Dehydrated castor oil (DCO) is castor oil heated with an acid catalyst so that water is removed from each ricinoleic acid chain. This creates a second double bond, often conjugated, which turns a non-drying oil into a drying oil that cures in air.
DCO is used in alkyd resins, paints, varnishes, printing inks, linoleum and some coatings. It dries faster than linseed oil and yellows less, and it has long been used as an alternative to tung oil.
How grades differ
DCO is sold in several grades. The main differences are how far the dehydration has gone and whether the oil has been bodied, which means heated further to raise its viscosity. Plants usually describe grades by viscosity and colour.
- Iodine value shows how many double bonds there are; it rises as dehydration goes further.
- Hydroxyl value shows how much ricinoleic acid is left unreacted; it should be low.
- Viscosity shows how much the oil has been bodied, often on the Gardner scale.
- Acid value shows free fatty acid, which affects resin making and pigment wetting.
- Colour, often on the Gardner scale, matters for white and light paints.
What can go wrong
Incomplete dehydration leaves a higher hydroxyl value and a slower-drying oil. Overheating darkens the colour and can raise viscosity beyond the grade. Residual catalyst can affect the resin cook.
Ask for a drying time test as well as the chemical figures if drying speed matters to you. Then run your own resin or paint trial on a sample before you approve the plant. Our guide on why quality drops after the first container explains why the first lot is not proof of the rest.
Shipping
DCO is a liquid and ships in drums, IBCs or flexitanks. It can skin over if exposed to air, so containers should be filled and sealed properly. See how castor derivatives are packed and shipped.
DCO and other drying oils
Paint and resin makers usually compare DCO with linseed oil and tung oil. Linseed oil is widely available but tends to yellow. Tung oil dries fast but supply is narrower. DCO sits between them, with good colour retention and fast drying when well made.
Switching oils in an alkyd recipe is not a one-to-one swap. Viscosity, acid value and drying behaviour differ, so the resin cook usually needs adjusting. Plan a few trial cooks before you commit to a new oil or a new plant.
Ask the plant whether it makes DCO from its own castor oil or buys oil in. As with other derivatives, feedstock quality shows up in colour and consistency.
Next step
We find DCO producers, have samples tested against your grade and supervise filling and loading. Book a free sourcing review to start.