METHOD FOR CRYSTALLIZATION AND SEPARATION OF LOW-MOLECULAR COMPONENTS FROM A GRANULATE OF A CRYSTALLIZABLE THERMOPLASTIC MATERIAL AND DEVICE THEREFOR

Abstract

A method may facilitate the crystallization of granules of a crystallizable thermoplastic material in conjunction with removal of low molecular mass components contained in the thermoplastic material. The crystallizable thermoplastic material may have a crystalline melting temperature of at least 130 C. According to the method, a crystallization stage and a removal stage may be performed at different temperatures of the granules. Often the crystallization may occur at a lower temperature than the removal of the low molecular mass components. In the crystallization and removal stages, a flow of gas may pass countercurrent to a direction along which the granules are conveyed. Further, example devices disclosed herein may be utilized to perform the exemplary methods disclosed herein.

Claims

1.-23. (canceled)

24. A method for crystallizing granules and removing low molecular mass components from the granules of a crystallizable thermoplastic material having a crystalline melting temperature of at least 130 C., the method comprising: at least partially crystallizing the granules of the crystallizable thermoplastic material in a crystallization stage at a first temperature; and at least partially removing the low molecular mass components from the at least partially crystallized granules in a removal stage at a second temperature that is different than the first temperature.

25. The method of claim 24 wherein the first temperature is at least 20 K below the crystalline melting temperature of the crystallizable thermoplastic material, or the second temperature is higher than the first temperature and up to a maximum of 5 K below the crystalline melting temperature of the crystallizable thermoplastic material.

26. The method of claim 24 wherein in the crystallization and removal stages the granules are traversed by a flow of gas that passes countercurrent to a direction along which the granules are conveyed,

27. The method of claim 26 wherein the gas is fed into the removal stage and after flowing through the granules in the removal stage is withdrawn and fed into the crystallization stage where the gas flows through the granules in the crystallization stage.

28. The method of claim 26 wherein the gas is nitrogen or dried air.

29. The method of claim 26 wherein the gas has a dew point of less than 20 C.

30. The method of claim 26 wherein the gas is a first gas, the method further comprising at least one of cooling the first gas or mixing the first gas with a second gas having a lower temperature than the first gas before the granules are traversed in the crystallization stage by the first gas or a mixture of the first and second gases, wherein the first gas or the mixture of the first and second gases is adjusted to a temperature below 20 K below the crystalline melting temperature of the crystallizable thermoplastic material.

31. The method of claim 24 further comprising feeding the granules into a cooling stage where the granules are cooled to a temperature of less than 80 C. after the granules pass through the removal stage.

32. The method of claim 31 wherein the granules are cooled in the cooling stage either indirectly in a shell and tube heat exchanger with at least one of a gas or a liquid heat transfer medium that has a temperature that is lower than a temperature of the granules, wherein the granules flow in tubes of the shell and tube heat exchanger and the at least one of the gas or the liquid heat transfer medium flows cross-countercurrent around the tubes; or by causing a flow of gas that has a temperature that is lower than a temperature of the granules to traverse the granules.

33. The method of claim 32 further comprising: heating the gas used in the cooling stage to a temperature between 20 K below the crystalline melting temperature of the crystallizable thermoplastic material and a maximum of 5 K below the crystalline melting temperature of the crystallizable thermoplastic material; and feeding the gas that has been heated into the removal stage.

34. The method of claim 33 wherein a mass flow of the gas fed into the removal stage or the cooling stage is 2.0-5.0 times a mass flow of the granules fed in, or a selected heat capacity of the gas flow fed into the removal stage, as calculated as an arithmetic product of a mass flow and a specific heat capacity of the gas, is greater than a heat capacity of a flow of the granules, as calculated as a product of a mass flow and a specific heat capacity of the crystallizable thermoplastic material, wherein a ratio of the heat capacity of the gas flow to the heat capacity of the flow of the granules is adjusted to between 1.25 and 2.5.

35. The method of claim 24 wherein the granules reside for between 0.5 to 5 hours in the crystallization stage or are crystallized to a degree of crystallization of 20% to 80%; and reside for between 1 to 30 hours in the removal stage.

36. The method of claim 24 further comprising moving the granules mechanically in the crystallization stage.

37. The method of claim 26 wherein the granules are moved mechanically by way of stirring.

38. The method of claim 24 wherein the low molecular mass components are removed down to a level of below 0.2% by weight.

39. The method of claim 24 wherein the low molecular mass components are removed down to a level of below 0.5% by weight.

40. The method of claim 24 wherein the low molecular mass components are removed down to a level of below 1.0% by weight.

41. The method of claim 24 wherein the crystallizable thermoplastic material is comprised of poly-L-lactic acid having a minimum D-lactic acid unit content of 6%, poly-D-lactic acid having a maximum L-lactic acid unit content of 6%, or copolymers of lactic acid, wherein the low molecular mass components are comprised of L-lactide, D-lactide, meso-lactide, lactic acid, or comonomers.

42. The method of claim 24 further comprising producing the granules as amorphous granules before feeding the granules into the crystallization zone, wherein the granules are produced as the amorphous granules by a polymerization reaction or a polycondensation reaction in a melt and subsequent granulation of a resulting polymer.

43. The method of claim 42 further comprising partially removing the low molecular mass components contained in the melt prior to the granulation.

44. The method of claim 42 further comprising partially removing the low molecular mass components contained in the melt prior to the granulation by way of a falling strand evaporator, under pressure reduced relative to standard conditions.

45. The method of claim 24 further comprising feeding the granules into a cooling stage where the granules are cooled to a temperature of less than 80 C. after the granules pass through the removal stage, wherein in the crystallization and removal stages the granules are traversed by a flow of gas that passes countercurrent to a direction along which the granules are conveyed, the method further comprising purifying the flow of gas after withdrawing the gas from the crystallization stage or the removal stage, wherein the purifying comprises at least partially removing low molecular mass material from the flow of gas.

46. The method of claim 45 wherein the purified gas is fed into the removal stage or the cooling stage and/or the removed lower molecular mass material is used to produce the crystallizable thermoplastic material.

47. A device for carrying out crystallization and removal of low molecular mass components from granules of a crystallizable thermoplastic material, the device comprising: a crystallization zone for the granules of the crystallizable thermoplastic material, the crystallization zone including an inlet and an outlet for the granules, wherein crystallization of the granules occurs at a first temperature; and a removal zone for removing the lower molecular mass components from the granules of the crystallizable thermoplastic material, the removal zone including an inlet and an outlet for the granules, wherein the removal zone is downstream of the crystallization zone, wherein removal of the lower molecular mass components occurs at a second temperature that is lower than the first temperature.

48. The device of claim 47 wherein the removal zone comprises a supply line for heated gas that is disposed at or near the outlet of the removal zone.

49. The device of claim 47 wherein the crystallization zone and the removal zone are in fluidic communication such that the granules are transportable from the outlet of the crystallization zone to the inlet of the removal zone and such that gas is transportable from the removal zone to the crystallization zone.

50. The device of claim 47 further comprising a granules cooler, wherein the outlet of the removal zone opens into an inlet of a granules cooler, the granules cooler comprising a feed for a cooling gas, the granules cooler further comprising in a region of the inlet a take-off facility for gas that opens into a supply line for heated gas in the removal zone, which is disposed at or near the outlet, the device further comprising a gas heater disposed upstream of the supply line, wherein the take-off facility opens into the gas heater.

51. The device of claim 47 wherein the crystallization zone further comprises means for mechanically moving a bed of granules of a crystallizable thermoplastic material located in the crystallization zone.

52. The device of claim 51 wherein the means for mechanically moving the bed of granules is a granules stirrer.

53. The device of claim 47 further comprising: a granulating device disposed upstream of the inlet of the crystallization zone, the granulating device being configured to produce granules from a melt of a cystallizable thermoplastic material, wherein an outlet of the granulating device is in fluid communication via a granules line with the inlet of the crystallization zone.

54. The device of claim 53 further comprising a reactor for producing the melt of the crystallizable thermoplastics material disposed upstream of the granulating device.

55. The device of claim 54 further comprising a device disposed between the reactor and the granulating device for partial removal of low molecular mass components from the melt of the crystallizable thermoplastic material.

56. The device of claim 47 wherein the crystallization zone includes a gas outlet that opens into a scrubbing device for removing low molecular mass components from a gas stream, wherein downstream units free the gas stream of condensables, wherein the gas stream following removal of the condensables is fed into a granules cooler or the crystallization zone.

57. The device of claim 47 wherein either the crystallization zone and the removal zone are disposed jointly in a tower apparatus, wherein the crystallization zone is in communication with the removal zone via a perforated plate having a conical design and a central opening via which the granules are transferable from the crystallization zone into the removal zone, or the crystallization zone and the removal zone are separated from one another and are in fluidic communication with one another via a granules line and a gas line.

Description

EXAMPLE 1

Laboratory Scale Demonomerization

[0142] Laboratory experiments were carried out on the demonomerization of PLA granules in a stream of inert gas. The apparatus consisted of gas washing bottles with an inset frit, which had been introduced into an oven heated with temperature-conditioned oil. The granules took the form of a fixed bed with a height of about 30 mm on the frit, through which preheated inert gas flowed from below. A thermometer for measuring the granules temperature was passed through the lid of the gas wash bottle and placed with its lower end in the middle of the bed of granules. The stream of inert gas was adjusted to 2 Nl/g/h using a flow meter, this amount ruling out any limitation on the demonomerization rate as a result of the amount of gas. The inert gas stream departing the apparatus was taken off to the environment via a liquid closure. This prevents moist air from the environment penetrating the apparatus and affecting the granules.

[0143] Inert gases used were synthetic air, and also nitrogen, both at 5.0 purity. Any effect of moisture can therefore be ruled out.

[0144] The gas wash bottle filled with granules is inserted into the preheated apparatus and the inert gas stream is commenced. The heating time of the granules to the intended temperature was 45 minutes. The start of the experiment was therefore defined as 45 minutes after the introduction of the bottles into the oven. The error produced as a result is negligible. Amorphous granules were used, in order to prevent release of monomer from the granules as a result of prior crystallization, with a consequent falsification of the result. During heating, therefore, there was agglomeration, which was reversed by shaking the bottle after the heating operation. After a predetermined time, the gas wash bottles were removed from the apparatus and the granules, after cooling to ambient temperature, were analyzed for remaining lactide content. Table 1 contains the results.

TABLE-US-00001 TABLE 1 laboratory scale demonomerization Experiment No 1 2 3 4 Inert gas Nitrogen Nitrogen Nitrogen Air Granules 130 140 150 150 temperature ( C.) Lactide content/melting point (%)/( C.): At start of experiment 0.37/160 0.37/160 0.37/160 0.37/160 After 3 h 0.29/159 0.26/158 0.22/163 0.25/164 After 6 h 0.27/159 0.25/161 0.15/164 0.22/165 After 24 h 0.17/162 0.14/162 0.10/171 0.10/170 Experiment No. 5 6 Inert gas Air Nitrogen Granules 150 150 temperature ( C.) Lactide content/melting point (%)/( C.): At start of experiment 3.5/160 3.5/160 After 2 h 1.86/161 1.74/162 After 4 h 1.06/164 0.86/163 After 6 h 0.57/165 0.56/164 After 24 h 0.12/170 0.11/171

[0145] It can be seen that the rate of demonomerization increases with the temperature of the granules. For optimum operation, therefore, working as close as possible to the melting point of the granules is desirable. Nitrogen and dry air are equally suitable as inert gases.

[0146] At constant temperature, the melting point of the granules increases during the demonomerization. There is therefore no likelihood of melting or softening in the time profile of the demonomerization. A higher temperature level at the demonomerization also results in a higher melting point. Consequently, there is also no risk of melting or softening as a consequence of temperature increase during the demonomerization, provided the melting point measured prior to treatment is not exceeded.

EXAMPLE 2

Demonomerization According to Embodiment c

[0147] The example shows the demonomerization of PLA granules with a preliminary demonomerization in a falling strand apparatus with subsequent demonomerization in a vertical tower apparatus according to embodiment c), FIG. 3. The results are contained in Table 2 under experiment numbers 1 to 5.

TABLE-US-00002 TABLE 2 pilot scale demonomerization Experiment No. 1 2 3 4 Embodiment c) c) c) c) Granule melting point ( C.) 170 160 160 150 Lactide content after 1.5 1.8 1.6 1.9 granulation (%) Temperature in the 150 145 140 140 crystallizer ( C.) Residence time in the 3.0 3.0 3.0 3.0 crystallizer (h) Agglomeration in the crystallizer no yes no yes Temperature in the 150 145 140 140 demonomerization zone ( C.) Residence time in the 5.0 5.0 5.0 5.0 demonomerization zone (h) Lactide content after 0.18 0.25 demonomerization (%) Experiment No. 5 6 7 8 Embodiment c) a) a) b) Granule melting point ( C.) 150 160 170 160 Lactide content after 1.7 3.4 3.0 3.2 granulation (%) Temperature in the 135 120 120 120 crystallizer ( C.) Residence time in the 3.0 6.0 6.0 2.0 crystallizer (h) Agglomeration in the crystallizer no no no no Temperature in the 135 150 160 150 demonomerization zone ( C.) Residence time in the 5.0 10.0 10.0 16.0 demonomerization zone (h) Lactide content after 0.35 0.20 0.17 0.13 demonomerization (%)

[0148] In a continuous pilot plant, PLA was prepared by ring-opening polymerization of lactide, which was demonomerized in the tower apparatus according to FIG. 3. Monomer-containing melt was drawn off by means of a gear pump from the falling strand apparatus serving for preliminary demonomerization, at a volume flow rate of 40 kg/h, and this melt was supplied for underwater hot cut pelletization. This pelletization produced virtually spherical PLA granules having an average diameter of 2.5 mm. The granules were in the amorphous state, evident from the transparency of the particles.

[0149] With a throughput of 40 kg/h, the tower apparatus offers a residence time of 3 h in the crystallization zone and of 5 h in the demonomerization zone. The inert gas used was dried air with a dew point of 40 C. The stirrer ran at a rotary speed of 2/ minute. The granules temperature was set equally in the two zones. The lactide content after granulation was between 1.5 and 2%. Experiment settings No. 1 and 3 showed that the crystallization can still be operated without agglomeration when the difference between PLA melting point and granules temperature is 20 C. The stirrer was able to break up the agglomerates formed, reliably and in a short time. With a temperature difference of 15 C. (Experiment 2) and particularly at a temperature difference of 10 C. (Experiment 4), irreversible agglomeration occurred in the crystallization zone, leading to the blocking of granule flow and to the termination of the experiment. Experiment 1 indicates that within a residence time of 8 h in the tower apparatus (total made up of the residence times in the crystallization zone and in the demonomerization zone), a lactide content of below 0.2% is achievable only with PLA having a melting point of 170 C. Only in that case is the crystallization temperature of 150 C. sufficient for the demonomerization as well. Experiments 3 and 5 in conjunction with 2 and 4 show that PLA melting points of 160 C. and below permit only crystallization temperatures and demonomerization temperatures which are not sufficient to establish a lactide content of below 0.2%. As long as the granules temperature in the crystallization zone is the same as in the demonomerization zone, agglomeration prevents the establishment of higher temperatures at which it might be possible to attain a residual monomer content of less than 0.2%.

EXAMPLE 3

Demonomerization According to Embodiment a

[0150] The example shows the demonomerization of PLA granules in a tower apparatus without preliminary demonomerization in the melt. The results are contained in Table 2 under experiment numbers 6 and 7.

[0151] In a continuous pilot plant, PLA was prepared by ring-opening polymerization of lactide, which was demonomerized according to FIG. 1. Monomer-containing melt was drawn off by means of a gear pump from the polymerization reactor, at a volume flow rate of 20 kg/h, and this melt was supplied for underwater hot cut pelletization. This pelletization produced virtually spherical PLA granules having an average diameter of 2.5 mm. The granules were in the amorphous state, evident from the transparency of the particles.

[0152] For a throughput of 20 kg/h of PLA granules, the tower apparatus offers a residence time of 6 h in the crystallization zone and of 10 h in the demonomerization zone. As in example 2, inert gas supplied was dried air having a dew point of 40 C. In view of the lack of preliminary demonomerization, the lactide content after granulation was somewhat more than 3%. The stirrer ran with a rotary speed of 2/ minute.

[0153] The granules temperature was adjusted in the crystallization zone to 120 C., a temperature at which roughly the maximum of the crystallization rate of PLA is situated. In the subsequent demonomerization zone, the granules temperature was increased, through a suitable choice of the temperature and of the quantitative flow rate of the supplied air, to a temperature which lay 10 C. below the melting point of the respective PLA grade. As a result of this choice of temperature, there was no agglomeration in the crystallization zone, and the demonomerization was able to be operated at a sufficiently high temperature. The results of experiments 6 and 7 show that in spite of the higher initial concentration, a lactide content of 0.20% or less in less than 20 h residence time was achievable.

EXAMPLE 4

Demonomerization According to Embodiment b

[0154] The example shows the demonomerization of PLA granules without preliminary demonomerization in the melt, by crystallization in a separate, horizontal crystallizer with subsequent demonomerization in a vertical tower apparatus according to embodiment b). The crystallizer used was a horizontally disposed rotating drum, rotating about its axis, with internal infrared lamps to heat the granules. On its inside, the drum was provided with a welded-on spiral belt in order to guide the granules. The residence time of the granules was adjusted through the choice of the rotary speed to 2 h. The results are contained in table 2 under experiment number 8. As in example 3, the melt did not undergo preliminary demonomerization.

[0155] In a continuous pilot polymerization, PLA was produced by ring-opening polymerization of lactide, and was demonomerized in the same tower apparatus as in examples 2 and 3. Monomer-containing melt was drawn off in a melt flow rate of 20 kg/h from the polymerization reactor, by means of a gear pump, and was passed for underwater hot cut pelletization. The pelletization generated approximately spherical PLA granules having an average diameter of 2.5 mm. The granules were in the amorphous state, apparent from the transparency of the particles.

[0156] With a throughput of 20 kg/h, the tower apparatus offers a residence time of 6 h in the crystallization zone and of 10 h in the demonomerization zone. In this example, both zones were operated at the same temperature and used for the demonomerization. The crystallization zone of the tower apparatus was operated without a stirrer. As in example 2, dried air with a dew point of 40 C. was supplied as inert gas. Owing to the lack of preliminary demonomerization, the lactide content after pelletization was somewhat more than 3%.

[0157] The granules left the pelletization at 50 C. and were adjusted to 120 C. in the outflow, through the heating in the rotary drum crystallizer, this temperature of 120 C. being the approximate location of the maximum crystallization rate of PLA. The granules temperature in the tower apparatus was adjusted, by a suitable choice of the temperature and of the volume flow rate of the air supplied, to a level which, at 150 C., was 10 C. below the melting point of the PLA granules produced. As a result of the crystallization in the upstream rotary drum crystallizer, it was possible to maintain this temperature throughout the tower apparatus. In spite of operation without the stirrer, there was no agglomeration. Accordingly, the demonomerization could be operated at a sufficiently high temperature. The results of experiment 8 show that in spite of the greater initial concentration, this apparatus arrangement as well allows a lactide content of 0.20% to be achieved in less than 20 h residence time.