Toughened Glass - An Overview

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For many applications requiring large spans of glass, such as large buildings, toughened glass is the only acceptable alternative to standard float glass. This is due in part to the fact that these glasses have exceptional strengths compared to standard annealed float glasses. Toughened glass is also known as tempered glass.

How is Toughened Glass Produced?

The improved properties of toughened glass are a result of the stress profile that is induced in the glass by the toughening heat treatment process. When performed correctly, the glass surface is in compression, while the center is in tension. This stress profile in the glass is successful as most failures start at the surface from tensile loads. In toughened glass, the applied tensile load must overcome the compressive stress at the surface before the surface can go into tension and fail.

The heat treatment process involves heating annealed (stress-free) glass up to a temperature between its glass transition temperature and its softening point. Its surface is then rapidly cooled. This is usually achieved using air jets. This process freezes the surface while the interior may still be molten, and consequently, there is a temperature differential across the thickness of the glass. The hotter, core section then contracts at a faster rate compared to the outside until an isothermal state is reached.

Initially, the rapid cooling of the surface tends to induce tensile stress in the surface. This is reversed in the latter stages of cooling, resulting in compressive stresses in the surface.

Key Properties

There are a number of properties that make toughened glass such a durable and versatile type of glass that has found applications across daily life. Some of the fundamental properties include:

  • Superior strength compared to annealed glass
  • When a failure occurs, glass panels shatter into small pieces that rather than sharp shards, as is the case with annealed glass
  • Good chemical resistance
  • Attacked by hydrofluoric acid (HF).

Toughened glass is often used for high-pressure and explosion proof windows, as well as in lighting and indoors for both industrial and domestic builds. It is suitable for these uses because of its ability to shatter into small, often circular pieces that pose a low risk of additional injury to those cleaning up shattered glass. It is also resistant to scratches and can withstand harsh weather conditions, and its overall durability allows it to meet many fire regulations, and as such sees it finding applications in scientific and medical fields, as well as in street furniture, cookware, and buildings.

Applications

Toughened glass is used throughout our daily lives, and a number of industries make regular use of toughened glass. Below are some examples of the applications for toughened glass in different sectors.

Applications of Toughened Glass in the Transport Industry

Toughened glass is used in all forms of transport from cars to boats and ships. In all cases (and in most countries), there are regulations in place pertaining to properties such as strength, thickness, and light transmission.

Applications of Toughened Glass in the Building and Construction Industry

Toughened glass finds many applications in the building industry including both internal and external uses. Internally, toughened glass may be used in frameless shower screens and office partitions, while externally it can be used for large window spans and balustrading.

Toughened glass panels can be manufactured to almost any shape, with the limiting factor often being specific furnace dimensions (at both ends of the scale). The glass can also be machined, with holes, beveled edges, etc. However, all machining must be carried out prior to toughening, and all edges must be polished. Unpolished edges may act as sites for crack propagation during the heat treatment process.

Household Applications for Toughened Glass

Windows made out of toughened glass offer increased security from burglary as it is difficult to break through. It is also more resistant against breakages from direct impact from footballs, for example.

Shower doors are also often made out of toughened glass. If a person was to slip in the bath or shower, the risk of serious injury is significantly reduced with toughened glass as it breaks into small pieces rather than large, sharp shards.

The International Residential Code (IRC) has strict guidelines about when toughened glass has to be used. Some of the situations in which toughened glass has to be used include:

•    When windows panes are larger than nine square feet
•    Within five feet of a shower, bath, pool, or similar
•    In stairways, ramps, or landings with glass nearby.

Summary

Toughened glass is an incredibly versatile material that finds uses across everyday life. It is used extensively in building and construction due to its enhanced strength and safety regarding breakages when compared with annealed glass. However, despite its overall durability, the edges of toughened glass are comparatively weak, and direct impact to the edges of toughened glass can result in breakages.

Sources and Further Reading

This article was updated on 7th February, 2019.

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