Measurement of internal stresses in coatings using time resolved fluorescence

This paper describes a new nondestructive technique for detecting internal stresses in coatings using time resolved fluorescence. The measurement principle is based upon an experimental result that the decay time of fluorescence from poly(3-octylthiophene), P3OT, dispersed in uniaxially-stretched polymer films decreases with increasing tensile stress acting on the films. Internal stresses in the clear coat and in the base coat of a multilayer structure, which was composed of electrodeposited coat, surface coat, base coat, and clear coat, were estimated from the decay time of fluorescence from P3OT in both coats. The order of internal stresses in the clear coat and base coat of the multilayer system was 1 MPa. When the coatings were piled up, the internal stress decreased as the distance from a metal substrate was increased. It was found that moisture and temperature influenced internal stresses in the clear coat rather than in the base coat. Internal stress in the clear coat, which was one layer coated on a metal substrate, was measured by the traditional bimetallic method or by the time resolved fluorescence technique. Comparing both methods, it was concluded that the time resolved fluorescence technique gave reliable values for internal stresses in coatings.     

Process control for thin organic coatings using fluorescence dyes

The fluorescence dyes rhodamine B and auramine O were added to a coating material based on hybrid polymers (ORMOCER^®s) in concentrations of 10 ppm each. The fluorescence intensity of the auramine depends on the curing state of the lacquer while rhodamine is almost insensitive to this variation of the molecular environment. With the combination of both these dyes, the curing state as well as the thickness of the coating can be measured.Due to the low concentration the dyes do not degrade the properties of the lacquer coating.     

Analysis of the microemulsion-polymerization of acrylamide by time resolved fluorescence

The polymerization of acrylamide in an inverse microemulsion has been studied by time resolved fluorescence measurements. The reaction was initiated by thermal and by photochemical decomposition of the initiator AIBN. A decrease of the fluorescence lifetime of the probe molecule pyrenetetrasulfonic acid sodium salt was observed. In case of the photochemically initiated reaction two distinct lifetimes could be separated. The corresponding pre-exponential factors showed a good correlation with the gravimetrically obtained conversion. During the thermally initiated polymerization the fluorescence decays as well as the intensity of the scattered light were measured on-line and analyzed by a mono-exponential decay law. Received: 3 November 1997/Accepted: 20 November 1997     

In-line monitoring of the conversion in photopolymerized acrylate coatings on polymer foils using NIR spectroscopy

Near-infrared (NIR) reflection spectroscopy was used to monitor the conversion of double bonds in acrylate coatings after irradiation with UV light or electron beams. Quantitative analysis of the spectroscopic data was performed either with a chemometric method on the basis of the PLS algorithm or according to the Beer-Lambert law. FTIR spectroscopy was used for calibration. In-line monitoring of the conversion in pilot-scale was carried out on clear and pigmented coatings, which were applied to polymer foils or paper by roll coating. Useful data were obtained from layers with a thickness from 4 g/m² upwards and at line speeds of at least up to 120 m/min. It was shown that any change of the irradiation dose or other parameters such as inertization leads to an immediate response in the conversion record. Similar investigations were also performed on layers of UV-curable adhesives on the basis of acrylic hot-melts.     

Characterization of photocurable coatings using fluorescence probes

In this paper, we describe a novel method for monitoring the degree of cure and coating thickness of photocurable resins using intramolecular charge transfer (ICT) fluorescence probes such as 5-dimethylaminonaphthalene-1-sulfonyl-n-butylamide (DASB) and 6-propionyl-2-(dimethylamino)-naphthalene (PRODAN). As the curing reaction proceeded, the fluorescence emission spectra of the ICT probes were found to exhibit hypsochromic spectral shifts because of the increase in matrix microviscosity, which made it more difficult for the excited molecule to relax to its twisted charge transfer state. The spectral changes of the probes as a function of the degree of the cure were monitored using a fluorescence intensity ratio method, which enabled an internal calibration of fluctuations in lamp intensity and sample thickness. It was found that the ratios of the fluorescence emission intensities at two different wavelengths were correlated linearly with the degree of polymerization for different types of acrylic or unsaturated polyester resins using either UV or visible (VIS) initiators. The coating thickness of photopolymers was measured based upon the nascent fluorescence from the coating substrate by virtue of an emission (from the substrate) and adsorption (by the coating) mechanism. The thicker the coating, the less the nascent fluorescence emission that can be observed. So the intensity of the nascent fluorescence from the substrate can be correlated to the coating thickness.     

In situ monitoring of sorption and drying of polymer films and coatings: self-referencing, nearly temperature-independent fluorescence sensors

Sorption and drying processes were monitored in situ in polymer films by a fluorescence rotor probe, 4-tricyanovinyl-[N-(2-hydroxyethyl)-N-ethyl]aniline (TC1), a solvatochromatic fluorescence probe, 4-(N,N-dimethylamino)-4′-nitrostilbene (DANS), and pyrene. Taking advantage of an intensity ratio to monitor sorption, these fluorescence probes were found to serve as self-referencing sensors of water sorption in polyvinylacetate, with DANS being the most sensitive followed by TC1 and pyrene. Additionally, the shapes of the emission spectra (and thus intensity ratios) for TC1 and DANS were independent of temperature over a range of reasonable expected use temperatures. Covalent attachment of these fluorescence dyes was shown to enable the determination of sorbate levels within particular layers of multilayer films or coatings. Finally, these probes were also shown to provide sensitivity to desorption or drying of both water and organic sorbates.     

Stress reduction mechanisms during photopolymerization of functionally graded polymer nanocomposite coatings

From the experimental analysis of the photocuring process in terms of reaction kinetics as well as modulus and shrinkage build-up, the residual stresses arising during the photopolymerization of functionally graded composite coatings based on an acrylate matrix and Fe₃O₄@SiO₂ core@shell nanoparticles are evaluated through a Finite Element Modeling approach. Owing to the monotonous variation of volume fraction of the constituent phases that influences the local conversion of the polymeric matrix, these coatings are able to decrease the residual stresses at the coating/substrate interface by as much as ≈25% compared to those encountered in composites with homogeneous compositions, and by as much as ≈40% compared to those arising in the pure polymer. The influence of substrate stiffness, nanoparticle stiffness and conversion degree of the polymer matrix was also analyzed, providing further information for the optimization of the stress reduction mechanism in graded nanocomposite coatings.     

Measurement of residual stresses in injection‐molded polymer parts by time‐resolved fluorescence

Time‐resolved fluorescence properties of 9‐methylanthracene (9MAn) dispersed in film of polyvinylchloride (PVC) containing carbon black were studied under tensile loadings. The fluorescence lifetime of 9MAn decreased from 5.70 to 5.55 ns, whereas the stresses acting on the films increased from 0 to 3 MPa. The change in fluorescence lifetimes of 9MAn during the stress relaxation process showed that the fluorescence lifetimes were correlated with the stresses, not with the strains. The results suggest that 9MAn is a useful probe for monitoring stresses acting on the matrix. With the use of the fluorescence properties of 9MAn, the residual tensile stresses on the skin‐layer of PVC injection‐molded test pieces were estimated. The estimated residual stresses were about ～ 1 MPa. The residual stresses were relaxed to 0 MPa with annealing at 100°C. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2600–2603, 2002     

Adhesion promotion and corrosion prevention using thin anisotropic coatings

Using sol-gel technology, thin organic/ceramic (ceramer) coatings have been applied to metal surfaces to enhance such surface properties as adhesion promotion and corrosion prevention. Isotropic coatings have been found to be effective in certain applications such as corrosion prevention, but the formation of anisotropic (functionally gradient) coatings permits greater flexibility over the resulting properties. Isotropic coatings derived from tetraethoxysilane, for example, have been found to effectively inhibit corrosion while being only 100-1000 A thick. These coatings do not, however, promote adhesion. Thin coatings made from traditional silane adhesion promoters alone are unable to prevent corrosion of metallic substrates. Using monomers with appropriate reactivities permits the single-step synthesis of anisotropic coatings that can both promote adhesion and prevent corrosion. These types of anisotropic coatings allow the physical and chemical properties of a coating to be varied as a function of the distance from the substrate and confer properties to the substrate that would not be possible from a single isotropic coating. The principle behind the construction of these anisotropic coatings is general enough that it can be used in many applications where microengineering of surface structures is important.     

Surface modification of coil coatings with thin plasma polymer films structure and stability

Plasma deposition of a thin top layer with tailored properties is an effective strategy of modification of the organic coating surface. Thin plasma polymer layers are candidates and can provide superior hardness, scratch resistance, modified surface hydrophobicity and easy to clean properties.The present work studies the stability of thin plasma polymer films deposited as top layer on polyurethane coil coating systems. Microwave, hollow cathode and radio frequency plasma polymerization reactors were employed in order to deposit a thin SiO_x based plasma polymer layer.The plasma film stability was studied using surface analysis techniques, ex situ and in situ atomic force microscopy and scanning electron microscopy. Energy dispersive spectroscopy, FTIR spectroscopy and optical measurements confirm the composition and plasma layer properties. The structure of the plasma layers was investigated by means of transmission electron microscopy.The surface morphology together with composition evolution allows the study of the stability of the different coatings. The structure examination of the formed plasma polymer film offers good clarification for coating failure. Decrease of the operating pressure during plasma polymerization and oxygen concentration in precursor mixture lead to formation of compacter layer with higher stability. Introduction of fluorine-containing precursor also increases the anti-weathering performance of the plasma polymer films.     

Stress development at the cement paste-steel interface for thin cement paste coatings

A novel technique for determining the development of stress at the steel-cement interface for thin cement paste coatings is briefly described. A series of interfacial stress-hydration time curves are presented for three matrix systems: cement + silica fume, cement + calcium carbonate, and tricalcium aluminate + gypsum. Results are discussed in terms of possible mechanisms of stress development.     

Measuring Tg in ultra‐thin polymer films with an excimer fluorescence technique

An excimer fluorescence technique has been applied to the measurement of T_g of ultra‐thin polystyrene films. This technique utilizes an excimer‐forming molecule with fluorescent emission in two wavelength bands. The intensity ratio of these bands is a sensitive measure of local viscosity. This technique has been applied to five polystyrene films in the thickness range of 25 nm to 200 nm supported on quartz substrates. The observed T_g for the five ultra‐thin polymer films was similar to the bulk T_g with no observed dependence upon thickness. Additionally, the T_g determined for each film did not show any dependence upon thermal history.     

Evaluation of thin defect-free epoxy coatings using electrochemical impedance spectroscopy

The water sorption of thin defect free epoxy films and coatings on aluminium electrodes was studied using the gravimetric method and electrochemical impedance spectroscopy. The results show that the double layer capacitance of the wetted surface of the aluminium electrode under the epoxy film is considerably smaller than that of a bare aluminium electrode, except in the early period of immersion. The resistivity of the solution absorbed in the epoxy coating is much higher than that of the bulk solution. A method for approximately determining the equilibrium water sorption of the epoxy coatings on the metal surface from the double layer capacitance is reported.     

In-line monitoring of dielectric and fluorescence spectroscopy during polymer/filler compounding

Abstract

A new multipurpose instrument has been used to obtain real-time dielectric and fluorescence spectroscopy from polymer resins compounded with inorganic fillers. The instrument, which is mounted at the exit of an extruder, contains a flow-through slit channel that provides a constant geometry platform for dielectric and fluorescence sensors and for other sensors that can be added as needed. The results of real-time monitoring of Nylon 6 and ethyl vinyl acetate copolymer (EVA) compounded with organo modified montmorillonite clays will be presented. Real-time dielectric data are corrected for electrode and conductivity effects before analysis yields information about dielectric relaxation phenomena. Significant differences in dielectric dispersion parameters were observed for clay nanocomposites in the aggregate, intercalated and exfoliated states. Fluorescent dyes, doped into EVA copolymer, are being used as molecular probes to study the effects of microstructure conformation on their spectra.     

Promoting the adhesion of high‐performance polymer fibers using functional plasma polymer coatings

Plasma‐copolymerized functional coatings of acrylic acid and 1,7‐octadiene were deposited onto high strength, high modulus, poly‐p‐phenylene benzobisoxazole (PBO) fibers. X‐ray photoelectron spectroscopy (XPS) with trifluoroethanol derivatization confirmed that the PBO fibers were covered completely with the plasma copolymer and that the coating contained a quantitative concentration of carboxylic acid groups. Microdebond single filament adhesion and interlaminar shear strength (ILSS) tests were used to evaluate the interfacial strength of epoxy resin composites containing these functionalized PBO fibers. Both the interfacial shear strength (IFSS) obtained from single filament tests, and the ILSS of high volume fraction composites were a function of the surface functionality of the fibers so that there was a good correlation between ILSS and IFSS data. The tensile strengths of single fibers with or without coating were comparable, demonstrating that the fiber surface was not damaged in the plasma‐coating procedure. Indeed, the statistical analysis showed that Weibull modulus was increased. Therefore, plasma‐polymerized coatings can be used to control the interfacial bond between PBO fibers and matrix resins and act as a protective size for preserving the mechanical properties of the fibers. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Real-time monitoring of diffusion in polymer films using fluorescent tracer

A method for real-time nondestructive monitoring of small molecules diffusion in polymeric films was developed. The method was based on detection of a fluorescent tracer eluting from the investigated polymer film into the solution in which this film was immersed. The kinetics of the tracer elution, monitored by the increase in solution fluorescence intensity, was used to deduce tracer diffusivity in polymer film. The data were treated using a straightforward mathematical model, describing diffusion from an infinite plane of a certain thickness immersed into a finite solvent bath. Fluorescent 7-diethylamino-4-methyl coumarin was used as a tracer. The diffusion of this tracer within plasticized poly(methyl methacrylate) and styrene-isoprene-styrene block copolymer matrices was monitored. The diffusion coefficients equal to 2 × 10^?9 cm²/s and 1 × 10^?9 cm²/s, respectively, were obtained. © 1995 John Wiley & Sons, Inc.     

Stress development in drying coatings

As a solvent‐cast polymeric coating dries, each part reaches a concentration at which it solidifies and develops elastic modulus. Thereafter, as further solvent departs, that part shrinks out‐of‐plane, but not in‐plane, if the coating adheres to its substrate. Hence, it develops in‐plane elastic stress. If the stress grows large enough, the stress‐free state may yield, which reduces the final stress level. A theoretical model of diffusion and mass transfer, large shrinkage‐induced deformation, and elastic stress, together with yielding and postyielding viscous deformation, was developed to predict stress evolution in one‐dimensional drying of polymer coatings. Concentration varies only perpendicularly to the substrate, the coating shrinks only in that direction, and the stress varies only in that direction but is in‐plane isotropic. The predictions are compared with measurements of evolving stress in various solvent‐cast polymer coatings and aqueous gelatin coatings by a cantilever‐deflection method. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1000–1013, 2001     

Spatial depth profiling of chromophores in thin polymer films using photopyroelectric spectroscopy

Frequency domain photopyroelectric effect spectrometry (FD-PPES) is a recent spectroscopic technique that may be used to recover depth profiles of optical absorption in thin films on a noncontact and nondestructive basis. In this work a series of well characterized thermally homogeneous samples were prepared that exhibited continuous or discrete subsurface profiles of optical absorption. Multifrequency FD-PPES measurements made on these samples were compared to theory using a heat transfer model, which we have developed for a multilayer thermally homogeneous absorber. Excellent agreement between experiment and theory was obtained for these model studies. While both phase and magnitude channels of the FD-PPES response give depth profiling information, the phase response is especially sensitive to variations in the subsurface profile of optical absorption. By means of the phase response it is possible to determine the number and spatial positions of absorbers in a series of discrete absorbing layers. The FD-PPES. technique has excellent potential for analysis and nondestructive evaluation of polymer films.