Millable urethanes

Millable urethanes are processed on standard rubber-processing machinery. They may be either peroxide or sulfur cured. The sulfur-cured varieties have some chemicals added to enable sulfur curing to take place. These polyurethanes also have the properties of castable polyurethanes but need to be processed on standard rubber machinery. 

Thanecure . [T E] Zinc chloride/ benzothiazyl disulfide vulcanization activator for sulfiir-curable millable urethane elastomers. 

Adaphax. [Am. Cyanamid] Sulfurless v anized vegetable oils extender for millable urethane and CR formula-... 

Peroxides are vulcanizing agents for elastomers, which contain no sites for attack by other types of vulcanizing agents. They are useful for ethylene-propylene rubber (EPR), ethylene-vinyl acetate copolymers (EAM), certain millable urethane rubbers, and silicone mbbers. They generally are not useful for vulcanizing butyl rubber (poly[isobutylene-co-isoprene]) because of a tendency toward chain scission, rather than crosslinking, when the polymer is subjected to the action of peroxide. 

It is important to note that with millable urethane elastomers the conventional use of reinforcing fillers, e.g. carbon blacks, silicas, clays, whitings, etc., and specialized plasticizers, flame retardants, etc., can be practised, thus greatly widening their applications. 

Such millable urethane block copolymers cover the hardness range 30 to 90IRHD. Their technology, being similar to dry rubber technology, can be represented by the following notes ... 

It will be realized from Chapter 1 that in urethane elastomer formation from liquid components there is the possibility of several reactions occurring simultaneously during a prepolymer or one-shot process, and that the relative proportion of one to the other will affect the overall properties of the final polymer. Thermoplastic and millable urethanes are not, during their processing and fabrication stages, subjected to the type of catalysis discussed in this chapter during their polymer synthesis operations, however, reaction rate-structure considerations will apply. 

Millable urethane elastomers are usually based on low molecular weight polymers, about 20000-30000 molecular weight, which are essentially linear in nature some branching is permissible but not to the extent that a three-dimensional structure is produced. In some grades a very small amount of branching is introduced for processing purposes. It is essential that the polymers used for millable urethanes contain no reactive isocyanate sites, otherwise storage instability results in the raw elastomer hence these materials are usually hydroxyl end-blocked for shelf stability purposes. 

A typical commercial versatile millable urethane is Adiprene CM, which is polyether-based and whose processing and compounding technology will now be described. No special handling precautions are necessary with respect to moisture exclusion. 

Unlike liquid urethane elastomers, millable urethane rubber requires reinforcing fillers to develop optimum strength and abrasion resistance. However, pronounced reinforcement is obtained with low concentrations of carbon black and to a lesser extent with certain mineral fillers. 

Millable urethane responds to various types of carbon-black loading in the same manner as does SBR (Styrene-Butadiene Rubber). The qualitative effects of various carbon blacks and non-black fillers on several properties of vulcanizates of millable urethanes are shown in Table 7.9 and Figs 7.3 and 7.4. 

Solid elastomeric urethanes are of two kinds, liquid cast and millable raw-gum types. llie former is prepared by reacting excess diisocyanate with polyd ... 

Method D appears to be possibly the most important type of isocyanate-based adhesive system. It is similar to Method B in that a preformed, fully reacted, high molecular weight polymer is employed as a vehicle in the adhesive formulation. The strength of the vehicle holds adherend members in exact position after assembly until the full bond has formed. Method D differs from Method B in that its vehicle polymer is a polyurethane. A further difference is that the inherent adhesive character and strength of the polyurethane vehicle frequently enables its use without added di- or poiyisocyanate. This strength may be realized in essentially amorphous compositions such as the thermoplastic polyurethane elastomers or millable gums. Or it may be achieved with crystallizing urethane adhesive polymers. 

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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