Rubbers millable gums

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. 

A drawback to the cast elastomers is limited shelf-life and a need to store them in the absence of moisture. As a result, millable elastomers were developed. These are produced by first forming hydroxy-terminated linear polyurethanes through reactions of linear aliphatic polyesters or polyethers with diisocyanates. The prepolymers are rubber-like gums that can be compounded on rubber mills with other ingredients and crosslinked. Crosslinking is accomplished by adding either more diisocyanates, or sulfur, or peroxides. Diisocyanates dimers that dissociate at about 150 °C are often used ... 

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... 

ST polysulfide rubber is a millable gum elastomer prepared from bis (2-chloroethyl) formal and sodium polysulfide. Vulcanizates exhibit outstanding resistance to a broad spectrum of oils and solvents, to gas permeability, low temperature, ozone and weathering. Typical uses include gas meter diaphragms, rollers, printing blankets, o-rings, and many specialty molded items. 

Polyurethane in the rubber industry can be used in (1) the thermoplastic elastomer form (TPE), discussed earlier, (2) a two-part liquid system in reaction injection molding (RIM), (3) the cast molding of rubber parts, or (4) as a millable gum that can be processed on a two-roll mill and cured with agents such as peroxides or sulfur, just as with conventional rubber. Although different versions of polyurethane elastomers must be tailor-made for each of these four common applications, the basic chemistry used is very similar in all. 

Millable polyurethane elastomers are elastomers to which the conventional techniques of mill compounding and vulcanization can be applied. In this approach, stable (hydroxyl-terminated) polymers are prepared by the reaction of linear polyesters (commonly adipates) or polyethers (commonly poly(oxytetramethylene) glycol) with diisOcyanates. These polymers are rubber-like gums which may be compounded on two-roll mills with other ingredients. Vulcanization may be effected by several types of reagents, but isocyanates, sulphur systems and peroxides are the most widely used. 

The millable raw-gum type is made by reacting approximate equal molar quantities of polyol and diisocyanate, attaining a high molecular weight. Additional polyisocyanate is added to the rubber on a mill, and cross-linking is accomplished by heating in a mold under pressure. 

Ex. 7. Bostik 7070 (100 parts) with Boscodur No. 1 (5 parts) appears to be another example of a two part millable polyurethane gum adhesive system. It is recommended by the manufacturer as a laundering and dry cleaning resistant cement for bonding urethane sponge and urethane rubber. It is also recom-... 

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