Tires
1. 70% of all synthetic elastomers used in tires in U.S. Over 260 million tires made per year.
2. Four basic parts to tire:
a. The tread, which grips the road.
b. The sidewall, which protects the side of the tire.
c. The liner, which prevents air loss.
d. The carcass, which holds the layers together.
3. 50% of tire is rubber by weight. Carbon (reinforcing agent) and tire cord in carcass (glass, nylon, polyester, or steel) make up rest of weight.
Tire Parts
1. Tread must have good "grip" to the road. Grip inversely related to elasticity, so SBR favored over natural.
a. Auto tires 3:1 SBR:polybutadiene.
b. Truck tire treads need less grip and must avoid heat buildup, so are 65-100% natural rubber. Aircraft tires are 100% natural rubber.
2. Carcass requires flexing properties, at least 60% natural rubber with SBR.
3. Sidewalls have only 0-50% natural rubber.
4. Liner made of butyl rubber because of its impermeability to air.
Types of Tires
Cross-Ply, Belted Bias-Ply, Radial-Ply
1. Change in market:
1972 radials held only 8% of tire market
1977 radials grew to 50%
1992 radials had 89% of market
2. Carcass contains "plies" of rubberized fabric.
3. In cross-ply cords cross tire at an angle.
4. In belted bias-ply cords cross tire at an angle and additional belt of fabric placed between plies and tread.
5. In radial-ply cords run straight across tire and additional belt of fabric included.
6. Radials have better wear and road-holding ability. Radials more easily damaged on side and less comfortable ride.
Vulcanization
1. Discovered by Charles Goodyear in 1839 by heating with sulfur (S8).
2. Other chemicals used in the vulcanization process:
a. Benzothiazoles and sulfenamides as accelerators.
b. ZnO and fatty acids (R-COOH) for complexation.
3. If S < 5%, elastic. If S > 5%, ebonite.
4. 100-160C typical vulcanization temperature.
Reinforcing Agents
1. 1912 Diamond Rubber Co. found that carbon strengthens the mechanical properties of rubber.
2. Carbon reinforced rubber tires last 10x as long.
3. Glass, polyester, nylon, and esp. steel also used.
4. 20% reinforcing agent increases tensile strength 40%.
Antidegradants
1. To prevent degradation by heat, oxygen, ozone, UV, and moisture.
2. Prevents brittleness, stickiness, cracking, etc.
3. Over 150 million lb of antidegradants per year.
4. Most are amines, phenols, phosphites, or sulfur compounds.
Development of Synthetic Rubber
1. 1910 structure of natural rubber found to be cis-1,4-polyisoprene.
2. Until 1930 only rubber from tropical rubber tree.
3. 1931 Du Pont introduced polychloroprene and Thiokol Corp. marketed polysulfide. But unsatisfactory for tires.
4. 1937 polyisobutylene marketed for tire innertubes.
5. WWII influenced speedy development of natural rubber substitutes.
6. 1940s Government Rubber Reserve Co. developed styrene-butadiene rubber (SBR) for tires.
7. Mid-1950s introduced stereoregular elastomers by Ziegler-Natta catalysis.
SBR vs. Natural Rubber
1. SBR is 40% of U.S. synthetic rubber market, 26% of total rubber market. U.S. production = 1.88 billion lb in 1990.
2. In 1940 natural rubber had 99.6% of U.S. market. In 1950 synthetics passed natural rubber. In 1990 natural rubber had 30% of market. U.S. imports were 2 billion/lb natural rubber in 1990.
3. Natural rubber making a comeback:
a. Increased agricultural yields.
b. Petrochemical shortages for SBR.
c. Trend toward radial-ply tires.