Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1999-11-08
2002-07-02
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S486000, C427S201000, C427S326000, C427S391000, C162S136000, C162SDIG009
Reexamination Certificate
active
06413591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the manufacture of paper and paperboard. In particular, the present invention concerns a novel method for coating paper, paperboard and similar cellulosic and lignocellulosic webs. Generally, such a method comprises the steps of applying a coating composition containing coating particles to the surface of a paper or paperboard web containing cellulosic fibers, and drying the coated web to produce a coated product.
2. Description of Related Art
Paper and paperboard are coated for many different reasons, e.g. for improving smoothness and opacity, for grease proofing, for providing release properties, and for achieving barrier properties against many different substances, such as oxygen and aroma. The surface of a base paper is always rough and in many cases the purpose of coating is to fill the unevennesses and to achieve better quality of the surface for printing and possibly for a secondary functional coating.
The weight of the coating layer is normally only 5 to 20%, in some cases, however, up to 40%, of the mass of the base paper. Very often the coating layer contains, some pigments in order to improve printability, brightness and opacity. Normally, all these functionary chemicals and pigments are bound to the surface of base paper sheet with some organic polymers, which often are applied in the form of latexes, e.g. as aqueous polymer emulsions.
Coated papers are frequently calendered in order to obtain a smoother finished surface for printing and for coating.
The paper coating and finishing methods used nowadays have some considerable drawbacks, such as waste production from secondary usage of the papers and deinking problems. In particular, the reuse of the paper would be much easier if polymer materials in the paper or paperboard could be avoided. Some polymer materials may even shorten the shelf life of the paper as a document paper.
Although calandering will improve surface smoothness and gloss of the coated papers, tests with newsprint and normal LWC papers carried out at paper mills in Finland using calenders with line-pressures in the range of 100 to 120 kN/m have shown that calendering decreases the tear strength of the paper considerably, up to 31%, and it will also decrease the opacity of paper by up to 35%. Similarly, the burst strength drops up to 38%.
SUMMARY OF THE INVENTION
It is an object of the invention to eliminate the problems related to the prior art and to provide a novel method for coating paper and paperboard and similar cellulosic webs.
This and other objects, together with the advantages thereof over known methods, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
It is known in the art that paper must be refined before sheet forming. Refining will liberate fibrils (tiny, thin parts of the fibre) from the fibres while still keeping most of them in the original fibre. These fibrils improve the strength of the paper sheet. In fact, paper web formation on a paper machine is, to a substantial extent, based on hydrogen-bonding between adjacent fibres and fibrils. The bonds are created when the wet web formed on a wire is dried. There are also some hydrogen bonds present between the surface of a paper or paperboard web and the coating applied thereto.
The invention is based on improving and enhancing hydrogen bonding between the coating particles and the cellulosic or lignocellulosic fibres of the web by increasing the surface area in the interface between the coating particles and the fibres. As a result, the coating particles will become more strongly bonded to the surface of the web so that coating is possible even without the use of polymeric binders. Preferably, the increased surface area in said interface is formed by using finely divided fibrous matter in the interface. This matter can stem from the web or be a part of it and/or it can comprise fibrous coating particles. Thus, an increased surface area in the interface between the coating particles and the cellulosic or lignocellulosic web can be provided by selecting a web which has an enlarged surface area and/or by selecting coating particles which have a greater surface than corresponding conventional particles and/or fibres.
Considerable advantages are achieved by the present invention. Thus, the present invention provides anchoring of pigment particles to the base web without any polymer usage and entirely without any foreign or secondary materials being introduced into the paper or, in certain cases, by using only relatively small amounts of polymer. Thus, because the present invention employs no or essentially no polymer binding material, the desired pigmenting function can be made using smaller amounts of pigments than in cases where polymers are used as binding aids.
The repulping of this paper web, where the coating is made with pigments and fines alone, is much easier than repulping of paper where polymers are used to bind the pigment on the web.
It is well known that the scattering coefficient of pigments and the opacity provided by said coefficient is dependent on the light refraction index between the pigments and the intimate material of the pigments, like polymers etc. During recirculation of a material of the present kind, the opacity of the recirculated products is not lowered as is the case with waste paper containing large amounts of polymeric materials.
The invention will improve the smoothness of the surface without addition of foreign polymer binding agents to the web. This opens up new dimensions for paper making. A smooth surface can now be achieved more easily than with other known paper surface finishing methods (e.g. calandering), while the strength and optical properties are kept almost on the original level. By mechanically treating the surface it can be made very even with the use of small amounts of pigments only.
The present method will offer great possibilities for rebuilding even older paper machines for novel use and for more flexible product range, making different specialities.
DETAILED DESCRIPTION OF THE INVENTION
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 chemithermomechanical pulping (CTMP).
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 present invention can be employed for coating of any desired paper or paperboard web to obtain a coated product having an increased opacity and brightness and improved printability. 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 diametre of less than about 0.5 mm, typically in the range of 0.001 to 0.2 mm and the “fibrils” and “fibres” are materials having a length to cross-section diameter ratio of more than about 6. In particular, the fibrous or fines material comprises fibres whose size is les
Ahonen Heikki
Dettling Bernhard
ISI-HEAD Oy
Parker Fred J.
Smith-Hill and Bedell
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