Materials
Generally, the growth years of a palm tree are from seven to eight years, and they grow approximately 1.2m high. A palm tree in the botanical garden of Nanjing Forestry University (China) was chosen for this study. As was shown in Fig. 3a, the tree stem used was straight, with a height of 1.3m and perimeter of approximately 0.6m. The tree trunk was wrapped by fibers, and the top part of the tree contained palm leaves. The trunk gradually shrank toward the apex of the stem of the tree and appeared as a pagoda shape.
As was shown in Fig. 3b, the stalk was spirally arranged from the root of the brown stem to the apex. The PPFs were extracted from stalks. The sheath fiber, from which PLSFs were extracted, was found in a cross-mesh arrangement, supporting and protecting the trunk of the palm tree (Fig. 3c).
After peeling off the sheath fiber, the stem of the palm was exposed, whose color was bright yellow and white, just like fresh bamboo shoots. There were altogether 47 pieces of leaf sheath after dissection, which were numbered from one to forty-seven. Sample No.1, No. 8, No.15, No.22, No.29, No.36, No.43, and No.47 are shown below (Fig. 4).
During dissection, it was found that each layer of leaf sheath wrapped the stem of the trunk firmly, with the fibers crossed and overlapped. Leaf sheath fiber was the one found close to the old leaf sheath near the root, as was shown in Figs. 4a and 4b. The color of this fiber was dark brown. Because of a long exposure to air, the dark brown gradually turned black, but the fibers still had good robustness. The color of the middle part of the leaf sheath was nankeen, with fibers firmly interwoven (Figs. 4c, 3d, and 4e). At the bottom of the upper part of the leaf sheath was still lively, mature fiber bundles at the top of the fiber gradually separated as the peeling of parenchyma (Figs. 4f, 4g, and 4h).
Methods
Tensile testing was performed on a universal mechanical testing machine(Shimadzu AGS-X20KN, Kyoto, Japan, Fig. 5a). The length of the fiber specimen used was 50mm, and the speed of machine was set at 20min/min. Latex was used on the clamp surface to avoid slipping(Fig. 5b). During the test, the fiber specimens were fixed on the central line of the clamping device.
To ensure valid data, the measurements included were only of the fractured part of the test specimens held between the grippers. A total of 50 valid measurements was collected for each sample group.
According to statistics, there were a total of 483 fibers in No. 22, among which 205 were oriented clockwise and 205 were anti-clockwise. Also, in the outer layer, there were 201 fibers, while the middle layer contained 77 fibers and the inner layer had 205 fibers.
Length Distributions. The average length of the fibers was 326.66mm. The average length of clockwise oriented fibers was 336.77mm, and the average length of the anti-clockwise ones was 316.55mm. Also, the length of the outer layer fibers was 353.50mm on average, that for the middle fibers was 331.78mm, and that for the inner ones was 297.98 mm.
The length of a single PLSF was determined to be between 200and 350mm, accounting for 50.3% of the total specimens. Among this, most were 250 to 300mm, occupying 19.9%. According to Fig. 6a, the length distribution of PLSFs was marked by a wave crest, which gradually decreased.
Weight Distributions. The average weight of a PLSF was approximately 0.0086g. The average weight in clockwise orientation was 0.0091g, and the average weight in anti-clockwise was 0.0082g. The average weight of the outer layer was 0.0086g, the middle layer 0.0186g, and inner layer 0.0048g.
According to Fig. 6b, the weight of PLSF was concentrated between 0.002 and 0.006g, accounting for 49.5%, after which the average weight decreased.
Fineness Distribution. As a kind of natural fiber, the cross-section of the palm fiber is irregular, so the weight of fiber at a certain length or the length of fiber at a certain weight are often used to represent the fineness. According to an international rule, the gram number per 1000m long fiber is denoted as the tex number (T). The tex number is proportional to the fiber's thickness, that is to say, when the fiber is thicker, the tex number is larger.
1
where T refers to the tex number of the fiber, m(g) refers to the weight of the fiber, and l (m) denotes the length.
The measured average fineness of PLSFs was 26.56tex. The average fineness in clockwise orientation was 27.49tex, and that in anti-clockwise was 25.63tex. The fineness of the outer layer was 22.82tex on average, the middle layer was 64.18tex, and the inner layer measured 15.75tex. The middle layer turned out to be the thickest.
According to Fig. 6c, the fineness of PLSFs was focused between 10 and 20tex for 52% of the fibers, while 20 to 30tex accounted for 20%, above which it substantially decreased.