Abstract
Rice husk has a water repellent wax layer on its surface and has high pH and buffering capacity, which inhibit surface adhesion and result in the poor strength and water resistance of rice husk particleboard bonded with water-borne urea formaldehyde (UF) resin. In this work, rice husks were pretreated with liquid hot water at different temperatures (130, 140, 150, and 160 °C) and residence times (10 and 20 min). Pretreated rice husks were used to produce particleboard with UF resin. The effects of liquid hot water pretreatment temperature and residence time on the wax contents and pH values were investigated. In addition, the buffering capacities of the rice husks and the mechanical properties and the water resistance of the rice husk particleboards were investigated. The results indicated that liquid hot water pretreatment remarkably reduced the wax contents, pH values, and buffering capacities of the rice husks. In addition, it improved the mechanical properties and water resistance of the rice husk particleboards. Liquid hot water pretreatment effectively improved bondability between rice husks and water-borne UF resin, and it enhanced the performance of the rice husk particleboard.
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Effect of Liquid Hot Water Pretreatment on Selected Properties of Rice Husk and Its Particleboard
Peng Luo,* Chuanmin Yang, Mengyao Li, and Yueqi Wang
Rice husk has a water repellent wax layer on its surface and has high pH and buffering capacity, which inhibit surface adhesion and result in the poor strength and water resistance of rice husk particleboard bonded with water-borne urea formaldehyde (UF) resin. In this work, rice husks were pretreated with liquid hot water at different temperatures (130, 140, 150, and 160 °C) and residence times (10 and 20 min). Pretreated rice husks were used to produce particleboard with UF resin. The effects of liquid hot water pretreatment temperature and residence time on the wax contents and pH values were investigated. In addition, the buffering capacities of the rice husks and the mechanical properties and the water resistance of the rice husk particleboards were investigated. The results indicated that liquid hot water pretreatment remarkably reduced the wax contents, pH values, and buffering capacities of the rice husks. In addition, it improved the mechanical properties and water resistance of the rice husk particleboards. Liquid hot water pretreatment effectively improved bondability between rice husks and water-borne UF resin, and it enhanced the performance of the rice husk particleboard.
Keywords: Rice husk; Particleboard; Liquid hot water pretreatment; Urea formaldehyde resin
Contact information: College of Mechanical Engineering, Tianjin University of Commerce, Tianjin 300134, China; *Corresponding author: pengluo@yeah.net
INTRODUCTION
Each year about 596 million tons of rice are produced globally (Li et al. 2010). Rice husks, the by-product of the rice milling process, which are readily available in large quantities, have not been fully utilized (Ciannamea et al. 2010; Ayrilmis et al. 2012; Kang et al. 2012; Kwon et al. 2013; Tabata et al. 2017). Currently, only a small portion of rice husks are recycled for application in agriculture, livestock, and other sectors; the remaining portion are mainly burnt or used for landfilling (Kwon et al. 2013; Wood et al. 2016), which wastes precious natural resources and causes serious air pollution (Fernandez et al. 2001; Wood et al. 2016). Therefore, the development of new technologies to efficiently utilize rice husk is of great importance (Kang et al. 2012). Rice husk contains 25% to 35% cellulose, 8% to 21% hemicelluloses, 26% to 31% lignin, 15% to 17% silica and waxes, and 2% to 5% of other soluble substances (Ciannamea et al. 2010). Because the basic chemical components of rice husks are similar to those of wood and they are readily available at relatively low cost, rice husks have the potential to serve as the raw material for particleboard production (Ciannamea et al. 2010; Ayrilmis et al. 2012; Kang et al. 2012). Urea formaldehyde resin, which has a relatively high bonding strength and is easily applicable and inexpensive, is the most used adhesive in wood-based composite panel production (Abdolzadeh et al. 2011). However, UF is not suitable to produce rice husk particleboards due to its weak strength properties, the poor dimensional stability of the resultant panels, and the high resin content requirement (Ciannamea et al. 2010). The deficiencies are largely due to the outer surface of rice husk being covered with smooth, water-repellent wax layers, which hinder the further penetration and diffusion of UF (Ciannamea et al. 2010; Ayrilmis et al. 2012; Kwon et al. 2013) and results in poor adhesion between the UF and rice husk particles. In addition, the high pH and buffering capacity of the crop materials inhibit the curing of pH-sensitive UF resin, which leads to greatly increased resin gel time and weak bonding between crop materials and UF resin (Han et al. 2010; Li et al. 2010). As a result, the rice husk particleboard has poor mechanical properties and water resistance. Removal of the water-repellent wax layers can enhance the adhesion properties of crop materials to adhesives and improve the performance of the particleboard made from the crop materials (Ndazi et al. 2007; Han et al. 2009, 2010; Li et al. 2011). Research works showed that chemical (Han et al. 1999; Li et al.2011), steaming (Han et al. 2009; Li et al. 2011; Baskaran et al. 2013), and steam explosion (Han et al.2010) pretreatment of crop materials are capable of reducing the water-repellent wax content, which enhances the crop materials’ wettability and affinity to UF.
Liquid hot water pretreatment, which uses pressure to maintain the water in a liquid state at elevated temperatures for several seconds to several minutes, is applied mainly to pre-hydrolyze and delignify lignocellulosic fiber for bioethanol production (Mosier et al. 2005; Wan and Li 2011; González et al. 2014; Rezania et al. 2020). Water under high pressure and temperature can penetrate the biomass and is able to separate lignocellulosic fiber components while minimizing cellulose hydrolysis (González et al. 2014). Acetic acid and other organic acids generated from autohydrolysis can catalyze further hydrolysis (Wan et al. 2011). Liquid hot water pretreatment, which is an environmentally friendly process, omits the need for neutralization and conditioning chemicals and corrosion resistant materials for the reactor because no chemicals are added (Mosier et al. 2005; Wan and Li 2011; González et al. 2014). The effects of liquid hot water pretreatment on detaching lignocellulosic fiber components strongly depend on pretreatment severity, which is determined by pretreatment temperature and residence time (Mosier et al. 2005; Wan and Li 2011; Rezania et al. 2020). Increasing temperature and/or prolonging residence time may increase the severity of liquid hot water pretreatment. Severe pretreatment conditions result in accretion of organic acids and, as a result, an acidic environment, which promotes the hydration of cellulose and removal of hemicellulose and a portion of lignin; the degradation of fermentable sugars dissolving in the liquid fraction to hydroxymethyl furfural, furfural, formic acid, and levulinic acid is also promoted (Wan and Li 2011). The liquid hot water pretreatment is thought to be capable of destroying the hydrophobic wax layer on rice husk surfaces and improving the bondability between the rice husks and UF resin. There is currently no information regarding the utilization of liquid hot water to pretreat rice husks for rice husk particleboard manufacturing. This work employed a liquid hot water process to pretreat rice husks at different temperatures and residence times. The purpose of crop material pretreatment for particleboard production is to efficiently remove the hydrophobic wax layers covering the surface of crop materials and minimize the degradation of cellulose, hemicellulose, and lignin. To avoid loss of thermal instable hemicelluloses that start to solubilize and hydrolyze at pretreatment temperatures above 150 C (Han et al. 2009; Talebnia et al. 2010), less severe pretreatment conditions were employed in this study. The objective of this work was to investigate the effect of pretreatment conditions on the chemical properties of rice husks and rice husk particleboard bonded with UF resin.
EXPERIMENTAL
Raw Materials
The rice husks were acquired from Huiguan Village in the Xiaozhan Township of the Jinnan District (Tianjin, China). Its initial moisture content was approximately 7.16%. A screen with 0.3-mm mesh was used to manually shed undersized rice husks and dusts.
Liquid Hot Water Pretreatment
Liquid hot water pretreatment was conducted in a custom-built 15-L batch rotary stainless steel cylindrical autoclave (Model ZQS; Tongda Light Industrial Equipment Corporation, Xianyang, China) with a design pressure of 4 MPa (Fig. 1). The autoclave, connected to a rotary shaft, was equipped with an electrical heater, a motor actuator, a safe valve, a steam releasing valve, a pressure meter (0 MPa to 6 MPa), and a temperature gauge (0℃ to 300 ℃). The autoclave was loaded with 1.1731 kg of rice husks (dry weight) and tap water. The solid to water ratio inside the autoclave was 1:7 (g/mL, dry weight). The autoclave was tightly closed and simultaneously heated and rotated at 1 rpm to ensure appropriate agitation and uniform contact of water with the rice husks. The pretreatment temperatures were 130 ℃, 140 ℃, 150 ℃, and 160 ℃, and the residence times were 10 min and 20 min. The residence time for each batch was counted when the temperature gauge registered the target temperature. Heating was stopped, and steam was released gradually by opening the steam releasing valve at the end of each treatment. The autoclave was discharged when reaching atmospheric pressure. Both the pretreated rice husks and the liquid became light brown to dark brown in color depending on pretreatment severity (Fig. 2).The pulp was filtered to separate the solid residues (pretreated rice husks) from the liquid fraction. The pretreated rice husks were oven dried at 100 ± 3 ℃ to reach 2% to 3% moisture content and placed into plastic bags for future utilization.
Fig. 1. Image of the autoclave
Measurement of Chemical Composition, pH, and Buffering Capacities of the Raw and Pretreated Rice Husks
The ethanol/benzene extractives contents, the pH values, and the acid and alkali buffering capacities of the raw and pretreated rice husks were measured according to the China National Test Standards of GB/T 2677.6 (1994), GB/T 6043 (2009), and GB/T 17660 (1999), respectively.
Particleboard Preparation
Commercial UF resin (Zhengzhou Kaibang Chemical Products Co., Ltd., Zhengzhou, China) was used in this study. The UF resin was dispersed with water to reach a solid content of 55%. The UF resin was sprayed onto rice husks in a blender (Model HWJ 25;Xiaoshan Commercial Equipment Corporation, Zhejiang, China) through a pneumatic atomizing nozzle. The resin application level was 14% based on the oven-dried weight of the rice husks. To evaluate the water resistance of particleboards made of liquid hot water pretreated rice husks, no water-repelling chemicals were utilized.
The single layer particleboard mats were formed manually using a laboratory mold. The mats were prepressed at 1.5 MPa for 30 s. Then, the mats were hot-pressed in accordance with a three-phase hot press schedule at 130 °C. During the first phase, the mat was pressed at 3.5 MPa for 180 s, and during the second and third phases, the mat was pressed under 2 MPa for 120 sand 1 MPa for 60 s, respectively. The board size was 500 mm×500 mm×6 mm with a target density of 800 kg/m3. Three panels were made for each condition. The particleboards made from untreated rice husks acted as controls.
Particleboard Evaluation
After manufacture, the rice husk particleboards were conditioned at 20 °C and 65% relative humidity for 2 weeks. Edges of the panels were trimmed to a final dimension of 450 mm×450 mm×6 mm. Then, the panels were sawed into test specimens and tested according to the China National Test Standard GB/T 17657 (1999) for density, internal bonding strength (IB), modulus of rupture (MOR), modulus of elasticity (MOE), 24 h water absorption (WA), and thickness swelling (TS). Analysis of variance (ANOVA) and Duncan’s mean separation tests were used to statistically analyze the data obtained with SPSS software (SPSS Inc., Version 19, Chicago, IL, USA).
RESULTS AND DISCUSSION
Table 1. Effect of Liquid Hot Water Pretreatment on Extractives Ethanol/Toluene, pH, and Buffering Capacity of Rice Husks
