Preparation and Performance of Short Carbon Fiber and Flake Graphene Reinforced Polycarbonate Composites: Effects of Different Tougheners

Different tougheners including methyl methacrylate-butadiene-styrene terpolymer (MBS, core-shell type), maleic anhydride (MAH) grafted ethylene-octene copolymer (EOM), and MAH grafted polyethylene wax (PEM) were investigated for toughening the polycarbonate (PC) composites reinforced by short carbon fiber (SCF) and flake graphene (FG). The effects of tougheners on the preparation, thermal conductivity and mechanical properties of PC composites were studied. Scanning electron microscopy was used for characterizing the impact fracture surfaces of the composites. The results showed that introducing tougheners into the carbon reinforced PC composites was beneficial to improving the processability, and PEM was more effective than EOM and MBS. Meanwhile, the through-thickness and the in-plan thermal conductivity decreased to some degree due to the isolated island effects of tougheners. Moreover, the brittle PC composites with high flexural stress could be easily toughened by tougheners. In contrast, PEM had better toughening function than EOM and MBS, and correspondingly, the stiffness of the composites was the lowest for the PEM toughened systems. The fractography revealed that dense and uniformly distributed carbon fillers dispersed in matrix PC and circular cavities coexisted in the composites. The naked fiber length gradually increased as the ductility of composite materials improved.


Introduction
Carbon materials, such as carbon fiber, carbon nanotube, graphene, etc., have been commonly studied for enhancing the mechanical, electrical and thermal properties of thermoplastic or thermosetting resins. These modified composites generally exhibit low thermal expansion, light-weighting, good heat dissipation, high stiffness, and can be potentially applied in integrated circuits, satellite devices, electrnonics packaging and encapsulation, and thermal management fields where higher thermal conductivity required [1] [2] [3] [4] [5].
Generally speaking, factors determining the thermal and mechanical properties of composites include the surface treatment and interfacial structure, alignment and packing structure, aspect ratio, volume fraction, size, shape, purity, polydispersity and intrinsic conductivity of fillers, etc. [6] [7] [8] [9]. However, the surface of pristine carbon materials is non-polar and chemically inert. As a result, the interfacial force between short carbon fibers and polymer matrix is quite weak, which tends to decrease the thermal and mechanical properties of composites. As a result, filler modification, matrix modification and interfacial modification approached were developed and found to effective in remarkably improving the properties of polymer composites [10] [11] [12]. At the same time, heterogeneous or hybrid fillers have been demonstrated as another effective method for enhancing the thermal and mechanical properties of polymer composites [6] [10] [13]. Two or more fillers with different size, shape showed a positive synergic effect on thermal conductivity and mechanical strength of composites [14] [15] [16]. Polycarbonate-based thermoplastic composites have been prepared and studied using different carbon materials as modifying fillers [17] [18] [19]. However, they usually showed low impact strength and obviously brittle behaviors [15] [16] [19]. In this work, MBS, EOM and PEM were used for toughening the PC composites reinforced by SCF and FG. The preparation, thermal conductivity, mechanical properties, and fracture mechanism of PC composites were studied.

Preparation of Polycarbonate Composites
First, confirm that you have the correct template for your paper size. This template has been tailored for output on the custom paper size (21 cm * 28.5 cm   the PE wax as toughening agent. In fact, PE wax is typically applied as a compatilizer or surface modifier in the composite materials. In this work, we found that PE was can effectively improve the processability of carbon materials filled PC composites. The lower parameter values of PE was system relative to EO copolymer system was mainly due to the difference in the melting points of PE wax and EO copolymer. The lower the melting temperature of toughener, the lower the parameter values, and the easier the composite preparation process. Moreover, a slightly reduced parameter values of MBS system as compared to the control case are probably due to the poor heat resistance properties of MBS.

Thermal Conductivity of PC Composites
For a polymer composite material to possess good thermal conductive properties, it needs an effective thermal conductive paths and network in its interior structures. The through-thickness and in-plane thermal conductivity data of the PC composites are shown in Figure 3. The values of the control sample filled by SCF and FG were 0.61 and 3.08 W/mK, respectively. The through-thickness thermal conductivity is much lower than the in-plane thermal conductivity. This indicates that PC composite is an anisotropic thermal conductive material. The key reason is that carbon fibers successively arrange along the flow direction, this is beneficial to the in-plane conductivity and leading to a decreased probability of inter-linking or contacting in through-thickness direction. Even though a certain amount of flake graphene is applied and filled into carbon fibers, the contribution to the through-thickness conductivity is insufficient [15]. When different tougheners are used, not only the through-thickness thermal conductivity but also the in-plan thermal conductivity decreased to some degree relative to the control case. This is largely attributed to the incompatibility between PC and tougheners. And the latter disperses in the PC matrix in the form of isolated islands and disrupts heat transfer process. Note that the PEM toughening system, it showed a relative higher through-thickness thermal conductivity than MBS and EOM, this is due to more insequent and transverse carbon fibers arrange in the matrix material and improve the through-thickness transmission capability. This will be discussed later.  Figure 4 shows the Charpy notched impact strengths of different PC composites with or without tougheners. As for the control sample, the impact strength is about 4.82 kJ/m 2 . In comparison, as different tougheners are added to the composites, the impact strength properties were clearly improved. Among them, the PEM was most effective in enhancing impact strength, followed by EOM and then by MBS. The corresponding impact strengths were increased by 63.3%, 25.7% and 7.7%, respectively. As shown in Figure 5, the flexural stress of the control sample increased linearly with the strain. The maximum stress reached 134.0 MPa when the strain at failure was 1.7%, and it showed an obviously brittle fracture mode. As for the MBS toughened system, as the strain increased, the flexural stress increase was slightly lower than the control sample, the maximum stress was 117.3 MPa. While the MAH grafted polyolefin tougheners were introduced, the PC composites exhibited a classical ductile bending deformation behavior. There existed an evident plateau region, that is, the stress remained constant as the strain increased. In comparison to EOM system having a maximum flexural stress of 71.8 MPa, the more ductile PEM system displayed a lower maximum stress of only 61.8 MPa. However, a lowest strain at maximum stress for the PE wax system is probably caused by its low molecular weights and weak deformation ability and orientation under external forces.

Fractography
In order to explain the structure-property relationship observed above, Figure 6 shows the fractopraphic images of PC composites with or without toughening modification by observing fractural surface after impact tests. As can be seen in Figure 6(a), the carbon fibers dispersed uniformly in the PC matrix. And the fracture surface of the control sample was relative flat, indicating relatively brittle fracture mode. Besides, there existed many circular cavities, which could be resulted from fiber breaking or peeling from matrix under the external force [16]. However, the fracture surfaces of different toughener systems showed distinct morphologies from the control case in addition to cavitation. The length of the naked carbon fibers gradually increased when toughener changed from MBS, to EOM, and subsequently to PEM (Figures 6(b)-(d)). This can be explained by the decreased torque values of the toughened PC composite systems during extrusion. The smaller torque means lower shearing forces. Consequently, the damage to carbon fibers was reduced and the fibers kept relatively longer length. Massive longer fibers breaking and peeling from matrix inevitably absorbed more impact energy, and as a result, it displayed a correspondingly ductile fracture mode. Furthermore, Figure 6(d) also shows many insequent and transverse carbon fibers orientation on the surface of PE wax toughening system, which identified the enhanced through-thickness thermal conductivity.

Conclusion
Core-shell type and grafting type tougheners were applied for enhancing the