Electrical properties and electromagnetic interference shielding effectiveness of polypropylene/carbon fiber composite foams A. Ameli, P.U. Jung, C.B. Park * Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 ARTICLE INFO Article history: Received 1 February 2013 Accepted 13 April 2013 Available online 26 April 2013 ABSTRACT Foamed and solid polypropylene/carbon fiber (PP–CF) composites containing various CF contents (0–10 vol.%) were injection-molded. Foamed composites were achieved using dis- solved pressurized nitrogen gas. The effects of foaming on the fibers inter-connectivity and orientation, electrical percolation threshold, through-plane electrical conductivity, longitu- dinal and transversal in-plane conductivities, dielectric permittivity, and electromagnetic interference (EMI) shielding effectiveness (SE) were investigated. Cell growth increased the fibers inter-connectivity by biaxial stretching of the matrix and also changed the fiber orientation. The introduction of foaming reduced the density of the injection-molded sam- ples by 25%, lowered the volume fraction of the percolation threshold from 8.5 to 7 vol.% CF, enhanced the through-plane conductivity up to a maximum of six orders of magnitude, increased the dielectric permittivity and resulted in the increase of the specific EMI SE up to 65%. Moreover, the uniformity of in-plane and through-plane conductivities as well as EMI SE along the injection-molded samples was greatly improved by foaming. The rela- tionships between the microstructure and electrical properties were also established. The results reveal that lightweight conductive products with lower fiber content and enhanced electrical and EMI shielding properties can be fabricated with the aid of injection foam molding for applications in electronics, aerospace and automotive industries. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Electronic devices radiate and are affected by electromagnetic interference (EMI). Protecting such devices from incoming EMI is essential to maintain their functionality and integrity, as well as controlling their EMI emission level is required to achieve product acceptance in complying with electromag- netic compatibility standards imposed by governmental agencies [1]. Conductive coatings, metal cabinets, foil lami- nates and conductive polymer composites (CPCs) are the means of EMI shielding. Metal-based shields, as the most widely used EMI protectors [2] suffer from drawbacks of being heavy, prone to corrosion, and expensive processing. CPCs containing carbon fiber, carbon black and stainless steel fibers have been extensively investigated in an attempt to overcome the shortcomings of metal-based shields [3–14], and more recently, the EMI shielding capabilities of CPCs with carbon nanotubes (CNT) and carbon nanofibers (CNF) have been studied [15–26]. Compared to the conventional CPCs, composites with carbon nanofillers present lower percolation threshold and superior electrical properties. The applications of such conductive additives are however limited due to their 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.04.050 * Corresponding author: Fax: +1 416 978 0947. E-mail address: park@mie.utoronto.ca (C.B. Park). CARBON 60 (2013) 379 – 391 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon