Volume Swelling Elastomers

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Volume Swelling Elastomers

General information

The volume change of elastomers and thermoplastic elastomers refers to the processes of volume swelling and volume shrinkage, which cause an increase or decrease in volume.

The volume change of elastomers and thermoplastic elastomers as a reaction to liquids/media (e.g. oils or greases) represents a quantifiable indicator for characterizing the chemical resistance to media. A media-chemically induced attack on the macromolecular structure and the additives used often leads to a change in the physical-mechanical properties or the use properties. The type of material (see also: Polymer & structure), the acting medium, their mutual compatibility as well as the media exposure time and the temperature influence the migration behaviour of liquids into the elastomer material.

Depending on the chemical resistance/solubility to liquids/media, volume changes occur in the following forms:

• Volume swelling/increase
• Volume shrinkage/ processing shrinkage/decrease
• complete material decomposition (chemically induced decomposition of the cross-linking structure (link to cross-linking elastomers))

Volume swelling is when the medium acts as a kind of solvent, penetrates the macromolecular structure and is deposited there, resulting in an increase in volume compared to the initial state.

Volume shrinkage, on the other hand, is characterized by the extraction of soluble additives/components and an associated decrease in volume.

Determination of volume swelling elastomers

Depending on the area of application of the elastomer materials, it is necessary to know the compatibility with media such as greases, oils or fuels so that damage due to a change in volume can be avoided.

The volume change of an elastomer material in a liquid/medium can be determined in accordance with ISO 1817 [1]. The test liquid must be at least 15 times the total volume of the completely immersed samples. The immersion time of the elastomer material in the liquid/medium is usually not limited to a predetermined end time and should instead be extended beyond reaching the absorption maximum. Within the framework of control tests and the empirical values already obtained in preliminary tests, a single immersion time can be selected in which the absorption maximum is reached.

To determine the volume change, the masses are determined before and after immersion in a liquid/medium, whereby the weighings are carried out in air and in deionized water, but also alternatively in a test liquid. This procedure corresponds to the determination of density using the immersion method (buoyancy method). The balance used for weighing must not exceed a maximum measurement uncertainty of 1 mg.

If the masses are weighed in deionized water, the percentage change in volume is calculated according to equation (1).

$\Delta V_{100}=\left({\frac {m_{i}-m_{i,m}+m_{s,w}}{m_{0}-m_{0,w}+m_{s,w}}}\right)$

(1)

where

Δ V100		is the percentage change in volume;
m0		is the initial mass of the sample;
mi		is the mass of the sample after immersion;
m0,w		is the initial mass of the sample in water (with additional weight, if used);
mi,w		is the mass of the sample after immersion in water (with additional weight, if used);
ms,w		is the mass of the additional weight in water, if used.

If, on the other hand, the masses are weighed in the test liquid used, the percentage change in volume is calculated according to equation (2).

$\Delta V_{100}=\left({\frac {m_{i}-m_{i,liq}+m_{s,liq}}{m_{0}-m_{0,w}+m_{s,w}}}\right)$

(2)

where

m0		is the density of the liquid
mi		is the mass of the sample after immersion;
mi,liq		is the mass of the sample (with additional weight, if used) in the liquid;
ms,liq		is the mass of the additional weight in the liquid, if used.

The minimum number of 3 samples per test series used to determine the volume change in accordance with the standard should have the same initial thickness (2 mm ± 0.2 mm) and a volume of 1 cm³ to 3 cm³, whereby the time interval between vulcanization and testing must be at least 16 hours, but must not exceed 3 months in the case of product testing.

The result of the percentage volume change is given as a median value for at least three test samples.

Explanation of terms

The term ‘test piece’ is used in the standard for determining the change in volume, whereas the term ‘sample’ is used without exception in the context of this lexicon in plastics analysis in contrast to the term ‘ test specimen’. The use of the term ‘test piece’ is limited to the reproduction of the titles of the standards.

See also

• Ageing elastomers
• Thermosets
• Density
• Degree of cross-linking elastomers
• Vulcanization

References

[1]	ISO 1817 (2024-03): Rubber, Vulcanized or Thermoplastic – Determination of the Effect of Liquids

Standards information

• ISO 23529 (2016-11): Rubber – General Procedures for Preparing and Conditioning Test Pieces for Physical Test Methods
• ISO 175 (2010-10): Plastics – Methods of Tests for the Determination of the Effects of Immersion in Liquid Chemicals
• ISO 13226 (2018-06): Rubber – Standard Reference Elastomers (SREs) for Characterizing the Effect of Liquids on Vulcanized Rubbers