GRADUATE SCHOOL OF SCIENCES & ENGINEERING
MATERIALS SCIENCE AND ENGINEERING
PhD THESIS DEFENSE BY AATIF IJAZ
Title: Functional Composites
Speaker: Aatif Ijaz
Time: November 02, 2018, 10:00
Rumeli Feneri Yolu
Thesis Committee Members:
Prof. Dr. A. Levent Demirel (Advisor, Koç University)
Dr. Annamaria Miko (Co-Advisor, Koç University)
Assoc. Prof. Dr. Funda Acar Yağcı (Koç University)
Assoc. Prof. Dr. Uğur Ünal (Koç University)
Prof. Dr. Nergis Arsu (Yıldız Technical University)
Assoc. Prof. Dr. Bülent Akgün (Boğaziçi University)
Functional composite coatings consisting of nano- and micro- filler particles are suitable candidates to meet the property and performance requirements in applications ranging from aerospace to biomedical implants. In this thesis work, several novel composites having anti-icing or anti-corrosion functions have been designed and realized. In all composites, silica based porous nano- and/or micro- filler particles were used due to their chemical stability, biocompatibility, easy surface functionalization and easy-to-tailor morphologies. Porous silica based fillers loaded with anti-icing agent or corrosion inhibitor were incorporated either in SBS matrix or in Fe-P electrodeposited films to obtain composite coatings with unique anti-icing or anti-corrosion properties based on either the release of the agents filled into porous silica-based fillers in the SBS matrix or the ability of silica based fillers to anodically polarize the metallic Fe-P matrix.
Mesoporous silica assemblies having different pore structures and morphologies were synthesized by sol-gel method using different types of non-ionic surfactants (polyethylene oxide-based block copolymers) and tetra ethyl orthosilicate (TEOS) precursor in highly acidic region and at room temperature. High specific surface area of 200-600 m2/g was achieved without any heat treatment or aging of the samples at elevated temperature. The morphology of the mesoporous silica assemblies was successfully controlled between spherical particles and monoliths by surfactant/TEOS ratio and acid catalyzer concentration. The spherical silica particles (800-4300 nm in diameter) were composed of primary particles (20-30 nm in diameter) within which cylindrical mesopores were aligned, but the alignment direction of each primary particle was random.
Diatomaceous earth (DE) or mesoporous silica loaded SBS based composites were developed as novel anti-icing agent releasing coatings. 30% by weight silica (DE or mesoporous silica) loaded SBS composites were post filled by keeping the composites in anti-icing agent (EG or
PEG) solutions in diethyl ether/acetone binary mixtures. The dissolved anti-icing agents penetrated into the swollen SBS matrix in the binary mixture. Porous silica particles served as the anti-icing agent carrier in the hydrophobic SBS matrix. The amount of anti-icing agent retained in the composite was increased by increasing the concentration of the anti-icing agent in the binary mixture, which resulted in longer freezing times of water droplets on the composite. The effective anti-icing mechanism was shown to be the release of the anti-icing agent upon contact with water and subsequent decrease of the water freezing temperature. The release of PEG in the inner DE pores was achieved by cutting the composite films into smaller pieces and increasing the water/composite interfacial area. This shows that the developed composites maintain their anti-icing activity for longer times in the presence of scratches and wear. Scratches and wear allow the anti-icing agent filled pores of silica particles buried in the SBS matrix to be exposed to the top surface with the possibility of new anti-icing agents being released when in contact with water.
The similar release mechanism was shown to work for anti-corrosive coatings on metals. Environmentally friendly corrosion inhibitor (Chitosan or PEG) loaded silica-based fillers (DE, mesoporous silica or halloysite nanotubes) embedded in SBS matrix were applied to copper substrates by dip coating method. The anti-corrosion properties of the coatings were electrochemically measured in 0.5 M NaCl solution. Unloaded SBS coating acted as passive barrier by protecting the metal from dissolution. The Tafel plots and cyclic voltammograms clearly showed that the corrosion current density and passive current density of copper coated by the composite were ~63% lower compared to unloaded SBS. The passive current density, the corrosion current density and the extent of pitting corrosion decreased with increasing amount of inhibitor loaded silica-based containers in the composite coatings.
The incorporation of corrosion inhibitor loaded or unloaded silica-based fillers (mesoporous silica particles or halloysite nanotubes) into electrodeposited Fe-P layers was shown to enhance the corrosion resistance of the films. The corrosion rate of Fe-P layers decreased by as much as a factor of 14 from 0.56 mm/year to 0.04 mm/year by incorporating silica-based fillers in the Fe-P matrix.