A comparative analysis on tensile strength of dry and moisture absorbed hybrid kenaf/glass polymer composites

Abstract

Economic and environmental concerns lead the researchers toward development of sustainable and renewable materials of which reinforced composites are part of. The abundantly available natural fibers have attracted the researchers to study their performance as reinforcements and feasibility for making automobile components. The performance of composite materials is mainly assessed through their mechanical properties. However, natural fibers to date were mainly used as reinforcements to create bulk composite components with reduced cost rather than improved mechanical performances. Among the methods available for improving mechanical properties of the natural fiber composites, combined mercerization treatment, hybridization, and incorporation of fly ash fillers in the matrix are the best solutions. Therefore, the objective of this research is to evaluate the tensile properties of hybrid kenaf/glass composites with and without fly ash particulate filler as per ASTM standards. Moisture absorption behavior and its effect on the tensile properties of hybrid composites are also investigated. The results revealed that the addition of 10wt % fly ash particles with natural fiber composites increased the tensile strength of composites while hybridization with glass fibers reduced the water absorption properties.

Keywords

Fly ash hybrid composite kenaf fabric mercerization moisture

Introduction

The increasing need of materials leads to new inventions. One of the most favorable inventions is the concept of composites [1]. The traditional composite materials manufactured by synthetic glass and carbon fibers as reinforcement are extensively being used in automotive applications due to their less weight, specific strength, and stiffness [2]. Despite these advantages, synthetic glass fiber composites lack in various aspects like recycling, reusability, and biodegradability problems at the end of their useful life. Hence, new environmental regulations have forced the researchers to search for new materials that can substitute the synthetic fibers. Natural materials are one of the most prominent alternatives for synthetic fibers for making automobile components due to their advantages of eco-friendliness, lightweight, renewability, and low emission of pollutants [3]. As a result, the demands for natural fibers have increased drastically due to its desired characteristics when compared to synthetic fibers. One of the distinguished natural fibers in India is kenaf fiber. The ease of availability and extraction of kenaf fibers attracts the attention of researchers to study its performance and feasibility as reinforcement, particularly in making automobile components [4 –6]. The performance of natural fabric composite may be affected due to the reduction in fiber/matrix compatibility [7]. When natural fiber composites are exposed to a humid environment, water molecules enter and get attached onto the hydrophilic groups of fiber, forming intermolecular hydrogen bonding with fiber and reduce fiber–matrix compatibility [8].

The tensile and moisture absorption properties of natural fiber polymer composites are significantly influenced by the interfacial bonding between the fiber and the matrix. In order to enhance the interfacial bonding between them, adhesion has to be improved. Fiber surface modification is one of the simplest methods to improve the adhesion properties. Fiber treatments using alkaline solutions have been applied by several researchers [9–10] to improve the fiber’s surface adhesion characteristics by eliminating impurities such as hemicellulose, lignin, and pectin. However, researchers focused on the effect of alkali treatment on unidirectional natural fibers [11–12], but research on woven fabric is very rare. Mylsamy and Rajendran [13] analyzed the effects of alkali treatment (Mercerization) on the mechanical properties of short agave (Sisal) fiber reinforced epoxy composite. They reported that alkali-treated fiber composites exhibited higher tensile strength than the one with untreated fibers. Bakar et al. [14] investigated the influence of alkali treatment to the mechanical properties of the kenaf/epoxy composite. The results revealed that the impact strength improved by 14% and flexural strength by 24%, respectively. Though numerous studies were available for surface modification of natural fiber, in addition there are various factors that also affect the interfacial properties such as maturity of fiber, harvest, and climatic conditions. Hence an understanding of alkali treatment on woven fabric is necessary for developing the natural fabric reinforced composites with enhanced interfacial bonding.

The synthetic fibers have better resistance to moisture absorption, whereas the natural fibers aren’t. Hence a natural fiber can be combined with a synthetic fiber so as to take the best advantage of the properties of both the fibers as well the advantages of one type of fiber could compensate for the lapse in the other one. Consequently, desired performance can be achieved through proper material design. Raghavendra et al. [15] have experimentally evaluated the tensile properties for glass–jute–glass–jute hybrid and glass–jute–jute–glass hybrid composites and they observed that the tensile strength of the specimens with the glass fibers at extremities showed higher values than the other specimens considered. Ghani et al. [16] conducted an experimental study on the effect of alkali treatment on the tensile properties of kenaf/glass reinforced hybrid composites and revealed that the alkali-treated hybrid composites showed improved tensile strength because of the removal of hemicellulose content from the kenaf fibers. From the literature,it is found that the enhancement of tensile properties depends on fiber weight/volume fraction, stacking sequence of the fiber layers, treatment of fiber, and the effect of environmental conditions. However there is limited research on considering the influence of these factors on the improvement tensile properties of the hybrid composites. Against this background, this research is proposed with an objective to explore the potential of hybrid kenaf/glass composites and the effects of hybridization on the tensile properties of composites.

In addition, the utilization of agro wastes [17] and industrial wastes [18] as filler reinforcements in hybrid composite has shown promising effect on the improvement of mechanical properties of the composite. All over the world, waste management is an evident problem nowadays [19]. The accumulation and disposal of industrial wastes are of national and international concerns. Fly ash is hazardous to the environment and improper disposal can cause serious ailment like lung cancer [20]. Due to its harmful effects, considerable research works have been undertaken worldwide to find its alternate usage. Over the last few decades, efforts have been made to utilize fly ash in huge quantities for road construction, land filling, and producing building materials [21]. In recent years, the utilization of fly ash as a filler in metal matrix composites and polymer matrix composites has received appreciable attention. This leads to lower consumption of costly matrix material and also helps in altering the overall properties of the composite.

Sengupta and Shubhalakshmi [22] studied the effect of fly ash fillers on the tensile properties of polypropylene composites and found that tensile strength reduced due to fly ash addition. Gupta et al. [23] studied the effect of fly ash on impact resistance and compressive strength of fiber reinforced fly ash composite and concluded that with a small amount of fly ash, compressive strength decreased but increased fire resistance. From the above research works, it was noticed that the concentration of fly ash fillers had both positive as well as negative effects on improving composite properties. Therefore, it is important to ascertain the limitation of fly ash concentration for its effective usage to get the desired properties. However, there were very few research works [24,25] which examined the effect of fly ash on mechanical properties of hybrid polymer composites. This study is focused on the hybridization of kenaf/glass fabric with the addition of fly ash fillers to enhance the desired tensile properties of the green composite by reducing the synthetic fiber usage and encourage the natural materials consumption.

In the present research, hybrid kenaf/glass polymer composites are fabricated with and without reinforcing fly ash particles. Pure kenaf fabric composite is also fabricated for comparison purpose. The combined effect of mercerization treatment and the addition of fly ash particles on the tensile and moisture absorption properties of the hybrid kenaf/glass polymer composites have been investigated experimentally.

Materials

Woven kenaf fabric

Kenaf (Hibiscus cannabinus L.) is a new renewable source of industrial purpose in those countries that have better economies. It is a well-known cellulosic source with economic and ecological advantages. It exhibits low density, nonabrasiveness during processing, specific mechanical properties, and biodegradability. The plain-woven kenaf fabric with 300 GSM was purchased from M/s. Anakaputhur Weavers Association, Chennai, India. The properties of kenaf fabric are summarized in Table 1 [26].

Table 1.

Properties of kenaf fabric [26].

	Value
Chemical composition
Lignin (%)	20.1
Cellulose (%)	44.4
Ash (%)	4.6
Mechanical properties
Tensile modulus (GPa)	53
Tensile strength (MPa)	350–930
Elongation at break (%)	1.6

Woven E-glass fabric

E-glass fabric is one of the most commonly used synthetic fabrics, manufactured with the raw materials such as limestone, silica, clay, fluorspar, and dolomite [2]. These ingredients are melted and extruded through bushings which have multiple small orifices to obtain filaments. A commercially available bidirectional E-glass fabric with 610 GSM was purchased from M/s. Sakhti fiber Glass Ltd, Chennai, India. The properties of E-glass fabric are presented in Table 2 [26].

Table 2.

Properties of E-glass fabric [26].

Property	Value
Density (g/cm³)	2.62
Tensile strength (MPa)	3450
Tensile modulus (GPa)	70
Tensile elongation (%)	2.5–3.1

Epoxy resin

To achieve the better reinforcing effect in the composite, it is necessary to have good adhesion between the fabrics and resin. Epoxy resin is known to form covalent cross-links with plant cell walls via –OH groups with low viscosity that can cure at room temperature. Moreover, it has low polymerization shrinkages during cure, good mechanical strength, excellent resistance to chemicals/solvents, and excellent adhesion to fibers. Epoxy LY556 with the hardener HY951 were used as matrix materials in this study. The properties of the epoxy resin are given in Table 3 [2].

Table 3.

Properties of epoxy resin and hardener [2].

Sl. No.	Properties	Epoxy resin	Hardener
1.	Appearance	Clear liquid	–
2.	Viscosity at 25℃ (mPa s)	10,000–12,000	50–100
3.	Density (g/cm³)	1.15–1.20	1.20–1.25
4.	Tensile strength (MPa)	83–93	–
5.	Tensile modulus (GPa)	0.3–0.6	–

Fly ash

The fly ash particles utilized in this study were procured from the Ennore Thermal Power Plant, Chennai, India. The chemical composition of fly ash is presented in Table 4. The predominant elements present in the fly ash comprise silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), and calcium oxide (CaO). Lesser amounts of the oxide elements such as potassium, magnesium, and manganese were also observed. As the total content of silica and alumina together is more than 70%, the fly ash can be confirmed as the class-F category (ASTM C618). The fly ash particles were sieved using 500 µm sieve by a sieve shaker. The particles passing through sieve were collected and used for preparing composites with varying weight percentage.

Table 4.

Chemical composition of class-F fly ash.

Elements	SiO₂	CaO	Al₂O₃	Fe₂O₃	MgO	Na₂O	K₂O	SO₃	MnO	SiO₂ + Al₂O₃ + Fe₂O₃
% by mass	63.6	2.57	28.2	2.99	1.54	0.05	0.003	0.26	0.03	94.78
ASTM C618 class-F	–	<10	–	–	5 (Max.)	–	–	–	–	70 (Min.)

The morphology of fly ash particle was examined through the scanning electron microscope (SEM). Figure 1 shows SEM micrograph of the fly ash sample with magnification 1500X. The SEM micrograph revealed that the particles were composed of nearly spherical shape which may be called cenospheres. These particles range from 10 to 15 µm in diameter.

Figure 1.

SEM micrograph of the fly ash. SEM: scanning electron microscope.

Methods

Mercerization of kenaf fabric

Alkaline treatment also known as mercerization is one of the most commonly used chemical treatments of natural fibers. This treatment removes a certain amount of hemicellulose, lignin, wax, and oils covering the external surface of the fiber cell wall [9]. A study [10] on mercerization treatment of kenaf fabrics with different concentrations (3, 6, and 9%) of sodium hydroxide (NaOH) solution revealed that 6% NaOH yields the optimum concentration for the mercerization treatment. This result directed the authors to treat the kenaf fabrics with an alkaline solution of 6% NaOH prior to fabrication of composite laminates.

NaOH aqueous solution of 6% in wt was prepared by dissolving 120 g of NaOH pellets in 2 l of water in a tray. The kenaf fabric to be treated was kept in the prepared alkaline solution for 3 h and then dried at room temperature (27℃) in a clean chamber for 24 h. The kenaf fabric was then washed with distilled water and left to dry at room temperature overnight followed by hot air oven drying for 8 h at 60℃.

Composite manufacturing

There are many techniques available in the industry for manufacturing composites such as hand layup [25], compression molding [27], vacuum molding [28], pultruding [29], and resin transfer molding [30]. Among these, the hand layup technique is one of the simplest, economical, and easiest methods for manufacturing composites [2]. In this study, all composite specimens were manufactured using hand layup process. Initially the epoxy resin was mixed with micron size fly ash particles at various weight percentages (5, 10, and 15%) and was physically blended by stirring for 5–10 min. Then the hardener was added in the ratio of resin:hardener (10:1) and it was stirred for 10 min to prepare a homogeneous mixture. Wooden mold made of plywood of size 310 mm × 310 mm × 5 mm was used for the preparation of composite laminates. An initial layer of the mold was filled with the prepared homogeneous mixture and fabric was laid over them. The above process was repeated for four layers. The layering sequence of the prepared natural and hybrid laminates is shown in Figure 2. The prepared composite laminates were cured at ambient temperature for a day by applying compression pressure using a dead weight of 25 kg at the top of the mold.

Figure 2.

Layering sequence of natural and hybrid composites.

During fabrication of fiber reinforced composites, the trapped air or other volatiles may exist in the composites which lead to the reduction in tensile properties. The percentage of voids (V_c) in the composite was calculated by using equation (1) according to ASTM: D2734 standard where ρ_ct and ρ_ce are the theoretical and experimental density of the composite samples, respectively.

The theoretical density $(ρ_{ct})$ of the composites was calculated using a modified form of Agarwal equation [31] (2)

ρ_{ct} = \frac{1}{(\frac{W_{f}}{ρ_{f}} + \frac{W_{r}}{ρ_{r}} + \frac{W_{fi}}{ρ_{fi}})}

(2)

where W_f, W_r, W_fi represent the weight fraction and

ρ_{f}, ρ_{r}, ρ_{fi}

represent the density of fiber, resin, filler, respectively. The experimental density of the composite samples and kenaf fabric was conducted according to ASTM: D792 standard.

Table 5 shows the void content of various fabricated composite samples reinforced with fly ash particles. From the results, it was observed that the experimental density of the composites was low when compared to the theoretical density for all composite samples. Moreover, the values of experimental density increased with increasing fly ash content.

Table 5.

Void content of kenaf/glass hybrid composite samples.

Sl No.	Designation	Weight fraction (%)			Theoretical density ρ_ct (g/cc)	Experimental density ρ_ce (g/cc)	Void content V_c (%)	Thickness of composite (mm)
Sl No.	Designation	Fiber	Resin	Filler	Theoretical density ρ_ct (g/cc)	Experimental density ρ_ce (g/cc)	Void content V_c (%)	Thickness of composite (mm)
1.	KNM	20	80	0	1.202	1.180	1.83	4.5
2.	KM	20	80	0	1.202	1.181	1.74	4.6
3.	KM5	20	75	5	1.228	1.205	1.87	4.7
4.	KM10	20	70	10	1.236	1.209	2.18	4.8
5.	KM15	20	65	15	1.262	1.226	2.85	4.8
6.	2KG	35	65	0	1.262	1.251	0.79	4
7.	2KG5	35	60	5	1.269	1.256	1.02	4.5
8.	2KG10	35	55	10	1.273	1.262	0.86	4.7
9.	2KG15	35	50	15	1.295	1.264	2.39	4.7

Note: KM: mercerized kenaf fabric composite without fly ash; KM5: kenaf fabric composite with 5% fly ash; KM10: kenaf fabric composite with 10% fly ash; KM15: kenaf fabric composite with 15% fly ash; KNM: nonmercerized kenaf fabric composite without fly ash; 2KG: kenaf/glass fabric hybrid composite; 2KG5: kenaf/glass fabric hybrid composite with 5% fly ash; 2KG10: kenaf/glass fabric hybrid composite with 10% fly ash; 2KG15: kenaf/glass fabric hybrid composite with 15% fly ash.

The results also revealed that KM15 and 2KG15 composites showed higher void content. Higher weight fraction of fly ash particles resulted in increasing of void content of composites. This might have happened due to improper mixing of fly ash fillers with the epoxy matrix. However, the percentage of void content in hybrid composites is less when compared to natural composite samples. Similar kind of result was observed by Dalbehera et al. [25] in the fabrication of jute/glass/fly ash composites.

Testing

Moisture absorption test

Moisture absorption test is used to predict the durability of composites in a climate change environment. The effect of moisture absorption on prepared composites was studied in accordance with ASTM: D 570 standard. The tensile specimens cut from composite samples were dried in an oven at 105℃ for 12 h and were subsequently cooled for an hour. The initial weight of the dried specimens was measured using a digital electronic balance. The specimens were then immersed in distilled water at room temperature (∼27℃). The weight of the dipped specimens was measured for every 24 h until saturation. The percentage of water absorption was determined by using the formula given in equation (3) where W_a and W_b are the weight of the composite samples after and before soaking in water.

Tensile test

The tensile tests were conducted on flat dog-bone specimens in Instron Computerized Universal Testing Machine of 60 tons capacity. Tests were carried out at room temperature and each test was performed until a fracture occurred. The applied tensile load and extension were recorded during the test for the calculation of stress and strain.

The samples used for tensile tests were machined according to the ASTM D: 638 standard. The tests were performed with a constant strain rate of 5 mm/min and a gauge length of the specimen was 50 mm. For each test, five samples were tested and the average value was taken for analysis. The standard test specimen and few of the tested samples are depicted in Figure 3. In the macroscopic point of view, when kenaf/glass hybrid composite is subjected to tensile loading, kenaf fiber had stretching followed by fracture whereas in the glass fiber, fiber pullout due to stretching was observed. Conversely, the addition of fly ash particles in the hybrid composite, both the kenaf and glass fiber had a fracture with negligible deformation. The tensile test was also carried out in few specimens that were involved in the water absorption test to understand the effect of moisture on the tensile strength of the composites.

Figure 3.

Tensile test specimens. (a) Standard tensile test specimen, (b) KM specimen before testing, (c) KNM specimen after tensile test, (d) 2KG specimen after tensile test, (e) KM10 specimen after tensile test, and (f) 2KG10 specimen after tensile test. KM: mercerized kenaf fabric composite without fly ash; KM10: kenaf fabric composite with 10% fly ash; KNM: nonmercerized kenaf fabric composite without fly ash; 2KG: kenaf/glass fabric hybrid composite; 2KG10: kenaf/glass fabric hybrid composite with 10% fly ash.

Results and discussions

The combined effect of mercerization treatment and the addition of fly ash particles on the tensile and water absorption properties of the kenaf/glass hybrid polymer composites have been investigated experimentally and discussed as follows.

Effect of mercerization on tensile properties

To improve the tensile properties of kenaf fabric composites by enhancing the surface adhesion properties, kenaf fabrics were subjected to mercerization treatment at room temperature. These mercerized kenaf fabrics (KM) were used to fabricate composite specimens. Kenaf composite with nonmercerized fabrics was also fabricated for comparison purpose. These KM and KNM composites were tested under tensile loading and results were compared.

Figure 4(a) and (b) displays the stress–strain curves for KNM and KM composites, respectively. The stress–strain curve for both composites exhibited brittle behavior, characterized by a linear relationship between deformation and stress until failure, without noticeable plastic deformation. The strain to failure of the mercerized kenaf composite (KM) was slightly higher than the nontreated composites. This increase in strain indicates the introduction of ductility into the kenaf fabrics after the mercerization treatment [32]. Figure 5 shows the comparison of tensile strength and young’s modulus of mercerized (KM) and nonmercerized (KNM) kenaf fabric composites.

Figure 4.

Stress–strain curves of (a) nonmercerized and (b) mercerized kenaf fabric composites.

Figure 5.

Comparison on tensile properties of nonmercerized and mercerized kenaf fabric composites.

From the histogram, it is observed that KM composite specimens showed higher tensile properties as compared to KNM composite specimens. The tensile strength of KM specimens was 30 MPa while it was 23 MPa for nonmercerized, which is an increase of 30%. This is due to the reason that the mercerization treatment modified the surface properties of kenaf fabric which leads to improved interfacial bonding and mechanical interlocking between the mercerized fabric and matrix resulting in better tensile properties. The mercerization treatment improved the fiber surface adhesion characteristics by removing artificial impurities, thereby producing a rough surface structure [33]. Fiore et al. [12] studied the influence of mercerization treatment (6wt %) on tensile properties of kenaf fiber/epoxy composites and the authors reported that the mercerization treatment improved the tensile strength to 36% when compared to untreated kenaf fiber composites. The young’s modulus of KM specimen also increased about 21% for the KNM composite through mercerization treatment. This is due to the fact that the strong fiber/matrix interface bonding increased the load transfer between fiber and matrix [34]. Fourier transform-infrared characterization helps to understand the removal of undesirable materials from the kenaf fiber by the mercerization treatment.

The FTIR spectra depicted in Figure 6 are evident through the changes of functional group of KNM and KM. The increased intensity of the OH peak (3291.52 cm⁻¹) on mercerized fabric compared to the nonmercerized fabric is clearly seen. The characteristic 1732.91 cm⁻¹ peak corresponds to the carbonyl (C=O) stretching vibration in hemicellulose [33]. The peak at 1239.51 cm^–1 is relative to the (C–O) vibration of esters, ethers, and phenolic groups attributed to the presence of lignin on the fiber surface [35]. The disappearance of peaks at 1732.91 and 1239.51 cm^–1 in mercerized fabric indicates the elimination of hemicellulose and lignin as a result of mercerization treatment. The same observation was also attained by the researchers [35 –37].

Figure 6.

IR spectra of nonmercerized and mercerized kenaf fabric. IR: infrared.

Effect of fly ash fillers on tensile properties

The effect of micro sized fly ash particles (5, 10, and 15%) on the tensile strength and young’s modulus of KM composites was studied. Figure 7 illustrates the stress–strain curve for KM10 composite specimens under tensile loading conditions. All the specimens exhibited a linear elastic behavior until breakage and the slope of the stress–strain curve increased upon addition of fly ash. The incorporation of fly ash particles in KM10 composite showed improved tensile properties than the KM composites. Figure 8 shows the comparison of fly ash addition on the tensile strength and young’s modulus of KM composites.

Figure 7.

Stress–strain curves for KM10 specimens. KM10: kenaf fabric composite with 10% fly ash.

Figure 8.

Comparison of tensile properties of kenaf fabric composites with fly ash fillers.

It can be inferred from the plot that KM5 (5 wt. of fly ash) composites improved the tensile strength (about 10%) and young’s modulus (about 17%) compared with the KM composites. With further increase of the fly ash content to 10%, the tensile strength and tensile modulus were increased to 32 and 40%, respectively. This is due to the fact that the addition of micro fly ash filler would restrain movement of the epoxy matrix which is responsible for elongation. In essence, the epoxy matrix transfers some of the applied stress to the fly ash particles, which bear a fraction of the load. This subsequently increases the tensile strength and stiffness of kenaf fabric composites. The observed results were in good agreement with other research work involving bamboo fiber/epoxy/fly-ash hybrid composites [24] and jute fiber/epoxy/fly-ash particulate composites [25]. However, the tensile properties decreased beyond 10% of fly ash addition. The decrease in the tensile properties of the KM15 composites was probably caused due to the poor interaction between fly ash filler and epoxy matrix. A similar observation was found by Jacob et al. [37] in case of sisal/oil palm reinforced hybrid natural rubber composites.

The surface characteristics of the tensile fractured composite specimens were studied through SEM analysis. Figure 9 shows the SEM micrograph of a KNM composite specimen. The micrograph of KNM composite shows notable interface voids between fiber and matrix. These voids occurred due to debonding during a tensile loading or because of poor contact occurred during composite fabrication [38,39]. In response to applied tensile loading, kenaf fibers in KNM composite got detached from the surface of the epoxy matrix which leads to formation of voids between fiber and matrix. The presence of these voids indicates poor fiber/matrix adhesion. This poor interfacial bonding could not provide an efficient stress transfer from the matrix to the fiber ultimately leading to lower tensile properties [37].

Figure 9.

SEM micrograph of tensile fractured KNM specimen. KNM: KNM: nonmercerized kenaf fabric composite without fly ash; SEM: scanning electron microscope.

Figure 10 shows the SEM micrograph of the tensile fracture surface of KM composite specimen. It is noticed from the image that no gaps were seen between fiber and matrix. The interface bonding between fiber and matrix confirms the strong fiber–matrix adhesion [35]. Furthermore, mercerization treatment has enhanced the surface roughness of the kenaf fiber and gives a good interfacial bonding between the fiber and matrix, thereby it also increases the tensile strength of the composite. Similar kind of morphological studies was conducted by various researchers [40 –42] and they have suggested that changes of fiber surface topography could affect the fiber/matrix interfacial adhesion.

Figure 10.

SEM micrograph of tensile fractured KM specimen. KM: KM: mercerized kenaf fabric composite without fly ash; SEM: scanning electron microscope.

Effect of hybridization on tensile properties

The effect of hybridization on the tensile properties of fly ash reinforced kenaf/glass fabric hybrid composites was studied experimentally. The typical stress–strain behavior of tested specimens presented in Figure 11(a) and (b) exhibited a linear elastic behavior until breakage, the slope of the stress–strain curve increased with increasing content of fly ash. The comparative results of the different hybrid composite specimens tested are shown in Figure 12.

Figure 11.

Stress–strain curves of (a) 2KG and (b) 2KG10 specimens. 2KG: kenaf/glass fabric hybrid composite; 2KG10: kenaf/glass fabric hybrid composite with 10% fly ash.

Figure 12.

Comparison of tensile properties of hybrid composites with fly ash fillers.

It can be inferred from the plot that 2KG5 (5 wt. of fly ash) composites improved the tensile strength (about 10%) and young’s modulus (about 19%) compared with the 2KG composites. With further increase of the fly ash content to 10%, the tensile strength and young’s modulus were increased to 39 and 30%, respectively. The results indicated that 2KG10 specimen exhibited better tensile properties than the other types of composites considered. The fly ash particles incorporated in the polymer matrix act as efficient stress transfer agent in the KM10 composites, inducing plastic deformation into the epoxy, and increase the tensile strength [42]. Furthermore, the agglomerated fly ash particles that occurred for the KM15 composites with high fly ash content (15 wt.) were believed to be responsible for the decrease in tensile strength.

The SEM micrograph of the tensile fractured surface of the 2KG specimen is shown in Figure 13. The image showed greater amounts of fiber pullout, fiber breakage, and matrix cracks. A stretching was observed in kenaf fabric whereas fiber pullout was observed in glass fabric.

Figure 13.

SEM micrograph of tensile fractured 2KG specimen. SEM: scanning electron microscope. 2KG: kenaf/glass fabric hybrid composite.

Table 6 shows the comparative results of the tensile strength and young’s modulus of hybrid kenaf/glass composites and plain kenaf fabric composites, respectively. The results indicated that 2KG composites improved the tensile strength and young’s modulus by 103 and 89%, respectively, compared with the KM composites. This is because of the reinforcement of higher modulus and load-carrying ability of the glass fiber in the plain kenaf fabric composite. With further addition of 10wt % fly ash particles to kenaf/glass (2KG10) composite had a significant influence on tensile strength and young’s modulus, with an increase of about 113 and 75% when compared with KM10 composite. The maximum value of tensile strength and young’s modulus are 85 MPa and 6.3 GPa (for 2KG10 composite) which is greater than that of all composite tested.

Table 6.

Comparative tensile properties of hybrid composites.

Composite designation	Tensile strength (MPa)	Young’s modulus (GPa)
KM	30	2.56
KM5	33.5	3
KM10	40	3.6
KM15	36.5	3.2
2KG	61	4.84
2KG5	67	5.75
2KG10	85	6.3
2KG15	73	5.8

KM: mercerized kenaf fabric composite without fly ash; KM5: kenaf fabric composite with 5% fly ash; KM10: kenaf fabric composite with 10% fly ash; KM15: Kenaf fabric composite with 15% fly ash; 2KG: kenaf/glass fabric hybrid composite; 2KG5: kenaf/glass fabric hybrid composite with 5% fly ash; 2KG10: kenaf/glass fabric hybrid composite with 10% fly ash; 2KG15: kenaf/glass fabric hybrid composite with 15% fly ash.

Effect of moisture absorption on tensile strength

The moisture absorption behavior of natural and hybrid composite samples was studied by immersing the composite specimens in distilled water for a period of 288 h. Figure 14 shows the moisture absorption curves of various composite specimens immersed in distilled water at room temperature.

Figure 14.

Moisture absorption test results.

The comparison of moisture absorption behavior of natural fabric composites and hybrid composites with fly ash fillers revealed that the moisture absorption of the specimens increased exponentially with the immersion time at the initial stage and reached steady state following a Fick’s diffusion process [43]. The moisture absorption of KM composite was low compared with KNM composite. This is because of the mercerization process which reduced hydrophilicity of kenaf fiber and improved fiber–matrix compatibility. This finding is similar to previous research works concerning other types of natural fiber reinforced composites [44,45]. It is also evident that the rate of moisture absorption increases with the reinforcement of fly ash fillers into the KM and 2KG composites.

From Figure 14, it is clear that hybridization of glass fiber showed the reduction in moisture absorption rate of the 2KG10 composite. The hybridization of glass fiber not only reduces the moisture absorption but also increased the mechanical properties of the natural fabric composites. This phenomena shows similar trend with Zamri et al. [46].

The comparison of the tensile strength of dry and moisture absorbed samples is shown in Figure 15. The tensile strength of KNM composites was 23.90 MPa on dry condition and the same sample had 19 MPa after absorbing moisture content. Similarly KM10 composites had 40 MPa on dry condition and it was 36 MPa at wet condition. Tensile strength of moisture absorbed 2KG10 samples was reduced about 8% compared to dry 2KG10 samples. When natural fiber composites are exposed to a humid environment, water molecules enter and get attached onto hydrophilic groups of fiber, forming intermolecular hydrogen bonding with fibers and reduce fiber–matrix compatibility [47]. This leads to micro-cracks in the matrix around swollen fibers and promotes capillarity and then transport through micro-cracks. This is evidenced from the SEM image of the moisture absorbed 2KG10 specimen as shown in Figure 16.

Figure 15.

Comparison of tensile strength of dry and moisture absorbed samples.

Figure 16.

SEM micrograph of moisture absorbed 2KG10 specimen. SEM: scanning electron microscope. 2KG10: kenaf/glass fabric hybrid composite with 10% fly ash.

Water soluble substances start leaching from fibers and eventually lead to debonding between fiber and matrix. This debonding is initiated by the development of pressure pockets at the surface of fibers due to the leaching of water soluble substances from the surface of the fiber. The mechanism of water absorption has been summarized in Figure 17. Overall the experimental results showed that the moisture absorption reduced the tensile strength of the composites. Similar findings have been reported in the literature [48 –50].

Figure 17.

Water absorption mechanism.

Conclusion

This study was conducted to determine the combined effect of mercerization treatment, fly ash filler addition, and the hybridization with glass fabric on improving the tensile strength of kenaf fabric-based natural composites. The findings of this research could be summarized as below:

The results showed that mercerization treatment had an influence on surface modification in fiber, which resulted in better fiber–matrix adhesion. Due to this modified surface properties, the tensile strength of the mercerized fabric (KM) composite was 30% higher than the nonmercerized fabric (KNM) composite.

The tensile strength of hybrid kenaf/glass/fly ash (2KG10) composite was 33% higher than the tensile strength of kenaf/glass composite without fly ash (2KG) composite.

The tensile strength of the hybrid kenaf/glass/fly ash (2KG10) composite decreased by 8% when the composite was exposed to atmosphere containing moisture content.

As per the test and outcomes, the usage of agro waste (kenaf) and industrial waste (fly ash) could improve the tensile properties of hybrid composites. Due to this significant improvement in tensile properties, fly ash can be used as the best and cheapest filler instead of the traditional filler materials. At the same time, the solid waste disposal also happens by consuming the waste for industrial wealth, thus the approach is a noteworthy process.

Overall, this study is encouraging the use of hybrid composites in automotive industries due to their improved tensile properties. Thus, it can be concluded that the developed hybrid composite will act as a lightweight and eco-friendly composites to be used as a material for making automobile components on account of their better tensile properties. As the developed hybrid composite is inconsistent with moisture atmosphere, care must be taken while making ship hull structures as it is always linked to water sources.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Dhingra

. Metals replacement by composites. JOM – J Min Met Mat 1986; 38: 17–17.

Chawla

. Composite materials: science and engineering, Verlag, New York: Springer Science and Business Media, 2012.

Mueller

Krobjilowski

. New discovery in the properties of composites reinforced with natural fibers. J Ind Text 2003; 33: 111–130.

Chand

Rohatgi

. Natural fibers and their composites, New Delhi, India: Periodical Experts Book Agency, 1994.

Nishino

Hirao

Kotera

et al.

Kenaf reinforced biodegradable composite. Compos Sci Technol 2003; 63: 1281–1286.

Holbery

Houston

. Natural-fiber-reinforced polymer composites in automotive applications. JOM – J Min Met Mat 2006; 58: 80–86.

Gomes

Matsuo

Goda

et al.

Development and effect of alkali treatment on tensile properties of curaua fiber green composites. Compos Part A 2007; 38: 1811–1820.

Singh

. Effect of water absorption on interface and tensile properties of jute fiber reinforced modified polyethylene composites developed by Palsule process. App Polym Compos 2013; 2: 113–124.

Cantero

et al.

Effects of fiber treatment on wettability and mechanical behaviour of flax/polypropylene composites. Compos Sci Technol 2003; 63: 1247–1254.

10.

Edeerozey

AMM

. Chemical modification of kenaf fibers. Mater Lett 2007; 61: 2023–2025.

11.

Yousif

Shalwan

Chin

et al.

Flexural properties of treated and untreated kenaf/epoxy composites. Mater Des 2012; 40: 378–385.

12.

Fiore

Di Bella

Valenza

. The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites. Compos Part B 2015; 68: 14–21.

13.

Mylsamy

Rajendran

. Influence of alkali treatment and fiber length on mechanical properties of short Agave fiber reinforced epoxy composites. Mater Des 2011; 32: 4629–4640.

14.

Bakar

Ahmad

Kuntjoro

. The mechanical properties of treated and untreated kenaf fibre reinforced epoxy composite. J Biobased Mater Bio 2010; 4: 159–163.

15.

Raghavendra

Ojha

Acharya

et al.

A comparative analysis of woven jute/glass hybrid polymer composite with and without reinforcing of fly ash particles. Polym Compos 2014; 8: 1–10.

16.

Ghani

Salleh

Hyie

et al.

Mechanical properties of kenaf/fiberglass polyester hybrid composite. Procedia Eng 2012; 41: 1654–1659.

17.

Panneerdhass

Gnanavelbabu

Rajkumar

. Mechanical properties of luffa fiber and ground nut reinforced epoxy polymer hybrid composites. Procedia Eng 2014; 97: 2042–2051.

18.

Sabarinathan

Rajkumar

Gnanavelbabu

. Mechanical properties of almond shell-sugarcane leaves hybrid epoxy polymer composite. Appl Mech Mater 2016; 852: 43–48.

19.

Jacobi

Besen

. Solid waste management in Sao Paulo: The challenges of sustainability. Estudos Avancados 2011; 25: 135–158.

20.

Borm

PJA

. Toxicity and occupational health hazards of coal fly ash (CFA). A review of data and comparison to coal mine dust. Ann Occup Hyg 1997; 41: 659–676.

21.

Gines

Chimenos

Vizcarro

et al.

Combined use of MSWI bottom ash and fly ash as aggregate in concrete formulation: Environmental and mechanical considerations. J Hazard Mater 2009; 169: 643–650.

22.

Sengupta S, Pal K, Ray D, et al. Furfuryl palmitate coated fly-ash used as filler in recycled polypropylene matrix composites. Compos Part B 2011; 42: 1834–1839.

23.

Gupta

Brar

Woldesenbet

. Effect of filler addition on the compressive and impact properties of glass fibre reinforced epoxy. Bull Mater Sci 2001; 24: 219–223.

24.

Jena

Pandit

Pradhan

. Effect of cenosphere on mechanical properties of bamboo–epoxy composites. J Reinf Plast Compos 2013; 32: 794–801.

25.

Dalbehera

Acharya

. Effect of cenosphere addition on the mechanical properties of jute-glass fiber hybrid epoxy composites. J Ind Text 2016; 46: 177–188.

26.

Design

. CES EduPack Ver. 6.2. 0 materials selection software, Cambridge: Granta Design Limited, 2010.

27.

Pinto

Lopez-Gonzalez

Jimenez-Martin

. Polymer composites prepared by compression molding of a mixture of carbon black and nylon 6 powder. Polym Compos 1999; 20: 804–808.

28.

Summerscales

Searle

. Low-pressure (vacuum infusion) techniques for moulding large composite structures. Proc IMechE, Part L: J Materials: Design and Applications 2005; 219: 45–58.

29.

Van de Velde

Kiekens

. Thermoplastic pultrusion of natural fiber reinforced composites. Compos Struct 2001; 54: 355–360.

30.

Sreekumar

Thomas

Marc Saiter

et al.

Effect of fiber surface modification on the mechanical and water absorption characteristics of sisal/polyester composites fabricated by resin transfer molding. Compos Part A 2009; 40: 1777–1784.

31.

Agarwal

Broutman

. Analysis and performance of fiber composites, 2nd edn. New York: John Wiley and Sons, 1990.

32.

George

Bhagawan

Thomas

. Improved interactions in chemically modified pineapple leaf fibre reinforced polyethylene composites. Compos Interface 1998; 5: 201–223.

33.

Anuar

Ahmad

Rasid

et al.

Mechanical properties and dynamic mechanical analysis of thermoplastic-natural-rubber-reinforced short carbon fiber and kenaf fiber hybrid composites. J Appl Polym Sci 2008; 107: 4043–4050.

34.

John

Francis

Varughese

et al.

Effect of chemical modification on properties of hybrid fiber biocomposites. Compos Part A 2008; 39: 352–363.

35.

Alavudeen

Rajini

Karthikeyan

et al.

Mechanical properties of banana/kenaf fiber-reinforced hybrid polyester composites: Effect of woven fabric and random orientation. Mater Des 2015; 66: 246–257.

36.

Goriparthi

Suman

KNS

Rao

. Effect of fiber surface treatments on mechanical and abrasive wear performance of polylactide/jute composites. Compos Part A 2012; 43: 1800–1808.

37.

Jacob

Thomas

Varughese

. Mechanical properties of sisal/oil palm hybrid fiber reinforced natural rubber composites. Compos Sci 2004; 64: 955–965.

38.

Then

Ibrahim

Zainuddin

et al.

Influence of alkaline-peroxide treatment of fiber on the mechanical properties of oil palm mesocarp fiber/poly (butylene succinate) biocomposite. BioRes 2015; 10: 1730–1746.

39.

Rayung

Ibrahim

Zainuddin

et al.

The effect of fiber bleaching treatment on the properties of poly (lactic acid)/oil palm empty fruit bunch fiber composites. Int J Mol Sci 2014; 15: 14728–14742.

40.

Datta

Kopczynska

. Effect of kenaf fibre modification on morphology and mechanical properties of thermoplastic polyurethane materials. Ind Crops Prod 2015; 74: 566–576.

41.

Sathishkumar

Navaneethakrishnan

Shankar

et al.

Investigation of chemically treated randomly oriented Sansevieria ehrenbergii fiber reinforced isophthallic polyester composites. J Compos Mater 2014; 48: 2961–2975.

42.

Khan

Shaikh

Alam

et al.

Effect of chemical treatments on the physical properties of non-woven jute/PLA biocomposites. BioRes 2015; 10: 7386–7404.

43.

Sathishkumar

. Influence of cellulose water absorption on the tensile properties of polyester composites reinforced with Sansevieria ehrenbergii fibers. J Ind Text 2015; 45: 1–15.

44.

Saw

Akhtar

Yadav

et al.

Hybrid composites made from jute/coir fibers: Water absorption, thickness swelling, density, morphology, and mechanical properties. J Nat Fibers 2014; 11: 39–53.

45.

Venkateshwaran

ElayaPerumal

Alavudeen

et al.

Mechanical and water absorption behaviour of banana/sisal reinforced hybrid composites. Mater Des 2011; 32: 4017–4021.

46.

Zamri

Akil

Bakar

et al.

Effect of water absorption on pultruded jute/glass fiber-reinforced unsaturated polyester hybrid composites. J Compos Mater 2012; 46: 51–61.

47.

Cheour

Assarar

Scida

et al.

Effect of water ageing on the mechanical and damping properties of flax-fibre reinforced composite materials. Compos Struct 2016; 152: 259–266.

48.

Yang

Kim

Park

et al.

Water absorption behavior and mechanical properties of lignocellulosic filler–polyolefin bio-composites. Compos Struct 2006; 72: 429–437.

49.

Dhakal

Zhang

Richardson

MOW

. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos Sci Technol 2007; 67: 1674–1683.

50.

Munoz

Garcia-Manrique

. Water absorption behaviour and its effect on the mechanical properties of flax fibre reinforced bioepoxy composites. Int J Polym Sci 2015, pp. 1–10.