Design Properties for Engineers: Water and Moisture Absorption 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
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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