FEATURE
FASTENINGS & ADHESIVES
strUctUraL aDHesiVes HISTORY IN THE MAKING...
There is little doubt that the advent of structural adhesives changed the course of modern industry. Today, various types of structural
adhesives are available, many of which have evolved considerably over the years, leading up to the
patented hybrid technologies of today. Bob Goss senior technology specialist at Henkel, comments
A
polymers) adhesives are today available in both single and two-part versions, catering for fast and slow cure times, as well as large gaps.
lthough a multitude of definitions are available, general opinion suggests that a structural adhesive is a substance
that fastens together elements to produce high modulus, high strength, permanent bonds. Such an adhesive must be capable of transmitting structural stress without loss of structural integrity (within design limits). From a LOCTITE perspective, the origins
of structural adhesives extend back over half a century, when the first anaerobic products came to market. However, anaerobic adhesives such as LOCTITE 306, 307, 308 and 310 were only suitable for small rigid joints – and typically not plastics at that time. Furthermore, all of these products required an activator and were relatively slow curing. The solution to slow curing arrived with
LOCTITE 312, the so-called ‘minute-bonding’ adhesive. One drawback, however, was its limited cure-through volume, so it was only suitable for use with very small gaps. This all changed with the development of 326, which offered both fast bonding, better impact resistance and good cure- through volume, tolerating gaps up to 0.5mm. Ongoing product development meant the
company’s range was able to address an increasing number of structural applications. For instance, LOCTITE 330 and 329 Multibond were the first toughened acrylic adhesives, enabling the joining of flexible metal panels, while LOCTITE 340 and 341 were able to facilitate the passing of bonded parts through a paint bake system. Today, an even more diverse range of toughened acrylic products is available, including clear products for display work. Structural adhesives such as epoxies and
polyurethanes were available prior to their introduction into the Henkel portfolio, but these too have evolved over time. Notably, the addition of epoxies in the past 20 years provided new structural bonding opportunities thanks to their increased chemical resistance and gap-fill properties, while PU (polyurethane) and MS (silane-modified
42 DESIGN SOLUTIONS JULY/AUGUST 2021 ongoing DemanD
The popularity of structural adhesives across industry continues to grow as more and more design engineers come to appreciate the opportunities available. Using adhesives in comparison to fasteners or welds is likely to deliver more uniform stress distribution over a larger area, produce sealed joints, reduce the risk of corrosion and provide the ability to join dissimilar materials. But which structural adhesive is best? The
answer depends on many factors, particularly as each adhesive in this category has its own set of attributes. Today’s structural bonders include epoxies, acrylics, PU polymers, MS polymers and silicones, which range from extremely strong to extremely elastic in order to meet demand for both rigid and flexible bonds.
Rigid bonds are mainly required to
accommodate high mechanical force transfers, typically when replacing traditional assembly methods. In contrast, elastic adhesives are finding growing use in applications where there is high dynamic stress resulting from temperature differences or component movement, for example.
Differentiating factors
When addressing the various structural adhesives in order of highest strength/least flexibility, epoxies top the list. These can deliver strength of circa 35 MPa for high-performance bonding applications, but offer a low percentage of elongation in comparison with other types of structural adhesives. For bonding rigid substrates, epoxies are ideal. Aside from delivering maximum strength, these adhesives provide excellent resistance to moisture and chemicals, as well as heat resistance up to 220°C for one hour. Application examples include golf clubs (bonding the composite head to the shaft), bicycle front forks (composite to titanium or aluminium) and heat exchanger end caps. It is also worth noting that both conducting and insulating epoxies are available. Acrylics offer high strength (albeit less
than epoxies) on a wide range of substrates. These products cure on demand under heat, light or ambient conditions (depending on the adhesive selected) and some need no mixing. Acrylics also offer gap-filling, corrosion resistance and high heat resistance properties (30 minutes at 200°C), and are excellent at bonding dissimilar substrates. Applications include electric motors and loud speaker magnet bonding. PU adhesives represent a great balance
between strength, durability and flexibility, supported by ambient curing. These products are particularly ideal for suppressing noise and vibration, with excellent adhesion on both plastics and metals. Potential uses are wide ranging and include trailers, wind turbine blades, skylights, electrical potting and filter
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