Affects of cold temperature cycling on PolyCarbonate

[Phraedrique]

This comes from a Sport Fencing discussion group. Apparently there was a recent failure of a polycarbonate fencing mask in europe, and one of the possible claimed causes of this failure was the cyclical cooling and re-warming of the material from airline storage to room temperatures. This seemed to me rather counter-intuitive that a PC plate would become brittle due to what I would consider rather normal temperature cycles. While I understand that many thermoplastics do become more brittle at lower temperatures, it seems to me that their elasticity shuold return once the material has been allowed to return to 'room temperature' conditions. In this case there was a catastrophic failure of a (supposed) lexan (tm?) plate used as part of a fencing mask.
A picture of the shattered face plate can be seen on the fencing.se web site:

http://www.fencing.se/nyheter/allvarligt-tillbud-med-nya-masken-i-odense

So to get to the point:
1: could exposure to cold temperatures found in aircraft baggage compartments cause lexan plates to become brittle and remain brittle after tehy have returned to normal room temperatures?
2: based soley on the picture on the web site, what would be the most likely explination of this kind of catastrophic failure of what is SUPPOSED to be a lexan (or PC) plate?

 

[Pud]

IMO:

1) No. The reason PC has a high impact strength is that it has a Tg of around 135ºC, so it is "rubbery" below this.

2) Are there silvery streaks which appear to be inside the material just above the hole? From what I can make our these look like environmental stress cracks (ESC). PC is very prone to this effect - virtually any fats/solvents (which may include polishes/cleaners/even natural grease off fingers) usually are a death knell. Any chance of a close-up of this area?

Micro scratches on the surface will also degrade the impact performance greatly.

The failure exhibits brittle fracture which leads me to the above hypotheses.

Just a thought - is the visor injection moulded or is it from a flat sheet that has been curved?

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[Phraedrique]

Unfortunately (or fortunately as the case may be)I have no direct connection to the attached picture as it was originally posted by the ?danish? fencing discussion group, so I will not be able to get a better view of the damage unless they decide to post something better ontheir web site (included link...).
You mention that any kind of fats or solvents would degrade the material.. would that include animal fats and solvents normally present in perspiration?
Since this device is obviously used in a sport where the mask is constaly exposed to strikes with a metal object, sometimes at very high velocities, would that be counterindicated for use in this situation?

 

[Pud]

It would indeed include bodily secretions.

Continued strikes will also cause scratches which will act as notches or stress raisers. PC is not good at notches.

I would guess that this sort of device needs to be changed on a regular basis. I recall reading somewhere that the cheaper PC motorcycle crash helmets should be changed at least every couple of years.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[Pud]

I have just enlarged the image with Photoshop and the visor is badly scratched all over. Some are deep gouges. Must have had difficulty seeing!

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

PC failed regularly as face shields for cricket batsmen. This was due to its vulnerability to solvent stress crack. Exposure to aromatic hydrocarbons will send it brittle, especially at areas already stressed even lightly.

Some of these aromatics hydrocarbons are very common in in everyday items like aerosols, insecticides, cosmetics, food, body excretions, sun screen, detergents etc.

My recommendation for a tough RELIABLE reasonably scratch resistant reasonably clear plastic is cellulose propionate.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Demon3]

Just to give you some idea. You can add one drop of acetone (nail polish remover) to a piece of polycarbonate and it will instantly form cracks and be ruined. If the pieces was under stress, it can "explode" as the solvent hits. Cleaning fluids, hand lotions and other liquid can do the same albeit perhaps it will take a little longer. So yes, environmental stress cracking is by far the most likely culprit and polycarbonate is famous for it. Put one sticker on a polycarbonate motorcycle helmet and all the impact resistance is gone.

Chris DeArmitt

http://www.phantomplastics.com

Consulting to the plastics industry