Characterisation of natural cellulosic fibre from Pennisetum purpureum stem as potential reinforcement of polymer composites

    Characterisation of natural cellulosic fibre from Pennisetum purpureum stem as potential reinforcement of polymer composites M.J.M. Ridzuan, M.S. Abdul Majid, M. Afendi, S.N. Aqmariah Kanafiah, J.M. Zahri, A.G. Gibson PII: DOI: Reference:

S0264-1275(15)30631-6 doi: 10.1016/j.matdes.2015.10.052 JMADE 791

To appear in: Received date: Revised date: Accepted date:

28 May 2015 12 October 2015 13 October 2015

Please cite this article as: M.J.M. Ridzuan, M.S. Abdul Majid, M. Afendi, S.N. Aqmariah Kanafiah, J.M. Zahri, A.G. Gibson, Characterisation of natural cellulosic fibre from Pennisetum purpureum stem as potential reinforcement of polymer composites, (2015), doi: 10.1016/j.matdes.2015.10.052

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ACCEPTED MANUSCRIPT Characterisation of natural cellulosic fibre from Pennisetum purpureum stem as potential reinforcement of polymer composites

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M.J.M. Ridzuana, M.S. Abdul Majida*, M. Afendia, S.N. Aqmariah Kanafiaha, J.M. Zahrib A.G. Gibsonc a

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School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, 02600 Arau, Perlis, Malaysia b Mechanical Engineering Department, Sultan Abdul Halim Mu’adzam Shah Polytechnic (POLIMAS), Bandar Darul Aman, 06000 Jitra Kedah, Malaysia c School of Mechanical and Systems Engineering, Stephenson Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK *

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Author’s email: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

Abstract

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Pennisetum purpureum (PP) fibres were comprehensively characterised to assess their potential as reinforcing materials in polymer composites. The fibres were treated with 5, 7, 10, 12, and 15% sodium hydroxide wt% concentration for 24 h. The fibres were subjected to

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single fibre tensile tests, thermo-gravimetric analysis (TGA), Fourier transform infrared

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spectroscopy (FTIR), and scanning electron microscopy (SEM). The average diameter of the untreated fibres was 0.24 ± 0.02 mm, and the treated fibres had an average diameter of less

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than 0.21 ± 0.03 mm, yielding a 12–45% reduction in the diameter. The moisture content of the treated fibres decreased as the concentration of the alkali increased. The morphological observation demonstrated that as the alkali concentration increased, the fibre becomes more

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compressed due to collapse the cellular/lumen structure, the void content decreased, and its surface became rougher. The 5% alkali-treated fibre achieved an average maximum ultimate tensile stress of 141± 24 MPa. Young’s modulus on the other hand, decreased from an average of 5.68 ± 0.14 GPa for untreated fibre to only 0.55 ± 0.17 GPa as the alkali concentrations increased from 5-15%. Keywords: Pennisetum purpureum fibre Mechanical properties Infrared spectroscopy Thermogravimetric analysis Natural fibre

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ACCEPTED MANUSCRIPT 1. Introduction The current growth in environmental awareness has generated increasing interest in the use of natural fibres as alternative reinforcement materials for polymer composites. This is

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largely owing to their low environmental impact, low cost, and relatively good specific

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properties. Scientists have been striving to develop biodegradable composites using

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renewable agro-based materials [1]. Natural fibres derived from plants demonstrate great potentials for use in plastic, automotive, and packaging industries because of their excellent characteristics such as low density, high specific stiffness, good mechanical properties, biodegradability, eco-friendliness, toxicologically harmless, good thermal and acoustic

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insulation [2,3]. In addition, these cellulosic fibres can reduce the overall material costs than the starting polymer [4]. Comprehensive reviews conducted by numbers of publications [5–

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10] have outlined the differences of natural fibres with regards to their mechanical properties and its applications. Several authors documented the use of natural fibres such as bamboo[10], flax [11], coir [12], arundo donax (giant reed) [13], okra [14], jute [15,16],

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wheat straw [17,18] and alfa [19] as reinforcement in composite materials. However, there are concerns regarding the attributes of natural fibres such as their

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hydrophilic nature, high moisture absorption, poor reactivity, and poor compatibility with polymeric matrices, all of which influence their mechanical properties [20–22]. The hydrophilic nature of natural fibre is known to produce weak interfacial adhesion in polymermatrix composites [23]. The type of natural fibre can also affect the biological performance

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of the composites, for example, a composite manufactured from abaca fibre has a much greater moisture content compared to flax reinforced composites [24]. These problems can be rectified through modification such as alkali treatment to enhance the interfacial adhesion between natural fibres and composite matrices, in addition to enhancing the mechanical, physical, and thermal properties of the fibres [25]. Other modifications during acetylation can modify the surface of the fibres and enhance their hydrophobicity [24]. Pennisetum purpureum fibre, also locally known as Napier grass (Rumput Gajah), is composed of 46% cellulose, 34% hemicellulose, and 20% lignin [26]. The purpose of the alkali treatment is to remove the hemicelluloses, split the fibres in the fibrils, and produce a closely packed cellulose chain owing to the release of the internal strain, which consequently improves the mechanical properties of the fibre [27]. Following the alkali treatment, the fibrillation of the fibres also increases the effective surface area available for wetting by the 2

ACCEPTED MANUSCRIPT resin, and enhances the bonding between the fibre-matrix interfaces within the polymer composites. The alkali treatment also breaks the hydrogen bonds and increases the number of free hydroxyl groups of the fibre, thus increasing the fibre reactivity [28]. The alkali

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treatments of various lignocellulosic fibres such as jute, hemp, kapok, sisal [29], banana [30],

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coir [31], and Napier grass [32] have been previously investigated. Haameem and colleagues recently determined that the maximum ultimate tensile stress of Pennisetum purpureum

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single fibres was achieved with 10% alkali treatment. However, this is contradictory to the results of Reddy et al. which determined that the maximum ultimate tensile stress of Pennisetum purpureum fibre was achieved with 5% alkali treatment [26]. The modulus of

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jute fibres improved by 12%, 68 %, and 79% following 4, 6, and 8 h of alkali treatment, respectively. The tenacity of the fibre improved by 46% following alkali treatment for 6 and

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8 h, and the breaking strain was reduced by 23% following an 8 h treatment. [33]. Liu et al., Rao et al. and Thakur et al. all demonstrated that the natural fibres exhibit great potential for use as an alternative to artificial glass and carbon fibres during the production of

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thermosetting or thermoplastic composites [34–36].

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In this paper, a comprehensive characterisation of Pennisetum purpureum fibres including their mechanical and thermal properties, as well as their physical and morphology prior to

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and following the alkali-treatment are reported. To the best of our knowledge, this is the first time an extensive characterisation of this fibre has been conducted and this study should provide new information to the research community.

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2. Materials and methods

2.1. Extraction of Pennisetum purpureum fibres Pennisetum purpureum plants were harvested from a local plantation in Bukit Kayu Hitam, Kedah, in northern peninsular Malaysia. Following the water retting process, the fibres were manually extracted from the stem internodes [37]. To separate the fibre strands, the stems were initially cleaned and subsequently crushed into small parts with a mallet. Subsequently, the short plant stems were immersed under running tap water for a few weeks to facilitate the separation process. Finally, the fibres were cleaned with distilled water and subsequently dried under the sun to remove the moisture content [38]. The Pennisetum purpureum plants and its extracted fibres are shown in Fig. 1.

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(b)

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(a)

Fig. 1: (a) Pennisetum purpureum grass and (b) the extracted Pennisetum purpureum fibres

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2.2 Alkali treatment

To treat natural fibres, previous researchers have employed alkali (sodium hydroxide)

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solutions of various percentages (2, 5, 7, 10, and 15%) and treatment for various immersion times (1, 3, and 24 h) [39]. Reddy et al. used alkali solutions of 2 and 5% to treat Pennisetum

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purpureum fibres at room temperature [26]. Considering this information, alkali solutions of 5, 7, 10, 12, and 15% were applied to the Pennisetum purpureum fibres for 24 h at room

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temperature during this investigation. A liquor ratio of 40:1 was used to remove the hemicelluloses and surface impurities of the fibre. Finally, the fibres were cleaned using

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distilled water and dried at room temperature. 2.3 Physical measurement The minimum and maximum diameters, average length, and average mass of both the treated and untreated fibres are presented in Table 1. Prior to tensile testing of the single fibres, their diameter, length, and mass were measured to ensure that the properties of the specimens were recorded for analysis purposes. Fibres of 150–170 mm in length were selected for the untreated and alkali-treated fibres. The mass of the fibres was between 0.0022–0.0028 g. The diameter of the fibre was measured using a metallurgical microscope (MT8100) with a magnification of 50×. Twenty fibre samples were randomly selected for each alkali concentration, and the diameter of each fibre was measured in three positions, as shown in Fig. 2 [13]. Following the alkali treatment, the average diameter relating to each alkali concentration was calculated in order to determine the variation in the diameter.

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ACCEPTED MANUSCRIPT Table 1: Physical properties of treated and untreated Pennisetum purpureum fibres. (20 samples for each concentration) Maximum Diameter (mm)

Average Length (mm)

Untreated 5% 7% 10% 12% 15%

0.21 0.16 0.14 0.13 0.12 0.12

0.27 0.26 0.25 0.18 0.15 0.16

158 163 154 154 155 158

0.0025 0.0024 0.0028 0.0025 0.0023 0.0022

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2.4 Determination of moisture content

Average Mass (g)

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Minimum Diameter (mm)

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Concentration

Bundles of single fibres bound together from each alkali concentration were weighed and

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dried in an air oven at 100°C for 24 h, weighed using a balance with 4 decimal places accuracy (± mg) and recorded as Md. The samples were then placed in humidity chamber at 70% relative humidity (RH) at 21°C for 24 h [40]. The humidity chamber was set up using

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distilled water and held at room temperature. Subsequently, the fibres were weighed again

using eq. (1):

sample.

x 100 %

(1)

= mass of the sample after exposing it in humidity and

= mass of the dried

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where

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Moisture content (MC) =

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after exposure in humidity and recorded as Mh. Finally, the moisture content was determined

2.5 Single tensile test

The tensile properties of the fibre were determined through the single fibre tensile testing method in accordance with ASTM D3822-07 [41]. Prior to the test, the treated and untreated fibres were dried in an oven at 100 °C for 24 h [26]. The treated and untreated fibres were weighed using an analytical balance and the length of the fibres was measured. The average diameters of the fibres were used to estimate the cross-sectional areas of the fibres. The fibre was mounted and glued onto a tab-shaped piece of paper with a gauge length greater of 100 mm. For each alkali concentration, twenty fibre samples were prepared, measured and subjected to tensile testing using a micro universal testing machine (INSTRON 5848) with a crosshead speed of 1 mm/min and a load cell of 2 kN. The stress was then computed from the ratio of the force and estimated cross-sectional area. 5

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Fig. 2: Diameter measurement of a single Pennisetum purpureum fibre using a standard

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optical microscope

2.6 Fourier transforms infrared spectroscopy (FTIR)

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The Perkin Elmer Spectrum 400 FTIR spectrophotometer was used to derive the FTIR

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spectra of the untreated and treated Pennisetum purpureum fibres. All the spectra were recorded in the wavenumber range of 650–4000 cm-1, operating in ATR (attenuated total

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reflectance) mode.

2.7 Thermo-gravimetric analysis (TGA)

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The thermal stability behaviour of the Pennisetum purpureum was assessed by TGA using the Gas Controller GC 200 STAR System analyser (Model METTLER TOLEDO). To prevent oxidation, the TGA analyses were performed under a nitrogen atmosphere at a flow rate of 20 ml/min. The samples, weighing 6–8 mg, were crushed and placed in an alumina crucible to avoid any temperature variation in the thermocouple measurements. The heating rate was maintained at 10 °C/min during heating between 30–500 °C [32]. 2.8 Surface morphology of Pennisetum purpureum by Scanning Electron Microscopy (SEM) The surface and cross-sectional morphologies of the untreated and treated Pennisetum purpureum fibre strands were examined using an SEM (HITACHI Model TM3000). The scanning images were obtained with an accelerating voltage of 15 kV and magnification of 1000 – 1200 ×. The layers were not surface coated prior to scanning.

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ACCEPTED MANUSCRIPT 3.0 Results and discussion 3.1. Effect of alkali treatment on diameter variation

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The diameter variation for the untreated and treated fibres is shown in Fig. 3. It can be

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observed that the diameters of the treated fibres are smaller than the untreated fibres. The average diameter of the untreated fibres was 0.24 ± 0.02 mm, and the treated fibres had an

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average diameter of less than 0.21 ± 0.03 mm. Following the alkali treatment, there was a 12– 45% reduction in the diameter. This reduction is owed to the removal of the amorphous components, collapse the cellular/lumen structure and the reduction in the hemicelluloses

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fraction as a result of the alkali treatment [26]. The hemicelluloses are amorphous and hydrophilic, and they are soluble in alkali solution [43]. Furthermore, lignin is amorphous

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and hydrophobic in nature, and also soluble in alkali solution [43]. Therefore, both lignin and hemicelluloses on the fibre surfaces can be removed by the alkali treatment. In our study, the

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chemical treatment resulted in a smaller and a more uniform diameter.

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0.25

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0.15

0.10

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Diameter (mm)

0.20

0.05

0.00

Untreated

5% Treated

7% Treated

10% Treated

12% Treated

15% Treated

Fig. 3: Variation in average diameter of fibres The fibres treated with a higher percentage concentration of alkali solution exhibited improved removal of lignin and hemicelluloses. There are no specific standards regarding the percentage concentration of alkaline solution for use in the chemical treatment of natural fibres. Hence, most studies regarding the chemical treatments of natural fibres are based on trials, which use varying amounts of alkali solutions. Our study on Pennisetum purpureum fibres demonstrated that the fibres treated with 5 and 7% alkali solutions exhibited a better 7

ACCEPTED MANUSCRIPT physical appearance than the untreated and other treated fibres. The untreated fibre was observed with surface impurities whilst higher than 10% treated fibres revealed damage texture. Reddy et al. investigated the treatment of Pennisetum purpureum fibres with 2–5%

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alkali solution and determined that as the percentage concentration of the alkali solution

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increased, the diameter of the fibre decreased [26].

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3.2 Moisture content

Moisture content of untreated and treated Pennisetum purpureum fibres are presented in Fig. 4. It can be observed that the untreated fibre exhibits the highest moisture content of 13.9 ± 1.1 %. The moisture content was reduced by over 48%, 7.2 ± 1.0 and 7.1 ± 0.9 % for 5 and

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7% alkali-treated fibres respectively. The moisture content was further reduced to 3.7 ± 0.8, 3.1 ± 0.7 and 1.7 ± 0.5 % for 10, 12 and 15 % alkali-treated fibres respectively. The reduced

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moisture content of the fibres is related to the fact that the treatment had caused the removal of hemicelluloses which reduced the amount of bonding sites and compressed the lumen structure of fibres. Moisture content is a very important parameter as it affects the dimension

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and physical properties of the fibres. They influence the dimensional stability, electrical

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resistivity, tensile strength, porosity and swelling behaviour of natural fibre reinforced composites [44]. Low moisture content of Pennisetum purpureum fibres is preferable in

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fabricating Pennisetum purpureum fibre reinforced polymer composites due to their lesser ability to hold up water molecules.

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16 14

Moisture Content (% )

12 10 8 6 4 2 0

Untreated

5% Treated

7% Treated

10% Treated

12% Treated

15% Treated

Fig. 4: Moisture content of Pennisetum purpureum fibres 8

ACCEPTED MANUSCRIPT 3.3 Mechanical properties of Pennisetum purpureum fibre Table 2 shows the mechanical elastic properties of untreated and alkali-treated Pennisetum purpureum fibres. The 5% alkali-treated fibre exhibits the highest ultimate tensile

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stress, followed by 7% treated, untreated, 10% treated, 12% treated, and 15% treated fibres,

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respectively. A maximum ultimate tensile stress of 141± 24 MPa was achieved with the 5%

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alkali treatment; this stress was confirmed by a previous study [26]. The ultimate tensile stress significantly increased from 73± 6 MPa for the untreated fibre to 141 ± 24 MPa for 5% alkali-treated fibre, which is an increase of over 90%. However, with a further increase in the alkali concentration to 7%, the ultimate tensile stress of the fibre decreased to 79 ± 14 MPa;

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however, the ultimate tensile stress of this fibre was 8% higher than that of the untreated fibres. The 10 and 12% alkali-treated fibres exhibited ultimate tensile stress of 50 ± 7 and 49

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± 9 MPa, respectively. The fibre treated with 15% alkali exhibited the lowest ultimate tensile stress, 31± 5 MPa. These findings confirmed that the fibre treated with 5% alkali exhibited a

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higher ultimate tensile stress than the untreated fibre and the other treated fibres.

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Table 2: Mechanical properties of treated and untreated Pennisetum purpureum fibres. (20 samples for each concentration)

Ultimate Tensile Stress (MPa)

Strain at break %

Young’s Modulus (GPa)

Untreated 5% 7% 10% 12% 15%

73 ± 6 141 ± 24 79 ± 14 50 ± 7 49 ± 9 31 ± 5

1.40 ± 0.23 2.30 ± 0.18 2.27 ± 0.21 4.43 ± 0.25 4.81 ± 0.38 6.40 ± 0.22

5.68 ± 0.14 4.86 ± 0.22 3.77 ± 0.11 1.00 ± 0.17 1.02 ± 0.32 0.55 ± 0.17

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Concentration

The inconsistencies in the length, mass, and diameter of all the tested samples consequently affect the percentage strain at break of the fibres with different respective strength. The results demonstrate that the untreated fibre exhibited the lowest strain at break, 1.40 ± 0.23 %. The strain at break drastically increased by more than 90% to reach approximately (2.30 ± 0.18) – (2.27± 0.21) % for the 5 and 7% alkali-treated fibres. Subsequently, the strain at break exhibited a significant increase to 4.43 ± 0.25 and 4.81± 0.38 % for the 10 and 12 % alkali-treated fibres respectively. Finally, the maximum strain at break, 6.40 ± 0.22 %, was achieved with the 15% alkali-treated fibre. Other researchers also found that treated fibres exhibited an increased strain at break [26,45].

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ACCEPTED MANUSCRIPT Furthermore, there was a reduction in the Young’s modulus of the treated fibres. The untreated fibre exhibited the highest Young’s modulus of 5.68 ± 0.14 GPa. The 5, 7, 10, 12, and 15% alkali-treated fibres exhibited values of 4.86 ± 0.22, 3.77 ± 0.11, 1.00 ± 0.17, 1.02 ±

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0.32 and 0.55 ± 0.17 GPa, respectively. Considering these results, the alkali treatment seems

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to enhance the elasticity of the fibre and hence a larger strain to failure can be achieved. Goda et al. produced similar results for ramie fibres; the elastic modulus decreased as the alkali

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treatment concentration increased [45]. Fig. 5 shows the stress-strain responses for selected of untreated and alkali-treated Pennisetum purpureum fibres. 140

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5% Treated Fibre

120

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Stress (MPa)

100 80 Untreated Fibre

7% Treated Fibre

60

12% Treated Fibre

20

0

1

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0

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40

10% Treated Fibre

2

15% Treated Fibre

3

4 Strain (% )

5

6

7

Fig. 5: Stress-strain responses for selected untreated and alkali-treated Pennisetum purpureum fibres.

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The treated fibres exhibit a higher ultimate tensile stress and strain to failure because of the changes in the cellulose crystallinity following the 5 and 7% alkali treatments [24]. The process involves the removal of the weak amorphous component so that the fibre only retains the crystalline components. The removal enables the fibril to rearrange themselves in a more compact manner, thus enhancing the tensile strength of the fibre [46]. Following the 10, 12, and 15% alkali treatments, there is an increase in the stiffness and brittleness of the fibres and this results in the decrease in the tensile strength and Young’s modulus. In the presence of the 10, 12, and 15% alkali solutions, the main structural components of the fibre were attacked, resulting in the formation of more grooves on the surface of the fibre. Consequently, this leads to further weakening of the fibre strength, resulting in a decrease in the ultimate tensile stress [42]. A similar observation was also reported by Reddy et al. [26]. Therefore, it can be concluded that the fibre treated with 5% alkali exhibits the maximum ultimate tensile stress,

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ACCEPTED MANUSCRIPT and the 10, 12, and 15% alkali treatments reduced the strength and Young’s modulus of the fibres but increased their fracture strain.

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3.4 FTIR analysis

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The FTIR spectra of the untreated and alkali-treated fibres are presented in Fig. 6. From Fig. 6 (a), it can be observed that the untreated fibre exhibits well-defined bands at

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approximately 665, 897, 1033, 1160, 1242, 1371, 1425, 1512, 1604, 1729, 2905, and 3345 cm-1 within its spectra. The small peaks at 665 cm-1 associated with the C-OH out of plane bending [47] and the peak at 897 cm-1 can be attributed to the presence of β-glycosidic

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linkages between the monosaccharides [14]. The intense band, centred at 1033 cm-1, is associated with the C-O stretching modes of the hydroxyl and ether groups in the cellulose

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[19]. The peak at 1160 cm-1 is associated with the C-O-C stretching vibration of the pyranose ring in the polysaccharides [48].

The absorbance peak centred at 1242 cm-1 is owed to the C-O stretching vibration of the

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acetyl group in the lignin [49], whilst the peak at 1371 cm-1 is attributed to the bending

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vibration of the C-H group of the aromatic ring in the polysaccharides [50]. The absorbance at 1425 cm-1 is associated with the CH2 symmetric bending [51]. The next peak at 1512 cm-1

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is attributed to the C=C stretching of the benzene ring of the lignin [14,52]. The peak centred at 1604 cm-1 indicates the C=C aromatic stretching with conjugated C-C bond and this peak is attributed to lignin content of the fibre [20]. The absorption band centred at 1729 cm-1 can

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be attributed to the C=O stretching vibration of the acetyl groups in the hemicelluloses [53]. The peak at 2905 cm-1 is a characteristic band for the C-H stretching vibration of CH and CH2 in the cellulose and hemicelluloses components [19]. The final peak at 3345 cm-1 is owed to the presence of O-H stretching vibrations and the hydrogen bond of the hydroxyl groups [48,54,55].

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ACCEPTED MANUSCRIPT 100

90

1729

897

Untreated Fibre

2905

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1512 665

3345

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80

1425

70

1371

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Transmittance(%)

1604

1242 1160

60

50

1000

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1033

1500

2000

2500

3000

3500

4000

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Wavenumber (cm-1 )

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a) Untreated fibre

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100

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665

897

80

70

1158

7% Treated Fibre

1368

2905

1425 1650

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Transmittance (%)

90

5% Treated Fibre

60

3342

1031

50

1000

1500

2000

2500

Wavenumber

3000

(cm-1 )

b) 5% and 7% treated fibre

12

3500

4000

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100

665

12% Treated Fibre 10% Treated Fibre

847

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80

1019

70

1000

1500

2000

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60

50

3458

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1692 1459

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Transmittance (%)

90

15% Treated Fibre

2500

3000

3500

4000

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Wavenumber (cm-1 )

c) 10, 12 and 15% treated fibre

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Fig 6: FTIR (a) Untreated fibre, (b) 5 and 7% treated fibre, (c) 10, 12, and 15% treated fibre

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FTIR is used to determine the compositional changes that occur during the alkali treatment. The bands for the 5 and 7% alkali-treated fibres are at approximately 665, 897,

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1031, 1158, 1368, 1650, and 3342 cm-1, as shown in Fig. 6(b). The bands for the 5 and 7% alkali-treated fibres are relatively similar to those of the untreated fibre with little difference. The bands at 1242, 1512, 1604, and 1729 cm-1 had disappeared for the alkali-treated fibres;

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this indicates that compositional changes had occurred. The bands at 1242, 1512, and 1604 cm-1 were present for the untreated fibre; these correspond to the lignin. However, the band at 1729 cm-1 is associated with the hemicelluloses [56]. Fig. 6(c) shows the FTIR spectra for the 10, 12, and 15% alkali-treated fibres with bands at 665, 847, 1019, 1459, 1692, and 3458 cm-1. The bands are almost completely different to those of the untreated fibre; the most significant difference is that the peaks are smaller than those for the 5 and 7% alkali-treated fibres. The bands at 897, 1160, 1242, 1371, 1604, and 2905 cm-1 disappear; these primarily correspond to the hemicelluloses, cellulose, and lignin [56]. Therefore, the FTIR studies demonstrate that the composition of the fibres can change as the alkali concentration increases. This observation agrees with the poor ultimate tensile stress of the 10, 12, and 15% alkali-treated fibres, as discussed in Section 3.3.

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ACCEPTED MANUSCRIPT 3.5 TGA results The TGA and the derivative of thermo-gravimetric (DTG) curves for the untreated and alkali-treated Pennisetum purpureum fibres are presented in Fig. 7. Table 3 shows the data of

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thermo-gravimetric analysis (TGA) for the untreated and alkali-treated Pennisetum

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purpureum fibres. The purpose of this analysis is to observe the thermal degradation curves

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of the hemicelluloses, cellulose and lignin. Fig. 7(a) shows the typical thermal decomposition for the cellulose and lignin portion of the untreated fibre. Up to a temperature of 91.5 °C, the initial curve indicated a moisture loss with a weight change of approximately 5.26%. At temperatures above 230 °C, degradation of the hemicellulose occurred. Up to 400.06 °C, the

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percentage of residue remaining was 26.62%, and approximately 73.38% had decomposed. The decomposition of the untreated Pennisetum purpureum fibre occurred in two stages. The

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decomposition of the cellulose initiated between 250–380 °C and most of the volatile materials decomposed between these temperatures. Therefore, above this temperature range, the remaining residues are considered to be char. The rapid decrease in mass is owed to

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cellulose volatilization and this was followed by a slow decrease in the mass of the lignin. In

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the DTG, it can be observed that rapid decomposition occurs at 364.7 °C; the fibres start to burn and the cellulose degrades [57,58]. A peak at approximately 306.6 °C indicates the

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decomposition of the hemicelluloses. Typically, it is more difficult for the lignin to decompose than the cellulose, since part of the lignin consists of benzene rings [59,60]. DT G

TG

5.26 %

Weight Change (%)

0

TG

27.74 %

80

Peak 44.8(°C)

-2 DT G

60

-4

Untreated Fibre 40

-6

40.38 %

Peak 306.6 (°C)

-8

2.62 % 20

- 10

Peak 364.7(°C) 0 100

200

300

Temperature (°C )

a) Untreated fibre

14

400

24.36%

- 12

500

Derivative Weight (%/min)

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100

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TG

7% Treated Fibre

4.13 %

100

0

TG

45.7 %

60

-2

T

48.57 %

DT G

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10.13 %

5% Treated Fibre 40

Peak 314.3 (°C)

-4 -6

10.21 % Peak 356.4 (°C)

20

Derivative Weight (%/min)

Peak 43.94 (°C)

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Weight Change (%)

80

-8

37.09 % 40.0 4 % - 10

100

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0 200

300

- 12

400

500

Temperature (°C )

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b) 5% and 7% treated fibre

TG

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3.62 % 3.02 % 3.34 % Peak 43.33 (°C)

0

45.77%

DTG

45.98% 45.66%

-2 -4

10% Treated Fibre

60

12 and 15% Treated Fibre

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Weight Change (%)

80

DTG

Derivative Weight (%/min)

TG 100

7.24 %

Peak 310.13 (°C) 8.73 %

-6

40

-8

10% Treated Fibre

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43.76%

41.88%

20

12 and 15% Treated Fibre

-10

-12

0

100

200

300 Temperature (°C)

400

500

c) 10, 12 and 15% treated fibre

Fig. 7: TGA-DTG curves (a) Untreated fibre, (b) 5 and 7% treated fibre, (c) 10, 12, 15% treated fibre Table 3: Thermo-gravimetric analysis (TGA) of Pennisetum purpureum fibre Sample

Untreated 5% Treated 7% Treated 10% Treated 12% Treated 15% Treated

Initial Degradation IDT (°C) % wt. loss 91.5 104.0 104.0 104.0 104.0 104.0

5.26 4.13 4.13 3.62 3.34 3.02

Final Degradation FDT(°C) % wt. loss 400.06 365.50 386.30 385.40 385.40 385.40

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73.38 52.70 49.83 49.39 49.00 49.00

Final residue (%) 26.62 47.30 50.17 50.61 51.00 51.00

ACCEPTED MANUSCRIPT Fig. 7(b) demonstrates the TGA-DTG curves for the 5 and 7% alkali-treated fibres. The initial curve between 30–104 °C corresponds to the decomposition of the moisture, and the weight difference is approximately 4.13%. This demonstrates a decrease of approximately

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21.5% for the moisture content of the 5 and 7% alkali-treated fibres. There was 47.3 % and

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50.17 % of residue remaining for the 5% (up to 385.5 °C) and 7% (up to 386.3 °C) alkalitreated fibres, respectively. This indicates that the 5 and 7% alkali-treated fibres had

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decomposed by approximately 52.7 and 49.83 %, respectively. This implies that the reduction of the residue is owed to the increase in the alkali concentration. The DTG demonstrates the differences in the peaks of the 5 and 7% alkali-treated fibres. For the 5% alkali-treated fibres,

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two peaks were recorded at 309.4 and 356.4 °C while only one peak was recorded at 314.3 °C for the 7% alkali-treated fibre. This occurred because an increase in the alkali

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concentration caused a reduction in both the hemicelluloses and some part of the lignin in the fibres.

Fig. 7(c) shows the TGA-DTG curves for the 10, 12, and 15% alkali-treated fibres. The

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moisture contents of the 10, 12, and 15% alkali-treated fibres were 3.62, 3.34, and 3.02%,

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respectively. It was confirmed that the moisture content of the fibres decreased as the alkali concentration increased. There was approximately 50–51% residue remaining for the 10, 12,

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and 15% alkali-treated fibres at 385.4 °C. The DTG demonstrates that only two peaks appeared at 43.33 and 310.13 °C. The TGA-DTG curves for all the Pennisetum purpureum fibres demonstrated a reduction in the moisture content and residue following the burning of

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the fibres. 3.6 SEM analysis

The SEM micrographs of the surfaces of the untreated and alkali-treated Pennisetum purpureum fibres are shown in Fig. 8. It was important to study the surface morphology of the fibres to observe the changes that occurred on the surface of the Pennisetum Purpureum fibre because of the alkali treatment. The SEM images revealed that the Pennisetum purpureum fibre had a multi cellular structure. Fig. 8(a) shows the SEM images of the untreated fibre. The white layer within the images represents the presence of impurities on the surface of the fibre. Fig. 8(b) shows the SEM image of the 5% alkali-treated fibre. It can be observed that this fibre is cleaner than the untreated fibre because of the removal of surface impurities by the treatment. However, some surface impurities can still be observed. The surface impurities on the fibres are soluble in the 16

ACCEPTED MANUSCRIPT alkali solution and this explains the cleansed surfaces [61]. Fig. 8(c-d) shows the 7 and 10% alkali-treated fibres. The surfaces of the 7 and 10% alkali-treated fibres had a similar

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Untreated fibre

b) 5% treated fibre

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a)

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appearance to the 5% alkali-treated fibre surface but they were cleaner with less impurities.

c)

7% treated fibre

e)

12% treated fibre

d) 10% treated fibre

f) 15% treated fibre

Fig. 8: (a–f) Surface morphologies of untreated and alkali treated Pennisetum purpureum fibres 17

ACCEPTED MANUSCRIPT Fig. 8(e-f) shows the 12 and 15% alkali-treated fibres. The damaged textures on the surfaces of the fibres can be observed. The 15% alkali-treated fibres demonstrated a more twisted and rougher surface than the 12% alkali-treated fibres. If these fibres were used as

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reinforcement materials in composites, these rough surfaces would be expected to promote

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good interfacial bonding between the fibres and the matrix [29]. However, if the surfaces were rougher than a desired level, this could reduce the mechanical properties of the fibres

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[62]. Following the elimination of the impurities as a result of the alkali treatment, it was apparent that there was a reduction in the mass of the fibre and an improvement in the surface area of the fibre.

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Fig. 9 shows the cross-sectional morphologies of the untreated and treated Pennisetum purpureum fibres. As mentioned above, the Pennisetum purpureum fibres exhibit a multi-

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cellular structure, which indicates a porous structure. A hollow cavity known as a lumen exists inside the unit of the fibre; this can be observed in Fig. 9(a). Fig. 9(b-f) demonstrates that the cellular/lumen structures disappear as the percentage of alkali treatment increases.

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The fibres treated with 15% alkali are shown in Fig. 9(f). They exhibit a compressed

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cellular/lumen structure without a void content. Furthermore, the alkali treatment destroyed the cellular/lumen structure of the fibre, and hence reduced the void content of the fibres.

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This can result in lower water absorption and explain the reduction of diameter for alkalitreated fibres. Therefore, the alkali treatment improves the mechanical and physical properties of the Pennisetum purpureum fibre, and thus facilitates its applications in

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composite structures.

a) Untreated fibre

b) 5% treated fibre

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d) 10% treated fibre

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c) 7% treated fibre

e) 12% treated fibre

f) 15% treated fibre

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Fig. 9: (a–f) Cross-sectional morphologies of untreated and alkali treated Pennisetum purpureum fibres

3.7 Comparison between the mechanical properties of Pennisetum purpureum and betel nut, coir and kenaf fibres. Table 4 shows a comparison on the diameter, ultimate tensile stress, Young’s modulus, strain at brake and moisture content of untreated and alkali treated Pennisetum purpureum fibres with betel nut, coir and kenaf fibres reported by Yusriah et al. [44]. These fibres are known locally to have good mechanical properties when used as reinforcement materials in polymer composites. The data presented suggested the mechanical properties of the 5% alkali treated Pennisetum purpureum fibre are comparable to these agricultural crops. These findings imply that the Pennisetum purpureum fibre could be used as reinforcement in polymer composites.

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ACCEPTED MANUSCRIPT Table 4: Comparison of Pennisetum purpureum fibre on span length, diameter, ultimate tensile stress, Young’s Modulus and strain at break with other natural fibres [20,44,63].

100

0.21-0.26

100

0.18-0.24

100

0.19-0.25

100

0.14-0.17

100

0.12-0.15

100

0.13-0.16

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Strain at break (%)

1.40

141

4.86

2.30

79

3.77

2.27

50

1.00

4.43

49

1.02

4.81

31

0.55

6.40

1.28 4-6 22-60

22.49 17-47 2.7-6.9

73

5.68

124 106-175 295-930

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57 0.3-3.5 1.4-11

Young’s modulus (GPa)

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Untreated Pennisetum purpureum 5% Treated Pennisetum purpureum 7% Treated Pennisetum purpureum f 10% Treated Pennisetum purpureum 12% Treated Pennisetum purpureum 15% Treated Pennisetum purpureum Betel nut (Areca catechu) Coir (Cocos nucifera) Kenaf (Hibiscus cannabinus)

Ultimate tensile stress (MPa)

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Fibre diameter (mm)

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Span Length (mm)

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Type of Fibre

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4.0 Conclusion

The physical, mechanical, thermal, compositional, and morphological properties of

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Pennisetum purpureum fibres were investigated. The results confirmed that there was a variation in the fibre diameter following the alkali treatment. The untreated fibre exhibited the largest diameter and the diameter further decreased as the concentration of the alkali

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treatment increased. The untreated Pennisetum purpureum fibres exhibited the highest moisture content as a result of their multi-cellular structures. Following an increase in the concentration of the alkali treatment, there is a reduction in the moisture content because the alkali treatment compresses the multi cellular structures and reduces the void contents of the fibres. This was confirmed by the observation of the cross-sectional morphologies. The crosssections of the Pennisetum purpureum fibres were observed. The untreated fibres exhibited large lumen structures, and there was a change in the structure following the alkali treatment. The surface morphologies of the untreated Pennisetum purpureum fibres demonstrated the presence of impurities on the surfaces of the fibres. The removal of impurities by the alkali treatment results in the fibrillation and cleansed surfaces of the fibre. The SEM observation of the Pennisetum purpureum fibres demonstrated that the alkali treatment increased the surface roughness of the fibre. The TGA-DTG results indicate that the alkali treatment removed the hemicelluloses of the fibres. This was further supported by the FTIR 20

ACCEPTED MANUSCRIPT spectrometry analysis. The 5% alkali-treated fibre achieved the maximum ultimate tensile stress. The 10, 12, and 15% alkali treatments reduced the ultimate tensile stress of the fibres; this was confirmed by SEM observations, which demonstrated the damaged textures and the

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twisted fibres. The study further supported the feasibility of utilizing Pennisetum purpureum

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fibres as reinforcing materials in polymer composites.

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Acknowledgement

The authors would like to thank the Universiti Malaysia Perlis and the Ministry of Education, Malaysia, for providing financial assistance (RAGS No.: 9018-00072). The

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authors are also thankful for the support of the facility during the course of the research. In addition, the Mechanical Engineering Department of Sultan Abdul Halim Mu’adzam Shah

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Polytechnic (POLIMAS), are acknowledged for their fruitful discussions and input to the project.

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Single fibre test (Tensile)

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Pennisetum purpureum stems and extracted fibres

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Graphical Abstract

Morphology of fibre

Stress-strain response of Pennisetum purpureum fibres

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ACCEPTED MANUSCRIPT Highlights

1. The treated fibres had an average diameter of less than 0.21 ± 0.03 mm.

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2. Moisture content of treated fibres reduced with increased alkaline concentration.

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3. Morphological study shows that the treated fibre’s surface became rougher.

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4. The 5% alkali-treated fibre yields average maximum tensile stress of 141± 24 MPa.

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