NIR is a widely used technology in recycling for the identification of plastics. As it cannot detect black plastic, this article presents alternative devices and technologies to solve this problem.
NIR spectroscopy (near infrared spectroscopy - NIRS) is a widely used identification technology in plastics recycling because it measures non-destructively and quickly, is easy to use and is therefore widely used. We offer NIR-based solutions, from the portable, easy-to-use trinamiX plastic scanner to our own Solid Scanner solution, which works like an NIR minilab and offers the possibility to create customized databases, to very fast HSI camera systems suitable for real-time inline process control. What they all have in common is that NIR cannot detect soot-colored plastics. Unfortunately, black plastic is everywhere, but it is difficult to identify and therefore difficult to sort and recycle. For this reason, plastics containing carbon black are often thermally recycled, which should be avoided.
This article therefore presents some alternative technologies for identifying and sorting black plastics, all of which have their own advantages and disadvantages.
Why is black plastic so difficult to sort?
NIR is used as a sorting process in practically all large sorting plants. The technology is fast, simple and reliable. However, due to its physical mode of operation, NIR cannot detect soot. Soot absorbs the entire NIR signal. Consequently, plastics colored with carbon black, i.e. black plastics, sometimes even (dark) gray plastics, cannot be detected and distinguished with the help of NIR.
Why is carbon black still used in plastics?
Carbon black is still a cheap and reliably available colorant. In other cases, carbon black is the only way to achieve the desired mechanical properties. Even if a solution is found today to replace carbon black, there will still be plastics colored with carbon black on the market for a long time to come (e.g. the many black components in the automotive industry), which will still have to be recycled years from now. Black plastics are and remain a pressing problem.
Innovative solutions for identifying black plastic
This article presents technical solutions based on the following working principles:
- FTIR spectroscopy
- MIR spectroscopy
- Electrostatic charge
- Laser spectroscopy
- VIS object recognition
Overview of the properties of alternative technologies in direct comparison to NIR
FTIR
- Can measure black plastics and many other materials and quantify the composition directly
- Also portable, significantly larger than portable NIR devices
- More expensive to purchase
- Measuring time approx. 20-30 seconds
- Only point measurements possible
- Technology is mainly used in laboratory equipment
MIR
- Can recognize black plastics
- Lighting in the MIR area requires considerable power, so additional fire protection is required
- No portable devices available
- Expensive to purchase
- Can also measure two-dimensional surfaces
Electrostatic charge
- Simple, mechanically robust technology
- Developed for the separation of binary mixtures
- High production output possible
- No portable devices available
Laser spectroscopy
- The comparatively most expensive technology
- No portable devices available
- High production volumes possible
VIS object recognition
- Lower hardware costs compared to other technologies due to VIS sensor technology
- Only recognizes material for which it is trained
- Requires regular updating of the AI-controlled database
3 (more or less) portable solutions for identifying black plastic
FTIR: Agilent 4300 handheld & Agilent Cary 630 desktop system
The measuring time is usually between a few seconds and several minutes. Sample preparation depends on the type of sample and the measurement method. Solid samples can be measured either as pellets or as thin layers that can transmit IR light. To be able to work with an FTIR device, you also need some specialist knowledge of the basics of infrared spectroscopy, how the interferometer and detector work, how to select the appropriate measurement method and sample preparation, and how to interpret the IR spectra.
The Agilent 4300 handheld FTIR spectrometer combines ease of use, robustness and flexibility in one system. Weighing only ~2 kg, it is ideal for mobile non-destructive testing in the field and outside laboratories. It can analyze different samples without the need for adjustments. It is used for the analysis of polymers, coatings, composites, bulk materials, cleaning verification and even artifacts and artwork.
The Agilent Cary 630 FTIR spectrometer is a desktop system weighing approx. 4 kg.
Sliding spark spectrometer: mIRoSpark
Sliding spark spectroscopy is a technique in which high-voltage sparks are used to vaporize and ionize a small amount of the plastic surface. The emitted radiation is then analyzed to identify the different atoms present in the material. The main advantages of this method are
- It can detect additives such as flame retardants and heavy metals that are not visible with near-infrared spectroscopy.
- It can identify plastics of any color, size and texture, including black plastics, films, foils, granules, solids, foams, carpets and textiles.
- It is fast, portable and easy to use, with a measurement time of one second and a simple gun-like device that can be pressed onto the sample.
Some of the main disadvantages of this method are:
- It requires contact between the measuring head and the sample surface, which can damage or contaminate the sample.
- It requires sample preparation to remove dust, dirt or paint from the sample surface.
- It may not be able to distinguish between similar types of polymers that have the same atomic composition but different molecular structures.
Sliding spark spectroscopy can also be used to analyze metals, alloys, ceramics and other inorganic materials.
The mIRoSpark device identifies plastics in various materials such as household waste, electronic waste, carpets and textiles. It weighs 14 kg and has a non-destructive mIRo part for measurements. With its SSS2 part, it can also identify black plastics. The measuring time is less than 1 second and it can measure films and granules. The device can also detect significant halogenated flame retardants and heavy metal additives.
4 industrial (inline) solutions for the identification of black plastic
MIR: Steinert UniSort black
MIR spectroscopy is a technique that uses mid-infrared radiation to analyze the molecular structure and composition of plastics. Some of the main advantages of MIR spectroscopy are:
The plastic analysis method is fast, accurate and non-destructive. It is able to identify different types of plastics, including additives, fillers and impurities that can affect the properties and performance of the plastic. It can also provide quantitative data on the concentration and distribution of the components in the plastic sample.
Some of the main disadvantages of MIR spectroscopy are:
To perform this task, expensive and advanced equipment and software as well as qualified operators and analysts are required. It can also be difficult to distinguish between certain plastics with comparable infrared spectra, e.g. PET and PBT.
The Steinert UniSort Black sorts various materials, including black ones, effectively reducing sorting residues and increasing the plastic yield. It uses NIR and MIR-HSI camera technology to accurately identify all materials. The classification system is designed to detect dark or black objects that can be overlooked by conventional NIR sorters, ensuring reliable detection.
Electrostatic plastic sorting: Hamos EKS
Electrostatic plastic sorting is a process that makes use of the effect that different plastics can be charged with different positive or negative charges. The charged plastics are then separated by an electric field. This procedure has some advantages and disadvantages.
Advantages:
- It can also separate black plastics that are not visible to optical sorters.
- High purities of the individual plastic fractions can be achieved.
- It can separate different plastics that do not differ in density or shape.
Disadvantages:
- Only certain plastics that differ in their electrical conductivity can be separated.
- It cannot process heavily soiled, painted or damp plastics.
The EKS electrostatic separators from Hamos can separate mixed plastics. This results in a purified plastic fraction in a dry process with low processing costs. The systems can efficiently separate material mixtures of different plastics regardless of their color, including black plastics. The polymer mixtures to be separated must be dry and dust-free and have a particle size of 2 mm to 10 mm.
Laser spectroscopy: UniSensor Powersort 200
Sorting machines with laser spectroscopy in plastics recycling are devices that can identify and separate different types of plastic based on their optical properties. They use laser beams to measure the spectrum of the light reflected or transmitted by the plastic flakes or granules and compare it with a database of known spectra for various polymers. In this way, they can sort plastics by type (such as PET, HDPE, PVC), color or quality.
Some of the advantages of sorting machines with laser spectroscopy in plastics recycling are
- They can detect and remove impurities such as metals, rubber or other foreign substances that could impair the quality of the recycled plastic.
- They can sort plastics that are difficult to distinguish using conventional methods, e.g. dark or black plastics or polymers of the same color (such as PVC, PP and PE).
Some disadvantages of sorting machines with laser spectroscopy in plastics recycling are:
- They can require high initial investment and maintenance costs as they work with complex and sophisticated technology.
- They may not be able to sort plastics that have similar spectra, such as some bioplastics or blends.
- They can generate waste or emissions from the laser process, e.g. heat, dust or ozone.
The UniSensor Powersort 200 can sort up to 3 tons per hour according to density. With a powerful signal processor and high-resolution optics, it generates and evaluates up to 1 million spectra per second. It can even sort particles up to a size of 2 square millimeters.
VIS object recognition: AMP Robotics & Max-AI
VIS object recognition requires the use of artificial intelligence (AI) and robotic systems to sort and recycle plastic waste. The technology uses computer vision and deep learning to identify different types of plastic based on their shape, color, texture and labeling. Each system is only able to recognize plastics that have been trained beforehand. The costs for the VIS sensor and the light source are much lower than the hardware costs of other sensor technologies. This solution is best suited for large plastic parts, i.e. the solution can only be used before the packaging has been shredded.
AMP Robotic ’s system consists of a vision system that analyzes the material composition, color, clarity, opacity and form factor of plastics, and a delta-shaped robot that picks up the plastics and places them in the designated containers. Its object recognition database must be updated regularly. The database currently includes water bottles, milk jugs, lids, cups, mugs, cartons, clamshells, coffee pods and thin films as well as materials such as PET, HDPE, LDPE, PP and PS.
Max-AI is another provider of this technology.
What developments are there in sensor technology that can detect black plastics?
A few years ago, the Fraunhofer BlackValue project attracted a lot of attention. The aim was to develop an automated solution for industrial use. The focus here was on terahertz sensor technology. However, there have been no industrially usable results to date.
A new project at the SKZ is currently being planned and aims to develop a portable solution based on AI.
What alternative methods are there to detect black plastics?
Another option is the use of special markers or tracers, which are added to the black plastics and glow or fluoresce under certain conditions. These can then be detected by optical sensors. Polysecure, for example, has developed a technology with which materials and products can be invisibly marked and tracked. The company has also developed TBS (Tracer-Based Sorting) technology, with which many material and waste streams can be efficiently and reliably sorted into any desired fraction. However, these markers must be customized for each polymer type and require close cooperation between manufacturers, recyclers and authorities.
There are also ideas for applying markings that indicate the type of plastic, e.g. barcodes or RFID tags. However, these markings must be standardized and legible and must not impair the recycling process.
In principle, there are also many ideas for replacing carbon black or increasing the NIR reflection with suitable additives and other colorants, such as color pigments. We offer appropriate solutions for testing the NIR detectability of plastics as early as the development phase.
For these reasons (lack of standardization, disruption of the recycling process and costs), these solutions have not yet become established.
Conclusion
Black plastics continue to pose a major challenge for recycling. NIR enables portable and easy-to-use devices. However, NIR-based devices cannot detect black plastics, let alone sort them. Alternative technologies are available. However, technical solutions for black plastics based on technologies such as FTIR, MIR, visual detection, laser spectroscopy or sliding spark spectroscopy are considerably more expensive to purchase, are generally not portable, are more complex to operate or can only separate two plastics per step. This article shows some commercially available technical solutions for the detection of black plastics and gives an insight into past and current research projects on this topic.
Detecting black plastics is a challenging but rewarding endeavor for companies looking to improve their efficiency and sustainability. The technical issues, the cost implications and the difficulty of changing habits are all obstacles that need to be considered, but they can be overcome with the right approach. If all these factors are taken into account, companies can successfully identify black plastics and reap the associated benefits.
About us - Solid Scanner
Let’s take responsibility and recycle more plastics - ask us for suitable solutions. Our portfolio includes solutions ranging from small, portable solutions to individual solutions based on hyperspectral camera systems for simple, automated identification of plastics in the sorting process and for inline process control, e.g. for homogeneity.
About trinamiX - the sensor technology company
trinamiX GmbH, based in Ludwigshafen, was founded in 2015 as a wholly owned subsidiary of BASF SE. As a start-up within the company, it is not only operationally independent, but also has unique access to the expertise and experience of the entire BASF Group.
Its patent-pending technologies enable people and machines to capture the hidden and invisible world around them to make better decisions and increase safety.