JP2005097798A - Bleached kraft pulp having improved thermal color fading resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bleached kraft pulp having a whiteness of 82-90%, bleached by chlorine-free bleaching method without using elemental chlorine and hypochlorous acid and having a thermal color fading resistance comparable or superior to chlorine-bleached pulp. <P>SOLUTION: The chlorine-free bleached kraft pulp having improved thermal color fading resistance comparable or superior to conventional chlorine-bleached pulp having a Δb<SP>*</SP>value of 2.20 is produced by digesting a lignocellulose substance by kraft digestion method, subjecting the produced pulp component to alkali oxygen delignification treatment and washing and screening treatment, bleaching the pulp by chlorine-free bleaching method without using elemental chlorine and hypochlorous acid and comprising the 1st bleaching step with ozone or chlorine dioxide and the final bleaching step with chlorine dioxide or hydrogen peroxide under a condition to get bleached pulp having a whiteness of 82-90%, a Kappa number of ≤2.20 and a carboxy group content of ≤22.0 meq/100 g BD pulp. The produced chlorine-free bleached kraft pulp has a chromaticity difference (Δb<SP>*</SP>) of the International CIE system of ≤2.20 after the thermal color fading test at a hot air drier temperature of 105°C for 12 hours. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  In the present invention, lignocellulosic material is digested by Kraft method, then alkaline oxygen delignification and washing selection are sequentially performed, and then treated with a chlorine-free bleaching sequence that does not contain elemental chlorine and hypochlorite. It relates to chlorine-free bleached kraft pulp with improved thermal fading.

  The pulp obtained by digesting the lignocellulosic material by the Kraft method is subjected to alkaline oxygen delignification and washing selection in sequence, and then proceeds to the bleaching step. There are mainly three bleaching processes, the conventional chlorine bleaching method, the chlorine-free bleaching method (Elementary Chlorine Free: ECF), and the complete chlorine-free bleaching method (Totally Chlorine Free: TCF). In the chlorine bleaching method, pulp is bleached with a combination of chemicals such as elemental chlorine, hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide, and caustic soda. The use of chlorine has been avoided due to the recent increase in environmental protection, and there is also a concern that chloroform generated by the use of hypochlorite may adversely affect the human body. , Canada, Germany, Scandinavian countries, etc.) Since the regulation of organochlorine compound load is legally enforced and there are many cases where laws and regulations prohibiting the use of hypochlorite have been established, ECF and TCF bleaching, which does not contain chlorate, has become the main method. In that case, as chemicals commonly used, chlorine dioxide, hydrogen peroxide, ozone by ECF bleaching method Oxygen, caustic soda, etc. are chelating agents, hydrogen peroxide, ozone, oxygen, caustic soda, and peracetic acid in the TCF bleaching method. Is the current mainstream.

In order to reduce the cost of equipment when converting from chlorine bleaching to ECF bleaching, it is common practice to use the old equipment of chlorine and hypochlorite, thereby replacing the chlorine stage with the initial chlorine dioxide. stage (D ₀ stage substantially) changed stage hypochlorite stage has a hydrogen peroxide stage (P stage substantially) or chlorine dioxide stage (D stage approximately). Recently, there is also an example of introducing an ozone stage, which is located at the beginning or middle of the bleaching sequence.

  ECF bleached pulp with chlorine dioxide has a higher fading property during distribution and storage than that with chlorine bleaching, and it is generally considered that the fading property is due to heat rather than light. JP 2002-266271, 2003-96680 According to JIS P8206, the potassium permanganate value is 1.5 or less, the PC value is 10.0 or less, and in order to improve the thermal fading (80 ° C.) of ECF bleached pulp based on chlorine dioxide, It is said that hexeneuronic acid (abbreviated as HexA) needs to be 10 mmol / kg BD (BD: bone dry = absolutely dry) pulp or less.

Furthermore, JP-A-2003-105684 uses P stage, which is carried out on the acidic side (pH: 1.5 to 4.5) as an alternative to D ₀ stage in chlorine-free bleaching of kraft pulp after alkaline oxygen delignification. In addition, when the HexA of the pulp after the acid P stage satisfies the condition of 15 mmol / kg BD pulp or less, the thermal fading property of the bleached kraft pulp is improved.

JP 2002-266271 A

JP2003-96680 JP2003-105684A

  In order to obtain the potassium permanganate value (1.5 or less), PC value (10.0 or less), HexA (10 mmol / kg BD pulp or less) described in the above-mentioned publicly known Japanese Patent Application Publication, Although it has been shown that it is necessary to select conditions, there was no consideration for the quality of pulp that met these conditions, and the potassium permanganate value was the lignin index of the finished bleached pulp There is still a question of whether or not HexA is appropriate and whether HexA is the cause of thermal discoloration.

  This invention is made | formed in order to improve the above subjects in bleached kraft pulp (ECF bleached kraft pulp), and consists of the following means.

In the present invention, in improving the thermal fading of (ECF) bleached kraft pulp, pulp components obtained by cooking lignocellulosic material by kraft cooking method are subjected to alkaline oxygen delignification and washing and selection in sequence, It is the same as the conventional method for producing unbleached kraft pulp until it is treated (or not treated) in an acidic aqueous solution. In the case of ozone bleaching in the ECF bleaching process, the acid treatment is followed by a high-concentration ozone bleaching stage without a washing stage, and the pulp is not washed after the ozone treatment, but is moved to a weak alkaline extraction stage with a low pH and a short residence time. (See Japanese Patent Application No. 2003-308517) Or, in the case of chlorine dioxide bleaching, it is preferable to wash the pulp after the acid treatment, perform the first stage chlorine dioxide treatment, and then shift to the conventional high pH alkali extraction stage. In any case, the finished bleached pulp having a final whiteness of 82 to 90% is produced by the subsequent treatment with chlorine dioxide and hydrogen peroxide, and these ECF bleached pulps have a kappa number of 2.2 or less, When the carboxyl group is 22.0 meq / 100 g BD pulp or less, the difference in chromaticity b ^* values (Δb ^*) of the international CIE system before and after thermal bleaching was performed at 105 ° C. for 12 hours in a hot air dryer ^. ) Is 2.20 or less, and a chlorine-free bleached kraft pulp with improved thermal fading is obtained with Δb ^* = 2.20 or less of conventional chlorine-bleached pulp.

  Hereinafter, the present invention will be described in more detail.

  The potassium permanganate value described in the above patent document is generally a method of measuring the residual lignin of unbleached and semi-bleached pulp, and this value varies depending on the sample amount of the measured pulp and the amount of potassium permanganate added, As a result, since it is not an appropriate method for measuring the amount of residual lignin in the finished bleached pulp, the present invention uses the kappa number as an index of the residual lignin in the finished bleached pulp, and its known correlation equation is: Lignin (%) = Kappa number × 0.147. The kappa number measurement method is more accurate than the potassium permanganate number method because it adjusts the sample amount of the measurement pulp so that half of the added amount of potassium permanganate is consumed. The pulp technology association uses it as a standard method (Dence, CW, In “Methods in Lignin Chemistry”, Eds. Lin, SY, Dence, CW, Springer-Verlag, Berlin, 1992, p. 48-52).

  In recent years, it has been found that HexA consumes potassium permanganate, and the potassium permanganate value and kappa number include not only the residual pulp lignin index but also the HexA index (Gellerstedt, G. , Li, J., Carbohydrate Res., 194: 41 (1996); Yoichi Tomimura, Kengo Masashi, Mitsuo Ishihara, Shuji Hosoya, 1997 Paper Pulp Research Presentation, p. 90; Jiang, ZH, Van Lierop , B., Berry, R., Tappi J., 83 (1): 167 (2000)), the effect of potassium permanganate or kappa number on the thermal fading of the finished bleached pulp is dependent on the residual lignin in the pulp. And the effect of both HexA.

  4-O-methylglucuronic acid residue is present in the side chain of xylan, which is hemicellulose of lignocellulosic material, and undergoes a hydrolysis reaction during the cooking process of kraft pulp, and further undergoes a demethoxylation reaction. (Buchert, J., Teleman, A., Harjunpaa, V., Tenkanen, M., Viikari, L., Vuorinen, T., Tappi J., 78 (11): 125 (1995)). HexA reacts with ozone to produce oxalic acid and react with calcium in the pulp and liquor to produce calcium oxalate, which becomes a scaling problem for equipment. On the other hand, since there is an enol ether group in the HexA structure, HexA is simply hydrolyzed with sulfuric acid to give 2-furancarboxylic acid and 5-formyl-2-furancarboxylic acid. Various methods for quantitative determination of HexA have been developed based on these degradation substances, and chemicals that cause this hydrolysis reaction are enzymes (VTT method), sodium formate (HUT method), mercury acetate (KTH method) ((Tenkanen, M ., Gellerstedt, G., Vuorinen, T., Teleman, A., Perttula, M., Li, J., Buchert, J., J. Pulp Paper Sci., 25 (9): 306 (1999)), Sulfuric acid (Jiang, ZH., Audet, A., Sullivan, J., Van Lierop B., Berry, R., J. Pulp Paper Sci., 27 (3): 92 (2001)), mercury chloride and sodium acetate・ There is 3 moisture (Chai, XS., Zhu, JY, Li, J., J. Pulp Paper Sci., 27 (5): 165 (2001)).

  In the known JP 2002-266271, JP 2003-96680, and JP 2003-105684, HexA is quantified based on the HUT method, but the hydrolysis of the pulp is 110 ° C. for 5 hours (HUT method: 110 ° C., 1 hour), and 2-furancarboxylic acid and 5-formyl-2-furancarboxylic acid in the filtrate were not subjected to the HUT method 245 nm absorption method. Performance Liquid Chromatography: High-performance liquid chromatography) was used to determine the amount of HexA. The reason for selecting HPLC is that the absorption of lignin and other sugars, which are impurities, affects the absorption at 245 nm, resulting in a high HexA content (Ayano Kawamura, Kaori Hamaguchi, Makoto Iwasaki, Shinya Yamamoto, Tadashi Ishii, Masashi Horino, 2001 Paper Pulp Research Presentation Abstracts, p. 66). On the other hand, according to the authors who developed the HUT method (Tenkanen, M., Gellerstedt, G., Vuorinen, T., Teleman, A., Perttula, M., Li, J., Buchert, J., J. Pulp Paper Sci., 25 (9): 306 (1999)), because other substances do not interfere with UV (ultraviolet) absorption of HexA solution, and only 80-90% of HexA is degraded under the set hydrolysis conditions. The HexA content by the HUT method accounts for 75 to 86% of the result by the KTH method. For these reasons, in the examples of the present invention, the KTH method was selected as a method for measuring the HexA content (Gellerstedt, G., Li, J., Carbohydrate Res., 194: 41 (1996)).

  According to the latest ASTM (American Society for Testing and Materials) recommended method for thermal fading, when the thermal fading is performed at a temperature of 100 ° C., a residence time of 5 days, or a temperature of 90 ° C., a residence time of 14 days, and a glass tube. Although results equivalent to natural fading can be obtained (ASTM's Paper Aging Research Program, 2002, ASTM International, PA), there is no standard test method for thermal fading until this method exists. In JP-A-2003-96680 and JP-A-2003-105684, the bleaching property of the finished bleached pulp is measured under the conditions of a temperature of 80 ° C., a relative humidity of 65%, and a residence time of 48 hours. The thermal fading test was performed under the conditions of a hot air dryer temperature of 105 ° C. and a residence time of 12 hours.

The difference in whiteness before and after fading (ΔR457) or PC (Post color) value is a general known index of fading, but strength and whiteness are important quality items in paper, while chromaticity is also the quality of paper. It is an important quality item. Furthermore, bleached pulp and high-quality paper have a yellowish appearance when subjected to thermal fading, and the first stage chemicals (eg, chlorine dioxide, ozone, chlorine, etc.) in the bleaching sequence and the final stage Since the b ^* value of the finished bleached pulp often changes with the use of chemicals (for example: chlorine dioxide, hydrogen peroxide, etc.) while the final whiteness is the same, the present invention uses an index of thermal fading of the bleached pulp. The chromaticity b ^* difference (Δb ^* ) before and after thermal fading is used, where b ^* is from the International CIE L ^* a ^* b ^* system. In the present invention, the correlation between the CIE chromaticity b ^* value and the Hunter chromaticity b value is as follows.

CIE chromaticity b ^* value = Hunter chromaticity b value × 0.9429 + 0.0541
Correlation coefficient R ² = 0.9997
According to the known JP-A Nos. 2002-266271 and 2003-96680, it is a requirement item that the discoloration of a finished bleached pulp obtained from multi-stage chlorine-free bleaching without using elemental chlorine and hypochlorous acid is improved. Means for satisfying potassium manganate value (1.5 or less), PC value (10.0 or less), HexA (10 mmol / kg BD pulp or less) are: (1) Chlorine dioxide addition rate at initial chlorine dioxide stage is 0 0.5-1.5%, preferably 0.6-1.0% (vs. BD pulp) (paragraph [0021] of JP-A-2002-266271) and (2) HexA content of unbleached pulp in the cooking step 35 -45 mmol / kg BD pulp and kappa number 18-23 (paragraph [0055] of JP-A-2003-96680). The HexA content in the pulp depends on the chip type. For example, the HexA content of Portuguese eucalyptus globulae is 45-60 mmol / kg BD pulp. Furthermore, in the cooking process, the amount of HexA produced increases as the delignification reaction proceeds, but the cooking temperature (150 → 170 ° C.), An investigation result was found that the amount of HexA produced decreased with increasing liquid ratio (4 → 8) etc. (Pedroso, AI, Carvalho, MG, J. Pulp Paper Sci., 29 (5): 150 (2003); Chai, X.-S, Luo, Q., Yoon, S.-H, Zhu, JY, J. Pulp Paper Sci., 27 (12): 403 (2001); Daniel, AID, Neto, CL, Evtuguin, DV, Silvestre, AJD, Tappi J., 2 (5): 3 (2003)).

Therefore, in the present invention, regardless of the type of lignocellulosic material, the cooking method and conditions of kraft pulp, the copper value of unbleached pulp, the ECF bleaching sequence, etc., the thermal bleaching Δb ^* of the finished bleached pulp is 2.2 or less, It is found that when the copper is 2.2 or less and the carboxyl group is 22.0 meq / 100 g BD pulp or less, the thermal fading of the ECF bleached pulp is equal to or less than that of the conventional chlorine bleached pulp.

The comparative value of thermal fading Δb ^* in the present invention is the value (2.20) of the conventional chlorine bleached pulp containing chlorine (abbreviated as C) and sodium hypochlorite (abbreviated as H), and Δb of ECF bleached pulp. ^{* The} value will be compared with this comparison value.

In the following examples of the present invention, ozone (abbreviated as Z) or chlorine dioxide was used in the first stage as the ECF bleaching method, and chlorine dioxide or hydrogen peroxide was used in the last stage. In the range of whiteness of finished bleached pulp from 82 to 90% Hunter, ECF bleached pulp having a Δb ^* value of 2.20 lower than that of conventional chlorine bleached pulp, ie, improved thermal fading, A sequence using chlorine dioxide after acid treatment is adopted. The results are shown in Table 1. The details of the bleaching conditions are described below. Here, the horizontal bar (-) represents the washing stage, a represents pulp pH adjustment and metal ion removal treatment, A represents high-temperature acid treatment stage, and E represents alkali extraction. (See Japanese Patent Application No. 2003-308517) The conditions for the a-stage treatment are as follows: pulp concentration: 10%; pH: 2.7 (adjusted with 4N dilute sulfuric acid); temperature: 50 ° C .; The conditions for the stage treatment were pulp concentration: 10%; pH: 3.1 (adjusted with 4N dilute sulfuric acid); temperature: 90 ° C .; residence time: 150 minutes.

An ECF bleached pulp equivalent to a Δb ^* value of 2.20 of the conventional chlorine bleached pulp (Comparative Example 1 in Table 2) to be compared with the kappa number of 1.28 of the present invention. Since Example 8) in Table 2 has a kappa number of 2.18, it is found that thermal bleaching is improved if the ECF bleached pulp has a kappa number of 2.20 or less. The results are shown in Table 2.

In Table 2 above, the correlation coefficient (R ² ) between the HexA content of ECF bleached pulp and Δb ^* is slightly high at 0.7469, but the HexA content of ECF bleached pulp with Δb ^* value of 2.20 or less is 17.1 mmol. Since the HexA content of ECF bleached pulp having a Δb ^* value of 2.20 or more is 17.1 mmol / kg BD pulp or less (Comparative Examples 2 and 3) with respect to / kgBD pulp (Example 8), the HexA content is heat fading. It seems to be wondering if it will affect.

As described above, the kappa number includes not only the residual lignin index of the pulp but also the HexA content. The lower the kappa number, the larger the ratio of HexA in the kappa number (Chai, X.- S, Luo, Q., Yoon, S.-H, Zhu, JY, J. Pulp Paper Sci., 27 (12): 403 (2001)), while the correlation coefficient between the kappa number and Δb ^* (R ² ) Since 0.9689 is higher than the correlation coefficient between HexA content and Δb ^* (R ² = 0.7469), the correlation coefficient between residual lignin amount and Δb ^* is higher than the correlation coefficient between HexA content and Δb ^*. It was found to be expensive. That is, it is found that residual lignin has a greater influence on thermal fading than HexA.

Since HexA has a problem of whether it affects thermal fading, in the present invention, the carboxyl group of the finished bleached pulp was measured and the relationship with thermal fading was investigated. The carboxyl group includes HexA, 4-O-methylglucuronic acid, 4-O-methyliduronic acid, galacturonic acid, glucuronic acid, and the like. As shown in Table 3 below, Δb ^* value 2 The carboxyl group of ECF bleached pulp of 20 or less is 22.0 meq / 100 g BD pulp or less, and conversely, the carboxyl group of ECF bleached pulp of Δb ^* value of 2.20 or more is also 22.0 meq / 100 g BD pulp or more. It has been found that the thermal bleaching property of ECF bleached pulp should be equal to or lower than that of conventional chlorine bleached pulp (Δb ^* ≦ 2.20) as long as the carboxyl group of the finished bleached pulp is 22.0 meq / 100 g BD pulp or less. Is done. The correlation coefficient (R ² ) between the carboxyl group content and Δb ^* was 0.6617.

  As the lignocellulosic material used in the present invention, in addition to wood materials (hardwood, conifers, etc.), non-wood materials (for example, kenaf, bagasse, bamboo, firewood, etc.) can also be used. There are conventional conventional or improved kraft cooking methods, but the cooking solution is white liquor (main components: caustic soda and sodium sulfide) or orange liquor (polysulfide liquor) alone, or orange liquor at the first stage of cooking, and after the cooking stage. White liquor may be used.

  In the alkaline oxygen delignification and cleaning / selection process described in the specification of the present invention, conventionally known equipment and operation methods can be used. Alkaline oxygen delignification may be a conventionally known one-stage system or two-stage system (for example, Dualox, OxyTrac two-stage system, etc.).

  The ECF bleached kraft pulp with improved thermal fading according to the present invention described above has the following effects.

(1) In bleached kraft pulp with improved thermal fading according to the present invention, thermal bleaching was performed under the conditions of a hot air dryer temperature of 105 ° C. and a residence time of 12 hours in the range of a finished whiteness of 82 to 90%. When the difference between the chromaticity b ^* values before and after (Δb ^* ) is 2.20 or less, it is equal to or less than that of conventional chlorine-bleached pulp. The paper quality in the next papermaking process can be protected and the cost can be reduced.

(2) Moreover, when the copper number of bleached kraft pulp is 2.20 or less, the thermal fading (Δb ^* ≦ 2.20) of these pulps is improved, and long-term distribution and storage in warehouses are possible. This has the effect of reducing running costs.

(3) Furthermore, when the carboxyl group content of the ECF bleached kraft pulp is 22.0 meq / 100 g BD pulp or less, the thermal fading (Δb ^* ≦ 2.20) of these pulps is improved and the pulp can be used. The amount of lignocellulosic material (for example, hardwood, coniferous and non-wood chips) that is a raw material can be reduced, and there is an effect of protecting the global environment.

  Although the above-described ECF bleached kraft pulp has improved thermal fading, the problem is what happens to these pulps compared to conventional chlorine bleached pulp in terms of other qualities (for example, strength, etc.). Therefore, the pulp strength and the like were measured, and the results are shown below.

  The chemical addition rate in each of the following bleaching sequences is% by weight per BD pulp, and a horizontal bar (-) between each stage indicates a washing stage. Other than the high concentration Z stage, the other bleaching stages were carried out in polyethylene bags.

(Examples 1-3, 7)
Bleaching temperature, pulp concentration, reaction time for the sequence of (AZE) -D ₁ -D ₂ , (AZE) -DP, (AZE) -PD, AD ₀ -ED ₁ -D ₂ Details of the chemical addition rate are as follows.

(Example 1)
(AZE) -D ₁ -D ₂ bleaching (1) Pulp used: Sampling of mixed eucalyptus hardwood kraft pulp (abbreviated as LUKP) with an alkaline oxygen delignification and washing of the actual production line with a copper number of 11.7 Stored at 5 ° C. while using.

(2) Stage A (acid treatment stage): LUKP having a pulp concentration of 10.5% was prepared, and then the pH of the pulp slurry was adjusted to about 3.1 with a 4N sulfuric acid solution to adjust the final pulp concentration to 10.0%. . (Sulfuric acid addition rate: 1.10%, pH: 3.10) This pulp was treated in a constant temperature bath at 90 ° C. for 150 minutes, and then washed to a pulp concentration of 38.7% in a centrifuge without washing. Dehydrated. (Copper number: 8.0)
(3) Stage Z (high-concentration ozone treatment stage): The pulp with a concentration of 38.7% after stage A was placed in a 2 liter vertical flask at 60 g BD / batch and attached to a laboratory evaporator. 10% ozone gas was injected into the flask to react ozone and pulp (ozone addition rate: 0.46%). The high-concentration ozone treatment was performed in a constant temperature bath at 50 ° C. Ozone bleached pulp was collected and mixed without washing.

  (4) Stage E (weakly alkaline extraction stage): 0.9% of caustic soda was added to the 38.7% concentrated pulp after the Z stage, and the pulp was held at 70% for 30 minutes at a pulp concentration of 14%. The extracted pulp was washed with tap water (2 liter / 200 g BD pulp) and dehydrated and concentrated to a pulp concentration of 18%.

(5) D _1st stage (chlorine dioxide bleaching stage): 0.2% of chlorine dioxide was added to the pulp after stage E, and chlorine dioxide treatment was performed at a pulp concentration of 14% and 75 ° C. for 117 minutes. The pulp filtrate pH after the reaction was 5.5. The obtained pulp was washed with tap water (2 liter / 200 g BD pulp) and dehydrated to a pulp concentration of 18%.

(6) D ₂ stage (chlorine dioxide bleaching stage): D ₂ stage pulp concentration, treatment temperature, reaction time, filtrate pH after reaction, and pulp washing method are all the same as D ₁ stage, but chlorine dioxide Only the addition rate of 0.1% was low. As a result, a finished pulp by the (AZE) -D ₁ -D ₂ sequence was obtained.

(Example 2)
(AZE) bleaching -D-P sequence (1) (AZE) stage: Using been (AZE) -D ₁ -D ₂ sequence (AZE) stage pulp described above.

(2) D stage (chlorine dioxide bleaching stage): Pulp after D ₁ stage of the above (AZE) -D ₁ -D ₂ sequence was used.

  (3) P stage (hydrogen peroxide bleaching stage): 0.15% of hydrogen peroxide was added to the pulp after stage D, and after performing hydrogen peroxide treatment at a pulp concentration of 14% and 70 ° C. for 120 minutes, tap water The pulp was washed and dehydrated with water (2 liter / 200 g BD pulp) to obtain a finished bleached pulp. The pulp filtrate pH after stage P was 9.5.

(Example 3)
(AZE) bleaching with -P-D sequence (1) (AZE) stage: Using been (AZE) -D ₁ -D ₂ sequence (AZE) stage pulp described above.

  (2) P stage: It implemented on the conditions of P stage of said (AZE) -DP sequence, and the hydrogen peroxide addition rate was 0.55%.

(3) D stage: The conditions of this bleaching stage are the same as those of D ₂ stage of the (Aze) -D ₁ -D ₂ sequence, but the chlorine dioxide addition rate is 0.18%, and the obtained pulp Was washed and dehydrated with tap water to obtain a finished bleached pulp.

(Example 7)
Bleaching with _{A-D 0 -E-D 1} -D 2 Sequence (1) A stage: take the pulp after A stage of the described (AZE) -D ₁ -D ₂ sequence, were washed with tap water, Dehydrated to a pulp concentration of 18%.

(2) D ₀ stage: 0.366% chlorine dioxide is added to the pulp after stage A, the pH is adjusted to 3.5 with 4% pulp concentration, 4N dilute sulfuric acid, temperature 58 ° C., time 25 minutes After the treatment, the pulp was washed and dehydrated with tap water (2 liter / 200 g BD pulp).

(3) E stage (alkali extraction stage): 0.72% of caustic soda was added to the pulp after D ₀ stage, and the pulp concentration was 14% and kept at 70 ° C. for 66 minutes. The extracted pulp was washed with tap water and concentrated to a pulp concentration of 18%.

(4) D ₁ stage and D ₂ stage (chlorine dioxide bleaching stage): Pulp after E stage is the same as the conditions of D ₁ stage and D ₂ stage of (AZE) -D ₁ -D ₂ sequence described above was carried out, were each chlorine dioxide addition rate 0.06% (D ₁ stage) and 0.15% (D ₂ stages). The obtained pulp was washed and dehydrated with tap water to obtain a finished bleached pulp.

(Examples 4 to 6)
The bleaching temperature, pulp concentration, and reaction time of the sequences (aZE) -D ₁ -D ₂ , (aZE) -DP, and (aZE) -P-D are the same as those in Examples 1 to 3 above, and The processing stage a is described in paragraph [0021] above. The chemical addition rates of Examples 1 to 6 are shown in Table 4 below.

(Example 8)
A-D ₀ bleaching -E-D ₁ -P Sequence (1) A stage (acid stage): Creates a pulp concentration of 10.5 percent LUKP, then the pH of the pulp slurry with 4N sulfuric acid solution of about 3. 1 and the final pulp concentration was adjusted to 10.0% (sulfuric acid addition rate: 1.10%, pH: 3.2). This pulp was treated in a constant temperature bath at 90 ° C. for a reaction time of 150 minutes, washed with tap water, and dehydrated to a pulp concentration of 18%. (Kappa number: 8.1; original pulp copper number: 11.3).

(2) D ₀ stage: 0.335% of chlorine dioxide is added to the pulp after stage A, the pH is adjusted to 3.5 with a pulp concentration of 4.5% and 4N dilute sulfuric acid, and the temperature is 58 ° C. for 25 hours. After a minute treatment, the pulp was washed and dehydrated with tap water (2 liter / 200 g BD pulp).

(3) E stage: caustic soda was added 0.741% on D ₀ stage after concentration of 18% of the pulp, pulp consistency 14%, was held 65 minutes at 70 ° C.. The extracted pulp was washed with tap water (2 liter / 200 g BD pulp) and dehydrated and concentrated to a pulp concentration of 18%.

(4) D stage ₁ : 0.07% of chlorine dioxide was added to the pulp after stage E, and chlorine dioxide treatment was performed at a pulp concentration of 14% and 75 ° C. for 65 minutes. The pulp filtrate pH after the reaction was 5.5. The obtained pulp was washed with tap water (2 liter / 200 g BD pulp) and dehydrated to a pulp concentration of 18%.

(5) P stage: hydrogen peroxide was added 0.25% on pulp after D ₁ stage, pulp consistency 14%, was adjusted with caustic soda so that the initial pH 10.6, at a constant temperature bath at 75 ° C. Hydrogen peroxide treatment was performed for 60 minutes. The filtrate pH after the reaction was 9.3, and the treated pulp was washed with tap water and concentrated. Thus, to obtain a finished pulp by _{A-D 0 -E-D 1} -P sequence.

(Comparative Example 1)
Conventional chlorine bleaching by C-E-H-D sequence The treatment temperature, reaction time, and pulp concentration of C-stage, E-stage, and D-stage are the D of the A-D ₀ -E-D ₁ -P sequence of Example 8 above. _Although it is equivalent to ₀ stage (corresponding to C stage), E stage, and D ₁ stage (corresponding to D stage), the chemical addition rate is as follows. Chlorine in stage C: 1.13%; Caustic soda in stage E: 0.865%; Chlorine dioxide in stage D: 0.35%. For stage H, hypo treatment was performed under the conditions of treatment temperature: 75 ° C .; reaction time: 35 minutes; pulp concentration: 14%; hypo addition rate: 0.25%. The pulp after each treatment stage was washed with tap water.

(Comparative Example 2)
Bleaching with the aD ₀ -ED ₁ -D ₂ Sequence The a treatment stage is described in paragraph [0021] above. The pulp concentration, reaction temperature, and reaction time of the D ₀ stage, E stage, D ₁ stage, and D ₂ stage are the same as the conditions of the corresponding stages of the A-D ₀ -ED ₁ -D ₂ sequence of Example 7 described above. Similarly, the chemical addition rate is as follows. Chlorine dioxide in D ₀ stage: 0.961%; sodium hydroxide in E stage: 1.52%: chlorine dioxide by D ₁ stage: 0.06%; chlorine dioxide in D ₂ stage: 0.15%. The pulp after each treatment stage was washed and concentrated with tap water.

(Comparative Example 3)
Bleaching with the A-D ₀ -E-P-D ₁ sequence The A treatment stage is described in paragraph [0021] above. Other D ₀ stage, E stage, P stage, a pulp concentration of D ₁ stage, the reaction temperature, reaction time, and conditions of the corresponding stage of the _{A-D 0 -E-D 1} -P sequence described above in Example 8 Similarly, the chemical addition rate is as follows. Chlorine dioxide in D ₀ stage: 0.335%; sodium hydroxide in E stage: 0.741% hydrogen peroxide in P stages: 0.45%; chlorine dioxide by D ₁ stage: 0.238%. The initial pH and final pH of the P stage were 10.8 and 10.4, respectively. The pulp after each treatment stage was washed and concentrated with tap water.

(Comparative Example 4)
Bleaching by D ₀ -ED ₁ -D ₂ sequence (1) Pulp used: Mixed hardwood kraft pulp (LUKP) with eucalyptus and oak chips with a kappa value of 12.5 kappa from an actual production line subjected to alkaline oxygen delignification and washing (Omitted) was sampled and used.

(2) D ₀ stage: 0.57% chlorine dioxide is added to LUKP, the pulp concentration is 4.5%, pH is adjusted to 3.5 with 4N dilute sulfuric acid, and the temperature is 58 ° C. for 25 minutes. Then, the pulp was washed and dehydrated with tap water (2 liter / 200 g BD pulp).

(3) E stage: 1.13% of caustic soda was added to 18% density pulp after D ₀ stage, and the pulp concentration was 14% and held at 70 ° C. for 65 minutes. The extracted pulp was washed with tap water (2 liter / 200 g BD pulp) and dehydrated and concentrated to a pulp concentration of 18%.

(4) D stage ₁ : 0.06% chlorine dioxide was added to the pulp after stage E, and chlorine dioxide treatment was performed at a pulp concentration of 14% and 75 ° C. for 117 minutes. The pulp filtrate pH after the reaction was 5.4, and the obtained pulp was washed with tap water (2 liter / 200 g BD pulp) and dehydrated to a pulp concentration of 18%.

(5) D ₂ stage: D ₂ stage pulp concentration, reaction temperature and reaction time are the same as D ₁ stage, chlorine dioxide addition rate is 0.15%, and after bleaching, the pulp is washed and D ₀ -E- Completed pulp was obtained according to the D ₁ -D ₂ sequence.

(Comparative Example 5)
Bleaching by D ₀ -E-P-D ₁ sequence Pulp used: Same as Comparative Example 4.
D ₀ stage, E stage, pulp consistency of D ₁ stage, the reaction temperature and reaction time, D ₀ stage D ₀ -E-D ₁ -D ₂ sequence of Comparative Example 4, E stage, similarly to the _first stage D The P stage was treated under conditions of a pulp concentration of 14%, a reaction temperature of 70 ° C., and a residence time of 120 minutes. The chemical addition rate is as follows. Chlorine dioxide at D ₀ stage: 0.57%; Caustic soda at E stage: 1.13%; Hydrogen peroxide at P stage: 1.33%; Chlorine dioxide at D ₁ stage: 0.20%. The pulp after each bleaching stage was washed with tap water (2 liter / 200 g BD pulp).

(Comparative Example 6)
Bleaching by D ₀ -ED ₁ -P sequence Pulp used: Same as Comparative Example 4.
The pulp concentration, reaction temperature and reaction time in the D ₀ stage, E stage, D ₁ stage and P stage are the same as in the D ₀ -E-P-D ₁ sequence of Comparative Example 5, and the chemical addition rate is as follows. It is. Chlorine dioxide at D ₀ stage: 0.57%; Caustic soda at E stage: 1.13%; Chlorine dioxide at D ₁ stage: 0.08%; Hydrogen peroxide at P stage: 1.25%. The pulp after each bleaching stage was washed with tap water (2 liter / 200 g BD pulp) to obtain a finished pulp by the D ₀ -ED ₁ -P sequence.

(Comparative Example 7)
Bleaching by D ₀ -E-P ₁ -P ₂ sequence Pulp used: Same as Comparative Example 4.
The pulp concentration, reaction temperature and reaction time of the D ₀ stage, E stage, P ₁ stage and P ₂ stage are the D ₀ stage, E stage and P stage of the D ₀ -EPD ₁ sequence of Comparative Example 6 above. The chemical addition rate is as follows. Chlorine dioxide at D ₀ stage: 0.57%; Caustic soda at E stage: 1.13%; Hydrogen peroxide at P ₁ stage: 1.33%; Hydrogen peroxide at P ₂ stage: 2.00 %. The pulp after each bleaching stage was washed with tap water (2 liter / 200 g BD pulp) to obtain a finished pulp by the D ₀ -E-P ₁ -P ₂ sequence.

<Method for measuring pulp quality>
Each finished pulp is beaten with a PFI mill described in “JIS P8221-2”, and after measuring freeness with a Canadian standard freeness (freeness) described in “JIS P8121,” “JIS P8222” and “JIS P8223”. A handsheet at each freeness level was prepared by the method described, and then subjected to a paper quality test. The measurement quality items were kappa number, tear length, specific rupture degree, specific tear degree, folding resistance, whiteness and viscosity. For each intensity item, a correlation diagram between the measured value and the freeness was created, and the quality value at a freeness of 500 ml was determined.

  Copper number: The copper number was measured by the method described in “JIS P8211”.

  Breaking length: The breaking length was determined based on the method described in “JIS P8113”.

  Specific rupture degree: indicated as a specific rupture degree using the method described in “JIS P8112”.

  Specific tear degree: The specific tear degree was measured by the method described in “JIS P8116”.

  Folding strength: Folding strength was determined using the method described in “JIS P8115”.

  Whiteness: Hunter whiteness was determined based on the method described in “JIS P8123”.

  Viscosity: Pulp viscosity was measured in accordance with the method described in “Tappi Standards T230 om-89”.

<Other pulp quality measurement methods>
The measurement of b ^* , HexA, and carboxyl group shown in Tables 1 to 3 was performed by the following method.

  Hand-colored sheet for thermal fading: A hand-sheet for measuring whiteness was used according to “JIS P8123”.

  Thermal fading test: Each hand-sheet was subjected to a thermal fading test under the conditions of standing in a commercial hot air dryer in the vertical direction at 105 ° C. and a treatment time of 12 hours.

b ^* measurements of: was measured b ^* of the CIE system in the Macbeth Color-eye testing machine.

  Measurement of carboxyl group: The carboxyl group in the finished bleached pulp was measured based on the method described in “Tappi Standards T237 om-93”.

  Measurement of HexA: HexA in the finished bleached pulp was measured by the method described in detail in the following document. Gellerstedt, G., Li, J., “A HLPC Method for the Quantitative Determination of Hexeneuronic Acid Groups in Chemical Pulps”, Carbohydrate Res., 194: 41 (1996). HexA was measured with an ultraviolet spectrophotometer UV-265FS manufactured by Shimadzu Corporation.

In the present invention, Examples 1 to 8 having a finished whiteness of 82 to 90% in the range of Hunter are inferior to the conventional chlorine bleached pulp of Comparative Example 1 in that the viscosity of the pulp is inferior due to the use of ozone. Since the tearing, folding resistance, etc. are almost the same results, these finished bleached pulps have no problems in terms of physical properties, and have a kappa number of 2.20 or less and a carboxyl group content of 22.0 meq / 100 g BD pulp. Below, the difference (Δb ^* ) in chromaticity b ^* values before and after thermal bleaching under the conditions of a hot air dryer temperature of 105 ° C. and a residence time of 12 hours is 2.20 or less. It was found that the thermal fading property of ECF bleached kraft pulp was improved to be equal to or less than Δb ^* .

Claims (5)

Pulp components obtained by cooking lignocellulosic material by kraft cooking method are subjected to alkaline oxygen delignification and washing and selection in sequence, and with or without acid treatment stage to treat unbleached pulp in acidic aqueous solution. Bleaching in the range of 82-90% hunter of final whiteness by chlorine-free bleaching method containing no chlorine and hypochlorous acid, and thermal fading under conditions of hot air dryer temperature 105 ° C and residence time 12 hours A chlorine-free bleached kraft pulp with improved thermal fading, characterized in that the difference (Δb ^* ) between the chromaticity b ^* values of the international CIE system before and after the test is 2.20 or less.   The chlorine-free bleached kraft pulp with improved thermal fading according to claim 1, wherein the bleached pulp has a copper number of 2.20 or less.   3. The chlorine-free bleached kraft pulp with improved thermal fading according to claim 1, wherein the bleached pulp has a carboxyl group content of 22.0 meq / 100 g BD pulp or less.   The bleached pulp is improved in thermal fading according to claim 1, 2 or 3, wherein the first stage of the multi-stage chlorine-free bleaching sequence is ozone or chlorine dioxide, and the last stage is chlorine dioxide or hydrogen peroxide. Chlorine-free bleached kraft pulp. The chlorine-free bleached kraft pulp with improved thermal fading according to claim 1, 2, 3 or 4, characterized in that the gnocellulose material comprises hardwood chips, softwood chips or non-wood.