TABLE OF CONTENTS Melt flow rate (MFR) is commonly used to measure the viscosity of a melt and is defined as the weight of a polymer melt that can be extruded through a defined orifice in a given time at a defined temperature and pressure. It is inversely related to M w. Easy flowing grades are generally less tough than those of higher M w and stiffer flow. PE, a crystallising polymer, is soluble only at elevated temperatures. Since most methods for determining molecular weight and MWD require the dissolution of the polymer, these determinations must also be carried out at elevated temperatures, certain modifications to the conventional methods being used for determining molecular weights. The methods for measuring molecular weight and characterising MWD of PE include classic methods, such as light scattering, osmometry (with special semi-permeable membranes to withstand high temperatures, such as cellophane and Ultracella filters), viscometry, and fractionation, and also newer ones, such as temperature rising elution fractionation (TREF) and the combination of size exclusion chromatography (SEC) with various molecular weight sensitive detectors.
Some useful data for such measurements are given in Tables 3.2 and 3.3. PE are soluble only in the vicinity of their crystalline melting points, generally >100 C, where they are susceptible to thermo-oxidative degradation. Temperatures of about 120 140 C are normally used with HDPE. Ultra high molecular weight PE (UHMWPE) is especially slow to dissolve. The most suitable solvent is nitrogen-sparged, stabilised (0.1% Santonox R) Decalin; the...
