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Rubber, Natural 387

G. Softened or Peptized Rubber J. New Rubber Processes
Softened or peptized rubber is prepared by adding a small While the standard methods of preparing and packing nat-
quantity of a softening agent or peptizer to latex, which ural rubber have worked well in the past, the synthetic
is then coagulated and made into sheet or crepe. It has rubber industry by 1955 had set high standards of presen-
the advantage that it can be easily broken down to a suit- tation of synthetic rubber in small, plastic film-wrapped
able plasticity as a first step in the manufacture of rubber bales, prepared and sold to technical specifications.
articles. Dr. L. Bateman began to alert the industry to the weak-
nesses and backwardness of the visual grading system.
By 1965, Malaysian research groups were ready to unveil
H. Superior Processing Rubbers Standard Malaysian Rubber (SMR) (Table VI).

Various types of superior processing rubbers are available, Other producing countries, such as Indonesia,
for example, SP smoked sheet, SP crepe, SP air-dried Singapore, Thailand, and Sri Lanka have similar versions
sheet, SP heveacrumb, SP brown crepe, PA 80, and PA of standard technically specified rubbers. Latex concen-
57. The first four mentioned are made by mixing 20% by trate has been made to technical specifications for many
weight of vulcanized latex with 80% by weight of un- years.
vulcanized latex. The mixtures are coagulated and the SMR technical specifications were first developed in
coagula processed and dried in the normal manner. SP 1965. Current technical specifications were revised in
brown crepe is made by first coagulating a mixture of 1970 (Table VII).
80% vulcanized and 20% unvulcanized latex; one part of
the resultant wet crumb is mixed with three parts of wet 1. SMR-5L is the same as SMR-5, except the former is
scrap on power mills and processed as estate thin brown extra light in color.
crepe. 2. SMR-EQ is also available as a special, extra-pure
PA 80, a concentrated form of SP rubber, is made by grade, above SMR-5L.
3. SMR-5CV is a constant viscosity type.
drying the coagulum of 80% vulcanized and 20% unvul-
canized rubber. The dry crumb is finally pressed into a
Normally, all natural storage hardens with time. This may
block. PA 57, another concentrated form of SP rubber, is
be at least 10–20 Mooney units in a few months. Since
made by drying the coagulum produced from a mixture of
70 parts of latex consisting of 80% vulcanized and 20% such stiff rubber requires premastication prior to factory
unvulcanized latex rubber together with 30 parts of a non- use, the viscosity stabilized form of SMR-5 was devel-
staining processing oil. The oil is added to give an easier oped. By treatment of latex with as little as a few tenths
processing concentrated SP rubber. of 1% of a monofunctional amine, such as hydroxylamine
SP rubbers must conform to the technical specification (hydrochloride salt), aldehyde sites along the rubber chain
of swell on compound extrusion and Mooney viscosity be-
fore they may be sold. Their special value lies in improved
TABLE VI Annual Tonnage
extrusion and calendering properties.
Standard Malaysian Rubber
Year Metric tons
I. Technically Classified Rubber
1966 9,000
Technically classified rubber, TC rubber, is supplied in
1967 24,000
three classes marked with a blue, yellow, or red circle,
1968 83,000
respectively. The rate of vulcanization of samples of a
1969 140,000
consignment is measured in an ACS 1 test compound prior
1970 228,000
to dispatch. Slow-curing rubbers are marked red, rubbers
1971 319,000
with a medium rate of cure yellow, and fast-curing rubbers
1972 280,000
blue. In the ACS 1 test compound with additional stearic
1973 364,000
acid, and in carbon black compounds, these differences are
1974 405,000
diminished and natural rubber is then of high uniformity
1975 433,000
regarding rate of cure. TC rubber reduces the variability
1977 542,000
in rate of cure in gum-type compounds and eliminates the
1979 576,000
extremes of fast-curing and slow-curing rubbers. It can be
1985 820,000
made available in any grade but present output is mainly
2000 1,100,000
confined to No. 1 RSS.