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Yellowed vac canopy - clearing suggestions?


Scott Hemsley

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39 minutes ago, Roger Holden said:

As far as I understand it, the yellowing variety were made from cheap PVC like industrial packing.  But Falcon and others use better quality PETG which contains some anti-UV agents.

Behaviour of PVC like that is a lot down to the plasticiser. Bisphenol-A and similar plasticisers added to it will oxidise with time to turn yellow and not-plasticise as well so the PVC becomes more brittle. Might be able to get it clear again but trimming would be a sod to do as the chances of splitting would rocket

 

15 minutes ago, Black Knight said:

 

I use PETG plastic, usually re-cycled vac formed packaging for my vac canopies

Just two weeks ago I went through my box of spare clear plastic and threw out about half of it as that amount had yellowed. In a closed card box stored in my flat, it went yellow.

 

Probably for the best. Even if you were able to UV bleach them to break down the colour there is already oxygen damage done to break down some of the polymer chains so it’d have be less formable and more likely turn yellow again after heating

Edited by LostCosmonauts
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Thanks again gentlemen. This has turned out to be an interesting thread, but after all is said and done, it looks like I'll be getting some proper materials to make a master and vac a new one.

 

Scott

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8 minutes ago, 303sqn said:

Biphenol-A is not a plasticiser, it is used to manufacture hard plactics such as polycarbonate.

 

http://www.rheothing.com/2016/06/bpa-is-not-plasticizer.html

True, I was trying to tie back to a chemistry that most modellers know and trying to say that the plasticisers in PVC (probably a phthalate ester) will age and yellow similarly to an epoxy. I worded the comparison clumsily and agree Bis-A and F are indeed mainly used as the diglycidyl esters to make polycarbonates, epoxies and in hybrid resins with polyesters and polyurethanes.

 

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Got a lovely CMK Attacker in the stash with the canopies still sealed in plastic, but they have severely yellowed nonetheless. Luckily I have nice and clear replacement canopies, but I'm going to try the sunlight trick on the originals just as an experiment.

I did some vacforming with acetate sheet, and noticed that my home-made canopies come out way thinner than any aftermarket ones. So thin in fact that I could use the original canopy (polished up and all) as a master.

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I just read 2 VERY interesting methods for treating yellowed clear vacforms on a 1/43 car model FB page.  One is to soak the part in a 50/50 mix of water and bleach for a few days.  The other is to soak it in hydrogen peroxide.

There are several people saying they've had good results....

 

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7 hours ago, Roger Holden said:

I just read 2 VERY interesting methods for treating yellowed clear vacforms on a 1/43 car model FB page.  One is to soak the part in a 50/50 mix of water and bleach for a few days.  The other is to soak it in hydrogen peroxide.

There are several people saying they've had good results....

 

How long would it stay in the hydrogen peroxide?

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4 hours ago, Roger Holden said:

Not sure; a while I think.  It was a method being advocated by some French guys.

I'll have to give both a try, but I did read somewhere that they can't be cleaned and to make new ones. Just have to try I guess.

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I tried both Hydrogen Peroxide and bleach (and Ammonia on a separate occasion) on yellowed canopies some time ago with no effect. It may be that the peroxide concentration I had (medical use from a pharmacy) wasn't strong enough, but that's all I had access to.

 

I think that once they've gone, that's it. It's a shame, as I have a lot of old, unusable canopies knocking about.

 

Cheers,

Mark,

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