Millable gum

Another approach has been adopted by the Du Pont Company with Adiprene C. This is a urethane-type polymer with unsaturated groups in the polymer. Because of the unsaturation the polymer may be vulcanised with sulphur, the standard vulcanising agent of the rubber industry. This is a clear-cut example of a product being modified to suit the processor rather than that of a processor adapting himself to meet new products. Whereas Adiprene C has poor tensile strength when unfilled, the use of carbon black leads to appreciable reinforcement (as is the case with SBR and to some extent natural rubber. 

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The millable gums are today of limited importance for two reasons ... 

The millable gums, which will be the only type considered here, would generally contain 5,000-9,000 of the dimethyl siloxane unit. 

The diisocyanates and polyols are reacted to form a high molecular weight hydroxyl terminated millable gum. These millable gums are compounded and processed as conventional elastomers, both sulphur and peroxides being used to cure the polymers. Here again, polyether and polyester types are available, and the differences between these two types referred to above also apply here. 

Polyurethane is also available in millable gum form which can be handled on the same conventional rubber processing equipment and cured in similar molds. 

Lack of knowledge, poor technical advice from suppliers and consequently the wrong choice of grade or improper processing are the main causes of comparatively less use of millable gum polyurethanes in the rubber industry. 

In addition to the liquid polysulfides, polysulfide millable gum elastomers have been used for many years with conventional rubber processing equipment. Compounds based on solid elastomers are being used in the manufacture of paint spray hose, rollers, and printing blankets for the graphic arts industry. New applications for these versatile materials are still being developed. 

Uses Used in PU industry for mfg. of prepolymers, thermoplastic elastomers. coatings and adhesives, dispersing agents, microcellularshoe sole systems, millable gums, cast elastomers... 

Prepolymers One-shot systems Thermoplastic Millable gums Castable liquids Reaction injection-moulded and cast systems Melt-processed by injection moulding or extrusion Conventional rubber processing... 

Three essential requirements of a millable gum are that it should have sufficient molecular weight to give sufficient strength for handling on mills and other processing equipment, stability so that it does not harden on storage and a suitable cure site. 

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