Heat stabilizers trends

Stabilizers. Heat stabilizers (qv) are included in PVC compounds to counteract the internal generation of hydrogen chloride as well as the external degradative effect of heat. Due to environmental considerations, there is a trend toward decreasing and even avoiding the use of stabilizers based on heavy metals, eg, lead. 

A continuing trend has been to polymeric retainers. Laminated phenolic cages have often been used for high speeds at temperatures up to 130°C. Heat stabilized nylon-6,6 has come into broad use in small ball bearings, both with and without glass reinforcement (39). Polyimide and PTFE are used up to 250°C. 

Simple primary amides such as stearamide, oleamide, and erucamide are widely used in polyolehns and styrenics as lubricants, but mainly in vinyl as release agents, discussed in the following. Aliphatic amides are strong enough nucleophiles (at process temperatures) to detract from the heat stability of PVC, and are therefore used at low levels or in situations where heat aging is not critical. When the latter is the case, the above amides are excellent multifunctional lubricants in flexible film and sheet. Stearamide is used mainly in filled applications, oleamide in clear or translucent sheet. Although it is feasible to formulate stabilizer systems that overcome effects on heat stability, there has been a trend towards replacement with ester lubricants. 

Another factor that affects trends in the stability constants of complexes formed by a series of metal ions is the crystal field stabilization energy. As was shown in Chapter 17, the aqua complexes for +2 ions of first-row transition metals reflect this effect by giving higher heats of hydration than would be expected on the basis of sizes and charges of the ions. Crystal field stabilization, as discussed in Section 17.4, would also lead to increased stability for complexes containing ligands other than water. It is a pervasive factor in the stability of many types of complexes. Because ligands that form tt bonds... 

At the other end of the temperature spectrum, with high thermal stability of siloxane-modified poly(arylene carbonates) also a desired property, the onset of thermal decomposition (40) for polymers 1-12 was found to be in the range of 385-456°C (as determined from TGA curves obtained by heating polymer samples in nitrogen at a heating rate of 20°C/min.). There does not appear to be any pronounced trend in regard to variation of the thermal stability with structure in polymers 1-12. The small differences in the values of T for these polymers can be due... 

One of the most successful applications of crystal field theory to transition metal chemistry, and the one that heralded the re-discovery of the theory by Orgel in 1952, has been the rationalization of observed thermodynamic properties of transition metal ions. Examples include explanations of trends in heats of hydration and lattice energies of transition metal compounds. These and other thermodynamic properties which are influenced by crystal field stabilization energies, including ideal solid-solution behaviour and distribution coefficients of transition metals between coexisting phases, are described in this chapter. 

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