A process for acylating a material, called solid, hydroxylated material , bearing hydroxyl groups (-OH), called reactogenic hydroxyls, accessible and capable of reacting with at least one fatty acid chloride in gaseous state, wherein: - said hydroxylated material is continuously run through an enclosure, called acylation enclosure , delimiting an internal space under a gaseous atmosphere; - a fatty acid chloride in liquid state is applied to the surface of said continuously run hydroxylated material ; - the gaseous atmosphere of said acylation enclosure is heated so that the temperature of the gaseous atmosphere in contact with said continuously run hydroxylated material in said acylation enclosure is at a temperature, called grafting temperature (Tg), lower than the vaporization temperature of the fatty acid chloride and sufficient to allow an acylation of said hydroxylated material passing through said acylation enclosure ; - a flow of a gaseous composition from downstream to upstream in said acylation enclosure countercurrently to said continuously run hydroxylated material is maintained; whereby: ∘ a continuously run material, called acylated material , is formed due to a covalent grafting acylation reaction occurring in said acylation enclosure between at least a portion of said reactogenic hydroxyls and fatty acid chloride in gaseous state, and; ∘ hydrochloric acid produced due to the acylation reaction is run from downstream to upstream by the flow of gaseous composition; characterised in that the flow of the gaseous composition is adjusted so that fatty acid chloride is maintained in the gaseous atmosphere of said acylation enclosure at a concentration such that the fatty acid chloride in gaseous state is present in the gaseous atmosphere of said acylation enclosure with a partial pressure substantially equal to the saturated vapor pressure of the fatty acid chloride at said grafting temperature (Tg) . - said hydroxylated material is continuously run through an enclosure, called acylation enclosure , delimiting an internal space under a gaseous atmosphere; - a fatty acid chloride in liquid state is applied to the surface of said continuously run hydroxylated material ; - the gaseous atmosphere of said acylation enclosure is heated so that the temperature of the gaseous atmosphere in contact with said continuously run hydroxylated material in said acylation enclosure is at a temperature, called grafting temperature (Tg), lower than the vaporization temperature of the fatty acid chloride and sufficient to allow an acylation of said hydroxylated material passing through said acylation enclosure ; - a flow of a gaseous composition from downstream to upstream in said acylation enclosure countercurrently to said continuously run hydroxylated material is maintained; ∘ a continuously run material, called acylated material , is formed due to a covalent grafting acylation reaction occurring in said acylation enclosure between at least a portion of said reactogenic hydroxyls and fatty acid chloride in gaseous state, and; ∘ hydrochloric acid produced due to the acylation reaction is run from downstream to upstream by the flow of gaseous composition; <EOS>