CLTE vs Moisture Absorption
When thinking about the dimensional stability of a finished application, one needs to consider the impact of Moisture Absorption and the Coefficient
of Linear Thermal Expansion (CLTE) that corresponds to the operating conditions of the application in its environment.
Moisture Absorption measures the weight gain of a plastic specimen in immersion for 24 hours and to full saturation. (ASTM D590) For the most part,
moisture exists in a free state in nano-sized pockets within the polymer itself. While 24 hour immersion is useful to gain an understanding of “rate of
absorption” saturation mainly applies to applications that will exist in a saturated environment. The practical moisture absorption of a specific plastic
material is dependent on reaching equilibrium within the environment which it exists. A material will not reach saturation unless it is immersed in liquid.
CLTE is a measure of the change in length of a plastic material per unit temperature. Every material is unique and every material has its own coefficient
that can be used to calculate expected growth over a temperature range. For engineering plastic materials, a common test range is -30°F to 300°F to
determine the coefficient.
So, the question is, which property has a larger impact on growth of a material? If we take a completely dry PEEK to saturation and compare it to PEEK
tested for growth at a range from 0°F to 150°F we find that the CLTE over 150°F has nearly seven times the impact of moisture from completely dry to
full saturation. A 50mm PEEK bar with a CLTE of 2.6 x 10-5 in/in/°F grew 0.2mm in length compared to the same PEEK bar growing only 0.03mm from dry
to saturation with a measured moisture saturation value of 0.5%. It is important to note that every material will differ in the results.
It is critical to understand how these properties can effect your finished applications. Port Plastics is your trusted neutral source of expertise for all
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