White parts that have yellowed over the years can be sanded/polished or repainted, but where they are otherwise "as new", treating them with dilute Hydrogen Peroxide is a less invasive option.
The theory is that Bromine (Br2, which is a reddish/brown colour) added to the plastic as a fire retarder gradually finds its way to the surface, making white plastic look yellow. Exposing old plastic to peroxide and ultraviolet light causes a reaction that effectively cleans off the yellowing, and produces some bubbles to let you know it's working. Of course it's more complicated than that, but my Chemistry GCSE was a long time ago ...
Safety note: model making (especially the more advanced techniques) involves tools and chemicals that can injure you - so be sure to read, understand & follow the appropriate MSDS or manual, and have adult supervision if necessary.
You will need:
- a quantity of Hydrogen Peroxide - 3% or 6% depending on light levels*;
- glass jar large enough to get your parts in;
- PPE, e.g. eye protection & gloves at minimum;
- a bright sunny day.
*: 3% aqueous H2O2 seems to be the recommended concentration (along with very long exposures) in sunny climates. That's not an option for me here in the UK, so I bought a half litre of 12% to try to compensate for the lower light levels. I found the reaction in the early afternoon sun (on one of the 4 or 5 days so far this year where the sunshine could be described as "bright") was a little on the vigorous side, not dangerously so but bubble were visibly rapidly forming, so I suggest 6% (or watering down stronger solutions) is more suitable for less suitable climates.
The "before" image (above) shows a vintage Willy's Wheeler bumper holder - note the distinctly yellow colour.
This is simple enough - put item in jar, carefully add enough H2O2 to cover, and leave somewhere safe (away from kids, animals, sources of ignition etc) with direct sun exposure.
Agitate/turn the parts regularly to knock the oxygen bubbles off , and once you're satisfied with the colour, dispose of the solution carefully & leave the part soaking in clean water in case the plastic is porous & any H2O2 remains.
In this example, after 4 hours the colour was vastly improved, and after 5 it was obvious no further improvement was going to be seen - although the reaction was still ongoing. If I'd used a weaker solution (I really would recommend 6% for the UK) the reaction would have been gentler & it wouldn't have mattered if it had taken all day.
After rinsing/soaking and drying, I used a little Novus "Plastic Clean & Shine" to protect the plastic. It's possible the yellowing will come back in time (retardant will keep migrating to the surface), but I'm quite happy no additional damage has been done by this - the plastic still felt very smooth, flexibility the same as before, and the tiny cracks that were already present are no worse.
The "after" images (pics above) show the part is considerably whiter :)
Written by TB member Jonny Retro