Precursor coating compositions containing water and an organic coupling solvent suitable for spraying with supercritical fluids as diluents

The present invention relates to precursor coating compositions containing water and at least one organic solvent which are particularly suitable for being admixed with at least one supercritical fluid used as a viscosity reduction diluent and then spraying this resultant liquid mixture of supercritical fluid and precursor coating composition onto a substrate to be coated. Processes for spraying this precursor mixture are also disclosed.

RELATED PATENT APPLICATIONS 
This application contains subject matter related to application Ser. No. 
133,068, filed Dec. 21, 1987, which application is a continuation-in-part 
of application Ser. No. 883,156, filed Jul. 8, 1986, now abandoned. This 
application also contains subject matter related to U.S. patent 
applications Ser. No. 218,896, filed Jul. 14, 1988; and Ser. No. 218,910, 
filed Jul. 14, 1988. 
FIELD OF THE INVENTION 
This invention, in general, pertains to the field of coating compositions. 
More specifically, the present invention relates to precursor coating 
compositions, containing water and at least one organic coupling solvent, 
which are particularly suitable for being admixed with at least one 
supercritical fluid used as a viscosity reduction diluent. The resultant 
admixed liquid mixture of supercritical fluid and precursor coating 
composition can then be sprayed onto a substrate to be coated. 
BACKGROUND OF THE INVENTION 
Prior to the present invention, the liquid spray application of coatings, 
such as lacquers, enamels and varnishes, was effected solely 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 painting and finishing operations. For this 
reason there has been a great deal of emphasis placed on the development 
of new coatings technologies which diminish the emission of organic 
solvent vapors. A number of technologies have emerged as having met most 
but not all of the performance and application requirements, and at the 
same time meeting emission requirements and regulations. They are: (a) 
powder coatings, (b) water-borne dispersions, (c) water-borne solutions, 
(d) non-aqueous dispersions, and (e) high solids coatings. Each of these 
technologies has been employed in certain applications and each has found 
a niche in a particular industry. However, at the present time, none has 
provided the performance and application properties that were initially 
expected. 
Powder coatings, for example, while providing ultra low emission of organic 
vapors, are characterized by poor gloss or good gloss with heavy orange 
peel, poor distinctness of image gloss (DOI), and poor film uniformity. 
Moreover, to obtain even these limited performance properties generally 
requires excessive film thicknesses and/or high curing temperatures. 
Pigmentation of powder coatings is often difficult, requiring at times 
milling and extrusion of the polymer-pigment composite mixture followed by 
cryogenic grinding. In addition, changing colors of the coating often 
requires its complete cleaning, because of dust contamination of the 
application equipment and finishing area. 
Water-borne coatings are very difficult to apply under conditions of high 
relative humidity without serious coating defects. These defects result 
from the fact that under conditions of high humidity, water evaporates 
more slowly than the organic cosolvents of the coalescing aid, and as 
might be expected in the case of aqueous dispersions, the loss of the 
organic cosolvent/coalescing aid interferes with film formation. Poor 
gloss, poor uniformity, and pin holes unfortunately often result. 
Additionally, water-borne coatings are not as resistant to corrosive 
environments as are the more conventional solvent borne coatings. 
Coatings applied with organic solvents at high solids levels avoid many of 
the pitfalls of powder and water-borne coatings. However, in these systems 
the molecular weight of the polymer has been decreased and reactive 
functionality has been incorporated therein so that further polymerization 
and crosslinking can take place after the coating has been applied. It has 
been hoped that this type of coating will meet the ever-increasing 
regulatory requirements and yet meet the most exacting coatings 
performance demands. However, there is a limit as to the ability of this 
technology to meet the performance requirement of a commercial coating 
operation. Present high solids systems have difficulty in application to 
vertical surfaces without running and sagging of the coating. Often, they 
are also prone to cratering and pin holing of the coating. If they possess 
good reactivity, they often have poor shelf and pot life. However, if they 
have adequate shelf stability, they cure and/or crosslink slowly or 
require high temperature to effect an adequate coating on the substrate. 
Clearly, what is needed is an environmentally safe, non-polluting diluent 
that can be used to thin very highly viscous polymer and coatings 
compositions to liquid spray application consistency. Such a diluent would 
allow utilization of the best aspects of organic solvent borne coatings 
applications and performance while reducing the environmental concerns to 
an acceptable level. Such a coating system could meet the requirements of 
shop- and field-applied liquid spray coatings as well as factory-applied 
finishes and still be in compliance with environmental regulations. 
Such a needed diluent has now been found and is discussed in the 
aforementioned related applications which teach, among other things, the 
utilization of supercritical fluids, such as supercritical carbon dioxide 
fluid, as diluents in highly viscous organic solvent borne and/or highly 
viscous non-aqueous dispersions coatings compositions to dilute these 
compositions to application viscosity required for liquid spray 
techniques. 
U.S. patent application Ser. No. 133,068, filed Dec. 21, 1987, to Hoy, et 
al., disclose processes and apparatus for the liquid spray application of 
coatings to a substrate that minimize the use of environmentally 
undesirable organic diluents. The broadest process embodiment of that 
application involves: 
(1) forming a liquid mixture in a closed system, said liquid mixture 
comprising: 
(a) at least one polymeric compound capable of forming a coating on a 
substrate; and 
(b) at least one supercritical fluid, in at least an amount which when 
added to (a) is sufficient to render the viscosity of said mixture of (a) 
and (b) to a point suitable for spray application; and 
(2) spraying said liquid mixture onto a substrate to form a liquid coating 
thereon. 
That application is also directed to a liquid spray process in which at 
least one active organic solvent (c) is admixed with (a) and (b) above 
prior to the liquid spray application of the resulting mixture to a 
substrate. The preferred supercritical fluid is supercritical carbon 
dioxide. The process employs an apparatus in which the mixture of the 
components of the liquid spray mixture can be blended and sprayed onto an 
appropriate substrate. The apparatus contains 
(1) means for supplying at least one polymeric compound capable of forming 
a continuous, adherent coating; 
(2) means for supplying at least one active organic solvent; 
(3) means for supplying supercritical carbon dioxide fluid; 
(4) means for forming a liquid mixture of components supplied from (1)-(3); 
and 
(5) means for spraying said liquid mixture onto a substrate. 
The apparatus may also provide for (6) means for heating any of said 
components and/or said liquid mixture of components. U.S. patent 
application Ser. No. 133,068 demonstrates the use of supercritical fluids, 
such as supercritical carbon dioxide fluid, as diluents in highly viscous 
organic solvent borne and/or highly viscous non-aqueous dispersions 
coatings compositions to dilute the compositions to application viscosity 
required for liquid spray techniques. It further demonstrates that the 
method is generally applicable to all organic solvent-borne coatings 
systems. 
Copending U.S. application Ser. No. 218,910, filed Jul. 14, 1988, is 
directed to a liquid coatings application process and apparatus in which 
supercritical fluids, such as supercritical carbon dioxide fluid, are used 
to reduce to application consistency, viscous coatings compositions to 
allow for their application as liquid sprays. The coatings compositions 
are sprayed by passing the composition under pressure through an orifice 
into the environment of the substrate. 
In particular, the process of U.S. application Ser. No. 218,910 for liquid 
spray application of coatings to a substrate comprises: 
(1) forming a liquid mixture in a closed system, said liquid mixture 
comprising: 
(a) at least one polymeric component capable of forming a coating on a 
substrate; and 
(b) a solvent component containing at least one supercritical fluid, in at 
least an amount which when added to (a) is sufficient to render the 
viscosity of said mixture to a point suitable for spray application; and 
(2) spraying said liquid mixture onto a substrate to form a liquid coating 
thereon by passing the mixture under pressure through an orifice into the 
environment of the substrate to form a liquid spray. 
U.S. application Ser. No. 218,895, filed Jul. 14, 1988, is directed to a 
process and apparatus for coating substrates by a liquid spray in which 1) 
supercritical fluid, such as supercritical carbon dioxide fluid, is used 
as a viscosity reduction diluent for coating formulations, 2) the mixture 
of supercritical fluid and coating formulation is passed under pressure 
through an orifice into the environment of the substrate to form the 
liquid spray, and 3) the liquid spray is electrically charged by a high 
electrical voltage relative to the substrate. 
In particular, the process of U.S. application Ser. No. 218,895 for 
electrostatic liquid spray application of coatings to a substrate 
comprises: 
(1) forming a liquid mixture in a closed system, said liquid mixture 
comprising: 
(a) at least one polymeric component capable of forming a coating on a 
substrate; and 
(b) a solvent component containing at least one supercritical fluid, in at 
least an amount which when added to (a) is sufficient to render the 
viscosity of said mixture to a point suitable for spray application; 
(2) spraying said liquid mixture onto a substrate to form a liquid coating 
thereon by passing the mixture under pressure through an orifice into the 
environment of the substrate to form a liquid spray; and 
(3) electrically charging said liquid spray by a high electrical voltage 
relative to the substrate and electric current. 
The use of supercritical fluids as a transport medium for the manufacture 
of surface coatings is well known. German patent application 28 53 066 
describes the use of a gas in the supercritical state as the fluid medium 
containing the solid or liquid coating substance in the dissolved form. In 
particular, the application addresses the coating of porous bodies with a 
protectant or a reactive or nonreactive decorative finish by immersion of 
the porous body in the supercritical fluid coupled with a pressure drop to 
effect the coating. The most significant porous bodies are porous 
catalysts. However, the applicant characterizes fabrics as porous bodies. 
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 
4,734,451, patented Mar. 29, 1988, describe forming a supercritical 
solution which includes a supercritical fluid solvent and a dissolved 
solute of a solid material and spraying the solution to produce a 
"molecular spray." A "molecular spray" is defined as a spray "of 
individual molecules (atoms) or very small clusters of the solute." The 
Smith patents are directed to producing fine films and powders. The films 
are used as surface coatings. 
Coating formulations are commonly applied to a substrate by passing the 
coating formulation under pressure through an orifice into air in order to 
form a liquid spray, which impacts the substrate and forms a liquid 
coating. In the coatings industry, three types of orifice sprays are 
commonly used; namely, air spray, airless spray, and air-assisted airless 
spray. 
Air spray uses compressed air to break up the liquid coating formulation 
into droplets and to propel the droplets to the substrate. The most common 
type of air nozzle mixes the coating formulation and high-velocity air 
outside of the nozzle to cause atomization. Auxiliary air streams are used 
to modify the shape of the spray. The coating formulation flows through 
the liquid orifice in the spray nozzle with relatively little pressure 
drop. Siphon or pressure feed, usually at pressures less than 18 psi, are 
used, depending upon the viscosity and quantity of coating formulation to 
be sprayed. 
Airless spray uses a high pressure drop across the orifice to propel the 
coating formulation through the orifice at high velocity. Upon exiting the 
orifice, the high-velocity liquid breaks up into droplets and disperses 
into the air to form a liquid spray. Sufficient momentum remains after 
atomization to carry the droplets to the substrate. The spray tip is 
contoured to modify the shape of the liquid spray, which is usually a 
round or elliptical cone or a flat fan. Turbulence promoters are sometimes 
inserted into the spray nozzle to aid atomization. Spray pressures 
typically range from 700 to 5000 psi. The pressure required increases with 
fluid viscosity. 
Air-assisted airless spray combines features of air spray and airless 
spray. It uses both compressed air and high pressure drop across the 
orifice to atomize the coating formulation and to shape the liquid spray, 
typically under milder conditions than each type of atomization is 
generated by itself. Generally the compressed air pressure and the air 
flow rate are lower than for air spray. Generally the liquid pressure drop 
is lower than for airless spray, but higher than for air spray. Liquid 
spray pressures typically range from 200 to 800 psi. The pressure required 
increases with fluid viscosity. 
Air spray, airless spray, and air-assisted airless spray can also be used 
with the liquid coating formulation heated or with the air heated or with 
both heated. Heating reduces the viscosity of the liquid coating 
formulation and aids atomization. 
In general, coating compositions are formulated to help minimize the 
coating defects that may occur after the coating composition has been 
sprayed by any of the above means onto the substrate and then dried. Such 
defects include, but are certainly not limited to, orange peel conditions, 
runs or sags, pin holing and solvent pops, fish eyes, blistering, and the 
like, all of which are well known to those skilled in this art. 
Indeed, some coating formulations are provided in concentrated form, that 
is, with a relatively high solids content, so that they may be custom 
tailored on site by the user. Thus, the user adds an appropriate amount of 
a particular solvent mixture to accommodate a particular end use spraying 
condition which may include variable wind conditions, ambient 
temperatures, drying conditions, humidity, and other such spraying 
condition factors. 
While the above-noted related patent applications all utilize supercritical 
fluids as a diluent to help reduce the viscosity of highly viscous organic 
solvent-borne and/or highly viscous non-aqueous dispersions coating 
compositions so as to facilitate the application of these compositions by 
liquid spray techniques, and by doing so, desirably reduce the amount of 
organic solvent which would otherwise be used, the overall objective, of 
course, is still to obtain a coated substrate having a uniform, smooth, 
continuous coating and substantially none of the above-noted defects. 
For obvious reasons, none of the prior art coating compositions have been 
formulated with the intent of having these compositions combined with a 
supercritical fluid as a diluent and then spraying the resultant admixed 
liquid mixture through an orifice and onto a substrate to form a liquid 
coating which is then dried and/or cured. 
Indeed, prior to the inventions described in the above-noted related 
applications and the present invention, it was unknown how a high 
concentration of highly volatile supercritical fluid, such as 
supercritical carbon dioxide fluid, would affect formation of a liquid 
spray containing a solids fraction; a diluent fraction in which said 
solids fraction is dissolved, suspended or dispersed, and a portion of the 
supercritical fluid. A spray mixture undergoes a large and rapid drop in 
pressure as it goes through the orifice. Accordingly, one of ordinary 
skill in the art could theorize that the supercritical spray mixture would 
produce a foam like shaving cream instead of a spray, because nucleation 
to form gas bubbles would be so rapid and intense. Alternatively, one of 
ordinary skill in the art could also expect that the spray mixture would 
produce a mist or fog of microdroplets instead of a spray, because 
atomization would be so intense. Another result that one could theorize is 
that the spray mixture would produce a spray of bubbles instead of 
droplets. Furthermore, even if a spray were formed, it would have been 
expected that the sudden and intense cooling that accompanies rapid 
depressurization and expansion of a supercritical fluid would cause the 
liquid droplets to freeze solid. For example, it is commonly known that 
the spray from carbon dioxide fire extinguishers produces solid dry ice 
particles. 
In the event that formation of a liquid spray were achieved, there is no 
assurance that the spray could be used to produce quality coherent 
polymeric coatings on a substrate. One of ordinary skill in the art could 
surmise that the liquid droplets would be so small or have so little 
momentum that they could not be deposited well onto the substrate. One 
could also theorize that foaming droplets or supercritical fluid dissolved 
in the coating would produce a layer of foam on the substrate or a coating 
full of bubbles when these characteristics were not desired in the 
coating. The liquid coating droplets that are deposited onto the substrate 
would have a much higher viscosity than the material that was sprayed, 
because they would have lost most of the supercritical fluid diluent and 
they would be at a lower temperature. Furthermore, the coating material 
would contain less volatile organic solvent than normal. Therefore, it is 
not unreasonable to expect that higher viscosity would prevent or hinder 
coalescence of the deposited droplets to form a coherent liquid coating; 
that it would reduce how much the droplets spread out on the substrate, so 
that thin coatings could not be produced; and that it would reduce the 
surface flow that produces a smooth coating. One can further theorize that 
moisture would condense onto the droplets and harm the coating, because 
the spray would be cooled below the dew point. 
Surprisingly, however, it has been shown, as discussed in application Ser. 
No. 883,156 noted above, that liquid sprays can indeed be formed by using 
supercritical fluids as viscosity reduction diluents and that such sprays 
can be used to deposit quality coherent polymeric coatings onto 
substrates. 
However, after admixing the highly viscous organic solvent borne and/or 
highly viscous non-aqueous dispersions coating compositions with 
supercritical fluids as a diluent to help reduce the viscosity, it may 
still be desirable to reduce the viscosity even further but keep the 
overall amount of supercritical fluid used substantially the same. 
Alternatively, it may also be desirable to maintain (or lower) the 
viscosity of the admixed coating composition and maintain the overall 
amount of supercritical fluids used substantially the same, but still want 
to reduce even further the amount of organic solvent in the admixed 
coating composition. 
More specifically, there may be coating compositions whose initial 
viscosity is so high that the amount of supercritical fluids that can be 
admixed with such compositions, without undesirably causing a two phase 
separation, is insufficient to reduce the viscosity to the point where 
such composition can properly be sprayed. 
Alternatively, since it is known that high molecular weight polymers 
generally provide finished coatings having better exterior durability, 
toughness, strength and solvent resistance, it may be desirable to use 
such a high molecular weight polymer in a coating composition in lieu of a 
similar but lower molecular weight polymer that may be there. However, the 
use of such a high molecular weight polymer introduces an increase in the 
overall viscosity of the coating composition. This increase in viscosity 
may be such that the amount of supercritical fluids now needed to reduce 
the viscosity of the composition to a point suitable for spray application 
may not be obtainable without breaking up the composition into two phases. 
Still further, for a given highly viscous coating composition containing a 
particular amount of polymeric component and an organic or non-aqueous 
solvent, respectively, it may be desirable to reduce the amount of such 
volatile solvents even further. Of course, such a reduction in solvent 
would inherently result in a corresponding increase in the overall 
viscosity of the coating composition. Here again, the increase in 
viscosity may be such that the amount of supercritical fluids needed to 
now reduce the viscosity of the composition to a point suitable for spray 
application may not be obtainable. 
Clearly, a need exists to be able to accomplish all of the above 
objectives. Preferably, these objectives should be able to be carried out 
without the necessity of adding supercritical fluid in an amount which is 
greater than that originally utilized, such that the expected diluent 
effect of the supercritical fluids can be expected to remain substantially 
about the same. Of course, if desired, more than the original amount of 
supercritical fluid may be used, if such amount does not cause the 
excessive breakup of the composition into two phases. 
Accordingly, the present invention provides a means by which the above 
noted goals may indeed be achieved and, more particularly, provides 
precursor coating compositions in which those goals have been manifested. 
Moreover, a need also exists to provide precursor coating compositions 
which in addition to achieving the above objectives are also formulated 
to: 
(a) be particularly compatible for subsequent admixture with a 
supercritical fluid diluent; 
(b) be particularly suitable, once admixed with the supercritical fluid, to 
help minimize any of the phenomena that may occur which are peculiarly 
associated with the utilization of such supercritical fluid, which 
phenomena may interfere with proper atomization of the admixed liquid 
mixture and/or proper diffusion of the supercritical fluid once atomized; 
and 
(c) provide the necessary coating characteristics such that once sprayed 
onto a substrate, it will help provide the necessary coalescence of the 
deposited droplets to form a coherent liquid coating while still not 
causing sagging or runs and help minimize any of the other defects noted 
above while at the same time, still allow for the release of any residual 
supercritical fluid that may be present after the coating has been applied 
to the substrate. 
Accordingly, the present invention provides such precursor coating 
compositions which not only fulfill the goals of (1) having an even lower 
viscosity and/or (2) having even less organic solvent, but which are also 
particularly suitable for subsequent admixture with at least one 
supercritical fluid which admixture is then sprayed through an orifice, 
such as airless spray or air-assisted airless spray methods, to apply an 
admixed coating composition onto a substrate which results in a substrate 
having a substantially uniform, continuous and substantially defect-free 
coating. 
SUMMARY OF THE INVENTION 
In the more broader aspects of the present invention, it has unexpectedly 
been found that water may actually be added to an organic solvent-borne 
coating composition such that when admixed with supercritical fluids, the 
water acts as an additional viscosity reduction diluent providing a 
composition having an even lower viscosity. Most importantly, however, the 
amount of supercritical fluids that are miscible with this 
water-containing coating composition remains at least substantially the 
same as in the composition in which no water is present. 
This discovery is quite surprising in that it has been found that materials 
such as liquid carbon dioxide or supercritical carbon dioxide are only 
sparingly miscible with water or water-borne polymer mixtures. Yet, when 
in the presence of at least one organic coupling solvent, quite 
surprisingly, a relatively large amount of water may be added to the 
organic solvent-borne coating composition under supercritical conditions 
while still retaining the supercritical fluid miscibility characteristics 
of the original composition. In general, up to about 30 percent by weight 
of water, based on the total weight of solvent/diluent present in the 
composition, may be added with substantially no reduction in the amount of 
supercritical fluid contained in the composition. 
Accordingly, in the illustration noted earlier in which not enough 
supercritical fluid could be added to a viscous coating composition so as 
to reduce its viscosity to a point suitable for spraying, this problem can 
now be solved by simply adding enough water to the composition (up to 
about 30 percent by weight of the total solvent/diluent present), so as to 
reduce the initial viscosity of the composition, while still keeping the 
amount of supercritical fluid that is capable of being admixed with the 
composition the same. In other words, the addition of the water to the 
composition serves to act as a further diluent to reduce the viscosity of 
the composition but does not substantially reduce the miscibility of the 
now water-containing composition with the supercritical fluids. Most 
importantly, such a viscosity reduction is achieved without adding organic 
solvent over and above that which was originally present. While a coupling 
solvent is desirably added to the composition in conjunction with such 
water addition, as will be more fully discussed hereinbelow, such coupling 
solvent may be used to replace some or all of the organic solvent present 
in the original composition such that the total amount of organic solvent 
in the water-containing composition is less than or equal to the amount 
contained in the original composition. With such a viscosity reduction in 
the new water-containing composition, the amount of supercritical fluids 
that can be admixed with this composition is generally enough to reduce 
the viscosity further to a point suitable for spraying. 
Similarly, in the illustration noted above in which it would be desirable 
to replace a low molecular weight polymer with a similar polymer having a 
higher molecular weight, but the amount of supercritical fluids that can 
be added to the new formulation cannot be increased to compensate for the 
increase in viscosity, that too can now be accomplished by adding water to 
the system. The water acts as a further diluent, and in conjunction with 
the supercritical fluids (the total amount of which remains substantially 
the same in both the original composition and in the composition 
containing water), the viscosity of the reformulated composition 
containing the higher molecular weight polymer is now reduced to the point 
that the amount of supercritical fluids that can be admixed with the 
composition is now enough to reduce the viscosity to a point at which it 
can be sprayed. 
Most significantly, in contrast to the above two illustrations in which 
water is typically added to a composition so as to actually increase the 
overall amount of solvent/diluent that is present, the present invention 
has also recognized that water may also be used to actually replace some 
of the organic solvent in the original composition. In this manner, while 
keeping the overall amount of solvent/diluent in the composition 
substantially about the same, it is possible to reduce even further the 
amount of volatile organic or non-aqueous solvent that is present in the 
coating composition so as to accommodate, if needed, the ever increasingly 
stringent guidelines that are being imposed. 
Of course, the present invention recognizes that it is not necessary to 
start with one composition formulation and then modify it by the addition 
of water. The present invention clearly encompasses the formulation of an 
initial composition which is formulated with water in accordance with the 
present invention. 
Still further, the present invention is also directed to coating 
compositions which are intended for admixture with at least one 
supercritical fluid and then subsequently sprayed onto a substrate as a 
liquid coating, particularly coating compositions containing water and 
organic solvent in accordance with the present invention, and formulated 
so as to provide such coating compositions with physical and/or chemical 
characteristics which make them eminently suitable for such intended use. 
In particular, by optimizing one or more specific physical and/or chemical 
properties of the coating composition, a number of factors influencing and 
affecting the overall coating process, which includes the utilization of a 
supercritical fluid, are significantly improved. Such factors include, but 
are not limited to, for example, (1) the ease with which the supercritical 
fluid is admixed with such compositions; (2) the amount of supercritical 
fluid that is capable of being admixed with the composition while still 
desirably maintaining a single phase; (3) the ease with which the 
resulting liquid admixture is sprayed; (4) the quality of the atomized 
spray droplets that are produced; (5) the ability of the supercritical 
fluid to rapidly diffuse from the atomized sprayed droplets; (6) the 
ability of the atomized liquid droplets to be deposited efficiently onto 
the substrate; (7) the ability of the atomized liquid droplets, once 
applied to the substrate, to sufficiently coalesce and form a coherent 
liquid coating; (8) the ability for any residual supercritical fluid still 
remaining in the coating applied to the substrate to effectively diffuse 
and escape; (9) the ability to help form an essentially defect free 
coating; and the like, all of which are affected, at least in part, by the 
characteristics of the precursor coating composition. 
It is recognized that variables other then the coating composition per se 
may have an effect on some or all of the above noted factors. For example, 
the spray temperature, the spray pressure, the particular supercritical 
fluid being used, the amount of supercritical fluid admixed with the 
precursor coating composition, the temperature and pressure of the 
environment in which the substrate is present, the distance between the 
spray orifice and the substrate, and the like, all have an effect upon the 
spraying process and the coating results that are obtained. Generally, 
however, assuming that all of such process variables are kept constant, 
the formulation of the precursor coating composition will still play a 
significant role in the overall spraying process and the resulting coating 
that is obtained. 
In particular, the invention comprises a precursor coating composition 
comprising a liquid mixture of: 
(a) a solids fraction containing at least one polymeric compound capable of 
forming a coating on a substrate; and 
(b) a solvent fraction containing at least one coupling solvent in which 
said at least one polymeric compound is at least partially soluble and 
which is at least partially miscible with water; and 
(c) water, which is present in an amount of less than about 30% by weight 
based on the weight of the solvent fraction; 
said liquid mixture having: 
(i) a viscosity of less than about 6,000 centipoise and having less than 
about 650 grams of the solvent fraction per liter of mixture; 
(ii) a solubility with at least one supercritical fluid, above the critical 
temperature and pressure of the supercritical fluid, of greater than 5% by 
weight of supercritical fluid in said mixture; 
(iii) a viscosity of less than about 300 centipoise when admixed with a 
sufficient amount of the at least one supercritical fluid, above the 
critical temperature and pressure of the supercritical fluid, so as to 
render the mixture suitable for spray application; and 
(iv) a solubility with the supercritical fluid in the non-supercritical 
state, at 25.degree. C. and one atmosphere absolute pressure of said 
fluid, of less than about 0.8% by weight of fluid in said mixture. 
As used herein, the "critical temperature" is defined as the temperature 
above which a gas cannot be liquefied by an increase in pressure. Also, as 
used herein, the "critical pressure" is defined as that pressure which is 
just sufficient to cause the appearance of two phases at the critical 
temperature. 
The invention is also directed to precursor coating compositions as 
described above to which pigments, pigment extenders, metallic flakes, 
fillers, drying agents, anti-foaming agents, anti-skinning agents, wetting 
agents, ultraviolet absorbers, cross-linking agents, and other coating 
additives are admixed with (a) and (b). 
The present invention is also directed to processes for the liquid spray 
application of the coatings discussed above to a substrate wherein the use 
of environmentally undesirable volatile organic solvents and non-aqueous 
diluents may even further be diminished than that realized in the above 
noted related applications. Accordingly, the process of the present 
invention comprises: 
(a) forming a liquid mixture in a closed system, said liquid mixture 
comprising: 
(i) a solids fraction containing at least one polymeric compound capable of 
forming a coating on a substrate; 
(ii) a solvent fraction containing at least one coupling solvent in which 
said at least one polymeric compound is at least partially soluble and 
which is at least partially miscible with water; 
(iii) water, which is present in an amount of less than about 30% by weight 
based on the weight of the solvent fraction; and 
(iv) at least one supercritical fluid, in at least an amount which when 
added to (i), (ii), and (iii) is sufficient to render the viscosity of 
said mixture to a point suitable for spray application; and 
(b) spraying said liquid mixture onto a substrate to form a liquid coating 
thereon. 
As used herein, the terms "liquid spray", "liquid droplets" or "liquid 
coating" is meant to define a spray, droplet, or coating containing a 
portion of the solids fraction, a portion of the solvent fraction, a 
portion of the water, in addition to any entrained supercritical fluid 
that may still be present in such spray, droplet or coating. 
In both the precursor coating composition as well as in the process using 
such precursor composition to apply the same to a substrate, in addition 
to the water, what is generally also desirable is the use of a coupling 
solvent. Such coupling solvent enables the presence of a single phase in 
the water-containing composition such that the components of the 
composition, namely, the polymeric components, the water, and the organic 
solvent all are at least partially miscible with one another. If desired, 
all of the organic solvent in the composition may be a coupling solvent. 
At the very least, the precursor composition contains polymeric component, 
water and such coupling solvent. The presence of an active solvent which, 
as used herein, is an organic solvent in which said polymeric compound is 
at least partially soluble and which is also at least partially miscible 
with the supercritical fluid, while desirable, is nevertheless optional. 
Such active solvent would be used in conjunction with a coupling solvent 
or, may actually be one and the same. 
As used herein, it is understood that the phrase "precursor coating 
composition" is a composition which is primarily intended and particularly 
suitable for admixture with at least one supercritical fluid for 
subsequent liquid spraying onto a substrate to provide a liquid coating 
thereon which, when dried or cured, helps produce a substantially uniform, 
continuous, substantially defect-free coating. However, if desired, this 
precursor coating composition may, of course, be utilized for an entirely 
different purpose although such use would not constitute the preferred 
objective of the present invention. Thus, the precursor coating 
composition may be utilized, if desired, by simply adding a suitable 
solvent to the composition (other than a supercritical fluid) and then 
using such a diluted composition as a coating medium in any conventional 
manner. It is to be understood that the scope of the present invention is 
not narrowly limited to using the precursor coating composition only with 
supercritical fluid and then spraying the resulting admixture. The 
precursor coating composition of the present invention is believed to be 
unique regardless of the manner in which it is eventually used. 
Also as used herein, it is understood that the phrases "admixed coating 
composition" or "admixed liquid mixture" are intended to mean a sprayable 
mixture of the precursor coating composition and at least one 
supercritical fluid. 
It should be noted and stressed that the above noted instances in which it 
would be desirable to utilize water as an additional viscosity reducing 
diluent are only exemplary. Other situations may arise, when using 
supercritical fluids as a viscosity reduction diluent, in which the use of 
water as yet an additional diluent would be applicable. Of course, the 
scope of the present invention includes such additional applications as 
well.

DETAILED DESCRIPTION OF THE INVENTION 
Because of its importance to the claimed invention, a brief discussion of 
relevant supercritical fluid phenomena is warranted. 
The supercritical fluid phenomenon is well documented, see pages F-62-F-64 
of the CRC Handbook of Chemistry and Physics, 67th Edition, 1986-1987, 
published by the CRC Press, Inc., Boca Raton, Fla. At high pressures above 
the critical point, the resulting supercritical fluid, or "dense gas", 
will attain densities approaching those of a liquid and will assume some 
of the properties of a liquid. These properties are dependent upon the 
fluid composition, temperature, and pressure. As used herein, the 
"critical point" is the transition point at which the liquid and gaseous 
states of a substance merge with each other and represent the combination 
of the critical temperature and critical pressure for a given substance. 
The compressibility of supercritical fluids is great just above the 
critical temperature where small changes in pressure result in large 
changes in the density of the supercritical fluid. The "liquid-like" 
behavior of a supercritical fluid at higher pressures results in greatly 
enhanced solubilizing capabilities compared to those of the "subcritical" 
compound, with higher diffusion coefficients and an extended useful 
temperature range compared to liquids. Compounds of high molecular weight 
can often be dissolved in the supercritical fluid at relatively low 
temperatures. An interesting phenomenon associated with supercritical 
fluids is the occurrence of a "threshold pressure" for solubility of a 
high molecular weight solute. As the pressure is increased, the solubility 
of the solute will often increase by many orders of magnitude with only a 
small pressure increase. The solvent capabilities of the supercritical 
fluid, however, are not essential to the broad aspects of the invention. 
Near-supercritical liquids also demonstrate solubility characteristics and 
other pertinent properties similar to those of supercritical fluids. The 
solute may be a liquid at the supercritical temperatures, even though it 
is a solid at lower temperatures. In addition, it has been demonstrated 
that fluid "modifiers" can often alter supercritical fluid properties 
significantly, even in relatively low concentrations, greatly increasing 
solubility for some solutes. These variations are considered to be within 
the concept of a supercritical fluid as used in the context of this 
invention. Therefore, as used herein, the phrase "supercritical fluid" 
denotes a compound above, at, or slightly below the critical temperature 
and pressure (the critical point) of that compound. 
Examples of compounds which are known to have utility as supercritical 
fluids are given in Table 1. 
TABLE 1 
______________________________________ 
EXAMPLES OF SUPERCRITICAL SOLVENTS 
Boiling Critical Critical 
Critical 
Point Temperature 
Pressure 
Density 
Compound (.degree.C.) 
(.degree.C.) 
(atm) (g/cm.sup.3) 
______________________________________ 
CO.sub.2 -78.5 31.3 72.9 0.448 
NH.sub.3 -33.35 132.4 112.5 0.235 
H.sub.2 O 100.00 374.15 218.3 0.315 
N.sub.2 O -88.56 36.5 71.7 0.45 
Xenon -108.3 16.6 57.6 0.118 
Krypton -153.2 -63.8 54.3 0.091 
Methane -164.00 -82.1 45.8 0.2 
Ethane -88.63 32.28 48.1 0.203 
Ethylene -103.7 9.21 49.7 0.218 
Propane -42.1 96.67 41.9 0.217 
Pentane 36.1 196.6 33.3 0.232 
Methanol 64.7 240.5 78.9 0.272 
Ethanol 78.5 243.0 63.0 0.276 
Isopropanol 
82.5 235.3 47.0 0.273 
Isobutanol 
108.0 275.0 42.4 0.272 
Chlorotri- 
-31.2 28.0 38.7 0.579 
fluoromethane 
Monofluoro- 
-78.4 44.6 58.0 0.3 
methane 
Cyclohexanol 
155.65 356.0 38.0 0.273 
______________________________________ 
Due to the low cost, environmental acceptability, non-flammability, and low 
critical temperature of carbon dioxide, supercritical carbon dioxide fluid 
is preferably used with the precursor coating compositions of the present 
invention. For many of the same reasons, nitrous oxide (N.sub.2 O) is a 
desirable supercritical fluid for admixture with the precursor coating 
compositions of the present invention. However, any of the aforementioned 
supercritical fluids and mixtures thereof are to be considered as being 
applicable for use with the precursor coating compositions. 
The solvency of supercritical carbon dioxide is substantially similar to 
that of a lower aliphatic hydrocarbon and, as a result, one can consider 
supercritical carbon dioxide as a replacement for the hydrocarbon solvent 
of a conventional coating formulation. In addition to the environmental 
benefit of replacing hydrocarbon solvents with supercritical carbon 
dioxide, there is a safety benefit also, because carbon dioxide is 
non-flammable. 
To better understand the phenomenon that is occurring when a supercritical 
fluid, such as supercritical carbon dioxide, is added to a precursor 
coating composition and the problems that may be encountered, reference is 
made to the phase diagram in FIG. 1 wherein the supercritical fluid is 
supercritical carbon dioxide fluid. In FIG. 1, the vertices of the 
triangular diagram represent the pure components of an admixed coating 
composition which for the purpose of this discussion contains no water. 
Vertex A is an organic solvent, vertex B is carbon dioxide, and vertex C 
represents a polymeric material. The curved line BFC represents the phase 
boundary between one phase and two phases. The point D represents a 
possible composition of a coating composition in which supercritical 
carbon dioxide has not been added. The point E represents a possible 
composition of an admixed coating composition, after admixture with 
supercritical carbon dioxide. Generally, the addition of supercritical 
carbon dioxide reduces the viscosity of the viscous precursor coating 
composition to a range where it can be readily atomized through a liquid 
spray apparatus. 
Thus, after atomization, a majority of the carbon dioxide vaporizes, 
leaving substantially the composition of the original viscous coating 
composition. Upon contacting the substrate, the remaining liquid mixture 
of the polymer and solvent(s) component(s) will flow, i.e., coalesce, to 
produce a uniform, smooth film on the substrate. The film forming pathway 
is illustrated in FIG. 1 by the line segments EE'D (atomization and 
decompression) and DC (coalescence and film formation). 
However, the addition of supercritical carbon dioxide to a viscous coating 
composition does not always result in a viscosity reduction which is 
sufficient to allow for spraying of the composition. Such viscosity 
reduction is limited to the amount of supercritical fluid, such as 
supercritical carbon dioxide, that can be admixed with the coating 
composition. If not enough supercritical fluid can be added, then the 
viscosity of the composition is not lowered enough to make it suitable for 
spray application. This limitation as to the amount of supercritical fluid 
that can be added to the composition is generally a function of the 
miscibility of the supercritical fluid with the coating composition and 
can best be visualized by again referring to FIG. 1. 
As can be seen from the phase diagram, particularly as shown by arrow 10, 
as more and more supercritical carbon dioxide is added to the coating 
composition in an attempt to reduce its viscosity sufficiently, the 
composition of the admixed liquid coating mixture approaches the two-phase 
boundary represented by line BFC. If enough supercritical carbon dioxide 
is added, the two-phase region is reached and the composition 
correspondingly breaks down into two phases. Sometimes, it may be 
desirable to admix an amount of supercritical fluid (in this case, 
supercritical carbon dioxide) which is even beyond the two phase boundary. 
Generally, however, it is not preferable to go much beyond this two phase 
boundary for optimum spraying performance and/or coating formation. This 
two-phase region may be reached, however, prior to achieving the necessary 
viscosity reduction. Any additional supercritical carbon dioxide added to 
the system beyond this point will generally not aid in any further 
viscosity reduction. 
Viscosity reduction brought about by adding supercritical carbon dioxide 
fluid to viscous coatings composition is illustrated in FIG. 2. There, the 
viscous coating composition of 65 percent polymer solution in methyl amyl 
ketone, which corresponds to point D in FIG. 1 has a viscosity of about 
300 centipoise and the solution is unsprayable. Adding supercritical 
carbon dioxide fluid to the coating composition reduces the viscosity such 
that a liquid mixture that contains 28 percent supercritical carbon 
dioxide fluid, which corresponds to point E in FIG. 1, is formed. This 
liquid mixture now has a viscosity of less than 30 centipoise and readily 
forms a liquid spray by passing it through an orifice in an airless spray 
gun. The pressure utilized in FIG. 2 is 1250 psi at a temperature of 
50.degree. C. The polymer is Acryloid.TM. AT-400, a product of Rohm and 
Haas, which contains 75 percent of non-volatile acrylic polymer dissolved 
in 25 percent methyl amyl ketone. 
In accordance with the present invention, however, water may be added to a 
highly viscous coating composition as a further viscosity reducing 
diluent, which water-containing composition may then be admixed with 
supercritical fluids. Regardless of how the coating composition has come 
to have an unusually high viscosity, whether it be due to the nature of 
the polymer itself (e.g., its molecular weight) or the nature and/or 
amount of the solvent present in the composition, the addition of the 
water will generally aid in reducing the viscosity to the extent such that 
when admixed with the supercritical fluids, the resulting admixed 
composition containing such water and supercritical fluids will be 
suitable for spraying. 
Generally, the amount of water that is present in the composition is up to 
about 30 percent by weight based on the weight of the total solvent 
fraction contained in the composition. Preferably, the amount of water is 
less than about 20 percent by weight on that same basis. 
Higher quantities of water than those noted above may be not be desirable 
for a number of reasons. For one, too much water may also result in a 
phase separation, i.e., the composition breaks down into a water phase and 
an organic solvent phase. Such a phase separation, as in the case of a 
phase separation caused by an excessive amount of supercritical fluid, may 
result in poor spraying performance and/or poor coating formation. 
Thus, without wishing to be bound by theory, once such a phase separation 
takes place, the separate water phase may attract supercritical fluid and 
organic solvent leaving less organic solvent present in the separate 
organic solvent phase. This may result in a viscosity imbalance between 
the two phases which may very well hinder or prevent the spray application 
of the coating composition. 
So too, it is also believed, again without wishing to be bound by theory, 
that the concentration of the organic solvent in the water phase may be 
such that even if the composition were sprayed, there may be an excessive 
evaporation of such solvent resulting in an insufficent amount of solvent 
on the substrate to allow for proper coalescence of the atomized particles 
resulting in a poor coating. 
For similar reasons, in the case where water is added to a coating 
composition so as to replace a portion of the organic solvent present 
therein and thereby further reduce the overall organic solvent content, 
there should still be enough solvent present which will facilitate the 
proper flow-out and coalescence of the sprayed particles on the substrate 
to form a desirable coating thereon. 
Accordingly, it is generally desirable to add as much water as may be 
necessary so as to provide a precursor coating composition whose viscosity 
may be reduced to a point suitable for spray application by subsequent 
admixture with supercritical fluids. 
Although the above discussion has focused upon those cases in which the 
amount of supercritical fluid added to a viscous composition is 
ineffective to reduce the viscosity to the extent necessary in order to 
spray such composition and adding water to these compositions in 
accordance with the present invention so as to be able to spray them, it 
is understood that the addition of water to a coating composition is not 
limited to only those conditions. Indeed, water may be added to a coating 
composition for subsequent admixture with supercritical fluids even when 
the viscosity of the original composition is such that the addition of 
supercritical fluids is indeed capable of reducing the viscosity to a 
point suitable for spraying. 
For example, in the case where it is desirable to simply reduce the amount 
of volatile organic solvent present in a first coating composition where 
such first coating composition could be sprayed when admixed with 
supercritical fluids, it is clear that when water is used to replace some 
of the volatile solvent contained therein, there is no concern here as to 
phase separation caused by an excessive amount of supercritical fluid. 
As briefly discussed earlier, even after water is added to a coating 
composition, it has been found by virtue of the present invention that the 
amount of supercritical fluid that is capable of being admixed with the 
now water-containing composition, in absolute terms, remains substantially 
the same. In other words, if, for example, 100 grams of supercritical 
carbon dioxide were able to be admixed with a particular composition, 
after adding water to this composition, approximately 100 grams of 
supercritical carbon dioxide can still be admixed with the now 
water-containing composition. 
That is not to say, however, that it is necessary to add the same amount of 
carbon dioxide in the new water-containing composition as was capable of 
being added to the original composition. Although it is beneficial to 
maximize the amount of supercritical fluids that are utilized as diluents 
for viscosity reduction to thereby keep the solvent fraction, particularly 
the organic solvents and/or non-aqueous diluents to a minimum, there is no 
criticality as to the amount of supercritical fluids that are used other 
than using an amount which will produce a sprayable composition. 
However, the capability of the water-containing composition to be admixed 
with the same amount of supercritical fluids that was able to be used in 
the "dry" composition is significant inasmuch as the concomitant diluent 
effect that is obtained by the supercritical fluid can be expected to also 
remain substantially constant. 
In addition to the water that is present in the precursor coating 
compositions of the present invention, which water is used as a further 
viscosity reducing diluent, it is generally also desirable to have a 
coupling solvent present in the precursor coating composition as well. Of 
course, if a coating composition to which the water is added already 
contains an organic solvent which may be characterized as a coupling 
solvent, no further addition of such a coupling solvent need be made. 
Similarly, if a precursor composition is being initially prepared with 
water, at least one of the organic solvents used for such composition 
should desirably be a coupling solvent. 
A coupling solvent is a solvent in which the polymeric compounds used in 
the solids fraction is at least partially soluble. Most importantly, 
however, such a coupling solvent is also at least partially miscible with 
water. Thus, the coupling solvent enables the miscibility of the solids 
fraction, the solvent fraction and the water to the extent that a single 
phase is desirably maintained such that the composition may optimally be 
sprayed and a good coating formed. 
Coupling solvents are well known to those skilled in the art and any 
conventional coupling solvents which are able to meet the aforementioned 
characteristics, namely, those in which the polymeric components of the 
solid fraction is at least partially soluble and in which water is at 
least partially miscible are all suitable for being used in the present 
invention. 
Applicable coupling solvents which may be used in the present invention 
include, but are not limited to, ethylene glycol ethers, propylene glycol 
ethers, chemical and physical combinations thereof; lactams; cyclic ureas; 
and the like. 
Specific coupling solvents (which are listed in order of most effectiveness 
to least effectiveness) include butoxy ethanol, propoxy ethanol, hexoxy 
ethanol, isopropoxy 2-propanol, butoxy 2-propanol, propoxy 2-propanol, 
tertiary butoxy 2-propanol, ethoxy ethanol, butoxy ethoxy ethanol, propoxy 
ethoxy ethanol, hexoxy ethoxy ethanol, methoxy ethanol, methoxy 
2-propanol, and ethoxy ethoxy ethanol. Also included are lactams such as 
n-methyl-2-pyrrolidone, and cyclic ureas such as dimethyl ethylene urea. 
In addition to a coupling solvent, it may also be desirable to add (or have 
present) an active solvent as well. An active solvent, as used herein, is 
meant to include those solvents which have particularly good solubility 
for the polymeric compounds that are used in the composition in addition 
to having at least partial miscibility with supercritical fluids. 
Suitable active solvents which may be utilized in the precursor coating 
compositions of the present invention include ketones such as acetone, 
methyl ethyl ketone, methyl isobutyl ketone, mesityl oxide, methyl amyl 
ketone, cyclohexanone and other aliphatic ketones; esters such as methyl 
acetate, ethyl acetate, alkyl carboxylic esters; ethers such as methyl 
t-butyl ether, dibutyl ether, methyl phenyl ether and other aliphatic or 
alkyl aromatic ethers; glycol ethers such as ethoxy ethanol, butoxy 
ethanol, ethoxy 2-propanol, propoxy ethanol, butoxy 2-propanol and other 
glycol ethers; glycol ether esters such as butoxyethoxy acetate, ethyl 
3-ethoxy propionate and other glycol ether esters; alcohols such methanol, 
ethanol, propanol, iso-propanol, butanol, iso-butanol, amyl alcohol and 
other aliphatic alcohols; aromatic hydrocarbons such as toluene, xylene, 
and other aromatics or mixtures of aromatic solvents; aliphatic 
hydrocarbons such as VM&P naphtha and mineral spirits, and other 
aliphatics or mixtures of aliphatics; nitro alkanes such as 
2-nitropropane. A review of the structural relationships important to the 
choice of solvent or solvent blend is given by Dileep et al., Ind. Eng. 
Che. (Product Research and Development) 24, 162, 1985 and Francis, A. W., 
J. Phys. Chem. 58, 1099, 1954. 
Of course, there are solvents which can function both as coupling solvents 
as well as active solvents and the one solvent may be used to accomplish 
both purposes. Such solvents include, for example, butoxy ethanol, propoxy 
ethanol, and propoxy 2-proponal. Glycol ethers are particularly preferred. 
When using both a coupling solvent as well as an active solvent in the 
precursor coating compositions of the present invention, the ratio of 
coupling solvent to active solvent by weight is generally in the range of 
from about 1:1 to 4:1, preferably about 2:1 to 3.5:1, and most preferably 
about 2.5:1 to 3:1. Generally, the ratio of coupling solvent to active 
solvent will be dependent, among other things, on the hydrophobicity of 
the active solvent. 
The presence of too much coupling solvent may interfere with the 
dissolution of the polymer while the presence of too much active solvent 
may interfere with proper miscibility of the water. 
While the polymeric compounds that are suitable for use in the present 
invention as coating materials generally include any of the polymers which 
are well known to those skilled in the coatings art, there are preferred 
polymers which are particularly desirable due to their possessing specific 
characteristics which make them generally more suitable for (1) admixture 
with supercritical fluids followed by (2) spraying such admixture onto a 
substrate so as to help obtain a defect free coating. 
Generally, the polymers which may be used in the present invention must be 
able to withstand the temperatures and/or pressures which are involved 
when they are ultimately admixed with the at least one supercritical 
fluid. Such applicable polymers include thermoplastic or thermosetting 
materials or may be crosslinkable film forming systems. 
In particular, the polymeric components include vinyl, acrylic, styrenic, 
and interpolymers of the base vinyl, acrylic, and styrenic monomers; 
polyesters, oil-free alkyds, alkyds, and the like; polyurethanes, 
oil-modified polyurethanes and thermoplastic urethanes systems; epoxy 
systems; phenolic systems; cellulosic esters such as acetate butyrate, 
acetate propionate, and nitrocellulose; amino resins such as urea 
formaldehyde, melamine formaldehyde, and other aminoplast polymers and 
resins materials; natural gums and resins; and enamels, varnishes, and 
lacquers. Also included are mixtures of the above coating materials 
commonly used and known to those skilled in the art that are formulated to 
achieve performance and cost balances required of commercial coatings. 
One characteristic which is possessed by particularly preferred polymers 
that are used in the present invention are those having a low elastic 
component of viscosity. A discussion of the components of viscosity can be 
found in, for example, "Rheological Measurement for Quality Control" by S. 
B. Driscoll, Rubber World (December, 1980), pages 31-34, the contents of 
which are incorporated by reference. Thus, where a number of polymers may 
provide a precursor coating composition having essentially the same 
viscosity, the most preferred polymer would be the one having the least 
elastic component of viscosity. Such polymers having a low elastic 
component of viscosity are generally those having a structural and 
molecular weight distribution which, in solution, minimizes chain 
entanglement. Particularly, the high molecular weight distribution of the 
polymeric compound should be minimized. A useful and conventional measure 
for the high molecular weight fraction of the molecular weight 
distribution is the ratio of the weight average (Mw) molecular weight of 
the polymeric compound to the number average (Mn) molecular weight of that 
polymeric compound, i.e., Mw/Mn. Reference is made to, for example, 
"Introduction to Polymers and Resins", Federation Series On Coatings 
Technology (1986), pages 26-31, which discusses molecular weight 
determination of polymers. the contents of which are incorporated herein 
by reference. Generally, for a given number average molecular weight, Mn, 
the higher the ratio of Mw/Mn, the greater the high molecular weight 
fraction that is present in the polymer, and the greater the elastic 
component of viscosity possessed by such polymer. 
Preferably, the predominant polymeric compound used in the precursor 
composition of the present invention has a Mw/Mn ratio of less than about 
4, and preferably less than about 3, and most preferably less than about 
2. 
The higher the elastic component of viscosity possessed by the polymer, the 
more elastic a polymer is, the more difficult it is to atomize an admixed 
precursor coating composition containing supercritical fluid made from 
such a polymer. Generally, as an admixed coating composition is released 
through the spray orifice, shearing forces act upon the material causing 
it to tear itself apart into fine atomized droplets. When, however, the 
polymer has a high elastic component of viscosity, such tearing apart is 
hindered and the composition does not atomize as well. 
Particularly desirable polymers having a relatively low elastic component 
of viscosity include those set forth in Table 2 below. 
TABLE 2 
______________________________________ 
Polymer M.sub.n M.sub.w M.sub.w /M.sub.n 
______________________________________ 
Alkyd 25,000-50,000 50,000-200,000 
2-4 
resins 
Epoxy 350-4,000 350-7,000 1.0-2.5 
resins 
Acrylic, 
25,000-350,000 
40,000-600,000 
1.5-3 
thermo- 
plastic, 
solution 
polymer 
Acrylic, 
500,000-2,000,000 
650,000-2,500,000 
1.1-1.8 
thermo- 
plastic, 
emulsion 
polymer 
Acrylic 1,000-2,000 1,200-2,200 1.1-1.5 
thermo- 
setting, 
oligomer 
Poly- 2,000-5,000 2,100-5,200 1.05-1.1 
butadiene, 
anionic 
poly- 
merized 
______________________________________ 
In addition to the polymeric compound that is contained in the solids 
fraction, additives which are typically utilized in coatings may also be 
used. For example, pigments, pigment extenders, metallic flakes, fillers, 
drying agents, anti-foaming agents, and anti-skinning agents, wetting 
agents, ultraviolet absorbers, cross-linking agents, and mixtures thereof, 
may all be utilized in the precursor composition of the present invention. 
In connection with the use of the various additives noted above, it is 
particularly desirable for pigments to be present in the precursor 
composition inasmuch as it has been found to aid in the diffusion of the 
supercritical fluid from the sprayed composition resulting in improved 
atomization. 
The solvent fraction of the precursor composition of the present invention 
includes at least one coupling solvent in which the at least one polymer 
compound is at least partially soluble and which is at least partially 
miscible with water, as discussed earlier. Optionally, an active solvent, 
as noted above, may also be employed. While the solvents that are suitable 
for use in the present invention generally include (as long as there is a 
coupling type solvent present) any solvent or mixtures of solvent which is 
capable of dissolving, dispersing or suspending the solids fraction when 
admixed with the supercritical fluid, as with the solids fraction 
discussed above, there are preferred solvents and solvent mixtures which 
are particularly desirable. Such preferred solvents, either coupling 
and/or active, possess certain desirable characteristics which make them 
generally more suitable for admixture with a supercritical fluid followed 
by spraying such admixture onto a substrate material. 
As is quite apparent, the selection of a particular solvent or solvent 
mixture will generally be dependent upon the particular polymeric 
compounds being used. 
In general, solvents suitable for the present invention should have the 
desired solvency characteristics as aforementioned and also the proper 
balance of evaporation rates so as to ensure coating formation. In other 
words, the solvent fraction should have a proper blend of fast and slow 
solvents. 
More specifically, solvents having fast evaporation rates are needed to 
help solubilize the solids fraction, as well as help reduce viscosity, and 
to substantially leave the admixed coating composition once it has been 
sprayed and before the atomized droplets contact the substrate. 
Correspondingly, solvents having slow evaporation rates are also needed to 
help solubilize the solids fraction, but these solvents are primarily 
needed to be present on the substrate after the atomized droplets have 
been applied so as to provide a suitably low viscosity to produce enough 
flow-out to obtain a uniform and continuous coating. 
However, too much of the fast solvent will produce a dry coating not having 
enough flow-out. Conversely, too much of the slow solvent will produce a 
coating having sagging and running defects and will not readily be dried 
thereby hindering the early handling of such a coated substrate. 
Based on a relative evaporation rate (RER) to a butyl acetate standard 
equal to 100 using ASTM Method D3599 at 25.degree. C. and one atmosphere 
pressure, the solvent fraction desirably has the following composition of 
fast and slow solvents as represented by corresponding RER values: 
______________________________________ 
Wt. % of Total Solvent Fraction 
RER 
______________________________________ 
30-100% &lt;50 
0-70% 50-100 
0-40% 101-250 
&lt;10% &gt;250 
______________________________________ 
More preferably, the solvent fraction has the following composition: 
______________________________________ 
Wt. % of Total Solvent Fraction 
RER 
______________________________________ 
40-100% &lt;50 
0-60% 50-100 
0-30% 101-250 
&lt;5% &gt;250 
______________________________________ 
Another characteristic of the solvent fraction which desirably is optimized 
so as to make it particularly suitable for admixture with a supercritical 
fluid with subsequent spraying thereof is the surface tension of the 
solvent fraction. Specifically, the less surface tension that a solvent 
has, the more desirable it is. 
Accordingly, solvents having low surface tension provide good atomization 
of the admixed precursor coating composition providing a fine aerosol. 
Such a fine aerosol desirably facilitates the escape of the supercritical 
fluid from the sprayed admixed coating composition prior to its contacting 
the substrate. 
Moreover, solvents having low surface tension also facilitate the formation 
of good coatings on the substrate by aiding in the diffusion of any 
residual supercritical fluid that may still be entrapped within the 
applied coating. So too, low surface tension solvents also help to 
suppress the formation of any bubbles caused by such residual 
supercritical fluid as it escapes. Still further, solvents having low 
surface tension desirably provide fast wetting and spreading 
characteristics which also aid in the formation of a defect-free, uniform 
coating on the substrate. 
Preferably, the solvent fraction has a surface tension at 25.degree. C. 
which is desirably less than 35 dynes/centimeter. More preferably, the 
surface tension is less than 30 dynes/centimeter, for example, 23-28 
dynes/centimeter. 
It is understood that it is not necessary that the solvent or solvent 
mixture possess such surface tension characteristics per se. While such 
solvents do certainly exist, as exemplified by such solvents as toluene, 
VM&P naphtha, butyl acetate, pentyl propionate, glycol ethers, methyl 
PROPASOL.RTM. acetate (manufactured by Union Carbide Corp., Danbury, 
Conn.), UCAR.RTM. Ester EEP (manufactured by Union Carbide Corp., Danbury, 
Conn.), and the like, there are conventional additives which may be added 
to the precursor composition which contains the solvent and solids 
fractions so as to suppress the surface tension of the composition as a 
whole. Such additives include surface active materials, commonly known as 
surfactants, which are well known to those skilled in the art and which 
are applicable to be used in both the solvent fraction of the present 
invention as well as in the precursor coating composition as a whole. 
Still further in connection with the solvent fraction, as the admixed 
coating composition containing the mixture of polymer, solvent, water and 
supercritical fluid is sprayed, the vaporization of the fast solvent 
contributes to the overall cooling of the sprayed composition. Inasmuch as 
the solubility of most supercritical fluids, particularly carbon dioxide, 
is higher at cooler temperatures, such cooling manifestly hinders the 
diffusion of the supercritical fluid from the sprayed composition. It is 
desirable to have essentially all of the supercritical fluid escape from 
the admixed coating composition, once it has been sprayed, such that 
essentially none of the supercritical fluid is left once the atomized 
liquid droplets contact the substrate. 
In order to help minimize the cooling effect caused by evaporation of the 
fast solvent, it is desirable that the solvent fraction have an overall 
low heat of vaporization. Preferably, the solvent fraction has an overall 
heat of vaporization of less than 110 calories per gram of solvent 
fraction and more preferably, less than about 100 calories per gram of 
solvent fraction. Desirably, the overall heat of vaporization of the 
solvent fraction is in the range of from about 65 to 95 calories/gram. 
Keeping in mind the underlying primary objective of the present invention, 
namely, to minimize the unnecessary release of solvent vapors into the 
atmosphere during the spray application of the admixed coating 
compositions, it is clear that the amount of solvent used in the precursor 
coating compositions should be less than that required to produce a 
mixture of polymeric compounds and solvent having a viscosity which would 
permit its application by liquid spray techniques. In other words, the 
inclusion of the solvent fraction should be minimized such that the 
diluent effect due to the presence of the supercritical fluid diluent is 
fully utilized. 
However, reducing the amount of solvent in the coating composition is 
beneficial not only for the purpose of minimizing environmental pollution, 
but as recognized by the present invention, such reduction in the amount 
of solvent is also desirable to help facilitate the spraying of the 
coating composition once it has been admixed with supercritical fluid as 
well as improving the appearance of the coating that is ultimately formed 
on the substrate. 
More specifically, as the amount of solvent fraction present in the 
precursor coating composition is increased and after such composition has 
been admixed with supercritical fluid and sprayed, the rate of diffusivity 
of such supercritical fluid through the sprayed composition generally 
decreases. This typically results in an increase of residual supercritical 
fluid retained in the coating after it has been applied to the substrate 
which may result in the formation of coating defects. In order to 
compensate for such decreased diffusivity, the temperature of the sprayed 
composition is generally raised so as to lower the solubility of the 
supercritical fluid. However, such an increase in spray temperature may 
have an effect on the particle size and the atomization quality of the 
sprayed composition. 
Still further, as the solvent fraction in the precursor coating composition 
is increased, there is a corresponding loss in the distinctness of image 
gloss (DOI) in the resulting coating. This is believed to be caused by a 
concomitant increase in the amount of residual supercritical fluid that is 
retained in the applied coating which gradually fizzles out of the 
coating. 
Furthermore, an excessive solvent fraction in the precursor coating 
composition diminishes the effectiveness of the supercritical fluid, 
because atomization is intensified such that deposition of the liquid 
spray onto the substrate becomes poorer than when the solvent fraction is 
not excessive. That is, the transfer of liquid coating to the substrate 
becomes less efficient. Therefore, more spraying must be done to build up 
coating thickness, with the result that more solvent is released to the 
environment and more precursor coating composition is used, which 
increases cost. In addition, because more spray passes must be used to 
build the desired coating thickness, a higher proportion of slow solvent 
is lost from the coating during the application process, so that less slow 
solvent is available to aid reflow of the coating to give a smooth surface 
once the desired coating thickness has been achieved. Without wishing to 
be bound by theory, it is believed that viscosity reduction caused by the 
excessive solvent fraction combines with viscosity reduction caused by the 
supercritical fluid to give droplets that have insufficient mass to be 
deposited efficiently onto the substrate from the spray. Instead, the 
droplets follow the gas flow into the environment. Using less solvent 
allows full utilization of the viscosity reduction of the supercritical 
fluid and gives droplet sizes that deposit efficiently onto the substrate. 
Moreover, an excessive solvent fraction produces a greater wet coating 
thickness for a desired dry coating thickness and a lower coating 
viscosity on the substrate, which increases the tendency for the coating 
to sag or run during application, drying, and curing. In addition, as the 
excessive solvent evaporates from the coating, the coating shrinks to a 
greater degree during the drying process and flow currents can be induced 
inside the coating which disrupt the leveling to give a smooth surface. 
Drying time is also increased. 
Consequently, in accordance with the present invention, the amount of 
solvent fraction that is present in the liquid mixture comprised of a 
solids fraction, a solvent fraction and water is no greater than about 650 
grams of solvent per liter of liquid mixture. More preferably, the amount 
of solvent fraction contained in the liquid mixture is less than about 450 
grams of solvent per liter of liquid mixture. Most preferably, the amount 
of solvent fraction is in the range of from about 200 to about 400 grams 
of solvent per liter of mixture. 
The liquid precursor coating composition of the present invention 
comprising the solids fraction, the solvent fraction and water should have 
a viscosity of less than about 6,000 centipoise, and preferably less than 
about 3,000 centipoise, and most preferably in the range of from about 500 
to about 2,000 centipoise. Obviously, a major factor contributing to the 
viscosity of the liquid mixture is the amount of solvent fraction and 
water contained therein, which variable was discussed above. Hence, it is 
apparent that the amount of solvent fraction present in the precursor 
coating composition should be considered hand-in-hand with the desired 
viscosity that is to be obtained. 
The viscosity of the precursor coating composition should be low enough 
such that there is enough solvent and water present to provide proper 
coalescence upon the substrate once the composition is sprayed while still 
being high enough to allow for a reduction in solvent usage so as to 
maximize the utilization of the supercritical fluid diluent and to 
concomitantly facilitate good atomization and coating formation. 
The viscosity of the precursor coating composition should also be such that 
when supercritical fluid is added, it is possible to add enough of the 
supercritical fluid, without entering the two phase region, such that the 
viscosity is lowered to less than about 300 centipoise, above the critical 
temperature and pressure of the supercritical fluid, so as to render the 
mixture suitable for spray application. More preferably, the admixed 
liquid coating composition has a viscosity which is less than about 150 
centipoise and preferably has a viscosity in the range of from about 5 to 
150 centipoise. Most preferably, the viscosity of the admixture of solids 
fraction, solvent fraction, water and supercritical fluid is in the range 
of from about 10 to about 50 centipoise. 
Yet another factor which the precursor coating composition must address is 
the solubility of the supercritical fluid both at supercritical conditions 
and at the conditions of the substrate, i.e., after the composition has 
been sprayed. 
The solubility requirements for these two sets of conditions are totally 
antithetical to one another. Thus, when admixing the supercritical fluid 
with the liquid precursor composition, it is desirable to have a 
composition which has a high solubility for the supercritical fluid at the 
supercritical conditions. In contrast, once the composition has been 
sprayed through the orifice, it is desirable that the solubility for the 
supercritical fluid at the conditions present in the environment of the 
substrate be as low as possible. 
More particularly, in accordance with the present invention, the liquid 
precursor coating composition should have a solubility for the 
supercritical fluid, above the critical temperature and pressure of the 
supercritical fluid, of at least 5% by weight of the supercritical fluid 
in the liquid mixture. Preferably, the solubility should be at least 15% 
by weight of the supercritical fluid in the mixture and, more preferably 
about 20 to 50% or greater by weight of supercritical fluid in the 
mixture. Most preferably, it is in the range of from about 25% to about 
35% by weight. 
If the precursor coating composition has a solubility which is less than 
that noted above, there simply would not be enough of a diluent effect 
provided by the supercritical fluid. This would result in an insufficient 
viscosity reduction such that the composition could not properly be 
sprayed. 
Once the composition is admixed with supercritical fluid and sprayed, it is 
desirable to have the supercritical fluid diffuse through the sprayed 
composition as quickly as possible such that there is very little residual 
supercritical fluid left in the coating once it has come into contact with 
the substrate. Accordingly, the fluid, which of course is no longer 
supercritical, should have a solubility in the liquid precursor coating 
composition of less than about 0.8% by weight of the fluid in the 
non-supercritical state in the precursor coating composition. Preferably, 
the solubility of the fluid is less than about 0.6% by weight in the 
composition. Most preferably, the fluid should be soluble in the precursor 
coating composition in an amount of less than about 0.4% by weight. As 
used herein, it is to be understood that the solubility of the fluid in 
the non-supercritical state is measured at 25.degree. C. and in one 
atmosphere absolute pressure of the fluid. 
A still further characteristic which the precursor coating composition 
should desirably possess is a high diffusivity for passing the 
supercritical fluid out of the composition once it has been sprayed with 
such supercritical fluid into the environment of the substrate. Such high 
diffusivities are desirable to help the supercritical fluid quickly escape 
from the sprayed liquid mixture. This helps prevent the presence of any 
residual supercritical fluid in the liquid coating applied to the 
substrate and thereby helps ensure the formation of a uniform, defect-free 
coating. 
Accordingly, the precursor coating composition should desirably possess an 
apparent gas diffusion rate (based on a sprayed particle average velocity 
of about 2 to about 8 m/sec and a spraying distance of about 30 cm at 
25.degree. C. and one atmosphere pressure) from the time it has been 
sprayed with supercritical fluid to the time it impacts on the substrate 
of at least about 7 to about 26 grams of supercritical fluid per gram of 
presursor coating composition per second. 
The precursor coating composition, after having been admixed with 
supercritical fluid, is sprayed onto a substrate to form a liquid coating 
thereon containing solids fraction, a portion of the solvent fraction, a 
portion of the water and any residual supercritical fluid that may be left 
by passing the admixed liquid mixture under pressure through an orifice 
into the environment of the substrate to form a liquid spray. 
An orifice is a hole or an opening in a wall or housing, such as in a spray 
tip of a spray nozzle on an electrostatic spray gun, through which the 
admixed liquid coating composition flows in going from a region of higher 
pressure, such as inside the spray gun, into a region of lower pressure, 
such as the air environment, outside of the spray gun and around the 
substrate. An orifice may also be a hole or an opening in the wall of a 
pressurized vessel, such as a tank or cylinder. An orifice may also be the 
open end of a tube or pipe or conduit through which the mixture is 
discharged. The open end of the tube or pipe or conduit may be constricted 
or partially blocked to reduce the open area. 
Spray orifices, spray tips, spray nozzles, and spray guns used for 
conventional electrostatic airless and air-assisted airless spraying of 
coating formulations such as paints, lacquers, enamels, and varnishes, are 
suitable for spraying the precursor coating compositions of the present 
invention when admixed with supercritical fluids. Spray guns, nozzles, and 
tips are preferred that do not have excessive flow volume between the 
orifice and the valve that turns the spray on and off. The spray guns, 
nozzles, and tips must be built to contain the spray pressure used. 
There are a broad variety of spray devices that one may use in spraying the 
precursor coating composition of the present invention after it has been 
admixed with supercritical fluid. Essentially, any spray gun may be used, 
from conventional airless and air-assisted airless spray devices to 
electrostatic spray devices. The choice of spray device is dependent upon 
the kind of application that is contemplated. 
The material of construction of the orifice through which the admixed 
coating composition is sprayed must possess the necessary mechanical 
strength for the high spray pressure used, have sufficient abrasion 
resistance to resist wear from fluid flow, and be inert to chemicals with 
which it comes into contact. Any of the materials used in the construction 
of airless spray tips, such as boron carbide, titanium carbide, ceramic, 
stainless steel or brass, is suitable, with tungsten carbide generally 
being preferred. 
The orifice sizes suitable for spraying the admixed liquid mixture should 
generally range from about 0.004-inch to about 0.072-inch diameter. 
Because the orifices are generally not circular, the diameters referred to 
are equivalent to a circular diameter. The proper selection is determined 
by the orifice size that will supply the desired amount of liquid coating 
and accomplish proper atomization for the coating. Generally, smaller 
orifices are desired at lower viscosity and larger orifices are desired at 
higher viscosity. Smaller orifices give finer atomization but lower 
output. Larger orifices give higher output but poorer atomization. Finer 
atomization is preferred in the practice of the present invention. 
Therefore, small orifice sizes from about 0.004-inch to about 0.025-inch 
diameter are preferred. Orifice sizes from about 0.007-inch to about 
0.015-inch diameter are most preferred. 
The designs of the spray tip that contains the spray orifice and of the 
spray nozzle that contains the spray tip are not critical. The spray tips 
and spray nozzles should have no protuberances near the orifice that would 
interfere with the spray. 
The shape of the spray is also not critical to being able to spray the 
admixed coating composition. The spray may be in the shape of a cone that 
is circular or elliptical in cross section or the spray may be in the 
shape of a flat fan, but the spray is not limited to these shapes. Sprays 
that are flat fans or cones that are elliptical in cross section are 
preferred. 
The distance from the orifice to the substrate is generally at a distance 
of from about 4 inches to about 24 inches. A distance of 6 inches to 18 
inches is preferred. A distance of 8 inches to 14 inches is most 
preferred. 
Devices and flow designs that promote turbulent or agitated flow in the 
liquid mixture prior to passing the liquid mixture under pressure through 
the orifice may also be used. Such techniques include but are not limited 
to the use of pre-orifices, diffusers, turbulence plates, restrictors, 
flow splitters/combiners, flow impingers, screens, baffles, vanes, and 
other inserts, devices, and flow networks that are used in electrostatic 
airless spray and air-assisted airless spray. 
Filtering the liquid mixture prior to flow through the orifice is desirable 
in order to remove particulates that might plug the orifice. This can be 
done using conventional high-pressure paint filters. A filter may also be 
inserted at or in the gun and a tip screen may be inserted at the spray 
tip to prevent orifice plugging. The size of the flow passages in the 
filter should be smaller than the size of the orifice, preferably 
significantly smaller. 
Electrostatic forces may be used to increase the proportion of precursor 
coating composition that is deposited onto a substrate from the spray. 
This is commonly referred to as increasing the transfer efficiency. This 
is done by using a high electrical voltage relative to the substrate to 
impart an electrical charge to the spray. This creates an electrical force 
of attraction between the spray droplets and the substrate, which causes 
droplets that would otherwise miss the substrate to be deposited onto it. 
When the electrical force causes droplets to be deposited on the edges and 
backside of the substrate, this effect is commonly referred to as wrap 
around. 
Preferably the substrate is grounded, but it may also be charged to the 
opposite sign as the spray. The substrate may be charged to the same sign 
as the spray, but at a lower voltage with respect to ground, but this is 
of less benefit, because this produces a weaker electrical force of 
attraction between the spray and the substrate than if the substrate were 
electrically grounded or charged to the opposite sign. Electrically 
grounding the substrate is the safest mode of operation. Preferably the 
spray is charged negative relative to electrical ground. 
The method of charging the spray is not critical to the practice of the 
invention provided the charging method is effective. The precursor coating 
composition can be electrically charged by applying high electrical 
voltage relative to the substrate and electrical current (1) within the 
spray gun, by direct contact with electrified walls or internal electrodes 
before leaving the orifice; (2) after the spray emerges from the orifice, 
by electrical discharge from external electrodes located near the orifice 
and close to the spray; or (3) away from the orifice, by passing the spray 
through or between electrified grids or arrays of external electrodes 
before the spray is deposited onto the substrate. Methods (1) and (2), 
individually or in combination, are preferred. Method (2) is most 
preferred. 
In charging method (1) above, the spray gun must be electrically 
insulating. The high voltage and electrical current is supplied to the 
admixed liquid mixture inside the gun by direct contact with an internal 
surface that is electrically conducting and electrified. This may be part 
of the wall of the flow conduit inside the gun or internal electrodes that 
extend into the flow or a combination of electrified elements, including 
the spray nozzle. The contact area must be large enough to transfer 
sufficient electrical charge to the admixed liquid mixture as it flows 
through the gun. This internal charging method has the advantage of having 
no external electrode that could interfere with the spray. A disadvantage 
is that if the admixed liquid mixture is not sufficiently electrically 
insulating, electrical current leakage can occur through the admixed 
liquid mixture to a grounded feed supply tank or feed delivery system. 
This reduces the amount of charge going to the spray. If current leakage 
is too high, then the feed supply tank and feed delivery system must be 
insulated from electrical ground, that is, be charged to high voltage. 
Current leakage can be measured by measuring the current flow from the 
high voltage electrical power supply without fluid flow. The current 
charging the spray is then the difference between the current with fluid 
flow and the current without fluid flow. The leakage current should be 
small compared to the charging current. 
In charging method (2) above, the spray is electrically charged after it 
emerges from the orifice or in the vicinity of the orifice. The spray gun 
and spray nozzle must be electrically insulating. The electrical charge is 
supplied from external electrode(s) close to the spray tip and adjacent to 
the spray. Under high electrical voltage, electrical current is discharged 
to the spray. The preferred electrodes are one or more metal wire(s) 
positioned adjacent to the spray. The electrodes may be either parallel to 
the spray or perpendicular to it or any orientation in between such that 
the electrical current issuing from the sharp point is favorably directed 
to the spray. The electrode(s) must be positioned close enough to the 
spray, preferably within one centimeter, to effectively charge the spray 
without interfering with the flow of the spray. The electrodes may be 
sharp pointed and may be branched. For planar sprays, one or more 
electrodes are preferably located to the side(s) of the planar spray so 
that electrical current is discharged to the face(s) of the spray. For 
oval sprays, one or more electrodes are located adjacent to the spray 
around the perimeter. The electrode(s) are located to effectively charge 
the spray. One or more auxiliary electrodes, which may be at a different 
voltage than the primary electrode(s) or electrically grounded, may be 
used to modify the electrical field or current between the primary 
electrode(s) and the spray. For example, a primary charging electrode may 
be on one side of the spray fan and a grounded insulated auxiliary 
electrode may by on the opposite side of the spray fan. Charging method 
(2) has the advantage of less current leakage through the admixed liquid 
mixture than charging method (1). Admixed liquid mixtures that are 
sufficiently conductive must have the feed supply and feed line insulated 
from electrical ground. In charging method (3) above, the spray is 
electrically charged farther away from the orifice and is more fully 
dispersed than in method (2). Therefore a larger system or network of 
external electrodes is usually required in order to effectively charge the 
spray. Therefore the method is less safe and less versatile. Also the 
distance between the electrodes and spray must be greater to avoid 
interfering with the spray. Therefore the charge applied to the spray is 
likely to be lower. But current leakage through the supply lines is 
eliminated. The spray is passed through or between electrified grids or 
arrays of external electrodes before the spray is deposited onto the 
substrate. The spray droplets are charged by ion bombardment from the 
electrical current discharged into air from the electrodes. The 
electrified grid may be one or several wire electrodes that extend across 
the spray area. Current can discharge from along the length of the 
electrodes. The electrified array may be one or several wire or pointed 
electrodes positioned around the spray area and which extend close to or 
into the spray such that current discharges from the ends of the 
electrodes. 
The present invention can be used with high electrical voltage in the range 
of about 30 to about 150 kilovolts. Higher electrical voltages are favored 
to impart higher electrical charge to the spray to enhance attraction to 
the substrate, but the voltage level must be safe for the type of charging 
and spray gun used. For safety reasons, the voltage of hand spray guns is 
usually restricted to less than 70 kilovolts and the equipment is designed 
to automatically shut off the voltage when the current exceeds a safe 
level. Generally for hand spray guns the useful range of electrical 
current is between 20 and 200 microamperes and optimum results are 
obtained with coating formulations that have very low electrical 
conductivity, that is, very high electrical resistance. For automatic 
spray guns that are used remotely, higher voltages and electrical currents 
can be safely used than for hand spray guns. Therefore the voltage can 
exceed 70 kilovolts up to 150 kilovolts and the current can exceed 200 
microamperes. 
These methods of electrostatic charging are known to those who are skilled 
in the art of electrostatic spraying. 
For electrostatic spraying, the substrate is preferably an electrical 
conductor such as metal. But substrates that are not conductors or 
semiconductors can also be sprayed. Preferably they are pretreated to 
create an electrically conducting surface. For instance, the substrate can 
be immersed in a special solution to impart conductivity to the surface. 
The method of generating the high electrical voltage and electrical current 
is not critical to the practice of the current invention. Conventional 
high voltage electrical power supplies can be used. The power supply 
should have standard safety features that prevent current or voltage 
surges. The electrical power supply may be built into the spray gun. Other 
charging methods may also be used. 
The spray pressure used is a function of the precursor coating composition, 
the supercritical fluid being used, and the viscosity of the liquid 
mixture. The minimum spray pressure is at or slightly below the critical 
pressure of the supercritical fluid. Generally, the pressure will be below 
5000 psi. Preferably, the spray pressure is above the critical pressure of 
the supercritical fluid and below 3000 psi. If the supercritical fluid is 
supercritical carbon dioxide fluid, the preferred spray pressure is 
between 1070 psi and 3000 psi. The most preferred spray pressure is 
between 1200 psi and 2500 psi. 
The spray temperature used is a function of the precursor coating 
composition, the supercritical fluid being used, and the concentration of 
supercritical fluid in the liquid mixture. The minimum spray temperature 
is at or slightly below the critical temperature of the supercritical 
fluid. The maximum temperature is the highest temperature at which the 
components of the liquid mixture are not significantly thermally degraded 
during the time that the liquid mixture is at that temperature. 
If the supercritical fluid is supercritical carbon dioxide fluid, because 
the supercritical fluid escaping from the spray nozzle could cool to the 
point of condensing solid carbon dioxide and any ambient water vapor 
present due to high humidity in the surrounding spray environment, the 
spray composition is preferably heated prior to atomization. The minimum 
spray temperature is about 31.degree. centigrade. The maximum temperature 
is determined by the thermal stability of the components in the liquid 
mixture. The preferred spray temperature is between 35.degree. and 
90.degree. centigrade. The most preferred temperature is between 
45.degree. and 75.degree. centigrade. Generally, liquid mixtures with 
greater amounts of supercritical carbon dioxide fluid require higher spray 
temperatures to counteract the greater cooling effect. 
If supercritical carbon dioxide fluid is employed as the supercritical 
fluid diluent, it preferably should be present in amounts ranging from 
about 10 to about 60 weight percent based upon the total weight of the 
admixed coating composition containing the solids fraction, the solvent 
fraction, water and supercritical carbon dioxide, thereby producing a 
mixture having viscosities from about 5 centipoise to about 150 centipoise 
at spray temperature. Most preferably, it is present in amounts ranging 
from about 20 to about 60 weight percent on the same basis, thereby 
producing a mixture having viscosities from about 10 centipoise to about 
50 centipoise at spray temperature. 
The spray temperature may be obtained by heating the liquid mixture before 
it enters the spray gun, by heating the spray gun itself, by circulating 
the heated liquid mixture to or through the spray gun to maintain the 
spray temperature, or by a combination of methods. Circulating the heated 
liquid mixture through the spray gun is preferred, to avoid heat loss and 
to maintain the desired spray temperature. Tubing, piping, hoses, and the 
spray gun are preferably insulated or heat traced to prevent heat loss. 
The environment into which the admixed coating composition is sprayed is 
not critical. However, the pressure therein must be less than that 
required to maintain the supercritical fluid component of the liquid spray 
mixture in the supercritical state. Preferably, the admixed liquid coating 
composition is sprayed in air under conditions at or near atmospheric 
pressure. Other gas environments can also be used, such as air with 
reduced oxygen content or inert gases such as nitrogen, carbon dioxide, 
helium, argon, xenon, or a mixture. Oxygen or oxygen enriched air is not 
desirable, because oxygen enhances the flammability of organic components 
in the spray. 
Generally, liquid spray droplets are produced which generally have an 
average diameter of one micron or greater. Preferably, these droplets have 
average diameters of from about 5 to 1000 microns. More preferably, these 
droplets have average diameters of from about 10 to about 300 microns. 
Small spray droplets are desirable to vent the supercritical fluid from 
the spray droplet before impacting the substrate. Small spray droplets 
also give higher quality finishes. 
The process of the present invention may be used to apply coatings by the 
application of liquid spray to a variety of substrates. Examples of 
suitable substrates include but are not limited to metal, wood, glass, 
plastic, paper, cloth, ceramic, masonry, stone, cement, asphalt, rubber, 
and composite materials. 
Through the practice of the present invention, films may be applied to 
substrates such that the cured films have thicknesses of from about 0.2 to 
about 6.0 mils. Preferably, the films have thicknesses of from about 0.5 
to about 2.0 mils, while most preferably, their thicknesses range from 
about 0.7 to about 1.5 mils. 
If curing of the coating composition present upon the coated substrate is 
required, it may be performed at this point by conventional means, such as 
allowing for evaporation of the active and/or coupling solvent, 
application of heat or ultraviolet light, etc. 
Compressed gas may be utilized to assist formation of the liquid spray 
and/or to modify the shape of the liquid spray that comes from the 
orifice. The assist gas is typically compressed air at pressures from 5 to 
80 psi, with low pressures of 5 to 20 psi preferred, but may also be air 
with reduced oxygen content or inert gases such as compressed nitrogen, 
carbon dioxide, helium, argon, or xenon, or a mixture. Compressed oxygen 
or oxygen enriched air is not desirable because oxygen enhances the 
flammability of the organic components in the spray. The assist gas is 
directed into the liquid spray as one or more high-velocity jets of gas, 
preferably arranged symmetrically on each side of the liquid spray to 
balance each other. The assist gas jets will preferably come from gas 
orifices built into the electrostatic spray tip and/or nozzle. The assist 
gas may also issue from an opening in the spray tip or nozzle that is a 
concentric annular ring that is around and centered on the liquid orifice, 
to produce a hollow-cone high-velocity jet of gas that converges on the 
liquid spray, but this creates a larger flow of assist gas that is not as 
desirable. The concentric annular ring may be divided into segments, to 
reduce gas flow rate, and it may be elliptical instead of circular, to 
shape the spray. Preferably, the flow rate and pressure of the assist gas 
are lower than those used in air spray. The assist gas may be heated to 
counteract the rapid cooling effect of the supercritical fluid diluent in 
the spray. The preferred temperature of heated assist gas ranges from 
about 35.degree. to about 90.degree. centigrade. The most preferred 
temperature ranges from about 45.degree. to about 75.degree. centigrade. 
The precursor coating composition of the present invention may be admixed 
with a supercritical fluid and then sprayed onto a substrate by a spray 
apparatus such as that shown in either FIGS. 3 or 4. 
The following illustrates apparatus that may be used to obtain the admixed 
coating composition of precursor coating composition and supercritical 
fluid and spray it in a batch mode in the practice of the present 
invention. The supercritical fluid illustrated is supercritical carbon 
dioxide fluid. 
Table 3 contains a listing of the equipment used in conducting the 
procedure described for the batch mode. 
TABLE 3 
______________________________________ 
Item # Description 
______________________________________ 
1. Linde bone-dry-grade liquid carbon dioxide 
in size K cylinder with eductor tube. 
2. Refrigeration heat exchanger. 
3. Hoke cylinder #8HD3000, 3.0-liter volume, 
made of 304 stainless steel, having double 
end connectors, 1800-psig pressure rating, 
mounted on scale; carbon dioxide feed tank. 
4. Circle Seal .TM. pressure relief valve 
P168-344-2000 set at 1800 psig. 
5. Vent valve. 
6. Sartorius 16-kilogram scale with 0.1-gram 
sensitivity. 
7. Hoke cylinder #8HD2250, 2.25-liter volume, 
made of 304 stainless steel, having double 
end connectors, 1800-psig pressure rating; 
pump supply tank. 
8. Zenith single-stream gear pump, model 
#HLB-5592-30CC, modified by adding a thin 
Grafoil .TM. gasket to improve metal-to-metal 
seal. 
9. Zenith gear pump drive model #4204157, with 
15:1 gear ratio, and pump speed controller 
#QM-371726F-15-XP, with speed range of 6 to 
120 revolutions per minute. 
10. Drain from circulation loop. 
11. Kenics static mixer. 
12. Cooling water heat exchanger. 
13. Hoke cylinder #8HD2250, 2.25-liter volume, 
made of 304 stainless steel, having double 
end connectors, 1800-psig pressure rating; 
spray supply tank. 
14. Airless spray gun. 
15. Bonderite .TM. 37 polished 24-gauge steel 
panel, 6-inch by 12-inch size. 
16. Vent valve. 
17. Liquid feed valve. 
18. Jerguson high-pressure sight glass series 
T-30 with window size #6 rated for 2260 psig 
pressure at 200.degree. F. temperature. 
19. Grove back-pressure regulator #5-90-W, rated 
for 2000 psig at 200.degree. F. temperature; dome is 
charged with pressurized nitrogen to desired 
spray pressure. 
20. Bypass valve. 
21. Pressurized nitrogen to set Grove 
back-pressure regulator and to purge unit. 
22. Nitrogen purge valve. 
23. Nitrogen vent valve. 
24. Ruska rolling-ball high-pressure viscometer 
#1602-811-00 with temperature controller and 
electronic timer. 
25. Pyncnometer, double-valved one-quarter-inch 
high-pressure tubing. 
______________________________________ 
The apparatus listed in Table 3 above is assembled as shown in the 
schematic representation contained in FIG. 3. Rigid connections are made 
with 1/8-inch diameter high-pressure tubing for gas flows and with 
1/4-inch diameter high-pressure tubing for liquid flows, using 
Swagelok.TM. fittings. The spray gun is connected to the tubing by using 
two Graco flexible 1/4-inch static-free nylon high-pressure hoses model 
#061-214 with 5000-psi pressure rating. Check valves are used to prevent 
back flow to the carbon dioxide feed tank (3) and bulk supply tank (1) and 
to the nitrogen cylinder (21). The circulation loop and carbon dioxide 
feed tank are protected from overpressurization by pressure relief valves 
(4). 
The apparatus consists of a circulation loop, a carbon dioxide feed system, 
and a side loop to measure viscosity and density. The circulation loop 
contains a pump supply tank (7), a gear pump (8) to provide circulation 
and maintain constant spray pressure, a static mixer (11), a cooler (12) 
to remove excess heat, a spray supply tank (13), an airless spray gun 
(14), a sight glass (18), and a pressure regulator (19) to maintain 
constant spray pressure. The pressure regulator (19) is set by using 
compressed nitrogen (21) regulated to the desired flow pressure. The 
carbon dioxide feed system contains a carbon dioxide bulk supply cylinder 
(1), a refrigeration heat exchanger (2), and a carbon dioxide feed tank 
(3) mounted on an electronic scale (6). The feed and exit lines to the 
feed tank (3) are coiled so that the force of the tank moving on the scale 
does not affect the scale reading. The side loop contains a viscometer 
(24) and pyncnometer (25) for measuring the viscosity and density of the 
admixed coating composition, if desired. 
All flow lines and tanks are lined with electrical heat tape and covered 
with insulation to heat the solution to spray temperature. The heat tape 
is divided into several circuits that are controlled independently: 
Circuit #1 Pressure regulator (19), bypass line (20), sight glass (18), and 
connecting lines. 
Circuit #2 Pump supply tank (7), gear pump (8), and line in circulation 
loop to carbon dioxide feed point. 
Circuit #3 Line in circulation loop from carbon dioxide feed point to 
cooler (12). 
Circuit #4 Spray supply tank (13). 
Circuit #5 Line from spray supply tank (13) to flexible hose connected to 
spray gun (14). 
Circuit #6 Carbon dioxide feed tank (3). 
Thermocouples located within the tanks and lines measure temperature. 
Admixed coating composition temperature is kept uniform around the loop by 
rapid circulation and by adjusting the heat tapes. 
The batch spray unit is filled by the following procedure. The unit is 
evacuated through the circulation loop vent (16) and a weighed amount of 
precursor coating composition is added through the feed valve (17) with 
the gear pump (8) circulating the material at a slow rate through the 
pressure regulator bypass valve (20). The carbon dioxide feed tank (3) is 
evacuated through the vent valve (5) and filled with liquid carbon dioxide 
from the carbon dioxide supply cylinder (1). To make filling the feed tank 
(3) easier, the carbon dioxide is passed through a refrigeration heat 
exchanger (2), so that the vapor pressure in the feed tank (3) is lower 
than the vapor pressure in the supply tank (1). The desired mass of carbon 
dioxide is pressurized into the circulation loop by heating the carbon 
dioxide feed tank (3) and valving in the desired amount as read on the 
balance (6). 
The spray pressure is generated by filling the unit with precursor coating 
composition and carbon dioxide to the required overall density and then 
heating it to the spray temperature. Prior to spraying, the pressure 
regulator (19) is bypassed (20) and the loop is at a uniform pressure. To 
prepare for spraying, the bypass (20) is closed so that the flow goes 
through the pressure regulator (19), which is adjusted to the flow 
pressure. During spraying, the spray pressure is kept constant by the gear 
pump (8) and the pressure regulator (19). The gear pump (8) pumps solution 
into the spray supply tank (13) from the pump supply tank (7) at a high 
circulation rate. The pressure regulator (19) discharges excess solution 
back into the pump supply tank (7). The pump supply tank (7) loses 
inventory and pressure, but the spray supply tank (13) is kept full and at 
spray pressure. 
The following illustrates apparatus that may be used to obtain the admixed 
coating composition of precursor coating composition and supercritical 
fluid and spray it in a continuous mode in the practice of the present 
invention. The supercritical fluid illustrated is supercritical carbon 
dioxide fluid. 
Table 4 contains a listing of the equipment used in conducting the 
procedure described for the continuous mode. 
TABLE 4 
______________________________________ 
Item # Description 
______________________________________ 
1. Linde bone-dry-grade liquid carbon dioxide 
in size K cylinder with eductor tube. 
2. Refrigeration heat exchanger. 
3. Hoke cylinder #8HD3000, 3.0-liter volume, 
made of 304 stainless steel, having double 
end connectors, 1800-psig pressure rating. 
4. Circle Seal .TM. pressure relief valve 
P168-344-2000 set at 1800 psig. 
5. Vent valve. 
6. Nitrogen gas supply. 
7. Graco double-acting piston pump model 
#947-963 with 4-ball design and Teflon .TM. 
packings mounted in #5 Hydra-Cat Cylinder 
Slave Kit #947-943; pump and feed line are 
refrigeration traced; carbon dioxide pump. 
8. Graco standard double-acting primary piston 
pump model #207-865 with Teflon .TM. packings; 
coating concentrate pump. 
9. Graco Variable Ratio Hydra-Cat .TM. 
Proportioning Pump unit model #226-936 with 
0.9:1 to 4.5:1 ratio range. 
10. Graco President air motor model #207-352. 
11. Utility compressed air at 95 psig supply 
pressure. 
12. Graco air filter model #106-149. 
13. Graco air pressure regulator model #206-197. 
14. Graco air line oiler model #214-848. 
15. Graco pressure relief valve model #208-317 
set at 3000 psig. 
16. Graco pressure relief valve model #208-317 
set at 3000 psig. 
17. Graco two-gallon pressure tank model 
#214-833. 
18. Graco air pressure regulator model #171-937. 
19. Graco pressure relief valve model #103-437 
set at 100 psig. 
20. Graco high-pressure fluid heater model 
#226-816. 
21. Graco high-pressure fluid filter model 
#218-029. 
22. Graco check valve model #214-037 with 
Teflon .TM. seal. 
23. Graco check valve model #214-037 with 
Teflon .TM. seal. 
24. Graco static mixer model #500-639. 
25. Graco high-pressure fluid heater model 
#226-816. 
26. Graco high-pressure fluid filter model 
#218-029. 
27. Kenics static mixer. 
28. Graco fluid pressure regulator model 
#206-661. 
29. Jerguson high-pressure sight glass series 
T-30 
with window size #6 rated for 2260 psig 
presure at 200.degree. F. temperature. 
30. Airless spray gun. 
31. Bonderite .TM. 37 polished 24-gauge steel 
panel, 6-inch by 12-inch size. 
32. Zenith single-stream gear pump, model 
#HLB-5592-30C, modified by adding a thin 
Grafoil .TM. gasket to improve metal-to-metal 
seal, with pump drive model #4204157, 
with 15:1 gear ratio, and pump speed 
controller model #QM-371726F-15-XP, with 
speed range of 6 to 120 revolutions per 
minute. 
33. Circle Seal .TM. pressure relief valve 
P168-344-2000 set at 2000 psig. 
34. Drain from circulation loop. 
______________________________________ 
The apparatus listed in Table 4 above is assembled as shown in the 
schematic representation contained in FIG. 4. Rigid connections were made 
with Dekuron 1/4-inch diameter, 0.036-inch thick, seamless, welded, type 
304 stainless steel hydraulic tubing ASTM A-269 with 5000-psi pressure 
rating, using Swagelok.TM. fittings. The pressure tank (17) is connected 
to the pump (8) using a Graco 3/8-inch static-free nylon high-pressure 
hose model #061-221 with 3000-psi pressure rating. All other flexible 
connections are made using Graco 1/4-inch static-free nylon high-pressure 
hoses model #061-214 with 5000-psi pressure rating. 
The precursor coating composition and carbon dioxide are pumped and 
proportioned by using a Graco Variable Ratio Hydra-Cat.TM. Proportioning 
Pump unit (9). It proportions two fluids together at a given volume ratio 
by using two piston pumps (7 and 8) that are slaved together. The piston 
rods for each pump are attached to opposite ends of a shaft that pivots up 
and down on a center fulcrum. The volume ratio is varied by sliding pump 
(7) along the shaft, which changes the stroke length. The pumps are driven 
on demand by an air motor (10). Pumping pressure is controlled by the air 
pressure that drives the air motor. The pumps are double-acting; they pump 
on upstroke and downstroke. The primary pump (8) is used to pump the 
precursor coating composition. It is of standard design, having one inlet 
and one outlet. It fills through a check valve at the bottom and 
discharges through a check valve at the top. A third check valve is 
located in the piston head, which allows liquid to flow from the bottom 
compartment to the top compartment when the piston is moving downward. 
This type of pump is designed to be used with low feed pressure, typically 
below 100 psi. The precursor coating composition is supplied to the 
primary pump (8) from a two-gallon pressure tank (17). After being 
pressurized in the pump to spray pressure, the precursor coating 
composition is then heated in an electric heater (20) to reduce its 
viscosity (to aid mixing with carbon dioxide), filtered in a fluid filter 
(21) to remove particulates, and fed through a check valve (22) into the 
mix point with carbon dioxide. The secondary pump (7) on the proportioning 
pump unit (9) is used to pump the liquid carbon dioxide. A double-acting 
piston pump (7) with a four-check-valve design is used because of the high 
vapor pressure of carbon dioxide. The pump has an inlet and an outlet on 
each side of the piston; no flow occurs through the piston. The proportion 
of carbon dioxide pumped into the admixed coating composition is varied by 
moving the secondary pump (7) along the moving shaft. Bone-dry-grade 
liquid carbon dioxide is pumped from cylinder (1) through refrigeration 
heat exchanger (2) to secondary pump (7). For measuring the carbon dioxide 
uptake rate, the carbon dioxide is pumped from Hoke cylinder (3) through 
heat exchanger (2) to pump (7). The liquid carbon dioxide is refrigerated 
in heat exchanger (2) in order to lower the vapor pressure, to prevent 
cavitation in pump (7). The Hoke cylinder (3) is filled from cylinder (1). 
Air or gaseous carbon dioxide in the cylinder (3) is vented (5) during 
filling. The Hoke cylinder (3) is mounted on a 16-kilogram Sartorius 
electronic scale with 0.1-gram sensitivity so that the amount of carbon 
dioxide in it can be weighed. After being pressurized to spray pressure in 
pump (7), the liquid carbon dioxide is fed unheated through check valve 
(23) to the mix point with the precursor coating composition. After the 
precursor coating composition and carbon dioxide are proportioned together 
at the mix point, the admixed coating composition is mixed in static mixer 
(24) and pumped on demand into a circulation loop, which circulates the 
admixed coating composition at spray pressure and temperature to or 
through the spray gun (30). The admixed coating composition is heated in 
an electric heater (25) to obtain the desired spray temperature and 
filtered in a fluid filter (26) to remove particulates. Fluid pressure 
regulator (28) is installed to lower the spray pressure below the pump 
pressure, if desired, or to help maintain a constant spray pressure. A 
Jerguson sight glass (29) is used to examine the phase condition of the 
admixed coating composition. Circulation flow in the circulation loop is 
obtained through the use of gear pump (32). 
The pressure tank (17) is filled with the precursor coating concentrate and 
pressurized with air to 50 psig. The primary pump (8) is primed by opening 
a drain valve on the bottom of filter (21) until air was purged from the 
line. 
The carbon dioxide secondary pump (7) is positioned along the pivoting 
shaft to give the desired percentage of maximum piston displacement. The 
refrigeration flow is adjusted to a temperature of -10.degree. C. and 
circulated through the refrigeration heat exchanger (2) and the 
refrigeration tracing on pump (7). The carbon dioxide feed line and 
circulation loop are filled with carbon dioxide and vented through valve 
(34) several times to purge air from the system. Then the valves to the 
mixing point are closed and the carbon dioxide feed line is filled to 
prime pump (7). 
The air pressure regulator (13) is adjusted to supply the air motor (10) 
with air at the desired pressure to pressurize the feed lines. The valves 
to the mix point are opened and the circulation loop filled with material. 
With the circulation loop return valve closed, to give plug flow around 
the circulation loop with no backmixing, material is drained from valve 
(34) until a uniform composition is obtained. Heater (20) is adjusted to 
give a feed temperature of 37.degree. C. The circulation heater (25) is 
adjusted to give the spray temperature. The circulation loop return valve 
is opened and the spray mixture is circulated at a high rate by adjusting 
the gear pump (32). The carbon dioxide content of the admixed coating 
composition is measured by measuring the carbon dioxide uptake rate from 
Hoke cylinder (3) and the precursor coating composition uptake rate from 
pressure tank (17) while spraying through the spray gun. Then the carbon 
dioxide feed is switched back to supply cylinder (1). 
An alternative method of proportioning the precursor coating composition 
and supercritical fluid in a continuous mode uses a mass proportionation 
apparatus instead of the volumetric proportionation apparatus illustrated 
above. The variable ratio proportioning pump unit (9) with pumps (7) and 
(8) shown in FIG. 4 is replaced with an apparatus having the following 
elements. For pumping the carbon dioxide, the double-acting four-ball 
piston pump (7) is driven individually on demand by attaching air motor 
(10) directly to it instead of being driven by the moving beam. 
Alternatively, the carbon dioxide can be pumped by using an air-driven 
cryogenic pump such as Haskel model DSF-35, which is a single-acting pump 
that utilizes a three-way cycling spool that is designed for pumping 
liquefied gases under pressure without requiring refrigeration to avoid 
cavitation. The pressurized carbon dioxide is then passed through a 
pressure regulator, which is used to control the desired spray pressure, 
and then through a mass-flow meter, such as Micro Motion model D6, which 
measures the flow rate of carbon dioxide as it is pumped on demand. For 
pumping the precursor coating composition, the standard double-acting 
primary piston pump (8) is replaced with a variable speed gear pump, such 
as the Zenith gear pump (32) that is used in the circulation loop. The 
gear pump pumping rate is controlled by a signal processor that receives 
the instantaneous carbon dioxide flow rate from the mass flow meter and 
then controls the gear pump revolution rate to pump the precursor coating 
composition at the proper flow rate to give the desired proportion of 
precursor coating composition and carbon dioxide in the admixed coating 
composition. An accumulator, such as Tobul model 4.7A30-4, may be 
installed in the circulation loop to increase the loop capacity and to 
minimize pressure pulsations in the loop when the spray gun is activated. 
EXAMPLES 
Examples 1-3 
In Examples 1-3, the organic solvent of a precursor coating composition 
containing a water-reducible alkyd as the polymeric coating component is 
progressively replaced with larger amounts of water to determine the 
amount of supercritical carbon dioxide that can be added to each of these 
compositions and to demonstrate that this amount, even after water 
addition, remains substantially the same. 
EXAMPLE 1 
As a control case containing no water, 135.2 grams of a water-reducible 
tall oil fatty acid alkyd resin (Cargill 7451, manufactured by Cargill, 
Inc. which is supplied as a 70% solution in butoxy ethanol) containing 
94.6 grams of polymer and 40.6 grams of butoxy ethanol; an additional 10.8 
grams of butoxy ethanol is added to increase the coupling solvent used in 
the composition (total butoxy ethanol coupling solvent=51.4 grams); and 
16.8 grams of Cymel.RTM. 303, a cross-linker manufactured by American 
Cyanamid Co. are admixed with supercritical carbon dioxide over a pressure 
range of 1,100 psi to 2,600 psi and a temperature range of 35.degree. C. 
to 55.degree. C. 
The maximum amount of supercritical carbon dioxide that can be added to 
this "dry" admixture and still provide a clear, single phase is 27% by 
weight based on the total weight of all of the components of the 
admixture. 
EXAMPLE 2 
In this example, 10% by weight of the organic solvent present in the 
precursor composition of Example 1 is replaced with water. 
Accordingly, Example 1 is repeated with the exception that 5.0 grams of 
distilled water is substituted for 5.0 grams of butoxy ethanol. 
The maximum amount of supercritical carbon dioxide that can now be added 
and still provide a clear, single phase is 26% by weight based on the 
total weight of all of the components of the admixture. 
EXAMPLE 3 
In this example, 20% by weight of the organic solvent present in the 
precursor composition of Example 1 is replaced with water. 
Accordingly, Example 1 is repeated with the exception that 10.0 grams of 
distilled water is substituted for 10.0 grams of butoxy ethanol. 
The maximum amount of supercritical carbon dioxide that the solution can 
tolerate and avoid phase separation due to saturation with supercritical 
carbon dioxide is 27% by weight based on the total weight of all 
components. Unlike Examples 1 and 2, substantial clouding of the solution 
develops above 12% by weight of supercritical carbon dioxide indicating a 
phase separation beginning to occur. However, the onset of such clouding 
does not hinder the spraying performance of this composition. 
Examples 4-7 
In Examples 4-7, the effects of (1) replacing organic solvent with water, 
and (2) adding water to the solvent fraction existing in a precursor 
composition containing a water-reducible polyester as the polymeric 
coating component are demonstrated. 
EXAMPLE 4 
As a control case containing no water, 100.0 grams of a water-reducible oil 
free polyester resin (Cargill 7203, manufactured by Cargill, inc.. which 
is supplied as a 75% solution in 2-butanol:butoxy ethanol in a ratio of 
2.97:1) containing 75.0 grams of polymer, 18.7 grams of 2-butanol and 6.3 
grams of butoxy ethanol; an additional 25.0 grams of butoxy ethanol is 
added to increase the coupling solvent used in the composition (total 
butoxy ethanol coupling solvent=31.3 grams); and 25.0 grams of Cymel.RTM. 
303 are admixed with supercritical carbon dioxide over the same 
temperature and pressure ranges set forth in Example 1. 
The maximum amount of supercritical carbon dioxide that the solution can 
tolerate, remain clear, and avoid phase separation due to saturation with 
supercritical carbon dioxide is 31 wt % based on the total weight of all 
components. 
EXAMPLE 5 
As yet another control case containing no water, Example 4 is repeated with 
the exception that the total amount of butoxy ethanol is reduced by 5.0 
grams. The maximum amount of supercritical carbon dioxide that the 
solution can tolerate, remain clear, and avoid phase separation due to 
saturation with supercritical carbon dioxide is 30% by weight based on 
total weight of all components. 
EXAMPLE 6 
In this example, 10% by weight of the organic solvent present in the 
precursor composition of Example 4 is replaced with water, which example 
is also equivalent to adding 11% by weight of water based on the total 
weight of organic solvent to the precursor composition of Example 5. 
Accordingly, Example 4 is repeated with the exception that 5.0 grams of 
distilled water is substituted for 5.0 grams of butoxy ethanol. Example 5 
is also repeated except that 5.0 grams of water is added to the 
composition. 
In each instance, the maximum amount of supercritical carbon dioxide that 
the solution can tolerate, remain clear, and avoid phase separation due to 
saturation with supercritical carbon dioxide is 29% by weight based on the 
total weight of all of the components. 
In the example, 20% by weight of the organic solvent present in the 
precursor composition of Example 4 is replaced with water. 
Accordingly, Example 4 is repeated with the exception that 10.0 grams of 
distilled water is substituted for 10.0 grams of butoxy ethanol. The 
maximum amount of supercritical carbon dioxide that the solution can 
tolerate, remain clear, and avoid phase separation due to saturation with 
supercritical carbon dioxide is 25%. Unlike Examples 4-6, substantial 
clouding of the solution develops above 15% by weight of supercritical 
carbon dioxide indicating phase separation beginning to occur. However, 
the onset of such clouding does not hinder the spraying performance of 
this composition. 
Examples 8-11 
In Examples 8-11, the organic solvent of a precursor coating composition 
containing a conventional polyester resin as the polymeric component is 
progressively replaced with larger amounts of water to determine the 
amount of supercritical carbon dioxide that can be added to each of these 
compositions and to demonstrate that this amount, even after water 
addition, remains substantially the same. 
EXAMPLE 8 
As a control case containing no water, 75.0 grams of an oil free polyester 
resin (Cargill 5780, manufactured by Cargill, Inc. in solvent free form); 
50.0 grams of butoxy ethanol; and 25.0 grams of Cymel.RTM. 303 are admixed 
with supercritical carbon dioxide over the same temperature and pressure 
ranges set forth in Example 1. 
The maximum amount of supercritical carbon dioxide that the solution can 
tolerate, remain clear, and avoid phase separation due to saturation with 
supercritical carbon dioxide is 32 wt % based on the total weight of all 
components. 
EXAMPLE 9 
In this example, 10% by weight of the organic solvent present in the 
precursor composition of Example 8 is replaced with water. 
Accordingly, Example 8 is repeated with the exception that 5.0 grams of 
distilled water is substituted for 5.0 grams of butoxy ethanol. 
The maximum amount of supercritical carbon dioxide that can now be added 
and still provide a clear, single phase is 32% by weight based on the 
total weight of all of the components of the admixture. 
EXAMPLE 10 
In this example, 20% by weight of the organic solvent present in the 
precursor composition of Example 8 is replaced with water. 
Accordingly, Example 8 is repeated with the exception that 10.0 grams of 
distilled water is substituted for 10.0 grams of butoxy ethanol. 
The maximum amount of supercritical carbon dioxide that the solution can 
tolerate, remain clear, and avoid phase separation due to saturation with 
supercritical carbon dioxide is 28 wt % based on the total weight of all 
components. Moderate clouding of the solution occurs above 23% by weight 
of supercritical carbon dioxide. This indicates that a very small amount 
of phase separation is occurring. 
EXAMPLE 11 
In this example, 30% by weight of the organic solvent present in the 
precursor composition of Example 8 is replaced with water. 
Accordingly, Example 8 is repeated with the exception that 15.0 grams of 
distilled water is substituted for 15.0 grams of butoxy ethanol. 
The maximum amount of supercritical carbon dioxide that the solution can 
tolerate and avoid phase separation due to saturation with supercritical 
carbon dioxide is 26% by weight based on the total weight of all 
components. Clouding of the solution develops upon the first addition of 
supercritical carbon dioxide indicating that an undesirable phase 
separation is occurring. 
Examples 12-14 
Examples 12-14 illustrate the desirability of using a coupling solvent 
(butoxy ethanol) in the precursor composition when water is present. 
EXAMPLE 12 
This example shows the effect of replacing 25 wt % of the coupling solvent 
in Example 10 with a non-coupling active solvent, ethyl 
3-ethoxypropionate. 
Example 10 is repeated with the exception that 10.0 grams of ethyl 
3-ethoxypropionate is substituted for 10.0 grams of butoxy ethanol. The 
maximum amount of supercritical carbon dioxide that the solution can 
tolerate and avoid phase separation due to saturation with supercritical 
carbon dioxide is 28% by weight based on the total weight of all 
components. Substantial clouding of the solution develops above 26% by 
weight of supercritical carbon dioxide indicating that phase separation is 
beginning to occur. 
EXAMPLE 13 
This example shows the effect of replacing 50 wt % of the coupling solvent 
in Example 10 with a non-coupling active solvent, ethyl 
3-ethoxypropionate. 
Example 10 is repeated with the exception that 20.0 grams of ethyl 
3-ethoxypropionate is substituted for 20.0 grams of butoxy ethanol. The 
maximum amount of supercritical carbon dioxide that the solution can 
tolerate and avoid phase separation due to saturation with supercritical 
carbon dioxide is 25% by weight based on the total weight of all 
components. Substantial clouding of the solution develops upon the first 
addition of supercritical carbon dioxide indicating an undesirable phase 
separation is occurring. 
EXAMPLE 14 
This example shows the effect of replacing all of the coupling solvent in 
Example 10 with a non-coupling active solvent, ethyl 3-ethoxypropionate. 
Example 10 is repeated with the exception that 40.0 grams of ethyl 
3-ethoxypropionate is substituted for all 40.0 grams of butoxy ethanol. A 
two phase mixture forms before any supercritical carbon dioxide is added. 
It is known that ethyl -ethoxypropionate unlike butoxy ethanol is 
immiscible in water; however, it is a solvent structurally related to 
2-ethoxyethyl acetate which has significant water solubility. Thus ethyl 
3-ethoxypropionate is polar enough to be close to the boundary separating 
water miscible and immiscible solvents. The maximum amount of 
supercritical carbon dioxide that the two phase mixture can tolerate and 
avoid additional phase separation due to saturation with supercritical 
carbon dioxide is 26% by weight based on the weight of all components. 
The above examples show that despite the low water solubility of carbon 
dioxide, it is possible to replace some of the organic solvent with water 
in coatings formulations using both water-reducible and conventional 
resins and obtain mixtures which are suitable for admixture with 
supercritical fluids, such as supercritical carbon dioxide. 
Both advantages of water addition are feasible. Water can be added to a 
coating formulation to reduce the viscosity while maintaining the amount 
of organic solvent constant (e.g., Examples 4 and 5). Moreover, water can 
also be substituted for organic solvent to lower the amount of overall 
solvent present in a coating while still maintaining the viscosity level 
(e.g., Examples 4 and 6). 
A sufficient amount of a coupling solvent is desired to couple the water 
into the organic coating, but once there is a sufficient amount of such 
coupling solvent, the water can be used to further dilute the coating for 
viscosity reduction to enable spraying.