Why Car Windows Are Not Shattered?
Different methods are applied to increase the durability of the glasses and to prevent the glass pieces from being dangerous. In the production of these materials, which are called safe glass, two methods are mostly preferred. In the first method, a thin film layer made of polyvinyl butyral, a polymeric material, is placed between the glass sheets. This process prevents the dangers by keeping the parts together even if the glass is broken. It also allows the glass to break more difficult.
In the second method, the glass is first heated to temperatures above the glass transition temperature. The glass transition temperature can be defined as the temperature at which amorphous materials become harder and more brittle than fluid when the atoms do not form a regular crystalline structure. The heated glass is then cooled down quickly. This process is called Tempering.
While the tempered glass can withstand 300 ° C thermal shock, the non-tempered glass can break when it is exposed to about 40 ° C thermal shock. However, in this process, the temperature is not the same all over the material. While the temperature of the outer part of the glass is below the glass transition temperature, the temperature inside the glass is higher than the glass transition temperature. For this reason, the fluidity of the outer surface of the glass is lower than that of the inner one. In addition, unlike ordinary glasses, the moment they break is not ice with salt. They break into relatively large pieces.
Then all of the material is cooled to room temperature. At this stage, since the change in the temperature of the interior is higher than on the outer surface, the interior becomes more tightened than the surface. The difference between the forces acting on the inner and outer parts of the glass causes small and unsharpened pieces to form when the glass is broken.