APPLICATOR FOR DIRECTING COATING MATERIALS AT A SUBSTRATE

Abstract: A process and apparatus for directing coating materials at a substrate with reduced production of mist and enhanced range of coating thickness and uniformity of coverage. A flow of coating liquid or fluid (821) is directed toward a substrate, and attenuated (826) in transit by a co-flowing impingement fluid (823). The impingement fluid is capable of attenuating liquid in the coating stream into droplets that form a fine mist (827). The mist is propelled toward the substrate (826) by the impingement fluid and deposited on the substrate. The aqueous liquid is preferably less than 100 DEG C, and directed through an outlet under a low pressure, for example, less than 12 psi (82 kPa, such that the liquid velocity is low (e.g. less than 1 meter/second). Process parameters may be varied to reduce grainy or streaky coatings, thereby assuring thorough coverage of a substrate even with very thin coatings.

Description: APPLICATOR FOR DIRECTING COATING MATERIALS AT A SUBSTR A T E CROSS REFERENCE TO RELATED CASES This is a Continuation-in-Part of co-pending United States Patent Application Serial No. 07/531,481 filed May 30, 1990, Serial No. 07/647,186 filed January 24, 1991, and Serial No. 07/692,861 filed April 29, 1991. All of these applications are incorporated herein by reference. BACKGROUND OF THE INVENTION The present invention relates to an apparatus and process for coating a substrate. More particularly, it concerns an apparatus and method for depositing a uniform coating of liquid, or a liquid containing particulates, on a broad variety of substrates such as paper, cloth and organics. GENERAL DISCUSSION OF THE BACKGROUND Many manufacturing processes coat products with a liquid film to preserve them or improve their physical properties. Starch, for example, is often applied to the surface of paper, preservatives and additives are sprayed on foods, surface treatments are placed on wood products, and tanning chemicals are applied to leather. It is often preferable to apply a uniform coating of such liquids, or at least to apply a thorough coating that does not leave bare any part of the surface to be treated. Conventional spray applicators have been used in such processes, but suffer from poor uniformity of application that sometimes leaves portions of the substrate untreated, undertreated or overtreated. Applicator rolls and blade coaters provide a more uniformly coated substrate, but are unwieldy and unable efficiently to apply liquids to non¬ linear surfaces. The drawbacks of several of these specific systems are illustrated below in connection with coating a paper web substrate. Paper webs are frequently treated to increase their surface strength and enhance their printability by providing a smooth printing surface on the paper. Paper coating is often performed by applying an excess amount of coating material onto an applicator roll for transfer to the web. Alternatively, the coating liquid is applied directly to the web in excess, and then metered to the correct thickness with a blade or rod. Although roll and blade coating systems apply relatively uniform layers on a substrate, such systems suffer from the drawback of requiring an expensive piece of heavy machinery that occupies a large amount of space. A typical roll coating system in a paper mill, such as a conventional two-roll size press or a gate roll system, can cost millions of dollars and require an in-line space of 10 to 30 meters (30 to 100 feet) . Placing a roll coating system within an existing line of equipment also requires removal and relocation of existing equipment, which greatly increases the installation costs. Spray systems are a less expensive and more compact alternative to roll coaters. In a typical spray applicator, pressure is applied directly to liquid in the spray head. Passage of the liquid through a constricted orifice in the spray head breaks the liquid into droplets of many sizes. In spite of their convenience, spray systems do not uniformly apply material to a substrate. The resulting coated product is streaky and blotched, rendering it less appealing to consumers. The irregular surface coverage may also diminish the appearance of printing on the surface. Another drawback with spray systems is that the droplets they produce tend to become airborne as a mist, and the mist is carried throughout the area adjacent the spray nozzle, where it builds up on the spray system and surrounding equipment. The mist can also pose a health or hygiene problem to workers in the vicinity who come into contact with the mist or inspire it. A cross-section of a typical prior art pressure spray head is shown in FIG. 1. The spray head 10 has a body 11 with a circular horizontal cross-section and a central interior bore 12 that tapers in the direction of a small cylindrical spray orifice 13. The liquid material is forced under pressure through the tapering central bore and out of the orifice at a high velocity to produce liquid droplets. The design of the central bore 12 and the orifice 13, in combination with the internal pressure on the material, determines the pattern of spray produced by the nozzle. The size distribution of the resulting droplets varies across a broad range, and the spray is difficult to control or direct. It also deposits unevenly across the surface width of a paper sheet or other object being coated. A typical lateral mass distribution of material from a conventional spray head is shown in FIG. 2. The applied coating is markedly non-uniform with two peaks 14 and 15 spaced laterally from the center line of the spray head. The volume flow at each of the peaks 14 and 15 is approximately twice the volume flow at the center 17 of the spray pattern, and approximately seven times the flow at the outer edges 18 and 19 of the pattern. The flow at the center 17 is itself approximately four times the flow at the edges 18 and 19. This lateral non-uniformity of application causes undesirable streaking of the coating on the substrate with thicker and thinner application of the material across its width. It is accordingly an object of this invention to provide an improved apparatus and method that can deposit liquid on a substrate more evenly than conventional spray coating. It is also an object of the invention to provide such an improved apparatus and method that more completely or thoroughly covers a substrate than do spray nozzles. Another object of the invention is to provide an apparatus and method that is less expensive and space consuming than roller or other conventional applicators, and which can be easily retrofitted into existing production lines. Yet another object is to provide an improved system for applying liquid coatings to a variety of substrates having many different topographies. Yet another object is to provide an improved application system that produces less ambient mist than conventional sprays. Finally, it is an object of the invention to provide an improved applicator that is capable of depositing very thin liquid coatings as well as thicker coatings. These and other objects of the invention will be understood more clearly by reference to the following detailed description and drawings. SUMMARY OF THE INVENTION The process of the present invention uniformly deposits a coating of a material on a substrate by directing a flow of an elongated array of the material from an outlet, such as multiple orifices or a slot, toward the substrate. A fluid (such as a gas) is impinged against the array to attenuate the flow into droplets that can deposit a uniform coating on the substrate. The substrate and array move relative to one another as the array is attenuated into droplets such that a coating may be evenly or thoroughly deposited over an area of the substrate. The flow rates and velocities of the coating material and impingement fluid can be varied over a broad range to alter the degree of attenuation of the array and the resulting uniformity of droplet deposition on the substrate. In some preferred embodiments, the coating material is a liquid, such as an aqueous liquid that is (by definition) at less than 100°C (212°F) . In other preferred embodiments, the liquid is non-aqueous, for example, an isocyanate such as PMDI, or acrylics, styrene- maleic anhydride, and epoxy resins. The low viscosity of the liquid renders it flowable at room temperature, hence the coating process can be carried out entirely at ambient temperature (15°C-40°C or 60°F-100°F) . The viscosity of the liquid can vary over a broad range, for example 1 - 2000 cP (0.001-2 Pa-s) , and the low viscosity of the liquid allows it to be directed through an outlet toward the substrate under low pressures, such as 5 - 25 psi (35 - 175 kPa) or even as low as 1 psi. In such low pressure embodiments the liquid moves at relatively low velocities from the outlet toward the substrate, and is impinged by a fluid, such as a gas, that moves at a greater velocity than the liquid. Although variable, the gas temperature is preferably less than 100°C, and preferably is ambient temperature. The gas may be humidified to help reduce the drying and build-up of water soluble coating materials inside the applicator head or at the outlet slot. Moisture or other additives in the gas stream may also be used to catalyze or modify the liquid in the attenuated array as it travels to the substrate. Plural fluid or gas streams may be spaced varying distances from the outlet. By including catalysts in the stream spaced by another intervening gas stream from the outlet, the possible catalyzation of the liquid at the outlet is minimized. The elongated liquid array emerging from the outlet has opposing faces, and the fluid can be impinged against either one or both faces of the array at a wide range of velocities, from 200 feet per second (60 m/s) to supersonic velocities. Attenuation of the liquid into increasingly finer droplets occurs as the fluid velocity is increased, for example, as it approaches sonic velocity. Much lower fluid velocities are also suitable for many applications where large droplet size can be tolerated. The present invention can be used to coat a broad variety of substrates, such as cellulosic, fiber, organic, synthetic, rubber, cloth, wood, leather, food, and plastic substrates. A wide variety of coating materials can also be applied to substrates using this method. The coating material may preferably be a liquid at room temperature such that it can be sprayed on the substrate in a liquid form without having first solidified before reaching the substrate. The coating fluid may contain particulate matter that is also to be deposited on the substrate. Alternatively, particulate matter can be introduced into the liquid by the impingement fluid. In alternative embodiments of the process, the coating liquid is dispersed into droplets before the impingement fluid encounters it. In such embodiments, the liquid is turned into a mist electrostatically or ultrasonically. The mist is then directed toward a moving substrate by an impingement fluid, which may be directed at the substrate under low pressure. The apparatus of the present invention includes an applicator, movement means for establishing relative movement between the substrate and applicator, and an outlet in the applicator that directs a flow of an elongated array of coating material toward the substrate. A fluid outlet in the applicator impinges a fluid, such as a gas, against the array to attenuate the array into droplets or direct it toward the substrate to deposit a coat of liquid on the moving substrate. A nozzle portion of the applicator head contains the outlet through which the coating material is ejected under pressure to form the liquid array. One or more impingement fluid slots may extend along the applicator adjacent the coating material outlet to provide a curtain of fluid, such as a gas, that is propelled under pressure against the array of coating material. The described apparatus is capable of depositing a uniform coating of coating material (such as a liquid) on the substrate, and the thickness of the coating can be varied from very thin to quite thick. In other preferred embodiments the applicator includes a cleaning means that removes a build-up of matter from the applicator head. In some embodiments this cleaning means includes a movable portion of the applicator that covers an internal passageway leading to the outlet. The movable portion may be hinged to the applicator to permit the movable portion to swing away from its closed position and open the applicator. The opened applicator provides access to its interior to permit the liquid passageway and liquid outlet slot to be cleaned. In other embodiments, the applicator is a head made of matable bipartite portions that meet to define an internal coating material passageway that communicates with an outlet. The portions of the head are matable, and may optionally be selectively separated by a power actuated arm that moves the matable portions apart to expose the internal passageway and outlet for cleaning. The cleaning means may also be an internal or external wiper that moves along or through the head to remove solids build-up. Solubilizing materials, such as humidified air, can also be added to the impingement fluid or gas to dissolve and remove water soluble solids from the head and outlet. Other solvents may also be included in the impingement fluid for cleaning purposes. The solvents may be selected to target and remove the dried coating material. The accumulation of agglomerated coating material in the head may also be diminished by coating the surfaces of the head with a low surface energy material that reduces adhesion of the coating liquid to the head and outlet. Examples of such materials include polytetrafluoroethylene, amorphous carbon or polycrystalline diamond. Adhesion to the head is also diminished by providing sharp edges around the outlets or orifices from which the coating material and impingement fluid emerge. The coating apparatus may also include a mist collection device. The mist is preferably collected with a pressure differential, for example, by providing a suction pressure from a hood adjacent the applicator. An air curtain may be directed toward the substrate between the hood and moving substrate to prevent escape of mist between the substrate and hood. Alternative collection devices include electrostatic directors that govern the movement of the mist. The director may be, for example, a repulsion plate or bar spaced from the substrate and charged to repel oppositely charged mist droplets toward the substrate. Alternatively, the mist may be collected by grounding the substrate to attract charged mist particles. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a prior art conventional spray nozzle. FIG. 2 is a graph representing the lateral flow distribution of liquid from the prior art spray nozzle of FIG. 1. FIG. 3 is a perspective view of the apparatus of the present invention in use coating a moving substrate. FIG. 4 is a view taken along view lines 4-4 of FIG. 3. FIG. 5 is an enlarged cross-sectional view of the head of FIGS. 3 and 4. FIG. 6 is an enlarged view of the central apex of the head, showing the liquid orifices. FIG. 7 is a perspective view of the central portion of the head taken along view lines 7 - 7 of FIG. 5. FIG. 8 is an enlarged view of the liquid passageway portion of the head circled in FIG. 7. FIG. 9 is an alternative embodiment of the applicator head. FIGS. 10 - 13 are alternative embodiments of the nozzle portion of the head shown in FIG. 9. FIG. 14 is a view similar to FIG. 4 showing another embodiment of the head in which the liquid outlet is a slot, an enlarged portion of the slot being shown in the circle. FIG. 15 is a cross-sectional view of an alternate slotted outlet nozzle portion of the head. FIG. 16 is another embodiment of the slotted head. FIG. 17 is an alternative embodiment of the head of the present invention. FIGS. 18A - D are several other embodiments of the nozzle portion of the head illustrating the wide variety of angles with which the fluid stream impinges the liquid. FIG. 19 is a side elevational view of an alternative embodiment of the invention showing a power means for opening the bipartite head about a pivot point. FIGS. 20 - 22 show alternative embodiments of the head of FIG. 19 having different means for opening the head to clean it. FIG. 23 shows an alternative embodiment of the head having a replaceable tip. FIG. 24 is a view along section lines 24-24 of FIG. 23. FIG. 25 is a perspective view of an alternative modular embodiment of the head. FIG. 26 is a schematic view of an alternative embodiment of the invention in which liquid is pre- atomized before being directed at a substrate. FIG. 27 is a schematic cross-sectional view of an electrostatic atomizer for dispersing liquid into droplets. FIG. 28 is a view similar to FIG. 3 showing an alternative embodiment of the applicator in which a collection hood surrounds the applicator. FIG. 29 is a cross-sectional view of the applicator taken along section lines 29-29 of FIG. 28. FIG. 30 is a cross-sectional and schematic view of an air scrubber for removing liquid droplets from the exhaust of the hood of FIG. 28. FIGS. 31 and 32 are cross-sectional and schematic views of other embodiments of the hood in which secondary flows of air are introduced into the hood. FIGS. 33 and 34 are schematic views of applicators applying a coating to substrates moving in different planes, wherein the coating on each side of the substrate can be different materials. FIGS. 35 - 37 are photographs prepared from high speed videotapes of liquid arrays impinged with gases at increasingly greater gas velocities. FIGS. 38 - 43 are photographs prepared from high speed videotapes of liquid arrays impinged with a gas at varying velocities, the photographs being taken at varying distances from the head. FIG. 44 is a photograph showing a grainy distribution of iodine stained coating liquid on a paper substrate coated with the present invention. FIG. 45 is another photograph showing a streaky distribution of iodine stained coating liquid. FIG. 46 is a series of photographs of iodine stained coating liquid on sheets of paper demonstrating the effect of air pressure and application rate on coverage uniformity with the applicator of the present invention. FIG. 47 is a series of photographs of iodine stained coating liquid on sheets of paper demonstrating the effect of air pressure and air gap width on coverage uniformity with the applicator of the present invention. FIG. 48 is a column average and single line grey intensity profile for a gate roll coated sample of paper. FIGS. 49 - 51 are column average and single line grey intensity profiles for materials coated with the apparatus and method of the present invention, illustrating variations in product quality as a function of process parameters, for Table II runs K7, S12C1, S7, and S6, respectively. FIGS. 52 - 65 are graphs showing column average and single line grey intensity profiles for Table II runs S3A, S16G, S16F, S18D, S15A, S13B, S12B, S5C, S5Q, S19E, S19G, S19K, S20A, and S21A respectively. FIGS. 66 - 69 are single line grey intensity profiles in the direction of substrate movement for the runs from Table II referenced on the face of the tracing. FIG. 70 is a cross-sectional view of an alternative embodiment of the applicator. FIG. 71 is a fragmentary view of the head taken along section lines 71-71 of FIG. 70, the central portion of the elongated head having been omitted from the drawing. FIG. 72 is a view similar to FIG. 70 showing yet another embodiment of the applicator in which a pair of parallel spaced fluid impingement slots extend along the head along both sides of the liquid outlet. FIGS. 73 and 74 are schematic diagrams of the apparatus of FIG. 70 showing several possible locations of filter screens. FIG. 75 is an alternative embodiment of the apparatus of FIG. 70 wherein some of the filter screens have been replaced with a porous fiber. FIG. 76 is a schematic view showing one possible mode of attenuation of the liquid into droplets. DETAILED DESCRIPTION OF SEVERAL PREFERRED EMBODIMENTS One preferred embodiment of the apparatus 56 of the present invention is shown in FIGS. 3 - 8 to include an applicator head 58 suspended by a mechanical arm 59 above a paper web substrate 60 that is moving below head 58 over rollers 61 in the direction of arrow 63. Of course, relative motion between the head and substrate can also be accomplished in other ways, such as by moving the head over a stationary substrate. Head 58 is shown in greater detail in FIG. 5 to be a bipartite head with a central portion that in cross-section defines an equilateral triangle. The central portion has mating, complementary wedge halves 82, 84 that meet along opposing faces to form a linear junction 86 that bisects an apex of the triangular cross-section. Each wedge 82, 84 has a notch 88 along the opposing junctional faces that, in combination with the corresponding notch from the other half portion of the head, forms a liquid chamber 90 along the length of head 80. The cross-sectional width of chamber 90 widens and then tapers along junction 86 to communicate with a plurality of narrow liquid passageways 92 (FIGS. 5, 7 and 8) that extend through head 80 along junction 86 to the apex of the head. Each passageway 92 terminates in a circular cross-section orifice 93 (which may also be square or diamond-shaped in section) that is machined to sharp edges 95, as shown in FIGS. 6 and 8. The faces 101, 103 of the head meet along a sharp apex 97 and each hemi- orifice extends in the plane of the face and is outlined by the sharp edges 95. The sharp edges 95 (for example, radius ≤.002 inch) help diminish build-up of coating material at the liquid outlet. Alternatively, one or more continuous elongated linear slots or other outlet configurations could replace the plurality of orifices 93, such as the slot described below in connection with FIG. 14. Such a slot is easier to manufacture and clean than a multiple orifice configuration. Complementary mating wedges 82, 84 are selectively held together by bolts 94, 96 that extend through bores 98, 100 in the wedges. Bore 98 communicates with an outer face 101 of wedge 84 and includes a land 102 against which the head of bolt 94 rests. Bore 98 communicates with the opposite side face 103 of head 80 formed by wedge 82, and bore 100 similarly has a land 104 against which the head of bolt 96 abuts. A notch 106 in wedge 84 of head 80 seats an elastomeric seal 108 to enhance the fluid tight nature of junction 86. An enclosure channel 116 is bolted to wedge 84 to form a fluid chamber 118 that extends along face 101 of wedge 84. Channel 116 is secured to portion 84 by a bolt 120 that extends through a bore 122 in channel 116 and an aligned bore 124 in wedge 84. Channel 116 includes an upper segment 126 that abuts tightly against face 101 of portion 84 and forms a relatively fluid tight seal therewith. Middle segment 128 and lower segment 130 of the channel extend downwardly and inwardly toward face 101 in the direction of the tapering end of head 80. Segment 130 terminates just short of face 101 in a flat face that extends parallel to face 101 and forms a narrow fluid passageway slot 132 that communicates at one end with fluid chamber 118 and at the other end forms a fluid outlet 134. Fluid passageway 132 travels along face 101 at a 30 degree angle to liquid passageways 92 such that fluid emerging from slotted fluid outlet 134 impinges the liquid array from outlets 93 at a 30 degree angle. The embodiment of FIG. 5 also includes a second air channel 116 attached to face 103 of wedge 82. A second fluid passageway is formed along face 103 such that the impingement fluid strikes the array of liquid from outlets 93 at about a 30 degree angle. Hence the liquid array is attenuated by fluid striking both faces of the array. Such bi-planar attenuation has been found to be acceptable but not essential to droplet deposition. Hence the second air channel 116 may be omitted, especially when attenuating low viscosity liquids. In operation, a coating liquid 72 (FIG. 3) is supplied under pressure to conduit 70 that communicates with chamber 90 such that the liquid distributes evenly across the length of the head. The pressurized liquid is propelled through the plurality of orifices 92 (FIG. 8) and emerges as a linear curtain or array 78 of liquid (FIG. 3) that extends across the width of substrate 60. Pressurized air 66 enters conduits 62, 64 such that each communicates with an air chamber 118 and the air is distributed through chambers 118 along the length of head 58 into passageways 132. The air emerges at slot 134 to impinge against liquid array 78 and attenuate the flowing liquid into smaller loops or ligaments of liquid and finally into droplets. By the time the liquid reaches substrate 34, it has been attenuated into small droplets that substantially completely cover and adhere to the top surface of the substrate. The degree of liquid attenuation can vary depending on the viscosity and flow rate of the coating liquid, and the attenuating gas velocity. It is frequently desireable for reasons of economy, appearance and function, to attenuate the liquid into a fine mist that deposits a thin uniform coating on the surface of the substrate. Multiple heads may be placed sequentially along a line to provide multiple coats of the same or different liquids on the substrate. If a thick single coating is desired, the operating parameters of the head may be changed, for example, to increase the volume flow of liquid. If less uniformity is required, the impingement fluid velocity may be reduced to decrease the liquid attenuation. Larger droplets will reach the substrate and form a thicker, less uniform coating. Another embodiment of the applicator head, which in this embodiment is rectilinear in cross-section, is shown in FIG. 9. Applicator head 150 is bipartite and includes mating, complementary square body portions 152, 154. Portion 152 includes a top wall 156, side wall 158, an upright liquid partition 160 that extends downwardly from top wall 156 parallel to side wall 158, and a horizontal partition 162 extending from a distal end of partition 160 away from side wall 158. Portion 154 of head 150 includes a complementary, mirror-image structure with a top wall 164, side wall 166, upright partition 168 and horizontal partition 170. Mating heads 152, 154 cooperatively form an elongated liquid chamber 171 therebetween that spans the length of head 150. Top walls 156, 164 abut along a fluid tight junction 172 that contains an elastomeric seal 174 for maintaining tightness of junction 172 and preventing escape of liquid from chamber 171 during use. Horizontal partitions 162, 170 do not abut, however, but instead stop short of one another such that their opposing faces 175, 177 form an elongated manifold 176. The manifold communicates with a nozzle member 178 that extends the length of head 150 below manifold 176. A plurality of tubular liquid passageways 180 extend through nozzle member 178 and each passageway terminates in a liquid orifice 182 at the tip of nozzle 178. The orifices may be shaped and sized like the outlets 93 of FIGS. 3 - 8. In alternative embodiments, a single slot 180 extends through the passageway to form a continuous linear outlet 182 instead of a plurality of orifices. A pair of elastomeric seals 184, 186 extend along nozzle member 178 the length of head 150 to provide a more fluid-tight seal. A pair of complementary, mirror image air plate members 200, 202 jointly form a nozzle portion of head 150. Air plate member 200 is an L-shaped member having a flange 204 that mates with a bottom face of side wall 158, and a plate that extends parallel to top wall 156 and horizontal partition 162. Member 200 forms an air chamber 208 in cooperation with walls 156, 158 and 160 of portion 152. An elastomeric seal 209 extends along the length of head 150 between side wall 158 and flange 204 to provide a fluid-tight seal. Air plate member 202 similarly includes a flange 210 that abuts the bottom of side wall 166, and a member 210 that extends parallel to top wall 164 and horizontal partition 170 to form, in cooperation with walls 164, 166 and 168 an air chamber 212 that extends along the length of head 150. An elastomeric seal 213 extends along the length of head 150 between side wall 166 and flange 210. Plates 200, 202 are each secured to head 150 by bolts 214, 216 that are respectively placed in recessed bores 218, 220 through flanges 204, 210 into side walls 158, 166. In operation, air is introduced under pressure into chambers 208, 212 and pressurized liquid is introduced into liquid chamber 171. Air emerges from slots 226, 228 substantially parallel to a linear liquid array emerging from outlet 182. The co-flowing airstream attenuates the liquid array into droplets for deposition on a substrate. An alternative embodiment of the nozzle portion of head 150 is shown in FIG. 10, wherein like parts have been given like reference numerals. In this embodiment, the air plates 230, 232 are substantially flat instead of L-shaped. Each plate 230, 232 is respectively secured to side walls 158, 166 by bolts 234, 236 through a slightly angled portion 235, 237 of each plate. The angled portion of each plate fits flush against the bottom face of walls 158, 166 and is sealed by elastomeric seals 209, 213. Because of the slight angle between portions 235, 237 and plates 230, 232, the plates extend at about a 30 degree angle downwardly from walls 158, 166 toward the center of head 150. Each plate extends centrally toward an elongated nozzle 238 through which pass a plurality of liquid passageways 180 that terminate in a row of orifices 182 aligned longitudinally along the length of head 150. Each plate 230, 232 terminates adjacent the orifices 182 of nozzle member 238 to form a pair of elongated slots 242, 244 that extend along the length of the head adjacent the plurality of orifices 182 in nozzle 238. Each slot has substantially parallel walls formed by the tip of nozzle 238 and the abutting walls of plates 230, 232, such that substantially co-planar flows of air and liquid emerge from the slots and orifices. Another embodiment of the invention, shown in FIG. 11, is similar to that shown in FIG. 9 except the tip of nozzle 238 tapers externally such that the edges of liquid orifices 182 are sharp edges. Each plate 230, 232 similarly tapers at its distal medial end to a narrower cross-sectional width than the remainder of the plate, and ends in angled edges 246, 248. The external walls of the tapering tip of nozzle 238 meet at an included angle of about 30 degrees and edges 246, 248 of the plates are parallel to the tapering tip of nozzle 238. Hence impinging gas that emerges from the air slots impinges liquid from outlets 182 at about a 30 degree angle against the liquid array. The sharp edges of the orifices 182 diminish build-up of coating material on the tip of the nozzle. FIG. 12 shows yet another embodiment of the nozzle portion of the head in which nozzle 238 includes a wide base 254 and a tapering body 256 that has a triangular cross-section. Body 256 tapers to an included angle of about 30 degrees. Base 254 and body 256 define a plurality of substantially co-planar, parallel liquid passageways 180 that extend through nozzle 238. Each passageway 180 opens into one of a plurality of aligned liquid outlets 182 that extend along the length of applicator head 150. Each air plate 230, 232 tapers to a sharp edge 258, 260 adjacent liquid orifices 182 to form elongated slots 262, 264 that extend adjacent and parallel the plurality of orifices. Tapering faces 266, 268 define planes that intersect at an included angle of about 90 degrees such that the air chambers 208, 212 narrow in the vicinity of tapering body 256 and direct a flow of impingement fluid at an acute angle against the linear array of liquid emerging from orifices 182. The sharp edges 258, 260 of the plates and the sharp edges of nozzle 238 inhibit accumulation of dried coating material in ' these areas. Finally, FIG. 13 shows a nozzle head similar to FIGS. 9 - 12, with like parts given like reference numerals. The liquid nozzle 238 includes a flat base 270, tapering body 272, an elongated slot forming member 274, and a plurality of tubular extensions 276 each defining a circular orifice 182 surrounded by an annular sharp edge. Air plates 230, 232 taper toward nozzle 238, and each is provided with a flat extension plate 278, 280 that tapers toward nozzle 238 to a very sharp edge that is spaced from yet parallel to the plurality of orifices 182 defined by tubular extensions 276. The sharp edges around orifice 182 and on plates 278, 280 diminish dried build-up of coating material dispensed from applicator head 150. In alternative embodiments of the invention, the liquid passageway 180 and liquid orifices 182 are a continuous elongated slot 282, as best seen in FIG. 14. That drawing is similar to FIG. 4, and like parts have been given like reference numerals. A magnified slotted portion of the head is shown in the circle. In this embodiment the liquid emerges from slot 282 as a curtain array, and is attenuated by air emerging from the slots defined by edges 134. Another embodiment of the slotted head is shown in FIG. 15, wherein only the nozzle portion of the head is shown. The nozzle portion 290 includes a triangular, fixed central wedge portion 292 that is triangular in cross-section and tapers to an included angle 294 that is defined by the intersection of flat faces 296, 298. A similar wedge-shaped fixed head portion 300 also tapers to an included angle 302 of about 30 degrees, the angle being defined by the intersection of flat faces 304, 306. Faces 298, 304 are adjacent and substantially parallel to one another defining therebetween a space having a width 308 of about 4 mils (0.004 inch or 100 μm) . Each head portion 292, 300 tapers to a sharp edge 310, 312 that defines an air gap 314 therebetween that is delimited by the sharp edges 310, 312 and is about 4 mils wide. A swingable wedge portion having a triangular cross-section is positioned adjacent central portion 292 and includes flat faces 322, 324 that taper to an included angle 326 of about 30 degrees to define a long, sharp edge 328. Face 322 is adjacent and parallel face 296 at a uniform distance 330, that in this embodiment is about 8 mils (0.008 inches or 200 μm) . The sharp edges 310, 328 thereby cooperatively define an elongated slot that serves as a liquid outlet. In operation, the liquid to be applied to the substrate is introduced from a pressurized reservoir (not shown) into the liquid passageway at 332. The liquid moves through the slotted space defined by faces 296, 322, and exits slotted outlet 331 to form an elongated linear array of liquid. Simultaneously, air 336 from a pressurized reservoir (not shown) is introduced into the slotted space defined by faces 298, 304 and is propelled under pressure out of air gap 314 to impinge against the liquid array from outlet 331 and attenuate it into droplets for even distribution onto a substrate. The included angle 294 of central wedge portion 292 is about 30 degrees in the disclosed embodiment, hence the air from gap 314 impinges the liquid array at an angle of about 30 degrees. This angle can vary from near zero to near 90 degrees, as long as the two streams are co-flowing. After a period of use, the flow of liquid and gas through the head is stopped and portion 320 is capable of swinging open in the direction of arrow 338 to permit access to the liquid passageway for cleaning of faces 296, 322 and edges 310, 328. In some embodiments portion 320 may also be movable in the direction of arrow 340 such that edge 320 moves in the plane of arrow 340 to permit selective protrusion or recession of edge 310 relative to edge 328. Yet another embodiment of the slotted outlet nozzle is shown in FIG. 16 wherein nozzle 350 includes a fixed side plate 352 that tapers at its distal end to a sharp edge 354 defined by tapering face 356 and flat face 358. A liquid passageway 360 is formed between fixed plate 352 and a central plate 362 that has parallel, flat faces 366, 368. Central plate 362 tapers at its distal end to a sharp edge 364 defined by flat face 366, and an inclined face 370 that forms an included angle of about 30 degrees with face 366, and an angle of about 150 degrees with face 368. Central plate 362 is retractable and adjustable along the axis shown by double arrow 372. Plates 352, 362 define liquid passageway 360 therebetween, which has the shape of a slotted enclosure that communicates with a liquid outlet slot 373 between sharp edges 354, 364. Nozzle 350 also includes a swingable plate 374 having flat faces 376, 378 that meet along a flat, blunt edge 380 that is perpendicular to faces 376, 378. A distal portion of face 376 runs parallel to inclined face 370 to form an air gap 381 that communicates along its length with an air chamber 386. Plate 374 is attached to a hinge 382 such that it swings outwardly around the axis of hinge 382 in the direction of arrow 384 to permit access to air chamber 386 between plates 362, 378. In operation, liquid 390 is introduced into the liquid passageway 360 from a pressurized reservoir, and flows out of liquid slot 373 to form a linear array of liquid. Air is simultaneously introduced into air chamber 386 under pressure and propelled out of air gap 381 to impinge the linear array of liquid at about a 30 degree angle and attenuate the liquid array into droplets for uniform deposition on a substrate. Central plate 362 can be moved in the axis of arrow 372 to recess or protrude edge 364 and simultaneously vary the distance of the air gap formed between faces 370, 376. After use, nozzle 350 can be cleaned by swinging plate 374 in the direction of arrow 384 to permit greater access to air chamber 386, central plate 362, and edges 354, 364. Attenuation of the liquid need not occur external to the applicator head. This principle is illustrated in FIG. 17, wherein a nozzle 400 of an applicator head is shown to include a flat plate 402 having parallel faces 404, 406. An end edge 408 intersects face 404 at a right angle, but then curves toward face 406 to form an arcuate junction with face 406. An internal, wedge-shaped member 410 includes flat faces 412, 414 that intersect along a sharp edge 416 at an included angle of about 30 degrees. Faces 404, 412 are parallel to one another and spaced 5- 15 mils (0.005 - 0.015 inches or 130 - 400 μm) apart to form a gas slot or passageway 418 therebetween. Also included in nozzle 400 is a plate 420 having flat parallel faces 422, 424 and flat face 426 which is co-planar with face 412 and forms an included angle of about 30 degrees with face 424 and an included angle of about 150 degrees with face 422. A slot or liquid passageway 427 is formed between faces 414, 422 and intersects slot 418 at a 30 degree angle. A wedge-shaped member 430 is positioned below plate 420 and includes a top interior face 432 and bottom exterior face 434 that taper toward each other and meet along a sharp edge 436 to define an included angle of about 20 degrees. Faces 424, 432 thereby define a slot or passageway 438 that decreases in width as it approaches edge 436 and terminates in an air gap 440. Finally, a wedge 442 has a flat interior upper face 444 and lower exterior face 446 that taper to a sharp edge 448 at an included angle of about 45 degrees. Exterior face 446 is co-planar with exterior face 434 of wedge 430. Faces 406, 444 define an air chamber 450 therebetween that tapers in width in the direction that air flows through the chamber until it forms an air gap 452 between edges 408, 448. In operation, a fluid such as air 454 is introduced under pressure into passageway 418. A liquid 456 to be coated on a substrate is simultaneously introduced through liquid passageway 427 such that the gas 454 impinges liquid 456 at about a 30 degree angle in an impingement zone 458 that is partially bounded by faces 404 and 426. The linear array of liquid emerging from liquid slot 427 is thereby attenuated by gas 454 into droplets which are then propelled out of nozzle 400 in the direction of gas 454 toward a substrate. A secondary flow of pressurized gas, such as air, can be expelled at low pressure from either or both of gaps 440, 452 to further direct the flow of attenuated droplets toward the substrate. Many different nozzle configurations are possible in accordance with the present invention. This is particularly true because the angle of impingement between the fluid and liquid can vary widely. Several different embodiments of the invention are shown in FIG. 18 to illustrate some different impingement angles that are possible with the present invention. In FIG. 18A, for example, a slotted liquid passageway 462 is defined between the parallel faces of external plate 464 and internal member 466. An inner fluid chamber 468 is formed between member 466 and an external face member 470. Liquid slot 462 is shown substantially vertical, while air chamber 468 tapers to a gap 472 that takes an arcuate path from horizontal to vertical to form an arcuate slot 476. The arcuate slot 476 is formed by complementary radiused portions 478, 480 on members 466, 470. Arcuate passageway 476 arcs through an angle of about 90 degrees from its proximal entrance to its distal exit to become almost parallel to liquid slot 462. Hence fluid passing through passageway 476 impinges the linear array of liquid 462 at an angle approaching zero degrees. FIG. 18B shows a similarly radiused passageway 476 in which the passageway arcs through about 90 degrees from a vertical to a horizontal orientation to become almost perpendicular to slot 462 and impinge gas on the emerging liquid array at an angle approaching 90 degrees. FIG. 18C shows a passageway 476 which diverges and then converges to increase the velocity of gas impinging against the liquid array. FIG. 18D shows a similar configuration in which passageway 476 first converges and then diverges, again to increase the impingement velocity of the fluid. The fluid impinges the liquid array at an angle of about 45 degrees in the embodiments of FIGS. 18C and 18D. Cleaning Means The present invention also includes a cleaning means for removing build-up of solidified coating material in the head. The cleaning means includes head designs which open to allow easier access to internal passageways and nozzle orifices. Other examples of cleaning means include external wipers, internal wipers, cleaning additives in the fluid and liquid streams, and flushes of pressurized water or other solvent for removing build-up of solidified coating material from the head. A flush pan of a cleaning fluid (such as water or another solvent) can also be brought into contact with the fluid and liquid outlets of the head to clean it. One particular embodiment of a cleaning means is shown in FIG. 19 in which a head 500 is shown that is similar to that shown in FIG. 5, but it includes an air channel on both sides of the head. Head 500 has complementary wedge portions 502, 504 that mate along a common junction 506 along which a liquid slot or series of liquid passageways are formed. The liquid passageway or slot terminates in a series of liquid orifices 508 or a continuous slot at the tip of nozzle head 500. An air plate 510 is secured to an outer face of wedge 502 to form an air chamber exterior to wedge 502 that tapers to an air slot or gap 512 adjacent liquid orifices 508. Another air plate 514 is similarly secured to and carried by an outer face of wedge 504 to form a tapering air chamber that terminates in an air slot or gap 516. A right angle flange 518 on wedge 502 includes first and second legs 520, 522. Leg 522 is attached along its exterior length to a top face 524 of wedge 502. Leg 522 is mounted on a pivot rod 526 such that portion 502 and plate 510 are free to pivot together about pivot 526 away from wedge 504 and plate 514. Leg 520 extends upward perpendicularly from face 524, and its distal end is pivotally mounted at 528 to the piston 530 of a pneumatic cylinder 532. The cylinder 532 is in turn pivotally mounted at 534 between a pair of parallel, upright flanges 536 (only one shown in FIG. 19) that are attached to a top face of wedge 504. Flanges 536 may in turn be secured to a support tube (not shown) that suspends nozzle head 500 above a substrate to be coated. In operation, fluid is impinged against a liquid emerging from nozzle head 500, as described in connection with FIG. 5 above. Once the coating process is completed, introduction of fluid and liquid through head 500 stops. The cylinder and piston assembly 530, 532 is then actuated to retract piston 530, and pivot wedge 502 and plate 510 away from wedge 504 and plate 514. In this manner, the liquid passageways or liquid slots along junction 506 will be exposed, which allows ready access for cleaning. FIG. 20 shows a nozzle head 500 similar to that shown in FIG. 19 with like parts being given like reference numerals. FIG. 20 differs, however, in that piston 530 is directly attached to a pivot rod 538 such that retraction of piston 530 opens the head for cleaning. FIG. 21 shows a similar arrangement in which both wedges 502, 504 have pivot rods 526 mounted on them near junction 506. A pair of parallel pivot rods 538 are secured to the top of each wedge 502, 504 parallel to rods 526 but spaced away from junction 506. A piston 530 is in turn connected to each of the rods 538. Retraction of pistons 530 swings each of the head portions about pivot rods 526 as jaw members away from each other to allow unobstructed access to the interior of the head for cleaning. FIG. 22 shows another alternative embodiment of the cleaning means in which the portions of the head are moved apart linearly instead of arcuately. An L-shaped flange 540 is mounted to wedge 502 with a plate 542 secured to top face 524, and an upright plate 544 projecting perpendicularly away from face 524 adjacent and parallel to junction 506. A pair of guide rails or plates 546, 548 are mounted on and project perpendicularly away from plate 544 parallel to top face 524. A U-shaped guide channel 550 is mounted to the face 524 and includes a top channel 552 that rides along rail 546, while a bottom channel 554 rides along bottom rail 548. During cleaning operations, wedge 504 is translated along rails 546, 548 to move wedge 504 away from wedge 502 and expose junction 506 for cleaning. Another embodiment of the cleaning means is shown in FIGS. 23 and 24, wherein the head includes a replaceable tip. The head 560 is again bipartite and includes portions 562, 564 pivotally mounted to one another along a pivot rod 566. A top face 567 of portion 562 is inclined at about a 45 degree angle to junction 568 to allow the halves of the head to move through an arc of 45 degrees about the hinge without mechanical interference. Complementary faces of portions 562, 564 meet along junction 568, and the faces contain cooperative angular notches that form an elongated internal liquid chamber 570 that communicates with a liquid supply line 572. Head portions 562, 564 also include complementary recessed arcuate faces that form guide channels to allow a replaceable tip 574 to slide longitudinally in and out of the nozzle portion of head 560. Tip 574 includes enlarged cross-section support member 576 that has radiused edges to facilitate its sliding in and out of the channels in head 560. Tip 574 includes two complementary, opposing, members that are hinged together along a longitudinal edge to form a liquid passageway 578 therebetween that terminates at sharp edges to define a slotted liquid orifice 586 therebetween. Patency of the passageway 578 is maintained by a plurality of lands 579 on one or both of the opposing faces of the bipartite tip 574. Lands 579 are diamond shaped, preferably elongated diamond shapes, with the lesser included angles of the diamond pointed along the axis of flow of liquid to minimize turbulent interference with liquid flow. An air plate 580 defines an air chamber 582 that tapers to an air slot 584 that extends along the length of tip 574 adjacent the sharp edges of the tip that define liquid slot 586. Air leaves chamber 582 at slot 584 to impinge upon a linear array of liquid (not shown) emerging from slot 586. When a build-up of material occurs in liquid passageway 578 or at slotted orifice 586, the replaceable tip 574 can be removed from head 560 by sliding the tip out of the head and opening it about its hinge to clean it. When the tip is damaged or worn, it is replaced with another module tip 574. A build-up of coating material inside the head can also be diminished by coating the interior of the head, or at least the portions that contact the coating liquid, with a low surface energy material that reduces adhesion of materials that contact it. Examples of such materials include polytetrafluoroethylene, polycrystalline diamond and amorphous carbon coatings. A suitable polycrystalline diamond coating can be obtained from Diamonex of Allentown, Pennsylvania. The head may be coated with amorphous diamond to a thickness of 50θA by a chemical vapor deposition process at 800°C (1470°F) . Such a coating has the advantage of being microscale smooth, closely replicating the surface it is applied to, and is chemically bonded to the surface. Modular Heads One of the advantages of the present invention is that it can be used to apply a uniform coating across very wide substrates, from several feet to several hundred feet wide. The applicator head in such instances may be quite long, unwieldy and heavy. These problems can be minimized by constructing the head from a plurality of aligned modules that can be individually removed or replaced. An example of such a module 590 is shown in FIG. 25. Module 590 includes a nozzle head 592 having a flat top 594, a pair of side walls 596 perpendicular to top 594, and a nozzle 598 that tapers to form a pair of air slots. An impingement gas is propelled against a linear liquid array formed at the tip of the nozzle as already described in connection with FIGS. 3 - 18. Protruding from top face 594 is a module attachment member 600 having a flat top face 602 and inwardly tapering faces 604, 606. Attachment member 600 slides into a complementary shaped recessed channel along a support member (not shown) to suspend head 592 above a substrate. A plurality of modules 590 can in this manner be positioned end to end with fluid and air supply lines 608, 610 aligned for communication along the length of the apposed modules. Many other modular arrangements are possible. It may be advantageous, for example, to mount the modules so that they can be removed by being pulled vertically downward, or slide out in a transverse instead of a longitudinal direction. These module arrangements would permit the removal of individual modules in the middle of a line without having to remove any of the other modules. Once removed, the modules can be cleaned or simply replaced by other mass-produced, identical modules. Alternative Attenuation Means The present invention does not necessarily require attenuation of the liquid stream by impingement of a fluid. FIG. 26, for example, illustrates an alternative embodiment of the invention in which an elongated liquid chamber 616 tapers to a slot 618. Chamber 616 is divided by plates 620, 622 from a low pressure air chamber 624, 626 on either side of liquid chamber 616. Wedges 628, 630, which respectively form the floors of chambers 624, 626, taper toward slot 618 to form narrow gaps 632, 634 that run adjacent and parallel to liquid slot 618 along its length. In operation, the liquid to be coated on a substrate is pre-atomized into fine particles 635 by electrostatic, ultrasonic or high pressure means before it enters liquid chamber 616. The atomized liquid exits chamber 616 at slot 618, and is directed toward a substrate by low pressure air emerging from slots 632, 634. Examples of electrostatic dispersion of liquids into droplets are described in Castle et al., IEEE Transactions on Industry Applications, f1:476-477 (May/June 1983) and Bailey, "Electrostatic Spraying of Liquids" (John Wiley & Sons, Inc. 1988) . A schematic representation of such an electrostatic dispersion device is shown in FIG. 27 in which an air conduit 637 is provided through which air flows in the direction of arrow 638. The flowing air encounters an air shear nozzle 639 that is supplied with liquid through supply line 640. Air 638 disperses the liquid from the nozzle into a fine mist of droplets that is then electrically charged by an induction electrode 641. The induction charging produces an electrostatic force on the droplets that counteracts surface tension forces and produces smaller, more uniform sized droplets. The uniform charge on the droplets produce a more dispersed entrainment of mist in the air, and the charge on the droplets can be used to attract the droplets to an oppositely charged or grounded substrate. Ultrasonic attenuators use high frequency vibrators or sound waves to vibrate a liquid and disperse it into droplets. An example of a suitable ultrasonic atomizer is the Ultrasonic Atomizing Nozzle systems available from Sono-Tek Corporation of Poughkeepsie, New York. A liquid stream is broken into a spray of tiny droplets by subjecting it to high frequency vibrations concentrated on an atomizing head of a titanium nozzle. The vibrations are generated by ceramic piezoelectric crystals in the nozzle body. Other suitable pre- dispersion systems would include Cool-Fog Systems from Cool-Fog Systems, Inc. of Stamford, Connecticut, or the Ultrasonic Spray Nozzles available form Heat Systems Ultrasonics of Farmingdale, New York. In other alternative embodiments, a supply of humidified air, steam or other vapor laden gas is directed through the coating outlet and impingement slot. A very thin coating of material can by applied to the substrate in this manner. If steam, for example, is propelled through both outlet 93 and impingement slot 132 in FIG. 5, a very thin yet thorough coating of water can be applied to the substrate. Collection Device A serious problem with many coating or spraying systems is that they produce a fine mist that deposits on machinery and workers in the vicinity of the applicator. This is a particular problem with materials such as starch that form a thick, solid deposit on almost any surface with which it comes in contact. Another serious problem is presented by systems that apply corrosive or biologically harmful materials, such as isocyanates, that have to be contained for environmental or health reasons. The applicator system of the present invention represents a substantial advance over the prior art, because it produces less mist than conventional spray nozzles. There are some applications, however, for which it is desireable to reduce the amount of ambient mist even further. An embodiment of a collection device for reducing the amount of ambient mist is shown in FIGS. 28 - 29, which show an applicator head 642 suspended above a moving substrate 643. A liquid inlet 644 introduces a liquid to be coated into a liquid chamber inside head 642. Air conduits 645, 646 convey pressurized air into head 642 for attenuating the liquid as it emerges from the head. A linear array of liquid 647 emerges from head 642 along its length, and the liquid is attenuated by an impinging gas which directs the liquid toward the substrate 643. A collection hood is suspended over substrate 643 spaced from head 642 on each side along an axis of movement 648 of the substrate. The hood on each side of the head includes an elongated tubular collector 650 with a collection slot 652 facing downwardly. Each tubular collector is oriented perpendicular to axis 648. Slot 652 subtends an arc of about 45 degrees to 60 degrees below a horizontal diameter 653 of tubular collection 650. A rectangular cover panel 654 extends from the upper edge of slot 652 and angles down toward substrate 643 at about a 15 degree angle. Cover panel 654 spans the width of substrate 642, and extends part of the distance to head 642 before terminating along a distal edge 656 that is parallel to the liquid array 647. The distal edges 656 of the two panels define an open area therebetween into which the liquid array is directed at substrate 643. Another rectangular panel 657 extends from a lower edge of the opening 652 and projects downwardly toward substrate 643 to provide a mist barrier. An upright wall 658 closes the free ends of each tubular collector 650, and extends between the collectors 650 to form a continuous barrier along a portion of one longitudinal edge of substrate 643. A similar wall 660 extends between the collectors 650, but does not close the end faces of each collector. Instead, exhaust tubes 662, 664 communicate with collectors 650 and extend away from substrate 642. A negative relative pressure (such as a vacuum suction) is provided in each tube 662, 664 to withdraw a mixture of mist and impingement gas out of the collectors, as indicated schematically by arrow 666. The enclosure formed by collectors 650, panels 654, 657 and walls 658, 660 is suspended slightly above substrate 643 to permit free movement of the substrate beneath the enclosure. Suspension of the enclosure thereby creates a small separation 670 between the bottom of the enclosure and the surface of substrate 643 that would normally permit some of the mist to escape from the enclosure. Most of the mist tends to spread out along the substrate, in both directions from head 642, along the axis of arrow 648. The majority of the mist is directed toward separation 670 in the direction 648 of movement of the substrate because the substrate carries the mist along with it. Hence the majority of the mist passes under barrier 657 in the direction 648. An air curtain is directed below the bottom edge of each barrier 657 to diminish the amount of mist that escapes from the enclosure underneath the edge. As best seen in FIG. 29, a tubular conduit 674 is mounted across the width of substrate 643 below each collector 650 on the outside face of each panel 657. The conduit 674 contains an air slot 675 that extends the length of the conduit, and communicates with an air directing member 676 that propels air downwardly at substrate 643 at an angle of about 45 degrees to the surface of the substrate. Air 678 (FIG. 28) is supplied to each conduit 674 such that a curtain of air is propelled out of member 676 and forms an air curtain 680 (FIG. 29) between the bottom of the enclosure hood and the surface of the substrate to diminish the amount of mist that escapes from the enclosure. FIG. 29 shows that the mist inside the hood rises to form a cloud 682 inside the enclosure. Upward recirculation 683 of the mist can direct currents of mist back toward head 642, and form a stagnant cloud below top panels 654. Development of this cloud can lead to deposition of coating material on the undersurface of panels 654, and growth of stalactites from the panels. The stalactites serve as foci from which drips of coating material drop onto the substrate to disrupt uniformity of the deposited coat. Such drops also impair the appearance of the sheet. Hence the inventors have allowed or introduced a secondary flow of air into the hood adjacent the head to disrupt formation of the undesirable cloud. The secondary flow is shown schematically by arrows 684 in FIG. 29, and can be any external source of air directed into the hood adjacent the head. A specific device for developing the secondary flow is illustrated in FIG. 31 wherein a matrix of air outlets are provided on a downwardly inclined face 685. A flow of air emerges from the outlets along the length of the hood to redirect any upward circulation of mist back down toward the substrate and into an excess air collection hood 686. Alternatively, as shown in FIG. 32, an air supply chamber 687 can cover the open area between the head and hood. The lower face of the chamber 687 contains a matrix of air outlets across its area to direct a blanket of air down at cloud 682 to prevent accumulation and stasis of mist .near the hood. Electrostatic Collection It would be possible to reduce environmental mist and improve the deposition of the liquid on the substrate by grounding the substrate, as shown in FIG. 28. A grounding member 690 is illustrated extending below substrate 643 transverse to the direction of movement 648 t - the substrate. Grounding member 690 can be, for example, a piece of metallic tinsel or a conductive brush that is in electrical contact with a ground 692. Charged particles of mist would be attracted to the grounded substrate to thereby reduce their dispersion into the environment and enhance their re-deposition on the surface of the substrate. An alternative or additional electrostatic repulsion member is shown at 694. Many types of electrostatic members can be used, including flat or arcuate plates that extend transversely across the substrate. The particular embodiment shown in FIG. 28 shows a bar having the shape of an inverted U in cross- section. The bar is negatively charged from a conventional charger (not shown) to propel toward the substrate any negatively charged droplets that pass between the member and the substrate. The bar could alternatively be negatively charged to propel positively charged droplets toward the substrate. These electrostatic collection methods can be enhanced by charging the droplets with an induction electrode, as shown in FIG. 27. Scrubbing and Venting the Mist It is desireable to vent the exhaust stream 666 (FIG. 28) from the hood into the environment to dispose of the large volume of gas and entrained liquid droplets that are produced by the liquid attenuation. Such venting to atmosphere is possible when the mixture of gas and liquid consists of an environmentally benign material, such as a mist of water. More frequently, however, the exhausted mist contains materials such as starch or isocyanates that cannot be exhausted into the atmosphere. Starch mist, for example, would deposit a film of starch on objects in the vicinity of the vent. Even more seriously, exhausting isocyanates into the atmosphere would expose people to undesirable biological consequences. In such situations, the mixture of gas and air 666 may be conducted into a scrubber 700 (FIG. 30) where the liquid mist is disentrained from the gas. Scrubber 700 is a container 702 that has a top panel 704 and a bottom panel 706. A pair of parallel spaced baffles 708, 710 project downwardly from top panel 704 across the entire width of container 702 and extend toward bottom panel 706 without reaching it. A pair of interdigitating, parallel baffles 712, 714 project upwardly from bottom panel 706. The baffles 708-714 form a circuitous pathway from a spray chamber 716 to a gas outlet 718. An array of conventional spray nozzles are provided in a spray plate 720 at the top of chamber 716 to disentrain droplets from the gas. A gas pump 724 communicates with gas outlet 718 to draw gas out of scrubber 700 and exhaust it to the environment at 726. A liquid pump 728 communicates with a liquid outlet 730 near the bottom of scrubber 700 to remove liquid that accumulates on the bottom of the scrubber. In operation, water is introduced at 734 into spray plate 720 to produce a matrix of downwardly directed water sprays 736. The sprays impinge against liquid droplets in the incoming stream 666, and help propel entrained liquid droplets toward the bottom of scrubber 700 where they col ' act in a liquid pool 738 with the water from sprays 736. Che gas and any remaining entrained liquid is drawn through the interdigitating baffles 708- 714 by pump 724 in the direction indicated by arrows 740- 744. The gas emerges at 746 and is drawn into gas outlet 718 by pump 724. The gas is substantially free of liquid and can be exhausted to the atmosphere at 726. Liquid pool 738 includes both water from sprays 736 and entrained liquid droplets removed from flow 666. Hence, scrubber 700 removes harmful or undesirable entrained liquids from the hood exhaust such that the high volumes of air or other gas removed from the hood can be exhausted to the atmosphere. Entrained liquid droplets from the stream of gas and mist are diluted in pool 738 for disposal or recirculation. Alternative System Designs Another embodiment of the invention is an apparatus 829 (FIGS. 70 and 71) designed in accordance with the present invention. A central bore 830 extends through head 831, and a series of spaced cylindrical passages 832 communicate with and extend downwardly from the central bore 830. The size and shape of the passages 832 may be changed through the use of plugs 833 that have central passages 834. The passages 834 and the plugs 833 aid the even distribution of material along the length of the apparatus. The passages 834 enter the top of a central distribution chamber 835. A screen 836, which is designed to be easily removed and cleaned, extends transversely across the central distribution chamber 835. A triangular cross-section nozzle or tip 837 is attached to the head 831, and the central distribution chamber 835 extends through the nozzle 837. At the lower end of the tip and at a corner of the triangle are a series of passages 838 communicating with the central distribution chamber 835. Each passage 838 terminates in an orifice 821 such that a series of linearly aligned orifices 821 are present along the length of the head. In some designs the central distribution chamber 835 may be omitted and the passages 838 or several passages 838 would be connected directly with a passage 832. The nozzle 837 may be of metal, such as aluminum, brass or stainless steel, and may be covered with a lubricating substance or a coating that prevents the buildup of material around the orifice 821 or on the nozzle 837. The lubricating substance coating may be Teflon (polytetrafluoroethylene) or another low surface energy coating. The nozzle 837 may also be made of Teflon. The orifices 821 may have a diameter in the range of 0.005 inch to 0.050 inch and be spaced in the range of 2 per inch to 30 per inch. A preferred diameter is in the range of 0.012 inch to 0.035 inch and a preferred spacing is in the range of 3 per inch to 24 per inch. The actual diameter and spacing may be varied depending on the product requirements and the coating material being applied. A pair of side plates 840 are attached parallel to and spaced from the sides of the central head 831, covering the sides of the central head 831 and the nozzle 837. The side plates 840 define, in cooperation with the sides of head 831 and nozzle 837, a gas or air passage 841 having openings 824 adjacent the orifices 821. The air passages 841 and the openings 824 may be continuous along the apparatus or may be broken into a series of openings by ridges formed in the passages 841. The ridges could be on the head 831 and nozzle 837 or on the inner face of plates 840. Air or other gas is supplied to the passages 841 through pipes 842. A cylindrical screen 843 inside pipe 842 removes any dirt or debris from the gas supply and aids in the even distribution of gas along the length of the apparatus. A gas distribution chamber 844 extends the length of head 831 between the pipes 842 and the passages 841. There are openings, either continuous or discontinuous, between the pipes 842 and the chambers 844. The chambers 844 in turn communicate directly with the passages 841. The chambers 844 may be filled with a porous material to assist distribution of the gas. A second pair of side plates 846 would be used if two streams of air are required, as shown in FIG. 72. The plates 846 are attached parallel to and spaced from the plates 840 and form second air passages 847 having openings 848 spaced from the air passage openings 824. At the upper end of each side plate 846 is a housing 849 that forms the second gas distribution chamber 850. The gas distribution chambers 850 may be connected to chambers 844 or distribution pipes 842 or have their own supply of air or other gas. Water vapor or steam may also be added to the gas passing through passages 847. The location of the screens may be modified, as shown in FIGS. 73 and 74. Apparatus 829 in FIG. 73 contains a material screen 836 in the distribution chamber 835 and the gas screen 843 is in the pipe 842 as has been described. FIG. 74 is a diagram of an alternative apparatus 829 which illustrates the exterior of the screens. The screen 836' is in housing 851. The coating material is pumped through pipe 852 into the interior of housing 851, and then passes outwardly through screen 836 and pipe 853 into central bore 830. The screen 843' is in housing 854. The air or gas is pumped through pipe 855 into the interior of housing 854. The gas then passes outwardly through screen 843 ' and through pipe 856 that carries the gas to pipes 842. The filters may be replaced by a bulky, porous material, as shown in FIG. 75, to gather particulate contaminants and also aid in the even distribution of the gas throughout the length of the apparatus 829. The screens in pipe 842, for example, are replaced by a porous material 857 that is placed in the pipes 842 along their entire length. Additional porous material 845 may, in alternative embodiments, be placed in the distribution chambers 844. The process, similar to earlier described embodiments, utilizes air streams to attenuate the fluid stream to a diameter smaller than the diameter of the orifice 821. Streams of air or other gas 823 (FIG. 76) pass through openings 824 adjacent the orifices 821 to attenuate the liquid stream to a lesser diameter than it emerged from each orifice 821. The liquid stream 822 eventually breaks up into ligaments 826 that have a diameter smaller than the diameter of the orifice 821. Because of the liquid surface tension and related properties these ligaments 826 form droplets 827 also having a diameter smaller than the orifice 821. The air directs the droplets 827 downwardly toward the substrate 828 and also creates crossflowing turbulence in the region below the head outlet that results in a more uniform deposition of the droplets 827 onto the substrate 828. In experiments using the apparatus at pressures on the liquid in the range of 0.05 to 10 psi and air-to- liquid mass ratios of 0.03:1 to 7.7:1 there was no visible misting. This compared to highly visible misting using typical spray heads or meltblown heads operating at meltblown conditions. The air or gas in the gas streams 823 may include steam or water vapor to prevent the coating material in the fluid stream 822 from drying out before it is placed on the substrate. Coating Process The present invention also includes a process for uniformly or thoroughly depositing a coating of a liquid or other coating material on a substrate by directing a fine mist of the liquid or material toward the substrate. Formation and propulsion of the mist may be simultaneously achieved by directing a flow of an elongated array of liquid from an outlet toward the substrate. The elongated array can be any shape that provides for distribution of the mist on the substrate across a desired swath. The array can, for example, be linear, arcuate, or chevron shaped, or sequential applicators may be used to form desired arrays. A fluid (such as a gas) is impinged against the liquid array to attenuate the liquid flow into droplets and deposit a uniform coating on a substrate that is moving relative to the attenuated array. More uniform arrays, such as a row of linearly spaced nozzles or a slot, can more readily deposit the liquid uniformly on the substrate in applications where uniformity is desired. In a typical application, paper is coated by directing a linearly aligned curtain or series of columns of liquid toward a substrate from a coating head. The flow of liquid is attenuated by gas emerging from a slot on one or both faces of the liquid curtain. The liquid can have a wide range of viscosities but typical coating liquids have relatively low viscosities and are liquids at room temperature. The melting point of the liquid may preferably be below room temperature to reduce or prevent solidification of the liquid before it reaches the substrate. In preferred embodiments, the coating liquid is an aqueous liquid, such as an aqueous solution of starch, carboxymethylcellulose, polyvinyl alcohol, latex, a suspension of bacterial cellulose, or any aqueous material, solution or emulsion. The aqueous liquid is dispersed from an applicator head at less than 100°C (212°F) , because by definition an aqueous liquid would boil above that temperature and no longer be in a liquid phase. It is not necessary for the aqueous liquid temperatures to be as high as 100°C (212°F) , and they can be sprayed at temperatures less than 70°(160°F), or even at ambient temperatures (25°C - 40°C or 77°F - 104°F) . The aqueous liquid does not solidify before reaching the substrate, hence the aqueous process should be performed above about 0° (32°F) . It may be preferable with some liquids, such as those that contain starch, to warm the liquid to 40° - 70°C (104°F -158°F) to prevent precipitation of the starch in the applicator. The process of the present invention can also be used to deposit non-aqueous liquids on substrates. In specific examples, this process can apply slurries of particulate materials or organic liquids, such as polymeric methylene diphenyl diisocyanate (PMDI) or emulsifiable polymeric methylene diphenyl diisocyanate (EMDI) . The use of PMDI and EMDI is disclosed in co-pending United States Patent Application Serial No. 07/692,861 that is incorporated herein by reference. Low viscosity of the liquids allows it to be directed through a series of preselected orifices, elongated slots or other outlets in the head at low pressures. Liquid pressures are typically less than 25 psi (170 kPa) , for example 5 - 12 psi (34 kPa - 82kPa) or less than 5 psi. Liquid pressure is directly related to the velocity with which liquid leaves the head, hence the liquid velocities can also be quite low, for example less than about 1 meter/second (3.28 feet/second). Attenuation of the flow of liquid into small droplets is achieved by impinging a fluid against the liquid array to break it into smaller segments, and eventually into fine droplets that have a diameter, for example, of about 100 μm or less. The diameter of droplets emerging from the orifices is equal to or slightly less than the diameter of the orifice, or less than the width of the slot. Hence the diameters of droplets emerging from an outlet having an effective diameter or width of 500 μm will be smaller than 500 μm after attenuation. The sizes of smaller droplets are difficult to measure, and although the inventors do not wish to be bound by theoretical computations or estimates, the size of many of the droplets appears to be 5 - 50 μm in diameter. The droplets are not necessarily uniform in diameter, and usually have a broad distribution of diameters. Some of the droplets may exceed 100 μ diameter. The importance of the droplet size is that the droplets of a particular liquid have a range of diameters that are sufficiently small to thoroughly coat a desired swath on a substrate. Small droplets of the present invention form a more uniform coating with less graininess, as defined below. In preferred embodiments, the droplets are small enough to provide a thin, uniform coat on a substrate. Thin coatings in the range of 0.11 - 0.19 g/m" (approximately 4.9 - 8.3 lbm/ton) can be provided on a surface of the substrate. The impingement fluid can be any substance that tends to flow or conform to the outline of its container. Examples of such fluids include gases, liquids, and solid particulates (such as sand or silicon) carried by another gas or liquid. Specific examples of impingement fluids are water, water or other types of vapor, acidic liquids for acid catalyzable coating materials, basic liquids for base catalyzable coating materials, carbon particles, dry pigment particles (such as TiO, , CaC0 3 ) , air, oxygen, nitrogen gas or gases that may participate in catalyzing or reacting with the coating liquid. Any of the coating liquids can also be used as impingement fluids, including liquid solutions or suspensions of starch, PVA, bacterial cellulose or latex. The fluid need not be heated, and may be any temperature between, for example, 25°C - 100°C (7°F - 212°F) , or ambient temperatures between 25°C - 40°C (77°F - 104°F), or even lower. The impingement fluid and liquid should preferably be co-flowing, and the velocity of the liquid is less than the velocity of the impingement fluid. Very good attenuation has been observed when the mass ratio of an impingement gas to coating material is in the range of from 0.03:1 to 7.7:1 and most preferably in the range 0.2:1 to 5:1. The relative velocities and flow rates of the impingement fluid and coating material can be varied over a wide range to achieve a desired mass ratio of impingement fluid to coating material that attenuates the liquid into droplets of a sufficiently small size to deposit a thorough or a uniformly thorough coating on the substrate. The examples in Table I and II provide guidance about varying these parameters to deposit a coating having minimal graininess or streakiness. Some applications do not require uniform coatings, and these parameters need not be followed. Minimal graininess is optimally illustrated by the images and grey intensity profile graphs of FIGS. 48 and 49. The liquid array has opposing faces, and the impingement fluid can be impinged against one or both of the faces of the array to attenuate the array into small droplets. The desired velocity of the impingement fluid varies depending on the viscosity and flow rate of the liquid. For many applications, however, the fluid is impinged against the liquid at a fluid velocity of 200 - 1600 feet/second (60 - 335 meters/second) . The greatest attenuation of the liquid occurs as the velocity of the fluid approaches sonic speeds (335 meters/second or 1100 feet/second) , and has not been observed to improve significantly beyond these velocities. Theoretically, attenuation continues to increase beyond sonic velocities, but measurement limitations make it difficult to determine changes in droplet diameters at these small dimensions. Although no deterioration in the degree of attenuation has been noted beyond sonic velocities, it may be undesirable in some situations to increase the impingement velocity beyond a sonic range because of the resulting increased flow of fluid that must be collected. Coating Materials One of the advantages of the present method is that it can be used to apply a wide variety of coating materials to a broad variety of substrates. Practically any material can be coated on a substrate using the present method. Even high viscosity liquids, such as thermoplastic material, can be applied in a thin, uniform layer to a substrate by heating the thermoplastic material and attenuating it to a sufficient degree to produce fine droplets that deposit uniformly on a surface to be coated. In other applications, liquids of lower viscosity are coated on the substrate. Materials such as starch (ethylated and other types of starch) , polyvinyl alcohol (PVA) , pigmented coatings, carboxymethylcellulose (CMC) , water, cellulose suspensions, latex and PMDI are applied to substrates such as paper and container board. The viscosity of these enumerated liquids is typically less than 2000 cP (2 Pa-s) at ambient temperature, more usually less than about 900 cp (0.9 Pa-ε) , and sometimes less than 50 or 100 cP (0.05 - 0.1 Pa-s) at ambient temperature. The coating process is facilitated by providing material which is a liquid at ambient temperature, thereby removing the need for heating the material to lower its viscosity and permit its extrusion from an applicator. Examples of coating materials include ethylated corn starch, such as that available from Cargill, Inc. of Cedar Rapids, Iowa; Penford Gum starches, such as PG200, 220, 230, 240, 250, 260, 270, 280, 290, 295, 300, 330, 360, or 380 available from Penford Products Co. of Cedar Rapids, Iowa; Airvol polyvinyl alcohol from Air Products and Chemicals, Inc. of Allentown, Pennsylvania; and clay pigments such as those that can be obtained from Englehard Corporation of Edison, New Jersey under the names Exsilon, Ultra Gloss 90, Ultra White 90, Lustra, Ultra Cote, HT, Gordon and S-23. Substrates The present method is versatile enough to direct an array of attenuated liquid at one or both faces of a substrate moving in many different planes. FIG. 33, for example, shows a paper web 750 moving in a horizontal plane below a head 752. An attenuated liquid array 754 is directed downwardly at web 750 to deposit a coating 756 on its surface. Simultaneously, a second head 758 is positioned below the substrate pointing upwardly such that an attenuated liquid array 760 is directed upwardly at the substrate and deposits a coating 762 on the undersurface of the paper web 750. An alternative embodiment is shown in FIG. 34 in which the paper web 766 is moving in a vertical plane in the direction of arrow 767 between a pair of heads 768 770 that are spraying each side surface of the web. The heads are positioned to spray the attenuated liquid in a generally horizontal direction on the vertically moving substrate. Although the substrate 766 shown in FIGS. 33 and 34 is a paper web, the method of the present invention is suitable for coating many types of substrates, including cellulosic, fiber, organic and synthetic substrates. Examples of cellulosic substrates include finished paper, pulp mats, liner boards, newsprint and already coated papers. Organic substrates can include foods being coated with additives or spices, or plants being coated with insecticide. Other examples of substrates include formed non-cellulosic fiber mats, rubber, cloth, wood, leather and plastic. The substrate can even be metallic, and need not be planar, for example, a transfer roller that in turn transfers the liquid to a substrate. The angle at which the head directs the liquid array toward the substrate is preferably a normal angle. Better coverage with enhanced uniformity of deposition is observed when the liquid is directed at a right angle to a flat surface being coated. Other angles are possible, especially when coating objects with irregular, non-planar surfaces. Another aspect of the invention is that more than one head can be placed sequentially along the substrate, such that layers of coating are applied one on top of the other on a single face of the substrate. A similar plurality of heads can be placed in coating relationship to another surface of the substrate such that multiple layers are applied to both surfaces. A paper web, for example, can have multiple coatings applied to each of its flat faces. The distance between the substrate and head can vary widely, but very thorough and uniform deposition occurs with the liquid emerging from the applicator head at a distance of 1 - 12 inches (2.5 cm - 30 cm) from the surface of the substrate, more preferably 1 - 3 inches (2.5 cm - 7.5 cm). When a uniform coating is desired, the head should preferably be at least far enough away from the substrate to permit the liquid to break substantially entirely into droplets. This distance will vary depending on such variables as the viscosity of the liquid and the flow rate and velocities of the liquid and impingement streams. It is possible to ascertain whether the liquid has been broken sufficiently into droplets by determining the thoroughness and uniformity of deposition on the substrate, as discussed in connection with FIGS. 48 - 69 below. Several hundred examples of the process are also provided in Tables I and II below to illustrate the effects of these and other variables on coating quality. Liquid Attenuation The process of the present invention uses a fluid stream, such as a curtain of air, to attenuate co-flowing liquid to a diameter or width that is smaller than an orifice from which the liquid emerged. An example of this process is shown in FIGS. 35 - 37, which is a sequential series of photographs of a bacterial cellulose suspension emerging from a multiple orifice head, such as the one shown in FIGS. 3 - 4. In FIG. 35 the liquid is emerging from a row of linearly aligned circular orifices having a diameter of 20 mils (0.020 inches or 500 μm) . The liquid emerges from the orifices to form a linear array that in this example is a series of downwardly directed co-planar columns of liquid having an initial diameter essentially the same as the orifice (20 mils) . Air emerges from a pair of parallel slots or air gaps adjacent the array. The slots are parallel to the plane of the array and direct a curtain of air at an acute angle toward the array. As a stream of air, or another gas or fluid passes through gaps adjacent the orifices, they begin to attenuate the fluid stream as shown in FIG. 35. As the gas, moving at a greater velocity than the liquid, impinges against the liquid, it causes an oscillation in the width or diameter of each columnar stream of the array. As the air velocity increases, the liquid stream eventually starts to form loops oriented in several planes, as shown in FIG. 36. The diameters of the loops become increasingly smaller as the velocity of the impingement gas and the distance from the orifice increases until the loops break into droplets of various sizes that are smaller than the orifice from which the liquid stream originally emerged (FIG. 37) . The air co- flowing impingement gas stream directs the droplets downwardly toward the substrate and also creates a cross- flowing turbulence in the region below the head outlet that results in a more uniform deposition of the droplets onto the substrate. The impingement stream may also be used to help clean the applicator head or alter the liquid flow. The impingement air stream may, for example, be humidified to solubilize water soluble materials that coat the interior of the head and build up around the air gaps or liquid orifice. The gas may be humidified to 70% - 100% relative humidity, or more preferably 90% - 100%. Alternatively, the gas may include an additive that modifies the liquid. Humidified air, for example, provides moisture that catalyzes the polymerization of PMDI during coating. When using a catalyst, such as moist air, it is preferable first to impinge a dry impingement fluid against the liquid to prevent initiation of polymerization at or near the outlet. In such a situation a pair of parallel impingement slots are provided adjacent the liquid outlet such that one slot is closer than the other to the liquid outlet. The slot closer to the outlet impinges dry gas against the liquid to initiate attenuation, while the second slot impinges the catalyzing fluid to initiate catalyzation at a distance from the outlet. In alternative embodiments, moisture may be harmful to the coating liquid, in which case the impingement gas is used to purge moist air from the applicator head. Purging is achieved by introducing a dry gas, such as nitrogen gas, through the applicator and outlets. The process of liquid attenuation and mist deposition on the substrate will be understood better by reference to the following examples. EXAMPLE I The trials of this example were designed to study the attenuation of the liquid array into droplets, and illustrate the effects of different process parameters on attenuation and deposition of liquids. During these trials the liquid flow pattern was recorded with a high¬ speed video system using an image intensifier camera from Visual Data Systems of Chicago. The intensifier allowed images to be obtained with a 10 μ second exposure time, effectively freezing the motion of the liquid for each video frame record. The framing rate for these trials was typically 1000 frames per second. Each video session corresponded to a particular set of operating conditions. The operating conditions consisted of: the liquid type (water, 6% CMC solution, or 10% starch solution) , the air slot gap (5, 15, or 23 mils) (125, 375 or 585 μm) , the head air plenum pressure, and the head liquid plenum pressure. Previous and subsequent calibration of the air and liquid flows was used to calculate the air and liquid flow rate for each operating condition. - 46/1 - 47 - - 47/1 - - 48 - Table I lists the operating conditions for each video session. Except for the geometry and raw pressure data, all but the attenuation value is calculated based on the flow calibrations. Attenuation is the term used to describe the decrease in liquid stream diameter as it is accelerated by the surrounding high-velocity air. No direct measure of attenuation was taken during these trials, though estimates could be made from some of the video pictures. The number listed under "Attenuation" is a very approximate value based on a Conservation of Energy technique proposed by Professor R.L. Shambaugh of the University of Oklahoma in "A Macroscopic View of the Melt- blowing Process for Producing Microfibers" in Meltblown Technology Today (Miller Freeman Publications, San Francisco, California 1989) . This method is not highly accurate with respect to precise droplet size, but seems useful in identifying operating conditions conducive to good attenuation. The lower the percentage, the greater the degree of attenuation. The physical process of attenuation and the relative effect of the operating conditions is shown in the still photographs of video images (FIGS. 38 - 43) . Figure 38 shows a split-screen image of a CMC run (Session 56 in Table I) . The image is horizontal because the camera has been rotated onto its side; the actual spray direction was downward. The screen image is split, showing the liquid streams emerging from four holes at sequential times. The bottom half of the screen is an image taken 1/2000 of a second after the image on the top half of the screen. This can be seen by the displacement of the "loop" in the top stream between the image on the top half of the screen and the image on the bottom half of the screen. In FIG. 38, the liquid and air flow from left to right. The width of the liquid stream at the left-most position is a good indication of the hole size (0.020 mils or 500 μm) . The hole-to-hole spacing is 1/12 of an inch (2.1 mm). The streams are considerably thinner as they - 49 - pass out of the image on the right-hand side of the screen. They are approximately half the size of the original stream width, which represents an attenuation of 50%. FIG. 39 shows an image for Session 57 (Table I) . This image was taken under the same operating conditions as Session 56 but with the camera moved down one inch (2.5 cm) from the head. The process of attenuation of the streams continues as the surrounding high-speed air accelerates the liquid streams and causes them to "loop" as they progress. In the middle of the screen, the streams appear to be approximately 1/5 the original size of the streams as they exited from the orifices in FIG. 38. In the dark region on the right of FIG. 39, larger liquid bundles are seen. After the liquid streams break, they may "snap back" on themselves to form larger elements than the final attenuated streams. This can also be seen by the single large drop near the center of the bottom image. FIG. 40 is a split image of Session 58 under the same conditions as Session 56 but with the camera moved down from the head by 2 . inches (6 cm) . Now the original streams have largely broken into elongated ligaments and droplets. Eventually, substantially all the liquid is in the form of droplets as the distance from the head increases. The distance at which the liquid is substantially all in the form of droplets varies considerably with the viscosity of the liquid, and the flow rates and velocities of the liquid and gas. But the distance is easily ascertainable by examining the uniformity of deposition of liquid on the substrate or by high speed photographs of the type shown in this Example. The amount of liquid in the image of FIG. 40 appears to be less than in the image of Session 56, right up next to the head. The amount of liquid at this distance from the head is actually the same, but appears less because the resolution of the camera does not record - 50 - smaller droplets. Also, the liquid spray broadens out of the field of focus of the camera as it moves away from the orifices and is not recorded on the videotape. An approximate estimate of the ultimate liquid attenuation, based on the Conservation of Energy calculation for Session 58 is 19%. This means that the final droplet size is approximately 1/5 the original liquid hole size, or about 100 μm for the conditions of Sessions 56 - 58. For the above sessions 56 - 58, the air velocity was 200 feet/second (67 m/s), the liquid velocity was 0.38 feet/second (0.13 m/s), and the air-to-liquid ratio was 2.5:1. Typical values for meltblown head processes are 490 feet/second (150 m/s) air velocity, 0.01 m/s (0.033 feet/second) liquid velocity, and 25 to 125 air-to-liquid ratio. FIG. 41 shows an image of a CMC spray taken at a low liquid flow rate in Session 60. The attenuation shown here is one of discontinuous ligament formation in which a stream of liquid is rapidly accelerated away from the head by an air flow. This liquid stream becomes progressively thinner until it breaks. Small attenuated droplets are formed from the ligaments for deposition on the substrate. FIG. 42 shows a split image for starch video Session 66. Compressed into a single image is the acceleration of a fluid lump producing "loopy" ligaments that snap back to produce a size distribution of droplets. The largest droplet is approximately the size of the original liquid hole, while the finest are only blurry specs on the image. Note from the table that this is a relatively unfavorable operating condition with a high liquid velocity of 1.35 m/s (4.4 feet/second) , a low air- to-liquid ratio of 0.57, and a calculated attenuation of 57% that is less desireable for thin uniform coatings. FIG. 43 is for Session 42 using water. This figure shows the breakup mechanism for water when the air stream has too little energy to effect attenuation. The air-to-liquid ratio is low at 0.37 and attenuation is calculated at only 73%. None of the usual attenuation - 51 - mechanism is apparent in this figure and the liquid streams simply travel out (down) from the head at nearly uniform width. At a certain distance, the width begins to oscillate. Eventually, a neck is formed which breaks and produces mostly uniform droplets approximately 50% larger than the holes, along with a small fraction of satellite droplets approximately 1/5 the size of the hole. This is classic liquid jet breakup. This is an example in which substantially all the droplets have not been attenuated, i.e., most of the droplets have a greater diameter than the width of the opening through which the liquid emerged to form the array. In contrast, the present invention envisions attenuating substantially all of the droplets, or at least more of the droplets than not, to a diameter less than the width of the opening from which the liquid emerged. EXAMPLE II The trials reported in this Example were carried out with four different head configurations and examine operating parameters in addition to those already discussed in Example I. Two configurations were based on a multiple orifice head design, such as that shown in FIGS. 3 - 8, in which a plurality of linearly aligned orifices produce an array of regular columns of coating liquid. Two additional configurations were based on a slot head design, such as shown in FIG. 14. The basic features of this design, and the specific Examples disclosed in Table II, are a slow moving liquid stream located between two fast, co-flowing gas (air) streams. The fast moving air stream draws the liquid stream down to a smaller dimension than its initial characteristic dimension near 500 μm. The liquid issues from either a slot or a series of closely spaced holes arranged in a straight line. The air issues from two gaps located on either side of and immediately adjacent the liquid slit or line of holes. The typical air gap dimension (the width of the gap through which the air emerges) is about 250 μm (0.010 inches) . - 52 - The main head configuration used in these trials was one with 0.024 inch equivalent diameter holes (24 mils or 610 μm) spaced 18 per inch (i.e. center-to-center spacing of 0.056 inches which is 56 mils or 1.4 mm) and a total length of 4 inches (10 cm) . The air gap for this head was varied from 0.005 to 0.015 inches (5 mils to 15 mils or 125 μm to 375 μm) . The second configuration used a similar but longer head. This second head was 12 inches (30 cm) long with 0.020 inch equivalent diameter holes (20 mils or 0.5 mm) spaced 787 per meter or 20 per inch. For the purposes of identification below these heads will be referred to as the 4-inch MOH (multiple orifice head) and the 12-inch MOH, and are described as "H" type (i.e., "hole" type) heads. Another set of heads substituted a single slot for the plurality of holes such that the liquid emerged from the head as a continuous curtain array. This type of head is referred to as an "S" type (i.e., "slot" type) head. The seven main parameters that specify the operating conditions for this series of runs are set forth in Table II. These parameters are: 1) the liquid velocity; 2) the air velocity; 3) the air gap (i.e. the air quantity) ; 4) the head-to-paper separation distance; 5) the head orientation with respect to the direction of paper sheet travel; 6) the coating formulation; and 7) the air plate setback. Other parameters such as air and liquid temperature or air humidity can also affect optimum head performance, but were not evaluated in this set of trials. - 53 - - 53/1 - Head Air (H)olaHα S Uq. Haatar Aft- Λppaannca-O Good. S Bad) Press. Gap or Slza Anglo Tamp. Tamp. AH. Uq. % Viscosity 1-Uβch. Rof. No. h H20 mils (S)lot mils dβg *f *F % Solids Brookii β ld Streaky Wormy Grainy Streak 1 5 H 20 0 60 1.14 . 2 5 H 20 0 60 1.14 3 5 H 20 0 60 1.14 4 5 H 20 0 60 1.14 5 5 H 20 0 60 1.14 6 5 H 20 0 60 1.14 7 S H 20 0 60 1.14 8 5 H 20 0 60 1.14 9 5 H 20 0 60 1.14 10 S H 20 0 60 1.14 11 5 H 20 0 60 1.14 12 5 H 20 0 60 1.14 13 5 H 20 0 60 1.14 1 5 H 20 0 60 1.14 15 5 H 20 0 60 1.14 1 8 5 H 20 0 60 1.14 17 S H 20 0 60 1.14 18 5 H 20 0 60 1.14 19 S H 20 0 60 1.14 20 S H 20 0 61 1.14 21 S H 20 0 61 1.14 22 5 H 20 0 61 1.14 23 5 H 20 0 61 1.14 24 5 H 20 0 61 1.14 25 5 H 20 0 81 1.14 28 S H 20 0 61 1.14 27 S H 20 0 81 1.14 20 S H 20 0 61 1.14 29 5 H 20 0 61 1.14 30 5 H 20 0 61 1.14 31 5 H 20 0 61 1.14 32 5 H 20 0 61 1.14 33 5 H 20 0 61 1.14 34 S H 20 0 61 1:14 35 5 H 20 0 61 1.14 36 10 H 20 0 60 1.14 37 10 H 20 0 60 1.14 38 10 H 20 0 60 1.14 39 10 H 20 0 60 1.14 40 10 H 20 0 60 1.14 41 10 H 20 0 60 1.14 42 10 H 20 0 60 1.14 43 10 H 20 0 60 1.14 44 10 H 20 0 60 1.14 45 10 H 20 0 60 1.14 46 10 H 20 0 60 1.14 47 10 H 20 0 60 1.14 41 10 H 20 0 60 1.14 40 10 H 20 0 60 1.14 50 10 H 20 0 60 1.14 Si 10 H 20 0 60 1.14 52 10 H 20 0 60 1.14 53 10 H 20 0 60 1.14 5 10 H 20 0 60 1.14 55 10 H 20 0 61 1.14 - 54 Appearance (1 Good. 5 Bad) Head Air (H)ole H o S Llq. Heater Air Uq. % Viscosity i-Meeft. Press. Gep or Size AngteTemp. Temp. RH Solids Broαklleld Streaky Wormy Grainy Streak F RβL tta. *n H20 mils (S)lot m ilt deg " 61 1.14 56 10 H 61 1.14 57 H H 61 1.14 51 61 1.14 59 H 61 1.14 60 H H 61 1.14 61 H 41 1.14 62 48 1.14 63 H 48 1.14 64 H 48 1.14 65 H H 48 1.14 66 48 1.14 67 H 20 30 48 1.14 66 H 20 0 48 1.14 69 H 20 0 48 1.14 70 H 20 0 48 1.44 71 H 20 0 60 1.08 72 H 20 0 0 60 1.01 H 20 73 60 1.01 74 H 20 0 60 1.01 75 H 20 0 60 1.01 76 H 20 0 60 1.01 77 H 20 0 60 1.0> 78 H 20 0 60 1.01 79 H 20 0 B0 1.01 80 H 20 0 BO 1.08 81 H 20 0 60 1.08 •2 H 20 0 60 1.08 83 H 20 0 58 1.12 84 H 20 0 58 1.12 85 H 20 0 58 1.12 86 H 20 0 58 1.12 87 H 20 0 58 1.12 81 H 20 0 58 1.12 89 H 20 0 SB 1.12 90 H 20 0 0 58 1.12 91 H 20 0 59 1.12 92 H 20 0 59 1.12 93 H 20 59 1.12 94 H 20 0 95 H 20 0 59 1.12 96 H 30 0 60 1.14 0 60 1.14 97 H 30 91 H 30 0 80 1.14 99 H 30 0 80 1.14 100 H 30 0 60 1.14 0 60 1.14 101 H 30 102 H 30 0 60 1.14 103 H 30 0 60 1.14 104 H 30 0 60 1.14 14 105 H 30 0 60 1. 106 H 30 0 60 1.14 1.14 107 H 30 0 60 101 H 30 0 60 1.14 14 109 H 30 0 60 '.. 110 H 30 0 60 1.14 14 111 H 30 0 ' SO 1. .4 112 H 30 0 60 1. 113 i s 6 0 0.41 91/03830 - 55 - SUBSTITUTE SHEET Appearance (1 Good. S Bad) Viscosity -Mech. Brooklleld Streaky Wormy Grainy Streak 2 3 3 3 3 3 3 1 2 2 3 1 2 3 3 2 2 3 2 2 1 2 1 2 1 3 2 2 4 1 1 1 3 3 4 3 2 2 3 2 3 2 2 1 1 1 1 1 1 1 2 2 2 2 2 2 - 56 - SUBSTITUTE SHEET - 56/1 - SUBSTITUTE SHEET - 57 - 91/03830 1-Uech. SUBSTITUTE SHEET - 58 - The liquid velocity refers to the velocity of the liquid immediately before it exits from the holes or slit and comes in contact with the air stream or streams. This velocity is typically somewhat less than 3 ft/s (l m/s) . The air velocity is the velocity of the air as it exits the air gaps immediately prior to the zone of initial air/liquid impingement. The air velocity ranges from 200 ft/s to 1100 ft/s (61 m/s to 335 m/s or Mach number of 0.2 to 1.0), where 1100 feet/second is sonic velocity. The air gap is the dimension of the slit formed between the air plate and the main body of the head which contains the liquid passages and orifices. Typically the slit width of the air gap is between 5 mils and 20 mils (125 μm - 500 μm) , and extends about 0.5 inches beyond the line of liquid orifices on both ends. The head-to-paper separation distance was typically between 1 inch and 10 inches (2.5 cm to 25 cm) . The orientation of the head is defined by the angle between the plane in which liquid flows out of the line of liquid orifices in the head, and the plane of travel of the paper being coated. Typically the head is oriented such that the liquid is normal to the plane of paper travel. Some tests were conducted with the head rotated such that the plane of the liquid array was about 45° to the plane of travel of the paper. The coating formulation can vary widely in concentration, temperature, constituents, and batches. Typical formulations used with the MOH have been CeUulon with CMC at 0.5% to 1.5% concentration, starch (PG290) at 10% concentration and 120°F, and PMDI at 100%. Several other constituents and several variations in concentration and batch have also been tried. The air plate setback is the distance between the end of the air plate and the end of the liquid orifices. Typically the air plate sets back from the liquid orifice tip about 10 to 15 mils (0.010 to 0.015 inches; 250 μm to 380 μm) . Air plate setback values are not shown in Table II. - 59 - Only the liquid and air velocities, the air gap and the head to paper separation distance were tested during the trials reported in Table II. The coating formulation for runs 1 - 117 was a 0.8% CeUulon/0.2% CMC mixture with 1100 ppm sorbic acid in water. This material was homogenized in the Gaulin Homogenizer (from APV Gaulin, Inc. of Hilversum, Holland) for three passes through the cell disruptor (CD) valve, followed by one pass through a 150 μm filter and one pass through a 125 μm filter. The head orientation was normal to the direction of travel of the paper and the air plate setback was constant at 15 mils (0.015 inches or 380 μm) . The liquid velocity for these trials was selected based on coating application rate. Two levels of application were used, 3 Ibm/ton/side and 5 Ibm/ton/side. These coverages correspond approximately to 0.11 g/irr/side and 0.19 g/m 2 /side for a 50 lbm/3300 sq.ft. sheet. Because of the differences in liquid hole size and number per inch this resulted in differences in actual liquid velocity for the two heads at the same level of coverage. Both air flow rate and air pressure were measured during the runs so air velocity could be calculated. Air velocity is specified in terms of air pressure because air velocity and air pressure are directly related. The nominal or equivalent air pressure was varied from 5 psig to 30 psig in 5 psig increments. In addition, the air gap for various runs was set at one of three values: 5, 10, or 15 mils (125, 250 or 380 μm) . Most of the trials were conducted with a head- to-paper separation distance of 3 inches, but a few runs were conducted with 1 inch and 10 inch distances. - 60 - The range of variables for these trials is expressed in Table III. TABLE III The fluid preparation and handling system consisted of a conical storage tub, a Moyno pump, a wire mesh filter, a spray collection tub, and a return pump. All tubing and fittings between the filter and the head were of food-grade quality to ensure freedom from orifice pluggage. Liquid flow was adjusted with a hand valve based on a pressure reading of the liquid at the head. Timed discharge rates from the head were also taken at the beginning and end of each set of runs and these were the basis for determining the correct pressure reading during the runs. To simulate the motion of the paper sheet on a paper machine a sled system was constructed to move single sheets of paper under the head at high speed. The sled consisted of a frame and a set of rails along which a pair of runners traveled. A platen to hold the paper sheet was attached to the runners, and bungee cords were used to propel the platen/runner combination along the rails. The head was suspended from a framework above the rails at the location where the platen/sled reached its maximum velocity. High-speed video data was used to determine that this velocity was approximately 1800 ft/min. After - 61 - passing under the head location the platen/runner combination was slowed and stopped with an arresting wire. The paper sheets were removed after one exposure to the coating spray, thus simulating the exposure that would be obtained on a paper machine at a similar speed. The paper samples were allowed to dry without further treatment and were stored in loose bundles. For most of these trials the paper used was a sized printing grade, although some unsized newsprint was also used. Visual comparison of these two types of sheets under the same spray conditions showed no apparent difference. The data from the two types of paper are not differentiated in Table II. Coating Uniformity Defects It is convenient to visualize the "coating" as lying on top of a smooth, flat substrate. In this circumstance the uniformity of the coating thickness is a measure of coating uniformity. Most substrates are not smooth and flat on the scale of nominal coating thickness (-0.5 to 10 μm) . It is therefore more appropriate to measure the quantity of dry coating applied per unit area instead of coating thickness. The selected units for coat weight are grams per square meter (g/m 2 ) . Because a continuous coating is usually sought, the scale over which the coat weight is measured is small, less than 1 mm by 1 mm. The variation in coat weight per unit area on this small scale is a measure of the coating uniformity. Under most conditions, CeUulon and starch coatings applied to papers are transparent. To obtain information about the coating uniformity, a fluorescent dye was added to the CeUulon or CMC coating mixtures used in these trials. Under ultraviolet light this rendered the coating distribution on the paper samples visible. The dye is distributed in the liquid phase of the coating, hence it is actually the uniformity of the distribution of the liquid phase that is visible. Although this distribution may not correspond exactly to the - 62 - distribution of the coating solids, it is an adequate approximation for these studies. The images and graphs in FIGS. 48 - 69 were generated from test sheets obtained during the starch runs. A starch run consisted of spraying starch under prescribed conditions onto a sheet of paper attached to a sled which moved under the head. The coated sheet was allowed to air dry. The sheet designation (e.g. S12C1) corresponds to that used in Table II where the operating conditions are presented. Starch is a clear coating so either a fluorescent dye or a staining agent must be used to make the coating visible. In the case of FIGS. 48 - 69 an iodine stain was used to produce a dark brown color wherever starch was applied. The stain is darker where there is more starch, so the intensity of the color can be used to judge the coat weight uniformity of the starch. To obtain a quantitative measure of the coat weight uniformity, the color intensity of the test sheets was digitized using a color scanner. This device measures the darkness or intensity at each location in the stained area of the test sheet using very small sample areas. The size of the sample areas is specified in terms of the number of dots or pixels per inch. In this case, 75 dots per inch (dpi) was used, resulting in a sampled area size of about 0.33 mm square. For a typical test sheet, the stained area was about 100 mm x 100 mm, so a total of about 90,000 intensity samples were taken per test sheet. The intensity range was broken into 256 levels of grey with a value of 0 (zero) corresponding to black and a value of 255 corresponding to white. All other levels of grey are in between these two extremes. The images shown in FIGS. 48 - 51 are printouts of the scanned test sheets usi g a Macintosh computer and a LaserWriter printer. The graphs of FIGS. 48 - 69 were produced using one of many available image analysis programs for grey- intensity images. The program was a public domain program called "Image 1.22y" that resulted from work carried out for the National Institutes of Health. Two types of - 63 - graphs are shown, both of which are grey intensity profile graphs. The bottom graph is a line profile, which represents the variation in the grey intensity along a line drawn on the image. All the lines use here were drawn near the mid-point of the stained area in the cross direction, i.e. the line is drawn perpendicular to the direction of motion of the test sheet when it was coated. The top graph is also a grey intensity profile, but represents the "column average" values for grey intensity. For this plot, the average of the grey intensities along a column of sampled areas in the machine direction was taken. The variation of these values in the cross direction was then plotted. This type of graph eliminates some of the point-to-point variations, but shows any streakiness in the coat weight variations. For the MOH coating applicator there are two principal coating uniformity defects: graininess and streakiness. Graininess is a non-uniformity on the scale of approximately 1 mm. With poor attenuation the individual droplets from the MOH head are relatively large, approximately the dimensions of the orifices, or 500 μm. When these droplets strike the paper they spread out to form circular or elliptical spots of coating surrounded largely by uncoated paper. The result is a localized non-uniformity. An example of a sample with a very grainy coating is shown in Figure 44. The MOH coating applicator may also produce non- uniformities on a larger scale that are generally aligned with the direction of paper travel. The whole paper sample is coated, but the coating is noticeably thinner in some areas than in others. The thin areas typically are approximately 1 cm wide and may be continuous in length, though streaks of 3 or 4 inches are more common. An example of a sample with severe streakiness is shown in Figure 45. As a reference for the streak duration, a three inch streak in a paper sample traveling 1800 ft/min (471 m/min) corresponds to non-uniformity for 83 milliseconds. - 64 - Data from the MOH coating uniformity trials are presented with respect to three variables: 1) effect of coating application rate and air pressure 2) effect of the air gap width 3) effect of the head-to-paper separation distance FIG. 46 is a set of twelve photographs of samples for both 3 and 5 Ibm/ton/side application rates (approximately 0.11 to 0.19 g/m 2 ) and for nominally 5 and 25 psig (35 kPa - 170 kPa) air pressures at an air gap of 10 mils (250 μm) using the 4-inch MOH head with 3 inch (7.5 cm) head-to-paper separation. These photographs compare coating uniformity for the given range of application rate (3 to 5 Ibm/ton/side) and air pressure (5 - 30 psi) . From the comparison in FIG. 46 it appears that the coating application rate of the Cellulon/CMC affects the density of the image but not the general character of the coating uniformity either in terms of graininess or streakiness. Increased air pressure significantly reduces graininess and somewhat reduces streakiness for this set of trials with this given coating material. Shown in FIG. 47 is a set of photographs of samples for 5 mil and 15 mil (125 μm and 375 μm) air gaps at 5 Ibm/ton/side application rate and for 5 to 30 psig air pressure. From the comparison in FIG. 47 it appears that the air gap width at constant pressure only modestly affects coating uniformity except at the lowest air pressure. Increased air pressure significantly reduces graininess and somewhat reduces streakiness. Photographs of samples for head-to-paper separation distances of 1-j, 3, and 10 inches (3.75, 7.5 and 25 cm) for the 12-inch MOH head with a 10 mil (250 μm) air gap at 3 Ibm/ton/side and for nominally 7.5 psig (110 kPa) air pressure were also taken, but are not shown. From the comparison in those trials it appears that increased head-to-paper separation distances increase the streakiness of the Cellulon/CMC coverage. Streakiness is - 65 - not very pronounced at 1 . and 3 inches (3.75 and 7.5 cm) head-to-paper separation. At a 10 inch (25 cm) separation the coverage is grainier and large scale non-uniformities become apparent. Initial spray trials with 1% Cellulon/CMC coating show that graininess can be most effectively eliminated with increased air pressure (air velocity) . Streakiness is also reduced by increased air pressure, but not as significantly or as consistently. Under the best conditions, the visual data indicate that Cellulon/CMC can be coated on a paper sheet with good uniformity of coverage, at least up to 1800 ft/min paper speed. The runs 118 - 260, and other tests and observations, suggest that liquid pressure in the head affects the uniformity of flow at various locations along the slit or series of orifices. It therefore may affect streakiness. Based on these data and observations, the liquid passage length is selected which results in a relatively larger pressure drop along the flow path, thus providing uniformity of liquid flow from one end to another and avoiding streakiness in particular applications where streaks are not desired. Head pressures above 20 kPa (3 psi) appear to be adequate for many coating materials tested so far. Different materials have been found to have less graininess at different preferred air flows. In practice, each new material or liquid flow condition is started with relatively low air pressure, approximately 27 kPa (4 psi) . The coating pattern is observed for graininess and the air pressure is increased until graininess is eliminated, if graininess is not desired. So far, air pressures less than 100 kPa (15 psi) have been sufficient to reduce graininess. Desireable attributes for some applications are illustrated by the photographs and graphs of FIGS. 48 and 49. The single line grey intensity profile (bottom graph) in FIG. 48 is always below 200, demonstrating no discontinuities in the coating made with a conventional - 66 - gate roll. A comparable single line density graph in FIG. 49 is similarly always below 200 and has no coating discontinuities. Of note is the lesser amplitude of variation of the single line density in FIG. 49, illustrating that the coating is even more uniform than with the prior art gate roll. A low amplitude of variation of the single line density graph corresponds visually to a low level of graininess. A high amplitude (as in FIG. 52) reflects excessive graininess. Variation from baseline of the column density graph is associated with streakiness of the coating. FIG. 54, for example, shows an undulating column density line that is reflected in the high streaky score (3) in Table II. Displacement of the graph away from 200 toward 80 reflects the amount of coating on the substrate. Hence process parameters may be assessed and selected for a wide variety of materials by determining their column average and single line densities. In a general sense, higher liquid flows produce less wormy coatings, higher air flows produce a less grainy distribution, and higher coating liquid pressure produces less streaky coatings. EXAMPLE III There is an optimum (but not required) relationship among the diameters of the orifices in the MOH, the viscosity of a coating fluid, and several other operating parameters. These relationships are illustrated by the equation for pressure drop along the liquid flow orifices: 32 μ v 1 d 2 where: P = pressure drop, Pa μ = apparent liquid viscosity, Pa-s v = liquid velocity, m/s 1 = orifice length, m - 67 - d = orifice effective diameter, m The apparent viscosity, μ, can be related to the Brookfield viscosity at 100 RPM, μ__ by the expression: μ = μ jj 20 s with: 8 v where: μ--, = Brookfield viscosity at 100 RPM, Pa-s = shear rate, 1/s s = exponent, unitless For orifices which are not circular the effective value of the diameter is : where : = orifice flow area , m ~ - 69 - TABLE IV Film Thickness (One Side Only) COVERAGE WET FILM 36% SOLIDS DRY FILM 2 PVA 0.05 grams/meter 0.83 microns .050 microns SG 1.023 10% solids 0.10 grams/meter 1.66 microns .010 microns WET FILM 3 10% SOLIDS DRY FILM PG 290 Starch 0.25 grams/meter^ . 2.4 microns .024 microns SG 1.05 a 10% solids 0.40 grams/meter 3.8 microns .038 microns The thin films of the present invention usually result in coverages of 1 g/m 2 or less, such as 0.40 g/m 2 , preferably 0.25 g/m 2 , most preferably .05 to 0.25 g/m 2 . Much thicker coatings can also be applied, and substrates can also be soaked by applying an excess of coating material. EXAMPLE V The present invention can be used to apply bacterial cellulose (cellulose produced by bacteria) to paper webs. A suitable bacterial cellulose is disclosed in United States Patent No. 4,861,427, which is incorporated herein by reference. This bacterial cellulose is available commercially as CeUulon. It would be possible to apply the bacterial cellulose at web speeds of 2000 feet/minute or more. The bacterial cellulose could be applied at concentrations in the range of 0.5% to 2.0%. A preferred range is 0.5% to 1.3%. Mixtures of bacterial cellulose and CMC in a weight ratio in a range of 2:1 to 10:1 bacterial cellulose to CMC, having a solids concentration in the range of 0.25% to 2.0%, could be applied to the substrate. A preferred concentration would be in the range of 0.25% to 1.3%. All concentrations are on a weight basis. - 68 - Some examples of typical values for flow conditions and geometry are: Some typical values for the fluid parameters are: The target value for pressure drop for many applications is between 13,000 Pa and 250,000 Pa depending on head size and expected flow range. Sharp edges, such as edges 95, 97, 328, 312, 354, or 364 may have a radius of .002 inch or less. Sharp edges can diminish build-up of coating material at or around the outlet for the coating material. EXAMPLE IV Several examples of film thickness are calculated below to illustrate some very thin coatings that can be achieved with the present invention. - 70 - EXAMPLE VI The process and apparatus of the present invention can also be used to enhance the strength of corrugated board packaging materials. This strength enhancement is achieved by applying relatively low amounts of selected isocyanate compounds to the corrugated packaging board. One suitable isocyanate resin compound is polymeric methylene diphenyl diisocyanate (PMDI) . Another is an e ulsifiable polymeric methylene diphenyl diisocyanate (EMDI) . These chemical compounds in liquid form, or in the form of an emulsion in the case of EMDI, may be sprayed onto a fluted container board medium (over a selected width) thereby coating all surfaces of the fluted medium, or it may be applied by a flute tip roll coater only in the tips. Using the present application to spray 5% on a weight basis of the medium of either PMDI or EMDI, and allowing a cure time of about 5 days for PMDI, the short column or top-to-bottom stacking strength improvement of the container will approximate 33%. The EMDI cures more quickly, needing only two days to cure. If the application is 10% by weight of these materials, strength is improved approximately 40%. It is believed that strength enhancement will occur as the isocyanate resin compound is added in an amount within a range of from 0.5% - 50% by weight of the medium. Other suitable chemical compounds that may be utilized to provide a stiffer fluted medium are various acrylics, polyvinyl acetates/alcohols, various latexes, styrene-maleic anhydride, epoxy resins, and others. EXAMPLE VII An optimum definition of coating uniformity is that it produces a column average and single line grey intensity profile graph similar to that shown in FIGS. 48 and 49 (gate roll run and #S12C1) . The aspect of these graphs that most indicates uniformity of coating is the low amplitude variation of the grey intensity columns and lines. The column intensity profile of these graphs - 71 - varies no more than about 10 units of intensity, while the single line intensity varies no more than about 30 - 50 units of intensity. Consistent values below 200 on each graph indicate completeness of coverage; the farther below 200 the line remains, the more likely there will be no discontinuities of coverage on the sheet. The lines in FIGS. 48 and 49 are at about 80 - 150, preferably below 125, and indicate a high probability of thorough coverage across a desired swath of substrate being coated. Having illustrated and described the principles of the invention in many preferred embodiments, it should be apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the following claims.

Claims: - 72 - CLAIMS 1. Apparatus for uniformly depositing a liquid on a substrate, comprising: an applicator; movement means for establishing relative movement between the substrate and applicator; a liquid outlet means in the applicator for directing a flow of an elongated array of a liquid toward the substrate; and a fluid outlet means in the applicator for impinging a fluid upon and attenuating the array into droplets that deposit a uniform coat of liquid on the substrate. 2. The. apparatus of claim 1 wherein the liquid outlet and fluid outlet means are means for depositing a thin, uniform coat of liquid on the substrate. 3. The apparatus of claim 1 further comprising cleaning means for removing particulate matter from the applicator. 4. The apparatus of claim 3 wherein the cleaning means comprises a movable portion of the applicator that covers an internal passage that leads to the liquid outlet. 5. The apparatus of claim 4 wherein the movable portion comprises a hinged portion of the head that pivots open to expose the internal passage. 6. The apparatus of claim 1 wherein the applicator comprises a fixed portion and a swingable portion, the fixed and swingable portions defining therebetween a liquid passageway and the liquid outlet means. 7. The apparatus of claim 3 wherein the cleaning means comprises a movable portion of the applicator covering an internal passageway that leads to the gas outlet. 8. The apparatus of claim 1 wherein the liquid outlet means is circumscribed by sharp edges. - 73 - 9. The apparatus of claim 1 wherein a sharp edge separates the liquid outlet means and fluid outlet means. 10. The apparatus of claim 1 further comprising a supply of a liquid that solidifies at or below room temperature, the supply of liquid communicating with the liquid outlet means. 11. The apparatus of claim 10 further comprising pressure means for directing the flow of liquid under pressure out of the liquid outlet means at 0.3 psi to 25 psi (2 kPa to 170 kPa) . 12. The apparatus of claim 6 wherein the liquid outlet means comprises a liquid outlet and the fluid outlet means comprises a fluid outlet, and the head further comprises a fluid chamber that communicates with the fluid outlet means, and a partition that separates the fluid chamber and liquid passageway, an edge of the partition demarcating the liquid outlet from the fluid outlet. 13. The apparatus of claim 12 wherein the partition edge tapers to a sharp edge toward the liquid passageway. 14. The apparatus of claim 12 wherein the partition is selectively recessible away from the fluid and liquid outlets. 15. The apparatus of claim 14 wherein the swingable portion comprises a wall of the fluid chamber. 16. The apparatus of claim 6 wherein the swingable portion is recessible away from the fluid and liquid outlet means. 17. The apparatus of claim 3 wherein the applicator comprises first and second selectively separable portions that mate to form a central liquid chamber that communicates with the liquid outlet means, and a fluid chamber exterior to the liquid chamber and communicating with the fluid outlet means. 18. The apparatus of claim 17 wherein the fluid outlet means comprises a fluid outlet and the liquid outlet means comprises a liquid outlet, the fluid outlet - 74 - and liquid outlet being separated by a sharp inner edge of the housing. 19. The apparatus of claim 4 wherein the applicator comprises first and second matable portions that meet to define the internal passage, and the cleaning means comprises opening means for selectively separating the portions of the applicator. 20. The apparatus of claim 4 wherein the applicator comprises first and second matable portions that meet to define the internal passage, and the cleaning means comprises a hinge about which at least one portion pivots. 21. The apparatus of claim 20 further comprising a power means for moving at least one portion of the applicator about the hinge. 22. The apparatus of claim 4 wherein the applicator comprises first and second matable portions that meet to define the internal passage, and a retractor that selectively moves at least one portion of the applicator to expose the internal passage for cleaning. 23. The apparatus of claim 3 wherein the cleaning means comprises an external wiper configured to slide into the liquid orifice and wipe it clean. 24. The apparatus of claim 1 further comprising a coating of a material on the surfaces of the head that form the fluid and liquid outlet means, the material comprising a substance that reduces formation of a coating in the head. 25. The apparatus of claim 24 wherein the material is polytetrafluoroethylene. 26. The apparatus of claim 24 wherein the material is amorphous carbon. 27. The apparatus of claim 24 wherein the material is polycrystalline diamond. 28. The apparatus of claim 1 wherein the movement means comprises means for moving the substrate past the liquid array. - 75 - 29. The apparatus of claim 1 wherein the liquid outlet means directs the flow of liquid downwardly against the substrate. 30. The apparatus of claim 1 wherein the liquid outlet means directs the flow of liquid upwardly against the substrate. 31. The apparatus of claim 1 wherein the substrate has opposing faces, and the liquid outlet means directs a flow of liquid at the opposing faces of the substrate. 32. The apparatus of claim 1 wherein the liquid outlet means directs the flow of liquid sidewardly against the substrate. 33. The apparatus of claim 1 wherein the liquid outlet means comprises a plurality of adjacent outlets. 34. The apparatus of claim 33 wherein the adjacent outlets are aligned. 35. The apparatus of claim 1 wherein the fluid outlet means is a slot in the applicator. 36. The apparatus of claim 35 wherein the liquid outlet means is a plurality of aligned openings in the applicator, and the fluid outlet means is an elongated slot extending adjacent the opening. 37. The apparatus of claim 1 wherein the fluid outlet means comprises means for impinging fluid at an angle approaching zero degrees to the direction of flow of the liquid array. 38. The apparatus of claim 1 wherein the fluid outlet means comprises means for impinging fluid at an angle approaching ninety degrees to the direction of flow of the liquid array. 39. The apparatus of claim 1 wherein the fluid outlet means comprises a converging air nozzle. 40. The apparatus of claim 1 wherein the fluid outlet means comprises a diverging air nozzle. 41. The apparatus of claim 1 wherein the liquid outlet means comprises a removable bipartite module - 76 - defining a liquid passageway between the parts of the module. 42. The apparatus of claim 41 comprising a plurality of land areas on opposing faces of the module that maintain patency of the liquid passageway. 43. The apparatus of claim 1 wherein the applicator comprises a plurality of removable adjacent applicator modules. 44. The apparatus of claim 1 further comprising a secondary air outlet that impinges the liquid array. 45. An apparatus for uniformly depositing a liquid on a substrate, comprising: a head comprising first and second portions, at least one of the portions being selectively movable relative to the other portion; a liquid outlet means in the head for directing an elongated array of liquid toward the substrate and forming a thin, uniform coat on the substrate, the liquid outlet means comprising a liquid outlet defined by a sharp edge; a liquid passageway through the head communicating with the liquid outlet, the movable portion of the head covering the liquid passageway and liquid outlet such that movement of the movable portion permits access to the liquid passageway and liquid outlet for cleaning; and a fluid outlet means in the head adjacent the liquid outlet for impinging a fluid upon and attenuating the flow of a liquid into droplets for uniform deposition on the substrate. 46. The apparatus of claim 45 wherein at least one of the portions is a hinged portion mounted on a hinge for movement away from the other portion. 47. The apparatus of claim 46 further comprising a power actuated means for pivoting the hinged portion of the head about the hinge. 48. The apparatus of claim 47 wherein each portion of the head comprises a liquid conveying part and - 77 - a fluid conveying part separated by a common wall, the liquid conveying part of each head having an open face that communicates with the liquid conveying part of the other portion of the head to form the liquid passageway and liquid outlet therebetween, the fluid conveying part of each head defining an enclosed area exterior to the liquid conveying part, the fluid conveying part forming an air gap that defines the fluid outlet. 49. The apparatus of claim 45 wherein the liquid outlet comprises a plurality of nozzles in the head. 50. The apparatus of claim 47 wherein the fluid outlet means comprises an elongated slot positioned adjacent the nozzles. 51. The apparatus of claim 50 wherein the slot comprises an air gap of 9 - 15 mils (125 - 380 μm) . 52. The apparatus of claim 51 wherein the air gap is 5 mils (125 μm) . 53. The apparatus of claim 45 wherein the liquid outlet is 1 - 10 inches (2.5 - 25 cm) away from the substrate. 54. The apparatus of claim 53 wherein the liquid outlet is 1 - 3 (2.5 - 7.5 cm) inches away from the substrate. 55. Apparatus for uniformly depositing a liquid on a substrate, comprising: an applicator; a liquid outlet in the applicator that directs a flow of an elongated array of a liquid toward the substrate as the substrate and array move relative to one another; and a fluid outlet in the applicator that impinges a fluid upon and attenuates the array into droplets that deposit a uniform coat of liquid on the substrate. 55. An apparatus for applying a uniform coating on a substrate, comprising: an applicator that directs a flow of liquid in a linear array toward the substrate; - 78 - a mover that moves the flow and substrate relative to each other; and a fluid impinger that impinges a fluid on the linear array of liquid and attenuates the linear array into droplets that deposit uniformly on the substrate. 56. A process for uniformly depositing a coating of a liquid on a substrate, comprising the steps of: providing a supply of the liquid; directing a flow of an elongated array of liquid from an outlet toward the substrate; moving the flow and substrate relative to each other; and impinging a fluid against the liquid stream to attenuate the liquid flow into droplets that deposit a uniform coating on the substrate. 57. The process of claim 56 wherein the step of directing a liquid flow comprises directing a liquid which is a liquid at ambient temperature. 58. The process of claim 57 wherein the directing step comprises directing an aqueous liquid. 59. The process of claim 56 wherein the directing step comprises directing a liquid at less than 100°C. 60. The process of claim 59 wherein the liquid is at less than 70°C. 61. The process of claim 60 wherein the liquid is at 25°C - 40°C. 62. The process of claim 56 wherein the directing step comprises directing the liquid through the outlet under pressure at less than 25 psi (170 kPa) . 63. The process of claim 62 wherein the directing step comprises directing liquid through the outlet under pressure at 5 - 12 psi (35 kPa - 82 kPa) . 64. The process of claim 62 wherein the directing step comprises directing the liquid through the outlet under pressure at less than 5 psi (35 kPa) . 65. The process of claim 56 wherein the providing step comprises providing an aqueous liquid. - 79 - 66. The process of claim 56 wherein the directing step comprises directing a flow of liquid from the outlet at a liquid velocity of less than about 1 meter/second. 67. The process of claim 56 wherein the impinging step comprises impinging a fluid which is at a fluid temperature less than 100°C. 68. The process of claim 67 wherein the impinging step comprises impinging a fluid at a fluid temperature between 25°C - 100°C. 69. The process of claim 67 wherein the impinging step comprises impinging a fluid at a fluid temperature between 25°C - 40°C. 70. The process of claim 67 wherein the impinging step comprises impinging a fluid that is at ambient temperature. 71. The process of claim 56 wherein the impinging step comprises impinging a fluid at ambient temperature. 72. The process of claim 56 wherein the impinging step comprises impinging a fluid that is a gas. 73. The process of claim 56 wherein the impinging step comprises impinging a fluid that is a liquid. 74. The process of claim 56 wherein the liquid array has opposing faces and the impinging step comprises impinging the fluid against only one face of the liquid array. 75. The process of claim 56 wherein the liquid array has opposing faces and the impinging step comprises impinging the fluid against both opposing faces of the liquid array. 76. The process of claim 56 wherein the impinging step comprises impinging fluid at a fluid velocity of 200-1600 ft./second (60 - 490 m/s). 77. The process of claim 56 wherein the step of providing a liquid comprises providing a liquid which has - 80 - a viscosity of less than 2,000 cP (2 Pa-s) at ambient temperature. 78. The process of claim 77 wherein the providing step further comprises providing a liquid which has a viscosity of less than about 900 cP (0.9 Pa-s) at ambient temperature. 79. The process of claim 78 wherein the providing step comprises providing a liquid which has a viscosity of less than 100 cP (0.1 Pa-s) at ambient temperature. 80. The process of claim 79 wherein the providing step comprises providing a liquid having a viscosity of less than about 50 cP (0.05 Pa-s) at ambient temperature. 81. The process of claim 56 wherein the providing step comprises providing a liquid which is a liquid at ambient temperature. 82. The process of claim 56 wherein the directing step comprises directing the flow of liquid toward a cellulosic substrate. 83. The process of claim 56 wherein the directing step comprises directing the liquid against a substrate having two faces, and the flow of liquid is directed at both faces. 84. The process of claim 56 wherein the directing step comprises directing the flow of liquid downwardly at the substrate. 85. The process of claim 56 wherein the directing step comprises directing the flow of liquid upwardly at the substrate. 86. The process of claim 56 wherein the directing step comprises directing the flow of liquid both upwardly and downwardly at the substrate. 87. The process of claim 56 wherein the substrate is an upright orientation and the step comprises directing the flow of liquid εidewardly at the substrate. - 81 - 88. The process of claim 87 wherein the flow of liquid is directed sidewardly from two opposing directions. 89. The process of claim 56 wherein the directing step comprises directing the liquid at a fiber substrate. 90. The process of claim 89 wherein the directing step comprises directing the liquid at an organic substrate. 91. The process of claim 89 wherein the directing step comprises directing the liquid at a synthetic substrate. 92. The process of claim 89 wherein the directing step comprises directing the liquid at a coated paper substrate. 93. The process of claim 56 wherein the directing step comprises directing the liquid at a metal substrate. 94. The process of claim 56 wherein the directing step comprises directing the liquid at a rubber substrate. 95. The process of claim 89 wherein the directing step comprises directing the liquid at a pulp mat substrate. 96. The process of claim 89 wherein the directing step comprises directing the liquid at a liner board substrate. 97. The process of claim 56 wherein the directing step comprises directing the liquid at a substrate selected from the group consisting of cloth, wood, leather, and plastic. 98. The process of claim 56 wherein the impinging step comprises impinging a fluid stream on the liquid with the fluid stream moving at a greater velocity than the liquid stream. 99. The process of claim 56 wherein the step of impinging the fluid further comprises issuing fluid from a gap adjacent the liquid outlet. - 82 - 10G The process of claim 99 wherein the impinging step comprises issuing a gas from a gap adjacent the liquid outlet wherein the gap has a width of 5 - 20 mils (125 - 500 μm) . 101. The process of claim 56 wherein the directing step comprises directing the flow of liquid toward the substrate from a distance of 1 - 12 inches (2.5 - 30 cm) . 102. The process of claim 56 wherein the directing step comprises directing the liquid at a normal angle toward the substrate. 103. The process of claim 56 wherein the providing step comprises providing a liquid that comprises a suspension of cellulose fibers. 104. The process of claim 103 wherein the providing step comprises providing a liquid that contains carboxymethyl cellulose. 105. The process of claim 104 wherein the carboxymethyl cellulose is 0.5 - 1.5%. 106. The process of claim 56 wherein the providing step comprises providing a starch containing liquid. 107. The process of claim 106 wherein the starch is an ethylated starch. 108. The process of claim 103 wherein the providing step comprises providing bacterial cellulose. 109. The process of claim 108 wherein the directing step comprises directing homogenized bacterial cellulose at the substrate. 110. The process of claim 109 wherein the providing step comprises providing bacterial cellulose homogenized in a cell disrupter. 111. The process of claim 56 wherein the providing step comprises passing the liquid through a filter prior to directing the flow of liquid. 112. The process of claim 56 wherein the impinging step comprises depositing a thin, uniform coating on the substrate. - 83 - 113. The process of claim 112 wherein the thin coating comprises a coating of 0.11 - 0.19 g/m 2 /side on the substrate. 114. The process of claim 56 wherein the directing step comprises providing a plurality of nozzles in communication with the supply of liquid and through which the flow of liquid is directed at the substrate. 115. The process of claim 114 wherein the nozzles are aligned in a row, and the impinging step comprises providing an elongated slot that defines a fluid gap extending adjacent the nozzles. 116. The process of claim 56 further comprising the step of humidifying the gas with a sufficient amount of moisture to reduce accumulation of the liquid at the outlet. 117. The process of claim 116 wherein the humidifying step comprises humidifying the gas to 70% - 100% relative humidity. 118. The process of claim 56 further comprising the step of supplying an additive to the fluid that modifies the liquid. 119. The process of claim 72 further comprising the step of purging the gas of moisture prior to impinging the gas against the liquid. 120. The process of claim 119 wherein the purging step comprises flowing an inert gas through the outlet. 121. A process for depositing a thin, uniform coating of an aqueous liquid on a substrate, comprising the steps of : providing an aqueous liquid that is a liquid at room temperature; directing a flow of unheated liquid from an outlet toward the substrate to be coated; impinging an unheated gas against the liquid stream to attenuate the liquid flow into droplets for depositing a thin, uniform coating on the substrate; and - 84 - moving the substrate and flow relative to each other. 122. The process of claim 120 wherein the liquid is directed through the outlet under pressure at less than 25 psi (170 kPa) , and the gas is impinged against the liquid stream at 200 - 1100 feet/second (61 - 335 m/s) . 123. The process of claim 121 wherein the impinging step comprises moving gas through an elongated slot toward the liquid. 124. The process of claim 121 wherein the impinging step comprises depositing a coating of 0.11 - 0.19 g/m"/side on the substrate. 125. A process of depositing a uniform coating of a liquid on a substrate, comprising the steps of: providing an elongated array of liquid droplets; moving the substrate and flow relative to each other; and impinging a fluid against the array to direct a thin coating of liquid toward the substrate. 126. The process of claim 56 wherein the providing step comprises an aqueous liquid. 127. The process of claim 56 wherein the providing step comprises providing liquid PMDI. 128. The process of claim 56 wherein the providing step comprises providing liquid EMDI. 129. The process of claim 56 wherein the liquid is an acrylic. 130. The process of claim 56 wherein the liquid is styrene-maleic anhydride. 131. The process of claim 56 wherein the liquid is an epoxy resin. 132. A process for depositing a thin, uniform coating of an aqueous liquid on a substrate, comprising the steps of: providing an aqueous liquid that is a liquid at room temperature; directing a flow of unheated liquid from an outlet toward the substrate to be coated; - 85 - impinging an unheated gas against the liquid stream to attenuate the liquid flow into droplets for depositing a coating on the substrate; and moving the substrate and flow relative to each other. 133. The process of claim 132 wherein the liquid is directed through the outlet under pressure at less than 25 psi (170 kPa) , and the gas is impinged against the liquid stream at 200 - 1100 feet/second (61 - 335 m/s) . 134. The process of claim 132 wherein the impinging step comprises moving gas through an elongated slot toward the liquid. 135. The process of claim 132 wherein the impinging step comprises depositing a coating of 0.11 - 0.19 g/m 2 /side on the substrate. 136. A process of depositing a uniform coating of an aqueous liquid on a substrate, comprising the steps of: providing an elongated array of aqueous liquid droplets; moving the substrate and array relative to each other; and impinging a fluid against the array to direct the liquid droplets toward the substrate and deposit them on the substrate as a uniform coating. 137. The method of claim 132 wherein the providing step comprises providing a liquid comprising polyvinyl alcohol. 138. The method of claim 132 wherein the providing step comprises providing a liquid consisting essentially of water. 139. A process for depositing a coating of an aqueous liquid on a substrate, comprising the steps of: providing an elongated array of an aqueous fluid; moving the substrate and array relative to each other; and impinging the same or another fluid against the array to direct the aqueous fluid toward the substrate. - 86 - 140. The process of claim 139 wherein the providing step comprises providing a humidified gas array. 141. The process of claim 139 wherein the impinging step further comprises attenuating the array. 142. A method of applying a uniform coating on a substrate, comprising the steps of: directing a flow of a liquid in an elongated array toward the substrate as the flow and substrate move relative to each other; impinging a fluid on the array of liquid to attenuate the array into droplets that form a fine mist that deposits uniformly on the substrate; and collecting at least a portion of the mist that does not deposit on the substrate. 143. The method of claim 142 wherein the collecting step comprises collecting the mist with a pressure differential. 144. The method of claim 143 wherein the collecting step comprises collecting the mist with a hood. 145. The method of claim 144 wherein the collecting step further comprises providing a suction pressure in the hood to attract the mist into the hood. 146. The method of claim 145 wherein the collecting step further comprises suspending the hood above the substrate with a proximal end of the hood adjacent the linear array of liquid and a distal end of the hood spaced away from the linear array along the path of the substrate, such that a space is formed between the substrate and hood, and directing a flow of air into the space. 147. The method of claim 146 wherein the collecting step further comprises suspending a second hood above the substrate along the path of the substrate on an opposing side of the linear array from the other hood. 148. The method of claim 142 wherein the mist has a predominate charge, and the collecting step comprises electrostatically collecting the mist. - 87 - 149. The method of claim 148 wherein the collecting step comprises propelling the mist toward the substrate with a member having a charge opposite the predominate charge of the mist. 150. The method of claim 148 wherein the step of electrostatically collecting the mist comprises grounding the substrate. 151. The method of claim 148 further comprising the step of electrostatically depositing the mist on the substrate. 152. The method of claim 142 further comprising the step of separating the collected mist into a liquid phase and a gas phase. 153. The method of claim 152 further comprising the step of exhausting the gas phase to the atmosphere. 154. The method of claim 144 wherein the hood is suspended above the substrate, and an air curtain is directed toward the substrate below the hood. 155. The method of claim 154 further comprising the step of introducing a secondary flow toward the substrate to direct the mist toward the substrate and into the hood. 156. The method of claim 143 wherein the substrate has opposing faces, and the method further comprises the step of balancing the pressure differential on the opposing faces of the substrate. 157. The method of claim 142 wherein the substrate has opposing surfaces, and the mist is collected on the same side of the surface from which the flow of liquid is directed. 158. An apparatus for applying a uniform coating on a substrate, comprising: an applicator that directs a flow of liquid in a linear array toward the substrate as the flow and substrate move relative to each other; a fluid impinger that impinges a fluid on the linear array of liquid and attenuates the linear array - 88 - into droplets that form a fine mist and deposit uniformly on the substrate; and a collector adjacent the substrate for collecting at least a portion of the mist that does not deposit on the substrate. 159. The apparatus of claim 158 wherein the collector is an enclosure on the same side of the substrate as the applicator. 160. The apparatus of claim 159 wherein the enclosure is a hood spaced away from the substrate. 161. The apparatus of claim 160 wherein the hood is a negative pressure hood. 162. The apparatus of claim 160 wherein the hood comprises an elongated negative pressure collection member extending across and above the substrate. 163. The apparatus of claim 160 further comprising a top panel extending over the substrate from the collection member toward the applicator. 164. The apparatus of claim 160 further comprising an air curtain directed toward the substrate between the hood and substrate. 165. The apparatus of claim 159 further comprising a secondary flow of air, on the same side of the substrate as the enclosure, that directs the mist toward the substrate and into the enclosure. 166. The apparatus of claim 158 wherein the applicator is positioned above a moving substrate. 167. The apparatus of claim 166 wherein a pair of applicators is positioned, one on each side of the applicator, along an axis of movement of the substrate. 168. The apparatus of claim 158 wherein the collector comprises an electrostatic director that electrostatically governs the movement of the mist. 169. The apparatus of claim 168 wherein the mist comprises charged droplets having a predominant charge, and the electrostatic director is a repulsion plate spaced from the substrate and charged to repel the mist droplets toward the substrate. - 89 - 170. The apparatus of claim 169 further comprising a charger that charges the droplets with a predominant charge. 171. The apparatus of claim 159 wherein the collector further comprises an electrostatic director in combination with the enclosure. 172. The apparatus of claim 168 wherein the electrostatic director comprises an electrical ground in electrical contact with the substrate. 173. The apparatus of claim 159 wherein the collector further comprises an electrical ground in combination with the enclosure. 174. The apparatus of claim 159 wherein the applicator comprises a head having a plurality of orifices through which the liquid is directed under pressure to form the liquid array. 175. The apparatus of claim 159 wherein the applicator comprises a head having an elongated slot through which the liquid is directed under pressure to form the liquid array. 176. The apparatus of claim 174 wherein the liquid impinger comprises an elongated slot adjacent the plurality of orifices through which fluid is directed under pressure. 177. The apparatus of claim 175 wherein the liquid impinger comprises an elongated slot adjacent the plurality of orifices through which fluid is directed under pressure. 178. A method of applying a coating on a substrate, comprising the steps of: providing an elongated array of liquid droplets; moving the substrate and array relative to each other; impinging a fluid against the array to direct the liquid droplets toward the substrate and deposit them on the substrate as a uniform coating; and collecting at least a portion of the mist that does deposit on the substrate. - 90 - 179. The method of claim 142 wherein the liquid is a diisocyanate. 180. The method of claim 179 wherein the diisocyanate is polymeric methylene diphenyl diisocyanate. 181. The method of claim 179 wherein the diisocyanate is emulsifiable polymeric methylene diphenyl diisocyanate. 182. The method of claim 178 wherein the step of providing an array of droplets comprises attenuating a flow of liquid by impinging a co-flowing fluid against it.

Inventors: ADAMS TERRY N, LEBLANC HENRY A, LORCK FLEMMING L, MCDERMOTT TED, STEPHENS R SCOTT, WALLICK SCOTT ALAN, WATKINS JOHN J, WATT WALTER D, WESTLAND JOHN A, YANCEY MICHAEL J, ADAMS, TERRY, N., LEBLANC, HENRY, A., LORCK, FLEMMING, L., MCDERMOTT, TED, STEPHENS, R., SCOTT, WALLICK, SCOTT, ALAN, WATKINS, JOHN, J., WATT, WALTER, D., WESTLAND, JOHN, A., YANCEY, MICHAEL, J.
Applicants: WEYERHAEUSER CO, WEYERHAEUSER COMPANY
Application year: 1991
Main classification: B05D1
IPC codes: B05B7, B05B13, B05B15, B05D1