Melt break

A few rheometers are available for measurement of equi-biaxial and planar extensional properties polymer melts [62,65,66]. The additional experimental challenges associated with these more complicated flows often preclude their use. In practice, these melt rheological properties are often first estimated from decomposing a shear flow curve into a relaxation spectrum and predicting the properties with a constitutive model appropriate for the extensional flow [54-57]. Predictions may be improved at higher strains with damping factors estimated from either a simple shear or uniaxial extensional flow. The limiting tensile strain or stress at the melt break point are not well predicted by this simple approach. 

Purple cloud of potassium permanganate diffusing outward as a solid piece dissolves in a beaker of water, seen from above. The solid piece (black) takes up much less space than the solution of potassium permanganate ions. This demonstrates the principle of diffusion, where a liquid or gas will spread out to fill the available space. This in turn demonstrates the fundamental differences between a solid and a gas or liquid. A solid is a rigid, constrained structure, which contrasts with the fluid, chaotic nature of a liquid or gas. The molecules of a solid are bonded, but solvation or melting breaks the bonds between the molecules. (Courtesy of Andrew Lambert Photography/Science Photo Library)... 

Figure 7-5. Generalized flow curve with the first Newtonian region (N), pseudoplastic region (st), second Newtonian region (N2), dilatant region (d), and onset of turbulence or melt break (t).

The same effect is observed in melts of macromolecular substances. Since, in this case, the liquids are elastic, however, additional elastic oscillations of small liquid particles occur. The uneveness of the oscillations causes an elastic turbulence. This occurs at much lower flow velocities than the normal turbulence, i.e., at low Reynolds numbers. Elastic turbulence can also be recognized by the fact that the flow rate increases much more sharply with increasing pressure in the elastic turbulence region than in the laminar flow region in normal liquids the increase in the flow rate is less in the turbulence region than in the laminar flow region. Elastic turbulence becomes apparent in the processing of plastics at what is called the melt break. 

The heating effect of an electric current is also the principle upon which a fuse gives protection to a circuit. The fuse element is made of a metal with a low melting point and forms a part of the electrical circuit. If an excessive cument flows, the fuse element overheats and melts, breaking the circuit. 

From the relation L=f(vij) it is obvious that the spinnability is governed by two processes, namely the cohesive break (or the swell effect) and the melt break (capillary break, melt fracture). According to Section 11.3.1, a certain amount of elastic energy can be stored in all viscoelastic fluids. This phenomenon leads, among others, to the Barus effect. 

A major area of negotiation in the development of the specification concerned the Drum Friction Test. This is a test which simulates a stalled belt. The NCB test in force had evolved after the Creswell Colliery disaster of 1953. A stalled rubber belt had remained in contact with its drive drum which continued to run for several hours. The frictional heat caused a fire which resulted in several fatalities. The drum friction test to emerge afterwards stipulated that the belt shall break, in addition to drum temperature and belt flow limitations. In effect this meant a textile-reinforced PVC belt in which the frictional heat melts the PVC and then melt-breaks the synthetic-fibre fabric as it makes contact with the drum. Such a belt meets the drum friction requirements and the NCB used this type of belt for many years. The steel-cable reinforced rubber-belt needed for the single-belt installation, first at Cadley Hill, and then at... 

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