NOTE: The turbulent behaviour of a fluid can produce falsely high results in viscosity tests.
Normally, turbulent behaviour is due to an excessively high rotation speed in relation to the
viscosity of the sample (see detailed Warning further on).
FACTORS AFFECTING VISCOSITY
There are many variables that affect the rheological properties of products, so it is very important to take
the following factors into account.
Temperature
Temperature is one of the most obvious factors affecting rheological behaviour.
It is essential to consider the effects of temperature on viscosity in the evaluation of materials that are
subject to changes in temperature during its use or other processes. Some examples of this are motor oils,
greases and adhesives.
Shear Rate
When a fluid is subjected to variations in the speed gradient during its process or use, it is essential to know
its viscosity at the projected speed gradients.
Examples of materials, which are subjected to and affected by important variations in speed gradient during
its process or use, are: paints, cosmetics, liquid latex, some food products such as ketchup and blood in the
human circulatory system.
Measurement conditions
The measurement conditions of a material during its viscosity reading can have a considerable effect on the
results of this measurement. Consequently, it is important to be careful and control the environment and
conditions of any sample subjected to analysis.
Variables such as the type of viscometer, the speed/spindle combination, the sample’s container, the
absence or presence of a spindle protector, the temperature of the sample and the sample preparation
techniques, etc, can affect not only the precision of the reading but also the real viscosity of the sample.
Time
Ageing under the same speed gradient conditions affects tixotropical and reopectical fluids.
In some fluids the action of time combined with the proportion of the shear is very complex. In these cases,
one can observe, with time, a return to the original fluid state.
Previous conditions
The conditions that the sample is subjected to before the viscosity reading can significantly affect the
results, especially with heat-sensitive fluids or ageing.
Thus, the storage condition and the sample preparation techniques should be conceived to minimize effects
on the viscosity measurements.
Composition and additives
A material’s composition is a determining factor in its viscosity. When the composition is altered, whether
this is by changing substance proportions that compose it or adding other substances, important changes
can be observed in their viscosity.
For example, adding solvent to printing ink reduces the viscosity of the ink and other types of additives are
used to control the rheological properties of paints.
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