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Copyright 2002 Plastics in End-of-Life Vehicles www.plastics-in-elv.org Page 1 of 3

Automatic identification and sorting ofplastics from different waste streams Infrared Spectroscopy (IR) - a status report

Background

Infrared spectroscopy is one of the most

widely used analytical techniques for

identifying different types of polymers and,

in some cases, different grades and/or

additives within the same polymer family. It

is considered to be highly accurate and

precise, but it has been traditionally used

mostly in laboratory settings by highly

trained technicians or scientists.

Groups of atoms in molecules vibrate with

sharply defined frequencies that are

predictable and unique. When a sample is

exposed to a beam of infrared radiation itabsorbs energy at frequencies characteristic

of specific atomic arrangements and bonds.

When the radiation that is transmitted

through or reflected from a given plastic

sample is examined, it will be changed based

on those frequencies which were most highly

absorbed by the material. The resulting

spectrum is a pattern of peaks and valleys

corresponding to

frequencies of the

radiation that werehighly absorbed.

Infrared spectra are

considered so

characteristic of

molecular

arrangements that parts of it are referred to as

the polymer fingerprint region.

Instruments specifically developed for

plastics identification operate in the short

wave near infrared range (SWNIR) from

700-1 100 nm wave length, in the near

infrared (NIR) range from 700-2500 nm or in

the mid infrared range (MIR) from 2500-

15000 nm.

While MIR is the most surface-sensitive ofthe three, none can see through typical paints,

metallic coatings, labels or most other

common surface coverings on plastic items.

Coatings must therefore be removed, or an

area free of coatings must be examined for

these techniques to be useful.

Short wave near infraredspectroscopy (SWNIR)

This technology measures overtones and

combinations of the fundamental molecular

vibrations that occur for plastics in the mid

infrared region. The SWNIR range is less

rich in spectral information than either NIR

or MIR, and the range of polymer types that

such a system can identify is therefore

limited. However, fibre optic probes, fixed

ratings and charge coupled device

detector arrays can be used in this range

of infrared frequencies. This can makesuch instruments very compact, robust,

easy to use and of relatively low in cost.

Although the existing technology has

been designed to differentiate between

ABS and HIPS, it may be able to identify

more plastic types by means of

improvements of the software and the

reference library.

Black and darkly pigmented plastics can notbe identified using SWNIR because too little

Authors: Bruno Krummenacher, Patrick Peuch, APME, Michael Fisher, APC,Michael Biddle, MBA Polymers, Inc.

IR spectroscopy has been

traditionally used mostly inlaboratory settings byhighly trained techniciansor scientists

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radiation is reflected back to the sensors at

this frequency range. Furthermore, SWNIR

will probably never be able to differentiatebetween more than a few polymers at a

time because of the limited information it

extracts.

Near infrared spectroscopy(NIR)

Near infrared spectroscopy is the most

commercially available technology that

offers effective polymer identification inless than one second (and less than 100

milliseconds in some cases). It operates

in the spectral range of 700-2500 nm,

which, like SWNIR, is dominated by

overtone and combination vibrations.

NIR, however, captures more unique

spectral characteristics, and some

systems have demonstrated a

remarkable ability to identify a wide

range of plastics quickly and accurately.

A wide range of photo-detectors with short

response times and high selectivity are

available in the NIR range. Quartz optical

fibres with low attenuation enable remote

sensing. Identification from a distance of

more than 10 cm is also possible with some

systems. Acousto-optical tuneable filters

(AOTF), where the optical diffraction of a

crystal is changed by acoustic frequencies

generated by piezo-electric transducers,have been developed. Instruments

employing AOTF technology can be made

containing no rotating gratings or other

moving parts, allowing them to be compact

and rugged. The primary drawback of such

systems is their difficulty in identifying

black pigmented materials, because carbon

black both absorbs and scatters highly at

NIR frequencies. Their use in an integrated

system with other methods could therefore

be beneficial. Prices of NIR instrumentsare in the range of 20,000 - 70,000.

A portable plastics identification

instrument was recently developed usingan AOTF crystal. This instrument,

controlled from a laptop PC, has a weight

of only 1.8 kg and is able to identify many

types of standard polymers with nearly

100% accuracy and within a measuring

time of 50-500 milliseconds (except for

black and dark plastics, of course). The

price of the instrument is expected to

eventually fall to around 10,000.

Numerous automated identification and

sorting systems

have been

developed for

plastic bottles,

and many of

these use NIR

spectroscopy for

at least part of

the

identification step. NIR works well in thisapplication because it can obtain spectra of

the plastics without touching the parts, it is

fast (tens of milliseconds), it only needs to

identify about five different plastics at the

most, and black plastics are not common in

bottles. Recently a German company

developed an automated NIR system for

durable goods, specifically computer and

business equipment parts. While the parts

are quite different from bottles from a size

and shape standpoint, they are mostlylightly coloured and are typically made

from only six or seven different types of

plastic.

Fourier transform midinfrared spectroscopy(FTMIR)

Fourier transform mid infrared (FTMIR)

instruments generate spectra in the

Automatic identi fi cat ion and sort ing of plast ics from di fferent waste st reamsInfrared Spectroscopy (IR) - a status report (Cont.)

Near inf rared spectroscopyis the most commerciallyavailable technology thatoffers effective polymeridentification in less thanone second...

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"fingerprint" region, which contains the

fundamental vibrational frequencies of the

functional groups within molecules. Someinstruments operate in diffuse reflectance

(DR) mode, where the light is reflected

from the

material in a

scattered

manner.

This

approach

usually

requires that a powder sample be placed in

the spectrometer. (Powder samples from

plastic items are usually obtained by

scraping with an abrasive pad.) While this

technique is faster and simpler than many

that preceded it (for example making salt

crystals from powders or casting films

from polymer solutions), it is still rather

time consuming compared with other

reflectance techniques.

Most new spectrometers that have been

developed for the rapid identification of

plastic samples use a specular reflectance

(SR) technique. In this mode, the infrared

light is reflected back from the material

surface like visible light from a mirror.

Although only a small portion of the light

is reflected back, this results in high

spectral contrast. A relatively smooth

surface is therefore required for good

measurements, which is the primarydrawback of this approach. The main

advantage of SR is that it usually requires

little or no sample preparation. The part is

simply brought into contact with a conical

probe affixed to the instrument, a switch is

activated (often with a foot), and the

identification displayed on a computer

screen within a few seconds. As with most

techniques, there is a balance between

speed and sampling ease on the one hand

and accuracy and precision on the other.

In most cases fibre optics are generally not

used because the only instruments

available in the MIR range are expensive,somewhat delicate, and do not cover some

of the important frequency ranges. In most

of the cases using MIR, the sensor must

be in close proximity to the plastic

item (within a few centimetres) and it

must be essentially motionless for at

least one second. Within

approximately two further seconds a

clear identification is usually made by

comparing the spectra with a reference

library, or by using some type of software

manipulation and analysis of the spectra.

Japanese automotive researchers (the same

group that developed the hand-held

dielectric clamp) have developed a

pyrolysis FTIP approach to facilitate the

identification of painted plastics. They use

a high-intensity infrared light to pyrolyse

the surface of a polymeric sample, sweep

the resulting vapours to a gas cell situatedinside a normal FTIK spectrometer, and

perform a vapour-phase analysis. The

researchers chose an infrared source

instead of a laser to perform the pyrolysis

for safety and cost reasons. The instrument

is capable of identifying first the paint,

then with subsequent scans identifying the

underlying plastic substrate. The probing

and analysis time is similar to that with SR

systems; a few seconds.

The main overall advantage of MIR

systems is their precision and accuracy,

especially their ability to identify blends,

dark and black pigmented polymers, a

range of fillers and even some additives.

Most FTMIR instruments use fairly

delicate interferometers, which makes the

somewhat expensive and less robust than

systems Without moving parts.

Nevertheless, prices have now reached as

low as around 30,000 for commercialunits.

Automatic identi fi cat ion and sort ing of plast ics from di fferent waste st reamsInfrared Spectroscopy (IR) - a status report (Cont.)

Japanese automotiveresearchers have developed apyrolysis FTIP approach tofacilitate the identification ofpainted plastics.

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