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Glass fibers

In the broad composites industry, the vast majority of all fibers used are glass. Glass fibers are the oldest and, by far, the most common reinforcement used in nonaerospace applications to replace heavier metal parts. Glass weighs more than carbon but is also more impact-resistant than carbon. Depending upon the

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Posted on: 11/1/2007
Source: CompositesWorld

In the broad composites industry, the vast majority of all fibers used are glass. Glass fibers are the oldest and, by far, the most common reinforcement used in nonaerospace applications to replace heavier metal parts. Glass weighs more than carbon but is also more impact-resistant than carbon. Depending upon the glass type, filament diameter, sizing chemistry and fiber form, a wide range of properties and performance levels can be achieved.

Fiber properties are determined by the fiber manufacturing process and the ingredients and coatings used in the process. During glass fiber production, raw materials are transformed into delicate and highly abrasive filaments, ranging in diameter from 3.5 to 24 micrometers. Silica sand is the primary raw ingredient, typically accounting for more than 50 percent of the glass fiber weight. Metal oxides and other ingredients can be added to the silica and the processing methods can be varied to customize the fibers for particular applications.

Glass filaments are supplied in bundles called strands. A strand is a collection of continuous glass filaments. Roving generally refers to a bundle of untwisted strands, packaged like thread on a large spool. Single-end roving consists of strands containing continuous, multiple glass filaments that run the length of the strand. Multiple-end roving contains lengthy but not entirely continuous strands, which are added or dropped in a staggered arrangement during the spooling process. Yarns are collections of strands that are twisted together.

Electrical or E-glass, so named because its chemical composition makes it an excellent electrical insulator, is particularly well suited to applications in which radio-signal transparency is desired, such as aircraft radomes, antennae and computer circuit boards. However, it is also the most economical glass fiber for composites, offering sufficient strength in most applications at a relatively ow cost. Over time, it has become the standard form of fiberglass, accounting for more than 90 percent of all glass-fiber reinforcements. At least 50 percent of E-glass fibers are silica oxide; the remainder are composed of oxides of aluminum, boron, calcium and other compounds, including limestone, fluorspar, boric acid and clay.

When greater strength is desired, high-strength glass, first developed for military applications in the 1960s, is an option. Variously known as S-glass in the U.S., R-glass in Europe and T-glass in Japan, its strand tensile strength is 700 ksi, with a tensile modulus of 14 Msi. S-glass has appreciably higher silica oxide, aluminum oxide and magnesium oxide content than E-glass, and is 40 to 70 percent stronger than E-glass. Both E-glass and S-glass lose up to half of their tensile strength as temperatures increase from ambient to 538°C/1000°F, although both fiber types still exhibit generally good strength in this elevated temperature range.

While glass fibers have relatively high chemical resistance, they can be eroded by leaching action, when exposed to water. For instance, an E-glass filament 10 microns in diameter typically loses 0.7 percent of its weight when placed in hot water for 24 hours. The erosion rate, however, slows significantly as the leached glass forms a protective barrier on the outside of the filament; only 0.9 percent total weight loss occurs after seven days of exposure. To slow erosion, moisture-resistant coatings, such as silane compounds, are applied during fiber manufacturing.

Corrosion-resistant glass, known as C-glass or E-CR glass, loses much less of its weight when exposed to an acid solution than does E-glass. C-glass and S-glass show good resistance to sulfuric acid. However, E-glass and S-glass are much more resistant to sodium carbonate solution (a base) than is C-glass. A boron-free glass fiber, with performance and price comparable to E-glass, demonstrates greater corrosion resistance in acidic environments (like E-CR glass), higher elastic modulus and better performance in high-temperature applications than does E-glass.

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