# Effect of Variable Conditions of Exposure on the Physical and Mechanical Properties of Blockboards
## 1 Introduction  
Wood-based composites are comprised of a large diversity of boards, including the blockboard category. Blockboard is considered to have aplywood structure due to its design of at least three layers (usually thin veneers for the faces and thick ones for the core). The veneers are oriented perpendicular to the core grains' orientation. The thickness of the face layers ranges from 2 mm to 3.5 mm and they are glued to the core under high pressure. However, blockboard differs from plywood as the core is made of strips of solid timber (resinous or softwood species) cut from low-grade logs or short timber, contributing, thus, to the more efficient use of wood and wood-waste recovery. Total production of plywood, blockboards and LVL together represented 40% of all wood-based panel production in 2018 with a production of 163 million m\({}^{3}\)[1], of which 145,000 m\({}^{3}\) represents only the blockboards production of Holzindustrie Schweighofer, the world's largest blockboard plant [2]. In recent years, there has been a growing demand for wood, and the price of logs has experienced a huge increase. For this reason, some of the timber industry has been forced to efficiently use existing wood resources, including the wood with defects or small-sizedwood resulting from wood processing, fast-growing wood species [3] or recycled wood, such as post-consumer wood [4] or wood from construction sites [3; 5]. In the furniture industry, there has been an increase in the demand for the manufacture of blockboards due to their structural and strength advantages. Blockboard is preferred over plywood for furniture manufacturing, especially in applications such as long shelves, top tables and side panels, because of the high resistance to warping or twisting and its durability. Blockboard is a highly appreciated, sustainable material used mainly for furniture, combining its durability with light weight. The actual research work aims to find solutions to lower the weight of blockboard by using light cores from resinous wood with various machined profiles and resistant face layers [6]. The blockboard panels are preferred by the furniture manufacturers because they can replace some solid wood elements with this material, lowering the product weight and maintaining the same properties and quality. Blockboard is also an ecological material with a very low formaldehyde emission level [7; 8]. Most studies were oriented to the study of physical and mechanical performances of blockboards constructed with various cores and faces. A part of the research works was oriented to the possibility of recycling wood resulting from construction sites for the core of blockboard with and without bonding and investigating the influence of core material on the mechanical properties of the boards [5; 9]. Innovative methods without adhesive, such as dowel welding of blockboard core strips with 0-20\({}^{\circ}\) dowel insertion angle, resulted in good mechanical properties of the boards [9]. A part of the studies investigated new designs of the structures [6; 10] and various raw materials for the core [11; 12; 13]. Other studies were oriented to the joint types used for the core stripes, to their lengths and their influence on the properties of blockboards [9; 14; 15]. There are few studies related to blockboard properties and structures. The influence of moisture content or of the relative humidity of air on the physical and mechanical properties of wood-based composites were also in the attention of researchers, but only in the case of particleboards and fibreboards [16; 17; 18]. There are also studies with regard to weathering stability and durability of plywood [19] and studies on lightweight, stabilised blockboard panels, a newly introduced product on the market, that show a decrease in internal bond and bending strength with the increase of the relative humidity of the air from 30% to 85% [6].