Name: experimental implementation of a new composite fabrication method: exposing bare fibers on the composite surface by the soft layer method
Uploaded: 2017-10-06T12:30:00.000+00:00
Duration: 6 min 26 s
Description: Watch this Scientific Journal Video about Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method at JoVE.com

This research was supported by the Climate Change Research Hub of KAIST (grant No. N11160012), the Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (grant No. 2011-0030065), the Leading Human Resource Training Program of Regional Neo Industry through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (grant no. NRF-2016H1D5A1910603). Their support is greatly appreciated.

1. Material Preparation
<ol>
	<li>Preparation of the composite material 
	NOTE: CAUTION, Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in these methods may be toxic and carcinogenic. Nanomaterials may have additional hazards compared to their bulk counterparts. Please use all appropriate safety practices when performing an experiment, including the use of engineering controls (fume hood, glove box) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes). 
	NOTE: Depending on the application, the type of reinforcing fiber can be one or a combination of the following: unidirectional fiber, woven fabric, non-woven felt, chopped fiber.
	<ol>
		<li>Unidirectional fiber type
		<ol>
			<li>Use pre-impregnated composite material (prepreg), as it is the most convenient to use.</li>
			<li>Stack the prepreg in a stacking sequence containing both 0 &#176; and 90 &#176; to avoid splitting. For example, stack into [03/903]s.</li>
		</ol>
		</li>
		<li>Woven fabric type
		<ol>
			<li>Prepare the woven carbon fabric and film-type epoxy resin. If using a prepreg, skip steps 1.1.2.1 to 1.1.2.6.</li>
			<li>Cleanse the fabric with 99.5% acetone or another solvent for degreasing. Take caution when handling the fabric after cleaning to avoid contamination. Place the fabric on a clean surface or lint-free wipe.</li>
			<li>Remove the solvent by drying under ambient conditions for 10 min.</li>
			<li>Peel off the backup film of the epoxy resin and attach 1 ply of film-type epoxy to 1 ply of carbon fabric.</li>
			<li>Place the epoxy-attached carbon fabric on a hot plate that is pre-heated to 70 &#176;C for 10 s for pre-impregnation.</li>
			<li>Cool the prepared prepreg in ambient conditions for 10 min and peel off the other backup film.</li>
			<li>Stack the fabric with the desired stacking sequence; for example, stack into [0]3.</li>
		</ol>
		</li>
		<li>Non-woven felt
		<ol>
			<li>Prepare the non-woven felt.</li>
			<li>Cleanse the felt with 99.5% acetone or another solvent for degreasing. Take caution when handling the felt after cleansing to avoid contamination. Place the felt on a clean surface or lint-free wipe.</li>
			<li>Peel off the backup film of the epoxy resin and attach 3 plies of film-type epoxy to 1 ply of carbon felt on each side.</li>
			<li>Place the epoxy-attached carbon felt on a hot plate that is pre-heated to 70 &#176;C for 10 s for pre-impregnation.</li>
			<li>Cool the prepared prepreg in the ambient condition for 10 min and peel off the other backup film.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Preparation of the soft layer 
	NOTE: For the soft layer, a fluoropolymer such as polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP), a polyolefin such as polyethylene or polypropylene, or a synthetic rubber such as silicone rubber or a fluoroelastomer can be used. In this protocol, FEP film is adopted, and its yield strength drops drastically over 120 &#176;C. 25-&#181;m-thick FEP is suitable for unidirectional fiber and non-woven felt composites, whereas a thicker 100-&#181;m-thick FEP is appropriate for woven fabric type composites10.
	<ol>
		<li>Cleanse the soft layer with 99.5% acetone. Handle with care to avoid wrinkles and pinholes.</li>
		<li>Wipe off the acetone on the soft layer with lint-free wipes. Ensure that there is no contaminant on the soft layer because it will be transferred to the composite during the curing process. Always keep the soft layer away from dust and small particles because these may damage not only the composite but also the compression mold.</li>
	</ol>
	</li>
</ol>
2. Composite Fabrication
<ol>
	<li>Installation of the compression mold
	<ol>
		<li>Prepare a compression mold with a cavity of size 120 mm &#215; 120 mm.</li>
		<li>Apply the mold release to the compression mold. Simply paste or spray the mold release to the mold, and wipe with lint-free wipes until only a thin layer of mold release remains.</li>
		<li>Cut the prepared composite laminate to a size of 118 mm &#215; 118 mm.</li>
		<li>Place 1 ply of 25 &#181;m-thick FEP film on the lower mold.</li>
		<li>Place the composite laminate on the FEP film and place another FEP film on the laminate.</li>
		<li>Flatten the soft layer and remove the air bubbles that are entrapped between the soft layer and composite laminate.</li>
		<li>Close the mold for the compression molding.</li>
	</ol>
	</li>
	<li>Compression molding
	<ol>
		<li>Heat the hot press to 150 &#176;C. 
		NOTE: The temperature of the specimen inside the mold is 140 &#176;C in this condition. The use of a lower temperature is also possible if an elastomer or polyolefin is adopted for the soft layer. Consider both the curing temperature of the composite and the softening temperature of the soft layer to determine the curing temperature.</li>
		<li>Place the mold in the hot press.</li>
		<li>Apply pressure using the hot press; the curing schedule and pressure depend on the composite type.
		<ol>
			<li>For a unidirectional fiber composite, apply a constant pressure of 20 MPa for 30 min; no additional process is required.</li>
			<li>For a woven fabric type composite, apply 20 MPa. After 4 min and 8 min, release the applied pressure to zero and immediately apply 20 MPa again. 
			NOTE: This process is called purging, and its purpose is to remove excessive resin and entrapped air bubbles. The number of purging steps may be increased depending on the size of the composite; larger-size composites require more purging.
			<ol>
				<li>However, after the viscosity of the resin starts to increase, do not purge. Cure for 30 min in total.</li>
			</ol>
			</li>
			<li>For a non-woven felt type composite, apply 3 MPa for 30 min. Beware of pressure overshoot, which will result in voids and defects in the final product. Increase the pressure slowly to avoid pressure overshoot.</li>
		</ol>
		</li>
		<li>Cool the compression mold in the hot press without releasing the pressure to below 120 &#176;C, which is the glass transition temperature of the fabricated composite.</li>
		<li>Release the pressure and remove the compression mold from the hot press.</li>
		<li>Demold the final product from the compression mold.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Hwang, I. U., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bipolar+plate+made+of+carbon+fiber+epoxy+composite+for+polymer+electrolyte+membrane+fuel+cells.">Bipolar plate made of carbon fiber epoxy composite for polymer electrolyte membrane fuel cells.</a> J Power Sources. 184 (1), 90-94 (2008).</li><li>Avasarala, B., Haldar, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+surface+roughness+of+composite+bipolar+plates+on+the+contact+resistance+of+a+proton+exchange+membrane+fuel+cell.">Effect of surface roughness of composite bipolar plates on the contact resistance of a proton exchange membrane fuel cell.</a> J Power Sources. 188 (1), 225-229 (2009).</li><li>Yu, H. N., Lim, J. W., Kim, M. K., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasma+treatment+of+the+carbon+fiber+bipolar+plate+for+PEM+fuel+cell.">Plasma treatment of the carbon fiber bipolar plate for PEM fuel cell.</a> Compos Struct. 94 (5), 1911-1918 (2012).</li><li>Lim, J. W., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+composite-metal+hybrid+bipolar+plates+for+PEM+fuel+cells.">Development of composite-metal hybrid bipolar plates for PEM fuel cells.</a> Int J Hydrogen Energy. 37 (17), (2012).</li><li>Kim, B. G., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electromagnetic-carbon+surface+treatment+of+composite+bipolar+plate+for+high-efficiency+polymer+electrolyte+membrane+fuel+cells.">Electromagnetic-carbon surface treatment of composite bipolar plate for high-efficiency polymer electrolyte membrane fuel cells.</a> J Power Sources. 195 (6), 1577-1582 (2010).</li><li>Kim, B. G., Lim, J. W., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+single-type+aluminum/composite+hybrid+bipolar+plate+with+surface+modification+for+high+efficiency+PEMFC.">A single-type aluminum/composite hybrid bipolar plate with surface modification for high efficiency PEMFC.</a> Int J Hydrogen Energy. 36 (4), 3087-3095 (2011).</li><li>Yu, H. N., Lim, J. W., Suh, J. D., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+graphite-coated+carbon+fiber+epoxy+composite+bipolar+plate+for+polymer+electrolyte+membrane+fuel+cell.">A graphite-coated carbon fiber epoxy composite bipolar plate for polymer electrolyte membrane fuel cell.</a> J Power Sources. 196 (23), 9868-9875 (2011).</li><li>Kim, K. H., Kim, B. G., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+carbon+composite+bipolar+plate+(BP)+for+vanadium+redox+flow+battery+(VRFB).">Development of carbon composite bipolar plate (BP) for vanadium redox flow battery (VRFB).</a> Compos Struct. 109, 253-259 (2014).</li><li>Lee, D., Lim, J. W., Nam, S., Choi, I., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gasket-integrated+carbon/silicone+elastomer+composite+bipolar+plate+for+high-temperature+PEMFC.">Gasket-integrated carbon/silicone elastomer composite bipolar plate for high-temperature PEMFC.</a> Compos Struct. 128, 284-290 (2015).</li><li>Lee, D., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electro-mechanical+properties+of+the+carbon+fabric+composites+with+fibers+exposed+on+the+surface.">Electro-mechanical properties of the carbon fabric composites with fibers exposed on the surface.</a> Compos Struct. 140, 77-83 (2016).</li><li>Lee, D., Lim, J. W., Nam, S., Choi, I., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Method+for+exposing+carbon+fibers+on+composite+bipolar+plates.">Method for exposing carbon fibers on composite bipolar plates.</a> Compos Struct. 134, 1-9 (2015).</li><li>Lee, D., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Carbon+composite+bipolar+plate+for+high-temperature+proton+exchange+membrane+fuel+cells+(HT-PEMFCs).">Carbon composite bipolar plate for high-temperature proton exchange membrane fuel cells (HT-PEMFCs).</a> J Power Sources. 327, 119-126 (2016).</li><li>Lee, D., Choe, J., Nam, S., Lim, J. W., Choi, I., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+non-woven+carbon+felt+composite+bipolar+plates+using+the+soft+layer+method.">Development of non-woven carbon felt composite bipolar plates using the soft layer method.</a> Compos struct. 160, 976-982 (2016).</li><li>Lee, D., Lim, J. W., Lee, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cathode/anode+integrated+composite+bipolar+plate+for+high-temperature+PEMFC.">Cathode/anode integrated composite bipolar plate for high-temperature PEMFC.</a> Compos Struct. 167, 144-151 (2017).</li><li>Lee, D., Oh, Y., Nam, S., Choe, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adhesion+Characteristics+of+Fiber-exposed+Glass+Composites.">Adhesion Characteristics of Fiber-exposed Glass Composites.</a> Compos Struct. 165, 9-14 (2017).</li></ol>

The authors have nothing to disclose.

The soft layer method provides significant advantages compared with the conventional methods, and with a lower manufacturing cost. All three types of composites manufactured by the soft layer method show unique characteristics in terms of the electrical properties, mechanical properties, gas permeability, and adhesion properties.
For the measurement of the electrical property, a four-point probe method was used. ASR was measured 5 times and the average value was taken as a representative value for that bipolar plate. A total of five bipolar plates were measured, and the maximum and minimum ASR values were used for the error bar.
The electrical conductivity in the through-thickness direction increases significantly due to the exposed carbon fiber (Figure 2a) and satisfies the DOE target (Department of Energy, United States) of ASR &#60;20 m&#937;&#8729;cm2 under a compaction pressure of 1.38 MPa. For the measurement of the mechanical property, tensile tests were performed according to ASTM D3039. Nine specimens were tested and the average value was taken as a representative value while the maximum and minimum values were used for the error bar.
The tensile strength of the unidirectional carbon fiber composite does not change much, but the carbon fabric and carbon felt type composites show significant increases in the tensile strength of 22% and 15%, respectively, when the soft layer method is applied. The tensile strength increases because the soft layer can apply a uniform pressure on the entire surface. For this reason, the gas permeability of the composite is improved as well10,14. In addition, the adhesion characteristics are improved due to the rough surface generated by the fibers15.
Although the soft layer provides incomparable advantages, care should be taken in the implementation to achieve the best result. First, use a soft layer without pores or defects. Resin will bleed out through the hole, which will result in dents after curing as well as contamination to the mold and composite. Minor wrinkles will disappear under high temperature and pressure, but holes will not. Second, the thickness of the soft layer must be taken into account when designing a mold, such as in the design of a channel-shape mold for a fuel cell. Measure the thickness of the soft layer after applying an identical pressure and temperature to what will be used to cure the composite; this thickness shall be adopted for the mold design. Third, multiple plies of the soft layer are possible, but great care must be taken, as when the number of soft layers increases, the capability to remove resin increases. However, wrinkles may appear on the composite surface. This is especially noticeable for non-woven carbon felt composites.
If the fibers are not well exposed, there are four options to choose from: increase the curing pressure; increase the curing temperature; select another soft layer that has lower mechanical properties or thermal properties; or provide a cavity for the excess resin. Because the basic mechanism of the soft layer method lies in the deformation of the soft layer under the applied pressure, modifying the curing pressure or temperature may improve the results.
In conclusion, the soft layer method brings numerous benefits that were not possible with other methods when implemented with proper care. Compared to the conventional methods to expose fibers on the surface, the soft layer method does not require any post surface treatment, making it an ideal method for large-area industrial applications where productivity is a crucial factor. This method can be further expanded to a general composite fabrication method or a general composite surface treatment method.

The bipolar plate is a multi-functional key component of energy conversion systems and energy storage systems such as fuel cells and batteries. The key functional requirements of the bipolar plate are as follows: high electrical conductivity in the through-thickness direction to reduce ohmic-loss, high mechanical properties to withstand high compaction pressure and external impacts, and high productivity for mass production.
Compared with the graphite and metals that were conventionally adopted as materials for the bipolar plate, carbon fiber/epoxy composites have a higher specific strength and stiffness, which indicates that the weight of the system can be greatly reduced by replacing the conventional bipolar plate materials with composites1. However, conventional carbon/epoxy composites have poor electrical conductivity in the through-thickness direction, which results in a large areal specific resistance (ASR), due to the resin-rich layer that is formed on the composite surface. The insulating resin-rich layer prevents direct contact between the conductive carbon fibers and adjacent components, such as another bipolar plate, gas diffusion layer (GDL), and carbon felt electrode (CFE). 
Many studies were conducted to resolve the high ASR due to the resin-rich layer. The first approach was surface treatment methods to selectively remove the resin-rich layer. For example, mechanical abrasion was attempted to remove the resin on the surface2. However, the carbon fibers were also damaged, which resulted in a poor ASR. Plasma treatment3,4 and microwave treatment methods5,6 were also developed to avoid fiber damage, but they resulted in low productivity and uniformity. The second approach, conductive layer coating methods, includes expanded graphite coating7,8. This method successfully reduced the ASR and has been regarded as a standard method to manufacture a composite bipolar plate. However, it is costly and has durability and delamination issues due to the low mechanical strength.
In this study, the "soft layer method", a novel manufacturing method that can expose carbon fibers on the composite bipolar plate surface, is demonstrated. The main purpose of this method is to obtain a low ASR with a low manufacturing cost. The soft layer method adopts a thin soft layer such as a polymer release film between the compression mold and bipolar plate. After curing in the compression mold and the detaching of the soft layer, the fabricated bipolar plate displays carbon fibers exposed on the surface without any post-surface treatment. This method not only decreased the ASR but also significantly increased the mechanical properties and solved the gas permeability issue. This method can be applied for many other purposes: the development of an electrically conductive plate, the manufacture of a thin composite, and the fabrication of an adhesive joint without surface treatment.

<table><tbody><tr><td>Unidirectional carbon/epoxy prepreg</td><td>SK Chemicals</td><td>USN020</td><td>Used to fabricate unidirectional carbon composite</td></tr><tr><td>Plain weave carbon fabric/epoxy prepreg</td><td>SK Chemicals</td><td>WSN 1k</td><td>Used to fabricate fabric carbon composite</td></tr><tr><td>Plain weave carbon fabric</td><td>SK Chemicals</td><td>C-112</td><td>Used to fabricate fabric carbon composite</td></tr><tr><td>Non-woven carbon felt</td><td>Newell</td><td>Graphite felt 3 mm</td><td>Used to fabricated felt carbon composite</td></tr><tr><td>Film type epoxy resin</td><td>SK Chemicals</td><td>K51</td><td>Used as a matrix of the composite</td></tr><tr><td>Acetone 99.5%</td><td>Samchun</td><td>67-64-1</td><td>Used to cleanse the carbon fiber and the soft layers</td></tr><tr><td>Mold release</td><td>ShinEtsu</td><td>KF-96</td><td>Used to coat the mold</td></tr><tr><td>Release film</td><td>Airtech</td><td>A4000V</td><td>Used as a soft layer</td></tr><tr><td>Compression mold</td><td>N/A</td><td>N/A</td><td>Machined in lab. Material: NAK80</td></tr><tr><td>Hot press</td><td>Hydrotek 100</td><td>N/A</td><td>Used to apply pressure and heat</td></tr><tr><td>Scanning electron microscope</td><td>FEI Compnay</td><td>Magellan 400</td><td>Used to investigate the surface of the composite</td></tr></tbody></table>

experimental implementation of a new composite fabrication method: exposing bare fibers on the composite surface by the soft layer method

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional component that should have high electrical conductivity, high mechanical properties, and high productivity.
In this regard, a carbon fiber/epoxy resin composite can be an ideal material to replace the conventional graphite bipolar plate, which often leads to the catastrophic failure of the entire system because of its inherent brittleness. Though the carbon/epoxy composite has high mechanical properties and is easy to manufacture, the electrical conductivity in the through-thickness direction is poor because of the resin-rich layer that forms on its surface. Therefore, an expanded graphite coating was adopted to solve the electrical conductivity issue. However, the expanded graphite coating not only increases the manufacturing costs but also has poor mechanical properties.
In this study, a method to expose fibers on the composite surface is demonstrated. There are currently many methods that can expose fibers by surface treatment after the fabrication of the composite. This new method, however, does not require surface treatment because the fibers are exposed during the manufacture of the composite. By exposing bare carbon fibers on the surface, the electrical conductivity and mechanical strength of the composite are increased drastically.

The fabricated specimens are observed using scanning electron microscopy (SEM) (Figure 1). Because the resin-rich layer that covers the top of the fibers is only a few micrometers thick, an optical microscopic image observed at the top of the specimen is not appropriate. A SEM image observed by tilting the specimen by 5&#176; provides a more representative image. Compared to the composites fabricated by conventional compression molding, which has its surface covered with resin, bare fibers are exposed without defects when the composites are fabricated by the soft layer method. The soft layer method was applicable to the unidirectional carbon composite, carbon fabric composite, and carbon felt composite.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/55815/55815fig1.jpg" /> 
Figure 1: SEM images of the fabricated specimen. (a) Unidirectional fiber composite with conventional method11; (b) Unidirectional fiber composite with soft layer method11; (c) Woven fabric composite with conventional method12; (d) Woven fabric composite with soft layer method12; (e) Non-woven felt composite with conventional method13; (f) Non-woven felt composite with soft layer method13. All referenced images have been reprinted with permission from original publishers. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55815/55815fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/55815/55815fig2.jpg" /> 
Figure 2: Performance of the composite bipolar plate. Here, the average value was taken as a representative value, while the maximum and minimum values were used for the error bars. (a) Electrical conductivity in the through-thickness direction, area of specific resistance (ASR) is shown; (b) Tensile strength. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55815/55815fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method at JoVE.com

Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method

A protocol to expose bare fibers on the composite surface by eliminating resin rich area is presented. The fibers are exposed during fabrication of the composites, not by the post surface treatment. The exposed carbon composites exhibit high electrical conductivity in the through-thickness direction and high mechanical property.

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional ...

experimental-implementation-new-composite-fabrication-method-exposing

Nanyang Technological University

Research

JoVE Journal

Engineering

8.2K Views.  Korea Advanced Institute of Science and Technology. A protocol to expose bare fibers on the composite surface by eliminating resin rich area is presented. The fibers are exposed during fabrication of the composites, not by the post surface treatment. The exposed carbon composites exhibit high electrical conductivity in the through-thickness direction and high mechanical property.

Video: Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Process of Making Three-dimensional Microstructures using Vaporization of a Sacrificial Component

Vascular structures in natural systems are able to provide high mass transport through high surface areas and optimized structure. Few synthetic material fabrication techniques are able to mimic the complexity of these structures while maintaining scalability. The Vaporization of a Sacrificial Component (VaSC) process is able to do so. This process uses sacrificial fibers as a template to form hollow, cylindrical microchannels embedded within a matrix. Tin (II) oxalate (SnOx) is embedded within poly(lactic) acid (PLA) fibers which facilitates the use of this process. The SnOx catalyzes the depolymerization of the PLA fibers at lower temperatures. The lactic acid monomers are gaseous at these temperatures and can be removed from the embedded matrix at temperatures that do not damage the matrix. Here we show a method for aligning these fibers using micromachined plates and a tensioning device to create complex patterns of three-dimensionally arrayed microchannels. The process allows the exploration of virtually any arrangement of fiber topologies and structures.

The Vaporization of a Sacrificial Component (VaSC) process is used to fabricate microvascular structures. This procedure uses sacrificial poly(lactic) acid fibers to form hollow microchannels with precise 3D geometric positioning provided by laser micromachined guide plates.

Vascular structures in natural systems are able to provide high mass transport through high surface areas and optimized structure. Few synthetic material ...

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

The integration of conductive nanoparticles, such as graphene nanoplatelets, into glass fiber composite materials creates an intrinsic electrical network susceptible to strain. Here, different methods to obtain strain sensors based on the addition of graphene nanoplatelets into the epoxy matrix or as a coating on glass fabrics are proposed.

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are ...

Microfluidic Fabrication of Polymeric and Biohybrid Fibers with Predesigned Size and Shape

A &#8220;sheath&#8221; fluid passing through a microfluidic channel at low Reynolds number can be directed around another &#8220;core&#8221; stream and used to dictate the shape as well as the diameter of a core stream.&#160;Grooves in the top and bottom of a microfluidic channel were designed to direct the sheath fluid and shape the core fluid.&#160;By matching the viscosity and hydrophilicity of the sheath and core fluids, the interfacial effects are minimized and complex fluid shapes can be formed.&#160;Controlling the relative flow rates of the sheath and core fluids determines the cross-sectional area of the core fluid.&#160;Fibers have been produced with sizes ranging from 300 nm to ~1 mm, and fiber cross-sections can be round, flat, square, or complex as in the case with double anchor fibers. Polymerization of the core fluid downstream from the shaping region solidifies the fibers.&#160;Photoinitiated click chemistries are well suited for rapid polymerization of the core fluid by irradiation with ultraviolet light.&#160;Fibers with a wide variety of shapes have been produced from a list of polymers including liquid crystals, poly(methylmethacrylate), thiol-ene and thiol-yne resins, polyethylene glycol, and hydrogel derivatives. Minimal shear during the shaping process and mild polymerization conditions also makes the fabrication process well suited for encapsulation of cells and other biological components.

Two adjacent fluids passing through a grooved microfluidic channel can be directed to form a sheath around a prepolymer core; thereby&#160;determining both shape and cross-section. Photoinitiated polymerization, such as thiol click chemistry, is well suited for rapidly solidifying the core fluid into a microfiber with predetermined size and shape.

A &#8220;sheath&#8221; fluid passing through a microfluidic channel at low Reynolds number can be directed around another &#8220;core&#8221; stream and ...

Bioengineering

Microfluidic-based Synthesis of Covalent Organic Frameworks (COFs): A Tool for Continuous Production of COF Fibers and Direct Printing on a Surface

Covalent Organic Frameworks (COFs) are a class of porous covalent materials which are frequently synthesized as unprocessable crystalline powders. The first COF was reported in 2005 with much effort centered on the establishment of new synthetic routes for its preparation. To date, most available synthetic methods for COF synthesis are based on bulk mixing under solvothermal conditions. Therefore, there is increasing interest in developing systematic protocols for COF synthesis that provide for fine control over reaction conditions and improve COF processability on surfaces, which is essential for their use in practical applications. Herein, we present a novel microfluidic-based method for COF synthesis where the reaction between two constituent building blocks, 1,3,5-benzenetricarbaldehyde (BTCA) and 1,3,5-tris(4-aminophenyl)benzene (TAPB), takes place under controlled diffusion conditions and at room temperature. Using such an approach yields sponge-like, crystalline fibers of a COF material, hereafter called MF-COF. The mechanical properties of MF-COF and the dynamic nature of the approach allow the continuous production of MF-COF fibers and their direct printing onto surfaces. The general method opens new potential applications requiring advanced printing of 2D or 3D COF structures on flexible or rigid surfaces.

Microfluidic-based Synthesis of Covalent Organic Frameworks COFs: A Tool for Continuous Production of COF Fibers and Direct Printing on a Surface

We present a novel microfluidic-based method for synthesis of covalent organic frameworks (COFs). We demonstrate how this approach can be used to produce continuous COF fibers, and also 2D or 3D COF structures on surfaces.

Covalent Organic Frameworks (COFs) are a class of porous covalent materials which are frequently synthesized as unprocessable crystalline powders. The ...

Chemistry

Microfluidic Dry-spinning and Characterization of Regenerated Silk Fibroin Fibers

The protocol demonstrates a method for mimicking the spinning process of silkworm. In the native spinning process, the contracting spinning duct enables the silk proteins to be compact and ordered by shearing and elongation forces. Here, a biomimetic microfluidic channel was designed to mimic the specific geometry of the spinning duct of the silkworm. Regenerated silk fibroin (RSF) spinning doped with high concentration, was extruded through the microchannel to dry-spin fibers at ambient temperature and pressure. In the post-treated process, the as-spun fibers were drawn and stored in ethanol aqueous solution. Synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) technology was used to investigate the microstructure of single RSF fibers, which were fixed to a sample holder with the RSF fiber axis normal to the microbeam of the X-ray. The crystallinity, crystallite size, and crystalline orientation of the fiber were calculated from the WAXD data. The diffraction arcs near the equator of the two-dimensional WAXD pattern indicate that the post-treated RSF fiber has a high orientation degree.

A protocol for the microfluidic spinning and microstructure characterization of regenerated silk fibroin monofilament is presented.

The protocol demonstrates a method for mimicking the spinning process of silkworm. In the native spinning process, the contracting spinning duct enables ...

Manufacturing Of Robust Natural Fiber Preforms Utilizing Bacterial Cellulose as Binder

A novel method of manufacturing rigid and robust natural fiber preforms is presented here. This method is based on a papermaking process, whereby loose and short sisal fibers are dispersed into a water suspension containing bacterial cellulose. The fiber and nanocellulose suspension is then filtered (using vacuum or gravity) and the wet filter cake pressed to squeeze out any excess water, followed by a drying step. This will result in the hornification of the bacterial cellulose network, holding the loose natural fibers together.
Our method is specially suited for the manufacturing of rigid and robust preforms of hydrophilic fibers. The porous and hydrophilic nature of such fibers results in significant water uptake, drawing in the bacterial cellulose dispersed in the suspension. The bacterial cellulose will then be filtered against the surface of these fibers, forming a bacterial cellulose coating. When the loose fiber-bacterial cellulose suspension is filtered and dried, the adjacent bacterial cellulose forms a network and hornified to hold the otherwise loose fibers together.
The introduction of bacterial cellulose into the preform resulted in a significant increase of the mechanical properties of the fiber preforms. This can be attributed to the high stiffness and strength of the bacterial cellulose network. With this preform, renewable high performance hierarchical composites can also be manufactured by using conventional composite production methods, such as resin film infusion (RFI) or resin transfer molding (RTM). Here, we also describe the manufacturing of renewable hierarchical composites using double bag vacuum assisted resin infusion.

We present a novel method of manufacturing rigid and robust short natural fiber preforms using a papermaking process. Bacterial cellulose acts simultaneously as the binder for the loose fibers and provides rigidity to the fiber preforms. These preforms can be infused with a resin to produce truly green hierarchical composites.

A novel method of manufacturing rigid and robust natural fiber preforms is presented here. This method is based on a papermaking process, whereby loose ...

Magnet Assisted Composite Manufacturing: A Flexible New Technique for Achieving High Consolidation Pressure in Vacuum Bag/Lay-Up Processes

This work demonstrates a protocol to improve the quality of composite laminates fabricated by wet lay-up vacuum bag processes using the recently developed magnet assisted composite manufacturing (MACM) technique. In this technique, permanent magnets are utilized to apply a sufficiently high consolidation pressure during the curing stage. To enhance the intensity of the magnetic field, and thus, to increase the magnetic compaction pressure, the magnets are placed on a magnetic top plate. First, the entire procedure of preparing the composite lay-up on a magnetic bottom steel plate using the conventional wet lay-up vacuum bag process is described. Second, placement of a set of Neodymium-Iron-Boron permanent magnets, arranged in alternating polarity, on the vacuum bag is illustrated. Next, the experimental procedures to measure the magnetic compaction pressure and volume fractions of the composite constituents are presented. Finally, methods used to characterize microstructure and mechanical properties of composite laminates are discussed in detail. The results prove the effectiveness of the MACM method in improving the quality of wet lay-up vacuum bag laminates. This method does not require large capital investment for tooling or equipment and can also be used to consolidate geometrically complex composite parts by placing the magnets on a matching top mold positioned on the vacuum bag.

A new technique for applying consolidation pressure on the vacuum bag lay-up to fabricate composite laminates is described. The goal of this protocol is to develop a simple, cost-effective technique to improve the quality of laminates fabricated by the wet lay-up vacuum bag method.

This work demonstrates a protocol to improve the quality of composite laminates fabricated by wet lay-up vacuum bag processes using the recently developed ...

Cutting Procedures, Tensile Testing, and Ageing of Flexible Unidirectional Composite Laminates

Many body armor designs incorporate unidirectional (UD) laminates. UD laminates are constructed of thin (&#60;0.05 mm) layers of high-performance yarns, where the yarns in each layer are oriented parallel to each other and held in place using binder resins and thin polymer films. The armor is constructed by stacking the unidirectional layers in different orientations. To date, only very preliminary work has been performed to characterize the ageing of the binder resins used in unidirectional laminates and the effects on their performance. For example, during the development of the conditioning protocol used in the National Institute of Justice Standard-0101.06, UD laminates showed visual signs of delamination and reductions in V50, which is the velocity at which half of the projectiles are expected to perforate the armor, after ageing. A better understanding of the material property changes in UD laminates is necessary to comprehend the long-term performance of armors constructed from these materials. There are no current standards recommended for mechanically interrogating unidirectional (UD) laminate materials. This study explores methods and best practices for accurately testing the mechanical properties of these materials and proposes a new test methodology for these materials. Best practices for ageing these materials are also described.

The goal of the study was to develop protocols to prepare consistent specimens for accurate mechanical testing of high-strength aramid or ultra-high-molar-mass polyethylene-based flexible unidirectional composite laminate materials and to describe protocols for performing artificial ageing on these materials.

Many body armor designs incorporate unidirectional (UD) laminates. UD laminates are constructed of thin (&#60;0.05 mm) layers of high-performance yarns, ...

Fabrication and Design of Wood-Based High-Performance Composites

Delignified densified wood is a new promising and sustainable material that possesses the potential to replace synthetic materials, such as glass fiber reinforced composites, due to its excellent mechanical properties. Delignified wood, however, is rather fragile in a wet state, which makes handling and shaping challenging. Here we present two fabrication processes, closed-mold densification and vacuum densification, to produce high-performance cellulose composites based on delignified wood, including an assessment of their advantages and limitations. Further, we suggest strategies for how the composites can be re-used or decomposed at the end-of-life cycle. Closed-mold densification has the advantage that no elaborate lab equipment is needed. Simple screw clamps or a press can be used for densification. We recommend this method for small parts with simple geometries and large radii of curvature. Vacuum densification in an open-mold process is suitable for larger objects and complex geometries, including small radii of curvature. Compared to the closed-mold process, the open-mold vacuum approach only needs the manufacture of a single mold cavity.

Delignified densified wood represents a new promising lightweight, high-performance and bio-based material with great potential to partially substitute natural fiber reinforced- or glass fiber reinforced composites in the future. We here present two versatile fabrication routes and demonstrate the possibility to create complex composite parts.

Delignified densified wood is a new promising and sustainable material that possesses the potential to replace synthetic materials, such as glass fiber ...