Keywords

1 Introduction

With good corrosion resistance, low thickness and low fabrication cost, composite graphite bipolar plates (CGPs) have good potential for application in proton exchange membrane fuel cells. However, due to its poor electrical and mechanical properties and long production cycle, CGPs have not yet been wildly adopted.

Those CGPs prepared based on expanded graphite plates combined with impregnation progress tend to form a better conductive network between graphite particles, which in turn leads to higher electrical conductivity [1]. In addition, impregnation would reduce the porosity within the CGPs, which in turn improves the mechanical strength and air tightness of the CGPs [2]. However, the key for improving the air tightness and mechanical properties of CGPs through the impregnation process is that the graphite plates were filled by resin adequately. A number of methods have been developed to enhance the resin filling efficiency, mainly by reducing the viscosity of the impregnating solution [3] and coupling the impregnation and molding process [4, 5]. However, the factors influencing the flow behavior of resin during impregnation and the resin filling efficiency have not been fully discussed yet.

In this study, graphite plates with different pore characteristics were prepared, and the penetration rate and upper limits of resin penetration into these graphite plates by various types of resins were measured. Based on Darcy’s law and boundary layer theory, the influence of pore structure and resin viscosity on the resin penetration process was analyzed.

2 Experiment

The graphite plates were produced by molding the mixture of flake graphite (median particle size of 21.4 μm) and expanded graphite (300 times expansion) with a mass ratio of 4:1 and pure expanded graphite, respectively, and the molding pressure was 80 MPa. These graphite plates were named as flake graphite plate (FGP) and expand graphite plate (EGP). To investigate the effect of the viscosity of the resin on the impregnation process, epoxy resin solution and acrylic acid-based solution were prepared separately. Among them, the epoxy resin was 6002-bisphenol A type epoxy resin (purchased from Sinopharm), abbreviated as EP; with glycidyl methacrylate (purchased from Sinopharm) as diluent, abbreviated as GMA; while the acrylic resin used DB41812 type microporous filler (purchased from Huizhou Docbond New Material Co., Ltd.), abbreviated as AR.

The pores size within the graphite plates were tested using the mercury-pressure method according to Mauran [6]. The viscosity of the resin solution was tested using a rotational viscometer. The surface tension of the resin solution and its contact angle to the graphite plate were measured using a dynamic surface tensiometer (DCAT-25, DataPhysics Instruments GmbH). The resin penetration rate was recorded using the mechanism shown in Fig. 1(a), moving the liquid pool until the liquid surface was in horizontal contact with the lower surface of the graphite plate, and recording the mass growth rate of the graphite plate using a microgram balance. The size of graphite plates was 30 * 15 * 0.65 mm. The viscosity, contact angle and penetration rate of EP were tested at 20 °C and 120 °C, respectively. In addition, the penetration rate of deionized water at 20 °C and 75 °C was also recorded.

Fig. 1.
figure 1

Schematic diagram of resin penetration process into graphite plate: (a) resin penetration process testing mechanism; (b) resin penetration into the pores inside the graphite plate; (c) the effect of capillary force and boundary layer on the penetration process.

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3 Result and Discussion

3.1 Pore Characteristics of Graphite Plates and Resin Penetration Behavior

The test results of the pore size distribution of FGP and EGP based on the mercury-pressure method were shown in Fig. 2. The pores within the graphite plates were mainly distributed between 27–700 μm and 5 nm–4.2 μm. EGP exhibited higher porosity and was mainly reflected in those pores with the size less than 1 μm.

Fig. 2.
figure 2

The pore size distribution of FGP and EGP

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The penetration rates of different liquids at different temperatures and their physical parameters were illustrated in Fig. 3, where the rapid rise in the initial stage was related to the surface tension of the resin liquid surface. As the temperature increases, the viscosity and surface tension of the EP decreased significantly, while its penetration rate and upper limit mass of saturation absorption were increased. When 15 wt% GMA was added as diluent, its viscosity decreased from 63.28 to 35.66 mPa s, however, the contact angle with graphite changed to 96.12°, while EP was unable to penetrate into the graphite plates. The viscosity of AR was 7.5 mPa s and exhibited better wettability to graphite, showing higher penetration rate and higher upper limit mass of saturation absorption, for FGP reaching saturation absorption of 30.74 mg at ~ 3600 s and EGP reaching 15.94 mg at ~ 4800 s. In addition, the penetration process of deionized water on graphite plates at 20 and 75 °C was recorded, as shown in Fig. 3(e)–(f). The viscosity of water was much lower than that of resin, while exhibited a higher penetration rate as well as an upper limit mass of saturation absorption. Comparing the penetration rates for different graphite plates, the penetration rates of almost all liquids were higher for FGP than for EGP, except for the penetration rates of water at 75 °C. A significant higher penetration rate and saturation absorption were obtained in EGP when deionized water was heated at 75 °C.

Fig. 3.
figure 3

Penetration rates of different types of liquids into the graphite plate at different temperatures: (a) EP @20 °C; (b) EP @120 °C; (c) EP + 15 wt% GMA @120 °C; (d) AR @20 °C; (e) H2O @20 °C; (f) H2O @75 °C

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Combining the porosity characteristics of these graphite plates and the penetration behavior of different types of resins, it can be seen that the lower viscosity of the resin was not only beneficial to raise the penetration rate, but also to be more fully filled in the graphite plates. The pore structure of the graphite plate also has a direct impact on the penetration behavior, and EGP has more pore structures with a size less than 1 μm, which exhibited a lower penetration rate. Despite the higher porosity in EGP, it still exhibited a lower upper limit of saturation absorption. In addition, when the viscosity of water decreased to 0.38 mPa s, it exhibited a rapid increase in both the penetration rate and the upper limit of saturation absorption.

3.2 Penetration Behavior Analysis

The penetration behavior of liquids in porous structures is usually described using Darcy’s law, as shown in Eq. (1) [6]:

$$ \overline{u} = - \frac{K}{\mu }\nabla p $$
(1)

where \(\overline{u}\) represented the volume flow rate; K was the permeability coefficient related to the pore structure; μ was the liquid viscosity and p was the pressure driving the liquid flow. In the penetration process shown in Fig. 1(a), gravity was a resistance to flow and there was no other pressure gradient, so the effect of capillary forces (Pc) on the resin penetration behavior needs to be considered. In addition, the forces formed by the solid-liquid interaction, such as capillary forces, have also been confirmed to be an important component in driving/obstructing the penetration process, which also makes it exhibited nonlinear characteristics [7]. Pc was calculated by Eq. (2):

$$ Pc = \frac{2\sigma \cdot \cos \theta }{r} \cdot \pi r^{2} = 2\pi r \cdot \sigma \cdot \cos \theta $$
(2)

where σ was the surface tension of resin or water; θ was the contact angle and r was the radius of the pore. Based on Eq. (2), the capillary force for the penetration of EP into the pore with a pore size of 20 μm at 120 °C was 7 kPa, and the capillary force for the penetration of AR into this pore size at 20 °C was 8.5 kPa. The capillary forces for water at 20 °C and 75 °C were 5.0 kPa and 5.6 kPa, respectively. When the contact angle was greater than 90°, the resin would not penetrate into the graphite plate effectively. This indicated that the capillary force was the main driving force. However, the higher capillary force of the resin exhibited a lower permeation rate and upper limit of saturation absorption, indicating that flow resistance has a more remarkable effect on the penetration process.

Boundary layer was a phenomenon that occurs at the solid-liquid interface, where the liquid flow velocity was significantly reduced due to intermolecular interaction forces. It has been confirmed that when the pore size reached the micron-meter level, the effect of the boundary layer on the resistance of liquid flow was not negligible, and the liquid flow in the pore was distinguished into a low flow rate region near the solid-liquid interface and a uniform flow rate region located in the middle region [8, 9], as illustrated in Fig. 1(c). And as the pore size decreased, the proportion of the boundary layer in the flow channel gradually increased, and the flow resistance would rise significantly [10]. It is worth noting that when the pore radius was equal to the thickness of the boundary layer, the liquid in the pore cannot flow through the pore under the influence of the boundary layer, and the pressure needs to be further increased to reduce the thickness of the boundary layer so that the liquid could flow through the pores [11, 12].

When we focused on the resin penetration process in graphite plate, the boundary layer gradually became thinner as the viscosity decreased, and the penetration rate increased significantly. At the same time smaller pores could be penetrated and the graphite plate showed a higher upper limit of absorption by the resin or water. In contrast, EGP possessed a more abundant microporous structure, the boundary layer forms a higher flow resistance, and more pores were not permeable, so it exhibited a lower permeation rate compared to FGP, and a lower saturation absorption. The impermeable pores at high viscosity changed to be permeated when the viscosity of water decreases to 0.38 mPa s, so it exhibited a significantly higher saturation absorption.

4 Conclusion

In this study, the penetration rates and upper limits of absorption of epoxy resins, acrylic resins and water into graphite plates with different pore structures were analyzed. The influence factors of penetration behavior were also analyzed in combination with capillary forces and boundary layers. The results showed that the penetration rate and the upper limit of absorption into the graphite plate increased significantly with the decrease of resin viscosity. As the EGP possessed more pores less than 1 μm, the flow resistance was significantly increased under the influence of the boundary layer and more pores were impermeable, thus exhibiting a lower penetration rate as well as an upper limit of absorption. Further, in order to achieve a more adequate impregnation of EGP by resin, the volume fraction of small pores should be reduced by adjusting the graphite particles, applying a lower viscosity resin as the infiltration agent as well as higher infiltration process pressure should also be concerned.