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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers 2 MANUFACTURING PROCESSES, CHEMICAL COMPOSITION, AND CLASSIFICATION THE Navy's report Man-Made Vitreous Fibers, discusses three principal classes of manufactured vitreous fibers (MVF): fibrous glass, mineral wool (slag and rock wool), and refractory ceramic fibers. The manufacturing processes and chemical composition of each fiber class varies depending on their specific end use; however, categorizing fibers into these classes does not capture the diversity of fibers in each class or the different potential hazards associated with each class. The last several years have seen an increase in the use of MVF in a variety of insulation and other industrial applications, including use in the Navy, most often as a replacement for asbestos. The Navy documentation on MVF discusses their manufacture and use in only general terms. The Navy does not indicate any applications that are specific to its needs other than a reference to the use of fibers in thermoplastics reinforcement for aircraft and marine hulls (NEHC 1997a). Subsequent information on exposure monitoring conducted by the Navy, informally received from the Navy Environmental Health Center, indicated that the Navy uses fibrous glass and wool primarily as insulation and, for continuous fibrous glass, in advanced composite material applications as reinforcement (P. Krevonick, Navy Environmental Health Center, Personal Commun., October 7, 1999). The monitoring data suggest the presence of MVF in boat structures, piping, acoustic panels, and lagging, although this information was not included in the Navy's original documentation. The lack of information on the specific materials that contain MVF and the quantities used by the Navy is of concern to the subcommittee; it makes it difficult to determine the extent of potential exposure of Navy personnel. These exposure standards cover all naval civilian and military
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers personnel but do not apply to Navy contractors, which are regulated by the Occupational Safety and Health Administration or applicable state regulatory agencies. The Navy does present an overview of the chemical composition of the fiber classes. Table 2-1 shows the chemical composition of various classes of fibers, expressed as percentages. From Table 2-1, it is evident that all MVF contain silica (SiO2), but they vary widely in their other components, both between and within classes of fibers. It should be noted that although the Navy refers briefly to the use of special purpose fibers, it does not include any information on their composition or sizes or on what distinguishes them from other MVF. The manufacturing processes for and composition of each fiber type are discussed below. GLASS FIBERS The primary ingredient in glass fibers is naturally occurring silicon dioxide; it also contains small amounts of other minerals. Permutations are made by adding other substances, such as oxides of aluminum, titanium, and zinc as stabilizers and oxides of magnesium, lithium, barium, calcium, sodium, and potassium as modifiers. By varying the amounts and types of stabilizers and modifiers, one can alter the physical properties of glass fibers. Stabilizers contribute to chemical durability; the intended use determines the amount of stabilizer added. Glass fibers are produced by mixing and melting the raw materials in high temperature furnaces and then processing them with various methods that depend on the end product. A continuous filament process is used for textile fibers, a rotary spray process for glass wool, and a flame attenuation process for making special purpose glass fibers. In the manufacture of textile fibers, molten glass is continuously drawn from the melting pot through bushings. This process allows for little variation in the preset average fiber diameter, which typically ranges from 3 to 25 µm. These continuous glass filaments are used in various applications, including textiles, and as reinforcements for plastic composites, such as boat hulls and automobile body parts. Glass wool is manufactured with a rotary process that consists of pouring the molten glass through a spinner that fiberizes the glass into discontinuous fibers. Fiber diameters vary widely: some are as small as
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers TABLE 2-1 Typical Chemical Composition of Some Commercial MVFa Composition (%) Nameb 11 A C 21 F G 22 RCF-1 X-607 Insofrax Class Glass Glass Glass Rock Rock Rock Slag RCF RCF sub.c RCF sub.c Components SiO2 63.40 65.00 61.70 46.30 56.30 60.10 38.40 47.70 58.30 76.20 Fe2O3 0.30 0.10 0.10 13.20 0.30 6.10 0.00 1.00 0.10 0.30 TiO2 0.06 0.02 0.02 2.60 0.10 0.05 0.50 2.10 0.05 0.08 Al2O3 3.90 1.90 1.00 13.50 3.20 0.40 10.60 48.00 1.30 1.40 CaO 7.40 7.40 7.20 10.00 26.10 18.80 38.00 0.07 38.70 0.20 MgO 2.80 2.60 2.90 9.10 6.40 8.30 9.90 0.08 0.40 21.50 Na2O 15.40 16.10 16.10 3.10 3.20 5.50 0.40 0.00 0.30 0.07 K2O 1.30 0.70 0.60 1.40 0.70 0.20 0.50 0.20 0.10 0.10 B2O2 4.50 4.70 9.20 0.00 0.00 0.00 0.00 0.01 0.00 0.00 P2O5 0.00 1.10 1.10 0.40 2.90 0.08 0.00 0.10 0.40 0.03 SO3 0.30 0.03 0.20 0.00 0.00 0.05 1.80 0.00 0.00 0.00 Cr2O3 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.03 0.00 0.00 MnO 0.01 0.00 0.01 0.20 0.00 0.00 0.70 0.00 0.00 0.01 ZrO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.00 0.00 Total 99.40 99.60 100.00 99.80 99.10 99.50 100.80 99.40 99.30 99.90 aData derived from Bernstein et al. (1996), Maxim et al. (1999a), and McConnell et al. (1994, 1995, and 1999); Material Safety Data sheets for Isofrax fibers from the Unifrax Corporation, Niagara Falls, NY. bName: 11, Certain Teed B glass wool fiber; A, new glass wool; C, new glass wool; 21, rock wool; F, rock wool; G, rock wool; 22, slag wool; RCF-1 - kaolin-based refractory ceramic fiber; X-607, rock wool produced by Unifrax; Isofrax, refractory ceramic fiber. cSubstituted RCF. 1 µm and the average is 3-15 µm. The glass wool fibers are bound together with such agents as urea-phenolic resins, which undergo a heat curing process that converts the binders to insoluble polymers. Other agents, such as lubricants and antistatic and wetting agents, can be added in the production process. Glass wool is used in industrial and commercial insulation applications—such as batts, blankets, and blowing wool—and for air ducts, ceiling panels, and acoustic panels.
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers Special purpose glass fibers are produced with a flame attenuation process. The hot, molten glass is poured in front of a high temperature gas flame; this results in fibers with a mean diameter of less than 3 µm. Various types of binders can be added in the process, depending on the intended end use applications. Special purpose glass fibers are typically used in applications that require high thermal and acoustic insulation, as in the aircraft industry, and for filtration media. MINERAL WOOL In the United States, most mineral wool (slag wool and rock wool) production uses slag as a raw material. Slag is formed during the reduction of iron ore to pig iron. Modern slag wool is composed of calcium, magnesium, and aluminum silicates with trace amounts of other oxides; raw materials—including clay, sand, and limestone—can be added to the coke-fired cupola or melted in an electric or gas heated furnace. Rock wool is produced through the same process with basaltic rock, limestone, clay, and feldspar, and small amounts of other additives. The production of slag wool and rock wool includes a wheel centrifuge process that results in discontinuous fibers averaging 3.5-7 µm in diameter. As with fibrous glass, the manufacturing process produces a range of fiber diameters, including respirable fibers. The addition of urea-phenolic resin produces bonded wool that is typically used for insulation batts, boards, blankets, and pipe covering. Nonbonded mineral wool is used as blown insulation or in the production of ceiling tiles. REFRACTORY CERAMIC FIBERS RCF comprise 1-2% of the worldwide production of MVF and are used in high temperature specialty applications. RCFs are used as bulk fibers, blankets, boards, paper, and textile products. They are produced by melting and spinning or blowing calcined kaolin or a mixture of alumina and oxides of zirconium, boron, or titanium. The average diameter of RCFs is 1-5 µm. RCF are unique in that although they are initially
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers amorphous, they can be partially converted to a crystalline form such as mullite or cristobalite, when heated to above 1800°F. CONCLUSIONS Each of the three categories of fibers contains fibers with different chemical compositions and different sizes. Animal and human studies that address potential health consequences of exposures to MVF should be based on knowledge of the chemical and size characteristics of the fibers under study. The Navy's review of the production, use, and chemical and physical properties of MVF covers technologies only through 1993. Of particular concern to the subcommittee is the Navy's understanding of the effects of time and temperature on the composition of MVF. The Navy does indicate in the Section “Chemical and Physical Properties” in Man-Made Vitreous Fibers, that MVF have high melting points, which make them good candidates for some applications, such as high temperature insulation, but it does not cite any studies on the wearing of these fibers and what happens to them when they are exposed to high temperatures. Since the anticipated exposure of Navy personnel is primarily to worn fibers, the subcommittee believes it would be helpful if the Navy included any relevant references on this topic or indicated that relevant data were not available. Because of the dynamic nature of the development of “new” fibers, which are being used in a myriad of applications, one can expect that MVF in the future will be different from those in use or in production today. Therefore, the Navy will have to be cognizant of those differences both with regard to current and future use, but also, and just as importantly, with regard to “tear out” and replacement of older fibers. More recent advances in the production of MVF are not included in the Navy's documentation (Maxim et al. 1999b). New uses for MVF and the properties of the fibers may have a substantial impact on the types of exposures that may be anticipated for Navy personnel, now and in the future.
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