Synergistic Effect of Plasma-Modified Halloysite Nanotubes and Carbon Black in Natural Rubber—Butadiene Rubber Blend Minna Poikelisp € a € a, 1 Amit Das, 1,2 Wilma Dierkes, 1,3 Jyrki Vuorinen 1 1 Plastics and Elastomer Technology, Department of Materials Science, Tampere University of Technology, P.O. Box 589, FI-33101 Tampere, Finland 2 Composite Materials, Leipniz Institute of Polymer Research Dresden, D-01069, Germany 3 Elastomer Technology and Engineering, University of Twente, NL-7522 AE Enschede, The Netherlands Correspondence to: M. Poikelisp€ a€ a (E-mail: minna.poikelispaa@tut.fi) ABSTRACT: Halloysite nanotubes (HNTs) were investigated concerning their suitability for rubber reinforcement. As they have geo- metrical similarity with carbon nanotubes, they were expected to impart a significant reinforcement effect on the rubber compounds but the dispersion of the nanofillers is difficult. In this work, HNTs were surface-modified by plasma polymerization to change their surface polarity and chemistry and used in a natural rubber/butadiene rubber blend in the presence of carbon black. The aim of the treatment was to improve the rubber–filler interaction and the dispersion of the fillers. A thiophene modification of HNTs improved stress–strain properties more than a pyrrole treatment. The surface modification resulted in a higher bound rubber content and lower Payne effect indicating better filler–polymer interaction. Scanning electron microscopy measurements showed an increased compati- bility of elastomers and fillers. As visualized by transmission electron microscopy, the thiophene-modified HNTs formed a special type of clusters with carbon black particles, which was ultimately reflected in the final mechanical properties of the nanocomposites. The addition of HNTs increased loss angle. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 000: 000–000, 2012 KEYWORDS: plasma polymerization; polymer–filler interactions; nanocomposites; halloysite Received 11 January 2012; accepted 17 May 2012; published online DOI: 10.1002/app.38080 INTRODUCTION Carbon black and silica are conventionally used fillers in rub- bers. Their primary particles have dimensions in the nanometer range, and structural reinforcing units have dimensions of a few hundreds of nanometers. In the last decade, in which nanotech- nology has been developed and implemented, nanofillers have extensively been investigated as fillers for polymers. They have been found to carry many benefits compared to conventional fillers: They have been reported to, for example, reduce viscosity and increase tensile strength, hardness, modulus, abrasion resist- ance, electrical conductivity as well as chemical resistance. 1–11 Nanoclays are widely studied materials used with rubbers. Typi- cally, they are layered silicates, and usually phyllosilicates such as montmorillonite, hectorite, and saponite are used. Recent studies 1,2,12–14 have focused on halloysite nanotubes (HNTs), which are 1 : 1 alumosilicates, a naturally occurring clay mineral with the empirical formula Al 2 Si 2 O 5 (OH) 4 2H 2 O. HNTs have a double layered crystalline structure. There are SiAO groups on the surface of the outer layer and Al(OH) 3 - groups on the inner side and edges of the tube. 13 The OH- groups have interparticle affinity which complicates dispersion of clay materials, but the filler–filler interactions are lower with HNTs than with layered silicates. 13 Furthermore, HNTs do not require exfoliation. Because of the tubular form, HNTs have a higher aspect ratio than other clay materials. The aspect ratio of HNT can vary between 10 and 130 depending on the dimen- sions of the tubes. The high aspect ratio and surface area of HNTs is expected to result in a special reinforcing effect on the polymer matrix. Conventional fillers are generally processed by direct incorpora- tion into the rubber matrix, eventually with the help of a cou- pling agent. The incorporation of nanofillers is more challeng- ing, and it is difficult to achieve a good dispersion by direct incorporation. Therefore, a successful application of nanofillers depends mainly on a good dispersion of the fillers. Besides dispersion, the interaction between rather hydrophobic polymers and hydrophilic fillers cause a challenge. The compati- bility can be improved by changing the polarity of the fillers. Dif- ferent kinds of coupling agents, such as silanes, are typically used to change the surface properties of the fillers. The coupling agents V C 2012 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2012, DOI: 10.1002/APP.38080 1