Multi-Modal Stand-Off Detection of Plastics for Sorting Application

Abstract

Plastics are extremely valuable to our society and are widely used in many industries such as food, beverage, textile, healthcare, automotive, aerospace, construction, agriculture, oil, etc. Since plastics do not degrade easily with time, the discarded plastics pose a grave environmental problem. Only less than 10% of plastics is recycled in the US. Therefore, there is an urgent need to increase the recycling of plastic products. At present, sorting of plastic waste is carried out manually which is time consuming, costly and manual labor intensive. To address this problem, a novel multi-modal sensor platform capable of improving the accuracy of automated sorting of various plastics by using molecular identification has been developed. This multimodal technique of standoff photothermal-photoacoustic detection is a combination of mid-infrared (MIR) photo-thermal spectroscopy and photoacoustic spectroscopy for characterizing surfaces. Since MIR is free of overtones, this region is known as the molecular fingerprint region. In this technique, the plastic surface is illuminated with a pulsed laser beam from a tunable quantum cascade laser (QCL). The photothermal detection is accomplished by collecting the scattered light from the plastic surface which contains the molecular vibration spectrum of the sample. A silicon bi-material microcantilever, fabricated by depositing a thin metal film on one of its sides is used as a sensitive uncooled infrared detector. Monitoring the bi-material cantilever bending as a function of illumination wavelength provides the IR absorption spectrum of the plastic material. Simultaneously, the photoacoustic signal generated by the absorption of light is monitored by a sensitive microphone. A highly sensitive microfabricated acoustic resonator is used for collecting the photoacoustic signal. Monitoring the microphone response as a function of wavelength produces photoacoustic spectrum (PAS) of the sample. This non-destructive characterization technique will help in enhancing the quality of plastic sorting process at materials recovery facilities and other industries by making the process inexpensive and time efficient.