The Polymer Degradation Process

This indicated that the polymer degradation process was likely sensitized by monomeric rather than polymeric rose bengal. 

Many phosphorous additives act as flame retardants in this way in hydroxyl containing polymers such as cellulose. During the polymer degradation process, phosphorus acids are produced which lead to char via phosphorylation and dehydration reactions. Relatively low quantities of phosphorus compounds are needed to impart a reasonable degree of flame retardancy. Several boron additives behave in a similar way by the promotion of carbonaceous char through esterification and dehydration reactions. 

It is an observed fact that heating PVC at temperatures above 70°C has a number of adverse effects on the properties of the polymer. At processing temperatures used in practice (150-200°C) sufficient degradation may take place during standard processing operations to render the product useless. It has been found that incorporation of certain materials known as stabilisers retards or moderates the degradation reaction so that useful processed materials may be obtained. 

Because the polymer degrades before melting, polyacrylonitrile is commonly formed into fibers via a wet spinning process. The precursor is actually a copolymer of acrylonitrile and other monomer(s) which are added to control the oxidation rate and lower the glass transition temperature of the material. Common copolymers include vinyl acetate, methyl acrylate, methyl methacrylate, acrylic acid, itaconic acid, and methacrylic acid [1,2]. 

All of the factors involved in weathering, including both the amount of intensity of sunlight, vary both seasonally and geographically. To understand fully and predict the weathering behaviour of any polymer requires information about exactly how these factors vary and how they then contribute to the overall degradation process. 

The release behavior depends on both the choice of polymer and on the formulation procedure. The best results were obtained with injection-molded samples. For prototypical drugs like p-nitroaniline, the drug release pattern followed closely that of the polymer degradation over a period of 9 months for PCPP. The correlation between release and degradation was still maintained in the more hydrophilic PCPP-SA, 20 80, where both processes were completed in 2 weeks. Compression molding can also be used for these polymers, but the correlation between drug release and polymer degradation is not as... 

The addition of heat shifts the equilibrium concentrations away from the products and back towards the reactants, the monomers. This is one reason why processing these types of polymers is often more difficult than processing products of chain growth mechanisms. The thermal degradation process can be dramatically accelerated by the presence of the low molecular weight condensation products such as water. Polyester, as an example, can depolymerize rapidly if processed in the presence of absorbed or entrained water. 

As discussed in Chapter 10, a wide variety of additives is used in the polymer industry. Stabilizers, waxes, and processing aids reduce degradation of the polymer during processing and use. Dyes and pigments provide the many hues that we observe in synthetic fabrics and molded articles, such as household containers and toys. Functional additives, such as glass fibers, carbon black, and metakaolins can improve dimensional stability, modulus, conductivity, or electrical resistivity of the polymer. Fillers can reduce the cost of the final part by replacing expensive resins with inexpensive materials such as wood flour and calcium carbonate. The additives chosen will depend on the properties desired. 

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

This is the primary degradation process accompanying processing of the polymer. The early stage of the dehydrochlorination process is uncomplicated by interfering processes. The only product observed by evolved gas analysis is hydrogen chloride (scheme 1). The sample... 

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