CA1289305C - Production of high strength linerboard with oxygen and alkali - Google Patents

Abstract

PRODUCTION OF HIGH STRENGTH LINERBOARD WITH OXYGEN AND ALKALI
Abstract of the Disclosure Pulp for the production of high strength linerboard is obtained by digesting wood chips in alkaline cooking liquor, defibering, treating with oxygen and alkali, and refining.
Linerboard pulp produced by this method has better paper strength properties and is easier to refine than pulp not treated with oxygen and alkali. The treatment is conducted in its best mode at temperatures below 100°C to minimize pulp yield losses.

Description

1~89305 This invention relates to a method for increasing strength properties and refinability of high yield chemical wood pulp by oxygen and alkali treatment. The enhanced properties of the pulp are advantageous for the manufacture of linerboard paper.
Sulfate pulp with a lignin content corresponding to a Kappa number of from about 60 to about 120 is conventionally used for the production of unbleached linerboard. Linerboard pulp manufactured in this way has good strength properties at relatively high yields (55-60%). The dry weight of washed fibers which are recovered after pulping is generally reported as a percentage of the weight of dry lignocellulosic material which was charged to the digestion process. This percentage is termed "yield".
Any decrease in yield caused by loss of lignocellulosic materials is undesirable in papermaking. Two of the more important strength properties of linerboard are burst and edgewise compressive strength. To obtain the desired burst and compressive strength, pulp is refined before the linerboard is formed. The action of refining fibrillates and collapses the pulp fibers, allowing them to form a more strongly bonded and dense board. Linerboard density is strongly correlated with burst and compressive strength levels. However, the pulp cannot be refined too severely since this will cause the pulp to drain poorly on the linerboard machine, resulting in low production rates.
Broad density is therefore achieved by a combination of refining and wet pressing on the paper machine.
It is known generally that delignification of pulp with oxygen and alkali is a commercially acceptable process. The process is usually applied to low yield chemical pulps as a pre-bleaching stage, before final bleaching with chlorine-containing chemicals. The Kappa number of the pulp is usually reduced from 30-35 to 15-20, signifying a reduction in lignin content of 40-50%.
- Reduction in lignin content to such a degree would result in paper of insufficient strength properties for linerboard manufacture.
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t~05 Kleppe et al ("Delignifying highyield pulps with oxygen and alkali," TAPPI, vol. 68, no. 7, p. 71, 1985) teach that sulfate pulp having a Xappa number within the range of 140-150 can be delignified with oxygen and alkali to pulp with a Kappa number of 110. In both of these treatments, however, oxygen, alkali, and pulp are reacted at a temperature (105C) and pressure (0.5 mPa, 58 psig) which were optimized for the removal of lignin from the pulp. Delignification rates and strength levels of highyield soda pulps are strongly influenced by temperature during oxygen and alkali treatment. Thus, reaction temperatures above 100C increase the extent and rate of delignification and promote oxidative degradation of wood carbohydrates.
Because of the relatively severe conditions of the above treatments, the pulps are stabilized against carbohydrate degradation by treatment with magnesium salts (0.05-0.15%, based on o.d. (oven dried) pulp) . These salts, however, reduce the yield loss associated with the carbohydrate fraction of the pulp allowing for further delignification.
It is an object of this invention, therefore, to provide an improved method of producing high strength kraft linerboard while reducing the refining energy required for its manufacture.
Accordingly, the invention provides a method for producing linerboard paper from high yield chemical wood pulp by treating the pulp with oxygen and alkali at a temperature of from about 50C to about 100C and a pressure of up to about 150 psig in the absence of protectors to increase the refinability of the pulp and the strength properties of the paper by limiting the reduction in lignin and carbohydrate content in the pulp as determined by a Kappa number reduction of no greater than 25% in the treated pulp.
Thus, the instant invention achieves the above objective by an improved linerboard manufacture method which uses oxygen and alkali as a means to chemically ~2 L~

,5 modify residual lignin present in high yield sulfate pulp without a substantial decrease in pulp yield. Hence, the process conditions utilized in this invention are much less severe (preferably, 74~C, 0.13 mPa, and 15 psig) than those used in prior art oxygen and alkali delignification processes resulting in minimized lignin and carbohydrate loss.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 shows graphically the relationship between the oxygen and alkali treatment of pulp and Williams Slowness at different beating times;
Figure 2 show graphically the relationship between the oxygen and alkali treatment of pulp and linerboard compressive strength at different beating times;
Figure 3 shows graphically the relationship between the oxygen and alkali treatment of pulp and linerboard burst factor at different beating times; and Figure 4 shows graphically the relationship between the oxygen and alkali treatment of pulp and linerboard tensile breaking lengths at different beating levels.
The figures thus present graphs which illustrate the ability to control linerboard pulp properties by regulating the beating time of pulps treated according to embodiments of the present invention.
It has been discovered that a high strength kraft linerboard can be accomplished by subjecting industrially prepared kraft pulp to a mild oxygen and alkali treatment.
The oxygen and alkali treatment of a high yield (55-60%) pulp produced from wood chips cooked in an alkaline cooking liquor allows production of a linerboard grade of paper with higher densities and physical strength levels than conventionally prepared linerboard. Upon refining, the oxygen and alkali treated pulp reach a given Williams Slowness and strength level more quickly than untreated pulp, indicating the treated pulp is easier to refine than .5 conventional linerboard pulp. (The Williams Slowness is the amount of time in seconds for one liter of water to drain through a three-gram sample of pulp~. The oxygen and alkali treatment is carried out on a pulp of medium consistency (8-20%, preferably 12%) at lower temperatures and pressures than those used in conventional oxygen delignification processes and in the absence of cellulose protectors. Employment of the pulp produced by this process in linerboard results in linerboard strength properties (burst, density, compressive strength) significantly higher than that measured in linerboard employing conventional kraft pulp of the same Kappa number.
The process has the effect of modifying the residual lignin present in high yield kraft pulp rather than substantially reducing pulp yield through lignin dissolution as is conventionally practiced with oxygen and alkali processes.
Embodiments of the invention are described in more detail with reference to the following Examples which summarize laboratory experiments in which industrial and laboratory prepared linerboard pulps were treated with oxygen and alkali.
Control ExamPle A
This pulp was an industrially produced kraft southern pine pulp with a Kappa number of 96.5. The pulp was washed in the laboratory which reduced the Kappa number to 87.7. The pulp was then beaten in a Valley Beater to various Williams Slowness levels and test handsheets were made.
Control ExamPle B
The same pulp as in Control Example A was treated in a laboratory oxygen reactor for one hour at 78C in the absence of oxygen. The pulp consistency was 12% and the initial pH was 10.9. After the treatment the pulp was washed and the Kappa number determined. The pulp was then beaten in a Valley Beater to various Williams Slowness levels, and test handsheets were made.
Example 1 The same pulp as in Control Example A was mixed with sodium hydroxide solution and sufficient water to bring the pulp consistency to ]2%. The sodium hydroxide charge was 1~ based on the o.d. weight of the pulp. The initial pH of the pulp was 12.1. The pulp was then treated in a laboratory oxygen reactor for one hour at 78C with an oxygen pressure of 15 psig. After the treatment, the pulp was washed, and the Kappa number was determined to be 81.1.
The pulp was then beaten in a Valley Beater to various Williams Slowness levels and test handsheets were made.
The pH of the pulp after the treatment was 10.3.

:~, ..~ ,,~

:

Ca~e 'Po~ket 110. ~IIR 85-l~

Exam~ 2 The same p~lp as in Control ExaInple A was mix~d wi~h sodium hy~ro,ci~1e solution and suf~icien~ wat~r to bring the pulp consisteney to 12%. The ,sodium hydro~ide charge was 2% based ~n o.d. pulp welght. Tbe initial pl~ of the pulp was 12.2, The pulp was then tre~ted in a la'boratory oY~ygen reac~or for one ho~r at 78~C with an oxygen pres~s~re ~f 15 psig. A~ter treat~ent the pulp was washed al1d the Kappa number de~er~lned to be 77.l. The pulp t~as then beaten ln a Valley Bcater ~o various Williams Slowness levels, and test handsheets ~7ere made~ The pH o~ the pulp af~er ~he treat~ent was lO,9.

.Y~ample 3 The same pulp as in Cont~ol E~ample A was ~ixed with sodium h~dro:~ide solu~ion al~d s-~fficien~ wate~ to bri~g the pulp conslstency ~o 12%. The sodium hydroxi~e ~har~e was 5~. ~ased on o,d. pulp wci~h~. The lnitial pll o~ the pulp was 13,0. The pulp was then ~rea~ed in a labora~ory oxygen reactor ~or one hour at 7~C and ~n oxy~cn prcssure o~ 15 p.si~. Ater tre~tment the p~lp was washed and the K~ppa num~cr determined ~o be ~8.2. The pulp was then beaten in a Valley Benter to various Williams Slo-~ness levcls, ~nd ~es~ handsheet,s ~e~e made.
The be~ting ti~es, Williams Slo~mess, handsheet densitles, ~nd pulp strength propertie~ ~r~ shown in Table I.

~ase ~o~ke~ No. CH~ ~5-18 TABLE I
EFFECT OF OXYGEII-AIi~ALI ON SlRE~IGTII PROPERTIES O~ FT Pl~iE PULPS

STFI ~ns~le Be~ting l~ ms Handshee~ Compressive Breakin~
Time Slown~ss Densi~y Strength ~urst Len~th (~n.) ~sec.) (k~Lm ) (lb,/1n.~ _Factor (10~ ~) C~ntrol 0 ~.3 40~ 11.5 24,0 39.1 Exa~ple A10 5.~ 483 15.8 36.0 5~,4 6.0 510 16.9 41.7 67.9 6.3 $4g 17.5 44.2 67.2 8.~ 610 19.2 57.0 75.3 10.9 64S 1~.5 58.5 73.3 Control 0 4.6 444 12,9 22,7 44.8 Exa~ple ~10 5.~ 500 17.8 3R,8 61.~
6.4 541 1~.~ 43.3 70.3 7.2 571 19,~ ~4.7 72.5 1~.1 602 1~.9 54.0 79.G
11.6 G45 20.~ 5~.7 7~.4 Example 1 0 5.5 4~5 14.4 ~.7 44.6 1~ 6.8 552 lg.3 46,~ 72.8 - ~5 7.8 5~5 l9.S 50.4 72.1 ~.~ G06 19.9 55.2 77.7 1~.7 ~67 22,3 64.3 86.5 33.0 6~5 2~.6 ~6.7 91.8 E~m~le 2 0 5.1 467 1~.2 ~8.7 48.7 1~ 6.5 S49 1~ 4.5 69.~
7.5 592 20.Z 4~.3 74.3 9.9 ~21 20.3 54.Z ~3.5 19.3 G76 2~.3 64.6 g~.S
33.4 699 2Z.2 ~8.0 86.8 Example 3 0 5.0 505 15.9 33.g 50.7 7.9 ~33 Z0.0 55.3 70.1 13.~ 6g9 21,~3 ~3.8 ~4.0 27 1 733 22.9 6~.8 8G.0 57 0 769 23.2 71.G 95.3 As s~en from the examples, ~e~tments of p~lp with oxy~en and - alkali prod~ed pulps wi~h highe~ shee~ densities and strcngth prope~tle~ in ~he unbe~n state (O minu~es beat~ng time) ~h~r.
untrc~ted pulp~, Figure 1 shows the ~eating ~i~es plotted ~gain~t Williams glown~ess. Upon a s~udy of Fi~ure 1 i~ becomes .
:

Case ~oc~et No. ~tlR ~5-ld eviden~ ~ha~ t~)e ~,y~en and alkali ~re~t~ent allo~s the p~lp to rc~ch a ~iven slowness with ~ lo~er amo~nt of beAt~ng, ~n an ind~stri~l scale, this result translates into decreased refinin~
en~r~y for equivalent pulp ~lowness levels. Con~rol E~A~P1e B
show~ ~h~t sorn~ o~ the stren~h increases are due to mechanical treatment ~eceivcd ~y thc pulp in the l~or~ory oxygen reactor.
~lowever, ~he.~e inereases are signi~icantly lower than those foun~
after thc additic)n o oxy~en and alkali.
In~rc~sc~ in ~he sodium hydroxide char~e in the pxesence of oxy~en i.mproved p~lp strcngth properties and 10~7ered ~he ~ea~ing ~m~s required tn ac~lieve c~ ~iven stren&th and slowness levcl. l'his ~an be deLermined fro~ a study of - Fi~ures 2, 3, and 4. Thc ~os~ ~ignificant improvements were ~bseLvecl ~iLh ~I caustic application of 5% based on ~h~ o.d.
~eight o~ p~lp.
Another rcsult o the oxy~en-alkali tre~cnt was a reduc~ion in p~llp happa nuMber. As shown in the ~ollowing examples, the ~c~ree of ]~appa number reduc~ion ~ direc~l~
relatèd to tlle so~lium hydroxide char~e.
A comparison of the s~ren~h properties of oxygen and ~lk~li tre~ted l~b~ratory pulp with the sa~le pulp cooked ~o a Kappa nu~ber similar ~o that o~ the o~ygen and ~lkali treated pulp is 6hown in Table I~.

: Control ~xam~le C
Thi~ pulp is a la~oratory prepared kra$t southern pine pulp with a waslle~ l~appa nurnber o~ 98.1. The pulp ~s then beaten ill ~ Vallcy Beater to v~rious Williams Slowness level~ and te~t hsndsheets wcre made.

i 0.5 C~se ~o~l~et l~l~. C~t~ U5-18 Control Example ~
This p~lp ls a labvratory prepared kr~ft southern pine pulp wlth a ~ashed K~ppa nu~er o~ 6~.6. ~'he pulp was then beaten in a V~lle~ ~eater co various Williams Slo~mess lcvels and ~est handsheet.s wcre made.

Ex;3mple 4 The same pulp a3 in Control Example C was mixed wi~h sodium l~ydroxide solution an~ su~icient ~ater to brin~ ~he pulp consistency to 12%. The sodium hydroxide ch~rge was 5% based on o.d. pulp weight. The ini~lal pll of the p~lp was 13Ø The pulp was then ~reated in ~ laboratory reactor for one hour at 78C
wi~l ~n oxy~en p~ssure of 15 psig. Af~r the treatmen~ thc pulp as washed ~tld the K~ppa number was de~e~min~d to bç 75.5, The p~lp was then be~ten in a Valley Beater ~o vnriou3 Williams Slo~mess levels, and tes~ handshee~s werc rnade. he pl~ o~ the p~lp after treatmen~ was 11.5.

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3.~0S ~e Do~ket ~lo. C11R ~5-1~

TA~LE II
CQ,IP~RISON OF OXYGEN AND ALKALI TREATE~ PIN~ P~LPS ~ H
KRAFT PULPS OF Sl~lILAR AND DIFFERENT KAPPA NU~BER~
Treatment Cond~tlons;
Labor~tt~ry Plne Pulp Prepared fr~m Charleston Pine ~hips 12~ Consistency One Hour Rcaction Time 5% NaOH ~pplied to Oxygen-Alkal~ Treated Pu1p 15,psig Oxygen ..

~TFl Tens~le Beating Williams Sheet Compres~ive Breaking ~re~tmentTim~ Slowlles~ Penslty Strength Len~h Burst Tear Uescription~In7n.) (sec.) (g/cc) (lb./in.) (lO ) Factor Factor Control Example C 0 5.l 0,3~ lO.l 32.1 17.3 214.3 (KappA No. 9B.1) 10 5.7 0.485 14.6 57.1 37.~ 262.~
5.90.516 l~.Z 63.3 41.5 263.4 2~ 6.g0.558 17.3 7~.2 47.5 236.9 ~ 37 20.1 86.~ ~8.3 2Z0.
l~.Z~.66S 20.5 86.5 G3. 5 20~.~
Control E~ample D 0 5.3 0.445 ll.7 3~.4 21.9 272.0 (Kappa l~o. 68.6) 10 5.6 0.550 l6.2 ~5.~ 41,3 30q.7 6.60.610 7~.4 73.1 50.1 272.3 7.90.64~ 19.9 80.2 58.6 259.Z
14.10.7l3 21.5 g2.2 69,9 2Z4.3 ~5 24.80,~32 ~2.4 98.6 73.0 Z15.8 Oxygen-Alkali O ~.10.492 14.5 40.5 32.0 Z94.6 Treated ~x~mple 4 lO 6.6 O.S99 18,~ 69.0 51.0 250.4 (Kappa No. 75.5) 15 7.8 0.65~ 19.7 77.3 59.9 Z30.7 ~0 9.70.680 20.8 ~2.1 ~3.5 210,5 23,50.743 22.6 94,7 69.8 196.1 3~ 40.50.77g 23.7 99.6 75.0 1e5.5 , As ~een from Ta~le Il, t.he oxygen and alkali tr~.ted pulp~was significan~ly higher in compressive stren~h, burst factor, brcaking lcngth, and handshee~ density when compar~d ~o ~he two ~raft pulps at constant beating ti~e. It is evident, ~he~efo~e, that stren~h propertie~ are more f~vorably enhanced by oxygen and alkali trea~ment than by an equlvalent red~ction in pulp Kappa number achieved ~hrough k~aft p~lping ~hanges.

- 10 - ' 1~ 05 Case ~ocket No, ~HK ~5-1~

l~hile this invention l--~s been described arld illustrated herein by re~erence ~o various fipecific m~terials, proced~res and .
cxamples, it is understood that the inVenti.OTI is ~ot restricted to ~he particular materi~ls, combina~ions of materlalsl and procedures selected for tlla~ purpose. ~lumerous variations of such details can be ~mployed, as will be appreciated by those skilled in ~he ar~.

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Claims (11)

1. A method for producing linerboard paper from high yield chemical wood pulp by treating the pulp with oxygen and alkali at a temperature of from about 50°C to about 100°C and a pressure of up to about 150 psig in the absence of protectors to increase the refinability of the pulp and the strength properties of the paper by limiting the reduction in lignin and carbohydrate content in the pulp as determined by a Kappa number reduction of no greater than 25% in the treated pulp.

2. The method of claim 1, wherein the pulp is a sulfate pulp.

3. The method of claim 2, wherein the high yield pulp has a Kappa number from about 60 to about 120.

4. The method of claim 3, wherein the treated pulp has a Kappa number of from about 45 to about 90.

5. The method of claim 1, wherein the chemical wood pulp is prepared from the digestion of lignocellulose materials in alkaline cooking liquor.

6. The method of claim 1, wherein the alkali is charged to the pulp prior to reaction with oxygen.

7. The method of claim l, wherein the alkali is added in an amount of from about 0.5% to about 5%, based on the oven dried weight of the pulp.

8. The method of claim 6, wherein the temperature is about 78°C and the pressure is about 15 psig.

9. The method of claim 1, wherein the pulp has a consistency of 8-20%.

10. In a method of delignifying chemical wood pulp with oxygen and alkali, the improvement comprising producing a treated pulp of improved refinability and a paper product of improved strength properties by limiting the reduction in lignin and carbohydrate content in the pulp as determined by a Kappa number reduction of no greater than 25% in the treated pulp by treating the pulp with oxygen and alkali, in the absence of protectors, at a temperature of about 78°C and a pressure of about 15 psig.

11. The method of claim 10, wherein the chemical wood pulp is of medium consistency.