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Auto Windshields that are Tougher
By gguild | July 21, 2008
Auto windshields that won’t get broken
Ouch! I just caught another rock!
Just after the road debris hits the auto windshield, most of us have a thought occur to us similar to “We can send a ship into outer space so why can’t we find auto windshields that won’t get broken?”
The simple answer is that we could, but for one of several reasons most people wouldn’t like it. Ironically, much of the technology that would allow such auto windshields comes from the space-age exploration. The time for commonly-used, break-proof auto windshields is not yet upon us, but we have learned a lot about adapting auto windshield glass to be much more effective in specific situations.
Bullet-resistant glass vs auto windshields glass
Bulletproof glass is a really a misnomer as glass is not bulletproof and what is bulletproof is not glass. Bullet-resistant glass is a more appropriate term. Probably not too surprisingly, bullet-resistant glass is made in much the same way as normal, modern auto windshields with only slightly different materials and techniques. First of all, both bullet-resistant and windshield replacement glass is actually a sandwiching of glass and polycarbonate thermoplastic materials.
What is different about bullet-resistant glass?
In auto windshields the polycarbonate layer is laminated between two pieces of glass. You may recognize a few of the polycarbonate brand names such as: Lexan, Cyrolon and Tuffak. In bullet-resistant glass, there could be several sandwiches of glass and polycarbonate. The polycarbonate material in bullet-resistant glass is usually a tougher form such as polyvinyl butyral, polyurethane, or ethylene-vinyl acetate. Instead of single layers, the layered pairs may be stacked four, five, or more times. Military grade bullet-resistant glass is typically up to 5 inches thick, or about 20 times thicker than normal, auto windshields.
Does windshield glass and bullet-resistant glass function the same way?
As a matter of fact, they work almost exactly the same way. The major difference in whether the auto glass can stop a projectile is one of mass—first, of the glass itself, and then the mass and speed of the projectile. Think of auto windshields as made of rock-resistant material as opposed to bullet-resistant material. It is a matter of degree. The larger the caliber of bullet (mass), the higher the velocity (speed), the more resistance will be needed to provide the stopping power required. It wouldn’t make sense to make auto windshields any thicker or heavier than is necessary to resist the projectiles normally encountered in normal use.
Why can’t we just install bullet-resistant windows in our cars?
Price is one reason. Another is that it doesn’t shape and curve like regular auto windshields. The thicker layers have to be custom made and molded to the outer, curved glass layer. Another reason is weight. With so much glass and polycarbonate that can be several inches thick, mounting the glass to a molded frame becomes extremely difficult. Without a tight, secure molding the auto windshields integrity is compromised. Essentially, the car would have to be designed around the auto windshield.
Why does the outer layer have to be glass that breaks? Couldn’t it be the polycarbonate, or Lexan, material?
It could, but that isn’t the ideal situation for automobiles. For one thing, in the hot, summer sun, plastics craze, fog, and yellow and even cleaning a windshield adds to that. This cuts visibility considerably. Second, windshield repair is not possible, as the outer layer of glass breaks outward as the polycarbonate flexes underneath, so striking it is more prone to shattering the outer layer.
What might be next in the future of windshield replacement?
We probably won’t see a standard automotive application until the era of transparent metals and ceramics, such is aluminum oxynitride (ALON)—a transparent ceramic made up of aluminum, oxygen and nitrogen—becomes practical. ALON is currently under development by the United States Air Force as a much lighter and much stronger bullet-resistant material. At the present time, ALON is extremely expensive to produce.
Many successful, specialized applications for auto windshields and other laminated safety glass demonstrate the strong advances made in making better and tougher glass. Anyone could find what he or she wants in damage-resistant glass windshields but often the weight, thickness or expense makes it impractical to use.
For now, our auto windshields are subject to breakage by the simplest and smallest of road debris given the right circumstances. You must trust your NGA certified windshield replacement technicians to keep you protected from the road debris.
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