# Borosilicate glass

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 (~3 × 10−6 /°C at 20°C), making them resistant to thermal shock, more so than any other common glass. Such glass is less subject to thermal stress and is commonly used for the construction of reagent bottles. Borosilicate glass is sold under such trade names as Borcam, Simax, Suprax, Kimax, Pyrex, Endural, Schott, or Refmex.

## 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, the name 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, TGI and Simax all refer to borosilicate glass when discussing laboratory glassware.

In addition to the quartz, sodium carbonate, and aluminum oxide traditionally used in glassmaking, boron is used in the manufacture of borosilicate glass. The composition of low expansion borosilicate glass such as those laboratory glasses mentioned above is approximately 80% silica, 13% boric oxide, 4% sodium oxide, and 2-3% aluminum oxide. Though more difficult to make than traditional glass due to the high melting temperature required (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.

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. Glass sales and supplies were brought to the industry by ABR Imagery in 1996 and glassblowing as a hobby became mainstream within about two years of this occurrence. From there, glassblowing became a household hobby amongst bead makers, lamp workers, and others.

## Manufacturing process

Borosilicate glass is created by adding boric oxide[4] to the traditional glassmaker's frit of silica 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.

## Physical characteristics

The common type of borosilicate glass used for laboratory glassware has a very low thermal expansion coefficient (3.3 x 10−6/K),[5] about one-third that of ordinary soda-lime glass. This reduces material stresses caused by temperature gradients which makes borosilicate a more suitable type of glass for certain applications (see below). Fused quartzware is even better in this respect (having a fifteen times lower thermal expansion than soda-lime glass), however the difficulty of working with fused quartz makes quartzware much more expensive; borosilicate glass is a low-cost compromise. While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subjected 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).

The softening point (temperature at which viscosity is approximately $10^{7.6}$ poise) of type 7740 Pyrex is 820 °C (1,510 °F).[6]

Borosilicate glass is less dense (at about 2.23g/cm3) than typical soda-lime glass due to the low atomic weight of boron.

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).

## Usage

Borosilicate glass has a wide variety of uses ranging from cookware to lab equipment as well as a component of high quality products such as implantable medical devices and devices used in space exploration

### Healthcare

Virtually all modern laboratory glassware is borosilicate glass. It is widely used in this application due to its chemical and thermal resistance and good optical clarity, but the glass can be reacted with sodium hydride to produce sodium borohydride, a common laboratory reducing agent. Fused quartz is also found in some laboratory equipment when its higher melting point and transmission of UV are required (e.g. for tube furnace liners and UV cuvettes), but the cost and difficulty of working with quartz make it excessive for the majority of laboratory equipment.

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 is widely used in implantable medical devices such as prosthetic eyes, Artificial hip joints, bone cements, dental composite materials (white fillings)[7] and even in breast implants

Many implantable devices benefit from the unique advantages of borosilicate glass encapsulation. Applications include veterinary tracking devices, neurostimulators for the treatment of epilepsy, implantable drug pumps, cochlear implants, and physiological sensors.[8]

### Electronics

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.

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.

### Cookware

Glass cookware is another common usage. Borosilicate glass is used for measuring cups, featuring screen printed markings providing graduated measurements, which are widely used in American kitchens.

Borosilicate glass is sometimes used for high-quality beverage glassware. Borosilicate glass lends kitchen- and glassware increased durability along with microwave and dishwasher compatibility.

### Lighting

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.

Several types of high-intensity discharge (HID) lamps, such as mercury vapor and metal halide lamps, use borosilicate glass as the outer envelope material.

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."[9] 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.

Lighting manufacturers use borosilicate glass in their refractors.

Organic light emitting diode (for display and lighting purposes) also uses borosilicate glass (BK7). The thicknesses of the BK7 glass substrates are usually less than 1 millimeter for the OLED fabrication. Due to its optical and mechanical characteristics in relation with cost, BK7 is a common substrate in OLEDs. However, depending on the application, sodalime glass substrates of similar thicknesses are also used in OLED fabrication.

### Optics

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. The maximum working temperature is 515 Fahrenheit (268.3 °C). While it transitions to a liquid starting at 550 degrees Fahrenheit(just before it turns red-hot), it is not workable until it reaches over 1000 fahrenheit. That means that in order to industrially produce this glass, oxygen/fuel torches must be used. Glassblowers borrowed technology and techniques from welders.

### Other

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.

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.

Borosilicate is also a material of choice for evacuated tube solar thermal technology, because of its high strength and heat resistance.

The thermal insulation tiles on the Space Shuttle were coated with a borosilicate glass.[10]

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.[11] 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.[citation needed] 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.[12]

## In lampworking

Borosilicate, or "boro" (as it is often called), is used extensively in the glassblowing process lampworking; the glassworker uses a burner torch to melt and form glass, using a variety of metal and graphite tools to shape it. Borosilicate is referred to as "hard glass" and has a higher melting point (approximately 3,000 degrees Fahrenheit / 1648 degrees Celsius) than "soft glass," which is preferred for glassblowing by beadmakers. 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 able to work with precision. Lampworking is also done as art and common items made include goblets, paper weights, pendants, compositions and figurines.

In recent years, with the resurgence of lampworking as a technique to make handmade glass beads, boro has become a popular material in many glass artists' studios. Boro for beadmaking comes in thin, pencil-like rods. Northstar, Momka's Glass, Trautman Art Glass, and Glass Alchemy are popular manufacturers, although there are other brands available. The metals used to color boro glass, particularly silver, often create strikingly beautiful and unpredictable results when melted in an oxygen-gas torch flame. Because it retains heat longer than soft glass, and because it is stronger than soft glass, boro is particularly suited for sculpting and creating large beads, including figurines. The tools used for making glass beads from boro glass are the same as those used for making glass beads from soft glass. Colored borosilicate lampworking glass is most often considerably more expensive than "soft" lampworking glass, and this is usually reflected in the selling price of handmade artisan borosilicate beads and other creations.

Zerodur replaced borosilicate glass in many optical applications

## References

1. ^ Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994), ISBN 3-540-57572-3
2. ^ Pyrex History
3. ^ "Exploding Pyrex". Retrieved 2009-10-27.
4. ^ "What is Borosilicate Glass?". qorpak.com. Retrieved 2012-11-02.
5. ^ "Borosilicato". refmexgl.com. Retrieved 2012-11-02.
6. ^ Weissler, G. L. (1979). Vacuum Physics and Technology (2 ed.). Academic Press. p. 315. ISBN 978-0-12-475914-5.
7. ^ R Wananuruksawong et al 2011 IOP Conf. Ser.: Mater. Sci. Eng. 18 192010 doi:10.1088/1757-899X/18/19/192010 Fabrication of Silicon Nitride Dental Core Ceramics with Borosilicate Veneering material
8. ^ http://medicaldesign.com/materials/encapsulating-smaller-and-smarter-implantables-glass-act
9. ^ danK. "Glass Industry: Then and Now" Glass Aficionado 2008: Issue 1 Print Page 54
10. ^ "SPACE SHUTTLE ORBITER SYSTEMS THERMAL PROTECTION SYSTEM". Retrieved 2009-07-15.
11. ^ M. I. Ojovan and W.E. Lee. An Introduction to Nuclear Waste Immobilisation, Elsevier, Amsterdam, 315pp. (2005)
12. ^ Chemical & Engineering News Vol. 86 No. 37, 15 September 2008, "Making Borosilicate nanoparticles is now possible", p. 35