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FEATURE
FASTENINGS & ADHESIVES
OVERCOMING REACH RESTRICTIONS
Adhesive bonding has become key in the manufacture of parts that offer superior performance and durability in rail vehicles and infrastructure applications. But with legislation in place to improve control of the use of diisocyanates, Henkel is offering alternatives that overcome REACH restrictions, as Ruairi O’Kane head Global Strategy Aviation, Space & Rail, Henkel, explains
F
or decades, high performance bonding in the rail sector was achieved in part with adhesive solutions containing diisocyanates,
a family of chemical building blocks mainly used to make polyurethane products. Polyurethanes were preferred for their high initial tack, exceptional tensile strength, elongation (up to 600%), and high fatigue resistance. They also show effective resistance to rail cleaners, with no loss of mechanical properties, and do reasonably well in UV exposure for exterior applications. However, the use of these compounds will one day be restricted in Europe, and eventually elsewhere, due to the Registration, Evaluation, Authorisation and Restriction of Chemicals initiative, or REACH. REACH calls for the progressive substitution of specific chemicals such as diisocyanates (referred to as ‘substances of very high concern’ or SVHC) when suitable alternatives have been identified. Having entered into force in 2007, REACH provisions are being phased-in over 11 years. To determine REACH compliance, companies
must identify and manage the risks linked to the substances they manufacture or market to the European Union. They have to demonstrate to ECHA (European Chemicals Agency) how they’ve determined a substance can be safely used without harming the end customer. While the current aim of the legislation is
not to ban the use of diisocyanates, but to improve control over their use, the REACH restriction poses a challenge for rail industry OEMs. In addition, many individual local governments/plants/customers are choosing to ban them based on the negative health effects. Diisocyanates, in combination with various
polyols, are essentially the ‘backbone’ of a polyurethane material, making them almost impossible to eliminate. As a result, rather than
22 DESIGN SOLUTIONS FEBRUARY 2022
cope with regulations that lay down specific migration limits (the maximum permitted quantities of a substance classified as hazardous to health), OEMs are seeking alternative solutions to polyurethanes – and so are turning to Henkel.
SILANE MODIFIED POLYMERS Henkel’s Silane Modified Polymers (SMPs) are structural adhesives for applications where high elasticity, primerless adhesion on various substrates, and high fatigue resistance are needed. Like polyurethane adhesives, they were primarily designed and developed to replace welds, rivets and other mechanical fasteners, enabling the assembly of non-metallic substrates with the added benefit of contributing to lighter weight designs, also called lightweighting. However, they contain no solvent, isocyanates or silicone, and thus are not restricted by REACH. Applications for Henkel SMPs include
structural bonding of panels, walls, roofs and floors, as well as bonding windscreens and side windows to the train body. These materials eliminate the need for rubber seals, resulting in improved vehicle appearance, and are superior to polyurethanes in outdoor applications where UV resistance is required. The SMP materials have an advantage in
process robustness. Polyurethanes work well but require a multiple step exacting process of cleaning and priming that have very specific time limitations. For best results, SMPs require the cleaning and priming, but the process is less sensitive and tolerant of variations. In some cases, primerless adhesion is possible. In addition, Henkel SMP materials are
compatible with most paint systems, to allow seamless blending with the exterior finish for enhanced vehicle aesthetics. These solutions provide excellent anti-flutter qualities when
bonding inner to outer panels, and impressive UV resistance. Assembly line adjustment would be minimal for this transition, as SMPs can be dispensed by hand or automatically using a manufacturer’s current equipment.
OEM SOLUTIONS There are currently no categories of diisocyanate chemicals outside Europe and Asia that are subject to similar limitations, but that may not always be the case. There is already legislation in some states in the US, which follows similar legislation to the REACH and SVHC requiring additional restrictions. A renewed focus on environmental issues may eventually inspire the adoption of REACH regulations at the federal level in the U.S. and elsewhere. In fact experts believe that, in time, REACH
will lead to the phasing out of several substances deemed hazardous. Replacing adhesive solutions containing
diisocyanates with Henkel SMPs preserves the qualities OEMs expect from this preferred bonding method: noise, vibration, harshness (NVH) damping, corrosion prevention, improved aesthetics, even stress distribution, reduction of water leaks and efficient assembly time. Henkel provides a complete range of
technologies that comply with REACH restrictions. They are appropriate for a variety of assembly applications and maintain elasticity for improvement of torsional stiffness and accommodation of differential thermal expansion. The result is stronger, lighter, more economical vehicles with increased performance.
Henkel
www.henkel-adhesives.co.uk
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