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Abstract

The automotive industry increasingly demands lightweight, strong materials capable of large-scale production. Therefore, the objective of this work is to develop polypropylene (PP) composites reinforced with short carbon fibers (SCF) and graphene nanoplatelets (GnP) that can be processed by injection molding. This study evaluates the effect of incorporating this hybrid reinforcement on the performance of PP composites. The response surface methodology (RSM) was used to model the mechanical properties as a function of the composition of the composites obtained. A co-rotating twin-screw extruder was used to prepare the set of binaries (PP/SCF and PP/GnP) and hybrid (PP/GnP/SCF) composites, followed by the injection molding process. Melt flow index (MFI) tests were performed to obtain preliminary information of the flow behavior of the composites. Conventional characterization techniques were used to determine the properties of the composites, such as thermogravimetrical analysis (TGA), flexural and impact strength tests. The GnP were characterized using Raman spectroscopy, the Brunauer-Emmett-Teller (BET) method, field emission gun-scanning electron microscopy (FEG-SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The results suggest that the graphene used has a sparse number of stacked layers and defects, classifying it as a better-quality material. Both the GnP and SCF contributed to increasing the thermal stability and flexural properties of the composites produced. Only the PP/SCF composites showed superior impact properties than PP. The 75/5/20 PP/GnP/SCF hybrid composites showed the best flexural properties.

Keywords

Polypropylene composite short carbon fiber graphene nanoplatelets mechanical properties thermal properties automotive industry

Introduction

Short-fiber composites are increasingly used in applications for the automotive industry, which require lightness, while maintaining the structural strength and safety characteristics of vehicles. Mordor Intelligence¹ predicts a compound annual growth rate (CAGR) of 4.4% between 2023 and 2027 for the short-fiber composites market targeting this sector. These types of composites produced with thermoplastic matrices are remarkably interesting for mass production, due to their compatibility with conventional processing techniques, such as extrusion and injection molding, with great recycling possibilities.^2–5

PP is the most used thermoplastic in the automotive industry. In 2022, it was the most prominent polymer in the sector, accounting for more than 32% of market revenue.⁶ PP has low density, easy processing and excellent mechanical properties combined with low cost. However, due to the high demands of vehicle performance and safety, it is often necessary to optimize certain properties, such as viscosity, impact resistance and rigidity. For this purpose, additives and/or reinforcing fillers, such as carbon fiber (CF), glass fiber (GF), talc, among other fillers must be added to the polymer.^7,8 The growing demand for PP composites reinforced with CF or GF has increased the consumption of this material.^9,10

The mechanical properties of short fiber composites are a function of several parameters, such as: properties of the constituent elements, the length and orientation of the fibers; dispersion and distribution of the fibers in the matrix, the processing conditions, among others.^11–13 The properties of the finished parts depend on the final length of these fibers. Höftberger¹⁴ observed that the compound step produces higher rate off fiber length reduction than injection molding process. The work by Fueta1¹⁵ shows that the length and average aspect ratio of the fiber determine the tensile strength values, while the SCF volumetric fraction significantly affects the modulus values.¹²

The interface between the matrix and the reinforcement phase plays a decisive role in the mechanical properties of composites. An adequate interfacial bond between the fibers and the matrix allows the transfer of load from the matrix to the reinforcement and presents the fibers from pull-out.^16–19 The interfacial adhesion of the CF to the PP matrix is extremely low, due to the presence of polar groups and low surface energy of the CF.^4,19 Therefore, fiber/matrix interface modification treatments become necessary. Commercial SCF have a sizing (coating) that improves interfacial adhesion with the polymer matrix and protects fiber surface. Surface treatment of fiber and/or use of compatibilizing agents have been proposed to reach this objective and optimize performance of composites.^4,11,20–26 However, the performance of the materials obtained does not fully meet the requirements of the industry, especially the automobile industry.¹¹

Hybrid composites are receiving a lot of attention, because the incorporation of nanoparticles not only strengthens the matrix but also improves the polymeric matrix-SCF surface interactions.^25–29 Published paper shows that the incorporation of nanofillers in PP/SCF composites leads to obtaining a satisfactory mechanical behavior. The reason for this is that the nanoparticles are distributed among the CF and increase the interfacial shear strength (ISS). Arao et al.²⁷ combined nanofillers (carbon nanotubes, alumina, and silica) with PP/SCF composites containing the compatibilizer, PP grafted with maleic anhydride (PP-g-MAH). The hybrid composites obtained showed higher strength and modulus.

Graphene, fullerene, and carbon nanotubes have shown interesting properties for use in nanocomposites. Graphene is an allotrope of carbon (sp² hybridization) with a two-dimension al structure and is defined as a single layer of graphite. Pure graphene, however, is not yet mass-produced, and consists of a few layers of graphene stacked together.^30,31 There is a growing interest in the use of GnP as a reinforcement element in polymeric nanocomposites.²⁹ Graphene has a high aspect ratio and surface area. When incorporated into polymeric matrices, graphene generates materials with excellent optical, electrical, thermal, and mechanical properties superior to conventional materials.^32–37 For these reasons, the standardization of graphene and reliable quality control for industrially produced graphene materials, has now become one of the most critical prerequisites for the growing graphene industry, where the fight against “fake graphene” in the market is one of the top priorities.³⁸

There is a significant demand for the development and implementation of reliable, simple, and low-cost analytical methods that provide information on the bulk properties of industrially produced graphene materials. Several analytical methods, such as Raman spectroscopy, surface area (Brunauer-Emmett-Teller [BET] method), field emission gun scanning electron microscopy (FEG-SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), particle size distribution (PSD), and pH titration, can meet these requirements. These methods can provide valuable information on the properties of graphene powders, characterizing their crystalline and graphitic structure, surface area, chemical composition, impurities, and functional groups.^39,40

There are studies published on PP/GnP composites,^41–43 involving or not the presence of compatibilizing agents. These studies show that, in general, the incorporation of GnP to PP promotes improved mechanical and thermal properties. There is a lack of published papers on PP/GnP/SCF hybrid composites. Junaedi⁴⁴ verified that the incorporation of 5 wt% of graphene to the PP/10 SCF composites promoted the improvement of the interfacial adhesion between PP and SCF and that the development of a hybrid composite PP/10SCF/2.5 GnP/2.5TiO2 resulted in increased flexural modulus. Ashori et al.¹⁸ show an improvement in the mechanical and thermo-mechanical properties of PP/SCF resulting from coating SCF with exfoliated GnP due to increased interfacial adhesion between fibers and PP.

This work presents the flexural and impact properties of PP/SCF and PP/GnP binary composites, as well as of PP/SCF/GnP hybrid composites. The composites properties were described as a function of the composition using the RSM. This work also presents the structural properties and chemical composition of the GnP used, such as number of layers, surface area and defects.

Materials and methods

The materials and methods used in this work are discussed below.

Materials: PP, SCF and GnP

PP homopolymer (PP H503) manufactured by Braskem (Brazil), was used as the polymeric matrix. Melt flow index (MFI) of PP is 3.0–3.8 g/10 min, at a temperature of 230°C and load of 2.16 kg, and density of 0.905 g/cm³. To increase the dispersion of the SCF and the GnP in the matrix, before the use, the PP pellets were subjected to a micronization process, resulting in particles with a maximum average size of l mm. SCF, Tenax®-A/J HT C804, manufactured by Toho Tenax American (USA) and supplied by Parabor (Brazil), were used as reinforcing element. These fibers have a nominal diameter of 6 mm, length/diameter ratio of 1000, and density of 0.450 g/cm³. GnP, supplied by Sigma Aldrich, with a surface area of 750 m²/g and volumetric density of 0.2–0.4 g/cm’, particle size less than 2 μm, were used as filler. Irganox 1010 manufactured by BASF, in a proportion 1.0 %w/w, was the antioxidant used.

Characterization of GnP

The techniques and methodologies used for characterizing the GnP are described below.

The Raman Spectrometer from the Laboratory of the National Institute of Metrology, Quality and Technology (INMETRO) was utilized in this study. Raman spectra were captured using a Witec Alpha 300 spectrometer with a 514.5 nm laser line, using a microscope with a 100x objective for all samples. The laser power was maintained at 0.5 mW.

N₂ adsorption–desorption isotherms of the prepared material were obtained at the boiling point of liquid nitrogen (77 K) using a St2 Nova 600 surface analyzer. BET method was conducted at the Laboratory of the National Institute of Metrology, Quality and Technology (INMETRO). The specific surface area was evaluated using the BET (Brunauer, Emmett and Teller) multipoint method.⁴⁵

The chemical and topographical characteristics of GnP were evaluated through Field emission gun-Scanning electron microscopy (FEG-SEM) using the JEOL JSM-7100FT FEG-SEM at the Nanofab Laboratory of the State University of Rio de Janeiro.

X-ray diffraction measurements, using the powder method, crucial analysis to characterize GnP were performed to identify phases in the samples, and detect potential contaminating phases introduced during powder production. For this purpose, a Bruker-AXS D8 Advance Eco diffractometer with Cu Kα radiation (40 kV/25 mA) was used at the Multiuser Laboratory of Technological Characterization (LMCT) of the Mineral Technology Center (CETEM). The experimental setup spanned an angular range of 5° to 80° (2θ), scanning at 0.01° per 92 s per step with a state-of-the-art (energy discriminating) LynxEye XE silicon drift position-sensitive linear detector.

FTIR spectroscopy analysis of the GnP samples was performed on a Perkin-Elmer Frontier FTIR spectrometer at the Biomaterials Laboratory of the Polytechnic Institute of the State University of Rio de Janeiro. An attenuated total reflectance (ATR) accessory, the Pike Miracle Single Reflection ATR from Technologies, was used for all analyses. The FTIR spectrum of the samples was obtained with a resolution of 4 cm⁻¹ and 256 scans, in the spectral range of 4000 to 700 cm⁻¹.

Composites preparation

A set of PP composites was formulated using RSM that consists of fitting a polynomial mathematical model to a response surface according to a statistical mixture design. The Minitab19 software (option-simplex lattice design for three components) was used for planning the formulation of the composites mixture and evaluating their mechanical behavior. The proportion (percent in mass) of each mixture components PP, SCF and GnP were the input variables (independent factors). The components of the mixtures were subjected to the following restrictions: 0.75 ≤ PP ≤1.00, SCF ≤0.2 and GnP ≤.05.

Different formulations of materials, Table 1, were processed using a Leistritz® twin-screw extruder (model ZSE 18 Maxx - 40D). A screw rotation speed of 500 rpm, a feed rate of 5 kg/h, a temperature profile of 200/210/190/190/190/190/200/220/220/230°C (feeding zone to die zone) and a feed dosage control set at 50% were used. After extrusion, the samples were granulated and oven dried at 80°C for 24 h. The specimens for the mechanical tests were obtained by injection molding in an Arburg model Allrounder 270S injection molding machine. The following experimental conditions were used: temperature profile: 160/175/185/195/205°C; switching volume - 3 cm³; injection pressure and speed - 1200 bar and 15 cm³/s respectively; mold cooling time - 30 s; discharge pressure and time - 600 bar and 2 s, respectively.

Table 1.

Formulation of materials defined by the surface response methodology.

Code	PP (wt%)	GnP (wt%)	SCF (wt%)
A	100	0	0
B	90	5	0
C	80	0	20
D	75	5	20
E	97.5	2.5	0
F	90	0	10
G	77.5	2.5	20
H	85	5	10
I	87.5	2.5	10
J	93.75	1.25	5
K	91.25	3.75	5
L	83.75	1.25	15
M	81.5	3.75	15

Characterization of composites

The techniques and methodologies used for characterizing the composites are described below:

Mechanical properties (flexural properties, impact strength)

Flexural and impact properties of the materials were evaluated. Bending tests were performed on a Shimadzu Brasil AG-X 100 kV Universal Testing Machine, in accordance with ASTM D790, using a standard three-point bending device. Test specimens measuring 127 mm long, 12.7 mm wide, and 3.2 mm thick were used. For each composition, a minimum of seven (7) test specimens were tested. Izod impact tests were performed with a 0.5 J pendulum on a CEAST 9050 impact tester (Instron Brasil) in accordance with ASTM D256. Test specimens measuring 63.5 mm long, 12.7 mm wide, and 3.2 mm thick were used. The injection-molded specimens were notched at 25°C using a CEAST drive milling cutter.

Melt flow index (MFI) and thermal properties (TGA)

Determination of melt flow index (MFI) was carried out to obtain information about the flow behavior of the composites. Tests were performed using the extrusion plastometer, melt flow index tester CEAST 7021(Instron Brazil). MFI measurements of the samples in pellet form were performed at 230°C, under a load of 2.16 kg and with a cutting time of 60 seconds, in accordance with ASTM D1238. The melt flow index (MFI) value was calculated using the arithmetic means of the results of 5 test specimens for each sample.

Thermogravimetric analysis was used to analyze the effect of GnP and/or SCF in corporation on the thermal stability of PP and PP composite. These analyses were performed on a STA 600 Multiple Analyzer Instrument (Perkin-Elmer). The TGA analysis consisted of heating the sample in pellet form from room temperature to 600°C at a rate of 10°C/min under a nitrogen atmosphere. TGA analyses were performed at the Materials Laboratory of the State University of Rio de Janeiro.

Results and discussion

Characterization of GnP

Structural features and surface area of the GnP obtained by Raman scattering spectrum and BET method

Figure 1 shows the first and second order Raman scattering spectrum of GnP. An intense D peak can be observed at approximately 1325 cm⁻¹, indicating that there are many defects in the crystalline structure of the GnP resulting from functional groups associated with oxidation processes, edge effects, vacancies, among others. The presence of a D band indicates disorder in the crystalline structure and does not occur in perfect crystals. The 2D band occurs at approximately 2675 cm⁻¹ and is related to the layers present in GnP, indicating its nature. According to the literature, the absence of symmetry in the 2D peak suggests the presence of multilayer flakes.⁴⁶

Figure 1.

Raman spectrum of GnP.

The maximum intensity of the G peak occurs at approximately 1563 cm⁻¹. The G peak is characteristic of ordered graphitic structures and, in the literature, a D-band to G-band intensity ratio (I_D/I_G) close to zero is indicative of crystalline graphite. As this value increases, the material loses its crystallinity and becomes structurally more disordered. The near-zero I_D/I_G ratio can also be attributed to the reduction in the average size of sp² domains after the reduction of graphene oxide (GO), due to the formation of several new graphite domains in the reduced graphene oxide (rGO). These new domains, although smaller in size than those found in GO, before reduction, are more numerous.^47,48

The I_D/I_G ratio of 0.96 obtained from the Raman spectrum of GnP used in this work, a value higher than that attributed to crystalline graphite, indicates the presence of more defects and the occurrence of chemical oxidation of graphite in GO. On the other hand, the I_2D/I_G ratio of 0.32 obtained suggests the presence of more than two graphene layers. This result indicates that the GnP sample has a reduced number of graphene layers, and it is plausible to infer that GnP can be classified as a better quality few-layer graphene (FLG).^47–49

The analysis of the Raman spectrum of GnP showed a greater predominance of the D and G bands, due to the presence of oxygenated groups in the crystal lattice.^50,51 Similar results were found by CHEN et al. (2010) and LIU et al. (2013) for graphene oxide, where a greater intensity of the G band was observed, suggesting that the material analyzed is graphene oxide.^52,53

The surface area of GnP is considered larger than that of other carbon-based nanomaterials, which contributes to maximizing the stress transfer from the polymer matrices to the filler. The specific surface area of the GnP used in this work and obtained by the multipoint BET method was 687 m²g⁻¹. This value was lower than that provided by the supplier, which was 750 m²g⁻¹. However, there is no information on the method used for this determination. In any case, the value obtained is within the expected range for this parameter.

Field-emission gun scanning electron microscopy (FEG-SEM) images of GnP

Flexibility is a striking feature of graphene.⁵¹ Figure 2 shows the micrographs of GnP, obtained by FEG-SEM. The folds along the graphene sheets indicate the flexibility of this material. The morphological profile indicates pronounced wrinkling due to the incorporation of sp³ carbon atoms, which produces the breakage of the sp² hybridized flat graphene nanosheets.^54,55

Figure 2.

FEG-SEM of GnP.

Analysis of X-ray diffraction (XRD) Spectrum of GnP

Figure 3 shows the XRD spectrum of GnP. The peak at the 2θ angle of 26.30° is an indication of the (002) crystalline plane. In addition, a presence of a second peak at the 2θ angle of 43.50°, corresponding to the (101) plane, is attributed to the specific orientation of the graphene layers.⁵⁶ In fact, the diffraction pattern of graphene does not present well-defined peaks, but rather bands, due to the spacing in the crystal lattice of graphene oxide that is not well defined, like that of highly ordered graphite. In addition, the XRD spectrum was also used to calculate the number of graphene layers, indicating that the sample consists of graphene with nine to 10 layers, i.e., few-layer graphene (FLG). This result is in good agreement with those obtained from the Raman spectra of GO.^57,58

Figure 3.

X-ray diffraction spectra of GnP.

Analysis of the fourier transform infrared spectrum (FTIR) of GnP

Figure 4 depicts the FTIR spectrum of GnP. The following absorption bands, corresponding to the oxygenated groups derived from the graphite oxidation process were observed: 3416 cm⁻¹ assigned to (O-H) hydroxyl group stretching vibration; 1647 cm⁻¹ assigned to (C = O) carbonyl group and (C = O) double carboxylic bond (C = O) stretching vibrations; 1530 cm⁻¹ assigned to carbon aromatic (C = C) vibrations; 1380 cm⁻¹ assigned to (OH) carboxyl group stretching vibrations; 1023 cm⁻¹ attributed to alkoxy groups vibrations.^38,47,59 Therefore, there is evidence that the acquired nanoplatelets have functional groups.

Figure 4.

Infrared absorption spectra of GnP.

Composites characterization

Melt flow index determination

Figure 5 shows the melt flow index (MFI) of the materials obtained. PP presents an MFI value of 3.4 g/10 min. This value matches that indicated by the manufacturer, considering the error.

Figure 5.

MFI of the PP and PP composites.

All composites showed lower MFI than PP. Considering that the viscosity is inversely proportional to MFI, these MFI data indicate that the effect of adding GnP or SCF in a PP matrix promotes an increase in the viscosity of the composites. The reduction in MFI values, in relation to PP, was 35% and 16% presented by the (95/5)wt% PP/GnP and (97.5/2.5)wt% PP/GnP composites, respectively. This strong effect of GnP on PP melt flow index reduction is not suitable for the injection molding process and restricts the mix design region of the experiments. On the other hand, the increase in the viscosity of composites can hinder the graphene dispersion in the matrix, negatively affecting the mechanical performance of composites.³² Other studies also show the decrease in MFI of polymeric matrices with the incorporation of micro or nano sized - fillers.^44,60,61 Junaedi et al.⁴⁴ obtained results like those presented in this work. The maximum reduction in MFI was presented by the composites containing 5 wt% of GnP.

Thermal stability of PP and PP composites

Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (dTG) of PP and composites were carried out to determine the effects of adding SCF and GnP on the thermal stability of PP. Figure 6 shows the TGA curves for all materials and Figure 7 shows the respective dTG curves. Table 2 shows the thermal stability parameters obtained from TGA and dTG curves of these materials.

Figure 6.

TGA thermograms of PP and PP composites.

Figure 7.

dTG curves of PP and PP composites.

Table 2.

Parameters obtained from TGA curves respective to PP and PP composites.

Sample	^T_onset (^oC)	T_max (°C)	^T_end (^oC)	Residual mass (%)
100PP	395	450	479	0
95/5PP/GnP	439	459	484	4.7
80/20 PP/SCF	428	453	494	19.9
75/5/20PP/GnP/SCF	445	464	492	26.8
97.5/2.5/ PP/GnP	440	457	500	2.7
90/10 PP/SCF	417	446	485	10.1
77.5/2.5/20PP/GnP/SCF	424	459	490	15.8
85/5/10PP/GnP/SCF	450	464	489	20.6
87.5/2.5/10PP/GnP/SCF	440	461	487	12.9
93.75/1.25/5PP/GnP/SCF	438	460	487	5.6
91.25/3.75/5PP/GnP/SCF	439	458	492	9.1
83.75/1.25/15PP/GnP/SCF	440	460	494	16
81.75/3.75/15PP/GnP/SCF	439	459	493	18.8

Thermal degradation of PP results from a random scission process promoted by free radicals.⁴ PP degradation starts at 395°C and ends completely at 479°C. These temperature values are close to those in the literature, which indicate that PP range degradation is between 300°C–475°C⁴. In general, the incorporation of GnP in polymeric matrices increases their thermal stability.⁶² According to Jun et a1.,⁶³ nanoparticles act as a barrier to degradation, promoting the removal of free radicals. GnP also contribute to thermal conduction and to the uniform distribution of heat scattered in the matrix, promoting an increase in the thermal stability of the polymer. Qiu et a1.⁶⁴ and Zhao et a1.⁶⁵ also reported a high increase in the thermal stability of PP with addition of small concentration of graphene (0.1%–2%). On the other hand, the incorporation of CF to the PP matrix increases the degradation temperature of the composites due to the greater heat absorption capacity of the CF.⁶⁶

The degradation range of the PP/GnP composites developed in this work is 438°C–500°C, while that of PP/SCF composites is 416°C–493°C. Ternary composites PP/GnP/SCF also showed greater thermal stability than PP. The increased thermal stability of the PP composites is also demonstrated by the T_max values, which are higher than that of PP.

The hybrid composites (85/5/10) PP/GnP/SCF and (75/5/20) PP/GnP/SCF were the samples that presented the greatest thermal stability with T_onset at 450°C and 445°C, respectively; both materials presented T_max at 464°C. The residual masses are consistent with the formulated filler composition for each sample.

Impact strength

According to Ashori et a1.,¹⁸ several factors such as deformation of matrix and fiber, fracture of matrix and fiber, fiber pull out and fiber-matrix adhesion affect the impact properties of short fibers composites. Interfacial adhesion cannot be too low, promoting crack propagation towards a poor interface, nortoohigh, restricting themobility of polymeric chains and making the composite brittle. These authors attributed the increase in impact resistance caused by the incorporation of SCF to the PP matrix to the inhibitory effect on crack propagation that these fibers exert during fracture. At the time of impact, some amount of crack propagation energy is used to drive the SCF out of the matrix and this loss of energy results in an increase in impact resistance. Rezaei et a1.⁶⁶ observed an increase in impact strength with an increase in the content of SCF incorporated in to the PP matrix. However, this increase was smaller as the composite was processed with shorter fibers. Similar results were obtained in this work.

Figures 8 and 9 show the impact strength of PP and PP composites. The impact strength of PP is 29.83 ± 1.35 J/m. The PP/SCF (90/10) and PP/SCF (80/20) composites were the only samples that showed higher impact strength than PP, 32.43 ± 1.26 J/m and 48.74 ± 2.24 J/m, respectively. These results show that SCF play a significant role in the fracture energy.

Figure 8.

Impact strength of PP and PP composites in the mixture design.

Figure 9.

Impact strength of PP and PP composites.

Patra et a1.⁶⁷ observed an increase in impact resistance with the incorporation of GnP into a PP matrix. This increase was greater when larger-sized GnP were used. The increase in impact strength was attributed to a reduction in crack propagation promoted by GnP. Al-Saleh et a1.⁴² observed a decrease in the impact strength with the incorporation of graphene to PP in the absence of a compatibilizer. This result was attributed to the incompatibility between the nanoparticles and the PP matrix.⁶⁸

In this present work, the incorporation of GnP had a deleterious effect on the impact strength of PP and PP/SCF composites. The hybrid composites (77.5/2.5/20) PP/GnP/SCF and (83.75/1.25/15) PP/GnP/SCF present impact properties similar to that presented by PP. The other hybrid composites showed lower impact resistance than PP. An evaluation of the fracture surface morphology must be performed to explain the results obtained.

Equation (1) shows the relationship between the impact resistance [IMPACT] and the components proportion of the PP/GnP/SCF composites obtained using a quadratic model with 95% of confidence. The coefficient of determination was 0.91.

I M P A C T = - 29.99 [PP] + 456.78 [SCF] + 6682.91 [GnP] - 432.40 [PP] [SCF] - 7068.75 [PP] [GnP] - 9166.32 [SCF] [GnP]

(1)

Equation (1) shows that in higher graphene concentration, the impact resistance increases and at the same content, the effect of graphene on the impact properties is higher than that of SCF. The interactions between PP and GnP and between SCF and GnP are not good and contribute to decrease the impact resistance. Further investigation is necessary to evaluate the results.

Flexural properties

The incorporation of GnP or SCF to PP should promote an increase in the modulus of elasticity of the composites produced, since these two types of reinforcement elements have a modulus of elasticity higher than that of PP. The fiber length and the fiber content incorporated in to the PP matrix influence the mechanical properties of PP/SCF composites.

Rezaei et al.⁶⁶ observed that the flexural strength and flexural modulus of PP/SCF composites increased with increasing weight fraction and fiber length. Junaedi et al.¹⁸ observed that the incorporation of GnP or SCF improves the PP flexural properties. At the same content of filler incorporated in to the matrix 5 wt%, the effect of graphene in the flexural properties was superior to that of short fibers. The PP/GnP/SCF hybrid composites also present superior flexural properties. Figures 10 and 11 show the flexural modulus of PP and PP composites.

Figure 10.

Flexural modulus of PP and PP composites in the mixture design.

Figure 11.

Flexural modulus of PP and PP composites.

Figure 10 shows the flexural modulus of PP and PP composites as a function of the components of the system. The results show that the incorporation of GnP exerts a greater influence on the modulus of PP than these of SCF. The data obtained, however, show that in the experimental region evaluated the composites with higher concentration of SCF presents superior modulus. PP shows a flexural modulus of 1560 ± 74 MPa. The sample that shows the higher modulus value was the (75/5/20) PP/GnP/SCF, respectively, 6380 ± 98 MPa.

Equation (2) shows the relationship between the flexural modulus [FLEXMOD] and the components proportion of the PP/GnP/SCF composites obtained using a quadratic model with 95% confidence. The coefficient of determination was 0.98.

F L E X M O D = - 1557.1 [PP] + 19483.3 [SCF] + 40789.1 [GnP] - 909.8 [PP] [SCF] - 29461.5 [PP] [GnP] + 61375.3 [SCF] [GnP]

(2)

The surface response for the flexural strength, Figure 12, shows that the incorporation of CF has a more pronounced effect on the flexural strength. Figure 13 shows that all composites have a higher flexural resistance than polypropylene. PP shows a flexural strength of 53.5 ± 0.4 MPa. The composite (75/5/20) wt% PP/GnP/SCF was the material that showed the higher flexural strength, respectively, 80.1 ± 0.8 MPa.

Figure 12.

Flexural strength of PP and PP composites in the mixture design.

Figure 13.

Flexural strength of PP and PP composites.

Equation (3) shows the relationship between the flexural strength [FLEXSTRENGTH] and the components proportion of the PP/GnP/SCF composites obtained using a quadratic model with 95% confidence. The coefficient of determination was 0.97.

F L E X S T R E N G T H = 54.37 [PP] - 12.57 [SCF] - 1530.99 [GnP] + 197.56 [PP] [SCF] + 1856.4 [PP] [GnP] + 1822.97 [SCF] [GnP]

(3)

Conclusion

In this study, the effect of incorporating GnP and SCF into a PP matrix on the mechanical and thermal stability of the polymer was evaluated using a RSM, and the following outcomes are arrived.

(1) The incorporation of GnP and SCF into PP increased the thermal stability of PP. The PP/GnP/SCF ternary composites also exhibited greater thermal stability than PP.

(2) With the same filler content incorporated in the matrix (5% by weight), the effect of graphene on flexural properties was greater than that of SCF. The PP/GnP/SCF hybrid composites also exhibited superior flexural properties.

(3) The results show that the incorporation of GnP exerts a greater influence on the modulus of elasticity of PP than the incorporation of SCF. The data obtained, however, show that, in the experimental region evaluated, the composites with a higher SCF concentration exhibited a higher modulus of elasticity.

(4) The surface response for the flexural strength shows that the incorporation of SCF has a more pronounced effect on the flexural strength and all composites have higher flexural resistance than PP.

(5) Superior impact properties were obtained when the polymer was reinforced with SCF. The results show that the SCF play a significantrole in fracture energy.

(6) From this study, it can be concluded that flexural properties and thermal stability of PP were improved by incorporating both reinforcement elements.

(7) The characterization results of GnP suggest that the graphene used has a sparse number of stacked layers and defects, which classifies it as a better-quality material.

Footnotes

Acknowledgements

The authors would like to thank the funding agencies FAPERJ and CNPq, and the Research Support Program of the State University of Rio de Janeiro (PAPD Pesquisa), for their financial support for this work.

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 study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) – Finance Code 001. The authors would like to thank the Laboratory of the National Institute of Metrology, Quality and Technology (INMETRO) for Raman spectroscopy and BET method tests, the Biomaterials Laboratory of the Polytechnic Institute of the Rio de Janeiro State University for Fourier transform infrared spectroscopy (FTIR) test support, the materials Laboratory of the Rio de Janeiro State University for thermal test, the Multiuser Laboratory of Nanofabrication and Characterization of Nanomaterials (NANOFAB) for assistance in scanning electron spectroscopy (FEM-SEM) tests, the Multiuser Laboratory of Technological Characterization (LMCT) of the Mineral Technology Center (CETEM) for assistance in X-ray diffraction (XRD) tests. DSc. Marisa Cristina Guimarães Rocha would like to thank CNPq for the financial support from process n^o 401982/2023-8. DSc. Ana Lúcia Nazareth da Silva would like to thank FAPERJ for the financial support from process n^o SEI-260003/015793/2021 - APQ1.

ORCID iDs

Marisa Cristina Guimarães Rocha

Valéria Dutra Ramos

Nancy Isabel Alvarez Acevedo

Helson Moreira da Costa

Ana Lúcia Nazareth da Silva

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Mechanical and thermal properties of carbon fibers and graphene nanoplatelets reinforced polypropylene hybrid nanocomposites

Abstract

Keywords

Introduction

Materials and methods

Materials: PP, SCF and GnP

Characterization of GnP

Composites preparation

Characterization of composites

Mechanical properties (flexural properties, impact strength)

Melt flow index (MFI) and thermal properties (TGA)

Results and discussion

Characterization of GnP

Structural features and surface area of the GnP obtained by Raman scattering spectrum and BET method

Field-emission gun scanning electron microscopy (FEG-SEM) images of GnP

Analysis of X-ray diffraction (XRD) Spectrum of GnP

Analysis of the fourier transform infrared spectrum (FTIR) of GnP

Composites characterization

Melt flow index determination

Thermal stability of PP and PP composites

Impact strength

Flexural properties

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iDs

References