PTFE bonded coatings for elastomer seals

Bonded coatings have proven effective as dry lubricants on profiled seals for many years and are gaining increasing importance in automotive O-ring seals. Other possible applications are energy storage diaphragms in electro-hydraulic brakes (reduction of gas permeation), or sealing rings in the food-processing industry.

Compared to classical lubricating oils or greases, PTFE-based bonded coatings offer a lot of advantages in the lubrication of elastomer components, for example by reducing friction and noise generated by stick-slip and preventing premature wear of highly stressed parts. Bonded coatings also make O-ring assembly easier in many applications:

Clean lubrication: Once hardened, the bonded coating forms a dry, clean and non-drip film
Easy separation of parts in automatic feeding: The bonded coating forms a separating layer thus preventing sticking of parts
Consistent quality: The components are covered with a uniform, adhesive sliding film
Less permeation of gases and fluids: Enhanced barrier effect on fluid and gaseous agents
Reduced assembly forces: by up to 40% with sealing rings

Selecting the right bonded coating
The selection of bonded coatings is based on the requirements of the O-ring seal. There are two major distinctive features:

O-rings subject to dynamic load requiring high wear and chemical resistance
O-rings subject to quasi-static load or no/low dynamic load where chemical resistance is of minor or no importance at all

O-ring seals used in motor vehicles have to offer good resistance to hydraulic, brake or motor fluids. A bonded coating serves to reduce assembly forces of the O-ring and ensure chemical stability. However, using a bonded coating does not eliminate the need for the component itself to be resistant to the material in contact. Therefore, the properties of both the material and the bonded coating have to be finely tuned.

The same applies to bonded coatings used in the food-processing industry for seals which may come into contact with foodstuffs. Compliance with national and international (e.g. FDA) regulations governing the use of products in context with food products is of vital importance in this field.

New requirements
Seals and diaphragms are often made of elastomer materials. Ever-increasing demands and specifications require new paths to be taken in the field of material development and after-treatment of materials by means of, for instance, bonded coatings. Permeation of gases and fluids into an elastomer components can be reduced by a bonded coating. This barrier effect is achieved by increasing the layer thickness. In conventional applications, where the bonded coating is used as a mere assembly aid, layer thickness has to be between 5 and 15 痠. Reduction of gas or fluid permeation requires layers of > 30 痠.

Coating process for O-ring seals
Apart from the bonded coating, the process technology used for applying the dry lubricant is another important parameter for attaining optimum sliding surfaces. Sticking of O-ring seals during the coating process affects their sealing affect. In conventional immersion processes drops may form and lead to defects on the O-ring surface. As a consequence, the seals produced may leak. Modern coating technologies, such as spray tumbling, solve these problems by processing complex multicomponent systems drip-free and without sticking of surfaces. On critical materials, plasma pretreatment may improve the adhesion of the bonded coating.