No abstract

Combinations of wood and metal are interesting hybrid composite materials, joining together the low density of wood with the stiffness and strength of metals. Different types of adhesives are used to connect wood and metal elements, but the compatibility between adhesives used and load-bearing materials must be sufficient, which often is challenging. In adhesive bonding technology, surface treatments are a crucial step in the process. In this study, an atmospheric plasma discharge was employed to enhance the adhesion strength of joints between common beech (Fagus sylvatica L.) wood, metals (steel and aluminum alloy), and four different types of adhesives. The optical properties of plasma discharges and its influence on treated substrates' surface morphology depended on the inherent properties of the treated materials. X-ray photoelectron spectroscopy revealed the surface oxidation of all the materials after plasma treatment. Consequently, the surface free energy of all materials increased as well. The positive effect of the plasma treatment on the tensile shear strength of single-lap joints shows a high potential of atmospheric plasma treatment technology for enhancement of adhesives strength of joints combining wooden elements, wood and steel, or wood and aluminum alloys. In addition to that, expensive epoxy and polyurethane adhesives could be replaced by more affordable polyvinyl acetate and melamine-ureaformaldehyde adhesives, and still perform at equal levels if the plasma was applied prior to bonding.

The curing process of two biobased adhesives: pine tanninhexamine (TH) and organosolv lignin non-isocyanate polyurethane (NIPU), suitable for interior nonstructural use, were compared with commercial urea-formaldehyde (UF) adhesive. Changes in chemical structure before and after the curing process were observed with Fouriertransform infrared spectroscopy (FTIR). The process of adhesive curing was monitored with differential scanning calorimetry (DSC) and the automated bonding evaluation system (ABES). Both DSC and ABES measurements confirmed UF as the fastest and NIPU as the slowest curing adhesive observed. Taking into account the ABES results, the optimal pressing parameters for the TH adhesive would be 4 min at 175°C, for the NIPU adhesive 7 min at 200°C and for the UF 1.5 min at 100°C. Strong linear correlation was observed between mechanical and chemical curing for the UF and NIPU adhesives, whereas lower correlation was observed for the TH adhesive. At all observed adhesives, the DSC measurements were underestimating the curing process determined by ABES in the first part and overestimating it at the end. The underestimation was the most evident with the TH adhesive and the less with the UF adhesive. When comparing the uncured and cured FTIR spectra of all three types of adhesives, a drastic decrease in the characteristic band of -OH groups at 3330-3400 cm −1 and an increase in the signal intensity at 2920 cm −1 of aliphatic -CH 2 -groups were observed. For the UF adhesive, the C=O stretching frequency has shifted from 1632 cm −1 for uncured to three different bands at 1766, 1701, and 1655 cm −1 for cured UF. The sharp band for phenolic alcohols at 1236 cm −1 of C-O stretch and hydroxyl O-H functional group at 1009 cm −1 and at 684 cm −1 of uncured TH adhesive diminished during curing, which indicates that a crosslinking reaction occurs via -OH groups. The peak of the C=O group of urethane bridges at 1697 cm −1 for uncured NIPU shifted to lower wavenumber at 1633 cm −1 for cured NIPU.

The characterization of the curing process allows the determination of the optimal pressing parameters, which is essential for the economical production of wood-based composites. In this study, an automated bonding evaluation system (ABES), dielectric analysis (DEA), and dynamic mechanical analysis (DMA) were used to determine the curing parameters of biobased pine tannin-hexamine adhesive at five temperatures ranging from 75 to 175 °C. This study aimed to compare the three above methods and to find correlations between them. All methods showed the same trend of the curing process, which became faster with increasing temperature. Due to various heating rates among the different methods, the curves representing the degree of cure were shifted to the left for the period in which nearly isothermal conditions were reached. It was determined that these methods could be mutually comparable. The ABES was regarded as the reference method; the DEA was regarded as a method that overestimates the curing process and that describes the beginning of the curing process more precisely; and the DMA method was regarded as a method that underestimates the curing process and that describes the end of the curing process more precisely. Linear trend lines were found between the observed methods.

A local tree-ring chronology of sessile oak (Qercus petraea) was constructed for the site Klanec pri Kozini (KLA), Slovenia (45.59° N, 13.92° E, 450 m a.s.l.) located on the Karst edge on the transition from the sub-Mediterranean climatic to temperate Continental zones. The chronology is 93 years long and covers the period 1925-2017. A comparison with four local oak chronologies from Slovenia and 38 from the surrounding countries (distance 65-220 km) showed that KLA agreed best with the local oak chronology of Rožnik, Ljubljana (ROZ), and two other oak chronologies from central Slovenia (the surroundings of Novo mesto and Ljubljana), one from Croatia and one from Austria. Dendroclimatological analysis showed that the main factor affecting tree-ring variation is June temperature (negative effect) and March precipitation (positive effect), which to a great extent explain the relation to other chronologies. The negative effect of June (maximal) temperature has become increasingly significant in the last few decades, which can be ascribed to warming related to ongoing climatic change.