With so much of today's specialized chemicals and pharmaceutical manufacture dependent on bespoke production equipment it is surprising that engineers often overlook the opportunity to specify customised elastomer seals. Yet developments in seal compounding technology make it easier than ever to produce customised seals.
With a wide choice of elastomers it should be possible to find a good match for any process conditions. However, for some high value, difficult to process chemicals, good is not enough. An emerging trend in chemical and pharmaceutical processing is the requirement for small changes to existing seal grades that have big effects on a seal's mechanical properties. There are many types of elastomer and an infinite number of performance enhancements can be realised by changes in compounding, allowing seals to be ‘fine-tuned' to meet the needs of specific applications.
Selecting a rubber seal begins with meeting the requirements for chemical resistance and operating temperature of a process. It needs to be borne in mind that synthetic elastomer materials generally consist of an organic polymer and inorganic reinforcing filler systems. Although the polymer back-bone may be similar, thus determining many of the physical properties, see Figure 1, there can be significant differences to the cross linking and filler systems, creating many of the differences in physical properties and hence sealing efficiency.
Rubber seal customisation is possible for the physical properties of an elastomer. By making changes to the filler system it is possible to optimise the physical properties of a particular grade of material when compared to others within the same grade.
The reinforcement effect of a filler is complex and dependent upon its structure, particle size and chemical make-up of the particles themselves.
Carbon black, for instance, has a very irregular surface, which makes the reinforcement particularly effective. However, some synthetic silicas are perfectly spherical, offering very little in terms of reinforcement. In order to achieve specific physical properties from a material the correct combination of reinforcing and non-reinforcing fillers must be selected.
Fillers can be classed as reinforcing or non-reinforcing, depending upon whether they arrest crack propagation to a greater extent than they raise stresses, or vice versa. Within a polymer the filler can have two effects, they may act as stress raisers, reducing the energy at break, or may arrest crack propagation to increase the energy required for breakage.
Engineers can use the ‘filler effect' to select the quadrant of the ‘Physical Properties Box' best suited to their process equipment. The modulus of a material is related to its hardness. As the modulus increases then so does the hardness; O-rings of high hardness are more capable of withstanding extrusion to higher pressures. The ultimate tensile strength, elongation to break and hysteresis loss of a material are useful indicators of the elasticity of an elastomer. Although seals by their very nature are typically used in compression, their elastic properties can result in the development of tensile stresses within the body of the seal when subjected to compression or shear stress.
Typical dynamic applications include sealing against a reciprocating or rotating shaft or bore. The compression of the material, combined with shear frictional forces, can result in tensile stresses that exceed the ultimate tensile strength of the material causing a tensile failure. In this case using a large surface area, small particle size filler would be a better choice than the standard reinforcing filler. However process- ability may suffer and scorch may result. It is often the case that a compromise between property enhancement and processability has to be made.