Adhesion

Adhesion of Elastomers

The term adhesion is derived from the Latin adhaerere, which means “to adhere”. It describes the physical state between substances that come into contact with each other, regardless of whether they are solids or liquids. In simple terms, adhesion is the mechanical adhesion to the contact surfaces of two substances. In elastomer terminology, the term describes the strength of the bond or connection between two elastomer surfaces or insert materials in a vulcanized or unvulcanized state.

How does adhesion occur?

Adhesion in elastomers is caused by physical and chemical interactions at the contact surface between the elastomer and another material. These interactions can be caused by van der Waals forces, hydrogen bonds or even covalent bonds. A simple example of a fairly light, unstable adhesion from everyday life is the adhesion of water drops to the leaves of a plant. But the effect of adhesion also comes into play with paints, varnishes or adhesives. In elastomers and the counter material, the specific surface properties such as roughness, chemical composition and flexibility determine the strength and quality of the adhesion.

Where and when is adhesion desired?

In sealing applications, for example in areas such as automotive or mechanical engineering, adhesion is desired to ensure effective seals between different components or stable rubber-metal connections . Good adhesion of the seal prevents leaks and protects against the ingress of moisture and dirt, as the sealing materials adhere firmly to the adjacent surfaces and prevent leaks. It is also desired where an object or its surface must remain stable or adhere even when subjected to vibrations or impact, such as in protective coatings or buffers.

Where and when is adhesion not desired?

However, during the manufacture or processing of elastomers, undesirable adhesion can cause problems, for example when materials stick to tools. Also, in environments where elastomers are exposed to heavy contamination, adhesion can cause dirt and small particles to stick to the surface of the sealing product, reducing the functionality and lifespan of the material. If we look at molded or injection-molded rubber parts , where the friction properties on a surface must be taken into account - for example when other objects act on it through friction, movement or pressure - low adhesion or varying adhesion properties may also be desirable. Examples of this are rubber feet of an object that should be easy to slide on a smooth surface.

Factors influencing adhesion

The adhesion behavior of seals made of elastomer materials can be influenced by various factors - for example the surface texture. Rougher surfaces increase the contact area and can improve adhesion.

In addition, environmental conditions such as temperature and humidity also play an important role. High humidity can reduce adhesion by forming water films between the materials, while extremely low or high temperatures can change the adhesion behavior of the elastomer in different ways.

In addition, the adhesion force is influenced by the interaction of the chemical composition of the elastomer and the contact material. Contact between incompatible materials can lead to poorer adhesion.

To improve adhesion, surface treatments such as roughening or coating with adhesion promoters can be applied. In addition, the chemical composition of the elastomers can be adjusted to optimize the adhesion characteristics.

Examples of elastomers that have high or low adhesion are:

Elastomer materials with high adhesion

• Natural rubber (NR): Natural rubber has very good adhesion to various materials and is often used in applications where strong adhesion is required, such as adhesives and sealants.
• Polyurethane (PU): Polyurethane is known for its excellent adhesion to many different surfaces, including metals, wood and some plastics. It is also widely used in coatings, sealants and adhesives.
• Styrene-butadiene rubber (SBR): This synthetic rubber adheres well to many surfaces and is a common choice for adhesive applications and tire manufacturing.

Elastomer materials with low adhesion

• Polytetrafluoroethylene (PTFE) : Although not traditionally classified as an elastomer, this material, best known under the trade name Teflon from DuPont, is often used in similar contexts due to its extremely smooth surface and chemical inertness. PTFE has a very low adhesion rate, making it ideal for non-stick applications and seals where bonding is not desired - such as O-ring coatings .
• Silicone Rubber: Silicone generally has lower adhesion compared to many other elastomers, making it ideal for applications where the material must be easily removed from a surface, such as in baking pans or in applications where tack-free seals are required.
• Fluororubber (FKM, Viton): Fluororubber has a relatively low tendency to adhere to many materials and is often used in applications where, for example, chemical resistance or resistance to high temperatures in O-rings or seals is more important than adhesion.

The choice of elastomer depends largely on the specific application. In some cases, such as sealing or adhesives, high adhesion is desired, in others, such as molded parts that are not intended to stick to each other, low adhesion is advantageous. It is also possible to modify the adhesion properties through surface treatments and additives to optimize the performance of an elastomer in a particular application.