Conveyor belts for use in the underground coal mines of the U.S. must be flame resistant. The test for flame resistance is the SSFT as mentioned in preceding sections. Generally, conveyor belts are made from a rubber or plastic compound combined with one or more layers of fabric material or different diameters of steel cables. A conveyor belt may be made from polyvinyl chloride and fabric such as polyester or styrene-butadiene rubber and several layers of polyester or nylon fabric. Flame-retardant compounds used in the manufacture of conveyor belts, such as antimony oxide, enhance the ability of conveyor belts to pass the required flame test. Sometimes carbon black is added to the conveyor belt compounding and may enhance the undesirable feature of afterglow, depending on the amount in the formulation. Figure 13.3 shows the SSFT used to test for the flame resistance of conveyor belts.
More details on conveyor belts and their construction may be found in the Rubber Manufacturers Association Handbook (RMA, 1989). A list of conveyor belt manufacturers with assigned acceptance numbers for conveyor belts that has met the CFR30, 18.65 requirements is available on MSHA's website. The SSFT is also used to determine the flame resistance of some components of conveyor belt haulage systems. Components such as conveyor belt roller covers, belt scrapers and lagging material for use on belt drives are constructed in a different manner than conveyor belts. The formulation for these items can also be different. For example, belt scraper material may be made from hard plastic which is shaped in a manner to remove debris from a conveyor belt. These materials are recommended to be flame resistant for use in underground coal mines, although not mandated by regulation. The same flame test and criteria for flame resistance is applied to these materials as for conveyor belts. A list of the manufacturers of these materials and their assigned acceptance number is also available on MSHA's website.