One of the most reliable thermochemical cycles in the world, this process requires an in-depth knowledge of chemistry, rooted in sectoral studies and scientifically proven facts. Talking about chemical strengthening by ion exchange is like “delving” into a chemistry book and browsing the pages packed with formulas and principles stemming from cutting-edge studies. But let's take a step back: why is chemical strengthening used? When glass is subjected to tensile stress, the presence of microscopic cracks on the surface intensifies the tension at the extremities, thus extending the cracks and causing the glass to break. These cracks are the main reason for the low mechanical resistance of glass to stress, which is why it is important to prevent the cracks from forming in the first place.
How to prevent window cracking
Chemical strengthening by ion exchange is not necessarily the only choice. Once we have understood that a prestressed surface layer needs to be introduced into the glass to make it more resistant, this can be achieved in two ways: thermally or chemically. But while thermal tempering is used at high temperatures, the use of chemistry makes it possible to overcome certain limitations of thermal tempering and therefore obtain a higher performing product. Chemical strengthening is appreciated for its ability to create glass with a reduced thickness, regardless of its intended shape. It also maintains a perfect surface geometry, which leads to significantly higher strengthening levels.
If glass has a specific thickness, has complex curves and needs to withstand higher mechanical stress than normal, then thermal tempering is not the best choice, with chemical strengthening delivering more resistant and durable glass.
Ion exchange
So what is chemical strengthening by ion exchange? With chemical strengthening, glass is immersed in a bath of molten potassium salts at a temperature higher than 380°C, resulting in an exchange between the surface sodium ions present in the glass and the potassium ions contained in the salt. Potassium ions are larger than sodium ions, and this results in the establishment of a system of residual stress characterised by compression stresses on the surface counterbalanced by tensile stresses within the glass. Basically, chemically strengthened glass has a higher surface tension, i.e. a higher resistance than thermally tempered glass. In the event of breakage, thermally tempered glass pulverizes into small, non-sharp fragments, while chemically tempered glass breaks into larger pieces that are less sharp than with untempered glass. But breaking chemically tempered glass requires stresses 10-to-15 times greater than thermally tempered glass, whose tolerated stress is 4-to-5 times greater than normal glass. Given the greater resistance of chemically tempered glass, are you still in doubt about which to choose?