In the glue biz, we use the term acrylic adhesive loosely, and in doing so, we can create some confusion. There are many types of adhesives based on acrylic chemistry in the form of one and two-component adhesives as well as no-mix resin and activator, even solvent or water-based adhesives, not to mention a variety of acrylic-based pressure-sensitive adhesives (tapes). There are also many adhesives that have an acrylic nature that aren’t categorized as acrylic adhesives.
Rest assured; Permabond and other adhesive manufacturers do not avoid calling these adhesives acrylic adhesives to be deceptive – it’s simply that due to the variety of acrylic adhesives available, the cure mechanism is often a better descriptor to understand if that particular type of adhesive is suitable for an application.
Consider that for anaerobic acrylic adhesives, both substrates should be metal, and the adhesive/sealant must be confined between two surfaces or cured with an activator. Often anaerobic adhesives are used as weld sealants, which seems contra-indicated as weld sealants are brushed on a warm surface and not confined between two surfaces. When using anaerobic adhesives for weld sealants, the parts are warmed, and low-viscosity adhesive/sealant is applied and allowed to wick into the small pores in the welds. This does, in fact, deprive the adhesive of enough air to cure. After cure, wipe away the excess or spray it with an activator to cure it off.
Another application in which the anaerobic acrylic adhesives leave a residue of adhesives that isn’t between two surfaces is munition sealing. The adhesive that is between the bullet and the shell is cured anaerobically; however, due to the high-speed manufacturing required, spraying with an activator or wiping away residue is not possible. Permabond developed specialty acrylic adhesives with NATO approval. These cure anaerobically as well as with UV light – so is this adhesive an acrylic, an anaerobic, or a UV curable adhesive? Yup, you got it – it is all three!
For industrial bonding applications, structural acrylic adhesives, many manufacturers sub-categorized by application. We define Structural Adhesives as adhesives that can support a structural load when fully cured.
They also provide:
There are several types of structural acrylic adhesive. In this article, we’ll discuss…
Methyl methacrylate adhesives (MMAs) have the most memorable smell. Not only is the smell offensive, it reminds many of the dentist – which doesn’t help most to find it pleasant. If an adhesive specialist recommends an MMA as the best type of product for your application, ventilating the area is recommended. MMAs are very good for bonding plastic. Permabond has both two-component and surface activated MMA adhesives.
Surface-activated or surface-initiated acrylic adhesives. Create strong bonds between metals, ferrites, glass, and some plastic. Generally, the activator is applied to one surface and a bead of adhesive to the other surface, and upon joining, enough mixing occurs to form a very strong bond. Some of these products can be used wherein the adhesive is applied to the same substrate as the activator – but care must be given to assemble the products before cure begins.
Bead–on–bead structural acrylic adhesives. For this type of adhesive, you place a bead of one part on one surface and a bead of the other part on top of it. If you need more time to assemble the parts you can apply the second bead to the other surface. Specialty products are available in which the two beads free-fall together onto a part, providing assembly is done consistently within the time frame allowed.
You can measure and mix two component adhesives by hand; however, the most consistent way to get sufficient mixing without incorporating air is through static mix nozzles. We don’t advise mixing bead-on-bead adhesives through static mix nozzles as the cure is too fast.
Most structural acrylic adhesives do not have a strong odor. However, specialty low-odor grades are available.
Here are the questions an adhesive specialist may ask:
Most acrylic adhesives will bond well to metals. MMAs have good adhesion to most plastics. New specialty grades even bond polyolefins.
Surface-activated and bead-on-bead types likely have less gap-filling ability than the two-component mixed products. Keep in mind that the maximum gap fill listed for the two-component mixed adhesives merely indicates the best shear strengths; you may fill larger gaps with less strength. However, for the surface-activated products, maximum gap fill is key. If you require more depth of cure, you may need to activate both surfaces.
There may be several options, so knowing what is preferred is ideal.
Often I’ve seen adhesive specialists ask engineers for two lists of adhesive requirements, one being the ‘must haves’ the second being the ‘nice to haves’. The selection between structural acrylics and structural epoxies fall into two categories.
MMAs or methyl methacrylates are classified as flammable; solvent-based grades may also be. However, there is a large variety of solvent-free, non-flammable structural acrylics available.
For more information on Permabond acrylic adhesives, contact our technical team.
