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Rubber Specification
Other Types and
Grades of Dry Natural Rubber
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The types and grades of natural rubber described in Part II cover the full range of grades for which international visual standards have been established. The rubbers described in Part IV A cover general purpose types sold to a technical specification. There are in addition certain special purpose rubbers available of which the following are general descriptions. It is recommended that for these "other grades" the seller should furnish a specific sample or specification, and that the buyer should carefully ascertain the quality of the delivery against such specific sample or specification. Air Dried Sheet (Pale Amber Unsmoked Sheet) This is a light colored sheet prepared under closely
controlled conditions in the same way as Ribbed Smoke Sheet but dried by air
usually in a shed or tunnel without smoke or additives other than those
generally accepted, such as sodium bisulphate and paranitrophenol. Anticrystallizing Rubber This form of natural rubber isomerised by chemical treatment is used in low temperature applications and is available on special request. Cyclized Rubber Masterbatch This rubber is prepared by heating stabilized latex with strong sulphuric acid, mixing with untreated latex containing an equal quantity of rubber, and coagulating. This coagulum is washed, machined and dried in the usual way. Cyclized rubber masterbatch is useful in the preparation of stiff vulcanizates. Stocks are normally limited but quantities can be supplied to special order. Heveaplus MG Rubber These rubbers are made by polymerizing methyl methacrylate monomer in situ in latex so that polymer chains are attached to the rubber molecule. The resultant latex is coagulated and the coagulum made into a crepe. Two products are available against specific orders, MG 30 containing 30% of methyl methacrylate, and MG 49 containing 49%. Their special value is in adhesives for bonding rubber to plastics. Partially Purified Crepe (PP Crepe) This rubber contains less than half the normal amount of protein and mineral matter present in pale crepe and is prepared from latex which has been centrifuged to remove some of the naturally occurring non-rubber substances. Rubber Powder There are several types of rubber powder which are made in different ways. They are usually in the form of granules about 1/32 inches (0.0794 mm) in diameter. They may be slightly vulcanized and contain appreciable quantities of dusting powder to prevent massing on storage. Skim Rubber When latex is concentrated by centrifuging, the byproduct skim latex is coagulated and made into smoke sheet, thick crepe or granulated rubber. It contains a higher proportion of non-rubbers than ordinary sheet or crepe and is rapid curing. Softened or Peptized Rubber This rubber is prepared by adding a small quantity of a softening agent or peptizer or latex which is then coagulated and made into sheet or crepe. It has the advantage that it can be easily broken down to a suitable plasticity as a first step in the manufacture of rubber articles. Superior Processing Rubbers Various types of superior processing rubbers are available; e.g. Smoked Sheet, SP Crepe, SP Air Dried Sheet, SP Heveacrumb, SP Brown Crepe, PA 80 and PA 57. The first four mentioned are made by mixing 20% by weight of vulcanized latex with 80% by weight of unvulcanized latex. The mixtures are coagulated and the coagula processed and dried in the normal manner. SP Brown Crepe is made by first coagulating a mixture of 80% vulcanized and 20% unvulcanized latex; one part of the resultant wet crumb is mixed with three parts of wet scrap on power mills and processed as Estate Thin Brown Crepe. PA 80, a concentrated form of PA rubber, is made by drying the coagulum of 80% vulcanized and 20% unvulcanized rubber. The dry crumb is finally pressed into a block. PA 57, another concentrated form of SP rubber, is made by drying the coagulum produced from a mixture of 70 parts of latex consisting of 80% vulcanized and 20% unvulcanized latex rubber together with 30 parts of a non-staining processing oil. The oil is added to give an easier processing concentrated SP rubber. SP rubbers must conform to the technical specifications of swell on compound extrusion and Mooney viscosity before they may be sold. Their special value lies in improved extrusion and calendaring properties. Technically Classified Rubber Technically classified rubber (T.C. rubber) is supplied in three classes marked respectively with a blue, yellow, or red circle. The rate of vulcanization of samples of a consignment is measure in an ACS 1 test compound prior to dispatch. Slow curing rubbers are marked - red, rubbers with a medium rate of cure - yellow, and fast curing rubbers - blue. In the ACS 1 test compound with additional stearic acid, and in carbon black compounds, these differences are diminished and natural rubber is then of high uniformity as regards rate of cure. T.C. rubber reduces the variability in rate of cure in gum type compounds and eliminates the extremes of fast curing and slow curing rubbers. It can be made available in any grade but present output is mainly confined to No. 1 RSS. Informational Addresses Technical particulars and the names of producers or sellers of these grades described in Parts IV A and IV B and be obtained from the following Natural Rubber Research Institutes, organizational and/or Government offices:
Malaysian Rubber Bureau 4th Floor, Woodlands House 5-7 Hall Street, Moonee Ponds Victoria 3039, Australia Austria Republic of Cameroons England The Malaysian Rubber Producers' Research Association France Germany Malaysian Rubber Bureau India Italy Ivory Coast Japan Liberia Malaysia The Rubber Research Institute of Malaysia The Netherlands New Zealand Nigeria Philippines Spain Singapore Sri Lanka (Ceylon) United States Malaysian Rubber Bureau Malaysian Rubber Bureau
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