It is now well known that lignocellulosic fibers (belonging to a highly aromatic material family having complex carbohydrate polymers of lignin, xylose and glucose) have undoubtedly contributed to the world’s economic prosperity and sustainability in the daily life of people. In view of their known unique characteristics including functional and environmental advantages, they are preferred as replacement to synthetic fibers in the development of composites. Among the lignocellulosic fibers available to make composites, bamboo fibers stand out prominent in view of their physical and mechanical characteristics as well as large availability and low cost, besides their uses in many diverse industrial applications such as textiles, paper, and construction [1]. In addition, primary target to replace or reduce the use of nonrenewable synthetic fibers such as glass fiber in composite development for various conventional and even nonconventional applications has revived new interest in bamboo fibers for such applications. Furthermore, bamboo fibers in general have tensile strength comparable to that of steel, besides good adherence to polymer matrixes [2,3].

Bamboo termed as ‘Mother Nature’s magic material’ is one of the abundantly available and fast-growing plant material that grows in most developing countries and thus readily available natural material [4, 5]. Another advantage of bamboo is its effectiveness in curbing carbon emissions making it a better and sustainable building material. Besides being sustainable and strong with specific strength equivalent to that of steel (250-625 MPa/g/cm3) [5], it can replace steel in places where it is available in plenty, particularly in construction sector (as portioning wall, ceiling, roof and other areas of engineering) where heavy load-bearing is not required [6]. There are about 1450 bamboo species from 70 genera found in diverse climates [7]. Brazil has about 80 varieties of bamboo grown particularly in subtropical and temperate zones with the Northeast of the country being the largest commercial cultivator [8] compared to more than 400 species of bamboo grown in China [1]. Some of the varieties grown in Brazil include: Bambusa taquara, abundant in the state of Rio Grande do Sul; Bambusa vulgaris, which is widely used for reforestation in the northeastern region; Guadua refracta, a very common in Goiás; Nastus barbatus is grown plentifully in São Paulo and Paraná; Guadua weberbaueri, is found throughout the country, while Merostachys skvortzovii Sendulsky,known as taquara-lixa grows mainly in Paraná with its distribution is widespread, mostly along the “Floresta Ombrófila Mista” (Araucaria forest) [9]. This plant, belonging to the family of bamboo, has about 18 genera with about 155 taquaras species and bamboos [9,10]. As is the case with other varieties of bamboos, this variety is also used in construction sector starting from beams and light sleepers, light fences, in making baskets and as pipes (indigenous musical instrument of Northeast Brazil) [11].

There are various methods for the preparation and separation of different varieties of bamboo fibers (macro, micro and nano size) from raw bamboo, which are reviewed by Liu et al and Zakikhani et al [1,12]. There is also a novel mechanical extraction process developed to obtain long bamboo fibers by Osorio et al [13]. Another method of extraction has also been reported which uses combined chemical and mechanical methods, with the latter using compression molding and roller mill techniques for the mechanical separation of the fibers [14].

However, it is not known whether the above methods are applicable to taquara-lixa bamboo. However, there is only one extraction process reported for the taquara-lixa bamboo using an alkali treatment [15]. One of the objectives of the present study is to develop another method of extraction, which is described later in the paper under Extraction of fibers.

Although a large number of reports are available on the properties of bamboo fibers in general, which are reviewed from time to time [1,12,16,19] and also specific studies dealing with the structure of bamboo and their mechanical properties with the aim of their biological applications [20], there seems to be very limited studies on the structure and properties of taquara-lixa bamboo fibers [15,21,22]. All the above studies have also reported development of composites using the bamboo fibers.

In view of the limited studies on taquara-lixa fibers, a detailed study was undertaken as part of a doctoral program of the first author wherein starting from the extraction of fibers, their characterization and development of epoxy based composites containing as-received fibers. (NaOH). For example, Alves Jr., working with composites of taquara fibers with polyester resin, has tried to find out possible way of treatment of these fibers with NaOH and its effect on the mechanical properties [21]. He used pull out tests and also determined the tensile properties of composites to understand the effect of NaOH treatment.

Considering the limited literature on the composites of these fibers, this paper presents the extraction of the fibers and their characterization in respect of chemical composition, morphology, crystallinity index and thermal stability of fibers with and without alkali treatment with the objective of development of composites using these fibers with epoxy matrix and their characterization. This part is being presented in another publication (Borges Neto et al, Unpublished).

Materials

The taquara-lixa bamboo in its ‘green’ state (natural humidity >30%) was collected from the municipality of São Bento do Sul, Santa Catarina state (Brazil). All chemicals used in this work were reagent grade. Sodium hydroxide (NaOH) used for alkali treatment was purchased from local market. In fact, this treatment was given to the fibers only to compare some results of other authors as mentioned earlier.

Methods

Extraction of Fibers

First, stem of taquara-lixa bamboo, which are shown in Figure 1(a) was sawed to eliminate the nodes and diaphragms and selected those internodes having lengths ranging between 200 to 350 mm. It may be noted that the nodes are regions of the stems with the highest concentration of fibers. Therefore, they were eliminated in order to obtain a material with greater homogeneity and better workability. The internodes were then fractured longitudinally in the form of "twigs" 10 mm wide. Wall thickness of the taquara-lixa bamboo was between 2 and 3 mm. These sticks were then passed between the rolls of an improved calendaring machine specially developed in Polymers Laboratory of University of the State of Santa Catarina (UDESC), Joinville (SC-Brazil). This machine allows adjustment in the distance between the rollers and they still have grooves and a small difference (1 mm) in the development of their surfaces, facilitating the greater homogeneity and better workability. The internodes were then crushing of "twigs" with varying thicknesses between 1 and 3 mm. For all the materials, 2-3 passes were made between the rollers, and with each new passage the distance between them was reduced, which at the end of the process gave a very perceptible separation with the naked eye, in relation to the original state of the fibers. During manual processing, the material was kept submerged in clean water (to keep the average moisture content of about 30% during the process). Fibers were then stored in air and allowed the moisture content to reach the value the same as that of the ambient humidity (15 to 20%).

Figure 1(a) shows the macro-photographs of taquara-lixa samples without leaves, while Figure 1(b&c) show their appearance after the first and second calendaring process. Then, the fibers were extracted manually from the samples after second calendaring (Figure 1(c&d) shows the macrophotograph of these fibers, while Figure 1(e) shows final fibers obtained from the stacks after the calendaring process, which is used for chemical treatment and characterization.

Figure 1: (a)-Stacks of Taquara-lixa (Merostachys skvortzovii Sendulsky); Samples: (b)- after first calendaring; (c)- after second calendaring; (d)-manually separated samples after second calendaring; (e)-final fibers obtained from the stacks after calendaring.

Process of obtaining the fibers described in this study was different from the earlier reported ones [15,21] where an alkaline treatment was given to taquara-lixa. In the process of Reis, more severe conditions were used before the calendaring process. This process consisted of immersion of the material for 90 min in a 10% w/w NaOH solution before each calendaring step, reduced spacing between the rollers, followed by treatment with hydrogen peroxide solution for additional despite the greater flexibility of the material, the manual separation of the fibers was still difficult [15]. Not only the use of NaOH generates effluents that must be treated but also the preparation of the composites becomes more difficult.

Therefore, the aim to compare NaOH treated and untreated taquara-lixa fibers is to evaluate if the fiber could be used untreated in order to facilitate the composite preparation and turn the process into a more environment friendly one.

Chemical Treatment of Fibers

Some of the extracted fibers were treated with a 10% (w/w) NaOH solution for 1 h and then they were washed in distilled water and dried in an oven (Quimis make, Model Q819V2) having precision of ± 5 °C) at about 100oC for 24 hours.

Characterization of Fibers

Chemical Composition

The sampling and preparation of the material for the analyses of chemical composition were carried out as per the procedures given in the TAPPI standard T257-cm02 and the TAPPI T264-cm97. The method for moisture determination was as per the TAPPI T550 om-03 standard, while the ash content was determined by applying the method described in the TAPPI T211-om02 standard. The solubility in both hot and cold water was measured according to the TAPPI T207 cm-99 standard. The pH values of the samples were determined following the TAPPI 252 om-12 standard. The assays to determine the total content of extractives were carried out according to the TAPPI T204 cm-97 standard. Finally, the quantity of acid-insoluble lignin in the samples was measured following the TAPPI T222 om-02 standard. The concentration of holocellulose, or total cellulose (α-cellulose + β-cellulose + γ-cellulose), was determined by difference, i.e., 100% - (% lignin + % total extractives). Details of the methods followed to determine each of the components of the fiber are given below:

For determining the moisture content of the fibers with and without alkali treatment, TAPPI T550 om-03 was used.

Approximately 2 g of each type were weighed in previously weighed beakers. These samples were oven-dried at 105 °C for 24 hours. Then, the samples were taken out of the oven and placed in a desiccator to cool and to avoid contact with the humidity of the environment. The samples were again weighed and the moisture content was calculated by the difference in the weights of the fiber before and after drying. The whole procedure was repeated thrice and the average of these was taken as the final moisture contents in each of the fiber types.

To determine the ash content of both types of fibers was about 5 g of each type of fibers were taken in previously incinerated and weighed porcelain crucibles. The samples were then kept in muffle furnace maintained at about 525 °C for 3 hours. Then, the crucibles were removed from the muffle and kept in a desiccator at room temperature and then they were weighed again. The ash content was calculated using the following equation:

AC = (Ma/Mb) x 100-----------(Eqn. 3)

where, AC is the ash content in the fibers, Ma and Mb are the initial weight and final weight of the crucible before and after heating respectively (both in g).

Solubility in cold water was determined by first mixing about 2 g each type of fibers with 300 ml of distilled water and this was transferred to 400 ml beakers. The samples were kept at room temperature for 48 hours with frequent shaking. Then, the material was transferred to a pre-weighed filter crucible. The material was washed with distilled water, filtered by suction and then kept in an oven for drying at 105 °C. The samples were then removed from the oven and kept in a desiccator until constant weight was observed. Then solubility in cold water was calculated using the following equation:

WSC = [(Mi-Mf)/Mi] x 100-----------(Eqn. 4)

where, WSC is the percentage of products soluble in cold water, Mi and Mf are the initial and final weights of the samples (both in grams).

On the other hand, for determining the solubility in hot water, about 2 g of each type of the fibers were mixed with 100 ml of hot distilled water and the mixed solution was taken in 250 ml Erlenmeyer’s beakers. The beakers were then kept in a warm bath for about 3 hours. Then, the samples were filtered in filter crucibles, washed with hot water and kept in an oven until dry. After drying, the samples were kept in desiccator and for weighed till constant weight was observed. Then the solubility in hot water was calculated using the equation 2 as mentioned above.

Total extractive contents were determined using a solvent sequence having different polarities. Ethanol-toluene, ethanol and water were used as the solvent in steps 1, 2 and 3 respectively. Each time, the material remaining in the extraction cartridge was air dried and subjected to the extraction by the next step. The final product, which defines the total extractive content, is the dry sample retained in the filter crucible. Then, the content of total extractives was calculated using the same equation (Equation 2) as mentioned above.

For determining the insoluble lignin content in the fiber samples of the present study, residues of fibers were weighed in different beakers and then 40 ml of 72% sulfuric acid was added of to each beaker and stirred using glass rods. The mixtures were covered with watch glasses and kept for 2 hours. About 300 ml of distilled water was added and the mixture was transferred to Erlenmeyer beakers to make total volume to about 575 ml. The mixtures were then kept in the hot bath for 4 hours, then removed and kept the beakers overnight tilted position over the table so that the insoluble material could precipitate. The samples were then filtered using filter crucible without stirring, since the precipitated lignin is a very delicate material and could again get into the solution. Finally, the samples were washed with hot water, kept in an oven for drying and then transferred into desiccators. Then the containers were weighed in analytical balance, which gives the value of lignin content.

Finally, holocellulose (hemicelluloses + α-cellulose + β-cellulose) content of the fibers was determined by the difference of subtracting the sum of lignin and total extractives by 100. In general, α-cellulose indicates the non-degraded, high molecular weight cellulose contained in the fiber, while β-cellulose indicates degraded cellulose.

Figure 2: Micrographs of the taquara-lixa fibers (a)- Untreated fiber; (b)- Alkali treated Fiber.

The pH of the fiber was determined following TAPPI 252 om-12 standard. First, about 2g of the powder of fibers that passed through the 40 mesh sieve, but retained in the 60 mesh sieve were poured into 100 ml hot water and kept for 3 h. The Soxhlet / thermal blanket was used so that there was no loss of water vapor and consequent variation of concentration of the analyzed extracts. The fibrous material was then transferred to taquara-lixa single fibers. Fibrils and small nodes can be observed, similar to those found in taquara-lixa before the calendaring process. The morphology of these fibers probably contributed to their high reinforcement potential, due to the possibility of mechanical entanglement of the fibrils.

Chemical Composition

Table 1 lists the chemical composition and pH values of the untreated and alkali-treated taquara-lixa fibers. It can be seen from the Table 1 that in the case of untreated fibers the moisture content is lower than many other lignocellulosic fibers (8.5 to 11.7%). Lignin content of untreated taquara-lixa fibers is found to be in the range of the value for pine and sugarcane bagasse, but lower lignin content than that of coir fibers (35.4%) and average of values of pine wood and sugarcane fibers (30 and 23%) [26,27]. These differences are understandable as it is well known that chemical composition of various plant fibers and even the same material may present variation in its chemical composition due to differences in climate, plant age, soil type, extraction method, etc. [28, 29]. Relatively high lignin content of taquara-lixa fibers may also be the reason for the difficulty in processing the stalks of this, which exhibit high resistance.

It can be seen that the treatment with NaOH reduced the moisture and lignin contents and also the solubility in cold and hot water. The moisture level of the untreated sample is lower than the water loss found in the thermal analysis studies (to be discussed later), because this refers theta configuration (In this system the sample surface is only to the water absorbed from the environment, not the hydration water. Also, considering moisture contents of 5.6 and 3.3% for untreated and alkali-treated taquara-lixa respectively, being very small, one may conclude that alkali treatment is not justifiable as it generates substantial amounts of environmentally harmful effluents.

The alkaline treatment partially extracts the lignin from the fibers, which also results in the fiber to be whiter and more flexible. In fact, it is reported that NaOH treatment increases the surface area of lignocellulosic fibers, which can facilitate the fiber-matrix adhesion of polymer composites. Also, it promotes structural changes, reducing their density and affecting their mechanical resistance [30,31]. It is also reported that depending on the contact time, temperature and concentration of the NaOH solution, mechanical resistance of these fibers can be impaired due to possible breakage in the polysaccharide chains. Also, there is always an optimal alkali concentration to attain the best fiber/matrix adhesion in the preparation of its composites with polymers without reducing their mechanical resistance [32,34]. Similarly, Beltrami et al have observed removal of hemicelluloses and lignin from the fibers for the alkali treatment of curauá fibers in FTIR spectra [33].

Table 1: Chemical composition and pH of untreated and NaOH treated taquara-lixa fibers.

Fibers were obtained from the stem of taquara-lixa bamboo by various steps including sawing of stems, two step calendaring process and final extraction of single fibers.

• Chemical treatment of taquara-lixa fibers with NaOH solution resulted in fibrils and small nodes on the surface of the fibers, reduction of moisture and lignin contents (whiter and more flexible fibers) and also the solubility in cold and hot water. On the other hand, holocellulose content increased slightly.
• The pH of both untreated and alkali treated taquara-lixa fibers did not show any significant changes in the fiber crystallinity index showing that the chemical treatment is not essential.
• X-ray diffraction profiles of both the samples were found to be identical suggesting the alkali treatment has little effect on the internal structure of the fiber. This is further underlined by almost similar value of calculated crystallinity index (56% and 60%).
• Taquara-lixa fiber (both alkali treated and without treatment) is thermally stable up to temperature of 250 oC, which can be attributed to observed high crystallinity index of the fiber (56%) and relatively high lignin contents (22-27%) of the fiber.
• The use of taquara-lixa fibers untreated instead of NaOH treated fibers avoids the production of effluents and therefore is a more environment friendly and cheaper approach to composites preparation.

Funding: The second author thanks for the fellowship received from National Council for Scientific and Technological Research (CNPq, Process No.02459/2023-5).

Acknowledgements

The authors would like to thank Prof. Ricardo Pedro Bom from University of the State of Santa Catarina (UDESC), Center of Technological Sciences, Joinville, Santa Catarirna (Brazil) for the donation of fibers used in this study. Dr. K.G. Satyanarayana would like to express sincere thanks to Poornaprajna Institute for Scientific Research (PPISR), Bengaluru, with whom he has been associated, for their encouragement.

Conflicts of interest

There are no conflicts of interest that exist with any of the authors of this paper.

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