Discussing Polymer Viscosity

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.

Melt Viscosity

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.

Solution Viscosity

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.