Acid etch resistance of automotive clearcoats. I: Laboratory test method development

This paper reports on the development of a laboratory test procedure for the evaluation of the environmental etch resistance of clearcoats. The test evaluates the bulk acid hydrolysis resistance of clearcoats by gravimetrically following material weight loss as a function of exposure time to a sulfuric acid solution, under conditions that simulate outdoor exposure. The bulk hydrolysis resistance of five production clearcoat technologies including acrylic melamine, acrylic melamine-silane, carbamate, acrylic urethane, and epoxy acid were evaluated. Results from the weight loss measurements were consistent with those anticipated based on the coating systems bulk chemistry and inherent hydrolysis resistance, for clearcoat systems processed under nominal processing conditions. The relative rankings from the laboratory test were found to correlate with field etch ratings. The test method is inexpensive, quantitative, and generates repeatable results that are not subject to environmental variations associated with current field etch testing. Ford Research Laboratory, Dearborn, MI 48121-2053.     

Degradation of organic coatings. I. Hydrolysis of melamine formaldehyde/acrylic copolymer films

The hydrolysis of melamine formaldehyde crosslinked acrylic copolymer films has been studied using infrared spectroscopy. It has been found that during hydrolysis, crosslinks between the acrylic copolymer and the melamine crosslinker are broken and that crosslinks between melamine molecules are formed. The rate of hydrolysis depends on the following: (1) the type of crosslinker used (partially alkylated melamines hydrolyze faster than fully alkylated melamines); (2) the amount and strength of acid catalyst used and whether or not the catalyst can be extracted from the coating during hydrolysis; (3) the initial crosslink density; (4) the hydrolysis temperature. In some formulations, it can be expected that virtually all of the acrylic–melamine crosslinkes will be hydrolyzed. However, due to the formation of melamine–melamine crosslinks, the overall crosslink density does not necessarily decrease significantly as a result of this hydrolysis.     

The weathering performance of acrylic melamine automotive clearcoats containing hydrophobic nanosilica

This work is an attempt to study the effect of weathering on various properties of a typical acrylic melamine clearcoat containing various loads of nanosilica. It was found that nanosilica caused an incomplete curing process of the acrylic melamine clearcoat, leading to a lower crosslink density and also less Young’s modulus of clearcoat compared to the pure clearcoat. Thermo-mechanical analyses showed that incomplete curing of nanocomposites induced post-curing reactions during weathering, leading to an increase of crosslinking density and Young’s modulus of nanocomposites. Viscoelastic studies showed that incorporation of nanosilica into acrylic melamine resulted in more plastic behavior and higher elongation at break of clearcoats during weathering. In addition, gloss retention, surface topology, and chemical structure investigations revealed that nanosilica particles enhanced weathering performance of clearcoats. This was explained by UV–Visible spectroscopy which confirmed the ability of silica to absorb the harmful incident rays, protecting the clearcoat against weathering degradation.     

The role of basecoat pigmentation on the biological resistance of an automotive clearcoat

This work aims to study the effect of various natural and artificial biological compounds on an automotive acrylic/melamine clearcoat applied over silver and black basecoats containing pigments. The visual performance of the coating system was evaluated at different aging conditions. To this end, analytical techniques including optical microscopy, scanning electron microscopy, gonio-spectrophotometery, gloss measurement, ATR-FTIR spectroscopy, and DMTA analysis were utilized to investigate the optical and mechanical response of the system upon exposure to the biological materials. Results indicated different effects produced by gums and bird droppings on both silver and black systems at all aging processes. In addition, a more severe effect of biological attacks was observed on the clearcoat samples applied on the black basecoat which had experienced postaging conditions. However, it was found that pancreatin and bird droppings influence the coating systems more severely compared to the natural and synthetic Arabic gums.     

水性丙烯酸酯汽车涂料树脂的制备与粘度的研究

合成了用于配制水性汽车罩光清漆的丙烯酸酯涂料树脂,研究了丙烯酸单体、含芳香酯基团单体、引发剂、链转移剂和聚合温度对树脂水溶性和粘度的影响,并用红外光谱表征了树脂的结构。结果表明,增加丙烯酸单体的用量会提高丙烯酸酯树脂的水溶性和粘度,而增加含芳香酯基团的甲基丙烯酸苄酯（BNMA）单体、引发剂和链转移剂的用量和提高聚合温度可降低树脂的粘度。该树脂与氨基树脂（HMMM）固化剂配制的漆膜性能优良,可以用作汽车原厂漆（OEM）。     

Mechanism and kinetics of the reaction of acrylic polyols with hexa (methoxymethyl) melamine

New procedures have been developed to study the mechanism and kinetics of the crosslinking of acrylic polyols with hexa (methoxymethyl) melamine. Both the rate of gel formation and alcohol release are found to be affected by the concentration and the nature of the catalyst and to be proportional to the hydrogen ion concentration. The rate of gel formation can be modeled by using the Miller and Macosko procedure with the input consisting of the polymer composition and the rate of release of reaction products. The correlation between calculated and experimentally determined quantity of gel formed is good when the reactions are slow, but becomes poor when gel formation is rapid, suggesting that the formation of large rings is favored by rapid reaction. The rate of gel formation shows much less variation among different polymer systems than does the rate of release of reaction products from the crosslinking reaction.     

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.     

Investigating the degradation resistance of silicone-acrylate containing automotive clearcoats exposed to bird droppings

This work reports preparation of acrylic/melamine based clearcoats containing various loads of a reactive polysiloxane additive. The additive was incorporated into the clearcoat formulations up to 8 wt% to enhance its resistance against bird droppings. Contact angle measurements, ATR spectroscopy, energy dispersive X-ray analysis, appearance measurements together with different microscopic techniques were utilized to reveal the effects of additive on the properties of the coating prior and after exposure to pancreatin, the synthetic equivalent of natural bird droppings. Appearance measurements, as well as optical microscope images obviously indicated that the additive improved the clearcoats resistance against pancreatin. Results revealed that surface free energy of the films was reduced in the presence of additive. It was also found that at high concentrations of additive, a significant part of it remained in the bulk, forming a second phase. This resulted in a lack of sufficient reactive groups and lead to an under-cure state at the clearcoat surface.     

The effect of TiO2, pigmentation on the hydrolysis of amino resin crosslinked epoxy can coatings

Pigmented (TiO₂), amino resin crosslinked coatings, designed for applications in can coatings’ internal lacquers, were formulated, characterized, applied, and cured. Three grades of a pigmentary form of TiO₂ were characterized in terms of their particle size, their particle morphology, their zeta potential, and their moisture retention behavior. Epoxy coatings that were crosslinked using one of several, different amino crosslinkers were prepared. The effect of the presence of the TiO₂ pigments on the hydrolysis of the cured coatings was monitored via the controlled retorting of the coatings. The different grades of TiO₂ pigment were selected, to establish whether or not they could be used interchangeably with respect to hydrolysis and to melamine release. Also, the effects of the aging of the fluid coatings on the amount of melamine released from the coatings (after curing and retorting) were monitored. Storage under laboratory conditions for 2, 20, and 40 weeks was used for this purpose. The TiO₂ pigment contributed significantly to the hydrolysis behavior of the epoxy coatings in that their presence substantially reduced the amount of melamine released and the extent of crosslinker hydrolysis. Typical results show that excluding the TiO₂ pigment particles from the formulation results in there being 50% more hydrolysis of the crosslinker to melamine. With respect to the melamine release and crosslinker hydrolysis, the different grades of the pigment gave similar results.     

Attributing the resistance against simulated tree gum of an acrylic/melamine film loaded with an active silicone additive to its surface free energy

The present work reports preparation of acrylic/melamine based clearcoats with different loadings of a functionalized polysiloxane additive. The resulting films were applied over a black basecoat automotive coating system. Dynamic mechanical thermal analysis (DMTA), contact angle measurement, optical microscopy and gonio-spectrophotometery were utilized to investigate the mechanical (viscoelastic/viscoplastic), surface and optical properties of the coating system before and after exposure to Arabic gum (simulated tree gum). The resistance of film surface against gum attack showed high dependence on gum adhesion to the films. It was revealed that depreciation of surface free energy of additive loaded films could greatly improve their behavior against gum, apparently due to their enhanced non-stick characteristic. Tg and loss peak height also increased, indicating that additive did not negatively influence coating curing state. However, at loading contents exceeding 4 wt% both Tg and loss peak height decreased, revealing that a phase separation had occurred. The improved resistance of films against Arabic gum was corresponded to the low clearcoat shrinkage and crack formation. Weaker interactions between gum and clearcoat surface was seen in presence of additive. The shape and size of defects (of gum exposed films) were also significantly changed.     

The effect of weathering and thermal treatment on the scratch recovery characteristics of clearcoats

The scratch and scratch recovery characteristics of two clearcoats, a polyurethane and an acrylic/melamine/silane clearcoat, were measured by a variety of methods. On most size scales tested, the polyurethane possessed superior scratch and mar resistance. The polyurethane also possessed improved scratch recovery after warming for 2 h at 60°C. After 2000 h of accelerated weathering, the scratch recovery characteristics of the polyurethane were largely maintained, while the acrylic/melamine/silane showed a significant drop in scratch recovery performance. Performance of both coatings was related to T _g, hardness, and crosslink density.     

Effect of polysiloxane additives on the scratch resistance of an acrylic melamine automotive clearcoat

Many attempts have been carried out to increase the scratch resistance of clearcoats using various appropriate additives. These additives may increase surface hardness or surface slippage, and/or enhance the bulk mechanical strength of the clearcoat. In the present study, the influence of various added loads of three differently structured polysiloxane additives on the scratch resistances of an acrylic melamine clearcoat was investigated. A series of analytical instruments, such as a laboratory carwash simulator, a scanning electron microscope, an optical microscope and an attenuated total reflectance Fourier transform infrared spectroscope, and others were used to compare the effects of the various added contents of polysiloxane additives on changes in the viscoelastic properties and scratch morphologies of the resultant clearcoats. The results illustrated that all polysiloxane additives improve the scratch resistance of such clearcoats, yet the optimum load varied for each individual additive. There is also a good indication that polyether-modified polysiloxanes improve scratch resistance by increasing surface hardness, while the fluorocarbon-modified polysiloxane tends to change the viscoelastic properties of the clearcoats. It was also found that scratch resistance varied linearly with Micro-Vickers’ hardness for brittle clearcoats.     

Accelerated clearcoat weathering studied by dynamic mechanical analysis

Automotive basecoat/clearcoat (BC/CC) systems of the acrylic polyol/melamine formaldehyde (ACR/MF) type were analyzed by dynamic mechanical analysis (DMA) before and after exposure to QUV. DMA changes after 500 h exposure were compared to gloss retention after five year Florida exposure or 2500 h QUV exposure. DMA readily distinguished between ‘poorer’ and ‘better’ BC/CC films received in a blind study. Crosslink density and T_g both increased during QUV. DMA changes depended strongly on the presence or absence of hindered amine light stabilizer (HALS). Variations in the basecoats had only minor effects on DMA results. Poly(urethane) clearcoats (without basecoats) were also studied. Differences in weathering behavior of films prepared with three widely used poly(isocyanates) are reported. Crosslink density decreased and T_g increased during QUV exposure. Stabilizers were very effective in the poly(urethane) clearcoats. DMA results are discussed in relation to FT-IR spectra obtained before and after QUV.     

Polycarbonate-modified acrylic polymers for coating materials

Reactivities of six-membered cyclic carbonates (1) were studied and a carbonate-modified acrylic polymer was synthesized, which was expected to show good water resistance. The carbonates (1) reacted with several alcohols under acidic conditions giving the corresponding alkyl 3-hydroxypropyl carbonates, quantitatively. The reactivities of 1 with alcohol depended on their steric hindrance, on the acidity of the catalyst and on the basicity of the solvents. ¹³C NMR analysis indicated that the reaction proceeded by interaction between the CO group of 1 and the acid catalyst, followed by ring-opening of 1 via the nucleophilic attack by the alcohol on the CO group. This reaction of 5,5-dimethyl-1,3-dioxane-2-one (1a) with n-butanol produced polymers (2) at 50°C in toluene, using methane sulfonic acid as a catalyst. The molecular weights ranged from 2530 to 9310 and ¹H NMR showed that the polymerization was initiated from n-butanol. Also the ring-opening reaction enabled the production of both a carbonate-modified acrylic polymer (4a) containing carbonate units in their side chains by the reaction of 1a with poly(styrene-co-2-hydroxyethyl methacrylate) (3a) and methacrylic monomers (HEMAC) containing 3-hydroxyneopentyl carbonate units by reaction of 1a with 2-hydroxyethyl methacrylate (HEMA). Free radical copolymerization of HEMAC also produced carbonate-modified acrylic polymer (4c). A carbonate-modified acrylic polymer showed excellent hydrolysis resistance when compared with -caprolacton-modified acrylic polymer (5a). Waterborne coatings, consisting of a carbonate-modified acrylic polymer (4c) and a melamine resin, showed excellent storage stability at 40°C when compared with a similar coatings based on an -caprolactone-modified acrylic polymer (5b), and the 4c melamine coating showed nearly the same curability to that of the 5b melamine coating when baked at 140°C for 30 min.     

Novel polyurethane polyols for waterborne and high solids coatings

Recently developed oligomeric β-hydroxyalkyl urethane polyols are finding application as modifiers and crosslinkers for waterborne and high solids coatings. In waterborne coatings, urethane polyols can be used as modifiers for acrylic, polyester or alkyd melamine resin crosslinked coatings to replace the cosolvent. In high solids coatings, polyurethane polyols are being employed to raise the application solids, increase film hardness and water resistance, and exterior durability.

There are also applications for polyurethane diols as resin intermediates and in the preparation of blocked isocyanate crosslinkers. β-Hydroxyalkyl urethane diol or polyols can be prepared by an isocyanate reaction with a diol, such as 1,2-propylene glycol, but also by non-isocyanate processes. The non-isocyanate routes to urethanes can utilize carbon monoxide, carbon dioxide, urea or organic carbonates as a carbonyl source for the carbamic acid ester.

It is possible to split these urethanes to the isocyanate, but interest has concentrated on using the urethane intermediates directly in coating applications without going through the isocyanate. The structures reported are bis-hydroxyalkyl urethane intermediates and derivatives of these compounds.     

Crosslinking with malonate blocked isocyanates and with melamine resins

Automotive clearcoats with improved acid etch resistance are being formulated using a combination of a dialkyl malonate blocked polyisocyanate, a melamine crosslinker, and an acrylic polyol,^1,2 These coatings contain lower levels of melamine crosslinker compared to conventional acrylic/HMMM systems and show excellent acid etch resistance. We explored the reaction mechanism of this complex crosslinking system and found explanations for the good chemical resistance properties. Presented at the 25th International Waterborne, High-Solids, and Powder Coatings Symposium, February 18–20, 1998, New Orleans, LA. Science Rd., Norwalk, CT 06852; E-mail: AlexHe@Kingindustries.com and Wblank@Kingindustries.com.     

Implementing DOE to study the effect of paint application parameters, film build, and dehydration temperature on solvent pop

This research investigated popping when automotive clearcoat was applied over waterborne basecoats using design of experiments (DOE). The study was conducted using melamine crosslinked acrylic latex basecoat and acrylic acid/epoxy clearcoat. The focus of the project was directed toward the paint application and dehydration stages. The study was conducted in four phases. In Phase 1, three paint application parameters, flow rate (FR), atomizing air (AA), and distance of the applicator tip from the substrate (D), were investigated. In Phase 2, FR and target distance were investigated in relation to popping while keeping other parameters as well as film build constant. In Phase 3, the effect of film build on popping was investigated by selecting three levels of film build. Phase 4 was focused on the possible effect of dehydration oven temperature on popping. Results indicated that FR does not affect popping directly. However, it significantly affects the film build. Both film build and dehydration oven temperature were found significant in relation to popping occurrences. This work suggests that dehydration oven temperature and film build affect solvent popping. Although this study identified the significant factors causing waterborne basecoat popping, conditions for a pop-free paint surface were not established.     

Interactions between basecoats and clearcoats in car refinish systems

The introduction of VOC compliant waterborne basecoats and solventborne basecoats on one hand and the introduction of VOC compliant clearcoats on the other hand forces to look to the interactions between the two systems. As well known from studies in the automotive OEM area small molecules like HALS and UV-absorbers additives diffuse from the clearcoat into the basecoat. This influences the durability performance of the system.The main question addressed in this paper is the interaction of the binder system of the basecoat and clearcoat. In general crosslinkers (e.g. poly-urethane coatings) are rather low molecular weight materials which may diffuse between layers as well. Also the lower molecular weight functional resins of the binder may move throughout the system.The interactions of basecoat/clearcoat combinations were investigated by means of chemical and physical techniques. The effect of a medium solid clearcoat, a VOC compliant p-urethane clearcoat and UV-clearcoat on interpenetration, physical and paint properties were studied.Also some attention is paid to the diffusion of HALS and UV-absorbers from the clearcoat into the basecoat.     

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.     

Cross-linked coatings by co-reaction of isocyanate–methoxymethyl melamine systems

Two-components methoxymethyl melamine/isocyanate and three-components methoxymethyl melamine/isocyanate/polyol coating systems were studied. Methoxymethyl melamine was shown to react with isocyanate groups through the carbocation formed by splitting within the methoxymethyl group. In three-components systems, methoxymethyl melamine/polyol and methoxymethyl melamine/isocyanate rections appear to occur, but not isocyanate/polyol reactions. This was ascribed to the release from the methoxymethyl melamine/polyol reaction of methanol which reacts more rapidly than the polyol with the isocyanate group. A blocked isocyanate was then used whereby the methoxymethyl melamine reacts at an earlier stage and urethane formation is delayed. Introducing the blocked isocyanate, however, required significant increase in cure temperature for the isocyanate/polyol reaction, narrowing the cure temperature window of the system. © 1995 John Wiley & Sons, Inc.