How To Processing of Block Natural Rubber

Since the 1960s, new processes have been developed to produce technically graded rubber from latex and field coagulum.  The new processes convert wet raw rubber into a granular form by fast, continuous, flow-line techniques at the plantation facilities.  In its final form the dried crumb or granule is compacted into blocks of solid rubber.  Hence, the name block rubber applies to its new forms.

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 converted into crumb by an extruder.  Alternatively, incompatible oil is added to the latex in the coagulation trough.

Figure 30.  Flowchart of processing block rubber

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.

TECHNICAL SPECIFICATION

The introduction of Technically Classified (TC) rubber in 1965 was a first step in this inspection. With this as well as with the following schemes it was realized that, with a diversity of rubber producing units of widely varying sizes, the conditions for a successful specification scheme are for a limited number of easily applied tests, which shall guarantee a high level of purity and at the same time act to raise the general level of quality.

It was also desirable that the tests should provide some measure of raw rubber quality and also have relevance to consumers’ factory processes. With these criteria in mind Malaya introduced the Standard Malaysian Rubber (SMR) release in March 1965.  All SMR is graded by technical standards with visual grading restricted by a subdivision of the top class.

Although a prime object of the scheme was to unify the collecting of the new forms of NR, nevertheless conventional types, i.e. smoked sheet, brown and blanket crepes can all be graded this way if they are supplied in wrapped, compressed bales not exceeding 50 kg (112 lbs) in weight and meet with the technical requirements.

The 1965 SMR scheme was as follows:

1. Standard Malaysian Rubber (SMR) is natural rubber supplied from the states of Malaya and such as other parts of Malaysia as may be approved.
2. SMR is packed in compressed block form, the bales not exceeding 50 kg (112 lbs). The bale is wrapped in polythene sheet or other approved material and is free of any other bale coating.
3. SMR is graded by technical specifications, not by the conventional visual standards (as set out in the “Green Book” of the International Rubber Quality and Packing Conference).
4. The three qualities are as under: (maximum permitted content, less than: )





5. Colour forms no part of this specification. However, light coloured rubber (as certified by the Rubber Research Institute of Malaya (RRIM) – see below) falling within the SMR 5 classes, but no other, may be specifically designated as SMR 5L.  Rubber prepared directly from latex by processes similar or equivalent to that now used for producing pale ADS will normally meet this colour standard; other rubber will normally not do so.
6. Suppliers of SMR of any quality are required to register with the Malayan Rubber Exports Regulation Board (MRERB) and with the RRIM.  For registration and use of the symbol the following is required: Typical samples of proposed SMR must be submitted; The SMR specification which these intend to meet must be stated; The RRIM will then determine and specify the quality standard to which the samples conform and, when so requested, whether the 5L standard is satisfied; Arrangements for quality testing and control must be approved by the RRIM; and Baling and packing procedures must be stated.
7. Bales of SMR must be marked with the registered ESEMAR symbol of the authorized quality standards, the preparation source of the rubber declared (e.g. smoked sheet, crumb, brown crepe), the weight and the supplier’s identity. Advice notices must be similarly explicit.
8. SMR will be assessed only on the above stated standards and not by conventional International Type grading.  Complaints and disciplinary action will be considered by the MRERB, with technical advice from the RRIM, on this basis alone.
9. It is intended to introduce an oxidizability test into the SMR specifications at a later date. In the initial stages, the RRIM reserves the right to test SMR rubbers for such a property, and if found necessary, to reject any consignment showing unsatisfactory behaviour.
10. The RRIM, in consultation with MRERB, reserves the right to amend the specifications after due notice has been given.

The 1965 SMR scheme lacked the essential element of a test for the inherent quality of the rubber; however, a test for raw rubber quality was developed, both to characterize getting visual market grades and to assess the technological content of the test in relation of increasing behaviour and vulcanize performance of rubbers.  This test involves measuring in Wallace Plasticity (WP) of a prepared sample, before and after aging for 30 minutes at 140 oC, under accuracy controlled conditions, and expressing the results as an index, Plasticity Retention Index, of percentage plasticity retained.

Plasticity Retention Index (PRI) =  (WP aged / WP initial) x 100%

The test is simple and quick to operate and results are reproducible to within about results when the test is operated under controlled conditions.  From the inception of the SMR scheme the PRI test has been used to control the quality of the rubber under test in order to prevent the inclusion of undesirable material.

Posted by Teguh

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