Coating processes – With post-treatment of coating or coating material – Swelling agent or solvent applied to treat coating
Reexamination Certificate
2001-12-31
2003-07-29
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Swelling agent or solvent applied to treat coating
C427S337000, C427S339000, C427S008000
Reexamination Certificate
active
06599575
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for the production of a durably hydrophilic, non-leaching coating and the product of such a process.
BACKGROUND OF THE INVENTION
The use of carbohydrates to form coatings is well known in the art. For example, carbohydrates such as hydroxyethycellulose can be derivatized wherein the derivative is dissolved in water and used as a coating. However, a coating formed from this type of carbohydrate derivative is water soluble and has to be further reacted to form a water insoluble coating.
Alternatively, a water insoluble cellulose derivative may be used to form a water insoluble coating. For example, cellulose acetate may be dissolved in a suitable solvent, such as acetone, to produce a coating. Unfortunately, there are no known derivatives which can be dissolved in an organic solvent to form a hydrophilic coating without further treatment of the material after coating.
In other cases, to avoid the difficulties inherent in working with flammable solvents, the cellulose derivative may be formed into an aqueous emulsion and the emulsion used to form a coating. This approach is taught in U.S. Pat. No. 3,565,669. Another means of forming a cellulosic coating is to use a water or organic solvent soluble cellulose derivative and then insolubilize the coating as taught in AU 8,317,253 and JP 5,210,893.
What is needed in the art is a method of forming a water insoluble carbohydrate coating using a one step process in which the carbohydrate may be dissolved directly in a water-based solution, without the formation of a derivative. Such a process would offer significant economic and environmental advantages over the prior art. Direct dissolution of the carbohydrate avoids the additional cost of derivatizing a polysaccharide and further chemically modifying the derivative.
SUMMARY OF THE INVENTION
The present invention solves one or more of the above described problems by providing a method for the production of a carbohydrate coating on a substrate. The method includes forming a carbohydrate-salt mixture by at least partially dissolving a water insoluble carbohydrate in an aqueous salt solution to form a carbohydrate-salt complex. The carbohydrate-salt mixture is applied to a substrate, and the substrate is rinsed with a solvent to form a coating. The coating is desirably hydrophilic and non-leaching.
More particularly, the aqueous salt solution contains a salt having a Hammett acidity between approximately +2 and −3, such as zinc chloride. The carbohydrate is desirably cellulose, starch, pectin, alginic acid, chitin or chemical derivatives thereof. In addition, the substrate may be a non-woven or woven material. Desirably, the substrate is hydrophobic.
The method of the present invention may further include adding a solvent to the carbohydrate-salt mixture to control the orientation and degree of aggregation of the carbohydrate-salt complex. Further, the method may include adding a surfactant to the carbohydrate-salt mixture to improve the application of the mixture to the substrate. The surfactant may be a cationic surfactant, an anionic surfactant or a nonionic surfactant.
Still more particularly, the method of the present invention includes heating the carbohydrate-salt mixture to a temperature between approximately 15° C. and 85° C. to allow optimal dissolution of the carbohydrate in the aqueous salt solution.
The present invention is also directed to a coated substrate made according to the method of the present invention and absorbent structures comprising the coated substrate. For example, the coated substrate of the present invention is particularly useful in personal care products such as diapers, feminine pads, panty liners, incontinence products, and training pants.
Other objects, features and advantages of this invention will become apparent upon reading the following detailed description in conjunction with the claims.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method of the present invention provides a process of making a durably hydrophilic, non-leaching coating. The process includes at least partially dissolving a water insoluble carbohydrate in an aqueous salt solution of low Hammett acidity to form a carbohydrate-salt complex. The complex in solution may be adjusted for optimum properties as a coating for a particular substrate through control of the concentration of the salt and the molecular weight and concentration of the water insoluble carbohydrate. If necessary, the surface tension of the mixture is also adjusted to better wet the substrate. The substrate is treated with the carbohydrate-salt mixture and then washed in a solvent to remove excess salt and regenerate the water insoluble carbohydrate as a hydrophilic, non-leaching coating.
In contrast to the prior art, the method of the present invention provides for at least partial dissolution of the carbohydrate directly in a water-based solution without the use of a solvent, and without the need for the formation of a derivative which must be insolubilized in a subsequent step.
Carbohydrates, as defined in this invention, are polymers containing linked sugars. Despite being composed of sugars, which are water-soluble as individual molecules, the larger carbohydrates are water insoluble due to extensive internal hydrogen bonding between the alcohol substituents of the sugar monomers. These molecules have hydrophilic and hydrophobic regions, usually based on the degree of sidedness for the hydroxyl substituents of the sugar ring. Any carbohydrate that is water insoluble due to internal hydrogen bonding may be used in the method of the present invention. Carbohydrates suitable for use in the, present invention include, but are not limited to cellulose, starch, pectin, alginic acid, chitin or chemical derivatives thereof. Desirably, the carbohydrate is cellulose.
Depending upon the carbohydrate used, the carbohydrate-salt mixture may contain from about 0.1% to about 50% by weight carbohydrate. Desirably, the carbohydrate-salt mixture contains from about 0.1% to about 10% by weight carbohydrate.
In the present invention, metallic salts of sufficiently low Hammett acidity, such as zinc and calcium ions, are used to disrupt the internal hydrogen bonding of the carbohydrates. The metallic salts form water soluble metal complexes with the water insoluble carbohydrates and alter the arrangement of the hydrophobic and hydrophilic regions of these carbohydrates once in solution. The aqueous salt solution typically contains at least 30% by weight salt, and desirably contains from about 60% to about 80% by weight salt. Examples of salts useful in the present invention include, but are not limited to, zinc thiocyanate, zinc halides such as zinc chloride, zinc bromide and zinc iodide, cadmium thiocyanate, cadmium halides such as cadmium chloride, cadmium bromide and cadmium iodide, titanium thiocyanate, titanium halides such as titanium chloride, titanium bromide and titanium iodide, zirconium thiocyanate, zirconium halides such as zirconium chloride, zirconium bromide and zirconium iodide, lithium thiocyanate, and lithium halides, such as lithium chloride, lithium bromide and lithium iodide, calcium thiocyanate, calcium halides, including calcium chloride, calcium bromide, and calcium iodide, magnesium thiocyanate, magnesium halides, including magnesium chloride, magnesium bromide, and magnesium iodide, strontium thiocyanate, strontium halides, including strontium chloride, strontium bromide, and strontium iodide, potassium thiocyanate, potassium halides such as potassium chloride, potassium bromide and potassium iodide, guanadinium thiocyanate, N-methyl morpholine oxide, or mixtures thereof. Desirably, the salt is zinc chloride because of its low cost and safety for human contact.
Hammett acidity is a measurement which is used for acidic solvents of high dielectric constant. The dielectric constant is a measure of the ion-solvating ability of the solvent. The Hammett acidity, H
0
is defined as:
H
0
=
pK
AH
W
+
+
log
[
A
Chen Li-fu
Reeves William G.
Barr Michael
Kilpatrick & Stockton LLP
Kimberly--Clark Worldwide, Inc.
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