Short carbon fiber reinforced polycarbonate composites

Composites: Part B 62 (2014) 230–235

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Short carbon fiber reinforced polycarbonate composites: Effects of different sizing materials Cem Ozkan a, N. Gamze Karsli b, Ayse Aytac a,b,⇑, Veli Deniz a,b a b

Department of Polymer Science and Technology, Kocaeli University, Turkey 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 30 July 2013 Received in revised form 27 February 2014 Accepted 5 March 2014 Available online 13 March 2014 Keywords: A. Carbon fiber A. Polymer–matrix composites (PMCs) B. Mechanical properties

a b s t r a c t In this paper, effects of the sizing material type and level on the mechanical, electrical and morphological properties of the short carbon fiber (CF) reinforced polycarbonate (PC) composites were investigated. Unsized CF and the CFs which were sized with epoxy/phenoxy(EPO_PHE), polyimide(PI) and phenoxy(PHE) were used as reinforcement materials. Fiber length distribution analysis indicated that sizing protected the CFs breakage into the smaller lengths during the processing. Effects of the sizing material type and level on the mechanical properties of CF reinforced PC composites were investigated by means of tensile and izod impact strength tests. Tensile test results revealed that tensile strength and modulus values of sized CF reinforced PC composites were higher than that of unsized CF reinforced PC composites. Besides, effect of the sizing material level on the tensile properties of composites changed with respect to the sizing material type. It was also found that the measured effects of the sizing agent type and level on the notched izod impact strength of composites were not so significant. In addition to this, it was found that sized CF reinforced PC composites had higher electrical conductivity values than unsized CF reinforced PC composites. Also, PHE sized CF reinforced composites had the highest electrical conductivity value among the other composites. Better interactions between EPO_PHE and PHE sized CF and PC matrix were observed from the scanning electron microscope analysis. As a result of this study, PHE and EPO_PHE sized CFs can be suggested as proper reinforcements for PC matrix. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Carbon fibers (CFs) have been widely used as reinforcement materials in composite manufacturing due to their exceptional properties such as high specific modulus, strength, stiffness, electrical properties and low density. While chemical and thermal properties of composites mainly depend on matrix materials, mechanical properties of composites such as strength depend on properties of carbon fiber and fiber/matrix interfacial adhesion strength. If there is good fiber/matrix adhesion strength, the applied load can be transferred from matrix to fiber more efficiently [1]. However, generally adhesion between carbon fiber and thermoplastic polymer matrix is poor because of the inert characteristics of fiber surface and matrix material [2,3]. Polycarbonate (PC) is one of these polymers and it is used in the short fiber reinforced advanced composites [4]. Different modification ⇑ 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). http://dx.doi.org/10.1016/j.compositesb.2014.03.002 1359-8368/Ó 2014 Elsevier Ltd. All rights reserved.

techniques have been applied to fiber surface to improve the interfacial adhesion between thermoplastic matrix and carbon fiber due to the lack of adhesion between them [1,2,4,5]. Plasma oxidation, radiation and chemical treatments are some of various methods which were applied to carbon fiber surface [6–13]. Another efficient method for fiber surface modification is sizing or coating of fiber surface with a thin polymeric layer. Sizing method prevents the fiber from the breakage during filament winding, prepreg, weaving and other composite manufacturing processes [7,8]. This method also improves the interfacial adhesion between fiber and matrix, since sizing material includes functional groups which can react with constituents of composite [2]. Besides, chemical structures of the sizing material and matrix should be similar due to the ‘similar dissolve mutually theory’ [6]. Consequently, different matrix materials require proper sizing materials and it is important to choose the ideal sizing material for polymer matrix material in order to obtain better composite properties. To our knowledge, there has been no study in which the effects of the sizing agent type and/or level on the properties of carbon fiber reinforced polycarbonate composites were investigated together. But there are a few studies which are investigated the

C. Ozkan et al. / Composites: Part B 62 (2014) 230–235

adhesion between CF and PC matrix. Danyadi et al. [4] studied the effects of four different coupling agents on the properties of CF reinforced PC composites. These coupling agents were containing epoxy, anhydride and isocyanate functional groups. They found that while the coupling agents which contain epoxy and isocyanate reacted with PC matrix, coupling agents which contain anhydride functional groups did not. Kushwaha et al. [14] studied the properties of polycarbonate composites which were reinforced with nickel coated carbon fiber. Their results showed that tensile, flexural properties and abrasion resistance of composites improved with the surface coating of fibers. Kim et al. [15] analyzed the effects of fiber length, fiber content, screw speed and fiber sizing on rheological and mechanical properties of polycarbonate/carbon fiber composites. They showed that the final fiber length of sized carbon fibers was greater than that of unsized carbon fibers after the extrusion process and they concluded that the final fiber length has a strong effect on the rheological and mechanical properties of composites. Raghavendran and Drzal [3] investigated the adhesion between PC matrix and CF on which two types of polymer was grafted to create covalent linkages. These polymers were low molecular weight PC and PMMA. They performed interfacial shear strength adhesion measurements and observed that the level of interfacial adhesion was improved by using polymer grafted carbon fiber in the composites. Their results showed that the improvement in interfacial adhesion was ranged from 20% to 80%, when polymer grafted carbon fibers were used in composites. In addition to these studies, there are some studies in the literature which investigated the effects of sizing agent properties such as molecular weight and concentration. Zhang et al. [2,16] studied the influence of different molecular weight sizing agents on the properties of carbon fibers and their composites. Their study revealed that interfacial shear strength and hydrothermal ageing decreased in the case where high and low molecular weight sizing agent was used. On the other hand, interfacial shear strength and hydrothermal ageing improved when moderate molecular weight sizing agent was used. In another study [17], the effect of sizing agent concentration on the performance of CF reinforced epoxy based composites was investigated. In this study, three levels of sizing agent concentration were studied and it had been found that the optimum level of sizing agent was 1.5 wt.%. In this study, it was aimed to investigate the effects of sizing material type and level on the properties of carbon fiber reinforced polycarbonate composites. For this purpose, unsized and CFs which were sized with three different types of sizing agent were used as the reinforcement material. Tensile test and izod impact test were carried out to investigate the effect of sizing material type and level on mechanical properties of carbon fiber reinforced PC composites. In addition to this, thermal stability of sizing materials was investigated by thermogravimetry analysis (TGA). Optical microscope analysis was performed to determine the fiber length distribution. Scanning electron microscope (SEM) was also used to analyze the fracture surfaces of composites.

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ing temperature, 100 rpm screw speed and 3 min mixing time. After the extrusion process, all compounds were molded by using a laboratory scale injection molding machine (DSM Xplore 12 ml Micro-injection Molder) with 295 °C barrel temperature, 100 °C mold temperature; and 10 bars injection pressure. Samples were burned in an ash oven for 30 min at 600 °C to investigate the fiber length distribution of composites. The residual ash was dispersed in water and then CFs were isolated from the composite. After that CFs were transferred to glass slides and images of fibers were obtained from optical microscope. These images were analyzed by using Image JÒ in order to determine the fiber length distribution. Tensile properties were investigated according to ISO-527 by using Shimadzu 100 kN model universal testing machine. Dimension of the test samples were 10 mm width, 4 mm thickness and 106 mm length. Tensile strength at yield and strain at break values of composites were determined by using 5 mm/min crosshead speed. Notched izod impact strength test was performed according to ISO 180/1A by using a Ceast machine with a 5.5 J hammer and 3.46 m/s impact velocity. After these tests, average of five measurements was reported with standard deviations. Thermogravimetric analysis (TGA) was conducted to determine the thermal stability of CFs which were unsized and 3 wt.% sized with different sizing materials at the processing temperature. Thermal analysis was performed by using a Perkin Elmer TGA Instrument. For isothermal TGA study, temperature was increased from 30 °C to 295 °C as quickly as possible and held at this temperature for 3 min under atmospheric conditions similar to conditions during composite preparation. Weight loss data at constant temperature were collected as a function of time by using special software. Electrical resistivity values of composites were measured with 2-point-probe technique by using Haoyue M890G Digital Multi Meter. For a good electrical contact in two point probe technique, copper wires were attached to both ends of the test specimen with silver paste. Resistivity measurements were performed by contacting probes with these copper wires, after the hardening of silver paste. After obtaining the resistance values of each sample, electrical conductivity values of composites were calculated [18,19] and average results of five measurements were reported for each prepared composite:

rðS=cmÞ ¼

Sample Thickness ðcmÞ Electrode Area ðcm2 Þ Resistance ðXÞ

ð1Þ

Tensile fractured surface morphology of composites was observed by using scanning electron microscope (SEM) (JEOL JSM-6510). All the sample surfaces were sputter coated with gold and palladium before the observation. 3. Results and discussion 3.1. Fiber length distribution

2. Materials and methods Polycarbonate (Wonderlite PC 110) was used as matrix material and supplied from Kempro (Istanbul). PAN-based carbon fibers (Aksaca), which were unsized and sized with three different materials, were supplied by Akkök Group (Turkey) and used as the reinforcement materials. Sizing materials were epoxy/phenoxy (EPO_PHE), phenoxy (PHE) and polyimide (PI). Level of sizing materials on CF surface were 1%, 2% and 3% by wt.%. Carbon fiber content in composites was kept constant at 30% by wt. and the initial fiber length was 6.0 mm. Composites were prepared by using a laboratory scale DSM Xplore micro-compounder at 295 °C process-

It is well known that, in the extrusion process, an excessive amount of shear stress is applied to composite during composite preparation and then composite is transferred to injection machine for molding. Meanwhile, fibers in the composite are broken and the fiber length distribution in the composite changes [20,21]. In this study, the effect of sizing material on the ultimate fiber length distribution in PC composites was investigated by using Image JÒ analyzing program. Ultimate fiber length distribution curves on number average basis for unsized and sized CF reinforced PC composites were given in Fig. 1. It can be seen from Fig. 1 that the measured ultimate fiber lengths are in the range from 25 to 500 lm. Results of the fiber length distribution analysis shows that while

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Fig. 1. Fiber length distribution of PC/CF composites.

number average fiber length is found to be 91 lm for unsized and PI sized CFs reinforced composites, it is 100 lm for EPO–PHE sized and PHE sized CFs reinforced composites. Moreover, after 50 lm fiber length, the highest number of fibers is observed for PHE sized CF reinforced composite among the other composites. In addition, in the range of 0–25 lm fiber length, while the maximum number of fiber is observed for unsized CF reinforced composites, PHE sized fibers give the lowest number of fiber. This means that unsized carbon fibers can be broken into smaller pieces than PHE sized carbon fibers; because, the bonding force between PHE sized carbon fiber and PC matrix may be higher than that of unsized carbon fibers and PC matrix. This might also be due to the protection effect of the sizing materials on fiber breakage. Similar results have been reported by Kim et al. [15]. 3.2. Tensile test of composites Effect of the sizing material type and level on tensile strength of CF reinforced PC composites were given in Fig. 2. It can be seen from Fig. 2 that addition of CF to the neat PC matrix (58.7 MPa) increases the tensile strength as expected. Besides, tensile strength values of sized carbon fiber reinforced PC composites are always higher than that of unsized CF reinforced composite. This figure also shows that PHE and EPO_PHE sized carbon fiber reinforced composites have higher tensile strength values than PI sized carbon fiber reinforced composites. Tensile strength values of the composites are affected by the fiber–matrix interaction at the interface. If interaction between fiber and matrix is poor, fibers

Fig. 2. Effect of sizing agent type and level on the tensile strength values of PC/CF composites.

easily separate and pull out from the matrix. If fibers are sized with proper sizing material which can interact with the matrix, this interaction hinders the separation and pull out of fibers from the matrix. In our study, higher tensile strength values and number average fiber lengths with PHE and EPO_PHE sized CF reinforced composites, can be attributed to the better interaction between fiber and matrix. This better interaction may result from the transesterification reactions between PC and phenoxy which takes place at the temperatures higher than 230 °C. As a result of these reactions, graft or cross linked copolymers occur and these copolymers act as a bridge between fiber surface and polymer, thus provide better stress transfer and result in better tensile properties [22,23]. Tensile modulus of composites with respect to the sizing agent type and level were given in Fig. 3. It can be seen in this figure that, tensile modulus of neat PC matrix (3.2 GPa) increases with the addition of carbon fibers as expected. The amount of increment can be estimated by the rule of mixture theory. According to this theory; modulus of a composite is given in the equation below [24]:

Ec ¼ g1 g0 V f Ef þ V m Em

ð2Þ

In this equation, g1 is the correction factor and it is used for lengths of fiber which are not fully contributing to the stiffness of the composite, g0 is the fiber orientation factor, E is the tensile modulus, V is the volume fraction and subscripts c, f and m represent composite, fiber and matrix, respectively [20,25]. In our study, fiber orientation factor g0 was taken as 1 because fibers are aligned

Fig. 3. Effect of sizing agent type and level on the modulus values of PC/CF composites.


along the flow direction during the injection molding process. Since Em, Vm, Ef and Vf values were constant and the same for all composites, tensile modulus was considered to be dependent on the change in g1 values of composites. In this study, tensile modulus values of sized carbon fiber reinforced PC composites are higher than that of unsized carbon fiber reinforced composites. This difference may be due to the lower average fiber length or g1 value of unsized CF reinforced composites. In addition, PHE and EPO_PHE sized CF reinforced PC composites have the highest tensile modulus (see Fig. 3), resulting from the highest number average fiber length or g1 value. Effects of the selected sizing material level on the properties of carbon fiber reinforced composites are different. While low level of sizing material on CFs may lead to weak coating performance, high level of sizing material may enhance the moisture adsorbing capacity of the sizing material and this makes the process ability of CFs worse. Therefore, sizing material type and level plays a key role on interfacial adhesion [17]. Results of the sizing agent level on tensile strength and modulus values of PC matrix composites were given in Figs. 2 and 3. It can be seen from these figures that, tensile strength value of EPO_PHE sized CF reinforced composites reached to the highest value at 1 wt.% sizing level, on the other hand, PHE and PI sized CF reinforced PC composites reached to the highest values at 2 wt.% sizing level. The highest modulus values were observed at 3 wt.% sizing level for all sizing material type. As seen from figures, effect of sizing material level on tensile properties of composites changes depending on the sizing material type. 3.3. Notched izod impact test of composites High impact strength properties of PC are based on the carbonate groups which exist in its structure and these groups provide high flexibility to PC. Notched izod impact strength of neat PC (11.81 kj/m2) is higher than that of its composites [26]. Generally it can be said that the addition of fiber into the ductile polymer matrix makes it brittle and this leads to decreasing the impact strength of fiber-reinforced composites [27–30]. Notched izod impact strength of sized and unsized carbon fiber reinforced polycarbonate composites was given in Fig. 4. As it can be seen from Fig. 4, notched izod impact strength value of neat PC decreases with addition of CF. Besides, while the highest notched izod impact strength value for EPO_PHE carbon fiber reinforced composites is observed at 2 wt.% sizing level, the highest notched izod impact strength value for PHE sized carbon fiber reinforced composites is observed at 1 wt.% sizing level respectively. In addition to this, notched izod impact strength values of EPO_PHE and PHE sized CF reinforced composites increased as 15% and 6%, respectively, when compared

Fig. 4. Effect of sizing agent type and level on the notched impact strength values of PC/CF composites.

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with unsized CF reinforced composite. It can be inferred from these results that the effect of sizing agent type on the notched izod impact strength of composites, is not so significant. 3.4. Thermogravimetric analysis (TGA) of carbon fibers Isothermal TGA was conducted to determine the weight loss of sizing material on the carbon fiber surface during processing. TGA thermograms of unsized and sized carbon fibers under isothermal conditions are given in Fig. 5. It can be seen from Fig. 5 that while the weight loss percentage of PI sizing material is measured as 0.1 wt.%, this value changed for EPO/PHE and PHE sizing materials as 0.8 and 0.7 wt.% respectively. In our previous study we investigated the thermal stability of these sizing materials on carbon fiber surface by using nonisothermal TGA [27]. We have found that while weight loss percentage of PI sizing was 2 wt.%, weight loss percentage of EPO/PHE and PHE sizing materials were 7 and 10 wt.% respectively until 450 °C. According to these results, it can be concluded that the weight loss during this processing, in other words, at processing temperature and for this cycle time, is negligible. It means that our processing condition has no effect on the interfacial adhesion between fiber and matrix. This result is also confirmed by mechanical test results of PC composites. Besides, it can be seen from Fig. 5 that a weight increase for unsized CF is seen at processing temperature and cycle time. This weight increase is explained in the literature as a result of the chemical reaction tendency of oxygen with aliphatic side groups on carbon fiber surface under atmospheric conditions, during processing [31]. It can be inferred from this result that the sizing process is also an efficient method for protection of carbon fiber surface from oxidation. 3.5. Electrical resistivity measurement Polycarbonate exhibits outstanding electrical insulation property and this property makes polycarbonate and its composites a prime material for electrical and electronic components. For this reason, effects of sizing agent type on the electrical properties of carbon fiber reinforced PC matrix composites were evaluated in this study. As it is known, electrical conductivity of CF reinforced composites increase with increasing amount of CF in those particular composites. Electrical behavior of polymer composites alter from insulator to semi-conductor at critical CF loading level and this point is known as ‘percolation threshold’. After this point, a continuous network of CF forms along the polymer matrix and allows the transition of the electrons from one CF to other by over-crossing the gap between fibers [26]. Electrical conductivity values of composites are given in Fig. 6. It can be seen from Fig. 6 that electrical conductivity values of composites are about 103 S/cm. This value is surprisingly higher than expected. This case may be explained as a result of the conduction mechanism of fibers which are comes into the contact with each other in the

Fig. 5. TGA curves of carbon fibers coated with different sizing agents.

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3.6. Scanning electron microscopy

Fig. 6. Electrical conductivity of the composites.

PC matrix. In other words, it can be concluded that our CF loading level (30 wt.%) is higher than percolation threshold point of PC composites [19]. We have found that sized CF reinforced PC composites have higher electrical conductivity than unsized CF reinforced PC composites. Moreover, in the case PHE sized carbon fiber, electrical conductivity values of composite reaches to the maximum; electrical conductivity values of EPO_PHE sized carbon fiber reinforced PC composites are slightly lower than that of PHE sized carbon fiber filled composites. On the other hand, PI sized carbon fiber has lower effect on the electrical conductivity when compared with EPO_PHE and PHE sized carbon fibers. Choi et al. found that while electrical conductivity of CF reinforced phenolic resin composites does not depend on the surface treatment methods, it depends upon the dispersion of carbon fibers in the matrix [19]. The bonding force between unsized CF and PC matrix was lower than that of sized CF and PC matrix. So in the case of unsized CF, phase separation and agglomeration occurs in matrix [15]. Therefore the ultimate fiber length is smaller for unsized CF reinforced composites. In our study, since EPO_PHE and PHE sized carbon fiber reinforced composites exhibit the longer ultimate fiber length distribution than other composites; electrical conductivity values of these composites were higher than that of other composites.

Interface studies were carried out to investigate CF surfaces, CFs pull out and CFs–PC interface by using SEM analyzes. SEM micrographs of the tensile fracture surfaces of unsized and sized carbon fiber reinforced composites were given in Fig. 7(a–d). It can be seen from figure that in case of unsized and PI sized carbon fiber, fiber surfaces were clean and smooth. Besides, pullout of fibers from the matrix can be seen from these figures for unsized and PI sized CF reinforced composites. This can be interpreted as an evidence for poor adhesion between unsized or PI sized carbon fiber and PC matrix. On the other hand, it can be seen from Fig. 7(b and c) that fibers are covered with a polymeric layer. This case can be attributed to the better interaction between EPO_PHE and PHE sized carbon fiber and PC matrix. This better interfacial interaction increases mechanical and electrical properties of composite which was confirmed by the results of performed tests. 4. Conclusion In this study, effect of different sizing material on the properties of CF reinforced PC composites was studied. Mechanical, electrical and morphological properties of prepared composites were investigated. Besides, thermal stability of sizing materials was determined at processing conditions by using isothermal thermogravimetric analysis (TGA). As a result of the fiber length distribution analysis, we have found that carbon fibers were protected by sizing materials during processing. Also, EPO–PHE and PHE sized CFs remained longer than unsized carbon fibers in the polymer matrix after processing. It was observed that tensile strength and modulus values of sized carbon fiber reinforced PC composites were higher than tensile strength and modulus values of unsized carbon fiber reinforced PC composites. Also, effect of sizing material level on the tensile properties of composites changed with respect to used sizing material type. Notched izod impact strength results showed that the effect of sizing agent type and level on the notched izod impact strength of composites was not so significant. Isothermal TGA analysis results showed that the weight

Fig. 7. SEM micrographs of tensile fracture surfaces of PC/CF composites, (a) unsized (1000), (b) EPO_PHE (1000), (c) PHE (1000), and (d) PI (1000).


loss amount of sizing materials was negligible and this means that sizing materials are thermally stable at the processing conditions. According to electrical resistivity test results, sized CF reinforced PC composites have higher electrical conductivity values than unsized CF reinforced PC composites. In addition, electrical conductivity value of PHE sized CF reinforced composites was the highest among the other composites. It has been seen from SEM micrographs that, while unsized and PI sized carbon fibers pulled out from the matrix, there was a better interaction between EPO_PHE and PHE sized carbon fibers and PC matrix. Thus, it can be inferred from these results that PHE and EPO_PHE sized CFs are more proper reinforcing materials for PC matrix. Acknowledgement This study supported by Ministry of Science, Industry and Technology of Turkey under the Project number 01020.STZ.21011-2. References [1] Lee JS, Kang TJ. Changes in physico-chemical and morphological properties of carbon fiber by surface treatment. Carbon 1997;35:209–16. [2] Zhang RL, Huang YD, Su D, Liu L, Tang YR. Influence of sizing molecular weight on the properties of carbon fibers and its composites. Mater Des 2012;34:649–54. [3] Raghavendran VK, Drzal LT. Fiber–matrix interfacial adhesion improvement in carbon fiber–bisphenol – a polycarbonate composites by polymer grafting. J Adhes 2002;78:895–922. [4] Danyadi L, Gulyas J, Pukanszky B. Coupling of carbon fibers to polycarbonate: surface chemistry and adhesion. Compos Interface 2003;10:61–76. [5] Tang LG, Kardos JL. A review of methods for improving the interfacial adhesion between carbon fiber and polymer matrix. Polym Compos 1997;18:100–13. [6] Liu J, Ge H, Chen J, Wang D, Liu H. The preparation of emulsion type sizing agent for carbon fiber and the properties of carbon fiber/vinyl ester resin composites. J Appl Polym Sci 2012;124:864–72. [7] Dai Z, Shi F, Zhang B, Li M, Zhang Z. Effect of sizing on carbon fiber surface properties and fibers/epoxy interfacial adhesion. Appl Surf Sci 2011;257:6980–5. [8] Luo Y, Zhao Y, Duan Y, Du S. Surface and wettability property analysis of CCF300 carbon fibers with different sizing or without sizing. Mater Des 2011;32:941–6. [9] Moran MAM, Alonso AM, Tascon JMD, Paiva MC, Bernardo CA. Effects of plasma oxidation on the surface and interfacial properties of carbon fibers/ polycarbonate composites. Carbon 2001;39:1057–68. [10] Ramanathan T, Bismarck A, Schulz E, Subramanian K. The use of a single-fiber pull-out test to investigate the influence of acidic and basic surface groups on carbon fibers on the adhesion to poly(phenylene sulfide) and matrixmorphology-dependent fracture behavior. Compos Sci Technol 2001;61:1703–10. [11] Zhao F, Huang Y. Grafting of polyhedral oligomeric silsesquioxanes on a carbon fiber surface. Novel coupling agents for fiber/polymer matrix composites. J Mater Chem 2011;21:3695–703.

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