Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1999-11-30
2003-05-06
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S475000, C427S482000, C118S627000
Reexamination Certificate
active
06558754
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for even distribution of dry hard particles to the surface of a continuously fed paper web, a process for providing the surface of a decor paper or an overlay paper for an abrasion resistant laminate with an even layer of small, hard particles and a particle coated decor paper or overlay paper produced by the process.
Products covered with a decorative thermosetting laminate are frequently used today. They are mostly used where the demands for abrasion resistance are high, but also where a resistance against different chemicals and humidity is required. Floor boards, floor skirtings, table tops and wall panels can be mentioned as examples on such products.
2. Description of the Related Art
Decorative thermosetting laminates are often made of two to seven Kraft paper sheets impregnated with phenol-formaldehyde resin and a decor paper sheet impregnated with melamine-formaldehyde resin or another thermosetting resin. The decor paper sheet can be monochromatic or patterned for instance with a wood pattern or a fancy pattern and placed as a top layer in the laminate.
Often one or more so-called overlay sheets of &agr;-cellulose usually impregnated with melamine-formaldehyde resin are placed on top of the decor paper to protect the decor paper sheet from abrasion.
There are also laminates consisting of a base layer of particle board or fibre board provided with such a decor paper sheet and possibly an overlay sheet. These sheets can be laminated towards the base layer under heat and pressure. If a decor paper only is used and no overlay sheet, the decor paper sheet can be glued towards the base layer instead.
To further increase the abrasion resistance of the decor paper sheet and/or the possible overlay sheets these may be provided with a coating of hard particles. These particles can be applied to the paper by mixing them into the thermoseting resin used for impregnating the paper. The particles can also be added to the wet cellulose fibers on the wire of a paper machine. Finally it is known to coat the resin impregnated paper with hard particles by applying the hard particles onto the paper before drying the resin.
The first method is illustrated for instance in U.S. Pat. No. 4,473,613. This method results in an uneven distribution of the hard particles and thereby an uneven abrasion resistance of the decorative laminate. The reason is that it is very difficult to disperse average size and bigger particles in a resin solution since these due to their higher density will sink to the bottom of the vessel used for storing the resin. Such a dispersion will therefore be practically unusable since the number of hard particles per surface unit will vary as time goes. This problem can partly be counteracted by increasing the viscosity of the resin solution by an addition of a thickener. However, such additives will deteriorate the properties of the resin and give a worse end result. In addition, even with a thickener it will be difficult to change the amount of hard particles per surface unit if so desired since also the resin content will be changed.
The second method mentioned above can be illustrated by the U.S. Pat. No. 3,798,111. The method disclosed in this patent is generally used for the production of overlay paper of &agr;-cellulose. The hard particles for instance of aluminium oxide are then spread over a layer of wet &agr;-cellulose fibers on the wire of a paper machine. With this method, the hard particles are distributed more or less irregularity within the whole fibre layer. Some of the particles even pass through the wire and cause serious pollution problems in the paper making machine. In the overlay paper obtained the hard particles will be distributed in an uncontrollable way. It is impossible to get an even distribution of the hard particles on the surface of the paper, where they give the best effect against abrasion.
In the above U.S. Pat. No. 3,798,111 a decor paper is made with the method disclosed, whereupon a decor is printed on top of the produced paper. Since the hard particles are situated below the decor they cannot possibly give an increased abrasion resistance. In spite of the disadvantages mentioned the method is videly used commercially for the production of abrasion resistant overlay sheets.
The third method mentioned above can be illustrated by our own U.S. Pat. No. 4,940,503, where the hard particles are applied to a continuous decor paper or an overlay paper which is impregnated with a liquid solution of a thermosetting resin. The resin is wet when the particles are coated on the paper. The paper is dried when the particles have been added.
The particles are distributed by means of a device comprising a container containing the hard particles and a rotating doctor-roll with an uneven surface placed under the container, whereby the particles are intended to fall from the container to the doctor-roll and then be evenly distributed on the paper web fed under the doctor-roll. The device usually contains an air knife intended to get the particles to come loose from the doctor-roll at a constant amount per unit of time.
Decorative thermosetting laminates produced for flooring boards where at least one overlay has been provided with hard particles by said method have been tremendeously successful.
The method is by far the best commercial one for production of highly abrasion resistant decorative thermosetting laminates. The particles are distributed very evenly on the paper web.
However, sometimes you find clusters of particles sticking together on the surface of the coated paper resulting in patchy or hazy areas. Between these clusters there are minor areas lacking particles. If the formation of such clusters could be avoided by an even better distribution of the particles the abrasion resistance would increase without addition of a higher amount of hard particles. A decrease of the cluster formations would also improve the decorative effect of the decorative laminate. Thus, there is a need for improvement of this process for even distribution of hard particles to the surface of a continuously fed paper, especially an overlay paper for abrasion resistant laminates. These laminates constitute the top layer of flooring boards which usually have a base layer of particle board or fibre board to which the laminate is glued. The flooring boards are furnished with groove and tenons in the side edges as ordinary flooring board of wood.
SUMMARY OF THE INVENTION
According to the present invention it has been possible after an extensive development work lasting for years to meet the above need. Thus, the present invention relates to an apparatus for even distribution of small hard particles to the surface of a continuously fed paper web impregnated with a liquid thermosetting resin composition, the resin being wet at the distribution of the hard particles. The apparatus includes a feed hopper containing the hard particles. The hopper has an outlet extending transversely of said fed paper web. A rotating doctor-roll preferably with an uneven surface is placed under the feed hopper and is in communication with said outlet for reception of hard particles therefrom. The doctor-roll is in spaced substantially parallel relation to said paper web fed under the doctor-roll. The apparatus also has a means for releasing the hard particles from the doctor-roll and distributing them evenly on the fed paper web. This means comprises an electrode arrangement placed between the feed hopper and the downwards directed vertical tangent (T) of the doctor-roll. The electrode arrangement is preferably enclosed by a casing provided with a downwards directed sliding plate, whereby the hard particles are lifted from the doctor-roll and fluidized by means of an electric field between the electrode arrangement and the doctor-roll resulting in an even amount of particles falling down on the paper web fed under the doctor-roll.
As mentioned the surface of the doctor-roll is preferably uneven. Sui
Bengtsson Jan-Olof
Frosthagen Henrik
Perkins Brian
Velin Per-Erik
Parker Fred J.
Pergo (Europe) AB
Stevens Davis Miller & Mosher L.L.P.
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