[Luk5566]

I'm using plant based oils in my hobby metalworking workshop (mostly rapeseed, also sunflower oil) as cutting oil. Both as brushed on oil and in a mist setup. I like it because it is gentle on my skin, I'm not that worried if I breathe some of its mist and I'm being environmentally conscious.

I'm also planning to make a cutting emulsion with it when the time comes to replace the one I'm currently using based on mineral oil.

However, I'm expecting to have issues with oxidation fairly soon (sticky varnish like stuff on machines etc). It is not that big of a deal as it washes away with more oil, but if possible I'd like to avoid that problem.

Does anyone know of any additives one can buy or make in a hobby setting that would prevent oil oxidation?

I searched for research articles that might help, but it seems the only thing that is being described is converting natural oils to esters (like in bio diesel). This isn't really a viable option for me because methanol has recently been put on "very difficult to buy" list... So hobby biodiesel production is now dead in my country.

The same topic interests me regarding recycling of old hydraulic oil. One can possibly filter out particulates and some varnishes, dry water out, but then one needs some antioxidants. However in this case the simplest is probably to mix in some new hydraulic oil that already has those additives. There is no such option for edible plant oils (unless one wants to loose their biggest advantage - non toxicity etc).

Edit: One more thing, the additive can't be corrosive. So things like vitamin C are probably unsuitable (vitamin C being acidic). Are there any chemicals that prevent oxidation and are basic or neutral?

There is always a possibility of water condensation on the machine covered with that oil that includes the additive. So the additive needs to be basic when dissolved in water, or be neutral/insoluble

[Enthalpy]

Hi Luk5566, welcome here!

It's nice to be environmentally conscious... BUT rapeseed oil isn't quite healthy for YOU if it can oxidize, which does happen rather easily. On air contact, at light, it will oxidize. That's why it has opaque hermetic bottles and a conservation limit. I'd trust mineral oil better than rapeseed oil.

Sunflower oil is less bad in this aspect. But the oils least sensitive to oxidation are the most saturated ones (the ones unhealthy in your stomach), especially palm oil, coconut oil (which are solid fats usually) and the more fluid palm kernel oil. Refining eliminates their odour. Thick animal fats are stable too and they excel in some metalworking operations.

An other possibility is to saturate any vegetable oil with hydrogen, as is done for industrial food production. I haven't seen any "hydrogenated oil" available in small amount, and it's solid, as pastries want it.

I'm not convinced that an antioxidant would protect rapeseed oil over time as a thin layer in the air under light. Choosing the proper oil is much easier.

A more extreme example, linseed oil:
https://en.wikipedia.org/wiki/Linseed_oil
General ideas:
https://en.wikipedia.org/wiki/Triglyceride
https://en.wikipedia.org/wiki/Triglyceride#Saturated_and_unsaturated_fats
https://en.wikipedia.org/wiki/Fatty_acid
No need to learn all that! Just understanding the difference between mineral and organic oil, and what the saturation of the acids does to the triglyceride properties, would help you.

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Biodegradable transformer oil has about the same demands as yours, only more stringent. It's strongly processed from vegetable oils. It tends to be more fluid than metal working oils, but you could just try what operations it fits. This one is rather easily available.

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Emulsions rot easily even with mineral oil. With a vegetable one, I feel the challenge is too difficult as a hobby. You might check how the food industry protects and stabilizes its mayonnaise and other sauces, but I wouldn't even try.

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Biodiesel: oxidation act on the unsaturated (double) bonds of the fatty acids that make triglycerides (organic oils and fats). The transformation to biodiesel acts on the ester function to make the fuel fluid. Probably no relationship.

Any cheap alcohol can make biodiesel. Ethanol is usually restricted, so if methanol becomes scarce where you live, you can use isopropanol for instance. The reaction may need stronger pushing than with methanol, and you need a bigger mass of alcohol for the same mass of fatty acids.

[Luk5566]

Thank you for the reply :-)

Regarding rapeseed oil I think I might have confused it with canola oil. I thought it is essentially the same thing so I used rapeseed and canola to refer to the same plant oil. Here in my part of Europe it is so common if you buy a bottle of a generic "vegetable oil" in a grocery store it is more likely than not it is in fact canola oil. I read up on the difference between the two and if rapeseed oil toxicity you referred two was caused by erucic acid then today's canola should be safe. I'd say I prefer sunflower, but I do have quite a bit of canola too (that's what I use for cooking). Palm, coconut oil as you said are solids at room temperature.

Regarding other biodegradable oils you mentioned (animal fats). They have been used in machining in the old days, but vegetable oil (especially unrefined and unfiltered version) is substantially better from lubricity and ability to make heavy cuts point of view.

For that reason it is sometimes even used as an additive to mineral cutting oils.

Probably the only reason why it is not used in the industry is the oxidation problem...

Regarding hydrogenation. Are not hydrogenated canola and sunflower oils solids at room temperature too? Or does it depend on "how much" they are hydrogenated?

Thank you for the references. I did once known all the basics, but I have to say I forgot them very quickly.

In case of making biodiesel using isopropanol. This is very interesting. However one most likely needs dry isopropanol for it. There was time 2~3 years ago one could buy a 5l can of dehydrated methanol for very cheap. Unfortunately people seem to confuse it with ethanol, they drink it with predictable consequences and because of that it is very heavily restricted. For some reason no seller now wants to even touch it. I bought restricted items before, for example potassium permanganate. I just filled one form to state what I need it for and that's it. No such way with methanol... So I'll likely soon be looking for how to dry isopropyl alcohol.

Ethanol isn't restricted here. In fact almost every grocery store has 95% ethanol on its shelves, but it is very expensive compared to other alcohols. And 95% is still not anhydrous...

There is exception for methanol sales as rc fuel. Unfortunately they usually mix it with some nitromethane and oil. Depending on how hard it is to make anhydrous iso extracting methane from that may be an option.

Thank you for the tip on the biodegradable transformer oil.

Regarding emulsions, I believe they mostly rot due to lack of oxygen. An oil separates creating a film on the surface. This prevents oxygen reaching the liquid and non-oxygen using bacteria takes over. Simply aerating unused mineral emulsion every other day has been enough for me to avoid it. No doubt vegetable oil emulsion might be more challenging.

[Enthalpy]

All strongly unsaturated oils ("siccative") get unhealthy at air contact or under light, quicker if they were heated. It's not only a matter of erucic acid. Rapeseed and canola differ little on that point. I don't use them for cooking.

More saturated fatty acids make triglycerides more solid. The choice of the triglyceride, or its hydrogenation, adjusts that. But the more saturated oils are also the more stable at heat and in air. Palm oil is very stable in the pan, olive oil far less especially if native (unrefined), while walnut or linseed oils are considered unsuitable for cooking and dangerous then.

So you need some trade-off between stability and fluidity. Or you can improve both if the fatty acids are short, that's why I recommended palm kernel oil, which you can still hydrogenate for more stability.

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I bought 1L of 99.9% isopropanol for 10.90€ including shipment there
www.ebay.de/itm/185376195971 (replace .de by the desired language)
Pure methanol is as expensive, 8€/L
www.ebay.it/itm/163412224861
Is denatonium a worry to make biodiesel? You could just use stove alcohol.
www.ebay.de/itm/175739654576 100%, with denatonium, 4€/L
And why anhydrous? I expect the esterification to produce water anyway.