Combined effects of optimized face milling and surfactant-based primers on the delamination resistance of one-component polyurethane bonds in beech, Douglas fir and larch

Bonding emerging Central-European timber species with one-component polyurethane (1C-PUR) adhesives is limited by poor delamination resistance under the wet conditions of EN 302-2. This study quantified how optimized face milling, compared with conventional peripheral milling, governs the delamination resistance of 1C-PUR bonds in European beech (Fagus sylvatica L.), Douglas fir (Pseudotsuga menziesii Franco) and European larch (Larix decidua Mill.), and assessed species-specific surfactant primers as a complementary treatment. Glulam specimens were produced from a single tree per species and tested according to EN 302-2 (n = 4 specimens, 40 bond lines per variant). Replacing peripheral milling with optimized face milling reduced delamination from 85.4% to 7.3% in beech, from 32.2% to 4.8% in larch and from 13.4% to 1.1% in Douglas fir; a two-way analysis of variance confirmed significant effects of species, machining method and their interaction (p < 0.001). Face milling alone brought Douglas fir and larch below the 5% normative threshold. Adding a species-specific primer (polysorbate-based for beech and Douglas fir, polyethylene-glycol-based for larch) further reduced delamination to 0.2% (beech), 0.5% (Douglas fir) and 3.7% (larch). Fluorescence microscopy showed deeper adhesive penetration after face milling than after peripheral milling, consistent with the improved bond durability.