Manufacturing of Recycled Carbon Fiber Reinforced Polypropylene Composites by High Speed ThermoKinetic Mixing for Lightweight Applications
Lutfiye Altay,1 Metehan Atagur,2 Orhan Akyuz,2 Yoldas Seki,3 Ibrahim Sen,4 Mehmet Sarikanat Kutlay Sever2 1 Mechanical Engineering Department, Ege University, Bornova, Izmir, Turkey
,1
_ Faculty of Engineering and Architecture, Izmir Katip Celebi University, C¸igli, Izmir, Turkey
2
3
Faculty of Science, Dokuz Eylul University, Buca, Izmir, Turkey
4
Izmir Makine Sanayi A.S¸, Turgutlu, Manisa, Turkey
Recycled carbon fiber reinforced polypropylene (PP) composites were fabricated by using high speed thermo-kinetic mixer. The effect of recycled carbon fiber on the morphological, chemical, physical, thermal, and mechanical properties of PP composites was investigated. The results indicate that the tensile and flexural strength of PP increase with addition of recycled carbon fiber up to 20 wt%. When 40 wt% recycled carbon fiber was added into PP, the tensile and flexural moduli were increased by about 177 and 359%, respectively. The coefficient of thermal expansion was found to be decreased by increasing recycled carbon fiber weight fraction. The addition of low weight fraction of recycled carbon fibers (up to 5 wt%) increased the nucleation process on PP crystallization. C 2017 Society of PlasPOLYM. COMPOS., 00:000–000, 2017. V tics Engineers
INTRODUCTION Thermoplastic based composites offer various advantages such as low density, high toughness, relative ease of fabrication, and recyclability. Polypropylene (PP) is one of the commodity polymers produced in large quantities due to its versatile applications in different fields [1]. PP based composites have been already used in many industrial applications, mainly in the automobile industry, in the manufacture of components of covering panels, lateral doors of trucks, as well as in the other internal covering parts of automobiles [2]. Glass fiber, natural Correspondence to: M. Sarikanat; e-mail:
[email protected] DOI 10.1002/pc.24394 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2017 Society of Plastics Engineers V
POLYMER COMPOSITES—2017
fiber, carbon fiber, carbon black, graphite, and talc are commonly used as fillers/reinforcements in PP based composites [3–9]. Carbon fiber is one of the most widely used reinforcing fibers for polymeric composite materials owing to its specific strength, specific modulus, and thermo-physical properties [10]. Carbon fiber reinforced polymer (CFRP) composites exhibit desirable properties including lightweight, high strength, high modulus, high electrical conductivity, high thermal conductivity, dimensional stability, and high fatigue resistance. They have been widely used in aerospace industry, sporting goods, performance automobiles, civil engineering, etc [11]. Over the last decade, disposal of the waste related to carbon fiber products has become a primary concern since CFRP composite usage has grown rapidly. The polymeric composites placed in a landfill are not biodegradable and take hundreds of years to decompose [11]. Current EU Directive on Landfill of Waste (99/31/EC) classifies CFRP as chemical waste, which has led to their high disposal costs [10, 11]. In addition, high energy requirements for its manufacturing result in high cost of virgin carbon fibers. Recycled carbon fiber, with its much lower price, could open up new markets and new opportunities in different application areas in aerospace, automotive, and sport industries where mechanical performance and weight savings are important [11]. Therefore, recycling and recovery of CFRP wastes has drawn attention for potential economic benefits. Recycled carbon fiber has been investigated either as a filler or a reinforcement in thermoset based composites [12, 13], injection molded thermoplastic compounds [14, 15], and construction [16–18]. The high stiffness, high strength, and low density of recycled carbon fibers allows
them to be re-used in new engineering materials for use in various applications [19]. Stoeffler et al. [20] investigated the mechanical properties (tensile, flexural, and izod impact) of polyphenylene sulfide (PPS) composites reinforced with recycled carbon fibers at different weight fractions. The recycled carbon fiber reinforced PPS composites showed similar mechanical properties compared to composites reinforced with virgin carbon fibers. Wong et al. demonstrated the effect of maleic anhydride as a coupling agent on the properties of recycled carbon fiber reinforced PP composites. Addition of coupling agent improved fibermatrix adhesion and thus the overall mechanical properties [21]. In Jiang’s work, PP and polyamide 6 were reinforced with the recycled carbon fiber using twin screw co-rotating extruder and injection molding. For comparison, polymeric composites reinforced with virgin carbon fibers were also fabricated. Different reinforcing effects on two polymers were observed due to differences in surface roughness, surface bonding, and fiber aspect ratio between virgin and recycled carbon fibers [11]. A modified carding and wrap spinning process is used for producing recycled carbon fiber reinforced PP yarns. It was concluded that thermoplastic composites made from recycled carbon fibers have a good potential to be used as low cost materials due to exhibiting comparable mechanical properties as virgin carbon fiber reinforced thermoplastics [10]. The objective of this study is to examine the influence of recycled carbon fibers on the morphological, physical, mechanical, chemical, and thermal properties of recycled carbon fiber reinforced PP composites. In the study, recycled carbon fiber was used as a reinforcement material in PP composites. Recycled carbon fiber with various weight fractions was blended with PP using a high speed thermo-kinetic mixer. The high speed thermo-kinetic mixer is designed to disperse particles/chopped fibers within the polymer by heating, mixing, and compounding, in a period of a few minutes to minimize any thermal degradation of the material [22]. The novelty of this study is using the high speed kinetic mixer method to fabricate recycled carbon fiber reinforced PP composites. This process is easy of fabrication and fairly cost effective for fabrication of composites and has not been explored in literature in detailed. The thermal, mechanical, chemical, physical, and morphological characterization of recycled carbon fiber reinforced PP composites were performed. MATERIALS AND METHODS
Composite Manufacturing PP based composites with 5, 10, 15, 20, 25, 30, and 40 wt% of recycled carbon fiber (5 CF, 10 CF, 15 CF, 20 CF, 25 CF, 30 CF, and 40 CF) were prepared by using a high speed thermo-kinetic mixer (G€ulnar Makina-Turkey). The thermo-kinetic mixer was operated at a shaft speed of 2000 rpm until the material reached 1908C. A quantity of 70 g of material was fed into the thermo-kinetic mixer for each batch and resulting mixing time in the melt state was about 20 s. One of the main advantages of the thermo-kinetic mixer is its ability to obtain optimum dispersion of fibers in a short time [23]. In a thermo-kinetic mixer, blades on a high-speed shaft accelerate the chopped fibers/particles and impart them high kinetic energy, which is converted to thermal energy when they hit the chamber wall [22, 23]. Manufacturing of Composite Plates Composite plates (200 3 200 3 2 mm) were prepared by compression molding technique (G€ulnar Hot Press with controlled heating, cooling and pressure, Turkey). The composites were produced by pressing the blends between the hot plates of a compression press at 1708C for 4 min at 70–120 bar pressure. Density Measurement Density measurement of the specimens was done according to the ASTM D792 using Densimeter MD-200S. The density of each composite was obtained by calculating the average of the density of specimens. FTIR-ATR Analysis FTIR-ATR analyses of the recycled carbon fiber, PP, and recycled carbon fiber reinforced PP composites were carried out by using Bruker Alpha spectrometer. The analyses were performed in the range of 400–4000 cm21, with a resolution of 4 cm21. Thermogravimetric Analysis (TGA) Thermogravimetric analyses of the recycled carbon fiber, PP and recycled carbon fiber reinforced PP composites were conducted by using thermogravimetric analyzer (TA Instruments, TGAQ500) at a heating rate of 108C/ min in the range of 30–6008C under nitrogen atmosphere.
Materials The matrix material used in the study, which was supplied from Petkim-SOCAR–Turkey, is homo-PP with a density of 0.90 g/cm3 and a melt flow index of 5 g/10 min. PAN based chopped recycled carbon fiber, having a fiber diameter of 7.5 mm and length of 6 mm, were purchased from ELG Carbon Fiber. 2 POLYMER COMPOSITES—2017
X-Ray Diffraction (XRD) Analysis The crystallographic structures of the recycled carbon fiber, PP, and recycled carbon fiber reinforced PP composites were determined by X-ray diffractometer (Panalytical Empyrean) with a LynxEye detector. Specimens were scanned from 5 to 808. The scan step size and time per DOI 10.1002/pc
TABLE 1. Density of PP, recycled carbon fiber, and recycled carbon fiber reinforced PP composites. Sample Density (g/cm3)
PP
CF
5 CF
10 CF
15 CF
20 CF
25 CF
30 CF
40 CF
0.900
1.770
0.925
0.951
0.959
0.999
1.012
1.029
1.109
step used in this study were 0.0248 and 0.2 s, respectively. Differential Scanning Calorimeter (DSC) Analysis Thermal properties were studied by using a DSC (TA Instruments, DSCQ20). Specimens were heated from 508C to 2008C at a rate of 108C/min under a nitrogen atmosphere and held for 10 min to destroy any residual nuclei before cooling at 208C/min. The heat of fusion was determined by integration of the DSC endotherm recorded at 1608C.
performed using a Shimadzu Autograph AG-IS Series universal testing machine at room temperature. The tests were carried out at a constant crosshead speed of 2 mm/ min and span length of 32 mm. Izod Impact Testing. Notched and Unnotched Izod Impact testing was conducted using ISO 180 on Izod/Charpy Impact Tester. The specimens in 3 mm thickness were cut into 12 mm width and 62 mm length. 2.55 mm notch was made by CNC machine. Tests were operated at room temperature with 5 J stricker and span length of 70 mm. Scanning Electron Microscopy (SEM) Observations
Dynamic Mechanical Analysis (DMA) The storage modulus and loss modulus of PP and recycled carbon fiber reinforced composites were evaluated using a dynamic mechanical analyzer (TA Instruments, DMA Q800). Single cantilever was used and multi frequency-strain modulus mode was selected to analyze all specimens between the temperatures of 25 and 1308C in air atmosphere.
Gold was deposited on the surface of recycled carbon fiber reinforced PP composite specimens by using plasma sputtering apparatus. Then, morphology of recycled carbon fiber reinforced PP composites were investigated by using scanning electron microscope (Carl Zeiss 300 VP) operated at 3 kV. RESULTS AND DISCUSSION
Thermomechanical Analysis (TMA)
Density Measurement
The thermal expansion coefficients (CTE) of PP and recycled carbon fiber reinforced PP composites were determined by using a TMA (TA Instruments, TMA 400) for two regimes below and above the glass transition temperature (Tg). TMA was carried out at expansion mode. Specimens (10 3 5 3 3 mm) were heated from 240 to 1208C at a rate of 58C min21.
Density values of PP, recycled carbon fiber and recycled carbon fiber reinforced PP composites are given in Table 1. Results show that the density of the composites increased with increasing fiber weight ratio. The density of the recycled carbon fiber (1.77 g/cm3) is relatively higher than that of PP (0.900 g/cm3); thus, the incorporation of recycled carbon fiber into PP increases the density of the unreinforced PP. Therefore, recycled carbon fiber, being twice stronger and 30% lighter compared with glass fiber (2.55 g/cm3), has a great potential to be used in lightweight applications instead of traditional materials where weight reduction is required [24–26].
Mechanical Testing Tensile Testing. The mechanical properties of PP and recycled carbon fiber reinforced PP composite specimens were tested at room temperature using a Shimadzu Autograph AG-IS Series universal testing machine equipped with a video extensometer system (SHIMADZU Noncontact Video Extensometer DVE-101/201) at a crosshead speed of 50 mm/min according to ASTM standard D63810. The average values of seven tests for tensile strength, elongation at break, and Young’s modulus were reported for each specimen. Flexural Testing. Three-point bending tests were conducted to characterize the flexural properties of the PP and recycled carbon fiber reinforced PP composite plates by following the ASTM D790 standard. The tests were DOI 10.1002/pc
FTIR-ATR Analysis Figure 1 shows the FTIR spectra of 5 CF, 10 CF, 15 CF, 20 CF, 25 CF, 30 CF, and 40 CF. The peaks at 1452 and 1375 cm21 may be attributed to symmetrical bending vibration of CH2 and CH3 groups, respectively. These peaks have not changed after incorporation of recycled carbon fiber into PP. The broad band at 1244 cm21 corresponds to the bending vibrations of CH2 group of PP. These bands cannot be seen in the spectra of recycled carbon fiber reinforced PP samples containing higher weight fraction of recycled carbon fiber. In the spectrum of recycled carbon fiber reinforced PP involving the POLYMER COMPOSITES—2017 3
FIG. 1. FTIR spectra of (a) PP and recycled carbon fiber reinforced PP composites with various recycled carbon fiber contents and (b) recycled carbon fiber. [Color figure can be viewed at wileyonlinelibrary.com]
greatest weight fraction of recycled carbon fiber, the intensity of all peaks of PP decreased remarkably. X-Ray Diffraction Analysis The effect of recycled carbon fiber weight fraction on crystallographic properties of composites was studied by
FIG. 3. TGA thermograms of PP, recycled carbon fiber, and recycled carbon fiber reinforced PP composites with various recycled carbon fiber weight fractions. [Color figure can be viewed at wileyonlinelibrary.com]
XRD analysis. Figure 2 shows XRD patterns of the PP, recycled carbon fiber, and recycled carbon fiber reinforced PP composites. PP was characterized by five main peaks at about 14.48, 15.98, 16.98, 18.558, and 21.258, corresponding to the (110), (010), (040), (130), and (041) planes, respectively. The diffraction pattern of recycled carbon fiber shows a broad diffraction peak near 2h 5 258, corresponding to the (002) reflection of a carbon structure of fiber. The broad diffraction peak (2h 5 15–308) of recycled carbon fiber can be attributed to the amorphous carbon structures [27]. As recycled carbon fiber content increases in PP, (002) reflection becomes stronger demonstrating higher content of recycled carbon fiber in PP compared with the unreinforced PP. Thermogravimetric Analysis
FIG. 2. XRD patterns of of PP, recycled carbon fiber and recycled carbon fiber reinforced PP composites with various recycled carbon fiber weight fractions.
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Thermogravimetric analyses of PP, recycled carbon fiber, and recycled carbon fiber reinforced PP composites with different weight fractions (ranging from 5 wt% up to 40 wt% recycled carbon fiber) are shown in Fig. 3. TGA results were summarized in Table 2. Decomposition temperature is an important parameter to characterize the thermal stability of polymeric materials. Onset degradation temperature, maximum degradation temperature, and temperature of the end of degradation can be determined from TGA curves. Onset degradation temperature is defined as the temperature when the weight loss reaches up to 5% of the total weight of the sample [28]. The TGA results show that the PP undergoes thermal degradation beginning at 420.348C and with a total mass loss of 99.87%. There is a small amount of inert residue remaining (0.13%). It can be seen from the Table 2 that onset degradation temperature of the recycled carbon fiber reinforced PP composites increases roughly with increasing recycled carbon fiber weight fractions compared with that of PP. DOI 10.1002/pc
TABLE 2. TGA data for PP, recycled carbon fiber, and recycled carbon fiber reinforced PP composites. Sample
Onset temperature (8C)
Max degradation temperature (8C)
End temperature (8C)
Degraded weight (%)
PP CF 5 CF 10 CF 15 CF 20 CF 25 CF 30 CF 40 CF
420.34 329.08 435.95 435.73 436.64 438.17 437.34 440.31 440.46
452.52 383.26 457.85 458.36 459.04 460.55 460.94 461.12 462.28
465.64 413.64 469.34 469.56 470.62 471.08 471.16 472.69 474.71
99.87 1.98 97.24 91.76 86.34 78.40 77.03 71.22 58.86
Maximum value of the mass loss rate occurs at the maximum degradation temperature, which can be obtained from derivative of TGA curve. Table 2 shows the variation of maximum degradation temperature of PP and recycled carbon fiber reinforced PP composites. Similarly, the values of the maximum degradation temperature increase with increasing recycled carbon fiber weight fraction. Maximum degradation temperatures for PP, 5 CF, 10 CF, 15 CF, 20 CF, 25 CF, 30 CF, and 40 CF were obtained to be 452.52, 457.85, 458.36, 459.04, 460.55, 460.94, 461.12, and 462.288C, respectively. TGA results for recycled carbon fiber in nitrogen atmosphere show that weight loss is 1.98% up to 465.648C. Weight loss is 58.86% up to 474.718C for 40 CF. It can be noted that as the weight fraction of recycled carbon fiber in PP was increased weight loss at maximum degradation temperature decreased. When decomposition is completed, corresponding temperature is defined as the end degradation temperature. In general, increase in degradation temperature is due to the fact that heat absorption capacity of carbon fiber is higher than that of PP [29]. These results demonstrate that recycled carbon fiber reinforcement significantly improves the thermal properties and thermal stability of the PP. Differential Scanning Calorimetry The effect of recycled carbon fiber on the crystallization of PP are analyzed by nonisothermal DSC experiments. Crystallization temperature (Tc), melting temperature (Tm), melting enthalpy (DH), and crystallinity (Xc) of PP and recycled carbon fiber reinforced PP composites are given in Table 3. All Tc values of the recycled carbon fiber reinforced PP composites are higher than that of PP. For 5 CF, Tc are increased by about 7.4% with respect to that of PP. However, there is no significant increase in Tc when the recycled carbon fiber weight fraction increases up to 40%. The incorporation of recycled carbon fiber into PP is observed to have little effect on the melting temperature of PP. When 5 wt% recycled carbon fiber is added, crystallinity increases significantly. However, further increase in recycled carbon fiber weight fraction causes a decrease in the overall rate of crystallization. Recycled carbon fiber can act as effective nucleating agents by increasing the rate of nucleation and narrowing the DOI 10.1002/pc
crystallite size distribution [30]. For 5 CF, the nucleating effect is significant. However, Xc is found to decrease with the further increase of the fiber weight fraction. When the weight fraction of the recycled carbon fiber is low (5 CF), crystallization rate and degree of crystallinity of the composites increased because of the mobility of the PP chains. When recycled carbon fiber loading is increased, the fillers act as the restriction sites to prevent the mobilization of the PP macromolecular chains from obtaining ordered crystal lattice alignment resulting in decreased degree of crystallinity [31]. The melting enthalpy is increased by adding small amount of recycled carbon fiber. With 5 wt% recycled carbon fibers, the melting enthalpy is increased by 47%. The melting enthalpy change is an indicator of the variation in the degree of crystalinity. The degree of crystalinity of composites was calculated from the crystallization enthalpy according to Eq. 1. Xc 5
DH 3100 /p 3DH0
(1)
where /P is weight fraction of PP in the composite and DH0 is the melting enthalpy of the 100% crystalline homo-PP, which is reported to be 209 J g21 [32]. The observed crystallization behavior shows that the addition of low weight fraction of carbon fibers (up to 5 wt%) increased the nucleation process on PP crystallization and higher recycled carbon fiber weight fraction has adverse effect on the crystallization of PP.
TABLE 3. Thermal parameters of PP and recycled carbon fiber reinforced PP composites with various recycled carbon fiber weight fractions.
PP 5 CF 10 CF 15 CF 20 CF 25 CF 30 CF 40 CF
Tm (8C)
Tc (8C)
DH (J/g)
Xc (%)
163.74 166.10 166.70 167.79 166.05 167.18 167.79 164.89
114.84 123.36 123.45 123.02 123.85 124.09 123.87 124.5
111.90 164.70 104.67 90.89 89.29 78.57 71.16 61.79
53.54 82.95 55.66 51.16 53.40 51.41 48.63 49.27
POLYMER COMPOSITES—2017 5
FIG. 4. Storage modulus (E0 ) of PP and recycled carbon fiber reinforced PP composites with various recycled carbon fiber weight fractions. [Color figure can be viewed at wileyonlinelibrary.com]
Dynamic Mechanical Analysis DMA looks at the modulus of elasticity or the ratio of mechanical stress to relative deformation. The storage modulus (E0 ) and the loss modulus (E00 ) were determined from the DMA tests. E0 represents the stiffness of a viscoelastic material, which is proportional to the elastic energy stored elastically and is reversible. Storage moduli of PP and recycled carbon fiber reinforced PP composites versus temperature as a function of recycled carbon fiber weight fractions are given in Fig. 4. The storage modulus decreases with increasing temperature due to an energy dissipation phenomenon involving cooperative motions of the polymer chain [31, 32]. Comparing the PP and the recycled carbon fiber reinforced PP composites with different recycled carbon fiber weight fractions at temperatures from 37 to 1258C, it is easy to find that the modulus of the composites containing 40 wt% recycled carbon fiber was higher than the other specimens, although it decreased dramatically by increasing temperature. It was observed that the stiffness
FIG. 6. CTE values of PP and recycled carbon fiber reinforced PP composites for T > Tg. [Color figure can be viewed at wileyonlinelibrary.com]
of the composites increased with increasing weight fraction of recycled carbon fiber, and the storage modulus of the composites was much higher than that of PP. The reason would probably be that the motions of the polymer chains are restricted by the recycled carbon fiber with the increment of temperature [31]. Figure 5 displays the loss modulus values of the PP and recycled carbon fiber reinforced PP composites. Loss modulus (E00 ) is a measure of viscous response of a material. It measures the energy dissipated as heat [31]. As was seen in Fig. 5, the E00 of all composites was much higher than that of the PP in the whole temperature range. Generally, the viscosity of recycled carbon fiber reinforced PP composites decreases gradually with the increase of temperature. The loss modulus continued to increase as the recycled carbon fiber weight fraction was increased. The relaxation transition peak around 708C represents the transition region from the glassy state to the rubbery state, as was shown in Fig. 5. Thermomechanical Analysis CTE was determined for two temperature regimes, that is, for T < Tg and T > Tg for PP and recycled carbon fiber
FIG. 5. Loss modulus (E00 ) of PP and recycled carbon fiber reinforced PP composites with various recycled carbon fiber weight fractions. [Color figure can be viewed at wileyonlinelibrary.com]
6 POLYMER COMPOSITES—2017
FIG. 7. CTE values of PP and recycled carbon fiber reinforced PP composites for T < Tg. [Color figure can be viewed at wileyonlinelibrary.com]
DOI 10.1002/pc
FIG. 8. Tensile strength of PP and recycled carbon fiber reinforced PP composites at various recycled carbon fiber weight fractions. [Color figure can be viewed at wileyonlinelibrary.com]
FIG. 11. Tensile modulus of PP and recycled carbon fiber reinforced PP at various recycled carbon fiber weight fractions. [Color figure can be viewed at wileyonlinelibrary.com]
FIG. 9. Effect of recycled carbon fiber content on flexural strength of PP and recycled carbon fiber reinforced PP. [Color figure can be viewed at wileyonlinelibrary.com]
FIG. 12. Effect of recycled carbon fiber content on flexural modulus of PP and recycled carbon fiber reinforced PP. [Color figure can be viewed at wileyonlinelibrary.com]
reinforced PP composites, as shown in Figs. 6 and 7, respectively. In particular, for temperatures above Tg, it was found that CTE values of PP, 5 CF, 10 CF,15 CF, 20 CF, 25 CF, 30 CF, and 40 CF specimens were determined to be 122.50, 107.55, 101.85, 93.80, 88.00, 82.74, 75.26, and 63.6 mm/(m8C), respectively. At temperatures below Tg, CTE of PP is 74.95 mm/(m8C). The CTE was found
to be decreased by increasing recycled carbon fiber weight fraction; 5 CF, 10 CF, 15 CF, 20 CF, 25 CF, 30 CF, and 40 CF composites exhibited 3, 11, 18, 23, 30, 34, and 37% decreases in CTE compared with that of PP, respectively. Decrease of the CTE of recycled carbon fiber reinforced PP composites was observed for all
FIG. 10. Elongation at break of recycled carbon fiber reinforced PP at various recycled carbon fiber weight fractions. [Color figure can be viewed at wileyonlinelibrary.com]
DOI 10.1002/pc
FIG. 13. Notched and Unnotched Izod Impact strength of PP and recycled carbon fiber reinforced PP comprising various weight fractions of recycled carbon fiber. [Color figure can be viewed at wileyonlinelibrary. com]
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FIG. 14. SEM images of (a) CF, (b) 5 CF, (c) 10 CF, (d) 15 CF, (e) 20 CF, (f) 25 CF, (g) 30 CF, and (h) 40 CF composites. [Color figure can be viewed at wileyonlinelibrary.com]
recycled carbon fiber weight fractions at both below and above Tg. The effect of recycled carbon fiber reinforcements on CTE for T < Tg regime was less dramatic. This reflects the high degree of alignment of the carbon fibers and the fact that recycled carbon fiber is stiffer compared with PP. 8 POLYMER COMPOSITES—2017
Mechanical Properties Tensile and Flexural Properties. Figures 8 and 9 show the variations of the tensile and flexural strength as a function of fiber weight fraction (wt%) for composites, DOI 10.1002/pc
respectively. Tensile strength values of PP, 5 CF, 10 CF, 15 CF, 20 CF, 25 CF, 30 CF, and 40 CF composites were obtained to be 19.52, 28.42, 33.88, 39.75, 45.78, 31.54, 27.55, and 18.01 MPa, respectively. As shown in Fig. 9, the flexural strength values for PP, 5 CF, 10 CF, 15 CF, 20 CF, 25 CF, 30 CF, and 40 CF were determined to be 34.68, 64.82, 68.75, 97.71, 64.67, 57.65, and 54.87 MPa, respectively. The results indicate that the tensile and flexural strength of PP increase with addition of recycled carbon fiber up to 20 wt%. According to Thomason and Vlug (1996), tensile strength increases with fiber loading. When the recycled carbon fiber reinforced PP composite is subjected to load, the recycled carbon fiber acts as the load carrier where stress is being transferred from the PP matrix to the recycled carbon fiber. This leads to an efficient and uniform stress distribution within the composites [33]. It has clearly shown that the tensile strength decreased as the weight fraction of recycled carbon fiber increased from 25 to 40 wt%. The addition of the recycled carbon fiber into PP reduced the ability of the composites to transfer applied stress. This is due to the possible agglomeration of the recycled carbon fiber, which can act as a stress concentration point. This would increase the ability of the composites to initiate cracks causing poor interfacial adhesion [4, 34]. Figure 10 shows elongation at break values of recycled carbon fiber reinforced PP composites. The elongation at break value of PP was about 196%. The elongation at break decreased when the recycled carbon fiber weight fractions was increased. These results indicate that PP becomes more brittle with the incorporation of recycled carbon fiber into PP. The effects of recycled carbon fiber weight fractions on the tensile and flexural moduli of recycled carbon fiber reinforced PP composites are given in Figs. 11 and 12, respectively. It can be seen that the tensile and flexural moduli of the PP increase with the addition of recycled carbon fiber in the studied weight fraction range (5–40 wt%). When 40 wt% recycled carbon fiber was added into PP, the tensile and flexural moduli were increased by about 177 and 359%, respectively. This is due to the addition of recycled carbon fiber into PP where the recycled carbon fiber acts as a load carrier in the composite [35]. Thus, the stiffness of PP increases with recycled carbon fiber incorporation. Impact Properties. The results from the unnotched and notched Izod impact testings are given in Fig. 13. The curves show that the unnotched and notched Izod impact properties of PP increased gradually with the increase of recycled carbon fiber weight fraction. The highest notched and unnotched impact strength for 20 CF was 42 and 74 J/ m, respectively (Fig. 13). This might be due to increased interfacial region between PP and recycled carbon fiber as a result of increased amount of recycled carbon fiber weight fraction [35]. However, the impact strength decreased with 25, 30, and 40 wt% recycled carbon fiber loading. These results showed that the dispersion of the DOI 10.1002/pc
reinforcements/fillers has a strong influence on mechanical and morphological properties of composites [4]. SEM Observations The SEM micrographs of the tensile fractured surfaces of recycled carbon fiber reinforced PP composites and carbon fibers are shown in Fig. 14. Recycled carbon fibers are observed to be dispersed uniformly in the PP and no obvious agglomerations of recycled carbon fibers were found in the experiments. Figure 14b–d show the SEM images of the PP composites containing 5, 10, 15 wt% recycled carbon fibers respectively. The recycled carbon fibers are wrapped well with PP, which indicates a good adhesion between the PP and recycled carbon fibers, though some pull-out recycled carbon fibers were seen on the recycled carbon fiber reinforced PP composite fracture surfaces with different lengths of recycled carbon fibers protruding outside the PP, as seen in Fig. 14b–h. This could be attributed to the poor interfacial adhesion between recycled carbon fiber and PP. To overcome this issue, the interfacial characteristics between the PP and recycled carbon fibers need to be improved through surface treatment of the recycled carbon fiber or introduction of a compatibilizer in the system [33, 34]. Surface modified recycled carbon fibers should bring a further increase in strength and modulus to the polymer composites. As the amount of recycled carbon fibers increases in the composites due to increased recycled carbon fiber weight fraction (Fig. 14f–h) degree of the alignment of fibers along direction of deformation increases. CONCLUSIONS The production of recycled carbon fiber reinforced PP composites for lightweight applications were investigated. The main conclusions drawn from this study are as follows: Density of the composites increased as recycled car-
bon fiber weight fraction increases from 5 to 40%. Change in melting temperature in PP composites
was not significant. Crystallinity increased up to 5% recycled carbon fiber weight fraction and decreases with further increase of recycled carbon fiber loading. The loss and storage moduli increased with increasing recycled carbon fiber weight fraction. CTE of recycled carbon fiber reinforced PP composites decreased for all recycled carbon fiber weight fractions at both below and above Tg. Improvement in tensile and flexural strength with weight fraction up to 20% was observed. The highest tensile and flexural strength are obtained at 20 wt% recycled carbon fiber loading. The tensile and flexural modulus of the PP increases with the addition of recycled carbon fiber in the studied weight fraction range (5–40 wt%). POLYMER COMPOSITES—2017 9
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DOI 10.1002/pc
