Coating processes – Spraying
Reexamination Certificate
1998-11-18
2001-04-24
Beck, Shrive (Department: 1762)
Coating processes
Spraying
C239S398000
Reexamination Certificate
active
06221435
ABSTRACT:
FIELD OF THE INVENTION
This invention, in general, pertains to the field of spraying polymeric coating compositions with reduced emissions of volatile organic solvent. More specifically, the present invention relates to the use of subcritical compressed fluids, such as subcritical compressed carbon dioxide or ethane, to reduce viscosity and to enhance atomization when spray applying coating compositions to substrates.
BACKGROUND OF THE INVENTION
Prior to the use of supercritical fluids, such as supercritical carbon dioxide, to replace volatile solvents that cause air pollution, the liquid spray application of coatings, such as paints, lacquers, enamels, and varnishes, was effected through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from coating operations.
While the utilization of supercritical fluids as viscosity reducing diluents in coating applications has met with much success and is effectively able to accomplish the primary objective of reducing the emission of volatile organic compounds into the atmosphere while simultaneously providing the stringent performance characteristics of the applied coating that is required by the coatings industry, a desire has nevertheless arisen to determine whether such viscosity reducing diluent effects can be obtained with suitable materials at conditions which are below the supercritical fluid state, i.e., with subcritical compressed fluids. In particular, this desire has been generated by the realization that there are coating materials which may contain components that are highly temperature sensitive, such as highly reactive cross-linking coating systems and two-package coating systems that are employed in high-solids coatings. As such, it may be undesirable to subject such components to the temperatures required to keep the viscosity reducing supercritical fluid in its supercritical state. By spraying at significantly lower temperature, it may be possible to spray a two-package coating using a conventional single-feed coating system instead of having to feed the two reactive components separately as required at elevated spray temperatures. In cross-linking coating systems, the undesirable increase in spray viscosity that results from premature reaction in the heated spray equipment can therefore be avoided or minimized.
Furthermore, it is common for sprays with supercritical fluids to be heated to relatively high temperatures, typically 50 to 60 C. or higher, to offset the cooling effect that occurs as the supercritical fluid expands from the spray as a free gas. This requires either the use of a circulating flow of the heated spray mixture, which is undesirable for spray operations that use color change or highly reactive coating systems, due to increased volume and residence time, or the use of a specially heated and thermostated spray gun and feed line in order to maintain the proper spray temperature at all times including at start-up. This increases the amount of equipment that must be operated and maintained in the spray operation, which increases equipment and labor costs and makes the operation more susceptible to interruptions due to equipment failure or loss of temperature control.
So too, generally depending on the particular coating composition to be sprayed, at the typically higher pressures needed to maintain the viscosity reducing supercritical fluid in its supercritical state, i.e., above its critical pressure, and at the typically still higher pressures required to obtain high solubility of the supercritical fluid at the elevated temperatures, more wear and tear may be experienced on the spray coating equipment, particularly the spray gun nozzle tips. Moreover, there is a general desire, particularly for safety reasons, to work with a process at a pressure which is as low as possible while still realizing the overall benefits of such a process.
Lower pressures also produce a lower velocity spray, which is advantageous for obtaining higher transfer efficiency in depositing the coating composition onto a substrate and in particular for making electrostatic deposition more effective. Lower spray velocity can also improve coating quality by reducing the amount of fine air bubbles that become entrapped in the liquid coating as the spray strikes the substrate, which may cause undesirable haze to occur in clear coatings and may promote solvent popping during baking. Lower pressures also make it possible to obtain lower spray application rates without having to use extremely small orifice sizes that are susceptible to becoming plugged and are difficult to manufacture. There is also a general desire to spray at the lower pressures at which air-assisted airless spraying is practiced, which are generally below supercritical fluid pressures.
SUMMARY OF THE INVENTION
By virtue of the present invention, the above needs have now been met. Processes have now been discovered which are able to provide a viscosity reducing effect and enhanced atomization of coating compositions by using a suitable environmentally acceptable material to replace volatile organic solvents while simultaneously reducing the pressure and/or temperature needed to achieve such a viscosity reducing diluent effect.
The present invention provides processes for spraying a liquid coating composition comprising polymeric compounds to form a coating on a substrate. In general, the processes comprise the steps of:
(1) forming a liquid mixture in a closed pressurized system, said liquid mixture containing a coating composition and a subcritical compressed fluid, wherein (a) the coating composition contains at least one polymeric compound having a number average molecular weight (M
n
) of less than about 5,000 and (b) the subcritical compressed fluid is miscible with and forms a liquid mixture with said coating composition and is present in the liquid mixture in an amount which renders the viscosity of said liquid mixture suitable for spraying and being capable of forming a choked flow when sprayed, and wherein said subcritical compressed fluid is a gas at standard conditions of 0° C. temperature and one atmosphere pressure (STP);
(2) spraying said liquid mixture by passing the liquid mixture through an orifice in a spray tip at a temperature and pressure that produces choked flow of the liquid mixture being sprayed which forms the spray; and
(3) applying said spray containing said coating composition to said substrate to form a coating thereon.
As used herein, it will be understood that a “supercritical fluid” is a material which is at a temperature and pressure such that the material is at, above, or slightly below its “critical point”. As used herein, the “critical point” of a material is the transition point at which the liquid and gaseous states of that material merge into each other and represents the combination of the critical temperature and critical pressure for that material. The “critical temperature”, as used herein, is defined as the temperature above which a gas cannot be liquified by an increase in pressure. The “critical pressure”, as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature.
Also as used herein, it will be understood that a “subcritical fluid” is a material which is at a temperature and/or pressure such that the material is below the temperature and pressure conditions at which the material achieves a supercritical fluid state. Such a subcritical fluid may be (i) at a temperature below at which the material achieves a supercritical fluid state while being at a pressure above at which the material achieves a supercritical fluid state, or (ii) at a pressure below at which the material achieves a supercritical fluid state while being at a temperature above at which the material achieves a supercritical fluid state, or (iii) at a temperature below at which the material achieves a supercritical fluid state and at a pressure below at wh
Beck Shrive
Calcagni Jennifer
Rooney G. P.
Union Carbide Chemicals & Plastics Technology Corporation
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