LCR7000 & RHEOSIGHTS P/N: 974194 |
Rev: A
|
ECO: 52468
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In general, polymeric materials tend to decrease in viscosity with an increase in
temperature, provided they are chemically stable. (Unstable materials may increase in
molecular weight through polymerization or crosslinking, producing a rise in viscosity.
Alternatively, it can break down via chain scission causing a larger than expected drop in
viscosity with temperature.)
A common method of representing the dependence of the viscosity on temperature is
through the use of the Arrhenius Equation, where the log of the viscosity (often the zero
shear viscosity value) vs. 1/Temperature is fitted to a straight line. Sometimes a log
(viscosity) vs. Temperature curve may be used. Both give similar good curve fits.
The WLF (Williams-Landel-Ferry) dependence is also used sometimes. It is found to be
particularly useful at temperatures closer to the glass transition temperature of the
material.
Suggested test temperatures for various materials are displayed in a table in the ASTM
D3835 test method.
Generally, inorganic fillers such as carbon black, CaCO3, TiO2 etc., increase the viscosity of
polymeric materials based on their volume fraction concentration. They also tend to
reduce extrudate swell (rocks don't swell).
Many materials adsorb (surface) and absorb (internal) moisture. While at room
temperature this often has little effect on the material, at elevated temperature even small
amounts of moisture can have dramatic effects on the viscosity of the material. This is
especially true for condensation polymers. (For example, it is known that the fiber strength
of PET is a strong function of the moisture in the material before the spinning process.)
Viscosity is Shear Stress over Shear Rate:
•
=
γ
τ
η
Where:
η
is viscosity,
τ
is the shear stress and
γ
is the shear rate.
5.
Effect of Temperature on Viscosity
6.
Effect of Fillers on Viscosity
7.
Effect of Moisture on Viscosity
8.
Equations and Corrections
