VISORS

The use of plastic laminations to protect polycarbonate blast protection visors

Project Driver:

Blast protection visors are the last line of defence to protect the face and eyes of deminers carrying out manual detection and clearance. The Visors are made of opaque polycarbonate sheet that is thermally bent to shape. Over time and with regular use, accumulated scratching and abrasion to the outerskin of the polycarbonate skin reduces reduces visibility and thus creates a relatively dangerous situation for the deminer. There are documented incident reports of accidents where deminers sustained facial injuries while prodding at landmines with the visor up because their vision was impaired due to excessive scratching of the polycarbonate skin.

To improve the service life of the visors we undertook to the investigate the best means of providing antiscratch resistence for visors. Our work looked into film laminations as well as sacrificail layers that would be easily and inexpensively replaceable.

Laminated Film Option.

Summary:

The 3M film SCLARL 150 was found to be the most suitable product to protect the polycarbonate visors. It meets the requirements of:

Supplying scratch protection for the polycarbonate
Not reacting with the polycarbonate.
Being simple to apply and remove.
Having high optical clarity.
Being able to withstand the conditions in the field.

Helmet with visor with anti-scratch film applied. Click image/helmet/helmet-uwa.jpg for full-size image.

Application of the film:

Equipment:

Rubber squeegee (a strip of rubber in a rigid holder - see figure 1)
Lint-free cloth (such as cotton) - it must be soft and absorbent.
Spray bottle with a solution of mild detergent (dish-washing detergent or baby shampoo) and water (the solution must not be too weak)

A squeegee

Procedure:

Cut an area of film to size using the knife or razor blade. It is recommended that no sharp corners are present on the film as these are usually the first areas to lift. Use approximately a 10mm radius on corners.
Clean visor thoroughly with the solution of detergent and water. All grease and dirt particles must be removed.
Lightly spray the visor with detergent and water solution where film is to be placed.
Remove backing from adhesive film and spray the adhesive side of film lightly with detergent and water solution.
Position plastic film (the detergent and water should make it easy to do this).
When the film is positioned, press the solution out from underneath one edge. This edge should then adhere lightly to the visor.
>Lubricate the top of the film with a light spray of the detergent and water solution then starting from the adhered edge, use the squeegee to squeeze the solution out from underneath and soak up the excess solution with the cloth. During this step be careful to avoid air bubbles under the film and remove any dirt that is still present.
Once all the solution is squeezed from underneath the film, and it has adhered to the surface, leave the film to dry until all the cloudiness has disappeared from the film. This may take as long as 14 days, depending on the weather conditions.

Note: If the detergent mixture is not strong enough the film may not adhere properly to the visor.
To remove the film, simply peel off. If any residue is left behind from the film, this can be easily removed with ethanol (methylated spirits) and a soft cloth. A new film can then be laid following the steps given above.

Notes on the SCLARL 150 film:

Manufacturer:: 3M
Type:: 3M Scotchshield^TM Ultra Safety and Security Window Film
Cost:: Approximately A$35.50 per square metre
Meets requirements for:: Flame resistance; Tear resistance; Abrasion resistance; Shatter resistance; Weatherability
Composition:: An optically clear micro-layered polyester film with an acrylic abrasion resistant surface coating.

The film should not lift, discolour or admit moisture underneath.
There should be no bubbles underneath, especially at the edges, as this will impair visibility and encourage the film to lift.
The film doesnít stretch well, so complex shapes should be avoided.
If the adhesive side of the film is exposed to air for lengthy periods it can impair the adhesion of the film to the visor.

Cost Breakdown:

SCLARL 150 is available in three roll widths: 1.016m; 1.27m; 1.525m. All are 30.4m in length. Partial roll lengths can be bought, though a nominal cutting fee will be charged. The following cost breakdown shows material costs only in Australian dollars without tax.

Roll width (m) Cost per roll $A Roll length (m) Area on roll (m²) 0.20m x 0.25m areas Full visor (0.45m x 0.3m)

No. visors cost per unit $A No. visors Cost per unit $A

1.016 $1096 30.4 30.88 608 $1.80 201 $5.45

1.27 $1370 30.4 36.61 760 $1.80 268 $5.11

1.525 $1643 30.4 46.36 912 $1.80 335 $4.90

Plastic sheet overlay option:

Construction:

A 1mm sheet of transparent acrylic plastic was bolted to the blast protection visor using the brackets that connect the visor to the helmet. Acrylic plastic was used because it was immediately available. However it has the advantage over polycarbonate of being cheaper and more scratch resistant.

The sheet of plastic was flat and then bent around the visor and bolted in place. This means that the skin is constantly under tension and trying to lift itself off the visor. If the plastic skin were pre-bent to the right shape it would eliminate the tension and provide a better seal. It was decided that this would be difficult to do with the level of accuracy required to eliminate large gaps between the skin and the visor.

The first problem encountered in the construction was in the cutting of the acrylic. The 1mm thick plastic sheet is too thin to cut with a band saw as it flexes and cracks badly. It is also too thick to cut with shears without it cracking. It was decided to trial a hot wire method of cutting the acrylic.

A nichrome wire was heated by passing an AC current through the wire. The use of AC current meant that the voltage could be easily adjusted by using a variable transformer. Two wire diameters were used, 0.66mm and 0.94mm. The 0.66mm diameter wire was found to cool down too much where it touched the plastic. The thicker 0.94mm wire had fewer difficulties with this.

It was found that the 1mm acrylic could be cut with the hot wire. However the temperature had to be carefully regulated. Too hot and the acrylic starts to char and brown, too cold and it stays very viscous and sticky. Polycarbonate appears to cut better than acrylic, as it seems less susceptible to temperature variations. Only 3mm polycarbonate was tested, which was far too thick to cut effectively, no thinner polycarbonate was available to test. Based on the stability of the 3mm polycarbonate under varying temperatures, polycarbonate will cut better than acrylic of an equivalent thickness.

Thick plastic sheets tend to have much hotter temperatures towards the outside of the sheet being cut. This means that the surface of the sheet can be charring, while the inside is barely melting. This appears to be caused by areas of different temperatures over the wire. The thick polycarbonate also tended to weld itself back together after the passage of the wire – part of this problem stems from the wire having to move too slowly through the thick sheet.

A 68mm section 0.66mm thick nichrome wire cut the acrylic best at 1.83V, with an unknown current draw. A 150mm section of the 0.94mm thick nichrome wire reached the optimum temperature at 2.50V and 4.85A, for a power level of 12.1W.

From these tests it was determined that a hot wire is suitable for cutting thin acrylic and polycarbonate sheeting of about 1mm thickness. Thicker plastic sheet would be best cut with a saw. It should be noted that these tests were to ascertain the feasibility of using a hot wire to cut the plastic, not to gain data on the optimum setup.

When the sheet of acrylic was bolted to the visor using the helmet mounting holes, it was noted that the edges tended to lift. More mounting points were required to hold it down. Four holes were drilled, one in each corner, and the acrylic skin was bolted down at these points. This stopped the edges of the acrylic lifting.

One of the concerns with the sacrificial skin is dirt and condensation being trapped between the two sheets of plastic. It is unknown exactly how much of a problem this could be, it was assumed that it would occur frequently and severely impact on the vision through the visor. To overcome this it was suggested that a watertight seal be employed between the plastic sheets. A large o-ring was dismissed as being too difficult to place and silicon rubber was used. The silicon rubber is however difficult to apply between the sheets neatly and achieve a good seal with, the plastic easily lifts away from the silicon rubber.

Another alternative is gluing the skin to the visor, providing an excellent seal and ease of application. The adhesive has to be easy to apply and remove, have a strong enough bond to curve the skin onto the visor and must provide a watertight seal.

If the plastic skin were made to clip onto the visor, the quick removal and fitting would mean that any contamination behind the plastic skin could be easily removed. The suitability of this system would depend on how often the space between the plastic sheets becomes contaminated and hence how often it has to be cleaned.

Conclusion:

Attaching a 1mm thick skin of plastic to the visor will provide excellent scratch protection and will provide additional impact resistance. Although the weight of the plastic skin is negligible, alternative protection methods add less weight. The main concerns are the possibly of contaminants such as water and dirt entering between the two pieces of plastic and affecting the visibility. To effectively seal against these contaminants is a difficult process that adds additional weight and complications.

A hot wire is a suitable method of cutting thin plastic, though the method needs some refining. It isn´t suitable for thick sheets of plastic, where a mechanical saw is a better option.

Therefore this report concludes that attaching a sheet of sacrificial plastic to the visors is not the most suitable method of provide scratch protection. A thinner film bonded on with an adhesive or electrostatic attraction would be far simpler to apply and remove and be less prone to contamination between the surfaces since the gap would not exist once the film was applied.

For more information on any of these send us E-mail at demining@mech.uwa.edu.au
All graphics by Demining Research Team. Aug 1998.
Last modified: 12:48:52 Wed 23 Sept 1998 by Brian McLean