Andy Smith
MIne-action specialist
 

Blast visors, maintenance and UV

 


Reproduced from UNMAS Technical Note 10.10 / 02, Issue 1.0 - 01 July 2004
, derived from advice prepared by Andy Smith

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” is 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 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 prodding/excavating).

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. Once 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 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 and compromises 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.

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My own view...

I was recently 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 AP 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 properties, and should not be recommended without extensive and independent trials.

"Exposure to 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").

"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 ]