The term acetate is used to describe fibres made from cellulose acetate. Cellulose is swollen in acetic acid and then converted to cellulose acetate by reaction with acetic anhydride follwed by dissolution in acetone. The resulting viscous solution is pumped through spinneretes to form fibres. Two categories are recognised, primary cellulose acetate (triacetate) and secondary cellulose acetate (diacetate). The differnce between them resides in the number of cellulose hydroxyl groups (coloured blue in Figure 3) that are replaced by an acetyl group (coloured green), a process termed acetylation. Triacetate has at least 92% acetylation whereas diacetate has less than 92% but at least 74% acetylation.