A comparative study of the microstructure and water permeability between flattened bamboo and bamboo culm
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The objective of this study is to investigate the microstructure, water permeability and the adhesion of waterborne coating on the flattened bamboo. The flattened bamboo was obtained by softening bamboo culm at 180 °C followed by compression. The microstructure and chemical component of flattened bamboo were investigated by scanning electron microscopy, Fourier transforms infrared spectroscopy, and X-ray diffraction. The adhesion and interface structure of waterborne coating onto flattened bamboo surface were also examined. The result indicated that the parenchyma cells in flattened bamboo were compressed, and starch in the parenchyma cell was extracted during the softening and flattening process in which the main chemical component did not change significantly. The water permeability of both flattened bamboo and bamboo culm is dependent on the direction: longitudinal direction > tangential direction > radial direction. However, the water permeability in all three directions in flattened bamboo was higher than those in the untreated bamboo. In addition, alkali dye solution was found to more easily permeate through the flattened bamboo when compared to acid dye solution, and the permeability varied depending on alkali dye or acid dye concentration. The adhesion of water-based polyurethane coating on the flattened bamboo can reach the second level.
KeywordsFlattened bamboo Microstructure Water permeability Adhesion
bamboo in the longitudinal direction
bamboo in the radial direction
bamboo in the tangential direction
flattened bamboo in the longitudinal direction
flattened bamboo in the radial direction
flattened bamboo in the tangential direction
In the 1980s, Maori  and Zhang et al.  invented the method of flattening bamboo culm to the bamboo board. However, the apparatus and technology were not good enough at that time, especially the apparatus, which limited its industrial production. Recently, as both the technology and the apparatus were improved, flattening of bamboo has been studied increasingly in academia and produced in the industry. Some researchers have reported how to flatten the bamboo culm into bamboo board effectively, such as softening in hot oil  or in saturated high-pressure steam , with different moisture contents , different press process , etc. Currently, one of the most efficient approaches is softening bamboo culm with saturated high-pressure steam at high temperature in a sealed container; the temperature and the pressure could be 180–190 °C and around 1.2 MPa .
As the flattening of bamboo developed, there have been enormous potentials for furniture and interior decoration application in which the water permeability is of great importance and has been taken into account prior to its practical application. The water permeability affects the modification, adhesive bonding properties, adhesion of coatings on the flattened bamboo surface, etc. For example, as used in furniture and interior decoration, the flattened bamboo needs to be treated differently to obtain the high durability, different colors, and high-quality coatings, which are closely related to water permeability. At present, only a few researches on the water permeability of bamboo culm [14, 15, 16] and the effect of different treatments on the water permeability of bamboo culm [17, 18] have been reported. The water permeability of bamboo culm was determined by the structure in different directions and can be improved by the hydrochloric acid treatment, microwave treatment, and freeze-drying treatment.
Flattened bamboo is a new material which is ready to be produced in large scale. However, there has been no reported research to evaluate the possibility of using flattened bamboo in furniture and interior decoration, especially about the water permeability of flattened bamboo, as well as the adhesion of the waterborne coating on its surface. Therefore, the objective of this study is to investigate the microstructure, chemical component, water permeability, and adhesion of the waterborne coating on the flattened bamboo, which will provide technical data for the application of flattened bamboo in furniture and interior decoration.
Materials and methods
The outer layer and inner layer of bamboo culm were first removed. Then the bamboo and flattened bamboo were ground into a powder, passed through a 200 mesh and dried in the oven at 103 °C for 2 h for Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) testing.
Microstructure and chemical component test
Flattened bamboo and bamboo were selected randomly to observe the morphology of the cross section and radial section with a field emission scanning electron microscope (FE-SEM, XL30 ESEM FEG, FEI Company, OR, USA), the acceleration of which was 7 kV.
The FTIR spectra of flattened bamboo and bamboo were measured in a spectrometer (VERTEX 80V, Bruker, German) within the range of 4000–400 cm−1, with a resolution of 4 cm−1 and 64 scans. KBr pellet consisting of KBr and flattened bamboo and bamboo powder was prepared with a weight ratio of 100:1.
The crystal structure of cellulose in the flattened bamboo and bamboo was characterized using a X-ray diffractometer (Ultima IV, Rigaku, Japan). The XRD patterns of the flattened bamboo and bamboo were obtained in the diffractometer with a CuKα radiation source (X-ray wavelength k = 0.154178 nm). 2θ was from 5° to 45°. The current and voltage for X-ray generation were 30 mA and 40 kV, respectively. The crystallinity index of cellulose was calculated from the height ratio between the intensity of the crystalline peak (I002–IAM) and total intensity (I002).
Water permeability test
Rectangular samples of flattened bamboo and bamboo culm were cut to 70 mm (longitudinal) × 25 mm (tangential) × 5 mm (radial). The water permeability was investigated in acid dye (brilliant crocein) solution and alkali dye (basic red) solution as the two dye types are usually used to obtain different colors. Brilliant crocein was bought according to GB/T 25816-2010, and basic red reached the requirement in HG/T 2551-2007.
The specimens were weighed and then put on the wire mesh in radial direction in a container with brilliant crocein and basic red solution with the concentration of 1, 3, and 5%, respectively. After 1, 2, 4, 7, 12, and 24 h, all the bamboo and flattened bamboo were weighed, respectively. Five specimens were tested for each type.
Adhesion of waterborne coating on the flattened bamboo test
The flattened bamboo was cut into 15 cm (longitudinal) × 12 cm (tangential) × 1 cm (radial). The waterborne polyurethane coatings were applied by brush on the flattened bamboo as one layer. It was heated at 50 °C and kept for 2 h. The coated samples were dried in a cool, dry environment for 7 days.
The degree of adhesion of the coating film was classified according to GB/4893.4-85 standard methods by a cross-cut test. Classification 1: the edges of the cuts are completely smooth; none of the squares of the lattice is detached. Classification 2: there is detachment of small flakes of the coating at the intersections of the cuts; slight detachment along the edges of the cuts. Classification 3: the coating has flaked partly or wholly discontinuously or continuously along the edges of the cuts. Classification 4: the coating has flaked partly or wholly on different parts of the squares. A cross-cut area not greater than 50% is affected. Classification 5: some squares have flaked partly or wholly. A cross-cut area greater than 50% is affected.
Results and discussion
Microstructure and chemical components of flattened bamboo
Permeability of flattened bamboo in different directions
For flattened bamboo, the water permeability in three directions was higher than that in bamboo. As shown in Figs. 5 and 7, the extraction of starch and the cracks in parenchyma cell wall helped to improve the water permeability [17, 18], which happened in flattened bamboo. Also, the decreased crystallinity in cellulose may be in part attributed to improving the water permeability of flattened bamboo compared with unflattened bamboo.
Permeability of flattened bamboo with different liquids
In basic red solutions, the water absorption weight and rate of flattened bamboo and bamboo decreased when the concentration increased. With increase in the concentration, the increased dye molecules block up the pits in the cell wall and the pores between parenchyma cells. When the concentration increased from 3 to 5%, the water absorption weight and rate of flattened bamboo were similar when the absorption time was less than 12 h. A similar phenomenon happened in the bamboo when the absorption time was less than 7 h.
With the same concentration, the water permeability of flattened bamboo was higher in basic red solution compared with that in brilliant crocein solution. By contrast, the water permeability of bamboo in basic red solution was lower than that in brilliant crocein solution when the concentration was lower (1% and 3%). As the concentration increased up to 5%, the water permeability of bamboo in brilliant crocein solution was higher than that in basic red solution. Regardless whether in basic red or brilliant crocein solution, with the same concentration, the water permeability of flattened bamboo was higher in comparison to that of unflattened bamboo.
Adhesion of waterborne coating film on the flattened bamboo
Flattened bamboo was produced for improving the utilization ratio of tubular bamboo culm and widening the applications. In this paper, the flattened bamboo produced with one of the most popular technologies in China was studied. The microstructure, chemical components, water permeability, and the adhesion and interface of waterborne coating on the surface of flattened bamboo were investigated. The results obtained are as follows: (1) the microstructure of flattened bamboo changed significantly, especially the parenchyma cells in which the shape and the lumen, the cell wall, and the starch changed. The chemical components in flattened bamboo were almost unchanged, but the crystallinity index of cellulose slightly decreased. (2) The water permeability of flattened bamboo, as well as that of bamboo, was dependent of the direction: longitudinal direction > tangential direction > radial direction. Whichever the direction, higher water permeability of flattened bamboo was obtained in comparison with bamboo. The water permeability was affected combined with the concentration and the kind of the solution. Flattened bamboo has higher permeability in alkali solution compared with that in acid solution, while that of bamboo depends on the concentration. (3) The adhesion classification of waterborne polyurethane coating on the surface of both outer layer and inner layer of flattened bamboo is two, even though the interfaces between fiber and coating as well as between coating and parenchyma cell were different.
The authors thank Dr. Xiubiao Zhang from International Centre for Bamboo and Rattan for providing the bamboo, and Dr. Yan Wu from Nanjing Forestry University for providing the polyurethane coating. The authors also thank Shuting Tang from Nanjing Forestry University for help in measuring the adhesion of coating on the flattened bamboo.
HC performed the SEM examination and was a major contributor to data analysis and writing the manuscript. YZ and XY performed FT-IR and XRD test, and YZ drew part of the images. HJ did the permeability test. TZ partly analyzed the data and wrote the manuscript. GW designed and financed the research. All authors read and approved the final manuscript.
This work was financed by the National Natural Science Foundation of China (31770598).
The authors declare that they have no competing interests.
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