Viscous flow processing

We have to take the viscous flow processes happening simultaneously into account. For the irreversible molecular movements Eq. (33) yields... 

Softening Behavior. The softening behavior of the PS control, a sodium C-PS, and a sodium S-PS are illustrated in Figure 4. These softening curves were obtained on materials that were compression molded. Both ionomers contained about 5 mol % ionic functionality and at that level are extremely difficult to melt process. Compression molding can be effected at temperatures of 250°C for the S-PS and 200°C for C-PS, provided that the molding is effected over a sufficiently long time to permit the viscous flow processes to occur. A Du Pont Thermo Mechanical Analyzer was used (10°C/min) under the same conditions for all three materials. 

Plastic materials are those that are formed from synthetic compounds e.g., polymers or natural compounds that have previously been modified, for example hydrocarbons refined from crude oil, natural gas or derivatives of ethane, methane and naphtha. By definition the manufacturing of a plastic component should include a viscous flowing process that usually requires heat and pressure, for example extrusion or injection moulding. 

In many materials, the mechanical response can show both elastic and viscous types of behavior the combination is known as viscoelasticity. In elastic solids, the strain and stress are considered to occur simultaneously, whereas viscosity leads to time-dependent strain effects. Viscoelastic effects are exhibited in many different forms and for a variety of structural reasons. For example, the thermoelastic effect was shown earlier to give rise to a delayed strain, though recovery of the strain was complete on unloading. This delayed elasticity is termed anelastic-ity and can result from various time-dependent mechanisms (internal friction). Figure 5.9 shows an example of the behavior that occurs for a material that has a combination of elastic and anelastic behavior. The material is subjected to a constant stress for a time, t. The elastic strain occurs instantaneously but, then, an additional time-dependent strain appears. On unloading, the elastic strain is recovered immediately but the anelastic strain takes some time before it disappears. Viscoelasticity is also important in creep but, in this case, the time-dependent strain becomes permanent (Fig. 5.10). In other cases, a strain can be applied to a material and a viscous flow process allows stress relaxation (Fig. 5.11). 

The activation energy for the growth of phlogopite crystals and for the viscous flow process can be determined from Fig. 1-43. As both curves are almost parallel, the value for both processes comes to approximately 314 kj/mol (Holand et al., 1982a). 

As a result, the activation energy for the crystal growth of phlogopite is the same as for the viscous flow process. Thus it can be concluded that the viscous flow process determines the rate of crystal growiih. 

Glass-ceramic microstructures like those depicted contrast in Fig. 2-52 (a) and (b) were produced in the viscous flow process at 1000 C (Table 2-25). The special SEM-composition technique (Fig. 2-52 (b)) revealed small, lightly colored crystals, shown by X-ray diffraction to be Zr02 crystals. The rod-shaped cavities visible in Fig. 2-52 can be attributed to Li PO crystals that were dissolved by the etching conditions selected. 

Dental crowns and bridges are fabricated according to the viscous flow process described in Section 4.4.2.3.B, in which the biomaterial IPS EMPRESS is introduced. The basic ifabrication principles, therefore, remain unchanged. As a result, dental technicians can fabricate precision restorations quickly and easily. The quality of the IPS EMPRESS 2 material, however, provides several benefits as far as processing is concerned. The special features of the material are illustrated in the following example of the fabrication of a three-unit bridge. 

On the basis of the results shown in Table 13.1, Berry concluded that the largest contribution to the surface energy of a glassy polymer comes from a viscous flow process that in PMMA, he suggested [14], was related to the interference bands observed on the fracture surfaces, as seen in Figure 13.6. He proposed that work was expended in the... 

---