Patent DE-19941194-A1 (DE)
Title: Coating of moving paper/cardboard web surfaces uses a liquid or paste coating medium which is foamed for application and which retains its foam structure after drying
Inventor: TIETZ MARTIN
IPC: D21H23/46
Filed: 1999-08-30
Abstract: To coat a moving web (12) of paper or cardboard, with a liquid or paste coating, the coating medium (16) is applied as a foam which retains its foam structure in the finished coating (14).  The coating medium (16) is foamed by the introduction of a gas, and pref. air, delivered by a number of tubes into the medium (16), across the width of the paper/cardboard web (12). The gas feeds can open into a channel across the web (12) width. The gas can also be delivered mechanically into the coating medium (16), using a rotating shaft with radial projecting beating units, which form a gap between their ends and the surrounding housing, and they can be moved through gap between projecting units and/or between neighboring stirring shafts. The medium can also be foamed by a chemical reaction. The content of the gas within the coating medium (16) is 20-80 vol% and pref. 30-60 vol%. The solid content of the foamed coating medium (16) is 5-70 wt%. The gas bubbles (14a-14c) within the coating medium (16) have a mean size of max. 10  mu m and pref. 1-10  mu m, and the max. bubble (14a-14c) is about 200  mu m. The bubbles (14a-14c) are diminished by a shear action on the foamed coating (16). The medium contains a stabilizing agent such as a tenside or stearate. The applicator has a short application time pref. with an application chamber and immediate ready dosing, or it has a long application time followed by a station to smooth and/or finally dose the applied coating layer (14). The applicator applies the coating medium (16) to the supporting web (12) in the required coating weight, by free spray jets. The coating is foamed directly before application, or the foamed medium has a given dwell time before it is coated on to the web (12). The applicator system is divided into sections, across the moving web (12), where the coating application can be adjusted individually within each section to set the foaming action and/or the foam treatment. The applied foam coating (16) is smoothed and/or finally dosed by a profiled doctor rod and/or a doctor rod with a smooth surface and/or a doctor blade. The applied coating layer (14) is dried by an infra red dryer and/or an air dryer. A covering layer (26) is applied over the coating (14), using the coating medium which has not been foamed. The coating medium (16) is a glue and/or starch and/or a dispersion of pigments and/or offset coating inks and/or gravure coating inks. An Independent claim is included for an applicator assembly to coat a moving paper or cardboard web (12) directly or indirectly with a liquid or paste coating medium (16), such as by a transfer roller. The coating medium (16) is foamed, and the coating layer (14) retains its foam structure in the finished product. Preferred Features: At least one gas feed delivers a gas, and pref. air into the coating medium (16), pref. with a number of gas feeds across the machine width which open into a channel. The gas can be introduced into the coating medium (16) by mechanical agitation or a chemical reaction. The foamed medium can be held in a reservoir for a given dwell time, before it is applied to the web surface. The thickness of the dried coating layer (14) is 10-200  mu m and pref.20-80  mu m.