ISBN: - 978-93-88936-09-5 4 .5 (L) Properties of Rubber Natural rubber is soft and translucent at 200 c, when chilled to 0-100c it becomes hard and opaque at the temperature of liquid air. It is brittle and transparent like glass. At temperatures exceeding 250c, rubber loses elasticity and becomes sticky. Rubber melts to a viscous fluid at about 2000 c. rubber is insoluble in water and is unaffected by alkalise or moderately strong acids. It is soluble in benzene, naphtha, carbon disulphide either chloroform or chlorinated hydrocarbon. Rubber is one of the best insulating and dielectric materials available. 4 .5 (M) Processing of the Crop The main crop from the rubber tree is latex, a milky white dispersion of rubber in water, which is harvested by the process of tapping. Coconut shells and polythene cups are used as containers in most of the Indian estates. Latex collected in the cups is transferred to clean buckets, two /three hours after tapping. Around 80 percent of the crop from plantation is in the form of latex. Latex is coagulated in suitable containers into thin slabs of coagulum and sheeted through a set of smooth rollers followed by a grooved set, and dried to obtain sheet rubber. Depending upon the drying method, sheet rubbers are classified into two: Ribbed smoked sheets and Air dried sheets. 4 .5 (N) Smoking The sheets, after four to six hours of dripping in shade, are put in the smoke house where the temperature is maintained between 400c and 600c. In the smoke house, sheets are dried gradually whereby blisters are avoided. It is preferable to smoke the sheets on the first day at a low temperature. For the subsequent days the sheets are to be dried at a higher temperature and fairly low relative humidity. Four days of smoking is generally sufficient under normal conditions but during the rainy season five to six days are required for a satisfactory drying. 4 .5 (O) Drying of rubber sheets Drying of the crumbs, pellets or granules produced in all the new processes is carried out at about 100 c. Drying time depends upon the size of the particles. Usually 4 to 8 hours are required for complete drying. The tunnel drier commonly used consists of a movable tray fitted under a stationary hood which contains an air circulating duct fan and heat exchanger. 4 .5 (P) Grading The completely dried sheets are shifted to the packing shed where they are carefully inspected and graded according to the standards as per IS-15361-2003. This standard provides for six grades of rubber smoked sheets, viz RSS IX, RSS 1, RSS 2, RSS 3, RSS 4 and RSS 5. 4 .5 (Q) Packing The sheets after grading are packed in bales of 50kg. In the international market a bale weight is usually III-II kg. The grades are marked on the bales and marketed by BIS. 107
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