Frozen stress

Spontaneous depolarization spectroscopy is like TSPC except that no electric field is applied whatsoever during the entire course of the experiment. The electrometer measures any current generated in the sample as a result of dipolar motion over the temperature range of the experiment. This method can be used, for example, to study the internal stress frozen into a sample during processing. 

Residual stress (frozen-in stress) n. Stress remaining in a part that has been chilled quickly during or after molding, extrusion, or forming. It remains because there was too little time for the stress to relax while the material was soft. Over time, high residual stress can cause parts to warp and shrink. It can be relieved and rendered harmless by annealing residually stressed parts while restraining them in fixtures. 

A local order of about 1 nm in length may exist in the amorphous state. Structure and properties are isotropic, in the absence of stress. An anisotropic behaviour is possible, if the amorphous polymer was stretched below Tg even after removing the stress (frozen-in residual stress). 

Stress, frozen-in Undesirable frozen-in or residual stresses developed during processing. 

The flow-induced stress plays an important role especially in thin-wall molding, for the incomplete relaxation of polymer molecules that freeze too fast and the less shrinkage that generates less the thermal-induced stress. Frozen layers on the surface of mold cavities act as poor thermal conductors. The molecular orientation in the hot core is allowed to relax. Yet the whole parts are cooled and frozen in a very short period and thus could not relax completely in thin-wall molding. Therefore, the flow-induced stress is a significant amount for thin-wall parts, while the thermal-induced stress dominates in thick-wall parts. 

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... 

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). 

Cooling rates can affect product properties in a number of ways. If the polymer melt is sheared into shape the molecules will be oriented. On release of shearing stresses the molecules will tend to re-coil or relax, a process which becomes slower as the temperature is reduced towards the Tg. If the mass solidifies before relaxation is complete (and this is commonly the case) frozen-in orientation will occur and the polymeric mass will be anisotropic with respect to mechanical properties. Sometimes such built-in orientation is deliberately introduced, such as... 

As previously stated, molecular orientation occurs during melt processing of polymers. On removal of the deforming stresses the molecules start to coil up again but the process may not go to equilibrium before the polymer cools to below its Tg. This leads to residual orientation (frozen-in strain) and corresponding frozen-in stresses. 

As with thermoplastics melt processes, the setting is achieved by cooling. It will be appreciated that such cooling is carried out while the polymer is under stress so that there is considerable frozen-in orientation. This can be maintained throughout the life of the article. It is possible with the higher molecular weight materials to heat shapes made from blanks many years previously and see them return to the original shape of the blank. 

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

The reason for the activity of the above named classes of liquids is not fully understood but it has been noted that the most active liquids are those which reduce the molecular cohesion to the greatest extent. It is also noticed that the effect is far more serious where biaxial stresses are involved (a condition which invariably causes a greater tendency to brittleness). Such stresses may be frozen in as a result of molecular orientation during processing or may be due to distortion during use. 

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 number of materials exist which neither attack the polymer molecule chemically nor dissolve it but which cannot be used because they cause cracking of fabricated parts. It is likely that the reason for this is that such media have sufficient solvent action to soften the surface of the part to such a degree that the frozen-in stresses tend to be released but with consequent cracking of the surface. 

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

Second-order stress is difficult to observe and much less extensively studied. The causes of internal stress are still a matter for investigation. There are broad generalisations, e.g. frozen-in excess surface energy and a combination of edge dislocations of similar orientation , and more detailed mechanisms advanced to explain specific examples. 

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