Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1997-11-06
2001-10-09
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S136000, C427S385500, C427S407100, C427S508000, C427S558000, C427S559000, C522S004000
Reexamination Certificate
active
06299944
ABSTRACT:
The invention concerns a method of curing aqueous coating compositions involving the use of radiation, and to radiation-curable aqueous coating compositions.
Methods involving the use of microwave radiation for curing waterborne coatings on substrates have been known for some years. For example, a method of forming a film of a water containing paint on a temperature sensitive substrate and then irradiating the coated substrate with microwaves to cure the coating is disclosed in WO 90/02613. The method is described to provide rapid curing of both zinc silicate paints and emulsion paint systems on temperature sensitive substrates that would otherwise be damaged by the stoving conditions normally required for curing, and to enable paint films to be cured very rapidly in on-line apparatus.
As documented in WO 90/02613, the vast majority of waterborne coatings contain some volatile plasticizers known in the art as coalescents. These coalescents are needed to ensure that during the drying process the polymer is soft enough to form a proper film and then later they evaporate and leave behind a hard resistant coating. Though microwave treatment of such coatings produces a rapid evaporation of the water, the slower evaporating coalescents tend to stay behind in the coating which, until they have evaporated from the coating, leave it insufficiently hard for the coated substrates to be stacked and stored shortly after treatment, otherwise they stick together so causing considerable damage when they are eventually separated again. Even though the problems of blocking and poor stackability as a result of using emulsion paints containing a coalescent material to reduce the minimum film forming temperature is mentioned in WO 90/02613, and this prior art appears to be directed to solving this problem, it particularly concentrates on providing a method of curing zinc silicate paint systems. We have found that adequate results for emulsion type coating compositions containing coalescents cannot be obtained by following the teaching of WO 90/02613; the coated substrates stick together when stacked a short time after microwave treatment.
UV curable compositions have been used industrially for some time, including as compositions for coating substrates. These compositions may be high solids compositions which contain low quantities or no volatile components, or lower solids, diluent or solvent-based compositions which contain significant quantities of volatile components such as organic solvents or water. The UV curable component may be, for example, an unsaturated pre-polymer. It has been recognized that the use of such UV curable unsaturated pre-polymers in aqueous coating compositions is particularly advantageous for environmental and ease of application reasons since, with water as the diluent, the viscosity can be regulated as much as is desired without having to add a polluting, volatile organic solvent and the inherent non-polluting nature of a UV-curing coating is not diminished by adjusting its viscosity. Waterborne UV-curing coatings can be easily and safely applied by spraying (automatic or manual), curtain coater, flow coater or roller coater. Further, because of the evaporation of the water during the drying process (and the resulting film shrinkage this entails), the gloss of these coatings is readily controlled by the addition of low amounts of standard flatting agents known in the art (e.g. amorphous silicas). Nevertheless, even with these advantages, waterborne UV-curing coatings have an important constraint: the water contained in the freshly applied film must be nearly completely evaporated before the coating is UV-cured. If it is not, water will be permanently trapped in the film and this will compromise the stain resistance of the coating and adversely affect the appearance of a transparent coating (introduction of haze). This preferred requirement of evaporating the water prior to UV-exposure means that the drying phase of a waterborne UV-curing coating takes longer than that of a 100% non-volatile UV-curing coating. As an example, it is common for the time between application and stacking of a 100% non-volatile UV-curing coating to be as short as 1 minute while for a waterborne UV-curing coating this same time might be 10 minutes or more. The industrial use of waterborne UV-curing coating thus suffers from a loss of productivity in comparison to the 100% non-volatile UV-curing coating. Productivity of a modern industrial process is extremely important and thus even though waterborne UV-curing coatings offer several advantages over 100% non-volatile UV-curing coatings, waterborne UV has not been able to realize its full potential due to its lower productivity.
The object of the present invention, therefore, is to provide a method of curing coating compositions which is quick and which provides a cured coating composition which is sufficiently hard as to allow handling, stacking and storage of the substrates shortly after coating whilst eliminating or at least significantly reducing the amount of blocking damage to the surface of the cured coated substrate. The present invention also aims to provide an efficient method suitable for curing coating compositions which do not contain a coalescent.
Thus, the present invention provides a method of curing a waterborne coating composition comprising the steps of:
a) applying the waterborne coating composition, which comprises polymer solids of which at least 5% by weight thereof is UV curable, to a substrate;
b) irradiating the coated substrate with microwave radiation; and
c) irradiating the coated substrate with UV radiation.
The combination of irradiating a coating with microwave and UV radiation advantageously overcomes the productivity problems associated with using microwave treatment alone and the productivity problems of using conventional UV curable coatings (either a composition containing in part a UV-curable component or a composition formed from 100% UV containing thermoset coating). Microwave drying by itself suffers from poor productivity (the panels are not stackable after the evaporation of the water) and UV-curing waterborne coatings suffer from poor productivity (they typically require up to 10 minutes or more drying time after application before they can be UV cured) yet, when combined, these two techniques surprisingly offer a highly productive coating process, enabling coated substrates to to handled and stacked shortly after treatment without fear of the substrates sticking together. The process can enable the coating on the substrate to be cured sufficiently enough for the substrate to be used, handled, stacked and/or stacked approximately 90 seconds after the coating was applied: microwave drying of the coating can take as little as 60 seconds from application, and UV-curing of the dried coating can take as little as 30 seconds.
Preferably the coating composition comprises at least 25% by weight, based upon the total weight of polymer solids in the coating composition, of a UV curable component, and particularly preferable is a composition which comprises polymer solids of which at least 50% by weight is UV curable.
The UV curable component may be selected from one of two main categories: 1.) free radical polymerised (meth)acrylate functionalised polymers and 2.) cationically polymerised epoxies, which categories are well known and well documented in the art. (Meth)acrylate functionalised polymers generally comprise (meth)acrylate-functional oligomers and monomers combined with a photoinitiator to facilitate UV cure. These (meth)acrylate functional oligomers are typically prepared by a) reaction of difunctional epoxies with (meth)acrylic acid, b) the reaction product of difunctional isocyanates with hydroxy-functional (meth)acrylates, or c) the condensation product of (meth)acrylic acid and hydroxyl groups on a polyester backbone, or an hydroxy(meth)acrylate with residual acid groups on a polyester backbone. Cationic systems tend to be based on cycloaliphatic epoxies and a photoinitiator which decomposes to g
Pianalto Bernard
Rohm and Haas Company
Stauss Karl
LandOfFree
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