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What plastic is it? New ways for recyclers to solve the mystery.

At MBA Polymers Inc. in Richmond, Calif., researchers know something about waste plastics that most recyclers can only guess at - namely what resins the scrap is made of. The largest recycling research facility in the U.S. is exploring leading-edge plastics identification technologies that promise unprecedented speed, compactness, portability, and accuracy. The new technologies may be suitable for shop-floor identification or QC of incoming materials, says Dr. Michael Biddle, president of MBA. Equipment being tested at MBA represents the first significant steps toward automated incline identification of recycled plastics other than bottles, he adds.

MBA's research is sponsored by the American Plastics Council (APC) in Washington, D.C. MBA is developing new sorting and separation technology on a proprietary basis. It is also evaluating identification technologies that a number of other firms have brought to the commercial or near-commercial stage.

Smaller, faster, smarter

New developments in ID technology include the advent of handheld or portable traits for production use and compact benchtop models for the lab or shop floor.

Speedier identification is another significant development. At least four new models in MBA's lab can detect the type of plastic in a sample within fractions of a second. Previous models took up to 5 sec, says Biddle, and just a few years ago the task took many minutes. In addition, electronic "libraries" of IR spectra are improving in accuracy.

Most of the instruments are based on infrared (IR) spectroscopy. They generally work by exposing the plastic sample to a specific frequency range of IR radiation emitted from a probe or gun, and then analyzing the energy that is transmitted or reflected by the sample. Some of the radiation is absorbed by the plastic - different molecular bonds absorb at different frequencies - so the composition and structure of a polymer generates its own characteristic IR spectrum.

Five of the identification units in MBA's lab use the near-IR (NIR) spectrum, while others use the mid-IR (MIR) range, most commonly referred to as Fourier Transform Infrared (FTIR) technology. NIR instruments operate in the 700-2500 nanometer (nm) IR spectrum. Their advantages include non-contact sensing, low cost, high speed, low surface sensitivity (they can identify a material with a rough or smooth surface), and ability to use fiber-optic probes. "The chief limitations are its inability to identify darkly pigmented parts and the fact that the information in the NIR spectral region is less rich than in the other IR regions," says Biddle.

On the other hand, FTIR covers a broader frequency range of 2500 to 10,000 nm or more, hence it gathers a richer amount of spectral information. It performs better than NIR at identifying black-pigmented plastics, many polymer blends, and some additives. FTIR's down side is that most commercial instruments use specular reflectance sensors, so it has higher surface sensitivity, requires larger instrumentation, and has slower recognition speed than NIR detectors.

New NIR technology

One of the newest units is the Kusta 2012, an automated model for identifying plastics in durable goods. It comes from Laser Labor Adlershof (LLA) of Berlin, Germany. The Kusta 2012 "reads" the sample in only 10 millisec through the use of a laser plasma-emission spectrometer. It uses pulsed laser light that creates a microplasma discharge in the polymer. MBA is about to install this system in an automated sorting line. The lab also has LLA's manual benchtop model, which uses a hand-held fiber-optic probe and has a 60-millisec identification speed. This older unit costs around $38,000.

Eliminating the flexible "umbilical cord" will add remote sensing capability to Plastiscan NIR technology, which comes from OPT Research Inc. of Tokyo and Infrared Fiber Systems Inc., Silver Spring, Md. The device uses an Acousto-Optic Tunable Filter system, which eliminates the need for moving parts and saves space. Up to 20 different materials reportedly can be identified in 0.1 sec with over 99% accuracy. Plastiscan is priced under $30,000.

"Factor-filter" NIR technology from Buhler Inc., Minneapolis, can identify up to 10 materials in under 0.1 sec. The device is designed to filter out unwanted resonances for a truer reading, and it has no moving parts. The technology has been commercially available in Europe through Buhler of Switzerland. The firm offers the unit here but recently said it is not actively pursing its application in plastics recycling.

Developments in FTIR

The first commercial FTIR pyrolysis unit for plastics recycling is from the Toyota Central Research and Development Labs Inc. in Japan and Nicolet Instruments Corp. in Madison, Wis. The portable system vaporizes a portion of a plastic part with a handheld pyrolysis probe. The vapor is sucked in and blown with a nitrogen carrier into the FTIR spectrometer, which shines a beam through the gas to obtain a spectral reading. Though its response time is a relatively slow 2-3 sec, the unit is not very surface sensitive, hence it can read a part that's been painted or one containing carbon black pigment. The handheld probe must contact the part surface, which requires a fiat area at least 20 mm in diam. The unit costs around $35,000.

Information on "hit quality," or the degree to which the sample's IR spectrum matches one in an ID device's electronic library, is a new feature of a benchtop FTIR instrument with a new specular reflectance (SR) probe. The hit quality tells you how likely the ID is to be accurate. The instrument, which comes from Bruker Analytik GmbH, Karlsruhe, Germany, also reports its second-best guess at an ID and how far this guess is in hit quality from the first determination. The unit takes 3-5 sec to analyze the spectral data, and the model in MBA's lab has a library of 730 samples to draw on.

Other ID units in the works

Short-wave NIR (SWNIR) technology distinguishes HIPS from ABS, which can be confused in other IR detection methods because beth materials contain butadiene. With the help of the APC and MBA, D-squared Development in LaGrande, Ore., developed a portable instrument that costs under $10,000, which is less than most other instruments, according to Biddle, though it is limited in the types of materials it can identify. It has a handheld probe that is connected to the instrument body by a flexible fiberoptic cable. SWNIR is comparatively slow (3-sec response time).

Toyota's Central R&D Labs have developed a fast and portable instrument for auto bumpers that identifies only polypropylene or polyurethane by dielectric sensing, says senior researcher Atsushi Murase. Not yet commercial, it consists of a pair of electrodes and a potentiometer to measure the electrostatic capacitance and the thickness of the part. The two contact probes reportedly read through paint and aren't sensitive to other surface conditions. The unit gives a result in less than 1 sec.
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Author:Knights, Mikell
Publication:Plastics Technology
Date:Apr 1, 1998
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