Bio-based photopolymer resins

Vat photopolymerisation builds a part by curing thin layers of liquid resin with light. Its three characteristic problems all follow from what the reaction physically does.

The reaction, and how deep it goes

A photoinitiator absorbs light and fragments into radicals. Each radical attacks the double bond of an acrylate or methacrylate monomer, which becomes a radical in turn and attacks the next. Because the monomers carry two or more acrylate groups, the growing chains crosslink into a rigid network rather than separate molecules.

Cure depth is governed by light absorption. Intensity falls exponentially with depth by the Beer–Lambert law, so a layer cures down to where the delivered dose drops below the threshold for gelation. This gives the working curve relating exposure to cure depth, and explains why light absorbers are added deliberately: reducing penetration depth sharpens the boundary between cured and uncured resin and improves resolution, at the cost of more exposure.

Shrinkage is not a defect, it is the reaction

Before curing, monomer molecules sit at van der Waals distances from one another, roughly 3–4 Å. After curing, the ones that reacted are joined by covalent bonds at roughly 1.5 Å. The material therefore occupies less volume than it did — typically several per cent by volume for acrylates.

The consequence is shrinkage stress. Early in the cure the resin is liquid and can flow to accommodate the contraction. Past the gel point it cannot, so all subsequent shrinkage builds internal stress — curl, warping, delamination, cracking in thick sections. Nothing removes this; it is managed by slower cure profiles, fillers, and chemistries with smaller volume change.

Cationic epoxy and oxetane systems shrink less, because ring-opening polymerisation converts a strained ring into a chain without bringing distant molecules together, and oxygen does not inhibit them. They cure more slowly and are sensitive to humidity, which is why radical acrylates remain dominant.

Oxygen stops the reaction, and one machine uses that

Molecular oxygen is a diradical and reacts readily with carbon-centred radicals, converting them into much less reactive peroxy radicals. The polymerisation stalls wherever oxygen can reach.

In practice this means the top surface of an exposed resin layer stays tacky — an inhibition layer tens of micrometres deep where the cure never completed.

Continuous liquid interface production turns the nuisance into the mechanism. The vat window is oxygen-permeable, maintaining a thin oxygen-rich “dead zone” where resin cannot cure. The part never adheres to the window, so the peel step that limits conventional bottom-up printing disappears and motion becomes continuous.

Where the bio-based versions sit

Acrylated epoxidised soybean oil is the workhorse: the oil’s double bonds are epoxidised and then opened with acrylic acid to install acrylate groups. It inherits the limitation described under bio-based polyols: the acrylate groups sit mid-chain, leaving dangling fatty segments that plasticise the network. Parts are tough but soft, with a low glass transition, so AESO is usually a component rather than the whole resin.

Isosorbide, itaconic acid and vanillin derivatives supply what the oils cannot: rigid or aromatic structures that raise the glass transition. Vanillin is obtainable from lignin, which makes it one of the few renewable aromatic routes at this scale.

One honest point belongs on any resin page. Uncured acrylate monomers are skin sensitisers and irritants, and a bio-based acrylate is still an acrylate. The hazard belongs to the reactive double bond, not to the feedstock, and it disappears only when the monomer is fully cured — which is why post-curing and glove discipline are part of the process rather than optional care.