About Different Types of Acrylic Adhesives
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. Some examples follow:
- Anaerobic adhesives are based on acrylic chemistry but are classified as anaerobics, not acrylics. Anaerobic adhesives are used for threadlocking, thread sealing, retaining, and gasketing metal components. Curing these materials occurs when in contact with metal and not in contact with oxygen (when the adhesive is confined between two metal surfaces.)
- UV curable adhesives may be of acrylic nature but like anaerobics are generally categorized by the cure mechanism (UV) not the adhesive composition.
- Cyanoacrylate adhesives are acrylic but are classified as cyanoacrylate or instant adhesives. These are also called super glues or crazy glues.
- Methylmethacrylate adhesives are a type of acrylic adhesive that may be grouped into an acrylic category or separated out as MMA’s.
Rest assured 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 to 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 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, the excess can be wiped away or sprayed 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 activator or wiping away residue is not possible. Specialty acrylic adhesives have been developed and NATO approved which 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 are generally sub categorized by application. Structural Adhesives are defined as adhesives which can support a structural load when fully cured.
They also provide:
- impact and vibration resistance
- high upper-temperature limits
- durable bonds in harsh environments
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 and are available both as two component and surface activated adhesives.
Advantages of Methyl Methacrylates
- Excellent adhesion to many substrates including; thermosets, thermoplastics composites, and metal
- Fast cure at room temperature
- High shear and peel strength
- Good impact strength
- Good chemical resistance
Limitations Methyl Methacrylates
- Strong odor
- Accurate mixing is imperative
- Limited pot life once mixed
Surface activated, or surface initiated acrylic adhesives. Create strong bonds between metals, ferrites, glass, and some on 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. A bead of one part is applied to one surface, and a bead of the other part applied on top of it. Should more time than available be required prior to joining, the second bead might be applied to the other surface. Specialty products are available in which the two beads can free fall together onto a part, providing assembly is done consistently within the time frame allowed.
Two-component Acrylic- These structural acrylic adhesives are mixed – the most consistent way to get sufficient mixing without incorporating air is through static mix nozzles. It isn’t advisable to mix bead on bead adhesives through static mix nozzles as the cure is too fast.
Surface activated, Bead on Bead and Two-Component Acrylics have similar advantages listed below.
- Rapid room temperature cure – no ovens required
- 100% solids – Environmentally friendly
- Nonstringing – Clean, efficient process
- Thixotropic viscosity –Easy to dispense
- Lower odor – Worker comfort
- Most are Non-flammable – Reduced shipping and handling costs
- High shear strength – Strong
- Toughened – Impact, vibration, and peel resistance
- Durable in high moisture environments
- High-temperature resistance up to 390°F
The limitations of each type vary
Limitations of Surface Activated
- Performance degrades with excess gap
- Adhesive squeezed out of the bond line will not cure
- Most grades are not flammable (MMA surface activated is flammable.)
- Two-part application
Limitation of Bead on Bead – the short open time.
Limitation of external mix system – an accurate dispensing system is required.
Most structural acrylic adhesives do not have strong odor. However, specialty low odor grades are available.
How to decide which type is best? Here are the questions an adhesive specialist may ask:
- What are the substrates and required strength?
Most acrylic adhesives will bond well to metals. MMAs have good adhesion to most plastics. New specialty grades even bond polyolefins.
- What is the gap?
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, larger gaps may be filled. For the surface activated products, maximum gap fill can be related the depth of cure provided by activating one surface.
- What is the preferred assembly process?
There may be several options, so knowing what is preferred is ideal.
Why use a structural acrylic adhesive instead of an epoxy?
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 a two categories.
- Adhesion – although both epoxies and acrylics bond most metals, there are acrylics with excellent adhesion to plastics.
- Fixture speed – The cure speed of heat cure epoxies is dependent on the temperature, but they take 15 minutes or more to cure – (some exceptions for induction curing apply.) Two-part epoxies are available in a variety of cure speeds, but the quickest take 4-6 minutes to fixture. Many acrylics provide a good amount of handling strength in 30 seconds.
Aren’t all acrylic adhesives flammable?
MMAs or methylmethacrylates 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.