Valorization of Spent Coffee Grounds and Brewer’s Spent Grain Waste Toward Toughening of a Biodegradable PBAT/PHBH Blend

Plastic pollution from packaging waste is driving the development of biodegradable composites for sustainable packaging. In this work, poly(butylene adipate-co-terephthalate)/poly(3-hydroxybutyrate) (PBAT/PHBH) blends (50/50 wt.%) were reinforced with agro-industrial waste fillers—spent coffee grounds (SCG), brewer’s spent grain (BSG), and cellulose powder (CP)—at 1–15 wt.% loading. The effects of these fillers on tensile properties, impact strength, and thermal stability were examined and supported by scanning electron microscopy (SEM) of fracture surfaces and thermogravimetric analysis (TGA). The neat PBAT/PHBH blend exhibited balanced stiffness and ductility. Low BSG loadings (≤5 wt.%) produced the greatest toughening, with impact strength increasing by ~92% and elongation at break significantly improving over the neat blend. SEM analysis indicated crack deflection and particle pull-out as dominant energy-dissipation mechanisms at low BSG loading. At higher BSG loading (15 wt.%), particle clustering and larger voids acted as stress concentrators, reducing impact performance. SCG improved ductility at low loading (1 wt.%), whereas increasing SCG content led to progressive reductions in tensile strength and elongation due to increased debonding and microvoid formation. In contrast, CP exhibited minimal reinforcement efficiency within the investigated range (1–5 wt.%). Overall, filler addition generally reduced tensile strength and, in several cases, tensile modulus, reflecting limited interfacial compatibility between the hydrophilic lignocellulosic fillers and the hydrophobic polyester matrix. TGA indicated a modest improvement in thermal stability at higher BSG loadings, reflected by shifts in T5% and Tmax1 (PHBH) toward higher temperatures. Overall, this study demonstrates that upcycled coffee and beer waste fillers can impart specific toughness benefits to biodegradable PBAT/PHBH blends, but interfacial incompatibility currently limits their reinforcement efficiency. The findings highlight the potential and challenges of these biocomposites for sustainable packaging applications and suggest that interface engineering (e.g., compatibilizers) will be key to unlocking optimal performance.