October 6, 1998
Following a catastrophic explosion(2) and fire, the Methuen Area Office has brought to our attention a potentially serious safety hazard associated with a process known as "flocking". It was discovered that under certain conditions(3), dusts produced from processed and treated nylon fiber may be combustible, ignitable and explosive as defined in 29 CFR 1910.399 and the National Fire Protection Association 70, National Electrical Code (NEC). There is a direct correlation between particle size and its explosive hazard. The smaller the particle, the more reactive the dust. As the materials become smaller, they disperse and remain suspended more easily, increasing the potential for ignition and propagation of the reaction.
Flocking is the precision application of finely-cut fibers to various substrates for decorative and functional uses. These substrates can be such common items as upholstery and drapery fabrics, carpeting, toys, belts, ribbon, vinyl, picture frames, rubber seals, metals, foam, cardboard, paper, and automotive components. The fibers are usually cotton, acrylic, polyester, rayon, or nylon. Some functional uses are noise reduction, insulation, friction modification, and surface protection.
In general, the industry process entails dyeing the raw fiber, cutting the fibers to product specifications, chemically treating to prevent the individual fibers from clumping/sticking together, creating a fiber/air mixture, and dropping the fibers through an electrical field. Fine dust may be produced during processing and handling of fibers. Coatings applied to fibers may be dislodged, and if allowed to accumulate, may contribute to a dust explosion. The ignition source can be arcs from the corona associated with the electrical grid or arcs from the high voltage equipment used in the production process. There are indications that small fires can develop in the process area from fiber accumulation on the electrical grid or equipment which can then become the ignition source. Static discharge may also be an ignition source.
To minimize the potential for personal injury or loss of life and property damage, we recommend the following:
Please distribute this bulletin to all area offices and appropriate labor and industry groups.
American Society for Testing and Materials, Committee E-27, Hazard Potential of Chemicals Annual Book of ASTM Standards, Volume 14.02, 1997.
Cashdollar, K. L., and M. Hertzburg, U.S. Department of Interior, Bureau of Mines, 20-l Explosibility Test Chamber for Dusts and Gases, Rev. Sci. Instrum., Vol. 56, 592-602, 1985
U.S. Department of Interior, Bureau of Mines, RI 5624, Laboratory Equipment and Test Procedures for Evaluating Explosibility of Dusts.
National Academy of Sciences, National Materials Advisory Board, NMAB 353-3-80 Classification of Combustible Dusts in Accordance with the NEC.
National Fire Protection Association, NFPA 70, National Electrical Code.
National Fire Protection Association, NFPA 497M, Manual for Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations.
National Fire Protection Association, Fire Protection Handbook, Seventeenth Edition, Section 3, Chapter 12.
1 The Directorate of Technical Support issues Hazard Information Bulletins (HIB) in accordance with OSHA Instruction CPL 2.65 to provide relevant information regarding unrecognized or misunderstood health hazards, inadequacies of materials, devices, techniques, and safety engineering controls. HIBs are initiated based on information provided by the field staff, studies, reports and concerns expressed by safety and health professionals, employers, and the public. Bulletins are developed based on a thorough evaluation of available facts and in coordination with appropriate parties.
2 In the instance where there was an explosion, the dust involved was from treated and processed nylon, 1.3mm(0.051 in.) long and 1.3 denier. The fiber was dropped through an electrical field grid, using direct current at 15,000 - 30,000 volts at less than 3 milliamps, onto a latex-coated woven fabric.
3 The OSHA Salt Lake Technical Center (SLTC) conducted tests on dusts produced from treated and untreated nylon flock using SLTC's 20-liter explosibility test chamber and also NMAB 353-3-80. The test results indicated that the dust associated with some treated nylon flocking materials can be explosive. As for the dust from the untreated nylon flock, it could not be positively concluded that it is not explosive.
4 Class II dust is defined by NMAB 353-3-80, Classification of Combustible Dusts in Accordance with the National Electrical Code, as having an ignition sensitivity equal to or greater than 0.2 or explosion severity equal to or greater than 0.5.
Ignition sensitivity and explosion severity are defined by the NMAB 353-3-80 as follows:
|Ignition sensitivity =||(TcxExMc)1|
|Explosion severity =||(PmaxxP)2|
|where||Pmax=||Maximum Explosive Pressure;||P = Maximum Rate Pressure Rise;|
|Tc =||Minimum Ignition Temperature;||E = Minimum Ignition Energy;|
|Mc =||Minimum Explosive Concentration.|
1 in the equation above refers to the appropriate values for Pittsburgh seam coal, the standard dust used by the Pittsburgh Research Laboratory (formerly part of the Bureau of Mines) which is part of the National Institute for Occupational Safety and Health, Department of Health and Human Service.
2 refers to the values for the specific dust in question.
Materials which have an ignition sensitivity greater than, or equal to 0.2 or an explosion severity of greater than or equal to 0.5 are class II dusts, and as such, trigger the requirements found in 29 CFR 1910.307 "Hazardous (classified) locations)."
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