In this study, beech wood (Fagus silvatica L.) has been chemically modified with citric acid (Acidum citricum) and sodium hypophosphate (SHP) as the catalyst and gradually thermo-condensed in the dryer. Afterwards, wetting angle, surface energy, and shear strength of glued joints of modified and unmodified wood were determined. Testing of the bond strength according to standard EN 204 and comparison between modified and unmodified samples were executed. The adhesive used for bonding samples was polyvinyl acetate (PVAC), commonly used for gluing solid wood panels. Testing material was divided into three groups (dry, wet, and wet conditioned samples), within which statistical analysis was performed, and the significance of the differences between the modified and unmodified samples was determined. Surface energy is correlated with the bond strength, indicating that modification with citric acid negatively affects the adhesive properties of beech wood. A reduction in the bond strength of modified wood glued with PVAC glue compared to unmodified wood was determined. All the results indicate that the modified samples do not meet the minimum requirements for EN 204 bonded with PVAC glue. Therefore, it will be necessary to conduct further studies using other types of adhesives to investigate whether modified wood might be suitable for gluing.

This study analysed the bonding mechanisms and strength between wood and non-wood implants in producing laminated oak-wood beams. The suitability of different types of adhesives, namely for load-bearing and general purpose, was also analysed. Three different types of non-wood implants—carbon fibres, glass fibres, and aluminium were glued with epoxy resin (ER), thermoplastic 1-k polyurethane adhesives (PUR), structural polyurethane adhesives (PUR 2 and PUR 3), and polyvinyl acetate (PVAc) adhesives and bonds were tested for shear strength (SS) according to ISO 6238:2018. Results of the bond quality expressed as the ultimate load to failure and displacement were recorded using the universal mechanical testing machine in combination with the digital image correlation (DIC) method. Before the shear test, all the samples were conditioned in dry and wet climatic conditions. Test results indicated that the application of PUR adhesives for bonding carbon and glass fibres with oak wood could sufficiently replace two-component ER, which is generally recommended for such purposes but is very challenging to utilise in industrial conditions. PVAc adhesives proved efficient only for combination with AL implants and in dry conditions. Aluminium sheets were shown to require surface pre-treatment, such as sanding and degreasing or a different type of adhesive to achieve sufficient adhesion.

This paper presents the influence of natural surface ageing in indoor conditions on bonding quality of thermally modified wood used in structural laminated products. Two unmodified and thermally modified wood species were used for the experiment: oak and beech. Samples were planed and glued with MUF adhesive 2 hours, 1, 2, 6, 10, and 18 days after planing. Properties of laminated beech and oak beams, namely shear strength, delamination and contact angle, were measured in order to detect 1) suitability of wood species for lamination process and 2) influence of extended storage time after planing on properties of laminated wood. Generally, both native and thermally modified beech exhibited better results of shear strength and delamination and had lower contact angles compared to oak and thermally modified oak. Results of the delamination test (total delamination) indicate time dependence of surface ageing. Both unmodified and thermally modified beech may be successfully laminated at least up to 2 days after planing, whereas neither oak nor thermally modified oak are suitable for lamination process due to excessive delamination. Results of delamination may be related to contact angle measurements. Shear strength of glue lines did not show any influence on natural surface ageing. However, whereas beech and thermally modified beech samples exhibited almost the same values of the shear strength regardless of the duration of surface ageing, there is an obvious difference in shear strength of oak and thermally modified oak samples.

Steaming beech wood is one of the most commonly used eco-based processes for wood color equilibration. In addition to color equalization, steaming has also been noticed to stabilize the final product (solid wood panels). The beech wood samples used in this study were steamed for two different periods. PVAc and polyurethane (PU) adhesives were used to analyze bonding strength and durability. The bonding strength was measured according to the EN 13354 standard. The samples were treated before testing according to the first part of the standard, i.e., immersion in water. The durability of the bonded joint was tested according to the ISO 9142 standard. The samples were treated before testing with two methods. The results of the bonding strength show the influence of the steaming process on the bonded joint. Short exposure to steam decreased bonding strength, and prolonged exposure increased bonding strength. From the results given and the statistical analysis, it can be concluded that a prolonged steaming period increases the stability of the beech wood and thus the durability of the bonded joints.

Various surface modification techniques have been developed to improve synthetic polymer surfaces’ wetting, adhesion, and printing by adding various functional (polar) groups. UV irradiation has been proposed as a suitable procedure to achieve adequate surface modifications of such polymers, which can be of further use to bond many compounds of interest. The activation of the surface, the favourable wetting properties, and the increased micro tensile strength of the substrate after short-term UV irradiation suggest that such pretreatment can also improve the bonding of the wood-glue system. Thus, this study aims to determine the feasibility of UV irradiation for pretreatment of wood surfaces before gluing and to determine the properties of wooden glued joints prepared in this way. UV irradiation was used to modify variously machined pieces of beech wood (Fagus sylvatica L.) before gluing. Six sets of samples were prepared for each machining process. Samples prepared in this manner were exposed to irradiation on a UV line. Each radiation level had a certain number of passes through the UV line, the more passes, the stronger the irradiation. Thus, the radiation levels were as follows: 1, 5, 10, 20, and 50 passes. The dose (energy delivered on the wood surface) in one pass was 2.36 J/cm2. A wetting angle test with glue, a compressive shear strength test of lap joints, and designation of main failure patterns were used to evaluate the properties of wooden glued joints. Wetting angle test was performed according to EN 828, while the compressive shear strength test samples were prepared and tested following the ISO 6238 standard. The tests were conducted using a polyvinyl acetate adhesive. The study found that UV irradiation before gluing improved the bonding properties of variously machined wood.

SAŽETAK • U konstrukcijama namještaja ugaoni se sklopovi često izvode sastavljanjem uz pomoć dvonavojnih ("trio")

The use of resins, whose curing reaction takes place by high temperature and hardener addition, is inevitably involved in particleboard manufacturing process. In addition to commercial hardeners, such as ammonium sulphate, with the aim of optimizing the production process and reducing the production costs, a certain percentage of hardener can, among other things, be replaced with price affordable bio-based materials. Tartaric acid, that is its salts (tartrates), which are commercially produced for the needs of wine and food industries, are a part of the aforementioned group of products. Since tartaric acid is a relatively inexpensive, readily available, weak diprotic and aldaric acid, the question arises whether it can be used as a component of the hardener system for curing urea-formaldehyde resins that are commercially used in particleboard production. For that reason, in this paper, the influence of partial replacement of ammonium sulphate hardener with tartaric acid on the mechanical properties (bending strength, modulus of elasticity and internal bond) and free formaldehyde content of experimentally produced particleboards was examined. Boards thickness, density and moisture content were also determined. The test results suggest that tartaric acid has a beneficial effect on the above particleboard properties, but they also indicate that the extent of that effect is strongly dependent on panel press time.

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