The present invention concerns a process according to the preamble of claim 1 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 according to the preamble of claim 13 and a process according to the preamble of claim 14 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 surface of the paper a very dense coating layer which cannot be penetrated by steam vapourizing 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 afore-described problem is called blistering and the required internal bond strength (z-directional strenght) 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 and 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.
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/mm2, when the ScottBond-strength of the chemical pulp is 400 J/m2. 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 mainly characterized by what is stated in the characterizing part of claim 1.
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 an comparative pulps. The present base paper can therefore be used for production of double-coated fine papers which in particular require greated 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 brigthness 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 cooking (Superbatch cooking).
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 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, the chemical pulp and, e.g., hardwood groundwood, will get very different elastic moduli (chemical pulp gets very high stiffness), which is undesirable as far as the toughness of a mixture produced of these pulps is concerned. This problem is not encountered in the present invention. For this reason, by means of the present invention, a mixture of hardwood groundwood and chemical pulp is obtained which is excellent as a base paper of fine papers.
According to a preferred embodiment, the chemical pulp used for preparing a base paper is produced by a cooking method known as a modified batch-type cook (Superbatch Cook). This cook is discribed in the literature [cf. for example Malinen, R Paperi ja Puu (Paper and Timber), 75 (1993) 14-18]. The cook in question is a modified cooking method which utilizes an alkaline cooking liquor just as the sulphate cook, but wherein delignification has been enhanced so that the kappa number of the chemical pulp is lowered without a significant reeduction of viscosity. Typically with a Superbatch process, pulp is cooked to a kappa number of 20 or less.
According to a preferred embodiment of the present invention, a softwood pulp produced by batch cooking is bleached with TCF bleaching. The following examples of suitable bleaching sequences can be mentioned:
Pn=several successive peroxide treatment stages
Q=treatment with complexing agent
An acid pretreatment at elevated temperature (an A stage) can be performed between the oxygen delignification (O-stage) and a bleaching step carried out with an oxidizing chemical (i.e. a Z-stage).
It is particularly preferred to carry out the bleaching of the pulp with two ozone stages and at least two peroxide stages. Between the stages carried out with oxidizing chemicals, it is possible to extract the pulp during various alkaline stages (such as E and E0) and/or to wash it with water.
Following the above-described treatment a pulp is obtained having an internal bond strength which is better than that of comparative pulps. It typically contains at least 40 mmol carboxylic acid groups/kg dry pulp. Preferably the elastic modulus of the chemical pulp used according to the present invention is below 6000 N/mm2, in particular below 5000 N/mm2 when ScottBond strength is 400 J/m2.
As mentioned above, the base paper can be produced from chemical pulp by combining it with aspen groundwood, by slushing the obtained fibrous base material, by forming a web from the stock and by drying the web on a paper machine in order to form a base paper. Generally, the pulp can be produced from any mechanical pulp made of a tree of the Populus family. Suitable species are, for example, P. tremula, P. tremuloides, P balsamea, P. balsamifera, P. trichocarpa and P. heterophylla. A preferred embodiment comprises using aspen (trembling aspen, P. tremula; an aspen known as Canadian aspen, P. tremuloides), or aspen varieties known as hydride aspens produced from different base aspens by hybridizing as well as other species produce by recombinant technology, or poplar. It is preferred to use groundwood (CTW), pressure groundwood (PGW) or thermomechanical pulp (TMP) manufactured from aspen, hydride aspen or poplar.
Preferably the mechanical aspen pulp contains about 10 to 20% of +20 . . . +48 mesh fibers, which confer mechanical strength to the pulp. In order to maximize light scattering, the portion of +100, +200 and −200 fractions should be as large as possible. Preferably they stand for distinctly more than 50% of the whole pulp. In particular their proportion of the whole pulp is over 70%, preferably over 80%. On the other hand, the amount of the smallest fraction, i.e. the −200 mesh, should not be too large, because then dewatering on the paper machine would become more difficult. Preferably the proportion of this fraction is smaller than 50%, in particular 45% or less.
Due to the excellent mechanical properties of the pulp according to the present invention the proportion of the mechanical pulp can be even up to 70 weight-% of the dry matter of the stock without the strength of the paper essentially suffering. Typically, the proportion of the mecanical pulp is at least 20% and in particular it is 30 to 60 weight-%.
Based on what is stated above, according to the invention the composition of a particularly preferred base paper is the following: 30 to 60 weight-% of the fibrous matter comprises mechanical pulp produced from aspen and 70 to 40 weight-% comprises softwood chemical pulp. The ScottBond strength of the chemical softwood (in particular pine) pulp is at least 400 J/m2 at a light scattering coefficient of 22 m2/kg and it contains at least 40 mmol carboxylic acid groups/kg dry pulp.
From the base paper according to the present invention it is possible to produce high-quality fine paper by coating it preferably twice, the first coating for example being carried out by a method known as the film press method, and the second coating is performed by blade coating. The amount of coating colour applied to the web by the film press method is typically about 5 to 50 g coating colour/m2, whereas the corresponding amount for doctor blade coating is 10 to 60 g coating colour/m2. The indicated amounts of coating have been calculated from the dry matter of the coating colour.