Water Absorption in Plastics
Today’s topic is “Water Absorption in Plastics.”
Do plastics actually absorb water?
What do you think?
The answer is: it depends on the type of plastic.
Some absorb only a little, some absorb a lot, and some absorb virtually no water at all. The degree to which a plastic absorbs water is expressed as its water absorption rate.
This rate is determined by standardised test methods such as JIS or ISO, which define the sample size, immersion time, water temperature, and other factors.
What happens when plastics absorb water?
Just like dried food—such as fu (wheat gluten) or dried seaweed—plastic materials that absorb moisture can swell and deform. Let’s look at how various plastics are often grouped by their water absorption rate (indicative ranges):
● Water Absorption Rate: 1% or higher
- Cellulose-based resins
- Nylon (polyamides)
- Urethane resins
● Water Absorption Rate: 0.1–1%
- Phenolic resins
- Polyether ether ketone (PEEK)
- Polyacetal (POM)
- Polyethylene terephthalate (PET)
- Acrylic resins (PMMA)
- Polycarbonate (PC)
● Water Absorption Rate: below 0.1%
- Teflon (PTFE)
- Polyethylene (PE)
- Polypropylene (PP)
- Polyphenylene sulfide (PPS)
- Polyvinylidene chloride (PVDC)
(Note: Depending on the data source and test method, actual values may vary.)
Why do some plastics absorb more water?
Plastics with low water absorption are often composed primarily of elements such as carbon (C) and hydrogen (H). In contrast, materials with higher absorption tend to contain oxygen (O) or hydroxyl groups (OH). Since water is H2O, it has a natural affinity for oxygen-containing groups. This is a general trend (not an absolute rule) that helps explain differences between materials.
In the plastics industry, nylon is well known for its water absorption. Even within nylon, there are multiple types:
- Nylon 6
- Nylon 66
- Nylon 11
- Nylon 12
When sorted by typical absorption, a common order is:
Nylon 6 (≈1.3–1.9%) > Nylon 66 (≈1.5%) > Nylon 11 (≈0.4%) > Nylon 12 (≈0.25%)
This shows that even within the same family, water absorption can differ by more than 1%. There is also monomer-cast nylon (MC nylon), often used in bearings and machined components, which is a modified nylon with improved performance characteristics.
In other words, not all nylons are equally absorbent. Nylon 6 and Nylon 66 have comparatively high absorption and are widely used in textiles—where some moisture uptake and, more importantly, fibre structure can aid comfort. By contrast, polyester (PET) generally has lower intrinsic water absorption than nylon; in apparel, its moisture management relies largely on fibre geometry and capillary wicking rather than true absorption.
Sponges often use highly absorbent materials such as cellulose or urethane foams, and their structure is designed with many small pores to maximise water retention.
Another example is superabsorbent polymer (SAP), which can absorb hundreds of times its own weight in water. SAPs are used in applications like:
- Disposable diapers
- Air fresheners
- Soil moisture retainers for horticulture
- Contact lenses
These are examples where high water absorption is desirable. In many industrial applications, however, the opposite is true—low water absorption is preferred or even essential.
Take Teflon (PTFE), for example. This material essentially does not absorb water. As mentioned in a previous article, it is used in the roof of the Tokyo Dome, where even small amounts of water could add unwanted weight and stress to the structure.
So, some plastics absorb a lot of water, others very little, and some virtually none. That doesn’t mean high absorption is always good or low absorption is always bad. Each material has strengths suited to specific applications.
At Kashima Bearings, higher water absorption can sometimes be a disadvantage—especially for plain bearings.
Plain bearings require a small clearance between the bearing’s inner diameter and the shaft. If a water-absorbing plastic swells in a humid or wet environment, that clearance may shrink, leading to rotation problems.
That’s why, in applications where moisture exposure is expected, we choose low-absorption materials or account for the expected expansion in our design to ensure proper performance.
