Paper making and fiber liberation – Processes and products – Plural fiber containing
Reexamination Certificate
1998-09-16
2002-06-18
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Plural fiber containing
C162S141000, C162S135000, C162S147000, C162S149000
Reexamination Certificate
active
06406592
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention concerns a process for preparing a base paper that can be used as base paper in particular for manufacturing coated fine papers or printing paperboards. A paper or paperboard of this kind comprises bleached chemical pulp.
The present invention also concerns a base paper for fine papers and a process for manufacturing fine paper.
The special problem of coated, in particular double-coated, fine papers is that the paper web tends to split in the dryer of the printing machine when water from the printing colour and similar solvents are removed by drying. The problem is caused by the fact that double-coating forms on the source of the paper a very dense coating layer which cannot be penetrated by steam vapourizmg from the base paper. The steam primarily stems from the normal 4 to 5% moisture content of paper and the bubbles formed from the moisture break the paper, if the strength properties of the base paper are not sufficient for resisting this steam pressure.
The aforedescribed problem is called blistering and the required internal bond strength (z-directional strength) of the paper is measured by the ScottBond value.
Traditionally, a reduction of the blistering of the base paper of fine papers has been aimed at by increasing the beating of the chemical pulp, in order to obtain more bonds between the fibers. This solution comprises the disadvantage that an increase of the beating does not enhance the bonding strength expressed by the ratio of strength-to-bonding surface area. Increased beating causes a number of problems. First, when the beating is increased, dewatering of paper is impaired. Therefore, the water content of the paper is disadvantageously high when the paper after web forming is transferred to the wet press section of the paper machine and then onwards to the drying section. As a result, it becomes more likely that the paper will adhere to the rollers of the wet press and drying sections, and the risk of web breaks increases. Further, the strength of the web is small at higher water contents and this already increases the risk of web breaks.
Secondly, also the properties of dry paper change in a an undesireable way if the pulp is subjected to extensive beating. When the beating is increased the density of the paper grows and as a result the stiffness of the paper decreases. This causes runability problems in the paper machine due to wavy edges. When paper density grows, the fibers of the chemical pulp are more arid more tightly bonded so that the elastic modulus increases. Then the paper becomes brittle and and its toughness is not sufficient to meet the strain caused by the paper and printing machines.
It should be mentioned that the unsufficient internal bond strength of paper causes problems also during sheet offset printing although no separate dryer is used in that printing technique. In sheet offset printing the problem is formed because the printing colours are sticky. When the paper is released from the printing nip, the surface of the paper and the wet printing colour are stuck together. If the internal bond strength of the paper is not large enough in comparison to the internal cohesion forces of the printing colour, the surface of the paper will accompany the printing colour and the paper will split in the middle of the sheet. Increased beating of the chemical pulp has been used in attempts to solve this problem also.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems of the prior art and to provide an entirely novel method for producing a paper web which can be used as a base web for coated fine papers. In particular it is an object of the present invention to provide a paper web having excellent formation and with a capacity of forming particularly strong bonds.
The present invention is based on the idea of forming the base paper from a mixture of mechanical and chemical pulp, the chemical pulp used comprising a chemical softwood pulp incorporating in combination a large ScottBond strength and a elastic modulus which is relatively small for chemical softwood pulp. Preferably the elastic modulus is less than 6000 N/mm
2
, when the ScottBond-strength of the chemical pulp is 400 J/m
2
. Thus, a paper produced from a mixture of mechanical pulp and chemical pulp will simultaneously have high ScottBond strength and large toughness.
More specifically, the solution according to the present invention is a process for producing base paper for fine paper, wherein the base paper is produced from a mixture of mechanical pulp and chemical pulp, whereby the chemical pulp used comprises softwood pulp having an elastic modulus which is close to the elastic modulus of elasticity of the mechanical pulp and exhibiting a large bonding strength.
Considerable advantages are obtained by the present invention. Thus, the pulp produced according to the invention has at the same amount of surface bonding, i.e. at the same light scattering, a better bonding strength than comparative pulps. The present base paper can therefore be used for production of double-coated fine papers which in particular require greater bonding strength of the base paper. Other fiber components whose internal bond strength in itself is not sufficient can be incorporated into the base paper. As a specific example reference can be made to the manufacture of fine paper from mixtures of aspen groundwood and chemical softwood pulp, whereby a strong paper is obtained as a finished product, said paper having good brightness and opacity and a very smooth surface. Thanks to the good bonding strength of the chemical softwood pulp, aspen groundwood can be used even in amounts up to 20 to 60% of the dry matter of the pulp.
The technical solution according to the present invention comprises using a chemical pulp which has been produced by chemical pulping which will protect the fibers, whereby their strength remains good. The cooking should be selective in the sense that it selectively removes lignin and spares the carbohydrates of the fiber. In connection with the present invention it has been found that these objects can be obtained by using batch cooking, a particularly preferred embodiment comprising extended batch cooling (SuperBatch™ cooking). SuperBatch™ (Trademark of Sunds Defibrator). The superbatch method is a modified batch method in which one or more SuperBatch™ digesters are used to prepare pulp. The superbatch method is a method in which hot cooking liquor is fed to the chips from the bottom of the digester. The hot cooking liquor is then circulated, with or without a heat exchanger, and steam is added to the liquor to raise the temperature thereof. After the desired defibration point is reached, the hot cooking liquor is displaced by warm liquor. The digested pulp is discharged from the SuperBatch™ digester by pumping.
As regards the strength of the chemical pulp, the pulping method is not as such a sufficient criterion, but the chemical pulp produced according to the invention should have enough bonds between the fibers. In connection with the present invention is has been found that by bleaching softwood pulp produced by batch cooking with chlorine free (TCF) bleaching comprising bleaching stages with peroxide and ozone particularly good strength properties are obtained. Said oxidizing chemicals form carboxylic groups on the fibers and these groups improve the strength of the bleached pulp.
The importance of the acid groups for forming bonds between the fibers has been discussed in Barzyk, D. et al. Journal of Pulp and Paper Science, 23 (1997) J59-J61. According to that article the bonding strength is based on carboxylic groups. In the present invention it has, however, been found that it is not only the amount of acid groups that is decisive, but the conditions of the cook and the bleaching sequences are also of importance.
As discussed above, when attempts are made to regulate the properties of the pulp by beating, i.e. when the ScottBond is raised by a high degree of beating, th
Leskelä Markku
Niskanen Kaarlo Johannes
Nygård Stina
Pitkänen Maija
Fortuna Jose
Kubovcik & Kubovcik
M-real Oyj
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