end cap/pleat bonding (including medical filters). One specific
example involving caravan assembly has seen 8000 screws replaced with LOCTITE PU adhesive. The move cut caravan weight and eliminated leaks, while providing a bonded
construction with 10 times more strength. As a result, the customer was able to introduce a 10-year body shell warranty. PU adhesives are also favoured for bonding car windscreens into place. This proven application is safety-critical as the joint has to withstand the force of a head-on collision and air-bag deployment. MS polymers are highly elastic and offer primer-less adhesion to most
substrates with no read-through on thin-gauge metals. Other characteristics include great UV stability and elastic behaviour, along with good health and safety attributes thanks to the absence of solvents, isocyanates and silicone. Typical applications include bonding solar panels into their frames and adhering train flooring into position. UV stability also ensures MS polymers are perfect for light fittings, where joining glass to metal is a common task. Last but not least, silicones deliver exceptional levels of stretch (up to
several hundred percent), but relatively low strength (up to 2MPa). As well as flexibility, silicones also provide temperature resistance up to 300°C (350°C intermittent) along with excellent UV and moisture resistance, and very good chemical resistance. Common uses include electrical potting, as well as hobs and oven doors (glass panel into metal frame).
Alongside these products there is now a new type of universal structural bonding adhesive in the mix, one that is already heralding a new era in product design and assembly. Patented hybrid technology first developed by Henkel in 2017 is yet another milestone for the LOCTITE brand. As the name suggests, hybrids combine the qualities of different
adhesives to achieve bond strength, speed and durability, providing improved performance on a variety of substrates and the versatility to solve even more design and assembly challenges. Many view the product as the perfect blend of durable structural adhesives and instant-bonding cyanoacrylates. Any application requiring both speed and structural integrity can therefore benefit. LOCTITE HY 4090, for example, provides exceptional adhesion and
strength on plastic/metal combinations, as well as rubber. Fast fixturing reduces assembly time, while curing is quick and robust, even at low temperature and with relatively large gaps between substrates. LOCTITE HY 4090 also offers good impact, vibration, moisture, temperature and chemical resistance. In addition, a number of important health and safety advantages make it safer to handle than traditional structural bonders. The balanced properties of LOCTITE HY 4090 ensure its suitability for general
assembly applications involving mixed substrates or poorly fitting parts. For example, it has proved ideal for a company making aluminium outdoor brochure holders with a clear acrylic window. The product cures quickly, causes no ‘bloom’ on the window and can withstand the outdoor elements.
Of course, every joint is different and subject to a multitude of different forces. Tensile, shear, compression, cleavage and peel are just some of the common force types that act upon mechanical joints, and these factors naturally influence product selection. Tensile force is the force applied to an adhesively bonded joint when
pulling the assembly apart perpendicularly to the bond line and adjoining substrate, while shear force is the force applied when substrates in an bonded joint are pulled parallel from one another along a plane. Most adhesive qualification and testing procedures evaluate the structural properties of an adhesive in the shear plane. Compression is the optimal force to place on a bonded assembly, and is applied to a joint when the bonded substrates are pushed together perpendicularly by an outside force. Peel and cleavage forces are similar to each other and are the least desirable forces for a bonded assembly. These forces apply to the leading edge
43 In caravan assembly,
8000 screws have been replaced with LOCTITE PU adhesive
of a bonded joint, delivering uneven stress distribution to the edge of the materials. Once the adhesive begins to pull apart along the leading edge, the fractures created in the adhesive may begin to propagate through the bond line. Despite the challenges presented by various forces, thinking about where
stress occurs in a joint means that simple design measures make it possible to maximise the opportunity for using structural adhesives and tap into the many benefits available. Thousands of application examples across several decades have shown that great advantages arrive from consulting with an adhesive expert such as Henkel, not just on joint design but other important factors such as surface preparation and adhesive application. Structural adhesives are clearly far more than just another footnote in
the history of engineering, they have helped to shape the course of modern design and bring countless benefits to industrial product manufacture.
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