# Non-biocidal wood modification

Why decay needs moisture above the fibre saturation point, the Fenton chemistry brown rot uses to get past the size limit, and how acetylation, furfurylation and thermal modification each lower equilibrium moisture content.

Brown-rot fungi attack wood with hydroxyl radicals, not enzymes, because radicals are small enough to enter a cell wall an enzyme cannot. Modern wood modification answers that by removing the water instead of poisoning the fungus.

Source: https://en.bioecon.ru/docs/biochem-industrial/polymers-materials/bio-impregnation-for-wood/
Updated: 2026-08-25



Wood preservation used to mean making wood toxic. The modern alternatives work on a different variable, and understanding decay explains why that variable is the right one.

## Decay is limited by water, not by fungal access

Wood-decay fungi need liquid water. Below roughly 20 per cent moisture content there is essentially no decay; decay becomes active above the **fibre saturation point**, around 30 per cent, at which the cell walls are saturated and free water begins to occupy the cell lumens.

This is the pivot. **Moisture content is the controlling variable**, and a material that cannot reach the threshold cannot rot regardless of what organisms are present.

Wood takes up water because its polysaccharides are covered in **hydroxyl groups** that hydrogen-bond water into the cell wall. Hemicelluloses are the most hygroscopic component, cellulose's amorphous regions next, and lignin least. Sorbed water also swells the wall, which is the same mechanism behind dimensional instability — the movement, cupping and checking that makes untreated wood difficult outdoors.

## How brown rot gets past a size limit

Brown-rot fungi degrade cellulose and hemicellulose and leave lignin behind, giving the crumbling brown residue the name describes. The mechanism is worth knowing because it is an elegant solution to a physical constraint.

**Cellulolytic enzymes are too large to penetrate an intact, lignified cell wall.** The pores in a sound cell wall are smaller than the enzyme. So early brown-rot attack is **not enzymatic**: the fungus deploys **Fenton chemistry**, secreting compounds that reduce iron and generate hydrogen peroxide, so that Fe(II) and H₂O₂ react to produce the **hydroxyl radical** — an extremely reactive, non-specific oxidant that is a small molecule.

Being small, it diffuses into the cell wall where enzymes cannot go, and depolymerises the polysaccharides there. Only after this loosening can the enzymes act. This is the same access problem seen in polymer biodegradation, solved in the opposite direction: rather than waiting at the surface, the organism sends in a reagent small enough to enter.

## Three ways to lower equilibrium moisture content

**Acetylation** reacts wood with acetic anhydride, converting accessible hydroxyls to **acetyl esters**. Two effects follow at once: there are fewer hydroxyls to bind water, and the bulkier acetyl groups occupy space in the cell wall, keeping it in a permanently swollen state so it cannot take up as much more. Equilibrium moisture content falls substantially — enough to sit below the decay threshold in normal exterior exposure.

**This is not a biocide.** No toxicant is added; the wood is chemically altered so that the conditions decay requires do not occur. Acetic acid is the byproduct, and residual acetic odour is the practical nuisance.

**Furfurylation** impregnates wood with **furfuryl alcohol** — made from pentose sugars in agricultural residues such as bagasse and corn cobs — and polymerises it in situ. The polymer bulks the cell wall, reduces moisture uptake and increases hardness. The feedstock is genuinely a residue stream, which is unusual in this field.

**Thermal modification** heats wood to roughly 180–230 °C in a low-oxygen atmosphere, degrading the **hemicelluloses** preferentially — the most hygroscopic fraction and a preferred fungal substrate. Equilibrium moisture content and swelling both drop.

Its cost is honest and should be stated: **the same degradation reduces strength**, particularly impact resistance and bending strength, so thermally modified timber is used in cladding and decking rather than in structural members. Every one of these methods buys durability with a property traded away — acetylation with cost and process complexity, furfurylation with weight and darkening, thermal modification with strength.