Selecting the perfect water absorbent material can be challenging. When considering a water-absorbing polymer material 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 — although they aren't the best water-absorbing material.
Differences in Water Absorption Rates in Plastic
Various plastic compounds absorb varying amounts of water and the presence of absorbed water in plastics can significantly affect their performance in different ways. Here are two:
1. Electrical conductivity of plastic
The water absorption rate can cause some plastics to be preferred over others for applications that include underwater use or in high humidity.
For example, polyethylene is preferred in many applications requiring a dielectric because the electrical properties in plastics noticeably change with water absorption and polyethylene absorbs virtually no water.
2. Dimension changes
In addition to affecting 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 acetals are preferred because of their low rate of water absorption.
How to Test Plastics for Water Absorption
Utilizing the ASTM D570 plastic test standard, there are three basic water absorption test procedures that can determine percentage increases in weight after plastic compounds are exposed to water under specified conditions:
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 a low or zero rate of water absorption is crucial for your plastic parts, in addition to polyethylene (UHMW) and acetal, consider polypropylene and polytetrafluoroethylene (Rulon and Teflon). They're extremely low water absorbent materials.
