Assembly EndUser: Crash Course on Choosing the Right Adhesive
There is little argument that using automotive adhesives is becoming a popular collision-repair method for metal bonding, plastic bonding, metal-to-plastic bonding, plastic repair, seam sealing, and foam application. All provide reduced repair cycle time, increased corrosion resistance and the reassurance of returning customers' vehicles back to pre-accident condition. But, with all of these repair options, how does one choose the right adhesive for each application? By gaining an understanding of each option and its intended use, selecting the right adhesive can be easy and efficient.
Hit the BooksIt may sound trite, but the obvious first step to understanding adhesives lies with gathering information. Begin by asking your paint and body supply representative for product catalogs and brochures; this literature will explain the various types of materials. Also, visit adhesive manufacturers' websites - many are loaded with useful tips - or call their technical-support hotlines for a more personal approach. While reviewing these materials, you may also learn about hands-on "how to" clinics. This type of interaction is an excellent way to get educated quickly on automotive adhesives.
Back to the Basics: ChemistryMany adhesive manufacturers spend vast resources on laboratory and crash testing of their products, as well as on hands-on collision-repair trial work. They engineer their repair-specific products to perform undetectable, long-lasting repairs, and they have the data to back it up. However, though some adhesives are capable of performing a variety of different jobs, it is recommended to use the best product for a specific repair. Since there is no one product that meets all needs, it is important to clearly understand each product's merits. As such, it is imperative to work with OEM-approved products to develop the best solution for each vehicle.
Understanding the type of chemistry being used is important as well. Acrylics, urethanes and epoxies are the most widely used collision-repair adhesives, but all have unique attributes. As one of the strongest adhesives (with shear strength at over 3,000 psi), acrylics are tough, require only minimal surface preparation and are an excellent choice for metal bonding. Urethane products are also strong, with 2,000+ psi shear strength, and are flexible. Further, urethanes work with most repair applications (with the exception of structural composite plastic repair), and some are sandable. Epoxy adhesives work well at both high and low temperatures, have similar shear strengths when compared to urethane products, and do a good job of filling gaps. Despite having a reputation for being rigid, epoxies perform well for undetectable structural repairs on composite plastics like SMC and fiberglass. Epoxies can be used with most other collision-repair applications as well.
With a basic understanding of product resources and associated chemistry, the next step is to review applications for automotive collision repair, specifically metal bonding, metal-to-plastic bonding, plastic bonding, plastic repair, seam sealing, and foams.
Metal BondingMetal-adhesive bonding is a method that has been approved by several automotive companies for non-structural metal joining. In fact, a recent GM technical bulletin recommends metal bonding as an alternative to welding. Chrysler and Ford have repair recommendations using metal joining adhesives with door skin replacement and also promote these types of adhesives when weld bonding other non-structural and structural body panels. With these types of approvals, body shop owners, managers and technicians have grown confident in selecting metal bonding as a solution.
Selecting the right adhesive for metal bonding - available in acrylic, epoxy and urethane formulas - requires the consideration of a number of factors, including panel size, necessary working time, handling strength time, and the desired curing time. While working time refers to the duration necessary for a technician to apply the adhesive, install the part and clamp or fasten the part, handling strength can be explained as the time required before an adhesive-bonded part can be moved.
Most adhesive companies offer two or three metal-bonding product choices: fast-set, medium-set and slow-set. The fast-set product typically works well with a small patch panel or with the repair of a loose hem flange. This type of adhesive typically provides 5-8 minutes' working time and cures to handling strength within 45 minutes to an hour. Medium-set formulas are ideal for door skins, average-size quarter panels and most roof skins because they allow 40-50 minutes of working time and cure to a handling strength in as fast as 11⁄2 to 2 hours, while slow-set products work well for large quarter panels and large roof skins. Further, the slow-set metal bonder normally provides longer work time and, subsequently, more cure time.
Plastic-to-Metal Bonding and Plastic BondingPlastic-to-metal and plastic-to-plastic adhesives have opened the door for automotive and truck makers to design previously unimaginable body styles. With the ability to securely bond two dissimilar materials, designers are able to help engineer lightweight plastic exterior body panels. This type of bonding also serves as a viable repair option. Urethane and epoxy adhesives are the most popular choices for these jobs.
Choosing the proper adhesive for this type of bonding also includes analyzing handling strength and cure time. Both fast- and slow-set options exist for plastic-to-metal and plastic bonding. The fast-set products allow about 5-7 minutes' working time and work well with small panel bonding, spoilers, and emblems. The slow set allows 50-60 minutes' working time and is ideal for installing large ground effects, plastic/SMC fenders, plastic/SMC door skins, and large truck plastic panels.
It is important to follow the repair directions supplied by the adhesive company since bonding techniques may differ from manufacturer to manufacturer.
While some adhesives will work on bare metal, others only work on painted or primed metals. Further, panel-bonding preparation is key for a failure-free repair.
SMC, Fiberglass and Carbon Fiber RepairThe ability to make successful rigid plastic repairs has advanced the collision industry. Technicians can now emulate the characteristics of rigid plastics with the combinations of epoxy adhesives, fiberglass cloth and heat.
Two options exist for repair of rigid plastics: cosmetic and structural repair. For cosmetic repairs such as nicks, chips, scratches and gouges, epoxy adhesives work well and are usually only needed in fast-set formulas. The fast-set epoxy adhesive will fill the shallow imperfections on the rigid panel and sand to a perfect featheredge. A structural repair, however, requires more work, but if procedures are followed, a long-lasting, undetectable repair is possible.
For structural repairs, such as cracks and holes, a heat-set epoxy formula in combination with several layers of fiberglass cloth is recommended. The repair area should be coved out to a smooth taper on the cosmetic side, and the backside should be roughened up a little. A backing patch of fiberglass cloth or a backer panel of similar composition with an adhesive coating is suggested, and the cosmetic side should be layered with strips of fiberglass cloth and adhesive. The repair area should then be heat-set to shrink the repair for about 10 minutes at 180°F; sanding may be performed after cool down. A skim coat of the fast-set or heat-set adhesive can be applied for final finishing, if desired.
Plastic Repairs (Non-Rigid)Advanced non-rigid plastic adhesives now adhere and repair most non-textured, non-rigid automotive exterior plastics. The majority of adhesives currently in use for these jobs are urethane or epoxy formulas. With non-rigid plastics, such as bumper covers, the technician simply needs to choose between structural and cosmetic products. Some adhesive companies suggest the use of a surface-modifier spray to promote adhesion and take the guesswork out of plastic identification.
The structural adhesive will repair cuts, rips, small holes, and mounting tabs, and usually can be used as a cosmetic repair adhesive. Further, it is a good idea to build a backing patch with structural adhesive and vinyl mesh on the non-cosmetic side to ensure structural integrity. In most cases, the cosmetic adhesive is used to repair small nicks, scratches and gouges in conjunction with a finishing, skim-coat putty over the top of the structural adhesive.
Seam SealersSeam sealers, found wherever there is a hem or a joining of metal, are the most prevalent vehicle adhesives in use today. Sealers are typically applied to seams at quarter panels, roof panels, door skins, rear body panels, frame rails, rocker panels, floor pans, pillars and aprons. Available in several chemical varieties, seam sealers are offered as 1K or 2K products, and are typically composed of urethane, epoxy, or MS polymer. The adhesive can be specified as factory-match, controlled-flow, self-leveling or sprayable.
Key to selecting the correct seam sealer is knowing the specified application as well as desired work and full cure times. Although a factory-match seam sealer can be used to duplicate most factory seams with creative tooling, several working and cure times are available, making selection tricky. Typical 2K sealers have accelerated working/cure times that can set up quick in 5-15 minutes, are paintable in 10-30 minutes and are fully cured in 24 hours. These types of adhesives are popular for door skins, quarter panels and body panels, and work well for technicians who want fast performance. The 1K factory match sealers take up to an hour to set and fully cure in about 72 hours. These one-component products are typically less expensive and paintable in about 10 minutes.
A controlled-flow seam sealer is a good solution for sloped surfaces, roof channels, drip rails and truck-bed floor seams. The controlled-flow formula enables technicians to make a smooth bead without tooling, has fast working/cure times and gives body technicians the ability to sand to a professional featheredge after 30 minutes of adhesive application.
Although self-leveling seam sealers are a relatively new technology, they are already widely used as a solution for the latest vehicles. These product are used mostly on roof ditch areas, but other uses include core supports to aprons, trunk seams and the quarter panel to roof deck areas. The sealer levels itself within its applied area and does not usually require any further tooling beyond brushing. Furthermore, in most cases, this type of product sets up fast and can be painted within 30 minutes.
Sprayable sealers are growing in popularity and are commonly used to spray textured trunk floors and wheelhouse areas. They also work well as a stone guard coating for rocker panels and lower portions of exterior body skins. The stone guard can then be immediately painted over with a preferred basecoat/clearcoat system. With atomized pneumatic spray guns, the technician can control the desired thickness and texture of the dispensed product. Sprayable seam sealers can also be used to lay traditional beads if necessary.