Effect of knot types on the quality of Eucalyptus urophylla×E. grandis veneer

To explore the effect of knot types on the quality of Eucalyptus urophylla×E. grandis veneer, and to provide a theoretical basis for improving the rational and efficient utilization of Eucalyptus urophylla×E. grandis wood and improving product quality.

Using 6-year-old Eucalyptus urophylla×E. grandis clear wood and knots wood as raw materials, the veneers were rotary-cut and processed. Thickness, back crack rates, surface roughness, wettability and tensile strength of the veneers were measured respectively, and tensile test of the veneers was carried out by digital image correlation (DIC) technology to analyze the effect of knot types on the quality of the veneers.

1) The actual average thicknesses of knotless, live knot, and dead knot veneers were 1.70, 1.71 and 1.72 mm, respectively. The average thickness of knotted veneers was greater than that of knotless veneers; 2) The average back crack rates of knotless, live knot, and dead knot veneers were 35.11%, 44.97% and 63.27%, respectively. Compared with the knotless veneers, knotted veneers exhibited a higher number of back cracks, greater crack depth, and an increased back crack rate; 3) The arithmetic mean deviation of the profile (Ra) and the maximum height of the profile (Rz) were selected as the parameters for evaluating the surface roughness. The Ra of knotless, live knot, and dead knot parts were 49.22, 80.43 and 120.00 μm, respectively, and the Rz were 221.89, 470.02 and 753.78 μm, respectively. The surface roughness of knotted parts was significantly higher than that of knotless parts; 4) Knots affect the wettability of veneers. The average contact angles of urea-formaldehyde resin (UF) on knotless, live knot, and dead knot parts within 1-60 s were 62.01°, 100.81°, and 55.41°, respectively. Compared with the knotless parts, the wettability of live knot parts decreased, while that of dead knot parts increased. 5) The average longitudinal tensile strengths of knotless, live knot, and dead knot veneers were 60.78, 22.31, and 16.21 MPa, respectively, and the average transverse tensile strengths were 0.57, 0.48, and 0.43 MPa, respectively. The tensile strength of knotted veneers was significantly lower than that of knotless veneers, and the decrease range of longitudinal tensile strength was significantly higher than that of transverse tensile strength.

Knots have a significant negative impact on the quality of veneer, and the influence of dead knots is greater than that of live knots.