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ASBESTOS FIBER .Vol. 6 Fibers, Inorganic 671 General References II. F. Arledter, "Metal Fibers," in O. A. Battista, e<l., Synthetic Fibers in Papermaking, Interscience Publishers, a division of John Wiley & Sons, Inc., New York, 1964. H. F. Arledter and S. E. Knowles, "Ceramic Fibers," in O. A. Battista, ed., Synthetic Fibers in Paper making, Interscience Publishers, a division of John Wiley & Sons, Inc., New York, 1964. C. Z. Carroll-Porczynski, Advanced Materials, Astex Publishing Co., Guildford, England, 1962. W. J. Eakins, Glass/Resin Interface: Patent Survey, Patent List and General Bibliography, Plastic Rept. 18, DeBell & Richardson Inc., Plastics Technical Evaluation Center, Picatinny Arsenal, Sept. 1964. L. R. McCreight, H. W. Rauch, Sr., and W. H. Sutton, Ceramic and Graphite Fibers and Whiskers, Vol. 1, Academic Press Inc., New York, 1965. C. F. Phillips, Glass: Us Industrial Applications, Reiuhold Publishing Corp., New York, 1960. H. F. Arledter The Mead Corporation ASBESTOS The term "asbestos" refers to a large group of inorganic minerals which have fibrous characteristics. The individual minerals which fall within the scope of the term asbestos vary widely with respect to chemical composition, physical structure, and properties. The more commonly encountered asbestos minerals are actinolite, amosite, anthophyllite, crocidolite, chrysotile, and tremolite. There is some disagree ment among mineralogists as to whether amosite constitutes a separate mineral or is merely a variant of anthophyllite with a higher content of iron ions in the crystal structure. However, since amosite is an accepted designation for a fiber that has com mercial applications, it will be treated as a separate mineral entity in this discussion. The theoretical formulas of the common asbestos minerals are listed in Table 1. The theoretical formulas shown in the table are seldom, if ever, encountered in the natural minerals. The asbestos minerals vary from the ideal formula as a result of a Table 1. Theoretical Formulas for Common Asbestos Minerals Name actinolite amosite anthophyllite crocidolite chrysotile tremolite Formula CasfMgFelsSisOaCOH), FesMgSuCWOH)* (MgFe hSisOM(OH )2 Na^Fe$Sig022(OH )s Mg4(OH)8SuOii) Ca2MgsSi,022(0H)2 phenomenon called "isomorphous substitution." This type of substitution occurs when ions of approximately the same size substitute randomly for each other within a crystal lattice structure. For example, within certain limits, the Al3+ ion can substitute for Si4+ in mineral structures, because the physical volume occupied by these two ions is similar. However, since the charges on the ions are different, electroneutrality of the overall structure is maintained by the simultaneous incorporation into the mineral structure of an ion like Na+ or K+ along with the Al3+. In other words, an Al3+/Na+ pair is equivalent to an Si4+ ion. In this way it is possible for an asbestos mineral to retain a given crystal structure and yet vary rather widely in chemical composition. Encyclopedia of Polymer Science and Technology* vol. 6* 1967 Vol. 6 Fibers, Inorganic 673 magnetite, FesCfi. Magnetite is one of the most common "impurities" in many chrysotile asbestos deposits, and is frequently present in commercial fiber products. Structure The structures of the asbestos minerals are characterized by infinitely repeating structural units based on silicon-oxygen tetrahedral subunits. With the exception of chrysotile, all the asbestos minerals are classed as "amphiboles." This term is used to identify a structure composed of [Si8022]12- units infinitely repeated in a linear fashion. In the case of the amphibole asbestos minerals, the [SigO^]12- unit appears to be as sociated always with two OH- groups. Thus the basic amphibole asbestos unit be comes [Si8022(OH)2]l4~ The ionic charge on these units is compensated by cations such as Na+, Mg2+, Fe2+, Fe3+, and Ca2+. Thus, asbestos minerals such as actinolite, crocidolite, anthophyllite, and tremolite all have the same basic structure with different cations incorporated into it. Table 1 illustrates this point. Fig. 1. Electron micrograph of chrysotile asbestos. Chrysotile is somewhat unique in the asbestos family in that it has a structure based on [Si40io]4- repeating units. This is the same unit which characterizes the clay minerals, and chrysotile is, in fact, the magnesium analog of the clay mineral kaolin Al2C>3-2 Si02.2 H20. However, electron micrographs such as that shown in Figure 1 have demonstrated that the fiber exists in the form of tubes about 250-300 A in di ameter. Bates and co-workers (2) were the first to point out this peculiar structure and offer an explanation for it. At first it was thought that chrysotile asbestos fibers were analogous to thick-walled soda straws with an overall diameter of about 300 A and a hole down the center with a diameter of 100-200 A. However, subsequent studies of the pore sizes in bundles of fibers indicated that the majority of pores were well below 100 A in diameter, and it is believed that the tubular fibers of chrysotile have either Vol. 6 Fibers, Inorganic 675 Chemical Properties Although the asbestos minerals are often considered to be relatively inert, non reactive substances, their chemical behavior can sometimes be important when they are combined with organic polymers. The dominant chemical feature of chrysotile is its strongly basic nature. The hydroxyl groups in the structure behave much like those in magnesium hydroxide. Suspensions of chrysotile in water usually yield pH values from 9.0 to 10.3, and the fiber is reactive with acidic substances. If the composition of chrysotile is written as Mg6(OH)8Si4Oio, it tends to reflect both the unit-cell composition of the fiber and the basic nature of the hydroxyl groups in the structure. Considered as a base, chrysotile has an equivalent weight of 40.2 grams. Complete reaction of the fiber with a strong acid produces an amorphous, hydrated silica residue with a high surface area; this residue is pseudomorphic after the original fiber. The reaction is shown in equation 2. The silica pseudomorph is friable and the fibers have little or no tensile strength. Mgs(OH)sSiiO,o --'--- 6 Mg2 + + 4 SiO,.x H,0 HjO amorphous solid (2) The fact that chrysotile tends to react wfith acidic groups should be borne in mind when the fiber is used in combination with plastics. Polymers with acidic func tional groups may exhibit some interaction with the surface of the fiber and this may, under certain circumstances, enhance the interfacial bond between the fiber and the polymer. On the other hand, if chrysotile is used with polymers that are subject to degradation by strong bases, the composite may undergo undesirable side reactions unless the fiber is pretreated in some w'ay. In certain respects the amphibole asbestos minerals are the chemical antithesis of chrysotile. These fibers contain relatively few hydroxyl groups, and the groups they do contain are so buried within the structure that they have little influence on the chemical behavior of the fiber. Aqueous suspensions of crocidolite and amosite are almost neutral, and these fibers are very resistant to acids. Thus, if asbestos is to be used in a plastic composite that will be subjected to a strong acid environment, one of the amphibole fibers such as crocidolite is often a much better choice than chrysotile. On the other hand, the amphiboles do show some reaction in the presence of strong bases, and chrysotile usually is a better choice for such environments. Asbestos for Plastics Asbestos fibers have been used in combination with plastics for more than forty years (10). The early applications in the 1920s were in asphalt floor tile, phenolicmolding compositions, and asphalt coatings. These were followed by vinyl-asbestos floor tile in the mid-1980s, and chemical-resistant equipment in 1933. Today, as in the past, the largest quantity of asbestos fiber used in plastics consists of the shorter grades, which function primarily as fillers rather than as reinforcing fibers. However, research efforts of the past ten to twenty years have been directed towrard the development of products and technology that use reinforcing asbestos fibers in plastics. One of the earliest uses of asbestos fiber as a sole reinforcement for plastics was made in England for aircraft structures during and shortly after World War II. Today, the use of as bestos-reinforced plastics has expanded to products for the electrical and the aerospace Vol. 6 Fibers, Inorganic 677 Bulk fiber, frequently called "loo.se" liber, is the most widely used form of asbestos in combination with plastics. Bulk fibers ordinarily are shipped to the customer from the producer or distributor in 100-lb pressure-packed paper or loosely packed jute bags. Chrysotile is used in the largest quantities in plastics, with crocidolite, anthophyllite, tremolite, and amosite used in lesser amounts. Chrysotile classified according to the QAMA test in grades 1 through 5 is normally used for its reinforcing properties. Grades 6 and 7 fibers are used for their thixotropic characteristics (to control flow), heat resistance, dimensional stability, and low cost. The specific properties imparted by the use of bulk fibers of both reinforcing and filler types will be discussed in more detail later. The second form of asbestos that is used in many different plastic applications is paper. Paper for use with plastic requires a high-bulk factor, or degree of openness, to allow the resin to penetrate into the paper rather than merely coating it. Asbestos papers usually are made either on a cylinder paper machine or with a special process and equipment. The most common type of paper is made with chrysotile fiber, but recently high-bulk crocidolite paper has become available. Asbestos paper is generally available in a range of thicknesses from 3 to 20 mils. Millboard is a product related to paper, but it is thicker and is available in MV> 34'-, and Iq-in. sections. Mats and felts are a third form in which asbestos fiber is used in plastic applica tions. The terms felt and mat are often used interchangeably in the plastics field, but felt usually refers to a thin product and mat refers to a thick product. Felts range in Table 5. Some Major Producers and Suppliers of Asbestos Company Johns-Manville Asbestos Fibre Div. Canadian Johns-Manville Co., Ltd; Turner & Newall Company Cape Asbestos Company Raybestos-Manhattan, Inc. H. K. Porter Company, Inc. Lake Asbestos The Flintkote Company Asbestos Corporation Limited The Philip Carey Company Nieolet Industries', Ine. National Gypsum Company Australian Blue Asbestos Pty., Ltd. Pacific Asbestos, Ltd. Union Carbide Corporation Atlas Asbestos Company The Ruberoid Company Powhatan Mining Company Smith & Kanzler Company Victor Manufacturing A Gasket Co. Armstrong Cork Company Congoleum-Nairn, Inc. American Asbestos Textile Corporation United States Rubber Company Tallman-McCluskey Fabrics Company Roving, fabric, and/or cloth. Bulk X X X X X X X X X X X X X X X X X Paper X X X X X Mat and felt X X X X X X X X X X X X X X Textiles' X X X X X X X X X Vnl. 6 Fibers, Inorganic 679 Fluorine-containing polymers; Tetrafluoroethylene polymers), polypro pylene (see Propylene polymers), and polyethylene (see Ethylene polymers). Miscellaneous. Asphalt (see Bituminous materials), polymerizable oils, and combinations with rubber and alkyd resin are other polymeric materials used with as bestos. Applications Aerospace. The use of asbestos-reinforced plastic parts in aircraft dates back to work in England during and following World War II. Applications have expanded for similar materials today to include parts for missiles, rockets, and space vehicles. Three forms of asbestos are used predominantly for these high-temperature-resistant applica tions: felt, paper, and bulk fibers. Chrysotile fibers are used in most applications al though crocidolite fibers have some limited applications. Phenolic or modified phenolic resins are the predominant polymers used with asbestos for aerospace applications. Silicones, high-temperature-resistant polyesters, epoxy resins, and polyimides (qv) are used to a lesser extent. Asbestos-reinforced phenolic compositions offer the following advantages for aero space and aircraft applications: high erosion and ablation resistance, relatively strong structure remaining after charring, excellent retention of strength at elevated tempera tures, good resistance to chemicals and water, low thermal diffusivity, high modulus of elasticity, good machineability, and relatively low cost. The material used most frequently in this field is a line of asbestos felts 0.010 in. thick or multiples thereof, which is supplied with 30 to 50 percent by weight hightemperature-resistant phenol-formaldehyde resin. Usually, the impregnated felt is supplied to molders and fabricators in a B-stage or prepreg form. The material is available in broad goods (width up to 50 in.), but it may be slit to relatively narrowwidth tapes. Thin papers (0.010 in.) are also available. The resin-impregnated felts and papers are generally used to make flat laminates, moderately complex shapes, and to tape-wind conical and other symmetrical shapes (see Filament winding). An advantage of this class of products is the low pressure required to obtain high strength and a good bond. Molding pressures used range from a few psi up to 500 psi (200 psi is used most often). Temperatures used are approxi mately 280 to 325F for approximately 30 to 60 min. Sections greater than Y> in. in thickness generally require additional time at pressure. The two types of molding compounds available are those that are manufactured by macerating prepreg felts and papers, and conventional dry-mixed compounds. These compounds are cured under conditions similar to those used for the felts and papers except that higher pressures are required (500 to 5000 lb/in.2). In order to avoid high tooling costs, many prototype parts are made for evaluation by machining the proper shape and design from large billets. The costs of these asbestos-phenolic resin products range from $0.60/lb to $5.00/Ib. Asbestos-reinforced plastics, with their excellent retention of physical properties at elevated temperatures, are suitable for flame shields, insulation for rocket nozzles back-up, blast-tube liners, fairings, aerodynamic surface parts, electrical components, and mounting devices. Table 7 lists the significant properties specified by a military specification covering asbestos-reinforced phenolic products. The numbers and titles of several other military specifications which have been drafted on these same groups of Vol. 6 Fibers, Inorganic 681 1'ahlc 8. Government and Military Specifications on Asbcstos-Rciiiforccri Plastics Designation MIL-M-14F MIL-Mt21556 (SHIPS) MIL-T-8U41A (WEP) MIL-I-81255 (WP) MIS-10049 MIL-P-25770A MIL-P-S059A MIL-A-81264 Title Molding Plastics and Molded Plastic Parts, Thermosetting Molding Plastic and Molded Plastic Parts, Asbestos-Fiber Filled, Arc and Flame-Resist ant Phenolic Resins Tape, Asbestos Felt, Impregnated with Phenolic Resin Insnlating Compound, Asbestos Phenolic Interim Resin Impregnated Mat and Felt Plastic Materials, Asbestos Base, Phenolic Resin, Low-or High-Pressure Laminates Plastic Material, Laminated, Thermosetting, Sheets and Tubes, Asbestos Base, Phenolic Resin Asbestos Felt or Mat, Resin Impregnated products are shown in Table 8. Table 9 provides data on specially developed hightemperature-resistant materials based on felt, paper, and bulk fibers. Another aerospace application for asbestos fibers is in combination with elas tomeric materials for use as internal insulation. Chemical-Resistant Structures. One of the most rapidly growing areas of ap plication for asbestos-reinforced plastics is in the field of corrosion-resistant structures, although such materials were used as early as 1933. Asbestos fibers offer outstanding resistance to chemicals when combined with thermosetting resin systems. Asbestos in the form of mats, papers, and bulk fibers is used to make reinforced corrosion-resistant plastic structures. Crocidolite asbestos fiber is used frequently where a broad range of chemical resistance is required, and it is almost always used where excellent acid resist ance is needed. The asbestos-reinforced plastic products have broad chemical resist ance to acids, alkalies, neutral salts, and organic solvents. The polymers usually used for these chemical-resistant applications are epoxy resins (qv), polyesters (qv), furan polymers (qv), and phenolic resins (qv). Owing to the small diameter of the individual fibril and the large fiber-resin inter face obtained on thorough wetting of the fibers, a better bond is usually achieved between the resin system and the asbestos fiber than is ordinarily obtained with glass fibers. Although the small diameter and wettability are beneficial to all applications, the advantages are probably greater for chemical resistance in that the wetting and encapsulation of fibers by resin ensures that the system will be more resistant to those chemicals to which the resin is resistant. This is in contrast to coated continuous glass libel's, in which the chemical agent can travel the length of the glass fiber inside the coating, much the same as in a wick. The advantages of a better bond and superior chemical resistance have led to the development of several products using an asbestosreinforced plastic liner inside a glass-reinforced structure. Most of these applications use an asbestos mat or felt (paper may be used) with a wet-layup process. The asbestos mats are used in a procedure similar to that currently used for glass mats. Asbestos-fiber materials can be used as the sole reinforcement for chemical-resistant structures. In the case of corrosion-resistant equipment, a slurry or paste-like molding compound is commonly used. In this process, material is placed around the Vol. 6 miO iC o O 4 i 4< O ** CO oIi c tO co to ! 01o bC 2 5 V 3 c o to 3 O <N i dd a 0. 50 *1 f Oh1l O os !N5 d d I OJ X^ rOnl d^ 04 a, 00 *o d - sa "3 d V ft. c be '<D 00 tO Oi 7O'! i Tto c* ^ fO o toI (XMI to 4 )o 1 1 .hOO aJ) X 'Ja3 X .-- X |a>, a xCOs ?x2 ~ Vo J= 7 o- a50 2XQ **g x n r* C iHo Tfi .i| - 93 " W 5 5 *M=3 --2.^M3 -0s! : "" 'ji z >SS " -- x _ 32 5 C 0J i _X i3r= 52 'X' I s r= 2 1 Ca, c<c k* 5S-=>w' 4) 2 _ * "M3Ol "33" --wJs s *5> 35 5g C V 3 * o a bC 3 r>> J2 S t** o xi .O htO c >m S.^ Q ^Ch XH OS _ - to X < S 4) to S3W >>.s O * *5s J>2 a** *> Js2i . 'I 2 ! 8QQ .15S Fibers, Inorganic 683 MTC 000220 Vol. 6 Fibers, liiorgunic 685 (d) Fig. 2. (a) Crocidolite asbestos-rein forced phenolic, epoxy, and polyester pipe and fitting system, (b) Typical asbestosreinforced plastic electrical components, (c) Parts made from asbestos-reinforced high-friction materials. (d) Asbestosreinforced plastic gaskets, (e) Asbestosfilled polypropylene parts. Courtesy Avisun Corporation. (e) MTC 000222 j T able 12. E le c tric a l P roperties o f Asbestos L a m in a te s (10a) MTC 000224 Vol. 6 s3l aS .<o6 85, w3a oL S 2=? X2I rCOw tVo V*--- d "O >> .1aX 1Pt CO <N i <cA* Fibers, Inorganic 687 C9 r- lipH t*. i <A *4 *+ w us CI -J>o3 wo foe a3 'O * . .a aafst 05 i i Hx ip ^s G OG 2 <N i> C IO 1C qss t-* H H rt ^ UJ cacao &s -c & E_ oO <N I 'H i5 . _sa > jVS H e 12 j=s* o .5 g S -^ 3 -5. XI > > 2P u aS a?) a XO 81 r o -O>> i S5vC--2.^_Xy ^_X2O^r ---%rXX 1 8 JLi >> i; 2 J) _ % " rX . Jc "s%5Z'Z^.'ss*.2jg>Z2p*;%^ g B 4S a .2 *03 & Vol. 6 Fibers, Inorganic 689 but chrysotile fibers usually are more desirable because of their softness, durability, silkiness, and slipperiness. Asbestos fiber provides the low-friction resistance, chemical inertness, and high-temperature resistance required by molded friction parts. Figures 2c and 2d illustrate various friction-material products and gaskets made from asbestosreinforced plastics. Reinforced Thermoplastics. Asbestos-reinforced thermoplastics have been developed and used successfully within the past five years. Both extrusion- and injec tion-molding grades are being used. Although nylon and vinyl resins have been rein forced with asbestos, the polymer used most frequently is polypropylene, and the as bestos fiber used is anthophyllite. Most other types of asbestos apparently cause deg radation of the polypropylene. Asbestos-reinforced polypropylene has a higher modulus of elasticity and a higher deflection temperature than unreinforced polypropylene. A typical molding compound contains approximately 30 to 40 percent asbestos fiber. Table 13 shows the results of Table 13. Properties of Asbestos-Reinforced and Unreinforced Polypropylene Property specific gravity modulus in flexure, psi X 10'1 tensile modulus, psi X 10~* deflection temperature at 66 psi, F coefficient of linear expansion, 0-150F, in./in./F X 10s tensile yield strength, psi ASTM test method D 79260T D 79061 D 63861T D 64856 D 69644 D 63861T Asbestos reinforced 1.24 450-550 300 280-290 2.1 4800 Unreinforced 0.90-0.91 170 160 230 3.8 5000 asbestos reinforcement of polypropylene. The cost of these reinforced compounds (25 to 50fl/Ib) is only slightly higher than that of the corresponding unreinforced polymer. Figure 2e illustrates some of the parts that have been made with asbestos-reinforced polypropylene. A high potential growth rate is forecast for this type of asbestospolymer product. Miscellaneous Applications. Asbestos fiber is used as a filler and flow-control agent for polyester premixed molding compounds. A typical premix formula contains approximately 2 to 5 percent asbestos by weight. Group 7 chrysotile asbestos fibers or floats are used most often. Compression-molded parts arc made for many different industries including the automotive, electrical, appliance, marine, and recreational t rades. Typical parts include heater housings, motor housings, industrial light shades, and transformer cases. Recent developments have demonstrated that premix molding compounds can be used with asbestos as the sole reinforcement or as the major reinforcing fiber. The fiber is also used to control the flow. Asbestos reinforcing provides stiffness, better surface and smoothness, and better heat resistance; asbestos-reinforced compounds exhibit less shrinkage than conventional glass-fiber-reinforced products. Another large application of asbestos products is the combination of asbestos with asphaltic compositions for the coating of steel pipe and for use in roads. Ordinarily, for coatings on pipe, an asbestos felt having high tensile strength is applied into an excess of molten asphalt on the pipe. The material imparts strength and integrity to