Uses of Borates in Glass
It should be noted that there were two sources of glass both natural and man-made. Naturally occurring glass was formed from volcanic glass called obsidian and has been used by many societies during the Stone Age which predates its development by humankind. These early societies used this unique glass as sharp cutting tools and used them for bartering when trading with other groups.
The development of man-made glass took place much later. The history of this glass dates back to 3500 BC in the regions of Egypt, Mesopotamia and Syria as a result of archeological excavations.
It is estimated the first use of borates in glass was by Otto Schott, a German glass producer, in the late 19thcentury, developed under the brand name Duran. In 1915, Corning Glass offered its version under the brand name Pyrex. While borosilicate glass is typically colorless, colored glass was developed around 1986 through the work of Paul Trautman for the studio art glass world. Since borosilicate glass has a higher melting temperature than other silica glasses, it historically became more challenging to bring to various industrial applications. The problem of coming up with new furnaces and burner designs was eventually overcome.
Mineral wool developed by John Plyer in 1870 was the first fiberglass material. The fibers were finely woven and were displayed at the 1893 World’s Fair in the form of an elaborate dress.
It was not until 1930 that fiberglass in an insulative form was manufactured by the Chance Brothers of Glasgow, Scotland, using a process developed by the Austrian scientist Drs. Pollack and Neumann.
In the late 1930s fiberglass insulation was produced in the US by Owens Corning Company, and it is believed that this is when borates were initially used in fiberglass.
This category will cover 3 types of glass: borosilicate, insulation (IFG) and textile fiberglass (TFG). Boron oxide, both alkaline and/or non-alkaline, is important in their production and in the finished glass product. Insulation and textile glass could use mineral borates, both Ulexite and Colemanite, respectively.
Borate benefits in these glass categories
- Improved glass fiberization (strength) and resistance to moisture (IFG & TFG)
- Improved transparency, brightness, plus heat and chemical resistance
- Reduced glass batch melting point, viscosity, thermal expansion coefficient
- Inhibited glass devitrification at working temperatures
- Improved decompressibility of IFG materials
- Powerful flux in the melting process
The primary constituents of this type of glass are silica, soda ash, lime and boric oxide in various concentrations. However there are more oxides.
All of these oxides are melted in a natural gas fired or oxy-gas furnace; however, some furnaces use electric boost in the melting process. Electric furnaces can be used as well due to their greater efficiency.
The addition rate for borosilicate glass can be anywhere from 5 – 20% B2O3 of the total glass batch. It is added to the batch in the form of both alkaline borates, sodium borate called Borax 5 Mol also known as Etibor 48, and anhydrous borax, known as Etibor 68. Non-alkaline borates, including boric acid and boric oxide (anhydrous boric acid) have been used in this glass. Mineral-based borates have also been used and include Ulexite and Colemanite.
Borosilicate Glass Chemistry – Physics
When glass is heated in a glass-melting furnace, the atoms within the glass vibrate. As the heat increases the atoms vibrate more, moving apart and together with greater speed. If the glass is heated unevenly, then these atoms expand at an uneven rate and the glass will expand and could shatter. Using the right amount of boric oxide helps to moderate these vibrations.
Boric oxide helps to control the expansion and shrinking of the glass to where the net movement is close to zero. Thus there is little movement and the glass does not shatter.
One of the key value points of this glass is its ability to withstand thermal shock due to its low coefficient of expansion delivered by boric oxide. Another value attributed to these boron-based glasses is their high chemical durability or ability to seal to the metal itself. This sealing quality is appropriate, for example, in light bulbs where the metal bottom screw cap meets the glass bulb.
The density of borosilicate glass is lower than soda-lime glass due to the atomic weight of boron. It should be noted that while borosilicate glass withstands thermal shock it can still crack into large chunks (rather than shattering like other glasses) should there be uneven and rapid variations in temperature.
The uses of borates in glass are well known.
Applications for borates in glass
Name brand products where borosilicate glass is found in laboratory glassware include, but are not limited to, Bomex, Duran, Pyrex, Simax and TGI for its thermal shock value, which inhibits cracking.
There is commercial lighting, such as sodium and mercury vapor lights and metal hydride lamps, where the outer covering is borosilicate-based. Even light emitting diodes (LEDs) and optical light emitting diodes (OLEDs) use this form of glass.
Additional higher-tech applications include telescope glass and its mirrored pieces plus the tubes in solar thermal technology, because of their high strength and heat resistance. During the early days of the US Space Shuttle program, the outer insulation tiles where coated with borosilicate glass.
The semiconductor world uses silica wafers coated with borosilicate glass to produce micro-electromechanical systems. The medical industry has a need for this glass due to its ability to resist chemical mixing of sodium ions from the glass that would mix with other drugs found in ampoules, vials and pre-filled syringes.
Low levels of radioactive waste can be encapsulated with borosilicate glass to vitrify the waste and thus enhance neutron absorption value as a safety feature to humans and other life.
Glass microspheres (solid & hollow) provide value for roadway and airport taxiway striping where a high refractive index is required when mixed with paints. They can also be found as functional filler in paints to add strength and as an extender.
As boron technology continues to expand, research into borosilicate nanoparticles is becoming highly investigated. It has been proposed that such technology be used in the life science areas. In addition other end uses could include the production of photonic band gap devices for high optical contrast and use in ultrasonic microscopy or chemical filtration membranes.
Home & Other uses
Home use includes glass cookware, such as measuring cups and ovenware and on occasion the more expensive beverage glasses.
Art glass is very popular with several colors included in the borosilicate glass. This type of glass is used in the glassblowing phase, creating unusual pieces of art which are stronger than most other glass materials.
Another more common use (which has grown significantly) is flat panel display glass used in TVs that formerly used cathode ray tube technology. Borates have offered strength and transparency to this consumer and commercial based glass.
General Fiberglass Chemistry
Fiberglass glass chemistry is similar whether it be Insulation Fiberglass (IFG) or Textile Fiberglass (TFG). Below are some of the major oxides found; however, other raw material could be added to adjust the glass properties. Oxides not captured in this table could include, but are not limited to, feldspar, sulfate, titanium, and zirconium.
Insulation Fiberglass (IFG)
Fiberglass insulation is by definition an insulation material made of glass fibers. The purpose is to entrap air and reduce the transmission of infrared radiation thereby reducing the transference of heat and sound. Furnace types include Gas/Air and Oxy-gas with and without electric boost. Electric furnaces are also used due to their high melting efficiency.
The boric oxide addition rate is approximately 4 – 7% of the glass batch. It is added to the batch using sodium tetraborate pentahydrate, also known as Etibor 48; however, mineral borates in fiberglass such as Ulexite have also been used as well.
Used in the construction industry for its insulative value, some rolls will be “faced” offering a paper backing applied to this insulation roll for wall or unfaced for attic installation. Chopped fiberglass is yet another variation of this insulation product also applied in attics. Still other forms of compressed fiberglass can be formed into tubing for duct work and still other heating and air conditioning systems.
Textile Fiberglass (TFG)
Textile fiberglass is long filamentous strands of glass used to produce structural products. Furnace types used to produce TFG include Gas/Air and Oxy-gas with and without electric boost.
One of the more common textile fiberglass products produced is e-glass. This is used in printed circuit boards due in part to its low sodium content using non-alkaline borate compounds. Textile fiberglass can also be used as a reinforcement fiber in composite materials.
There are various diameter glass fibers which are classified as A, C, E, and S. E glass has the greatest use in the composite products that are captured in the application section below.
The boric oxide addition rate is approximately 5 – 7% of the glass batch. It is added to the batch as boric acid or boric oxide; however, mineral borates in fiberglass such as Colemanite could also be used.
The uses of borates in fiberglass particularly textile fiberglass is used in but not limited to aerospace, air conditioning filters, appliances, asphalt roofing shingles, automobile bodies, bath tubs, boat hulls and components, concrete reinforcement, doors, flooring including reinforced PVC tiles, gypsum wall board, housewares, power tools, printed circuit boards, pumps, shower stalls, sporting goods, switches, tanks, valves and window frames.
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