Feb. 25 — A windshield can be as important to auto safety as seat belts, air bags and anti-lock brakes.

More than just protection from wind, rain and debris, auto glass can save lives by preventing the driver and passengers from being thrown out of a vehicle in a collision. And, today more than ever, vehicles depend on the glass to add to its structure and strength as a whole. So, for example, the windshield can be an essential part of an air bag system or prevent the roof from caving in.

But installation is key: The difference between a properly affixed windshield and a poor job can be the difference between life and death.

Getting the Job Done Right

The National Glass Association is an industry group that has certified more than 10,000 installers nationwide. 20/20’s Arnold Diaz spoke to experts who suggest sticking with installers certified by the NGA; while it’s no guarantee of getting a quality job, you can at least increase your chances of getting your windshield safely replaced.

To help ensure your family’s safety, follow these tips from the NGA:

- Insist that a strong automotive grade adhesive be used to bond the glass.

- Don’t let the glass be installed on corroded metal.

- Follow — to the letter — the directions given by your technician.

- Drive the vehicle only when the adhesive has fully cured (dried and hardened).

- Always wear seat belts and insist that all passengers wear them too.

Just How Complicated Can It Be?

A good technician must be familiar with the specifications for hundreds of automobile and truck models, and understand the correct replacement glass installation techniques for each. The wrong adhesive, or the right adhesive applied in the wrong amount, can leave a windshield vulnerable to summer heat, winter cold, the stresses of high-speed driving and the sudden impact of collision. Improper handling of the windshield, such as with greasy hands, is another mistake that is commonly made.

What is Automotive Grade Urethane?

It’s the most widely used adhesive. It needs to be strong and flexible, able to bond to smooth and rough surfaces, withstand adverse weather conditions, yet still be workable. Its strength is measured in two ways: tensile and lap shear.

Tensile strength is measured by the pounds per square inch (PSI) of force necessary to pull apart two hard surfaces until the bond fails. If you adhere your coffee cup to the kitchen table and pull it vertically off the surface, for example, the strength needed to separate it from the table would be called tensile strength. Automotive urethane usually has more than 1000psi of tensile strength.

Lap shear strength is also measured by pounds per square inch until failure. To test lap shear strength, objects are pulled in a diagonal fashion. If you were to pull the same coffee cup off the table diagonally, the strength needed to separate it from the table would be lap shear strength. Automotive urethane has more than 500psi of lap shear strength. This force adds to the structural strength of the vehicle body and is especially important in collisions when a rollover occurs.

Flexibility is equally important. Urethane must have flexibility to allow for vehicle body movement. While a car bends and flexes with the roadway, the glass is rigid. So if the adhesive is too inflexible, the glass would break over a rough surface or other expected road conditions.

Urethane must also adhere to different types of surfaces. Glass is very smooth, yet the metal frame of the vehicle is rough. Automotive adhesives must be able to bond, with equal strength, to all of the various surfaces used in the manufacture of motor vehicles.

The Curing Process

One unusual thing about urethane is the way it cures or hardens. Unlike paint or other adhesive products that use a solvent carrier, which dissipates into the air to dry or cure, urethane uses moisture that’s present in the air to cure the product to its strong, rubber-like consistency.

If humidity is low, as may be the case in cold, dry climates like Arizona or the northern states, the cure rate of urethane products may be slowed down. To promote the process, moisture may need to be added. Simply adding water after the installation could give the urethane enough moisture to begin the cure; however, it may be necessary for the installation to take place under controlled conditions, like a heated garage or repair facility, and not as part of mobile service.

There are new adhesives on the market that require no moisture to cure, but they do need some warmth. These products are usually a little more expensive than urethane, but the quick release of the vehicle may sometimes be worth the cost.

Windshields: The Origin

Prior to 1919, “horseless carriages” were very few in number and operated at relatively low speeds. 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 windshield.

But this did not solve the problem completely. While the new windshields made the drive more comfortable, they did not adequately protect from flying debris. Objects would frequently fracture early windshields, causing dangerous glass fragments to shower the occupants.

In 1919, Henry Ford addressed the problem by using a new technology founded 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 PVB must be .030 of an inch thick to provide the strength needed to hold the occupants in the vehicle and the flexibility to reduce injury if a person is thrown into the glass. All currently fabricated windshields must meet this specification to satisfy federal motor vehicle safety standards.