Material Science and Engineering

Biography

Biography: Baris Kaynak

Abstract

The coatings of molds and dies in polymer industry, especially in extrusion and injection moulding sectors, have to fulfil several particular requirements. Coatings should primarily exhibit wear-reducing properties and anti-adhesive behaviour. These properties are required for many applications, and are also important to reduce demolding forces. Currently, the coating of tool surfaces is based on vacuum techniques such as physical vapour deposition, which is used to coat the metal surface with a thin inorganic layer (e.g. CrN and TiN) with a thickness in the micrometer range. However, vacuum processes are rather complex and costly procedures, and for purely inorganic coatings the generation of anti-adhesive effects is limited due to the high surface tension of such coatings. In this work, metallic tools are surface modified with various alkyl and perfluoroalkyl silanes from the liquid phase. In particular, these molecular coatings are investigated with respect to adhesive properties, durability and long-term stability. Tool steel surfaces are treated with acid (dilute HNO3) and corona discharges to create hydroxyl groups on the surface (“activation”). The modification of the metal surface is achieved by subsequent reaction of the surface hydroxyl groups with organosilanes. The characterization of the modified surfaces is performed by infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. It is shown that the surface energy of steel surfaces can be reduced significantly by application of the organosilanes. Using physic-chemical methods, the thermal stability of the silane layers is studied at 290°C in order to evaluate the applicability of such coatings for molds and dies used in the processing of thermoplastics (e.g. PC and PA). The stability is examined as a function of the structure of the modifying organosilane, layer thickness, and absence / presence of oxygen during the ageing test. When appropriate molecular structures and application techniques are selected, anti-adhesive silane layers exhibit a surprising thermal stability which makes them suitable candidates for the coating of tool steel. Most important, a renewal of silane coatings can be done in a convenient process without employing critical or toxic chemicals. The results are discussed with respect to applications in polymer processing.