Paper making and fiber liberation – Processes and products – Non-uniform internal structure
Reexamination Certificate
1996-11-18
2001-01-30
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform internal structure
C162S109000, C162S199000, C162S202000, C162S216000, C162SDIG004
Reexamination Certificate
active
06179963
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for influencing the breaking length cross-machine profile of a running fibrous material web in a paper machine by use of a consistency controlled flowbox which discharges the stock suspension onto a wire or between two wires.
Flowboxes of this type are known, for example from:
(1) DE 35 14 554 A1
(2) U.S. Pat. No. 5,196,091
(3) DE 40 19 593 C2
Essential quality features of paper webs are the uniformity of paper thickness, the strengths in the machine and cross-machine directions, the fiber orientation and the basis weight, in each case viewed across the web width. The basis weight profile is of decisive importance.
DE 40 19 593 C2 discloses measures to make the profiles of these properties largely uniform. The paper stock suspension is fed to the flow chamber of the flowbox through a plurality of channels which are located alongside one another in parallel and are distributed uniformly over the machine width. A mixer is connected upstream of each channel by which the concentration and the throughput in the channel may be set variably. This avoids need for adjusting spindles at the lips of the outlet channel of the flowbox. The lips are kept straight by their more or less stiff construction, so that the outlet gap between the lips has a constant height, viewed across the width in the gap. In other constructions, for example, U.S. Pat. No. 5,196,091, the outlet gap is also adjusted.
The breaking length represents a measure of the breaking strength of the paper. It indicates the length of the paper strip that will break under the load of its own weight.
A breaking length can be determined in the machine direction of the paper web or else in the cross-machine direction. The so-called breaking length ratio presents a particular problem. From this breaking length, it is possible to determine the ratio R
L
/R
Q
(=breaking length ratio). Measurements have shown that the breaking length ratio, measured across the width of the paper web, is of variable size. In the central region across the web width, it is to a certain extent constant, whereas it decreases towards the edges. As a result, the breaking length ratio, represented over the width of the paper web, resembles the internal contour of a bathtub which is open upward or open downward.
A web having a large ratio of R
L
/R
Q
is able to absorb pronounced tensile forces in the machine direction.
This is of particular interest during processing in fast running printing machines. In the latter, R
L
should therefore be large in relation to R
Q
.
By contrast, in the case of so-called format papers, as well as for many packaging papers, a strength which, as far as possible, is equal in the machine direction and in the cross-machine direction is desired. If this is not present and if the cross-machine strength is low at the web edges, then tearing of the web occurs when it is loaded in the cross-machine direction.
SUMMARY OF THE INVENTION
The invention has the object of configuring the papermaking process to influence the cross-machine profiles of breaking length ratio and basis weight so that they primarily run linearly, in that the above-mentioned bathtub effect is suppressed to a large extent, and so that the profile of the breaking length ratio, viewed over the web width, becomes as linear as possible.
The inventors hereof knew that the breaking length ratio is directly correlated with the fiber alignment or fiber orientation in the paper web. But the fiber orientation is not solely responsible for the breaking length ratio. This fiber orientation can be measured off line and on line. The off line method is intended to be described below. For this, one takes various samples across the width of the web from a finished paper reel at the end of the paper machine. On these samples, one identifies the web running direction, the machine long sides (front side and drive side) and a measure X, of the distance of the sample from the left-hand edge, viewing the paper from above.
This off line measuring method gives a good overview of the fiber orientation of the paper weight currently being produced, since the production processes are sufficiently stationary with time.
In order to assess the fiber orientation, an imaginary coordinate system is placed onto the paper sample. In this case, the Y axis points in the web running direction and the X axis points correspondingly transversely to that direction. Irrespective of the measuring method used, the breaking length, for example, is determined as a function of the measuring angle (positive angle measured from the positive Y axis in the direction of the positive X axis and negative angle measured from the positive Y axis in the direction of the negative X axis). The representation of the measurement results, presented in the form of vectors having starting points located at the origin of the imaginary coordinate system, yields a semi-ellipse, having major axes which may not coincide with the coordinate axes. The orientation of the major axes has been defined in the web running direction or at right angles thereto in the clockwise direction.
It has become established practice to mirror the semi-ellipses symmetrically about a point and to represent them as complete ellipses, although this is not correct for the reason that a measured value at a specific angle alpha naturally yields the identical, only repeating itself, measured value at the angle alpha + or −180°. These ellipses are called fiber orientation ellipses below.
The ratio of the location vector in the Y direction to the location vector in the positive X direction then yields the so-called breaking length ratio. This value is dimensionless. The angle between the maximum location vector and the positive Y axis indicates the fiber orientation angle.
This property of the direction dependent strength value of the paper is called anisotropy. Quantitatively, it is expressed, for example, using the breaking length ratio.
In order to represent the properties of fiber orientation and strength properties across the machine width, that is, the cross-machine profile, one needs at least three diagrams (machine direction breaking length, cross-machine direction breaking length, fiber orientation angle or breaking length ratio, one breaking length, fiber orientation angle). These diagrams then depict the evidence of theoretically infinitely many fiber orientation ellipses.
The inventors hereof knew from practice that a paper web shrinks not only in the machine direction but also in the cross-machine direction during drying. Indeed, it shrinks particularly severely in the edge regions. The resulting cross-machine shrinkage profile likewise has a curve which is similar to the bathtub shape.
Although paper fibers shrink more severely transversely to their longitudinal extent than in their longitudinal direction during drying, the more severe cross-machine shrinkage effect at the edges of a paper web cannot be explained merely by a fiber orientation which is typical for the edge region.
Moreover, the inventors also knew that any measures for influencing fiber orientation and hence for influencing the breaking length ratio must not make the basis weight cross-machine profile worse.
The object of the invention is achieved by setting the web property cross-machine profile by the lay of the fibers such that the ratio of breaking length in the machine direction to the breaking length in the cross-machine direction is essentially constant over the width of the web.
The inventors have recognized that, in spite of the large number of parameters which have to be taken into account and some of which partially cause others, it is possible to set an essentially flat basis weight cross-machine profile and a breaking length ratio cross-machine profile at the same time.
Since the invention of consistency controlled flowboxes, a good basis weight cross-machine profile and a good fiber orientation cross-machine profile may be set at the same time.
If such a flowbox additionally has a sectionall
Begemann Ulrich
Guggemos Adolf
Chin Peter
Ostrolenk Faber Gerb & Soffen, LLP
Voith Sulzer Papiermaschinen GmbH
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