Production, classification and properties of NR

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

The natural rubber processing factories obtain the raw material collected from trees either in large plantations or from smaller independent holdings, in one of two forms, field coagula or field latex (figure 4). Field coagula consist of cup lump from the collection cups and tree and cup lace obtained, for example, from the rim of the cup (table 1). The lower grades of material are prepared from cup lump, partially dried small holder's rubber, rubber tree lace and earth scrap after cleaning. Iron-free water is necessary to prevent polymer oxidation. Field coagula and latex are the base raw materials for the broad range of natural grades to be described in this review.

[FIGURE 4 OMITTED]

Fresh Hevea latex has a pH ranging from 6.5 to 7.0 and density of 0.98 (refs. 8 and 9). The traditional preservative is ammonia (in concentrated aqueous solution), which is added in small quantities to the latex when collected from the cup. Tetramethylthiuram disulfide (TMTD) and zinc oxide are also used as a preservative due to their greater effectiveness as a bactericide. Most latex concentrates are produced to meet the International Standard ISO 2004 (ref. 13). This standard defines the minimum content for total solids, dry rubber content, non-rubber solids and alkalinity (as N[H.sub.3]).

Natural rubber products and grades

Natural rubber is available in six basic forms: Sheets; crepes; block rubber, technically specified; technical specification for sheet rubber; preserved latex concentrates; and specialty rubbers which have been mechanically or chemically modified.

Among these six types, the first three represent nearly 90% of the total natural rubber produced in the world. In the commercial market, these three types of dry natural rubber are available in over 40 grades consisting of ribbed smoked sheets, air dried sheets, crepes (which include latex-based, and field coagulum-derived estate brown crepes), remilled crepes and technically specified rubber in block form. Among the three major types, crepes are now of minor significance in the world market, accounting for less than 75,000 tons per year. Field coagulum grade block rubbers have essentially replaced brown crepes, except in India. Only Sri Lanka and India continue to produce latex crepes. Figure 4 presents a simplified schematic of the process followed in the production of natural rubber (ref. 3).

Sheet rubber

From the end-user's perspective, two types of sheet rubbers are produced for the commercial market, namely ribbed smoked sheets (RSS) and air dried sheets (ADS). Among these two types, ribbed smoked sheet is the most popular.

Ribbed smoked sheet rubbers are made from intentionally coagulated whole field latex. They are classified by a visual evaluation. To establish acceptable grades for commercial purposes, the International Rubber Quality and Packing Conference has prepared a description for grading, with details given in the Green Book (ref. 15). Whole field latex used to produce ribbed smoked sheet is first diluted to 15% solids, and then coagulated for around 16 hours with dilute formic acid. The coagulated material is then milled, water removed and sheeted with a rough surface to facilitate drying. Sheets are then suspended on poles for drying in a smokehouse for one to seven days. Only deliberately coagulated rubber latex processed into rubber sheets, properly dried and smoked can be used in making RSS. A number of prohibitions are also applicable to the RSS grades. Wet, bleached, undercured and original rubber and rubber that is not completely visually dry at the time of buyer's inspection are not acceptable (except slightly under-cured rubber as specified for RSS5). Skim rubber made of skim latex cannot be used in whole or in part in the patches as required under packing specifications. Prior to grading RSS, the sheets are separated, inspected and any blemishes are removed by manually cutting and removing defective material.