Sealing applications

Graphites (natural or synthetic) are black semireinforcing fillers used almost exclusively in rotary shaft seal applications where surface lubricity and abrasion resistance must be enhanced (see Carbon, natural graphite). 

There are many rubber eompounds used in industry as o-ring seeondary seals. Some elastomerie eompounds are only found in one or two sealing applications in one industry. We have listed the following four eompounds, which find broad popularity in almost any production plant. 

For most applications, the balanced, o-ring cartridge seal will adequately handle every pump, liquid, and condition in a modern industrial process plant. There are, however, some industrial pumping applications that will present problems to even the best of mechanical seals. Should one of these applications cause the seal to give less than desirable performance, the next step to take in extending the service life of the seal (and ultimately the pump) is to install some type of environmental control to protect and isolate the seal components from the fluid. Let s consider some difficult sealing applications. 

Dry Face Seal Application, Atmospheric Bearing Housing (Onshore Application)... 

The rubbers may be vulcanised by conventional accelerated sulphur systems and also by peroxides. The vulcanisates are widely used in petrol hose and seal applications. Two limiting factors of the materials as rubbers are the tendency to harden in the presence of sulphur-bearing oils, particularly at elevated temperatures (presumably due to a form of vulcanisation), and the rather limited heat resistance. The latter may be improved somewhat by Judicious compounding to give vulcanisates that may be used up to 150°C. When for the above reasons nitrile rubbers are unsatisfactory it may be necessary to consider acrylic rubbers (Chapter 15), epichlorohydrin rubbers (Chapter 19) and in more extreme conditions fluororubbers (Chapter 13). 

The low hardness has led to uses in printers rollers and stereos. It is, however, to be noted that when the material has been used to replace cellular rubbers or flexible polyurethane foams in sealing applications, problems have arisen where it has not been appreciated that although the rubber is very soft it is for practical purposes incompressible. 

Nitrile rubber, due to its oil resistance, is widely used in sealing applications, hose liners, roll coverings, conveyor belts, shoe soles and plant linings. Nitrile rubber is also available as a latex. 

The main use for acrylates is in sealing applications where the improved heat stability over nitrile rubber is the benefit. Automotive transmission seals are probably the major use, other automotive applications include O-rings, hose, tubing, cable covering and spark plug boots. Also used in adhesive formulations. 

The main uses of the fluorocarbons are in sealing applications. Hose, tubing and expansion joints are another area of use. 

Because PIB is fully saturated, it is cured as a thermoset elastomer through inclusion of about l%-2% isoprene that supplies the needed double bonds used in the curing process. Other materials including brominated paramethyl styrene are replacing isoprene for this use. PIB is also used in sealing applications and medical closures and sealants. 

The principal uses of ECO are in automotive applications, eg, for fuel line and fuel vapor recirculation. There are also some downhole seal applications where ECO is used. About 7000 t/yr of epichlorohydrin are used in the United States. This is all supplied by the Zeon Corporation. 

Aqueous-based EVA polymer emulsions suited for the use in heat seal applications are produced at a comparatively low pressures process, less than 14 M Pa (2). 

It has been found that in the development of EVA polymers for heat seal applications by emulsion polymerization that the concentration of vinyl acetate and ethylene in the polymer is not solely responsible for its use as a heat seal adhesive (2). 

J.J. Rabasco, C.L. Daniels, D.W. Horwat, M.S. Vratsanos, and R.H. Bott, Semi-crystalline ethylene vinyl acetate emulsion polymers for heat seal applications, US Patent 7189461, assigned to Air Products Polymers, L.P. (Allentown, PA), March 13,2007. 

Silicone rubber has both excellent low temperature and high temperature properties. It can withstand temperatures up to 315°C and workable at -65°C. Poor performance with low tear strength and abrasion resistance limit their use in most applications. Liquid silicone compounds LTV which are room temperature vulcanizable are useful for small repairs and sealing application and have been used for poured-in-place gaskets. 

Softening is apparent over a wide range, while the melt is strong and elastic. This gradual melting is beneficial in heat-sealing applications. 

The ability of the material to recover is important in sealing applications, where the compression forces may change due to temperature fluctuations. 

Epoxies can cure in deep sections and are useful in potting and deep-section sealing applications. They adhere well to different substrates and therefore are used in the general assembly of many medical devices. A clear, medical-grade, low-viscosity epoxy adhesive has proved useful in the fabrication of access ports that are implanted beneath the skin of patients who require multiple infusions.21... 

Perfluoroelastomers (ASTM designation FFKM) are essentially copolymers of two perfluorinated monomers, TFE and PMVE with a CSM, which is essential for cross-linking. Perfluoroelastomers can be cured by ionizing radiation without any additives. The advantage of radiation cured FFKM is the absence of any additives, so that the product is very pure. The disadvantage is the relatively low upper-use temperature of the cured material, typically 150°C, which limits the material to special sealing applications only [32]. 

Hot-melt adhesives are usually used for box-sealing applications for coated paperboard products. Migration of mineral hydrocarbons from these adhesives into foods was investigated in a survey. No mineral hydrocarbons were detected but several other substances were present in the adhesives. These were proposed to be natural resins such as polyterpene resins, tall oil rosin esters, and sterol-like natural products.Examples of each of these natural products are described in ref. 13 as being used for hot-melt adhesives. 

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