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Growing demand for non-PU structural adhesives after EU diisocyanates rule
Adhesives containing more than 0.1% diisocyanates can no longer be used in the European Union without following strict training requirements after a new REACH rule took effect on 24 August 2023. The announcement of the rule three years ago kicked off a wave of innovation in alternative structural adhesives.
Isocyanates, along with polyols, are the essential building blocks of polyurethanes, the plastic material used for foams as well as adhesives. Toluene diisocyanate (TDI), the most common isocyanate used for flexible polyurethane (PU) foam, is a toxic chemical that can cause irritation of the skin and eyes and affect the respiratory and central nervous systems. PU-based adhesives generally contain less hazardous methylene diphenylisocyanate (MDI), but MDI could still cause allergic reactions or lead to occupational asthma.
Regulators on both sides of the Atlantic have had diisocyanates in their crosshairs. The European Commission in 2020 added restrictions on diisocyanates to their Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulation that took effect on 24 August after a three-year transition period. Materials with over 0.1% of the stuff by weight can only be used if workers handling them undergo extensive training and certification every five years. In the United States workers protection agency OSHA is implementing comparable rules.
The rules are driving isocyanate replacement. For many companies, in particular the ones with high staff turnover or many temp workers, the training requirements and operational disruptions that come with them are unaffordable and unworkable. In response, there has been a growing demand in recent years for alternative adhesives that don’t contain isocyanates or polyurethane. The wave of innovation that this has unleashed is making PU-based adhesives look increasingly outdated, with newer technologies providing greater performance, higher strength, and faster curing.
For non-structural applications, bio-adhesives based on cellulose, starch, lignin, vegetable oil, or protein-based silanes can be a solution. However, increasingly structural adhesives are used to replace costly mechanical fasteners, join dissimilar substrates, speed up manufacturing processes, achieve more even stress distribution, and contribute to lightweighting and sound dampening. Industries like automotive and aerospace still commonly use structural adhesives based on synthetic polymers from fossil fuel sources.
Leading adhesives formulators over the last decade have worked on improving their polyurethane-based structural adhesives. Henkel developed a solvent-free polyurethane bicomponent product, with very low volatile organic compounds (VOCs), containing less than 0.1% diisocyanate. Regular PU hotmelt contains as much as 2-5% diisocyanate. Marketed as a MicroEmission solution, Henkel’s products let users avoid the training requirements now mandated under the REACH regulation, even though they are still PU adhesives.
The most used alternatives to structural PU adhesives are two-part epoxy and acrylic adhesives. A resin and hardener have to be mixed to initiate a cure at room temperature. Epoxy adhesives do not shrink during curing, unlike acrylic adhesives, and are more resistant to vibrations. Acrylic adhesives are easier to use in many applications and can bond various metals and plastics often without a primer. Companies focusing on innovation in acrylics, like 3M, claim their products can replace metal fasteners for both static and dynamic loads in automotive, aerospace, and construction.
A third isocyanate-free, non-polyurethane alternative is cyanoacrylate adhesives, known in the consumer market as superglues. They are easy-to-use, single-component, fast-curing adhesives that can bond a wide variety of substrates, but are traditionally not as strong as polyurethane, epoxy, or acrylic adhesives; great for plastics, and less effective on metals. Improved formulations with viscosity modifiers, rubber tougheners, and rheology modifiers to alter flow have made cyanoacrylates the second-largest engineering adhesives category after silicones.
But should cyanoacrylates be considered for structural applications? For 70 years since the introduction of cyanoacrylate glues by Eastman Kodak, later becoming Permabond, the basic production process of cyanoacrylates has not changed much. Henkel, 3M, HB Fuller, and others only introduced incremental improvements over the decades. The Knoevenagel process that everybody used involves a step of thermal cracking to break long molecules down into smaller ones, requiring very high temperatures and using lots of energy with low to moderate yields for most monomers. Only Ethyl Cyanoacrylate (ECA) delivers decent yields with this process.
A new crackless process enables cyanoacrylate monomers to be manufactured without cracking the polymer, making the process more efficient and sustainable. The new generation of cyanoacrylate adhesives is generally based on Methoxyethyl Cyanoacrylate (MECA), which unlike ECA is odourless and doesn’t cause blooming, the white glaze that appears when ECA cures. They are easier to manufacture with a higher yield and the resulting product has a higher impact, temperature, moisture, and solvent resistance and develops structural bonding to steel, aluminum, and other substrates.
The crackless process was developed by Ramón Bacardit, a research executive at Henkel who left to form his own company Afinitica, which patented the process in 2014 and was then acquired by Bostik, now Arkema. Henkel collaborated with Afinitica, but eventually decided not to pursue the crackless technology. In January 2021, Arkema formed a joint venture with Taiwanese cyanoacrylate maker Cartell Chemical to form CMC (Crackless Monomer Company), ‘to accelerate the development and production of high value-added engineering adhesives’.
Hybrid adhesives are another category that could benefit from the EU’s diisocyanates rule. Henkel with the Loctite brand built a strong position in patented structural hybrid adhesives that combine cyanoacrylate and epoxy, combining the curing speed and substrate compatibility of cyanoacrylate adhesives with the increased bond strength and durability of epoxy adhesives. Elsewhere there is innovation going on in hybrid adhesives based on modified polymer technology, combining the performance of silicones and polyurethanes.