Melt-flow number

The width of molecular weight distribution (MWD) is usually represented by the ratio of the weight—average and the number—average molecular weights, MJM. In iadustry, MWD is often represented by the value of the melt flow ratio (MER), which is calculated as a ratio of two melt indexes measured at two melt pressures that differ by a factor of 10. Most commodity-grade LLDPE resias have a narrow MWD, with the MJM ratios of 2.5—4.5 and MER values in the 20—35 range. However, LLDPE resias produced with chromium oxide-based catalysts have a broad MWD, with M.Jof 10—35 and MER of 80-200. 

Studies of melt flow properties of polypropylene indicate that it is more non-Newtonian than polyethylene in that the apparent viscosity declines more rapidly with increase in shear rate. The melt viscosity is also more sensitive to temperature. Van der Wegt has shown that if the log (apparent viscosity) is plotted against log (shear stress) for a number of polypropylene grades differing in molecular weight, molecular weight distribution and measured at different temperatures the curves obtained have practically the same shape and differ only in position. 

In practice there are a number of other factors to be taken into account. For example, the above analysis assumes that this plastic is Newtonian, ie that it has a constant viscosity, r). In reality the plastic melt is non-Newtonian so that the viscosity will change with the different shear rates in each of the three runner sections analysed. In addition, the melt flow into the mould will not be isothermal - the plastic melt immediately in contact with the mould will solidify. This will continuously reduce the effective runner cross-section for the melt coming along behind. The effects of non-Newtonian and non-isothermal behaviour are dealt with in Chapter 5. 

The temperature of the screw was measured by several investigators [29-32]. The measurements were performed by mounting thermocouples in an axial hole bored in the center of the screw or by protruding the thermocouples into the melt flow. The sensor signals were then transmitted to a chart recorder using an electrical rotary union. The technology available at the time of these measurements limited the number of sensors in the screw and the quality of the data. 

The certificate also gives limiting viscosity numbers in 1-chloronaph-thalene, 1,2,4-trichlorobenzene, and decahydronaphthalene. Density and melt flow viscosity under specified ASTM conditions are also included. Hence SRM 1475 may be utilized in the calibration of a variety of instruments used for characterizing polyethylene. A series of papers describing the methods used to characterize this sample have been published (7). 

The cell density (number of cells per unit cross-section area or volume) is also used to characterize the coarseness or fineness of foam. Foamed products can feature a deliberately created inhomogeneous (nonuniform) morphology. An example is when a foamed core is sandwiched between solid skins as in so-called structural foams, or in elastomeric products with so-called integral skins. With cells elongated in the direction of foam rise or melt flow, the process will give an anisotropic structure and properties (Chapter 15). 

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