ALKALI METAL SILICATE AND OXY-BORON COMPOUND COMPOSITION FOR PAPERMAKING

Abstract: A reinforcing agent for paper is disclosed comprising a solution of silicate and an oxy-boron compound. The solution, which is prepared by combining suitable sources of SiO2, B2O3, Na2O and water, is used to coat and/or impregnate paper to provide increased strength. The impregnated paper also does not lose strength when exposed to moisture.

Description: ALKALI METAL SILICATE AND OXY-BORON COMPOUND COMPOSITION FOR PAPERMAKING Background of the Invention Our invention pertains to strengthening paper by impregnating it with a siliceous agent. More specifically, * our invention relates to treating paper with the reaction product of a mixture of soluble silicate and an acid reacting oxy-boron compound. Soluble silicates have a wide variety of applica¬ tions in the paper industry for the production of containers and other materials. These applications include the use of soluble silicates as an adhesive in the manu¬ facturing of spiral tubes and convolute drums, as an adhesive in the lamination of aluminum foil to paper, and as an impregnating material to improve the edgewise crush strength of kraft liner board used in corrugated containers. Soluble silicates perform well in these applications under most conditions. Problems occur when the silicate-treated materials are exposed to high humidity or come in contact with water. When this happens the silicate rehydrates and loses its adhesive properties and rigidity. Also, water or high humidity conditions usually cause the alkali component of the silicate to migrate through the paper, causing a discoloration or mottling effect. To date, silicate users have had to tolerate these limitations, since there has been no effective way to prevent moisture pickup and rehydration of silicate films under high humidity conditions. Unexamined Japanese Patent Disclosure 62-223391, dated October 1, 1987, teaches that combinations of sodium silicate and a boron compound with a surfactant provide some improvement in the moisture resistance of paper strengthened with silicate. That disclosure teaches that mixtures of boric acid, borates or borax and sodium sili¬ cate that gel and then redissolve provide such improvements when combined with a surfactant. It is an object of our invention to provide a sili¬ cate modified with an acid reacting oxy-boron compound that does not require gelling and redissolving prior to use. An additional object of the invention is to provide such a composition of the proper rheology to penetrate the paper. It is a further objective to provide a composition that sets and develops water resistance without exposure to elevated temperatures. Slightly elevated temperatures provide a shorter set time but are not required to promote water resistance. Summary of the Invention We have found that compositions prepared by com¬ bining solutions of alkali metal silicate and certain acid reacting oxy-boron compounds can be used to reinforce paper without requiring gelling or addition of a surfactant. Careful control of the composition and concentration pro¬ vides a product that sets or cures at ambient or slightly elevated temperatures. High temperature treatments are not required to provide moisture resistance when the material is applied to paper. The prior art teaches that combinations of sodium silicate and potassium metaborate provide water resistant binders when treated at high temperatures for extended periods of time. See U.S. Patent 4,171,986. The metaborate solutions described in this patent are neutral and do not induce setting and do not provide water resistance at room temperature. Instead the combination of potassium metaborate and silicate provides water resistance when treated at high temperatures through a coacervation effect. The patent recites potassium meta- borate because high levels of borate salt can be achieved at low temperatures. The compositions of our invention are prepared by mixing solutions of alkali metal silicates and acid reacting oxy-boron compounds in the proportions to provide solutions that are sufficiently stable to prepare, ship, store and use, but gel rapidly upon application and pene- tration of paper. When applied under pressure, the solution enters the pores of the paper and upon setting provides increased stiffness and strength. Paper treated with the product of our invention actually absorbs slightly more water than untreated paper, but surprisingly retains significantly more strength upon exposure to high humidity. The Invention Our composition is prepared by combining aqueous solutions of sodium silicate and an acid reacting oxy-boron compound with any water required to provide the concentra¬ tion desired. Sodium silicate solutions provide SiO~ and a 2 0. Silica sols, silica gels, precipitated silica and the like provide Si0 2 and can be combined with a silicate solution or dissolved in NaOH. Sodium silicate, a useful source of Si0 2 and Na 2 0, should have 2.6 to 3.75 parts by weight (pbw) of SiO_ for each pbw of Na 2 0. The oxy-boron compound used in the composition of our invention can be any readily soluble compound that provides boron as an anion and provides acid reactivity after dissolution and/or hydrolysis. Such materials include boric acid, sodium pentaborate, potassium penta- borate, borax (sodium tetraborate) and mixtures thereof. Sodium and potassium metaborate are not suitable since their solutions are neutral. These materials are dissolved in water and used in the most concentrated form possible, since water can be added during preparation of the composition. Our composition is prepared by combining the ingre- dients in the following manner. Any silica source or caustic not included with the sodium silicate can be added to it. . For example, if some additional silica is required, silica gel is dissolved in the desired sodium silicate. The boron compound is dissolved in water at an elevated temperature. The silicate is agitated vigorously to provide high shear sufficient to prevent concentration gradients. The boron containing solution is added, thereby providing the stable composition of our invention. Other means of mixing are also useful. Nozzle mixing with suf¬ ficient shear can be efficient. Such mixing devices with incremental addition of the boron containing material are more desirable. The composition contains about 20 to about 40% solids calculated as Na 2 0, B 2 0 3 and SiO,. We prefer a solids content of about 27 to 37% for reinforcing paper. Useful solutions also contain more than 2 pbw of Na_0 for each pbw of B 2 °3' We P refer 2 « 5 to 3 « 5 P° w of Na ? Per pbw of B 2° 3 • τhe P roduct of our invention has the follow¬ ing composition: By controlling the composition and the concentration of the silicate boron containing solution and maintaining the agitation, gelling is avoided and the product has suf¬ ficient stability for its purpose. Eventually the modified silicate solution will increase in viscosity and gel. If the solution is subjected to shear prior to gelation, the desirable properties and results upon application to paper are retained. Despite the relatively high alkalinity of our product, more than 2 pbw of Na 2 0 per pbw of B 2 0 o . the product sets very quickly upon application to paper. It appears that the acidity of the boron compound is en¬ hanced when in contact with the paper, thereby promoting setting of the modified silicate. The paper impregnated with our composition can be set using a temperature of less than 150°F. We prefer to let the material set at ambient temperature. The product of our invention is applied and pene- trates paper easily. It can be applied under pressure in any way convenient, but is usually placed in the nip of drums or rollers wherein the paper pulls the solution through the nip under pressure. Paper impregnated with the composition does pick up water when exposed to moisture but retains a more significant amount of strength than paper treated by other treatments or untreated paper. Compare the following results. When kraft liner board is impregnated with 2 lbs/ 2 1000 ft of normal 3.22 ratio sodium silicate, the edge- wise crush strength is improved by 19% or more. When this treated paper is exposed to 92% relative humidity at 77°F for one week the entire strength increase is lost, and the kraft is approximately 5% lower in strength than untreated kraft conditioned in the same manner. When a product prepared with a 3.22 ratio sodium silicate and enough sodium tetraborate to produce a 3:1 a 2 0:B 2 0 3 ratio is applied at a loading of 2 lbs/1000 ft , the edgewise crush strength is improved by 25%. When con¬ ditioned at 100% relative humidity at 90°F for 5 days the treated kraft still retains 19% greater strength than untreated kraft conditioned in the same manner. Not only does the use of our product provide in¬ creased strength and moisture resistance to paper, but other properties are also improved. Silicate-impregnated paper is difficult to glue. Starch adhesives do not bond with such materials. The product of our invention appears to provide a different surface upon impregnation of paper so that starch adhesives bond more readily to the surface, providing greater strength. Examples The following examples illustrate certain embodi¬ ments of our invention. These examples are not provided to establish the scope of the invention, which is described in the disclosure and recited in the claims. The propor¬ tions are in parts by weight (pbw) or percent by weight (% wt/wt) unless otherwise indicated. Example 1 A solution was prepared by dissolving 31 pbw borax in 109 pbw of water at about 185°F. A vessel equipped with an intensive mixing device was charged with 430 pbw of a sodium silicate solution containing 8.90% a 2 0 and 28.7% Si0 2 . High shear was applied and the borax solu¬ tion was added at 1.2 pbw/minute. After the borax solution was added mixing was continued for 30 minutes. The final product had 3.0 pbw of Na 2 0 per pbw of B 2 ° 3 • This material had excellent stability. Example 2 The material prepared as described in Example 1 was used to impregnate or coat 42 pound kraft liner board at 2 various loadings between about 2 and 9 lbs/1000 ft . A sodium silicate solution was also used as a comparison, 2 and the loading was 10 lbs/1000 ft . Several advantages were noticed when the borosilicate was used as compared to the use of ordinary sodium silicate. The borosilicate penetrated the kraft paper at a lower pressure than that required for the sodium silicate solutions. The borosili¬ cate set much faster or more easily than the sodium silicate. The amount of heat required to set the paper impregnated with the composition was at least an order of magnitude less than that required to set paper impregnated with sodium silicate solution. Tests for ring crush strength were run on paper impregnated as previously described. These tests were done on the dry paper and some samples that were steamed for 30 minutes. This is a crude but quick way of deter¬ mining strength retained upon exposure to moisture. The results of these tests are as follows. Table 2 *Samples 2 and 3 were prepared by coating the paper. **Samples 4 to 6 were prepared by impregnating the paper. A sample of paper impregnated with sodium silicate at 2 1100 llbbss//11000 ft retained 37% of its strength when steamed. Example 3 A laboratory procedure was developed to approximate impregnation of paper using a commercial machine. Paper is soaked in a diluted borosilicate solution for about 20 minutes. The borosilicate solution is absorbed by the paper. The borosilicate solids retained in the paper are directly proportional to the concentration of the boro¬ silicate solution. If a solution containing 2.5% sodium borosilicate solids is used, the paper contains about 3.7 pounds of the solids per thousand square feet (TSF) . Paper was treated as indicated with a solution with a ratio of Na 2 0:B 2 0 3 of 3.0. The paper was dried and then exposed to 50 or 90% relative humidity for seven days. The exposed paper was tested for moisture pickup and ring crush strength. The results are as follows: Table 3 Moisture (%) Ring Crush Strength (Machine Direction) (lbs) These results indicate that even though the paper treated with borosilicate picked up more moisture than the un¬ treated paper, the treated paper retained greater strength than the untreated paper. Example 4 A group of borosilicate solutions having Na 2 0:B,0_ ratios of 2.5, 3.0 and 3.5 were diluted to 2.5% solids, and paper was treated as described in Example 3 and exposed to 90% relative humidity prior to testing for ring crush strength. The results are as follows. Table 4 Ring Crush Strength (Cross Direction) Na 2 0:B 2 0, lbs. 2.5 50 3.0 50 3.5 40 These results indicate that a 2 0:B 2 0 3 ratios of about 2.5 to about 3.0 provide the best ring crush strength. Solutions that have Na 2 0:B 2 0 3 ratios of about 2.4 or less are unstable. Example 5 A borosilicate solution was made using potassium metaborate as described in U.S. Patent 4,171,986 diluted to 2.5% solids and used to impregnate paper as described in Example 3 and exposed to 90% relative humidity prior to testing for ring crush strength. A borosilicate solution made with sodium tetraborate was also tested. The results are as follows. Table 5 Ring Crush Strength at 90% Relative Humidity (lbs) Agent Potassium metaborate Sodium tetraborate These results indicate that the borosilicate materials made according to the composition of our invention are significantly superior to the materials made by the prior art described in U.S. Patent 4,171,986.

Claims: Claims We claim: 1. A method of treating paper consisting of the steps of applying an aqueous solution of alkali metal silicate and an acid reacting oxy-boron compound under sufficient pressure to force said solution between the fibers of the paper, and allowing said solution to set at a temperature of 150°F, whereby the treated paper absorbs more water than untreated paper but retains greater strength than untreated paper when exposed to moisture. 2. A method of treating paper consisting of the steps of applying an aqueous solution of sodium silicate and an acid reacting oxy-boron compound under sufficient pressure to force said solution between the fibers of the paper, and allowing said solution to set at a temperature of 150°F or less, said solution containing 20 to 40% by weight dissolved solids and more than two pbw of Na 2 0 for each pbw of B _0 3 , whereby the treated paper absorbs more water than untreated paper but retains greater strength than untreated paper when exposed to moisture wherein the oxy-boron compound is selected from the group consisting of boric acid, sodium pentaborate, potassium pentaborate, borax (sodium tetraborate) and mixtures thereof. 3. The method of Claim 2 wherein the setting tem¬ perature is ambient temperature. 4. The method of Claim 2 wherein the solution con¬ tains 5 to 13% by weight Na 2 0, 15 to 27% by weight Si0 2 , and 1.3 to 4% by weight B 2 °3' the remainder to 100% by weight being water. 5. The method of Claim 4 wherein the solution con¬ tains 6.5 to 10% by weight Na 2 0, 17.5 to 25% by weight Si0 2 , and 1.5 to 3.5% by weight S2°3' the balance to 100% by weight being water. 6. The method of Claim 3 wherein the solution con¬ tains 5 to 13% by weight Na 2 0, 15 to 27% by weight SiO_, and 1.3 to 4% by weight B 2 ° 3 ' ** ne remainder to 100% by weight being water. 7. The method of Claim 6 wherein the solution con¬ tains 6.5 to 10% by weight Na 2 0, 17.5 to 25% by weight Si0 2 , 1.5 to 3.5% by weight B 2 °3' tne balance to 100% by weight being water. 8. The method of Claim 1 wherein the setting tem¬ perature is ambient temperature. 9. A modified silicate solution for treating paper prepared by reacting a sodium silicate solution with an acid reacting oxy-boron compound whereby the modified silicate solution contains 20 to 40% by weight of dissolved solids and more than 2 pbw of a 2 0 per pbw of B 2 0,3 wherein the said oxy-boron compound is selected from the group consisting of boric acid, sodium pentaborate, potassium pentaborate, borax (sodium tetraborate) and mix¬ tures thereof. 10. The modified silicate solution of Claim 9 wherein said modified silicate solution contains 5 to 13% by weight of a 2 0, 15 to 27% by weight of Si0 2 and 1.3 to 4% by weight of B 2 0 3' tne balance to 100% being water. 11. The modified silicate solution of Claim 10 wherein said modified silicate solution contains 6.5 to 10% by weight a 2 0, 17.5 to 25% by weight Si0 2 , and 1.5 to 3.5% B,0 3/ the balance to 100% by weight being

Inventors: KRUMRINE PAUL H, NEHRING ROBERT J JR, PETERS JOHN F, REIFSNYDER RICHARD H, KRUMRINE, PAUL, H., NEHRING, ROBERT, J., JR., PETERS, JOHN, F., REIFSNYDER, RICHARD, H.
Applicants: PQ CORP, PQ CORPORATION
Application year: 1990
Main classification: D21H19
IPC codes: D21H17