Abstract
The aim of this study was to investigate the dimensional stability and mold resistance of slivers from the outer and inner layers of bamboo treated with alkali solutions at various concentrations. The microstructure of the bamboo slivers considerably changed as the parenchyma cells collapsed after alkali treatment followed by a drying process. The water absorption of the treated bamboo slivers increased, while the dimensional stability decreased, especially for the slivers from the inner layer of bamboo. The alkali treatment removed starch from the parenchyma cells in the bamboo slivers treated with a 2% to 15% alkali solution, resulting in a considerable improvement in the mold resistance. The mold resistance performance of inner bamboo slivers was greatly improved when treated at a low concentration (2%). No mold on the bamboo slivers was found even in a high humidity environment for a long period of time, i.e. 87 days. As the concentration increased up to 25%, alkali only removed starch from parenchyma cells that were near the surface of the bamboo slivers and caused partial damage to the parenchyma cells in the outer bamboo slivers. The wettability of the alkali-treated bamboo slivers was higher than that of the untreated samples due to the removal of lignin and a rougher surface. Based on the test results, alkali treatment is a simple yet highly effective method for improving mold resistance but would cause a reduction in the dimensional stability of the bamboo slivers.
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Dimensional Stability and Mold Resistance of Bamboo Slivers Treated by Alkali
Hong Chen,a,b Jieyu Wu,a,b Jiangjing Shi,a,b Ge Wang,c and Wenfu Zhang d,*
The aim of this study was to investigate the dimensional stability and mold resistance of slivers from the outer and inner layers of bamboo treated with alkali solutions at various concentrations. The microstructure of the bamboo slivers considerably changed as the parenchyma cells collapsed after alkali treatment followed by a drying process. The water absorption of the treated bamboo slivers increased, while the dimensional stability decreased, especially for the slivers from the inner layer of bamboo. The alkali treatment removed starch from the parenchyma cells in the bamboo slivers treated with a 2% to 15% alkali solution, resulting in a considerable improvement in the mold resistance. The mold resistance performance of inner bamboo slivers was greatly improved when treated at a low concentration (2%). No mold on the bamboo slivers was found even in a high humidity environment for a long period of time, i.e. 87 days. As the concentration increased up to 25%, alkali only removed starch from parenchyma cells that were near the surface of the bamboo slivers and caused partial damage to the parenchyma cells in the outer bamboo slivers. The wettability of the alkali-treated bamboo slivers was higher than that of the untreated samples due to the removal of lignin and a rougher surface. Based on the test results, alkali treatment is a simple yet highly effective method for improving mold resistance but would cause a reduction in the dimensional stability of the bamboo slivers.
DOI: 10.15376/biores.17.2.2827-2848
Keywords: Bamboo slivers; Alkali treatment; Microstructure; Dimensional stability; Mold resistance
Contact information: a: College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037 China; b: Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China; c: International Center for Bamboo and Rattan, Beijing 100102 China; d: Zhejiang Academy of Forestry, Hangzhou 310023 China;
* Corresponding author: zhangwenfu542697@163.com
INTRODUCTION
Bamboo, one of the fastest-growing plants, is usually processed into multi-scale units for different end-uses, e.g., bamboo strips, bamboo slivers, bamboo bundles, and individual bamboo fibers (Yu et al. 2014; Yu et al. 2015; Deng and Wang 2018; Yang et al. 2020). The structure and properties of bamboo units at different scales vary, as bamboo has a gradient structure (Chen et al. 2015). Bamboo is traditionally split into bamboo slivers for bamboo weaving products. Recently, the use of bamboo slivers has been expanded to the field of bio-composites (Chen et al. 2019; Shi et al. 2019; Chen et al. 2021c). There are many modification methods for improving the properties of bamboo strips and bamboo fibers, including hot oil, densification with or without delignification, filling resin, and alkali treatment (Das and Chakraborty 2006; Dixon et al. 2016; Chen et al. 2017; Salvati et al. 2018; Kalali et al. 2019; Tang et al., 2019; Chen et al. 2020a; Li et al. 2020; Wang et al. 2021). However, few studies have reported the effect of modification on bamboo slivers, especially in terms of a comparative study between the slivers from the outer and inner layers of bamboo. Alkali treatment is a popular method for modifying bamboo fibers and other plant fibers to obtain a high-performance reinforcement effect in composites (Cai et al. 2015; Huang and Young 2019; Kathirselvam et al. 2019; Akinyemi et al. 2020; Xia et al. 2020). The structure and properties of the treated bamboo fibers were determined according to the alkali treatment conditions, i.e., concentration, time, temperatures, and the size of the bamboo unit itself (Das and Chakraborty 2006; Liu and Hu 2008; Chen et al. 2017; Chen et al. 2018). According to the previous research, the water absorption of bamboo fiber composites decreased from 61% to 26% when treated with 5% NaOH for 0.5 h, which was due to the reduction of the hydroxyl group in the cell wall of the fiber molecule by chemical treatment (Kushwaha and Kuma 2010). The wettability of bamboo fibers was enhanced when treated by NaOH solution with lower concentrations, while it was reduced with alkali treatment at high concentration (25%) (Chen et al. 2018). It was shown that the water absorption and wettability of bamboo fiber were improved after alkali treatment.
The effect of the alkali treatment on bamboo strips is different from its effect on bamboo fibers, in terms of the microstructure, mechanical properties, and cellulose crystallinity (Ray et al. 2005; Salvati et al. 2018). This could be due to the fact that a bamboo sliver is a specific unit consisting of fibers surrounded by parenchyma matrix, which is not as thick as a bamboo strip. As a result, a gradient structure can be avoided, as suggested in Fig. 1. In previous studies, it was found that parenchyma cells reacted differently with alkali in comparison to fibers (Chen et al. 2021b; Yuan et al. 2021). The bamboo slivers obtained from the outer layer and inner layer of bamboo have different parenchyma cell volume fractions (Fig. 1b and Fig. 1c). Although processing bamboo into thin bamboo slivers could avoid a gradient structure in a bamboo product, the difference in the parenchyma cell volume fractions of the slivers from the outer and inner layers of bamboo might still cause a discrepancy in the structure and properties.
Fig. 1. The structure of the bamboo units at multi-scale: (a) bamboo culm; (b) a sliver from the outer bamboo; (c) a sliver from the inner bamboo; (d) a bamboo fiber bundle; and (e) an individual bamboo fiber
Besides, there are nutritional substances, i.e., starch, sugar, and protein in bamboo, and these are ideal for mold growth when bamboo is stored in a warm and humid environment. The mold can cause discoloration, which has a serious impact on the use, storage, and transportation of bamboo products. Hence, the treatment of bamboo against mold is very important. Many scholars have improved the mold resistance of bamboo by oil heat treatment, acid treatment and ZnO/PMHS coatings, and achieved good results (Chen et al. 2019; Hao et al. 2021; Yu et al. 2022). However, there have been few reports on the influence of alkali treatment on the mold resistance performance of bamboo. A two-step NaOH treatment has been reported to improve mold resistance of bamboo strips, in which the bamboo strips were soaked in 1% NaOH solution at 140 °C after hydrothermal treatment at atmospheric pressure (0.2 to 0.3 MPa), and then the treated bamboo strips were baked at 180 °C for 2 hours (Cheng et al. 2013).
In a previous study, the tensile strength and the flexibility of bamboo slivers treated with alkali were drastically improved, especially for the slivers from the inner layer of bamboo, which widely broadens the application of bamboo slivers (Chen et al. 2021a). This study further investigated the effect of alkali treatment on the dimensional stability and mold resistance of bamboo slivers. In this research, the microstructure, dimensional stability, water wettability, and mold resistance were examined for bamboo slivers from the outer and inner layers of bamboo and were treated with different alkali solution concentrations.
EXPERIMENTAL
Materials
Sample preparation
Moso bamboo (Phyllostachys pubescens) was obtained from Zhejiang, China, and dried to a moisture content of 8% to 12%. The bamboo was cut into bamboo slivers with a length of approximately 120 mm, a width of approximately 4 mm, and a thickness of approximately 1 mm from the outer and inner layers of the bamboo culm, respectively. Then the bamboo slivers were immersed in sodium hydroxide (NaOH) solutions with various concentrations, i.e., 2%, 5%, 10%, 15%, and 25%, for 2 h at room temperature. The treated bamboo slivers were then washed until neutral. All the treated bamboo slivers were dried in an oven at a temperature of 50 °C for 8 h.
Characterization
The surface morphology of the untreated and treated bamboo slivers and the cross-sections of the untreated and treated bamboo slivers after being immersed in water for 12 h were observed with an environmental scanning electron microscopy (ESEM) (FEI Quanta 200, Hillsboro, OR). The pore structure of the bamboo slivers treated with 15% NaOH was obtained via micro-computed tomography (Micro-CT) (NanoVoxel 3000, Sanying Precision Instruments Co., Ltd, Tianjin, China). The voltage and current were set at 80 Kv and 60 μA, respectively. The scanning time varied from 1.5 to 3 h, depending on the dimensions of the samples. The water absorption (WA), thickness swelling (TS), width swelling (WS), and volumetric swelling (VS) of the treated and untreated slivers from the outer layer and inner layer of bamboo were calculated according to GB/T standard 1931 (2009), GB/T standard 1934.1 (2009), and GB/T standard 1934.2 (2009), respectively, as shown in Eqs. 1 through 4,
where m0, t0, w0, and v0 are the weight (g), thickness (mm), width (mm), and volume (g/cm3) of the untreated and alkali-treated bamboo slivers before being immersed in water for 12 h, respectively, while m1, t1, w1, and v1 are the weight (g), thickness (mm), width (mm), and volume (g/cm3) of the corresponding samples after being immersed in water for 12 h. The untreated and alkali-treated bamboo slivers were stored in an ambient environment (a temperature of 20 to 28 °C and relative humidity (RH) of 54% to 72%) for 3 months to examine mold resistance. The contact angles of distilled water on the untreated and treated bamboo slivers were determined with a Krüss DAS100S (Hamburg, Germany). The CCD cameras recorded the process of a water droplet dropping on the bamboo sliver surface. The volume of the water droplet was 2 µL.
RESULTS AND DISCUSSION
Microstructure
Figure 2 shows that the color of the bamboo slivers was altered when treated at different alkali concentrations. The color became increasingly darker as the alkali concentration increased. In addition, the color of the slivers from the inner layer of bamboo was more pronouncedly altered compared to the outer layer of the bamboo when treated at the same alkali concentration.
Fig. 2. Photographs of the untreated and treated bamboo slivers showing the change in the color: (a) the inner layer; and (b) the outer layer
Figures 3 and 4 show the parenchyma cells in the slivers from the outer layer and the inner layer of the bamboo, respectively. The collapse of the parenchyma cells was increasingly more severe as the alkali concentration continued to increase, but the damage of the cell walls within the parenchyma cells only occurred when treated at an alkali concentration of greater than 5% and 10% for the slivers from the inner and outer layers of the bamboo, respectively. This indicated that the parenchyma cells in the slivers from the bamboo inner layer were easier to collapse during the alkali treatment in comparison to the parenchyma cells from the outer layer. Meanwhile, the fibers in the slivers were not considerably altered, as shown in Figs. S1 and S2 (see Appendix). The interface between the fibers was degraded when treated with 25% NaOH and the fibers partly separated from the sliver surface, as shown in Fig. S3. The severity of the damage to the parenchyma cells and fibers was different due to the differences in their chemical components, microstructure, and properties, which lead to them reacting differently when treated under the same alkali conditions (Huang et al. 2015; Zhang et al. 2018; Jin et al. 2019; Lian et al. 2020; Chen et al. 2021b).
Fig. 3. Cross-section of the parenchyma cells in the slivers from the inner layer of the bamboo treated with different concentrations (scale bar = 50 μm)
Fig. 4. Cross-section of parenchyma cells in the slivers from the outer layer of the bamboo treated with different concentrations (scale bar = 50 μm)
The density of the bamboo slivers treated with a 15% NaOH solution increased up to the maximum values of 1.7 and 1.45 g/cm3 for the slivers from the inner and outer layer of the bamboo, respectively (the untreated samples were 0.87 and 1.22 g/cm3, respectively). Figure 5 shows the corresponding microstructure observed via Micro-CT. The parenchyma cells collapsed in a greater amount in the slivers from the inner layer of the bamboo compared to the outer layer. In addition, the parenchyma cells near the surface of the slivers collapsed more pronouncedly than the parenchyma cells in the middle part, especially in the slivers from the outer layer of the bamboo. This was probably due to the fact that the alkali solution could more easily interact with the parenchyma cells near the surface than those in the middle part. The porosity of the untreated slivers from the outer and inner layer of bamboo was 35.52% and 43.64%, respectively. The 15% NaOH treatment resulted in a considerable decrease in the porosity, leading to reductions of 64% and 80%, respectively. This indicated that the parenchyma cells in the inner bamboo samples changed more with respect to the alkali treatment than the outer bamboo samples did.
Fig. 5. The cross-sectional morphology and pore structure of the bamboo slivers: (a) and (e) the untreated inner bamboo; (b) and (f) the 15% NaOH treated inner bamboo; (c) and (g) the untreated outer bamboo; and (d) and (h) the 15% NaOH treated outer bamboo
Water Absorption and Dimensional Stability
The water absorption and dimensional stability are shown in Fig. 6. Relevant data are shown in Table S1-S4. The water absorption of the untreated slivers from the inner layer of bamboo was much higher than the water absorption of the untreated slivers from the outer layer of bamboo. The former had a greater amount of parenchyma cells that possessed a larger lumen and thinner cell wall as well as a higher hemicellulose content; as such, they more easily swelled in water (Jin et al. 2019; Chen et al. 2020b; Lian et al. 2020). When treated at various alkali concentrations, the water absorption by the bamboo slivers was higher than the water absorption by the untreated slivers, except for the 15% NaOH treated slivers from the inner layers of the bamboo. This was primarily due to the partial removal of lignin and hemicellulose during the alkali treatment, which led to greater accessibility of the hydroxyl groups (Chen et al. 2021b). The increase in the water absorption by the treated slivers from the inner layer of the bamboo was more pronounced than the increase in the water absorption by the treated slivers from the outer layer of bamboo due to the higher volume fraction of parenchyma cells within the inner layer of bamboo.
Fig. 6. The water absorption and dimensional stability of the bamboo slivers: (a) water absorption; (b) width swelling; (c) thickness swelling; and (d) volumetric swelling t
The width and thickness swelling amounts of the treated bamboo slivers were higher than the width and thickness swelling of the untreated slivers. The treated bamboo slivers from the inner layer of the bamboo swelled considerably more compared to the treated slivers from the outer layer of the bamboo, as the former had more parenchyma cells, which collapsed more easily during alkali treatment and the subsequent drying process. As shown in Figs. 7 and 8, the parenchyma cells in the treated slivers still collapsed even after the 12-h immersion in water when compared with the untreated slivers. The parenchyma cells had greater swelling in the treated slivers from the inner layer compared to the parenchyma cells from the outer layer of the bamboo after the immersion in water. As shown in Figs. S4 and S5, there was no considerable change in the fibers, which was probably due to the fact that there were more fibers in the slivers from the outer layer of the bamboo compared to the slivers from the inner layer of bamboo. This prevented the parenchyma cells from swelling in the water, as the fibers governed the deformation and movement of the other cell lumens (Chen et al. 2020b). Therefore, the parenchyma cells in the slivers from the inner layer of the bamboo collapsed during the alkali treatment and had more pronounced swelling when immersed in water compared to the slivers from the outer layer of the bamboo. The thickness, width, and volume swelling of the slivers from the outer layer and the inner layer of the bamboo reached their maximum when treated with 5% NaOH, which was likely due to the fact that the parenchyma cells collapsed the most severely and there was less damage to the parenchyma cell walls compared to the samples treated at other alkali concentrations.
Fig. 7. Parenchyma cells in the slivers from the outer layer of the bamboo after a 12 h immersion in water