Swelling

Swelling results from the interaction between a polymer and a substance that penetrates the material. Common causes are:

• Chemical interaction: polymers absorb substances such as oils, solvents or water, which affect their molecular structure.

• Temperature: higher temperatures accelerate swelling, as they promote the diffusion of substances into the material.

• Polarity: polymers tend to absorb substances of similar polarity (e.g. polar solvents in polar polymers).

Swelling is usually described as the percentage increase in volume of a material:

Degree of swelling (%) = [(volume after swelling - initial volume) / initial volume] × 100

Alternatively, the increase in mass can be measured, particularly for liquid absorption:

Degree of swelling (%) = [(mass after swelling - initial mass) / initial mass] × 100

Swelling is influenced by various factors:

• Type of material: elastomers such as NBR (nitrile rubber) are particularly susceptible to oil absorption, while EPDM is resistant to water and steam.

• Medium: polar substances such as water or alcohols affect polymers differently from non-polar substances such as oil or petrol.

• Temperature: a higher temperature accelerates diffusion and increases swelling.

• Contact time: the longer the polymer is exposed to the medium, the greater the swelling.

Swelling is an important factor in material selection and product development:

Positive applications

• Seals: controlled swelling in O-rings and seals can improve sealing, as the material completely fills the gap.

• Active materials: in hydrogels, swelling is used to store substances such as medicines and release them in a controlled way.

Negative effects

• Loss of shape: excessive swelling can lead to deformation and impaired function.

• Mechanical weakening: absorbing liquids can cause the material to lose strength.

• Material failure: prolonged swelling can damage the material irreversibly or change its chemical composition.

A material’s resistance to swelling depends on its chemical structure:

Material	Resistant to	Sensitive to
EPDM	Water, steam	Mineral oils, fuels
NBR	Oils, greases	Water, ozone
FKM (Viton®)	Oils, chemicals	Polar solvents
Silicone	Temperature, water	Oils, solvents

Controlling and predicting swelling is often difficult:

• Choice of material: requires precise knowledge of the interaction between material and medium.

• Long-term effects: long-term swelling can lead to material fatigue and failure.

• Temperature dependence: different operating conditions make planning complex.

• How is swelling tested? Typically by immersing a material in the relevant medium and measuring the change in volume or mass.

• Which materials are particularly resistant to swelling? Materials such as EPDM and FKM are resistant to many media, while NBR is particularly resistant to oils.

• Can swelling be reversed? Swelling is usually reversible, as long as the material has not been chemically attacked or damaged.

• How can swelling be minimised? By choosing materials suitable for the specific medium and by using coatings or barriers.

Swelling is a central aspect in the rubber and plastics industry and can have both positive and negative effects on material performance. Careful material selection and precise control of operating conditions are crucial to use or avoid swelling effectively. Knowing the swelling behaviour of a material is essential to ensure the function and longevity of products.