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glass fiber composite flow frame for a vanadium redox flow battery (VRFB)
Accepted Manuscript Development of a fluoroelastomer/glass fiber composite flow frame for a vanadium redox flow battery (VRFB) Soohyun Nam, Dongyoung Lee, Jinwhan Kim, Dai Gil Lee PII: DOI: Reference:
S0263-8223(16)30093-9 http://dx.doi.org/10.1016/j.compstruct.2016.02.052 COST 7265
To appear in:
Composite Structures
Please cite this article as: Nam, S., Lee, D., Kim, J., Lee, D.G., Development of a fluoroelastomer/glass fiber composite flow frame for a vanadium redox flow battery (VRFB), Composite Structures (2016), doi: http:// dx.doi.org/10.1016/j.compstruct.2016.02.052
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Development of a fluoroelastomer/glass fiber composite flow frame for a vanadium redox flow battery (VRFB) Soohyun Nam, Dongyoung Lee, Jinwhan Kim and Dai Gil Lee* School of Mechanical Aerospace & Systems Engineering, KAIST (Korea Advanced Institute of Science and Technology) ME3221, 291 Daehak-ro, Yuseong-gu, Daejeon-shi, Korea 305-701 *Corresponding author. Tel.:+82-42-350-4481; Fax: +82-42-350-5221. E-mail: [email protected] Abstract The stack of a vanadium redox flow battery (VRFB), which is a promising energy storage system (ESS), is composed of flow frames (FFs), carbon felt electrodes, bipolar plates (BPs) and membranes. The components of VRFB are assembled and compacted using flat gaskets or O-rings to seal the highly concentrated vanadium sulfuric electrolyte. The reliable sealing of the stack is a crucial issue because the highly concentrated sulfuric acid will damage the system when it leaks. In this work, a gasket-less FF composed of fluoroelastomer/glass fiber composite was developed to prevent leakage without using gaskets or O-rings. The fluoroelastomer layer, which has high chemical stability under an acidic environment, was formed at the surface of the composite, which worked as a sealant of the FF owing to its resilience. Glass fiber reinforcement into the matrix was employed to provide mechanical properties for use as a flow frame structure under the compaction pressure. The surface treatment and fabrication methods for the fluoroelastomer/glass fiber composite were developed. Based on the measured mechanical properties and sealability test of the composite with respect to the curing conditions, the zero-leakage compaction pressure for the gasket-less FF was investigated.
Keywords: vanadium redox flow battery (VRFB), flow frame (FF), fluoroelastomer composite, gasket-less.
1
1. Introduction The energy storage system (ESS) is widely considered to be an effective approach to improve the reliability, power quality, and economy of renewable electrical energy. A vanadium redox flow battery (VRFB) is the most promising technology among the various ESSs such as lithium-ion, lead-acid, and supercapacitors [1]. VRFBs have several advantages including non-explosiveness, high energy efficiency, short response time, low self-discharge and a long lifetime. In addition, the most important advantage is that the power generation capacity and electricity storage capacity can be designed independently, which simplifies scale-up for large-scale ESS [2]. As shown in Fig. 1, a unit cell of VRFB consists of electrodes, an ion exchange membrane, bipolar plates (BPs) and positive and negative electrolytes circulated by pumps. The energy conversion between the chemical potential of vanadium electrolytes and electrical energy occurs at the electrodes when liquid electrolytes are flowing through the cell. Because VRFBs are operated by the viscous flows of strong sulfuric acid-based electrolytes, the carbon felt electrodes are compacted less than 0.1 MPa to provide flow permeability, which makes the sealability of FFs difficult [3]. Therefore, avoiding the leakage of electrolytes is a main concern when constructing the stack because leakages of electrolytes result in an unbalanced concentration between positive and negative electrolytes, which reduces energy efficiency. A typical unit cell of a VRFB stack is illustrated in Fig. 2(a); it is composed of carbon felt electrodes, flow frames (FFs), BPs, a current collector and an ion-exchange membrane. Additionally, rubber gaskets and O-rings are inserted between all components to seal the stack, and the whole stack is compressed by endplates and tiebars as shown in Fig. 1. However, the achievement of perfect sealing is difficult with conventional gaskets or O-rings because a VRFB stack consists of several hundred unit 2
cells that require a high level of alignment of the components. The rubber gaskets and O-rings should be replaced regularly during long-term operation (10–20 years) to maintain the sealability against creep deformation under the compaction pressure. In this work, the concept of gasket-less composite FFs was developed to overcome the technical problems of rubber gaskets and O-rings. A schematic drawing of the developed gasket-less composite FF structure is illustrated in Fig. 3, which is composed of fluoroelastomer matrix and glass fabric reinforcement. The fluoroelastomer layers, which are resin rich layers, are formed at the surfaces of the composite structure, whose resilience allows it to function as a gasket. The glass fiber fabric reinforcement provides required mechanical properties for the composite to be used as a structure under the compaction pressure and to retard creep deformation. With the gasket-less composite FF concept, the number of components in a stack can be reduced by half as shown in Fig. 2(b). The components of VRFBs have been studied by many researchers. Carbon cloth, carbon paper and carbon felts have been investigated and suggested for the electrode materials [4-6], and pretreatment methods to increase the performance of the electrodes were also investigated [7-9]. Including the conventional graphite plate BPs, recently developed carbon composite BPs have been widely investigated by many researchers [10-16]. Research has been conducted on perfluorinated membranes owing to the high conductivity and good chemical stability in the oxidizing electrolyte environment [1719]. Regarding the FF structure, most research has focused on the channel design and materials that can endure the oxidizing electrolyte environment. Nam et al. studied the FF-BP assembly by co-curing carbon/epoxy and glass/epoxy composite structures to reduce the leakage path between the components [20]. Research on the fluoroelastomer composite has been performed primarily to 3
improve the properties of the fluoroelastomer such as its thermal resistance, electrical properties, wear resistance, crack retardation and curing time [21-23]. Previous studies on the components of VRFBs have mostly focused on the electrode, BP and membranes, which are directly related to the electrochemical performance of the stack. Most fluoroelastomer composites have been studied to increase the thermal, electrical and mechanical properties as sealing materials rather than for use as structural materials. In this work, a fluoroelastomer/glass fiber composite structure was developed for the FFs of VRFBs. To increase the bonding between fluoroelastomer matrix and glass fiber fabric reinforcements, surface treatment methods were investigated. A cyclic pressurizing cure process was introduced to impregnate the highly viscous fluoroelastomer into the glass fiber fabrics. The fluoroelastomer/glass fiber composite FF specimens were fabricated with respect to the number of glass fiber fabric layers and curing pressure to investigate the composition of the composite. The mechanical properties of fabricated specimens were measured. In addition, the sealability test was performed by measuring the zero-leakage compaction pressure of each fabricated specimen. Based on the mechanical properties and sealing performance, the fabrication condition of fluoroelastomer/glass fiber composite for the FFs of VRFBs was investigated.
2. Experimental 2.1 Concept and material selection The concept of gasket-less FFs is illustrated in Fig. 3. During the fabrication process of the composite structure, the elastomer layer, which is a resin rich layer, is formed on the surface, and the glass fiber reinforcements provide the required 4
mechanical properties. Materials for VRFB components should be chemically stable owing to the highly oxidizing environment under strong,
acid-based
electrolytes.
In this work,
fluoroelastomer (Dai-El G327, Daikin, Japan) was selected because of its high chemical stability in acidic environments. Plain weave glass fiber fabric (GEP126, SK Chemicals, Korea) of 0.3 mm in thickness was used as reinforcement to increase the mechanical properties. Generally, fluoroelastomer compounds are composed of a curing agent such as bisphenol, metal oxides (magnesium oxide and calcium hydroxide) and fillers such as carbon black, silica or carbon nanotubes. Although fillers are used to improve the mechanical or electrical properties of fluoroelastomer, they increase the viscosity of the compound. In this work, fillers were not adopted when fabricating the fluoroelastomer compound to reduce the viscosity of the compound.
2.2 Surface treatment methods Co-curing of the fluoroelastomer compound and glass fibers is difficult owing to the fluorine in the compound. From the preliminary experiment, it was found that the glass fiber fabric could not be impregnated properly without surface treatment on the glass fiber as shown in Fig. 4. In this work, flame treatment and silane coupling agent treatment on glass fibers were adopted to increase the bonding between the fluoroelastomer and glass fibers. The surface treatment methods performed in this work are shown in Fig. 5. First, acetone-cleansed and dried glass fiber fabrics were flame treated for 20 s with propane gas and the temperature of flame was 900°C. Flametreated glass fiber fabrics were then immersed into a 0.1% silane coupling agent solution and dried at 120°C. Among the various silane coupling agents, coupling agents in the amino and 5
mercapto functional groups are known for increasing the bonding performance of elastomeric resins [24]. In this study, the amino functional group (KBM-803, ShinEtsu, Japan) and mercapto functional group (KBM-903, ShinEtsu, Japan) were investigated as coupling agents for the fabrication of the fluoroelastomer/glass fiber composite.
2.3 Fabrication of fluoroelastomer/glass fiber composite Generally, fluoroelastomer compounds are produced in the form of thick slabs (5 mm thick), as shown in Fig. 6(a). To impregnate the fluoroelastomer resin into glass fiber fabric layers, the thick slab compounds were compressed into sheets (0.5 mm thickness), as shown in Fig. 6(b). The glass fiber fabrics and fluoroelastomer sheet compound were stacked and placed in the compression mold for curing, and they were cured at 190°C for 20 min as shown in Fig. 7. The glass fiber fabric can be torn by a large shear force applied by a highly viscous fluoroelastomer compound at the beginning of the curing cycle. In this work, the cyclic pressurizing process was adopted during the first 2 min of the curing cycle until the viscosity of the fluoroelastomer decreased enough to prevent tearing of the glass fiber fabrics. To provide liquid electrolytes in the VRFB stack, the FF has flow channel structures, which are thinner than the surrounding parts as shown in Fig. 8(a). The dimensions of the fluoroelastomer/glass fiber composite structure can be controlled by changing the number of glass fiber fabrics and the curing pressure. In this work, fluoroelastomer/glass fiber composite FF specimens were fabricated with 4-ply and 8ply glass fiber fabrics by varying the curing pressure (10 MPa, 15 MPa and 20 MPa). The stacking sequences of glass fiber fabrics and the fluoroelastomer sheet compound are shown in Fig. 8(b) and (c). All specimens were fabricated with same surface
6
treatment methods and heating condition.
2.3 Mechanical properties and sealability tests The FF structure should endure the compaction pressure for long cycles of operation, which requires high mechanical properties. Because the tensile strength of fluoroelastomer is only 10–20 MPa, the fluoroelastomer itself cannot be used as the FF structure. The tensile strength and flexural strength were measured and compared with respect to the surface treatment methods to evaluate the bonding properties between fluoroelastomer and glass fiber fabrics. Additionally, the tensile strengths of fluoroelastomer/glass fiber composites fabricated with various thickness and curing pressure were measured and compared. Tensile strengths and flexural strengths were measured based on ASTM D3039 and ASTM D790, respectively, and the dimension and configuration of each specimen are described in Fig. 9. The sealing performance of developed composite FF specimens were evaluated by measuring the zero-leakage compaction pressure. The schematic diagram of sealability testing equipment and FF specimens are shown in Fig. 10. The equipment was separated into two chambers sealed with two FF specimens. Air pressure of 0.1 MPa was applied to chamber 1, and leaked air inside chamber 2 was detected by pressure sensor 2. The zero-leakage compaction pressures were measured by reducing the compaction pressure from the initial pressure of 1.0 MPa.
3. Results and discussion 3.1 Fabrication methods for fluoroelastomer/glass fiber composite The tensile and flexural strengths of the fluoroelastomer/glass fiber composite are shown in Fig. 11. Both the tensile and flexural strength of the specimens with mercapto 7
functional group silane coupling agent surface treatment showed higher values than those of the amino functional group silane coupling agent by 10% and 22%, respectively. Metal oxides such as magnesium oxide and calcium hydroxide were dispersed into the fluoroelastomer compound for curing activation and acid acceptors. The mercapto functional group in the silane coupling agent could bond to the metal oxide, which would increase the bonding between fluoroelastomer and glass fibers. Therefore, the mercapto functional group silane coupling agent was found to be more effective for fabrication of the fluoroelastomer/glass fiber composite structure than the amino functional group silane coupling agent. To investigate the effect of the cyclic pressurizing process during curing, the cross section images of fabricated specimens with and without the cyclic pressurizing process were observed by optical microscope (VHX-700FE, Keyence, Japan). The mercapto silane coupling agent was used in the surface treatment for both specimens, and the curing pressure was 20 MPa. As shown in Fig. 12(a), the glass fibers were not fully wetted without cyclic pressurizing; however, the glass fibers were fully wetted with cyclic pressurizing as shown in Fig. 12(b), from which the effect of cyclic pressurizing was verified. Cross-section images of specimens fabricated with 4-ply and 8-ply glass fiber fabrics are shown in Fig. 13, and the thicknesses of fabricated fluoroelastomer layers at the surface, total thicknesses and fiber volume fractions are summarized in Table 1. As the curing pressure increased, the fluoroelastomer layers at the surface became thinner. The decrease in thickness was larger in the 8-ply case than in the 4-ply case because the fluoroelastomer sheet compound was inserted between the glass fiber fabric layers in the 8-ply case. The total thickness and fiber volume fraction showed a similar trend. The thicker fluoroelastomer layer at the surface would require a lower compaction pressure 8
for sealing; however, it would have inferior mechanical properties.
3.2 Mechanical properties and sealability tests The tensile strengths of specimens fabricated with varying curing pressures are shown in Fig. 14. The tensile strength increased from 78 MPa to 100 MPa in the 4-ply case and increased from 68 MPa to 118 MPa in the 8-ply case. As expected from the observation of cross section images, the tensile strengths increased with increasing curing pressure and were proportional to the fiber volume fractions as expected. Compared with the strength of PVC (~65 MPa), which is the conventional FF material for VRFBs, the fabricated fluoroelastomer/glass fiber composites had higher strength, which verified that the developed fluoroelastomer/glass fiber composites could be successfully applied for FFs of VRFBs. The zero-leakage compaction pressure with respect to the curing pressure and stacking configuration is shown in Fig. 15. The required zero-leakage compaction pressure increased from 0.24 to 0.26 MPa in the 4-ply case and increased from 0.23 to 0.3 MPa in the 8-ply case as the curing pressure increased. Although the zero-leakage pressure increased as the curing pressure increased, the required zero-leakage compaction pressures were lower than a typical compaction pressure for a VRFB stack, which was 0.5 MPa for all cases. The higher curing pressure resulted in a thinner fluoroelastomer layer at the surface and a higher fiber volume fraction, which increased the tensile strength. Even with the thin fluoroelastomer layer at the surface, the required zero-leakage compaction was lower than that of the conventional method. Therefore, the fluoroelastomer/glass fiber composite fabricated with a curing pressure of 20 MPa would be desired for gasket-less FF application. 9
4. Conclusion Gasket-less flow frames (FFs) for vanadium redox flow batteries (VRFBs) were developed with fluoroelastomer/glass fiber composite, which had a fluoroelastomer layer (resin rich layer) at the surface for sealing performance. To fabricate the fluoroelastomer/glass fiber composite structure, the surface treatment methods for the glass fiber fabric were investigated. From the experimental results, the flame treatment and silane coupling agent, which included mercapto functional group treatment, were found to be effective methods. Additionally, the cyclic pressurizing curing cycle was adopted to impregnate the highly viscous fluoroelastomer compound into glass fibers. The fluoroelastomer/glass fiber composite structures for FFs of VRFBs were investigated by varying the number of plies of glass fiber fabrics and the curing pressure. The tensile strength of fluoroelastomer/glass fiber composites fabricated with a curing pressure of 20 MPa was 118 MPa in the 8-ply case, and the corresponding zero-leakage compaction pressure was 0.3 MPa, which represented improved mechanical and sealing performance. Therefore, the gasket-less FF structure for VRFBs could be developed properly with the fluoroelastomer/glass fiber composite.
Acknowledgment This research was supported by the Climate Change Research Hub of KAIST (grant No. N11160012). Their support is greatly appreciated.
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References [1] Lee BS, Gushee DE. Renewable power: not yet ready for prime time. Chemical Engineering Progress. 2009;105:22. [2] Li L, Kim S, Wang W, Vijayakumar M, Nie Z, Chen B, et al. A Stable Vanadium Redox‐Flow Battery with High Energy Density for Large‐Scale Energy Storage. Advanced Energy Materials. 2011;1:394-400. [3] Leung P, Li XH, de Leon CP, Berlouis L, Low CTJ, Walsh FC. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Advances. 2012;2:10125-56. [4] Skyllas‐Kazacos M, Grossmith F. Efficient vanadium redox flow cell. Journal of the Electrochemical Society. 1987;134:2950-3. [5] Leung P, Li X, De León CP, Berlouis L, Low CJ, Walsh FC. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Advances. 2012;2:10125-56. [6] Aaron D, Liu Q, Tang Z, Grim G, Papandrew A, Turhan A, et al. Dramatic performance gains in vanadium redox flow batteries through modified cell architecture. Journal of Power Sources. 2012;206:450-3. [7] Kim KJ, Kim YJ, Kim JH, Park MS. The effects of surface modification on carbon felt electrodes for use in vanadium redox flow batteries. Materials Chemistry and Physics. 2011;131:547-53. [8] Gao C, Wang NF, Peng S, Liu SQ, Lei Y, Liang XX, et al. Influence of Fenton's reagent treatment on electrochemical properties of graphite felt for all vanadium redox flow battery. Electrochimica Acta. 2013;88:193-202. [9] Wu X, Xu H, Xu P, Shen Y, Lu L, Shi J, et al. Microwave-treated graphite felt as the 11
positive electrode for all-vanadium redox flow battery. Journal of Power Sources. 2014;263:104-9. [10] Kim KH, Kim BG, Lee DG. Development of carbon composite bipolar plate (BP) for vanadium redox flow battery (VRFB). Composite Structures. 2014;109:253-9. [11] Choe J, Kim KH, Lee DG. Corrugated carbon/epoxy composite bipolar plate for vanadium redox flow batteries. Composite Structures. 2015;119:534-42. [12] Choe J, Lim JW, Kim M, Kim J, Lee DG. Durability of graphite coated carbon composite bipolar plates for vanadium redox flow batteries. Composite Structures. 2015;134:106-13. [13] Kim KH, Choe J, Nam S, Kim BG, Lee DG. Surface crack closing method for the carbon composite bipolar plates of a redox flow battery. Composite Structures. 2015;119:436-42. [14] Lee D, Lim JW, Nam S, Choi I, Lee DG. Method for exposing carbon fibers on composite bipolar plates. Composite Structures. 2015;134:1-9. [15] Lim JW, Lee DG. Carbon fiber/polyethylene bipolar plate-carbon felt electrode assembly for vanadium redox flow batteries (VRFB). Composite Structures. 2015;134:483-92. [16] Lee D, Lee DG. Electro-mechanical properties of the carbon fabric composites with fibers exposed on the surface. Composite Structures. 2016;140:77-83. [17] Chieng S, Kazacos M, Skyllas-Kazacos M. Modification of Daramic, microporous separator, for redox flow battery applications. Journal of Membrane Science. 1992;75:81-91. [18] Lu Y, Goodenough JB. Rechargeable alkali-ion cathode-flow battery. Journal of Materials Chemistry. 2011;21:10113-7. [19] Vafiadis H, Skyllas-Kazacos M. Evaluation of membranes for the novel vanadium 12
bromine redox flow cell. Journal of Membrane Science. 2006;279:394-402. [20] Nam S, Lee D, Choi I, Lee DG. Smart cure cycle for reducing the thermal residual stress of a co-cured E-glass/carbon/epoxy composite structure for a vanadium redox flow battery. Composite Structures. 2015;120:107-16. [21] Kader MA, Lyu MY, Nah C. A study on melt processing and thermal properties of fluoroelastomer nanocomposites. Composites Science and Technology. 2006;66:143143. [22] XU T, YANG J, LIU J, FU Q. Surface modification of carbon nanotube and its influence on the conductivity property of carbon nanotube/fluoro-elastomer composite. Acta Materiae Compositae Sinica. 2010;3:002. [23] Bhattacharya SK, Bhowmick AK, Singh RK. Crack-Growth Resistance of Fluoroelastomer Vulcanizates Filled with Particulate and Fiber Filler. Journal of Materials Science. 1995;30:243-7. [24] Plueddemann EP. Silane coupling agents: Springer Science & Business Media, 2013.
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Table 1 Dimensional properties of specimens with different fabrication condition. Fabrication condition
Total thickness
Surface fluoroelastomer
Fiber volume
(mm)
layer thickness (mm)
fraction (%)
10
1.55
0.37
13
15
1.53
0.36
15
20
1.38
0.23
21
10
3.14
0.45
11
15
2.48
0.24
19
20
2.15
0.20
23
Number
Curing
of ply
pressure (MPa)
4-ply
8-ply
14
Endplate
Electrolyte tank
Stack
Pump
Electrode
Bipolar plate
Anode: VO2++2H++e- VO2++H2O Cathode: V2+ V3++eMembrane
Fig. 1 Schematic diagram of a VRFB system and the chemical reactions in the unit cell.
15
Current collector Bipolar plate Sheet gasket Gasket-less Flow frame
Flow frame Electrode Sheet gasket Membrane
O-ring
(a)
(b)
Fig. 2 Components of VRFB unit cell: (a) conventional components; (b) reduced components with gasket-less composite flow frame.
16
Fluoroelastomer layer
Glass fiber fabric
Fig. 3 Concept of gasket-less composite flow frame for VRFB
17
10 mm
Fig. 4 Shredded glass fiber fabrics when fabricated without surface treatment methods.
18
1. Flame treatment for 20-sec Glass fabric
2. Silane coupling agent treatment
0.1% silane solution
120 oC
Torch
Fig. 5 Surface treatment methods for fluoroelastomer/glass fiber composite.
19
the
glass fabric
to
fabricate
the
5 mm
0.5 mm (a)
(b)
Fig. 6 Fluoroelastomer: (a) as received slab compound; (b) fabricated sheet compound.
20
Compression mold Specimen
Temperature (° C)
(a)
Pressure (MPa)
190oC Cyclic pressure
0
2
20
Time (minute)
(b) Fig. 7 Fabrication method of the fluoroelastomer/glass fiber composite: (a) schematic diagram for compression molding; (b) curing condition. .
21
Flow channel
(a)
Glass fiber fabric Fluoroelastomer sheet compound 4-ply 4-ply
4-ply (b)
(c)
Fig. 8 Cross section of flow channel part of FF and stacking sequences of fluoroelastomer sheet compound and glass fabric: (a)cross section of flow channel part; (b) 4-ply of glass fabric (c) 8-ply of glass fabric.
22
50
100
10
15 10
Tab
unit: mm
32 (a)
(b)
Fig. 9 Dimensions and configurations for mechanical property tests to measure: (a) tensile strength; (b) flexural strength (Dimensions in mm).
23
Pressure sensor 1
Pressure Pressure sensor 2
Butyl sealant
Air Chamber 2 Pressure Chamber 1 8 60
Flow frame specimen
Fig. 10 Schematic diagrams of sealability test and flow frame specimen (Dimensions in mm).
24
Flexural strength (MPa)
Tensile strength (MPa)
120 100 80 60 40 20 0 Amino KBM-803 functional group
15 12 9 6 3 0 Amino KBM-803
Mercapto KBM-903 functional group
functional group
(a)
Mercapto KBM-903 functional group
(b)
Fig. 11 Effects of the silane coupling agent on fluoroelastomer/glass fiber composite: (a) tensile strength; (b) flexural strength.
25
50 µm
50 µm
Unwetted fibers
(a)
(b)
Fig. 12 Effect of the cyclic pressurizing process: (a) without cyclic pressurizing process; (b) with cyclic pressurizing process.
26
1 mm
1 mm
1 mm 0.37 mm
0.36 mm
(a) 1 mm
(b) 0.45 mm
(d)
0.23 mm
(c) 1 mm
1 mm 0.24 mm
(e)
0.20 mm
(f)
Fig. 13 Cross section images of fabricated fluoroelastomer/glass fiber composite specimens: (a) 4-ply cured at 10 MPa; (b) 4-ply cured at 15 MPa; (c) 4-ply cured at 20 MPa; (d) 8-ply cured at 10 MPa; (e) 8-ply cured at 15 MPa; (f) 8-ply cured at 20 MPa.
27
4-ply
Tensile strength (MPa)
140
8-ply
120 100 80 60 40 20 0 10
15
20
Curing pressure (MPa) Fig. 14 Tensile strengths with respect to the curing pressure and stacking sequences.
28
4-ply
8-ply
Zero-leakage pressure (MPa)
0.6 Typical compaction pressure
0.5 0.4 0.3 0.2 0.1 0 10
15 Curing pressure (MPa)
20
Fig. 15 Zero-leakage compaction pressure with respect to the curing pressure and stacking sequences.
29
S0263-8223(16)30093-9 http://dx.doi.org/10.1016/j.compstruct.2016.02.052 COST 7265
To appear in:
Composite Structures
Please cite this article as: Nam, S., Lee, D., Kim, J., Lee, D.G., Development of a fluoroelastomer/glass fiber composite flow frame for a vanadium redox flow battery (VRFB), Composite Structures (2016), doi: http:// dx.doi.org/10.1016/j.compstruct.2016.02.052
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Development of a fluoroelastomer/glass fiber composite flow frame for a vanadium redox flow battery (VRFB) Soohyun Nam, Dongyoung Lee, Jinwhan Kim and Dai Gil Lee* School of Mechanical Aerospace & Systems Engineering, KAIST (Korea Advanced Institute of Science and Technology) ME3221, 291 Daehak-ro, Yuseong-gu, Daejeon-shi, Korea 305-701 *Corresponding author. Tel.:+82-42-350-4481; Fax: +82-42-350-5221. E-mail: [email protected] Abstract The stack of a vanadium redox flow battery (VRFB), which is a promising energy storage system (ESS), is composed of flow frames (FFs), carbon felt electrodes, bipolar plates (BPs) and membranes. The components of VRFB are assembled and compacted using flat gaskets or O-rings to seal the highly concentrated vanadium sulfuric electrolyte. The reliable sealing of the stack is a crucial issue because the highly concentrated sulfuric acid will damage the system when it leaks. In this work, a gasket-less FF composed of fluoroelastomer/glass fiber composite was developed to prevent leakage without using gaskets or O-rings. The fluoroelastomer layer, which has high chemical stability under an acidic environment, was formed at the surface of the composite, which worked as a sealant of the FF owing to its resilience. Glass fiber reinforcement into the matrix was employed to provide mechanical properties for use as a flow frame structure under the compaction pressure. The surface treatment and fabrication methods for the fluoroelastomer/glass fiber composite were developed. Based on the measured mechanical properties and sealability test of the composite with respect to the curing conditions, the zero-leakage compaction pressure for the gasket-less FF was investigated.
Keywords: vanadium redox flow battery (VRFB), flow frame (FF), fluoroelastomer composite, gasket-less.
1
1. Introduction The energy storage system (ESS) is widely considered to be an effective approach to improve the reliability, power quality, and economy of renewable electrical energy. A vanadium redox flow battery (VRFB) is the most promising technology among the various ESSs such as lithium-ion, lead-acid, and supercapacitors [1]. VRFBs have several advantages including non-explosiveness, high energy efficiency, short response time, low self-discharge and a long lifetime. In addition, the most important advantage is that the power generation capacity and electricity storage capacity can be designed independently, which simplifies scale-up for large-scale ESS [2]. As shown in Fig. 1, a unit cell of VRFB consists of electrodes, an ion exchange membrane, bipolar plates (BPs) and positive and negative electrolytes circulated by pumps. The energy conversion between the chemical potential of vanadium electrolytes and electrical energy occurs at the electrodes when liquid electrolytes are flowing through the cell. Because VRFBs are operated by the viscous flows of strong sulfuric acid-based electrolytes, the carbon felt electrodes are compacted less than 0.1 MPa to provide flow permeability, which makes the sealability of FFs difficult [3]. Therefore, avoiding the leakage of electrolytes is a main concern when constructing the stack because leakages of electrolytes result in an unbalanced concentration between positive and negative electrolytes, which reduces energy efficiency. A typical unit cell of a VRFB stack is illustrated in Fig. 2(a); it is composed of carbon felt electrodes, flow frames (FFs), BPs, a current collector and an ion-exchange membrane. Additionally, rubber gaskets and O-rings are inserted between all components to seal the stack, and the whole stack is compressed by endplates and tiebars as shown in Fig. 1. However, the achievement of perfect sealing is difficult with conventional gaskets or O-rings because a VRFB stack consists of several hundred unit 2
cells that require a high level of alignment of the components. The rubber gaskets and O-rings should be replaced regularly during long-term operation (10–20 years) to maintain the sealability against creep deformation under the compaction pressure. In this work, the concept of gasket-less composite FFs was developed to overcome the technical problems of rubber gaskets and O-rings. A schematic drawing of the developed gasket-less composite FF structure is illustrated in Fig. 3, which is composed of fluoroelastomer matrix and glass fabric reinforcement. The fluoroelastomer layers, which are resin rich layers, are formed at the surfaces of the composite structure, whose resilience allows it to function as a gasket. The glass fiber fabric reinforcement provides required mechanical properties for the composite to be used as a structure under the compaction pressure and to retard creep deformation. With the gasket-less composite FF concept, the number of components in a stack can be reduced by half as shown in Fig. 2(b). The components of VRFBs have been studied by many researchers. Carbon cloth, carbon paper and carbon felts have been investigated and suggested for the electrode materials [4-6], and pretreatment methods to increase the performance of the electrodes were also investigated [7-9]. Including the conventional graphite plate BPs, recently developed carbon composite BPs have been widely investigated by many researchers [10-16]. Research has been conducted on perfluorinated membranes owing to the high conductivity and good chemical stability in the oxidizing electrolyte environment [1719]. Regarding the FF structure, most research has focused on the channel design and materials that can endure the oxidizing electrolyte environment. Nam et al. studied the FF-BP assembly by co-curing carbon/epoxy and glass/epoxy composite structures to reduce the leakage path between the components [20]. Research on the fluoroelastomer composite has been performed primarily to 3
improve the properties of the fluoroelastomer such as its thermal resistance, electrical properties, wear resistance, crack retardation and curing time [21-23]. Previous studies on the components of VRFBs have mostly focused on the electrode, BP and membranes, which are directly related to the electrochemical performance of the stack. Most fluoroelastomer composites have been studied to increase the thermal, electrical and mechanical properties as sealing materials rather than for use as structural materials. In this work, a fluoroelastomer/glass fiber composite structure was developed for the FFs of VRFBs. To increase the bonding between fluoroelastomer matrix and glass fiber fabric reinforcements, surface treatment methods were investigated. A cyclic pressurizing cure process was introduced to impregnate the highly viscous fluoroelastomer into the glass fiber fabrics. The fluoroelastomer/glass fiber composite FF specimens were fabricated with respect to the number of glass fiber fabric layers and curing pressure to investigate the composition of the composite. The mechanical properties of fabricated specimens were measured. In addition, the sealability test was performed by measuring the zero-leakage compaction pressure of each fabricated specimen. Based on the mechanical properties and sealing performance, the fabrication condition of fluoroelastomer/glass fiber composite for the FFs of VRFBs was investigated.
2. Experimental 2.1 Concept and material selection The concept of gasket-less FFs is illustrated in Fig. 3. During the fabrication process of the composite structure, the elastomer layer, which is a resin rich layer, is formed on the surface, and the glass fiber reinforcements provide the required 4
mechanical properties. Materials for VRFB components should be chemically stable owing to the highly oxidizing environment under strong,
acid-based
electrolytes.
In this work,
fluoroelastomer (Dai-El G327, Daikin, Japan) was selected because of its high chemical stability in acidic environments. Plain weave glass fiber fabric (GEP126, SK Chemicals, Korea) of 0.3 mm in thickness was used as reinforcement to increase the mechanical properties. Generally, fluoroelastomer compounds are composed of a curing agent such as bisphenol, metal oxides (magnesium oxide and calcium hydroxide) and fillers such as carbon black, silica or carbon nanotubes. Although fillers are used to improve the mechanical or electrical properties of fluoroelastomer, they increase the viscosity of the compound. In this work, fillers were not adopted when fabricating the fluoroelastomer compound to reduce the viscosity of the compound.
2.2 Surface treatment methods Co-curing of the fluoroelastomer compound and glass fibers is difficult owing to the fluorine in the compound. From the preliminary experiment, it was found that the glass fiber fabric could not be impregnated properly without surface treatment on the glass fiber as shown in Fig. 4. In this work, flame treatment and silane coupling agent treatment on glass fibers were adopted to increase the bonding between the fluoroelastomer and glass fibers. The surface treatment methods performed in this work are shown in Fig. 5. First, acetone-cleansed and dried glass fiber fabrics were flame treated for 20 s with propane gas and the temperature of flame was 900°C. Flametreated glass fiber fabrics were then immersed into a 0.1% silane coupling agent solution and dried at 120°C. Among the various silane coupling agents, coupling agents in the amino and 5
mercapto functional groups are known for increasing the bonding performance of elastomeric resins [24]. In this study, the amino functional group (KBM-803, ShinEtsu, Japan) and mercapto functional group (KBM-903, ShinEtsu, Japan) were investigated as coupling agents for the fabrication of the fluoroelastomer/glass fiber composite.
2.3 Fabrication of fluoroelastomer/glass fiber composite Generally, fluoroelastomer compounds are produced in the form of thick slabs (5 mm thick), as shown in Fig. 6(a). To impregnate the fluoroelastomer resin into glass fiber fabric layers, the thick slab compounds were compressed into sheets (0.5 mm thickness), as shown in Fig. 6(b). The glass fiber fabrics and fluoroelastomer sheet compound were stacked and placed in the compression mold for curing, and they were cured at 190°C for 20 min as shown in Fig. 7. The glass fiber fabric can be torn by a large shear force applied by a highly viscous fluoroelastomer compound at the beginning of the curing cycle. In this work, the cyclic pressurizing process was adopted during the first 2 min of the curing cycle until the viscosity of the fluoroelastomer decreased enough to prevent tearing of the glass fiber fabrics. To provide liquid electrolytes in the VRFB stack, the FF has flow channel structures, which are thinner than the surrounding parts as shown in Fig. 8(a). The dimensions of the fluoroelastomer/glass fiber composite structure can be controlled by changing the number of glass fiber fabrics and the curing pressure. In this work, fluoroelastomer/glass fiber composite FF specimens were fabricated with 4-ply and 8ply glass fiber fabrics by varying the curing pressure (10 MPa, 15 MPa and 20 MPa). The stacking sequences of glass fiber fabrics and the fluoroelastomer sheet compound are shown in Fig. 8(b) and (c). All specimens were fabricated with same surface
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treatment methods and heating condition.
2.3 Mechanical properties and sealability tests The FF structure should endure the compaction pressure for long cycles of operation, which requires high mechanical properties. Because the tensile strength of fluoroelastomer is only 10–20 MPa, the fluoroelastomer itself cannot be used as the FF structure. The tensile strength and flexural strength were measured and compared with respect to the surface treatment methods to evaluate the bonding properties between fluoroelastomer and glass fiber fabrics. Additionally, the tensile strengths of fluoroelastomer/glass fiber composites fabricated with various thickness and curing pressure were measured and compared. Tensile strengths and flexural strengths were measured based on ASTM D3039 and ASTM D790, respectively, and the dimension and configuration of each specimen are described in Fig. 9. The sealing performance of developed composite FF specimens were evaluated by measuring the zero-leakage compaction pressure. The schematic diagram of sealability testing equipment and FF specimens are shown in Fig. 10. The equipment was separated into two chambers sealed with two FF specimens. Air pressure of 0.1 MPa was applied to chamber 1, and leaked air inside chamber 2 was detected by pressure sensor 2. The zero-leakage compaction pressures were measured by reducing the compaction pressure from the initial pressure of 1.0 MPa.
3. Results and discussion 3.1 Fabrication methods for fluoroelastomer/glass fiber composite The tensile and flexural strengths of the fluoroelastomer/glass fiber composite are shown in Fig. 11. Both the tensile and flexural strength of the specimens with mercapto 7
functional group silane coupling agent surface treatment showed higher values than those of the amino functional group silane coupling agent by 10% and 22%, respectively. Metal oxides such as magnesium oxide and calcium hydroxide were dispersed into the fluoroelastomer compound for curing activation and acid acceptors. The mercapto functional group in the silane coupling agent could bond to the metal oxide, which would increase the bonding between fluoroelastomer and glass fibers. Therefore, the mercapto functional group silane coupling agent was found to be more effective for fabrication of the fluoroelastomer/glass fiber composite structure than the amino functional group silane coupling agent. To investigate the effect of the cyclic pressurizing process during curing, the cross section images of fabricated specimens with and without the cyclic pressurizing process were observed by optical microscope (VHX-700FE, Keyence, Japan). The mercapto silane coupling agent was used in the surface treatment for both specimens, and the curing pressure was 20 MPa. As shown in Fig. 12(a), the glass fibers were not fully wetted without cyclic pressurizing; however, the glass fibers were fully wetted with cyclic pressurizing as shown in Fig. 12(b), from which the effect of cyclic pressurizing was verified. Cross-section images of specimens fabricated with 4-ply and 8-ply glass fiber fabrics are shown in Fig. 13, and the thicknesses of fabricated fluoroelastomer layers at the surface, total thicknesses and fiber volume fractions are summarized in Table 1. As the curing pressure increased, the fluoroelastomer layers at the surface became thinner. The decrease in thickness was larger in the 8-ply case than in the 4-ply case because the fluoroelastomer sheet compound was inserted between the glass fiber fabric layers in the 8-ply case. The total thickness and fiber volume fraction showed a similar trend. The thicker fluoroelastomer layer at the surface would require a lower compaction pressure 8
for sealing; however, it would have inferior mechanical properties.
3.2 Mechanical properties and sealability tests The tensile strengths of specimens fabricated with varying curing pressures are shown in Fig. 14. The tensile strength increased from 78 MPa to 100 MPa in the 4-ply case and increased from 68 MPa to 118 MPa in the 8-ply case. As expected from the observation of cross section images, the tensile strengths increased with increasing curing pressure and were proportional to the fiber volume fractions as expected. Compared with the strength of PVC (~65 MPa), which is the conventional FF material for VRFBs, the fabricated fluoroelastomer/glass fiber composites had higher strength, which verified that the developed fluoroelastomer/glass fiber composites could be successfully applied for FFs of VRFBs. The zero-leakage compaction pressure with respect to the curing pressure and stacking configuration is shown in Fig. 15. The required zero-leakage compaction pressure increased from 0.24 to 0.26 MPa in the 4-ply case and increased from 0.23 to 0.3 MPa in the 8-ply case as the curing pressure increased. Although the zero-leakage pressure increased as the curing pressure increased, the required zero-leakage compaction pressures were lower than a typical compaction pressure for a VRFB stack, which was 0.5 MPa for all cases. The higher curing pressure resulted in a thinner fluoroelastomer layer at the surface and a higher fiber volume fraction, which increased the tensile strength. Even with the thin fluoroelastomer layer at the surface, the required zero-leakage compaction was lower than that of the conventional method. Therefore, the fluoroelastomer/glass fiber composite fabricated with a curing pressure of 20 MPa would be desired for gasket-less FF application. 9
4. Conclusion Gasket-less flow frames (FFs) for vanadium redox flow batteries (VRFBs) were developed with fluoroelastomer/glass fiber composite, which had a fluoroelastomer layer (resin rich layer) at the surface for sealing performance. To fabricate the fluoroelastomer/glass fiber composite structure, the surface treatment methods for the glass fiber fabric were investigated. From the experimental results, the flame treatment and silane coupling agent, which included mercapto functional group treatment, were found to be effective methods. Additionally, the cyclic pressurizing curing cycle was adopted to impregnate the highly viscous fluoroelastomer compound into glass fibers. The fluoroelastomer/glass fiber composite structures for FFs of VRFBs were investigated by varying the number of plies of glass fiber fabrics and the curing pressure. The tensile strength of fluoroelastomer/glass fiber composites fabricated with a curing pressure of 20 MPa was 118 MPa in the 8-ply case, and the corresponding zero-leakage compaction pressure was 0.3 MPa, which represented improved mechanical and sealing performance. Therefore, the gasket-less FF structure for VRFBs could be developed properly with the fluoroelastomer/glass fiber composite.
Acknowledgment This research was supported by the Climate Change Research Hub of KAIST (grant No. N11160012). Their support is greatly appreciated.
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References [1] Lee BS, Gushee DE. Renewable power: not yet ready for prime time. Chemical Engineering Progress. 2009;105:22. [2] Li L, Kim S, Wang W, Vijayakumar M, Nie Z, Chen B, et al. A Stable Vanadium Redox‐Flow Battery with High Energy Density for Large‐Scale Energy Storage. Advanced Energy Materials. 2011;1:394-400. [3] Leung P, Li XH, de Leon CP, Berlouis L, Low CTJ, Walsh FC. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Advances. 2012;2:10125-56. [4] Skyllas‐Kazacos M, Grossmith F. Efficient vanadium redox flow cell. Journal of the Electrochemical Society. 1987;134:2950-3. [5] Leung P, Li X, De León CP, Berlouis L, Low CJ, Walsh FC. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Advances. 2012;2:10125-56. [6] Aaron D, Liu Q, Tang Z, Grim G, Papandrew A, Turhan A, et al. Dramatic performance gains in vanadium redox flow batteries through modified cell architecture. Journal of Power Sources. 2012;206:450-3. [7] Kim KJ, Kim YJ, Kim JH, Park MS. The effects of surface modification on carbon felt electrodes for use in vanadium redox flow batteries. Materials Chemistry and Physics. 2011;131:547-53. [8] Gao C, Wang NF, Peng S, Liu SQ, Lei Y, Liang XX, et al. Influence of Fenton's reagent treatment on electrochemical properties of graphite felt for all vanadium redox flow battery. Electrochimica Acta. 2013;88:193-202. [9] Wu X, Xu H, Xu P, Shen Y, Lu L, Shi J, et al. Microwave-treated graphite felt as the 11
positive electrode for all-vanadium redox flow battery. Journal of Power Sources. 2014;263:104-9. [10] Kim KH, Kim BG, Lee DG. Development of carbon composite bipolar plate (BP) for vanadium redox flow battery (VRFB). Composite Structures. 2014;109:253-9. [11] Choe J, Kim KH, Lee DG. Corrugated carbon/epoxy composite bipolar plate for vanadium redox flow batteries. Composite Structures. 2015;119:534-42. [12] Choe J, Lim JW, Kim M, Kim J, Lee DG. Durability of graphite coated carbon composite bipolar plates for vanadium redox flow batteries. Composite Structures. 2015;134:106-13. [13] Kim KH, Choe J, Nam S, Kim BG, Lee DG. Surface crack closing method for the carbon composite bipolar plates of a redox flow battery. Composite Structures. 2015;119:436-42. [14] Lee D, Lim JW, Nam S, Choi I, Lee DG. Method for exposing carbon fibers on composite bipolar plates. Composite Structures. 2015;134:1-9. [15] Lim JW, Lee DG. Carbon fiber/polyethylene bipolar plate-carbon felt electrode assembly for vanadium redox flow batteries (VRFB). Composite Structures. 2015;134:483-92. [16] Lee D, Lee DG. Electro-mechanical properties of the carbon fabric composites with fibers exposed on the surface. Composite Structures. 2016;140:77-83. [17] Chieng S, Kazacos M, Skyllas-Kazacos M. Modification of Daramic, microporous separator, for redox flow battery applications. Journal of Membrane Science. 1992;75:81-91. [18] Lu Y, Goodenough JB. Rechargeable alkali-ion cathode-flow battery. Journal of Materials Chemistry. 2011;21:10113-7. [19] Vafiadis H, Skyllas-Kazacos M. Evaluation of membranes for the novel vanadium 12
bromine redox flow cell. Journal of Membrane Science. 2006;279:394-402. [20] Nam S, Lee D, Choi I, Lee DG. Smart cure cycle for reducing the thermal residual stress of a co-cured E-glass/carbon/epoxy composite structure for a vanadium redox flow battery. Composite Structures. 2015;120:107-16. [21] Kader MA, Lyu MY, Nah C. A study on melt processing and thermal properties of fluoroelastomer nanocomposites. Composites Science and Technology. 2006;66:143143. [22] XU T, YANG J, LIU J, FU Q. Surface modification of carbon nanotube and its influence on the conductivity property of carbon nanotube/fluoro-elastomer composite. Acta Materiae Compositae Sinica. 2010;3:002. [23] Bhattacharya SK, Bhowmick AK, Singh RK. Crack-Growth Resistance of Fluoroelastomer Vulcanizates Filled with Particulate and Fiber Filler. Journal of Materials Science. 1995;30:243-7. [24] Plueddemann EP. Silane coupling agents: Springer Science & Business Media, 2013.
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Table 1 Dimensional properties of specimens with different fabrication condition. Fabrication condition
Total thickness
Surface fluoroelastomer
Fiber volume
(mm)
layer thickness (mm)
fraction (%)
10
1.55
0.37
13
15
1.53
0.36
15
20
1.38
0.23
21
10
3.14
0.45
11
15
2.48
0.24
19
20
2.15
0.20
23
Number
Curing
of ply
pressure (MPa)
4-ply
8-ply
14
Endplate
Electrolyte tank
Stack
Pump
Bipolar plate
Anode: VO2++2H++e- VO2++H2O Cathode: V2+ V3++eMembrane
Fig. 1 Schematic diagram of a VRFB system and the chemical reactions in the unit cell.
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Current collector Bipolar plate Sheet gasket Gasket-less Flow frame
Flow frame Electrode Sheet gasket Membrane
O-ring
(a)
(b)
Fig. 2 Components of VRFB unit cell: (a) conventional components; (b) reduced components with gasket-less composite flow frame.
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Fluoroelastomer layer
Glass fiber fabric
Fig. 3 Concept of gasket-less composite flow frame for VRFB
17
10 mm
Fig. 4 Shredded glass fiber fabrics when fabricated without surface treatment methods.
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1. Flame treatment for 20-sec Glass fabric
2. Silane coupling agent treatment
0.1% silane solution
120 oC
Torch
Fig. 5 Surface treatment methods for fluoroelastomer/glass fiber composite.
19
the
glass fabric
to
fabricate
the
5 mm
0.5 mm (a)
(b)
Fig. 6 Fluoroelastomer: (a) as received slab compound; (b) fabricated sheet compound.
20
Compression mold Specimen
Temperature (° C)
(a)
Pressure (MPa)
190oC Cyclic pressure
0
2
20
Time (minute)
(b) Fig. 7 Fabrication method of the fluoroelastomer/glass fiber composite: (a) schematic diagram for compression molding; (b) curing condition. .
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Flow channel
(a)
Glass fiber fabric Fluoroelastomer sheet compound 4-ply 4-ply
4-ply (b)
(c)
Fig. 8 Cross section of flow channel part of FF and stacking sequences of fluoroelastomer sheet compound and glass fabric: (a)cross section of flow channel part; (b) 4-ply of glass fabric (c) 8-ply of glass fabric.
22
50
100
10
15 10
Tab
unit: mm
32 (a)
(b)
Fig. 9 Dimensions and configurations for mechanical property tests to measure: (a) tensile strength; (b) flexural strength (Dimensions in mm).
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Pressure sensor 1
Pressure Pressure sensor 2
Butyl sealant
Air Chamber 2 Pressure Chamber 1 8 60
Flow frame specimen
Fig. 10 Schematic diagrams of sealability test and flow frame specimen (Dimensions in mm).
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Flexural strength (MPa)
Tensile strength (MPa)
120 100 80 60 40 20 0 Amino KBM-803 functional group
15 12 9 6 3 0 Amino KBM-803
Mercapto KBM-903 functional group
functional group
(a)
Mercapto KBM-903 functional group
(b)
Fig. 11 Effects of the silane coupling agent on fluoroelastomer/glass fiber composite: (a) tensile strength; (b) flexural strength.
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50 µm
50 µm
Unwetted fibers
(a)
(b)
Fig. 12 Effect of the cyclic pressurizing process: (a) without cyclic pressurizing process; (b) with cyclic pressurizing process.
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1 mm
1 mm
1 mm 0.37 mm
0.36 mm
(a) 1 mm
(b) 0.45 mm
(d)
0.23 mm
(c) 1 mm
1 mm 0.24 mm
(e)
0.20 mm
(f)
Fig. 13 Cross section images of fabricated fluoroelastomer/glass fiber composite specimens: (a) 4-ply cured at 10 MPa; (b) 4-ply cured at 15 MPa; (c) 4-ply cured at 20 MPa; (d) 8-ply cured at 10 MPa; (e) 8-ply cured at 15 MPa; (f) 8-ply cured at 20 MPa.
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4-ply
Tensile strength (MPa)
140
8-ply
120 100 80 60 40 20 0 10
15
20
Curing pressure (MPa) Fig. 14 Tensile strengths with respect to the curing pressure and stacking sequences.
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4-ply
8-ply
Zero-leakage pressure (MPa)
0.6 Typical compaction pressure
0.5 0.4 0.3 0.2 0.1 0 10
15 Curing pressure (MPa)
20
Fig. 15 Zero-leakage compaction pressure with respect to the curing pressure and stacking sequences.
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