Quantification of lactide amounts in a polymeric matrix

Abstract

The present invention relates to a method for quantification of the amount of lactide in a lactide-based polymeric matrix by means of Infra Red Spectroscopy measurement. According to the invention the quantification is based on measurements performed on absorptions in the near Infra Red region of the electromagnetic spectrum. The invented method allows a rapid, easy and cheap quantification of lactide in a polymeric matrix, especially in PLA.

Claims

1. Method comprising quantification of the amount of lactide, which is a cyclic dimer of lactic acid, in a lactide-based polymeric matrix by means of Infra Red Spectroscopy measurements, wherein the amount of lactide is measured in a reaction mixture in which said lactide is polymerized into said lactide based polymeric matrix, which polymeric matrix comprises polylactic acid (PLA) or PLA-containing copolymers, wherein the quantification is based on measurements performed on absorption spectra recorded by a spectrometer comprising at least one probe disposed in the reaction mixture and operated in the range between 12000 cm.sup.1 and 4000 cm.sup.1 in the near Infra Red region of the electromagnetic spectrum so as to capture molecular overtones and combination vibrations of said lactide, and wherein said quantification of the amount of lactide further includes converting said measurements performed on said absorption spectra using a calculation model that is validated by the use of recorded near Infra Red spectra of mixtures having known amounts of lactide in PLA.

2. Method according to claim 1, wherein the polymeric matrix comprises PLA.

3. Method according to claim 2, wherein the polymeric matrix essentially consists of PLA.

4. Method according to claim 2, wherein the PLA has a M.sub.n of at least 10.000 kg/mol.

5. Method according to claim 1, wherein the amount of lactide is measured in the final polymerization product of the polymeric matrix.

6. Method according to claim 1, wherein the polymerization process is a batch process.

7. Method according to claim 1, wherein the polymerization process is a continuous process.

8. Method according to claim 7 wherein the amount of lactide is measured simultaneously at different stages of the polymerization process.

Description

(1) The present invention is described in more detail and elucidated by different examples and a drawing, in which

(2) FIG. 1 shows several nIR spectra of lactide, both in the absence and presence of a polymeric matrix,

(3) FIG. 2 depicts a series of near IR spectra measured with different concentrations of lactide in a PLA polymeric matrix,

(4) FIG. 3 shows a calibration curve of measured and calculated data of lactide concentrations in a PLA matrix,

(5) FIG. 4 shows a plot in which the amount of lactide in a lactide-to-PLA polymerization batch process is measured, and

(6) FIG. 5 shows a schematic diagram of a continuous lactide-to-PLA polymerization process indicating the points of interest where near Infra Red measurements can be performed

(7) In FIGS. 1 and 2, nIR spectra are shown, in which the absorption A is depicted as a function of the wave number WN. In more detail, FIG. 1-A shows nIR spectra of L-lactide (Puralact Polymer Grade), both measured in pure form (solid line, sample 1) and measured in a polymeric matrix (dotted line, sample 2). The spectra were recorded in reflective mode over the range between approximately 12000 and 4000 cm.sup.1. Sample 2, having lactide in a polymeric matrix of PLA, contained 48% by weight of L-lactide in 52% by weight of PLA. The M.sub.n of the PLA was higher than 10000 kg/mol. The measurements were performed with a Bruker MPA IR spectrometer. The peaks of interest for the quantification method according to the present invention are located in the spectral range between 6100 and 5100 cm.sup.1. FIG. 1-B shows in more detail the spectra of both samples 1 and 2 over this smaller spectral range.

(8) FIG. 2 shows how the near IR spectrum of lactide changes as a result of its conversion into PLA. The conversion started with 100% of weight of lactide and stopped after 52% by weight of the lactide was polymerized into PLA. This polymerization has a notable effect on especially two absorptions. First, the peak around 5830 cm.sup.1 appears to increase in time during the polymerization process. Secondly, a decrease and shift is observed of the peak around 5270-5220 cm.sup.1. This part of the nIR spectrum can be used to fill the model that will finally enable calculating lactide amounts from observed spectra. Different samples were drawn from the reaction medium and quenched after which the lactide amount was measured by off-line techniques such as Gas Chromatography (GC) or Liquid Chromatography (LC). These lactide amounts are assigned to the relevant spectra when building the calculation model.

(9) FIG. 3 shows a so-called cross-validation curve of measurements of lactide in a polymeric matrix. In order to determine this curve, a series of samples with different concentrations of lactide in a polymeric matrix have been prepared. The amount of lactide was varied between 0.05 and 1.5 wt %. In the present case, PLLA (M.sub.n larger than 10000) was used as the polymeric matrix. The lactide-concentration C was measured as a function of the known lactide-concentration C in the samples. Based on the cross-validation curve it was concluded that very low amounts of lactide can be reproducibly measured in a lactide-containing polymeric matrix. The root mean square of the error cross validation was shown to be 0.04. This small value allows to measure lactide concentration as low as approximately 0.05% by weight in a reproducible manner by means of the invented method.

(10) FIG. 4 shows a plot of the lactide concentration C which was online quantified by means of the method according to the present invention as a function of time t. In the course of this experiment, 400 g of L-lactide (Puralact Polymer Grade) was added to a reaction vessel, which was heated to 180 C. and the lactide was allowed to melt under continuous stirring. A nIR probe was positioned in the vessel for monitoring the lactide-to-PLA conversion. After the lactide was molten in the vessel, 60 mg of catalyst (Tin-octoate) and 4.12 g of initiator (n-hexanol) were added.

(11) The moment of adding the catalyst and the initiator was chosen as t=0. At that moment, the lactide concentration was 100%. Due to the rapid ring-opening polymerization (ROP) of lactide into PLA, the lactide concentration dropped rapidly after t=0. After 6 min, the lactide concentration reached an equilibrium value of approximately 3%. The small deviations on the line are attributed to the presence of temperature fluctuations within the vessel. These fluctuations are expected due to the temperature sensitivity of molecular vibrations. Incorporation of temperature effects into the model results in a significant improvement in the accuracy of the measurement.

(12) FIG. 5 shows schematically an interesting application of the presently invented quantification method in the course of a continuous process for lactide-to-PLA polymerization. For this purpose, tube reactor 1 is fed at reactor entrance site 2 with lactide. The added lactide is either in the liquid phase or fed as a solid mass, which mass is heated in the tube so that the lactide becomes liquid. An appropriate amount of catalyst (Tin-octoate) and initiator (n-hexanol) is added, either just before the adding of the lactide or just after it was entered at site 2. The entered lactide is transferred by means of pressure through the length of the reactor body to reactor exit site 3 in the direction indicated by arrow 4. During the transfer, the lactide is converted into its polymerization product PLA. Care is taken that the reactor is kept at a temperature at which both lactide and PLA are in the liquid phase.

(13) The tube reactor contains at various locations four probes 5, 6, 7 and 8 for measuring the nIR spectrum of the reaction mixture which is transferred through the reactor. These probes are connected by means of optical fibers to a nIR measuring apparatus 9. In the apparatus, the measured spectra values are converted to amounts of lactide in the polymeric matrix of PLA. This conversion is based on a calculation model that is validated by the use of recorded nIR spectra of reaction mixtures with known amounts of lactide in PLA. Critical measuring points in the reactor are the reactor entrance 2 and the reactor exit 3.

(14) Tube reactor 1 may comprise a devolatilisation unit at location 10 (not shown in detail), which unit is aimed at the removal of unconverted lactide or other (volatile) impurities from the polymer. In such situation, critical measurement points 7 and 8 are preferably located in the tube reactor 1 just before and just after such devolatilisation unit 10.

(15) With the presently invented lactide quantification method, small amounts of lactide can be determined online in a polymeric matrix in a relatively simple manner.

(16) While the invention has been illustrated and described in detail in the foregoing description, such description is to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments and experiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the disclosure and the appended claims.

(17) In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.