Multi-Purpose Utilization of Kapok Fiber and Properties of Ceiba Pentandra Tree in Various Branches of Industry

ABSTRACT Ceiba pentandra (L.) Gaertn. is a member of the Malvaceae family. The plant is also known by other common names such as: kapok tree, silk cotton tree, Java cotton and many others. It grows in South and Central America, Mexico, the Caribbean, India, Thailand, Indonesia. The plant is best known because of the fiber produced by tree fruits and unique fiber structure. Interestingly, the plant, in addition to a specific fiber with multiple uses, is used as food and its parts also exhibit medicinal activity. This paper attempts to give an overview of the current knowledge of C. pentandra (L.) Gaertn. This review provide a brief and important information concerning kapok tree and kapok fiber industrial applications as well as on reports of its use in nutrition and traditional medicine.


Introduction
In recent years, in the era of European Green Deal, issues such as recycling, energy and organic valorization, waste prevention have received much attention worldwide. The use of biomass and bioenergy in various industries is increasing worldwide. These industries produce biofuels, biogas, biocatalysts, biocomposites, cellulosic textiles and many others. However, there are still many issues to be resolved and important questions regarding the use of biomass that need to be investigated. In recent years also, there has been an increase in environmental, water and air pollution derived from synthetic industrial waste. Pollution of wastewater and landfills with textile waste has become one of the most disturbing problems in virtually every country, on every continent. The industry is beginning to recognize these problems and is increasingly willing to fold toward the search for raw materials of natural origin, environmentally friendly and easy to use. Alternatives are being sought to replace or reduce the use of synthetic products. In many ways, kapok is an interesting plant and a practical source of plant fiber with specific beneficial properties. Ceiba pentandra (L.) Gaertn. is a tree with multiple uses (Nkouam et al. 2017). Due to the beneficial properties of its fibers, the plant has been used in the textile, paper, aviation and method. The results obtained by the authors suggest strong reducing and radical scavenging activity by the compounds present in the kapok oil such as phenols, flavonoids, alkaloids and tannins.

Kapok fiber and its characteristics
As previously mentioned, natural fibers are obtained from plants or animals. Among them plant fibers are natural reinforcing and filling materials used in polymer composites. This is because of their good availability, low price and renewable nature (Miedzianowska, Masłowski, and Strzelec 2018). Recently, many researchers investigated the mechanical and chemical properties of the materials based on natural fibers (Yıldızhan et al. 2018).
Kapok belongs to a small group of seed fibers, of which cotton (Gossypium) is the most industrially and economically important. Seed fibers are unicellular fibers that cover the inside of the fruit bags. Kapok fiber is relatively inexpensive and easily biodegradable.
The species of the genus Ceiba produces fruits with fibers with a high content of cellulose and nanocellulose (Gómez-Maqueo and Gamboa-DeBuen 2022). The fiber is also composed of hemicellulose, lignin, pectin and wax (Futalan et al. 2022;Prachayawarakorn et al. 2013).
Kapok is called the woolly fiber of the fruit of kapok trees. Kapok is an easily breakable, nonspinnable, seed fiber in the form of a puff. Figure 1 shows the appearance of kapok fibers. Kapok fiber have large inner spaces filled with air (hollow structure) and have a fairly high fat content, which makes them waterproof. The fibers are thin, soft and shiny and also light, very resistant to moisture, light yellow to light brown in color. A single kapok fiber is a cylindrical microtube with hollow tubular structure with diameter ranging from 10-25 mm and large air-filled lume (Sangalang 2021). Kapok, due to its unique fiber structureabundant hollow microtubes, contains as much as 80% air and is therefore more than 8 times lighter than cotton (Zheng and Wang 2014). The kapok seed fiber is covered by a hydrophobic waxy layer Peng et al. 2021).
Kapok fibers are formed by single cells having a very thin wall and a wide inner channel filled with air (Figures 2 and 3). Kapok fiber differs from other natural fibers by its high clearance and thin cell walls. The wall thickness is of the order of about 1-2 µm, the fiber diameter is 15-35 µm. The channel occupies about 80-90% of the fiber surface (Liua et al. 2022;Xiang et al. 2013). Regarding the chemical composition of kapok fiber, it contains: 35% cellulose, 22% hemicellulose and about 22% lignin (Yıldızhan et al. 2018). A detailed percentage description of the chemical composition of kapok fiber was given in his work by Sangalang (Sangalang 2021). Figure 2 present microphotograph of the longitudinal view of kapok fibers. Figure 3 present microphotograph of cross section of the kapok fibers. Both figures were taken at 500× magnification using  a Hitachi S 3400N scanning electron microscope in high vacuum SE mode. In longitudinal view, kapok fibers are straight, round or finely oval, smooth without clearly defined features. In cross section, the fibers have an uneven oval shape, flattened with a very well-defined inner channel.

Kapok fiber-based composites
Polymer composites reinforced with plant-based natural fibers are an excellent alternative to synthetic materials. Their production is associated with reduced costs and increased biodegradability. The work on polymer composites is becoming more and more advanced. Renewability and sustainability are the most important advantages of biocomposite materials. An additional important advantage is their biodegradability, making biocomposites likely to dominate the future of industry. Rangappa et al. performed studies on the physical, mechanical, and thermal properties and morphological structure of composites prepared using chicken feather fiber and Ceiba pentandra bark (Rangappa et al. 2022). In this study a new innovative way to utilize waste chicken feather fibers and Ceiba pentandra bark as reinforcement materials in polymer composites was successfully presented (Rangappa et al. 2022). The authors proposed the introduction of bioepoxy resin in the production of composites and presented an idea to improve the properties of composites using carbon fabrics as shielding layers with plant and animal fibers. As a result of the experiments, hybrid bioepoxy composites reinforced with chicken feather fiber and Ceiba pentandra tree bark with carbon layers were prepared and their properties were analyzed (Rangappa et al. 2022).
The high air content in kapok fiber means that the fiber can easily absorb surrounding heat. This makes kapok fiber possess an excellent thermal insulation properties. The material produced from it is airy and breathable. Thus, natural fiber based unwoven fabric are utilized for thermal insulation applications. Jabbar and colleagues developed of kapok/recycled-PET blended needle-punched thermal waddings. The effect of percentage of kapok fibers on thermal resistance and air permeability was analyzed in the prepared non-woven needled wadding. The study showed that the thermal resistance decreased significantly with increasing % of kapok fibers, indicating good thermal insulation properties of kapok fibers. The prepared nonwovens may find applications in textile industry (Jabbar et al. 2020). Pividal and Rocha developed nonwoven fabrics containing raw kapok and flax fibers for use in agro-textiles/geotextiles (Pividal and Rocha 2021). While Kumar developed nonwoven fabrics with flax and low-melting PET for acoustic and thermal insulation applications (Kumar et al. 2021).
In the paper of Wang et al. C. pentandra fibers were chemically modified in 3-D crosslinked structure. Kapok fiber structure was coated with dopamine through a three-dimensional (3-D) polymerization. This technology made kapok fiber has shown antibacterial activity. Silver ions from kapok fiber coated with poly-DOPA with silver (KF-DOPA/Ag) were shown to alter cell morphology. The produced composite proved effective in deactivating bacterial cells (Wang et al. 2016).

Kapok as an absorbent material
Due to its structure, kapok fiber (hollow inside and wax coated outside) has ideal properties to be used as a filter to separate oil from water (Rengasamy, Das, and Karan 2011). The process of oil-water separation in kapok filters is divided into the following steps: infiltration, separation, displacement and equilibration (Huang and Lim 2006). Oil is absorbed due to hydrophobic interactions and Van der Waals forces between the oil and the wax-coated fiber surface, and absorbed by the fiber due to capillary movement. By increasing the surface roughness of the fiber, its hydrophobicity can be further increased (Wang, Zheng, and Wang 2012a, 2012b, Wang, Zheng, and Wang 2013a, 2013b. The authors of this study suggested that acetylation of kapok fiber and coating it with a hydrophobicoleophilic polymer, can be a very good way to minimize water absorption, especially in the case of oil removal from emulsions (oil in water -wastewater problem) or oil from marine water bodies (Quek, Ngadi, and Zaini 2020;Wang, Zheng, and Wang 2013a).
Chen et al. tested a biotemplated catalytic tubular micromotor consisting of natural kapok fiber matrix and manganese dioxide nanoparticles deposited on the outer and inner walls of the mentioned above fiber. Authors of the paper demonstrated its applications for rapid removal of methylene blue in real-world wastewater. It can be considered as self-propelled microcleaners for wastewater treatment (Chen et al. 2021). In another experimental study, Chen and his team described a new monolithic solar steam generator derived from kapok fiber-based MXene composite aerogel. It has been made by dipping the aerogels which composed of kapok fiber and sodium alginate (SA) as substrates in the suspension of MXene. With the use of suitable hydrophilic and oleophobic chemical modifications, the composite aerogel based on kapok fibers can be widely used in oily wastewater . A similar superhydrophobic aerogel based on natural kapok fibers and sodium alginate was tested by Tian and his research team. It was obtained as a result of directional freeze-drying and chemical vapor deposition. The described aerogel has been shown to effectively collect oil contaminants from the surface and bottom of the water and can provide an alternative to membrane-based treatment of oil wastewater through filtration (Tian et al. 2022). In a subsequent scientific work researchers have produced a unique single-molecule bio-interfacial entanglement as an absorption layer to capture the catalyst for ZnO electroless deposition on the surface of natural hollow-structured kapok fibers, where ZnO is loading on both the outer and inner walls of a fiber-structured hollow. The authors of the paper have suggested that using porous substrates to fix and load ZnO is a promising technical method to improve the water purification efficiency and recycling durability of ZnO ).
In the experimental study of Wang, Zheng, and Wang (2012d), it was observed that PBMA (polybutylmethacrylate) coated kapok fiber showed higher absorption capacity than crude kapok fiber as well as better reusability and oil retention capabilities (Wang, Zheng, and Wang 2012d). In similar work, a new kind of oil sorbent based on kapok fiber was also prepared by the coating of the mixture of polybutylmethacrylate and hydrophobic silica on fiber surface. Generally, the fiber produced in this way had a higher sorption capacity than the crude fiber in the oil/water mixture (Wang et al. 2013c). Kapok fiber was also used to produce a carbon microtube aerogel (CMA) with hydrophobicity, strong adsorption capacity and thermal stability. Thus, the CMA can be used as an efficient and environmentally friendly adsorbent (Song et al. 2020). It was found that the hollow tubular structures allow the resulting carbonized kapok fiber composites with microwave-absorbing abilities. In the study of Long and colleagues lightweight and sustainable microwave absorbers with a high absorption capacity and broad effective absorption bandwidth were obtained. It was showed that due to the balanced impedance matching and attenuation capacity, carbonized kapok fiber demonstrate the best absorption performance, where the RL min reaches−49.46 dB at 16.48 GHz for CKF-30 with a thickness of 2.3 mm (Long et al. 2022).
In the experimental work of Yunos and team superoleophilic-hydrophobic kapok bundles were synthesized via one-step carbonization in 300°C. Their utility as effective oil sorbents was investigated. It has been shown that by varying the carbonization temperature, the surface roughness and internal graphitic phase of kapok bundles can be controlled. Such action increases their oil sorption and retention. In addition, the carbonized kapok fibers can be reused (Yunos et al. 2022). According to the literature, different surface modifications of kapok fiber are used depending on the type of absorbent material. These issues are summarized in the Table 1. (Zheng and Wang 2014).

Other important application examples of kapok fiber in various industries
Due to the valuable properties of kapok fiber such as hydrophobicity (due to the wax layer), oleophilicity, and biodegradability, kapok fiber has been used as filling for pillows (Sangalang 2021;Yıldızhan et al. 2018), quilts, and soft toys. Moreover, it is used for production of hygroscopic gauze, insulating materials and in upholstery or like floating material in life jackets (Sangalang 2021).
Biofuels are produced by processing biomass, which can be plants, animal substances or microorganisms. Currently, the most commonly used biofuels are so-called type I biofuels, which are produced from plants that are also used for food production. Liquid biofuels are biodiesel and bioethanol, known as renewable energy sources. Biofuels have a low carbon intensity, so they don't actually directly affect global warming. This is in line with the Green Deal, which promotes the transformation toward climate neutrality. Bioethanol can be produce from biomass with high level of lignin, starch, or cellulose that has been processed into glucose. Because of high glucose content, kapok fiber may be a potential resource for the production of bioethanol (Zheng and Wang 2014). A study performed by Tye et al. based on chemical composition analysis showed that the cellulose content of C. pentandra fiber was 50.7%, and the glucose content was 59.8%. The high glucose level indicated that kapok fiber is a potential substrate for bioethanol production. However, it requires pretreatment of the fiber. After water, acid and alkaline pretreatments, the yield of reducing sugar was increased to 39.1%, 85.2% and 100%, respectively (Tye et al. 2012). In the study conducted by Budiyono et team the effect of  Wang (2013b,2012d) hydrogen peroxide acetic acid pretreatment of kapok fruit peel waste on the cellulose, hemicellulose, and lignin content produced were examined. The authors of the paper also evaluated the effect of fermentation pH on the levels of bioethanol produced (Budiyono et al. 2022). Budiyono et al. discovered that the pretreatment of HPAC was able to remove 90% of the lignin content in kapok fruit peel waste. The highest yield of bioethanol was on 8.78% level (Budiyono et al. 2022). In the innovative study, biodiesel was derived from kapok oil via ultrasonication (US)-assisted catalytic transesterification method. The authors of the paper reported that the maximum biodiesel yield obtained from kapok oil was 93 ± 1.04% by catalytic trans-esterification reactions (Pasawan et al. 2022).
There has been carried out the research work on methods to obtain activated carbon fibers based on natural plant fibers such as kapok fiber. Kapok fiber can also be a desirable candidate for catalyst carriers and recovery.
Currently, a large part of the products applied as acoustic insulation are made of synthetic or artificial materials. The most commonly used are mineral wool, glass wool, polyurethane foams (PUR), polyester. Recently, natural raw materials are becoming an important, ecofriendly alternative to commonly used sound absorbers due to reduced production costs. The results of ongoing work confirmed that kapok fibers due to their natural structure and hollow space (channel) inside show very good sound absorption properties moreover, they can be an ideal, natural alternative to commonly used sound absorbing materials (Liua et al. 2022;Xiang et al. 2013). Kapok fibers can be used as constituent of valuable materials prepared for the sound absorption activity (Zheng and Wang 2014;Rijavec 2008;Xiang et al. 20132013). Moreover, as a result of subsequent experimental work multifunctional composites based on kapok fiber with flame retardant and sound-absorption properties were obtained .
Polysaccharides and high content of pentosan detected in this tree are favorable for dense and good pulping. These properties make the tree suitable for pulp and paper manufacture (Walia et al. 2009;Leh et al. 2021). Experiments performed show that the incorporation of kapok pulp into the mixed pulps is beneficial for the improvement of tensile and burst strengths of the sheets (Zheng and Wang 2014). In one of the more recent works conducted by Peng and his team, the effect of various pulping methods like mechanical, chemi-mechanical, semichemical and chemical pulping on kapok fiber properties was investigated. After analysis, it was concluded that kapok pulp could be a valuable source of reinforcing fibers with high recycling potential (Peng et al. 2021).
Kapok is also applied as a substrate for active materials in supercapacitors. Hard carbon micronano tubes derived from kapok fiber were successfully used as anode materials for sodium-ion batteries. Moreover, electron paramagnetic resonance and thermogravimetric analysis were applied to investigate the Na-ion storage mechanism in the hard carbon micro-nano tubes (Yu et al. 2020). In another paper a multi-level composite structure formed by the micro-nano materials based on selfassembled molybdenum disulfide (MoS2) nanoflowers, Mxene and hollow carbonized kapok fiber was developed. Construction of heterostructures by stacking different two-dimensional materials is able to improve electrochemical performance. Therefore, construction of MoS2/Mxene heterostructure on stress-modulated kapok fiber can be useful for high-rate sodium-ion batteries. It was found, that this system had higher specific capacity than pure MoS2, and has better rate performance and stable structure .
In the paper performer by An and colleagues, for the first time were reported the form-stable phase-change materials with exceptional latent heat by employment of sugar alcohol, in this case erythritol and mannitol, as organic phase-change materials and carbonized kapok fiber as porous supporting materials for thermal energy storage. The use of kapok fiber as a carrier material by physically combining it with sugar alcohols is of great technological importance due to its high energy storage capacity, high loading values and enhanced thermal conductivity (An et al. 2019). Furthermore, the unique structure, good conductivity and excellent physical properties exhibited by kapok fiber nominate it as a highly preferred cocatalyst for solar energy processes (Zerga and Tahir 2022).
Composites formed on the basis of kapok fibers are also used in the adsorption of heavy metal ions and dyes from aqueous solutions. The introduction of various surface modifications and the synthesis of kapok fiber-based composites results in higher adsorption capacities for the selective removal of heavy metals and dyes through these procedures (Futalan et al. 2022).

Kapok as a food source
Kapok trees begin to bear fruit at 3-8 years of age. Harvesting the fruit from the kapok tree takes place in late June and early July, or during the dry season after the fruit has fallen to the ground. The leaves are the most commonly used part of the plant. For the people of Nigeria and Cameroon they are a valuable vegetable, especially during the dry season when access to other plants is limited. Research conducted by Friday and his team found that the leaves of C. pentandra contain nutrients and minerals that may be useful in nutrition. The leaf samples studied were obtained from Nigeria and other West Africa countries (Friday et al. 2011). The bark and roots are also eaten by ethnic groups living in Africa. Men, in particular, chew the roots and bark of kapok, as they believe in its fertility-enhancing effects. The seeds contained in the fruit of C. pentandra are consumed by people living in Cameroon and Nigeria and nearby countries in the region. Roasted seeds with salt, are a substitute for peanuts as a snack.
Kapok seeds are also used in animal feed, the crushed seeds are used both as animal feed and for oil production (Nkouam et al. 2017). In addition to the seeds, the leaves are also often eaten by animals. A study by Hendrig et al. examined the use of kapok tree (Ceiba pentandra) as a food source for vertebrates during the dry season in Madagascar. Twenty-one native vertebrate species (5 lemurs, 5 bats, and 11 birds) were found to forage in or within the kapok tree during periods of low food availability (Hending et al. 2021). The nectar of C. pentandra is a food source for many species of bats (Cynopterus sphinx, Pteropus giganteus or Rousettus leschenaulti) (Singaravelan and Marimuthu 2004), mammals, birds, and small invertebrates (Gribel, Gibbs, and Queiroz 1999;Hending et al. 2021).
The presence of anti-nutritional compounds may be responsible, for decreased appetite, weight loss, stunted growth of broilers and change in egg color in laying hens fed kapok tree seed meal (Kadirvel, Natanam, and Udayasurian 1986;Narahari and Rajini 2003). It was examined, whether the content of these compounds also affects the weight gain of rabbits (Wafar, Yakubu, and Lalabe 2018). In this study, feed was enriched with the addition of crude kapok seed meal containing 89.51% dry matter, 17.45% crude fiber, 22.59% crude protein, 2.53% tannin, 3.34% alkaloids, 2.48% phenol, 2.95% flavonoids, 1.3% saponins, 17.97% trypsin inhibitors, 1.69% hemagglutinin and 1.12% oxalates. Unfortunately, the addition of kapok seed matzah also caused a decrease in growth performance and feed digestibility in rabbits. It was concluded that in rabbit feeding, the addition of kapok seed meal should not exceed 10% (Wafar, Yakubu, and Lalabe 2018). Wafar et al. subjected kapok seed meal used for rabbit feeding to heat treatment (cooking, toasting) and fermentation. They observed that cooked as well as the other processes decreased the values of antinutritional compounds, and complete inactivation of trypsin inhibitors. However, the final body weight and total feed intake of rabbits fed diets with treated kapok matzah were significantly (P < .05) lower than those of the control group. Comparing the different treatments of kapok meal, it was noted that cooking was the most effective processing method in this group of animals had better growth parameters than the groups fed toasted or fermented meal (Wafar et al. 2018). In a study performed by Irie (Irie 1990), pigs were fed a feed supplemented with copper or kapok meal, or both, and the properties of spare fat were studied. Pigs were given a control diet plus 200 mg·kg −1 copper; 3% kapok meal; 100 mg·kg −1 iron and 100 mg·kg −1 zinc. The results indicated that the addition of kapok meal significantly increased the melting point, C18:0 content and C18:0/C18:1 ratio in pig fats and decreased the C16:1 content. In another study, pigs were fed kapok oil at a rate of 0.5% addition of kapok oil ozociclopropenoid fatty acid (0.01% CPFA) per feed. It was found that although there was no effect of kapok oil supplementation on growth performance and intramuscular fat content in muscle, there was an increase in saturated fatty acids in subcutaneous and intramuscular fat and a decrease in monounsaturated fatty acids (P < .05). Unfortunately, meat consumption of the kapok oil-supplemented group showed a reduction in pork palatability due to a decrease in monounsaturated fatty acids, but the tenderness, juiciness, texture, and flavor intensity of the LM chops were similar in both groups (Maeda et al. 2017).

Properties of kapok used for medicinal purposes
Kapok is counted among the herbs of Ajuverdian medicine, it shows astringent and diuretic properties, reduces fever, relaxes spasms and inhibits bleeding. The leaves exhibit abortive, diaphoretic, emollient, laxative and sedative properties. Externally, the crushed leaves are used as a dressing for ulcers, sprains, tumors, abscesses, vitiligo and the squeezed juice of the leaves is used for skin infections. The leaves can be harvested at any time during the growing season and used fresh or dried. The roots and stems are attributed with emetic and antispasmodic properties. Macerates of the bark are considered a remedy for heart problems and hypertension, and are attributed with stimulant and anthelmintic properties. The bark is usually harvested during the dry season. The flowers have emollient properties and are used as a remedy for constipation. The results performed in Cameroon showed that almost all parts of kapok are used in curing many diseases such as sexually transmitted illnesses, fever, skin problems, eyes infections. Sometimes the plant is treated as aphrodisiac (Nkouam et al. 2017).
There are reports that extracts from some parts of the plant show hepatoprotective, hypolipidemic, anti-inflammatory, antihypertensive and sugar-lowering activities in type II diabetes ). The fruits have emollient properties. C. pentandra was found to exhibit nephroprotective effect against metotrexate-induced kidney damage. This was the first report of this in the scientific literature. The authors of the study hypothesized that a mechanism in which the plant's antioxidant, antiapoptotic and anti-inflammatory properties play an important role is most likely responsible for this effect ). Kumar and his team studied the properties of bark extracts from C. pentandra. They concluded that extracts examined possess cytotoxic and antitumor activity (Kumar et al. 2016). One of the more recent papers describes newly detected flavonolignans (ceibapentains A and B; and cinchonains Ia and Ib) that were isolated from the aerial parts of Ceiba pentandra. Their beneficial effects in Alzheimer's disease were described (Abouelela i wsp. 2020b). In a recent published study, silver nanoparticles (Ag-NPs) were synthesized using leaf and bark extract of Erythrina suberosa Roxb. and Ceiba pentandra L. and their antioxidant and antibacterial activities were checked. Ag-NPs possessed high potential to fight and eradicate biofilm forming bacteria and showed significant antioxidant activity (Afzal et al. 2022). These properties can be used to treat bacterial infections.

Conclusions
Kapok is a very useful, biodegradable, organic fiber. Kapok fiber is widely used. It is applicable in textiles, buoyancy, acoustical and support materials, thermal insulation, oil absorbing materials, paper production, biofuel and kapok-derived activated carbon fibers. The plant and its preparations are also used in medicine and as a food. From the point of view of the possibility of using this plant in many industrial fields, it can be concluded that it is a very valuable plant with beneficial biological and many other properties. This plant is of increasing interest, especially by combining its higher hollowness of fibers and hydrophobic -oleophilic properties. The fact that the plant and the properties of its fibers are of great interest is evidenced by the recently published scientific papers. However, further in-depth research on the industrial applicability of this plant and its properties as well as new technologies is still needed.
• Wide range of industrial uses: It is used for filling mattresses, upholstery and as a warming lining. For filling floats of life jackets. As a raw material in the acoustic industry As a material for removing fats (oils) from water In furniture industry, paper industry • The seeds yield oil Oil is used to make soap As an feed additive • As food and feed (leaves, roots) Human food Animal feed • Source of biologically active compounds used in phytotherapy