Effect of ionizing radiation on structural variations in the systems rubber-polyethylene and rubber-polystyrene

Effect of r a d i a t i o n on r u b b e r - p o l y m e r systems

1283

17. S. A. GLIKMAN and V. I. KLENIN, Tez. dokl. V koll. konf. Izd. Akad. N a u k . S S S R , Moscow, 222, 1962 18. S. A. GLIKMAN a n d Ye. P. KORCHAGINA, Dokl. vyssh, shkoly 1: 147, 1959 19. S. A. GLIKMAN, Ye. P. K O R C H A G I N A and L. L. SEV'YANTS, Vysokomol. soyed. 3: 353, 1961

EFFECT OF IONIZING RADIATION ON STRUCTURAL VARIATIONS IN THE SYSTEMS RUBBER-POLYETHYLENE AND RUBBER-POLYSTYRENE* G. A. BLOKH, V. A. ZHURKO, fix. P . M E L E S H E V I C H ,

M. A. VYAZANKINA,

F. V. BRONSHTEIN

M . fix. V A S ' K O V S K A Y A ,

and E. V. TSIPENYUK

D n e p r o p e t r o v s k Chemico-Technological I n s t i t u t e

(Received 4 November 1961) C O M P L E X structural changes connected with the formation of free polymeric radicals whose recombination may lead to the appearance of three-dimensional space systems are known to occur in polymers under the influence of ionizing radiation [1-5]. The present work is devoted to the study of the structural changes which occur under ionizing irradiation in the following combined systems: rubber (SKB, SKS-30, NK)--plastic (low-molecular weight polyethylene PEND and high-molecular weight PEVD, polystyrene PS), taken in different ratios. The r a d i a t i o n dose was b e t w e e n 1 a n d 50-100 megarads. The b o m b a r d m e n t was carried out on a s°Co a p p a r a t u s in 1600 g. equiv, radium. The rubber was m i x e d w i t h the plastic, on rollers for 10 m i n u t e s at 150 ° for low-molecular weight p o l y e t h y l e n e a n d at 110 ° for 10 m i n u t e s for the high-molecular weight one. T h e n the rubber-plastic m i x t u r e was pressed in moulds. The resulting 5 m m thick strips were used to m a k e cylindrical specimens 16 m m in d i a m e t e r for b o m b a r d m e n t a n d for t a k i n g the t h e r m o m e c h a n i c a l curves. Specim e n s were m a d e f r o m 2 m m thick strip for b o m b a r d m e n t and subsequent d e t e r m i n a t i o n of the physical a n d m e c h a n i c a l properties of the b o m b a r d e d systems. The b o m b a r d m e n t was carried out in air w i t h o u t h e a t i r g . The s t r u c t u r a l changes occurring in the systems were assessed f r o m the t h e r m o m e c h a n i c a l curves in a wide t e m p e r a t u r e range which, as is well known, show the physical states of p o l y m e r systems and the degree of cross-linking. The therm o m e c h a n i c a l curves were t a k e n on specimens 5 m m thick a n d 16 m m in diameter. A load of 250 g was applied to the specimen for 15 seconds w h e n the t e m p e r a t u r e c h a n g e d t h r o u g h 10 °. The d e f o r m a t i o n u n d e r this load was n o t e d w i t h a micrometer. A f t e r 15 seconds the load was r e m o v e d a n d the specimen h e a t e d up to a t e m p e r a t u r e 10 ° higher t h a n the previous one, after which t h e load was again applied and d e f o r m a t i o n measured, and so on. * Vysokomol. soyed. 5: No. 4, 605-613, 1963. This w o r k was carried out in the D e p a r t m e n t of R u b b e r T e c h n o l o g y of the D n e p r o p e t r o v s k Chemico-Technological I n s t i t u t e in collaboration w i t h the I n s t i t u t e of Physical Chemistry, A c a d e m y of Sciences Ukr. S S R and the Kiev Reclaimed Rubber Factory.

1284

G.A. BLOK~ et al.

Besides this, we determined the concentration of cross-links arising as a result of the ionizing radiation in the space lattice, using the swelling maximum method with appropriate calculation according to Flory's equation [6-7]. Figures 1-4 show the results o b t a i n e d in the investigation of the t h e r m o mechanical properties of the systems. I t can be seen from the t h e r m o m e c h a n i e a l curves t h a t there is a considerable difference in the t e m p e r a t u r e dependence of deformation for b o m b a r d e d and n o n - b o m b a r d e d systems. I n all cases the u n b o m b a r d e d polymers a n d their mixtures show transition to the viscous flow state u n d e r t e m p e r a t u r e conditions where these are a b s e n t after b o m b a r d m e n t . THERMOMECHANICAL PROPERTIES

Figure 1 shows the t h e r m o m e c h a n i c a l curves for pure s o d i u m - b u t a n e r u b b e r S K B a n d its m i x t u r e s with the low-molecular weight p o l y e t h y l e n e P E N D . I t can be seen from these d a t a t h a t even after a dose of 1 m e g a r a d b o t h the S K B a n d its m i x t u r e in P E N D have completely e x h a u s t e d the transition to the viscous flow state, while with a P E N D c o n c e n t r a t i o n o f 5 0 % or more some elasticity is still preserved.

,/ ~o

.~.

2o-

~

2 i

eo

i

i

i

i

I

1oo

140

C

!

co I00 c~

2 3

,

6'0

~

2O

3

io

le0

'

.

Ibo

'

I, o

I

2 80

I20

16'0

200 100 Temperature, °C

'

2 I40

180

220

:FIG. l.IThermomeehanical curves of irradiatedsystems S K B - P E N D : a-- SKB; b--SKB-PEND (80 : 20); c--SKB-PEND (50 : 50)' d--SKB-PEND i~0 : 80). ~tadiation doses: •--0; 2--1 megarad; 3--3 megarads. Figure 2 gives the t h e r m o m e c h a n i c a l curves for S K B m i x t u r e s with PS. The changeover o f the s y s t e m to the viscous flow state is a l m o s t c o m p l e t e l y e x h a u s t e d at 3 m e g a r a d with 200/o polystyrene. The c h a r a c t e r of the curve

Effect of radiation on rubber-polymer systems

1285

changes however, with higher polystyrene concentrations. At 15% PS and 3 megarad the highly elastic properties of the system are preserved, and at 80% polystyrene and 3 megarad the specimen has still not lost its capacity for going over to the viscous flow state. Bombardment of the polystyrene alone shows t h a t even at 5 megarad it does not lose its capacity for deformation and changeover to the viscous flow state. This behaviour is natural for systems with polystyrene as the presence of the phenyl nuclei in the side groups of the polymer chain make it more resistant to radiation. Figure 3 gives the thermomechanical curves of systems composed of butadiene styrol rubber SKS-30 and PEVD. I t can be seen from these figures t h a t for rubber SKS-30 and its mixtures with polyethylene higher doses are required for the structural processes which lead to the formation of a space lattice and exhaustion of the viscous flow properties at the same t~mperatures. SKS-30 itself does not lose its capacity for changing over to the viscous flow state until a dose of 5 megarad. I t can be seen from this figure t h a t as the PEVD concentration increases the nature of the curves changes, the capacity of the bombarded polymers for deformation increasing with temperature. For instance, for the system SKS-30 and 50% PEVD, or SKS-30 and 80% PEVD the capacity for deformation is not exhausted even at 10 megarads, although a dose of 5 megarads is sufficient for the systems to lose their ability to pass over to the viscous flow state. At the same time PEVD alone loses its ability to pass over to the viscous flow state after a bombardment of 10 megarads. :oo

.g

b

a

20

2

2

120

160

200

100

140 180 Temper~fure , °C

100

f40

180

FIG. 2. Thermomeehanieal curves of the irradiated systems SKB-PS: a--SKB; b--SKB-PS (80:20); c--SKB-PS (50:50); d--SKB-PS (20: 80); e--PS. Doses: •--0; 2--1 megarad; 3 -- 3 megara&u; 4 -- 5 megarads.

G . A . BLOKH et al.

1286

"

•

r~

i

r

I

I

80

120

c

I

i

I

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r

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I

I

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I

I

I

120

d

5

160

e

;,l y9.,.l/l" gO

120

160

100 I40 180 Temper~tupe , °C

100

140

180

FIe. 3. Thermomeehanical curves of the irradiated systems SKS-30-PEVD: a--SKS-30; b - - S K S - 3 0 - P E N D (80 : 20); c - - S K S - 3 0 - P E N D (50 : 50); d - - S K S - 3 0 - P E N D (20 : 80); e - - P E N D . Doses: •--0; 2--1 megarad; 3--3 megarads; 4--5 megarads; 5--10 megarads; 6 -- 21.4 megarads. F i n a l l y , Fig. 4 gives the t h e r m o m e c h a n i c a l curves of S K S - 3 0 - P E N D m i x t u r e s . I f t h e s e c u r v e s a r e c o m p a r e d w i t h t h o s e for S K S - 3 0 - P E V D i t c a n b e s e e n t h a t the SKS-30-PEND systems, and particularly PEND alone, have been subject t o c o n s i d e r a b l y m o r e c r o s s - l i n k a g e t h a n t h e P E V D , i.e. t h e y are less r e s i s t a n t t o radiation.

FORMATION OF SPACIAL NETWORKS ON BOMBARDMENT Ttm density of the cross-links is known to be one of the most important characteristics of the degree of cross-linking. The concentration of cross-links can be determined experimentally from equilibrium swelling data. The equilibrium stage can be found from F]ory's equation [7]; which can also be used to find the concentration of the cross-links:

1 ~ 1 ,~, 1 (Q~ + ])-']=o, ln(l+~_)-(Q +,)-'-/~(O+,)--mc[(O_+,)-~ where Q ~ is the equilibrium degree of swelling given by the ratio of the increassed volume of the swollen specimen to the volume of the dry specimen after swelling m e = (Mc/Pk)Vp, where Vp is the molar volume of the solvent, ema/g.mol; Pk is the density of the rubber, g/cm 3 a n d Mc is the molecular weight of the portion of chain contained between neighbouring sites of the space network; p is a parameter which characterizes the molecular interactiorl between the rubber and the solvent:

Effect of radiation on rubber-polymer systems

28

1287

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160

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120

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Tempepature , °O

FIG. 4. Thermomechanical curves of irradiated systems S K S - 3 0 - P E N D : a - - S K S - 3 0 ; b-S K S - 3 0 - P E N D (80 : 20); c - - S K S - 3 0 - P E N D (50 : 50); d - - S K S - 3 0 - P E N D (20 : 80); e - - P E N D . Doses: •--0; 2--1 Mrad; 3 - - 3 Mrads; 4 - - 5 Mrads; 5--10 Mrads.

vp ('~K--Jp)~ RT where gk and 6p are the square root of the specific cohesion energy of the rubber and solvent, which are accordingly called the solubility parameters, and ~s = 1/8 where 7 is the number of solvent molecules surrounding one segment of a rubber molecule. For rubbers 7 ~ 4 is assumed[S]. According to Scott's data [9] for natural rubber Jk (eal/cma) ~, is 8"35 for natural rubber 8.45 for sodium butadiene, 15% for butadiene styrene (15~o styrene) 8-60 for the same with 25~o styrene a n d 8.70 with 40~/o styrene. Thus the transition from ~ K rubbers to S K B and SKS with 40~o styrene concentration only has a very slight effect on the solubility parameter Jk. I n this papers [7, 10] Shvarts has worked out nomograms which can be used to determine the p parameter for a given type of rubber and solvent. According to the data in [7] the rubber-solvent interaetion parameter is 0.347 for a system of natural rubber and m-xylene, 0.318 for sodium butadiene rubber and m-xylene and 0.292 for butadiene styrene rubber and m-styrene. I t would seem that in taking these ~ parametr values for systems of rubber with a 20~o filling of polyethylene on polystyrol we have actually allowed a small error which, however, has not altered the character of the erosslinking observed under the influence of bombardment. F r o m the resulting experimental data on the weight swelling of irradiated rubber-plastic systems the equilibrium swelling was determined in that work, on the basis of the equation

1288

G . A . BLOKH et al.

Q~

Pob~-- Pi.,~

Pk

Pinit " F

pp

where Q ~ is the equilibrium degree of swelling, Pobs is the weight of the swollen specimen, Pinit is the weight of the initial specimen, Pk i8 the rubber density, g/era a, pp is the solvent density, g / c o a, F is the rubber concentration in the original specimen. F r o m the nomogram relating the equilibrium swelling Q and p the interaction parameter, the m c value is found [7] a n d after this the molecular weight of the part of the rubber chain between the space network sites is calculated: M¢=m~ .pk.vp

~5~

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FIG. 5. Effect of ionizing irradiation on the variation in plasticity (solid curves) and swelling (dashed curves) in gasolene after 24 hrs: 1--SKS-30; 2 - - S K S - 3 0 P E N D (80 : 20); 2 " - - S K S - 3 0 - P E N D (80 : 20); 3 - - S K S . 3 0 - P E N D (50 : 50); 3 " - - S K S - 3 0 - P E N D (50 : 50).

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FIG. 6. Ini~uence of ionizing irradiation on the variation in wear resistance (solid curves) and swelling (dashed) in gasolene after 24 hrs: 1 - - S K S ; 2 - - S K S P E N D (80 : 20); 3 - - S K S - P E N D (50 : 50); 4 - - S K S - P E N D (20 : 80); 5 - - P E N D .

Effect of radiation on rubber-polymer systems

1289

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FIG. 7. Effect of ionizing radiation on the physico-mechanical properties of: 1 - NK; 2 - - N K PS (80 : 20); NK-PS (50 : 50) 4 - - N K - P E N D (80 : 20); 5 - - N K - P E N D (50 : 50); 6 - - N K - P E N D (20 • 80). ON the basis of these figures the cross-link concentration was determined according to n c ~-

NA m l _ 1 2v¢

where vc is the effective molar volume of the section of molecule between the neighbouring sites of the space network(vc--Mc/p k) and NA is the Avegadro number which is 6.023 × 1023. The method described is widely used both in Russian and foreign investigations for studying the structures which arise in rubbers both under sulphur [11, 12] and radiation [13] vulcanization. It has been found that it can be used for a characteristic of the formation of special structures both for unfilled and filled systems [14]. It would naturally be interesting to study formation of special structures in a mixture of rubber and plastic under the influence of ionizing radiation. I f the data obtained for real rubbers and for rubbers with polyethylene or polystyrol are compared this might give an indication of the formation of spacial networks between rubber and plastic. The T~ble gives the results of the e x p e r i m e n t a l d e t e r m i n a t i o n s of the crosslink c o n c e n t r a t i o n s in N K , SKS-30 a n d S K B s y s t e m s with p o l y e t h y l e n e a n d polystyrene. I t can be seen t h a t , depending on the t y p e od r u b b e r a n d the conc e n t r a t i o n o f plastic, the cross-link c o n c e n t r a t i o n rises considerably with the irradiation dose. C o m p a r i n g the cross-link c o n c e n t r a t i o n in i r r a d i a t e d N K a n d in its m i x t u r e s with 2 0 % p o l y e t h y l e n e or 2 0 % p o l y s t y r e n e , the following can be seen: with a dose of 50 megarad, in N K with 2 0 % P E V D a p p r o x i m a t e l y three times more cross-links (6.25 × 10 TM m1-1) are f o r m e d t h a n with pure N K (1.98 × 10 TM ml-]); with 2 0 % p o l y s t y r e n e on the o t h e r h a n d the cross-link c o n c e n t r a t i o n (1.90 × 10 TM m1-1) is at the same level as for the N K . I n none of these eases do

1290

G . A . BLOKH et a l .

EFFECT

OF I O N I Z I N G R A D I A T I O N ON T H E F O R M A T I O N OF CROSS-LINKS I N T H E SYSTEMS: R U B B E R -

POLYETHYLENE, RUBBER--POLYSTYRENE Irradiated system

Dose, Mrad

Q~

~/c

vc

Cross-links concentration nc × 1019 m l - 1

NK *

0 50 100

-6.32 4.24

-14 000 6 520

-15 200 6 780

None 1.98 4.25

NK-t- 20% PEVD

0 10 25 50 100

-6.63 4.97 3.30 2.36

-14 600 8 750 4 430 2 690

-15 900 9 200 4 820 2 920

None 1.90 3.18 6.25 10.30

NK+20°/o

0 25 50 100

-12.50 6.53 4.40

-45 600 14 600 7 350

-49 600 15 900 8 000

None 0.61 1.90 3.78

SKS-30?

0 25 100

-11.20 5.18

-30 900 7 940

-33 600 8 600

None 0.90 3-50

S K S - 3 0 + 20~o P E N D

0 10 25 50 100

9.16 6-80 5.18 3.14

-21 0 0 0 12 850 7 940 3 500

-22 800 14 0 0 0 8 600 3 810

None 1.33 2.17 3.50 7.95

SKS-30-{-20~o PEVD

5 10

10.10 9.27

25 100 21 000

27 300 22 900

1.10 1.32

SKS-30+20%

10 25 50

16-30 9.42 8.00

54 700 21 0 0 0 16 700

59 500 22 900 18 100

0.51 1.32 1.66

10

4.50

6 760

7 360

4.10

1 5

4.08 1.97

5 600 1 750

6 100 1 900

0-50 15.80

PS

PS

SKB$ SKB-20~/o PEND

-

-

• ~ = 0-347. 1" # = 0 . 2 9 2 . ~: /~ ffi 0.318.

the unirradiated systems have any cross-links. The cross-link concentration also rises with a radiation dose in SKS-30, b u t the amount is less than with NK. While 100 megarads produces 4.25 × 1019 ml -x in NK, for SKS-30 it is 3.5 x 1019 m1-1. After 10 megarads, 1.32 × 1019 m1-1 cross-links arose in SKS-30 with 20% PEVD, which is considerably less than the similar system of N K with 20% P E V D (1.90 × 1019 ml-1). I f the 20% PEVI) is replaced b y 20% polystyrene in the SKS-30 mixture the cross-link concentration falls from 1.32 to 0.51 x l019 m1-1 with 10 megarads. These examples confirm what has already been observed,

Effect of radiation rubber-polymer systems

1291

i.e. that radiation structural changes occur more slowly and require higher irradiation doses in polymers containing phenyl groupings. The greatest strueturization is observed in the case of SKB. At a dose of 10 megarads the same number of cross-links, 4.10 x 1019 m1-1 is observed as in N K after a dose of 100 megarads (4.25 x 1019 ml-1). I f a mixture of SKB with 20% P E N D is irradiated with 5 megarads, 15.80 × 1019 m1-1 cross-links are formed. This is considerably more than in the analogous system on SKS-30 base (7.95 x 1019 m1-1) and N K (10.30 × 1019 m1-1) irradiated with 100 megarads. It is interesting to note that sulphur vulcanisates have almost the same number of cross-links as those obtained in our study. According to Dogadkin and Tarasova for instance [11], natural rubber vuleanisates have a cross-link concentration (determined b y the same method) of 7-8 x 1019 m1-1, and those of butadiene styrene rubber SKS-30 have 10-11.5 x 1019 m1-1. In the studies by Shvarts and Buiko [12] the cross-link concentration was determined for rubber filled with 50% channel black on the basis of the isoprene rubber S K I in dependence on vulcanization time. The cross-link concentration in the rubber vulcanized under the optimum condition of 133 ° was 9.05 × 1019 m1-1, at 143 ° it was 8.55 x 1019 and in rubber vulcanized at 163 ° it was 7.70 × 10 ]9 m1-1. For the unfilled rubbers the cross-link concentration was 5.7-5.5-4.09 x 1019 m1-1. Thus, our analysis of the thermomechanical curves and data on the formation of cross-links, as determined by the Flory swelling method [7], shows that a space network is formed under ionizing radiation and that it is due to the interaction of the rubber polymer radicals with the corresponding radicals of the polyethylene or polystyrene. This means that we have observed the co-vulcanization of rubber and polyethylene or rubber and polystyrene. The formation of this joint space structure is accompanied b y a change in the physical and mechanical properties, namely: the plasticity of the irradiated system falls considerably, hardness, strength and wear resistance increase and the swelling index falls, and so on [5]. The results of these tests, carried out b y the usual methods used for rubbers, are shown in Figures 5-7. Thus it can be seen that if rubber is combined with a thermo-setting plastic in appropriate proportions and subjected to a high energy ionizing radiation, materials can be obtained which have the required association of physical and mechanical properties. CONCLUSIONS

(1) The influence of ionizing radiation on structural changes in systems rubber (SKB, SKS-30, NK)-plastie (polyethylene, polystyrene) has been studied at radiation doses of 1-100 megarads. (2) It has been found that co-vulcanization of the rubber and plastic occurs as a result of the ionizing radiation, to produce materials with improved physical and mechanical properties. TransZated by V. ALFORI)

1292

V. YE. EsKr~ a n d T. I. VOLKOV

REFERENCES 1. V. L. KARPOV, K. S. KUZ'MINSKII and Yu. S. LAZURKIN, Izotopy i izluch, v khimii. (Isotopes and Radiation in Chemistry.) Izd. Akad. Nauk. SSSR, 139, 1958 2. A. N. PRAVEDNIKOV and S. F. MEDVEDEV, T r u d y I Vses. soy. po radiats, khimii, Izd. Akad. Nauk. SSSR, 269, 1958 3. Z. N. TARASOVA, M. Ya. KAPLUNOV, B. A. DOGADKIN, V. L. KARPOV and A. Kh. BREGER, K a u c h u k i rezina, 5, 14, 1954 4. G. A. BLOKH, V. L. KARPOV, Yu. M. MALINSKII, L. P. OL'SHANSKII and M. S. KHLOPLYANKINA, Tezisy dokl. n a II, Trudy I Vses. soy. po radiats, khimii, Izd. Akad. Nauk. SSSR, 81, 1960 5. G. A. BLOKH, V. A. ZHURKO and M. A. VYAZANKINA et al., Tezisy dokl. n a I n-t soy. po khim. i tekhn, kauchuka i reziny, Dnepropetrovsk, 3, 1961 6. A. G. SHVARTS, K a u c h u k i rezina, 7, 31, 1957 7. P. FLORY, J. Chem. Phys. 18: 108, 1949 8. D. GEE, Article in coll. Khim. bol'sh, molekul. For. Lit. Publ. Hse. 1, 1948 9. R. SCOTT and M. MAGAT, J. Polymer Sci. 4: 555, 1949 10. A. G. SHVARTS, Kolloidn. zh. 19: 376, 1957 11. B. A. DOGADKIN and Z. N. TARASOVA, Sb. Vul'kanizatsiya rezin. (Rubber Vulcanization.) Goskhimizdat 1954 12. A. G. SHVARTS and G. N. BUIKO, K a u c h u k i rezina, 1, 1959 13. L. A. OKSENT'YEVICH, T. S. NIKITINA and A. S. KUZ'MINSKII, K a u e h u k i rezina, 5, 21, 1959 14. G. KRAUS, Rubber Warld 135: 254, 1956

LIGHT SCATTERING AND VISCOSITY OF POLY-2,5-DICHLOROSTYRENE SOLUTIONS IN DIOXANE* V. YE. ESKIN and T. I. VOLKOV Institute of Polymers, U.S.S.R. Academy of Sciences (Received 4 November 1961)

THE parallel study of a polymer in an ideal and a good solvent considerably extends the range of information which can be obtained regarding the properties and behaviour of its macromolecules in solutions. In [1] we gave the results of the study of solutions of poly-2,5-dichlorstyrol (PDCS-2,5) in an ideal solvent. I n this paper we give the results of the measurement of light scattering and viscosity for solutions of PDCS-2,5 fractions in a good solvent, dioxane. POLYMER, FRACTIONATION, SOLVENT I n [1] we said that of the eight test fractions obtained as a result of the first fractionation of the original PDCS-2,5 specimen, only half showed a sufficient degree of monodis* Vysokomol. soyed. 5: No. 4, 614-621 (1963).