Identification of Recyclable Polymers with a Handheld Near-Infrared Spectrometer

Recycling of plastics and polymers, while in existence for some time, has seen significant growth driven by a combination of increasing regulation, escalating disposal costs, rising feedstock prices, and the rapid expansion of economies in China and India. Effective recycling requires that materials be rapidly and positively identified so that they can be processed in an appropriate manner. In addition, this material identification needs to be performed outside a laboratory, typically in a dirty, dusty, industrial environment.

As an example, a significant amount (ca. 2.5 million tons) of waste carpeting is sent annually to landfills in the United States, while less than 5% is recycled.1 Typical carpet fibers are polyethylene terephthalate (PET), polypropylene (PP), Nylon 6 (N6) (polycaprolactam), Nylon 66 (N66) (formed from hexamethylene diamine and adipic acid), and wool; these can be recycled if the facility can rapidly identify and separate these materials. In recent years, many outlets have been developed for recycled resins; for instance, polypropylene is used in leachfield drainage chambers2; “plastic lumber” products such as fence posts, decking, and railroad ties3; while Nylon 6 may be depolymerized back to caprolactam.1 Carpet recycling, therefore, has multiple benefits: less material in landfills, lower consumption of petroleum feedstocks, and economically viable production of industrial and consumer products.4

Figure 1 - The NIR spectra of polypropylene (PP), polyethylene terephthalate (PET), and Nylon 66 (N66) carpet fibers demonstrate clear spectroscopic differences between these materials, and thus the ability to identify them using a handheld spectrometer.

Rapid, nondestructive identification of fibers and polymers can be readily performed using near-infrared (NIR) spectroscopy, since the first overtone region (1350–1800 nm) demonstrates a unique fingerprint for each polymer. For example, the spectra of PET, PP, and N66 (Figure 1) clearly show that these different polymers can be readily distinguished. While NIR spectroscopy is an ideal technique, most NIR spectrometers are quite large, designed for use in a controlled laboratory environment, and are cost prohibitive for this application.

The Anavo™ analyzer (Axsun Technologies, Billerica, MA) addresses exactly this class of application. The analyzer is a self-contained, handheld, NIR analyzer designed around a microelectromechanical system (MEMS)-based microoptical platform,5 specifically intended for use in the field. It provides a rapid and cost-effective method to identify materials such as polymers and carpet fibers, as well as many other materials with an NIR signature.

Experimental

Figure 2 - Anavo NIR material ID and analysis system.

The Anavo analyzer (Figure 2) collects the spectrum of a sample, compares it to an on-board library, and provides a positive material identification, all in less than 1 sec. It measures the diffuse NIR reflectance spectrum of the sample using a chip-scale MEMS-based microspectrometer,3 employing a widely tunable NIR laser. Real-time wavelength and amplitude referencing is done on-board, ensuring calibration transfer among systems. The spectrometer is housed in a rugged, field-worthy enclosure, intended for use in heavy industrial environments. The analyzer is battery-powered, dustproof, and withstands a 3-ft drop onto a concrete floor.

Figure 3 - To identify a carpet fiber, the Anavo analyzer is pressed up against the sample, and an answer is given in less than 1 sec.

The operation is extremely straightforward, as shown in Figure 3; the operator simply holds the sample in front of the window and presses a trigger to initiate the measurement. The data collection process and analysis is automatic, and takes less than 1 sec. The name of material identified is displayed and is announced in the local language through an on-board speaker or to an earpiece. The on-board library includes the spectra of the polymers that are commonly used in carpeting in a wide variety of physical forms and colors. This library can be expanded with additional materials of interest, as needed.

The analyzer is powered by a commercially available rechargeable battery that can provide service for more than 4 hr. If desired, it can also be operated using line power; this configuration makes the analyzer extremely useful as part of an on-line testing system for real-time automated sorting or verification of raw materials or final product. For portable and on-line measurements, data can be sent to an external datalogger or spreadsheet application via a wired or wireless connection.

Results and discussion

The Anavo analyzer is specifically designed for carpet fiber recycling, and is targeted at PP, PET, N6, N66, and wool. With this very limited set of materials, its display is a group of different color-coded light-emitting diodes (LEDs). For general-purpose material identification, which can be in fields as diverse as plastics recycling, raw material identification, and pharmaceutical counterfeiting, the Anavo analyzer can identify hundreds of materials, displaying the results on its built-in display. Quantitative information can also be obtained and displayed.

Figure 4 - Comparison of ABS spectra, showing the effects of flame-retardant additives.

The spectra of PP, N6, and N66 presented in Figure 1 clearly demonstrate that the different polymers can be readily identified. Engineering resins used in electronic equipment (personal computers, monitors, copiers, etc.) may contain flame-retardant additives, and these can pose a problem in recycling, depending on the particular additives used. Additives can include organophosphates, hydrated minerals (such as magnesium and aluminum hydroxides), and halogenated organics (such as polybrominated diphenyl ethers [PBDEs]). Figure 4 shows that there are significant spectroscopic differences between acrylonitrilebutadiene-styrene (ABS) resins with and without flame-retardant additives.

The Anavo analyzer can be supplied with a number of built-in spectral libraries, representing a large number of compounds, in different physical forms. For instance, in postindustrial and postconsumer plastics recycling, some 50 different base materials are commonly encountered, with many additives and blends. Sometimes, proprietary compounds, or a different class of materials, are of interest. In that case, a fresh model can be created using standard chemometric software (Pirouette, Infometrix, Bothell, WA) and downloaded to the analyzer. This flexibility ensures that the analyzer will continue to meet requirements as new materials are encountered.

Conclusion

Recycling of carpet fibers and plastics is a growing, economically viable business, and it depends on the accurate identification of the materials to be recycled. Rugged, handheld near-infrared spectrometers, providing “point-and-shoot” operation, fulfill this need. In addition to carpet recycling, the technology can be used to identify polymers in electronics recycling (e.g., PCs and monitors), identify polymer pellets and films in a postindustrial setting, and sort scrap automobile parts for recycling. This type of instrumentation can also address other portable material identification applications as varied as raw material identification and pharmaceutical counterfeiting.

References

  1. McCoy, M. Finding new life for old carpets. Chem. Eng. News2006, 84(43), 33–8.
  2. For more information on these products, see the Infiltrator Systems Web site, www.infiltratorsystems.com
  3. Krishnaswamy, P.; Lampo, R. Recycled-plastic lumber standards: from waste plastics to markets for plastic lumber bridges. ASTM Standardization News, Dec 2001.
  4. For more information on carpet recycling, see the Carpet America Recovery Effort Web site, www.carpetrecovery.org.
  5. Crocombe, R.A.; Flanders, D.C.; Atia, W. Micro-optical instrumentation for process spectroscopy. Proc. SPIE2004, 5591, 11–25.

The authors are with Axsun Technologies, 1 Fortune Dr., Billerica, MA 01821, U.S.A.; tel.: 978-262-0049; fax: 978-262-0035; e-mail: [email protected].

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