WO2003085386A1 - Polymer characterisation - Google Patents

Abstract

The invention relates to a method for the characterisation of a polymer or a polymer mixture comprising at least PET, PEN, PBT, PA, PC or the copolymers thereof, said polymer or polymer mixture containing at least one type of end group or functional group and being processed during a process involving at least one of the following steps: melt extrusion, crystallisation, drying and solid phase post-condensation. The inventive method comprises the following steps: a) a polymer sample is taken from the polymer or polymer mixture in at least one of the steps of the process, b) said polymer sample is prepared for its characterisation, and c) the respective polymer sample is characterised by means of infrared spectroscopy. According to the invention, the sample taken is prepared in step b) in such a way that it is spread out to form a layer, and the characterisation is carried out in step c) on the sample spread out to form a layer.

Description

 Polymer Characterization

The invention relates to a method for characterizing a polymer or an at least polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), polyamide (PA), polycarbonate (PC) or their copolimers containing polymer mixture according to the preamble of claim 1. In addition, it relates to a method for preparing a sample taken from a process according to the preamble of claim 40.

The characterization of polymers is carried out, for example, for quality control and for process monitoring, for example in melt extrusion, crystallization, drying or solid phase post-condensation of such polymers. The parameters for the polymer characterization are, for example, the intrinsic viscosity (IV) of the polymer or the concentration of certain functional groups in the polymer molecules. The end group concentration is particularly significant in the case of polymers which are formed from monomers by polycondensation reactions. Examples here are amino groups (-NH ₂ ), hydroxyl groups (-OH) and carboxyl groups (-COOH), which are used in the polycondensation of polyamides and of polyesters as functional groups for linking two molecules with the elimination of an H ₂ O molecule are effective. Both the intrinsic viscosity and the end group concentration of a polymer sample allow conclusions to be drawn about the average molecular weight and thus about the expected mechanical and thermal properties of the polymer.

The end group concentration of the polymer is determined, for example, by means of infrared spectroscopy. The polymer sample taken is irradiated with infrared, after which the absorption spectrum obtained is evaluated. The type and the concentration of the end groups present in the sample can be determined from the position and the height of the absorption bands in the frequency spectrum which are characteristic of the end groups. The invention has for its object to provide a method for the reliable characterization of a polymer during and / or after its processing, which can be carried out partially or fully automatically.

This object is achieved according to claim 1 by a method for characterizing a polymer or a polymer mixture comprising at least PET, PEN, PBT, PA, PC or their copolymers, which in a process comprising at least one of the steps of melt extrusion, crystallization, drying and solid phases - Has post-condensation, is processed and contains at least one type of end groups or functional groups, the method comprising the following steps: a) taking a polymer sample from the polymer or polymer mixture in at least one of the steps of the process; b) preparing the polymer sample taken for its characterization; c) characterizing the polymer sample taken in each case by means of infrared spectroscopy; the preparation of the sample taken in step b) is carried out by spreading the sample taken into a layer and characterizing the sample in step c) on the sample spread to a layer.

The spreading of the sample to a layer is reproducible and particularly advantageous for radiography by means of IR spectroscopy. In addition, the sample spread into a layer can be dissolved more quickly for IV determination than a grainy sample. A large number of samples can therefore be taken in a short time and evaluated to characterize the polymer.

In addition to the IR spectroscopic characterization of the polymer, a determination of the intrinsic viscosity is preferably carried out, in particular by dissolving a certain amount of the polymer sample taken in a certain amount of a standard solvent to form a standard solution, the viscosity of which being a measure of the intrinsic viscosity by comparison is determined with a reference solution serving as a reference sample. For the characterization of the polymer sample taken, the concentration of the at least one end group or functional group in the sample is preferably determined, the following steps in particular being carried out:

d) irradiating the defined sample taken with IR radiation over a frequency range in which the specific absorption bands of the at least one end group type are contained, and determining the IR absorption spectrum for the polymer sample taken; c2) comparing the determined IR absorption spectrum of the taken defined sample with the IR absorption spectrum of a defined reference sample which has at least one substance in which the at least one end group type is also contained; and c3) calculating the concentration of the at least one end group type in the sample based on the result of the comparison in step c2).

The invention also relates to a method for preparing a polymer sample or polymer mixture sample taken from a process for processing / processing a polymer or polymer mixture, on the basis of which the polymer or the polymer mixture is to be characterized, the preparation of the sample being taken thereby that the sample taken is spread into a layer.

The sample taken is expediently spread out to form a layer of a certain thickness, the thickness of which is preferably 25 μm to 500 μm, in particular 200 μm to 400 μm. On the one hand, these layer thicknesses are relatively stable and still sufficiently easy to handle and, on the other hand, allow enough radiation, in particular IR radiation, to pass through for the spectroscopy in order to obtain a sufficiently strong IR signal for spectral analysis.

The sample is preferably taken in solid form and spread to the thin layer by pressing, a sample of a pellet of the polymer or polymer mixture being taken in particular and pressed by means of a press. Through the Forming the sample in the solid state below the melting temperature largely prevents the measurement result from being falsified, for example by thermal degradation or by hydrolysis of the polymer molecules.

A pellet of the polymer or polymer mixture is expediently taken as a sample and rolled by means of a rolling mill. Alternatively, the pellet pressed using the above-mentioned press can then be rolled using a rolling mill.

In a particularly advantageous embodiment, the sample is taken in solid form and rolled by several successive pairs of rolls, the gap width of the roll nips becoming increasingly smaller from roll pair to roll pair and the gap width of the last roll pair corresponding to the specific layer thickness of the sample. The gradual reshaping in increasingly smaller nips prevents cracking in the sample layer. Individual cracks in the sample are at least disturbing for the spectroscopic examination and especially for the IR spectroscopic examination. In addition, the deformation of the sample in several steps minimizes the mechanical damage to the chain molecules of the polymer.

Alternatively, the sample can also be taken in solid form, pulverized and spread on a carrier material which is transparent, in particular IR-transparent, for the spectroscopy to be used. Here the spread powdery sample has a lot of "cracks", which can be taken into account when evaluating the measured spectrum.

The powdered sample spread on the carrier material can also be melted, so that a thin, coherent sample film is formed. Since the powder particles are very small, the entire pulverized sample can be melted relatively quickly, with all sample particles of the powder being in the melted state at a relatively high temperature only for a short and relatively uniform period of time. This also largely limits the thermal damage or hydrolysis of the chain molecules. If characterization of the polymer is to take place in a section of the process in which the polymer is in any case in the form of a melt, the sample is preferably taken in molten form and spread by dropping it onto an IR-transparent carrier material until it solidifies. Here too, the rapid "freezing" of the liquid sample largely precludes any falsification of the sample by thermal, mechanical or hydrolytic degradation of chain molecules.

The processing of the polymer or polymer mixture in the process, which has at least one of the steps of melt extrusion, crystallization, drying and solid phase post-condensation, is preferably carried out continuously. In the process, at least one sampling according to the invention takes place in at least one of the steps of the process. Depending on the sampling point in the process, one of the sampling and sample preparation procedures listed in the previous paragraphs is carried out for sampling.

The polymer can be, for example, polyethylene terephthalate (PET), and the at least one end group type can be carboxyl groups (-COOH) and hydroxyl groups (-OH). The polymer can also be polybutylene terephthalate (PBT) and the at least one end group type can be carboxyl groups and hydroxyl groups. However, the polymer can also be polyamide (PA), for example, and the at least one end group type can be amino groups (-NH ₂ ) and carboxyl groups (-COOH).

With these polymers, the sample is spread into a layer by reshaping the PET, PBT or PA, preferably at a temperature between 30 ° C and 170 ° C.

For the calibration of the measurement, reference samples are used which have a known concentration of the respective end groups to be determined:

For the characterization of PET or PBT, the reference sample preferably has a substance that consists of only one type of molecule and contains at least one OH group per molecule. Particularly advantageous as a reference sample here is the Bishydroxy ethylene terephthalate (BHET) containing two OH groups per molecule is used as a solution in tetrahydrofuran (THF).

For the characterization of PET or PBT, the reference sample preferably alternatively has a substance that consists of only one type of molecule and contains at least one COOH group per molecule. The use of terephthalic acid (TPS), which contains two COOH groups per molecule, as a solution in tetrahydrofuran (THF) is particularly advantageous as a reference sample.

For the characterization of PET or PBT, the reference sample can also have a first substance that consists of only one type of molecule and contains at least one OH group per molecule, and a second substance that consists of only one type of molecule and at least one COOH group contains per molecule. For this, e.g. BHET and TPS in the same number of moles dissolved in THF.

For the characterization of PET or PBT, the reference sample preferably alternatively has a substance that consists of only one type of molecule and contains at least one OH group and at least one COOH group per molecule.

For the characterization of PA, the reference sample preferably has a substance that consists of only one type of molecule and contains at least one NH ₂ group per molecule.

For the characterization of PA, the reference sample preferably alternatively has a substance that consists of only one type of molecule and contains at least one COOH group per molecule.

For the characterization of PA, the reference sample can also contain a first substance that consists of only one type of molecule and contains at least one NH ₂ group per molecule, and a second substance that consists of only one type of molecule and at least one COOH group per Contains molecule. For the characterization of PA, the reference sample preferably alternatively has a substance that consists of only one type of molecule and contains at least one NH ₂ group and at least one COOH group per molecule.

Suitable solvents for the reference samples mentioned (BHET and / or TPS dissolved in THF, for example for PET or PBT) are hydrophobic organic solvents which themselves have no OH groups and / or COOH groups and in the spectral range of IR absorption OH groups and / or the COOH groups have no IR absorption. In addition, these solvents must be chemically inert at least to the OH groups and / or the COOH groups in order not to react with them. The same applies to the reference samples for PA, the solvent here also having to be inert to the NH ₂ groups of the reference sample molecules for PA.

In a particularly advantageous embodiment of the method according to the invention, the IR absorption spectrum of a first sample taken and the IR absorption spectrum of a second sample taken and subsequently deuterated are subtracted from one another in step d) to determine the IR absorption spectrum. As a result, the absorption bands of the end groups of interest obtained in IR spectroscopy are "exposed". When the deuteration of the macromolecules in heavy water (D ₂ O) is long enough, only the hydrogen (H atoms) at the end groups of the macromolecules is practically completely replaced by deuterium (D atoms), while the hydrogen on the skeleton of the macromolecules is practically completely replaced is retained, ie there is practically no exchange. The IR absorption spectrum of the deuterated sample thus differs from the IR absorption spectrum of the non-deuterated sample in the position of the absorption bands of the end groups. The absorption bands of the COOD end groups and the OD end groups are shifted towards lower frequencies with respect to the absorption bands of the COOH end groups and the OH end groups, while the absorption background which extends over a wide frequency range of the IR absorption spectrum at deuterated Subtraction of the two spectra from one another removes the interfering background without removing the COOH and OH absorption bands of interest. which frees them from the background obscuring them and becomes accessible for a quantitative evaluation.

In order to determine the concentration of the OH groups, the COOH groups and the NH ₂ groups, the IR absorption peaks assigned to the OH groups, the COOH groups and the NH ₂ groups are evaluated, the evaluation preferably by detecting the Height of the IR absorption peaks and / or the area under the IR absorption peaks. The concentration of the respective end group is proportional to the height of the absorption peaks (peaks) and also proportional to the area under the absorption peaks (peaks).

Incidentally, the IR absorption spectra of the reference samples are obtained in the same way as the IR absorption spectra of the polymer sample.

In the process, at least one sample can be taken from at least two of the steps of the process, in particular a characterization is carried out for the at least one sample taken from the at least two steps of the process and the results of the characterizations are compared with one another. When processing PET, e.g. Samples are taken from the melt in the extruder, samples from the crystallizer and samples from the SSP reactor. By determining the end group concentration and the IV of these respective samples, one obtains an insight into the efficiency of the entire preparation process.

Continuous sampling in the melt extrusion step can e.g. by branching off a continuous melt flow parallel to the melt extrusion step and passing through a measuring cell.

The continuous or at least quasi-continuous discrete sampling in the crystallization, drying or solid phase post-condensation step can be carried out, for example, by continuously branching off at least one pellet. In the process for processing the polymer in a process step in which the polymer or the polymer mixture is in the form of a melt, the following steps can also be carried out particularly advantageously:

51) irradiating a defined melt volume in the polymer melt process space with IR radiation over a frequency range in which the specific absorption bands of the at least one end group type are contained, and determining the IR absorption spectrum for the defined irradiated melt volume;

52) comparing the determined IR absorption spectrum of the defined irradiated melt volume with the IR absorption spectrum of a defined reference sample which has at least one substance in which the at least one end group type is also contained; and

53) Calculate the concentration of the at least one end group type in the defined melt volume based on the result of the comparison in step S2).

The irradiation of a defined melt volume in the process space having polymer melt with IR radiation in step S1) eliminates the need to branch off the melt and is particularly well suited for the continuous characterization of the polymer melt.

In a particularly preferred embodiment of the method according to the invention, in step d) or in step S1) to determine the IR absorption spectrum, the IR absorption spectrum for a first sample taken or a first defined melt volume and the IR absorption spectrum of a second taken and subsequently deuterated sample subtracted from one another, the IR absorption spectra of the samples made of deuterated polymer intended for the subtraction preferably being stored for different types of polymer.

Further advantages, features and possible applications of the invention will become apparent from the following description, which is not to be interpreted in a restrictive manner, with reference to the attached drawing, in which: Figure 1 is a schematic view of a first device for preparing a polymer sample taken for its characterization; and

Figure 2 is a schematic view of a second device for preparing a taken polymer sample for its characterization; and

Fig. 3 illustrates a device for the deuteration of a taken polymer sample or a reference sample.

The method according to the invention is described below by way of example on the basis of the determination of the concentrations of the OH end groups and the COOH end groups, as described e.g. for PET, PBT or PEN.

A first device for preparing polymer samples 9 according to the invention, which have been taken from a process for polymer processing, is shown schematically in FIG. 1. The polymer samples taken from the process in the solid state are in the form of pellets 9, the diameter or largest dimension of which is, for example, between about 2 mm and 5 mm. After removal (not shown), the pellets 9 arrive on a conveyor belt 1, which transports them to a filling funnel 2, into which they fall. The pellets 9 are passed through the hopper 2 into a press 3 in which they are pressed. The preliminary pressing in the press 3 is a first step in the shaping to a flat, spread-out sample. After the preliminary pressing in the press 3, the somewhat flattened pellets fall into a rolling mill 4, in which they are pressed further. The final pressing that takes place in the rolling mill 4 leads to rolled-out samples 9 'of small thickness. The rolling mill 4 is adjusted as required so that polymer samples 9 ′ suitable for irradiation with infrared radiation are obtained in the form of thin platelets or in the form of a thin film with a thickness of approximately 25 μm to 500 μm, in particular 200 μm to 400 μm. The polymer samples 9 'then migrate through an alignment funnel 5 in which they are aligned. The samples 9 'rolled out to form the thin plate or thin film are removed from the alignment funnel 5, fixed in a sample holder 8 and irradiated with infrared radiation which is generated in an infrared source 6. The fixie tion of the polymer samples 9 'in the sample holder 8 is done manually or semi-automatically. The IR radiation passing through the rolled sample 9 'is spectrally broken down in an infrared detector 7. Fourier transform infrared spectroscopy is preferably used for this.

A second device for preparing the polymer samples according to the invention is shown schematically in FIG. 2. The device and procedure of FIG. 2 differ from the device and procedure of FIG. 1 in that instead of the sample holder 8 (see FIG. 1), in addition to the first conveyor belt 1, a second conveyor belt 10 is provided, the belt of which consists of an IR permeable material, which has no absorption at least in the spectral range in which the expected IR absorption bands of the polymer samples 9 'are to be expected. All elements of FIG. 2 which correspond to or are identical to those of FIG. 1 have the same reference numerals as in FIG. 1. The samples 9 ′ which have been rolled out to form a thin plate or a thin film are placed on the second conveyor belt 10 , The thin polymer samples 9 'lying on the conveyor belt 10 are irradiated with infrared radiation by means of the conveyor belt 10 during their transport. This second device or second procedure for sample preparation can take place fully automatically. The sampling (not shown) is carried out by gravity by letting the pellets 9 fall out of a section of the process. After transport through the conveyor belt 1, the further transport of the pellets 9 also takes place by gravity through the hopper 2, the press 3, the rolling mill 4 and the alignment hopper 5, from which the rolled polymer samples 9 'are then also deposited on the conveyor belt 10 by gravity become.

To characterize the prepared polymer samples 9 ', the IR absorption bands of the polymer samples 9' are determined. To determine the IR absorption spectrum of the polymer sample 9 ', its IR absorption spectrum is compared with the IR absorption spectrum of a sample 9 "taken from the process at the same location, prepared in the same way by rolling and then deuterated (see FIG. 3). Alternatively, the polymer sample 9 'which was previously removed, prepared and irradiated for the IR spectroscopy can be deuterated after it has been irradiated in order to then undergo the same IR spectroscopic examination again.

3 shows a device for the deuteration of a polymer sample 9 ". The initially non-deuterated polymer samples 9 'are placed in an Erlenmeyer flask 11 filled with heavy water D ₂ O, which has a filler neck 11a and a suction neck 11b. The Erlenmeyer Piston 11 is then sealed at its filler neck 11a with a filler seal 12, through which a tube 12a passes. The suction port 11b of the Erlenmeyer piston 11 is provided with a suction nozzle 13. The piston 11 filled with the polymer samples 9 "to be interpreted is kept in a vessel 14 at about 50 ° C in a water bath that is constantly heated and stirred. A combination device 16 is used for this purpose, which on the one hand has a heating plate 16a and on the other hand sets a magnetic stirrer 15 in rotation. In order to prevent a reaction of D ₂ O with atmospheric H ₂ O in the space above the D ₂ O bath, the piston 11 is flushed with N _2, for example. Depending on their thickness, the rolled-out polymer samples 9 "must remain in the D ₂ O bath over a more or less long period (several days). For this reason, a particularly small thickness, for example of only 10 μm, is preferably used for the deuterated samples Thickness and a temperature of D ₂ O of 50 ° C. can result in complete deuteration of the hydroxyl groups (-OH) and carboxyl groups (-COOH) at the end, for example in the case of PET, PBT or in the case of PEN to form OD groups or COOD groups can be reached within about 24 hours, but the other hydrogen atoms of the molecular skeletons are not exchanged.

The spectrum of the non-deuterated polymer sample 9 'and the spectrum of the deuterated polymer sample 9 "are subtracted from one another. With different layer thicknesses of the non-deuterated polymer sample 9' (eg 50 μm) and the deuterated polymer sample 9" (eg 10 μm), one may have to be added of the two spectra are multiplied by a compensation factor in order to compensate for the different layer thicknesses before the subtraction is carried out. Alternatively, several of the generally thinner deuterated polymer samples 9 ″ can be pressed together in order to obtain the same layer thickness as that of the non-deuterated sample 9 ′ (for example 5 × 10 μm). However, care must be taken to ensure that there are no air pockets between the one - individual layers are present. As a further alternative, the same layer thicknesses can be used for both the non-deuterated polymer samples 9 ′ and for the deuterated polymer samples 9 ″ (for example 20 μm in each case). A compromise is made here, on the one hand, with longer deuteration times than with a 10 μm layer thickness of the samples 9 "and, on the other hand, must work with weaker spectrum signals than with a 25 μm layer thickness of the samples 9 '.

By subtracting the IR absorption spectra of the non-deuterated polymer sample 9 'and the deuterated polymer sample 9 "from one another, the" background "of the two spectra, which extends over wide frequency ranges and is caused by IR absorption due to vibrations that occur the CC bonds, CH bonds and C ₆ rings of the molecular skeletons of the polymer molecules and based on more or less strong interactions between the polymer molecules. Since the absorption bands (peaks) of the OD groups and the COOD groups with respect to the absorption bands (peaks) of the OH groups and the COOH groups are shifted to lower frequencies, these absorption bands do not cancel out during subtraction, so that in the difference spectrum the OH bands and the COOH bands with their original height remain on a low baseline and can be quantitatively can be evaluated by looking at the height of these bands and / or below them l the area used as a measure of the concentration of the respective end group.

Since the absorption bands of H ₂ O molecules lie in the region of the absorption bands of the OH groups and the COOH groups and partially cover them, it is important that the non-deuterated polymer sample 9 'is dried before IR spectroscopy. A vacuum oven operating at approximately 50 ° C. is preferably used for this. However, the pellets 9 are expediently removed from a section of the process in which they are largely dried, ie at the end of the solid phase post-condensation.

In order to obtain absolute values for the concentrations of the OH groups and the COOH groups, reference samples must be examined. As reference samples for calibration for determining the concentration of the OH end groups and the COOH End groups are used as reference samples which have a known concentration of the respective end groups to be determined. For the characterization of PET or PBT, the reference sample preferably has a substance that consists of only one type of molecule and contains at least one OH group per molecule. The use of bishydroxy ethylene terephthalate (BHET), which contains two OH groups per molecule, as a solution in tetrahydrofuran (THF) is particularly advantageous as a reference sample. For the characterization of PET or PBT, the reference sample preferably alternatively has a substance that consists of only one type of molecule and contains at least one COOH group per molecule. The use of terephthalic acid (TPS), which contains two COOH groups per molecule, as a solution in tetrahydrofuran (THF) is particularly advantageous as a reference sample. For the characterization of PET or PBT, the reference sample can also have a first substance that consists of only one type of molecule and contains at least one OH group per molecule, and a second substance that consists of only one type of molecule and at least one COOH group contains per molecule. BHET and TPS in the same number of moles dissolved in THF are suitable for this. For the characterization of PET or PBT, the reference sample preferably alternatively has a substance that consists of only one type of molecule and contains at least one OH group and at least one COOH group per molecule.

Suitable solvents for the reference samples mentioned (BHET and / or TPS dissolved in THF, for example for PET or PBT) are hydrophobic organic solvents which themselves have no OH groups and / or COOH groups and in the spectral range of IR absorption OH groups and / or the COOH groups have no IR absorption. In addition, these solvents must at least be chemically inert to the OH groups and / or the COOH groups in order to avoid any reactions and interactions with them. Tetrahydrofuran fulfills all of these criteria and is therefore particularly suitable as a solvent for the reference samples based on BHET and / or TPS. LIST OF REFERENCE NUMBERS

Conveyor belt for taken polymer samples Filling hopper Press Rolling mill Alignment funnel IR source Detector Sample holder Polymer sample before preparation (pellet) 'Polymer sample after preparation (plate, film) "deuterated polymer sample 0 Conveyor belt for prepared polymer samples 1 Erlenmeyer flask for D ₂ O bath (deuteration ) and N ₂ flushing 1 a filler neck 1 b suction nozzle 2 filler seal 3 suction nozzle 4 vessel for H ₂ O bath (temperature control) 5 magnetic stirrer 6 combination device (heating and stirring) 6a heating plate

Claims

claims 1. A method for characterizing a polymer or a polymer mixture comprising at least PET, PEN, PBT, PA, PC or their copolymers, which is processed in a process which comprises at least one of the steps of melt extrusion, crystallization, drying and solid phase post-condensation, and at least contains a type of end groups or functional groups, the method comprising the following steps: a) taking a polymer sample from the polymer or polymer mixture in at least one of the steps of the process; b) preparing the polymer sample taken for its characterization; c) characterizing the polymer sample taken in each case by means of infrared spectroscopy; characterized in that the preparation of the respectively taken sample in step b) takes place in that the taken sample is spread out into a layer and the characterization of the sample in step c) takes place on the sample spread out into a layer. 2. The method according to claim 1, characterized in that the intrinsic viscosity is determined to characterize the polymer sample taken. 3. The method according to claim 2, characterized in that the determination of the intrinsic viscosity by dissolving a certain amount of the polymer sample taken in a certain amount of a standard solvent to a standard solution and the viscosity as a measure of the intrinsic viscosity by comparison with a reference solution serving as reference sample is determined. 4. The method according to claim 1, characterized in that the concentration of the at least one end group or functional group in the sample is determined to characterize the polymer sample taken. 5. The method according to claim 4, characterized in that the concentration of the at least one end group or functional group is determined by the following steps: d) irradiating the defined sample taken with IR radiation over a frequency range in which the specific absorption bands of the at least one type of end group is contained, and determining the IR absorption spectrum for the polymer sample taken; c2) comparing the determined IR absorption spectrum of the defined sample taken with the IR absorption spectrum of a defined reference sample which has at least one substance in which the at least one end group type is also contained; and c3) calculating the concentration of the at least one end group type in the sample based on the result of the comparison in step c2). 6. The method according to any one of claims 1 to 5, characterized in that the sample taken is spread out to a layer of certain thickness. 7. The method according to claim 6, characterized in that the determined thickness is 25 microns to 500 microns, in particular 200 microns to 400 microns. 8. The method according to any one of claims 1 to 7, characterized in that the sample is taken in solid form and spread by pressing to the thin layer. 9. The method according to claim 8, characterized in that a pellet of the polymer or polymer mixture is taken as a sample and pressed by means of a press. 10. The method according to claim 8, characterized in that a pellet of the polymer or polymer mixture is taken as a sample and is rolled by means of a rolling mill. 11. The method according to claim 8, characterized in that the pellet pressed by means of the press is then rolled by means of a rolling mill. 12. The method according to any one of claims 1 to 7, characterized in that the sample is taken in solid form and rolled by a plurality of successive pairs of rolls, the gap width of the roll nips from roll pair to roll pair becoming increasingly smaller and the gap width of the last pair of rolls of the determined layer thickness corresponds to the sample. 13. The method according to any one of claims 1 to 7, characterized in that the sample is taken in solid form, pulverized and spread on an IR-transparent carrier material. 14. The method according to claim 13, characterized in that the powdered and spread on the support material sample is melted, so that a thin coherent sample film is formed. 15. The method according to any one of claims 1 to 7, characterized in that the sample is taken in molten form and spread by dropping onto an IR-transparent carrier material and cooled until solidification. 16. The method according to any one of claims 1 to 15, characterized in that the processing of the polymer or polymer mixture in the process, which has at least one of the steps melt extrusion, crystallization, drying and solid phase post-condensation, is carried out continuously. 17. The method according to any one of claims 1 to 16, characterized in that in the process at least one sample is taken in at least one of the steps of the process. 18. The method according to any one of claims 1 to 17, characterized in that the polymer is polyethylene terephthalate (PET) and the at least one end group type are carboxyl groups (-COOH) and hydroxyl groups (-OH). 19. The method according to any one of claims 1 to 17, characterized in that the polymer is polybutylene terephthalate (PBT) and the at least one end group type are carboxyl groups and hydroxyl groups. 20. The method according to any one of claims 1 to 17, characterized in that the polymer is polyamide (PA) and the at least one type of end groups are amino groups (-NH ₂ ) and carboxyl groups (-COOH). 21. The method according to any one of claims 18 to 20, characterized in that the sample is spread into a layer by shaping the PET, PBT or PA at a temperature between 30 ° C and 170 ° C. 22. The method according to any one of claims 18 or 19, characterized in that the reference sample has a substance which consists of only one type of molecule and contains at least one OH group per molecule. 23. The method according to any one of claims 18 or 19, characterized in that the reference sample has a substance which consists of only one type of molecule and contains at least one COOH group per molecule. 24. The method according to any one of claims 18 or 19, characterized in that the reference sample has a first substance which consists of only one type of molecule and contains at least one OH group per molecule, and has a second substance which consists of only one Molecular type exists and contains at least one COOH group per molecule. 25. The method according to any one of claims 18 or 19, characterized in that the reference sample has a substance which consists of only one type of molecule and contains at least one OH group and at least one COOH group per molecule. 26. The method according to claim 20, characterized in that the reference sample has a substance which consists of only one type of molecule and contains at least one NH ₂ group per molecule. 27. The method according to claim 20, characterized in that the reference sample has a substance which consists of only one type of molecule and contains at least one COOH group per molecule. 28. The method according to claim 20, characterized in that the reference sample has a first substance which consists of only one type of molecule and contains at least one NH ₂ group per molecule, and has a second substance which consists of only one type of molecule and contains at least one COOH group per molecule. 29. The method according to claim 20, characterized in that the reference sample has a substance which consists of only one type of molecule and contains at least one NH ₂ group and at least one COOH group per molecule. 30. The method according to any one of claims 5 to 29, characterized in that in step d) to determine the IR absorption spectrum, the IR absorption spectrum of a first sample taken and the IR absorption spectrum of a second sample taken and then deuterated are subtracted from one another. 31. The method according to claim 30, characterized in that to determine the concentration of the OH groups, the COOH groups and the NH ₂ groups, the IR groups assigned to the OH groups, the COOH groups or the NH ₂ groups. Absorption peaks can be evaluated. 32. The method according to claim 31, characterized in that the evaluation is carried out by detecting the height of the IR absorption peaks and / or the area under the IR absorption peaks. 33. The method according to any one of claims 17 to 32, characterized in that in the process at least one sample is taken from at least two of the steps of the process. 34. The method as claimed in claim 33, characterized in that a characterization is carried out for the at least one sampling in each case at the at least two steps of the process and the results of the characterizations are compared with one another. 35. The method according to any one of claims 1 to 34, characterized in that the continuous sampling in the melt extrusion step is carried out by branching off a continuous melt flow parallel to the melt extrusion step and passing through a measuring cell. 36. The method according to any one of claims 1 to 34, characterized in that the continuous sampling in the step of crystallization, drying or solid phase post-condensation is carried out by continuously branching off at least one pellet. 37. The method according to any one of claims 4 to 36, characterized in that in the process for processing the polymer in a process step in which the polymer or the polymer mixture is present as a melt, the following steps are carried out: S1) irradiating a defined melt volume in the polymer melt process space with IR radiation over a frequency range in which the specific absorption bands of the at least one end group type are contained, and determining the IR absorption spectrum for the defined irradiated melt volume; 52) comparing the determined IR absorption spectrum of the defined irradiated melt volume with the IR absorption spectrum of a defined reference sample which has at least one substance in which the at least one end group type is also contained; and 53) Calculate the concentration of the at least one end group type in the defined melt volume based on the result of the comparison in step S2). 38. The method according to any one of claims 5 to 37, characterized in that in step d) or in step S1) to determine the IR absorption spectrum, the IR absorption spectrum for a first sample taken or a first defined melt volume and the IR Absorption spectrum of a second sample taken and then deuterated are subtracted from each other. 39. The method according to any one of claims 5 to 38, characterized in that the IR absorption spectra of the samples of deuterated polymer intended for subtraction are stored for different types of polymer. 40. Method for preparing a polymer sample or polymer mixture sample taken from a process for processing / processing a polymer or polymer mixture, on the basis of which the polymer or the polymer mixture is to be characterized, characterized in that the preparation of the sample taken is carried out thereby that the sample taken is spread into a layer. 41. The method according to claim 40, characterized in that the sample taken is spread to a layer of a certain thickness. 42. The method according to claim 41, characterized in that the determined thickness is 25 μm to 500 μm, in particular 200 μm to 400 μm. , 43. The method according to any one of claims 40 to 42, characterized in that the sample taken is in solid form and is spread to the thin layer by pressing. 44. The method according to claim 43, characterized in that the sample taken is in the form of a pellet of the polymer or polymer mixture and is pressed by means of a press. 45. The method according to claim 43, characterized in that the sample taken is in the form of a pellet of the polymer or polymer mixture and is rolled by means of a rolling mill. 46. The method according to claim 43 or 44, characterized in that the previously pressed pellet is then rolled by means of a rolling mill. 47. The method according to any one of claims 40 to 46, characterized in that the sample is taken in solid form and rolled by a plurality of successive pairs of rolls, the gap width of the roll gaps from roll pair to roll pair becoming increasingly smaller and the gap width of the last pair of rolls of the determined layer thickness corresponds to the sample. 48. The method according to any one of claims 40 to 46, characterized in that the sample is taken in solid form, pulverized and spread on a carrier material transparent for a spectroscopic examination. 49. The method according to claim 48, characterized in that the pulverized and spread on the carrier material sample is melted, so that a thin coherent sample film is formed. 50. Method according to one of claims 40 to 46, characterized in that the sample is taken in molten form and spread by dropping onto a support material which is transparent for a spectroscopic examination and is cooled to solidification.