Important properties for a structural adhesive in an aerospace application are strength, toughness, and flexibility, alongside temperature and environmental resistance. When choosing an adhesive, methyl methacrylate structural adhesives (MMAs) may be a good candidate to test, as these adhesives offer strength, toughness, and ease of use when bonding a broad range of materials, including metals, plastics, and composites.
Compared with alternative chemistries like epoxies and polyurethanes (PUs), the benefits and properties of MMAs as structural adhesives are perhaps less widely known. Because they bond to a wide range of substrates, and have choices of cure speed, MMAs are quite versatile. They are excellent for bonding metals (steel, stainless steel, aluminium), thermoset composites (both carbon fibre and glass fibre reinforced), and thermoplastics (acrylics, ABS, polycarbonates, PVC, and other engineered plastics). They are favourable in applications where fast cure speed, strength, toughness, and environmental resistance are important. Typical aerospace bonding applications for MMAs include repair and maintenance, sealing and gasketing, and for bonding interiors.
MMA fundamentals
MMAs are two-part reactive acrylic adhesives made up of a resin and an activator, but additives may include elastomeric tougheners and impact modifiers. Mixing at room temperature causes a free-radical reaction that enables fast curing. MMAs are typically available in two mix ratios, 1:1 and 10:1. To suit different process requirements, MMAs are available with a range of working times (5 minutes up to 120 minutes) and fixture times (6 minutes up to 4 hours).
Methyl methacrylate adhesives are readily and commonly available in two-part cartridges from 50 ml to 490 ml in size, and can be supplied in bulk for larger volume applications. They are applied through static mixing nozzles, which comfortably give the adhesives an adequate mix.
Due to the fast exothermic reaction that forms them, MMAs fully cure and reach their full strength at room temperature. Compared with other adhesives, their cure is less susceptible to ambient conditions like temperature and humidity. Importantly, the time between mix/application and fixture strength (enough cured strength to allow further processing, sometimes called green strength or handling strength) is short. This offers key processing advantages by reducing work in progress and enabling faster throughput.
Typically, less surface preparation is required when using MMAs compared with other adhesives, as the chemistry is able to cut through some surface contamination. A solvent wipe to remove any light oils on metals or composites, or a dry wipe on plastics, is often all that is required with an MMA, although some projects will require the use of a solvent wipe or abrasion. Some MMA products are available that bond metal without the need for a primer, which creates a leaner process, removes the risk of error, and can improve health and safety.
Some disadvantages
Acrylic adhesive technology does have a stronger odour than other chemistries, although all SCIGRIP MMAs have GREENGUARD certification for low VOC and chemical emissions. MMAs have lower operating temperature ranges than, say, epoxy adhesives. In some cases, where there is larger adhesive mass or thicker bond lines, understanding of the exothermic heat produced during fast cure is required to avoid potential substrate discolouration or distortion.
MMAs in light aircraft and glider manufacturing
One company using MMAs in the manufacture of high-performance motor gliders is Papiorek. Initially, the company used a myriad of chemical compounds to bond the pilot canopies to the structure of the glider, a time consuming process. Technicians experienced issues with bubbling on application, and had to wait up to 12 hours for the materials to cure.
To overcome the process challenges, Papiorek trialled SCIGRIP’s SG42, a two-component, clear, reactive adhesive, and MMA SG5000, a fast curing, primerless metal bonder. SG42 was used to bond Plexiglass substrates in the aircraft’s cockpit, resulting in a seamless bond line and clear joint that enabled perfect visibility though the pilot’s window. SG5000 was used to secure the Plexiglass canopy to the aircraft’s epoxy frame.
There were several benefits of working with these two MMAs: the range of working times enabled greater flexibility throughout the assembly process, and the rapid cure time helped Papiorek reduce production times, while eradicating the bubbling problems their technicians had encountered with their previous system.
It is important to note that there is no perfect or universal adhesive, and the benefits and disadvantages of each chemistry should be weighed up against the demands of the application and processing environment. Overall, MMAs bring many process and technical advantages to applications in aerospace manufacturing, and may be a good candidate to trial when reviewing structural adhesive chemistries.
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