Compressibility of the kapok fibrous assembly

Abstract

In this paper we study the compressibility of kapok fibrous assemblies via scanning electron microscopy (SEM) image observations on kapok microstructures and measurements obtained from Kawabata Evaluation System (KES) compression testing. The assemblies, made of slightly carded kapok fibers, were treated in different levels of relative humidity (dry or wet) and pressure (0 or 100 kPa) conditions. The SEM images of the treated samples revealed that kapok fibers were initially hollow and circular, but could be crushed partially or totally to become thin ribbons after being pressed with a 100 kPa pressure. Over 80% of the fibers in the wet-pressure-treated assembly appeared to be crushed. In the KES testing, the compressional resilience, bulkiness and other parameters of the kapok assemblies were calculated from the compression curves. The results showed that the compressional resilience of the dry-treated kapok assemblies was better than that of the wet-treated assemblies; the bulkiness of both the dry- and wet-treated assemblies was reduced after the pressure treatment, but in the wet assemblies kapok hollow structures and interspaces among fibers were much easier to be squeezed than those of the dry assemblies.

Keywords

kapok fibrous assembly compressibility

The growing awareness of healthy and environmentally friendly textiles in everyday life has fostered more research on the utilization of natural fibers. Kapok is a natural fiber that possesses unique properties, such as wide lumina and a thin cellulosic wall, which make it valuable for many special needs. The color of the fiber appears yellowish or light-brown, and has a silk-like luster.¹ An uncrushed kapok fiber has a cylindrical shape with a lumen degree being as high as 80–90%,² and thus is light, buoyant, and excellent in thermal insulation.^3,4 Efforts have been made to explore the performance and utilizations of kapok fibers in regular situations,^4,5 usually at a room temperature of about 21℃ and humidity of about 65%. Kapok's macromolecular chains of cellulose were studied by differential scanning calorimetry, and the crystallinity and the average refractive index were measured by X-ray diffraction.⁶ The fine structure model⁷ and dyeing properties⁸ were established previously.^7,8 It was determined that the main components of kapok fiber are cellulose, lignin and xylan by chemical dissolution.⁹ Kapok utilization research was focused on the industrial use and home use. The former was mainly in kapok fiber assemblies used as a buoyancy material,^10–13 sound and heat insulation materials,¹⁴ oil-absorbing materials,^15–19 and reinforcement composites.²⁰ Home use of kapok was mainly for the wadding in high classical bedding, pillows and cushions, and the raw material of lightweight clothing. The spinnability of the kapok fiber and the properties of its blended yarn were also explored.^21–24 However, kapok lumens can be easily crushed in textile processing, and therefore the ultimate performance may be quite different from what is expected. It is important to understand the impact of environmental factors, such as humidity and pressure, on the performance of kapok fibers.

This paper reports the results of a study of the compressibility of kapok fibrous assembly under different humidity and pressure treatments by analyzing scanning electron microscopy (SEM) images of kapok fibers and by using the compression tester of the Kawabata Evaluation System (KES). The tasks included observing changes of kapok fiber structures with different treatments, understanding the differences in compression resilience, bulkiness, work and other parameters of kapok assemblies brought out by the treatments, and establishing a pipe-piling model for estimating the hollow status of kapok fibers in a pressed assembly.

Experiment

Sample preparation

The kapok fibers used in the study were selected from the family of Ceiba pentandra growing in Indonesia. For the compressibility test, the kapok fibrous samples were prepared using the following steps:

Kapok fibers were pre-conditioned at standard temperature 20 ± 2℃ and relative humidity (RH) of 65 ± 5% for 24 hours.

The fibers were carded slightly into bundles so that fibers were straightened and parallel (see Figure 1(a)).

A specimen was weighed to be 100 mg using a torsion balance, and uniformly packed in a 30 × 30 × 15 mm³ paper container to achieve a planar density of approximately 1.11 g/mm²; the bottom of the container had an opening of 20 × 20 mm² that permits only fibers to be compressed in the test (see Figure 1(b)).

A total of 20 specimens were made and marked from D1 to D10 and W1 to W10.

Figure 1.

Sample preparation.

Sample treatments

The kapok samples were heated in an oven at a temperature of 45 ± 2℃ for 1 h to reduce moisture content firstly, and then treated using three different conditions: dry (15% RH), wet (99% RH) and pressure (100 kPa) treatments. In the dry treatment, the sample containers were placed into a desiccator with silica-gel desiccants at the bottom to control RH at 15% for 24 h (see Figure 1(c)). In the wet treatment, the sample containers were placed in the desiccator where distilled water was used to control RH at 99% for 24 hours. The humidity was measured by a hygrometer inside of the desiccator. Both the dry- and the wet-treated samples were pressed by a weight of 4.08 kg to the bottom surface of 20 × 20 mm² that produced 100 kPa pressure (see Figure 2) on the samples for 15 s immediately after the sample was taken out of the desiccator. The sample treatments were performed in the same temperature (20 ± 2℃). The pressure of 100 kPa was chosen to imitate the possible severe impacts on the fiber that could be experienced during textile processing and use.

Figure 2.

Pressure treatment of an assembly by weight.

The 20 specimens were divided into two groups. Group W, specimens from W1 to W10, underwent the wet and pressure treatments, while group D, from D1 to D10, undertook the dry and pressure treatments. Details of the treatments are shown in Table 1.

Table 1.

Sample treatments

Specimens		Conditions
Treatment	Number	RH (%)	Pressure (kPa)	Pressure time (s)
Dry	D1–D10	15	0	0
Dry-pressure	D1–D10	15	100	15
Wet	W1–W10	99	0	0
Wet-pressure	W1–W10	99	100	15

Compression test

The compression tests of the kapok fibrous assemblies were performed in a constant temperature and humidity laboratory by the KES (Kawabata Evaluation Systems)-FB3 tester²⁵ in a standard temperature and humidity laboratory. The tester with a 2 cm² round feeler pressed the specimen at a speed of 0.05 cm/s, and stopped automatically once it reached a maximum pressure at 50 Pa. The KES-FB3 generated a compression curve that depicted the change in thickness of the specimen with the exerted pressure.²⁶ Figure 3 is one example of the compression curves of the kapok assembly. A few important parameters can be extracted from the curve to describe the compressibility of the tested material:

T_o (cm): the thickness at the initial pressure of 0.5 Pa, i.e. original thickness of the specimen.

T_m (cm): the thickness at the maximum pressure p_m = 50 Pa.

W_c (cN·cm/cm²): the compressional work per unit area when the specimen was compressed by the feeler. If p is the compressing pressure, W_c = $\int_{T_{o}}^{T_{m}} p d T$ . The larger the compressional work is, the bulkier the sample is.

W_c′ (cN·cm/cm²): the compressional work per unit area when the feeler released the load from the specimen.

R_c (%): this is the resilience measure, expressed as a percentage of W_c′ in W_c, i.e. R_c = W_c´/W_c × 100%. This index represents the compressional resilience of the samples. The larger R_c is, the better the compressional resilience is.

L_c: this measures the flexion of compression curves, i.e. L_c = 2W_c / p_m (T_o - T_m), and it reflects the compression complexity of the material.

Two additional parameters can be derived from the measured parameters:²⁷

B (cm³/g): this measures the bulkiness, that is, the volume of the unit mass material at the original thickness T_o. Here B = T_o/M, where M (g/cm²) is the area density of the specimen.

ΔT (cm): this is the compressed height, i.e. ΔT = T_o – T_m. It reflects the loss in interspaces of the fluffy structure after compressing.

Figure 3.

The compression curves of the kapok assembly.

Results and discussion

SEM observations

To examine the hollow status of kapok fibers in an assembly, the micro-structures of kapok fibers were observed by using a scanning electron microscope (model: JSM-5600LV). Figure 4 shows the cross-section and longitudinal images of three different kapok fibers from a wet-pressure-treated assembly (99% RH and 100 kPa). To prevent further deformation caused by the cross-sectioning, liquid nitrogen was used to “freeze” the kapok fibers before the cross-sectioning. Fibers were emerged in liquid nitrogen for 15 minutes to become extremely brittle and rigid, and then were fractured easily by bending them. Figure 4(a) displays one of the uncrushed fibers in the assembly which has circular and exceedingly hollow cross-section, and uniform longitudinal surface. The high degree (80–90%) of hollowness of kapok allows the cellulose capsule to retain more still air than other types of fibers, making kapok exceptionally good for thermal insulation. Figure 4(b) is an image of a partially squeezed fiber which has an elliptical cross-section and longitudinal convolutions, indicating that its cellulosic capsule loses much air space. Figure 4(c) is the image of an entirely crushed fiber, which has a very flat cross-section and a ribbon-like body. This fiber essentially squeezed out all of the inner air.

Figure 4.

SEM photos of the cross-sections and the longitudinal surface of kapok under the wet-pressure treatment.

We also took eight SEM images randomly on the surfaces of the wet-treated and wet-pressure-treated kapok assemblies, respectively. Figure 5(a) is one of the images that show the kapok fibers in the wet-treated assembly, and Figure 5(b) is one from the wet-pressure-treated assembly. It was found that over 90% of fibers in the wet-treated assemblies (unpressured) retained circular shapes, and therefore were uncrushed. On the other hand, more than 80% of the fibers in the wet-pressure-treated assemblies seemed apparently squeezed. The pressure treatment (100 kPa) greatly changed the hollow status of most kapok fibers.

Figure 5.

SEM photos of kapok fibrous assembly under different treatments.

Compressibility of kapok fibrous assembly under different treatments

Figures 6 and 7 show the compressibility differences of kapok fibrous assembly under dry (15% RH), wet (99% RH), dry-pressure [15% RH + 100 kPa (15 s)] and wet-pressure [99% RH + 100 kPa (15 s)] treatments. The statistics of the defined compressibility indices of the samples are also included in Table 2.

Figure 6.

Compressibility of kapok fibrous assemblies under different treatments: R_c and L_c.

Figure 7.

Compressibility of kapok fibrous assemblies under different treatments: W_c, B, ΔT and T_m.

Table 2.

Statistics of compressibility indices of the samples under different treatments

Treatment	W_c (cNċcmċcm⁻²)	R_c (%)	B (cm³ċg⁻¹)	ΔT (mm)	T_m (mm)	L _c
Average	I: RH 15% + 0 Pa	9.07	41.83	132.94	11.49	3.26	0.32
	II: RH 15% + 100 kPa (15 s)	5.46	46.44	97.06	8.09	2.69	0.27
	III: RH 99% + 0 Pa	8.49	24.91	125.28	10.92	2.99	0.31
	IV: RH99% + 100 kPa (15 s)	2.35	44.77	35.50	2.24	1.70	0.42
CV	I: RH 15% + 0 Pa	5.41	3.79	5.86	5.91	8.39	4.52
	II: RH15% + 100 kPa (15 s)	7.22	3.23	5.68	5.51	7.16	5.21
	III: RH99% + 0 Pa	1.67	7.95	3.6	5.26	7.67	5.55
	IV: RH99% + 100 kPa (15 s)	10.41	4.72	11.76	17.33	5.72	13.81

When the dry-treated samples D1–D10 are compared with the wet-treated samples W1–W10 (data without the 100 kPa treatment in Table 2), it is evident that the compressional resilience (R_c) of the wet-treated samples (24.91%) is much lower than that of the dry-treated samples (41.83%). This means humidity treatment could seriously affect the compressional resilience (R_c) of the samples without any pressure. While the linearity (L_c) of compression curves of both groups are similar (approximately 0.3), L_c reflects the compression complexity of the material. This means that the bulkiness of the wet-treated assemblies would diminish easily because of their lower compressional resilience.

However, when both groups of samples were treated with the extra pressure of 100 kPa before the KES tests, the R_c increased when compared with their unpressured counterparts (Figure 6(a)). The R_c of the wet-pressure-treated samples (44.77%) were almost doubled compared with those of the wet-unpressured samples (24.91%). The linearity (L_c) of the dry-pressure-treated samples descended a little while that of the wet-pressure-treated samples increased greatly. This reflects the extra pressure lowering the fraction of interspaces among fibers in the wet-treated samples effectively, because porous materials should obtain higher compressional resilience and linearity at the lower pore fraction.

The other four compressional parameters (compressional work per unit area W_c, the bulkiness B, compressed height ΔT and the thickness at the maximum test load T_m) are plotted in Figure 7(a), (b), (c) and (d). The wet-treated assemblies were all a little lower than those of the dry assemblies. While being treated with the extra pressure, these four parameters were all reduced, and the declining extents of the dry-pressure-treated samples were much less than those of the wet-pressure-treated samples. Compared with the wet-pressure-treated samples, the dry-pressure-treated samples also had much higher values in those four parameters.

These results show that in dry-pressure-treated assemblies, the loss of the interior interspaces among fibers and the hollow structure of kapok fiber was less than that of wet-pressure-treated samples. The wet-pressure-treated samples were easier to squeeze. In addition, a small variation of the dry-pressure samples’ compression curves, which resulted in a small reduction in L_c, indicates that the pressure treatment might not cause deformation of the hollow structure of dry kapok fibers but actually narrows the interior interspaces among fibers.

Table 3 shows the significance tests of experimental data to help us to further understand the compressibility of the samples under different treatments. In the table, tests I–IV refer to the different treatments: I, 15% RH + 0 Pa; II, RH 15% + 100 kPa (15 s); III, RH 99% + 0 Pa; IV, RH 99% + 100 kPa (15 s). The major compressibility parameters exhibit significant differences among differently treated assemblies, especially W_c between the dry-treated samples and the wet-pressure-treated samples, R_c between the dry-treated samples and the wet-treated samples, and B, ΔT and L_c between the dry-pressure-treated samples and the wet-pressure-treated sample. These differences show that the humidity and pressure can affect the kapok fibers’ hollow structure and thus the assembly’s fluffy structure seriously, causing distinct variations in their compression curves.

Table 3.

Tests for statistical significance of compressibility indexes of samples under different treatments

Treatment	W_c (cN·cm·cm⁻²)	R_c (%)	B (cm³·g⁻¹)	ΔT (cm)	T_m (cm)	L _c
I & II: t-test	19.68(++)*	5.71(+)	15.07(++)	15.46(++)	6.01(+)	5.27(+)
I & III: t-test	3.66(+)	32.60(++)	2.72(+)	1.86(+)	6.44(+)	0.75(−)
I & IV: t-test	55.92(++)	4.78(+)	42.74(++)	42.51(++)	26.27(++)	8.26(+)
II & III: t-test	22.36(++)	20.78(++)	11.70(++)	11.15(++)	3.40(+)	6.32(+)
II & IV: t-test	28.10(++)	2.74(+)	50.29(++)	59.06(++)	17.39(++)	15.81(++)
III & IV: t-test	51.43(++)	20.74(++)	44.62(++)	37.79(++)	24.43(++)	8.65(+)

The significance level is 0.1, where t_0.95(9) = 1.8311.

“−”

No significant difference where t < 1.8311.

“

+”Significant difference where 1.8311 < t < 10.

“

++”Extremely significant difference where t > 10.

Theoretical analysis of the hollow status of kapok fibers in assemblies

It is necessary to have a model that can be used to estimate compressibility of kapok fibers in an assembly. This study attempted to establish an initial model to describe only geometrical changes of a kapok assembly when being compressed. Assume that kapok fibers in an assembly are ideally parallel, tightly packed and uncompressed. The structure of kapok fiber assemblies used in the study can be simulated using a simple pipe-piling model as depicted in Figure 8. The annulus cross-section of round hollow kapok fibers would be squeezed to be a flat ellipse sheet once pressed by the large external load without the consideration of slippage among layers, as shown by comparing Figure 9(a) and (b). The thickness of the assembly would change from T_max to T_min, and its width would become from l to (l + Δl). Therefore, the compression status of hollow kapok fibers in the assembly can be evaluated by analyzing the deformation of the cross-sections from the initial status to the collapsed status.

Figure 8.

Fiber pile model.

Figure 9.

Two statuses of a kapok cross-section.

Cut a small cube with a base area being l × l out of a collapsed assembly and estimate the mass per unit area m by weighing the cube. Let V_annulus denote the volume of the annulus wall of the fiber, ρ_b the average density of a kapok fiber, D the fiber diameter, and h the thickness of the fiber wall (see Figure 8). The number n of fibers in the unit area can be calculated in equation (1) as follows:

n = \frac{m}{ρ_{b} \times V_{annulus}} = \frac{m}{ρ_{b} \times \frac{π}{4} [D^{2} - (D - 2 h)^{2}] \times l}

(1)

When the assembly is compressed, the assembly loses its height and gains its width. The ratio of the assembly’s initial width to the width of the collapsed assembly can be calculated by the change in width of a single fiber in equation (2) as follows:

λ = \frac{l}{l + Δ l} = \frac{D}{2 h + (π D - 2 π h) / 2} = \frac{n}{N} = \frac{m}{M}

(2)

where N is the number of fibers in the unit area of the initial assembly and M is the mass per unit area of the initial assembly.

The thickness of the kapok cube can be estimated for its initial and collapsed statuses, T_max and T_min, and used to calculate the compressed cross-section area ΔS which could indicate the space loss during the pressure treatment as follows:

T_{max} = \frac{N \times D^{2}}{l}

(3)

T_{min} = \frac{N \times h (π D - 2 π h + 4 h)}{(l + Δ l)}

(4)

\begin{matrix} Δ S = T_{max} \times l - T_{min} \times (l + Δ l) = N \times D^{2} \\ - N \times h (π D - 2 π h + 4 h) \end{matrix}

(5)

For the kapok fibers used in this paper, the fiber diameter (D) is 17.84–22.11 µm, the thickness (h) of fiber wall is about 1 µm,³ and the density of fiber wall (ρ_b) is 1.33 g/cm³ and the mass per unit area (m) of the 20 assemblies is 18.41 × 10⁻³g/cm². By deducing from equations, we could figure out the estimated numbers of fibers in one assembly N = n/λ = 1.5129n, and the compressed area of the assembly. Table 4 shows comparative results between the test results and the calculated values. From the pipe-piling model, ΔS, the loss of the assembly’s cross-section area after the compression was calculated to be 0.099 cm². This is very close to the measured result of ΔS, 0.098 cm². It is reasonable to deduce kapok fibers after wet-pressure treatment were squeezed to be flat, and the hollow structure of kapok fiber could be kept well in assemblies without a pressure treatment. The reason why the test averages are clearly larger than the theoretical values is that kapok fibers could not be kept entirely parallel and tight in an actual arrangement. This non-ideal arrangement also contributed to the bulkiness of the assemblies.

Table 4.

Test and theoretical thickness of kapok fibrous assembly

Initial	Collapsed	ΔS
Calculated average total fiber number - N	N = 3.5147 × 10⁴
Calculated average thickness (cm) - T	0.140	0.031	0.099
Calculated average cross section area (cm²) - S	0.140	0.041	0.099
Measured thickness (cm) - T	0.326	0.170	0.098
Measured cross-sectional area (cm²) - S	0.326	0.228	0.098

Conclusions

SEM photos of kapok fibrous assemblies indicate that over 90% fibers in the wet-treated assemblies (unpressured) remained circular shapes, but more than 80% of fibers in the wet-pressure-treated assemblies seemed apparently crushed. The pressure treatment (100 kPa) greatly changed the hollow status of most kapok fibers.

The compression tests demonstrate that the wet-treated kapok fibrous assembly possesses a much lower compressional resilience than the dry-treated samples, and the bulkiness of the wet-treated kapok fibrous assembly after repeated pressure treatment diminishes. Although the bulkiness of the dry-treated and the wet-treated kapok fibrous assemblies both descend after the pressure treatment, the loss of interspaces among fibers and the hollow structure of kapok fiber in the dry-pressure-treated assemblies is much less than that of the wet-pressure-treated samples. The wet-pressure-treated samples are easier to be squeezed. Both humidity and pressure can significantly affect the assembly’s fluffy structure and the kapok fibers’ hollow structure.

A simple pipe-piling model was adopted to simulate the fiber arrangement in the kapok fibrous assembly. This model appears to be effective in estimating the changes of the assembly’s cross-sectional area. The calculated results from the theoretical model were consistent with the measured values. But the model ignores many important factors, such as variations in fiber orientation and residual pores in the fibrous mass, and therefore its use is limited.

Footnotes

Funding

The authors acknowledge the financial supports from the Doctoral Innovation Funding of Donghua University (NO. 12D10136).

References

Fengel

Przyklenk

. Studies on kapok-1. Electron microscopic observations. Holzforschung 1986; 40: 137–141.

Xiao

Shi

. Structures and performances of the kapok fiber. J Textil Res 2005; 26(4): 4–6.

Liu

Wang

. Kapok Battings and its Thermal Insulation Properties, Shanghai: College of Textiles, Donghua University, 2011, pp. 18–22, 50–52.

Xiao

Shi

. Characters and application prospects of kapok fiber. J Donghua Univ (Nat Sci Ed) 2005; 31(2): 121–125.

Liu

Wang

. Research on the kapok fiber and its application. Adv Textil Technol 2009; 4: 55–57.

Nakamura

. Studies on bound water of cellulose by differential scanning calorimetry. Textil Res J 1981; 51: 607–613.

Mishra

Elangovan

. Chemical processing of kapok fiber. Colourage 1994; 41(9): 31–34.

Xiao

. Structures and properties of Kapok Fiber and the Wettability and Buoyancy Characteristics of Kapok Fibrous Assemblies, Shanghai: Donghua University, 2006.

Sunmonu

Abdullahi

. Characterization of fibres from the plant Ceiba pentandra. J Textil Inst 1981; 2: 273–274.

10.

Defense Technical Information Center. Accelerated Wear Test to Life Jackets. US Defense Report AD-787115, Defense Technical Information Center, 1946.

11.

Mauersberger

. Matthews' Textile Fibers: Their Physical, Microscopic, and Chemical Properties, New York: Chapman & Hall, 1947, pp. 398–405.

12.

Defense Technical Information Center. Field Test of Lifejacket Flotation Materials. US Defense Report AD-A119344, Defense Technical Information Center, 1982.

13.

Natarajan V. Studies on textile applications of unconventional natural cellulosic fibres as fibres for the next millennium. In Proceedings of the 80th World Conference of the Textile Institute, Manchester, UK, 2000.

14.

. Kapok-wool thermal insulation building composite materials. J Indust Textil 1998; 17(7): 20–20.

15.

Wen

H-R

. Oil-absorption material and its application. Jiangsu Chem 1998; 26(3): 43–44.

16.

Lim

. In situ oil/water separation using hydrophobic-oleophilic fibrous wall: a lab-scale feasibility study for groundwater cleanup. J Hazard Mater B 2006; 137: 820–826.

17.

Lim TT. Evaluation of hydrophobicity/oleophilicity of kapok and its performance in oily water filtration: comparison of raw and solvent-treated fibers. Indust Crops Products 2007; 26(2): 125–134.

18.

Huang

. Performance and mechanism of a hydrophobic-oleophilic kapok filter for oil/water separation. Desalination 2006; 190: 295–307.

19.

Lim

. Evaluation of kapok (Ceiba pentandra (L.) Gaertn.) as a natural hollow hydrophobic-oleophilic fibrous sorbent for oil spill cleanup. Chemosphere 2007; 66: 955–963.

20.

Leonard

. The performance of cotton–kapok fabric–polyester composites. Polym Test 1999; 18: 181–198.

21.

Danda

BMD

. Processibility of Nigerian kapok fibre. Indian J Fibre Textil Res 2003; 28: 147–149.

22.

Sun

Wang

. Test and analyses of Java cotton and cotton blending yarn property. Cotton Textil Technol 2005; 6: 34–36.

23.

Zhao

Wang

. Test analyses of twist and elongation and strength of Java cotton cotton blended rotor yarn. Cotton Textil Technol 2007; 3: 26–29.

24.

Sun

. Study on the Properties of Java Cotton and Cotton Blending Yarn and Fabric, Shanghai: Donghua University, 2006.

25.

Wang

. Properties Design of Garment Fabric, Shanghai: Donghua University Press, 2005, pp. 11–13.

26.

Lou

Wang

Liu

. Compressibility of kapok wadding. J Textil Res 2007; 28: 10–13.