Guide to bumper cover plastics

Plastics used in bumper covers

A variety of bumper cover plastics are used to form bumpers.

All late-model cars and truck bumpers are constructed with a plastic bumper cover, energy-absorbing cushion, and a lateral steel or aluminum bumper. The energy-absorbing cushion is often a high-density polyethylene foam, honeycomb or egg-crate shaped plastic.

Three-piece bumper construction

Plastic bumper cover material

If you’re going to perform a repair on the bumper cover, you must know the type of plastic used by the carmaker. Carmakers use a variety of different plastics like polycarbonates, polypropylene, polyamides, polyesters, polyurethanes, and thermoplastic olefins or TPOs to form their bumper covers. Many bumper covers contain a combination of these different materials. Manufacturers mix these plastics with glass fibers or other materials to strengthen the bumper. Some plastics require an adhesion promoter prior to adding any filler material, primer or paint, while other plastics can be melted together.

Two categories of plastics used in bumpers

Thermoplastic bumper material

Thermoplastic material is made from a plastic polymer which becomes soft, moldable and formable when heated and hard when cooled. Thermoplastic materials can be rewarmed, reformed and remolded several times without any change in their chemistry or mechanical properties. When thermoplastics are heated to their melting point, they melt to a liquid.

Thermo-set bumper material

A thermo-set bumper material is made by mixing two or more components. The two components cross-link together during the curing process to form an irreversible chemical bond. Body filler (Bondo) and hardener and two-part epoxies are examples of thermos-set plastics. Once catalyzed, thermo-set plastics can’t be melted back or together to form a repair. In other words, you can’t use heat alone to fuse a crack in a thermos-set polyurethane bumper cover. In fact, if you attempt to use heat to melt and fuse a crack in a PUR bumper cover, the plastic will instead liquefy, bubble and smoke and will cool sticky to the touch. This is because the base material has broken down under the heat.

Most common types of plastics used in bumper covers

Polyurethane (PUR), Reaction Injection Molding (RIM), Reinforced reaction injection molding (RRIM), Thermoset polyurethane — Most common on domestic vehicles. Yellow or grey in color. Bubbles and smokes when you attempt to melt. Thermo-set PUR sands powdery and can be smoothed with a grinder as long as you don’t use high pressure or high speed that causes the plastic to liquefy.

PUR and most thermo-set plastics are repaired by heat fusing and grinding a “V” groove and filling with urethane filler material and structural support (stainless steel screen) to the backside of the cracked material.

Thermoplastic Olefin (TPO), Thermo-Elastic Olefin (TEO)— TPO and TEO are a combination of polypropylene, an elastomer or rubber, and a mineral filler such as calcium carbonate or talc.

TPO bumper material sands in chunks melts like butter with high-speed grinders, feels like wax and gets stringy when hot, so keep this in mind when trying sand or grind a TPO bumper cover.

TPO bumper covers are the most difficult to repair because they contain 3% to 5% wax-based mold release within the material. The embedded wax makes it difficult for filler, adhesive, primer and paint to stick. Body shops prefer to discard a TPO bumper because the repair is so difficult

However, TPO IS repairable as long as you know how to prepare it. You must use a cleaner and adhesion promoter prior to adding any filler material.

Thermoplastic Poly Propylene— PP is semi-flexible, melts & smears when grinding, waxy or greasy feel. Repaired by heat fusing and grinding a “V” groove and filling with polypropylene filler material and structural support (stainless steel screen) to the backside of the cracked material.

©, 2020 Rick Muscoplat