The effects of weathering on the mechanical performance of automotive paint systems

The durability of automotive paint systems continues to be a great concern to both auto companies and their coating suppliers. Recent advances in assessing the durability of coatings by measuring weathering-induced chemical composition changes have greatly increased our ability to discern superior from inferior coatings. However, different coatings will likely tolerate different amounts of weathering-induced chemical composition changes while still maintaining their mechanical integrity. Thus, a means of linking chemical composition changes to changes in relevant mechanical properties would be highly desirable. The fracture energy, the amount of mechanical energy required to propagate a crack in a material, is a sensitive measure of the brittleness of a material and is relevant to a number of potential failure mechanisms in automotive paint systems. The fracture energy of clearcoats can vary widely depending on the formulation of the clearcoat (initial chemical composition and additive package) and on the amount of weathering. Weathering embrittles most coatings. Weathering-induced changes in the fracture energy are related to chemical composition changes occurring in the clearcoat. Because the brittlest materials will not crack without an applied stress, the stress distribution in complete paint systems as a function of weathering must also be known to accurately anticipate mechanical failures. Measuring thermoelastic constants of individual layers allows for computation of the stresses in complete paint systems. Stresses tend to increase with weathering. The presence of flaws in the clearcoat changes the stress distribution dramatically. Coupled with fracture energy measurements, the stress measurements provide additional insight into paint system failure mechanisms.     

Studying the effects of the chemical structure of an automotive clearcoat on its biological degradation caused by tree gums

The effects of artificial and natural tree gums on the mechanical, chemical, and aesthetic performances of two automotive acrylic/melamine clearcoats were studied. To this end, two clearcoats with different acrylic/melamine ratios were investigated. Biological experiments were performed under post-aging conditions using an accelerated weathering test. Analytical techniques including optical microscopy, scanning electron microscopy (SEM), gloss measurement, FTIR, and DMTA analyses were utilized to reveal the responses of the coating system upon exposure to the aforementioned biological materials. Contact angle measurements were also conducted to estimate the surface energy of the coatings. Greater crosslinking density, together with a higher T _g and damping behavior of the clearcoat, indicative of a greater degree of cure, were obtained as the ratio of melamine crosslinker increased. It was shown that both Arabic and natural tree gums could strongly attach to the clearcoats’ surface, imposing a significant stress during the drying process, thereby leading to a physical failure. In addition, the acidic nature of these biological materials leads to a chemical alteration in the clearcoats’ structure. The greater crosslinking density and lower hydrophilicity of the clearcoats containing higher melamine crosslinker were responsible for the weaker interaction of gums with the surface. This showed a greater capability for stress damping. Small surface cracks with fracture morphology on the coatings exposed to biological materials at higher exposure times (in the xenon test) were also observed. This is discussed based on the adhesion of the coatings to gums at longer exposure times, because of significant stress.     

Resistance of paint coatings to multiple solid particle impact: effect of coating thickness and substrate material

A novel technique has been developed to determine the resistance of paint coatings to multiple solid particle impact (i.e. solid particle erosion). The effect of paint layer thickness on erosion resistance was evaluated for two acrylic automotive topcoats. These coatings displayed a two-stage response to erosion. Initially, their thickness was reduced progressively, but once a critical thickness was reached the remaining coating was removed by individual impacts. A simple model is proposed to describe this behaviour. A new measure, specific erosion resistance, which takes account of the coating thickness, is defined to allow coatings with different thicknesses to be compared and has been applied to several industrially sprayed automotive clearcoats on both steel and polymer (TPO) substrates. The clearcoats exhibited significantly higher specific erosion resistance when applied to polymer substrates.     

Characterization of clearcoats containing phosphoric acid-functionalized acrylic polyols for automotive precoated metal sheet coatings

In this study, the various physical and mechanical properties of clearcoats prepared through a new crosslinking method were investigated. The method was aimed at developing clearcoat systems to improve the deep-draw processing and formability performance in precoated metal (PCM) sheets for automotive applications. From phosphoric acid-functionalized acrylic polyols (PAFAPs) first synthesized in this study and glycidyl methacrylate-modified acrylic copolymer (GMAMAC), phosphoric acid-GMA modified acrylic polyols (PAGMAPs) were newly prepared as new binders in automotive clearcoats. Several clearcoats were formulated with different molecular weights and hydroxyl contents from PAFAPs and GMAMAC. Using clearcoats themselves, the crosslinking reactions for these clearcoats were compared by evaluating the curing behaviors with a rigid-body pendulum test (RPT) and the changes of chemical structures via attenuated total reflectance FTIR spectroscopy. The mechanical properties of the clearcoats were systematically characterized, using dynamic mechanical analysis (DMA) and universal testing machine analysis (UTM). Also, various tests were carried out using PCM sheets by depositing clearcoats above the same PCM-based primers and basecoat layers on galvanized steel. The fracture and deformation patterns related to surface damages on the clearcoat surface were visualized using a nano-scratch test, in association with atomic force microscopy. In particular, deep-draw processing tests, based on forming process simulations, were employed to scrutinize the effect of clearcoats developed in this study on the forming feature in PCM sheets. From the results of RPT, DMA, and UTM tests, the primary crosslinking networks of PAGMAPs from the synthesized PAFAPs and GMAMAC, and also succeeding secondary crosslinking networks between PAGMAPs and blocked isocyanates, were closely correlated with the degree of crosslinking (X _c), in accordance with the molecular weight between crosslinks (M _c), and glass transition temperature (T _g). As a result, the presented clearcoats with a long pendulum period, a low rubbery modulus, and a large tensile strain value, which are the significant factors for developing automotive PCM sheet technology, have truly demonstrated more superior formability during the deep-draw process. It is confirmed that properties of clearcoats with toughness and flexibility could be optimally controlled by PAGMAPs for automotive coatings.     

On the combined use of UVA, HALS, photooxidation, and fracture energy measurements to anticipate the long-term weathering performance of clearcoat/basecoat automotive paint systems

Transmission UV spectroscopy measurements of clearcoat ul traviol et light absorber (UVA) disposition, electronspin resonance (ESR) spectroscopy measurements of clearcoat and basecoat Active hindered amine light stabilizer (HALS) disposition, and transmission Fourier transform infrared (FTIR) measurements of photooxidation have been carried out on 5 μm thickslioes of clearcoat/basecoat/primer/e-coatpaint systems on steel panels as a function of outdoor exposure. These analysis results are combined with clearcoat fracture energy measurements to assess the possibility that a clearcoat/basecoat paint system will resistcatastrophic cracking/peeling failure at long times. Taken together, all results indicate that these nontraditional paint weathering performance metrics should be added to the existing repertoire of paint weathering performancemetrics to ensure that inferior clearcoat/basecoat automotive paint systems are not introduced into service. Ford Research Laboratory, MD 3182SRL, P.O. Box 2053. Dearborn, MI 48124     

Effects of weathering,scale, and rate of loading on polycarbonate fracture toughness

The fracture toughness of polycarbonate specimens of 3–9 mm thickness obtained from an actual aircraft canopy, were studied under accelerated weathering conditions and different rates of loading. Although no significant effects of thickness and loading rate on the critical stress intensity factors were observed, two different failure modes, brittle fracture and ductile fracture triggered by “pop-in,” were observed. The mode of failure was a random event and the probability of ductile failure associated with pop-in increases with the weathering time. More insight to material characteristics are gained through analysis of the specific fracture energy (SFE). The average values of SFE decrease monotonically with accelerated weathering time. This effect is ascribed to physical aging of the PC in the weatherometer that was corroborated through increases in density. The values of SFE seem to correlate with the probability for ductile fracture. This information can be used to establish conservative critical stress values for design. © 1994 John Wiley & Sons, Inc.     

Evaluation of the weathering performance of basecoat/clearcoat automotive paint systems by electrochemical properties measurements

The aim of the present study was to investigate the possibility of evaluating the weathering performance of a basecoat/clearcoat automotive paint system through the determination of its electrochemical properties. To this end, the electrochemical properties of a basecoat/clearcoat automotive paint in a 3.5% solution of NaCl in deionized water were measured at different weathering exposure times. A constant phase element (CPE) was used for describing the electrochemical behavior of the coatings under test. The values of the CPE parameters, i.e. Y₀ (the CPE constant) and n (the CPE power) were subsequently correlated to the extent of photo-oxidation (as measured by appearance, surface roughness, FTIR, surface tension and adhesion measurements) of clearcoat at the surface, in the bulk and at the interface between the basecoat and the clearcoat. The result showed that the electrochemical parameters Y₀ and n provide ready means for comparing the weathering performances of basecoat/clearcoat automotive paint systems. Increases in the value of Y₀ together with decreases in the value of n with increasing weathering exposure times suggest increased possibilities for the onset of cracking in the clearcoat itself in addition to its propagation towards the basecoat. Additionally, sudden variations in the values of Y₀ and n are indicative of increases for the clearcoat to peel off.     

Temperature dependent first matrix cracking stress model for the unidirectional fiber reinforced ceramic composites

Based on a kind of equivalence between heat energy and fracture energy, assuming that there is a constant maximum storage of energy that includes both heat energy and fracture energy, a new temperature dependent fracture surface energy model is developed. Using the new model and the classical ACK theory, a temperature dependent first matrix cracking stress model is obtained for the fiber reinforced ceramic composites. According to the model, the temperature dependent first matrix cracking stress of materials can be easily predicted using some basic material parameters such as matrix fracture surface energy and Young’s modulus. The model is verified by comparison with experimental data of SiC fiber reinforced reaction-bonded Si₃N₄ composites at different temperatures. Good agreement is obtained between predicted and experimental data of first matrix cracking stress. The dependency of first matrix cracking stress on fracture surface energy and interfacial shear strength is systematically analyzed.     

Curing and Photostabilization of Thermoset and Photoset Acrylate Polymers

The light‐stability of thermosetting acrylate and UV‐cured acrylate polymers has been tested in an accelerated QUV weatherometer. Infrared spectroscopy was used to monitor in situ the chemical changes occurring upon both curing and photoageing of 35 μm thick clearcoats. The curing was shown to proceed more extensively in photoset than in thermoset acrylate polymers which contain 6 and 20% unreacted functional groups, respectively. During photoageing, the loss of the binder structural groups is accompanied by the production of oxidation products (carbonyl and hydroxyl groups). The UV‐cured polyurethane‐acrylate coating was shown to be more resistant to accelerated weathering than the melamine/acrylate thermoset currently used as automotive finishes. The addition of a hydroxyphenyltriazine UV‐absorber and a hindered amine light stabilizer (HALS) radical scavenger was found to increase substantially the light stability of acrylate clearcoats.     

Cracking and loss of adhesion of Si3N4 and SiO2:P films deposited on Al substrates

Tensile tests in a scanning electron microscope have been performed to study the mechanical stability of different film/substrate systems consisting of films of Si₃N₄ and SiO₂ :4.5 wt% P deposited on Al substrates. Successive stages of crack development were observed: transverse through-thickness cracking of the film precedes its loss of adhesion and buckling, induced by the transverse contraction of the substrate. It was observed that the presence of a thermally grown Al₂O₃ interlayer improved the adhesion of the films by delaying the de-adhesion process. The influence of roughness on the interfacial strength was analysed from the observation of the de-adhesion of a SiO₂ :4.5 wt% P film deposited on a scratched Al substrate. The critical strain for the through-thickness cracking of each film was calculated. Then the multiple film cracking was analysed through the evolution of the crack density with the longitudinal strain. Finally, by using the point at which the film de-adhered, an interfacial fracture energy was calculated for each system.     

Weathering performance of thermoset and photoset acrylate coatings

Thermosetting and photosetting acrylate polymers have been tested for their resistance to accelerated weathering. The chemical changes occurring both upon curing and upon photoaging have been followed by infrared spectroscopy. In the photodegraded polymers some functional groups are lost (ether, amide, CH groups) and oxidation products are formed (carbonyl and hydroxyl groups). The UV-cured polyurethane-acrylate coatings were found to be more resistant to accelerated weathering than the melamine/acrylate thermosets currently used as automotive finishes. The light stability of both thermoset and photoset acrylate clearcoats was substantially increased by the addition of a hydroxy-phenyltriazine UV-absorber and a HALS radical scavenger. The weathering resistance of painted metallic panels has been markedly enhanced by means of properly stabilized UV-cured acrylic coatings. Département de Photochimie Générade (UMR-CNRS No. 7525), 3, rue Werner - 68200 Mulhouse (France). Additives Division, BU Imaging and Coating Additives, CH-4002 Basle, Switzerland.     

Chemical depth profiling of automotive coating systems using slab microtome sectioning with IR/UV-VIS spectroscopy and optical microscopy

Chemical surface/near-surface analysis and depth profiling of high performance automotive coating systems can be done efficiently by commonly available infrared (IR), ultraviolet-visible (UV-VIS) and optical microscopy (OM) measurement technologies. The different stages of mass loss for acrylic/melamine-crosslinked clearcoats modeled after those used in high performance automotive coating systems were explored as a function of exposure time/conditions. Two approaches were used. Transmission mode IR experiments were conducted to monitor mass loss in isolated clearcoats. OM was used to monitor mass loss in clearcoats that are part of actual multilayered coating systems. UV-screener (UVA) permanence and interphase mixing (i.e., between clearcoat and basecoat layers) as well as general chemistry depth profiling studies of model acrylic/melamine-crosslinked and acrylic/melamine+silane-crosslinked clearcoats were done as a function of system preparation and exposure time/conditions. UVA depth profiles were obtained using slab microtomy sectioning co-planar to the surface, followed by solvent extraction and UV-VIS solution analysis to monitor UVA content as a function of section mass and thickness. General chemistry depth profiles were obtained using slab microtomy sectioning (as mentioned previously), followed by ATR (IR) analysis to monitor changes in the photooxidation index values and the spectral envelop in the 2600–3800 cm⁻¹ range. The approaches described herein do not require expertise/equipment that is available to only a few, thus increasing the potential utility of such tools. Marshall R&D Laboratory, Philadelphia, PA, 19146.     

Analysis of scratch characteristics of automotive clearcoats containing silane modified blocked isocyanates via carwash and nano-scratch tests

The aim of this study was to investigate the scratch characteristics of automotive clearcoats based on an acrylic polyol resin, with butylated melamine and silane modified blocked isocyanates, using car-wash and nano-scratch tests.To scrutinize the effect of silane modified blocked isocyanate on the chemical and mechanical properties of clearcoats, with respect to changes in the crosslinking networks inside the clearcoats due to the curing reaction, dynamic mechanical analysis (DMA) and FT-IR analysis were performed. The scratch behaviors were analyzed via Amtec–Kistler car-wash and nano-scratch tests, accompanied with scratch images simultaneously visualized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The basic properties of various automotive clearcoats such as impact resistance, pencil hardness, solvent resistance, and stone-chip resistance, were also compared. The results showed that a close correlation existed between the scratch resistance data obtained from the car-wash and nano-scratch tests for clearcoats made from acrylic polyol resin, with melamine and silane modified blocked isocyanates. Also, all the mechanical properties, including scratch resistance, noticeably improved due to the increased crosslinking networks via the formation of urethane bonds, when the portion of silane modified blocked isocyanates was increased. This was verified from the surface profiles and images of the scratched clearcoats captured using AFM and SEM.     

Evaluation of Mode-II Fracture Energy of Adhesive Joints with Different Bond Thickness

A study on the mode-II edge-sliding fracture behaviour of aluminium-adhesive joints was carried out. Compact pure shear (CPS) adhesive joints of different bond thickness were produced using a rubber-modified epoxy resin as the adhesive. An analytical model was developed to calculate the stress distribution along the bond line of the joint. A crack-closure technique was used to evaluate the mode-II strain energy release rate. G _II, as a function of the adhesive bond thickness. The results indicated that for a given applied load, G _II increased gradually with the bond thickness. A finite element model (FEM) was also developed to evaluate the stress state along the bond line and the strain energy release rate of the CPS specimens. Consistent results were obtained between the theoretical model and finite element analysis. Scanning electron micrographs of the fracture surface illustrated a mainly interfacial fracture path between the adherends and the adhesive for all adhesive joint specimens. The critical fracture load increased very rapidly with bond thickness in the range 0.02 mm to 0.1 mm but remained constant thereafter. However, the mode-II critical fracture energy rose more gradually as the bond thickness was increased.     

Surface-tension-driven flows of coatings: Bondline readout formation

Bondline readout (BLRO) is a coating defect frequently observed on adhesively bonded, polymeric automotive body panels. This paper addresses ridging BLRO in clearcoats, not optical and mechanical BLRO, which are characterized by metalflake orientation (dark/light effects) in basecoats and by distortion in substrates, respectively. Ridging BLRO is due to film thickness differences and results from Marangoni-type, surface-tension-driven flows. In this study, the effects of several parameters on BLRO are investigated experimentally. These parameters include initial film thickness, heating rate, viscosity, solvent-to-resin surface-tension ratio, and solvent volatility. The experiments clearly demonstrate three modes of BLRO flow—formation, flow-out, and reformation—that result from competing surface-tension-gradient forces (temperature- versus concentration-induced). Experimental results are used to validate a proposed BLRO mechanism and, in subsequent work, a BLRO-predicting numerical code. R & D Center, MD 480-102-000, 30500 Mound Rd, Warren, MI 48090-9055, email: rick.blunk@gm.com. Department of Chemical Engineering, Dow Building, Ann Arbor, MI 48109-2136, email: wilkes@umich.edu.     

Dual-curing behavior and scratch characteristics of hydroxyl functionalized urethane methacrylate oligomer for automotive clearcoats

UV–thermal dual-curable, hydroxyl- and methacrylate-functionalized urethane oligomers with different contents of unsaturated double bonds and hydroxyl groups have been synthesized and incorporated into automotive clearcoats to investigate their curing and scratch behaviors. Dynamic mechanical analyses (DMA) and FT-IR analyses were performed to observe the variation of the crosslinking networks that resulted from the chemical reactions by UV and thermal dual-curing operations with varying curing conditions, such as UV dose, thermal curing time, thermal curing temperature, and curing sequence. The scratch behaviors of dual-cured automotive clearcoats were analyzed via nano-scratch tests, accompanied with scratch images simultaneously visualized using scanning electron microscopy (SEM). The mechanical and chemical properties, such as impact resistance, pencil hardness, acid-etch resistance, and stone-chip resistance, of dual-curable clearcoats were also compared with those of UV mono-cure and 1 K thermal-cure clearcoats. The results clearly showed that the dual-curing process induced a considerably higher degree of crosslinking for the cured clearcoats prepared from the dual-curable oligomers, melamine crosslinkers, and photoinitiators. Their mechanical properties including scratch resistance were also noticeably improved via the UV–thermal curing sequence, which led to an increased conversion rate of double bonds compared with clearcoats produced using the thermal–UV curing sequence. The best conditions for high crosslinking density as well as high hardness and modulus were 2400 mJ/cm² at 150 °C for 10 min in the UV–thermal curing process. This result was corroborated from the reaction kinetics and surface images of the scratched clearcoats captured by SEM.     

Mode-l Fracture Behaviour of Adhesive Joints. Part I. Relationship Between Fracture Energy and Bond Thickness

The fracture properties of adhesive joints of aluminium were investigated using a rubber-modified tough epoxy resin system (G_IC = 2.76 kJ/m²) as adhesive material. Compact tension (CT) adhesive joints were manufactured for a wide range of bond thickness t (from 0.05mm to 10mm) and fracture tests conducted under static load. Scanning electron microscopy (SEM) was used to examine the fracture surface morphology. A large deformation elastic- plastic finite element model was developed to evaluate the J-integral value for different bond thickness. The fracture energy, J_c , was found to be highly dependent on the bond thickness and was lower than that of the bulk adhesive. As the bond thickness was increased J_c also increased, though not monotonically, towards the fracture energy of the bulk adhesive. This result was caused by the complicated interactions between the stress and strain fields, plastic deformation of the adhesive around the crack tip, constraint from the adherends and the failure path. It was shown that values of J_c as a function of bond thickness correlated well with the variation of plastic zone height. Scanning electron micrographs from the fracture surfaces of the CT adhesive joints illustrated that the failure path was mainly cohesive through the centre-plane of the adhesive layer. Brittle fracture mechanisms were observed for thin bonds (0.04mm < t< 0.5 mm) but tough fracture mechanisms were identified for thick bonds (t > 1 mm).     

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.     

Characterization to the weathering extent of LLDPE/LDPE thin film

A kind of LLDPE (linear low density polyethylene)/LDPE (low density polyethylene) thin film for farm applications was subjected to accelerated and natural weathering. Carbonyl group, melting point, tensile elastic modulus, and high‐temperature shearing modulus of weathered films were investigated as function of weathering time. Two kinds of carbonyl index, I₁ and I₂, which result from infrared spectroscopy (IR) spectra of the weathered films, were defined to characterize the weathering extent of the LLDPE/LDPE thin film. Based on I₁ and I₂, a correlation is made between the artificial and natural weathering of the film: 1 h of the artificial weathering is equivalent to about 10.73 h of the natural weathering. The difference between the accelerated weathering and the natural weathering was also discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 12–16, 2003     

Conductive and transparent multilayer films for low-temperature TiO2/Ag/SiO2 electrodes by E-beam evaporation with IAD

Conductive and transparent multilayer thin films consisting of three alternating layers (TiO₂/Ag/SiO₂, TAS) have been fabricated for applications as transparent conducting oxides. Metal oxide and metal layers were prepared by electron-beam evaporation with ion-assisted deposition, and the optical and electrical properties of the resulting films as well as their energy bounding characteristics and microstructures were carefully investigated. The optical properties of the obtained TAS material were compared with those of well-known transparent metal oxide glasses such as ZnO/Ag/ZnO, TiO₂/Ag/TiO₂, ZnO/Cu/ZnO, and ZnO/Al/ZnO. The weathering resistance of the TAS film was improved by using a protective SiO₂ film as the uppermost layer. The transmittance spectra and sheet resistance of the material were carefully measured and analyzed as a function of the layer thickness. By properly adjusting the thickness of the metal and dielectric films, a low sheet resistance of 6.5 ohm/sq and a high average transmittance of over 89% in the 400 to 700 nm wavelength regions were achieved. We found that the Ag layer played a significant role in determining the optical and electrical properties of this film.