How does moisture affect the properties of PA?

Polyamide (PA), colloquially also known as nylon, are among the most frequently used engineering thermoplastics. They offer a good combination of strength, toughness, abrasion resistance and chemical resistance. However, one critical factor significantly influences the performance of

PA

:

moisture

.

Hygroscopic behaviour of PA

Polyamides are characterised by hygroscopic behaviour – this means they actively absorb moisture from their surroundings. This property arises from the chemical structure of the plastic: the amide groups in the polymer network can form hydrogen bonds with water molecules. Particularly

PA6

and PA66 are known for absorbing water relatively quickly in a humid environment.

Moisture absorption occurs through diffusion and depends heavily on humidity and temperature. In everyday engineering use, the water absorption range for PA6 is around 2 to 3 % (under standard climate conditions), but can reach values of 8 to 10 % at full saturation. PA66 absorbs somewhat less moisture, but also remains distinctly hygroscopic. The process is reversible: in a dry environment, the

material

releases the moisture again, which likewise influences its properties.

This behaviour makes PA sensitive to fluctuations in the operating environment. It is therefore important to take the moisture content into account during design, processing and storage – especially in applications with tight dimensional requirements or mechanical loading.

Effects on mechanical properties

The absorbed water molecules act like a plasticiser within the polymer network. This changes the mechanical properties:

• Tensile strength decreases as moisture content rises
• Elongation at break increases significantly
• Stiffness (modulus of elasticity) is reduced
• Impact strength improves

These effects are reversible: if the component dries out again, the properties approach their original state.

Dimensional change and dimensional stability

A key effect of moisture absorption is the change in volume of the material. The incorporated water molecules cause the polymer to expand, which can lead to a noticeable dimensional change. These dimensional changes should not be neglected – depending on component geometry and water absorption, the increase in volume can amount to several percent. This effect is particularly noticeable with PA6.

In practice, this means that tight tolerances can hardly be reliably maintained after processing or where ambient humidity fluctuates strongly. Structural stresses or distortion are possible if parts are permeated with moisture to differing degrees or absorb more water at certain points than in other areas. Press-fitting or assembling PA components into other assemblies can also lead to problems if the dimensional change has not been factored in.

A further aspect

: These deformations occur with a delay – moisture absorption is a time-dependent process. PA components can therefore still change their shape weeks after production, which is critical in safety-relevant or functional applications.

Targeted

conditioning

of the components to a defined moisture level is therefore advisable, along with a corresponding

design with dimensional allowances

. In particularly sensitive cases, the use of less hygroscopic PA types such as PA12 can also be a suitable solution.

Thermal properties

The thermal properties of polyamides are significantly influenced by the presence of water. Moisture reduces, among other things, the glass transition temperature (Tg), i.e. the temperature at which the

plastic

transitions from a solid to a rubber-like state. This effect occurs because water acts as a molecular plasticiser, increasing the mobility of the polymer chains.

In practice, this means

: a moisture-permeated PA component becomes soft and deformable at significantly lower temperatures than a dried one. This can lead to problems in operation, particularly if the component is mechanically loaded or thermally stressed. Heat deflection temperature also falls as water content rises, which restricts the range of applications for thermally stressed components.

Furthermore, moisture can alter the thermal expansion behaviour of PA. The coefficient of linear expansion rises with increasing moisture, which must also be taken into account in addition to the dimensional change. In combination with fluctuating ambient temperatures, this results in complex deformation patterns that must absolutely be considered when selecting the material and designing the component.

For applications with permanently

elevated temperatures

or strongly fluctuating humidity, either an alternative material or targeted pre-conditioning is therefore advisable – including validation of the components under real operating conditions.

Electrical properties

Water acts as an electrical conductor – this has a negative effect on the insulating properties of PA:

• Dielectric strength decreases
• Tracking resistance deteriorates
• Specific resistance is reduced

In electrical applications (e.g. connectors, housings), dry storage and processing are therefore crucial.

Influence on processing

Moist PA granules lead to the following during injection moulding:

• Bubble formation
• Surface defects
• Reduced strength

Drying before processing

(typically 80 °C, 4–12 h) is therefore absolutely essential. The permissible residual moisture content is usually <0.2 %.

Measures for dealing with moisture

Selection of less hygroscopic PA types

Not all polyamides are equally sensitive to moisture. PA12, for instance, shows a significantly lower absorption capacity than PA6 or PA66. For applications where dimensional accuracy or electrical properties are critical, choosing a less hygroscopic PA type can be an effective measure.

Component design with tolerances for dimensional change

Since dimensional change due to moisture cannot be completely avoided with PA, tolerances should already be taken into account at the design stage. This means: choose dimensions such that the function remains guaranteed even in the moist state. Alternatively, a clearance can be provided for compensation.

Conditioning of parts to a defined moisture level

Targeted storage in an environment with controlled humidity allows a desired moisture content to be achieved in the component. This conditioning helps to ensure stable properties – particularly in applications where PA components are precisely matched to one another.

Drying before processing and assembly

Before PA granules are processed or components assembled, thorough drying is necessary. Only in this way can quality problems such as voids, bubbles or reduced strength be avoided. Industrial dryers with defined temperature and time profiles are essential for this.

Storage in dry, air-conditioned rooms

Even after production, PA components should not be stored unprotected in a humid environment. Ideally, storage takes place in airtight containers or air-conditioned rooms to prevent unwanted re-moistening. This safeguards quality until final processing or delivery.

Conclusion

Moisture influences PA in almost all property areas: mechanical, thermal, electrical and dimensional. Anyone designing or processing PA components must actively take the hygroscopic properties into account – through material selection, design, storage and processing. Only in this way can

durably functional and dimensionally accurate

components be realised.