Polymer stabilization physical factors

Stabilizers are physically lost from the polymer by evaporation and leaching and their concentration in the polymer drops. As a consequence, polymer articles are no more chemically protected. Physical factors limiting the lifetime of stabilizer doped polymers due to the physical impermanence of stabilizers are explained in Sect. 1.3.1. Factors influencing the efficacy of the two permanent and impermanent stabilizers due to the physical interaction with the polymer matrix are given in Sect. 1.3.2. 

It must be therefore taken into consideration that the physical loss of stabilizers may influence the r ults of the accelerated ageing test This is true mainly in tests where the polymer degradation is accelerated by physical factors having very different intensities to those applied under practical conditions. 

The fundamental questions are What is the microscopic mechanism or driving force for the transition, and what physical factors are important Two distinct possibilities have been advanced side-chain crystallization (5, 6, 17-19), which is postulated to induce polymer backbone ordering, and conformation-dependent polymer-solvent interactions that arise explicitly from electron delocalization and that stabilize an ordered rodlike conformation (20-24). Side-chain crystallization remains a qualitative suggestion that has not been developed to the point where it has predictive power and can be critically tested. However, in the solid state, the enhanced importance of packing effects makes such a mechanism more plausible (18, 19). 

As mentioned earlier, both chemical (catalyst, surfactants, stabilizers) and physical (fluid dynamics, energy dissipation rates, circulation time and so on) factors control the performance of the suspension polymerization reactor. It is first necessary to examine the available experimental data to clearly understand the role of these chemical and physical factors. The available data indicates that the yield of usable polymer beads in laboratory scale reactor is more than 85%. Laboratory experiments were then planned to examine the sensitivity of the yield to various parameters of the polymerization recipe under the same hydrodynamic conditions. These experiments showed that the yield is relatively insensitive to small deviations in the chemical recipe. Analysis of the available data on pilot and plant scale indicated a progressive decrease in the yield of usable polymer beads from laboratory to pilot to plant scale. This analysis and some indirect evidence suggested that it may be possible to re-design the plant-scale reactor hardware to generate better fluid dynamics and mixing to increase the yield of particles in the desired size range. 

Ash et al. (1983) studied the chemical stability of xanthan and one novel heteropolysaccharide which they do not identify. They note the earlier findings of Davison and Mentzer (1980), who observed less than 30% loss in viscosity over 500 days at 90 "C in sea water, and the results of Sutherland et al (1986), who found little loss in viscosity at 112" C in 6% brine over 42 days. Ash et al (1983) found considerable variation in stability of xanthan products obtained from different sources. They, like other workers, noted an increase in viscosity over the first few days of the test which has been ascribed to delayed hydration of the polymer. They also noted that the stability of one of the xanthan samples depended strongly on the salinity of the test brine as shown in Figure 4.8. The data in Figure 4.8 refer to stability at 70°C, but a similar pattern is found at 90°C. This increased stability with increasing salinity gave one clue to one of the physical factors which may influence stability—namely the order-disorder (or helix-coil) transition, as... 

Besides the performance there are many other factors that determine the choice for a stabilizer system. As stabilizers have to protect polymers for long times, it is important that these stabilizers stay in a polymer over the lifetime, which is related to several stabilizer-related physical factors. Stabilizers are normally added in mixtures or together with other additives possible interaction between these additives might have an influence on the performance of stabilizers. Reactions with other chemicals from the environment of the plastic can lead to a deactivation of the stabilizer and a reduced lifetime. In many cases another requirement for stabilizers is that they are not colored or discolored. If polymers are used that can come into contact with food, an indirect food contact approval is required. As stabilizers have to be added to a polymer there are requirements for toxicity (which is not equivalent to indirect food contact approval) and dosability. A number of these requirements are discussed in the following. 

Antioxidants and stabilizers were used on a trial-and-error basis more than 100 years ago. Real scientific and technological progress is relatively recent. It was ushered in by the development of a basic understanding of the underlying mechanisms of polymer degradation. This has made it possible to ascribe specific chemical and physical functions to antioxidants and stabilizers according to their mode of action in the oxidative process. An important aspect in the development of antioxidants is that their effectiveness does not only depend on the chemical inhibition process but also on physical factors such as solubility of antioxidants and their compatibility with the polymer, diffusion and migration phenomena within the polymer matrix, volatility and extractability of antioxidants. Some historical perspectives of this development has been reviewed recently. ... 

The structures of sol-gel-derived inorganic polymers evolve continually as products of successive hydrolysis, condensation and restructuring (reverse of Equations 1-3) reactions. Therefore, to understand structural evolution in detail, we must understand the physical and chemical mechanisms which control the sequence and pattern of these reactions during gelation, drying, and consolidation. Although it is known that gel structure is affected by many factors including catalytic conditions, solvent composition and water to alkoxide ratio (13-141, we will show that many of the observed trends can be explained on the basis of the stability of the M-O-M condensation product in its synthesis environment. 

Polyvinyl alcohol (PVA), which is a water soluble polyhidroxy polymer, is one of the widely used synthetic polymers for a variety of medical applications [197] because of easy preparation, excellent chemical resistance, and physical properties. [198] But it has poor stability in water because of its highly hydrophilic character. Therefore, to overcome this problem PVA should be insolubilized by copolymerization [43], grafting [199], crosslinking [200], and blending [201], These processes may lead a decrease in the hydrophilic character of PVA. Because of this reason these processes should be carried out in the presence of hydrophilic polymers. Polyfyinyl pyrrolidone), PVP, is one of the hydrophilic, biocompatible polymer and it is used in many biomedical applications [202] and separation processes to increase the hydrophilic character of the blended polymeric materials [203,204], An important factor in the development of new materials based on polymeric blends is the miscibility between the polymers in the mixture, because the degree of miscibility is directly related to the final properties of polymeric blends [205],... 

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