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Some basic information on
architectural glass:
Chemically strengthened glass
is a type of glass that has increased strength. When
broken it still shatters in long pointed splinters similar
to float (annealed) glass. For this reason, it is not
considered a safety glass and must be laminated if a
safety glass is required.
Chemically strengthened glass is typically six to eight
times the strength of annealed glass.
The glass is chemically strengthened by submerging the
glass in a bath containing a potassium salt (typically
potassium nitrate) at 450 °C. This causes sodium ions in
the glass surface to be replaced by potassium ions from
the bath solution.
These potassium ions are larger than the sodium ions and
therefore wedge into the gaps left by the smaller sodium
ions when they migrate to the potassium nitrate solution.
This replacement of ions causes the surface of the glass
to be in a state of compression and the core in
compensating tension. The surface compression of
chemically strengthened glass may reach up to 690 MPa.
There also exists a more advanced two-stage process for
making chemically strengthened glass, in which the glass
article is first immersed in a sodium nitrate bath at 450
°C, which enriches the surface with sodium ions. This
leaves more sodium ions on the glass for the immersion in
potassium nitrate to replace with potassium ions. In this
way, the use of a sodium nitrate bath increases the
potential for surface compression in the finished article.
Chemical strengthening results in a strengthening similar
to toughened glass, however the process does not use
extreme variations of temperature and therefore chemically
strengthened glass has little or no bow or warp, optical
distortion or strain pattern. This differs from toughened
glass, in which slender pieces can often be significantly
bowed.
Also unlike toughened glass, chemically strengthened glass
may be cut after strengthening, but loses its added
strength within the region of approximately 20 mm of the
cut. Similarly, when the surface of chemically
strengthened glass is deeply scratched, this area loses
its additional strength.
Chemically strengthened glass was used on some fighter
aircraft canopies. |
