Fillers, rubber Tear strength

The physical properties of natural rubber and synthetic rubber compounds are affected greatly by the type and amount of fillers used. Carbon black is the most commonly used filler. Increasing amounts of carbon black increases the hardness and modulus of the vulcanizates. Resilience and resistance to impinging type abrasion decrease along with elongation. Tensile strength and tear strength... 

A filler with a high surface area increases the interaction with the matrix and thus increases tear strength (Figure 8.23) When rubber is filled with silica the large surface area of the silica interacts with the rubber and adheres to it. This adhesive interaction allows energy to be stored or dissipated. 

Silicone polymers, crosslinked by radiation in the absence of fillers, form rubbers with poor tensile and tear strength. Much better properties can, however, be obtained for small doses by the inclusion of a filler such as silica or carbon black [425]. 

Rather peculiar to the rubber industry is the use of the fine particle size reinfordng fillers, particularly carbon black. Fillers may be used from 50 phr to as high as 100-120 phr or even higher proportions. Their use improves such properties as modulus, tear strength, abrasion resistance, and hardness. They are essential with amorphous rubbers such as SBR and polybutadiene that has Kttle strength without them. They are less essential with strain-crystallizing rubbers such as NR for many applications but are important in the manufacture of tires and related products. 

The example chosen here to illustrate this type of composite involves a polymeric phase that exhibits rubberlike elasticity. This application is of considerable practical importance since elastomers, particularly those which cannot undergo strain-induced crystallization, are generally compounded with a reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers and silica to polysiloxane elastomers. The advantages obtained include improved abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat buUd-up) and compression set (permanent deformation). 

The particles of carbon black are not discrete but are fused clusters of individual particles. The reinforcement conferred by the black is not influenced to any extent by the size of the unit but predominantly by the size of the particles within the unit. The primary particle typically has cross-sectional dimensions" of 5-100 nm. It is well established that the most appropriate way of describing the size of the primary particles is to express it as speciflc surface area/weight Particle size of itself has relatively little effect on the modulus. But tensile and tear strengths are affected by the particle size and both properties are normally enhanced as the surface area increases (i.e. surface area increases with decreasing particle size). The high surface area enhances the ability of the filler to wet the rubber and thus enhances the interaction at the rubber filler interface. It is the enhancement of the filler-rubber interface that provides the desired reinforcement in filled vulcanized rubber. 

NR fiUed polyisoprene-coated sihca showed better tensile strength, 300 % modulus and abrasion resistance due to better filler-rubber interation which was confirmed by SEM. However lower elongation at break, tear strength, hardness and specific gravity were also observed. 

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