Borosilicate glass
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Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion (~5 × 10−6 /°C at 20°C), making them resistant to thermal shock, more so than any other common glass.
History
Borosilicate glass was first developed by German glassmaker Otto Schott in the late 19th century[1] and sold under the brand name "Duran" in 1893. After Corning Glass Works introduced Pyrex in 1915, it became a synonym for borosilicate glass in the English-speaking world.
The European manufacturer of Pyrex, Arc International, uses borosilicate glass in its Pyrex glass kitchen products;[2] however, the U.S. manufacturer of Pyrex kitchenware uses tempered soda-lime glass.[3] Thus Pyrex can refer to either soda-lime glass or borosilicate glass when discussing kitchen glassware, while Pyrex, Bomex, Duran and Kimax all refer to borosilicate glass when discussing laboratory glassware.
Most borosilicate glass is colorless. Colored borosilicate, for the studio glass trade, was first widely brought onto the market in 1986 when Paul Trautman founded Northstar Glassworks.[citation needed] In 2000, former Northstar Glassworks employee Henry Grimmett started Glass Alchemy and developed the first cadmium Crayon Colors and aventurine Sparkle colors in the borosilicate palette.
In addition to the quartz, sodium carbonate, and calcium carbonate traditionally used in glassmaking, boron is used in the manufacture of borosilicate glass. Typically, the resulting glass composition is about 70% silica, 10% boron oxide, 8% sodium oxide, 8% potassium oxide, and 1% calcium oxide (lime). Though somewhat more difficult to make than traditional glass (Corning conducted a major revamp of their operations to make it), it is economical to produce; its superior durability, chemical and heat resistance finds excellent use in chemical laboratory equipment, cookware, lighting and, in certain cases, windows.
Manufacturing process
Borosilicate glass is created by adding boron to the traditional glassmaker's frit of silicate sand, soda, and ground lime. Since borosilicate glass melts at a higher temperature than ordinary silicate glass, some new techniques were required for industrial production. Borrowing from the welding trade, burners combining oxygen with natural gas were required.
Composition and physical characteristics
Borosilicate glass has a very low thermal expansion coefficient, about one-third that of ordinary glass. This reduces material stresses caused by temperature gradients, thus making it more resistant to breaking. This makes it a popular material for objects like telescope mirrors, where it is essential to have very little deviation in shape. It is also used in the processing of high-level radioactive waste, where the waste is immobilised in the glass through a process known as vitrification (contrast with Synroc).
The softening point (temperature at which viscosity is approximately poise) of type 7740 Pyrex is 820 °C (1,510 °F).[4]
Borosilicate glass is less dense than ordinary glass.
While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subject to rapid or uneven temperature variations. When broken, borosilicate glass tends to crack into large pieces rather than shattering (it will snap rather than splinter).
Optically, borosilicate glasses are crown glasses with low dispersion (Abbe numbers around 65) and relatively low refractive indices (1.51–1.54 across the visible range).
Fraction by weight
| Element | Atomic number |
Fraction |
|---|---|---|
| B | 5 | 0.040064 |
| O | 8 | 0.539562 |
| Na | 11 | 0.028191 |
| Al | 13 | 0.011644 |
| Si | 14 | 0.377220 |
| K | 19 | 0.003321 |
Physical characteristics
Density = 2.23 g/cm³
Mean excitation energy = 134.0 eV
Usage
During the mid-twentieth century, borosilicate glass tubing was used to pipe coolants (often distilled water) through high power vacuum tube–based electronic equipment, such as commercial broadcast transmitters.
Glass cookware is another common usage. Borosilicate glass is used for measuring cups, featuring screen printed markings providing graduated measurements, are widely used in American kitchens.
Aquarium heaters are sometimes made of borosilicate glass. Due to its high heat resistance, it can tolerate the significant temperature difference between the water and the nichrome heating element.
Many high-quality flashlights use borosilicate glass for the lens. This allows for a higher percentage of light transmittance through the lens compared to plastics and lower-quality glass.
Specialty marijuana and tobacco pipes are made from borosilicate glass. The high heat resistance makes the pipes more durable.
Most premanufactured glass guitar slides are also made of borosilicate glass.
New lampworking techniques led to artistic applications such as contemporary glass marbles. The modern studio glass movement has responded to color. "The availability of colors began to increase when companies such as Glass Alchemy introduced the Crayon Colors, which brought a whole new vivacity to the glass industry."[5] Borosilicate is commonly used in the glassblowing form of lampworking and the artists create a range of products ranging from jewelry, kitchenware, to sculpture as well as for artistic glass tobacco pipes.
Borosilicate glass is sometimes used for high-quality beverage glassware. Borosilicate glass lends kitchen- and glassware increased durability along with microwave and dishwasher compatibility.
Most astronomical reflecting telescope glass mirror components are made of borosilicate glass because of its low coefficient of expansion with heat. This makes very precise optical surfaces possible that change very little with temperature, and matched glass mirror components that "track" across temperature changes and retain the optical system's characteristics.
The optical glass most often used for making instrument lenses is Schott BK-7 (or the equivalent from other makers), a very finely made borosilicate crown glass[citation needed]. It is also designated as 517642 glass after its 1.517 refractive index and 64.2 Abbe number. Other less costly borosilicate glasses, such as Schott B270 or the equivalent, are used to make "crown glass" eyeglass lenses. Ordinary lower-cost borosilicate glass, like that used to make kitchenware and even reflecting telescope mirrors, cannot be used for high-quality lenses because of the striations and inclusions common to lower grades of this type of glass.
Borosilicate is also a material of choice for evacuated tube solar thermal technology, because of its high strength and heat resistance.
Borosilicate glasses also find application in the semiconductor industry in the development of microelectromechanical systems (MEMS), as part of stacks of etched silica wafers bonded to the etched borosilicate glass.
The thermal insulation tiles on the Space Shuttle are coated with a borosilicate glass[6].
Lighting manufacturers use borosilicate glass in their refractors.
Additionally, borosilicate tubing is used as the feedstock for the production of parenteral drug packaging, such as vials and pre-filled syringes, and is also used for the production of ampoules and dental cartridges. The chemical resistance of borosilicate glass minimizes the migration of sodium ions from the glass matrix thus making it well suited for injectable drug applications. This type of glass is typically referred to as USP / EP JP Type I.
Borosilicate glasses are used for immobilisation and disposal of radioactive wastes. In most countries high-level radioactive waste has been incorporated into alkali borosilicate or phosphate vitreous waste forms for many years and vitrification is an established technology [7]. Vitrification is a particularly attractive immobilization route because of the high chemical durability of the vitrified glass product. This characteristic has been used by industry for centuries. The chemical resistance of glass can allow it to remain in a corrosive environment for many thousands and even millions of years.
Borosilicate nanoparticles
It was initially thought that borosilicate glass could not be formed into nanoparticles, since an unstable boron oxide precursor prevented successful forming of these shapes. However, in 2008 a team of researchers from the Swiss Federal Institute of Technology at Lausanne were successful in forming borosilicate nanoparticles of 100 to 500 nanometers in diameter. The researchers formed a gel of tetraethylorthosilicate and trimethoxyboroxine. When this gel is exposed to water under proper conditions, a dynamic reaction ensues which results in the nanoparticles.[8]
In lampworking
Borosilicate, or "boro" as it is often referred to, is used extensively in the glassblowing process lampworking, which involves using a burner torch to melt and form glass, using a variety of metal and graphite tools. Borosilicate is referred to as "hard glass" and has a higher melting point than "soft glass" which is used in glassblowing formed in large furnaces and large rods. Raw glass used in lampworking comes in glass rods for solid work and glass tubes for hollow work tubes and vessels/containers. Lampworking is used to make complex and custom scientific apparatus; most major universities have a lampworking shop to manufacture and repair their glassware. For this kind of "scientific glassblowing," the specifications must be exact and the glassblower must be highly skilled and precise. Lampworking is also done as art and common items made include goblets, paper weights and pendants.
References
- ^ Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994), ISBN 3540575723
- ^ Pyrex History
- ^ "Exploding Pyrex". Retrieved 2009-10-27.
- ^ Weissler, G. L. (1979). Vacuum Physics and Technology (2 ed.). Academic Press. p. 315. ISBN 9780124759145.
- ^ danK. "Glass Industry: Then and Now" Glass Aficionado 2008: Issue 1 Print Page 54
- ^ "SPACE SHUTTLE ORBITER SYSTEMS THERMAL PROTECTION SYSTEM". Retrieved 2009-07-15.
- ^ M. I. Ojovan and W.E. Lee. An Introduction to Nuclear Waste Immobilisation, Elsevier, Amsterdam, 315pp. (2005)
- ^ Chemical & Engineering News Vol. 86 No. 37, 15 Sept. 2008, "Making Borosilicate nanoparticles is now possible", p. 35
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