Abstract
The morphology, physical, and mechanical properties were investigated for single-layer particleboard made with various proportional contents of wood and rape stalk particles glued with urea-formaldehyde resin. Fine crushed rape stalk particles were used for the experimental particleboards. The weight ratios of rape-to-wood particles were 0:100, 10:90, 30:70, 50:50, and 70:30. Mixed beech and spruce wood particles with percentages of 30% for beech wood and 70% for spruce wood were considered for the configuration. Urea-formaldehyde resin with a solid content of 66 ± 1% was added to the single-mat configuration at a level of 12%, based on the weight of the particles. Physical (density, water absorption, and thickness swelling) and mechanical (modulus of elasticity, bending strength, internal bond strength) investigations on the particleboards obtained in the laboratory conditions were conducted. The results were compared to the requirements of the EN 312 (2004) standard. In addition, morphological observation at macro- and micro-scale and vertical density profile analysis was conducted on particleboards in order to characterize the interaction between wood-rape particles and resin. Variations of the structure and density were observed relative to thickness, providing information concerning to the internal bond performance of the panels.
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Morphology, Physical, and Mechanical Properties of Particleboard Made from Rape Straw and Wood Particles Glued with Urea-Formaldehyde Resin
Camelia Cosereanu a and Camelia Cerbu b
The morphology, physical, and mechanical properties were investigated for single-layer particleboard made with various proportional contents of wood and rape stalk particles glued with urea-formaldehyde resin. Fine crushed rape stalk particles were used for the experimental particleboards. The weight ratios of rape-to-wood particles were 0:100, 10:90, 30:70, 50:50, and 70:30. Mixed beech and spruce wood particles with percentages of 30% for beech wood and 70% for spruce wood were considered for the configuration. Urea-formaldehyde resin with a solid content of 66 ± 1% was added to the single-mat configuration at a level of 12%, based on the weight of the particles. Physical (density, water absorption, and thickness swelling) and mechanical (modulus of elasticity, bending strength, internal bond strength) investigations on the particleboards obtained in the laboratory conditions were conducted. The results were compared to the requirements of the EN 312 (2004) standard. In addition, morphological observation at macro- and micro-scale and vertical density profile analysis was conducted on particleboards in order to characterize the interaction between wood-rape particles and resin. Variations of the structure and density were observed relative to thickness, providing information concerning to the internal bond performance of the panels.
Keywords: Particleboard; Rape stalk; Wood particles; Physical properties; Mechanical properties
Contact information: a: Transylvania University of Brasov, Faculty of Wood Engineering, Department of Wood Processing and Wood Products Design, 29 Eroilor Blvd, 500036 Brasov, Romania; b: Department of Mechanical Engineering, Faculty of Mechanical Engineering, Transilvania University of Brasov, 29 Eroilor Blvd, 500036, Brasov, Romania; *Corresponding author: cboieriu@unitbv.ro
INTRODUCTION
Rapeseed crop is gaining popularity every year among Romanian farmers, becoming a profitable crop due to relatively strong market viability and stable yields. With a planted area of approx. 600,000 hectares and an increased production reaching around 1750 tonnes, Romania is one of the main rapeseed cultivators and exporters in Europe according to GAIN Report RO1709 (2017). Oilseed rape is used for biodiesel production. At present, rape straws are used for burning in heat or electricity production, for animal bedding or composting if the plants are not chemically treated. Habashescu and Cerempei (2012) showed that two thirds of the harvested rape plants are represented by stems and leaves, and only one third by seeds. Thus, rape straw (RS) shows considerable promise as crop residue biomass.
Chemical content of rape straw stems consists of cellulose, hemicellulose, and lignin (Dziurka et al. 2005; Potůček and Milichovský 2011; Huang et al. 2016), similar to wood (Szczepkowski et al. 2007). This is the reason that much research is conducted nowadays for using this renewable resource as an alternative to wood in particleboard and fiberboard manufacturing.
Particleboard production in Romania reached 2,800,000 cubic meters in 2017 (Faostat 2017), and wood demand for this sector is high, while great efforts are being made to save forests. In this context, particleboard manufacturers are interested to find alternative raw materials for replacing wood as much as possible, without affecting the technological process. Rape straw could be an alternative to wood as a raw material to produce boards.
In recent years, research on using rape straw to produce boards with synthetic resins was conducted. Dziurka et al. (2005) made experimental boards with 100% rape straw particles using four types of binding agents, namely urea-formaldehyde (UF), phenol-formaldehyde (PF), melamine-phenol-formaldehyde (MPF), and diphenyl diisocyanate (pMDI). The best mechanical performance was obtained for the panels glued with pMDI (8% and 10%). The other three adhesives with ratios of 12% and 14% recorded values in the limits imposed by standard EN 312 (2004) for panels of general use.
Another research direction (Dukarska et al. 2006) was conducted using mixed wood (W) and rape straw (RS) particles in various ratios for single layer particleboard preparation. Three types of adhesives, i.e., PF (10%), MPF (10%), and pMDI (8%) were used for the experimental panels. Even if the results of mechanical tests were satisfactory for rape straw particles ratio up to 75% and all types of adhesives, water absorption and thickness swelling after 24-h water immersion were not in the upper limits of standard EN 312 (2004).
Dziurka and Mirski (2013) investigated low density panels (350 kg/m3 to 550 kg/m3) made from beech (1.7 mm thick) veneered rape straw particleboards using pMDI. Only panels with a higher density (550 kg/m3) fulfilled all the limits required by EN 312 (2004); the other structures recorded values below accepted limits for bending strength tests. Other boards made from wood chips, rape straw, and expanded polystyrene bonded with MUF resin and having a density of 600 kg/m3 met the mechanical requirements for boards intended for interior design and furniture (Dziurka et al. 2015). Dukarska et al. (2017) studied the possibility of using RS particles glued with hybrid pMDI/PF resin in the weight ratio 70:30. Boards with densities between 450 kg/m3 and 650 kg/m3, improved strength properties and water resistance. Three-layer particleboards made in semi-industrial conditions from rape straw (50% of the core) showed enhanced technological properties (Kowaluk et al. 2007) in terms of reduced friction forces during the cutting process.
Even if pMDI resins proved to be the most suitable for gluing straws because they are able to absorb these adhesives, creating a good adherence at the bonding interface (Mo et al. 2001; Boquillon et al. 2004), they have the great disadvantage of being highly toxic if inhaled, thus affecting the respiratory system and causing skin allergies. Long-term exposure to these adhesives causes chronic respiratory diseases and asthma (Tan 2012). Thus, even though isocyanate has the advantage of reducing formaldehyde emissions when it is incorporated as a binder, it has other drawbacks affecting human health, especially when it is used on an industrial scale.
The objective of the present paper is to provide information about the optimum proportional content of rape straw into the single mat configuration of wood-rape particleboard bound with urea-formaldehyde resin (12 wt%) and to make inference on the performance of mechanical properties in correlation with the percentage of rape straw in the structure and on its influence on the vertical profile density (VPD) and on the morphological structure of the composite. The panels’ target density of 640 kg/m3 is similar to that of the particleboards manufactured at the industrial scale. The wood raw materials were provided by a Romanian producer and consist of 70% spruce wood particles and 30% beech wood particles. Since UF resin is still used by particleboard manufacturers on a large scale, the information provided by the present study has a practical application. In an effort to predict the mechanical performance of the composites, this work involved macro- and microscopic investigation of the cross sections of the experimental panels and at the level of the interface between particles, together with VDP analysis.
EXPERIMENTAL
Materials
Mixed beech and spruce wood particles (30% for beech wood and 70% for spruce wood) used for the experimental panels were provided by a Romanian particleboard manufacturer. The moisture content of wood particles were determined by the gravimetric method and ranged from 7.9% to 8.1%. The wood chips sizes were between 5 mm and 20 mm in length, 2 mm and 6 mm in width, and 0.5 mm to 2 mm in thickness. Rape straw stems with moisture content between 11% and 12% were cut into short pieces and crushed by hammer milling. The obtained particles were sieved using a horizontal screen shaker with sieves of 2 mm2 x 2 mm2, 1 mm2 x 1 mm2, and 0.5 mm2 x 0.5 mm2 to remove the dust and to obtain the fractions necessary for the mat. Only particles retained in the 1 mm2 x 1 mm2 and 0.5 mm2 x 0.5 mm2 sieves were used to make particleboard. A total of 87.5% of the used rape straw particles remained in the 1 mm2 x 1 mm2 sieve, and 6% of the particles remained in the 0.5 mm2 x 0.5 mm2 sieve. The remaining 6.5% of the particles were removed as dust (Fig. 1). The sizes of the rape straw particles varied from 1 mm to 5 mm in length, 0.5 mm to 2 mm in width, and 0.1 mm to 0.5 mm in thickness.