ADHESIVES
MANAGING
BLOOMING
One downside of Cyanoacrylates’ fast curing
speed is that they are quite volatile, which can
cause blooming. Here Peter Swanson, Managing
Director of adhesives specialist Intertronics, shares
his tips on how to manage and prevent blooming
in your process.
If you have ever used a
cyanoacrylate adhesive
(often known as “instant
adhesive” or “superglue”),
you may have seen blooming
— a chalky, white residue
usually around the edge of
the bondline on the surface
of your part. Importantly,
blooming does not affect the
integrity of the adhesive bond,
but it will be undesirable
if aesthetics are important
to you, or if the presence
of blooming might imply
a quality problem to your
customer. If you are making
medical devices, jewellery,
electronic instrumentation,
consumer electronics or
loudspeakers, for example,
you may understandably be
concerned about blooming
compromising the appearance
of your products.
WHY DOES IT OCCUR?
CAs cure very rapidly, usually
by exposure to the moisture
on the surface of the parts.
They achieve fixture strength
in seconds and full cure in up
to 24 hours. The chemistry
tends to be quite volatile, and
blooming is a side effect of
this volatility and fast cure.
The energy from the cure
chemical reaction can promote
the release of unreacted
cyanoacrylate monomers into
the air, which condense and
PROCESS
Another way of reducing
blooming is to minimise
the size of adhesive fillets.
Dispensing cyanoacrylate
adhesives is often a challenge
because of their fast reaction
time, but it is worth optimising
for two reasons. In general,
cyanoacrylate adhesives are
more effective when used
sparingly – cure is faster,
bonds are stronger and there
is material economy. They
are also optimal when the
bond is completely interfacial
– between two surfaces – as
the cure is a surface reaction.
Excess adhesive squeezed out
fall back onto the surface
of the part in the form of
white crystals or flakes. The
resulting hazy by-product,
most noticeable around the
periphery of the bondline, is
known as blooming, frosting
or blushing.
LOW ODOUR, LOW BLOOM
There are a number of
cyanoacrylate monomers
which are used to formulate
cyanoacrylate adhesives.
Ethyl (ECA) monomers are
the most commonly used in
both industry and consumer
products; they are effective
and cure quickly. Their
volatility means that they also
more prone to both blooming
effects and a harsh odour,
which may be irritating and
require local ventilation
during use.
Cyanoacrylate adhesives
based on methoxyethyl
(MECA) monomers are less
volatile than ECA products,
and are often marketed as
“low odour, low bloom”
formulations. They are less
susceptible to blooming
and have a more acceptable
smell. However, they can
compare poorly with ECAbased
products in terms of
cure speed, strength and
cost, and so their use involves
compromises.
More recent advances in
MECA-based cyanoacrylate
adhesives have enabled
the production of products
which are low odour and
low blooming without
compromising cure time or
physical performance. For
example, the Born2Bond
Ultra range of cyanoacrylate
adhesives combine the fast
cure associated with ECAbased
CAs with the low
blooming characteristics
of MECA-based CAs. Their
inherently low volatility
means less odour, and they
are less irritating; there are
no CLP hazard symbols on
the labels.
24 Issue 4 2020