Retarding effect of carboxyl-containing dissolved organic matter derived from the thermochemical conversion of spent coffee grounds on formaldehyde-free water-based coatings

Abstract

This study employed thermochemical conversion and heterogeneous blending techniques to optimize a formaldehyde-free water-based composite coating incorporating carboxyl-containing dissolved organic matter (DOM). DOM with >90% aqueous solubility, derived from spent coffee grounds (SCG), is incorporated into a calcium silicate-based matrix to form a composite coating. This study evaluated the physicochemical properties of carbon black (CB), SCG, pyrolyzed SCG, and DOM as coating additives, along with the performance of their composite coatings. The effect of DOM concentration is further examined, and the impact of the additives on adhesion, hygroscopicity, and thermal insulation is compared. Results show that carboxyl-containing DOM acts as a set retarder, suppressing the agglomeration and improving the adhesion of calcium silicate hydrate. The water-based composite coating incorporating 3% DOM (3%-300DOMC) achieves adhesion comparable to that of a commercial coating, as verified by ISO 2409 testing. Compared with the blank, 3%-DOMC exhibited superior moisture resistance and provided the most effective thermal insulation performance. Specifically, at a hot plate temperature of 90 °C, the surface of 3%-DOMC exhibited a temperature differential of up to 30 °C relative to the hot plate. Overall, the valorization of SCG into formaldehyde-free water-based composite coatings aligns with the concept of circular economy and represents a feasible and promising pathway toward environmental sustainability.