Hi Peter:
> A manufacturer has an IEC 950 type equipment with a polymeric enclosure
> meeting the fire enclosure requirements, in this case, enclosure is flame
> rated min. 94V-1. Assuming he now painted the enclosure to give it a better
> look, will this have an effect on the original flame rating? I would assume
> that the paint/enclosure combination could degrade the flammability rating.
I believe the answer is that the change to the
flame rating will be a function of the paint
itself, i.e., there is no general answer. The
paint may improve the rating, it may not affect
the rating, or it may degrade the rating.
I suggest that you perform an A-B comparison
test. Cut 1/2-inch by 5 inch bars from the same
part of two enclosures, one with paint and one
without paint. Then perform the vertical burning
test per UL 94.
Since you are performing a comparison test, it
is not necessary to have the precise UL 94 test
set-up. The only important test criteria are:
the bars be held in a vertical position;
the flame is the same for both painted and
unpainted tests.
For the purposes of such testing, you can hold
the bars with pliers above a candle flame. The
bar is removed from the flame after 10 seconds,
and the time of flaming is recorded.
Degradation for V-1 to V-2 only requires that
the bar drop flaming drops. All other burn
criteria remain the same.
Upgrading from V-1 to V-0 requires very much
less burning time, 30 seconds to 10 seconds.
The paint is heat-sunk by the plastic. Assuming
a thin coat of paint, the temperature of the
paint is the same temperature as the surface of
the plastic.
Furthermore, the paint prevents oxygen from
reaching the surface of the plastic. The
plastic cannot burn until the paint has burned
or is otherwise damaged to allow oxygen to the
surface of the plastic.
If the paint burns at a higher temperature than
the plastic, then the paint will retard the
rate of burning, and the sample could go from
V-1 to V-0.
If the paint burns at a lower temperature than
the plastic, then the paint will quickly burn
off the surface, and the plastic will burn as
if there is no paint. Because the paint is
thin compared to the plastic, the paint will
not contribute much thermal energy to the
plastic, and should not increase the duration
of burning by a large amount, i.e., enough to
exceed the allowed 30-second burn time for V-1.
Also, I don't believe the paint will contribute
to the plastic dropping flaming drops, which is
the difference between V-1 and V-2.
In my opinion, a thin coat of paint should not
degrade the flame rating, and may improve the
flame rating. If the paint is a thick coat,
then the burning characteristics of the paint
will have a much larger effect and will not fit
this analysis.
If the paint is only on one side of the plastic,
then I believe the plastic will burn "normally"
on the unpainted side, and the paint will have
no effect on the flame test (althought it might
have an effect on the end-product).
Remember that the UL 94 test is done with bars;
the edges of the bars will not have paint. The
flame is applied to the bottom edge of the bar,
to the unpainted plastic. I believe that this
point of ignition and the subsequent burning
will overcome the effects of the paint where the
paint ignition temperature is less than that of
the plastic.
Best regards,
Rich
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