Mechanical properties of sisal fibre-reinforced ...

Composite Interfaces

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Mechanical properties of sisal fibre-reinforced polymer composites: a review Idowu David Ibrahim, Tamba Jamiru, Emmanuel R. Sadiku, Williams Kehinde Kupolati, Stephen C. Agwuncha & Gbenga Ekundayo To cite this article: Idowu David Ibrahim, Tamba Jamiru, Emmanuel R. Sadiku, Williams Kehinde Kupolati, Stephen C. Agwuncha & Gbenga Ekundayo (2016) Mechanical properties of sisal fibre-reinforced polymer composites: a review, Composite Interfaces, 23:1, 15-36, DOI: 10.1080/09276440.2016.1087247 To link to this article: http://dx.doi.org/10.1080/09276440.2016.1087247

Published online: 21 Sep 2015.

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Date: 18 January 2016, At: 05:33

Composite Interfaces, 2016 VOL. 23, NO. 1, 15–36 http://dx.doi.org/10.1080/09276440.2016.1087247

Mechanical properties of sisal fibre-reinforced polymer composites: a review Idowu David Ibrahima, Tamba Jamirua, Emmanuel R. Sadikub, Williams Kehinde Kupolatic, Stephen C. Agwunchab and Gbenga Ekundayob Department of Mechanical Engineering, Tshwane University of Technology, Pretoria, South Africa; Department of Chemical, Metallurgy and Materials Engineering (Polymer Section), Tshwane University of Technology, Pretoria, South Africa; cDepartment of Civil Engineering, Tshwane University of Technology, Pretoria, South Africa

Downloaded by [Tshwane University of Technology] at 05:33 18 January 2016

a

b

ABSTRACT

There has been a growing interest in the utilization of sisal fibres as reinforcement in the production of polymeric composite materials. Natural fibres have gained recognition as reinforcements in fibre polymer–matrix composites because of their mechanical properties and environmental friendliness. The mechanical properties of sisal fibre-reinforced polymer composites have been studied by many researchers and a few of them are discussed in this article. Various fibre treatments, which are carried out in order to improve adhesion, leading to improved mechanical properties, are also discussed in this review paper. This review also focuses on the influence of fibre content and fabrication methods, which can significantly affect the mechanical properties of sisal fibre-reinforced polymer composites. Abbreviations: ASF alkali-treated sisal fibre; AMSF alkali and maleic anhydride treated sisal fibre; ASTM American Society for Testing and Materials; C3H6N6 melamine; CM compression moulding; FTIR Fourier transform infrared; GMA glycidyl methacrylate; H2SO4 hydrogen teraoxosulphate VI; IFSS interfacial shear stress; KMnO4 permanganate treatment; KOH potassium hydroxide; LDPE low density polyethylene; MA maleic anhydride; MAPE maleic anhydride-grafted polyethylene; MAPP maleic anhydride-grafted polypropylene; MMA methylmethacrylate; MPP the ratio of polypropylene and maleic anhydride-grafted polypropylene; MSF maleic anhydride-treated sisal fibre; NaOH sodium hydroxide; OBDC O-hydroxybenzene diazonium chloride; UT untreated; PE polyethylene; phr per hundred of resin; PLA poly(lactic acid); PMPPIC poly[methylene poly(phenyl isocyanate)] treatment; PP polypropylene; PPG-TDI polypropyleneglycol derivative of toluene diisocyanate treatment; RTM resin transfer moulding; SEM Scanning electron microscope; SF sisal fibre; SrTiO3 strontium titanate; wt% weight percent; ZnB zinc borate; %v/v percentage volume per volume.

CONTACT Idowu David Ibrahim © 2015 Taylor & Francis

[email protected]

ARTICLE HISTORY

Received 6 May 2015 Accepted 24 August 2015 KEYWORDS

Sisal fibre; chemical treatment; fibre content; composite fabrication; mechanical properties

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I. D. Ibrahim et al.


Introduction Sisal fibre belongs to the family of Asparagaceae, with the botanical name agave sisalana. It is a specie of agave with origin in southern Mexico, but could be found in other countries like Brazil, Tanzania, China, Ethiopia, Angola, South Africa and Morocco.[1] The commercial use of sisal in composites has increased due to its strength, low density, environmental friendliness and cost-effectiveness. Sisal fibre can be combined with other materials such as polymers to produce a composite. A composite is the combination of two or more component materials in which the major component is the matrix and the minor component is the reinforcement.[2] Generally, composite material consist of reinforcement (fibres, particles, fillers, etc.) which are embedded in a matrix.[3] The reinforcement is held by the matrix, while the reinforced material provides the overall mechanical properties of the composite. A composite is named based on the type of matrix used, which could be metal, ceramic or polymer. When it is a polymer, it is termed polymer matrix composite. A matrix on its own may not have the required properties base on the area of application, but the properties can be enhanced by reinforcing it with fibres. Natural fibres have certain benefits, such as low density, high specific strength, modulus and relatively non-abrasive and have no health hazards to the environment when used as reinforcing elements.[4] Natural fibres are gaining prominence in terms of fibre reinforcement in fibre/polymer composites and find application in many fields, such as concrete,[5,6] asphalt [7] and automotive.[8] The use of fibre as reinforcement with polymer has been a success to the scientists and engineers based on proven high qualities in various fields of engineering application. Many researchers have used natural fibres, such as wood fibre,[9–12] bamboo,[13–15] flax,[16–21] silk,[22,23] hemp,[24,25] sisal,[26–29] jute,[30–32] kenaf [33,34] and banana [35,36], as reinforcements in thermoplastics and thermosets. The high cost of synthetic fibres and their hazardous environmental impact have genuinely necessitated the investigation of natural fibres. The interesting aspect about natural fibres, e.g. hemp, jute, kenaf, coir, sisal, banana and pineapple, is that they are renewable resources and have little or no negative impact on the environment since it is biodegradable and biostable. Their environmental friendliness has given them the edge over synthetic fibres. Outstanding effort has been made by researchers on how to extract fibre,[37] improve mechanical properties,[38,39] thermal stability [40] and water absorption.[41] The aim of this review is to identify the optimum way of improving the mechanical properties, thermal stability and water absorption characteristics of agave-sisalana fibre. The mechanical and chemical properties of agave sisalana vary, based on the location, age and method of extraction. Table 1 shows both the chemical and mechanical properties of sisal fibre and Table 2 shows different concentrations of alkaline treatment of sisal fibres and the effect on mechanical properties, based on different reports by researchers. Based on the researches that have been conducted by various researchers, Kaewkuk et al. [42] reported that tensile strength and Young’s modulus increased as fibre content increase from 10 to 30 wt%, Dwivedi and Chand [43] also reported that compatibilizer improved mechanical properties and greatly influenced the wear resistance. According to Mylsamy and Rajendran [44], fibre/epoxy composite having fibre length of 3 mm has better wear resistance than 5 and 7 mm fibre length. In other researches, fibre treatment and fillers were reported to have improved fibre tensile strength and hardness as reported by Oladele et al. [45] and Ismail


17

Table 1. Chemical and mechanical properties of sisal as reported by some researchers.

Origin of fibre – _ Kenya – India India China


India

Density (kg/m3) 1450 1030 – 1400 1450 1450 1290

Diameter (μm) 50–300 – – – 100–300 100–300 100–200

1450

100–300

Tensile strength (MPa) 530–640 500–600 347 450–700 530–630 400–700 345.9– 436.3 400–700

Tensile modulus (GPa) 9.4–22 16–21 14 7–13 17–22 9–20 14.1–15.9

Maximum strain (%) 3–7 3.6–5.1 5 4–9 3.64–5.12 5–14 2.2–2.8

Cellulose (%) 67 – – – 70 85–88 65.8

Lignin (%) 12 – – – 12 4–5 9.9

Reference [47] [48] [49] [50] [51] [52] [53]

9–20

5–14

85–88

4–5

[54]

Table 2. Alkaline treatment of sisal fibres with various concentrations and result obtained. NaOH conc. (%) 2 5 20 18 2

Immersion time (min) 240 30 120 60 120

Tensile strength (MPa) 380.5 – 76.40 78.22 26

Tensile modulus (GPa) – – 1.96 –

Flexural strength (MPa) 311.5 56 82.3 138.782 –

Flexural modulus (GPa) 23.75 3.51 5.32 –

Reference [53] [44] [55] [56] [42]

Figure 1. Structure of (a) lignin and (b) cellulose.

et al. [46], respectively. In this review, the effect of fibre modification and fibre loading on the mechanical properties, morphological and water absorption characteristics of sisal fibre/ polymer composites are discussed. Natural fibre such sisal are lingo-cellulose in nature and contain cellulose, hemicellulose, wax and lignin. The chemical composition of the fibre varies based on location, age,

18



species and method of cultivation. These chemical compositions affect the properties of the composite. Khan et al. [57] indicated that sisal fibre has 77–85% holocellulose, 65–73% cellulose, 9–11% hemicellulose, 5–6% lignin, 0.9–1.2% wax and 9–11% moisture; Fávaro et al. [58] reported 43–56% cellulose, 21–24% hemicellulose, 7–9% lignin and 0.6–1.1% ash. The presence of cellulose made the fibre to be hydrophilic in nature, having high affinity for water, which make compatibility difficult with hydrophobic polymer matrix. Figure 1 shows the lignin and cellulose structures of natural fibres. In order to create an improved fibre/polymer composite, the compatibility between fibre and polymer must be worked upon. This process is achievable through fibre surface modification which could be chemical treatment [59,60] or/and heat treatment [61].

Chemical treatments for fibre surface modification Chemical treatment helps to improve the interfacial interaction between the fibre and the matrix by removing hemicellulose, lignin, wax and other impurities, leading to good fibre/ polymer matrix bonding. Before chemical treatment is done, the sisal fibres could be washed with mild detergent solution and then reined in distilled water to remove dust and other impurities that may be present on the fibre. There are different methods for fibre surface treatment (as discussed below) and Figure 2(a–e) shows the different chemical treatments for sisal fibres.

Figure 2. Chemical treatments for sisal fibre: (a) NaOH (b) acetylation (c) strontium titanate (d) glycidyl methacrylate (e) O-hydroxybenzene diazonium chloride.[55,62]


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Table 3. Effect of surface treatment on interfacial shear strength of henequen/unsaturated polyester resin composites using various treatment methods and media. S/N 1 2 3 4 5 6 7

Treatment method/media Untreated Soaking/tap water Ultrasonic/tap water Soaking/1 wt% NaOH Soaking/6 wt% NaOH Ultrasonic/1 wt% NaOH Ultrasonic/6 wt% NaOH

Interfacial shear strength (MPa) 5.5 7.5 8.7 6.6 9.0 5.5 8.2


Source: Adapted from Cho et al. [63]. NaOH: Sodium hydroxide.

Mercerization treatment of sisal fibre Mercerization is the treatment of fibre with alkali, e.g. sodium hydroxide (NaOH). Alkali treatment of sisal fibres with sodium hydroxide is usually carried out by immersing the fibre in a certain concentration of NaOH solution for a specified period of time at room temperature or at an elevated temperature. Fibres can be treated with various concentrations and immersion time, and these parameters (concentration, temperature and time) influence the overall result. Following immersion in alkali, the fibres are washed with distilled water in order to remove the soapy content on the fibre and then neutralized with mild acid e.g. acetic acid in order to remove excess alkali. The fibre is further washed with water and left to dry in air or in an oven at a controlled temperature.[55,62] It is the most commonly used method of treatment, since it is economical. Cho et al. [63] reported that after treatment, the topography of the fibre changed, making it rough and therefore improving the interfacial shear stregnth as shown in Table 3. Huda et al. [64] also reported similar observation with pineapple leaf fibre. In a similar report, alkali-treated henequen fibre which is a close relative of sisal fibre was treated with 10% potassium hydroxide (KOH) for 1 h before curing. The result showed improved properties, such as tensile strength and elongation-at-break of 300 and 290% over untreated fibre, respectively.[65] It was also reported by Kim and Netravali [66] that mercerized sisal fibre (under tension) improved the fracture stress and stiffness of sisal fibre-reinforced composites with soy protein resin by 12.2 and 36.2%, respectively. Acetylation treatment of sisal fibre This is a mixture of acetic acid and acetic anhydride in a ratio of 1:1. The fibre is immersed in the mixture with 1 ml concentration of H2SO4 as a catalyst. The fibre is removed from the mixture and washed thoroughly with distilled water in order to completely remove any unreacted reagents and then dried in an oven at regulated temperature.[62] The mixture ratio of acetic acid and acetic anhydride and the catalyst concentration can vary, depending on the desired outcome. Similar treatment method was employed by Alvarez et al. [67]; it was observed that the treatment improved the interfacial shear stress, irrespective of the polymer when compared with the untreated fibre. Mohanty et al. [68], who also used this method of treatment, reported that acetylation is a well-known esterification mode of treatment of natural fibres by introducing plasticization to cellulosic fibres. Acetic anhydride is chosen over acetic acid during acetylation treatment as there is no sufficient reaction with cellulose at a later stage (when acetic acid

20


was used). The cellulosic fibres are first immersed in acetic acid before treatment with acetic anhydride because acetic anhydride is a poor swelling agent for cellulose-base fibres. The aim of the treatment is to reduce the hygroscopic nature of natural fibres, which will result in enhanced interfacial adhesion between the fibre and the matrix since there is no swelling, even though there is moisture absorbed by the composite.


Strontium titanate (SrTiO3) fibre treatment After washing the fibre in distilled water or mild detergent, the fibre is allowed to dry in air until a constant weight is obtained. The dried fibre is immersed in ethanol solution using 2% strontium titanate as a coupling agent. Ethanoic acid is added while the pH is maintained between 4.5 and 5.5. The fibre can further be washed with ethanol and then dried in an oven.[62] This process leads to bond durability and improved fibre/polymer interfacial adhesion, resulting in improved composite properties. Glycidyl methacrylate (GMA) sisal fibre treatment Pre-washed and dried sisal fibres are dipped in a solution of glycidyl methacrylate. In order to minimize free pervasive reaction, due to unsaturation at the end of GMA molecules, hydroquinone of about 2% of GMA is added. The fibre is drained and rinsed with excess acetone in order to remove completely any GMA residue and then dried in a vacuum for about 24 h.[62] This process improves the interfacial interaction between the fibre and polymer matrix. O-hydroxybenzene diazonium chloride (OBDC) fibre treatment Water is mixed with solution of OBDC, prepared in the ratio of 90:10. The sisal fibre is then immersed in the solution for treatment. The fibre is washed with distilled water and dried in an oven for 4 h. There is increased stress transfer as a result of the significant improvement in the interfacial bonding adhesion between the fibre and polymer matrix.[62] This treatment method improves the overall properties of the composite. Thermal treatment The fibres are cleaned in order to remove dirt or impurities and the fibre is then placed in an oven and heated to an elevated temperature below the degradation temperature of the fibre for a period of time. Sreekumar et al. [60] heated the fibre to a temperature of 100 °C continuously for 2 h in an air circulating oven. While Huang et al. [69] heat-treated at different temperatures (150, 200, 220 and 250 °C) and times (5, 10, 15, 20 and 30 min), the best tensile strength of 74 MPa at 220 °C for 10 min and flexural strength of 117 MPa when heat-treated at 150 °C for 15 min were reported. Benzoylation treatment This process involves the use of NaOH solution and benzoyl chloride. Clean fibres are completely immersed in a solution of NaOH and agitated with benzoyl chloride. The fibre



21

is allowed to remain in the solution for a period of time and later soak in ethanol in order to remove unreacted benzoyl chloride. Sreekumar et al. [60] agitated the immersed sisal fibre with 50 ml benzoyl chloride for 15 min and soaked in ethanol for 1 h. In a similar treatment method, Luz et al. [70] added 200 ml of 10% NaOH solution and 100 ml of benzoyl chloride to previously swollen 20 grams of bagasse with 18% NaOH solution. The chemical modification with benzoylation was done for 1 h at 55 °C, after which the fibre was washed with ethanol and distilled water in order to remove excess reagent and sodium chloride formed. The chemically modified fibre was characterized by FTIR and absorption peaks were observed around 1300 cm−1, which indicate the presence of ester groups and at 710, 1600 and 1950 cm−1, which also indicate the presence of aromatic groups. Zhou et al. [71] treated sisal fibre with three different methods and observed the surface topography of the fibre. From the SEM images, it was observed that the fibres contain bundles of cells, which are held together by node-like material. The surface of the untreated sisal fibre appeared to be smooth as a result of waxes on the fibre surface. The treatment with NaOH made the surface rougher which can be attributed to the removal of hemicellulose, wax and other impurities from the surface of the fibre. The rougher surface enhances good interlocking adhesion between the fibre and the matrix, leading to improved mechanical properties, thermal stability and lower water absorption. The treatment with (i) alkali and 3-aminopropyltriethoxysilane and (ii) alkali and 3-aminopropyltrimethoxysilane is observed to make the surface of the fibre almost as smooth as the untreated fibre. This could be because the silane-coupling agent completely covered the surface of the fibre after reaching adsorption equilibrium.

Methods for the fabrication of fibre-reinforced polymer composites Fibre-reinforced polymer composites are fabricated by injection, extrusion and compression moulding (CM) methods. Figure 3 shows an assembly of an injection, extrusion and CM machines. Each of the moulding methods has its areas of application. When fabricating unidirectional (long) fibres, the method of extrusion cannot be used, however it is applicable for short fibre-based polymer composites. Extrusion is the process or an instance of forcing semi-soft material which could be metals or polymers through a die in order for it to take the shape of the mould. This method has been used by many researchers that worked with short fibres.[72,73] Injection moulding is a manufacturing process in which heated plastic materials (thermoplastics or thermosetting) are forced under pressure into a closed desired mould.[3] The following working parts can be found on a complete injection mould assembly: hopper, heaters, nozzle, mould, platen, screw gear and tie bar. This fabrication method is used for thermoplastics and thermosets, where high-volume and low-cost components are manufactured. This method is limited to short fibres only, just like the extrusion method. This method has been used by many researchers in the past.[74,75] CM is among the oldest manufacturing methods used for composite fabrication. The assembly process has two match steel dies in which the deformable material is placed between them and held under pressure at high temperature. Development of voids is eliminated due to the high pressure. The CM method can be used to produce small-dimensional materials. In addition, different kinds of shapes, sizes and complex materials can be fabricated by CM. Long unidirectional and short-fibre-based polymer composites can be


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Figure 3. Fabrication methods (a) compression (b) extrusion and (c) injection moulding machine. Table 4. Mechanical properties of permanganate treated sisal textile-reinforced vinyl ester composites made by RTM and CM methods. Preparation method RTM CM

Tensile strength (MPa) 31.80 30.38

Flexural strength (MPa) 72.70 57.49

Impact strength (kJ/m2) 28.81 25.13

Source: Adapted from Li [79]. RTM: Resin transfer moulding, CM: Compression moulding.

produced, hence another merit of CM over the other methods mentioned earlier. Various researches have been carried out using this method by material researchers.[76–78] Li [79] worked on two different methods in order to prepare sisal textile materialreinforced composites. The effects of fibre content and fibre treatment on the permeability of sisal textile were investigated. Two different chemicals were used for treating the fibres, namely γ-methacryloxpropyl silane (CH2CH3CCOO(CH2)3Si(OCH3)3) and permanganate (KMnO4). The effect of both treatments were investigated on the tensile, flexural and impact strength of sisal textile-reinforced composites prepared by CM and resin transfer moulding (RTM). It was observed that RTM method produced a better result than CM method, as shown in Table 4. RTM is an efficient and economical method to make sisal textile-reinforced composites. The chances of void formation in the composites are very low, given rise to better mechanical properties compared to CM.

Mechanical properties of fibre/polymer composites Mechanical properties have always been the concern of many researchers when it comes to fibre-reinforced composites. Different methods, such as tensile strength,[53,80] flexural


23


strength [81] and impact strength,[82] have been adopted in order to improve the mechanical properties. For the sake of this review, the following mechanical properties, tensile strength, tensile modulus, flexural strength, flexural modulus and impact strength, will be discussed extensively. Tensile strength is the maximum stretching force (or stress) that a material can withstand before breaking or tearing apart. It is measured in terms of force per unit area. The tensile strength of fibre can be determined theoretically by the following equations:

𝜎f =

Fc Af

(1)

Af =

m 𝜌L

(2)

where, 𝜎f is the tensile strength of fibre, Fc is the force at failure, m is the mass of fibre, 𝜌 is the density of fibre, Af is the average cross-sectional area of fibre and L is the length of fibre. The overall tensile strength of a composite can be determined when the strength of the fibre and the matrix are known, using the mixture rule for composites,[83,84] as shown in Equation (3).

𝜎c = 𝜎f Vf + 𝜎m (1 − Vf )

(3)

where 𝜎c the strength of the composites is, 𝜎f is the strength of the fibre, 𝜎m is the strength of the matrix and Vf is the volume fraction of fibre. Flexural strength is the ability of a material to resist deformation under a bending load and it is measured in terms of stress. It represents the highest stress experienced by the material at the rupture load.[85]

Flexural strength =

3wL 2bd 2

(4)

Flexural modulus =

mL3 4bd 3

(5)

where b is the breath of the specimen, d is the thickness of the specimen, L is the length of the specimen, m is the slope of a load–deflection curve and w is the maximum load. Flexural strength is also known as bending strength, while flexural modulus is also known as bending modulus. There are two methods used for the determination of the flexural properties of a material, namely three-point and four-point loading systems. Equation (4) shows the formula for rectangular samples under a three-point load set-up, according to American Society for Testing and Materials (ASTM) D638.[86,87] The impact strength of a composite material is its ability to absorb and dissipate energy in the form of creating of new surfaces under shock or sudden blow.[88] According to Shubhra et al. [3], there are two frequently used testing methods for determining impact strength; Charpy test and Izod test. These methods can be employed according to ASTM D6110 and ASTM D256, respectively. Chattopadhyay et al. [89] used notched Izod impact tester with a notch angle of 45° and depth of 2.54 mm. The unit of impact strength is kJ/m2.


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Figure 4. Tensile strength of sisal fibre/matrix treated with various concentrations of NaOH solution.

Figure 5. Tensile modulus of sisal fibre/polymer treated with various concentrations of NaOH solution.

The mechanical properties of different sisal fibres used in polymer composites have been studied by many researchers and are summarized in Figures 4–7. Kaewkuk et al. [42] had worked on sisal fibre/polypropylene composite, Boopalan et al. [55] worked on jute and sisal fibre-reinforced polymer composite, Khanam et al. [62] also worked on sisal/polypropylene



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Figure 6. Flexural strength of sisal fibre/polymer treated with various concentrations of NaOH solution.

Figure 7. Flexural modulus of sisal fibre/polymer treated with various concentrations of NaOH solution.

composites, Nayak et al. [56] worked on sisal fibre-recycled polypropylene composites, while Rong et al. [53] worked on unidirectional sisal-reinforced epoxy composites, their findings are summarized in Figures 4–7. They all treated the fibre chemically with alkali (NaOH) at different concentrations in order to improve the interfacial bonding between


26


the fibre and the polymer matrix used. Kaewkuk et al. [42] used sisal fibre obtained from Thailand and treated the fibre with 2% NaOH for 2 h and then dried it in an oven at 60 °C overnight. Three different fibre contents viz: 10, 20 and 30% sisal fibre with polypropylene were studied. The 30 wt% fibre content gave the best property in terms of tensile strength and Young’s modulus, with values of 26 MPa and 0.99 GPa, respectively. Boopalan et al. [55] compared the tensile and flexural strengths of jute and sisal fibre treated with 20% NaOH. The treated sisal fibre gave a marginally better flexural strength of 82.3 MPa, which is ~2.75% more than jute fibre. Joseph et al. [90] worked on short sisal fibre-reinforced polypropylene composites. It was reported that different treatments influence the tensile strength and tensile modulus. Sisal fibre was treated with alkali (NaOH), polypropyleneglycol derivative of toluene diisocyanate (PPG-TDI), poly[methylene poly(phenyl isocyanate)] (PMPPIC), maleic anhydride (MAPP), benzoyl chloride treatment (benzoylation) and permanganate (KMnO4) treatment. The best result was observed when MAPP treatment method was done, followed by PMPPIC. With the treatment methods, NaOH showed the least tensile strength and tensile modulus, but better than the untreated sisal fibre-reinforced polypropylene composites. It was also reported that increase in fibre loading led to increase in tensile strength and tensile modulus. For each treatment method used, optimal strength was observed with 30 wt% fibre loading. Rajesh et al. [91] treated sisal fibre with 10% NaOH and prepared composites of sisal fibre/ PLA at different fibre content (5, 10, 15, 20 and 25 wt%). It was reported that composites with 20 wt% treated fibre content showed better tensile strength and flexural strength of 30 and 17% higher than neat PLA, respectively, while the impact strength was 107% higher than the neat PLA. Increasing fibre content of treated fibre resulted in a decrease of the impact strength. The decrease in impact strength can be attributed to enhanced interfacial adhesion between the fibre and the polymer, instead of fibre pull-out, the composite fractured on receiving mechanical shock. Jeencham et al. [92] reported that the addition of 40 phr zinc borate (ZnB) to untreated sisal fibre/PP composites improved the mechanical properties, only slightly, when compared with untreated fibre without ZnB. They reported the tensile strength, tensile modulus and impact strength of: 32.62 ± 0.53 MPa, 2.25 ± 0.10 GPa and 12.96 ± 0.20 kJ/m2, respectively. ZnB was added in order to enhance the flame retardancy of the composites, but recorded only a slight improvement in the mechanical properties and thermal stability of the composites. This work could be improved upon by treating the fibre and then adding the flame retardant. In a similar research with treated sisal fibre, there was no significant difference in the mechanical properties of alkali (NaOH)-treated fibre and untreated fibre/ PP composites.[93] From Figures 4–7, it is clearly shown that unidirectional sisal fibre/epoxy composite gave the best tensile strength in a report by Rong et al. [53], who treated the fibre with 2% NaOH alkali with a 58 wt% fibre content and using CM method to fabricate the composite. They had better result in terms of tensile modulus, flexural strength and flexural modulus of 5.55 GPa, 311.5 MPa and 23.75 GPa, respectively. Khanam et al. [62] also recorded high mechanical properties; however this was not as high when compared with Rong et al.’s [53] data, as seen in Figures 5–7. The huge disparity in the results may be due to the fibre content, chemical treatment, mode of fabrication or the type of matrix used and an admixture of any of these.


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Methods of improvement of mechanical properties of fibre/polymer composites


Fibre treatment Belgacem and Gandini [94] categorized fibre treatments into three major parts, namely physical treatments, physicochemical treatments and chemical grafting. Physicochemical treatments include corona, plasma, laser, vacuum-ultraviolet and γ-ray treatment with the aim to purify, oxidize or activate the surface of the lignocellulose fibres. The reinforcing ability of natural fibres is usually enhanced by fibre treatment, which leads to increases in the properties of fibre-reinforced resulting polymer composites. Fibre treatment is very necessary, since there is a weak interfacial reaction and poor wettability that exist between untreated natural fibres and polymer matrix. Natural fibres are generally hydrophilic in nature because of the presence of high –OH groups in the fibres and thermoplastics, on the other hand, are hydrophobic. Fibre treatment is carried out in order to improve the fibre/ polymer interfacial adhesion and hence increase the interfacial reaction between fibres and matrix. Many works have been done on fibre treatment as a means of improving the mechanical properties. Figures 8 and 9 show the influence of chemical treatment on water absorption of fibre/polymer composites. Kanny and Mohan [95,96] treated sisal fibre with 40 g of NaOH–clay solution. The solution was prepared by adding clay to 40 g of NaOH and stirred vigorously until the clay dissolved completely in the solution. They immersed sisal fibre in the resulting solution for 1 h and then dried for 4 h at 60 °C. They found out that the tensile strength, tensile modulus and strain of sisal/polypropylene increased by 14, 18 and 14%, respectively. NaOH–clay-treated fibre led to improvement in water uptake from 12.5 to 10.3% when compared with untreated fibre.

Figure 8. Effects of sisal fibre content on the water absorption property of sisal fibre/urea-formaldehyde resin composites. Source: Adapted from Zhong et al. [100].


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Figure 9. Water absorption of sisal fibre/polymer treated with various concentrations of NaOH solution.

Kaewkuk et al. [42] worked on two methods of treatment. Firstly, treated sisal fibre with 2 wt% sodium hydroxide for 2 h; after 2 h, the fibres were washed with distilled water in order to remove every traces of alkali and dried overnight at 60 °C. Secondly, they heattreated the untreated sisal fibres at a temperature of 170 °C under atmospheric pressure, in the presence of air for 30 min in an oven. The untreated sisal fibre/PP composite produced the least tensile strength, Young’s modulus and impact strength, while the heat-treated sisal fibre/PP composite gave the best result in terms of tensile strength, Young’s modulus and impact strength. NaOH-treated fibre/PP had the least water absorption during a 90-day immersion time, while the untreated sisal fibre/PP composite had the highest water absorption. Mishra and Naik [97] treated sisal fibre with maleic anhydride (MA) in order to improve the mechanical properties. It was reported that the treatment improved the tensile strength, tensile modulus and flexural modulus, as show in Table 5. The treated sisal fibre/polystyrene composites also showed better impact strength of ~17 Nm/cm2; similar result was obtained with the hardness (shore-D) test of ~78. For the samples prepared, there was a general trend in the properties; as the fibre content increased from 40 to 55 wt%, the mechanical properties decreased, which could be attributed to too many fibre–fibre contacts. Wu et al. [98] prepared untreated and different treated sisal fibre-reinforced composites by novel vane extruder. This treatment was observed to have improved the tensile strength, as presented in Table 6. All the treatment methods were shown to have improved the flexural strength, except for ASF/PP and MSF/PP composites. Young’s modulus of the composites was enhanced for all composites with the exception of MSF/PP and MSF/MPP when compared with untreated sisal fibre composite. The impact strength was observed to have been improved by MSF/PP, SF/MPP and MSF/MPP treatments, except for ASF/PP, AMSF/PP, ASF/MPP and AMSF/MPP, which showed lower impact strength compared to the untreated SF/PP composites. The fibres became so fragile due to the removal of impurities from the


29

Table 5. Mechanical properties of untreated and treated sisal fibre/polystyrene composites. Composition polystyrene/ fibre (wt%) 100/00 60/40 55/45 50/50 45/55 60/40 55/45 50/50 45/55

Samples Polystyrene Untreated sisal fibre Treated sisal fibre with MA

Tensile strength (MPa) 20.64 8.63 6.97 3.30 2.79 9.89 7.85 5.51 4.16

Tensile modulus (MPa) 1226 1308 900 660 466 1705.66 1274.91 852.78 555.11

Flexural modulus (MPa) 3416 3047 2896 2793 2470 4565 4246 3747 3217

Elongation (%) 3.35 3.00 2.75 2.50 2.00 2.75 3.25 2.75 2.75


Source: Adapted from Mishra and Naik [97].

Table 6. Effect of surface treatment on the mechanical properties of the sisal fibre-reinforced composites. Composites SF/PP ASF/PP MSF/PP AMSF/PP SF/MPP ASF/MPP MSF/MPP AMSF/PP

Tensile strength (MPa) 25 ± 0.78 30 ± 0.79 26 ± 0.06 29 ± 1.20 33 ± 0.20 35 ± 0.60 31 ± 0.70 37 ± 0.80

Young’s modulus (MPa) 566 ± 15 797 ± 11 513 ± 26 588 ± 31 681 ± 40 839 ± 39 412 ± 50 903 ± 44

Flexural strength (MPa) 39.0 ± 1.2 38.1 ± 0.7 38.3 ± 0.7 40.7 ± 0.8 46.0 ± 1.0 44.2 ± 0.5 44.1 ± 1.1 44.1 ± 0.8

Flexural modulus (GPa) 2.320 ± 0.075 2.000 ± 0.049 1.819 ± 0.055 1.742 ± 0.050 1.928 ± 0.033 2.159 ± 0.060 1.992 ± 0.070 2.251 ± 0.057

Source: Adapted from Wu et al. [98]. SF: sisal fibre; ASF: alkali-treated sisal fibre; MSF: maleic anhydride-treated sisal fibre; AMSF: alkali and maleic anhydride-treated sisal fibre; PP: polypropylene; MPP: the ratio of polypropylene and maleic anhydride-grafted polypropylene (7:1) by weight.

surface of the fibre, which is as a result of the alkali treatment. Untreated fibres maintained their natural properties, which were better than treated fibre. Due to the impurities on the fibre surface, the fibre makes relative displacement within a certain range on the fibre–polymer interface. As a result of this, the energy-absorbing capacity of the composites is greatly increased. Li et al. [99] reported that silane and permanganate treatments improved the tensile strength, tensile modulus, flexural strength and flexural modulus, but reduced the impact strength of sisal textile-reinforced vinyl-ester. Fibre content It has been observed that the change in the fibre content often leads to change in properties of fibre-reinforced polymer composites. Most times, an increase in the fibre content will lead to an increase in the strength and modulus,[26] but this can also lead to more water uptake of the fibre-reinforced polymer composites, due to the hydrophilic nature of the fibres. Zhong et al. [100] worked on sisal fibre/urea-formaldehyde thermoset resin composites and varying the fibre content thus: 30, 40, 50, 60 and 70 wt%. Treatment of sisal fibres with 2% NaOH was done in water at a controlled temperature of 22 ± 2 °C for 24 h and then washed with distilled water, left to dry at room temperature and later dried in an oven at 70 °C for 15 h. Melamine (C3H6N6) was added to improve the interfacial bonding


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between the fibre and polymer matrix. CM method was used for the fabrication of the composite. The flexural strength, flexural modulus and water absorption were: 58.58 MPa, 7.63 GPa and 0.98%, respectively, when 30 wt% of fibre content was used and this presented the best result. The highest impact strength of 9.42 kJ/m2 was recorded when 50 wt% sisal fibre content was used. Chow et al. [101] worked on sisal/PP composites in order to confirm the effect of fibre content on the tensile strength, Young’s modulus and water uptake. It was observed that the more the fibre content in the composite for prolonged immersion time in water at 90 °C, the greater the decrease in tensile strength and Young’s modulus. Increase in fibre content also led to an increase in water uptake, due to the hydrophilic nature of natural fibres. Joseph et al. [102] worked on three different thermosets (polyester, epoxy and phenolformaldehyde), varying the fibre content and compared these thermosets with LDPE. It is observed that varying the fibre content showed increases in the mechanical properties of all composites prepared. Sisal/phenolic composites showed better tensile and flexural strength among the thermoset group, while sisal/polyester composite was observed to have higher impact strength. The flexural strength of fibre/thermoset-based composites was observed to decrease at high fibre content above 45% volume, which could be explained due to high level of fibre–fibre interaction. Based on the comparison, sisal fibre-reinforced thermoset composites showed better tensile strength than the SF/LDPE composite. Bakare et al. [103] also observed the same trend in sisal fibre-reinforced rubber seed oil-based polyurethane composites. In a recent research, Zhao et al. [104] observed increases in the tensile strength and tensile modulus, while impact strength decreased with increasing fibre content (10, 20 and 30 wt%) for untreated sisal fibre/PE composites that were prepared through pre-impregnation method. In a composition of PE/30%SF/15%MAPE, the tensile strength, tensile modulus and impact strength were 39.3 ± 0.9 MPa, 3.53 ± 0.22 GPa and 14.4 ± 0.5 kJ/m2, respectively. The decrease in impact strength is due to the improvement in the interfacial shear strength. The interfacial shear strength lowers the impact strength and improves the tensile strength.[105] Sreekumar et al. [77] prepared sisal/polyester composites using two methods of composite fabrication, viz. RTM and CM. From their report, it was shown that increase in fibre content showed increased tensile strength and Young’s modulus for both methods employed for the fabrication. Four different fibre loading for RTM (19, 27, 43 and 50 wt%) and CM (24, 34, 42 and 48 wt%) were used and observed that the tensile strength and Young’s modulus increased when there was 43 and 42 wt% fibre loading, respectively. The tensile strength and Young’s modulus for RTM was 67 ± 2.33 MPa and 2196 ± 5.42 MPa, respectively, while that of CM was 57 ± 2.06 MPa and 1868 ± 1.28, respectively. The fibre length was maintained at 30 mm. The least strength observed at the least fibre content is due to poor stress transfer by the fibre acting as the reinforcement. At high fibre content above 42 wt% (CM) and 43 wt% (RTM), the properties start to drop, which is as a result of strong fibre–fibre interaction, leading to void and poor wettability between the fibre and the resin. For both methods employed for the fabrication, it was observed that RTM method showed better mechanical properties when compared to CM method, due to the possible elimination of voids in the composites during preparation. Sangthong et al. [106] worked on admicellar-treated sisal fibres-reinforced unsaturated polyester composites. Similar trend was observed as fibre content increased from 10, 20 to 30 %v/v, there were increases in tensile strength, tensile modulus, flexural strength and flexural modulus. Further increase in fibre content showed decrease in the mechanical


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properties of all composites, except for hardness and impact strength. Fibre treatment and fibre content were observed to have influenced the strength of the composites. The increase in the mechanical properties with increase in fibre content could be attributed to the good stress transfer of the reinforcing fibre [77] and having similar polar behaviours.[107] Kanny and Mohan [96] reported that increase in fibre content increases the tensile strength and tensile modulus of SF/PP and SF/epoxy composites, while the water absorption of the SF/PP and SF/epoxy composites also increased with increasing fibre content and immersion time. PP is known to be more hydrophobic in nature than epoxy resin, thus results in lower water absorption of SF/PP composite.

Conclusion Natural fibres are considered as possible replacement for synthetic fibre in fibre/polymer composite. Natural fibres have significant advantages over synthetic fibres, in terms of the positive environmental impact, low cost, low density and their biodegradability. The major problems of natural fibres are their affinity for water (hydrophilic nature) and their incompatibility with thermoplastic and thermoset resins. The surface of fibres can be modified in order to improve the interfacial bonding and adhesion between the fibre and the polymer matrix. The fibres are, most often, subjected to various chemical treatments, such as alkalization, acetylation, glycidyl methacrylate and strontium titanate, in order to enhance fibre–polymer adhesion, which will normally bring about improvements in the mechanical properties, water absorption and thermal stability. Except for sodium hydroxide, the other treatment methods are rarely reviewed. Increase in fibre content leads to improved mechanical properties, but causes more water uptake, due to the hydrophilic nature of natural fibres, generally. Future research should focus on further improving the properties of sisal fibre-reinforced polymer composites by adopting various treatment methods (if need be an admixture of two or more chemicals), such as alkalization/mercerization, acetylation, oxidation, heat-treatment and diazotization, for better interfacial adhesion between sisal fibres and polymer matrix. In addition, future work is needed on the methods of fabrication for adequate and proper impregnation and fibre loading for optimum improvement of the mechanical properties of fibre/polymer composites.

Disclosure statement No potential conflict of interest was reported by the authors.

Funding This research received no specific grant from any funding agency in the public, commercial or notfor-profit sectors.

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