| 摘要 |
Polyvinyl acetals are prepared by adding under agitation a liquid aldehyde, having a water solubility not greater than 20 gms. per 10 gms. of water at 20 DEG C. under atmospheric pressure, to an aqueous solution of a polyvinyl alcohol at below 50 DEG C. in the presence of an acid catalyst, continuing the agitation whilst a polyvinyl acetal is precipitated and, further agitating, whilst the mixture is heated to a temperature above 50 DEG C. in the presence of a dispersing agent. The polyvinyl alcohol must be such that the residual vinyl ester does not exceed 30 per cent by weight of the polyvinyl alcohol, preferably the amount of residual ester is in the range 0.5-6.0 per cent. The dispersing agent may be added prior to the initial reaction. An accelerating agent may also be present. Preferably, the initial reaction is carried out at 20-30 DEG C. and the final treatment at 50-75 DEG C. Specified aldehydes are propionaldehyde, butyraldehyde, isobutyraldehyde, valeraldehyde and isovaleraldehyde. Dispersing agents listed are gelatine, methyl cellulose, starch, alkali metal salts of monoalkyl sulphuric acid esters of long chain fatty acids, e.g. " Duponal ME " (Na salt of a sulphonated mixture of normal primary aliphatic alcohols with lauryl alcohol predominating), " Avirol " (Registered Trade Mark) (salt of a mixture of cetyl and stearyl sulphates), long-chain sulphonic acids and their alkali metal salts, e.g. " Alkanol " (tri-isopropyl beta-naphthalene sulphonic acid), and " Tergitol " (Registered Trade Mark) (a secondary alcohol sulphate of 17 carbon atoms). Specified acid catalysts are sulphuric acid and sulphonic acids. Accelerating agents listed are the substances sold under the names " Duponal, " " Avirol, " " Alkanol " and " Tergitol " described above. Examples are given. |
