Bio-based adhesives from spent coffee grounds: A sustainable pathway using medium chain length polyhydroxyalkanoates

This study explores the synthesis of medium chain length polyhydroxyalkanoates (mcl-PHA) from Spent Coffee Grounds (SCGs) oil, and its application as a bio-based Pressure Sensitive Adhesive (PSA) for food packaging materials. The mcl-PHA was produced through the bioreactor fermentation of Pseudomonas resinovorans, yielding 40 % (gpolymer/gCDW) under single pulse modality. The polymer showed heterogenous monomer composition from C6 to C18, endowed with 20 % unsaturation. As a PSA, the material was evaluated on stainless steel, low-density polyethylene (LDPE), cellulose (CELL), and mixed LDPE/CELL substrates. Tack testing revealed strong adhesion yet relatively low adhesive cohesion. Cyclic debonding tests demonstrated full recovery of adhesive strength, consistent with the polymer's viscoelasticity. Lap shear and peel tests on flexible plastics substrates highlighted strong affinity to cellulose (peel strength: 37.2 N m−1) attributed to the high polar component of the surface free energy of mcl-PHA (σSP = 24.9 mN m−1). Weaker adhesive performances were noted on LDPE (peel strength: 3.8 N m−1). Recovery of mcl-PHA was achieved through acetone dissolution, enabling its potential reuse. The results highlight SCGs-derived mcl-PHA as a sustainable PSA candidate, particularly for cellulose-based substrates, while underscoring challenges in bonding non-polar surfaces like LDPE.