Viscous melt flow

Modification of BPA-PC for adaptation to the conditions of production of CD and CD-ROM disks, and of substrate disks for WORM and EOD was necessary. BPA-PC standard quaHties for extmsion and injection mol ding have, depending on molecular weight, melt flow indexes (MEI), (according to ISO 1130/ASTM 1238 in g/10 min at 300°C/1.2 kg, between less than 3 g/10 min (viscous types) up to 17 g/10 min. For CDs and optical data storage disks, however, MEI values exceeding 30 g/10 min, and for exceptionally short cycle times (5—7 s) even >60 g/lOmin are demanded at an injection mass temperature of 300°C (see Table 5). 

The mechanical behavior of plastics is dominated by such viscoelastic phenomena as tensile strength, elongation at breaks, stiffness, and rupture energy, which are often the controlling factors in a design. The viscous attributes of plastic melt flow are also important considerations in the fabrication of plastic products. (Chapter 8, INFLUENCE ON PERFORMANCE, Viscoelasticity). 

Freezing action Because of the heat exchange between the flowing TP melt and the mold walls, the flow may freeze (solidify) before the product is completely filled. Products that have alternate sections with thick and then thin walls can cause problems in flow and cooling that make them difficult to fill. In some cases the plastics that have been selected for the end use requirement are too viscous to flow properly in a mold cavity, and this makes the manufacture difficult. 

Shear rate When, a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner and cavity, or die wall, it is subject to a shearing force. As the screw speed increases, so does the shear rate, with potential advantages and disadvantages. The advantages of an increased shear rate are a less viscous melt and easier flow. This shear-thinning action is required to move the melt. 

Rheodynamics of non-linear viscous fluids flowing in circular channels with moving walls is described most comprehensively in 1S-34). With respect to the above conclusion (see sect 2.2.1) that the high elasticity of a melt influences insignificantly flow rate parameters of a flow, the combined shear is discussed in 24128-30,341 on the basis of a general approach to the analysis of viscosimetric flows developed by B. Colleman and W. Noll. 

Heating and cooling often take place while the polymer melt flows, making viscous dissipation an influencing factor during the process. However, since most plastic parts are thin, the conduction often occurs only across the thickness and the viscous heating is a result of shear within the narrow gap of a die or mold cavity. For such cases, the equations reduce to,... 

The viscous contribution to the total entrance pressure loss is very small. [C. D. Han, Influence of the Die Entry Angle in the Entrance Pressure Drop, Recoverable Elastic Energy and Onset of Flow Instability in Polymer Melt Flow, AIChE. J., 17, 1480 (1970).]... 

When heating an amorphous polymer it eventually reaches a temperature designated as its flow temperature. This is a very viscous transition state and further heating leads to a viscous melt. 

For blends containing 30-40 wt% of g-PO, the value of MFI is between 0.2 and 0.8 g/10 min, which is 10-26 times lower than that of pure PA6 the level of values corresponds with the requirements imposed on extrusion-processed materials (74). It is of interest that irrespective of g-PO type, at a concentration of 30-40 wt%, the MFI values of PA blends are quite similar despite a great difference in melt viscosities of g-POs used for blend preparation (Table 18.1). This can probably be explained by the fact that the continuous phase, during melt flowing of a blend, is formed by lesser viscous PA6 (76), which dominates in the blend. The decisive influence on the flow development of such blends comes from interphase interactions that are alike for all... 

The most commonly used type of inhibited films is produced by joint processing of thermoplastic polymers and highly dispersed Cl powders. When the melting points, as well as the sublimation and thermal destruction temperatures, of the Cl exceed the viscous flow temperature of the thermoplast, the film acquires the structure of a polymer composite filled with solid inhibitor particles [101]. The particles may partially dissolve in the polymer melt and exert a plasticizing effect on the polymer. Introduction of 1-2% of NBA into PE elevates melt flow by 1.5-1.8 times [10]. Aggregation of solid Cl particles may present essential difficulties for their joint processing with polymer melts and result in structural defects or impair the quality of the film as a whole [4]. 

The results observed in this ehapter emphasize that the microcomposites rheology description models do not give adequate treatment of melt viscosity for particulate-filled nanocomposites. The correct description of the nanocomposites rheological properties can be obtained within the frameworks of viscous liquid flow fractal models. It is significant, that such an approach differs principally from the used ones to describe microcomposites. So, nanofiller particles aggregation reduces both melt viscosity and elastic modulus of nanocomposites in the solid-phase state. For microcomposites, melt viscosity enhancement is accompanied by elastic modulus increase. 

The indicated discrepancy forces to search on other approach for the description of nanocomposites PP/CNT melt viscosity. And as earlier, the fiactal model of viscous liquid flow (the Eq. (49)) [68] was chosen as such approach. As it is known through Ref. [77], CNT by virtue of tiieir strong anisotropy (in case of the used carbon nanotubes of mark Taimite the ratio of length to diameter is larger than 45) and low transversal stiffiiess form ring-like structures with radius which can be determined with the aid of the equation [78] ... 

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