Thermal degradation viscosity

When cured with room temperature curing system these resins have similar thermal stability to ordinary bis-phenol A type epoxides. However, when they are cured with high-temperature hardeners such as methyl nadic anhydride both thermal degradation stability and heat deflection temperatures are considerably improved. Chemical resistance is also markedly improved. Perhaps the most serious limitation of these materials is their high viscosity. 

Polycondensation of highly viscous polyesters in the melt phase is limited. The removal of the volatile by-products becomes more difficult due to diffusion inhibited by the increased viscosity of higher-IV polyesters. In addition, undesirable side reactions due to thermal degradation impede the growth of the molecular chains. As a consequence, the reaction rate decreases and decomposition reactions dominate, thus resulting in a decrease in the melt viscosity [2], As it is able to address these limitations, SSP has become the method of choice and is therefore so popular. 

Fiber orientation uniformity is also affected by small-scale or timewise variations in polymer viscosity, related to breakage of polymer chains during the extrusion process. The degradation occurs as a result of residual moisture that immediately reacts to break chains, and by thermal degradation that occurs more gradually over time. Different residence times and temperature histories within the laminar flow streamlines lead to different viscosities, and hence different average orientation levels in the different fibers. 

Figure 13.20 The rate of thermal degradation of PET as a function of the intrinsic viscosity measured at different temperatures 1, 280 °C 2, 290°C 3, 300°C 4, 310°C [29b, 29c, 39], From Thermal degradation of PET. A kinetic analysis of gravimetric data, Covney, J. D., Day, M. and Wiles, D. M., J. Appl. Polym. Sci., 28, 2887 (1983), copyright (1983 John Wiley Sons, Inc.). Reprinted by permission of John Wiley Sons, Inc.

These polymers have reduced viscosities up to 1.13 dL/g and electric conductivity as high as 10 11 10 12 S/cm. All the polymers are insoluble in common organic solvents but soluble in cone H2S04. Thermal degradation begins around 400°C in air and nitrogen according to tga. 

The calendering process is commonly used for shaping high melt viscosity thermoplastic sheets and is particularly suitable for polymers susceptible to thermal degradation or containing substantial amounts of solid additives. This is because the calender can convey large rates of melt with a small mechanical energy input (compared to an extruder). 

Bradley, T. D., and Mitchell, J. R. (1988). The determination of the kinetics of polysaccharide thermal degradation using high temperature viscosity measurements. Carbohydr. Chem. 9 257-267. 

The difficulty with these polymers is that they usually do not melt without heavy thermal degradation and can hardly be dissolved. If melting or dissolving would be successful an extreme viscosity of the melt or solution above the clearing temperature had to be expected. 

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