# Bonding of Selected Hardwoods with PVAc Adhesive
## 1 Introduction
[1] found better penetration of the water-based urea-formaldehyde (UF) adhesive into beech wood in the tangential direction at pressure application, while no significant difference between penetrations in the tangential and radial directions occurred when pressure was not applied. The roughness of wood surfaces depends, first of all, on the wood anatomy and the mode of its machining [19; 21; 22; 23]. Recognition and quantification of the surface roughness are important from the viewpoint of wood bonding and surface treatment, as the wood surface morphology significantly influences the wood wetting with film-forming materials and the adhesion of these materials to the wood substrate. The circular rotary saw usually causes a higher surface roughness of woods, in comparison with their planning or sanding [22; 23]. [PERSON] and [PERSON] [14] inspected four Japanese wood species--sugi, hinoki, hiba, and karamatsu--and determined that their adhesion strengths with the PVAc adhesive achieved greater values if their surfaces were pre-finished with 80-grit sandpaper, compared to those surfaces pre-finished with finer 120- and 240-grit sizes. [PERSON] et al. [20] found out that a greater roughness of Calabrian pine wood surfaces was caused by their machining in this order, from most to least influential: sawing with a circular ripsaw, sanding, and planning. The shear strengths of specimens bonded with the PVAc adhesive were better for sanded or sawed wood surfaces compared with planned ones. [PERSON] et al. [5; 24] also documented the increased roughness of wood surfaces resulting from using sandpapers with lower grit sizes and the better wood surface adhesion with the PVAc adhesive. However, the experiments of [PERSON] and [PERSON] [19] showed that higher adhesion with various adhesive types had smoother planed wood surfaces than those prepared by band or circular sawing. The high polarity and good wettability of wood surfaces is given mainly by the presence of hydroxyl, carbonyl, and carboxyl groups in the lignin-polysaccharide matrix of the cell walls. This results in the formation of strong physical bonds with various polar adhesives. Wood surfaces with higher polarities are more wettable with water-basedadhesives [16; 25]. The consequence is a higher penetration of the adhesives through the lumens of cell elements on the wood surface. However, the penetration rate can partly be limited by the formation of Van der Waals interactions, dipolar interactions, and hydrogen bonds of polar adhesives with the lignin-polysaccharide matrix of wood cell walls [26]. The wettability of wood, stability of the adhesive systems, and quality of the final adhesion can negatively or positively be influenced by wood extractives and also by preservatives or other excipients added to the wood [27; 28; 29; 30]. Polar and nonpolar extractives play a major role in wood bonding processes, as they can contribute to or determine the relevant bonding properties of wood, such as acidity (pH value), wettability (contact angle, surface free energy), or even permeability (clogging of lumens by crystals). Extractives of a high acidity accelerate the curing of acid curing urea-formaldehyde(UF) and melamine-urea-formaldehyde(MUF) resins, decelerate bonding with alkaline hardening phenol-formaldehyde(PF) resins, or degrade PUR adhesives [9; 31; 11]. Starch and monomeric sugars, which belong to the primary polar water-soluble extractives present in all wood species, have a negative effect on the bonding of wood with cement, MUF, and PF adhesives [11; 31]. Secondary extractives specifically occur in various hardwood and softwood species. These extractives, which are typically situated in the heart zones of some European and several tropical hardwood species, contain either various polar polyphenols with a hydrophilic character, such as flavonoids, tannins, sterols, flobafenes, rubrenolide, rubrynolide, and quinones or coumarins, as well as various nonpolar or semi-polar waxes, fats, and oils with a hydrophobic characteristic [32; 33; 34; 35].