Automotive Windshield, Car Window Glass & Safety Glass

Silica glass is used for manufacturing automotive windshield and windows.

Typical approximate composition of automotive windshield:

72% SiO2 as vitrifier
14% Na2O as flux
10% CaO as stabilizer
4% MgO as stabilizer

Car window is made of flat glass which is poured using the float glass process. Fusion occurs at a temperature of 1550 degrees Celsius; the melt then passes through a refining zone of 1500 degrees C to 1100 degrees C. The melt is then poured onto the so-called float bath (liquid tin) and is cooled to a temperature of 600 degrees C. It is at this stage that the two plane-parallel surfaces of exceptionally high-quality are produced (tin bath surface below, fire-finished surface above). After passing through a cooling zone, the glass is then cut to pieces of 6.10 m x 3.35 m.

The glass can now be further processed in one of two ways:

- As single-pane toughened safety glass (TSG) or
- As laminated safety glass (LSG) which is primarily used for automotive windshield.

TSG car glass window differ from LSG glass windows in that they exhibit higher mechanical and thermal strength as well as different breakage and shattering behavior. TSG glass panes pass through a toughening process which greatly pre stresses the surface of the glass. In case of breakage, these glass windows break into many small dull-edged pieces.

The LSG car window, on the other hand, exhibits a normal shattering pattern. When an LSG car window is cracked, it is easier to see through than a TSG auto window glass. Laminated glass is made of two panes of glass bonded to each other by means of an intermediate film or plastic (polyvinylbutyral).

This type of window design helps to reduce danger of injury because the glass shatters into many small, blunt pieces which are held in place by the plastic film. In the case of the Sekuriflex window (a refinement of the LSG window), an additional plastic film with excellent optical and mechanical properties (e.g., abrasive and stratch resistance) is applied to the inside surface of the window.

Green or bronze-colored windows are used for heat absorption because they block the transmisson of infrared light more strongly than light of shorter wave lenghts. On the other hand, light transmission in the visual spectrum is reduced from 90% to 80%.
 

Optical properties

The optical properties of TSG and LSG car window is roughly the same, because the optical properties of the PVB film are very nearly the same as those of window glass.

Heatable safety glass

In the case of heatable safety glass (TSG), heating conductors are applied to the car window (not yet prestressed) using the silk-screen process, and are sintered in place during the prestressing process. Subsequent galvanization increases the strength of the heating conductors, tempers them and protects them against environmental influences. Window areas treated in this way can be kept free of mist by heating, and even of ice if the heating power is high enough.

Heated window areas must never be cleaned using caustic or scouring cleansers (e.g. chlorine, ammonia, sand or acids).

In the case of LSG car windows, very thin, nearly invisible heating wires are applied in wavy lines to the plastic adhesive film. They are connected in series and/or parallel in order to achieve the intended electrical resistance. The thin wires provide even better heating coverage. The heating power of passenger car rear window heating systems is approximately 3...5 W/dm2.