Abstract
Lignin phenol formaldehyde (LPF) resols were produced using depolymerized lignin fractions at various levels of phenol substitution (50 to 70 wt %). To produce monomeric-rich (BCD-oil) and oligomeric (BCD-oligomers) bio-based phenolic compounds, softwood kraft lignin was base-catalysed degraded. These base-catalysed depolymerized (BCD) building blocks were further used to substitute phenol in the synthesis of phenolic resins and were characterized in detail (such as viscosity, free formaldehyde and phenol content, chemical composition, curing and bonding behaviour). The adhesive properties were compared to a phenol formaldehyde (PF) reference resin and a LPF with untreated kraft lignin. The resins synthesized with the two depolymerized lignin types differ significantly from each other with increasing phenol substitution. While with LPF-BCD-oligomers the viscosity increases and the bonding strength is not effected by increasing lignin content in the resin, a reduction of these properties could be observed with LPF-BCD-oil. Furthermore, LPF-BCD-oil showed similar curing behaviour and ultimate strength as the reference LPF. Adhesive bonds made using LPF-BCD-oligomers exhibited similar strength to those made using PF. Compared to the reference resins, it has been demonstrated that modified renewable lignin based phenolic components can be an equally performing alternative to phenol even for high degrees of substitution of 70%.
Highlights
Synthetic resins such as formaldehyde-based polycondensation adhesive systems are of great importance for the production of wood-based materials
The goal of this study is to describe the influence of base-catalysed depolymerized kraft lignin (BCD) on the resin properties of lignin-based phenolic resins
Gravimetric evaluation of isolated base-catalysed lignin fractions resulted in a recovery of 16 kg BCD-oligomers (40 wt % of lignin) and 6 kg base-catalysed lignin oil (BCD-oil) (15 wt % of lignin) out of 40 kg precipitated kraft lignin after pilot scale depolymerisation, separation and purification
Summary
Synthetic resins such as formaldehyde-based polycondensation adhesive systems are of great importance for the production of wood-based materials. A clear increase in research activity to improve the share of adhesives from bio-resources is evident [1]. This transition is reflected in the prioritization of conferences such as the international conference on wood adhesives (Atlanta, GA, USA, 2017) [2]. Lignin—present in all vascular terrestrial plants—is a complex, amorphous polymer with phenylpropane units linked together by carbon–carbon or ether bonds. It is the second major compound in wood following cellulose and represents 30% of all non-fossil organic carbon on
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