The Problem With Carbon Black

October 3, 2023
3 min read
George Hill
Sagitto Ltd

Carbon black is a common black pigment, traditionally produced from charring organic materials such as wood or bone. It appears black because it reflects very little light in the visible part of the spectrum.

For example, the intense blacks in the painting by Edouard Manet, Music in the Tuileries, are derived from carbon black produced from charred ivory.

Recycling and Carbon Black

The problem with carbon black is that not only is it an excellent absorber of light in the visible spectrum, but also it absorbs a great deal of light in the near-infrared. This creates a problem when near-infrared spectroscopy is used to identify the composition of items such as textiles and plastics for recycling.

Carbon Black and Textiles

Many of Sagitto's customers use our miniature NIR spectrometer to test the precise fibre composition of textiles destined for recycling. Some black textiles - in particular black denims - are dyed using carbon black. And the absorbance signal that our spectrometer receives from these black denims is often stripped of so much information that it is difficult for our machine learning models to interpret them. This is best illustrated with an example.

An Example : Levi 511 Black Denim Jeans

If we scan an almost-new pair of Levi 511 black denim jeans, and compare the NIR absorbance spectra against a dark navy denim from The Fabric Store, we can clearly see the difference.

Heavy-weight dark navy denim (left) ;Levi 511 black denim jeans, outside (right)
Carbon black makes the technque for identifying the fibre compostion of black denim a challenge.
NIR absorbance spectra for black and navy denim

Fortunately if we scan the inside of these jeans, the effect of the carbon black dye is less pronounced. Although the spectrum from the inside of the jeans is a long way from that of a regular navy denim, there is enough information remaining for us to make a good prediction. The label says that the jeans are 98% cotton, and 2% elastane. We predict approximately 96% cotton, and note the presence of elastane, without trying to quantify it.

Predictions for Levi511 black denim jeans, outside (left) and inside (right).

Our textile recycling customers, like Renewcell, know to scan the inside of any black denim factory off-cuts that they test, in order to test the textile sample's fibre composition. But this isn't a practicable option when sorting textiles in high volumes. In these cases, textiles coloured with carbon black often have to be assigned to the category of 'known unknown' fibre composition.

Carbon Black and Plastic Polymers

Just as with textiles, the use of carbon black as a colour additive to plastic polymers can create difficulties for sorting systems using NIR. Each plastic polymer has a unique NIR absorbance fingerprint, and these are the basis for NIR sorting in plastic recycling facilities. Once again, this is best illustrated with an example.

NIR absorbance spectra provide an excellent an excellent technique for identifying polymers
Each plastic polymer has a unique NIR absorbance fingerprint
An Example : KitchenAid Basting Spoon

This KitchenAid Classic basting spoon is made of two types of plastic polymer - ABS for the red handle, and glass-fibre reinforced Nylon66 (also known as PA66) for the body.

Sagitto's technique of using NIR and machine learning enable us to accuracy identify the ABS polymer in this household items
KitchenAid Classic basting spoon made from PA66 with carbon black colour additive, and red ABS

When we scan the two parts of the basting spoon with Sagitto's miniature NIR spectrometer, and analyse the data with our plastic resin model, we're easily able to identify the ABS in the handle but are unable to identify the black part of the spoon.

Examples of the impact of carbon black in identifying polymers using NIR
Results from Sagitto's Resin Identification Code machine learning model

The inability to identify the carbon black part of the spoon makes sense, when we look at its NIR absorbance spectra compared to Nylon66 without carbon black as a colourant. The addition of carbon black has stripped the absorbance spectrum of any of the distinguishing features that we see in a normal NIR absorbance plot for PA66 - the blue line.

NIR can easily identify two o the three polymers shown here
NIR absorbance spectra for ABS with red colourant, and PA66 with (and without) carbon black colourant

Innovation In Masterbatch Products

Masterbatches are solid additives used to color raw plastic polymers.

REC-NIR Black™ masterbatch allows scanning by NIR

One of Sagitto's customers, Ampacet Corporation, has developed a range of masterbatch products specifically to address the problem of carbon black. Their REC-NIR Black™ masterbatch allows polymers to be black in the visible part of the spectrum while not interfering with the polymer's unique NIR absorbance fingerprint. This means that black HDPE, PP and PET plastics can be sorted for recycling using NIR.


This blog post is intended to illustrate - using NIR absorbance plots of real world items - why carbon black is challenging for the recycling industry. Fortunately most black fabric is not dyed with carbon black. And innovations such as Ampacet's REC-NIR Black™ masterbatch are giving black plastic packaging a second life too.

Subscribe to Sagitto's Blog

Get industry insights that you won't delete, straight in your inbox.
We use contact information you provide to us to contact you about our relevant content, products, and services. You may unsubscribe from these communications at any time. For information, check out our Privacy Policy.
George Hill
Sagitto Ltd
Sagitto's founder, George Hill, first started working with artificial intelligence during the 1980s, while developing 'expert systems' within Bank of America in London. On returning to New Zealand, he undertook part-time study with the University of Waikato's Machine Learning Group while working for Hill Laboratories, a well-known New Zealand commercial testing laboratory. This led to the formation of Sagitto Limited, dedicated to combining the power of artificial intelligence and machine learning with spectroscopy.

More news

Vendor Lockin

You Should Be Free To Leave

We believe that our customers should subscribe to our services willingly, because of the value that they receive and not because they are locked in to using us. That is why we take particular care to provide a smooth pathway, should our corporate customers decide to no longer use Sagitto's services.

Read Article

Know Your Lavender Oil

'English lavender' oil is extracted from the flowers of Lavandula angustifolia, while 'Lavandin' oil is made from Lavandula x. intermedia, a hybrid cross between Lavandula angustifolia and Lavandula latifolia (Spike Lavender). Near infrared spectroscopy not only gives a very rapid and inexpensive method to tell the difference between these two types of oil, but also allows the composition of oils to be accurately measured.

Read Article
Data Science

Benchmark Against Machine Learning Models

The success of generative AI applications such as ChatGPT and DALL-E has increased public awareness of the power of artificial intelligence software. Sagitto's Benchmarking Service allows users of infrared spectroscopy instruments to benchmark their current models against models generated by machine learning.

Read Article