Laminated Glass and PVB
Laminated glass is made by sandwiching a layer of polyvinyl butyral (PVB) between two pieces of glass. The glass and the PVB are sealed by a series of pressure rollers and then heated. This combination of pressure and heat chemically and mechanically bonds the PVB to the glass. The mechanical bond occurs through the adhesiveness of the PVB, while the chemical bond is created through hydrogen bonding of the PVB to the glass.
That inserted layer of PVB is what allows the glass to absorb energy during an impact and gives the glass resistance to penetration from flying projectiles. It also deflects up to 95 percent of ultraviolet (UV) rays from the sun [source: Reuters]. Laminated glass can break and be punctured, but it will stay intact because of its chemical bond with the PVB.
The strength of laminated automotive glass allows it to perform two very important functions in cars. First, it allows the passenger-side air bag to deploy correctly. Driver's side air bags tend to fly straight toward the driver from the steering wheel, but when the passenger air bag is deployed, it bounces off the windshield toward the passenger. An air bag deploys with incredible speed --1/30th of a second -- and can withstand 2,000 pounds (907 kilograms) of force. The windshield has to absorb both the speed and force of the air bag in order to protect the passenger in an accident. Because of its strength, laminated glass can keep occupants inside the car during an accident. In the past, occupants could be ejected through the windshield because the glass wasn't strong enough, but today's windshields provide more security.
In addition to absorbing the force of deployed air bags and keeping passengers inside the vehicle, laminated windshields also provide strength to a car's roof. Windshields keep the roof from buckling and crashing down on passengers completely during a rollover. Without the rigidity and strength of laminated glass windshields, many roofs would pose greater risks to passengers in certain kinds of accidents.
If you happen to find a small chip in your windshield, don't worry. You don't have to go out and get your entire windshield replaced to retain its strength. Some small chips can be quickly and easily repaired with a windshield chip repair kit. Most automotive stores carry these kits for around $10 and allow you to inject resin into the trouble spot and remove excess air in the problem area. Once you've repaired the glass, you'll barely be able to notice your patch job.
Let's move on to the next page to find out how tempered glass is made and how it protects.