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

The overall goal of this procedure is to fabricate a fiber-reinforced composite with bare fibers exposed on the surface without any surface treatment. This technique can expose bare fibers on a composite surface without the use of expensive and inefficient surface treatments. The main advantage of this technique is that it improves the electrical conductivity and mechanical strengths of the composite.

This technique also has high productivity. To prepare unidirectional carbon fiber fabric, pre-impregnated with epoxy, stack squares of prepreg with a fiber volume fraction of 50 to 60%in a sequence including zero degree and 90 degree orientations to avoid splitting of the laminate. To prepare each square of prepreg plain-weave carbon fiber fabric, first cleanse the fabric with acetone or ethanol.

Place the fabric on a clean surface or on a lint-free laboratory wipe, and allow the acetone to evaporate under ambient conditions. Peel off one backing of a sheet of film-type epoxy resin with a curing temperature of 125 degrees Celsius. Apply a single sheet of epoxy resin film to the clean carbon fabric.

Place the fabric epoxy side down on a hot plate at 70 degrees Celsius. Heat the fabric for 10 seconds to pre-impregnate the fabric with epoxy. Allow the prepreg fabric to cool under ambient conditions for 10 minutes.

Then, remove the remaining backing sheet from the epoxy-coated side. Stack the plain-weave prepreg fabric in the desired stacking sequence to form the plain-weave carbon fiber fabric laminate. To prepare non-woven carbon fiber felt laminate, apply triple-layered epoxy sheet to each side of a piece of clean, dry carbon felt.

Heat the epoxy-coated sides of the felt at 70 degrees Celsius for 10 seconds each with mass applied on the top. Allow the prepreg felt to cool for 10 minutes, and then peel off the remaining backing sheets. Next, wash sheets of 25 micrometer thick, non-perforated, fluorinated ethylene propylene release film with ethanol or acetone.

Carefully dry the FEP sheets with lint-free wipes without causing wrinkles or pinholes. Ensure that the sheets are free of contaminants and particulate matter. Then, place each clean, dry sheet between lint-free wipes, and store the sheets in a dust-free, clean, plastic case.

To begin the composite fabrication, apply silicone mold release to the compression mold. Wipe the mold with lint-free wipes to leave only a thin layer of mold release. Then, trim a piece of prepared carbon fiber laminate to be slightly smaller than the mold cavity.

Place a sheet of 25 micrometer thick clean FEP film on the lower mold. Place the laminate on the mold, and cover it with 25 or 100 micrometer thick FEP film, depending on the fiber type. Carefully smooth out the film so that no air bubbles remain between the soft layer and the laminate.

Then, close the compression mold. Heat a hot press to 150 degrees Celsius, and place the compression mold in the press. For unidirectional fiber composite, constantly apply 20 megapascals for 30 minutes.

For a woven composite, apply 20 megapascals for four minutes, release the pressure, and immediately reapply 20 megapascals. Purge the material in this way until the viscosity of the resin begins increasing. Then, hold the pressure at 20 megapascals until the composite has cured for 30 minutes in total.

For a felt composite, slowly increase the pressure to three megapascals, being careful not to overshoot. Cure the material at three megapascals for 30 minutes. Once the sample is cured, without releasing the pressure, cool the compression mold to below 120 degrees Celsius.

Then, release the pressure, remove the mold, and de-mold the fabricated specimen. Scanning electron microscopy images acquired at a five-degree tilt showed that fibers were visible in greater detail in composites fabricated with the soft-layer method than with conventional compression molding. This was attributed to the elimination of the resin-rich layer on top of the composite.

Despite the elimination of the epoxy resin layer, the fibers did not show visible defects. The through-thickness electrical conductivity was greater in composites fabricated with the soft-layer method, with a corresponding decrease in area-specific resistance to below 20 milliohms per square centimeter at a compaction pressure of 1.38 megapascals. The measured tensile strengths of soft-layer composites were comparable to those fabricated by the conventional compression method.

The woven fabric and felt composites showed increases of 22%and 15%respectively. These increases were attributed to the soft layer applying a uniform pressure across the composite surface. By following this procedure, you will be able to fabricate a fiber-exposed composite without difficulty.

The mechanism is simple. And with a good understand of your material, the implementation is even simpler. While attempting this procedure, remember to fine tune it to fit your specific application.

Consider the curing condition of the composite and the thermal properties of the soft layer, and adjust the procedure accordingly to obtain the desired results.