Abrading – Abrading process
Reexamination Certificate
2000-07-11
2002-10-22
Morgan, Eileen P. (Department: 3723)
Abrading
Abrading process
C451S054000, C451S167000, C451S184000, C029S090100, C029S090500
Reexamination Certificate
active
06468133
ABSTRACT:
The present invention relates to fibrous web finishing. In particular, the invention concerns a method according to the preamble of claim
1
for increasing the smoothness of paper and board webs by mechanical treatment.
Paper is normally manufactured by the wet method. According to that method fibres are suspended in water to form a fibrous furnish and a wet web is formed from the furnish on a wire screen. The web is then dried step by step using different mechanical and thermal systems to a preselected state of dryness.
In conventional technology, the fibrous furnish is maintained in turbulent state before web formation in order to avoid orientation of the fibres. However, as a result of the turbulence, there will be formed flocks in the web, having a fibre density larger than that of the surrounding parts of the web.
For the purpose of all printing operations, the surface of the paper should be as smooth and/or homogeneous as possible. The same is true for papers coated with mineral particle layer and latex binding materials. Therefore, very often (base) papers are calendered before coating and also papers containing mineral fillers are treated with a calender for achieving a smoother surface. Calendering is in particular necessary for certain paper qualities because of the above-mentioned flock formation.
There are numerous types of calenders, but all of them even the surface by mechanical pressing and sliding forces. Conventional calendering is hampered by some considerable disadvantages. After remoisturing, a surface smoothened by calendering will totally or partially regain its original form. It is also known that papers loose up to 35-40% of its strength properties and 25-35% of its original opacity as a result of calendering. Further, the original tenacity of the paper web will remarkably decrease.
In view of the above problems related to calendering, great efforts have been made to avoid said flock formation and to find some different methods for surface smoothening.
U.S. Pat No. 2,349,704 discloses a method for polishing the surface of a paper web with a cloth polishing roll. The surface of the roll contains a powdered abrasive which is bound to the surface with the aid of a binder. The object is to press and polish paper to the same extent as is made by the supercalendering process, and according to specification of the patent, the density of the treated paper is the same as after a supercalendering process and gloss, measured by a Baush & Lomb glossmeter, is 10 points higher than before the treatment.
U.S. Pat. No. 5,533,244 discloses another method, somewhat similar to the one mentioned above, for polishing paper with a woven belt which slides at different speed over the paper web than the web itself, producing frictional action.
A soft calender device which acts as a rubbing friction device on paper surface is disclosed in U.S. Pat. No. 4,089,738. The device will smoothen the paper surface in the same way as original supercalenders.
None of the prior art method will provide for a satisfying removal of high density flocks from the paper surface. Further, it is apparent that the strength properties of the paper deteriorate during the application of the known methods.
It is, therefore, an object of the present invention to eliminate the disadvantages of the prior art and to provide a novel method for treating the surface of a fibrous web, in particular a paper or board surface in order to improve its smoothness while substantially retaining the mechanical properties of the web.
The present invention is based on the surprising finding that the surface of many fibrous webs can be smoothen by grinding off only the most protruding parts of the web with a grinding means, such as a grinding belt or viberating grinding device or rotating grinding cylinder, to provide a smoothened surface having unaltered or even improved properties of mechanical strengthness. In particular, the presention comprises grinding in the dry state (“dry grinding”) only the higher parts of fibrous web (in cross section) while pressing the surface against the grinding surface so little that no noticeable increase of density of the web can be found.
More specifically, the invention is mainly characterized by what is stated in the characterizing part of claim
1
.
The present invention provides a number of advantages. Surprisingly, it has been found that, e.g., ground paper had a better tensile strength and also better bursting strength that the original paper. Although we do not wish to be bound by any particular theory, it would appear that this phenomenon is based on the forces inside the stressed web becoming more evenly distributed when the strength of the parts having the highest strength is decreased. Initially, because of the poor evenness (formation) of the paper web, the forces are not so strong at the thinnest part of the paper. However, grinding will redistribute the adhesion forces within the web matrix. Another possible explanation is that fines generated obviously during the grinding process and also fibrils, one end of which still sticks to the original fibre, are reassembled on the surface.
During the surface grinding process of the present invention, very limited amounts of loose fibres and dust are formed. This is probably because the grinding friction of the present invention will release some water vapour from the surface and it will condense on the paper leaving the grinding process part of machinery. This condensed water will bind fines back to the surface.
Next the invention will be examined in more detail with the aid of the following detailed description and with reference to a working example.
Within the scope of the present invention, the terms “cellulosic” and “lignocellulosic” are used to designate materials derived from cellulose and lignocellulosic materials, respectively. In particular “cellulosic” refers to material obtainable from chemical pulping of wood and other plant raw material. Thus, a web containing “cellulosic fibres” is made for example from kraft, sulphite or organosolv pulp. “Lignocellulosic” refers to material obtainable from wood and other plant raw material by mechanical defibering, for example by an industrial refining process, such as refiner mechanical pulping (RMP), pressurized refiner mechanical pulping (PRMP), thermomechanical pulping (TMP), groundwood (GW) or pressurized groundwood (PGW), or chemithermo-mechanical pulping (CTMP) or any other method for manufacturing a fibrous material which can be formed into a web and coated.
The terms “paper” and “paperboard” refer to sheet-formed products containing cellulosic or lignocellulosic fibres. “Paperboard” is synonymous with “cardboard”. The grammage of the paper or paperboard can vary within broad ranges from about 30 to about 500 g/m
2
. The roughness of the web which is to be treated in about 0.1 to 30 &mgr;m, preferably about 1 to 15 &mgr;m. The present invention can be employed for treating any desired paper or paperboard web. As a practical matter, the term “paper” or “paper web” is herein used to designate both “paper” and “paperboard” and “paper web” and “paperboard web”, respectively.
The terms “fines”, “fibrils” and “fibres” denote finely divided material having a cross-sectional diameter of less than about 10 &mgr;m, typically in the range of 0.001 to 2 &mgr;m and the “fibrils” and “fibres” are materials having a length to cross-section diameter ratio of more than about 6.
The “roughness” of the web which is to be coated is generally given as “microns” (&mgr;m). The print-surf surface roughness at 1000 kPa can be measured according to, for example, ISO 8791-4:1992 (E). Typically the roughness of paper webs is in the range of 8 to 2 microns. As discussed below and shown in the working examples, by subjecting the surface of a paper or paperboard web to a grinding treatment according to the invention, it is possible to reduce the roughness of the web by at least 20%, preferably about 40 to 60% while maintaining the mechanical properties of the web.
The present invention comprises the step
Ahonen Heikki
Dettling Bernhard
IDI-Head Oy
Morgan Eileen P.
Smith-Hill and Bedell
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