Plastic Part Design Properties for Engineers - Water Uptake of Aliphatic Polyamides

Hello and welcome back to a new post. Today we discuss the water and moisture uptake of aliphatic short and long chain Polyamides. In a previous post I discussed the water uptake for high performance polymers - check it out here. Here you can find a collection of all my "Design Properties for Plastics Engineering" posts. 

Properties of Polyamides

In general, Polyamides are often used as engineering material due to their high thermal stability, very good strength and hardness, combined with high mechanical damping characteristics and good chemical resistance. However, all Polyamides are hygroscopic due to the polar amide groups which form hydrogen bonds with water. Water absorption (at a given temperature and relative humidity) is proportional to the amount of amorphous part of the Polyamide. As a consequence, the water acts as a plasticizer and lowers the mechanical properties. At higher temperatures, hydrolysis can take place too. 

How much is the water uptake of aliphatic Polyamides? 

Figure 1 shows the water uptake situation of the most used aliphatic Polyamides at equilibrium in 50% relative humidity and at equilibrium in complete saturation.

Figure 1: Water uptake data of most used aliphatic Polyamides

Long chain aliphatic Polyamides such as PA 6.10, PA 6.12, PA 11, and PA 12 show a lower water absorption compared to PA 6, PA 6.6, and PA 4.6.  Higher dimensional stability, together with low variation in the properties during ambient humidity changes are the result. Major reason for the lower water uptake is the relatively long hydrocarbon chain length (limiting the amide groups to form hydrogen bonds with water).

Important during material selection is the consideration of the  behavior of Polyamides when they are exposed to water (part immersion) or humid environment. The part dimensions need to be still kept within the specified tolerance. 

Thank you for reading and #findoutaboutplastics

Greetings, 

Herwig 

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Literature:

[1] https://www.findoutaboutplastics.com/2020/12/design-properties-for-engineers-water.html

[2] https://www.sciencedirect.com/science/article/pii/S2590048X20300911

[3] https://www.hanser-elibrary.com/doi/book/10.3139/9783446437296