It offers fast green strength and fast cures at room temperature, sticks to metals, plastic and composite materials, and provides high-strength bonds once cured. What’s more, new products continue to be developed, reports Will Dalrymple
MMAs consist of a methyl methacrylate monomer with amines and a peroxide-based activator, according to IPS Adhesives, whose range is sold under the Scigrip brand. When mixed, the peroxide reacts with the amines to produce reactive free radicals, which interact with a double bond in the monomer, causing an exothermic (heat-releasing) cure reaction.
IPS Adhesives global new product director Thomas Race explains that MMAs tend to form molecular links in a free-form manner during cure, unlike epoxies and PUs, which tend to be linear. That helps to make them innately tougher than epoxies, as their linear structure makes it easier for a crack caused by an impact to propagate through the matrix, leading to a fracture. (Rubber additives also increase toughness).
Two-part MMAs cure in a different way to epoxies and PUs, which Rebecca Wilmot, business, marketing and product manager at Permabond Engineering Adhesives, pictured, compares to hooks and eyes: every molecule of epoxy resin must match up with one of hardener in a strict ratio, or the performance of the adhesive goes wrong. In contrast, she describes the MMA cure as a chain reaction, with distinct initiation, propagation and termination phases. This means that strict ratios are not required to achieve good performance; pressing together one surface with a bead of part A on to the other surface prepared with a bead of part B constitutes sufficient mixing.
Permabond offers several different forms of MMA product, including single-part, two-part with brush-on initiator and separate resin, and two-part initiator-resin premix that is dispensed with a static mixing nozzle.
Scigrip provides two main families of MMAs: in addition to the broad 1:1 ratio, it also offers a 10:1 range, which provide better elongation, at the expense of losing some strength (see box). A recent product is SG800, aimed at the transportation sector to replace mechanical fasteners. Both manufacturers continue to develop new MMAs.
If MMAs offer flexibility around their mix ratios, their manufacturers absolutely must formulate two-part systems to separate the reactive ingredients to provide a product that is stable – which means a 12-month shelf life.
INTENSE
Once the parts mix, the reaction is intense. Wilmot says that it can formulate adhesives to offer fixture time in less than two minutes, suitable for fast-moving production lines, such as bonding magnets on rotors or stators in motor manufacturing.
Such a quick cure can get very warm indeed; temperatures depend not just on the ratio of activator to resin, but also the volume of adhesive, says Race. That means temperatures of at least 100ºC even for slower-curing products. This heat is an advantage in that MMA bonding requires only room-temperature conditions to perform. No extra heating cycles are required or even encouraged. On the other hand, the heating can affect some substrates by creating bond line shadows. One way to prevent that is minimising the bond line; for example, Scigrip’s 1:1 products are limited to 2mm bond line. Having said that, Scigrip offers gap-fill products, as well as thixotropic mixtures that hold their shape.
Both manufacturers work through distributors. Biesterfeld is one of the main Scigrip distributors; its primary focus on plastics is complementary, contends Julian Ferries, its rubber and chemicals UK sales manager. “We have in-depth technical knowledge of substrates, and have the backup of Scigrip as a big brother,” he says.
Although MMAs tend to cure quickly (minutes rather than hours, although specific times vary) both manufacturers have tweaked the formulations to provide multiple opening times ranging up to about an hour. Wilmot at Permabond admits the longer open times are difficult to achieve with a 1:1 MMA formulation – although it achieves 60 minutes with one product.
COMPLICATIONS
Another potential issue affecting MMAs with longer open times is so-called ‘oxygen inhibition’ in which the adhesive’s surface becomes permanently tacky (the adhesive underneath may cure properly). Race blames that on an incorrect mix ratio leading to an overlong cure time; Wilmot, who calls the phenomenon quite common, offers a particular Permabond adhesive said to be particularly resistant to that.
Potential customers should also understand that MMAs have a strong smell. Both manufacturers recommend adequate ventilation, either natural or assisted by a powered fume cabinet or fume hood. While admittedly smelly, Scigrip’s MMAs have such low release of volatile organic compounds that odour is not a risk to health; they are certified to Greenguard certification, a US qualification for low chemical emissions primarily for furniture and commercial building materials.
In addition to odour, some MMA formulations have hazard warnings. Both manufacturers are working to remove hazard labels and improve the safety of their products, they say.
Potential users of MMAs should also know that they are flammable when in their uncured state, which means they require additional care during transport, since they are classified as hazardous materials. Once cured, however, they burn just as normal plastics do. (One exception is Permabond’s new TA4230 product, which contains a fire-retardant filler, and has been approved to UL94-V0). To mitigate the risk during production, Permabond makes batches of MMAs in a special ATEX area. “You don’t want a spark when volatile material is airborne. Ensuring adequate ventilation and not using mobile phones around volatile materials is good practice.”
On the other hand, risks are relative, she points out. Compare working with a flammable MMA to the standard method of making a structural repair of metal, say a handrail, using an oxyacetlyene welding torch. What with the gas and the naked flame, that application has a much higher flammability risk, she points out – so high that such work is prohibited in some places, such as oil and gas environments.
That product demonstrates how flexible the MMA platform is to new designs. IPS Adhesives Europe sales director Andy Pace adds: “There are still many opportunities that we have in our product pipeline. The need to develop new, lighter and stronger substrates and the demand for quicker and stronger sustainable adhesives remains high. We see this as a growth market, and we are continue to invest and support the relevant markets.”
BOX: MMAS IN FIGURES
A Scigrip white paper on MMAs – see www.is.gd/urazip –contends that its MMAs offer performance in between that of epoxy and PU adhesives.
In particular, its 1:1 range offers maximum tensile strength of 35-40MPa, and maximum tensile elongation of 5-10%. Its 10:1 range offers maximum tensile strength of 30-35MPa, and maximum tensile elongation of 30-150%.
In comparison, epoxy adhesives generally exhibit tensile strength of 27-36MPa and elongation of 2-6%, whereas PUs have tensile strength of 6-14MPa and elongation of 100-300%.
BOX: NEW PRODUCTS
New products are expanding the range of adherence to challenging materials.
Later this autumn, Scigrip is launching a low-surface energy 10:1 ratio MMA adhesive, SG400LSE, designed for bonding polypropylene not to require any surface pretreatment, and is said to be a big breakthrough.
Permabond’s TA4027 (pictured) is said to break new ground in adhesion, not to low surface energy plastics, but for metals without surface preparation. It includes an adhesion promoter said to cut through the oxide layer on metals, and any surface grime, to bond to the parent metal.
