Nylon Moisture Absorption: How Water Affects PA6 and PA66 Properties

Nylon absorbs water from the air — up to 9% of its own weight at saturation for PA6. This property fundamentally changes the material’s dimensions, strength, and stiffness in service. Parts designed using dry-as-molded data sheet values will behave completely differently after weeks of ambient exposure. This guide explains exactly what happens and how to design for it.

The Numbers: Dry vs Conditioned

Scientific illustration showing hygroscopic nylon part absorbing water molecules

Недвижимость	PA6 Dry	PA6 at 50% RH (2.5% H2O)	PA6 Water-Saturated (9%)
Прочность на разрыв	80 МПа	50-55 MPa	35-40 MPa
Модуль упругости	2,800 MPa	1,200 MPa	500 MPa
Ударная прочность	5 kJ/m²	15-25 kJ/m²	>50 kJ/m² (NB)
Dimensional Change	Baseline	+0.3-0.5%	+1.0-1.5%

Design Implications

Nylon bearing installed in metal housing: left image dry-as-machined fit loose c

The stiffness loss is the biggest practical challenge — a nylon part that feels rigid and strong straight out of the mold becomes noticeably more flexible after several days in ambient air. For structural brackets, design using conditioned properties (50% RH values), not dry data sheet numbers. The impact toughness increase, however, is a benefit — dry nylon is brittle, but conditioned nylon absorbs impact without shattering, which is why nylon zip ties are flexible and tough rather than stiff and brittle.

Часто задаваемые вопросы

Three engineering plastic samples in humidity chamber: nylon showing moisture ab

How long does it take nylon to reach moisture equilibrium?

Thin-walled parts (under 2mm) reach 50% RH equilibrium in 24-48 hours. Thick sections (over 6mm) can take weeks. Water immersion accelerates the process to hours. For production parts tested for dimensional compliance, always note the conditioning state on the inspection report.

Is nylon dimensionally stable after it absorbs moisture?

It stabilizes — but at a larger dimension than dry. A 100mm PA6 part grows to 100.3-100.5mm at 50% RH equilibrium. If your assembly clearance relies on the dry-as-molded dimension, the parts will interfere after conditioning. Design with the conditioned dimension as the nominal.

Can I seal nylon to prevent moisture absorption?

No coating or surface treatment permanently prevents nylon moisture absorption — water molecules diffuse through the polymer matrix at the molecular level. For applications where moisture absorption is unacceptable, switch to acetal (POM) or PBT, which absorb negligible moisture.

Does glass fiber reduce moisture absorption?

Yes — glass fibers don’t absorb water, so PA66-GF30 absorbs roughly 2/3 of the moisture of unfilled PA66 (because 30% of the volume is inert glass). This is one reason glass-filled nylon is preferred for under-hood automotive applications where humidity varies dramatically.

Concerned about nylon moisture effects in your design?
We can recommend alternative materials or design compensations. Acetal (POM), PBT, and PEEK all offer excellent dimensional stability without the moisture sensitivity of nylon.