Float Glass - Composition & Manufacture

All float glass manufactured by Saint-Gobain Glass UK is done so in accordance with EN 572-9:2004 [1], as required by CE marking.

Composition

Soda-Lime-Silicate (SLS) glass is required to have the following composition, as defined by EN 572-1:2012 [2];

Compound

Proportion

Silicon Dioxide (SiO2)

69% to 74%

Calcium Oxide (CaO)

5% to 14%

Sodium Oxide (Na2O)

10% to 16%

Magnesium Oxide (MgO)

0% to 6%

Aluminium Oxide (Al2O3)

0% to 3%

Others

0% to 5%

The batch composition that ultimately produces the SLS glass will typically be composed of formers, fluxes and stabilisers. Formers, in this case silica sand, makes up the bulk of the glass, with fluxes, such as sodium carbonate, allowing the silica sand to be melted at lower temperatures.

Stabilisers, such as calcium carbonate, make the glass resistant to attack by moisture in the atmosphere, or direct contact with water.

Metal oxides can also be added to the batch to colour the glass, as with SGG PARSOL.

Manufacture

Float glass, such as SGG PLANICLEAR, is manufactured using a process with 5 key stages;

  • Batch Mixing
  • Melting and Refining
  • The Float Bath
  • Annealing
  • Cutting

Batch Mixing

The batch must be carefully controlled in order to generate a glass with a composition which meets the requirements of EN 572-1:2012 and conforms to SGG UKs own internal standards.

All of the raw materials used by SGG UK in the production of float glass are responsibly sourced. SGG UK is the first UK glass manufacturer to be awarded BES 6001 [3] accreditation.

SGG UK also utilises, on average, 30% recycled glass (cullet) in the batch, which has the additional benefit of lowering the melting point of the batch, resulting in a more efficient process.

Melting and Refining

Within the furnace, a continuous melting process takes place.

The batch is automatically charged into the furnace, and the materials are melted at approximately 1550oC. Whilst in the furnace, the molten glass is homogenised and refined, with bubbles being removed.

The molten glass will exit the furnace at approximately 1000oC

The Float Bath

The molten glass is floated onto a bath of molten tin, forming a perfectly flat surface, and a ribbon that is naturally approximately 6.5 mm thick. The width and thickness of the glass ribbon is controlled by the rate at which the glass is pulled through the tin bath.

The ribbon will be cooked as it travels through the tin bath, leaving at approximately 600oC.

Annealing

The glass ribbon is continually cooled to approximately 250oC, at a controlled rate, in order to prevent the generation of stresses within the glass. Any excessive stress present in the glass may cause fractures whilst cutting, handling or processing. The annealing process allows the glass to be cut and worked without such issues.

Cutting

The glass monitored using an online defect detection system, and is cut to optimise around any defects present. The resultant glass is produced in 3 main sizes, plus standard stock sizes;

Size Name

Other References

Dimensions (mm x mm)

Plateau Largeur Fabrication

Jumbo/PLF

3210 x 6000

Dimension a Largeur de Fabrication

LES/DLF

3210 x 2550

3210 x 2250

Standard Stock Size

End Cap/SSS

Variable

Cut glass is then stacked for shipping to customers, coating or laminating.

Physical Properties

Annealed glass is an elastic material that does not exhibit any permanent deformation as a result of applied forces. However, due to its brittle nature, if subjected to excessive levels of stress, it will fracture.

The below properties are considered typical for Soda-Lime-Silicate glass, manufactured in accordance with EN 572-9:2004 and EN 572-1:2012 [2, 1].

Characteristic

Symbol

Value

Density

ρ

2500 kg/m3

Young’s Modulus

E

70 GPa

Poisson’s Ratio

μ

0.2

Coefficient of Linear Thermal Expansion

(20–300 oC)

α

9 x 10-6 /K

Thermal Conductivity

λ

1.0 W/m.K

Specific Heat Capacity

cp

0.72 x 103 J/kg.K

Resistance to Temperature Differential1

 

40 K

Characteristic Bending Strength2

 

45 N/mm2

Note 1: The resistance to a temperature differential is dependent on the quality of the cut edge.

Note 2: The characteristic bending strength applies to quasi-static loading for short durations, with a 5% probability of breakage at the lower limit of the 95% confidence interval.

 

Heat Treated Glasses

Glass can be heat strengthened [4] or thermally toughened [5, 6] in order to generate compressive surface stress and increase the resistance of glass to surface tensile stresses and thermal stresses.

Heat treated clear float glass has the stated properties in accordance with their associated standards;

Characteristic

Heat Strengthened

Thermally Toughened

Resistance to Temperature Differential1

100 K

200 K

Characteristic Bending Strength2

70 N/mm2

120 N/mm2

Other properties, such as density, Young’s modulus, etc, will remain unchanged.

 

Referenced Documents

[1]

European Committee for Standardization, EN 572-9:2004 - Glass in building. Basic soda lime silicate glass products. Evaluation of conformity/Product standard, CEN, 2004.

[2]

European Committee for Standardization, EN 572-1:2012 - Glass in building. Basic soda lime silicate glass products. Definitions and general physical and mechanical properties, CEN, 2012.

[3]

BRE, BES 6001 - Responsible Sourcing of Construction Products, BRE, 2014.

[4]

European Committee for Standardization, EN 1863-2:2004 - Glass in building. Heat strengthened soda lime silicate glass. Evaluation of conformity. Product standard, CEN, 2004.

[5]

European Committee for Standardization, EN 12150-2:2004 - Glass in building. Thermally toughened soda lime silicate safety glass. Evaluation of conformity/Product standard, CEN, 2004.

[6]

European Committee for Standardization, EN 14179-2:2005 - Glass in building. Heat-soaked thermally-toughened soda lime silicate safety glass. Evaluation of conformity/product standard, CEN, 2005.