Abstract
The objective of this research was to characterize the cotton stalk resource. This paper also focused on the manufacturing of formaldehyde-free particleboards using whole cotton stalk. The effects of opening particle sizes and cotton ball residue ratios on performance properties of manufactured particleboard was assessed. Modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), and thickness swelling (TS) were characterized. Particleboards manufactured from four different opening particle sizes (6, 8, 10, and 20 mm) using an emulsifiable polymeric isocyanate adhesive (eMDI) were investigated. Similarly, the same performance properties were evaluated using four different cotton ball residue ratios (0, 5, 10, and 15 wt%). Results showed that the stem length and diameter of cotton stalks cultivated in Queensland (Australia) vary greatly and range from 160 to 890 mm and 5.03 to 10.88 mm, respectively. The highest weight proportion of the resource is the cotton stem making up 46 wt% of the available resource. The highest average values of MOR, MOE, IB, and the lowest TS were observed for boards with an opening particle size of 8 mm. The cotton ball residues had detrimental effects on the mechanical properties with a decrease in properties observed with increasing cotton ball residue load.
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Homogenous Particleboard Made from Whole Cotton (Gossypium hirsutum L.) Stalk Agricultural Waste: Optimisation of Particle Size and Influence of Cotton Residue on Performance
Thanh Tung Nguyen,a,c,* Henri Bailleres,d Adam Redman,d William Leggate,d Luigi-j Vandi,b,c and Michael Heiztmann a,c,*
The objective of this research was to characterize the cotton stalk resource. This paper also focused on the manufacturing of formaldehyde-free particleboards using whole cotton stalk. The effects of opening particle sizes and cotton ball residue ratios on performance properties of manufactured particleboard was assessed. Modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), and thickness swelling (TS) were characterized. Particleboards manufactured from four different opening particle sizes (6, 8, 10, and 20 mm) using an emulsifiable polymeric isocyanate adhesive (eMDI) were investigated. Similarly, the same performance properties were evaluated using four different cotton ball residue ratios (0, 5, 10, and 15 wt%). Results showed that the stem length and diameter of cotton stalks cultivated in Queensland (Australia) vary greatly and range from 160 to 890 mm and 5.03 to 10.88 mm, respectively. The highest weight proportion of the resource is the cotton stem making up 46 wt% of the available resource. The highest average values of MOR, MOE, IB, and the lowest TS were observed for boards with an opening particle size of 8 mm. The cotton ball residues had detrimental effects on the mechanical properties with a decrease in properties observed with increasing cotton ball residue load.
Keywords: Cotton stalks; Emusifiable polymeric isocyanate (eMDI) adhesive; Particleboard
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; *Corresponding authors: thanhtung.nguyen@uq.edu.au; m.heitzmann@uq.edu.au
INTRODUCTION
The global demand for wood and wood-based panels has considerably increased in recent decades and it is projected to continue to grow. According to the Food and Agriculture Organization of the United Nations’ (FAO) yearbook of forest and forest products (FAO 1997, 2002, 2007, 2012, and 2017), the world consumption of wood-based panels was 139 million cubic meters in 1995 and has grown to 393.2 million cubic meters by 2015. The global consumption of particleboards increased from 60.9 million cubic meters to 110 million cubic meters for the same period. Furthermore, the international average annual growth rate of sawn wood consumption for the period of 2020 to 2030 is predicted to be approximately 1.4%, and the figure for wood-based panels is estimated at 2.9% (FAO 2009). This leads to high pressures on diminishing forest resources. Therefore, research into alternative fiber resources for the manufacturing of wood-based panels in general and particleboards in particular has gained popularity. Agricultural waste, such as rice straw, rice husk, sorghum stalks, and cotton stalks, have been considered as a potential lignocellulosic material source for particleboard production. The development of particleboards based on agricultural waste presents a significant opportunity, particularly for regions without access to sustainable timber resources. Utilizing these alternative resources not only reduces pressure on forest resources but also helps in minimizing environmental impacts by preventing incineration as a mode of reducing trash between crop rotations. For the particleboard production, a particularly promising source is cotton stalk. As shown in Table 1, cotton stalks have similar hemicellulose and cellulose content to the most common hardwood species commonly used in the particleboard industry.
Table 1. Chemical Composition of Cotton Stalks and Hardwood Species
In Australia, cotton (Gossypium hirsutum L.) is cultivated only in Queensland and New South Wales with the five-year average (2011 to 2016) being 372,000 hectares (ABARES 2017). One hectare of cotton generates roughly three tons of dry stalks (Gomes et al. 1997). This indicates that approximately 1.2 million tons of cotton stalks are annually produced in Australia. At present, almost all of this waste is either burnt in the field leading to air pollution (Zhou et al. 2010; Kargarfard and Latibari 2011) and a decrease in soil biological activity (Alma et al. 2005) or plowed into the soil, which provides an ideal living place for pests during winter (Seedahmed 2014). Therefore, burning cotton stalk waste or incorporating this waste into the soil wastes a potentially underutilized resource.
A small number of studies on the utilization of cotton stalks for the manufacture of particleboards have been conducted. Particleboards made from cotton stalks without bark and other impurities, using urea formaldehyde resin were investigated by Guler and Ozen (2004). Their results showed that cotton stalks are a valuable raw material and that this resource can be used as alternative material for particleboard production. An investigation on the properties of cotton carpel-based particleboards by applying urea-formaldehyde and melamine urea-formaldehyde (Alma et al. 2005) reported that all studied properties of boards nearly met the minimum requirements according to TS EN 312-2 (1999) for general particleboards. Kadja et al. (2011) studied particleboards made from the stems of cotton plants using bone resin at the pressing temperature of 140 °C and revealed that the boards met the requirements of standard ANSI A 208.1 (1999) in terms of modulus of rupture (MOR) and modulus of elasticity (MOE) values with the rate of bone resin application of 10% and 12.5%. Mixing cotton stalks with paulownia and industrial wood at different rates for particleboard production was conducted by Khanjanzadeh et al. (2012). The results indicated that the highest mechanical properties were achieved at cotton stalk/industrial wood and paulownia wood/industrial wood rates of 50 wt% and 70 wt%, respectively.
The influence of a number of production parameters on the particleboard performance of boards manufactured from cotton stalks without bark was conducted by Nazerian et al. (2015). The outcome revealed that the MOR, MOE, and IB values of boards increased with increasing board density and press temperature. The increase in press closing speed results in increasing MOR but decreasing IB. Additionally, water absorption and thickness swelling decreased with increasing panel density and press temperature, while press closing speed had an insignificant effect on water absorption and thickness swelling. Optimization of the implementation process and physical properties of particleboard manufactured from bark-less cotton and kenaf stems using bone adhesive at different temperatures and rates of bone adhesive (Soulama et al. 2015) showed that the optimal temperature and resin content were 140 °C and 12.5%, respectively. The panels’ properties meet the requirements of the standard ANSI A 208.1 (1999) in terms of the MOR and MOE values, but TS did not meet the standard. Thickness swelling was almost four times higher than the standard requirement of 8% after 2 h of immersion in water. Nazerian et al. (2018) investigated blending cotton stalks with poplar wood for the core layer at different ratio levels and different ratios of melamine formaldehyde resin to urea formaldehyde as well as shelling ratios for particleboard production. The results of this investigation revealed that the maximum values of MOR, MOE, and IB were achieved when the ratio of melamine formaldehyde resin to urea formaldehyde was 18.3%, the shelling ratio was 30.3%, and the weight rate of poplar to cotton stalk in the core layer was 10.7%.
The prior studies conducted on the manufacturing of cotton-stalk particleboards discussed above only considered stems of cotton stalk without bark and therefore did not account for the other important components of cotton stalks such as branches, bark, carpels, and cotton ball residues. The average bark of cotton stems has been shown to be 26 wt% of cotton stem mass (Li et al. 2012) representing a significant potential additional biomass resource. Removing bark from thin cotton stems is also technically difficult and adds cost. Moreover, the resin used in those studies was formaldehyde-based resin, which can release free formaldehyde during utilization. This will have detrimental impacts on human health. Consequently, a study on the manufacture of formaldehyde-free particleboards from whole cotton stalks is necessary.
The aim of this study is to: 1) Characterize cotton stalk morphology; 2) Investigate the effects of opening particle sizes (20, 10, 8, and 6 mm) on the mechanical and physical properties of homogenous particleboard made of whole cotton stalks; and 3) Evaluate the influence of cotton ball residues at four different loadings (0, 5, 10, and 15 wt%) on particleboard performance.
EXPERIMENTAL
Materials
Resource characterization
The cotton stalks used in this research were harvested after less than 1 month since the cotton ball was 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%.
Optimization of opening particle sizes
After air-drying, whole cotton stalks were converted into chips using a commercial chipping machine (Hansa C7; Hamilton, New Zealand). The chips were milled into four different particle size categories by a laboratory cutting mill (SM 100; Retsch, Haan, Germany) using the four different sieve sizes: 20, 10, 8, and 6 mm (Fig. 1). Following this, the particles were shaken with a 0.3-mm sieve to remove fine particles with a size smaller than 0.3 mm.
Fig. 1. Four different sieves: (a) 20 mm, (b) 10 mm, (c) 8 mm, and (d) 6 mm
The influence of cotton ball residues on performance of particleboards
Cotton ball residues were removed from the cotton stalks, and then the best performing opening size particles (8 mm) from the optimization of the opening particle size procedure was chosen to manufacture particles for this study. The cotton ball residues were added according to four different ratios (0, 5, 10, and 15 wt%) during the process of particle manufacturing.
Methods
Resource characterization
In order to characterize cotton stalk morphology, 100 cotton stalks were randomly selected and then conditioned at 20 °C and 65% relative humidity (RH) to reach a moisture content of 12%. Subsequently, some physical properties were measured including the number of branches, stem length, stem diameter, stalk mass, and each cotton stalk component mass. The following protocol was followed:
The overall length of each stalk was measured using a measuring tape. The diameter of each cotton stem was measured at the middle of the stem using a digital caliper. The largest diameter d1 and the perpendicular diameter d2 (perpendicular to d1), were measured. The average of each stem was calculated according to Eq. 1,
D (mm) = (d1 + d2) / 2 (1)
where D is the average stem diameter (mm), d1 is the largest diameter stem mid-point (mm), and d2 is the diameter perpendicular to d1 (mm).
To determine the weight proportion of each cotton stalk component, firstly the entire cotton stalk and each component of a cotton stalk were weighed. Eventually, the weight proportion of each cotton stalk component (stem, branches, carpel, and ball) was calculated according to Eq. 2,
P (%) = (Cotton component mass / Cotton stalk mass) × 100 (2)
where P is percentage of the cotton component
Optimization of opening particle sizes and analysis of the distribution of particle sizes
After conditioning to a moisture content of 12%, a sieve analysis was performed for each particle category with a laboratory shaker (Fig. 3). This was done to observe the particle size distribution and to understand the influence of particle size on particleboard properties. Each particle category was sorted into five sieve groups using four different metal sieves consisting of 3.15, 2, 1, and 0.5 mm (Fig. 2) opening sizes. Then, 100 g of particles were shaken for a duration of 10 min and five 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 total mass of particles (%), wi is the mass of particles on the sieve i (g), and w is the total mass of particles (g).
In order 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. 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)
Fig. 2. Laboratory shaker
Particleboard production and test
After the particles were separated into size groups and the fines smaller than 0.3 mm were removed, all four types of particles were conditioned to 16% moisture content. The four types of homogenous particleboard, A, B, C, and D, according to the opening particle sizes of 20, 10, 8, and 6 mm, respectively, were manufactured with a target thickness of 12 mm and a density of 0.75 g/cm3.
A wooden mold with the dimension of 28 cm × 32 cm was used for mat forming. Three replications of each type were made. An emulsifiable polymeric isocyanate adhesive (eMDI) I-BOND® PB EM 4352 provided by Huntsman (Huntsman Polyurethanes Australia, Hillsdale, Australia) was applied such that 6% of adhesive was used as a ratio of the oven-dry weight of the particles. The resin was sprayed directly onto the particles using an atomized spray nozzle during the initial stage of the 5 min long mixing process. Four board types were manufactured using a hot press (Carver 3856, Wabash, IN, USA) with board production parameters shown in Table 2. The eMDI resin was applied in this research. He and Yan (2005) investigated effect of moisture content on curing kinetics of pMDI resin and wood mixture. Results indicated that the curing rate increased noticeably with increasing moisture content of raw materials. Mat moisture content between 10.8% and 18.5% can be applied isocyanate resins (Cai et al. 2006). Frazier (2003) suggested that mat moisture content level ranging from 12% to 20% will yield acceptable board performance with pMDI resin. Moreover, the higher moisture content levels mean that the particles become bonded together under moisture conditions that are closer to the those under which they will be used (in-service moisture content of the particleboard). This can be advantageous for reducing the level of residual stresses, which originate from the moisture uptake after manufacturing. In addition, when moisture content of raw materials is low, the particle will absorb more adhesive, which can lead to lack of adhesive on particle surface. Consequently, mat moisture content of 16% was chosen for this study. It is important to note that eMDI, unlike UF adhesive, does not contain any water. Hence, the moisture content of the mat should not be directly compared with the raw material moisture content reported in studies using UF adhesives, as these values do not include the additional water that is added as part of the adhesive application process.
Table 2. Experimental Design for Optimization of Opening Particle Sizes
The press pressure was released slowly over the duration of one minute after 3 minutes of pressing time. The slow pressure release was necessary to avoid delamination due to a rapid release of the steam pressure. It is important to note that the boards produced in this research are relatively small (280-mm x 320-mm). When upscaling the manufacturing process, care should be taken to adequately manage the steam pressure. Upscaling the process may require changes to the overall pressing time and un-moulding process.
After being taken from the hot press, the boards were placed in the ventilation area for at least two weeks for the balancing of the moisture content, relieving internal stress, and allowing full resin cure, before trimming the edges and cutting test pieces.
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 reach approximately 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, State College, PA, USA).
The influence of cotton ball residues on performance of particleboards
After the particles were generated using a sieve size of 8 mm, with all the fines smaller than 0.3 mm removed, all particles with different cotton ball residue ratios (0, 5, 10, and 15 wt%) were conditioned to 16% moisture content. Four single layer particleboard types were prepared for this study with a target thickness of 12 mm and a density of 0.75 g/cm3 as shown in Table 3. Three replications of each type were made.
Table 3. Experimental Design for the Influence of Cotton Ball Residues
The testing methodology described in “Optimization of opening particle sizes” was followed again for this investigation.
RESULTS AND DISCUSSION
Resource characterization
The results obtained from the measurement of 100 cotton stalk samples demonstrated that the minimum and maximum length of stems were 160 mm and 890 mm, respectively. The smallest average diameter at the middle of the stem was 5.03 mm and the largest was 10.88 mm. The resulting mean values are shown in Table 4.
Table 4. Some Physical Properties of Cotton Stalks
The average percentage of each component of the cotton stalk by mass is illustrated in Fig. 3 and shows that the largest proportion is made up of the cotton stem (46 wt%), followed by branches (33 wt%), and the smallest is cotton ball residues (5 wt%). The ratio of cotton bark to total mass ratio was reported at 26 wt% of the cotton stem mass (Li et al. 2012). Consequently, in the event that only the cotton stem without bark is used, the total utilization of biomass is less than 50 wt%. This is not only uneconomic, but the removal of bark from thin cotton stalks is also technically challenging and adds to the overall processing costs.