Article - Issue 68, September 2016
HOW DOES THAT WORK? - Toughened glass
Commercial glass is generally made up of three main components: sand (also referred to as silicon dioxide), limestone and sodium carbonate. Toughened glass is a type of safety glass that is physically stronger having been reinforced by either thermal or chemical treatments. It is used in a range of applications, from car windows and buildings to mobile phones, tablet devices and TVs.
Tempering the glass to compress the outer surface and expand the inner layers balances the tension in the glass to make it tougher. This also causes the glass to crumble into small granular chunks instead of jagged shards when broken, making it less likely to cause injury, and means that the surface is more resistant to cracks and scratches.
With thermal tempering, the glass passes through a furnace that heats it above its transition temperature of 564°C to around 620°C. The glass surface is then cooled quickly with forced air while the inner portion is able to flow freely and contract.
Chemical toughening produces a layer of compressive stress on the surface of the glass by exchanging the sodium ions in the surface with larger potassium ions that take up more room and are pressed together when the glass cools. This is done by immersing the glass in a bath of molten potassium nitrate. Glass that has been chemically strengthened is tougher than glass that has been tempered thermally.