Lignin useful in various fossil free glues

Professor Mojgan Nejad from Michigan State University (MSU) and her team have, in close collaboration with global adhesive producers, developed various lignin-based adhesives. In a webinar at the BioGlue competence centre she presented three promising technologies for utilizing lignin in adhesives.

Phenolic Adhesives

In North America, it is still common to use phenol-formaldehyde resin when producing plywood or OSB boards. Dr. Nejad’s group was the first to fully replace phenol with unmodified lignin in the formulation of phenolic adhesive. The resulting resin had 50-80% less formaldehyde by weight. Later, in collaboration with Hexion, they utilized kraft softwood lignin produced by West Fraser to create lignin-based resin, focusing on methods to reduce its curing time.

“We were seeing 100% wood failure after the lap-shear test, and this is exactly what is required from a high-performance adhesive,” says Nejad.

They subsequently replaced 100% of formaldehyde with a bio-based dialdehyde (glyoxal) to formulate a fully bio-based phenolic adhesive.

Although the lignin-glyoxal adhesive demonstrated good water resistance, it disintegrated when soaked in boiling water, as required by the plywood standard associations in the U.S., which stipulate that the board must withstand soaking in boiling water for four hours.

“That's why we recommend this fully bio-based adhesive for interior-grade plywood,” she says.

Polyurethane Adhesives

Mass timber products, such as cross-laminated timber (CLT) and glulam, are manufactured using polyurethane adhesives. Dr. Nejad’s group has developed a lignin-based polyurethane adhesive. Their patented technology (2018) demonstrated that they can replace 100% of the petroleum-based polyols with what they refer to as liquid lignin polyols, which will then be used to prepare two-component (2K) PU adhesives.

“All the formulated lignin-based polyurethane adhesives are outperforming commercial PU adhesives in terms of both wet and dry adhesion strengths. This adhesive also had excellent adhesion to both plastics (HDPE) and aluminum,” she says.

Epoxy Resins

Dr. Nejad’s lab has also developed a non-toxic, bisphenol A-free lignin-based epoxy resin using unmodified, commercially available lignins, along with bio-based epichlorohydrin (ECH) from AGC VinyThai, to prepare epoxidized lignin. This epoxidized lignin can later be cured with a bio-based curing agent made from cashew nutshell. This fully bio-based epoxy resin could be used for adhesives, coatings, and bio-composite applications.

“When we added 25% bamboo fiber to this fully bio-based epoxy resin, the lignin-based epoxy composite performed significantly better than the BPA-based composites,” says Nejad.