Wednesday, 9 December 2020

Design Properties for Engineers: Water and Moisture Absorption of High Performance Polymers


Design Properties for Engineers: Water and Moisture Absorption of High Performance Polymers


In this blog post, we discuss the water and moisture of high performance polymers.

Important for material selection is the behavior of polymer compounds when they are exposed to water (part immersion) or humid environment. Contact of plastics with water and humidity is possible; however water can diffuse into the plastic and change its physical and dimensional properties. This is depending on the contact time and geometry of the plastic part.

Depending on the polymer, water remains on the surface or it diffuses into the polymer. Diffusion lowers the inter-molecular binding forces and in turn increases the chain mobility. As a consequence, mechanical strength is reduced. Furthermore, electrical and other physical properties are reduced too. Also, dimensional changes occur. Polymers which show a strong volume change (high diffusion rate) are called hygroscopic.

The good message is that most of those processes are physical nature and are reversible by applying a proper drying process. However, when polymers are often exposed to water vapor, the risk of hydrolysis (=chemical reaction in which water molecules rupture one or more chemical bonds and can lead to chain breakage) is much higher compared to normal water exposure.

Measurement standards

Classification of water and moisture uptake is done by using ISO 62 (water absorption 24 hours, 23°C) and/or ASTM D570.

Low water uptake

Among the high performance polymers, fluoropolymers such as PTFE and PVDF take up very low amounts of water. The same is valid for Polyphenylene Sulfide (PPS). Polysulfones (PSU, PESU, PPSU) take up a limited amount of water. Polyarylketones (PEEK, PEK, PEKEKK) absorb low amounts of water too.  Hydrolysis resistance of the aforementioned materials is outstanding. Water vapor sterilization is several times possible without compromising on the properties.

Hygroscopic high performance polymers

On the other hand, Polyimides are hydroscopic. They take up high amounts of water already in normal climate conditions (50% humidity in air). Direct contact with water results in even more water uptake (e.g. PBI: 14%). Hydrolysis resistance is lower compared to PEEK or PPS and as a result cracks are formed over time when exposed to water. If the plastic part is wet and will be rapidly heated (in case of high temperature applications), expansion of water turning into vapor can cause damage to the part.

A small water uptake of PAI is influencing the physical properties immediately: elongation increases more than 10% at 2% water uptake. Impact strength increases 20% compared to the starting point. Sometimes, changes due to water uptake can be an advantage too. There are lots of parts which need to be mounted and in such cases it is beneficial to have more elongation due to water uptake compared to a dry part.

Polyphthalamide (PPA), especially PA6T/6I and long chain PPA (PA9T and PA10T) show much lower water and moisture uptake compared to aliphatic polyamides. 

A word on moisture uptake and dimensional changes after moulding 

Moisture absorption begins in the moment the part leaves the mould (in particular for hygroscopic materials).

As moulded, the moisture content of the part is approximately equal to that of the pellets that went into the moulding machine. Let us assume that e.g. an aliphatic Polyamide such as PA 6.6 is properly dried before moulding, this would place the moisture content for a part produced from unfilled PA 6.6 below 0.20% (referred to as dry-as-moulded).

Water molecules force the polymer chains to increase and this leads to volumetric expansion. The part size increase can be equal to 0.5-0.6% in an unfilled PA 6.6 (at room temperature; higher temperatures results in higher changes). However, glass fiber reinforced compounds can reduce the dimensional changes down to 0.1%.

Thanks and #findoutaboutplastics

Greetings,

Herwig

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Literature:

[1] Erwin Baur, Tim A. Osswald, Natalie Rudolph: Saechtling Kunststoff Taschenbuch, Hanser Munich 

[2] https://www.ptonline.com/articles/dimensional-stability-after-molding-part-4

 



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