Ligneous resole adhesives for exterior-grade plywood

Masoumeh Ghorbani,Hendrikus W G Van Herwijnen,Pia Solt,Johannes Konnerth,Falk Liebner

doi:10.1007/s00107-017-1249-9

Abstract

The performance of lignin–phenol–formaldehyde (LPF) resole adhesives was investigated for exterior-grade plywood manufacture using commercial pine kraft lignin to replace 40 wt% of phenol. Furthermore, the implementation of a lignin methylolation step prior to resole cooking was tested aiming to activate lignin towards the electrophilic aromatic addition of formaldehyde and to improve thus the performance of the respective adhesive. Comprehensive monitoring of the resole cooking process and characterization of the obtained resins confirmed a significant network expanding effect of lignin. This is evident from the fast increase of viscosity which occurs, however, at the expense of cross-link density, as reflected by the somewhat longer B-time measured for both types of ligneous PF resins. Evaluation of bonding strength development using a bond strength development test confirmed that the less dense network of LPF resins translates into delayed bond formation compared to the PF reference resin. Methylolation of lignin prior to resole cooking had no significant impact on bond formation as evident from the very similar behaviour during the bond strength development test. However, both the studied LPF resins turned out to be suitable adhesives for the manufacture of plywood panels for outdoor usage. Specimen prepared from beech veneers fulfilled the respective standards as determined by shear strength and bending strength tests.

Highlights

Phenol–formaldehyde (PF) resoles constitute a major class of adhesives that finds wide use in the manufacture of exterior-grade plywood, laminated veneer lumber and glue laminated timber
Lignin methylolation is frequently recommended for pine kraft lignin (Zhao et al 1994; Hu et al 2011) due to the higher content of activated aromatic moieties for electrophilic aromatic substitution at moderate sterical hindrance, and higher content of free phenolic hydroxyl groups (Tejado et al 2007; Malutan et al 2008a; Passauer et al 2011)
Earlier studies comparing the reactivity of different types of commercial lignins towards electrophilic addition of formaldehyde have shown that the obtained ligneous A-stage resoles featured significantly higher contents of free formaldehyde not consumed during resin cooking compared to their lignin-free counterparts (Ghorbani et al 2016)

Summary

Introduction

Phenol–formaldehyde (PF) resoles constitute a major class of adhesives that finds wide use in the manufacture of exterior-grade plywood, laminated veneer lumber and glue laminated timber. For the most predominant kraft process, Lora (2008) identified only one mill producing kraft lignin located in the US This situation might change as a couple of mills have announced serious interest in providing kraft lignin, and lignin extraction technologies on an industrial scale become commercially available (e.g. LignoBoost technology, Valmet, Espoo, Finland). It is well known that lignin is inferior to phenol with regard to reactivity (Pizzi 2013), i.e. towards electrophilic aromatic addition of formaldehyde. This is mainly due to fewer reactive sites per aromatic moieties available in lignin, lower amount of free phenolic groups and the sterical hindrance of lignin molecules that altogether

Methods

Results

Conclusion