Rayon and cotton were considered unsuitable because of the poor 

 mildew resistance and high percentage of water absorption of these 

 fibers [7]. 



Weatherability was another factor considered in the selection of 

 substrate material. In a direct-exposure weatherability test conducted 

 by Dupont [7], acrylic (Orion) was found to be relatively unaffected by 

 both long and short wavelengths of ultraviolet light and nylon and 

 polyester (Dacron) have very good light and weather-resistance properties. 



Acrylic fibers, by definition, contain at least 85% by weight of 

 acrylonitrile units. Acrylic fibers vary in their physical and chemical 

 properties, depending on their intended application. They are produced 

 in high tensile strength yarns that are in the same range as cotton. 

 They have good resistance to deformation both dry and wet and retain 

 their strength after prolonged exposure to high temperatures. Acrylic 

 fibers are unaffected by detergents, oils, or biological attack and have 

 excellent resistance to sunlight. However, under high temperature and 

 stress conditions, acrylics do not perform well and have not found much 

 industrial application [8,10,11,12]. 



Nylon fibers, in addition to their high tensile strength, are 

 extremely tough and resilient, retain their properties at elevated 

 temperatures, and generally have good resistance to organic solvents and 

 chemicals [14,15,16]. The water absorption rate of nylon is considerably 

 higher than polyester or acrylic. It is also not as dimensionally 

 stable as polyester. 



Modacrylic fibers are copolymer fiber containing less than 85% 

 acryonitrile. The fibers are strong and tough and feel similar to wool. 

 The fibers are not as good as nylon or polyester in abrasion resistance 

 [7,17]. The fibers retain most of their tensile properties over a broad 

 temperature range and have excellent chemical resistance and resistance 

 to insects, fungi, and micro-organisms. One problem with the modacrylic 

 fibers is the softening range. However, this would be more of a produc- 

 tion rather than an operational problem since the softening temperature 

 is above the temperature that would be expected during use. Because of 

 this softening around temperatures of 250 F to 300 F, modacrylics are not 

 as widely used as other fibers and, therefore, may not be as readily 

 available. 



Polyethylene and polypropylene are members of the generic class of 

 fiber known as polyolefins and by definition have at least 85% by weight 

 of ethylene, propylene, or other olefin [17]. Polypropylene fibers have 

 a lower specific gravity than any other commercial fiber, and highly 

 oriented fibers have high tensile strength. The major drawback to 

 polypropylene is its rapid degradation in weather [4]. 



Polyethylene is more suitable for plastic film rather than fiber 

 applications. The fibers are not strong and can be elongated excessively. 

 Polyethylene also has an extremely low melting point and poor weathera- 

 bility; the fibers shrink when exposed to elevated temperatures in the 

 presence of water. Polyethylene is inert to most chemicals but is not 

 easy to coat unless chemically modified. 



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