Cooling Elasticity

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

Plasticizers. It was found in 1926 that solutions of PVC, prepared at elevated temperatures with high boiling solvents, possessed unusual elastic properties when cooled to room temperature (137). Such solutions are flexible, elastic, and exhibit a high degree of chemical inertness and solvent resistance. 

The mechanical properties of wood tend to increase when it is cooled and to decrease when it is heated (6,18). If untreated wood heated in air is not exposed to temperatures of more than - 70° C for more than about 1 year, the decrease in properties with increasing temperature is referred to as immediate or reversible ie, the property would be lower if tested at the higher temperature but would be unchanged if heated and then tested at room temperature. The immediate effect of temperature on strength and modulus of elasticity of clear wood, based on several different loading modes, is illustrated in Figures 4—6 (6). 

Higher temperatures result in permanent degradation. The amount of this irreversible loss in mechanical properties depends upon moisture content, heating medium, temperature, exposure period, and, to some extent, species. The effects of these factors on modulus of mpture, modulus of elasticity, and work to maximum load are illustrated in Figures 6—9 (6). The effects may be less severe for commercial lumber than for clear wood heated in air (Fig. 10). The permanent property losses shown are based on tests conducted after specimens were cooled to - 24° C and conditioned to a moisture content of... 

Reversible HydrocoUoids (Agar). The agar-based impression materials are thermally reversible, aqueous gels (230,231), that become viscous fluids in boiling water and set to an elastic gel when cooled below 35°C. The popularity of agar-based impression materials has diminished with the introduction of elastic impression materials such as alginate-based, polysulfide, silicone, and polyether impression materials, but agar [9002-18-0] materials are still used in substantial quantities. 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

A metal tumbler would not crack because its elastic limit must be exceeded considerably before it fails. However, repetitions of thermal stress, when some plastic flow occurs on both heating and cooling cycles, can result in either cracking or so much deformation that a part becomes unserviceable. 

By linking the chain ends of different molecules they form a type of network structure as long as the domains remain glassy. As the polymer is heated above the of the domain polymer block the domain molecules become mobile and on application of a stress the material flows like a thermoplastic. On cooling, new domains will be formed, thus regenerating the elastic state. 

The next stop is to cool the nylon below its Tg without removing the stress, retaining its molecular orientation. The nylon becomes rigid with a much higher elastic modulus in the tension direction [15,000 to 20,000 MPa (2 to 3 x 106 psi)]. This is nearly ten times the elastic modulus of the unoriented nylon-66 plastic. The stress for any elastic extension must work against the rigid backbone of the nylon molecule and not simply unkink molecules. This procedure has been commonly used in the commercial production of man-made fibers since the 1930s via DuPont. 

The passage of a sound wave along a tube, so that no energy is dissipated by friction, is an example of a compressional wave of permanent type, and Newton applied his equation (1) to determine the velocity of sound in air. For this purpose he took e as the isothermal elasticity of air, which is equivalent to assuming that the temperature is the same in all parts of the wave as that in the unstrained medium. Since air is heated by compression and cooled by expansion, the assumption implies that these temperature differences are automatically annulled by conduction. Taking the isothermal elasticity, we have ... 

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