Strength heat degradation effects

An increased degradation effect has been observed with an increase in the temperature at which cellulose was irradiated. Spirora and Flyate (88) observed that changes caused by UV radiation are about the same as those caused by thermal aging. Desai (89) reports that the wet strength of papers exposed to UV increases as does that of heat-treated pulp. Launer and Wilson (33), using irradiation at wavelengths of 330 nm to 440 nm, found that papers made of cotton and wood fibers with... 

J. X. Li, Y. Matsuo and S. Kimura, Heat-exposure effects on the crystalline development and the strength degradation of silicon carbide fibers, Adv. Composite Mater., 2 [2], 103-116 (1992). 

In some cases, several of these processes occur simultaneously, depending on the sample size, the heating rate, the pyrolysis temperature, the environment, and the presence of any additives. Although polymer degradation schemes can be greatly altered by the presence of comonomers, side-chain substituents, and other chemical constituent factors, the ultimate thermal stability is determined by the relative strengths of the main-chain bonds. Many additives and comonomers employed as flame retardants are thermally labile and as a result the thermal stability of the polymer system is reduced. In order to reduce the observed effects of the flame-retardant additives on the thermal stability of the polymeric materials, more thermally stable and hence inherently flame-resistant polymers are of increasing interest. 

Diluents will also affect the performance properties of the adhesive. Diluents generally lower the degree of crosslinking and degrade the physical properties of the cured epoxy. This reduction in crosslink density increases the resiliency of the adhesive, but it also reduces tensile strength as well as heat and chemical resistance. These effects are more pronounced at elevated temperatures than at room temperature. The degree of these effects will depend on whether the diluent has epoxy functionality (reactive diluents) or whether the diluent is incapable of reacting with the epoxy system (nonreactive diluents). 

According to Waller and coworkers, the loss of strength on heating the yarn in the presence of oxygen and 12 % of moisture is not uniform with time, and appears to go through three distinct phases. These are an induction period, an interval of rapid degradation, and, finally, a phase of rather slow deterioration. The existence of an induction period which increases as the temperature is decreased indicates that the reaction products accelerate the degradation process. The effect of the decomposition products, accumulated in... 

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