Page 28 - POLYMER TECHNOLOGY

The coefficient of thermal expansion for plastic and polymer materials is calculated
as:

α = ΔL / (L0 * ΔT)
Where:
 = coefficient of thermal expansion
ΔL = changes in length of test specimen due to heating or to cooling
L0 = original length of specimen
ΔT = temperature changes

Therefore, α is obtained by dividing the linear expansion per unit length by the
change in temperature. The thermal expansion difference develops internal
stresses and stress concentrations in the polymeric material, which allows
premature failure to occur.

c. Thermal stability
Thermal stability of polymeric material is defined as the ability of the material to
resist deformation under temperature change and maintain its properties, such as
strength, toughness, or elasticity. In other word thermal stability is the ability of
polymeric material to withstand thermal stress resulting from the change of
temperature.
For example, a plastic water bottle will not withstand at high temperatures, the
bottle will change in shape and will lose its properties.

Figure 3.2: Plastic bottle will distort at high temperature.

Heat distortion temperature testing of plastics (HDT) is a standard test used to
determine the temperature at which a polymeric materials specimen deforms
under a given flexural load. The heat distortion temperature is also known as the
or heat deflection temperature (HDT).
The properties of polymeric materials depend on temperature. This takes on
special importance for mechanically stressed parts, which are exposed in use to
high temperatures. The heat distortion temperature properties of a plastic

24 | P a g e