Method for the continuous production of biodegradable polyesters

A biodegradable polyester composed of:

• A) an acid component made of
• a1) from 35 to 60 mol % of at least one aliphatic dicarboxylic acid or ester thereof, or a mixture thereof, selected from the group consisting of:
• succinic acid, adipic acid, and sebacic acid;
• a2) from 65 to 40 mol % of terephthalic acid or ester thereof, or a mixture thereof,
• where the molar percentages of components a1) and a2) give a total of 100%, and;
• B) a diol component (b1) selected from the group consisting of:
• 1,4-butanediol or 1,3-propanediol, or a mixture thereof;
• b2) from 0.05 to 1% by weight, based on components A and B, of glycerol,
• D) a component
• d1) from 0.1 to 2% by weight, based on components A and B, of hexamethylene diisocyanate;
• and with an acid number, measured to DIN EN 12634, which is smaller than 1.0 mg KOH/g, and with an MVR to ISO 1133 which is smaller than 6 cm3/10 min (190° C., 2.16 kg weight);
• wherein the biodegradable polyester is obtained by a continuous process, the process comprising
• mixing the aliphatic dihydroxy compounds, the aliphatic and aromatic dicarboxylic acids, and, optionally, further comonomers (component C), without addition of a catalyst, to give a paste, or, as an alternative, the liquid esters of the dicarboxylic acids are fed into the system, as also are the dihydroxy compound and, optionally, further comonomers, without addition of any catalyst, where
• i) in a first stage, this mixture, together with the entire amount or with a portion of the catalyst, is continuously esterified or, respectively, transesterified;
• ii) in a second stage, the transesterification or, respectively, esterification product obtained in i) is continuously precondensed to an intrinsic viscosity of from 20 to 70 cm3/g to DIN 53728;
• iii) in a third stage, the product obtainable from ii) is continuously polycondensed to an intrinsic viscosity of from 60 to 170 cm3/g to DIN 53728, and
• iv) in a fourth stage, the product obtainable from iii) is reacted continuously with a chain extender D in a polyaddition reaction to an intrinsic viscosity of from 150 to 320 cm3/g to DIN 53728;
• where stage ii) is carried out in a tower reactor, and the product stream is conducted concurrently by way of a falling-film cascade, and the reaction vapors are removed in situ from the reaction mixture;
• and
• where, in stage ii), the transesterification product or esterification product is precondensed to an intrinsic viscosity of from 25 to 55 cm3/g to DIN 53728.