shoulder/valve area which caus-
es leakage. And on 18 cans, the
valve blew off.
The weakest part is the connec-
tion between the valve and the
can. This test shows that cans
currently on the market do not re-
sist extreme temperatures, but the
temperature limits are in reality
much lower than 70°C. This is
why bursting cans have hardly
ever been reported by stores. On-
ly when exposed to direct sun-
light (which is against common
practice) might cans reach ex-
treme temperatures.
Fig. 2 shows the pressure of dime-
thyl ether as a function of temper-
ature. Pure dimethyl ether is used
in ice sprays against warts. It is
also used as a single propellant in
many spray applications in hair
care. Those cans normally require
a 12 bar-rated can because the
pressure of the filling does not ex-
ceed 8 bar at 50°C, when normal-
ly applied. Under certain condi-
tions, e.g. cans in which mainly
propellant is left, the pressure
reaches about 10 bar at 50°C,
which an indicated 12 bar-rated
can would withstand. But at 60°C
it would exceed the 12 bar limit,
which could cause the can to
burst. This is one of the reasons
why 15 bar-rated cans or even
higher-rated cans would be used.
At 65°C the pressure might lead
to a burst anyway.
Combined with the knowledge
that temperatures can go up to
65°C
8
, the intrinsic safety limits
that metal cans withstand by their
nature can be defined. This has to
be kept in mind when designing
plastic containers; they should
provide the same level of safety as
metal cans, and should be able to
withstand temperatures of at least
60–65°C. This measure is impor-
tant in order to maintain the safe-
ty record of the aerosol industry –
which has zero accidents relating
to failures in the safety standards,
as reported in RAPEX
9
.
Design criteria for
plastic aerosols
These are the design criteria for
plastic aerosol packs:
• Material shall not splinter on
burst
• Temperature resistance, but as
water bath testing is difficult
for plastic containers, particu-
larly when drying with hot air
afterwards, alternative testing
methods to hot water bath are
recommended. An additional
high temperature test, e.g. at
60–65°C, is not included in le-
gal proposals (to maintain
alignment of legal require-
ments with those for metal
cans). However, it reflects the
expected temperature resist-
ance during production and
when used by consumers
• Long term stability against en-
vironmental impact, including
stability tests on each individu-
al aerosol product
These design criteria are essen-
tial. But it is important to mention
that this is only one part of the de-
cision-making process. It is even
more important how to overcome
the disadvantages of plastics or to
leverage their unique properties.
Transparency might be an obsta-
cle to consumers, but it is a way
to make filling levels and dosage
clearly visible, which is some-
thing very difficult or virtually
impossible to obtain with metal
cans at low cost. Applications in
which dosage makes the differ-
ence are colourants, especially
mousses, or pharma applications.
Dosing valves should still be con-
sidered even though making sure
that the aerosol pack is kept in the
appropriate direction is also an
option.
Chemical resistance can be one of
the issues where plastic contain-
ers offer some advantages. De-
spite restrictions due to permea-
bility, there are still many
applications in which plastic is
the material of choice, and there
are plastic aerosol packs that sur-
vived over decades, stable and at
full functionality. It is just a mat-
ter of cost and not forgetting
whether the application is really
sustainable.
The biggest disadvantage of plas-
tic aerosol packs is the lack of full
recyclability. Metal cans are prov-
en to be fully recyclable. This is
not the case with plastic cans as
they tend to be composite packag-
ings. Normally a plastic container
is linked to a metal valve, which
is a composite in itself.
For metal cans this is not a prob-
lem, as minor plastic parts, coat-
ings and imprints do not cause
any problems in metal recycling,
but in plastic recycling, metal
pieces remain a challenge. This is
an important aspect for the aero-
sol industry to keep in mind, par-
ticularly when considering that
plastic pollution is a concern
passed on to future generations.
* More details on the results as well as the
reference list can be found on the Internet –
see download panel
The filling level is visibly low
“Plastic aerosol cans can survive
over decades at full functionality”
l
53
