Name: wicking tests for unidirectional fabrics: measurements of capillary parameters to evaluate capillary pressure in liquid composite molding processes
Uploaded: 2017-01-27T13:00:00
Description: Watch this Scientific Journal Video about Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes at JoVE.co

Caution: Consult all relevant material safety data sheets. Chemicals used for the tests are toxic and carcinogenic. Use personal protective equipment (safety glasses, gloves, lab coat, full-length pants, and closed-toe shoes).
1. Setup for Tests
<ol>
	<li>Preparation of samples
	<ol>
		<li>Cut strips of fabric along the direction perpendicular to the fibers (in order to test wicking in the fiber direction). 
		NOTE: The lengths of the strips are calculated in order to o...

<ol>
	<li>Lawrence, J. M., Neacsu, V., Advani, S. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modeling+the+impact+of+capillary+pressure+and+air+entrapment+on+fiber+tow+saturation+during+resin+infusion+in+lcm.">Modeling the impact of capillary pressure and air entrapment on fiber tow saturation during resin infusion in lcm.</a> Compos Part A: Appl Sci Manuf. 40 (8), 1053-1064 (2009).</li><li>Ravey, C., Ruiz, E., Trochu, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+the+optimal+impregnation+velocity+in+resin+transfer+molding+by+capillary+rise+experiments+and+infrared+thermography.">Determination of the optimal impregnation velocity in resin transfer molding by capillary rise experiments and infrared thermography.</a> Compos Sci Technol. 99, 96-102 (2014).</li><li>Verrey, J., Michaud, V., M&#229;nson, J. -. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamic+capillary+effects+in+liquid+composite+moulding+with+non-crimp+fabrics.">Dynamic capillary effects in liquid composite moulding with non-crimp fabrics.</a> Compos Part A: Appl Sci Manuf. 37 (1), 92-102 (2006).</li><li>Abouorm, L., Moulin, N., Bruchon, J., Drapier, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Monolithic+approach+of+Stokes-+Darcy+coupling+for+LCM+process+modelling.">Monolithic approach of Stokes- Darcy coupling for LCM process modelling.</a> Key Eng Mater. 554, 447-455 (2013).</li><li>Washburn, E. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Note+on+a+method+of+determining+the+distribution+of+pore+sizes+in+a+porous+material.">Note on a method of determining the distribution of pore sizes in a porous material.</a> Proc Natl Acad Sci USA. , 115-116 (1921).</li><li>Pucci, M. F., Liotier, P. -. J., Drapier, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+effects+on+flax+fibers-modification+and+characterization+of+the+wetting+dynamics.">Capillary effects on flax fibers-modification and characterization of the wetting dynamics.</a> Compos Part A: Appl Sci Manuf. 77, 257-265 (2015).</li><li>Darcy, H., Dalmont, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Les fontaines publiques de la ville de Dijon: exposition et application. , (1856).</li><li>Pucci, M. F., Liotier, P. -. J., Drapier, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+wicking+in+a+fibrous+reinforcement-orthotropic+issues+to+determine+the+capillary+pressure+components.">Capillary wicking in a fibrous reinforcement-orthotropic issues to determine the capillary pressure components.</a> Compos Part A: Appl Sci Manuf. 77, 133-141 (2015).</li><li>Pucci, M. F., Liotier, P. -. J., Drapier, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+wicking+in+flax+fabrics+-+effects+of+swelling+in+water.">Capillary wicking in flax fabrics - effects of swelling in water.</a> Colloids Surf A: Physicochem Eng Aspects. 498, 176-184 (2016).</li><li>Nguyen, V. H., Lagard&#232;re, M., Park, C. H., Panier, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Permeability+of+natural+fiber+reinforcement+for+liquid+composite+molding+processes.">Permeability of natural fiber reinforcement for liquid composite molding processes.</a> J Mater Sci. 49 (18), 6449-6458 (2014).</li><li>Stuart, T., McCall, R., Sharma, H., Lyons, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modelling+of+wicking+and+moisture+interactions+of+flax+and+viscose+fibres.">Modelling of wicking and moisture interactions of flax and viscose fibres.</a> Carbohydr Polym. 123, 359-368 (2015).</li></ol>

The critical steps in the protocol relate to the preparation of the samples. Firstly, the rolled sampled have to be tight in order to make the assumption of a homogeneous fiber volume ratio. If there is a tightness gradient in the sample, the Washburn equation5,6 cannot be used to fit the wicking curves. In addition, the boundary conditions between the fabric and the sample holder are difficult to control. Thus, the filter paper (1.1.3) must also be carefully ins...

During impregnation of fibrous reinforcements in liquid composite molding (LSM) processes, the resin flow is driven by a pressure gradient. Capillary effects have an additional effect that can compete with the pressure gradient, depending on the process parameters. Their influence on the process thus has to be evaluated1,2. This can be done by defining an apparent capillary pressure, Pcap, modifying the initial pressure gradient3. This parameter may subsequently be inserted into numerical models in order to simulate flows during processes and to accurately predict void formation4.
The spontaneous impregnation of a fabric by a liquid (wicking) can be described by the Washburn equation5. Originally, the Washburn equation described the capillary rise of a liquid in a tube. This equation was then extended for porous structures, such as fibrous reinforcements, that can be approximated to a capillary tube network. Considering a cylindrical sample holder with a radius, R, filled with a porous medium, the Washburn equation was modified in the form of squared mass gain (m&#178;(t)) over time, as follows6:
<img alt="Equation 1" src="/files/ftp_upload/55059/image001.jpg" />&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;(1)
where c is a parameter that accounts for tortuosity, &#7773; is the mean pore radius, and &#949; = 1-Vf is the porosity (Vf being the fiber volume ratio). All parameters in the square brackets concern the morphology and configuration of the porous medium, and they can be consolidated into a constant, C, referred to as the &#34;geometric porous medium factor.&#34; The other parameters express the dependence of wicking on the interactions between the medium and the liquid (through &#961;, &#951;, and &#947;L, which are, respectively, the density, viscosity, and surface tension of the liquid, and through &#952;a, an apparent advancing contact angle).
In parallel, the flow through a porous medium is usually modelled with the well-known Darcy law7, which relates an equivalent fluid velocity, vD, to the pressure drop through the permeability of the medium, K, and the liquid viscosity, &#951;. This equation also allows for the expression of the mass gain over a square root of time and thus for the consideration of the equivalence between the two equations. From this equivalence between the Washburn equation and the Darcy law, the capillary pressure was then defined as follows8:
<img alt="Equation 2" src="/files/ftp_upload/55059/image002.jpg" />&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;(2)
Here, the main focus is to describe the experimental procedure to measure the geometric factors and the apparent advancing contact angles for unidirectional fabrics, with the aim of determining the capillary pressure. This method relies on using a tensiometer to perform wicking tests (Figure 1). A tensiometer is a microbalance with a resolution of 10 &#181;g that measures the liquid mass either forming a meniscus around a solid or ascending a fibrous medium. Wicking tests were carried out considering a one-dimensional characterization (direction along the fibers)8,9. Quasi-unidirectional fabrics used to validate the procedure were carbon uni-directional (UD) fabrics at a Vf = 40%. Once the method was validated, flax fabrics were submitted to a thermal treatment that modifies the wetting behavior of fibers6, and wicking tests were performed with different fiber volume ratios (from 30% to 40%) for both untreated and treated flax fabrics. To determine morphological and wetting parameters, at least two wicking tests are mandatory: the first one with a totally-wetting liquid, like n-hexane, to determine C (Equation 1), and the second one with the liquid of interest, to determine the apparent advancing contact angle once C is known. In the first approach, water was used to evaluate the procedure.
This method can be applied to different fabrics and liquids, allowing for the evaluation of the influence of material geometry (morphology of fabrics), porosity (different fiber volume ratios), and viscosity and surface tension of liquid on the capillary impregnation phenomena. It is obvious that the procedure according to the Washburn theory (Equation 1) can be adopted only if wicking curves (m&#178;(t)) recorded by the tensiometer have a linear trend. This means that the parameters in Equation 1 must remain constant during the entire wicking process. If this is not the case, as for flax reinforcements in water, because fibers undergo swelling10,11, the Washburn equation should be modified to include the effect of swelling in order to describe the tests properly9. Treated fabrics were found to be less sensitive to water sorption9. Geometric factors and wetting parameters can be measured from linear fits, allowing for the calculation of the capillary pressure, Pcap.

<table border="0" cellpadding="0" cellspacing="0" width="700">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Carbon UD fabrics</td>
			<td style="width:167px;">Hexcel&#160;</td>
			<td style="width:151px;">48580</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Flax UD fabrics</td>
			<td style="width:167px;">Libeco</td>
			<td style="width:151px;">FLAXDRY UD 180</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">n-Hexane</td>
			<td style="width:167px;">Sigma Aldrich</td>
			<td style="width:151px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sulfochromic acid</td>
			<td style="width:167px;">home made</td>
			<td style="width:151px;"></td>
			<td>toxic and corrosive</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Filter paper</td>
			<td style="width:167px;">Dataphysic</td>
			<td style="width:151px;">FP11</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Tensiometer</td>
			<td style="width:167px;">Dataphysic</td>
			<td style="width:151px;">DCAT11</td>
			<td></td>
		</tr>
	</tbody>
</table>

wicking tests for unidirectional fabrics: measurements of capillary parameters to evaluate capillary pressure in liquid composite molding processes

During impregnation of a fibrous reinforcement in liquid composite molding (LCM) processes, capillary effects have to be understood in order to identify their influence on void formation in composite parts. Wicking in a fibrous medium described by the Washburn equation was considered equivalent to a flow under the effect of capillary pressure according to the Darcy law. Experimental tests for the characterization of wicking were conducted with both carbon and flax fiber reinforcement. Quasi-unidirectional fabrics were then tested by means of a tensiometer to determine the morphological and wetting parameters along the fiber direction. The procedure was shown to be promising when the morphology of the fabric is unchanged during capillary wicking. In the case of carbon fabrics, the capillary pressure can be calculated. Flax fibers are sensitive to moisture sorption and swell in water. This phenomenon has to be taken into account to assess the wetting parameters. In order to make fibers less sensitive to water sorption, a thermal treatment was carried out on flax reinforcements. This treatment enhances fiber morphological stability and prevents swelling in water. It was shown that treated fabrics have a linear wicking trend similar to those found in carbon fabrics, allowing for the determination of capillary pressure.

Curves of mass gain during wicking obtained with the tensiometer for carbon and untreated and treated flax fabrics are shown in Figures 2 and 3. All curves are shown after the subtraction of both weights of the external meniscus due to the sample holder and filter paper and are shifted to zero.
It is possible to observe from the plots in Figure 2 that, with both n-hexane and wat...

Watch this Scientific Journal Video about Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes at JoVE.co

Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes

An experimental method to measure geometrical parameters and the apparent advancing contact angles describing capillary wicking in unidirectional synthetic and natural fabrics is proposed. These parameters are mandatory for the determination of the capillary pressures that must be taken into account for liquid composite molding (LCM) applications.

During impregnation of a fibrous reinforcement in liquid composite molding (LCM) processes, capillary effects have to be understood in order to identify ...

The overall goal of this procedure is to assess the fiber capillary pressure values required for designing liquid composite molding processes. This method can help answer a key question in the field of composite manufacturing by liquid composite molding processes, about formation of void and dry-spots in large and complex composite parts. The advantage of this technique is that it can be used with a standard tensiometer and any kind of reinforcement for composites.Demonstrating this procedure will be Monica Francesca Pucci, a Ph.D student at the George Friedel Laboratory, here in the Ecole Mines Saint-Etienne. To begin sample preparation, cut a fabric strip perpendicular to the grain that is 20 millimeters wide and has a fiber volume of 40%Roll the strip into a tight cylinder, ensuring that it is evenly rolled throughout. Wrap the sample in filter paper no thicker than 0.1 millimeters.Carefully insert the sample into the sample holder, ensuring that the fabric is uniformly separated from the holder by the filter paper. Screw in the sample holder caps to compact the fabric sample under the piston. After preparing the fabric sample, clamp the sample holder to the tensiometer.Fill a 70 millimeter wide borosilicate glass container with n-hexane to a height of 12 millimeters. Place the container on the tensiometer elevation platform. In the tensiometer software, set the liquid container translation speed to 0.5 millimeters per second and the surface detection threshold to eight milligrams.Then, start the sicking test. Once the liquid contacts the sample, monitor the liquid mass gain over time in the instrument software. When the liquid reaches the top of the sample, as shown by no further increase in mass, end the test.Fit the linear segment of the sicking curve with the Washburn equation. And determine the geometric constant using zero degrees for the advancing contact angle. Remove the sample holder and the container of hexane from the tensiometer.Open the sample holder and remove the fabric sample. Then, immerse the sample holder in sulfochromic acid for 30 seconds. Rinse the sample holder with distilled water and dry it under a stream of compressed air.Prepare an identical rolled fabric sample wrapped in filter paper. And place it in the clean, dry sample holder for the advancing contact angle measurement. Close the sample holder.Place a 70 millimeter wide borosilicate glass container filled with water, to a depth of 12 millimeters, in the tensiometer. And clamp the sample holder in the tensiometer. Set the tensiometer parameters and perform the sicking test.Fit the sicking curve to the Washburn equation using the previously calculated geometric factor. And determine the advancing contact angle. Remove and clean the sample holder with sulfochromic acid and distilled water.And dry the holder with compressed air. Place a piece of filter paper in the sample holder as a control to evaluate the liquid weight contribution from the sample holder and filter paper. Secure the sample holder in a container of hexane in the tensiometer.Perform the sicking test and fit the curve to the Washburn equation. Subtract the squared mass gain from the previous hexane test value. Correct the geometric constant from the mass gain from liquid uptake by the filter paper and sample holder.Clean the sample holder and insert another piece of filter paper. Place the sample holder in a container of water in the tensiometer. Perform the sicking test and fit the curve to the Washburn equation.Subtract the squared mass gain value from the previous water test value, and correct the advancing contact angle. Carbon fabric was evaluated with this procedure. In both the hexane and water tests, the sicking curve is linear.Using the apparent advancing contact angle and values determined from the geometric factor, the capillary pressure can be determined. When treated and untreated flax fabrics were evaluated with hexane, both fabrics showed a linear trend in sicking. However, when evaluated with water, the untreated flax samples did not follow a linear trend, preventing determination of the advancing contact angle.This difference was attributed to the untreated fibers swelling during the sicking process, affecting the final equilibrium weight and the sicking speed. A modified Washburn equation that accounts for swelling, was used to determine the capillary pressure in untreated flax. After watching this video, you should have a good understanding of how to estimate for capillary pressure in order to prevent void formation or dry-spots in composite parts manufactured by liquid composite molding processes.Once mastered, this technique can be done in 30 minutes if it is performed properly. While attempting this procedure, it is important to pay attention to fiber orientation. Following this procedure, tests in the other fiber direction can be performed in order to answer additional questions like the interaction between the fabric reinforcements autotrophy, and the autotrophy of capillary effects.After this development, this technique paved the way for research in the field of capillary effect characterization, to explore the capillary pressure jumps in hybrid composites. Don't forget that working with sulfochromic acid can be extremely hazardous. And precautions such as appropriate ventilation, gloves, and safety glasses, must always be taken while attempting this procedure.

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