Seal Science

A seal is used to act as a barrier between spaces containing different materials or different pressures. Seal requirements are governed by the application. Principal applications are locations where material losses (e.g., lubricants) have to be eliminated and the penetration of foreign materials (e.g., dust) into the system must be prevented.

In addition to these requirements, seals must provide reliable operation during a prescribed service life. That can be a challenge because they are subjected to wear due to movement, temperature or chemical attack. Also, it is important for the seal to have as low power consumption (friction) as possible and that it can be removed for maintenance.

In addition, there are several areas of concern regarding seals and lubricants, including swelling and shrinkage, and the impact of using mineral, PAO and silicon oils.

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Swelling and shrinkage: Swelling and shrinkage are the most important changes experienced by seal materials after contact with lubricants.

Swelling is caused by diffusion into the surface areas of the seal material until it becomes saturated. Swelling behaviour depends not only on the chemical structure of the oil and of the polymer material, but also on the degree of crosslinking of the polymer.

In most cases, shrinkage is the result of the loss of plasticizer in the polymeric seal material.

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Mineral oils: It should be kept in mind that lubricants with a mineral oil base and a high aromatic compound content generally have poorer seal compatibility than highly refined oils. Ester oils are also known to have relatively poor compatibility. With oils having a similar chemical structure, the tendency toward swelling decreases as the viscosity rises.

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Polyalphaolefins (PAOs): PAO oils are synthetic hydrocarbons and unlike mineral oils, which consist of a large number of different molecules, are constructed from the same molecule. Their behaviour with respect to compatibility with elastomers is similar to that of mineral oils.

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Silicone oils: By virtue of their chemical structure, which differs greatly from that of many elastomers, silicone oils often exhibit good compatibility with seal materials.

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By nature, elastomer seals are relatively soft and elastic. Due to these properties, a rubber material assumes the shape of its counterpart. This can result in the two parts sticking together. In such cases, it is important to use a lubricant as a separating medium.

As a result of swelling or shrinkage, elastomers undergo a change in modulus of elasticity, tensile strength and hardness. This means that in the case of incompatibilities, the mechanical properties of elastomers can be greatly altered.

In any case, it is advisable to conduct compatibility tests between the lubricant and the seal material, since elastomers in the same class can exhibit different swelling behaviours. The reasons for this may be different plasticizers, plasticizer content or the degree of polymerization.

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