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Composites 101: Glass Fibers

Glass fibers are made by pulling molten glass through devices called bushings with thousands of tiny holes. Because the glass is pulled and stretched or elongated just before it cools, the fibers are even smaller than the tiny holes the glass flows through. Fibers typically range from 10 to 22 microns in diameter, which is thinner than most human head hair (17 to 100 microns).

The raw materials used for the glass will determine a variety of factors such as the temperature needed to melt the glass and the strength of the resulting fibers. Without getting too technical, glass compositions with higher silica content and metallic oxides will require a higher melting temperature and be harder to make into fibers, and thus more expensive, but they will also impart higher performance characteristics to the fibers.

As soon as each filament of glass emerges from the bushing and cools, a chemical sizing is applied to the surface of the glass. The sizing has two functions: keep the fibers from abrading and breaking, and help the fibers bond securely with the resin.

If the glass is melted at a higher temperature and is less viscous, and/or the glass is pulled faster by the forming winder, the glass fibers will be thinner. Lower temperatures and/or slower pull will produce thicker glass fibers. Fiber glass producers, like textile makers, have for centuries referred to fiber diameter with terms like yield or tex. For example, a product with a 450 yield will have 450 yards of fiber in a pound.

The strands of glass are wound onto a temporary core to form a package that can be shipped to a composite fabricator.

Pound for pound, glass fibers are stronger than steel. They have a high ratio of surface area to weight and an amorphous structure that gives them the same properties both along the fiber and across the fiber. Key performance attributes include tensile strength, stiffness, modulus and fatigue resistance. While other fibers may have one attribute that is better than glass, glass often provides the best combination of properties for the price. For example, carbon is known for being light and strong, but it can also be brittle in some applications. Compared to carbon, glass can undergo more elongation before it fails.


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