Blown castor oil is castor oil that has been thickened by blowing air through it at a raised temperature. Oxygen links the oil molecules together, which raises viscosity, density and polarity.
It is used as a non-migrating plasticiser in nitrocellulose lacquers, printing inks, adhesives and leather finishes. It is also used in lubricants, hydraulic fluids, polyurethane systems and some cosmetics.
How grades are described
Blown oils are sold by viscosity. A plant can stop the blowing at any point, so it offers a range from slightly thickened to very heavy oils. Gardner-Holdt letters or a viscosity in mPa·s at a set temperature are common.
Make sure you and the plant use the same scale and temperature. A grade name alone is not a specification. These are the figures to agree:
- Viscosity at a stated temperature, with a tolerance band.
- Acid value, which rises as blowing goes further.
- Hydroxyl value and iodine value.
- Colour, usually on the Gardner scale.
- Specific gravity and moisture.
Why it is used
Blown castor oil mixes well with nitrocellulose and many resins. Because its molecules are larger, it migrates less than simple plasticisers. That gives films that stay flexible longer.
In leather and textiles it softens and adds water resistance. In inks it improves flexibility and adhesion. In lubricants it adds film strength and is useful where a polar oil is needed.
What to watch
Viscosity drifts between lots if blowing is not controlled. Colour can darken if the oil is overheated. Ask how the plant controls the end point and whether it measures viscosity on every batch.
Order a sample of the exact grade and test it in your own formulation. Then check later lots against that approved sample. Our guide on castor derivative quality issues lists other common problems.
Blown oil versus other castor oils
Blown castor oil is sometimes confused with dehydrated castor oil and with bodied oils. They are different products. Blown oil is thickened by oxidation and keeps most of its hydroxyl groups. Dehydrated castor oil has lost its hydroxyl groups and dries in air, which blown oil does not do to the same extent.
So a lacquer or ink formula written for blown oil will not behave the same with DCO, and the reverse is also true. Name the product clearly on your order, and include viscosity and acid value limits so the plant cannot substitute a nearby grade.
Blown oil keeps well in sealed drums but can thicken slowly with air exposure. Ask the plant to fill drums close to the brim and seal them tightly.
Next step
We find plants that make blown castor oil, test samples against your viscosity band and supervise filling. Book a free sourcing review to start.