The present invention relates to a method for spray-coating aqueous paint, whereby coating defects raised by change in surrounding conditions, such as temperature and humidity are avoided.
The term xe2x80x9callowable volume absolute humidityxe2x80x9d in a unit of g/m3, means a difference between saturated volume absolute humidity and absolute humidity at a given temperature. The saturated volume absolute humidity means a maximum amount of water contained in gaseous form in the air of a unit volume.
Aqueous paint (water-borne paint) mainly contains water as a solvent. Therefore, aqueous paint is not hazardous to human body in coating conditions, and can easily treat when compared to solvent based paint (solvent-borne paint). Aqueous paint is advantageously recycled by collecting an over-spray paint that has not been coated, with an aqueous solvent, on an article to be coated, filtering and concentrating the collected paint, and adjusting the paint formulation for recycle use. The recycling of aqueous paint reduces paint waste and saves resources. Therefore, aqueous paint has been widely used for industrial coating, such as automotive and home electric apparatus coating.
In a coating line for automotive bodies, coating aqueous paint is generally conducted by spray-coating; wherein the aqueous paint is sprayed onto an article employing a spray gun, to form a thin and uniform film coating on the article.
Aqueous paint, when spray-coated, is deposited onto an article, allowing evaporation of some of the solvent (i.e. water in the air), to result in forming a wet coating. The wet coating is then dried or baked to form a dry coating on the article.
Appearance of the dry coating significantly depends on both the amount of water evaporated from the aqueous paint during spray-coating and setting, which is the time between spray-coating and drying or baking, and the flowability (i.e. viscosity) of the wet coating. The amount of water evaporated generally depends on the coating conditions, such as temperature and humidity. For example, when the coating temperature is too low and humidity is too high, evaporation of water from aqueous paint is so slow that viscosity of the wet coating deposited on the article is lowered, and flowability is elevated, resulting in xe2x80x9csaggingxe2x80x9d of the coated film. In addition, when the coating temperature is too high and the humidity is too low, evaporation of water from the aqueous paint is so accelerated that the wet coating becomes very viscous, with poor flowability, resulting in xe2x80x9csurface blemishesxe2x80x9d of the coated film.
It is also known in the art that the viscosity of a wet coating increases as the non-volatile content of aqueous paint increases, and that the non-volatile content of wet coating changes degree of water evaporation from aqueous paint during coating. In order to prevent surface defects, such as sagging or surface blemishes, the viscosity of a wet coating should be controlled not only by adjusting the amount of water evaporated from the aqueous paint indirectly, but also by adjusting the non-volatile content of the aqueous paint directly, in accordance with changing coating conditions, such as temperature and humidity.
Coating conditions of aqueous paint in the prior art are generally controlled to a surrounding temperature of 15 to 35xc2x0 C. and a relative humidity of 60 to 90%. However, it is considered very difficult and expensive to optimize theh non-volatile content of aqueous paint with a change of coating conditions, because coating conditions actually change based upon the time of day (morning, day time or evening) and with the seasons. Even if non-volatile content of aqueous paint is optimized, it should be utilized in coating conditions where the amount of water evaporated is constant. This may be performed only in facilities where temperature and humidity are kept constant and where a hood is used to cover both the portion introducing the aqueous paint into a spray gun and the portion coating the paint on articles. These facilities seem cost consuming.
The present invention is to provide a method for spray-coating aqueous paint wherein the non-volatile content of aqueous paint is adjusted in accordance with a change of coating conditions (temperature and humidity), and wherein the amount of water evaporated from the aqueous paint is controlled, without complicated and expensive operations; resulting in the formation of coatings having good appearance, without surface defects, such as sagging and surface blemishes. ?
As the result of studying a relation between paint viscosity and non-volatile content (NV) in paint, the present inventors have found that excellent appearance would be obtained by controlling a temperature of aqueous paint (paint temperature) during spray coating such as a difference (xcex94NV=NV2xe2x88x92NV1) between NV(NV1) of aqueous paint during spray coating and NV(NV2) of wet coating after one minute setting is within the range of 3 to 8%.
The NV2 for wet coating also changes in accordance with change of surrounding temperature and humidity. For example, NV2 becomes higher at a condition of high temperature and low humidity, in comparison with a condition of low temperature and high humidity, because wet coating is dried much more. The change of NV2 in turn changes xcex94NV. In view of the above, the present inventors have now introduced concept of allowable volume absolute humidity that is calculated from surrounding temperature and humidity, in order to adjust paint temperature, whereby xcex94NV is adjusted within preferred ranges (3 to 8%). The introduction of allowable volume absolute humidity reduces parameters from two, i.e. temperature and humidity, to one.
Accordingly, the present invention provides a method for spray-coating aqueous paint, characterized in that a portion of a spray gun, especially a gun tip, is cooled or heated to adjust a temperature of aqueous paint passing through the spray gun to a suitable range within allowable volume absolute humidity during spray coating, so that the temperature of aqueous paint maintains in optimum range in accordance with change of both surrounding temperatures and surrounding humidities during spray coating.
In addition, the present invention provides that the temperature of paint is controlled within a range satisfying the following equations:
aX2+bX+cxe2x89xa6Yxe2x89xa6dX2+eX+f
xe2x80x8310xe2x89xa6Xxe2x89xa680
1xe2x89xa6Yxe2x89xa615 
wherein X shows a temperature of aqueous paint, Y shows an allowable volume absolute humidity, and a, b, c, d, e and f are coefficients that are specific to the aqueous paint employed and experimentally obtained.