Study of vulcanization by radiation of polyorganosiloxane elastomers in the presence of pore forming compounds

0032-3950/78/0101-0248507.50/0

Polymer ScienceU.S.S.R.Vol. 20, pp. 248-253. (~) Pergamon Press Ltd. 1978. Printed in Poland

STUDY OF VULCANIZATION BY RADIATION OF POLYORGANOSILOXANE ELASTOMERS IN THE PRESENCE OF PORE FORMING COMPOUNDS* V. T. KozT.ov, V. I. PAS~INI~, A. G. YEVSWrWV, B. G. YERSHOV and Z. N. TARASOVA Scientific Research Institute of the R u b b e r I n d u s t r y

(Received 28 April 1977) Studies were made of structural radiation and chemical conversions of 2,2-azobis-isobutyronitrile, azodicarbonamide, di-nitrozopentamcthylene tetramine, benzenesulphohydrazide (BSH) a n d SKT, SKTV-1, S K T F V siloxane rubbers in the presence of these "pore formers". I t was shown b y E P R that pore formerst have a marked effect on the yield of intermediate paramagnetie reaction centres (PMC) and radiation crosslinking of the rubbers studied, however, the effect of pore formers varies a n d is determined by structure. I t was shown that the yield of PMC in rubbers in the presence of pore formers is 5-10 times ]ower than the yield of crosslinks; yield increases with a n increase in the a m o u n t of pore former. I t was concluded t h a t a high proportion of erosslinks are formed directly during radiolysis (at 77°K).

IlCVESTIGATIOI~of vulcanization by radiation of polyorganosiloxanes [1-3] enabled us to develop an effective met hod for the manufacture of crude rubber products from polysiloxane elastomers [4]. Further investigation produced porous polyorganosiloxane rubbers prepared by radiation vulcanization [5]. Investigation of the physical and chemical aspect of radiolysis of polyorganosiloxane mixtures containing pore forming compounds are not only essential to control the radiation stage of structure formation, but also to regulate pore formation itself - - obtaining pores of a given type. The type of pores is determined by the degree of radiation vulcanization, i.e. structural end c o n v e r s i o n s - crosslinking and breakdown, which are determined by processes of formation and interaction of intermediate active centres--radicals, ions, etc. Polydimethyl-, polydimethylvinyl-, polydimethylvinylphenylsiloxaneswere examined b y the addition of various poreformers: 2,2-azo-bis-isobutyronitrile (ChKhZ-57), azodicarbonamide (ChKhZ-21), dinitresopentamethylene tetramine (ChKhZ-18) and benzenesulphohydrazide (BSH). Elastomers were chosen from a very broad class of polyorganosiloxanes, produced on an industrial scale. Organic pore formers were selected from the main classes of chemical compounds produced on an industrial scale in the U.S.S.R. A 8nCo apparatus with a dose rate of 0.5-2.5 Mrad/hr was used as source of y-radiation. The formation a n d identification of active centres was examined by E P R . E P R spectra Vysokomol, soyed. A20: No. l , 215-219, 1978. t Pore f o r m e r = Porofor, a blowing agent.

*

248

Study of vulcanization by radiation of polyorganosfloxane elastomers

249-

were measured and subjected to riadiolysis at the temperature of liquid nitrogen by methods previously described [6]. The yield of crosslinks was determined by equilibrium swelling and calculated using equations previously derived [7, 8]. The purification of elastomers involved the extraction of crushed elastomer samples by hot acetone in nitrogen for 16 hr, followed by two fold reprecipitation of the r u b b e r with methanol from benzene solution. • W i t h a n increase of the dose of r a d i a t i o n to 35 Mrad, E P R s p e c t r a of p o r e f o r m e r s (Fig. 1) change little, it is o n l y t h e i n t e n s i t y of the signal which i n c r e a s e s .

a

b

FIG. 1

FIG. 2

FIG. 1. E P R spectra of pore formers studied during low temperature (77°K) radiolysis: 1--ChKhZ-21; 2--ChKhZ-18; 3--BSG; 4--ChKhZ-57; dose of radiation 1 (a) and 10 Mrad (b). FIG. 2. E P R spectra of SKT, SKTV-1 and SKTFV rubbers and the effect of pore forming additives in low temperature (77°K) pyrolysis: 1--SKT or SKTV-1; 2-4--same with addition of 5 parts by wt. of one of the pore formers: ChKhZ-21, ChKhZ-18, ChKhZ-57 (2); 10 (3) and 20 parts by wt. ChKhZ-21 (4); 5--SKTFV; 6 - - S K T F V + 5 parts by wt. ChKhZ-18 or ChKhZ-57. Dose of radiation 1 (a), 5 (b) and 10 Mrad (c). S p e c t r a of ChKhZ-21, C h K h Z - 1 8 a n d B S H (Fig. 1) Porofors t a k e t h e f o r m o f singlet lines or lines w i t h a w e a k l y resolved super-fine s t r u c t u r e (SFS) w i t h a w i d t h o f ~ 7 0 4 - 1 . 0 ; 3 0 ± 1 . 0 a n d 6 0 ± 1 - 0 0 e , respectively; t h e E P R s p e c t r u m o f C h K h Z - 5 7 is a q u i n t e t w i t h s e p a r a t i o n of 2 4 ~ : 1 0 e , each c o m p o n e n t h a v i n g

250

V . T . KOZLOV ef al.

a triplet SFS with a separation of ~ 1 4 ± 1 0 e and m a y be due to an anion-radical o f the type

CH3

CH.

I

J

CH3--C--N'---N--C--CHa

i

CN

I

CN

with a quintet structure of the E P R spectrum with an intensity ratio of 1 : 2 : 3 : : 2 : 1, as shown b y Fig. 1 (spectrum 4). Further separation of each of the components is, apparently, the consequence of separation on the nitrogen atom of the CN group. The E P R spectrum of ChKhZ-21 is a broad ( ~ 7 0 Oe) singlet line, typical of paramagnetic centres (PMC), in which the unpaired electron is effectively delocalized for the molecule. I t follows from previous studies [9, 10] that similar structures have a low ionization potential and can readily emit electrons to form cation radicals of the type HzN--C---N + ' - - N - - C - - - N H . ,

in which the unpaired electron is deloealized for the molecule as a result of ~-a conjugation. For ChKhZ-18 arid BSG free radicals are probably formed as a result of the separation of hydrogen atoms from CI-I2 and N H groups, respectively. Results of radiation yields of PMC and crosslinks in low temperature radiolysis (77°K) of pore formers are tabulated. The highest yield of paramagnetic eentres calculated from initial linear sections of curves showing the formation of PMC are observed in radiolysis of ChKhZ-21 and ChKhZ-g7 pore formers (GpMc----5"3 and 2.5). For ChKhZ-18 and BCG pore formers the yield of PMC is much lower and is 0.6 and 0.3, respectively. Symmetrical and cyclic structure with NO groups in the case of ChKhZ-18 and the presence of an aromatic and S02 group in the case of B S H suggests marked radiation stability of these compounds. The yield of various radiolytic products is therefore considerably reduced in the polymer system, including the radiation yield of structure formation [11]. For compounds ChKhZ-21 and ChKhZ-57 it is assumed that intermediate reaction centres are c~efly f o r m e d - - a cation radical for the first and an anion radical for the second compound--which may, apparently, infuence the yield of the radiolytic end product--structure formation of the polymer system when adding to it compound of this type. In the system of SKTV-1 and ChKhZ-21 pore formers a linear increase is observed in yields of PMC with an increase in the amount of pore former added, i.e. an additive yield of paramagnetic centres is observed in the polymer system,

Study of vulcanization by radiation of polyorganosiloxane elastomers

251

w h i c h indicates t h e absence of i n t e r a c t i o n b e t w e e n PMC f o r m e d in t h e p o r e f o r m e r a n d r u b b e r in l o w t e m p e r a t u r e radiolysis. W h e n a d d i n g a C h K h Z - 5 7 pore f o r m e r t o S K T V - 1 r u b b e r , in which t h e v a l u e of GpMc is twice as high as in r u b b e r , t h e yield of PI~IC decreases. T h i s m a y be e x p l a i n e d b y t h e effective i n t e r a c t i o n of PMC of r u b b e r a n d P o r o f o r e v e n u n d e r conditions of low t e m p e r a t u r e radiolysis. T h e anion radical p r o p o s e d in case of radiolysis of C h K h Z - 5 7 , being a good t r a p for positively c h a r g e d holes o f t h e p o l y m e r s y s t e m , a p p a r e n t l y , reduces t h e yield of radiolytic p r o d u c t s , including p r o d u c t s of crosslinking. COMPARISON

OF RADIATION YIELDS

OF RADICALS G R AND CROSSLINKS Gc IN

PRESENCE

OF PORE

SKTV-1

IN THE

FORB~ERS

(Radiolysis at 77°K) Rubber Porofor SKTV- 1 ChKhZ-21 SKTV-1 ChKhZ-57 SKTV- 1 GhKhZ - 18 SKTV- 1 BSG SKTV- 1 ChKhZ -21 SKTV- 1 ChKhZ-57 SKTV- 1 ChKhZ - 18 SKTV- 1 BSG SKTV- 1 ChKhZ-21 S KTV - 1 ChKhZ-57 SKTV- 1 ChKhZ-18 SKTV- 1 BSG

G

GR

Concentration of pore formers in the mixture (parts by wt.) 10 15 20 1.5 I 5 1-2 5"3 1"2 2"5 1"2 1"2

Go

1.2

1.4

1.7

1"1

1.0

0.8

0.7

0.9 0'9

0.7

0.6

0"5

6'3

6"3

6.3

6.2

6"1

6"3

5"8

3.8

2.5

6"3

GR/2Go

2"2

1.3

3.8

6"3

4"8

1.6

1.1

0"9

0"1

0-1

0.11

0.14

0'18

0"1

0"09

0.13

0-16

0"1 0"1

0.12 0"09

0.22

0.26 m

0.27

0.28

E P R s p e c t r a of r u b b e r s in t h e presence of p o r e f o r m e r s (Fig. 2) confirms this a s s u m p t i o n concerning t h e possible n e u t r a l i z a t i o n o f anion-radicals w i t h positive holes f r o m t h e p o l y m e r s y s t e m , since e v e n w i t h considerable c o n c e n t r a tions (15 p a r t s b y wt.) o f t h e C h K h Z - 5 7 p o r e f o r m e r n o E P R signal is o b s e r v e d

252

V . T . KozLov

et al.

in t h e mixture, typical of pure ChKhZ-57 pore former (Figs. 1 and 2) and attributed by us to the anion radical. This interpretation of results is also in agreement with the assumption concerning the formation of cation radicals in radiolysis of ChXhZ-21. In this case ChXhZ-21 cation radicals do not impede ion-molecular reactions in the polymer system: entrapping electrons decomposed by heat they even contribute to more effective reactions taking place directly "by radiation", as described previously. Spectra of SXTV-1, SKT, SKTFV rubber with pore foiming [12] additives ChKhZ-18, ChXhZ-21, SKhZ-57 and BSG in a proportion of up to 5 parts by weight are close to spectra of pure rubber (Fig. 2), except for the SKTFV mixture with 5 Farts by weight ChKhZ-21, where the singlet of ChKhZ-21 is superimposed on the slcectI~m of the ~ublcer, which also eonfiims the additivity o f the yield of ]~MC in the rubber in the presence of ChKhZ-21. On adding a Fore foimer (ChXhZ-21) to SKTV-1 rubber in the proportion of 10 lcarts by weight the spectrum changes (Fig. 2). With a dose of radiation of 1 Mrad the singlet of ChXhZ-21 pore former is superimposed on the spectrum of pure rubber. With doses higher than 20 Mrad spectra of rubber -- pore forming mixtures are similar to spectra of pure rubber. The Table shows that the yield of I ~ C in rubbers in the presence of pore foimeIs is 5--10 times lower than the yield of cross]inks, the proportion of t h e effect of ] ~ C on erosslin'king increasing with an increase in the amount of pore former. I t m a y therefore be concluded that during radiolysis of polysiloxane + p o r e fo~ming systems a large proportion of the crosslinks are directly formed during radiolysis, i.e. "under radiation". This conclusion is in agreement with experimental results of quantitative determinations of crosslinks, formed in post-radiation [13, 14]. In particular, in the case of radiolysis of polysiloxanes SXTV and SKT [14] "under radiation" crosslinks are formed in a proportion of 75--90%. One of the most important processes taking place directly during radiolysis, as indicated [11], is the ionic-molecular reaction of regrouping ions [15] with the foImation of new chemical bonds--crosslinks. On adding to SXTV-1 rubber a Fore foImer (ChKhZ-57) in the proportion of 1-5-18 Farts by weight (Table), the yield of PMC decreases in proportion to the content of ChXhZ-57 Fore former. The yield of crosslinks also decreases in a linear manner with an increase in the content of ChKhZ-57 pore former. These results eonfiim the assumption concerning the redistribution of radiation energy [ e i ~ e e n mixture CCmlconents as a result of accepting positively chaIg(d "holes" of the l=olymer system by anion radicals of the ChKhZ-57 pore fo liner. In view of this process anion radicals of the pore foimer are neutralized on the one hand and the general proportion of I~MC in the system decreases with an increase of the content of CbKhZ-57 Fore foimer in rubber (Table); on t h e other hand, acceptance of positive "holes" from the polymer system by anionradicals of the 1core foincer reduces the yield of ionic-molecular reactions of cross-

Study of vulcanization by radiation of polyorganosiloxane elastomers

253

linking, which is m a n i f e s t e d in a r e d u c t i o n of values of Gc w i t h a n increase in t h e c o n t e n t of C h K h Z - 5 7 p o r e f o r m e r . An increase in t h e p r o p o r t i o n of ChKhZ-21 p o r e f o r m e r in S K T V - 1 r u b b e r increases t h e overall a m o u n t of P1KC. T h e yield of crosslinks is p r a c t i c a l l y u n c h a n g e d (6.3 w i t h o u t ChKhZ-21; 6.1 w i t h 20 p a r t s b y weight ChKhZ-21). As indicated, cation radicals are f o r m e d in this case which e v i d e n t l y do n o t h i n d e r ion-molecular reactions of t h e p o l y m e r s y s t e m , which is expressed in t h e c o n s t a n c y o f values of Gc w i t h a n increase in t h e c o n t e n t of ChKhZ-21 p o r e former. A significant f a c t o r is t h e absence of solubility of ChKhZ-21 p o r e f o r m e r in e l a s t o m e r s , w h i c h a c c o u n t s for t h e i n t e r a c t i o n w i t h t h e e l a s t o m e r o n l y on t h e interface. As a result of these e x p e r i m e n t s relations were established b e t w e e n t h e effect o f p o r e f o r m e r s studied on t h e f o r m a t i o n a n d i n t e r a c t i o n of i n t e r m e d i a t e r e a c t i o n centres (PMC) a n d on t h e f o r m a t i o n of radiolytic e n d p r o d u c t s (crosslinks). Translated by E. SEM~I~

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