The term used to describe fibres of cellulose ethanoate (acetate) wherein between 74% and 92% of the hydroxyl groups of the original cellulose are ethanoylated (acetylated). Purified cellulose is ethanoylated (acetylated) by ethanoic anhydride (acetic anhydride) in the presence of a catalyst (such as sulphuric acid or perchloric acid) in a solvent such as dichloromethane (methylene chloride) or ethanoic (acetic) acid. The reaction proceeds until primary cellulose acetate containing 60% of combined ethanoic acid is formed.
Secondary cellulose acetate is formed from the primary acetate by partial hydrolysis. It is obtained by adding water in excess of that required to react with the residual ethanoic anhydride, which thus allows the hydrolysis to take place.
Textile Resource (http://www.textile.org.uk)
Yarn and fabrics made of Cellulose Acetate