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Description  |
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FIELD OF THE INVENTION
This invention pertains to mixing and proportioning a compressible fluid
and a non-compressible fluid. In a preferred 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.
BACKGROUND OF THE INVENTION
Coating compositions are complex 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
formation 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 coating's formulation, there has been a considerable effort by the
coating formulators and applicators to reduce VOC emissions for both
economical and environmental reasons.
A great deal of emphasis 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 technologies has
been employed in certain applications, and each has found a niche in a
particular 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. Pat. No. 4,923,720 discloses methods and apparatus for the production
of the high solid coating formulation in which substantial mounts of the
liquid solvent component have been removed and replaced with a non-toxic
and environmentally compatible supercritical fluid, such as supercritical
carbon dioxide. This coating composition is then sprayed onto a substrate
at which time the supercritical carbon dioxide vaporizes to assist spray
atomization. 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. However, one requirement of U.S. Pat. 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 volume of the
supercritical carbon dioxide is varied depending upon the system pressure
and temperature. This can result in deviation of the supercritical carbon
dioxide concentration in the coating formulation, resulting in
inconsistent spray characteristics.
U.S. Pat. No. 5,215,257 discloses an improved method and apparatus for
forming and dispensing a coating material formulation or solution
containing a fluid coating composition and a fluid diluent, such as a
supercritical carbon dioxide. The control system opens and closes the
supply of supercritical carbon dioxide 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 dioxide concentration in the coating 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 pressure, temperature and coating
formulation. Furthermore, with respect to compositions having both liquid
and gas components in a multiple phase solution, it has been found that
controlling carbon dioxide concentration is difficult. The signal from the
capacitance sensing circuit produces a relatively widely fluctuating
signal due to the formation of bubbles. Another deficiency of the
apparatus is that the device requires the feed coating capacitance
information of formulation before carbon dioxide addition to calculate
control set point values with respect to carbon dioxide concentration.
Aforementioned U.S. Pat. No. 4,923,720 discloses an apparatus capable of
pumping and proportioning a coating formulation and liquid carbon dioxide.
In one embodiment, volumetric proportioning of the coating formulation
stream and the supercritical carbon dioxide 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 coating formulation which is slaved to another reciprocating pump
which is used to pump the liquid carbon dioxide. The piston rods for each
pump are attached to opposite ends of a shaft that pivots up and down on a
center fulcrum. The volume ratio is varied by sliding one pump along the
shaft, which changes the stroke length.
However, liquid carbon dioxide is relatively compressible at ambient
temperature, the temperature at which it is typically stored in a
pressurized container. Such compressibility may undesirably cause
fluctuations and oscillations of the amount of carbon dioxide that is
present in the admixed coating formulation that is to be sprayed. This
occurs due to the incompatible pumping characteristics of the relatively
non-compressible coating formulation and the relatively compressible
liquid carbon dioxide. With the coating formulation, pressure is
immediately generated in the reciprocating pump as soon as its volume is
displaced. Inasmuch as the liquid carbon dioxide is substantially
compressible, a larger volume is needed to be displaced in order to
generate the same pressure. Because mixing occurs when the flow of the
coating formulation and of the liquid carbon dioxide are at the same
pressure, the flow rate of carbon dioxide 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 changing
direction during its cycle. Thus, if the driving force declines, the
pressure in the coating formulation flow declines even more rapidly, due
to its non-compressibility, than the pressure in the liquid carbon dioxide
flow.
Accordingly, the pressures generated in both flows may be out of phase
during the pumping. U.S. Pat. No. 4,621,927 discloses a mixture control
apparatus controlling a flow rate of a second fluid to be mixed 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.
Pat. 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.
SUMMARY OF THE INVENTION
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 mixture comprised of
a non-compressible 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 compressible
fluid. This invention simply and accurately proportions the fluids because
it has been surprisingly discovered that the density of the
non-compressible fluid and compressible fluid mixture does not vary
significantly in many systems as long as the solubility limit of the
compressible fluid in the non-compressible fluid mixture is not exceeded.
As used herein, the phrase "compressible 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 and 0.degree. C. unless otherwise noted.
More specifically, the present invention in its broader embodiment
comprises an apparatus for continuously mixing a substantially
compressible fluid and a substantially non-compressible fluid in a
predetermined proportion which includes:
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 volumetric 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 measuring the volumetric 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
mixture; and
h) means for controlling the flow rate of the substantially compressible
fluid in response to the signals generated in (d) and (g).
As used herein, the phrases "coating formulation" or "coating composition"
are understood to mean a typical, conventional coating composition which
does not have any supercritical fluid admixed therewith. Also as used
herein, the phrases "admixed liquid mixture " or "admixed coating
formulation" are meant to include an admixture of a coating formulation
with at least one supercritical fluid.
The present invention also comprises a method for forming a mixture of a
substantially compressible fluid and a substantially non-compressible
fluid in a predetermined proportion which comprises:
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 the non-compressible fluid such that
the density of the resulting mixture behaves substantially as a
non-compressible fluid;
e) measuring the volumetric flow rate of the mixture; 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-compressible fluid" is understood to
include a mixture whose density is unaffected by a change in concentration
of the components in the mixture of less than about 10%, preferably of
less than 5%, and most preferably of less than 2%.
By measuring the volumetric flow rate of the non-compressible fluid and
compressible 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 eliminated. In a preferred
embodiment of the invention the density of the resulting fluid mixture is
also measured to ensure that the fluid mixture is behaving substantially
as a non-compressible fluid.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 is a phase diagram for a supercritical carbon dioxide, polymer and
solvent system.
FIG. 2 is a graph of the density versus composition of ethanol/water and
isopropyl alcohol/water systems.
FIG. 3 is a graph of the density versus composition of a dimethyl
sulfoxide/acetone system.
FIG. 4 is a graph of the density versus composition of an acrylic
polymer/methyl aryl ketone solution.
FIG. 5 is a graph of the density versus composition of a polymeric coating
composition/carbon dioxide solution.
FIG. 6 is a diagram of the apparatus suitable for proportioning and
spraying a compressible fluid and non-compressible fluid.
FIG. 7 is a diagram of the apparatus used to conduct the experimental
trials described herein.
FIGS. 8-11 are graphical representations of flow rate versus time for the
spray application of various coating mixtures.
FIGS. 12 and 13 are graphs of the density versus composition for two
coating compositions in carbon dioxide.
FIGS. 14 and 15 are graphs of the density versus composition for two
coating compositions in ethane.
DETAILED DESCRIPTION OF THE INVENTION
It is to be understood that while the following discussion will primarily
focus upon providing a proportionated admixed liquid mixture of a coating
formulation and supercritical fluid, such as carbon dioxide, which is
suitable for being sprayed onto a substrate, the present invention is in
no way limited to this embodiment. As is readily apparent from the
foregoing discussion, the present invention encompasses the
proportionation of any compressible and non-compressible fluid to form a
desired mixture for any intended subsequent use.
The coating compositions employed in this invention are broadly defined to
include paints, lacquers, adhesives and the like. Such coating materials
may also include those that are typically utilized in the agricultural
field such as, but not limited to, fertilizers, herbicides and
insecticides.
The coating compositions employed in the present invention typically
comprises a solids component containing at least one polymeric component,
pigments, 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 discussion of the components found in coating compositions can be
found in U.S. Pat. No. 5,171,613.
Supercritical fluid phenomenon is well documented, (see pages F-62-F-64 of
the CRC Handbook of Chemistry and Physics, 67th Edition, 1986-1987,
published by the CRC Press, Inc., Boca Raton, Fla.). At high pressures
above the critical point, the resulting supercritical fluid, or "dense
gas", will attain densities approaching those of a liquid and will assume
some of the properties of a liquid. These properties are dependent upon
the fluid composition, temperature, and pressure. As used herein, the
"critical point" is the transition point at which the liquid and gaseous
states of a substance merge into each other and represents the combination
of the critical 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 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 compressibility of supercritical fluids is great just above the
critical temperature where small changes in pressure result in large
changes in the density of the supercritical fluid. The "liquid-like"
behavior of a supercritical fluid at higher pressures results in greatly
enhanced solubilizing capabilities compared to those of the "subcritical"
compound, with higher diffusion coefficients and an extended useful
temperature range compared to liquids. Compounds of high molecular weight
can often be dissolved in the supercritical fluid at relatively low
temperatures. An interesting phenomenon associated with supercritical
fluids is the occurrence of a "threshold pressure" for solubility of a
high molecular weight solute. As the pressure is increased, the solubility
of the solute will often increase by many orders of magnitude with only a
small pressure increase. The solvent capabilities of the supercritical
fluid, however, are not essential to the broad aspects of the present
invention.
Near-supercritical liquids also demonstrate solubility characteristics and
other pertinent properties similar to those of supercritical fluids. The
solute may be a liquid at the supercritical temperatures, even though it
is a solid at lower temperatures. In addition, it has been demonstrated
that fluid "modifiers" can often alter supercritical fluid properties
significantly, even in relatively low concentrations, greatly increasing
solubility for some solutes. These variations are considered to be within
the concept of a supercritical fluid as used in the context of this
invention. Therefore, as used herein, the phrase "supercritical fluid"
denotes a compound above, at, or slightly below the critical temperature
and pressure (the critical point) of that compound.
Examples of compounds which are known to have utility as supercritical
fluids are listed in aforementioned U.S. Pat. No. 4,723,920.
Due to the low cost, environmental acceptability, non-flammability and low
critical temperature of carbon dioxide, supercritical carbon dioxide fluid
is preferably used with the coating formulations. For many of the same
reasons, nitrous oxide (N.sub.2 O) is a desirable supercritical fluid for
admixture with the coating formulations. However, any of the supercritical
fluids and the mixtures thereof are to be considered as being applicable
for use with the coating formulations.
The solvency of supercritical carbon dioxide is substantially similar to
that of a lower aliphatic hydrocarbon and, as a result, one can consider
supercritical carbon dioxide as a replacement for the hydrocarbon solvent
of a conventional coating formulation. In addition to the environmental
benefit of replacing hydrocarbon solvents with supercritical carbon
dioxide, there is a safety benefit also, because carbon dioxide is
non-flammable.
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
form a liquid spray, which impacts the substrate and forms a liquid
coating. In the coatings industry, three types of orifice sprays are
commonly used; namely, air spray, airless spray, and air-assisted airless
spray.
Air spray, airless spray, and air-assisted airless spray can also be used
with the liquid coating formulation heated or with the air heated or with
both heated. Heating reduces the viscosity of the liquid coating
formulation and aids atomization. The present invention can also be
applied by electrostatic applications as described in U.S. Pat. No.
5,106,650.
In essentially every process in which a mixture is prepared for a
particular purpose, the constituents of that mixture usually need to be
present in particular, accurately proportionated amounts in order 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 formulation 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 retaining the ability to effectively spray the liquid mixture
of coating formulations and supercritical fluid and also obtain a
desirable 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 recognizable when the admixture
containing 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 FIG. 1 wherein the supercritical fluid is supercritical carbon
dioxide fluid. In FIG. 1, the vertices of the triangular diagram represent
the pure components of an admixed coating formulation which for the
purpose of this discussion contains no water. Vertex A is an organic
solvent, vertex B is carbon dioxide, and vertex C represents a polymeric
material. The curved line BFC represents the phase boundary between one
phase and two phases. The point D represents a possible composition of a
coating formulation in which supercritical carbon dioxide has not been
added. The point E represents a possible composition of an admixed coating
formulation, after admixture with supercritical carbon dioxide.
Thus, after atomization, a majority of the carbon dioxide vaporizes,
leaving substantially the composition of the original coating formulation.
Upon contacting the substrate, the remaining liquid mixture of the polymer
and solvent(s) component(s) will flow, i.e., coalesce, to produce a
uniform, smooth film on the substrate. The film forming pathway is
illustrated in FIG. 1 by the line segments EED (atomization and
decompression) and DC (coalescence and film formation).
However, the amount of supercritical fluid, such as supercritical carbon
dioxide, that can be mixed with a coating formulation is generally a
function of the miscibility of the supercritical fluid with the coating
formulation as can best be visualized by referring to FIG. 1.
As can be seen from the phase diagram, particularly as shown by arrow 10,
as more and more supercritical carbon dioxide is added to the coating
formulation, the composition of the admixed liquid coating mixture
approaches the two-phase boundary represented by line BFC. If enough
supercritical carbon dioxide is added, the two-phase region is reached and
the composition correspondingly breaks down into two fluid phases.
Sometimes, it may be desirable to admix an amount of supercritical fluid
(in this case, supercritical carbon dioxide) which is even beyond the two
phase boundary. Generally, however, it is not preferable to go much beyond
this two phase boundary for optimum spraying performance and/or coating
formation.
In addition to avoiding the two-phase state of the supercritical fluid and
the coating formulation, proper proportionation is also desirable to
provide optimum spraying conditions, such as, formation of desired admixed
viscosity, formation of desired particle size, formation of desired
sprayed fan shape, and the like.
Accordingly, in order to spray liquid coating formulations containing
supercritical fluid as a diluent on a continuous, semi-continuous, and/or
an intermittent or periodic on-demand basis, it is necessary to prepare
such liquid coating formulations in response to such spraying by
accurately mixing a proportioned mount of the coating formulation with the
supercritical fluid. However, the compressibility 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 supercritical
fluid.
The non-compressible fluid in the present invention is typically in the
liquid state. The liquid state is characterized by the strong interaction
of the molecules, which distinguishes liquids from gases, and the state of
disorder of the molecular motion, which distinguishes liquids from solids.
The behavior of liquids are generally well understood and their properties
tend not to vary significantly over discrete ranges.
However, no known liquid solutions are exactly ideal. Solutions of highly
similar components may only show slight deviations, whereas greater
deviation are observed in almost all other solutions, 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 differences in the molecules. One of the easiest ways to
characterize the differences in behavior of liquid mixture is to measure
the density of the mixture.
FIG. 2 is a plot of liquid density versus composition of water and ethanol
and water and iso-propyl alcohol at atmospheric pressure at 20.degree. C.
With the addition of ethanol or isopropyl alcohol to the mixture, the
density of the mixture gradually decreases to the density of the undiluted
alcohol. FIG. 3 demonstrates a similar result with a plot of the liquid
density of dimethyl sulfoxide and acetone at atmospheric temperature and
pressure.
Some polymers in liquid solvents also behave similarly. Referring to FIG.
4, an acrylic polymer (AT954, Rohm & Haas Go.) and n-methyl aryl ketone
(MAK) were mixed at atmospheric pressure and 25.degree. C. With increasing
MAK levels, the density of the mixture decreased gradually to the density
of pure MAK.
Surprisingly, it has been discovered that in contrast to the above mixtures
wherein the density of the mixture compositions uniformly decreases,
mixtures of polymeric compositions, solvents and compressible fluids
undergo a period wherein the density is relatively constant. This
relatively constant density mixture remains until a two phase mixture is
created at which point the density of the mixture changes rapidly.
Referring to FIG. 5, a plot of mixture density of the components listed in
Table 1 below, in carbon dioxide is presented.
TABLE 1
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COMPONENTS WEIGHT PERCENT
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Alkyd, Reichhold 6255-03
21.6%
Nitrocellulose, 6.0%
Plasticizer 2.4%
Urea, Bettle 80 resin
10.0%
Solvents 60.0%
(mixture of MAK, i-propyl alcohol,
n-butanol, and ethylethoxy propionate
(EEP))
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With the addition of carbon dioxid | | |
