Glass Fibers

GLASS FIBERS are among the most versatile industrial materials known today. They are readily produced from raw materials, which are available in virtually unlimited supply (Ref 1). All glass fibers described in this article are derived from compositions containing silica. They exhibit useful bulk properties such as hardness, transparency, resistance to chemical attack, stability, and inertness, as well as desirable fiber properties such as strength, flexibility, and stiffness (Ref 2). Glass fibers are used in the manufacture of structural composites, printed circuit boards and a wide range of special-purpose products (Ref 3). Fiber Forming Processes. Glass melts are made by fusing (co-melting) silica with minerals, which contain the oxides needed to form a given composition. The molten mass is rapidly cooled to prevent crystallization and formed into glass fibers by a process also known as fiberization. Nearly all continuous glass fibers are made by a direct draw process and formed by extruding molten glass through a platinum alloy bushing that may contain up to several thousand individual orifices, each being 0.793 to 3.175 mm (0.0312 to 0.125 in.) in diameter (Ref 1). While still highly viscous, the resulting fibers are rapidly drawn to a fine diameter and solidify. Typical fiber diameters range from 3 to 20 lm (118 to 787 lin.). Individual filaments are combined into multifilament strands, which are pulled by mechanical winders at velocities of up to 61 m/ s (200 ft/s) and wound onto tubes or forming packages. This is the only process that is described in detail subsequently in the present article. The marble melt process can be used to form special-purpose, for example, high-strength fibers. In this process, the raw materials are melted, and solid glass marbles, usually 2 to 3 cm (0.8 to 1.2 in.) in diameter, are formed from the melt. The marbles are remelted (at the same or at a different location) and formed into glass fibers. Glass fibers can also be down drawn from the surface of solid preforms. Although this is the only process used for manufacturing optical fibers, which are not discussed in this Volume, it is a specialty process for manufacturing structural glass fibers such as silica or quartz glass fibers. These and other specialty processes are highlighted wherever appropriate but not discussed in full. Additional details about fiber forming are provided in the section “Glass Melting and Fiber Forming” in this article. Sizes and Binders. Glass filaments are highly abrasive to each other (Ref 4). “Size” coatings or binders are therefore applied before the strand is gathered to minimize degradation of filament strength that would otherwise be caused by filament-to-filament abrasion. Binders provide lubrication, protection, and/or coupling. The size may be temporary, as in the form of a starch-oil emulsion that is subsequently removed by heating and replaced with a glass-to-resin coupling agent known as a finish. On the other hand, the size may be a compatible treatment that performs several necessary functions during the subsequent forming operation and which, during impregnation, acts as a coupling agent to the resin being reinforced.