Subject plasticization effect

The resistance to plastic flow can be schematically illustrated by dashpots with characteristic viscosities. The resistance to deformations within the elastic regions can be characterized by elastic springs and spring force constants. In real fibers, in contrast to ideal fibers, the mechanical behavior is best characterized by simultaneous elastic and plastic deformations. Materials that undergo simultaneous elastic and plastic effects are said to be viscoelastic. Several models describing viscoelasticity in terms of springs and dashpots in various series and parallel combinations have been proposed. The concepts of elasticity, plasticity, and viscoelasticity have been the subjects of several excellent reviews (21,22). 

Polycarbonate appears to an almost unique extent to be subject to an anti-plasticizing effect on the addition of low molecular weight compounds. It is reported that the derivatives of l,l-bis(4-hydroxyphenyl)-2,2-propane operate in this context by tightly filling the free volume between the polymer molecules. Charge-transfer complexes , for example tetracyanoethylene-A-stilbene s, antiplasticizing action has been explained in terms of aandyff relaxations. ... 

Besides the brittle elastic behavior, when a gel is subjected to a tensile load, under a compressive load the porous network can be irreversibly transformed. This plasticity effect depends strongly on the volume fraction of pores, but is also clearly affected by macropores and by the OH content. In fact, either under tension or compression, the gel material is not stable and its structure and mechanical features evolve. 

Figure 26.7 shows the chemical structures of an NLO chromophore (APAN) and an epoxy-based polymer (BPAZO) where NLO moieties are attached to the backbone [81]. Both the dye and the polymer are functionalized with thermally cross-linkable acryioyl groups. As the dye-doped polymer is subjected to heat as part of the simultaneous poling/curing process, the inter- and intramolecular cross-linking reactions occur simultaneously (Fig. 26.8). The 7g of the cross-linked polymer-dye network is lower than that of the undoped polymer network because of the plasticizing effect of the dissolved dye. However, the temporal stability at 100°C of the polymer-dye network is better than that of the undoped polymer network (Fig. 26.9) as a direct result of the increased cross-linking density in the cross-linked guest-host system. Therefore, the addition of the thermally cross-linkable NLO dye not only increases the...

In water-wall incinerators. The internal walls of the combustion chamber are lined with boiler tubes that are arranged vertically and welded together in continuous sections. When water walls are employed in place of refrac toiy materials, they are not only useful for the recovery of steam but also extremely effective in controlling furnace temperature without introducing excess air however, they are subject to corrosion by the hydrochloric acid produced from the burning of some plastic compounds and the molten ash containing salts (chlorides and sulfates) that attach to the tubes. 

Since the mid-1950s several materials have been found effective in combating ozone-initiated degradation, in particular certain p-phenylenediamine derivatives. The actual choice of such antiozonants depends on the type of polymer and on whether or not the polymer is to be subject to dynamic stressing in service. Since antiozonants are not known to have any use in plastics materials, even those which may have certain rubber particles for toughening, they will not be dealt with further here. Anyone interested further should consult references 3-5. 

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and superimposed on this is an alternating axial load of 100 N. If the fatigue limit for the material is 13 MN/m and the creep rupture strength at the equivalent time is 40 MN/m, estimate a suitable diameter for the rod. Thermal effects may be ignored and a fatigue strength reduction factor of 1.5 with a safety factor of 2.5 should be used. 

A plastic beam is to be subjected to load for a period of 1500 hours. Use the 1500 hour modulus values given below and the data in Table 1.5 to decide which of the materials listed would provide the most cost effective design (on a stiffness basis). 

From this relatively simple test, therefore, it is possible to obtain complete flow data on the material as shown in Fig. 5.3. Note that shear rates similar to those experienced in processing equipment can be achieved. Variations in melt temperature and hypostatic pressure also have an effect on the shear and tensile viscosities of the melt. An increase in temperature causes a decrease in viscosity and an increase in hydrostatic pressure causes an increase in viscosity. Topically, for low density polyethlyene an increase in temperature of 40°C causes a vertical shift of the viscosity curve by a factor of about 3. Since the plastic will be subjected to a temperature rise when it is forced through the die, it is usually worthwhile to check (by means of Equation 5.64) whether or not this is signiflcant. Fig. 5.2 shows the effect of temperature on the viscosity of polypropylene. 

Most polymer processing methods involve heating and cooling of the polymer melt. So far the effect of the surroundings on the melt has been assumed to be small and experience in the situations analysed has proved this to be a reasonable assumption. However, in most polymer flow studies it is preferable to consider the effect of heat transfer between the melt and its surroundings. It is not proposed to do a detailed analysis of heat transfer techniques here, since these are dealt with in many standard texts on this subject. Instead some simple methods which may be used for heat flow calculations involving plastics are demonstrated. 

In the preparation and processing of ionomers, plasticizers may be added to reduce viscosity at elevated temperatures and to permit easier processing. These plasticizers have an effect, as well, on the mechanical properties, both in the rubbery state and in the glassy state these effects depend on the composition of the ionomer, the polar or nonpolar nature of the plasticizer and on the concentration. Many studies have been carried out on plasticized ionomers and on the influence of plasticizer on viscoelastic and relaxation behavior and a review of this subject has been given 119]. However, there is still relatively little information on effects of plasticizer type and concentration on specific mechanical properties of ionomers in the glassy state or solid state. 

Wooden racks used in sea-water tests are likely to be subject to severe damage by marine borers. The wood used, therefore, must be treated with an effective preservative, for example creosote applied under pressure, if the test is to extend for several years. Organic copper compound preservatives may suffice for shorter tests, for example 2 or 3 years. Since the leaching of such preservatives may have some effects on corrosion, metal racks fitted with porcelain or plastics insulators have an advantage over wooden racks. 

If the wire is to be used to carry much higher frequency currents, the design problem in geometry and plastic selection becomes more complicated. The dielectric constant and dielectric loss values for the plastics become important in the design. At a frequency of one megahertz the effect of the dielectric on the power transmission behavior of the wire is substantial and, even at frequencies of 10 to 100 kilohertz, the insulation on the wire must be considered in the design as a major electrical element in the circuit. More on the subject of insulation will be following this section. 

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