Unique microstructure of kapok fibers in longitudinal microscopic images

Abstract

This paper is focused on investigating the microstructure of kapok fiber through the observation of residual deformation and fracture morphology of kapok fibers in longitudinal microscopic images. By comparing the morphology of kapok fibers before and after different treatments, unique weak spiral lines inside the cell wall were found. The spiral lines that divide the cell wall into a wound ribbon appear to be the weak locations (e.g., the fracture or bending points) of the fiber. The damage mechanism of kapok fibers in assemblies was summarized. Details of structures inside the ribbon were also examined to reveal that macro-fibrils of 0.2 µm in diameter and >1.0 µm in length were packed neatly along the fiber axial on the surface. A framework of the multi-assembly structure of kapok fiber was summarized.

Keywords

kapok fiber macro-fibril microstructure spiral trace weak link

Kapok is a natural cellulosic fiber that possesses unique properties, such as low density, high hollowness (80–90%) and antibacterial function, and is an ideal material for thermal insulation, oil-absorbing, buoyancy, bedding and sound-absorption.^1–10 Kapok plants can grow in mountainous areas and therefore do not need to be planted on cultivated lands. Furthermore, the utilization of kapok fibers will not cause disputes within the food supply chain as kapok fiber is non-edible. However, kapok fiber is also distinguished by inferior mechanical properties – low strength, high stiffness and fragility, which make it extremely unsuitable for textile processing.¹¹ For example, lots of fiber fragments can fall off at the beginning use of a kapok fiber towel. After being used for a period of time, fiber fragments can be gathered at the corner of the cover of kapok fiber padding materials. Such phenomena are closely associated with the unique microstructure of kapok fiber, especially in the fiber length direction.

The microstructure of nature fibers usually includes four structure units: protofibrils, microfibrils, fibrils and macrofibrils.¹² Previous research has revealed that the cell wall of kapok fiber is laminated, with five layers labeled as S, W1, W2, W3 and IS from outer to inner, as shown in Figure 1. Each layer contains packed crystalline cellulose.¹² Fibrils with a diameter of 2–5 nm were found, and the angle of the fibrils in relation to the fiber axial in each layer showed obvious differences.^11,13 However, no prior study has explored the structural units within kapok fiber at a scale greater than that of the fibril inside the fiber. In addition, a description of the assembled forms of structural units of kapok fiber at different scales has not been reported in the literature.
Figure 1.
Diagram of the lateral cell wall structure of kapok fiber.

This study was aimed to investigate how different structural units are assembled along the fiber axis inside the kapok fiber, to develop a better understanding of its damage mechanism to explain phenomena that happen in kapok fiber products, and to provide fundamental morphological information on the microstructures for its utilization. Optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were the tools used to observe sizes and morphologies of fiber microstructures.

Experimental details

Materials

The kapok fiber samples used in this study were taken from three different sources, as listed in Table 1. Both intact kapok fibers and fibers subjected to various degrees of damage as would occur during textile processing are included in these samples.
Table 1.
Source of material

Classification Source

Fibers Raw kapok fibers from Hainan Island

Fabrics Knitted fabrics of blended yarns (40 s^a, kapok/viscose/cotton 30/45/25)

Fiber fragments Kapok fiber fragments fallen from padding materials

a
s (yarn linear unit): number of 840 yards yarn in one pound.

Sample preparation

Dewaxing treatment

To observe the fiber structure clearly, the fat, wax and other impurities on the fiber surface were first removed.¹⁴ All of the samples were treated with 6 g/L NaOH, 0.6 g/L JFC, 1.5 g/L Na₂SO₃, 1.5 g/L Na₂SiO₃ and 0.5 g/L Na₃PO₄ aqueous solution for 2 h at 90℃ with a solid-to-liquid radio of 1:10 (g/mL).

Hollowness restoration

Kapok fibers are easy to squash during the spinning process. The flattening deformation distorts the original structure of the fiber. It is necessary to restore the hollowness of the fiber in order to observe the original distribution of structural units in the cell wall. The method used to restore the hollowness of the fiber is illustrated in an existing patent.¹⁵ Briefly, fibers were first dewaxed, as described in the previous dewaxing treatment procedure, and then treated in a relatively high-concentration alkali solution (180–280 g/L) for 60–80 s.

Ultrasonic treatment

Ultrasonic treatment is an essential method used to remove impurities and break weak sections of the fiber.¹⁴ Once fibers have received ultrasonic treatment, the characteristics of their structural units are easier to observe. Kapok fibers were soaked in distilled water (concentration: 2% in mass) and then treated for 6 min with 80% power by an ultrasonic processor (VCX750, SONIC&MATIRIALS, INC.).

Microscopic observation

The morphology of kapok fibers was examined using OM, SEM and AFM. OM (BEIOM MC2001, LaBo America Inc.) was used to examine the breakage and deformation of kapok fibers. SEM (Quanta-250, FEI) was used to investigate the size and morphology of longitudinal structure in kapok fibers and AFM (NanoScope IV, Veeco) was used to observe the finer morphology of the fibers.

Results and discussion

Spiral traces on raw kapok fibers

As shown in Figure 2(a), a raw kapok fiber has a relatively smooth surface under SEM. Only vague spiral concave and convex traces of the fiber can be detected (marked by arrows). The spiral trace can be more visibly seen in the AFM image (Figure 2(b)), which shows a spiral groove with a width of about 200 nm (calculated by contrasting with the calibrated scale, the arrow points to the fiber axial).
Figure 2.
Images of raw kapok fibers: (a) scanning electron microscopy image; (b) atomic force microscopy image.

Figure 3 displays spiral grooves with a width of less than 150 nm in the surface a kapok fiber, taken from a blended yarn. Mechanical forces exerted on kapok fibers in textile processing can impact weak locations within the fiber and make them much more visible than on a raw kapok fiber.
Figure 3.
Spiral grooves in the surface of a kapok fiber from blended yarns.

All spiral traces on the kapok fibers in Figures 2 and 3 suggest that kapok fibers have axial weak links yielding uneven mechanical properties.

Axial weak links of kapok fiber after dewaxing

Raw kapok fibers have a smooth surface and no structural details can be observed under OM. However, horizontal fine lines and uneven distortions appeared in flatten kapok fibers after the removal of fat and wax from the surface, as shown in Figure 4. Compared with cotton fiber, which has over 90% cellulose, kapok fiber has quite high contents of lignin (13–22%) and hemicellulose (22–45%).^1,9,16 Both lignin and hemicellulose are amorphous materials and alkali soluble.^17–19 Hence, the sodium hydroxide (NaOH) solution (the dewaxing treatment solution) can also help to remove those amorphous materials for clear observation of cellulose structures.
Figure 4.
Optical microscopy images of kapok fibers: (a) raw fiber; (b) fiber with horizontal fine lines after removing fat and wax from the surface; (c) fiber with horizontal fine lines and uneven distortions after dewaxing.

Figure 5(a) shows a SEM image of dewaxed kapok fibers in blended fabrics that gives clearer structure details than an OM image. It shows transverse seams with uniform spacing (marked by arrows), some of which were fractured (circled). While Figure 5(a) represents the overall morphology of kapok fiber, Figure 5(b) shows a close-up view of a seam area that reveals multiple wrinkles around a seam.
Figure 5.
Scanning electron microscopy images of kapok fibers in blended fabrics after dewaxing treatment: (a) overall morphology; (b) partial enlargement at the seam.

Figure 6 shows dewaxed kapok fibers with their hollowness been restored. Spiral seams with a pitch of 8–10 µm were found and are indicated by arrows in the figure. This means that the initial forms (uncompressed) of the surface seams are spiral curves (Figure 6) that then become flattened lines when fibers are compressed (Figure 5). Furthermore, both left-hand (Figure 6(a)) and right-hand (Figure 6(b)) spiral directions are observed. Microns attached to the fiber surface in Figure 6(a) are mostly NaOH crystals deposited when fibers were washed with cold water.
Figure 6.
Spiral traces on kapok fibers from blended fabrics after hollowness restoration.

Degradation of kapok fibers after ultrasonic treatment

As shown in Figure 7, kapok fibers could be degraded at the spiral traces by an ultrasonic treatment and part of the fiber became a spiral belt. The width of the belt is rather uniform on the same fiber, but varies among different fibers. This finding further suggests that the spiral trace is the weak link along the axis of the cell wall of a kapok fiber, and is the location where kapok fibers within a product tend to break or deform. This also explains why the cell wall of a kapok fiber could form a screwed ribbon after a mechanical treatment in a previous study.¹¹
Figure 7.
Optical microscopy images of kapok fibers after ultrasonic treatment.

Finer surface structure of kapok fibers

Figure 8(a) shows that the surface of kapok fiber is filled with slender units (named macro-fibrils in this paper) of about 0.2 µm in diameter and over 1.0 µm in length, which are arranged tightly along the axis. Figure 8(b) shows an amplification of the circled area in Figure 8(a), indicating where a macro-fibril passes through the weak line. The arrangement of macro-fibrils is neat, and highly oriented with the fiber axis. The fibrils within the inner layers of a kapok fiber could be assumed to be arranged in a highly ordered manner, and there also may be larger structural units than fibrils present, such as macro-fibrils.
Figure 8.
Kapok fibers after dewaxing treatment: (a) scanning electron microscopy image; (b) atomic force microscopy image.

Damage mechanism of kapok fiber and statistical analysis of the weak link

Figure 9 shows damages of kapok fibers captured in different microscopic images. Figure 9(a) shows a SEM image of kapok that presents a torn strip unit along the helix after a brittle fracture occurred following liquid nitrogen refrigeration.²⁰ The fracture portion (circled) shows the layered radial structure of the cell wall. Figure 9(b) shows an OM image that contains debris of a kapok fiber from a fabric that has multiple transverse cracks. Figure 9(c) shows a SEM image of kapok fiber fragments that had fallen from padding materials, which display transverse wrinkles spaced about 20 µm apart. From various observations, the damage mechanism of kapok fibers may be summarized into two scenarios: (1) if a fiber remains hollow, stress would always be built up at the spiral weak links first, and the cell wall would be partially or completely untwist along the spiral line to be a loose belt; (2) if a fiber has been squashed, it will be bent by a force at the weak links to form transverse wrinkles and then gradually rupture at the wrinkles.
Figure 9.
Kapok fiber fragments: (a) brittle fracture after frozen by liquid nitrogen in scanning electron microscopy (SEM); (b) fragments in optical microscopy (×600); (c) fragments in SEM.

Some detailed information about weak lines present in kapok fibers were collected and are summarized in Table 2. For kapok fibers used for stuffing or blended yarns, the spiral weak lines suffered from small repeated stresses and became wrinkles spaced 20–40 µm apart. This space range is similar to that of dewaxed kapok fibers observed under SEM. When impacted by large mechanical forces (e.g., frozen by liquid nitrogen) or strong alkali solutions, the seam space of kapok fibers was reduced to 5–10 µm. With the ultrasonic treatment, the spacing of the weak lines could be further reduced to 3 µm. Finer seam lines with 0.7–2 µm spacing could be observed under AFM. This minimal spacing may be the width of the spiral units of a kapok fiber. The data in Table 2 can be useful when processing kapok fibers for different commercial applications.
Table 2.
Forms and spacing of spiral seams in kapok fibers from different materials

Materials Forms of weak link Instruments Space (µm) Observed fibers

Raw kapok fibers Wrinkles SEM 30–40 6

Fragments fallen from kapok stuffing materials Wrinkles SEM 20–25 10

Brittle fracture of kapok fiber frozen by liquid nitrogen Spiral fractures SEM About 10 10

Blended yarns Wrinkles SEM 25–30 10

Blended yarns and fabrics Occasional spiral defects SEM About 6 50

Kapok fibers after dewaxing treatment Spiral seams SEM 30–35 20

AFM 0.7–2 10

Kapok fibers with restored hollowness Spiral seams SEM 8–10 20

Kapok fibers after ultrasonic treatment Spiral fractures OM 3–15 10

SEM: scanning electron microscopy; AFM: atomic force microscopy; OM: optical microscopy.

Model of the microstructure of kapok fiber

Figure 10 illustrates the multi-assembly structure of the cell wall of kapok fiber. Based on the observed evidences, we propose to use a tubular model composed of tightly wound ribbon, whose width may vary from 0.7 to 2 µm, to represent the cell wall of a kapok fiber, as shown in Figure 10(a). The ribbon contains a network of woven macro-fibrils that are arranged neatly along the axial with a diameter of 0.2 µm and a length of above 1.0 µm (Figure 10(b)). Since smaller cellulose units (fibrils) with a diameter of 3.2–5 nm were reported in previous studies of kapok fiber,^11,13 we infer that a macro-fibril may consist of parallel fibrils (Figure 10(c)). The angles of the fibrils relative to the axial vary in different layers.^11,13 As shown in Figure 10(d), inside the fibril, cellulose chains have a strong preference to assemble into crystals; in regions between fibrils, hydrogen bonds form a non-crystalline structure within the disordered assembly of chain segments.²¹
Figure 10.
Diagram of the cell wall structure of kapok fiber.

Conclusion

Through the observation of the microstructures of both raw and treated kapok fibers in various microscopic images, we found that strip units are twisted along the fiber axial to form the tubular structure in the main layer of a kapok fiber, and that the traces of these strips are spiral weak lines inside the cell wall of the fiber. Forms of the weak lines in kapok fibers varied under different conditions. The minimum width of the strip units observed is about 0.7 µm. Macro-fibrils that are 0.2 µm in diameter and >1.0 µm in length are arranged neatly along the fiber axis. Based on the findings, a model of a multi-assembly structure is proposed to deepen the understanding of the unique microstructures of kapok fibers.

Classification	Source
Fibers	Raw kapok fibers from Hainan Island
Fabrics	Knitted fabrics of blended yarns (40 s^a, kapok/viscose/cotton 30/45/25)
Fiber fragments	Kapok fiber fragments fallen from padding materials

Materials	Forms of weak link	Instruments	Space (µm)	Observed fibers
Raw kapok fibers	Wrinkles	SEM	30–40	6
Fragments fallen from kapok stuffing materials	Wrinkles	SEM	20–25	10
Brittle fracture of kapok fiber frozen by liquid nitrogen	Spiral fractures	SEM	About 10	10
Blended yarns	Wrinkles	SEM	25–30	10
Blended yarns and fabrics	Occasional spiral defects	SEM	About 6	50
Kapok fibers after dewaxing treatment	Spiral seams	SEM	30–35	20
AFM	0.7–2	10
Kapok fibers with restored hollowness	Spiral seams	SEM	8–10	20
Kapok fibers after ultrasonic treatment	Spiral fractures	OM	3–15	10

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Scientific and Technological Innovation Open Project of the Key Laboratory in Shaanxi province of China (grant number 2014SZS13-K03).

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