Kapok Fiber-Structure, Characteristics and Applications: (A Review)

Kapok fiber is a light, cottony, pale yellow to brown fiber obtained from Kapok fruit. The fiber is characterized by a large, hollow lumen and is generally hydrophobic due to its waxy surface. The fiber is difficult to spun into thread and it is commonly used as filling in mattresses, pillows, stuffed toys and upholstery. Compared to other natural fiber, kapok’s use is limited in application. However, it is now gaining attention in the scientific community as an important material due to its outstanding properties. This review will provide a detailed overview on the structure, composition and morphology of kapok fiber. It will also discuss the recent researches conducted on kapok fiber including its application as adsorbent material, biocomposite, biotemplate, electrode material and others.

natural fiber is related to the nature of cellulose and its crystallinity 17 . Its considerable wax content 4,9,14 is thought to be responsible for its hydrophobicity and oil absorbency properties 10,19 . Infrared absorption characteristics of the raw fiber show well defined broad peak at about 3200-3400 cm -1 which indicates the O-H stretching vibration of the intramolecular hydrogen bonds in cellulose 9,15,20,21 . The peak about 2898-2917 cm -1 corresponds to the asymmetric and symmetric C-H stretching of aliphatic (CH 2 and -CH 3 ) groups 12,15,[20][21][22] . This denotes the presence of plant wax which is generally consists of n-alkanes with small fraction of alcohols, fatty acids, aldehydes, and esters. The peaks observed at about 1720-1737 cm -1 are associated with presence of carbonyl (CO) group stretching of ester, aliphatic aldehydes esters and ketones of plant wax and C-O stretching of ketones, carboxylic groups and esters in lignin and acetyl groups in xylan 9,12,15,20,21 . The C-C stretching vibration in the aromatic structure of lignin is observed at 1504-1511 cm -1 9,15,21 while the peak at 1425-1428 cm -1 denotes CH bending vibration in lignin and cellulose 15,21. The absorption bands at 1243-1250 cm -1 and 1155-1160 cm -1 is ascribed to the stretching of aryl-alkyl ether group in lignin and hemicellulose 4,9,15 and to the asymmetric bridge stretching vibration of C-O-C group in the cellulose, respectively 12 . Glucose ring stretching in hemicellulose and cellulose is indicated by absorption peak at 897-900 cm - 1 15,21 . Elemental analysis revealed that kapok fiber is composed mainly of C (75.3%) and O (24.2%). Abundance of C1 and C2 in the C1 spectra denotes that kapok fiber is consists mainly of cellulose and lignin 23 .
Analysis of the fiber under scanning electron microscope (SEM), reveals its very smooth surface without any ripple and no coarse structure on its surface. The smoothness of the surface can be attributed to the wax that is adhered on the fiber 4,24,26 . A single kapok fiber can be viewed as a cylindrical microtube with a nano-scaled wrinkle on its surface. It exhibited hollow tubular structure with diameter ranging from 10-25 mm and large air-filled lumen 4,11,12,[23][24][25][26][27][28][29][30] .
The fiber has high degree of hollowness as indicated from the observed porosity of 77% 11,12,13 . This characteristic makes the fiber as good material for insulation 12 . Considering its large lumens, the fibers have a bulk density of 0.305 g/cm 3,11 . X-ray diffraction pattern of the raw kapok fiber reveals characteristic peaks of cellulose to appear in the position of 2H close to around 15.5° and 22.6° corresponding to the (110), (101) and (200), (002) crystallographic planes. The former represents amorphous materials in cellulosic fibers while the latter represent the crystalline material in the cellulosic fibers 20,23,26,31,32 .
The index of crystallinity (CrI) of the raw kapok fiber is about 35% 4,9,26 and is found to be 59% crystalline region and 41% amorphous region 33 . The specific surface area of the fiber, calculated by the Brunauer-Emmett-Teller (BET) method, was 5.5 m 2 /g 31 . The fiber's conductivity ranges from 0.03 to 0.04 W/mK for density which varies between 5 and 40 kg/m 3 . Its average thermal diffusivity is 17.1 x 10 -7 m 2 /s which makes kapok as a good heat insulator 34 . Kapok fiber exhibits low wettability as manifested by its high contact angle on water about 120 o 4,22,26 to as high as 155 o -158 o 21,23,35 denoting that the surface is nonpolar and highly hydrophobic 4,22 . Table 2 shows the comparison of the mechanical strength of kapok fiber with other natural plant fibers. From the table, it can be deduced that the mechanical property of kapok fiber is not outstanding in comparison with other plant fibers-low strength, low stiffness and high fragility 36 . This is the reason why kapok is not suitable for spinning into yarn or textile applications due to its brittleness, slippery and smooth texture 9,30 . The chemical composition of plant-based fibers has a great effect on its mechanical property. This is due to the presence of cellulose, hemicellulose and lignin which is responsible for the bond behavior and degradation of natural fibers in composite 38 . The amount of cellulose and the orientation of its microfibrils also affects the strength and stiffness of the fiber 27 . Due to high crystallinity orientation index (COI) of kapok (71.5 %), the fiber tends to exhibit poor flexibility which may cause problems during the spinning process 9 . It is also reported that the processing of fiber affects its mechanical properties. Mechanical forces exerted on kapok fibers may affect the weak locations present within the fiber. Traces of spiral lines present in the cell wall suggests that kapok fiber possesses axial weak links which yields uneven mechanical properties 30 . Fibers produced by chemical extraction are stronger than those produced mechanically since it can soften the fibers and may also increase its surface area 26 . When utilized in papermaking, NaOH-treated kapok fiber provided higher tensile strength due to increased fiber bonding as lignin was removed and fibers became more conformable 1 .

Surface Modification Process of Kapok Fiber
The waxy surface and hollow structure of kapok fiber can be altered and the crystallinity of fiber wall matrix can be changed to improve its inherent properties. Different surface modification process, either chemical treatment and/or physical treatment, may improve the physical and chemical interactions at the fiber's interface 40 .

Chemical Treatment of Kapok Fiber
The use of solvents to modify the surface structure of kapok is the most popular method being employed.
Superhydrophobic kapok fiber was developed in order to improve the hydrophobic and oleophilic characteristics of the fiber. In a study by Tigno et al., 35 , TiO 2 nanoparticles were immobilized onto the kapok fiber's surface via hydrothermal method. To tune the wettability of the TiO 2 -based kapok nanocomposite, functionalization using 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PTES) was then performed. Surface morphology analysis via SEM of the raw kapok fiber and TiO 2 -based kapok nanocomposite before and after the PTES functionalization revealed changes from smooth fiber walls to rough and wrinkly surface as evident due to deposition of TiO 2 nanoparticles on the surface. FTIR spectra clearly depicted the presence of -OH groups which are attributed to the presence of cutin, cellulose and water molecules in the fiber and its disappearance after TiO 2 nanoparticles have been embedded. The prepared nanocomposite became superhydrophobic as shown from the high water contact angle of 151 o .
By incorporating tetraethylorthosilicate (as source of silica nanoparticles) onto kapok fiber via sol-gel method, followed by hydrophobic modification using hydrolyzed dodecyltrimethoxysilane (DTMS), a superhydrophobic and oleophilic oil sorbent was successfully fabricated. Due to silica nanoparticles, existence of uniform nanoscale roughness with low surface energy was observed after surface modification as confirmed by FTIR spectroscopy, SEM and through wettability studies of water and oil on the fiber surface. Oil sorption studies on the prepared sorbent indicated that the coated fiber exhibited excellent oil sorption capacity owing to its superhydrophobic surface 41 .
In another study, kapok fiber was treated with five different solvents: water, NaOH solution, HCl solution, NaClO 2 solution and chloroform 26 . After the treatment, the samples were filtered, washed several times with distilled water and dried at 70 o C. The structures of untreated and treated kapok fibers were investigated and compared using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Chemical treatment, particularly CHCl 3 treatment, successfully removed the waxy surface as implied by the increase of absorption bands at 3410 cm -1 and an intensive band at 2914 cm -1 . It also denotes the increase of cellulose hydroxyl groups in fiber walls due to the breaking of hydrogen bond. Another notable observation is the deesterification of lignin as observed from the significant decrease in stretching vibration at 1730 cm -1 which is ascribed to the C=O stretching vibration of ketones, carboxylic acid and esters in lignin 42 . NaOH-treated fiber showed reduction of band intensity at 1740 cm -1 and 1245 cm -1 which suggests that the strong alkali promotes deesterification of kapok fiber and complete removal of all the esters linked with aromatic ring of lignin. On the other hand, bleaching of kapok fiber leads to cleavage of the aromatic ring in lignin or delignification which is denoted by the disappearance of bands around 1602 cm -1 and 1504 cm -1 , respectively. By delignification, the amorphous portion of the cellulose increases and a decrease in hydrophobicity as observed in the reduction of static water contact angle 1,21,42 . The surface of kapok fiber, which is inherently smooth, is altered after chemical treatment. Treatment of strong alkali on the fiber resulted in the collapsed of the hollow tube of the fiber and coarseness of the surface which increased its surface area 15 . Changes in the surface structure enables better adherence of oil to the outer surface and better penetration at the inner lumen, thus increase oil absorbency for the solvent treated kapok than the untreated fiber. In terms of crystallinity, changes in crystallinity index (CrI) have been observed after the chemical treatment. Crystallinity index analysis revealed that the CrI is 35.34%, 33.93%, 22.17%, 32.00%, 26.97% and 27.17% for untreated, water-treated, HCl-treated, NaOH-treated, NaClO 2 -treated, and chloroform-treated kapok fiber, respectively. Changes in the aggregate structure and expansion of amorphous portion present in kapok fiber is noted after treatment of fiber with HCl, NaClO 2 and chloroform however NaOH-treatment does not significantly alter the fiber's crystallinity. Subjecting the fiber to low-alkali concentration at a short time does not alter the crystalline region of lignocellulose significantly. XPS analysis of raw and treated kapok revealed that NaClO 2 treatment increased the O/C ratio which is attributed to the decomposition of lignin and increase in cellulose content of kapok fiber. Chemical treatment is indeed very effective in changing the inherent characteristic of the fiber 8,26 .

Physical Treatment of Kapok Fiber
Cold plasma treatment on the surface of kapok fiber was done as a means to modify the surface composition of the fiber and improve wettability 13 . Changes in the FTIR spectra, thermal behavior and formation of irregular surface texture, as confirmed by SEM, was evident in plasma-treated fiber as opposed to untreated fiber. Improvement in wettability was also noted due to removal of hydrophobic groups such as oil and wax on the kapok surface and exposure of openings and pores which causes liquids to penetrate easily 13 . With these procedures, it can be concluded that plasma treatment is a good approach for the surface activation of kapok fiber and may be used for the enhancement of fiber-matrix adhesion.
High pressure treatment can also be used to modify the structure of the fiber 43 . In a study conducted, kapok samples were heated in an oven at 45+/-2 o C for 1 h and then treated under three different conditions: dry with 15% relative humidity (RH), wet with 99% RH and pressure of 100 kPa. SEM image of the treated fiber revealed that the once hollow and circular structure were crushed partially or totally to flat, thin ribbon-like thread after the application of 100 kPa pressure. Pressing the fibers devoid the cellulosic capsule of air space. The pressure treatment greatly changed the hollow structure of most kapok fibers.
Ultrasonic treatment is also used as an essential method of removing the impurities and breaking the weak sections in the fiber 30 . Treatment of soaked kapok fiber (in distilled water) with ultrasonic power for 6 min and 80% power resulted in a degraded fiber at the spiral traces with some parts became a spiral belt. The consistency on the width of the belt varies among different fibers. It was suggested that the spiral belt is the weak link along the cell wall of the fiber and the location wherein breakage or deformation may originate.

Applications and Uses of Kapok Kapok Fiber-based Composites
Reinforcement of kapok with other plant fibers or materials in composites is promising due to its ecological and environmental advantage and attractive mechanical properties.
Bionanocomposite films based on sodium alginate and kapok nanofibrils were prepared to enhance the potential of kapok fiber 44 . Cellulose nanofibrils (CNF) were extracted from the fiber after subjecting it to alkaline treatment, bleaching and acid hydrolysis. The obtained CNFs were subjected to TEMPO (2,2,6,6-tetramethyl-piperidinyl-1-oxyl) mediated oxidation to yield surface-modified cellulose nanocrystals (TOCNC). Solution casting technique was used to prepare bionanocomposite films based on sodium alginate and nanocellulose (CNFs and TOCNCs). Analysis of the prepared bionanocomposite films revealed that ultrasonication of TOCNCs has significant effect in the reduction of surface roughness of the nanocomposite and contributed in the improvement of the mechanical strength of the polymer. Improved dispersion of TOCNCs particles in the alginate matrix was noted than those containing CNCs. It was demonstrated that nanocomposite films based on TOCNCs presents better reinforcement in alginate biopolymer and may have potential applications in tissue engineering, structural and packaging applications.
A phase change composite material utilizing kapok fiber-based aerogel to form a porous material as support framework was fabricated 45 . A lightweight kapok fiber-based aerogel was made by oxidizing the fiber and combining it with sodium alginate and hydroxylated boron nitride which will be used to encapsulate polyethylene glycol (PEG) and prevent it from leaking. Phase change materials were then prepared by vacuum-assisted impregnation of PEG into the aerogels producing a highly thermally viable material with stable shape. The stability of the composite material's shape is noticeable and is reported without leakage even if it is heated to the PEG's melting point.
Surface activated kapok fiber using plasma treatment was added with recycled polyethylene to produce a composite 46 . Addition of plasma-treated fiber, as opposed to raw kapok fibers, were found to possess enhanced mechanical performance of the composite. Reduction in flow rate of the polymer was evident after the addition of the fiber as indicated by melt flow measurements. Furthermore, improvement in fiber/matrix adhesion was seen after the addition of plasma-treated fiber.
An iodine-doped polyaniline/kapok fiber (Pan/KF) composite is developed using the semidry method with iodine vapor as both dopant and oxidant 23 . The prepared composite exhibited good hydrophilicity compared with raw kapok fiber. The contact angle of the Pan/KF composite was reduced from 107 O to 86 O and 4 h increase in the reaction time suggests that the surface has transformed from hydrophobic to hydrophilic. Surface morphology of the composite revealed a rough surface with wrinkles and groves. It was noted that the degree of wrinkles and grooves increased with increasing polymerization time of aniline. Also, the hollow structure of the fiber is preserved during the process of medication with polyaniline. The prepared composite demonstrated good adsorption performance towards Au (III) and methylene blue.
A reinforced thermoplastic cassava starch composite (TPCS) using jute fiber and kapok fiber was fabricated 9 . Different ratios of jute and kapok fiber was added into the TPCS matrix and shaped by compression molding machine. Results revealed that increasing the concentration of cellulose load increased the stress at maximum load and Young's modulus of the TPCS. It was noted that the stress at maximum and Young's modulus were higher in TPCS/kapok than TPCS/jute which may be attributed due to the former's lower density. Another reason cited is the evenness in the distribution of kapok fiber as compared with jute fibers. IR shift was detected in the composite which may be due to the formation of new hydrogen bond which is formed after the addition of the jute or kapok fibers. Water absorption of the TPCS matrix, particularly the TPCS/jute, was reduced after the incorporation of the fibers. Thermal stability of the composites was improved however the thermal degradation temperature of the TPCS matrix were not changed by the addition of the jute fibers whereas it decreased after the addition of kapok fiber. Indeed, the overall property of the thermoplastic starch was improved after the addition of jute and kapok fiber.

Kapok Fiber as Oil Absorbing Material
Kapok fibers, which is naturally oleophilic 13 , have been extensively investigated for its ability to absorb oil in water systems 2,41,47 . Table 3 lists the oil sorption studies conducted on kapok fiber. Due to its low surface energy (about 31-40 mN/m), adhesive forces between oil molecules and kapok fiber surface are higher, thus oil droplets are easily adhered on the fiber's surface 10,11,47 . The surface energy of the fiber is close to the surface energy of the oils (20-30 mN/m) 10,11,47 while lower than that of water (72 mN/m) 47 . The kapok fiber is hydrophobic with water contact angle as high as 158 21,23,35,47 . SEM and OM results revealed the presence of smooth surface due to wax content and hollow structure 52 . On the other hand, IR studies confirmed the waxy characteristic of the surface due to the presence of aliphatic, carbonyl stretching of ester and C-O stretch of acetyl group 12,15,[20][21][22] . As stated by the above-mentioned characteristics, kapok fiber is an excellent absorbent for oil in water systems. However, observations on the low oil retention capacity of the fiber have been an issue as attributed to its smooth surface. To address this, modification of the surface morphology by polymer coating 48,52 and solvent treatment 26 was ventured. The rough surface of the modified kapok fiber, together with its low surface energy, prevented more oil from escaping the surface of the fiber thus increasing its oil sorption capacity. Sorption coefficient is greatly influenced by temperature at the packing density of 0.1 g/cm 3 . It was also reported that the wettability of oil to the fiber's surface and the adhesion work of kapok fiber to oil weakens as temperature increases 53 .

Kapok Fiber as Adsorbent for dyes and Heavy Metals
The use of kapok and other plant fibers as material for the development of inexpensive, high-capacity adsorbent has been pursued due to its renewability and broad spectrum of functional groups that can be involved in chemical modification. Modifications on the structure and/or composition of the fiber has been done in order to enhance its properties 21,22,28,42 . Studies on the utilization of kapok fiber as adsorbent for heavy metals and dyes were explored to further expand the use of the fiber in water treatment technology. Due to the detrimental effects posed by heavy metals and dyes on the water systems, complete removal of it is deemed necessary 22,54 . Table 4 shows the sorption studies on dyes and heavy metals being undertaken using kapok fibers.

Kapok Fiber as biomass Material for Catalyst and Electrodes
The use of natural biomass material like kapok in the preparation of hollow ceramic material and hollow metal oxide fiber was being developed due to its diverse use as photocatalyst for dye degradation 32,42,64,65 , electrocatalytic reduction and degradation of metals and other compounds such as Au (III), bisphenol A, paraquat, CO 2 and phenol 28,[66][67][68][69] , and for hydrogen gas generation 32,70,71 . One of the methods employed for the utilization of kapok fiber as biomaterial is through template approach. Template approach is a simple process which involves the use of natural biomass material as template followed by impregnation of metal oxide and finally, heat decomposition to release the hollow metal oxide fiber. Due to its hollow structure and large number of surface functional groups such as -OH and C=O, metal cations are easily adsorbed into the template which are then converted to hollow ceramic structure upon heat decomposition 72,73 . In other cases, pyrolysis of the chemically modified-fiber at high temperature is employed. Kapok fiber not only serves as carbon precursor but also transforms into activated carbon substrate during calcination 65,67 .
Similarly, kapok fiber has been utilized as micro-tube biomaterial or biotemplate for electrodes in fuels cells or batteries [74][75][76][77] and supercapacitor [78][79][80] . Synthesis using simple hydrothermal method followed by calcination at high temperature and inert atmosphere was the method employed to produce an electroactive hybrid composite. Other method involves the synthesis via micro-tube method to prepare kapok fiber as a micro-tube biotemplate and bio-carbon source to synthesize metal oxide/C composites 75 . In another study, in-situ polymerization of pyrrole on oxidized kapok fiber followed by carbonization is done to produce an ultralight carbon aerogel 78 . Due to the fiber's unique hollow tubular structure, faradaic reactive sites are more accessible and the transport length for ions and charges in real capacitive process is shortened 74 . In addition, because of fibers large surface area, highly defective carbon matrix and the synergistic effect of the hybrid electroactive components, high specific capacitance was obtained in the electroactive hybrid composites 78 .

Other Uses of Kapok Fiber
Kapok fiber has been explored for variety of uses apart from its conventional use. The potential use of kapok fiber as pulp source for paper making was explored 1 . Sodium hydroxide treatment (10% optimum concentration) of the fiber reduced the amount of lignin and increased the yield of the pulp. Addition of kapok pulp to commercial softwood pulp, increased the tensile and burst strength of the blended sheets, however low tear resistance and elongation was observed due to the brittleness of the pulp. Kapok pulp can be used as additive to commercial softwood pulp for the manufacture of commercial packaging paper due to its light weightiness, high strength, and excellent waterresistant characteristics.
Kapok fibers also found use in biomedical applications. Hydroxyapatite (HAp) nanotubes were synthesized by hydrothermal process using kapok fiber as natural templates 81 . HAp nanotubes with average inner radius of 10 nm and length of several hundred nm were successfully developed and found to be biocompatible with HeLa cell lines. Hollow HAp nanotubes can be used as a nanocarrier for bone tissue engineering applications. On the other hand, zinc oxide-embedded paper with antimicrobial property was fabricated from kapok, in situ. The ZnO embedded paper exhibits superior antimicrobial index against Gram-positive S. aureus than Gram-negative E. coli 82 .
Kapok fiber was used as biotemplate for the green synthesis of cellulose-based copper oxide nanomaterial via simple sol-gel derived solvo-thermal method. The obtained nanocomposite exhibited effective antimicrobial activity against Gram positive and Gram negative bacteria, as well as good heterogenous catalyst for the production of biodiesel from oleic acid and ethanol. Its outstanding specific capacitance performance can be utilized further in super capacitor applications 83 .
Porous composite membrane (PCM) utilizing kapok fiber membrane as basement membrane was fabricated and studied 84 . Facile synthesis of porous organic polymers (POPs) with macroporous structure were synthesized via click chemistry. The fabricated PCM exhibited porous structure, outstanding flexibility and excellent mechanical strength. The fabricated PCM was used for particulate matter capture and exhibited outstanding capture efficiency for both PM 2.5 (≥99.80±1.58%) and PM 10 (≥99.89±0.89%) in 30 h period.
Kapok fiber was also used as bioreinforcement material to virgin bitumen. The fiber surface, which is inherently smooth, underwent alkali-treatment to increase the surface' roughness which enhanced the interfacial bond strength between the fiber and matrix. In addition, treatment with silane coupling agents reinforces adhesion of the fiber and bitumen via surface grafting effect. Addition of kapok fiber increased the viscosity of bitumen and increased its permanent deformation resistance while decreasing its fatigue damage resistance. However, slight negative effect on the stress relaxation ability of bitumen was observed after the addition of kapok fiber 85 .
Utilization of kapok fiber for the synthesis of a super insulating composite aerogel from polymethylsilsesquioxane-kapok fiber have been made as a promising candidate for uses wherein embedded mass is critical e.g., space application 86 . Methyltrimethoxysilane (MTMS) was used as a silica precursor while kapok fibers were used as a reinforcing additive owing to its homogenous hollow structure and intrinsic low density. The synthesized aerogel can be made easily rollable like standard silica blanket and noted to possessed outstanding characteristic in comparison with commercially available aerogel composites. It has a density as low as 0.053 g/cm 3 , thermal conductivity of 0.018 W/mK and a flexural strength (at maximum stress) of 108±kPa 21 .

CONCLUSION
Kapok fiber transitioned from a simple stuffing material used for toys, pillow and lifejacket to a material of diverse uses in water treatment technology, packaging technology, papermaking and in the electronics industry. Due to its remarkable surface morphology and fiber structure, numerous applications were explored and investigated. However due to its poor mechanical properties and smooth surface, the use of fiber has been limited to some extent. To overcome this, reinforcement of kapok fiber with other polymer matrices was done to increase the strength of the material. Pretreatment of the fiber by physical or chemical process has been done to alter the surface characteristics and chemical composition of the fiber. By utilizing these approach, various research and applications on kapok fiber has expanded and ventured until today.

ACKNOwLEdgEMENT
The author would like to extend his gratitude and appreciation to his colleagues in the College of Arts and Sciences of Batangas State University.