Polymer processing oxygen stabilization

Need for PA material stabilization arises at the stage of polymer processing, as in this case the polymer, as a rule, should be transferred into flowing state. Stabilization of polymer materials plays a very important role, first of all, to the action of light and heat, oxygen and air. 

In most polymer applications oxygen is present. Consequently, the stability is less than in a vacuum (Fig. 10.1) or that in an inert gas. Many oxidation studies use films, a few jxm thick, in which there is a uniform oxygen concentration. The oxidation of polyolefins is auto-catalytic, since the main product (hydroperoxides) initiates the reaction (Section 10.2.1). An induction period is followed by a constant rate of oxygen consumption (Fig. 10.3). In the latter, the rates of hydroperoxide destruction and formation are equal. Antioxidants increase the induction period, but they are eventually consumed. The process is exactly the same as in the melt, except that the rate is lower The activation energy for the maximum oxidation rate in polyethylene is 146 kj mol . It appears that all hydrogen atoms on the chain are equally vulnerable to oxidation. 

Stabilization also plays as important a role in applications of polymer materials, especially stabilization against the combined action of light and oxygen and the effects of continuous mechanical action. Even though the variety of mechanisms of polymer breakdown is great, chain processes of oxidation and chain decomposition play a basic role. Hence investigations related to the processes of the appearance of free radicals, the elucidation of their role as initiators of various chain processes of... 

In common with other silicone products, the fluids have outstanding heat resistance. Methyl silicone fluids are stable in air up to 150°C for long periods in the absence of air this temperature is raised to 250°C. The presence of phenyl groups in the polymer enhances heat stability and the aforementioned temperatures are increased by about 100 deg. C. The changes that occur in air and in an inert atmosphere are different. In air, cross-linking occurs and the fluid viscosity increases. The process is initiated by oxygen attack on methyl groups and some of the reactions which may be involved are shown below [6] ... 

The keypoint in this process for formation of anisotropic polymer carbon from polymer fibres with a C/C back bone which is preorientated in direction parallel with the fibre axis is the length constancy or even elongation of the polymer fibre during stabilization treatment by mechanical-means (34). This process parameter is the same as it has been applied for the not inflammable textile fibres (for instance black ORLON) before (35). Also today, stabilized PAN is used as a non inflammable textile. For this purpose minimum oxygen contents of 5-7 % are needed for inflammibility. 

Oxygen also influences the mechanochemistry of saturated polymers (Fig. 3.30) [2, 13, 17-19, 50]. Oxygen can play a role in polymer processing considering that air is present in conventional processing equipment [19]. Moreover, relatively high temperatures and stresses are involved. The results obtained during polystyrene extrusion at different temperatures with and without air are shown in Fig. 3.31. The difference is dramatic. The temperature of maximum shear stability for extrusion in air (see Section B of this chapter) is clearly evident. 

From an industrial stand-point, a major virtue of radical polymerizations is that they can often be carried out under relatively undemanding conditions. In marked contrast to ionic or coordination polymerizations, they exhibit a tolerance of trace impurities, A consequence of this is that high molecular weight polymers can often be produced without removal of the stabilizers present in commercial monomers, in the presence of trace amounts of oxygen, or in solvents that have not been rigorously dried or purified, Indeed, radical polymerizations are remarkable amongst chain polymerization processes in that they can be conveniently-conducted in aqueous media. 

Stabilizers counter the effect of the high temperatures and the oxygen rich atmosphere experienced by the resin during the rotational molding process. Since some rotational molded parts require up to one hour of residence time in the oven, such stabilizers are essential. Without them, the polymer would lose its inherent properties, becoming unfit for the final application. 

Polymer structure and formulation. As an example, Woo et al. [7] measured OIT values for series of commercial PVC resins and polyester thermoplastic elastomers (TPEs). The researchers used the ASTM D3895-80 procedure, but substituted air as the oxidising gas instead of pure oxygen. A dependency on thermal processing history of the TPE film samples appeared to influence the measured OIT in the PVC study, chemically different chain ends affected polymer stability and hence OIT values. 

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