An efficient solution to determine surface energy of powders and porous media: Application to untreated and treated lignin

Abstract : The estimation of solid surface energy is generally obtained through the determination of liquid/solid contact angles. Many questions arise in the literature about the appropriate contact angle to use (dynamic, quasi-static or static, measured at a macro or a micro-scale). Theoretically, equilibrium contact angles should be inserted into equations to determine surface energy components. However, for powders or porous materials more issues arise, notably due to the imbibition of the liquid into the medium. An apparent contact angle can be obtained using wicking tests and the modified Washburn equation. Nonetheless, Washburn hypotheses are not always respected or no longer valid during wicking meaning that this procedure could not allow to determine contact angles. This study has the aim to propose a new simple method to determine the surface energy components of powders and porous media. An experimental protocol coupled to a modified Jurin law is proposed. This protocol was applied to lignin particles used as reinforcement in polylactic acid (PLA). Some treatments were performed on lignin to improve the adhesion with PLA and the modifications due to the treatments were characterized. Reliable surface energy and components of untreated and treated lignin were obtained revealing the efficiency of the proposed method.