The material in the windshield plays a key role in durability and safety. Glass in windshields must meet safety requirements, but it should also have high impact resistance to stones and other road debris, as well as high scratch resistance to avoid unwanted glass breakage and scratching.
Compared with other glass materials like more typical soda-lime glass, borosilicate glass features high strength for impact and scratch resistance. This makes borosilicate float glass like Borofloat® 33 from SCHOTT a good material for windshields. This specialty glass is manufactured since 1993 in Germany, is globally available and a standard in various industries. The outer glass needs this durability to withstand the impact of debris. To reduce costs, a functional solution is to use borosilicate glass for the outer pane and soda-lime glass for the inner pane.
However, there are challenges in the bending process due to the material property differences between borosilicate and soda-lime glass. Material properties like viscosity and the thermal expansion coefficient play a vital role in glass bending in the heat-treatment process. The viscosity differences make glass creeping happen at different temperatures, meaning borosilicate needs a higher temperature than soda-lime for the same viscosity. At high temperature changes, thermal expansion coefficient differences also affect how different types of glass expand in windshield bending. This can easily cause defects in the visual quality of the windshield, and good control in the bending process is needed from the bending furnace to avoid such defects.