Thermal stability depolymerization

The thermal stability of polymers of types (1) and (2) is also dependent on the nature of the substituents on phosphoms. Polymers with methoxy and ethoxy substituents undergo skeletal changes and degradation above about 100°C, but aryloxy and fluoroalkoxy substituents provide higher thermal stability (4). Most of the P—N- and P—O-substituted polymers either depolymerize via ring-chain equilibration or undergo cross-linking reactions at temperatures much above 150—175°C. 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

These techniques help in providing the following information specific heat, enthalpy changes, heat of transformation, crystallinity, melting behavior, evaporation, sublimation, glass transition, thermal decomposition, depolymerization, thermal stability, content analysis, chemical reactions/polymerization linear expansion, coefficient, and Young s modulus, etc. 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

The two main brominated flame retardants used commercially in PET are PyroChek 68PB (see Figure 14.18) and Saytex HP-7010 (Albemarle). Both of these flame retardants are based on brominated polystyrene. While there are similarities between these flame retardants, they are not equivalents. There are quality and performance differences between these two products as they use different raw materials (i.e. polystyrenes) and the process for bromination is different. Saytex HP-7010 has better thermal stability and colour control than does PyroCheck 68 PB. However, if higher flow characteristics are a necessary property of the FR-PET, then Pyrocheck 68 PB would be the product of choice. Sodium antimonate is the appropriate synergist in PET since it is more stable at the higher processing temperatures required of PET and does not cause depolymerization of this polyesters. 

Zn(R-dtp)2 complexes have been characterized and their thermal stabilities investigated 173,184,190,297-299,301-305) Zn(R-dtp)2 compounds are thermally degraded to volatile olefins and non-volatile residues and this serves as the basis for gas chromatographic determination of the compounds 304,30s) Several papers describing pyrolyses of Zn(R-dtp>2 complexes have discussed mechanisms for formation of olefins, sulfides, and other products 173,184,190,298,299, 304) Dakternieks and Graddon i8s,283)35 mentioned earlier, have reported thermodynamic measurements for depolymerization and adduct formation reactions of zinc, cadmium and mercury R-dtp compounds. 

One of the most prominent features in the heterogeneous copolymerization of trioxane is the occurrence of two different kinds of active centers—dissolved and crystalline copolymer cations. They have different copolymer reactivity ratios and different tendencies to depolymerize, i.e., different formaldehyde equilibrium concentrations. At first the formation of soluble copolymer with high dioxolane content did not raise much hope for obtaining a crystalline copolymer of good thermal stability from trioxane and dioxolane but the gradual depolymerization of the soluble copolymer proved to be a useful side reaction which greatly improved the situation. Eventually, the entire complicated process turned out to be quite favorable for the formation of a stable crystalline copolymer with the desired random distribution. 

Cjc/o-sulfur rings are yellow or pale orange, low-melting solids (Table 6) of variable stability. For example, S9 decomposes above 0°C, while Se and Sy are only moderately stable at room temperature. Their thermal stability decreases in the order S12, Sig, S20 (years) > Se, S9, Sio, Su, Si3 (days) Sy (minutes). Their interconversion reactions (S Sm, m f n) have been studied in some detail. Thermal as well as photochemical reactions are possible, and a number of mechanisms have been proposed (Figure 7). (1) Radical, consisting of ring opening (S" - " S ), polymerization (S ham and depolymerization (S" " mixture... 

Diffusion of solutions of Cr, Co and Mn ions through a PTFE membrane allows for separation of Cr from the remaining ions. Thermal stability of polymeric materials is a significant consideration in many poly(phosphazene) applications. The kinetics of the thermal degradation of PTFE are best fit with a model requiring a two step initiation for depolymerization. These steps involve formation of defect units, such as =P(0)NH- and =P(0)N(CH2CFj)-, which become active centers for depolymerization. Mixed... 

Various other studies on thermal stability of these resins are reported in literature [21-23], Particular interest was given to the thermooxidative decomposition of these resins. The process occurs by two different paths. One path takes place with depolymerization and significant weight loss ... 

These reactions have been reported for phosphonium carboxylates (4, 5). This is, in fact, one of the major factors contributing to the high thermal stability of polymers prepared with phosphines as initiators. A competing reaction is depolymerization via an unzipping reaction. 

This section briefly describes the thermal behaviour and conversion of other plastics, including materials such as PET, that are condensation polymers. As described in Chapter 2, condensation polymers are best depolymerized by chemolysis. However, the knowledge of both their thermal stability and the products derived from their thermal decomposition is of interest because in many cases they are present as contaminants in wastes containing addition polymers. 

These depolymerization reactions are complicated by side reactions. For example, if both P—Cl and P—OCH2CF3 or P—OCgHj units are present in the same reaction system, thermolysis causes elimination of CF3CH2CI or C5H5CI with simultaneous crosslinking of the chains . Clearly, depolymerization and elimination reactions are important in the context of the practical thermal stability of polyphosphazenes. Thus,... 

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