Originally asbestos fibers were mainly used because of their fire resistance and chemical compatibility; in the 1980’s because of government restrictions other fibers such as aramid, acrylic, carbon, graphite and other mineral fibers have replaced asbestos. In the manufacturing process a single type or combination of these fibers, elastomers and fillers are mixed with suitable solvents to form dough that is then run through two-roller calendar called a “Sheeter” to form sheets. This dough is continuously run through these rollers; the larger steam heated and the smaller water cooled. The dough sticks to the heated roller and is pressed to the appropriate thickness by the smaller cool roller. When the correct thickness is achieved the material is cut from the larger roll treated with an anti-stick coating and then trimmed into sheets.
Today there are many types of non-asbestos materials to choose from for sealing applications. From time to time new materials are added to our list; since there are so many we will address the most common types with concentration on compressed gasket materials.
Non-asbestos materials include the following types:
Beater Addition - This material is basically manufactured like paper products in a wet process. Binders such as SBR, Nitrile, and Chloroprene are used to bond the fibers together as well as increase the strength or improve the heat and/or chemical resistance.
Cellulose Fiber - This material is also known as Vegetable Fiber because it is made of processed plant fiber. It is held together by a glue-glycerin binder and is available in thicknesses from .003" to 1/16" in homogeneous form and thickness above 1/16" to 1/4" in a laminated form. This material is used in low temperature applications and requires low bolt load to seal. It is an excellent general purpose material for sealing petroleum products, air, gases and solvents of many kinds.
Cork Composition - This material is made from the bark of the cork oak tree. The bark is ground into granules and bound together with protein or synthetic resins formed into blocks or mats, which are sliced into sheets. Cork sheets have high compressibility, good crush resistance, good recovery and high degree of impermeability under low flange loads. Cork material can be used where temperature does not exceed 250 degrees F and low internal pressure up to 50 PSI. Cork has a predisposition to relax and a tendency to stick to flanges.
Cork and Rubber Composition - Cork and Rubber composition is made like the Cork Composition but is bonded with various types of synthetic rubber to provide the compressible characteristics of cork and the durability of rubber. This material is available in sheet and roll form and has a better shelf life compared to cork composition material.
Sheet Sizes - The most common size sheets are 150" x 150" and 120" x 120" coming off the “sheeter”.
These large sheets can be cut down to other common sizes which are 60" x 120", 60" x 60" and 50" x 50". Common thicknesses of these sheets are 1/64", 1/32", 1/16" and 1/8"; some manufactures also offer 1/4" thickness on particular styles. Wire inserted sheets are still available but are not very common today.
Gaskets from these materials can be furnished in one piece up to 150" in diameter eliminating splices giving a more effective seal and longer gasket life. Before using any material it is recommended that the operating conditions be discussed with suppliers to provide the appropriate material for the service giving the most effective result and longer gasket life.