Rubber and Elastomers
Elastomers are polymers that can be stretched to several times their original length and then snap back to their original shape when the force is released. This extraordinary elastic behavior — unmatched by metals, ceramics, or other polymer classes — arises from a unique combination of molecular architecture: long, flexible, coiled chains connected by a sparse network of crosslinks. Rubber, the most familiar elastomer, has been a transformative material since the 19th century.
Natural Rubber
Natural rubber is harvested as latex, a milky suspension of polymer particles tapped from the bark of the rubber tree (Hevea brasiliensis), primarily grown in Southeast Asia. The polymer is cis-1,4-polyisoprene, a chain of isoprene (C₅H₈) units in which the cis configuration around each double bond forces the chain into a coiled, irregular shape that resists crystallization at room temperature.
Raw natural rubber has serious limitations: it becomes sticky in hot weather, brittle in cold weather, and dissolves in hydrocarbon solvents. These problems made rubber products unreliable until Charles Goodyear accidentally discovered vulcanization in 1839.
Vulcanization
Vulcanization is the process of crosslinking rubber chains with sulfur. When natural rubber is heated with 1-5% sulfur at 140-160 degC, sulfur atoms form short bridges (-S-S- or -S-Sx-S-) between adjacent polymer chains. These crosslinks create a three-dimensional network that:
- Prevents chains from sliding permanently past one another (eliminating stickiness)
- Maintains elasticity over a wide temperature range (-40 to 100 degC)
- Dramatically improves mechanical strength and abrasion resistance
- Prevents dissolution in solvents (the network can swell but not dissolve)
The degree of vulcanization controls the material's properties. Lightly vulcanized rubber (2-3% sulfur) is soft and highly elastic, suitable for rubber bands and gloves. Heavily vulcanized rubber (25-30% sulfur) becomes ebonite (hard rubber), a rigid material once used for bowling balls and fountain pens.
Modern vulcanization uses accelerators (organic compounds that speed the reaction and reduce the sulfur needed) and activators (zinc oxide and stearic acid) to achieve precise crosslink density in minutes rather than hours.
Synthetic Rubbers
The strategic importance of rubber — especially during World War II, when Japan cut off Southeast Asian supplies — drove intensive research into synthetic alternatives. Today, synthetic rubbers account for roughly 60% of global rubber consumption (about 15 million tons per year).
Styrene-Butadiene Rubber (SBR) is the most widely produced synthetic rubber, made by copolymerizing styrene (~25%) with 1,3-butadiene (~75%). SBR offers good abrasion resistance and is the primary rubber in passenger car tires. It lacks the resilience and low heat buildup of natural rubber, making it less suitable for truck and aircraft tires.
Polybutadiene Rubber (BR) is second in production volume. Its high resilience and low-temperature flexibility make it an essential component in tire treads (blended with SBR or natural rubber) and the cores of golf balls.
Neoprene (Polychloroprene), invented by DuPont in 1931, was the first successful synthetic rubber. The chlorine atom on the polymer backbone provides excellent resistance to oil, ozone, and weathering. Neoprene is used in wetsuits, automotive belts, gaskets, and electrical cable jackets.
Nitrile Rubber (NBR) is a copolymer of acrylonitrile and butadiene. The polar nitrile groups provide outstanding resistance to oils and fuels, making NBR the standard material for disposable examination gloves, fuel hoses, and O-rings.
EPDM (Ethylene-Propylene-Diene Monomer) rubber resists ozone, UV light, and weathering exceptionally well. It is the dominant rubber for automotive seals, roofing membranes, and garden hoses.
The Molecular Basis of Elasticity
Elasticity in rubbers arises from entropy. At rest, polymer chains adopt random, coiled conformations that maximize entropy. Stretching the rubber forces chains into extended, ordered arrangements — a low-entropy state. When the stretching force is removed, the chains spontaneously recoil to their disordered, high-entropy state, just as a compressed spring returns to its relaxed length.
Crosslinks are essential: without them, the chains would simply slide past one another during stretching and would not recover their shape. With too many crosslinks, chain segments cannot uncoil, and the material becomes rigid rather than elastic. The optimal crosslink density for rubber is roughly one crosslink per 100-200 monomer units.
Rubber in Tires
The tire industry consumes roughly 70% of all rubber produced worldwide. A modern radial tire is an engineering marvel containing multiple types of rubber:
- Tread — SBR/BR/natural rubber blend for grip and abrasion resistance
- Sidewall — natural rubber for flex fatigue resistance
- Inner liner — butyl rubber (polyisobutylene-co-isoprene) for air impermeability
- Bead filler — hard rubber compound to anchor the tire to the rim
A single large truck tire contains about 32 kg of rubber, 4.5 kg of carbon black (reinforcing filler), 0.5 kg of sulfur, and 5.5 kg of steel wire.
Applications Beyond Tires
Elastomers serve critical roles across industries: vibration dampers in machinery, seals and gaskets in plumbing and engines, flexible tubing in medical devices, shock-absorbing soles in athletic shoes, and earthquake-resistant bearings under bridges and buildings. In every case, the fundamental requirement is the same — large, reversible deformation.