Comparative study on the role of kaolin, calcined kaolin and chemically treated kaolin in alkyd‐based paints for protection of steel

Abstract

PurposeKaolin is a soft white mineral which has a large array of uses. It is most commonly referred to as “China clay”. Sources of this mineral can be found all over the world, its uses are multiple and diverse. It is a mineral belonging to the group of alumino‐silicates with the chemical composition Al2Si2O5(OH)4. It is white, soft, plastic clay mainly composed of plate‐like particles. It is a unique industrial mineral, which remains chemically inert over a relatively wide pH range. The purpose of this paper is to study the difference in performances of untreated or natural kaolin, thermally treated kaolin or so‐called “calcined kaolin” and chemically treated kaolin using ammonium molybdate to convert the γ‐Al2O3 naturally found in kaolin to α‐Al2O3 which possesses one of the most known stable crystalline phases (corundum) in alkyd based anticorrosive paint formulations used for protection of steel.Design/methodology/approachThe different kaolins were characterized using different analytical and spectro‐photometric techniques. The pigments were characterized using X‐ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Evaluation of these pigments using international standard testing methods (ASTM) was estimated. The extender pigments were then incorporated in solvent‐based paint formulations based on medium oil‐modified soya‐bean dehydrated castor oil alkyd resin. The physico‐mechanical properties of dry films and their corrosion properties using accelerated laboratory test in 3.5% NaCl for 28 days were tested.FindingsThe results of this work revealed that calcined kaolin was better in its performance in protection of steel than kaolin, while chemically treated kaolin varied in its performance according to the concentration of modifier or dopant.Practical implicationsKaolin has a large array of uses. These pigments can be applied in other polymer composites, e.g. rubber and plastics as reinforcing agent.Originality/valueThe untreated and treated kaolins are environmentally friendly pigments which can impart high anticorrosive behaviour to paint films with concomitant cost saving.