Microstructure and chemical components of flattened bamboo
The photographs of bamboo culm and flattened bamboo are presented in Fig. 4. There is slight difference in the color and the grain between the inner layer and outer layer of flattened bamboo. It mainly resulted from the chemical component and structure of bamboo.
The microstructures of bamboo and flattened bamboo in cross section and radial section are shown in Fig. 5. The basic units in bamboo are vascular bundles where bamboo fibers exist, and parenchyma consists of parenchyma cells. After softening and flattening, the parenchyma was compressed to a certain extent, and the starch in the parenchyma cells was extracted during the steam treatment (Fig. 5a, b). There was not significant change in the fibers in vascular bundles, although a few small cracks between fibers were expected (Fig. 5c, d), as the interface between fibers was weak [19]. However, a large number of cracks were developed in the parenchyma cell wall, including both in the layers and the interface between layers as shown in Fig. 5d. Moreover, the damage in the interface between the layers of the parenchyma cell wall was more pronounced when compared with the layers themselves. From the radial section (Fig. 5e, f), it also can be observed that the starch was removed, and the parenchyma cells were compressed where lots of cracks appeared the same as observed in the cross section.
The chemical structure of bamboo and flattened bamboo are evaluated with FTIR spectra from 4000 to 400 cm−1 as shown in Fig. 6. The peaks in the FTIR spectra at 1605, 1510 and 1261 cm−1 were due to lignin [20, 21], and the peak at 1737 cm−1 in was due to C=O in hemicellulose [22], which were similar in both types of bamboo. As reported by Zhang et al. [10], the hemicellulose in flattened bamboo steadily decreased with the increase of softening temperatures and disappeared when treated at 160 and 180 °C for 8 min. In our study, the bamboo was steam heated at 180 °C for 1–3 min; it indicated if the treatment time was short, the hemicellulose would not degrade much even when the temperature was up to 180 °C.
The XRD patterns of bamboo and flattened bamboo are shown in Fig. 7. There were three characteristic peaks in the XRD patterns of bamboo and flattened bamboo around 15.76°, 22°, and 34.74°, attributed to (110) and (200) and (004) reflections of the crystalline structure in cellulose Ι, and the reflection (004) is related to the longitudinal structure of cellulose [23]. Similar XRD patterns of cellulose in flattened bamboo were observed to that in bamboo, showing the similar cellulose Ι structure. It indicated that the softening and flattening process did not change the crystal form of cellulose. While the intensity of (200) reflection in flattened bamboo was weaker than that in bamboo, it emphasized the decrease in the fraction of crystalline cellulose [24]. Also, the crystallinity index of cellulose in bamboo and flattened bamboo was 61.8% and 50.4%, respectively. The combination of steam treatment at 180 °C and compression in the flattening process might account for the reduction in the crystallinity index, which needs be further studied.
Permeability of flattened bamboo in different directions
The water absorption weight and rate of flattened bamboo and bamboo are presented in Fig. 8. The water absorption weight and rate were different in different directions in both bamboo and flattened bamboo. In the longitudinal direction, the water absorption weight and rate were much higher than that in the other two directions, for both flattened bamboo and bamboo. The water absorption weight and rate in tangential direction were higher compared with that in the radial direction for both types of bamboo. This was in accordance with the water vapor diffusion resistance of bamboo in the longitudinal direction being remarkably lower than that in the tangential and radial directions [16].
The water permeability was dependent on the structure of bamboo and flattened bamboo in different directions. Bamboo consists of parenchyma cells, with embedded vascular bundles composed of fibers, metaxylem vessels, and sieve tubes with companion cells [25]. As shown in Fig. 9, in the longitudinal direction, there were a number of vascular bundles, the structure of which was beneficial for water permeability, especially vessels and sieve tubes. However, in the tangential and radial direction, there was no tissue with straight conduits and interconnectivity, and water transport mainly relied on the pits in fibers, parenchyma cell wall and the pores between parenchyma cells. The vascular bundles consisting of fibers prevented the water transport [16], as there were much fewer pits in the fiber and almost no pores between fibers in comparison to those in the parenchyma cell wall (shown in Fig. 9d). Therefore, the water permeability of flattened bamboo in different directions was similar to that of bamboo: longitudinal direction > tangential direction > radial direction, which was in accordance with the previous research on the water permeability of bamboo [14].
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
The water absorption weight and rate of flattened bamboo and bamboo in different liquids with different concentrations are investigated in Fig. 10. In the brilliant crocein solutions, the water absorption weight and rate of bamboo decreased with increase in concentration, while those of flattened bamboo in the solution with concentration at 3% and 5% were similar when the absorption time was less than 12 h. Up to 24 h, the water absorption weight in 5% brilliant crocein solution was higher than in 3% solution. This might be because the damaged cell wall in flattened bamboo was more easily affected by the acid solution with higher concentration, as acid pretreatment enhanced the water permeability [17].
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
Figure 11 shows the images of coated samples and coating films after the cross-cut test. After the cross-cut test, a little coating film had flaked along the edges and at the intersections of the cuts on both the outer layer and inner layer of the flattened bamboo (Fig. 11a, b). It indicated that the adhesion classification of the waterborne coating on both the outer layer and inner layer was 2. Figure 11c, d show the coating film off from the samples; there was a slightly more bamboo tissue on the coating film from the outer layer than from the inner layer. It might suggest that the interface between the outer layer and coating was different from that between the coating and inner layer.
Furthermore, as the shape of bamboo culm was tubular, the adhesion cannot be measured with the cross-cut test, and the interface between bamboo and coating was observed compared with that between flattened bamboo and coating in Fig. 12. For both bamboo and flattened bamboo, there were two kinds of interface: one was the interface between fiber and coating, and the other was the interface between parenchyma cell and coating. The interface between fiber and coating was greater in the outer layer because there were a large number of vascular bundles (Fig. 11a, c). But in the inner layer, the interface between parenchyma cell and the coating was more dominant because of the presence of a large amount of parenchyma there (Fig. 11b, d). Even in some areas of the inner layer, there was no interface between fiber and coating, but only interface between parenchyma cell and coating (Fig. 12d). From the FE-SEM observation, in flattened bamboo, the interface between fiber and coating was similar to that in bamboo, whereas the interface between parenchyma cell and coating was very different in the two substrates. The interface between parenchyma cell and coating included the interface between the outer surface of parenchyma cell and coating, and between the inner surface and coating. The parenchyma cell in flattened bamboo was compressed and the starch was extracted, and the cell lumen decreased and the cell shape changed. When coated, the interface between parenchyma cell and coating in flattened bamboo was very different from that in bamboo, as shown in Fig. 12b, d. In fact, the interface and the bonding between bamboo and coating are very complicated and very important in the industry and need to be further studied in the future.