general terms, slowing down
the cure will allow more time
for volatile monomers to be
released. On the other hand,
making the cure a lot faster
will generate a higher exotherm
(heat energy) in the adhesive,
encouraging the release of
volatiles. Controlling the speed
of cure is one potential way of
minimising blooming.
If exposed fillets are
an inevitable outcome of
the bondline design, then
accelerating their cure should
restrict the opportunity for
blooming. One method is to
use an activator - a chemical,
usually applied by spray, which
quickens the cure. Application
might need to be carefully
controlled, as there is a risk
that this chemical reaction is
too fast, creating an exotherm
which drives off unreacted
monomers to give frosting or
blooming. Another, possibly
more elegant, approach is to
use a light cure cyanoacrylate
adhesive, like Born2Bond Light
Lock. Any interfacial adhesive
is cured by the normal
cyanoacrylate mechanism,
but exposed fillets are cured
tack-free by exposure to low
intensity UV light. The light
cure of only a few seconds
eliminates the possibility of
blooming because the volatiles
never have a chance to become
mobile. It is suitable for use in
assembling medical devices,
since it passes ISO 10993
testing. These light cure CAs
can replace an activator in a
bonding process, removing a
chemical and the associated
health & safety, environmental
and cost overhead.
Cyanoacrylate adhesives
will become slower with
age, and so more prone to
blooming. Practice good stock
control and housekeeping to
preclude the use of materials
which are out of shelf life.
ENVIRONMENTAL FACTORS
Simple local ventilation and
increased airflow will help to
into fillets around the bondline
will cure more slowly and will
provide exposed surfaces from
which blooming can occur,
so eliminating or reducing
this will help with a blooming
issue. Where joint size or
configuration means that
fillets are inescapable, a low
bloom, fast cure formulation
allows manufacturers a wider
dispensing tolerance.
CURE SPEED
A number of factors can
influence the cure speed
of cyanoacrylate adhesives,
beyond the inherent reaction
time of the formulation. In
blow the volatilised monomers
away from the parts before
they can settle on the surface.
Blooming will also occur if
the bonded parts are placed
too quickly into a sealed
environment such as a bag or
shipping container, where any
volatility is trapped with the
parts. Because CAs usually
cure by a reaction with surface
moisture, relative humidity
(RH) can be important. If RH
is too low, the adhesive will
take longer to cure. If it is
too high, the curing reaction
may occur too quickly and
the resultant larger exotherm
may encourage blooming.
Monitoring of relative
humidity is advised for critical
processes involving moisturerelated
cures (single part RTV
silicone is another example
where RH is important).
Higher ambient temperatures
encourage the volatility of
the CA monomers, leading
to a higher likelihood of
blooming. Conversely, lower
temperatures slow the reaction
mechanism, causing prolonged
cure time and opportunity
for volatiles to be released
into the air. Best practice
would suggest keeping the
relative humidity to 40% -
60% and the temperature at
around 21⁰C; consistency of
environmental conditions will
inform consistency of process
times and outcomes.
SUBSTRATES
Acidic surfaces like wood
or leather will cause
cyanoacrylate adhesives to
cure more slowly, increasing
the risk of blooming. In these
cases, it is recommended that
a “surface insensitive” CA
is used. They have additives
which neutralise the acid
surface and promote cure.
Alternatively, Born2Bond Ultra
offers a fixture time on leather
of 5-10 seconds, with a low
odour, low bloom formulation.
CONCLUSION
If cyanoacrylate adhesive
blooming is an issue, consider
changing to one of the latest
innovative technologies, which
can alleviate the problem
without conceding adhesive
performance. Low odour, low
bloom CAs are now available
which are fast cure or offer an
additional light cure property.
Dispensing and environmental
control are also important to
help get consistent results.
Manufacturers who have
previously avoided CAs due
to their downsides may be
able to benefit from these new
options, and manufacturers
who have struggled to manage
blooming may finally have a
solution.
Intertronics
www.intertronics.co.uk
01865 842842
www.fastening-solutions.co.uk 25
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