A common measurement used to characterize thermoplastic polymers is called melt index. It is essentially an indirect, and inversely proportional, measure of the viscosity of the polymer when molten. One measures the mass of polymer melt which will flow through an orifice in a given amount of time under defined conditions of temperature, pressure, and geometry. The larger the melt index value, the lower is its viscosity, and therefore, the average molecular weight of the polymer is lower. Higher molecular weight polymers will be more viscous and less will flow under the same conditions so the melt index will be a smaller number. The melt index is typically expressed in terms of grams of polymer which flow out in a ten minute period, thus g/10 min or dg/min.
Different polymer types often report melt index at differing conditions. For example, polyethylenes typically report melt index at 190 degrees Centigrade whereas polypropylenes are typically reported at 230 degrees, due in part to their differing melting points. Therefore, melt index values are not always directly comparable between polymer types.
There are standardized methods for melt index under ASTM and ISO, for example, ASTM D1238. Such standard methods specify the geometry and other constraints on the device used as well as the combinations of conditions. The device is essentially an upright, narrow cylindrical barrel fitted with a plunger and a removable (for cleaning) orifice at the bottom. The barrel is temperature controlled and a defined weight is placed on the plunger to provide the prescribed force and thus pressure on the plunger, which drives the polymer melt through the orifice. Typically, polymer pellets are loaded into the barrel and allowed to come to the measurement temperature, well above the polymer melting point, then the weight is applied to the plunger, forcing polymer through the orifice. The extrudate is measured via literal weighing or by volumetric methods (plunger travel) using known melt density.
Different weights may be used on the plunger for different polymer types or for different molecular weight ranges within products of a given type. For example, blow molding grades of HDPE might report a melt index value using a 21.6 kg weight, due to the high viscosity of such grades, while blown film extrusion grades of LLDPE or LDPE generally use a 2.16 kg weight.
Terminology also varies among polymer types and can be a source of confusion. Melt index, melt flow index, and melt flow rate are generally synonymous but often connote different measurement conditions and are frequently associated with different polymer type. Ratios of melt flows measured using two different weight loadings are also sometimes used to characterize the degree of shear-thinning behavior of the polymer. As the force increases, the apparent viscosity decreases and the flow is higher than expected, thus the melt flow ratio can differ between two polymers when expressed as the ratio of melt index measured at high loading to that at low load for each polymer. Changes in melt flow ratios usually reflect differences in molecular weight distribution and/or levels of long chain branching between polymer grades.
Melt indices are useful for comparing different grades and average molecular weights for the same polymer type. Suitability of a particular grade for a particular processing method can often be stated in terms of the range of melt index values suitable for use in that method, such as for injection molding versus blown film extrusion. For example, for polyethylenes, typical melt index values for blown film extrusion might range from 0.5 to 2 dg/min, for cast film extrusion from 2 to 6, for extrusion coating from 6 to 20, and for injection molding from 20 to 60, all measured at 190 C using a 2.16 kg weight.
Thus, a common metric for understanding ethylene vinyl acetate copolymers (EVA) is the melt index, which is inversely related to the average molecular weight. As discussed in an earlier post, the molecular weight of EVA can be varied over a rather wide range, so the range of attainable melt index values is large. That product envelope spans the melt viscosity range necessary to process the polymer in a wide variety of methods from blow molding to extrusion to injection molding and all the way to those applicable to handling hot melt adhesives, up to and including your “hot glue gun” used for arts and crafts at home.