Automotive glass is prone to breakage on impact during an accident or from airborne objects.
However, it is probably the only component on a car that does not succumb to wear and tear. Automotive glass only ever needs replacing if it is broken.
HOW IT’S MADE
The raw material of glass is silica or silicon dioxide, available in abundance from sand.
Silica is heated to beyond 1500 deg C before it becomes a liquid and then cast over a layer of molten tin.
The liquid silica floats on the tin to form an even, flat layer, which is then cooled to create the solid glass panel. The float glass, as it is known, is subsequently cut into the curved shapes as required.
In the past, automotive glass was similar to that in the windows or facades of buildings. However, there were dangers whenever the glass shattered during accidental breakage.
The resulting shards of broken glass can be sharper than knife blades. This makes them potentially life-threatening projectiles when the fragments fly into the car’s cabin.
MODERN AUTOMOTIVE GLASS
Automotive glass today is of a variety categorised as “safety glass”. However, this does not mean it is immune to breakage from bending or foreign-object impact.
The term is used because of its drastically reduced potential to cause bodily cuts.
Hence, safety glass is a mandatory requirement for the windscreen and windows on every motor vehicle. There are two basic types of automotive safety glass.
The more common “toughened safety glass” is made from a single glass pane. When molten silica is cast, metal oxides in precise portions are added. This enhances the physical properties of the material.
After this, the automotive glass is cut, ground, shaped or (where required by design) drilled for mounting brackets. Then, it undergoes a secondary thermal process that prestresses it.
This results in a structure with evenly distributed internal stresses.
Once toughened, this automotive glass cannot be cut or drilled. It is also not easy to break or damage.
WHAT HAPPENS WHEN IT’S HIT?
Toughened glass, when subject to impact or torsion, will shatter into small fragments with blunt edges.
You can actually scoop up pieces of the broken safety glass with your bare palms and not end up with a single scrape.
Try throwing a brick at a windscreen – the brick is likely to bounce back! “Laminated safety glass” is the other variation of automotive glass, and it’s usually used for windscreens and glass sunroofs.
The lamination in question comprises a clear or coloured (tinted) polymer resin called polyvinyl butyral or PVB sandwiched between two glass sheets.
This glass is bonded to the flexible PVB that is inherently resistant to cracking. So, any impact on either glass surface will not result in loose glass fragments.
Even if the entire glass surface is shattered, the PVB maintains its bonding with the glass pieces.
Laminated glass is now the standard for windscreens. Some high-end cars utilise a variation of the laminated glass that substitutes the PVB layer with a vacuum.
The gap acts as an insulator against both heat and noise. The two layers of toughened glass are adhesive-bonded around the edges.
Adhesive-bonding is also the modern method of attaching (front and rear) windscreen glass to the frame of the car body.
Because glass is inherently very stiff, it contributes to the overall stiffness of the structure, specifically the roof and its supporting pillars.
A more complex version of the laminated glass is used in the making of bullet-proof glass. Certain companies offer specially built armoured cars for VVIPs in politics or business who are at risk of being shot at.
Bullet-proof glass will prevent the penetration of ammunition from most firearms. It is made by building up multiple layers of polymer materials and glass.
Hence, the windscreen and window panels could be as thick as 35mm.
It requires extensive and costly modifications to the mounting frames and doors. But to those who need the extra protection on the road, it would be money well spent.