Bio-based flame retardants

Polymer combustion is a self-sustaining cycle, and a flame retardant is a deliberate interruption of it. Which point it interrupts determines what it can and cannot protect.

The loop that has to be broken

Solids do not burn; their decomposition products do. Radiant heat from a flame pyrolyses the polymer, cracking chains into small volatile fragments. Those volatiles mix with air and burn in the gas phase, in a branching radical chain reaction whose key carriers are H· and OH· radicals. The heat released radiates back to the polymer surface and drives more pyrolysis.

Three inputs sustain this: fuel, oxygen and heat. The two practical intervention points are the gas-phase radical chain and the solid-phase fuel supply.

Gas phase: scavenging the radicals

Halogenated retardants act in the flame itself. Under heat they release halogen radicals, which react with the highly reactive H· and OH· carriers and replace them with far less reactive halogen species. The branching chain slows, and the flame is starved of the radicals that propagate it. This is efficient, works across polymer types regardless of whether the polymer chars, and needs relatively little additive.

It is also being phased out, and the reasons are environmental rather than technical: persistence and bioaccumulation of several brominated compounds, and the formation of brominated dioxins and furans under uncontrolled combustion. The bio-based alternatives do not replace this mechanism — they work in the other place.

Solid phase: making char instead of fuel

Phosphorus chemistry acts on the polymer before it becomes fuel. On heating, phosphorus compounds release phosphoric and then polyphosphoric acid, which dehydrates the polymer — stripping water from a carbon backbone rather than letting it fragment into volatiles.

The result is char: a carbonaceous, thermally stable layer that does two things at once. It is a physical barrier slowing heat into the material and volatiles out of it, and it is carbon that did not become fuel, removing mass from the combustion stream entirely.

This mechanism is far more effective on polymers that char readily — cellulosics, polyurethanes, polyesters, materials with oxygen and hydroxyls in the backbone. It works poorly on polyolefins, whose pure hydrocarbon chains depolymerise cleanly to volatiles with almost no char residue. That is the honest limit of the phosphorus route, bio-based or not.

Why phytic acid and DNA are genuinely interesting

An intumescent system swells into a thick insulating foam, and it needs three components: an acid source to dehydrate, a carbon source to char, and a blowing agent releasing gas to expand the char into a foam. Conventionally these are three separate additives — ammonium polyphosphate, pentaerythritol, melamine.

Phytic acid — myo-inositol hexakisphosphate, the main phosphorus store in plant seeds and abundant in rice bran — is roughly 28% phosphorus by mass, which is very high for an organic molecule, and carries a polyol ring. It is therefore an acid source and a carbon source in one.

DNA contains all three, which is why it has been studied seriously as a flame retardant rather than as a curiosity. Its phosphate backbone is the acid source; the deoxyribose sugars are the carbon source; and the nucleobases, rich in nitrogen, release ammonia on heating and act as the blowing agent. A single molecule carries the complete intumescent triad. Cost keeps it a laboratory demonstration, but it is a clean illustration that the required chemistry already exists in biology.

Other routes follow the same logic: phosphorylated lignin adds phosphorus to a naturally char-forming aromatic polymer, and tannins and other polyphenols are used as bio-based carbon sources in intumescent formulations.