US2598580A - Bleaching or brightening a web of mixed fibers - Google Patents

Description

latented May 27, 1952 JRUSS tittttitl ilit.

BLEACHING OR BRIGHTENING A WEB OF MIXED FIBERS Robert L. McEwen, Williamsville, and Fred R. Sheldon, Buffalo, N. Y., asslgnors to Buffalo Electro-Chemical Company, a corporation of New York Inc., Buifalo, N. Y.,

No Drawing. Application October 8, 1947, Serial No. 778,746

2 Claims. (Cl. 92-4) This invention relates to bleaching or brightening a web or sheet of pulp or paper and more particularly such a Web or sheet of pulp or paper comprising a mixture of a chemical wood pulp and a wood pulp containing substantial amounts of ligneous and non-cellulosic material, such as groundwood or mechanical pulps, pretreated groundwood or semi-chemical wood pulps.

Chemical wood pulps, whether mad by the soda, sulfate or sulfite method, have been treated chemically to obtain relatively pure cellulose fibers from the original wood and contain very small amounts of lignin, generally below about 3-4% of the total weight. Since such chemical wood pulps are composed of relatively pure cellulose fibers, such pulps are not difficult to bleach, many methods of bleaching such chemical pulps with hypochlorite, hydrosulfite, hydrogen peroxide, etc. having been commercially employed. However, while such pulps produce a permanently bright, high strength paper, such paper is relatively costly because of the low yield of chemical pulps from the original wood and some arerelatively harsh, non-absorbent, non-resilient, and lack opacity.

On the other hand, groundwood or mechanical wood pulps, pretreated groundwood and semichemical wood pulps contain up to about 35% lignin based on the weight of the pulp and also contain other non-cellulosic materials. Wood pulps containing lignin in amounts greater than about 5-6% by weight are referred to herein as wood pulps containing substantial amounts of lignin and non-cellulosic material. Because of the presence of such substantial amounts of lignin and non-cellulosic material, such pulps are difficult to bleach. Further, because of the shortness of the fibers, groundwood or mechanical wood pulps are of low strength. However, paper made from such groundwood or mechanical pulps, pretreated groundwood, and the like, is relatively low in cost because of the high yield from the original wood and possess superior qualities to chemical pulp papers in point of printability, bulk, softness, resiliency, absorbency and opacity.

By blending proper proportions of chemical pulps with groundwood pulps, pretreated groundwood or semi-chemical wood pulps in the furnish beater, a paper having the desired specific physical properties can be produced. However, producing high brightness paper from such blended pulps presents special difliculties. Thus, for example, when groundwood pulp is bleached as pulp and then blended with chemical pulps, such as bleached or unbleached sulfite, sulfate or soda pulps, the brightness of the mixed furnish is well below the arithmetic mean of the brightness values of the pulps so-blended and has been found to be below even the brightness values predicated by the empirical formulae of mixed pulp furnish brightness. These formulae and actual investigation demonstrate that the duller pulp predominates in producing the resultant brightness of a mixture of pulps. This loss of brightness, upon blending or mixing pulps, is

more pronounced the higher the individual brightness values of the pulps prior to blending and is also more pronounced the greater the differential between the brightnesses of these pulps.

It is the principal object of the present inven tion to provide a method of bleaching or raising the brightness of a web or sheet containing a mixture of a chemical wood pulp and a wood pulp containing substantial amounts of ligneous and non-cellulosic material to a higher value than is now commercially feasible to obtain and in many cases is now impossible to obtain.

Another object is to produce economically a high groundwood content web or sheet of high brightness and suitable vfor printing or other uses where the valuable physical properties of groundwood pulp are desired.

Another object is to bleach such mixture to such brightness levels as to permit large amounts of low cost groundwood pulp to be used, thus placing high groundwood content papers in competition with bleached soda and coated papers prepared from bleached chemical pulps.

Another object is to provide a method of bleaching a web or sheet of Wood pulp containing substantial amounts of ligneous and non-cellulosic material which compensates for the loss in brightness occurring in accordance with the empirical formulae of mixed pulp furnish brightness.

Another object is to bleach such web or sheet on existing paper making equipment.

A further object is to provide such a method which can be practiced on commercial paper making equipment without substantial alteration or addition thereto or without afiecting the rate of production of paper produced by the paper brightening a web or sheet of pulp mixtures containing a substantial proportion of the high yield 7 but impure groundwood, pretreatedgroundwood or semi-chemical pulps. T

In an attempt to produce high brightness paper composed of a mixture of chemical wood pulp, such as sulfite, and a pulp containing substantial amounts of ligneous and non-cellulosic materials, such as groundwood, the blending of a bleached sulfite pulp with a bleached groundwood pulp might be undertaken in the furnish beater. The sulfite pulp can have been economically bleached to a brightness of, say, 85.7 units, measured by a General Electric reflectometer, by following the usual process of chlorination followed by alkali extraction and hypochlorite bleach, and the groundwood pulp can have been economically bleached to a brightness of, say, 64.8 units, by the addition to the groundwood pulp of 2.0% by weight of a 27.5% hydrogen peroxide solution on a dry pulp basis.

When these two bleached pulps, i. e., bleached 75.2 units, i. e., the arithmetic mean value of the bleached pulp brightnesses. The empirical formulae of mixed furnish brightnesses known to those skilled in the art, however, predicts --a brightness of 72.2 units for this bleached sulfite and bleached groundwood mixture. However, by

actual experience, this mixture of bleached sulfite and bleached groundwood in the amounts and of the brightnesses above specified is found to reach only 69.9 brightness units. Moreover, further loss in brightness occurs as the mixed sulfite and groundwood pulp is formed into a web or sheet and passes through the paper making machine. Thus, the brightness of the finished paper containing 50% groundwood and 50% sulfite is 68.6 units instead of the 69.9 units of the blended pulp furnish. This loss of brightness in the paper making machine is due in part to the processing water used which may introduce darkening material, and also to the necessary heat during the drying operation which prematurely ages and further darkens the paper.

This low brightness of the 68.6 units very materially restricts the uses, demand and value of the finished paper, and it has been found desirable to provide a paper which contains substantial proportions of groundwood, pretreated groundwood or semi-chemical pulps and which has a brightness value much higher than 68.6 units. The groundwood, pretreated groundwood or semi-chemical pulp content of a paper is of value in increasing its printability, bulk, softness, resiliency, absorbency, and opacity as well as reducing its cost, but the low brightness levels of such papers as now made, and as represented by the above brightness value of 68.6 units for a 50% sulfite, 50% groundwood paper, is a distinct deterrent to the uses and demand for such paper.

Since the duller pulp brightness predominates in the resulting brightness of pulp mixtures, it is obvious that by bleaching the groundwood pulp to a higher brightness value, finished paper of a higher brightness can be produced. Thus, with the use of 4.0% by weight of a 27.5% hydrogen peroxide solution on a dry pulp basis the brightness of the groundwood pulp can be raised to 70.0 units as compared with the brightness of 64.8 units obtained with the use of 2 .0% of such hydrogen peroxide solution. When this brighter groundwood pulp is mixed in equal proportions with the sulfite pulp having a brightness of, for

instance, 85.7 units, the pulp mixture has a brightness of 71.3 units. On passing through the paper making machine this brightness drops so that the finished paper has a brightness of 70.8 units. However, this brightness value of 70.8 units indicates the level above which brightnesses are generally not obtainable with sulfite and groundwood pulps mixed in the assumed proportionfollowing the above. procedure. When bleaching groundwood as pulp, if more than 4.0% by weight of 27.5% hydrogen peroxide on a dry pulp basis is used in bleaching the groundwood pulp, little increase in brightness of the groundwood pulp is achieved since the efiiciency of brightening groundwood pulps with peroxide falls off very rapidly as higher brightness levels are approached. In effect, therefore, it is practically impossible to obtain a brightness value of a paper composed of 50% sulfite and 50% groundwood much above 70.8 units by the above procedure, this brightness level still materially restricting the uses and value of a paper of this and similar compositions.

We have found that a web or sheet of paper formed on a paper making machine from a furnish containing a chemical wood pulp, such as bleached or unbleached sulfite pulp, and a bleached pulp containing substantial amounts of ligneous and non-cellulosic material, such as bleached groundwood, can be bleached to brightness levels which are impossible to obtain by the separate bleaching of the pulps prior to blending in the furnish beater, as previously described, and in particular far in excess of the value of about 70.8 units which represents the economical brightness ceiling for a mixture of bleached sulfite and bleached groundwood pulps in equal proportions as set forth in the previous illustration. Further, we have found that the practice of the present invention is commercially practicable in producing such higher brightness levels. This bleaching to higher brightness values, in accordance with the present invention is, accomplished by first bleaching at least the" groundwood, pretreated groundwood or semi-chemical pulp component before blending it with the chemical pulp component in the furnish beater and thereafter, as a separate bleaching step, applying a suitable bleaching solution to the web or sheet of paper being formed from the blended pulps and at such time that the bleaching solution will be dried into the paper through heat subsequently applied to the web or sheet, or through the sensible heat in the web or 'sheet at the time the bleach solution is applied. We prefer to employ an alkaline aqueous peroxide solution in this second or sheet bleaching step or phase. In this second or sheet bleaching phase, the alkaline peroxide bleaching solution is applied to the web or sheet after the natural brightness losses have occurred as a result of the blending or mixing of the pulp furnish constituents and therefore under conditions where further brightness losses resulting from mixing the pulps are impossible. In this manner the web or sheet formed from such mixture of pulps can be simply, economically and practically bleached to brightness levels which are impossible or commercially prohibitive through excessive bleaching of the pulp furnish constituents separately prior to blending or mixing.

In practicing the invention it is not essential that the chemical pulp component be bleached prior to blending with the groundwood, pretreated groundwood or semi-chemical pulp in the furnish DHUQQ REI'UULWML beater, although. of course, the. maximum brightness value ofthe finished paper is not achieved without such separate bleaching of the chemical pulp. However, even with the use of unbleached chemical .pulp brightness values close to 70.0 units can be achieved thereby to provide papers containing substantial amountsofgroundwood, pretreated groundwood and semi-chemical pulps which are'brig'hter than those heretofore produced by combining unbleached chemical pulps with bleached groundwood. Nevertheless, since chemical pulps. are'relatively easy and economical to bleach, it is preferred in the practice of the present invention to employ a bleached chemical pulp component so that a finished paper having a maximum brightness may be produced.

In relation to the first phase of the bleaching procedure of the .present invention, namely, the separate bleaching of the chemical pulp or 'pu'lps and the pulp or pulps containing substantial amounts of ligneous and non-'cellulosic materials, it is'our intent that our invention shall comprehend any method suitable for the separate brightening of the pulp component containing substantial amounts of 'ligneous and non-cellulosic material and, when desired, for the separate brightening 'of the chemical pulp component, Bleaching of the groundwood, pretreated groundwood, or semi-chemical pulp component is required, and any suitable chemical bleach or bleaching procedure can be utilized for this purose. As to this bleaching of the groundwood, pretreated groundwood, or semi-chemical pulp component, we prefer that an alkaline aqueous peroxide solution be used, the exactcomposition of which can be in accordance with any of the well known prior art practices. 'The pulp can be bleached at low density or at high density with or without the pretreatment described in the copending applications of Robert L. McEwen, serial No. 765,647, filed August 1, 1947, now Patent No. 2,465,327, and Serial No. 576,359, filed February 5, 1945, now abandoned. Likewise, if desired, the chemical component, being in the form of relatively pure cellulose fiber, can be readily bleached with any one of a wide variety of chemical bleaches and bleaching procedures.

For the second phase of the bleaching-procedure of ourmethod, we require that the bleaching solution be applied to the web or sheet of mixed pulps under such conditions that water is evaporated from the web or sheet after the application of the bleaching solution thereto, or, in other words, the chemical residues are left in the web or sheet. Any bleaching solution which, when applied under these conditions, will produce the desired increase in brightness of the web or sheet can be employed, although we prefer to employ an alkaline, aqueous solution of hydrogen peroxide. The concentration of this hydrogen peroxide bleaching solution and the quantity applied to the sheet will, of course, be

governed by the degree of bleaching desired. In this second or sheet bleaching phase it is essential that the alkaline aqueous hydrogen peroxide solution contains a water soluble phosphate such as trisodium phosphate, a pyrophosphate, a Y

tetraphosphate, or various mixtures of these phosphates. Other agents, such as silicate of soda or magnesium sulfate, may be added in small amounts to the hydrogen peroxide solution containing the phosphate in order to stabilize the solution for commercial use, but are not essential to the practice of the invention. The alkaline and-stabilizing additions should be such asfjto provide the solution with a pH of from 9:0 to- 15.. Al hou h the use oi-:-hydr sen sp ox deas such is preferred, it will be understoodthat aqueous peroxide solutions may be employed which are prepared by the use of alkali metal peroxides. alkaline earth metal peroxides, and other percornpounds, which in aqueoussolution produce hydrogen; peroxide or may be converted into hydrogen peroxide by acidification. I

The: bleaching solution canbe applied to the web or sheet in any suitable. manner, as-by pad-,- ding with. applicator rolls,,spraying;or the like. ltlmay be applied to either. one or both surfaces of the web orsheet, and'at any appropriate place. on the machine so long-as theweb or sheet will. thereafter besubjected to. the heatof atfleasta,

portion of the. dryers of the paper machine, or to the sensible heat within theipaperbeforethe solution is applied, for a .long enough period to produce the bleaching action. When a web or sheet of pulp. is treated in accordance with the invention, the bleaching solution is applied to thesurface of the web orsheetpreferablyeither at the size press or between the last wetpress and the first dryer of the paper making machine. However, it may be applied aheadof or within the dryerpart of the machine or at any other appropriate place so long as the application is. far enough 'fromthe dry, end, of, thepaper may permit water tov mg 2.0 byweight of 'a'27.5=% hydrogen peroxide solution on a dry pulp basis and stabilized with 6.0% ofsilicate-of-soda on a'dry pulp basis.

The separately bleached sulfite and ground wood pulps were then mixed in equal proportionsina water suspension in the furnish beater preparatory to the formation therefrom ofa web,

or sheet on a conventional paper making ma. chine. The furnish resulting from-this mixing or blending of the separately bleached sulfite and groundwood pulps had a brightness of 6959 units, as determined by brightness measurements of hand sheets prepared from the furnish, this drop being due to the duller groundwood pulp predominating and lowering the brightness of the furnish considerably below the arithmetic mean of the brightnesses of the furnish constituents. It will be noted that up to this point the procedure as so described is identical with that previously described and hence without the full practice of the present invention the paper formed on the paper making machine would have a brightness of 68.6.

In practicing the second phase of the invention, an alkaline aqueous peroxide solution containing hydrogen peroxide and tetrasodium pyrophosphate was applied to the 50% bleached sulfite50% bleached groundwood web as the sheet went to the size press during its passage through the paper making machine. Application was effected by spraying the bleaching solution onto OE-Fifi bil EiUUllll wit ss-ease:

tioii, left'thersize press in a moist, evenly'saturatedconditiom An analysis 'of the 'sheet leavingithesize press-showed that -3.1'%-'by weightof a -27i5 aqueous-solution" of hydrogen peroxide? and -3. 1% by weight ofr'tetrasodium pyrophosphateWere applied to the moist sheet ona dry sheet'basis'.

The-pH of'the solution so applied "to the sheet was 913; this pHbeing; provided 'by the presence of thetetrasodiuni'pyrophosphate; lid-other. addi tions being m-ade to adjust the pHTo' this" value.

A ,Shfi of a paper j continuously saturated with I bleaching solution wasthus prepared andthereafter dried; on the; steam; h ated dryers o'fj'the paper machine; A T e dl'iedj bleached, sheet; so roduced .1T8dfa bj1i hthess' of 76.4 units of felted wood pulp of improved brightness: and containing a*-b1endof a 'sulfite wood pulp and a substantialproportion of a groundwood'pu-lp" which contains a'substantialamountiof ligheous and other non-cellulosic material, said. method consisting essentially of the steps" of "bleachingsaid groundwood pulp component by; itself with an alkaline? aqueous peroxide? solution before blendingjwithisaid sulfitepwoodfpulp-component; blending"saidiicomponents, inia Water suspension, felting the blended pulpsf into. a webiorn sheet, applying to sald wjeb or sheet an aqueouslperoxi'clev bleaching solution containing hydrogen peroxide and a, water soluble phosphate asthe essential ingredients, and having an. initial pHiinthe ran e of'from 9.0m 11.5, and thereafterevaporating waterfromsaidweborlsheetr,

2., The method of producing a, w.eb..o1;,. sheet. of felted, woodipulp .of improvedlnbrightness land It ll f e 5 1 "that the practice offthejilive f *containir'llg av blend,ofiaflsulfiteawoodlpul-p and ,a,

tiQ i rfirs'tlbleaohifig the ,sulfite and grounded wo od p ilps separately, andtherjeafter zmplirifie'v a hydrogen!peroxide bleaching, solution to.the weborsheet of mixed 'm lp's at such time that the hydro enperoxidebleaching solution is dried intov the:sheet-(of. paper; pa ssinglthroughl the paper making machine, prodl'icednabbrightness increase of 7.8.junits as compared withpaper on whichionlythefirs'tbleatzhing namely, the separate bleache substantial.v proportion of I a. groundwood, pulp.- which loontainsla substantial amount oftligneous and other non-cellulosicl material, said "method consisting essentially. oi. thew-steps of bleaching,

said :groundwood. pulp ,-component-by1-itself; with; an alkaline aqueous. peroxide solution before blending with said sulfi-tewood pulp component,:

bleaching :s aid-rl sulphiter-wood -pulp componentiby 7. itself! before blendin glwith saidsecond: :wood pulp:

ingj oilthe, sulfiteiandgroundwood pulps;Jwas caricdmponemhblendmg; saridlcompgnenhs ma water,

ried out. p v in any other commercially feasible mannerwhenpapers containing; pulps having, substantial amounts of, JigneoiisandI n0n-cellulosic} materials are Cconsidered; It will benoted in the-procedure as above outlined in carryingioutthe inyentionthat, exclusivelof the\cheini-calpulp the bleaching to a'tbrightness of,76.4a 1nits was effected with the; expenditure -of: 4.,1,% ,of, a,v2'1.5 hydrogen peroxide solution based on the -;dry weight-7M the:

finished papen pi this 51.0% beingeapplied .from

a 2 0%; bleach ,to the groundwood, component which nemprises 0%-:Q hefinished ane As used-lathe appendedn a ms,bya u ou pe oxide solution is mean asolutioncontaining water and hydrogen; peroxide its. 1 water soluble salts. n ot er e m o n s wh chinqueo s-sow: tionsv yield hydrogen; peroxide o1wmay: be: con: rteds t 1 hydreeen: peroxide by fi ation;

It will be @npar ntthat chan es and modifier:

This bri h Ssr v l is runobtainabls suspension', feltingfzthe, blendedlpulpsrinto.alweb' or sheet,-japp1yin vtosaidWebpr sheet an aqueous peroxide bleachingg clution'con-taininghydnogem peroxides-and a water; soluble: phosphate .asethe essential ingredients andhayingiaminitial the range of from; 9.01m; 1L5, and; thereafter evaporatingzxvater. from isaidr webazor; sheet;

SHELDONA r REF-EaENoEs-CITEDQ UNITEDQSTATESIP'A'IENTS .1

Number- 1 Date 'Name; Re.719,4770 Hirschkind "Feb: 19,-; 1935 2,150;9,26 I Kaulimanaetualw;h-lMarlr2le 1939s OTHER; "REFERENGES Chemical Engineering; Ootoben; 1947, ,pages 92-96 and,,14O- 143.,