Nylon and Moisture Absorption
Nylon resins absorb moisture from the air. The properties of nylon moldings are directly affected by the amount of moisture they contain. Moisture acts as a plasticizer in nylon and therefore reduces strength and stiffness properties but increases elongation and toughness. In general, as moisture content rises, significant increases occur in impact strength and other energy absorbing characteristics of the material.
Why Do Polyamides Absorb Moisture?
Nylons, having a ‘polar’ structure attract moisture. The amount of moisture absorbed depends on several factors. The temperature and relative humidity, the exposure time, the type of nylon and the part thickness, all influence the moisture absorption characteristics for a given nylon. In general, a nylon part that has a higher crystallinity will absorb less moisture than the same part that was molded with less crystallinity.
In hydrogen bonding the hydrogen atom on one polymer chain wants to bond to an oxygen atom on another polymer chain. This hydrogen bonding is one of the reasons that nylons have good thermal and mechanical properties.
Moisture absorbed into nylon also wants to hydrogen bond. The hydrogen atom in the water bonds to an oxygen atom in the nylon polymer chain. At the same time, the oxygen atom in the water bonds to the hydrogen atom in the nylon polymer chain. This hydrogen bonded water molecule acts like a plasticizer, allowing the nylon polymer chains to slide easily along one another. This is why nylon drools and flashes if it is injection molded wet. Moisture absorbed onto the surface of the nylon resin or part can generally be removed by drying for 3 or 4 hours. However, for moisture that has absorbed more deeply, a much longer drying time is needed, sometimes up to 24 hours or more. In order for this moisture to be removed it is necessary to not only break the hydrogen bonds but to allow sufficient time for the water molecules to diffuse to the surface.
Effects of Moisture Absorption on Properties
The water molecules that hydrogen bond between the nylon polymer chains cause the to move farther apart from each other. This causes a ‘loosening’ of the molecular attraction which causes a reduction in mechanical properties and an increase in fluidity. The glass transition temperature reduces, and can drop by 20°C or more. The strength and stiffness decrease while the impact and toughness increase. The amount of reduction in mechanical properties is dependent on the amount of moisture absorbed.
How Much Moisture is Absorbed?
Amco Polymers offers a range of products within the various Polyamide sub-families. These include Nylon 11, Nylon 12, Nylon 46, Nylon 6, Nylon 66, Nylon 612, PARAs and PPAs. A wide range of additives, fillers and reinforcements can be incorporated to further enhance physical properties. We can also develop customized compounds.
Let Amco engineers help you or your company properly design your Nylon parts while taking into account your end-use requirements and how the moisture absorption characteristics may affect part performance. We offer a range of tools to help you design your part. Let us help!
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