Temperature effects mechanical properties

This section is organized as follows the basic physical characteristics are introduced, followed by discussions of uniaxial room temperature mechanical properties, the effect of long term thermal exposure, creep behavior, notch sensitivity, and fatigue performance. The significantly different off-axis, matrix-dominated properties are described in the final section. 

The same authors fabricated PLA/starch blends with various concentrations of two natural antioxidants, a-TOC and resveratrol, by melt blending and compression moulding processes. The sheets showed a yellowish colour and a significant reduction in the glass transition and melting temperatures. The addition of these agents enhanced mechanical properties, an effect which is attributed to a compatibilization effect between PLA and starch chains. A detailed study of the release process showed that blending with starch accelerated the release of resveratrol and a-TOC from PLA into ethanol. 

Cracks and kernel integrity are not the only criteria used for describing rice quality, because color, glass transition temperature, and so on are even easier to measure with instruments. Sensory characteristics, such as smell, taste, texture, are less easy to evaluate. In this chapter, only the main criteria, accessible by means of repeatable instrumental measurements, are detailed, namely the mechanical properties. The effect of drying on both paddy and parboiled paddy rice is discussed. 

The kinetic nature of the glass transition should be clear from the last chapter, where we first identified this transition by a change in the mechanical properties of a sample in very rapid deformations. In that chapter we concluded that molecular motion could simply not keep up with these high-frequency deformations. The complementarity between time and temperature enters the picture in this way. At lower temperatures the motion of molecules becomes more sluggish and equivalent effects on mechanical properties are produced by cooling as by frequency variations. We shall return to an examination of this time-temperature equivalency in Sec. 4.10. First, however, it will be profitable to consider the possibility of a thermodynamic description of the transition which occurs at Tg. 

Chemical Properties. A combination of excellent chemical and mechanical properties at elevated temperatures result in high performance service in the chemical processing industry. Teflon PEA resins have been exposed to a variety of organic and inorganic compounds commonly encountered in chemical service (26). They are not attacked by inorganic acids, bases, halogens, metal salt solutions, organic acids, and anhydrides. Aromatic and ahphatic hydrocarbons, alcohols, aldehydes, ketones, ethers, amines, esters, chlorinated compounds, and other polymer solvents have Httle effect. However, like other perfluorinated polymers,they react with alkah metals and elemental fluorine. 

The shallow penetration of ion implantation would in itself make it appear useless as a technique for engineering appHcations however, there are several situations involving both physical and chemical properties in which the effect of the implanted ion persists to depths fat greater than the initial implantation range. The thickness of the modified zone can also be extended by combining ion implantation with a deposition technique or if deposition occurs spontaneously during the ion implantation process. In addition, ion implantation at elevated temperatures, but below temperatures at which degradation of mechanical properties could occur, has been shown to increase the penetration depths substantially (5). 

The water hberated during the cure has no apparent effect on the composite properties. Glass-filled composites prepared in this manner retain mechanical properties at elevated temperatures as well as solvent and flammabiUty resistance (88). PhenoHc-graphite-fiber composites that exhibit superior mechanical properties have also been prepared by this process. 

The effect of temperature on PSF tensile stress—strain behavior is depicted in Figure 4. The resin continues to exhibit useful mechanical properties at temperatures up to 160°C under prolonged or repeated thermal exposure. PES and PPSF extend this temperature limit to about 180°C. The dependence of flexural moduli on temperature for polysulfones is shown in Figure 5 with comparison to other engineering thermoplastics. 

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