Process for preparing a paper web

Paper making and fiber liberation – Processes and products – Non-fiber additive

Reexamination Certificate

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Details

C162S183000, C162S184000, C162S009000, C162S013000, C162S175000, C162SDIG004

Reexamination Certificate

active

06436238

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a process claim
1
for producing a paper web.
2. Description of Related Art
According to a process of this kind a fibrous raw material is slushed and the stock obtained is formed into a web which is dried.
In paper making, mechanical pulps are used for, e.g., newsprint and SC and LWX papers. Compared to chemical pulp the particular advantage of mechanical pulps are their lower production costs and greatly yields. Entirely new fields of applications are also being found for mechanical pulps, in particular hardwood pulps. In connection with the present invention it has been found that by using mechanical pulps made from aspen it is possible to manufacture qualities of fine papers that are essentially better than the present ones. At the same time, this leads to conditions of web formation that are, however, distinctly more difficult than for conventional fine papers and for newsprint prepared from spruce. The present invention relates to the control of these web forming conditions.
In comparison to chemical pulps there are some considerable problems relating to mechanical pulps, such as high concentrations of LC substances (liquid and colloidal substances), amounting to 2000 to 8000 mg/l (when purely chemical pulps are used the corresponding amounts are 500 to 1000 mg/l). The high concentrations of disturbing substances increase the risk of disturbances to runability. The concentrations of lipophilic extractives which are particularly troublesome as regards runability are four times, or even up to seven times larger in bleached mechanical pulps than in chemical pulps. The lipophilic extractives are the main cause for most of the precipitates, stains and hole on a paper machine.
The brightness of a paper containing mechanical pulps is lower and the brightness stability poorer than for a traditional fine paper containing solely chemical pulps. The large concentration of fines in the mechanical pulps in both an advantage and a disadvantage. It renders the paper a good bulk, because it has a large scattering capacity, and it gives the paper high opacity, but for the runability of the paper machine a high fines concentration is a disadvantage. The fines have a large specific surface which consumes a lot of the various paper chemicals, both process chemicals and functional chemicals. It is also necessary to use large amounts of chemicals in webs containing abundant amounts of fines in order to control dewatering. The fines have a large specific surface and they impair dewatering of the web during paper manufacture.
The risk of pitch trouble increases in particular in the presence of bivalent or multivalent metal ions. These are capable of precipitating pitch already at small concentrations. In particular calcium gives rise to problems in processes wherein calcium carbonate has been used as a filler or a coating pigment and when mechanical pulp is employed.
Mechanical pulps are produced from softwood, primarily spruce, and to a lesser extent from hardwood, such as aspen (lat.
Populus tremula
). The pulps produced from different wood species contain varying amounts of lipophilic extractives. The extractives concentration of bleached spruce groundwood (PGW) is about 0.30 to 0.35% and of bleached aspen pressure groundwood 0.60 to 0.70%. Also the composition of the extractives is different for hardwood and softwood. Thus, spruce groundwood contains large amounts of free resinous acids, free fatty acids, fatty acid esters and free and esterified esters. The composition of the lipophilic extractives of aspen groundwood differs from that of spruce. In the extractives fraction of aspen groundwood the fatty acid esters dominate, there are only small amounts of free fatty acids and no resin acid can be found in the extractives of hardwood.
Because of the high concentration of extractives in mechanical aspen pulp, mechanical pulps produced from aspen have been considered to involve great runability risks.
There are also other problems associated with the use of aspen in paper making. The stone cells of the inner layer of the bark, i.e. the sclerides, have made it difficult to use aspen in chemical pulps and problems have also been caused by bark trash of aspen pressure groundwood. Furthermore, some 24 to 26% of the aspen cells are vasculum cells, which increase the risk of runability problems when using aspen pulp. The small vasculum cells of aspen have been found to cause spot and stain formation on the web.
In comparison to spruce, hardwood have a higher water retention (WRW) and the smaller fibres give a denser web. Both these factors reduce the dewatering of the web. The benefit of aspen compared to spruce is its lower lignin concentration which gives a higher brightness and improved brightness stability. Aspen would therefore be an interesting wood raw material in pulp making but for the above reasons and because of the smaller strength of aspen in comparison to spruce, the use of aspen has not increased in mechanical pulps. The relative of aspen, the poplar (lat.
Populus balsamea
), is to some extent employed for production of groundwood pulps in Northern America, but the same kind of runability problems also appear in connection therewith.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems related to the prior art and to provide an entirely novel solution for utilizing mechanical pulp produced from aspen in paper making, in particular for production of a base paper for a new generation of fine papers. In particular, the present invention concerns a solution for controlling the clearly more difficult web forming conditions of aspen. Higher brightness and brightness stability are required of fine papers. On a higher brightness level it is more critical to control e.g. the formation of stains and spot caused by pitch.
DETAILED DESCRIPTION OF THE INVENTION
The invention is based on the following surprising findings, which have made it possible substantially to improve the runability of aspen mechanical pulp on a paper machine.
The amounts of organic matter dissolving from groundwood and other mechanical pulp and the variations of these amounts can be restricted by maintaining the pH of the paper machine relatively low and within a relatively narrow range. In addition to the low pH it is further possible to limit the amount of soluble total organic matter (COD) and the amount of organic anionic impurities by maintaining a relatively high conductivity.
The relatively low pH and the relatively high conductivity are particularly advantageous as regards the pitch substance which disperses into water from aspen groundwood. According to Sundberg et al. (K. Sundberg, J. Thornton, R. Ekman, B. Holmbom, Nord. Pulp Pap. Res. J. 9(1994) 2, 125-128) dispersion of pitch from pulp to water is promoted by the fact that the carbohydrates dissolving in water from pulp (spruce groundwood—TMP) sterically stabilize the pitch dispersion. When operating a process according to our invention the high conductivity restricts the solubility of the organic substance and when the concentration in water of the substance which stabilizes the pitch dispersion drops, it reduces the amount of pitch dispersing into water.
We have also found that, depending on the specific application, the peroxide dosage needed for bleaching aspen groundwood is substantially smaller than the peroxide dosage for spruce groundwood, which also by itself reduces the amount of water-soluble substance. Surprisingly the concentration of extractives on the surface of the bleached aspen groundwood (coverage of extractives) is not higher than for spruce groundwood, even if the total concentration of extractives in aspen groundwood is about twice that of spruce groundwood.
Concerning runability it is also essential to control dewatering. The use of aspen groundwood involves a significant risk in this aspect because its fiber size is small and generally the water retention of hardwood pulps is greater for them because of th

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