F. Bellucci

The principles of dielectric measurements for in situ monitoring of composite processing

Abstract The fundamental concepts of dielectric behavior of polymers and the utilization of dielectric measurements for in situ monitoring of cure of polymers and composites are discussed. Information is presented on currently used dielectric sensors and the procedure for calculation of dielectric parameters from the monitored signal. The review is written to accommodate both the fundamental and the pragmatic aspects of dielectric monitoring of cure. In the final part of the review, a critical assessment is offered of the advantages and disadvantages of dielectric measurements for the in situ monitoring of processing of polymers and composites.

Impedance spectroscopy of reactive polymers : correlations with chemorheology during network formation

An investigation was carried out of correlations between dielectric, spectroscopic, and rheological properties during reactions in several epoxy/amine formulations of both polymer‐forming and nonpolymer‐forming nature. Dielectric results obtained from impedance spectroscopy were compared with results from near‐infrared spectroscopy, high performance liquid chromatography, and steady shear and dynamic mechanical measurements. Reaction kinetics obtained from dielectric, spectroscopic, and chromatographic results were in excellent agreement. Gelation and vitrification times of multifunctional formulations, determined by dielectric and rheological measurements, agreed surprisingly well, despite the empirical nature of such correlations. Nonetheless, a realization of the full potential of dielectric impedance spectroscopy in monitoring the progress of chemophysical changes in reactive polymers necessitates the development of fundamental scientific correlations between dielectric and chemorheological phenomena during cure.

The behaviour of iron and low alloy steels in anhydrous organic solvents—methanolic solutions

Abstract The data supplied by electrochemical methods such as potentiodynamic polarization curves and potentiostatic anodic transients show that in methanolic solution a region of unstable passivity for Armco iron and Ni-Cr-Mo low alloy steels results from the overlapping of certain ranges of solution composition (e.g. acidity up to 10 −3 M as sulphuric acid, chlorides 10 −4 to 10 −3 M, water 0.01–0.5%). Slow strain rate tests indicated that the instability of the protective oxide films can induce susceptibility to stress corrosion cracking in the presence of applied stresses.

Effect of water content on the corrosion behaviour of Ni in methanol (MeOH) solutions

Abstract The effect of water content on the corrosion behaviour of nickel in methanolic solutions have been investigated tracing the potentiostatic curves at room temperature in a standard electrochemical cell. Preliminary results reported in this communication, shown that the electrochemical behaviour of Ni in MeOH is strongly influenced by the water content of the solution and that traces of water (of the order of 0.5%) are able to guarantee the passivity of nickel.

Impedance spectroscopy of reactive polymers. 5. Impedance as a measure of chemical and physical changes in glass formers

A study was conducted aimed at establishing the nature of chemical and physical phenomena in polymeric and nonpolymeric glass formers that can be observed by impedance measurements. Various systems were investigated that undergo a temporal evolution of structure as a result of chemical reactions and physical processes such as crystallization, vitrification, or phase separation. Distinct and systematic changes in impedance during crystallization and vitrification confirmed that these events could be monitored by impedance spectroscopy. Of particular interest was the potential use of impedance measurements in detecting gelation in crosslinking polymers. It was shown that the experimentally observed knee in imaginary impedance during reaction shifts with frequency and, hence, cannot be used to measure gelation. But a new insight at the molecular level was obtained by employing a novel experimental approach based on simultaneous dielectric-infrared measurements. Evidence was generated to support the formation of a hydrogen-bonded complex in the vicinity of gel point in polymer networks, which affords a vehicle for the migration of intrinsic charges and provides a contribution to the overall conductivity. This finding should be explored further because it suggests the possibility of correlating dielectric response with gelation.

The Effect of Post‐Cure Annealing on the Protective Properties of Polyimides on Chromium Substrates

The effect of post-cure annealing on the protective properties of thin (2.6-2.8 μm) polyimide films on chromium metallic substrates was investigated in aerated neutral 0.05, 0.5, and 5 M aqueous NaCl solutions at ambient temperature (20°C). The study was carried out using the technique of electrochemical impedance spectroscopy to monitor film degradation as a function of immersion time in the test solutions. Results obtained indicate that when exposed to aqueous NaCl solutions for extended periods, a majority of the nonannealed coated substrates failed ; however, the performance of the annealed samples was significantly better. At face value, these results suggested improved protective properties of the polyimide due to annealing. However, a more in-depth analysis of the data revealed that post-cure annealing leads to the development of a thicker oxide on the chromium substrate. The change in the oxide occurring during the post-cure annealing treatment contributes to the improved corrosion resistance of the polyimide/Cr system.

Protective Properties of Polyimide (PMDA‐ODA) on Aluminum Metallic Substrate

The protective properties of thin polyimide (PI) films on aluminum metallic substrates were studied using the impedance spectroscopy technique. The effects of storage time, film thickness (1.2 and 2.4 μm), NaCl concentration (up to 0.5 M) and the synergistic effect of temperature (80 o C) and relative humidity (85%) on the lifetime of the PI were investigated

THE INFLUENCE OF TARTARIC ACID ADDITIONS ON THE ANODIZING BEHAVIOUR OF AA2024-T3 ALLOY IN SULPHURIC ACID

AA2024-T3 Al alloy was anodized in a 0.46 Μ sulphuric acid solution and in a potential alternative sulphuric (0,46M) + tartaric (0,53M) acid solution. The anodising process was carried out in both solutions at constant voltage, following an Alenia S.p.a Patent. The Al alloy was pre-treated by etching in NaOH at 40°C, and desmutting in 0.3 % vol HN03 at room temperature. The morphologies of the anodic films generated in the selected anodising solutions were examined by TEM and FEG-SEM: the electrochemical behaviour of the anodised 2024-T3 alloys was investigated using DC and AC electrochemical techniques. AC measurements were performed in 0.6 Μ NaCl, and 0.5 Μ Na2.S04, and 0,1 Μ ΝΗ4Β5θ84Η2θ solutions, while DC measurements Were carried out in 0.46 Μ sulphuric acid and in 0.46 Μ sulphuric + 0.53 Μ tartaric acid solutions at room temperature. The FEG-SEM probing of the as received alloy revealed rolling lines and the presence of second phase particles; after anodizing, the porous film morphology was readily revealed at the surface of the anodic film. Transmission electron microscopy of ultramicrotomed sections of the as received AA2024-T3 alloy revealed the presence of relatively thin disturbed layer of fine-grained alloy. TEM observations of the anodic films, showed a porous layer of about 1400 nm thickness for both anodising conditions and relatively thin barrier layer thicknesses of 17.2 and 23 nm for films formed in sulphuric and in the mixed acid solutions respectively. Results obtained by d.c. measurements indicated that the electrochemical

The Correlation Between Substrate Mass Loss and Electrochemical Impedance Spectroscopy Data for a Polymer-Coated Metal

Research has been carried out to evaluate the correlation between substrate mass loss and data generated by electrochemical impedance spectroscopy (EIS). Experiments were carried out as a function of exposure time for both polymer-coated and hare samples. Mass loss was determined in situ and in real time with a vibrating sample magnetometer (VSM). In all cases, the sample was held potentiostatically at the open circuit potential (OCP) while EIS data were collected. Mass loss data calculated from electrochemical impedance (El) spectra were then compared to equivalent data generated on the VSM. Although previous research has investigated the correlation for bare samples [W. J. Lorenz and F. Mansfeld, Corros. Sci., 21, 647 (1981)] establishing this relationship for a coated substrate is more convoluted. The difficulty results from errors associated with solution uptake within the polymer, and corrosion product entrapment. Such errors obviate the use of traditional gravimetric techniques and, until now, the relationship between substrate mass loss and EIS has not unequivocally been established for polymer-coated samples. Although the current research indicates that EIS can accurately define the mass loss for a bare sample, the correlation for coated samples has, generally, not been as good.

Moisture Uptake in Organic Coatings Monitored with EIS

Although EIS is widely applied as a technique for monitoring the degradation of polymeric coatings when exposed to acqueous environments, a rigourous theoretical framework has not yet been fully developed. In this paper a mathematical model relating the kinetics of moisture uptake to the changes in the dielectric properties of organic coating is presented and a correlation between the extent of absorbed water and changes in the coating capacitance is developed. Thus, it is possible to measure both water diffusivity and solubility from capacitance data, making this technique very attractive as an alternative to the gravimetric method for investigating the barrier properties of organic coatings. The model developed in this paper was then employed to interpret the behaviour of industrial electrogalvanized iron sheet coated with a water based epoxyphenolic paint when exposed to an aqueous sodium chloride solution at room temperature.