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

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.