Stress freezing

The plastic material is usually extended in the melt flow direction when it is injected in a mold. The molecular orientation and thermal stress freezes when the mold temperature goes below the matoial glass transition tenqrerature. The residual stress in the molded part is a critical issue causing part warp and birefiingence, which significantly reduces the yield rate for part production. 

Gelation. A sol becomes a gel when it can support a stress elasticaUy, defined as the gelation poiat or gelation time, C A sharp iucrease ia viscosity accompanies gelation. A sol freezes ia a particular polymer stmcture at the gel poiat (27). This frozen-ia stmcture may change appreciably with... 

Air-Entrainment Agents. Materials that are used to improve the abiUty of concrete to resist damage from freezing are generally known as air-entrainment agents. These surfactant admixtures (see Surfactants) produce a foam which persists in the mixed concrete, and serves to entrain many small spherical air voids that measure from 10 to 250 p.m in diameter. The air voids alleviate internal stresses in the concrete that may occur when the pore solution freezes. In practice, up to 10% air by volume may be entrained in concrete placed in severe environments. 

Supercritical and Freeze Drying. To eliminate surface tension related drying stresses in fine pore materials such as gels, ware can be heated in an autoclave until the Hquid becomes a supercritical fluid, after which drying can be accompHshed by isothermal depressurization to remove the fluid (45,69,72) (see Supercritical fluid). In materials that are heat sensitive, the ware can be frozen and the frozen Hquid can be removed by sublimation (45,69). 

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

A further source of stress may arise from incorrect mould design. For example, if the ejector pins are designed in such a way to cause distortion of the mouldings, internal stresses may develop. This will happen if the mould is distorted while the centre is still molten, but cooling, since some molecules will freeze in the distorted position. On recovery by the moulding of its natural shape these molecules will be under stress. 

The rather rigid molecules and high setting temperatures are conducive to molecules freezing in an oriented position with consequent high frozen-in stresses. 

Fujikawa, S. Miura, K. (1986). Plasma membrane ultrastructural changes caused by mechanical stress in the formation of extracellular ice as a primary cause of slow freezing injury in fhiit-bodies of basidiomycetes (Lyophyllum ulmarium [Fr.J KOhner). Cryobiol. 23,371-382. 

Obviously, one looks for causes. That declines in one or another species have natural factor etiologies is unequivocal. The demise of American elms and of the chestnut were due to natural factors. Insect infestations, bacterial and fungal diseases, hurricanes, floods, freezes, droughts and many other stresses can cause extensive tree death (5). But in such declines typically only a single species is affected or climatic events caused decline in a delimited area. In almost all declines caused by natural events, the causal factors can be identified we know their precise etiologies. Natural events are always part of the natural environment and must be factored in when evaluating forest declines (Table I). 

Ikeda, 1. (1982). Freezing injury and protection of citrus in Japan. In Plant Cold Hardiness and Freezing Stress, vol. 2, ed. P.H. Li A. Sakai, pp. 575-89. London Academic Press. 

A more general role of ABA in stress tolerance has been found in carrot cells. When a suspension culture of carrot cells was exposed to ABA and then selected for tolerance to freezing, the ABA-treated cells were found to be more tolerant to the stress (Reaney Gusta, 1987). These results provide further evidence for the presence of common mechanisms conveying tolerance to many of the environmental stresses. 

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