1. Field of the Invention
The present invention relates to a method of coating and more particularly relates to a coating method in which the coating is cured at a high temperature for a short period of time such as coil coating.
2. Brief Description of the Prior Art
Coil coating involves the coating of a continuous length of flat metal sheet. The sheet which is usually thin gauge steel or aluminum is coiled over a spool which is continuously unwound and passed to a coating station where the sheet is coated usually by direct or reverse roll coating in a continuous manner as it passes through the station. After the coating is applied, the coated coil is passed to a baking station for curing.
Many commercially employed exterior coil coating compositions use organic solvent-based acrylic polymers as the resinous binder. However, these compositions present environmental problems with regard to solvent emissions. In attempts to overcome these problems, there have been suggestions in the prior art to use water-based acrylic polymers.
The water-based acrylic polymers which have been suggested are the thermosetting high molecular weight emulsion polymerized latex-type polymers which provide hard, durable coatings necessary to provide the long-term warranty requirements associated with coil coating uses such as home and industrial siding. Unfortunately, it has been found that many of the commercially available thermosetting water-based acrylic polymers do not provide commercially acceptable coatings for coil line use.
As mentioned above, after the metal coil has been coated, the substrate is passed to a baking station for curing. Since industrial coil coating lines operate at very high speeds, for example, about 300 feet a minute, the coil passes through the baking station very quickly, generally in less than 60 seconds. During this short residence time, the coating must be completely cured. Therefore, the baking stations are generally operated at very high temperatures, that is, about 450.degree. F. (232.degree. C.) and above so that the metal substrate will reach a peak metal temperature of at least 190.degree. C. in less than 60 seconds. Under these rigorous curing conditions, many of the commercial thermosetting water-based acrylic coating compositions solvent pop, that is, develop bubbles just beneath the coating surface or actually develop holes in the coating.
Although not intending to be bound by any theory, it is believed that since many commercially available acrylic latices have a fairly wide distribution of particle sizes, the smaller particles become closely packed when the coating composition coalesces over the substrate. This close packing seals off the water (and any organic cosolvent) preventing them from evaporating. During the baking operation, the coating composition will crosslink before all the water and organic cosolvent has volatilized resulting in the remaining water and cosolvent accumulating as tiny bubbles beneath the exposed surface of the coating or bursting through the coating leaving a permanent hole.
It has been found that the particle size and distribution of the particle is a critical parameter in controlling solvent popping. Latices of uniform particle size of 2000-3000 A with a deviation of .+-. 300 A eliminate solvent popping and are stable. Latices of uniform particle size larger than 3000 A will not solvent pop but are unstable and form a hard, irreversible sediment on standing.
Latices of uniform particle size smaller than 2000 A are stable but are extremely prone to solvent popping.
It is believed that the large uniformly sized latex particles do not pack together closely enough during coating, baking and coalescence to prevent the water and organic cosolvent from evaporating before the coating composition crosslinks.