TITLE

PROCESS FOR REPULPING WET-STRENGTH BROKE BACKGROUND OF THE INVENTION

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 papermakmg 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.

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.

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.

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°C to about 70 ⁰ C. 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. Any persulfate salt, typically sodium persulfate, Na2S2θg, 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 US Patent 5,972,164 describes the repulping of wet-strength broke using a dry mix of a persulfate (e.g., the sodium salt, Na2S2θg) and base.

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.

US Serial Number 10/963,932 filed October 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. 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

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

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.

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".

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

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. 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.

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. 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°C to about 80°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. 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".

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. 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.

Within the temperature range of about 25 to about 80°C, most commercial processes are operated at about 6O ⁰ 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. The process of the present invention provides a method for the complete repulping of wet-strength broke containing resins. The process of the present invention provides a method for repulping wet-strength broke that requires much lower active halogen levels, or, in the case of mixtures of potassium monopersulfate and cyanuric acid, eliminates halogen use entirely. As noted > above, reaction products from the use of higher levels of active halogen are environmentally undesirable.

Progress in the repulping of wet-strength broke is measured in the industry by determining the Voith Index (from Voith Co., Heidenheim, Germany). The Voith Index provides a visual method for measuring the extent of repulping versus time, thus expressing the Voith Index as a function of repulping time provides an excellent measure of the repulping process. The Voith Index has values of from 1 (corresponding to about 41% of the paper not repulped) to 9 (corresponding to about 0.05% of the paper not repulped).

At equivalent active chlorine levels, anhydrous sodium dichloroisocyanate is the fastest chlorine-containing oxidant, with sodium hypochlorite slightly slower. In the industry, repulping times of 15 minutes or less (to a Voith index of 5) are acceptable, since other processing factors make shorter times of marginal value. For example, 3-bromo-l-chloro-5,5-dimethylhydantoin (Comparative Example H) provides the fastest rate among all compositions tested, but any marginal advantage of this fast rate is more than offset by the higher cost of the chemical. Extending the repulping to reach a Voith Index of 8 or 9, however, provides a sensitive measure of repulping speed. The lower cost of sodium hypochlorite makes it the industry standard.

Potassium monopersulfate alone, which is chlorine free, enables wet-strength broke repulping, but the repulping rate is significantly slower. The repulping speed of potassium monopersulfate/active halogen and potassium monopersulfate/cyanuric acid blends, however, shows an unexpected and non- linear relationship between blend composition and process rate. As the potassium monopersulfate concentration in a potassium monopersulfate/active halogen agent blend is raised from zero to about 80%, the process rate is only slightly slower than for the active halogen agent alone, and is as fast as sodium hypochlorite alone. Above 80% potassium monopersulfate, the rate slows, but even at 95%, the repulping rate is only marginally slower. The percent of potassium monopersulfate content in a potassium monopersulfate/active chlorine agent blend is a direct measure of the reduction in active chlorine. Thus the potassium monopersulfate/active halogen agent blends provide an effective means for the reduction of active chlorine without an adverse impact on the repulping rate. Substituting blends of potassium monopersulfate/sodium hypochlorite to replace potassium monopersulfate/active halogen agent does not provide a similar nonlinear effect of relative concentration on repulping rate.

Additionally, blends of potassium monopersulfate/ anhydrous sodium dichloroisocyanate are stable together, and thus can be premixed and stored under dry conditions, permitting pre-blending and prepackaging.

Sodium persulfate is known to be a slower repulping agent for wet strength paper compared with OXONE. Comparative Examples A (OXONE) and I (sodium persulfate) in Tables 1 and 2 demonstrate this difference. In contrast to

the monopersulfate mixtures of the invention, blends of sodium persulfate with anhydrous sodium dichloroisocyanate do not show the synergistic effects obtained with blends of OXONE and sodium dichloroisocyanate.

The process of the present invention is useful for repulping wet-strength broke containing resins at lower or no levels of halogen without significantly reducing the repulping rate. This provides the environmental advantage of reducing halogen emissions.

MATERIALS AND TEST METHODS The following materials and test methods were used in the examples herein.

Potassium monopersulfate as the stable mixed triple salt 2KHSO ₅ .KHSO ₄ .K ₂ SO ₄ and is available from E. I. du Pont de Nemours and Company, Wilmington DE, under the tradename OXONE. Anhydrous sodium dichloro-s-triazinetrione (ASDC) is available from

Aldrich (Milwaukee WI). Commercial quantities are available from Occidental Chemical Corporation (OxyChem, Dallas TX) and Shikoku Chemicals Corporation (Kagawa, Japan and Los Angeles CA).

Cyanuric Acid (CYA) and 3-bromo-l-chloro-5,5-dimethylhydantoin (BCDMH) are available from Aldrich (Milwaukee WI).

1, 3-Dichloro-5,5-dimethylhydantoin (DCDMH) is available from Alfa Aesar (Ward Hill MA).

Sodium persulfate (SPS) is available from J. T. Baker (Phillipsburg, NJ).

Test Method 1 - Repulping Procedure

A 4-gallon FORMAX MAELSTROM laboratory scale repulper (from Adirondack Machine Corp., Queensbury, NY) was equipped with a ¹ A hp (186 W) Dayton motor (from Dayton Electric Mfg. Co., Niles, IL). To the repulper was added BOUNTY paper towels (Proctor & Gamble, Cincinnati OH, 220 g, precut into 2" x 2" pieces) and 3.7 L of deionized water at 60 ⁰ C. With the agitator on, the pH was adjusted to pH 10.5 with a 10% NaOH solution. Immediately, the preweighed repulper chemicals (the monopersulfate mixtures of the invention)

were added to the repulper in an amount equal to 2%, or as otherwise specified in Table I ₅ of the dry paper weight loading.

At various times the extent of repulping or fiber separation was assessed using a modified Voith Speck Index Method (Voith Paper, Wet Lab Testing Procedure, VSTM 113.0, see below). The Method Modification was to combine steps 3.1 and 3.2 onto a single step, thus wet pulp (20 g) was mixed with water (500 mL) using a laboratory mixer for 10 s. Repulping procedures were terminated when a Voith Index of 9 was obtained, or at 120 min., whichever was the sooner. Test Method 2 - Voith Paper, Wet Lab Testing Procedure, VSTM 113.0.

Details are available from the Voith Company, Voith Company,

Heidenheim, Germany. In Table 1, where the repulping process was relatively slow (as in Comparative Examples C, E, F, and I) it was practical to sample the repulping bath at intervals to provide the repulping time in minutes required to reach any specific Voith Index. In such cases actual measured Voith readings versus time are shown in Table 1. When the repulping occurred faster, such timing was difficult. In these faster cases, the collected data at various times were subjected to second order polynomial regression analysis. This analysis provided a quadratic equation from which the Voith indices at exact times were calculated, together with the r^-value showing the "fit" between the data and the equation.

An r^-value of 1.0 denotes a perfect fit between data and equation. An r^- value of 0.95 or greater was accepted as an adequate fit for the purposes of evaluating Examples 1 - 9 and Comparative Examples A, B, D, G, and H. Voith readings in Table 1 are rounded to whole numbers.

EXAMPLES Example 1

Using a repulper as described in Test Method 1 above, a blend comprising OXONE Monopersulfate Compound (3.52 g, 80 wt. %) and anhydrous sodium dichloro-s-triazinetrione (0.88 g, 20 wt. %) was weighed and added to the repulper. At the time of addition, the repulper contained BOUNTY paper towels (Proctor & Gamble, Cincinnati OH, 22Og precut into 2" x 2" pieces) and 3.7L of

deionized water at 60°C and adjusted to pH 10.5 with a 10% NaOH solution. After adding the chemical, the pH was immediately readjusted to pH 10.5 if necessary. At various times the extent of repulping or fiber separation was assessed using a modified Voith Speck Index method (Voith Paper, Wet Lab Testing Procedure, VSTM 113.0) as described in Test Method 2 above. The performance results are presented in Table 1 below. It can be seen that excellent repulping was achieved in 15 minutes or less time.

Examples 2 - 5, Comparative Examples A - E and I

Various blends of OXONE monopersulfate compound and anhydrous sodium dichloro-s-triazinetrione (Examples 2 - 5) were prepared, used in the repulping process and evaluated as described in Example 1 with the component, grams added and weight percents as listed in Table 1. Controls of OXONE monopersulfate compound, 100 wt. % (Comparative Example A); anhydrous sodium dichloroisocyanate, 100 wt. % (Comparative Example B); and sodium persulfate, 100 wt. % (Comparative Example I) were processed and evaluated in the same manner. Comparative Example C contained no repulping aid. Comparative Example D contained sodium hypochlorite containing the active halogen equivalent of Comparative Example B. Comparative Example E contained a combination of OXONE (80wt. %) and sodium hypochlorite containing the active halogen equivalent of Example 1.

The sodium hypochlorite source (Hypo) contained 5.81% active halogen, while the sodium dichloro-s-triazinetrione source contained 60.63% active halogen.

In Comparative Example D, Hypo (45.91 g) replaced, on an equal active halogen basis, the ASDC (4.4 g) in Comparative Example B.

In Comparative Example E, Hypo (9.18 g) replaced, on an equal active halogen basis, the ASDC (0.88 g) in Example 1.

In Comparative Example I, sodium persulfate (4.4 g) replaced, on an equal weight basis, the OXONE (4.4 g) in Comparative Example A. These comparative Examples were processed and evaluated as in

Example 1.

The data in Table 1 showed that the repulping performance of all blends of OXONE monopersulfate compound with anhydrous sodium dichloroisocyanate (Examples 1-5) was improved versus that for OXONE alone (Comparative Example A) and a combination of OXONE and sodium hypochlorite (Comparative Example E) with equivalent active halogen content to that in

Example 1. At every stage of repulping, as judged by the Voith reading, the time for repulping is reduced using a blended composition of OXONE and anhydrous sodium dichloroisocyanate. Even at a low anhydrous sodium dichloroisocyanate content (e.g., 5 - 10%, Examples 2 and 3), excellent repulping was achieved in less or equal to about 37 to 32 minutes; a 16% to 27% time reduction for complete repulping (Voith = 9) versus the 100% OXONE control (Comparative Example A). Sodium persulfate (Comparative Example I) repulping times were approximately double the corresponding times for OXONE (Comparative Example A) on an equal weight basis. Examples 6 and 9 and Comparative Example F

Example 6 was a blend of 3.52 g OXONE monopersulfate compound and 0.52 g cyanuric acid, containing the same molar equivalent of cyanuric acid to that of anhydrous sodium dichloroisocyanate in Example 1. Example 9 was a mixture of 3 components, 3.52 g OXONE monopersulfate compound, 0.66 g anhydrous sodium dichloroisocyanate and 0.13 g cyanuric acid. The cyanuric acid was applied at the molar equivalent to that of sodium dichloroisocyanurate. Comparative Example F contained the molar equivalent of cyanuric acid (2.59 g) to that of anhydrous sodium dichloroisocyanate as in Comparative Example B. Using the molar equivalent of cyanuric acid to that of anhydrous sodium dichloroisocyanate provides a direct mole-to-mole comparison of the performance of cyanuric acid to anhydrous sodium dichloroisocyanate.

In Example 6, CYA (0.52 g) replaced, on an equimolar basis, the ASDC (0.88 g) in Example 1.

In Example 9, CYA (0.13 g) replaced, on an equimolar basis, one-quarter or 0.22 g of the ASDC in Example 1.

In Comparative Example F, CYA (2.59 g) has replaced, on an equal molar basis, the ASDC (4.4 g) in Example B.

These compositions were used in the repulping process of Example 1 and evaluated periodically as in Example 1 using Test Method 2. The resulting data is in Table 1.

The data in Table 1 showed that the repulping performance of Examples 6 and 9 was improved versus that for OXONE alone (Comparative Example A) as well as for cyanuric acid alone (Comparative Example F). Example 6 performed similarly to Example 3, while containing no active halogen component.

Examples 7 and 8 and Comparative Examples G and H

Example 7 was a blend of 3.52 g OXONE monopersulfate compound (80wt. %) and 0.79 g of dichlorodimethylhydantoin, such blend containing the same molar equivalent of anhydrous sodium dichloroisocyanate in Example 1. Example 8 was a blend of 3.52 g OXONE monopersulfate compound (80wt. %) and 0.97 g of bromochlorodimethylhydantoin, such blend containing the same molar equivalent of anhydrous sodium dichloroisocyanate as in Example 1. In Example 7, DCDMH (0.79 g) replaced, on an equimolar basis, the

ASDC (0.88 g) in Example 1.

In Example 8, BCDMH (0.97 g) replaced, on an equimolar basis, the ASDC (0.88 g) in Example 1.

In Comparative Example G, DCDMH (3.96 g) has replaced, on an equal molar basis, the ASDC (4.4 g) in Comparative Example B.

In Comparative Example H, BCDMH (4.84 g) has replaced, on an equal molar basis, the ASDC (4.4 g) in Comparative Example B.

These compositions were used in the repulping process of Example 1 and evaluated periodically as in Example 1 using Test Method 2. The resulting data is in Table 1.

It can be seen in Table 1 that Examples 7 and 8 performed better then OXONE monopersulfate compound alone (Comparative Example A). Thus, the halogenated hydantoins were identified as another source of active halogen for repulping.

Table 1

(a) For Comparative Example C, the Voith Index was 1.5 after 15 min.

(b) For Comparative Example E ₅ the Voith Index was 2.5 after 10 min.

(c) Abbreviations:

ND: Not determined; repulping was terminated after 120 min. OXONE: OXONE Monopersulfate Compound. ASDC: Anhydrous sodium dichloro-s-triazinetrione. Hypo: Sodium hypochlorite.

CYA: Cyanuric Acid.

DCDMH: 1, 3-Dichloro-5,5-dimethylhydantoin. BCDMH: 3-Bromo-l-chloro-5,5-dimethylhydantoin. SPS: Sodium persulfate. (d) Voith per minute readings were determined from regression analysis of the collected data as described in Test Method 2.

Table 1 shows that blends of OXONE/ASDC containing 80% or less by weight of OXONE (Examples 1, 4, 5) provided faster wet-strength repulping than Hypo itself (Comparative Example D) and were comparable with ASDC itself (Comparative Example B). Such OXONE/ASDC blends contributed markedly less active halogen than ASDC or Hypo. Higher ratios of OXONE in OXONE/ASDC blends (Examples 2, 3) had even lower active halogen contribution and still showed wet-strength repulping rates higher than OXONE itself (Comparative Example A). Conversely, a blend of OXONE/Hypo (Comparative Example E) did not show any synergistic effect.

Although CYA itself (Comparative Example F) provided a rate only marginally better than a control with no additive (Comparative Example C), blends of CYA with OXONE (Example 6) also showed a significant increase in wet-strength repulping rate compared with OXONE alone (Comparative Example A), and in OXONE/CYA there was still no active halogen contribution.

Blends of OXONE with DCDMH and BCDMH (Examples 7, 8), and blends of OXONE with ASDC and CYA (Example 9) also enhanced wet-strength repulping rates compared with OXONE alone (Comparative Example A). For instance, a blend of OXONE/DCDMH in a 3.52:0.97 weight ratio (Example 7) was as effective as DCDMH alone (Comparative Example G) and had but 27.5% of the active halogen.

Table 2 below represents a rearrangement of Table 1 showing the time to reach a Voith Index of 6 (a measure of repulping rate) and the relative active halogen content based on the sodium hypochlorite Comparative Example D.

Table 2

(1) Examples of the present invention denoted by numbers and Comparative Examples denoted by letters as described above.

(2) Abbreviations are the same as for Table 1.

(3) Data from Table 1.

The results in Tables 1 and 2 showed that the blends of OXONE with ASDC ₅ DCDMH, BCDMH, and CYA provided effective wet-strength repulping aids that sharply reduce or eliminate active halogen from the repulping process.