The Lotus Effect on Synthetic Leather

We love the Rock’n Roll look of leather bombers, but the material is not always comfortable, especially if it is made of synthetic leather. A few rays of sunshine in the spring and the parts that touch the skin become sticky and unpleasant. Scientific research found a solution that corrects this imperfection and, in addition, makes the material self-cleaning. A team from Ohio State University explored the microscopic structures of a living organism to construct a hydrophobic synthetic leather. This property makes liquids like water and oil—the main cause of moisture and dirt—roll off the material’s surface.

Today’s synthetic leather is permeable like natural leather but becomes stickier than leather when exposed to heat. The nanotechnology created by the two researchers was inspired by the lotus leaf, a superhydrophobic and self-cleaning aquatic plant. Combining these two properties produces what is referred to as the lotus effect.

This video shows the nanometric roughness of the lotus leaf surface

Manufacturing Water Repellent and Oil Repellent Synthetic Leather

The study entitled “Fabrication of bioinspired superliquiphobic synthetic leather with self-cleaning and low adhesion,” co-authored by Dev Gurera and Bharat Bhushan, was published on May 20, 2018, in the scientific journal Colloids and Surfaces A: Physicochemical and Engineering Aspects.

Bhushan, an expert in biomimicry, and his colleague Howard D. Winbigler, a professor of mechanical engineering, had already developed water-repellent and oil-repellent materials. Their technique involves spraying a silica nanoparticle coating on the material’s surface. But Bhushan and his student Gurera found that this method was not suitable for synthetic leather because of its chemical composition.

As the researchers explained, synthetic leather is made of about 55% polymer—usually polyurethane (PU) or polyvinyl chloride (PVC)—and 40% plasticizers, which are added to make the material more flexible and more resistant. However, these plasticizers prevent nanoparticles from adhering to the surface, especially inside the grooves. To neutralize the effect of this component, the surface is subjected to prior treatment with ultraviolet light commonly used in the manufacture of computer chips. After applying silica nanoparticles, the researchers sealed the layer with a silicone resin. The resulting coating is transparent and does not alter the color and texture of the synthetic leather.

Efficiency and Mechanical Strength Tests

The treatment improved the material’s mechanical properties and kept them intact even when the material was heated to 70° C. Hydrophobicity tests on PU and PVC proved the technology’s effectiveness. In fact, water poured over the synthetic leather rolled at an incline of 2 degrees and oil, which is lighter, rolled at an incline of 4 degrees. A material is considered hydrophobic when the inclination needed is less than 10 degrees.

The coating is also wear-resistant. The researchers scratched the treated material’s surface 100 times with a sapphire and repeated the tests. When the droplets were placed on the worn part, they rolled at an angle of 7 degrees. Even in this state of wear, the synthetic leather retained its hydrophobicity.

To test the self-cleaning property, the researchers sprayed black silicon carbide powder onto the synthetic leather and measured the amount of powder that could be absorbed by a single drop of water. On untreated material, water absorbed about 10% of the powder, while on treated synthetic leather it removed 90% of the powder.

Synthetic leather use in the areas of sportswear and accessories, automobile, leather goods, furniture, etc. will be more varied and it will perform better under extreme conditions.