Melt Flow definition

If you trace the word resin back far enough, you ll find that it was originally defined as a low molecular weight, natural polymer that is an exudate of (it exudes from) vegetable or non-vegetable matter. Examples are rosin (from pine trees), shellac (from insects), and both frankincense and myrrh (aromatic gums from an East African and an Asian species of tree). Resins like these do not flow if heat and pressure are applied, like plastics do. They decompose or melt. (This definition of resin is obsolete in commerce today.)... 

A true melt fracture by definition must involve some kind of bulk failure in melt flow. Either massive chain disentanglement or chain scission or both must occur in the bulk. Such a real cohesive breakdown away from the surface may also produce extrudate distortion. In general, other forms of irregular extrudate distortion do occur at high stresses. Only direct flow visualization may reveal the origin of such extrudate distortions. 

Products The cascade technology enables the manufacturing of tailor-made products with a definite MWD from narrow to broad MWD. The melt flow index may vary from 0.2 (bimodal product) to over 50 (unimodal product). Homopolymers and copolymers are used in various... 

Table 2.1 shows the density and melt flow index values for low-, medium-, and high-density polyethylenes. By definition, these polymers are characterized by a certain range of density. Each of them can have practically any melt flow index, hence, MFI is independent of the respective density category. Table 2.1 shows only the most applicable range of MFI, though it varies between 0.01 and 100 and wider. 

Different properties of plastics are preferred for different fabrication methods. The viscosity of the resin melt is typically measured against a standard test referred to as the melt flow index (MFI). The MFI and resin density characteristics of some of the primary applications are shown in Table 1.4. As can be seen, the MFI and density properties vary with fabrication method. To allow for high tolerances and precise definition, injection molding utilizes a resin melt which is runny (MFI=5-400) in comparison to blow mold grade resin (MFI=0.01-0.2) or extrusion grade resin (MFI=0.3-2). Blow mold resin tends to be more taffy like so that it will retain thickness while pressure is applied inside a mold. This viscosity difference limits the type of recycling production methods possible because different melt viscosity resins do not mix homogeneously. If different melt viscosity resins are not separated pror to reuse, the resin properties will not be uniform in production. 

The apparatus for determining MFI is basically a circular orifice rheometer. By its very definition, MFI represents a point at specific shear rate and shear stress values on the viscosity versus shear rate curve. The expressions for shear stress T and shear rate y in the melt-flow apparatus are given by the well-known conventional forms discussed in Sec. 3.2.1 as follows ... 

In the example for a polyamide 12, the code 22-030 of data block 3 indicates a polymer with an intrinsic viscosity of 210 ml/g ( cell 22 ), and a modulus of elasticity of 280 MPa ( cell 030 ). On the other hand, 20-D050 in data block 3 for a poly(ethylene) tested according to DIN 16776 means a material with a density within cell 020 (e.g., 0.918 g/cm ) and a melt flow rate within the cell 050 (e.g., 4.2g/min), measmed under condition D (a temperature of 190°C under a load of 2.16 kg). The definition of cells for each material and testing procedure can be found in special tables. 

Before we are in a position to discuss the viscosity of polymer melts, we must first give a quantitative definition of what is meant by viscosity and then say something about how this property is measured. This will not be our only exposure to experimental viscosity in this volume—other methods for determining bulk viscosity will be taken up in the next chapter and the viscosity of solutions will be discussed in Chap. 9—so the discussion of viscometry will only be introductory. Throughout we shall be concerned with constant temperature experiments conducted under nonturbulent flow conditions. 

In the capillary method, the time required for a liquid to flow through a capillary tube is determined. The melt under investigation flows with a constant rate through a tube with a small, definite cross-sectional area, such as a cylindrical capillary. The viscosity can be measured in an absolute way from the pressure drop. This method can yield the most reliable absolute data, the viscosity being given by a modified Hagen-Poiseuille equation ... 

The simplest definition of a first-order transition is one in which heat flows into or out of the material with no change in temperature. Examples are melting and boiling and their reversals, crystallization and condensation. 

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