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Borosilicate glass is a specialized form of glass that uses boric acid as a component in its fabrication. The result of the addition of the element boron is a type of glass that is very resistant to thermal shock and exhibits a much lower coefficient of thermal expansion than that of common silicate glass. In this article, a review of borosilicate glass will be presented, including its development, properties, and uses.

To learn more about other types of glass, review our topic article on understanding glass.

Development of Borosilicate Glass

Back in 1882, a German chemist named Otto Schott was interested in experimenting with ways to create glass that had the ability to withstand sudden changes in temperature or exposure to uneven temperatures without shattering. In that year, he made the discovery that ushered in the creation of borosilicate glass. Schott discovered that the addition of the element Boron to the glass fabrication process resulted in a heat-resistant form of glass.

Later work by chemists W.C. Taylor and Eugene Sullivan at Corning Glass refined the efforts of Otto Schott and expanded the temperature resistant properties of borosilicate glass. As a result of these innovations, customized glass fabrication grew, where there is now over a million different formulations of glass that can be customized for specific product needs by enhancing the desired physical and mechanical properties of glass.

Perhaps the most well-known application for borosilicate glass grew from further research at Corning Glass. After joining the company in 1914, physicist Jesse Littleton was given the task of testing and evaluating the physical properties of the newly created glass formulation. After his wife’s ceramic casserole dish accidentally broke, she suggested that perhaps this newly developed heat resistant glass might prove to be a useful product for baking. After she tested the notion by baking a cake in a sample glass container that Littleton had brought home, a new use for borosilicate glass was born – glass cookware. Corning Glass introduced a line of product known as Pyrex^®_[1], which was for many years of its manufacturing run produced using borosilicate glass.

Properties of Borosilicate Glass

Borosilicate glass is generally chemically resistant, but perhaps its most remarkable physical property is its low coefficient of thermal expansion, which explains why the glass can resist shattering under sudden rapid changes in temperature. Glass generally is a poor conductor of heat, so when you take hot glass and immerse it in cold water, the exterior of the glass cools rapidly while the interior does not. The stresses caused by the temperature differential cause the glass to shatter.

With borosilicate glass, the addition of boric acid (H₃BO₃) to the formulation results in a glass that has a low coefficient of thermal expansion, which means that when the glass is heated or cooled, it does not expand or contract very much. This dimensional stability is what enables borosilicate glass to be capable of withstanding rapid and extreme temperature changes without cracking.

The chemical composition of borosilicate glass typically consists of around 81% silicon dioxide (SiO₂) and 13% boron trioxide (B₂O₃) with lesser concentrations of sodium oxide and aluminum oxide. (Note that the concentrations of boric oxide can vary, 5-13% is typical). The element Boron is what provides the glass with its dimensional stability so that the material doesn’t shrink or grow as the temperature to which it is exposed changes.

Table 1 below summarizes the typical properties of borosilicate glass. Note from the table the value for the Coefficient of Linear Thermal Expansion (3.3 x 10^-6 ) and the temperature range over which that value applies 20^oC, 300^oC. This value is about 1/3 that of traditional soda-lime glass.

Table 1 – Properties of Borosilicate Glass

	Material Property	Value
General	Density (@ 25oC)	2.23 g/cm3
Mechanical	Young’s Modulus	64 GPa
	Poisson’s ratio (μ)	0.2
Thermal	Maximum rated use temperature	500oC
	Transformation temperature	525oC
	Thermal Conductivity (@90oC)	1.2 W/(moK)
	Coefficient of mean linear thermal expansion (@ 20oC, 300oC)	3.3 x 10-6 /oK
Electrical	Volume resistance	1015 Ω cm
	Dielectric Constant	4.6
	Dielectric Strength	30 kV/mm
Optical	Refractive Index (@ λ =587.6 nm)	1.473
	Stress-optical coefficient	4.0 x 10-6 mm2/N

Applications of Borosilicate Glass

The initial problem that was attempting to be solved at the time that Otto Schott began experimenting with glass formulations was to create a glass that could stand up against extreme temperature exposure. For example, the glass that was used in lanterns at that time would end up shattering or cracking in rainy conditions because the cold rain on the exterior surface of the glass caused a large temperature gradient when compared with the temperature of the hot interior face.

Once borosilicate glass became available, its applications became immediately apparent. Some of the common uses for borosilicate glass include:

• Laboratory glassware
• Scientific lenses and hot mirrors
• Bakeware and cookware
• Thermal insulation
• High-intensity lighting products
• Sight glass
• Aircraft exterior lenses
• Aquarium heaters
• Electronics
• Rapid prototyping

Laboratory glassware

The high dimensional stability and ability to tolerate exposure to different temperatures at the same time make borosilicate glass a natural material choice from which to create laboratory glassware, also called labware. Petri dishes, microscope slides, bottles, beakers, flasks, test tubes, funnels, and measuring instruments such as graduated cylinders are all common examples. Besides the favorable thermal properties, borosilicate glass is very resistant and non-reactive to most chemicals.

Scientific lenses and hot mirrors

Borosilicate glass can be molded into high precision optical components such as lenses for use in telescopes and other precision optical devices. The low coefficient of thermal expansion for borosilicate glass means that the optical properties of the lenses will be stable over changes in temperature as the glass lens will not significantly change its dimensions. The glass is also ideal for use in hot mirrors that reflect infrared light.

Bakeware and cookware

Among its first and most common uses is in the creation of household cookware and bakeware. The thermal properties of borosilicate cookware allow it to be transported from a hot oven to a cool countertop without fear of cracking or shattering. It is also used in products such as measuring cups and is safe for use in microwave ovens and dishwashers.

Thermal insulation

The thermal properties of borosilicate glass were used to fabricate the thermal tiles that protected the space shuttle from the heat of reentry into earth’s atmosphere.

High-intensity lighting products

Stage lights and lighting products used in the film industry make use of borosilicate glass lenses, as these lights can reach high temperatures when operated continuously for hours on end. Other lamp products that use high-intensity discharge (HID), such as mercury vapor lamps or metal halide lamps, will make use of borosilicate lenses or outer envelopes.

Sight glass

In industrial processes, tanks make use of sight glasses that are often fabricated from borosilicate glass. These sight glasses allow for the visual monitoring of substances and processes without the need to open the tank or storage vessel, and without interrupting the process.

Aircraft exterior lenses

The exterior lamps on an aircraft use borosilicate glass lenses due to the clear optical properties (transmissibility) and the ability to withstand the temperature differential experienced during high altitude flight. Figure 1 below shows an example of the optical transmission characteristics of one type of borosilicate glass. Note the very flat performance across the full spectrum of wavelength from 300 – 1200 nm.

Image credit: https://www.schott.com/d/tubing/9a0f5126-6e35-43bd-bf2a-349912caf9f2/schott-algae-brochure-borosilicate.pdf

Aquarium heaters

Aquarium heaters are a very common application for borosilicate glass, where the heater tube is exposed to a thermal gradient from the interior nichrome heating element and the much colder water in the tank.

Electronics

Borosilicate glass is used in the semiconductor fabrication industry for modern electronics where silicon wafers may be bonded to etched borosilicate glass.

Rapid prototyping

In fused deposition modeling, a form of 3D printing or additive manufacturing, borosilicate glass is an ideal material for use upon which the heated and extruded plastic materials are deposited during the build process.

Summary

This article provided a brief review of what borosilicate glass is, its development, the material properties that make this material useful, and some of the more common applications that utilize it. For information on other products, consult our additional guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products. 

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