GB2264558A

GB2264558A - Method of identifying polymer materials

Info

Publication number: GB2264558A
Application number: GB9303163A
Authority: GB
Inventors: Evangelos Theocharous
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1992-02-26
Filing date: 1993-02-17
Publication date: 1993-09-01
Also published as: GB9303163D0; GB9204064D0

Abstract

At least one material capable of fluorescing in the far red or near infrared region is added to a polymer. A polymer sample 2 may be subsequently identified by illumination from a laser source 1 at wavelengths in the far red or near infrared suitable for exciting the fluorescent material, and determining the presence of the material by spectroscopic means 6 to identify the polymer. The polymer may be label led with at least two fluorescent materials, the materials being present in a predetermined weight ratio whereby the ratio of the fluorescence intensities observed for each material is proportional to the predetermined weight ratio. The fluorescent materials may be dyes or rare earth compounds added to the polymer in solution or as sub-micron particles in a concentration from 0.001 to 1 ppm. <IMAGE>

Description

METHOD OF IDENTIFYING POLYMER MATERIALS The present invention relates to a method of identifying polymer materials in particular to a method of identifying polymers by use of near infrared (NIR) fluorescence spectroscopy.

Polymer waste particularly plastic waste forms approximately 7% of domestic waste and can be higher in some industrial areas. A major problem with the recycling of plastic waste is the absence of precise and efficient systems to separate out individual plastic materials from a mixed waste stream.

Plastic waste is diverse and may include coloured or opaque high density polyethylene bottles or containers, clear or tinted polyethylene terephthalate cups as well as a diversity of polyvinyl chloride bottles.

At present various methods for identifying and sorting plastic waste have been used. Procedures which rely on manual sorting tend to be unreliable due to eyesight, poor attention span and slowness.

More recently techniques involving electromagnetic spectroscopy have been developed but these tend to be complex and costly and impractical for commercial use.

To enable efficient recycling there is a need to identify a low cost, fast and reliable method of identifying plastics which can be used to separate various families of plastics from mixed waste.

Fluorescence spectroscopy is a well established analytical technique capable of providing high sensitivity combined with good selectivity. Fluorescent light has a different wavelength compared to the excitation light. The fluorescent light can therefore be spectrally filtered and monitored above a zero background level.

This allows fluorescence spectroscopy to be used to detect very low concentrations.

Unfortunately in some circumstances many species fluorscence when excited by light of high photon energy, for example UV light, and this can be a serious drawback in signal detection because the detection of a fluorescence signal cannot be unambiguously assigned to the species being detected.

Polymers such as polyethylene do not fluoresce when excited with far red or near infrared (NIR) light.

We have now found that by using materials which fluoresce in the far red or near infrared region of the spectrum polymers may be labelled to enable their identification. In this way the recycling of polymer waste may be facilitated by using very low concentrations of such fluorescent materials.

Thus according to the present invention there is provided a method for identifying a polymer, said method comprising the steps of: (a) adding at least one material capable of fluorescing in the far red or near infrared region to the polymer, (b) exposing the polymer to a laser source at wavelengths in the far red of near infrared suitable for exciting said material, and (c) determining the presence of said material by spectroscopic means to identify said polymer.

Suitable materials include dyes which fluoresce at wavelengths in the range 600 to 1000 nm, in particular in the range 700-900 nm.

Other suitable materials include rare earth compounds.

Diode lasers are particularly suitable for use in the method of the present invention having the advantages of low cost, small size and high reliability.

The presence of fluorescing materials in a polymer may suitably be detected by means-of a silicon photodetector.

The fluorescence detected from the polymer may suitably be displayed as a spectrum of fluorescence signal against wavelength.

The method of the present invention is particularly suitable for use in the labelling of plastic materials for example in the recycling of plastics.

Since plastic materials do not fluoresce significantly when illuminated with near infrared radiation the method of the present invention has the advantage of requiring very low concentrations of dyes for labelling applications.

Typical concentrations are those in the range 0.001 to 1 ppm.

For recycling applications where it is necessary to be able to identify a number of polymers from a group of polymers the preferred number of fluorescent materials present in each polymer is two.

For this application the fluorescent materials chosen are those in which their fluorescence spectra do not overlap. The resulting spectra may be separated by suitable optical filtering and the fluorescence strength of each material measured.

Each polymer may be labelled with the same fluorescent materials but at different relative concentrations. The ratio of the fluorescence intensities of the two materials may be correlated with the ratio of the weights of the materials present in the polymer and can be used to provide unambiguous labelling of a large number of polymers.

In this way different polymers, blends of polymers or polymer compositions may each be given a unique label based on the relative concentrations of the fluorescent materials present.

The presence of the labelled polymer or labelled composition may thereby be detected by the method of the present invention.

Thus according to another aspect of the present invention there is provided a method for identifying a polymer said method comprising the steps of (a) labelling said polymer by adding at least two materials capable of fluorescing in the far red or near infrared region to the polymer, said materials being present in a predetermined weight ratio, (b) exposing the polymer to a laser source at wavelengths in the far red or near infrared suitable for exciting said materials, and (c) determining the presence of said labelled polymer by spectroscopic means whereby the ratio of the fluorescence intensities observed for each material is proportional to said predetermined weight ratio.

If required three or more fluorescent materials may be incorporated in each polymer and identified in a similar way.

By using more than one fluorescent material, factors such as the obscuration of the polymer sample due to dirt or container scattering, fluctuations in the excitation source or changes in the position of the sample which may give rise to a reduction in the strength of the fluorescent signal, may be reduced.

Since the fluorescent materials employed have broad absorption bands the fluorescence of two materials can be excited by use of a single source emitting at a suitable wavelength.

The fluorescent materials may suitably be added to the polymer in solution before the polymer solidifies. Alternatively they may be first incorporated in solid particles of sub-micron diameter which are subsequently added to the polymer solution.

The present invention is further illustrated with reference to the accompanying Figures wherein: Figure 1 represents an arrangement suitable for identifying a polymer, and Figure 2 represents an arrangement whereby the presence of two fluorescent materials in a polymer may be identified.

In Figure 1 a diode laser (1) emitting at a wavelength of 670nm is used to illuminate a polymer sample (2) incorporating a suitable dye. The laser is recollimated by a glass lens (3) before entering the sample. Some of the fluorescence generated along with some of the reflected and scattered laswer light is recollimated by lens (3) reflected by an aluminium coated mirror (4) positioned at 45 to the laser output, and focused by means of lens (5) onto a silicon photodetector (6). A long pass filter (7) transmitting above 720nm is used to block the 670nm reflected and scattered light.

The photodetector output is amplified using a two-stage single chip amplifier.

In Figure 2 the fluorescence spectra from a sample incorporating two dyes are recorded by using two detectors.

The output from the laser (11) after reflection by the sample (12) and mirror (13) is passed through lens (14) and split by the beamsplitter (15) before passing to the photodeectors (16) and (17).

Claims

Claims:

1. A method for identifying a polymer, said method comprising the steps of: (a) adding at least one material capable of fluorescing in the far red or near infrared region to the polymer.

(b) exposing the polymer to a laser source at wavelengths in the far red of near infrared suitable for exciting said material, and (c) determining the presence of said material by spectroscopic means to identify said polymer.

2. A method according to claim 1 in which the material capable of fluorescing in the far red or near infrared region to the polymer is a dye or a rare earth compound.

3. A method according to claim 2 in which the dye or rare earth compound is present in a concentration of from 0.001 to lppm.

4. A method according to any one of the preceding claims in which the laser source is a diode laser.

5. A method according to any one of the preceding claims in which the presence of said material is determined by means of a silicon photodetector.

6. A method for identifying a polymer, said method comprising the steps of (a) labelling said polymer by adding at least two materials capable of fluorescing in the far red or near infrared region to the polymer, said materials being present in a predetermined weight ratio.

(b) exposing the polymer to a laser source at wavelengths in the far red or near infrared suitable for exciting said materials, and (c) determining the presence of said labelled polymer by spectroscopic means whereby the ratio of the fluorescence intensities observed for each material is proportional to said predetermined weight ratio.

7. A method according to claim 6 in which the materials capable of fluorescing are added to the polymer in solution.

8. A method according to claim 6 in which the materials capable of fluorescing are added to the polymer as solid particles of sub-micron diameter