Water Absorption

Sprachauswahl/Language selection

Dieser Artikel ist auch auf Deutsch verfügbar Wasseraufnahme

A service provided by
Polymer Service GmbH Merseburg
Tel.: +49 3461 30889-50 E-Mail: info@psm-merseburg.de Web: https://www.psm-merseburg.de
Our further education offers: https://www.psm-merseburg.de/weiterbildung
PSM on Wikipedia: https://de.wikipedia.org/wiki/Polymer Service Merseburg

Water absorption

Fundamentals

Several different effects occur in plastics that are subjected to medial stress by water, e.g:

• dimensional changes (swelling) caused by water absorption,
• extraction of water-soluble components and
• changes in various properties.

From a physical point of view, water absorption is a diffusion process.

Experimental methods

The methods for determining water absorption are described in ISO 62 [1]:

• Method 1: Determination of water absorption in water at 23 °C
• Method 2: Determination of water absorption in boiling water
• Method 3: Determination of water-soluble components
• Method 4: Determination of water absorption after storage at 50 % relative humidity (see: standard climate)

According to the standard, exposure to humidity, immersion in water at 23 °C and boiling water can cause completely different material behaviour. Immersion in water at 23 °C and storage at 100 % humidity are practically equivalent in terms of their effects [2]. The amount of water absorbed when the equilibrium state is reached can be used to compare different plastics. The carefully controlled, non-equilibrium exposure of test specimens made of plastics with precisely defined dimensions can be used to compare different charges of the same material and to determine the diffusion constant of the material to be tested [1].

Physical-chemical processes

The possibility of chain cleavage by reaction with water (hydrolysis) exists for polymers that contain ester, amide or similar functional groups in the main chain. Acids and alkalis can act as catalysts here and favour chain cleavage, especially in hot water, which thus represents the reverse reaction of polymer synthesis. Due to their hydrophobic phenyl groups, aromatic polyesters only absorb very few aqueous solvents, so that hydrolysis is only observed at elevated temperatures and generally have good cold water resistance [3].

Physico-chemical processes in connection with hydrolysis can be, for example, the local change in crystallinity, the release of additives or softening when water is absorbed.

Typical mechanical processes are, for example, the formation of residual stresses as a result of locally different swelling processes or the abrasive damage of plastic surfaces in the presence of aqueous solutions with high flow rates, e.g. during the transport of liquids or natural weathering.

Determination of characteristic values

The principle of the quantitative description of water absorption consists of immersing test specimens in distilled water at 23 °C or in boiling distilled water or exposing them to climates with 50 % relative humidity at specified temperatures and for a specified period of time [1]. The amount of water absorbed is calculated by determining changes in mass using commercially available weighing instruments (error limit ± 0.1 mg).

The test specimens used are square or round panels, tubular or rod-shaped test specimens or test specimens made of prefabricated profiles, extruded parts, panels and laminates.

The water absorption c is described as the difference between the mass of the test specimen after immersion m₂ (swollen test specimen) and the mass of the test specimen after drying m₁ (extracted test specimen)

$c\,=\,m_{2}-m_{1}\ \left[mg\right]$

or as a relative change in mass as a percentage (%)

$c\,=\,{\frac {m_{2}-m_{1}}{m_{1}}}\cdot 100\ \left[\%\right]$

or

$c\,=\,{\frac {m_{2}-m_{3}}{m_{1}}}\cdot 100\ \left[\%\right]$

where are:

m1		Mass of the test speciemen after the first drying and before immersion in mg
m2		Mass of the test specimen after immersion in mg
m3		Mass after immersion and final drying in mg

Tab. 1: Water absorption of various plastics in cold water [2]

Material	Water absorption after 4 d (mg)
CA	100–250
PA	50–800
PC	5–10
PE	ca. 0
PF , Typ 31	100–180
PF, filler free	10–20
PMMA	30–40
POM	20
PS	2–5
PVC-copolymerisate	5–20
UF, Typ 131	200–300
Vulkanized fibre	1500

See also

• Ageing
• Standard atmospheres
• Test climate
• Plastography
• Moulding compound test

References

[1]	ISO 62 (2008-02): Plastics – Determination of Water Absorption
[2]	Stoeckhert, K., Woebcken, W. (Eds.): Kunststoff-Lexikon. Carl Hanser Munich Vienna (1998), 9th Edition, p. 605, (ISBN 3-446-17969-0) (see AMK-Library under G 3)
[3]	Franck, A.: Kunststoff-Kompendium. Vogel Buchverlag, Würzburg (2000) 5th Edition, p. 258 (ISBN 3-8023-1855-2)