Paper making and fiber liberation – Processes and products – Mineral fiber
Reexamination Certificate
2000-04-05
2002-07-09
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Mineral fiber
C162S158000, C162S184000, C427S424000, C427S391000
Reexamination Certificate
active
06416624
ABSTRACT:
FIELD OF THE INVENTION
This invention, in general, pertains to the field of applying additive compositions to sheet materials, such as paper, textile, and flexible sheets in the manufacture of sheet material products. More particularly, the present invention is directed to methods for the spray application of an additive composition to sheet materials in the manufacture of sheet material products by using a compressed fluid to enhance atomization and spray formation at lower volatile solvent levels and higher viscosities.
BACKGROUND OF THE INVENTION
Many industrial processes spray liquid compositions that contain volatile solvent to apply coatings, adhesives, and additives, or to spray dry materials. The solvent performs a variety of functions, such as to dissolve materials, provide a carrier medium for emulsions and dispersions, reduce viscosity for spraying, and to give proper flow characteristics upon application, such as film formation on a substrate or penetration into a porous or absorbent material. However, organic solvents are a major source of workplace and environmental air pollution and can be a fire hazard in manufacturing processes.
Therefore, water is often used as a solvent to avoid these problems. But, water can also have unfavorable characteristics which make it desirable to minimize its use in manufacturing processes. Many materials that are sprayed do not dissolve in water, so chemical agents such as surfactants must be used to emulsify or disperse the material into water in a stable form. Water has a relatively low evaporation rate and a high heat of evaporation, so drying the product can be slow and energy intensive, and often it must be heated to temperatures that can cause degradation in order to increase the drying rate. Because less water evaporates in the spray than with volatile organic solvents, the sprayed composition is often deposited with a viscosity that is too low for proper application, so performance can deteriorate. Furthermore, some substrates do not tolerate water; they are degraded by water absorption, which can cause swelling or weaken cohesiveness, or a water-borne composition does not properly wet the substrate, because water has a high interfacial tension, or the material is hydrophobic.
Successful and economical spray application of compositions also depends upon the spray properties produced by the spray method, in addition to the volatile solvent and viscosity characteristics of the composition sprayed. It is highly desirable that the spray method produces a spray that has a favorable droplet size, which will depend upon the application, and a narrow droplet size distribution that minimizes both overly large droplets, which typically give poor application quality, and overly small droplets, which typically become overspray and give inefficient deposition and increased material usage. It is also desirable for the spray pattern to have a uniform interior and tapered edges so that the composition is applied uniformly during application. The spray should not have an excessively high or low velocity or be exceedingly turbulent. The spray fan should have a proper width for a given application and provide good pattern control so that the composition is applied in the amounts and locations intended. It is also desirable for the spray method to be able to atomize high viscosity compositions in order to minimize or eliminate the use of volatile solvents.
Conventional spray methods such as air spray and airless spray methods, while each having certain favorable properties, also have undesirable characteristics that can limit their usefulness for applying compositions in manufacturing processes. Air spray methods provide an adjustable, uniform, feathered spray fan and fine atomization, but they require low viscosity, typically 50 to 100 centipoise, so they use a large proportion of volatile solvent. Air sprays are also highly turbulent and they produce a very broad droplet size distribution that has a large proportion of overly small droplets that become overspray and give low application efficiency. Airless spray methods can atomize higher viscosity materials with less solvent, but they typically produce coarse atomization and an overly large droplet size that is unsuitable for many applications. Airless sprays also produce nonuniform tailing or fishtailing spray patterns which make it difficult to apply compositions uniformly.
The conventional atomization mechanism of airless sprays is known in the art. In general, the material exits the orifice at ambient pressure as a liquid film that becomes unstable from shear induced by its high velocity relative to the surrounding air. Waves grow in the liquid film, become unstable, and break up into liquid filaments that likewise become unstable and break up into droplets. Atomization occurs because cohesion and surface tension forces, which hold the liquid together, are overcome by shear and fluid inertia forces, which break it apart. Often the liquid film extends far enough from the orifice to be visible before atomizing into droplets. The sprays are generally angular in shape and have a fan width that is close to the fan width rating of the spray tip. Viscous dissipation markedly reduces atomization energy, so higher viscosity gives coarser atomization. As used herein, the terms “liquid-film spray” and “liquid-film atomization” are understood to mean a spray, spray fan, or spray pattern in which atomization occurs by this conventional mechanism. Liquid-film sprays characteristically form a “tailing” or “fishtail” spray pattern, wherein material is distributed unevenly in the spray. Surface tension often gathers more liquid at the edges of the spray fan than in the center, which can produce coarsely atomized jets of material that sometimes separate from the spray. Examples of liquid-film sprays are shown photographically in FIGS. 4a, 4b, 4c, 4d, 10a, 11a, 12a, and 12b of U.S. Pat. No. 5,057,342 and in FIGS. 3a, 3b, 3c, 9a, 9b, and 9c of U.S. Pat. No. 5,009,367.
Supercritical fluids or subcritical compressed fluids, such as carbon dioxide or ethane, can produce a new airless spray atomization mechanism, which can produce fine droplet size and a feathered spray needed to apply high quality coatings. Without wishing to be bound by theory, the atomization is believed to be produced by the dissolved compressed fluid, such as carbon dioxide, becoming supersaturated as the spray mixture suddenly drops in pressure in the spray orifice. This creates a very large driving force for gasification, and very fine carbon dioxide gas bubbles convert the solution to a gas-liquid mixture. This is believed to alter the flow pressure by lowering the speed of sound to where it equals the flow velocity, which chokes the flow, so instead of dropping to atmospheric pressure, the flow exits the orifice at a relatively high pressure. This creates a pressurized zone outside the orifice in which the spray mixture expands freely to atmospheric pressure. The carbon dioxide gas released expands and produces an expansive force that overwhelms the liquid forces that would normally bind the fluid flow together. The expansion is constrained only by a groove cut across the outlet, which shapes the spray into a flat or oval fan. The spray width is adjusted by changing the pitch of the groove. A different atomization mechanism is evident because atomization appears to occur right at the spray orifice instead of away from it. No liquid film is visible at the orifice. Furthermore, the spray typically leaves the nozzle at a much wider angle than normal airless sprays and produces a “feathered” spray with tapered edges like an air spray. This frequently produces a rounded, parabolic-shaped spray fan, instead of a sharp angular fan. The spray typically has a wider fan width than conventional airless sprays produced by the same spray tip. As used herein, the terms “decompressive spray” and “decompressive atomization” are understood to mean to a spray, spray fan, or spray pattern that has these characteristics as described herein. Examples of
Baumert Duane F.
Cesaretti Richard S.
Goad Jeffrey D.
Nielsen Kenneth A.
Chin Peter
Hug Eric
Union Carbide Chemicals & Plastics Technology Corporation
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