# Carbon Fiber Papers Prepared by Wet-Laid Technique Using PVB/PF Composite Fibers as the Binders
## 3 Results and Discussion  
### PVB/PF Composite Fibers  
PVB/PF composite fibers with different compositions were prepared by a dry spinning method. The spinnabilities of these dopes are shown in Table 1. In these samples, with the increase of the PF content, the dopes gradually lose their spinnability due to the PF's low molecular weight and brittleness. When the mass ratio of PVB and PF reached 5:5, the spinnability was rather poor, but after reducing the dope velocity to 0.1 mL/min and enhancing the spinning temperature to 320 \({}^{\circ}\)C, the dopes regained good spinnability. When the content of PF was more than 60 wt%, the dopes were hardly spinnable under the existing equipment and conditions. Figure 2 shows the TGA curves of PVB and PF. It can be seen that the residual carbon rate of PVB was very low (1.41%), which meant PVB merely played the role of binding during the papermaking process, and its residual carbon effect on connecting CFs, was negligible after heat treatment. On the contrary, the PF had a relatively higher carbon residual rate (33.12%), indicating the residual carbon from PF provided the primary binding structure, ensuring the strength of CFP after heat treatment. Therefore, more PF is expected in the mat to increase the content of residual carbon after heat treatment, which can improve the strength of CPF. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Evaluation of Spinnability** & **Evaluation of Spinnability** \\
**PVB:PF** & **(Spinning Temperature of** & **(Spinning Temperature of** \\  & **260 \({}^{\circ}\)C)** & **320 \({}^{\circ}\)C)** \\ \hline
10:0 & 5 & 5 \\
9:1 & 5 & 5 \\
8:2 & 4 & 5 \\
7:3 & 4 & 5 \\
6:4 & 3 & 4 \\
5:5 & 2 & 4 \\
4:6 & 1 & 1 \\ \hline \hline \end{tabular} The spinnabilities of these dopes were classified into five levels under optimized stable conditions. Levels 5 to 2 meant the spinnabilities were excellent, good, fair, and poor, respectively. The numbers of fibers’ breaking during spinning within 12 h were 0, 1 to 3, 4 to 8, and more than 8, respectively. Level 1 meant it was not spinnable.  
\end{table} 
Table 1: Spinnabilities of PVB/PF ethanol dopes with different compositions. Figure 2: TGA curves of PVB and PF. Figure 3 shows the macroscopic and microcosmic morphologies of PVB/PF composite fibers with a mass ratio of 5:5. The continuous golden PVB/PF filaments (more than 2000 m) indicated the good spinnability of this dope (Figure 3a). It can be seen in Figure 3b that the diameter of the prepared fiber was about 80 \(\upmu\)m, its surface was very smooth, and there was no obvious phase separation under the naked eye. In order to investigate the phase structure in the composites, PVB/PF fibers were immersed in ethylene glycol, which can dissolve PF for 48 h to etch the composite fibers. After etching, many concave structures evenly appeared on the surface (Figure 3c), indicating the PF was uniformly distributed in the PVB matrix. On the cross-section of etched fiber, honeycomb structures can be observed because of the removal of PF (Figure 3d), which further confirmed that in the PVB/PF composite fibers, PVB existed as a continuous phase, while PF as dispersion phases with a size of about 2-3 \(\upmu\)m.  
The interaction and miscibility between PF and PVB were also investigated by DSC and FTIR. Figure 4 shows the DSC curves of PVB/PF composite fibers as a function of composition. The T\({}_{\text{g}}\)s of pure PVB and PF were 68.41 \({}^{\circ}\)C and 33.05 \({}^{\circ}\)C, respectively. For the PVB/PF composite fibers, all of the samples exhibited a single T\({}_{\text{g}}\) lying between the glass transition temperatures of the pure components, indicating that PVB and PF were miscible to some degree. Meanwhile, the T\({}_{\text{g}}\) shifted gradually to a lower temperature with the increase of PF content. The T\({}_{\text{g}}\) deviation of the fibers reflected the change of inter-association strength that depended on the number of functional groups in the composites.