Written by Mark Jones
The term “acrylic” is used to describe two things: the polymer structures present in some one-part adhesives and the cure chemistry of some two-part, reactive structural adhesives. The most common acrylic adhesives found in a general hardware store are caulks and pastes. Typically, these are acrylic latexes made of acrylate polymers suspended in water or other solvents.
Although it’s possible to employ additional reactive curing chemistry, these adhesives usually form a bond as the water evaporates. The adhesion occurs, thanks to the entanglement of the acrylic polymers. Examples include caulks, carpet and tile adhesives, and construction adhesives.
These systems are generally quite safe. Aqueous systems emit few vapors and are not particularly flammable.
Two-part structural acrylic adhesives are high-performance formulas rivaling, if not surpassing, the performance of epoxies and polyurethanes. Like all structural adhesives, acrylates can support a structural load when fully cured.
They also exhibit:
- Good impact and vibration resistance
- Durable bonds that withstand most environments
- Adequate temperature stability
Many two-part, reactive acrylic adhesives use methyl methacrylate (MMA) as the reactive species. MMA smells potent and somewhat offensive. It’s also a volatile, flammable material, so care must be taken to ensure adequate ventilation and elimination of the ignition sources.
Despite the foul odor, acrylate adhesives find several uses because they offer excellent adhesion to various substrates. They bond well to many plastics, including thermosets, thermoplastics, composites, and metals. The cured resin has high shear, peel strength, impact strength, and chemical resistance.
The cure chemistry of acrylate adhesives is fast, even at room temperature. The resin’s ability to wet and adhere to several materials means there are opportunities to add fillers and tougheners, depending on the application.
Additionally, acrylics can be formulated to snap cure. After mixing, there’s an induction time where no noticeable thickening occurs. This induction period is followed by an extremely rapid cure or snap cure. This is advantageous for complicated structures or larger assemblies.
However, there are also drawbacks. Aside from the foul odor and flammability concerns, the fast cure chemistry means limited pot-life of the mixed resin. Although fast curing is often a blessing, it can also be a curse.
Reactive acrylic adhesives are typically sold in a twin cartridge configuration, requiring a special cartridge gun and static mixer nozzle. Some are sold as two separate containers for bead-on-bead application. One part is directly applied over the other, and then the surfaces are pushed into the adhesive for bonding. No mixing is required.
Certain industrial-setting formulations cover one surface with resin and the other with an initiator. Pressing the surfaces together is all that’s necessary to trigger the cure chemistry. The pot life is not an issue in this case because there’s no mixing.
UV-cure acrylic adhesives are specialty materials used in various manufacturing and dental applications. A photosensitive initiator is present in a one-part formulation. Exposure to UV light creates a reactive species initiating the cure chemistry. The cure chemistry is free-radical polymerization.
Many UV-cure dental resins use MMA (you might associate its unpleasant odor with the dentist). The UV light used to cure these adhesives can damage the eyes, so protective glasses are essential.
Structural and specialty acrylic adhesives can offer several advantages and are widely used in industrial settings.