Influence of Polyurethane Binders on the Surface Properties of Finished Leathers Part 1 – Function as a Sealing Coat

Abstract

Finishing is the final stage of the leather-making process, where the surface of the leather is enhanced to improve its durability, appearance, and texture. The sealing coat (used predominantly in corrected grain leathers for leather accessories and footwear industries) is the first step that forms the bottom layer of the leather finishing film before the base coat. The goal of the sealing coat is to keep the leather surface reactive and prevent the pigmented base coat from sinking completely into the grain of the leather. The chemicals used in the sealing coat provide the leather surface with a charge and condition (addition of oil/fat and/or polyurethane -PU) to ensure that the chemicals (pigments, binders, and other additives) used in the base coat of the finishing process are not completely absorbed into the grain layer of the crust leather. In the present study, the influence of PU as an additive to the sealing coat formulation was evaluated through measurement of surface properties, such as contact angle, surface energy, and work of adhesion. The values of the contact angle between the liquids (water/ dimethyl sulfoxide/ hexadecane) and the leather surface were used to evaluate the leather’s surface energy and work of adhesion. The water contact angle of the uncoated crust, cationic oil ground (COG)-coated crust, and COG-PU-coated crust was measured as 77.3, 93.6, and 109.0 degrees respectively. The increase in contact angle indicates an increase in the hydrophobicity of the leather’s surface on coating with COG and COG+PU. The surface energy component (γs) of the crust leather surface without coating, COG-coated, and COG-PU-coated leathers were 35.2, 28.3, and 24.8 m/Nm, while the work of adhesion of the above leathers for water were 88.8, 68.2, and 49.1 mJm-2, respectively. The experimental values indicated that PU plays an important role as an additive to the sealing coat and enhances the required properties in the finishing process for the desired final leather.