Filler toughening of plastics. Part 1—The effect of surface interactions on physico-mechanical properties and rheological behaviour of ultrafine CaCO 3 /HDPE nanocomposites A. Lazzeri a, * , S.M. Zebarjad b , M. Pracella c , K. Cavalier d , R. Rosa d a Center for Materials Engineering, University of Pisa, Via Diotisalvi 2, 56126 Pisa, Italy b Department of Materials Science and Engineering, Ferdowsi University, Mashhad, Iran c CNR, Institute for Biomedical and Composite Materials, Section of Pisa, Via Diotisalvi 2, 56126 Pisa, Italy d Solvay SBU Advanced Functional Minerals, PCC Research Center for Polymers, Plastisols and Sealants, 13129 Salin de Giraud, France Received 17 October 2004; received in revised form 29 November 2004; accepted 30 November 2004 Available online 19 December 2004 Abstract Precipitated CaCO 3 (PCC)/High Density Polyethylene (HDPE) composites were prepared on a twin screw mixer-single screw extruder with a particle content of 10 vol%. The average particle size was 70 nm. The influence of surface treatment of the particles, with and without stearic acid (SA), on the physico-mechanical and rheological properties was studied. The experiments included tensile tests, impact tests, differential scanning calorimetry (DSC), microscopy and rheology experiments. The addition of 10 vol% calcium carbonate to HDPE causes a rise in Young’s modulus and yield stress of its composites and is accompanied by a sharp drop in impact strength. The addition of SA has the effect of slightly decreasing both Young’s modulus and yield stress of the composites compared to the uncoated PCC composites, while the impact strength progressively increases. During the tensile test filled HDPE composites showed stress whitening zones appear and develop along the gauge length. Volume measurements during tensile tests showed an increase in volume strain with deformation, due to the matrix-particle debonding phenomenon, while pure HDPE showed actually a decrease in volume with elongation. At constant deformation, for the composites with coated PCC, it can be observed that an increase in the SA content leads to a slight decrease in volume change. The microscopical evaluation showed cavities and voids due to debonding and deformation bands in the stress whitened areas. DSC experiments have shown that uncoated PCC particles have a very small nucleating effect on HDPE. q 2004 Published by Elsevier Ltd. Keywords: Nanocomposites; Filler toughening; Surface agents 1. Introduction The toughness of both commodity and engineering plastics at extreme conditions such as impact loading and low temperatures can be improved by means of the incorporation of rubber particles, albeit at the expense of a reduction in the elastic modulus of the material [1]. The use of rigid fillers to toughen polymers has also received considerable attention in recent years [2–9]. This approach would, in principle, lead to rigid and tough composites. An early paper reported toughening of isotactic polypropylene (iPP) using ultrafine calcium carbonate (average particle size 70 nm) [9]. In that work, the use of nanosized particles, suggested by the concept of a critical interparticle distance [10], could obtain a limited increase in fracture toughness, attributed to crack-pinning [11–14], while at higher filler loadings a good dispersion of the particles was not achieved. At the time when that early paper [9] was written, a thorough understanding of the principles governing tough- ening of polymers was still lacking. Recently Lazzeri and Bucknall have elucidated the mechanism for rubber toughening in non-crazing polymers [15–17]. They showed that these particles can facilitate the 0032-3861/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymer.2004.11.111 Polymer 46 (2005) 827–844 www.elsevier.com/locate/polymer * Corresponding author. Tel.: C39 050 511207. E-mail address: a.lazzeri@ing.unipi.it (A. Lazzeri).