Abstract
The suitability of using milled sunflower husks as a wood substitute for producing medium-density particleboard was investigated. Additionally, the impact of the adhesive type and the amount used on the properties of the panels were evaluated. Urea-formaldehyde (UF) in three commercial variants (UCL, U96, and AG), phenol-formaldehyde (PF), modified melamine urea-formaldehyde (VM), and polymeric diphenylmethane diisocyanate (pMDI), as well as mixtures of VM/AG and of PF/pMDI, were used to manufacture the panels. The adhesive content was varied between 3% and 6% for pMDI, and from 9% and 12% for the other adhesives. Higher thickness swelling (TS) and water absorption (WA) values were observed with the UF panels compared with the PF and pMDI panels. The lowest mechanical strength properties were observed for the UF panels, with the commercial variants ranking (from highest to lowest): UCL > VM/AG > U96. Increasing the adhesive content level resulted in better dimensional stabilities and mechanical properties for the pMDI and PF panels, which met some of the performance requirements for interior uses prescribed by the relevant standard.
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Influence of Adhesive Type and Content on the Properties of Particleboard Made from Sunflower Husks
Octavia Zeleniuc, Luminita-Maria Brenci,* Camelia Cosereanu, and Adriana Fotin
The suitability of using milled sunflower husks as a wood substitute for producing medium-density particleboard was investigated. Additionally, the impact of the adhesive type and the amount used on the properties of the panels were evaluated. Urea-formaldehyde (UF) in three commercial variants (UCL, U96, and AG), phenol-formaldehyde (PF), modified melamine urea-formaldehyde (VM), and polymeric diphenylmethane diisocyanate (pMDI), as well as mixtures of VM/AG and of PF/pMDI, were used to manufacture the panels. The adhesive content was varied between 3% and 6% for pMDI, and from 9% and 12% for the other adhesives. Higher thickness swelling (TS) and water absorption (WA) values were observed with the UF panels compared with the PF and pMDI panels. The lowest mechanical strength properties were observed for the UF panels, with the commercial variants ranking (from highest to lowest): UCL > VM/AG > U96. Increasing the adhesive content level resulted in better dimensional stabilities and mechanical properties for the pMDI and PF panels, which met some of the performance requirements for interior uses prescribed by the relevant standard.
Keywords: Sunflower husks; Urea-formaldehyde; Phenol-formaldehyde; Melamine urea-formaldehyde; Polymeric diphenylmethane diisocyanate; Particleboard; Panels; Mechanical strength
Contact information: Department of Wood Processing and Wood Products Design, Faculty of Wood Engineering, University Transilvania Braşov, Braşov, Romania; *Corresponding author: brenlu@unitbv.ro
INTRODUCTION
Forests not only supply wood for the rapidly growing composite industry, but they also help to support a healthy ecosystem and a sustainable environment. Hence, it is important to preserve forest resources while at the same time to develop new building products.
Global wood panel production has increased rapidly, with a 123% increase in 2016 (416 million m3) when compared with 2000 (FAO 2017). China, the USA, Russia, Canada, and Germany represented the five largest producers and consumers, which accounted for 69% of global output in 2016 (FAO 2016). In Europe, wood-based panel production grew after 1990, from approximately 30 million m3 to 86 million m3 because of the development of new products, such as medium-density fiberboard (MDF) and oriented strand board (OSB), as well as new investments made in Eastern Europe. These investments have contributed to a 70% increase in Eastern Europe’s panel production in 2017 when compared with 2000 (FAO 2017).
Romania is one of the Eastern European countries that have made significant investments in wood-based panels after 2008. In the last few years, new capacities were developed, which increased wood panel production to 6 million m3 (2016), compared with 1.5 million m3 in 2008 (FAO 2017). This increased production has placed some strain on the existing forest resources. In the near future, it is anticipated that available forest resources will be insufficient to satisfy all the wood demands while maintaining requirements for forest sustainability. This would require the promotion of new raw materials, such as agro-waste resources.
Short rotation wood could compensate for the forest resource deficits in Romania, but the areas harvested with these plantations at the national level represent less than 5% (Nicolescu and Hernea 2018). A more preferable alternative to wood is renewable agricultural residues. Currently, Romania has the potential for high-yield agricultural production; Romania is a typical agricultural country, with 62% of the arable land area (8.8 million ha) dedicated to agricultural activities (UNECE 2012). The most important agricultural crops are maize (corn), wheat, barley, sunflower, and soybean (soya). Agricultural residue wastes, such as straw, stalks, stems, and corn cobs, have the potential to be used as raw material for wood-based composites.
In Romania the annual crop residue amounts are between 10.2 and 27.0 dry Mt/year, of which approximately 4.7 to 12.6 dry Mt/year are collected (Scarlat et al. 2011). More than 50% of these unused residues are disposed into landfills, which contributes to environmental pollution. Another proportion of the unused residuals (approximately 46%) are used in animal feed and as fuel pellets for heat production. Sunflower (Helianthus annuus L.) has relatively a short growth cycle and it is easy to adapt at different soil conditions, thus is cultivated worldwide on a surface of 26.2 million hectares, reaching a production of 47.34 million tons in 2016 (Soare and Chiurciu 2018). Sunflower is the most important agricultural crop cultivated in Romania over 1 million hectares, followed by rapeseed and soybean. Sunflower production has increased in recent years, which has made Romania a leader in the European Union (EU); it accounted for approximately 24% of the total EU production in 2016 and 2017 (i.e., 1.95 to 2.25 million metric tons) (Dobrescu 2017). In 2018 Romania has maintained its first place in the harvested sunflower production, with 1.785 million tons from 7.906 million tons of EU 28 production. At the world level, Romania contributed with 4.29% to the world production, being on the 5th place among Ukraine, Russia, Argentina and China (Eurostat 2019). The sunflower is basically used for the oil production, in EU amounting about 7.6 million tons of the crushed seed (Nazlin et al. 2017). The sunflower oil produced in Romania is expected to increase slightly, reaching 0.338 million metric tons (2017/2018) (Dobrescu 2017) and 3.800 million tons in Europe, in 2019 (Krautgartner et al. 2019). The percentage of husks in sunflower seeds varies between 10% and 30%, which depends upon the dehulling process used (Wan et al. 1979; Isobe et al. 1992; Heinrich 2017; Kumar 2018). The density of the husks is very low (212 kg/m3 at 10% moisture content) (Gamea 2013). Based on the husks ratio in the seeds it can be estimated that approximately 3.42 million tons of husks annually become available from the dehulling process in EU. As a consequence, a large area of storage is necessary for oil producers, therefore the wastes of sunflower husks could be degraded in time and pollute the environment. Generally, these husks are used for fuel pellets, briquettes, xylose extraction, fertilizer, and animal feed. The husks are high in fiber and low in protein, and therefore have a very low commercial feed value (Le Clef and Kemper 2015). Their use in the particleboard production is very scarce. Taking in consideration the pressure put on the forest resources, the intensifying of trees harvesting to meet the production demand, these husks (by-products) could be a potential new resources for the particleboard production. The principal constituents of the husks are cellulose (27.43%), lignin (24.23%), hemicellulose (29.04%), and extractives (9%) (Popescu et al. 2013), similar to those of hardwood species.
A literature review indicates that research has been conducted to make composites from sunflower residues (e.g., stalk, husks, and by-products obtained after oil extraction from the seeds). These residuals have been combined with aspen wood particles (Gertjejansen et al. 1972), cement (Sisman and Gezer 2013), cotton waste (Binici and Aksogan 2014), Calabrian pine, poplar wood particles (Bektas et al. 2005; Guler et al. 2006), polypropylene (Kaymakci et al. 2013), and chitosan (Mati-Baouche et al. 2014) to form composites. Most of the developed composite panels that utilize sunflower wastes are combined with a variety of other raw materials, such as agricultural wastes (corn, rice, wheat), wood particles (poplar, pine, aspen), and inorganics (plaster and concrete). The use of these raw material mixtures in particleboard manufacturing involves the time and cost for collecting, storing, milling, defibration, sorting, heat-treating, and pressing operations.
According to recent data it could be estimated that the adhesives used in Europe for particleboard production are: ureo-formaldehidic (UF) (90 to 92%), melamino-ureo-formaldehidic (6 to 7%), and polymeric diphenylmethane diisocyanate (pMDI) (1 to 2%) (Kutnar and Burnard 2013). Phenolic resin (PF) is the second important bonding adhesive after UF, employed in the manufacture of wood based panels (Athanassiadou et al. 2015; Sandberg 2016). Their amount required by technology is between 9 and 12% (Ayrilmis and Nemli 2017; Laskowska and Mamiński 2018). The physical and mechanical properties of sunflower-based particleboard are lower than those of wood particleboard when UF and PF adhesives are used (Bektas et al. 2005; Kwon et al. 2014; Guler 2017). PMDI and emulsified pMDI in water (EMDI) have been found to be good substitutes for formaldehyde-based adhesives, leading to improved mechanical properties for the panels formed (Franke et al. 1994; Tongboon et al. 2002; Papadopoulos et al. 2002; Preechatiwong et al. 2007; Garay et al. 2009; Dukarska et al. 2017).
The objectives of this research were to produce particleboards made from 100% sunflower husks that possessed physical and mechanical properties approaching that of wood particleboards, as well as to evaluate how various adhesives and their usage levels affect the physical and mechanical properties of the resulting panels.
EXPERIMENTAL
Lignocellulosic Material
Sunflower husks (Helianthus annuus L.) were used as a raw lignocellulosic material to manufacture particleboards. The husks from the sunflower seed dehulling process were obtained from a Romanian sunflower oil manufacturer. The hammer-milled husks were sieved through 4- and 0.5-mm mesh screens to remove oversized and undersized particles. The accepted fraction had particles with lengths from 2.55 to 4.76 mm, widths from 1.05 to 2.3 mm, and thicknesses of 0.2 mm. The screened husks were dried to 4% moisture content.
Adhesives
The following adhesives were used to manufacture sunflower husk particleboards: urea-formaldehyde (UF), phenol formaldehyde (PF), modified melamine-formaldehyde (VM), and polymeric diphenylmethane diisocyanate (pMDI). Three commercial variants of UF were tested (UCL, U96, and AG); these variants differed from one another regarding the synthesis method and the formaldehyde/urea (F/U) molar ratio (1.15, 0.96, and 1.09, respectively). Mixtures of VM/AG (20:80 wt. ratio) and PF/pMDI (70:30 wt. ratio) were also used. PMDI today is generally applied in the European OSB industry (Stroobants and Grunwald 2014). The level applied is different depending on the product. In the OSB manufacturing the adhesive content ranges between 1.5% to 5%, and for particleboard an accelerator is added to the UF, and the combination in the core layer amounts to a percentage of 0.3% to 0.5% (Mantanis et al. 2017). Some research employed pMDI at rates of 1%, 2%, 3%, 4% and 6% for particleboard manufacturing (Papadopoulos et al. 2002; Korai and Ling 2011). Generally the hot pressing temperature varies from 180 °C to 240 °C (Papadopoulos et al. 2002) and the pressure time from 3 min to 6 min (Papadopoulos et al 2002; Korai and Ling 2011; Dukarska et al. 2017; Solt et al. 2019). A higher temperature was used for boards with pMDI, to reach sufficient temperature to allow the resin to cure. PMDI provides high bond strength, faster reaction time and superior resistance to water (Dunky 2003), thus it was employed beside UF adhesives. The choice of adhesives for the experimental tests was based on the data provided by the literature (Papadopoulos et al. 2002; Ressel 2008; Mendes et al. 2009; Korai and Ling 2011; Ayrilmis and Nemli 2017; Dukarska et al. 2017; Laskowska and Mamiński 2018; Solt et al. 2019). The adhesive types and the pressing schedule are presented in Table 1.
The solid resin content was based on the oven-dry weight of the husk particles. Ammonium chloride (NH4Cl) was used a hardener for urea-formaldehyde resins and was added at 1.5% (based on the weight of the dry resin). All adhesives were obtained from Viromet SA (Victoria, Romania).
Table 1. Adhesive Content Level and Pressing Parameters
Panel Forming and Pressing
The milled husk particles were weighed and mixed with the selected adhesive in a blender. Panels with lateral dimensions of 420 mm x 420 mm were manually formed with a homogenous single-layer structure. Panels were hot-pressed at 2.45 N/mm2 to obtain a target density of 600 kg/m3. The pressing conditions are presented in Table 1. Two replicates were made for each panel type. After pressing, the panels were conditioned at 20 °C and 65% relative humidity until they reached equilibrium moisture content; the conditioned panels were trimmed to nominal lateral dimensions of 400 mm x 400 mm, with a thickness of 16 mm.
Physical and Mechanical Strength Characterization
Water absorption (WA) and thickness swelling (TS) tests were performed in accordance with the EN 317 (1993) standard. The density of the panels was measured in accordance with the EN 323 (1993) standard. The density profile was measured using a compact X-ray density profile analyzer (DPX300; Imal S.R.; Modena, Italy). The mechanical tests were performed using a Zwick/Roell Z010 universal-testing machine (Zwick/Roell; Kennesaw, GA) that was equipped with a ±10-kN load cell. The modulus of rupture (MOR), modulus of elasticity (MOE), and internal bond strength (IB) were evaluated in accordance with the EN 310 (1993) and EN 319 (1993) standards, respectively. Ten measurements were performed for each property tested; the reported values are the average of the measurements. One-way analysis of variance (ANOVA, using Microsoft Excel) was performed to evaluate the statistical effects of adhesive type and content level on the properties of the panels. A statistical significance level of α ≤ 0.05 was selected.
RESULTS AND DISCUSSION
Morphological Characterization
The panels produced with sunflower husk particles appeared rigid and strong. The particles showed good cohesion and were not easily detached (Fig. 1).
Fig. 1. Outside appearance of the experimental panels
More compact structures and uniform distributions of particles were observed for the panels made with 6% pMDI, 12% PF, and 12% VM/AG. It appeared that these adhesives were uniformly distributed over the surface of the particles and filled the voids between the particles to provide adequate adhesion among the particles. Moreover, pMDI penetrated into the amorphous components of the husk cell wall at the molecular level and led to plasticization, improving the thickness swelling resistance of panels (Frazier 2003). The internal morphologies of the various panels are more clearly observed in the high-definition photographs shown in Fig. 2 (originally 4800 dpi resolution).
For 3% pMDI, the panel structure was less compact and the adhesive partially adhered to some particles, resulting in the formation of localized agglomerations. The heterogeneous distribution of husk particles and adhesive led to more voids in the structure when urea-formaldehyde adhesives were used.
Physical and Mechanical Characterization
Figure 3 shows the vertical density profile (VDP) of the panels. These graphs exhibited a typical density profile caused by mat densification. A higher peak density was found 1 mm and 3 to 4 mm from the surface areas for the UF panels and PF/pMDI panels, respectively (Fig. 3; a through f, g through k). The core of the panels had a more pronounced U-shaped profile in the cases of UCL and U96 adhesives (Fig. 3; a, b, d, and e.).