Raw natural rubber (NR) is processed into two physical forms: the dry rubber and the liquid rubber (Latex concentrate). The two main grades of dry rubber are sheet and crumb rubber. crumb rubber accounts for over 90% of Nigeria’s total production.
Processing of Latex Grade Crumb Rubber
The processing of latex grade NR can be regarded as a 6 – stage operation. This consists of bulking, coagulation, size reduction, drying, bailing and packaging/pelletizing
Bulking/Chemical Treatment: This is the first stage in the processing of latex grade NR. The field latex is strained through a 60 mesh sieve into the bulking tank. Here the latex is homogenized and its Dry Rubber Content (DRC) measured. At this stage antioxidants such as sodium metabisulphite (which prevents enzymic darkening) is added at the level of 0.04% w/w on DRC and 10% w/w aqeous solution.
Coagulation: Coagulation is carried out in aluminium tanks or coagulation pits using acetic acid.
Size Reduction: The final size reduction is effected by acreeper/hammer mill after the coagulum has been passed successively through crusher, macerator and creeper units.
Drying: There are a variety of driers. These driers are equipped with thermostatic control facilities. The rubber crumbs are placed in drying boxes and fed into the dryer maintained at 100 – 1100C.Drying period is about 3 hours.
Baling: Dry rubber crumbs from the drier are cooled to at least 500C before baling using a hydraulic press operating at pressures of about 1.500 psi (105 kgf/cm2). Each bale weighs 331/3 kg and has a nominal size of 680mm x 340mm x 150mm.
Packaging/Pelletization: Each bale is wrapped in a transparent polythene film of minimum thickness 0.03mm. To obviate the need for unwrapping film before use, most tyre manufacturers specify the use of dispersible polythene films of melting point 1150C maximum and specific gravity 0.92 maximum. The grade and company name are to be written on the wrapper or on the colour code marking.
Thirty (30) bales are packed into wooden pallets measuring about 110mm x 1425mm x 915mm to give a lot of rubber with a net weight of 1.0 – 1.2 metric tons. A second type of packaging technique in current use is the shrink-wrapped unit; the essential components being the base pallet, a unit forming box or mould and shrink wrapping equipment.
Processing of Coagula Grade Crumb Rubber
Processing of latex grade Crumb Rubber involves:
- bulking,
- coagulation,
- size reduction,
- drying,
- bailing
- packaging/pelletizing
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