Boron in Glass and Textile Fiberglass

Boric oxide in glass manufacturing

In the glass and textile fiberglass manufacturing process, boric oxide functions as both a flux and network former. It significantly lowers melting temperature and inhibits crystallization of the glass, both of which greatly facilitate processing. Generally, boric oxide lowers viscosity, controls thermal expansion, and inhibits devitrification—increasing durability and chemical resistance and reducing susceptibility to mechanical or thermal shock.

Borates can also positively affect the properties of the final product. For example, borates act as a powerful flux and lower glass batch melting temperatures during fiberglass manufacturing, aiding the fiberizing process and improving durability in use. And when those glass fibers are used in electronics or aerospace applications, borates enable control of dielectric properties—one reason textile fiberglass made with boric oxide is used in the manufacture of printed circuit boards, microelectromechanical systems, and thermal insulation tiles like those on the U.S. space shuttle.

Textile fiberglass

Textile fiberglass comes in several types (A, C, E-CR, D, E, R, or S) and forms (rovings, yarns, chopped strands, milled fibers, or woven and mat textiles). Most (90-95%) textile fiberglass products are E-glass (aka electronic glass). Originally aimed at electrical applications, E-glass is now primarily used to reinforce thermoset and thermoplastic polymer composite structures, known as fiber reinforced plastic (FRP) or glass fiber reinforced plastic (GFRP).

In the textile industry, E-glass used for printed circuit boards and aerospace applications must contain 5-10% boric oxide (B₂O₃). E-glass for general reinforcement purposes can vary from 0-10% B₂O₃. Low-dielectric textile glass fibers, used in high-frequency electronics applications, have a higher B₂O₃ content than E-glass, reducing the dielectric constant.

With a diameter of a few microns and coated in silane (an inorganic compound) to improve compatibility with matrix material, continuous strand textile glass fibers provide a powerful reinforcement for applications including boats, wind turbine blades, pipes, and lightweight composite structural components in cars, trucks, trains, and aircraft.

Borates for increased strength and chemical resistance

As a component of borosilicate glass, borates improve resistance to thermal shock, increase aqueous durability and mechanical strength, imbue electrical neutrality, and structurally modify the glass to make it resistant to heat and chemicals. Viewing this page on a tablet or mobile phone? Alkali-free borosilicate glass is used in all thin film transistor liquid crystal display (TFT LCD) substrate glass and some types of scratch-resistant, protective outer layers for touchscreens.

In sealed headlights, lamp covers, halogen bulbs, and fluorescent tubes, borosilicate glass provides high electrical resistance, strength, and durability. Ampoules and vials for medicine, as well as vacuum flasks, rely on borates for increased chemical resistance and aqueous durability. Pharmaceutical (or neutral) glass can be engineered so that, in contact with aqueous solutions, it creates the same pH as is found in the human body. And, borosilicate glass maintains optimum brilliance. Along with its low coefficient of thermal expansion, this makes the material suitable for optics such as astronomical reflecting telescopes and eyeglasses.

Borate-containing glass beads are used in plastics as reinforcement-extenders. Hollow microspheres are used to manufacture automotive parts and patching compounds. Their low-density, high-compressive strength, and insulation from heat and sound make them ideal as light-weight fillers for polymeric materials. Cover glass and substrate glass for flat photovoltaic cells have specific quality and performance requirements that sometimes need to be met by specialized borosilicate glasses, including high strength-to-weight ratio, impact resistance, and surface compatibility with electronics materials.