Fiberglass At 50
Boric oxide and the uses of glass fiber in war and peace
by K. L. Loewenstein Ph.D.
September 1996

Fiberglass really came into the public domain in 1946, though its pre-history goes back a further half century. Items made of a coarse glass cloth were exhibited at Chicago in 1893. German scientists developed the first form of glass wool during World War I when they were cut off from supplies of asbestos. In the U.S. during the 1930s, Corning Glass and Owens-Illinois took fiberizing technology from laboratory experimentation to factory-scale production, so that an infant industry was in place there just before World War II. Fiberglass became an important military material during the war, and the infant industry grew rapidly to manhood.

The war's end brought greatly reduced military procurement. Instead of cutting back, the fiberglass makers set out to show the world that their new material was as good as - or better than - many traditional materials in a vast range of non-military applications. It is reasonable to claim that they could never have achieved their sensational success without boric oxide (B2O3). Glass fibers are now in everyday use all around the world. For it is essentially boric oxide that enables glass fibers to be produced at relatively low temperatures, with controlled viscosity, tolerable corrosion levels, and minimal devitrification. Without it the processes would be hazardously hot and uneconomi-cally corrosive. B2O3 may be one of the more expensive constituents in the fiberglass batch, but arguably it is the most critical.

Two types of fiberglass - one borate-assisted success

The first type consists of short curly fibers made in the form of a blanket, bound by an adhesive. It is used for thermal and acoustic insulation, and its application in construction, housing, and industry is now vast.

The second type of fiberglass consists of mechanically drawn straight continuous fibers used as such or made into textile-type products - woven or non-woven fabrics, yarns, rovings, mats. This type of fiberglass has given rise to the new industry of glass reinforced plastics, first of a range of engineering composites using different polymers as matrix materials and a variety of fibers.

Work on synthetic thermosetting resins for electrical insulation in the late 1930s led accidentally to their reinforcement by glass. The resultant reinforced plastic possessed properties which were of immediate application in radomes, the plastic housing sheltering warplane radar antenna assemblies. For these to be successful their glass reinforcement had to have high electrical resistance. This meant that unlike most other types of glass it could not contain any significant amount of alkali oxide, i.e. sodium or potassium oxide.

That was the first problem. The second was to formulate a glass that had a temperature/viscosity relationship which made fiber forming on an industrial scale technically and commercially viable. At the temperature used for glass forming, the glass had to be stable, and not devitrify (crystallize). Both problems were solved by formulating a calcium aluminum borosilicate glass which could be made by the then available technology.

The composition as patented (Brit. Pat. GB 520 247 [1940]) tried to keep the use of boric oxide secret, simply specifying "up to 10% of non-alkaline auxiliary fluxes". However, it is difficult to conceive of a non-alkaline flux other than boric oxide.

Because of its electrical properties the new glass was originally called E-glass; the first glass used for insulation was called C-glass, because of its chemical resistance. In general parlance, the two types may now be referred to as textile fiberglass (TFG) and insulation fiberglass (IFG). Most IFG is now sodium borosilicate rather than the original C-glass, with typical composition ranges of

Silica: 55-70 weight percent
Boric oxide: 3-12

Aluminum oxide: 0-7
Sodium oxide: 3-18

For the last 50 years or so, boric oxide has remained the faithful ally of the fiberglass industry. From an initial production of a few hundred tons per year, the TFG industry now produces in excess of two million tons per year, and its applications in glass reinforced plastic have spread into most parts of everyday life, often without the user being aware of it. Its general properties of low weight, corrosion resistance, high strength, high adaptability, and low cost when mass produced have led to the phenomenal growth.

From radomes, fiberglass soon went into other military applications after the war - armour, aircraft parts, rocket motor cases, rifle butts. For civilian use it went into parts for cars and motorcycles, boats, circuit boards, shower stalls, bath tubs, and sink units. Fiberglass reinforced plastics rapidly became more and more important in many forms of transportation - vehicle cabs, railway coaches, car and special vehicle bodies, and boat hulls. Russian icebreakers have reinforced plastic hulls, as have navy minesweepers.

Other uses include translucent roofing sheets, special custom-fabricated structures like church steeples and airport roofing, furniture, piping, cylindrical tanks for all sorts of liquids, sports equipment (e.g. skis, fishing rods, wind surfers), cement reinforcement, and electrical insulators. Thermoplastics (PVC, polyethylene, nylon etc.) are reinforced with chopped fiberglass, and their use is increasing rapidly. Glass reinforcement of plasterboard enables the fire resistance of partitioning to be raised, thus increasing the safety of buildings.

Not all attempted diversifications were equally successful however. Much was invested in the fiberglass reinforcement of vehicle tires, but these efforts failed, together with all other attempts to reinforce rubber. Fiberglass raincoats and wedding dresses never made the grade either.

In IFG, the borosilicate compositions have remained substantially unchanged over the years as they provide the required level of resistance to attack by the atmosphere, and therefore give adequate life to the insulation material - unlike soda lime silica glass fibers, for example. While the main applications are still thermal and acoustic insulation, their market size and significance are changing with the increasing need for energy conservation and reduction of noise pollution.

Boric oxide enables the fiberizing temperature to be minimized while maintaining aqueous durability. Otherwise atmospheric moisture could attack and weaken the fibers during storage under compression, and prevent the insulation batt from recovering to its correct thickness. The presence of boric oxide is essential in maintaining the product's insulation value. Boric oxide also increases the infra-red absorption of the fibers thus increasing the insulation value of the product, and enabling its density to be reduced.

Chopped continuously-drawn IFG and TFG are both used in the manufacture of roofing mat, sometimes called tissue, by a process akin to the manufacture of paper from wood pulp. In conjunction with bitumen, this mat is used to make roofing sheet (shingles), to waterproof flat concrete roofing, and for the protection of underground steel pipes. TFG mat is also beginning to find application in printed circuit boards.

Behind all these diversifying applications there has, of course, been massive technological improvement and change in the glass fiberizing process. Here we can but list some landmark developments: direct-melt furnaces; press moulding for the mass production of identical items; filament winding for pipes and cylinders; rotary fiberizing; pre-impregnation of fiberglass materials; pultrusion of fibers impregnated with uncured resin; furnaces and refractories with ever higher corrosion resistance and longer life; chemical coupling to give improved adhesion between glass and resin; sizing to protect newly made fibers during subsequent processing; new types of platinum enabling furnaces to carry up to 6,000 nozzles each; direct-melt furnaces with outputs of nearly 200 tonnes per day.

Environmental improvements include reduction of emissions, electric melting, the use of oxygen instead of air to avoid the formation of acidic gases, and effective recycling.

But without the availability of boric oxide, the development of the fiberglass industry might not have taken place at all, could have followed a different path, or could have been delayed to an extent that other materials might have been developed in place of glass fiber.

Former managing director of Fibertech Ltd., Dr. Loewenstein is author of 'The Manufacturing Technology of Continuous Glass Fibers', and holds numerous patents in the glass fiber field.