Document ommggqw06mgnRwbg7YMEn8v8E

<- n JGSlOS \' i D G E 'S By Robert E. Byrne Jr* Asbestos baa been used in plastics in large quantities since the development of phenol-formaldehyde resins in 1909. The longer grades have traditionally been used for reinforcement in the form of fibers, yarns, or textiles. The shorter grades are used as Alters or extenders and, to some extent, for viscosity control. The utility of asbestos has been demonstrated in most types of resins and in myriad applications. . Asbestos, a comparative newcomer in industry, is one of the few important non-metallic minerals that the United Slates im ports from foreign sources. Less than 10% of our an nual requirement is current ly produced from domestic deposits. The remainder comes mainly from Canada. Since I960, however, domes tic production has increased rapidly to an estimated 1968 production valued at more than $6 million. The name asbestos is a broad, commercial term that has been applied to a num ber of natural fibrous silicate minerals that are incombus tible and can be separated by mechanical or chemical means into fibers of vari ous lengths and thicknesses. They differ in chemical com position and other proper ties. Low cost, exceptional me chanical properties, and re sistance to heat account for much of the growth in the use of asbestos in plastics products. It has recently face of a chrysotile been recognized that the as asbestos fibril. bestos unit fibril is a natu rally occurring "whisker", i.e., an inorganic crystal grown under controlled pressures and temperature. Because of its high strength, stiffness, and aspect ratio, the asbestos fibril is an excellent whisker reinforcement. Maximum utiliza tion of its desirable reinforcing characteristics requires a high degree of fibril liberation and the absence of extraneous gangue material. Asbestos research is expanding the variety of products avail able, with a strong trend toward more highly refined fiber with higher standards of quality control. High performance, chemi cally modified asbestos products with altered surface properties are now on the market. The industry is working to keep pace with the increasing sophistication of the plastics industry. Chem ically "designed" asbestos products will be part of custom en gineered plastics systems of the future. Section Manager, Research and Development Dept., CaJidria Asbestos, Union Carbide Corp., Niagara Falls, N.Y. The Ecbestosmirserris The six generally recognized varieties of asbestos include the fibrous form of serpentine, known as chrysotile, and five miner als of the amphibole group; anthophyliite, crocidoiite, actinolite, tremolite, and amosite. Their comparative properties are shown in Table I. Chrysolite is the most plentiful and useful form of asbestos. Of the more than two billion lbs. of asbestos used each year by the plastics industry. 90-plus % is chrysotile. Its positive surface charge, softness and flexibility, and its low refractive index set it apart from other asbestos. Chrysotile's fiber diameters, less than 0.03 micron, are the smallest among commonly known fibers, and the material has exceptional mechanical strength, as indicated by the evidence listed in Table 1, p. 388. Chrysotile's crystal structure is similar to that of the kaolinite minerals, consisting of alternate layers of tetrahedrally co-ordi nated silica and octahedrally co-ordinated magnesium hydroxide joined into a composite sheet. The outer surface of the chrysotile fibril, essentially magnesium hydroxide, is similar to the mineral brucite. Fig. 1 is a representation of this structure showing the hydroxyl outer surface of the fibril. The magnesium hydroxide surface of chrysotile, with its resultant cationic charge, deter mines its chemical affinity for many resin systems. Because of a mismatch of the brucite and silica layers within the chrysotile structure, curved composite sheets are formed that result in closed cylinders of extreme length-to-diameter ratio (often as high as 1000:1). Evidence for the cylindrical structure of chrysotile has been obtained from X-ray and electron diffrac tion studies,.and from electron micrographs. Recent high resolu tion electron microscopic studies in Japan have shown the average outside diameter of the chrysotile fiber to be about 250 A, and its inside diameter to be about 140 A- Chrysotile occurs only in serpentinite rock bodies, most com monly as cross-fiber veins. These cross fibers, an electron micro graph of which is shown in Fig. 2, vary in length up to 2 in. and are usually shortened during processing. In most mining operations with ore containing less than 15% asbestos, the fiber must be separated from its host rock by crushing and milling, with subsequent grading. Chrysotile is graded by a Quebec stan dard according to fiber length, from the longest. Grade 3, to the shortest, Grade 7. Fiber purification is accomplished by conventional dry mill ing techniques as well as by newly developed hydraulic proc esses, Figure 2 shows the fibril liberation possible through pneu matic and hydraulic processing, respectively. The high purity colloidal fiber products, represented by Fig. 3, resulting from hydraulic milling are one example of the trend within the asbes tos industry toward controlled, higher purity products. Recently, commercial products based on chrysotile have been introduced which, by chemical modifications of the normal chrysotile fiber, have novel surface properties. These materials are used mainly for viscosity control in anti-sag and thixotropic applications. Compared with chrysotile, amphibole fibers are larger in cross section, harsh and hard. The amphibole asbestos minerals are anionic in surface charge and are more resistant to acids than is chrysotile. A 17598 UCC 023059 Fig. 2 (left): Electron micrograph of chrysotile asbestos fibers. Fig. 3: Purified colloidal asbestos fiber products after hydraulic milling. Aruhophyllite fibers usually occur as fibrous masses contain* ing randomly oriented cubic blocks of fiber up to an inch in length. They are generally used in insulating materials, as filter ing media, and as fillers in plastics, paints, and asbestos-cement products. Asbestos-reinforced polypropylene contains anlho- phyllite almost exclusively. . Crocidolite, or blue asbestos, is found principally in South Africa. Needle-like in structure and possessed of exceptionally high mechanical strength, it is used in plastics where its pro nounced color is not detrimental. TremoKte and actinotite are very similar, except for the high er iron content of the latter. Both are in relatively short supply and are used mainly as filter media or fillers. Amosite is a long yellowish-brown fiber which is harsh and brittle, and is used mainly in thermal insulation. Thcrmoplcstics applications The largest single use for asbestos in thermoplastics is in vinyl-asbestos floor tile. It is also used in numerous sealants and mastics for the automotive and construction industries, in compounds based on butyl rubber, vinyl plastisols, and asphalt. Commercial thermoplastic molding compounds using asbestos include polypropylene, nylon, rigid PVC, and linear polyethyl ene. Short fiber chrysotile is incorporated into vinyl asbestos floor tile to improve heat resistance and dimensional stability. Since 1965, the consumption of asbestos-reinforced polypro pylene by the automotive industry has increased rapidly. This trend toward (he use of short fiber products for reinforcement is dependent on the inherent strength and fibril geometry of asbes tos. Table II shows the increased stiffness, heat resistance, and other properties of polypropylene containing 40% anthophyllite. Lowering the asbestos content to 20% is said to double or tri ple impact strength, and improve stability and gloss while sacri ficing only 23% in flexural modulus and 25' F. in heat distortion temperature. The volume of short fiber chrysotile used in mastics, sealants, and adhesives of various types is increasing. From 5 to 60% as bestos is used in adhesives to reduce crazing, shrinkage, and embrittlement. Asbestos also reduces cost, increases bulk, and provides controlled penetration and resistance to heat in such adhesives as phenolics, urea, resorcinol, epoxy, nitrile and re claimed rubber, and silicates. In mastics and sealants, asbestos provides toughness, flow and penetration control, and reduced cost. The colloidal grades of chrysotile are reported to contrib ute additional performance and handling improvements to this type of product. Teble I: CompcrEtive propsrties cf asbestos minersts Property Chrysotile Artthophyllilo Crccidillic TremoMte / rr.C::tc Composition MgjSliOs(OH)i Mg,Si.O,,(OH)> Na,Fe,S1.0,.(OH), Ca,FeiSiiOit(OH)t Mg,Ca,SI,0,,(0H), (Fe, Mg)jSltO,j(OH)i Crystal habit Fine, easily Prismatic Long brittle fibers Prismatic crystals Prismatic crystals Prismatic crystals separable crystals and and fibers and fibers and fibers fibers fibers Veining Cross-, slip-. Slip- & massive- Cross-fiber Slip- & massive- Slip- & massive- Cross-fiber & massive- fiber fiber fiber fiber Color White, gray, green Gray-white Blue Green White, gray-white, Ash gray or brown green Luster Silky Vitreous to Silky to dull Silky Silky Vitreous to pearly pearly Texture Soft to harsh, Harsh Soft to harsh Harsh Harsh Harsh Flexibility Hardness also silky Very good 2.5-4 Poor 5.5-6 Good 4 Poor 6 Poor 5.5 Good 5.5-6 Fiber diameter, A Tensile strength, p.s.l. 200-400 300,000-600,000 600-900 600-900 <4000 400,000-600,000 600-900 <1000 600-900 1000-8000 600-900 100,000-200,000 Modulus of elasticity, p.s.i. Specific heat, Btu/lb./' F. Specific gravity . Refractive Index (mean) pH 23X10* 0.266 2.4-2.6 1.51-1.55 9.5-10.3 -- 0.210 2.8-3.1 1.61 9.4 27x10* 0.201 S.2-3.3 1.7 9.1 -- 0,217 3.0-3.2 1.63 9.5 -- 0.212 2.9-3,2 1.61 9.5 24x10* 0.193 3.1-3.3 1.64 9.1 Electric charge In aqueous slurry Resistance to acids Positive Poor Negative Very, good Negative Good Negative Fair Negative Good Negative Good UCC 023060 A i 7599 T:ifc!= II: Fny-rer.i prcpcrticE of cob dittos reinio.'^ L i:iCrrAcphrliiS'1 Prop' rly Fciyrropyirne E3_;e 43% rf-:n eshe'tos ri'/icr. (wilh 30% ;BlbEitCE.', 6 "" C/10 6/C Tensile strength, p.s.I.xlO"* Ultimata elongation, % Flexural strength, p.s.l.xlO-* Flexural modulus, p.s.l.xlO~* Izod Impact, ft,lb./ln. notched unnotched Heat deflection temp., * F. 66 p.s.l. 264 p.s.l. Deformation under load, 24 hr., @ 2000 p.s.l., 122* F,, % Specific gravity Water absorption, 24 hr., % Coefficient of linear thermal expansion, 10-* in./in./* F. Mold shrinkage, in./in. Dielectric strength, S/T, VPM Volume resistivity, ohms/cm. Arc resistance, sec. Dielectric constant, @ 60 Cyc. @10" Cyc. Dissipation factor, @ 60 Cyc. @10" Cyc. 5.0 200 _ 0.18 0.4 -- 230 -- 3.5 0.91 <0.01 3.8 0.015 650 1.0x10" 136 2.2 2.2 0.0002 0.0002 4.8 20.0 5-- . 30.0 0.65 1.5 0.5 0.9 -- 13 290 -- 385 1.6 1.24 -- 0.02 _ 2.1 0.01 -- 450 450 1.5x10'1 6.9x10" 121 127 2.75 4.3 2.6 3.5 0.007 0.053 0.002 0.019 a--Comparison* with propeftia# of Unreinlorced nylons may be mads by consulting ths Plastics Properties Charts in Section 5 of this Encyclopedia. 14.0 20.0 ---- 21.0 30.0 1.2 1.5 0.8 0.8 14 12 340 465 ---- _, ---- 450 450 9.6x10" 7.6x10" 123 126 4.0 4.1 3.3 3.5 0.031 0.018 0.032 0.017 Recently, several manufacturers have introduced asbestosreinforced nylon and polyolefin molding compounds. The me chanical and electrical properties of several nylon compounds containing 30% chrysotile are also shown in Table ll. The per formance of colloidal chrysotile iii this application is illustrative of the type of whisker reinforcement which occurs through opti mum natural affinity of the fiber and the polymer matrix. High levels of reinforcement can be attained in resins lacking this af finity by Using high purity colloidal chrysotile and technology to improve fiber wetting and fiber dispersion. Recently disclosed "cerapiast" products, for example, exhibit threefold increases in tensile strength and modulus with 40% asbestos in linear poly ethylene. The fineness of chrysotile fibers permits dense struc tures with maximum resin fiber adhesion enhanced by the high surface area, ca. 60 mVg, of the asbestos. An extension of this technology is reported to produce a number of products useful when transparency or translucency is desired in the composite. Transparent resins such as vinyls and inonomers can, in many cases, be reinforced with little loss in clarity since the diameter of colloidal asbestos is well under the wave-length of visible light. . I<. rm;;.: apc'icr-' i-:':3 Asbestos-filled or reinforced plastic products originated with phenolic molding compounds. However, virtually every type of thermosetting resin, including urea-formaldehyde, melamines, furanes, epoxies, polyesters, and silicones, may be asbestos-filled. Asbestos shorts and floats are used in molding compounds to produce desirable molding characteristics, good surface finish, and improved physical properties in the molded pieces to be employed as electrical insulation, heat insulation, or high fric tion brake and clutch facings. Polyester premixes used to fabricate automotive parts, tractor and marine engine housings, and materials handling bins and containers often contain short fiber chrysotile to provide flow control, improved surface appearances, reduced shrinkage, and low cost. The new low profile systems utilize asbestos for con trolled shrinkage, improved mechanical strength, and heat re sistance. The amount of asbestos needed varies with the particular res in system and the fiber length and degree of openness of the asbestos. Each system has an optimum level for maximum me chanical strength. For example, in phenolics about 23% is typi cal for asbestos floats, or as low as 5 % colloidal chrysotile. Ibis level may vary up to 60%, however, depending on the levels of chemical, thermal, and impact resistance, and the electrical properties desired. Asbestos-filled phenolic and urea-formalde hyde resins find use in many electrical applications where heat resistance is also required. Asbestos-filled polyester and diallyl phthalate compounds are also used in electrical applications. The asbestos-containing compounds offer the capability of mold ing with exceptional accuracy, molding-in metal inserts easily, and mass producing intricate shapes difficult to fabricate from other dielectric materials. Phenolic-asbestos laminates used for rocket flame tubes are one example of the increased demand for asbestos-reinforced systems brought about by space age requirements for heat, chemical, and shock resistance.--End in pJcslics C: rC'':'.'i.rc:'rc JiLir Higher modulus Some improvement in tensile Plow control in mold Good surface finish Heat stability Dimensional stability Reduced creep Chemical resistance Higher hardness Arc resistance Low water absorption Discdvtmlr.gcs increase in viscosity of resin fair electrical resistance somewhat abrasive possible polymer degration high density As o re'.nicrcinc tificr ASvcnirgss Low cost Easy flash removal Row control in mold Minimum liber degration Good flexural strength Low water absorption Heat resistance Good electrical properties Chemical resistance Dissever,leges fair impact strength dark color possible mixing difficulties UCC 023061 A I730C