PROJECT SUMMARY

Electrically and thermally conductive resins are commercially produced today in limited quantities. However, demand for thermally and electrically conductive resins is rapidly growing due to more stringent regulation on electronic noise, as well as the increased need for smaller, more densely packed electronic components. The vast majority of the conductive resins available today or studied in the past utilize only one type of conductive filler. In prior work, the PI has noted a positive synergistic effect (reduces electrical resistivity by a factor of 100 ohm-cm, which is two orders of magnitude) when three different carbon-based conductive fillers are combined in nylon 6,6 matrix material. Hence, in this proposed study, a factorial design approach will be used to determine the effect of four different commercially available and cost effective conductive fillers  on the thermal and electrical conductivity of a polycarbonate and a nylon 6,6 based conductive resin. The fillers include an electrically conductive carbon black, pitch-based carbon fiber, polyacrylonitrile (PAN)-based carbon fiber, and a high purity synthetic graphite.  These conductive resins will be further studied to determine the variables that affect thermal and electrical conductivity, such as conductivity of the constituents, percent crystallinity of the matrix, length of conductive filler in the composite, and distance between conductive fillers. This information will be used to develop thermal and electrical conductivity models that accurately predict the thermal and electrical conductivity of conductive resins containing combinations of different conductive fillers. As a result of this proposed activity, new synergistic effects will be discovered that will allow conductive resins to be produced in a variety of thermoplastics that are more electrically and thermally conductive and cost effective than those currently used. It will also be possible to use the thermal conductivity and electrical conductivity models developed from this proposed activity to efficiently conduct material designs without extensive, costly, experimental work. Therefore, society will benefit from the development of improved cost effective conductive resins that meet the demands of the electronics industry in the 21st century. In addition, the polymers/composites community will benefit from enhanced understanding of the mechanisms of thermal and electrical conductivity in conductive resins. 

INTRODUCTION

Most polymer resins are thermally and electrically insulating. Increasing the thermal and electrical conductivity of these resins opens large, new markets. The advantages of conductive resins as compared to metals includes improved corrosion resistance, lighter weight, and the ability to adapt the conductivity properties to suit the application needs. For example, a thermally conductive resin is ideally suited for heat sink applications, such as lighting ballasts and transformer housings. An electrically conductive resin is used in static dissipative, semi-conducting (e.g., fuel gages, etc.), or EMI (Electromagnetic Interference)/RFI (Radio Frequency Interference) shielding applications (e.g., computer and cellular phone housings, etc.)

Typical thermal conductivity (TC) values for some common materials are 0.2 to 0.3 for polymers, 1 for carbon black, 10 for PAN (polyacrylonitrile)-based carbon fiber, 100 to 800 (depends on the heat treatment temperature) for petroleum pitch -based carbon fiber, 234 for aluminum, 400 for copper, and 600 for graphite (all values in W/mK). Electrical resistivity (ER) values for various materials are 10¹⁴ to 10 ¹⁶ for polymers, 10^-2 for electrically conductive carbon black, 10^-3 for PAN-based carbon fiber, 10^-4 for pitch-based carbon fiber, 10^-5 for graphite, and 10^-6 for metals such as aluminum and copper (all values in ohm-cm). One approach to improving thermal and electrical conductivity of a polymer is through the addition of a conductive filler material, such as carbon and metal. Conductive resins with a TC from approximately 1 W/mK to 30 W/mK can be used in heat sink applications. Conductive resins with ER ranging from approximately 10⁸ ohm-cm to 10³ ohm-cm are used for static dissipative applications. Conductive resins with ER ranging from approximately 10² ohm-cm to 10^-1 ohm-cm are used for slightly electrically conducting applications. Those with ER approximately 10^-2 ohm-cm are used for EMI/RFI shielding applications.

There are many references in the literature concerning adding a conductive filler to a polymer matrix in order to produce a more thermally and electrically conductive material. For example, metal fibers/particles (aluminum, steel, iron, copper, silver) and nickel coated glass fibers have been used. These metallic fillers have several disadvantages, relative to carbon, which include higher cost, higher density and weight, and greater susceptibility to oxidation. Carbon particles have been effective conductive fillers.  Carbon black and carbon fiber have also been used. Carbon black fillers have been successfully used to improve electrical conductivity, but these materials often have relatively low thermal conductivity. Carbon fibers, on the other hand, have performed well to improve the thermal and electrical conductivities. 

For this study, two different polymers were used as matrix materials. These were DuPont Zytel 101 NC010, an unmodified, semi-crystalline nylon 6,6 and GE Plastics Lexan HF1110-111N (polycarbonate), an amorphous engineering thermoplastic.  The first filler material was Ketjenblack EC600JD, an electrically conductive carbon black available from Akzo Nobel. Carbon black has a large surface area and hence, can contact a large amount of polymer.  The next fillers was Thermocarb^TM Specialty Graphite, a high quality synthetic graphite that is available from Conoco, Inc.  Thermocarb^TM was used due to its high thermal conductivity and moderately high electrical conductivity. The third filler was ThermalGraph DKD X, a pitch-based milled (200 microns long) carbon fiber available from BP/Amoco.  

This particular project is a 2³ factorial design, with the two levels being high/low loadings of the filler and the three factors will be the three different conductive fillers.  In addition, composites were produced that contained varying amounts of a single filler. These materials were extruded and injection molded into test specimens. Electrical (download PDF file) and thermal conductivity (download PDF  file) testing  are complete, along with analysis of results and modelling.  Shielding effectiveness results and analysis are also complete (download PDF file).  Tensile and notched Izod impact testing and analysis are ongoing.