1. Field of the Invention
In at least one aspect, the present invention relates to methods of coating a substrate with a clearcoat and compositions thereof and, in particular, to methods of coating a substrate with a clearcoat by applying to the substrate a dual curable composition that is first photocurable and then thermally cured.
2. Background Art
Typically, the painted surfaces of an automobile are protected by coating with a clearcoat. Clearcoats protect the vehicle from the deleterious effects of sunlight. Accordingly, these coatings typically have light stabilizers, usually consisting of a combination of UV absorbers and free radical scavengers. The absorbers prevent the energetic rays of the sun from causing permanent damage to the polymer matrix of the clearcoat and the underlying coats, including pigments. The free radical scavengers deactivate the highly reactive species that arise as a result of unwanted breakdown processes, and act to promote further breakdown.
Currently, there are two main categories of clearcoat compositions that are used to form clear coatings. These categories are medium solid coating systems and high solid coating systems. Solid as used in this context refers to components that are not volatile organic compounds (VOC) including liquids with low vapor pressure. Medium solid coating systems typically contain volatile organic solvents in amounts over 70 weight percent. Accordingly, these systems are undesirable because of environmental and health concern. Moreover, such high solvent systems are subject to government regulations in many countries. High solid systems are more desirable because such systems contain much less volatile organic solvents. In the high solid systems, solvents are typically replaced by liquid oligomers or liquid monomers.
Although high solid coating systems are desirable because of the relatively low amounts of VOCs, coatings from such systems often produce coatings marred by sag. Sag refers to the phenomenon of runs and drips that occurs in paint coating. The tendency of a coating to sag results from several factors. For example, sag may occur from edge effects generated from localized high film build around edges, holes in the substrate, character lines, and the like. Sag may also result from the increased surface tension due to solvent evaporation on two surfaces at an edge and by Faraday's Cage effect. Moreover, sag is observed to be thickness dependent. In the absence of flow control agents, reducing the film thickness by a factor of two reduces the sag by a factor of four. Additionally, for a coating containing 3% microgel a similar reduction in coating thickness results in a 12-fold reduction in sag.
Coating reaction kinetics is another factor that needs to be considered in minimizing sag. The maximum temperature reached by a coating prior to gel is an important parameter because it essentially determines the minimum viscosity of the coating after solvent evaporation. Therefore, with respect to sag, it is desirable to utilize coatings with higher rates of reaction. These systems become cross-linked sooner, building molecular weight which increases until the coating gels, thereby avoiding or limiting sag. The temperature at gel is higher for high solids coatings versus medium solids coatings because of the higher extent of reaction at gel for high solids coatings. Accordingly, high solid systems have an increased sag potential due to this phenomenon.
Coating viscosity and cure conditions are additional important factors in controlling sag. If the viscosity is high just after and during cure, then sag may be avoided. However, for high solids coating systems, the low molecular weight resin typically used in these systems and the extent of cure at gel makes sag somewhat inevitable. A significant difference in fluidity between medium solids (“conventional”) clearcoat and high solids clearcoat has been observed. Specifically, during a thermal cure cycle the medium solids clear maintained limited fluidity over the range of heating rates, whereas the high solids clear are significantly affected by the heating rate (lower heating rates—resulting in greater fluidity). Similarly, higher molecular weight systems produce limited fluidity as compared to lower molecular weight systems.
Typically, in medium solid coating systems, sag is minimized by the use of large amounts of VOCs during the application and cure of the coating. That is, the high molecular weight resin used in these systems require large amounts of organic solvent(s) to reduce the high molecular weight resins viscosity within the wet coating. High solid coatings use lower molecular weight resins to bring down the viscosity. In doing so, thermal cure sag tolerance has been compromised. In high solids coating systems, reduction of sag depends on rheological control agents (“RCA”) to modify the flow and deformation of the liquid coating system. Key characteristics that are sought when adding Theological control agents are to limit settling within the coating, to improve atomization by shear thinning during spray application, and to avoid sag during the thermal cure cycle of the coating by quickly reestablishing a high viscosity after application.
Accordingly, there exists a need from an improved clearcoat composition that contains low amount of volatile organic solvents and produces a coating with low sag.