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 don’t absorb any 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, which define the sample size, immersion time, water temperature, and other factors.
But 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 take a look at how various plastics are classified by their water absorption rate:
● Water Absorption Rate: 1% or Higher
- Cellulose-based resins
- Nylon
- Urethane resins
● Water Absorption Rate: 0.1–1%
- Phenolic resins
- Polyether ether ketone (PEEK)
- Polyacetal (POM)
- Polyethylene terephthalate (PET)
- Acrylic resins
- Polycarbonate (PC)
● Water Absorption Rate: Below 0.1%
- Teflon (PTFE)
- Polyethylene (PE)
- Polypropylene (PP)
- Polyphenylene sulfide (PPS)
- Polyvinylidene chloride (PVDC)
(Please note that depending on the data source and test method, actual values may vary.)
So, do high or low absorption rates tell us anything about a material’s properties?
It turns out they do—thanks to some basic chemistry.
Plastics with low water absorption are often composed primarily of elements such as carbon (C) and hydrogen (H). In contrast, materials with higher absorption rates tend to contain oxygen (O) or hydroxyl groups (OH). Since water is H₂O, it has a natural affinity for other oxygen-based groups. This is a general trend, not a strict rule, but it helps explain why some materials are more water-absorbent than others.
In the plastic industry, nylon is well known for its water absorption. And even within nylon, there are various types, such as:
- Nylon 6
- Nylon 66
- Nylon 11
- Nylon 12
When sorted by absorption rate, the order looks like this:
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’s also monomer-cast nylon, which is often used in bearings and machined components. It’s a modified type of nylon with improved performance characteristics.
In other words, not all nylons are equally absorbent.
Nylon 6 and Nylon 66 have particularly high absorption and are often used in textiles, where this property is an advantage—for example, in absorbing sweat.
You’ve probably also seen polyester listed on clothing labels. Like nylon, polyester has a relatively high water absorption rate and is commonly used in apparel. In addition to the material itself, design features like fine gaps between fibres are used to further enhance absorption. Think of how a sponge works.
Sponges often use highly absorbent plastics such as cellulose or urethane, and their structure is designed with many small pores to maximise water retention.
Another example is superabsorbent polymer (SAP), which can absorb 100 to 1,000 times its own weight in water. That’s a huge capacity. 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. But in many industrial applications, the opposite is true—low water absorption is preferred or even essential.
Take Teflon (PTFE), for example. This material essentially does not absorb water at all. 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.
As you can see, some plastics absorb a lot of water, others very little, and some none at all. But that doesn’t mean high absorption is good or low absorption is bad. Every material has its strengths, and each is suited to specific applications.
In our case at Kashima Bearings, high 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 during use in a humid or wet environment, that clearance may disappear, leading to rotation problems.
That’s why, in applications where moisture exposure is anticipated, we choose low-absorption materials or calculate the expected expansion in advance to ensure proper performance.
