Natural Cellulose Fibers from Renewable Resources

Keywords

Pineapple leave • Yarn processing • Yarn properties

10.1 Extracting Fibers from Pineapple Plant Residues

Pineapple (Ananas comosus) belongs to the Bromeliaceae family and is a short tropical plant that grows to 1-2 m in height and width. About 21.9 million tons of pineapples were produced in the world in 2011 [14FAO]. As seen from Fig. 10.1, pineapple plants consist of a rosette of 20-30 leaves that are generally 6 cm wide and up to 1 m long. About 96-100 tons of fresh leaves are generated per hectare. Fresh pineapple leaves yield about 2.5-3.5 % fibers that are white, creamy, and silk like with a soft texture and also absorb high amounts of moisture [12Nad]. Fibers obtained from pineapple leaves are composed of relatively higher amounts of cellulose (74 %), lignin (10.4 %), and ash (4.7 %) [12Nad]. Pineapple leaf fibers (PALF) fibers are reported to have a microfibrillar angle of 14° that results in lower elongation. Traditionally, fibers are manually extracted from pineapple leaves by scrapping the outer layers. During the last decade, decorticating machines have been developed that can process about 35 kg of fibers per 8 h shift.

In a study, the influence of various chemical modifications on the morphological and physical properties of pineapple leaf fibers was studied [12Man]. To extract finer fibers, pineapple leaves were cut to 25 cm length, soaked in 10 % NaOH at 30 °C for 2 h, and later washed in water. Fibers were also bleached using 0.4 % sodium hypochlorite at pH4 at 85-90 °C for 90 min. In addition, acetylation and grafting of acrylonitrile were also done [12Man]. Fibers with diameters between 30 and 40 pm were selected and tested for their tensile properties. Table 10.1 shows the properties of the PALF before and after various modifications. Treating the fibers with acetic acid resulted in drastic loss in tensile properties, whereas grafting improved the tensile strength and elongation. Morphologically, fibers became

Fig. 10.1 Various parts of a pineapple plant

smooth after the chemical/physical modifications. Unmodified fibers had a crystallinity of about 66 %, but grafting with acrylonitrile resulted in a decrease of crystallinity to 41 %. Table 10.2 lists some of the properties of pineapple leaf fibers.

Fibers were extracted from the pineapple crown by treating with 1 % alkali solution for potential use as reinforcement for polypropylene [11Sip]. Untreated and treated fibers had % crystallinity of 42 % and 38 %, respectively, and showed peaks belonging to cellulose I and cellulose IV [11Sip]. In a study to understand the

influence of variety of plant on fiber properties, leaves from six varieties of pineapple grown in Brazil were manually extracted and studied for their morpho­logical and physical properties. Considerable variations in properties were found for the fibers extracted from the different varieties of pineapples [13Net]. Some of the varieties of pineapple plants and the properties of the fibers obtained and their variations are listed in Table 10.3. Strength and modulus of the fibers vary by more than three times, mainly due to the different cross-sectional areas. Fibers that had larger cross-sectional areas tend to have lower strength and modulus due to the higher probability of defective places. In addition, the composition of the fibers would also have played a role in determining the tensile properties, and it may not be appropriate to compare fibers with different compositions and unit areas. In a similar study, it was found that alkali treatment to various extents resulted in the fineness of the fibers to vary from 20 to 15 denier, and tenacity ranged from 2.2 to 4.7 g/den [11Li].

Pineapple leaves were mechanically and chemically treated, and the changes in diameter of the fibers were related to tensile properties. As seen in Table 10.4, finer fibers (72.7 pm) obtained by mechanical abrasion with grade 100 abrasive paper had 250 % higher strength than the non-treated fibers. Similarly, bleaching with hypochlorite solution reduced fiber diameters but also decreased the tensile properties. Increase in fiber crystallinity was thought to be the major contributing factor to increase in tensile strength after various treatments.

Ananas erectifolius, a plant similar to that of pineapple and commonly called as curaua, has been used to extract natural cellulose fibers [09Spi]. The plant is commonly found in the Amazon regions of Brazil and grows 1.5—1.7 m long and

4 cm wide leaves which are used to extract fibers. The density of the fibers was between 1.1 and 1.2 g/cm3, average diameter of the fibers was 60 pm, and moisture absorption varied from 9.2 to 12.1 %. The strength of the fibers varied from 2.7 to 6.9 g/den, modulus ranged from 115 to 308 g/den, and elongation varied from 3 to

5 % depending on the treatments used for the fibers [09Spi].