Response Surface Methodology for Formulating PVA/Starch/ Lignin Biodegradable Plastic

The use of petroleum-based plastics has raised environmental issues as more plastic waste enters and accumulates in the environment. It has led to the development of biodegradable plastics. Starch is one of the potential materials to make biodegradable plastic, but starchbased plastic has poor mechanical strength. Blending starch with poly(vinyl alcohol) (PVA) and lignin is expected to improve the mechanical properties of the plastic. Biodegradable plastic films from PVA/starch/lignin blends with glycerol as a plasticizer were prepared using an internal mixer for compounding and a hot press molding machine for film making. The percentage of lignin (2-10%), glycerol (25-65%), and mixing temperature (190-230 C) were varied according to the three levels of the Box-Behnken design. The ANOVA evaluation revealed that glycerol had the most significant effect on the mechanical properties of the film. Then, three models for the estimation of tensile strength, elongation at break, and tear resistance were developed. As expected, the models satisfactorily predict the effect of all input variables on the response variables. The optimum conditions for preparing the film were acquired from the equations, namely 197.6 C for the temperature, 10% for lignin, and 45.1% for glycerol. The biodegradable plastic prepared using the optimum conditions possessed a tensile strength of 8.46 ± 1.08 MPa, an elongation at break of 139.00 ± 8.59%, and a tear resistance of 69.50 ± 2.50 N/mm. These values are in good agreement with the predicted values.


Figure 1. Molecular structure of lignin [10]
The simplicity has made the solution casting method becomes the most applicable method for biodegradable plastic preparation [7,8,13,14]. However, this method is not suitable for industrial-scale production due to its requirement of the removal of a large amount of water (up to 90% of the solution). This dehydration step is an energy-intensive and time-consuming operation. Therefore, this process is considered to be expensive, inefficient, and not environmentally benign. These problems encourage researchers to develop an efficient, cost-effective, and environmentally friendly process through the melt extrusion technique [20].
The objective of this work was to investigate the influence of lignin and glycerol concentration, as well as mixing temperature on the mechanical properties of PVA/starch/lignin plastic for biodegradable plastic preparation via the melt extrusion method. The Box-Behnken experimental design was applied in this study to determine the optimum conditions to produce biodegradable plastic with the best mechanical properties. It is expected that the resulting biodegradable plastic is suitable for food packaging material with the mechanical properties that meet the quality standard set by the Board of Indonesian National Standard through a standard document of SNI 7818:2014. This report is organized as follows. Section 1 briefly describes the background and the objective of this research. The details of the materials and methods used in this research are explained in Section 2. The research results are presented in Section 3 along with their in-depth discussion. Finally, the conclusion of this study is presented in Section 4.

2-1-Materials
Materials used in this work were PVA, alkaline lignin, cassava starch, and glycerol. The PVA (POVALTM JP-18) with a degree of hydrolysis of 87-89% was obtained from Japan Vam & Poval Co., Ltd. Alkaline lignin (product number L0082) was purchased from Tokyo Chemical Industry. The lignin has a methoxy group of 12% and a water content of 7.4%. Cassava starch and glycerol (99.7% purity) were obtained from local suppliers, namely PT. Budi Acid and PT. Wilmar Nabati Indonesia, respectively. Before biodegradable plastic preparation, all materials, except glycerol were dehydrated at 105C for 4 hours in an electric oven (Binder, Type: 1815300002020).

2-2-Experimental Design
This work employed response surface methodology (RSM) to analyze the effect of parametric quantity inputs (independent variables) on the outcome responses (dependent variables). Different experimental design models can be used in the RSM analysis, which includes the central composite designs (CCD), Doehlert matrix (DM), and the Box-Behnken design (BBD) as the more complicated three-level designs [21]. The Box-Behnken is selected in this work for three chosen independent variables at three levels.
This work studied the influence of three selected independent variables, namely mixing temperature (T), lignin (L), and glycerol (G), on the tensile strength (TS), elongation at break (EB), and tear resistance (TR) of the biodegradable plastic. The independent variables T, L, and G were ranged from 190 to 230C, 2 to 10%, and 25 to 65%, respectively. For the sake of statistical analysis, the independent variables are transformed to dimensionless coded variables (Xi) according to Equation 1, where the subscripts high and low indicate high and low levels, respectively. The coded and actual values of the parameters used in this work are listed in Table 1. The response and the dimensionless input parameters involved in this study can be correlated according to the secondorder polynomial equation as presented in Equation 2 [22]: where Y represents the predicted response, β0 is a constant, βi is the coefficient of the linear effects, βii is the coefficient of the square effects, βij is the coefficient of interactions between two independent input variables, and ε is a random error. Fifteen experiments were formulated using MINITAB Version 19. The experimental design with dimensionless and actual variables is presented in Table 2.

2-3-Preparation of Biodegradable Plastic Films
The PVA and starch were blended with a composition of 60% and 40%, respectively. This mixture was then used in the entire experiment. Lignin was added to the PVA/starch blend with various compositions (see Table 2) and mixed to obtain a uniform blend. A predetermined amount of glycerol (see Table 2) was then added and further mixed to the attainment of a homogeneous mixture. The mixture was then let to stand in the oven at 50C for 24 hours to ensure the incorporation of glycerol. The mixture was compounded using Haake Rheomix 3000P Internal Mixer with a counterrotating Banbury rotor with a rotation speed of 100 rpm. The compounding process was conducted at various temperatures (see Table 2) for 5 minutes. The compounded mixture was then extruded to form a film by using a hot press molding machine (Toyoseiki Mini Test Press-10) at 180-200C and 100 MPa for 20 minutes. A schematic diagram of the biodegradable plastic film processing is depicted in Figure 2.

2-4-Characterization of Films
The films were tested for TS, EB, and TR. The measurement of TS and EB were conducted following the ASTM D882, while TR measurement was conducted following the ASTM D1004. All the measurements were conducted using Universal Testing Machine (Shimadzu AGS-10kNG) as shown in Figure 3.

3-1-1-Effect of Mixing Temperature on Film Properties
The effect of mixing temperature (T) on the TS, EB, and TR of the film is presented in Figures 4-a, 4-b, and 4-c, respectively. It is observed in Figures 4-a and 4-b that as the temperature rises from 190C to 210C, the TS of the film increases from 6.44 MPa to 7.14 MPa, while the EB increases from 22.9% to 134.3%, but at 230C both the TS and the EB decrease to 6.30 MPa and 103.3%, respectively. A similar trend is also observed in Figure 4-c in which the TR of the film at 190C, 210C, and 230C are 54.74 N/mm, 62.25 N/mm, and 56.49 N/mm, respectively. The increase of the TS and the EB of PVA/starch film at higher temperature was reported by Zanela et al. [4] who used five types of PVA with different degrees of hydrolysis. They extruded the PVA/starch blend at two different temperatures (170 and 200C). They stated that the tensile strength was influenced by the extruding temperature and the type of the PVA. For four types of PVA they used (Selvol 523, 540, 107, and 325) they found that the TS of the film extruded at 170C varied from 1.46 MPa to 2.2 MPa, while that at 200C was found to be 2.46 -4.56 MPa. The EB followed the same trend as the TS, in which it increased from 66 -166% (for film extruded at 170C) to 95 -391% (for film extruded at 200C).
The increase of the TS, EB, and TR of the film from 190 to 210C might be related to a more homogeneous blend as mixed at higher temperatures. With increasing temperature from 190 to 210C, the lignin and other components become softer, and the composite is more homogeneous [23] permitting a better interaction among the components [4]. As a result, the film shows higher TS, EB, and TR as the temperature rises from 190 to 210C. However, the TS, EB, and TR of the films become smaller at mixing temperature of 230C. It could be related to the thermal degradation of the components. When lignin is heated, some molecular rearrangement associated with softening of lignin starts to take place at 127-193°C [23,24]. At higher temperatures, starting at 230C, lignin undergoes decomposition to smaller molecules with the main products of Guaiacol and Syringol [25]. Another molecule susceptible to degradation is PVA. The thermal decomposition of PVA starts at 200C [26] with the most significant decomposition occurring at 240-340C [27,28]. Meanwhile, starch decomposes at 300-400C [29]. At 230C it is most likely that lignin and PVA start to decompose, while starch is still stable. Therefore, a lower value of TS, EB, and TR at 230C could be related to the thermal decomposition of lignin and PVA.    Figure 5-a that increasing lignin from 2% to 6% and 10% results in the increase of the TS by 13.5% and 16.8%, respectively. Similar results were reported by other researchers who used the solution casting method [13][14][15][16][17]25] and melt extrusion method [18] in preparing the film. Adding 1% to 20% of lignin to starch was able to increase the tensile strength of the starch-based film by 2.9 to 86.3% [15][16][17][18]. Meanwhile, the tensile strength of PVA-based film could be increased by 16.7 to 62.5% when lignin as much as 1 to 20% was added to the PVA [12][13][14]. The enforcement effect of lignin could be related to the strong interaction between lignin and other components in the film. Having hydroxyl groups, lignin can interact with both starch [30] and PVA [31] molecules through hydrogen bonds forming a rigid continuous three-dimensional network [17]. Hence, the film has a higher TS with the increase of lignin. Figure 6 schematically depicts the possible interaction between PVA, starch, and lignin through hydrogen bond in which starch is represented by amylose [30,31].

3-1-2-Effect of Lignin on Film Properties
The effect of lignin (L) on the EB is shown in Figure 5-b, in which the EB of the film with 6% and 10% of lignin increases by 282% and 597% compared to that of the film with 2% of lignin. Several groups of researchers reported a

T (°C)
similar trend that incorporating 1 to 20% of lignin could increase the EB by 2.9 to 900% [12][13][14][15]18]. However, the negative effect of lignin on the EB of the film was also reported by other researchers, that adding 1 to 20% of lignin led to the decrease of the EB by 1.3 to 38.7% [14,16,17]. The different effects of lignin on the EB of the film might be due to the different composition and molecular weight of lignin, as well as the preparation methods of the plastics [32]. It was reported that lignin with high molecular weight led to brittle plastics, while low molecular weight lignin resulted in plasticizing effect [15]. Therefore, it is possible that the lignin used in this work has low molecular weight so that it has a plasticizing effect on the plastics. Figure 5-c presents the effect of lignin on the TR of the film. The value of TR increased by 18.8 and 29.3% when the lignin addition was increased from 2% to 6 and 10%, respectively. Previous research found a similar result in which the tear resistance of polyurethane-based biodegradable film increased by 75.7% when the addition of lignin was increased from 24.18 to 43.15% [33]. It was explained that the positive effect of lignin on the TR was related to the role of lignin as the crosslinking agent where the hydroxyl group of the lignin reacted with the isocyanate group of the polyurethane [34]. However, it is presumable that there is no such reaction between PVA, starch, and lignin in the biodegradable plastic preparation in this work. The positive effect of lignin on the TR could be related to the strong interaction through hydrogen bonds among the components of the plastics, thus forming a rigid continuous three-dimensional network [17].  Figure 7 shows the effect of glycerol on the mechanical properties of the film. It is shown in Figures 7(a, and b) that the TS decreases while the EB becomes higher as glycerol increases. Other groups of researchers found similar results [12,35,36]. Sue et al. [12] varied the amount of glycerol from 3.3 to 20.8% in making PVA/lignin film. They found that as the percentage of glycerol increased from 3.3 to 20.8%, the TS of the film decreased from 35 to 26 MPa while the EB increased from 300 to 440%. Other groups of researchers manufactured a biodegradable film from durian and jackfruit seed flour with various ratios [35]. For a biodegradable plastic film made of mixed flour with a jackfruit to durian seed flour ratio of 1:1, the TS of the film decreased from 4.39 to 3.35 MPa, while the EB increased from 29.3 to 37.2% when the glycerol used in the film making increased from 30% to 40%. Tarique et al. [36] reported that increasing glycerol from 0 to 45% in making a biodegradable film from arrowroot starch caused a decrease of TS from 16.48 to 1.95 MPa and an increase of EB from 2.49 to 27.33%.

3-1-3-Effect of Glycerol on Film Properties
The influence of glycerol on the TS and EB of the film can be explained in terms of molecular interaction between the components. Cassava starch is composed of amylose and amylopectin. Both starch components molecules, and polyvinyl alcohol, have hydroxyl groups which allow the interaction between molecules through hydrogen bonds. With a smaller molecule, glycerol fills the space between starch and PVA, thereby reducing the intermolecular attraction between starch and PVA molecules, and hence, decreasing the TS of the film. The effect of glycerol on the EB is related to the presence of glycerol in between PVA and starch molecules which makes the film more flexible. As a consequence, the EB increases with the addition of glycerol [35,36].  [37,38]. Jusoh et al. [38] made blown film from polypropylene with crude palm oil as a plasticizer. They found that both the transverse-direction (TD) and machinedirection (MD) TR of the film increased with the increase of the plasticizer. Kim et al. [37] also made blown film from poly(butylene adipate-co-terephthalate) with poly(lactic acid) and adipic acid ester as the plasticizer. They also found that both TD and MD-TR were positively influenced by the addition of the plasticizer. The positive effect of plasticizer on the TR could be related to the small size of the plasticizer molecule compared to those of the polymer. The plasticizer molecules fill the spaces between the polymer molecules which enhance the interfacial adhesion between polymer molecule layers resulting in the reduction of defects such as void. Any stress resulting in tearing action can be absorbed and distributed to the neighboring polymer chains. Hence, the plasticizer increases the TR [37]. However, Figure 7-c shows that at higher concentration of glycerol (65%) the TR decreases. Although the film could be more homogeneous, it is so flexible and the distance between polymer molecules layers is apart that the interaction becomes weaker. As a result, the TR of the film with 65% glycerol is lower than that with 45% glycerol.

3-2-Statistical Analysis
The experimental results along with the predicted values for the mechanical properties are presented in Table 3. The data were statistically analyzed to fit the model (Equation 2) for the TS, EB, and TR using response surface methodology.

3-2-1-Tensile Strength
The  [47]. The difference in the TS obtained by various researchers could be related to different types of starch, plasticizer, and reinforcement, as well as the formulation of the film. The results of the analysis of variance for the TS are presented in Table 4. The contribution percentage of the input variables on the response variables is defined as the ratio of each adjusted sum of squares (linear, square, and 2-way interaction) to the total adjusted sum of squares. The analysis can depict the major contributors to the response. In addition, the F-value can also be utilized to find the significant contributors for the corresponding response. The contribution of any input variable is considered significant if its F-value is greater than 4 [48]. Multiple regression analysis with a confidence interval of 95% was used to find the correlation between each response and the independent variables. The mathematical model for the TS is presented in Equation 3.  Table 4 show that the model for the TS has an R 2 value of 91.96% which means that the model can describe 91.96% variations in response to the independent variables. Furthermore, the p-value of the model is 0.028 (< 0.05) which indicates the significance of the developed model and the parameters [49]. Equation 3 is used to calculate the mechanical properties of the film, and the results are presented in Table 3. The relation between the actual and predicted values of TS is shown in Figure 8. The straight line represents the perfect predictive ability of the models. It is shown in Figure 8 that the experimental data which are represented by blue solid circles are close to the line, meaning that the model can predict the TS well.

Figure 8. Actual versus predicted plot of tensile strength
The correlation and interaction between the input variables and the TS are depicted in three-dimensional response surface plots as shown in Figures 9 and 10. The results presented in Table 4 show that glycerol (G) has the most significant effect on the TS with a contribution percentage of 60.29%, while G 2 only contributes as much as 0.42%. The dependency of TS on G can be evaluated based on the percentage of contribution of G and G 2 . As the contribution percentage of G (linear effect) is much higher than that of G 2 (quadratic effect), it can be concluded that the effect of G on the TS is approximately linear. It is observed in Figure 9 that the TS decreases almost linearly with the increase of glycerol. It is in good agreement with the experimental data shown in Figure 7-a. Sue et al. [12] also reported that the TR of the PVA-based film decreased almost linearly from 35 to 26 MPa as the amount of glycerol increased from 3.3 to 20.8%. The second and third most significant effects are the mixing temperature (T) and the T 2 with the contribution percentage of 13.41%, and 13.06%, respectively. This finding suggests an almost equal contribution of both linear and quadratic effects of the mixing temperature on the TS. Both Figures 9 and 10 also clearly demonstrate a non-linear effect of the mixing temperature on the TS. Initially, the TS increases as the temperature increases from 190 to 210C. However, the TS value decreases as the temperature further increases beyond 210C. It is also confirmed by the experimental data presented in Figure 4-a. The significance of G, T, and T 2 are confirmed by their p-values which are less than 0.05 (0.002, 0.040, and 0.042, respectively). The increase of TS of PVA-based film as the extruding temperature increased from 170 to 200C was reported by Zanela et al. [4]. However, the information about the effect of temperature higher than 200C on the mechanical properties of biodegradable plastic film is not found in the literature.
Although the effect of lignin on the TS is not significant, it can be analyzed by using the ANOVA results presented in Table 4. Both L and L 2 have contribution percentages of 0.33 and 2.58%, respectively. Hence, the model predicts that the effect of lignin on the TS is not linear. The TS slightly increases with the increase of lignin from 0 to 10%, as depicted in Figure 10, which is in agreement with the actual data shown in Figure 5-a. Other researchers also found that the effect of lignin on the TS was positive and non-linear [12,13,17]. Table 3 shows that the EB of the biodegradable plastic films varies from 9.04% (run 9) to 168.7% (run 13), which is in the same range as reported by other researchers who manufactured PVA/starch-based biodegradable plastic films [14,17,18]. The EB values of the films obtained in this work are also comparable to LDPE, HDLPE, mixed LDPE-HDPE [42], and HDPE-based degradable films [43]. While researchers reported lower values of the EB [15,16], many others found higher values of the EB [12,13]. The different values of EB might be due to the different source and molecular weight of the lignin, as well as the difference in the formulation of the biodegradable plastic. Table 5 presents the analysis of variance for the EB as the response variable. The model developed for the EB is written in Equation 4. The mechanical properties of the film calculated using Equation 4 are presented in Table 3. The relation between the actual and predicted values of the elongation at break is shown in Figure 11.

Figure 11. Actual versus predicted values of the elongation at break
Three-dimensional response plots (Figures 12 and 13) clearly present the correlation between the input variables and the EB. As tabulated in Table 5, glycerol (G) provides the most significant effect on the EB with a contribution percentage of 72.76% and a p-value of 0.001, while the G 2 contributes only 2.77% to the variation of the EB. Therefore, the model estimates that the glycerol effect on the EB follows an almost linear function. Accordingly, Figure 12 also presents that the EB increases almost linearly with glycerol. The experimental data depicted in Figure 7-b confirm the effect of glycerol on the EB in which the EB almost linearly increases with the increase of G. Several researchers reported similar results, i.e., a positive effect of G on the EB [12,35,36]. Sue et al. [12] found that the effect of glycerol on the EB was linear for the percentage of glycerol ranged from 10 to 21%, while Tarique et al. [36] who used 15 to 45% of glycerol reported that the effect of glycerol on the EB was not linear. The different effects (linear or non-linear) of G on the EB could be due to different formulations of the biodegradable plastic.
The second most significant effect on the EB is the interaction between the mixing temperature and lignin (TL) with a contribution percentage of 15.90% and a p-value of 0.016. When the effect of 2-way interaction is significant and the value of the coefficient of interactions is negative, the response surface is twisted as shown in Figure 13. At 190C, the EB increases with the increase of lignin, which is exactly shown in Figure 5-b. Meanwhile, at 230C the EB decreases as the lignin increases. The experimental data presented in Table 3 demonstrate the same trend in which at 230C and 45% of glycerol, the EB decreases from 103.30% to 57.65% as lignin increases from 2 (run 10) to 10% (run 6). The negative effect of L on the EB at 230C could be related to the decomposition of lignin at 230C [25,50].
The effect of the mixing temperature on the EB is not significant. However, as seen in Figures 12 and 13, the effect of T on the EB is not linear, because the contribution percentage of T 2 (3.57%) is higher than that of T (1.42%). It increases as the temperature increases from 190 to 210C, but it then decreases as the temperature further increases from 210 to 230C. The experimental data depicted in Figure 4-b also exhibits the same phenomenon.

3-2-3-Tear Resistance
The TR of the film obtained in this work are ranged from 38.07 N/mm (run 4) to 70.76 N/mm (run 14). This result cannot be compared to the TR of other PVA/starch-based biodegradable plastic film, as such data are not found in the literature. However, the TR of the film obtained in this work is comparable to that of polyurethane-based bioplastics (25.66 -45.09 N/mm) [33], LDPE, and mixed LDPE-HDPE films (45 -57 N/mm) [42]. It is even higher than the TR of poly(butylene adipate-co-terephthalate) blown film plasticized by poly(lactic acid) and adipic acid ester both in machine direction (4.63 -8.67 N/mm) and transverse direction (13.19 -16.16 N/mm) [37]. The analysis of variance for the effect of the mixing temperature, lignin, and glycerol on the TR is presented in Table 6. The model correlating the input variables to the TR is written in Equation 5.  Table 3. The relation between the actual and predicted values of the TR are shown in Figure 14.

Figure 14. Actual versus predicted values of tear resistance
The correlation between the input variables and the TR is presented in three-dimensional response plots (Figures 15  and 16). Table 6 shows that G 2 and G have the most significant effect on the TR with the contribution percentage of 43.55% (p-value = 0.004) and 26.42% (p-value = 0.010), respectively. Therefore, the model predicts that the effect of G on the TR is non-linear as depicted in Figure 15, in which the TR increases and then decreases as G increases from 25% to 65%. It is the same as the experimental data shown in Figure 7-c. Jusoh et al. [38] reported that the MD-TR of polypropylene blown film plasticized by crude palm oil increased from 279 g (neat polypropylene film) to 400 g (polypropylene + 5% plasticizer), while the TD-TR increased from 469 g (neat polypropylene film) to 673 g (polypropylene + 5% plasticizer). Kim et al. [37] found that incorporation of 10% of poly(lactic acid) as the plasticizer in manufacturing of poly(butylene adipate-co-terephthalate) blown film could increase the MD-TR from 4.63 to 8.67 N/mm, and the TD-TR from 13.19 to 16.16 N/mm.
Although being less significant, the effect of the mixing temperature (T) on the TR can be analyzed by using the ANOVA results presented in Table 6. The contribution percentage of T is 1.00%, while that of T 2 is 7.06%. Therefore, it can be concluded that the effect of T on the TR is non-linear, as is seen in Figures 15 and 16. A similar effect is  (Figure 4-c). Meanwhile, it is shown in Figure 16 that the effect L on the TR is almost linear. It is because the contribution of L is much higher than that of L 2 , i.e., 2.6% and 0.52%, respectively. Experimental data depicted in Figure 5-c shows that the TR increases almost linearly with the increase of L. Zhang et al. [33] found that the TR of polyurethane-based biodegradable film increased from 25.66 to 45.09 N/mm when lignin was increased from 24.18 to 43.15%. The increase in TR with lignin could be due to the hydrogen bond among the PVA, starch, lignin, and glycerol molecules, and thus forming a strong three-dimensional network [17].

3-3-Optimization of Film Formulation
The optimum conditions for the preparation of biodegradable plastic film were estimated by the response optimizer by which the influence of both single and interacted parameters can be obtained [51]. This step resulted that the optimum conditions being 197.6C for temperature, 10% for lignin, and 45.1% for glycerol. Applying these conditions to the model would result in 7.93 MPa for the TS, 143% for the EB, and 68.50 N/mm for the TR.
Model validation was also conducted by performing a verification experiment at the optimum condition. The mechanical values of the verification sample were 8.46 ± 1.08 MPa, 139.00 ± 8.59%, and 69.50 ± 2.50 N/mm for the TS, the EB, and the TR, respectively. The TS and the TR values are slightly higher than the predicted values, while the value of EB is lower than the predicted value. Nevertheless, they are still within 95% confidence interval, i.e., 7.120 -9.798 MPa, 128.34 -149.66%, and 63. 29 -75.71 N/mm for the TS, the EB, and the TR, respectively. Therefore, the models are capable to predict reasonable responses. Table 7 shows the TS, the EB, and the TR of biodegradable plastic bags according to SNI 7818:2014 along with the biodegradable plastic made with the optimum condition in this work. Among the three properties, only the TR value exceeds the standard. Hence, further research is required to enhance the TS and the EB, while maintaining the TR.

4-Conclusion
Biodegradable plastic films from PVA/starch/lignin blends incorporating glycerol as a plasticizer were successfully manufactured through melt compounding and hot press molding machines. Three models were developed for the tensile strength, the elongation at break, and the tear resistance. The results show that glycerol, lignin, and mixing temperature affect the mechanical properties. The glycerol percentage was found to be the most significant factor. The optimum conditions to produce the biodegradable plastic film with good mechanical properties are 197.6 °C for temperature, 10% of lignin, and 45.1% of glycerol. The mechanical properties of the biodegradable plastic obtained at the optimum condition predicted using the proposed model are 7.93 MPa, 143%, and 68.50 N/mm for tensile strength, elongation at break, and tear resistance, respectively. Model validation was performed by model verification with experimental data at the optimum condition. The tensile strength of the verification sample was 8.46 ± 1.08 MPa, the elongation at break was 139.00 ± 8.59%, and the tear resistance was 69.50 ± 2.50 N/mm. These mechanical properties are within a 95% confidence interval. Furthermore, the models can predict the effects of individual and two-way interactions of the input variables on the response variables. Therefore, the models offer reasonable results to predict the response.

5-1-Author Contributions
Conceptualization, R.R. and R.W.; data collection R.W.; data validation A.C.K. and H.H.; data processing R.R. and R.W.; data analysis and interpretation R.R., R.W., A.C.K., and H.H.; writing-original draft preparation, R.W. and R.R.; writing-review and editing, R.R. and A.C.K.; project funding and administration, R.W. All authors have read and approved to the final manuscript.

5-2-Data Availability Statement
The data presented in this study are available in article.

5-3-Funding
The authors received no financial support for the research, authorship, and/or publication of this article.

5-5-Conflicts of Interest
The authors declare that there is no conflict of interests regarding the publication of this manuscript. In addition, the ethical issues, including plagiarism, informed consent, misconduct, data fabrication and/or falsification, double publication and/or submission, and redundancies have been completely observed by the authors.