Asbestos Textiles: the history, manufacturing process & uses of asbestos textiles, fabrics, and fibers used in such products as fireproof blankets, safety clothing, packings, clutch facings, brake linings, plastics, and filters. Page top photo: an asbestos textile weaving operation showing standard magazine creel and loom creel - 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. 130-141 
Next to the asbestos-cement industry, probably the most important use of asbestos is in the manufacture of yarn and cloth. These textiles are used in such different products as safety clothing, packings, clutch facings, brake linings, plastics, and filters.
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The process of manufacturing asbestos textile products follows the process that is used for other textiles. The nature and characteristic of asbestos fibers differ from organic fibers.
The asbestos cloth vibration dampener shown at left is discussed
at ASBESTOS DUCT VIBRATION DAMPERS.
Cotton fibers are rough, twisted and irregular; wool fibers are covered with scaly bands. However, asbestos fiber has none of the characteristics which enable the individual fibers to cling to each other. This lack of spinning bond can make the manufacture of pure asbestos yarns difficult and costly, especially if fine yarns are required. Special opening and blending processes for asbestos fibers are necessary. The fibers are processed in carding machines similar to those used for wool.
The main reason for the use of asbestos fiber in the manufacture
of textiles is because of its fire and heat resistance,
acid resistance, and durability.
See ASBESTOS TEMPERATURE PROPERTIES
Asbestos textiles, being mineral, are durable even under severe service conditions; e.g., they are used as padding and cover cloths on flat work ironers in commercial laundries. Another example of the durability of asbestos is its use as belting for the conveying of hot materials. In the manufacture of wire, or galvanized materials, other materials could probably be used for the hot-metallic wiping process but they would need constant replacement. Asbestos lasts much longer.
The largest quantities of asbestos textile fibers are used for friction materials, industrial packing, electrical insulation and thermal insulation, applications where heat resistance and durability are required.
The utility of fibers, filaments and yarns, both natural and man made, as textile raw materials depends principally upon the physical properties of the fibers, which include mechanical, thermal, optical and electrical properties. Among the properties which make asbestos interesting is its modulus of elasticity which is 25 x 106 psi.
The asbestos cloth pipe or duct wrap shown at left is discussed at ASBESTOS DUCT VIBRATION DAMPERS.
The longer and better grades of asbestos fiber, which are the Quebec Standard Crudes No. 1 and 2, and Group 3 are generally the only fibers used for the manufacture of asbestos textiles. Combining or blending fibers is considered an art; it involves mixing different grades of asbestos.
Chrysotile is used predominantly. Crocidolite and amosite are sometimes blended with chrysotile. Blue asbestos textiles are manufactured for special acid-resistant applications. In these applications, crocidolite (blue) is generally used by itself. Coarse yarns are manufactured which are considered highly specialized products.
Asbestos is the only mineral that may be fabricated into finished products by using spindles, looms, and other textile equipment. Amosite asbestos, even though it has long fiber and fairly high tensile strength, is very difficult to fabricate into textile products because of the coarse nature of the fiber. It has a tendency to pulverize in the textile operation.
The fibrous structure of long asbestos permits its use by itself in textile equipment. The long asbestos fibers as well as other types of fibers which have length can provide the carrying action for shorter asbestos fibers such as Group 3. The carrier fibers are definitely required for handling shorter asbestos fibers. When cotton is used with short asbestos fibers, low cost products can be produced which still provide certain degrees of heat resistance and good physical properties.
Most of the asbestos yarns manufactured are plied yarns. Single yarns are not a very satisfactory product because asbestos fibers are lacking in uniformity. Uniformity can be achieved by such special processes as chemically dispersing asbestos. The fault of nonuniformity in mechanically opened fibers is overcome by doubling two or more strands of yarn. In twisted yarns, there are two important points to be considered; i.e., uniformity and strength. Uniformity identifies the diameter and weight of yarn. The strength of the yarn depends principally upon the fiber length and the grade of fibers used.
Standards for numbering asbestos yarn are used. For example, ASTM (D299-52) pertains to asbestos yarns and describes methods of identifying the yarn. The asbestos textile industry uses a numbering system for the yarn; it designates the cut, number of plies, and whether the yarn is plain or metallic. In a four digit figure, the first two digits indicate the cut, the next digit indicates the number of plies and the final digit indicates the number of metallic strands. If the final digit is a zero, it indicates a plain yarn. In a three digit system, only the first digit indicates the cut.
The term "cut" is derived by cutting (dividing) 7,000 grains into linear mass units by the grain weight of one hundred yards of a single yarn. The number of such units expresses the number of the cut and when multiplied by one hundred, indicates the nominal yards per pound.
The term "ply" identifies the number of strands of single asbestos yarn twisted together to form a heavier yarn. Ply is a term used principally in combination with a number to designate the strands of single yarn twisted together— 2 ply, 3 ply, etc.
Plain, single asbestos yarn is generally identified by cut numbers ranging from 5 to 50 cut. A 5 cut yarn represents 500 yd of yarn weighing one pound. For a 10 cut yarn, 1,000 yd weighs one pound. The 50 cut yarn involves 5,000 yd per pound. Examples of yarn designations are No. 931 yarn (9 cut, 3 ply, 1 wire yarn) and No. 1420 yarn (14 cut, 2 ply, plain yarn).
Various methods of identifying woven asbestos cloth are used. ASTM Specification (D677-50) pertains to asbestos cloth. An example of the Asbestos Textile Institute designation for cloth is No. 20P28. The digits that precede the letter identify the weight of the cloth in ounces per square yard. The letter identifies the weave and the remaining digits identify the cut of the yarn used in weaving.
The asbestos content of an asbestos textile is the principal and sole criterion upon which the grade of the asbestos product is based. Within each grade there may be a variety of constructions, weights, weave designs and insertions which may serve to yield fabrics having different properties for different applications. The ASTM has established standards which classify asbestos products in accordance with the percent of asbestos content. Table 8.1 lists these percentages.
TABLE 8.1. PERCENTAGE OF ASBESTOS CONTENT IN ASBESTOS TEXTILE PRODUCTS
|ASTM Grade||Asbestos Content, % by wt|
|Commercial||75 up to but not including 80|
|Underwriter||80 up to but not including 85|
|Grade A||85 up to but not including 90|
|Grade AA||90 up to but not including 95|
|Grade AAA||95 up to but not including 99|
|Grade AAAA||99 up to and including 100 % asbestos|
|Adapted from Rosato (1959) p. 133|
One practical method of determining chrysotile asbestos content of an asbestos textile product which is not treated with resins or other foreign matter is to determine the cotton or organic content of the product in accord with ASTM procedures. This method describes that one test specimen, weighing not less than 5 grams taken from each sample roll, dried to constant weight in an oven at 105 to 110°C, and the weights of the dried specimens reported. The specimens are placed in an electric oven and heated for not less than one hour at 800 to 8100C.
After removal from the oven they are cooled to room temperature in a desiccator, and then weighed. The weight of the residue is divided by the factor 0.86 in order to determine the original weight of the asbestos content. This weight of asbestos content is divided by the weight of the dried specimens in order to obtain the percentage of asbestos. The average of the determination shall be the asbestos content. In this particular example, the average of 14 per cent water of crystallization is used for chrysotile fiber. In other examples the per cent would of course vary.
In Table 8.2 the tensile strength of asbestos cloth at room temperature, with a 24 hr exposure at 800°F, is reported. All tensile tests were conducted at room temperature after the exposure. Tests were conducted in accord with ASTM (D577-52, Method D39, Section 10). This test is often called the tensile grab test.
These data are presented for general information based on different weaves. Temperature test data under different conditions are also reported in the chapter on Properties of Asbestos. The weave specification affects many properties of the finished textile.
TABLE 82. TENSILE STRENGTH OF ASBESTOS CLOTH VS. TEMPERATURE*
* Asbestos Textile Institute, Philadelphia, Penna., Report No. 23 ( Dec., Tests conducted at room temperature after temperature exposure.)
Adapted from Rosato (1959) p. 135
[Note that apparently the order of the two right-most columns were reversed in the original text; also we inserted decimal places missing in the original text - Ed.]
Table of Asbestos Textile Samples & Properties Used in the Tensile Strength Tests Above
|Sample No.||Type||Asbestos Content %||Grade||Warp-Grab Tensile Strength
|Adapted from Rosato (1959) p. 135|
Manufacturing Processes Basically the first step in the textile mill involves fiberizing asbestos and freeing it from rock impurities. The fibers are first carded into a sheet, then separated into rovings which are wound on jack spools, and spun into yarn. Yarn can be made into thread, cord or rope. Metallic yarns are made by adding strands of fine brass, copper or lead wire to the strands of asbestos. The yarns can be put into looms and woven into fabrics by dry or wet processes using textile operations. See Figure 8.1 below.
Crude fiber is generally prepared for spinning at the factory where it is to be spun. Preliminary treatment of fiber is usually accomplished in a pan crusher. This type of equipment consists of steel wheels approximately 1 1 /2 to 2 ft in diameter with a flat surface on the periphery which in turn rolls around in a circular pan.
Crude fiber is fed into the pan and scrapers push .the fiber under the rotating wheel. This operation is very critical since it can mechanically break the fibers. Special techniques and such engineered rolls as rotary-toothed cylinders have been devised to reduce damage of fibers.
After crushing, the fibers are transferred to opening devices which actually fiberize the product further. The opened or fiberized fibers pass over shaking screens or through trommel screens where they are again cleaned. Final operation generally involves lifting of suitable asbestos fibers by air suction.
Blending of such fibers as asbestos with cotton can be preformed in the preliminary mixing operation prior to the carding operation. However, the blending is generally performed during the carding operation. Carding rolls are fitted with a series of sharp steel bristle rotating brushes which comb the fibers into specific positions. During this combining operation short fibers and minute contaminating products such as rocks are removed. The result of carding is that opened asbestos fiber is formed into a loose continuous sheet or blanket.
Figure 8.2. A line of up-to-date carding machinery. Courtesy Davis & Furber Machine Co.
he carding process involves fiber material which is fed from a hopper to a weighing pan on the lifter apron of the carding machine. See Figure 8.2 above. A comb which passes over the apron spreads the moving mass and prevents accumulation of fibers. The weighing pan continuously supplies a predetermined amount of fibers. The fibers drop onto a conveyor apron of the carding table which carries it to the "licker-in" rolls.
In the carding machine, the fibers are combed by passing them between the main cylinder and the worker cylinders. The combining action is provided by the excess surface speed of the main cylinder over that of the worker The fibers are rubbed into loosely compact strands between oscillating surfaces. These strands or rovings are then wound on jack spools for spinning.
Figure 8.3. Asbestos roving is being wound on jack spools after asbestos sheets leave carding machines. Courtesy Johns-Manville Corporation.
When fibers such as cotton are added to asbestos, they are added during the carding process. By means of a weighing scale, different percentages of fibers can be made into a web.
The web is lifted from the main cylinder of the carding machine by the doffer comb. From there it is divided into narrow ribbons corresponding to the number of rovings. Figure 8.3 shows roving being wound on jack spools as it leaves an asbestos carding machine.
The rovings are condensed in a rub apron and are spooled singly on paper cores or together on a jack spool. These rovings obtain strength by a spinning process.
The strength which asbestos roving lacks is supplied by twisting in the spinning frames which are standard textile equipment. After the spinning operation, it can be plied with similar rovings. Asbestos yarns can be made up with such different core as fine metal wires, cotton, glass and nylon. The uniformity of the asbestos yarn is principally directly related to the carding operation.
In the manufacture of asbestos textiles, the carded blanket or web is cut into narrow strips identified as slivers. Asbestos lap is made by combining in a parallel layup varying numbers of slivers to form different weights. Asbestos lap is wound into roll form which is supplied to fabricators which recard them for insulating electrical wires and cables. These narrow strips of the carded web (or lap) when rubbed mechanically into untwisted strands are called rovings.
Yarn can be made as plain and metallic (or wire-inserted yarn). The standard insert is one or more strands of 8-mil brass wire but for special purposes wire of different diameter may be used. Other metals or alloys of copper, zinc, nickel, "Nichrome," "Inconel," and "Monel" are used in place of the brass.
When yarns are wetted with water during the weaving operations, the finished product is harsher and more open than cloth woven dry. Numerous patents have been issued for improving strength properties of asbestos yarn. American Cyanamid Company has British Patent 563,678 (1944) which specifies that in order to strengthen asbestos yarn, the yarn is impregnated with a melamine resin.
Figure 8.4. Flue cleaning hose is covered with a braid of heat resistant asbestos cord. Such a cover protects the hose from the hot walls of flues and resists the transfer of heat to the hands of the operator. (Courtesy The Gates Rubber Co.)
Such varied methods have been developed to produce new textile products as combining asbestos fibers with glass or ceramic fiber strands. Another interesting process is the use of short asbestos fiber with no long fiber carriers to produce yarn. Asbestos, paper strips are used to produce the yarn.
The fiber is separated in a beater and mixed with a liquid to form a pulp. The pulp is transferred to a paper machine where the fibers adhere to moving rolls. An endless blanket picks up the matted fiber and carries it on into a continuous sheet. The sheet is peeled off, dried and wound in a large roll. Then it can be cut into narrow strips andwith special machines it can be spirally wound into single yarns.
Other textile products include braided and knitted fabrics, wick for caulking and seals, rope for boiler expansion joints and gaskets, loose carded fiber for filtration, felts for belting and packing, tape for electrical and thermal barriers, tubing or sleeving for electrical cables, and mechanical abrasion protection of items subject to heat.
See Figure 8.4 above. They provide for parachute pack covers, theater curtains, firesmothering blankets, fire fighting suits, fuel and oil hose, conveyor belts, clothing for industrial plants, ironing pads, and others as listed in the introductory chapter. Asbestos fabrics coated with metallic or rubber base compounds have been developed as heat and flame reflectors or insulators.
Various ASTM and Federal Specifications have been prepared which pertain to these different textile products.
Asbestos fabrics are also used in and discussed at ASBESTOS PACKINGS & GASKETS.
Continue reading at ASBESTOS PLASTICS or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
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