Heat stabilizers, additive organic compound

Heat stabilizers protect polymers from the chemical degrading effects of heat or uv irradiation. These additives include a wide variety of chemical substances, ranging from purely organic chemicals to metallic soaps to complex organometaUic compounds. By far the most common polymer requiring the use of heat stabilizers is poly(vinyl chloride) (PVC). However, copolymers of PVC, chlorinated poly(vinyl chloride) (CPVC), poly(vinyhdene chloride) (PVDC), and chlorinated polyethylene (CPE), also benefit from this technology. Without the use of heat stabilizers, PVC could not be the widely used polymer that it is, with worldwide production of nearly 16 million metric tons in 1991 alone (see Vinyl polymers). 

PVC, a polymer that is widely used because of its low gas permeability and high fire retardancy, is sensitive to light and heat. The degradation leads to dehydrochlorination and color formation [Chapiro, 1962 Bradley, 1984 Thomas et al., 1986]. To process the resin it is necessary to add heat stabilizers. The most common additives include metal organic compounds, acid acceptors and alkalis, dioctyl phthalate, and dioctyl sebacate [Bradley, 1984]. PVC is only slightly crystalline. However, it has been used to produce heat-shrink products [Bradley, 1984]. 

Lead heat stabilizers are used for wire and cable applications as primary additives. They may be based on organic compounds including stearates and phthalates or inorganic salts such as sulphates, phosphates and carbonates. Although knowledge of the toxicity of lead has initiated the search for its replacement, no suitable, cost-effective material has yet been identified. 

Heats of hj drogenation have been measured for a wide variety of unsaturated organic compounds, to give information about strain and stabilization (resonance) energies. However, this is only one type of addition reaction for which thermochemical data have been accumulated. Lacher and his co-workers have measured heats of halogenation of ethylenic compounds, for example the chlorination of the homologous series of perfluoroalkenes, viz.,... 

These examples only hint at the analysis of heats of formation of organic compounds that is possible. Benson and co-workers summarized the methods and data for calculations for the major functional groups in organic chemistry. ° In addition, the data allow calculation of heat capacities and entropies of these compounds in the same marmer in which heats of formation are determined. Heats of formation are valuable reference points in discussing the stabilities of various isomers or products of reactions, whether they are calculated by bond increments or group increments or are derived as part of a theoretical calculation. 

However, these effects can be prevented or reduced by antioxidants, i.e. organic compounds which help protect the plastics imder hostile conditions. Other additives called heat stabilizers help stop plastics, particularly PVC, from decomposing dimng processing. They are often compoimds based on epoxies, or on calcium, zinc, tin and other metals. 

---