Yellowed vac canopy - clearing suggestions?

[Scott Hemsley]

Title say it all.  I've 'discovered' 2 slightly yellowed Aeroclub Harvard canopies in my canopy stash.  Any suggestions on to losing the yellow and restore them to their original clarity?  

 

Scott

[72modeler]

...the entire modeling world wants to know!

Mike

[Seawinder]

The only answer I've ever read to that question is that there's no way to fix it since it's due to a chemical change in the material.

[Black Knight]

Wot they say. Can't be done

What I do is to pour special modelling plaster into the canopy then vac form new ones over the plaster buck

[Ed Russell]

3 hours ago, Black Knight said:

pour special modelling plaster into the canopy then vac form new ones over the plaster buck

That works fine.

 

Alternatively, make a silicon mould of the exterior aspect of the canopy then use resin (plus or minus metal powder) to make a master for vacforming. You are using a female shape as a male mould but given the negligible  thickness of the vacform material it sometimes gives better detail. It depends on how the canopy was originally made.

 

I have seen a method that does work for clearing  bromide pigment from solid plastic but it appears more trouble than it is worth and is not guaranteed to work on acetate or butyrate clear material. It uses a mixture of hydrogen peroxide and sodium carbonate and is done under UV light.

NB - This seems like a procedure which should only be attempted after becoming familiar with the techniques and having an appreciation of the hazards!

 

[politicni komisar]

 

Not much help to solve the problem. This happens with all VAC old canopies.

In stash 

i have some Classic Airframes and MpM kit  and aftermarket Falcon,Aeroclub with this problem age around 20+ years.

 

No work for them are all on hold ....even the union cannot help them.🤔

 

 

I have some from RobTaurus that are still transparent after ten years.

 

P.k

[dragonlanceHR]

Rob Taurus uses PETG plastic which does not yellow. Also, they are more rigid and that is a big plus for me.

 

@ comrade PK

I never had a Falcon/Squadron vac canopy go yellow, and I have some sets that are close to my age.

 

Cheers,

Vedran

[Work In Progress]

Me too, my Falcon stuff is all perfect and I've had most of that for between 20 and 25 years. It depends what the base plastic is.

Edited July 9, 2020 by Work In Progress

[Graham Boak]

From another current thread, Jay models sell replacement "long rear" Harvard canopies, originally Falcon.

[Black Knight]

11 hours ago, Black Knight said:

What I do is to pour special modelling plaster into the canopy then vac form new ones over the plaster buck

This was done for my Shorts SC1.

Kit maker could not supply a new canopy

 

 

 

 

[Scott Hemsley]

_{Thanks for all the replies/suggestions, gentlemen.  I had an email from a fellow modeller with plenty of vac experience and he basically told me the same thing - it can't be done and that I should consider vac'ing  a new one using the old to make a master.  Of course there is another option … use the Heller kit canopy, remove the required frames, polish and replace the rear 'Texan' portion with the longer 'Harvard' portion from the Falcon canopy.  They should match up quite well as the Heller canopy was obviously used in making the Falcon masters.} 

 

_{Graham.  Thanks for the suggestion of a Falcon canopy, but I have a few of those.  However, the Falcon canopy is of the earlier model of Harvard canopy with the 'extensive' vertical framing. - good for a Yale, Harvard Mk.I and a Harvard Mk.II.   My eventual goal is to make the canopy as seen on the post-war Harvard Mk.IV, which has even less vertical framing than the T-6G.}

 

_Scott

[qn30jEkPz7]

If you’re lucky it might be like dark-yellowing in alkyd paints where you get oxygen reacting with the plastic and yellow/brown discolouration. A bit of (gentle, through glass) exposure to daylight might see them lighten quite a bit as the chromophores absorb sunlight and break down

[Crimea River]

Well, I'll be the outlier and suggest that you put it out in the sun for a day or two. Nothing to lose and sunshine is cheap. A member on another forum tried this and had some of the yellow disappear.

[qn30jEkPz7]

https://www.justpaint.org/what-is-dark-yellowing/

 

[Rick Brown]

You can get cheep UV sources for doing nails (don't ask...).

I tried this technique on some old decals.

And failed!

Still, my nails now look beautiful.

Rick.

[Graham Boak]

Sunshine has been tried and doesn't work.

[John Thompson]

If you want to take the time, one of the members on Hyperscale Plane Talking using the name Hofner Bassman offers a service whereby he takes your yellowed canopy, makes a mold from it, then vacuforms a crystal-clear replacement. In fact, there's a chance he might have done the canopy you need for someone else and already have a master available, so check him out via the members list on HS and drop him a PM if you want to investigate further.

 

John

[Ed Russell]

Unfortunately, putting it out in the sun is more likely to increase the yellowing rather than have no effect. Similarly bleach, alkaline solutions and acid solutions have all been shown not to work. There is actually a reasonable amount of scientific study in this field due to such yellowing in medical devices being a flag for possible component failure.

There are no known cures - the method outlined in my post above may work on clear film - it does apparently work on solid plastics. As I said, it's easier to make a new one.

[Crimea River]

5 hours ago, Graham Boak said:

Sunshine has been tried and doesn't work.

Well, here's a case where it did: https://ww2aircraft.net/forum/threads/1-48-xp-55-ascender-prototype-weird-aircraft-trainers.46670/page-3#post-1540894

 

Vac form canopy before:

 

 

After two weeks on a sunny window sill:

 

[Black Knight]

Interesting.

I wonder what sort of plastic was used that it cleared. 

[qn30jEkPz7]

33 minutes ago, Ed Russell said:

There is actually a reasonable amount of scientific study in this field due to such yellowing in medical devices being a flag for possible component failure.

Yup, I worked in paints and coatings field and can vouch for the amount of time and effort that went into looking at the chemistry of yellowing, ageing and weathering. If it were predictable, consistent and always remediable there’d be a lot of chemists out of work  

[qn30jEkPz7]

10 minutes ago, Black Knight said:

Interesting.

I wonder what sort of plastic was used that it cleared. 

Could be a polycarbonate, some polyester or a styrene-acrylonitrile 

[303sqn]

The main cause of of yellowing in plastics is oxidation, heat and UV light. They cause desaturation and conjugation. When all the available bonds on a carbon atom are occupied by hydrogen atoms it is ‘saturated’. When there are carbon to carbon double bonds, C=C, then the molecule is said to be unsaturated. That is the difference between saturated and unsaturated fats. The formation of these double bonds results in conjugation. Conjugation is simply a chain of alternating double and single bonds like so, C-C=C-C=C-C=C-C=. In conjugated systems the π bonding electrons delocalise. That means that the electrons are no longer associated with a single carbon atom. Delocalised electrons absorb light in the visible spectrum and it is that that leads to the plastic looking yellowish. The colour of many dyes is derived from this mechanism.

 

Most flame retardants are safe but aliphatic bromine compounds are sensitive to heat and UV light and breakdown to release bromine. This accelerates the yellowing process.

 

Some manufacturers add a blue dye to counteract the yellowing.

[Roger Holden]

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.

[Black Knight]

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.

My Falcon canopies feel more like a vinyl