Curiosity made me do it...interesting read.
"History of Auto Glass
Prior to 1919, “horseless carriages” were very few in number and operated at relatively low speeds. When they first appeared on the scene, they weren’t equipped with windshields. For protection against bad weather, insects, and other road debris, drivers and passengers alike used goggles. Just imagine driving in your car today without a windshield. The wind in your face, blowing through your hair, while you wisss down the road at high speeds. Sounds pretty neat huh.
As motorized vehicles grew more common and their speed increased, the wind and debris thrown into the faces of drivers became a more serious matter. To alleviate this concern, manufacturers included glass as, literally, a wind shield. While the new windshields made the drive more comfortable, they did not adequately protect from flying debris. These first windshields were hand cut from plate glass. The problem with these plate glass windshields was they easily shattered into large, dangerously sharp pieces upon impact.
In 1919 Henry Ford addressed the problem by using a new technology, developed in France, called glass laminating. Windshields made using this process were actually two layers of glass with a cellulose inner layer that held the glass together. Between 1919 and 1929 Ford ordered the use of laminated glass on all of his vehicles.
Today, windshields are no longer held together with cellulose, but with a high-strength vinyl called polyvinyl butyral (PVB). This type of glass is ideal for automobile windshields because of it's strength and reliability.
Today most windshield damage is caused by small rocks that get caught between the treads of vehicle tires. As the car builds up speed these rocks are flung into the air... and onto your windshield. When this happens the glass, depending on the angle of the hit and how fast you are going, can become 'dinged'. Upon impact, even if the glass shatters, it sticks to the inner plastic laminate instead of showering the occupants with pieces of flying glass. The laminated glass also provides a cushioning effect during collisions. Because of its strength, this plastic layer absorbs much of the shock upon impact and keeps the occupants from being ejected through the windshield.
The glass in the rest of the car is different. Around the 1950's the door glass and the back glass changed to a tempered glass. It is just one piece of glass that is sent into an atmospheric oven that heats and quenches the glass to harden it. This tempered or “toughened” glass is also considered safety glass. It is strengthened through the application of heat and pressure. Upon impact it crumbles into rounded glass pebbles instead of shattering into large dangerous pieces.
Glass is very susceptible to quick changes in temperature. Rapid expansion or contraction will cause it to crack. This can happen in the winter when your defroster rapidly heats a frozen windshield, or in summer when a hot windshield comes in contact with the cold water of a car wash.
No matter how it gets there, if you have either a 'ding' or cracked windshield, your safety is being compromised. Even though the windshield won't cave in on you, it isn't exactly safe. All cars must pass roll over tests, and as the car manufacturer lightened the weight of the vehicle, they made the windshield an important factor in roll over safety. If you have a cracked, or 'dinged' windshield you are compromising the structural integrity of your vehicle.
Let me give you an example of this that your science teacher might have used back in school. Take an egg. Place two fingers on the top with your thumb on the bottom of the egg. Now squeeze with all your might... what happened... nothing right? Mother nature created the perfect non-breakable structure so when the mother hen sat on her nest the eggs wouldn't break. Okay... now take a needle and poke a hole in the egg... you have just compromised it's structural integrity. Next, hold the egg in your hand like before and squeeze hard... :-) You probably have egg all over your hand... right? If your windshield has a crack or rock chip in it, the structural integrity has been compromised much like the egg with a needle hole in it.
The good news is that laminated safety glass can usually be repaired, however, while scratches can be buffed from tempered glass, it cannot be repaired. The reason for different types of glass in vehicles today is safety. We can not have tempered glass for windshields on the roads in the United States. Although harder to break than a windshield, tempered glass can be very unstable. If the skin is broken, it might explode in your face while you are driving. The real reason for this product is in case of rollover. It is very difficult to cut through laminated glass. So if you are ever trapped in your car, its easier to get through tempered glass than laminate.
Today’s windshields have evolved into complex, engineered glass systems with heating elements to melt ice, or, coatings to repel ultra-violet radiation. Shade bands, radio & telephone antennas, satellite uplinks, heads-up holographic instrument displays, or even sensors to activate windshield wipers or close convertible tops are now common.
The modern windshield is also thinner and lighter to reduce overall vehicle weight and thereby contribute to increased fuel economy. In short, consumer demands have influenced engineers to design enhancements into the glass and, in so doing, have made windshields more costly to produce.
Increased windshield performance comes at the cost of higher prices. In the early 1990's, increased prices created an economic incentive to repair rather than replace windshields. Prior to 1990, fewer than 500,000 damaged windshields were repaired every year. Today millions of Americans chose repair annually. The reason… average windshield replacements cost between $250 and $1,000; the average repair is between $50 and $75, depending on the number of 'dings'. "
