that a change occurring in the features and mechanism of oxidation at 260-270 ° leads to a reduction in the effective activation energy from 80-100 down to 8 kJ/mole, while the oxidation rate, which below 200 ° is only slightly dependent on the O2 pressure, becomes a linear function of the pressure. The mechanism proposed to account for the observed relations includes stage of peroxide radicM decay and is as follows O oo.

~ CI:I2--CHO+ "OH The low molecular weight "OH radicals generated in reaction (I) may, according to the proposed mechanism, participate in chain transfer (reaction of * Vysokomol. soyed. A24: No. 4, 808-813, 1982. 902

Kinetios of ~hermaloxidation of poly~imo~hylphenylonc oxide

' 903

R H ~ - ' O H ) a n d c h a i n t e r m | n a t i o n R'-}-'OH), E v i d e n c e in s u p p o r t of' t h e mechanism includes t h e f o r m a t i o n o f e p o x i d e groups i n P E oxidized a t t e m p e r a t u r e s a b o v e 270 ° . I n c r e a s e d c o m p l e x i t y o f t h e p o l y m e r s t r u c t u r e complicates t h e m e c h a n i s m o f t h e o x i d a t i o n reaction. A r e d u c e d n u m b e r o f aliphatie groups a n d increased rigidity o f t h e chain on going f r o m P E to more c o m p l e x p o l y m e r s is b o u n d t~) lessen t h e role o f t h e chain t r a n s f e r r e a c t i o n RO~+RH-.ROOH%R', (II) p r e d o m i n a t i n g a t low t e m p e r a t u r e s , a n d will accordingly increase t h e role o[ reactions o f p e r o x i d e r ~ i c ~ l d e c a y [1]. I n o t h e r words a t r a n s i t i o n t o a high ~ m p e r a t u r e m e c h a n i s m for p o l y m e r s o f m o r e c o m p l e x s t r u c t u r e containing aliphatic groups will o c c u r a t lower t e m p e r a t u r e s t h a u for t h e simplest polyolefins. I t is h o w e v e r to be e x p e c t e d t h a t a t ~ 200 °, w h e n no significant role is p l a y e d b y reactions o f o x i d a t i o n o f a r o m a t i c a n d heterocyclic f r a g m e n t s [2, 3], t h a t o x i d a t i o n will t~ke place b y a m e c h a n i s m similar to t h a t of p o l y e t h y l e n e , a n d t h a t low molecular weight radicals will p a r t i c i p a t e in it. T o shed light on t h e main features of the mechanism of high-temperature oxidation of polymers c o n t a i n i n g aliphatic groups, along w i t h a r o m a t i c a n d o t h e r groups, we investig a t e d t h e o x i d a t i o n o f poly-2,6-dimethyl- 1,4-phenylene oxide (PPO) a t 230-270~. The study objects were PPO (M----40,000) film specimens of thickness 2-150 pro. Th~ reaction was run in equipment involving circulation of On and freezing of volatile reactiol~ pi~duets, making it possible to record O= absorption uncomplicated by the liberation o:l" reaction products. The basic measurements were carried out at 240° at O= pressure of 2 X 1()~ Pa. To analyze volatile products an annular reactor [5] was used for the oxidation rea~tiol~: ~ho products were analyzed chromatographically, and the consumption and accumulatio[t of functional groups were investigated by infrared. ~/Iolecular weights were determined from the intrinsic viscosities of solutions of PPO in chloroform [6].

NOz~ rnole/kq l.qq

I'0- 3 0.6I-~

u,m=~.10~ mo/e/~j.~e c 6

2

0.2 0

1

2

T/me 1o-~,~ec

0

g

10

15

20

~,

FIG. 1

FIG. 2

FIG. 1. Kinetic curves of e l absorption during oxidation of PPO at 240° under pressures of 0.7 X 10' (1), 1.0 X 10~ (2), 1-3 X 10' (3), 1.6 X 10' (~) and 2.0 X 10' Pa (5). F]G. 2 .Maximum rate of O= absorption vs. preesuro at 230 (1), 240 (2) and 250° (3).

904

I. A. S~--ffiovA and Yu. A. Sm,Y ~ m x o v

Figure 1 shows the curves of O~ absorption b y P P O fiimR of thickness 20 pm a t 240 ° with O= pressure a t different levels. The curves are autoeatalytie in character; the duration of the autoeatalytic reactions is ~ 4 0 min. The m a x i m u m rate of oxidation Wm=z at 230-250 ° i s a linear function of pc0, t h o u g h t a t 2500 the straight line intercepts the Y-axis at a point other than zero (Fig. 2). Thus a dependence of the oxidation rate on lao. typical for the high temperature process appears already at 250 ° in the oxidation of PPO. T h e effective activation energy was determined from the temperature dependence of the rate of O= absorption. Below 230 ° it w a s 74.8 k J / m o l e , and above 250 °, 85.4 k J / m o l e . The Arrhenius law does not hold in the interval 230-250 °, apparently because transition over T= is accompanied b y a change in the mobility of the macromoleeules. A relatively low activation energy is typical for high temperature oxidation processes [1, 2]. io9

CRf~H

ca,. /elk9 3

1

0.6 2

o

o.q 0'2

0.5

~

O.J O.l 0T/me 1234 f, lO-~sec I

0

2

q t*lO'~sec Fie. 3

I

i

I

17m. 4

FIG. 3. K i n e t i c s of b r e a k d o w n of P P O hydroperoxide a t 110 (1), 120 (2) a n d 130 ° (3). :FIG. 4. Kinotics of change i n c o n c e n t r a t i o n of c a r b o n y l groups d u r i n g oxidation of P P O at~ 240 (1), 250 (2) a n d 270 ° (3).

The time of the autocatalytic reaction is known to be of an order that is equal to the lifetime of branching products. To investigate the nature of the product initiated oxidation of P P O was carried out a t 80 ° in the presence o f azoisodibutyronitrile (ABN, 0.1 mole/kg) and the amount of P P O hydroperoxide accumulating was equal to 3.7 × 10 -3 mole/kg. As is seen from Fig. 3 the hydroperoxide decomposes under vacuum at 110-130 ° , and the kinetic curves m a y in this case be described b y a first order equation, while the effective activation energy of breakdown is 105.6 kJ/mole; on extrapolating the breakdown constant in line with these data to 240 ° we obtain a value of 0.33 sec -z, which makes the average lifetime of the hydroperoxide groups N 3 see. Thus the hydroperoxide lifetime is too short to account for the long times of autoeatalysis.

Kinetics of thermal oxidation of polydlmethylphenylene oxide

905

Autocatalysis normally appears in the high temperature oxidation of polymers containing CHs groups [7], b u t is absent in the ease of polymers containing methylene groups [2, 8]. According to scheme (I) decay of peroxide radicals generated from CH 3 groups may result in aldehydes ~CH~--0--0"~ ~ CT/~H~'0H,

(HI)

which in turn are an additional source of active radicals (aldehyde branching of the chain) ~ ~ O + O,-~ ~ CO"+ HO~

(IV)

In line with these assumptions carbonyl groups that are unstable under the reaction conditions are formed in the oxidizing PPO, and the concentration of these groups reaches a certain limit, after which it is reduced (Fig. 4). If we say for simplicity that the rate at which aldehyde groups are formed, w~, is constant during the oxidation, and t h a t the rate of their consumption is proportional to their current concentration ca, we obtain

dc. - -

dt

=

w~

--

k,,c~

(1)

I t is obvious that wa and ka are proportional to the 02 concentration since aldehyde groups are formed and consumed in oxidation reactions. A solution of equation (1) with c~----Otakes the form 0

Wa

c~------~a (l--e-*'*)

(2>

I t can be seen from Fig. 4, where experimental points have been plotted on a curve calculated on the basis of formul~ (2) that when wa= 9.7 X 10- ' mole/kg. •sec and ka=4.5 x I0 -4 sec-I the experimental d a b axe in good agreement with

the calculated values. Removal of aldehyde groups in PPO oxidized in ammonia results in restoration of autocatalysis, whereas these groups remain intact at 240 ° under vacuum. I t therefore appears proven that the mechanism of autocatalysis of the high-temperature oxidation of polymers containing CH s groups is an aldehyde mechanism. Primarily methyl groups are consumed in the oxidation of PPO, although aromatic and ether groups disappear quite rapidly along with the methyl groups (Fig. 5a). Volatile degradation products (water a n d CO) are formed in the oxidation process (Fig. 5b). The ratio of the rates of consumption of the methyl, ether and aromatic groups is 6 : 5 : 1 at 240 °. Thus free radicals generated during

906

I. A ; S~B~.t~COVA a n d Y ~ . A , S ] s c s ~ r ~ o v

oxidation o f CHs groups initiate breakdown of t h e m o r e stable groups [2]. The sum total o f volatile produot~ amounts to ~60°//0 on the a m o u n t of absorbed oxygen, b u t this value varies in the course of the reaction, in view of which a pressure change in the reaction system without elimination of volstiles is n o r a reliable characteristic of the process.

c, mo/e/kg 6 8 ~

CL

c, mole/k9 a ~ 3 16

c , r,7o/e/kq

c. mZ,~~ole/kq "~

I'5

o

-

3 2

i

-

I

0

I

.... I

,, ]

,i

i

Z

q

6

8

10

14

I

0.5

12.

2

I

0 t,lO'~,sec

3

~io. 5. Kinetics of consumption of methyl (1), ether (~i and phenyl groups (3) (=) and of accumulation of water (6) and carbon monoxide (5) (b) in the oxich~tion of PPO; 240°, Po,=2X 10' Pa. The oxidation of PPO is accompanied b y cros,liuking of the n~cromolecules: the MVf of the polymer increases, and a t a certain degree of oxidation (20 rain a t 240 ° and 2 × 104 Pa) an insoluble fraction is formed. Since the crossllnlclng probability of the molecules increases with increasing MW, a reduction in MW is observed for the soluble residue (Fig. 6) after the onset of formation of the insoluble fraction.

mole/kq,sec mole/cruZ.see

.M, 10`4 % %

10 - 5

o-

o!

0

~

.....

5

/0

3'0

10

2.0

E

1"0

0

0

I,,

15

Time J', lO'~,.~ec Fzo. 6

15

]

20

1.

I.

60

~ ..... t

,oo

t,ar,',

FIG. 7

Fie. 6. Kinetics of change in MW (1) aud in accumulation of the insoluble fraction e (2) during the oxide~ion of PPO; 240°, Pc== 2 x 10' Pa, FIe. 7. Maximum rate ofPPO oxid~bion referred to unit mass wm (1) and to unit surface w~ (2) vs. thiekneas of s p e c i m e n s ; 2 4 0 °, P o = f 2 X 10 ~ Pa.

Kinetics of thermal oxidation of polydimethylphenylene oxide

907

Evidence favouring participation of low molecular weight free radicals at the propagation and termination stages in the high temperature oxidation of PPO is seen in the complex dependence of the rate of O 2 absorption on the thickness of the oxidizing sample I (Fig. 7). In the interval up to 7 pm the true rate of oxidation of the polymer, referred to unit mass wm increases with t h e thickness of the specimen, and is then rapidly reduced, after which it remains constantin the interval 20-70/nn. Of particular interest is the fact that the rate, referred to unit surface wa decreases in the interval 7-12/~m with increasing thickness of the sample, i.e. the stream of oxygen passing through a unit surface into the volume of the oxidizing sample is diminished. So complex a dependence could not be accounted for in terms of the diffusion of oxygen. One must allow for the fact that there are in the oxidizing polymer several types (at least two) of light free radicals. This radical has a path length of 2 = D~(/CR[RH]+ ~ k([X~])-~, (3) where kR is the constant for interaction of free radicals with groups in the chain; kt is the constant for interaction of free radicals with products formed during oxidation; D is the diffusion coefficient for free radicals; [RH] is the polymer concentration; [X(] is the concentration of products formed in the oxidation reaction. Thus the higher the reactivity of the radical the shorter is the path traversed by the latter from generation to decay. If A becomes comparable with sample thickness a significant fraction of radicals will pass from the sample into the gas phase. The release of inactive radicals, participating mainly in termination reactions, becomes a significant factor at greater thicknesses than for the more active radicals, that participate mainly in chain transfer. This is why the rate increases with thickness of the sample in the range of small thicknesses (up t o 7 gm), and it likewise accounts for the lower rate in the case of medium thicknesses

(7-20 #m). A significant amount of peroxide compounds (4.2× 10 -2 mole/kg o~dized PPO) appearing in volatile products of oxidation of PPO and accumulating in the course of 180 rain of oxidation are attributable to light peroxide radicals o f of type H0~ or CH30~ interacting with other low molecular weight products. Supplementing the high temperature oxidation mechanism in [1] with steps of chaiu branching on the aldehyde group and recombination of macroradicals required to explain the formation of an insoluble fraction, we obtain a high-temperature oxidation scheme for a polymer containing methyl groups as the sole aliphatic fragments (X denotes the polymer chain) (a) X--CHs~-0s->X--CH:+H0~ X--CH~÷ 0s-~X--CHI--0~ (b) (o) X-- OH1-- O~~X-- OttO-b'OH (d) X--~,--O~-FX--~,-~X--CH,-- OOHTX-- ~Hg" X-- CH, q-'OH-~X-- CH~+HIO

908

I. A. S ~ r E o v A and Yv. A. S~sY~IKov X--CH~ +'OH-~X--CH,-- OH

(/) ~J)

X--CH, + HO~-~X-- CH~+ HIOz

(g)

X--CH~+HO~-~X--CH,OOH

(h)

"OH-~released into the volume

(i)

HO~-*released into the volume

(k)

X--CH i - OOH-*X-- CHi-- 0" +'O H

(l)

X--CHI-- OOtt-~X--CHO +HIO

(m)

X--CHO+O,~X--'CO+HO~

(~)

x-oo ~x+oo

(o)

X--CHI--O~+X--CHI--O~(X--CHt, X')-~ohain t~rmination

(p)

Despite its a p p a r e n t complexity the scheme includes only a minimum n u m b e r o f steps necessary to account for observed features of the process investigated. Step (a) m scheme (V) a p p a r e n t l y has a high ac~vat i on energy and, starting a t a certain temperature, competes successfully with steps of chain branching (l) an d (n). This could account for a change in the dependence of the oxidation rate on th e Oi pressure observed at 250 ° (Fig. 2). The scheme does n o t include reactions o f t y p e R" + R1H-*R~, the result of which is the disintegration of a~omatic groups during the oxidation process, or reactions of isomerizstion of radicals, since r e a ~ i o n s o f b e t h these types take place without altering the total n u m b e r of free radicals. We assume t h a t the proposed mechanism could well be applicable to the oxidation of other polymers containing CH3 groups along with more stable ar o matic an d heteroeyclic fragments. ,REFERENCES

1. V. V. YEDEMSKAYA, V. B. M~.I.ER and Yu. A. SHLYAPNIKOY, Dokl. Akad. Nauk SSSR 196: No. 5, 1121, 1971 2. I. A. SERENKOVA, V. 1~. KULAGIN, G. ~/L TSEITLIN, Yu. A. SHLYAPNEKOV and Y. V. KORSHAK, Vysokomol. soyed. B16: 493, 1974 (Not translated in Polymer Sci

U.S.S.R.) 3. P. G. KELLEHER and R. B. GASSIE, J. Appl. Polymer Sci. 11: 134, 1967 4. B. A. GROMOV, V. Y. YEDEMSKAYA, Ye. S. TORSUYEVAand Yu. A. SHLYAPNIKOY, Plast. massy, No. 10, 85, 1967 ft. S. (L KIRYUSHK][N, A. P. MARYIN and Yu. A. SHLYAPNEKOV, Vysokomol. soyed. A22: 1428, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 6, 1570, 1980) 6. M. MASLOVSKIand A. GRUPPON, Zesz. nauk. Plodz., No. 219, 139, 1975 7. S. S. DASHEVSKAYA, M. S. AKUTIN and Yu. A. SHLYAPNIKOV, Vysokomol. soyed. B16: 353, 1974 (Not translated in Polymer Sci. U.S.S.R.) 8. H. H. G. JETJ.T~IEK and S. N. LIPOVAC, Ma~romolecules, 3: 231, 1970 9. D. A. F R A N X - ~ I E N E T 8 ~ Diffuziya i f~ploperedacha v khim. kinetike (Diffusion and Heat Transfer in Chemieal Kinetics). 1 st ed., p. 102, Nauka, Moscow, 1967