Sun bloom ! .

[Don149]

Anyone had a problem with clear canopies getting a blue " bloom " after a model has been stood in the sun for some time ? .

In my case it is the nose and stbd waist gun position on a HK B 25 . I`ve removed the " greenhouse " with some delicate surgery

and polished the inside with metal polish but with only limited success ! . I`ve thought about dipping it in Klear , but wondered

if anyone , first of all , has had this prob and has come up with a better cure , apart from " don`t leave them in the sun " .

                                                                                                                                                                                                Don .

[MilneBay]

I have,, although I've  had it develop without being in direct sunlight as well. It was particularly noticeable in some Monogram kits I built about 20 years ago - I put it down to a bad batch of the clear plastic as other Monogram kits didn't suffer from it. I also tried the Future route as well but that wasn't a cure. Eventually I replaced the canopy on one of the affected models with an example from a similar scale Tamiya kit.  

[3DStewart]

It may be halogen based fire-retardants aging in the sun. Many old computer cases go yellow for this reason.  It's distributed through the material so polishing/future dipping will have no effect. I believe there are chemicals that can be used to reverse the process on the surface, which works for opaque plastics but not canopies. I think replacement is the only solution.

[Don149]

Thanks guys , looks like I`ll have to live with it . Funny thing is , it hasn`t affected the top turret ! .

[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.

 

But you say that the plastic has a blue colour. That seems to be rather unusual and I do not know of a mechanism that would cause that. The only thing that I can think of is that some manufacturers add a blue dye to counteract the yellowing. Maybe something is happening in the degradation of the plastic  causing the blue colour of the dye to manifest itself.   

 

Edited December 6, 2018 by 303sqn

[Don149]

Thanks for the chemical analysis , 303 , but I`m still not able to cure the damn problem .

                                                                                                                                          Ah well ! .

                                                                                                                                                          Don .

[MilneBay]

I also note that that slight blueish bloom is also present the cheaper varieties of gin and vodka which should be a clear spirit normally. Therefore it appears to be some product of the constituents of what the substance is made. In the case of the cheap alcohol I suspect that it would be some additive that allows the basic fluid to be "stretched" to save cost. 

[Ed Russell]

@303sqn 's analysis is spot-on and in vac-forms there is no cure.  however I assume the HK B-25 has polystyrene clear parts.

I have seen plenty of yellow vac-form ones but not a polystyrene blue one. A picture would be interesting.

 

Hmm......... vodka in canopies? Chemical Engineering PhD for someone in that I think!

[MilneBay]

14 hours ago, Ed Russell said:

@303sqn (Can't edit that out - odd)

 

Hmm......... vodka in canopies? Chemical Engineering PhD for someone in that I think!

 

Well cheap vodka and also gin - I first noticed that years ago while working in the ME and it is a common thing in the cheap spirits here. Obviously there is some common component in some alcohol applications and some clear styrene products that causes it.

[JWM]

 

On 12/6/2018 at 8:53 PM, Don149 said:

Thanks for the chemical analysis , 303 , but I`m still not able to cure the damn problem .

                                                                                                                                          Ah well ! .

                                                                                                                                                          Don .

The explanation point to reason - the structural changies in the volume of plastic. So no hope at all