Green strength rubber

Whilst the blend has a good green strength it is usual to vulcanise the rubber by an accelerated sulphur system using a higher than usual accelerator sulphur ratio. 

The copolymerization of butadiene in trans configuration with suitable comonomers represents a second route for obtaining a wide range of strain induced crystallizable elastomers, with melting point tailorable in a wide range of temperatures. These copolymers can be used, in particular, in blends with other crystallizable rubbers (e.g. synthetic cis-l,4-polyisoprene) in order to improve their "green strength". 

Figure 7. Green strength of different rubbers. Elongation rate, 0.3 m/min compound, 55 parts carbon black N 330 NR, natural rubber IR, synthetic cis-1,4-...

Their crystallization behavior compares with natural rubber, as follows (1) their rate of crystallization is more rapid and (2) their amount of crystallinity is temperature dependent, but considerably less strain dependent. These experimental rubbers have excellent green strength and building tack. 

Since the tack and green strength of these rubbers may depend on their ability to undergo strain induced crystallization, this behavior was studied and compared with that shown by NR. Information on the strain induced crystallization behavior of these experimental rubbers was obtained from x-ray diffraction measurements and from rheo-optical studies. 

Nitrile rubbers are copolymers of butadiene and acrylonitrile which are produced by emulsion polymerisation hot and cold polymerised types are available. The hot polymerised types generally have higher green strength and are slightly harder to process than cold copolymers. 

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. 

Initial attempts to make synthetic rubber similar to NR date back to mid 1800s. Modern synthetic polyisoprene is designed to be similar to natural rubber in structure and properties. Although it has lower green strength, slower cure rates, lower hot tear, and... 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

Green strength -1) The resistance to deformation of a rubber stock in the uncured state. 2) Uncured adhesion between plied or spliced surfaces. 

ISO 9026 1991. Raw rubber or unvulcanised compounds - Determination of green strength. 

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