Lignin-containing cellulose nanofibril application in pMDI wood adhesives

Recently, reinforced and functional polymers have attracted research attention owing to their application in numerous areas. Depending on the desired applications and material properties, different functional polymers have been developed. In particular, cellulose nanofibrils have been extensively used in the development of polymer materials owing to their excellent properties. Usually, production of the cellulose nanofibrils requires lignin removal to facilitate a thorough fibrillation of cellulose fibers to cellulose nanofibrils given that lignin acts as a binder between fibrils. Nonetheless, recent research shows that having some lignin component remain is advantageous in lowing the impact on the environment, cutting down the production cost and optimizing the performance efficiency (enhancing the thermal stability and increasing the compatibility with hydrophobic polymers). Therefore, lignin-containing cellulose nanofibrils (LCNF) without completely removing lignin have been developed in this context.

While a stringent regulation has been applied on the use of formaldehyde-based commercial wood adhesives taking into consideration of the potential harmful impact of formaldehyde, polymeric diphenylmethane diisocyanate (pMDI) is identified as a promising formaldehyde-free solution. Despite its advantages, several challenges especially during the manufacturing process, render pMDI wood adhesives unsuitable for applications that require a continuous bondline. Even though studies regarding enhancement of the mechanical performance of adhesives is well covered in existing literature, the effect of LCNF on pMDI wood adhesives remains a research area.

To this note, University of Toronto researchers: Heyu Chen (PhD candidate), Dr. Sandeep Nair, Dr. Prashant Chauhan and led by distinguished professor, Prof. Ning Yan, investigated the effect of LCNF on the reinforcing performance of pMDI wood adhesives. The authors tested either solvent-exchange or freeze-dried LCNF aqueous suspension before mixing with the pMDI. The solvent-exchange helped in replacing water with low polarity organic solvent while the freeze-drying helped in removing water from the aqueous solution without losing the original fibril structure. Specifically, 3wt% of solvent-exchanged LCNF and 3wt% of freeze-dried LCNF were added to the pMDI, respectively. Eventually, the performance of the two methods was compared based on the bonding performance of the resulting adhesives. The work is published in Chemical Engineering Journal.

For the solvent-exchanged and freeze-dried method, Prof. Ning Yan and her research team observed enhanced dry bonding strength by 109% and 69% and wet bonding strength by 119% and 84% respectively. This was attributed to the highly reactive nature and reaction accessibility of the lignin on the nanocellulose fibril. Furthermore, it was necessary to conduct curing kinetics analysis. Curing pMDI with LCNF required higher activation energy than that with the moisture in wood. The fact that neat pMDI performed poorly in applications require adhesive retention at the bonding interface was due to the inconsistent and partially starved bondline. However, this pMDI limitation was effectively solved with a small amount of LCNF addition by drastically enhancing the gap-filling properties thus forming continuous bondline. Therefore, sustainable LCNF from renewable biomass will advance the development of high-performance pMDI adhesives for wider practical applications.