How Automotive Glass Works

During the early 20th century, horseless carriages started using glass to protect drivers from harsh winds. However, the standard form of glass used in those times didn't adequately protect occupants from flying debris. It also posed a risk to the occupants if an object struck the glass or if the vehicle was involved in an accident.

In 1903, French chemist Edouard Benedictus stumbled upon the secret to shatter-resistant glass when he dropped a glass flask filled with a dried collodion film. He found that the glass coated with the film cracked but kept its original shape. However, this laminated glass wouldn't be implemented in automobiles until the 1920s [source: Time].

Automakers used laminated glass in their windshields to optimize occupant safety during accidents and to protect passengers from projectiles during normal driving conditions. For all its benefits, though, the first types of laminated glass offered limited puncture resistance. Today's laminated glass consists of a thin layer of polyvinyl butyral (PVB) inserted between two layers of solid glass.

In addition to laminated glass, automakers began to use tempered glass in the late 1930s. This type of glass is used in the vehicle's side and back windows and gains its strength through a heating and rapid cooling process that strengthens the glass' outer surface as well as its core.

By the 1960s, the American public had become increasingly aware that automobiles needed to be designed for more than just looks. This realization derived partly from consumer crusader Ralph Nader's work exposing the dangers posed by certain vehicles and the need for government safety standards. In response, the U.S. government formed the National Highway Traffic Safety Administration (NHTSA) in 1970, which President Lyndon Johnson had signed into law in 1966 [source: Zanona].

Since then, NHTSA has implemented regulations affecting all areas of vehicle safety, including automotive glass. Some of the Federal Motor Vehicle Safety Standards (FMVSS) for automotive glass include:

Now that we know how automotive glass came to be, let's find out how it's made.

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. Laminated glass can break and be punctured, but it will stay intact because of its chemical bond with the PVB. The density of the material also helps it deflect road noise and help quiet down a vehicle's cabin [source: AIS Glass].

The strength of laminated automotive glass allows it to perform two very important functions in cars. First, it allows the passenger-side airbag to deploy correctly. Driver's side air bags tend to fly straight toward the driver from the steering wheel, but when the passenger airbag is deployed, it bounces off the windshield toward the passenger. An airbag deploys in under 30 milliseconds, using about 5 pounds (2.3 kilograms) per square inch 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 to $20, 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. It's crucial to spot and repair a chip as soon as possible. If left alone, a small nick can easily spread and turn into a larger crack, requiring a more expensive windshield replacement.

Tempered glass is just as important to a vehicle's safety as laminated glass, but it differs greatly in both form and function. This type of glass is used for the surrounding windows of vehicles (also called the sidelites) and the back window (or backlite). Tempered glass is created by heating and then rapidly cooling the glass to room temperature by ushering it through a system of blowers.

The surface of the glass cools much faster than the center of the glass and contracts, causing compressive stresses, while the center of the glass expands because of its temperature, producing tensile stresses. What does that mean? Imagine a piece of glass that could be pulled or stretched to a certain length (tensile stress), while being pushed down and compressed (compressive stress) simultaneously. Both the pulling and pushing stresses achieved through the heating and cooling process give tempered glass its tensile and compressive strength. The differences between these two give the glass five to 10 times the amount of strength it originally had.

The edges on a typical piece of tempered glass are very weak. This is caused in part by the rapid release of heat during the cooling phase of the tempering process. To help compensate for this weaker area, the glass is ground down on the edges. When tempered glass breaks, it shatters into small, dull pieces. The differences between the compressive and tensile stresses are what enable the glass to break in this way. The pulling and the pushing of the glass produce a significant amount of energy during the tempering process. When the glass breaks, this energy is released and causes the glass to break into small pieces [source: AIS Glass Solutions].

Because of its strength, tempered glass can withstand the daily use of automobile driving. Without it, our cars would be filled with glass every time we encountered a pothole, got into a fender bender or closed a door.

The modern vehicle is incorporating more glass than ever before, including larger windshields, sunroofs, and even glass panels that replace the roof completely. As such, strength and safety standards have to keep up with the industry. One such innovation in this field is Gorilla Glass, a proprietary laminate material from global glass supplier, Corning. The new compound promises to be both stronger and lighter than traditional laminated windshields, as well as more durable against rock chips and hail damage. Manufacturers like Chrysler and McLaren have begun using Gorilla Glass on their new factory models [source: Corning].

Some new vehicles are also getting large panoramic glass built into their roofs, either fixed in place, or with electric sliding mechanisms. These glass roofs can give passengers a great view out of the vehicle, but they also add to weight, complexity and manufacturing cost [source: AIS].

European glass manufacturer AGC has also begun to implement silver-coated windshields, which help combat extreme hot and cold conditions. In heat, the transparent glass allows visible light to pass through, while the embedded silver particles reflect infrared light away from the cabin. AGC claims that this process can reduce interior temperatures by nearly 60 degrees Fahrenheit on hot days. During winter, the silver also acts as an electrically conductive heating element. This allows the car's climate control system to heat up the entire windshield and melt away ice without any need for intrusive wires. In 2017 , Volkswagen began offering the silver treatment as an option on many of its European market vehicles.

Automotive glass isn't only being designed for safety and comfort. Glass makers and car manufacturers are also trying to find avenues for recycling the glass. Composite materials like tempered and laminated glass have been historically difficult to recycle, thanks to their chemical treatments and use of plastics. However, automakers and glass suppliers have begun to introduce recycling programs for windshields, using specialized machines that can separate the pure glass from its synthetic ingredients [source: Thompson].

Glass manufacturers continue to explore new ideas for making glass stronger, safer and adaptable for new vehicles. You might not think about it much, but our vehicles wouldn't be as safe as they are without modern tempered and laminated glass.