JP2008292300A - Apparatus and method for sorting out plastic material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To sort out a plastic material simply and precisely. <P>SOLUTION: When sorting out the plastic material, a far-ultraviolet spectrum of an article made of the plastic material is measured, and then the material of the article is sorted out from a spectrum property near an absorption edge of an absorption band of the far-ultraviolet spectrum. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to spectroscopic analysis of plastics.

  In recent years, there are concerns about environmental changes and resource shortages on a global scale, and the need for recycling of daily necessities is increasing. In order to reuse discarded plastic materials, the plastic materials must be sorted and collected by material. In one method, the plastic material is irradiated with light in the near-infrared region, and the material is discriminated by spectroscopic analysis of reflected light or transmitted light from the plastic material (for example, Japanese Patent Application Laid-Open No. 2002-214136 and Japanese Patent Application Laid-Open 2002-267601).

  Examples of plastic materials used as plastic bottles and sheets include polystyrene, polyethylene, polyethylene terephthalate, polypropylene, and polyvinyl chloride. These absorption spectra in the near-infrared region are different as shown in FIG. 3 of JP-A-9-89768 and FIG. 3 of JP-A-2002-214136. Because they are hydrocarbon polymers, their spectra are similar. In particular, commercially available products contain various additives for each product due to processability and durability, making it difficult to distinguish them. In order to reduce misjudgment, multivariate analysis such as principal component analysis or a technique such as a neural network is used. Therefore, it is desired to improve the accuracy of plastic material discrimination at the plastic sorting and recovery site.

  By the way, although the present invention relates to spectroscopic measurement in the ultraviolet region, it is well known that absorption by electronic transition generally occurs in the ultraviolet region of organic substances. However, until now, no attempt has been made to compare and measure the absorption spectra of plastic materials in the ultraviolet region, and to use the difference in the characteristics of the spectra to identify and distinguish plastic materials. In particular, in the wavelength region of 200 nm or less, which is referred to as the vacuum ultraviolet region, since the ultraviolet absorption of oxygen in the air is large, the measurement environment is set to a vacuum, or an inert gas (nitrogen or argon having no ultraviolet absorption in the wavelength region). In general, ultraviolet spectroscopy has been avoided for industrial use. As described above, the application of far-ultraviolet spectroscopy was not common because the measurement environment was limited by strong absorption of oxygen, but in recent years, far-ultraviolet spectroscopic measurement has been performed by using a nitrogen purge instead of evacuation. It has become easier. However, far ultraviolet spectroscopy has not received much attention as an analysis method.

Examining conventional techniques for analysis using far-ultraviolet spectroscopy, Japanese Patent Application Laid-Open No. 11-51751 uses the ultraviolet spectrum to measure the coating amount associated with coating styrene or butadiene latex on paper. Is described. Here, the spectrum is measured in the ultraviolet region where the absorption is larger than that in the infrared region, and the coating weight is obtained from the reflectance at specific wavelengths (for example, 220 nm and 260 nm).
JP 2002-214136 A JP 2002-267601 A JP-A-9-89768 JP-A-11-51751

  An object of the present invention is to provide a simple and highly accurate plastic material discrimination method and apparatus.

  In the plastic material discrimination method according to the present invention, the ultraviolet spectrum of an article made of a plastic material is measured, and then the material of the article is classified based on the spectral characteristics near the absorption edge of the absorption band of the ultraviolet spectrum. The spectral characteristic in the vicinity of the absorption edge is, for example, a cutoff wavelength. The material includes, for example, at least one of polystyrene, polyethylene, polyethylene terephthalate, polypropylene, and polyvinyl chloride.

  The plastic material discriminating apparatus according to the present invention includes an ultraviolet light transmissive plate member capable of placing an article made of a plastic material as a measurement object, a light source that generates ultraviolet light, and an ultraviolet light detection that detects incident ultraviolet light. And in an environment that does not contain oxygen gas, the ultraviolet light generated by the light source is dispersed and guided to the plate member, and the reflected light from the article on the plate member is reflected on the ultraviolet light detector. And a signal processing unit for determining the material of the article from the signal input from the ultraviolet light detector. In the plastic material discrimination device, the plate member is, for example, a window plate that refracts incident light. Alternatively, the plate member is, for example, a total reflection attenuating optical probe that totally reflects incident light. Preferably, the display device further includes a display device for displaying the determination result by the signal processing unit.

By using a spectral spectrum appearing in the ultraviolet region of a plastic material, material discrimination can be made much more accurately than using a conventional spectral spectrum in the near infrared region. Spectroscopic measurement and analysis are easy, and qualitative analysis can be performed with high sensitivity and selectivity.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
A spectrum in the ultraviolet wavelength region of 350 nm or less is measured to distinguish the material of an organic compound such as a plastic material. These materials absorb far ultraviolet light due to the σ-σ ^* transition of the C—C bond, but the spectral shape near the absorption edge on the high wavelength side changes for each material, so the material is based on the spectral data. High sensitivity and clear discrimination. This will be described in more detail below.

FIG. 1 shows the transmittance | permeability in the 120-300 nm ultraviolet wavelength area | region of 6 types of commercially available food packaging wrap films as an example. Here, samples 1 and 2 are films mainly composed of polyvinylidene (PVDC), sample 3 is a film mainly composed of polyvinyl chloride (PVC), and samples 4, 5 and 6 are polyethylene. It is a film mainly composed of (PE). These samples are manufactured by different companies. The thickness of the sample is about 10 μm. As can be seen from FIG. 1, the absorption is broad in the spectra of these samples. Polyethylene films (samples 4, 5, 6) are widely absorbed at wavelengths shorter than 170 nm. Also, the spectrum of the PVC film (sample 3) produces a broad absorption from a wavelength higher than that of PE, and the PVD film (samples 1 and 2) produces a broad absorption from a higher wavelength. These absorptions are due to the σ-σ ^* transition of the C—C bond.

  The six plastic samples shown in Fig. 1 are generally commercially available film products, and various additives should have been added during the manufacturing process, but nonetheless added to the absorption edge characteristics of each material. There is no big difference between things. In general, polyethylene resins are known to have side chains in the carbon bonds in order to control their hardness and stretchability, but the small change in the absorption spectrum near 185 nm of samples 4 to 6 is the type of these side chains. It is thought that it is caused by the difference in numbers. Even if there is such a difference for each product, the basic spectral shapes in the vicinity of the absorption edge on the high wavelength side of the absorption band agree well with materials having the same main component.

  As described above, since the spectrum shape near the absorption edge is determined by the main component, the plastic material can be identified only by looking at the raw spectrum data. The six samples shown in FIG. 1 can be clearly classified into three groups having different spectral shapes near the absorption edge only from raw data in the region of 120 to 300 nm. For example, a cut-off wavelength may be used as an amount representing the spectral shape near the absorption edge. The cutoff wavelength is defined as a wavelength having a transmittance of 5%, for example.

  Moreover, FIG. 2 shows the transmittance | permeability in the 150-450 nm wavelength range of the material of the main plastic products marketed. The thickness of polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC) samples is about 10 μm, and the thickness of polystyrene (PS) and polyethylene terephthalate (PET) samples is about 30 μm. Since these materials have different spectral shapes in the vicinity of the absorption edge as in the example of FIG. 1, the materials can be discriminated based on the spectral shape in the vicinity of the absorption edge only with raw data in the region of 150 to 350 nm.

  For comparison, in the conventional near-infrared spectrum, the absorption bands of these materials have a small absorbance and a large overlap of the absorption bands. For this reason, it was difficult to distinguish the material by simply measuring the spectral spectrum. FIG. 3 shows reference near-infrared spectra of various plastics shown in FIG. 3 of JP-A-2002-214136. Comparing this near-infrared spectrum with the ultraviolet spectrum, it can be seen that in the ultraviolet spectrum, materials that were difficult to distinguish in the near-infrared spectrum can be clearly distinguished by the spectral shape near the absorption edge.

When light in the ultraviolet region is applied to an organic compound, in the case of a single bond, electrons in the ground state (σ orbital) transition to a high energy state (σ ^* orbital). The bond energy of the C—C bond in the organic compound varies depending on the substituent at the C atom. For this reason, the σ-σ ^* transition is very sensitive to substituents, and the far-ultraviolet spectrum of an organic compound varies depending on the type and number of substituents when substitution is performed. Fig. 4 shows the carbon cross-linked structure of polyethylene (PE), polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC), but some of the hydrocarbon bonds in the plastic material in the order of PE, PVC and PVDC are chlorine. Has been replaced. Therefore, in the ultraviolet absorption spectrum of each plastic material, PE (samples 4, 5, 6), PVC (sample 3), PVDC (samples 1, 2), and hydrocarbon bonds are replaced with chlorine. The cutoff wavelength of the absorption band is displaced to the longer wavelength side.

As the above measurement example shows, the absorption spectrum in the ultraviolet region of plastic shows a large change following the change in the cross-linked structure such as hydrocarbon. The plastics shown in FIG. 1 and FIG. 2 show a large spectral difference in the wavelength region of 160 to 260 nm. This difference is due to the difference in the bond energy of the C—C bond due to the substituent. The spectral shape in the vicinity of the absorption edge of the absorption band due to the σ-σ ^* transition of the C—C bond is very sensitive to the composition of the plastic and has high selectivity, so that it can be used for the classification of plastic materials. Therefore, by using the spectral spectrum having such properties appearing in the ultraviolet region of the plastic material, it becomes possible to perform material discrimination much more accurately than using the conventional spectral spectrum in the near infrared region.

  Next, a specific configuration of the plastic discrimination device will be described. In this embodiment, the plastic material is spectroscopically analyzed in the ultraviolet region of 160 to 350 nm. At that time, light having a short wavelength of 200 nm or less is absorbed by oxygen in the air, so that the measurement environment is inert such as vacuum or nitrogen. It is necessary to replace with gas. FIG. 5 shows an example of the apparatus configuration. In the above measurement example, the transmittance is measured by placing a sample in an environment that does not contain oxygen. However, it is desirable to be able to discriminate samples in the air at the site of plastic separation collection. Therefore, the reflected light from the sample is measured and sorted.

FIG. 5 shows an example of an ultraviolet spectrometer. The ultraviolet spectroscopic device includes an N ₂ purge area 10 that does not contain oxygen and a non-purge area 30 that is in normal air. In the ultraviolet light path disposed in the N ₂ purge area 10, the internal air is removed by purging with nitrogen gas, argon gas or the like. Further, it may be evacuated to a vacuum. The measurement sample 40 outside the N ₂ purge area 10 is in the air and is placed in contact with the sapphire window plate 16. Ultraviolet light generated from an ultraviolet light source (for example, a deuterium lamp) 32 provided in the non-purge area 30 immediately enters the N ₂ purge area 10, passes through the grating 12 that is a monochromator, and is reflected by the mirror 14. Then, the light enters the sapphire window plate 16, passes through the window plate 16, and strikes the sample. The incident angle is set appropriately. The wavelength is changed by controlling the grating 12. Note that the material of the window plate 16 may be synthetic quartz, magnesium fluoride, calcium fluoride, or the like having good ultraviolet transmission characteristics instead of sapphire. The reflected light from the measurement sample 40 isolated on the window plate 16 enters the window plate 16, is reflected by the mirror 18, enters the ultraviolet light detector 20, and is detected. The spectrum data detected by the ultraviolet light detector 20 is processed by the signal processing unit 34 in the non-purge area 30. The measurement wavelength is, for example, 160 to 210 nm. In such an apparatus, while the measurement sample 40 is in a normal air environment, the spectroscopic measurement can be performed in a vacuum ultraviolet environment from which oxygen has been removed. It becomes possible by destruction. When this apparatus is used, for example, at a plastic separation / recovery site, the accuracy of plastic material discrimination can be improved as compared with conventional infrared spectroscopy.

  As shown in FIG. 6, instead of the window plate 16, an attenuated total reflection optical probe 16 'made of sapphire or synthetic quartz may be installed. Total reflection attenuation is used to measure the absorption spectrum of a material with very high absorption, and it is a soaking of light in the wavelength order that is formed when light is totally reflected on the surface of an optical probe (evanescent wave). Measure the amount of light absorption in the sample. Incident light is incident on the optical probe 16 'at an incident angle that causes total reflection and is totally reflected. At this time, the sample in contact with the attenuated total reflection type optical probe 16 ′ is impregnated with light of a wavelength order generated on the total reflection surface in contact with the sample and then reflected. This reflected light is detected by the ultraviolet light detector 20. Thereby, the absorption of ultraviolet light by the sample is measured.

  The signal processing unit 34 includes a CPU that controls the whole, sets the wavelength of ultraviolet light by controlling the grating 12, and inputs a measurement signal from the ultraviolet light sensor 20. The signal versus wavelength graph thus obtained can be displayed on the display device 36. Further, the determination result of the material may be displayed on the display device 36. Although not shown, the determination result may be printed on a printer.

  FIG. 7 shows a flowchart of material determination in the signal processing unit 34. A table of absorption characteristics of far ultraviolet light for the material to be measured is stored in advance in the storage device in the signal processing unit 34 (S10). First, the grating 12 and the ultraviolet light detector 20 are controlled to record the detection value of the ultraviolet light sensor 20 in the measurement wavelength range (S12). An absorption edge characteristic (for example, a cutoff wavelength) is obtained from the obtained measurement data (S14), a material is specified with reference to the absorption edge characteristic table (S16), and a discrimination result is displayed or printed (S18). If an unmeasured sample remains (YES in S20), the process returns to step S12 and the above-described determination process is repeated.

Graphs of the far ultraviolet spectrum of six commercially available food packaging wrap films Graph of the far ultraviolet spectrum of five plastics Graph of spectra of various plastics in the conventional near infrared region for comparison Diagram showing the carbon cross-linked structure of polyethylene (PE), polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC) Diagram of ultraviolet spectrometer Figure of modification of ultraviolet spectrometer Material judgment flowchart

Explanation of symbols

10 N ₂ purge area, 12 grating, 16 window plate, 16 ′ attenuated total reflection optical probe, 30 non-purge area, 32 ultraviolet light source, 34 signal processing unit, 36 display device.

Claims (7)

Measure the ultraviolet spectrum of articles made of plastic materials,
A plastic material discrimination method in which the material of the article is separated from the spectral characteristics near the absorption edge of the absorption band of the ultraviolet spectrum.   2. The plastic material discrimination method according to claim 1, wherein the spectral characteristic in the vicinity of the absorption edge is a cutoff wavelength.   2. The plastic material discrimination method according to claim 1, wherein the material includes at least one of polystyrene, polyethylene, polyethylene terephthalate, polypropylene, and polyvinyl chloride. An ultraviolet light transmissive plate member capable of placing an article made of a plastic material as a measurement object;
A light source that generates ultraviolet light;
An ultraviolet light detector for detecting incident ultraviolet light;
In an environment that does not contain oxygen gas, the ultraviolet light generated by the light source is dispersed and guided to the plate member, and the reflected light from the article on the plate member is guided to the ultraviolet light detector. Optical system,
A plastic material discriminating device comprising: a signal processing unit that discriminates the material of the article from a signal input from the ultraviolet light detector.   5. The plastic material discriminating apparatus according to claim 4, wherein the plate member is a window plate that refracts incident light.   5. The plastic material discriminating apparatus according to claim 4, wherein the plate member is a total reflection attenuation optical probe that totally reflects incident light.   The plastic material discrimination device according to any one of claims 4 to 6, further comprising a display device for displaying a discrimination result by the signal processing unit.

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