Influence of excessive filler coating on the tensile properties of LDPE – calcium carbonate composites Maged A. Osman, Ayman Atallah, Ulrich W. Suter * Department of Materials, Institute of Polymers, ETH Zu ¨rich, CH-8092 Zu ¨rich, Switzerland Received 17 April 2003; received in revised form 17 October 2003; accepted 2 December 2003 Abstract Calcium carbonate fillers are usually coated with stearic acid to reduce their surface energy and improve their dispersion in polymers. Commercial products are often over-coated and contain an excess of surfactant. It was found that stearic acid linearly increases the modulus and yield stress of LDPE but reduces its tensile strength, yield strain, and ultimate elongation. The influence of surfactant excess on the tensile properties of low-density polyethylene (LDPE) – CaCO 3 composites was investigated. Compounds of LDPE and optimally coated filler or with excess surfactant were prepared and their properties compared. CaCO 3 increased the stiffness and yield stress of the polymer but reduced all its other tensile properties. Over-coating the filler did not lead to linear accumulation of the effects of filler and stearic acid on the polymer matrix. In fact, surfactant excess amplifies the reinforcing effect on the stiffness but reduces all other mechanical properties of the composite. Calcium stearate, which is sometimes used as acid scavenger, lubricant or processing aid, has the same effect on the polymer properties as stearic acid, but to a smaller extent. It is concluded that it is most advantageous to coat the filler with the optimal amount of surfactant necessary to cover its surface with an organic monolayer unless the influence of excessive coating is required for a certain application. Care must also be taken in interpreting some of the published results, where the quality of the filler coating was not investigated. q 2003 Elsevier Ltd. All rights reserved. Keywords: Tensile properties; Over-coating; Polyethylene–calcite composites 1. Introduction Thermoplastic polymers and especially polyolefins are produced and consumed today in vast quantities. However, they are seldom used as neat polymers and are usually compounded with mineral fillers. Initially, fillers were used as ‘extenders’ for polymers to reduce cost but as the polymer price decreased and the requirements of modern applications increased, attention has been more and more focused on functionality enhancement [1–3]. Nowadays, ‘functional fillers’ find application in the polymer industry almost exclusively, e.g. to improve stiffness, toughness, dimensional-stability, electric-insulation or to decrease the dielectric-loss. A prerequisite for functional fillers is full dispersion (break-up of agglomerates into their primary particles) and uniform spatial distribution in the polymer matrix because agglomerates entrap air and act as sites for fracture initiation, thus leading to premature material failure [4–6]. Calcium carbonate is one of the most abundant materials on our planet and has been quite early used in ground form to produce polymer composites. There are no less than three minerals or phases of CaCO 3 (calcite, aragonite and vaterite), but calcite is that most widely found in nature. In contrast to precipitated calcium carbonate, ground natural calcite is usually micron-sized (easier to disperse) with a broad size distribution and irregular shape. To reduce its high surface energy and its particle – particle interactions, which lead to agglomerates, it is often coated by a variety of surface modifiers such as fatty acids, phosphates, silanes, titanates or zirconates [2,3,7,8]. The most widely used calcite coating is the surface treatment with stearic acid or one of its salts. As a result, an ultra thin layer of hydrophobic alkyl chains is chemically bonded to the surface. The coated organic film represents the interface between the filler and the polymer matrix, and hence influences the wetting and adhesion properties of the two phases involved. It also influences the growth of the interphase and consequently 0032-3861/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2003.12.020 Polymer 45 (2004) 1177–1183 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 41-1-632-3127; fax: þ41-1-632-1096. E-mail address: uwsuter@eth.ch (U.W. Suter).