Many advancements in adhesive chemistry occurred during the WWII era, as researchers sought new materials that could be used to support the war effort. Acrylics are no exception. Their strong bond strength, combined with fast cure speeds, made them a continued focus for product development in the latter half of the 20th century.
Acrylic adhesives are known to have a lot of odor and dispense a paste-like viscosity, while being non-sag and easy to dispense by hand. They change color during curing, giving a visual indication of curing progress. Acrylic adhesives are usually packaged in dual cartridges containing two separate components in the unreacted state—one an acrylic and one an accelerator. They tolerate small variations in mix ratio without compromising performance. Many formulas contain small, spherical glass beads to help maintain the desired thickness when the material is dispensed, resulting in optimum bondline thicknesses.
Advantages to Using Acrylic Adhesives
Acrylic adhesives can have a long open time but once they begin to react, they quickly reach handling strength. This cure profile is ideal for many manufacturing operations since parts can be assembled quickly and moved from the product floor. The adhesives are available in a range of open times, from several minutes to about an hour, and reach full cure at room temperature, although curing can be accelerated with moderate heat of less than 140°F (70°C).
These formulas require little-to-no surface preparation, unlike some other chemistries that require scuffing, sanding or primers. They will bond through light surface oils and rust preventatives. Elimination of preparatory steps translates to process efficiencies and cost savings. Additional process and cost benefits result from acrylics’ ability to withstand post-processing and e-coat temperature exposures up to 400°F (200°C). Not only are transportation costs to and from coaters’ sites reduced, but higher structural strength bonds are achieved when adhesives can be applied to the pre-painted surface as opposed to bonding directly to the paint layer or masking/sanding the paint layer to expose a bare metal surface after coating.
Installed acrylic adhesives offer excellent impact, shear and peel strengths. They maintain strength and adhesion after long term exposure to multiple environments, such as salt, fog, humidity, thermal cycling and various chemical exposures.
Things to Know About Acrylic Adhesives
Making their good cure times even more flexible, acrylic adhesives can be used with a variety of accelerators, although it’s important to note that these products can affect mix ratio, viscosity, color change and application method. Acrylics’ surface cure is inhibited by the oxygen present in air, which can result in a thin layer of surface tack on exposed areas.
Some formulas have reduced bondline readthrough (no visible outline/marking from the adhesive on the substrate) making them a good choice for thin substrates or aluminum composite panels (ACM).
Acrylic adhesives provide excellent adhesion to unprepared or minimally prepared metals, composites and most thermoplastics. They do not, however, bond well to porous surfaces (substrate that has openings or holes). They have moderate elongation of about 40 percent and withstand temperatures up to about 300°F (150°C).
For more in our series, check out “Part 1: Adhesive Basics” and watch for upcoming installments on epoxy and urethane adhesives.