Paper and paperboard sheet is coated with the help of different techniques, each one of them having its advantages and disadvantages. Spray, jet and curtain coating tech- niques have in common that the coating mix is applied to the surface of the web being treated by way of allowing the coating mix to float freely within the space delineated by the applicator feed opening and the web surface. Accordingly, the coating mix is not transferred by pressing to the web surface, but rather, it may spread in a self-contained fashion on the web under its relative low kinetic energy.

Since the surface smoothness and freedom of pores of the applied coat are imperative factors as to the coat quality, the coating mix should spread evenly over the entire surface being treated. Up to now, good spreading capability has been aimed at by adjusting the viscosity and solids content of the coating mix inasmuch as these parameters have a strong effect on the behavior of a coating mix droplet or film on the base sheet surface. It has been found, however, that nevertheless some coating formulations or base sheet grades develop rough coats or small pores in the coating.

Such coatings undermine the printing result, because these defects permit the print- ing ink to penetrate into the porous and absorptive base sheet thus allowing the ink to spread over a wider area than actually intended. As a result, the print becomes blurred. This problem is particularly difficult to overcome in spray coating, wherein the individual droplets should spread into contact with each other in order to form a sufficiently uniform coat.

It is an object of the present invention to provide a method capable of improving the spreading of the coating mix in application techniques based on free spreading of coating mix on the web surface during application. It is a particular object of the invention to improve the quality of applied coat in spray coating.

The goal of the invention is achieved by virtue of matching the surface energies of the base sheet and the coating mix with each other so that the surface energy of the coating mix or the liquid phase separated therefrom is lower or equal to the surface energy of the surface being treated.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention provides significant benefits.

The invention excels over the prior art by virtue of its improved capability of control- ling the behavior of coating mix on the surface being treated, whereby the quality of the finished web is elevated and, above all, the quality parameters of the product being manufactured become more predictable. Due to the improved predictability of the behavior of the selected combination of coating base and base sheet, coating formulations and the base sheet quality may now be varied in an easier fashion than in the prior art so that a desired product grade will result. As a result, the number of required test runs is thus reduced in the development of a new product or, alternative- ly, the qualities of a given product may be optimized to a greater extent. A sheet surface of excellent printability is attained, since the coating mix droplets can be made to coalesce into a coating film of sufficiently small pore size such that the pigment particles of the printing ink will not penetrate therethrough. If the pores are additionally adjusted to give good permeability to the thinner or solvent of the printing ink, the setting of the printing ink on the print surface becomes fast thus further improving the printability of the sheet.

Next, the invention is described in more detail with the help of exemplary embodi- ments.

In the text below, the term coating mix covers not only formulations employed in actual pigment coating mixes, but also surface size furnishes and other formulations

used in the surface treatment of paper and paperboard webs such that can be applied as freely and unsupportedly jetted sprays and droplets to the surface of a web. While a paper web is discussed in the following exemplary embodiment, obviously the same operating principle of the invention is also functional on a web of paperboard.

The invention is aimed to handling of webs made of fibrous materials. The web can be pretreated with a suitable material, but a clear fibrous structure must be recognizable on the surface thereof on which the coating mix is to be spread. By coating mix it is understood pigment coating mixes in connection of this invention. If there is a first pigment coating layer on the web, a further coating layer can be manufactured on its surface by the method according to the invention. In all it is essential that there is no sealing membrane-like layer between the fiber web and the coating layers that separates them from each other.

The basis of the invention is the fact that incompatible surface energies of the base sheet and the coating mix prevent the formation of a uniform liquid film on the sheet surface, but instead, the coating mix sets forming droplets in the same fashion as water applied to a waxed surface of a car. As a result, the coat layer becomes grainy thus excluding the use of the sheet as a printing substrate due to the inferior print quality caused by the grainy coat. The applied coating mix forms droplets only if the surface energy of the coating mix is greater than that of the substrate on which the coating mix lands, that is, the surface energy of the base sheet surface in this exemplary case.

Surface energy can be derived from the contact angle of a liquid substance droplet with the surface of a supporting substrate. Suitable measurement equipment is avail- <BR> <BR> able from, e. g. , FIBRO System AB, Sweden. Typically, the measurement is carried out by dropping a small droplet of the liquid or mix to be measured onto the surface to be treated, while the behavior of the droplet on the surface is simultaneously imaged. Subsequently, the droplet-to surface contact angle, droplet volume and bottom diameter are determined from the image data as a function of time. From

these data, the average value and variance of measurement results can be computed in a conventional fashion. As the contact angle is determined by the surface energies of the droplet and the supporting surface, these surface energy values can be comput- ed from the measurement data. The numerical surface energy values mentioned in this patent application are average values obtained by measurements. The surface energy of the coating mix may be measured using the coating mix as such or, alterna- tively, by first separating the liquid phase of the mix and then determining the surface energy of this phase.

The surface energy of the coating mix may be in the order of 30 nN/m or less, while the surface energy of the base sheet typically used in a typical embodiment of the present invention is about 35-45 nN/m. Hence the difference between these two surface energies is greater than 5 nN/m. By virtue of adjusting the difference between the surface energy values of the coating mix and the base sheet equal or greater than this value, invention provides good spreading of the coating mix on the sheet surface, whereby good surface quality of the applied coat results. Maximally, the coating mix may have a surface energy equal to the surface energy of the base sheet. If the surface energy of the coating mix is greater than that of the base sheet, the applied coating mix forms droplets thus rendering inferior surface quality.

The surface energy values of the coating mix and base sheet can be modified in various ways. The surface energy of the coating mix may be modified by surface- active agents that lower the surface energy of the coating mix thus promoting the spreading of the applied droplets or film of coating mix on the base sheet surface.

Such agents include, among others, sodium lauryl sulfate and different alcohol derivatives. Modification of the base sheet surface energy, however, is often easier to perform and, in fact, offers greater operating latitude. For instance, the base sheet surface energy may be modified chemically with the help of different polymer compounds, sizes and rosins. Among such web treatment agents, typical are starch, AKD and ASA. The base sheet surface energy can also be altered electrically by means of, e. g. , a corona discharge treatment or mechanical rubbing with the help of rotating brushes, for instance. Mechanical treatment may also be carried out so that the base sheet surface gains an electrical charge or the surface structure is physically modified.