NON-FOSSIL DYES

A dyestuff is usually an organic molecule that absorbs light and then emits that light at a frequency that is detectable to the human eye. In other words, it emits light in the visual light spectrum - this is why people say they are coloured. Technically, a dyestuff can be uncoloured, and it is called coloured because it emits light in the ultraviolet or infra-red range.

Before the invention of synthetic dyes by William Henry Perkin in 1856, the dyestuffs used were natural dyes harvested from plants and animals. Many of those natural dyestuffs exist today and can be used albeit for specialist purposes, and generally they are lower performance. The major shift from natural dyes to synthetic is for supply and demand reasons (which ultimately affects price). Natural dyes cannot be produced at the scale that synthetic dyes need to be produced.

Synthetic dye production
Petrochemicals and coal lie at the heart of synthetic dye production with some key ingredients coming out as by-products of oil production or out of the conversion of coal into coke. Coal tar and benzene are leading sources of dyestuffs, and they can safely be called fossil carbon.
Carbon sources can be divided into fossil origin and biogenic origin. Fossil carbon originates from sources within the earth that have existed for millions of years - they are part of the long carbon-cycle, with some fossil carbon not existing in short-lived carbon cycles for millions of years. Biogenic carbon originates from the upper atmosphere of the earth and has been made only recently. One possible theory as to why CO2 levels have increased to 400 ppm in the atmosphere is because ancient fossil carbon has been released. Scientists have shown that a short carbon-cycle (leaving fossil carbon in the ground) can allow a net-zero or net negative effect on carbon in the atmosphere. In other words, having only biogenic carbon circulating results in most of that carbon being stored resulting in a decrease of carbon in the atmosphere.
If fossil carbon is made into dyestuffs, they will remain in the form of how they are used until the dyestuff (and the product it is in) reach the end-of-life and are released via degradation back into the atmosphere. Ultimately, fossil carbon in dyestuffs will make its way back to the air and will contribute to CO2 increase.

Measurement of fossil
Fossil carbon can be detected in the material they are in by using a test that can calculate the fossil to biogenic ratio. If a material is largely made of fossil sources (oil and coal) then the material will be largely fossil carbon. If the material is made of natural sources, then the carbon within will be almost entirely biogenic.
Modern manufacturers of shoes, handbags, and upholstery are interested in the measurement of the carbon type so they can make claims about fossil-free and low impact products. Sourcing renewable materials will need to be largely natural materials that will automatically allow the product to be biogenic, fossil-free. Except manufacturers that are seeking a colour to their product will need to include dyestuff. Here is where the fossil content will inevitably increase because the producer will include a synthetic dye.

Modern biogenic dyestuffs
Not that recently, commercial production of natural dyes was given a commercial boost with the use of logwoods. Haematoxylum campechianum is a species of plant that produces compounds that show a range of colours (given a specific set of conditions). Firstly, they change colour according to the pH. Secondly, they can also change colour if they are in the presence of other compounds (e.g., ammonia). These kinds of plants can be grown at huge scale on non-arable land and the products of that farming can then be used in a range of products. The most interesting thing about logwoods is that they exhibit good performance, even light fastness.
So herein lies the challenge. How can tanners and product manufacturers obtain large quantities of biogenic dyes that are affordable, renewable, and at the performance quality over their rivals - the synthetic dyes? Fully biogenic leathers will be difficult if fossil-based dyes are included.

Candidates
It is also not impossible to imagine the concept where the biogenic dyestuff is made synthetically from natural starting materials. Benzene is most commonly extracted from petroleum sources, but it is at the heart of most aromatic chemistry and is found almost everywhere from pine resins to the lignins found in wood. It is not unfeasible that any sort of phenolic aromatic found naturally can be converted into its benzene form and then the usual dye synthesis can take place.
It is also not beyond the realm of reason to take waste natural substances and start looking for cheap ways in which they can be modified into synthetic dyes at scale. Normally, the dye synthesis starts with the chemist taking a chromogen (like benzene) and then adding a chromophore like a nitro group. A natural source of biogenic benzene can be sourced and then a mixture of nitric and sulfuric acid can result in that biogenic benzene becoming a nitrobenzene. Nitrobenzene then easily formed into azo dyes which is a very common class of dyes. To complete the synthesis for the pedant, the last step is usually the addition of an auxochrome, of which there are many natural sources, like chlorine or amines (fish smell).
So, the production of biogenic synthetic dyes is not impossible, but is not yet commercial and there is no significant demand for them just yet. However, as the world weans itself off fossil fuels these will be the kinds of solutions that chemical manufacturers are looking for. The advantage of thinking of these ways of sourcing biogenic dyes is that it ensures the production of high-performance dyes that the industry knows well. The natural analogues of synthetic dyes will have the identical performance as the source of the incoming molecules does not change the ultimate performance of a molecule of a particular chemical structure. The difference lies in the principle of where those atoms were derived. Going forward the industry will prefer biogenic origins and the industry should aim for this target.
A final thought, the real winner in the race for biogenic “anything” is the synthesis of the compound of interest from CO2 out of the atmosphere. If chemical industries in the future take atmospheric CO2 and they synthesise it into organic compounds using renewable energy, then the atmospheric CO2 is decreasing and humans are getting a source of renewable organics. Many examples of this type of chemistry are already underway with the use of modified sugarcane fibre going into the Allbirds tekkie! These concepts have been converted into commercial scale reality and will only continue to grow.

In the next issue: Croc leather - is it the secret to their survival? The American alligator has been saved by the conservation effort of over 40 years. Whilst the Nile Crocodile has never achieved the same levels of threat as the alligator, how does the farming of crocodiles contribute to a healthy croc population and their future?