Many polymer materials that are closely related to our lives are susceptible to heat, such as floors, wall covers, toys, coated films, and fabrics, etc. Fortunately, heat stabilizers protect the properties of the overall polymer (appearance, strength, elasticity, durability, etc.) from heat damage, either during the manufacturing process or during normal use of the finished product. While different polymers have varying degrees of heat resistance, heat stabilizers are added to virtually all polymer types.
Heat stabilizers are widely used in polymer materials to protect them from thermal stress during processing and during the life of the finished product. By far the most common polymer requiring heat stabilizers is polyvinyl chloride (PVC). Therefore, we take PVC as an example to introduce the practical application of heat stabilizers. Because of the properties inherent in the PVC i.e., its low cost and high performance, combined with the wide range of products that can be obtained from different processing conditions and techniques, PVC has become a universal polymer. Without the use of heat stabilizers, PVC could not be the widely used polymer that it is today. The instability in PVC is due primarily to the presence of allylic chloride atoms. Prompted by the exposure to heat, the chemical breakdown results in the release of hydrogen chloride, which leads to discoloration, polymer degradation, and potential corrosion of processing equipment (Figure 1). In PVC, heat stabilizers must accomplish several functions, including neutralizing hydrogen chloride, replace weakened carbon-chlorine bonds, prevention of autoxidation, and disruption of polyunsaturated sequences.
Figure 1. Thermal degradation of PVC
The working principle of heat stabilizers is basically similar. Effective stabilization of PVC against thermal stress can be achieved by adding the following heat stabilizers:
Mixed metal salts (formerly Ba/Cd, now largely replaced because of toxicity concerns by Ba/Zn and Ca/Zn salts of fatty acids or phenolates, mainly used in flexible and semirigid PVC).
Organotin stabilizers (e.g., dibutyltin maleate or din-alkyltin mercaptides, preferentially used for rigid PVC).
Lead stabilizers (tribasic, dibasic, or neutral lead stearate, dibasic lead phthalate; inorganic lead sulfates, carbonates, or phosphites, mainly used in wire and cable applications).
Primary heat stabilizers may advantageously be used in combination with organic co-stabilizers, such as β-aminocrotonates, β-diketones, or β-ketocarboxylic esters, or inorganic co-stabilizers, such as zeolites or hydrotalcites.
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