No abstract

No abstract

Silica particles were treated by silane coupling agents to study the effects of surface chemistry of silica particles on the secondary structure formed by silica particles in styrene-butadiene rubber (SBR). The relation between the size of secondary structure and tensile properties of silica-filled SBR vulcanizates was also investigated. The water molecules adsorbed on the silica surface enhanced both the reactivity of the coupling agents with silanol groups on the silica surface and the condensation reaction between coupling agents. The averaged size of one agglomerate in the filled vulcanizate (S agg ) decreased with the increase of the hydrophobic character of silica particles, which were obtained by the introduction of coupling agents with alkyl groups. The initial slope of stress-strain curves for silicafilled vulcanizates decreased with the decrease of S agg . Further, a clear Payne effect was observed in all silica-filled vulcanizates, with this tendency more prominent in the larger S agg .

ABSTRACT:The effects of surface chemistry of silica particles on the mechanical properties were studied for silica filled styrene-butadiene rubber (SBR) systems. The samples were prepared from different kinds of silicas and of silane coupling agents. The breakdown of the agglomerate formed by silica particles was successfully detected by transmission electron microscopy (TEM) observations when the strain was applied to silica filled vulcanizates. The degree of breakdown of the agglomerate of silica particles by the strain was more prominent in the larger one of which size was controlled by the number of silanol group per unit surface area of silica particles. The amount of entrapped rubber within the agglomerate seemed to be decreased with the decrease in the agglomerate size. Also, the initial agglomerate size and the change of agglomerate by the strain became small by the introduction of silane coupling agent, such as bis(3-triethoxysilylpropyl)tetrasulfane (TESPT). At a given degree of vulcanization, the initial modulus of silica filled vulcanizates was governed by the size of agglomerate formed by silica particles and the amount of entrapped rubber phase. On the other hand, at a larger strain, the tensile strength of the filled vulcanizates increased by the introduction of interfacial combination between silica particles and rubber matrix by TESPT. These results indicate that the stress-strain behavior of filled vulcanizate is affected by the agglomerate of the fillers and the interactions between filler and rubber matrix.KEY WORDS Silica / Styrene-Butadiene Rubber / Silane Coupling Agent / Transmission Electron Microscopy (TEM) / Agglomerate / Mechanical Properties / Silicas have been used as fillers for rubber reinforcement. 1-3 However, the utilization of silica caused two serious problems. One was a bad processability, such as a lower dispersiblity of silica particles in rubber matrix and a high viscosity of silica filled unvulcanizates. [2][3][4][5][6][7][8] The other one was a poor reinforcement, such as a low tensile strength, low abrasion resistance and so on, in comparison with carbon black filled rubber composites. [1][2][3][4][5][6][7][8] The chemical environment of silica particle is quite different from that of carbon black due to the existence of silanol groups in the particles. Thus, the primary discussion on the properties of silica filled rubber composites has been concerned with the interactions between silica particles and the interactions between silica particles and rubber molecules. Wolff and Wang studied the effects of surface energies of fillers on rubber reinforcement, and reported 7 that the surface energies of silica were characterized by a dispersive and specific components. The higher specific component led to the higher viscosity of the rubber composites due to the strong interactions among silica particles. On the other hand, the lower dispersive component caused weak filler-rubber interactions, leading to the lower † To whom correspondence should be addressed. content of bound...

SYNOPSISThe structure and formation mechanism of carbon gel in carbon black filled polyisoprene composites were studied by the pulsed NMR technique. The composites were prepared from a wide range of molecular weights by a solution blend. The carbon gels were extracted from the composites by a solvent-extraction method. The content of carbon gel was not governed by the molecular weight of rubber but was controlled by the viscosity of rubber solutions which were used for the blend. Three rubber phases, having different spin-spin relaxation times, were detected in all the carbon gels. The increase of carbon gel content in the composites was mainly from the increase of highly mobile rubber phase, and the gel became soft with the development of this phase. On the other hand, the content and structure of glassy rubber phases were not affected by the size of the carbon gel, and they showed almost a constant value despite the large change in the carbon gel content. A part of the highly mobile rubber phase in the gels could be removed by solvent extraction a t high temperature. These results suggest that the formation of carbon gel is primarily governed by two factors: One is the well-known rubber-carbon black interaction, and the other is a physical crosslink between the carbon gel and unbound rubber molecules during blend. 0 1996 John Wiley & Sons, Inc. I NTRO DU CTlO NThe incorporation of carbon black into elastomers develops the so-called bound rubber around carbon blacks, and such structural developments are believed to enhance the physical and mechanical properties of carbon black/rubber composites. Thus, numerous studies have been reported and reviewed on the carbon black/rubber interaction and its effect on reinforcement. It is generally accepted that the carbon black filled rubber system is composed of two rubber phases: One is the so-called bound rubber (carbon gel) in which rubber molecules are adsorbed on the carbon black surface, and the other is a free rubber (nonbound rubber) which is soluble in common organic solvents. The amount of carbon gel is * To whom correspondence should be addressed. Journal of Applied Polymer Science, Vol. 61, 1345Vol. 61, -1350Vol. 61, (1996 0 1996 John Wiley & Sons, Inc.CCC 0021 -8995/96/081345-06 affected by many factors such as chemical structure' and the molecular weight of the surface area of the carbon black,5 and the technique for the dispersion of carbon black in the r~b b e r .~.~ Although these results have been explained by a chemical or physical interaction between rubber molecules and carbon black, complete understanding is still lac king.Pulsed NMR studies have revealed that carbon gel has a multicomponent phase whose structure depends on the extent of interaction between carbon black and r~b b e r .~.~-" Thus, it is expected that structural analyses of carbon gel prepared under various conditions would give us useful information on the formation mechanism of carbon gel.In this study, structural analyses were carried out by the pulsed NMR method for carbon gels p...