1. Introduction

Environmental and economic concerns are stimulating research in the development of eco-friendly composites for several engineering applications. In this regard, researchers put their attention towards the utilization of low-cost lignocellulosic fibers, such as bamboo, jute, sisal, PALF, coir, ramie, etc. as reinforcements in petroleum-based thermoplastics. Owing to their exceptional features such as lightweight, renewability, biodegradability, easy availability, high specific strength & stiffness, good acoustic, thermal, and electrical insulation make them as promising candidates compared to inorganic fibers (Ali et al., 2017; Ali et al., 2018; Basu et al., 2017; Bodirlau et al., 2013; Holt et al., 2014; Khalil et al., 2007; Latif et al., 2015; Li et al., 2016; Qaiss et al., 2014; Salasinska et al., 2016; Shi et al., 2013).

Among all the natural cellulosic fibers, the PALF has significant potential to be used as a reinforcing agent in fiber or powder form. This is because of the presence of high cellulose content (70-85%), low-density, and low microfibrillar angle. Moreover, PALF is a waste product of pineapple cultivation and therefore relatively inexpensive which can be employed for industrial purposes (Arib et al., 2004; Mishra et al., 2001; Pavithran et al., 1987). The fibers derived from pineapple leaves have excellent characteristics for use in automotive, building & construction, packaging, and general engineering applications. According to the statistical database (2017) of ‘Food and Agricultural Organization’, the total production of pineapple fruit in the entire world is 25.8 million tons (FAOSTAT, 2017).

Studies showed that the PALF could be used effectively for polypropylene (PP), polyethylene (PE), thermoplastic starch, and rubber reinforcement (Kengkhetkit & Amornsakchai, 2012). In addition, its carbon footprint is much lower than that of other cultivated natural and synthetic fibers (Kengkhetkit & Amornsakchai, 2014). Although cellulosic fibers have number of advantageous features, they exhibit some problems such as hydrophilic nature, limited thermal stability, and poor dispersion characteristics within the non-polar thermoplastic matrix (Bledzki et al., 1998; Cantero et al., 2003; Kazayawoko et al., 1999; Raj et al.,1989). These limitations can be remedied by several methods like grafting functional moieties onto the fibers, introduction of coupling agents, or pre- treatment of fibers with suitable agents. There have been numerous studies conducted for improving the interfacial adhesion between natural fibers and the matrix.

Table 1 reported that the chemical treatment has profound influence on the mechanical properties of fiber-reinforced composites (FRCs). It was observed that the use of a coupling agent containing maleic anhydride, polyethylene-grafted-maleic anhydride (MAPE) has not been addressed significantly. Therefore, in the present communication, the effect of maleic anhydride coupling agent on mechanical properties of PALF/HDPE/MAPE biocomposite was investigated. A review of the existing literature (Table 2) revealed that the fiber length and loading are the other critical parameters that can affect the thermo-mechanical properties of biocomposites. It was observed from literature review that most of the work includes one parameter effect at a time while keeping other at a fixed level. The interaction effect of various parameters has not been discussed so far. Therefore, to solve this problem, the design of experiment (DOE) method was used (Ashenai Ghasemi et al., 2016; Mhalla et al., 2017; Rostamiyan et al., 2014; Rostamiyan, Fereidoon, Mashhadzadeh et al., 2015; Rostamiyan, Fereidoon, Rezaeiashtiyani et al., 2015; Rostamiyan, Fereidoon, Nakhaei et al., 2017; Subasinghe et al., 2016; Ghasemi et al., 2016).

Several optimization techniques such as RSM, Taguchi method, full factorial, fractional factorial, ANN, fuzzy logic, and GA are available which have significant potential to optimize the performance parameters (Mohamed et al., 2015). Furthermore, these techniques have significant potential to develop models that can predict and established the relationship between different inputs and response variables. RSM is a promising analytical tool to determine the significance of interactions and square terms of parameters, 3D response surface generation, and optimize the parameters. A three-level BBD is economical and popular in industrial research for modeling and optimizing the parameters to satisfy the defined desirable response variables. Therefore, in this study, the Box-Behnken design (BBD) which is a subset of RSM was employed as a DOE method for optimizing the mechanical properties of HDPE/PALF/MAPE biocomposite. The individual and simultaneous effects of the fiber length, PALF loading, and MAPE compatibilizer content on tensile, flexural, and impact properties were investigated.

2. Experimental

2.1. Materials

High-density polyethylene [HDPE, density=950 kg/m³, melt flow index (MFI)=12.3 g/10 min] and the coupling agent, polyethylene maleic anhydride (MAPE, Mw 26000) was obtained from M/s Solvay Chemicals Ltd. Kerala, India. The pineapple leaf fiber (PALF) with diameter ranging in between 60 to 100 µm was collected from M/s Go Green products, Chennai, India. The collected fibers were thoroughly washed and then dried in an oven at 60^oC to a final moisture content of 5%. The elemental composition and SEM micrograph of the studied PALF is shown in Table 3 and Figure 1, respectively.

2.2. Fabrication of biocomposite samples

Biocomposite samples of each formulation (Table 4) were obtained through melt-blending in a twin-screw extruder (Brabender plasticorder, Germany), which has a mixing chamber of 69 cm³ volumetric capacity. To improve the interfacial bonding strength between fiber and matrix resin, a pre-impregnation technique was utilized. The obtained pre-pregs were cut into 6 mm long pellets and further blended with pure HDPE to obtain the desired composition. The mixing was conducted at 190^oC and 35 RPM for 10 min. The pre-mixed pellets were molded in TS-270 injection molding machine (Windsor, India) under the temperature setting of injection hopper and nozzle at 170^oC and 180^oC, respectively. Specimens were conditioned in a laboratory atmosphere of 23±5^oC and 50% RH prior to testing.

2.3. Mechanical tests

Tensile and flexural (three-point bending) tests were conducted on a calibrated Tabletop Tinius Olsen Horizon H50KS, a Universal testing machine as per ASTM D638 and D790 standard, respectively. A digital pendulum impact tester was employed to determine impact strength. Four specimens for each composite sample were evaluated and their average values were reported.

2.4. Response surface method

RSM is a mathematical and statistical technique that can assess the effect of individual parameters and the interaction of parameters on response variables. It has significant potential in modeling and analysis of problems with a small number of experimental data points (Myres et al., 2016). In RSM, a three-level Box Behnken design (BBD) was used to study the effect of linear, quadratic, cubic, and cross product models of three performance parameters and to develop an experimental design matrix. The design was built in Design-Expert software (version 6.0.8) with consideration of three critical variables i.e., fiber length (A), fiber loading (B), and MAPE content (C). The range and levels of the selected parameters are shown in Table 5. In this work, the second order polynomial equation was used for fitting the experimental data and find out the relevant model terms.

Where y represents the predicted response, b_o , regression equation constant; b_i , linear coefficient; b_ii , square coefficient of each parameter; and b_ij , first order interaction coefficient. To estimate the desired responses with reliable measurements, a total number of 17 experiments were conducted with 12 factorial points. Furthermore, the experimental sequence was randomized to minimize the effect of uncontrollable parameters. The regression (R²), adjusted (R²), predicted (R²), pure error sum of squares, lack of fit, response plots, and adequate precision were used in the determination of the robustness of the developed model.

3. Results and discussion

3.1. ANOVA and regression models

The experimental test results were used to generate the mathematical models for each response variable. Among different mathematical models (linear, quadratic, cubic and 2FI), the quadratic was selected according to three different tests - the sequential model sum of squares, adequacy, and lack-of-fit (Table 6). The sum of square and mean square in ANOVA results (Tables 7-11) also revealed the adequacy of quadratic model for fitting the experimental data. The second-order equations (ii-vi) express the overall predictive model in terms of variables:

where A, B, and C are fiber length, fiber loading, and MAPE content, respectively. The significance of each model term was explained in terms of probability value (p-value). To eliminate non-effective terms and make regression model best fitted, ANOVA was repeated, and the results are presented in Tables 12-14. A fitted regression model with statistical significance is presented in the following equations:

validation of regression models at 95% confidence interval was explained in terms of p, F, and R² values. The insignificant “Lack-of-Fit” and p-values less than 0.05 confirmed the rejection of null-hypothesis. The model F (107.22) and p (<0.0001) values for tensile strength, F (560.03) and p (<0.0001) values for tensile modulus, F (475.55) and p (<0.0001) values for flexural strength, F (36.64) and p (<0.0001) values for flexural modulus, and F (53.86) and p (<0.0001) values for impact strength showed that the quadratic model was significant. The regression model was further analyzed by evaluating R², adjusted R², and predicted R² which indicates the proportion of total variation in response variable predicted by model. The higher correlation coefficients confirm the suitability and correctness of the model. The adjusted R² can be used to prevent probability error while predicted R² indicates how well the model predicts responses for new observations. The adjusted R² and predicted R² are in reasonable agreement with the values of 0.9755 and 0.9433 for tensile strength, 0.9968 and 0.9816 for tensile modulus, 0.9963 and 0.9820 for flexural strength, 0.9304 and 0.8446 for flexural modulus, and 0.9586 and 0.9256 for impact strength, respectively. The R² value for response variables implying that a high correlation exists between observed and predicted values. In addition, the adequacy of the model was confirmed using normal probabili-ty plots of the residuals, predicted versus actual values, and residuals versus predicted values plots. Figures 2-4 revealed that the error points are normally distributed along a straight line, implying that the model is adequate, and it represents the experimental data. The correlation between predicted response values and the actual values is shown in Figures 5-7, presenting uniformly distributed data points around the mean of the response variables. The linear regression fit is obtained with an R² values of 0.9847, 0.9986, 0.9984, 0.9565, and 0.9767 for the tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength respectively, indicating that the model is accurately describing the experimental observations. In the end, the final plots of residuals versus predicted values (Figures 8-10) concluded that the residuals scattering for all mechanical responses were not significant which confirms that the proposed model was suitable. The maximum error (Tables 15-17) lies in between predicted and measured values were 4.10%, 0.26%, 1.51%, 1.01%, and 4.02% for the tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength, respectively.

3.2. Effect of design parameters on tensile properties of biocomposites

Figure 11 (a)-(d) illustrate the 3D surface response plots for tensile strength and modulus as a function of fiber length and content, keeping the MAPE load fixed at 1 and 5 wt.%. The tensile strength was increased to a maximum value and then decreased slowly with the increase of fiber length. However, the tensile modulus was varied inversely with fiber length, and it was due to the severe fiber breakage and fibrillation of longer PALF during composite compounding. Irrespective of fiber length, the incorporation of PALF beyond a specified limit did not produce any significant reinforcing effect. This was because of the agglomeration and poor interfacial adhesion of fibers within the polymer matrix.

The variation in tensile properties with fiber length and compatibilizer content is shown in Figure 12 (a)-(d). The tensile strength and stiffness were increased to maximum values and then decreased with the increase of MAPE content. It was due to the excess removal of lignin compound in highly concentrated solution. Figure 13 (c)-(d) reveals the significant effect of fiber loading and MAPE content on tensile strength and modulus. The compatibilizer effect was pronounced only under high fiber loading condition (24 wt.%) which results in an increase of tensile modulus with the increase of MAPE content. It was due to the formation of rigid networks which constraint the movement of polymeric chains. The 3.6 wt.% MAPE compatibilized composite having 17% fiber content with 13 mm fiber length exhibits maximum tensile strength (Table 18).

The main effect plot of each parameter on tensile properties of biocomposites is shown in Figure 14. Among all three critical variables, the fiber loading has highest degree of influence on tensile properties. The strength and stiffness of biocomposites were increased to maximum values and then decreased with the increase of fiber content.

3.3. Effect of design parameters on flexural properties of biocomposites

The interaction effect of fiber length and content on flexural properties of biocomposites is shown in Figure 15 (a)-(d). Like tensile strength, the flexural strength as well as flexural modulus were increased to maximum values and then gradually decreased with the increase of fiber length. The flexural modulus of biocomposites was increased with the increase of fiber content and it was due to the presence of hydroxyl groups in PALF which facilitate in binding of polymeric chains and the chains entanglement.

Figure 16 (a)-(d) reveals the combined effect of fiber length and MAPE content on flexural properties of biocomposites. It was observed that for all fiber length, the flexural strength and modulus were increased with the increase of compatibilizer load, and it was due to the formation of covalent linkages in between anhydride and hydroxyl (-OH) groups of fibers. The interaction effect of fiber loading and MAPE content on flexural properties of biocomposites is shown in Figure 17 (a)-(d). It is worth to noted that the reinforcement efficiency of PALF was significantly increased after the addition of compatibilizer. The interaction between fiber loading and MAPE content significantly affects the flexural strength (p-value BC<0.05). The 4.5 wt.% MAPE compatibilized composite having 22% fiber content with 14 mm length exhibits maximum flexural strength (Table 18).

Figure 18 illustrates the main effect plot of each individual parameter on flexural strength and modulus. It was observed that the fiber volume content has highest degree influence on the flexural behavior of developed composites. The addition of fibers beyond a limiting volume could not increase the strength of composite. However, the flexural modulus was increased progressively with the increase of fiber volume fraction. The fiber aspect ratio (length to diameter ratio) is a crucial parameter which dictates the reinforcing efficiency of fiber. The bending strength and stiffness were increased to maximum values and then decrease with the increase of fiber length. This kind of behavior was found in kenaf fiber-filled poly (butylene succinate) biocomposites (Thirmizir et al., 2011).

3.4. Effect of design parameters on impact strength of biocomposites

Figure 19 (a)-(b) illustrates the effect of fiber length and MAPE content on impact strength of biocomposites, keeping the fiber content was fixed at 8 and 24 wt.%. It was observed that the composite of higher aspect ratio (L/D) yields better impact toughness than the composite of low L/D ratio. Moreover, the low aspect ratio results in stress concentration and poor dispersion of fibers in HDPE matrix (Gamstedt et al., 2007). It is worth noted that the impact fracture strength of composites was decreased with the increase of compatibilizer content. This was attributed to the predominant action of fiber-fracture than the fiber pull-out.

In fiber composites having strong interfacial adhesion, fiber fracture is more common and dissipates lesser energy than the fiber pull-out (Wambua et al., 2003). The overall impact toughness of a fiber-reinforced composite (FRP) depends on the nature of the constituent elements, internal structure and geometry of the composite, fiber morphology, chemical composition, and interfacial adhesion between filler element and the matrix.

The interaction effect of fiber loading and MAPE content on impact strength of biocomposites is shown in Figure 20 (a)-(b). It was observed that the impact strength of composites was decreased with the increase of PALF content. This implies that the loading of PALF results in transition from ductile to brittle behavior. The important toughening mechanism in FRP is crack bridging by fibers associated with frictional sliding during fiber pull-out. It is worth noted that the composite shows poor mechanical strength at high fiber content (24 wt%)

The 1 wt.% MAPE compatibilized composite having 9% fiber content with 14 mm length exhibits maximum impact strength (Table 18).

The main effect plot of each individual parameter on impact strength of biocomposites is shown in Figure 21. The PALF content has highest degree of influence on impact strength. Moreover, the F value of fiber content (140.29) at 0.05 probability level confirms its highest significance on impact strength. The decrement in impact strength with increase of PALF content may be due to the agglomeration of fibers. Table 14 reported that the compatibilizer content is another crucial parameter with a significant F value (112.98). It was observed that the impact strength was decreased with the increase of MAPE content. The stronger interfacial adhesion results in the major occurrence of fiber fracture than that of fiber pull-out under loading condition.

3.5. Optimization of response variables

The numerical optimization method was employed to generate the optimal condition for each response variable. The goal was to maximize strength and modulus (Table 19); therefore, the target was set at highest value obtained from experimental results. The pre-defined goal, importance level, optimum and desirable values of each parameter are shown in Figures 22-23. To validate the Box-Behnken design model, a minimum of four samples were developed at the optimum condition of performance parameters. It was observed that the obtained experimental results (Table 20) are within a 5% difference from predicted values which proves the adequacy, reliability, and significance of current model.

4. Conclusions

The primary objective of this work was to optimize the mechanical properties of biocomposites using Box-Behnken RSM technique. The effects of fiber length, fiber content, and compatibilizer load on mechanical properties of PALF/HDPE biocomposites as summarized below.

• The quadratic model was suitable and showed reasonable agreement in correlation coefficients (R², adj. R², and pred. R²) for predicting the mechanical responses. Moreover, the verification of model fitness using statistical ANOVA technique confirmed its adequacy and reliability for navigating the design space.

• The experimental and statistical results showed the significance of all three influential variables, i.e., fiber length, fiber loading, and MAPE content on tensile, flexural, and impact properties. Furthermore, the interaction of fiber loading and MAPE were highly related to the mechanical properties of biocomposites.

• Based on final ANOVA results, the PALF loading had the greatest effect on modulus of rupture (MOR) and modulus of elasticity (MOE) of HDPE/PALF/MAPE biocomposites.

• The most improvements of tensile and flexural properties (strength and modulus) were obtained in the medium levels of fiber length and high levels of fiber loading and MAPE content. However, the maximum impact strength was attained in low levels of fiber load and MAPE content and a medium level of fiber length. Under the synergistic combination of these optimal conditions; the tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength were predicted to be 32.35 MPa, 1475 MPa, 49.21 MPa, 1659.04 MPa, and 58.24 J/m, respectively.

Acknowledgments

The authors extend their gratitude to the Delhi Technological University, Delhi (India) for supporting this research work.

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Notes

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*Corresponding author. E-mail address: mohit.30mittal@gmail.com (M. Mittal).

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