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

Area of Science:

• Biotechnology and Materials Science
• Sustainable Polymers
• Adhesive Technology

Background:

• Spent coffee grounds (SCGs) represent a significant waste stream with potential for valorization.
• Medium chain length polyhydroxyalkanoates (mcl-PHAs) are biodegradable polyesters with tunable properties.
• Developing bio-based adhesives is crucial for sustainable packaging solutions.

Purpose of the Study:

• To synthesize mcl-PHA from SCGs oil.
• To evaluate the performance of SCGs-derived mcl-PHA as a bio-based pressure-sensitive adhesive (PSA) for food packaging.
• To investigate the adhesive properties on various substrates and assess reusability.

Main Methods:

• Bioreactor fermentation of *Pseudomonas resinovorans* using SCGs oil.
• Characterization of mcl-PHA monomer composition and unsaturation.
• Evaluation of PSA properties including tack, cohesion, viscoelasticity, lap shear, and peel strength on stainless steel, LDPE, and cellulose.

Main Results:

• mcl-PHA was produced with a yield of 40% (g_polymer/g_CDW) and a heterogeneous monomer composition (C6-C18) with 20% unsaturation.
• The PSA exhibited strong adhesion and viscoelastic recovery, with notable high peel strength (37.2 N m^-1) on cellulose due to high polar surface energy.
• Weaker adhesion (3.8 N m^-1) was observed on low-density polyethylene (LDPE).

Conclusions:

• SCGs-derived mcl-PHA is a viable sustainable PSA candidate, particularly for polar substrates like cellulose.
• Challenges remain in achieving strong adhesion on non-polar surfaces such as LDPE.
• The ability to recover mcl-PHA via acetone dissolution supports its potential for reuse in packaging applications.