METHOD FOR PRODUCING POLYACRYLAMIDE COMPOSITION

Abstract

Disclosed is a method for producing an aldehyde crosslinked polyacrylamide composition useful for strengthening paper. The method includes steps, wherein a) an aldehyde crosslinker and polyacrylamide are mixed in water to form an aqueous solution of aldehyde crosslinked polyacrylamide prepolymer; b) acid is added to the aqueous prepolymer solution to adjust the pH of the solution to a value from 1 to 5; c) storing and transporting the prepolymer solution to the location where the final glyoxalated polyacrylamide composition will be used; and d) adding a base to the stabilized prepolymer solution to adjust the pH of the solution to a value from 5.5 to 12, and e) allowing the aldehyde crosslinker and polyacrylamide contained in the aldehyde crosslinked polyacrylamide prepolymer solution to react further and form aldehyde crosslinked polyacrylamide composition useful for strengthening paper.

Claims

1. A method for producing an aldehyde crosslinked polyacrylamide composition useful for strengthening paper, wherein the method comprises following steps: a. mixing aldehyde crosslinker and polyacrylamide in water to form an aqueous solution of aldehyde crosslinked polyacrylamide prepolymer, b. adding acid to said aqueous prepolymer solution to adjust the pH of the solution to a value from 1 to 5, and thus forming a stabilized aldehyde crosslinked polyacrylamide prepolymer solution, whereafter the stabilized aldehyde crosslinked polyacrylamide prepolymer solution has a solid content from 20% to 50%, c. storing said stabilized aldehyde crosslinked polyacrylamide prepolymer solution and transporting it to the location where the final aldehyde crosslinked polyacrylamide composition will be used, d. adding a base to the stabilized prepolymer solution to adjust the pH of the solution to a value from 5.5 to 12, and e. allowing the aldehyde crosslinker and polyacrylamide contained in the aldehyde crosslinked polyacrylamide prepolymer solution to react further and form aldehyde crosslinked polyacrylamide composition useful for strengthening paper.

2. A method according to claim 1, wherein the aldehyde crosslinker is formaldehyde, paraformaldehyde, glutaraldehyde or glyoxal.

3. A method according to claim 1, wherein the formed aldehyde crosslinked polyacrylamide composition is further diluted by the addition of water.

4. A method according to claim 1, wherein the formed aldehyde crosslinked polyacrylamide composition is directly added as a strength enhancer to fiber slurry at a paper mill, with or without the further dilution by water.

5. A method according to claim 1, wherein the formed aldehyde crosslinked polyacrylamide composition is stabilized by adding acid to said composition and adjusting the pH to value from 1 to 5, and storing said stabilized composition for later use as a strength enhancer, with or without the further dilution by water.

6. A method according to claim 5, wherein the formed aldehyde crosslinked polyacrylamide composition is stabilized by adding acid to said composition and adjusting the pH to value from 1.5 to 4.5, preferably to value from 2 to 4.

7. A method according to claim 1, wherein the pH adjustment during the stabilization is done with a combination acid and base addition.

8. A method according to claim 1, wherein the stabilized aldehyde crosslinked polyacrylamide prepolymer solution has a viscosity from 5 to 600 cps.

9. A method according to claim 1, wherein the steps d) or e) further comprises the addition of additional glyoxal and/or polyacrylamide to the solution.

10. A method according to claim 1, wherein a glyoxalated polyacrylamide composition useful for strengthening paper is prepared, and wherein the method comprises following steps: a. mixing aqueous solutions of glyoxal and polyacrylamide to form an aqueous solution of glyoxalated polyacrylamide (GPAM) prepolymer, b. adding acid to said aqueous prepolymer solution to adjust the pH of the solution to a value from 1 to 5, and forming a stabilized GPAM prepolymer solution having a solid content from 20% to 50%, c. storing said stabilized GPAM prepolymer solution and transporting it to a location where the final glyoxalated polyacrylamide composition will be used, d. adding a base to the stabilized GPAM prepolymer solution to adjust the pH of the solution to value from 5.5 to 12, and e. allowing the glyoxal and polyacrylamide contained in the GPAM prepolymer solution to react further and form glyoxalated polyacrylamide composition useful for strengthening paper.

Description

DESCRIPTION OF THE INVENTION

[0014] An object of the present invention is thus to provide a method to alleviate the above problems. The present invention provides a method that can be used to alleviate the shelf-life and gelling problems that are caused by premature crosslinking of the strength improving compositions. Especially the present invention is directed to method that lowers the shipping costs associated with this kind of paper making additives. The objects of the invention are achieved by a method which is characterized by what is stated in the independent claim. The preferred embodiments of the invention are disclosed in the dependent claims.

[0015] Accordingly the present invention provides as a first aspect a method for producing an aldehyde crosslinked polyacrylamide composition useful for strengthening paper.

[0016] The present invention is a combination of traditional off-site production of paper additives and on-site production. The typical prior art production methods for aldehyde crosslinked polyacrylamide compositions have been off-site methods, which means the crosslinked polyacrylamide compositions have not been produced at paper mill, but on the manufacturing plant of the chemical provider. The crosslinked polyacrylamide compositions have been prepared so that they are stable during shipping and storing, and therefore a significant dilution to as low as 8.0% active solids concentration has been needed.

[0017] In order to avoid the high shipping costs of a very dilute solutions of the additives, some on-site production methods has been disclosed more recently. In such methods the crosslinked polyacrylamide compositions have been produced at the paper mill from the starting materials. However, because paper mills often have limited space and personnel for on-site chemical production, a simplified on-site production approach is always favored by papermakers. The present invention provides a solution for this.

[0018] This invention provides a new on-site production method for aldehyde crosslinked polyacrylamide compositions. According to this method, aldehyde crosslinked polyacrylamide compositions are prepared in two stages. In the first stage, aldehyde crosslinker and polyacrylamide basepolymer are mixed to produce a concentrated prepolymer product with a solid content of at least 20%. The prepolymer can be a simple blend and stabilized under acidic conditions. Alternatively, aldehyde crosslinker and polyacrylamide basepolymer can react to a predetermined viscosity range under alkaline conditions before being stabilized by acid. In the second stage, the prepolymer is diluted by water and the aldehyde crosslinking reaction is reactivated by adjusting the solution to alkaline pH conditions. Once reaching a predetermined final viscosity range, the product can be applied to the paper machine directly with or without further dilution by water.

[0019] In more detail, the present invention provides a method for producing an aldehyde crosslinked polyacrylamide composition useful for strengthening paper, wherein the method comprises following steps: [0020] a. mixing aldehyde crosslinker and polyacrylamide basepolymer in water to form an aqueous solution of aldehyde crosslinked polyacrylamide prepolymer, [0021] b. adding acid to said aqueous prepolymer solution to adjust the pH of the solution to a value from 1 to 5, and thus forming a stabilized aldehyde crosslinked polyacrylamide prepolymer solution, whereafter the stabilized aldehyde crosslinked polyacrylamide prepolymer solution has a solid content from 20% to 50%, [0022] c. storing said stabilized aldehyde crosslinked polyacrylamide prepolymer solution and transporting it to the location where the final aldehyde crosslinked polyacrylamide composition will be used, [0023] d. adding a base to the stabilized aldehyde crosslinked polyacrylamide prepolymer solution to adjust the pH of the solution to a value from 5.5 to 12, [0024] e. allowing the aldehyde crosslinker and polyacrylamide contained in the aldehyde crosslinked polyacrylamide prepolymer solution to react further and form aldehyde crosslinked polyacrylamide composition useful for strengthening paper.

[0025] In the above disclosed method steps a) and b) form the first stage of the method, and steps d) and e) form the second stage of the method. Between these two stages the prepolymer is stored and shipped (transported) to a paper mill where it will be used. Storing may be conducted by using shipping containers, and thus, shipping and storing can also be understood to be a single procedure without any separate storing step.

[0026] For the purpose of the present invention aldehyde crosslinker refers to a compound containing one or more aldehyde (CHO) groups, where the aldehyde groups are capable of reacting with the amino or amido groups of a polymer comprising amino or amido groups as described herein. Exemplary aldehydes can include formaldehyde, paraformaldehyde, glutaraldehyde, glyoxal, and the like. Glyoxal is preferred.

[0027] For the purpose of the present invention polyacrylamide refers to a copolymer containing ionic monomers and acrylamide-containing monomers. Examples of suitable acrylamide-containing monomers include acrylamide and methacrylamide. Ionic monomers can be anionic or cationic. Examples of suitable anionic monomers include acrylic acid, methacrylic acid, methacrylamide 2-acrylamido-2-methylpropane sulfonate (AMPS), styrene sulfonate, and mixtures thereof as well as their corresponding water soluble or dispersible alkali metal and ammonium salts. Examples of suitable cationic monomers are diallyldimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, dimethylaminoethylmethacrylate sulfate, methatcrylamido-propyltrimethylammonium chloride, methacryloyloxyethyltrimethylammonium chloride, and mixtures thereof. Polyacrylamide basepolymers can be cationic, anionic, or amphoteric. The net charge of amphoteric basepolymers can be cationic, anionic, or neutral.

[0028] Polyacrylamide basepolymer refers to polyacrylamide used as a starting material in the present invention.

[0029] For the purpose of the present invention aldehyde crosslinked polyacrylamide prepolymer solution refers to an aqueous solution of aldehyde crosslinker and polyacrylamide basepolymer, which have been allowed to react to predetermined viscosity, which is lower than the viscosity of the final aldehyde crosslinked polyacrylamide obtained from ste e) of the present method, assuming that no further dilution of the composition by addition of water is made. It should be noted that the aldehyde crosslinked polyacrylamide that will be used at the paper mill might be diluted with the addition of water and such dilution would lower the viscosity of the composition.

[0030] Because aldehyde crosslinked polyacrylamide prepolymer solution is not stable as such, it has to be stabilized. In the present invention the stabilization can be done by adjusting the pH of the solution to a value from 1 to 5, and thus forming a stabilized aldehyde crosslinked polyacrylamide prepolymer solution. In an embodiment of the invention the pH of the solution is adjusted to a value from 1.5 to 4.5. Preferably the pH is adjusted to a value from 2 to 4.

[0031] For the purpose of the present invention aldehyde crosslinked polyacrylamide refers to the final reaction product, which is obtained from the further reaction of the aldehyde crosslinked polyacrylamide prepolymer solution. The prepolymer has been allowed to react to predetermined viscosity and thus forming aldehyde crosslinked polyacrylamide, which is the additive designed for improving paper dry strength and/or wet strength. An example of an aldehyde crosslinked polyacrylamide is glyoxalated polyacrylamide (GPAM), which is one of the preferred embodiments of the invention.

[0032] The inventive concept of the present invention is based on the realization that aldehyde crosslinked polyacrylamide prepolymer solutions can be stabilized by adjusting the pH to a value from 1 to 5 without the need to significantly dilute the solids concentration. Although the stability of aldehyde crosslinked polyacrylamide compositions has been intensively studied, no one had earlier thought that such stabilized prepolymers could be used to solve the stability problem and that their use would lead to a very simplified overall process.

[0033] An advantage of the present invention is that a concentrated aqueous solution of aldehyde crosslinked polyacrylamide prepolymer can be produced and stabilized, and then shipped to the paper mill. This reduces the costs related to shipping and intermediate storing remarkably. In the method of the present invention aldehyde crosslinker and polyacrylamide basepolymer can be mixed at a solid content as high as 50% and shipped to papermaking mills without gelling before carrying out the second step reaction.

[0034] In an embodiment of the present invention the stabilized prepolymer solution has a solid content from 20% to 50%. Preferably the solid content of the stabilized prepolymer solution is from 25 to 50% and more preferably from 30 to 40%.

[0035] In one embodiment of the present invention the stabilized prepolymer solution has a solid content from 20% to 50% and a viscosity of 5 to 600 cps. If not indicated otherwise, all viscosity values in this application have been measured at room temperature (22 C.) using Brookfield LVT viscometer.

[0036] It is an advantage of the present invention that the prepolymer can react to a predetermined viscosity value before being shipped to paper mills for the second stage reaction. This approach will reduce on-site reaction time and further simplify on-site production complexity. Preferably, the on-site production can be carried out in a continuous fashion with reduced reaction time.

[0037] It is another advantage of the present invention that the prepolymer can be shipped and handled as a single product. In comparison, crosslinker and polyacrylamide are shipped and handled separated in conventional on-site production approaches. This new inventive approach can eliminate the on-site mixing step and also reduces the number of storage vessels or alternatively can use these added facets to further improve the resulting product performance if appropriate.

[0038] The present invention is not limited to methods where the prepolymer is used alone as a single product in the second stage of the method. In an embodiment of the present invention it is also possible that the ratio of aldehyde crosslinker to polyacrylamide basepolymer is adjusted by adding additional aldehyde crosslinker and/or polyacrylamide basepolymer in the second stage of the inventive method. The amount of additional aldehyde crosslinker may be as high as 90% as calculated from the total amount of the aldehyde crosslinker in the composition after the addition. The amount of additional polyacrylamide basepolymer may be as high as 90% as calculated from the total amount of the polyacrylamide basepolymer in the composition after the addition. It is however preferred embodiment that further addition of aldehyde crosslinker and polyacrylamide basepolymer can be avoided in the second stage of the method.

[0039] In the second stage, during steps d) and e), the prepolymer is diluted by water and the aldehyde crosslinking reaction is reactivated by adjusting the solution to alkaline pH conditions or at least to pH values ranging from 5.5 to 12. Once reaching a predetermined final viscosity range, the product can be applied to the paper machine directly with or without further dilution by water. Preferred viscosity ranges are 10 to 100 cps for an 8% final product using a Brookfield LVT viscometer.

[0040] In an embodiment of the invention, the method comprises a further step, wherein the formed aldehyde crosslinked polyacrylamide composition from step e) is further diluted by addition of water. The solid content of the composition after this additional dilution step can be from 1% to 25%, preferably from 4% to 20%, more preferably from 7% to 15%.

[0041] In another embodiment of the invention, the method comprises a further step, wherein the formed aldehyde crosslinked polyacrylamide composition from step e) is directly added as a strength enhancer to fiber slurry at a paper mill, with or without the further dilution by water.

[0042] In an embodiment of the invention, the method comprises a further step, wherein the formed aldehyde crosslinked polyacrylamide composition from step e) is stabilized by adding acid to said composition and adjusting the pH to a value from 1 to 5, preferably to a value from 1.5 to 4.5, more preferably to a value from 2 to 4, and storing said stabilized composition for later use as a strength enhancer, with or without the further dilution by water. In one embodiment of the invention the formed aldehyde crosslinked polyacrylamide composition from step e), which may optionally be further diluted and/or stabilized, has a solid content of 1 to 20% and a viscosity of 5 to 100 cps.

[0043] In the present invention the pH adjustment during the stabilization can be done with a combination of acid and base addition. Preferred acids include sulfuric acid, hydrochloric acid, hydrobromic acid, phosphoric acid, nitric acid, formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, oxalic acid, lactic acid, malic acid, citric acid, carbonic acid, and adipic acid. Preferred bases include sodium hydroxide, potassium hydroxide, lithium hydroxide, cesium hydroxide, barium hydroxide, ammonium, magnesium hydroxide, aluminum hydroxide, sodium silicate, sodium carbonate, sodium bicarbonate, lime, and calcium hydroxide. Combination of acid and base may be used to provide a puffer to the solution, which further stabilizes it.

EXAMPLES

[0044] Raw materials 40% glyoxal aqueous solution was obtained from BASF. The cationic polyacrylamide sample was a 40% aqueous solution of copolymer of acrylamide and diallyldimethylammonium chloride (DADMAC).

Examples 1-8

Prepolymers by Blending Glyoxal and Polyacrylamide

[0045] In these examples, glyoxal and polyacrylamide were mixed and stabilized under low pH conditions. Conventional GPAM products often build up excessive viscosity under storage. In many cases, they gelled completely and became un-usable when stored over an extended period of time, especially at elevated temperatures. The objective of this invention is to ensure that glyoxal/polyacrylamide blends remain fluid during the shipping period. Consequently, the second stage of on-site reaction can be carried out successfully.

[0046] Table 1 presents product viscosity change as a function of time at an elevated temperature of 35 C. Product solid contents were adjusted by adding additional de-ionized water. Final pH was adjusted using 10% sulfuric acid solution or 10% sodium hydroxide solution. After 8 days of aging, all samples remained fluid. The viscosity value of 40% blend increased only from 68 cps to 96 cps and the viscosity value of 25% blend remained almost unchanged at around 15-17 cps. This study shows that glyoxal and polyacrylamide can be mixed at a solid content as high as 40% and shipped to papermaking mills without gelling before carrying out the second stage reaction.

TABLE-US-00001 TABLE 1 Examples 1-8, stability as a function of time at 35 C. Gly- Viscosity (cps) PAM oxal Water Solid Day Day Day Day Day Ex. (g) (g) (g) content pH 0 4 8 10 21 1 77 23 0 40% 2.5 68 74 96 2 77 23 0 40% 3.5 68 75 96 3 67.3 20.2 12.5 35% 2.5 34 35 NA 46 61 4 67.3 20.2 12.5 35% 3.5 34 34 NA 46 60 5 57.7 17.3 25 30% 2.5 23 23 NA 25 29 6 57.7 17.3 25 30% 3.5 22 22 NA 24 27 7 48.1 14.4 37.5 25% 2.5 15 16 NA 16 17 8 48.1 14.4 37.5 25% 3.5 15 16 NA 16 17

Examples 9-12

Prepolymers by Reacting Glyoxal and Polyacrylamide to a Pre-Determined Viscosity Value

[0047] In these examples, glyoxal and polyacrylamide were mixed and pH was increased to 9.0 using the dilute NaOH solution. The weight ratio of glyoxal to polyacrylamide was adjusted at 0.23. The solid content was 23%. Once the solution viscosity increased to a pre-determined value, dilute sulfuric acid was added to quench the reaction and stabilize the product. As shown in Table 2, all samples were stable for more than one week at 35 C. Additionally, the shelf life of prepolymer was also a function of final viscosity and final pH. Low viscosity and lower pH led to longer shelf life.

TABLE-US-00002 TABLE 2 Examples 9-12 stability as a function of time at 35 C. Initial Final Solid Final viscosity viscosity Days Ex. content pH (cps) (cps) to gel 9 23% 2.5 10.3 15.6 29 10 23% 2.5 10.3 20.4 22 11 23% 2.5 10.3 28.1 14 12 23% 3.5 10.3 15.7 9

Examples 13-14

GPAM

[0048] Examples 13 and 14 were GPAM products with the same composition and properties. However, Example 13 was prepared by the conventional one-step reaction, whereas Example 14 was prepared by the two-step reaction. The objective of this study was to investigate whether the two-step approach would affect product strength performance or not.

[0049] For Example 13, glyoxal, polyacrylamide, and water were first added to a glass vessel and mixed using a magnetic stirring bar. The weight ratio of glyoxal to polyacrylamide was set at 0.3 and the solid content was set at 11.5%. pH was then adjusted to 9.0 to initiate the reaction. Once the viscosity value reached 21 cps, dilute acid was used to stabilize the product. For Example 14, glyoxal, Example 11 (prepolymer), and water were added to a glass vessel and reaction was initiated by adjusting solution pH to 9.0. Once the viscosity value reached 21 cps, dilute acid was stabilize the product.

[0050] Table 3 compares wet tensile strength performance between Example 13 and Example 14. In summary, these two products provided similar initial wet tensile and also permanent wet tensile. This result indicates that the two-approach reaction has no significant impact on GPAM strength properties.

TABLE-US-00003 TABLE 3 Wet tensile strength Initial wet tensile Permanent wet tensile (lb/in) (lb/in) Blank 0.65 0.18 Example 13 1.62 0.73 Example 14 1.56 0.71