CA2173600C - Method and apparatus for proportioning and mixing non-compressible and compressible fluids - Google Patents

Abstract

The present invention is directed to methods and apparatus for effectively proportioning a mixture of compressible and non-compressible fluids, wherein the resulting mixture has an almost constant density. The present invention is particularly useful for admixing supercritical fluids with polymeric coating compositions for various spray applications.

Description

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D-17147 217360~

lVlli~THOD A~l~ APPAR ~ FOR PROPORTIONING A~l) G NON COMP~h~ R~ AI~T) COMPRl;~ Rl.F~ uIn~c:

Field of The ~nv~ntion This invention pertains to mi~ing and proportioning a compressible fluid and a non-compres~ible fluid. In a ~refe,.ed embodiment of the invention the compressible fluid is a supercritical fluid, the non-compressible fluid is a coating composition, and the resultant mixture is applied to a substrate by spraying techniques.

Rack~ound Of The Inven~;on Coating compositions are comple~ mixtures which often include binders, pigments~ surfactants, flow-control agents, and organic solvents. Organic solvents serve a variety of purposes related to viscosity reduction, film foImation and adhesion. In spraying paints and coatings, organic solvents reduce their viscosity. This viscosity reduction is needed to enable atomization when the material is sprayed and also to facilitate droplet coalescence on the surface, thus giving a coherent, uniform film. Spray atomization requires a very low viscosity to produce the fine droplets needed for high-quality coatings.
Despite the important role of volatile organic compounds ("VOC") play in the co~ting~s formulation, there has been a considerable effort by the coating fo~nulators and applicators to reduce VOC
emissions for both economical and envirol-me~tal reasons.
A great deal of emph~cis has been placed on the development of new coating technologies which will reduce 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 having met the emission requirements and regulations. They are (a) powder, (b) waterborne, dispersion, (c) waterborne, solution, (d) non-aqueous dispersion, and (e) high solids coatings. Each of these tecnnologies has beer~ employed in certain applications, and each has found a niche in a particular ~ -- ) D-17147 ~173~

industry. In a majority of cases, the coatings from these new technologies are inferior to the old in one or more important application or performance features.
U.S. Patent No. 4,923,720 discloses Dhethods and apparatus for the production of the high solid co~tin~ formulation in which substantial amounts of the liquid solvent component have been removed and replaee~ w~th a w~-~xic an~ o~ çnt~lly comp~t;hle supercritical fluid, such as supercritical carbon dio~de. Thig co~tine composition is then sprayed onto a substrate at which time the supercritical carbon dioa~ide vaporizes to assist spray ~tomi7~tion. In order to produce a coating material solution or formulation with the desired application characteristics, the relative proportion of the liquid composition and supercritical carbon dioxide should be maintained at a predetermined ratio or within a predetermined range. Howev~l, one requirement of U.S. Patent No. 4,923,720 is to control the relative proportion of liquid coating composition and supercritical fluid. The liquid coating composition and supercritical fluid are each introduced into the system by a separate pump. However, the volllme of the supercritical carbon dio~ide is varied depending upon the system pressure and temperature. This can result in deviation of the supercritical carbon dioxide concelltlalion in the coating formulation, resulting in inconsistent spray characteristics.
U.S. Patent No. 5,215,257 discloses an improved method and apparatus for fo~ning and dispensing a coating material formulation or solution con~inin~ a fluid coating composition and a fluid diluent, such as a su~elc.;lical carbon dioxide. The control system opens and closes the supply of supercritical carbon dio~cide and/or liquid coating composition in accordance with variation of capacitance in the formulation. The devices requires predetermined set point values to control supercritical carbon dio~ide concentration in the co~t;n~
formulation. However, the correlation between the carbon dioxide concentration in the coating formulation and the values obtained by capacitance sensor can vary significantly depending upon system D-17147 ~ ~173600 pressure, temperature and coating formulation. ~urthermore, with respect to compositions having both liquid and gas components in a multiple phase solution, it has been found that controlling carbon dio~ide concentration is difficult. The signal fro~ tbe capacitance ~encing circuit produces a relativeiy widely fluctuating signal due to the formation of bubbles. Another deficiency of the apparatus is that the device requires the feed ~a~g ~ps~citance ~ormation of form~ tiQn before carbon dioxide addition to calculate control set point values with respect to carbon dioxide concentration.
Aforementioned U.S. Patent No. 4,923,720 discloses an apparatus capable of pumping and proportioning a co~ting formlll~tion and liquid carbon dioxide. In one embodiment, volumetric proportioning of the coating formulation stream and the supercritical carbon dio~ide stream is carried out by means of reciprocating pumps which displace a volume of fluid from the pump during each one of its pumping cycles. One reciprocating pump is used to pump the co~t;ng formulation which is slaved to another reciprocating pump which is used to pump the liquid carbon dio~ide. The piston rods for each pump are attached to opposite ends of a shaft that pivots up and down on a center fulcrum. The voll~me ratio is varied by sliding one pump along the shaft, which changes the stroke length.
However, liquid carbon dioxide is relatively CO~pl esfiible at ambient temperature, the temperature at which it is typically stored in a pressurized cont~iner. Such comprescihility may undesirably cause fluctuations and osçill~tio~.c of the amount of carbon dioxide that is present in the ~rlmiYed co~ting formulation that is to be sprayed. This occurs due to the incompatible pumping characteristics of the relat*ely non-compressible coating formulation and the relatively co~-essible liquid carbon dioxide. With the coating formulation, pressure i5 immediately generated in the reciprocating pump as soon as its volume is displaced. Inasmuch as the liquid carbon dioxide is subst~r~ti~lly compressible, a larger volume is needed to be displaced in order to generate the same pressure. Because mi~ing occurs when the fiow of 2173~00 the coating formulation and of the liqu d carbon dioxide are at the same pressure, the flow rate of carbon dio~ide lags behind the flow rate of the coating formulation.
This oscillation is further accentuated if the driving force operating the pump varies during the operating cycle, such as an air motor rh5~nging direction dunng its cycle. Thus, if the driving force declines, the pressure in the co~1;ng f~mtll~t;nn flow de~l~bes even more rapidly, due to its non-co~ylessihility~ than the pressure in the liquid carbon dio~ide flow.
Accordingly, the pressures generated in both flows may be out of phase during the pllmping. U.S. Patent No. 4,621,927 discloses a ~L~LL~ e control apparatus controlling a flow rate of a second fluid to be mL~ed with a first fluid so as to prepare a third fluid having a predetermined concentration. A set point variable of the flow rate of the second fluid is calculated in accordance with the flow rate of the third fluid so as to improve controllability of the apparatus. However, the invention in U.S. No. 4,621,927 cannot control the mixture of compressible fluid(s) and non-compressible fluid(s) because the thermodynamic properties of the fluids are influenced by variables such as pressure, temperature, and concentration.

~umm~rv of The ~nv~n~on By virtue of the present invention, the above deficiencies have now been overcome. Methods and apparatus have been discovered which are capable of accurately and continuously providing a proportioned mi~ture comprised of a non-co~lessible fluid and a compressible fluid.
In particular, the present invention measures the volumetric flow of the non-compressible fluid stream before and after the addition of compressible fluid to determine and to control the amounts of co~)~ressible fluid. This invention ~imply and accurately proportions the ~uids because it has been surprisingly discovered that the density of the non-compressible fluid and compressible fluid mi~ture D-17147 ~1736û0 does not vary significantly in many systems as long as the solubility limit of the compressible fluid in the non-compressible fluid ~ix l,u}e is not e~ceeded.
As used herein, the phrase "compres`sible fluid" is meant to include a material whose density is affected by a change in pressure to an extent of at least about 5%. As used herein, all fluids are understood to be at one atmosphere pressure a~ Q~ ess otherw~ie aoted.
More specifically, the present invention in its broader embodiment comprises an apparatus for continuously miYir~ a 6ubstantially compressible fluid and a subst~nt~ y non-co~ essible fluid in a predetermined proportion which includes:
a) means for supplying substantially Co~ essible fluid;
b) means for supplying substantially non-compressible fluid;
c) means for measuring the voll~metric flow rate of the substantially non-compressible fluid;
d) means for generating a signal based upon the volumetric flow rate of the substantially non-compressible fluid;
e) means for forming a mLL~U~ e of the measured substantially non-co~ressible fluid and subst~nti~lly compressible fluid, 6uch that the density of the resulting mixture behaves substantially like a non-co~ ressible fluid;
f) means for measuring the volumetric flow rate of said mixture;
g) means for generating a signal based upon the flow rate of the substantially com~ressible fluid and substantially non-coll-y, essible fluid ~ . e; and h) means for controlling the flow rate of the subst~nti~lly - compressible fluid in response to the sir~ls ~enerated in (d) and (g).

- t D-17147 ~ ~l73ç;ao As used herein, the phrases "coating formulation" or "coating composition" are understood to mean a typical, collvelltional co~ting composition which does not have any supercritical fluid admiged therewith. Also as used herein, the phrases "~ ed liquid ....~ . e" or "admiged coating formulation" are meant to include an admisture of a coating form~ tion with at least one supercritical fluid.
The present invention also comprises a method for formin~
a mixture of a substantially compressible fluid and a subst~nti~lly non-compressible fluid in a predetermuned proportion which comyl;ses:
a) providing a non-compressible fluid;
b) measuring said non-compressible fluid's volumetric flow rate;
c) providing a colllylessible fluid;
d) mimng the compressible fluid witb the non-comyressible fluid such that the density of the resulting migture behaves substantially as a non-compressible fluid;
e) measuring the volumetric flow rate of the migture; and f) controlling the flow rate of the compressible fluid based upon the volumetric flow rate of said mixture.
As used herein "substantially as a non-co"~ essible fluid"
is understood to include a mixture whose density is unaffected by a change in concentration of the components in the m~ re of less than about 10~c, preferably of less than 5%, and most preferably of less than 2%.
By measunng the volumetric flow rate of the non-compressible fluid and ~oml)ressible fluid/non-compressed fluid mixture and then controlling the flow rate of the compressible fluid pump, the difficulties associated with handling a compressible fluid are substantially elimin~ted. In a preferred embodiment of the invention the density of the resulting fluid mixture is also measured to ensure D-17147 - ~ ~ 7 3 6 0 0 that the fluid mi~tllre is behaving substantially as a non~ essible fluid.

netailed nescr~Dtion Of The n~win~s Figure 1 is a phase diagr~m for a supercritical carbon dio2ide, polymer and solvent system.
Figure 2 is a graph of the density versus ~mMSitiDn of ethanol/water and isopropyl alcohol/water systems.
Figure 3 is a graph of the density versus composition of a dimethyl sulfoxide/acetone system.
Figure 4 is a graph of the density versus composition of an acrylic polymer/methyl aryl ketone solution.
Figure 5 is a graph of the density versus composition of a polymeric coating composition/carbon dioxide solution.
Figure 6 is a diagram of the apparatus suitable for proportioning and spraying a compressible fluid and non-compressible fluid.
Figure 7 is a diagram of the apparatus used to conduct the experimental trials described herein.
Figures 8-11 are graphical representations of flow rate versus time for the spray application of various coating mi~ es.
Figures 12 and 13 are graphs of the density versus composition for two coating compositions in carbon dio2ide.
Figures 14 and 15 are graphs of the density versus composition for two coating compositions in ethane.

netailed Descrintion Of The InvenRon It is to be understood that while the following discussion will primarily focus upon providing a proportionated ~mised liquid mi~ture of a coating formulation and supercritical fluid, suc~ as carbon dio~ide, which is suitable for being sprayed onto a substrate, ~he present invention is in no way limited to this embodiment. As is readily apparent from the foregoing discussion, the present invention .

3~0 encompasses the proportionation of any compressible and non-compressible fluid to form a desired ~lu~ e for any intended subsequent use.
The co~hne compositions employed~in this ~lvel~tion are broadly defined to include paints, lacquers, adhesives and the like.
Such coating materials may also include those that are typically ltili7e.
in the agricultural field such as, but not limited to, fertilizers, herbicides and insecticides.
The coating compositions employed in the present invention typically co~l;ses a solids component cont~inine at least one polymeric component, piem~nts~ melting agents, cross-linking agents, ultraviolet light stabilizers. In addition to the solids component, a solvent fraction is also employed, including active solvents, coupling solvents and water. Other liquid components often found in coating compositions may also be used such as curing agents, plasticizers, surfactants and the like. The components of both the solvent fraction and the liquid fraction of coating compositions are well known to those with skill in the art. A more thorough ~liscllcsion of the components found in coating compositions can be found in U.S. Patent No.
5,171,613.
Supercritical fluud phenomenon is well doc~lmente~, (6ee pages F-62 - F-64 of the CRC Handbook of Ch~mistry and physics~ 67th Edition, 1986-1987, pl~h1isher~ by the CRC Press, Inc., Boca Raton, Florida). At high pressures above the critical point, the resulting supercritical fluid, or "dense gas", will attain densities appro~-hir~g 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 subst~nc~
merge into each other and represents the comhin~tion of the cFitical temperature and critical pressure for a given substance. The critical temperature", as used herein, is defined as the temperature above which a gas cannot be liquefied by an increase in pressure. The "critical D-17147 21731i00 pressure", as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the "critical temperature".
The compressihility 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 gréatly enh~nced soll~hili7ing capabilities compared to those of the "subcritical" compound, with higher diffusion coefflcients and an extended useful temperature range comp~red to liquids. Compounds of high molecular weight can often be dissolved in the supercritical fluid at relatively low temperatures. An interesting phenomeno~ associated with supercritical fluids is the OCCul 1 ellce 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 present invention.
Near-supercritical liquids also demonstrate solubility characteristics and other pertinent properties ~imil~r to those of supercritical fluids. The solute may be a liquid at the supercritical temperatures, even though it is a solid at lower tem~ al~6. In addition, it has been demonstrated that fluid "modifiers" can often alter supercritical fluid properties significantly, even in relatively low concentrations, greatly increasing 601ubility for 60me 601utes. These variations are considered to be within the concept of a supercritical fluid as used in the conte~t of this invention. Therefol e, as used herein, the phrase "supercritical ~uid" denotes a compound above, at, or slightly below the critical temperature and pressure (the critical point) of that compound.
~ Ys~mples of compounds which are known to have utility as supercritical fluids are listed in aforementioned U.S. Patent No.

4,723,920.

D-17147 ~ ~1~3600 Due to the low cost, enviro~mental acceptability, non-flAmm~qhility and low critical tempel at~l~e of carbon dioxide, supercritical carbon dio~ide fluid is preferably used with the coating formulations. For many of the same reasons, nitrous o~ide (N20) is a desirable supercritical fluid for a~mirtt~re with the coating formulations. However, any ofthe supercritical fluids and the mi~tvres eof are to be-considered as being applicable for use with the coating formulations.
The solvency of supercritical carbon dioxide is substantially simil7~r to that of a lower aliph~t;c hydrocarbon and, as a result, one can consider supercritical carbon dioxide as a replAcemPnt for the hydrocarbon solvent of a conventional coating formulation. In addition to the enviromnental benefit of replacing hydrocarbon solvents with supercritical carbon dioxide, there is a safety benefit also, because carbon dioxide is non-flAmmAhle.
Due to the solvency of the supercritical fluid with the coating formulations, a single phase liquid mixture is formed which is capable of being sprayed by airless spray techniques.
Coating formulations are commonly applied to a substrate by pAssing the coating formulation under pressure through an orifice into air in order to for_ a liquid spray, which imp~ s the substrate and forms a liquid co~ting. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray, airless spray, and air-assisted airless spray can also be used with the liquid coatin~ forml~lat;on heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid coating formulation and aids ato_ization. The present invention can also be applied by electrostatic applications as described in U.S. Patent No. 5,106,650.
- In essentially every process in which a miYtllre is preparedfor a particular purpose, the constituents of that mi~ture usually need to be present in particular, accurately proportionated amounts in order D-17147 ~173~iO0 for the mixture to be effective for its intended use. In the aforementioned related patent, the underlying objective is to reduce the amount of organic solvent present in a coating form~ tion by the use of supercritical fluid. Understandably, with this objective in mind, it is generally desirable to utilize as much supercritical fluid as possible while still ret~ining the ability to effectively spray the liquid ~ e of coating fc~ulations and supercritical fluid and also obtain a ~eRi coating on the substrate. Accordingly, here too, it is particularly preferred that there be prescribed, proportionated amounts of supercritical fluid and of coating formulation present in the liquid coating mixture to be sprayed.
Generally, the preferred upper limit of supercritical fluid addition is that which is capable of being miscible with the coating formulation. This practical upper limit is generally recogni7~b1e when the a~mi~zture cont~inin~ coating formulation and supercritical fluid breaks down from one phase into two fluid phases.
To better understand this phenomenon, reference is made to the phase diagram in Figure 1 wherein the supercritical fluid is supercritical carbon dio~ide fluid. In Figure 1, the vertices of the triangular diagram represent the pure components of an ~miYe~
coating formulation which for the purpose of this discussion cont~inc no water. Vertex A is an organic solvent, vertex B i8 carbon dioxide, and vertex C represents a polymeric material. The curved line BFC
represents the phase boundary between one phase and two ph~ces. The point D represents a possible composition of a coating formulation in which supercritical carbon dio~de has not been added. The point E
represents a possible composition of an ad_ixed coating formulation, after admi~ture with supercritical carbon dioxide.
Thus, afler ~tomi7~tion, a majority ofthe carbon dio~ide vaporizes, leaving subst~nt,;~11y the composition of the original co~ting formulation. Upon contacting the substrate, the rem~ining li4uid e of the poly_er and solvent(s) component(s) will flow, i.e., coalesce, to produce a unifonn, smooth film on the substrate. The film D-17147 217360~

forming pathway is illustrated in Figure 1 by the line seEments EED
(at~mi7~tion and decG~ ression) and DC (coalescence and film formation).
However, the ~mount of supercritica~ fluid, such as 6upercritical carbon dio~ide, that can be rnLYed with a coating formulation is generally a fllnctio~ of the miscibility of the 6upercritical fluid with the coating formulation as can be-st b~gllr~i7e~ by ~ g to Figure 1.
As can be seen from the phase diagram, particularly as shown by arrow 10, as more and more supercritical carbon dioYide is added to the coating formulation, the composition of the ~miYe~ liquid coating mixture approaches the two-phase boundary represented by line BFC. If enough supercritical carbon dioside is added, the two-phase region is reached and the composition correspondingly breaks down into two fluid phases. Sometimes, it may be desirable to admis an amount of supercritical fluid (in this case, supercritical carbon dio~ide) 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.
In addition to avoiding the two-phase state of the supercritical fluid and the coating formulation, proper proportionation is also desirable to provide o~ ,m spraying conditions, 6uch a6, formation of desired s~lmiyei3 viscosity, formation of desired particle size, formation of desired S~l ayed fan shape, and the like.
Accol ~i..gly, in order to spray liquid co~ing formulations cont~ining supercritical fluid as a diluent on a continuous, semi-continuous, and/or an intennittent or peAodic on-demand basis, it is necess~ to prepare such liquid co~t;n~ fonnulations in response to such 6l 1 ayh.g by accurately miYing a proportioned amount of the coating formulation with the 6upercritical ~uid. Ho~ve~e~, the compressihility of supercritical fluids is much greater than that of liquids. Consequently, a small change in pressure results in large changes in the density of the 6upercritical fluid.

D-17147 ~173600 The non-coml..essible ~uid in the present invention is typically in the liquid state. The liquid state is characterized by the strong interaction of the molecules, which di~ uishes liquids from gases, and the state of disorder of the molectlkt-`motion, which distinguishes liquids from solids. The behavior of liquids are generally well understood and their properties tend not to vary ~i~tificstntly over dis~rete ranges.
However, no known liquid solutions are esactly ideal.
Solutions of high]y ~imilstr components may only show slight deviations, whereas greater deviation are observed in almost all other 601utions, where the components differ in size, mass and chemical nature. It has been observed that polymers do not easily blend to form true solutions.
As a result, polymers separate into distinct phases when brought together if there are appreciable di~el ellces in the molecules. One of the easiest ways to characterize the differences in behavior of liquid mi~ture is to measure the density of the ~ e.
Figure 2 is a plot of liquid density versus composition of water and ethanol and water and iso-propyl alcohol at atmospheAc pressure at 20C. With the addition of ethanol or isol.ro~yl alcohol to the mi~ture, the density of the mixture gradually decreases to the density of the undiluted plcohol. Figure 3 demonstrates a ~jmils~r result with a plot of the liquid density of dimethyl sulfoxide and ~cet4~e at atmospheric temperature and pressure.
Some polymers in liquid solvents also behave ~imil~ly.
Referring to Figure 4, an acrylic polymer (AT954, Rohm & Haas Co.) and n-methyl aryl ketone (MAK) were mi~ced at atmospheric pressure and 25C. With increasing MAK levels, the density of the ~ e decreased gradually to the density of pure MAK
Surprisingly, it has been discovered that in contrast to the above ~l~es wherein the density of the ~ e compositions uniformly decreases, ~l~ es of polymeric compositions, solvents and compressible fluids undergo a period wherein the density is relatively constant. This relatively constant density mi~ture rem~inc until a two D-17147 ~173600 phase mi~ture is created at which point the density of the mi~ture changes rapidly.
Referring to Figure 6, a plot of ,.lule density ofthe components listed in Table 1 below, in carbon dioxide is presehted .

T~T F l COMPONF.~TS W~IGHT PF~CF~T
Alkyd, Reichhold 6256-03 21.6%
Nitrocellulose, 6.0%
Plasticizer 2.4%
Urea, Bettle 80 resin 10.0%
Solvents 60.0%
(mi~ture of MAK, i-propyl alcohol, n-butanol, and ethyletho~y propionate (EEP)) With the addition of carbon dioxide (up to 30 weight percent) to the composition in Tab!e 1, the mixture density decreased less than 1.2 percent. With the addition of more than 40 percent carbon dio~ide, mixture densities decreased siFnifis~ntly and two distinct ph~ses were created, 8 carbon dioxide rich phase and a polymer rich phase.
Without wishing to be bound by any theory, we believe that the arrangement of the polymer and solvent molecules change with the continuing addition of carbon dio~nde to a polymer-solvent ~ e such that the coating forrmll~t;on maintains a constant ratio between the total mass and the total volume (the density of the system). Such effect would be a result of an çnh~ncetl interaction between the solvent and the polymer due to the presence of carbon dio~ide. The large free volume contributed to the system by the carbon dioxide would l,el~t a better solvent and polymer interaction, resulting in a smaller density reduction than expected.
Referring now to Figure 6, apparatus is depicted which by taking advantage of the relative constant density, is capable of ~ : } ~
D-17147 ~173600 pllmping, pressurizing, proportioning, heAting, and miYing a co~ting composition with carbon diogide to form an a-~mixed liquid ~ ure through only volumet;ric measurem~nts The co~ c~mposit;on and supercritical carbon dioYide is provided at the precisely desired proportions ready for being sprayed. The a~& allls depicted herein is able to simply and e!egantly p~ ,Ol ,ion the liquid miYture by m~king use of-~e constant density pherlomen~ described herein. As notes~
above, while this ~i~cllcsion is focused on carbon dioxide it is not limited to this material and the present invention may include any compressible fluid.
In particular, carbon dioxide is supplied as a liquid from any suitable source (9), such as a t~nk or cylinder. Preferably, the liquid carbon diogide is supplied on a continuous basis. The carbon diogide is then fed to carbon dioxide feed pump (7) through an optional 0-3000 psi pressure indicator (8). The carbon dioxide is sent to a control valve (10) then heated to about 30-80C in the prehe~ter (3) and then sent to mixer (5). Refernng now to the coating composition, the co~1;n~
is supplied by a pllmp (1) through an optional pressure gauge (2), through a flow meter (4) to the preheater (3). The coating composition is then sent to the miYing unit (5) to form the a~mixed liquid ~LI~e.
The flow rate of the coating composition and carbon dio~ide are then measured by the 6econd flow meter (11). An optional thermocouple (6) is preferably provided. An optional density meter (17) is preferably provided to monitor the density of the ~miyer7 coating formulation. In a most preferred embo~ime~t a density meter is employed to ensure that the flow rate of carbon dioYide does not becoTne so large as to create a significant change in the density of the ~miye~
coating formulation. A sight gauge (18) is ~.efelsbly employed for phase analysis. The prlmiYetl co~ting formulation miYture can then be adjusted to desired final temperature by an optional heater (not shown) and provided through a conduit (13) to the spray gun (14). Tbe mixture of coating and carbon dioYide also r~n be recirculated through the heater (12) and recirculation pump (16) to m~int~in constant spray D-17147 il73600 temperature if desired. A multi-ch~nnel flow ratio co~ u~er (15) receives the cign~l~ of the flow rates from both the flow meters and is used to output Eignal to control the flow rate of tbe carbon dio~cide via control valve (10).
The specific equipment items employed in Figure 1 are listed in Table 2 below.

TART ~ ~.

ITF~l~I n~c~rpTIoN
Coatings feed pump, Graco Model 205-630 2 Pressure in~ tQr, range from 0 to 3000 psi 3 Nordson H-400 series paint beater 4 Precision gear meter, ZHM-01, AW. Co.
Sparger and static KenicsTM mixer 6 Thermocouple, k-type 7 Carbon dioxide feed pump, Haskel Model No.
DSF-25 witb 51050 Spool 8 Pressure indicator, range from 0 to 3000 psi 9 Carbon dioxide Cylinder Jordan control valve, Model 708, 0.002cv, linear trim 11 Precision gear meter, ZHM-01, AW. Co.
12 Nordson paint heater 13 High pressure ~pray hose 14 High pressure spray gun Multi-ch~nne1 flow ratio computer, EMO-1005 16 Ross MF-24-11-10-AAAA Recirculation pltmp 17 Micromotion Model No. D40HSS
Density meter 18 JergensonTM sight gauge -_ ~ Q

The type of volumetric flow meter used in the present invention is not critical. Any suitable volumetric flow meter such as gear meters, turbines and rotameters and the like may be used of which gears meters are preferred.
Whereas, the exact scope of the present invention is set forth in the appended ~ imc~ the following specific eY~mples illustrate oertain aspects o~e present invention and more particularly, point o~t =
methods of evaluating the s~me. However, the examples are set forth for illustration only and are not to be construed as limitations on the present invention as set forth in the appended claims. All parts and percentages are by weight unless otherwise` specified.

T~XAMPT,T~ 1 Apparatus suitable for studying the controll~hility of compressible fluid, specifical~y carbon dio~ide, was constructed and is depicted in Figure 7. The unit was comprised of feed pumps for coating formulation (101) and carbon dioxide (107), two ~ow gear meters (104) and (111), a control valve (110), heaters (103), a micro-processor based flow controller (115) and a homogeneous mi~ing unit for the two fluids (105). The co~ting material was fed from a cor~iner, and pressurized to 1500-2200 psig at room temperature by an air-driven liquid co~ting pump. The coating material was preheated to 30-40C through a heater (103). The flow rstes of co~qting materisl were measured by a pre~i~iQn gear meter (104). Liquid carbon dioxide was fed from a cylinder, and pressurized to 1500-2200 psi at room temperature by an air-driven carbon dioside liquid pump (107). Then carbon dio~ide was preheated to 30-40C through a heater (103). These two ~uids were mixed through a ~nising unit (105), which was comprised of a ~a,~el, and two Kenies~M mixers.
The flow rates of the ~l~ e of coating material and carbon dio~ide were measured by a precision gear meter (11~,), and heated in heater (112) to 45-60C before spray application. The D-17147 - ~173600 mi~ture of coating and carbon dioxide were re-circ~ te-l through the spray gun (114) to m~int~in constant spray tempelstula.
A multi-ch~nnel flow ratio co~u~er (115) ~ac~ived ~i~n~l~
of the flow rates from both gear meters, displayed the totaled flow rates, and was used to manipulate the position of a carbon dionde control valve (110) to control a required carbon dioxide cQncentration in the coating mixture. ~or the data analysis, the flow rate of carbon dioxide was also monitored with a mass flow meter (109), and the data from gear meters (a) and (b) were interfaced to a computerized data acquisition system (116).
The specific items listed in Figure 7 are as follows:

TART F. 3 ITEM n~SCR~PTION
101 Coatings feed pump, Graco Model 206-530 102 Pressure indicator, range from 0 to 3000 psi 103 Nordson H-400 series paint heater 104 Precision gear meter, ZHM-01, AW. Co.
105 Sparger and static KenicsTM mixer 106 Thermocouple, k-type 107 Carbon dioxide feed pump, ~kel 108 Pressure indicator, range from 0 to 3000 psi 109 Mass flow meter, Micro Motion meter Model No. D6 110 Jordan con-trol valve, Model 708, 0.002cv, linear trim 111 Precision gear meter, ZHM-01, AW. Co.
112 Nordson heater H-400 113 High pressure spray hose 114 High pressure spray gan 115 Multi-channel flow ratio computer, EMO-1005 116 Computerized data acquisition system Cole Palmer, D-17147 ~173600 Figure 8 is a plot of co~tingS flow rate versus time (120 seconds) for continuous sl,~ay~lg of an A~mi~ed coating form~ t;on from a spray apparatus depicted in Figure 7. The coAt;n g form~ t;oI~
was a mi~ture of acrylic and mel~mine polymers~and organic solvents.
Point #1 in tbe Figure 8 is the coatings flow rate measured by a precision gear meter (104). Point #2 in the Figure 8 is tbe flow rate of the admi~ed coatings formula~ion flow rate measured by a precicion gear meter (111). From the disclosure of tbis invention, ca~l,oll dioxide flow rate is the difference between tbe re~rlin~ of gear meter (111) and the readings of gear meter (104).

~XAl~
Figure 9 illustrates carbon dioxide flow rates from a spray unit using the coating formulation described in Table 1 in an appa atus ~imil~r to Figure 7 determined by two methods; 1) calc~llAti~ the differences in flow rate between the two flow gear meters from Figure 7, and 2) actual carbon dioxide flow rates measured by the mas6 flow meter (109). The differences in the graphs is believed to be c~-lce~ by response time delays and the effect of data averaging in the mas6 flow meter, because it requires 0.2-0.6 second time delays for the flow calculations to be conducted However, overall flows for 120 6ec~nrlR
were 89.0 cubic centimeters (cc) from this invention, and B9.~ gram6 from mass flow meter, indicating that the:
1. Density of the mixture of coAtings and carbon dioxide is close to 1.0 grams/cc, which is almost the s~me as the density of co~tin~
material alone; and 2. The combir~tion of two volumetric flow meters can be used to measure and accurately control carbon dio~nde con~entrations.

A~ .~ 3 Fi~re 10 shows three plots of flow rates; l)co~tin~
composition,; 2) mi~ture of coating composition and carbon dio~cide; and 3)carbon dio~ide for a 90 second continuous spray interval using the D-17147 ~173~00 ~-coating formulation in ~Y~mple 1. Apparatus simil~r to that disclosed in Figure 7, without a recirculation loop was used. Line #1 in Figure 10 indicates co~tings flow rates measured by a precision gear meter. Line #2 in Figure 10 was the coatings and carbon dio~de mi~cture flow rates measured by a precision gear meter. Line #3 in Figure 10 carbon dio~ide flow rates calculated from the differences between the re~iing~
of the gear meter and the reP~ings of the flow meters. Overall these plots follow the same trends, and the totalized flow rates of coatings and ~lul e of coatings and carbon dio~ide for 90 seconds were 219.~ cc and 310.9 cc, respectively. Therefore, carbon dioxide flows for 90 seconds were 91.4 cc from the method of the present invention, and 92.0 grams as measured by the mass flow meter, indicating that the comhin~tion of the two volumetric flow meters can be used to accurately measure and control carbon dioxide flowrates.

~Al~ ~ 4 Figure 11 shows two flow rates: 1) coatings; and ~miYe~
coating formulations intermittently sprayed for 800 seCo~lc from a spray unit described in Figure 7 without a recirculation loop. Point #1 in the Figure 11 indicates coatings flow rates measured by a precision gear meter 104. Point #2 in the Figure 11 inrli~tes the flow rate of coating and carbon dio2ide flow mixture measured by a pre~i~ion gear meter 111. Overall, these plots followed the same trends, and the totalized flow rates of coatings and ~lmi~ed coating formulation for 800 ~econds were 219~ cc and 3109 cc, respectively. Therefore, carbon dio~ide flows for 800 seconds were 914 cc as measured by the method of the present invention, and 920 grams from mass flow meter. The Example once again demonstrates that the comhin~tion of two -volumetric flow gear meters can be used to measure and control carbon dioxide concentrations accurately.

D-17147 2 i 73600 ~XA~ ~ 5 In this e~ample, the total flowrate of carbon dioxide obtained from a carbon dioxide mass flow meter and two volumetric gear meters were compared at the different process conditions.
Apparatus depicted in Figure 7 was employed to make the comparisons.

T~A~, CONrlITION~ l2 E
Conditions of at mi~ing, Temperature (C) 33 36 40 45 50 Pressure(psi) 1600 1600 1600 1600 1600 CO2, measured from mass meter (grams) 101 115 105 96 86 C2 measured(cc) 105 121 112 120 130 from flow meters Relative error (~c) 3.5 5.2 6.7 25 51 Relative error is defined as (CO2 from this invention-CO2 from mass meter)/CO2 from mass meter.
As expected with increasing temperatures, the density of the admixed coating formulation changes. The c~nging density of the e results in a larger l,e~ ce~-tage error when relying on volumetric measurements.

~XAl~IP~,~ 6 The total amount of carbon dio~ide mixed with a coating formulation and sprayed from apparatus depicted in Figure 7 was measured. The amount of carbon dio~ide used was measured using a mass flow meter and two voIumetric gear meters. The coating formulation consisted of 69 weight percent AT-954 Acrylic, available D-17147 ~3600 from Rohm & Haas, and 31 weight percent MAK. The pressure and temp~ at~ll e at the coatings and carbon dioxide mi~ing unit were maintained at 1600 psi and 36C, respectively.

T~TA~ .
coNrlITIoN~
CO2, concentration in the formulation 15% 27.6% 39%
CO2, measured from mass meter (grams) 31 50 71 CO2, measured from volumetric meter (cc) 30.5 60.0 74.9 solution appearance clear clear haze Relative error (~) 1.6% 0.05'o 5.5%
Relative error is defined as (C2 from this invention-CO2 from mass meter)/CO2 from mass meter A clear solution appearance is indicative of a single phase solution. A hazy appearance indicates that the solution is in two distinct phases. This ~mple demonstrates the highly accurate proportionation of the fluids when the single phase solution is maintained. When a two phase solution is created, the density of the 60lution typically bègins to change rapidly and the accuracy of the proportionation apparatus is liminished.

l;~Al~p~,~ 7 The apparatus of ~mple 1 was used to 6pray the formulation of Table 1 with carbon dio~ide at 1500 psi and 50C.
At 1500 psi and 50C, densities of coating formulation a~d supercritical carbon dio~ide were 0.9652g/cc and 0.3978 g/cc, respectively. With the addition of carbon dio~ide into the form~ ;on (a) up to 30 percent, the mixture densities decreased les6 than 1.2 percent. However, with the addition of more than 40 percent~ arbon dio~de, mixture densities decreased significantly, and the co~ting D-17147 ~ 3 ~ 0 0 formulation displayed two distinct ph~es; a carbon dioxide rich phase and a polymer rich phase.

~ XAl~qP~J~ 8 Figure 13 is a graph o~ miYture density-of the coating formulation listed below with carbon diogide at 1500 psi and 50C as measured by the spray unit depicted in Figure 6. At 1500 psi and 50C, the densities of the coating formulation and supercritical carbon dioxide were measured as 0.9700 g/cc and 0.3978 g/cc, respectively.
Adding carbon diogide into the admi~ed coating formulation up to levels appro~çhing 40 percent, the a~rni~ed coating fonnulation mixture density decreased less than 1.5 percent. However, with the addition of more than 45 percent carbon dio~ide into the ~rlmiye~
coating formulation, the miYture density significantly decreased, and the mi~ture separated into distinctive two phases.

Components Weieht ~er~nt Alkyd, 6255-03 20.6%
Nitrocellulose, 5.7%
Plasticizer 9.5%
Water 4.8%
Solvents 57.1%
(miYture of MAK, i-propyl alcohol, n-bllt~nol, EEP) ~XAl~p~ ~ 9 Figure 14 is a plot of mi~ture density of co~tin~
formulation from ~Y~mple 8 with supercritical ethane at 1500 psi and 50C measured from a spray unit in Figure 5. At 1500 psi and 50C, the densities of the coating formulation and supercritical ethane were measured as 0.9652 g/cc and 0.203 g/cc, respectively. With the addition D-17147 217 360n of ethane into the formulation at levels up to about 25 percent, the mixture was a fiingle clear phase. However, the mi~ture density decreased more than 19 percent.

~Al~P~,~ 10 Figure 15 is a plot of ~l.u e density of coating formulation from F~y~mple 7 with supercritical ethane at 1500 psi and 50C measured from the fipray unit depicted in Figure 6. At 1500 psi and 50C, the densities of 100 percent of coating formulation and supercritical ethane were measured as 0.9652 g/cc and 0.203 g/cc, respectively. With the addition of ethane into the formulation (a) up to 17 percent, the mi~ture showed a fiingle clear phase. However, the mi~ture densities decreased more than 11.7 percent.
Figures 12-15 clearly demonstrate that the une~pected constant density properties of the admi~ed coating formulations especially when using supercritical carbon dioxide as a viscosity reducing agent. However, when a two phase solution is created, tbe density of the mixture can vary significantly witb increasing COlll~l essed fluid levels. The ethane mixtures did not exhibit a fiubst~nti~lly constant density region when admi~ed, therefore the present invention would not be fiuitable for accurately proportionately these mi~tures.

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for preparing a mixture of a compressible fluid and a non-compressible fluid comprising:
a) providing a non-compressible fluid;
b) measuring said non-compressible fluid's volumetric flow rate;
c) providing a compressible fluid;
d) mixing the compressible fluid with a sufficient amount of the non-compressible fluid such that the density of the resulting mixture behaves substantially like a non-compressible fluid;
e) measuring a volumetric flow rate of the mixture;
and f) controlling the flow rate of the compressible fluid based upon the flow rate of said mixture.

2. The method of claim 1 wherein the non-compressible fluid is a polymeric material and the compressible fluid is a supercritical fluid.

3. The method of claim 2 wherein the resulting polymeric material/supercritical fluid mixture is sprayed onto a substrate.

4. The method of claim 3 wherein the polymeric material/supercritical mixture is recirculated prior to spraying.

5. The method of claim 1 wherein a second non-compressible fluid is provided to the non-compressible fluid prior to being measured by the volumetric flow meter.

6. The method of claim 1 wherein precision gear meters are employed to separately measure the flow rate of the non-compressible fluid and the flow rate of the fluid mixture of the non-compressible fluid and compressible fluid.

7. The method of Claim 1 wherein the flow rate of the compressible fluid is controlled by the volumetric flow rate of the non-compressible fluid and volumetric flow rate of the fluid mixture of the compressible fluid and non-compressible fluid.

8. An apparatus for mixing a substantially compressible fluid and a substantially non-compressible fluid in a predetermined proportion which comprises:
a) means for supplying substantially compressible fluid;
b) means for supplying substantially non-compressible fluid;
c) means for measuring the volumetric flow rate of the substantially non-compressible fluid;
d) means for generating a signal based upon the flow rate of the substantially non-compressible fluid;
e) means for forming a mixture of the measured substantially non-compressible fluid and substantially compressible fluid such that the density of the resulting mixture behaves substantially like a non-compressible fluid;
f) means for volumetrically measuring the flow rate of said mixture;
g) means for generating a signal based upon the flow rate of the substantially compressible fluid and substantially non-compressible fluid; and h) means for controlling the flow rate of the substantially compressible fluid in response to the signals generated in (d) and (g).

9. The apparatus of claim 8 wherein the means for measuring the flow rate of the substantially non-compressible fluid is a positive displacement pump.

10. The apparatus of claim 8 wherein the means for measuring the volumetric flow rate of the mixture of the substantially non-compressible fluid and substantially compressible fluid is a gear meter.

11. The apparatus of claim 8 wherein the means for measuring the flow rate of the substantially non-compressible fluid is a gear meter.

12. The apparatus of claim 8 wherein the means for measuring the flow rate of the substantially non-compressible fluid is a gear meter, turbine meter or rotameter.