The physics of windshield repair

As a windshield repair technician, you know how to repair a windshield break, but do you know what causes the glass to break in the first place? Understanding the science behind windshield repair can help you not only perform successful repairs but also educate your customers about the process.

What makes glass break?
Several forces and contributing factors cause glass to break. First, let’s look at the stress associated with flex. In a car or truck, the windshield flexes along with the vehicle body. Under normal conditions, this flex does not cause the glass to crack and fail. For the glass to break, other factors must contribute.

Next, the stress of an object hitting the windshield sometimes forms a chip. Other times, the glass remains intact.

Uneven heating and cooling also can cause glass to crack. Known as thermal shock, this too can result in significant damage or no damage at all.

To understand these inconsistencies, we need to look at the surface of the glass microscopically. Under stress, microscopic surface cracks in the glass grow until they reach a critical size, at which point the glass fractures catastrophically. Humid environments and moisture reduce glass strength even more. In a weakened state, glass can fail without warning if subjected to stress.

As a result, some areas of the glass are weaker than others. This explains why a small stone can chip a windshield in one area, while a larger rock may cause no damage to another.

Evacuating the damage
To perform a successful windshield repair, you must evacuate the damage of air and moisture. Then, you must fill all areas of the break with resin. This may seem pretty straightforward, but it is not as simple as it sounds.

Let’s look at the stone chip pictured above. There is an impact point, a cone of glass and small cracks or legs radiating from the damage center. You must remove the trapped air in all of these areas to allow the resin to flow into them. Ideally, you should remove the air from the break before any resin comes in contact with the damage. If this is not possible, you must pull the air through the resin. To do so, the force acting upon the air must be stronger than the surface tension of the resin in its way.

It is also crucial that you remove excess moisture from the damage before any resin enters the break. Excess moisture can not only weaken the glass but also cause the repair to fail over time.

Choosing a resin
When evaluating windshield repair resin, look at it from a performance standpoint. First, the resin must be able to enter and reach all areas of the damage. It should also have enough tensile strength to withstand any flexing, expansion or contraction the windshield may experience.

Researchers agree that the theoretical strength of soda-lime glass—the type used in windshield construction—is 1 million to 2 million PSI. However, the actual tensile strength is around 10,000 PSI due to micro-cracks and other surface flaws.

Windshield repair resin needs to approach these tensile strengths or have enough strength to withstand the expansion and contraction of the glass. Two characteristics make up the total tensile strength of the resin: adhesive strength and cohesive strength. Adhesive strength is the ability of the resin to adhere to the glass, and cohesive strength is the resin’s ability to hold itself together.

Another important characteristic is the resin’s refractive index after it has cured. The refractive index determines how much a light wave “bends” when entering and leaving the surface of the resin. Ideally, it should be the same as that of the windshield glass, so that any light passing through the resin and glass acts the same as if it were passing through glass only. This makes the resin in the damage invisible.

In a perfect world, the refractive index of the windshield and resin would match exactly, but in reality, it varies from windshield to windshield due to their different densities. The typical range is from 1.52 to 1.62. This makes it difficult to match the refractive index of the cured resin to the glass exactly, and why you may still be able to see the resin after it has cured.

Curing the resin
Resin cures as the result of a photochemical process in which monomers harden or cure upon exposure to ultraviolet radiation. A specifically formulated monomer will bond with other monomers—or polymerize—when it is exposed to UV light and isolated from oxygen molecules. This UV “curable” monomer has a photo initiator that absorbs UV energy and initiates a polymerizing reaction in the monomer. In order for this reaction to take place, the UV radiation must be in the correct wavelength—usually 365 to 380 nanometers—and be of sufficient intensity to completely polymerize the resin within a few minutes.

For the purpose of resin curing, intensity is measured in microwatts per square centimeters, µW/cm2. The sun is capable of producing UV radiation in the correct wavelength to cure the resin. However, its intensity varies depending on the time of day, season, amount of particulates in the air, cloud cover and other variables.

Different resins cure at different rates and require varying levels of intensity. It is important to use a UV lamp with a known and consistent intensity and to match that to the resin you are using. Consistency is important, otherwise, you won’t know when the resin is completely cured, and a weak repair will result.

An exact science
While many claim that windshield repair is not an exact science, the facts say otherwise. If you take the time to research the science, and change your technique, equipment and materials as a result, you will only improve your quality of service.