Float glass

0.25-5 usd/kg
Circularity potential
Very high
Strength
Medium
Production energy
Low
Stiffness
High
Embodied CO2
Low
Density
Medium

Float glass is soda lime glass produced as flat sheet, which can be further processed by bending, strengthening and laminating. It is named after the method of production: molten glass is floated on a pool of liquid tin to produce a perfectly flat and smooth sheet.

Its technical strength is far higher than its practical strength. Glass is very brittle and any imperfections (scratches or flaws, for example) act as a stress concentrator, causing premature failure when even a relatively small amount of tension is applied, such as by bending. So even though the theoretical strength is in the order of GPa, tiny imperfections reduce the strength down to around 14 MPa. The mechanical properties are enhanced with strengthening and tempering, which can raise tensile strength to 40 and 70 MPa, respectively.


Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products


The process of controlled heating and cooling are used to strengthen and temper glass. Heat strengthening improves the thermal and mechanical properties of glass. This is accomplished by heating the glass, and then rapidly cooling the surface. Because the core of the material cools more slowly, it pulls on the solidified surface and so puts it under compression. The tension generated results in an increase in breaking strength – any thermal shock or impact would have to overcome the pre-tensioned load before the glass is affected. Heat strengthened glass has a surface compression of 25-50 MPa, and tempered glass over 70 MPa.

An advantage of tempered (also called toughened, or safety glass) glass is that it shatters into small particles if damaged, which increases safety. All of the pre-tensioned energy is released at once. However, this means that once tempered, the glass cannot be cut or altered. Tempered glass is used in the side and rear windows of cars. However, in some cases, laminated glass is preferred for enhanced safety and strength. In public transport, toughened glass is preferred, because it can be broken through in the even of an accident.


Design properties
Cost usd/kg
0.85-3
Embodied energy MJ/kg
20-25
Carbon footprint kgCO2e/kg
1.8
Density kg/m3
2490
Tensile modulus GPa
70
Tensile strength MPa
70-80
Flexural strength MPa
120-200
Shear modulus GPa
28.5
Hardness Mohs
5.5
Poissons ratio
0.22
Thermal expansion (µm/m)/ºC
8.8
Melt temperature ºC
1000
Thermal conductivity W/mK
1
Temperature min-max °C
-40 to 300
Thermal
insulator
Electrical
insulator