Epoxies date back to the 1930s, when a Swiss researcher was experimenting with materials to use in making dentures. Since then, these versatile compounds have served as adhesives, coatings, and structural resins in everything from surfboards to airplane frames—wherever toughness and durability really matter. J-B Weld adhesive was invented by a truck mechanic as an alternative to torch welding, but this staple of DIY toolkits and Car Talk punch lines will form an industrial-strength bond on many surfaces. It comes in two separate tubes: a resin and a curing agent. Squeeze out equal amounts of each, mix, and let the chemical magic begin.
The resin in J-B Weld epoxy is bisphenol-A (yep, that BPA) combined with garlicky-smelling epichlorohydrin. Chemically, the molecule is a chain with little carbon-and-oxygen triangles, called epoxide rings, on the ends. That’s where the sticky action happens.
Basically quartz. It’s added to the resin for body and viscosity, without which the goop would be too fluid to adhere and set properly—more like J-B Melt.
The name says it all. Made up of sooty bits often produced by burning hydrocarbons, it gives the resin its inky color.
A cheap filler, found in both the resin and the hardener. Like the silica, it gives the product more volume while decreasing the cost per ounce.
This is the curing agent. It contains amine groups that break open the epoxide rings so their carbons can hook up with the amine’s nitrogen. One TEPA molecule can lock onto four epoxy resin chains, and the other ends of those chains can bond to other TEPA molecules. All that cross-linking forms a super-strong network structure—a thermoset polymer. When it hardens, J-B Weld can withstand forces of nearly 2 tons per square inch and temperatures up to 550 degrees Fahrenheit.
The curing agent on its own is kind of lackadaisical at opening the rings, so epoxies often use accelerators like this one. DMP-30 is short for 2,4,6-tris(dimethylaminomethyl)phenol. It has a reactive hydroxyl group (–OH) hanging off the side that helps rip those epoxide rings open like Christmas presents.
This colorless liquid modifies viscosity and acts as a solvent to help the curing agent disperse during mixing. Like DMP-30, benzyl alcohol also has a hydroxyl group that can trigger curing, so it’s put in the hardener tube.
This stuff adds more body to the mix, and it turns the curing agent a color-coded white. Mix with the black resin and you get that famous gray.
More filler. BaSO4 is radiopaque, which is why they make you swallow it to x-ray your innards. Pro tip: Do not attempt this with J-B Weld.