Coating apparatus – Solid member or material acting on coating after application – Web or belt member
Reexamination Certificate
1999-06-14
2001-11-06
Edwards, Laura (Department: 1734)
Coating apparatus
Solid member or material acting on coating after application
Web or belt member
C118S119000, C118S126000, C162S281000
Reexamination Certificate
active
06312520
ABSTRACT:
The present invention relates to a coating blade for applying coating material onto a travelling web, particularly coating liquid or paste onto a travelling paper web.
The traditional blade coating technology is based on the fact that the blade can deflect, i.e. that the blade in its cross-direction will be subject to deflection when the blade by one longitudinal edge thereof engages a travelling web. Contrary hereto Swedish patent 468 344 discloses a technology where the blade rather than being deflected has a movable attachment enabling movement of the blade towards and away from the travelling web. By using this technology it is possible to use coating blades which essentially have no bending capacity.
Since the primary function of the coating blade is to even and remove excess of a coating liquid containing hard particles, for example titanium dioxide, calcium carbonate and the like, the edge section of the blade engaging the travelling web will be subject to continuous wear. Such wear can be caused also by particles found in the paper web proper. As soon as the blade has reached a certain degree of wear it has to be replaced. Such replacement of blade involves high costs, on the one hand caused by the cost of the blade per se, on the other hand also and essentially by the production interruption caused by the change of blade.
An important property of coating blades is thus that the resistance to wear or abration is as high as possible. Since the blades which are used in the traditional blade coating techniques also shall have springing properties which to some extent result in a lower resistance to wear the blade quality becomes a compromise between spring properties and wear resistance.
In addition to the fact that the wear resistance of the blades is dependent on the material of the blade it is also known to coat the engagement surface of the blade with wear-resistant materials, for example ceramic materials or chromium. In view of the fact that such wear resistant layers shall be applied to elongate and thin materials, often steel bands, limitations in production will arise resulting in high production costs.
The present invention has for its main object to provide a coating blade of improved resistance to wear. Another object of the invention is to provide coating blades which can be manufactured at a low cost.
Yet another object of the invention is to provide blades which give advantages in a quality with regard to the coated layer. For these and other objects which will be clear from the following disclosure there is provided by the present invention coating blades for applying a coating material onto a travelling web. Although the invention is not restricted hereto it Will in the following be described particularly in connection with the application of a coating liquid onto a travelling paper web. The coating blade according to the invention includes a band of steel or other form-stable material which, along the edge section intended to engage the web, is provided with a wear-resistant coating. In accordance with the present invention it has been surprisingly found that if said coating is constituted by a material of a relatively low hardness, namely a hardness of about 10 to about 100 measured in accordance with Shore A, substantial advantages are gained in connection with the use of the coating blade.
The fact that the use of a coating blade having a coating of a relatively soft material resulted in a high wear resistance in combination with important operational advantages was completely surprising to the skilled artisan. As acknowledged in the conventional art one has looked for coating with hard materials, for example cerams, metals or the like, for the purpose of obtaining a high wear resistance, and the relatively complicated techniques that have to be used for the application of this type of hard coatings has involved high production costs and thereby an undesired high price on the final product.
In connection with the present invention it is preferred that the coating applied to the coating blade has a hardness according to Shore A of about 30 to about 80, particularly about 40 to about 70. It is particularly preferred that the hardness according to Shore A lies within the range of about 50 to 60.
The surprising observation on which the present invention is founded, namely the possibility of using a relatively soft material as a coating, means that said coating material can be constituted by an organic polymer. Examples of useful polymers are polyurethanes, styrene-butadiene polymers, i.e. polymers of rubber type, and polyolephins.
A particularly preferred type of polymer is polyurethanes, the building stones of which in a conventional manner are constituted by polyoles and diisocyanates. Usual diisocyanates for polyurethane systems are toluene diisocianate, diphenylmethane diisocyanate and naphtalene diisoocyanate. Also less common diisocyanates are available, such as hexamehtylene diisocyanate and isoforon diisocyanate. Polyurethanes are available for example as esterurethanes, etherurethanes and urethanes based on hydroxyl-terminated polybutandienes.
What type of pure polyurethane that is used in the present invention is not critical for the practical result as long as the hardness thereof lies Within the ranges given above.
Experiments carried out while using the techniques according to the present invention have shown that the material results in high wear-resistance or abrasion-resistance but also other advantages, such as improved fiber coverage and operability, i.e. low failure frequency, have been recorded. The soft coating gives an improved complience vis-à-vis the paper surface and thereby results in a lower puttying effect compared to traditional hard blades or blade coatings. The improved operability is probably due to the fact that the soft coating at the edge of the blade more easily allows passage of the particles which are present in the paper web or in the coating paste in view of the fact that the soft material will be elasticly deformed by the particle. Immediately after the passage of a particle the coating material then regains its starting position.
In view of the elasticity of the coating at the edge of the blade in engagement with the travelling web the blade edge also to some extent adapts to the contour of the paper surface which results in the desired effect that the coating layer covers the fibers of the base paper in a more even manner than in traditional coating using hard blades or blade coatings. This effect has been shown by extensive pilot experiments, where the result with regard to failure frequency and fiber coverage has been studied. In comparison with regard to rupture frequency the number of ruptures of the web occurring at the blade have been counted. The runs have been carried out at equal conditions with regard to base paper quality, running time, coating machine variables etc.
In the comparison with regard to fiber coverage the results have been studied by so called “burning tests” and colouring with an absorbing type of colour “kroda”. The burning test is carried out by moisturing the coated paper surface with an about 10% ammonium chloride solution. This chemical results in darkening of the cellulose fibers when the paper is heated to about 300-400° C. using a heat pistol or in a furnace. The white coating layer then appears in contrast against the dark substrate.
This test shows quite clearly how the coating layer has been distributed on the surface of the paper. The result of this comparison shows clearly that a polymer-coated blade in accordance with the invention results in a coating layer of a more even thickness as compared to a steel blade of conventional type which results in a flamy effect and thus greater amount of coating in the “valleys” of the paper surface and less coating on the “tops” of the paper surface.
The colouring test also shows the same result in that the colour is absorbed more on the sections where the coating layer is thinner. In view of the fact that the “valleys” of the paper surf
Eriksson Tore
Karlsson Hakan
BTG Kalle Inventing AB
Burns Doane Swecker & Mathis L.L.P.
Edwards Laura
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