L. Nicodemo

Water Transport in Organic Coatings

Abstract The electrochemical impedance spectroscopy (EIS) technique was used to evaluate the water transport (diffusion and equilibrium water uptake) and the dielectric properties of freestanding polyimide (Kapton® and PMDA-ODA) and polyethyleneterephthalate (PET) membranes at 25 and 40°C, respectively, and in supported PMDA-ODA-coated metals. Permeability and diffusion coefficients of freestanding films were also obtained by using the Payne cup method and the MacBain quartz spring balance in order to assess the reliability of the EIS method when compared to other techniques. Results from this work show that the diffusivity of water in polyimide films varies from 1.42 × 10−9 to 3.53 × 10−9 cm2 s−1 for thickness between 2.4 and 125 μm, while the equilibrium water uptake varies from 2.31 to 4.63% by weight for the same range of thickness. The average calculated dielectric constant of the freestanding Kapton® films is 4.5. Water diffusion coefficient in PET varies from 2.11 × 10−9 to 12.61 × 10−9 cm2 s−1 for...

Protective properties of epoxy-based organic coatings on mild steel

The protective properties of epoxy-based organic coatings on mild steel substrates have been studied using the electrochemical impedance spectroscopy (EIS) technique. The effects of coating composition, film thickness and curing temperature on the lifetime of the epoxy coating were investigated. The results obtained indicate that the film composition dramatically affects the rate of corrosion of the metallic substrate. Coatings with low and high content of curing agent exhibit early failure when exposed to an air-saturated 0.5 M aqueous solution of NaCl. Extended lifetimes were observed for samples with a content of the curing agent equal to 8.5% by weight. An increase in lifetime was also observed on increasing the film thickness and the curing temperature. The failure mechanism seems to occur in two steps: the first step is related to water uptake in the film while the second step is related to Cl− ion diffusion through the coating. The results have been interpreted in terms of a model in which the dielectric properties (capacitance and resistance) of the coating depend strongly on the coating composition and are affected by both the water and salt uptake into the film.

A study of corrosion initiation on polyimide coatings

Abstract The protective properties of thin polyimide (PI) films on aluminum and iron metallic substrates were studied using a.c. and d.c. electrochemical techniques. The effects of storage time, film thickness, and the synergistic effect of temperature (80°C) and relative humidity (85%) on the lifetime of the PI were investigated. The results obtained can be summarized as follows: (i) early failure was observed for the PI/AI samples tested directly after curing, while extended lifetime was observed for samples stored in a desiccator for 2 months before testing, (ii) early failure was also observed for the PI/Fe samples even after storage in a desiccator before testing, (iii) the lifetime increases by increasing the film thickness, and (iv) temperature and relative humidity (RH) dramatically affect failure. A mathematical model is presented to relate coating failure to salt uptake by the PI. By applying this model to the experimental findings, an estimate of the heterogeneities or defects in the PIs is given.

THE EFFECT OF FILM THICKNESS ON OXYGEN SORPTION AND TRANSPORT IN DRY AND WATER-SATURATED KAPTON POLYIMIDE

Abstract Sorption and transport of oxygen in dry and water saturated Kapton® polyimide films of different thickness were investigated at 25°C with pure oxygen using: (i) a manometric permeation apparatus (gasmembranegas configuration), (ii) a high pressure gas sorption equipment, and (iii) an electrochemical technique based on the oxygen Clark electrode (liquid-membrane-liquid configuration), respectively. A sharp increase of permeability, solubility, and diffusivety coefficient as thickness increased in the range 13–50 μm was observed for the oxygen transport through both dry and water-saturated Kapton. The variation of the oxygen parameters in the range of thicknesses investigated were small in the case of dry polyimide compared to the case of the water-saturated polymer. An interpretation is presented in which the oxygen transport parameters in both dry and water-saturated conditions are analyzed in terms of ‘dual-sorption—dual-mobility’ model and related to the different polymer morphologies. such as crystallinity level, crystallinity orientation and excess free volume content, characterizing samples of different thickness.

Electrochemical characterisation of multilayer organic coatings

Abstract The protective properties of eight high performance commercial multilayer organic coatings for aeronautical use (based on polyurethane, epoxy and polyurethane-compatible epoxy resins) on anodised 2024-T3 Al alloy were evaluated in neutral aerated 3.5% NaCl aqueous solution using the electrochemical impedance spectroscopy (EIS) technique. The investigation was performed on specimens supplied and prepared from Alenia in accordance with their technical specifications. Results obtained in this paper show that all coating systems exhibit excellent protective properties even after prolonged immersion in the test solution (one year). Although the dielectric properties of all multilayer coatings are quite similar, EIS was proved to be able to discriminate among them, making possible a rank of these protective systems to prevent or to reduce corrosion. Capacitance measurements performed on the same coatings allow their behaviour to be forecasted.

Recoil kinetics of uniaxially oriented polystyrene

SummaryRecovery experiments aboveTg have been performed on highly oriented polystyrene sheets obtained from extrusion and subsequent hot drawing. A shift procedure has been applied to recoil data using the WLF equation on two different types of commercial polystyrene. A variableλ defined as the fractional distance from equilibrium is able to reduce all the data obtained on sheets drawn at different draw ratios to a single master curve. The change in molecular weight affects only the value of the glass transition and the relaxation kinetics but has no influence on theC1 andC2 constants of the WLF equation. Moreover the molecular orientation does not modify theTg of the drawn polymers.ZusammenfassungEs werden Kriecherholungsversuche oberhalb der GlastemperaturTg an hochorientierten Polystyrolplatten durchgeführt, die durch Extrusion und nachfolgende Heißverstreckung hergestellt worden sind. Die Schrumpfdaten von zwei verschiedenen kommerziellen Polystyrolen werden mittels der WLF-Gleichung übereinander geschoben. Durch eine Variableλ, welche den relativen Abstand von der Gleichgewichtskonfiguration beschreibt, kann man sämtliche verschiedenen Verstreckungsverhältnissen zugeordnete Meßwerte auf einer einzigen Master-Kurve vereinigen. Der Unterschied im Molekulargewicht beeinflußt lediglich die Glastemperatur und die Relaxationsgeschwindigkeit, hat aber keinen Einfluß auf den Wert der KonstantenC1 undC2 der WLF-Gleichung. Darüber hinaus wird die Glastemperatur durch die molekulare Orientierung infolge der Verstreckung nicht verändert.

The effect of composition on the performance of epoxy paints

Water-diffusion coefficient water solubility and oxygen permeability measurements have been performed on free-standing membranes of epoxy-based organic coatings for different values of paint composition. Concomitant direct (d.c.) and alternating (a.c.) current measurements were carried out on coated mild steel in order to investigate the protection exhibited by the coatings in neutral aerated 0.5 M NaCl. A general model has been used to analyse the a.c. data in terms of reactions occurring during the interaction of the coated metal with the aggressive environment. A correlation between transport properties of free films and the protective properties of paints has also been attempted. No relationship was found between the water permeability and the coating performance over exposure time, whereas the oxygen permeability seems to be responsible for coating effectiveness at later stages. The electrical resistivity of the coatings and its dependence on both the composition and water content has been related to early failures.

Transport of water dissolved oxygen in polymers via electrochemical technique

Abstract Transport properties of polymer membranes for dissolved oxygen in a liquid-membrane-liquid configuration were investigated by using an electrochemical technique based on the use of the Clark oxygen electrode. The oxygen permeability and the diffusivity coefficient in a polymer-water system can be determined accurately by appropriate choice of the experimental conditions. Polymers that do not absorb water such as polyethyleneterephtalate (PET) and bioriented polypropylene (OPP) were tested for permeation experiments in both gas-membrane-gas and liquid-membrane-liquid configuration. Data obtained in the liquid-membrane-liquid configuration with the electrochemical technique show excellent agreement with those obtained with the gas-membrane-gas configuration. The permeability of water saturated Kapton® polyimide (PI) for dissolved oxygen was also evaluated and compared with data obtained in the gas-membrane-gas configuration.

Iron dissolution in acid water-methanol mixtures

Abstract Dissolution rates of Armco iron in de-aereated and aereated acid (10−3 M H2SO4) water-methanol mixtures have been evaluated by accurate weight loss measurements, by extrapolation of Tafel lines to the corrosion potential and by using the linear polarization method. The corrosion rate was found to decrease and then to increase at increasing molar fraction of methanol both in aereated and aereated solutions, showing a minimum at an intermediate solvent composition which is significantly different from the values observed in pure solvents. Furthermore the corrosion rate in de-aereated acid pure methanol is greater than that in pure water by a factor of nearly two. The experimental results have been plotted in terms of r s r w ratio versus methanol mole fraction, where rs is the corrosion rate in the mixture and rw that in pure water. A behaviour was found quite similar to that exhibited by the ratio of exchange current density for the hydrogen evolution reaction. This similarity leads to a theoretical expression for the ratio r s r w as well as to a physical explanation for the greatest iron dissolution rate in de-aereated acid methanol solution compared to that in pure water which is in satisfactory agreement with experimental results.

The kinetics of surface craze growth in polycarbonate exposed to normal heptane

A variable radius of curvature strain-bending apparatus has been used to investigate the propagation of surface crazes in polycarbonate exposed to n-heptane. It was found that for crazes growing in a zone free of other crazes, the growth rate was linear at constant temperature and exponentially dependent upon temperature. An activated-state theory was developed to describe the kinetics of growth in terms of two parameters,B−1, a characteristic strain constant that was independent of test variables, andA′, a characteristic growth-rate parameter that varied exponentially with temperature. A strain-temperature equivalence principle was adopted to predict the crazing behaviour over a range of temperatures and strains. A critical strain was defined in terms of a maximum allowable craze growth rate.