Process for repulping wet-strength broke

Abstract: A repulping process for wet-strength broke comprising a wet-strength resin and cellulosic fiber, said process comprising contacting an aqueous slurry of said broke with a mixture of 1) potassium monopersulfate as an oxidizing agent and 2) at least one of an active halogen agent, cyanuric acid, or a mixture thereof is disclosed. (end of abstract)

Agent: E I Du Pont De Nemours And Company Legal Patent Records Center - Wilmington, DE, US
Inventors: Jennifer Ann Boettcher, Min Chao, Thomas Peter Tufano
USPTO Applicaton #: #20070062661 - Class: 162191000 (USPTO)
Related Patent Categories: Paper Making And Fiber Liberation, Processes And Products, Reclamation, Salvage Or Reuse Of Materials, Broke Or Trim

Process for repulping wet-strength broke description/claims

The Patent Description & Claims data below is from USPTO Patent Application 20070062661, Process for repulping wet-strength broke.

Brief Patent Description - Full Patent Description - Patent Application Claims

BACKGROUND OF THE INVENTION

[0001] Paper is recycled via a process called repulping, wherein the cellulose fibers that comprised the original sheet are separated. These fibers can be cleaned, treated, redispersed, and prepared into a pulp slurry essentially similar to that used to make the original sheet. The normal papermaking process is then followed to form a sheet made from recycled fibers. The process of repulping involves mixing, under shear, in water. Chemicals may be added to accelerate the process; and elevated temperatures are often used.

[0002] Paper is made to provide specific functional properties. Chemicals are often added to impart and/or enhance these properties. Among the more widely used additives are wet-strength resins. These chemicals act to provide strength to wet paper and are used in, among other paper products, paper towel and packaging.

[0003] Repulping paper containing a wet-strength resin is difficult because the resin (such as a polyamide-epichlorohydrin resin) is added during paper production to enhance the strength of the paper produced so that the paper does not fall apart when used under wet conditions. The wet-strength resin binds the cellulose fibers together, impeding the repulping process of separating the cellulose fibers. Typically, paper treated with wet-strength resins will retain at least 15% of the dry strength of the paper when wet. Paper without wet-strength resin generally retains only 2 to 7% of its dry strength when wet.

[0004] Oxidation facilitates the breakdown of the wet-strength resin to permit separation of the cellulose fibers. Hypochlorite, particularly sodium hypochlorite, is typically used by paper mills in the repulping of wet-strength paper to oxidize the wet-strength resin to facilitate fiber separation. Hypochlorite oxidizes the wet-strength resins within a narrow, carefully maintained pH range and within a temperature range of from about 50.degree. C. to about 70.degree. C.

[0005] Environmental issues have been raised concerning the use of hypochlorite for repulping. These concerns relate to the formation of chlorinated organic compounds that are adsorbed by the pulp, chloroform emission, and the problem of adding chlorinated hydrocarbons to the effluent stream. For these reasons, non-halogen-containing compounds, such as persulfates have been used more recently to oxidize wet-strength resin during the repulping process.

[0006] Any persulfate salt, typically sodium persulfate, Na.sub.2S.sub.2O.sub.8, can be used. This material can also be used with alkali metal, alkaline earth metal, or ammonium salts of carbonate, bicarbonate or sesquicarbonate to enhance repulping performance. Mixtures of persulfate and carbonate, bicarbonate or sesquicarbonate can be used which exhibit substantially increased handling safety over persulfate alone. Gelman, et al., in U.S. Pat. No. 5,972,164 describes the repulping of wet-strength broke using a dry mix of a persulfate (e.g., the sodium salt, Na.sub.2S.sub.2O.sub.8) and base.

[0007] In general, however, systems employing persulfate salts are slower than hypochlorite or similar systems. Moreover, the introduction of additional heat, beyond that used with hypochlorite, is generally required for repulping suspensions employing such persulfate salts, in order to accelerate the degradation of the wet-strength resins to a commercially acceptable rate.

[0008] U.S. Ser. No. 10/963,932 filed Oct. 13, 2004, discloses compositions comprising a stable anhydrous mixture of an oxidizing agent and an active halogen agent wherein the oxidizing agent is potassium monopersulfate and the active halogen agent is an alkali metal salt of dichloro-s-triazinetrione, halogenated dimethylhydantoin, or mixtures thereof. These compositions are disclosed for use in sanitizing water, but use in repulping paper is not suggested.

[0009] It is desirable to provide a process for the repulping of wet-strength broke that reduces the active halogen required without significantly reducing the repulping rate. The present invention provides such a process.

SUMMARY OF THE INVENTION

[0010] The present invention comprises a repulping process for use with wet-strength broke comprising a wet-strength resin and cellulosic fiber, said process comprising contacting an aqueous slurry of said broke with a mixture of 1) potassium monopersulfate as an oxidizing agent, and 2) at least one of an active halogen agent, cyanuric acid, or a combination thereof.

DETAILED DESCRIPTION

[0011] Tradenames are shown herein in upper case. All compositions shown as percent are weight percent unless otherwise expressly specified. The invention relates to paper reprocessing, and more particularly to the repulping of wet-strength broke, and to a composition and process for the repulping of wet-strength paper.

[0012] The process of the present invention comprises contacting an aqueous slurry of wet-strength broke with an aqueous oxidizing composition comprising a mixture of 1) at least one oxidizing agent, which is potassium monopersulfate, and 2) at least one of (A) an active halogen agent selected from the group consisting of anhydrous sodium dichloroisocyanate, dichlorodimethylhydantoin, bromochlorodimethylhydantoin, or mixtures thereof, (B) cyanuric acid, or (C) mixtures of (A) and (B). Such oxidizing compositions are hereinafter termed "monopersulfate mixtures of the invention".

[0013] A commercial source of potassium monopersulfate is OXONE, available from E. I. du Pont de Nemours and Company, Wilmington, Del. The name "potassium monopersulfate" is commonly used in the trade to refer to the mixed triple salt 2 KHSO.sub.5.KHSO.sub.4.K.sub.2SO.sub.4, a crystalline salt of enhanced solid-state stability.

[0014] In the monopersulfate mixtures of the invention, the proportion of potassium monopersulfate is from about 1% to about 99%, preferably about 50% to about 98%, and more preferably from about 80% to about 95%. The remainder of the mixture, to make up to 100% total, is cyanuric acid, or at least one chemical selected from the group comprising anhydrous sodium dichloroisocyanate, dichlorodimethylhydantoin, and bromochlorodimethyl hydantoin. The oxidizing agent and active halogen agent or cyanuric acid are present at a weight ratio of oxidizing agent to active halogen agent or cyanuric acid of from about 98:2 to about 50:50.

[0015] The concentration of the monopersulfate mixtures of the invention in the aqueous slurry of the broke is from 0.5% to about 5% based on the dry weight of the pulp and preferably from about 1% to about 2% based on the dry weight of the pulp. The ratio of cyanuric acid to one or more of the active halogen sources is from 0:100 to 100:0.

[0016] Of particular interest is a monopersulfate mixture of the invention comprising potassium monopersulfate and cyanuric acid without an active halogen agent. The proportion of potassium monopersulfate is as described above, from about 1% to about 99%, preferably about 50% to about 98%, and more preferably from about 80% to about 95%. The remainder of the mixture is cyanuric acid. Such compositions contain no active halogen agent, an environmentally desirable characteristic.

[0017] The present invention provides a process for the improved repulping of wet-strength broke, comprising the use of the monopersulfate mixtures of the invention. The aqueous repulping process of the present invention is conducted at a temperature of from about 25.degree. C. to about 80.degree. C. The process is conducted at a pH of about 7 to about 11, and preferably about 10 to about 11. The pH is adjusted to this range with a suitable base, such as an alkali metal hydroxide, alkali metal carbonate, alkali metal bicarbonate, alkaline earth metal hydroxide, alkaline earth metal carbonate, or alkaline earth metal bicarbonate. Sodium hydroxide is preferred. The use of carbonates and bicarbonates limits the accessible pH range. Anhydrous and non-hygroscopic bases such as the carbonates and bicarbonates listed above may be premixed with the monopersulfate mixtures of the invention. However, premixing hygroscopic bases, such as the alkali metal hydroxides, is not recommended. As the pH value is lowered, higher operational temperatures in the range recited above are preferred.

[0018] The repulping process is conducted in a slurry contained and agitated in a mechanical repulper. The sequence of addition of the paper, the monopersulfate mixtures of the invention, and the base is optional. The components of the monopersulfate mixtures of the invention can be added separately or in a premixed form. The potassium monopersulfate and active halogen/cyanuric acid component may be added separately or in a premixed form. The base used for pH adjustment can be added before, with, or after the addition of the monopersulfate mixtures of the invention. However, a final pH adjustment after the paper and the monopersulfate mixtures of the invention have been added may be needed. Thus, addition of the base last is preferred. The liquid phase in which the repulping occurs is termed the "repulping bath".

[0019] The process of repulping paper to obtain recycled pulp fibers is carried out by any mechanical action that disperses dry pulp fibers into an aqueous pulp fiber suspension. Conditions for repulping, as well as equipment commercially used, are discussed in "Handbook for Pulp &Paper Technologists, Second Edition" by G. A. Smook, Angus Wilde Publications, 1992, Chapter 13, pp. 194-195 and 211-212. The repulping process and its control are known to those skilled in the art.

[0020] Other additives known to those skilled in the art may be added before, with, or after the addition of the repulping aids used in the present invention. Examples of other additives include but are not limited to ink particle collectors and removers, defoamers, biocides, complexing agents, fixation and conditioning agents, and surfactants.

[0021] Within the temperature range of about 25 to about 80.degree. C., most commercial processes are operated at about 60.degree. C. Faster repulping rates potentially allow operation at lower temperatures providing energy savings. Alternatively, faster repulping to higher Voith indices (greater than 5, indicating greater fiber separation) would reduce or eliminate the need for subsequent mechanical processing steps prior to preparing recycled paper from the repulped slurry.

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