Rubber restoration- an old laboratory trick for DIY tinkerers

About rubber restoration for old perished items to give them a new lease of life using caustic drain cleaner.

We were off to the ocean for a holiday after Covid19 lockdown and rummaging around for some snorkelling gear. I found a sad-looking pair of neglected flippers that were distorted, stiff and perishing. Rather than throwing them away, I thought that I would try to restore the rubber. There are complex treatments such as in this video and this post using boiling and all sorts of commercial chemicals and plasticisers.

However, I remembered an old and simple laboratory trick that we used to restore rubber stoppers that had gone hard and useless. It was simple, involved little effort and cost almost nothing.

The laboratory restoration process involved soaking the rubber in a caustic soda solution and it was effortless and made a miraculous recovery of the rubber properties.

I was uncertain about the type of rubber that was in the flippers, but I knew it was not silicone rubber (It does not perish and is the subject of some of my DIY silicone rubber posts). However, I thought that the caustic soda soaking trick might just work and I had nothing to lose if it did not work. Sodium hydroxide (drain cleaner) is featured in my tinkering posts such as making DIY sodium silicate refractory. For this purpose, it is best to be very pure (preferably from an unopened jar). However, for rubber restoration, this purity is of no concern. Here is an example of pure sodium hydroxide and if you purchase the pure product it can be used for many other tinkering tasks. Otherwise, any left-over dodgy caustic drain cleaner will do.

The rubber restoration treatment

Caution: Caustic soda (sodium hydroxide) is a simple, cheap and most useful chemical for a tinkerers kit. It is not a sneaky toxic chemical. It is just honestly nasty if you get on your skin and eyes (made of only sodium, oxygen and hydrogen). So use gloves and goggles and take great care and don’t let children, pets or others have access to the caustic in solid or liquid form.

Hint for using rubber restoration solutions

When spilt on skin caustic solution will make a slimy soapy feel on your skin that is difficult to remove with water alone. “It will tell you if you have contaminated yourself as it makes soap out of your protective skin oils and bites in deeper!”

When using caustic solutions or even mildly alkaline substances, I also make a bulk weak solution of vinegar in water. It can be instantly used to quickly neutralise any spilt or splashed caustic. It will immediately and safely neutralise any alkali on your skin and stop that slippery biting feeling. It also can be used to finish the alkaline rubber restoration and for the final clean up of contaminated tools etc. Lastly, the residual caustic liquid can go safely down your kitchen sink to clean your drains. Let it sit there overnight to let it have the maximum cleaning effect.

The rubber resoration liquid

I used a 2L bucket to make the caustic pickling solution and put this inside a much larger bucket that provides stability and support for the blades of the flippers. It will also contain any spills. Using the small bucket also meant that the ‘shoes’ of the flippers can be flooded with less caustic liquid volume. I do this work outside as the reaction with the perished rubber makes an unpleasant smell that will dissipate quickly outside. “The smell is not harmful and is just like the smell of alkaline degreasing agents when used on well-used kitchen walls prior to painting renovation work.”

The amount of sodium hydroxide used is not critical, but less is better as it avoids the hazardous of boiling and splashing while the sodium hydroxide dissolves and emits a lot of heat. It just means that the weaker caustic solution will take a little longer to work, but it is a powerful chemical that will do its job soon enough and the weaker is much safer.

The magic rubber resoration dunking

Put enough water in the small bucket to flood the shoe portion of the flippers and THEN carefully add approximately two dessert spoons of sodium hydroxide to the cold water. Done this way the water will get hot, but not hot enough to become a boiling splash hazard. I used a silicone rubber cooking spatula to gently mix and dissolve the sodium hydroxide pellets.

The foul smell of sweet success with rubber resoration

I knew I had a fair chance of success as the gungy oxidized surface of the rubber dissolved away and made the caustic solution go dark and murky and gave off that unpleasant kitchen wall degreasing smell. I was a bit impatient, as usual, and used the spatula to rub and scrape off the gunge layer. “But it would have come off with no effort, by me, in good time.”

When the soaking wash finished (after about 30 min), I rinsed off the gunge with a big volume of water in the large bucket and used a dishwashing brush to remove the adhering slimy gunge.

After such treatment, the rubber was left soft and alkaline and had a slimy feel and was still a little bit smelly. So I stopped this sliminess and smell by neutralizing the alkali by soaking the flipper shoes in dilute vinegar and water. It takes some time to neutralize the caustic because the caustic that has penetrated deep into the rubber needs to also leach out over time to react and be neutralized by the acetic acid from the vinegar.

When the cleaning and neutralizing was finished, I packed some newspaper in plastic bags to make shoe lasts to hold the shoe shape open while the restored rubber cured. The restored flipper shoes have a smooth, shiny, soft, stretchy and odourless finish once more and have been given a new lease of life.

Note: Some other posts on rubber restoration seem to treat the containers, pot and tools as somehow contaminated. “This is chemical nonsense!” When caustic solutions are washed off and neutralized properly, they will leave the items cleaner than they have been for a long time and there should be no residuals to be concerned about. Even tiny traces of alkali will eventually find carbon dioxide from the air to neutralize itself and make harmless carbonates that are part of the stuff-of-life on planet earth. “This is the same reaction that slowly spoils a poorly sealed container of drain cleaner and leaves the white fuzzy deposit around the lid.”

The silicone rubber kitchen spatula that I used in the caustic solution had some dark stains on/in the blade and I noticed that the solution also removed most of this staining, so there is another use for good old caustic soda. “I will still happily use it in the kitchen.”

Conclusion

Strong caustic soda solution can restore the softness and stretch of perished rubber snorkelling flippers and probably other rubber ware. The caustic chemical should be used with care with suitable safety precautions.

Tim