Rubber Terms Links

However some non-rubber compounds are also called elastomers if they exhibit a nondeforming elastic property similar to rubber at room temperature, even if the compound is relatively hard. The two main groups of non-rubber elastomers are thermoplastics, e.g., polyvinyl chloride, polypropylene and thermosets, e.g., ethylene propylene rubber, cross-linked polyethylene. These two groups are also covered by the term plastic . 

Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

The term ABS was originally used as a general term to describe various blends and copolymers containing acrylonitrile, butadiene and styrene. Prominent among the earliest materials were physical blends of acrylonitrile-styrene copolymers (SAN) (which are glassy) and acrylonitrile-butadiene copolymers (which are rubbery). Such materials are now obsolete but are referred to briefly below, as Type 1 materials, since they do illustrate some basic principles. Today the term ABS usually refers to a product consisting of discrete cross-linked polybutadiene rubber particles that are grafted with SAN and embedded in a SAN matrix. 

Vulcanisation is the term used for the process in which the rubber molecules are lightly crosslinked in order to reduce plasticity and develop elasticity. It was originally applied to the use of sulfur for this purpose, but is now used for any similar process of cross-linking. Sulfur, though, remains the substance most widely used for this purpose. 

Stress upturn in the stress-strain relation of carbon black-fiUed rubbers can be reasonably revealed in terms of the non-Gaussian treatment, by regarding the distance between adjacent carbon particles as the distance between cross-links in the theory. 

The large scale molecular motions which take place in the rubber plateau and terminal zones of an uncross-linked linear polymer give rise to stress relaxation and thereby energy dissipation. For narrow molecular weight distribution elastomers non-catastrophic rupture of the material is caused by the disentanglement processes which occur in the terminal zone, e.g., by the reptation process. In practical terms it means that the green strength of the elastomer is poor. 

For optimum cross-linking efficiency, a combination of accelerators is used. Table 6 shows the increase in load-bearing capability of vulcanizates based on different rubbers as the ratio of two accelerators is changed (14). The term 300% modulus represents the strain at 300% stress and is not a true modulus because rubber gives nonlinear stress—strain behavior. For polymers with primary allylic carbon atoms, the use of two accelerators gives significandy higher 300% modulus than either accelerator used alone. When the mbber polymer consists of secondary allylic carbon atoms, the modulus is level until the sulfenamide OBTS becomes the principle accelerator. 

In the rubbery region, which is just above (in terms of temperature) the leathery region, polymer chains have high mobility and may assume many different conformations, such as compact coils, by bond rotation and without much disentanglement. When these rubbery polymers are elongated rapidly, they snap back in a reversible process when the tension is removed. This elasticity can be preserved over long periods of time if occasional cross-links are present, as in vulcanized soft rubber, but the process is not reversible for linear polymers when the stress is applied over long periods of time. 

For peroxide cross-linking, organic peroxides, such as dicumyl, di-t-butyl, and benzoyl peroxides, are used in amounts 1 to 3 phr (parts per hundred parts of rubber). Typical cure cycles are 5 to 10 min at temperatures 115 to 170°C (239 to 338°F), depending on the type of peroxide used. Each peroxide has a specific use. A postcure is recommended to complete the cross-linking reaction and to remove the residues from the decomposition of peroxide. This improves the long-term heat aging properties.62... 

Presented in this paper are the results of an investigation concerning the link between structure and properties of rubber-toughened plastics. An attempt has been made to assess the importance of the spatial distribution of rubber particles in terms of their effectiveness in controlling craze initiation and growth. Also studied in particular were the effects of rubber particle size on the mechanical properties of HIPS materials. A... 

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