Recently, automotive construction technology has changed dramatically. Over the past few years, advances in technology have made construction of lighter, stronger and more fuel efficient vehicles possible, in ways only dreamt of in the minds of science fiction writers. Plastics, fiberglass, carbon fiber and aluminum are now commonplace in today's automobiles.
Along with the advances in construction techniques and exotic materials, repair technology has been forced to advance as well. Many of these newer materials are glued or bonded to a substrate or substructure. Since conventional glues were not strong enough to withstand the daily rigors automobiles undergo, newer formulas were developed. Many adhesives are developed with specialized functions, and must be used exclusively for the application for which they were developed, or they will fail.When repairing a customer's vehicle, structural failure is not an option. As a repair professional, you must be aware of the proper usage of technology. To understand which adhesives to use for specific applications, you must first understand the basics.
What is glue?
Glue generally refers to proteins prepared from boiled animal tissue, bones and blood, which, when rendered, produce a sticky residue referred to as natural glue. Historically, the word glue has been extended to any type of glue-like, sticky substances that are used to attach one material to another.
Glue is an adhesive compound that adheres or bonds two items together. Adhesives may come from either natural or synthetic sources. Generally, in the automotive industry, synthetic adhesives are the norm.
The first adhesives were plant derivatives. Archaeologists have found 6,000-year-old ceramic vessels that were broken and had been repaired using ancient plant adhesives. The Egyptians used glues to adhere wooden and stone tombs, furniture, ivory and their early versions of paper, papyrus. Glues and adhesives have long been part of everyday life. Modern science, however, has made adhesives even stronger and more versatile through advanced chemistry.There is a huge array of adhesives in the marketplace today. The following are some of the most common examples in the automotive arena.
Cyanoacrylate
Cyanoacrylate is the generic name for substances typically sold under the names Super Glue and Krazy Glue. These glues are very useful in the case of minor tab repairs, like on a tail lamp or mauling. Some of my older technicians refer to this product as "salvage" glue, since they are able to fix so many things with it. Used normally, it dries very quickly, but it will dry almost instantly when used in conjunction with a spray-type adhesion promoter or drying agent.
Again, this type of adhesive is used for minor, non-structural types of repairs. Cyanoacrylate glues will not work well on porous materials, nor will they bridge gaps in the two materials to be glued. For best results with this type of adhesive, the mating parts should fit tightly together before gluing, and should be made from a nonporous material like plastic or metal.
Bonding glues
Bonding glues are, as the name states, used for bonding panels to a specific substrate. They are usually epoxy type adhesives, but can be other types of synthetics like methacrylate or acrylic. These types of glues are extremely strong, and dry by a chemical reaction between the hardener component and the adhesive component. Drying times, or curing times, are affected by temperature, and they usually work best at roughly 77 degrees Fahrenheit (25 degrees Celsius).Panel bonding adhesives usually have working times of about 60-90 minutes, so that the technician will have ample time to apply the adhesive and fit the panel. These types of glues usually cure and set in about four hours. Acrylics sometimes dry much faster, and are better suited for smaller areas.
Different types of adhesives have different bonding characteristics and work better with certain substrates. Always refer to the adhesive manufacturers' recommended usage when bonding panels. Although most of these glues will adhere almost anything to anything else, for safety issues, you should always follow the recommended procedure. Doing so will insure you of quality results. Also, since most major adhesive manufacturers offer a written warranty on their products, following the recommended use and procedure will enforce the warranty in the event of a failure years down the road.
Altering the application will usually get you into trouble. Even if it looks good right after bonding, it may not hold properly. Because bonding requires "bond line control" or a specific thickness of material on a bonded joint, most quality adhesives contain glass beads, which will maintain the proper bond thickness when clamped. The glass beads in the mixture will prevent the joint from becoming too tight when clamped by filling the void along with the epoxy mixture, keeping the joint at roughly 0.005-0.010 inches (0.127-0.254 mm).
Application
Again, make sure to refer to the instruction provided by the adhesives manufacturer. Minimal surface preparation is required for most glues. It's important, however, that both surfaces are clean, dry and free from contamination.
Always use a static mixer when using bonding adhesives. Do not try to mix the components manually and apply them to the panel. A static mixer will insure proper mix consistency, and will allow you to control the adhesive much more easily. A static mixer is merely a plastic tube with an insert inside that looks like a screw. As the material is forced into the tube, this screw-like insert turns, pulling the adhesive together with its other component.
You should always "equalize" the product when using a static mixer, by allowing a portion of the mixed material to flow out of the tube, about an inch or so, before applying the adhesive to the panel. This ensures proper mixture. Because this product dries by a chemical reaction, you will notice it getting hot when drying. This is normal. Use enough adhesive during application to fill the joint. Make sure you mate the two surfaces within the adhesives listed working time.
Remember, the application temperature can alter the working/dry time. If it's really warm, it will tend to dry faster. Keep this in mind so you don't end up with a mess. Once the surfaces are bonded, clean up any excess glue immediately with a solvent. Most adhesive manufacturers sell some type of product for this purpose.
Do not disturb the bonded components after clamping until they are cured. Wait overnight. These products can be force dried, if you want to work with them more quickly. At about 150 degrees, most bonding adhesives will cure in about an hour. Be careful with these products, however, as they are flammable in their uncured state.
Follow Inter-Industry Conference On Auto Collision Repair (I-CAR) recommendations when bonding panels, apply the glue to the areas recommended and weld the remainder. Most of these adhesives can be used in conjunction with traditional welding methods.
When using these materials, always follow the recommended safety precautions. These glues contain powerful chemicals that should be handled with the proper caution. Breathing their fumes can be dangerous.
Urethane adhesives
Another useful adhesive is urethane adhesive. It is mainly used for repairing plastic components like bumper brackets and tabs, headlamps, grilles or backing panels in bumper repair systems. It also may be used for emblems, molding and mounting.
The drying time for urethane adhesives is usually much faster than the epoxy glues, approximately 30 seconds of working time. Setting time is about 30 minutes, with a full cure in about an hour. This super-fast dry time makes it a great choice for most minor plastic repairs.
It is heavily bodied, and unlike Krazy Glue, will fill gaps as well. Although it is mixed in the same manner as an epoxy or acrylic, it requires different surface preparation than those adhesives do.
Application
Generally, the surfaces to be joined must be cleaned with soap and water first. Once dried, they should be cleaned with plastic prep or cleaner. The areas to be joined are then sanded with 180-grit paper, and then re-cleaned with the cleaning solvent after sanding is completed. If the product being repaired is made from thermo plastic, such as TPO or EPDM, it may need to be treated with an adhesion promoter prior to bonding.
Identifying the type of plastic a component is made from is very important in determining the proper repair methodology. Most adhesive manufacturers can offer you tips on identifying a component's makeup. In fact, they have wall charts with breakdowns of different plastics and their identifiers, which they will give you if you ask for them. Again, revert back to manufacturer recommendations to assure a proper bond.
Once prepped, dry fit both components that are to be bonded. Once satisfied with the fit, apply the product to one side of the items to be glued. Quickly bond the two surfaces and clamp or hold them together until the adhesive is set.
All of these products will have a much greater shelf life if they are kept in a cool storage place. Exposure to extreme heat, like a shop in the summer, substantially lessens their shelf life. Store them in your office if it is climate controlled.
Final word
There are various miscellaneous glues that are used every day in a collision shop, such as weather-strip adhesives, double-sided tapes, trim adhesives and emblem glues. These products are usually one component, "old school" synthetics that are easy to use, but not very strong or wear-resistant in comparison to the more modern chemical concoctions.
With the improvement in glues, we may see some radical changes in the collision repair industry in the future. It is not out of the question to assume more and more structural panels can and will be bonded, and the potential exists for entire vehicles to be bonded together. As unbelievable as this sounds, it isn't really that far-fetched.
The strength a glued joint can achieve is phenomenal. In actuality, the glued joints using today's technology, in many cases, are stronger than the material being bonded. If we think about how rapidly the modern adhesives have evolved, just think of what can happen in the next ten years or so. Many other industries have embraced adhesive technology, and are immersed in it. We are just touching the tip of the iceberg in our field. So get your glue guns ready.
