This article series about asbestos plastics & molded materials describes the history, manufacturing process & uses of asbestos plastics and molded materials such as asbestos reinforced handles, the Vanguard rocked nose cone, automobile parts & housings, electronic equipment (radar scanner), asbestos-filled Teflon, rocket motor parts, plastic drop tanks for the Hawker Sea Hawk, and hundreds of other products.
Page top photo: the asbestos-plastic drop tank for the Hawker Sea Hawk - Adapted from Rosato (1959) .
This articles series about the manufacture & use of asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials.
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The following text is Adapted from Rosato (1959) p. 142-177  © 2013 InspectApedia.com
Introduction Asbestos reinforced and filled plastics have played an important role in the progress of the growth of industry since the turn of the century.
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Since the development of phenol-formaldehyde resins in 1909, asbestos has been used in large quantities as a reinforcement, additive, and filler. There have been large quantities of short asbestos used in plastics such as those shown in Figure 9.1. Figure 9.2 shows the use of long fiber asbestos in asbestos-reinforced plastic rocket nose cones.
Figure 9.1. Typical difficult molding of long handle which has a hollow center to accommodate the electrical thermostat and wiring for Dominion skillet. - Courtesy Durez Plastics Div., Hooker Electro-Chemical Co.
Plastic materials are available which can be reinforced with such different fibers as asbestos, glass fiber, glass flakes, cotton, and Fiberfrax. These varied fibers provide for either different characteristics or in some cases, they can produce equivalent properties for specific applications. The review in this chapter concerns only asbestos reinforced plastics.
Generally, when reviewing the subject of asbestos filled or reinforced plastics, phenolic resins or condensation type resins are discussed. However, asbestos is used with such other resins as those listed in Table 9.1. Asbestos fibers, and in particular short fibers, are used in combinations with such other fibers (including long asbestos fibers) as polyester premix compounds.
Figure 9.3 below shows an automobile air conditioning housing which is made up of 55 per cent by weight of asbestos shorts or floats, 10 per cent chopped glass fiber, and 35 per cent polyester resin (with styrene). An extremely large quantity of asbestos fibers is used with these types of compounds but in most cases they are not identified as containing asbestos.
Plastics is one of the few billion dollar industries in the United States. It is one of the fastest growing industries with approximately a 300 per cent increase in production in the last ten years. The term "plastics" pertains to many different products. Asbestos is used in many of them. In most applications it provides for meeting property requirements at low cost. As the plastic industry grows, more use of asbestos also develops both in thermoplastic and thermosetting resins.
|SOME USES OF ASBESTOS IN PLASTIC COMPOSITIONS * TABLE 9.1|
|Type of Asbestos-Containing Resin||Products & Applications Using This Asbestos Plastic Resin||Method of Fabrication of the Asbestos-Plastic Resin Product|
|Rosin, with or without waxes,
gilsonite, creosote, shellac,
mineral oils, phenolic resins,
|Hot molding, chilling|
|Shellac, with rosin, creosote,
||Electrical insulations and miscellaneous
||Hot molding, with or without
|Asphalts, tars and pitches, with chlorinated rubber, oils, solvents, etc.||Heat insulator coatings for metal pipes Parts for electrical and heat appliances||Brushing Drying, cold molding, using waterproofing molding|
|Waxes (Montan, carnauba, etc.) with shellac, natural resins, gilsonite, modified rosins, mineral oils, etc.||Casting, coating, molding
Box toe compositions
|Hot molding, chilling
Hot molding, chilling
Hot pressing, chilling
|Polymerizable oils||Joints, coating compositions, etc.||Brushing, trowelling, etc.,
|Rubber or chlorinated rubber, with or without alkyd resins, etc. with or without organic filler and asbestos||Brake linings, clutch facings, valve discs,
gaskets, packings, etc.
||Hot molding, curing
|Phenolics, alone or with natural resins, rubber, inorganic filler (silica or mica) and asbestos, floats, long fiber, paper, yarn or fabric||
|Molded heat resistant compositions
Impregnated sheets, panels, or boards,
gaskets, valve discs, etc.
Asbestos fabric pressed as blocks
Asbestos sheet as spacers for heat insulators
Asbestos yarns, etc. for clutch facings, etc.
Laminated under heat and
Heat and pressure
|Cellulose esters||Asbestos and cellulose esters used to in-
crease strength and toughness of
glycerol phtholate molding compounds
|Alkyd resins||Commutator rings
Door stripes, electrical spools, etc.
Wood veneer panels
Acid proof coatings
|Hot molding, chilling, curing
Hot molding, chilling
Cold molding, curing
|Phenolic varnishes||To increase adherence to coated surface,
to increase flexibility of undercoatings,
on metal surfaces, also used in
packings, putties and boards
||Varies depending on application
|Sheet protection for metal surfaces||Asbestos paper or sheet is connected to metal surface by phenolic resin and
then coated with the same material
||Curing by heat and pressure|
|* A. B. Cummins, "Asbestos in Plastic Compositions" Modern Plastics.|
Figure 9.2. Vanguard rocket nose cone is made of asbestos-phenolic material. It is subjected to aerodynamic heating, shock heat, and structural loads. The two-piece cone splits apart and drops off after the rocket leaves the atmosp
Typical commercial applications of asbestos-plastic materials include pulleys, casters, electrical circuit breakers, pipes, roller and sleeve bearings, metal bearing retainers, bushings, containers, ducts, and washing machine agitators. Architects and engineers are developing more uses for asbestos based plastics too, inasmuch as they provide for increased resistance to fire and increase structural strength.
Information concerning the impact-porosity resistance of - plastics is applicable to doors or paneling, containers, boats (hull and floor), radomes, pressure bottles or tubes, plywood surface, rocket tubes, decorative panels, and ducts. Asbestos sheet products can be used on the surface, in the center or interleaved with other reinforcements to provide for high impact-porosity resistance.
The impact-porosity resistance of glass fabric or glass woven roving-polyester resin laminates is increased threefold when glass fabric is interleaved and/or surfaced with asbestos felts or papers. *
One of the more important and spectacular applications of asbestos reinforced plastics involves its use in missiles. Its insulation, ablation, and fine structural characteristics are all required in order that parts may function properly. Special asbestos-base plastics have been developed for use in direct rocket blasts which in turn have helped solve some of the high temperature problems.
Parts which are involved in the use of this type of product include nose cones, thermal - insulation barriers between propellant and steel motor bodies, rocket exhaust tubes, electrical conduits, aft insulators, deflector plates, rocket motor pressure plugs, turbine wheels, ducts, bulk head, fins, shrouds, and sliver traps. See Figures 9.8 through 9.10 inclusive.
* U.S. Navy, Bureau of Ships, Final Report, "The Use of Asbestos Fiber Materials in Glass Reinforced Plastic Laminates," Project 4860-Q-27 NS034-045 (Aug., 1957).
Figure 9.8. The Nike-Hercules sustainer motor contains asbestosphenolic insulation from the forward end to the exhaust end. - Courtesy The Thiokol Corporation. Click this or any image at InspectApedia to see a detailed, enlarged version.
Figure 9.9. A rocket motor tube tailpipe fabricated from asbestosphenolic plastic. -Courtesy The Bristol Aeroplane, Ltd.
Figure 9.10. Rocket motor body asbestos-plastic exhaust cone and ring.
Experimental tests using temperatures as high as 20,000°F have been conducted on various asbestos products. The materials are very poor conductors of heat and therefore help to solve some of the thermal problems. During their exposure to extremely high temperatures for short periods of time their surfaces will oxidize or carbonize and produce a protective outer surface. Ablation or errosion occurs at a relatively slow rate.
Various parts have been made by such different techniques as compression molding, vacuum bag, autoclave, and transfer molding. These plastic parts can be easily mass produced. They have been used in modern development and production devices.
Figure 9.11. Asbestos base plastic 75-gal drop tank for the Hawker Sea Hawk. This tank is the first plastic drop tank in the world to have received full design approval.
The Bristol Aeroplane Company, Ltd., Filton, Bristol, England, has now received world wide acceptance of its asbestos-phenolic external auxiliary fuel tanks for aircraft.
This is the first world wide acceptance of any reinforced plastic part. These drop-tanks are shown in Figure 9.11.
They are required in large numbers and they form an important and costly item in modern air force inventory. The Bristol tank capacities are varied—SO, 100, 150, 200, 300, and 500 gal.
The Bristol tanks have been developed as a logical sequence to the company's work on plastics for primary structures. The production techniques used, i.e., the molding of cylindrical fuselage sections by the autoclave process and the hydraulic press methods used for central sections and frames, have both been found to be readily adaptable to the manufacture of drop-tanks; the former process is used for the shell and the latter for the internal structure.
Newly developed [1950's - Ed.] high strength asbestos reinforced honeycomb is now in pilot-plant production. This new type of core material is the first of its kind where good physical properties at room and elevated temperatures are developed with asbestos. The presently available type utilizes a heat resistant phenolic resin.
Summary of preliminary properties of this type material identified as Hexcel Type XHRP % 6 —Asb.- 9.0 are shown in Table 9.11. A structural honeycomb utilizing silicone resin with asbestos reinforcement is in an early stage of development.
* Hexel Co.
In 1951, a Delta Aircraft wing made of asbestos-phenolic laminate was displayed at the Exhibition of the Society of British Aircraft Constructors at Farnsborough, England. The Royal Aircraft Establishment, Ministry of Supply provided the exhibit in order to show progress on reinforced plastic aircraft parts. The wing was approximately 8 ft from the fuselage with approximately an lift chord at the root. The shell of the wing continued around a leading edge and over the whole area back to the ailerons. There were no outer skin joints.
Fabrication of the wing was accomplished by two techniques; i.e., zexo pressure and vacuum technique. In the zero pressure process, asbestos-phenolic sheet material (Durestos) was saturated with warm water producing a very soft sheet which could be hand rolled into simple curvatures.
By applying heat, these sheets would tend to delaminate and to produce low physical properties. To eliminate this problem, the softened sheets were treated with a water-soluble, coldsetting resorcinol resin. This resin penetrates the material and produces a solid compressed sheet. During the cure, the resorcinol resin shrinks. Inasmuch as one side of the molded material will be exposed to the air, water is free to evaporate. This method of fabrication is a specialized art; it is not a standard procedure.
The vacuum process is a more desirable method of.fabrieating parts at low pressure, inasmuch as it provides for the elimination of condensation formed during the curing of the phenolic resin. The phenolic preimpregnated asbestos sheet material is manufactured so that suitable flow and resin conditions exist for this type of fabrication. A male or female mold is used in conjunction with a vacuum bag which is generally made of rubber.
Two different basic internal structures were manufactured for the Delta wing construction. One was designed to provide compartments for various equipment. The other design involved internal fuel tanks. Asbestos-phenolic plastics were used in the manufacture of a high performance experimental wing for a glider by the Plastics Division of F. G. Miles, Ltd., Shoreham Airport, Sussex, England (1953).
The wing was of the high-performance type, being designed for laminar flow and having an aspect ratio of 18. The wing had a 60 ft span, a 5 ft chord at the root, and weighed 155 lb. This plastic type of wing had been manufactured inasmuch as a high degree of accuracy was required in the contour of the wing. This accuracy permitted the production of a low cost item as compared with the conventional methods of manufacture.
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