Method and apparatus for identifying articles with a luminescent marker for recycling

Abstract

The composition of articles can be identified using luminescent codes printed or coated onto the articles (or labels on the articles) by means of an ink which contains long persistence luminescent materials. Radiation is used to excite the luminescent materials and this is then extinguished before the article passes beneath a spectrometer. An infrared beam then strikes the article in order to stimulate emission from the long persistence luminescent materials, via a detrapping mechanism, and the spectrometer detects the emission in order to identify the article.

Claims

1. A method for identifying an article marked with a luminescent marker, including carrying out the following steps in order: (a) providing an article marked with a luminescent marker which identifies said article, (b) exciting the marker with a first type of electromagnetic radiation in order to store luminescence in said marker, (c) ceasing said exciting step, (d) waiting for a period of time, (e) applying a second type of electromagnetic radiation to the marker in order to release said stored luminescence, and (f) detecting said released luminescence in order to identify said article.

2. A method as claimed in claim 1, wherein: (a) the first type of electromagnetic radiation has a wavelength from 200 nm to 700 nm, and (b) the second type of electromagnetic radiation has a wavelength from 700 nm to 3000 nm.

3. A method as claimed in claim 1, wherein the luminescent marker includes one or more phosphor markers.

4. A method as claimed in claim 3, wherein said one or more phosphor markers has a phosphor emission intensity 50 ms after the excitation is terminated that is at least 37% of its intensity ims after the excitation is terminated.

5. A method as claimed in claim 1, wherein the first type of radiation is applied either by moving the article through a static field of said radiation or by scanning the article with said radiation.

6. A method as claimed in claim I, wherein the second type of radiation is applied either by moving the article through a static field of said radiation or by scanning the article with said radiation.

7. A method as claimed in claim 1, wherein the article is moved along a path between step (b) and step (f).

8. A method as claimed in claim 7, wherein the article is moved on a conveyor belt.

9. A method as claimed in claim 1, including the step of making a decision regarding recycling of said article based on the identity of the article.

10. A method as claimed in claim wherein the period of time in step (d) is from 1 ms to 5 s.

11. A method as claimed in claim I, including the steps of providing a detector for step (f) and shielding said detector so that it only detects radiation from step (e).

12. A method as claimed in claim 1 wherein: (a) the marker is disposed on the article itself, or (b) the marker is disposed on a substrate and the substrate is disposed on the article.

13. A method as claimed in claim 12 in which: (a) the luminescent marker is disposed on the article, or (b) the substrate disposed on the article, using a binder that is water or alkali soluble.

14. Apparatus for identifying an article marked with a luminescent marker, including: (a) a conveyor for conveying an article along a path from a first path position to a second path position, (b) a source of a first type of electromagnetic radiation, for supplying said first type of radiation to a first radiation position between the first path position and the second path position, wherein the first type of radiation induces storage of luminescence in the article, (c) a source of a second type of electromagnetic radiation, for supplying said second type of radiation to a second radiation position between the first radiation position and the second path position, wherein the second type of radiation induces release of the stored luminescence from the article, (d) a detector, for detecting radiation emitted from the article when it is between the second radiation and the second path position and thereby identifying the article, and (e) a shield, for shielding the detector from any radiation emitted by the article before it reaches the second radiation position.

15. Apparatus as claimed in claim 14, additionally including: (f) a diverter, for diverting the article from the path depending on its identity.

16. Apparatus as claimed in claim 14, wherein: (a) the first type of radiation has a wavelength from 200 nm to 700 nm, and (b) the second type of electromagnetic radiation has a wavelength from 700 nm to 3000 nm.

17. Apparatus as claimed in claim 14, wherein the conveyor is a conveyor belt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A number of preferred embodiments of the invention will now be illustrated with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of prior art apparatus for sorting polymeric articles;

(3) FIG. 2 is a graph showing the decay in luminescence from a Y.sub.2D.sub.2S: Eu 3+ phosphor;

(4) FIG. 3-5 illustrate apparatus for sorting polymeric articles in accordance with the invention.

DETAILED DESCRIPTION OF EXEMPLARY VERSIONS OF THE INVENTION

(5) The great majority of OBAs and luminescent inks used in commerce are organic materials and have the characteristic that they cease to emit luminescence very quickly after the exciting radiation is switched off, often within nanoseconds. Some inorganic phosphors also respond very quickly like this, but most are slower and their luminescence ceases for example some milliseconds after the excitation source is switched off. The decay time of inorganic phosphors is largely dependent on the activator used with for example Eu.sup.3+ having decay times (defined as the time to reach 1/e of their initial brightness) of ˜1 millisecond, whereas phosphors doped with Eu.sup.2+ activator are more than an order of magnitude faster. For example FIG. 2 illustrates the decay in luminescence from a Y.sub.2O.sub.2S:Eu.sup.3+ phosphor.

(6) Codes may be written using luminescent materials (typically inorganic phosphors) that have relatively slow decay times and the exciting radiation is cut off shortly before the article passes beneath the analyser, such that the stray luminescences will have ceased to emit, whereas the luminescent codes are still emitting. Many useful phosphors have decay times in the milliseconds range and at belt speeds of 3 m/s this implies truncating the exciting radiation a matter of a few millimetres before the analyser and given the heterogeneous nature of the articles passing down the line, however, this is not easy to engineer.

(7) Preferably therefore so called long persistence phosphors are used. Such materials have been known for many years, although the precise mechanism by which they work is still not fully understood. They can have decay times of, for example from seconds to several hours. This permits the radiation to be terminated for example 10 cm before the articles pass beneath the analyser, and this is much easier to engineer. Many long persistence phosphors are known that can be excited using visible light, such as violet or blue light. The use of visible excitation sources has a number of substantial advantages. It is much less damaging to human tissue than UV. Because we can see this light it is much easier to prevent excessive exposure. In many cases ambient light will be sufficient to generate luminescence, although exposure to brighter sources will improve signal strength. Because visible light is used in mass market lighting applications, suitable sources are available at relatively low cost. Additionally, visible light, for example blue, does not excite most OBAs efficiently and so these interferences are substantially reduced.

(8) Long persistence phosphors are in some cases formulated by adding additional dopants to known phosphor compositions. These new dopants are believed to form trapping centres, which effectively store the energy injected by the exciting radiation and release it slowly over a period of time. In other cases, these trapping centres are thought to be formed by defects in the phosphor matrix itself.

(9) A problem with these materials, however, is that too little luminescence is emitted during the ˜2-3 milliseconds that the article passes beneath the analyser, and so signal levels are low necessitating the use of higher quantities of phosphor, which increases costs. It is known, however, that the luminescent emission of some of these long persistence phosphor materials can be stimulated promptly using infrared radiation. If they are scanned using an infrared beam they immediately emit the energy stored in the traps as an intense burst of luminescence. It has been realised that this is very convenient in the current application, where a beam of broad band infrared radiation is already scanned across the articles, to facilitate the NIR measurements, and so no new equipment is required to implement the invention.

(10) There are two ways of implementing this process. The first is to print the luminescent code onto the label that is nearly always present. The labels are subsequently removed prior to recycling and so this has the advantage that the recycled material is not contaminated by luminescent material (which might compromise the quality of the recycled material). The label removal process, however, is not 100% efficient and a certain percentage of labels get through.

(11) The second approach that has been promoted is to print the code directly onto the article. As mentioned above, this suffers from the drawback of potential build-up of the luminescent materials in the waste stream. It is also more difficult to read the code if there is a label on top of it. It does have significant advantages, however. A certain percentage of the bottles lose their labels before sorting. For example, shrink wrap labels can come off when the plastic bottles are crushed. Additionally, some articles do not have labels: the product information may be printed directly onto the article.

(12) After sorting the recycled materials are washed in a hot alkali solution. It is common for labels to be attached using alkali-soluble adhesives which are designed to dissolve immediately during this treatment. If the phosphor code is contained within a binder that dissolves in this process then it will be washed out of the material at this stage, which is helpful to both processes. It may also help the valuable components in the phosphors to be recovered and recycled. It is advantageous to use alkali soluble resins as the binder for the luminescent materials so that they can be easily washed off and separated from the material to be recycled.

(13) There are many long-persistent phosphors known in the art. For example, the Phosphor Handbook 2.sup.nd Edition (Edited by W. M. Yen, S. Shionoya, and H. Yamamoto, published by CRC Press) lists more than 70 known phosphors with long persistence properties. Ideally the present invention may be applied by making small modifications to existing sorting equipment, and in the following section some examples of how it may be implemented are shown.

(14) A common arrangement, for example widely used by Tomra Sorting plants worldwide, is to use a spot of light from and quartz halogen lamp which is scanned across the conveyor belt. In this case there are two convenient ways in which the invention can be implemented.

(15) FIG. 3 illustrates the first in which an area is bathed in UV light of suitable wavelength to excite the phosphor(s) being used. In the diagram 1 is the conveyor belt, 2 is the area bathed in static (i.e. unscanned) UV radiation, 3 is an area that is not bathed in either UV or infrared radiation, 4 is an area scanned by the infrared beam and 5 is an array of air-jets that are used to eject articles identified as having the correct composition.

(16) FIG. 4 illustrates a second example of how the technology may be implemented on a system utilising a scanned infrared beam. In this case a spot of UV (6 in the diagram) is scanned in unison, but slightly behind the infrared beam (so that the UV strikes the articles first), ideally utilising the same mirror system employed to scan the infrared beam. Once again the UV beam does not overlap with the infrared beam. In both cases the spectrometer is shielded so that it cannot receive radiation directly from the UV irradiated area, but only from the infrared radiated area.

(17) Quartz halogen lamps are widely used as sources of infrared. These sources also produce visible radiation. Ideally the visible component should be removed before the beam strikes the articles using a long pass filter, which are readily available in commerce (e.g. from Edmund's Optics), otherwise the scattered visible component will compete with the luminescent signals.

(18) FIG. 5 illustrates another example, in this case for a different sorting system that is in use commercially at the current time. In this case the articles are funneled into channels and irradiated with static infrared beams, 6. The simplest approach in this case is for a static area to be bathed in UV just before the infrared irradiated area is bathed with infrared. Once again there must be a gap between these two areas that is not bathed in either type of beam, and the detection spectrometers must be shielded from luminescence coming directly from the area bathed in UV.

(19) In the foregoing for brevity it has been assumed that the exciting radiation is UV. The exciting radiation is chosen so as to be compatible with the phosphors to be detected and may in fact be UV-A, UV-B, UV-C or indeed visible light since some long persistence phosphors, such as CaS:Eu, may be excited using visible light. Indeed, there are some long-persistence phosphors that are excited by ambient lighting and it is possible to use the infrared stimulated emission process described herein without the need for an addition excitation source.

(20) If phosphors that can be excited using visible light were used then ambient lighting could provide the first radiation source. In practice, however, since the intensity of the light stimulated by the second radiation source is closely related to the excitation intensity then it would be advantageous to use much brighter visible light than is normally present in ambient lighting for the first radiation source. Nevertheless, there are advantages (health and safety, cost) in using visible light rather than ultraviolet.

(21) All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

(22) The disclosures in UK patent application number GB1706291.0 from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.