DE19820948C1 - Machine controlling plastic granule quality under realistic conditions - Google Patents

Abstract

The metering extruder (1) feeds a head (2) blowing a flat test sheet (5). A controlled tempering and cooling unit (3, 4) feeds the test sheet through a measurement chamber (6) containing an IR light source (7). An FTIR (Fourier transform infra red ) spectrometer gathers and resolves the IR light. An instrument processes the signals from the detector (8). An Independent claim is included for the corresponding method of quality control. Sheet is made from a sample of granules in the machine, and the infra red absorption spectrum is evaluated, to determine properties of the granules. Preferred features: Between measurement volume and source, the light is linearly-polarized. A roller unit includes a sheet positioner with two guide rollers. This maintains IR incidence on the sample sheet, at the Brewster angle. The positioner includes a drum in bearings. This carries further bearings, mounting the guide rollers. All three axes are mutually parallel and lie in one plane. The drum axis is central to the two rollers, and lies between detector and IR source. The source wave length lies in the mid infra red spectrum.

Description

The present invention relates to a device and a method for quality control of plastic granules lat.

Such devices are used to material properties plastic granules and, if necessary, due to the Review information obtained when manufacturing the Plastic granules to be considered.

For the investigation of material properties of plastic for example infrared spectroscopy is used so that on optical ways are tried, properties of plastics ascertain. So it is known from DD 273 175 A3, from a sample taken from a reactor or granulator Manufacture press film and this with the help of infrared spec to examine microscopy. A continuous review of the However, the properties of the granules to be checked are here not possible because the production of the press film and the pre preparation of the spectroscopic measurements too much time in Takes up. An online review of the quality of her provided plastic granulate is therefore also not possible Lich.

It is also known from DE 33 06 476 A1 for qualification plastic granules in a supply line to provide a branch line to a processing extruder, through which a small proportion of granulate to a measuring extruder is directed. To check the quality of the granules melt that emerges from the measuring extruder, the pressure before a measuring nozzle of the measuring extruder, the range of fluctuation there measured pressure and the elasticity of the measuring nozzle emerging melt strand measured. Material properties of plastic granulate, such as its chemical composition, are not measurable with the device described there.  It is also from the publication "Orientation Measurement provides tensile strength online "by W. Michaeli and J. Hauck in Kunststoffe, 87 (1997) 5, known for online review of the The tensile strength of PE-LD and PS tubular films is an optical one Use measuring device with which the birefringence of investigated film is measured and from it to the mecha niche film properties shrink and tensile strength is closed. The device described there permits the online check of a mechanical property a plastic film, but here too there is no possibility given the chemical composition of the film online to deliver.

Furthermore, WO 91/07650 describes a method for measuring the Degree of curing of a resin material in a ver bundle material, known as a prepreg, in the IR spectro copy is used. Using an IR light source and the absorption of the composite material is measured by an IR spectrometer than measured for two different IR wavelengths. From the If two absorption values are obtained, their ratio will be calculates and to derive the degree - for the healing of the Resin used. To carry out the process, a Device used in which a flat material to Imprä For purposes of decoration, it is coated with resin and that emerging composite material between IR light sources one on the one hand and an FTIR spectrometer on the other hand so that individual absorption values can be measured. Identification of individual chemical compounds or Substances contained in the composite material are not possible because only with regard to the IR spectrum Absorption values can be measured. The procedure also serves ren and the device described in WO 91/07650 are to carry out measurements from which on the quali plastic granules could be concluded.

Methods are also known from DE 195 07 750 A1, with which the color of plastic extrudates immediately  can be determined after or during their manufacture. In one individual should be determined whether those used for coloring Pigments and dyes added to the plastic extrudates are available in sufficient quantities. This will be a thermoplastic melt produced in an extruder by means of a Calendered into a band and the band through one with a liquid-filled vessel into which the Immersed measuring head of a spectrophotometer. With this too Device can be especially the structure of an art quality inspections related to the fabric tape to lead.

Furthermore, the German interpretation 1 648 635 described an optical component with which optical measurement flat material, especially for finding defects in being simplified. To adjust the angle, under which is a flat material against an incident light beam is a device with two side by side Arranged roles provided that swivel together axis can be pivoted. It runs over the roles Flat material. By swiveling the rollers around the swivel The angle of incidence of the incident light beam can be axis changed compared to the flat material.

Proceeding from this, the invention is based on the object an apparatus and method for fast, comprehensive To create quality control of plastic granules.

This object is achieved by a device for quality control of plastic granules

• - A measuring extruder with a blowing device for continuous Lichen manufacturing a substantially flat pro ben film made of plastic granulate,
• - A cooling device for a defined temperature cooling the extruded sample film,
• - A roller device for continuous guiding  the sample film through a measuring volume,
• An IR light source for irradiating the measurement volume,
• - an FTIR spectrometer with detector for spectral resolved collection of IR light from the measurement volume and
• - An evaluation device for processing Signa len supplied by the detector.

The fast or even online quality account trolls of plastic granules, for example in one Main stream is fed to a processing extruder  thereby allowing the measuring extruder, the branched Plastic granulate can be fed from a main stream, together with the blowing device and the cooling device continuously produces a flat sample film based on opti can be investigated.

The cooling device brings about a defined temperature cooling len the extruded sample film, so that it the property reproduces the granulate as accurately as possible and / or the Properties of the sample film, including its thickness, if possible similar to those made from the main granulate stream are foil. Are preferred for cooling the extruded Sample film cooling rollers of predetermined temperature used.

For the optical examination of the properties of the sample film an IR light source and an FTIR spectrometer work together, for example for an Ab recorded in transmission sorption measurement can be arranged.

The sample film is passed through a measuring volume which is between the IR light source and the detector of the FTIR spectro meters is arranged.

The evaluation device processes the signals from the detector with which the FTIR spectrometer is equipped. The Spectrometer delivers the intensity of the spectrally resolved IR light transmitted through the sample. To do this, the Transmission signals that are absorbed in the sample film Light calculated as an infrared absorption spectrum. This Absorption spectrum contains for those contained in the sample Substances characteristic intensity courses. By ver equal to the absorption spectra with previously obtained Kalibra tion spectra, for example for individual or for several substances to be expected in the plastic granulate can absolute concentrations of chemical components of the sample Foil made from the plastic granulate, firmly be put.  

With this device, a recording time for a Absorption spectrum with sufficient resolution for identification cation of certain substances within, for example, 3 Realize seconds. This recording time is enough to usual speeds of the processing extruder Online quality control of the plastic granulate ren. To increase the measuring accuracy, several Spectra recorded and averaged.

It is preferred between the measurement volume and the IR light source a polarizer for generation in the plane of the sample film linearly polarized IR light arranged, the roller Device a film positioning device with two Füh tion rollers, which are arranged so that the IR light in the measurement volume essentially in compliance with the Brewster Incidence on the sample film. In this execution form of the invention results in a significant improvement in Quality of the IR absorption spectra since the occurrence of Interference phenomena in certain areas of the specter trums is minimized. Interference rings occur when on the more or less good film surfaces the light in the The film is partially reflected and interacts with other beams overlaid with an optical phase shift. The avoid that of these interference rings simplifies and accelerates the Evaluation of the absorption spectra.

The film positioning device preferably has a rotatably mounted drum, the axes of rotation of the drum mel and the guide rollers are parallel to each other. In in this way the fact that Sample foils made of different granulate material differed Liche refractive indices, which is known for the value for the Brewster angle is changed. The respective setting The Brewster angle can be adjusted by rotating the drum be added, for example manually, without the Roller device in total requires any modifications become.  

The Brewster angle setting is then particularly one fold if the axes of rotation of the guide rollers and the drum lie in one plane and the axis of rotation of the drum in the Middle between the axes of rotation of the guide rollers and between the spectrometer inlet and the IR light source is arranged.

An IR light source is preferably used, the one in the middle Infrared (MIR) emitted. Examples of such light sources are a heated ceramic that may stabilize performance is, a W lamp, a Nernst spotlight, a mercury bo gen lamp or an Oppermann body. The MIR spectral range is cheap because it is good for the following plastics, for example identifiable absorption spectra provides: PP, PS, LDPE, LLDPE, HDPE, PPMMA, EVA, Nylon, PC, ABS, PB, PET, EPDM, PE / PP, PC / ABS, PPO / PS / nylon, PET / PE.

For this IR spectral range, DTGS (deu terized triglycine sulfate), TGS, PbS, InSb or HgCdTe be set.

With the help of the MIR absorption spectra, the chemical Determine the composition of resins, for example the Ana perform lysis of functional groups, or even Determine the composition of copolymers. The quanti tative determination of anti-oxidants, UV absorbers, Lubricants, stabilizers, fillers, material stretchers, Impact resistance agent, machining help, flame retardants u. a. is possible. Even multi-layered Structures can be determined in terms of their thickness, if the density of the material and thus its absorption coefficient is known from calibration measurements. Leave it physical properties such as overall Determine density and meltflow index.

To obtain the absorption spectra for transmission measurements sung it is necessary that from the to be checked  Granulate-made film in the MIR spectral range has sufficient transmission. The above example mate rialien meet this requirement. However, it can also examine other plastics with the device, see above if there is sufficient transmission in the MIR spectral range exhibit.

With the above-described embodiments of a device for quality control of plastic granules, the following method can be carried out:

• - Branch off a sample amount of plastic granulate a plastic granulate main stream;
• - Continuous production of a film from the abge branched plastic granulate;
• - Guide the film to a measuring volume using IR light is irradiated;
• - Continuous recording of IR absorption spectra in transmission of Ab cut the film; and
• - Evaluation of the IR absorption spectra for determination of material properties of the plastic granulate.

In this method, IR light from the MIR spectra is preferred tralbereich used, this light in the film plane can be linearly polarized and at the Brewster angle to the Slide in.

To evaluate the IR absorption spectra, identifi decoration of certain substances in the sample film a ver immediately carried out with calibration absorption spectra the. This speeds up the identification of the substances and is preferred with a computer using more suitable Evaluation software carried out.

A preferred embodiment of the invention is as follows described with reference to the drawings. Show it:  

Figure 1 is a schematic view of a device for rapid quality control of plastic granules.

Figure 2 is a schematic view of an optical measuring arrangement for receiving IR absorption spectra.

Fig. 3 is a schematic view of a modification of the optical measuring arrangement shown in Fig. 2;

Fig. 4 is a schematic view of a film positioning device;

Figure 5 is a graphical representation of MIR absorption spectra of low density polyethylene (LDPE).

As can be seen in Fig. 1, a device for quality control of plastic granules has a measuring extruder 1 , to which a sample amount of the plastic granules to be examined, which is derived from a main stream, is supplied. A flat sample film 5 is produced by means of a downstream blowing device 2 and cooling rollers 3 , 4 . The cooling rollers 3 , 4 are used in particular to prevent chemical reactions which are possible in principle due to the heating of the granules in the measuring extruder, but which falsify the measurement results of the quality control by changing the chemical composition of the sample film 5 .

The sample film 5 is guided to a measuring volume 6 into which an IR light source 7 of a Fast Fourier Transform (FFT) IR spectrometer radiates light in the range from 350 to 6000 cm ^-1 . In the figures, the IR light source 7 is shown a simplicity of representation because of a detector 8 arranged opposite. In practice, however, a coupling of the IR light takes place via a deflection mirror which is positioned opposite the detector 8 .

For example, the interferometer of a Bomem MB 100 spectrometer can be used as the FTIR spectrometer. The sample film is continuously produced by the measuring extruder 1 and the Blasvor device 2 , passed through the measuring volume 6 and then wound up.

The speed of production of the sample film can range from 1 m / min to 100 m / min. The IR light of different wavelengths is absorbed by the sample film 5 to different extents, the size of the absorption being determined by means of the detector 8 . The recording time for a spectrum is in the range of three seconds for a spectral resolution of 4 cm ^-1 . The resolution mentioned is sufficient to carry out an identification of substances in the sample film 5 with sufficient certainty. To achieve such a resolution, multiple absorption spectra can be averaged, with an average of 10 m sample film being obtained for the recording of 64 spectra at a conventional sample film speed.

For averaging over several spectra and thus over one certain sample film length is decisive with how much averaged absorption spectra for identification of a material required in the sample film Resolution of the absorption spectra is achieved.

The recorded, possibly averaged absorption spectra are compared with calibration spectra for a quantitative Density determination for certain materials. It is also possible, with known density for certain mate rialien thickness measurements for the sample film.

Fig. 5 shows an example of MIR absorption spectra recorded with the device for quality control of plastic granules in transmission. The plastic granulate to be examined contains LDPE here.

The density determination for LDPE or other materials  proceeds as follows:

First, absorption spectra on foils of example sample material provided by the manufacturer for potash used for purposes of known density. From theory tical approaches it is known which molecules in the samples ent have a decisive influence on the density of the samples and in what areas of MIR absorption occurs.

For samples to be checked, their density can be determined for certain Materials, for example LDPE, by quantitative comparison of the characteristic absorption bands with the potash brations spectra can be determined. It can be enough Absorption measurements only over sections of the MIR area to be carried out, provided the material is clearly assigned is guaranteed.

Figs. 2 and 3 show optical measuring arrangements for receiving infrared transmission absorption spectra. A polarizer 13 generates linearly polarized light from the unpolarized IR light, the vector of which vibrates for the electric field strength in the plane of the drawing.

The sample film 5 is arranged in the measuring volume 6 while observing the material-specific Brewster angle Θ _p , for which it is known that:

tan Θ _p = n ₁ / n ₀

With
n ₁ = refractive index of the sample film and
n ₀ = refractive index of the medium surrounding the sample case (here air with n ₀ = 1).

In FIGS. 2 and 3, the sample film runs parallel to the plane of oscillation of the vector of the electric field strength of the IR light and the Brewster angle is between the film normal and the optical axis of the IR light source. In this way, the occurrence of interference rings in the measured IR absorption spectra is suppressed.

FIG. 4 shows the positioning device 9 used to set the Brewster angle for the sample film 5 with respect to the optical axis of the IR light source 7 .

The positioning device has a rotatably mounted drum 10 on which two guide rollers 11 , 12 are rotatably mounted, with which the sample film is guided through the measuring volume 6 . The axes of rotation of the guide rollers 11 , 12 and the drum 10 lie in one plane, the axis of rotation of the drum 10 being arranged in the middle between the axes of rotation of the guide rollers 11 , 12 .

The axis of rotation of the drum 10 lies within the plane of the sample film 5 and intersects the optical axis of the IR light source 7 . The axis of rotation of the drum 10 is therefore arranged between the detector 8 and the IR light source 7 .

With this arrangement of the positioning device 9 , the guide rollers 11 , 12 can be pivoted about the axis of rotation of the drum 10 by, for example, manual or computer-controlled rotation of the drum 10 , the angular position of the sample film 5 in the measuring volume 6 changing. By simultaneously recording absorption spectra and rotating the drum 10 about its axis of rotation, the suitable angle for absorption spectra that are largely free of interference rings can be found for each sample film material.

Claims (9)

1. Device for quality control of plastic granules lat, with

• 1. a measuring extruder ( 1 ) with a blowing device ( 2 ) for the continuous production of an essentially flat sample film ( 5 ) from the plastic granulate,
• 2. a device ( 3 , 4 ) for cooling the extruded sample film in a defined temperature,
• 3. a roller device ( 11 , 12 ) for continuously guiding the sample film ( 5 ) through a measuring volume ( 6 ),
• 4. an IR light source ( 7 ) for irradiating the measurement volume,
• 5. an FTIR spectrometer with detector ( 8 ) for spectrally resolved collection of IR light from the measurement volume ( 6 ) and
• 6. an evaluation device for processing signals, which are supplied by the detector ( 8 ).

2. Device according to claim 1, in which between the Meßvo lumen ( 6 ) and the IR light source ( 7 ) a polarizer ( 13 ) for generation in the plane of the sample film ( 5 ) linearly polarized IR light is arranged and Roller device ( 3 , 4 , 11 , 12 ) has a film positioning device ( 9 ) with two guide rollers ( 11 , 12 ) which are arranged so that the IR light in the measuring volume ( 6 ) essentially in compliance with Brewster angle falls on the sample film ( 5 ). 3. Device according to claim 2, wherein the positioning device ( 9 ) has a rotatably mounted drum ( 10 ) on which the two guide rollers ( 11 , 12 ) are rotatably mounted, the axes of rotation of the guide rollers ( 11 , 12 ) and the drum ( 10 ) are parallel to each other. 4. The device according to claim 3, wherein the axes of rotation of the guide rollers ( 11 , 12 ) and the drum ( 10 ) lie in one plane and the axis of rotation of the drum ( 10 ) in the middle between the axes of rotation of the guide rollers ( 11 , 12 ) and between the detector ( 8 ) and IR light source ( 7 ) is net angeord. 5. Device according to one of claims 1 to 4, wherein the wavelength of the IR light source ( 7 ) is in the range of the MIR. 6. Process for quality control of plastic granules, comprising the steps:

• 1. Branch a sample amount of plastic granules from a plastic granulate main stream;
• 2. Continuous production of a film from the branched-off plastic granulate;
• 3. Guide the film to a measurement volume that is irradiated with IR light;
• 4. Continuous recording of IR absorption spectra in transmission of sections in the measurement volume from the film; and
• 5. Evaluation of the IR absorption spectra to determine the material properties of the plastic granulate.

7. The method of claim 6, wherein the IR light from the MIR Area is used. 8. The method according to any one of claims 6 or 7, in which in the film plane linearly polarized IR light, which in Brewster angle falls on the slide is used. 9. The method according to any one of claims 6 to 8, in which Evaluation of the IR absorption spectra, a comparison with Calibration measurements are carried out.