Carbon Fiber Papers Prepared by Wet-Laid Technique Using PVB/PF Composite Fibers as the Binders

Abstract

Carbon fiber paper (CFP) is one of the most important units of gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). The binder used in the wet-laid technique has a significant effect on the properties of CFP. In this work, the polyvinyl butyral/phenol-formaldehyde resin (PVB/PF) composite fibers firstly prepared by a dry spinning method were applied for CFP fabrication to replace traditional binders during the papermaking process and remove the PF impregnation process. In the composite fibers with a mass ratio of 5:5, PF phase with a size of about 2~3 μm evenly distributed in PVB matrix. PVB and PF were miscible to some degree, which was beneficial for their binding effect during hot-press. These composite fibers can successfully bind carbon fibers (CFs) during the papermaking process, and their residual carbon efficiently welded the CFs after heat treatment. The content and length of composite fibers in the mat affected the binding structure among CFs, which influenced the properties of CFP, increased the composite fibers’ content and reduced their length, significantly improving the strength of CFP. Therefore, the application of this solid fiber binder could enhance the comprehensive properties of CFP by adjusting the fibers’ parameters in the mat and also make the fabrication of CFP more environmentally friendly and low-cost.

1. Introduction

Carbon fiber paper (CFP), with the properties of high porosity, good electrical conductivity, excellent mechanical strength, outstanding corrosion resistance, etc. [1], has become the ideal material as a gas diffusion layer (GDL) of proton exchange membrane fuel cells (PEMFCs) and provides reactant and product permeability, transport of electrons, and heat and support of membrane electrode assembly (MEA) [2,3,4].

The first commercially available CFP was produced by the wet-laid technique, and the process normally consisted of four steps: papermaking, impregnation of phenol-formaldehyde resin (PF), compression molding, and heat treatment. In the first step, binders like polyvinyl alcohol (PVA) and polyvinyl butyral (PVB) are dispersed in water with the chopped carbon fibers (CFs). These binders are also required to absorb more PF during the impregnation process to increase the residual carbon content after carbonization and finally improve the CFP’s tensile strength, electrical properties, etc. Typically, the mass ratio of PVA fibers and CFs in the dispersion is fixed to 1~1.5:10 [5,6], for a low addition of PVA may not play the role of binding CFs and absorbing much PF, while too much PVA will lead to floccules. However, these binders should be dissolved in some solvents to obtain dispersion with chopped CFs for papermaking, which causes severe environmental problems. PVA fibers are in a half-dissolved state in water, so the addition of PVA in the mats after filtration is relatively low and uncontrollable. Meantime, the residual PVA in the water causes serious water pollution and huge water-treatment cost. The dispersion then goes through a wire screen to remove the water and the resultant mats are dried to absorb more PF. The subsequent PF impregnation is usually processed in a PF/ethanol solution and always leads to an uncontrollable distribution of resin and uniformity of mats due to the gravity of fluid during the drying process. After impregnation with PF, the mats are heated to approximately 150 °C for solvent evaporation. The solvents are difficult to recycle, resulting in an unfriendly operating environment. For further absorption of PF to achieve better strength of CFP, the mats are performed with impregnation and compression molding process several times, which costs a lot of time and energy [7].

In recent years, many efforts have focused on improving the CFP’s properties rather than the green and low-cost manufacturing techniques, including product permeability, mechanical properties, electrical conductivity, etc. [8,9,10,11,12,13]. Functional components like graphite [6,14], carbon nanotubes (CNTs) [15,16,17,18,19,20,21], and carbon black (CB) [22] were introduced into the CFP in the process of PF impregnation. However, the increase of one property always accompanies sacrificing others in these methods, and these processes are much more complicated. Considering the wet-laid technique is still the most mainstream method, green manufacturing approaches, for example, to reduce water pollution and simplify the preparation process to lower production costs, are urgently needed. In order to address this issue, some bio-materials were developed as substitutes for CFs and traditional binders. For example, Dang et al. used natural wood as the carbon substrate precursor. The unique, accessible perpendicular channels and the presence of a microporous layer of this carbon substrate after heat treatment provided a possibility for practical application [23]. Similarly, inexpensive, natural fiber-based papers and fabric selected as the carbon substrates by Leonard et al. also showed the potential to lower the cost of CFP making [24]. As for binders, cellulose fibers [25,26], the most abundant renewable material, as a substitution for conventional binders, were reported without using polluting, volatile organic compounds. As an alternative to PF, a sucrose aqueous solution [27] was used during the impregnation process instead of PF alcohol solution to lower the carbonization cost and reduce the pollution of the environment. Nevertheless, natural materials usually have a very low residual carbon rate which limits the properties of CFP. As far as low-cost manufacturing is concerned, some fabricating methods have also been proposed. For instance, CFP could be prepared by a dry-laying method of CFs and expanded graphite in the PF without water using [28] or by a molding method [29,30]. However, the uniformity of these prepared CFPs is hard to control, and the properties are unsatisfactory.

In this work, PVB, traditionally used as the binder in the wet-laid papermaking process, and PF as the precursor of residual carbon in the resin impregnation process, were combined as the solid binder to prepare the CFP. The binders were insoluble in water, so that water pollution could be reduced. The PF impregnation process was removed to provide a greener operating environment, and the uniformity of CFP was improved without fluid impregnation. The solid binders of PVB/PF composite fibers were prepared by a dry spinning method, and the CFP using these binders was fabricated by the wet-laid technique. The effects of the content and length of composite fibers on the properties of CFP were discussed.

2. Materials and Methods

2.1. Materials

PVB (aviation class), polyethylene oxide (PEO, AR), and ethanol (99.7%) were supplied by Sinopharm Chemical Reagent (Shanghai, China). Novolak-type PF was obtained from Zhengzhou Hongxing Chemical (Henan, China). Ethylene glycol (99%) was provided by Kunshan Jinke Microelectronics Material (Jiangsu, China). CFs with a length of 6 mm were directly purchased from Sinopec Shanghai Petrochemical (Shanghai, China).

2.2. Methods

2.2.1. Preparation of PVB/PF Composite Fibers

PVB/PF composite fibers were obtained by a dry spinning method. PVB/PF spinning dopes were prepared by dissolving PVB and PF in ethanol at 55 °C, and the mass fraction of PVB and PF in the spinning dopes was 50%. The spinning dopes were extruded from a spinneret and then went through a stainless-steel pipeline (inner diameter = 12 cm and length = 1 m) to form the fibers, and the air in the pipeline was heated to 260~320 °C. Then, the fibers went into a water bath at 25 °C and were cooled to prevent them from sticking, and were then rolled up. The spinneret was 50 holes with 200 μm in diameter. The dope velocity was 0.1~0.2 mL/min, and the roller winding speed was 1.5~3 m/min. Six samples with different ratios of PVB and PF (10:0~5:5) were obtained, and resultant fibers were cut into lengths of 2 mm, 5 mm, and 8 mm, respectively, for further use.

2.2.2. Preparation of CFP Using PVB/PF as Binder

First, PVB/PF fibers and chopped CFs (6 mm in length) were mixed in a 2 L aqueous solution of 0.1 wt% PEO and stirred at 3000 r/min to form a homogenous slurry (25 °C). The total fiber amount was fixed to 2 g. The slurry was then filtered throughout a round copper mesh mold (22 cm in diameter and 75 μm in pore size) to form a mat. The resultant mat was dried at 40 °C for 24 h. The PVB/PF fiber/carbon fiber mats prepared via the wet-laid technique contained 22, 29, 36, 41, and 45 wt% PVB/PF fibers, respectively. Second, the fiber mat was then compression molded between two flat steel molds in air, and the temperature, time, and pressure were 180 °C, 10 min, and 20 MPa, respectively. Third, the hot-pressed mat was then heat treated by carbonization and subsequent graphitization under a nitrogen atmosphere. The conditions are shown in Figure 1.

2.2.3. Characterization

The morphologies of PVB/PF composite fibers and CFP were characterized by scanning electron microscope (SEM, JEOL, JSM-IT300, Tokyo, Japan). All samples were dried at 60 °C for 2 h. After that, the samples sprayed with gold were used for the SEM experiment. The functional group information of PVB, PF, and the composite fibers and interactions between PVB and PF were obtained by Fourier-transform infrared spectroscopy (FTIR, Thermo Fisher Scientific, Nicolet iS5, Waltham, MA, USA). Samples were thoroughly mixed with KBr in an agate mortar to obtain a mixture with a ratio of 1:99 (composite fiber/KBr). Then, discs for recording the spectra were prepared with the resultant mixture in a hydraulic press. The infrared spectrum range was 550~4000 cm^–1 with 32 scans and a resolution of 4 cm⁻¹ under a nitrogen atmosphere. Thermogravimetric analyses (TGA, TA, Q5000IR, New Castle, DE, USA) of the samples were performed in N₂ flow with a heating rate of 10 °C/min from room temperature up to 700 °C. T_gs of the PVB/PF composite fibers were determined by differential scanning calorimetry (DSC, Mettler-Toledo, Q20, Columbus, OH, USA), and the scan rate was 5 °C/min within a temperature range of 20 to 150 °C. The thermal shrinkage behavior of PVB/PF composite fibers was observed on a hot plate microscope (Guangzhou Weizhi Information Technology, US300, Guangzhou, China). All samples were heated from 20 °C to 200 °C at a heating rate of 10 °C/min.

As for the properties of CFP, the tensile strength was measured by a universal tensile testing machine (Instron, Instron5969, Boston, MA, USA). The samples were cut into 1 cm × 3 cm rectangular sheets for the strength test, and the loading rate was 5 mm/min. The resistivity was determined by the Four-probe Tester (Guangzhou fourth probe Technology, RTS-8, Guangzhou, China). The average pore size and air permeability were measured by a microfiltration membrane pore size analyzer (Gaoqian Functional Materials, PSDA-30M, Nanjing, China) with a test area of 0.79 cm² and nitrogen atmosphere.

3. Results and Discussion

3.1. 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. Spinnabilities of PVB/PF ethanol dopes with different compositions.

PVB:PF	Evaluation of Spinnability (Spinning Temperature of 260 °C)	Evaluation of Spinnability (Spinning Temperature of 320 °C)
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

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.

. 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 °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.

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 μm, 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 μm.

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_gs of pure PVB and PF were 68.41 °C and 33.05 °C, respectively. For the PVB/PF composite fibers, all of the samples exhibited a single T_g lying between the glass transition temperatures of the pure components, indicating that PVB and PF were miscible to some degree. Meanwhile, the T_g shifted gradually to a lower temperature with the increase of PF content. The T_g deviation of the fibers reflected the change of inter-association strength that depended on the number of functional groups in the composites.

In order to further explore the interaction between PVF and PF, the IR spectra of PVB, PF and PVB/PF composites were analyzed, respectively. As can be seen in Figure 5a, the peaks appearing at 3430 cm⁻¹, 1740 cm⁻¹, and 1130 cm⁻¹ were characteristic peaks of PVB, which were ascribed to a hydroxyl group (OH), carbonyl group double bond (C=O) and ether bond (C–O), respectively, while the peaks within the range of 2870~2970 cm⁻¹ were related to the aliphatic groups in the polymer. As for PF, the peaks of OH, carbon–carbon double bond (C=C) and a methylene group (CH₂) were observed at 3320 cm⁻¹, 1600 cm⁻¹ and in the range of 1360~1460 cm⁻¹. The peaks of C–O were observed at 1075 cm⁻¹ and 1243 cm⁻¹. Meanwhile, the infrared peaks of PVB/PF composite fibers (mass ratio of 5:5) were the superposition of the infrared peaks of pure PVB and pure PR, indicating that no chemical bond was formed between PVB and PF. However, it can be seen in Figure 5b with the increase of PF’s content, the hydroxyl vibration absorption peak gradually turned wider and shifted from 3430 cm⁻¹ to 3382 cm⁻¹, which indicated the formation of hydrogen binding between PVB and PF and its force was gradually increasing.

3.2. Binding Behavior of PVB/PF Composite Fibers

In order to evaluate the adhesive properties of the binders, the binding behavior of the PVB/PF fibers (mass ratio of 5:5) was characterized by a hot-stage microscope technique (Figure 6). As the temperature increased to 105 °C, the fibers were softened and shrunk by 23% in the direction of length. At 150 °C, the fibers significantly shrunk and accumulated towards the cross point of the two bundles of CFs due to surface tension. When they were heated to 170~190 °C, the PVB/PF binders completely changed from solid to liquid and wrapped the cross point of the two bundles of CFs, exhibiting good wettability to the carbon fiber and playing a significant role in binding. At the same time, there was no obvious phase separation in the whole heating process due to good miscibility between PVB and PF, which was favorable for improving the binding efficiency between CFs.

A hot-pressed mat with 41 wt% PVB/PF fibers (2 mm in length, mass ratio of 5:5) was prepared to track the binding structure. It can be seen in Figure 7a that the cross points among CFs (circled in Figure 7) were bound by the PVB/PF after the hot-press, and the composites were concentrated at the cross points between CFs, which indicated that this kind of structure could improve the strength of the mat while not sacrificing the porosity. In the process of heat treatment, the removal of non-carbon elements of binders was mainly in the form of gas. Finally, the residual carbon from the binder was gained. Figure 7b shows the morphology of CFP prepared using this mat. It can be clearly seen that most of the cross points between CFs were bound, and some pores were blocked by the residual carbon. This is mainly because too many and shorter PVB/PF fiber binders were added in the preparation process.

Figure 8 shows the tensile strength of hot-pressed mat with different lengths and content of PVB/PF composite fibers (mass ratio of 5:5), and it has important significance for subsequent processing and continuity of the production technology. When the length of the binder fibers was the same, the tensile strength of the hot-pressed mat increased with the content of binders due to more CFs being bound by the PVB/PF. When the length of binder was 2 mm, the tensile strength of hot-pressed mat with 45%wt binders was 39.3 MPa, which was 11.2 times the that of the hot-pressed mat with 22 wt% binders. While the length of the binder was fixed to 8 mm, the tensile strength of the hot-pressed mat with 45%wt binders was 18.6 MPa, which was 1140% higher than that of the hot-pressed mat with 22 wt% binders. The length of PVB/PF fiber also affected the strength. As the length increased, the strength of the hot-pressed mat tended to decrease under the same additional amount. For example, when the binder addition was 45 wt%, the tensile strength of the hot-pressed mat using 2 mm fiber binders was 1.15 times and 2.1 times that of the hot-pressed mat using 5 mm and 8 mm fiber binders, respectively. This is because when the content of binders is the same, the use of shorter fiber binders means the number of bound CFs is larger, resulting in the enhancement of strength.

3.3. 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 mΩ·cm, which was lower than that of CFP prepared with a mat using fiber binders in lengths of 5 mm (20.8 mΩ·cm) or 8 mm (13.7 mΩ·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 mΩ·cm, lower than that of CFP prepared with a mat using 41 wt% (12.1 mΩ·cm) or 36 wt% (15.6 mΩ·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 (Figure 9b and Figure 10).

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. However, due to the melting of longer fiber binders, which participate more in the connection of the mat network and their movement along CFs during hot-press, the space where the fiber binder previously existed was largely vacated. Thus, longer fiber binders in the mat led to larger average pore size and higher air permeability of CFP, and this corresponded to the measured average pore size (27, 28, and 30 μm with binders in lengths of 2, 5, and 8 mm) and air permeability (927, 1012, and 1089 mL·mm·cm⁻²·hr⁻¹·mmAq⁻¹ with binders in lengths of 2, 5, and 8 mm) data of CFPs prepared with a mat using 41 wt% binders in Figure 9c,d. Meanwhile, under the same length of fiber binders, the average pore size of CFP decreased with the content of the binder. As the length was 2 mm, the average pore size of CFP prepared using a mat with 36 wt% binder was 29 μm, and it turned to 28 μm with 41 wt% content and further decreased to 25 μm under the content of 45 wt%. The air permeability of CFP prepared using a mat with a 2 mm-long binder also showed a downward trend (1050, 927, and 789 mL·mm·cm⁻²·hr⁻¹·mmAq⁻¹ with 36, 41 and 45 wt%, respectively). This is because that more residual carbon obtained from more content of fiber binders is easier to occupy more gaps among CFs and block more pore structures (Figure 10c,d), resulting in a smaller average pore size which further made the air permeability lower. Some properties of prepared CFPs and commercial CFPs are listed in

Table 2. Properties of prepared CFPs and commercial CFPs.

Properties	Strength (Mpa)	Resistivity (mΩ·cm)	Average Pore Size (um)	Air Permeability (mL·mm·cm−2·hr−1·mmAq−1)
Commercial (EP40)	11.1	5.9	34	618
Sample 1 Sample 2	8.1 4.3	11.1 12.1	25 24	789 985

Sample 1 and 2 represents CFPs prepared using mats with 45 wt% binders in lengths of 2 mm and 5 mm, respectively.

. Samples 1 and 2 have higher air permeability and resistivity than commercial CFPs, and lower strength. As for CFPs used for PEMFCs, the requirements of comprehensive performance are relatively high than single properties. CFPs prepared by a wet-laid technique using PVB/PF composite fibers as the binders in this study can meet the application through adjustment of PVB/PF fiber parameters.

4. Conclusions

A solid PVB/PF fiber binder, applied to replace the traditional binders in the papermaking process and remove the process of PF impregnation for CFP fabrication, was successfully developed. The diameter of the PVB/PF fiber with a mass ratio of 5:5 was about 80 μm. PVB existed as a continuous phase, while PF existed as a dispersion phase with a size of about 2~3 μm. The shrinkage caused by melting made this binder move along CFs and accumulate around the cross points of CFs during compression molding, and there was no obvious phase separation in the whole heating process. This behavior was favorable for efficient binding between CFs, which further improved the properties of CFP after heat treatment. The length and content of PVB/PF fibers in the mat had a very significant influence on the binding structure among CFs which further affected the CFP’s properties. Results showed that the properties of CFPs can be controlled by adjusting the binder’s parameters. This paper provides a novel method to prepare CFP with excellent properties of being low in cost and environmentally friendly.