Effects of NCO:OH ratio on the mechanical properties and chemical structure of Kraft lignin–based polyurethane adhesive

ABSTRACT

This work aimed to evaluate the influence of the aliphatic and aromatic hydroxyl level on the polyurethane adhesive property and chemical structure. This adhesive was obtained through the reaction of technical Kraft lignin (TKL) as polyol with diphenylmethane diisocyanate (MDI). Thus, lignopolyurethane adhesives were obtained with NCO:OH ratios of 0.8:1.0, 0.9:1.0, 1.0:1.0, 1.1:1.0, and 1.2:1.0. Initially only the TKL aliphatic hydroxyl level was taken into consideration in the stoichiometry in order to define the mass ratio between MDI and polyol. Subsequently, lignopolyurethane adhesive was obtained using the same NCO:OH ratios considering TKL total hydroxyls’ level, and aromatic and aliphatic hydroxyls. The chemical structures of the synthesized adhesives were analyzed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (¹³C NMR). The mechanical property of the adhesively bonded joints, comprising wood substrates and synthesized adhesives, was measured using single lap shear tests. Results illustrated that by increasing the NCO:OH ratio, there is an increase in the free isocyanate content leading to higher shear strength values. Higher free isocyanate content leads to MDI dimer formation in the lignopolyurethane structure.

KEYWORDS:

• Adhesives for wood
• lignin
• polyurethane
• wood

Funding

The authors thank FAPESP (São Paulo Research Foundation) and CNPq (National Counsel of Technological and Scientific Development) and acknowledge UFABC (Federal University of ABC) for the doctoral fellowship of A.M. Nacas and N.M. Ito and financial support.

Additional information

Funding

The authors thank FAPESP (São Paulo Research Foundation) and CNPq (National Counsel of Technological and Scientific Development) and acknowledge UFABC (Federal University of ABC) for the doctoral fellowship of A.M. Nacas and N.M. Ito and financial support.