Use of Boron in Glass
Glass, in its various forms, represents the largest single outlet for boron products. Boron is a powerful flux and also confers high chemical resistance for glasses in general. As a flux it works by reducing melting point, viscosity, thermal expanding coefficient, and increasing breakage index, transparency and brightness, and heat resistance.
In both insulation fiber glass (IFG) and reinforcement fiberglass (RFG), use of boron improves the fluxing capabilities of the batch, reduces glass batch melting temperatures and increases the fiberizing efficiency by lowering the viscosity. It controls the relationship between temperature, viscosity and surface tension to create optimal glass fiberization. Boron also reduces the tendency of crystallization and increases the strength of the fibers and resistance against moisture.
Insulation Fiberglass and Fiberglass Reinforcement
Insulation fiberglass (IFG) is the single largest use of borates worldwide. Etibor 48 (Borax Pentahydrate) is the main borate used in the manufacture of insulation fiberglass where it performs a powerful flux and lowers the batch melting temperature. It also helps control the relationship between melt viscosity, surface tension and temperature to create the optimum conditions for fiberization. The result is strong fibers that are resistant to water and chemical attack while being biosoluble (dissolving in the lungs if inhaled).
Insulation fiberglass, also known as glass wool insulation, works by trapping air in its fibers to reduce heat transfer. In addition to its usage for thermal insulation in both commercial and residential buildings, it is used as acoustic insulation. As a thermal insulator, insulation fiberglass helps reduce energy use and carbon dioxide emissions from the built environment. The most important role of borates in the glass fibers is the increase in infrared radiation absorbance, significantly increasing the insulation performance of the material.
Insulation fiberglass can be used as batts (pre-cut slabs), as a blanket (roll) or as loose fill (blown fibers) in buildings. Minor uses include wrapping heating, ventilation and air conditioning systems and duct and pipes for refrigeration systems. Another important role of boron in IFG is to impart decompressibility. When the finished product is transported, it is firmly compacted into bales in order to minimize freight cost. When it is used in the construction industry, the main application area for IFG, it must be decompressed in order to provide the good air pockets/layers essential for insulation. Specifically in IFG, incorporation of boron reduces viscosity of the melt and thereby assists fiberization, as well as inhibiting the leaching of fluxes.
Constituents of fiberglass may vary with the type of production. ’’E-glass’’, which has a low alkaline property, is the most widely consumed type. It accounts for about 90 percent of fiberglass consumption in the world since this type of fiber is less likely to break during the application process. E-glass has boron oxide content up to 12% and is produced in a number of forms like filaments and chopped strands as per the end uses. Other boron-based Fiber Glass types are increasingly used to reinforce concrete as a substitute for steel and aggregates.
Borosilicate glass is one of the major applications for borates in the glass industry. The most significant properties borosilicate glass renders to the end products are resistance to thermal shocks and chemical attacks, withstanding scratches and high endurance to impacts.
Thanks to these properties, borosilicate glasses are used in many glass products like laboratory glasses, pharmaceuticals, cookware, solar energy systems, fluorescent tubes and lamp covers and automotive lighting assemblies. Other uses include products where a glass-to-metal bond is required in metal vapor discharge lamps for street lighting (sodium vapor), tungsten filament lamps and radio valves. Neutral glasses for vacuum flasks, ampoules and medicine vials rely on the chemical and aqueous resistance and durability. Cosmetic containers need chemical resistance and optical clarity. Solid microspheres are used in runway reflector systems while hollow microspheres are often used to manufacture automotive parts with their low-density, high compressive strength and good heat and sound insulation properties. Borates can also be used in the manufacture of optical glasses, art-glass, lenses, prisms, space protection glass, telescope mirrors, opal glassware and optical communications products.
Borosilicate glasses have boron oxide (B2O3) content between 5-30%. Anhydrous borax and Borax Pentahydrate are the borate products most often preferred for borosilicate glass.
Flat display panel glass production, like LCD, is one of the major boron-consuming areas that has been growing recently. Flat panel glass production has increased dramatically since consumers’ preferences have shifted from cathode-ray tube (CRT) TVs to flat panel screens. Generally, 11-13% boron oxide is used in flat panel glass production. Alkaline materials, like sodium, are unwanted in flat panel glass production because alkaline ions degrade “the thin film transistor (TFT) property” of the glass by getting mixed with liquid crystal material. Therefore, alkaline-free boric acid is used as boron source in flat panel glass.
Boron is also used in fiber optics, which enable luminary photons to be transferred effectively in communication systems. Fiber optics are formed of two different parts: an inner core and outer sections. The inner core is made of glass with high index of refraction, whereas the outer section is made of glass with low index of refraction. Inner core is generally a produced silicate molten with borosilicate glass. In addition, borosilicate glass consumption in solar energy systems is gaining momentum because such systems are popular alternatives used in response to rising fossil energy cost and green energy policies.
Photovoltaic cells, with their high strength to weight ratio, impact resistance and compatibility with electronic materials benefit, from specialized borosilicate glasses. Evacuated solar collector tubes for solar water heating and solar power generation stations use large arrays of borosilicate collector tubes to gather the reflected radiation from mirrors to drive steam turbines to generate electricity.
Textile fiberglass (TFG) is a series of continuous strand glass fibers used to reinforce various materials. In textile fiberglass manufacture, borates act as a powerful flux and lower the batch meting temperatures. Non-sodium borates such as Boric acid and Colemanite are the borate products used in this application.
Textile fiberglass is produced in several fiber types: A, C, D, E, E-CR, R and S, with 90-95% being considered E-glass. These fibers can be in the form of yarn, roving, chopped strand, woven and mat textiles and milled fibers. The textile fiberglass industry has standardized E-glass into two categories: general reinforcement applications (0-10% B203) and printed circuit board and aerospace applications (5-10% B203).
E-glass used to reinforce thermoset and thermoplastic polymer composites structures is often known as fiber-reinforced plastic (FRP) or glass-fiber-reinforced plastic (GFRP). Important applications of products made with FRP or GFRP are light-weight composite components for cars, aircraft, trucks and trains; wind turbine blades; boats; pipes and sports equipment.
E-glass for printed circuit boards and aerospace uses often require low dielectric fibers which have a higher B2O3 content than regular E-glass for high frequency electronics applications.