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Rubber, Natural 391

A. Test Recipes TABLE XI Product Usage of Natural Rubber a
The Crude Rubber Committee of the Rubber Division of Product Percentage
the American Chemical Society has recommended test
recipes. These are used by the consumer as standard ref- Tires and tire products 68.0
erence compounds. The ACS-I test recipe is as follows: Mechanical goods 13.5
Latex products 10.0
Footwear 3.0
Natural rubber (test sample) 100. Adhesives 1.5
Zinc oxide (Amer. Proc.—low lead) 6. Miscellaneous 2.0
Stearic acid (Comm.—double pressed) 0.5 Total 100.0
Mercaptobenzothiazole (MBT, Captax) 0.5 a It is interesting to note that synthetic rubber con-
Sulfur (rubber makers’ grade) 3.5 sumption is also distributed among products in much the
Total 110.5 same percentages.
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Cure: 20, 30, 40, 60, 80 min at 127 C (260 F)
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Tests: Modulus at 500, 600, and 700% elongation; B. Distribution by Product
Tensile strength, breaking elongation on all cures
The percentage distribution of natural rubber usage by
product category has remained remarkably steady for the
Some rubbers tend to be slower than normal in cure rate,
past 20 years (Table XI).
usually because of insufficient naturally occurring activa-
tors. In such cases, the same Crude Rubber Committee C. Latex Products
recommended a test recipe with higher fatty acid content
(ACS-II test recipe): Natural rubber latex is often used in blends with vari-
ous synthetic rubber latices. This is because of conferring
properties of superior wet gel strength, tear resistance,
Natural rubber 100. strength, and wet tack. Product uses are in foam back-
Zinc oxide 6. ing for carpets, foam for bedding and upholstery, dipped
Stearic acid 4. goods, surgical goods, gloves, drug sundries, adhesives,
Mercaptobenzothiazole 0.5 and thread. Compounding is done by the addition of aque-
Sulfur 3.5 ous dispersions of rubber chemicals.
Total 114. Recently, latex allergy sensitivity has been claimed,
Cure: 5, 10, 15, 20, 30, 45, 65, 100, especially with latex gloves. Latex containing high pro-
◦
◦
150, 225 min at 141 C (286 F) tein levels are particularly targeted. In addition, powdered
gloves have been reported to provide an aerosol to carry
Pure gum vulcanizates exhibit erratic test results. In the allergens. Extra safety has been provided by double-
addition, MBT is no longer the most common cure accel- centrifuging the field latex and also by enzyme treatment
erator for natural rubber. Also, there is some merit in using to destroy 95% of the proteins.
carbon-black-loaded vulcanizates, to accommodate syn-
thetic rubber comparisons. For these reasons, the Crude D. Tire Products
Rubber Subcommittee of D-11, ASTM has recommended
Asageneralruleofthumb,thelargerthetire,thehigherthe
the following test recipe (ASTM test recipe 1-I):
percentage content of natural rubber. In passenger tires,
natural rubber is primarily used in the carcass, for hot
strength properties, low heat build-up in service, supe-
Natural rubber (or cis-polyisoprene) 100.
rior building tack, and better ply adhesion. In radial tires,
Zinc oxide 5.
much higher amounts of natural rubber are used, espe-
Stearic acid 2.
cially for essential green stock strength during the forming
N-tert-Butyl-2-benzothiazole sulfenamide 0.7
and building operations.
Oil furnace black 35.
It is the commercial vehicle in which the majority of
Sulfur 2.25
natural rubber is consumed. This includes aircraft, truck,
Total 144.95
tractor, and any off-the-road application. These use natural
rubber in the tread and sidewall, as well as the carcass. Re-
Depending upon the ultimate product usage, either or both sistance to blowout, tread groove cracking, and rib tearing
the pure gum and black loaded recipe may be tested. are the outstanding features.