Influence of the Spent Coffee Ground-to-Ammonium Lignosulfonate Ratio on the Performance of Wood-Fiber Biocomposites

Spent coffee grounds (SCGs) and ammonium lignosulfonate (ALS) are promising renewable constituents for wood-fiber biocomposites, but the effect of their relative proportion within the binder phase remains insufficiently understood. This preliminary study evaluated wood-fiber panels produced at a constant dry-matter matrix-to-fiber ratio of 50:50 while varying the SCG:ALS ratio from 70:30 to 30:70. The panels were hot-pressed and characterized for their physical and mechanical properties, while the corresponding SCG-ALS matrix formulations were evaluated by simultaneous TGA-DSC analysis. Increasing the SCG fraction generally improved dimensional stability and mechanical performance, whereas formulations containing 50% or more ALS showed a marked deterioration in water resistance. The SCG-rich 70:30 formulation provided the highest bending performance, with MOE and MOR values of 2156.5 and 12.83 N·mm^-2, respectively. Thermal analysis revealed ratio-dependent changes in mass loss and in the principal heat-flow event, which occurred between approximately 113 and 136 °C, confirming that the SCG:ALS ratio altered the early thermal behavior of the binder system. Overall, the SCG:ALS ratio was identified as a key formulation parameter, with SCG-rich systems providing the most favorable balance between mechanical performance, dimensional stability, and internal cohesion.