Factors Affecting Rubber Elasticity

D. FACTORS AFFECTING RUBBER ELASTICITY 1. Effect of Degree of Crosslinking... 

Temperature and other environmental factors affect the mechanical behavior. Thus, polymers can show all features of a glassy brittle solid, an elastic rubber, or a viscous liquid depending on the temperature and time scale of measurements. At low temperatures or high frequencies, a polymeric... 

ML value is an index of the mechanical property of synthetic rubber. It is related to the averaged molecular weight of synthetic rubber products. In butadiene rubber production, the ML value should be controlled within 43 to 47. Too high ML values correspond to insufficiency of elasticity, while too low ML values correspond to low tensile strength of products. Table 14.4 illustrates some data of production records about the factors affecting ML values in production. It has been found that there are five chief factors influencing the ML values in production X (feed rate of butadiene monomer), X2 (feed rate of solvent oil), Xj (feed rate of catalyst), X4 (temperature of feed), X5 (temperature of the lower part of the first reactor). 

The modulus of an ideal rubbery network depends only on the length of the chain segment between associations, and should be affected by the molecular weight of the uncrosslinked molecule only insofar as the chain ends do not contribute to the network. Consequently, the shift factor b should reflect this free chain end effect. This has been tested using the simplest expression for the modulus in the theory of rubber elasticity. 

Under different deformation conditions, natural rubber (NR) may exhibit predominant viscous flow, elastic or viscoelastic behaviour. Thus, the time for the movement of the NR molecular chains, i.e. relaxation time, is vastly affected by those deformation rate and NR types. The variation of NR types such as smoked rubber sheet, rubber blocks such as skim block, STR 5L and STR 20, is another factor that influences rheological properties and processing of NR due to their different Mooney viscosity, molecular weight distribution and gel content. Types of NRs are based on the different production processes of NR in which rubber smoked sheets are produced from NR in the latex form whereas block types are produced from various types of NR sources such as from latex in the case of STR 5L, from rubber scrap in the case of STR 20 and from skim latex in the case of skim block. 

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