Repulping paper and paperboard

Abstract: A composition for repulping a cellulose fiber paper broke in an aqueous slurry comprising a mixture of a non-chlorinating oxidizing agent and a water-soluble buffering salt that is capable of maintaining the Ph between about 7 and about 12 in the reaction mixture of the wet-strength broke and the oxidizing agent, the use of the composition in a process for repulping the paper broke, particularly for repulping paper containing wet strength resins, and the process for repulping the paper broke.

Description: The present invention relates to repulping paper and paperboard products. Paper and paperboard waste materials, commonly called "broke," are difficult to repulp without special chemical treatment when they contain wet-strength resins, such as polyamide-epichlorohydrin resins, which are widely used in the manufacture of paper and paperboard because in the absence of such agents, paper products normally retain only 3 to 5% of their dry strength after being wetted. Wet-strength resins allow paper to retain 10 to 50% of its dry strength when wet. This property is essential in paper and paperboard grades such as toweling, sanitary tissue, coffee filters and milk cartons. Wet-strength resins are added to paper and paperboard at the time of manufacture. Polyamide-epichlorohydrin wet-strength resins are disclosed in US-A-No. 2,926,116 and are commercially available from Hercules Incorporated as KYMENE® wet-strength resins. US-A-3,427,217 (Miller, 2/1969) teaches the use of oxidizing salts such as sodium hypochlorite, ammonium persulfate, and others to repulp wet-strength broke containing resins such as polyaminopolyamide-epichlorohydrin resins, urea-formaldehyde and melamine-formaldehyde resins. Schmalz, in [ Tappi , vol 44, no. 4, pp. 275-280 (April, 1961)], also teaches the repulping of polyamide-epichlorohydrin resin-containing broke with hypochlorite, and alternatively with strong alkali. However, the alkali method is relatively slow, and although the hypochlorite salts effectively repulp wet-strength broke, they can form environmentally undesirable organochloride-containing degradation products in effluents from the process. Espy and Geist, in [ Proceedings, TAPPI Papermakers Conference , Washington, DC, April, 1990, pp 147-149], describe the repulping of polyamide-epichlorohydrin resin-containing broke with persulfate salts, and disclose that the process is more rapid at high Ph values produced by adding a strong alkali such as sodium hydroxide. However, during the repulping operation, the pH originally established by the strong alkali can fall as far as the acid range, slowing the further course of repulping. JP-A-56020692 (Honshu Paper Manufacturing KK) discloses adding to aqueous paper pulp inorganic or organic oxidizing agents, including hydrogen peroxide, ammonium persulphate, potassium persulphate, sodium perborate, sodium nitrite, sodium nitrate, sodium chlorate, sodium chlorite, butyl hydroperoxide, peracetic acid and perbenzoic acid. Preferably phosphate is also added, including sodium tripolyphosphate, sodium pyrophosphate and sodium hexametaphosphate. The amount of the oxidizing agent added is 0.01-1 wt.% and the amount of phosphate added is 0.01-1 wt% based on dry waste pulp. There is a need for a process for repulping wet-strength broke, particularly paper products composed of cellulose fibers that contain a wet-strength resin such as a polyamide-epichlorohydrin resin or its equivalent, by using a non-chlorinating oxidizing agent and ensuring continuously efficient repulping. According to the invention, a composition for repulping a cellulose fiber paper broke in an aqueous slurry, comprising a non-chlorinating oxidizing agent and an alkali as defined in the present claim 1. Preferably the composition is further characterized in that it comprises a mixture of the non-chlorinating oxidizing agent and the alkali in an aqueous solution or as a dry blend. In the dry blend, preferably the amount of the oxidizing agent is from 10 to 95% by weight and the amount of the buffer is from 5 to 90% by weight. The repulping composition is further characterized in that the buffering salt is a water-soluble ammonium, alkali metal, or alkaline earth metal carbonates ans phosphates and optionally their conjugate acid salts conjugate acid salt. A further preferred composition comprises one of said salts of a dibasic or tribasic inorganic acid as the buffering salt, in combination with from 0 to about 95 mole % of its conjugate acid salt. Also according to the invention, a process for repulping a paper product composed of cellulose fibers comprises slurrying the paper in an aqueous solution of a non-chlorinating oxidizing agent and an alkali as defined in the present claim 5. The more preferred buffering salts yield a pH between about 8 and about 11, and include sodium and/or potassium carbonate and/or carbonate-bicarbonate salts, phosphate-monohydrogen phosphate or monohydrogen phosphate-dihydrogen phosphate (as their sodium and/or potassium salts), boric acid-borax, or borax-sodium borate, and the like, particularly a combination of carbonates and bicarbonates. The most preferred buffering salt is sodium carbonate. The oxidizing agent is an organic hydroperoxide, an organic peracid, hydrogen peroxide, or an ammonium, alkali metal, or alkaline earth metal salt of persulfuric acid or monopersulfuric acid. The tendency of the aqueous medium to become more acidic increases the need for the buffering capability to enhance the speed of the reaction, and therefore the amount of buffering salt added should be sufficient to maintain the desired alkaline pH as the repulping proceeds. Preferably, the pH should be about 10 to achieve lower repulping times. Wet-strength resins susceptible to repulping degradation by the persulfate-buffer combinations of the invention include polyamide-epichlorohydrin resins such as those described in US-A-2,926,116 and 2,926,154, and US Reissue Patent 26,018; polyamine-epichlorohydrin resins such as bishexamethylenetriamine-epichlorohydrin, C 6 -diamine-epichlorohydrin resins; polyamine-epoxyalkyl organosulfonate and polyamide-epoxyalkyl organosulfonate resins such as those described in US-A-5,082,527. Examples of the preferred non-chlorinating oxidizing agents include the organic hydroperoxides t-butyl hydroperoxide and cumene hydroperoxide, and the organic peracids peracetic acid and perbenzoic acid. Among the ammonium, alkali metal and alkaline earth metal salts of persulfuric acid and mono-persulfuric acid, sodium persulfate is the most preferred. Buffering salts with an effective pH between 7 and 12 are listed in standard laboratory handbooks, and can be readily chosen by chemists skilled in the art. If buffers consisting of a combination of dibasic or tribasic acid salts and their conjugate acid salts are used, such as carbonate-bicarbonate, phosphate-monohydrogen phosphate, monohydrogen phosphate-dihydrogen phosphate, etc, they may be prepared directly from a salt of the dibasic or tribasic acid and a conjugate acid. Alternatively, they may be formed in situ by partial neutralization of the salt of the dibasic or tribasic acid with the decomposition products from the repulping process, and/or by another acid, such as a mineral acid like sulfuric acid, phosphoric acid, or boric acid, or even by exposure to atmospheric carbon dioxide. Conversely, they may also be formed in situ by partial neutralization of the conjugate acid, such as bicarbonate salt, a monohydrogen phosphate salt, borax, etc., with a strong alkali, such as an alkali metal hydroxide or an alkaline earth metal hydroxide. Preferred buffers are carbonate and the carbonate-bicarbonate systems. The particularly preferred buffer, sodium carbonate, is commonly known as soda ash. The proportion of non-chlorinating oxidizing agent to buffer salts normally ranges from 95:5 to 10:90 by weight, and depends on operating conditions. For example, in laboratory screening experiments where paper slurry concentrations are relatively dilute, e.g. about 1.0 to 1.5%, higher proportions of buffer may be used to maintain the pH of the relatively large amount of water present; that is, the ratio of oxidizing agent to buffer will be lower. In commercial equipment where higher slurry concentrations are common (from about 3% to about 15%), the proportion of oxidizing agent to buffer salts may be high. Preferably the proportion of oxidizing agent to buffer is from about 90:10 to about 35:65, and most preferably it is from about 85:15 to about 75:25. Pulp concentrations are those used in normal commercial practice in available commercial defibering and repulping equipment, ranging from about 1% to about 20%. In industrial practice concentrations between about 3% and 15% are usually preferred. Operating temperatures range between about 50°C and about 95°C in vented equipment at atmospheric pressure and between 95°C and about 120°C in pressurized equipment. Temperatures above about 70°C are preferred because they produce shorter reaction times. The amount of non-chlorinating oxidant used will depend on the kind of pulp used in the paper, the kind and amount of wet-strength resin employed, and the degree of wet strength, and can be readily determined by routine experiment. Generally, it will be from 0.5% to 10% by weight, preferably from 1% to 8% and most preferably from 1% to 5%, based on the dry weight of paper to be repulped. Ordinarily in commercial practice, the amount of buffering salt is between 0.01% and 0.25% by weight, based on water in the paper slurry, preferably between 0.02% and 0.1%. For a given grade of wet-strength paper or paperboard, the optimum amounts of oxidizing agent and buffer salt(s) can be determined by experiment, using the usual criteria of speed and cost. The wet-strength paper or paperboard to be repulped, water, non-chlorinating oxidizing agent, and inorganic buffer salts may be added in any convenient order. The oxidizing agent and buffer salts can be added to the water either as separate solutions, premixed in solution, separate dry ingredients, or a dry blend. It is convenient to adjust the water Ph, and to dissolve the oxidizing agent and buffer chemicals, before adding the broke, in order to facilitate good dispersion. A preferred process mode of addition, for convenience in operation, is to add a stable solution or a dry blend of oxidizing agent and buffer salt. Such combinations may comprise one of the oxidizing agents disclosed above and one or more inorganic buffer salts yielding an alkaline Ph. Preferred examples of dry blends include sodium carbonate or a mixture of sodium carbonate or potassium carbonate and sodium bicarbonate or potassium bicarbonate, blended with sodium persulfate. Most preferred is a blend of sodium carbonate and sodium persulfate. The time required for complete defibering of the wet-strength paper depends on the kind of pulp comprising the paper, the type and amount of wet-strength resin, the wet strength of the paper product, and the aging and/or heat curing history of the paper. Defibering of paper is considered complete when the paper has disintegrated to a suspension of individual fibers in water, free of knots or bundles of fibers. Combinations of non-chlorinating oxidizing agents and alkaline buffers can sometimes be helpful in dispersing paper that does not contain a wet-strength resin, but of course, most grades of paper without wet-strength resins are relatively easily dispersed in water by mechanical forces alone. The preferred use of the oxidizing agents and buffers of this invention is in wet-strength grades of paper, which means it has broad industrial application in the paper industry. The following procedures, preparations and examples illustrate the practice of the invention. Unless otherwise indicated all quantities given are in parts by weight. REPULPING PROCEDURE Repulping tests were carried out according to TAPPI method T205 OM-88. The apparatus is a cylindrical reaction vessel 15.2 cm diameter and 19.2 cm high, with spiral baffles on the walls, and furnished with a three-bladed propeller agitator. A 20-g sample of wet-strength paper, cut into 2.5-cm square pieces, is suspended in 1.5 liters of heated water. The reagents are added, and the mixture is agitated at 2800 rpm with heating. Small samples of the slurry are withdrawn after 5, 10, 20, 30, 40, 50, and 60 minutes. The progress of repulping is judged by comparison with standard samples at different stages of dispersion. Progress is expressed on a scale of 1 to 6, where 1 signifies un-attacked paper and 6, complete defibering. In the examples, times in minutes refer to reaching stage 6 - complete defibering. PREPARATIONS A-D Wet-strength paper was made from a 50/50 blend of bleached hardwood and bleached softwood kraft pulps, refined to about 500 Canadian Standard freeness, on a Noble-Wood handsheet machine, in water containing 100 ppm calcium hardness and 50 ppm alkalinity, at pH about 7.5. To the proportioner of the machine was added 0.5% (as resin solids based on dry fiber) of a commercial polyamide-epichlorohydrin resin, known under the trade name KYMENE® 557H wet-strength resin. Handsheets were dried for about 40 sec. at about 115°C, and were aged about 2 weeks before testing. PREPARATIONS E-J AND L-N Wet-strength paper was made from a 70/30 blend of bleached hardwood and bleached softwood kraft pulps, refined to about 500 Canadian Standard freeness, on a continuous sheet former, in water containing 100 ppm calcium hardness and 50 ppm alkalinity, at pH about 7.5. To the machine chest of the machine was added 0.5% (as resin solids based on dry fiber) of a commercial polyamide-epichlorohydrin resin, known under the trade name KYMENE® 557H wet-strength resin. The paper was dried over a series of driers between about 83°-89°C to between about 4% and about 5% moisture content. Paper was aged about 2 weeks before testing. PREPARATION K Paper in this example was made similarly to that in Preparations E-J and L-M, except that the wet-strength chemicals added to the machine chest comprised 0.35% by weight of dry fiber of KYMENE® 557H wet-strength resin and 0.15% of sodium carboxymethylcellulose (CMC) having a degree of substitution = 0.7 (commercially available as Aqualon CMC-7M). PREPARATION O Paper in this example was made similarly to that in Preparation K, except that the wet-strength chemicals added to the machine chest comprised 0.25% by weight of dry fiber of Kymene® 557H wet-strength resin and 0.10% of sodium carboxymethylcellulose (CMC) having a degree of substitution = 0.7 (commercially available as Aqualon CMC-7M). EXAMPLE 1 This example is a series of laboratory tests that were run to repulp Preparations A-O. Table 1 summarizes the combination of oxidant and buffering salt used and the results obtained. Preparation A through J illustrate the operability of various buffers, using various samples of wet-strength paper made at different times. Preparation K through M illustrate faster repulping in the presence of buffers, in paired tests with the same batch of paper. Preparation M also shows that the oxidant-buffer combination is superior to buffer alone, as well as to oxidant alone. Preparation N and O illustrate the use of hydrogen peroxide in the presence of buffer with two different wet strength chemical combinations. COMPARISON A Using the procedure of Example 1 paper containing KYMENE® wet-strength resin was repulped in 5 minutes at 60°C except that 3 percent by weight sodium hypochlorite was used. As a result the repulped paper contained an objectionable organic chloride content. EXAMPLE 2 A dry blend was prepared by mixing 0.52 g potassium persulfate with 0.86 g sodium carbonate and 0.91 g sodium bicarbonate. This produced a powdered mixture that was added to 20g shredded paper in 1.5 liter water in a high shear laboratory mixer. The paper contained a KYMENE® wet-strength resin. The repulping mixture was maintained at a temperature of 59°C for 20 minutes. It was observed that the paper was repulped (redispersed). EXAMPLE 3 Example 2 was repeated with a dry blend containing 0.52 g potassium persulfate, 0.86 g sodium carbonate and 0.91 g sodium bicarbonate. Using a temperature of 71°C the paper was redispersed in 10 minutes at a pH of 9.5. EXAMPLE 4 Example 2 was repeated except that ammonium persulfate was substituted for sodium persulfate. Satisfactory repulping was obtained. EXAMPLE 5 Experiments can be run similar to Examples 1-4 except using sodium, potassium or ammonium persulfate as from 50 to 90% and sodium carbonate or a mixture of sodium carbonate/sodium bicarbonate as from 10 to 50% of either a concentrated solution or dry blend to be added in an amount of from 0.5 to 5.0% by weight solids based on the weight of the paper being repulped. Depending on the paper being repulped from 20 to 60 minutes high shear agitation at 60 to 80°C will be required. These experiments illustrate it is possible to use any of the described combinations for repulping. The use of sodium persulfate and sodium carbonate would be preferred on the basis of lower cost when used as either a dry blend or in the form of a concentrated solution. EXAMPLE 6 Experiments can be run similar to Example 4 except that sodium persulfate is used as the oxidizing agent in the dry blend. Inorganic salts suitable other than sodium carbonate and sodium carbonate sodium bicarbonate are: trisodium phosphate, borax, sodium borate, potassium carbonate, potassium bicarbonate and disodium monohydrogen phosphate. Suitable buffering activity can be obtained in the pH 8 to 11 range as long as the amount of inorganic salt in the dry blend does not exceed 50% by weight along with the remainder being persulfate. EXAMPLE 7 A dry blend was prepared with 0.60 g OXONE® (DuPont Trademark) potassium monopersulfate and 3.40 g sodium carbonate and 3.60 g sodium bicarbonate. Using the procedure of Example 1 a laboratory sample was repulped in 10 minutes at a temperature of 71°C. EXAMPLE 8 An experiment was run similar to Example 7 except that sodium persulfate available from FMC was used in place of DuPont OXONE® potassium monopersulfate. Similar results were obtained. EXAMPLE 9 Example 8 was scaled up to repulp a commercial size batch. Results similar to laboratory experiments were obtained. This illustrates how effectively the invention can be put into practice once operational parameters have been established on a smaller trial scale.

Claims: A composition for repulping a cellulose fiber paper broke in an aqueous slurry comprising a non-chlorinating oxidizing agent and an alkali, characterized in that the oxidizing agent is an organic hydroperoxide, an organic peracid, hydrogen peroxide or an ammonium salt, alkali metal salt, or alkaline earth metal salt of persulfuric acid or monopersulfuric acid; the alkali is a water-soluble buffering salt that is capable of maintaining the pH between about 7 and about 12 in the aqueous reaction mixture of the wet-strength broke and the oxidizing agent and the buffering salt is selected from the group consisting of ammonium, alkali metal and alkaline earth metal carbonates and phosphates and optionally their conjugate acid salts. A composition as claimed in claim 1, further characterized in that it comprises a mixture of the non-chlorinating oxidizing agent and the alkali in an aqueous solution. A composition for repulping a paper product as claimed in claim 1 or 2, further characterized in that the buffering salt is a water-soluble ammonium, alkali metal, or alkaline earth metal carbonate or phosphate, or one of said salts in combination with its conjugate acid salt. A composition for repulping a paper product as claimed in claim 3, further characterized in that the buffering salt is one of said carbonates or phosphates in combination with from 0 to about 95 mole % of its conjugate acid salt, and the said buffering salt is capable of maintaining the pH between about 8 and about 11 in the aqueous reaction mixture of the wet-strength broke and the oxidizing agent. A process for repulping a paper product composed of cellulose fibers, comprising slurrying the paper in an aqueous solution of a non-chlorinating oxidizing agent and an alkali, characterized in that the oxidizing agent is an organic hydroperoxide, an organic peracid, hydrogen peroxide or an ammonium salt, alkali metal salt, or alkaline earth metal salt of persulfuric acid or monopersulfuric acid; the alkali is a water-soluble buffering salt that maintains a pH of from 7 to 12 during the repulping operation and the buffering salt is selected from the group consisting of ammonium, alkali metal and alkaline earth metal carbonates and phosphates and optionally their conjugate acid salts. A process as claimed in claim 5, further characterized in that the process comprises dissolving the oxidizing agent and the buffering salt in water, adding the paper to the aqueous solution to form a slurry, and agitating the slurry. A process as claimed in claim 5 or 6, further characterized in that the buffering salt is one of said carbonates or phosphates in combination with from 0 to about 95 mole % of its conjugate acid salt, and the said buffering salt maintains a pH between about 8 and about 11 during the repulping operation. A process for repulping a paper product as claimed in claim 5 or 6, or 7, further characterized in that the buffering salt is a sodium or potassium carbonate with or without a sodium or potassium bicarbonate salts, or a sodium or potassium phosphate-monohydrogen phosphate or monohydrogen phosphate-dihydrogen phosphate. A process for repulping a paper product as claimed in any one of claims 5 to 8, further characterized in that the oxidizing agent is hydrogen peroxide or an ammonium salt, alkali metal salt, or alkaline earth metal salt of persulfuric acid or monopersulfuric acid. A process for repulping a paper product as claimed in any one of claims 5 to 9, further characterized in that the oxidizing agent is present in an amount of from about from about 0.5% to about 10% by weight, based on the dry weight of paper to be repulped. A process for repulping a paper product as claimed in claim 10, further characterized in that the oxidizing agent is present in an amount of from about from 1% to 8%. A process for repulping a paper product as claimed in any one of claims 5 to 11, further characterized in that the amount of the buffering salt is between 0.01% and 0.25% by weight, based on water in the paper slurry. A process for repulping a paper product as claimed in claim 12, further characterized in that the amount of the buffering salt is between 0.02% and 0.1% by weight, based on water in the paper slurry. A process for repulping a paper product as claimed in any of claims 5 to 13, further characterized in that the proportion of non-chlorinating oxidizing agent to buffering salts is from 95:5 to 10:90 by weight in a slurry containing from 3% to 15% of paper pulp. A process for repulping a paper product as claimed in any one of claims 5 to 14, further characterized in that the cellulose fibers are bonded together by a wet-strength resin. A process for repulping a paper product as claimed in claim 15, further characterized in that the wet-strength resin is a polyamide-epichlorohydrin resin. A process for repulping a paper product as claimed in any one of claims 5 to 16, further characterized in that the temperature of the slurry is maintained at from about 20 to about 120°C.

Inventors: ESPY HERBERT H, ESPY, HERBERT H.
Applicants: HERCULES INC, HERCULES INCORPORATED
Application year: 1993
Main classification: D21C5
IPC codes: D21C5, D21C3