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Christy, E. O., Soemarno, Sumarlan, S. H., and Soehardjono, A. (2021). "Binderless bark particleboard made from gelam (Melaleuca viridiflora Sol. ex Gaertn.) bark waste: The effect of the pressing temperature on its mechanical and physical properties," BioResources 16(2), 4171-4199.

Abstract

This study investigated the effects of the pressing temperature on the mechanical and physical properties of binderless bark particleboard made from Gelam bark waste and the improvement of those properties. In addition, the thermal insulation properties of the particleboard were determined. Four different temperatures (140 °C, 160 °C, 180 °C, and 200 °C) were used to make single-layer binderless bark particleboard with a target density of less than or equal to 0.59 g/cm3. Results revealed that the pressing temperature affected the mechanical properties (modulus of rupture, modulus of elasticity, and tensile strength perpendicular to panel surface), which increased as the temperature increased, and the physical properties (thickness swelling and water absorption), which decreased as the temperature increased. Based on the Tukey test, increasing the temperature from 180 to 200 °C did not significantly affect the mechanical or physical properties, except for the tensile strength perpendicular to panel surface. None of the mechanical properties met SNI standard 03-2105-2006 (2006); however, the 12% maximum thickness swelling requirement was met for binderless bark particleboard hot-pressed at 200 °C. Binderless bark particleboard hot-pressed at 200 °C had high water resistance, regardless of its low strength, and a thermal conductivity value of 0.14 W/m∙K.


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Binderless Bark Particleboard Made from Gelam (Melaleuca viridiflora Sol. ex Gaertn.) Bark Waste: The Effect of the Pressing Temperature on Its Mechanical and Physical Properties

Eva Oktoberyani Christy,a,b,* Soemarno,c Sumardi Hadi Sumarlan,d and Agoes Soehardjono e

This study investigated the effects of the pressing temperature on the mechanical and physical properties of binderless bark particleboard made from Gelam bark waste and the improvement of those properties. In addition, the thermal insulation properties of the particleboard were determined. Four different temperatures (140 °C, 160 °C, 180 °C, and 200 °C) were used to make single-layer binderless bark particleboard with a target density of less than or equal to 0.59 g/cm3. Results revealed that the pressing temperature affected the mechanical properties (modulus of rupture, modulus of elasticity, and tensile strength perpendicular to panel surface), which increased as the temperature increased, and the physical properties (thickness swelling and water absorption), which decreased as the temperature increased. Based on the Tukey test, increasing the temperature from 180 to 200 °C did not significantly affect the mechanical or physical properties, except for the tensile strength perpendicular to panel surface. None of the mechanical properties met SNI standard 03-2105-2006 (2006); however, the 12% maximum thickness swelling requirement was met for binderless bark particleboard hot-pressed at 200 °C. Binderless bark particleboard hot-pressed at 200 °C had high water resistance, regardless of its low strength, and a thermal conductivity value of 0.14 W/m∙K.

Keywords: Mechanical properties; Dimensional stability; Thermal conductivity; Self-bonding; Lignocellulosic material

Contact information: a: Postgraduate Program, Faculty of Agriculture, Brawijaya University, Malang 65145 Indonesia; b: Department of forestry, Faculty of Agriculture, Palangka Raya University, Palangka Raya, Central Kalimantan 74874 Indonesia; c: Department of Soil Science, Faculty of Agriculture, Brawijaya University, Malang 65145 Indonesia; d: Department of Agricultural Engineering, Faculty of Agricultural Technology, Brawijaya University, Malang 65145 Indonesia; e: Department of Civil Engineering, Faculty of Engineering, Brawijaya University, Malang 65145 Indonesia; *Corresponding author: eochristy28@gmail.com

INTRODUCTION

Lignocellulosic materials from agricultural waste products, forestry residues, and other non-wood products can be used as alternative raw materials for the production of composite panels, e.g., particleboards and fiberboards. A similar trend is also apparent in the use of adhesive-free panel technology. These tendencies are driven by the scarcity of wood resources and the formaldehyde emissions associated with the production of particleboards (Wang et al. 2018). Formaldehyde emissions from formaldehyde-based adhesives are quite detrimental to human health, as they may lead to illness, e.g., leukemia (Golden 2011; Zhang and Lin 2016). Numerous studies have addressed this global problem. These studies encourage the production of binderless boards from waste generated in the production of rattan furniture (Ahmad et al. 2019), also from agricultural waste such as unripe coconut husks (Araújo Junior et al. 2018), wheat straw residues (Domínguez-Robles et al. 2020), rice husk (Ferrandez-Garcia et al. 2017), sunflower bark and flax shives (Mahieu et al. 2019), banana trunk waste (Nadhari et al. 2019), and almond residues (Ferrandez-Villena et al. 2019). There are also binderless boards made from other natural resources, i.e., Totora (Schoenoplectus californicus (C.A. Mey) Soják) stems (Hidalgo-Cordero et al. 2020) and Arundo donax L. rhizomes (Ferrandez-Villena et al. 2020).

Wood bark is a lignocellulosic-based forestry residue waste product that could be considered for the production of binderless particleboards (Romaní et al. 2020). Chen and Yan (2018) pointed out that bark is the outermost layer of a tree trunk. The primary chemical composition of tree bark is quite similar to wood, i.e., it consists of cellulose, hemicellulose, and lignins; however, tree bark is also rich in extractives such as tannins, suberins, rosins, etc. Chen and Yan (2018) also stated that tannins and lignins have adhesive properties. With regard to these two components, Chow (1972, 1975) argued that a high-density bark board can be made without synthetic resin, since both extractives and lignins, which are phenolic materials, can function as an adhesive and therefore may contribute to the self-bonding process of bark particles. Nitu et al. (2017) pointed out that the chemical composition of a lignocellulosic material is an important consideration and determines its suitability in the making of binderless composites.

In the case of manufacturing panels from tree bark without synthetic adhesives, the authors learned that high-temperature pressing is more favorable because a temperature higher than 180 C will improve the physical and mechanical properties of the board. At that temperature (greater than 180 C), thermal reactions, i.e., polymerization and partial degradation of the chemical components of the bark, will occur. The polymerization of the phenolic extractives and possibly lignins will produce a strong bond between the bark particles (Chow 1972, 1975). In addition, it should be noted that oven-dried samples comprised of wood and bark begin to soften at 180 C (Chow and Pickles 1971). In regard to the parameters of binderless particleboard production via the hot-pressing process, Gupta et al. (2011) argued that the pressing temperature is one of the most important parameters in producing particleboards without synthetic resin (bark board), since the binding of wood bark particles without synthetic adhesive is believed to occur due to a thermal effect. Gupta et al. (2011) found that all the properties of the bark board made from beetle-infested lodgepole pine (Pinus contorta) bark drastically increased as the pressing temperature increased, from 170 to 230 C. In the production of adhesive-free boards using other materials, Boon et al. (2013) maintained that the role of the pressing temperature in terms of improving the mechanical properties of binderless particleboards made from palm oil trunk is more important than any other parameters.

Gelam bark waste (GBW) is a lignocellulosic material, and Xiao et al. (2014) mentioned that the bark from the Melaleuca tree is rich in lignins. It is an abundant waste material, which is generated by peeling the bark off a Gelam log with a diameter of less than 10 cm. The Gelam tree has multi-layered bark, and it is one of the Melaleuca species grown in Central Kalimantan. According to Sakasegawa et al. (2003), this tree is locally referred to as Gelam in Indonesia. Supriyati et al. (2015) mentioned that the Melaleuca species naturally and abundantly grows in Indonesian territory, especially in the peat swamp forests of Central and South Kalimantan, and along the southern coast of Sumatera. Usually, bark waste is simply burned, used as land filling, or thrown into the river, which obviously creates an environmental problem. Given the chemical content of GBW, the authors believe that it is suitable to proposing the use of GBW to produce a low-density binderless bark particleboard (BBP) via the hot-pressing process with the pressing temperature as a variable parameter. To the best of the authors’ knowledge, there is only limited information on the making of binderless bark particleboards from Gelam bark waste. A previous study by Sato (2008) explored the possibility of producing high-density bark binderless boards from Melaleuca bark with the hot-pressing temperature at 180 °C. However, there is no information about the effect of the pressing temperature on low-density binderless bark particleboard properties made from Gelam bark using a high-pressing temperature at 180 °C and 200 °C in the production of its. Therefore, this recent study investigates the effect of the pressing temperature on the mechanical and physical properties of BBP made from GBW. The mechanical and physical properties evaluated included the modulus of rupture (MoR), modulus of elasticity (MoE), tensile strength perpendicular to panel surface (TSPtPS), density, moisture content (MC), thickness swelling after 24 h of soaking (TS24h), and water absorption after 24 h of soaking (WA24h). Also, the chemical properties of the raw materials were determined by conventional chemical analysis. Furthermore, Fourier-transform infrared spectroscopy was used to observe any changes in the FTIR spectra between the raw material and the BBP, which were pressed at different temperatures. Observation via scanning electron microscopy equipped with energy-dispersive X-ray analysis was also performed to study the microstructure and quality of the bonding was formed in the BBP. Besides, according to Lakreb et al. (2018) there has been growing interest in using bark particleboard as a thermal insulation material in recent years. As in studies by Pásztory et al. (2017) and Pásztory et al. (2019), the resulting bark particleboard has a low density, resulting in good thermal insulation properties. Regarding BBP, which also made with the low-density target. So that thermal insulation properties also need to be determined in view of its use as an insulating material.

EXPERIMENTAL

Raw Materials

Gelam (Melaleuca viridiflora Sol. ex Gaertn.) bark waste used in this study was obtained from Central Kalimantan, specifically from a local Gelam wood seller in the village of Garung, Jabiren Raya, in the Pulang Pisau Regency (location coordinates 11412’29.99”E and 238’14.96”S). The GBW was obtained by stripping Gelam wood logs with a diameter of less than 10 cm and an approximate thickness of 3.5 mm. Both parts of the bark, i.e., inner and outer, were used in the study. The bark was manually cut using a machete to a length of approximately 1 cm to 4 cm (Fig. 1a), then air-dried for approximately three weeks until the moisture content decreased to the range 13% to 15%. Afterward, these small pieces were mashed using a wood crusher, and the particles that passed through the 10 mesh sized filters were used to make the BBP (Fig. 1b). Finally, the particles were air-dried until the moisture content decreased to 5% to 7%. The irregular shapes of the bark particles, which occurs in both the outer and inner barks, can be seen in Fig. 1c.

Fig. 1. Photograph of the barks: (a) small cut bark; (b) bark particles; and (c) SEM micrographs of the bark particles at different magnifications.

Chemical Analysis of the Raw Materials

The chemical content of the GBW was analyzed according to the following standards: SNI standard 8401:2017 (2017)/Identical with TAPPI standard T204cm-07 (Alcohol-benzene and dichloromethane Extractives), SNI standard 01-1305-1989 (1989) (Solubility in hot water), SNI standard 14-1838-1990 (1990) (Solubility in 1% NaOH), Wise methods (Wise 1946) (Holocellulose), ASTM standard D1103-60 (1977) (α cellulose), SNI standard 0492-2008 (2008) (Klason Lignin), and SNI ISO standard 776:2010 (2010) (Ash). All chemical analyses were repeated three times.

Manufacturing and Testing

There are four types of BBP based on the pressing temperature used during manufacturing, i.e., the boards pressed at 140, 160, 180, and 200 C (five replicates for each temperature treatment), for a total of 20 single-layer boards measuring 300 mm x 300 mm x 10 mm with a target density of less than or equal to 0.59 g/cm3. To make the boards, 540 g of bark particles were first manually molded into a mat shape by placing and trampling it on a rectangular wooden forming box, whose base was covered with an aluminum sheet. A thickness bar was placed on top of the aluminum sheet, in the form of a wood frame measuring 300 mm long and 10 mm thick. The dimensions of the wooden forming box were 300 mm x 300 mm with a height of 100 mm. After the mat was molded, its upper surface was covered with another aluminum sheet (as shown in Fig. 2). Then the mat was cold-pressed for 5 min, followed by hot pressing at four different temperatures (for each temperature sample set) with a pressure of 30 kg/cmfor 20 min using a hydraulic hot press (Carver Laboratory Press, Carver Inc., Wabash, IN). To avoid blowing and blistering as well as ensuring the continuity of pressing the board with a hot-pressing machine, without turning off the tool, the board was immediately removed from the hot-pressing machine and transferred into a clamp to be cooled for 24 h, after which the clamp was removed. Then, the board was conditioned for two weeks via air-drying at a temperature of 25 C to 30 C with a relative humidity of 60% to 65%. Finally, the board was ready to be cut into a test sample.

Fig. 2. The manual formation of a mat

Five replicate test samples for each physical and mechanical properties test were prepared for each pressing temperature. All tests were carried out according to SNI standard 03-2105-2006 (2006). The physical properties tests included determining the density, moisture content (MC), thickness swelling after 24 h of soaking (TS24h), and water absorption after 24 h of soaking (WA24h) of the boards. The SNI standard 03-2105-2006 (2006) does not set standards for the water absorption of particleboard. However, the water absorption needs to be tested to determine how resistant the boards were to water, particularly for exterior use.

For the density and MC tests, samples measuring 100 mm x 100 mm were prepared. For these two tests, the same samples were used, because the density test did not damage the samples. The density test was carried out under air-dry conditions. First, the sample was weighed, then its volume was calculated by measuring the average length and width of the boards from two different measurement points; the thickness of the boards was determined by measuring the average thickness from four measurement points. The density of the boards was obtained by dividing its weight by its volume. Meanwhile, the MC was calculated by subtracting the initial weight of the board from the final weight of the board after it was dried in an oven at 103 C 2 C until it reached a constant weight.

The TS24h and WA24h values of the boards were determined using test samples measuring 50 mm x 50 mm. The tests were carried out by submerging the samples horizontally underwater at a temperature of 25 C 1 C for 24 h. Before submerging the samples, the authors made a note of the initial weight and thickness of the samples. After soaking, the weight and thickness of the samples were remeasured. The measurement method for determining the thickness of the samples, before and after soaking, was taken at the same location, i.e., all four corners that were located 10 mm from its actual corners (at the point of intersection of the length and width measurement).

When testing the mechanical properties of the boards, i.e., its modulus of rupture (MoR) and modulus of elasticity (MoE), samples were prepared that measured 200 mm x 50 mm, and the test was conducted using an Iber Test universal testing machine (Model MIB20AM, Madrid, Spain) under dry conditions. The test samples were placed horizontally on the two supports (the length of the support span was 150 mm), and the load was applied at the center of the samples with a loading speed of 10 mm/minute. The deflection was recorded, and the load application was continued until it reached its maximum load. In principle, the MoR refers to the ability of the adhesive-free particleboards to withstand a centrally applied load in a dry state. The tensile strength perpendicular to panel surface (TSPtPS) test, which is also known as the internal bonding strength (IB) test, is intended to measure the strength of adhesive-free particleboard in terms of sustaining an upright tensile load on its surface. The testing was carried out on a test sample that measured 50 mm x 50 mm. First, the length and width of the sample were measured and recorded. Then the sample was glued to two iron blocks and left to dry for 24 h. Afterward, the sample was pulled vertically with a loading speed of 2 mm/min.

Three pieces of BBPs that were pressed at 200 C were selected for the thermal conductivity tests (λ) at room temperature using a Kemtherm QTM-D3 thermal conductivity meter equipped with QTM PD3 probe (Kyoto Electronics Manufacturing Ltd, Kyoto, Japan). The basic principles of testing used the transient hot-wire method. Samples measuring 145 mm x 55 mm were prepared for the test. The probe was connected to the measuring device (the cable is connected to a 220-volt power supply), and the device was then heated for 30 min. The value of the heater current was 1 A2, which was based on the conductivity of the sample being tested. The value of the constant was adjusted to the value on the probe constant table; the probe was then placed on top of the sample. After a count down from 60 to 0 s, the thermal conductivity value would be shown on the digital display.

The data collected from testing the mechanical and physical properties were statistically analyzed using a single-factor analysis of variance (ANOVA) in Microsoft Excel for Windows, followed by Tukey’s (HSD) post hoc test with α equal to 0.05.

Fourier Transform Infrared Spectroscopy (FTIR)

The samples used in the analysis were the raw materials as well as the boards that were hot pressed at four different pressing temperatures (these samples were previously used in the bending strength tests). The FTIR analysis was carried out with a Shimadzu IR Prestige-21 Infrared spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The test samples were prepared in the form of KBr-pellets, and then the IR spectra were taken within the range of 4000 to 500 cm-1 and recorded with 40 scans at a resolution equal to 4.0 cm-1.

Scanning Electron Microscopy (SEM)

For this analysis, the BBP samples were cut into parallel-sections and cross-sections. Then, a microstructure observation was performed using a Carl-Zeiss (Evo MA 10, Cambridge, United Kingdom) scanning electron microscope (SEM), equipped with a Bruker (Quantax, Bruker Nano GmbH, Berlin, Germany) energy dispersive X-ray spectroscopy (EDX) detector, which was operated using an accelerated voltage of 20 kV. Before observation, the samples were coated with gold-palladium for 60s using an Emitech sputter coater (SC7620, Quorum Technologies Ltd, Lewes, United Kingdom). The SEM micrographs were taken at 60 x, 300 x, and 1000 x magnifications for each surface and cross-section. The EDX analysis for the cross-sections was taken at 300 x magnifications.

RESULTS AND DISCUSSION

All BBPs, i.e., the samples made at different pressing temperatures, were made without delamination. The board pressed at a temperature of 200 C had a smooth surface. Moreover, all the boards pressed at different pressing temperatures showed varying surface colors, ranging from light brown to dark brown with respect to the lowest temperature to the highest (as shown in Fig. 3). The material also emitted a distinctive odor. Similar results were also noticeable in the binderless boards made using the following raw materials: bagasse (Panyakaew and Fotios 2011), palm oil trunk (Boon et al. 2013), unripe coconut husk (Araújo Junior et al. 2018), wheat straw (Wang et al. 2019), soybean straw (Song et al. 2020), jute stick (Nitu et al. 2020), and densified wood (Shi et al. 2020); this could be the result of the modification of the chemical components that occurs during the heat treatment (Panyakaew and Fotios 2011; Wang et al. 2019). The presence of hemicellulose degradation and extractive movement may be the cause of the darkening of the color (Shi et al. 2020). Furthermore, Pintiaux et al. (2015) mentioned that the color change in the specimen was a sign of degradation, as reported by Araújo Junior et al. (2018) biomass components, namely hemicellulose, decompose at temperatures of 170 C or higher, while cellulose decomposes at 200 C or higher. Meanwhile, lignin decomposes more slowly in the range of 200 to 500 C.

Fig. 3. Appearance of the BBP surface at various pressing temperature

Chemical Properties of the Raw Material

The results of the GBW chemical analyses are shown in Tables 1 and 2. As can be seen in Table 1, the average holocellulose value for GBW was 78.8%, which is higher than the holocellulose values of woods from Borneo (Pettersen 1984). The GBW holocellulose value was also relatively higher than the holocellulose value found in a study by Ozgenc et al. (2017). The high holocellulose content of the bark is probably the result of the bark being peeled from the trunk using a commercial log-peeling machine. Tree bark stripped with commercial log-peeling machines often contains a large amount of actual wood, with less lignins and extractives and more cellulose than only bark (Geng et al. 2006). This explanation seems plausible because it is quite likely that wood could have been ripped away along with the bark when the bark was stripped from the trunk; this is also true for the Gelam bark stripped in the traditional way using a machete. Another explanation for this is the fact that holocellulose still contains lignin residues (Santana and Okino 2007). As shown in Table 1, the holocellulose content is corrected by lignin residues (Harun and Labosky 2007) and showed a lower value than GBW. Meanwhile, the average alpha-cellulose value of GBW was lower than the alpha-cellulose value of the woods from Borneo. The ash content of GBW (1.23%) is not high compared to the ash content of shagbark hickory (7.8%) but was higher than that of Melaleuca sp. wood (1.04%).

Table 1. The Main Components and the Ash Content of GBW Compared with Data from Literature Reviews on Wood and Other Tree Barks

The Klason lignin content of GBW (47.7%) is lower than the Klason lignin content of white pine bark but higher than the Klason lignin content of shagbark hickory (Harun and Labosky 2007). In addition, the Klason lignin content of GBW is higher than the Klason lignin contents of alder, chestnut, and beech bark, after an alcohol-benzene only dissolution and an alcohol-benzene and NaOH 1% dissolution (Ozgenc et al. 2017). The Klason lignin content of GBW is also higher than the Klason lignin content of Melaleuca sp. wood. From these results, the authors concluded that the Klason lignin content of GBW was high. However, Santana and Okino (2007) claimed that the method for determining lignin content has some flaws, which may give the impression of a higher lignin content than the actual figure (most common) or reduce it (rare). In the case of tree bark, Dou et al. (2018) argued that Klason lignins possibly also include other components other than lignins, even though the samples have been extracted in succession with several different solvents using commonly used methods. These components include condensed and hydrolysable tannins, and suberins that give the impression of a higher lignin content instead of the actual value (Harkin and Rowe 1971; Harun and Labosky 2007; Rowell et al. 2012). For the results of misleading from standard lignin analysis, Harkin and Rowe (1971) mark the word “lignin” which consists of a mixture of true lignin and suberized phlobaphene ranging from 40 to 50% for hardwood bark.

Table 2. Extractive Contents/solubility of GBW Compared with Data from Literature Reviews on Wood and Other Tree Barks