Relationship between viscoelasticity and scratch morphology of coating films

In order to make clear the relationship between viscoelasticity of coating film and its scratch resistance, a number of clear coatings were examined by surface observation and measurements of their mechanical properties.

The scratches can be classified into two types by means of optical microscopic observation with attached polarizing filter, scraped fracture-type and plastic deformation-type. The ratios of the damaged areas to the whole observed areas were determined by image analysis as a measure of scratch resistance.

The comparison between the relaxation spectrum of the coatings and the ratio of the scratched area showed that the degree of surface damage caused by a scratch tester which simulated car washing machine correlated with the storage modulus of the coatings G_sr, at a relaxation time of 1 s. Moreover, the degree of plastic deformation by scratches depended also on S_pls, the minimum strain which could cause plastic deformation in the film.     

UV-photopolymerisation of poly(methyl methacrylate)-based inorganic-organic hybrid coatings and bulk samples reinforced with methacrylate-modified zirconium oxocluster

In this work, we developed and optimised a synthetic route which enables to produce by spray-coating hard, transparent and stable inorganic-organic hybrid coatings for a wide variety of different substrates (e.g. stone, stainless steel, polymethylmethacrylate, polyethylene, wood, anodized aluminum). Chemically and thermally stable acrylate-based hybrid materials embedding the zirconium oxocluster Zr₄O₂(OMc)₁₂, OMc(CH₂C(CH₃)C(O)O) were prepared and UV-cured.The coatings of different compositions on different substrates were characterized by numerous analytical and spectroscopic methods such as FT-IR spectroscopy, Angle Resolved-XPS (AR-XPS), Secondary Ion Mass Spectrometry (SIMS), X-ray Absorption Spectroscopy (XAS), Environmental Scanning Electron Microscopy (ESEM) and Energy Dispersive X-ray analysis (EDX). Bulk samples were also prepared for additional characterizations. The bulk samples were analysed by FT-IR, whereas the cross-linking degree was qualitatively evaluated by swelling experiments. As far as the mechanical properties are concerned, the shear storage modulus (G′) and loss modulus (G″) were measured by Dynamical Mechanical Thermal Analysis (DMTA) technique. Moreover, the best conditions for the curing and cross-linking processes of the hybrid materials were studied up to 200 °C by using Differential Scanning Calorimetry (DSC). The thermal stability of the hybrid samples was evaluated by Thermogravimetric Analysis (TGA).     

Fracture behaviour of microcrack-free alumina–aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries

Alumina + 10 vol.% aluminium titanate composites were obtained by colloidal filtration and reaction sintering of alumina and titania. The materials were dense with aluminium titanate grains of average sizes 2.2–2.4 μm located mainly at alumina triple points. The reaction sintering schedule promoted the formation of additional nanometric grains, identified as aluminium titanate using STEM–EDX analysis between the alumina grains. This special microstructure led to a change of the toughening mechanism from the typical crack bridging reported for microcrack-free composites fabricated from alumina and aluminium titanate powders to microcracking.

The identification of microcracking as the main toughening mechanism was done from the analysis of stable fracture tests of SENVB samples in three points bending and fractographic observations. Monophase alumina materials with similar grain sizes were used as reference.

Different fracture toughness parameters were derived from the load–displacement curves: the critical stress intensity factor, K_IC, the critical energy release rate, G_IC, the J-Integral and the work of fracture, γ_WOF, and the R curves were also built. The comparison between the linear elastic fracture parameters and the non-linear ones revealed significant toughening and flaw tolerance.     

Organically modified montmorillonite hyperbranched polyurethane–urea hybrid composites

The synthesis and characterization of hyperbranched polyurethane/clay (HBPu/o-clay) hybrid coatings prepared from HB aliphatic polyester polyols (HPs) and isophorone diisocyanate (IPDI) is reported. The HPs are prepared by reacting adipic acid (AA) separately with pentaerythritol (PE), trimethylol propane (TMP) and glycerol (Gly) in different mole ratios of 0.6:1.0, 0.8:1.0 and 1.0:1.0, respectively. K⁺-montmorillonite (K10) modified with cetyltrimethyl ammonium bromide (CTAB) is used to increase the compatibility between the clay and polymer matrix. The clay is well-dispersed into the polyester matrix by ultrasonication method. Later on, NCO capped HBPU prepolymer (NCO–HBPu) is synthesized from the clay-dispersed polyester by reacting with IPDI at NCO/OH ratio of 1.6:1.0. In the next step, excess NCO content in the binder is cured with moisture. The properties of different HBPu/o-clay hybrid coatings were studied by FTIR, TGA and DMTA instruments and a structure to property correlation is drawn. The deconvolution technique is used to identify the amount of hydrogen bonding present in the hybrid coatings. The TGA and DMTA result showed an enhancement of the thermal stability, room temperature modulus (E′) and glass transition value (T_g) for the clay modified hybrid coatings, and increases with increasing clay content. TGA results also suggest that the PE-based HBPu/o-clay hybrid coatings have higher thermal stability than TMP or Gly-based systems, which are in close agreement to the FTIR analysis.     

Electrochemical synthesis: a novel technique for processing multi-functional coatings

Electrochemical synthesis is a powerful tool for surface modification, substrate cleaning and formulation of thin films and bulk materials. It is especially suited for surface modification of fibers, metals and films. In the past decade electrochemical method has become the preferred technique for in situ passivation, and coating of commodity metals such as aluminum, zinc, copper and steel.

We have successfully synthesized different kinds of conducting polymers, including polypyrrole (PPy)–polyaniline (PANi) composites. The processability and corrosion performance of PPy/PANi, composite coatings are significantly better than those for either PPy or PANi, coatings.

In this paper, we will discuss the use of electrochemical technique in the synthesis and characterization of multi-functional corrosion resistant conducting polymer coatings for aerospace and automotive applications.     

Rheological methods for characterising modern paint systems

In this paper the most important measuring methods for studying the rheological behaviour of modern paint systems will be presented (oscillation tests, determination of low yield points). In a first part the different methods were used to describe the rheological/physical properties of model polyurethane microgels. A second part deals with rheological investigations of effect development (light–dark flop) in waterborne pearlescent/coloured pigment systems. The measured values for storage modulus G′, phase shift δ and yield point were compared with flop-index values. The results show good correlations between the observable effect and the rheological parameters of the liquid paint. The third part presents measurements of powder and electrodeposition coatings and of clearcoats with the oscillation technique to determine the viscosity–temperature behaviour. The results were correlated with technical properties of the final films.     

Rheological behavior of mixed system of ionic liquid [C8mim]Br and sodium oleate in water

We report on the rheological behavior of wormlike micelles constructed by ionic liquid surfactant [C₈mim]Br (1-octyl-3-methylimidazolium bromide) and anionic surfactant sodium oleate (NaOA) in aqueous solution. The effects of surfactant composition, total surfactant concentration, added salts, and temperature were investigated. The prevailing surfactant effect at lower concentration and the leading cosolvent effect at higher concentration of [C₈mim]Br may be the main reasons for appearance of well-established maximum in key rheological parameters with variation of surfactant composition and total surfactant concentration. The Cole-Cole plots demonstrate that the systems (total surfactant concentration falls within 0.17–0.35 mol·L^-1 and molar ratio 0.33≤R≤0.50) fit the Maxwell’s mechanical model as linear viscoelastic fluid. The addition of NaBr or sodium salicylate decreases significantly the viscosity and the relaxation time of the wormlike micelle solution but cannot change the value of plateau modulus G₀. The present system has low rheological tolerance to temperature. The increase of temperature decreases the average contour length and viscosity of wormlike micelles and thus strengthens the relaxation progress of diffusion and weakens the relaxation progress of reptation. Increasing the temperature also decreases the value of plateau modulus G₀ and shifts the minimum value of the loss modulus G^″_min to higher frequencies.     

Some catalytic properties of hydrothermally synthesised zinc aluminate spinel

Two samples of zinc aluminate were hydrothermally synthesised from zinc acetate and different aluminium sources: basic aluminium nitrate or aluminium hydroxide. The textural properties of the prepared ZnAl₂O₄ samples are different from these one of the zinc aluminate prepared by conventional way. Powder XRD and TEM measurements reveal that samples are single-phase material or mixture of ZnAl₂O₄ with small amount of γ-Al₂O₃, with morphology of quasi-spherical shape. Catalytic properties of the hydrothermally obtained zinc aluminate and Pt (Pd) catalysts supported on them were investigated in the reactions of cyclohexene isomerisation and combustion of trichloroethylene, respectively. It was evidenced that activity and selectivity of the investigated materials could be qualitatively correlated with the part of the strong acid centres measured by TPD of NH₃.     

Protective abilities of nanocomposite coatings containing Al2O3 nano-particles loaded by CeCl3

The protective ability of hybrid nano-composite oxysilane coatings, deposited via sol–gel method on AA2024-T3 – aluminium alloy, were studied by linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) methods in 0.05 M solution of NaCl. Cerium chloride (CeCl₃) was incorporated as an inhibitor into a sol–gel hybrid matrix in two different routes: directly and via filled porous Al₂O₃ nano-particle aggregates with diameters up to 500 nm. The influences of the inhibitor concentration, as well as the influence of nano-particles on the barrier properties and the susceptibility against corrosion, were evaluated and EIS spectra were fitted by appropriated equivalent circuits. The values for C_coat, R_coat, C_oxy and R_oxy were achieved and their evolution over time was investigated. The investigated coatings possess highly expressed barrier properties (10⁶ to 10⁷ Ω cm²). Despite of the chloride ions inside of the matrix, some samples illustrated a significant durability of over 4000 h during exposure to the corrosion medium before first signs of corrosion appeared. The electrochemical results were compared with the neutral salt spray test. Thus, it was proved that the potential of these coatings is to be used as anticorrosive protective materials and are candidate to replace Cr(VI)-based anti-corrosion coatings.     

Three-components organic–inorganic hybrid materials as protective coatings for wood: Optimisation,synthesis, and characterisation

The present work explores new solutions for the development of functional flame-resistant hybrid coatings for wood, by using oxocluster-reinforced hybrid materials. Hybrid coatings and bulk materials were produced by photopolymerisation of 3-methacryloxypropyltrimethoxysilane (MAPTMS) with methylmethacrylate (MMA) in the presence of the dimeric oxocluster (Zr₆O₄(OH)₄(OOCCH₂CHCH₂)₁₂(n-PrOH)]₂·4(CH₂CHCH₂COOH), (Zr12), characterised by the presence of 12 vinylacetate groups for each molecules. The molar ratios among silane, MMA monomer and oxocluster were changed to optimise the best performing formulation. The final molar ratio chosen for the spray deposition of the coatings under inert atmosphere and for the preparation of the bulk specimens was MAPTMS:MMA:Zr12 = 1:3:0.008. Attenuated Total Reflectance-Fourier Transform Infra Red (ATR-FTIR) spectroscopy and Differential Scannig Calorimetry (DSC) were used in time-resolved fashion to optimise the photopolymerisation time, resulting to be 20 min. The polymerisation of the organic part and the condensation of the siloxane groups were investigated by the combined use of DSC, Fourier Transform Infra-Red (FTIR) and solid state Nuclear Magnetic Resonance (NMR) spectroscopies, showing that, whereas a complete organic polymerisation degree was reached, the condensation of the silica component was not completed. Dynamical Mechanical Spectroscopy (DMS) evidenced that: (1) the copolymerisation of the silane with the Zr12 oxocluster without MMA yields materials with very poor mechanical thermo-properties; (2) the Zr12 oxocluster copolymerised with MMA gives very stiff but fragile hybrids; (3) the ternary system yields instead flexible materials, which are endowed with outstanding thermo-mechanical properties. The optimised formulation was used for the deposition of coatings on wood (Larex), which were analysed by Scanning Electron Microscopy (SEM), contact angles measurements and tested toward flame-resistance.     

Multifunctional,environmental coatings on AA2024 by combining anodization with sol-gel process

The present work proposes the development of multifunctional composite coatings on AA 2024 by combining anodization and sol-gel process. To render the surface of AA 2024 with maximum corrosion resistance, eco-friendly citric-sulfuric acid (CSA) electrolyte with low sulphur content was used for anodization at 20 V for a duration of 30 min. The obtained anodized layer was porous. Ambient curable hybrid sol-gel coating with 8-hydroxy quinoline (8-HQ) as corrosion inhibitor was used as a sealant for the porous anodized layer to enhance the corrosion resistance with self-healing properties. Surface pre-treatments were carried out using sand blasting and alkaline etching to ensure high surface activity prior to anodization. The surface morphology and chemical composition of samples with and without coatings were characterized by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopic analysis (EDX). Adhesion strength and wettability of the coatings were measured by tape adhesion test and water contact angle analysis respectively, which revealed excellent binding strength and hydrophobic nature. The corrosion resistance of the coatings was evaluated using electrochemical impedance spectroscopic (EIS), potentiodynamic polarization and salt spray tests. The results revealed improved corrosion resistance of anodized + 8-HQ sealed AA2024. Moreover, when the coated samples were scribed and exposed to the corrosive medium, the SEM/EDX mapping confirmed presence of corrosion inhibitors at the location of the defect, thereby confirming the self-healing property. Hence, the proposed system is a chromium-free, environment friendly multifunctional system exhibiting excellent self-healing corrosion protection and can be a promising substitute for chromic acid anodization.     

Plasma-Sprayed Aluminum and Titanium Adherends: II. Durability Studies for Wedge Specimens Bonded with Polyimide Adhesive

The durability of plasma-sprayed metals bonded with a polyimide adhesive has been studied. Metal adherend surfaces were prepared for adhesive bonding by plasma-spraying inorganic powders on aluminum and titanium. The plasma-sprayed materials included Al₂O₃, AlPO₄. MgO, and SiO₂ on aluminum, and TiO₂, TiSi₂, MgO, and SiO₂ on titanium. The coatings were sprayed at two different thicknesses. Durability studies of samples prepared in a wedge-type geometry were carried out. Bonded specimens were maintained in an environmental cycle that included exposure to the conditions; low temperature, - 20°C; relative humidity at elevated temperature, 70% RH at 66°C; elevated temperature (160°C) in air, high temperature (160°C) in vacuum (130 torr, 0.2 atm.), and room temperature. Crack growth rate and mode of failure were determined. The results of the durability tests indicate that thin coatings (25 μm) of plasma-sprayed materials perform better than thicker (150 μm) coatings. The crack growth rate for thin coatings (25 μm) of Al₂O₃, AlPO₄, SiO₂, and MgO plasma-sprayed on aluminum was equivalent to that for phosphoric acid anodized aluminum. Similarly, the durability performance for titanium samples prepared with a 25 μm-thick TiO₂, TiSi₂, and SiO₂ plasma-sprayed coatings was equivalent to that for a Turco®-prepared titanium surface. Although the evaluation of durability as a function of surface chemistry was an objective of the study, it was not possible to evaluate the effect, since most failures occurred within the adhesive (cohesive failure) during the environmental tests. That failure occurred in the adhesive indicates that the coating-adherend and the coating-adhesive interactions are sufficiently robust to prevent interfacial failure under the experimental conditions investigated.     

Effect of nano ZnO on the phase mixing of polyurethane hybrid dispersions

In the present study series of aqueous polyurethane (PU)/ZnO hybrid dispersions were prepared using dimethylolpropionic acid (bis-MPA) as an ionic center. For this, NCO terminated PU prepolymers with pendent acid groups were prepared first, then different concentrations of nano ZnO powder was incorporated into the PU matrix. The hybrid dispersions were prepared by adding required amount of triethylamine (TEA), water and chain extender. The prepared PU/ZnO hybrid dispersions were casted in a Teflon petri dish and the dried films were used for TGA, DMTA, SEM, gel content and contact angle measurements. The phase mixing behavior was studied from FT-IR peak deconvolution technique and DMTA analysis and the result suggests that phase mixing increases with ZnO content. The DMTA data suggest that the phase mixing and soft segment glass transition increases but storage modulus decreases with increasing with ZnO content. The FT-IR deconvolution result supports to the DMTA analysis. The coating properties like adhesive strength, water absorption, contact angle, gel content and corrosion resistance of the hybrid coatings were also evaluated.     

PACl: A simulation of the change in Al concentration and Al solubility in RO

Polyaluminium chloride (coagulant) is a very complex compound. The change in the ratio HCO₃/CO₂ during coagulation cause a change in pH in the subsequent RO unit (because CO₂ is not rejected by the RO membrane), and it was found that this change is constant for any given recovery, e.g. at 75% recovery, the pH change between the RO feed water and concentrate water is 0.6. The simulation of aluminium solubility and concentration along RO systems were performed assuming the presence of three aluminium species, Al₁₃⁷⁺, Al(OH)₄⁻, Al(OH)²⁺. The simulation of aluminium solubility was performed at two temperatures, 20°C to represent the summer period and 5°C to represent the winter period. Results showed that for winter (5°C) an RO feed water pH of at least 7.2 and for warm weather (20°C), a feed water pH of at least 6.7 are required respectively, to avoid scaling of aluminium on the RO membranes.

In conclusion, theoretical simulation of aluminium solubility and results of deposition factor showed that aluminium more than likely plays a role in fouling of the RO membranes and could be a reason for the frequent cleaning.     

Synthesis and characterization of non-oxide ceramic coatings on graphite substrates by solid–vapor reaction process

Silicon carbide (SiC) and silicon carbide/silicon nitride (SiC/Si₃N₄) were successfully synthesized on graphite substrates by the use of solid–vapor reaction (SVR) process. Layers of SiC and SiC/Si₃N₄ were synthesized on graphite substrate through the reaction between SiO and substrate the (SiO_(vapor) + 2C_{(from graphite)}) and N₂ and substrate the (3SiO_(vapor) + 2N_{2 (vapor)} + 3C_{(from graphite)}), respectively. With the increase of dwell time and synthesis temperature the thickness of SiC layers increases up to 1500 °C temperature. Also, with the increase of synthesis temperature hardness value of SiC coatings is increased, which is 10–15 times higher than the substrate. The critical load of SiC coatings for wear resistance is about 22 N, which was observed by scratch tests. The synthesized SiC coatings appear to consist of a β-SiC phase mixed with a minor amount of an -SiC phase, and its thickness is mainly affected by porosity of the substrate. The thickness of SiC/Si₃N₄ coatings is much thinner than that of SiC coatings, but gives higher value of surface hardness.     

New high performance waterborne organic–inorganic hybrid materials from UV curing

The third generation glycerol based hyperbranched polyester (HBPE-3G) was synthesized by reacting glycerol and 2,2-bis(hydroxymethyl)propionic acid in a stoichiometric molar ratio. This polyester was used as preparation for waterborne hyperbranched UV cured polyurethane–silica hybrid coatings. The acrylic terminated waterborne coatings were prepared by modifying some of the hydroxyl groups HBPE-3G to acidic groups by reacting with maleic anhydride. The remaining hydroxyl groups were reacted with NCO terminated acrylate. Then the prepolymer was neutralized with triethyl amine and dispersed in water. The waterborne hybrid formulations were prepared by mixing 3, 5 and 10% of trimethoxysilylpropyl methacrylate (TMSPM) into the acrylic terminated waterborne coatings. The films were casted and cured under UV light. The various film properties were studied by XRD, AFM, TGA and DMTA instruments. TGA result suggests that the onset degradation temperature and final mass residue increase with increasing TMSPM concentration. DMTA results show that the storage modulus and glass transition temperature increase for the hybrid formulations with increasing TMSPM concentration. The structural characterization of the hybrid coatings was performed using FTIR spectroscopy. The tensile test demonstrated that the mechanical properties improve with the increasing TMSPM content. Surface morphology was studied by atomic force microscopy (AFM). AFM study revealed that TMSPM was crosslinked homogenously through the polymer matrix. The TGA and DMTA data suggest that higher thermal stability and glass transition temperature (T_g) for the TMSPM hybrid films compared to their pure counterparts were obtained with increasing the TMSPM content.     

A macroscopically nondestructive method for characterizing surface mechanical properties of polymeric coatings under accelerated weathering

The surface of coatings and plastics is the first target in any degradation process initiated by ultraviolet (UV) radiation or mechanical stress (via scratch and abrasion). Surface damage can lead to changes in optical, morphological, and mechanical properties and can result in pathways for ingress of moisture and corrosive agents. Current test methods for monitoring performance of protective coatings focus on chemical properties and optical properties, such as color and gloss measurements, or invasive tests such as abrasion and cross-cut adhesion. In this study, a macroscopically nondestructive performance protocol using nanoindentation metrology via a well-controlled scratch test was applied to evaluate the scratch resistance and monitor the surface mechanical property changes in a protective coating under accelerated weathering. Polyurethane (PU) coatings with different polyol compositions were chosen for this study. Coating specimens were exposed to high-intensity UV radiation at 55°C and 75% RH conditions. Exposed specimens were removed at specified UV exposure times for surface modulus/hardness and scratch resistance characterization via nanoindentation and scratch test. The effect of polyol type and UV radiation dose on the scratch damage (scratch morphology) was investigated and correlated with the surface hardness and modulus of the materials.     

Synthesis and properties of lanthanum hexaaluminate ceramic La1−xCaxAl11−y−zMgyTizO18

The aim of the present work is to obtain ceramic materials with a hexagonal structure and high density, hardness and mechanical strength at lower synthesis temperature. Ceramic samples with nominal composition La_1−xCa_xAl_11−y−zMg_yTi_zO₁₈ (x=0–1; y=0–3; z=0–3,5) are prepared. The samples are sintered at temperature 1500 °C by one-stage and two-stage ceramic technology. By X-ray diffraction and scanning electron microscopy, predominant phase LaAl₁₁O₁₈ and second phases LaAlO₃ and -Al₂O₃ are identified. Ceramic materials are characterized with high physico-mechanical properties and may be find application for production of mill bodies and materials for immobilization of nuclear waste.     

On the electrochemical and structural behavior of biologically degraded automotive coatings; Part 1: Effect of natural and simulated bird droppings

Environmental factors can potentially deteriorate automotive coatings. These include UV radiation, humidity, hot–cold shocks as well as aggressive chemical compounds. In addition, natural occurring materials such as bird droppings and tree gums are also enable to affect the coatings. The present work aims to study the degradation of an automotive coating system exposed to natural and simulated bird droppings. To this end, structural analysis of samples was studied using FT-IR spectroscopy, optical and atomic force microscopes. Also, the mechanical and electrochemical behaviors of coatings were investigated by DMTA and electrochemical impedance spectroscopy (EIS). It was found that the biological materials significantly affect the mechanical and chemical properties of the coatings, resulting in a decrease in corrosion resistance. A two-time constant semicircle was observed for degraded coatings after a short time as a result of electrolyte diffusion into the coating/metal interface. The behavior of the coatings was then discussed based on the formation of surface defects.     

Yttrium Silicate Coatings for Oxidation Protection of Carbon–Silicon Carbide Composites

Yttrium silicate (Y₂SiO₅) coatings complement SiC coatings for protecting ceramic multilayer composite materials based on carbon-fiber-reinforced SiC composites (C-SiC). Thick (100 μm), dense Y₂SiO₅ coatings were prepared by dip coating, using concentrated aqueous slips. The resulting phases were studied by taking into account the simultaneous presence of oxide and non-oxide materials, which affected the chemical stability of the coatings. Thick, mechanically stable coatings were obtained by sintering in carbon crucibles and a SiC bed in an argon-flow furnace. Pure Y₂SiO₅ coatings completely separated from the SiC substrates. A high percentage of Y₂Si₂O₇ was necessary to fit the thermal expansion coefficients and ensure the stability of the coatings. Oxidation resistance of the coated substrates was investigated by isothermal and stepwise oxidation tests.