Abstract
Thermal stabilizers protect polymers from chemical degradation caused by heat or UV radiation. These additives include a wide variety of chemicals, from pure organic chemicals to metallic soaps to complex organometallic compounds. By far the most common polymer requiring the use of heat stabilizers is polyvinyl chloride (PVC). However, copolymers of PVC, chlorinated polyvinyl chloride (CPVC), polyvinylidene chloride (PVDC) and chlorinated polyethylene (CPE) also benefit from this technology. PVC is the most important class of halogenated polymers requiring these chemical additives.
In normal operation, PVC resin is intimately mixed with the desired ingredients under high shear mixing conditions to form a homogeneous dry powder compound. Heat stabilizers can be liquid or powder and are added early in the mix cycle to provide stabilization during this operation. Preheating the resin to approximately the glass transition temperature facilitates the adsorption of liquid additives, resulting in better powder flow properties and lower bulk density in the final compound. Post-compounding operations, such as extrusion pelletization, increase the overall thermal history of the polymer, so slightly higher levels of thermal stabilizers are required to compensate for this.
Organotin based heat stabilizers are the most effective and most commonly used PVC stabilizers. These are derivatives of tetravalent tin. The second most widely used stabilizer is the mixed metal combination. These products dominate flexible PVC applications in the United States. The commercially important alkali and alkaline earth metal based salts and soaps of calcium, zinc, magnesium, barium and cadmium used in these stabilizer systems. Other organic compounds such as phosphites, epoxides, polyols and beta-diketones can also be added to further enhance performance. Antimony mercaptan stabilizers are also used.
