CA2247542A1 - Process for preparing base paper for fine paper - Google Patents

Abstract

The present invention concerns a method for producing a base paper for fine papers, the base paper being produced from a mixture of a mechanical pulp and a chemical pulp.
The chemical pulp used comprises a chemical softwood pulp having a elastic modulus close to that of the mechanical pulp and a great bonding strength. It is preferred to use a chemical softwood pulp which produces a sheet having a elastic modulus of less than 6000 N/mm2 when the bonding strength is 400 J/m2. A pulp of this kind has a goodScottBond strength at the same light scattering. The base paper produced by the method can therefore be used in double-coated fine papers which require a large bonding strength of the base paper.

Description

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Pr~lc~ss for Preparing Base Paper for Fille Paper The present illvenhorl cnnc~rn~ a process according to the preamble of claim 1 for ~rCpa~ g a base paper th~t can be used as base pa~er ul paIticular for m~nllfA~lrin~
coated fine papers or printing ~a~ oards. A paper or paperbo~rd of this kind COI ~ ~p~ ;~es ble~rhP~l rhPn~iC~l pulp.

T~e present invention also cQn~ernC ~ base paper for f~ne papers ~rror~line to the preamble of claim 13 and a process according to the preamble of claim 14 for m~n-lf~tllnnf~i fin~:
paper.

Ihe special problem of coated, in particular double-coated, fine papers is that the paper web tends to split in the dIyer of the r intin& mz-~hin.- when ~ater from the pnntin~ colour 1~ and s~milar solvents are removed by drying The problem is caused by ~e fact that double-coating forms on the surface of the paper a ve~y dense coating layer which csnnot be p~.n~trat~rl by steam ~apounzing from ~:he hase paper. The steam rrimArily stems from the normal 4 to 5 % moisture co~tent of paper and the bubbles formed from the mni~h~re break the paper, if the s¢ength ~ ties ~f the base paper are not sllff~ nt for r~ei~tin~ t.his steam pressure.

The afore-~esrrihed problem is called blicterin~ and the required intemal bo~d strength (z-directinn~l strenght) of the paper is measured by the ScottBond value.

25 Tr~r~iti~ n~lly, a reduction of the bli~t~nn~ of the base paper of f~ne papers has been aimed at by increasing the beating of the r~ c~ I pulp, in order to obtain more bonds between the fibers. This solution comrri~Ps the disadvanta~e that an increase of the bea~ing does not enh~n-~e the bonding strength expressed by the ratio of strength-to-bonding surface area.
Increased beating causes a number of problems. Firs~, when the bea~irlg is increased, 30 dewateri~g of paper is ~ a~Led. Therefore, the water content ofthe paper is disadv~nt~eously hig~ when the paper aflcer web r~ lillg is tr~ncfe~ed ~o ~e wet press section of the paper m~rhinR arld thcn onwards to the dryin~ section. As a result, it becomes more likel~ that the paper will adbere ~o the rollers ofthe wet press and drying sec~i~ n~, and ~e risk of web breaks ~ncreases. Further, the str~ngth of the web is srnall at 35 higher water c~ntPntC and this already increases the risk of web breaks.

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CA 022'7542 1998-09-16 ~ 2 Secondly, also the ~lup.,.lies of dry ;~aper ehange in a an undesireable way if the pulp is subjected to exteIIsive be~tine When the beat~llg i~ increased d~e density of the paper ~rows and as a result the stiffn~c of the pc~per decreases. This causes n~nability problems ill the paper m~ inP due to wa~ edges. ~hen paper density grows, the fibers of the 5 chemical pulp are more and more ti~htly bonded so that the elastic modulus increases Then the paper becomes brittle and ~nd its tollghnPcs is not s~ nt to meet the strain caused by the paper and ~rinting m~hinP~

It should be men~ioned that the llncu firif t~l intemal bond s~erl~tb of pa3~er causes 10 problems also during sheet offset pri ltin~ although no s~ P dryer is used in that printinf~ technique. In sheet offset pr nting lhe problem is fonned bec~llee the prin~ng colours are sticky. When the paper i- released f~om the printing nip, 'he surface of the paper and the wet printing colour arr stuc1~ together. If the ulternal bond strength of the paper is not large enou~h in com~ ; on to ~e i~temal cohPcion forces of the printin~
1~ colour, the surface of the paper will -~cccm~ y the rrinting colour and the paper will split in the middle of ~e sheel. Incre ced '~eatir;g of the chPmir~l pulp has been used ~n attempts to solve this problem also. ;

It is an object of the present irlventio~ to çJ jmin~te the problems of ~e pnor art and to 20 provide arl entirely novel metl1od for produciIl~ a paper web v~hich can be used as a base web for coa~ed fine papers. In particular it ~3 an object of the present invention to provide a paper web having ~.~rf~llerlt formatio:l and ~vith a capaci~ of fc~ ing particularly saong bonds.

25 The present mvention is based on the idea ~.~,f forrrli~e the base paper from a rnixture of mPrhanj~l and chemical pulp, the chemic ~l pulp ~ed comrricin~ a rh.-mi~al softwood pulp ir~corlJolaLing in cnmbination a large .~cottBond strength and a elastic m~ 5 which is relatively small for rh~niC~l sofcwood pl~lp. Prefera~oly t~e elastic mo~ c is less than 6000 N/mm~, when the ScottBond-strength of the c~ mi~al pulp is 400 J/m2. 1 hus, a paper 30 produced from a mLXtUre of mPrh~ni~ pul~ d chemical pulp will Cimlllt~pously have high ScottBond strength and lar~e toughn~ s.

More sperifi~lly, the solution accordir~ the present invention is mainly characterized by what is stated in the chAr~ct~ri~.nF part of claim l.
t~.onci~1prable advantages are obtail,ed by t le present invenrion. Thus, the pulp produced .; . . .

accor~iing to the invention has a~ the same anlOUnt of surface bonding, i.e. at the same light t.orin~ a better bonding strength than comparative pulps. The present base paper can therefore be used for protlllrtinn of double-,coated fine papers which in p~rticular require greated bondin~ strength of the base paper. Other fiber components whose int~.rnAl bond S s~en~th in itself is not suffiçient can be ~nco~porated into the base paper. As a specifie eY~rnrle reference can be made to the m~n-~fA~nlre of fine paper ~om mixtures of aspen groundwood and cllPrnir~l softwood pulp, whereby a strong paper is obt~inP.l as a fini~h product, said paper having ~ood hrigthnPs~s and opacity and a very smooth sur~ace. ~hanks to the good bon~in~ strength of tl-e chem~c~l soft~ood pulp, aspen groundwood can be 10 used even in a~.n~unts up to 20 to 60 % of the dry matter of the pulp.

The tec~ni~-~l solution according to the present invention comrr-~s using a ~h~mi~l pulp which has beerl produced by rl~Prnir~l pulping which will protect the fibers, ~hereby their s~ength remains good. The cooking sbould be selective irl the sense that it selectively 15 removes lignin alld spares the carbohydrates of the fiber. In co~ulection with the present in~en~on it has been found that the~e objects can be obtained by using batch cookin~, a particularly l ,efe.,ed emboo~limPnt comrn~in~ extended batch coo~ tSuperbatch cooking).

20 As re~ards the stren~ of the r~ rni~ pulp, the pulping m~ tho~ is not as such a sllffiçierlt crjterion~ but the ~h~ l pulp ~r~,~luc~1 a~cordin~ to the i~vention should have enough bonds ~ the fibers. In corln~ction with the prese~lt in~cntion is has been found that by blea~h;~.~ softwood pulp produced by batch cooking with TCF hle~ in~ compris~ng blea~hiTIg stages with peroxide and ozone particu~arly good streng~ p.o~e.Lies are~5 obtained. Said o~ i7i~ chemi~-A1c form carboxylic groups on the fibers and these grollps p~Jve the strength of the ble~ch~cl pulp.

The i~oll~nce of the acid groups for forrr.ing bonds between the fibers has been riiscl~cse~
in Ba~k, D. et al. Joumal of Pulp and Paper Sci~n~e, 23 ( ~ 997) JS9-J6 1. According to 30 that article the bo~ n~ strength is based on,carboxylic groups. Inthe present invention it has, ~owever, been found that it is not only the ~nount of acid groups tha~ is decisive, but the con~lihor~s ofthe cook and Ihe blearllin~ sequences are also of i~ o,~ulce.
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As ~licclleeed above, ~vhen aLl~ are ma~e to regula[e the l~ro~cllies ofthe pulp by 3~ be~tirlg~ i.e. when the ScottBond is raised hy a high degree of be~tj~, ~e rh~ pulp and, e.g., hardwood grolmdwood, will ~et ~rery different elar,tic moduli (chemical pulp gets . .

very high stiff~l~ss)l which is undesirable as far as the to~hn~s of a mixture produced of these pulps is CQ~ e~P~ This problem is not encountered ill the prcsent inve~ntion For this reason, by means of the present invention, a ~ Lu~c of hardwood groundwood &nd che-mical pulp is obtained whicll is excellellL ~5 ~ basc paper of fine papcrs.
s According to a ~re~l~d eJnbodir:lent~ the eh~rnic~l pulp used for preparing a base paper is produced by a coolcine method known as ~ modified batch-type cook (Superbatch Cook).
Tllis cook is discribed in tlle lit~r~tllre [~f for example Malinen, R. Paperi ja Puu (Paper and Tirnber)~ 75 (1993) 1~18~. The cook in question is a modified coolcing method which 10 utilizes an ~Ik~line cooking liquor just as the sulphate cook, but wherein rl~li nifir~tinn has been enhanced so that the kappa number of the chemical pulp is lo~ered without asignificant reeduction of viscosity. Typically with a S~ alch process, pulp is cooked to a kappa number of 20 or less. -.
i 1~ According to a prefeIred embo~lim~nt of t-~le prescnt i~vention, a sof~wood pulp produced by batch cookiIlg is blc~r-h~d with TCF bl: ~chi~g. The f~llowing eY~mrles o~ sllit~hle hl~rllin~ s~que:nces can be mrnti~n (Q)-O-Z-P-Z-P
(Q)~~~Z~~~Pn O-(Q)-Z-E-P-Z-E-p O~Z~(Q)~Pn O-X~Z~Pn 25 O = oxyge~ treatrt~ t P = pe~oxide lrc~ -cnt Pn = several sllccessive pOEoxide trea~nent stages E = alkali step Q ~ t~nt With C~rnrl~yin~; agent 30 X = enzyme ~tment Arl acid ~,u~ nt at elevated L~ e.aL~Ire (ar} A slage) can be p~. I'n. ~ berween the oxygen ~1Pli~nifit~tion (O-stage) and a ble;~ching step carried out with an oyi~li7ing chtornic~l (i e a Z-slage).
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It is par~icularly p.e~llcd to carry out t~ ~ bl~ hirlg of lhe pulp with t~o ozone stages . ' '' ~

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, 5 and at least two peroxide stages. Between the s~ages carried out with ox~ n~
rhf~rnir~l.c, it is possible to extract Ih~ pul~ during variolls Au~ np stage_ (such as E a~d E0) and/or to wash it with ~rater. I

S Following the above~srrihed tre~trnPn~ a pulp is obtained having an intern~l bond s~rength which is better than that of col,.p~ative pulps. It Iypically cQIltAinc at least 40 mtnol carboxylic acid groups/kg dry pulp. Preferably t~e elastic m~ S of the ~h~rnirsll pulp used accordin~ to the present illvention is below 6000 N/~, in particular below 5000 N/mm~ when ~cortRon~1 streng~h is 400 J/m2.
As mPntil~nP~ above, the base paper can bé produced firom rhP~ir~l pulp by comhini-~ it wi~ a pen groundwood, by Clllchin~ the obtAin~d fibrous base material, by fom}~g a web from the stocl~ and by dry~ng the web on a paper m~ hinl~ iIl order to form a base paper. Generally, the pulp can 'oe produc~d from any m~ bqnie~l pulp made of a tree OI
15 the Popul7~s family. Sl~itA~le species are, 'for eYAmrle, P. ~entlcla, P. trem~loides, P
balsarnea, P. bals~",i~,a, P. trichocarpa aJld P. heterophylla. A ~r~ cd embor1imPnr comrn~es using aspen (trembling aspen, P. tremrlla; an aspen k~own as C~n~iAn aspen, P. trem~oi~s), or aspen ~ arieties know~ as hybride aspens produced from dilr~e lt base aspens by hybrid~zing as ~vell as o~her species prod~ced by recombinant technology, 20 or poplar. It is preferred to use groun~wood (GW), pressure g~uul~dwood (PGW) or th~ .,,P~hqnir~l pulp (~MP) m~mlf~r~lred from aspen, hydride aspen or popl~r.
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PrPfer~ y the mPehqnir~l aspen pulp c~ nt~in~ about 10 to 20 % of +20... +48 mesh fibers, which confer mPr~nir~l strength ;lo the pulp. In order to m~Yimi7~ lightsc~e ;~, the portion of +100, +200 and -200 f.acl;o~.~ should be as large as possible.
Preferably they stand for ~lictinrtly more ~lan 50 % of the whole pulp. III par~icular dleir ~7101JUlLiOII of the whole pulp is over 70 %, preferably over 80 %. On the otber hand, the amoun~ of tlle smallest ~action, i.e. the -200 _esh, should not be too large, because ~en dtw.lL~,ihlg on the paper m~rhinP vvould become more ~iffirolr Pre~erably Ihe proportio of this fr~tinn is smaller than 50 %, in particular ~5 % or less.

Due IO ~he esrP11PlU ~nrrk-~nirSll p,upe.~e~ of the pulp accord~l~g to ~e preseDt ill~,~Lion the ~u~urLion of the mP~h~nie-l pulp ca~ be even up lo 70 weight-% of the d~ matter of ~e s~ock ~ilhOul the s~eng~h of rhe paper essenti~lly ~urrL- ;u~. Typically, the ~opol-ion of the ~ c~-i- al pUlp iS at leas~ 20 % aIld ~n par~icular it is 30 to 60 weigh~-%

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Based on what is stated above, according~co the invention the CnmpoC;ti-n of a particu-larly lJr.,f~ d base paper is the followin~: 30 to 60 weight-% of rhe fibrous matter corc~rises mPrh~nirsl pulp prodllced ~rom aspen and 70 to 40 weigh~-% comprises softwood ~ Tn;~ pulp. The ScottRQn~ strength of the ch~--ir~l surLwuod (in parlicular S pine) pulp is at least 400 J/m~ at a light scauering coeffi~ient of 22 m~/kg and it cont~inc at leasl 40 mmol carboxylic acid groups/kg dry pulp From the base paper according to the pr~sent invention it is possible ro produce high-quality fine paper by coating it preferab. y twice, the first coating for PY~mpl~ be~g 10 carried out by a mPtho~ kno~n as ~he fi~r~l press mf~thntl, and the second coaung is ~e.fo.l.led by blade coating. The amoun; of coatin~ colour applied to the ~veb by the film press me~hod is t~pically about S to 50 g coating colour/m2, whereas the corresponding amount for doctor blade coating is 10 1o 60 g cûating cûlour/m2. The in~lic;~t~d amounts of coat~ng have been c~lr~ rPA ~om ~he ~Iry matter of the coating colour.
NeYt, the in rention will be eY~minpA more closely wiLh- ~he aid of a ~e~ ed descnpuon and with Lcfe.c.lc~ to the ~rhf~A drawin~Ts and working eY.~mples.

Figure 1 compares the pulps Ai~rk~sed in ~~ ~Y~mpl~os; the ScottRQ:~A al~cl~gth iS
20 inAir~ A on ~he y al~is as a fimc~ion of tho ligh~ âca~ i~ coefficient, Figure 2 ;nrlirzres the Scot~RonA strenghts of three mixed sheets as a filnrtiorl of the light scattering coPfhri~nt and Pigure 3 cont~inc a comr~ con of the ela~i~ic moduli of four ~ n~ al pUlpS as a ~mrtinn Of il~tpl~ bond stren~th.
The following mea~wc~ L standards ha-e been used in the examples:
- IS0 brightn~ss of the c~ c~l pulp~ C'M 11 and SCAN-P3 - light scal~.~ug coefficient: SCA~-C 27 - Scottl3ond s~ren~th; Tappi T833 - bri~htnPsc: SCAN-P3:93 (D65/10~) ..
- opacity: SCAN-P8:93 (C/2) - surface coar~ SCAN-P76:95 _ R~nAtsPn coarse~less: SCAN-P21;67 - ~loss: Tappi T480 (75~) and T653 ~2~~ ~
35 - elastic moA~ c meacurement: SCAN-P ~8 (slrip size and tensile velocity) " , , ~.

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For measurement of the elaslic mod~Tlll~ the sheet was prepared and tlle clryi~g was carried out according to standard SCA~-C 2~.

F.Y~np~r 1 5 TntPrn~l bond st~ength of r.hrm;r~l pulps T~e Sco~RQn~l streng~ of sheet produced from soft~ood ~h.~mir~l pUlp is infll~n~erl by the exteD~t of bo~d~g surface between the fibres and the stren~h of the bonds. The amoum oft~e bond~ng surface is, on i~s par~, s~roIlgly depeIldent on tbe degree of bealing lO of the rll~mir~l pulp used ~n ~h~etm~kin~ When beating is increased The bon-lin~ area and at the same time the bon~lin~ strength are increased. To ma~ce it possible to compare bond ~L~ s~ in ~his example the illterllal bo~d strellgth~ of dirr~c;ul chPTnir~l pulps are coù~pa~ed by ~.~...;nil~ them as a fi~ tion of the light sc~ coe-ffiri~nt in ~he same way as in the article by ~3arzyk et al~ Joumal of Pulp and Paper Sci~ e, 23 (1997) J59-J61, Figures 3 and 4, already referred to above. It is cou eit~ble tha~ wilh ch~mir~l soft~rood pulps the light sc~ coef~ciellr is a measure of the amount of bonding surface of the fibers, the greater the amount of bonding surface the sn~ller ~he ligh~
scatterirlg coPffiri~nt 20 In this ~esl ~e iMerrlal bond strength and light scarcer~ng coeffiri~nt of ch~ni~ ~l pulps have been mo~lifiPA by beating the pulps in an ~scher-Wyss-refiner at various energy amounts of 0 to 200 kWh/ton. The specifie edge load of dle beating was 3 Ws/m. The results are intlir~t~l in Figure 1. In that ~igure, the curve ~xre~ E to a higher level at the same amount of bonding sllrface, i.e. Iight scattering, stands for an irlcreased bon~ g 25 strength.

C~raphs 1 to 3 depict cellulosic pulps prod~lced by a contin~ batch cooking (Super-Batch) whicll have been subjected to chlo~ e-free b~ ~hin~e (TCE~) by using two ozone and two peroxide stages (ZPZP). Graphs~4 and 5 depict a pulp ~lo.l,lccd by a colltiml~lc COOl~lg m~thn~, ~hich also has bee~ sub~ected to rh1orin~free ble~hi~-~ (TCF) byusing one ozone and one peroxide stage (ZP). The coolLing result is, culu~.ued to the above mPnrionP~1 ba~:h cooking, more heterogenous and weal~er fibers are pro Ill~eA The fiber collapses more easily, and i~ looses ~s light sc ~ c~f~ient which moves ~ecurve to the left. The pulps produced by both mPth~ls 1 to 3 anLI 4 and 5 comain at least apprrJ~ tPly an equal ~mollnt~ of carboxylic acid groups (41 - 47 mekv./kg alld 42 - 46 mekv./kg, l.,s~ ely).

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~, Graphc 6 tO 9 show pulps which havee be~ subjected to a b'R--~ine without elemPn~l rhlorinP (ECF ble~chi~). The st~ng m~T~ri~l of ~o~ti~ 6 was a raw m~tPri~l obt~in~hl~ in t~e north of Finland Il compr~ses small s~ze fibers which give a large .~perlfir surface (m2/g fiber) and, ~erefore, it light scalterin~ coeffiriPnt is good. The S conr~onrration of carboxylic acid groups was 34 mekv./lcg. The raw material of cooldng 7 was ob~ained from F~1enl Finla~d and t~e chP-nir~l pulp had been prodused by batch cooking. Graphs 8 aIld 9 ,eplesell~ the ~nternal bonding ~ n~lh of pulps produoed by cnntin~ llC cookin~ aDd hll~rh~l by _CF t~le~o~ . The conr~ r,.linn of carboxylic groups was 27 to 34 mekv.lkg. The graT~}j~ show tha~ pulps 1 to 3 gi~e greater values for ehe bon~ing strengt~ thaD the other pulps i~ the same light SC~ coeffirient ~he dirr~ ces becomP more pron-)~mred wh~l the pulp have been subjec~ed tO extended beati~g.
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~ex~, three of the afore-mPnrinn~ pulps were sel~~r~A for a sheet formi7lg test. ,~1thoU~h ~he pulps were not from the same batches as above, pulp A cor,~o~de~ to pulps 1 to 3, pulp B co~,s~o-lded to pulp 6 and pulp C cor~es~onded to pulp 7. The pulps were refined ~n a l~hot~r~)ry Valley bea~er cO ~ the degree of bea~ing (drai~age) was CSF
380 ml. Then sheets were produced from ~e pulps so Iha~ in each ~est point the sheets cnn~inPf~ 60 % rhPmir~l pulp and 40 %: Lspen PGW pulp (aspen of Populus family).
When the bonding srrengths of the mixed sheets vs. light sc~ E coeffi~iPnt now were e~ .-unP-l, a reSUll according tO Figure ~ as obtained. Even if the dirrtrc.,ces are rather small, it is apparent that c}lPmir~l pulp A L~i~eS a beuer result than pulps 13 a3~d C. The ~end is the same as for pure pulp sheets; Ln o~er words: by the combination of batch cookin~ and TCF ble~hjn~ accordlng to the present invenrion a better bonding strength is obtain~d than for the co.~ ;ve pulps, even if these separately ~clude the partial ele~Pnt~ of the invention.

Finally, an analysis was made ro ~lPrPrmit e how ~he elas~ic InoAllllls develops as a filnrtil~n of ScouBond ~k~ h. This tes~ clllded pulps from three pro~llrti~)n batches (Al, A2 and A3), which co~es~on~ ro F~ulps 1 to 3 of Figure 1, an~ a pulp sample D
which c~ o~ ed to pulps 8 and 9 of .F i~,rure 1. Pulp ~rles Al and A2 had been refined to different beating degrees in a ~ icher-Wyss refuler and ~ s A3 arld Dagain ~ a Valley bealer. Figure 3 show~ lhar the elastic mod~ lc of pulp A was smaller than for pulp D, if the C0~1 ~150n iS carried out at t~e same .~cottRnn~l strength. Thus, it can be ~xre~rpd from ~he pulp A according to tbe invenlion ~at it gives a smaller elastic ~. .

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, CA 02247542 l998-09-l6 modlllllc tharl D and, accordingly, that a paper produced from pulp A is less bri~de. In o~er words, the paper is tougher ~an a paper made from pulp D. The superiority of pulp A is ~o-.ol~n-ed ~hen the pulps are beaten to a high degree of beati~g in order to obtain good ScottRond stre~gth. -Example ~
Production of a fine paper cnnt~inin~e aspen PGW

A base paper was produced from a mPch~nir~l aspen pulp (GW) and ~ht~ nir~l p~e pulp, 10 which were mixed at a wei~ht ratio of 40 to 60. Ground r~lrillrn carbonate was added as a filler to the suspen.ci~-n in an amount of about 10 % of the fibrous m~tPri~1 The base paper was produced on a gap former. The ylo~t:lties of the base paper were the following:

gpmm~e 53 3 gJm2 buLk 1.45 cm31g opacity 88 %
bri~hmPss 82.5 ~;
CQ~r~Pnrc~ 240 ml/mi~
porosi.~ 170 ml/min filler colltent I2 %

25 ~nmr~rative test carried out in u~nnPc-tion with the invelltion ha~e shown that the ~ lAI.. ~e of the base paper is at least lO % smaller than that of a base paper produced entirely from a b~ 1 ch~mi~l pulp arld having the col.esponding opacity and brl nhtnPss .

30 In order to produce the fine paper from t}le above~escribed base paper it was coated nvice, firsl with ~e film press method a~ the~ ~vith doctor blade coa~ing A r~lrillm c~LOIlaL~ pigment having ~he ~arlicle size distribution shown i~ Table 1 was used in Ihe coa~ing colours;

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Table 1. ~article size distribution ~f Ih.~ r~onate p;g~nt Ma~. par~icle s~zeCumulative pro; ,~ n of [llm] weight 2 gS

'' ' 0.2 10 The coatin~o colour was produced in a r~ ~r known per se b~ mixing together the pi~mPnt the binder and the other addi~ . Tl~e d~ r content of ~e preCQ~tinn colour was ~0 5~ and of the surface coa~i ~ r col~ur 61 % . The aboYe dcscl,bul colours 15 were used for coat~ng the afore-m~ltior~ ag2 pap!~r in the following conrl-tions:

Preco~ir~ by the film press Illeth~: 9 ~ p'~r sll~; ~d the surface coa.ting al a doctor blade sladon: 10 ~ g/m- per si~e al a S~;.'f ~ 1500 mJmin. The coated paper was ,cuper-c~l~n~l~red.
Tlle properties of the end prod~c~ were ~ rmine~ ~d compar~d to tho~e of ~wo c~ cially avaLlable finer ~ap~rs, v:i7. .L.mr~izrt ~ o) and Nopacoat (Nordl~
Papier). The results will ~ppear ftol?l T 1'.. ~':

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Table 2. Optical pl ~ye. lies of a double-coated f~e paper Paper~ I,umialt Nopacoat according to the inven~ion G~ammage [g/m ] 80 100 99 S BuL~c 0.85 0.83 0.78 Opaci~y [%] 94 92.7 92 6 Bri~htna~c [%] 94 91 96.7 ~ m-~othnrce pps 10 ~llmJ0.8 1.2 0.8 Gloss [%] 73 66 71 Table 2 shows that the propc-Lies of a fine paper produced by the inveIltion are better in all ~ ;L~ than those of c~..ra.,.rive papers having co,les~o.lding buL~c and ~.,3~-..-~age.
Thus thc yield ga~n on cqual levcl of opaci~ is over 20 %.

The ScottRnn~l bonding s~er~ of t~e fine paper ple~aled according to ~e Fx~mrle was 306 J/m2. This is also fillly co~?ar~ble to the ~lle~lh of a ~ irio~l fine paperc-~mpricing only rl~ ir~l pulp. Even if the inte~al bondi~g ~e~g~ of aspen PGW is i~feAo~ to ~al of e.~. c~P~ al b~rch pu!p, the present iIlvention has provided a paper ~0 which is strong cnough for usc as a fine paper.

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Claims (15)

1. A process for producing base paper for coated fine papere, characterized in that the base paper is produced from a mixture of mechanical pulp and chemical pulp, whereby the chemical pulp used comprises softwood pulp having a elastic modulus which is close to the elastic modulus of elasticity of the mechanical pulp and exhibiting a large bonding strength.

2. The process according to claim 1, characterized by using a chemical softwood pulp, which can be used for producing a sheet having a elastic modulus of less than 6000 N/mm2 when the bonding strength is 400 J/m2.

3. The process according to claim 1 or claim 2, characterized by using mechanical pulp produced from hardwood and bleached softwood cellulosic pulp produced by batch cooking.

4. The process according to claim 2 or claim 3, characterized by using softwood cellulosic pulp bleached with ozone and peroxide.

5. The process according to claim 4, characterized by using softwood cellulosic pulp bleached with at least two ozone stages and at least two peroxide stages.

6. The process according to any of the preceding claims, characterized by using a cellulosic pulp produced by the SuperBatch process.

7. The process according to any of claims 1 to 6, characterized by using cellulosic pulp having a ScottBond strength amounting to at least 400 J/m2 at a light scattering coeffcient of 22 m2/kg and containing over 40 mequivalents of carboxylic acid groups per kg of dry pulp.

8. The process according to any of the preceding claims, characterized by using a cellulosic pulp having a brightness of more than 82, preferably over 85 and inparticular over 88.

9. The process according to any of the preceding claims, according to which process - a stock is formed from a fibrous raw material, - the stock is formed into a web, and - the web is dried in order to form a base paper, characterized by - forming the stock from mechanical pulp produced from wood material from the Populus family and from TCF bleached softwood cellulosic pulp produced by batch cooking, the amount of mechanical pulp being 20 to 70 weight-% and that of the bleached softwood cellulosic pulp being 80 to 30 weight-% of the dry matter of the suspension.

10. The process according to claim 9, characterized in that 30 to 60 weight-% ofthe dry matter of the stock is formed by mechanical pulp and 70 to 40 weight-% by softwood cellulosic pulp.

11. The process according to claim 9 or 10, characterized in that the mechanicalpulp is produced from P. tremula, P. tremuloides, P balsamea, P. balsamifera, P.trichocarpa or P. heterophylla.

12. The process according to claim 11, characterized in that the mechanical pulpis pressure ground wood.

13. Base paper for fine papers, characterized in that 30 to 60 weight-% of its fibrous material consists of a mechanical pulp produced from aspen and 70 to 40 weight-% consists of chemical softwood pulp, the latter having a ScottBond strength amounting to at least 400 J/m2 at a light scattering coefficient of 22 m2/kg and containing over 40 mequivalents of carboxylic acid groups per kg of dry pulp.

14. A process for producing a coated fine paper, characterized in that a base paper produced according to any of claims 1 to 11 or a base paper according to claim 12 is coated with two coating colour layer, the first coating being conducted by the film press method and the second by doctor blade coating.

15. The process according to claim 14, characterized by applying on the web 5 to 50 g coating colour/m2 by the film press method and 10 to 60 g coating colour/m2 by doctor blade coating, the coating weights being calculated based on the dry matter of the coating colour.