Selecting the Right Material: Water Absorption

When considering a plastic compound to use for a custom machined part and a specific application, some typical properties to evaluate are temperature, impact strength, and wear resistance. Thinking about how much water a plastic component can absorb might not come immediately to mind.

But like many materials, plastics can absorb water – various plastic compounds absorb various amounts of water and the presence of absorbed water in plastics can significantly affect their performance in different ways. The rate of water absorption can cause some plastics to be preferred over others for applications that include use under water or in high humidity.

For example, polyethylene is preferred for use in many applications that require a dielectric because electrical properties in plastics noticeably change with water absorption and polyethylene absorbs virtually no water.

In addition to effecting changes in electrical conductivity, water can cause changes in a plastic product’s dimensions when absorbed in large amounts. When dimensional stability is required, plastics such as various types of acetal are preferred because of their low rate of water absorption.

Testing plastics for water absorption

Utilizing the ASTM D570 plastic test standard, there are three basic water absorption test procedures that can determine percent increases in weight after plastic compounds are exposed to water under specified conditions:

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Source: UL LLC

The first procedure above, which provides the percentage of water absorbed in a 24-hour period, is the most important when considering plastics to use in custom plastic machined parts that will be used in high humidity applications. As noted earlier, the more a plastic product absorbs water the more its dimensions can change which can result in a significant loss of performance or even failure in an application.

If low or zero rate of absorption is crucial for your plastic parts, in addition to polyethylene (UHMW) and acetal, consider polypropylene, polytetrafluoroethylene (Rulon and Teflon).