WO2022236331A1

WO2022236331A1 - Methods for controlling and measuring coating edge sharpness

Info

Publication number: WO2022236331A1
Application number: PCT/US2022/072183
Authority: WO
Inventors: Yong Han YEONG; Mikhail Khudiakov; Mehran Arbab
Original assignee: Prc-Desoto International, Inc.
Priority date: 2021-05-07
Filing date: 2022-05-06
Publication date: 2022-11-10

Abstract

Disclosed herein is a method of measuring edge sharpness of coating droplets deposited on a substrate. The method comprises selecting a maximum allowable droplet diameter for coating droplets deposited outside a target edge of the coating, selecting a maximum size distribution of the coating droplets deposited outside of the target edge of the coating, selecting a maximum surface edge roughness of a deposited edge of the coating, measuring an actual maximum droplet diameter of the coating droplets deposited outside the target edge of the coating and comparing the actual maximum droplet diameter with the maximum allowable droplet diameter, measuring an actual size distribution of the coating droplets deposited outside the target edge of the coating and comparing the actual size distribution with the maximum size distribution, and measuring an actual surface edge roughness of the deposited edge of the coating and comparing the actual surface edge roughness with the maximum surface edge roughness. Also disclosed herein is a method for controlling liquid coating droplets during deposition onto a substrate. The method comprises directing atomized liquid coating droplets in a flow path toward the substrate, and controlling at least one of droplet size, droplet size distribution, droplet flight path; droplet drift, and droplet splash.

Description

METHODS FOR CONTROLLING AND MEASURING COATING EDGE SHARPNESS

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Patent Application No. 63/185,525 filed May 7, 2021, which is incorporated herein by reference. This application also claims priority to PCT International Patent Application Serial No. PCT/US2021/031413 filed May 7, 2021, which is incorporated herein by reference.

FIELD

[0002] Methods for producing coatings with controlled edge sharpness and measuring coating edge sharpness are provided.

BACKGROUND

[0003] Coating deposition systems have been used to apply coatings onto various substrates. The systems include droplet generating devices including mass resonators, piezoelectric elements, wave concentrators and fluid ejectors. In such systems, it is desirable to achieve good edge sharpness.

[0004] PCT Application No. PCT/US2021/031413 entitled “Mask and Air Pressure Control Systems for Use in Coating Deposition”, which is incorporated by reference herein, discloses mask and air pressure control systems for improving edge sharpness in coatings produced by droplet generating devices.

SUMMARY

[0005] Disclosed herein is a method of measuring edge sharpness of coating droplets deposited on a substrate. The method comprises selecting a maximum allowable droplet diameter for coating droplets deposited outside a target edge of the coating, selecting a maximum size distribution of the coating droplets deposited outside of the target edge of the coating, selecting a maximum surface edge roughness of a deposited edge of the coating, measuring an actual maximum droplet diameter of the coating droplets deposited outside the target edge of the coating and comparing the actual maximum droplet diameter with the maximum allowable droplet diameter, measuring an actual size distribution of the coating droplets deposited outside the target edge of the coating and comparing the actual size distribution with the maximum size distribution, and measuring an actual surface edge roughness of the deposited edge of the coating and comparing the actual surface edge roughness with the maximum surface edge roughness.

[0006] Also disclosed herein is a method for controlling liquid coating droplets during deposition onto a substrate. The method comprises directing atomized liquid coating droplets in a flow path toward the substrate, and controlling at least one of droplet size, droplet size distribution, droplet flight path; droplet drift, and droplet splash.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Fig. 1 is a side view illustrating a coating droplet ejector positioned above a substrate to be coated, illustrating the flight of coating droplets from the ejector to the substrate.

[0008] Fig. 2 schematically illustrates operation of a droplet generating device in which monodisperse droplets are directed toward a substrate.

[0009] Fig. 3 schematically illustrates operation of a droplet generating device in which polydisperse droplets are directed toward a substrate.

[0010] Fig. 4 schematically illustrates a multiple-port fluid ejector.

[0011] Figs. 5A and 5B schematically illustrate fluid ejectors with in-line mixing features for multi-component liquid coatings.

[0012] Fig. 6 illustrates the operation of a coating droplet ejector to deposit a coating onto a substrate.

[0013] Fig. 7 is a photograph of an edge of a sprayed coating showing oversprayed droplets deposited outside the edge of the coating.

[0014] Fig. 8 is a photograph of an edge of a sprayed coating showing fewer oversprayed droplets deposited outside the edge of the coating in comparison with the sprayed coating edge shown in Fig. 7.

[0015] Fig. 9 is a graph of number of oversprayed droplets versus oversprayed droplet feret diameters.

[0016] Figs. 10 and 11 are graphs of numbers of oversprayed droplets versus oversprayed droplet diameters illustrating linear and parabolic edge sharpness criteria, respectively.

[0017] Fig. 12 includes the photograph of Fig. 8 and a magnified portion thereof showing a peak to valley distance at the edge of the sprayed coating. DETAILED DESCRIPTION

[0018] Methods of controlling coating edge sharpness for coating deposition devices including coating droplet generation systems are provided. The systems may include a coating droplet ejector that may be connected to a conventional mass resonator, piezoelectric elements and a conical wave concentrator. Methods for measuring coating edge sharpness are also provided.

[0019] The present methods may be used to spray deposit various types of coatings, such as solvent-based and/or water-based aerospace coatings, automotive coatings, architectural coatings, and the like. For example, solvent-based polyurethane coatings typically used for coating aircraft may be applied with the present mask and pressure control systems.

[0020] As shown in Fig. 1, a coating droplet ejector 30 may be positioned above a substrate S upon which a coating is to be applied. The coating droplet ejector 30 includes an ejector base 31 with a mounting hole 32 for attachment to a conventional system including a mass resonator, piezoelectric element and wave concentrator, as more fully described below. A coating delivery port 33 extends upward from the base 31. An ejector arm 34 extending laterally from the base 31 has a generally planar lower surface 35 terminating in an ejector edge 36. The ejector arm 34 includes a standard fluid ejection orifice centrally located on the underside of the ejector 30 adjacent the edge 36 through which paint and other coatings may be delivered. The ejector edge 36 of the ejector arm 34 is located vertically above the surface S, e.g., at a 45° angle in relation to the surface of the substrate S.

[0021] The coating droplet ejector 30 may be used to generate a coating droplet spray pattern D that is directed toward the substrate S. The droplet spray pattern D may include a particle size distribution including relatively small, medium and large coating droplets. The various droplet sizes shown in Fig. 1 are not drawn to scale, but are enlarged to more clearly illustrate the different droplet sizes in the spray pattern D. As shown in Fig. 1, the ejector edge 36 of the coating droplet ejector 30 is located an ejector distance D_E from the top surface of the substrate S. The ejector distance D_E may be adjusted as desired, and may typically range from 5 to 20 mm, or from 8 to 15 mm, or from 10 to 12 mm.

[0022] Multiple coating droplet ejector shapes and dimensions may be used, including wedge and anvil designs. The fluid coating is drawn to the flat ejector edge 36 closest to the fluid ejection orifice on the underside of the ejector via surface tension. The fluid is atomized and ejected at a single spot near the flat edge 36. The ejector 30 may be fabricated out of any suitable material such as polished titanium. The coating droplet ejector 30 provides minimal variance in ejection characteristics and clean operation. An ejector with multiple orifices may be used to eject higher volume of fluids. The coating droplet ejector may comprise a wedge design with a single orifice as shown in Fig. 1 or may be a double anvil design with multiple-orifices.

[0023] The mass resonator, piezoelectric elements and conical wave concentrator of the droplet generation system may be of any suitable design, such as disclosed in PCT Publication No. WO 2018/042165, which is incorporated herein by reference. Piezoelectric elements may be sandwiched between the mass resonator and wave concentrator. The coating droplet ejector 30 may be attached to the tip of the wave concentrator via the mounting hole 32. A temperature stabilization system (not shown) may be implemented to maintain the temperature of the resonating system at room temperature in order to stabilize the coating droplet ejection process.

[0024] Deposition of the coating droplets on the substrate S may be controlled, resulting in sharp coating edges through methods such as a negative or positive air pressure environment as the coating droplets travel toward the substrate S. As used herein, the term “air pressure control” includes the application of vacuum, i.e., sub-atmospheric pressures, and the application of air pressures above atmospheric pressure. For example, negative air pressure may force smaller coating droplets, which are more susceptible to advection, to be drawn and removed from the droplet spray pattern D. The smaller removed droplets may be collected, e.g., in a filter. The larger droplets, which have more momentum and inertia, continue on their flight paths to the substrate S. This mechanism may effectively reduce the coating droplet size distribution of the ejected droplets to minimize oversprayed droplets on the substrate S.

[0025] During coating operations as schematically shown in Figs. 2 and 3, coating edge sharpness may be affected by liquid coating droplets that deviate from their flight paths from the ejector 30 to the surface S. Excessive splashing/splatter from large droplets may occur on the surface S to cause rough edges. Fig. 2 illustrates a monodisperse spray pattern D_M, and Fig. 3 illustrates a polydisperse spray pattern Dp. Parameters such as droplet sizes, droplet size distributions, droplet flight paths, splash characteristics and coating formulations may be controlled to increase edge sharpness in monodisperse systems as shown in Fig. 2 and polydisperse systems as shown in Fig. 3. [0026] The properties of the coating formulation, e.g., surface tension, density, other rheological properties, can be optimized to promote droplet breakup that is more consistent and produces desired droplet sizes. The quality of the droplet sizes may depend on coating formulation and its atomizing mechanism. Coating optimization may therefore be coordinated with atomization strategies.

[0027] Coating atomization may be based on single droplet generation techniques or other techniques resulting in certain size distributions. For single droplet generation systems such as shown in Fig. 2, the droplet size may be reduced or increased, e.g., smaller droplets for less overspray and more edge sharpness, larger droplets to cover a larger surface area. As previously described, the coating formulation properties may also be adjusted to achieve different droplet sizes.

[0028] Droplet size distributions from polydisperse droplet generations such as shown in Fig. 3 may be narrowed or tightened. For example, a distribution may be centered close to a target droplet diameter and light-tailed. Increased amounts of droplets with intended diameters may improve consistency of coating film thickness and may also simplify the task of controlling droplet drifting and splashing.

[0029] Additionally, for polydisperse droplet generation, methods may be employed to control droplet sizes post-atomization. The droplets may be sorted to exclude extremely large/small droplets. This may be achieved by cyclone separators where centrifugal forces are created to sort the large and small droplets. The desirable droplet sizes can then be captured for ejection.

[0030] During flight, smaller droplets may encounter flow disturbances between the ejector and the surface. Such dynamic flow disturbances may be generated as the ejector accelerates to deposition velocity. Large droplets may carry higher momentum and inertia and may be generally unaffected by mild/small flow structures. However, smaller droplets may tend to follow the surrounding flow path and drift. To steer these wayward droplets, non-intrusive external forces may be used.

[0031] A negative or positive air pressure force may be generated to either remove and capture such wayward droplets or steer them toward the original flight path. Multiple vacuum and/or air pressure ports may be positioned around the spray patterns D_M and Dp with dynamic levels of pressure. [0032] Use of thermophoretic forces may be used to control droplet flight paths. A temperature gradient may be created at selected points of flight paths to steer droplets to their intended targets. Large droplets may tend to exhibit positive thermophoretic force, i.e., from hot to cold, while smaller droplets may exhibit negative thermophoretic force. Temperature gradients may be generated, e.g., through laser beam heating. However, exceeding coating temperature limits may be avoided.

[0033] Dielectrophoresis may be used to impose a non-uniform electric field to steer droplet flight path, in which case the droplets may not need to be charged. Electrostatic techniques may be used to charge the liquid coating droplets and ground the surface to draw the droplets to the correct landing position. Acoustic wave control may be used, wherein acoustical plates are placed between the droplet flight path and the substrate to impart sound pressure to manipulate droplet flight trajectory.

[0034] The impact of a liquid coating droplet on a surface may result in splash and breakup into smaller droplets. Splashing may produce rough coating edges and the smaller droplets may cause overspray. The magnitude of droplet splashing may depend on the coating properties such as density, surface tension and diameter, as well as travel velocity. This may be quantified by the Weber number. Reducing the Weber number will generally reduce droplet splashing. Since the Weber number is a function of velocity squared, it may be desirable to reduce droplet velocity. A decrease in droplet velocity may be achieved by increasing the drag imparted on the liquid coating droplet while in flight. Changing the surrounding air density by reducing ambient pressure or temperature may also achieve this effect. An enclosure around the coating deposition device may contact the surface to seal and provide an adjusted ambient environment for application of the liquid coating.

[0035] Changing the droplet impact angle may also reduce the degree of splashing. The nozzle may mechanically rotate to keep the angle between the surface and the nozzle constant. Dielectrophoresis may be applied in the opposite direction of droplet flight trajectory to produce additional drag on the droplets and reduce travel velocity. Changing the surface energy of the impacted surface to reduce droplet spread and splash may also be used.

[0036] The ability to coat large surface areas of aircraft and other substrates within a short time may be desirable for maskless applications on aircraft to be practical. The deposition device may atomize larger volumes of the liquid coating and distribute the atomized droplets to be deposited on larger surface areas.

[0037] The fluid ejector may be designed to accommodate multiple independent fluid delivery ports for liquid coatings, for example, a multi-port fluid ejector 130 as shown in Fig. 4 including four liquid coating supply lines and orifices. Liquid coatings may be delivered to the multiple ports and atomized at discrete locations at the tip of the ejector 130. When painting large surface areas such as aircraft, liquid coating may be delivered to the multiple ports for high deposition coverage. To achieve high coating edge sharpness, the feed can be independently controlled to limit delivery to an outer edge port. Different coating colors can also be delivered separately to each port to result in multi-color or tinted coatings.

[0038] As schematically illustrated in Figs. 5 A and 5B, in-line mixing may further automate the deposition process. Different components, for example, A, B and C, of the liquid coating can be fed independently for static mixing or to an impeller for dynamic mixing and then delivered to the fluid ejector 30 for atomization. Fig. 5A schematically illustrates a static mixer 40 upstream from the fluid ejector 30. Fig. 5B schematically illustrates a dynamic impeller mixer 41. In addition, the amount of the liquid coating that is delivered to each line can be changed in-situ to result in potentially different coating properties.

[0039] Atomization and deposition of a coating from a wedge design fluid ejector are shown in Fig. 6. At resonant frequency, the deflections of the ejector provide sufficient energy to draw the fluid to the flat edge of the ejector for atomization. The energy provided to the atomized droplets is also sufficient to overcome the effects of gravity, thereby allowing for not only vertical prints, but also horizontal prints.

[0040] Conventional evaluation processes for sharp coated edges are currently qualitative. The present disclosure may utilize quantitative criteria to meet for a visually sharp coating edge viewed at 0.5 meters / ~20 inches away from the panel. These quantitative criteria may determine different grades of print sharpness for different applications.

[0041] Examples of coating edge sharpness are shown in Figs. 7 and 8. A microscopic image of a coating edge with a field of view of 3.5 mm x 2.5 mm may be used. The feret diameter of oversprayed droplets, such as shown in the circled region in Figs. 7 and 8 of a sample image may be measured through standard image analysis. [0042] The coatings in Figs. 7 and 8 are polyester base coatings commercially available from PPG Industries under the designation Desothane HD 9008. The black coatings are spray applied onto aluminum substrates that are pre-coated with a conventional HVLP spray gun with Desothane HD 9008 white coatings. The coating may have an average dry film thickness (DFT) of 1 mil (25 pm), ±0.15 mil (3.8 pm), gloss units above 90 at 60°, and tension values above 14. The sharpness may be evaluated using the quantitative criteria described below.

[0043] Fig. 9 is a graph of number of oversprayed droplets versus oversprayed droplet feret diameters including an upper trace generated from the image of Fig. 7 that is used to establish empirically derived edge sharpness criteria, and a lower trace generated from the image of Fig. 8 that shows improved edge sharpness characteristics within the criteria established by the upper trace. None of the oversprayed droplets exceed 100 pm Feret diameter. The valley-to- peak distance of the coating edge was also below 100 pm. In addition, the region of oversprayed droplets beyond the coating edge was less than 1.5 mm.

[0044] The size distribution of the oversprayed droplets may be below other selected distribution curves. Exemplary distribution curves may be in a linear or parabolic form as shown in Figs. 10 and 11, respectively, or may be empirically derived such as described above and shown in Fig. 9. In addition, the maximum allowable Feret diameter of oversprayed droplets may be 100 pm, and the region of oversprayed droplets beyond the coating edge may not exceed 1.5 mm.

[0045] Fig. 12 includes a magnified portion of the coating edge shown in Fig. 8. The distance between a peak P and adjacent valley V is labeled Dpv. The peak-to-valley distance Dpv on a printed edge may not exceed, for example, 100 pm.

[0046] Using size distribution curves such as those shown above, the maximum allowable droplet diameter for coating droplets deposited outside a target edge of the coating and maximum size distribution of the coating droplets deposited outside of the target edge of the coating may be selected. In addition, the maximum surface edge roughness of a deposited edge of the coating may be selected. The actual maximum droplet diameter of the coating droplets deposited outside the target edge of the coating may be measured and compared with the selected maximum allowable droplet diameter. The actual size distribution of the coating droplets deposited outside the target edge of the coating may be measured and compared with the selected maximum size distribution. The actual surface edge roughness of the deposited edge of the coating may be measured and compared with the selected maximum surface edge roughness. The edge sharpness of the coating may be considered acceptable if the measured values are within the selected maximum allowable droplet diameter, maximum size distribution and maximum surface edge roughness.

[0047] For purposes of the detailed description, it is to be understood that the disclosure may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers such as those expressing values, amounts, percentages, ranges, subranges and fractions may be read as if prefaced by the word “about,” even if the term does not expressly appear. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Where a closed or open-ended numerical range is described herein, all numbers, values, amounts, percentages, subranges and fractions within or encompassed by the numerical range are to be considered as being specifically included in and belonging to the original disclosure of this application as if these numbers, values, amounts, percentages, subranges and fractions had been explicitly written out in their entirety.

[0048] Notwithstanding that the numerical ranges and parameters setting forth broad scope are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0049] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0050] As used herein, unless indicated otherwise, a plural term can encompass its singular counterpart and vice versa, unless indicated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances.

[0051] As used herein, “including,” “containing” and like terms are understood in the context of this application to be synonymous with “comprising” and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, ingredients or method steps. As used herein, “consisting of’ is understood in the context of this application to exclude the presence of any unspecified element, ingredient or method step. As used herein, “consisting essentially of’ is understood in the context of this application to include the specified elements, materials, ingredients or method steps “and those that do not materially affect basic and novel characteristic(s)”.

[0052] As used herein, the terms “on,” “onto,” “applied on,” “applied onto,” “formed on,” “deposited on,” “deposited onto,” mean formed, overlaid, deposited, or provided on but not necessarily in contact with the surface. For example, an electrodepositable coating composition “deposited onto” a substrate does not preclude the presence of one or more other intervening coating layers of the same or different composition located between the electrodepositable coating composition and the substrate.

[0053] Whereas particular examples of this disclosure have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present disclosure may be made without departing from what is defined in the appended claims.

Claims

WE CLAIM:

1. A method of measuring edge sharpness of coating droplets deposited on a substrate, the method comprising: selecting a maximum allowable droplet diameter for coating droplets deposited outside a target edge of the coating; selecting a maximum size distribution of the coating droplets deposited outside of the target edge of the coating; selecting a maximum surface edge roughness of a deposited edge of the coating; measuring an actual maximum droplet diameter of the coating droplets deposited outside the target edge of the coating and comparing the actual maximum droplet diameter with the maximum allowable droplet diameter; measuring an actual size distribution of the coating droplets deposited outside the target edge of the coating and comparing the actual size distribution with the maximum size distribution; and measuring an actual surface edge roughness of the deposited edge of the coating and comparing the actual surface edge roughness with the maximum surface edge roughness.

2. The method of claim 1, further comprising determining whether the actual maximum droplet diameter provides an acceptable maximum droplet diameter.

3. The method of claim 2, wherein the acceptable maximum droplet diameter does not exceed 100 pm Feret diameter.

4. The method of claim 1, further comprising determining whether the actual size distribution provides an acceptable size distribution.

5. The method of claim 4, wherein the acceptable size distribution is derived from a distribution curve of number of oversprayed droplets versus oversprayed droplet diameter.

6. The method of claim 5, wherein the distribution curve is empirically derived.

7. The method of claim 5, wherein the distribution curve is linear.

8. The method of claim 5, wherein the distribution curve is parabolic.

9. The method of claim 1, further comprising determining whether the actual surface edge roughness provides an acceptable surface edge roughness.

10. The method of claim 9, wherein the acceptable surface edge roughness has a valley-to-peak distance that does not exceed 100 pm.

11. The method of any of claims 1-10, further comprising: selecting a maximum allowable oversprayed droplet distance; and measuring an actual oversprayed droplet distance.

12. The method of claim 11, further comprising determining whether the actual oversprayed droplet distance provides an acceptable oversprayed droplet distance.

13. The method of claim 12, wherein the acceptable oversprayed droplet distance does not exceed 1.5 mm.

14. A method for controlling liquid coating droplets during deposition onto a substrate, the method comprising: directing atomized liquid coating droplets in a flow path toward the substrate; and controlling at least one of droplet size, droplet size distribution, droplet flight path, droplet drift, and droplet splash.

15. The method of claim 14, comprising applying a vacuum or pressurized air to at least a portion of the atomized liquid coating droplets in the flow path.

16. The method of claim 15, comprising applying a vacuum to the atomized liquid coating droplets in the flow path.

17. The method of claim 15, comprising applying pressurized air to the atomized liquid coating droplets in the flow path.

18. The method of any of claims 14-17, wherein droplet size is controlled.

19. The method of any of claims 14-18, wherein droplet size distribution is controlled.

20. The method of any of claims 14-19, wherein droplet flight path is controlled.

21. The method of any of claims 14-20, wherein droplet drift is controlled.

22. The method of any of claims 14-21, wherein droplet splash is controlled.

23. The method of any of claims 14-22, wherein the atomized liquid coating droplets comprise a monodispersion of droplets having substantially the same size.

24. The method of any of claims 14-23, wherein the atomized liquid coating droplets comprise a polydispersion of droplets having different sizes and a size distribution.

25. The method of any of claims 14-24, comprising using multiple coating feed lines in fluid communication with the fluid ejector to generate multiple flow paths of the atomized liquid coating droplets toward the substrate.

26. The method of any of claims 14-25, comprising using a mixer upstream from the fluid ejector to mix multiple coating components into a coating formulation prior to contact with the fluid ejector.

27. The method of claim 26, wherein the mixer comprises a static mixer.

28. The method of claim 26, wherein the mixer comprises a dynamic mixer including an impeller.

29. The method of any of claims 14-28, further comprising varying velocity of the droplets in the flow path.

30. The method of any of claims 14-29, further comprising adjusting droplet impact angle of the droplets impacting the substrate.