Polymer, branched Decompose

Steps (iv) and (v) are thought to be the main chain branching steps in combustion. Several flame retardants act by inhibiting this vapour phase combustion. They produce decomposition products (at the same temperature as the polymer substrate decomposes) which interfere with the chain branching steps in combustion. Less reactive radicals are formed, which inhibit the combustion process. 

There are a number of methods of classifying polymers. One is to adopt the approach of using their response to thermal treatment and to divide them into thermoplastics and thermosets. Thermoplastics are polymers which melt when heated and resolidify when cooled, while thermosets are those which do not melt when heated but, at sufficiently high temperatures, decompose irreversibly. This system has the benefit that there is a useful chemical distinction between the two groups. Thermoplastics comprise essentially linear or lightly branched polymer molecules, while thermosets are substantially crosslinked materials, consisting of an extensive three-dimensional network of covalent chemical bonding. 

Grant and Paul Chemical Shifts. The technique of obtaining branch content information from NMR for polymers utilizes an empirical relationship given by Grant and Paul [29,79,80]. The Grant and Paul empirical relationship [29,79,80] can be used to calculate the values of the chemical shifts for carbon atoms in the vicinity of a branch point in a hydrocarbon polymer. The empirical relationship was obtained from NMR studies on alkanes. The chemical shift of any carbon atom in a 13C-NMR can be decomposed as a sum of contributions from its nearest five neighboring carbon atoms. The value of the chemical shift for any carbon atom C, is given as,... 

The polymers thus formed are very heat-resistant. Polyphenylsilsesquioxane, for example, starts to decompose above 600 °C. At 900 °C only loss of phenyl groups occurs, the silicon-oxygen framework of the macromolecule remaining intact . This sharply differs the above polymers from linear and branched polyphenylsilses-quioxanes. 

Both linear and branched polymers are typically thermoplastics, meaning they can be melted before they undergo decomposition. However, cross-linked three-dimensional or network polymers are thermosets, meaning they decompose before they melt. The crosslink density can vary from low, such as found in a rubber band, to high, such as found in ebonite (Figure 2.3). 

Oxidation of solid polymers is radical - chain process with marked branching and square break of kinetic chains, the main branching product being hydroperoxide. Fast - decomposing hydroperoxide is localized in amorphous phase, being more stable in crystals [302]. 

Some synthetic polymers can be recycled and some cannot. So-called thermoplastic polymers, usually composed of hnear or only shghtly branched molecules, can be heated and formed and then reheated and reformed. Therefore, they can be recycled. On the other hand, thermosetting polymers, which consist of molecules with extensive three-dimensional cross-hnking, decompose when heated, so they cannot be reheated and reformed. This makes them more difficult to recycle. 

The rate of the thermal decomposition of polyolefins is related to the presence of branches and side substituents in the polymer backbone. Accordingly, the following order of thermal degradation is usually observed HDPE < LDPE < PP < PS. Depending on the temperature, up to four fractions can be produced by the thermal decomposition of these plastics gases, oils, waxes and a solid residue. PVC degradation follows a different pathway HC1 is first removed at a low temperature to yield a polyene residue, which is then decomposed at a higher temperature. 

---