Paper coating method and apparatus

A method of applying coating liquid to a moving web of paper with a coater having a limited coating liquid application zone defined by a pressure chamber having an outlet slot with front and rear edges, side edges substantially sealed to the web and a doctor blade at the rear edge engaging the web, and with the front edge being spaced from the web, comprising the steps of

• applying coating liquid through the limited application zone under a pressure in the order of about 7 to about 150 inches of water to one surface of the moving web of paper,
• forming and maintaining a reservoir of coating liquid under pressure on the web in the application zone between the doctor blade and the front and side edges of the pressure chamber,
• doctoring the coating on the web at the rear edge of said application zone with the doctor blade within about 0.0004ths to about 0.0100ths of a second of the pressure application of the coating liquid to the web and while the coating liquid is maintained under said pressure,
• maintaining said pressure in said application zone and at the doctor blade by establishing a liquid seal in a gap defined between the web and the front edge of said application zone which extends substantially continuously across the width of the web, and
• establishing and maintaining said liquid seal by spacing said front edge of said pressure chamber less than one inch from the surface of the web and continuously flowing coating liquid under said pressure reversely of the direction of web travel through the gap between the moving web and the front edge of said pressure chamber to substantially completely and continuously fill said gap with coating liquid for sealing off the front edge of the application zone and preventing entry of air and foreign matter through the gap into the zone and for purging the application zone.

A method of applying coating liquid to a moving web of paper with a coater having a limited coating liquid application zone defined by an outlet slot from a pressure chamber and having a front edge spaced from the web, end spaces substantially sealed to the web and a doctor blade at the rear edge engaging the web, comprising the steps of:

• establishing and maintaining a liquid seal in a gap between the web and the front edge of the limited application zone which extends substantially continuously across said zone;
• locating the doctor blade immediately adjacent the rear of the liquid seal in engagement with the web;
• sealing the end spaces between the liquid seal and the doctor blade;
• forming and maintaining a reservoir of coating liquid on the web in the application zone between the liquid seal, doctor blade and sealed end spaces;
• pressurizing the coating liquid in the reservoir to a pressure of at least 7 inches of water and causing said liquid to seal off the front edge of the application zone for preventing entry of air and foreign matter through the gap into the zone;
• applying the coating liquid at substantially said pressure to the web;
• doctoring the coating liquid from the web within 0.0100th of a second or less of its application to the web and while the coating liquid is under said pressure,
• whereby the pigment of the coating liquid is applied primarily to the surface of the web with a minimum penetration of the coating liquid into the web.