Compressional resilience of the kapok fibrous assembly

Abstract

Kapok fiber is light, buoyant, and excellent in thermal insulation because of its high degree of hollowness. However, it is difficult to preserve the lumen structure of kapok fiber under repetitive compression in the course of manufacturing and utilization, and then the fiber may lose its unique features quickly. Thus, it is critical to know the resilience of kapok under repeated compression and how it can be affected by environmental factors. In this paper, we studied the compressional resilience of fibrous assemblies made of slightly carded kapok fibers by using an Instron compression tester. The nonlinear compressional behavior of the assemblies was observed in the repeated transverse compressing cycles with a constant compressional strain, and three characteristic stages of the entire compressing cycles—linear elastic, metamorphosis, and densification—were identified. The plastic and the visco-elasto-plastic compressional deformations were characterized from the compressional stress–strain curve. It was found that the conditioning humidity and the number of compressing cycles could affect the compressional resilience of the kapok fibrous assembly, and that the dry-treated kapok fibrous assemblies possessed better resilience and higher strains than the wet-treated ones.

Keywords

kapok fibrous assembly compressional resilience

The compressional resilience is a mechanical property critical for textile materials used as padding, seal, and insulation products. The real compressional behavior of a loose fibrous assembly is normally affected by fiber bending, partial stretching and twist, inter-fiber friction and slippage, assembly density, fiber arrangement, fiber mechanical and surface properties, etc.^1,2 Many researchers have investigated the compressional mechanical behavior of fibrous assemblies by establishing classical theories, models and numerical methods to estimate the compressional performance. Van Wyk³ proposed a model based on the bending of randomly distributed fiber elements and derived the press–volume relationship for the compression stroke and the number of contacts in the assembly. Stearn⁴ gave out the calculating method of the compressed volume and derived the correction factor for the number of contacts in an initially random assembly. Takashi and Kunio⁵ generalized Stearn’s approach by introducing an orientation density function. Pan^6,7 proposed the microstructure theory and steric restriction on fibrous assembly. Later, Lee’s analytic expression of compressional strain and poison ratio,^8,9 Carnaby and Pan’s slippage standard,¹⁰ Lee et al.’s energy method,¹¹ and other work^12–19 on the fibrous assembly greatly promoted the study of compressional properties of textiles, which inspired the present study on compressional resilience of the kapok fibrous assembly (KFA).

Kapok is a natural fiber that possesses unique properties that make it valuable for many special needs. The color of the fiber appears yellowish or light-brown, and has a silk-like luster.²⁰ Kapok fiber is 16–24 mm in length and 17–22 µm in diameter. The average breaking tenacity of single kapok fiber is 1.44–1.71 cN and the range of elongation at break is 1.8–4.2%.²¹ More uniquely, kapok fibers are hollow fibers with lumen degrees being as high as 80–90%, and thus are light, buoyant, and great in thermal insulation.²² However, the lumen structure of kapok fiber is difficult to preserve under multiple compression during the manufacturing and utilization process.²³ In other words, some of kapok’s unique properties may decline quickly in the manufacturing process. It is important to understand the resilience of kapok under repeated compression and how its resilience can be affected by environmental factors. In this project, we aimed at understanding the mechanical behavior of KFAs under repeated compression, and investigating the compressional resilience of dry- and wet-treated samples on an Instron compression tester. The KFA samples were made of slightly carded kapok fibers, which could be considered as a transversely isotropic network of fiber and air. The compression load was exerted perpendicularly to the surface of the assembly, and transverse compressing cycles were repeated with a constant compressional strain, which included creep and stress relaxations. The recorded stress–strain (S-S) curves were used to characterize the visco-elasto-plastic compressional deformations of the assembly. The impacts of the conditioning humidity and the number of compressing cycles on the compressional resilience of the KFA were also studied.

Experimental details

Sample preparation

The kapok fibers used in the study were selected from the family of Ceiba pentandra grown in Indonesian. For the compressional resilience test, the kapok fibrous samples were prepared as our previous study,²⁴ using the following steps:

kapok fibers were 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 Figures 1(a) and (b));

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 approximately 1.11 g/mm²; the bottom of the container had an opening of 20 mm × 20 mm that permits only fibers to be compressed in the test (see Figure 1(c));

a total of 10 specimens were made and marked from D#1 to D#5 and W#1 to W#5.

Figure 1.

Sample preparation.

Sample treatments

The kapok samples were heated in the oven at a temperature of 45 ± 2℃ for 1 h to reduce moisture content firstly, and then treated in two different conditions: dry (15% RH) and wet (99% RH) 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(d)). In the wet treatment, the sample containers were placed in the desiccator where distilled water was used to control RH at 99% for 24 h (see Figure 1(e)). The humidity was measured by a hygrometer inside the desiccators.²⁴ The sample treatments were performed in the same temperature (20 ± 2℃).²⁵ The five dry specimens and the five wet specimens were labeled from D#1 to D#5 and from W#1 to W#5, respectively. These specimens were split for testing into two compressing cycles—30 and 50, as arranged in Table 1.

Table 1.

Sample treatments

Specimen		Condition
Treatment	Numbers	RH (%)	Compressing cycles
Dry	D#1–D#3	15	30
Dry	D#4–D#5	15	50
Wet	W#1–W#3	99	30
Wet	W#4–W#5	99	50

RH: relative humidity.

Compressional resilience test

The compressional resilience tests of the KFA were performed on an Instron Compression Tester 5500 series in a temperature- and humidity-controlled laboratory (20 ± 2℃, 65 ± 4%).²⁶ The gauge length of the tester was set at 2.5 cm (see Figure 2(a)) and the compressional load ranged from 0 to 2097.2 N. The 15-cm² round feeler of the tester continuously pressed the specimen at the speed of 0.06 cm/s, and stopped once it reached the maximum force at 2097.2 N (see Figure 2(b)).

Figure 2.

Instron Compression Tester 5500 series: (a) initial position; (b) pressing position.

The compressional feeler then returned to its original position automatically at speed of 0.2 cm/s. During the return, the lifting speed was faster than the recovery of the KFAs and the feeler did not maintain a good contact with the surface of the specimen. Therefore, the data of the compression recovery were not collected.²⁷ It took five minutes to complete the stress relaxation on the KFAs before the next compressing cycle. In each compressing cycle, the tester recorded the compressional data including compressional stress, strain, load, extension, and time. Two compressing levels (30 and 50 cycles) were used to compress the samples, as listed in Table 1.

Figure 3(a) shows one of the complete S-S curves of the KFA, and Figure 3(b) shows a highlighted region circled in Figure 3(a). From Figure 3(b), three characteristic phases of the S-S curve, indicated by points 1, 2, and m (point of the maximum compressional load), can be observed. A few important parameters can be extracted from the curve to describe the compressional resilience of the tested material.

T_o (cm): the thickness at the initial stress (ɛ₀) of 0.0588 N/cm² of each compressing cycle.

T_i (cm): the thickness at the specific point of the S-S curve (Figure 3(b)), i = 1, 2, m.

ΔT_i (cm): the compressed height at phase i (i = 1, 2, m), that is, ΔT₁ = T₁ – T₀, ΔT₂ = T₂ – T₁, ΔT_m = T_m – T₂. It reflects the loss in interspaces of the fluffy structure after compressing.

Wc_j (N cm/cm²): the compressional work per unit area in the jth compressing cycle. If δ is the compressing stress, Wc_j = $\int_{ɛ_{o}}^{ɛ_{m}} δ (t) d ɛ (t)$ . The larger the compressional work is, the bulkier the sample is.

λc_j (%): the compression work descending index of the jth compressing cycle expressed as λc_j = (Wc_j – Wc_j₊₁)/Wc_j × 100%. This index represents the compression work descending trend of samples. The larger λc_j is, the smaller the retention of the compression work is. It reflects the compressional resilience indirectly.

Figure 3.

The compressional stress–strain curves of the kapok fibrous assembly (KFA): (a) a complete compressing stress–strain curve of the KFA; (b) characteristic phases of the stress–strain curve.

Results and discussion

Single stress–strain curve change of the KFA

The compressibility of the KFA was scrutinized with the S-S curve in one compressing cycle. As shown in Figure 3, the S-S curve of a dry-treated sample revealed that the compressional deformations can be divided into two stages: plastic compressional deformation and visco-elasto-plastic deformation. At the beginning of the compression, the KFA was in a low volume fraction, and had few contact fibers points and plenty of air among fibers. The inter-fiber space was easily squeezed out when subject to the external load. The fibrous assembly underwent plastic deformation with an extremely low compressional modulus. As the external load increased, it compacted the assembly to some compactness, which made the fibers contact more tightly and ideally like fiber piling or solid cellular material. The KFA was transferred into the visco-elasto-plastic compressional deformation stage, highlighted in the circled strain range in Figure 3(a). This circled range is amplified in Figure 3(b) and it was easy to find that in this stage, the KFA experienced the visco-elastic compressional deformation phase when the kapok fiber in the assembly could still keep the hollow structure well, as shown in the segment from the original position 0 to the inflection point 1 of the S-S curve (see Figure 3(b)), and then yielded into the elasto-plastic compressional deformation phase when plenty of the fibers of the KFA started to be squashed as ribbon-like and some fiber cell walls were even crushed after inflection point 2.

As demonstrated in our previous study,²³ the compressional S-S curve in a compress–release cycle could not follow the same path, and would generate the hysteresis loop. Since the lifting speed of the compressional feeler of the tester was faster than stress relaxation of the KFA, the true compression recovery of the sample could not be measured. Hence, we did not record the recovery data. However, the irrecoverable compressional deformations of both dry- and wet-treated KFAs were clearly increased as the compressing cycles increase, as shown in Figure 4. The compression strains of both S-S curves consistently shift rightward, changing from 99.11% to 65.77% for the dry sample and 98.03% to 28.22% for the wet sample after the 50 compressing cycles. This phenomenon indicates the samples lost some of the thickness in each of the cycles, and the starts of the visco-elasto-plastic compressional deformation on the S-S curves were delayed as the repeated compressing cycles continued, especially during the previous 25 cycles. This is more clearly shown in the S-S curves of every five compressing cycles in the bottom figures in Figure 4.

Figure 4.

The stress–strain curves of the wet-treated kapok fibrous assembly (KFA) of 50 compressing cycles: (a) the dry-treated KFA; (b) the wet-treated KFA.

Characterization of multiple compression on the kapok fibrous assembly

To assess the compressional resilience of KFA, the multiple cycles of compression were repeated with creep and stress relaxations. The mechanical properties of the KFA in compression tests were highly nonlinear, resulting from the slippage, friction, and entanglement among fibers and the collapse of the hollow structure of fibers. As shown in Figures 5 and 6, the thickness and the thickness difference at the specific phases of the compression were fluctuated with the compressing cycles, which displayed that both the dry- and wet-treated KFAs could maintain in a stable thickness range, (13.37, 17.32) mm for the dry ones and (4.91, 8.26) mm for the wet ones. The dry KFAs could return to the original thickness level and maintain the consistency of the thicknesses after stress relaxations during the prior 30 compressing cycles, while the thicknesses of the wet KFAs could keep well only for 12 cycles.

Figure 5.

The thickness of dry- and wet-treated kapok fibrous assembly (KFA) of 50 compressing cycles.

Figure 6.

The thickness difference of dry- and wet-treated kapok fibrous assembly (KFA) of 50 compressing cycles. (Color online only.)

As defined by the thickness difference ΔT_i, ΔT₁ reflected the compressed height between the thickness at the original position 0 and the inflection point 1 (see Figure 3), which was mostly from the loose inter-fiber space of the KFA. ΔT₂ was the difference between the thickness at points 1 and 2 (see Figure 3), which was mostly from the loss of inner space of fibers in the KFA. ΔT_m was the difference between the thickness at point 2 and the maximum compressional load, which was from the rest space in the KFA. As the nonlinear fitting curves show in Figure 6, with the compressing cycles increased, ΔT₁ (black in Figure 6) descended and the descending rate slowed down gradually. At the beginning of the compression, the descending rate of the dry-treated KFAs was obviously larger than that of the wet-treated ones, and it was confirmed by the fact that the dry KFAs with a bulkier original thickness was easier to squash than the wet ones, and certainly the dry KFAs had more inter-fiber space. However, ΔT₂ (red in Figure 6) of both the dry- and wet-treated KFAs were consistent throughout the compressing cycles, which reflected that the inner space of the dry or wet fibers of each sample KFA was extremely similar. ΔT_m (blue in Figure 6) and the descending rate of the wet-treated KFAs was apparently larger than that of the dry one because of the influence of the conditioning humidity, which meant that the dry KFAs have a relatively better compressional resilience.

Figure 7 displays the work per area (W_c) and the compressional work descending index (λ _c ) of different compressing cycles. It was observed that the W_c and λ _c of both dry and wet KFAs plunged from the initial values at the beginning cycle, and stabilized quickly after the 10th cycle. However, the conditioning humidity in the wet KFAs made fibers swell, increased inter-fiber pressures, and made the structural change more mild (see Figures 7(b) and (d)).

Figure 7.

The compressional work per area and its descending index of dry- and wet-treated kapok fibrous assembly (KFA).

The compressional strains of each compression at both 30- and 50-compression levels were also investigated, and three characteristic stages can be observed in Figure 8: linear elastic (Stage A); metamorphosis (Stage B); and densification (Stage C).

Figure 8.

The compressional strain-compressing cycle curves at 30 and 50 times levels.

Stage A refers to the curve in the beginning cycles, that is, the first 13 (or 19) cycles in the 30-compression (or 50-compression) level. In this stage, the KFAs deformed in an approximately linear elastic manner with a relatively low modulus compared with the two following stages. The compressional strain declined quickly with the number of compressing cycles because the loose space among the fibers in the KFA was squeezed out easily and the fibers were compacted by the external load from the repeated compression. Therefore, the KFA strain deformation in elastic stage was mainly caused by the slippage, friction, and entanglement among fibers.

After 13 (or 19) cycles in the 30-compression (or 50-compression) level, the strain-cycle curves exhibited an apparent yield point and started instable changes because in the elastic stage most of the inter-fiber space had been squeezed out, the fibers suffered more direct collisions, and the fiber’s hollow structure started to collapse in the current metamorphosis stage—Stage B. Lumens collapsed due to elastic bulking, yielding, or brittle crushing as the compression continued.²⁸ Through the unsteady changes in this metamorphosis stage, the KFAs were adjusted to another stable stage by the compacting, collapse, and contact of fibers. However, before the onset of collapse of fiber hollow structures in a larger scale, the fibers with the transversely compressional stiffness relieved part of the solid collisions randomly. At the end of the metamorphosis stage, the KFA was supposed to deform at a nearly constant compressional modulus with the relatively stable network of fibers and air.

Stage C covers the curve section after the 18 (or 30) cycles in the 30-compression (or 50-compression) level (see Figure 8). In this stage, fibers in the KFA were in good contact and compactness, and undertook the main resistance to compression, which made the change of strain slow down drastically. As reported in our previous study²³ on the KFA compressibility, the hollow kapok fiber could be crushed into a ribbon-like body or cell walls under a repeated compression load, especially for the wet-treated KFA.

The three characteristic stages of the compressional resilience of the KFA were apparent both in the 30- and 50-compression levels, whereas the proportions of the metamorphosis stage in the compression levels were much different. As shown in Figure 9, the metamorphosis stage in the 30-compression level underwent about five compressing cycles and accounted for 16.67% of the entire cycles, while the same stage in the 50-compression level underwent 14 compressing cycles and accounted for 28% of the entire cycles. It showed that, in the higher compression level, the metamorphosis stage would take more cycles to complete the metamorphosis of fibers in the KFA. As shown in Figure 8, the transition of the compressional strain from the elastic stage to the metamorphosis stage of each sample was more distinct than the transition from the metamorphosis stage to the densification stage. The reason was that the resistance to compression in the elastic stage mainly consisted of fiber slippage, friction, and entanglement, whereas the main resistance to compression after the metamorphosis stage arose from the deformation of the fibers’ hollow structure, which led to the collapse of hollow kapok fibers in the densification stage.

Figure 9.

The percentage of three characteristic stages in two compression levels.

Influences of conditioning humidity and compressing cycles

As mentioned above, the conditioning humidity had a great influence on the structural change and the compressional resilience of the KFAs, especially on their stability. The reversion phenomenon of the compressional strain was found during the repetitive compression tests in which the compressional strain of a later compressing cycle was larger than that of a previous one. We defined the reversion numbers in total cycles to reflect the frequency of the reversion and to describe the stability of the compressional resilience of the KFAs under different humidity, as shown in Table 2. It was clear that the average of the reversion numbers of the wet-treated KFAs were lower than the dry-treated ones in both compression levels, which reflected that the dry KFAs might perform a lower resilience stability.

Table 2.

The reversion number of the kapok fibrous assembly.

Treatment	30 Compressing cycles			50 Compressing cycles
Treatment	#1	#2	#3	#4	#5
Dry	13	13	13	19	21
Wet	8	6	6	11	12

Table 3 and Figure 10 show the averages and standard deviations in the parenthesis of the compressional strains of the KFAs in the three characteristic stages. Compared with the wet-treated KFAs, the dry-treated ones possessed higher compressional strains and lower coefficients of variation. The average compressional strains of the KFAs under treatments I, II, and III declined gradually from the elastic, metamorphosis, to densification stages, but not those under treatment IV. The largest strain (0.85) was from the KFA in the elastic stage under II treatment and the smallest one (0.47) was from the KFA in stage B under IV treatment. In the metamorphosis stage, the strain of dry-treated KFAs with 50 compressing cycles (0.78) was almost doubled compared with that of wet-treated KFAs with 50 compressing cycles (0.47). The dry-treated KFAs could remain a higher compressional strain in the final stage compared with the wet one. The conditioning humidity (99%) lowered the strain of KFAs efficiently because the vapor in KFAs made fibers swell and increased inter-fiber pressures, which led to more irrecoverable lumen collapses. Under the repetitive compression tests, the wet-treated KFAs would accelerate the collapse of fiber cell wall, be densified quickly, and start to consolidate, resulting in the rapid stress increase as the compressing continued.²⁸

Figure 10.

Three-dimensional bars of the compressional strain statistics of the kapok fibrous assembly in three characteristic stages.

Table 3.

Compressional strain of the kapok fibrous assembly in three characteristic stages.

Treatment	Elastic stage	Metamorphosis Stage	Densification Stage
I: Dry+30 cycles	0.80 (0.12)	0.73 (0.14)	0.69 (0.05)
II: Dry+50 cycles	0.85 (0.09)	0.78 (0.06)	0.72 (0.10)
III: Wet+30 cycles	0.74 (0.18)	0.67 (0.10)	0.62 (0.09)
IV: Wet+50 cycles	0.55 (0.18)	0.47 (0.09)	0.53 (0.16)

To examine the compressing cycle effect on the compressional resilience of KFAs, the results showed that increasing the number of compressing cycles could lower the compressional strain of the wet-treated KFAs, but increase the compressional strain of the dry-treated ones to some extent. As demonstrated by the t-test results of the compressional strain statistics of the KFAs in the three stages shown in Table 4, the compressional strains of the dry- and wet-treated KFAs under 30 cycles showed a significant difference in which the experimental t values were close to the respective given t values, while under 50 compressing cycles the experimental t values of the dry- and wet-treated KFAs were much larger than their respective given t values.²⁹ More compressing cycles widened the compressional strain difference between the dry- and wet-treated KFAs.

Table 4.

t-test of the strain statistics of the kapok fibrous assembly in the three characteristic stages^a.

Treatment	Stage
Treatment	Elastic	Metamorphosis	Densification
I & III	1.6991 (1.6849)	2.2554 (2.0150)	3.8242 (1.7823)
II & IV	10.2012 (1.7291)	17.0421 (1.7595)	5.3694 (1.7247)

The significance level is 0.1. The numbers in parenthesis are the t-values from the t Distribution Critical Values Table.

Conclusions

Two main phases, plastic and visco-elasto-plastic deformations, in the compression of a KFA were observed from the compressional S-S curve. The plastic compressional deformation was mostly caused by squeezing the inter-fiber space out and the irrecoverable slippage and friction among fibers. After the fibers were compacted to some extent, the compression of KFA was transformed into the visco-elasto-plastic deformation, which included both visco-elastic and elasto-plastic deformations.

The compressional resilience test on KFAs revealed the KFAs’ nonlinear compressional behavior and three characteristic stages in the entire compressing cycles: elastic, metamorphosis, and densification. The strain plunged in the elastic stage due to its low elastic modulus, which was determined mainly by the loss of the inter-fiber space in the KFA. In the metamorphosis stage, the strain appeared unsteady because the resistance came mostly from the change of the intra-fiber space. In addition, the KFAs took more compression cycles to complete the metamorphosis stage in the 50-compression level than in the 30-compression level. In the stage of densification, the changes in the compressional strain of the KFAs in both levels became moderate. These findings on the compressional resilience of the KFA could be helpful for selecting more reasonable processing parameters during the manufacture in order to preserve KFAs’ unique properties.

The conditioning humidity, with which the KFA was treated, lowered the KFA’s compressional strain drastically because the vapor in the KFA made fibers swell, increased inter-fiber pressures, and ultimately caused more irrecoverable collapse of lumen. Increasing the number of compressing cycles could lower the strain of the wet-treated KFAs but increase the strain of dry-treated KFAs to some extent. In addition, more compressing cycles could widen strain differences between the dry- and wet-treated KFAs.

Footnotes

Funding

The research was partially supported by a scholarship from China Scholarship Council (NO.2011663014) and the Doctorial Innovation Fund of Donghua University (NO.12D10136).

Acknowledgement

The authors would like to thank Alfonso H. Huerta García and Eric E. Calloway at the School of Human Ecology, the University of Texas at Austin for their assistances in the compression tests.

References

Morton

Hearle

JWS

. Physical properties of textile fiber, 3rd ed. Manchester: The Textile Institute, 1993.

Chu

. Textile physics, 2nd ed. Shanghai: Donghua University Press, 2009.

Van Wyk

. Notes on the compressibility of wool. J Textil Inst 1946; 37: T285–T292.

Stearn

. The effect of anisotropy in the randomness of fiber orientation on fiber to fiber contacts. J Textil Inst 1971; 62: 353–360.

Takashi

Kunio

. Numbers of fiber to fiber contacts in general fiber assemblies. Textil Res J 1977; 47: 13–17.

Pan

. A modified analysis of t he micro structural characteristics of general fiber assemblies. Textil Res J 1993; 63: 336–345.

Pan

. Fiber contact in fiber assemblies. Textil Res J 1995; 65: 6–18.

Lee

Carnaby

. Compressional energy of the random fiber assembly part I: theory. Textil Res J 1992; 62: 185–191.

Lee

Initial compressional behavior of fiber assembly. In: Kawabata

Postle

Niwa

(eds). Objective measurement: Applications to product design and process control, Osaka: The Textile Machinery Society of Japan, 1985, pp. 613–622.

10.

Carnaby

Pan

. Theory of the compression hysteresis of fibrous assemblies. Textil Res J 1989; 59: 275–284.

11.

Lee

Carnaby

Tandon

. Compressional energy of the random fiber assembly part II: evaluation. Textil Res J 1992; 62: 258–265.

12.

Dunlop

. Characterizing the compression properties of fiber masses. J Textil Inst 1974; 65: 532–536.

13.

Dunlop

. Acoustic emission from wool during compression. J Textil Inst 1979; 70: 364–366.

14.

Dunlop

. On the compression characteristics of fiber masses. J Textil Inst 1983; 74: 92–97.

15.

Neckar

. Compression and packing density of fibrous assemblies. Textil Res J 1997; 67: 123–130.

16.

Sebestyen

Hickie

. The effect of certain fiber parameters on the compressibility of wool. J Textil Inst 1971; 62: 545–560.

17.

Samson

. The compression of wool fabrics. Textil Res J 1972; 42: 272–273.

18.

Batra SK and Mohamed MH. Some thoughts on the compressional behavior of fiber assemblies. In: international seminar on nonwovens, BTRA, India, 1990.

19.

Matsuo

Lee

. Compressional behavior of fiber assembly mass (pressure deformation characteristics). J Fed Asian Prof Assoc 1996; 1: 31–36.

20.

Fengel

Przyklenk

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

21.

Lou

Liu

. Analysis of the tensile property of kapok fiber. J Donghua Univ 2009; 35.

22.

Liu W and Wang FM. Kapok battings and its thermal insulation properties. Shanghai: College of Textiles, Donghua University, 2011: 5–8 and 13–22.

23.

Yan

Wang

. A study on the quality of kapok blended yarns through different processing methods. J Textil Inst 2013; 104: 675–681.

24.

Yan

Fang

Wang

. Compressibility of the kapok fibrous assembly. Textil Res J 2013; 83: 1020–1029.

25.

Bao

. Experimental technology of textile material, Beijing: China Textile Press, 2004.

26.

ASTM D 6571-01e1:2008. Standard test method for determination of compression resistance and recovery properties of highloft nonwoven fabric using static force loading.

27.

Instron Company. Reference Operation Manual of Instron Compression Testers 5500 Series, 2003.

28.

Ribeiro

Costa

. Modeling and simulation of the nonlinear behavior of paper: a cellular materials approach. Chem Eng Sci 2007; 62: 6696–6708.

29.

Zhuang

. Applied mathematical statistics, Guangzhou: South China University of Technology Press, 2006.