Coating processes – Spraying
Reexamination Certificate
2001-08-30
2002-10-15
Cameron, Erma (Department: 1762)
Coating processes
Spraying
C427S422000
Reexamination Certificate
active
06465047
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for applying a polymer dispersion in a precise manner to a substrate, through the use of airless spray technology. The method provides application flexibility for applying the polymer dispersion on the surface of a substrate, or with controlled penetration into the substrate. The method is particularly useful in applying a polymer dispersion to the surface of a porous substrate, and especially useful for applying a binder or a surface treatment to a non-woven substrate.
BACKGROUND OF THE INVENTION
Aqueous polymer dispersions can be applied to a substrate in many ways, including brushing, immersion (saturation), foaming, and spraying. Spray application is known in the industry as an efficient and high-speed means of applying liquid coatings to substrates.
Spray application of polymeric dispersions is used in the production of non-woven materials. The polymeric dispersions are used as binders to consolidate the base-sheet, used for control linting in Multi Bonded Airlaid (MBAL) materials, or used as surface treatments. Current spray processes used in the production of non-woven materials require that the polymer dispersion be diluted with water, making accurate control of binder distribution within a fiber matrix difficult. Typical spray bonding processes involve dilution of a polymer dispersion to between 10 and 35 percent non-volatile content in order to achieve satisfactory atomization. Application of polymer resin to the non-woven occurs using a spray boom, containing nozzles positioned across the web profile, which are fed from a sealed pressure vessel.
Current polymer spray systems work well for through fiber coating of the non-woven material, resulting in a coating of both the surface and core of the non-woven substrate. Current methods do not allow for surface application only, primarily due to the low viscosity of the diluted dispersion. For precision application, a higher solids dispersion must be used in order to minimize penetration of the substrate, and must be used in conjunction with a system that offers superior atomization of the wet dispersion. Current spray bonding is also prone to over-spray and line contamination.
U.S Pat. Nos. 4,515,836 and 5,573,429 describe the use of airless spray technology for the application of a coating to plastic bottles. In the described airless spray process, higher solids polymeric dispersion (typically from 40 to 60 percent) are applied to plastic bottles.
There is a need for a high-speed process for applying a polymer dispersion in a controlled manner onto, or into, a porous substrate, particularly in the non-wovens industry.
Surprisingly it has been found that polymer dispersions, having viscosities of between 10 and 5,000 m·Pas can be applied in a precise manner to porous substrates using an airless spray process. One advantage of the process is the ability to use commercial polymer dispersions without the need for dilution. Another advantage is that at the higher solids levels, the polymer dispersion can be applied to the surface only. This precise application of polymer leads to less polymer usage, and the higher solids dispersions require less time and energy to dry, thereby reducing manufacturing costs.
SUMMARY OF THE INVENTION
The present invention is directed to a method for applying an aqueous polymer dispersion to a substrate comprising:
a) forming a polymer dispersion having a viscosity of from 10 to 5,000 m·Pas;
b) applying said dispersion to a substrate by use of airless spray at a pressure of from 100 to 1500 psi.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method for applying a polymer dispersion in a precise manner to a substrate, through the use of an airless spray.
An aqueous polymer dispersion, as used herein, refers to polymer particles dispersed in water. The dispersion may be formed by any means known in the art, such as emulsion polymerization, inverse emulsion polymerization, suspension polymerization, and the dispersion of polymer particles into water which are either self-dispersing, or are stabilized using colloid stabilizers, surfactants, or both. Natural polymers may be dispersed with or without the use of stabilizers and/or surfactants.
By airless spray, as used herein, is meant the application of an aqueous polymer dispersion to a substrate using hydraulic pressure instead of air to atomize and spray liquids. This technology involves forcing a polymer dispersion through a small, precise orifice at a pressure of from 100 to 1500 psi. This process results in the dispersion forming a fan pattern containing virtually no air turbulence, and thereby minimal overspray. The absence of air also reduces the droplet velocity, thereby minimizing the disturbance of the web.
Cross-cut nozzles, dome nozzles, and other types of nozzles can be used in the airless spray process. A cross-cut nozzle capable of providing precise fan patterns is preferred over standard airless nozzles. As a cross-cut nozzle wears, the fan pattern widens creating an acceptable problem of over-lapping coverage. As a standard nozzle wears the fan pattern narrows, creating unacceptable gaps between adjacent fan patterns.
The airless spray system ideally includes a high quality filtration system to minimize the risk of nozzle clogging. A high pressure pump is used to pressurize the dispersion to between 100 and 1500 psi. It is noted that a standard air driven piston pump can cause momentary interruptions in sprays as the pump shifts strokes, which could lead to uneven distribution of the polymer dispersion. This problem can be eliminated with appropriately designed fluid circuits using constant pressure pumps or surge chambers, and/or fluid regulators.
Pre-atomization may optionally be used in the present invention. Pre-atomization essentially involves the generation of turbulence within the polymer dispersion prior to entering the nozzle, which can assist in optimizing atomization.
The aqueous polymer dispersion may include any polymer or a mixture of polymers known in the art, and can be made by means known in the art. Polymers useful in the present invention include both natural and synthetic polymers; and homopolymers, copolymers, block polymers, multi-stage polymers, and star polymers. Examples of useful polymer chemistries include, but are not limited to, ethylene vinyl acetate polymers, acrylic homopolymers and copolymers, vinyl acetate homopolymers, starches, cellulosics, all with and without cross-linking capability. Stabilization of such dispersions can be achieved using a wide variety of chemical species, including low molecular weight emulsifying agents, such as anionic, nonionic, cationic, and amphoteric surfactants; high molecular weight protective colloids, such as polyvinyl alcohol, cellulosics, dextrines and starches; or a combination thereof. The polymer may be functionalized to improve performance characteristics after it is applied to the substrate. This includes cross-linking and self-cross-linking functionality, and the range of fully redispersible polymer dispersions to water repellent hydrophobic polymer dispersions. Preferably, the dispersion is not excessively thixotropic in behavior when used with a cross cut nozzle. Polymer dispersions having Newtonian flow characteristics can be used with dome nozzles without preatomization.
The key parameters of the dispersion are the viscosity and solids level. The solids level is preferably maximized to reduce the amount of water that must be removed following application. The viscosity dictates the degree of atomization. Useful viscosities are from 10 to 5,000 m·Pas, and preferably 100 to 2,000 m·Pas at the temperature of application. Optimum atomization is obtained using dispersions having a viscosity of from 100 to 2000 m·Pas. Dispersions with viscosities in excess of 2000 m·Pas can be used with the airless spray system, but require additional air pressure and possible warming. The viscosity of the dispersion must be within a range that can be handled by the airless spray equipment. War
Scott Paul
Sol Andre
Cameron Erma
National Starch and Chemical Investment Holding Corporation
Roland, Esq. Thomas F.
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