Polymer melt surface roughness

For primary insulation or cable jackets, high production rates are achieved by extmding a tube of resin with a larger internal diameter than the base wke and a thicker wall than the final insulation. The tube is then drawn down to the desked size. An operating temperature of 315—400°C is preferred, depending on holdup time. The surface roughness caused by melt fracture determines the upper limit of production rates under specific extmsion conditions (76). Corrosion-resistant metals should be used for all parts of the extmsion equipment that come in contact with the molten polymer (77). 

However, there is not much information available about plastic properties besides density and melt flow index (MFI). Having no consistent grade or/and source of plastics, a manufacturer faces from time to time a significant variation in the flowability of a polymer that is already purchased, sometimes in amounts of rail cars. These variations lead to improper process settings for production. This in turn results in a variety of defects on the surface of extruded profiles, such as sharkskin (fish skin), surface roughness, edge tearing, and so forth, and other property inconsistency in the final product. 

Equation (2.17) allows us to make a number of conclusions. So, at the conditions mentioned previously, conservation increase, that is, initial nanoparticles aggregation intensification, results to nanocomposite melt viscosity reduction, whereas enhancement, i.e., increasing the nanoparticles degree of surface roughness, raises the melt viscosity. At = 2.0, i.e., the nanofiller particles have a smooth surface, the melt viscosity for the matrix polymer and the nanocomposite will be equal. It is interesting that the extrapolation of the MFl dependence, obtained experimentally, and for the one calculated using Eq. (2.19), values give the value of MFl = 0.602 g/10 min at d = 2.0, that is practically equal to the experimental value of MFI = 0.622 g/10 min. The indicated factors, critical ones for nanocomposites, are not taken into consideration in continuous treatment of melt viscosity for polymer composites (Eq. (2.8)). 

The elastic effects in polymer melts are associated with the molecular coil deformation shown in Fig. 3.9. The effects include die swell, a diameter increase when the melt exits from a die and flow instabilities such as melt fracture (causing a rough surface). One measure of the elastic effects is the tensile stress difference — a-yy that occurs in shear flow in the xy axes. There can be a tensile stress in the direction of flow, or a compressive stress (Tyy on the channel walls, or a combination of the two. Figure 5.7 shows that, as the shear rate increases, the value of m... 

At temperatures above the melting point, water reacts rapidly with certain polymers such as nylon, polycarbonate, polybutylene tereph-thalate (PBT), and polyethylene terephthalate (PET). This reaction results in a decrease in the molecular weight. At the same time, absorbed water can form steam that results in surface roughness, splay, and internal bubbles. The reaction between water and the molten polymer is accelerated by prolonged exposure to temperatures above the melting point. 

Oliver and Mason [292] have used replica methods to assess the effect of surface roughness on the spreading of liquids and to measure contact angles. For stationary studies, small beads of poly(methyl methacrylate) (PMMA) were melted and the molten droplets spread and solidified on the surfaces. Dynamic studies involved polymer melts mounted on a remotely controlled hot stage stub in the SEM and the experiments were video recorded. 

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