Abstract
The compaction behavior of cotton stalk particle mats, temperature profile inside the particle mats, and influence of surface particle size were studied relative to the properties of three-layered cotton stalk particleboards. Modulus of rupture (MOR), modulus of elasticity (MOE), internal bond, and thickness swelling were used as a measure for mechanical and physical performance. Two types of cotton stalk particleboard were manufactured. Results indicated that compression stiffness of the particle mat increased with increasing particle size; however, it decreased with increasing mat moisture content and temperature. At mat moisture contents of 12% and 18%, the plateau temperature at the centerline was not significantly different between boards having coarse and fine particles. However, the plateau time of boards with coarse particles was significantly lower than that of boards with fine particles. Additionally, thickness swelling of boards with a surface particle size of 2 mm was significantly lower than that of boards with surface particle size of 4 mm. Boards with a surface particle size of 2 mm had MOR and MOE values 15% and 10% higher, respectively, than boards with surface particle size of 4 mm. Internal bond decreased 6.5% with decreasing surface particle size from 4 mm to 2 mm.
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Processing Parameters Optimization of Cotton Stalk (Gossypium hirsutum L.) Particleboards with Emulsifiable Polymeric Isocyanate Adhesive
Thanh Tung Nguyen,a,c,e* Adam Redman,d William Leggate,d Luigi-j Vandi,b,c Henri Bailleres,d and Michael Heitzmann a,c,*
The compaction behavior of cotton stalk particle mats, temperature profile inside the particle mats, and influence of surface particle size were studied relative to the properties of three-layered cotton stalk particleboards. Modulus of rupture (MOR), modulus of elasticity (MOE), internal bond, and thickness swelling were used as a measure for mechanical and physical performance. Two types of cotton stalk particleboard were manufactured. Results indicated that compression stiffness of the particle mat increased with increasing particle size; however, it decreased with increasing mat moisture content and temperature. At mat moisture contents of 12% and 18%, the plateau temperature at the centerline was not significantly different between boards having coarse and fine particles. However, the plateau time of boards with coarse particles was significantly lower than that of boards with fine particles. Additionally, thickness swelling of boards with a surface particle size of 2 mm was significantly lower than that of boards with surface particle size of 4 mm. Boards with a surface particle size of 2 mm had MOR and MOE values 15% and 10% higher, respectively, than boards with surface particle size of 4 mm. Internal bond decreased 6.5% with decreasing surface particle size from 4 mm to 2 mm.
Keywords: Cotton stalks; Mat compaction behavior; Temperature behavior; Particleboards
Contact information: a: School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia; b: School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia; c: Centre for Advanced Materials Processing and Manufacturing, The University of Queensland, Brisbane, QLD 4072, Australia; d: Queensland Department of Agriculture and Fisheries, Horticulture and Forestry Science, Salisbury Research Facility, 50 Evans Rd, Salisbury, QLD 4107, Australia; e: Research Institute of Forest Industry, Vietnamese Academy of Forest Sciences, 46 Duc Thang, North Tu Liem, Ha Noi, Vietnam;
* Corresponding authors: thanhtung.nguyen@uq.edu.au; m.heitzmann@uq.edu.au
INTRODUCTION
A global shortage of wood materials along with continuing population growth is motivating research in the utilization of agricultural residues as a new source of raw material to reduce pressure on diminishing forest resources. Cotton stalk residues have a comparable composition and structure to the most common hardwood species (Tutus et al. 2010; Soulama et al. 2015). Therefore, cotton stalks can be used as an alternative lignocellulosic material for the manufacturing of particleboard. Particles are subjected to different kinds of stress during the hot-press period. There are three main parameters, which have effects on the hot-pressing process, namely the mat moisture content, press closing rate, and press temperature. These factors have influences on the curing rate of resin, the development of the vertical density profile (density profile in the thickness direction), and bond quality; consequently, they will have impact on the mechanical and physical properties of the particleboard (Wong et al. 1998). Moreover, the combination of elevated temperature and high moisture content of the mat increase the chance of delamination, caused by excess steam pressure inside the board during pressing, when the press pressure is released (Dai and Wang 2004).
It is well known that the particle mat structure of three-layered particleboard results from a combination of three factors: (1) particle shape and dimension in the surface and core layers, (2) moisture content, and (3) resin content in the surface and core layers. These factors considerably influence the formation of the vertical density profile of the particleboard. Moreover, the density of wood materials has noticeable effects on the stress-strain relationship in the mat during hot-press application and hence leads to changes in the vertical density profile (Wang and Winistorfer 2000). In addition, wood type and particle size are known as factors that affect the performance of the particleboard, and they also determine the temperature behavior in the mat of particleboard during the hot-press period (Rofii et al. 2014).
In order to optimize the production process of particleboard made from cotton stalks, it is of critical importance to understand the influence of cotton stalk particle sizes, particle mat moisture content, and temperature on mat compaction behavior as well as temperature behavior in the mat during the hot-press stage. There are a number of published studies on manufactured particleboards from cotton stalk residues. Guler and Ozen (2004) investigated some properties of particleboards made from cotton stalk; however, the research did not mention the particle sizes used for the surface and core layers, with the results showing only that cotton stalks can be used as an alternative and valuable raw material for the manufacturing of particleboard. Kadja et al. (2011) studied the utilization of bone adhesive to produce particleboard from stems of cotton plants at a pressing temperature of 140 °C. Again, the particle sizes were not mentioned in this study; the results revealed that the boards met requirements of the standard ANSI A208.1 (1999) in terms of MOR (modulus of rupture) and MOE (modulus of elasticity) values with the rate of bone resin applied at 10% and 12.5%. The effects of some technological production variables on mechanical and physical properties of particleboard manufactured from cotton stalks were studied by Nazerian et al. (2015). In this investigation, the authors only mentioned that the cotton stalk was ground using a disc mill into particles usable for production of particleboard and the results showed that the MOR, MOE, and IB (internal bond) of boards increased with increasing board density and press temperature. Additionally, the increase in press closing speed resulted in increasing MOR but decreasing IB. Water absorption and thickness swelling decreased with increasing panel density and press temperature, while the press closing speed had an insignificant effect on water absorption and thickness swelling. Furthermore, all previous studies conducted on the manufacture of cotton-stalk particleboards mentioned above only considered cotton-stalk stems without bark as raw materials. The average stem (with bark) proportion accounted for just 46 wt% of the whole mass of cotton stalk (Nguyen at al. 2020). The average bark of cotton stems has been reported to be 26 wt% of cotton stem mass (Li et al. 2012). Removing bark from very thin cotton stems is also technically difficult and adds cost. Therefore, the utilization of whole cotton-stalk (including bark and branches) as raw materials represents a significant potential additional biomass. To the authors’ knowledge, no studies have been conducted to understand the influence of cotton stalk particle sizes, particle mat moisture content, and temperature on mat compaction behavior as well as temperature behavior in the mat during the hot-press stage.
The aims of this research were to investigate the effects of opening particle sizes (the size of the sieve was used to generate particles as mentioned in Fig. 1). For example, opening particle sizes of 8 mm means that particles had passed through sieve size of 8 mm. Studies were conducted to determine the effects of particle size, particle mat moisture content, and temperature on the compression behavior of the particle mat. Further tests were done to evaluate the influences of particle sizes and moisture content of the particle mat on temperature behavior inside the mat of particleboard, and to optimize surface particle size.
EXPERIMENTAL
Materials
The cotton stalks used in this research were obtained less than 1 month after the cotton balls were harvested at Goodar, Queensland, Australia (WGS-84 coordinates: -28.4775 S, 150.2455 E). After harvesting, the cotton stalks were air-dried for two months to a moisture content of 20%. Cotton ball residues have negative impacts on properties of boards (Nguyen at al. 2020). Therefore, cotton ball residues were separated from the cotton stalks. Subsequently, those stalks (stalks without cotton ball residues) were converted into chips using a commercial chipping machine (Hansa C7, Hansa Products Ltd., Hamilton, New Zealand). Subsequently, the chips were milled to three different particle sizes by a laboratory cutting mill (SM 100, Retsch Technology GmbH, Haan, Germany) using three different sieves: 8 mm, 4 mm, and 2 mm (Fig. 1). Subsequently, these particles were shaken by using a 0.3-mm sieve to remove particles with a size smaller than 0.3 mm.
Fig. 1. Three different sieves: (a) 8 mm, (b) 4 mm, and (c) 2 mm
Methods
Particle mat compaction behavior
Particles that had passed through three different opening particle sizes (8 mm, 4 mm, and 2 mm) were used to investigate their compaction mat behavior at different moisture contents (12%, 18%, 24%, and 30%). The average moisture content at fiber saturation point (FSP) is approximately 30%, and the change of moisture content below FSP has effects on wood properties. The experimental also involved different temperatures (room temperature, 50, 75, 100, 125, 150, and 170 °C). The particles were first conditioned at a temperature of 20 °C and 65% relative humidity (RH) to reach an equilibrium moisture of 12 ± 1%. The particles with the moisture contents of 18%, 24%, and 30% were obtained by spaying water on particles containing a moisture content of 12% with the calculated amount of water added in accordance with individual target moisture content and then placed in a sealed plastic bag for a week.
In order to study the particles’ compression behavior, a compression mold was designed as shown in Fig. 2 with a plunger and cylinder diameter of 50 mm. Holes with diameter of 1 mm were made on the sides of cylinder to ensure air and gases could escape easily during compression tests (Fig. 2).
Fig. 2. Compression mold
For the compression at room temperature, 16.5 g of particles at different sizes and moisture contents were loaded into the compression mold, which was put in the compression cage (Fig. 3). The compression cage was set up inside the environmental chamber that allows a static temperature to be set. For the compression at higher temperatures, 16.5 g of particles at different sizes and moisture content were wrapped in aluminium foil and placed in the environmental chamber with the temperature set to the desired set-point to heat particles to the desired temperature. As soon as the particles reached the desired temperature, they were placed in the compression mold and the compaction behavior was measured.
Fig. 3. Set-up for particles compression tests
The compression test was conducted by using a universal testing machine (Instron 4505, Instron, Wycombe, England) (Fig. 3). The maximum load of 10 kN was applied with a loading rate of 15 mm/min. Three replications were performed for each type of test.
During tests, the force and displacement were recorded. The specific pressure at the specific time during the testing period was calculated based on the recorded force at that moment and the plunger base area according to Eq. 1,
P (N/mm2) =F/S (1)
where P is specific pressure (N/mm2), F is force (N), and S is plunger base area (mm2).
The density of the compacted mat was calculated based on the mat weight, plunger base area, and distance between the plunger and bottom cylinder according to Eq. 2,
(g/cm3) = m/Sl (2)
where is density of compacted mat (g/cm3), m is mat weight (g), S is the plunger base area (mm2), and l is the distance between the plunger and bottom cylinder (cm). The compression behavior of particle mats is visualized as a pressure – density diagram.
Temperature behavior inside the mat of boards
The aim of this investigation was to find the effect of particle size and mat moisture content on temperature profile according to the thickness direction, the nominal mat moisture content at 12%, 18%, 24%, and 30%. Particles were conditioned at a temperature of 20 °C and 65% relative humidity to reach an equilibrium moisture of 12%. The particles with the moisture contents of 18%, 24%, and 30% were obtained by spraying water on particles containing a moisture content of 12% with the calculated amount of water added in accordance with individual target moisture content and then placed in a sealed plastic bag for a week), and the three opening particle sizes 8 mm, 4 mm, and 2 mm (particles were generated by using sieve sizes of 8 mm, 4 mm, and 2 mm), where the thickness of the board was 12 mm, the target density was 0.75 g/cm3 and the board size was 28 cm x 32 cm. A press temperature and pressure of 170 °C and 3 MPa, were applied, respectively. Emulsifiable polymeric isocyanate adhesive (eMDI) I-BOND® PB EM 4352 (Huntsman Polyurethanes, Melbourne, Australia) was applied at a rate of 6% based on the oven-dry weight of the particles. The temperature behavior inside the mat according to thickness direction was measured at the surface and core (centerline) of the mat for both homogenous boards and three-layered boards. A type T thermocouple and a temperature measurement device (dataTaker DT80, Longtek Pty Ltd., Barcaldine, Australia) were used to conduct this investigation. Three replications were conducted for each type of measurement. The temperature curves displayed are the average ones.
For the three-layered particleboard, the surface layers were fine particles (2-mm opening particle size) and the core layers are coarse particles (8 mm opening particle size). The proportion between surface layers and core layers by particle weight was 45 wt% and 55 wt%, respectively, for a board thickness of 12 mm; following the recommended layup presented by Fu (1998)
Optimization of opening particle size for the surface layers
The aim of this study was to evaluate the influence of opening particle size in the surface layers on mechanical and physical properties of three-layered cotton stalk particleboards. The best performing opening size particles (8 mm) in terms of mechanical performance from four coarse opening particle sizes, including 20 mm, 10 mm, 8 mm, and 6 mm, was chosen for the core layer for this study, based on work previously performed by Nguyen et al. (2020). Two opening particle sizes including 4 mm and 2 mm were selected for the surface layers of this study. After being generated, the fine particles with a size smaller than 0.3 mm were removed for all three opening particle sizes (8 mm, 4 mm, and 2 mm) using a sieve size of 0.3 mm.
A sieve analysis was performed for two opening particle size categories of 4 mm and 2 mm after conditioning to a moisture content of 12% (in order to get particles with moisture content of 12%, the particles were conditioned at a temperature of 20 °C and relative humidity of 65% to reach an equilibrium moisture of 12%). Each particle category was sorted into five sieve groups using four different metal sieves consisting of 3.15, 2, 1, and 0.5 mm. Then, 100 g of particles were shaken for a duration of 10 min, and three replications were performed. Finally, the distribution of particle size was calculated as a percentage according to Eq. 3,
Pi (%) = (wi / w) × 100 (3)
where Pi is the percentage (%) of particles on the sieve i compared to the total mass of particles, wi is the mass (g) of particles on the sieve i, and w is the total mass of particles (g).
To determine the slenderness ratio of the particles, 100 particles were randomly selected from each sieve analysis group particle category, and their length and thickness were measured using a digital caliper (Mitutoyo 500-753-20; Mitutoyo Corporation, Kanagawa, Japan). The slenderness ratio of particles was calculated according to Eq. 4,
S = l / d (4)
where S is the slenderness ratio, l is the length of a particle (mm), and d is thickness of a particle (mm).
To manufacture boards, the particle size of 8 mm was conditioned to a moisture content of 12%, and the particle sizes of 4 mm and 2 mm were conditioned to a moisture content of 18%. The particles with a moisture content of 12% were obtained by conditioning particles at a temperature of 20 °C and 65% of relative humidity to reach an equilibrium moisture content of 12%. The particles with a moisture content of 18% were achieved by conditioning particles at a temperature of 20 °C and relative humidity of 85% to reach an equilibrium moisture content of 18%. The two types of three-layered particleboard were labelled type A, consisting of 8 mm particles for the core layer and 4 mm for surface layers; type B consisting of 8 mm particles for the core layer and 2 mm for the surface layers. The target thickness was 12 mm, and the target density was 0.75 g/cm3. The boards were manufactured with a mass ratio between surface layers and the core layer of 45 wt% and 55 wt% respectively as recommended by Fu (1998). A wooden mold with the dimension of 28 cm x 32 cm was used for mat forming. Three replications were manufactured for each type of particleboard.
As the adhesive, eMDI I-BOND® PB EM 4352, which was provided by Huntsman Polyurethane (Melbourne, Australia), was applied such that 8% and 6% of adhesive were used as a ratio of the oven-dry weight of particles for the surface layers and core layer, respectively. The resin was sprayed using an atomized spray nozzle on the particles in a blender for 5 min to obtain a homogenized mixture. Two board types were manufactured using a hot press (Carver 3856, Carver Inc., Wabash, IN, USA) with board production parameters displayed in Table 1.
Table 1. Experiment Design for the Optimization of Opening Face Particle Sizes
The press pressure was released slowly over the duration of 1 min after 3 min of pressing time. The slow pressure release was necessary to avoid delamination as a result of the rapid release of steam pressure.
The size of test pieces and methods of testing were compliant with Australian/New Zealand standard AS/NZS 4266.1 (2017). The test specimens were conditioned in a conditioning cabinet at 20 °C and 65% RH to equalize 12% moisture content. Both MOR and MOE were conducted according to Australian/New Zealand standard AS/NZS 4266.1 (2017) section 7, and the number of test specimens was 9 for each board type. The IB was performed in accordance with AS/NZS 4266.1 (2017) sections 8 and 9, and specimens were tested for each board category. Thickness swelling tests after 2 h and 24 h immersion in water on 9 test pieces was conducted for each type of board following AS/NZS 4266.1 (2017) section 10. The density of boards was determined in accordance with AS/NZS 4266.1 (2017) section 6.
The results of this research were analyzed using an analysis of variance (ANOVA) multiple comparison test with a confidence interval of 95% using Minitab Statistical Software (Minitab LLC, v.19, Minitab Inc., State College, PA, USA).
RESULTS AND DISCUSSION
Particle Mat Compaction Behavior
The influences of particle size and moisture content (MC) on compaction behavior of particle mats at room temperature are illustrated in Fig. 4. The data shows different particle sizes have different mat compaction throughout the entire compression process. The mat density at the same level of pressure and moisture content was increased considerably with decreasing particle size. For example, the mat density at a pressure of 5 N/mm2 and moisture content of 12% was increased from 0.7 g/cm3 to 0.8 g/cm3 with decreasing opening particle size from 8 mm to 2 mm, respectively.
The finding of increasing compression stiffness at room temperature of the mat with increasing particle size can be explained as particles with large size are able to bridge the gaps and contribute to compression resistance as a consequence of bending. Therefore, compression stiffness of mats containing larger particle size is higher than that of mats with smaller particle size.
Furthermore, it can also be seen from Fig. 4 that the moisture content of particles had an impact on the compaction behavior of the particle mat. The density of the particle mat increased with increasing the moisture content of the particles. The density of mats with a particle size of 8 mm increased from 0.7 g/cm3 to 0.83 g/cm3 at a pressure of 5 N/mm2 with the moisture content of particles increasing from 12% to 30%.
Fig. 4. Compaction behavior of the particle mat at room temperature at different MC of the mat with 8 mm particle size (a), mat with 4 mm particle size (b), and mat with 2 mm particle size (c).
The density values of mats with particle sizes of 4 mm and 2 mm increased from 0.77 g/cm3 to 0.9 g/cm3 and 0.8 g/cm3 to 0.92 g/cm3, respectively. This phenomenon can be explained by changes in the proportions of bound water in the wood. The average moisture content at the fiber saturation point (FSP) is approximately 30% (Glass and Zenlinka 2010). When the moisture content is below the FSP, the change in moisture content has an effect on wood strength. Increasing the amount of bound water leads to reducing hydrogen bonding between the organic polymers of the cell wall (Glass and Zenlinka 2010), which reduces the strength of wood.
The density of particle mats at temperatures of 50 and 75 °C and different moisture contents (MC) is shown in Fig. 5. Results revealed that the mat density of all particle sizes at all moisture contents increased considerably compared to those values at room temperature with increasing temperature of particle mats to 50 and 75 °C. For instance, the density of mat with a particles size of 8 mm at a moisture content of 12% and pressure of 5 N/mm2 was increased from 0.7 g/cm3 at room temperature to 0.8 g/cm3 at a temperature of 75 °C. Similarly, under the same conditions, the density of mats containing particle sizes of 4 mm and 2 mm were increased from 0.77 to 0.94 g/cm3 and 0.83 to 1.0 g/cm3, respectively. This indicates that increasing temperature resulted in decreasing compression stiffness of the particle mat. The explanation for this is that increased temperature creates an increase in the plasticity of lignin and an increase in spatial size. This decreases intermolecular contact leading to a loss in wood strength (Winandy and Rowell 2005).
Fig. 5. Compaction behavior at different MCs of the mat with 8 mm particle size at 50 °C (a), mat with 4 mm particle size at 50 °C (b), mat with 2 mm particle size at 50 °C (c), mat with 8 mm particle size at 75 °C (d), mat with 4 mm particle size at 75 °C (e), and mat with 2 mm particle size at 75 °C (f).
Furthermore, hemicellulose is amorphous and essentially thermoplastic (Back and Salmén 1982). This means that hemicellulose become more plasticized with the increase in temperature, which also contributes to wood strength reduction.
It can be seen from Fig. 6 that when the particle mat temperature was increased to 100 °C, the mat density for all particle sizes and moisture contents decreased compared to a temperature of 75 °C (Figs. 5d, through 5f). For example, the density of the mat with a particle size of 2 mm at a pressure of 5 N/mm2 and moisture content of 12% decreased from 1.0 to 0.9 g/cm3 with increasing temperature from 75 to 100 °C. This is not in line with the theory mentioned above that an increase in temperature results in increasing mat density (see Fig. 5 for when temperature increased from room temperature to 50 °C and then 75 °C). The potential reason for this is that at a temperature of 100 °C, water is starting to evaporate and generate water vapor. During pressing, the water vapor can become trapped, and the resulting steam pressure opposes the closing pressure of the press. This is in line with the results indicated in Fig. 9, which indicate that higher particle mat moisture content led to higher plateau temperature and plateau time. It is well known that higher plateau temperature leads to higher steam pressure and longer plateau time resulting in a longer duration of keeping that steam pressure. Furthermore, when pressing pressure reaches 5 N/mm2, the compression process is stopped but the water vapor still does not escape from the mat.
Fig. 6. Compaction behavior of the particle mat at 100 °C and different MC of mat with 8 mm particle size (a), mat with 4 mm particle size (b), and mat with 2 mm particle size (c).
Figure 7 shows the results of particle mat densities when the temperature of the mats was increased from 125 to 170 °C. This indicates that the mats’ density decreased with increasing moisture content when the temperature of the mats reached 125 °C and onward. This is the opposite trend compared to when temperature increased from room temperature to 100 °C. The reason for this is most likely that at an elevated temperature from 125 °C and above, a higher moisture content mat will generate higher vapor pressure that opposes the pressing pressure during the molding process. This is explained more clearly in the next section (temperature behavior inside the mat of the boards).
Fig. 7. Compaction behavior of the particle mat at different MC of mat with 8 mm particle size at 125 °C (a), 4 mm particle size at 125 °C (b), 2 mm particle size at 125 °C (c), 8 mm particle size at 150 °C (d), 4 mm particle size at 150 °C (e), 2 mm particle size at 150 °C (f), 8 mm particle size at 170 °C (g), 4 mm particle size at 170 °C (h), and 2 mm particle size at 170 °C (i).
The findings from particle mat compaction behavior indicate that to develop the best density profile in the thickness direction, the coarse particles with lower moisture content and fine particles with higher moisture content should be applied for the core layer and surface layers, respectively. The findings also emphasize that another factor that contributes to the best density profile development is a rapid closing rate, which should also be applied to ensure that at the moment the top plate of the hot press touches the stop bar, the temperature in the core of the mat is still around room temperature.
Temperature Behavior Inside the Boards
The results of surface temperature change during hot-pressing of homogenous boards with various particle sizes and moisture contents are shown in Fig. 8. This indicates that the temperature change was almost the same for all particle sizes and moisture contents. This means particle size and moisture content do not have an effect on the surface temperature profile as hot-pressing progresses. The reason for this is that the surface of the board is directly in contact with the aluminium sheet, so heat is rapidly transferred from the hot plate through the aluminium sheet to the surface of the board.