High-elasticity

The importance of polymer composites arises largely from the fact that such low density materials can have unusually high elastic modulus and tensile strength. Polymers have extensive applications in various fields of industry and agriculture. They are used as constructional materials or protective coatings. Exploitation of polymers is of special importance for products that may be exposed to the radiation or temperature, since the use of polymers make it possible to decrease the consumption of expensive (and, sometimes, deficient) metals and alloys, and to extent the lifetime of the whole product. 

The situation is complicated, however, because some of the drag on a skidding tire is due to the elastic hysteresis effect discussed in Section XII-2E. That is, asperities in the road surface produce a traveling depression in the tire with energy loss due to imperfect elasticity of the tire material. In fact, tires made of high-elastic hysteresis material will tend to show superior skid resistance and coefficient of friction. 

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

In false twist texturing (FTT), shown schematically in Figure 7, a device twists yam upstream of its location as the threadline passes across a heater, and the yam untwists downstream from the device and is wound up. If the yam is not heated downstream from the twist device, it has bulk and high elasticity (stretch). If the yam is heated downstream from the twist device, it has bulk, but much less stretch. FTT machines initially used two steps to sequentially draw and texture. Later machines combined those steps to simultaneously draw and texture (92). Initial machines used pin spindles as the false twist device. Texturing speeds were about 150 m/min and slowly increased to about 300 m/min. As POY quaUty and stmctural stabiUty improved, new... 

For most hydrardic pressure-driven processes (eg, reverse osmosis), dense membranes in hoUow-fiber configuration can be employed only if the internal diameters of the fibers are kept within the order of magnitude of the fiber-wall thickness. The asymmetric hoUow fiber has to have a high elastic modulus to prevent catastrophic coUapse of the filament. The yield-stress CJy of the fiber material, operating under hydrardic pressure, can be related to the fiber coUapse pressure to yield a more reaUstic estimate of plastic coUapse ... 

Al—Li. Ahoys containing about two to three percent lithium [7439-93-2] Li, (Fig. 15) received much attention in the 1980s because of their low density and high elastic modulus. Each weight percent of lithium in aluminum ahoys decreases density by about three percent and increases elastic modulus by about six percent. The system is characteri2ed by a eutectic reaction at 8.1% Li at 579°C. The maximum soHd solubiHty is 4.7% Li. The strengthening precipitate in binary Al—Li ahoys is metastable Al Li [12359-85-2] having the cubic LI2 crystal stmcture, and the equhibrium precipitate is complex cubic... 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

Many materials deposited by CVD have a high elastic modulus and a low fracture toughness and are therefore affected by residual film stresses. 

BeryUium is used in the space shuttle orbiter as window frames, umbUical doors, and the navigation base assembly. An important appHcation for beryUium is inertial guidance components for missiles and aircraft. Here the lightweight, high elastic modulus, dimensional stabUity, and the capabUity of being machined to extremely close tolerances are aU important. 

The high elastic modulus, compressive strength, and wear resistance of cemented carbides make them ideal candidates for use in boring bars, long shafts, and plungers, where reduction in deflection, chatter, and vibration are concerns. Metal, ceramic, and carbide powder-compacting dies and punches are generahy made of 6 wt % and 11 wt % Co ahoys, respectively. Another apphcation area for carbides is the synthetic diamond industry where carbides are used for dies and pistons (see Carbon). 

Materials having high elastic moduH deform less for a given stress. Typical E values are shown in Table 1. 

Coating solutions often exhibit a mixture of viscous and elastic behavior, with the response of a particular system depending on the stmcture of the material and the extent of deformation. Eor example, polymer melts can be highly elastic if a polymer chain can stretch when subjected to deformation. 

Blends of isobutylene polymers with thermoplastic resins are used for toughening these compounds. High density polyethylene and isotactic polypropylene are often modified with 5 to 30 wt % polyisobutylene. At higher elastomer concentration the blends of butyl-type polymers with polyolefins become more mbbery in nature, and these compositions are used as thermoplastic elastomers (98). In some cases, a halobutyl phase is cross-linked as it is dispersed in the polyolefin to produce a highly elastic compound that is processible in thermoplastic mol ding equipment (99) (see Elastomers, synthetic-thermoplastic). ... 

Dynamic explosion detectors use a piezoresistive pressure sensor installed behind the large-area, gas-tight, welded membrane. To ensure optimum pressure transference from the membrane to the active sensor element, the space between the membrane and the sensor is filled with a special, highly elastic oil. The construc tion is such that the dynamic explosion detec tor can withstand overpressures of 10 bar without any damage or effect on its setup characteristic. The operational range is adjustable between 0 and 5 bar abs. Dynamic explo-... 

Figure 3.2. Application of stress to a highly elastic body. Rate of chain uncoiling with time...

If we consider the total deformation (T>totai) occurring during flow to be almost entirely composed of a viscous flow (Dyisc) and a high elastic deformation due to chain uncoiling then we may write... 

Viscous deformations, at a fixed deforming stress, increase rapidly with temperature whereas elastic deformations change much more slowly. For this reason the high elastic deformation component tends to be more important at lower processing temperatures than at high processing temperatures. 

A deformation due to chain uncoiling which is not instantaneous and whose rate depends on temperature (high elastic deformation, Dhe)-... 

In Figure 9.7 stresses are imposed on a body showing ordinary elastic deformation only, a second body showing high elastic deformation only and a third body showing viscous deformation only. The stress is imposed at time to and held at a constant value until time t, when it is removed. Deformation... 

Figure 9.7. Types of deformalional response as a result of a fixed load being imposed betweem times to and ti (a), (b) Ordinary elastic material, (c) Highly elastic material, (d) Viscous material...

Figure 9.8. Deformation-time curves, (a) Material showing substantial ordinary elastic, high elastic and viscous components of deformation, (b) Material in which high elastic deformation...

One of the most important conclusions from this is that since both the viscous and the high elastic components of deformation depend on both time and temperature, the total deformation will depend on time and temperature. Since this fact has been shown to be an important factor affecting many polymer properties it is proposed to consider the background to this in greater detail in the following section. 

The price of these rubbers has become such that for many applications they have been replaced by less expensive alternatives. These include the use of EPDM rubbers for automotive parts not requiring oil resistance and plasticised PVC for applications where flexibility rather than high elasticity is required. 

High elasticity is also not utilised in the main application of chlorosulphonated polyethylenes, in wire and cable coating, which consume about 40% of output. The combination of heat and oil resistance has led to widespread use as sheathing for nuclear power cables, offshore oil rig cables and in diesel electric locomotives. Other uses include chemical plant hose, spark plug boots and as a base for flexible magnetic strips. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

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