Turning wood waste into adhesives

A substance in trees that is usually discarded by paper manufacturers has been used to create adhesives which perform as well as commercially-available products.

Pressure sensitive adhesives (PSAs) are used in objects such as labels, packaging, sticky notes, and plastic wraps. They are typically made from polymers derived from petrochemicals, but a team of researchers has developed a method of using lignin, a major component in trees and plants, to create a greener alternative.

Lignin is the most abundant source of aromatic building blocks in nature. It is a waste product in pulp and paper manufacturing that is usually discarded in landfills or burned. This makes it a renewable resource without having to cut down trees to obtain the lignin. Lignin is a natural polymer that has a similar structure and properties to petroleum-derived polymers such as polystyrene and polymethyl methacrylate which are commonly used as adhesives.

The aromatic molecules in lignin are linked by robust C–C and C–O bonds, which makes them difficult to deconstruct. Using a commercially-available catalyst, Ru/C, which is effective at C–O bond cleavage, the researchers were able to deconstruct poplar wood in methanol. The depolymerisation produced two aromatic compounds, 4-propylsyringol and 4-propylguaiacol, which were then polymerised to synthesise a PSA.

"We start with a biopolymer, and we end up with another polymer," said Dionisios Vlachos, director of the Catalysis Center for Energy Innovation (CCEI) and the Delaware Energy Institute. "Lignin is very hard, a solid part of the biomass that is the hardest to break down. Developing a catalyst and a process to actually crack these molecules is difficult."

"We can use the same separation, purification, polymerisation, and characterisation methods to make these materials as one can use to make the current commercial, and petroleum-based, analogues," said Thomas Epps, professor of materials science and engineering at the University of Delaware. "But we can get better properties, and we can use a much greener source."

Conventional tapes often have added tackifiers, which increase adhesion but decrease the lifetime of the materials. "The thing that we found a bit surprising and interesting is that our materials gave similar performance to Scotch tape and Fisherbrand tape without any additional formulation or other additives that are typically used in commercial materials to improve their performance," said Epps.

The team used lignin sourced from poplar wood, and future work will involve exploring the adhesives that can be made from lignin from other types of wood.

ACS Central Science http://doi.org/crxx