Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-02-14
2002-04-16
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168300, C162S181200, C162S183000, C162S175000
Reexamination Certificate
active
06372089
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to colloidal silica sols having high surface area and enhanced stability. It is also directed to processes for making such colloidal silica sols and to the use of such colloidal silica sols in the production of paper. The colloidal silica sols of the present invention uniquely exhibit this stability at such high surface area without modification of the surface with, for example, aluminum. Moreover, the colloidal silica sols of the present invention exhibit such enhanced stability at relatively high solids contents. Furthermore, the colloidal silica sols of the present invention advantageously exhibit excellent activity not only in alkaline furnishes but also in acid furnishes in papermaking. The silica sols of the present invention are useful, among other areas, in the papermaking industry, for example, as retention and dewatering aids.
U.S. Pat. Nos. 5,643,414 and 5,368,833 describe a colloidal silica microparticle consisting of high surface area, i.e., greater than 700 m
2
/g and an S-value between 20 and 40 which is useful in papermaking. These patents teach the need for surface treatment of the colloidal silica particle with aluminum in order to stabilize the surface area and thereby the product. U.S. Pat. No. 5,603,805 also describes a colloidal silica having a surface area less than 700 m
2
/g and with an S-value of 20 to 40 which can be used in the making of paper. This patent clearly teaches that surface areas of less than 700 m
2
/g are needed in order to obtain a stable colloidal silica product without aluminum surface treatment that is useful in the paper making application.
In contrast, the present invention provides a stable composition of colloidal silica that is useful in the application of making paper and has a surface area greater than 700 m
2
/g and has an S-value of 20 to 50. Contrary to the teachings of the above-referenced patents, the present invention advantageously provides a colloidal silica that remains stable without the addition of surface aluminum.
SUMMARY OF THE INVENTION
The present invention provides for a stable colloidal silica having a surface area of greater than 700 m
2
/g, preferably greater than 750 m
2
/g, and most preferably greater than greater than 800 m
2
/g, and an S-value of from 20-50, preferably from 20-40. Colloidal silicas of the present invention do not require surface treatment with surface treatment agents such as aluminum to achieve stability. As used herein, a stable colloidal silica is defined as one whose surface area remains above 700 m
2
/g and whose S-value remains in the range of from 20-50 as the concentrated product is aged at room temperature for a period of at least 30 days, preferably for a period of at least 60 days. The colloidal silica aquasols of the present invention can be produced and stored at concentrations of greater than 7 percent by weight SiO
2
solids, and even as high as 15 percent by weight SiO
2
solids or higher, and remain stable at room temperature for at least 30 days, typically for at least 60 days, compared to art-known silica aquasols. Moreover, colloidal silica sols of the present invention demonstrate advantageously improved performance over art-known colloidal silica sols in applications such as papermaking. For example, colloidal silica sols of the present invention which are made of only silica have shown enhanced activity not only in alkaline furnishes but unexpectedly also in acid furnishes in papermaking.
The present invention also provides processes for making a colloidal silica of the present invention having a surface area of greater than 700 m
2
/g and an S-value of from 20-50. One process comprises: (a) forming an initial composition (or heel) containing water, an alkali metal silicate having a mole ratio of SiO
2
to alkali metal oxide in the range of from 15:1 to 1:1 and a pH of at least 10, typically at least 11, and an acid (and/or a corresponding salt thereof), the alkali metal silicate and acid being initially present in a ratio by weight of at least 63:1, while maintaining the temperature of the initial composition below 100 degrees F., preferably below 85 degrees F., typically from 60-85 degrees F.: (b) slowly and continuously adding to the initial composition an aqueous silicic acid composition typically having a SiO
2
content in the range of from 5.0 to 7.2 percent by weight, preferably from 6.0 to 6.8 percent by weight, while maintaining the temperature of the composition below 100 degrees F., typically from about 60-85 degrees F., until from one-half to three-quarters of the silicic acid composition has been added to the initial composition: (c) thereafter, slowly increasing the temperature of the composition, for example over a period of from about 10-35 minutes, to from 115-125 degrees F. and maintaining the temperature until the addition of the silicic acid composition is complete; (d) optionally, maintaining the temperature of the composition below 125 degrees F., typically from 115 to 125 degrees F. for about an hour; and (e) thereafter discontinuing the heating and optionally removing water from the resulting composition until the solids content based on SiO
2
of the resulting composition is 7 percent by weight or higher, typically 11 percent by weight or higher.
An alternative process for preparing aqueous silica sols of the present invention entails the use of a cationic ion exchange resin to initiate the reaction of the alkali metal silicate (see Example 3 below). The reaction is controlled by the rate of addition (for example, from 0 to 30 minutes, typically less than 15 minutes) and the ratio of alkali metal silicate to ion exchange resin during the polymerization to produce the colloidal silica. The molar ratio of hydrogen ion in the cationic ion exchange resin to alkali metal ion in the alkali metal silicate ranges from 40 to 100 percent, preferably from 50 to 100 percent. The temperature during colloidal silica formation in this alternative embodiment of the invention generally ranges from 50 degrees F. to 100 degrees F., preferably from 70 degrees F. to 90 degrees F. Heat treatment of the colloidal silica product (i.e., post treatment) is optional in this embodiment of the process of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention provides for the manufacture a colloidal silica product having a surface area via Sears Method, see Anal. Chem., 28, 1981(1956), of greater than 700 m
2
/g, preferably greater than 750 m
2
/g, and most preferably greater than 800 m
2
/g, an S-value, see Iler and Dalton, J. Phys. Chem., 60, 955(1956), of less than 50, preferably from 20 to 50, and most preferably from 20 to 40, and a percent by weight solids concentration of between 7 and 20% silica, i.e., SiO
2
.
One process consists of the preparation of an initial composition (“heel”), followed by addition of a source of active silica, usually in the form of silicic acid or polysilicic acid, over a specified time. During the addition of the active silica, the reaction temperature is controlled within a specified reaction temperature profile. Once the prescribed amount of active silica is added, the mixture can be concentrated. The concentration process can be carried out using a variety of methods. Such methods may include, but are not limited to, evaporation and/or membrane separation techniques such as ultrafiltration or microfiltration. The water is removed until the final product contains from 7 to 20% by weight SiO
2
.
In the above process, the initial heel is composed of water, any of a number of commercially available silicates or alkali water glasses, and an acid and/or a corresponding salt thereof in a prescribed ratio. While the order of addition is not important, for purposes of ease of manufacture, it has been discovered that the acid should be added to the dilution water, prior to the addition of the silicate.
The alkali water glasses or silicates can be any number of conventional materials. These are normally potassium or sodium salts. The mole ratio of SiO
2
to Na
2
O or
Huang Cheng-Sung
Keiser Bruce A.
MacDonald Dennis I.
Nunn Maureen B.
Breininger Thomas M.
Brumm Margaret M.
Fortuna Jose
Nalco Chemical Company
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