Plastic Sorting Using a Mid-IR Linear Detector Array

In these times of increased product supply, the delivery of quality products and product presentation becomes even more important. Intelligent sorting is a mechanized solution that allows accurate sorting of products. Production capacity is increased by the automation of the sorting process. This directly affects profit and therefore the quality and capacity of the products delivered.

In the current climate, it is hard to imagine a world without plastics - plastic objects are used in all areas of our lives. Natural materials are much more expensive to produce than plastic products and thanks to advanced production methods, plastic is also much faster to manufacture.

Plastics are defined as materials that have been made by man from non-naturally occurring synthetic polymers or natural polymers (modified using appropriate additives). Colloquially, these types of materials are referred to as the collective name “plastic”, though it must be remembered that this is an over-simplification, which does not allow for variations between individual types of polymer materials.

The need for transparent, completely water-resistant, and with many other chemical factors raw materials turned out to be a turning point for the industry. This translated into single items having much greater availability and lower prices.

Many industries have been revolutionized by the development of effective methods for plastic objects mass production, but what about mass recycling methods?

Symbol	Polymer	Common products		Recycled products	Spectral range [μm]
	Polyethylene Terephthalate	Soda & water bottles, cups, jars, trays, clamshells		Clothing, carpet, clamshells, soda & water bottles	5 - 14
	High-Density Polyethylene	Milk jugs, detergent & shampoo bottles, flower pots, grocery bags		Detergent bottles, flower pots, crates, pipe, decking	3.3 - 14
	Polyvinyl Chloride	Cleaning supply jugs, pool liners, twine, sheeting, automotive product bottles		Pipe, wall siding, binders, carpet backing, flooring	7 - 16
	Low-Density Polyethylene	Bread bags, paper towels & tissue overwrap, squeeze bottles, trash bags, six-pack rings		Trash bags, plastic tumber, furniture, shipping envelopes, compost bins	3.3 - 14
	Polypropylene	Yogurt tubs, cups, juice bottles, straws, hangers, sand & shipping bags		Paint cans, speed bumps, auto parts, food containers, hangers, plant pots, razor handles	3 - 13
	Polystyrene	To-go containers & flatware, hot cups, razors, CD cases, shipping cushion, cartons, trays		Picture frames, crown molding, rulers, flower pots, hangers, toys, tape dispenses	3 - 18
	Other	Polycarbonate, nylon, ABS, acrylic, PLA: bottles, safety glasses, CDs, headlight lenses		Electronic housings, auto parts	-

There are numerous possibilities when using recycled materials. Recycled plastic can be used to make water bottles, furniture, pipes, auto parts, to name but a few. That being said, the demand for proper detection of plastic is increasing daily. Traditional plastic sorting deals with the issue of how to distinguish specific kinds of plastic. Even tiny portions of the plastic element can harm the effectiveness of recycling. Additionally, the problem of plastic sorting does not only affect land trash, but also marine debris.

One method of plastic sorting is to utilize a sorting lane [Figure 1]. To begin, the plastic must be cut into small fragments. Secondly, the small plastic parts are measured. Finally, correctly identified plastic parts are moved into a separate hole with air jets. This process can be repeated afterward, to differentiate multiple plastic types such as PETE, HDPE, PP, etc.

Figure 1. Example of sorting plastic by using VIGO detectors.

The simplest, yet extremely accurate technique for identification and classification can be performed with spectrophotometer methods such as FT-IR. In the FT-IR method, IR light illuminates the previously fragmented plastic parts. The spectrophotometer system collects reflected light, where a signal can be obtained thanks to detection by the VIGO detector. Later this signal is transformed with Fourier Transform to be able to gather specific information concerning the scanned material [Figure 2]. The analysis is fast and accurate.

Figure 2. The method of measuring plastic by FT-IR. The chart shows examples of absorption bands of various plastics.

Each kind of plastic has its own absorption bands which may be utilized in material identification. VIGO’s linear detector array can differentiate all kinds of plastic through the detection of these characteristic absorption bands [Figure 2]. The principle can be seen below.

The chart illustrates examples of absorption bands of different plastics.

By detection of each marked absorption band, the VIGO detector can “see” a type of the measured plastic. Organic compounds such as polymers can be observed with greater accuracy in the MWIR range in comparison to the NIR range. Utilizing multielement detectors in spectrophotometry enables the elimination of moving parts or filters. This simplifies the spectrophotometer and strengthens system reliability.

A 32-element linear array detector is recommended for the OEM spectrophotometer for each polymer absorption band. Key benefits from utilizing the linear array detector include:

• High-speed measurement
• Low-power-uncooled detector
• High separation accuracy due to high SNR ratio
• Elimination of moving parts and/or filters

MID-IR Multielement HgCdTe / InAsSb detector features

• High S/N ratio
• 3-14 µm wavelength band
• High-frequency operation
• A low drift of output signals

Module configuration options

• USB digital interface
• Microprocessor inside
• Customized mechanical layout

Figure 3. VIGO’s detector detectivity and some examples of absorption bands which can be used for plastic identification.

VIGO specializes in customized detectors and modules tailored to the client’s application. 32-element arrays are now available in production with dedicated preamplifiers.

The detectors line is a set of individual active elements and each one’s signal is output independently. Unlike a single-element detector, a multielement detector enables radiation of different wavelengths to be recorded at the same time. The majority of the multielement detectors produced in the VIGO System are based on HgCdTe (epitaxial HgCdTe heterostructure) photovoltaic detectors, thermoelectrically cooled.

Chart 1 shows examples of spectral characteristics and Table 1 - parameters of detectors optimized for various wavelengths.

Chart 1. Exemplary spectral detectivity.

Table 1. Detectivity and time constant of HgCdTe detectors.

Our technological capabilities also enable the production of multielement detectors with InAsSb (indium arsenide antimonide) using the MBE (Molecular Beam Epitaxy) method.

• High separation accuracy due to high SNR ratio
• High-speed measurement
• Elimination of moving parts and/or filters
• Low power – uncooled detector

These devices are compliant with the RoHS Directive. They are designed for applications where it must be ensured that they can handle tough operating conditions.

The biggest benefit advantage of VIGO System multielement detectors is that there is no need for cryogenic cooling. This leads to the size and weight of the device is reduced, and therefore a fall in power consumption.

Figure 4 illustrates the dimensions (unit: mm) of TO8 16pin (a) and flatpack 40pin (b) housings in which VIGO System multielement detectors are mounted.

Figure 4. Mechanical layout.

Key Product Features

Advantages:

• High-speed response
• High sensitivity
• Convenient cryogenic-free operation

The major advantages of VIGO System multielement detectors are extremely high accuracy and measurement speed. Accuracy to a single millikelvin is achieved in temperature measurements, even when measuring an object present in the field of view for only a few microseconds.

In spectrophotometry, these advantages enable high-quality measurements to be obtained in a short time. Compared to the time one-piece detectors require to perform scanning and full-spectrum analysis, measuring the entire spectral range at once cuts the measurement time.

Table 2 shows the parameters of VIGO System multielement detectors, chosen for the requirements of individual applications.

Table 2. Parameters.

VIGO System multielement detectors are on offer with a wide range of accessories. Accessories can be customized to the application needs and for integration with the user's system. Table 3 illustrates some examples below.

Table 3. Accessories.

Applications

Multielement detectors are utilized in point, non-contact temperature measurements of quick-moving elements. For example, the real-time monitoring of the temperature of internal and external wheel bearings and high-speed train brakes. Other applications include anomalies detection, temperature measurements on production lines, monitoring of cooling or combustion profiles, etc.

Spectrophotometers currently on the market typically use the near-infrared range of 0.8-2.5 μm. Organic compounds, greenhouse gases, hydrocarbons can be more accurately observed in the MWIR (3.0 - 8.0 µm) and LWIR (8.0 - 14.0 µm) ranges. Utilizing multielement detectors to eliminates the need for filters and allows for the use of moving mechanical elements for scanning the spectra or space, and, consequently, removes work-related errors.

VIGO System multielement detectors enable high-quality spectrophotometric measurements in a short time, and very low noise also enables operation with low-power sources: IR diodes or thermal.

Another application of multielement detectors is a high performance optical sorting systems. The detector line scans elements moving on the tape and allows their specific chemical composition to be identified. Optical sorting can be utilized in the chemical, food, mining, and pharmacological industries.

This information has been sourced, reviewed and adapted from materials provided by VIGO System.

For more information on this source, please visit VIGO System.