Page 320 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GQQ/GLT P2: GRB Final Pages
Encyclopedia of Physical Science and Technology EN014B-670 July 28, 2001 16:50

386 Rubber, Natural

commodity on a trading exchange. These grades are seg- rubber sheet is coated with a very light slurry of soapstone,
regated into eight types, depending on the preparation and talc, or whiting in water, preventing the bales from sticking
the raw scrap used. Each type is subdivided into grades in transit.
based on quality. In the past, quality has mainly been deter-
mined by visual examination. Of special concern are dirt,
IV. MODIFIED RUBBER
cleanliness (absence of foreign substances such as bark), AND DERIVATIVES
color, freedom from blemishes, and general uniformity.
The above types and grades are officially endorsed by
A. Anticrystallizing Rubber
organizations from consuming and producing countries.
In the United States, this means the Rubber Trade Asso- This form of natural rubber, isomerized by chemical treat-
ciation of New York and the Rubber Manufacturers’ As- ment, is used in low-temperature applications.
sociation of Washington, D.C. An official manual, titled
“International Standards of Quality and Packing for Natu-
B. Cyclized Rubber Masterbatch
ral Rubber Grades,” describes these rubbers. The man-
ual is commonly called “The Green Book.” The most This rubber is prepared by heating stabilized latex with
recent edition was approved at the Fourth International strong sulfuric acid, mixing with untreated latex contain-
Rubber Quality and Packing Conference (IRQPC) held in ing an equal quantity of rubber, and coagulating. The co-
Brussels, Belgium, in June 1968. agulum is washed, machined, and dried in the usual way.
Cyclized rubber masterbatch is useful in the preparation
of stiff vulcanizates.
B. Specifications
Natural rubber is a rather uniform material leaving the tree.
C. Heveaplus MG Rubber
It is the subsequent handling and processing that introduce
variability. Of the 35 official grades, there are really only These rubbers are made by polymerizing methyl
about eight major types. These are summarized in a visual methacrylate monomer in situ in latex so that polymer
grading scheme. chains are attached to the rubber molecule. The resultant
latex is coagulated and the coagulum made into a crepe.
Two products are available, MG 30 containing 30%
Type Source Grades methyl methacrylate, and MG 49 containing 49%. Their
special value is in adhesives for bonding rubber to
Ribbed smoked sheet Field latex 1×,1,2,3,4,5
plastics.
Pale crepe Field latex 1×,1,2,3
(thick and thin)
Estate brown crepe High grade 1×,2×,3× D. Partially Purified Crepe (PP Crepe)
natural coagulum (thick and thin)
This rubber contains less than half the normal amount of
Compo crepe Coagulum, tree scrap 1, 2, 3
protein and mineral matter present in pale crepe and is
Thin brown crepe Natural coagulum, 1, 2, 3, 4
unsmoked sheets prepared from latex that has been centrifuged to remove
some of the naturally occurring nonrubber substances.
Thick blanket crepe Natural coagulum, 2, 3, 4
unsmoked sheets
Flat bark crepe Natural coagulum, Standard and hard E. Rubber Powder
earth scrap
Pure smoked Remilled RSS There are several types of rubber powder that are made in
or cuttings different ways. They are usually in the form of granules
Blanket crepe about 1 in. in diameter. They may be slightly vulcanized
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and contain appreciable quantities of dusting powder to
C. Packing and Shipping prevent massing on storage.
Natural rubber was originally shipped in wooden cases,
F. Skim Rubber
similar to tea. Unfortunately, wood splinters often became
imbedded in the rubber. About 1930, burlap or jute bags When latex is concentrated by centrifuging, the by-
were used, but again the fibers were troublesome. The product skim latex is coagulated and made into smoked
standard practice finally became ”bareback” baling. Rub- sheet, thick crepe, or granulated rubber. It contains a
ber sheets are piled together and squeezed with a hydraulic higher proportion of nonrubbers than ordinary sheet or
3
press into a 5 ft bale, weighing 250 lb. The outer wrap of crepe and is rapid-curing.