CA2111794C - Method for spraying polymeric compositions with reduced solvent emission and enhanced atomization - Google Patents

Abstract

Methods are presented by which polymeric compositions, such as coating compositions, can be sprayed with compressed fluids, such as carbon dioxide, nitrous oxide, or ethane, at higher solids levels and with finer atomization to give improved spray application quality with reduced emission of solvent.

Description

~ .

OD FOR SPRAYING POLYMFRTC COMPOSITIONS WITu u~u SOLV~T EMISSION AND ENHAN~Fn ATOMIZATION

FIELD OF THE lhv~h,lON
This invention, in gen2,al, pertains to the field of spraying polymeric ~ _ ositions with re~ce~
emission of volatile organic 601vent. More particularly, the present invention i~ directed to methods for spraying polymeric ~ sitions using 6upercritical fluids or subcritical ~ ~essed fluids, such as carbon dioxide, under conditions that give enh~nce~ atomization.

BACKGROUND OF THE lhV~ ~lON
Many industrial ~ocesses ~pray compositions that contain viccous or solid polymeric ~ _nents, such as coatings, adhesives, release agents, additives, gel coats, lubricants, and agricultural materials. To spray such materials, it has been c- ~n practice to use relatively large r -un~s of organic solvents. The solvents perform a variety of functions, such as to dissolve the polymers; to reduce viscosity for spraying; to provide a carrier medium for dispersions; and to give proper flow when the composition is sprayed onto a substrate, such as coal~scence and leveling to form a smooth coherent coating film. However, the solvents released by the ~pray operation are a major source of air pollution.
There are several patents which disclose new spray technology that can markedly reduce organic solvent e~issions, by using envi~or -ntally acceptable supercritical fluids or ~ubcritical compressed fluids, such as carbon dioxide, to replace .
. .

the ~olvent fraction in solvent-borne c~ a~itions that i~ neede~ to obtain low spray viscosity: U.S.
Patent Nos. 4,923,720 and S,108,799 disclose methods for using supercritical fluids for the spray application of coatings. U.S. Patent No. 5,106,650 discloses methods for using supercritical carbon dioxide for the elec~los~atic spray application of coatings. U.S. Patent No. 5,009,367 discloses methods for using supercritical fluids for obtaining wider airless sprays. U.S. Patent No. 5,057,342 discloses methods for using supercritical fluids for obtaining feathered airless sprays. U.S. Patent No.
4,882,107 discloses ~ethods for using supercritical fluids to apply mold release agents, such as in the production of polyurethane foam. U.S. Patent No.
5,066,522 discloses methods for using supercritical fluids to apply adhesive coatings.
Smith, in U.S. Patent No. 4,582,731, issued April 15, 1986; U.S. Patent No. 4,734,227, issued March 29, l9B8; and U.S. Patent 4,734,451, issued March 29, 1988; disclosee methods for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in ~pe~,itical fluid solvents, which may contain organic solvents. The concer,t~ation of 6aid solutes are described as being quite dilute; on the order of 0.1 percent by weight. In conventional spray applications, the solute conc~ntration is normally 50 times or more greater than this level.
The molecular sprays disclosed in the Smith patents are defined as a spray "of individual molecules (atoms) or very small cluster of solute"
which are in the order of about 30 Angstroms in '~;
, .
i~. .
;.~.
;
,.~
~":

: ,:': , - : .

:

2ill794 D-16941 3 ~ ~ ;
~; r ~ ~er. These ndroplets~ ~re more than 10~ to 109 less massive than the droplets formed in con~ ional --methods that Smith refers to as "liguid spray"
applications.
The conventional atomization mechanism of airless sprays is well known and is discucse~ and illustrated by Dombroski, N., and Johns, W. R., Chemical Enaineerinq Science 18: 203, 1963. The coating exits the orifice as a liguid film that becomes unstable from ~hear induced by its h$gh velocity relative to the s~oun~;ng air. Waves grow in the liquid film, ~ec_ - unstable, and break up into liquid filaments that likewise bes~ - unstable and break up into droplets. Atomization oc~u~s because cohesion and surface tension forces, which hold the liquid together, are overcome by shear and fluid inertia forces, which break it apart. However, visco~ dissipation markedly reduces atomization energy, so relatively coarse atomization typically results. Liquid-film sprays are angular in shape and have a fan width that is about the fan width rating of the spray tip. They characteristically form a ~tailing" or "fishtail" spray pattern, wherein coating material is distributed unevenly in the spray. Surface tension often gathers more liquid at the edges of the spray fan than in the center, which can produce coarsely atomized ~ets of coating that sometimes separate from the spray. As used herein, the phrases "liguid-Silm atomization" and ~liquid-film spray" are understood to mean a ~pray, ~pray fan, or spray pattern in which atomization oc~u s by this conventional mechanism.
As disclosed in the aforementioned related ~;~

- ~-, ,, : . .. . -.: ~ , - , . . . .
rf~; " . ., . ~ ' ,, ~,. , ', - :
.,:: . . .. .
:~ ~ .. . . . .. . . .. . . .

-. ~ .

patents, ~u~e, CL itical fluids or subcritical c- ~egsed fluids such as carbon dioxide are not only effective viscosity reducers, they can produce a new airless ~pray atomization --~Anism, which can produce finer droplet size than by conventional airless spray methods and a feathered spray needed to apply high quality coatings. Without wishing to be bound by theory, the new type of atomization is believed to be produced by the dissolved carbon dioxide suddenly becoming ~cee~ingly supersaturated as the spray mixture enters the spray orifice and experiences a sudden and large drop in pressure.
This creates a very large driving force for gasification of the carbon dioxide, which overwhelms the cohesion, surface tension, and viscosity forces that oppose atomization and normally bind the fluid flow together.
A different atomization ~~hAnism is evident because atomization appears to occur right at the spray orifice instead of away from it. Atomization is believed to be due not to break-up of a liquid film from shear with the SUL L ounding air but, instead, to the force of the expandin~ carbon dioxide gas. Therefore, no liquid film is visible coming out Y~ of the nozzle. Furthermore, because the spray is no longer bound by cohesion and surface tension forces, ' it leaves the nozzle at a much wider angle than r normal airless sprays and produc~s a "feathered~
6pray with tapered edges like an air spray. This produces a ~Onl 'f ~ parabolic-shi~ped spray fan instead of the sharp angular fans typical of conventional airless sprays. The spray also typically has a much wider fan width than ' .

.

"." ,, ! . , , ' ' ' ' ' ' ~ ' " ' " ' '; ' ' ' ' ' ' ..:

conventional airless sprays produced by the same spray tip. As used herein, the phrases ~de~ essive atomization" and "de~: ,essive spray"
are understood to mean to a ~pray, spray fan, or spray pattern that has the prece~i ng characteristics.
Generally, the preferred upper limit of ~upercritical fluid addition i8 that which i6 capable of being miscible with the polymeric coating composition. This practical upper limit is generally ~ecoy..izable when the admixture containing coating ~ -~ition and supercritical fluid breaks down from one phase into two fluid phafies. To better understand this phenomenon, reference is made to the phase diagram in Figure 1, wherein the supercritical fluid is carbon dioxide. The vertices of the triangular diagram represent the pure c- ~-nents of a coating formulation admixed with carbon dioxide, which for the ~ ose of this ~isc~sion contains no water. Vertex A is solvent, vertex B is carbon dioxide, and vertex C represents a polymeric material. In this diagram, the polymer and the solvent are completely miscible in all proportions and the carbon dioxide and the solvent are likewise completely miscible in all portions, but the carbon dioxide and the polymer are not miscible in any portion, because the carbon diOxidê is a non-solvent for the polymer. The curved line BFC represents the phase boundary between one phase and two ph~ses. The point D ,eplesents a possible coating composition to which carbon dioxide has not been added. The point E
ep,esents a possible composition of a coating formulation admixture after addition of supercritical carbon dioxide. The added supercritical carbon -:

'''3~

.. , , : ' .

21117~

dioxide is fully dissolved and has re~uced the viscosity of the v;~co~ coating i- ~sition to a range where it can be readily atomized by passing it through an orifice 6uch as in an airless ~pray gun.
After atomization, the carbon dioxide vaporizes, leaving 6ubstantially the c- _sition of the original visco~c coating CD~,OSition. Upon contacting the 6ubstrate, the liquid mixture of polymer and solvent coalesces to produce a smooth coating film on the substrate. The film forming pathway is illustrated in Figure 1 by the line 6-, ~nts EE'D ~atomization and decompression) and DC (coalescence and film formation).
Although the supercritical fluid spray methods have been successful, one difficult problem that is created is that the reformulated polymeric ~ sition, which is called a concentrate, has increasingly higher viscosity as higher levels of 601vent are removed to further reduce solvent emissions. 'Concentrate viscosities typically increase from a conventional viscosity of about 100 centipoise to about 800 to 5000 centipoise or higher as more solvent is ,~ ,ved. Therefore, obtaining fine atomization bes es increasingly more difficult.
This limits the amount of ~olvent that can be ~ -ied and hence the solids level that can be used in the CQnCentrate. The poo~e, atomization gives pGo,eL
spray application quality such as poorer coatings.
Therefore a need clearly exists for methods by which atomization can be enhAnce~ when using &~pe,~,itical fluids or subcritical ~ p:essed fluids to ~pray polymeric compositions in order to reach higher solids levels and to obtain finer atomization to ;'. ., ~ . ' ' ' ' ' , ' '' ' ' ~ . . ' ' . 1,,: . . ' . ', ~ 2111794 obtain imp~oved spray application quality.

SUMMARY OF THE l NVh~ ~lON
By virtue of the present invention, methods have been discove.~d that are in~ee~ able to accomplish the above noted objectives. Polymeric c~. ~sitions can be sprayed with supe,~itical or 6ubcritical L~ _- essed fluids such as carbon dioxide, nitrous oxide, or ethane at higher solids levels and with finer atomization to give i ~oved spray application quality with reduced emission of solvent.
'In its broadest embodiment, the present invention is directed to a process for 6praying a ;~
polymeric c ~sition to form a ~pray of finely atomized liquid droplets, which comprises~
~-(1) forming a liquid mixture at temperature T~
'in a closed system, said mixture comprising:
(a) a nonvolatile materials fraction containing at least one polymeric CQ' ~Lnd and which is capable of being sprayed; and (b) a solvent fraction which is at least partially miscible with the nonvolatile materials fraction and contains at least one compressed fluid in an amount which when added to (a) is sufficient:
(i) to render the viscosity of said mixture to a point suitable for being sprayed; and ;~(ii) to enable said liguid mixture to form a liquid compressed fluid phase at temperature T~;
wherein the compressed fluid is a gas at standard conditions of 0~~ and one '' , ~ .

;

21117~

at -~phere pressure (STP); and (2) spraying said liguid mixture by passing the mixture at temperature ~ and spray pressure P~ into an orifice through which said mixture flows to form a liquid 6pray, wherein spray pressure Pl is above the minimum pressure P2 at which said liquid mixture forms a liquid compressed fluid phase at temperature r.
In a preferred embodiment, the spray pressure P~
is above or just below the maximum pressure P3 at which said mixture forms a liquid c_ iessed fluid . phase at temperature r.
In another preferred _ ho~ nt, the solvent :
fraction additionally contains at least one active solvent for the polymeric compound.
In yet another preferred embodiment, the ~~ _essed fluid is a supercritical fluid at temperature ~ and spray pressure P~.
In still another preferred . ~o~i ?nt, the compressed fluid is carbon dioxide, nitrous oxide, ethane, or a mixture thereof.
In another embodiment, the present invention is directed to a process for the spray application of polymeric coating compositions to a substrate, which ' comprises:
i (1) forming a liquid mixture at temperature r in a closed system, said mixture comprising:
(a) a nonvolatile materials fraction containing at least one polymeric __ ~ound capable of forming a coatinq on a substrate; and (b) a solvent fraction which is at least partially miscible with the nonvolatile materials fraction and contains at least one ~ essed fluid in an r ~U~ which when added to (a) is ~ufficient:
(i) to render the viscosity of said - -mixture to a point suitable for being sprayed; and (ii) to enable said liquid mixture to form a liquid compressed fluid phase at temperature r;-wherein the c Lessed fluid is a gas at standard conditions of o~C and one atmosphere pressure (STP); and -(2) spraying said liguid mixture onto a substrate to form a coating thereon by passing the mixture at temperature r and spray pressure P~ into an orifice through which said mixture flows to form a liquid spray, wherein spray pressure Pl is above the ini - pressure P2 at which said liquid mixture forms a liquid compressed fluid phase at temperature Here again, in a preferred embodiment, the spray pressure P~ is above or just below the maximum pressure P3 at which said mixture forms a liquid compressed fluid phase at temperature r .
In another preferred e ~ nt, the solvent fraction additionally contains at least one active solvent for the polymeric compound.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a triangular phase diagram for admi~Les of a polymeric coating s~. _sition and supercritical carbon dioxide.

~: 2111794 '':

Figure 2 is a phase diagram illustrating how phase relationships depend upon pressure and , compressed fluid concentration at constant temperature.
' Figure 3 i6 a phase diagram ~llustrating how phase relationships ~epend upon pressure and temperature at constant compressed fluid ~ concentration.
,~; Figure 4 is a pressure-temperature phase diagram ' for an acrylic polymeric c ,_sition illustrating how ~,' the phase relationships shift for 15%, 20%, 25%, and ~, 30% carbon dioxide by weight.
Figure 5 i8 a pressure-temperature phase diagram ~ for a polymeric coating c- ,_sition illustrating the "!,i, phase relationships at 35% and 43% carbon dioxide by ' weight.
''rl Figure 6 is a diagram showing how the density of ~ a mixture of an acrylic polymeric c- ,2sition and 28%
y'l carbon dioxide by weight varies with pressure at , temperatures of 24~, 38~, and 55~ Celsius.
Figure 7 is a pressure-temperature phase diagram for a polyester polymeric ~ sition illustrating phase relationships at 30% carbon dioxide by weight.

DETAILED DESCRIPTION OF THE INV~lION
It has been found that, by using the methods of !' the present invention, polymeric c ,~sitions can be ; sprayed with ~ essed fluids such as carbon dioxide, nitrous oxide, and ethane under conditions ' that e~Ance atomization. This allows the polymeric compositions to be sprayed at higher solids levels ~' and with finer atomization, which gives improved 6pray application quality and reduced emission of ''' ' . .

.
.

2111794 ~ ~

eolvent. The methods are particularly applicable to the ~pray application of coatings to a substrate.
As used herein, it will be understood that a n~_ ~essed fluid" is a fluid which may ~e in it~
g~ceo~ state, its liquid state, or a combination thereof, or i6 a ~upercritical fluid, depen~n~ upon (i) the particular temperature and pressure to which it is subjected, (ii) the vapor pressure of the fluid at that particular temperature, and (iii) the critical temperature and critical pressure of the fluid, but which is in its ~aseol~C state at standard conditions of 0~ Celsius temperature and one ~ -atmosphere absolute pressure (STP). As used herein, a "supercritical fluid" is a fluid that is at a temperature and pressure such that it is at, above, or slightly below its critical point.
C~ ounds which may be used as __ ~essed fluids in the present invention include but are not limited to carbon dioxide, nitrous oxide, ammonia, xenon, ethane, ethylene, propane, propylene, butane, isobutane, chlorotrifluoromethane, monofluoromethane, -~
and mixtures thereof.
Preferably, the compressed fluid has appreciable solubility in the polymeric composition and is envi,o l~ntally compatible, can be made environmentally compatible by treatment, such as by thermal deco asition or incineration, or can be readily ~ecove~ed from the spray environment, such as by absorption or adsorption. The utility of any of the above-mentioned compressed fluids in the practice of the present $nvention will depend upon the polymeric composition used, the temperature and pressure of application, and the inertness and 21117~4 ,. .

~tability of the compressed fluid.
; Due to envi~ ntal compatibility, low toxicity, and high solubility, carbon dioxide, ethane, and nitrous oxide are preferred compressed fluids in the present invention. Due to low cost, non-flammability, stability, and wide availability, carbon dioxide is the most preferred compressed fluid. However, use of any of the aforementioned c- - -c and mixtures thereof are to be considered within the scope of the present invention.
As used herein, the phrase "polymeric c -sition" is understood to mean conventional polymeric c- -sitions, materials, and formulations that have no c_ ~essed fluid admixed therewith. As also used herein, the phrases "coating s_ osition", "coating material", and "coating formulation" are understood to mean liquid _- tositions comprising conventional coating ca. _sitions, materials, and formulations that have no compressed fluid admixed < therewith.
As used herein, the term "solvent" is understood to mean conventional solvents that have no _ ~essed fluid ~ iYed therewith and which are in the liquid 6tate at conditions of about 25~C temperature and one atmosphere absolute pressure. As used herein, the phrase "active solvent" is understood to mean any 601vent or mixture of solvents that is miscible with the compressed fluid and is a good solvent for the polymeric compound.
The polymeric compositions that may be used with the present invention are generally comprised of a nonvolatile materials portion containing at least one polymeric compound and which is capable of being prayed. The polymeric ~m,_sitions, in addition to the nonvolatile materials portion, may also contain a 601vent portion which i8 at least partially mi6cible with the nonvolatile materials portion. A~ used herein, the phrase "nonvolatile material6" i6 understood to mean solid materials and liquid materials 6uch as 601id polymers, liquid polymers, and other c ~ ci that are nonvolatile at a temperature of about 25~ Celsius. In general, the nonvolatile materials portion is the portion of the polymeric composition that remains after the solvent portion, if any, has evaporated from the polymeric c~ ~ition. Examples of polymeric cc csitions that may be used include coating compositions, adhesives, release agents, additive fo, ~l~tions, gel coats, lubricants, non-aqueous detergents, and other compositions containing polymers, which are capable of being sprayed when admixed with compressed fluid.
The polymeric compositions that may be used include liquid ~_ ~ositions that are cor.ventionally sprayed using solvents and in which it is desired to reduce ~5 or eliminate the solvent content used for spraying.
Also included are polymeric ~c -sitions which heretofore could not be sprayed, or could not be prayed well, because the application or product requires that either no solvent or just a low level of solvent be present in the spray, with the --Yi - permitted 601vent level being too low to obtain 6ufficiently low vi6cosity to achieve good atomization of the composition or to obtain a well-formed spray. The polymeric composition may comprise a liquid polymer system that may contain other nonvolatile materials but which has no solvent, . .
,~
.~ .
,-~ .
~p i .~1 r~ . . ' ' 2~ 1179~

or a very low level of solvent.
Polymeric ~ ~sitions that may be used a~
polymeric coating compositions with the present invention typically include a nonvolatile materials portion contAining at least one polymeric compound which is capable of forming a coating on a substrate, whether 6uch c~ r- i6 a paint, enamel, lacquer, varnish, adhesive, chemical agent, release agent, lubricant, protective oil, non-aqueous detergent, an agricultural coating, or the like.
Generally, the nonvolatile materials used in the polymeric compositions of the present invention, such as the polymers, must be able to withstand the temperatures and pressures to which they are subjected after they are ultimately admixed with the ~- .essed fluid. Such applicable polymers include thermoplastic polymers, thermosetting polymers, crosslinkable film forming systems, and mixL~,es thereof. The polymers may be liquid polymers or solid polymers and they may be dissolved in solvent.
In particular, the polymeric c- p~ul.ds include vinyl, acrylic, styrenic, and interpolymers of the base vinyl, acrylic, and styrenic monomers;
polyesters; oil-free alkyds, alkyds, and the like;
polyurethanes, two-package polyurethane, oil-modified polyurethanes and thermoplastic urethanes systems;
epoxy systems; phenolic systems; cellulosic polymers such as acetate butyrate, acetate propionate, and nitrocellulose; amino polymers such as urea formaldehyde, melamine formaldehyde, and other aminoplast polymers and resins materials; natural gums and resins; silicone polymers such as polydimethylsiloxane and other polymers cont~ini~g .. .. ; ~, .~ ~ , 2~11794 ~ilicon; polymers containing fluorine; rubber-based adhesives including nitrile rubbers which are copolymers of unsaturated nitriles with dienes, styrene-butadiene rubbers, thermoplastic rubbers, neoprene or polychlo~oplene rubbers, and the like.
In addition to the polymeric c~ d, the nonvolatile materials portion of the polymeric ~ ition may alss comprise other materials such as ; waxes; nonvolatile organic compou,.ds such as antioxidants, surfactants, ultraviolet absorbers, whiteners, and plasticizers; and nonvolatile inorganic materials such as chemical agents, polymer additives, abrasives, and glass fibers; and the like.
The nonvolatile materials portion of polymeric coating compositions, in addition to the polymeric compound, may contain c~nver.tional additives which are typically utilized in coatin~s. For example, i~ pigments, pigment extenders, metallic flakes, fillers, drying agents, anti-foaming agents, anti-sk~nni~g agents, wetting agents, ultraviolet , absorbers, cross-linking agents, plasticizers, and ; ~ix~.e~ thereof, may all be utilized in the coating compositions to be used with the methods of the present invention.
~ In addition to the nonvolatile materials I portion, a solvent portion may also be employed in bl.' the polymeric c ~sitions. The solvent may perform ' a variety of functions, ~uch as to dissolve the i polymer and other com~oncnts, to reduce viscosity, to give proper flow characteristics, and the like. As ' used herein, the solvent portion is comprised of essentially any organic solvent or non-aqueous diluent which is at least partially miscible with the t'~
' .
. ,~,~,~ , . . .

. 21117~4 nonvolatile material6 portion. Preferably, the solvent portion contains at least one active colvent for the polymeric c~ _-~ '. The selection of a particular solvent portion ~or a given nonvolatile materials portion in order to form a polymeric coating co: -sition or formulation is well known to those skilled in the art of coatings. In general, up to about 30 percent by weight of water, preferably up to about 20 percent by weight, may also be present in the solvent portion provided that a coupling solvent i8 also present. All such solvent portions are suitable in the present invention.
A coupling solvent is a 601vent in which the nonvolatile materials such as polymers are at least partially soluble. Most importantly, however, such a coupling solvent i8 also at least partially miscible with water. Thus, the coupling solvent enables the miscibility of the nonvolatile materials, the solvent, and the water to the extent that a single liquid phase is desirably maintained such that the composition may optimally be sprayed and, Sor example, a good coating formed. The coupling solvent also enables miscilibity with compressed fluid.
Coupling solvents are well known to those skilled in the art of coatings and any con~el.~ional coupling solvents which are able to meet the aforementioned characteristics are suitable for being used in the present invention. Applicable coupling solvents include, but are not limited to, ethylene glycol ethers, propylene glycol ethers, and chemical and physical co~binations thereof; lactams; cyclic ureas;
and the like. When water is not present in the polymeric composition, a coupling solvent i5 not ~' .~ . ~ ~ ~ ; ' "' '"'..: ' -;, ,'''' ; ~ '~ ' ' .', ''"~ ' ',~,,':" ' , . .

21117~

neceS6A~y~ but may still be employed.
Other solvents wh$ch may be present in typical polymeric compositions, includ$ng coating compositions and the like, and which may be ut$1ized in the present invention include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl amyl ketone, cycloh~xAnone and other aliphatic ketones; esters ~uch as methyl acetate, ethyl acetate, and other al~yl carboxylic esters; ethers, such as methyl t-butyl ether, dibutyl ether, methyl phenyl ether and other aliphatic or alkyl aromatic ethers; glycol ethers such as ethoxy ethanol, butoxy ethanol, ethoxy 2-propanol, plOpOXy ethanol, butoxy 2-propanol and other glycol ethers; glycol ether esters such as butoxy ethoxy acetate, ethyl 3-ethoxy propionate and other glycol ether esters; alcohols such as methanol, ethanol, propanol, butanol, amyl alcohol and other aliphatic alcohols; aromatic hydrocarbons such as toluene, xylene, and other aromatics or mixtures of aromatic solvents; aliphatic hyd~ocarbons such as VM&P naphtha and mineral spirits, and other aliphatics or mix~u.es of aliphatics; and nitroalkanes such as 2-nitropropane.
C ressed fluids have been found to be good viscosity reducing diluents for polymeric compositions such as coating formulations, as disclosed in the aforementioned related patents. For example, consider an acrylic concentrate that has a vi w osity of 1340 centipoise (25~ Celsius). ~d~ing carbon dioxide to 30 weight percent concentration reduces the viscosity to below 25 centipoise.
For spraying the polymeric composition to form a spray of finely atomized l$quid droplets, the , .

polymeric composition is first A~' 1Yed with at least one c~ _essed fluid to form a liquid mixture at temperature T~ in a closed system, said mixture comprising (a) a nonvolatile materials fraction cont~ining at least one polymeric c -und and which is capable of being sprayed and (b) a solvent fraction which is at least partially miscible with the nonvolatile materials fraction and contains the at least one ~: essed fluid. As used herein, the phrase "nonvolatile materials fraction" is understood to comprise the nonvolatile materials portion of the polymeric o -sition. As used herein, the phrase "solvent fraction" is understood to comprise the at least one compressed fluid and the solvent portion of the polymeric c- _sition, if the polymeric c~ ~sition contains a solvent portion, or to comprise just the at least one compressed fluid, if the polymeric portion contains just nonvolatile materials with no solvent.
The solvent fraction contains the at least one compressed fluid in an amount which when added to the nonvolatile materials fraction is sufficient to ,ende~ the viscosity of the liquid mixture to a point suitable for being sprayed. Preferably, the -viscosity of the liquid mixture is less than about 200 centipoise, more preferably less than about 100 centipoise, and most preferably less than about 50 centipoise.
The solvent fraction also contains the at least one compressed fluid in an amount which when added to the nonvolatile materials fraction is sufficient to enable the liquid mixture to form a liquid compressed fluid phase at temperature T~. The liquid mixture is .

, .

211179~

sprayed by passing the mixture at temperature T~ and spray pressure Pl into an orifice through which said mixture flows to form a liquid 6pray, wherein ~pray pressure P~ is above the ~inimum pressure P2 at which 6aid liquid mixture forms a liquid compressed fluid phase at temperature T~.
When compressed fluid is admixed with a polymeric ss,,~sition at a given temperature T~, the number and type of ph7ee~ formed depends upon the pl25~U~e and the compressed fluid concen~ation in the admixture. To better understand this ph~n, -non, reference is made to the phase diagram in Figure 2, which illustrates the phase relationships for a typical liquid polymeric ~ ,~osition and c ,_essed fluid. For these discussions of phase diagrams, the polymeric cs ,~sition is understood to consist of a liquid solution containing polymer dissolved in solvent, with no dispersed nonsoluble materials therein. The phase relationships can be readily extended to polymeric c_ ,~sitions with dispersed nonsoluble materials, such as a pigmented polymeric coating composition comprising pigment dispersed in a clear polymeric vehicle, by considering the dispersed nonsoluble materials as comprising an additional inert phase. Polymeric c ,ssitions comprising liquid polymer6 with no solvent also have analogous phase relationships.
The phase diagram illustrated in Figure 2 at temperature T~ shows a liquid region (L), a liquid-vapor region (LV), and a liguid-liquid region (LL). The solid lines border regions having the ~ame ~ 'sr and types of ph~se6, although the c. ,:sitions and ~mounts of the ph~ses can change with location ' 2111794 ..
.

within the regions. In the liquid region (L), the compressed Sluid is completely dissolved in the polymeric c- ~sition. In the liquid-vapor r~gion (LV), which exists at pressures below the bubble point pressure curve A-K-B-2-C, the ~ ,essed fluid is not fully dissolved in the polymeric _ _sition;
the excess forms a vapor or gaseous compressed fluid phase that is Al -s~ entirely compressed fluid and contains very little solvent vapor. A portion of the bubble point pressure curve, se~ ?nt A-X-B, separates the liquid and liquid-vapor regions. The bubble point pressure is the pressure at which the first bubble of vapor or gi~seous compressed fluid is formed as the pressure is lowered from high pressure, at a -~
given compressed fluid level and temperature T~. The bubble point pressure generally increases with s .essed fluid concentration, but asymptotically approaches a limiting bubble point pressure as the _- _essed fluid concentration approaches 100%.
The liquid-liquid region (LL) is formed at pressures above the bubble point curve segment B-2-C
as the ~ ,essed fluid concentration is increased above the solubility limit curve B-3-D. We have discovered that two types of liquid-liquid regions can be formed. The type required for the present invention consists of a liquid polymeric phase and a liquid c- Lessed fluid phase. The liquid polymeric phase is substantially the polymeric composition 6aturated with dissolved compressed fluid. The liquid compressed fluid phase is primarily compressed fluid, but it can contain appreciable amounts of solvent extracted from the liquid polymeric phase.
It usually contains relatively little polymer, .

: ~ .
':' .; -because the __ lessed fluid is a non-solvent or very poor solvent for the polymers.
Se3 -nt ~-2-C of the bubble point pressure curve border_ the low pressure end of the liguid-l~quid reqion. A relatively narrow liquid-liquid-vapor (LLV) region lies just below it, which i5 generally not _hown because it is narrow. In the LLV region, the eY~ess col l~ssed fluid not dissolved in the liquid polymeric phase forms both a liquid __ ,essed fluid phase and a g ,A, ceous compressed fluid phase. As the pressure is lowered within the LLV region, the liquid compressed fluid is converted to gaseous compressed fluid.
Point G in the liquid region (L) in Figure 2 represents an admixture of polymeric composition and compressed fluid that is formed in a closed system at t~ -lature r. The admixture contains sufficient compressed fluid to render the viscosity suitable for spraying and to produce a feathered spray, as disclosed in the aforementioned related patents. The admixture is sprayed by passing it under pressure th-ou~h an orifice to form a liquid spray. In this illustration, the admixture contains about 2~%
compressed fluid and i6 sprayed at a pressure of 1500 psi. The dotted line G-X-H shows the path of the admixture as it undergoes rapid depressurization in the spray orifice. The admixture ,~ -inC liquid until it depressurizes to the bubble point pressure at point K, at which point the dissolved compressed fluid becomes supersaturated. As the pressure drops below the bubble point pressure, the supersaturated essed fluid nucleates to form g~seo~C compressed fluid, which eYrAn~C as it ~e~ e~ses and thereby .' .. ;

' 21117~

creates an eyr~ncive force for atomization and pro~es a wider, feathered 6pray.
The solubility limit curve B-3-D that 6eparates the liquid region (L) from the liquid-liquid region (LL), however, has been found not to be a vertical line that gives a single 601ubility limit concentration, that is, the points B and D do not occur at the same _ lessed fluid concentration.
Instead, the ~- ,essed fluid solubility limit has been found to increase relatively rapidly with higher pressure, that is, point D at high pressure has a higher compressed fluid concentration than point B at low pressure.
We have discovered that enhanced atomization can be obtaine~, when spraying polymeric compositions, for combinations of ~ p:essed fluid concentration, spray temperature, and spray pressure for which the liquid spray mixture passes through the liquid-liguid region during depressurization. Without wishing to be bound by theory, enh~nce~ atomization i8 believed to occur beca-~ce the dissolved compressed fluid, during dc~less~ization in the spray orifice, nucleates to form a liquid ~ ilessed fluid phase before forming g~o~c compressed fluid, instead of nucleating directly to a g~Fec~s compressed fluid phase. Nucleation to a liquid c _essed fluid phase is much more favorable energetically than to a gas compressed fluid phase. Therefore, nucleation should occur much more quickly during depressurization, that i6, at higher ~ess~e because much less ~upersaturation is required, and furthermore a much higher concentration of nucleation sites should form in the decompressing fluid. These liquid nucleation .
~ ' ~

~ .

~ y; ' ! ~
,,,. . , ,. , ' , ' ' "', ', ~, 2111~4 sites of liquid ~ ~essed fluid readily vaporize to gaseous c_ _essed fluid upon further depressurization, which creates an eYrAncive force that is greater and more widely distributed in the decompressive spray than if the ~ essed fluid nucleated directly to fewer gas phase cites at a higher degree of -upe.saturation, that is, at lower pressure. ~his higher level and better distribution of expansive force is therefore more effective at ove~ ing the cohesion, surface tension, and viscosity forces that oppose atomization. Therefore, more intense atomization can occur.
A liquid mixture of polymeric composition and ressed fluid that i6 sprayed in confo, -nce with the present invention is illustrated by Point E in Figure 2. In addition to having c~ ~essed fluid in an amount sufficient to render the viscosity to a point suitable for being sprayed and to give a feathered spray, the amount of compressed fluid is sufficient to enable the liquid mixture to form a liquid ~ ~essed fluid phase at temperature r.
Furthermore, the spray pressure P~ of point E is above the minimum pressure P2 ~nd the -Yj pressure P3 at which the liquid mixture forms a liquid ~_ ~essed fluid phase. Pressure P2 in general is the bubble point pressure, along segment B-2-C of the bubble point pressure curve, for the essed fluid concentration in the liguid mixture.
Pressure P3 in general is the solubility limit ~,ess~,e, along solubility limit curve B-3-D, for the compressed fluid concentration in the liquid mixture.
Therefore, during depressurization, the liguid mixture follows the path indicated by the dotted line .

.
,~
i .
?
i.~

21117~4 -~ ~

E-3-2-F, which p~s6es from the liquid region (L) through a portion of the liquid-liquid region ~LL) before entering the liquid-vapor region (LV) at lower pressure. Therefore, the dissolved c~ ~_essed fluid nucleates to form liquid s- p essed fluid that readily vaporizes to ~ceouC compressed fluid and thereby gives enhAnced atomization by virtue of the higher level and better distribution of tbe expansive force of the g~eo~C z p~2ssed fluid. In this illustration, the compressed fluid concentration is about 30%, spray pressure P~ at point E is about 1900 psi, pressure P3 at point 3 is about 1670 psi, and pressure P2 at point 2 is about 1380 psi. Of course, the A~S_ , ~ ession path E-3-2-F continues until depressurization reaches ambient pressure, as indicated by the arrow at point F.
It is understood that the phase diagram in Figure 2 illustrates the typical relationships between the ph~ses ~ the bubble point pressure curve, and the solubility limit curve, but that the actual pressure and compressed fluid concentration values of the curves will ~epend upon the polymeric composition -~
and compressed fluid used as well as the temperature r. It is also understood that the curve B-2-C
extends to higher compressed fluid concentrations and ~
that the curve B-3-D extends to higher pressures than ~ -shown. Such phase diagrams have been measured with carbon dioxide and ethane being the compressed fluid, with enh~nce~ atomization being obtained by the methods of the present invention. Nitrous oxide is expected to have similar solubility and spray characteristics to carbon dioxide, because its critical temperature and pressure ar- nearly the ~ame ~.', :
~ :

.j . ~.. .. : . .. : . . ..
.

- 211179~

as carbon dioxide, and it has the same molecular weight and si~ilar molecular structure.
The relationship between the p~h~ses, at a given compressed fluid concentration, can be shown as a function of temperature and pressure by using the phase diagram illustrated in Figure 3. The liquid region ~L), liguid-liguid region ~LL), and liguid-vapor region ~LV) co,,es~ond to those shown in Figure 2. The curve A-K-B-2-C i6 the CG~ ~ esponding bubble point pressure curve and the curve B-3-D i8 the corresponding solubility limit curve. ~he c- -essed fluid concentration is sufficient to form a liguid-liguid region having a liquid c. _essed fluid phase at temperatures above the temperature at point B, which in this illustration is about 55~
Celsius. Spray path G-K-H co,,e~onds to the depressurization path of the admixture sprayed in accordance with the aforementioned related patents.
Spray path E-3-2-F corresponds to the depressurization path of the liguid mixture of polymeric composition and _ ~essed fluid sprayed in accordance with the present invention.
The deplessurizations occur at substantially constant t~ - ature at pressures above the bubble point pressures, that is, segments G-K and E-3-2, because liguids undergo very little expansion cooling in comparison to the expansion cooling caused by ~Aseo~C compressed fluid at pressllrDs below the bubble point pressures. In fact, the bubble point pressu~a can be detected by measuring the temperature of a mixture as it depressurizes, such as in ~pparatus used to measure ~_ ~essed fluid solubility and phase diagrams, which have been described in the . !

' rr ~

;~
211179~i ~-16941 26 afo~. -ntioned related patents. The bubble polnt p~eSa~l~ is the pressure ~t which the temperature first begins to drop during the decompression.
s~iements show that very little, if any, cooling occurs during depressurization through the liquid-liquid region (LL) before the bubble point pressure is reached. Below the bubble point pressures, formation and exrAncion of the gaseous compressed fluid phase causes eYpAnsion cooling, as illustrated by segments K-H and 2-F. Of course, the depressurizations and expansion cooling continue until ambient pressure is reached, as indicated by the arrows at point H and F.
Here too, it is understood that the phase diagram in Figure 3 illustrates the typical relation~hips between the phA6es, the bubble point pressure curve, and the solubility limit curve, but ~; -that the actual pressure and temperature values of the curves will depend upon the polymeric composition ;-and ~ lessed fluid used as well as the compressed fluid concen~-ation. It is also understood that the es B-2-C and B-3-D extend to higher pressures and temperatures than shown. In general, higher compressed fluid concentration shifts the bubble point pressure curve A-K-B-2-C to higher pressure and shifts the solubility limit curve ~-3-D to lower temperature and lower pressure.
A pressure-temperature phase diagram for a polymeric ~ -sition that is a thermosetting coating conce..~.ate is shown in Figure 4. It contains acrylic and melamine polymers at a polymer level of 78 percent by weight dissolved in a blend of methyl amyl ketone, ethyl 3-ethoxypropionate, and isobutanol . ~ .

. .

..

. ,, . . - ~ ~ .

D-16s41 27 solvents. The c ,~ssed fluid i6 carbon dioxide.
The diagram shows how the phase rel~tion~h~ps shift for 15%, 20%, 25%, and 30% carbon dioxide by weight in the liquid mixture. At 15% and 20% carbon dioxide, the liquid-liquid region (LL) i8 formed only at very high temperature above 70~ Celsius and hence it is not shown on the diagram. At 25% carbon dioxide, there is sufficient c _essed fluid to form a liquid-liquid region at temperatures T~ above about 55~ Celsius and at pressures above 1400 psi. At 30%
carbon dioxide, the liquid-liquid region has shifted to much lower temperature and pressure, so that the liquid-liquid region is formed at temperatures T~
above about 20~ Celsius and at pressures above about 7~0 psi. This system forms a liquid carbon dioxide phase in the liguid-liquid region. Therefore, essurization from the liquid region into the liquid-liquid region causes carbon dioxide nucleation to form a liguid carbon dioxide phase.
A pressure-temperature phase diagram for a polymeric composition that is an air-dry lacquer coating concentrate is shown in Figure 5. It contains nitrocellulose and alkyd solid polymers at a polymer level of about 38 percent by weight dissolved in a blend of methyl amyl ketone and other solvents.
The compressed fluid is carbon dioxide. The diagram shows the phase relationchips for 35% and 43% carbon dioxide by weight in the liquid mixture. At carbon dioxide concentrations below about 30% there is insufficient compressed fluid to form a liquid-liquid region at temperatures below about 75~ Celsius. At 3S% carbon dioxide, there is sufficient c~ ~e6sed fluid to form a liquid-liquid region at t~ ~atures . . ' ' ' ' ' . ~ ' ' ' ' ' .. . . . . ~

r above about 55~ Celsius and at pressures above 1200 psi. At 43% carbon dioxide, the liquid-liquid region has shifted so that the liquid-liquid region i8 formed at temperatures T~ above about 29~ Celsius and at pressures above about 840 psi. This system forms a liquid carbon dioxide phase in the liquid-liquid region.
The difference in g~seous and liguid nucleation properties obtained by ~F._essurization across the bubble point curve and the ~olubility limit curve, respectively, can be visually observed in the afoL.- -ntioned apparatus used to measure compressed fluid solubility and phase diagrams.
Depressurization across the bubble point curve proAuce6 a mixture of fine gas bubbles dispersed in -~
the clear liquid polymeric phase. The mixture is readily identifiable as such, because the bubbles generally are large enough and few enough to be seen -individually by close eY- in~tion. They also have low density, ~o they are very buoyant. Identifying -the exact p~ 255~L e at which the first bubbles are formed usually requires careful eYr inAtion of the polymer solution, because the first bubbles are few -~
and tiny. They are more easily ~een as the pressure drops below the bubble point pressure, because the first bubbles formed become larger as more ~upersatured compressed fluid vaporizes into them.
Sometimes relatively few new bubbles are formed by additional nucleation. In contrast, depressurization across the solubility limit curve, from the liquid region to the liquid-liquid region, causes the clear ~olution to sharply and rapidly turn opaque, ob~inin~ the appearance of milk. Therefore, the ' . . .

~ 2111794 transition is commonly referred to as the "white point". The transit$on is rapidly reversible with 61ight changes is pressure. The nucleated l$quid droplets of ~- lessed fluid are so tiny that they can not be seen individually. The mixture turns opaque because the concentration of nucleation sites is very high compared to gas phase nucleation. As the pressure is lowered further, more liguid c- _~ssed fluid is formed, and the mixture be~_ -s a dispersion of larger droplets as the droplets grow and begin to agglomerate together. The droplets are much les~ buoyant than gas bubbles, but they are less dense than the polymeric phase. Therefore, as more liguid compressed fluid is formed, the droplets be~ ? large ~nough to be seen and to readily float upward to form a liquid level at the top of the mixture, after agitation is stopped. As the pressure is lowered, more liguid compressed fluid is formed until the the bubble point pressure is reached, at which point the liguid cor _e&sed fluid readily vaporizes into gaseous compressed fluid over a relatively narrow range of pressure.
The difference between depressurization across the bubble point curve and the solubility limit curve can also be seen be ex- ining how the density of the liquid mixture changes with pressure. This is shown in Figure 6 for the thermosetting acrylic polvmeric composition used in Figure 4, with a carbon dioxide concen~tion of about 28 percent by weight. The density was measured by circulating the liquid mixture continllo~sly through a sensitive densitometer. The c$rculation loop also contained a circulation pump, a heater to maintaln constant , temperature, and a piston-type ac_ lator, which was -used to vary the pressure of the liquid mixture, by ;
varying the pressure of c rssed nitrogen fed to the accumulator. The density profile was ~ red at temperatures of 24~, 38~, and 55~ Celsius as the liquid was depressurized from 2000 psi. At all three temperatures, the liquid mixture was essentially incompressible at pressures at which the mixture was a single liquid phase. At 24~ and 38~ Celsius, the density d~opped suddenly and linearly with pressure after the bubble point pressure was reached and g~seous carbon dioxide was formed. In contrast, at 55~ Celsius, as the mixture crossed the solubility limit at point A, a liquid carbon dioxide phase was formed which, being liquid, had a density much closer ~-to the polymeric phase than gas. Therefore, the density dropped much more slowly with pressure as the mixture passed through the liquid-liquid region, as seen by the curvature in the density profile. Only at much lower pressure, below the bubble point pressure, did the density drop much more rapidly with pressure as gas was formed.
Therefore, that the spray pressure Pl is above or within the liquid-liquid region of the phase diagram can be determined visually for clear polymeric compositions or by measuring the density profile for opaque polymeric ~ _ asitions, such as pigmented coating c -citions. To visually observe the phase condition of the liquid mixture, a high-pressure sight glass can be installed in the pray apparatus. To measure the density profile, a densitometer, such as a Micromotion densitometer, can ~-be installed in the spray apparatus. Then the phase , . . ~ .

.,-,:;, ~ . . . . . .
, ............. . .

211~794 condition and phase transition can be observed or detected as the spray pressure i6 lowered and ralsed.
That the liquid mixture of polymeric composition and ~_ _essed fluid forms a liquid c _essed fluid phase in the liquid-liquid region can be dete~ ~ne~
visually for clear or opague polymeric ~ _ sitions alike, or by using the clear vehicle of opaque ~_ -sitions that contain dispersed nonvolatile materials. A liquid compressed fluid phase can be dete, ineA to form upon depressurization across the solubility limit pressure, because the lower density of the liquid compressed fluid phase causes the agglomerated droplets to migrate to the top of the mixture and to form a clear inviscid liquid layer when agitation is stopped. Further depressurization to below the bubble point pressure can also be seen to cause the separated liquid phase to vaporize to gas.
Anotber temperature-pressure phase diagram for a polymeric c -sition that is a thermosetting coating co~centrate is shown in Figure 7. It contains polyester and melamine polymers at a polymer level of 67 ~elcen~ by weight dissolved in a blend of methyl amyl ketone, ethylene glycol butyl acetate ether, and isobutanol solvents. The c_ _essed fluid is carbon dioxide at 30 percent by weight. The liquid-liguid region for this mixture was dete, ined to not contain a liguid carbon dioxide phase. Therefore this system i~ not in accordance with the present invention.
Spraying the mixture with depressurization through the liquid-liquid region did not give e~h~nced atomization. ,~
In the practice of the present invention, the ,.

.
211179~

spray pressure P~ must be above the minimum pressure P2 at which the liquid mixture of polymer composition ~nd compressed fluid forms a liquid compressed fluid phase ~t t~ -~ature r. Preferably, the spray pressure Pl is above or ~ust below the maximum pressure P3 at which the liquid mixture forms a liguid compressed fluid phase at temperature l~, so that the liquid mixture, before being sprayed, contains little or no liquid ~_ Lessed fluid phase.
Most preferably, the spray pressure Pl is above the maximum p~essu -~ P3 at which the liquid mixture forms a liquid compressed fluid phase at temperature T~.
The liquid c ~essed fluid phase in the liquid-liquid region of the phase diagram has been found to be capable of extracting significant amounts of solvent from the liquid polymeric phase. This can significantly increase the viscosity of the polymeric phase, which can hinder atomization and give poor spray performance. For example, if the polymeric composition is a coating concentrate~ the solvent lost by extraction, which evaporates in the at -s~ere when sprayed, can significantly increase the viscosity of the deposited coating, which can cause poor coalescence and film formation.
An excessively high spray pressure P~ is not desirable, because the liquid mixture, when sprayed, must depressurize more in the spray orifice before the liquid mixture drops below the solubility limit pressure. Therefore, preferably, the spray pressure P~ is less than about 600 psi above pressure P3, more preferably less than about 300 psi above p~cssu~e P3.
Preferably the difference in pressure between .

, .
r ~

, ' : ' ' '' ~\

211179~

the maximum pressure P3 and the mlni pres~ure P2 at which the liquid mixture forms a liquid com~e~e~
fluid phase at temperature T~ i6 qreater than about 100 psi, more preferably greater than about 200 p8i-; The liquid mixture of polymeric composition and compressed fluid may be prepared for 6praying by any of the spray apparatus disclosed in the afo,. -ntioned related patents or other apparatus.
The spray apparatus may also be a UN'ICARB System Supply Unit manufactured by Nordson Corporation to .opo~-ion, mix, heat, and pressurize polymeric c _6itions with c- _essed fluids such as carbon dioxide for the spray application of coatings.
The liquid mixture is sprayed by passing the mixture at temperature ~ and spray pressure P~ into an orifice through which the mixture flows to form a liquid spray. An orifice is a hole or an opening in a wall or housing, such as in a spray tip. Spray orifices, spray tips, spray nozzles, and spray guns used for conventional and electrostatic airless an~
air-assisted airless spraying of coating foL lations such as paints, lacquers, enamels, and var~i~hes, are suitable for spraying the liquid mixtures of the present invention. Spray guns, nozzles, and tips are preferred 1) that do not have eYcessive flow volume between the orifice and the valve that turns the 6pray on and off and 2) that do not obstruct the wide ~ngle at which the spray typically exits the spray orifice. The most preferred 6pray tips and spray guns ~re the UNICARB~ spray tips and spray guns manufa~u.ed by Nordson Co.~,ation. Orifice sizes of from about .007-inch to about .025-inch ~ in~l diameter are preferred, although smaller and larger :~ 21117~4 orifice sizes may be used. Devices and flow designs, euch as pre-orifices or turbulence promoters, that promote turbulent or agitated flow in the liguid mixture prior to passing the mixture through the orifice may al80 be used. The ~.e o~ifice preferably does not create an ~Yce6sively large pressure drop in the flow of liquid mixture.
Spray droplets are produced which have an average diameter of one micron or greater, preferably from about 10 to about 100 microns. The optimal spray droplet size will Aepend upon the requirements of the spray application. For the spray application of coatings, preferably the spray droplets have an average diameter from about 15 to about 80 microns, more preferably from about 20 to about 50 microns.
Preferably, the compressed fluid has appreciable solubility in the polymeric composition. In general, for the compressed fluid to produce 6ufficient viscosity reduction and to provide a sufficient eYp~ncive force for atomization, the c lessed fluid, such as carbon diox$de or ethane, should have a solubility in the polymeric ~. _sition of at least about 5 weight percent, based upon the total weight of _ ressed fluid and solvent-borne &- ,_sition, preferably at least about 10 weight percent, more preferably of at least about 20 weight percent, and most preferably of at least about 25 weight percent.
Although high spray pressures P~ of 5000 psi and higher may be used, preferably the spray pressure P~
i8 below about 3000 psi, more preferably below about 2000 psi. Very low pressure is generally not compatible with high compressed fluid solubility in the polymeric composit~on. Therefore, preferably the .
:-:

,,. . :.

' ' . ' ~

spray pressure Pl is above about 50 percent of the critical pressure of the compressed fluid, more preferably ~bove about 75 percent of the critical pressure, and most preferably above, at, or slightly below the critical pressure.
Preferably, the spray temperature T~ of the liquid mixture is below about 150~ Celsius, more preferably below about 100~ Celsius, and most preferably below about 80~ Celsius. The temperature level that may by utili2ed will in general depend upon the stability of the polymeric system. Reactive systems must generally be sprayed at lower temperature than non-reactive systems like air-dry lacquers. Preferably, the spray te.~ature r of the liquid mixture is above about 20~, more preferably above about 25~, and most preferably .
above, at, or slightly below the critical temperature of the s- ~essed fluid. The liquid mixture is preferably heated to a temperature r that substantially compensates for the drop in spray temperature that OC~l S due to expansion cooling of the decompressing compressed fluid.
The pressure P~ and temperature r used for a given application will ~epen~ upon the particular properties of the CQ' , _ essed fluid and the polymeric composition. In particular, they will depend upon the conditions necessAry to form a liquid compressed fluid phase upon depressurization. The liquid mixture is preferably sprayed at a temperature r and pressure P~ at which the compressed fluid is a ~upercritical fluid. The spray is preferably a decompressive spray that is feathered and has a . ~
p :

21117~4 parabolic 6hape.
The polymeric ~c ssition preferably has a viscosity of about 500 to about 5000 centipoise (25~
Celsius) before admixed with the compressed fluid, more preferably from about 800 to about 3000 centipoi6e, although higher and lower viscosity may also be used with the present invention, depending upon the requirement6 of the spray application. For coating applications, the viscosity should be at a level that gives proper coalescence and film formation for a given application.
If a coating is deposited by the spray, the form of the coating and the c ~sition of the substrate are not critical to the present invention. If curing of the polymeric coating s~m,~sition present upon the coated substrate is reguired, it may be performed by conventional means, such as allowing for evaporation of solvent, application of heat or ultraviolet light, etc.
Electrostatics may be used to increase the deposition of coating material onto the substrate.
This is done by using a high electrical voltage in the range of about 30 to about 150 kilovolts to impart an electrical charge to the liquid mixture or the spray. ~ny of the methods disclosed in the aforementioned U.S. Patent No. 5,106,650 may be used in the practice of the present invention.
In general, polymeric coating compositions used in the present invention for coating applications should have a solvent portion containing solvents with the proper b~l~nre of evaporation rates ~o as to ensu~e proper coating formation and to minimize solvent los- by ev~poration in the spray bas-d on a .
!

~'.;;' ' .,~. ' ' ' '/"." . '' ~ ' ' 211~79q ~: D-16941 37 relative ~vapo,ation rate ~RER) to a butyl acetate standard equal to 100 using ASTM Method D3599 at 25~
Celsius and one at -s~'ere p~es~u,L, the solvent portion desirably has the following c -sition of fast and slow evaporating solvents as ep,esented by corresponding RER values:
Weight Percent of Total Solvent Portion RER
30 - 100% < 50 0 - 70% 50 - 100 :.
~ 0 - 40% 101 - 250 ~. < 10% > 250 More preferably, the solvent portion has the following composition:
~eight Percent of Total Solvent Portion RER
40 - 100% < 50 0 - 60% 50 - 100 0 - 30% 101 - 250 < 5% > 250 While preferred forms of the present invention have been described, it should be apparent to those skilled in the art that methods and apparatus may be employed that are different from those shown without ~ departing from the spirit and scope thereof.
.. :
,, ,!: .
.

,~ ' ! -i~ .
~ ' .

, .
(, . ~ ~ .. . ,, - ... .

A polymeric coating composition that gives a clear acrylic thermoset coating and has a polymer level of 78.0 percent by weight was prepared from Rohm & Hass-AcryloidTM AT-964 resin and American Cyanamid CymelTM 323 resin. AcryloidTM AT-954 resin contains an acrylic polymer with a weight-average molecular weight of 6,070 at a polymer level of 85 weight percent dissolved in methyl amyl ketone. CymelTM resin is a melamine polymer cross-linking agent with a weight average molecular weight of 490 at a polymer level of 80 weight percent dissolved in isobutanol. The polymeric composition had the following composition by weight: 59.0% acrylic polymer, 19.0%
melamine polymer, 10.4% methyl amyl ketone, 6.4% ethyl 3-ethoxypropionate, 4.8% isobutanol, and 0.4% Silwet(~ surfactant. The viscosity was about 2000 centipoise. The solvent portion had the following distribution of solvents by relative evaporation rate (RER):
22.0% RER of 74, 48.2% RER of 40, 29.8% RER of 11.
The liquid mixture of polymeric composition and compressed carbon dioxide fluid was prepared and sprayed on a continuous basis by using the proportioning and spraying apparatus disclosed in Figure 2 of U.S. Patent No. 5,105,843. Carbon dioxide supplied from a cylinder was pressurized by a pump and regulated to the desired spray pressure P1 by a pressure regulator. A mass flow meter measured the mass flow rate of carbon dioxide fed through a check valve to the mix point with the polymeric composition. The polymeric composition was supplied from a tank, pre-pressurized by a supply pump, and pressurized and metered by a precision gear pump. A gear meter measured the amount delivered through a check valve to the mix point with the carbon dioxide. The speed command of the gear pump was electronically controlled by an input signal from the mass flow meter by using a control system to automatically obtain the desired proportion of polymeric composition and carbon dioxide. The metering rate was electronically adjusted by a feedback signal from the gear meter to correct for pumping inefficiency. The liquid mixture of polymeric composition and carbon dioxide from the mix point was further mixed in static mixer and admixed with recycled liquid mixture in a circulation loop. The circulation loop contained a static mixer, a piston-type accumulator, a heater, a filter, a densitometer, a high-pressure sight glass, a spray gun, a circulation pump, and a second heater. The spray gun was a Nordson A7A automatic airless spray gun with a Binks spray tip #9-0950 with a Spraying Systems tip insert #15153-NY to reduce the void volume in the spray tip. The spray tip had a 9-mil orifice size.
The liquid mixture contained 25 percent carbon dioxide by weight. The phase diagram for this system is shown in Figure 4.
The liquid mixture was first sprayed at conditions that are not in accordance with the present invention. The spray temperature was 50~ Celsius and the spray pressure was 1380 psi. When the liquid mixture was depressurized, the sight glass showed that no liquid-liquid region was formed and that the bubble point pressure was 1270 psi, as shown in Figure 4. The spray was a feathered, decompressive spray having fine atomization. Spray droplet size was measured by laser diffraction by using a Malvern type 2600 spray and droplet sizer. The Sauter-mean-diameter average droplet size was about 31 microns.
The liquid mixture was then sprayed in accordance with the present invention. The spray temperature T~ was 65~ Celsius and the spray pressure Pl was 1760 psi. When the liquid mixture was depressurized, the sight glass showed that the m~ximum pressure P3 at which a liquid carbon dioxide phase was formed was 1660 psi and the minimum pressure P2 at which a liquid carbon dioxide phase was formed was 1540 psi, as shown in Figure 4. The spray was a feathered, decompressive spray having very fine atomization. The average droplet size was reduced to about 23 microns. This enhanced atomization had an average droplet volume that was about 41 percent of the average droplet volume of the first spray. This finer atomization gave higher quality coatings having better appearance and a smoother finish.

The same polymeric composition, spray unit, spray gun, and spray tip were used as in Example 1.
The liquid mixture contained 30 percent carbon dioxide by weight. It was sprayed in accordance with the present invention at a subcritical temperature. The spray temperature T~ was 30~ Celsius and the spray pressure Pl was 1625 psi. When the liquid mixture was depressurized, the sight glass showed that the m~ximum pressure P3 at which a liquid carbon dioxide phase was formed was 1520 psi and the minimum pressure P2 at which a liquid carbon dioxide phase was formed was 940 psi, as shown in Figure 4.
The spray was a decompressive spray having fine atomization. The average droplet size was about 33 microns.
For one comparison, the liquid mixture was sprayed at conditions not in accordance with the present invention. The carbon dioxide concentration was 25 weight percent. The spray temperature was also 30~ Celsius and the spray pressure was 960 psi. When the liquid mixture was depressurized, the sight glass showed that no liquid-liquid region was formed and that the bubble point pressure was about 880 psi, which was just slightly lower than the bubble point pressure at 30 percent carbon dioxide, as shown in Figure 4. This spray had very poor atomization, having an average droplet size of about 170 microns, despite having nearly the same bubble point pressure at which gaseous nucleation would occur.
For another comparison, another liquid mixture was sprayed, which has a liquid-liquid region at 30 percent carbon dioxide that is similar to that shown in Figure 4 for the acrylic polymeric composition, that is, the solubility limit pressure curves and the bubble point pressure curves are .~imil~r. This second polymeric composition contained a thermosetting polyester polymer that has low molecular weight, like the AcryloidTM AT-954 acrylic polymer, and which also contained CymelTM 323 resin as a cross-linking polymer. The phase diagram, at 30 percent carbon dioxide, is shown in Figure 7. The ~ D-16941 polymer level was 67 percent by weight and the viscosity was about 1000 centipoise. Therefore, because this polymeric composition has lower polymer level and viscosity, it should be more easily atomized than the acrylic composition. However, depressurization of the liquid mixture of the polyester polymeric composition and carbon dioxide, from the liquid region, at a temperature of 30~ Celsius and other temperatures, showed that this system is not in accordance with the present invention, because the liquid-liquid region formed does not have a liquid carbon dioxide phase. Therefore, nucleation to liquid carbon dioxide can not occur during depressurization in the spray orifice, and enhanced atomiztion is not produced. The polyester liquid mixture with 30 percent carbon dioxide was sprayed at a temperature of 30~ Celsius and a pressure of about 1600 psi. The atomization was very poor, having an average droplet size of over 150 microns.
~,nh~qnced atomization also did not occur at higher temperatures and pressures above the solubility limit curve.

An acrylic polymeric coating composition was prepared using the same polymers as in Example 1, but at a higher polymer level of 83.6 percent be weight. The polymeric composition had the following composition by weight: 64.1% acrylic polymer, 19.5% melamine polymer, 11.3% methyl amyl ketone, 4.9% isobutanol, and 0.2%
Silwet(~) surfactant. The viscosity was about 6500 centipoise. The same spray unit, spray gun, and spray tip were used as in Example 1.
The liquid mixture of acrylic polymeric composition and 23 weight percent carbon dioxide was first sprayed at conditions that are not in accordance with the present invention. The spray temperature was 65~ Celsius and the spray pressure was 1700 psi. When the liquid mixture was depressurized, the sight glass showed that no liquid-liquid region was formed and that the bubble point pressure was 1360 psi. The spray was decompressive spray having an average droplet size of about 57 microns. The spray produced a poor quality coating having poor appearance.
The liquid mixture was then sprayed with 27 weight percent carbon dioxide in accordance with the present invention. The spray temperature T~ was also 55~ Celsius and the spray pressure Pl was also 1700 psi. When the liquid mixture was depressurized, the sight glass showed that the mz~ximum pressure P3 at which a liquid carbon dioxide phase was formed was 1530 psi. The spray was a decompressive spray having fine atomization. The average droplet size was reduced to about 34 microns. This enhanced atomization enabled high quality coatings having good appearance and a smooth finish to be sprayed at this higher polymer level with reduced emission of solvent.

A polymeric coating composition that gives a clear air-dry lacquer coating was prepared by dissolving nitrocellulose and alkyd solid polymers in a blend of methyl amyl ketone and other solvents at a polymer level of about 38 percent by weight. The viscosity was about 850 centipoise. The same spray unit, spray gun, and spray tip were used as in Example 1.
The liquid mixture of polymeric composition and 35 weight percent carbon dioxide was first sprayed at conditions that are not in accordance with the present invention. The spray temperature was 45~ Celsius and the spray pressure was 1125 psi. When the liquid mixture was depressurized, the sight glass showed that no liquid-liquid region was formed and that the bubble point pressure was 1030 psi, as shown in Figure 5. The spray was a decompressive spray having fine atomization. The average droplet size was about 26 microns.
The liquid mixture was then sprayed in accordance with the present invention. The spray temperature T~ was about 65~ Celsius and the spray pressure P1 was about 1750 psi. When the liquid mixture was depressurized, the sight glass showed that the m~ximum pressure P3 at which a liquid carbon dioxide phase was formed was 1650 psi and the minimum pressure P2 at which a liquid carbon dioxide phase was formed was 1450 psi, as shown in Figure 5. The spray was a decompressive spray having very fine atomization. The average droplet size was reduced to about 16 microns. This enhanced atomization had an average droplet volume that was about 23 percent of the average droplet volume of the first spray.

The same polymeric composition, spray unit, spray gun, and spray tip were used as in Example 4.

The liquid mixture of polymeric composition and 35 weight percent carbon dioxide was first sprayed at conditions that are not in accordance with the present invention. The spray temperature was 35~ Celsius and the spray pressure was 965 psi. When the liquid mixture was depressurized, the sight glass showed that no liquid-liquid region was formed and that the bubble point pressure was 870 psi, as shown in Figure 5. The spray had an average droplet size of about 73 microns. The spray produced a poor quality coating having poor appearance.
The liquid mixture was then sprayed with 43 weight percent carbon dioxide in accordance with the present invention. The spray temperature T~ was also 35~ Celsius and the spray pressure Pl was 1600 psi. When the liquid mixture was depressurized, the sight glass showed that the m~ximum pressure P3 at which a liquid carbon dioxide phase was phase was formed was 1150 psi and the minimum pressure P2 at which a liquid carbon dioxide phase was formed was 930 psi, as shown in Figure 5. The spray had fine atomization. The average droplet size was reduced to about 28 microns. This enhanced atomization enabled a high quality coating to be sprayed having good appearance and a smooth finish.

A polymeric coating composition that gives a clear air-dry lacquer coating was prepared by dissolving an acrylic solid polymer in solvent at a polymer level of about 38 percent by weight. The same spray unit, spray gun, and spray tip were used as in Example 1.

The liquid mixture of polymeric composition and 45 weight percent carbon dioxide was first sprayed at conditions that are not in accordance with the present invention. The spray temperature was 40~ Celsius and the spray pressure was 1050 psi. When the liquid mixture was depressurized, the sight glass showed that no liquid-liquid region was formed and that the bubble point pressure was 950 psi. The spray had an average droplet size of about 86 microns.
The liquid mixture was then sprayed in accordance with the present invention at the same carbon dioxide concentration. The spray temperature was T~ was about 55~ Celsius and the spray pressure Pl was about 1475 psi. When the liquid mixture was depressurized, the sight glass showed that the m~ximum pressure P3 at which a liquid carbon dioxide phase was formed was 1375 psi and the minimum pressure P2 at which a liquid carbon dioxide phase was formed was 1225 psi. The average droplet size of the spray was reduced to about 54 microns.

Claims (20)

1. A process for spraying a polymeric composition to form a spray of finely atomized liquid droplets, which comprises:
(1) forming a liquid mixture at temperature T°
in a closed system, said mixture comprising:
(a) a nonvolatile materials fraction containing at least one polymeric compound and which is capable of being sprayed; and (b) a solvent fraction which is at least partially miscible with the nonvolatile materials fraction and contains at least one compressed fluid in an amount which when added to (a) is sufficient:
(i) to render the viscosity of said mixture to a point suitable for being sprayed; and (ii) to enable said liquid mixture to form a liquid compressed fluid phase at temperature T°;
wherein the compressed fluid is a gas at standard conditions of 0°C and one atmosphere pressure (STP); and (2) spraying said liquid mixture by passing the mixture at temperature T° and spray pressure P1 into an orifice through which said mixture flows to form a liquid spray, wherein spray pressure P1 is above the minimum pressure P2 at which said liquid mixture forms a liquid compressed fluid phase at temperature T°.

2. The process of Claim 1, wherein said spray pressure P1 is above or just below the maximum pressure P3 at which said mixture forms a liquid compressed fluid phase at temperature T°.

3. The process of Claim 2, wherein said spray pressure P1 is less than about 600 psi above said pressure P3.

4. The process of Claim 1, wherein the viscosity of the liguid mixture of (a) and (b) is less than about 200 centipoise.

5. The process of Claim 1, wherein the solvent fraction contains at least one active solvent for the polymeric compound.

6. The process of Claim 1, wherein the compressed fluid is a supercritical fluid at temperature T° and spray pressure P1.

7. The process of Claim 1, wherein the compressed fluid is carbon dioxide, nitrous oxide, ethane, or a mixture thereof.

8. A process for the spray application of polymeric coating compositions to a substrate, which comprises:
(1) forming a liquid mixture at temperature T°
in a closed system, said mixture comprising:
(a) a nonvolatile materials fraction containing at least one polymeric compound capable of forming a coating on a substrate; and (b) a solvent fraction which is at least partially miscible with the nonvolatile materials fraction and contains at least one compressed fluid in an amount which when added to (a) is sufficient:
(i) to render the viscosity of said mixture to a point suitable for being sprayed; and (ii) to enable said liguid mixture to form a liquid compressed fluid phase at temperature T°;
wherein the compressed fluid is a gas at standard conditions of 0°C and one atmosphere pressure (STP); and (2) spraying said liquid mixture onto a substrate to form a coating thereon by passing the mixture at temperature T° and spray pressure P1 into an orifice through which said mixture flows to form a liguid spray, wherein spray pressure P1 is above the minimum pressure P2 at which said liquid mixture forms a liquid compressed fluid phase at temperature T°.

9. The process of Claim 8, wherein said spray pressure P1 is above or just below the maximum pressure P3 at which said mixture forms a liquid compressed fluid phase at temperature T°.

10. The process of Claim 9, wherein said spray pressure P1 is less than about 600 psi above said pressure P3.

11. The process of Claim 8, wherein the viscosity of the liquid mixture of (a) and (b) is less than about 200 centipoise.

12. The process of Claim 8, wherein the solvent fraction contains at least one active solvent for the polymeric compound.

13. The process of Claim 8, wherein the compressed fluid is a supercritical fluid at temperature T° and spray pressure P1.

14. The process of Claim 8, wherein the compressed fluid is carbon dioxide, nitrous oxide, ethane, or a mixture thereof.

15. The process of Claim 8, wherein the at least one polymeric compound is selected from the group consisting of thermoplastic polymers, thermosetting polymers, crosslinkable film forming systems, and mixtures thereof.

16. The process of Claim 15, wherein the at least one polymeric compound is selected from the group consisting of enamels, varnished, lacquers, acrylic polymers, vinyl polymers, styrenic polymers, polyesters, alkyds, polyurethanes, two-package polyurethanes, epoxy systems, phenolic systems, cellulosic polymers, amino polymers, silicone polymers, polymers containing fluorine, and mixtures thereof.

17. The process of Claim 8, wherein the spray is a feathered, decompressive spray.

18. The process of Claim 8, wherein the liquid mixture or spray is electrically charged by a high electrical voltage.

19. The process of Claim 1, wherein the polymeric coating composition has a solvent portion composition of:
Weight Percent of Total Solvent Portion RER
30 - 100% < 50 0 - 70% 50 - 100 0 - 40% 101 - 250 < 10% > 250

20. The process of Claim 19, wherein the polymeric coating composition has a solvent portion composition of:
Weight Percent of Total Solvent Portion RER
40 - 100% < 50 0 - 60% 50 - 100 0 - 30% 101 - 250 < 5% > 250