May/June  2001    


The Inside Scoop
Resin Properties and Application

by Paul Syfko

Over the past several years, I have received many questions about resin properties and performance issues.

To begin to understand the function and properties of repair resins, one must first have a basic knowledge of a typical resin formulation. All resins consist of three primary components: oligomers, monomers and photoinitiators. 

The first component and the foundation of any resin comes from the oligomer. The oligomer is a high-molecular, weight-viscous material. Its function is to impart toughness and durability into the finished product. 

The second component of any resin is called a monomer. A monomer is usually low-viscosity and is used to dilute or thin the oligomer to the desired viscosity. The function of the monomer is to control viscosity, improve wetting and increase adhesion. Although both the oligomer and the monomer are liquid, they are 100 percent reactive and do not flash off during the curing process. The third component of any resin formulation is the catalyst. The catalyst or photoinitiator sets in motion a chemical reaction that converts the liquid resin into a solid adhesive. The photoinitiator type and level will determine the curing speed. It is the balance of these three components that yield the desired properties in any windshield repair resin.

Choosing a Resin
There are six basic properties that should be considered when selecting a windshield repair resin. The first two properties, viscosity and wetting, deal with the liquid phase of the resin. 

Viscosity: Property of a liquid that tends to prevent it from flowing when subjected to an applied force. A viscometer is typically used to measure the fluidís response to a force. High-viscosity resins tend to resist flow. 

Conversely, low-viscosity resins tend to flow easily. The application or uses of some resins are viscosity dependent. Generally, repair resins are low-viscosity providing for ease of filling into the damage. However, a high-viscosity resin is useful in pit fill and some edge crack applications. The intent of using a higher viscosity resin is to keep the material in place before you cure it. 

Wetting: The process in which a liquid spontaneously adheres to and spreads on a solid surface. The resinís ability to overcome the surface tension is key to the overall performance of the material. The wetting properties of a resin are many times more important than its relative viscosity. It is possible to have a high-viscosity resin flow into a break faster than a lower-viscosity resin with poor wetting characteristics. A resin with good wetting properties will also have greater adhesion than a resin with poor wetting properties. The wetting characteristics of any liquid can be measured by its contact angle on a given surface. The contact angle is simply how much the liquid forms droplets on the surface.

A Resinís Solid Aspects
The following four characteristics deal with the solid aspects of the resin: 

Cure Time: The cure time is how long it takes for the resin to completely turn from a liquid to a solid. The cure time depends on the following criteria: ultraviolet source, photoinitiator type/level and the glass type. The ultraviolet (UV) source is critical in determining the correct cure time. The sun typically has an output of 2.2 mw/cm2 and most UV lamps have an output of 6-to-16mw/cm2. In either case, both light sources would cure the surface at relatively the same speed. However, the cure or curing at the bottom of the break would be improved significantly by the higher intensity UV output. 

The physical properties of the cured resin are dependent upon the curing source. Some believe that it is more efficient to cure using the sun. Others believe that it is better to use the lamp as a UV source. In either case, you need to be aware of the fact that not all resins are compatible with both curing types. Thus, be sure to check with the manufacturer/distributor to ensure that the curing source you use is optimized for your resin. 

The photoinitiator type and level will also dictate the cure speed and surface cure at a given intensity. Finally, the glass type has an effect on the cure time. Some glass used in the manufacturing of windshields is solar-inhibitive. This type of glass usually has a UV inhibitor blended through out the glass. This inhibitor absorbs most of the UV and slows down the cure process. It is important to note the type of glass you are dealing with to ensure that you are using the best curing option.

Refractive Index: The refractive index is the ratio of the speed of light through one material over that of another. As light passes through most materials, it is bent, absorbed and reflected. A good example of the refractive index is the illusion created when you place a stick into water. It looks as though the stick bends. The human eye perceives this as distortion. 

There are inherent differences in refractive indexes between glass and repair resins. The key is to find a resin that in its solid or cured state closely matches the index of the glass laminate. This will minimize any optical deviation from the glass and provide for improved cosmetics of the finished repair. 

Adhesive Strength: The state in which chemical or physical forces hold two surfaces together. The degree of adhesion between that of the resin and the glass determines the structural strength of the repair. 

Adhesive failure can take place either adhesively or cohesively. Adhesive failure occurs when the adhesive pulls away from the substrate (glass).

Achieving Optimal Adhesion
Cohesive failure is when the actual adhesive pulls apart. There are typically two types of adhesive measurements that should be considered when examining repair resinsótensile and shear strength. Resins that have a high tensile strength are usually rigid and have no flexibility. Resins with high shear strength tend to be more elastic and softer. The application of the resin should determine the proper blend of adhesive strengths. In some circumstances a specific resin should have a high tensile strength while other applications demand high shear strength. 

Shrinkage: Typically, when the resin cures or transforms from a liquid to a solid there is a change in volume. This volumetric shrinkage from liquid to solid is very critical in the windshield repair process. When a stone damage occurs, there is glass separation and a void that needs to be filled. If the repair resin has excessive shrinkage the filled void will partially reappear after curing. It is also important to note that excessive shrinkage will negatively affect the adhesive strength. If you are experiencing breaks reappearing immediately after curing, it is most likely due to excessive shrinkage of the resin.

Weatherability: The durability of a repair is affected drastically by the resinís ability to withstand the environment. The most detrimental environmental effects are short wave UV light, extreme temperature changes and water intrusion. Exposure to a short wave UV source can degrade many resins. Degradation can take place as yellowing, chalking and even disintegration. The resin formulation should compensate for environmental effects. Excessive temperature changes can also negatively effect repair resins.

Change of Seasons
When temperatures change, there are differences between the expansion/contraction rates of glass and repair resins. This expansion/contraction rate may cause the resin to fail either adhesively or cohesively. Water intrusion can also be devastating on repair adhesives. The adhesion of some resins is negatively impacted by the presence of water. Water can also be adsorbed into some resins making them cloudy or milky looking. In both cases water can lead to failed repairs. 

There is a great amount of knowledge and technology that is involved in the formulation of a windshield repair resin and there is a fine balancing act between desired physical properties and performance requirements.

Note: The purpose of the preceding article is to provide a basic understanding of windshield repair resins and how they function rather than an outline for any specific design parameters.

Paul Syfko is the operations manager for Glass Medic of Columbus, Ohio.