Introduction

Over the last two decades, there has been a growing industrial interest in the development of lignocellulosic natural fibers as replacements for glass or other traditional reinforcement materials used in composites [1]. Natural fibers composites offer advantages of biodegradability, good accessibility, fast renewability, high specific properties, and low cost [2, 3]. However, a better understanding of the detailed morphology, structural, and mechanical properties of natural fibers is essential for evaluation of their potential as reinforcing materials in composites and optimization of the service life performance of composites [4].

Windmill palm (Trachycarpus fortunei) is the most widely cultivated species at the latitudinal palm range margin, and distinguished by its large and fan-shaped leaves [5]. Every year, a portion of the leaves of the palm dries out to form a mesh of coarse and brown leaf sheath fiber that clasps the tree trunk, making it appear to be wrapped in burlap [6] (Fig. 1a). The palm leaf sheath fiber is considered an abundant agricultural byproduct due to the necessary regular pruning process of the palm tree by removing loose mats to keep the tree attractive and safe. Currently, due to their high durability and strength characteristics, palm fibers are used to make a variety of by-products such as mattresses, sofas, marine ropes, sacks, and traditional raincoats [7]. However, these applications only utilize a small percentage of the total material produced, and the majority of the material is discarded directly as waste, causing serious environmental problems [8]. The possible use of palm fibers for composite reinforcement may provide a strategy for the efficient utilization of these waste fibers.

Fig. 1
figure 1

Photograph of a windmill palm tree covered with leaf sheath; b leaf sheath; c a small piece of leaf sheath showing the arrangements of different layers of fiber bundle, with inserted images showing the schematic view of fiber arrangement

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Each palm leaf sheath is composed of a middle layer of coarse fibers which is sandwiched between two layers of fibers with smaller diameters. There have been only preliminary studies of palm leaf sheath fibers. Zhang et al. [9] revealed a unique structure of palm fiber with sub-cylindrical shape containing abundant fiber cells in the cross section and Si-dots on the surface. Previously, the mechanical properties of palm fiber were mainly determined through monotonic tensile tests. Palm fiber exhibits excellent elongation properties, which is able to tolerate significantly higher stain than other natural fibers [10]. Chen et al. [11] investigated the influence of the chemical treatment of windmill palm fibers on its tensile properties, and found that alkalized fibers possessed higher tensile properties than untreated fibers. Nevertheless, none of these studies examined the potential variation in the physical and mechanical properties of palm fiber from different layers of the leaf sheath. Since there are three layers in each sheet of palm leaf sheath with some differences in terms of fiber morphology and tensile properties [12], it is important to assess the properties of these individual layers for a more comprehensive evaluation of the use of palm fiber as composite reinforcement material.

To facilitate reliable use of palm fiber in composites, a detailed investigation was performed of the mechanical properties of windmill palm fibers, separated from the three layers of leaf sheath, in relationship to their chemical composition and structural properties. First, the mechanical properties of palm fibers were determined at two scales: tensile tests at the fiber scale and nanoindentation tests at the cell wall scale. Next, microscopy (SEM) observation and X-ray diffraction were conducted to investigate morphology and structure (microfibrillar angle and relative degree of crystallinity), followed by chemical composition analysis. Finally, the results were considered in a detailed assessment of the relationships between the physical, chemical, and mechanical properties of the fiber.

Materials and methods

Sample preparation

Windmill palm tree trunk covered by leaf sheath was obtained from Anhui province, China (Fig. 1a). The leaf sheath fibers were mechanically separated from the petiole (Fig. 1b), and then washed with distilled water to remove impurities and dust and further air-dried to remove excessive water and moisture. Then, the cleaned leaf sheath was sorted into three layers, namely the outer, middle, and inner layers, according to their diameters and orientations (Fig. 1c). Comparison of sample preparation of different plant fibers is summarized in Additional file 1: Table S1.

Morphological and chemical analysis

The longitudinal and transverse surface morphology of the palm fiber were investigated through SEM (XL30, FEI, USA) operating at 7 kV. The tensile fracture surface of the palm fibers was also analyzed. Before SEM observation, the samples were sputter-coated with a thin layer of Pt using an Edwards Sputter Coater.

The diameter and length of each palm fiber and elementary fiber were measured with a digital biological microscope (XTS20G, Fukai Ltd., China). In order to extract elementary fiber, palm fibers were chemically treated using a mixture solution of hydrogen peroxide and glacial acetic as described by Yu et al. [13]. For measurement of elementary fiber cell wall thickness, samples were embedded with Spurr resin and cut to obtain smooth transverse section for SEM observation.

The chemical composition (α-cellulose, hemicellulose, lignin, and ash content) of the palm fibers in different leaf sheath layers was analyzed in accordance with the Technical association of Pulp and Paper industry (TAPPI) standards, as described by Khalil et al. [14].

X-ray diffraction

A Philips X-ray diffraction system (X’pert Pro, PANalytical B.V., Almelo, NL) was used to evaluate both the microfibrillar angle (MFA) and relative crystallinity index (CrI) of palm fibers. Before measurement, palm fibers of the same layer were arranged closely to form a mat. Here, evaluation of the entire fiber was performed instead of examination of the powder samples that are commonly used in order to preserve the internal structure of the fibers [15]. Using Eq. (1), MFA of the palm fibers was determined by application of the widely used 0.6T method [16]:

$${\text{MFA}} = 0.6 \times T,$$
(1)

where T-value was determined as half the width of the peak obtained in azimuthal distribution (Additional file 1: Fig. S1).

CrI was calculated as described by Segal et al. [17], as follows:

$${\text{CrI }} = \, (I_{00 2} - I_{\text{am}} )/I_{00 2} ,$$
(2)

where I002 is the maximum intensity of the 002 peak at 2θ about 22° and Iam is the minimum intensity of diffraction of the amorphous materials at 2θ about 18°.

Tensile tests

Tensile tests were first conducted in a universal mechanical testing machine (Instron Microtester 5848, USA) using a 500 N load cell. Before the tensile test, two palm fiber ends were glued with instant epoxy adhesive onto a piece of poplar veneer to avoid any specimen sliding from the clamps. A gauge length of 50 mm was selected and elongation was recorded with a non-contacting video extensometer by tracking two small black speckles marked on the sample. For each layer of fibers, at least 30 fibers were tested. Tensile tests were conducted under an environment of 20 °C and 30% relative humidity (RH). To calculate the tensile strength and modulus of palm fibers, the cross-sectional area of the samples was assessed with a digital microscope (XTS20G, Fukai LTD., China) and then measured with an image-processing software (Image-ProPlus 5.0, Media Cybernetics, USA).

Nanoindentation testing

For nanoindentation testing, palm fibers were embedded in Spurr resin and cured in a flat mold as previous described [13]. After curing, the embedded samples were polished with a glass knife and a diamond knife to prepare a smooth surface exposing a transverse section. The nanoindentation experiments were performed using a Triboindenter (Hysitron Incorporation, USA) with a diamond Berkovich indenter (tip radius less than 100 nm). The indentation procedure was performed under load control using a three-segment load ramp: reaching the peak load of 150 μN within 6 s, holding at the peak load for 6 s, and then unloading within 3 s. In total, 20–25 indentations were conducted for each layer of palm fibers. The elastic modulus and hardness were calculated from the experimental curves according to the method developed by Oliver and Pharr [18].

Results and discussion

Fiber morphology

A windmill palm trunk is covered with a large amount of leaf sheath (Fig. 1a), and each leaf sheath can be easily separated into three layers of fibers due to the orderly arrangements (Fig. 1c). The fiber size differs in different layers of palm fibers (listed in Table 1): the middle layer contained the largest size of fibers (516 ± 40 μm), followed by the outer layer (372 ± 75 μm), and the inner layer (230 ± 37 μm). Also, morphological differences were detected by cross-sectional SEM observation, as the fibers from the inner layer showed a homogeneous structure almost completely comprising numerous single elementary fiber (Fig. 2a), whereas the fibers from the middle and outer layers exhibited clear vessels and phloem tissue (conducting tissues) surrounded by elementary fibers (Fig. 2b, c). This specific organization of fibers in different layers is likely related to their different functions, as the small fibers in the inner layer, which closely clasps the palm trunk, mainly play a structural role, and the large fibers in the outer and middle layers facilitate the transportation of nutrients through the conductive tissues. Unlike the observed variability of fiber diameters and morphology, the dimensions of elementary fiber were similar among the different layers, with average diameters ranging from 12.1 to 13.1 μm, and fiber length ranging from 930 to 1019 μm, and cell wall thickness ranging from 1.7 to 2.2 μm. A similar observation was reported by Zhai et al. [12] previously.

Table 1 Fiber and elementary fiber sizes in different palm leaf sheath layers
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Fig. 2
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SEM images of palm fibers in different layers: a inner layer, b middle layers, c outer layer

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There was similar surface morphology of palm fibers in different layers. For example, in a fiber sample from the outer layer, the fiber was sub-cylindrical in shape (Fig. 3a) with a large number of wax and impurities on the rough surface (Fig. 3b). In some cleaner areas, the surface was covered by arrays of protrusions (Fig. 3c), identified as silica bodies embedding circular craters under higher magnification (Fig. 3d) which had been confirmed through energy dispersive X-rays spectrometry (EDS) [19]. Similar features were also observed in other palm trees, such as piassava palm fibers [20], date palm fibers [21], and sugar palm fibers [22]. The main function of these silica bodies is to protect the external surface of the fiber from pathogenic fungi and insects, but the presence of the silica may interfere with pulping and papermaking [21]. However, the silica bodies do not stick firmly onto the fiber and can be removed mechanically, leaving empty craters that may facilitate mechanical interlocking of the fiber and matrix due to the rougher surface [2].

Fig. 3
figure 3

Surface morphological characterizations of palm fibers in outer layer: a sub-cylindrical shape, b waxy and course surface, c arrays of protrusions embedded with d silica

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Chemical composition and structural properties

The chemical composition of windmill palm fibers in various leaf sheath layers is detailed in Table 2, and the composition of other palm family fibers and some commonly used fibers are presented for comparison. Like other natural fibers, analysis showed that the palm fibers were composed mainly of cellulose, hemicellulose, and lignin, with similar contents among the different layers of leaf sheath. The remarkable consistency of the composition was in contrast to the dramatic variations reported in fiber diameter and cross-sectional morphology. Windmill palm has a relatively lower cellulose content (34.3–35.5%) compared to other members of the palm family, such as oil palm (42.7–65.0%) [2], and sugar palm (43.8–44.5%) [23], but a level comparable with piassava palm (31.6%) [20]. Compared with flax, jute, and sisal fibers [24], windmill palm fiber has a relative higher lignin content that is comparable to that of other palm family fibers, except oil palm. The high lignin content may contribute to its brown color and useful properties such as weather, fungal, and bacterial resistance [19].

Table 2 Comparison of properties of windmill palm fibers with other natural fibers
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The MFA, the spiral angle between the cellulose fibrils and the fiber axial, is considered an influential structural parameter that affects the axial mechanical properties of natural fibers [25]. The measured MFA values of palm fibers were in the range of 38.8 ± 1.5° for the inner layer to 39.4 ± 1.0° for the outer layer, which were very similar and reproducible values (Table 2). The CrI of windmill palm fibers was found to vary slightly for the different layers, with the highest in the middle layers (49.2%), followed by the outer layers (46.7%), and finally the inner layer (45.7%). These results are similar to those reported by Zhai et al. [12], Zhang et al. [9].

Tensile properties

The typical stress–strain curves of palm fibers obtained from each leaf sheath layer are presented in Fig. 4, and their tensile properties are listed in Table 2. The curves show that the palm fibers all exhibited linear elastic at low stress, and then yielding point at around 2% strain followed by plastic deformation until breakage, and similar results have been reported for piassava fiber [19] and coir fiber [4]. There were some evident differences in the mechanical properties of palm fibers among the different leaf sheath layers, as shown in Table 2. Some large variations in the tensile properties of palm fibers can be found in all three layers, which are often observed for natural fibers published by other authors [7, 21]. In general, the average tensile modulus, stress, and failure strain of the fibers in the inner layer were much higher than those in fibers from the outer and middle layers. As the crystalline cellulose confers the stiffness and strength (around 135 GPa) [26], the superior tensile properties of natural fibers have been attributed to the high cellulose content and crystallinity, and the comparatively low MFA [27]. However, the cellulose content, CrI and MFA values were in similar ranges among different layers of windmill palm fibers (Table 2). Therefore, the differences in mechanical properties between layers may be more associated with the morphological parameters, as obvious variations in fiber morphology, particularly fiber diameter, were observed between different leaf sheath layers (Table 1). Fibers in the inner layers had the smallest diameter and exhibited superior mechanical properties among the three layers. Consistently, earlier studies showed significant correlations between mechanical properties and fiber diameter, and a decreased trend in mechanical properties was observed for increasing fiber diameters [12, 28]. As assayed here, windmill palm fibers exhibited higher tensile modulus, but lower failure strain compared with previously reported results [9, 12].

Fig. 4
figure 4

a Typical stress–strain curves of windmill palm fibers in different layers; fracture surface of fiber in b inner, c middle, and d outer layer of one windmill palm leaf sheath

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As shown in Table 2, windmill palm fibers exhibited significantly lower tensile modulus and strength compared with those of flax, jute, and sisal [24], but values that were much closer to that of other palm family fibers, such as date palm [21] and especially piassava fiber [20]. This is expected as windmill palm fibers have lower cellulose content (34.3–35.5%), lower CrI (45.7–49.2%), and higher MFA values (38.8°–39.4°). However, unlike these stiffer and stronger fibers that exhibit brittle failure at relatively low ultimate tensile strain (Table 2), windmill palm fibers showed ductile failure with a large failure strain of 10.0–17.6% with dissipated strain energy. The ductile behavior of windmill palm fiber was related to its fracture mode, as shown in Fig. 4b–d. The failure of the fibers is caused by the fracture of the cell wall with delamination, as shown in the inset in Fig. 4b. Similar intercellular fracture has also been reported for other fibers with a low cellulose content and high MFA, such as coir and piassava fiber [29].

The use of windmill palm fibers as reinforcement material in composites is perhaps not an obvious choice due to its relatively lower tensile modulus and strength compared to other widely used fibers. However, although the addition of windmill palm fiber may not enhance the stiffness and strength of a composite, it can improve the toughness of the material due to its high failure strain. Additionally, waste windmill palm fiber can be incorporated with a strong–brittle fiber (such as jute or flax) into a polymer matrix to produce a hybrid composite, in which the palm fibers exhibit high failure strain which allows better stress transfer, and provides large diameter material to increase the effective fiber–matrix adhesion area [30]. As mentioned above, the windmill palm fibers share strong similarities with coir and piassava fiber, and these two fibers have been successfully used as reinforcement materials [31, 32], which strongly suggests that the windmill palm may be an excellent reinforcement material to improve the properties of composites.

Nanoindentation testing

Nanoindentation is a powerful tool to characterize mechanical properties on a very small scale [33]. The typical load–depth curves of the windmill palm fibers in different leaf sheath layer were determined and shown in Fig. 5a. Fibers in the inner layers reached a much lower maximum depth (ca. 96 nm) compared to those from the middle and outer layers (ca. 145 nm), however, the final depth of these fibers were similar (ca. 70 nm) after the unloading process, which indicated different viscoelastic behavior among different layers. Figure 5b–d exhibits the indented areas of the fiber cell wall in three layers, and only indents located in the cell (marked by red arrows) were considered acceptable.

Fig. 5
figure 5

a Typical load–depth curves of nanoindentation tests in cell wall of windmill palm fibers in different layers; Residual indentations on cell wall of windmill palm fibers in a inner, b middle, c outer layer

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The longitudinal elastic modulus and hardness of palm cell walls in three layers are shown in Table 3. Palm fibers in inner layers showed much higher value in elastic modulus (13.2 GPa) and hardness (0.5 GPa) than that of palm fibers in the middle and outer layers. In general, the mechanical properties of the fiber cell wall are positively related to lignin content and negatively correlated with MFA values [34]. However, despite palm fiber in inner and outer layer exhibited no appreciable differences in chemical composition, MFA, CrI values (Table 2), as well as elementary fiber size and cell wall thickness values (Table 1), their nanoindentation properties exhibited significant differences. The diameter of fiber in inner layer was much smaller (230 μm) than that of fiber in outer layer (372 μm), which indicated that fiber diameter could be important contributors to the difference. Similar results have been observed for the two types of leaf sheath fiber identified in date palm, where small fiber bundles (70–120 μm) exhibited superior nanoindentation properties than large fiber bundles (600–1200 μm), which might be related to their mechanical role in leaf sheath [21]. The nanoindentation properties of windmill palm fiber were in the same range with those of crops and wood, and comparable to those of date palm [21], as shown in Table 2. This may be partly explained by the remarkable high lignin content in the windmill palm fibers, which makes the fiber cell wall rigid.

Table 3 Comparison of nanoindentation properties of windmill palm fiber with other natural fibers
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Conclusions

The fiber morphology, chemical composition, physical, and multi-scale mechanical properties of windmill palm fibers in three leaf sheath layers were investigated in this study. The tensile and nanoindentation properties of fine fibers in the inner leaf sheath layer were highest among the three layers, which may reflect their mechanical role in leaf sheath. The palm fibers are not as strong and stiff, but exhibit a high elongation at break. The nanoindentation results revealed that the modulus and hardness of windmill palm fiber were comparable to those of wood and crops fibers. These results will help promote the use of windmill palm fiber as a material from renewable resources with more high-value applications.