|Asbestos Overview and Handling Recommendations (GTZ, 1996)|
|Part III. Asbestos substitutes|
|3 Fiber substitutes for Asbestos fibers in the building area|
In the building area Asbestos primarily is applied in the form of Asbestos cement, so that the most effective substitution is with Asbestos-free fibers (hereafter referred to as substitute fibers - SF- or substitute fiber cement - SFC).
After trials with plant fibers, such as flax or hemp, it was found that such fibers are not very appropriate because of their swelling capability and their low resistance against microbes. Steel fibers, on the other hand, are not appropriate due to their poor dispersing ability in the cement matrix. Trials with polyamides, such as nylon or perlon, also had to be given up after a longer period.
Very promising trials have been performed with polypropylene fibers, which bind extensively with cement. Presently, the following are mainly used as reinforcement for fibrous cement: polyacrylic fibers, such as Dolan 10 fibers, plastic fibers made with polyvinylalcohol or also cellulose fibers. In addition, non-textile glass fibers and wollastonite fibers are used. The spectra of properties of these types of fibers are described in the following sections.
Glass fibers produced with diameters in the range of 0.1 pm to 20 pm have become particularly important. Since lengthwise fracturing is not to be expected, these fibers only conditionally lie within the range for penetration to the lungs. Based on their compatibility, the residence time in the organism is relatively short.
Application areas for these mineral fibers are in the loose form in stuffing insulations, mats, felts and sheets, as well as in fibrous filling materials in the areas of fire protection, thermal insulation, noise or vibration insulation, filtration, friction linings and also chemical products and fibrous cement.
Glass fiber concrete (Heidelberger Zementwerke AG) is made of a cement-bound matrix and alkali-resistant, highly firm glass fibers. This material produced in a mixing concrete process has very good properties, especially for facade elements, because of its high bending tensile strength, high impact resistance, corrosion resistance and its good ductility. Its relatively low weight and high fire resistance (Fire Class Al) enable the manufacturing of complicated and multi-functional building parts.
The inorganic natural fiber wollastonite is a calcium metasilicate with a chain structure consisting of at least 96.5 % CaO and SiO2. The fiber diameter ranges from 10 pm to 100 pm; under mechanical wear a fracturing into fibrous pieces with diameters as small as 0.1 pm is possible. Critical diameter ranges can be prevented through the use of appropriate production methods, however.
Wollastonite is used as filling material in the areas of fire protection (sheets), thermal and electrical insulation (sheets, fill for moulding material), friction linings, structural elements and other construction products, chemical products and others (paints, glues and moulding material).
Fibers made of cellulose are used as additives in building products to enable the dispersion of synthetic fibers in the cement matrix.
An example for the application of cellulose fibers which is under development is a so-called fiber cocktail, consisting of cellulose and cellulose fiber acting to disperse polyvinylalcohol or polyacrylnitrile fibers in the cement matrix (Eternit AG). Through special processing procedures, an orientation of the fibers is also achieved, which leads to an improvement in the mechanical properties.
Organic synthetic polyacrylnitrile fibers (PAN-fibers) can be manufactured for technical applications with a diameter above 18 pm. They therefore lie outside the range penetrable to the lungs. A lengthwise splitting under mechanical wear is only conditionally possible. PAN-fibers are used for reinforcement in the areas of filtration, friction linings, structural elements and other construction products (as reinforcement up to 2 % in fibrous cement), etc..
Organic synthetic polyvinylalcohol fibers (PVA - fibers, PVA fraction at least 85%) can be made water insoluble through combination with aldehydes. The fiber diameters typically lie between 10 pm and 20 pm and are therefore not penetrable to lungs. Data on the lengthwise splitting do not exist, however it may be assumed that this does not occur. PVA- fibers are applied in the building area for reinforcement of fiber cement products (Fraction 2 %).
Organic synthetic polypropylene fibers (PP - fibers) can be manufactured with diameters of 20 pm to 100 pm and are therefore not penetrable to lungs. Lengthwise splitting is not to be expected. The fibers are applied in the areas of gaskets and structural elements and other construction products, particularly fibrous cement. Trials with fiber mats and fiber fleeces showed, however, that due to the arising separation of layers between mats and cement paste, no satisfactory results could be achieved. In Europe only Moplefan (Italy) still manufactures fibrous cement sheets from short-fibered PP fleeces or PP - fiber and cement paste.
The fiber substitute technique has not yet reached the standard of quality of Asbestos fiber reinforcement. Deficiencies, such as frost uplifting and fine fissures from thinning of fibers, occurred particularly with corrugated SFC products.
Aside from several disadvantages which SF have over Asbestos fibers, such as reduced binding and higher price, the new SFC products also have advantages, such as higher elasticity and easier processing.
The mechanical resistance of fibrous cement products is dependent on the mixture of ingredients, which is different for each application area, and on the particular manufacturing process. The maximum temperature resistance lies around 150°C.
As with Asbestos fiber products, SFC products have generally limited resistance against moss formation, fungus collection and mold. This is true to the same extent for Asbestos containing and Asbestos-free products. They are also impaired by acids, vegetable oils and fats, magnesium salt solutions, sulfates, ammonium salts, iron chloride, warm distilled water and hot condensed water. Chlorine, sulfur dioxide and smoke also act destructively over long periods.
SFC products, however, resist alkalis, salts, alcohols, mineral oils (bitumen), and tar. They do not corrode and resist dry gases.
Cement contains free unbound alkali metals, which partly separate during the hardening process, and lead to the formation of hydroxyl ions in aqueous medium. (Cement reacts as an alkaline substance).
These characteristics also apply for housing construction and water pipelines, with the limitation that no adequate SFC pipe products have been found for high pressure requirements.
Aside from SF substitutes, other fiber-free substitutes are usable in the building area, and their application is especially gaining importance in housing construction and water pipeline construction. Their possible applications are therefore discussed in the next two sections.