How to Repair Thermoset Polyurethane

Thermoset polyurethane was commonly used on domestic bumpers back in the 1980's and 90's. It's pretty much been replaced by polypropylene now, but you'll still see the old yellow plastic out there once in a while.

Polyvance was formed in 1981 as Urethane Supply Company to address the needs of bumper recyclers and to supply products to repair urethane bumper covers. Around that time, many people did not think urethane bumpers could be repaired. We proved they can be repaired.

Polyurethane is a thermoset plastic, meaning that it is NOT meltable. The solid is formed by reacting two liquid components which crosslink in the mold. You will NOT be able to use the nitrogen welder on this type of plastic. You can repair it with the airless plastic welder. This will not provide a fusion weld. The urethane welding rod is used more like a hot melt glue.

You can use our Mini-Weld Model 7 or the integrated airless welder that is part of our nitrogen plastic welding systems to perform the repair. These two options are recommended because you need to be able to control the temperature of the welder.

This article can help you determine if your plastic part is thermoset polyurethane or a thermoplastic.

Always wear proper safety gear while working!

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Products Used:

Note: This is a list of the Polyvance products that can be used to make this repair. This list does not include sanders, grinders, or other common tools you will need. If you have any questions, please don't hesitate to call technical support at 800-633-3047.

• 1000 Super Prep Plastic Cleaner (several options available) OR 1001-4 EcoPrep Plastic Cleaner
• Aluminum Body Tape (several options available)
• 6125 Tapered Burr
• 2000 Flex Filler (several options available)
• Polyurethane Welding Rod and an Airless Plastic Welder(choose a method below):
• Polyurethane Welding Rod with the airless plastic welding side of a nitrogen plastic welder. Our current nitrogen plastic welders include the 6027HT Standard Welding Tip.
• 5700HT Mini-Weld Model 7 (230V kit available here). The 5700HT kit includes a 15 foot starter roll of polyurethane welding rod and the 6027HT Standard Welding Tip.
• Note: Make sure you are using either the 6027HT Standard Welding Tip or the 6029 Bigfoot Welding Tip when using the round polyurethane welding rods.
• You can buy our polyurethane welding rod separately here.

Video Notes:

• This type of repair can be done with the Mini-Weld Model 7 or the airless side of a nitrogen plastic welder.
• English Closed Captions have been provided for this video. Click on the CC button to turn them on or off.
• The length of this video is in no way representative of the actual time required to perform a complete repair and therefore should not be used for estimating purposes.

English Transcript:

Thermoset polyurethane was commonly used on domestic bumpers back in the 1980s and 90s. It's pretty much been replaced by polypropylene now, but you'll still see the old yellow plastic out there once in a while. Polyurethane is a thermoset plastic, meaning that it is not meltable. The solid is formed by reacting two liquid components which cross-link in the mold. You will not be able to use the nitrogen welder on this type of plastic. You can repair it with the airless plastic welder. When you repair urethane with the airless welder, you don't actually melt the base material. You use the melted welding rod like a hot melt glue. So, think of it like a brazing process.

First, apply aluminum tape to the outer surface to align the torn plastic. Then prepare the surface like you would for an adhesive repair. Grind a v-groove about halfway through the plastic. Rough up the plastic inside the v-groove and in the surrounding area with coarse sandpaper. Make sure there are no smooth spots or sharp corners. Blow the area dust free when you're done.

Set the temperature of the airless welder to the PUR setting. When the welder is set properly, the rod should come out of the bottom of the welding tip clear and melted, not smoking and bubbling. Adjust the temperature as necessary to achieve this. To do the weld, hold the welder tip slightly off the surface of the plastic. Feed the R01 urethane welding rod into the tip, and fill the v-groove with melted welding rod. Fill the v-groove for about an inch, then take the rod out of the welder tip. While the rod you deposited is still hot, press the melted rod down into the pores and smooth it out. You can touch the bumper, but don't linger the heat on it for long. Remember, this type of plastic is not meltable. Once you've finished smoothing the first bit of rod, continue the process inch by inch, until you've completed the entire repair. Fill the v-groove with melted rod and smooth it out while it is hot. If the bumper is torn to the edge, pile up the rod at the edge to strengthen the repair. Let the rod you welded on the back side cool completely.

Peel off the aluminum tape and repeat the process on the front side. First, grind a v-groove halfway through the plastic. Then, rough up the inside of the v-groove, and feather back with 50 grit paper. Feather back the paint with 80 grit. There should be no smooth spots or sharp corners when you're done.

Again, fill the v-groove for about an inch with melted welding rod, then smooth it out while it's still hot. As you weld, the heat gets sucked out of the welding tip, so you may need to increase the heat setting to maintain the proper temperature. Just weld in one-inch segments and try to keep from overheating the base material. When you're finished, let the weld cool completely.

On the front side, use coarse sandpaper again to grind the weld area flush. Flex the weld back and forth to make sure of its strength. Done properly, welds on polyurethane are very strong.

Because the welding rod doesn't feather well on the bumper, you'll need to apply a skim coat of flexible filler over the area. Prep the surface by sanding with 80 grit in a DA sander, making sure there are no shiny spots or sharp edges. Blow dust free. Finish by applying a skim coat of flexible epoxy filler, like our 2000 Flex Filler. Adhesion promoter is not needed on polyurethane.