Received: 14 May 2009, Revised: 25 June 2009, Accepted: 29 June 2009, Published online in Wiley Online Library: 5 August 2009 Thermal-crosslinked polyacrylonitrile fiber compacts E. Kalfon-Cohen a * , H. Harel a , M. Saadon-Yechezkia a , K. Timna a , T. Zhidkov a , A. Weinberg b and G. Marom a Polyacrylonitrile (PAN) textile fibers, in the form of fabrics or threads, were compacted in a heat-pressure cycle and crosslinked by nitrile polymerization to form a thermoset composite article, whose mechanical properties were found to surpass those of commercially available polypropylene (PP) fiber counterparts. Additional advantages of the PAN compacts included their significant thermal stability (>300-C, i.e., twice that of PP) in addition to their flame retardancy, thereby rendering them as the structural material of choice for applications where heat protection and fire resistance are essential. Copyright ß 2009 John Wiley & Sons, Ltd. Keywords: polyacrylonitrile; composites; crosslinking compaction; flame retardant INTRODUCTION The compaction of polymeric fibers, based on the sintering of skin-molten threads under high pressure, has emerged as a new production technology for highly reinforced composite materials. The best example is that of hot compaction of polyethylene (PE) fibers, originally developed by Ward and his co-workers, [1] consisting of the fusing-together of fibers in a processing cycle of a specific pressure-temperature sequence. The processing conditions are chosen so that optimum fiber melting occurs at the fiber skin, thereby allowing inter-fiber coalescence via the formation of a new matrix phase upon cooling. Lately, a modified process has been proposed. [2] This method utilizes the elevated melting point of the PE fiber under high pressure, giving rise to a spike of pressure reduction for a controlled period of time, and allowing a limited surface melting. Most of the original work on compacts has been performed with ultrahigh molecular weight polyethylene (UHMWPE) fibers and was initiated by Ward. [1] Subsequently, Cohen proposed a more effective temperature-pressure cycle [2] and Farris focused mainly on products (plates) for ballistic protection. [3] UHMWPE fibers were chosen, in the first place, due to their exceptional mechanical properties and low-specific gravity. They also present a skin-core structure, allowing skin melting at a relatively low-compaction temperature without conferring deterioration on the highly oriented crystalline core. [4] Following the initial studies of the hot compaction of melt-spun polyethylene fibers, further research has demonstrated that conditions can be found for the successful compaction of a very wide range of oriented fibers and tapes, including gel spun poly- ethylene fibers such as Dyneema [5] and Spectra, [6] polyethylene terephthalate fibers, [7] liquid crystalline polymer fibers, [8] and fibrillated polypropylene tapes. [9] The new highly oriented melt spun polyethylene tape denoted Tensylon and manufactured by Synthetic Industries was found to have exceptional properties when compacted into a sheet. Crosslinking is known to improve the adhesion at the fiber/ matrix interface and consequently enhances the mechanical properties and thermal stability of a composite. The crosslinking issue has been widely investigated in our group. Ratner et al. presented a unique combination of hot compaction and crosslinking, producing an LPE compact made of linear polyethylene (LPE) fibers treated with dicumyl peroxide (DCP). [10–12] At elevated temperatures, DCP dissociated to free radicals, thereby initiating crosslinking through inter-chain covalent bonding. The strong fiber/fiber inter-facial adhesion resulting from the network formation led to an improvement in the mechanical properties, in the order of 20%. Moreover, the use of high modulus fibers, such as Dyneema (UHMWPE), in crosslinked compacts generated an enhancement of Young’s modulus by approximately 36% while the tensile strength was improved by almost 55% as compared to the non-crosslinked compact. [10–12] Polyacrylonitrile (PAN) fibers are well known in a large range of textile applications including clothing, upholstery, and heavy textiles. In addition, the chemical structure of PAN, which is given in Fig. 1a, allows a stabilization stage by nitrile polymerization leading to the formation of a ladder polymer and/or crosslinking. This stabilization stage can be succeeded by a series of chemical reactions, which eventually lead to carbonization and graphitiza- tion. [13,14] The stabilization is of relevance to this study as it results in a conjugated imine system that is responsible for ladder (depicted in Fig. 1b) and crosslinked (portrayed in Fig. 1c) structures. Such a reaction can be evidenced by the progressive (wileyonlinelibrary.com) DOI: 10.1002/pat.1521 Research Article * Correspondence to: E. Kalfon-Cohen, Casali Institute of Applied Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel. E-mail: Estelle.Kalfon@mail.huji.ac.il a E. Kalfon-Cohen, H. Harel, M. Saadon-Yechezkia, K. Timna, T. Zhidkov, G. Marom Casali Institute of Applied Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel b A. Weinberg Shenkar College of Engineering and Design, 52526 Ramat-Gan, Israel Polym. Adv. Technol. 2010, 21 904–910 Copyright ß 2009 John Wiley & Sons, Ltd. 904