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Viscosity is an important characteristic for all materials, especially polymers. For liquids, viscosity will influence flow characteristics, heat transfer, and mass transfer. For polymers, viscosity is used as a measuring stick. Similar to putting a thermometer into a turkey to see if its been cooked long enough, viscosity can reveal how well a polymer is developed.
Imagine someone handing you a solid cube of polymer and asking you to determine its viscosity. Since polymer viscosities can range from 10-2 to 1012 poise, one testing method really doesn't cover them all. For this reason, two common methods are used: melt and solution.
Primarily used for higher viscosity polymers, the melt index is a common testing method. Test parameters such as compression weight and testing temperature are dependent on the type of polymer.
For example, polyethylene parameters include a temperature of 190 0C
and a weight of 10 kg. After a set amount of time (often 10 minutes) the polymer
that was extruded is weighed. The melt index would be expressed as grams polymer/10
minutes. This value corresponds to the polymer's average molecular weight.
Polyethylene with an average molecular weight of 150,000 g/mole would have a melt index
near 0.3 g/10 minutes. Melt indices above 15 g/10 minutes are desirable for process
points or final products where ease of processing in important. Melt indices below 1
g/10 minutes are favored when mechanical properties are important. The melt index is
not an intrinsic property of polymers (which limits its usefulness), but is nearly always
specified when a polymer is requested due to its ability to predict general behavior
properties of the polymer.
Testing by solution viscosity starts with dissolving a polymer in a solvent (usually accompanied by gentle heat). The solution is then timed as it travels through an appartus (often a pipette). The time for the pure solvent to flow through the appartus is also timed. These two values are compared to yield a relative viscosity. Relative viscosity is the ratio of the polymer viscosity to the solvent viscosity. Of greater interest to the polymer industry is a property known as intrinsic viscosity (IV). IV is proportional to the volume of the polymer and to the molecular weight.