Differences in the moisture absorption of engineering thermoplastics
How much moisture various engineering thermoplastics such as PA, POM, PC, PET and PEEK absorb and what effects this has.
Moisture absorption is one of the decisive properties of engineering thermoplastics and influences both the processing and the possible applications of a component. It determines how reliably a plastic maintains its dimensions, whether it remains mechanically stable in the long term and how well it is suited for electrical insulation purposes.
Even small differences in water absorption can significantly change performance – a detail that in industries such as mechanical engineering, electrical engineering, medical technology or aerospace decides the success of an application. For developers and designers, this means always including the hygroscopicity of a material in material selection and design planning.
Why moisture absorption matters
Thermoplastics are not completely closed but can absorb moisture from the environment. This process usually occurs through diffusion and depends on the chemical structure of the polymer. Polymers with polar groups (such as polyamides) absorb significantly more water than non-polar materials (such as POM or PE). Even a small uptake can change dimensional stability, influence mechanical strength and impair electrical insulating properties. In some cases, however, moisture also has positive effects, for example increased impact strength or reduced brittleness.
The consequences are diverse and affect both mechanical strength and electrical properties, dimensional accuracy, service life and even surface quality. If too much moisture is absorbed, this can reduce stiffness and strength, worsen creep resistance and at the same time diminish insulating properties. In addition, affected components tend to change their dimensions through swelling or shrinking, which leads to problems in precision applications. Surface quality can also suffer, as stress cracks may form or optical transparency may be impaired.
Differences by plastic type
Polyamide (PA)
Polyamide is one of the most hygroscopic engineering thermoplastics. Depending on the type, between 1.5 and 3 % water can be absorbed. This leads to significant swelling and dimensional change, which is problematic for precision components. At the same time, toughness increases sharply with rising moisture, which gives PA advantages in dynamic applications such as gears or plain bearings.
In a dry environment, PA is rather brittle, which makes correct conditioning in use particularly important. For designers, it is crucial to choose larger tolerances and to allow for longer conditioning times. In electrical applications, PA can only be used to a limited extent, as the insulating properties suffer greatly from water absorption. In addition, water absorption can influence the melting point and viscosity during processing, which is why careful pre-drying before injection moulding is indispensable.
Polyoxymethylene (POM)
POM absorbs very little moisture (below 0.5 %). As a result, it remains extremely dimensionally accurate and is preferred for precision parts. Typical applications are gears, valves, pump housings or bearings where reliability and constant dimensions are decisive. Its electrical properties also remain largely unaffected. Another advantage: POM remains stable even after prolonged storage in humid environments.
The combination of low water absorption, good sliding properties and wear resistance makes POM one of the most important engineering plastics in mechanical engineering. In humid environments or in contact with water, POM changes its dimensions only minimally, which makes it particularly suitable for components in pumps, fittings or in the sanitary sector. Pre-drying is usually not mandatory before processing, but it can improve surface quality.
Polycarbonate (PC)
Polycarbonate is in the middle range with 0.2–0.3 % moisture absorption. It is less critical than polyamide, but not as stable as POM or PET. Particularly in optical applications (e.g. housing parts or transparent covers), slight water absorption can lead to stress cracks or surface changes. In addition, moisture can influence impact strength and intensify ageing under UV exposure.
In mechanical engineering, PC is often used where high transparency and impact strength are required, for example in protective covers, displays or optical components. Outdoors, the combination of UV radiation and moisture can lead to microcracks, which is why UV-stabilised variants or coatings are recommended. Drying is advisable before processing, as residual moisture at high processing temperatures can cause hydrolysis and clouding.
Polyethylene terephthalate (PET)
PET absorbs about 0.2–0.4 % water. It is therefore dimensionally stable, but heat and prolonged moisture can influence its crystal structure. In plain bearings or in the food sector, PET plays to its strengths, as it offers a good balance between dimensional accuracy and wear resistance. For outdoor applications, the interaction between moisture and UV radiation should be taken into account.
PET has high hardness and wear resistance, combined with good chemical resistance to many media. Due to the relatively low water absorption, dimensional accuracy is well maintained even with fluctuating humidity, which is why PET is used in precision bearings, timing belt guides or in contact with food. However, pre-drying before processing is necessary, as moisture in combination with high temperatures can trigger hydrolysis processes.
Polyetheretherketone (PEEK)
PEEK absorbs virtually no moisture at below 0.1 %. It remains stable even at high temperatures or in aggressive environments. This makes PEEK a preferred material for medical technology, aerospace or highly stressed machine parts where precision and reliability are required under extreme conditions. Its excellent chemical resistance also helps to ensure that moisture has practically no effect.
PEEK combines high heat deflection resistance, excellent mechanical properties and minimal water absorption, which predestines it for applications where even the smallest dimensional changes would be unacceptable. Typical fields of application are implants, bearings in the high-temperature range or structural elements in aviation. During processing, drying is usually only a precautionary measure, but it contributes to optimum surface quality and process reliability.
Other relevant plastics
Other plastics such as PPS (polyphenylene sulphide), PEI (polyetherimide) or PSU (polysulphone) also exhibit very low moisture absorption and are therefore used in electronics and high-temperature applications. By contrast, materials such as ABS or PMMA also absorb moisture, albeit to a lesser extent, which can influence their optical and mechanical properties. Special blends or fibre-reinforced variants of these plastics can further reduce water absorption.
Comparison table: moisture absorption of selected thermoplastics
| Plastic | Typical water absorption (%) | Effects |
|---|---|---|
| PA (polyamide) | 1.5–3.0 | High swelling, better toughness, limited dimensional accuracy |
| POM | < 0.5 | Very dimensionally stable, constant electrical properties |
| PC | 0.2–0.3 | Moderate absorption, possible stress cracks |
| PET | 0.2–0.4 | Good stability, possible long-term changes |
| PEEK | < 0.1 | Excellent dimensional accuracy, hardly any effects even under extreme conditions |
| PPS / PEI | < 0.1 | Suitable for electronics & high-temperature applications |
Practical consequences for the application
- Design: For plastics with high moisture absorption such as PA, tolerances must be chosen more generously to compensate for dimensional changes.
- Storage: Hygroscopic plastics should be stored in a dry environment. Welded bags with desiccant or special drying cabinets are ideal.
- Processing: Before injection moulding or extrusion, pre-drying is indispensable. Even slight residual moisture can lead to hydrolysis and impair the mechanical properties.
- Operating environment: For precision parts or electrical applications, low-moisture materials such as POM, PET, PPS or PEEK are preferable. In dynamic applications where toughness is more important, however, PA materials can offer advantages.
- Service life and maintenance: For components made of hygroscopic plastics, dimensional changes must be expected during operation. Regular inspections and adapted design planning increase operational reliability.
- Coatings and modifications: In certain cases, materials can be specifically adapted to the operating conditions through coatings, glass fibre reinforcement or additives.
Summary
The differences in the moisture absorption of engineering thermoplastics are considerable and affect many properties of a component – from dimensional accuracy through mechanical resistance to electrical insulating properties. While polyamide binds large amounts of water and therefore swells strongly, high-performance plastics such as POM, PET, PEEK or PPS show very high stability.
The choice of material therefore always depends on the specific field of application: dynamic applications benefit from the toughness of moist PA materials, while precision parts rely on low-moisture polymers. For companies, this means carrying out a targeted weighing of toughness, resistance and dimensional accuracy already in the planning phase. Anyone who knows the relationships can not only avoid expensive mistakes but also decisively improve the service life and reliability of their products.
FAQ
Why does polyamide absorb so much water? The molecular structure of PA contains polar amide groups that form hydrogen bonds with water molecules. As a result, the material can absorb moisture particularly strongly.
How can moisture absorption be reduced? In addition to suitable storage and pre-drying, additives, fillers or special coatings can also reduce water absorption. In some cases, material substitution is the best solution.
Which plastics are suitable for precise gears? POM, PET and PEEK are ideal for precision gears due to their low moisture absorption and high dimensional accuracy.
How does moisture affect electrical properties? Moisture increases conductivity and reduces insulating capability. This can be critical, particularly in switchgear or connectors.
Are dry plastics also stable in the long term? Not necessarily. Even after pre-drying, hygroscopic plastics absorb moisture again during use. Material selection should therefore always take the real environmental conditions into account.
Are there standards or test methods for determining moisture absorption? Yes, common standards such as ISO 62 or ASTM D570 specify methods for measuring the water absorption of plastics and thus enable different materials to be compared.
