Characterization of a New Native Plant Textile,
Leaf Sheath from Cocos nucifera L., as Potential
Reinforcement of Polymer Composites

Vidil L; Potiron CO; Bilba K; Ars�ne MA

Research Article

Ann Agric Crop Sci. 2020; 5(1): 1056.

Characterization of a New Native Plant Textile, Leaf Sheath from Cocos nucifera L., as Potential Reinforcement of Polymer Composites

Vidil L, Potiron CO*, Bilba K and Arsène MA

Université des Antilles - Laboratoire COVACHIM-M2E EA 3592, France

*Corresponding author: Cristel Onésippe Potiron, Université des Antilles - Laboratoire COVACHIM-M2E EA 3592, UFR SEN, Campus de Fouillole. BP 250 97157 Pointe-à-Pitre, Guadeloupe, France

Received: January 31, 2020; Accepted: February 26 2020; Published: March 04, 2020

Abstract

Natural fibers in composite materials have received growing interest for the last decades. Few works have explored the study of long vegetable fibers reinforcing composites such as textile reinforcements to develop higher performance of bio-materials which could be used in load-bearing applications. Therefore, the identification of new natural textiles should turn into an active research challenge for use as reinforcement in the development of green composites. In this study, leaf sheaths from Cocos nucifera L. was investigated. This material presents the distinctive feature to be naturally available as a textile. Thus, coconut leaf sheath was characterized morphologically, chemically, physically and mechanically. Coconut textile displays low Helium density and water sensitivity, promising thermal stability and tensile properties in both fibers directions forming the sheath. Compared to natural and synthetic textiles, coconut leaf sheath appears as a suitable, promising and competitive textile reinforcement for the development of eco-friendly continuous fibers composites.

Keywords: Bio-textile; Vegetable fibers; Chemical properties; Physical properties; Mechanical properties

Introduction

The 20^th century was marked by an unprecedented industrial growth characterized by a material prosperity and a not responsible control of nature by humans neglecting the deleterious impact of this model on environment [1]. Faced with the alarming and undeniable observation bound to global warming and natural resource scarcity, the 21^st century is the bearer of many challenges being in line with a sustainable development process to resolve the current ecological crisis [2,3]. In this context, new environmental regulations [4] have been elaborated what has incited manufacturers and researchers to develop alternatives to materials made of non-renewable fossil and mineral resources.

One of these alternatives concerns the development of plant fibers composites as substitutes for synthetic fibers composites. Indeed, vegetable fibers show several advantages compared to their conventional synthetic counterparts like low cost, low density, low abrasiveness, high specific mechanical properties, abundant avaibility, biodegradability, renewability and sustainability [5-9].

Fibers length (continuous/discontinuous) and their orientation (random/oriented) are two major factors influencing mechanical composites properties [10,11]. Polymers reinforced with discontinuous fibers, generally randomly oriented to ensure mass production [12], are employed in non load-bearing structures. The development of continuous fibers composites is, however, essential to manufacture materials which can be used in structural applications [13,14,15]. These materials present higher mechanical properties in the fibers axis direction compared to non-aligned short fibers reinforced polymers [16,17]. The long fibrous reinforcements whose preferred placement is adapted to external stresses on the final piece, can be oriented in one direction or several directions. They form unidirectional and textile composites respectively [18]. Textile composites can be woven, knitted, braided or sewn [19,20]. They are known to exhibit better toughness, impact and inter-laminar shear resistance, damage tolerance, integrity and conformability than unidirectional laminates [20,21]. It is only recently that lignocellulosic textiles have been used as polymer reinforcements [22]. Their studies number is low compared to the number of works dealing with discontinuous plant fibers composites due to the innate short length of the lignocellulosic fibers [23]. To form long yarns which are interlaced to make textile structures, vegetable fibers need to be twisted [24].

In this overall context and in the continuity of these research areas falling within a sustainable development approach, this work intends to contribute to the identification of new natural textile reinforcements, and thus, to study the potential use of the leaf sheaths from Cocos nucifera L. in bio-composites fabrication. This resource, non exploited industrially until now, was chosen because in addition to be vegetable and available locally all the year. Coconut trees grow extensively in humid tropical regions where they occupy 12 303 924 ha worldwide whose 127 918 ha in the Caribbean and 85 ha in Guadeloupe [25]. A coconut tree can live more than 100 years and produces 18 leaf sheaths by year on average [26]. It shows the advantage to be a textile by nature freeing us from the interim step of reinforcement production in the form of textile. Thus, this reinforcement potential use will enable to minimize the production costs. In order to use this resource at best, coconut leaf sheath was extensively characterized by chemical composition analysis, infrared spectroscopy study, thermo-gravimetric experiment, morphological observations and physico-mechanical properties assessments (specific density, water absorption, tensile strength, Young modulus, strain and toughness). Its properties were compared to literature for other fibrous reinforcements.

Materials and Methods

Materials

The coconut leaf sheaths were collected manually from coconut trees Cocos nucifera L. in Guadeloupe, France (Figure 1). The sheaths, located at the petiole of the leaves, ensure their sticking on the pseudotrunk of coconut trees. The coconut leaf sheaths were cleaned with high-pressure water before being rinsed thoroughly with distilled water and then sun-dried for one week.

Figure 1: Pictures of a coconut tree Cocos nucifera L. in Guadeloupe (a) and its leaf sheaths (b).

    
    
    Figure 1: Pictures of a coconut tree Cocos nucifera L. in Guadeloupe (a) and
its leaf sheaths (b).

Methods of characterization

Morphology: The both faces structure of coconut leaf sheaths was observed using a binocular magnifier Nikon SMZ 1000.

A Scanning Electron Microscope (SEM) Quanta 250 coupled with an Energy Dispersive Spectrometer (EDS) were used to study surface morphology and cross section of coconut leaf sheaths fibers. Prior to the analysis of the fibers cross section, samples were coated in epoxy resin, and then polished.

Linear density: To evaluate the number of fibers for 10 cm as a function of their thicknesses, each coconut leaf sheaths face was first scanned with Canoscan 4400F and then measured with Image J software. Eight coconut leaf sheats faces were scanned three times each.

Chemical composition: Cellulose, hemicelluloses, lignins and extractives weight contents were determined according to Ouensanga and Picard methods [27]. Between two and four analysis per component were done.

Infrared spectroscopy analysis: Fourier Transform Infrared Spectroscopy (FTIR) in Attenuated Total Reflection (ATR) mode was carried out on untreated coconut leaf sheaths. IR spectra were recorded using the Bruker Tensor 27 spectrometer in the 4000-400 cm^-1 region with 20 scans with a resolution of 4 cm^-1. To check the results reproducibility, three analysis were performed.

Helium density: Helium density of coconut leaf sheaths was measured using a helium Pycnomatic Thermo Electron pycnometer. In this way, sheaths discs of the diameter of the nacelle were cut out. Five measurements were conducted for each sample.

Water absorption: For each sample, the coefficient of water absorption at saturation was estimated by equation 1:

WA(%)=	W_s-W_i	X100	equation 1
	W_i

Where :

W_s is the weight of the specimen saturated with water (g),

W_i is the initial weight of the specimen (g).

This test was carried out immersing 20 samples of coconut leaf sheaths (13×5)cm² in distilled water at (26±1)^oC. Before testing, coconut leaf sheaths specimens were dried at 105^oC until a constant weight was reached.

Thermal decomposition analysis: Thermogravimetric Analysis (TGA) was used to study the thermal degradation of coconut leaf sheaths. Thermograms were carried out using Perkin Elmer Pyris-1 TGA instrument from 50^oC to 800^oC at a heating rate of 10^oC/min in a nitrogen atmosphere. Before testing, fibers were chopped and dried at 105^oC until a sample constant weight was reached. To check the results reproducibility, three analysis were performed.

Mechanical properties: In both preferential fibers directions, coconut leaf sheaths were tensile tested at (24±3) ^oC and at (70±5) % relative humidity, according to ASTM D5035 method [28] using an universal machine Servohydraulic Test from System MTS Landmark equipped with a 2.5kN load cell (rate 10mm/min). For each samples set, eight specimens were tested. To prevent sliding of coconut leaf sheaths in clamping jaws, wood tabs were stuck on their extremities. For each tested direction, cross sectional area of each specimen was obtained adding up cross sectional areas of tested fibers. These were evaluated for each fiber, averaging cross sectional areas to the level and on both sides of failure zone, before the test. For each longitudinal thickness determined with Image J software, fibers cross sectional area was calculated using the equation of fibers cross sectional areas modelling displayed on Figure 2.

Figure 2: Modelling of fibers cross sectional areas as a function of their longitudinal thickness.

    
    
    Figure 2: Modelling of fibers cross sectional areas as a function of their
longitudinal thickness.

This modelling was obtained from 180unit fibers taken out from coconut leaf sheaths. The calculation used for fibers area is the one of an ellipse (cf. 3.1.2). The longitudinal thickness was evaluated using Image J software whereas the cross sectional thickness was estimated with a sliding calliper whose precision is 0.01mm.

Statistical analysis: The mechanical properties of coconut leaf sheaths were analysed statistically using analysis of variance (ANOVA) followed by a Tukey test to detect potential differences between the two fibers directions tested. The statistical tests were carried out at 5% level of significance.

Results and Discussion

Fibrous structure and morphology

The coconut leaf sheaths observations are visible on Figures 3, 4, 5 and 6 at different scales.

Macroscopic scale: The coconut leaf sheaths present the particularity to have a symmetry axis represented in yellow on Figure 3-a. The observation of this material at macroscopic scale reveals a textile composition shaped by three collinear fibers layers:

Figure 3: Pictures of a whole coconut leaf sheath (a), of its outer face (b) and of its inner face (c).

    
    
    Figure 3: Pictures of a whole coconut leaf sheath (a), of its outer face (b) and
of its inner face (c).

- The outer face, exposed to the environment, is made up of the tangle of both fibrous networks depicted by layers I and II on Figure 3-b. Their architecture is similar to the one of a biaxial braid whose structure is illustrated by Denniger et al. [29]. Indeed, fibers of layer II comprising big (1 mm ‹ thickness ≤ 3 mm), medium (0.2 mm ‹ thickness ≤ 1 mm) and fine fibers (thickness ≤ 0.2 mm) are kept parallel by fine fibers of the layer I (thickness ≤ 0.2 mm) (Figure 4) which are arranged diagonally to the direction of the layer II,

Figure 4: Linear density of coconut leaf sheaths.

    
    
    Figure 4: Linear density of coconut leaf sheaths.

- The inner face in contact with the coconut tree pseudotrunk is composed of a single layer of unidirectional fibers, the layer III (Figure 3-c), whose thickness ≤ 0.8 mm (Figure 4). The fibers of this layer are tied to the outer face by fibrous « seams ».

Fibers of layers I and III are placed at (146±8)^o in relation to fibers of layer II. This structure underlines the birectional organisation of the coconut leaf sheaths whose thickness is estimated at (2±1) mm.

The number of each fibers thicknesses class is informed on Figure 4, and this, for each layer.

Microscopic scale: The SEM pictures of coconut leaf sheaths cross sections on Figure 5 show the wide disparity of fibers highlighted on Figure 4. Each fiber reveals an approximate ellipsoidal section consisting of:

Figure 5: SEM pictures of cross sections of coconut leaf sheaths embedded in resin.

    
    
    Figure 5: SEM pictures of cross sections of coconut leaf sheaths embedded
in resin.

- A central or peripheral canal whose some are circled in on Figure 5,

- A bundle of elementary fibers bound together by an intervening layer, the middle lamella, consisting of lignins mainly [30]. The single fibers demonstrate homogeneity in their shape, rather ellipsoidal and in their dimensions : their cross sectional area is about (200±100) μm² ; they have a cell wall around (5±3) μm of thickness which surrounds an hollow space in their center, the lumen [31,32] whose diameter is (10±4) μm approximately.

Fibers surface of coconut leaf sheahts is illustrated on SEM pictures on Figure 6. It is characterized by the presence of many cavities spread uniformly, giving to the surface a rough feature. The surface roughness of coconut leaf sheahts should improve the fiber/ polymer adhesion by an increase of mechanical interlocking at the fiber/matrix interface and of fibers contact surfaces with the resin [6,33].

Figure 6: SEM images of surface morphology of coconut leaf sheaths fibers.

    
    
    Figure 6: SEM images of surface morphology of coconut leaf sheaths fibers.

Besides, grained protusions are noticed in numerous cavities; they are made up of silicon essentially, according to EDS analysis not shown here.

Chemical analysis

The chemical composition of leaf sheaths from Cocos nucifera L. compared to the ones of vegetable fibers commonly used as reinforcements is presented in Table 1.

Table 1: Chemical composition of coconut leaf sheaths compared to those of plant fibrous reinforcement.




  
    Fibers
    Cellulose (weight %)
    Hemicelluloses (weight %)
    Lignins (weight %)
    Extractibles (weight %)
    References
  
  
    Coconut leaf sheath
    39.22±1.45
    22.17±3.91
    29.62±0.74
    0.81±0.06
    Present study
  
  
    Flax
    64-76
    17-21
    2-3
    2-3
    
  
  
    Hemp
    70-75
    18-22
    4-6
    1-2
    
  
  
    Sisal
    65-66
    12
    10
    1
    
  
  
    Jute
    61-72
    12-20
    13-Dec
    0.7
    [60,61,32]
  
  
    Cotton
    83
    6
    -
    6
    
  
  
    Kenaf
    72-53
    20-34
    21-Sep
    -
    
  
  
    Ramie
    69
    13
    1
    2
    
  
  
    Banana
    60-65
    19
    5-10
    5
    
  
  
    Bamboo
    74
    12
    10
    3
    
  
  
    Abaca
    56-63
    20-25
    7-9
    3
    
  
  
    Coir
    32-43
    0.2-0.3
    40-45
    -
    
  
  
    Oil palm
    56-65
    28
    19-20
    4
    
  
  
    Tururi
    57
    12
    31
    -



Table 1: Chemical composition of coconut leaf sheaths compared to those of plant fibrous reinforcement.

The major botanical component of leaf sheaths is cellulose making them interesting potential candidates as polymers fibrous reinforcements. Indeed, cellulose provides strength and structural stability to the fibers owing to its semi-crystalline structure in the form of microfibrils constituted by hydrogen bonds between its glucose units [34]. The cellulosic reinforcing elements are embedded in an amorphous matrix composed of lignins, hemicelluloses and extractives mainly, the whole forming the plant cells wall [35].

The extractibles are constitued of waxes, fats and terpenes primarily, being removed from fibers by an organic solvents mix made of ethanol and toluene [36]. This component is in very low proportion in coconut leaf sheaths, present also in minority in other lignocellulosic fibers (Table 1).

The noticed hemicelluloses content is similar to the ones of most classic vegetable reinforcements (Table 1). The hemicelluloses, highly hydrophilic, should harm compatibility between fibers and most polymers which are hydrophobic, causing the decrease of composites mechanical properties [37,38].

The lignins percentage in leaf sheaths is of the same order of magnitude as the ones of fibers from other palm trees (oil palm, tururi) or other parts of Cocos nucifera L. such as coconut husk (coir). Nevertheless, lignins are less numerous in the other fibrous plant reinforcements (Table 1) predicting a better stiffness of coconut leaf sheaths fibers [39]. Besides, this stiff wall component prevents cellulose fibrils from reorienting in the load direction limiting the stress transfer [40].

However, improvement of fibers mechanical properties and fiber/polymer compatibility can be achieved by chemical, physical or biological treatments [41,42].

The additional non dosed part (8.18%) should correspond with pectins and inorganic constituents such as minerals [43] which are lesser components than cellulose, lignins and hemicelluloses.

Infrared spectroscopy

The infrared spectrum of coconut leaf sheaths is shown in Figure 7. The observed absorption bands are characteristic of the main lignocellulosic components of plant fibers (cellulose, hemicelluloses and lignins) constituted primarily by alkenes, ester, ketone, alcohol and aromatic groups.

Figure 7: FTIR spectrum of coconut leaf sheaths.

    
    
    Figure 7: FTIR spectrum of coconut leaf sheaths.

The broad peak at 3340 cm^-1 is characteristic of the hydrogen bonded O-H stretching whereas the absorption band at 2890 cm^-1 corresponds to alkyl C-H symmetric and asymmetric stretching. Besides, the band at 1729 cm^-1 can be attributed to C=O elongation of hemicelluloses carbonyl groups. The presence of moisture in leaf sheaths may be reflected in the peak at 1598 cm^-1 in addition to the one at 3340 cm^-1. The C=C stretching absorption of lignins aromatic skeletal appears at 1505 cm-1 which can be also assigned to proteins. The milder peaks at 1453 cm^-1 and 1422 cm^-1 could be associated with the CH₂ and CH₃ bendings in cellulose and lignins respectively. In addition, the absorbance at 1368 cm^-1 belongs to cellulose C-H vibrations. The well-defined peak at 1236 cm^-1 corresponds to C-O stretching of acetyl groups in lignins and hemicelluloses. At 1154 cm^-1 and 1030 cm^-1, the absorptions are attributed to cellulose C-O-C and C-O stretching vibrations. The small peak at 898 cm^-1 is due to vibrations of holocelluloses β-glycosidic bonds [44,45].

The FTIR spectrum obtained for coconut leaf sheaths shows a similar trend to the ones of other plant fibers [46,47].

Physical properties

Table 2 displays the Helium density and the water absorption coefficient found for coconut leaf sheaths and reported in literature for ordinary natural and synthetic fibrous reinforcements.

Table 2: Physical properties of coconut leaf sheaths and other lignocellulosic and synthetic fibers.




  
    Fibers
    Helium density (g/cm³)
    References
    Saturation water absorption    coefficient (%)
    References
  
  
    Coconut leaf sheath
    0.97±0.01
    Present study
    95±13
    Present study
  
  
    Flax
    1.4-1.5
    [62,63,9]
    -
    
  
  
    Hemp
    1,5
    160
    [64]
  
  
    Sisal
    1.3-1.5
    175-230
    [65,66]
  
  
    Jute
    1.3-1.5
    281
    [67]
  
  
    Cotton
    1.5-1.6
    -
    
  
  
    Ramie
    1.5
    -
    
  
  
    Banana
    1.4
    407
    [68]
  
  
    Bamboo
    0.6-1.25
    -
    
  
  
    Kenaf
    1.2-1.5
    285
    [67]
  
  
    Coir
    1.2-1.5
    100-127
    [65,66]
  
  
    Oil palm
    0.7-1.6
    -
    
  
  
    Date palm
    0.7-1.2
    133
    [69]
  
  
    Tururi palm
    1
    70-74
    [60]
  
  
    Glass
    2.5
    -
    
  
  
    Aramid
    1.4
    -
    
  
  
    Carbon
    1.4
    -



Table 2: Physical properties of coconut leaf sheaths and other lignocellulosic and synthetic fibers.

Coconut leaf sheaths indicate a lower density than most wellestablished plant (flax, hemp, sisal, jute, cotton, ramie, banana, kenaf, coir) and synthetic fibers (glass, aramid, carbon) usually used as reinforcement in polymers. Therefore, compared to these fibrous reinforcements, coconut leaf sheaths present the advantage of reinforcing more lightweight eco-friendly composites for the same fibers volume fraction.

Moreover, coconut leaf sheaths absorb less water at saturation than hemp, sisal, jute, banana and kenaf. Nevertheless, their ability to absorb water is comparable to the one of fibers from plants of the same family such as date palm, tururi palm or from other parts of the coconut tree like coconut husk for coir fibers. These observations could be correlated partly to the lignins content, similar in coconut leaf sheaths, palm trees and coir fibers but higher than in hemp, sisal, jute, banana and kenaf, as presented in 3.2. Indeed, lignins have an hydrophobic nature due to their complex and random structure made of hydrocarbons [48] and their low –OH-to-carbon ratio [9]. In addition, the lower water sensitivity of coconut leaf sheaths compared to hemp, sisal, jute, banana and kenaf should lead to a better dimensional stability of coconut leaf sheaths reinforced composites given the decrease of the phenomena shrinkage/swelling of coconut leaf sheaths fibers induced by the cellulose and hemicelluloses hydrophilic nature. Thus, the quality of the interfacial adhesion coconut leaf sheaths fibers/matrix should be better preserved as well as the mechanical properties of the resulting composites in the long term consequently [49,50,9].

Thermal degradation

The thermal degradation process of coconut leaf sheaths was studied by thermogravimetric analysis. Their mass percentage evolution (TGA) and their derivative (DTG) are represented as a fonction of temperature in Figure. The TGA curve profile, highlighting three weight loss steps, compares well with those reported in literature for other lignocellulosic fibers [51,52]. This curve attests to the presence of the main chemical components in coconut leaf sheaths as developed hereinafter.

The first degradation stage is observed for temperatures lower than 220^oC. In this step, the only very low weight loss around 100^oC represented by the small peak 1 on DTG curve (Figure 8) is assigned to the vaporization of water contained in fibers [22]. The water content around 1% doesn’t correspond to the natural moisture of coconut leaf sheaths, these fibers having been dried (105^oC for 24h) before the thermogravimetric test. Indeed, before composite manufacture, the natural fibers should be the most possible dried to avoid producing porous composites with decreasing mechanical performance [53].

		Origin	Textile structure	Loaded direction	Strength (MPa)	Young modulus (GPa)	Failure strain (%)	References
Native Textiles	Plant Textiles	Coconut leaf sheath	Bidirectional : Biaxial braid + sewn fibers	CLS-I-III	193±30	7.8±1.2	6.5±2.4	Present study
				CLS-II	133±30	7.4±0.7	5.1±1.9
		Tururi (Manicaria saccifera)	Unidirectional	Fibers direction	18±8	0.6±0.3	5.9±1.0	[70]
					71	2.2	5.3	[60]
		Ashok (Polyalthia cerasoides)	Unidirectional	Fibers direction	44	3.5	2.5	[71]


		Daalibuda (Hildegardia populifolia)	Unidirectional	Fibers direction	76	3.1	2.5	[72]

		Karaya (Sterculia urens)	Unidirectional	Fibers direction	10	0.6	2

		Dhaman (Grewia tilifolia)	Unidirectional	Fibers direction	65	4.5	1.6	[73]

		Indian cherry tree (Cordia dichotoma)	Unidirectional	Fibers direction	17±1	2.2±0.1	1.4±0.1	[74]
Man-made textiles
		Flax	Bidirectional : Woven	Warp	33	0.5	17.7	[75]
				Weft	41	1	7.1
		Jute	Bidirectional : Woven	Warp	57	-	10.4	[76]
				Weft	89	-	5.2
		Hemp	Bidirectional : Plain woven	Warp	22	0.5	9.3	[77]
				Weft	25	0.5	11.2
		Glass (commercial name : HexForce 1658)	Bidirectional : Plain woven	Warp	175	-	-	[78]
	Synthetic textiles			Weft	175	-	-
		Glass (commercial name : HexForce 4180)	Bidirectional : 4 Satin	Warp	164	-	-
				Weft	219	-	-
		Glass (commercial name : HexForce 7725)	Bidirectional : 2/2 twill	Warp	321	-	-
				Weft	263	-	-