Production, classification and properties of NR

Rubber World, August, 2005 by M. Brendan Rodgers, Donald S. Tracey, Walter H. Waddell

* Pure smoked blanket crepe. This grade is made by milling (on power wash mills) smoked rubber derived from ribbed smoked sheet (including block sheets), or ribbed smoked sheet cuttings. No other type of rubber can be used. Rubber of this type must be dry, clean, firm, tough, and also must retain an easily detectable smoked sheet odor. Sludge, oil spots, heat spots, sand, dirty packing and foreign matter are not permissible. Color variation from brown to very dark brown is permissible (table 5).

Technically specified natural rubber

The International Standards Organization (ISO) first published a technical specification (ISO 2000) for natural rubber in 1964 (ref. 16). Based on these specifications, Malaysia introduced a national Standard Malaysian Rubber (SMR) scheme in 1965, and since then, all the natural rubber-producing countries started production and marketing of technically specified rubbers based on the ISO 2000 scheme. Technically specified rubbers are shipped in 'blocks' which are generally 33.3 kg bales in the international market and 25.0 kg in India. All block rubbers are also guaranteed to conform to certain technical specifications as defined by the national schemes of by ISO 2000 (table 6).

The nomenclature describing technically specified rubbers consists of a three or four letter country code, followed by a numeral indicating the maximum permissible dirt content for that grade expressed as a hundredth's of one percent. In Malaysia, the TSR is designated as Standard Malaysian Rubber (SMR). In Indonesia, the designation given is Standard Indonesian Rubber (SIR). In Thailand, the TSRs are called Standard Thai Rubber (STR, and sometimes denoted as TTR). In India, the TSRs are designated as Indian Standard Natural Rubber (ISNR) (ref. 4). Grading is based on the dirt content measured as a weight percent. Dirt is considered to be the residue remaining when the rubber is dissolved in a solvent, washed through a 45-micrometer sieve and dried.

Technically specified rubber (TSR) accounts for approximately 60% of the natural rubber produced. The International Standards Organization has specified six grades of technically specified rubber.

TSR-CV

TSR-CV, the CV designating constant viscosity, is produced from field latex and is viscosity stabilized to a specified Mooney viscosity. The storage hardening of this grade of rubber is also to be within eight hardness units. It is shipped in a 1.2-metric ton pallet, which facilitates handling, transportation and storage space utilization. Each pallet consists of 36 bales of 33.3 kg net weight, and each bale is wrapped in a polyethylene bag which is dispersible and compatible with rubber when mixed in an internal mixer at temperatures exceeding 110[degrees]C, which would be typical in most rubber mixing facilities. TSR-CV rubbers are generally softer than conventional technically specified grades. Coupled with its constant viscosity feature, it can provide a cost advantage in eliminating pre-mastication. When used in open mills, the rubber forms a coherent band almost instantaneously, thus potentially improving milling equipment throughput. Additional claimed benefits of TSR-CV include: