Processing green strength

Elongation, tensile strength, hardness, snappiness, good flowability, easy vulcanization, weatherability. ozone resistance, heat resistance, skid resistance, abrasion resistance Flowability, injection molding processability easy vulcanization, weatherability, ozone resistance, heat resistance, skid resistance, abrasion resistance, snappiness Green strength, flowability, extmdability injection molding processability, weatherability, ozone resistance, heat resistance snappiness... 

Figure 16. Processibility and green strength of trans-butadiene-piperylene copolymers as a function of melting point and Mooney viscosity.

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. 

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. 

The next key component deals with the selection of a binder or adhesive system. In addition to the adhesive characteristic offered by the wetting agents and dispersants it is generally necessary to incorporate a synthetic or natural gum to provide good green strength and integrity in the finish product. The selection process is a trial and error procedure. 

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