Adhesives offer significant advantages to design engineers, but just
what product is the best for the job in hand? Bob Goss gives some general
guidance
The conditions under which a structure is intended to operate determine
the requirements of the adhesive selected to bond its components. Bonding
areas, too, are sized on the basis of the load capacities of the cured
adhesive. But on many occasions, vibration, bending and fatigue
resistance criteria prevail over high load transmission capacity, and for
this reason, structures are often assembled using tough elastic adhesives
with lower ultimate strength instead of rigid products with high
mechanical strength. Structural adhesives are often classified on the
basis of their modulus of elasticity.
Many epoxies and most cyanoacrylates fall into the category of 'rigid'
adhesives. From the point of view of structural integrity, epoxy-based
adhesives are most widely used, especially where a large gap-filling
capacity is needed. In recent times, epoxies have been used for
structural bonds in the bodies of lightweight vehicles where previously
welding would have been specified. Where assembly involves close-fitting
surfaces, then a cyanoacrylate is commonly the preferred method of
bonding. They are used primarily on plastic substrates where their cure
speed and excellent mechanical resistance make them ideal for the
assembly of small parts.
'Tough' adhesives can accommodate higher peel, cleavage, dynamic and
impact forces, compared with rigid adhesives. Nevertheless, their nominal
shear strengths can be noticeably lower. For this reason, the bond areas
must be somewhat larger to transmit equivalent loads. At the bonding
line, the adhesive should be thick enough to allow it to deflect and
dampen the load effects.
Where there is a high gap-filling requirement, then tough epoxies or
two-component acrylics should be considered. Acrylics are used less
frequently in structural applications, although their performance and
ease-of-application make them perfect solutions to many assembly
problems. One of the most developed applications is the bonding of
ferrites in electric motors and loudspeakers, where a balance between
structural strength and easy application is required.
In situations where there is a moderate gap-filling requirement,
activator-cured acrylics, structural anaerobics or ultra violet (UV)
light cured acrylics and anaerobics are a good choice. Anaerobics cure in
the absence of air when the two mating surfaces are brought together. The
result is a tough and very durable bond. One application where anaerobics
really come into their own is the joining of cylindrical metal parts.
Structures assembled with elastic adhesives can deform without
compromising the integrity of the components. One good example involves
vehicle bodies. Here, the glass windows contribute stiffness to the
assembly through the use of polyurethane-based elastic adhesives that
join them to the body. Traditionally, the choice here was between
silicones and polyurethanes, but recent advances in epoxy, cyanoacrylate
and modified silane technology have resulted in an even wider range of
flexible adhesives being available to the design engineer.