The technology behind Accoya® wood is based on wood acetylation, a process that has been studied by scientists around the world for more than 75 years. This method of improving wood has been proven to deliver such superior performance that it has long been used as the “gold standard” against which other methods are measured.

The physical properties of any material are determined by its chemical structure. Wood contains an abundance of chemical groups called "free hydroxyls" (represented as OH in the picture). Free hydroxyl groups adsorb and release water according to changes in the climatic conditions to which the wood is exposed. This is the main reason why wood swells and shrinks. It is also believed that the digestion of wood by enzymes initiates at the free hydroxyl sites - which is one of the principal reasons why wood is prone to decay.

Acetylation effectively changes the free hydroxyls within the wood into acetyl groups. This is done by reacting the wood with acetic anhydride, which comes from acetic acid (known as vinegar when in its dilute form). When the free hydroxyl group is transformed to an acetyl group, the ability of the wood to adsorb water is greatly reduced, rendering the wood more dimensionally stable and, because it is no longer digestible, extremely durable.

source: https://www.accoya.info/technology.html