For consumers who make purchases with animal welfare and the many chemicals used to make leather in mind, synthetic leather provides an alternative. The polyurethane or polyvinyl chloride material has a leatherlike feel and appearance.
But synthetic leather is not without drawbacks: Although the production process is more environmentally friendly than that used for natural leather, it’s still not green. Firms that make synthetic leather for consumer products have therefore been under pressure to come up with cleaner technology. Those efforts are bearing fruit.
Scientists and engineers at Bayer MaterialScience, for example, are introducing water-based manufacturing technology to make polyurethane synthetic leather for footwear and apparel. The approach avoids the toxic solvent dimethylformamide (DMF) that is currently necessary for making polyurethane synthetic leather. The Bayer process also uses significantly less energy and only a fraction of the water consumed in the conventional process.
“Polyurethane leather is a great product, but it has been in strong need of a process upgrade,” says Nicholas Smith, Bayer’s Shanghai-based global head of textile coatings.
The chemical-intensive process of converting animal hides into leather has been around for thousands of years. To remove hair and fats, modify collagen protein, and dye the material, leather makers need numerous chemicals, including detergents, enzymes, lime, ammonium chloride, sulfuric acid, brine, and chromium sulfate. Half of all leather is still made in developing countries where workers slosh hides by hand in a series of vats filled with these chemicals. When spent, the aqueous solutions are often released untreated into local streams.
Polyurethane is a versatile, inert material used for protective surface coatings on floors, adhesives for wood products, soft foam to pad furniture, hard elastomers for objects such as skateboard wheels, synthetic fibers such as spandex for stretchy clothes, and composites such as synthetic leather. Bayer’s polyurethanes were recently used in several layers of the Brazuca soccer ball prepared for this year’s World Cup.
Global annual production of polyurethane synthetic leather has reached more than 4 billion m2 per year, Bayer’s Smith says. It’s used for applications such as running shoes, clothing, and handbags in which high softness and varied surface texture are desired. Polyvinyl chloride leather tends to be used more for durable applications such as furniture and car seats. About 90% of the polyurethane leather made with the solvent-based processes is produced in China, where it stays and is converted into a variety of consumer goods.
Polyurethane synthetic leather avoids the chemical soup used in the leather tanning process. Polyurethanes are typically made using an amine catalyst to combine a diisocyanate (a toxic reagent that requires careful handling) with a diol.
DMF is miscible with water and works well as a solvent for making polyurethanes. But it is absorbed through the skin and has been linked to cancer and birth defects. As chemists have better understood those toxic effects, they have started avoiding the solvent in manufacturing processes.
One option they’re pursuing is switching from DMF-based polyurethane production to a water-based process. Polyurethane dispersion technology that generates polyurethane particles in aqueous solution has already caught on for making adhesives, coatings, and foams. Bayer is now the first to give it a go for making synthetic leather.
The chemistry of Bayer’s process is “impressive and welcome,” says Mila Aldema Ramos, chair of the American Leather Chemists Association’s Committee of Environmental Affairs and a research chemist at the Department of Agriculture’s Eastern Regional Research Center, in Wyndmoor, Pa. “Green and sustainable processing is the way to go if we want to save the Earth for future generations to enjoy,” Ramos says. “Although there is nothing like the real thing—natural leather—I still applaud their eco-friendly and noble method of obtaining synthetic leather by eliminating DMF and reducing the amount of wastewater discharged into the environment.”
Synthetic leather is a layered composite made by first coating a polyurethane paste onto a fabric substrate, Smith explains. Manufacturers coagulate the paste by soaking it in a water bath to help polyurethane chains align and precipitate, creating a porous base layer that simulates the feel of natural leather. A polyurethane skin layer with the texture of leather is prepared separately and heat-laminated to the base layer.
DMF has been essential to forming the base layer’s porous structure, Smith says. But as a volatile organic compound, it diffuses into the air throughout the production process, which includes multiple water washes, several drying steps, and the lamination step. Worker exposure to the solvent is a safety concern, he notes.
To make a water-based system work, Bayer researchers had to figure out how to make the porous base layer without DMF. The Bayer team learned how to mechanically generate a foam consisting of polyurethane nanoparticles of two different sizes, which leaves air pockets in the base layer and simulates the porous material made with DMF.
“This is a new type of chemistry for a new type of product,” Smith says. Among the advantages is eliminating worker exposure to DMF, he notes. Other advantages of eliminating DMF include reducing energy consumption by 55% by avoiding heating water, multiple drying steps, and DMF recovery. And although the new process is a water-based method, it still uses 95% less water than the DMF process.
The new Bayer process “is successfully moving the dial in favor of green chemistry,” says Richard P. Wool, a professor of chemical and biomolecular engineering at the University of Delaware. Wool received a 2013 Presidential Green Chemistry Challenge Award in part for his group’s invention of a synthetic leather made from soybean oil and natural fibers, including those from chicken feathers and flax.
Although DMF helps develop the polyurethane base layer’s porous substructure and gives the material its leatherlike soft touch, it’s a toxic solvent, Wool adds. “Smith’s team has shown that this same property can be achieved by water-stabilized polyurethane particles, thereby eliminating DMF and saving energy on drying,” Wool says. “Overall, this is a very big step.”