Page 319 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Rubber, Natural 385

TABLE V Effect of Ethylene on Yield The coagulum is passed through a series of several
grooved rolls on creping machines. After about eight
Treated
passes through such creping rolls, operating at a differ-
Clone Untreated (lb/year/acre)
ential speed, and sprayed continuously with water, the
Tjir 1 1280 2480 final sheet is thin, has a creped surface, and is slightly
PB 86 1300 2920 yellow–white in color. The sheets are then dried in well-
◦
PR 107 1890 2250 ventilated sheds or hot-air tunnel driers at 40–45 C. This
GT 1 2420 3030 may take up to 6 days to dry. Pale crepe is carefully in-
RRIM 501 2170 2640 spected and packed as 75–160 lb bales in multiwall paper
RRIM 605 1400 2650 bags or wooden boxes.
RRIM 623 1760 2900
RRIM 600 2950 5490
2. Smoked Sheets
Ribbed smoked sheets (RSS) are prepared much as is pale
dramatic yield increase is immediate. Renewed applica-
crepe. However, even speed rollers are used, with a final
tion is optimum at 2-month intervals.
embossing roll to provide the characteristic ribbed surface.
Since the largest components of operating (and final)
The rib pattern is intended to provide increased surface
costs on a rubber estate are tapping and collecting of latex,
area for a faster rate of drying.
the utilization of ethylene or newer generation stimulants
The ribbed sheets are placed on racks mounted on trol-
has tremendous economic and social impact.
leys, then placed in a smoke-house, then a drying chamber.
The total cycle takes 2–4 days, with entry temperature at
40 C and exit temperature at 60 C. Most estates convert
◦
◦
III. PREPARATION
80–85% of their total crop into RSS.
Smoking is done with wood fires. The smoke provides
Collected latex upon arrival at the producing factory is
the characteristic odor of RSS. Some people feel that the
strained and blended in large tanks holding about 700 gal-
cresols, phenols, and other components of the smoke con-
lons or more. The latex is then diluted with water to about
fer antioxidant properties to the rubbers. Most probably,
15% solids (for sheets) and 20–25% (for pale crepe). It is
some antiseptic effects are gained. Indeed, if the smoke
then transferred into coagulating aluminum tanks of 200–
is eliminated, air-dried sheets result. These are becoming
400 gallons capacity. To 100 parts of diluted latex may be
more popular, with their amber or light brown color.
added 5 parts of either a 0.5% formic acid solution or a
1% acetic acid solution.
The tanks are about 16 in. deep with removable alu- 3. Brown Crepe
minum partitions. Coagulation is complete in several
Such crepes constitute the largest single type of rubber
hours, but usually is done overnight. The soft, smooth, and
made from raw material other than field latex. Since the
gelatinous slabs are then processed into sheets or crepes.
raw material is so variable, that is, cup lump, tree lace, etc.,
it must be soaked overnight in a dilute solution of sodium
A. Grades and Types bisulfite to lighten the color and to remove surface dirt.
After maceration, the slabs are sheeted and treated as with
1. Pale Crepe
pale crepe. Due to varying degrees of enzyme darkening,
Only 5–10% of total field latex is made into pale crepe. color may be quite variable and mottled. Such crepes can
This is a premium rubber, noted for lightness of color. be baled bareback (as with RSS) in 250 lb cubical bales,
As such, it goes into white sidewall tires, drug sundries, bound with rust-proof bands, or wrapped in burlap.
surgical goods, and certain domestic products.
Prior to coagulation, latex is treated with 0.5% sodium
4. Blanket Crepe (Ambers)
bisulfite to prevent darkening of the rubber by enzymes. A
bleaching agent, such as 0.1% xylyl mercaptan, may also Such rubber type is made from RSS clippings and remilled
be utilized. Another method is fractional precipitation by cup lump.
first adding only 15% of the total coagulating solution,
which coagulates about 10% of the latex. The latter is
5. Standard Grades
skimmed off and contains most of the natural occurring
protein and β-carotene, the yellow coloring pigment in There are 35 grades of dry natural rubber sold on the mar-
natural rubber. This produces sole crepe. ket. The market operates as with any other agricultural