Oily residue on old toy rubbery tyres

Question

I have some sets made by well-known Danish plastic toy manufacturer from my childhood which I've been reviewing recently. Most of the elements was in a condition I was expecting (some yellowing, a little bit damage), but not with the smaller tyres. Originally, I found them quite oily/greasy by touch with some even leaving a bit of, what seemed to look like, liquid residue when removed from plastic wheels. Oh, and they were also full of domestic dust.

Didn't pay too much attention as I was going to clean them with the rest of bricks in washing machine using mild detergent. Upon drying I've check affected elements again and found that plastic elements exposed to rubber tyres were clean. These small tyres however, while their conditions was much better, they still had a bit of oily feeling by touch.

Tyres having this issues are up to 30 years old and rather small. Bigger ones are from the same time period but seem to be made of different material and did not show any signs of being oily even prior cleaning.

Affected tyres look like that if it matters (pictures just show the form, not the condition):

So I'd like to know few this around this issue:

• What is going on here?
• Is it possible to clean rubber tyres of this residue and get them back to normal condition? How long would it last?
• Is it safe for plastic elements made of ABS and poly-carbonate (two most common plastic types used by this manufacturer) to have contact with rubbery tyres exposing issue explained above? Any other possible hazards?
• Is there anything I could do to prevent from such issue happening to similar tyres in the future?

Disclaimer. I have not included a brand name for couple of reasons. First, my interest is purely from science/practical point of view. Next, I am not looking to accuse any one of some issues in case there are any. And last reason - idea was to hint a company, as I thought some may have better knowledge of exact materials used in above elements.

Answer 1

The "rubber", or elastomer, likely has plasticizers to soften the material and make it more elastic and less brittle. These often migrate to the surface as parts age. Some of these plasticizers for elastomers include:

• Sebacates, similar to animal fats
• Adipates, based on adipic acid, which is found in beets
• Phthalates, esters of phthalic anhydride Some are innocuous, but others, such as bisphenol A (BPA) may have long-term toxic effects. Washing the parts, as you have done, should remove them from the surface. You might need to repeat washing, using different detergents.

Another possibility is depolymerization, actual breakdown of the plastic. At room temperature, this process is slow, though it could contribute to that sticky surface layer.

As for safety, you might check on rules at the time and place the toys were made. Modern toys are made with safety in mind, but plastics have had a bad rap in the past, such as causing liver damage to workers in vinyl production. Caveat emptor.