DesIgn ProDuCT news - DPN Staff

10 Design Product News
dpncanada.com June/July 2007
Feature: Adhesives & Fasteners
Hot melt adhesive a
marine alternative to
mechanical fasteners
By Ross Noel
Until recently, manufacturers of
marine products have had limited
choices when it comes to joining
parts together without
trading off productivity,
performance, quality
and cost.
Traditional screws
and bolts require drilling
holes, which can provide an avenue
for water to penetrate into the substrate.
Holes created for
bolts allow water
to penetrate
These holes can also create pockets for
moisture and debris to accumulate, which
can cause leaks, corrosion, contamination,
and even loosening of the screws and
bolts, leading to loss of attachment. In
addition, bolted areas of
the substrate concentrate
tension into stress points
that can result in fatigue,
crazing and cracking.
Organic, one-component
room temperature vulcanization
(RTV) sealants are frequently used to fill
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Hot melt adhesives develop strength and adhesion very quickly, and can eliminate the bottlenecks in manufacturing
caused by waiting for sealants to cure.
the space in the cavity, and pre-formed
organic gaskets can be used as an interlayer
between substrates to maintain a
seal during differential thermal expansion
and stress. However, exposure to contaminants,
high temperatures and ultraviolet
(UV) rays can break down and weaken
organic materials.
More recently, heat- and UV-resistant
silicone sealants have provided longerterm
performance in keeping out moisture
and debris. Unfortunately, as with organic
RTVs, silicones require time to cure and
develop adhesion and strength; this limits
their use to filling space and being used as
gaskets.
Taking that technology a step further
are new, reactive hot melt silicone adhesive
sealants, which provide the high performance
of neutral cure silicone sealants,
the immediate adhesion of pressure sensitive
adhesives (PSAs) and the immediate
green strength of high-viscosity materials.
Because this hot melt adhesive develops
strength and adhesion very quickly, it can
eliminate the bottlenecks in manufacturing
that are caused by waiting for sealants
to cure. Also, the fast increase in viscosity
reduces the “squeeze out” mess that
comes with typical RTV sealants.
When applied at temperatures from
120º to 130ºC, the sealant has bonding
strength similar to double-sided tapes,
with adhesion between 3 and 11 psi. Once
cured, it has a tensile strength from 350 to
700 psi, yet remains flexible and stable at
temperatures from -45º to 150ºC.
These dual adhesion mechanisms, from
the PSA and the neutral cure sealant, have
resulted in the reduction or elimination of
surface preparations with solvents, priming
or surface activation that many RTVs
and tapes require. The silicone hot melt
adheres well to common substrates such
as metal, wood, painted surfaces and
glass, as well as low-energy surfaces such
as PVC, ABS, PE, Teflon and many other
plastics.
In addition to sealing strength, the cured
silicone provides flexibility that mechanical
fasteners cannot. Rather than the
focused bond points of mechanical fastening,
hot melt sealants create a bond line
that allows the stress to be evenly distributed.
It also absorbs some of the stress
caused by substrate movement from thermal
expansions and mechanical stress.
Because it is not heat-activated, silicone
hot melt has a long pot life and 15-minute
open time, so it works well with automated
and hand-applied equipment. In
addition, it has a very low volatile organic
content (VOCs), which makes it user- and
environmentally friendly.
Ross Noel is Senior Applications Engineer,
Dow Corning (ross.noel@dowcorning.
com).
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