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Determining the optimum conditions for silane treated rubberwood flour-recycled polypropylene composites using response surface methodology
Materials Today Communications 24 (2020) 100971
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Determining the optimum conditions for silane treated rubberwood flourrecycled polypropylene composites using response surface methodology
T
Sriwan Khamtreea, Thanate Ratanawilaia,*, Sukritthira Ratanawilaib a b
Department of Industrial Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla, Thailand Department of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla, Thailand
ARTICLE INFO
ABSTRACT
Keywords: Rubberwood flour Recycled polypropylene Surface treatment Silane concentration Mechanical properties Response surface methodology
The objective of this work was to determine the optimum combination of two materials in production of woodplastic composites, using response surface methodology (RSM). The two independent variables investigated were the wood content and the silane concentration. The experiments were conducted based on a central composite design. The findings showed that the wood content significantly affected the tensile and flexural properties of the composites as well as their water absorption and the silane concentration also significantly affected the water absorption of the samples. The optimum properties found by RSM were a content of 39.22 wt% wood, 60.78 wt% recycled polypropylene and a 3.44 % silane concentration. The findings from this work can be used to produce value-added and environmentally friendly panels for a variety of different applications.
1. Introduction In the past 25 years, wood-plastic composites (WPCs) have gained popularity and attracted attention due to their potential applications in many industries, including the construction and automobile industries as well as the manufacture of decking, railing, fencing, and building materials [1,2]. Such popularity of WPCs is due to their many advantages, including, high strength, durability and stiffness, low-density and low maintenance requirement, as well as being constructed from renewable resources and thus being environment-friendly [3]. WPCs are relatively new materials as compared to particle- or fiber-board which are the traditional types of wood composite panels [4]. In general, WPCs are made from small particles of lignocellulosic fiber reinforced by a polymer matrix. Spruce, pine, fir, eucalyptus, and oak are the most common wood species used in the manufacture of WPCs although non-wood resources such as palm and bamboo can also be utilized [5–8]. Rubberwood waste material from furniture and lumber production has also been used to manufacture experimental WPC panels in a number of previous studies [4–9] and the use of wood waste as a reinforcement in composites with a recycled polymer matrix creates value from waste materials and thus reduces the production cost of WPCs.
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Previous studies have reported that many variables influence the physical and mechanical properties of the final WPC product, including the polymer matrix, the wood species and content, and the coupling agent used [5]. Ratanawilai and Taneerat [10] investigated the influence of different types of polymers, including low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), polypropylene (PP), and polystyrene (PS) on the mechanical properties of WPCs, with the results showing that PP gave the best mechanical properties. The use of recycled plastic has added benefits such as the low cost of the material, high heat stability, and a reduction in the environmental impact [5,11]. Some studies have also reported that WPCs containing recycled polypropylene (rPP) exhibited higher tensile and flexural properties than those made from virgin PP [11]. Kaewkuk et al. [12] found that a fiber content of 10, 20, or 30 % by weight in sisal-PP composites increased the tensile and water resistance properties. Leu et al. [5] evaluated the optimization of four parameters in the manufacture of WPCs, finding that the mechanical and physical properties were optimized by the following values: (1) wood particle size: less than 125 μm; (2) coupling agent content: 3 %; (3) lubricant content: 3 %; (4) wood content: 50 % or less. In addition, chemical surface treatments such as dewaxing, alkaline,
Corresponding author. E-mail address: [email protected] (T. Ratanawilai).
https://doi.org/10.1016/j.mtcomm.2020.100971 Received 11 May 2019; Received in revised form 25 August 2019; Accepted 31 January 2020 Available online 01 February 2020 2352-4928/ © 2020 Elsevier Ltd. All rights reserved.
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silane, benzoylation, and acrylation treatments have been applied to promote the interfacial adhesion between the wood and the polymer and to improve the properties of composites [13–16]. Farsi [15] reported that alkaline treatment and the use of a coupling agent in beechwood-PP composites produced the optimal tensile and flexural properties while Asumani et al. [13] found that in a combined alkalinesilane treatment, a sodium hydroxide (NaOH) solution applied at a proportion of 6 % produced the highest tensile and flexural properties among values of between 1 and 8 % tested, in kenaf fiber-reinforced PP composites. Khamtree et al. [17] investigated the optimal wood content of rubberwood flour (RWF)-rPP composites after the wood was treated with a 2 % NaOH solution. The results showed that a wood content of 30 % produced the highest tensile and flexural properties relative to other wood contents between 20 and 50 %. Gwon et al. [18] explored the effect of silane concentrations ranging from 0.5 to 3.0 % on the properties of WPCs, finding that the concentration affected the tensile strength of wood fiber-PP composites. Similarly, Sepe et al. [19] studied the effect of silane concentrations of 1, 5, and 20 % on the mechanical properties of hemp-fiber reinforced epoxy composites with the results indicating that a concentration of 1 % optimized their properties. Meanwhile, Nopparut and Amornsakchai [20] tested hemp-HDPE composites with fiber contents varying between 20 and 50 % and reported that their stiffness increased with fiber contents of up to 40 % but decreased with a content of 50 %. Further, the silane-treated hemp-HDPE composites had higher thermal stability as compared to those treated with NaOH. In addition, Khamtree et al. [21] investigated the effect of the alkaline-silane treatment of RWF in rPP composites. They found that the silane concentration significantly affected the water absorption, tensile properties, flexural properties and hardness of the RWF-rPP composites and a 5 % silane concentration produced the optimal properties. This study adopted response surface methodology (RSM) as the method of determining the optimum parameters since RSM has been widely used in previous studies to optimize variables and responses with the minimum number of experiments [22,23]. There are three classifications of RSM, the Box-Behnken design, the three-level factorial design, and the central composite design (CCD), and CCD is, widely proposed as the most effective and popular design for the optimization of variables [23]. Several studies have conducted studies to statistically determine the optimum conditions to enhance the properties of WPCs. Homkhiew et al. [24,25] determined the optimal combination of wood, plastic, and chemical additives in RWF-rPP composites using a D-optimal mixture experimental design. They found that composite samples which were made based on a formulation with 50.3 wt% rPP, 44.5 wt% RWF, 3.9 wt% maleic anhydride-grafted polypropylene, 0.2 wt% UV stabilizer, and 1.0 wt% lubricant had good mechanical properties and water resistance. Zhao et al. [26] applied RSM with a Box-Behnken design to determine the significant manufacturing process variables and to optimize the processing conditions of rubber-crumb-wood particle reinforced cement-based composites. Further, Amiandamhen et al. [27] employed a CCD design with RSM to determine the optimum process conditions including the binder ratio and the fly ash and wood content of composite panels and to predict their properties. In addition, Almansoory et al. [28] applied a CCD design with RSM to determine the optimal conditions for producing biosurfactant with the minimum surface tension activity in a bacterial-mediated process and Kataria and Garg [23] determined the optimal process variables for Cd(II) and Pb (II) adsorption onto ZnO nanoflowers using CCD with RSM. As reviewed above previous studies have explored the optimal conditions and concentrations of chemical treatments to optimize the properties of WPCs and this study used RSM employing a CCD to determine the optimal RWF content and silane concentration and investigated their effect on the tensile and flexural properties and water absorption of RWFrPP composites. It is expected that results from this study would provide a better understanding of such composites so that they can be used for different applications with more efficient service life.
2. Materials and methods 2.1. Materials RWF was obtained from a local furniture factory in Trang, Thailand. It was screened through a standard 80 mesh sieve to produce a particle size smaller than 180 μm. The amount of cellulose, hemicelluloses, lignin, and ash in RWF were 39 %, 29 %, 28 %, and 4 %, respectively [24]. The rPP pellets utilized were supplied by Withaya Intertrade Co., Ltd, Samutprakarn, Thailand, and had a melt flow index of 11 g/10 min at 230 °C, under the trade name WT170. NaOH and Triethoxyvinylsilane (97 %) were purchased from Merck Co. (Germany) and Sigma-Aldrich (USA), respectively. NaOH was pellet form with molecular weight 40 g mol−1. The chemical formula, a molecular weight, the boiling point and a density of triethoxyvinylsilane were C8H18O3Si, 190.31 g mol−1, 160−161 °C, and 0.903 g mL−1 at 25 °C, respectively. 2.2. Surface treatment of raw material The RWF was treated with NaOH solution having 2 % weight by volume at a temperature 25 °C for 24 h then washed with distilled water until it became neutral, following dried in an oven at a temperature 80 °C for 24 h. The alkaline-treated RWF was treated with silane concentrations ranging from 1 to 5 % (by weight percentage relative to RWF) in an ethanol and water mixture (50:50 by volume). The pH of the silane solution was maintained between 4.5 and 5 by adding acetic acid and then stirring at room temperature for 1 h. Finally, the alkalinetreated RWF was immersed in the silane solution for 2 h and then filtered and dried at 80 °C for 24 h. 2.3. Processing of composite samples Treated RWF was mixed with rPP pellets using a twin-screw extruder (Model EMT-26, En Mach Co., Ltd., Nonthaburi, Thailand). The temperature of the 10 barrels of the extruder was controlled at 160−190 °C and the screw speed was set at 60 rpm. The extruded compound was pelletized and placed in a hot press at 190 °C for 15 min at 1,000 psi before cooling under a pressure of 250 psi at room temperature. 2.4. Experimental design The experiments were designed based on a CCD, using Design Expert software (version 8.0.6, Stat-Ease, Inc., Minneapolis, USA). The CCD was designed to statistically evaluate the effects of the wood content and silane concentration on the tensile and flexural properties of the composites and their water absorption in order to optimize these dependent variables using RMS. The two independent variables, which were considered for optimization were the wood content (x1), and silane concentration (x2). The intervals selected over which to conduct the experimental design were 30−50 wt% wood content and 1–5 % silane concentration. The total experiments conducted were 13 runs in a randomized order including eight runs coupled with five center-point runs as displayed in Table 1. The tensile strength, tensile modulus, flexural strength, flexural modulus, and water absorption of the WPCs were considered in the experimental design. 2.5. Mechanical properties of the composite samples The tensile and flexural properties of the samples were investigated employing a universal testing machine (Model 5582, Instron Corporation, Massachusetts, USA) according to ASTM D638, and D790, respectively. The tensile test of the samples was carried out at a 5 mm/ min crosshead speed while the crosshead speed was 2 mm/min for 2
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Table 1 Central composite design (circumscribed) matrix with experimental and predicted values of the responses. Run
Variables
Tensile Strength (MPa)
Tensile Modulus (MPa)
Flexural Strength (MPa)
Flexural Modulus (MPa)
Water absorption (%)
x1
x2
Actual
Predicted
Actual
Predicted
Actual
Predicted
Actual
Predicted
Actual
Predicted
1
40.0
3.0
19.75
30.0
1.0
4
40.0
3.0
5
40.0
3.0
6
30.0
5.0
7
40.0
3.0
8
54.2
3.0
9
40.0
0.2
10
25.9
3.0
11
40.0
5.8
12
50.0
1.0
13
50.0
5.0
7.61 (0.21) 7.42 (0.17) 5.58 (0.21) 7.67 (0.20) 7.35 (0.33) 4.05 (0.36) 7.42 (0.20) 11.51 (0.26) 7.75 (0.37) 2.98 (0.37) 6.59 (0.29) 10.91 (0.30) 10.16 (0.36)
7.48
3
2627.78 (92.94) 2593.57 (84.09) 2299.26 (88.72) 2629.51 (83.02) 2538.64 (91.64) 2130.86 (89.24) 2651.17 (86.56) 3396.60 (70.16) 2518.34 (53.98) 2071.11 (67.66) 2603.78 (51.24) 3240.90 (51.26) 3047.69 (65.77)
2642.04
3.0
37.82 (0.57) 36.84 (0.65) 38.96 (0.62) 36.02 (0.21) 35.22 (0.52) 36.74 (0.55) 35.50 (0.50) 25.64 (0.51) 32.57 (0.59) 39.09 (0.52) 34.63 (0.63) 29.61 (0.56) 31.16 (0.55)
35.75
40.0
916.11 (70.05) 934.92 (61.17) 861.11 (49.67) 894.56 (60.02) 938.96 (59.97) 863.99 (44.81) 938.96 (68.14) 933.93 (60.46) 937.51 (71.58) 802.48 (54.92) 917.62 (50.03) 984.55 (50.10) 962.82 (40.28)
918.92
2
19.88 (0.30) 19.44 (0.36) 22.03 (0.14) 19.41 (0.10) 19.47 (0.33) 20.96 (0.50) 19.15 (0.33) 13.40 (0.29) 18.69 (0.31) 22.39 (0.28) 19.67 (0.18) 15.15 (0.20) 16.75 (0.26)
19.75 22.21 19.75 19.75 21.41 19.75 14.13 19.16 22.36 19.94 15.05 16.97
918.92 863.20 918.92 918.92 851.44 918.92 1006.39 927.15 832.06 910.69 986.40 974.64
35.75 38.12 35.75 35.75 36.88 35.75 25.95 33.26 38.70 35.06 27.26 29.78
2642.04 2205.56 2642.04 2642.04 2145.32 2642.04 3304.61 2684.21 1984.13 2599.87 3138.76 3078.52
7.48 5.03 7.48 7.48 4.05 7.48 11.65 8.16 3.33 6.79 10.91 9.93
Note: The value in parentheses are standard deviations from five replicates.
flexural testing on samples with an 80 mm span length. Five replications were conducted for each condition at room temperature and a relative humidity of 50 ± 5 % controlled for more than 40 h prior to testing.
Table 2 The p-values obtained in the ANOVA analysis of the polynomial models.
2.6. Water absorption of the composite samples
Source
Tensile Strength
Tensile Modulus
Flexural Strength
Flexural Modulus
Water Absorption
Terms
Quadratic < 0.0001* < 0.0001* 0.079 0.004* 0.001* 0.456 0.211 0.990 0.983 0.949 0.33 1.74
Linear 0.0001* 0.0001* 0.369 – – – 0.653 0.907 0.889 0.846 17.68 1.92
Quadratic 0.0004* < 0.0001* 0.542 0.172 0.009* 0.059 0.276 0.941 0.899 0.718 1.24 3.58
Linear < 0.0001* < 0.0001* 0.358 – – – 0.051 0.957 0.948 0.913 88.55 3.35
Linear < 0.0001* < 0.0001* 0.001* – – – 0.050 0.989 0.987 0.977 0.28 3.74
x1 x2 x1 x2 x12 x22 Lack of fit R2 Adj-R2 Pred-R2 Std. Dev. CV (%)
The water absorption testing was carried out according to ASTM D570. The samples were conditioned at 50 °C for 24 h in an oven and cooled in a desiccator prior to the tests. The samples were soaked in distilled water at 25 °C for 10 weeks. Five replications for each condition were tested at room temperature. The weight was immediately measured to the nearest 0.001 g at the commencement of the experiment and thereafter at weekly intervals for the entire period of testing. 2.7. Morphological analysis of the composite samples
Note: * p-value less than 0.05 is considered significant.
The morphologies of fractured surfaces of the WPCs were evaluated using a scanning electron microscope (SEM; model FEI Quanta 400 Oregon, USA) operated at 20 kV. In order to avoid electrical charge accumulation during imaging, the samples were coated with a thin layer of gold before the micrographs were produced.
with a very small p-value (p < 0.0001). Moreover, the lack of fit for the tensile strength model was 0.211 which was clearly not significant. This result implied that the model performed well and was an excellent fit for the data derived from the experiments [25]. Additionally, the fit of these models was evaluated using coefficients of determination (R2), adjusted coefficients of determination (Adj-R2), predicted coefficients of determination (Pred-R2), and coefficients of variation (CV%). The extent to which variability in the data was accounted for by the model was indicated by the R2 and Adj-R2 values while the Pred-R2 was employed to take into account the number of covariates or predictors in the model. The R2 coefficient provided satisfactory outcomes for the quadratic model to the experimental data [28]. The R2 values of the tensile strength, tensile modulus, flexural strength, flexural modulus, and water absorption were 0.990, 0.907, 0.941, 0.957, and 0.989, respectively. Thus, for example, the quadratic model for the tensile strength response shows that 99.02 % of the variability observed can be explained by the two independent variables
3. Results and discussion 3.1. Statistical analysis of the response model The models were subjected to analysis of variance (ANOVA) to determine the goodness of fit of the model to the empirical data as shown in Table 2. The results clearly show that the model fit was statistically significant (p < 0.05). A quadratic model produced the best fit for the tensile and flexural strength whereas, a linear model produced the best fit for the tensile modulus, flexural modulus, and water absorption. As an example, the model for the tensile strength was highly significant 3
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investigated in the production of the WPCs and that only 0.98 % of the total variability cannot be explained. Moreover, the R2 values were all generally very close to 1, which demonstrates a good fit to the data [25]. The highest Adj-R2 value was found for the tensile strength (0.983), which also indicates high significance for the model [28] and the predicted coefficient of determination for that property was 0.949, further indicating the goodness of fit. The CV% is the ratio of the standard deviation to the mean and the CV% of the tensile strength, tensile modulus, flexural strength, flexural modulus, and water absorption were 1.74 %, 1.92 %, 3.58 %, 3.35 %, and 3.74 %, respectively, based on the replicated experiments. These low CV% values also indicate the high reliability of the data from the experiments [23].
with previous studies which have also reported that increases in the wood content decreased the tensile strength while the tensile modulus increased. Khamtree et al. [17] found that with the wood content at 20–30 %, the tensile strength increased but when the wood content was increased to 40 %–50 %, the tensile strength decreased. In addition, the tensile modulus increased with increases in the wood content of the composites. Moreover, Asumani et al. [13] noted that alkaline-silane treatment of WPCs resulted in improved tensile strength with a wood content of 20–30% but the tensile strength decreased with a 35 % wood content since the increased wood content resulted in poor compatibility between the wood and the plastic due to inhibition of the voids in the composites with a transfer of loading from plastic to wood [13,17]. However, the silane concentration had no significant effect on the tensile properties although when the degree of silane concentration used in the treatment of the wood increased, the tensile properties of the WPCs decreased. This result is in agreement with that of Gwon et al. [18] who reported that WPCs containing natural wood fiber treated with silane at a concentration of 2.0 % gave the highest tensile strength values, but the value decreased when the concentration was increased to 2.5 %. Those findings are in agreement with those determined in a previous study carried out by Sepe et al. [19] who studied composites manufactured with silane-treated hemp fiber. Their results showed that the tensile modulus of the composites treated with 5 % silane was lower than that of 1 % silane treated samples. Fig. 2 shows the model predictions versus the observations of tensile strength. The plot reveals the data to be relatively linear indicating that the model predictions are well matched to the empirical observations. Thus the model adequacy checking for the tensile strength response indicated that the fit of the model to data was accurate.
3.2. Evaluation of model adequacy The residual of the mathematical models for the response was analyzed. The normal % probability versus the residuals plots for the responses resembles a straight line and the residual data distribution is therefore approximately normal. Plotting the residuals in time order of data collection is helpful in detecting the correlation between the residuals. There is no reason to suspect any violation of independence or constant variance assumptions. Moreover, the plots of residuals versus the fitted values indicating the residuals are randomly scattered around the centerline of zero, with no obvious pattern. Similarly, the results of the model adequacy checking for all properties including tensile strength tensile modulus, flexural strength, flexural modulus, and water absorption did not indicate any problems with the fit of the models to the data. Although this type of checking cannot guarantee the predictive capability of a model, it can suggest that models are sound approximations for interpolating within the experimental range.
3.4. Effect of wood content and silane concentration on the flexural properties of the composite samples
3.3. Effect of wood content and silane concentration on the tensile properties of the composite samples
The results of the CCD experiments in predicting the flexural strength and modulus based on the observed responses are presented in Table 1. It was found that the highest flexural strength of 39.09 MPa in run number 10, was achieved with a 25.9 % wood content and a 3.0 % silane concentration while the highest flexural modulus of 3396.60 MPa was produced with a 54.2 % wood content and a 3.0 % silane concentration. The quadratic model for flexural strength and the linear model for flexural modulus are expressed in Eqs. (3) and (4) below:
The tensile strength and modulus responses were evaluated in a simulated run based on the CCD and the results are summarized in Table 1. The results revealed that run number 10, with a 25.9 % wood content and a 3.0 % silane concentration yielded the highest tensile strength of 22.39 MPa. Additionally, run number 12 with a 50 % wood content and a 1 % silane concentration exhibited the highest tensile modulus of 984.55 MPa. The quadratic model of the tensile strength and the linear model of the tensile modulus are shown as Eqs. (1) and (2) below:
Tensile Strength = 22.94 + 0.20x1
1.07x2 +
Flexural Strength = 29.33 + 0.77x1 0.26x 22
0.034x1 x2 –0.0074x12
0.025x 22 Tensile Modulus = 681.34 + 6.16 x1 – 2.94 x2
0.16x2 + 0.047x1 x2
Flexural Modulus = 820.82 + 46.66 x1 − 15.06 x2
(1)
0.017x12 (3) (4)
The equations for the flexural strength and modulus show positive coefficients for the wood content (x1 = 0.77 and 46.66) and negative coefficients for the silane concentration (x2 = −0.16 and −15.06) and exhibit a similar trend for the effect of these independent variables to that noted for the tensile properties of the composites. The three-dimensional (3D) response surface plots of the effect of the wood content and the silane concentration on the flexural strength and flexural modulus are shown in Fig. 1(c) and (d), respectively. It can be clearly observed that the flexural strength of the samples decreased with a wood content exceeding 35 %. This result is in agreement with the findings of several previous studies, Asumani et al. [13] reported that the flexural strength improved in a composite produced with alkaline-silane treatment with a 30 % wood content, while the strength decreased with a 35 % wood content. Further in a previous study of WPCs manufactured with decayed wood flour contents ranging from 30 to 50 %, it was found that the composites with a 30 % wood content had the highest flexural strength relative to those manufactured with 40
(2)
These model were found to be an adequate representation of the response. The equations show positive coefficients for the wood content (x1 = 0.20 and 6.16) which indicate that the wood content positively affected the tensile properties, whereas the negative coefficients for the silane concentration (x2 = −1.07 and −2.94) indicated a negative effect on these properties. Thus the tensile properties of the WPCs increased with an increasing wood content but decreased with an increasing silane concentration. The effect of the wood content and silane concentration in the composites on their tensile strength and tensile modulus is illustrated by the three-dimensional (3D) RSM plot shown in Fig. 1(a) and (b), respectively. When the wood content did not exceed 35 % this resulted in higher tensile strength but when the wood content increased above 35 %, the tensile strength of the samples decreased. This is in agreement 4
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Fig. 1. Response surface plots of the yield response: (a) tensile strength, (b) tensile modulus, (c) flexural strength, (d) flexural modulus, and (e) water absorption.
% and 50 % wood flour [3]. In the same study, however, increasing the decayed-wood content from 30 to 50 % resulted in an improvement in the flexural modulus of the composites [3,17,29]. Sepe et al. [19] studied the influence of the silane concentration on the flexural strength and modulus of hemp-plastic composites. The results showed that the flexural properties decreased with increases in the silane concentration, a trend which is similar to that found in the present study.
3.5. Effect of wood content and silane concentration on water absorption of the composite samples The water absorption results from the CCD are included in Table 1. Run number 10 with a 25.9 % wood content and a 3.0 % silane concentration exhibited the lowest water absorption of 2.98 %. The linear model of the water absorption is given as Eq. (5) below: 5
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and silane concentration on the water absorption of the WPCs is displayed in the three-dimensional RSM plot in Fig. 1(e). The increase in water absorption with an increasing wood content is in agreement with the result of Leu et al. [5]. Moreover, Gharbi et al. [30] reported that the water absorption of composites manufactured with silane-treated wood increased with wood contents of between 10 % and 30 % and in addition, the overall water absorption decreased with increases a silane concentration. Similar results were reported by Gwon et al. [18], who studied WPCs produced with silane treatment concentrations of between 0.5 and 3.0 %. The findings showed that above a 1.5 % silane concentration, the water absorption of the composites decreased and the lowest water absorption was found at a 3 % concentration. This finding may be due to wood being a hydrophilic lignocellulose material, consisting mainly of cellulose, hemicellulose, and lignin with extractive compounds on the surface [13,31] which absorbs moisture from the surrounding environment [32]. Silane treatment improves the interaction and adhesion between the wood and plastic [8,14,20] and the treatment of the wood with silane is able to improve water resistance as compared to untreated wood. However, the degree of water absorption by WPCs is also dependent on the wood content. Fig. 2. The plots of predicted versus actual values for tensile strength.
3.6. Optimization of the independent variables for the mechanical properties and water absorption of the composite samples
Table 3 Predicted and observed responses with the conditions optimized for all properties. variables
Predicted Observed RMSE*
The optimization of the tensile and flexural properties and water absorption of the WPCs was achieved by optimizing all the independent variables as shown in Table 3. The optimum value for the two independent variables was found to be a 39.22 % wood content (x1) and a 3.44 % silane concentration (x2). At this condition, the highest tensile strength (y1), tensile modulus (y2), flexural strength (y3), and flexural modulus (y4) were achieved of 19.74 MPa, 912.91 MPa, 36.60 MPa, and 2598.99 MPa, respectively whereas the minimum water absorption (y5) of 7.12 % was achieved. Additionally, in order to confirm the accuracy of the analysis using RSM, the predicted values were compared with the experimentally observed with root mean square error (RMSE). The SEM micrographs show that the WPCs with the optimum conditions at 39.22 wt% RWF and a 3.44 % silane concentration (Fig. 3a) had fewer voids as compared to the WPCs from condition at 50.0 wt% RWF and a 1.0 % silane concentration (Fig. 3b) which indicates that the WPC produced employing the optimum conditions showed good compatibility between the wood and plastic and provided superior tensile and flexural strength as well as lower water absorption.
Responses (MPa)
x1
x2
y1
y2
y3
y4
y5
39.22
3.44
19.77 19.41 0.88
912.91 939.45 73.81
36.00 35.77 1.56
2598.99 2574.63 175.62
7.12 6.99 0.84
* Root mean square error (RMSE) of predicted values and observed values.
Water absorption = −3.55 + 0.294 x1 −0.245 x2
(5)
The linear equation of the water absorption shows a positive coefficient for the wood content (x1 = 0.294), revealing that the wood content positively affected the water absorption, whereas the negative coefficient for the silane concentration (x2 = −0.245) shows its negative effect on the water absorption. The effect of the wood content
Fig. 3. Scanning electron micrographs of WPCs (magnification ×1000): (a) 39.22 wt% RWF at a 3.44 % silane concentration and (b) 50.0 wt% RWF at a 1.0 % silane concentration. 6
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Table 4 Predicted responses with the optimum conditions of each property. Responses
x1
x2
Predicted response
Desirability
y1 y2 y3 y4 y5
30.00 50.00 30.00 50.00 30.00
1.00 1.00 2.37 1.00 5.00
22.03 986.54 38.95 3138.97 4.03
0.960 0.961 0.990 0.806 0.876
density polyethylene, Compos. Part. B-Eng. 56 (2014) 137–141. [7] Y. Li, Characterization of acetylated eucalyptus wood fibers and its effect on the interface of eucalyptus wood/polypropylene composites, Int. J. Adhes. Adhes. 50 (2014) 96–101. [8] M.A. Flores-Hernandez, I.R. Gonzalez, M.G. Lomeli-Ramirez, F.J. Fuentes-Talavera, J.A. Silva-Guzman, M.A. Cerpa-Gallegos, S. Garcia-Enriquez, Physical and mechanical properties of wood plastic composites polystyrene-white oak wood flour, J. Compos. Mater. 48 (2) (2014) 209–217. [9] A. Ashori, S. Sheshmani, Hybrid composites made from recycled material: moisture absorption and thickness swelling behavior, Bioresour. Technol. 101 (12) (2010) 4717–4720. [10] T. Ratanawilai, K. Taneerat, Alternative polymeric matrices for wood-plastic composites: effects on mechanical properties and resistance to natural weathering, Constr. Build. Mater. 172 (2018) 349–357. [11] S.K. Najafi, Use of recycled plastic in wood plastic composites – a review, Waste. Manage. 33 (9) (2013) 1898–1905. [12] S. Kaewkuk, W. Sutapun, K. Jarukumjorn, Effects of interfacial modification and fiber content on physical properties of sisal fiber/polypropylene composites, Compos. Part. B-Eng. 45 (1) (2013) 544–549. [13] 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 (9) (2012) 1431–1440. [14] Y. Xie, C.A.S. Hill, Z. Xiao, H. Militz, C. Mai, Silane coupling agents used for natural fiber/polymer composites: a review, Compos. Part. A-Appl. Sci. Manuf. 41 (7) (2010) 806–819. [15] M. Farsi, Wood-plastic composites: influence of wood flour chemical modification on the mechanical performance, J. Reinf. Plast. Compos. 29 (24) (2010) 3587–3592. [16] L. Fang, L. Chang, W.J. Guo, Y. Chen, Z. Wang, Influence of silane surface modification of veneer on interfacial adhesion of wood-plastic plywood, Appl. Surf. Sci. 288 (2014) 682–689. [17] S. Khamtree, T. Ratanawilai, S. Ratanawilai, A comparison on untreated and alkaline treated rubberwood flour on physical and mechanical properties of wood plastic composites, Prog. Ind. Ecol. Int. J. 12 (3) (2018) 297–308. [18] J.G. Gwon, S.Y. Lee, S.J. Chun, G.H. Doh, J.H. Kim, Effects of chemical treatments of hybrid fillers on the physical and thermal properties of wood plastic composites, Compos. Part. A-Appl. Sci. Manuf. 41 (10) (2010) 1491–1497. [19] R. Sepe, F. Bollino, L. Boccarusso, F. Caputo, Influence of chemical treatments on mechanical properties of hemp fiber reinforced composites, Compos. Part. B-Eng. 133 (2018) 210–217. [20] A. Nopparut, T. Amornsakchai, Influence of pineapple leaf fiber and it’s surface treatment on molecular orientation in, and mechanical properties of, injection molded nylon composites, Polym. Test. 52 (2016) 141–149. [21] S. Khamtree, T. Ratanawilai, S. Ratanawilai, The effect of alkaline-silane treatment of rubberwood flour for water absorption and mechanical properties of plastic composites, J. Thermoplast. Compos. Mater. (2018), https://doi.org/10.1177/ 0892705718808556. [22] O. Rezaifar, M. Hasanzadeh, M. Gholhaki, Concrete made with hybrid blends of crumb rubber and metakaolin: optimization using Response Surface Method, Constr. Build. Mater. 123 (2016) 59–68. [23] N. Kataria, V.K. Garg, Optimization of Pb (II) and Cd (II) adsorption onto ZnO nanoflowers using central composite design: isotherms and kinetics modelling, J. Mol. Liq. 271 (2018) 228–239. [24] C. HomKhiew, T. Ratanawilai, W. Thonruang, The optimal formulation of recycled polypropylene/rubberwood flour composites from experiments with mixture design, Compos. Part. B-Eng. 56 (2014) 350–357. [25] C. HomKhiew, T. Ratanawilai, W. Thonruang, Optimizing the formulation of polypropylene and rubberwood flour composites for moisture resistance by mixture design, J. Reinf. Plast. Compos. 33 (9) (2014) 810–823. [26] J. Zhao, Y. Yao, Q. Cui, X.M. Wang, Optimization of processing variables and mechanical properties in rubber-wood particles reinforced cement based composites manufacturing technology, Compos. Part. B-Eng. 50 (2013) 193–201. [27] S.O. Amiandamhen, M. Meincken, L. Tyhoda, Magnesium based phosphate cement binder for composite panels: a response surface methodology for optimization of processing variables in boards produced from agricultural and wood processing industrial residues, Ind. Crops Prod. 94 (2016) 746–754. [28] A.F. Almansoory, H.A. Hasan, M. Idris, S.R.S. Abdullah, N. Anuar, E.M.M. Tibin, Biosurfactant production by the hydrocarbon-degrading bacteria (HDB) Serratia marcescens: optimization using central composite design (CCD), J. Ind. Eng. Chem. 47 (2017) 272–280. [29] I. Turku, A. Keskisaari, T. Karki, A. Puurtinen, P. Marttila, Characterization of wood plastic composites manufactured from recycled plastic blends, Compos. Struct. 161 (2017) 469–476. [30] A. Gharbi, R.B. Hassen, S. Boufi, Composite materials from unsaturated polyester resin and olive nuts residue: the effect of silane treatment, Ind. Crops Prod. 62 (2014) 491–498. [31] K. Kaewtatip, J. Thongmee, Preparation of thermoplastic starch/treated bagasse fiber composites, Starch-Starke 66 (2014) 724–728. [32] N. Lu, S. Oza, Thermal stability and thermo-mechanical properties of hemp-high density polyethylene composites: effect of two different chemical modifications, Compos. Part. B-Eng. 44 (1) (2013) 484–490.
Based on the model optimization of the wood content and silane concentration, the maximum mechanical properties and the minimum water absorption of the WPC are shown in Table 4. The predicted responses of the tensile strength (y1), tensile modulus (y2), flexural strength (y3), flexural modulus (y4), and water absorption (y5) were 22.03 MPa, 986.54 MPa, 38.95 MPa, 3138.97 MPa, and 4.03 %, respectively. Moreover, the desirability scores of each response were close to 0.90 which indicates that the optimum value of the variables was achieved [23]. 4. Conclusions This study sought to optimize the wood content and silane concentration to obtain the maximum tensile and flexural properties and the minimum water absorption of experimentally manufactured WPCs. The experiments were designed and analyzed with a CCD using DesignExpert software. The optimal conditions for the WPCs were found to be 39.22 wt% wood, 60.78 wt% rPP, and a 3.44 % silane concentration. The predicted responses based on the highest tensile strength, tensile modulus, flexural strength, and flexural modulus were 22.03 MPa, 986.54 MPa, 38.95 MPa, and 3138.97 MPa, respectively whereas the lowest water absorption was 4.03 % with the desirability function found to be close to 0.90. The results of this work will be useful in improving the properties of WPCs produced for different applications so that they can be used more efficiently. Declaration of Competing Interest None. Acknowledgments The authors would like to thank the Office of the Higher Education Commission for the financial support of Mrs. Sriwan Khamtree under the CHE-PhD Scholarship Program and the government budget of Prince of Songkla University (ENG-61-00-62-S), and also the Rubberwood Technology and Management Research Group (ENG-5427-11-0137-S) of Faculty of Engineering, Prince of Songkla University, Thailand. References [1] B. Lei, Y. Zhang, Y. He, Y. Xie, B. Xu, Z. Lin, L. Huang, S. Tan, M. Wang, X. Cai, Preparation and characterization of wood-plastic composite reinforced by graphitic carbon nitride, Mater. Des. 66 (2015) 103–109. [2] A.V. Kiruthika, A review on physico-mechanical properties of bast fibre reinforced polymer composites, J. Build. Eng. 9 (2017) 91–99. [3] N. Ayrilmis, A. Kaymakci, T. Gulec, Potential use of decayed wood in production of wood plastic composite, Ind. Crops Prod. 74 (2015) 279–284. [4] C. Srivabut, T. Ratanawilai, S. Hiziroglu, Effect of nanoclay, talcum, and calcium carbonate as filler on properties of composites manufactured from recycled polypropylene and rubberwood fiber, Constr. Build. Mater. 162 (2018) 450–458. [5] S.Y. Leu, T.H. Yang, S.F. Lo, T.H. Yang, Optimized material composition to improve the physical and mechanical properties of extruded wood-plastic composites (WPCs), Constr. Build. Mater. 29 (2012) 120–127. [6] S.M. Mirmehdi, F. Zeinaly, F. Dabbagh, Date palm wood flour as filler of linear low-
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Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm
Determining the optimum conditions for silane treated rubberwood flourrecycled polypropylene composites using response surface methodology
T
Sriwan Khamtreea, Thanate Ratanawilaia,*, Sukritthira Ratanawilaib a b
Department of Industrial Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla, Thailand Department of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla, Thailand
ARTICLE INFO
ABSTRACT
Keywords: Rubberwood flour Recycled polypropylene Surface treatment Silane concentration Mechanical properties Response surface methodology
The objective of this work was to determine the optimum combination of two materials in production of woodplastic composites, using response surface methodology (RSM). The two independent variables investigated were the wood content and the silane concentration. The experiments were conducted based on a central composite design. The findings showed that the wood content significantly affected the tensile and flexural properties of the composites as well as their water absorption and the silane concentration also significantly affected the water absorption of the samples. The optimum properties found by RSM were a content of 39.22 wt% wood, 60.78 wt% recycled polypropylene and a 3.44 % silane concentration. The findings from this work can be used to produce value-added and environmentally friendly panels for a variety of different applications.
1. Introduction In the past 25 years, wood-plastic composites (WPCs) have gained popularity and attracted attention due to their potential applications in many industries, including the construction and automobile industries as well as the manufacture of decking, railing, fencing, and building materials [1,2]. Such popularity of WPCs is due to their many advantages, including, high strength, durability and stiffness, low-density and low maintenance requirement, as well as being constructed from renewable resources and thus being environment-friendly [3]. WPCs are relatively new materials as compared to particle- or fiber-board which are the traditional types of wood composite panels [4]. In general, WPCs are made from small particles of lignocellulosic fiber reinforced by a polymer matrix. Spruce, pine, fir, eucalyptus, and oak are the most common wood species used in the manufacture of WPCs although non-wood resources such as palm and bamboo can also be utilized [5–8]. Rubberwood waste material from furniture and lumber production has also been used to manufacture experimental WPC panels in a number of previous studies [4–9] and the use of wood waste as a reinforcement in composites with a recycled polymer matrix creates value from waste materials and thus reduces the production cost of WPCs.
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Previous studies have reported that many variables influence the physical and mechanical properties of the final WPC product, including the polymer matrix, the wood species and content, and the coupling agent used [5]. Ratanawilai and Taneerat [10] investigated the influence of different types of polymers, including low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), polypropylene (PP), and polystyrene (PS) on the mechanical properties of WPCs, with the results showing that PP gave the best mechanical properties. The use of recycled plastic has added benefits such as the low cost of the material, high heat stability, and a reduction in the environmental impact [5,11]. Some studies have also reported that WPCs containing recycled polypropylene (rPP) exhibited higher tensile and flexural properties than those made from virgin PP [11]. Kaewkuk et al. [12] found that a fiber content of 10, 20, or 30 % by weight in sisal-PP composites increased the tensile and water resistance properties. Leu et al. [5] evaluated the optimization of four parameters in the manufacture of WPCs, finding that the mechanical and physical properties were optimized by the following values: (1) wood particle size: less than 125 μm; (2) coupling agent content: 3 %; (3) lubricant content: 3 %; (4) wood content: 50 % or less. In addition, chemical surface treatments such as dewaxing, alkaline,
Corresponding author. E-mail address: [email protected] (T. Ratanawilai).
https://doi.org/10.1016/j.mtcomm.2020.100971 Received 11 May 2019; Received in revised form 25 August 2019; Accepted 31 January 2020 Available online 01 February 2020 2352-4928/ © 2020 Elsevier Ltd. All rights reserved.
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silane, benzoylation, and acrylation treatments have been applied to promote the interfacial adhesion between the wood and the polymer and to improve the properties of composites [13–16]. Farsi [15] reported that alkaline treatment and the use of a coupling agent in beechwood-PP composites produced the optimal tensile and flexural properties while Asumani et al. [13] found that in a combined alkalinesilane treatment, a sodium hydroxide (NaOH) solution applied at a proportion of 6 % produced the highest tensile and flexural properties among values of between 1 and 8 % tested, in kenaf fiber-reinforced PP composites. Khamtree et al. [17] investigated the optimal wood content of rubberwood flour (RWF)-rPP composites after the wood was treated with a 2 % NaOH solution. The results showed that a wood content of 30 % produced the highest tensile and flexural properties relative to other wood contents between 20 and 50 %. Gwon et al. [18] explored the effect of silane concentrations ranging from 0.5 to 3.0 % on the properties of WPCs, finding that the concentration affected the tensile strength of wood fiber-PP composites. Similarly, Sepe et al. [19] studied the effect of silane concentrations of 1, 5, and 20 % on the mechanical properties of hemp-fiber reinforced epoxy composites with the results indicating that a concentration of 1 % optimized their properties. Meanwhile, Nopparut and Amornsakchai [20] tested hemp-HDPE composites with fiber contents varying between 20 and 50 % and reported that their stiffness increased with fiber contents of up to 40 % but decreased with a content of 50 %. Further, the silane-treated hemp-HDPE composites had higher thermal stability as compared to those treated with NaOH. In addition, Khamtree et al. [21] investigated the effect of the alkaline-silane treatment of RWF in rPP composites. They found that the silane concentration significantly affected the water absorption, tensile properties, flexural properties and hardness of the RWF-rPP composites and a 5 % silane concentration produced the optimal properties. This study adopted response surface methodology (RSM) as the method of determining the optimum parameters since RSM has been widely used in previous studies to optimize variables and responses with the minimum number of experiments [22,23]. There are three classifications of RSM, the Box-Behnken design, the three-level factorial design, and the central composite design (CCD), and CCD is, widely proposed as the most effective and popular design for the optimization of variables [23]. Several studies have conducted studies to statistically determine the optimum conditions to enhance the properties of WPCs. Homkhiew et al. [24,25] determined the optimal combination of wood, plastic, and chemical additives in RWF-rPP composites using a D-optimal mixture experimental design. They found that composite samples which were made based on a formulation with 50.3 wt% rPP, 44.5 wt% RWF, 3.9 wt% maleic anhydride-grafted polypropylene, 0.2 wt% UV stabilizer, and 1.0 wt% lubricant had good mechanical properties and water resistance. Zhao et al. [26] applied RSM with a Box-Behnken design to determine the significant manufacturing process variables and to optimize the processing conditions of rubber-crumb-wood particle reinforced cement-based composites. Further, Amiandamhen et al. [27] employed a CCD design with RSM to determine the optimum process conditions including the binder ratio and the fly ash and wood content of composite panels and to predict their properties. In addition, Almansoory et al. [28] applied a CCD design with RSM to determine the optimal conditions for producing biosurfactant with the minimum surface tension activity in a bacterial-mediated process and Kataria and Garg [23] determined the optimal process variables for Cd(II) and Pb (II) adsorption onto ZnO nanoflowers using CCD with RSM. As reviewed above previous studies have explored the optimal conditions and concentrations of chemical treatments to optimize the properties of WPCs and this study used RSM employing a CCD to determine the optimal RWF content and silane concentration and investigated their effect on the tensile and flexural properties and water absorption of RWFrPP composites. It is expected that results from this study would provide a better understanding of such composites so that they can be used for different applications with more efficient service life.
2. Materials and methods 2.1. Materials RWF was obtained from a local furniture factory in Trang, Thailand. It was screened through a standard 80 mesh sieve to produce a particle size smaller than 180 μm. The amount of cellulose, hemicelluloses, lignin, and ash in RWF were 39 %, 29 %, 28 %, and 4 %, respectively [24]. The rPP pellets utilized were supplied by Withaya Intertrade Co., Ltd, Samutprakarn, Thailand, and had a melt flow index of 11 g/10 min at 230 °C, under the trade name WT170. NaOH and Triethoxyvinylsilane (97 %) were purchased from Merck Co. (Germany) and Sigma-Aldrich (USA), respectively. NaOH was pellet form with molecular weight 40 g mol−1. The chemical formula, a molecular weight, the boiling point and a density of triethoxyvinylsilane were C8H18O3Si, 190.31 g mol−1, 160−161 °C, and 0.903 g mL−1 at 25 °C, respectively. 2.2. Surface treatment of raw material The RWF was treated with NaOH solution having 2 % weight by volume at a temperature 25 °C for 24 h then washed with distilled water until it became neutral, following dried in an oven at a temperature 80 °C for 24 h. The alkaline-treated RWF was treated with silane concentrations ranging from 1 to 5 % (by weight percentage relative to RWF) in an ethanol and water mixture (50:50 by volume). The pH of the silane solution was maintained between 4.5 and 5 by adding acetic acid and then stirring at room temperature for 1 h. Finally, the alkalinetreated RWF was immersed in the silane solution for 2 h and then filtered and dried at 80 °C for 24 h. 2.3. Processing of composite samples Treated RWF was mixed with rPP pellets using a twin-screw extruder (Model EMT-26, En Mach Co., Ltd., Nonthaburi, Thailand). The temperature of the 10 barrels of the extruder was controlled at 160−190 °C and the screw speed was set at 60 rpm. The extruded compound was pelletized and placed in a hot press at 190 °C for 15 min at 1,000 psi before cooling under a pressure of 250 psi at room temperature. 2.4. Experimental design The experiments were designed based on a CCD, using Design Expert software (version 8.0.6, Stat-Ease, Inc., Minneapolis, USA). The CCD was designed to statistically evaluate the effects of the wood content and silane concentration on the tensile and flexural properties of the composites and their water absorption in order to optimize these dependent variables using RMS. The two independent variables, which were considered for optimization were the wood content (x1), and silane concentration (x2). The intervals selected over which to conduct the experimental design were 30−50 wt% wood content and 1–5 % silane concentration. The total experiments conducted were 13 runs in a randomized order including eight runs coupled with five center-point runs as displayed in Table 1. The tensile strength, tensile modulus, flexural strength, flexural modulus, and water absorption of the WPCs were considered in the experimental design. 2.5. Mechanical properties of the composite samples The tensile and flexural properties of the samples were investigated employing a universal testing machine (Model 5582, Instron Corporation, Massachusetts, USA) according to ASTM D638, and D790, respectively. The tensile test of the samples was carried out at a 5 mm/ min crosshead speed while the crosshead speed was 2 mm/min for 2
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Table 1 Central composite design (circumscribed) matrix with experimental and predicted values of the responses. Run
Variables
Tensile Strength (MPa)
Tensile Modulus (MPa)
Flexural Strength (MPa)
Flexural Modulus (MPa)
Water absorption (%)
x1
x2
Actual
Predicted
Actual
Predicted
Actual
Predicted
Actual
Predicted
Actual
Predicted
1
40.0
3.0
19.75
30.0
1.0
4
40.0
3.0
5
40.0
3.0
6
30.0
5.0
7
40.0
3.0
8
54.2
3.0
9
40.0
0.2
10
25.9
3.0
11
40.0
5.8
12
50.0
1.0
13
50.0
5.0
7.61 (0.21) 7.42 (0.17) 5.58 (0.21) 7.67 (0.20) 7.35 (0.33) 4.05 (0.36) 7.42 (0.20) 11.51 (0.26) 7.75 (0.37) 2.98 (0.37) 6.59 (0.29) 10.91 (0.30) 10.16 (0.36)
7.48
3
2627.78 (92.94) 2593.57 (84.09) 2299.26 (88.72) 2629.51 (83.02) 2538.64 (91.64) 2130.86 (89.24) 2651.17 (86.56) 3396.60 (70.16) 2518.34 (53.98) 2071.11 (67.66) 2603.78 (51.24) 3240.90 (51.26) 3047.69 (65.77)
2642.04
3.0
37.82 (0.57) 36.84 (0.65) 38.96 (0.62) 36.02 (0.21) 35.22 (0.52) 36.74 (0.55) 35.50 (0.50) 25.64 (0.51) 32.57 (0.59) 39.09 (0.52) 34.63 (0.63) 29.61 (0.56) 31.16 (0.55)
35.75
40.0
916.11 (70.05) 934.92 (61.17) 861.11 (49.67) 894.56 (60.02) 938.96 (59.97) 863.99 (44.81) 938.96 (68.14) 933.93 (60.46) 937.51 (71.58) 802.48 (54.92) 917.62 (50.03) 984.55 (50.10) 962.82 (40.28)
918.92
2
19.88 (0.30) 19.44 (0.36) 22.03 (0.14) 19.41 (0.10) 19.47 (0.33) 20.96 (0.50) 19.15 (0.33) 13.40 (0.29) 18.69 (0.31) 22.39 (0.28) 19.67 (0.18) 15.15 (0.20) 16.75 (0.26)
19.75 22.21 19.75 19.75 21.41 19.75 14.13 19.16 22.36 19.94 15.05 16.97
918.92 863.20 918.92 918.92 851.44 918.92 1006.39 927.15 832.06 910.69 986.40 974.64
35.75 38.12 35.75 35.75 36.88 35.75 25.95 33.26 38.70 35.06 27.26 29.78
2642.04 2205.56 2642.04 2642.04 2145.32 2642.04 3304.61 2684.21 1984.13 2599.87 3138.76 3078.52
7.48 5.03 7.48 7.48 4.05 7.48 11.65 8.16 3.33 6.79 10.91 9.93
Note: The value in parentheses are standard deviations from five replicates.
flexural testing on samples with an 80 mm span length. Five replications were conducted for each condition at room temperature and a relative humidity of 50 ± 5 % controlled for more than 40 h prior to testing.
Table 2 The p-values obtained in the ANOVA analysis of the polynomial models.
2.6. Water absorption of the composite samples
Source
Tensile Strength
Tensile Modulus
Flexural Strength
Flexural Modulus
Water Absorption
Terms
Quadratic < 0.0001* < 0.0001* 0.079 0.004* 0.001* 0.456 0.211 0.990 0.983 0.949 0.33 1.74
Linear 0.0001* 0.0001* 0.369 – – – 0.653 0.907 0.889 0.846 17.68 1.92
Quadratic 0.0004* < 0.0001* 0.542 0.172 0.009* 0.059 0.276 0.941 0.899 0.718 1.24 3.58
Linear < 0.0001* < 0.0001* 0.358 – – – 0.051 0.957 0.948 0.913 88.55 3.35
Linear < 0.0001* < 0.0001* 0.001* – – – 0.050 0.989 0.987 0.977 0.28 3.74
x1 x2 x1 x2 x12 x22 Lack of fit R2 Adj-R2 Pred-R2 Std. Dev. CV (%)
The water absorption testing was carried out according to ASTM D570. The samples were conditioned at 50 °C for 24 h in an oven and cooled in a desiccator prior to the tests. The samples were soaked in distilled water at 25 °C for 10 weeks. Five replications for each condition were tested at room temperature. The weight was immediately measured to the nearest 0.001 g at the commencement of the experiment and thereafter at weekly intervals for the entire period of testing. 2.7. Morphological analysis of the composite samples
Note: * p-value less than 0.05 is considered significant.
The morphologies of fractured surfaces of the WPCs were evaluated using a scanning electron microscope (SEM; model FEI Quanta 400 Oregon, USA) operated at 20 kV. In order to avoid electrical charge accumulation during imaging, the samples were coated with a thin layer of gold before the micrographs were produced.
with a very small p-value (p < 0.0001). Moreover, the lack of fit for the tensile strength model was 0.211 which was clearly not significant. This result implied that the model performed well and was an excellent fit for the data derived from the experiments [25]. Additionally, the fit of these models was evaluated using coefficients of determination (R2), adjusted coefficients of determination (Adj-R2), predicted coefficients of determination (Pred-R2), and coefficients of variation (CV%). The extent to which variability in the data was accounted for by the model was indicated by the R2 and Adj-R2 values while the Pred-R2 was employed to take into account the number of covariates or predictors in the model. The R2 coefficient provided satisfactory outcomes for the quadratic model to the experimental data [28]. The R2 values of the tensile strength, tensile modulus, flexural strength, flexural modulus, and water absorption were 0.990, 0.907, 0.941, 0.957, and 0.989, respectively. Thus, for example, the quadratic model for the tensile strength response shows that 99.02 % of the variability observed can be explained by the two independent variables
3. Results and discussion 3.1. Statistical analysis of the response model The models were subjected to analysis of variance (ANOVA) to determine the goodness of fit of the model to the empirical data as shown in Table 2. The results clearly show that the model fit was statistically significant (p < 0.05). A quadratic model produced the best fit for the tensile and flexural strength whereas, a linear model produced the best fit for the tensile modulus, flexural modulus, and water absorption. As an example, the model for the tensile strength was highly significant 3
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investigated in the production of the WPCs and that only 0.98 % of the total variability cannot be explained. Moreover, the R2 values were all generally very close to 1, which demonstrates a good fit to the data [25]. The highest Adj-R2 value was found for the tensile strength (0.983), which also indicates high significance for the model [28] and the predicted coefficient of determination for that property was 0.949, further indicating the goodness of fit. The CV% is the ratio of the standard deviation to the mean and the CV% of the tensile strength, tensile modulus, flexural strength, flexural modulus, and water absorption were 1.74 %, 1.92 %, 3.58 %, 3.35 %, and 3.74 %, respectively, based on the replicated experiments. These low CV% values also indicate the high reliability of the data from the experiments [23].
with previous studies which have also reported that increases in the wood content decreased the tensile strength while the tensile modulus increased. Khamtree et al. [17] found that with the wood content at 20–30 %, the tensile strength increased but when the wood content was increased to 40 %–50 %, the tensile strength decreased. In addition, the tensile modulus increased with increases in the wood content of the composites. Moreover, Asumani et al. [13] noted that alkaline-silane treatment of WPCs resulted in improved tensile strength with a wood content of 20–30% but the tensile strength decreased with a 35 % wood content since the increased wood content resulted in poor compatibility between the wood and the plastic due to inhibition of the voids in the composites with a transfer of loading from plastic to wood [13,17]. However, the silane concentration had no significant effect on the tensile properties although when the degree of silane concentration used in the treatment of the wood increased, the tensile properties of the WPCs decreased. This result is in agreement with that of Gwon et al. [18] who reported that WPCs containing natural wood fiber treated with silane at a concentration of 2.0 % gave the highest tensile strength values, but the value decreased when the concentration was increased to 2.5 %. Those findings are in agreement with those determined in a previous study carried out by Sepe et al. [19] who studied composites manufactured with silane-treated hemp fiber. Their results showed that the tensile modulus of the composites treated with 5 % silane was lower than that of 1 % silane treated samples. Fig. 2 shows the model predictions versus the observations of tensile strength. The plot reveals the data to be relatively linear indicating that the model predictions are well matched to the empirical observations. Thus the model adequacy checking for the tensile strength response indicated that the fit of the model to data was accurate.
3.2. Evaluation of model adequacy The residual of the mathematical models for the response was analyzed. The normal % probability versus the residuals plots for the responses resembles a straight line and the residual data distribution is therefore approximately normal. Plotting the residuals in time order of data collection is helpful in detecting the correlation between the residuals. There is no reason to suspect any violation of independence or constant variance assumptions. Moreover, the plots of residuals versus the fitted values indicating the residuals are randomly scattered around the centerline of zero, with no obvious pattern. Similarly, the results of the model adequacy checking for all properties including tensile strength tensile modulus, flexural strength, flexural modulus, and water absorption did not indicate any problems with the fit of the models to the data. Although this type of checking cannot guarantee the predictive capability of a model, it can suggest that models are sound approximations for interpolating within the experimental range.
3.4. Effect of wood content and silane concentration on the flexural properties of the composite samples
3.3. Effect of wood content and silane concentration on the tensile properties of the composite samples
The results of the CCD experiments in predicting the flexural strength and modulus based on the observed responses are presented in Table 1. It was found that the highest flexural strength of 39.09 MPa in run number 10, was achieved with a 25.9 % wood content and a 3.0 % silane concentration while the highest flexural modulus of 3396.60 MPa was produced with a 54.2 % wood content and a 3.0 % silane concentration. The quadratic model for flexural strength and the linear model for flexural modulus are expressed in Eqs. (3) and (4) below:
The tensile strength and modulus responses were evaluated in a simulated run based on the CCD and the results are summarized in Table 1. The results revealed that run number 10, with a 25.9 % wood content and a 3.0 % silane concentration yielded the highest tensile strength of 22.39 MPa. Additionally, run number 12 with a 50 % wood content and a 1 % silane concentration exhibited the highest tensile modulus of 984.55 MPa. The quadratic model of the tensile strength and the linear model of the tensile modulus are shown as Eqs. (1) and (2) below:
Tensile Strength = 22.94 + 0.20x1
1.07x2 +
Flexural Strength = 29.33 + 0.77x1 0.26x 22
0.034x1 x2 –0.0074x12
0.025x 22 Tensile Modulus = 681.34 + 6.16 x1 – 2.94 x2
0.16x2 + 0.047x1 x2
Flexural Modulus = 820.82 + 46.66 x1 − 15.06 x2
(1)
0.017x12 (3) (4)
The equations for the flexural strength and modulus show positive coefficients for the wood content (x1 = 0.77 and 46.66) and negative coefficients for the silane concentration (x2 = −0.16 and −15.06) and exhibit a similar trend for the effect of these independent variables to that noted for the tensile properties of the composites. The three-dimensional (3D) response surface plots of the effect of the wood content and the silane concentration on the flexural strength and flexural modulus are shown in Fig. 1(c) and (d), respectively. It can be clearly observed that the flexural strength of the samples decreased with a wood content exceeding 35 %. This result is in agreement with the findings of several previous studies, Asumani et al. [13] reported that the flexural strength improved in a composite produced with alkaline-silane treatment with a 30 % wood content, while the strength decreased with a 35 % wood content. Further in a previous study of WPCs manufactured with decayed wood flour contents ranging from 30 to 50 %, it was found that the composites with a 30 % wood content had the highest flexural strength relative to those manufactured with 40
(2)
These model were found to be an adequate representation of the response. The equations show positive coefficients for the wood content (x1 = 0.20 and 6.16) which indicate that the wood content positively affected the tensile properties, whereas the negative coefficients for the silane concentration (x2 = −1.07 and −2.94) indicated a negative effect on these properties. Thus the tensile properties of the WPCs increased with an increasing wood content but decreased with an increasing silane concentration. The effect of the wood content and silane concentration in the composites on their tensile strength and tensile modulus is illustrated by the three-dimensional (3D) RSM plot shown in Fig. 1(a) and (b), respectively. When the wood content did not exceed 35 % this resulted in higher tensile strength but when the wood content increased above 35 %, the tensile strength of the samples decreased. This is in agreement 4
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Fig. 1. Response surface plots of the yield response: (a) tensile strength, (b) tensile modulus, (c) flexural strength, (d) flexural modulus, and (e) water absorption.
% and 50 % wood flour [3]. In the same study, however, increasing the decayed-wood content from 30 to 50 % resulted in an improvement in the flexural modulus of the composites [3,17,29]. Sepe et al. [19] studied the influence of the silane concentration on the flexural strength and modulus of hemp-plastic composites. The results showed that the flexural properties decreased with increases in the silane concentration, a trend which is similar to that found in the present study.
3.5. Effect of wood content and silane concentration on water absorption of the composite samples The water absorption results from the CCD are included in Table 1. Run number 10 with a 25.9 % wood content and a 3.0 % silane concentration exhibited the lowest water absorption of 2.98 %. The linear model of the water absorption is given as Eq. (5) below: 5
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and silane concentration on the water absorption of the WPCs is displayed in the three-dimensional RSM plot in Fig. 1(e). The increase in water absorption with an increasing wood content is in agreement with the result of Leu et al. [5]. Moreover, Gharbi et al. [30] reported that the water absorption of composites manufactured with silane-treated wood increased with wood contents of between 10 % and 30 % and in addition, the overall water absorption decreased with increases a silane concentration. Similar results were reported by Gwon et al. [18], who studied WPCs produced with silane treatment concentrations of between 0.5 and 3.0 %. The findings showed that above a 1.5 % silane concentration, the water absorption of the composites decreased and the lowest water absorption was found at a 3 % concentration. This finding may be due to wood being a hydrophilic lignocellulose material, consisting mainly of cellulose, hemicellulose, and lignin with extractive compounds on the surface [13,31] which absorbs moisture from the surrounding environment [32]. Silane treatment improves the interaction and adhesion between the wood and plastic [8,14,20] and the treatment of the wood with silane is able to improve water resistance as compared to untreated wood. However, the degree of water absorption by WPCs is also dependent on the wood content. Fig. 2. The plots of predicted versus actual values for tensile strength.
3.6. Optimization of the independent variables for the mechanical properties and water absorption of the composite samples
Table 3 Predicted and observed responses with the conditions optimized for all properties. variables
Predicted Observed RMSE*
The optimization of the tensile and flexural properties and water absorption of the WPCs was achieved by optimizing all the independent variables as shown in Table 3. The optimum value for the two independent variables was found to be a 39.22 % wood content (x1) and a 3.44 % silane concentration (x2). At this condition, the highest tensile strength (y1), tensile modulus (y2), flexural strength (y3), and flexural modulus (y4) were achieved of 19.74 MPa, 912.91 MPa, 36.60 MPa, and 2598.99 MPa, respectively whereas the minimum water absorption (y5) of 7.12 % was achieved. Additionally, in order to confirm the accuracy of the analysis using RSM, the predicted values were compared with the experimentally observed with root mean square error (RMSE). The SEM micrographs show that the WPCs with the optimum conditions at 39.22 wt% RWF and a 3.44 % silane concentration (Fig. 3a) had fewer voids as compared to the WPCs from condition at 50.0 wt% RWF and a 1.0 % silane concentration (Fig. 3b) which indicates that the WPC produced employing the optimum conditions showed good compatibility between the wood and plastic and provided superior tensile and flexural strength as well as lower water absorption.
Responses (MPa)
x1
x2
y1
y2
y3
y4
y5
39.22
3.44
19.77 19.41 0.88
912.91 939.45 73.81
36.00 35.77 1.56
2598.99 2574.63 175.62
7.12 6.99 0.84
* Root mean square error (RMSE) of predicted values and observed values.
Water absorption = −3.55 + 0.294 x1 −0.245 x2
(5)
The linear equation of the water absorption shows a positive coefficient for the wood content (x1 = 0.294), revealing that the wood content positively affected the water absorption, whereas the negative coefficient for the silane concentration (x2 = −0.245) shows its negative effect on the water absorption. The effect of the wood content
Fig. 3. Scanning electron micrographs of WPCs (magnification ×1000): (a) 39.22 wt% RWF at a 3.44 % silane concentration and (b) 50.0 wt% RWF at a 1.0 % silane concentration. 6
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Table 4 Predicted responses with the optimum conditions of each property. Responses
x1
x2
Predicted response
Desirability
y1 y2 y3 y4 y5
30.00 50.00 30.00 50.00 30.00
1.00 1.00 2.37 1.00 5.00
22.03 986.54 38.95 3138.97 4.03
0.960 0.961 0.990 0.806 0.876
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Based on the model optimization of the wood content and silane concentration, the maximum mechanical properties and the minimum water absorption of the WPC are shown in Table 4. The predicted responses of the tensile strength (y1), tensile modulus (y2), flexural strength (y3), flexural modulus (y4), and water absorption (y5) were 22.03 MPa, 986.54 MPa, 38.95 MPa, 3138.97 MPa, and 4.03 %, respectively. Moreover, the desirability scores of each response were close to 0.90 which indicates that the optimum value of the variables was achieved [23]. 4. Conclusions This study sought to optimize the wood content and silane concentration to obtain the maximum tensile and flexural properties and the minimum water absorption of experimentally manufactured WPCs. The experiments were designed and analyzed with a CCD using DesignExpert software. The optimal conditions for the WPCs were found to be 39.22 wt% wood, 60.78 wt% rPP, and a 3.44 % silane concentration. The predicted responses based on the highest tensile strength, tensile modulus, flexural strength, and flexural modulus were 22.03 MPa, 986.54 MPa, 38.95 MPa, and 3138.97 MPa, respectively whereas the lowest water absorption was 4.03 % with the desirability function found to be close to 0.90. The results of this work will be useful in improving the properties of WPCs produced for different applications so that they can be used more efficiently. Declaration of Competing Interest None. Acknowledgments The authors would like to thank the Office of the Higher Education Commission for the financial support of Mrs. Sriwan Khamtree under the CHE-PhD Scholarship Program and the government budget of Prince of Songkla University (ENG-61-00-62-S), and also the Rubberwood Technology and Management Research Group (ENG-5427-11-0137-S) of Faculty of Engineering, Prince of Songkla University, Thailand. References [1] B. Lei, Y. Zhang, Y. He, Y. Xie, B. Xu, Z. Lin, L. Huang, S. Tan, M. Wang, X. Cai, Preparation and characterization of wood-plastic composite reinforced by graphitic carbon nitride, Mater. Des. 66 (2015) 103–109. [2] A.V. Kiruthika, A review on physico-mechanical properties of bast fibre reinforced polymer composites, J. Build. Eng. 9 (2017) 91–99. [3] N. Ayrilmis, A. Kaymakci, T. Gulec, Potential use of decayed wood in production of wood plastic composite, Ind. Crops Prod. 74 (2015) 279–284. [4] C. Srivabut, T. Ratanawilai, S. Hiziroglu, Effect of nanoclay, talcum, and calcium carbonate as filler on properties of composites manufactured from recycled polypropylene and rubberwood fiber, Constr. Build. Mater. 162 (2018) 450–458. [5] S.Y. Leu, T.H. Yang, S.F. Lo, T.H. Yang, Optimized material composition to improve the physical and mechanical properties of extruded wood-plastic composites (WPCs), Constr. Build. Mater. 29 (2012) 120–127. [6] S.M. Mirmehdi, F. Zeinaly, F. Dabbagh, Date palm wood flour as filler of linear low-
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