Reproduced from UNMAS Technical Note 10.10 / 02, Issue 1.0 (I drafted this part.)
11.
Visors
11.1. The effects of sunlight on visors
Users should be aware that the polycarbonate material from which
blast-visors are made is adversely affected by prolonged exposure
to sunlight (Ultra Violet light: UV). The effect of sunlight
is to create hardened areas from which a crack may propagate.
Visors have “shattered” in some recorded accidents,
and a follow-up indicates that these visors had been in use
for several years. The number of hours and the intensity of
the UV to which they were exposed cannot be reliably estimated.
A visor that shatters in an anti-personnel mine blast event
can add to the wearer’s injuries. Eye loss has resulted.
Polycarbonate can be chemically treated during manufacture to
provide a measure of UV resistance. The effectiveness of the
various UV treatments is not known but some lead to a reduction
in the optical clarity of the material. Visors manufactured
with UV resistant properties should be subjected to NATO STANAG
2920 V50 fragmentation testing to ensure that the level of fragmentation
resistance is not lower than that achieved with untreated polycarbonate
and should also be checked for optical properties before purchase.
Drawing inferences from the available evidence in the DDAS [Database
of Demining Accidents], it is recommended that:
1) visors manufactured from untreated polycarbonate are replaced
annually or every 225 days of use in order to minimise risks
of degraded protection as a result of UV exposure;
2)
visors are marked with identifiers so that their use can be
recorded and audited, and replacement can be made at timely intervals.
These
recommendations do not necessarily apply to the visor’s
mounting, frame or helmet, which may have a longer service life.
11.2.
Using visors attached
to helmets
IMAS 10.30 requires that:
“eye
protection …. providing full frontal coverage of face
and throat as part of the specified frontal protection ensemble”
be provided.
Users
are advised that many short visors attached to helmets do not
provide full frontal coverage of the face and throat and it
is not only the lower face and throat that are at greater risk
as a result.
The DDAS has records of many accidents where the
victims were wearing helmets and visors with the visor 'down',
but they had tilted the helmet back and looked out beneath the
bottom edge of the visor. This provided a direct line of flight
for fragments from a detonation to enter their eyes and eye
loss has resulted.
Visors
attached to helmets that were designed for military use often
stand some distance from the face and flare towards the bottom.
This allows good ventilation, but was not designed to protect
against a threat that comes predominantly from below (the most
common demining accident occurs while excavating the ground).
This
helmet and visor were used in an accident involving an eye injury
Drawing inferences from the available evidence in the DDAS,
it is recommended that:
1) short visors attached to helmets are replaced by longer versions
that provide “full frontal coverage of face and throat”;
2)
helmet visors are always used in a fully closed position;
3) purchasers consider replacing combat helmets with alternatives
that provide ventilation while allowing the visor to be closer
to the wearer’s face;
4) purchasers consider buying visors that do not have hinges
that allow them to be raised.
11.3.
Visor maintenance
Polycarbonate visors are easily scratched, especially in dusty
environments. When scratched, vision is impaired.
Users are advised that polycarbonate is porous and the use of
chemical polishes and abrasives on a visor face may have unpredictable
results on the protective properties of the material. The use
of abrasives will reduce the thickness of the material and should
be avoided.
It is recommended that:
1) the only polish applied to untreated polycarbonate should
be a high-quality, smooth toothpaste. The cloth used should
be dust and grit free: the soft lint material used
for cleaning spectacles is usually suitable;
2) a regime of visor maintenance by washing with clean soapy
water and storing in soft, dust free bags (with a strong outer)
should be enforced; (note: use soap not detergent! Hand soap
may be ideal and liquid detergents are not a substitute);
3) appropriate means of protecting visors in transit should
be devised;
4) visors should be checked regularly and replaced whenever
their condition restricts visibility because this may compromise safety.
Polycarbonate can have its outer surfaced hardened. This makes
the material a little more resistant to the light scratching
that is common, but less flexible because the outer surface
is hard. Generally, visors with a hardened outer surface must
be thicker and heavier in order to provide the same level of
blast protection as an untreated example. The hardened surface
of a treated visor should only be polished using methods and
materials recommended by the manufacturer.
*********************
My
own view...
When asked to give an opinion about a 'visor-grinding/polishing'
kit being offered for sale by a visor manufacturer. This is
what I wrote in response:
Any
method of removing scratches that involves using various grades
of grinding paste on one or both sides of the visor MUST reduce its
thickness. If this is done with a visor that was originally
5mm thick, the polished visor will no longer comply with the
IMAS requirement for an employer to provide 5mm untreated polycarbonate
(or equivalent) facial protection during Humanitarian Demining.
If the grinding process is done at high speeds and without
concern about the friction heating the material, spot hardening
of the material is likely to occur, which can cause polymer
chains to align and the material to lose its flexibility. This
is potentially very dangerous because, under the pressures associated
with an anti-personnel blast mine detonation, cracks are likely to propagate
from the hardened areas and the entire visor may shatter.
Polycarbonate is actually porous and the pastes
used when grinding may be absorbed within it (depending on the
chemicals used in the paste). Any change to the chemicals present
in a visor may change its ability to flex. Pastes should not be used unless proven safe in independent trials.
Exposure to Ultra Violet light (UV) does harden polycarbonate. To minimise
the risk that UV hardened polycarbonate protection is used in
Humanitarian Demining, it is a UNMAS recommendation that visors
be replaced annually, or every 225 days of use (whichever is
sooner). In fact, UV intensity varies dramatically, as does
the length of time that a visor may be exposed to UV during
a working day (or even during storage), so no general
length of time can be rigidly applied. If it could, the UNMAS
recommendation would probably have become an IMAS requirement
(a 'shall' instead of a 'should').
Light scratches can be removed by playing a hot-air gun over the surface of a visor. Because the heat only penetrates to a shallow level, this is the same as heat-treating a visor to harden it and make it less likely to get scratched. In independent blast tests carried out in Canada, heat-treated visors shattered far more readily than those not treated. Surface heat treating is irresponsible and unsafe. Slowly reheating all of the material to its moulding temperature in a fan oven large enough for air to circulate all around the visor, then allowing it to cool in air can remove scratches from a visor without reducing its ability to flex. The oven required is far larger than most that would be available, so this revival of a scratched visor would not usually be cost effective.
In short, lengthening the life of a visor by thinning
the material from which it is made is potentially unsafe. Visor
life should be approached by preventing damage, not by seeking
to repair it.
There
are simple ways of testing whether visors have become brittle
which can be done in the field - contact me for informal advice. Email me at avs(at)nolandmines.com
See also Developing face and eye protection.
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