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Ligneous resole adhesives for exterior-grade plywood
- Ghorbani, Masoumeh, Liebner, Falk, van Herwijnen, HendrikusW. G., Solt, Pia, Konnerth, Johannes
- European journal of wood and wood products 2018 v.76 no.1 pp. 251-258
- Fagus, Lewis acids, adhesives, bending strength, cooking, formaldehyde, lignin, manufacturing, monitoring, phenol, plywood, resins, shear strength, viscosity
- 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.