PTFE is themoplastic. graphite filler which acts as conductive filler is mixed with PTFE.
You said graphite filler would be added as conductive filler; meaning you want your composite to be conductive. In order to acheive that you need to add graphite fiber at least upto the percolation limit at which these fiber will make a continuous path. You have to find the percolation limit of graphite fiber in PTFE.
As per my experience, I had to add 50 wt% graphite powder in Ethyle Methacrylate polymer to make it conductive. This we acheived by doing successive addition and testing.
In your question, you mentioned graphite fiber and the explanation below the question you mentioned graphite filler, the effect on conductivity of these materials are quite different. Since you indicated conductive filler, the description here will be limited to electrical conductivity alone. The conductivity of carbon added polymer depends on the aspect ratio (length divided by diameter) of the filler/fiber added. Thus, graphitic filler, such as carbon nanotube can achieve percolation at much less than 1% addition, whereas graphitic carbon powder needs to be added as much as 30%. The latter case is when the filler is not forming any special nanostructure/aggregate or selective concentration in a phase separated matrix. Even micrometer sized carbon fibers can achieve percolation at a few % loading if the aspect ratio of the fiber is high, say above 100. If your matrix is single phase and rather low aspect ratio filler is homogeneously distributed, large amount of filler is needed (20-30%). On the other hand, if the matrix is a blend of PTFE and somewhat hydrophilic polymer, the graphite filler/fiber can be placed selectively at the interface of the phase selected morphology, allowing significant reduction of percolation filler content. Some examples of further complex and new approaches are described in the papers shown below.
“A New Concept for Nanoparticle Distribution in SBR/NBR Blend Solution and Films via Molecular Confinement,” M. Kawazoe and H. Ishida, Macromolecules, 41, 2931-2937 (2008).
“Verification of Selective Adsorption of Polymer on Filler Surface in a Binary Immiscible Polymer Solution Blend Based on the Size Matching between the Radius of Gyration (Rg) of the Polymer and the Filler Structure: Changing the Aggregate Structure and the Surface Character of Carbon Black,” M. Kawazoe and H. Ishida, Macromolecles, 42, 6175 (2009).
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