For bonding composites, plastics or painted metal, urethane structural adhesives offer benefits such as low exotherm, low odor and high elongation. Despite these advantages, OEMs have often used methyl methacrylate adhesives (MMA) instead, because MMAs offer a fast rate of cure, which facilitates a part’s quick transition to the next step in a given manufacturing operation (such as cutting to size, removing flashing, inserting additional components, and painting of finished components).
Urethanes that have adequate open times also take a long time to cure, so one strategy has been to add heat to hasten the cure. For example, original heavy-duty truck and automotive equipment manufacturer assembly lines may use two-component polyurethane adhesives to achieve fast attachment of parts and resort to additional methods of heat (such as hot air impingement, heated tooling, or ovens) to accelerate the rate of cure and shorten manufacturing cycle times. This process, however, adds the cost of heated tooling to the manufacturing costs and can create potential quality issues with “cold spots.” (Cold spots are complex shapes and thicknesses of the component that are sheltered from the heat applied for curing or in the tooling itself.)
Fourth-generation structural polyurethane adhesives feature a cure profile similar to that of an acrylic and they cure at room temperature. They can be used in place of MMAs while retaining the short cycle times needed for productivity.
Properties of fourth-generation structural polyurethane adhesives include:
- Excellent adhesion when bonding GFRP, sheet molding compound (SMC), bulk molding compound (BMC), thermoplastics, and cross bonding composites to metals.
- Non-sag paste with excellent gap filling and good thixotropy, allowing for vertical application.
- Versatility in curing:
- Room temperature (ambient) cure.
- Higher capital cost systems are not required but may be used:
- Forced air and gravity convection ovens.
- Heated tooling and fixturing.
- IR heating elements.
- Gravity feed system for bulk dispensing: low viscosity A- and B- components allow for the material to utilize gravity to feed into metering pumps. This negates the need for the additional cost of pail or drum pumps for the metering system.
Testing of fourth-generation structural polyurethane adhesives have shown excellent adhesion on a variety of substrates, low exothermic temperature (which prevents softening of thermoplastics and potential read-through) and the short timespan needed to achieve processing strength.