Viscosity glass melts

At the macroscopic level a solid is defined as a substance that has both a definite volume and a definite shape. At the microscopic level, solids may be one of two types—amorphous or crystalline. Amorphous solids lack extensive ordering of the particles. There is a lack of regularity of the structure. There may be small regions of order separated by large areas of disordered particles. They resemble liquids more than solids in this characteristic. Amorphous solids have no distinct, melting point. They simply get softer and softer as the temperature rises, leading to a decrease in viscosity. Glass, rubber, and charcoal are examples of amorphous solids. 

The homogeneous glass, in which a few percent of alumina is added for better processing, is prepared from a melt at 1300-1500°C (Schnabel 1976) or 1000-1200°C/1450 C (McMillan 1980). It is important for the properties that the melt is as homogeneous as possible. Schnabel (1976,1978) produced glass capillaries or hollow fibers directly from the glass melt at a viscosity of 10 P. The phase separation was carried out by heat treatment between 500-800°C. 

Because of their novel topologies, polyrotaxanes have properties different from those of conventional polymers. Solubility, intrinsic viscosity, melt viscosity, glass transition, melting temperature and phase behavior can be altered by the formation of polyrotaxanes. The detailed changes are related both to the properties of the threaded cyclics and to the backbone and the threading efficiency. 

A new technology, based on impregnating continuous glass fibres with an extruded low viscosity PVC melt and subsequent application via the extrusion die of a standard PVC extrudate, has given PVC profiles with a 500% increase in stiffness (201). 

Because of the similarity between the mechanisms of viscous flow, diffusion and electrical conductivity, which are all activated processes, a relationship between these phenomena was sought. It has been established empirically that the temperature dependence of viscosity and resistivity of glass melts are often mutually dependent according to the relationship log t/ 3 log 2, or log = a log — b (cf. Morey, 1954). However, it should be borne in mind that mobility of cations is critical for transfer of electric charges while mobility of anionic structural units (network formers) is involved in the case of vi.scous flow. This is why the relation between the two quantities is difficult to interpret. 

If the entire temperature dependence of viscosity is to be measured, it is necessary to use several methods based on different principles. In the viscosity range 10 —10 dPa s, use is mostly made of rotary viscometers. A platinum cylinder rotates around its axis in the glass melt in a crucible, and the force required for revolving the cylinder at a certain speed is measured. In another arrangement, the external crucible is rotated while the internal one is suspended on a torsion wire. Within the same viscosity region, it is possible to measure with a counterbalanced sphere viscometer a plat inum sphere suspended on a thin wire from the balance arm is immersed in the glass melt in a crucible. The other balance arm is loaded and the speed at which the sphere is withdrawn from the melt is measured. 

The viscosity of melts in the system Na20 —SiOj is shown in Fig. 125. The composition of binary glasses can be estimated roughly from density using the diagram in Fig. 126. 

Figure 4. Melt viscosity-glass transition relationships for plasticized S-PS (1.78 mol %) samples based on various levels of DOP and glycerol (r = 2 X 105 dyn/cm2 220°C 1" X 0.05" capillary (D) DOP (A)...

Fluidity is the reciprocal of the viscosity. A melt with a large fluidity will flow readily, whereas a melt with a large viscosity has a large resistance to flow. While fluidity is often used in dealing with ordinary liquids, virtually all literature dealing with glass forming melts discusses flow behavior in terms of the viscosity. 

At low viscosities, glass forming melts usually behave as Newtonian liquids which immediately relax to relieve an applied stress. At extremely high viscosities, however, these liquids respond to the rapid application of a stress as if they were actually elastic materials. It follows that there must exist an intermediate range of viscosities where the response of these melts to application of a stress is intermediate between the behavior of a pure liquid and that of an elastic solid. Since this behavior has aspects of both viscous flow and elastic response, it is known as viscoelasticity, or viscoelastic behavior. 

Explain why the viscosities of melts which form network glasses often vary more slowly with temperature than do the viscosities of ionic melts. 

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