Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
1997-09-03
2001-09-18
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S158000, C162S181400, C162S181600, C162S177000, C162S168100, C162S169000, C162S164400, C162S183000
Reexamination Certificate
active
06290815
ABSTRACT:
The invention relates to a paper sheet containing grit particles giving it, in use, high abrasion resistance. This sheet is used, in particular, in the manufacture of laminates in which it gives their surface increased abrasion resistance. The invention also relates to the laminates containing it, as well as to the processes for manufacturing the sheets and the laminates.
For many years, laminates have been employed as materials in dwellings and commercial and industrial premises. Typical applications of such laminates are floor coverings, in particular those imitating a parquet floor, coverings for furniture, table tops, chairs and other articles. They are consequently subjected to repeated rubbing which abrades their surface; a desired property of these laminates is therefore a high abrasion resistance, and in the case of floors it must be extremely high.
Abrasion resistance can be characterized by the so called Taber resistance, measured according to the NF-EN 483-2 (1991) standard and expressed as a number of revolutions. The abrasion resistance of a laminate should correspond to a Taber resistance of at least 4000 revolutions in the case of intensive domestic usage, and should reach 10,000 to 12,000 revolutions in the case of usage in public places.
Several kinds of decorative boards exist: so called high-pressure boards, so-called low-pressure boards and boards covered with a sheet called a finished, adhesive sheet.
So called high-pressure laminates are produced from a core consisting of resin-impregnated sheets. These sheets are generally made of kraft paper and have been impregnated with a thermosetting resin, usually a phenolic resin. After having impregnated the sheets with resin, they are dried, cut and then stacked one on top of another. The number of sheets in the stack depends on the applications and varies between 3 and 9, but may be higher. Next, a decorative sheet is placed on the stack of sheets forming the core. Such a decorative sheet is generally a sheet of paper bearing a printed or colour pattern or containing decorative particles, and it is impregnated with a thermosetting resin which does not darken with heat, for example melamine-formaldehyde resins, benzoguanamine formaldehyde resins and unsaturated polyester resins; sometimes this sheet is not impregnated, resin being provided by creep of the resin from the sheets which surround it. In general, placed on top of the decorative sheet is a protective covering sheet which is unpatterned and transparent in the final laminate; in the art, this protective sheet is called an “over-lay” or “overlay”. Next, the stack of impregnated sheets is placed in a laminating press, the platens of which are provided with a metal sheet giving the laminate the surface finish. The stack is then densified by heating, at a temperature of about 110° C. to 170° C., and by pressing, with a pressure of about 5.5 MPa to 11 MPa, for approximately 25 to 35 minutes in order to obtain an integral structure. Next, this structure is fixed to a base support; for example, it is adhesively bonded to a particleboard.
The so-called low-pressure laminates are produced using only one decorative sheet impregnated with thermosetting resin and optionally an overlay sheet which is laminated directly onto the base support during a short cycle, the temperature being about 160 to 175° C. and the pressure 1.25 MPa to 1.6 MPa.
The third kind of decorative board consists of boards composed of a base support, generally a wood particleboard or a fibreboard, and of a decorative sheet of paper impregnated with a composition containing a binder or a melamine-formaldehyde resin and a polymer, this sheet being fixed to the support by means of an adhesive. In this case, the sheet of paper is a decorative sheet of uniform colour or one with decorative patterns. The color or the patterns are generally applied by printing on the sheet, before or after impregnation. In addition, a varnish or a lacquer is applied, for the purpose of protecting the surface of the sheet. An example of such a sheet, called a finished foil or impregnated foil, and the decorative board containing it are described in Patent Application WO-A-9517551.
The overlay protective sheet is conventionally manufactured by dewatering an aqueous suspension of very lightly refined cellulose fibers. This sheet has a low grammage, of between 10 and 50 g/m
2
, and is not opacified. It is impregnated with a thermosetting resin, which leads to it being transparent in the final laminate and enables the decoration of the laminate to be seen. The resin is usually chosen from melamine resins, urea resins or unsaturated polyester resins. This overlay sheet protects the surface of the laminate and, in particular, it increases its abrasion resistance as it provides additional thermosetting resin. In order to have a very high abrasion resistance, it is necessary to use a high grammage overlay sheet, but this impairs the good visibility of the decoration through this sheet. This is the most common way of increasing the abrasion resistance of laminates. However, this resistance may also be increased by the use of particles having a high abrasiveness, and therefore being abrasion resistant, which are called in the art grit particles. Their abrasive character is due to their hardness and to their morphology, which exhibits many sharp edges. These particles are present in or on the overlay sheet or on the decorative sheet itself. In order not to impair the transparency of the overlay sheet after impregnation, these particles, on the one hand, must be transparent to translucent or have a white to greyish-white colour and, on the other hand, must not have too high an average size, this being preferably less than or equal to 200 &mgr;m, and must not be present in too great a quantity. However, in order to have high abrasion resistance, it is preferable for the size of the particles to be the highest possible and/or for these particles to be added in great quantity.
The use of very hard inorganic particles, such as, for example, those of alumina or corundum, of silica, including quartz, boron nitride, silicon carbide, titanium carbide and tungsten carbide, their Mohs hardness being between 3 and 10, or even higher, has thus been described. These particles have been mentioned in numerous patents, in particular in Patents GB-A-1,139,183, GB-A-1,378,879, DE-A-2,107,091, FR-A-2,104,707, FR-A-2,139,990, U.S. Pat. No. 3,661,673, U.S. Pat. No. 5,141,799 and CA 836,522. These particles may be introduced into or placed on the sheet of paper:
by addition to the pulp stock during the manufacture of the sheet, although, since these particles have an abrasive character, the papermaking machine is rapidly worn; another drawback is that the particles are poorly retained in the paper;
or by deposition using a second head box on the papermaking machine, but wear of the machine still occurs;
or by impregnation or coating at the same time as the impregnation resin, after they have been mixed with the resin, and optionally after the sheet has been printed in the case of a decorative sheet; but with the drawback that it is difficult to make the mixture very homogeneous, segregation of the particles possibly occurring and thus giving rise to poor retention of the particles on the sheet;
or by deposition, in particular by electrostatic spraying, on the sheet pre-impregnated with resin, this process, described in Patent FR-B-2,104,707, filed in 1970, for helping to overcome the cited drawbacks of the prior art, reducing the number of manufacturing machines subjected to abrasion, although it is difficult to spray uniformly and spraying creates a dusty atmosphere.
Moreover, whatever the process for incorporating the grit particles, there is still rapid wear of the laminating machines, in particular of the metal sheets of the platens of laminating presses. One drawback of this wear of the metal sheets of the presses is that the surface finish which they give the laminates also deteriorates. This is particularly troublesome for obtaining high-gloss l
Caulet Pierre
Magnin Henri
Perrin Claude
Arjo Wiggins S.A.
Bacon & Thomas
Fortuna Jose
LandOfFree
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