Characterization and comparative evaluation of thermal, structural, chemical, mechanical and morphological properties of six pineapple leaf fiber varieties for use in composites

https://doi.org/10.1016/j.indcrop.2012.08.001Get rights and content

Abstract

Natural fibers are candidates to replace conventional mechanical reinforcements in composites. Six cultivars of fibers of different pineapples varieties were characterized by tensile tests, thermogravimetry, X-ray diffraction, scanning electron microscopy and infrared spectroscopy. The elastic modulus and tensile strength values were in the range of 15–53 GPa and from 210 to 695 MPa, respectively. The final volatile loss temperatures for the six varieties were in the range between 175 and 195 °C and the onset temperatures in the range of 240–260 °C. The high degree of cellulose crystallinity index influenced the mechanical properties, hence suitable for composite reinforcement. This study aims to add information value to the literature regarding pineapple leaf fibers and its characteristics for technical and engineering applications. It was demonstrated that within the pineapple family, there are intrinsic variabilities for natural materials, indicating different potential uses for each variety.

Highlights

► The six pineapple leaf fiber varieties meet the requirements to be used as fibrous reinforcement in composites. ► Young's modulus ranged from 15 to 53 GPa and tensile strength between 210 and 695 MPa. ► The mechanical properties had a direct relationship with cellulose crystallinity index. ► There is a inverse relationship between the area and the cellulose crystallinity index. ► The six pineapple leaf fiber varieties have satisfactory values of degradation temperature for use as reinforcement in some polymer matrices.

Introduction

The global population growth, corroborated with the average individual consumption increase has led to very high global consumption levels of raw materials and finished products. Moreover, there is an increasing search for “green” or “environmentally friendly” materials and methods.

Such trend and awareness to environmentally friendly behavior and increased demand have progressively driven the primary sector to seek substitutes for materials with polluting attributes and/or from non-renewable sources throughout the production process (Leao et al., 2009).

Lignocellulosic fibers have emerged as a reinforcement alternative in polymer matrices to obtain composites for a wide variety of applications (Sanadi et al., 1995, Leao et al., 2009, de Paoli et al., 2009). There are some advantages over synthetic fibers, namely: low densities and abrasiveness (relatively), high possible filling levels resulting in high stiffness and high specific properties, recyclability, high bending resistance, biodegradability, wide variety of fibers available worldwide, rural financial income generation and low cost (Sanadi et al., 1994, Sanadi, 2004, Santos, 2006, Shanks et al., 2006, Chattopadhyay et al., 2009). The main limitations are high moisture absorption with decrease of mechanical properties, occasional poor compatibility with hydrophobic resins, low processing maximum working temperature and seasonality (Santos, 2006).

Vegetable fibers have considerably complex structures, defined by a wide variety of organic compounds such as lignin, hemicellulose, waxes, fatty acids, fats, pectins, among others (Rowell et al., 2000, Martins et al., 2004). Lignocellulosic fibers are, in fact, bundles of smaller units: the fiber cells or ultimate fibers, which are held together by binder agents (predominantly lignin and hemicellulose), also found on the outside of the fiber bundles and leaves. The fiber cells are structured in different layers, formed essentially by groups of nano-scale cellulose chains (fibrils) extending helically along the axis of the fiber cells and interconnected by amorphous regions composed of lignin and hemicellulose. The helix angle between the fibrils in the secondary wall (S2) and the axis of the fiber cell is known as microfibrillar angle (Rowell et al., 1997, Martin, 2001, Silva et al., 2009).

The adequate mechanical properties of lignocellulosic fibers applied as reinforcement in polymers are largely attributed to their cellulosic fraction, given that this is responsible for the fiber's crystalline organization. The pineapple fibers studied in this work have potential for the aforementioned applications due to their high crystallinity (Reddy and Yang, 2005, Tomczak et al., 2007, Tomczak, 2010).

There are several factors that influence the properties of fibers such as number of fiber cells, cell wall thickness, microfibrillar angle, cellulose content, molecular structure (Mukherjee and Satyanarayana, 1986), cellulose crystallinity index (amount, orientation and the degree of polymerization) (Sanadi, 2004).

Cellulose is the main structural component of the lignocellulosic fibers, as it provides strength and stability to the cell walls and the fiber as a whole (Paster et al., 2003). Therefore, the cellulose content in a fiber or bundle of fibers influences its properties and consequently its applications. As an example of this cellulose role, it is known that pineapple and banana fibers have a higher cellulose content, which is probably related to the relatively higher weight of the fruit they support and the fact that they are less perishable. Other fiber sources such as corn stover, bagasse, wheat, rice and barley straw, and sorghum stalks all contain nearly the same amount of cellulose. Fibers in these crops support relatively smaller weights in comparison with bananas and pineapples (Reddy and Yang, 2005).

Fibers with a higher cellulose fraction are more suitable for fibrous applications, while the ones containing more hemicellulose are preferable for producing ethanol and other fermentation products because hemicellulose is relatively easily hydrolysable into fermentable sugars (Reddy and Yang, 2005). Mechanically, hemicellulose contributes little to the stiffness and strength of fibers or individual cells (Thompson, 1983). Lignin is a highly crosslinked molecular complex with amorphous structure and acts as binder agent between individual fiber cells and between the fibrils forming the cell wall (Mohanty et al., 2000). Along with chemical composition, as commented earlier, structural organization of the fiber influences properties. Mechanically, the main structural component, called “ultimate” or individual fiber cells, are longer in pineapple fiber than in most of other fibers, therefore these sources can produce long fibers. That said, it is important to notice that a lignocellulosic reinforcement's suitability for a given application or product is a result from different factors, such as chemical, structural and morphological aspects, as well as end use, and not the cellulose content alone.

With the interdependence of the many given features of the fiber system, it can be said that biofibers do not always show the general relationship between crystallinity and strength observed in pure cellulose fibers such as cotton and rayon, that is, the higher the crystallinity, the higher the strength, that being the reason why this relationship is relatively complex. The presence of substantial amounts of noncellulosics, mainly lignin, which contributes to the strength of fibers and the variations in the dimensions of unit cells, is the major reason for the absence of a good relationship between crystallinity and strength. However, biofibers with longer unit cells as pineapple's have higher strength (Reddy and Yang, 2005).

Despite the impossibility to establish a direct relation between cellulose content and individual structural aspects, generally, higher cellulose percentages, lower microfibrillar angles (Sukumaran et al., 2001), higher cellulose degrees of crystallinity and aspect ratios lead to better mechanical properties and lower extensibility. Thus, elongation of the fibers depends mainly on the orientation and degree of crystallinity of the cellulose and the angle of the microfibrils to the fiber axis.

Multiple types of lignocellulosic fibers are available according to their species, places of origin, seasonality and mode of obtainment. The following examples of lignocellulosic fibers are successfully used in the reinforcement of composites: jute, flax, coconut, sisal, cotton, bananas, curaua, acai, hemp, pineapple, soy, ramie sugar cane, piassava, among others. The world annual production of natural fibers in 2010 was of 28.4 million tons, of which 7.5 million tons were produced in India (FAOSTAT, 2010). In 2007, 12% of the research groups in the Metallurgical and Materials Engineering area were involved in research related to composites with lignocellulosic fibers (Satyanarayana et al., 2007). Notwithstanding this substantial growth in research on lignocellulosic fibers, there is still a considerable portion of untapped potential, such as the fibers obtained from pineapple leaves (Souza et al., 2007, Amaral et al., 2011).

There has been relatively little research carried out on fibers of the pineapple plant leaf, and its introduction in industrial uses is recent compared to other lignocellulosic fibers such as jute, flax and sisal. For this reason, the pineapple plant is not profitably planted, and most of the plant material, except for its fruit, is discarded due to the lack of knowledge of its economic potential. Brazil is the third largest producer with around 7% of the world production. Brazil's Northeast region has the highest production in the country. According to estimates made in 2004, the country's production was about 1.4 million tons of fiber, with exports of only 1% of this total. Studies indicated a yield of 1.22 tons per hectare, with the production of 40 leaves per pineapple, a mass of 0.065 kg per leaf and fiber income of 2%. The values could lead to a production price per hectare of US$ 434, considering the fiber price of US$ 0.36/kg (Satyanarayana et al., 2007).

In the work reported here, five accessions and an hybrid of APGB (Active Pineapple Germplasm Bank, located in Cruz das Almas, BA, Brazil, at Embrapa Cassava and Tropical Fruits), with more than 600 cultivars of the genus Ananas and, Bromeliaceae families (Cabral et al., 2004, Costa et al., 2011), were studied. They were characterized by morphology, cellulose crystallinity index and functional groups; evaluating the thermal and mechanical properties for use as reinforcement in composites with polymer matrices, emphasizing the different qualities and limitations of each variety.

Section snippets

Materials

Lignocellulosic fibers were taken from six botanical varieties of the genus Ananas (Bromeliaceae family) from APGB; and are listed as follows:

  • -

    A: Bromelia sp.

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    B: Ananas comosus var. comosus

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    C: Bilbergis sp.

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    D: Ananas comosus var. bracteatus

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    E: Ananas comosus var. erectifolius

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    F: Ananas macrodontes x Primavera

Initially, the leaves were immersed in water to be posteriorly calendered, so that the fibers could be separated from leaf cover without being deformed or broken. The fibers were manually

Results and discussion

The fiber sections showed with the SEM technique were randomly chosen among those that could be better focused and viewed, from the basal to the apical portion of the leaf. All fibers evaluated were manually collected and thus a large presence of mucilage material was observed in all cultivars (pulp and outer layers of the leaves) and also cementitious material/other components (Fig. 2). In PALF B and F, a separation formed by the parenchyma cells was observed in the presence of external matter

Conclusions

Six PALFs of APGB cultivars were comparatively characterized and evaluated for potential application as mechanical reinforcements in polymeric composites. The mechanical properties coupled with thermogravimetric analysis indicated that the six PALF meet the requirements to be used as fibrous reinforcement in a reasonable range of possible composite applications, especially when comparing those properties with some of the fibers traditionally used with this purpose.

Peaks relating to cellulose I

Acknowledgements

The authors would like to thank CNPq/PIBIC, CAPES, FINEP and EMBRAPA for the financial support.

References (49)

  • ASTM D 3379-75, 1978. Standard Test Method for Tensile Strength and Young's Modulus for High-Modulus Single-Filament...
  • S.H.P. Bettini et al.

    Investigation on the use of coir fiber as alternative reinforcement in polypropylene

    J. Appl. Polym. Sci.

    (2010)
  • S. Borysiak et al.

    Research into the mercerization process of beechwood using the waxs method

    Fibres Text. East. Eur.

    (2008)
  • Cabral, J.R.S., Castellen, M.S., Souza, F.V.D., Matos, A.P., Ferreira, F.R., 2004. Banco Ativo de Germoplasma de...
  • W.D. Callister

    Fundamentos da ciência e engenharia de materiais: uma abordagem integrada

    (2006)
  • S.K. Chattopadhyay et al.

    Influence of varying fiber lengths on mechanical, thermal, and morphological properties of MA-g-PP compatibilized and chemically modified short pineapple leaf fiber reinforced polypropylene composites

    J. Appl. Polym. Sci.

    (2009)
  • Correa, A.C., 2010. Preparação de Nanofibras de celulose a partir de fibras de curauá para o desenvolvimento de...
  • A.S. Costa et al.

    Use of response surface methodology for optimization of the extraction of enzymes from pineapple pulp

    Acta Hortic. Johor Baru.

    (2011)
  • M. Crestani et al.

    Das Américas para o Mundo – origem domesticação e dispersão do abacaxizeiro

    Cienc. Rural.

    (2010)
  • de Paoli, M.A., Fermoselli, K.K.G., Spinacé, M.A.S., Santos, P.A., Girioli, J.C., 2009. Process of Confection of...
  • FAOSTAT – Food and Agriculture Organization of the United Nations Statistical Database, 2010. Crops database....
  • P. Garside et al.

    Identification of cellulosic fibres by FTIR spectroscopy: thread and single fibre analysis by attenuated total reflectance

    Stud. Conserv.

    (2003)
  • M.L. Hassan et al.

    Utilization of lignocellulosic fibers in molded polyester composites

    J. Appl. Polym. Sci.

    (2003)
  • P.H. Hermans et al.

    Quantitative X-ray investigations on the crystallinity of cellulose fibers: a background analysis

    J. Appl. Phys.

    (1948)
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