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.
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.
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.
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.
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.
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.
Innovation In Masterbatch Products
Masterbatches are solid additives used to color raw plastic polymers.
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.
Conclusion
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.
