TECHNICAL DEVELOPMENT OF THE SUPERCRITICAL FLUID SPRAY PROCESS FOR THE APPLICATION OF COATINGS

Kenneth A. Nielsen Research Scientist

Union Carbide Corporation

John N. Argyropoulos Technology Manager

Union Carbide Corporation

R. Chris Clark Richard S. Cesaretti

Senior Coating Specialists Union Carbide Corporation

Jeffrey D. Goad Gregory C. Ross Senior Chemists

Union Carbide Corporation

Third Annual Advanced Coatings Technology Conference Dearborn, Michigan, November 9-1 1, 1993

ABSTRACT

Technical development and commercial implementation of the supercritical fluid spray process have produced the surprising result that coating appearance and performance can actually be improved by removing 60 to 80 percent of the organic solvent from solvent-borne coating formulations. Unlike normal diluent solvents and water, the decompressive release of supercritical carbon dioxide during spray formation produces a new type of spray that can have superior atomization characteristics compared to air, HVLP, air-assisted airless, and conventional airless spray methods. Not only can this improve coating quality, it can reduce material and operating costs by eliminating application steps and reducing the amount of coating solids sprayed per part. Furthermore, air toxic solvents such as l,l,l-trichlor~etha~e c a ~ he eliminated from the reformulated coating. The process is compatible with most types of polymer systems and is being applied to UV coatings and two-package reactive systems. It is undergoing commercial development and implementation in a variety of applications including wood furniture, plastics, automotive topcoats and components, general industrial, release surfaces, and adhesives. With continued polymer development, the process can be used as a liquid analog of powder coatings but have appearance and application advantages.

@ 1993 Union Carbide All Rights Reserved

TECHNICAL DEVELOPMENT OF THE SUPERCRITICAL FLUID SPRAY PROCESS FOR THE APPLICATION OF COATINGS

INTRODUCTION

The supercritical fluid spray process (1,2) was introduced in 1990 as a new pollution prevention technology for the spray application of coatings. Since then the process has undergone considerable technical development and evaluation as it has undergone commercial development and implementation. This has generated the surprising result that the process can actually improve coating appearance and application performance in addition to substantially preventing pollution. Indeed, commercial implementation activity in Europe is due primarily to appearance and performance improvements instead of pollution reduction.

The process uses supercritical carbon dioxide to replace volatile organic solvents in conventional and high solids coating formulations. Volatile organic compound (VOC) emissions have been reduced up to 80% and air toxic solvents have been totally eliminated for many coatings. More importantly, the decompressive release of carbon dioxide can produce superior atomization by a new atomization mechanism (3). This new type of spray has been shown to produce coatings having equal or improved quality to those applied by conventional spray methods with high- solvent formulations. Furthermore, improved application performance has been demonstrated by reducing application steps, reducing the amount of coating solids sprayed, or by enabling thicker coating films to be applied without sag. This paper discloses how coatings are reformulated and sprayed using supercritical carbon dioxide and reports coating application results obtained during commercial implementation.

COATING REFORMULATION

Conventional and high solids solvent-borne coating formulations can often be reformulated for spray application with supercritical carbon dioxide by adjusting the solvent level and solvent blend, with little or no change being made to the polymer system and pigments. Generally, the polymer system or pigments are adjusted only if too much medium or slow evaporating solvents must be removed from the coating formulation, in addition to the fast evaporating diluent solvents that are replaced by carbon dioxide, in order to meet emission requirements.

COATING SYSTEMS The process has been found to be broadly applicable to most types of coating systems. It has been demonstrated using air-dry lacquers, thermoplastic polymers, thermosetting polymers ultraviolet light cured polymers; and two-component polymer systems. A variety of clear, pigmented, and metallic coatings have been applied.

Polymers that have been used in single-package systems include acrylics, air-dry alkyds, baking alkyds, polyesters, melamines, melamine formaldehydes, nitrocellulose, silicones, vinyls, epoxies, phenolics, ureas, urethanes, and waxes. Acrylates have been used in ultraviolet light cured coatings. Polymer molecular weights have ranged from very low to very high.

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Reactive and catalyzed polymers have been used in two-package systems. Acid catalyzed alkyd urea and epoxy polymers have been used with high-ratio proportionation of 25:l to 50:l of polymer to catalyst. Reactive isocyanate and acrylic polymers have been used to apply polyurethane coatings with low-ratio proportionation of 1: 1 to 3: 1.

Pigments that have been used include titanium dioxide whites and pastels, carbon blacks, chrome type yellows, phalo blues, and organic reds. Inert fillers include silicas, clays, aluminum flakes, carbonates, and ceramics. Metallic coatings used include a clear metallic reflective coating and a black metallic coating. All pigments tested have been compatible with carbon dioxide.

SOLVENT PROFILE Ideally, the solvent composition of coatings sprayed conventionally and with supercritical carbon dioxide are the sqme once the coating is deposited onto the substrate. Therefore, the solvents eliminated from the reformulated coating are those that quickly evaporate in the spray or very soon after application, so they usually contribute little or nothing to film formation. These fast evaporating solvents are typically the major portion of the solvent blend in conventional coating formulations. They function primarily as diluent solvents to give the low viscosity needed for proper atomization by conventional spray methods. They must evaporate rapidly in the spray to deposit the coating at a high enough viscosity to prevent running and sagging. Therefore, they can be readily eliminated from the reformulated coating and be replaced by supercritical carbon dioxide in the spray mixture. Carbon dioxide can be considered to be an extremely fast evaporating solvent.

In practice, experience has demonstrated that medium evaporating solvents, that is, solvents with relative evaporation rates of 100 to 250 (butyl acetate RER = loo), can nearly always be totally eliminated in addition to fast solvents with relative evaporation rates above 250. The medium evaporating solvents evaporate significantly in the spray and contribute little to film formation when compared to slow evaporating solvents. Because they are not needed for atomization with supercritical carbon dioxide, the medium evaporating solvents needlessly increase solvent emissions and can contribute to running and sagging during deppsition. Therefore, solvents with relative evaporation rates above 100 are generally totally eliminated in the reformulated coating.

The spray application uses solvent most efficiently by minimizing solvent evaporation in tile spray. This is particularly important when the solvent level in the reformulated coating is reduced to minimize solvent emissions. The solvent level can only be lowered to the lowest level required for proper film formation. For a given solvent level, minimizing the solvent lost in the spray maximizes the solvent deposited with the coating to aid film formation. In atomization with supercritical carbon dioxide, solvent evaporation in the spray is not needed in order to increase the coating viscosity sufficiently to prevent running and sagging; carhon dioxide release can increase the viscosity to the proper level. Therefore, the ideal limit occurs when no solvent evaporates in the spray, so that all of the solvent in the coating formulation is deposited with the coating.

Experience has demonstrated that the best coating and best solvent efficiency can often be obtained by eliminating most of the slow evaporating solvents that have a relative evaporation rate above 50 from the reformulated coating, in addition to the medium and fast evaporating solvents.

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The reformulated coating will often have the following solvent profile, which is compared with a conventional coating:

Relative Evaporation Solvent Composition. Weight Percent Rate Range Reforrnu 1 a ted Conventional

< 50 65 - 100 25 50 - 100 0 - 35 35

100 < 0 40

For the supercritical fluid spray process, therefore, the definitions of fast, medium, and slow evaporating solvents are effectively different from those for conventional spray processes. They are compared below by relative evaporation rate:

Relative Evaporation Solvent EvaDoration Rate Rate Range Supercritical Conventional

< 50 Slow Very Slow 50 - 100 Medium Slow

100 - 250 Fast Medium 250 < Very Fast Fast

The reformulated coating therefore will often consist predominantly or entirely of slow evaporating solvents with relative evaporation rates below 50. This is radically different from the solvent profile philosophy used to formulate coatings for application by conventional spray methods. One conventional belief is that a significant amount of medium and sometimes even fast evaporating solvent must be deposited along with the slow evaporating solvent to obtain a high quality coating. A higher solvent level is thought to be required during coalescence and the first stage of film formation than is required later. However, this belief is in conflict with the observation that frequently coatings must be applied in stages to allow the fast and medium evaporating solvents to evaporate sufficiently between stages to avoid running and sagging problems.

Of course, conventional spray methods require a predominant proportion of fast and medium diluent solvents to obtain proper atomization viscosity. Therefore, it has not been possible to conventionally spray apply coatings without using fast and medium evaporating solvents. The high quality coating appearance and performance obtained by using supercritical carbon dioxide demonstrates that fast and medium evaporating solvents are not required for film formation when effective atomization can be obtained without them.

Although the reformulated solvent blend contains predominately slow evaporating solvents, considerable latitude remains for adjusting the overall evaporation rate profile in response to flow requirements, solvent flash requirements, oven and air-dry schedules, sagging, air entrapment, and solvent-popping characteristics. This is because the fast and medium evaporating solvents contribute proportionately much less to the overall evaporation rate of the solvent blend than the slow evaporating solvents.

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The optimum blend of slow evaporating solvents for film formation and curing of the reformulated coating is generally determined by spray tests that use actual application and curing conditions. In addition to evaporation rate, the solvents are chosen to be compatible with the chemical and physical properties of the polymers, pigments, and additives in the coating as well as with the substrate and other coatings in wet-on-wet applications. For example, alcohols used for stability in amino resin systems will still be required; however, a slower evaporating alcohol might be preferred. In electrostatic spray applications, the solvents must give the proper electrical resistivity. Economic, regulatory, and safety factors will also be important.

HAZARDOUS AIR POLLUTANTS The Clean Air Act now regulates individual air toxic solvents in addition to total volatile organic compound emissions. This includes 1 , 1, l-trichloroethane, which is widely used because it was previously exempt from VOC regulations. The hazardous air pollutants used in largest quantity in conventional coatings mainly have relative evaporation rates above 50. This includes hexane, methyl ethyl ketone, methanol, toluene, methyl isobutyl ketone, ethyl benzene, and xylene.

Hazardous air pollutants in solvent-borne coatings can be totally eliminated by reformulating the coating for spray application with supercritical carbon dioxide. Carbon dioxide has been demonstrated to be an effective substitute for 1, 1, l-trichloroethane and the other fast solvents.

SOLIDS LEVEL Solvent reductions achieved using the supercritical fluid spray process have exceeded initial expectations. When the process was first proposed, solvent reductions of 30 to 50% were expected, based upon the typical level of fast evaporating solvents in conventional coatings. This was increased to 30 to 70% when laboratory spray tests showed that medium evaporating solvents could often be removed as well. As the process has progressed through commercial development and implementation, the solvent reductions achieved have ranged from 50 to SO%, typically being 60 to 70%, as more has been learned about formulating coating systems to match the atomization properties of the spray.

The solids level that can be obtained, of course, depends upon the solids level of the conventional or high solids coating. This depends upon the molecular weight of the polymer. system. Higher molecular weight polymers are used at lower solids level to maintain a desirable viscosity for atomization and film formation.

Air-dry lacquers can often be used at substantially higher solids levels , without significantly changing the polymer system. For higher molecular weight lacquers, the solids level can generally be increased from a conventional level of 17 to 24% to a reformulated level of 38 to

I IuwLI IIIuILLulal wL1511L clIb llI+IbuI)b 6bILbLuI l lvl l l a !eve! ~f 25 to 30% to a reformulated level of 45 to 55 % . 48% t;Y weigh:. CAS 1 n w v v n . - - n l n n v ~ I - v q r i a ; m h t l c l o n i i ~ v c tha ;noronem ;r m a n o r n l b r frnm

Thermosetting, catalyzed, ultraviolet light, and two-component coating systems use lower molecular weight polymers than lacquers, so much higher solids levels can be achieved in the reformulated coating. A higher solids level is obtained from reformulated high solids coatings than from conventional coatings, because the polymers have lower molecular weight. Reformulated coatings with solids levels as high as about 85 % have been demonstrated.

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When the coating application is not limited to using the same polymer system or molecular weight, solids levels can potentially be increased to very high levels. The success achieved so far reinforces the belief that the technology can be extended to where the solvent content is severely reduced, or entirely eliminated, to give solids levels of 95 to 100% for many applications. This can be done by developing coatings from liquid polymer systems that have proper film forming and curing characteristics and significant carbon dioxide solubility. This has a!ready been demonstrated for a simple application that sprays a liquid polymer with 1 , 1,l-trichloroethane. The l,l,l-trichloroethane was totally replaced with carbon dioxide to spray a solvent-free coating. Development work has already begun on a more complex zero-VOC coating.

VISCOSITY The reformulated coating is called a coating concentrate because it has much higher viscosity than conventional and high solids coatings. The concentrate viscosity is typically between 800 and 3000 centipoise at a temperature of 25 Celsius, although viscosities below 500 and above 12,000 centipoise have been sprayed. This contrasts with high solids coatings, which have viscosities below 100 centipoise for air spray and air-assisted airless spray applications that require high quality appearance.

Spray tests are used to adjust the concentrate viscosity to give proper film formation and coating appearance for the desired film thickness. The deposition viscosity and concentrate viscosity become the same in the limit of no solvent loss in the spray. The concentrate viscosity must be lowered to the extent that solvent loss does occur, which can be found by measuring the solids level of the deposited coating. For concentrates that contain just slow evaporating solvents, low solvent losses have been measured that increase the solids level by one or two percentage points.

Thermosetting coating concentrates often have lower reaction stability because most of the diluent solvent that reduces reaction rate has been removed. They are also more sensitive to small increases in molecular weight from cross-linking reactions. Therefore, viscosity generally increases at a higher rate over time, which can reduce shelf and pot life. In practice, this need not cause much difficulty; single-package coatings have been used at very high solids levels. Often the higher solids level can be compensated for by increasing the concentration of inhibitor solvent or by adjusting the catalyst or cross-linking agent. At very high solids levels, spraying the coating as a two-package formulation is sometimes preferred.

REFORMULATION EXAMPLE How a high solids coating formulated for air spray application might be reformulated for application with supercritical fluid (SCF) is illustrated below. The high solids coating is a thermosetting acrylic-melamine formulation used in an automobile clear coat application. The coating concentrate is based on commercial &monsiraiioii experieilce, Gut it is i,oi ai1 opiimized foKli-up&~oil, -which is propiietaiy-.

The weight-percent compositions and properties of the formulations are compared in the tables below. The solids level has been increased from 58 to 84% by removing 74% of the solvent. The methyl ethyl ketone and xylene, which are hazardous air pollutants, and the butyl acetate have been eliminated by volatility considerations. The isobutanol has been kept in the same proportion to the melamine polymer because it is a reaction inhibitor. In order to reduce the volatile organic compounds,

The same polymers are used in the same proportion.

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including baking reaction products, to the desired level of 2.00 pounds per gallon, the proportion of methyl amyl ketone to polymer has been reduced by 25%. To compensate for this, the much slower solvent ethyl 3-ethoxypropionate has been substituted for some of the methyl amyl ketone. The overall evaporation rate would then be optimized for best coating appearance, as determined by spray trials, by adjusting the solvent ratios at the same total solvent level. The isobutanol level can be reduced by using a slower evaporating alcohol to inhibit the reaction.

Formulation Air SCF Component - -

Acrylic Polymer 38.60 Melamine Polymer 19.40 Methyl Ethyl Ketone 10.25 Butyl Acetate 10.23 Is0 butanol 3.88 Xylene 7.99 Methyl Amyl Ketone 9.65 Ethyl 3-Ethoxypropionate 0.00 Total Weight Percent 100.00

55.90 28.10 0.00 0.00 5.62 0.00 7.18 3.20

100.00

Air Spray Formulation

Solids Level 58.00 % Viscosity 100 CPS Density 8.50 LBS/GAL VOC (As Mixed) 3.57 LBS/GAL VOC (As Baked) 4.00 LBS/GAL

Solvent Blend -~ Air SCF

----- ----- 24.40 0.00 24.36 0.00 9.24 35.12

19.02 0.00 22.98 44.88 0.00 20.00

100.00 100.00

SCF Spray Formulation

84.00 % 2000 CPS

8.75 LBS/GAL 1.40 LBS/GAL 2.00 LBS/GAL

CARBON DIOXIDE SOLUBILITY

Supercritical fluids are unique solvents because their solubility characteristics strongly depend upon pressure and temperature. Carbon dioxide is a particularly useful supercritical fluid because it has considerable solubility in most coating formulations.

Carbon dioxide solubility in a coating formulation is primarily determined by the polymer system and solids level at a given temperature and pressure. The choice of solvent is of secondary impnrttnce fer mest ceatifigs, sc the sdvext blend caii be opti~ized without sig~ificmtb changing the carbon dioxide solubility. Pigments act as inert filler in calculating the solubility of a pigmented system from the solubility of the clear vehicle.

As the polymer level in the formulation increases, carbon dioxide solubility decreases. However, considerable solubility remains, even at high polymer levels. This can be seen from the triangular phase diagram for an acrylic polymer used in high solids coatings (Figure 1). The polymer has a weight-average molecular weight of 6000. Several solubility curves are superimposed to show how solubility increases with pressure at a temperature of 60 Celsius.

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FIGURE 1

CARBON DIOXIDE SOLUBILITY FOR ACRYLIC COATING 6OOO MOLECULAR WEIGHT AT 60 CELSIUS

100% SOLVENT

100% CARBON DIOXIDE POLYMER

Each curve divides compositions that give single-phase solutions from those that give two-phase mixtures. At a pressure of 1600 psi, the solubility is high enough that the polymer system can be sprayed at very high polymer levels. Even in the limit of 100% polymer, the solubility is 17% by weight. Solubilities are given below for common types of air-dry and thermosetting polymer systems, at a pressure of 1600 psi and a temperature of 60 Celsius.

Po 1 y me r Tyue Polymer Level Solubility

Acrylic #I 40 % 45.8 % Cellulosic #1 40 % 39.6 % Cellulosic #2 40 % 47.6 % Acrylic #2 50 % 40.9 % Acrylic #3 50 % 34.1 % Alkyd #1 50 % 31.1 % Cellulosic #3 50 % 44.0 % Acrylic #3 60 % 29.2 % Melamine #I 60 % 54.5 % Melamine #2 60 % 40.3 % Polyester # 1 60 % 31.0 % Acrylic #4 70 % 32.6 %

Acrylic #4 80 % 28.3 % Alkyd #2 80 % 23.4 %

Polyester #2 70 % 33.4 %

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The data suggest that a variety of liquid polymers could be developed with high carbon dioxide solubility to enable solvent-free coatings to be sprayed. Solubilities measured for conventional liquid polymer systems at very high polymer levels are given below. A few have the proper viscosity for spray application. Most would need lower molecular weights. Because they would need to be cured by reaction, they must be thermosetting, catalyzed, or two-package coatings.

Po 1 y mer Ty= Polymer Level Solubility

Urea-Formaldehyde 95 % 27 %

Melamine-Formaldehyde 98 % 26 % EPOXY 100 % 24 % Epoxidized Fatty Ester 100 % 36 % Silicone 100 % 40 %

Polyester 95 % 43 %

SPRAY CHARACTERISTICS

Unlike normal solvents and water, supercritical carbon dioxide does much more than dilute the coating formulation to a low spray viscosity. More importantly, the decompressive release of carbon dioxide gas during spray fomiation produces a new atomization mechanism and a new type of spray (2). This decompressive spray can have superior atomization and spray characteristics compared to air, HVLP, air-assisted airless, and conventional airless spray methods.

The decompressive expansion of carbon dioxide as it rapidly depressurizes in the spray orifice produces an expansive force that overwhelms the viscosity, cohesion, and surface tension forces in the coating material that oppose atomization. Furthermore, the expansive force applies energy to the atomizing fluid on a much smaller scale and much more homogeneously than is possible by blasting the fluid with air jets. Therefore, the energy is used more efficiently and uniformly to produce the fine atomization required for high quality coatings and efficient use of the coating material.

The decompressive spray is parabolic in shape and has the favorable "feathered" spray characteristics of the air spray methods. This allows uniform coatings to be applied without using the high volume of compressed air to atomize and shape the spray. The spray can be produced in any width from narrow to very wide by changing the spray width rating of the airless spray tip. A wide range of spray flow rates can be used as required by the spray application by changing the orifice size.

SPRAY CONDITIONS The supercritical carbon dioxide is used at a level that gives a decompressive spray with the desired atomization characteristics. This can vary from 10 to 50% by weight of the spray mixture, depending upon the solubility, polymer level, pigment level, and spray temperature and pressure. The viscosity of the spray mixture is generally reduced to below 50 centipoise. However, in practice, the spray viscosity is not measured.

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Solubility is usually not a limiting factor for spray application. Spraying is done below the solubility limit for the spray temperature and pressure, so that the carbon dioxide is fully dissolved. Excess carbon dioxide forms a second phase, but usually no harm results if a small excess is present. A large excess can be detrimental because solvent is extracted from the polymer solution into the excess carbon dioxide. This can increase the viscosity and reduce the amount of solvent available for film coalescence and leveling.

The proportion of carbon dioxide to solvent in the spray mixture increases with the solids level in the coating formulation and the carbon dioxide level. The spray mixture usually contains more supercritical carbon dioxide than organic solvent, often twice as much or more, so polymer solvation can be different than in normal solutions. Because carbon dioxide is a small molecule, it has much higher diffusivity and can penetrate polymer configurations better than normal solvents. It is an insulating solvent that can give the spray mixture the proper electrical resistivity for good electrostatic charging of the spray droplets.

The spray temperature generally ranges from 30 to 70 Celsius, often being 40 to 60 Celsius. The spray mixture is generally heated above the critical temperature to modify the atomization and to offset cooling that occurs from decompressive expansion of the carbon dioxide. The spray pressure generally ranges from 1100 to 2000 psi, often being 1200 to 1600 psi. The spray mixture is pressurized above the critical pressure to increase solubility. Higher pressure increases the spray flow rate, as in normal airless sprays. Lower spray temperatures and pressures have come to be used more, as more has been learned about coating formulation and spray generation.

DROPLET SIZE Decompressive atomization can produce fine sprays that are in the same size range as air spray systems and rotary atomizers. Laser light scattering measurements show that average droplet sizes are typically in the 20 to 50 micron range, although much smaller and larger sizes can be produced if desired. Furthermore, the droplet size distributions can be quite narrow, which is highly desirable to obtain both high quality and high transfer efficiency. Coating quality deteriorates significantly as the number and size of the largest droplets in the spray increase above a low level. The acceptable level depends on coating thickness. Generally, it is desirable to minimize the population of droplets above about 70 microns in size. Transfer efficiency drops rapidly as the droplet size drops below about 10 microns, because the droplets become too. small for inertial and electrostatic forces to be effective at transporting the droplets to the substrate. Therefore they are lost as overspray. The decompressive sprays often have 90 to 96% of the droplets between 12 and 65 microns in size.

The droplet size distributions illustrated for a high solids acrylic polymer (Figure 2) and a high molecular weight cellulosic polymer (Figure 3) both have relatively few droplets above 70 microns and below 10 microns. The thermosetting acrylic coating formulation has 79% polymer and a viscosity of 2000 centipoise, It would be used as an automotive body clear coat, It was sprayed with 28% carbon dioxide at a spray temperature of 55 Celsius and a pressure of 1650 psi. The droplet size distribution is quite narrow despite the high polymer level and viscosity. Only 3% of the droplets are below 12 microns and 1% above 65 microns. The spray has 96% of the droplets between 12 and 65 microns and 80% between 20 and 50 microns.

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FIGURE 2 DROPLET SIZE DISTRIBUTION

FOR ACRYLIC COATING 6ooo MOLECULAR WEIGHT FIGURE 3

I-

1 ° % ~ O% 5 10 20

(1;

DROPLET SIZE DISTRIBUTION FOR CELLULOSIC COATING

10%

20%% 0%

40 65 110 5 10 20 I,

40 65 110

DROPLET SIZE, MICRONS DROPLET SIZE. MICRONS

The cellulosic lacquer formulation has 38% polymer and a viscosity of 700 centipoise. It was sprayed with 45% carbon dioxide at 58 Celsius and 1750 psi. The spray has 95% of the droplets between 12 and 65 microns and 83% between 15 and 50 microns.

The droplet size distribution for the decompressive spray can be considerably narrower than the distribution obtained with air spray systems (Figure 4). The acrylic coating concentrate with 78% polymer was diluted with fast evaporating solvent to give two air spray coating formulations having low viscosity, but with higher (spray 1) and lower (spray 2) solvent levels. The diluted coatings were sprayed with an air spray gun and the sprays were adjusted to give the same average droplet size (35 microns) as the decompressive spray. This comparison shows that droplet size distributions for both air sprays are significantly broader than the decompressive spray. The air sprays have many more large droplets that cause orange peel and many more small droplets that cause overspray. Therefore, air sprays must typically be adjusted to have significantly lower average droplet sizes than are required with decompressive sprays, in order to obtain the same high coating quality. However, although using lower average droplet size is effective in eliminating the large droplets, it greatly increases the fraction of small droplets that cause werspray and pesr transfer efficieficy .

Furthermore, the decompressive atomization produces a spray that is highly spatially uniform in droplet size, that is, the droplet size varies very little with position in the spray. The spatial droplet size profile illustrated (Figure 5) is for the high solids acrylic coating (Figure 3), but it was sprayed at a temperature of 50 Celsius and a pressure of 1520 psi. The spray width is 12 to 14 inches. The droplet sizes were measured along the major axis of the spray at a distance of 30 centimeters from the spray tip. The droplet size was also uniform along the minor axis across the spray and did not change with distance from the spray tip.

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FIGURE 4 DROPLET SIZE DISTRIBUTION FOR SCF DECOMPRESSIVE SPRAY AND AIR SPRAYS AT HIGH (1) AND LOW (2) SOLVENT LEVELS

FOR THE SAME HIGH SOLIDS ACRYLIC POLYMER

0 o o 6 0 0 .

0 . AVERAGE

35 MICRONS O . loo ] DROPLET SIZE m u

E 80

60

ALL CASES

0

40

0 0 .

0 . O O

0 o SCFSPRAY

6 0 AIRSPRAY 1

2 0 d u E a- N H v)

10 100 1000

DROPLET SIZE, MICRONS

FIGURE 5

SPATIAL DROPLET SIZE DISTRIBUTION

40

30

25

.

20

15

10

5

0

-30 -20 -10 0 10 20 30

POSITION ON MAJOR AXIS, CM

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FIGURE 6

SPRAY VELOCITY PROFILES

35

30

25

20

15

10

5

0

DOWNSTREAM DISTANCE

0 10CM 0 20CM

3 0 C M + 40CM

0 20CM 3 0 C M

+ 40CM

0 5 10 15 20 25 30

DISTANCE FROM CENTER, MAJOR AXIS, CM

Spatial uniformity in droplet size is highly desirable for obtaining high quality coatings and high transfer efficiency, because it gives a narrower overall droplet size distribution for the entire spray. This eliminates portions of the spray that have too coarse or too fine atomization. Obtaining spatial uniformity can be a major problem with air spray systems, because droplet size often varies markedly with location within the spray.

The droplet size for a given coating can be made smaller or larger and therefore optimized by varying the carbon dioxide concentration and spray temperature and pressure and by other techniques. For given spray conditions, the droplet size can also be varied by changing the polymer level and coating viscosity. The width of the droplet size distribution can also be varied. Decompressive atomization is unusual in that if the solids level is too low, that is, too much solvent is used, then the spray can be over atomized because the droplet size becomes too Iew fer efficient depcsitlm. Therefme, spray applicatis:: cax semetimes acka!!j.. be impwed by removing solvent and increasing the solids level.

SPRAY VELOCITY The decompressive spray decelerates rapidly with increasing distance from the spray tip to give a soft spray and a low deposition velocity (Figure 6). The decompressing carbon dioxide causes the spray to leave the spray orifice at a very wide angle in a parabolic shape. This very rapid increase in spray width and cross-section creates a large braking force due to greatly increased shear with surrounding air.

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The spray velocity profiles illustrated are for the high solids acrylic coating (Figures 3 and 8). It was sprayed at a pressure of 1520 psi at a flow rate of 300 gramdminute. The velocity profile was measured at distances of 10, 20, 30, and 40 centimeters from the spray tip along the major axis of the spray by using a phase-doppler laser particle analyzer. The spray superficial velocity at the spray orifice is 82 meterdsecond. At typical spray distances of 30 to 40 centimeters from the spray tip, the maximum droplet velocity in the spray, which is at the centerline, has dropped to about 10 meterdsecond. The average velocity of the spray, of course, is significantly lower and drops to about 7 meterdsecond or less.

COATING PERFORMANCE

Commercial testing and production have demonstrated that the high quality atomization produced by the decompressive spray can improve coating appearance and performance. Superior atomization has allowed the same high quality appearance to be obtained with thinner coatings when compared to HVLP and air-assisted airless spray systems. Because little or no reduction in transfer efficiency occurs despite finer atomization, applicators have been able to reduce the amount of coating solids sprayed per part, so coating costs have been reduced. When coating thickness is the application criteria, coating quality has been improved by applying thicker coatings without sag. Furthermore, the application steps can often be reduced, so productivity can be increased and production costs lowered.

The supercritical fluid spray process can allow continued use of lacquer coatings with superior performance instead of having to convert to lower molecular weight reactive systems. The performance of baked high solids coatings can be improved by allowing higher molecular weight polymers to be used at higher solids levels. This can improve coating appearance, reduce baking temperature, and avoid the use of two-package coatings.

AIR-DRY LACQUER COATINGS Solvent-borne lacquer coatings depend upon high molecular weight polymers for coating appearance and performance and for ease of application. They provide repairability, versatility, and low cost, such as by avoiding bake ovens required by thermoset coatings. The supercritical fluid spray process allows lacquers to be applied at much higher solids levels with improved appearance and economics as well. Surprisingly, experience has shown that high solvent levels are not required to obtain a high quality coating when effective atomization is used. Furthermore, the process is compatible with all types of substrates, including metal, wood, plastics, and flexible substrates.

In one study, the application properties of the decompressive spray were compared to an HVLP spray system- under commercia! production conditions. The conventional lacquer contained 19 % solids and 53% air toxics. The reformulated coating contained 42% solids and no air toxic solvents, which gave a solvent reduction of 67 % . Transfer efficiencies were compared by hand spraying a series of flat parts that were 20 x 16 inches in size. The transfer efficiency of the decompressive spray was 5 to 8 percentage points lower than the HVLP spray because the droplet size was smaller. However, production spray comparisons showed that the finer atomization gave an improved coating appearance with a thinner coating, so the measured coating solids usage overall was actually reduced 27 % . Therefore, solvent emissions were

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reduced by 76% compared to the HVLP system. When objects surfaces, the overly wide spray pattern produced by the HVLP transfer efficiencies than the decompressive spray, which could be requirements.

were sprayed instead of flat system gave lower effective better matched to application

Reliable comparisons of the coating performance of the decompressive spray and air-assisted airless spray systems can be made by using commercial production experience for the same application. At one installation, the amount of solvent consumed per part is 60% less than the amount consumed by application of the conventional coating by air-assisted airless spray guns. This is only slightly less than the solvent reduction made in the reformulated coating, when compared on an equal solids basis. However, the decompressive spray gives a smoother finish and allows higher film build on vertical surfaces without running and sagging. Therefore, more coating solids are applied per part as is desired to improve coating appearance. These figures indicate that the transfer efficiency of the decompressive spray is therefore the same or better than that of the air-assisted airless spray. Furthermore, the coating is now applied in one application step in one spray booth, whereas two spray booths had been required before. The high quality also simplified another manufacturing step and reduced labor requirements. Therefore the new process has produced substantial savings and improved quality. A further benefit is that production can be doubled without increasing solvent emissions or requiring new permits.

At another installation, the reformulated coating contains 67 % less solvent than the conventional coating applied by air-assisted airless spray guns. In this application, the superior atomization produced by the decompressive spray enables the same coating appearance to be obtained with a much thinner film build. Overall solids usage per part has been reduced by about 40%, which gives substantial material cost savings. Solvent emissions have been reduced by about 80% overall. Furthermore, the coating is now applied in one step in one spray booth instead of two, which gives significant labor and capital investment savings.

An important aspect of air-dry lacquer applications is obtaining the proper dry time for the coating. Often the evaporation rate of a conventional coating has to be adjusted frequently to compensate for changes in ambient air temperature. However, with the decompressive spray, the evaporation rate of the reformulated coating need not be adjusted. Instead, the dry time of the applied coating can be varied by adjusting the carbon dioxide level or spray temperature in response to changes in the spray booth environment. This allows the solvent content of the deposited coating to be changed without changing the formulation.

BARED COATLVGS The cmting perfmxmce d baked coatkg system can best be i!!us:rated by demonstration spray tests made by a major automobile manufacturer with an automotive body clear coat. A commercial coating was reformulated by removing fast solvents and substituting slow solvents appropriate to the supercritical fluid spray process. The solvent level was reduced to give 2.0 pounds of volatile organic compound emissions per gallon of coating, including baking losses. The clear coat was spray applied wet-on-wet to a conventional solvent-borne base coat. However, no problem is foreseen in adapting to water-borne base coats. The coating was sprayed from multiple electrostatic spray guns mounted on reciprocating robotic applicators. A dry film thickness of about 2 mils was built in two coats. The coating was applied, flashed, and

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baked at production rates and conditions. The appearance quality of the coating equaled or exceeded current production requirements. Furthermore, because the coating was a reformulated conventional coating, the polymer molecular weight could be kept substantially higher than could be used in a high solids coating formulated to the same solvent level. Therefore, the solvent- borne coating performance should be retained.

In another test, the transfer efficiency was measured by foiling an entire automobile body and then spraying it the same way. The overall transfer efficiency of the decompressive spray was 80%.

Solvent emissions were also measured in the spray booth and bake oven as the automobile was sprayed, using air sampling at different locations. This confirmed that much less solvent is given off in the spray booth by the decompressive spray than when conventional clear coats are applied. Indeed much more solvent was given off in the bake oven than in the spray booth, as would be expected from the elimination of the fast solvents. Since the oven effluent gases are typically recycled back to the burner, these emissions are incinerated.

SYSTEM PERFORMANCE

The reformulated coating and carbon dioxide are accurately proportioned and sprayed by using commercial spray units and spray guns developed by Nordson Corporation. The compact spray unit uses electrical capacitance to automatically measure and control the concentration of carbon dioxide in the spray mixture at the desired level. The coating and carbon dioxide are pressurized by air driven pumps to the desired spray pressure, metered and mixed on demand, heated to the desired spray temperature, and circulated to the spray guns. The carbon dioxide is supplied from either gas cylinders or refrigerated tanks depending upon usage rate. The reformulated coating is supplied the same way as conventional coatings. Manual and automatic spray guns have been developed for both normal and electrostatic spraying. The spray guns and spray tips are designed for the unique properties of the spray mixture and the decompressive spray. The spray flow rate and fan width are controlled by the spray orifice size and angle.

The first commercial installation of the supercritical fluid spray process has been in continuous production now for two years applying a nitrocellulose lacquer on a wood chair line (4). This has demonstrated improved coating quality, manpower and material cost reductions, and favorable operating experience, so the installation is now being expanded to a wood table line (5). During the past year several other commercial installations have been made in the wood furniture industry. Commercial installations have also been made for plastics, bake pans, general metal applications, and for applying mold release agents in foam manufacture. An automatic line for applying automobile clear coat has also been installed. An installation for applying a pigmented coating to tractor cabins has recently begun production. More installations are pending in a variety of applications. Over 30 commercial spray units are now being used for production, commercial testing, and pilot scale demonstration.

This commercial production experience has demonstrated that the spray system can be readily retrofitted into existing spray operations and that process cost reductions can provide a relatively

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short payback period for the installation. Other process features have also been demonstrated. For example, the installations show that the viscous coating can be supplied to the spray booth by recirculation through long supply lines from a paint kitchen. At one installation, very high material utilization has been achieved by collecting 90% of the overspray and reconstituting it for reuse.

The technology has recently been extended by development of a commercial spray unit for the application of two-package reactive coatings at either high or low ratios.

SOLVENT RECOVERY & INCINERATION

For coating operations that use bake ovens or forced drying, the supercritical fluid spray process is useful in a systems approach to obtain greater reduction in volatile organic compound emissions than can be obtained by just reducing the VOC content of the coating formulation. Because the reformulated coating contains mostly slow evaporating solvents, a much smaller portion of the solvent evaporates inside the spray booth, where large volumes of air are required. A much greater portion is emitted in the dry line or oven space, where air flows are much lower, so solvent recovery or incineration are less costly.

The difference can be illustrated using the automobile clear coat reformulation example, where the VOC has been reduced from 4.00 to 2.00 pounds per gallon including baking losses, which corresponds to a reduction from 3.57 to 1.40 pounds per gallon in the formulation. For the conventional coating, the solvent emissions from deposited material occur 80% in the spray booth and 20% in the oven, whereas for the reformulated coating the emissions occur 20% in the spray booth and 80% in the oven. The transfer efficiency is assumed to be 80% for both cases. All of the solvent from the overspray is assumed to evaporate in the booth. The VOC emissions are compared below on an equal applied-solids basis:

Conventional SCF Spray VOC ChanPe

Solvent in Booth 3.00 lb 0.34 lb 89 % Reduction Solvent in Oven 0.57 lb 0.60 lb 5% Increase Solvent Total 3.57 Ib 0.94 lb 74 % Reduction

However, not all of the overspray solvent evaporates in the spray booth. If the overspray is captured by a water curtain, much of the slow evaporating solvent is captured by the water and does not evaporate. If this is taken into account, the reduction in booth emissions is even greater, as shown below:

Coiiveriiiuriai SCF Smay VOC C h a n g e

Solvent in Booth 2.86 lb 0.19 lb 93 % Reduction Solvent Captured 0.14 lb 0.15 lb 7% Increase

Therefore, whereas the VOC content of the coating has been reduced by 74%, the VOC emission in the spray booth has been reduced by 93%. But the solvent load to the oven is increased just 5 % , which an existing solvent recovery or incineration system could accommodate.

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CONCLUSIONS

Commercial production experience has demonstrated that the high quality atomization produced by the decompressive spray can improve coating appearance and application performance. Contrary to conventional belief, the results show that significantly reducing the use of fast and medium evaporating organic solvents can actually improve coating quality. Furthermore, material and operating costs can also be reduced by eliminating application steps and reducing the amount of coating solids sprayed per part. The process can be readily retrofit into a variety of spray operations.

The supercritical fluid spray process has been shown to be an effective pollution prevention technology that is applicable to most types of solvent-borne coating systems. Volatile organic compound emissions have been reduced up to 80% and air toxic solvents have been completely eliminated in many applications. Solvent emissions are expected to continue to be reduced as new coating systems are developed that have high carbon dioxide solubility. Eventually the technology is expected to become the liquid analog of powder coatings but to have better coating performance and significant application advantages.

Commercial use is expected to continue to expand because the process can be an effective replacement for conventional and high solids coating applications that presently use air, HVLP, air-assisted airless, airless, and rotary spray systems. Market areas include wood and metal furniture, plastics, automotive topcoats and components, general industrial, adhesives, release surfaces, aircraft, marine, drums, and appliances.

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REFERENCES

Nielsen, K. A. et al. Supercritical Fluid Spray Application Technology: A Pollution Prevention Technology for the Future. Journal of Oil & Color Chemists Association 74 (10): 362-368 (October 1991).

Nielsen, K. A. et al. Spray Application of Low-VOC Coatings Using Supercritical Fluids. Society of Automotive Engineers 1991 Transactions, Journal of Materials & Manufacturing 100 (5): 9-16 (1992).

Nielsen; K. A. et al. A New Atomization Mechanism for Airless Spraying: The Supercritical Fluid Spray Process. Pages 367-374 in Semerjian, H. G., Editor. Proceedings of the Fifth International Conference on Liquid Atomization and Spray Systems. NIST Publication 813, Gaithersburg, Maryland (July 1991).

Anonym". First Carbnn Dinxide Sdvent Productien System. 1,YdEStlYL.d F";Zlsfii;zg 67(11): 34-36 (1991).

Anonymous. Pennsylvania House Expands UNICANP' Use. Furniture Design & Manufacturing 92( 1 1) : 46-49 (1992).

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ACKNOWLEDGEMENTS

We acknowledge the valuable contributions of D. C. Busby, D. J. Dickson, R. A. Engleman, C . W. Glancy, B. L. Hilker, K. L. Hoy, A. C. Kuo, J. J. Lear, C. S . Lee, W. P. Miller, B. Mouden, K. M. Perry, N. R. Ramsey, J. D. Wines, and P. R. Zitzelsberger. We thank Professor M. D. Donohue of Johns Hopkins University for his assistance. We thank J. D. Colwell and Professor D. W. Senser of Purdue University for Figures 4, 5 , and 6.