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Song, W., Zhu, M., and Zhang, S. (2018). "Comparison of the properties of fiberboard composites with bamboo green, wood, or their combination as the fibrous raw material," BioRes. 13(2), 3315-3334.

Abstract

The potential of bamboo green (B), an abundant lignocellulosic residue from the bamboo processing industry, was evaluated to serve as an alternative fibrous raw material in the production of fiberboard. Urea-formaldehyde resin-bonded fiberboards were prepared from B, wood fiber (W), and a mixture of the two (BW). The board type depended on the mass fraction of B in fibrous raw materials (including B and W), which were 0%, 20%, 40%, 60%, 80%, and 100%. The analytical methods used to characterize fibers and fiberboards included X-ray diffraction, thermogravimetric analysis, dynamic mechanical analysis, contact angle analysis, physical-mechanical analysis, and scanning electron microscopy. Compared with W, B showed a higher crystallinity index and thermogravimetric stability, but lower surface hydrophilicity and weaker interactions with urea-formaldehyde resin. Compared with W fiberboards, B fiberboards possessed a lower interfacial adhesion but fibrous raw materials in B fiberboards were better dispersed; moreover, B fiberboard displayed a higher dynamic viscosity, thermogravimetric stability, surface wettability, water absorption, and flexural modulus, but lower thickness swelling and flexural strength. The fiberboards produced with BW had better performances than the fiberboards produced with B and W. The 40% B mass fraction resulted in BW fiberboards with the best physical-mechanical properties.


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Comparison of the Properties of Fiberboard Composites with Bamboo Green, Wood, or their Combination as the Fibrous Raw Material

Wei Song,a,b,c Minghao Zhu,a and Shuangbao Zhang a,b,c,*

The potential of bamboo green (B), an abundant lignocellulosic residue from the bamboo processing industry, was evaluated to serve as an alternative fibrous raw material in the production of fiberboard. Urea-formaldehyde resin-bonded fiberboards were prepared from B, wood fiber (W), and a mixture of the two (BW). The board type depended on the mass fraction of B in fibrous raw materials (including B and W), which were 0%, 20%, 40%, 60%, 80%, and 100%. The analytical methods used to characterize fibers and fiberboards included X-ray diffraction, thermogravimetric analysis, dynamic mechanical analysis, contact angle analysis, physical-mechanical analysis, and scanning electron microscopy. Compared with W, B showed a higher crystallinity index and thermogravimetric stability, but lower surface hydrophilicity and weaker interactions with urea-formaldehyde resin. Compared with W fiberboards, B fiberboards possessed a lower interfacial adhesion but fibrous raw materials in B fiberboards were better dispersed; moreover, B fiberboard displayed a higher dynamic viscosity, thermogravimetric stability, surface wettability, water absorption, and flexural modulus, but lower thickness swelling and flexural strength. The fiberboards produced with BW had better performances than the fiberboards produced with B and W. The 40% B mass fraction resulted in BW fiberboards with the best physical-mechanical properties.

Keywords: Fiberboard; Bamboo green; Urea-formaldehyde resin; Interfacial adhesion; Physical-mechanical property

Contact information: a: Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing 100083, China; b: MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University, Beijing 100083, China; c: MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Forestry University, Beijing 100083, China;

* Corresponding author: shuangbaozhangj5@163.com

INTRODUCTION

Because of merits such as good processability, a smooth surface, and high dimensional stability, fiberboards have become one of the most commonly applied natural fiber composites, and it is used extensively in construction, building, furniture, and interior decorating (Wang et al. 2016a; Guan et al. 2017). For example, in North America, particleboard and medium density fiberboard (MDF) are the most extensively applied engineered wood products used in the manufacturing of furniture components (Dettmer and Smith 2015). Typically, fiberboard consists of wood fiber and a binder, and it is fabricated by pressing at a high pressure and temperature (Hong et al. 2017). In recent years, the increase in population has raised the demand for these wood products substantially (Kusumah et al. 2017). A high rate of wood consumption has aggravated deforestation and environmental pollution around the world (De Almeida et al. 2017). Because limited wood resources cannot meet this growing need, there is an increasing interest in exploring alternative biomass materials that could be used to sustain fiberboard production (Klímek et al. 2018).

Every year, the agricultural and forestry production industries generate abundant lignocellulosic residues (Huang et al. 2016; Cao et al. 2017). Until now, most of these residues have been burned or buried, which results in environmental pollution and wasted resources (Fan et al. 2015; Theng et al. 2017a). Recently, utilizing these residues in the fabrication of fiberboard has been considered a promising strategy, from both environmental and economic perspectives (Jin et al. 2017; Zhang et al. 2017). For example, Basta et al. (2017) manufactured bagasse boards bonded with activated carbon-modified urea-formaldehyde resin. Cao et al. (2017) prepared isocyanate-glued boards from sodium hydroxide-treated and ammonia-treated wheat straw. Dukarska et al. (2017) fabricated rapeseed straw boards bonded with mixed isocyanate/phenol-formaldehyde resin. Araújo Jr. et al. (2017) investigated boards produced from unripe coconut husk. Klímek et al. (2018) evaluated boards produced from Miscanthus stalk. Kusumah et al. (2017) studied the effects of the hot-pressing process on boards produced from sweet sorghum bagasse. Lenormand et al. (2017) surveyed the thermal and acoustic behaviors of sunflower pith boards. Qu et al. (2017) observed the influence of composting on the properties of rice straw and straw boards. Sahin et al. (2017) measured the physical-mechanical properties of boards made from peachnut shell and glass flour. Sam-Brew and Smith (2017) prepared boards from flax shive and hemp hurd residues. Theng et al. (2017a) assessed boards fabricated from the rice straw that was modified by a digestion plus defibration process. Uitterhaegen et al. (2017) manufactured boards from the coriander straw that was treated with a twin-screw extrusion method.

As a perennial woody grass, bamboo is cultivated worldwide (Li et al. 2013; Deng et al. 2017). Because bamboo is highly productive and fast-growing, it has been applied in a wide range of industries, such as in board production and the pulp and paper industries (Fan et al. 2015; Ramage et al. 2017). For bamboo, the stem consists of skin, timber, and pith, and the timber is further divided into the green, meat, and yellow according to the density of the vascular bundle (Xin et al. 2015; Wang et al. 2016b). In the bamboo processing industry, bamboo green and yellow are usually removed from the timber and become waste (Li et al. 2014; Pan et al. 2017). The reason for removing them has been explained by some researchers. Zhang et al. (2013) pointed out that bamboo green and yellow have abundant hydrophobic substances, such as silica and wax, which adversely affect the surface wettability and gluability of bamboo during board production. Xu et al. (2016) noted that the abundant silica in these fibrous raw materials negatively influences the pulping and papermaking processes, particularly the operations that occur in the evaporator, recovery furnace, and lime kiln. Currently, the utilization rate of crude bamboo is still lower than 40% by weight, and removing bamboo green produces abundant lignocellulosic residues (Fan et al. 2015). For example, China is recognized as the “bamboo kingdom” because of its abundant bamboo resources; it possesses the largest plantation area and highest annual yield in the world (Song et al. 2015). In China, the bamboo processing industry generates approximately 50 million tons of lignocellulosic residues each year, which are mainly composed of bamboo green and yellow (Huang et al. 2016).

With the development of the bamboo industry, the utilization of bamboo green has received increasing attention. During the past few years, some studies have been conducted on converting bamboo green into various biochemicals and biofuels. Huang et al. (2015) characterized hemicellulose extracted from bamboo green. Huang et al. (2016) isolated ferulic acid from the cell walls of bamboo green. Li et al. (2014) studied the sulfite treatment of bamboo green for enzymatic saccharification. Li et al. (2015) assessed the impact of different reagents on the enzymatic hydrolysis of bamboo green. Xin et al. (2015) compared the influence of dilute acid and aqueous ammonia treatments on the physicochemical properties and enzymatic hydrolysis of bamboo green. Yang et al. (2016) employed visible-near infrared spectra to establish a fast method for analyzing the chemical composition and enzymatic digestibility of bamboo green. In addition to the above-mentioned reports, some researchers have investigated the surface properties of bamboo green and their effects on bamboo board production. Zhang et al. (2013) measured the impact of alkali treatment of bamboo green on the adhesion between the bamboo green strip and isocyanate. Deng et al. (2015) investigated the influence of the degree of bamboo green removal on the performance of laminated bamboo-bundle veneer lumber. Zhang et al. (2015) observed the surface chemistry and wettability of bamboo green that was modified by physical and chemical treatments. Although some fruitful research on bamboo green has been done, there are still many untouched issues that remain that need to be explored. For example, the literature does not currently contain any publications that report the effects of bamboo green on the properties of fiberboard. Therefore, it is still unknown whether bamboo green can serve as a desirable alternative material to sustain fiberboard production.

To address these unknowns, urea-formaldehyde resin-bonded fiberboards were prepared using bamboo green, wood, and a mixture of the two as the fibrous raw material in this research. The board type was determined by the mass fraction of the bamboo green in fibrous raw materials, which ranged from 0% to 100%. The analytical methods used to characterize the fibers and boards included X-ray diffraction, thermogravimetric analysis, dynamic mechanical analysis, contact angle analysis, physical-mechanical analysis, and scanning electron microscopy. Using these methods, the following analyses were conducted: (1) comparison of the properties of the bamboo green and wood; (2) comparison of the properties of the bamboo green fiberboard and wood fiberboard; and (3) analysis of the influence of the bamboo green mass fraction on the properties of the fiberboards made with a mixture of bamboo green and wood. The conclusions in this study can provide some new insights for the development of natural fiber composites from lignocellulosic residues.

EXPERIMENTAL

Materials

The bamboo green (species: Neosinocalamus affinis; main particle size: 20 mesh to 40 mesh) was purchased from Chitianhua Group (Guiyang, China). The wood fiber (species: Populus tomentosa; main particle size: 20 mesh to 40 mesh) and urea-formaldehyde resin (molar ratio of formaldehyde to urea: 1.1; solid mass fraction: 52%; pH: 8.5; viscosity: 40 cP; ammonium chloride curing agent mass fraction: 1% compared with solid resin) were purchased from Krono Wood-based Panels Co., Ltd. (Beijing, China).

Board Preparation

The bamboo green and wood were oven-dried to a moisture content of 3%. Using a laboratory blender, these fibrous raw materials were mixed with the resin. In fibrous raw materials (including bamboo green and wood), the mass fraction of the bamboo green ranged from 0% to 100%. In total materials (including fibrous raw materials and resin), the mass fraction of the resin was 17%. Next, the resinated fibers were manually formed into mats, which had a moisture content of 12%. The target density and thickness of the boards were 0.75 g/cm3 and 10 mm, respectively. Finally, the pre-pressed mats were hot-pressed at 180 °C for 2 min at a pressure of 2 MPa. Similar parameters for preparing boards have been used by other researchers (Kargarfard and Jahan-Latibari 2014; Lü et al. 2015; Hong et al. 2017; Kusumah et al. 2017; Theng et al. 2017b).

In this study, when the bamboo green mass fractions in fibrous raw materials were 0%, 20%, 40%, 60%, 80%, and 100%, the corresponding boards were labeled B0W1, B2W8, B4W6, B6W4, B8W2, and B1W0, respectively. It should be noted that when the bamboo green mass fraction was 0%, the fibrous raw material in the board was only wood and the corresponding board (B0W1) was a wood fiberboard; when this mass fraction was 100%, the fibrous raw material in the board was only bamboo green and the corresponding board (B1W0) was a bamboo green fiberboard. Three duplicate boards were produced for each level of bamboo green mass fraction in fibrous raw materials.

Analytical Methods

X-ray diffraction

The X-ray diffraction patterns of the fibers were collected with an X’Pert Pro X-ray diffractometer (PANalytical, Almelo, Netherlands) from the 2θ angles of 8° to 40° at a scanning rate of 2°/s. The diffractometer used Ni-filtered CuKα radiation (wavelength: 1.5406 Å) and was operated at 40 mA and 40 kV. The crystallinity index (Crl, %) of the cellulose was calculated with the following equation (Segal et al. 1959),

Crl = (I002 – Iam)/I002 × 100 (1)

where I002 (a.u.) and Iam (a.u.) represent the peak intensities corresponding to the crystalline and amorphous fractions of the cellulose, respectively.

Thermogravimetric analysis

The thermogravimetric curves of the fibers and boards were recorded using a TGA Q500 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA) from 100 °C to 500 °C at a heating rate of 10 °C/min and with a 60-mL/min nitrogen flow rate.

Dynamic mechanical analysis

The storage modulus and loss factor of the boards (dimensions: 55 mm × 12 mm × 3 mm) were measured using a DMA Q800 dynamic mechanical analyzer (TA Instruments) from 50 °C to 210 °C at a heating rate of 5 °C/min, under a dual-cantilever mode, and with a frequency of 1 Hz and amplitude of 30 µm.

Contact angle

The water contact angle with respect to the boards was recorded with an OCA20 contact angle meter (DataPhysics Instruments GmbH, Filderstadt, Germany). The measurement was repeated eight times.

Physical-mechanical properties

The water absorption of the boards was measured according to the Chinese national standard GB/T 17657 (2013). The thickness swelling, flexural strength, and flexural modulus of the boards were measured according to the Chinese national standard GB/T 11718 (2009). The water absorption and thickness swelling were measured after the boards were soaked in 20 °C water for 24 h. The flexural properties were measured with the three-point flexural test. The water absorption measurement was repeated three times, and the flexural measurement was repeated six times. The mechanical measurement was performed using an MWW-50 universal mechanical testing machine (Tayasaf Corporation, Beijing, China). The statistical analysis was done using the software SPSS 20 (IBM, Armonk, NY, USA).

Scanning electron microscopy

The flexural fracture surface of the boards was observed using an SU8010 scanning electron microscope (Hitachi, Tokyo, Japan) under an acceleration voltage of 5 kV. Before observation, the samples were sputter-coated with gold.

RESULTS AND DISCUSSION

Comparison of the Bamboo Green and Wood Properties

X-ray diffraction analysis

Figure 1a shows the X-ray diffraction patterns of the bamboo green and wood. Both fibers showed peaks at the 2θ angles of 16°, 22°, and 35°, which indicated that they possessed a crystalline structure of native cellulose I (Song et al. 2015).

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Fig. 1. X-ray diffraction pattern (a) and crystallinity index (b) of bamboo green (B) and wood (W)

Moreover, at the 2θ angle of 26°, there was a peak in the diffraction pattern of the bamboo green, but not in that of the wood. This peak was related to silica (Lee et al. 2017). Therefore, Fig. 1a demonstrates that, compared with the wood, the bamboo green contained more silica, which is a hydrophobic substance that will adversely influence the surface wettability and gluability (Zhang et al. 2013). Similar results can be found in previous reports. Deng et al. (2015) observed the contact angle of bamboo with different removal degrees of bamboo green, and found that increasing the removal degree decreased the contact angle of phenol-formaldehyde resin on the bamboo. Zhang et al. (2013) evaluated the bonding performance of sodium hydroxide-treated bamboo green, and found that the treatment enhanced the adhesion between the bamboo green strip and isocyanate. These authors explained that the surface of the bamboo green is usually covered with abundant silica and wax. Removing the bamboo green reduced the amount of these hydrophobic substances, and thus promoted the surface wettability of the bamboo. Moreover, alkali conditions dissolved these hydrophobic substances, which boosted the gluability of the bamboo green.

Figure 1b shows that the crystallinity index of the bamboo green was higher than that of the wood. This difference may have been related to their chemical composition. For example, the presence of some amorphous compounds, such as hemicellulose, lignin, and amorphous cellulose, can decrease the crystallinity index of lignocellulosic fiber (Song et al. 2017a). Therefore, Fig. 1b indicates that the bamboo green may have had a lower content of these amorphous compounds compared with the wood. In the future, the differences in the chemical composition between the bamboo green and wood should be further studied.

Thermogravimetric analysis

Figure 2 shows the thermogravimetric curves for the bamboo green and wood. When the temperature increased from 100 °C to 200 °C, the weight loss of the fiber was mainly caused by dehydration (Feng et al. 2012). Over this temperature range, the bamboo green lost less weight than the wood, which indicated that the bamboo green had a lower hygroscopicity and moisture content. This may have been because the surface of the bamboo green was covered with an abundant amount of hydrophobic substances, such as silica and wax (see Fig. 1 analysis). Moreover, this may also have been related to the higher crystallinity index of the bamboo green (see Fig. 1 analysis), which could have decreased the accessibility of hydrophilic hydroxyl groups in the bamboo green to water molecules (Song et al. 2017a).

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Fig. 2. Thermogravimetric curves of the bamboo green (B) and wood (W)

When the temperature was increased from 200 °C to 500 °C, the weight loss of the fiber was mainly attributable to the degradation of cell wall compounds, such as hemicellulose, cellulose, and lignin (Song et al. 2017b). Over this temperature range, the bamboo green lost less weight than the wood, which indicated that the bamboo green possessed a higher resistance to pyrolysis. This may have been because the bamboo green contained abundant silica (see Fig. 1 analysis), which hindered the heat transmission and enhanced the thermal stability (Liu et al. 2016). Similar results can be found in previous reports. Wang et al. (2011) observed the thermogravimetric curves of a starch-based wood adhesive and found that the silica-modified adhesive lost less weight from 200 °C to 500 °C. They explained that the silica enhanced the molecular structure of the adhesive, which improved its thermal stability.

Comparison of the Properties of the Bamboo Green Fiberboard and Wood Fiberboard

Dynamic mechanical analysis

Figure 3 shows the dynamic mechanical properties of B1W0 and B0W1. When the temperature increased from 50 °C to 210 °C, the storage modulus retention ratio of the boards (i.e., the ratio of the storage modulus at a given temperature to the storage modulus at 50 °C) decreased, but the loss factor increased. This was because increasing the temperature weakened the intermolecular forces in the composites, which decreased the stiffness and elasticity of the boards and increased their toughness and viscosity (Ren et al. 2014; Song et al. 2017b).

Fig. 3. Dynamic mechanical properties of B1W0 and B0W1: (a) storage modulus retention ratio and (b) loss factor

Using the dynamic mechanical data, the interfacial bonding of the natural fiber composites can be evaluated by analyzing the adhesion factor (A, a.u.), which can be calculated with Eq. 2,

A = 1/(1-Vf) × tanδb/tanδr – 1 (2)

where Vf represents the fiber volume fraction in the boards, and tanδb and tanδr represent the loss factor of the board and resin, respectively (Wang et al. 2017).

Because B1W0 and B0W1 possessed the same Vf and tanδr values, their A value was affected only by the tanδb value. Figure 3b shows the tanδb value of B1W0 was higher than that of B0W1 from 50 °C to 210 °C; thus, the A value of B1W0 was higher than that of B0W1. Typically, a lower A value reflects a stronger interaction between the fiber and resin, which leads to a higher interfacial bonding in the composites (Luo et al. 2017). Therefore, the A values indicated that the bamboo green had a weaker interaction with the resin and that B1W0 had a lower interfacial bonding. This may have been because the bamboo green contained abundant silica and wax (see Fig. 1 analysis), and these hydrophobic substances adversely affected its wettability and gluability.

Figure 3a shows B1W0 exhibited a lower storage modulus retention ratio than B0W1 from 50 °C to 210 °C, which indicated that the stiffness properties of B1W0 were more thermally unstable than those of B0W1 (Ren et al. 2014). This could be explained by the lower interfacial bonding in B1W0, which made the stress transfer not very efficient; hence, it was more likely for the stiffness properties of B1W0 to be negatively affected by the increase in temperature (Lu et al. 2014).

Figure 3b shows B1W0 gave a higher loss factor than B0W1 from 50 °C to 210 °C, which indicated that the viscosity and elasticity of B1W0 was higher and lower than that of B0W1, respectively (Song et al. 2017b). This result was also supported by the lower interfacial bonding in B1W0 (Ren et al. 2014). Typically, a higher loss factor means that a material possesses a greater damping capacity (Li and Wang 2017). Under loading, this material is more likely to convert energy into heat, which is dissipated within the material itself, rather than released into the air as noise (Ren et al. 2014). Therefore, Fig. 3b reveals that bamboo green could make the boards more efficient at absorbing sound or other undesirable vibrations (Zhu et al. 2017).

Thermogravimetric analysis

Figure 4 shows the thermogravimetric curves of B1W0 and B0W1. When the temperature increased from 100 °C to 200 °C, the weight loss of the fiberboard was primarily attributed to water evaporation (Song et al. 2017b). Over this temperature range, B1W0 lost less weight than B0W1, which indicated that B1W0 had a lower hygroscopicity and moisture content. This may have been because the bamboo green had a lower hygroscopicity and moisture content than the wood (see Fig. 2 analysis). Moreover, when the temperature increased from 100 °C to 200 °C, B1W0 lost less weight than the bamboo green and B0W1 lost less weight than the wood. These results indicated that the hygroscopicity and moisture content of the boards were lower than those of the corresponding fibrous raw materials. This may have been because these boards were prepared through a hot-pressing process, and the heat treatment reduced the hygroscopicity and moisture content of the materials (Song et al. 2017a).

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Fig. 4. Thermogravimetric curves of the bamboo green (B), wood (W), B1W0, and B0W1

When the temperature increased from 200 °C to 500 °C, the weight loss of the fiberboard was primarily attributed to the degradation of plant cell wall components (Song et al. 2017b). Over this temperature range, B1W0 lost less weight than B0W1, which indicated that B1W0 had a higher pyrolysis resistance. This may have been because the bamboo green possessed a higher pyrolysis resistance than the wood (Fig. 2). When the temperature increased from 200 °C to 360 °C, B1W0 lost more weight than the bamboo green and B0W1 also lost more weight than the wood, which indicated that the pyrolysis resistances of the boards were lower than those of the corresponding fibrous raw materials. This may have been because the urea-formaldehyde resin in the boards began to degrade after 200 °C and accelerated the degradation of the boards over this temperature range (Feng et al. 2012). When the temperature increased from 360 °C to 500 °C, B1W0 lost more weight than the bamboo green but B0W1 lost less weight than the wood, which indicated that the pyrolysis resistance of B1W0 was still lower than that of the bamboo green, but the pyrolysis resistance of B0W1 was higher than that of the wood. This may have been because the interaction between the resin and bamboo green was different from that between the resin and wood. It has been reported that urea-formaldehyde resin can enhance cross-linking between the main components of lignocellulosic fiber and inhibit the degradation of fiberboard over this temperature range (Feng et al. 2012). Therefore, the urea-formaldehyde resin noticeably inhibited the degradation of B0W1. However, the bamboo green had a weaker interaction with the resin (Fig. 3), and thus the urea-formaldehyde resin did not remarkably inhibit the degradation of B1W0.

Contact angle analysis

Figure 5 shows the initial water contact angle on B1W0 and B0W1. The contact angle of B1W0 was lower than 90° and that of B0W1 was higher than 90°, which indicated that the surface of B1W0 was hydrophilic and that of B0W1 was hydrophobic (Song et al. 2018). Because the fibrous raw materials in B1W0 and B0W1 were encapsulated within the urea-formaldehyde resin, the contact angle results indicated that the resin in B1W0 was more hydrophilic than that in B0W1. This could have been because when using urea-formaldehyde resin to prepare natural fiber composites, the hydrophilic active functional groups in the resin react with the fibrous raw materials, during which some of these groups are consumed (Chen et al. 2017; Zhou et al. 2017). When analyzing the results exhibited in Fig. 3, it was demonstrated that, compared with the wood, the bamboo green experienced a weaker interaction with the resin. Therefore, when preparing B1W0, many hydrophilic active functional groups in the resin could not be consumed, which made the resin in B1W0 more hydrophilic. Typically, to improve the physical-mechanical properties of fiberboards and to reduce their formaldehyde emissions, boards will be coated with some decorative surface materials, and a good board wettability is required when performing this treatment (Buyuksari et al. 2010). Therefore, as the results displayed in Fig. 5 indicated, the bamboo green was able to improve the wettability of the boards, and thus made them easier to be coated.

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Fig. 5. Initial water contact angle on B1W0 and B0W1

Physical-mechanical analysis

Figure 6 shows the physical-mechanical properties of B1W0 and B0W1. The water absorption of B1W0 was 6.3% higher than that of B0W1, which indicated that B1W0 absorbed more water during 24 h of water immersion. The thickness swelling of B1W0 was 9.8% lower than that of B0W1, which indicated that B1W0 exhibited a better dimensional stability after absorbing water. The flexural strength of B1W0 was 16.8% lower than that of B0W1, which indicated that B1W0 had a lower resistance to flexural fracture. The flexural modulus of B1W0 was 31.7% higher than that of B0W1, which indicated that B1W0 displayed a higher resistance to flexural deformation.

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Fig. 6. Physical-mechanical properties of B1W0 and B0W1: (a) water absorption, (b) thickness swelling, (c) flexural strength, and (d) flexural modulus

When analyzing the results displayed in Fig. 3, it was confirmed that B1W0 had a lower interfacial adhesion than B0W1. Therefore, the interface of B1W0 will have more gaps, which allowed more water to enter the boards during water immersion and negatively affected the stress transfer in the boards, causing B1W0 to show a higher water absorption and lower flexural strength (Song et al. 2017b; Xu and Fu 2017). Similar results can be found in previous reports. For example, Tang et al. (2017) observed that increasing the resin content improved the interfacial adhesion of poplar wood fiberboards, and thus reduced the water absorption of the boards. Kurokochi and Sato (2015) reported that rice straw contains abundant silica and wax; after removing these hydrophobic substances, the interfacial adhesion of the straw boards was enhanced, which increased the flexural strength of the boards.

When analyzing the results shown in Fig. 2, it was noted that the bamboo green possessed a lower hygroscopicity than the wood. During water immersion of the boards, the lower hygroscopicity prevented the bamboo green from absorbing water and expanding, which caused B1W0 to exhibit a lower thickness swelling (Chang et al. 2018). When analyzing Fig. 1, it was determined that the bamboo green displayed a higher crystallinity index than the wood. The higher value gave the bamboo green a higher stiffness and caused B1W0 to obtain a higher flexural modulus (Cao et al. 2017). Similar results can be found in previous reports. For example, Kurokochi and Sato (2015) determined that when the content of silica and wax in rice straw increased, the hygroscopicity of the straw decreased, which led to a lower thickness swelling for straw boards. Cao et al. (2017) found that a sodium hydroxide treatment for wheat straw increased its crystallinity index, and resulted in straw boards with a higher flexural modulus.

Scanning electron microscope analysis

Figure 7 shows the flexural fracture surface of B1W0 and B0W1. Compared with B0W1, B1W0 exhibited a surface with a more homogenous texture, which indicated that there was a better dispersion of the fibrous raw materials in B1W0 (Liu et al. 2014). This was because the surface of the bamboo green was coated with abundant silica and wax (Fig. 1), and these hydrophobic substances made the fibrous raw materials unlikely to agglomerate, which led to a higher fiber dispersion in the boards (Ahamad Nordin et al. 2017). Figure 7a shows that there were many small gaps on the surface of B1W0, which reflected a lower interfacial adhesion (Song et al. 2017b). Similar to the dispersion result, this result was also related to the abundant silica and wax in the bamboo green, as these hydrophobic substances negatively affected its wettability and gluability (Deng et al. 2015). In contrast, Fig. 7b displays fiber splitting and tearing on the surface of B0W1, which meant that stress was transferred from the resin to the fibers (Liu et al. 2014). This revealed a higher interfacial adhesion between the wood and resin (Ahamad Nordin et al. 2017).