| 摘要 |
When carrying out the acetylation of cellulose in stages by the introduction in succession of at least four acetylation baths, the earlier stages of the acetylation are effected with the aid of a strong acid catalyst such as sulphuric acid with or without the addition of halogen, and the later stages of the acetylation are effected after the introduction of a reagent capable of reducing the catalytic power of the sulphuric acid &c. The composition of acetylation baths with respect to acetic anhydride and sulphuric acid is so chosen as to give at the completion of the first two or three stages an acetate containing 10-15 per cent acetyl groups; and in the later baths the sulphuric acid is omitted, and in at least one of these the reagent for the partial neutralization of the sulphuric acid is introduced; and the temperature is maintained between 18-26 DEG C. during the earlier stages, between 25-40 DEG C. during the intermediate stages, and preferably between 40-50 DEG C. during the final stage. Suitable reagents are the salts of feeble acids with strong bases such as sodium or potassium acetates, and sodium lactate or benzoate, although the neutral sulphates of sodium and potassium may also be employed. The salts are added in solution or suspension in acetic acid. Quite small quantities of the salts suffice. The acetylation is preferably carried out in cylindrical acetylators provided with a water jacket and rotated about an axis perpendicular to its axis of symmetry. In an example, cotton is before subjection to the acetylation process, softened by treatment in the acetylator with acetic acid vapour as described in Specification 303,135; and at the completion of the acetylation when the mass is uniformly dispersed, the temperature is maintained at 35-40 DEG C. until the viscosity of the cellulose acetate has fallen to the desired value; thereupon the product is saponified in the manner described in Specification 303,135, [Class 2 (ii), Cellulose &c.].
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