Document G6xropgY1x0nJm1KRG2kMK5Kr

SYMPOSIUM monoial SUR L'AMIANTE 24, 25, 26, 27 Mai 1982 MONTREAL QuS., Canada Stance IV - 26 Mai L'amiante et les substituts WORLD SYMPOSIUM ON ASBESTOS May 24, 25, 26, 27, 1982 MONTREAL Qu., Canada Session IV - May 26 Asbestos and Substitutes THE FEASIBILITY OF SUBSTITUTION: "THE POTENTIAL FOR THE SUBSTITUTION OF AS3EST0S IN INDUSTRIAL APPLICATIONS" Andrew M. Pye (United Kingdom) WORLD SYMPOSIUM ON ASBESTOS Mav 25-27. Montreal Canada 1/ THE POTENTIAL FOR THE SUBSTITUTION OF ASBESTOS IN INDUSTRIAL APPLICATIONS by Andrew Pye, Deputy Editor, Design Engineering Morgan-Grampian Publishers Ltd., 30 Calderwood St., Woolwich, SE13 6QH. SUMMARY This paper is concerned with the replacement of asbestos in industrial engineering applications, which comprise industrial textiles, millboard, high temperature insulation, jointings and seals (including gaskets), friction materials, bearings, electrical insulation and fibre-reinforced plastic composites. It is recognised that construction industry applications, such as asbestos-cement, insulation boards and sprayed insulation comprise a significant proportion of historical asbestos usage; thus, for completeness, guidance is given on the state-of-the-art substitution potential and sources of further reference. It is emphasised that the substitution of asbestos containing materials will not be by a single, universal product; rather, each application must be considered individually. The resultant erosion of the supply infrastructure of the user industries evidently leads to cost penalties associated with the use of many substitutes. This is most apparent with the most arduous of applications. The costs of research and development into new materials and ox' the acquisition of new or modified processing machinery have, in some cases, to be borne by the user industries. However, the cost of asbestos has escalated considerably in recent years, whilst the cost of some substitutes has been reduced as they become more widely specified. 2/ This paper will not consider the possibility of associated health hazards which may conceivably arise out of the use of some substitute materials, which subject will be dealt with by other speakers. Never theless, it should be remembered that some mineral wool, ceramic fibre and other fibrillated or pulped materials do contain a proportion of fibres which may be loosely classified as 'respirable'; until such materials are afforded a clean bill of health, their use should be treated with the same degree of caution that is afforded to asbestos- based products. Although chrysotile is the most abundant and most widely used asbestos mineral, detailed differences in properties lead to the use in some areas of other types: amosite has superior heat resistance and is used in the production of insulating boards; crocidollte has the highest mechanical strength and best resistance to acids, but because it constitutes a greater health hazard than other types is subject to extensive national legislation restricting its use; anthophyllite finds use as an expensive filler and in specialised applications on-account of good heat and chemical resistance. 3/ Properties of asbestos The most widely appreciated property of asbestos is resistance to heat and fire, although this resistance is not as great as is popularly believed. Asbestos cannot be classed as refractory, although normally its properties are sufficient to withstand superheated steam and other high temperature industrial environ ments. Degradation of the crystal structure of asbestos and major loss of strength occur in the temperature range 300-500C. However, a useful performance can be obtained, especially where mechanical strength is not important, at higher temperatures. Specified working temperatures for some asbestos products may be as high as 600C, and in some cases asbestos can maintain its Q integrity up to 1700 C. Quoted values for the strength of asbestos fibres are very high. However, the fibres are of variable quality and, in any event, measurements of strength are derived from testing in controlled laboratory conditions. Discussion with suppliers of asbestos suggests tha the reliable strength of asbestos (chrysotile) fibres, as commercially available, is no higher than about TOOMNm --2 Various other properties have made asbestos a valuable material. Its resistance to chemical and biological attack is utilised in hostile environments; friction and wear characteristics (of chrysotile) lead to extensive use in friction clutches, brake linings and bearings; the high aspect ratio of asbestos fibres makes them useful as rein forcement in polymer and cement-based products, the latter having historically accounted for over 30'i of total asbestos consumption. Traditionally, this combination of properties has been obtainable at a price significantly lower than that of most competitive materials in specific applications, although there are several factors which exert upward pressure on prices: these include pressure on markets by the substitution of alternative materials; the cost of developing substitute materials and manufacturing processes; the cost of meeting lower fibre emission levels in manufacturing processes; the cost of meeting lower fibre emission levels in manufacturing; and the limited resource life- of asbestos supplies. A 18 m Thermal insulation and drv packings 4/ The use of asbestos for Insulation purposes encompasses three main areas: asbestos insulation board, asbestos spray and asbestos for lagging in high temperature applications. Insulation boards and asbestos spray are construction industry products and are outside the main scope of this paper, although it should be said that sprayed products have or will be banned by many countries and have largely been displaced by sprayed products based on mineral or ceramic fibres. Lagging products are normally blanket materials, which may be woven or unwoven. Dry asbestos packings are mostly used for conditions of dry heat and are useful as flame spread barriers and in static sealing applications. They may be based on woven or unwoven materials, the woven yarns, ropes and fabrics being preferred where sealing, as opposed to thermal insulation, is the primary requirement. Static seals are employed in many industries for furnace and kiln door seals, seals for railway exhaust manifolds, cathode ray tubes and floodlight lamps, etc. Other applications are heating element substrates, caulking of cracks in brickwork and wiping of wire for galvanising. Rope lagging made by wrapping loosely aligned fibre with asbestos yam is used as a thermally insulating pipe wire for general purpose pipe wrapping and as a static soft seal. Substitutes for bulk fibre and soraved asbestos in thermal insulation Packed or sprayed asbestos is replaceable by many of the s^m^he^ti^ ceramic fibre product forms currently available, such as bulk fibre, strips, rope, spray, pre-formed shapes, felts, mats and blankets. The thermal stability is generally superior to that of asbestos (1260-l600C) and the chemical stability is usually good. Asbestos has a superior chemical stability in some situations, such as in hot, reducing atmospheres and under hot, stressed conditions, but careful prediction of the service conditions usually allows a ceramic fibre to be selected that is adequate for the Job. Although the cost of these materials is higher than that of asbestos (in the case of true woven products considerably so) this may be offset by increased service life, lass maintenance, and increased process efficiency due to the option to operate at higher temperatures. A 18 168 5/ A considerable variety of ceramic fibres is available, the precise application determining which is necessary, In general, they all possess good thermal and chemical stability, low heat storage, resistance to thermal shock, low thermal conductivity and incombustability. Ceramic fibre products generally have a thermal conductivity comparable to corresponding asbestos products and, especially in blanket form, are being used increasingly for their acoustic properties, particularly at elevated temperatures, where vibration is a problem. However, their resistance to fluctu ating or flexural stresses is probably not as good as that of asbestos products. Ceramic fibres are in widespread use as re fractory insulation, seals and fire-protective components. Ceramic fibre textiles contain insert materials to increase the fabric-tensile strength. Allov wire inserts are available for obtaining maximum tensile strength at elevated temperatures; glass filament inserts are used in applications where metal is undesirable (e.g. dielectrics). Silica fibres are available in yam and various textile products. The mean fibre diameter is lOmu-m, about the same as glass, since these materials are made by leaching E-glass fibres. The material is speci fied for continuous use at 1000C, although a spe'cial grade containing chromia is available in fabric fora for continuous use temperatures up to 1^*00C. This is an extremely expensive product. Typical appli cations are for flexible electrical insulation in textile form up to 1000C and as flexible packings in bulk and felt form. Cheaper mineral fibre products are available in many product forms. These are usually cost competitive with asbestos but their mechanical and insulation properties may be inferior; although loose wool fibres can withstand a temperature of 830C in some applications, some products incorporating binders are not suitable for service above 300C. In some furnace lining applications where thick sections of insulation are required, the cost of an all-ceramic fibre installation could be prohibitive. An approach which has been employed successfully in situations such as this is to use a thick section of insulation based on the cheaper mineral vool products in the cooler areas and to face it with a thinner section of the relatively expensive ceramic fibre insulation in the critical hot spots. 6/ Various lime/silica and magnesia bonded products containing unspecified non-asbestos fibres are also widely available. Vermiculite for high temperature insulation is normally bonded with a high-alumina cement or fireclay and is therefore competitive vith ceramic fibres for hot-face lining applications. Vermiculite, which is the name given to a group of hydrated laminar silicates resembling mica in appearance, exfoliates on heating. Many vermiculite products can withstand temperatures of 1100C and bult materials prices are low. X similar material, expanded Perlite is used as an insulating cover on the surface of molten metal to prevent excessive heat loss during pourings. The material can also be used to make refractory bricks; it is used occasionally to 1100C, although above 923C the thermal conductivity increases appreciably. On thermal insulation performance along, it is difficult to better the performance of Microtherm. a microoorous ceramic comprising an ultrafine powder of amorphous silica, structured and bonded to give an extremely small cavity, or cell, size. 3ecause the cell size is smaller than the mean free path of an air molecule, gaseous molecular conduction'is inhibited to a degree that the thermal conductivity of Microtherm is less than that of still airi Typical applications for Microtherm, which is available in block, panel, glass cloth slatted, quilted or as a glass cloth sleeve-filled coil wrap, has been used for electric cooker ceramic hob insulation, insulating the panels of night storage heaters, and for electronic package protection. The low conductivity reduces the requisite thickness of insulation, so that on a performance-cost basis, it is usually cheaper than ceramic fibres, although the latter can perform up to 1600C (see above). Substitutes for asbestos textiles in static sealing These applications include those where the resiliency of asbestos yarn or rope is important in providing a seal. It is worth remembering that asbestos in these applications was itself a substitute for cotton, flax, hemp, jute and cellulose. Since asbestos is fine, fibrillar, flexible and tough, the textile processes used were designed around these characteristics. Class, although it is brittle and relatively thick, can, under the right manufacturing conditions, provide an excellent sealing material; abrasion resistance is usually inferior 7/ to asbestos, bur generally offers an acceptable lifetime. Glass yarns have replaced asbestos in some end uses and in fact some 70^ of the UK market has been captured by glass fibre and mineral wool, sometimes in combination. Impregnated packings Impregnated asbestos packings are used to provide a seal between two surfaces, one of which is moving. Impregnating lubricants include oils, greases and PTFE. The impregnation also removes the need to dust-st^press the product. The main outlets are compressors, pumps and valves; wherever a liquid or gas is moved through pipework, pumps and valves will be encountered. Compression packings for sealing fluids are made of asbestos, cotton, hemp. Jute, PTFE, carbon fibre, soft metal wires and foils and glass fibre (Table are two ways in which asbestos may be replaced: using compression packings made of other materials, or using alternative sealing systems which do not require compression packings. In practice, the latter alternative is possible only where the seal is in a rotary situation, mainly in rotary pumps, where mechanical seals have long been used instead of compression packings (albeit not universally so). This is not possible in reciprocating machines and valves. Therefore there remain some pumps, valves and compressors for which a large future demand for compression packings may be anti cipated. Other compression packings may be grouped into: (a) vegetable fibre packings, which are less than half the price of asbestos and satisfactory for oil and water service, but which would work out much more expensive in high temperature environments, where in many cases, the frequent replacement needed would be unacceptable; (b) more expensive solutions such as carbon fibre (costing two orders of magnitude more) or PTFI (one order mors). The latter is actually in widespread commercial use on account of its resistance to a wide range of highly corrosive chemicals and also in the food, drink and pharmaceutical industries where freedom from contamination is obligator;.'. Kevlar IT is a cross-plated aramid fibre construction with a special blend of lubricants. This offers a combination of strength, low thermal expansion and resistance to hydrocarbons, solvents, A 1817 1 8/ acids and alkalies in the pH ranje 2-13. It is particularly suitable for pump applications involving abrasive slurries and has also proven most successful on high pressure reciprocating water pumps when used with a special anti-extrusion support ring. It may be used to 230C. Aramid fibres have been reported as tough to machine and can abrade shafts. In general convenience of use, there is little or no difference between these products, but the very wide coverage of service conditions covered by asbestos makes it attractive from a stock holding point of view, so much so that asbestos packings are often used where vegetable packings could be technically satisfactory. According to some sources, asbestos fibre packings are also to be preferred on safety grounds, in that leakage from asbestos fibre packed glands is invariably slow at first and the rate of leakage increases, gradually. By contrast, if high heat is generated through shaft wear, for example, a PTFE packing can sinter and shrink, with resultant rapid leakage of dangerous fluids. Compressed asbestos fibre (CAF) CAP comprise high grade asbestos fibre well opened and intimately bonded with specially selected polymers. Suitable for temperatures up to 600C and pressures up to 2000 psi, the vast bulk of CAP is used for the manufacture of cut joints or gaskets; wherever two static metal surfaces are joined together in such a way as to be easily separated it is usual, if not essential, to have a gasket between the surfaces. If the joint is to be leak-free and the operating temperature is high then CAP is attractive. Consequently, the outlets for CAP are widespread, the major ones being; Automotive Pipework Valves and cocks Electric motors Electromagnets Industrial plant Cylinder heads, carburettors, exhaust manifolds, sumps, oil pumps, thermostats, timing covers; antisqueal in brakes; Chemicals, power stations, marine; Packing rings; Heat insulation; Heat insulation; Steam services. A 18 172 9/ ^ubst-ituteis_for_C^F Gasket or Joint products which may be considered as substitutes for CAF may be divided for convenience into groups: Grouo_A_^ Less expensive than CAS Rubber sheet and rubber insertion; cork and rubber (combined); glue/glycerine impregnated cellulose; pastes or compounds. Group A2 More expensive than CAS PTFE - cord, sheet, strip, envelopes around other cores; graphite fibre woven into soft packing, or expanded graphite used as a tape which can be compressed for gaskets or packings. Group A 5 Verv new or developmental products At least six European manufacturers are offering a non-asbestos 'CAS' based upon aramid fibre (Kevlar) pulp. A new, homogeneous, skeet prouucr intended as a re placement for asbestos gasketing, has been developed by Rogers Corporation under the trademark Nobestos/Duroid. A blend of reinforcing aaterials including polyester fibres and neoprene latex binder, it is produced in a number of colour-coded grades for specific exposure conditions, in cluding increased crush resistance and tensile strength to help prevent high pressure blow-outs. Two grades have a maximum service temperature of 205C, with one grade suitable for o C which is recommended for sealing halogen gases, oil, water, coolants and fuels. A non-asbestos material produced from high performance 'synthetic fibres' specially bonded with nitrile rubber is available from Richard Klinger under the trade name Klingersil C-llO. For extreme service conditions, particularly fluc tuating temperature and pressure, there is a grade containing high quality,' mild steel wire mesh reinforcement. Similarly, in the CSA, a product composed of acrylic fibres in a nitrile binder, is quoted for temperatures up to 570C and 1000 psi. This product, '31ue-Garil, is supplied by Gariock. Grouo__3 started sheet metal: machined metal Solid metal gaskets and pressed metal rings offer a highly reliable, but much more expensive solution than CAT, or its substitutes. A Is 10/ Group A materials may be considered as substitutes only in that for certain limited conditions in which CAP is normally used, they can be used instead and a good seal would be achieved. They are not, however, in any sense, universal replacements for CAP, but all have unique properties suitable for specific service conditions. Rubber sheet and rubber insertion sheeting tend to be regarded as general service materials for non-onerous Joint connections . Cork/rubber and cellulose are compounded mainly for resistance to specific oils at low temperatures and pressures and are ade quately compressible at low bolt loads. Cork is affected by acids and alkalies. PTPE joints have excellent acid and chemical resistance up to 200C. Graohite is self-lubricating, has a low thermal expansion, a wide temperature range and is resistant to most fluids except strong reducing agents. It is extremely expensive. Jointing compounds are applied as an extrusion from a tube or gun to very light flanges and usually require a setting, or cure, time before assembly. Also in this category are hard-setting pastes which should be usually regarded as a first-aid temporary stop-gap material until flange faces can be corrected. Collectively these materials can be regarded as substitutes for a small percentage only of total CAP consumption. Furthermore, there is little demand for asbestos substitute materials. In automotive use a particular problem is the highly critical cylinder head gasket field, although this may alternatively use a rubber-impregnated product or a sandwich construction of copper around asbestos paper or millboard. All of these products require resilience, resistance to the effects of hot oils, hot gases and hot water/antifreeze mistures and must cope with the severe working conditions of sealing high intermittent pressures, CAP is generally more expensive than Group A1 materials, but less expensive than Group A2 materials. Group A1 materials are in the same broad price range as CAP and the final gasket cost is relatively leas different because of the more or less constant value added in cutting (pastes excluded). A 18174 11/ Xt is worth noting that Lloyds safety requirements for marine and diesel installations and auxiliaries preclude the use of cork/ rubber and rubber sheet joints in oil pipes and water services because of vulnerability in fire conditions; CAT is approved for these applications. Proofed asbestos products Asbestos in fabric or plaited fora can be bonded and coated with various elastomers such as chloroprene (Neoprene), which can be selected for resistance to specific oils and solvents, possibly with the addition of other materials such as soft metals or rubber strips. A high (or low) temperature resistant material is created to sheet form, from which packings, rings or tapes can be cut or fabricated which have sufficient tensile strength to withstand pressure when used as gaskets. The ability to seal imperfections in metal flange faces by flow is also required, whilst maintaining clamping pressures sufficient to achieve leak-tight Joints. Typical uses are in flange joints and boiler connections, pump, valve and pipe joint seals in oil refinery and chemical plant, seals in in ternal combustion engines, oven and autoclave doors, tank seals and hydraulic systems. Substitutes Few early attempts were made by manufacturers of other materials and products to replace asbestos proofed products. This is attri butable to the minimal health hazard presented by these products in service, the natural affinity of asbestos for rubber, the resilience of the fibre as well as the rubber, which enables good seals to be achieved with very roughly cast flanges, lack of growth in the market. As with plaited packings, vegetable fibre proofed products may be used in many applications", but their life may be shorter. There is evidence that certain types of mineral fibre, such as Rockwool, may be suitable for incorporation into rubber-proofed products, although details of performance capabilities are not known. The rubber proofing of woven glass fabrics can result in a greatly enhanced sealing material which combines the strength and temperature resistance of glass fibres with the flexibility and sealing properties of rubber. These materials use silicone or neoprene-based proqfing applied to high quality glass fabrics. This provides a range of highly effective A 18 12/ non-flammable materials in thicknesses from 1mm to 3mm for general low pressure sealing or thermal insulation up to ^50C. In some cases, up to 530C "ay be considered. T3A Industrial Products (UK) offer a continuous filament, airtextured glass fibre, which has been proofed both sides by a natural rubber compound, formulated to withstand temperatures up to 230C and a wide range of liquids and gases. This has been developed as an alternative to proofed asbestos cloth and, in view of the high modulus of the glass base, it will also act as a direct substitute for metallic-reinforced asbestos cloth. The material can readily be fabricated into hand-made Joints and gaskets for boiler handholes and manholes, tank and pressure vessel covers and retort doors. It will seal against air, water, steam and alkalies to ^00 psi. Textiles for protective clothing Chrysotile fibre forms the basis raw material for almost all the activities of the asbestos textile industry. The length and flexibility of the longer grades of chrysotile are such that spinning into yam and cloth weaving are possible. X key point to grasp about applications of asbestos textiles is their diversity and many applications of products based upon asbestos restiles will be discussed in other sections of this paper. Garments for protection from fire and heat are manufactured from lined or 'backed' asbestos cloth, which may be aluminised to give a heat reflecting surface. Specific products include gloves, heat protection clothing and fire-fighting suits. In certain cases, protection is needed from accidental splash (e.g. hot metal) and special wool bler.ds or fire--retardent rav on may be considered as an alternative. These are suitable for direct heat and metal splash protection and has been tested for protection against molten steel splash at 1500C. .Humanised grades are available for greater protection against radiant heat. The materials are resistant to chemical attack and can be laundered without deterioration. The wool fibres are surprisingly resistant to ignition and flame-spread and the clothing is competitive in price with as bestos based products. A 18 1 76 13/ Vhere protection against hot metal or conductive heat is not required, leather gloves and pads provide short term protection against heat, provided they are not saturated with a flammable liquid. In addition, leather gloves resist sparks, chips and rough objects and also provide some cushioning against blows. There is effective competition to asbestos from leather and respective usage has varied with fluctuations in price which occur in response to shifts in raw materials costs. Aromatic oolvamide (aramid) fibres, manufactured by the Du Pont company under the trade names Xomex and Kevlar, although extremely expensive in raw materials terms, have high performance characteris tics which make them attractive as asbestos substitutes. The dis tinction between the two is that .Nomex has superior heat resistance, whilst Kevlar is a high (or very high) specific strength fibre, with high resistance to tearing and punching. Clothing made of Xomex temperature resistant aramid fibre has found applications in fire-fighting, foundry work and as protective under-clothing for racing car drivers. The materials in suitable form allow short-term protection against exposure to temperatures o up to 1370 C, this being achieved by adopting a 'two layer concept 1; Xomex chars, at high temperature, forming crystalline graphite this hard residue layer continues to protect, preventing the destruction of an inner 'thermal underwear' fabric layer which in cum extends the protection time against blisters. For pro tection against molten metal impingement, an aluminised surface may be necessary. Gloves of Xomex are suitable for use with contact temperatures up to 300C. T3A Industrial Textiles Division manufactures, under the Fortamid tradename, an aramid composite material which consists of an outer Xomex surface, needled through a fine polyester scrim to a layer of permanently flame-retarded rayon fibres. A double-sided aluminium film is bonded to the Xomex surface, resulting in a material combining outstanding protection against radiant heat, together with excellent insulation values. The outer Xomex surface of Fortamid is serviceable up to *00 o C, providing the first line of protection at elevated temperatures. The inner PFR rayon layer will withstand around 200 C. Fortamid is used for gloves, mitts, aprons and other clothing items. A 18177 14/ Nomex la suitable for protection against moat chemical hazards, with the exception of some strong acids and may be laundered with little deterioration in properties. Finished articles made'from Xomex tend to be of the order of three times as expensive as those made from asbestos, but the ability to launder indicates that their useful life may be higher. Heat protection textiles based on continuous silica fibres, but would, on account of high cost, be reserved for premium applications, nevertheless, they are obtainable in thinner sections than the textile forms of ceramic fibres (0.25 "" opposed to 3ma) and do not shed fibres under impact. Commercialised as the 'only truly flexible insulant which can operate at 1000C, silica textiles can withstand repeated applications of molten metal. Asbestos cloth, glass cloth and ceramic fibre textiles all fail this test, unless proprietary coatings be applied. The light weight makes silica fibre textiles particularly appropriate for the aerospace industry, for which these products were initially developed in the 19^0*s for insulating tail cones and pipes of the jet engine. It is understood that new processing methods are coming on stream for ceramic fibre textiles which will enable them to be produced in thinner sections, making these more competitive with silica fibre textiles. Vhere personnel protection and more modest heat resistance is specified, gloves made of Kevlar 29 fibre may be preferred, sometimes with rayon interior for added comfort. Kevlar is inherently flame retardant, thermally stable and will only start to char and decomoose at temperatures above ^25C. Limited contact exposure up to 535C is possible. Xn some circumstances, alumcnised fire suits have been made --sing Kevlar, this is believed to be due to substantlallv improved wearer comfort, relative to Nomex, as a result of the reduced weight of the finished article. spite of the potential of Kevlar in many applications, onlv a few of which will be discussed in this paper, the Du Pent company indicate no prospect of the price being reduced due to economise of scale - the fibre is currently selling at a commercial loss. A 18178 15/ Fire blankets, curtains and aprons In general, a suitable material may be selected from those derailed above. Commonly glass fibre (or a layer of ceramic fibres between two woven glass cloths), may be used for safety curtains and fire blankets. Compared with asbestos, glass is slightly less heat resistant and can fail more dramatically (it may melt). Where flexing may occur such in fire blankets, glass fabrics may be graphite/silicone oil lubricated for additional abrasion resistance. Normal glass fabrics will not resist continuous molten metal contact unless a proprietary 'weld-stop* coating is applied. Even ceramic fibres, which can be used to higher temperatures,require a special coating in order to resist molten metals unless coated. An alternative for asbestos curtains is glassfibre cloth coated with oil and abrasion-resistant Hypalon (chlorosulphonated polyethylene) rubber. Made by A_lpha Associates, New Jersey, USA, it is UL listed. At a thickness of 0.45-0.35mm it can withstand 3000 abrasive cycles and suffers slight damage in weld spatter tests. General purpose heat protection and insulation cloths These cloths, for which the main use is in secondary wrapping of primary insulation on steam pipes and boilers, may be substituted by glass cloths, which is inferior in terms of overall technical performance (although glass retains its mechanical strength at higher temperatures than asbestos) but mav be some ')0-4oe,; cheaper. Glass cloth is 'less convenient' to handle, being more of an irritant than a known health hazard. Millboard This is one of the most versatile asbestos materials used in industry. Features of millboard vhich contribute to this versatility are: ease of cutting of punching to size and shape; useful thermal insulation properties; ability to be impregnated with bonding agents and cements; ability to be wet-moulded; compressibility. Millboard finds applications wherever a low cost, relatively soft, low density board material with modest mechanical properties, nigh heat resistance and good thermal and electrical insulation charac- 16/ for the transport of hot saterials in the steel and glass industries, as formers for wire-wound electrical resistances, flange gaskets for joiats ia ducting and trunking used for high, volume/low pressure gas transport, cylinder head gaskets, iasulating linings to minimise heat losses from ovens and moulds, as plugs and stoppers for molten metal containers and, in resin impregnated fora, for clutch facings. Substitutes for millboard For many applications, millboards made from alun^ao^il_icate_fibres or mineral wool fibres bonded with a high temperature inorganic (silica) binder can provide a direct replacement, although tend to be more expensive. They are available in thicknesses up to 30mm and are made by a suction method from an aqueous slurry of fibres. TBA Sealing Materials Division, for example offer three grades in their Fireflv fange, which differ in mechanical and fire resistance properties according to end-use requirements. For thermal insulation applications, if the thermal and mechanical conditions are not too severe, one of the several types of mineral fibre block and slab products should be an effective substitution. A recent materials development for materials for handling hot glassware formed on automatic production lines is Cerberite: this is a oolvinide resin, reinforced virh carbon fibre and containing a graphite filler. This material has been used to substitute asbestos and metals, not because of health hazards, but because these materials were potentially damaging to the surface condition, and thus the strength of the product. The problem became acute with increasing economic pressure to reduce the wail thickness of glass bottles, demanding maximum obtainable strength. The condition of the glass surface was affected by the abrasive nature of asbestos or metal, and also the tendency of asbestos to absorb oil and dirt; since replacement by Cerberite, the mean glass strength of a 0.23 litre bottle (essentially independent of glass thickness) has been increased from 23-30MPa to 35--`5MPa. For further details, Johson Radley, a division of United Glass should be contacted. Electrical insulation 17/ Asbestos is widely used in the electrical industry in the form of paper, tape, cloth and board. It is frequently applied as a felted material and as a filler for natural and synthetic insulating1 resins. In dielectric applications, the most common impregnant is a solid resin. The impregnation of the asbestos with resin increase its dielectric strength, improves its mechanical properties and results in a moisture-proofing material in order to offset the hygroscopic properties of the asbestos itself. Laminates suitable for use up to 180-200C can also be made from felts or woven cloths with appropriate high temperature resins. They are used for low voltage transformers, armature slot vedges, furnace parts, domestic heating equipment and similar applications. Resin-bonded papers and boards use, for example, phenol formaldehyde, polyvinylacetal, epoxy, or silicone resins according to the temperature of service and may also contain other fibres or fillers. Asbestos papers and mat are made by methods similar to cellulose paper. Most cases in which asbestos textiles and papers are employed for electrical insulation also demand a degree,of thermal and/or chemical protection. Chrysotile asbestos and smaller amounts of amosite and tremolite are used as textiles for electrical insulation purposes. In this form asbestos is employed for insulating wires and cables, especially those which are designed for low voltage, high current use under severe temperature conditions, arcing barriers in switches and circuit breakers, and as braided sleevings for electrical appliances and insulated conductors where fire protection and resistance to mechanical abrasion are sought. Asbestos in the forms of cloth tape and sleeving may be reinforced with glass or natural fibres. Asbestos-cement products are used in the electrical industry for the construction of mechanically string and.heat-resistant rods, tubes, cylinders and plates. Applications include panel boards, arcing barriers and insulating tubes and cylinders used in the construction of air-cooled transformers. A 18 1 8 1 Cheap asbestos boards bonded with starch and calendarised are not very strong, but will withstand fairly high temperatures. Asbestos millboard is used for applications such as formers for wire-wound electrical resistances. 18/ For electrical use, the choice of asbestos free from iron and the removal of any iron oxide or metallic particles is important. Asbestos materials are not suitable for high-voltage insulation, nor for high frequency insulation because of high dielectric loss even whan dry. Their main electrical use is in low voltage, high temperature situations and for the confinement of arcs. Substitutes for asbestos products in electrical insulation A brief summary of the classes of electrical insulating materials as detailed in BS 2757:1956 is given in Table 2. Substitute electrical insulants Mica products offer outstanding voltage endurance, low power factor and excellent corona resistance. The principal application for mica products is for high voltage insulation, traditionally in the form of splittings, but increasingly in the form of resin-bonded mica paper, as confidence in the product grows. Mica paper is a cleaned-flake form, in which the flakes are held together by Van der Vaals forces, prior to the formation of a composite by resin impregnation. In some cases a woven glass backing layer is added to provide better handling strength. Whilst there is nothing to challenge the performance of mica at high voltages which will erode anything else! some erosion does occur with resin-bonded products (it is important to avoid voids), but these are still 'more than adequate' to withstand the discharge up to Class F. Resin-bonded products are much easier to use than mica splittings. Depending upon the amount of resin employed, the temperature performance of the micaglass tapes is more or less determined by the characteristics of the resin used. However, with certain in organic silicone-based binders, tapes are able to withstand severe electrical and mechanical overload conditions. High density mica paper permits designs with a minimum binder content (the weakest 182 19/ link thermally and in flame resistance) and designs that electrically retain 60-70# of original dielectric strength after exposure to temperatures of 650-800C. An outer jacket of a conventional in sulator for mechanical protection is used in such applications as wire and cables in skyscrapers, offshore oil platforms, aircraft and the like vhere circuit integrity is critical; mica paper protected cables have been shovn to withstand a direct flame with a temperature of 1000C for up to three hours. Silicone-bonded mica paper can replace electrical insulation applications of asbestos, which is only a moderate electrical insulant suitable for high temperatures. Although the resin degrades at about 300c, silica-type bonds are formed which sustain the in tegrity of the composite up to the degradation temperature of the mica. In the case of muscovite mica this is about 800 oC and for phlogopite mica, which is 50# more expensive, it is 1000C, Mica products, unless reinforced with glass, can be mechanically weaker than asbestos products, but in most cases this has not proven a limitation. Mica paper impregnated with a- silicone resin may be specially formulated to ensure that it remains flexible in its normal state. However, if post-cured at a suitable temperature, after forming to shape round an application structure, it will produce a rigid shape. This product has been used successfully as electrical and heat insulation . in refractory induction furnaces. It has also been used for band heater insulation, heat shields and protective applications in arc welding where high temperature resistance is important. It has also been used in the steel industry as a substitute for asbestos blocks, completely eliminating expensive machining operations and the dust which they create. For arc chutes and boxes, where asbestos cement has been tra ditionally employed, Micanite and Insulators have found that an electrical grade tolviter PMC, compression moulded as spacers, is satisfactory for millisecond exposure to l600C. Mloa-sillcone can also be used, but DMC offers additional manufacturing flexibility. M8 1 83 20/ Although asbestos fibres are more resistant than cellulose to o temperatures above 100 C, glass fibres are more resistant above 200C, Consequently, in applications such as insulation sleeving for cables and battery separators, direct replacement with glass is frequently satisfactory. Glass fabrics of many kinds (cloth, tape, tubes, cords) are woven from yarn and used in resin-bonded laminates. Glass chopped strand mat is also used for moulded or hand-lay composites. The fibres do not alter in mechanical properties up to 200C and their strength is roughly halved at 350C; they are not suitable for applications where severe flexing is involved. Glass fibres served onto wire and treated with suitable resins are used in high temperature windings. This type of insulation is more vulnerable to abrasion than most other wire coverings. It should be remembered that many of the mechanical and other properties of glass-bearing composites will be determined by the properties of the matrix resin. Aramid paper (Du Pont1s Nomex) is finding significant applications as a replacement for asbestos end glass fibres, or for improved performance to replace cotton or kraft paper. The product retains 65ro of room temperature tensile strength at 225C and has a life at this temperature exceeding ten years. However, it may bum under intense exposure to arcs. Saturable forms of Nomex paper are supplied to manufacturers of rigid laminates utilising various resins. In the flexible area, Xomex is laminated to commonly used films, fabrics, papers and metals. Polvmer Films Some polymers for electrical insulation are available in film form, notably irradiated, or crosslinked, polyethylene (105 C), polyester and polyimide. Polyester film, because of its high resistance to penetration and tearing, can withstand any amount of handling during assembly. Slot closures and coil end insulation are suitable applications for polyester films and in special electrical machines it can be wrapped around the conductor as the primary insulation. Polyester film has been allocated to Class Z (maximum working temperature 120C). A 18184 21/ This classification is considered by users to be very conservative and it is common for unsupported polyester to be used as a Class 5 material (130C) and its use in such conditions has been well vindicated. The use of polyimide film (such as Du Font's Kapton) permits, as in traction motors, a permissible windings temperature of 200C for the rotor and 220C for the stator (Class H). The manufacturer claims that some grades have been successfully used in applications at UOOC and, incidentally, also as low at _269C(Lk). Applications for polyimide film include wire and cable tapes, formed coil insulation, substrates for flexible printed circuits, motor slot liners, magnet wire insulation, transformers and capacitor insulation, magnetic and pressure sensitive tapes and tubing. Like all organic materials, polyimide film is attacked by corona and will ultimately fail die lectrically when exposed continuously to corona. At moderate levels of corona exposure, devices insulated with polyimide film have survived up to 3000 hours, but this cannot be guaranteed under all circumstances. Polyimide film of 0.12~mm thickness has replaced O.Lmm thick asbestos paper on the conductors of classes F and H squirrel cage motors at k 9 A5EA, Vasteras, Sweden, enabling the constructor to offer more- reliable motors. Because polyimide is much tougher than the material it replaced, built-in weak points due to rough handling during production are avoided. AS2A were already using polyester film for class and 3 slip ring motors, so were able to standardise their manufacturing processes for high speed film handling techniques. In the AI traction motors used in the new class 6E1 locomotive ordered for the South African 3000V DC system, the original design incorporated maximum iron to limit magnetic saturation of the armature teeth, resulting in a very narrow slot. Polyimide film was the only insulation available capable of allowing more copper to be added, because of its excellent dielectric strength. A mica-glass tape (see below) was replaced for insulation of the conductors by a 12.5mm wide FEP-coated polyimide film, wrapped with 50ec overlap and sealed around the conductors. For ground or slot insulation, polyimide film replaced a combination of mica and asbestos sheet. A 1818b 22/ High temperature polvmers The insulation of electrical cables normally involves the use of a high temperature polymer over the primary insulation. Commonly these have included silicone, fluoropolymer (e.g. FEE), EPM rubber. Some newer high temperature polymers are worthy of mention? these include polyimide and polyethersulphone, suitable for use as wire and cable coatings at temperatures, in excess of 150C. They offer the ability to withstand high temperatures with toughness and flexi bility, but are expensive. Polyethersulphone is thermoplastic but can withstand soldering temperatures for short periods. Polyetheretherketone fPEEKl. like polyethersulphone, is a thermoplastic material o offering an in-service temperature capability approaching 300 C. Offered initially as a high performance wire coating, this aromatic, amorphous polymer has outstanding combustion characteristics and chemical resistance. Friction materials Asbestos--based brake and clutch linings and pads are in widespread use, particularly but not exclusively in the automotive industry. Brakes and clutches are devised for stopping or transmitting motion, which involves the dissipation of energy and thus the production of heat. In the case of friction materials for brakes, temperatures in excess of 800 C may be experienced and with clutch materials tempera-- o tures range up to 300 C. The most important characteristics in clutches and brakes are: (a) the friction levels, which must maintain their design parameters over a wide range of operating speeds, pressures and surface t emperatures j (b) the wear of both the'friction material and its mating surface must be of an acceptable level in order to ensure a safe pedal travel reserve, adequate life and adequate mechanical strength. In order to maintain safety standards, braking regulations and tests covering new vehicles have been embodied in legislation in most Continental countries and in the United States. These are . basically more stringent versions of the MOT tests operative in the United Kingdom; they specify a duty which the friction material must be able to withstand and set minimum performance standards. These are related to the type, weight and performance characteristics of the vehicles concerned and the test procedures are both varied and 23/ complex. Teats of friction materials include water immersion, low and high temperature evaluations, and tests related to the material recovery following many applications of the brakes. Other requirements such as acceptable noise levels, the wear rates of the brake drum or disc, judder, squeal, etc., all have to be evaluated and contained within Motor Industry requirements. Asbestos in friction materials Friction materials originally used natural materials such as wood. cork and leather, and later, stun cotton was utilised, but all were unsuitable for the rapid development of motor vehicles. The advent of asbestos-based linings in 1908 set the pattern for friction materials for a quarter of a century and influenced vehicles brake drum designs for an even longer period. The improved friction and wear performances required over the years by vehicle design changes resulted in asbestos fibres being bound together by resins and other fibres and fillers. Most modem friction material linings and pads are moulded using short chrysotile fibre asbestos in amounts varying from 10-TOfi, plus various fillers designed to impart particular friction, wear or lubricating properties, and a binder system of synthetic resins and/or elastomers which are used both for their tribological properties as well as to polymerise and bind the whole mixture into the required shape. Some woven brake linings are used for industrial purposes and for transmissions or clutches, which use longer fibre asbestos spun into yam. Such textile-based linings may have asbestos contents of 60-80ci. In the case of clutches a woven asbestos cloth reinforced vioh brass wire and impregnated with phenolic resin may be used. Chrysotile asbestos performs in these applications a complex function which is not completely understood. The fibres stiffen and strengthen the filled phenolic resin matrix and can maintain these properties at the high temperatures of operation. Ihe friction and wear characteristics of asbestos and its thermal decomposition product, forsterite, influence the braking efficiency and service life of the products; if anything high operating temperatures tend to promote slightly higher friction. Forsterite is a non-fibrous silicate which is not believed to constitute a health hazard. In vestigations have established that the dust produced from the wear A 18187 24/ of asbestos friction materials contains only l-2fc of asbestos fibres. Repeated exposure to this dust may be a potential hazard to persons working in close proximity to these products (e.g. garage mechanics) if sensible precautions are not taken. Substitutes for asbestos in friction materials Various fibres have been used in phenolic binders, such as steel, glass, mineral wool and aluminosilicates. They all have drawbacks and none is yet as good as asbestos. However, a considerable volume of work has been carried out on the use of steel and glass fibres. Vehicle trials have latterly been carried and it is estimated that about 30i of the friction products market could be substituted in this way, the exception being the heavier duty materials. The first use of steel fibres in friction materials was by Germany, prior to and during World War IX, due to the difficulty of obtaining imported asbestos. It was found the brakes worked well, with little loss of efficiency when vet, but that the wear debris was abrasive and severely damaged the counterface. It has been found easier to use steel than glass in the manufacturing process, which uses the same volume fraction of steel as asbestos. The steel fibre is usually of the fine steel wool type and is about four times as expensive as the asbestos used in friction applications. Glass-fibre based friction materials also present a very abrasive wear debris with corresponding damage problems. At high temperatures, the glass can melt, which can produce a sudden undesirable loss of friction (fading), since the sliding surfaces can be effectively lubricated by the molten glass. Hence, latest developments, involving blends with other materials, are aimed at heavy duty vehicles, such as trucks, in preference to cars. Conventional E-glass fibre is about half the price of steel wool. Both glass and steel fibres behave satisfactorily in the wet-mix manufacturing process, but in the dry-mix process steel fibres tend to segregate and glass fibres orientate themselves in preferred directions, leading to loss of optimum properties. Brakes using both glass glass and steel tend to be considerably noisier than their asbestos counterparts. A 18188 25/ A more recent and promising candidate for the reinforcement of friction materials, because it does not have characteristics which could negatively influence performance is Kevlar (Du Pont). Not withstanding its cost, Kevlar is said to show several advantages over asbestos. Resistance to wear is three times that of asbestos and the friction characteristics remain stable at high temperatures because the fibre does not melt or soften. Even after analysis at A50C, the fibre will begin to carbonise. Kevlar gives a less aggressive friction surface and because it has toughness and re sistance to crack propagation, imparts a high level of burst and crack resistance at operating temperatures. Many forms of Kevlar fibre can be used to reinforce friction materials continuous filament gives the highest strength and level of overall performance and is used in heavy cuty clutch linings; crimped cut fibre from 6-100mm long is being tested in moulded brake linings and clutch plates and also as staple in spun yams; a new product called pulp, which is highly fibrillated, shorter form available as a dry fibre and as a vet pulp sheet, is being evaluated for possible commercial production. Pulp has ease-of-processing advantages in both wet and dry manufacturing methods established for the production of friction prbducts. Some sources suggest that Kevlar pulp contains a proportion of respirable fibre. In mining with resins and other fillers. Kevlar requires special attention because it is tough and the fibres do not break up easily; there is also a tendency to lump. Good dispersion, which is important for moulding performance and machinability is best obtained in a high speed, high shear miner using crimped fibre of 6-12mm length, and by the use of finely divided fillers. Phenolic resins, currently used in friction products, can be used to impregnate Kevlar yam by conventional techniques. Minor modifications of the dipping equipment to open the yam, and shorter curing times are needed to obtain a good bond. Machining clutch plates and brake linings reinforced with Kevlar fibre requires special attention because the fibre is inherently tough. Tools and procedures have been developed and evaluated; positive results have led to trials on large volume production equipment. A18189 26/ The characteristica of friction products reinforced with Kevlar have already found commercial applications in the automotive industry. Beral Bremsbelag KG in the Federal Republic of Germany markets clutch facings as original equipment to Porsche, Audi and Daimler Benz. Evaluation is underway at other major European car and truck manu facturers . Other clutch lining producers have also initiated fleet testing. Drum brake blocks for trucks are being actively developed. Fleet test results indicate that the performance is at least equal to asbestos reinforced products. A commercial adoption of drum brake blocks is expected within the next two years. Mixtures of Kevlar pulp with ceramic fibres are under development. Sintered metals, ceramics and metallised bonded products have been available for many years, evolved from powder technology. Because of their particular performance characteristics, use has been limited to heavy duty applications, such as off-the-road earth moving equip ment where they have been successful. The high thermal conductivity of these materials creates, in normal automotive applications, a risk of heating and boiling of brake fluid, which can cause erratic performance and inconsistent behaviour between hot and cold conditions. Silicon nitride ceramic was used for the brake pads in the Concorde prototypes 001 and 002. It was found to have a higher service life than asbestos, and a higher thermal conductivity, which in this application was desirable. The material is ex pensive, the powder costing over 5000 per tonne and the components were heavier than the carbon-reinfarced carbon composites eventually adopted. Carbon/carbon composites are used for the brakes on the production Concordes and have also been used on racing cars where they are more efficient than asbestos materials under the high service temperature conditions. The carbon-fibre reinforcement used is a cheaper type than conventional RAE types and is formed by charring wool, rayon, nylon or any organic materials in the farm of yarn, felt or cloth, This charred fibre is infiltrated with resin or pitch which is subsequently charred. Alternatively, an organic gas may be passed through the charred fibres while they are heated. These processes are repeated several times and the final product is denser than I most other composites of this type. It oxidises only very slowly ! at about S00C. The anisotropy of these materials produces uneven 27/ txe&C flow and thermal expansion coefficients and, additionally, they have relatively low tensile and impact strengths. Their high cost (development prices quoted exceed 10,000/tonne) restricts their application to specialised circumstances. Dry rubbing bearings X significant application of asbestos composite materials is in plain rolling bearings. In this application a matrix of thermo setting phenolic resin is used to impregnate asbestos cloth or yarn. The principal advantage of these materials is that, although they can be lubricated with oil or grease and in some cases are supplied im pregnated with up to 7% mineral oil, they are also able to function effectively without lubrication, or lubricated by in situ process fluids or seawater. The life of dry rolling bearings is comparable with that of lubricated bearings only at slow sliding speeds, in practice, principally in oscillating applications. Their use is generally advantageous for applications with sliding speeds below im/s and where the use of a lubricated bearing is impossible or unattractive. X detailed discussion of this area is carried in the written paper, which tine constraints prevent during this presentation. Most alternatives carry a cost penalty relative to asbestos, unless inferior performance be acceptable. Typical applications of dry rubbing bearings include automotive steering column bushes, brake and clutch pedal bushes, bearings for textile spinning frame spindles, food slicer bearings, copying machine rollers, bearings for conveyor rollers, compressor thrust washers, aircraft control and undercarriage bearings, dock equipment, lock gates, large railway bearings, and as seals and liners for metallic ball and roller bearings in, for example, railway axle boxes. Asbestos bearings in particular find extensive application in I^rge marine bearings, where high loads and low sliding speeds are 28/ encountered and where seawater may be used as a lubricant. Xaportant examples are rudder and steering gear bearings and stern shaft bearings. Historically, liwua vitae in the form of axial staves with a gunmetal bush was commonly used. This arrangement was also suitable for lubri cation by seawater, but the hard, oily wood suffered from a tendency to swell in water and the life expectancy was low when high loads were encountered. The next generation of marine bearings employed bronze or white-metal bearings, which are still in extensive use. Metallic bearings are suitable for a wide range of applications when cost is not of prime importance and constitute excellent bearings when loads are moderate and speeds sufficiently high to create a continuous lubri cant film. However, under the high load, low speed regimes- encountered in marine bearings, the consequent marginal, or 'boundary' lubrication can lead to excessive galling and seizure. Reinforced Plastics bearing materials also have other major advantages in this application: white metal is inherently weak under shock loading conditions; in marine environments galvanic corrosion is likely to occur with metal bearings; | and plastic matrices are less stiff (lever Young's modulus in compression) than bearing metals, which means that misalignment is a less serious problem. These advantages lead to reduced maintenance time and longer life expectancy, which, when combined with good machinability and ease of fitting, contribute to significant cost reductions in this application. Asbestos bearings additionally are tolerant of line and spot loading. This good performance as a bearing material is not well understood, fcut it is thought that the tendency of the flexible chrysotile fibres to 'brush out1 on the bearing surface may be sigtiifleant. A 18192 29/ Substitutes for asbestos-reinforced thermosets in bearing applications When materials are to be evaluated in connection with a specific problem, the test conditions required are unambiguous: they should simulate as closely as possible those of the applications. Pre liminary evaluation of materials on a wider basis, however, is much more difficult. For a reasonably complete evaluation, the following tests are required: (1) Determination of the coefficient of friction and wear rate at a moderate load and a relatively low speed in sliding for which frictional heating is negligible; (2) Determination of the maximum temperature limit at low speeds of sliding; (3) Determination of the critical load and speed limits at which either friction or wear become excessive as a result of frictional heating. The following factors are also important to specific applications and design: environment, thermal conductivity, expansion coefficient, resistance to shock loading and cost. Intelligent use of design options, for example, in the provision of seals against the ingress of corrosive fluids, can increase the range of suitable materials for specific applications. However, the remainder of this section is concerned briefly with the materials properties of potential substitutes for asbestos composite plain bearings, operating under high load, low speed conditions. Reinforced thermo set 3 Asbestos composite bearings belong to the reinforced thermosetfamily of plain bearing materials. Among the other members of this family which do not contain asbestos are: (a) polyester bonded textile laminates with molybdenum disulphide or graphite. (b) cellulose-fabric based phenolic laminates with uniformly distributed PTFE or graphite. The members of this family have broadly similar physical and chemical properties. However, the maximum operating temperature of asbestos-reinforced composite is generally higher - up to 175C compared with !00-130C for the other composites, and the coefficient of linear expansion is lower. A 18 193 30/ The substitution of asbestos-reinforced composites by either of these two classes of alternatives is therefore heavily dependent upon the exact nature of the application and whether the marginal loss in properties, particularly maximum operating temperature, is acceptable. For applications in which such substitutes are not acceptable, substitutes have to be sought from other families of plain rubbing bearing materials. Other plain bearing materials Some potential substitutes, again depending upon specific conditions are nolv^ide^, oolvamide-imides. woven and resin-bonded PTFE fibre, graphite-incremated metals. PTFS-impregnated metals. The last two products are generally based on powder matallurgy methods, but may not be preferred in marine applications for much the same reasons as white metal. The remaining materials in the list carry a heavy cost penalty relative to asbestos. Mention should also be made, for applications which are less demanding, of the nylons (polyamides - not to be confused with polyinidesi) and acetals, and it should be appreciated that many uses where'impregnated metals are economically and technically satisfactory could be adequately covered by the family of oil- j impregnated porous bearings. Very recently, at the 1391 L~ Design Engineering chow, Railko, a specialist company in the bearings field, unveiled a new general purpose bearing material, Railko NT. This is based upon 'an anti scuffing thermosetting resin incorporating a special system of organic fibres'. Vhen compared with asbestos reinforced materials, it is claimed to offer superior impact strength, combined with higher resilience plus improved wear and friction characteristics. It is available in a wide variety of moulded or machined components, rod, tube and sheet. Reinforced clastic composites Excluding such composite products that are dealt with m ether sections (friction materials, bearings, etc) there remains a vide range of asbestos/polyraer composites in use across the engineering spectrum. These are usually made with a thermoset matrix, although composites using thermoplastic matrices such as nylon, polypropylene 31/ and PVC are in fairly widespread use. Typical applications are for small machine parts, usually made by an injection moulding method, such as vehicle distributor caps, fans, fan shrouds, small casings and other similar products. In these applications, the fibres are randomly aligned in three dimensions. Composites using asbestos in various degrees of alignment have been made, such as those based on 'Durestos' and'Pyrotex* materials and using woven cloths of asbestos, which utilise the asbestos in an efficient manner and enable,a significant proportion of the ultimate properties of the fibres to be achieved in practice. However, these materials use the more expensive, longer fibre length grades of chrysotile and an expensive alignment process, so that it is often simpler and cheaper to sue continuous glass or other fibre for this type of high performance composite appli cation. Durestos (T3A) resinated felts are used in Defence work for applications requiring ablative properties and for automotive use. A recent grade is designed for use as valve seats, particularly in the refined oil products handling industries. The new material has been developed at the request of Shell to fill the need for a tough, strong material with low static friction and good sealing properties. This material has proven to be very satisfactory for bail valve seats for equipment carrying refined oil products in the cargo handling system. The material shows good flow characteristics and is suitable for processing by compression or transfer moulding. It also has a very low mould shrinkage and coefficient of expansion, stability factors which make components particularly suitable for valves on tankers which are subjected to a wide range ox' climatic temperature variations. A widespread application for an asbestos/thermoplastic sheet composite for junction, destination and mileage signs used on roads and motorways. These are manufactured from asbestos in a PVC matrix and consist essentially of a random array of asbestos fibres in two dimensions. Similar materials are used as cladding materials, having a degree of fire resistance, without the edge corrosion of some metal alternatives. A 18 19b Substitutes for asbestos composites 32/ Asbestos reinforced thermoplastics moulding compounds are part of a family of compounds) which compete with each other and with non-reinforced thermoplastics and thermosets and with metals in engineering components. In practice, each specific design has to be considered with respect to strength, stiffness, weight, thermal stability, heat and chemical resistance and cost, and it is the specific combination of these which will determine which material represents the best compromise. Very generally,asbestos reinforced thermoplastics score on stiffness, fire retardancy and low coefficient of expansion. There is no doubt that other metals of plastics with different. fillers can be used to replace these asbestos compounds. Class fibre provides an adequate replacement for asbestos in many of the injection moulded component applications. Longer fibre lengths are necessary with glass than with asbestos to achieve comparable aspect ratios and this necessitates careful manufacturing control to avoid fibre damage during component fabrication. Class fibres have a lover modulus than a asbestos; consequently, if component stiffness is important, the volume loading necessary may be higher. Cenerallv the maximum continuous operating temperature for a glass-reinforced composite is comparable to that of the corresponding asbestos material, al though with glass, mechanical properties may be retained at higher temperatures for longer periods of time. Perstorp Ferguson, for example, offer a full range of asbestosfree phenolic moulding compounds and according to the company 'we withdrew a range of materials (i.e. asbestos filled grades) that had extremely broad scope in a great number of applications. There is no filler system that can match asbestos in all its properties. 2ut ve have the logical alternative. Ve applied the same philosophy to our asbestos--free grades as we do to all out thermoset compounds a grade ideally suited to each application..!' In practise this amounts to grades containing glass and/or mineral fibres. A18 196 33/ For two-dimensional random sheet applications, glass chopped strand mat may be effective. The stiffness of glass-fibre sheets may be increased, if necessary, by 'hybridisation1, the addition of a higher modulus (usually carbon) fibre to the composite in small amounts. Carbon fibres are expensive, but often in this type of application, the weight saving possible for a particular design stiffness and the consequent economy of glass fibre make hybrid structures cost-effective with all glass composites. Synthetic fibres such as nylon or rayon, and natural fibres such as cotton and paper, impart less strength to a plastic matrix than do glass or asbestos. Maxi mini service temperatures are also signi ficantly reduced. They are generally employed when strengths in termediate between those of the plastic matrix and the reinforced composite are satisfactory. Cotton and paper reinforcements have been widely used for many years with phenolic and melamine resins for electrical and decorative purposes. Such materials as hirh-silica glass, cuartz. rrashite. and other unusual reinforcements are used to meet extreme high temperature demands. Typical applications include high-silica glass as insu lation in rockets, quartz for structures exposed to long-term elevated temperatures and graphite for abrasion-resistant in rockets. A 18197 34/ summary of the use of asbestos-based materials in buildings This section reviews the use of, and substitutes for, asbestosbased products which are used in buildings, excluding sprayed asbestos, which is now virtually obsolete as far as new constructions are concerned. Use is made of original material by Robert Derricott of the Greater London Council which appears in Asbestos, Properties, Applications and Hazards, ed. Michaels and Chissick (John Viley & Sons) 1979. Higher density hard-surface materials For asbestos-cement building products, about 12-15"i of chrysotile fibre is normally added to hydrated Portland cement; with'pipes a small proportion of amosite fibre is introduced also. The end product is then compressed into flat or corrugated sheets and other moulded goods in which the asbestos fibre is cementitiously bound in the resultant strong but brittle, durable, non-combustible composite. Semi-compressed asbestos-cement flat sheets are available in a naturally light grey colour and with a single smooth face. They are often specified for use as wall and ceiling linings, particularly in agricultural and sectional buildings. Fully compressed flat sheets available with a range of factory-applied coloured finishes and are specified for use in the facing of composite vailing panels, in weatherboarding, etc. Thermal conductivities should be checked with individual manufacturers, but the thermal conductivity is typically of the order of 0.65V/m,a and the density about 1200 and l600kg/m^ for semi-compressed and compressed asbestos-cement flat sheets respectively. Asbestos wood*, so named because of its workability and not its content, contains a higher proportion, believed to be about 2yi, of chysotile asbestos fibre in hydrated asbestos cement; it is used in general thermal insulation work and in the fire protection of structura steelwork. Semi- and fully compressed asbestos-cement flat sheets are also available with the addition of 'pulp fibre', giving the boards a light tan appearance, Uses include internal partitions and roof and wall linings in industrialised buildings. The density of fully compressed sheets is about IjOOkg/m^ and of semi-compressed and asbestos wood sheets about 1200kg/m . The thermal conductivity of all three types of board is about 0,3V/m.a. A 18198 35/ FuXly compressed asbestos-cement is also available In integrally coloured, extruded, hollow-section form. The variety of uses include cills and copings in an unfinished state for external application and internal skirting boards, shelves, supported working surfaces, etc., which if desired can be wax polished. UK Marinite, a rigid insulating composite of autoclaved amosite asbestos-reinforced hydrated lime and silica, has been developed for the manufacture of glazing channels by the Timber Research and Development Association (TRADA) under licence from Cape Boards and Panels Ltd., as Marinite TRADE Firecheck Channel. This material resists the retention of areas of glazing in fire conditions and may be finished with intumescent paint in the sealing of gaps in panels, screens, and door constructions. It is also available in board form for internal insulation and fire protection applications where boards thicker than standard asbestos insulating boards are required, or when veneering or edge screwing is specified. Lover density soft-surfaced materials For lower density asbestos-based thermal insulating boards, about 25-lOfs of amosite asbestos fibre is normally added to hydrated Portland cement. The composite is then semi-compressed into boards which, on account of their characteristic softness and higher asbestos fibre content in comparison with asbestos-cement products, are more susceptible to fibre release when handled or worked. This lower density (around TOO kg/mJ softness and higher asbestos fibre content give the boards an ability to offer not just fire protection but also insulation against heat (the thermal conductivity is about 0.1V/mK) and sound transmission. Being light, off-white, acid resistant, smooth-faced on one side, easy to cut to size or shape, the boards are often used for internal wall and celling linings, duct and door linings and in the fire protection of structural steelwork. Timber framed partitions faced with asbestos insulating boards will offer about 35 dS sound reduction, depending upon the board thickness, and the cavity dimensions and content. With a chrysotile asbestos fibre content of about ^5-98ci and with other non-combustible agents and fillers, asbestos millboard can also be used for thermal insulation applications. & 1 8 1 99 The substitution of asbestos In building boards 36/ (a) Hard surfaced materials Depending upon which or the properties of asbestos-cement, asbestos wood, US Marinite, etc., may be considered superfluous to the requirements of the Job in hand, wall and ceiling linings, partitions, fire-door constructions, cills skirtings and the like may alternatively be constructed with such hard-surfaced materials as cementitious composites reinforced with, in order of ascending extra cost, glass. steel, carbon, or constructed with glass-reinforced gvosum. metal sheeting, glass-reinforced plastic. In glass-reinforced cement (GRC), for example, about of re latively expensive alkali-resistant glass-fibres are normally added to hydrated cement and other fillers. The resulting composite can be manufactured by different processes including extrusion, hand or mechanical spray-up and moulding and can be vacuum dewatered. The composite was introduced by Majumdar and Nurse (197M and the Building Research Station (1976) reported, on a more recent assessment, that although there is a tendency to embrittle on wet storage, and long term strength forecasts are difficult to make, the material has a high impact strength and is non-combustible and has consistently satisfied ondicative fire tests. The commercial development of this composite has been taken up by Pilkingtons as Cem-FIL under licence from the National Research Development Corporation (NRDC). Various external finishes are available and the density is of the order of 2000kg/m^. A new form, Cemril 2, has glass fibre protected by a surface coating and is reported to have much improved long term durability. Glass-reinforced cement or reinforced concrete is also offered as a substitute for large diameter pipes which may otherwise be specified in asbestos cement; smaller diameter pipes are available in unplasticised (rigid) PVC and various metals. In each case the viability of the alternative material may depend upon its ability or failure to cover the properties of asbestos; with pipes embedded in the ground, soil contamination may restrict the choice available. During the investigations which led to the introduction of Cem-FIL by Pilkingtons, glass-reinforced gvtsuralGRP)was made using commercially available, cheaper E-glass fibre and a non-alkaline (sNPsuln) binding material. The NRDC is responsible for encouraging M820C 37/ its manufacture under licence. Again, the material offers a high impact strength, a density of about 1000kg/m^ and excellent fire resistance, although on account of the gypsum base, it is not available for external applications. Both GRC and GRG have a thermal conductivity of about 0.2V/mK and are offered for use as wall and celling linings, in fire door constructions, cills, skirtings, etc., GRG being restricted to internal use. As with asbestos cement, these materials are available in composite construction encapsulating thermal insulation. Sheet metals are also available for all of these applications; aluminium or steel, for example, in flat or moulded sections, being generally strong but susceptible to impact damage and twisting or bucking on heating, are offered for roof and wall-cladding materials, cills smd skirtings. Various finishes, including colour coating, are possible. As good thermal conductors, however, metals necessitate protection against cold bridges and the formation of condensation; the end result'is higher cost. GRP may be considered where non-combustibility is not a requirement, for example in flat or moulded wall and roof cladding panels, where its properties can be accommodated. Generally GRP has poor scratch resistance and low impact strength; it is thermosetting and thus chars on heating and can produce large amounts of smoke and noxious fumes on burning, although fire properties and weathering character istics vary greatly according to formulation. Allowance must be made for thermal expansion in properly detailing fire-stops and against the ingress of water. Apart from thermosets, thermoplastics such as acrylics also scratch relatively easily, but have higher impact strengths. They melt on heating, whereby smoke production and the release of molten droplets in fire conditions may be problems. (b) Lower density soft-surfaced materials Many leading building material manufacturers have now marketed alternatives to asbestos insulating boards. For example, Supalux (Cape) is said to be organic fibre-reinforced calcium silicate with 'other fillers'. In comparison with the asbestos-containing counter part, the material has higher density, different moisture movement characteristics lower impact and tensile strengths and a slightly greater thermal conductivity. The manufacturers offer independent test certificate to demonstrate fire performance. TAC Limpet insu- A18201 38/ lation board is said to be 'non-asbestos fibre reinforced Portland cement with density modofiers1. In comparison with asbestos counterparts it also has higher density and different moisture movement characteristics, but this time a higher impact strength, a lower tensile strength and a slightly greater thermal conductivity. Again, independent test certificates are available from the manu facturers in respect of performance in fire conditions. Ceramic fibre board is also available (see above). Cape Boards and Panels Ltd., offer Cape Monolux as an asbestosfree alternative to Marinite for applications requiring a rigid, non-combustible, monolithic, asbestos-free lining material for use in ventilation products and fire-door construction, which can be veneered and is resistant to attack by vermin and mould. Resin-bonded mineral wool fibres are available in various densities from 30-200kg/m"^ and various thicknesses and are used for the non-combustible sound and fire resistance of plant pipework and buildings. An example is Rockwool in slab form, designed for use at temperatures up to about 700C with a thermal conductivity of the order of 0.03-0. 15V/mk. It is formed from various stones into an inert rock wool fibre and is manufactured by ^he Rockwool Company (UK) Ltd. Vermiculite is also used with organic binders to provide asbestosfree non-combustible insulating boards, which are used, for example, for the protection of structural steelwork. They are relatively light and have a density of about lOOkg/m"'. The boards are rigid and resistant to chemical attack and rot. An example is Vicuclad, the manufacturer of which, Villiam Kenyon Sons (Vicuclad) Ltd., claims up to Lh fire resistance, depending upon thickness. Similarly a Cape product in this category is known as Vermiculux. Glass is also used to form rigid, non-combustible insulating products which are impervious and rot-resistant, but may be expected to soften at about T30C. Pittsburgh Coming, for instance, market the Poamglas cellular glass insulation range. Vith a density of *1 about 130kg/m'' and a thermal conductivity (depending upon temperature), of about 0.05'"/mk, the material is offered for use in such applications as roofing, curtain walling and pipe insulatcon. A 18202 39/ Perlite (volcanic rock expanded by heat) is also used together with mineral fibres and binders to form a rigid, strong, non combustible insulation board. Celo-thera, manufactured in the US by Celotex Ltd., is typical of this type; with a density of about 175kg/mJ and a thermal conductivity of 0.05V/mk, the board is offered as a roofing insulant. Conclusions It is rare in industry, given unlimited times and capital, to fail for technical reasons to find a material to substitute for another. In the asbestos industry the existence or threat of legislation has proven a formidable positive driving force towards the development of technically suitable substitutes far many applications. Vhat is not a technical feasibility is the ascendancy of a single universal substitute material; each application has to be considered individually. The resultant erosion of the established infrastructure of the asbestos supply and consuming industries evidently leads to cost penalties associated with the use of many substitutes. Further more the costs of research and development of new materials and of the acquisition of new or modified processing machinery have to be borne by the product manufacturers. Therefore, there is a dilemma of compromise between performance, and cost; in most cases there is a cost penalty to pay for the use of a substitute or, alternatively, inferior performance has to be accepted. This is most apparent with the most arduous applications. Nobody will doubt the desirability of minimising the consumption of a material vhich is without doubt a hazard to health. Nevertheless it should not be forgotten that asbestos products safeguard life in uses such as fire barriers and friction materials. It is clearly injudicious to accept inferior performance in such instances. Additionally, providing that the hazards in manufacture can be adequately controlled (have they always been?) the dangers from asbestos fibres encapsulated in a matrix must be minimal. There does not seem to be the same outer;/' over the use of beryllia ceramic heat sinks in electronic integrated circuits. Due regard should also be paid to any potential health hazards associated with fibrous substi tutes. &18203 40/ Consequently, restrictions on the use of asbestos must be selective, taking account of the technical capability of substitutes and customer needs and should allow time for both producer and con sumer industries to come to terms with the necessary capital invest ment and technological developments. Acknowledgement s The author is grateful for advice, assistance and information provided by the following organisations: Cape Industries Ltd., Turner 4 Newall Ltd., Railko Ltd., 3SA Croup Ltd. Marglass Ltd., Mandoval Ltd., Tilling Construction Services Ltd., Advanced Materials ^igineering Ltd.,. Consolidated 3eryllium Ltd., William Kenyon 4 Sons Ltd., Fibreglass Ltd., Morganite Ceramic Fibres L McKechnie Ceramic Fibres Ltd., Carborundum Co.Ltd., Multifabs Ltd., Fothergill 4 Harvey Ltd., Du Pont de Nemours, Courtauids Ltd., A Chemical 4 Insulating Co.Ltd., Imperial Chemical Industries Ltd., Micropore International Ltd., Bestobel Ltd., Sigri Flektrographit Gmbh. Fulmer Research Institute Ltd., 3TR Permali Ltd., Building Research Establishment, General Electric Corporation, Cogebi S/A, Greater London Council, Commission of the European Communities and to all others whose contributions have been left unacknowledged in error. ^ 18204 TABLE I EXAMPLES OF GLAND PACKINGS PRODUCT DESCRIPTION APPLICATION AREAS 41/ Max rsgp- Suitable pH range A Plaited white asbestos, S impregnated with PTFE B E S Asbestos yarn, plaited con T struction with special oil, 0 graphited s Vhite metal foil pleated round asbestos core. Con c tains lubricating oil and 0 graphite N T Inconel wire reinforced A I N asbestos yarn. Contains corrosion inhibitor, sacri ficial metal, graphite I N Range of packings contain ing asbestos yarn made by G chemical dispersion, and high temperature graphite lubricant General sex-vice, med/higt pressure pharmaceutical valves it pumps 290 Centrifugal pumps,rotary 250 shafts, valves. Resists mineral and vegetable oil s Reciprocating rods, rams, 275 rotary shafts 'Universal' valve gland packing. NOT for oxygen or concentrated acids. 650 Various types up to 650 pH 3-12 pH 3-12 pH 3-10 PH3-12 PH3-1U A Dry packing, braided E s glass continuous filament B E PTFE yarn, impregnated with 5 PTFE dispersion and lubri T cant 0 s Lubricated hemp F R Heavy duty flair, PTFE E impregnated E Cotton square braid, lubri cated and graphite finished Caulking, thermal insu lation etc. Valves and pumps handling corrosive media 600 290 low/medium duty in fresh| and sea water I Heavy reciprocating machinexy, abrasive media 120 150 low/medium duty in fresh! 120 and seawater | Cotton duck proofed with HR rubber impregnated and graphited Reciprocal rods and hydraulic services 150 Graphite filament yarn PTFE - impregnated aramid yarn PTFE - impregnated glass fibre yam Aluminium foil, moulded into rings High speed chemical packing. Med/high P. 350 j j Pumping aggressive fluids 288 || Various pump glands j 290 j Reciprocating and rotaxy shafts 5U0 Lead foil, moulded Into ring's Reciprocating and rotary shafts 260 PH5-11 pHO-li pK6-9 PH5-10 PH6-9 pHS-10 pHl-li PH3-12 pH**--12 - fcl820b