By A. Clare
The strive towards sustainable leather processing is causing the leather industry to address environmental issues and transition towards greener production processes. European directives in favour of the circular economy have increased the demand for environmentally friendly alternatives during leather processing. The circular economy model is based on the idea that any by-product produced within the leather industry is reused in the leather making process (or in other industries). Solid waste is generated during most stages of leather production (Figure 1) and has the potential to be transferred into valuable resources to allow sustainable leather production to be.
Figure 1 - Types of solid waste produced during leather production.
Utilising Keratin from Tannery Hair Waste:
Many studies have demonstrated success in reusing keratin waste in leather processing. Keratin can be extracted from hair waste using alkali hydrolysis and employed in chrome tanning and chrome exhaustion. The keratin hydrolysate improves the uptake of chrome by leather without significantly modifying the leathers physical properties.
Leather waste-derived keratin has also shown success when used in leather retanning. Since the 1960s, many researchers have demonstrated hydrolysed keratin can improve leather fullness, elasticity and dyeing performance when used as a filler during the retanning process.
Utilisation of Untanned Solid Waste:
Utilising Collagen Hydrolysate in Retanning:
Traditional powdered polymers are traditionally used as retanning agents; however, these significantly contribute to effluent salinity load. The leather industry is, therefore, looking for alternative retanning agents to reduce this environmental impact. Powdered collagen hydrolysate can replace formaldehyde as an environmentally friendly filler during the retanning stage. Collagen hydrolysate improves the properties of leather, such as fullness, grain tightness and dye brightness. Furthermore, the product improves tensile strength and tear strength, demonstrating it chemically interacts with the leather matrix. The retanning agent is also biodegradable and improves the exhaustion of post-tanning chemicals, thereby further increasing sustainability.
Utilising Collagen Hydrolysate to Improve Leather Dyeing:
Traditional acids, such as formic acid, are often used to fix dyes during leather processing due to their low cost and good performance. However, despite fixation, some of dye often remains in the waste effluent. Collagen hydrolysate has shown success when used to prepare a colour fixing agent, increasing the dye uptake rate to 97%. The product also improves dry- and wet-rub fastness by 0.5-1 grade and, the resulting leather has excellent softness. Utilising collagen hydrolysate in leather dyeing reduces environmental pollution by decreasing solid waste and wastewater contaminants.
Utilising Fats from Fleshing Waste as a Fatliquor:
Fats can be extracted from solid fleshing waste and characterised before being processed into a good quality sulfated fatliquor for leather lubrication. The resulting leathers show increased tensile strength and tear strength relative to those processed using commercial fatliquors. Furthermore, the leathers had a better appearance, and its quality was on par with those conventionally processed.
Utilising Chrome-containing Solid Waste:
Using Chrome Shavings to Produce Dehairing Protease:
Collagen hydrolysate can be extracted from chrome shavings and used in the production of de-hairing protease. Collagen hydrolysate provides an inexpensive carbon and nitrogen source when used as a substrate during enzyme production by Bacillus, compared to conventionally used wheat bran. Collagen hydrolysate stabilized the protease enzyme and resulted in a high level of enzyme production. Furthermore, the quality of leather de-haired using the resulting protease was on par with conventional leather. Collagen derived from chrome shavings can, therefore, be reused in an environmentally friendly dehairing process.
Utilising Chrome Shavings for Retanning:
Protein can be extracted from chrome shavings by alkaline hydrolysis and modified using acrylic monomers to produce a retanning product. The product can be applied to wet blue and has a strong affinity for collagen and easily penetrates the leather matrix. The retanning agent improves the physical characteristics of the leather, enhancing grain smoothness, softness, and fullness. Furthermore, the mechanical properties, including tensile strength and tear strength, were also improved. The graft protein polymer, derived from chrome shavings, can be used as greener chemistry to replace commercial tanning agents with no impact on leather quality.
Using Chrome Shavings to Produce Regenerated Leather:
Chrome shavings can be reused to produce a split leather substitute. The shavings can be soaked with an expansion softener and mechanically pulverized to produce leather fibres. A binder is added to the fibres before they are compressed and finished. Waterborne finishes can improve the mechanical properties of regenerated leather: hydrogen bonds form between the finish and the leather shavings, which increases tensile strength, heat storage ability and thermal stability.
Utilising Dechrome Collagen as a Finishing Agent:
Solvent-based polyurethane finishes have excellent properties, high performance, and low production cost; however, they emit volatile substances (VOC), contain many organic solvents, and have poor air permeability. A lot of research has, therefore, focused on developing environmentally friendly water-borne PU finishes, which are non-toxic and contain no VOC, however, their performance is incomparable to those which are solvent-based.
Water-borne polyurethane, modified with collagen hydrolysate, can be used to produce a finishing agent with improved performance. The hydrophilic groups on collagen hydrolysate can react with the isocyanate group on PU, resulting in the synthesis of a new PU-based finishing agent which can be applied to the leather. The resulting leather showed good extensibility and elongation at break, demonstrating the potential application for this modified polyurethane to be used in leather finishing.
Utilising Finished Leather Waste:
Finished Leather Waste for Preparation of Regenerated Leather Composites:
Finished leather waste includes leather trimming, crust leather buffing dust and finished leather scraps; however, it also includes used leather products. Regenerated leather composites can be prepared by combining fiberised finished leather waste with plant fibres, which can then be compressed into a regenerated leather material. Plant fibres include those derived from bananas, coconuts, sugar cane and corn silk, all of which offer advantages as a reinforcement material due to their low cost, high strength, and excellent mechanical properties. Fibres from coconuts have demonstrated excellent performance when used in ratios of 50:40 (plant fibre/leather). Coconut fibre/leather composites have good tensile strength, elongation at break, tear strength and flexibility. The prepared composites have shown promising applications in the footwear and leather goods industry, specifically in low-income countries where these materials are abundantly available.
Conclusion:
The cost-effectiveness of managing and treating solid waste has for long been recognised as an important issue in many industries. The leather industry is continuously working towards sustainable leather production by increasing profitability whilst simultaneously reducing its environmental impact. There is now an ongoing recognition that reusing leather industry waste offers an innovative management solution for reducing solid waste. Adding value to by-products presents a feasible case for sustainable leather production by reducing pollution, replacing hazardous commercial products, and reducing production costs.
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