RAPID-CURING LIGNIN-BASED EXTERIOR WOOD ADHESIVES .2. ESTERS ACCELERATION MECHANISM AND APPLICATION TO PANEL PRODUCTS

The accelerating effect of organic esters such as triacetin (glycerol triacetate), propylene carbonate, vinyl acetate and methyl formate on the curing of aqueous solutions of methylolated wood kraft lignin are shown by gel times determination. The mechanism involved appeared to be the same as already reported for synthetic phenol-formaldehyde resins. A difference noticed when methylolated lignin is involved is the presence first of partial methylolation at the meta position of the aromatic nuclei of the phenylpropane units. The ester species causing the additional cross-linking mechanism also appeared from model compounds studies to favour a meta attack when substituted phenolic nuclei are used. The MDI (4,4'-diphenylmethane diisocyanate) and ester acceleration mechanisms were shown not to depend from each other, and thus involved cross-linking through different reactive groups of the resin. Excellent exterior grade particleboard were prepared using MDI/PF/MKL/ester combinations, thus combining the MDI and ester acceleration effect, at wood kraft lignin resin solids contents of between 46% and 63%, and at pressing times much faster than those used for traditional synthetic PF resins. 40/60 PF/MKL adhesives, without MDI, and relying only on the ester acceleration also appeared to give good results but their pressing times were slower than those of the other formulations.