Borosilicate Glass: Composition, Properties and Uses

Borosilicate glass is a special type of glass containing a significant proportion of boron oxide (B₂O₃) in addition to silica and other glass-forming or modifying oxides. It is well known for its low coefficient of thermal expansion and excellent resistance to sudden changes in temperature.

Composition of Borosilicate Glass

The composition varies according to the particular grade and application. A typical borosilicate glass contains:

Constituent	Approximate Role
SiO2	Main glass-forming component; provides the basic silicate network.
B2O3	Important glass former/modifier that lowers thermal expansion and improves thermal and chemical resistance.
Na2O and K2O	Modify the glass network and help in melting and processing.
Al2O3	Can improve chemical durability and mechanical properties.

A commonly cited laboratory borosilicate glass composition is approximately 70–81% SiO₂, 7–13% B₂O₃, 4–8% Na₂O/K₂O, with smaller amounts of other oxides. The exact composition depends on the manufacturer and grade.

Manufacture

Borosilicate glass is manufactured by melting carefully selected raw materials containing silica, boron compounds, and suitable modifying oxides at high temperature. The molten glass is homogenized and then formed into the required shape. Controlled cooling and annealing are used to reduce internal stresses.

Important Properties

• Low thermal expansion: It expands only slightly when heated compared with ordinary soda-lime glass.
• High thermal-shock resistance: It can withstand relatively rapid temperature changes without easily cracking.
• Good chemical resistance: It has good resistance to water and many chemicals.
• Good heat resistance: It can be used at elevated temperatures within the limits of the particular glass grade.
• High transparency: Many grades are highly transparent over the visible region.
• Good dimensional stability: Its low thermal expansion makes it useful where accurate dimensions must be maintained during temperature changes.

Why Does Borosilicate Glass Resist Thermal Shock?

The major reason is its low coefficient of thermal expansion. When a glass article is heated unevenly, different parts of the article tend to expand by different amounts. In borosilicate glass, this difference is relatively small because of its low thermal expansion. Consequently, lower thermal stresses are produced, reducing the tendency of the glass to crack during rapid temperature changes.

Uses of Borosilicate Glass

• Laboratory glassware such as beakers, flasks, and test tubes
• Chemical processing equipment
• Glass tubing and sight glasses
• Cookware and heat-resistant household glassware
• Pharmaceutical containers and vials
• Lighting and high-temperature applications
• Optical and scientific instruments

Borosilicate Glass vs Soda-Lime Glass

Property	Borosilicate Glass	Soda-Lime Glass
Major special constituent	B2O3	Higher proportion of Na2O and CaO
Thermal expansion	Low	Higher
Thermal-shock resistance	High	Lower
Chemical resistance	Generally high	Generally lower than borosilicate glass
Common uses	Laboratory, pharmaceutical, chemical and heat-resistant applications	Windows, bottles, containers and general-purpose glassware

Key point: Borosilicate glass contains significant B₂O₃ and is particularly important because of its low thermal expansion, high thermal-shock resistance, and good chemical durability. Therefore, it is widely used for laboratory and other applications involving temperature changes and chemical contact.

Barasat Government College (West Bengal State University)