Kraft lignin-based polyurethanes: Bulk properties, stability and adhesion to native aluminum surfaces

Abstract

Lignin as a byproduct from pulp and paper manufacturing has been extensively investigated as raw material for polymer developments. Although, fundamental topics related to the adhesion mechanisms of lignin-based reactive polyurethanes (LPU) still remain unclear. In this work, LPU thin films on native aluminum surface were prepared and characterized. Diluted in THF, employed as the solvent, reactive mixtures of 4,4′-methylene diphenyl isocyanate (4,4′-MDI) and the soluble fraction of three lignins (liquid hydroxypropylated Kraft lignin and two powder Kraft lignins with different pH) were used for LPU thin film deposition via spin coating on aluminum (PVD layer on silicon wafer). The resulting film thickness ranged from 8 nm to several micrometers. The chemical state of the three LPU compositions was assessed in bulk by infrared attenuated total reflectance (IR-ATR) and in the thin films by infrared external reflection absorption spectroscopy (IR-ERAS). Binding energies of 4,4′-methylene diphenyl diisocyanate with aliphatic and aromatic hydroxyl groups were estimated using Density Functional Theory (DFT) simulations. Thus, besides the elucidation of the bulk chemical state of LPUs, the relation to adhesion and stability of LPUs to a native aluminum surface was evaluated. In addition, film topography and homogeneity were monitored by SFM. All lignin types form uniform and homogeneous films. Results revealed a higher consumption of isocyanate groups (NCO) in the formulation with the alkaline Kraft lignin than with the acid one, despite its lower hydroxyl content. A gradient of unreacted NCO was observed in LPU thin films obtained with the powder Kraft lignin types. Residual NCO is also found in thicker films, while the thinnest films did not contain unreacted NCO anymore, indicating the activating effect of the native aluminum surface on LPU formation.