Methyl methacrylate adhesives, also known as MMA adhesives or acrylic adhesives, are two-part reactive acrylic adhesives. Methyl methacrylate by itself cures to a hard, brittle film. Therefore, most MMA adhesives are formulated with modifiers that improve flexibility and act as strengthening agents. These modifiers greatly improve the MMA adhesive’s overall tensile strength, as well as its resistance to peeling and impact damage.
Methyl methacrylate adhesives cure differently than other two-part adhesives. What this means is that they are less sensitive to mixing ratios. The ratio for mixing can vary from 100:3 to 1:1, allowing the user to adjust how fast or slow the adhesive cures. Increasing the curing temperature can also speed up cure time, but the final strength will suffer significantly if the product is cured too quickly.
Most two-part adhesives require the resin and hardener to be mixed before the mixed solution is applied to the area to be bonded. This is where methyl methacrylate adhesives stand out from the crowd. When using MMA adhesives, the resin can be applied to one surface and the hardener to the other. Bringing the two surfaces together will initiate the curing process. This unique feature can greatly reduce waste while saving you time.
Methyl methacrylate adhesives are used in numerous industries, but they are becoming increasingly common in wearable and handheld devices. These tiny electronics – including smartwatches, Bluetooth earpieces, etc. – simply cannot be manufactured using traditional welding or riveting techniques. The companies that make these products use MMA adhesives because they create strong bonds without adding extra weight or bulk to the device.
WHAT ARE METHYL METHACRYLATE ADHESIVES AND HOW DO THEY WORK?
Compared to other products, like cyanoacrylates, epoxies, and polyurethanes, methyl methacrylate adhesives are relatively new. They consist of an adhesive resin component that is made of methacrylate monomers and amines mixed with additional toughening agents and rubbers. The other component is a peroxide-based activator paste. When the two components are mixed, the amines react with the peroxides to release reactive free radicals. These free radicals then interact with a double bond found in the methacrylate monomers, resulting in a rapid, exothermic polymerization reaction and the curing of the adhesive.
Unlike other structural adhesives, MMA adhesives work well when bonding dissimilar materials, such as flexible plastic and rigid metal. They combine exceptional flexibility with high strength to form bonds that are extremely durable yet flexible enough to work well for dissimilar surfaces. The fast exothermic reaction means that the time between curing and fixture is very short, making these structural adhesives ideal for fast-paced environments.
ADVANTAGES OF METHYL METHACRYLATE ADHESIVES
MMA adhesives offer numerous advantages, especially when it comes to the integrity of the materials bonded. While riveting or welding can result in stress points that lead to cracks in the structure, a thin layer of MMA adhesive distributes the load in a more even manner. This eliminates weak spots and their potential failures. Compared to welding and riveting, using a structural adhesive also looks nicer and improves the aesthetics of the finished product.
Because MMA adhesives are less sensitive to variations in mixing ratios than other two-component adhesives, users can adjust the cure profile to suit their needs. This means that the adhesive can be formulated to have a slower reaction rate after mixing to allow for a longer work life. It can also be formulated to cure faster to speed up the assembly process when needed. This level of flexibility is not possible with many other types of structural adhesive, so this is a huge benefit.
Bonds created using methyl methacrylate adhesives have excellent temperature tolerances and remain flexible at low temperatures. MMA adhesives have excellent resistance to environmental factors and solvents, as well as a high resistance to fatigue. Methyl methacrylate adhesives have high tensile strength, good impact resistance, excellent peel strength, and minimal bond shrinkage.
When using methyl methacrylate adhesives, there is no need to use a primer. MMA adhesives cure at room temperature and are extremely easy to apply. Compared to urethanes, they have a higher bond strength and offer better resistance to impact and fatigue.
TYPES OF MMA ADHESIVES
There are several different MMA adhesive products on the market. Because they can be easily modified, manufacturers like Henkel offer a wide range of MMA adhesives that are suitable for use in various applications. While these products are available from a number of leading brands, Henkel’s line of Loctite MMA adhesives are some of the best on the market. Let’s take a closer look at just a few of the many products that are currently available
Loctite H3000 is a high-strength structural adhesive that can be used to bond a wide variety of substrates. It is a general purpose product with a low viscosity and a medium fixture time. It has a work life time of 5 minutes and a shear strength of 4150 psi on steel.
Loctite HHD8000 (Also Known as Loctite HF8000R)
Loctite HHD8000 is a two-component non-sag methyl methacrylate adhesive that cures at room temperature and offers exceptional bond strength on a wide range of substrates, including metal and composites. It also offers excellent impact strength in cold temperatures and provides high elongation. Depending on how the product is mixed, it has a maximum working life of 4 to 8 minutes. Fixture time depends on what substrates are being bonded and ranges from 10 to 25 minutes. When cured completely, this product has a shear strength of 480 psi on ABS, 2650 psi on aluminum, and 2890 psi on stainless steel.
Loctite AA HF8025 (Also Known as Loctite HF8025)
Another two-component non-sag MMA adhesive that cures at room temperature, Loctite AA HF8025 works well on multiple substrates, including composites, plastics, and metals. It contains spacer beads that help to ensure that there is an adequate gap in the bond area to allow for optimal impact and shock performance. This feature makes this particular product particularly well-suited to used in wearable and handheld devices that are frequently exposed to impacts or hard vibrations, such as smartphones and tablets.
Loctite H8110 Speedbonder is a two-part methyl methacrylate adhesive system that works especially well with aluminum. It is the fast fixture version of another Loctite product – Loctite H8000. It contains glass spacer beads to prevent users from over-clamping assemblies and fixtures in 5 to 10 minutes on steel. It has a full cure time of 24 hours and once fully cured has a shear strength of 1500 psi on steel.
Loctite HHD8600 (Also Known as Loctite HF8600)
Loctite HHD8600 is a two-component MMA adhesive system that is formulated with low halogen content for structural bonding. It works especially well for bonding metal in situations where excellent impact and shock resistance is needed. By adding mild heat to bonded assemblies, it is possible to reduce cure times. The adhesive contains 0.127mm spacers to ensure that there is a sufficient gap in the adhesive area to allow for optimal impact and shock performance.
Loctite AA HF8075LV (Also Known as Loctite HF8075LV)
A super-tough, two-component methyl methacrylate system that cures at room temperature, Loctite AA HF8075 is specially designed to be used for the structural bonding of electronic assemblies. It complies with the low halogen content requirements of this particular task, and it does the job exceptionally well. Adding mild heat enables the user to reduce curing time, allowing for faster production. This product is most often used for the structural bonding of metal and plastic components where vibration and impact resistance are important. In the portable devices industry, it is commonly used to bond displays, structural frames, top cases, etc.
Loctite HHD 8150R
Specially designed for handheld device and laptop cover bonding, Loctite HHD 8150 R is a phthalate and hydrogen-free methyl methacrylate adhesive. It is formulated to deliver superior adhesion, fast fixture, and resistance to damp heat when used to bond PCABS blends, anodized aluminum, magnesium aluminum, and alloys.
Loctite HHD 8160
Loctite HHD 8160 is a two-component MMA adhesive that is REACH-compliant and free of halogen. It is specifically designed for medium to large format devices, such as laptop computers. It works well for adhering polycarbonate, anodized aluminum, and aluminum alloys. This product is formulated to offer low-temperature fast fixture processing abilities.
Loctite HHD 8900
Loctite HHD 8900 is phthalate free and halogen free. This product is a two-part methyl methacrylate system that is suitable for bonding small parts made from a wide range of materials, including glass, plastics, and metal. It is formulated to minimize curing in the mix nozzle. It can also be used for bonding large parts when long open time allows for maximum work time. By adding heat, fixture can be achieved quickly. It offers high temperature and humidity resistance as well as excellent tensile strength.
When you need to bond dissimilar materials and welding or mechanical fasters are not good options, a methyl methacrylate adhesive may be your best bet. These products come in a wide variety of formulations to meet a vast range of needs, and they work exceptionally well for bonding dissimilar surfaces. They offer exceptional strength and flexibility without adding bulk or significant weight to finished assemblies, making them ideal for things like handheld and wearable devices, such as smartphones, tablets, smartwatches, Bluetooth devices, etc.