Stabilizer

Any substance that tends to maintain the physical and chemical properties of a material. Degradation, that is, irreversible changes in chemical composition or structure, is responsible for the premature failure of materials. Stabilizers are used to extend the useful life of materials as well as to maintain their critical properties above the design specifications. Oxygen and water are the principal degradants, but ultraviolet radiation also can have a significant effect (photodegradation).

A wide variety of additives has been developed to stabilize polymers against degradation. Stabilizers are available that inhibit thermal oxidation, burning, photodegradation, and ozone deterioration of elastomers. Research in the chemistry of the low-temperature oxidation of natural rubber has revealed that hydrocarbon polymers oxidize by a free-radical chain mechanism. In pure, low-molecular-weight hydrocarbons, an added initiator is required to produce the first radicals. In contrast, initiation of polymer oxidation occurs in the complex molecules of elastomers through impurities already present, for example, hydroperoxides. See also Chain reaction (chemistry); Free radical.

Stabilization of hydrocarbon polymers can be accomplished with preventative or chain-breaking antioxidants. Preventative antioxidants stabilize by reducing the number of radicals formed in the initiation stage. Where hydroperoxides are responsible for initiation, the induced decomposition of these reactive intermediates into nonradical products provides effective stabilization. Suppressing the catalytic effects of metallic impurities that increase the rate of radical formation can also provide stabilization. Chain-breaking antioxidants interrupt the oxidative chain by providing labile hydrogens to compete with the polymer in reaction with the propagating radicals. The by-product of this reaction is a radical which is not capable of continuing the oxidative chain.

Stabilization of polymers against photooxidation, the principal component of outdoor weathering, is accomplished by addition of ultraviolet absorbers, and radical scavengers. Ultraviolet absorbers absorb and harmlessly dissipate damaging radiation. Another class of additives, known as hindered-amine light stabilizers, function by scavenging destructive radicals. See also Antioxidant; Inhibitor (chemistry); Photodegradation.

For other kinds of stabilizers, see Stabilizer.

In chemistry a stabilizer is a chemical which tends to inhibit the reaction between two or more other chemicals. It can be thought of as the antonym to a catalyst. It can be also a chemical that inhibits separation of suspensions, emulsions, and foams.

Some kinds of stabilizers are:

• antioxidants, preventing unwanted oxidation of materials
• sequestrants, forming chelate complexes and inactivating traces of metal ions that would otherwise act as catalysts
• emulsifiers and surfactants, for stabilization of emulsions
• ultraviolet stabilizers, protecting materials, especially plastics, from harmful effects of ultraviolet radiation
  • UV absorbers, chemicals absorbing ultraviolet radiation and preventing it from penetrating the materials; principally the same as sunscreens
  • quenchers, dissipating the radiation energy as heat instead of letting it break chemical bonds; often organic nickel salts, e.g. nickel phenolates
  • scavengers, eliminating the free radicals formed by ultraviolet radiation; often hindered amine light stabilizers (HALS)

See also

• Corrosion inhibitor

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