Abstract
Flexural properties were evaluated of blockboard with spruce (Picea abies Mill) core and faces made of 2.5-mm fromager (Ceiba pentandra) veneer and 3-mm high-density fiberboard (HDF). For these two types of structures, fiber glass, jute, gauze, and cotton fabrics, were separately bonded under the face layers to improve the strength performance. Flexural properties, modulus of rupture (MOR), and modulus of elasticity (MOE) were determined under laboratory conditions. Improved values were found for MOR and MOE tested in the parallel to core grain direction compared to those perpendicular-to-grain. They were 32% to 49% (MOR) and 39% to 95% (MOE) improvements in case of veneer faces and 142% to 161% (MOR) and 134% to 245% (MOE) improvements in case of HDF faces. The best results of MOR and MOE were obtained for glass fiber used as insertion material, the higher ones being reached for specimens tested in the parallel direction to grain, which were 56.1 N/mm2 (MOR) and 6704 N/mm2 (MOE) for HDF faces. Generally, the improvements were more evident on the blockboard structures with veneer faces oriented perpendicular-to-core grain (30% for MOR and 18% MOE) and for HDF faces with parallel core grain orientation (16% for MOR and 6% MOE).
Download PDF
Full Article
Flexural Properties of Blockboard Reinforced with Glass Fiber and Various Types of Fabrics
Mihai Ispas, Camelia Cosereanu, Octavia Zeleniuc,* and Mihaela Porojan
Flexural properties were evaluated of blockboard with spruce (Picea abies Mill) core and faces made of 2.5-mm fromager (Ceiba pentandra) veneer and 3-mm high-density fiberboard (HDF). For these two types of structures, fiber glass, jute, gauze, and cotton fabrics, were separately bonded under the face layers to improve the strength performance. Flexural properties, modulus of rupture (MOR), and modulus of elasticity (MOE) were determined under laboratory conditions. Improved values were found for MOR and MOE tested in the parallel to core grain direction compared to those perpendicular-to-grain. They were 32% to 49% (MOR) and 39% to 95% (MOE) improvements in case of veneer faces and 142% to 161% (MOR) and 134% to 245% (MOE) improvements in case of HDF faces. The best results of MOR and MOE were obtained for glass fiber used as insertion material, the higher ones being reached for specimens tested in the parallel direction to grain, which were 56.1 N/mm2 (MOR) and 6704 N/mm2 (MOE) for HDF faces. Generally, the improvements were more evident on the blockboard structures with veneer faces oriented perpendicular-to-core grain (30% for MOR and 18% MOE) and for HDF faces with parallel core grain orientation (16% for MOR and 6% MOE).
Keywords: Blockboard; Spruce core; Fromager veneer; HDF; Fabrics; Fiber glass; MOR; MOE
Contact information: Department of Wood Processing and Wooden Products Design, Transilvania University of Brasov, Faculty of Wood Engineering, B-dul Eroilor 29, 500036, Brasov, Romania; *Corresponding author: zoctavia@unitbv.ro
INTRODUCTION
Blockboard is an engineered wood product that is widely used in furniture making, where resistance to bending is required. Thus, it is preferred over plywood for long shelves or top tables. It is lightweight, exhibits high resistance to warping or twisting, is durable, and it is also suitable for interior design, such as wall panels, partitions, doors, or flooring.
In Europe, the total production of blockboard amounted to 268,000 m3 in 2004. The main blockboard producers are Germany, Italy, Poland, and Czech Republic (Bio-based News 2005). Romania became the leader in Europe with a blockboard production of 145,000 m3 in 2017, because of the investments made by Schweighofer Group.
Blockboard consists of a central layer (core) made up of solid wood strips that may contain defects undesirable for the face of the finished panels (Laufenberg et al. 2006). Otherwise, two faces from different wood-based composites, such as veneers, medium-density fiberboard (MDF), high-density fiberboard (HDF), or plywood, complete their sandwich structure, which is more stable and has a good resistance to warping. Adjacent veneers are oriented perpendicular to the core wood grains’ orientation. The thickness of the outer layers ranges from 2 mm to 3.5 mm and are applied on the core with an adhesive under high pressure. The core is usually made from low-grade logs and low-quality wood, small length timber, or timber wastes, which can increase the yield of the raw material used. Therefore, making a blockboard core represents a significant opportunity for increasing the use of unused materials (Bowyer and Stokke 1982).
Several studies have investigated the physical and mechanical properties of the blockboard made from various wood species and wood-based materials for the faces and core (Tang et al. 2001; Laufenberg et al. 2006; Gayda 2016; Teixeira and Firme de Melo 2017; Pinati et al. 2018; Nelis et al. 2019). Others studied the influence of adhesive type and content (Zanuttini and Cremonini 2002) and the type of joint (Teixeira and Firme de Melo 2017; Nazerian et al. 2018) on the mechanical properties of the blockboard. Non-destructive testing to detect defects in the blockboard structure (Wu et al. 2009; Yang and Qi 2011), laboratory decay, termite resistance evaluation (Kartal and Ayrilmis 2005), formaldehyde emission (Böhm et al. 2012), and fire performance (Laufenberg et al. 2006) tests have been also performed by researchers.
Regarding the mechanical properties, it was found that short core blocks up to approximately 20 cm could be used for blockboard manufacturing to obtain a good panel performance (Bowyer and Stokke 1982). No difference in static bending was observed for experimental panels made with core strips with and without glue on the edges (Zanuttini and Cremonini 2002). The bending strength performance increases when short slats are end-to-end mitered jointed or half-jointed instead of butt-jointed (Nazerian et al. 2018). Densities of the blockboard panels made from different cores (Paulownia, Picea abies, Pinus oocarpa, Castilla ulei, and Acrocarpus fraxinifolius) have influenced the values of bending strength and modulus of elasticity. The lighter core materials presented lower modulus of rupture (MOR), modulus of elasticity (MOE), and screw withdrawal resistance compared to pine (Pinati et al. 2018; Nelis et al. 2019). Additionally, it was shown that MOR and MOE tested in parallel core grain orientation recorded considerably higher values than those of perpendicular core grain orientation (Tang et al. 2001; Teixeira and Firme de Melo 2017; Haseli et al. 2018).
In the blockboard production the key items are represented by the cost of the core material and also by the strength properties when compared to plywood. To overcome this, one way is to use locally available low-grade wood material resources for the core (Nazerian et al. 2018) and make a suitable dispersion of defects in all areas of the panel (Colak et al. 2007). The other way is to reduce the negative effect of the defects, such as knots or cracks, which decrease the strength of the panel, is by combining the low quality cores with more resistant faces, such as MDF (Haseli et al. 2018) or HDF.
Today, the blockboard producers are confronted with the problem of raw material availability and prices. Therefore, they should maximize yield from low-grade and scrap wood and keep the cost at a low level required by the market, sometimes these affect the mechanical performance of the blockboard.
The present research investigates the flexural properties (MOR and MOE) of the blockboard reinforced with four types of inserts under the bounded face layers. Blockboard made of spruce core covered with HDF and veneer were used as reference. The properties of experimental blockboard structures were compared with those of reference panels.
EXPERIMENTAL
The outcome of the experimental set-up was flexural strength of the proposed structures without modification of the panel’s appearance and to maintain their weight and thickness within acceptable limits. The inputs considered as constant parameters were the raw materials for the core and face layers, the panel dimensions, and the technological process. The experiment was performed in the laboratory conditions and the proposed structures (with insertion materials) were compared to the reference panels (without insertion materials) manufactured also in laboratory conditions.
Materials
All raw materials for the experimental panels’ manufacturing were provided by the Romanian manufacturer Holzindustrie Schweighofer Group (HSG) (Comanesti, Romania) and consisted of 15-mm-thick spruce (Picea abies L.) assembled cores and two types of face layers (HDF 3-mm-thick and fromager veneer 2.5-mm-thick). Urea-formaldehyde adhesive (UF) (S.C. Viromet S.A., Victoria, Romania) was used for gluing the face layers to the core. As indicated by the manufacturer, urea formaldehyde resin (82.5%) was mixed with rye flour (11.5%) and ammonium chloride (6% dry mass based on the weight of dry resin) and applied at 200 g/m2 to 250 g/m2, as shown in Table 2.
Fig. 1. Specimens prepared for the tensile strength test; a) glass fiber; b) jute fabric, and c) performance of the tensile test at the universal testing machine
The strips of the core were edge-glued side-by-side with a polyvinyl acetate resin at the manufacturer’s location. Assembled cores and face layers were provided by HSG at the size of 500 mm × 500 mm (length × width). Glass fiber ISOMAT 712 (GF) (160 g/m2 weight, acquired from Izotech Services, Bucharest, Romania) and fabrics, such as gauze (G) (48 g/m2 weight, manufactured by Europoptex, Craiova, Romania), jute (J) (330 g/m2 weight manufactured by Textila, Iasi, Romania), and cotton (C) (155 g/m2 weight, manufactured by Sabaev, Bucharest, Romania) were used as insertion materials that were applied with UF under the faces. Prior panel manufacturing, three samples of 50 mm (W) (Figs. 1a and 1b) from each insert type were subjected to the tensile strength test (Fig. 1c) according to ISO 13934-1 (2013). The samples B1 through B3 were oriented with weft (transverse yarns inserted over-and-under the warp) along the force direction, and U1 through U3 samples were oriented with warp (longitudinal yarns) along the force direction. The results are shown in Table 1.
Table 1. Tensile Strength of the Insertion Materials Used for the Experimental Blockboard Structures
* Values in parenthesis represent the standard deviations
Table 2. Design of the Experimental Blockboards
Blockboard manufacturing
Ten types of blockboard structures were manufactured in the laboratory conditions (Table 2). Samples without inserts were used as reference for comparing the bending test results. First, the UF adhesive mixture was uniformly applied with a palette knife onto one side of the core. A sheet of HDF or veneer (oriented perpendicular to the core grain direction) was then applied. The operation was repeated for the other side of the core, and the sandwich obtained was hot-pressed at 6.5 MPa and 105 °C for 9 min (the pressing parameters were based on the HSG manufacturer recommendations). After hot-pressing, all panels were conditioned at room conditions (20 °C and 35% air relative humidity) for 72 h. For reinforcing materials applied between the core and face layers, the UF adhesive was applied both on the core and faces, so that the amount of adhesive increased, as shown in Table 2. The code of blockboard structures, their components, and mean density values are presented in Table 2. Figure 2 shows the design of the blockboard structures.
Fig. 2. Design of the manufactured blockboard structures: a) R-V; b) GF-V; c) G-V; d) J-V; e) C-V; f) R-HDF; g) GF-HDF; h) G-HDF; i) J-HDF, and j) C-HDF
Methods
Static bending properties (MOR and MOE) were tested according to EN 310 (1993). Sampling was taken from each experimental panel according to EN 326-1 (1994). Six specimens from each blockboard panel were cut and tested, three of them were parallel (Fig. 3a), and the other three perpendicular to the core grain orientation (Fig. 3b). Three panels of each structure were used, so the bending tests (MOR and MOE) were conducted on nine specimens of each type.
Fig. 3. Testing method: a) parallel and b) perpendicular to the core grain orientation
Prior to mechanical testing, the specimens with dimensions of 492 mm × 50 mm × 19 mm were conditioned for 14 days at the temperature of 20 ºC ± 2 ºC and 65% ± 2% relative humidity to reach the equilibrium moisture content of 12%. The universal testing machine (IBX 600; IMAL, San Damaso, Italy) was used for testing the samples. The cross-head speed of the equipment was adjusted so that the failure occurred within an average of 60 s ± 10 s.
RESULTS AND DISCUSSION
The results of flexural tests are presented in Fig. 4 for MOR and in Fig. 5 for MOE, both in parallel (//) (a, b) and perpendicular () (c, d) direction. In red are indicated the values for reference samples.
Fig. 4. MOR values: a) MOR // and veneer faces, b) MOR// and HDF faces, c) MOR and veneer faces, and d) MOR and HDF faces
The highest increase of MOR was reached by all structures reinforced with GF, followed by J. The lowest values were registered by structures reinforced with C and G fabrics. All inserts proved to be more efficient in improving bending properties when compared to both references R-V and R-HDF. This was more obvious for blockboards with veneer faces than those with HDF faces, for which MOR values parallel-to-core grains (MOR//) were 14% to 37.6% higher than R-V (Fig. 4a), whilst for HDF layers they were 0.4% to 26.6% higher than R-HDF (Fig. 4b). The same trend was noticed for MOR values perpendicular-to-core grain direction (MOR) (Figs. 4c and 4d). For veneer faces, MOR values were 21% to 36% higher than R-V (Fig. 4c), while for HDF faces they were 2.2% to 19.4% higher than R-HDF (Fig. 4d).
The MOE values demonstrated similar trends and performance to those for MOR. Generally, MOE// values were higher than MOE and increased for almost all structures when compared to the references (Fig. 5). The MOR// values were 1.32 to 1.49 times higher than MOR for blockboard with veneer faces and 2.42 to 2.61 times higher for blockboard with HDF faces. The results obtained were in agreement with other studies (Laufenberg et al. 2006; Teixeira et al. 2017; Haseli et al. 2018; Nazerian et al. 2018). Generally, the standard deviations of the results were higher for MOR than for MOE (Figs. 4 and 5). This was explained by the influence of the core strips’ defects on each specimen. Figure 6 represents the core formation (Fig. 6a) with longitudinal strips, and also their blue marking on the ends prior to blockboard manufacturing. Marks indicated the row of strips that had small lengths jointed end to end and helped the authors avoid these areas as much as possible during sampling for the bending tests.