ADHESION PROMOTERS:
An Insurance Policy for the Paint Department
Adding an extra step to your refinishing routine can eliminate adhesion failures and work
wonders for long-term customer satisfaction.
By CHARLIE BARONE, Contributing Editor
January 1999
Paint is often the biggest indicator of a repair job's quality--or at least that's how consumers tend to judge collision work. Does it match? Is it glossy? Does the finish endure, or is it starting to peel off that aftermarket bumper cover? Shop owners, who must be attentive to long-term customer satisfaction, must provide consumers with repairs that endure the tests of time, environmental hazards, weather elements and automatic car washes. Motorists can patronize whichever shops they choose, and their selections are often based more on quality than on price. Earning a reputation for long-lasting, quality work can make all the difference for consumers trying to make that decision.
Your business' stake in the endurance of paint work holds true well beyond the terms of any written or spoken warranty. Granted, there are instances of product failure and unforeseen environmental effects, but the majority of paint adhesion failures are preventable. And regardless of the cause, a peeling paint job is a liability to the party that applied it--just ask the North American auto makers, who have stripped and repainted several hundred thousand cars and trucks. This didn't happen out of legal obligation (in most cases). Rather, it was a business decision. Ford Motor Co.'s campaign against delamination was as much an effort to maintain its share of the light truck market as it was anything else. Had the company simply walked away from the paint failures, Ford likely would not be in the position it is today. The same principles--on a much smaller scale--hold true for body shops.
In Search of Some Assurance
As you follow up with customers whose cars were repaired at your shop several years ago (you do this, right?), are you hearing complaints related to paint adhesion? Is the paint peeling from around the moldings you weren't paid to R&I? Is it peeling from around the windshield to reveal molding you didn't bridge with nylon line? How are the lower body panels surviving? Are plastic parts the biggest source of paint delamination? If so, it sounds like you are in need of some insurance.
At minimal expense, you can protect your business from those pesky adhesion failure comebacks. More importantly, you can save your facility from the devastating effects of poor customer satisfaction. The cars that don't come back can do more damage than ones you have to repaint, because every time a customer reaches for the door handle on his or her car and notices a little more paint missing, it sends a subliminal message: The shop was negligent. Meanwhile, all the insurance you need to protect yourself from the above-mentioned hazards comes in a can: adhesion promoters.
The U.S. Environmental Protection Agency (EPA) defines these products as coatings designed to facilitate the bonding of a primer or topcoat on surfaces such as trim, moldings, door locks, door sills (where sanding is impractical), on plastic parts and on the edges of sanded areas. The operative word there is "facilitate." Adhesion promoters are not substitutes for proper prep work. "I wish there was a magic ingredient that would replace proper preparation, but that is just not the case," says Alan Craighead, a technical advisor at the Akzo Nobel Collision Repair Instruction Center (CRIC) in Norcross, Ga.
Lines of Defense
Adhesion characteristics can be divided into two categories, mechanical and chemical. Cleaning the substrate is the first step--and the foundation for--mechanical or chemical adhesion. If contamination is left on the substrate, neither the best sanding processes nor the chemical makeup of a product will overcome it.
Mechanical adhesion is then promoted by sanding the substrate. This actually increases the surface area the coating can contact. "Take a 3-in. piece of string in a straight line and lay it next to a piece of string in a straight line that spans that same 3-in. distance but that has been shaped with peaks and valleys," Craighead says. "The line that has been shaped with peaks and valleys represents the increased surface area that takes place when sanding a substrate. By simply straightening the shaped string, you can see that it is substantially longer than the straight line."
Now it's time for the second line of defense: chemical adhesion. "Chemical adhesion of a product is determined by three characteristics: surface tension, binder makeup and chemical composition," Craighead says. "For wetting purposes, which contribute significantly to adhesion, the surface tension of a coating must be equal to or lower than the substrate. Without proper surface tension qualities, poor adhesion will result.
"Binder types also contribute to adhesion. If you remember in the old days, switching from lacquer products (physically drying) to spraying alkyds/acrylic enamels (chemically reactive), the binder was very sticky and wanted to stick to everything. By matching the reactive/adhesive sights of the coating with the sights in the substrate, adhesion will take place. In lay terms, binder will determine adhesion qualities."
How do the chemicals work? What chemical has the ability to "soften" the substrate? Craighead explains: "Chemical make-up of a product covers a broad area, but let's look at one product and how it provides adhesion. Self-etching primers provide chemical adhesion through the acid, literally etching the metallic and non-metallic substrates. This creates a bond similar to Velcro, a hooking and interlocking of the coating and substrate. In addition, the phosphoric acid derivatives are capable of further reaction with these substrates, leading to metal-phosphorester linkages. In a nutshell, a chemical reaction takes place, creating a chemical bond outside of mechanical adhesion."
Most paint systems require the use of a self-etching primer over bare metal followed by a primer surfacer, wet-on-wet. Custom car painters often create the monochrome look by painting chrome bumpers and trim the same color as the body. "Looking at the bonding of primer or topcoat to trim moldings or door locks, we still recommend a two-step process when possible," Craighead advises. "Abrade the substrate through media blasting or sanding, being sure to clean the substrate thoroughly before and after, and utilize a self-etching primer or coating that has a surface tension equal to or lower than the substrate. The coating should be designed and recommended for those primed or unprimed substrates."
Rules and Procedures
To foster longevity in your paint jobs, establish a routine that includes adhesion promoters as standard practice. If your paint shop isn't working according to plan at present, you may want to establish simple guidelines for each step of the process. All paint manufacturers will provide their customers with outlines for establishing standard procedures, and every one of them will include specifics designed to eliminate paint-adhesion failures. The guidelines also contain procedures for painting plastic parts.
Most adhesion promoters have extended shelf life, so technicians can mix enough for the day's production and leave it in the gun. This added step usually only requires a minimum flash-off time, which typically amounts to about 15 minutes. Check with your paint supplier for the specifics on each product you use.
A large part of the problems plaguing plastic painting pertains to union issues: the union of two chemical compounds, acrylics and acrylics. The vast majority of paint adhesion failures are on plastic bumper fascia, ground effects, spoilers, etc. (ABRN October 1998, Paint Shop, "Painting Plastics: A Recipe for Success"). "Of all the materials that today's automotive refinisher must deal with, flexible plastic fascia represent perhaps the greatest challenge," says Dr. Stacey Balderson, product specialist for DuPont Automotive Finishes, in a recent issue of DuPont's Refinisher News. "That's because fascia present several potential problems. In addition to their flexibility, there is the potential incompatibility of the substrate and the coating (thermoplastic olefins (TPO) and the potential for poor wetting of the coating. Thus, surface preparation and selection of the finishing system become super critical. But even prior to those steps, the refinisher faces another daunting task: identifying the type of plastic and determining whether the substrate is unprimed, pre-primed or OEM-finished. This will dictate the type of surface preparation needed and the choice of paint system."
Unprimed plastic generally can be divided into two categories--thermoplastics and thermosets, Balderson says. Thermoplastics are moldable and can be deformed by heat, hence their ability to be recycled. They also are sensitive to solvent attack. This group includes acrylonitrile acrylic rubber styrene (ARS) and polyolefins, such as polypropylene and TPO. Thermosets are insoluble and infusible, making them difficult to recycle. Typical thermosets are non-polyolefins, including polyurethanes (PUR) and sheet-molded compounds (SMC).
"The refinisher needs to identify the plastic type in order to choose [the appropriate system] that is optimum for polyolefin or non-polyolefin products," Balderson says. "This can be done by checking the identification symbol imprinted on the back of the part [which is not feasible if the part is on the vehicle] or by using the float or abrasion test." The float test entails cutting a shaving of plastic from the back of the part and dropping the shaving into a glass of water. The shaving should be free of mold release agents and paint. If the shaving floats, it's polyolefin. If the shaving sinks or is submerged, it's non-polyolefin. The abrasion test involves sanding a spot near the damaged area with a Grade 36 Roloc disc. The polyolefin material melts or frays when sanded, but the non-polyolefin sands cleanly.
Plastic parts that come unprimed from the OEMs require special adhesion promoters/primers, Craighead says. "If the plastic is not thoroughly cleaned and all mold release agents [are not] removed, no type of adhesion primer will overcome that contamination," he says. "Plastic parts primers work through solvents and binders that burrow into the substrate. Low surface tension and 'stickiness' of the binder make the primer work."
Some coatings can be applied directly to new factory parts without sanding or adhesion promoters. "The reason we are able to do this is because the profile of an OEM e-coat provides ample tooth over a red scuff pad for mechanical adhesion," Craighead says. "Our test results, with thorough cleaning of the substrate, show no adhesion failures both in practical and analytical testing."
Added Benefits
Adhesion promoters also prove useful when blending panels. Because the color coat being tapered into the adjacent panel does not wet the substrate in the same way the material coats the primary panels, there is some risk of mottling or other failures. A chemical assist in this procedure will give you a better-looking blend and further assure adequate adhesion. But experts warn that adhesion promoters do not replace sanding.
Whether you are painting plastics, working on salvage parts or just trying your best to turn out a quality product, your shop's paint work is like a business card. Essentially, it's who you (as a business) are. Take these additional steps to ensure your shop's success--and build a reputation for long-lasting quality paint jobs.
