Considerations for characterizing moisture effects in coatings weathering studies

This article investigates the effects of weathering factors on moisture absorption of coatings and presents results from several projects including real-time and accelerated weathering studies of coatings moisture absorption. Moisture variable characteristics, observed in outdoor exposure environments, are reviewed. Data are presented from the experiments performed in end-use and accelerated weathering environments to characterize moisture characteristics on coatings. The information from natural outdoor environment characterizations and design of experiments (DOEs) indicates the new approaches for characterizing coatings weathering with regard to moisture effects. Considerations and new approaches for performing coating weathering studies are discussed. Presented at the 2007 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, October 3–5, 2007, in Toronto, Ont., Canada.     

Recovery of surface defects on epoxy coatings and implications for the use of accelerated weathering

Surface roughness, arising from photodegradation, increases overall during weathering but may relax and diminish during episodes when exposure is limited. Different ambient temperatures will change the balance between photodegradation defect size and recovery, depending on the value of the glass transition temperature of the polymer. Epoxy coatings were exposed to periods of ultraviolet irradiation, after which the recovery of the surface roughness was monitored at several temperatures, above and below their glass transition temperatures. Atomic force microscopy, as well as following the increase in roughness with exposure, showed that increased exposure made phase separated domains more distinct. Recovery of nanoindentation on un-damaged coatings produced a similar value of the glass transition temperature to that deduced from the degradation roughness recovery. This was significantly lower at the surface of the epoxy coatings than was measured for the bulk. Confocal Raman spectroscopy was unable to detect any chemical difference between the surface of any films and deeper in their bulk. This evidence suggests that the low glass transition temperature is not due to different curing chemistry at the surface of the coating, but hints that the surface of these crosslinked coatings may relax differently to the bulk or have a different physical structure. These results lead to questions about how to change accelerated testing to better serve the needs of coatings’ technology and how to make progress in the overall goal of service lifetime prediction.     

A nondestructive method of measuring losses in polymeric optical fibres

Conclusions A procedure has been described for nondestructive measurement of specific light losses in polymeric optical fibres, from lateral scattering. The procedure is applicable in the case of a nonuniform optical fibre in which the intensity of lateral scattering does not change exponentially along the fibre.To measure light losses in a specimen of lengthl, it is necessary to measure the intensity of the light scattering through the side surface of specimen sections of lengthl ₁ andl ₂ (l=l ₁+l ₂) twice: interchanging the places of the input and output ends of the fibre.Translated from Khimicheskie Volokna, No. 1, pp. 22–24, January–February, 1990.     

The effect of accelerated weathering on the degradation of polymeric films

The influence of artificial weathering on three general properties of films of high-impact polystyrene (HIP), poly(ethylene terephthalate) (PET), and poly(vinyl chloride) (PVC) was investigated, Degradation resulting from weathering was investigated in terms of changes in the water vapor permeation, mechanical properties, and optical transmission. The diffusivities and permeabilities of these three materials were significantly altered by weathering at two levels of exposure. Films of HIP were embrittled at both levels of exposure, whereas the PET films were embrittled only at the high level of exposure. Films of PVC showed embrittlement at the highest level of exposure and the lowest test temperature. Changes in the optical properties were less significant, amounting to a relatively small decrease in the transmission of the films in the visible range.     

Mechanical property changes and degradation during accelerated weathering of polyester-urethane coatings

Chemical changes, measured using spectrocopy, and crosslink density, measured by mechanical thermal analysis, were determined during accelerated weathering on a model polyester-urethane coating of known composition. The tensile modulus, measured above the glass transition temperature, and thus the crosslink density, decreased with exposure, as expected from the chemical changes. However, the tensile modulus, measured at room temperature, increased with exposure. Physical aging of the polymer network was found to occur concurrently with photodegradation and accounts for much of the increase in room temperature modulus. Increased hydrogen bonding in the increasingly oxidized polyester-urethane may also contribute to the increase in modulus at room temperature. Both physical and chemical changes must be determined if changes, and rates of change, in performance due to weathering are to be understood. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL     

Photooxidation of polypropylene under natural and accelerated weathering conditions

The environmental and accelerated photodegradation of two polypropylene (PP) films (high and low crystallinity) were investigated. FT-IR measurements coupled with derivatization reactions (NO and SF₄), elongation at break tests and gel permeation chromatography technique were used to monitor the degree of oxidation during the UV exposure. No stoichiometry changes were observed under both UV conditions for the PP films. The dominant photoproduct, i.e., ketone, was suggested to be produced from peroxy radicals. The low concentration of tert-alcohol detected questioned the validity of its formation from hydroperoxide decomposition as suggested by several authors. Under natural exposure, the effect of degradation on mechanical property was different for high and low PP. High PP was useless after a short exposure time; low PP retained its physical property for a longer period of time. In the latter case, the analysis of the changes in elongation at break, the polydispersity P = M_w/M_n, the number of chain scissions N_t, and the kinetic accumulation of photoproducts provided strong indications on the mechanisms of acid and ester production and shed some light on the competition between crosslinking and chain scission reactions. The estimated acceleration factors were 7.5 and 8.5 for high and low PP, respectively, values that were much lower than those obtained for linear low-density polyethylene. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2497–2503, 1997     

An improved accelerated weathering protocol to anticipate Florida exposure behavior of coatings

A new accelerated weathering protocol has been developed which closely replicates the performance of automotive and aerospace coating systems exposed in South Florida. IR spectroscopy was used to verify that the chemical composition changes that occurred during accelerated weathering in devices with a glass filter that produced a high fidelity reproduction of sunlight’s UV spectrum matched those that occurred during natural weathering. Gravimetric water absorption measurements were used to tune the volume of water absorption during accelerated weathering to match that which occurred during natural weathering in South Florida. The frequency of water exposure was then scaled to the appropriate UV dose. A variety of coating systems were used to verify the correlation between the physical failures observed in the accelerated weathering protocol and natural weathering in South Florida. The new accelerated weathering protocol correctly reproduced gloss loss, delamination, cracking, blistering, and good performance in a variety of diverse coating systems. For automotive basecoat/clearcoat paint systems, the new weathering protocol shows significant acceleration over both Florida and previous accelerated weathering tests. For monocoat aerospace systems, the new weathering protocol showed less acceleration than for automotive coatings, but was still an improvement over previous accelerated tests and was faster than Florida exposure.     

Effects of artificial weathering on the mechanical properties of paper‐based materials consolidated with polymeric materials

The study of polymer behavior over time represents the crucial stage in the setup of innovative methodologies for paper restoration. In this research, a series of laboratory simulations by accelerated aging and characterization tests by chemical and physical measurements were carried out on paper samples consolidated both by grafting polymerization with acrylic copolymers and by coating with waterborne polyurethanes with the aim of determining their harmlessness and long‐term effectiveness. In this way, our purpose was not only to verify possible risks for the paper materials but also to determine the advantages and potentialities of new restoration methods through an appropriate and essential working plan of investigations. From our evaluation, both the selected consolidation methods appeared unsuitable to be used in paper restoration because of the damage that they could cause on the paper materials because of their limited durability over time. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Patterns of erosion from acrylic and fluoropolymer coatings in accelerated and natural weathering tests

Commercial poly (vinylidene difluoride) (PVDF)/acrylic coatings show minimal gloss loss or bulk chemical change even after sustained Florida or accelerated weathering testing. Previously, we showed that “QUV-B” weight loss rates can be used as an early predictor of the weatherability of fluoropolymer/acrylic blend clearcoats. In this work, we study the microscopic basis for differences seen between various blends and pure acrylic coatings. Scanning electron microscopy (SEM) images reveal different characteristic patterns of degradation. 900 First Ave., King of Prussia, PA 19406.     

A nondestructive method to determine material properties in anisotropic plates

Material parameters in anisotropic rectangular plates are determined in a nondestructive way. Real-time, TV-holography is used to determine frequencies and shapes of the first five modes of vibration of plates with free-free boundary conditions. According to rules given in the paper, finite element analysis is then used to determine two effective Young's moduli, the shear modulus, and the Poisson's ratio.     

A nondestructive method to determine material properties in orthotropic plates

An electronic speckle pattern interferometer (ESPI) is used to determine modes of vibration in rectangular, orthotropic, free-free plates; that is using a noncontact, nondestructive, optical method. It is shown, using the finite element method (FEM), that each of the first three modes of vibration in rectangular orthotropic plates has a strong dependence upon only one of the main material parameters, namely the in-plane shear modulus and the two Young's moduli, respectively. With this one-variable dependence it is a simple task to determine the effective material parameters. This method has several obvious advantages compared to the use of test bars and it can be extended to give a measure of the damping parameters and probably also be used for production control. Preliminary results are presented and discussed.     

The renormalization group method in modelling of mechanical properties of heterogeneous polymeric materials

Summary The renormalization group technique, developed in the mean field theory, is applied here as a method for calculating of elastic or viscoelastic properties of heterogeneous materials with complex structures. The method consists in averageing of properties of small volume elements of the heterogeneous system by step-wise application of a recurrence relation which renormalizes properties of the group of neighbouring cells to properties of a new cell of larger dimensions. The recurrence relations are derived on the basis of the packet model — a group of eight elemental cells.     

Effect of surface treatments on rheological,mechanical and magnetic properties of ferrite-filled polymeric systems

Experimental data on the effect of the type and amount of surface treating agents on the rheological and mechanical properties of the ferrite-filled styrene-isoprene-styrene systems at high levels of loading has been presented. The viscosities of the treated ferrite systems are found to be lower than the untreated systems, with the extent in reduction depending upon the type and amount of surface treatment. It is also found that surface treatment decreases the tensile strength and percentage elongation in the present filler-matrix system, and the extent of the decrease in viscosity as well as mechanical properties is more in the presence of titanates than with silanes.     

Nanoindentation as an alternative to mechanical abrasion for assessing wear of polymeric automotive coatings

Mechanical abrasion followed by transmission delta-haze measurement is a standard means to assess wear in polymeric silicone automotive hard coatings. However, the drawbacks of this abrasion–transmission (A–T) technique (drift, variability, sample size, and test time) make an alternative measurement method desirable. Literature reports have shown that the ratio of hardness to modulus can successfully predict wear performance in ceramic and metallic nanocomposite coatings. This work studied measures from both nanoindentation and nanoscratch testing to determine which could be a viable alternative to the historical A–T test for a specific polymeric coating system. Both nanoindentation measures of hardness (H) and the ratio of hardness to modulus (H/E _r) showed high repeatability compared with the other measures evaluated in this study and compared with the historical test. Of these two measures, the ratio H/E _r with an exponential fit showed the strongest correlation with A–T delta-haze measurements. Key formulation and process factors affecting abrasion resistance in automotive coatings were analyzed in a designed experiment with historical A–T delta-haze and nanoindentation H/E _r as responses. Analysis showed significant benefits to the use of the H/E _r measure of abrasion resistance in modeling coating performance.     

Metrology for characterizing scratch resistance of polymer coatings

Current methods for scratch resistance assessment are often based on “relative but not quantitative” types of measurements, such as visual inspection, gloss changes, and changes in gray scale level or lightness. Most results are used for qualitative assessment purposes, which result in the lack of a repeatable and reliable standardized test method for the polymer materials community. To implement a scientifically based standardized test method for quantifying scratch resistance, it is vital to understand the relationships between material mechanical properties, morphology, and appearance (optical properties) of surface and subsurface deformation. In this article, preliminary results from a scratch testing protocol to identify the “onset” of plastic deformation in poly(methyl methacrylate) and poly(propylene) commercial samples are presented. Recent advances in optical scattering measurements to identify the onset of plastic deformation by analyzing specular and off-specular intensities are also presented. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL.     

Formation of coatings with tailored properties on polyperoxide-modified polymeric surfaces

Modification (peroxidation) of polymer surfaces, polypropylene, polyethylene, polyethylene terephthalate, nylon 6-6, poly(phenylene oxide), and ethylene–propylene copolymer were affected by the surface grafting of functional polyperoxides (FPPs) such as poly(5-tert-butylperoxy-5-methyl-1-hexene-3-yne-co-octylmethacrylate) (VEP-co-OMA), poly{N-[(tert-butylperoxy)-methyl]acrylamide-co-octyl methacrylate} (PO-co-OMA), and poly(tert-butylperoxy-methacrylate-co-octyl methacrylate) (PEst-co-OMA). The degree of surface modification was shown to be determined primarily by the structure of the polymer substrate. Using a peroxidized surface as an initiator of grafted copolymerization enabled the grafting of functional monomers (acrylic acid, acrylamide, 4-vinylpyridine, and hydroxyethyl methacrylate) and polysaccharides (heparin, dextran, etc.) and thereby imparted adhesive, antibacterial, haemocompatible properties to the polymer surface.     

Dynamic mechanical properties of some polymeric acid zinc salts

Mechanical properties, shear modulus, and damping of a series of polyacid divalent metal salts have been correlated with the degree of salt formation. The salts were prepared in situ by molding mixed powders of 94/6 acrylic acid–2-ethylhexyl acrylate and zinc oxide at temperatures of 200–300°C. and pressures of 5,000–10,000 psi. Zinc oxide consumption was followed by x-ray techniques. Compositions contained 25–200% of theory metal oxide as charged. Excesses, over theory, of metal oxide were shown to lead to the formation of substantial amounts of pendent half-salts which are high damping and have temperature-sensitive shear moduli. Only complete reaction as the di-salt, at 300°C. and 10,000 psi, leads to low damping products with temperature-insensitive high modulus. The modulus increase due to ionic bonding as the di-salt, over that expected from classical filler action alone, ranged from 40 to 80%, depending upon the theory chosen to calculate filler action. The pendent half-salt gives much smaller moduli increases and unreacted metal oxide appears to act as classical filler in an intertangled complex polyelectrolyte salt matrix. The modulus of the di-salt was found to be 6–7 times higher than moduli for normal organic rigid polymers.