Zhenghao Shi, M. W. Kwok, To Ngai
Abstract The use of leather in human history spans thousands of years, and the mass production of leather techniques also has a longstanding history over a century. The emergence of synthetic leather arose as a response to the market’s demand for an alternative due to a shortage of natural leather supply. However, the ongoing challenge of environmental pollution during the continuous development of synthetic leather to achieve comparable features to its natural counterparts has led the leather industry to deviate from its original goal of sustainability and environmentally friendliness, focused on “waste to worth” principle. Following the replacement of polyvinyl chloride (PVC) with polyurethane (PU) as the primary material for the surface layer in synthetic leather, waterborne polyurethane (WPU) and solvent-free polyurethane (SFPU) emerged as the principal environmentally friendly raw material for synthetic leather manufacturing. The free volume (FV) theory explains the relation between mass transfer in polymer films and increasing FV in polymer coatings, highlighting its benefit to water vapor permeability (WVP). The WVP of a synthetic leather is primarily influenced by the hydrophilicity and porous structure of the polymer coating, determined by the base fabrics and the raw polymer coating material. This paper reviews various methods for preparing porous structures to increase WVP of polymer films or coatings: chemical blowing, thermally expandable microspheres, laser drilling, hollow microspheres, surfactant-stabilized foam templates and Pickering aqueous foam templates. It also provides a concise outline of present issues and prospects in improving the breathability of synthetic leather derived from these approaches. Graphical Abstract