To obtain a formed item having excellent mechanical properties by forming a cellulose acetate having a specific value of an average acetylation degree, viscosity average polymerization degree of not less than a specific value and loss tangent of not less than a specific value.
This cellulose acetate has an average acetylation degree of 55-59%, viscosity average polymerization degree of not less than 220 and loss tangent tan δ of dynamic viscoelasticity of not less than 220 measured at 1 Hz and at 25°C. The cellulose acetate includes cellulose acetate butylate including a cellulose acetate as a major component and a mixture of a cellulose acetate and an inorganic acid. The cellulose acetate is manufactured by adding 9-18 pts.wt. of sulfuric acid, 200-400 pts.wt. of acetic acid anhydride and 300-600 pts.wt. of acetic acid to 100 pts.wt. of a pre-treated and activated cellulose acetate, reacting them under stirring at 50-85°C, neutralizing the mixture by adding a liquid mixture of acetic acid, water and magnesium acetate when the reaction is finished, introducing steam in a closed system and ageing thereof at 50-75°C for 90-240 minutes for saponification.
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