Melt film interface

Recalling the discussion earlier in this chapter, in most cases melting in the channel typically occurs at all four edges of the solid bed, with the majority of the melting occurring at the solid bed-melt film interface located between the solid bed and the barrel wall, as shown in Eig. 6.2. The newly molten resin from this location is then conveyed by the motion of the screw to a melt pool located at the pushing side of the channel. Eor very special and sometimes unpredictable conditions, the melting process can occur by a different mechanism. In these cases, the... 

The change in size of the solid bed over a small down-channel increment will depend on the rate of melting at the solid bed-melt film interface. Consider a small differential volume element, perpendicular to the solid-melt interface (Fig. 9.32). The solid bed has a local down-channel velocity Vsz and a local velocity component into the melt film of Vsy. The barrel surface velocity Vb is resolved into down-channel and cross-channel components Vb . and Vbx. 

Melting occurs primarily at the solid plug/melt film interface due to the heat flux (q) from the melt film into the solid plug. The mass rate of melting per unit interface area (ft) will be proportional to q. 

Material subjected to melting moves down the screw channel at the velocity of the material bed. Melting occurs at the surface where the material exposed to the solid bed-melt film interface undergoes phase change i.e., melt form and transferred as melt film. The molten material is accelerated and forced to undergo elongation with respect to the motion of the material inside the barrel relative to the screw. The material in the screw is considerably higher than that of the... 

The melt flows from the melt film towards the active flight flank. Only a small fraction of the material can flow through the clearance. As a result, the majority of the melt will flow into the melt pool. A circulating flow will be set up in the melt pool as a result of the barrel velocity. Since most of the viscous heat generation occurs in the upper melt film, it is generally assumed that all melting takes place at the upper solid bed-melt film interface. As melting proceeds, the cross-sectional area of the solid bed will reduce and the cross-sectional area of the melt pool will tend to increase. The melt pool, therefore, will exert considerable pressure on the solid bed. This reduces the width of the solid bed, while the melt film between the solid bed... 

The solid bed-melt film interface is assumed to be a distinct interface existing at the melting point, Tm, of the polymer. 

It is now useful to examine the melting energy at the four solid interfaces for this new model and for the historical Tadmor model [8]. The dissipation data from the simulations are summarized in Table 6.1 for a PE resin with a viscosity of 880 Pa-s. Examination of the table points out that the vectorial velocities (V)) for Zones C and D are very different for the assumption of barrel and screw rotation, as presented previously and as shown in Table 6.1. Eor the historic model, all energy is dissipated in the Zone C melt film, and the cumulative energy for melting was calculated... 

This analysis starts with the assumption that melting occurs in all four melt films that surround the solid bed. The initial analysis will be carried out for Film C in Fig. A6.1. The film is located between the barrel and the solid bed interface. This analysis describes the viscous energy dissipation in the film and the energy conduction from the barrel wall and how they relate to the melting flux at the solid bed-melt interface. 

Figure A6.2 Schematic for the two sources of energy (as a flux) for melting resin at the solid bed-melt Film C interface...

Returning to our pellet, we note the point where it reaches the end of the delay zone when the solid bed has acquired a small upward velocity toward the barrel surface. At some point in the extruder, our pellet will reach the melt film-solid bed interface, experiencing toward the end of this approach a quick (exponential) rise in temperature up to the melting point. After being converted into melt, our fluid particle is quickly swept... 

Fig. 9.32 A differential volume element perpendicular to the melt film-solid bed interface. Schematic view of temperature profile in the film and solid bed shown at right. Schematic views of velocity profiles (isothermal model) in the x and z directions are also shown.

Melt flow interface Melt flow interface 2 films 3 films... 

As a consequence of the concentration gradient in the melt film, a basicity gradient also exists from regions of high p 0 ) at the melt/oxide interface to regions of low p 0 ) at the melt/gas interface. By reaching regions with low p(0 ), the solute experiences a lower solubihty and precipitation of solid oxide occurs by the reversal of the dissolution reactions ... 

This precipitation creates a permanent sink for the solute, promoting the continuous flux of the dissolved oxide in the melt film. On the other hand, the consumption of oxidant gas O2 by the formation of 0 and subsequent dissolution reaction at the melt/oxide interface creates the gradient for oxidant diffusion. Figure 11 shows a schematic plot of the gradients and transport processes within the melt film. 

Fig. 11 Schematic plot of the Gas phase solubility gradient and transport processes within the melt film. At the melt/gas interface, the oxide precipitates.

The reaction Eqs. (63 and 65) are producing oxide ions at the melt/scale interface, whereas at the melt/gas interface, oxide ions (in form of C03 ) are consumed by the dissolution reactions involving O2 in the melt. This gives rise to a gradient in 0 concentration toward the melt film and a solubility gradient, as described in Sect. 6.1.4.1. 

A number of parameters determine whether or not the morphology of interest is adopted by a microphase separating copolymer melt. Of these, most important are the interaction parameter, the volume ratio of the blocks, the degree of polymerization, the individual block molecular weight distributions, the overall polydispersity, the interactions with the interfaces, and last but not least, the temperature. Only the latter two parameters are experimentally accessible and can be altered after the synthesis of the copolymer (disregarding polymer blends). Control over the self-assembly at the film interfaces becomes essential when the polymer films are intended to be used as templates. Meuler et al. recently published a comprehensive review on how these various parameters affect the formation of gyroid-like morphologies in polymeric materials [47]. 

Daoulas, K. C., Harmandaris, V. A., and Mavrantzas, V. G., 2005. Detailed atomistic simulation of a polymer melt/solid interface Structure, density, and conformation of a thin film of polyethylene melt adsorbed on graphite, Macromolecules,... 

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