The proven Kevlar and a new, soft, thermally conductive carbon fiber are combined with proprietary graphite powder, each fiber used depending on the requirements of the application. Wear components molded from the Kevlar BMC have been in the field for more than four years with no wear-related failures.
The new conductive carbon fiber and combined graphite powder version has been introduced to improve the chemical resistance over the Kevlar. This also provides thermal conductivity. The new composite BMC has passed house testing by several customers and is now being field tested.
The Kevlar parts are slightly more elastic for press fits while the carbon fiber is easier to machine. Both the Kevlar and the carbon fiber-filled molded parts offer similar physical properties. Kevlar also absorbs 7 percent water at ambient temperature. A processing mechanism was developed that reduced the water absorption of the molded part equal to glass-filled epoxy, which is the benchmark for fiber-filled compounds. Even after machining the molded part, the Kevlar does not absorb water. In the field, no indications of water absorption have been observed.
Although field tests have not been completed, it appears that the thermal conductivity of the carbon fiber will be especially valuable in higher temperature applications as the heat will pass through the molded part instead of building up with an insulating effect of normal composites.
Kevlar is attacked by strong bases while the carbon fiber is affected by galvanic corrosion in sea water. It appears each has challenges, but new fillers are constantly being tested in research and development. The new resin appears impervious to temperatures below 200 C. The latest effort is to test a special glass fiber combined with the ceramic spheres for sea water applications. It is known that the wear component will not wear, but it is not yet known if the new glass will wear the shaft or if the ceramic beads will maintain the needed separation and prevent wear.
The concept of using ceramic beads to reduce the surface contact area is new. The function of the molded part is totally dependent on the adhesion and dimensional stability found in the new resin. Because this BMC composite is new, addressing any failure mode is the focus. The goal is to define and recommend a product for each application and cover the widest possible temperature and chemical range with zero wear to the bushing or shaft. The resin seems to adhere to everything, so fillers that were never successfully used, such as Kevlar, can now be tested. Adhesion to polytetrafluoroethylene (PTFE) has even been successful.
As soon as any wear with the Kevlar parts in the water pumps is found, those limitations will be known. P&S
Randy Lewis is director of R&D at TitanAdvancedComposites.com. He has spent almost 40 years in every area of the plastics molding and compounding industry.