Sage Journals: Discover world-class research

Abstract

This study aims to fabricate Polylactic acid/Graphene/Clay (PLA/GPN/Clay) composite using the fused deposition modeling (FDM) process and simultaneously improve the yield and impact strength. Hence, Taguchi method and grey relational grade analysis are used to attain the optimal values of GPN content, clay content, print speed and nozzle temperature. SEM, TGA and DSC tests are used to analyze the fabricated samples. The results indicated that the addition of GPN and clay in the PLA led to an enhancement of the thermal stability of PLA/GPN/Clay composite. Taguchi method results also demonstrated that the impact of graphene content and nozzle temperature on the yield and impact strength is more than the other parameters. The grey relational grade analysis also indicated that the impact and yield strength of PLA/GPN/Clay composite were enhanced with a 2 wt% clay content, a 1 wt% graphene content, a 30 mm/s print speed and a 215°C nozzle temperature.

Keywords

PLA/GPN/clay composite fused deposition modeling thermal stability impact strength yield strength

Introduction

Polylactic acid (PLA) is a linear aliphatic polyester fabricated from renewable resources like cornstarch and sugar beet.¹ Due to its low toxicity, this thermoplastic material is biodegradable and biocompatible.² The advantages of PLA are high strength, low energy loss, biocompatibility, low carbon dioxide consumption during fabrication transparency, and resistance to fat and water penetration.^1,2 Therefore, it has been widely applied in food packaging,^3,4 protection of crops² and biomedical applications.^5,6 However, polylactic acid has disadvantages such as low thermal resistance, low impact resistance at room temperature, permeability to gases, slow degradation rate, hydrophobicity, and chemical neutrality.^2–4 Hence, different types of nano-fillers have been commonly used to enhance the thermal and mechanical behavior of PLA.^7–10 Among nanofillers, graphene (GPN) is widely used in polymer nanocomposites due to its extraordinary physical properties.¹¹ Graphene has attracted great attention in different technological fields like electronics, conversion, sensors, energy storage, composite materials, and capacitors.^12,13 Graphene has excellent properties such as high thermal conductivity,¹⁴ good elastic modulus, high surface area¹⁵ and excellent intrinsic mobility.¹⁶ Furthermore, it has the ability to withstand a current density of 108 Acm⁻² and also has high electrical conductivity and good optical transmittance.^14,17 Properties of polymer composites can be also enhanced by adding graphene nanoparticles. Azizli et al.¹⁸ reported that an increase in the graphene content up to 0.7 wt% in the PA6/PLA composite improved the tensile modulus and tensile strength. Paydayesh et al.¹⁹ stated that the thermal stability, elastic modulus and tensile strength of PLA based composite filled with 3 phr graphene improved compared to the neat blend. Camargo et al.²⁰ and Caminero et al.²¹ stated that the tensile and flexural strength of PLA/Graphene composite improved compared with pure PLA, but the impact strength reduced. Therefore, all mechanical properties of polymer composites are not improved by adding the graphene nanoparticles. Another excellent additive used as a nucleating agent in polymers is clay nanoparticles.^22–25 Clay nanoparticles have great potential for industrial applications due to their low price, wide availability, easy processability, and good performance.^24,25 Zawawi et al.²⁴ stated that the thermal and mechanical features of PLA/Clay composite improved by addition of 3 wt% clay nanoparticles. AbdulAmeer et al.²⁶ reported that by adding 3 wt% CNT into polypropylene (PP), the tensile and impact strength increased without sacrificing thermal stability. AbdulAmeer et al.²⁷ also observed a substantial improvement in the elastic modulus and flexural strength of the PP/Clay composite by adding 4 wt% nanoclay. Asadi et al.²⁸ reported that the tensile strength and elastic modulus of PLA/Clay composite improved by addition of 3 wt% clay nanoparticles. However, Kontou et al.²⁹ found that the tensile strength and elastic modulus of PLA/Clay composite reduced by addition of clay nanoparticles.

The results of previous research show that using only one filler generally does not improve all mechanical properties of polymer composites. Therefore, the idea of using two or more different fillers has been proposed. Batakliev et al.³⁰ observed that incorporation of both carbon nanotubes (CNT) and graphene (GPN) in the PLA/GNP/CNT composite, indicated better hardness and elastic modulus compared to the PLA/CNT composites loaded with a single carbon nanofiller. Kumar et al.³¹ reported that by addition of 1.5 wt% carbon nanotubes and 0.5 wt% graphene into PLA, the tensile strength, elastic modulus, yield strength and impact strength were greatly enhanced. Nimbagal et al.³² found that by incorporation of 0.2 wt% graphene and 0.3 wt% carbon nanotubes the tensile and flexural strength of PLA/epoxy composite improved. Khammassi et al.³³ incorporated silver and graphene nanoparticles into PLA to enhance the stiffness and elastic modulus. Therefore, the results of previous studies have shown that the use of two or more fillers simultaneously improves mechanical properties of polymer composites. Hence, in this study, two commonly used fillers, namely graphene and carbon nanotube, were used.

In recent years, 3D printers have been increasingly used to manufacture polymer composites. To achieve optimal mechanical properties of printed polymer composites, it is necessary to extract optimal parameter conditions. Taher et al.³⁴ obtained the optimum values of TiO₂ content, nozzle temperature and print speed to increase the impact resistance and elastic modulus of the PP/EPDM/TiO₂ composite produced by fused deposition modeling (FDM). Afshari et al.³⁵ stated that the best conditions for increasing the hardness and tensile strength of PP/GPN composites was obtained at graphene content of 1.5 wt%. Hardani et al.³⁶ found that a rise of print speed declined the impact and tensile strength, but a rise of nozzle temperature enhanced the impact and tensile strength. It was also reported by Hardani et al.³⁷ that the elastic modulus and bending strength of PLA/CNT composite were enhanced by nozzle temperature of 210°C, printing speed of 20 mm/s, and CNT content of 2.9 wt%. Xu et al.³⁸ observed that the elongation and strength of PP/EPDM/TiO₂ composite have increased with a 28 mm/s print speed, 2.5 wt% TiO₂ and 227°C nozzle temperature. Benfriha et al.³⁹ reported that the parameters of 3D printing have an important effect on the cooling time of printed parts and consequently affects the adhesive strength of adjacent filaments. Vanaei et al.⁴⁰ found that the impact of extruder temperature on thermal and mechanical properties of PLA printed by FDM process is higher than other process parameters. The impact of FDM parameters on the mechanical behavior of polymer composites was also investigated by other researchers.^41–49

A review of previous research indicated that in recent years, the fabrication of ternary composites has increased to simultaneously improve mechanical properties. However, in the previous research, the ternary combination of PLA, clay, and graphene has not been used to produce the PLA/Clay/GPN ternary composite. Feasibility experiments have shown that by selecting the optimal amounts of clay and graphene in the PLA/Clay/GPN composite, the mechanical and thermal properties of the produced composite can be improved. Moreover, the PLA/Clay/GPN composite has been made by FDM process, in which the mechanical properties of the fabricated composite can be improved by controlling the process parameters. Therefore, in this research, an attempt has been made to fabricate a PLA/Clay/GPN ternary composite using FDM process and to improve its mechanical and thermal properties. Then, the fabricated samples were analyzed by DSC, TGA, SEM and rheological tests. The Taguchi method has been chosen to study the effect of clay content, graphene content, print speed and nozzle temperature on the yield and impact strength of the composite with a smaller number of experiments. Finally, the optimal parameter conditions have been obtained using the gray relational grade analysis to simultaneously improve the impact and yield strength of the composite.

Materials and methods

Materials

In this study, powder particles of polylactic acid (PLA 3052D) were purchased from Plastika Kritis S.A. (Heraklion, Crete, Greece). The melt flow index and molecular weight of polylactic acid were 14 g/10 min (ASTM D1238) and 116,000 g/mol, respectively. The graphene nanoplatelets have a mean surface area of 700 m²/g, a thickness of less than 40 nm and a particle radius of lower than 1.25 μm. The functionalized graphene nanoplatelets have the functional groups such as hydroxyls, carboxyls, and ethers. Organically-modified montmorillonite (OMMT) nanoclay was purchased from Southern Clay Products Inc. Nanoclays have a d-spacing of 15 A and specific gravity of 1.66 4 kg/m³.

Fused deposition modeling (FDM)

In the present study, the PLA/Clay/GPN ternary composite was fabricated using FDM process. Before mixing the PLA/Clay/GPN powder compositions, they were dried in an oven at 80°C. A mechanical mixer was employed to blend PLA powders with Clay and GPN nanoparticles at different loadings. The production of filament with a radius of 0.75 mm has been done with an extruder (Filabot EX2, USA). For extruding filaments, a temperature of 210°C and a rotational velocity of 3 r/min were used. To check for the presence of holes or porosity in the produced filaments, their quality was visually inspected. For implementation of the FDM process, a 3D printer (Model Wanhao D12-230) was applied. The values of nozzle temperature and print speed were changed during FDM process. Table 1 indicates the constant parameters of FDM process.

Table 1.

Values of constant parameters of FDM.

Parameters	Nozzle diameter	Layer thickness	Air gap	Bed temperature	Built orientation
Values	0.4 mm	0.2 mm	0.05 mm	80°C	0°

Taguchi design

For design of experiments, the Taguchi design is used in this study to reduce the number of experiments. Minitab® 17 software is used to design the experiments. The input parameters were clay content, Graphene content, nozzle temperature and print speed, and the output responses were impact strength and yield strength. Each input parameter was considered in three levels. Thus, based on the orthogonal arrays table proposed by Taguchi design, an L9 orthogonal array was selected. The design matrix based on Taguchi method was presented in Table 2. The experiments were carried out with three repetitions.

Table 2.

Design matrix based on L9 orthogonal array.

Run	1	2	3	4	5	6	7	8	9
Clay (wt%)	0	0	0	2	2	2	4	4	4
Graphene (wt%)	0	1	2	0	1	2	0	1	2
Print speed (mm/s)	15	30	45	30	45	15	45	15	30
Nozzle temperature (°C)	185	200	215	215	185	200	200	215	185
Yield strength (MPa)	38.5	52	45.6	58.9	54.1	50.2	42.3	57.4	47.4
Impact strength (kJ)	1.7	2.6	2.1	1.9	1.8	1.7	1.4	2.2	1.5
Void fraction (%)	11.7	7.4	6.2	4.8	9.5	6.9	8.1	7.2	8.6

Thermal tests

Differential scanning calorimetry test (Netzch 200 F3, Maia) was conducted to obtain the melting (T_m) and crystallization (T_c) temperatures of produced samples. Differential scanning calorimetry test was carried out with a scanning rate of 10°C/min in a nitrogen environment. The test was performed with heating-cooling-heating cycles from 20 to 200°C. The percentage crystallinity (X_c) of produced samples was obtained by equation (1).

X_{c} = (\frac{{∆ H}_{m}}{{∆ H}_{L}}) \times 100 %

(1)where,

{∆ H}_{m}

and

{∆ H}_{L}

are the measured melting enthalpy and the enthalpy for a perfect crystal of PLA (93 J/g), respectively.

Mechanical tests

The FDM process was used to fabricate tensile and impact specimens. The specimens of tensile test were produced based on ASTM D-638 standard. A Zwick/Roell-Z100 machine has utilized to execute the tensile test with a 50 mm/min cross-head speed. After conducting the tensile test, the stress-strain curve for all samples were obtained, as shown in Figure 1. The Izod impact test was also performed according to ASTM D-256.

Figure 1.

Stress-strain curve for all samples.

Microstructure observation

To observe the microstructure of produced samples, scanning electron microscopy (Model VEGA-TESCAN-XMU) is applied. For preparation of the fracture surface of samples, they were cryofractured in liquid nitrogen. Then, an Agar Scientific sputter coater B7340 (United Kingdom) was used to coat the prepared surfaces with gold. The void fraction in the microstructure was also measured by ImageJ software, as shown in Table 2. The void fraction is quantified as a percentage representing the ratio of the void area to the total area of the sample.

Rheological tests

Dynamic rheological measurements were performed by using a strain-controlled Advanced Rheometrics Expansion System rheometer (ARES-TA Instruments) with a parallel-plate geometry (25 mm plate diameter) at 200°C. Frequency sweep tests were also conducted on the samples within their linear viscoelastic region, determined from strain sweeps, at 200°C.

Results and discussion

Analysis of thermal properties

In order to study the thermal properties of PLA/Clay/GPN composite the differential scanning calorimetry (DSC) analysis is employed. Figure 2 shows the results of DSC analysis for PLA/Clay/GPN composite. The values of melting temperature, crystallization temperature and percentage crystallinity of PLA/Clay/GPN composite were obtained from DSC test, as listed in Table 3.

Figure 2.

Results of DSC test.

Table 3.

Thermal properties of PLA/Clay/GPN composite obtained by DSC test.

Thermal properties	PLA + 0 wt% clay + 0 wt% GPN	PLA + 2 wt% clay + 1 wt% GPN	PLA + 4 wt% clay + 2 wt% GPN
Crystallization temperature (°C)	128.9	131.8	134.6
Crystallinity (%)	49.2	52.2	54.1
Melting temperature (°C)	167.7	171.4	173.5

From Table 3, it can be seen that a rise of clay and graphene in the PLA enhanced the crystallization and melting temperatures in the PLA/Clay/GPN composite. It should be noted that part of the heat is absorbed by clay and graphene nanoparticles, thus the crystallization and melting temperatures of the composite increased by addition of clay and graphene. Therefore, the thermal stability and heat transfer of the PLA/Clay/GPN composite improved by addition of clay and graphene in PLA. Furthermore, the results of DSC in Table 3 depict that the increase of the amount of clay and graphene nanoparticles improved the crystallinity percentage of the composite, because these nanoparticles act as nucleation agents in PLA.

For investigating the thermal properties of PLA/Clay/GPN composite, the thermogravimetric analysis (TGA) was conducted in the range of 0 to 600°C, as shown in Figure 3. Figure 3 shows that the thermal stability of PLA increased by addition of clay and graphene. When the amount of nanoparticles is 0 wt%, the thermal degradation of PLA starts at 299°C and its mass decreases continuously until 536°C. However, by adding nanoparticles, the decomposition temperature of PLA/Clay/GPN composite increases to 341°C.

Figure 3.

Thermogravimetric analysis of PLA/Clay/GPN composite.

It should be noted that the movement of polymer chains is restricted by nanoparticles, which improves the decomposition temperature of the PLA/Clay/GPN composite.^35,36 The thermal behavior of the composite is analyzed by the weight loss temperature at 10, 50 and 90%, as shown in Table 4. Based on Table 4, an increase in the Clay and graphene content enhanced the thermal stability of PLA.

Table 4.

Weight loss temperature at 10, 50 and 90 %.

Temperature	PLA + 0 wt% clay + 0 wt% GPN	PLA + 2 wt% clay + 1 wt% GPN	PLA + 4 wt% clay + 2 wt% GPN
T₁₀ (°C)	350	367	388
T₅₀ (°C)	413	441	452
T₉₀ (°C)	470	486	497

Microstructure analysis

The microstructure of the printed parts is studied to realize the dispersion of the nanoparticles within the PLA/Clay/GPN composite and evaluate the quality of the adhesive between the filament layers. Figure 4 shows the microstructure of tensile specimens for different amounts of clay and graphene content. It is obvious from Figure 4(a) that the fracture surface of pure PLA has a smooth and uniform microstructure. Figure 4(b) shows that when amounts of clay and graphene are 2 and 1 wt%, the nanoparticles are good distributed within the PLA/Clay/GPN composite. The interfacial interaction between the nanoparticles and PLA matrix increased at this condition, leading to an enhancement in the mechanical properties of the PLA/Clay/GPN composite. Moreover, when amounts of clay and graphene increased up to 4 and 2 wt%, the agglomeration of the nanoparticles has detected in the microstructure of the composite (Figure 4(c)). The agglomeration of nanoparticles results in their improper dispersion within the matrix, which leads to the worsening of the mechanical behavior of the composite.

Figure 4.

Fracture surface of the composite for (a) Clay = 0 wt%, GPN = 0 wt%, (b) Clay = 2 wt%, GPN = 1 wt%, (c) Clay = 4 wt%, GPN = 2 wt%.

The impact of print speed on the microstructure of the PLA/Clay/GPN composite was presented in Figure 5. According to Figure 5(a), the adhesion strength between the printed layers is low at print speed of 15 mm/s, because of nucleation of voids and cracks in the fracture surface. Thus, the mechanical properties of the composite reduced when the adhesion strength between the printed layers is low.

Figure 5.

Fracture surface of the composite for print speed of (a) 15, (b) 30 and (c) 45 mm/s.

It can be detected from Figure 5(b) that when the print speed raised to 30 mm/s, the presence of voids and cracks decreased in the fracture surface of printed sample, which shows that the adhesive strength between the printed layers improved. Figure 5(c) shows that a rise of the print speed to 45 mm/s has once again increased the existence of voids and cracks between the printed layers, since the cooling rate decreased and so the adhesion between printed materials reduced.

The impact of different nozzle temperatures on the microstructure of printed samples was presented in Figure 6. As clear from Figure 6(a), when the nozzle temperature is 185°C the adhesion strength between the printed layers is low, because the voids and cracks in the fracture surface of the printed samples are high. The lower adhesion strength at nozzle temperature of 185°C is because of high viscosity of the polymer. It should be noted that by increasing the viscosity of the filament and reduction of its fluidity, the deposition process does not occur effectively. Thus, the adhesion between adjacent filaments decreases because of the reduced interpenetration of polymer chains.^38,39 Figure 6(b) indicates that when the nozzle temperature increased to 200°C, the voids and cracks in the fracture surface reduced and bonding strength between the printed layers increased, which was owing to the increase of the viscosity of the printed filament. Moreover, according to Figure 6(c), by further increasing the nozzle temperature to 215°C, the presence of cracks and voids at the fracture surface reduced, which led to an improvement in the adhesion of the printed layers.

Figure 6.

Fracture surface of the composite for nozzle temperature of (a) 185, (b) 200 and (c) 215°C.

The effect of different parameters on the void fraction in the fracture surface was also measured, and the results are shown in Figure 7. As can be seen from Figure 7, an increase in clay content to 2 wt% led to void filling and a decrease in void fraction, but an increase in clay content to 4 wt% led to an increase in void fraction, which is probably due to particle agglomeration. Moreover, an increase in graphene content to 2 wt% resulted in a slight decrease in the void fraction, because graphene is at the nanoscale. Also, according to Figure 7, the lowest void fraction was obtained at a print speed of 30 mm/s. Finally, an increase in nozzle temperature to 215°C resulted in greater adhesion between filament layers and a significant reduction in void fraction. A reduction in the void fraction leads to an improvement in the mechanical properties of the samples.

Figure 7.

Effect of various parameters on void fraction.

Rheological measurement results

Figure 8 shows the storage (G′) and loss (G″) moduli of the PLA/Clay/GPN composite at different loadings at 200°C versus dynamic frequency sweep. It can be observed from Figure 8(a) and (b) that addition of GNP and Clay significantly enhances the moduli of PLA over the entire frequency range. At high frequencies (>10 rad/s), G′ and G″ increase monotonically with increasing GNP and Clay. It can be also observed from Figure 8 that at lower frequencies, when GNP and Clay contents are 1 and 2 wt%, the moduli significantly improves but when GNP and Clay contents are 2 and 4 wt%, a decrease is observed in both G′ and G″. Moreover, the greater increase in G′ of PLA with addition of GNP and Clay compared to that of G″ indicates the more pronounced effect of GNP and Clay on the elastic rather than the viscous response of PLA.⁵⁰

Figure 8.

(a) Storage and (b) loss moduli of PLA/Clay/GNP composites versus frequency.

It was also observed that at lower frequency range, the composites behaved more solid like, while at higher frequency range, where the hydrodynamic force is more evident, the composites behaved liquid like. Moreover, at lower frequency, G′ was more sensitive to the presence of GNP and Clay.

By examining the slope (n) of the G′-ω diagram, the distribution of nanoparticles in PLA can be investigated. For pure molten PLA, a terminal-like behavior has been observed for storage modulus in low frequencies and the slope (n) is proportional to 2(G′ ∼ ωⁿ).⁵¹ The values of n are determined by the slope of the theoretical curves, when they coincide with the experimental points, in the low-frequency region. The values of n compare the different dispersions and quantify the degree of dispersion of the nanoparticles. A decrease in the slope of the G′-ω diagram indicates a uniform dispersion of nanoparticles due to the good interaction of nanoparticles and polymer. Table 5 shows the calculated slope values (n) for the tested specimens, using the mathematical linear regression function results by the Matlab program.

Table 5.

Slope of G′ versus ω diagram in the low frequency region.

Composite	PLA + 0 wt% clay + 0 wt% GPN	PLA + 2 wt% clay + 1 wt% GPN	PLA + 4 wt% clay + 2 wt% GPN
Slope (n)	1.63	0.54	1.49

It can be seen from Table 5 that as the concentration of GPN and Clay increased up to 1 and 2 wt%, n value decreased significantly, which led to a raised degree of dispersion. Theoretically, a slope value of ∼ 0.5 is characteristic of “gel” type structures, which is usually associated with reaching the percolation threshold.⁵² However, the increase of GPN and Clay up to 2 and 4 wt% led to a reduction in the n value, which shows that dispersion of the nanoparticles affected by agglomeration.

Analysis of Taguchi design

To examine the effect of process parameters on the yield and impact strength of fabricated samples, the analysis of signal-to-noise ratio (S/N) is used. In the experimental design with Taguchi method, disturbing and controllable factors are considered. Disturbing factors (noise) influence the process but their control is not cost-effective, while controllable factors (signal) are under control. The S/N ratio of Taguchi technique can be applied instead of the output responses. Depending on the considered responses, the desired goal is divided into three categories: smaller is better, larger is better and nominal is better. Equations (2) and (3) are used to calculate the S/N ratio for “smaller is better” and “larger is better”, respectively.

\frac{S}{N} = - 10 \log 10 (\frac{1}{n} \sum_{i = 0}^{n} ({y_{i}}^{2}))

(2)

\frac{S}{N} = - 10 \log 10 (\frac{1}{n} \sum_{i = 0}^{n} (\frac{1}{{y_{i}}^{2}}))

(3)In these equations, n represents the repetition of experimental tests (n = 3) and y_i represents the recorded response for the i^th time. Since the maximum values of yield and impact strength are expected, equation (3) is used to obtain the S/N ratio. The S/N ratio and the optimal parameter level for each response are obtained by Minitab software, as given in Table 6. The values of yield and impact strength were also indicated in Table 6. The analysis of variance (ANOVA) for S/N ratio was also calculated, as shown in Tables 7 and 8 for yield and impact strength respectively. It can be seen from ANOVA results in Tables 7 and 8 that the clay content has the highest influence on the yield strength (33.3% contribution), but the graphene content has the highest influence on the impact strength (42.1% contribution).

Table 6.

S/N ratios for yield and impact strength.

Run	1	2	3	4	5	6	7	8	9
Yield strength (MPa)	38.5	52	45.6	58.9	54.1	50.2	42.3	57.4	47.4
S/N for yield strength	31.71	34.32	33.18	35.40	34.66	34.01	32.53	35.18	33.52
Impact strength (kJ)	1.7	2.6	2.1	1.9	1.8	1.7	1.4	2.2	1.5
S/N for impact strength	4.61	8.30	6.44	5.58	5.11	4.61	2.92	6.85	3.52

Table 7.

Results of ANOVA for yield strength.

Source	DF	Seq SS	Adj SS	Adj MS	Contribution
Clay	2	3.9956	3.99557	1.99779	33.3%
Graphene	2	3.7264	3.72638	1.86319	31.1%
Print speed	2	1.5518	1.55179	0.77589	12.9%
Nozzle temperature	2	2.7038	2.70381	1.35191	22.6%
Total	8	11.9776

Table 8.

Results of ANOVA for impact strength.

Source	DF	Seq SS	Adj SS	Adj MS	Contribution
Clay	2	6.3579	6.35787	3.17893	28.2%
Graphene	2	9.4972	9.49718	4.74859	42.1%
Print speed	2	1.4288	1.42880	0.71440	6.3%
Nozzle temperature	2	5.2968	5.29679	2.64839	23.4%
Total	8	22.5806

Figure 9 displays the S/N ratios in a plot for yield and impact strength. Figure 9(a) indicates that the greatest yield strength is attained for clay content of 2 wt% and graphene content of 1 wt%. According to SEM images in Figure 4(b), it can be concluded that the increase of yield strength at clay content of 2 wt% and graphene content of 1 wt% is owing to the good dispersion of nanoparticles in the PLA polymer. Moreover, the reduction of yield strength at higher amounts of clay and graphene content is due to the agglomeration of nanoparticles, which prevented proper distribution of nanoparticles in the PLA polymer. In addition, a decrease in the yield strength of the printed samples containing 4 wt% clay and 2 wt% graphene could potentially be attributed to the higher crystallinity of the PLA matrix. It worth mentioning that an increase in crystallinity enhances the internal rigidity of PLA but can occasionally lead to a reduction in the overall strength of the specimen. Similar results have also been reported in Refs. 18–21,26–29. Figure 9(b) indicates that the maximum impact strength is achieved with a 0 wt% clay and 1 wt% graphene. Since clay particles are larger in size, their presence in the PLA matrix can be a stress concentration site and worsen the impact strength of PLA/Clay/GPN composite.

Figure 9.

Effect of parameters on (a) yield strength, (b) impact strength.

Figure 9(a) and (b) also show that by increasing print speed to 30 mm/s the yield and impact strength of PLA/Clay/GPN composite improved, while by increasing print speed up to 45 mm/s the yield and impact strength decreased. According to Figure 5(b), the improvement of yield and impact strength at print speed of 30 mm/s can be because of the increase in the adhesive strength between the printed layers and consequently the reduction of voids and cracks between filament layers. The presence of holes and cracks in the composite structure can be a stress concentration site, which reduces yield and impact strength. The results obtained are in agreement with the results found in Refs. 34–39,53–57. It can be observed from Figure 9(a) and (b) that an elevation of nozzle temperature to 215°C enhanced the yield and impact strength of PLA/Clay/GPN composite, so that the highest yield and impact strength were obtained at nozzle temperature of 215°C. As can be seen from Figure 6(c), the greater adhesion between the printed layers caused by the lower viscosity of the PLA is responsible for the increased yield and impact strength at nozzle temperature of 215°C. The robust bonding between printed layers at this temperature minimizes the formation of cavities and cracks (Figure 6(c)), leading to improved mechanical properties in terms of both yield and impact strength. Additionally, as shown in the SEM image in Figure 6(a), defects in the microstructure caused by poor filament layer adhesion significantly degrade these properties at a nozzle temperature of 180°C. It is important to note that insufficient temperature during filament deposition results in weak interlayer bonds, primarily due to inadequate molecular chain fusion between the layers. These findings align well with the observations reported in other studies.^{34–39,54–56}

Grey relational analysis

To enhance both yield and impact strength concurrently, identifying the optimal parameter levels is essential. The findings derived from the Taguchi method were further refined using the grey relational grade technique to obtain the optimal parameter levels. This approach involves the normalization of S/N values, ensuring they fall within the range of 0 to 1.⁵⁸ The normalization process was carried out using equation (4).⁵⁸

Z_{i j} = \frac{Y_{i j} - \min (Y_{i j})}{\max (Y_{i j}) - \min (Y_{i j})}

(4)In equation (4), Z_ij denotes the normalized values, which fall within the range of 0 to 1, while Y_ij denotes the outcome of the i_th test for the j_th response.⁵⁸ The normalized signal-to-noise (S/N) values for each response was shown in Table 9.

Table 9.

Normalized values, gray relational coefficient and G_i coefficient for each response.

Run	Z_ij for yield strength	Z_ij for impact strength	GC_ij for yield strength	GC_ij for impact strength	G_i coefficient
1	0.0000	0.3141	0.3333	0.4216	0.377482
2	0.7073	1.0000	0.6308	1.0000	0.815385
3	0.3984	0.6543	0.4539	0.5912	0.522542
4	1.0000	0.4944	1.0000	0.4972	0.748614
5	0.7995	0.4071	0.7137	0.4575	0.585608
6	0.6233	0.3141	0.5703	0.4216	0.495977
7	0.2222	0.0000	0.3913	0.3333	0.362319
8	0.9404	0.7302	0.8935	0.6495	0.771489
9	0.4905	0.1115	0.4953	0.3601	0.427705

The gray relational coefficient (GC_ij) is attained by equation (5). To achieve this coefficient, the normalized values are first transformed into a deviation sequence (Δ_ij). The value of Δ_ij is achieved by measuring the difference between the target value (i.e. 1) and normalized values (equation (6)).⁵⁸

{G C}_{i j} = \frac{∆_{\min} + λ ∆_{\max}}{∆_{i j} + λ ∆_{\max}}

(5)

∆_{i j} = 1 - Z_{i j}

(6)In equation (5), Δ_min and Δ_max are equal to 0 and 1, respectively. The discrimination coefficient (λ), ranging between 0 and 1, is selected based on the specific target. In the present research, discrimination coefficient is set to 0.5.⁵⁸ To attain the value of GC_ij for each response, equation (6) was calculated, as the results were given in Table 9.

Once the gray relational coefficients were determined for all runs, the final coefficient (G_i) was calculated using equation (7). The experiment yielding the highest Gi value indicates the optimal parameter levels.⁵⁸

G_{i} = \frac{1}{m} \sum (α_{j} \times G C_{i j})

(7)In Equation (7), m denotes the cumulative impact coefficient of the responses, while αj shows the specific impact coefficient of an individual response. In this study, the same impact coefficient was used for all responses. Table 9 summarizes the G_i coefficients for all experiments. Based on the data in Table 9, the highest Gi coefficient for yield strength was observed in experiment 4, while for impact strength, it was achieved in experiment 2. Consequently, yield and impact strength were enhanced in samples 4 and 2, respectively. However, the optimal conditions for simultaneously improving both yield and impact strength were identified in experiment 2. These conditions included a graphene content of 1 wt%, clay content of 0 wt%, a print speed of 30 mm/s, and a nozzle temperature of 200°C.

To verify the accuracy of the achieved results, several validation experiments have been conducted under optimal parameter conditions (Clay = 0 wt%, Graphene = 1 wt%, Print speed = 30 mm/s, Nozzle temperature = 200°C). Also, several experiments were conducted in the initial conditions of the parameters (Clay = 0 wt%, Graphene = 0 wt%, Print speed = 15 mm/s, Nozzle temperature = 185°C) to compare with the optimal conditions. Results of experiments at initial and optimal conditions were shown in Table 10. As can be seen from Table 10, the yield and impact strength values in the optimal conditions have improved by 34% and 75%, respectively, compared to the initial conditions.

Table 10.

Results of experiments at initial and optimal conditions.

Run	Initial conditions	Optimal conditions
Clay (wt%)	0	0
Graphene (wt%)	0	1
Print speed (mm/s)	15	30
Nozzle temperature (°C)	185	200
Yield strength (MPa)	38.5	51.7
Impact strength (kJ)	1.7	2.8

Given that both responses of yield and impact strength improved at a nozzle temperature of 215°C, the nozzle temperature was increased to 215°C in optimal conditions to observe the change in responses. In addition, the yield strength improved at clay content of 2 wt% and the impact strength decreased slightly, so the clay content was also chosen at 2 wt% to examine the values of the responses obtained. Table 11 shows the results of experimental tests in the initial conditions and the new optimal conditions.

Table 11.

Results of experiments at initial and new optimal conditions.

Run	Initial conditions	Optimal conditions
Clay (wt%)	0	2
Graphene (wt%)	0	1
Print speed (mm/s)	15	30
Nozzle temperature (°C)	185	215
Yield strength (MPa)	38.5	61.4
Impact strength (kJ)	1.7	2.6

According to Table 11, the yield and impact strength values in the new optimal conditions have improved by 59% and 62%, respectively, compared to the initial conditions. Therefore, under the new optimal conditions, the yield and impact strength have improved significantly.

Conclusion

In this research, the yield and impact strength of the PLA/Clay/GPN composite enhanced using the Taguchi method and grey relational analysis. The analysis of thermal behavior of the PLA/Clay/GPN composite declared that the increase of clay and graphene content in the PLA was followed by an enhancement in the crystallization and melting temperatures of the PLA/Clay/GPN composite. Furthermore, the thermal stability of PLA increased by addition of clay and graphene, because part of the heat input is absorbed by clay and graphene nanoparticles. The microstructure analysis indicated that when amounts of clay and graphene are 2 and 1 wt%, the nanoparticles are well dispersed in the PLA/Clay/GPN composite, leading to an enhancement in the yield and impact strength of the PLA/Clay/GPN composite. However, when amounts of clay and graphene increased up to 4 and 2 wt%, the aggregation of the nanoparticles was detected in the microstructure of the composite. The Taguchi analysis indicated that the greatest yield and impact strength of the composite obtained with a print speed of 30 mm/s and nozzle temperature of 215°C due to the formation of a good adhesion bonding between the filament layers and consequently the reduction of the presence of voids and cracks in the microstructure. The grey relational analysis showed that the optimal conditions for simultaneously improving both yield and impact strength can be obtained by the following conditions: a graphene content of 1 wt%, clay content of 2 wt%, a print speed of 30 mm/s, and a nozzle temperature of 215°C.

Footnotes

Acknowledgements

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through large group Research Project under grant number RGP2/257/46.

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the King Khalid University (RGP2/257/46).

ORCID iD

Mohamed A. Diab

Data Availability Statement

The data supporting the findings of this study are available within the article.*

References

Ajaj

Al-Salman

HNK

Hussein

, et al. Effect and investigating of graphene nanoparticles on mechanical, physical properties of polylactic acid polymer. Case Stud Chem Environ Eng 2024; 9: 100612.

Sánchez-Rodríguez

Avilés

M-D

Pamies

, et al. Extruded PLA nanocomposites modified by graphene oxide and ionic liquid. Polymers 2021; 13(4): 655.

Dejene

Gudayu

. Achieving performance and functionality in PLA-based packaging: insights from hybrid composites with false banana (Enset) fiber and ZnO nanoparticles. J Thermoplast Compos Mater 2025; 38(6): 2138–2168.

Dejene

. Reviewing the manufacturing challenges and scientific debates: Insights into the antibacterial capabilities and potential applications of PLA/ZnO nanocomposites. J Thermoplast Compos Mater. 2024; 38: 08927057241292298.

Namasivayam Sukumaar

Arjunan

Pandiaraj

, et al. Experimental investigation of 3D printed polylactic acid and polylactic acid–hydroxyapatite composite through material extrusion technique for biomedical application. J Thermoplast Compos Mater 2025; 38(4): 1303–1333.

Kumar

Singh

, et al. ZnO nanoparticle-grafted PLA thermoplastic composites for 3D printing applications: tuning of thermal, mechanical, morphological and shape memory effect. J Thermoplast Compos Mater 2022; 35(6): 799–825.

Jagadeesh

Rangappa

, et al. Effect of infill pattern and filler ratio on the mechanical properties of 3D printed eco-friendly wollastonite/polylactic acid composites. J Thermoplast Compos Mater 2025;08927057251344247. OnlineFirst.

Luo

Fortenberry

Ren

, et al. Recent progress in enhancing poly(lactic acid) stereocomplex formation for material property improvement. Front Chem 2020; 8: 688.

Sessini

Palenzuela

Damian

, et al. Bio-based polyether from limonene oxide catalytic ROP as green polymeric plasticizer for PLA. Polymer 2020; 210: 123003.

10.

Alberts

Ballentine

Barnes

, et al. Impact of metal additives on particle emission profiles from a fused filament fabrication 3D printer. Atmos Environ X 2021; 244: 117956.

11.

Dubey

Kesarwani

Verma

. Incorporation of graphene nanoplatelets/hydroxyapatite in PMMA bone cement for characterization and enhanced mechanical properties of biopolymer composites. J Thermoplast Compos Mater 2023; 36(5): 1978–2008.

12.

Kumar

Duvedi

Sharma

, et al. Navigating the frontier: additive manufacturing’s role in synthesizing piezoelectric materials for flexible electronics. J Thermoplast Compos Mater 2025; 38(4): 1598–1636.

13.

Schedin

Geim

Morozov

, et al. Detection of individual gas molecules adsorbed on graphene. Nature 2007; 6: 652–655.

14.

Lee

Wei

Kysar

, et al. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008; 321: 385–388.

15.

Farjadian

Abbaspour

Sadatlu

MAA

, et al. Recent developments in graphene and graphene oxide: properties, synthesis, and modifications: a review. ChemistrySelect 2020; 5: 10200–10219.

16.

Bolotin

Sikes

Jiang

, et al. Ultrahigh electron mobility in suspended graphene. Solid State Commun 2008; 146: 351–355.

17.

Zhang

Zheng

Yan

, et al. Electrically conductive polyethylene terephthalate/graphene nanocomposites prepared by melt compounding. Polymer 2010; 51: 1191–1196.

18.

Azizli

Ghadami

Vafa

, et al. Compatibilization of immiscible PA6/PLA nanocomposites using graphene oxide and PTW compatibilizer for high thermal and mechanical applications. J Polym Environ 2023; 31: 4193–4209.

19.

Paydayesh

Mousavi

Estaji

, et al. Functionalized graphene nanoplatelets/poly (lactic acid)/chitosan nanocomposites: mechanical, biodegradability, and electrical conductivity properties. Polym Compos 2022; 43(1): 411–421.

20.

Camargo

Machado

ÁR

Almeida

, et al. Mechanical properties of PLA-graphene filament for FDM 3D printing. Int J Adv Manuf Technol 2019; 103: 2423–2443.

21.

Caminero

MÁ

Chacón

García-Plaza

, et al. Additive manufacturing of PLA-based composites using fused filament fabrication: effect of graphene nanoplatelet reinforcement on mechanical properties, dimensional accuracy and texture. Polymers 2019; 11: 799.

22.

Mihankhah

Azdast

Mohammadzadeh

, et al. Fused filament fabrication of biodegradable polylactic acid reinforced by nanoclay as a potential biomedical material. J Thermoplast Compos Mater 2023; 36(3): 961–983.

23.

Lopes Alves

de Tarso Vieira e Rosa

de Redondo Realinho

, et al. Single and hybrid organoclay‐filled PLA nanocomposites: mechanical properties, viscoelastic behavior and fracture toughening mechanism. J Appl Polym Sci 2021; 138(32): 50784.

24.

Zawawi

EZE

Hafizah

AHN

Romli

, et al. Effect of nanoclay on mechanical and morphological properties of poly (lactide) acid (PLA) and polypropylene (PP) blends. Mater Today Proc 2021; 46: 1778–1782.

25.

Kahraman

Özdemir

Gümüş

, et al. Morphological, rheological, and mechanical properties of PLA/TPU/nanoclay blends compatibilized with epoxy‐based Joncryl chain extender. Colloid Polym Sci 2023; 301(1): 51–62.

26.

AbdulAmeer

Altalbawy

Adhab

, et al. Simultaneous improvement of thermal and mechanical properties of PP/SEBS/clay nanocomposite produced by fused filament fabrication method. Polym Eng Sci 2025; 65: 1200–1213.

27.

AbdulAmeer

Altalbawy

FMA

Adhab

, et al. Response surface methodology to optimize the thermal stability, young modulus and flexural strength of PP/SEBS/clay nanocomposite produced by FFF method. J Compos Mater. 2025; 59: 00219983241313016.

28.

Asadi

Javadi

Mohammadzadeh

, et al. Investigation on the role of nanoclay and nano calcium carbonate on morphology, rheology, crystallinity and mechanical properties of binary and ternary nanocomposites based on PLA. Int J Polym Anal Char 2021; 26(1): 1–16.

29.

Kontou

Niaounakis

Georgiopoulos

. Comparative study of PLA nanocomposites reinforced with clay and silica nanofillers and their mixtures. J Appl Polym Sci 2011; 122(3): 1519–1529.

30.

Batakliev

Georgiev

Angelov

, et al. Synergistic effect of graphene nanoplatelets and multiwall carbon nanotubes incorporated in PLA matrix: nanoindentation of composites with improved mechanical properties. J Mater Eng Perform 2021; 30: 3822–3830.

31.

Kumar

GSP

Keshavamurthy

Panigrahi

, et al. Enhanced mechanical properties of CNT/Graphene reinforced PLA-based composites fabricated via fused deposition modelling. Results Eng 2025; 25: 104472.

32.

Nimbagal

Banapurmath

Sajjan

, et al. Studies on hybrid bio-nanocomposites for structural applications. J Mater Eng Perform 2021; 30: 6461–6480.

33.

Khammassi

Tarfaoui

Škrlová

, et al. Poly(Lactic acid) (PLA)-Based nanocomposites: impact of vermiculite, silver, and graphene oxide on thermal stability, isothermal crystallization, and local mechanical behavior. J Compos Sci 2022; 6(4): 112.

34.

Taher

Afshari

Houmani

, et al. Simultaneous enhancement of the impact strength and tensile modulus of PP/EPDM/TiO2 nanocomposite fabricated by fused filament fabrication. Colloid Polym Sci 2024; 302: 393–407.

35.

Afshari

Hardani

Hamounpeyma

, et al. Friction stir welding of polypropylene based graphene nanocomposites fabricated with 3-D printing: an investigation on the microstructure and mechanical properties. J Compos Mater 2023; 57(8): 1523–1538.

36.

Hardani

Afshari

Allahyari

, et al. Optimization of FFF process parameters to improve the tensile strength and impact energy of polylactic acid/carbon nanotube composite. Polym Eng Sci 2024; 64: 5047–5060.

37.

Hardani

Afshari

Samadi

, et al. An enhancement in the tensile modulus and bending resistance of polylactic acid/carbon nanotube composite by optimizing FFF process parameters. J Thermoplast Compos Mater 2024; 38(0): 1379–1403.

38.

Chen

Zheng

, et al. An improvement in the mechanical properties of polypropylene/ethylene-propylene-diene monomer/titanium dioxide nanocomposite obtained by fused filament fabrication. J Thermoplast Compos Mater 2025; 38(1): 115–139.

39.

Benfriha

Ahmadifar

Shirinbayan

, et al. Effect of process parameters on thermal and mechanical properties of polymer‐based composites using fused filament fabrication. Polym Compos 2021; 42(11): 6025–6037.

40.

Vanaei

Shirinbayan

Deligant

, et al. Influence of process parameters on thermal and mechanical properties of polylactic acid fabricated by fused filament fabrication. Polym Eng Sci 2020; 60(8): 1822–1831.

41.

Khan

Tariq

Abas

, et al. Parametric investigation and optimisation of mechanical properties of thick tri-material based composite of PLA-PETG-ABS 3D-printed using fused filament fabrication. Composites Part C 2023; 12: 100392.

42.

Martín

Auñón

Martín

. Influence of infill pattern on mechanical behavior of polymeric and composites specimens manufactured using fused filament fabrication technology. Polymers 2021; 13(17): 2934.

43.

Mustafa

Aloyaydi

Subbarayan

, et al. Mechanical properties enhancement in composite material structures of poly‐lactic acid/epoxy/milled glass fibers prepared by fused filament fabrication and solution casting. Polym Compos 2021; 42(12): 6847–6866.

44.

Katalagarianakis

Van de Voorde

Pien

, et al. The effect of carbon fiber content on physico-mechanical properties of recycled poly (ethylene terephthalate) composites additively manufactured with fused filament fabrication. Addit Manuf 2022; 60: 103246.

45.

Pei

Wang

Chen

C-B

, et al. Process-structure-property analysis of short carbon fiber reinforced polymer composite via fused filament fabrication. J Manuf Process 2021; 64: 544–556.

46.

Dul

Fambri

Pegoretti

. High-performance polyamide/carbon fiber composites for fused filament fabrication: mechanical and functional performances. J Mater Eng Perform 2021; 30: 5066–5085.

47.

Caminero

MÁ

Romero Gutiérrez

Chacón

, et al. Effects of fused filament fabrication parameters on the manufacturing of 316L stainless-steel components: geometric and mechanical properties. Rapid Prototyp J 2022; 28(10): 2004–2026.

48.

Luke

Soares

Marshall

, et al. Effect of fiber content and fiber orientation on mechanical behavior of fused filament fabricated continuous-glass-fiber-reinforced nylon. Rapid Prototyp J 2021; 27(7): 1346–1354.

49.

Hasanzadeh

Mihankhah

Azdast

, et al. Process‐property relationship in polylactic acid composites reinforced by iron microparticles and 3D printed by fused filament fabrication. Polym Eng Sci 2024; 64(1): 399–411.

50.

Kashi

Gupta

Baum

, et al. Phase transition and anomalous rheological behaviour of polylactide/graphene nanocomposites. Compos B Eng 2018; 135: 25–34.

51.

Mukherjee

Kao

Gupta

, et al. Evaluating the state of dispersion on cellulosic biopolymer by rheology. J Appl Polym Sci 2016; 133(12): app.43200.

52.

Ivanova

RADOST

Kotsilkova

RUMIANA

. Influence of graphene nanoplates and multiwall carbon nanotubes on rheology, structure, and properties relationship of poly (lactic acid). J Theor Appl Mech 2021; 51: 317–334.

53.

Hikmat

Rostam

Ahmed

. Investigation of tensile property-based Taguchi method of PLA parts fabricated by FDM 3D printing technology. Results Eng 2021; 11: 100264.

54.

Naveed

. Investigating the material properties and microstructural changes of fused filament fabricated PLA and tough-PLA parts. Polymers 2021; 13: 1487.

55.

Zakaria

Khan

Fee

MFC

, et al. Printing temperature, printing speed and raster angle variation effect in fused filament fabrication. IOP Conf Ser Mater Sci Eng 2019; 670: 012066.

56.

Bakhtiari

Nikzad

Tolouei-Rad

. Influence of three-dimensional printing parameters on compressive properties and surface smoothness of polylactic acid specimens. Polymers 2023; 15: 3827.

57.

Bayraktar

Uzun

Cakiroglu

, et al. Experimental study on the 3D-printed plastic parts and predicting the mechanical properties using artificial neural networks. Polym Adv Technol 2017; 28: 1044–1051.

58.

Ayaz

Daneshpayeh

Noroozi

. Enhancing the impact and flexural strength of PP/LLDPE/TiO2/SEBS nano-composites by using Taguchi methodology. Compos Sci Technol 2016; 129: 61–69.

Studying the influence of fused deposition modeling parameters on the yield and impact strength of PLA/graphene/clay composite using Taguchi method

Abstract

Keywords

Introduction

Materials and methods

Materials

Fused deposition modeling (FDM)

Taguchi design

Thermal tests

Mechanical tests

Microstructure observation

Rheological tests

Results and discussion

Analysis of thermal properties

Microstructure analysis

Rheological measurement results

Analysis of Taguchi design

Grey relational analysis

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

Data Availability Statement

References