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Color response of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and birch (Betula pubescens) subjected to heat treatment in capillary phase B. SundqvistClearwood of Scots pine, Norway spruce, and birch was subjected to heat treatment while capillary water was kept in the wood. The treatments were performed on initially green wood from 65°C to 95°C from 0 to 6 days, followed by drying at 35°C for 2 days. Color measurements, CIEL*C*h color space, were made on dry planed samples using a photoelectric colorimeter. Treatment time was more important than temperature for birch sapwood regarding the color responses, while time and temperature were of similar importance for pine and spruce. Birch sapwood became much redder and darker compared with pine and spruce. The darkening accelerated generally when treatment temperature exceeded approximately 80°C. Pine and spruce showed generally similar color responses, untreated and treated, except for pine heartwood untreated, which showed a more saturated color. Pine treated at 65°C and 80°C showed red-yellow shift and yellow-red shift for sap-and heartwood respectively, as time elapsed. The color homogeneity was less for birch sapwood than for pine and spruce, and the homogeneity was generally indicated to decrease with increasing treatment temperature. Farbreaktion auf die Hitzebehandlung in kapillarer Phase von Kiefern, Fichten, und Birkenholz Fehlerfreies Holz von Kiefer, Fichte und Birke wurde einer Hitzebehandlung unterworfen, wobei das Wasser im Holz in kapillarer Phase vorlag. Waldfrisches Holz wurde von 0 bis 6 Tagen bei 65°C und 95°C gehalten. Danach erfolgte eine zweitägige Trocknung bei 35°C. Farbmessungen (CIEL*C*h Farbraum) erfolgten an getrockneten und gehobelten Proben mittels eines photoelektrischen Colorimeters. Die Behandlungsdauer erwies sich als wichtiger als die Temperatur für die Farbreaktion des Birkensplintholzes, während beide für Kiefern-und Fichtenholz gleichbedeutend waren. Birkensplintholz verfärbte sich erheblich röter und dunkler im Vergleich mit Kiefer und Fichte. Dieser Prozeß beschleunigte sich, wenn die Temperatur etwa 80°C überschritt. Kiefern-und Fichtenholz zeigten allgemein ähnliche Reaktionen (behandelt oder unbehandelt) mit Ausnahme des Kieferkernholzes, welches gesättigtere Farbtöne aufwies. Kiefernholz zeigte nach Behandlung bei 65°C und 80°C eine rot-gelb bzw. gelbrot-Verschiebung in Splint-und Kernholz. Die Homogenität der Verfärbung war in Birkensplinzholz geringer als bei Kiefer und Fichte. Allgemein nahm sie mit steigender Temperatur ab.

As wood construction increasingly uses engineered wood products worldwide, concerns arise about the integrity of the wood and adhesives used. Bondline strength is a crucial issue for engineered wood applications, especially in cold climates. In this study, Norway spruce (Picea abies) joints (150 mm 9 20 mm 9 10 mm) were bonded with seven commercially available adhesives: polyurethane (PUR), polyvinyl acetate (PVAc), emulsion-polymer-isocyanate (EPI), melamine-formaldehyde (MF), phenol-resorcinol-formaldehyde (PRF), melamine-urea-formaldehyde1 (MUF1), and melamine-urea-formaldehyde2 (MUF2). Each adhesive was tested at six temperatures: 20, -20, -30, -40, -50 and -60°C. Generally, within the temperature test range, temperature changes significantly affected the shear strength of solid wood and wood joints. As the temperature decreased, the shear strength decreased. PUR adhesive in most cases resulted in the strongest shear strength and MUF adhesive resulted in the weakest. MF and PRF adhesives responded to temperature changes in a similar manner to that of the PUR adhesive. The shear strengths of wood joints with PVAc and EPI adhesives were more sensitive to temperature change. At low temperatures, the variability of shear strengths increased with all adhesives. Percent wood failures of joints bonded with different adhesives in most cases were not sensitive to temperature changes.

The impact of cold temperatures on the shear strength of Scots pine (Pinus sylvestris) joints glued with seven commercially available adhesives was studied in this work. The cold temperatures investigated were: 20, −20, −30, −40, and −50 °C. Generally, within the temperature test range, the shear strength of Scots pine solid wood and wood joints were more resistant to the effect of temperature than those of Norway spruce. As the temperature decreased, only some of the joints' shear strength significantly decreased. In most cases, PUR adhesive yielded the strongest shear strength and MUF adhesive yielded the weakest shear strength. MF adhesive responded to temperature changes in a similar manner to that of PUR and PVAc adhesives. The shear strengths of wood joints with PRF and EPI adhesives were more sensitive to temperature change. For dynamic tests of shear strength, the values for 12-h and 6-day tests under temperature cycles (−20 and 0 °C) were compared. The values for 6-day tests were lower than those for 12-h tests. Therefore, the duration of the samples subjected to the same temperature had a significant impact on shear strength. Our results indicate that PUR adhesive is the most stable; whereas the stability of MUF and PRF adhesives decreased significantly.