Stabilization of commercial polymers

A major problem with such high molecular mass antioxidants is their greater insolublity in the host polymer. Moreover, any minor differences in the repeat units can result in a two phase system. As a result this approach is generally less effective in the stabilization of commercial polymers. By contrast oligomeric antioxidants of sufficient solubility in the host polymer e.g. the commercial hindered piperidine, 32) were found to be very effective as UV stabilizers although they still suffer from physical loss by solvent leaching. 

Polymers with other pendant photosensitive moieties such as 0-furylacrylic ester (2) or / -styrylacrylic ester (5) are highly photosensitive and have even higher photosensitivity after the addition of photosensitizers. However, the thermal stability of these polymers is inferior to that of the polymer with pendant cinnamic esters (4). Polymers with pendant benzalacetophenone (5), styrylpyridinium (6), a-cyanocinnamic ester (7) or a-phenylmaleimide (8) have high photosensitivity but they can not be sensitized. In addition, the photosensitive moieties that are used in the syntheses of these polymers are not commercially available, in contrast to cinnamic acid. 

Processes that are essentially modifications of laboratory methods and that allow operation on a larger scale are used for commercial preparation of vinyhdene chloride polymers. The intended use dictates the polymer characteristics and, to some extent, the method of manufacture. Emulsion polymerization and suspension polymerization are the preferred industrial processes. Either process is carried out in a closed, stirred reactor, which should be glass-lined and jacketed for heating and cooling. The reactor must be purged of oxygen, and the water and monomer must be free of metallic impurities to prevent an adverse effect on the thermal stability of the polymer. 

Polymeric sulfur is produced commercially as insoluble sulfur (IS) and is used in the rubber industry [56] for the vulcanization of natural and synthetic rubbers since it avoids the blooming out of sulfur from the rubber mixture as is observed if Ss is used. The polymeric sulfur (trade-name Crys-tex [57]) is produced by quenching hot sulfur vapor in liquid carbon disulfide under pressure, followed by stabilization of the polymer (against spontaneous depolymerization), filtration, and drying in nitrogen gas. Common stabilizers [58] are certain olefins R2C=CH2 like a-methylstyrene which obviously react with the chain-ends (probably -SH) of the sulfur polymer and in this way hinder the formation of rings by a tail-bites-head reaction. In this industrial process the polymer forms from reactive small sulfur molecules present in sulfur vapor [59] which are unstable at ambient temperatures and react to a mixture of Ss and on quenching. 

Table 11 compares the effectiveness of a synergistic UV stabilizer (BHBM-B + EBHPT-B) with some commercial stabilizing systems for ABS added conventionally. The exceptional activity of the polymer-bound system is believed to be due to the fact that it is confined to the rubber phase of the polyblend (18), which is known to be more sensitive than the thermoplastic phase to the effects of both heat and light (36). This finding, if confirmed in other multiphase systems, could be of considerable importance for the stabilization of heterogeneous polymer blends. 

We will discuss in this section the various ways that can be used to improve the thermal stability of polymers. The synthesis and thermal behaviour of some typical heat-resistant polymers (sometimes commercially available) will then be given. The volatilization of these materials has very seldom been thoroughly studied orders of reaction, activation energies and pre-exponential factors have generally not been determined. Therefore the thermal stability of the polymers will be characterized in an arbitrary way for the purpose of comparison. It must be stressed, however, that the physical properties of a polymer are at least as important for use at high temperature as the volatilization characteristics an infusible polymer is very difficult to process, and a heat resistant polymer with a low softening temperature is often useless. The softening temperature corresponds to the loss of mechanical properties. It can be measured by the standard heat deflection test. 

Fluoropolymer manufacturers and suppliers have developed time-temperature-shear-rate data for melt viscosity or melt flow rate (index) to provide an assessment of the thermal stability of these polymers. Figures 6.1 and 6.2 show the melt viscosity of a few commercial grades of polyvinylidene fluoride as a function of temperature at a fixed shear rate. The relationships between melt viscosity and shear rate, and shear stress versus shear rate, are presented in Figs. 

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