Vegetable-oil printing inks

An ink is a colorant dispersed in a vehicle. The colorant is usually the same regardless of feedstock; the vehicle is where bio-based substitution happens, and it determines how the ink sets.

Four ways an ink stops being liquid#

Absorption. The vehicle soaks into a porous substrate, leaving pigment at the surface. This is how coldset web offset works on newsprint — the ink never truly dries, which is why newspaper rubs off.

Evaporation. A volatile solvent or water leaves, usually with heat. Heatset web offset, most flexographic and gravure printing work this way, and the volatile component is the source of the sector’s VOC emissions.

Oxidative polymerisation. The vehicle chemically cures by reacting with atmospheric oxygen. This is sheetfed offset, and it is where drying oils earn their name.

Radiation curing. UV or electron-beam energy initiates acrylate polymerisation, as in photopolymer resins. Nearly instant, no solvent, and the uncured monomers are sensitisers.

Why drying oils dry#

A drying oil’s fatty acids are polyunsaturated, and the reactive site is not the double bond itself but the bis-allylic carbon — the CH₂ sitting between two double bonds. That position’s C–H bond is unusually weak, because the radical left after abstracting the hydrogen is stabilised by delocalisation across the adjacent double bonds.

Autoxidation therefore proceeds readily: oxygen abstracts that hydrogen, the resulting radical adds O₂ to form a peroxy radical, which abstracts from a neighbouring chain, propagating the chain reaction. The peroxides decompose to further radicals, and the chains crosslink into a solid film.

The rate depends on how many bis-allylic sites the oil has. Linseed oil is rich in linolenic acid, with three double bonds and two bis-allylic positions per chain, and dries fast. Soybean oil is dominated by linoleic acid, with two double bonds and only one bis-allylic position, and dries appreciably slower.

That is the honest technical position on soy ink: substituting soybean oil into a sheetfed oxidative ink slows the cure, and the substitution is straightforward in coldset and heatset inks, where drying is by absorption or evaporation and the vehicle’s unsaturation matters much less. Metal-soap driers — cobalt historically, now increasingly manganese and iron complexes because of cobalt’s reclassification — catalyse peroxide decomposition and are used to restore speed.

Where the real advantages are#

Deinking. Recovered paper is deinked by alkaline flotation: the pulp is repulped with sodium hydroxide and surfactants, ink particles are detached from fibre, and air bubbles carry them off. Vegetable-oil vehicles contain ester bonds that saponify under those alkaline conditions, which helps the ink film break up and release from the fibre. Mineral-oil vehicles have no such bond. Cleaner detachment means brighter recycled pulp and less fibre lost with the rejects.

VOC reduction. Vegetable oils are non-volatile, so replacing mineral-oil distillates in a heatset vehicle directly reduces emissions.

Mineral oil migration, which is the sharpest regulatory driver. Mineral-oil vehicles contain MOSH and MOAH fractions — saturated and aromatic hydrocarbons. These persist through recycling into recycled paperboard, and from there they migrate through the board into dry foods. Some MOAH constituents include potentially genotoxic and carcinogenic structures. This is why food-contact board is a focus for vegetable-oil substitution, and it is a contamination argument rather than a carbon one.

Two honest caveats. The pigments are usually unchanged, including any heavy-metal-containing ones, so a “bio-based ink” claim concerns the vehicle. And bio-based does not mean fully renewable: the resins that provide gloss and rub resistance are frequently still petrochemical.