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Tensile and thermomechanical properties of short carbon fiber reinforced polyamide 6 composites
Composites: Part B 51 (2013) 270–275
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Tensile and thermomechanical properties of short carbon fiber reinforced polyamide 6 composites Nevin Gamze Karsli, Ayse Aytac ⇑ Department of Chemical Engineering, Kocaeli University, Engineering Faculty, 41380 Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 10 January 2013 Accepted 8 March 2013 Available online 22 March 2013 Keywords: A. Carbon fiber A. Polymer–matrix composites (PMCs) B. Mechanical properties
a b s t r a c t In this study, carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by using melt mixing method. Effects of fiber length and content, on the mechanical, thermal and morphological properties of CF reinforced PA6 composites were investigated. Fiber length distributions of composites were also determined by using an image analyzing program. It was seen that the maximum number of fibers were observed in the range of 0–50 lm. Mechanical test results showed that, increasing CF content increased the tensile strength, modulus and hardness values but decreased strain at break values of composites. DSC results showed that Tg and Tm values of composites were not changed significantly with increasing CF content and length. However, heat of fusion and the relative degree of crystallinity values of composites decreased with ascending CF content. DMA results revealed that storage modulus and loss modulus values of composites increased with increasing CF content. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Polymer composites are being increasingly employed in the plastic industry because of their good strengths and low densities [1]. Especially short fiber reinforced thermoplastic polymer composites are widely used in many field such as aircraft, aerospace or automotive industry [2,3]. Among the thermoplastic polymers, polyamide 6 (PA6) has become a strong competitor matrix owing to its good thermal stability, low dielectric constant and high tensile strength [3–5]. On the other hand, carbon fiber (CF) has good mechanical, thermal and electrical properties and its strength/density ratio is high [6]. These advantages make it one of the most commonly used reinforcing materials especially for polymer composites. Because of these properties, CF reinforced composites have attracted many researchers’ attention [1–8]. It is known that composite properties do not depend only on the types of the matrix and fiber, but also other factors such as manufacturing processes, fiber concentration, fiber length, fiber orientation and fiber matrix adhesion. All these factors are interconnected [1,6–8]. Processing techniques such as extrusion compounding and injection molding are frequently used to prepare carbon fiber reinforced polymer composites [9,10]. However, a significant amount ⇑ Corresponding author. Current address: Chemical Engineering Department, Kocaeli University, Engineering Faculty, 41380 Izmit, Kocaeli, Turkey. Tel.: +90 262 303 35 32. E-mail address: [email protected] (A. Aytac). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.03.023
of fiber breakage takes place during processing. Fiber breakage probably occurs due to interactions between fiber–polymer, fiber–fiber and fiber-processing equipment surface wall. Fiber breakage resulting from these interactions leads to decrease in fiber length and this decrement decreases fiber reinforcing efficiency [11]. This also affects the mechanical properties of composites. Therefore, the effects of fiber length and content on the mechanical properties of carbon fiber reinforced polymer composites, should be considered as a combined effect, and should be taken into consideration as two competing effects that determine the final mechanical properties of composites [12,13]. As far as we know, there is limited number of publications in the literature about the effect of fiber length and content on the properties of CF reinforced PA6 composites. Zhou et al. investigated the fiber breakage and dispersion for carbon fiber reinforced nylon 6/clay nanocomposites, and the effects of clay and processing conditions on fiber breakage and dispersion were taken into account respectively [14]. They concluded that the presence of organoclay could improve fiber dispersion. They also observed the bimodal distribution of fiber length in nanocomposites, which is parallel to that in conventional fiber reinforced composites. Li and Zhang studied the effects of HNO3-treated short carbon fiber (SCF) concentration on the tensile properties of ABS and ABS/PA6 matrix composites [15]. They found that when short carbon fiber amount increased in the ABS matrix from 5 to 30 wt%, the tensile strength and tensile modulus were enhanced. They also showed that the tensile strength and modulus values increased with an increase in the PA6 amount of the ABS/PA6/SCF systems owing to the improved adhesion at the interface. In another study made by Molnàr
N.G. Karsli, A. Aytac / Composites: Part B 51 (2013) 270–275
et al. short carbon fiber reinforced PA6 composites were prepared by homogenization of the components in a twin screw extruder and by injection molding [2]. They discussed the effect of composition and injection rate on the structure of composites in their study. They found that the average fiber length decreased with the increasing fiber content and processing rate. Also, both increased fiber content and injection rate lead to an increase in the stiffness and toughness. In this study, CF reinforced PA6 composites were prepared by extrusion compounding and injection molding techniques. The effects of fiber length and content on the mechanical, thermo mechanical, and morphological properties of CF reinforced PA6 were investigated. 2. Materials and methods PA6 used in this study was NylemÒ6, supplied from Emas Plastik (Turkey). Short carbon fibers (AKSACA) in three different lengths (0.6 cm, 1.2 cm and 1.9 cm) were supplied from AKSA (Turkey). Composites containing 0, 2, 4, 6, 8 and 20 wt% carbon fiber were prepared by melt mixing in a laboratory scale co-rotating twin-screw mini extruder (DSM Xplore) at 240 °C, 100 rpm and 3 min. All the compounds were subsequently injection molded into dumbbell-shaped tensile bars using a laboratory scale injection molding machine (DSM Xplore) with barrel temperature 240 °C and mold temperature 30 °C; and injection pressure was 8 bars. Tensile tests were applied according to ASTM D 638-10 [16] using a computer controlled Instron 4411 universal testing machine. Tensile strength, modulus, and strain at break values of composites were determined using five dumbbell-shaped samples for each composition at a constant crosshead speed of 5 mm/min. Rockwell hardness tests of composites were performed according to the ASTM D 785-08 [17] using a Brooks Model MAT 10/250 Hardness Testing Machine. Rockwell R scale was used with 12.7 mm diameter ball indenter, 10 kg minor load, and 60 kg major load. Glass transition temperature (Tg), melting temperature (Tm), heat of fusion (DHf) and the relative degree of crystallinity (Dv) values of the composites were measured by performing differential scanning calorimeter analysis (DSC) (Mettler Toledo DSC 1) under nitrogen atmosphere. The DSC analysis was carried out in the temperature range from 25 °C to 250 °C at a heating rate of 10 °C/min. The relative degree of crystallinity (percent improvement in crystallinity compared to as received) of the samples were calculated with the following expression:
Dv i ð%Þ ¼
DHf;i DH0 100 DH 0
ð1Þ
where DH0 and DHf,i are the heat of fusion of pure PA6 and the heat of fusion of composites, respectively [18]. After the tensile test, the morphologies of fractured surfaces of composites were examined using a scanning electron microscope (JEOL JSM-6335F). Before the examinations, tensile fracture surfaces of samples were sputter coated with gold and palladium. In order to determine the fiber length distribution, carbon fibers were separated from the PA6 matrix. During the separation process, CF reinforced PA6 composites were burned in an ash oven for about 10 min at 550 °C. The residual ash was dispersed in water and then the CFs were transferred to glass slides and investigated by an optical microscope. A minimum of 12 photographs were taken for each slide and approximately 50 fibers for each run were taken into account for fiber length distribution analysis. The fiber images obtained from the optical microscope were then analyzed by Image JÒ and the fiber length distributions and mean fiber length were determined.
271
Storage modulus (E0 ), loss modulus (E00 ) and tan delta (tan d) values of composites were determined by dynamic mechanical analysis (DMA). DMA was carried out by using a DMA 50 analyzer from 01 dB-Metravib working in the tension mode. Specimens were tested under the conditions of 0.001 N dynamic force and 1 Hz frequency. The scan was done from 30 to 150 °C at 3 °C/min rate. Specimen dimensions were 16 mm height, 2 mm thickness and 4 mm width.
3. Results and discussion Fiber length distribution analysis was performed on the samples, which contains CF in the same weight ratios but in different lengths, in order to investigate the effect of fiber length on the properties of CF reinforced PA6 composites. Fiber length distribution obtained from this analysis is shown in Fig. 1. It can be seen from Fig. 1 that the maximum number of fibers was observed in the range of 0–50 lm. These results also show that at the studied process conditions, fiber lengths are reached to an ultimate average value regardless of the initial fiber length values. Thus, it can be concluded that significant fiber breakage occurs during compounding by extrusion and injection molding and these effects should be taken into consideration for process optimization [14,19]. Figs. 2a–c and 3a and b show the influence of fiber content and length on the tensile strength at yield, tensile strength at break and modulus values of CF reinforced PA6 composites as a function of CF weight fraction (wt%), respectively. It was observed that increasing CF amount increases the tensile strength and modulus values of PA6 composites. On the other hand, fiber length at the studied range had no effect on the tensile strength and modulus values of these composites. This is in accordance with what was found in fiber distribution analysis. At the end of the process, the fiber lengths reach to an ultimate value regardless of their initial values. The same ultimate fiber length ensures the same efficiency factor for fiber reinforcements. This case explains why fiber length at the studied range has no effect on the tensile strength and modulus values of composites. The influence of fiber content and length on the strain at break values of CF reinforced PA6 composites as a function of CF weight fraction (wt%) was shown in Fig. 4a and b, respectively. It was observed that the strain at break values decreased with increasing CF content regardless of the used CF length. Similar findings have been shown in the literature and were explained by a failure mechanism [6,7,11,20]. According to this mechanism, under the tensile stress, the micro-cracks start at the end of fibers because of the higher stress concentration in these points. Then these microcracks move along the fiber length and spread across the matrix. When the cracks grow to a critical size, fracturing of the composite takes place. This result shows that the failure is closely related to the number of fiber ends. This explains why increasing CF content decreases strain at break values. On the other hand, it can be seen from Fig. 4b that increasing fiber length increases the strain at break values. This result may be explained with the decreasing number of fiber ends in the composites. Since CF reinforced PA6 composites containing 0.6 cm fiber include more fiber ends than that of CF reinforced composites containing 1.2 cm at the same sample weight, strain at break value of the former is lower. Correspondingly, more fiber ends mean more micro-crack and this effect decreases the strain at break values. The influence of fiber content and length on the hardness values of CF reinforced PA6 composites as a function of CF weight fraction (wt%) was shown in Fig. 5a and b, respectively. It was observed that increasing CF content increases the hardness values of composites in Fig. 5a. This increment in hardness of composites can be
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Fig. 1. Fiber length distribution of PA6/CF composites for 6% CF content.
Fig. 2. Effect of fiber content and length on the tensile strength at break values of PA6/CF composites, (a) tensile strength at yield, (b) tensile strength at break, and (c) effect of fiber length.
explained by following mechanism. When compressive force is applied to the composite, thermoplastic matrix phase and fiber phase are pressed in the mean time, where they touch each other and a resistance occurs. Because of this behavior, even if there is a poor bonding between fibers and matrix, the load can be transferred more effectively at the interface [21]. On the other hand, the fiber
length at the studied range had no effect on the hardness values of composites in Fig. 5b. The reason of this is obtaining almost the same fiber lengths in all of the prepared composite. The glass transition temperature (Tg), melting temperature (Tm), heat of fusion of composites (DHf) and the relative degree of crystallinity Dv (%) values of containing 1.2 cm CF reinforced PA6
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Fig. 3. Effect of fiber content and length on the E-modulus values of PA6/CF composites, (a) effect of fiber content and (b) effect of fiber length.
Fig. 4. Effect of fiber content and length on the tensile strain at break values of PA6/CF composites, (a) effect of fiber content and (b) effect of fiber length.
Fig. 5. Effect of fiber content and length on the Rockwell hardness values of PA6/CF composites, (a) effect of fiber content and (b) effect of fiber length.
composites were given in Table 1. As it is known, the precision and repeatability of Tg values strictly depend on the methodology used. We have measured Tg values by using both DSC and DMA
Table 1 Results of DSC analysis. CF content (wt%)
0 2 4 6 8 20 a
CF length (cm) 1.2 Tga (°C)
Tg (°C)
Tm (°C)
DHf (J/g)
Dv (%)
64.2 63.6 65.5 66.6 63.8 64.9
59.0 56.4 59.2 62.3 53.9 58.7
229.4 222.5 227.4 228.5 227.9 227.2
71.9 62.6 59.4 59.1 55.7 55.3
– 12.9 17.5 17.8 22.5 23.1
Tg values were obtained from DMA analysis.
techniques (Table 1). There are some differences between the Tg values obtained from DSC and DMA. Although there is slightly variation in the Tg values obtained from DSC, no changes were observed in the Tg values obtained from DMA with the increasing CF content. On the other hand, the heat of fusion and the relative degree of crystallinity values of composites decreased with the increasing CF content. The fibers generally affect the degree of crystallinity values of the composites by increasing the numbers of nucleation sites and crystal growth rate at low fiber concentration. But, as concentration of fibers in the composite is increased, fibers start to prevent the mobility of polymer matrix chains. This behavior, obstruct the crystal growth in the matrix [18,22]. SEM analysis was employed to observe the tensile fracture surfaces of short CF reinforced PA6 composite samples based on 1.2 cm fiber length with different fiber contents. The SEM micrographs of the fracture surfaces of short carbon fiber reinforced PA6 composites were given in Fig. 6a–c. It is seen that the surfaces
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Fig. 6. SEM micrographs of tensile fracture surfaces of PA6/CF composites, (a) 1.2 cm-4% (1000), (b) 1.2 cm-6% (1000), and (c) 1.2 cm-8% (1000).
of the fibers were not coated with the polymeric matrix and most of the fibers pulled out. This can be attributed to the poor interfacial adhesion between the fiber and matrix. Moreover, the stresses during tensile test are not high enough to cause the fiber failure after matrix fracture. For this reason, fibers pulled out from the matrix instead of fractured [7]. This also clearly shows that, there is a poor interfacial adhesion between the fiber and matrix for CF reinforced PA6 composites. Also there were dark rings occurring between the fibers and the matrix. The reason of those dark rings is most probably the local deformation of the matrix around the fibers [22]. It has been also seen that the fiber surfaces were clean.
This shows the lack of interaction between the carbon fiber and PA6 matrix [6]. Storage modulus (E0 ), loss modulus (E00 ) and tan delta (tan d) values of composites are shown in Fig. 7a–c respectively. It can be seen from Fig. 7a that storage modulus of composites increased with increasing CF content. The addition of fibers to polymer matrix allows greater stress transfer at the interface. The increase in storage modulus is mainly due to this stress transfer between the matrix and fibers. Besides, storage modulus of composites increase when the movements of polymer chains are restricted by interactions between the fiber–fiber and fiber–matrix [23–25]. It
Fig. 7. DMA test results of PA6/CF composites, (a) storage modulus, (b) loss modulus, (c) tan delta.
N.G. Karsli, A. Aytac / Composites: Part B 51 (2013) 270–275
can also be seen that storage modulus of composites decreased with increasing temperature. As temperature increases, components of composites become more mobile and start the move away from each other, so there are no stress transfer between fiber and matrix anymore [25]. It can be seen from Fig. 7b that loss modulus of composites increased with increasing CF content. Loss modulus indicates the material’s ability to dissipate energy in the form of heat or molecular rearrangements during the deformation [23]. The greater content of fibers was incorporated to matrix, the slower the flow and higher the loss modulus. But the maximum heat dissipation occurs at the temperature where loss modulus is at a maximum value [23,25]. After this maximum value, the loss modulus of composites decrease with increasing temperature. The loss modulus indicates the viscous nature of the polymer and generally as the temperature increases, viscosity of the materials decreases [23]. This trend can also be seen in Fig. 7b. Tan delta is the ratio of the loss modulus to the storage modulus. In thermoplastics, tan delta value of a polymer reaches a maximum when the polymer is heated up to the glass transition temperature [26]. It can be seen from Table 1 and Fig. 7c that Tg values of carbon fiber reinforced composites did not change with the increasing carbon fiber content. Also, Tg values obtained from DMA are a few degrees higher than Tg values obtained from DSC. This is mainly a result of the frequency effect in the DMA. Since frequency effect freezes the polymer chain movements so molecular relaxations of polymer chains can only occur at higher temperatures. This is the reason that higher glass transition temperatures values found with DMA when compared to DSC [27]. 4. Conclusions The aim of this study is to investigate the effects of fiber length and content on the mechanical, thermal and morphological properties of CF reinforced PA6 composites. Carbon fiber length distribution in the PA6 composites was determined by Image JÒ analyzing program. It was seen that the maximum number of fibers was observed in the range of 0–50 lm. According to mechanical test results, increasing CF content increases the tensile strength, modulus and hardness values but decreases strain at break values of composites. On the other hand, fiber length at the studied range had no effect on the tensile strength, modulus and hardness values but ascending fiber length increased the strain at break values of composites. It has been showed that Tg and Tm values of composites were not affected significantly with increasing CF content. But heat of fusion and the relative degree of crystallinity values of composites decreased with ascending CF content. It is seen in the SEM micrographs that most of the fibers were pulled out from the matrix. DMA results showed that storage modulus and loss modulus values of composites increased with increasing CF content. It was also observed from tan delta results that Tg values obtained from DMA are a few degrees higher than Tg values obtained from DSC. Consequently, it can be concluded that, initial fiber length at the studied range did not have any significant effect on the properties of composites due to the fiber breakage during processing. As a result, the process parameters should be optimized to reduce the fiber breakage and to obtain better composite properties. Acknowledgements The authors thank to AKSA Akrilik Kimya Sanayi A.S ß for providing short carbon fibers, Assoc. Prof. Dr. Yalçın Tanes for his valuable
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support in realization of this study and to Dr. Nurhan Vatansever for allowing us to use the mechanical testing laboratory in Vocational School of Kosekoy at Kocaeli University. References [1] Nie WZ, Li J, Zhang YF. Tensile properties of surface treated carbon fibre reinforced ABS/PA6 composites. Plast Rubber Compos 2010;39(1): 16–20. [2] Molnàr S, Rosenberger S, Gulyàs J, Pukànszky B. Structure and impact resistance of short carbon fiber reinforced polyamide 6 composites. J Macromol Sci B 1999;38(5–6):721–35. [3] Botelho EC, Figiel L, Rezende MC, Lauke B. Mechanical behavior of carbon fiber reinforced polyamide composites. Compos Sci Technol 2003;63: 1843–55. [4] Botelho EC, Rezende MC. Monitoring of carbon fiber/polyamide composites processing by rheological and thermal analyses. Polym Plast Technol 2006;45:61–9. [5] Li J. Interfacial studies on the O3 modified carbon fiber-reinforced polyamide 6 composites. Appl Surf Sci 2008;255:2822–4. [6] Li J, Cai CL. The carbon fiber surface treatment and addition of PA6 on tensile properties of ABS composites. Curr Appl Phys 2011;11:50–4. [7] Li J, Zhang YF. The tensile properties of HNO3-treated carbon fiber reinforced ABS/PA6 composites. Surf Interface Anal 2009;41:610–4. [8] Li J, Zhang YF. Tensile strength of ABS/PA6 composites reinforced with HNO3treated carbon fibers. Mech Compos Mater 2009;45(5):537–42. [9] Fu SY, Lauke B, Mäder E, Hu X, Yue CY. Fracture resistance of short-glass–fiberreinforced and short-carbon-fiber-reinforced polypropylene under Charpy impact load and its dependence on processing. J Mater Process Technol 1999;89–90:501–7. [10] Fu SY, Hu X, Yue CY. Effects of fiber length and orientation distributions on the mechanical properties of short-fiber-reinforced polymers. Mater Sci Res Int 1999;5(2):74–83. [11] Fu SY, Lauke B, Mäder E, Yue CY, Hu X. Tensile properties of short-glass–fiberand short-carbon-fiber-reinforced polypropylene composites. Composites Part A 2000;31:1117–25. [12] Fu SY, Mai YW, Ching ECY, Li RKY. Correction of the measurement of fiber length of short fiber reinforced thermoplastics. Composites Part A 2002;33:1549–55. [13] Bijsterbosch H, Gaymans RJ. Polyamide 6-long glass fiber injection moldings. Polym Compos 1995;16(5):363–9. [14] Zhou H, Qian Z, Meng X, Ding Y, Zhang S, Yang M. Fiber breakage and dispersion in carbon-fiber-reinforced nylon 6/clay nanocomposites. J Appl Polym Sci 2007;106:1751–6. [15] Li J, Zhang YF. The tensile properties of short carbon fiber reinforced ABS and ABS/PA6 composites. J Reinf Plast Comp 2010;29(11):1727–33. [16] Standard test method for tensile properties of plastics. ASTM D 638-10; 2010. [17] Standard test method for rockwell hardness of plastics and electrical insulating materials. ASTM D 785-08; 2008. [18] Ren X, Wang XQ, Sui G, Zhong WH, Fuqua MA, Ulven CA. Effects of carbon nanofibers on crystalline structures and properties of ultrahigh molecular weight polyethylene blend fabricated using twin-screw extrusion. J Appl Polym Sci 2008;107:2837–45. [19] Gupta VB, Mittal RK, Sharma PK. Some studies on glass fiber-reinforced polypropylene. Part I: reduction in fiber length during processing. Polym Compos 1989;10(1):8–15. [20] Fu SY, Lauke B, Mäder E, Yue CY, Hu X, Mai YW. Hybrid effects on tensile properties of hybrid short-glass–fiber-and short-carbon fiber-reinforced polypropylene composites. J Mater Sci 2001;36:1243–51. [21] Suresha B, Ravi Kumar BN, Venkataramareddy M, Jayaraju T. Role of micro/ nano fillers on mechanical and tribological properties of polyamide66/ polypropylene composites. Mater Des 2010;31:1993–2000. [22] Lozano K, Barrera V. Nanofiber–reinforced thermoplastic composites. I. Thermoanalytical and mechanical analysis. J Appl Polym Sci 2001;79:125–33. [23] Rezaei F, Yunus R, Ibrahim NA. Effect of fiber length on thermomechanical properties of short carbon fiber reinforced polypropylene composites. Mater Des 2009;30:260–3. [24] Abdullah SA, Iqbal A, Frormann L. Melt mixing of carbon fibers and carbon nanotubes incorporated polyurethanes. J Appl Polym Sci 2008;110: 196–202. [25] Jacob M, Francis B, Thomas S, Varughese KT. Dynamical mechanical analysis of sisal/oil palm hybrid fiber-reinforced natural rubber composites. Polym Compos 2006;27:671–80. [26] Yang S, Taha-Tijerina J, Serrato-Diaz V, Hernandez K, Lozano K. Dynamic mechanical and thermal analysis of aligned vapor grown carbon nanofiber reinforced polyethylene. Composites Part B 2007;38:228–35. [27] Gracia-Fernández CA, Gómez-Barreiro S, López-Beceiro J, Tarrío Saavedra J, Naya S, Artiaga R. Comparative study of the dynamic glass transition temperature by DMA and TMDSC. Polym Test 2010;29:1002–6.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Tensile and thermomechanical properties of short carbon fiber reinforced polyamide 6 composites Nevin Gamze Karsli, Ayse Aytac ⇑ Department of Chemical Engineering, Kocaeli University, Engineering Faculty, 41380 Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 10 January 2013 Accepted 8 March 2013 Available online 22 March 2013 Keywords: A. Carbon fiber A. Polymer–matrix composites (PMCs) B. Mechanical properties
a b s t r a c t In this study, carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by using melt mixing method. Effects of fiber length and content, on the mechanical, thermal and morphological properties of CF reinforced PA6 composites were investigated. Fiber length distributions of composites were also determined by using an image analyzing program. It was seen that the maximum number of fibers were observed in the range of 0–50 lm. Mechanical test results showed that, increasing CF content increased the tensile strength, modulus and hardness values but decreased strain at break values of composites. DSC results showed that Tg and Tm values of composites were not changed significantly with increasing CF content and length. However, heat of fusion and the relative degree of crystallinity values of composites decreased with ascending CF content. DMA results revealed that storage modulus and loss modulus values of composites increased with increasing CF content. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Polymer composites are being increasingly employed in the plastic industry because of their good strengths and low densities [1]. Especially short fiber reinforced thermoplastic polymer composites are widely used in many field such as aircraft, aerospace or automotive industry [2,3]. Among the thermoplastic polymers, polyamide 6 (PA6) has become a strong competitor matrix owing to its good thermal stability, low dielectric constant and high tensile strength [3–5]. On the other hand, carbon fiber (CF) has good mechanical, thermal and electrical properties and its strength/density ratio is high [6]. These advantages make it one of the most commonly used reinforcing materials especially for polymer composites. Because of these properties, CF reinforced composites have attracted many researchers’ attention [1–8]. It is known that composite properties do not depend only on the types of the matrix and fiber, but also other factors such as manufacturing processes, fiber concentration, fiber length, fiber orientation and fiber matrix adhesion. All these factors are interconnected [1,6–8]. Processing techniques such as extrusion compounding and injection molding are frequently used to prepare carbon fiber reinforced polymer composites [9,10]. However, a significant amount ⇑ Corresponding author. Current address: Chemical Engineering Department, Kocaeli University, Engineering Faculty, 41380 Izmit, Kocaeli, Turkey. Tel.: +90 262 303 35 32. E-mail address: [email protected] (A. Aytac). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.03.023
of fiber breakage takes place during processing. Fiber breakage probably occurs due to interactions between fiber–polymer, fiber–fiber and fiber-processing equipment surface wall. Fiber breakage resulting from these interactions leads to decrease in fiber length and this decrement decreases fiber reinforcing efficiency [11]. This also affects the mechanical properties of composites. Therefore, the effects of fiber length and content on the mechanical properties of carbon fiber reinforced polymer composites, should be considered as a combined effect, and should be taken into consideration as two competing effects that determine the final mechanical properties of composites [12,13]. As far as we know, there is limited number of publications in the literature about the effect of fiber length and content on the properties of CF reinforced PA6 composites. Zhou et al. investigated the fiber breakage and dispersion for carbon fiber reinforced nylon 6/clay nanocomposites, and the effects of clay and processing conditions on fiber breakage and dispersion were taken into account respectively [14]. They concluded that the presence of organoclay could improve fiber dispersion. They also observed the bimodal distribution of fiber length in nanocomposites, which is parallel to that in conventional fiber reinforced composites. Li and Zhang studied the effects of HNO3-treated short carbon fiber (SCF) concentration on the tensile properties of ABS and ABS/PA6 matrix composites [15]. They found that when short carbon fiber amount increased in the ABS matrix from 5 to 30 wt%, the tensile strength and tensile modulus were enhanced. They also showed that the tensile strength and modulus values increased with an increase in the PA6 amount of the ABS/PA6/SCF systems owing to the improved adhesion at the interface. In another study made by Molnàr
N.G. Karsli, A. Aytac / Composites: Part B 51 (2013) 270–275
et al. short carbon fiber reinforced PA6 composites were prepared by homogenization of the components in a twin screw extruder and by injection molding [2]. They discussed the effect of composition and injection rate on the structure of composites in their study. They found that the average fiber length decreased with the increasing fiber content and processing rate. Also, both increased fiber content and injection rate lead to an increase in the stiffness and toughness. In this study, CF reinforced PA6 composites were prepared by extrusion compounding and injection molding techniques. The effects of fiber length and content on the mechanical, thermo mechanical, and morphological properties of CF reinforced PA6 were investigated. 2. Materials and methods PA6 used in this study was NylemÒ6, supplied from Emas Plastik (Turkey). Short carbon fibers (AKSACA) in three different lengths (0.6 cm, 1.2 cm and 1.9 cm) were supplied from AKSA (Turkey). Composites containing 0, 2, 4, 6, 8 and 20 wt% carbon fiber were prepared by melt mixing in a laboratory scale co-rotating twin-screw mini extruder (DSM Xplore) at 240 °C, 100 rpm and 3 min. All the compounds were subsequently injection molded into dumbbell-shaped tensile bars using a laboratory scale injection molding machine (DSM Xplore) with barrel temperature 240 °C and mold temperature 30 °C; and injection pressure was 8 bars. Tensile tests were applied according to ASTM D 638-10 [16] using a computer controlled Instron 4411 universal testing machine. Tensile strength, modulus, and strain at break values of composites were determined using five dumbbell-shaped samples for each composition at a constant crosshead speed of 5 mm/min. Rockwell hardness tests of composites were performed according to the ASTM D 785-08 [17] using a Brooks Model MAT 10/250 Hardness Testing Machine. Rockwell R scale was used with 12.7 mm diameter ball indenter, 10 kg minor load, and 60 kg major load. Glass transition temperature (Tg), melting temperature (Tm), heat of fusion (DHf) and the relative degree of crystallinity (Dv) values of the composites were measured by performing differential scanning calorimeter analysis (DSC) (Mettler Toledo DSC 1) under nitrogen atmosphere. The DSC analysis was carried out in the temperature range from 25 °C to 250 °C at a heating rate of 10 °C/min. The relative degree of crystallinity (percent improvement in crystallinity compared to as received) of the samples were calculated with the following expression:
Dv i ð%Þ ¼
DHf;i DH0 100 DH 0
ð1Þ
where DH0 and DHf,i are the heat of fusion of pure PA6 and the heat of fusion of composites, respectively [18]. After the tensile test, the morphologies of fractured surfaces of composites were examined using a scanning electron microscope (JEOL JSM-6335F). Before the examinations, tensile fracture surfaces of samples were sputter coated with gold and palladium. In order to determine the fiber length distribution, carbon fibers were separated from the PA6 matrix. During the separation process, CF reinforced PA6 composites were burned in an ash oven for about 10 min at 550 °C. The residual ash was dispersed in water and then the CFs were transferred to glass slides and investigated by an optical microscope. A minimum of 12 photographs were taken for each slide and approximately 50 fibers for each run were taken into account for fiber length distribution analysis. The fiber images obtained from the optical microscope were then analyzed by Image JÒ and the fiber length distributions and mean fiber length were determined.
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Storage modulus (E0 ), loss modulus (E00 ) and tan delta (tan d) values of composites were determined by dynamic mechanical analysis (DMA). DMA was carried out by using a DMA 50 analyzer from 01 dB-Metravib working in the tension mode. Specimens were tested under the conditions of 0.001 N dynamic force and 1 Hz frequency. The scan was done from 30 to 150 °C at 3 °C/min rate. Specimen dimensions were 16 mm height, 2 mm thickness and 4 mm width.
3. Results and discussion Fiber length distribution analysis was performed on the samples, which contains CF in the same weight ratios but in different lengths, in order to investigate the effect of fiber length on the properties of CF reinforced PA6 composites. Fiber length distribution obtained from this analysis is shown in Fig. 1. It can be seen from Fig. 1 that the maximum number of fibers was observed in the range of 0–50 lm. These results also show that at the studied process conditions, fiber lengths are reached to an ultimate average value regardless of the initial fiber length values. Thus, it can be concluded that significant fiber breakage occurs during compounding by extrusion and injection molding and these effects should be taken into consideration for process optimization [14,19]. Figs. 2a–c and 3a and b show the influence of fiber content and length on the tensile strength at yield, tensile strength at break and modulus values of CF reinforced PA6 composites as a function of CF weight fraction (wt%), respectively. It was observed that increasing CF amount increases the tensile strength and modulus values of PA6 composites. On the other hand, fiber length at the studied range had no effect on the tensile strength and modulus values of these composites. This is in accordance with what was found in fiber distribution analysis. At the end of the process, the fiber lengths reach to an ultimate value regardless of their initial values. The same ultimate fiber length ensures the same efficiency factor for fiber reinforcements. This case explains why fiber length at the studied range has no effect on the tensile strength and modulus values of composites. The influence of fiber content and length on the strain at break values of CF reinforced PA6 composites as a function of CF weight fraction (wt%) was shown in Fig. 4a and b, respectively. It was observed that the strain at break values decreased with increasing CF content regardless of the used CF length. Similar findings have been shown in the literature and were explained by a failure mechanism [6,7,11,20]. According to this mechanism, under the tensile stress, the micro-cracks start at the end of fibers because of the higher stress concentration in these points. Then these microcracks move along the fiber length and spread across the matrix. When the cracks grow to a critical size, fracturing of the composite takes place. This result shows that the failure is closely related to the number of fiber ends. This explains why increasing CF content decreases strain at break values. On the other hand, it can be seen from Fig. 4b that increasing fiber length increases the strain at break values. This result may be explained with the decreasing number of fiber ends in the composites. Since CF reinforced PA6 composites containing 0.6 cm fiber include more fiber ends than that of CF reinforced composites containing 1.2 cm at the same sample weight, strain at break value of the former is lower. Correspondingly, more fiber ends mean more micro-crack and this effect decreases the strain at break values. The influence of fiber content and length on the hardness values of CF reinforced PA6 composites as a function of CF weight fraction (wt%) was shown in Fig. 5a and b, respectively. It was observed that increasing CF content increases the hardness values of composites in Fig. 5a. This increment in hardness of composites can be
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Fig. 1. Fiber length distribution of PA6/CF composites for 6% CF content.
Fig. 2. Effect of fiber content and length on the tensile strength at break values of PA6/CF composites, (a) tensile strength at yield, (b) tensile strength at break, and (c) effect of fiber length.
explained by following mechanism. When compressive force is applied to the composite, thermoplastic matrix phase and fiber phase are pressed in the mean time, where they touch each other and a resistance occurs. Because of this behavior, even if there is a poor bonding between fibers and matrix, the load can be transferred more effectively at the interface [21]. On the other hand, the fiber
length at the studied range had no effect on the hardness values of composites in Fig. 5b. The reason of this is obtaining almost the same fiber lengths in all of the prepared composite. The glass transition temperature (Tg), melting temperature (Tm), heat of fusion of composites (DHf) and the relative degree of crystallinity Dv (%) values of containing 1.2 cm CF reinforced PA6
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Fig. 3. Effect of fiber content and length on the E-modulus values of PA6/CF composites, (a) effect of fiber content and (b) effect of fiber length.
Fig. 4. Effect of fiber content and length on the tensile strain at break values of PA6/CF composites, (a) effect of fiber content and (b) effect of fiber length.
Fig. 5. Effect of fiber content and length on the Rockwell hardness values of PA6/CF composites, (a) effect of fiber content and (b) effect of fiber length.
composites were given in Table 1. As it is known, the precision and repeatability of Tg values strictly depend on the methodology used. We have measured Tg values by using both DSC and DMA
Table 1 Results of DSC analysis. CF content (wt%)
0 2 4 6 8 20 a
CF length (cm) 1.2 Tga (°C)
Tg (°C)
Tm (°C)
DHf (J/g)
Dv (%)
64.2 63.6 65.5 66.6 63.8 64.9
59.0 56.4 59.2 62.3 53.9 58.7
229.4 222.5 227.4 228.5 227.9 227.2
71.9 62.6 59.4 59.1 55.7 55.3
– 12.9 17.5 17.8 22.5 23.1
Tg values were obtained from DMA analysis.
techniques (Table 1). There are some differences between the Tg values obtained from DSC and DMA. Although there is slightly variation in the Tg values obtained from DSC, no changes were observed in the Tg values obtained from DMA with the increasing CF content. On the other hand, the heat of fusion and the relative degree of crystallinity values of composites decreased with the increasing CF content. The fibers generally affect the degree of crystallinity values of the composites by increasing the numbers of nucleation sites and crystal growth rate at low fiber concentration. But, as concentration of fibers in the composite is increased, fibers start to prevent the mobility of polymer matrix chains. This behavior, obstruct the crystal growth in the matrix [18,22]. SEM analysis was employed to observe the tensile fracture surfaces of short CF reinforced PA6 composite samples based on 1.2 cm fiber length with different fiber contents. The SEM micrographs of the fracture surfaces of short carbon fiber reinforced PA6 composites were given in Fig. 6a–c. It is seen that the surfaces
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Fig. 6. SEM micrographs of tensile fracture surfaces of PA6/CF composites, (a) 1.2 cm-4% (1000), (b) 1.2 cm-6% (1000), and (c) 1.2 cm-8% (1000).
of the fibers were not coated with the polymeric matrix and most of the fibers pulled out. This can be attributed to the poor interfacial adhesion between the fiber and matrix. Moreover, the stresses during tensile test are not high enough to cause the fiber failure after matrix fracture. For this reason, fibers pulled out from the matrix instead of fractured [7]. This also clearly shows that, there is a poor interfacial adhesion between the fiber and matrix for CF reinforced PA6 composites. Also there were dark rings occurring between the fibers and the matrix. The reason of those dark rings is most probably the local deformation of the matrix around the fibers [22]. It has been also seen that the fiber surfaces were clean.
This shows the lack of interaction between the carbon fiber and PA6 matrix [6]. Storage modulus (E0 ), loss modulus (E00 ) and tan delta (tan d) values of composites are shown in Fig. 7a–c respectively. It can be seen from Fig. 7a that storage modulus of composites increased with increasing CF content. The addition of fibers to polymer matrix allows greater stress transfer at the interface. The increase in storage modulus is mainly due to this stress transfer between the matrix and fibers. Besides, storage modulus of composites increase when the movements of polymer chains are restricted by interactions between the fiber–fiber and fiber–matrix [23–25]. It
Fig. 7. DMA test results of PA6/CF composites, (a) storage modulus, (b) loss modulus, (c) tan delta.
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can also be seen that storage modulus of composites decreased with increasing temperature. As temperature increases, components of composites become more mobile and start the move away from each other, so there are no stress transfer between fiber and matrix anymore [25]. It can be seen from Fig. 7b that loss modulus of composites increased with increasing CF content. Loss modulus indicates the material’s ability to dissipate energy in the form of heat or molecular rearrangements during the deformation [23]. The greater content of fibers was incorporated to matrix, the slower the flow and higher the loss modulus. But the maximum heat dissipation occurs at the temperature where loss modulus is at a maximum value [23,25]. After this maximum value, the loss modulus of composites decrease with increasing temperature. The loss modulus indicates the viscous nature of the polymer and generally as the temperature increases, viscosity of the materials decreases [23]. This trend can also be seen in Fig. 7b. Tan delta is the ratio of the loss modulus to the storage modulus. In thermoplastics, tan delta value of a polymer reaches a maximum when the polymer is heated up to the glass transition temperature [26]. It can be seen from Table 1 and Fig. 7c that Tg values of carbon fiber reinforced composites did not change with the increasing carbon fiber content. Also, Tg values obtained from DMA are a few degrees higher than Tg values obtained from DSC. This is mainly a result of the frequency effect in the DMA. Since frequency effect freezes the polymer chain movements so molecular relaxations of polymer chains can only occur at higher temperatures. This is the reason that higher glass transition temperatures values found with DMA when compared to DSC [27]. 4. Conclusions The aim of this study is to investigate the effects of fiber length and content on the mechanical, thermal and morphological properties of CF reinforced PA6 composites. Carbon fiber length distribution in the PA6 composites was determined by Image JÒ analyzing program. It was seen that the maximum number of fibers was observed in the range of 0–50 lm. According to mechanical test results, increasing CF content increases the tensile strength, modulus and hardness values but decreases strain at break values of composites. On the other hand, fiber length at the studied range had no effect on the tensile strength, modulus and hardness values but ascending fiber length increased the strain at break values of composites. It has been showed that Tg and Tm values of composites were not affected significantly with increasing CF content. But heat of fusion and the relative degree of crystallinity values of composites decreased with ascending CF content. It is seen in the SEM micrographs that most of the fibers were pulled out from the matrix. DMA results showed that storage modulus and loss modulus values of composites increased with increasing CF content. It was also observed from tan delta results that Tg values obtained from DMA are a few degrees higher than Tg values obtained from DSC. Consequently, it can be concluded that, initial fiber length at the studied range did not have any significant effect on the properties of composites due to the fiber breakage during processing. As a result, the process parameters should be optimized to reduce the fiber breakage and to obtain better composite properties. Acknowledgements The authors thank to AKSA Akrilik Kimya Sanayi A.S ß for providing short carbon fibers, Assoc. Prof. Dr. Yalçın Tanes for his valuable
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