Chemically tempered glass – The thin, strong product with the fine, hard surface
Chemically tempered glass – also known as chemically strengthened glass – is used i.e. as protective glass on electronic instruments, smartphones, cameras and in industrial solutions, where requirements demands thin glass with high strength and scratch resistance. This type of glass is known by many as “gorilla glass”, but “gorilla glass” is actually a specific raw material (Aluminosilicate glass), which is hardened and used for e.g. smartphones. We have similar raw materials that are also very suitable for chemical tempering. It is the chemical composition of the glass that determines its suitability.
It is possible to to use the process on most types of glass. However concerning glass types with a sodium content of less than 10% it difficult to document the effect of chemically tempering.
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Properties of chemically tempered glass
The optical quality of chemically tempered glass is extremely good because the surfaces are strengthened by the tempering process without being mechanically affected. This preserves the original optic properties of the surface.
Chemical tempering is particularly suitable for glass that is thinner than 3 mm. Glass all the way down to 0.1 mm in thickness can be processed with a chemical tempering. Glass that is thicker than 3 mm can also be chemically tempered, but since it is a process that strengthens the surface, the impact and bending strength becomes proportionally smaller the thicker the glass. However, you still get resistance to scratches and temperature differences.
Chemical stregthening adds following to the glass:
- Improved impact strength
- Improved flexural strength
- Increased resistance to temperature changes
- Improved scratch resistance
Processing of chemically tempered glass
Unlike thermally tempered glass, chemically tempered glass can be subsequently processed. You can grind, cut and drill in it, but the exposed edges will not be hardened, so the strength of the glass is reduced. It is also possible to chemically harden glass, which is already curved in different shapes.
Chemically tempered glass is not safety glass
Despite the increased strength against impact and bending, chemically tempered glass cannot be qualified as a safety glass. It does not granulate like thermally tempered glass, but instead breaks and shatters similar to ordinary glass (though in much smaller pieces). If the solution requires a safety glass that granulates upon breaking , one should choose thermally tempered glass instead, or 2 chemically tempered glasses that are laminated together, by laminating the glass together it becomes a safety glass.
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The chemically tempering process
At Mirit Glas, we primarily use two types of glass when we make chemically strengthened glass. It is plain soda-lime glass and aluminosilicate glass. The actual chemical tempering process is the same for both type of glass, but the raw material cost is different, leaving aluminosilicate glass more expensive, but faster to chemical temper, so depending on the size and quantity it must be decided which raw material to choose. Also aluminosilicate glass can be made stronger after chemical tempering than soda-lime glass and when aluminosilicate glass breaks is will break in much smaller pieces than soda-lime glass.
The main ingredient when chemically tempering is potassium nitrate, which is heated to about 400 degrees to make it liquid. The glass is chemically hardened by being immersed in the molten potassium nitrate bath for a certain amount of time.
Soda-lime glass must be immersed in the tank for 16 hours, while aluminosilicate glass can simply be immersed for four hours.
While the glass is immersed in the solution, a chemical process takes place in the surface of the glass, where sodium ions are replaced with the larger potassium ions. This causes the surface to be compressed and the core of the glass to compensate by stress tension. It makes the glass stronger, mores scratch resistant and even heat resistant up to 250°C. Chemically tempered aluminosilicate glass can reach a surface compression up to 5 times the values of normal thermal tempered soda-lime glass.
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