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Improved Mechanical Properties and Theoretical Prediction of Young’s Modulus of Polylactide Composites Reinforced with Sisal Fibers
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ScienceDirect Materials Today: Proceedings 5 (2018) 22494–22505
www.materialstoday.com/proceedings
ICASE_2017
Improved Mechanical Properties and Theoretical Prediction of Young’s Modulus of Polylactide Composites Reinforced with Sisal Fibers Prakash Krishnaiaha, Chantara Thevy Ratnamb, Sivakumar Manickama* a
Department of Chemical and Environmental Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, Semenyih, Selangor D.E, 43500, Malaysia b Malaysian Nuclear Agency, Selangor, Malaysia
Abstract The aim of this study is to evaluate the effect of different surface modifications on mechanical properties of sisal fiber reinforced PLA (SF/PLA) composites. Using theoretical models Young’s modulus of the SF/PLA composites has been predicted. Tensile properties of single fiber showed a reduction in the tensile strength and modulus by 25% and 26% respectively as compared to the untreated sisal fiber owing to surface treatments. Optimization of fiber loading has also been studied for SF/PLA composites. The tensile strength increased by 10% for 15 wt% of fiber loading with the combined treatment of alkali and HIU as compared to the untreated fiber reinforced PLA composites. Tensile modulus of 30 wt% of fiber loading showed 75.4% increment as compared to pure PLA. Young’s modulus of the composites has also been predicted by using the theoretical models which fit well to the obtained experimental values. This investigation confirms that the surface treatments of SF/PLA composites with alkali and HIU are effective to improve their mechanical properties. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility ofInternational Conference on Advances in Science & Engineering ICASE - 2017. Keywords: Sisal fibers; Surface treatments; Ultrasound; PLA Composites; Tensile properties
1. Introduction In recent years, an inevitable change in the environmental policies in the industrial sectors to reduce the usage of non-degradable related materials lead to the development of biodegradable polymer composite materials with
* Corresponding author. Tel.: +6 (03) 8924 8156; Fax: +6 (03) 8924 8017 E-mail address:[email protected] (S. Manickam) 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility ofInternational Conference on Advances in Science & Engineering ICASE - 2017.
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specified engineering properties[1]. Poly (lactic acid) (PLA) is emerging as one of the best biodegradable thermoplastics among many other biodegradable polymers available at present owing to its remarkable tensile properties, biodegradability, biocompatibility and its easy processing ability. More importantly, as compared to other conventional thermoplastic polymers such as polypropylene (PP), polyethylene (PE), PLA is an excellent alternative considering its mechanical properties especially Young’s modulus. However, higher brittleness and lower crystallization ability significantly limiting its industrial production for the specified engineering applications[2]. To overcome these drawbacks along with a reduction in the manufacturing cost, cellulosic fibers seem to be the best choice to utilize as reinforcing fillers in the generation of polymeric composites[3,4]. Natural fibers as fillers for the generation of polymer composites offer many advantages as compared to synthetic fibers such as light-weight, lower manufacturing cost, easy to handle, good thermal and acoustic insulation properties[5]. However, unlike many other synthetic fibers, natural fibers have limitations due to their hydrophilic nature which causes incompatibility with hydrophobic nature of polymer matrices to use them as reinforcing fillers. This is mainly due to the presence of amorphous materials such as hemicellulose, lignin, pectin and other waxy materials on the surface of natural fibers. These amorphous materials have the tendency of absorbing moisture which leads to a significant reduction in the mechanical, thermal and structural properties of natural fiber reinforced polymer composites[6]. Thus, it is important to remove the amorphous materials from the surface of natural fibers in order to achieve the targeted mechanical, thermal and structural properties depending on the end use applications. Surface modification is often the used method in the removal of amorphous materials which enhances the interfacial adhesion between the reinforcing fillers and polymer matrices[7]. Among the available surface treatments, alkali treatment is the most commonly used chemical treatment methods to remove the amorphous materials from the surface of natural fibers[7].Cai et al.[8]Investigated the influence of alkali treatment on the morphological and tensile properties of abaca fibers. They found that the alkali treated abaca fibers significantly removed the binding materials such as amorphous materials and enhanced the tensile strength by 41% as compared to the untreated fibers. High intensity ultrasound (HIU) treatment is emerging as an effective mechanical treatment for the removal of amorphous materials from the surface of natural fibers[9,10]. HIU is a mechanical process in which electrical energy is converted into acoustical energy. HIU produces cavitation which is the formation, growth and violent collapse of hundreds of bubbles in the solution simultaneously. These high energy cavitation bubbles produce high temperatures and pressures which lead to etching effects on the surface of fibers. It is reported that the application of HIU with alkali pre-treatment on the surface of natural fibers could be more effective for the removal of amorphous materials and to enhance the thermal stability and crystallinity significantly[10]. The mechanical and elastic properties of fibers reinforced polymer composites depend on many different parameters such as fiber length, fiber orientation, fiber geometry, fiber dispersion and interfacial adhesion between fiber and matrix. Young’s modulus is an important intrinsic property of the composite materials which solely depends on the volumetric fiber fractions of the composites, Young’s modulus of the matrix materials and intrinsic Young’s modulus of the fibers used for the composites. The Young’s modulus of the natural fibers has to be measured experimentally to predict the Young’s modulus of the composites [11–14]. In this investigation, sisal fibers were treated with an alkali solution (7 wt%), high intensity ultrasound (HIU) as well as with the combined treatments of alkali (7 wt%) and HIU. Their PLA composites were prepared by melt mixing and compression molding and the effect of surface treatments on tensile properties of the composites were studied. Different mathematical models (parallel, series, Hirsch and Bowyer-Baders) were used in order to predict the Young’s modulus of the SF/PLA composites which was then used to compare with the obtained experimental values. 2. Prediction of Young’s modulus with theoretical models The mechanical properties of fibres could be predicted by theoretical models which are based on different assumptions and experimental data. Several researchers were also used these theoretical models to predict the Young’s modulus of the fibres reinforced composites [12,15,16].
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2.1 Parallel and series model Parallel or Voigt model and Series or Reuss model are the simplest and widely used models to predict the experimental data of Young’s modulus of short fibres reinforced composite materials [12,13,16]. The Young’s modulus of the short SF/PLA composites could be calculated by using the following eqns. 1 and 2: Parallel model: = η . M . V + (1 − V ). M
(eqn. 1)
Series model:
=
M M .V M (
M
V ).
(eqn. 2)
Where Mc, Mm and Mf are the Young’s modulus of the composites, matrix and the fibres respectively and Vf is the volume fraction of the fibres. ηe is the fibre efficiency or compatibility factor for the strength of the composite which is used to correct the contribution of semi aligned short fibres in the composites. Fibre efficiency factor (ηe) is the product of fibre orientation efficiency factor (η0) and fibre length efficiency factor (η1) [17]. According to CoxKrenschel’s model, the fibre length efficiency factor is defined by the following eqn. 3 [18]:
η =1−
(
LF
)
(eqn. 3)
LF
Where LF is the weighted fibre length and β is the coefficient of stress concentration rate at the ends of the fibers according to the following eqn. 4:
(1 −
=
)
√
(eqn. 4)
Where r is the radius of the fibre, νm is the Poisson’s ratio of the matrix which is assumed to be 0.36 [19]. Fibre efficiency factor (η0) can be calculated by considering the fibre orientation across the matrix in the composites. It is assumed to be a rectangular or square distribution of discontinuous phase of short fibres inside the matrix phase. Fukuda and Kawata[14,20] studied the fibre orientation of short fibre reinforced thermoplastics. According to them, the function of orientation distribution is represented by the following eqn. 5: 1
, (0 ≤ ≤ (eqn. 5) 0, ( < ) Where α is the average orientation angle of the fibres inside the matrix and α0 is the fibre orientation limit angle. The Young’s modulus of the fibre reinforced polymer composites largely depends on the angle of fibre orientation inside the composite system. The angle of fibre orientations can be calculated by the following eqn. 6: η =
η =
+
(eqn. 6)
By using the eqns. 3, 4, 5 and 6, the fibre efficiency factor (ηe) can be calculated.
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2.2 Hirsch model: =
(
+ 1−
+ (1 − )
+
1−
(eqn. 7)
Where Mc, Mm and Mf are the Young’s modulus of the composites, matrix and the fibres respectively and Vf is the volume fraction of the fibres. Where x is an empirical parameter which is related to fibre-matrix interface 2.3 Bowyer and Baber model According to Bowyer and Baber model, the tensile properties of short fiber reinforced composites are the sum of contributions from subcritical and supercritical fibres and the matrix. According to this model, the Young’s modulus can be calculated by using the following eqn. 8: M = K K M . V + (1 − V ). M
(eqn. 8)
Where K1 is the fiber orientation factor which depends on the fiber orientation during the preparation of composite.K2 is the fibre length factor. According to Fu and Lauke[17], for short fibre composites, fibre length and fibre interface factor (K2) can be calculated by using the following eqns. 9 and 10: For fibre length L >Lc K = 1 − (L ⁄2L)
(eqn. 9)
For fibre length L
(eqn. 10)
Where, L and Lc represent the length and the critical length of the fibres respectively. Critical length of the fibre is defined as the minimum length of the fibre required for the stress to reach the fracture stress of the fibre. Fibres which are shorter than critical fibre length will not carry the maximum possible load and become inefficient [22]. 3. Experimental 3.1. Materials Polylactide (PLA) (trade name of IngeoTM2003D) was supplied by NatureWorks.Inc, USA. The melt index was 6 g/10 min (210 oC, 2.16 kg). Sisal fibres (SFs) were obtained from vibrant nature, Chennai, India. Sisal fibres were cut to an average size of 5 mm and then cleaned with distilled water and dried at 80 oC before usage. Analytical grade NaOH and acetic acid were purchased from R&M Chemicals and used as received for the alkali treatment. 3.2. Fibre surface treatments 3.2.1. Alkali treatment The alkali (NaOH) solution (7% w/v) was prepared using distilled water. The optimization of alkali treatment was carried out as described in our previous work [23]. At first, the cleaned and dried sisal fibres were soaked in an alkali solution at room temperature for 24 h with constant stirring. The fibres were then washed several times with distilled water containing 1% acetic acid to neutralize the remaining NaOH in the fibres and then dried at 100 oC for 24 h using hot air oven.
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3.2.2. Treatment with high intensity ultrasound (HIU) Untreated and SFs that were treated with 7% (w/v) concentration of alkali were subjected to HIU for 90 min as described in our previous work [23]. An ultrasonic processor (Hielscher UIP1000hd, transducer tip diameter of 24 mm) with the frequency of 20 kHz and an output power of 1000 W was employed. Distilled water was used as the medium and the ratio of fibres to water was maintained at 1:20 (w/v) during the treatment. The total volume of the solution employed was 1000 ml and the ultrasonic horn tip was dipped into the reactor approximately 50% inside the water medium. The temperature was maintained in between 25 to 30 oC by using an ice/water bath. After the treatment, the fibres were then washed with distilled water and dried at 100 oC in a hot air oven for 24 h and the resultant sisal fibres were then used for the fabrication of composites. 3.3. Fabrication of composites Different ratio of untreated and treated sisal fibres (10, 15, 20, 25 and 30 wt%) and PLA were mixed with an internal mixer (BrabenderPlasticoder PL2000-6 equipped with co-rotating blades and a mixing head with a volumetric capacity of 69 cm3). The rotor speed and the blending temperature were set at 50 rpm and 180 oC respectively. Before mixing, all the samples were dried under vacuum at 60 oC for 4 h to remove any moisture. Materials obtained from the internal mixer were subjected to compression moulding which involves 3 min of preheating without any pressure, 20 sec of venting and 3 min of compression under 100 bar at 180 oC using hot and cold pressing machine (LP-S-50 Scientific Hot and Cold Press). Then the hot moulded samples were cooled immediately between two platens of cold press at 20 oC for 2 min. 3.4. Extraction of fibres from the composites By employing a Soxhlet apparatus and dichloromethane as a solvent, reinforced sisal fibres were extracted from the composites by matrix solubilisation method. Composites were cut into small pieces and placed inside a cellulose filter and introduced into the Soxhlet equipment. After the extraction, the fibres were rinsed with acetone and then with distilled water to remove any solvent residue in the fibres. Fibbers were then dried at 100 oC for 24 h [24]. 4. Characterizations 4.1. Determination of density, length and diameter of the fibres The density of the composites (ρc) was calculated by using a pycnometer which was carried out according to ISO 118-3. Distilled water was used as a reference liquid for this test at 23 oC. The density of the composites was then calculated by using the following eqn. 11. ρ =
W (W ⁄ρ ) + (W ⁄ρ )
(eqn. 11)
Where Wc, Wm and Wf are the weights of the composites, matrix and fibres respectively, whereas ρm and ρf are the density of matrix and fibres respectively. Fiber length distribution and diameter of the extracted sisal fibres from the composites were analysed using optical microscopy (Model: Olympus SZX7). 4.2. Measurement of tensile strength of single fibre Tensile properties of untreated and surface treated sisal fibres were carried out according to ASTM D3379-75 standard test method for single filament. For this purpose, untreated and surface treated single fibre was fixed to the rectangular cardboard sheet with the dimensions of 50 mm × 20 mm (length × width) which has a 20 mm hole in the middle of the cardboard sheet as shown in Fig. 1. The single fibre was mounted on the cardboard sheet by using polyvinyl acetate glue. The mounted fibres were then placed in the grips of tensile testing machine (model: Toyoski tensile testing machine) and a cutter was used to cut the supporting sides of the cardboard. The tensile test was
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carried out at a cross head speed of 0.5 mm/min using 10 N-cell load. The fibres were assumed to be cylindrical and the diameter of the fibres was measured at five different points using optical microscopy (Model: Olympus SZX7). The obtained average diameter was then used to calculate the tensile properties of fibres considering twenty specimens.
Fig. 1 Schematic diagram of the method followed to determine the tensile strength of single fibre
4.3. Tensile properties of SF/PLA composites Dumb-bell shaped specimens were punched from the molded sheets of PLA, untreated, HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites after 24 h of storage. Ultimate tensile strength (maximum stress), elongation at break and Young’s modulus were measured according to ASTM D638 standard using Toyoski tensile testing machine at a constant crosshead speed of 5 mm/min. Six specimens were tested and at least five replicate specimens have been presented as an average of tested specimens. 4.4. Mechanical properties
600
20
500 400 300 200 100 0 UT
ULT ALK Fibre treatments
(A)
ALKULT
Tensile modulus (GPa)
Tensile strength (MPa)
4.4.1 Tensile properties of single sisal fibre Fig. 2shows the typical stress-strain curve obtained from the tensile tests. Similar to other natural fibres, sisal fibres showed brittle nature under tensile test conditions and a sudden drop in the load was observed when the fibres break [25,26].
15 10 5 0 UT
ULT ALK Fibre treatments
ALKULT
(B)
Fig. 2 Tensile properties of untreated (UT), HIU treated (ULT), alkali treated (ALK) and the combination of alkali and HIU treated (ALKTULT) sisal fibres: (A) tensile strength (B) tensile modulus
Fig. 2 (A) and (B) show the plot of tensile strength and modulus values against different surface treatments of sisal fibres. Tensile strength and modulus of HIU treated sisal fibres showed a slight decrease by 5.6% and 5%
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respectively as compared to the untreated fibres. However, a significant reduction in the tensile strength and modulus was observed for the alkali and the combination of alkali and HIU treated sisal fibres. The tensile strength and modulus reduced by 25% and 26% respectively for the combination of alkali and HIU treated sisal fibres as compared to untreated sisal fibres. This is mainly due to the removal of amorphous materials such as hemicellulose, lignin and other waxy materials from the surface of fibres.Hemicellulose and lignin are the two main components on the cell wall structure of natural fibres which are tightly bound to the cellulose. Removal of these amorphous materials due to alkali treatment causes fibre degradation and leads to the reduction in the mechanical properties of fibres[27–29]. Moreover, the application of HIU for alkali pre-treated SFs significantly damages the surface of fibres and reduces the fibre diameter which significantly reduces the mechanical properties [10,30]. These observations are in agreement with the obtained results of FE-SEM. 4.4.2 Tensile properties of SF/PLA composites Tensile properties of PLA composites reinforced with sisal fibres of untreated and different surface treated are evaluated and Fig. 3 shows the typical stress vs strain graph of 15wt% loading of SF/PLA composites. It may be observed that the curves show linear deformation followed by sudden fracture. Tensile strength of untreated, HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites was evaluated and illustrated in Fig. 4. It could be noted from this figure that the tensile strength decreased with an increase in the fibre loading as compared to pure PLA except for 15 wt% of fibre loading for which the tensile strength slightly increased to 54.24 MPa (10% higher than pure PLA). However, as compared to the untreated SF composites, surface treated sisal fibres composites showed a gradual increment in the tensile strength at all the percentages of fibre loading. Pure PLA ALKT
UT ALKTULT
ULT
Stress (GPa)
0.06 0.05 0.04 0.03 0.02 0.01 0 0.00
0.89
2.22 Strain (%)
3.54
4.86
Fig. 3Typical stress versus strain curves of untreated (UT), HIU treated (ULT), alkali treated (ALK) and the combination of alkali and HIU treated (ALKTULT) SF/PLA composites
Combination of alkali and HIU treated SF/PLA composites showed superior tensile properties. It is worth noted that the tensile strength of surface treated SF/PLA composites was superior than the surface treated single sisal fibres. This is due to the removal of amorphous materials which act as binding materials for cellulose component inside the fibres. Removal of these amorphous materials causes the voids on the surface of sisal fibres which leads to a reduction in the tensile strength. However, in case of composites, removal of amorphous materials enhances the adhesion between fibres and matrix materials which leads to increase the tensile strength. Similar observation has been reported by Orue et al [27] where they noticed that the tensile strength reduced by more than 50% for the combined treatment of alkali and silanes as compared to the untreated sisal fibres. However, they also reported that the tensile strength of treated SF/PLA composites showed superior tensile properties as compared to untreated SF/PLA composites. PLA composites with the fibre loading of 15 wt% showed higher tensile strength among all other fibre contents. The tensile strength of HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites were 43 MPa, 47 MPa and 54.24 MPa respectively as compared to the untreated sisal fibre
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composites, which were 10%, 20% and 33% higher than the untreated fibres composites with the same weight percentage (15 wt%) of fibre loading. Pure PLA
(a)
UT
ULT
ALKT
ALKT-ULT
Tensile modulus (MPa)
Tensile strength (MPa)
60 50 40 30 20 10 0 0
10
15
20
Fibre loading (wt%)
25
30
Pure PLA
(b)
UT
ULT
ALK
ALKULT
2000 1800 1600 1400 1200 1000 800 600 400 200 0 0
10
15
20
25
30
Fibre loading (wt%)
Fig. 4(a) Tensile strength and (b) modulus of untreated (UT), HIU treated (ULT), alkali treated (ALK) and the combination of alkali and HIU treated (ALKTULT) SF/PLA composites
This trend is almost the same at all fibre loading. An increase in the tensile strength for the treated sisal fibres composites as compared to the untreated sisal fibres composites indicates the removal of non-cellulosic (amorphous) materials and other waxy materials from the surface of sisal fibres during surface treatment [31,32]. These results further support the observations obtained from FTIR and FE-SEM from our previous work [23]. Ma and Whan[33] reported that the bamboo fibres reinforced PLA composites with different surface treatments such as Plasma, UV irradiation and direct silane treatments showed an increase in the tensile strength by 43.7%, 60.7% and 71.1% respectively. As compared to pure PLA, the tensile strength decreased for the SF/PLA composites with an increase in the fibre loading from 10 to 30% irrespective of the surface treatments. This may be due to an increase in the fibre agglomeration in the fibre-matrix interface which leads to poor interfacial adhesion between the fibres and matrix. Tisserat et al. [34] observed that the tensile strength of PLA composites decreased significantly by 20% with the addition of 25 wt% of paulownia wood filler as compared to pure PLA. They also indicated that the aspect ratio of filler materials play an important role in improving the mechanical properties of composite materials. Lee et al. [35] observed a reduction in the tensile strength by 10% with the addition of 20 wt% of sisal fibres in the PLA composites as compared to pure PLA. They also indicated that the poor interfacial adhesion between the fibre and matrix and the agglomeration of fibres across the matrix resulted in reducing the tensile strength. In contrast to the tensile strength, tensile modulus enhanced significantly with the incorporation of sisal fibres into PLA matrix at all the fibre loading as compared to pure PLA. It can be seen from Fig. 4 that sisal fibre loading of 30 wt % on the PLA composite increased the tensile modulus by 25%, 46%, 54% and 75.4% for the untreated, HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites respectively as compared to pure PLA. These results are in agreement with the observations of Ragoubi et al. [36] where they observed that the tensile modulus increased by 19% and 31% for miscanthus grass fibres reinforced PLA composites with and without fibre surface treatments respectively as compared to pure PLA. A significant enhancement in the tensile modulus is attributed to improved interfacial adhesion between the sisal fibres and PLA matrix which enhances the load transfer from the matrix phase to filler phase [33,35,37,38]. It is worth noted that the aspect ratio of the fibres playing an important role in altering the mechanical properties of fibre reinforced polymer composites. A significant reduction of percentage elongation that is more than 50% could be seen with an addition of even 10 wt% of untreated sisal fibres. It is further reduced by 20% with an increase in the fibre loading up to 30 wt%. Addition of fibre fillers onto the PLA matrix restricts the mobility of polymer chain and making it more resistant to elongation at break as compared to pure PLA. This is in agreement with the reports of Petinakis et al. [39] and other researchers [34–36,38].
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4.4.3 Theoretical predictions of Young’s modulus Practically, it is impossible to calculate the experimental values of tensile or Young’s modulus of natural fibre reinforced polymer composites accurately due to large variation in the matrix and fibres parameters such as aspect ratio of fibres, fibre orientation, surface treatments of fibres etc., However, intrinsic tensile modulus of the reinforcing fibres is the key parameter to predict the modulus or strength of the fibre reinforced composites [21]. Mathematical models are commonly used to estimate the theoretical values of the tensile or flexural modulus of natural fibre reinforced polymer composites [15]. Figs. 5-6show the comparison of Young’s modulus obtained from the experimental values of different surface treated (untreated, HIU treated, alkali treated and the combination of alkali and HIU treated) SF/PLA composites with the theoretical values calculated by using different mathematical models. It can be seen that Young’s modulus gradually increases with an increase in the volume fraction of fibres. It is well-known that the stiffness of the fibre reinforced composites increases with the addition of filler materials to the polymer matrix. The modulus mainly depends on the intrinsic rigidity of the fibres and the matrix [14,21]. The fibre efficiency factor (ηe) was calculated using the eqns. 3 to 6 which was 0.53. This theoretically calculated value of the fibre efficiency factor was found to fit with the experimental values.
Tensile modulus (GPa)
2.5
Parallel Hirsch
2 1.5 1 0.5 0 0.089
0.135
0.184
0.228
Volume fraction of fibres (Vf)
Experimental Series BBM
(b) Tensile modulus (GPa)
Experimental Series BBM
(a)
0.275
3
Parallel Hirsch
2.5 2 1.5 1 0.5 0 0.089
0.135
0.184
0.228
0.275
Volume fraction of fibres (Vf)
Fig. 5 Variations of theoretical and experimental values of tensile modulus for the (a) untreated (b) SF/PLA composites
It could be seen from Figs. 5-6 that at initial fibre loading there is a good agreement between the experimental and theoretical values in all the models. This may be due to low volume of fibre loading. It is well known that at low volume fractions of fibres, it can be assumed to be a uniform distribution of stress and strain which results in better load transform across the fibres from the matrix phase in the composite which leads to improving the mechanical properties of fibre reinforced polymer composites. However, in case of higher fibre volume fractions, there could be large variations between experimental and theoretical values. This may be due to more possibilities for the fibre agglomeration across the matrix which causes an uneven distribution of stress and strain and leads to a reduction in the mechanical properties. Kalaprasad et al. [12] studied the theoretical modelling of tensile properties of short SF/LDPE composites. Parallel and series models were commonly used for continuous fibre reinforced polymer composites, where the stress transfer mechanism is different as compared to short fibre reinforced polymer composites. In short fibre composites, the stress transfer largely depends on the fibre orientations inside the composite, stress concentration at the end of the fibres as well as on the critical fibre length. It could be seen from Figs. 5-6 that the experimental values of surface treated SF/PLA composites do not show much variation as compared to theoretical values. However, there is a marginal agreement between the experimental and theoretical values for the alkali treated and the combination of alkali and HIU treated SF/PLA composites (Figs. 5-6). Hirsch model is a combination of parallel and series models which shows a good agreement between theoretical and experimental values in all the cases. It is mentioned that the prediction of Hirsch model largely depends on the value of x in eqn 7[15,21]. The factors which control the x values are: fibre orientation, fibre length and the effect of stress concentration of the fibres. The stress transfer factor x is approximately 0.4 which is in agreement with the semialigned fibre reinforced polymer composites [12]. The theoretical values of Young’s modulus of modified Bowyer and Bader model (BBM) were calculated using the eqn. 8.
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Tensile modulus (GPa)
3
Parallel Hirsch
2.5 2 1.5 1 0.5
Experimental Series BBM
(b) Tensile modulus (GPa)
Experimental Series BBM
(a)
22503 Parallel Hirsch
2.5 2 1.5 1 0.5 0
0 0.089
0.135
0.184
0.228
Volume fraction of fibres (Vf)
0.275
0.089
0.135
0.184
0.228
0.275
Volume fraction of fibres (Vf)
Fig. 6 Variations in the theoretical and experimental values of tensile modulus of (a) alkaliand (b) the combination of alkali and HIU treated SF/PLA composites
The values of K2 and K1 were calculated to be 0.5 and 1 respectively by using eqns. 9 and 10. These values are in agreement with the observations of Joseph et al. [11].It was noticed that there is a marginal variation with the experimental and theoretical values when BBM model was applied for predicting the Young’s modulus of SF/PLA composites. Unlike other theoretical models, BBM model exhibits higher values as compared to the experimental values. This may be due to variations in the fibre orientation factor (K1) and fibre length factor (K2) for the untreated and HIU treated SF/PLA composites.From these observations, it is noticed that the values of Young’s modulus of theoretical models are in reasonable correlation with the experimental values in all the models except a little variation in the BBM model. A good agreement in the experimental and theoretical values of Young’s modulus of untreated and different surface treated SF/PLA composites reveals better utilization of SFs and PLA matrix for the generation of SF/PLA composites. 5. Conclusions In this work, the mechanical properties of SF/PLA composites were studied with different percentage of fibre loading. Tensile strength increased by 10% for 15 wt% of sisal fibre loading using the combined treatment as compared to the untreated fibres. However, a further increase in the fibre loading above 15% decreased the tensile strength. Tensile modulus for 30 wt% of fibre loading showed 75.4% increment as compared to pure PLA. The theoretical models such as series and parallel, Hirsch’s and Bowyer and Bader were used to predict the Young’s modulus of the untreated and different surface treated short SF/PLA composites to fit the obtained experimental values. Young’s modulus of the untreated and different surface treated short SF/PLA composites was presented as a function of volume fractions of the fibres. All the models except Bowyer and Bader model (BBM) show a reasonably good agreement with the experimental values particularly at lower fractions of fibres. The series and Hirsch’s models show very good correlations with the experimental values. The effect of surface treatments on the tensile properties was also investigated. The results of theoretical models revealed that the surface treatments of fibres influence the tensile properties of short SF/PLA composites. Overall, this investigation proves that the surface treatments of alkali and HIU are an effective way for improving the mechanical properties of SF/PLA composites. References: [1]
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overview, Compos. Part B Eng. 43 (2012) 2883–2892. and tensile properties of abaca fibers, Ind. Crops Prod. 65 (2015) 27–35. Biomacromolecules. 9 (2008) 1919–1923. [10] W. Chen, H. Yu, Y. Liu, P. Chen, M. Zhang, Y. Hai, Individualization of cellulose nanofibers from wood using high-intensity ultrasonication combined with chemical pretreatments, Carbohydr. Polym. 83 (2011) 1804–1811. [11] P. V. Joseph, G. Mathew, K. Joseph, S. Thomas, P. Pradeep, Mechanical properties of short sisal fiber-reinforced polypropylene composites: Comparison of experimental data with theoretical predictions, J. Appl. Polym. Sci. 88 (2003) 602–611. [12] G. Kalaprasad, K. Joseph, S. Thomas, C. Pavithran, Theoretical modelling of tensile properties of short sisal fibre-reinforced low-density polyethylene composites, J. Mater. Sci. 32 (1997) 4261–4267. [13] H. Cabral, Structure-properties relationship of short jute fiber-reinforced polypropylene composites, J. Compos. Mater. 39 (2005) 51–65. [14] L.A. Granda, X. Espinach, F. López, J.C. García, M. Delgado-Aguilar, P. Mutjé, Semichemical fibres of Leucaena collinsii reinforced polypropylene: Macromechanical and micromechanical analysis, Compos. Part B Eng. 92 (2016) 332–337. [15] A. Serrano, F.X. Espinach, J. Tresserras, R. del Rey, N. Pellicer, P. Mutje, Macro and micromechanics analysis of short fiber composites stiffness: The case of old newspaper fibers-polypropylene composites, Mater. Des. 55 (2014) 319–324. [16] S.K. Nayak, G. Dixit, K.K. Appu Kuttan, Mechanical properties of eco-friendly recycled polymer composites: A comparative study of theoretical and experimental results, Int. J. Plast. Technol. 17 (2013) 75–93. [17] S.Y. Fu, B. Lauke, Effects of fiber length and fiber orientation distributions on the tensile strength of short-fiber-reinforced polymers, Compos. Sci. Technol. 56 (1996) 1179–1190. [18] F. Vilaseca, A. Valadez-Gonzalez, P.J. Herrera-Franco, M.A. Pelach, J.P. Lopez, P. Mutje, Biocomposites from abaca strands and polypropylene. Part I: Evaluation of the tensile properties, Bioresour. Technol. 101 (2010) 387–395. [19] F.X. Espinach, F. Julian, N. Verdaguer, L. Torres, M.A. Pelach, F. Vilaseca, P. Mutje, Analysis of tensile and flexural modulus in hemp strands/polypropylene composites, Compos. Part B Eng. 47 (2013) 339–343. [20] Y. Sanomura, M. Kawamura, Fiber orientation control of short-fiber reinforced thermoplastics by ram extrusion, Polym. Compos. 24 (2003) 587–596. [21] J.P. López, S. Boufi, N.E. El Mansouri, P. Mutjé, F. Vilaseca, PP composites based on mechanical pulp, deinked newspaper and jute strands: A comparative study, Compos. Part B. 43 (2012) 3453–3461. [22] T. Lacroix, B. Tilmans, R. Keunings, M. Desaeger, I. Verpoest, Modelling of critical fibre length and interfacial debonding in the fragmentation testing of polymer composites, Compos. Sci. Technol. 43 (1992) 379–387. [23] P. Krishnaiah, C.T. Ratnam, S. Manickam, Enhancements in crystallinity, thermal stability, tensile modulus and strength of sisal fibres and their PP composites induced by the synergistic effects of alkali and high intensity ultrasound (HIU) treatments, Ultrason. Sonochem. 34 (2017) 729–742. [24] R. Reixach, F.X. Espinach, E. Franco-Marques, F. Ramirez De Cartagena, N. Pellicer, J. Tresserras, P. Mutje, Modeling of the tensile moduli of mechanical, thermomechanical, and chemi-thermomechanical pulps from orange tree pruning, Polym. Compos. 34 (2013) 1840– 1846. [25] A. Belaadi, A. Bezazi, M. Bourchak, F. Scarpa, C. Zhu, Thermochemical and statistical mechanical properties of natural sisal fibres, Compos. Part B Eng. 67 (2014) 481–489. [26] F.D.A. Silva, N. Chawla, R.D.D.T. Filho, Tensile behavior of high performance natural (sisal) fibers, Compos. Sci. Technol. 68 (2008) 3438–3443. [27] A. Orue, A. Jauregi, U. Unsuain, J. Labidi, A. Eceiza, A. Arbelaiz, The effect of alkaline and silane treatments on mechanical properties and breakage of sisal fibers and poly (lactic acid)/ sisal fiber composites, Compos. - Part A Appl. Sci. Manuf. 84 (2016) 186–195.
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[28] R. Mahjoub, J.M. Yatim, A.R. Mohd Sam, S.H. Hashemi, Tensile properties of kenaf fiber due to various conditions of chemical fiber surface modifications, Constr. Build. Mater. 55 (2014) 103–113. [29] M. Haameem J.A., M.S. Abdul Majid, M. Afendi, H.F.A. Marzuki, I. Fahmi, A.G. Gibson, Mechanical properties of Napier grass fibre/polyester composites, Compos. Struct. 136 (2016) 1–10. [30] A. Iskalieva, B.M. Yimmou, P.R. Gogate, M. Horvath, P.G. Horvath, L. Csoka, Cavitation assisted delignification of wheat straw: A review, Ultrason. Sonochem. 19 (2012) 984–993. [31] F.Z. Arrakhiz, M. Elachaby, R. Bouhfid, S. Vaudreuil, M. Essassi, A. Qaiss, Mechanical and thermal properties of polypropylene reinforced with Alfa fiber under different chemical treatment, Mater. Des. 35 (2012) 318–322. [32] Y. Dong, A. Ghataura, H. Takagi, H.J. Haroosh, A.N. Nakagaito, K.-T. Lau, Polylactic acid (PLA) biocomposites reinforced with coir fibres: Evaluation of mechanical performance and multifunctional properties, Compos. Part A Appl. Sci. Manuf. 63 (2014) 76–84. [33] H. Ma, C. Whan Joo, Influence of surface treatments on structural and mechanical properties of bamboo fiber-reinforced poly(lactic acid) biocomposites, J. Compos. Mater. 45 (2011) 2455–2463. [34] B. Tisserat, N. Joshee, A.K. Mahapatra, G.W. Selling, V.L. Finkenstadt, Physical and mechanical properties of extruded poly(lactic acid)based Paulownia elongata biocomposites, Ind. Crops Prod. 44 (2013) 88–96. [35] K.-Y.Y. Lee, P. Bharadia, J.J. Blaker, A. Bismarck, Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement, Compos. Part A Appl. Sci. Manuf. 43 (2012) 2065–2074. [36] M. Ragoubi, B. George, S. Molina, D. Bienaimé, A. Merlin, J.M. Hiver, A. Dahoun, Effect of corona discharge treatment on mechanical and thermal properties of composites based on miscanthus fibres and polylactic acid or polypropylene matrix, Compos. Part A Appl. Sci. Manuf. 43 (2012) 675–685. [37] O.M.L. Asumani, R.G. Reid, R. Paskaramoorthy, The effects of alkali–silane treatment on the tensile and flexural properties of short fibre non-woven kenaf reinforced polypropylene composites, Compos. Part A Appl. Sci. Manuf. 43 (2012) 1431–1440. [38] M.J. Cran, S.W. Bigger, I. S.M.A., Effect of kenaf fibre loading and thymol concentration on the mechanical and thermal properties of PLA/kenaf/thymol composites, Ind. Crops Prod. 61 (2014) 74–83. [39] E. Petinakis, L. Yu, G. Edward, K. Dean, H. Liu, A.D. Scully, Effect of matrix-particle interfacial adhesion on the mechanical properties of poly(lactic acid)/wood-flour micro-composites, J. Polym. Environ. 17 (2009) 83–94.
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ICASE_2017
Improved Mechanical Properties and Theoretical Prediction of Young’s Modulus of Polylactide Composites Reinforced with Sisal Fibers Prakash Krishnaiaha, Chantara Thevy Ratnamb, Sivakumar Manickama* a
Department of Chemical and Environmental Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, Semenyih, Selangor D.E, 43500, Malaysia b Malaysian Nuclear Agency, Selangor, Malaysia
Abstract The aim of this study is to evaluate the effect of different surface modifications on mechanical properties of sisal fiber reinforced PLA (SF/PLA) composites. Using theoretical models Young’s modulus of the SF/PLA composites has been predicted. Tensile properties of single fiber showed a reduction in the tensile strength and modulus by 25% and 26% respectively as compared to the untreated sisal fiber owing to surface treatments. Optimization of fiber loading has also been studied for SF/PLA composites. The tensile strength increased by 10% for 15 wt% of fiber loading with the combined treatment of alkali and HIU as compared to the untreated fiber reinforced PLA composites. Tensile modulus of 30 wt% of fiber loading showed 75.4% increment as compared to pure PLA. Young’s modulus of the composites has also been predicted by using the theoretical models which fit well to the obtained experimental values. This investigation confirms that the surface treatments of SF/PLA composites with alkali and HIU are effective to improve their mechanical properties. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility ofInternational Conference on Advances in Science & Engineering ICASE - 2017. Keywords: Sisal fibers; Surface treatments; Ultrasound; PLA Composites; Tensile properties
1. Introduction In recent years, an inevitable change in the environmental policies in the industrial sectors to reduce the usage of non-degradable related materials lead to the development of biodegradable polymer composite materials with
* Corresponding author. Tel.: +6 (03) 8924 8156; Fax: +6 (03) 8924 8017 E-mail address:[email protected] (S. Manickam) 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility ofInternational Conference on Advances in Science & Engineering ICASE - 2017.
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specified engineering properties[1]. Poly (lactic acid) (PLA) is emerging as one of the best biodegradable thermoplastics among many other biodegradable polymers available at present owing to its remarkable tensile properties, biodegradability, biocompatibility and its easy processing ability. More importantly, as compared to other conventional thermoplastic polymers such as polypropylene (PP), polyethylene (PE), PLA is an excellent alternative considering its mechanical properties especially Young’s modulus. However, higher brittleness and lower crystallization ability significantly limiting its industrial production for the specified engineering applications[2]. To overcome these drawbacks along with a reduction in the manufacturing cost, cellulosic fibers seem to be the best choice to utilize as reinforcing fillers in the generation of polymeric composites[3,4]. Natural fibers as fillers for the generation of polymer composites offer many advantages as compared to synthetic fibers such as light-weight, lower manufacturing cost, easy to handle, good thermal and acoustic insulation properties[5]. However, unlike many other synthetic fibers, natural fibers have limitations due to their hydrophilic nature which causes incompatibility with hydrophobic nature of polymer matrices to use them as reinforcing fillers. This is mainly due to the presence of amorphous materials such as hemicellulose, lignin, pectin and other waxy materials on the surface of natural fibers. These amorphous materials have the tendency of absorbing moisture which leads to a significant reduction in the mechanical, thermal and structural properties of natural fiber reinforced polymer composites[6]. Thus, it is important to remove the amorphous materials from the surface of natural fibers in order to achieve the targeted mechanical, thermal and structural properties depending on the end use applications. Surface modification is often the used method in the removal of amorphous materials which enhances the interfacial adhesion between the reinforcing fillers and polymer matrices[7]. Among the available surface treatments, alkali treatment is the most commonly used chemical treatment methods to remove the amorphous materials from the surface of natural fibers[7].Cai et al.[8]Investigated the influence of alkali treatment on the morphological and tensile properties of abaca fibers. They found that the alkali treated abaca fibers significantly removed the binding materials such as amorphous materials and enhanced the tensile strength by 41% as compared to the untreated fibers. High intensity ultrasound (HIU) treatment is emerging as an effective mechanical treatment for the removal of amorphous materials from the surface of natural fibers[9,10]. HIU is a mechanical process in which electrical energy is converted into acoustical energy. HIU produces cavitation which is the formation, growth and violent collapse of hundreds of bubbles in the solution simultaneously. These high energy cavitation bubbles produce high temperatures and pressures which lead to etching effects on the surface of fibers. It is reported that the application of HIU with alkali pre-treatment on the surface of natural fibers could be more effective for the removal of amorphous materials and to enhance the thermal stability and crystallinity significantly[10]. The mechanical and elastic properties of fibers reinforced polymer composites depend on many different parameters such as fiber length, fiber orientation, fiber geometry, fiber dispersion and interfacial adhesion between fiber and matrix. Young’s modulus is an important intrinsic property of the composite materials which solely depends on the volumetric fiber fractions of the composites, Young’s modulus of the matrix materials and intrinsic Young’s modulus of the fibers used for the composites. The Young’s modulus of the natural fibers has to be measured experimentally to predict the Young’s modulus of the composites [11–14]. In this investigation, sisal fibers were treated with an alkali solution (7 wt%), high intensity ultrasound (HIU) as well as with the combined treatments of alkali (7 wt%) and HIU. Their PLA composites were prepared by melt mixing and compression molding and the effect of surface treatments on tensile properties of the composites were studied. Different mathematical models (parallel, series, Hirsch and Bowyer-Baders) were used in order to predict the Young’s modulus of the SF/PLA composites which was then used to compare with the obtained experimental values. 2. Prediction of Young’s modulus with theoretical models The mechanical properties of fibres could be predicted by theoretical models which are based on different assumptions and experimental data. Several researchers were also used these theoretical models to predict the Young’s modulus of the fibres reinforced composites [12,15,16].
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2.1 Parallel and series model Parallel or Voigt model and Series or Reuss model are the simplest and widely used models to predict the experimental data of Young’s modulus of short fibres reinforced composite materials [12,13,16]. The Young’s modulus of the short SF/PLA composites could be calculated by using the following eqns. 1 and 2: Parallel model: = η . M . V + (1 − V ). M
(eqn. 1)
Series model:
=
M M .V M (
M
V ).
(eqn. 2)
Where Mc, Mm and Mf are the Young’s modulus of the composites, matrix and the fibres respectively and Vf is the volume fraction of the fibres. ηe is the fibre efficiency or compatibility factor for the strength of the composite which is used to correct the contribution of semi aligned short fibres in the composites. Fibre efficiency factor (ηe) is the product of fibre orientation efficiency factor (η0) and fibre length efficiency factor (η1) [17]. According to CoxKrenschel’s model, the fibre length efficiency factor is defined by the following eqn. 3 [18]:
η =1−
(
LF
)
(eqn. 3)
LF
Where LF is the weighted fibre length and β is the coefficient of stress concentration rate at the ends of the fibers according to the following eqn. 4:
(1 −
=
)
√
(eqn. 4)
Where r is the radius of the fibre, νm is the Poisson’s ratio of the matrix which is assumed to be 0.36 [19]. Fibre efficiency factor (η0) can be calculated by considering the fibre orientation across the matrix in the composites. It is assumed to be a rectangular or square distribution of discontinuous phase of short fibres inside the matrix phase. Fukuda and Kawata[14,20] studied the fibre orientation of short fibre reinforced thermoplastics. According to them, the function of orientation distribution is represented by the following eqn. 5: 1
, (0 ≤ ≤ (eqn. 5) 0, ( < ) Where α is the average orientation angle of the fibres inside the matrix and α0 is the fibre orientation limit angle. The Young’s modulus of the fibre reinforced polymer composites largely depends on the angle of fibre orientation inside the composite system. The angle of fibre orientations can be calculated by the following eqn. 6: η =
η =
+
(eqn. 6)
By using the eqns. 3, 4, 5 and 6, the fibre efficiency factor (ηe) can be calculated.
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2.2 Hirsch model: =
(
+ 1−
+ (1 − )
+
1−
(eqn. 7)
Where Mc, Mm and Mf are the Young’s modulus of the composites, matrix and the fibres respectively and Vf is the volume fraction of the fibres. Where x is an empirical parameter which is related to fibre-matrix interface 2.3 Bowyer and Baber model According to Bowyer and Baber model, the tensile properties of short fiber reinforced composites are the sum of contributions from subcritical and supercritical fibres and the matrix. According to this model, the Young’s modulus can be calculated by using the following eqn. 8: M = K K M . V + (1 − V ). M
(eqn. 8)
Where K1 is the fiber orientation factor which depends on the fiber orientation during the preparation of composite.K2 is the fibre length factor. According to Fu and Lauke[17], for short fibre composites, fibre length and fibre interface factor (K2) can be calculated by using the following eqns. 9 and 10: For fibre length L >Lc K = 1 − (L ⁄2L)
(eqn. 9)
For fibre length L
(eqn. 10)
Where, L and Lc represent the length and the critical length of the fibres respectively. Critical length of the fibre is defined as the minimum length of the fibre required for the stress to reach the fracture stress of the fibre. Fibres which are shorter than critical fibre length will not carry the maximum possible load and become inefficient [22]. 3. Experimental 3.1. Materials Polylactide (PLA) (trade name of IngeoTM2003D) was supplied by NatureWorks.Inc, USA. The melt index was 6 g/10 min (210 oC, 2.16 kg). Sisal fibres (SFs) were obtained from vibrant nature, Chennai, India. Sisal fibres were cut to an average size of 5 mm and then cleaned with distilled water and dried at 80 oC before usage. Analytical grade NaOH and acetic acid were purchased from R&M Chemicals and used as received for the alkali treatment. 3.2. Fibre surface treatments 3.2.1. Alkali treatment The alkali (NaOH) solution (7% w/v) was prepared using distilled water. The optimization of alkali treatment was carried out as described in our previous work [23]. At first, the cleaned and dried sisal fibres were soaked in an alkali solution at room temperature for 24 h with constant stirring. The fibres were then washed several times with distilled water containing 1% acetic acid to neutralize the remaining NaOH in the fibres and then dried at 100 oC for 24 h using hot air oven.
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3.2.2. Treatment with high intensity ultrasound (HIU) Untreated and SFs that were treated with 7% (w/v) concentration of alkali were subjected to HIU for 90 min as described in our previous work [23]. An ultrasonic processor (Hielscher UIP1000hd, transducer tip diameter of 24 mm) with the frequency of 20 kHz and an output power of 1000 W was employed. Distilled water was used as the medium and the ratio of fibres to water was maintained at 1:20 (w/v) during the treatment. The total volume of the solution employed was 1000 ml and the ultrasonic horn tip was dipped into the reactor approximately 50% inside the water medium. The temperature was maintained in between 25 to 30 oC by using an ice/water bath. After the treatment, the fibres were then washed with distilled water and dried at 100 oC in a hot air oven for 24 h and the resultant sisal fibres were then used for the fabrication of composites. 3.3. Fabrication of composites Different ratio of untreated and treated sisal fibres (10, 15, 20, 25 and 30 wt%) and PLA were mixed with an internal mixer (BrabenderPlasticoder PL2000-6 equipped with co-rotating blades and a mixing head with a volumetric capacity of 69 cm3). The rotor speed and the blending temperature were set at 50 rpm and 180 oC respectively. Before mixing, all the samples were dried under vacuum at 60 oC for 4 h to remove any moisture. Materials obtained from the internal mixer were subjected to compression moulding which involves 3 min of preheating without any pressure, 20 sec of venting and 3 min of compression under 100 bar at 180 oC using hot and cold pressing machine (LP-S-50 Scientific Hot and Cold Press). Then the hot moulded samples were cooled immediately between two platens of cold press at 20 oC for 2 min. 3.4. Extraction of fibres from the composites By employing a Soxhlet apparatus and dichloromethane as a solvent, reinforced sisal fibres were extracted from the composites by matrix solubilisation method. Composites were cut into small pieces and placed inside a cellulose filter and introduced into the Soxhlet equipment. After the extraction, the fibres were rinsed with acetone and then with distilled water to remove any solvent residue in the fibres. Fibbers were then dried at 100 oC for 24 h [24]. 4. Characterizations 4.1. Determination of density, length and diameter of the fibres The density of the composites (ρc) was calculated by using a pycnometer which was carried out according to ISO 118-3. Distilled water was used as a reference liquid for this test at 23 oC. The density of the composites was then calculated by using the following eqn. 11. ρ =
W (W ⁄ρ ) + (W ⁄ρ )
(eqn. 11)
Where Wc, Wm and Wf are the weights of the composites, matrix and fibres respectively, whereas ρm and ρf are the density of matrix and fibres respectively. Fiber length distribution and diameter of the extracted sisal fibres from the composites were analysed using optical microscopy (Model: Olympus SZX7). 4.2. Measurement of tensile strength of single fibre Tensile properties of untreated and surface treated sisal fibres were carried out according to ASTM D3379-75 standard test method for single filament. For this purpose, untreated and surface treated single fibre was fixed to the rectangular cardboard sheet with the dimensions of 50 mm × 20 mm (length × width) which has a 20 mm hole in the middle of the cardboard sheet as shown in Fig. 1. The single fibre was mounted on the cardboard sheet by using polyvinyl acetate glue. The mounted fibres were then placed in the grips of tensile testing machine (model: Toyoski tensile testing machine) and a cutter was used to cut the supporting sides of the cardboard. The tensile test was
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carried out at a cross head speed of 0.5 mm/min using 10 N-cell load. The fibres were assumed to be cylindrical and the diameter of the fibres was measured at five different points using optical microscopy (Model: Olympus SZX7). The obtained average diameter was then used to calculate the tensile properties of fibres considering twenty specimens.
Fig. 1 Schematic diagram of the method followed to determine the tensile strength of single fibre
4.3. Tensile properties of SF/PLA composites Dumb-bell shaped specimens were punched from the molded sheets of PLA, untreated, HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites after 24 h of storage. Ultimate tensile strength (maximum stress), elongation at break and Young’s modulus were measured according to ASTM D638 standard using Toyoski tensile testing machine at a constant crosshead speed of 5 mm/min. Six specimens were tested and at least five replicate specimens have been presented as an average of tested specimens. 4.4. Mechanical properties
600
20
500 400 300 200 100 0 UT
ULT ALK Fibre treatments
(A)
ALKULT
Tensile modulus (GPa)
Tensile strength (MPa)
4.4.1 Tensile properties of single sisal fibre Fig. 2shows the typical stress-strain curve obtained from the tensile tests. Similar to other natural fibres, sisal fibres showed brittle nature under tensile test conditions and a sudden drop in the load was observed when the fibres break [25,26].
15 10 5 0 UT
ULT ALK Fibre treatments
ALKULT
(B)
Fig. 2 Tensile properties of untreated (UT), HIU treated (ULT), alkali treated (ALK) and the combination of alkali and HIU treated (ALKTULT) sisal fibres: (A) tensile strength (B) tensile modulus
Fig. 2 (A) and (B) show the plot of tensile strength and modulus values against different surface treatments of sisal fibres. Tensile strength and modulus of HIU treated sisal fibres showed a slight decrease by 5.6% and 5%
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respectively as compared to the untreated fibres. However, a significant reduction in the tensile strength and modulus was observed for the alkali and the combination of alkali and HIU treated sisal fibres. The tensile strength and modulus reduced by 25% and 26% respectively for the combination of alkali and HIU treated sisal fibres as compared to untreated sisal fibres. This is mainly due to the removal of amorphous materials such as hemicellulose, lignin and other waxy materials from the surface of fibres.Hemicellulose and lignin are the two main components on the cell wall structure of natural fibres which are tightly bound to the cellulose. Removal of these amorphous materials due to alkali treatment causes fibre degradation and leads to the reduction in the mechanical properties of fibres[27–29]. Moreover, the application of HIU for alkali pre-treated SFs significantly damages the surface of fibres and reduces the fibre diameter which significantly reduces the mechanical properties [10,30]. These observations are in agreement with the obtained results of FE-SEM. 4.4.2 Tensile properties of SF/PLA composites Tensile properties of PLA composites reinforced with sisal fibres of untreated and different surface treated are evaluated and Fig. 3 shows the typical stress vs strain graph of 15wt% loading of SF/PLA composites. It may be observed that the curves show linear deformation followed by sudden fracture. Tensile strength of untreated, HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites was evaluated and illustrated in Fig. 4. It could be noted from this figure that the tensile strength decreased with an increase in the fibre loading as compared to pure PLA except for 15 wt% of fibre loading for which the tensile strength slightly increased to 54.24 MPa (10% higher than pure PLA). However, as compared to the untreated SF composites, surface treated sisal fibres composites showed a gradual increment in the tensile strength at all the percentages of fibre loading. Pure PLA ALKT
UT ALKTULT
ULT
Stress (GPa)
0.06 0.05 0.04 0.03 0.02 0.01 0 0.00
0.89
2.22 Strain (%)
3.54
4.86
Fig. 3Typical stress versus strain curves of untreated (UT), HIU treated (ULT), alkali treated (ALK) and the combination of alkali and HIU treated (ALKTULT) SF/PLA composites
Combination of alkali and HIU treated SF/PLA composites showed superior tensile properties. It is worth noted that the tensile strength of surface treated SF/PLA composites was superior than the surface treated single sisal fibres. This is due to the removal of amorphous materials which act as binding materials for cellulose component inside the fibres. Removal of these amorphous materials causes the voids on the surface of sisal fibres which leads to a reduction in the tensile strength. However, in case of composites, removal of amorphous materials enhances the adhesion between fibres and matrix materials which leads to increase the tensile strength. Similar observation has been reported by Orue et al [27] where they noticed that the tensile strength reduced by more than 50% for the combined treatment of alkali and silanes as compared to the untreated sisal fibres. However, they also reported that the tensile strength of treated SF/PLA composites showed superior tensile properties as compared to untreated SF/PLA composites. PLA composites with the fibre loading of 15 wt% showed higher tensile strength among all other fibre contents. The tensile strength of HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites were 43 MPa, 47 MPa and 54.24 MPa respectively as compared to the untreated sisal fibre
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composites, which were 10%, 20% and 33% higher than the untreated fibres composites with the same weight percentage (15 wt%) of fibre loading. Pure PLA
(a)
UT
ULT
ALKT
ALKT-ULT
Tensile modulus (MPa)
Tensile strength (MPa)
60 50 40 30 20 10 0 0
10
15
20
Fibre loading (wt%)
25
30
Pure PLA
(b)
UT
ULT
ALK
ALKULT
2000 1800 1600 1400 1200 1000 800 600 400 200 0 0
10
15
20
25
30
Fibre loading (wt%)
Fig. 4(a) Tensile strength and (b) modulus of untreated (UT), HIU treated (ULT), alkali treated (ALK) and the combination of alkali and HIU treated (ALKTULT) SF/PLA composites
This trend is almost the same at all fibre loading. An increase in the tensile strength for the treated sisal fibres composites as compared to the untreated sisal fibres composites indicates the removal of non-cellulosic (amorphous) materials and other waxy materials from the surface of sisal fibres during surface treatment [31,32]. These results further support the observations obtained from FTIR and FE-SEM from our previous work [23]. Ma and Whan[33] reported that the bamboo fibres reinforced PLA composites with different surface treatments such as Plasma, UV irradiation and direct silane treatments showed an increase in the tensile strength by 43.7%, 60.7% and 71.1% respectively. As compared to pure PLA, the tensile strength decreased for the SF/PLA composites with an increase in the fibre loading from 10 to 30% irrespective of the surface treatments. This may be due to an increase in the fibre agglomeration in the fibre-matrix interface which leads to poor interfacial adhesion between the fibres and matrix. Tisserat et al. [34] observed that the tensile strength of PLA composites decreased significantly by 20% with the addition of 25 wt% of paulownia wood filler as compared to pure PLA. They also indicated that the aspect ratio of filler materials play an important role in improving the mechanical properties of composite materials. Lee et al. [35] observed a reduction in the tensile strength by 10% with the addition of 20 wt% of sisal fibres in the PLA composites as compared to pure PLA. They also indicated that the poor interfacial adhesion between the fibre and matrix and the agglomeration of fibres across the matrix resulted in reducing the tensile strength. In contrast to the tensile strength, tensile modulus enhanced significantly with the incorporation of sisal fibres into PLA matrix at all the fibre loading as compared to pure PLA. It can be seen from Fig. 4 that sisal fibre loading of 30 wt % on the PLA composite increased the tensile modulus by 25%, 46%, 54% and 75.4% for the untreated, HIU treated, alkali treated and the combination of alkali and HIU treated SF/PLA composites respectively as compared to pure PLA. These results are in agreement with the observations of Ragoubi et al. [36] where they observed that the tensile modulus increased by 19% and 31% for miscanthus grass fibres reinforced PLA composites with and without fibre surface treatments respectively as compared to pure PLA. A significant enhancement in the tensile modulus is attributed to improved interfacial adhesion between the sisal fibres and PLA matrix which enhances the load transfer from the matrix phase to filler phase [33,35,37,38]. It is worth noted that the aspect ratio of the fibres playing an important role in altering the mechanical properties of fibre reinforced polymer composites. A significant reduction of percentage elongation that is more than 50% could be seen with an addition of even 10 wt% of untreated sisal fibres. It is further reduced by 20% with an increase in the fibre loading up to 30 wt%. Addition of fibre fillers onto the PLA matrix restricts the mobility of polymer chain and making it more resistant to elongation at break as compared to pure PLA. This is in agreement with the reports of Petinakis et al. [39] and other researchers [34–36,38].
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4.4.3 Theoretical predictions of Young’s modulus Practically, it is impossible to calculate the experimental values of tensile or Young’s modulus of natural fibre reinforced polymer composites accurately due to large variation in the matrix and fibres parameters such as aspect ratio of fibres, fibre orientation, surface treatments of fibres etc., However, intrinsic tensile modulus of the reinforcing fibres is the key parameter to predict the modulus or strength of the fibre reinforced composites [21]. Mathematical models are commonly used to estimate the theoretical values of the tensile or flexural modulus of natural fibre reinforced polymer composites [15]. Figs. 5-6show the comparison of Young’s modulus obtained from the experimental values of different surface treated (untreated, HIU treated, alkali treated and the combination of alkali and HIU treated) SF/PLA composites with the theoretical values calculated by using different mathematical models. It can be seen that Young’s modulus gradually increases with an increase in the volume fraction of fibres. It is well-known that the stiffness of the fibre reinforced composites increases with the addition of filler materials to the polymer matrix. The modulus mainly depends on the intrinsic rigidity of the fibres and the matrix [14,21]. The fibre efficiency factor (ηe) was calculated using the eqns. 3 to 6 which was 0.53. This theoretically calculated value of the fibre efficiency factor was found to fit with the experimental values.
Tensile modulus (GPa)
2.5
Parallel Hirsch
2 1.5 1 0.5 0 0.089
0.135
0.184
0.228
Volume fraction of fibres (Vf)
Experimental Series BBM
(b) Tensile modulus (GPa)
Experimental Series BBM
(a)
0.275
3
Parallel Hirsch
2.5 2 1.5 1 0.5 0 0.089
0.135
0.184
0.228
0.275
Volume fraction of fibres (Vf)
Fig. 5 Variations of theoretical and experimental values of tensile modulus for the (a) untreated (b) SF/PLA composites
It could be seen from Figs. 5-6 that at initial fibre loading there is a good agreement between the experimental and theoretical values in all the models. This may be due to low volume of fibre loading. It is well known that at low volume fractions of fibres, it can be assumed to be a uniform distribution of stress and strain which results in better load transform across the fibres from the matrix phase in the composite which leads to improving the mechanical properties of fibre reinforced polymer composites. However, in case of higher fibre volume fractions, there could be large variations between experimental and theoretical values. This may be due to more possibilities for the fibre agglomeration across the matrix which causes an uneven distribution of stress and strain and leads to a reduction in the mechanical properties. Kalaprasad et al. [12] studied the theoretical modelling of tensile properties of short SF/LDPE composites. Parallel and series models were commonly used for continuous fibre reinforced polymer composites, where the stress transfer mechanism is different as compared to short fibre reinforced polymer composites. In short fibre composites, the stress transfer largely depends on the fibre orientations inside the composite, stress concentration at the end of the fibres as well as on the critical fibre length. It could be seen from Figs. 5-6 that the experimental values of surface treated SF/PLA composites do not show much variation as compared to theoretical values. However, there is a marginal agreement between the experimental and theoretical values for the alkali treated and the combination of alkali and HIU treated SF/PLA composites (Figs. 5-6). Hirsch model is a combination of parallel and series models which shows a good agreement between theoretical and experimental values in all the cases. It is mentioned that the prediction of Hirsch model largely depends on the value of x in eqn 7[15,21]. The factors which control the x values are: fibre orientation, fibre length and the effect of stress concentration of the fibres. The stress transfer factor x is approximately 0.4 which is in agreement with the semialigned fibre reinforced polymer composites [12]. The theoretical values of Young’s modulus of modified Bowyer and Bader model (BBM) were calculated using the eqn. 8.
P. Krishnaiah.et.al.,/ Materials Today: Proceedings 5 (2018) 22494–22505
Tensile modulus (GPa)
3
Parallel Hirsch
2.5 2 1.5 1 0.5
Experimental Series BBM
(b) Tensile modulus (GPa)
Experimental Series BBM
(a)
22503 Parallel Hirsch
2.5 2 1.5 1 0.5 0
0 0.089
0.135
0.184
0.228
Volume fraction of fibres (Vf)
0.275
0.089
0.135
0.184
0.228
0.275
Volume fraction of fibres (Vf)
Fig. 6 Variations in the theoretical and experimental values of tensile modulus of (a) alkaliand (b) the combination of alkali and HIU treated SF/PLA composites
The values of K2 and K1 were calculated to be 0.5 and 1 respectively by using eqns. 9 and 10. These values are in agreement with the observations of Joseph et al. [11].It was noticed that there is a marginal variation with the experimental and theoretical values when BBM model was applied for predicting the Young’s modulus of SF/PLA composites. Unlike other theoretical models, BBM model exhibits higher values as compared to the experimental values. This may be due to variations in the fibre orientation factor (K1) and fibre length factor (K2) for the untreated and HIU treated SF/PLA composites.From these observations, it is noticed that the values of Young’s modulus of theoretical models are in reasonable correlation with the experimental values in all the models except a little variation in the BBM model. A good agreement in the experimental and theoretical values of Young’s modulus of untreated and different surface treated SF/PLA composites reveals better utilization of SFs and PLA matrix for the generation of SF/PLA composites. 5. Conclusions In this work, the mechanical properties of SF/PLA composites were studied with different percentage of fibre loading. Tensile strength increased by 10% for 15 wt% of sisal fibre loading using the combined treatment as compared to the untreated fibres. However, a further increase in the fibre loading above 15% decreased the tensile strength. Tensile modulus for 30 wt% of fibre loading showed 75.4% increment as compared to pure PLA. The theoretical models such as series and parallel, Hirsch’s and Bowyer and Bader were used to predict the Young’s modulus of the untreated and different surface treated short SF/PLA composites to fit the obtained experimental values. Young’s modulus of the untreated and different surface treated short SF/PLA composites was presented as a function of volume fractions of the fibres. All the models except Bowyer and Bader model (BBM) show a reasonably good agreement with the experimental values particularly at lower fractions of fibres. The series and Hirsch’s models show very good correlations with the experimental values. The effect of surface treatments on the tensile properties was also investigated. The results of theoretical models revealed that the surface treatments of fibres influence the tensile properties of short SF/PLA composites. Overall, this investigation proves that the surface treatments of alkali and HIU are an effective way for improving the mechanical properties of SF/PLA composites. References: [1]
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