The influence of felling season and log-soaking temperature on the wetting and phenol formaldehyde adhesive bonding characteristics of birch veneer

Abstract

Most adhesive studies employing wood veneer as the substrate assume that it is a relatively uniform material if wood species and veneer thickness are constant. In the present study, veneers from rotary cut birch (Betula pendula Roth) were produced from logs harvested in spring, autumn and winter, and soaked at 20°C and 70°C prior to peeling. Firstly, veneers produced from logs felled in autumn were dried at 103°C for 24 h and subsequently half of these veneers were heat-treated at 180°C for 3 h. In addition, veneers produced from logs felled in all three seasons were dried at 160°C for 3.5 min to simulate industrial drying. The wettability of veneers was evaluated goniometrically, and bonding strength was evaluated with an automated bonding evaluation system (ABES). The results show that soaking birch logs at 70°C rather than at 20°C before peeling, or harvesting trees in the spring rather than in the autumn or winter, gives rise to veneers with enhanced wettability and higher bond strengths with a phenol-formaldehyde adhesive. Changes in the preparation and history of a veneer surface may have a profound effect on the development of adhesive bonds.

Effect of Log Soaking and the Temperature of Peeling on the Properties of Rotary-Cut Birch (Betula pendula Roth) Veneer Bonded with Phenol-Formaldehyde Adhesive

“…The color measurements showed no statistical differences between hot-and cold-peeled veneer surfaces. It is important to note that the color of both the hot-and coldpeeled surfaces in this study was lighter and less yellow than veneers that were produced from material that was soaked and peeled at 20 °C (L* = 72.92 and b* = 28.53, unpublished data from the veneers obtained by Rohumaa et al (2014) Cooling the logs prior to peeling had no effect on the surface roughness, as measured by stylus (Table 1). The roughness parameters were similar on both surfaces, and there was no statistical difference between veneer surfaces that were peeled after cooling or when hot.…”

“…All subsequent measurements were carried out on the tight side (i.e., the opposite side to the lathe checks side) of the veneer surfaces to eliminate any influence from variation in the number, depth, or form of lathe checks in the veneer sheets. The material processing used in the present study was identical to that reported by Rohumaa et al (2014); this makes it possible to compare bond strength and wettability data obtained from the present study with data on veneer properties and bonding characteristics obtained from the researchers following log soaking at 20 °C and peeling at 20 °C (Rohumaa et al 2014).…”

“…Previous studies have shown that the temperature at which a birch log is soaked affects the strength of the bond developed with a phenol-formaldehyde (PF) resin (Rohumaa et al 2014); however, it has been still unclear as to the biggest contributing factor to the surface properties of veneer that subsequently affects bonding. Is it the elevated soaking temperature that alters the chemistry of the wood, or is it the temperature at which the wood is peeled that affects the physical properties of the veneer surface by softening the material?…”

“…Taking the wood polymers above their glass transition temperatures (Tg) (Salmen 1982) reduces the cutting forces and limits premature knife damage (Bailleres et al 2015). This subsequently affects the surface properties of the veneer such as roughness (Aydin et al 2006), wettability (Rohumaa et al 2014), and integrity . Elevated soaking temperatures also affect the color of wood (Mayer and Koch 2007;Yamamoto et al 2015a).…”

“…At higher temperatures, the formation of deep lathe checks is reduced (Meriluoto 1965;Dupleix et al 2013), which is beneficial because shallower checks are less detrimental to veneer strength perpendicular to grain (Kontinen et al 1992) as well as bond quality in plywood (Rohumaa et al 2013). When heating the logs to 70 °C instead of 20 °C, bond strength increases by approximately 50% (Rohumaa et al 2014) when it is measured between veneers where the lathe checks are not present (i.e., on the 'tight' side of the veneer).…”