Development of bonding strength of modified birch veneers during adhesive curing

This study investigated the bonding strength development of furfurylated, N-methylol melamine (NMM) modified and thermally treated birch veneers glued with hot curing phenol formaldehyde (PF) adhesive in different pressing (20, 160 s) and open assembly times (20 s, 10 min). For testing, the automated bonding evaluation system ABES was used with 2 applied pressure at 130°C. The bonding strength of both modified and unmodified samples increased significantly by prolongation of the pressing time from 20 to 160 s in all cases and for both open assembly times. A deviation was observed for the samples treated at 220°C and at 20 s open assembly time. With the exception of NMM modified veneers, bonding strength did not change significantly by increasing the assembly time in the case of 20 s pressing for both modified and unmodified samples. At 160 s pressing time, extension of the assembly time developed a better bonding for controls, NMM modified and thermally treated veneers at 180°C. The combination of 10 min assembly time and 160 s pressing time proved as the optimal bonding condition for controls, NMM modified and thermally treated veneers at 180°C while the highest bonding strength was noted in 20 assembly time and 160 s pressing time for furfurylated veneers. In most of the cases modification affected negatively the bonding performance of the veneers, in particular for furfurylated and NMM modified samples.

The effect of combined melamine-resin-colouring agent modification on water related properties of beech wood

Beech wood (Fagus sylvatica L.) was modified with aqueous solutions of methylated N-methylol melamine (NMM) and a metal-complex dye and the water related properties were determined. Wood blocks, treated to the highest weight percent gain (WPG), attained approximately 5 % cell wall bulking and 30 % anti-swelling efficiency (ASE) after ten cycles of water saturation and drying. The metal-complex dye was stably fixed in the resin matrix and was hardly washed out. The equilibrium moisture content of the modified samples related to the dry mass of untreated beech wood (EMCR) was not considerably reduced compared to the EMC of the control. The maximum swelling of the modified samples as a result of vapour sorption was only reduced above 65 % relative humidity compared to the control. Capillary water uptake of wood was significantly reduced by the resin modification. The results indicate the potential of the combined modification to improve the water related properties of wood.

Performance of coated and uncoated horizontal lap-joint members during 20 years of outdoor exposure

Horizontal lap-joint trials were set up using eleven different wood species representing a wide range of natural durability. Coated and uncoated lap-joint specimens as well as non-jointed reference specimens were exposed for up to 20 years and evaluated with respect to decay, formation of cracks and performance of the coating. The tropical wood species Tatajuba, Cedrorana, and Dark Red Meranti performed still very well and also some Europe-grown softwoods with coloured heardwood were still in good shape. The lap-joint set up turned out to be a method that can be used also for determining the durability and performance of untreated naturally durable wood, but suffered from several drawbacks such as time-consuming and costly specimen preparation, difficult to detect onset of decay, and generally long exposure times needed for a reliable durability assessment. Cracks were often the starting point for internal decay, but did not exclusively occur in the lap area.

Acetylation of plantation softwood without catalysts or solvents

This study explored acetylation of wood of Larix kaempferi (Lamb.) Carr. and Pinus sylvestris var. mongolica Litv. without catalysts or solvents. Both wood samples were impregnated with acetic anhydride and subsequently heated to 120°C for different reaction durations (0.5-8 h) in the esterification reagent. The extent of acetylation was measured by weight percent gain (WPG), which varied from 12.0% to 21.7% and 13.6% to 22.3% for both wood species. The cell wall bulking and anti-shrink efficiency (ASE) started to increase faster and then increase slower with increasing reaction time. As the WPG reached 19.2% and 17.8% or more separately, ASE of both acetylated wood were above 50% in any RH conditions. FTIR, CP/MAS 13C NMR, and XPS studies produced evidences for acetylation of both wood species. The degree of acetylation of wood cell wall polymers increased with increasing WPG, but during the process degradation of lignin and acid hydrolysis of carbohydrates occurred.