Repulp of wet strength paper

Repulping Wet-strenght Paper
For paper mills, waste paper fiber is an important source of raw materials, with economic advantages, paper mills can use fibril and secondary fiber copying and reuse paper and paper cut edges to reduce raw material costs. As the demand for secondary fibers increases, paper made using permanent wet strength resins such as polyamide polyamine-epichlorohydrin becomes an important fiber raw material. However, these papers, especially unbleached paper and waste corrugated cartons (OCC), are difficult to repulp. Sodium Hypochlorite is generally used for repulping of bleached wet strength papers because it cannot completely repulping unbleached wet strength papers and environmental regulations require paper mills to reduce the production of absorbable organic halogen (AOX) waste water produced, thus limiting the The use of sodium hypochlorite.
It has been reported that peroxydisulfate (M2S2O8) and potassium persulphate (active ingredient in trimeric salt 2KHSO5.KHSO4.K2SO4 is KHSO5) are effective agents for repulping of bleached wet strength paper. However, under the normal neutral or alkaline repulping conditions, this reagent is less effective for OCC wet strength paper and unbleached paper. We do not expect side reactions between oxidants and lignin to make unbleached paper difficult to re-slurry. An oxidant that breaks down into free radicals can extract hydrogen from lignin to form inactive radicals, leaving behind the largest amount of unoxidized The wet-strength resin with excess oxidizer can overcome the side reactions. However, this is expensive, inefficient, and has damage to the fiber.
After further study of the chemical properties of organic and inorganic peroxides, it was found to be important in the oxidation-reduction and polymerization processes. Under appropriate reaction conditions, the oxidant is activated by the metal or forms a radical by thermal decomposition. A large number of reactivated radicals can extract hydrogen or other atoms from saturated and unsaturated reactants, add to unsaturated reactants to combine with other radicals or polymerize into dimers, or undergo redox reactions. , including electron and ligand transfer processes. The reactants such as polyhexamethylene diamide (NyLon-6.6), a linear polyamide similar to the PAE backbone, are oxidized by H2O2 ions.
It has been reported that an oxidant catalyzed by a metal ligand undergoes a nucleophilic reaction with amines on the oxidized reactants by an oxidation reaction, cleaving the polypeptide. The reaction between hydroxyl groups in hydrogen peroxide and model compounds representing phenol and non-phenol nuclei in lignin was found to be dependent on the pH of the reaction. The above studies have shown that free radicals can effectively repulase unbleached wet-strength paper under conditions conducive to radical wet-strength resin reactions rather than free radical lignin reactions. In this study, inorganic oxidizers were used to test the repulping of wet strength paper. pH, time, temperature, rewetting agent, shear force and reagent concentration all had effects on repulping. The repulped pulp was screened with a vibrating screen having a gap of 0.15 mm, and the degree of repulping was quantitatively determined. In this study, dry tensile strength comparisons were made with repulped handsheets and base paper. In addition, the new repulping process performance was compared with alkaline unpigmented board repulping process in different paper mills. 1 Experiment 1.1 Re-pulping process 1.1.1 Alkaline process Industrial paper was cut into pieces of 1×1 in, diluted with tap water containing iron and copper of less than 0.15 ppm and 0.30 ppm to a concentration of 2%, and heated to Before the desired reaction temperature (70-90°C), the pH was adjusted to 11 with NaOH solution while gently stirring the pulp. If necessary, add the oxidizer and add NaOH to adjust the pH to 11. Then, the pulp was mixed at the reaction temperature for 60 min, sent to a fiber separator, and sheared at 3000 r/min for several minutes. The screen was screened for 20 min using a vibrating screen with a gap of 0.15 mm. The screen residue (non-distilled components) was dried at a temperature of 105°C. 1.1.2 Two-stage pH process The industrial paper was cut into 1×1 in pieces and diluted with tap water with iron and copper ion contents of less than 0.15 ppm and 0.30 ppm to a concentration of 2%. Optionally, a humectant may be added and the pH adjusted with a mineral acid, in the range of 3-7, while gently agitating, prior to heating at the desired reaction temperature (ie, 70-90°C). Add oxidizer and mix for at least 30 minutes at reaction temperature. The pH was then adjusted to 11 with NaOH solution and mixed for at least 30 min at the reaction temperature. The pulp mixture was sent to a bar mill and sheared at 3000 r/min for several minutes. A vibrating mesh sieve with a gap of 0.15 mm was used to screen for 20 min to obtain a paper sample, which was dried at 105°C. 1.2 Handsheet process The pH of the recovered pulp and puree (concentration 0.5%) was adjusted to 7 using a 8inMark IV type dynamic sheeter to make a handsheet of 4g in weight and rolled on the felt by hand. The wet paper sheet was pressed, dried on a 105°C Williams dryer for 10 minutes, aged in an oven for 10 minutes, and the dry tensile strength of the handsheets was measured using an Instron tester. 2 Discussion Paper samples from this study were taken from several paper mills in the United States and listed in Table 1. The paper sample contains PAE or polyamine-epichlorohydrin wet strength resin, and the amount of wet strength resin contained is provided by the factory. Paper samples C-F were also sized using 0.1% to 0.2% AKD. The (% wet/dry) wet strength of paper is greater than 20%. Table 1 Industrial paper used in the repulp study. All paper samples contained PAE wet strength resin, B used polyamine-epichlorohydrin paper paper wet strength agent, % unbleached paperboard A 0.6 unbleached surface. Cardboard B 0.4 Unflavored Paperboard C 1.0 Bleached Advertising Cardboard D 1.0 Unbleached Advanced Packaging Paper E 1.0 Unbleached Raisin Palette Paper F 1.0
The sample was treated with the reagent at a specific temperature and pH, placed in a sealed stainless steel container, and gently stirred with a three-blade propeller. Except that potassium persulfate (KHSO5 or MPS) is added as a trimeric salt, the oxidizing agent is active when added. During the repulping test, the stirring is carefully controlled to increase the performance of the reagents used in the process. After treatment, the pulp mixture (A-D) was cut in a rod fiber disintegrator at 3000 r/min for 3 min, and the paper samples (E, F) required 6 min. Paper samples E and F require greater shear force to completely disintegrate the fibers, probably due to the high content of AKD in the paper. 2.1 Alkaline repulping Bleached and unbleached paper containing permanent wet strength resins (PAE or polyamine-epichlorohydrin) is repulped at a pH of 11, despite the addition of various forms and amounts of oxidizing agents. The effect is still not good (see Table 2). The repulping of the unbleached paper B, using sodium hypochlorite as the oxidizing agent, requires at least 0.4% of the wet strength resin in the paper. Because the effect is very poor, it cannot be tested with sodium hypochlorite. The propoxylate-added linear alcohol (rewetting agent) reduces the surface tension of the repulping solution (from 60 to 28 dynes/cm), improving the wetting of the paper sample A, but this also does not improve the repulping effect. For bleached advertising cardboard containing 1% PAE resin and AKD sizing agent, the effect of the reagents used was also poor. Table 2 Alkaline pulping of bleached and unbleached paper containing PAE and polyamine-epichlorohydrin wet strength resin at a temperature of 70° C. After treatment with a reagent, shearing at 3000 r/min for 30 min, adding NaOH to adjust the pH To 11
Reagent Reagent Dosage, % pH Repulp Time, Min Repulp Rate, % Unbleached Paperboard A NaOH 7.0 11 60 50 Na2S4O8 1.0 11 60 66 KHSO5 1.0 11 60 66 KHSO5+0.1% 5.0 11 60 67 Add Moistening Unbleached board B NaOH 7.0 11 60 64 Na2S4O8 10.0 11 60 74 KHSO5 10.0 11 60 77 NaOCl 10.0 6 60 15 Unbleached board C NaOH 7.0 11 60 32 Na2S4O8 10.0 11 60 58 KHSO5 10.0 11 60 61 Bleached cardboard D NaOH 7.0 12 60 48 48 Na2S4O8 2.5 11 60 56 KHSO5 2.5 12 60 58 2.2 Two-stage pH repulping The paper (A-D) is repulped. At pH ≤7, the oxidizing agent is added and heated for 30 min before the pH is raised. Values ​​up to 11 and heating for 30 min, good pulping results (see Table 3). In any case, when the amount of oxidizer was reduced or the same, the repulping rate was high compared to alkaline (pH 11) repulping. Although lignin can be oxidized over a wide pH range, it can be seen from these results that, at low pH, it favors the oxidation of wet-strength resin (or oxidized cellulose) rather than oxidized wood. Prime reaction. When inorganic peroxides are used in wet-strength paper repulping, further analysis is required to determine the oxidized reactants (wet-strength resin, cellulose, or both) and the oxidation mechanism (free radical, nucleophilic, or electrophilic addition reaction). ). Table 3 Two-stage pH repulping, bleached and unbleached papers containing PAE and multistrand-epichlorohydrin wet strength resin, re-slurry at a temperature of 70°C, treated with low pH for 30 min before increasing the pH with NaOH, After treatment with reagent, cut at 3000r/min for 3min
Reagent Reagent Amount, % Process pH Re-Pulping Time, min Re-Pulling Rate, % Unbleached Noodles A Na2S4O8 1.0 4/11 60 79 KHSO5 1.0 4/11 60 75 H2O2 1.0 4/11 60 75 Unbleached Paperboard B H2O2 1.0 4/11 60 87 H2O2 5.0 4/11 60 87 KHSO5 10.0 4/11 60 88 Unflushed board C Na2S4O8 10.0 3/11 60 74 KHSO5 2.5 3/11 60 70 KHSO5 10.0 3/11 60 89 H2O2 10.0 3 / 11 60 84 Bleached advertising board D Na2S4O8 2.5 7/11 60 94 KHSO5 2.5 7/11 60 89 H2O2 2.5 7/11 60 88 2.3 Oxidation of wet strength resin 2.3.1 PAE
PAE resins contain secondary amines and quaternary amines that are easily oxidized. The oxidation of quaternary amines will be discussed in the next section. The oxidation of secondary amines (ie ammonium on the main chain of the PAE resin) is primarily via the free radical pathway. By a free radical attacking the α-C on the amide N atom, it can be activated by the electron pair of the N atom, followed by a resin decomposition reaction (C-N bond cleavage) to produce primary amines and aldehyde by-products. In order to confirm this reaction mechanism, an aqueous PAE resin solution for industrial use was reacted with Fenton's reagent at a pH of 4 and a temperature of 50°C. The aldehyde by-product in the amine structure was observed in the treated PAE solution by C13 nuclear magnetic resonance spectroscopy. And chemical shifts, which indicate that it is reasonable to generate oxidation by a free mechanism. 2.3.2 Polyamine-epichlorohydrin This resin contains oxidizable quaternary amines and secondary amine groups. The most likely oxidation pathway is a nucleophilic reaction. This reaction is achieved by the amine (i.e. peroxide cations or free radicals) on the electrophile in the resin, forming an amino oxide, followed by cleavage to give (chain) olefins and hydroxylamines. 2.4 Two-stage pH repulping with rewetting agent In the alkaline repulping process, the rewet does not affect the degree of re-pulping of the unbleached paper-like A, but in the two-stage pH process, the rewet can increase the total The repulping rate (see Table 4 and Figure 1). The completely moist paper can penetrate the oxidizer from outside to inside. The reactants can be more uniformly oxidized (PAE resin or cellulose), resulting in a higher repulping rate. The humectant is a surfactant that reduces the surface tension of the repulping fluid and reduces the time to wet the sheet. If the reweting agent is to be effective, the surface tension of the repulping fluid must be reduced to below 30 dynes/cm, and no foaming occurs under high shear forces. Typical rewet agents are linear or branched ethoxylate or propoxylate alcohols. Defoamers can also be used as rewetting agents, such as silicone-based latexes. Figure 1 Effect of rewet on pH repulping for two stages (2.5% MPS, 70°C). Table 4 In the two-stage pH repulping process, the unbleached paper containing PAE wet strength resin was treated with rewet. At a temperature of 70°C, the paper samples were treated at a lower pH for 30 minutes before the pH was increased with NaOH, and after the treatment with the reagents, the pulp was cut at 3000 r/min for 3 minutes.
Reagents Reagent amount for paper, % Rewetting for paper, % pH for re-slurry time, min Reslurry % Unbleached noodles A Na2S4O8 1.0 0.10 4/11 60 80 KHSO5 1.0 0.10 4/11 60 84 H2O2 1.0 0.10 4/11 60 82 H2SO4 9.0 0.10 4/11 60 70 NaOH Unbleached board B KHSO5 2.5 0.10 3/11 60 77 KHSO5 2.5 0.15 3/11 60 79 H2SO4 9.0 0.10 3/11 120 45 NaOH 2.5 Two-stage pH Process Optimum Process Conditions Figures 2-6 show the effect of oxidizer usage, process time, temperature, shear force, and the amount of wet strength resin in unbleached paper on the two-stage pH repulping process. These factors have a great influence on repulping. The optimum laboratory repulping conditions for the paper samples A-F are listed in Table 5. In the two-stage pH repulping process, higher repulping rates can be obtained with reasonable process conditions, times, and temperatures. These paper samples are re-pulped with an alkaline process. Even with the same process conditions (same oxidant content, time, temperature, and shear), the yield of repulping is much lower than that of the two-stage pH process repulping. Rate (see Table 6). Figure 2 Oxidizer

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