Influence of preparation procedure on pine tannin-based cold-set glulam adhesives

Preparation of tannin-resorcinol-formaldehyde cold-set adhesives for glulam and fingerjointing by the traditional rather than the honeymoon route, while very successful for the slower-reacting polyflavonoid tannins (mimosa and quebracho) due to the comparable reactivity towards formaldehyde of both the flavonoid unit A-ring and of the resorcinol chemical added, becomes problematic for fast rective tannins. This is so because the A-rings of pine tannins procyanidins and pecan tannins prodelphinidins are much more reactive than the resorcinol chemical added and as a consequence tannin-formaldehyde condensation and gelling is the favourite reaction. Although a few encouraging solutions one of which goes a long way in partially overcoming the problem are presented and tested, the problem of a pine tannin-resorcinol adduct used as a cold set wood adhesive is that due to the tannin contributing to an excessive proportion to final cross-linking, and due to the tannin bulky molecules, early immobilisation of the network during setting occurs and as a consequence cross-linking density per unit volume remains lower than for resins prepared with the slower reacting tannins.