BLEACHING GROUNIDWOCI) PULPS WITH IHYPOCIILOPITES

Transcription

1 BLEACING GROUNIDWOCI) PULPS WIT IYPOCIILOPITES February 1949 INFORMATION REVIEWED AND REAFFIRMED March 1956 No UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison 5, Wisconsin In Cooperation with the University of Wisconsin

2 BLEACING GROUNDWOOD PULPS WIT YPOCLORITES-l'a BY RALP M. KINGSBURY, Chemist FORREST A. SIMMONDS, Chemist and EARLE S. LEWIS, Physical Science Aid Forest Products Laboratory, 2 Forest Service U. S. Department of Agriculture am,.,n.1 Summary Bleaching experiments were made at the Forest Products Laboratory on groundwood pulps from 13 hardwoods, 6 softwoods, and also a mixture of 6 hardwoods alone and in a mixture with Eastern white pine, as occur naturally on Northeast farm wood lots. The results show that calcium hypochlorite is a satisfactory bleaching agent for the hardwood pulps, but probably is not satisfactory for the softwood pulp. In general, the brightness of the hardwood pulps, individually and in mixtures, was increased to the range of 7 to 79 percent with 1 percent available chlorine. Upon exposure to carbon arc light, the recession in brightness ranged from 4 to 11 points, Bleached pulp yields were 98 to 1 percent of the unbleached pulp. The only consistent effect of the bleaching on drainage and strength properties was an increase in tensile strength. In order to get a good bleaching effect with hypochlorite, its tendency to react very rapidly with groundwood pulp must be retarded considerably, especially at the start of the reaction. This was done by use of low density, low temperature, and a high alkalinity, with the latter being the most critical of the three. The lime requirement for control of alkalinity ranged from 3 to 6 percent, including the free lime in the bleaching liquor. Substitution of sodium silicate for a part of the lime in the bleaching of -Presented at TAPPI Mechanical Pulping Conference, Poland Spring ouse, Poland Spring, Maine, Sept , 1948.?The results included in this report are taken in part from an investigation undertaken with funds furnished under the Research & Marketing Act, -Maintained at Madison 5, Wis., in cooperation with the University of Wisconsin. Report No, R1736

3 some pulps further improved brightness up to 5 points. Final addition of sulfurous acid, or equivalent, to the stock had a similar effect. Although the hue of hypochlorite-bleached pulps was in the yellow range, in the case of hardwood pulps, it is believed to be light enough to be generally satisfactory. Experimental results indicated that pulps from mixtures of hardwoods can be bleached satisfactorily with hypochlorite and that the brightness of the mixtures will be proportional to that obtainable on pulps from the individual woods. Although the Eastern white pine pulp did not respond satisfactorily to hypochlorite, a 5-5 mixture of it and the mixed hardwood pulp was bleached to about 7 brightness with 1 percent available chlorine. Essentially the same result was obtained, however, by blending bleached mixed hardwood pulp with unbleached pine pulp. This procedure would be the more economical of the two. Introduction The use of local supplies of pulpwood is advantageous not only to the pulp manufacturer but also to any farmers of those regions owning stands of timber. owever, supplies of wood available on farm wood lots within easy reach of the mills, are, in many instances, of poor quality or not the kinds ordinarily used. If it can be shown how to produce satisfactory pulp from these species and from mixtures of them as they occur, the marketability of farm-lot wood will be increased. For example, these woods can be used to a greater extent for the production of groundwood pulps when practical ways of bleaching the pulps become available. The purpose of this work was to develop bleaching procedures suitable for groundwood pulps made from a variety of individual woods and from mixed species. The mixed species used was representative of certain stands on farm woodlots in the Northeast, The requirements for a process for bleaching groundwood pulps are that its chemical cost will be low, that it will effect an appreciable increase in brightness that will not recede too rapidly, that it will not develop a strong yellow hue, and, finally, that it will cause only a small loss in weight of pulp by chemical action. It appears that no one bleaching process meets these requirements for all goundwood pulps. The most promising agents for the commercial bleaching of groundwood pulps are hydrosulfites, peroxides, and hypochlorites (1, 4, 6, 7). Important Factors in the Use of ypochloritez The rate of reaction between calcium or sodium hypochlorite and groundwood with 1 percent available chlorine is normally extremely rapid, For satisfactory result in any instance, the density, temperature, and alkalinity must be adjusted so that the rate of reaction will be definitely retarded at the start. It appears, however, that the alkalinity is the most critical of the three variables. In general, density should not exceed 6 percent, and Report No

4 no apparent.benefit was observed for lower values. Temperature does not need to exceed 3 C., and alkalinity should be equivalent initially to the P range of 11 to 12 and at the end of the reaction, to not less than p 8. Examples of the influence of alkalinity and of the use of sodium silicate in the buffer system are given in a previous publication (4). Depending on the pulp, the use of sodium silicate as a part of the buffer system will give a somewhat better brightness than otherwise. It further appears that the reaction should be stopped a little short of complete chlorine consumption in order to realize the maximum brightness. The addition of sulfur dioxide at the end of the reaction is usually beneficial to brightness, but it does not improve its stability. Effects of Bleaching Brightness The brightness of the hardwood pulps reported. on here was increased from 8 to 23 points with calcium hypochlorite at the level of 1 percent available chlorine. It is shown in table.1 that those increases resulted in brightness values of 7 to 79 percent for the bleached pulps. In some instances, with, for example, the sugar maple and sweetgum pulps, sodium hypochlorite gave from 4 to 8 points higher brightness values than calcium hypochlorite. In general, however, calcium hypochlorite gave higher brightness values for the hardwood ground pulps than did sodium perioxide under the conditions used. Pulps made from two different lots each of quaking aspen, paper birch, yellow-poplar, and sugar maple woods were studied to note possible variations within species. Although little difference was observed between the lots of aspen and paper birch, the results obtained for the yellow-poplar and sugar maple pulps indicate that variation within some species is to be expected. The experience with the two lots of yellow-poplar was especially interesting. The fresh pulp from lot II in its natural condition did not respond to hypochlorite nor to sodium peroxide. When the pulp was extracted with alcohol, however, either before or after treatment with 1 percent chlorine as calcium hypochlorite, a brightness increase of 7 points resulted. The alcohol removed a yellow-colored material. After a portion of the unextracted pulp at about 25 percent density had been in storage about 5 days at 5 C., it did not respond to hypochlorite, but another portion at the same density after storage at room temperature during the same period did respond to the extent of 11 points in brightness. No difficulty was experienced in bleaching a pulp from a mixture of hardwoods comprising 29 percent each of red maple and paper birch, 16 percent each sugar maple and yellow birch, and 5 percent each of white ash and American beech by weight. On a volume basis, the percentage values were essentially the same as on the weight basis because of the similarity of the specific gravities. The optimum conditions for calcium hypochlorite bleaching were the same as for the other hardwood pulps. Uhen a brightness value was calculated for the mixture on the basis of direct proportionality from the experimental brightness values for the bleached pulps from the individual Report Not 4.736

5 woods, the calculated value agreed with the value determined experimentally. Although pulps from two of the component woods, white ash and American beech, contained numerous shives that were not bleached satisfactorily, there was no evidence of unbleached shives in the bleached pulp from the mixture. The brightness values obtained for the softwood mechanical pulps indicate that, as a class, they do not respond well to hypochlorites. Of five softwood pulps, only a white spruce pul p reached a brightness value possibly of practical significance when treated with 1 percent available chlorine as calcium hypochlorite. With sodium hy pochlorite equivalent to 1 percent chlorine, a southern yellow pine pulp reached only 63 brightness, with the increase being 8.5 points. The Eastern white pine pulp required 15 percent chlorine as calcium hypochlorite for a brightness approaching 7, and even then the bleached pulp was quite yellow in hue. The use of silicate in conjunction with lime as the buffer system with calcium hypochlorite gave no improvement in the brightness of the Eastern white pine pulp. The use of sodium hypochlorite or extractions with hot water, sodium hydroxide, or hydrochloric acid had little effect. Extraction with alcohol, however, removed a yellow material; and when the extracted pulp was treated with 1 percent chlorine as calcium hypochlorite and this was followed with a final treatment with sulfurous acid, a brightness increase of 4.2 points resulted. At the present time, hypochlorites show little promise as practicable agents for bleaching softwood mechanical pulps. stability of Brightness Relative stability of the brightness of the mechanical pulps after bleaching with hypochlorites is compared to that of those bleached with sodium peroxide by the recession values given in table 2. The values were obtained by exposure of test sheets to single-arc carbon arc light. Because of differences in exposure time, all pairs of pulps are not directly comparable, but it appears that the greater proportion of recession effected by carbon arc light occurs at the end of 1 hour (5). The data show that the brightness of pulps after bleaching with hypochlorite was less stable than after bleaching with sodium peroxide, The hardwood Pulps bleached with hypochlorite, including the mixed hardwood pulp, retained, however, 5 to 8 percent of the original brightness increase. Owing to a generally higher initial brightness than that of the peroxide-bleached pulps, they were at least as bright as the latter after both had been subjected to exposure. I ue Because of the hue of mechanical pulps, either bleached or unbleached, the brightness value alone is inadequate to characterize appearance. Consequently, the appearance of the pulps dealt with in this report is given in terms of brightness, dominant wave length, and purity (2), as shown in table 1. It was previously suggested (4) that a yellowness value (3) might be a sufficient supplementary value for a numerical comparison of the appearance of bleached groundwood pulps. Further experience, however, indicates that Report No

6 the usefulness of the yellowness value is limited, apparently because bleached groundwood pulps are too far removed from the degree of whiteness for which the yellowness value was originally intended to be used, The amber, green, and blue filters used in the tristimulus measurements had maximum transmission values of approximately 59, 555, and 156 millimicrons. The range of dominant wave length for all pulps, both bleached and unbleached, was from 562 millimicrons for the hypochlorite-bleached white spruce pulp to about 582 millimicrons for the unbleached yellow birch and sugar maple pulps. The range for green is considered to be 5 to 57 millimicrons, and that for yellow, 57 to 59 millimicrons (2). On that basis, the experimental pulps were all essentially yellow in hue. If 58 millimicrons are taken arbitrarily as yellow, the indication is that bleaching with hypochlorites consistently gives hues slightly toward the green, or lower, wave length. With sodium peroxide the trend toward the green is, in general, the same, but the effect is slightly less, In general, the purity of hue resulting from hypochlorite bleaching appeared to be about the same as that resulting from bleaching with sodium peroxide, The high purity values of the bleached yellow-poplar II (table 1), Western hemlock, and the Eastern white pine pulps after bleaching with 1 percent available chlorine accords well with their strong yellow appearance. Yield The yield values given in table 1, determined without loss of fiber, show that losses in weight due to the chemical action of calcium hypochlorite averaged.9 percent, with a maximum of 2.7 percent. Those results appear very favorable in comparison to those obtained with sodium peroxide. Freeness and Strength All of the experimental pulp were not made under grinding conditions for optimum freeness and strength. Consequently, the effect of bleaching on those properties is expressed as a percentage change in table 3. A decrease in freeness was obtained more frequently than an increase, with the changes ranging from a negligible to an appreciable amount. Although the percentage values for changes in bursting strength were large because the bases for calculation were all small, they represent only small actual changes in points. The same was true of the changes in tearing strength. Since there was no definite trend toward higher or lower values, it is apparent that there was no significant effect of bleaching on bursting and tearing strengths. There was a definite trend toward relatively large increases in tensile strength as a result of hypochlorite bleaching, although the tensile strength of the southern yellow pine and American beech pulps was decreased about 8 pounds per square inch, With those two pulps excluded, the average increase in tensile strength was 365 pounds per square inch, the maximum being 972 pounds per square inch, As previously reported (4), a somewhat Report No,

7 smaller increase in tensile strength can be expected when bleaching with sodium peroxide. ardwood and Softwood Mixtures The possibilities of processing mixtures of hardwoods and softwoods are of interest to manufacturers l of groundwood papers. Results are given in table 4 of hypochlorite-bleaching experiments on stocks composed of mixtures of an Eastern white pine groundwood with one made from a mixture of hardwoods. The failure of the Eastern white pine pulp to respond to a treatment of 1 percent available chlorine as hypochlorite, except for improvement in tensile strength, was discussed above. When mixtures of the pine and mixed hardwood pulps were treated with 1 percent available chlorine as calcium hypochlorite, the brightness was increased in proportion to.the amount of 'iardwood pulp in the mixture. This is shorn by the fact that the experimental values can be calculated almost exactly on a direct proportionality basis from the brightness values of the unbleached pine pulp and of the bleached mixed hardwood pulp. The indication is that when the mixture of the unbleached pine and mixed hardwood pulp was treated with hypochlorite, the effective rates at which chlorine was consumed by the two pulps were equal despite the large difference in the reaction times when the pulps were bleached separately. In the preparation of a mixed pulp in the brightness range of 65 to 73 from Eastern white pine and mixed hardwoods of the composition dealt with here, less chlorine would be required by mixing unbleached pine pulp with hypochlorite-bleached hardwood pulp than by bleaching the mixture with hypochlorite. Report No,

8 Literature Cited (1) Andrews, I.. Zinc ydrosulfite Treatment of Groundwood. Pulp & Paper Magazine of Canada 46(9)679-81,, Aug Pulp cc Paper Industry 19(8)58, 6, Aug (2) ardy, Arthur C. and staff. andbook of Colorimetry. The Technology Press, Cambridge, Mass (3) unter, Richard S. Photoelectric Tristimulus Colorimetry with Three Filters. Natl.Bureau of Standards Circ. C429, July 3, (4) Kingsbury, R. M. Simonds, F. A., and Lewis, E. S. Observations on Bleaching Groundwood Pulps, Paper Trade Jour. 126(24)49-59, June 1, Pulp & Paper Magazine of Canada, Oct (5) Lewis, arry F., Reineck,, Edward A., and Fronmuller Douglas. The Fading of Groundwood by Light. Paper Trade Jour. 12(8)44-48, Aug. 23, (6) McEwen, Robert L. igh Density of Mechanical Pulp with ydrogen Peroxide. Paper Trade Jour. 122(17), Apr. 25, (7) Reichert, J. S. Sodium Peroxide Bleaching of Mechanical Pulps. Paper Trade Jour. 118(11)45-52, Mar. 16, Report No

9 Table 1.--Influence et hvpochlorites and sodium peroxide on appearance and yield of mechanical pulp. _ Specie, Bleaching agent 1 - Stock Temper- Alkalinity Duration brightness-2 Dominant wave length purity Yield of density ature or.1 bleat.hqd Lind Amount Initial FinaltreatmentUnbleathedBleachedUnbleachedBleached UnbleachedBleaohed pulp4 I t i. pulp null, pulp pulp pulp pulp PeroentPercent. pa 2ff r. Min. Percent Percent Mmu. Mau. Percent pervlalfirclulk ardwood. Sweetgum os(o , , r Black tupelo 11.11,2; * , ' Water tupelo TaLF5 1} , ' '4 ' Yellow-poplar I lia3f2.g 11, Yellow-poplar.//tc11, ;,1.3 k li I Eastern ;;_11FO '34, z qur,itznaloped. I ; gfs1)2 2f I a ' 97.9 Quaking aspen IIIICIfgal)2;' , _ ' Paper birth I TX61)2; ' Paper birch II ; gfgl) i liv gm t Yellow birth ; 241?g61) 2 ; Z? I 53.8._ Sugar maple I ; 1=61) I 2 z MaC1, ,4 NaC12 I I e,,, 2 12 ;?i54 '; Sugar maple' II 1881) ,3 1842, ! ; 41 I sil I 1,4 9.3 I , Red maple 1.(1) t , Ash, white 14Fggli ; / teag1)2 American beech a , Mixed hardwooder81) 2; I I ' , Softwoods Eastern white zcs.coc1)2 1 Pine,c.(C ime Southern - yellow Ca(C1) 2 15 pine ,2 Tack pine te2coc1/2 152 White.prate IN,21 2! 2 12( (C ! 2 Western bowie,* a(1) 2. 1 znac ! i g2,..3 ; 2g! o , o , i , " I ! 15.8! [ 1Percentage values refer to weight of available chlorine, sodium peroxide, or other chemicals and are based on unbleached pulp. The calcium hypoohlorite solution contained.3 to.7 percent free alkali as hydroxide and required 1 to 6 percent additional lime for adjusting initial p. The sodium hypoohlorite contained 3.6 percent free alkali as hydroxide and required up to 2 percent additional caustic for adjusting initial p. The peroxide bleaching solution contained 2 percent sodium peroxide, 5 percent sodium silicate,.5 percent magnesium sulfate, and up to 1.5 percent sulfuric &old to adjust initial p. 2Measured on air-dry teat sheets. Except as noted, test sheets of the hypochlorite-bleached pulps were formed at p 7.5 from pulp washed with tap water of p 7.5. In the case of peroxide-bleached pulps a final adjustment of p to 5 was made with eulfurous acid and the test sheets were formed at p 5 from unwashed pulp. IBased on unbleached pulp. -This sample of pulp was freshly made. When bleached after 53 days. storage at room temperature the brightness obtained was 71.5 percent. IA final adjustment of p to 5 was made with sulfurous acid and test sheets were formed at p 5. from unwashed pulp. Two percent sodium meta silicate was added to the bleach mixture. 2Twenty-nine percent each of red maple and paper birch, 16 percent each of sugar maple and yellow birch, and 5 percent each of American beech and white ash on weight basis and essentially the same by volume. K P

10 Table 2.--Recession of brightness of bleached groundwood.ulps upon exposure to carbon arc light Pulp Bleaching agent' Brightness Duration Recession increase when of 2 bleached exposure- Points ours Points Paper birch I Ca(C1) Na Paper birch II! Ca(C1) Na Yellow birch Ca(C1) Na Sugar maple I -NaCl Sugar maple II Ca(C1) Na _ 2 _ Red maple Ca(C1) ' Na White ash ca(oc1)2, Na American beech Ca(C1) 2! 22, Na Mixed hardwood. Ca(C1) Ns _. _.2 _ Eastern cottonwood Ca(C1) ,7 Na Eastern white pine 2 --Ca(OC1) Na2 o White spruce 'Ca(C1) Ca(C1) Na The amount of hypochlorite was 1 percent expressed as available chlorine, except in the two instances footnoted, and the amount of sodium peroxide was 2 percent.?single-arc, carbon arc light. -Two percent of sodium metasilicate was added to the bleach mixture. -Twenty-nine 4 percent each of red maple and paper birch, 16 percent each of sugar maple and yellow birch, and 5 percent each of white ash and American beech on the weight basis. -Fifteen percent available chlorine. Report No

11 Pulp Table 3.--Change in freeness and strength resulting from bleaching groundwood pulps with hypochlorites. Sweetgum Ca(C1) 2-1 NaC1 Black tupelo I Ca(C1) 2 1 NaC1-11 Black tupelo II Ca(C1) 2-23 NaC1-16 Water tupelo Ca(C1)2-19 Yellow,Toplar I Ca(OC1) 2 27 Quaking aspen I Ca(C1) 2-35 Quaking aspen II Ca(C1) 2-2 Sugar maple I?NaC1-3 Red maple Ca(C1) 2 / 6 White ash Ca(C1) American beech,ca(c1) 2 ; 14 Bleaching Freeness BurstingTearing Tensile agent strengthstrengthstrength CanadianShopper- standard Riegler nn Mixed hardwood Ca(C1) Eastern white pine -Ca(C1) 2 5 Southern pine t Ca(C1) a(oc1) 2 ; r- Percent Percent Percent Percent Percent The amount of hypochlorite, except as noted, was 1 percent expressed as available chlorine.?two percent sodium metasilicate was added to the bleach mixture. -Fifteen percent available chlorine i2 3 ' 33 ' ; ; -4 ; ! ; 22, ' Report No

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