Last update: 06.08.2019
Safety & security
7.

Partially tempered glass

Production is the same as for fully tempered glass, but the cooling process is slower, which means that the stress differences in the glass are lower.

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Production is the same as for fully tempered glass, but the cooling process is slower, which means that the stress differences in the glass are lower. You could even rank heat strengthened glass between float and fully tempered glass. The values for thermo-shock resistance and the flexural tensile strength are proof of this.
Slower cooling down during the production process vastly reduces the potential risk of a spontaneous breakage by NiS inclusions.
 

Breakage pattern

The structure after a fracture occurs is similar to that of a float glass pane. The fracture radiates outward from the point where the impact/fracture occurred, to the edges of the pane. Typically, “islands“ (larger fragments) and “particles“ (small fragments) should be avoided by adjusting the process parameters in order to ensure that the glass remains in the frame after breakage.
The test procedures and limitations on “islands“ and “fragments“ are defined in EN 1863-1.

Important note: monolithic heat strengthened glass is not a safety glass!

Breakage pattern

 

Residual load capacity as component of laminated glass

Due to the fracture characteristics of heat strengthened glass, which are different from tempered glass, a single unit of laminated safety glass made from heat strengthened glass has excellent residual load-bearing properties.
In the event of failure of one of two heat strengthened panes in laminated safety glass, only minor deflection occurs, contrary to the sagging of laminated safety glass made from tempered glass. This is why heat strengthened glass is increasingly replacing tempered glass in laminated glass when increased flexural tensile strength and alternating temperature loading are required.
 

Fragmentation of heat strengthened glass

 

Tensile bending strength (EN 1863-1)

Heat strengthened glass made from float glass
σ = 55 Mpa

Heat strengthened glass made from patterned glass
σ = 55 Mpa

Heat strengthened glass made from enamelled glass, whereby the enamelled side is under tensile stress
σ = 45 Mpa

Thermo-shock resistance (EN 1863-1)

The failure strength of a heat strengthened glass against temperature differences within one glass pane is 100 K (annealed float glass: 40 K). This makes heat strengthened glass suitable for any application where there is increased risk of thermally induced breakage.