Coating apparatus – With vacuum or fluid pressure chamber – With means to apply electrical and/or radiant energy to work...
Reexamination Certificate
1999-08-06
2001-05-29
Edwards, Laura (Department: 1734)
Coating apparatus
With vacuum or fluid pressure chamber
With means to apply electrical and/or radiant energy to work...
C118S620000, C118S623000, C118S323000, C118S325000
Reexamination Certificate
active
06237525
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for coating a paper or board web or the like by an aerosol generated with the aid of an ultrasonic atomizer.
BACKGROUND OF THE INVENTION
The quality of paper board can be improved by, among other ways coating the paper or board web with a suitable coating mix. The coat is most commonly applied using brush applicators, nozzle applicators and/or short-dwell coaters, all of which are combined with an almost simultaneous control of coat thickness by means of a doctor blade. These methods all employ that a narrow slit formed prolimate to the moving web, whereby the amount of applied coat is controlled by the slit. The final control of the coat weight may be performed in a number of different manners. Using the above-mentioned methods under conditions optimized for each method, a good coat quality can be attained at relatively high web speeds. However, running the web at a speed above a certain limit may be difficult due to a number of reasons. During application, the web is subjected to a relatively severe strain, which particularly at high web speeds, readily causes web breakages. During coat application, the web is slightly elongated by the tensional strain and increased moisture content, whereby such elongation must be managed by the drive control system of the paper machine. Another shortcoming of conventional coaters in application at high web speeds is the uncontrolled flow of the coating mix at the instant of application resulting in uneven coat smoothing, which can be seen as blots and streaks on the web and as splashing around the machine. The severe strain imposed on the web during doctor blade coating causes a coating problem.
Because industry demand is for paper making machinery operating at higher web speeds, novel methods must be developed for the application of coating mix onto the web surface. One of these new methods is spray-coating in which the coating mix is blown from spray nozzles onto the web. One form of spray-coating is ultra-sonic coating in which the coating mix is atomized into small droplets with the help of ultrasonic energy and the droplets are guided to hit the web surface chiefly ejected by the ultrasonic atomizer. Alternatively, ultrasonic energy can be used merely for atomizing the coating mix into droplets. The principal benefit of ultrasonic coating is that an extremely small droplet size can be achieved, whereby the applied coat becomes homogeneous and no mechanical doctoring gap for coat weight control is required in the vicinity of the web. The ultrasonic nozzle has a nozzle channel ending at the nozzle tip surface, and the material to be atomized is fed into the channel. Piezoelectric elements are adapted about this tubular part, emitting ultrasonic vibrations capable of atomizing the material to be spray-coated into an aerosol.
Wider use of ultrasonic coating has been limited by the lack of sufficient atomizing output power in commercially available ultrasonic nozzles. For instance, if it is desired to coat the web with an amount of coating mix that results in 10 g/m
2
dry coat weight, the amount of coating mix applied onto each linear meter of a web running at 30 m/s must be 600 gs
−1
m
−1
when the coating mix contains 50% solids. Then, the required ultrasonic atomizing effect is 1000-3000 gs
−1
m
−1
. However, the output power of conventional ultrasonic nozzles is so low that the required number of nozzles readily becomes very high if the web speed and amount of applied coat is increased. This type of nozzle makes it difficult to achieve high-speed application onto a narrow area which would be advantageous in terms of minimal moisture absorption into the web. Nozzles with a circular orifice are also hampered by their spot-shaped hit area of the atomized droplets which makes it difficult to achieve a homogeneous coat as the hit areas of the nozzles cannot be merged with each other in a seamless manner. Difficult-to-pump mixes, for example, high-viscosity coatings are also difficult to atomize by means of conventional ultrasonic nozzles.
SUMMARY OF THE INVENTION
It is an object of the present invention to achieve a method for applying a coat to a paper or board web based on ultrasonic atomization of the coating mix.
The goal of the present invention is accomplished by applying the output energy of a plurality of ultrasonic transducers by means of a vibrated beam to a coating mix flow passing close to the web surface, whereby the ultrasonic energy is transmitted to the coating mix so as to atomize the upper layer of the coating mix flow into an aerosol which is emitted toward and deposited on the moving web.
The invention offers significant benefits.
Ultrasonic atomization makes it possible to achieve an extremely small aerosol droplet size and homogeneous size distribution of the droplets. The use of the vibrated atomizer beam according to the present invention achieves a smooth coat thickness over the width of the web, and moreover, permits profile control of the amount of applied coat. The droplet size can be controlled by varying the ultrasonic frequency, and the coat weight can be easily controlled by altering the thickness of the atomized coating flow and its distance from the web. An electric field may be readily applied between the coat mix flow and the web, thereby enhancing the transfer of the droplets onto the web. A number of vibrated atomizer beams can be employed within a relatively narrow application zone, whereby a sufficient atomizing energy density and amount of atomized coating mix is provided. Excess aerosol is easy to recover as the air layer travelling along with the running web forces the excess aerosol back to the coating mix flow. With regard to its mechanical construction, the applicator apparatus according to the present invention is extremely simple compared with conventional coaters as it requires no complex end seals, heavy framework or complex control arrangements. In its simplest embodiment, the only moving parts of the apparatus comprise elements for adjusting the distance between the surface of the coating mix flow and the web.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.
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Cohen, Pontani, Lierberman & Pavane
Edwards Laura
Valmet Corporation
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