Freshly tapped latex and field coagulum can be made into various rubber categories by different processing methods. Traditionally, NR has been made into sheets whose quality was visually inspected. In addition to sheets, NR has also been traded as crepes (similarly graded on a visual basis). The traditional methods of processing into sheet and crepe rubbers are still widely used, especially by smallholdings. Block rubber (graded by technical tests) was first introduced by Malaysia in the mid-1960s. Further technical developments included new processing techniques for latex concentrates with special applications.
For more detailed information on NR processing, please refer to the articles listed under the headings "Processing and Treatment", "Compounding Ingredients", "Properties and Testing" in the relevant section of the Rubber-Stichting site (select "Articles from Natuurrubber" on the left menu).
Ribbed smoked sheets (RSS) production from latex occurs in either factory-scale facilities (on estates and larger smallholdings) or in tiny field units (individual smallholdings). Despite its variability in scale, it always includes the following operations: blending, coagulation, milling, drying, and finishing. The layout for RSS production (factory-scale processing) is detailed in the figure below.
Ribbed smoke sheet plant
Latex from various sources is initially collected and
blended in a large latex bulding and blending tank. This blending operation
is important to ensure the production of uniform and consistent rubber.
Coagulation is effected by the addition of coagulants, such as formic
or acetic acid. In factory-scale processing it occurs in coagulation
troughs, where latex is first diluted with water. Individual smallholdings
blend and coagulate their latex in 4-5 litre pans. Milling involves
feeding slabs of coagulum to successive pairs of rollers ("sheeting
battery") to produce sheets of uniform thickness. Appropriately
grooved rollers imprint on each sheet "ribs", which expand
the surface area for drying. After sizing, the sheets are hung on lines
and dried in smokehouse at successively higher temperature ("ribbed
smoked sheets"). On individual smallholdings, sheets are often
dried by natural ventilation ("white unsmoked sheets") and
sold to a dealer who smokes and grades them. Blemishes are then removed
manually and sheets are visually graded. They are finally pressed into
bales with talc to prevent adhesion.
The process of making sheet rubber is relatively simple and it is still commonly used on smallholdings and smaller estates.
Rubber in crepe form is processed from either latex (pale and sole crepe) or cup lumps, scrap and poor quality sheet rubber (remilled crepes). The traditional method of rubber processing to produce crepe rubber is similar to that for sheet rubber. An important additional step in making crepe rubber is the removal of yellow carotenoid pigments in the latex. In addition, the latex is coagulated by "fractional" coagulation: the first fraction is the unstable, yellow fraction processed into off-colour pale crepe of relatively low grade; the remaining latex is a wither material. The coagulum formed is then washed and fed to pairs of rollers rotating at different speeds, which manufacture the rubber into thin crepes. The crepes are dried in hot drying chambers or tunnels or on drying floors.
Since the 1960s, new processes have been developed to produce technically graded rubber from latex and field coagulum. The production of block rubbers involves relatively sophisticated machineries and heavy power consumption. The basic layout of block rubber production from latex is detailed in the figure below (factory-scale processing).
Block rubber plant
In block rubber production from latex, latex arriving from different locations is first blended in a large bulking tank. Chemicals are added to control viscosity and affect colour. The latex containing the requisite chemicals is then coagulated by adding coagulants (formic acid) in a long coagulation trough. Solid latex coagulum is then processed into crumb by either physical or chemical means. In the physical case, the coagulum is first fed though the rotating rolls of a creping machine. The crepe is then mashed into small pieces through a hammer mill and finally concerted into crumb by an extruder. Alternatively, incompatible oil is added to the latex in the coagulation trough. The material is then fed though a creper and transformed into crumb. In both cases, the crumbs finally obtained are dried by hot air. Depending on the throughput, factories may use single-layer tray driers with direct firing burners, apron driers, or chamber driers. The dry rubber is finally baled (commonly by means of a hydraulic press) and wrapped in polythene to prevent adhesion between bales in the crate. When field coagulum rather than latex is being processed into block rubber, the materials involved include foreign matter and require pre-cleaning. Field coagulum materials are first forced through a macerator (to slice up large blocks) and then passed through a series of creeping rolls. There is obviously no need for coagulation.
Latex concentrates are processed by various methods, including centrifugation, evaporation, creaming, and electro-decantation.
Important steps in the manufacturing of rubber goods from latex and dry rubber include:
- Preparing (NR generally requires prior mastication and other preparatory operations before mixing with compound ingredients)
- Compounding (the mixing in the rubber inputs of materials additional to the base polymer or polymer blend and vulcanization constituents, to meet increasingly diverse requirements in rubber goods)
- Shaping (for example, the rubber mix may be heated and shaped into a mould of appropriate form, or frictioned on to a fabric)
- Vulcanisation: Vulcanization was one of the key
discoveries in the manufacture of rubber products. It involves a chemical
process whereby individual polymer molecules are linked to other polymer
molecules by atomic bridges (the actual chemical cross-linking is usually
done with sulfur, but there are other technologies, including peroxide-based
systems). Vulcanization makes the raw rubber harder, more durable and
more resistant to chemical attack.