Toughening
Background
For many applications such as buildings requiring large spans of glass, toughened glass is the only acceptable alternative. These glasses have exceptional strengths compared to standard annealed float glasses.

These improved properties 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 and then rapidly cooling the surface. 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 a tensile stress in the surface. This is reversed in the latter stages of cooling, resulting in compressive stresses in the surface.

Key Properties
  • Superior strength compared to annealed glass
  • When 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)
Applications
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.
Building and Construction Industry
Toughened glass finds many applications in the building industry from internal to external. 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, bevelled 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.