Get a Grip – Flex Agent and Adhesion Promoter

We recently shot a video in which one of our product mangers repainted the rear bumper of his car to test a new single-stage finish. One of the comments from that story asked why we hadn’t used a flex agent on that flexible plastic part. And the short answer was, “It wasn’t really necessary.”

This answer didn’t sit well with everyone, so we thought it would be worth discussing the nature of flex additive. And while we’re at it, let’s also talk about its close cousin, adhesion promoter. Both of these chemical treatments are designed to ensure paint adheres well to flexible plastic auto parts. However, they work differently and perform different functions.

Flex Agent

Let’s go back to the original question about additive flex agents – modifiers that are blended into paint before it’s applied. These were developed when vehicles began using soft plastic exterior parts like bumpers, grilles, and other body trim. Their role was to keep the cured paint finish flexible long enough to be assembled without flaking off or cracking. This was particularly necessary on post-production (i.e., body shop) repairs where flexible parts may be handled more off the vehicle.

Paint chemistry has been another factor in the need for additive flex enhancement. Acrylic enamel paints once commonly used in production shops tend to be brittle when cured, making flex agents a necessity. Today, acrylic urethanes are more common, and they adhere better under a variety of conditions. Flex agents are generally no longer considered a requirement for all jobs, including our GTI bumper.

It should be noted, however, that even flexible plastic body parts rarely flex much once they’re mounted to a vehicle. As a result, the properties that traditionally made flex additives desirable are temporary, lasting only weeks or months at most. Eventually the paint hardness will match the rest of the finish. Short of a severe impact, properly applied paint shouldn’t crack or peel from flexible parts.

Adhesion Promoter

Paint adhesion is a greater concern than flex when it comes to plastic parts. Several types of plastic used for automotive trim can be resistant to bonding with certain paints without an adhesion promoter. These chemicals act as a surface preparation for bare plastics, chemically “scuffing” them to enhance their acceptance of paint.

Not all plastics require such a product, but most thermoplastics will. These include TPO (Thermoplastic Olefin), TPRU (Polyurethane), ABS (Acrylonitrilebutadiene-Styrene), and SMC (Sheet Moldable Compound).

Adhesion promoter is typically applied directly to bare plastic rather than mixed into the paint or primer, sprayed either from an aerosol can or through a paint gun. Once applied, there is a window of time in which the surface will accept paint material. Each product will have its own active window depending on material and other factors like temperature.

Many pre-primed parts will have already included the use of an adhesion promoter. Always check with the parts provider to confirm. Generally speaking, a part that arrives finished in primer should be ready to paint without requiring additional adhesion promoter.