Fabrication of Bioinspired Superliquiphobic Synthetic Leather with Self-Cleaning and Low Adhesion

Dev Gurera and Bharat Bhushan
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLBB),
The Ohio State University, 201 W. 19th Avenue, Columbus, OH 43210-1142, USA

INTRODUCTION: Leather has many applications including in footwear, furnishings, automotive industry, clothing, bookbinders, gloves, sports gears, bags, and cases. In order to reduce cost and to tailor flexibility, the demand for synthetic leather has grown. For some the applications, leather requires liquid-repellency, self-cleaning, low adhesion to reduce stickiness at elevated temperatures. Surfaces are extremely liquid repellent if the liquid contact angle is over 150°, referred to as superliquiphobicity ¹. Surfaces are self-cleaning if the tilt angle is less than 10°, preferably 2°–4°.

Superliquiphobic and self-cleaning surfaces have been fabricated inspired by superhydrophobic lotus leaf ¹. The lotus leaf structure consists of a hierarchical structure with micropapillae superimposed with nanotubules that are formed by self-assembly of long-chain hydrocarbon wax. The wax makes the surface hydrophobic with a water contact angle of 95°–110°. The hierarchical structures make it superhydrophobic with a water contact angle of about 164° and self-cleaning with a tilt angle of about 3°.

In previous studies, an attempt was made to make natural leather superhydrophobic by using a spray coating of silica nanoparticles and an epoxy binder ². However, the coating lost its superhydrophobicity, from water contact angle 165°, to 130° in just one hour. In this study, synthetic leather surfaces were made superliquiphobic by using a multilayered nanoparticle composite structure ³. To demonstrate superliquiphobicity, distilled water and hexadecane droplets were used for contact angle (CA) and tilt angle (TA) measurements at ambient and elevated temperatures. Further, experiments were conducted for self-cleaning and mechanical durability of the coating.

METHODS:  Synthetic leathers used in this study were PU and PVC leather. A nanocomposite layer of hydrophobic silica nanoparticles and methylphenyl silicone resin was used to achieve hierarchical structure ³. The silica nanoparticles and methylphenyl silicone resin were chosen because of their high hardness and wear resistance. The nanocomposite layer was deposited using spray coating technique because it provides the desired surface as compared to other techniques. The nanocomposite layer was later coated with low energy fluorinated materials, because they are known to provide repellency to low surface tension liquids such as oils. Fluorinated materials are commonly used because fluorine is very electronegative and has a low polarizability.

The coatings were characterized for liquid repellency, self-cleaning at ambient and high temperature, and mechanical durability. Liquid repellency was measured using a goniometer. The self-cleaning characteristics were examined by contaminating the sample with silicon carbide particles and comparing the removal of particles by water droplets before and after the experiment. The mechanical durability of the surface was examined using a ball-on-flat tribometer ⁴.

RESULTS AND DISCUSSION:  Both the leathers are liquiphilic to start with. Untreated PU leather has water CA of 76°±5°, and hexadecane completely wets it as shown by the red dyed hexadecane drop in Fig. 1 (top). PVC leather has water CA of 62°±6° and hexadecane CA of 19°±2°. Both the leathers have TA greater than 90°. After depositing the nanoparticle-binder coating, 

both of the leathers become superhydrophobic, with TA about 2° and as expected, the coatings were superoleophilic. After the application of the fluorosilane coating, surfaces became superoleophobic with hexadecane CA of about 157° and TA of about 3°. As shown in Fig. 1 (bottom), the hexadecane droplet just rolls-off of a 10° tilted PU sample. However, the superoleophobicity was found to be non-uniform. It was because the plasticizers in the synthetic leather leach out to top of the surface via capillary action to make the surface chemically active ³.

Figure 1: Photographs comparing the hexadecane repellency on untreated and nanoparticle-binder/fluorosilane coated PU leather ³.

Further tests were performed on the coatings. The nanoparticle-binder coating removed about 90% of the particles in comparison to the 10% of the particles removed by the untreated leather. The untreated is less efficient because it is liquiphilic, hence a droplet will slide on it instead of rolling. Next, the CA of about 160° and TA about 2° remained the same up to 70°C. Around 80°C, the leather starts to lose its superhydrophobicity. It is believed that the polymer in the leather reaches its glass transition temperature, which is about 80°C for PVC ⁵. In addition, 80°C also happens to be a typical temperature inside an automobile on a hot day ⁶. That may be a reason why one feels leather being sticky on a hot day. Lastly, after a wear test, some burnishing was observed for the nanoparticle-binder coating, and some loss of TA. When a water droplet was allowed to roll across the wear track, TA increased from 2° to 7°. A TA of under 10° maintains self-cleaning properties.

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