Wednesday, 19 October 2016

Removing Yellowing From Old Plastic

article from Dave's Amiga Hardware Page

Many people have discovered that the cases of their old computers have become yellowed  after many years. This discolouration cannot be washed off as the plastic itself has been discoloured. It mostly affects ABS plastic, one of the most popular type of plastic used for computer cases, keyboards and other similar items.

Causes of Discolouration
There are a number of possible causes, which, on their own or in combination, which can cause this discolouration. I won't go into the fine details, but a technical explanation can be found here.

  1. The quality of the plastic: ABS is a mixture of several different polymers and stabilising agents. The stabilisers are intended to prevent yellowing, however they can be depleted by the oxidising process. Once depleted, yellowing can occur. It has been noted that individual keys on a keyboard can exhibit different amounts of yellowing, even though they are all exposed to the same environmental conditions. Some keys may have come from different batches with different amounts of stabiliser in them.
  2. Exposure to light: In many cases the discolouration almost seems like sunburn. Discolouration is worst where the item is exposed to light the most. Items that had been placed on top of the plastic, and left there for many years, can leave an imprint with little or no discolouration in the plastic underneath.
  3. Heat: Chemical reactions normally progress faster in the presence of heat. It is logical to assume this is also a factor causing or aggravating discolouration of plastics.
  4. Oxygen: Discolouration is the result of a chemical reaction between certain ingredients used in the plastic and the oxygen in the air.
  5. Surface condition: If the surface of the item is degraded (possibly due to exposure to UV light, harsh chemicals, or the result of certain ingredients gradually evaporating out of the plastic as a result of age), there may be microscopic pits or cracks in this surface. This increases the surface area of the plastic, meaning that more oxygen can get in to cause discolouration.

How To Remove Discolouration
It seems everyone has their own favourite recipe or method to achieve this. Basically we need to reverse the chemical reaction that caused the original discolouration. Usually these recipes involve hydrogen peroxide, a small quantity of laundry "booster" and a wetting or gelling agent which helps the solution stay on the plastic after being painted on. The item is then exposed to sunshine or UV light for several hours. Variations of this are known as 'Retr0brite' and can be found by using Google.

I have never really liked this approach. Having a thin layer of solution painted onto the plastic means that it's prone to drying out, which causes the reaction to stop. The solution then has to be reapplied. With the hot Australian sunshine here, I find I have to constantly apply the solution for several hours. Not ideal at all! Some people paint the solution on and then cover with cling wrap to stop it drying out. While effective, it's hard to 'paint' the solution on evenly and there have been cases of uneven results, with some areas with less discolouration removal than others.

I have found a method of removing discolouration that is simple, cheap, effective, and gives good results. It simply involves purchasing a jar of 'laundry booster' from the supermarket. An example is "Vanish Oxi Action". There are many other brands. Look for sodium percarbonate in the ingredient list. Dissolve it in water in a shallow container, and then fully submerge the item to be whitened in the solution. You may need to attach weights to keep the item submerged. The whole thing is left in the sun for several hours. Stir occasionally. I have found 500g of laundry booster for every 10l of water works well. You can experiment with different quantities of laundry booster, as it does not seem to be critical. I'm lucky in that I live in Australia with very strong sunshine, however I still get good results on cloudy days. The sodium percarbonate, when dissolved in water, creates the hydrogen peroxide that is needed to remove discolouration. It's worth noting that this solution is a milky white. I think this is an advantage as it helps diffuse the UV light, resulting in a much more even effect, without noticeably slowing the process.

At the time I was experimenting with using a UV lamp, however, this was not successful. Using sunlight is the best way to go

An item being retr0brited by this method

What To Expect

A badly yellowed VIC-20 case before treatment. Note variations in discolouration

The same VIC-20 case after treatment

Using this method ensures that that the effect is even over the entire object. Unfortunately results can sometimes still be patchy. Some people have noted some areas can get a patchy, chalky look to them. This is visible in the VIC-20 case shown below. Note the rectangular darker area. A label had previously been there, which I removed before retr0briting. As you would expect there was no yellowing under the label, and as a result, this area exhibited no change after the Retr0brite treatment and was not chalky. Given that the label would have protected the plastic surface underneath, I suspect that the surface not protected was damaged in some way. UV light is well known for damaging plastics, and it is quite likely the surface was degraded, possibly with microscopic holes and fissures invisible to the naked eye. This would increase the surface area, increasing exposure to oxygen in the air. In addition, more volatile components of the plastic could have evaporated over time, causing changes to the chemical balance of the plastic. These changes could make the plastic react differently to the Retr0brite, causing the uneven, chalky result. On the other hand, it's also quite possible the chalkiness is already there and is masked by the yellow discolouration.



Is it a permanent fix for yellowed plastics?
The short answer is no. While Retr0brite reverses the effects of plastic oxidation, it does nothing to prevent it happening again. Below is the same VIC-20 case 4.5 years after the retr0brite treatment. It is clearly becoming yellowed again. Given that heat, light and oxygen are all involved in the yellowing process, some people have tried to prevent this happening again. Methods tried include storage in dark areas and painting the item with lacquer to protect the item from oxygen in the air. So far these have been unsuccessful. Until some more time has passed, and we get more results from further experimentation, I believe the best solution for items becoming yellowed again is to repeat the retr0briting. The plastic will not be weakened or made brittle by retr0brite as it affects only the surface of the item. UV light is more likely to cause this kind of damage.


It's worth noting that the chalky areas don't seem to be yellowing as much again. I suspect the constituent of ABS that yellows is depleted, possibly as a result of the damage caused by the original UV exposure. I have also noticed that areas that were never yellowed in the first place (due to lack of exposure to UV) have now acquired a slight, even yellow tinge. Could it be caused by the stabilisers in the plastic being inactivated by oxidation during the retr0brite process?

As this particular VIC-20 case seems to be very prone to yellowing, I am going to try retr0briting it again, then cover parts of it with different coatings to try and exclude UV light, oxygen, or both in different areas. Once summer returns here in Australia (late 2015), I'll expose the case in the full harsh Australian sunshine for several months and see what happens.

First Published: August 19th 2015

1 comment:

  1. Wow good idea I will give this a try on some of my old computers

    ReplyDelete