METHOD FOR THE SEPARATION OF VOLATILE COMPOUNDS FROM VISCOUS PRODUCTS BY MEANS OF A THIN-FILM EVAPORATOR, AND POLYLACTIDE RESIN

Abstract

A method for removing compounds in the gaseous aggregate state from PLA-containing products in the viscous aggregate state by means of a thin-film evaporator. The compounds may be present in the liquid or solid aggregate state in the products under standard conditions. The invention further relates to a polylactide resin prepared in accordance with the method of the invention.

Claims

1.-18. (canceled)

19. A method for removing compounds in gaseous aggregate state from a polymer melt containing polylactide and/or a poly(co)lactide, said melt comprising said compounds in liquid or solid aggregate state, with a thin-film treatment apparatus comprising: a housing having a heatable and/or coolable housing jacket which surrounds a rotationally symmetrical treatment chamber extending in an axial direction, at least one inlet port, arranged in an inlet region of the housing, for introducing material to be treated into the treatment chamber, at least one outlet port arranged in an outlet region of the housing, for discharging the material from the treatment chamber, and a drivable rotor shaft arranged in the treatment chamber and extending coaxially therewith, for generating a film of the material on an inside face of the housing jacket and for conveying the material from the inlet region to the outlet region, the rotor shaft comprising a central rotor shaft body with rotor blades arranged on the periphery of said body, radially outermost ends of said blades being spaced a distance from the inside face of the housing jacket, and at least one temperature sensor arranged in the treatment chamber for measuring the temperature of the film of material, the method comprising: carrying the polymer melt in a viscous aggregate state into the treatment chamber via the at least one inlet port, generating a film of the polymer melt on the inside face of the housing jacket, converting at least a portion of the compounds present in the liquid or solid aggregate state in the polymer melt into the gaseous aggregate state, discharging at least a portion of the compounds in the gaseous aggregate state from the thin-film treatment apparatus, discharging the treated polymer melt via the at least one outlet port from the thin-film treatment apparatus, and carrying out a permanent or temporary determination of a local temperature value of the film at one or more location with the at least one temperature sensor.

20. The method of claim 19, including comparing the temperature value determined with a setpoint value and regulating at least one operating parameter depending on the deviation from the setpoint value.

21. The method of claim 20, including regulating the amount of heat to be removed from or added to the polymer melt at a corresponding location depending on the deviation from the setpoint value.

22. The method of claim 20, including regulating the amount of heat to be removed from or added to the polymer melt at least partly by way of the amount of heat to be removed from or added to the housing jacket and/or by way of a rotary speed of the rotor shaft.

23. The method of claim 19, wherein the compounds to be removed are selected from the group consisting of lactide, lactic acid, lactic acid dimers, and lactic acid oligomers that are in gaseous form under the prevailing operating conditions, water and additives of the polymer synthesis.

24. The method of claim 23 wherein the additives include one or more of catalysts, initiators, or stabilizers.

25. The method of claim 19, wherein the thin-film treatment apparatus is in the form of a thin-film evaporator, a thin-film drier, or a thin-film reactor.

26. The method of claim 19, including assigning a signal line to the at least one temperature sensor and passing a signal indicative of temperature from the at least one temperature sensor to an external signal processing apparatus.

27. The method of claim 19, including a plurality of temperature sensors distributed over the length of the treatment chamber.

28. The method of claim 19, wherein the rotor blades are operated with a rotary speed of about 0.1 to 10 m/s, and/or the rotor blades are used to set a shear rate of not more than 1000 1/s, the shear rate being the ratio of the peripheral speed to the distance of the rotor blades from the inside wall of the treatment chamber.

29. The method of claim 28, wherein the rotor blades are operated with a rotary speed of from 0.5 to 2 m/s.

30. The method of claim 28, wherein the rotor blades are used to set a shear rate of not more than about 200 to 300 1/s.

31. The method of claim 19, wherein the interior of the housing jacket includes a housing jacket cavity which is traversed by a flow of a heat transfer medium for the purpose of heating and/or cooling.

32. The method of claim 19, wherein the housing jacket comprises at least two housing jacket segments which are heated and/or cooled independently of one another.

33. The method of claim 32, wherein the housing jacket segments each surround a corresponding treatment chamber zone, and temperature sensors are distributed over different treatment chamber zones.

34. The method of claim 19, wherein the temperature value obtained from the polymer melt is used to adjust the temperature of the heatable and/or coolable housing jacket to a predetermined setpoint value, the housing jacket includes at least two housing jacket segments that each possess at least one temperature sensor, and a separate adjustment of the temperature takes place in each of the at least two housing jacket segments by means of a respective measurement value of the temperature of the film of the polymer melt in each respective housing jacket segment.

35. The method of claim 34, wherein a viscosity of the polymer melt carried is at least 300 Pa.Math.s in the treatment chamber via a vapour port, and further comprising: adjusting a pressure which is reduced relative to standard conditions to a pressure of below 100 mbar, discharging compounds in the gaseous aggregate state via the vapour port, and recovering the discharged compounds in a downstream apparatus configured to recover gaseous compounds from the gaseous aggregate state by condensation and/or desublimation, wherein the residence time of the polymer melt in the thin-film treatment apparatus is from about 2 to 4 min, and/or the temperature of each of the housing jacket segments is adjusted to a temperature lower than the temperature of the polymer melt fed into the treatment chamber, and conditioned to a temperature of 130 to 250 C.

36. The method of claim 19, wherein before the polymer melt is carried into the treatment chamber, one or more of deactivators, additives, stabilizers or mixtures are fed into the polymer melt via a static mixer which is engaged upstream of the thin-film treatment apparatus.

37. The method of claim 19, including feeding an inert gas into the atmosphere in the interior of the thin-film treatment apparatus.

38. The method of claim 19, wherein the amount of the compounds in the polymer melt after passage through the thin-film treatment apparatus is not more than 0.2 times the amount of the compounds before passage through the thin-film treatment apparatus, wherein the residual lactide content of the polymer melt after passage through the thin-film treatment apparatus is less than 0.5%, the yellowing of the polymer melt, measured as b* value, during the conduct of the method is increased by not more than 4 scale values, the yellow coloration of the polymer melt, measured as b* value, after passage through the thin-film evaporator is less than 15, and during the method a maximum of 50 black specks are formed per kilogram of discharged polymer melt, and/or the weight-average molecular weight of the polymer melt after conduct of the method and after demonomerization is not more than 20% lower than the weight-average molecular weight of the polymer melt fed into the thin-film treatment apparatus.

39. A polylactic acid resin prepared by the method of claim 19, having a weight-average molecular weight of between 50 000 g/mol and 500 000 g/mol, a yellow coloration, measured as b* value, of less than 15, a residual monomer content of less than 0.5 wt %, and a black specks content of less than 50 per kilogram.

Description

[0060] In the figures

[0061] FIG. 1 shows an embodiment of a thin-film treatment apparatus of the invention;

[0062] FIG. 2 shows another embodiment of a thin-film treatment apparatus of the invention;

[0063] FIG. 3 shows a combination of a thin-film treatment apparatus of the invention with a mixing element and also a condenser and/or desublimator.

[0064] In the sense of the elucidations that have been made, the definitions which apply are as follows:

[0065] PLA: A polymer consisting substantially or entirely of lactide structural units.

[0066] Monomer: This refers primarily to dilactide, or simply just lactide. Lactide is the cyclic diester of lactic acid. Lactide comprehends the L,L-dilactide, the D,D-dilactide, and also the meso-dilactide, consisting of one L- and one D-unit.

[0067] Demonomerization: Removal, or apparatus for removal, of monomer from a polymer by transfer of the monomer to the gas phase and separation of the monomer-containing gas phase from the polymer. In addition to the monomer, there are always other volatile components present in the polymer, such as lactic acid, cyclic and linear oligomers, and also products of thermal polymer degradation, which are removed together with the monomer. On account of their low concentration by comparison with the monomer, they are not identified further in the text and are always included in the term monomer.

[0068] Thin-film evaporator: A vertically standing, cylindrical apparatus with an internal wiper system, which distributes the incoming polymer melt uniformly over the surface, which is temperature-conditionable from the outside, and conveys the polymer melt downwards. As a result of the conveying action of the rotor blades, a continually self-renewing melt surface is generated. The residence time in the apparatus and the surface renewal rate can be adjusted by altering the rotary speed of the rotor. The discharge of the melt from the apparatus is assisted by the rotor shaft, which is specially shaped in the discharge cone.

[0069] Temperature-conditioning zone: Region of the thin-film evaporator in which the temperature can be adjusted by a heat transfer medium which flows in this zone within the jacket of the thin-film evaporator. A thin-film evaporator may possess up to five temperature-conditioning zones independent from one another.

[0070] Triple point: Point in the pressure-temperature diagram of a pure substance at which all three phasessolid, liquid, and vapourcoexist. The triple point is the meeting point of the solid/liquid, liquid/vapour and solid/vapour phase boundary lines.

[0071] For pure L-lactide, this point is situated at 96.9 C. and 1.4 mbar. For the purposes of this invention, this value should not be regarded as absoluteit depends on the composition of the lactide in the method presented. The triple point is affected both by the amount of the optical isomers L-lactide, meso-lactide and D-lactide, and also by by-products of the PLA polymerization, which evaporate or sublime together with the lactide in the demonomerization. These products include lactic acid and other cyclic or linear oligomers of PLA, and also degradation products from the PLA polymerization.

[0072] Desublimation: Direct transition of a substance from the vapour state into the solid state at pressures and temperatures below the triple point, i.e. without passing through the liquid state in between. The opposite of sublimation.

[0073] Stabilization: In order to prevent the monomer reforming after polymerization and demonomerization, and so impairing the product quality, the catalyst for the ring-opening polymerization must be deactivated by addition of suitable additives. Suitable substances for stabilization are described in the relevant literature.

[0074] Black specks: Small particulate solids, mostly carbon-based, which form through degradation of the polymer as a consequence of long residence times at high temperatures, and possess an (average) diameter of more than 100 micrometres, so that they are visible in the product to the naked eye. If an apparatus for the processing of polymers produces black specks, this is a sign of dead zones within the apparatus, in other words of regions which are not traversed by flow and in which polymer is able to deposit and degrade. The degraded polymer is then washed out of the apparatus from time to time as black specks with the polymer.

[0075] The invention relates in particular to a method for demonomerizing a PLA melt in a thin-film evaporator. It is applicable to any polymer melt which consists substantially of lactide structural units, independently of the enantiomer composition, and has been prepared in a ring-opening polymerization.

[0076] FIG. 1 shows an exemplary embodiment of a thin-film treatment apparatus of the invention in the form of a thin-film evaporator 1, as used in the method of the invention. In this thin-film evaporator 1, via an inlet port 2, for example, monomer-containing PLA can be introduced continuously into the thin-film evaporator 1, as a liquid or viscous melt, in particular directly after the continuous polymerization. The viscous, liquid-melt PLA is distributed uniformly over the temperature-conditionable inner surface of the treatment chamber 13 of the thin-film evaporator 1, by wiper blades 3 connected to a central rotor shaft 6, and is transported downwards in the treatment chamber 13. Another feature of the equipment is that the central rotor shaft 6 is fitted, in the conical outlet region, with specially shaped conveying elements 4, such as a conveying screw, for example, which force the viscous melt into the intake region of the discharge pump 5. This discharge pump 5 is a gear pump with a specially shaped, very wide intake region. The construction of the wiper blades 3 and the discharge screw 4 is such that a residence time of the melt in the thin-film evaporator as a whole does not exceed 20 minutes and there are no dead zones formed that lead to degradation of the product. The monomer vapour leaving the melt is drawn off from the thin-film evaporator 1 via the vapour port 7, in counter-current to the melt flow. Temperature conditioning of the equipment is accomplished by a heat transfer medium, which is circulated by means of a pump 12 within the jacket 10 of the thin-film evaporator 1, the temperature of this medium being adjustable. The thin-film evaporator 1 possesses at least one housing jacket segment 10. The temperature of the heat transfer medium must be able both to be increased actively by a heater 8 and also cooled actively by a cooler 9. The heater 8 is needed particularly for the start-up operation, so that the thin-film evaporator 1 can be heated to operating temperature (180 C.-220 C.). The cooler 9 is an essential element for operation. It ensures that the product temperature during demonomerization does not increase too sharply and that the product is not damaged. The thin-film evaporator 1 possesses three temperature sensors 11. In operation of the thin-film evaporator, the temperature sensors 11 are submerged into the film of the product. At different locations in the product mixture, accordingly, the temperature can be monitored continually. By the central rotor shaft 6, moreover, inert gas can be put into the treatment chamber 13 in order to improve the demonomerization performance.

[0077] The demonomerization is operated under reduced pressure. It holds true here that the lower the reduced pressure, the better the demonomerization performance. The thin-film evaporator is operated typically at a pressure of less than 10 mbar (absolute).

[0078] A further embodiment of a thin-film treatment apparatus of the invention is shown in FIG. 2. The thin-film evaporator 1 in this case is substantially identical with FIG. 1, the sole and essential difference being that the cooling or temperature-conditioning jacket comprises three separately conditionable housing jacket segments 10a, 10b and 10c. These individual housing jacket segments 10a, 10b and 10c are each supplied via a separate circuit A, B and C for the temperature conditioning of the respective housing jacket segments 10a, 10b and 10c of the heating/cooling jacket 10 of the thin-film evaporator 1. At different vertical locations in the thin-film evaporator 1, therefore, it is possible to select different temperature regions and/or to set targeted temperature gradients over the thin-film evaporator 1 as a whole. Each cooling/heating circuit A, B and C for the individual housing jacket segments 10a, 10b and 10c of the temperature-conditioned jacket 10 of the thin-film evaporator 1 possesses a separate pump 12 and also heating elements 8 and/or cooling elements 9. The housing jacket segments 10b and 10c possess temperature sensors 11; in the housing jacket segment 10b, for example, there are two temperature sensors accommodated; the housing jacket segment 10c has a further temperature sensor 11. The housing jacket segment 10a (discharge zone) does not possess a temperature sensor. Moreover, provision may be made for the housing jacket segment 10c to be subdivided furtherfor example, provision may be made for separate temperature-conditioning of the part below the input port 2, while the part above the input port 2, where the vapour port 7 is located, likewise possesses a separate temperature-conditioning circuit. In this case, provision may then be made for no temperature sensor to be present in the upper housing jacket segment (conditioning zone), whereas a temperature sensor 11 is present in the separate housing jacket segment formed below the input port 2.

[0079] FIG. 3 shows an embodiment of the thin-film evaporator of the invention with an upstream static mixer C and also with a downstream apparatus E for condensing the discharged gaseous compounds. The thin-film evaporator in this case comprises two temperature sensors 11. The static mixer C connects to the inlet for product (see reference symbol 2 in FIG. 1), while the condensation apparatus E connects with the vapour port (see reference symbol 7 in FIG. 1) of the thin-film evaporator.

[0080] Polylactide melt A is carried into the mixer C. The static mixer C here may be fed with a metered addition of additives B, especially stabilizers for deactivating any catalyst still present in the polylactide melt. This prevents further reaction of the polylactide melt in the static mixing element C and/or in the thin-film evaporator D. Discharged gaseous compounds, particularly lactide, can be condensed in a downstream condensation apparatus E, and the monomer can therefore be withdrawn in liquid form from the condensation apparatus E (reference symbol F).

[0081] The thin-film evaporator according to FIG. 3 comprises two temperature sensors 11 and 11, which are formed in accordance with the embodiments of the thin-film evaporators according to the above-described figures.

[0082] While the pressure is above the triple-point pressure of dilactide (1.4 mbar), the monomer can be recovered (F) by condensation (E). In order to achieve particularly low residual monomer levels in the melt, however, operation at a pressure below 1 mbar is needed. In that case the lactide must be recovered by means of desublimation (E).

[0083] Rather than by a reduction in the pressure to below 1 mbar, the evaporation of the monomer can be facilitated by introduction of an inert gas into the thin-film evaporator. The presence of the inert gas here acts like a pressure reduction in that it lowers the partial pressure of the monomer in the gas phase, thereby in turn increasing the driving concentration difference between melt and gas phase. In this manner, the demonomerization can take place above the triple-point pressure of dilactide, with very good demonomerization performance at the same time. If there is a desire to reduce the residual monomer content still further, the inert gas can also be added at a pressure in the thin-film evaporator of less than 1 mbar.

[0084] Finally, in order to acquire residual monomer levels that are as low as possible, it is necessary to mix the stabilizer (B) into the melt (A) for deactivating the catalyst ahead of the thin-film evaporation (D). For economic reasons, the mixing of stabilizer and melt is best done in a static mixer (C). Addition after the demonomerization (G), as described in EP 2698394 or else in EP 1070097, is not sufficient to lower the monomer content of the melt by a factor of at least 5.

List of Reference Symbols

[0085] 1 Thin-film treatment apparatus; thin-film evaporator

[0086] 2 Inlet port

[0087] 3 Rotor blade; wiper blade element

[0088] 4 Conveying element

[0089] 5 Discharge pump

[0090] 6 Rotor shaft body

[0091] 7 Vapour port

[0092] 8 Heater 9 Cooler

[0093] 10 (a,b,c) Housing jacket segments

[0094] 11 Temperature sensors

[0095] 12 Pump

[0096] 13 Treatment chamber

[0097] The present invention is elucidated in more detail by way of example with reference to the experiments below.

Analytical Methods

Residual Monomer Content of PLA

[0098] The PLA sample is dissolved in chloroform and precipitated with isopropanol. The precipitated PLA is removed by filtration to leave the low molecular mass constituents in the solution.

[0099] Following addition of pentamethylbenzene as internal standard, the solution is separated into its constituents in a gas chromatograph on a DB-5; 15/0.32 capillary column, and lactide is detected quantitatively with a flame ionization detector.

Molecular Weight (Weight Average)

[0100] The weighed quantity of polymer is dissolved in a defined volume of chloroform. In an Ubbelohde capillary viscometer, which sits in a thermostatic water bath adjusted to 20 C.+/0.1 C., measurements are made of the transit time of the solution and of the pure solvent. The ratio of these two is the relative solution viscosity. It is converted by the single-point method of J. Dorgan et al., J. Polym. Sci.: Part B: Polym. Physics, Vol. 43,3100-3111 (2005), into the intrinsic viscosity (I.V.). The connection between the I.V. and the weight-average molar mass of the polymer is that described by the equation known as the Mark-Houwink equation. For the PLA/chloroform duo, the equation is as follows (J. Dorgan, loc. cit.):

I.V.=K*M.sub.w.sup.a, where K=1.53*104, a=0.759

L*a*b* Colour Space

[0101] The coloristic values are determined on an amorphous polymer which has been ground to powder. The polymer may take the form either of strands (prior to the demonomerization) or amorphous granules (after the demonomerization). The coloristic values of the L*a*b* colour space are measured by means of a colour spectrophotometer which is calibrated against a white standard. The light from a standard light source is reflected by the powdered polymer. The intensity of the reflected radiation is determined by a photocell. The L* value indicates the luminance. The colour of the sample material is described by two axes, with the b* values indicating the discoloration in the direction of blue (negative b* value) or yellow (positive b* value). The a* value indicates the red or green tonality.

EXAMPLES

Example 1: Demonomerization of a PLA Melt of Low Molecular Weight in an Inventive Thin-Film Evaporator

[0102] In a pilot plant, dilactide is polymerized to PLA in a combination of stirred tank and tubular reactor, as described for example in EP 2188047. Subsequently, in a static mixer, a stabilizer is mixed in so as to deactivate the catalyst. The molecular weight of the polymer after the tubular reactor is 171 000 g/mol and the monomer content is 4.8%. The b* colour value was ascertained as being 5.4. The temperature at entry was 186 C. The melt enters a thin-film evaporator which is equipped with four conditioning zones. Mounted on the stirrer shaft are two temperature measurement points 11, which are able to directly measure the temperature of the melt in the thin-film evaporator. The temperature sensor 11 here is arranged at the height of the second conditioned zone, which adjoins the intake zone (with inlet for the product) at the bottom. The second temperature sensor 11 is arranged at the height of the third conditioned zone (below the second zone). Intake zones and discharge zones do not possess temperature sensors. The admission temperatures of the heat transfer media was set at 192 C. in all four conditioning zones. In the steady state, a product temperature of 203 C. at the upper temperature measurement point 11 and a product temperature of 205 C. at the lower temperature measurement point 11 were established. After the demonomerization, a monomer content of 0.16% was measured. The molecular weight was 168 000 g/mol and the b* colour value was unchanged. The numerical values are collated in the table below.

Example 2: Demonomerization of a PLA Melt of High Molecular Weight in an Inventive Thin-Film Evaporator

[0103] The procedure from example 1 was repeated. The PLA product, however, had a higher viscosity, causing a greater rise in temperature of the melt in the thin-film evaporator. To compensate this, the temperatures in the conditioning zones were reduced. The results are collated in the table.

[0104] From examples 1 and 2 it is clear that there is no marked reduction in molecular weight and no increase in the b* colour value under these conditions.

Example 3: Demonomerization of a PLA Melt of High Molecular Weight in an Inventive Thin-Film Evaporator Without Cooling

[0105] The procedure from example 2 was repeated, but the temperature in the conditioning zones was set at 210 C. This caused an increase in the melt temperature to 231 C. As can be seen from the data in the table, there was a marked reduction in molecular weight, and the b* colour value climbed from 6.4 to 8.2. Moreover, the residual monomer content is higher than in the two preceding examples. This demonstrates that in the absence of adequate temperature control in the thin-film evaporator, a product deteriorates in the course of demonomerization in the thin-film evaporator.

Example 4: Effect of the Stabilizer on the Monomer Content

[0106] The procedure from example 2 was repeated, except that no stabilizer was metered before the demonomerization. As can be seen from the data in the table, the monomer content after the thin-film evaporation rises to more than 1.5%. The monomer content is therefore reduced by a factor of only 2.7. It is therefore vital that the PLA melt is stabilized before the demonomerization.

TABLE-US-00001 TABLE Ex. 1 Ex. 2 Ex. 3 Ex. 4 Entry Monomer content 4.8 5.2 5.0 4.9 Molecular weight 171 000 230 000 239 000 222 000 Temperature 186 C. 186 C. 186 C. 190 C. b* colour value 5.4 6.2 6.4 6.8 Admission temperature.sup.1 192 C. 190 C. 210 195 C. Product temperature.sup.2 203 C. 208 C. 226 C. 209 C. Product temperature.sup.3 205 C. 213 C. 231 C. 213 C. Exit Monomer content .sup.0.16% .sup.0.18% .sup.0.31% .sup.1.42% Molecular weight 168 000 227 000 220 000 219 000 b* colour value 5.4 6.5 8.2 6.9 .sup.1Admission temperature of the heat transfer medium in all four temperature zones .sup.2Product temperature in the second conditioning zone (sensor 11) .sup.3Product temperature in the third conditioning zone (sensor 11)