The effect of surface preparation on the adhesion properties of reclaimed and fresh pine wood

This study examined the adhesion strength (or stress at failure), extensional behavior, and failure modes of fresh, reclaimed, and hybrid (fresh-reclaimed) Scots pine (Pinus sylvestris L.) adhesive joints prepared under two conditions, planed and sanded, using polyurethane adhesive. Adhesion strength was evaluated by lap-shear testing, while relative crosshead displacement was used as an additional indicator of joint deformation. Surface morphology was characterized through 3D topography, contact-angle measurements, surface free energy analysis, and scanning electron microscopy (SEM). Mechanical testing showed sanding wood improved adhesion strength, as resulting shear stresses increased from 6.21 N/mm² to 9.76 N/mm². Recorded relative crosshead displacement increased from 0.009 mm/mm in planed fresh-fresh to 0.013 to –0.016 mm/mm in sanded and hybrid joints. However, hybrid joints showed high variance among the samples, with a coefficient of variance of 53.8%, showing load transfer instability between reclaimed and fresh wood substrates. Planed reclaimed-reclaimed joints failed via wood failure, whereas fresh-fresh joints displayed adhesive failure. Hybrid joints revealed mixed failure dominated by wood failure on the reclaimed side. The results show that wood aging affects surface topography and adhesion performance, and that sanding improves adhesion in fresh and reclaimed wood but does not affect hybrid joints in the same way.