Unvulcanised Sheet Rubber

The process of controlled, gentle warming of unvulcanised calendered sheet rubber to reduce its anisotropy and prevent subsequent distortion of articles made from the sheet and cured by embedding in chalk in open steam. 

The lining sheet is prepared by calendering in thin plies up to 1 mm in thickness, followed by combining the individual layers on the calender to form a finished sheet, usually between 3 mm and 6 mm thick. Alternatively, lining sheet can be manufactured by the roller die process in which the sheet is extruded and then passed through rollers to give the finished thickness. Extrusion is also used for the preparation of unvulcanised rubber tubes suitable for lining pipes up to 200 mm in diameter. 

Change in dimensions of an unvulcanised rubber (calendered sheet or extruded section) on cooling from the processing temperature. Also the volume contraction of a moulded rubber product on cooling from vulcanising temperature. See Coefficient of Thermal Expansion (Volumej. Shrinking... 

Wet anti-tack agents can be soap or detergent solutions or suspensions of the dry agents in water. For example, dissolved polymers with non-ionic surface active agents which form a thin layer of polymer on unvulcanised rubber sheets magnesium stearate in water zinc stearate dispersion and aqueous dispersions of fatty acid salts. 

Except for work on complete products, a test piece must be formed before the test can be carried out. In many cases, the test piece can be directly moulded but, particularly when tests on finished products are concerned, the specimens need to be cut and/or buffed to some particular geometric shape (Figure 4.1). It is convenient to consider separately, first the mixing and moulding leading up to a vulcanised (or thermoplastic) test piece or test sheet, and secondly the preparation of test pieces from moulded sheets or products. The preparation of test pieces for tests on raw rubber and unvulcanised compounds will be considered integrally with those tests in Chapter 6. 

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