# Bonding of Selected Hardwoods with PVAc Adhesive
## 1 Introduction  
The strength and stability of glued joints are the priority properties of all construction and decorative composites based on metals, wood, glass, plastics, and also other traditional and modern materials. This also applies to glued solid wood products for industrial, building, and transport structures, furniture, musical instruments, sports equipment, and other uses. With regards to the glued wood products, not only is the initial strength of glued joints important, but also the stability of the joints during indoor and mainly outdoor exposures, causing one-off or cyclical changes in wood moisture and temperature. The most essential parameters influencing the overall bonding quality of wood products include the following: (a) the wood's species, density, chemical and anatomical structure, physical and strength characteristics, surface machining determining the surface roughness, grain orientation, moisture content, and pre-treatment with biocides or other additives, (b) the adhesive's chemical structure, weight solid, viscosity, surface tension, and mechanism of hardening, and (c) the bonding technology's pressure, time, and temperature [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. The low density, high permeability, and high surface roughness of the individual wood species are basic factors that play an important role in terms of better adhesive penetration depth, usually in connection with a positive impact on the bonding quality. However, [PERSON] et al. [8] found that the adhesion strength of bonded wood is not always most prominently connected with its density. When compared, experiments of severalresearchers who tested bonded hardwoods with density (\(\rho\)) in a range of 300-1000 kg/m\({}^{3}\) showed that the greater values of the adhesion strength were not in all cases found in specimens prepared from denser species (adhesion = 4.095 + 0.014 \(\rho\)/MPa/; R\({}^{2}\) = only 0.25), but almost always in those prepared from species characterized by a higher shear strength (adhesion = 0.628 + 0.912 \(\tau\)/MPa/; R\({}^{2}\) = 0.88). [PERSON] and [PERSON] [14] examined these tendencies and found out that the adhesion strength of bonded woods was greater in the denser karamatsu species than in the less dense sugi species. Similar tendencies for nine European wood species observed [PERSON] [15]. On the contrary, [PERSON] et al. [16] demonstrated a higher adhesion strength in bonded specimens made from the less dense _Paraserianthes falcataria_ tropical wood than from the denser _Acacia mangium_. Water-based adhesives, to which water dispersed adhesives also belong (e.g., polyvinyl acetate (PVAc) adhesives [17]), may reach an adhesion optimum when the water has totally penetrated into the wood substrate [18]. PVAc adhesives are commonly used in the wood industry for general assembly applications, film overlay and high-pressure lamination, edge gluing, wood veneer, and edge bonding [17]. They are safe, non-toxic, non-combustible, easy cleanable, without pollution, cure at room temperature, colorless, transparent and tough after curing, and give a high adhesion strength to bonded wood elements. [PERSON] and [PERSON] [19] determined that there was a greater adhesion strength for beech and Scotch pine woods bonded with PVAc adhesive along the tangential direction than the radial one. [PERSON] et al. [20] obtained similar results for Calabrian pine wood bonded with PVAc and polyurethane (PUR) adhesives and recommended performing the bonding process on the tangential surfaces with higher pressures. However, in spite of these results, it is well known that the penetration depth of liquid adhesives within the wood structure is influenced not only by the wood's anatomical direction, density, moisture, and final permeability, but also by the physical and chemical characteristics of the adhesive and the technological bonding conditions, such as pressure, temperature, and time. For example, [PERSON] et al.