Chemical resistance

The chemical compatibility list is intended to assist the user in determining the suitability of a variety of elastomers in many different chemicals.

The overview of chemical resistances helps the designer to select the material. The list is not exhaustive, but contains a very large number of chemicals. Their effect on materials such as EPDM, CR, PTFE or NBR is listed in 4 different categories.

Category 1 No to slight change, 0 to 5% volume swelling (very good)
Category 2 Slight to moderate change, 5 to 10% volume swelling (good)
Category 3 Moderate to strong change, 10 to 20% volume swelling (moderate)
Category 4 Not recommended/bad, as the material reacts very strongly
The note nn indicates that no values are available.

The basis is comprehensive laboratory tests, practical experience and expert opinions.

Volume swelling is only one indicator for determining the chemical resistance of elastomers. It is based solely on the influencing factor "solubility". A chemical attack on the polymer chain can also be characterized by a change in the physical properties, such as tensile strength, elongation at break or hardness.

In dynamic applications, a volume swelling of up to 15 or 20 percent is usually acceptable. However, it is important to note that higher compressions lead to

• a strong increase in friction and
• a decrease in resilience and
• a decrease in abrasion resistance

to the point where the use of a particular material becomes impossible.

Volume shrinkage is often caused by media that remove plasticizer from the rubber compound. A decrease in volume is usually accompanied by an increase in hardness. Just as swelling compensates for compression set, volume shrinkage intensifies the effect of compression set. In practice, this would be equivalent to pulling an O-ring seal away from the surfaces to be sealed - a leakage path is created.

It is therefore obvious that chemical shrinkage is far more critical than chemical swelling . More than 3 or 4 percent volumetric shrinkage can be a serious problem for dynamic seals.

Increased temperatures or longer exposure times can create more aggressive conditions. In some cases, specific compounds within the same material family may exhibit better resistance in certain applications than others.

Volume changes in rubber compounds occur through reactions of the chemicals with the polymer chain and the cross-linking system or through reactions with the fillers.