Mechanical properties of vulcanized natural rubber filled with a linear crystallizing hydrocarbon

:l~olymer Science U.S.S;R; VoL 31, No. 7, lap. 1612-1616, 1989

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MECHANICAL PROPERTIES OF VULCANIZED NATURAL RUBBER FILLED WITH A LINEAR CRYSTALLIZING HYDROCARBON* T. YE. GROKHOVSKAYA,A. L. VOLYNSKII and N. F. BAKEYEV M. V. Lomonosov Moscow State University (Received 7 January 1988)

Mechanical properties of systems composed of natural rubber of various degree of crosslinking and of a linear hydrocarbon (n-octadecane), prepared by spontaneous swelling of the rubber in the hydrocarbon melt, were investigated. In the filled systems, the dependences of the initial modulus and of strain at break on crosslinking density are of limiting character. The spatial topology of the rubber network determines the phase state of the low-molecular component, and this leads to a change in the way in which it affects the mechanical properties of the polymer: from reinforcement with a solid filler, to plastification by a liquid hydrocarbon. PREVIOUSLYit has been shown [1] that the system vulcanized natural rubber (NR)-crystallizing hydrocarbon, prepared by spontaneous swelling of the rubber in the hydrocarbon melt, undergoes a phase separation of cooling below the crystallization temperature of the solvent, in consequence of which it is transformed from a homogeneous into a two-phase system, consisting of the crystals of the low-molecular component dispersed in a continuous rubber matrix. The resulting solid system is unstable and phase separation proceeds further for a long time. In an approach to thermodynamic equilibrium, in these systems the lowmolecular component is redistributed in the sample, and it also separates out of the sample volume (sweating). It has been shown [2] that the observed processes to a considerable degree depend on the crosslink density of the rubber matrix, and on the structure of the generated network. As part of further studies of the structure and properties of such systems, in this work the mechanical properties of the composites were studied under conditions of uniaxial drawing at constant rate. Films of crosslinked NR 0"5 mm thick were used. The crosslinking agent used was dicumyl peroxide (DCP), added to the plastified rubber in doses indicated below. Components NR, plasticate, wt.p. DCP, wt.p.

NR-1 200 1

NR-2 200 2

* Vysokomol. soyed. A31: No. 7, 1471-1474, 1989. 1612

NR-3 200 3

NR-4 200 4

NR-5 200 6

NR-6 200 8

Mechanical properties of vulcanized natural rubber

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The conditions of N R vulcanization were described in [2], The samples for our studies were prepared by swelling of N R of various degree of crosslinking in n-octadecane melt at 50 ° to equilibrium (3 hr). The samples were then taken out of the liquid, cooled to room temperature, their surface was cleaned and the degree of swelling was determined gravimetrically. At the initial moment the samples contained 250 to 100 wt.% octadecane. Prior to the mechanical studied, the samples were stored at room temperature for 15-20 days. A change in the concentration of octadeeane in the composites was checked by washing in heptane after careful cleaning of the surface. Octadecane qualified as pure was used without further purifical, T~=28"2 ° Mechanical studies were performed with the "Instron" dynamometer at room temperature. The samples were cut in the shape of double-sided dumbbells, with the working part 10 ram in length and 4.2 mm in width. The deformation rate was 50 ram/rain. Calorimetric studies were performed with. the thermoanalyzer 1090 "DuPont", with DSC mark 910. The rate of temperature growth was 10 deg/min. T h e c h a n g e in relative c o n t e n t ~ o f the h y d r o c a r b o n in N R s a m p l e s o f v a r i o u s degrees o f c r o s s l i n k i n g d e p e n d i n g on storing t i m e at r o o m t e m p e r a t u r e is s h o w n in Fig. 1. I t is a p p a r e n t t h a t at t h e t i m e o f the m e c h a n i c a l studies (15-20 d a y s f r o m p r e p a r a t i o n ) , irrespective o f i d e n t i c a l history, s a m p l e s with different degrees o f c r o s s l i n k i n g c o n t a i n e d different a m o u n t s o f o c t a d e c a n e , a n d were f o u n d t o be at v a r i o u s stages o f a p p r o a c h to equilibrium. S i m u l t a n e o u s l y with m e c h a n i c a l studies, t h e p h a s e c o m p o s i t i o n o f the s a m p l e s

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FIe. 1. Dependence of octadecane content in N R samples of various degree of crosslinking on storage time at room temperature (equilibrium degree of swelling defined as 100%). Curve numbers correspond to NR sample designation. F2o. 2. Calorimetric melting curves of octadecane (OD) in NR of various degree of crosslinking: / - f r e e OD; 2 - N R - 1 ; 207~o OD; 3 - N R - 3 , 89% OD; 4 - N R - 4 , 63~/00D; 5--NR-5, 39y, OD; 6 - N R - 6 , 17Yo OD; 7 - N R - 6 , 107~. OD ("freshly prepared sample"). Dashed line indicates the temperature of mechanical studies.

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was determined by DSC. Calorimetric melting cur~,es for octadecane in NR samples of various degree of crosslinking are shown in Fig. 2. It may be seen that octadecane in weakly crosslinked samples (NR-1) exlaibits one melting peak, near to the temperature of the melting peak of the frec hydrocarbon. More highly crosslinked samples exhibit two octadecane melting peaks (a high-temperature and a low-temperature one). This is connected with the circumstance that phase separation has occurred in the more highly crosslinked samples, in consequence of which a considerable amount of octadecane has separated into the low-temperature fraction which at room temperature (temperature of the mechanical studies) occurs in a liquid solution state. The causes of such phase separation have been discussed in detail in [2]. With increasing degree of crosslinking of the rubber matrix, the relative amount of the liquid octadecane fi'action increases and at the end, at the maximum degree of crosslinking (NR-6, Fig. 2, curve 6) practically all hydrocarbon in the polymer is present in the liquid state at' room temperature. From Fig. 1 we may see that the composites on the basis of NR-6 exhibit the highest rate of phase separation. In consequence of this, such a system is the most removed from t.he initial state ("freshly prepared" sample) which in Fig. 2 is represented by curve 7. Comparing curves 6 and 7 we recognize that the loss of a considerable part of octadecane (sweating) leads to a lowering of its melting temperature from 31.3 to 11.5 °, i.e. by 20 °. In this way, the state of the hydrocarbon in NR at room temperature is completely changed: crystalline octadecane in the initial sample has been changed to liquid. Samples with other degrees of crosslinking contain a mixture of liquid and solid octadecane in various proportions (curves 2-5).

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FIG. 3; Deformation curves of NR samples of various degree of crosslinking: a-original rubber, curve number corresponds to sample designation; b-filled samples: 1-NR-I, 207~ OD; 2 NR-2, 157% OD; 3--NR-3, 89~o OD; 4-NR-4, 63Yo OD; 5-NR-5, 39yo OD; 6 - NR-6, 17~o OD; 7-NR-2, 157~ OD, sample drawn at 30°. Let us now discuss the mechanical properties of such systems. The drawing curves of NR samples and of their composites with octadecane are shown in Fig. 3. Pure NR exhibits the well known changes in mechanical properties with increase in crosslinking: an increase in the initial modulus and strength, and a lowering of strain at break. For

Mechanical properties of vulcanized natural rubber

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the filled systems, the effect of crosslinking is more complicated. Let us discuss this effect considering the dependence of initial modulus E (Fig. 4) and of ultimate strain eB (Fig. 5) on the concentration of the crosslinking agent c. EB

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The data presented indicate that both E and eB of pure NR change regularly in the whole range of DCP concentrations: E increases and eB declines with increasing crosslink density. A.t the same time, in filled systems both these dependences exhibit an limiting character. At small degrees of crosslinking, E is considerably higher in filled systems than in the original NR; with increasing degree of crosslinking, it relatively rapidly decreases to values below those for the original NR. On the contrary, ultimate strain is lower for filled samples than for the original NR; it increases with increasing degree of crosslinking, reaches the ~ values for original NR, then decreases again so that for samples with the maximum degree of crosslinking it lies below the value for the original rubber. These features can be explained based on an analysis of the phase state of the filler (octadecane) at room temperature-the temperature at which the mechanical properties are measured. Figure 2 indicates that at low degrees of erosslinking, at room temperature the main part of octadecane in the rubber matrix occurs m the crystalline state. In other words, in this case the rubber contains a solid low-molecular filler, and according to present notions [3], this is expected to increase the elastic modulus and decrease the ultimate strain of the material. With increasing network density, the total amount of the low-molecular component decreases; simultaneously also the fraction of the hydrocarbon which is solid at room temperature is observed to decrease, and the fraction of liquid hydrocarbon dissolved in NR to increase. A decrease in the amount of solid filler duly results in lowering of the modulus and growth of eB. At the highest degree of crosslinking, all

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T..YE. GROKHOVSKAYAe t aL

octadecane occurs in the dissolved state at room temperature. It is known from the literature [4] that N R samples containing a dissolved low-molecular component may exhibit considerably smaller ultimate strain values than the original material. Evidently, also in the presently studied case, the presence of a considerable amount of dissolved octadecane in the most highly crosslinked samples also leads to a lowering of eB, i.e. with increasing crosslinking of the NR, the reinforcing effect of the solid filler is changed to the plastifying effect of a liquid. It should be noted that a similar effect (lowering of eB and E) is observed for weakly crosslinked N R samples when the mechanical studies are canied out at 30 °, i.e. under conditions when all octadecane ~s transfoimed to the liquid state (Fig. 3b, curve 7). Thuswith a change in N R crosslinking, the phase state of the low-molecular filler is modified, and this in turn changes the way in which it affects the properties of the polymer; :from reinforcement by the solid filler, to plastification by the liquid plastifier. Translated by D. DOSKO~tLOV,~ REFERENCES

1. A. L. VOLYNSKII, T. Ye. GROKHOVSKAYA, A. SANCHEZ and N. F. BAKEYEV, Vysokotool. soyed. 1328: 373, 1986 (Not translated in Polymer Sci. U.S.S.R.) 2. A. L. VOLYNSKII, T. Ye. GROKHOVSKAYA, G. M. LUKOVKIN, A. SANCHEZ and N. F. BAKEYEV, Vysokornol. soyed. A30: 1932, 1988 (Translated in Polymer Science U.S.S.R. 30: 9, 2054, 1988) 3. Usilenie elastomerov (Reinforcement of Elastorners). p. 484, (Ed. G. Krans), Leningrad, 1968 4. Yu. Z. ZUYEV, Razrushenie polirnerov pod deistviem agressivnykh srcd (Polymer Destruction in Aggresive Media), 2rid ed., Moscow, 1972