# Carbon Fiber Papers Prepared by Wet-Laid Technique Using PVB/PF Composite Fibers as the Binders
## 3 Results and Discussion
### Effect of PVB/PF Composite Fiber Parameters on the Properties of CFP  
Figure 9 shows the properties of CFPs prepared using mats with different lengths and content of PVB/PF fibers (mass ratio of 5.5). The variation of the tensile strength of CFP is illustrated in Figure 9a. It was observed that the strength of CFP decreased with the prolonger of fiber binders under the same content. As the binder addition was fixed to 45 wt%, CFP prepared using a mat with 8 mm-long fiber binders exhibited a strength of 2.7 MPa, which was lower than the strength of CFP prepared with a mat using fiber binders in lengths of 5 mm (4.3 MPa) or 8 mm (8.1 MPa). On the other hand, the tensile strength of CFP always increased with the fiber binder content regardless of the length of the fibers. When using 2 mm-long fiber binders, the strength of CFP prepared using mats with 45 wt% binders reached 8.1 MPa, which was much higher than that of CFP prepared with a mat using 41 wt% (5.2 MPa) or 36 wt% (0.9 MPa) fiber binders. The tensile strength of used carbon fiber was 3000-3600 MPa which was much higher than that of CFPs. This is because the overall mechanical properties depended not only on the properties of the carbon fiber itself but also on the contacting area between CFs and residual carbon and the number of binding points. As can be explained in Figure 10a-c, when the content of the binder is the same, the longer the PVB/PF fibers are, the larger the contacting area between CFs and residual is, and the smaller number of the connecting points among CFs is. Under the combined influence of these factors, the strength decreased as the fiber length increased. From Figure 10c,d, as the length of fiber binders is fixed when the binder content is low, a large number of CFs' cross points are not bound, and the binding areas are very small. When the CFP is subjected to external forces, the stress cannot be fully transferred between CFs, leading to a low tensile strength. With the increase of binder content, more and more CFs were bound together, and overall tensile strength was enhanced. As is shown in Figure 9b, the effect of fiber binder parameters on resistivity is exactly the opposite of their effect on strength. When the addition of binder was 41 wt%, the resistivity of CFP prepared using a mat with 2 mm-long fiber binders was 12.1 m\(\Omega\cdot\)cm, which was lower than that of CFP prepared with a mat using fiber binders in lengths of 5 mm (20.8 m\(\Omega\cdot\)cm) or 8 mm (13.7 m\(\Omega\cdot\)cm). As the length was fixed to 2 mm, the resistivity of CFP prepared using a mat with 45 wt% fiber binder was 11.1 m\(\Omega\cdot\)cm, lower than that of CFP prepared with a mat using 41 wt% (12.1 m\(\Omega\cdot\)cm) or 36 wt% (15.6 m\(\Omega\cdot\)cm) fiber binders. For CFP, the electrical properties are dependent on the conductive network, which is formed by CFs and residual carbon. Therefore, the use of shorter or more content of fiber binders in the mat decreased the resistivity of CFP by providing more binding points between CFs, which helped build a more complete and dense conductive network (Figures 9b and 10). Figure 8: Strength of hot-pressed mat with different lengths and content of PVB/PF fibers. Figure 10: The binding effects of composite fiber binders of different parameters: a smaller number of (**a**) 8 mm-long fibers, (**b**) 5 mm-long fibers, (**c**) 2 mm-long fibers, and (**d**) a larger number of 2 mm-long fibers. Average pore size and air permeability are the critical properties of CFP, and they tend to be positively correlated. Figure 9c,d shows the two properties of CFP prepared with various fiber binder parameters. It can be seen that the average pore size and air permeability of CFP became larger with the prolonger fiber binders under the same content. When the addition of fiber binder in the mat was constant, the binding area was supposed to be the same.