METHOD FOR DIRECTING AN ELONGATED FLOW OF COATING MATERIALS TOWARD A SUBSTRATE

Abstract: A method for depositing a liquid on a substrate by directing an elongated linear flow of a liquid (647) toward a substrate (670), and impinging a flow of fluid against the liquid to atomize the liquid and deposit liquid droplets on the substrate. Excess mist is collected in a collector device (650), which is provided with a paddlewheel or auger to help move mist into and through the collector. Heated air may also be introduced into the collector. Even distribution of liquid out of an applicator is enhanced by flowing liquid through a row of orifices and into an impingement plate before flowing the liquid out of an outlet slot or orifices. A non-wetting or subliming powder may be included in the liquid or fluid that renders the coating porous on the substrate.

Description: METHOD FOR DIRECTING AN ELONGATED FLOW OF COATING MATERIALS TOWARD A SUBSTRATE 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. Another type of spray system is air assisted atomization, in which liquid emerges from a circular opening and is changed into droplets by an annular stream of air. Air assisted atomization has been used, for example, in spray painting devices. This technology has not been suitable for uniformly coating substrates because of nonuniformities in the coating and production of environmentally unacceptable misting. Moving spray painters close to surfaces being painted results in a less uniform coat of paint being deposited on the surface. 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 or airblast 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 or airblast atomizers. 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, and more particularly can combine the benefits of mist reduction with enhanced coating uniformity. 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 deposits a coating of a material on a substrate by directing a flow 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 flow to form droplets that can deposit a thin uniform coating on the substrate. The substrate and flow move relative to one another as the flow is changed 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 characteristics of droplet formation and the resulting uniformity of droplet deposition on the substrate. The droplets can be formed in various ways, depending on the coating material viscosity, fluid impingement velocity, and changes in viscosity of the material as the droplets are forming. At low impingement velocities or high material viscosities, the fluid may be attenuated into droplets by gradual elongation of liquid streams. At higher impingement velocities or lower material viscosities, airblast atomization occurs as the liquid is immediately changed from a confluent liquid into atomized droplets as the liquid emerges from its outlet. The coating material does not increase in viscosity after it leaves the outlet to such an extent that droplets will not form. This is a difference between the present invention and meltblown technology, because in meltblowing the viscosity of extruded material increases to a sufficient extent that networks of fibers form. The coating materials of the present invention include non- thermoplastic materials that do not form fiber networks of the meltblowing variety. Instead, the coating materials form droplets that are deposited on a substrate. The flow of coating material emerging from the applicator can take a variety of forms prior to being impinged with the impingement fluid. The flow of material can take the form of a series of columns emerging from a plurality of orifices, or a continuous curtain emerging from a slot. The plurality of orifices can be linearly aligned, arranged in a chevron or arcuate configuration, in staggered rows or any other shape that allows the impingement fluid to attenuate or blast the liquid into droplets. The slot can have a similarly wide variety of shapes, such as a continuous, straight linear slot, a series of discontinuous slots, an arcuate or chevron shaped slot, or staggered rows. The term "linear" is used in its usual technical manner to refer to the shape traced by a moving point, which can include a straight, arcuate, or even serpentine line. As used herein, the term "linear" does not include a circular shape. 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 l - 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 can be minimized or enhanced. The elongated liquid flow emerging from the outlet has opposing faces, and the fluid can be impinged against either one or both faces at a wide range of velocities, from 200 feet per second (60 m/s) to sonic or supersonic velocities. Attenuation of the liquid into increasingly finer droplets occurs as the fluid velocity is increased, for example, as it approaches sonic velocity. At high impingement velocities, the liquid is immediately blasted into droplets as the liquid emerges from the outlet. Much lower fluid velocities are also suitable for many applications where large droplet size can be tolerated. Immediate atomization can also be achieved at relatively low fluid velocities when the liquid has a low viscosity. 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 coating material toward the substrate. A fluid outlet in the applicator impinges a fluid, such as a gas, against the coating material to form droplets that are directed toward the substrate to deposit a coat of liquid on the moving substrate. A nozzle portion of the applicator head contains the outlet (which can be a plurality of outlets) through which the coating material is ejected under pressure. 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 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. A paddlewheel or auger can be placed in or adjacent the collector to help direct mist into the collector and out of a collection duct. 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 alternative 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. FIG. 20 is a schematic view of an alternative embodiment of the invention in which liquid is pre- atomized before being directed at a substrate. FIG. 21 is a schematic cross-sectional view of an electrostatic atomizer for dispersing liquid into droplets. FIG. 22 is a view similar to FIG. 3 showing an alternative embodiment of the applicator in which a collection hood surrounds the applicator. FIG. 23 is a cross-sectional view of the applicator taken along section lines 23-23 of FIG. 22. FIG. 24 is a cross-sectional and schematic view of an air scrubber for removing liquid droplets from the exhaust of the hood of FIG. 22. FIG. 25 is a cross-sectional schematic view of another embodiment of the hood in which secondary flows of air are introduced into the hood. FIGS. 26 - 28 are photographs prepared from high speed videotapes of liquid impinged with gases at increasingly greater gas velocities. FIGS. 29 - 30 are photographs prepared from high speed videotapes showing airblast atomization of liquid immediately as it emerges from the liquid outlet. FIG. 31 is a photograph showing a grainy distribution of iodine stained coating liquid on a paper substrate coated with the present invention. FIG. 32 is another photograph showing a streaky distribution of iodine stained coating liquid. FIG. 33 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. 34 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. 35A is an image while FIG. 35B is a column average and FIG. 35C is a single line grey intensity profile for a gate roll coated sample of paper. FIGS. 36A - 38A are images while FIGS. 36B - 38B are column average and FIGS. 36C - 38C are 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 S12C1, S7, and S6, respectively. FIGS. 39A - 52A are graphs showing column average intensity profiles while FIGS. 39B - 52B are 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. 53A-E, 54A-E, 55A-D and 56A-E 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. 57 is a schematic view showing airblast atomization of liquid as it emerges from the applicator. FIG. 58 is a schematic end view of a collector device in which a paddlewheel feeds mist into the collector. FIG. 59 is a schematic end view of a collector in which an augur extends along the length of the collector. FIG. 60 is a schematic cross-sectional view of another embodiment of the applicator in which liquid flows through a series of holes and on to a target plate before entering liquid outlets. FIG. 61 is yet another embodiment of the applicator in which a replaceable tip is provided inside the applicator. 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 58. 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 he i- orifice extends in the plane of the face and is outlined by edges 95. The sharp edges 97 (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 10l 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 liquid from outlets 93 at about a 30 degree angle. Hence the liquid is attenuated or atomized by fluid striking both faces of the emerging liquid. Such bi-planar attenuation or atomization has been found to be acceptable but not essential to droplet deposition. Hence the second air channel 116 may be omitted, especially when coating with 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 distribution 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 distribution 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. Instead of gradual liquid attenuation, a more rapid airblast atomization can occur. Airblast atomization would more typically occur at higher gas impingement velocities or elevated mass ratios of impingement gas to coating material mass. The coating material is blasted into droplets by high velocity air immediately as the material emerges from the applicator. The degree of liquid attenuation or atomization can vary depending on the viscosity and flow rate of the coating liquid, and the impingement gas velocity. It is frequently desireable for reasons of economy, appearance and function, to form 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 atomization or attenuation. Larger droplets will reach the substrate and form a thicker, less uniform coating. Application of larger droplets may be preferred when saturation is desired. 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 or elongated distribution of liquid emerging from outlet 182. The co-flowing airstrea attenuates or atomizes the liquid array into droplets for deposition on a substrate. A linear distribution of liquid is a liquid flow that has an elongated linear width. The width of the distribution is elongated in the direction that slot or orifices 82 extend. An elongated distribution is not necessarily linear (although it may be) . The elongated distribution has a width that extends in the direction perpendicular to the direction of flow of coating material from the outlet to the substrate, and typically is elongated in the direction that slot or plurality of orifices 82 extend. Although the liquid distribution from the embodiment of FIG. 9 is in the form of a straight line, a variety of shapes would be suitable, such as an elongated arcuate or chevron shaped distribution. Uniformity or symmetry of the array or distribution is not required. 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 distribution. 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. ir 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 or atomized 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 distribution 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 from outlet 331 and attenuate or atomize 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 distribution of liquid. Air is simultaneously introduced into air chamber 386 under pressure and propelled out of air gap 381 to impinge the linear distribution of liquid at about a 30 degree angle and attenuate or atomize the liquid distribution 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 or atomization 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 gas 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 distribution of liquid emerging from liquid slot 427 is thereby attenuated or atomized 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 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 distribution 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, gas 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. Alternative Attenuation Means The present invention does not necessarily require attenuation (including atomization) of the liquid stream by impingement of a fluid. FIG. 20, 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, fl: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. 21 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 be 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. 22 - 23, 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 gradually attenuating or immediately atomizing the liquid as it emerges from the head. A linear distribution of liquid 647 emerges from head 642 along its length, and the liquid is attenuated or atomized 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 643, 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. 23, 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. 22) 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. 23) 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. 23 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. 23, 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. 25 wherein a lid 690 or 691 extends the entire distance from each of collectors 650 to the top of the applicator 642 such that the lid is co-planar with a top face of the applicator. A row of orifices is provided through each lid 690, 691 adjacent the top face of applicator 642 to provide inlets for a secondary stream of air to redirect any upward circulation of mist back down toward the substrate and into an excess air collection hood. Alternatively, a hot air duct (not shown) can supply hot air (for example at 80°C) to redirect the mist downward and diminish formation of condensate inside the hood and on the faces of the applicator. 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. 22. A grounding member 690 is illustrated extending below substrate 643 transverse to the direction of movement 648 of 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. 22 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 positively 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. 21. Scrubbing and Venting the Mist It is desireable to vent the exhaust stream 666 (FIG. 22) 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. 24) 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 collect in a liquid pool 738 with the water from sprays 736. The 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. 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 (i.e. distribution) 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 or atomize the liquid flow into droplets and deposit a uniform coating on a substrate that is moving relative to the array. More uniform arrays, such as those produced by 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. Linear alignment refers to a curtain or series of columns that is capable of being intersected along substantially all its length by a line. The liquid flow is attenuated or atomized 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) . 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 to break it into smaller segments, and eventually into fine droplets that have a diameter, for example, of about 100 μm or less. Atomization of the liquid flow (which is a sub-category of attenuation) is achieved by impinging a fluid against the liquid with sufficient energy to immediately break the liquid into droplets without forming increasingly smaller segments. 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 or atomization. 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 can have a range of diameters that are sufficiently small to thoroughly coat a desired swath on a substrate, if such uniformity is desired. Small droplets of the present invention can selectively 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 2 (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 TiC^, CaCO j ), 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 (77°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 droplet formation 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 or atomizes 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. 35 and 36. The liquid distribution has opposing faces, and the impingement fluid can be impinged against one or both of the faces of the distribution to attenuate or atomize the liquid 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 minimum size droplet formation 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, droplet size may continue to decrease beyond sonic velocities, but measurement limitations make it difficult to determine changes in droplet diameters at these small dimensions. Although no deterioration of droplet formation 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 or atomizing 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, usually less than about 900 cp (0.9 Pa-s), 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 coat 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. A distribution of droplets 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 a distribution of droplets 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. Parallel plural liquid outlet slots can also be provided in the applicator to apply multiple coatings to the substrate. 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. 35 - 56 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 or Atomization The process of the present invention uses a fluid stream, such as a curtain of air, to attenuate (or more particularly in many instances atomize) co-flowing liquid to a diameter or width that is smaller than an orifice from which the liquid emerged. An example of the attenuation process is shown in FIGS. 26 - 28, 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. 26 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. 26. 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. 27. 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. 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. As already mentioned, droplets can be formed by immediate airblast atomization instead of a more gradual form of attenuation. Such atomization is shown in FIGS. 29 and 30, where liquid emerging from outlets is immediately blasted into droplets as the liquid emerges from the outlets. The runs shown in FIGS. 29 and 30 were performed with a slotted head four inches long, a 0.005 inch liquid slot opening, an air gap of 0.005 inch, and 78 inches of water pressure in the head. Air was impinged at 16 CFM at 30 psig against a Cellulon/CMC (4:1 ratio) that was 1.11% total solids. The impingement velocity at which atomization instead of attenuation occurs depends on the viscosity of the liquid. Coating materials with greater viscosities (e.g. starch) require higher impingement velocities for atomization than low viscosity liquids such a water. Atomization represents one end of the spectrum of attenuation in which ligament formation becomes vanishingly small or nonexistent. Atomization occurs in many of the runs reported in Tables I and II. 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 or atomization 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 formation of droplets, and illustrate the effects of different process parameters on droplet formation 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. TABLE I cont. LIQUID AIR LIQUID AIR AIR UQUID ENERGY ENERGY D.Di LIQUID AIR LIQUID ASSUMED MAX FINAL MI VIDEO AIR SLOT VOL FLOW VOL FLOW VELOCITY VELOCITY RATIO IN AIR IN LIQUID ATTEN- PRESS. PRESS. VOL FLOW ISCOSITY FLUID SESS. LIQUID MILS GPM CFM M/S M S W W UATION PSI PSI CC S CP VEL. M/S 36 WATER 23 0.11 7.60 0.71 30.24 0.62 1.92 0.002 54% 6.78 246 37 WATER 23 0.11 10.75 0.71 42.76 0.88 5.43 0.002 42% 6.78 407 2 38 WATER 23 0.11 13.17 0.71 52.37 1.07 9.98 0.002 36% 6.78 547 1 39 WATER 23 0.11 14.43 0.71 67.37 1.18 13.12 0.002 34% 8.78 6 26 1 40 WATER 23 0.11 13.17 0.71 62.37 1.07 9.98 0.002 36% 6.78 547 1 41 WATER 23 0.11 7.60 0.71 30.24 0.62 1.92 0.002 54% 6.78 248 2 42 WATER 23 0.18 7.60 1.18 30.24 0.37 1.92 0.006 73% 11.28 2 19 3 CO 43 WATER 23 0.18 10.75 1.18 42.76 0.53 5.43 0.008 60% 11,28 332 3 44 WATER 23 0.18 13.17 1.18 52.37 0.64 9.98 0.008 52% 11.28 4.37 2 01 45 WATER 23 0.18 14.22 1.18 56.57 0.70 12.58 0.008 49% 11.28 4.37 O 46 WATER 23 0.26 7.60 1.88 30.24 0.23 1.92 0.032 89% 17.90 238 4 H 47 WATER 23 0.28 10.75 1.88 42.76 0.33 5.43 0.032 78% 17.90 3 10 48 WATER 23 0.28 13.17 1.88 52.37 0.41 9.98 0.032 70% 17.90 363 C 49 CMC 23 0.06 10.75 0.38 42.76 1.63 5.43 0.000 26% 3 3.64 1000 548 1 • H 50 CMC 23 0.06 10.75 0.38 42.76 1.63 5.43 0.000 26% 3 3.64 1000 548 PI 51 CMC 23 0.08 10.75 0.38 42.76 1.63 5.43 0.000 26% 3 3.64 1000 548 52 CMC 23 0.08 13.17 0.38 62.37 2.00 9.98 0.000 23% 3 3.64 1000 7.41 1 O 53 CMC 23 0.06 15.21 0.38 60.47 2.31 15.36 0.000 20% 3 3.64 1000 920 1 54 CMC 23 0.06 17.00 0.38 87.61 2.58 21.47 0.000 19% 3 3.64 1000 1087 55 CMC 23 0.08 17.00 0.38 87.61 2.58 21.47 0.000 19% 3 3.64 1000 1087 66 CMC 23 0.08 17.00 0.38 67.61 2.58 21.47 0.000 19% 3 3.64 1000 1087 67 CMC 23 0.08 17.00 0.38 67.61 2.58 21.47 0.000 19% 3 3.64 1000 10.87 58 CMC 23 0.08 17.00 0.38 67.61 2.58 21.47 0.000 19% 3 3.64 1000 1087 59 CMC 23 0.02 17.00 0.13 67.C1 7.70 21.47 0.000 8% 1 1.22 1000 1876 60 CMC 23 0.02 17.00 0.13 67.61 7.70 21.47 0.000 8% 1 1.22 1000 18.76 4 81 CMC 23 0.02 15.21 0.13 60.47 6.89 15.36 0.000 9% 1 1.22 1000 1567 62 CMC 23 0.02 10.75 0.13 42.76 4.87 5.43 0.000 12% 1 1.22 1000 9.44 63 CMC 23 0.04 17.00 0.28 67.61 3.86 21.47 0.000 14% 2 2.43 1000 1329 64 CMC 23 0.04 17.00 0.26 67.61 3.86 21.47 0.000 14% 2 2.43 1000 1329 65 STARCH 23 0.20 12.02 1.35 47J1 0.52 7.59 0.012 60% 12.66 50 3.60 66 STARCH 23 0.20 13.17 1.35 52.37 0.57 9.98 0.012 57% 12.66 50 4.16 67 STARCH 23 0.30 13.17 1.96 62.37 0.39 9.98 0.036 72% 16.70 50 381 68 STARCH 23 0.34 13.17 2.26 52.37 0.34 9.98 0.055 77% 21.53 50 379 69 STARCH 23 0.34 13.17 2.26 52.37 0.34 9.98 0.055 77% 21.63 50 3 79 70 STARCH 23 0.34 13.17 226 52.37 0.34 9.96 0.055 77% 21.53 50 379 Table I lists the operating conditions for each video session. Except for the geometry and raw pressure data, all but the Df/Di value is calculated based on the flow calibrations. The Df/Di term is used to describe the decrease in droplet size as the liquid stream is accelerated or atomized by the surrounding high-velocity air. No direct measure of Df/Di was taken during these trials, though estimates could be made from some of the video pictures. The number listed under Df/Di 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 droplet formation. The lower the percentage, the smaller the diameter of the resulting droplets. A df/Di value of 50% would indicate a droplet having a diameter of one-half the initial diameter of the liquid column as the column emerged from the liquid outlet. 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 (such as a curtain or plurality of columns) located between two fast, co-flowing gas (air) streams. The fast moving air stream changes the liquid stream either gradually or instantaneously into droplets having a smaller dimension than its initial characteristic dimension (column diameter or curtain width) 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) . 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. TABLE U-A Coat Coat Uquid Head Head Head Air (H)oie t Press. Gap or in H20 mils (S)lot in H20 mils (S)lot TABLE ll-Aconi. Coat Coat Uquid Head Head Head Air (H)ole Appearance (1 Good, 5 Bad) iscosity 1-Mech. rookfield Streak Wormy Grainy Streak Appearance (1 Good, 5 Bad) ity 1-Mech. kfield Streaky Wormy Grainy Streak 1«Mech. TABLE U-B coπL HorS Uq. Heater Air Appearance (1 Good, 5 Bad) Rei. Size Angle Temp. Temp. RJi Uq.% Viscosity 1-Mech. No. mils deg •F •F Solids Brookfieid Streaky Wormy Grainy Streak 145 6 45 120 136 95.6 10 1 3 146 6 45 116 136 932 1 2 147 6 45 115 131 832 1 2 148 6 45 123 132 952 1 1 149 6 45 125 133 95.4 1 2 150 6 0 125 135 832 1 1 151 6 0 125 136 93.4 1 z 152 6 0 125 135 93.6 1 1 153 6 0 125 135 1 3 154 6 0 125 135 1 2 155 6 0 125 135 3 2 156 30 125 133 762 10 5 4 157 30 0 125 139 772 10 1 1 158 30 0 122 128 922 10 1 1 159 6 0 125 132 892 to 1 1 160 6 0 125 135 962 ' '10 4 3 3 161 6 0 125 136 822 10 5 4 3 162 6 0 125 136 91.7 10 1 2 2 163 6 0 123 133 98.7 10 5 3 2 164 6 0 123 133 93.4 10 4 3 2 165 6 0 125 132 942 10 1 2 1 166 6 0 122 142 832 10 1 1 1 167 6 0 120 131 9&4 10 1 1 1 168 6 0 120 137 972 10 1 1 1 469 6 0 122 139 83.7 10 1 2 2 170 6 0 133 164 832 10 1 2 2 171 6 0 133 161 86.1 10 3 2 2 172 6 0 132 162 972 10 5 3 3 173 6 0 132 161 942 10 2 2 1 174 20 0 100 135 392 10 2 2 2 175 20 0 100 135 392 10 1 1 176 20 0 100 135 392 10 1 1 177 6 0 115 113 982 15 3 3 178 6 0 110 109 872 15 3 3 179 6 0 110 107 972 15 2 3 180 6 0 100 117 704 272 2 2 181 6 0 100 116 97.0 272 2 2 182 6 0 100 166 912 272 2 2 183 6 0 100 116 86.4 272 2 2 184 6 0 100 116 100.0 272 3 2 185 6 0 100 116 884 272 2 2 186 6 0 100 115 84.6 272 3 2 187 6 0 100 115 83.1 272 2 188 6 0 100 120 782 272 2 189 30 0 90 101 812 14.7 135 1 190 30 0 90 100 772 14.7 135 2 191 30 0 90 100 862 14.7 135 1 192 30 0 120 115 85.7 192 130 ~3 TABLE II-B conL H orS Uq. Heater Air Appearance (1 Good, 5 Bad) Ref. Size Angle Temp. Temp. R.H. Uq.% Viscosity 1«Mβch. No. mils deg •F •F % SoBds Brookfieid Streaky Wormy Grainy Streak 193 30 0 120 122 772 202 189 3 2 194 30 0 117 121 83.4 202 247 3 1 195 30 0 118 122 922 212 306 3 3 196 30 0 120 124 10O0 222 364 3 2 197 30 0 118 123 834 222 423 3 2 198 30 0 116 119 85.7 232 482 3 2 199 30 0 120 134 75.0 242 540 2 2 200 30 0 120 134 83.6 252 599 201 30 0 117 124 84.0 25.7 658 202 30 0 117 121 872 262 716 203 30 0 116 120 862 272 775 204 30 0 114 118 724 27.7 833 205 30 0 114 117 562 284 892 206 30 0 118 123 952 .6.7 23 207 30 0 117' 121 85.1 6.7 24 4 208 30 0 117 122 73.7' ' 6.7 25 3 209 30 0 117 121 822 6.7 26 3 210 30 0 117 121 904 6.7 27 4 211 30 0 119 123 92.6 6.7 28 3 212 30 0 89 97 262 6.7 29 3 213 30 0 83 89 262 6.7 30 2 214 30 0 79 86 264 6.7 31 3 215 30 45 84 90 25.7 6.7 32 2 216 30 45 84 90 282 6.7 33 217 30 45 99 107 272 6.7 34 218 8 0 93 98 805 7 34 219 8 0 93 98 902 7 35 220 8 0 93 99 90.1 7 36 221 8 0 93 99 92.7 7 37 999 8 0 94 99 732 7 38 223 8 0 95 100 824 7 39 224 8 0 95 101 802 7 40 225 8 0 93 97 84.1 7 42 226 30 0 125 132 772 132 438 227 30 0 121 127 892 132 438 228 30 0 117 122 882 132 438 229 30 0 122 122 682 132 438 230 30 0 120 120 502 132 438 231 30 0 120 120 472 132 438 232 30 0 115 115 562 132 438 233 30 0 118 123 332 132 438 5 234 30 0 111 115 312 132 438 5 235 30 0 98 106 282 132 438 2 236 8 0 114 119 82.7 82 27 1 237 8 0 114 119 69.1 8.7 27 1 238 8 0 110 114 882 82 28 5 239 8 0 108 112 812 9.0 29 2 3 240 8 0 108 111 862 9.1 30 1 2 TABLE U-B co . H orS Uq. Heater Air Appearance (1 Good, 5 Bad) Rθi. Size Angle Temp. Temp. R.H. Uq.% Viscosity 1-Mech. No. mils deg •F β F Soilds Brookfieid Streaky Wormy Grainy Streak 241 8 0 108 112 89.7 92 31 2 242 8 0 107 110 832 9.4 31 3 243 8 0 109 112 822 9.6 32 1 244 8 0 109 112 614 92 33 1 245 8 0 110 114 772 92 34 2 246 8 0 112 116 823 10.1 35 3 247 8 0 113 117 862 102 35 2 248 8 0 113 117 782 104 36 2 249 8 0 109 11? 824 102 37 1 1. 250 8 0 109 113 862 10.7 38 4 3 251 8 0 118 121 972 10 85 3 3 252 8 0 115 119 96.1 12 85 4 3 253 8 0 109 114 852 132 85 2 2 254 8 0 111 116 942 142 85 3 3 255 8 0 114 117 352 102 29 1 1 256 8 0 113 116 892 ' 102 33 1 2 257 8 0 111 114 832 112 38 1 2 258 8 0 104 108 90.1 112 42 2 2 259 8 0 104 108 802 112 47 1 2 260 8 0 105 109 832 122 51 2 1 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 (1 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 Celiulon 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. 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% Celiulon/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 lbm/ton/side and 5 lbm/ton/side. These coverages correspond approximately to 0.11 g/nr/side and 0.19 g/irP/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. The separation distance is measured from the tip of the applicator where liquid emerges from the slot to the surface of the substrate. A distance of 3 inches was found to produce less mist than a 10 inch distance, while unexpectedly retaining uniformity of coating application. The liquid viscosities and gas impingement velocities in Table II are sufficient that immediate or almost immediate airblast atomization of the liquid probably occurred as the liquid emerged from the outlet. The range of variables for these trials is expressed in 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 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, Celiulon and starch coatings applied to papers are transparent. To obtain information about the coating uniformity, a fluorescent dye was added to the Celiulon 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 distribution of the coating solids, it is an adequate approximation for these studies. The images and graphs in FIGS. 35 - 56 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. 35A - 56A 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. 35A - 38A are printouts of the scanned test sheets using a Macintosh computer and a LaserWriter printer. The graphs of FIGS. 35 - 56 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 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 used 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 droplet formation 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 FIG. 31. 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 FIG. 32. 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. 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. 33 is a set of twelve photographs of samples for both 3 and 5 lbm/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 lbm/ton/side) and air pressure (5 - 30 psi) . From the comparison in FIG. 33 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. 34 is a set of photographs of samples for 5 mil and 15 mil (125 μm and 375 μm) air gaps at 5 lbm/ton/side application rate and for 5 to 30 psig air pressure. From the comparison in FIG. 34 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 \, 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 lbm/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 not very pronounced at l 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 FIGS. 35 and 36. The single line grey intensity profile FIG. 35C is always below 200, demonstrating no discontinuities in the coating made with a conventional gate roll. A comparable single line density graph in FIG. 36C 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. 36C, 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. 39B) reflects excessive graininess. Variation from baseline of the column density graph is associated with streakiness of the coating. FIG. 41, 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 P = d 2 where: P = pressure drop, Pa μ = apparent liquid viscosity, Pa-s v = liquid velocity, m/s 1 = orifice length, m d = orifice effective diameter, m The apparent viscosity, μ, can be related to the Brookfieid viscosity at 100 RPM, μ^ by the expression: β - β 20 s with: 8 v where: βg = Brookfieid 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: 4 A d = π where: = orifice flow area, m 2 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. In those embodiments that use a slotted head, the optimum relationship among the width of the slot and the other variables mentioned above is 12μvl P = w 2 where w is the width of the slot. EXAMPLE IV Several examples of film thickness are calculated below to illustrate some very thin coatings that can be achieved with the present invention. TABLE IV Film Thickness (One Side Only) COVERAGE WET FILM 36% SOLIDS DRY FILM VA 0.05 grams/meter^ 0.83 microns .050 microns SG 1.02 a 10% solids 0.10 grams/meter* 1.66 microns .010 microns WET FILM 310% SOLIDS DRY FILM PG 290 Starch 0.25 grams/meter' 2.4 microns .024 microns SG 1.05 S 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/ir?, 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 Celiulon. 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. 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 emulsifiable 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. 35 and 36 (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 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. 35 and 36 are at about 80 - 150, preferably below 125, and indicate a high probability of thorough coverage across a desired swath of substrate being coated. EXAMPLE VIII An alternative mist collection device is schematically shown in FIG. 58 wherein a substrate 750 moves in the direction of arrow 752 beneath an applicator 754. A pair of tubular collectors 756, 758 are placed above the substrate, one on each side of the applicator, and each collector presents a downwardly facing slot 760 or 762 into which mist is drawn by negative pressure. A paddlewheel cylinder 764 is provided adjacent slot 760 and extends the length of collector 756. Another paddlewheel cylinder 766 is provided adjacent slot 762 and extends the length of collector 758. Each paddlewheel rotates in the direction indicated by the arrows in FIG. 58 to direct mist into the collector. Yet another mist collector is shown in FIG. 59, in which like parts are given like reference numerals. The paddlewheels have been removed in this embodiment, and an auger 768 or 770 is placed within each collector such that the auger axis of rotation is coaxial with the longitudinal axis of the collector in which the auger rotates. Auger 768 rotates in the direction of arrow 772, and auger 770 rotates in the direction of arrow 774, to convey mist that enters slots 760, 762 to the ends and out of the collectors. EXAMPLE IX This Example concerns designing the liquid flow passage in the applicator to obtain uniform liquid distribution along the length of the applicator. This design keeps the flow velocity low so that both the dynamic head and the friction losses are small compared to the pressure drop across the exit slot or orifices. The design shown in FIG. 60 places a series of holes 780 and a target plate 782 within the liquid passage above the slot or orifice inlets 784. This series of holes and target plate separate the liquid passage into two sections. The "upper" section 786 is the passage intended for distributing the liquid along the entire length of the applicator. The "lower" section 788 is intended to distribute the liquid uniformly to the inlet of the slot or orifices. The size and number of holes 780 is selected to avoid pluggage and to present a total flow area much less than the slot or multiple orifice area. The series of holes represent a significant resistance to flow and aid in uniform distribution of liquid along the length of the head. The pressure drop across the slot or multiple orifices will then only have to distribute liquid uniformly over the dimension of the separation of the holes. The target plate 782 is preferably located a short distance below the series of holes at a distance approximately equal to the hole diameter. Its purpose is to dissipate the dynamic head of the liquid and redirect it away from the slot or multiple orifice inlet. EXAMPLE X Another embodiment of the applicator is shown in FIG. 61, and includes a top portion 790 and mating lower portion 792. A removable, triangular cross-section tip 794 is held in place between portions 790, 792. A central, tubular manifold chamber 796 extends the length of top portion 790 and distributes coating material through a passageway 798 to a slot 800 in tip 794, and eventually out of a narrow liquid outlot slot 801. A pair of longitudinally extending tubular fluid manifold chambers 802, 804 extend parallel to chamber 796 and introduce gas into passageways 806, 808 that communicate with passageways 810, 812 and provide an impingement fluid. Tip 806 may be selectively removed from the applicator by separating top and bottom portions 790, 792. The tip may be replaced when it is worn or when a different width outlet slot 801 is desired. EXAMPLE XI This Example illustrates the use of the applicator to produce a micro or macro porous coating or sheet by the use of non-wetting or subliming particles. In normal use the applicator can produce a uniform, continuous coating or sheet by the use of high speed gas stream impinging a co-flowing liquid stream. A powder may be added to the liquid or gas stream. The material of the powder may be either non-wetted by the liquid or may be a material which sublimes at a temperature below the melting or decomposition temperature of the remaining coating or web. The resulting coating would be porous or non- continuous. The size of the defects may be controlled by the selection of the size of the powder. Possible applications would be in the production of micro-porous material to allow the underlying web or substrate to "breathe" but otherwise provide a continuous coating. Other products that could be produced this way include microporous filter media and blister resistant coatings. EXAMPLE XII A pigmented coating was prepared and sprayed using the 4-inch slotted head. Sheets were run to test the coverage quality at varying liquid pressures in the head and varying air pressures. The coverage quality was determined using grey-scale image analysis on the sheets. The pigmented coating was applied using the 4- inch slotted head mounted on the testing sled. A Columbus formulation was used to make the pigmented coating (39% total solid) and is shown in Table V: TABLE V Pigmented coating formulation The air pressure and the fluid pressure in the head were varied at three levels each for a total of nine combinations. The levels for the air were varied at 5, 10, and 15 psi. The three fluid pressures were 80, 110, and 150 inches of water, corresponding to 6.8, 8.9, and 9.2 g/m 2 coat weights respectively. The air temperature, air humidity, and the head temperature were left ambient. The coverage quality was analyzed using grey- scale imaging. The sheets were dyed using Croda Red ink to make the pigmented coating visible. A grey-scale image was produced using an image scanner. The image was analyzed for graininess and the streakiness (or patchiness) in the same manner as described above. Small scale non-uniformities are called graininess while larger patches and streaks are called streakiness. Table VI shows the graininess and the streakiness for the nine different combinations. Visually, the best sheet was PCF13 (15 psi air, 6.8 g/m 2 ). It also has the lowest percentage of streakiness and a lower value for the graininess. The combination of high air flow and low coating flow seems to give the best coverage. This is probably because a minimum air flow is needed to atomize the fluid enough to get a good coverage quality. As the liquid flow is increased, the minimum air flow needed for acceptable atomization will also increase. TABLE II EXAMPLE XIII In the applicator described in connection with FIGS. 5-19, one or two high velocity gas streams impinge an adjacent liquid stream. In this example, the arrangement is not altered, but a fine powder is added to one or both of the air streams, or to the liquid stream. A fine powder would preferably be used that contains solid particles having a longest dimension preferably less than one-fifth of the air or liquid passage minimum dimension. The powder may be carried in the liquid stream as part of the coating formulation, for example, or in the gas stream. Carrying the powder in the gas limits contact of the powder with the liquid until mixing with the gas jet issuing from the applicator. Limited mixing of the powder and coating would control the degree of reaction or interaction of the solid powder with the liquid, or could control the relative location or degree of stratification of the final coating. One example of the application of a fine powder is the application of a pigmented coating color where the solid phase clay, CaC03, or whiteners and brighteners such as Ti02, or other powder-like material is entrained in the gas stream while the binder, SBR latex, starch or other material is applied with the liquid stream. This could provide a potential useful stratification of the two types of materials. Particles could be introduced by providing a particulate outlet between the impingement gas outlet and coating material outlet. Alternatively, particles would be suspended in the impingement gas by blowing the impingement air over wells containing particulate material prior to the gas emerging from the applicator outlet. When a porous coating is desired, the powder can be a non-wetting material such as Teflon (polytetra fluoroethylene) . The coating liquid will not adhere well to the particles, which will subsequently be removed to leave small holes in the coating. EXAMPLE XIV The potential uses for the present applicator are extremely broad and varied. A miniature spray coating applicator could be used in cooperation with a printer or plotter near the inking head to coat the paper surface before printing or seal a newly inked surface to avoid smearing or erasure. Varying sizes of applicators can be used to apply a controlled, uniform amount of acid or caustic to a moving substrate in an etching or cleaning operation. Acid or caustic applications could also be used to treat cloth, leather or wood materials. Tannic acid, for example, could be applied to leather. Hydrofluoric acid can be used to etch glass by spraying a uniform coating of the acid on exposed portions of a glass surface. The applicator of the present invention can also be used to distribute bleach or other chemical solutions uniformly onto washer drums, deckers, flumes, or conveyors. Such applications would be especially helpful where accurate amounts of the liquid are to be applied evenly across an expanse. Ceramics may also be made by mixing liquid epoxy resin with solid particles in separate co-flowing streams. Thin ceramic coatings may be applied to a substrate in this manner. Elevated epoxy temperatures could be used to fuse the mixed co-flowing epoxy and particulate streams. 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: CLAIMS 1. A method for depositing a liquid on a substrate, comprising: directing an elongated linear flow of liquid from a plurality of orifices or a slot toward a substrate at a first velocity; impinging a flow of fluid against the flow of liquid at a greater velocity than the first velocity to form droplets that deposit a coating on the substrate, wherein the impinging flow atomizes the flow of liquid immediately as the impinging flow of fluid meets the flow of liquid. 2. The method of claim 1 wherein the step of forming droplets comprises forming droplets that deposit a uniform coating on the substrate. 3. The method of claim 1 wherein the step of directing a flow of liquid comprises directing a flow of non-thermoplastic liquid. 4. The method of claim 1 wherein the step of directing a liquid comprises directing a plurality of columns toward the substrate. 5. The method of claim 1 wherein the directing step comprises directing a curtain of liquid toward the substrate. 6. The method of claims 4 or 5 wherein the directing step comprises directing a flow that is arcuate, chevron shaped, staggered, serpentine or discontinuous. 7. A method for depositing a liquid on a substrate, comprising: directing an elongated linear distribution of liquid toward a substrate; impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate; collecting droplets that are not deposited on the substrate by providing a collector adjacent the substrate;and directing the droplets into the collector with a paddlewheel. 8. A method for depositing a liquid on a substrate, comprising: directing an elongated linear distribution of liquid toward a substrate; impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate; and collecting droplets that are not deposited on the substrate by providing a collector adjacent the substrate, and directing the droplets through the collector with an auger within the collector. 9. A method for depositing a liquid on a substrate, comprising: directing an elongated linear distribution of liquid toward a substrate; impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate; collecting in a collector droplets that are not deposited on the substrate; and directing a flow of heated air into the collector. 10. The method of claim 1 further comprising the step of providing relative movement between the flow of liquid and substrate, wherein the flow of liquid has a width that is elongated transverse to the direction of flow of the liquid and transverse to a direction of relative movement between the flow of liquid and substrate. 11. The method of claim 1 further comprising the step of directing a flow of a second liquid toward the substrate and impinging a fluid against the flow of second liquid to form droplets that deposit on the substrate. 12. The method of claim l wherein the step of forming droplets comprises forming droplets with a Df/Di of at least about 50%. 13. The method of claim 1 wherein the impinging step comprises impinging the fluid at a subsonic velocity. 14. The method of claim 1 wherein the impinging step comprises impinging the fluid at substantially sonic velocity. 15. The method of claim 1 wherein the impinging step comprises impinging the fluid at supersonic velocity. 16. The method of claim 1 wherein the directing step comprises directing the flow from a distance of about 3 inches. 17. A method for depositing a liquid on a substrate comprising: directing an elongated linear distribution of liquid toward a substrate; impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate; and wherein the directing step comprises directing the flow from a plurality of orifices having a diameter of about: 32μvl d = 18. A method for depositing a liquid on a substrate comprising: directing an elongated linear distribution of liquid toward a substrate; impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate; wherein the flow is directed from a straight slot having a width of about 12μvl w = 19. A method for depositing a liquid on a substrate, comprising: flowing a liquid through a row of orifices and into an impingement plate, then directing an elongated linear distribution of the liquid out of an elongated outlet toward a substrate; and impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate. 20. A method for depositing a coating on a substrate, comprising: flowing a liquid through a first set of orifices and then a second set of orifices, the first set of orifices collectively having a lesser cross sectional area than the collective cross sectional area of the second set of orifices; then directing the liquid out of a plurality of orifices or a linear slot toward a substrate; and impinging a flow of fluid against the liquid as it emerges from the orifices or slot to form droplets that deposit on the substrate. 21. A method for depositing a liquid on a substrate, comprising: directing an elongated linear distribution of liquid toward a substrate; impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate; including in the flow of liquid or flow of fluid a material that renders the coating porous on the substrate. 22. The method of claim 21 wherein the material is a powder that sublimes below the melting or decomposition temperature of the remainder of the coating. 23. The method of claim 21 wherein the material is a non-wetting powder. 24. The method of claim 23 wherein the powder is polytetrafluoroethylene powder. 25. The method of claim 1 wherein the step of depositing a coating comprises depositing a pigmented coating on the substrate. 26. The method of claim 1 further comprising providing a powder in the flow of liquid or flow of fluid. 27. The method of claim 26 wherein the providing step comprises providing the powder in the gas stream. 28. The method of claim 27 wherein the step of providing the flow of liquid comprises providing a flow of binder for the powder. 29. The method of claim 27 wherein the providing step comprises introducing a powder into the gas stream by blowing the fluid over containers that contain the powder. 30. A method for depositing a liquid on a substrate, comprising: directing an elongated linear distribution of a caustic or acid liquid toward a substrate; and impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate. 31. A method for depositing a liquid on a substrate, comprising: directing an elongated linear distribution of a liquid toward a substrate selected from the group consisting of a washer drum, flume or conveyor; and impinging a flow of fluid against the flow of liquid to form droplets that deposit a coating on the substrate. 32. The method of claim 1, wherein the directing step comprises directing a flow of epoxy resin, and the impinging step comprises impinging a fluid that contains solid particles that mix with the epoxy resin to form a ceramic coating. 33. A coated substrate produced by the method of any of claims 1-32. 34. A coated substrate produced by the method of claim 1 and having a column intensity profile that varies no more than about 10 units of intensity. 35. A coated substrate produced by the method of claim 1 and having a single line intensity that varies no more than about 30-50 units of intensity. 36. A coated substrate produced by the method of claim 1 and that has a column intensity profile that varies no more than about 10 units of intensity, and a single line intensity that varies no more than about 30- 50 units of intensity.

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, B05B15, B05D1