The next area of leather finish materials is advanced in development. The traditional coating materials for leather have included: proteins (typically blood, casein, and albumin); acrylates; butadienes (rubbers and styrenes); polyurethane; and cellulose derivatives. Most of these are based on petrochemical derivatives and will ultimately be unsustainable if fossil fuel reserves are depleted.
The surface coating industry is continuously looking at plant-based origins and with the recent emphasis on coatings that avoid plastics, industry look to plant based bio-polymers or to waste materials that originate from food waste.
Chitin and chitosan are modified carbohydrates that link together as repeating units of N-acetyl-D-glucosamine or D-glucosamine linked via a ß-(1 to 4) linkage. The presence of an acetyl group changes the solubility of the polymer and determines whether the surface coating is water resistant or not.
Figure 1 shows the chemical structure of a section of the chitin polymer. The diagram illustrates the repeating unit of N-acetyl-D-glucosamine. The structure is largely unreactive due to the absence of ionic functional groups and is therefore an ideal material for the material use in insect skeletons. In fact, it is not only insects that use this versatile covering but will be included into other arthropods (shellfish and arachnids). The area where this is interesting is when the material is found in the waste pile at food companies. The shelling of seafood generates enormous amounts of waste and if an application can be found for this free substrate then the economics of surface coatings becomes lucrative.
Figure 1. The chemical structure of a chitin polymer.
The difficulty with chitin is that its lack of ionicity means that it is not water soluble. The majority of leather finishing is now water-based so to use a non-ionic chemistry makes the application based on emulsion-based water chemistry, or on dissolving the chitin into a solvent and then applying. This means that many chemical companies are resisting using unmodified chitin.
Older members of the tanning industry will recall that chitosan was touted in the leather industry before. In the 1990s the use of chitosan was either suggested for use in the wastewater plant or for use in the retanning, dyeing, and fatliquoring. The chitosan is found in the Arthropoda at lower concentrations, but more commonly is seen in the fungal kingdom. Together with chitin, the fungal cell walls and cells comprise largely of these two glucosamines. The structural difference between the two is that chitosan is deacetylated to leave the D-glucosamine, see Figure 2.
Figure 2. The chemical structure of chitosan polymer.
Figure 2 also shows that the structure can contain the acetylated group which appear randomly allowing the coating to be resistant to general solubility, but the presence of the amine group means that at low pH the molecule is soluble and can be dissolved in normal aqueous based finishing.
Chitosan and chitin blends
On their own, chitosan and chitin do not produce coatings that can match the properties of other leather finishes. Typically, scientists have used these carbohydrates in blends with traditional polymers. The latest generation of self-healing paints include chitosan, polyurethanes, and a catalyst that can upon UV activation will react with the -OH groups of the chitosan to reform the chemical links and maintain film integrity. The surface coating damage allows the release of the catalyst so that self-healing can proceed. Can next-generation leather finishes in high performance applications do the same?
The other main reason why these types of polymers are seeing more use is that coating specialists are looking progressively towards finish films that can breakdown in the biosphere. A performance coating that is highly cross-linked and that is derived from a petrochemical source can be quite resistant to the action of bacteria and fungi (and their enzymes). The reason for this is that the backbone of these polymers tends to be quite inert and requires special enzymes that can find their binding sites. Petrochemicals also tend to be quite poor in a balanced distribution of carbon, oxygen, and nitrogen which are vital for bacteria and fungal growth. Typically, petrochemicals are carbon and oxygen rich, but lack the nitrogen.
The inclusion of a glucosamine that contains additional nitrogen and is a biopolymer that bacteria can sink their teeth into creates an interesting opportunity. Surface coating technologies are littered with references to coating that include hard and soft segments, that is some parts of the coating which are prone to the degradative effects and parts that are highly resistant.
Soft segment degradation must not leave hard segment microplastics however, as scrutiny in the US and in the EU have resulted in regulation of products that are associated with microplastic pollution. Incorporating chitosan into the coating allows good film breakdown and the potential for healing finishes.
The additional benefit of glucosamines and proteins/sugars in general mean that the coating will have a much more natural feel. Leather finish research has seen the investigation into zein (protein from maize), keratin (protein from hair, feathers, and wool), gelatin, and even soluble collagens. These hydrolysates in the finish act like a protein filler would – they increase the water properties, dilute the cost of the main film, provide better flow-out and give a touch that can make the leather feel much more like an aniline finish.
Of course, the challenge is getting the performance of these coatings with the inclusion of these products. They often require cross-linking, and this can prove difficult in an age where many cross-linkers are highly regulated as a restricted substance or where worker safety is being carefully controlled.