Internal mixer deficiencies

Urethanes are processed as mbber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The Hquid prepolymers are cast using automatic metered casting machines, and the thermoplastic peUets are processed like aU thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensUe, and resistance to many oUs is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

The use of stabilisers (antioxidants) may, however, have adverse effects in that they inhibit cross-linking of the rubber. The influence of phenolic antioxidants on polystyrene-SBR alloys blended in an internal mixer at 180°C has been studied. It was found that alloys containing 1% of certain phenolic antioxidants were gel-deficient in the rubber phase.The gel-deficient blends were blotchy in appearance, and had lower flow rates compared with the normal materials, and mouldings were somewhat brittle. Substantial improvements in the impact properties were achieved when the antioxidant was added later in the mixing cycle after the rubber had reached a moderate degree of cross-linking. 

Butadiene rubbers may be processed and vulcanized in much the same way as natural rubber, but compounding operations are rather more difficult on standard equipment at the usual temperatures. Above about SO C these rubbers do not band well on mills and internal mixers are not satisfactory. This deficiency is overcome by blending with natural or styrene-butadiene rubbers. 

The mill mixing and the use of internal mixer have been the major modes of mixing of rubber. However, these methods are not without deficiencies. The mill mixing may be labour-intensive so that it is used when the cost of mixing is not a concern. Therefore, it tends to be used for research and development. In production, it is used when the cost, for other than mixing dominates or when there is a critical requirement for a good dispersion. 

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