Patent DE-10012256-A1 (DE)
Title: Curtain coating applicator for a moving paper/cardboard web has electrodes at given electrical potentials to generate magnetic fields to affect the coating medium flow between the applicator and web surface in direct or indirect coating
Inventor: MENDEZ-GALLON BENJAMIN
IPC: D21H23/48
Filed: 2000-03-14
Abstract: The applicator station (10), to apply a coating of a liquid or paste medium (16) on one or both sides of the web surfaces (U) of a moving web, uses a curtain coating action. A unit (30) to generate an electrical field is near the curtain coating applicator (12), to give a supporting force for the coating curtain (18) which falls from the applicator (12) to the web coating surface (U).  The coating medium (16) falls by gravity as a curtain (18) from the applicator (12) to the moving web coating surface (U). Or the applicator (12) is a free jet unit, where the ejection pulses carry the coating stream to the coating surface. The applicator (12) is held at a given electrical potential (V1), and a second electrical potential (VE or V2) is at the web coating surface (U), at least in the zone of the applicator (12). An electrode array (30) is at the upstream side of the applicator (12) and close to it, preferably at a gap from the web coating surface (U) and which is held at a third set electrical potential (V3). The electrode array (30) has at least one flat electrode, with a number of projections or needle points on the side towards the moving web, or it has a number of neighboring needle electrodes (32) across the web coating surface (U). A further electrode array (46) is downstream of the coating medium strike point (P) at the web coating surface (U), to generate an electrical field to give a force towards the coating medium (16) on the coated surface (U). The electrode array (46) is held at a fourth defined electrical potential (V4). The first and/or third and/or fourth electrical potentials (V1,V3,V4) have a value of 5-60 kV and preferably 30 kV. The second electrical potential is an earth potential (VE). The web (20 for coating is supported by a counter roller (22), at the application point where the coating medium is applied to the web surface directly or indirectly. The support roller (22) is in contact with an electrode, to hold it at the second electrical potential value (V2). The outer surface (22a) of the support roller (22) is in slip contact with the electrode (50), or the electrode is in slip contact with the roller shaft axis (A). Where the coating is applied directly, the web (20) passes around a web guide as an electrode (48), held at the second electrical potential (V2). A magnetic field generator can be used instead of or in addition to the electrode arrays (30) and/or (46), to affect the movement of the coating medium (16). A suction unit (34), to weaken the air layer (G) carried by the moving web surface (U), is in front of the electrode array (30) in the direction (L) of web travel. A drag scraper (36) is at the downstream end of the suction unit (34), with preferably a further electrode (40) between the scraper (36) and the downstream end of the suction unit (34), using a number of adjacent and preferably needle electrodes across the web or a flat electrode with a number of projections or needle points on the side towards the web, and at a gap of 2-30 mm from the web surface and with a floating electrical potential. The electrode array (40) is electrically insulated (42) against the suction unit (34). The applicator (12) and/or the suction unit (34) are held at a given electrical potential, and preferably earth potential (VE). An Independent claim is included for a web coating assembly with an edge guide to define the side edges of the coating curtain (18) over at least part of its falling movement through gravity, between the applicator (12) and the web coating surface (U). Preferred Features: At least one side edge guide has a surface structure so that the wetting angle set by the characteristics of the coating medium (16) and the guide surface is <= 90 deg . The curtain edge guide surface can be structured or roughened or toothed or has an outer spiral. At least one curtain edge guide is of glass or metal, and can slide laterally (Q) across the web coating surface (U). The curtain edge guide can be adjusted at an angle to the vertical. An electrode, at a given electrical potential, is near at least one coating curtain edge guide, and preferably parallel to it.