Blade coater

In 1933, the first roU coater was installed as an integral part of a paper machine. These on-machine coaters produce a low cost coated paper used largely for magazines. Coating of paper off-machine at speeds greater than paper machine manufacturing speeds is possible with the use of the various available blade coaters (17). 

Fig. 2. (a) Puddle coater (b) roll appHcator blade coater (c) fountain blade coater. 

In the rod-blade coater unit a rod is mounted at the end of the blade. This coater behaves more like the beveled blade coater than a flexible blade... 

Two-BIa.de Coaters. In order to coat both sides of a web simultaneously, two flexible blade coaters can be used back-to-back, ie, with both blades pressing against each other and the web between them. The web usually travels vertically upward. Different coatings can be appHed on each side of the web. The blades tend to be thinner and more flexible than the standard blades and the angle to the web is lower. The web has to have sufficient tensile strength to be pulled through the nip. 

Figure 1 is a schematic diagram of a typical blade coater. This is still the most common form of coating device used in the paper industry, although new... 

Roll coaters can apply a coating simultaneously to both sides of the sheet, while blade coaters generally treat one side at a time and require intermediate drying. Roll and blade systems have been combined in the Billblade coater, which has found use in the production of carbonless copy papers and for paper precoating.224... 

Figure 18.8 (a) A blade coater installation in a paper machine (b) schematic illustration of paper... 

The quantity of coating applied by a blade coater is determined by the roughness of the paper surface, the solids content and viscosity of the coating color, the speed of the coating operation, the geometry of the system, blade mechanical characteristics and the pressure applied at the blade.225,226 Calendering prior to coating application... 

Coating High De makes processing in high speed blade coater problematic De << 1 equiUbrium thermodynamics any work done isother-mally is immediately dispersed as heat. 

As a general rule of thumb, the doctor blade coater or knife coater and the "comma coater" or other round-bottomed coating tools are used for the manufacture of tape thicknesses from 50 to > 1000 pm (0.002 to > 0.040 in.). When thinner coatings are required, the industry has turned to lip coaters, microgravure coaters, and slot-die coaters. The lip coater is used to make multilayered ceramic capacitor tapes for thicknesses of 50 pm (0.002 in) or less. For tape thicknesses 25 pm (0.001 in.) and less, the microgravure and slot-die techniques have been used very effectively. 

Blade coaters are commonly used on pigmented coatings. They have the unique feature of troweling in the low areas in a paper web, thus producing a coated surface that has excellent smoothness and printing qualities. The backing... 

Most high-temperature units have a cooldown time of 1 to 1.5 min. Occurrence of a break means a significant fire risk if the hot surface is below the web. Since air-knife and blade coaters usually coat the surface, this is not a problem. 

Soybean protein is a very important binder type, especially in North America. It has properties very similar to those of casein. Isolated soybean proteins are hydrolyzed, isoelectric proteins. They are used in the form of alpha and delta proteins with four different viscosities (extra low, low, medium, and high). The viscosity refers to the dissolved soybean protein, the solvent of choice being aqueous ammonia (26 Be). Proteins dissolved in this way exhibit very low sensitivity to water after drying. like casein, soybean protein is mostly used as a mixture with polymer dispersions. This combination permits the preparation of coating colors with high sohds content and a relatively low viscosity. Solids contents ofca. 60%, suitable for blade coaters, can be achieved. 

Coating of graphic papers is more often performed off-line. Some machines with on-line blade coaters are in operation, but usually at moderate machine speeds. 

Unlike film and blade coaters, the application is contact-free. This considerably reduces the risk of web breaks and, therefore, improves runnability. It is expected that the significance of curtain coating will increase. 

Wood-containing papers are mainly single-coated (LWC). Coat weights are usually between 5 and 10 g m per side. Traditionally, the coat is applied with a blade coater. More modern installations use a film press for the single coat. 

Art papers represent one of the highest quality printing papers and are used for illustrated books, calendars, and brochures. The basis weight varies from 100 to 230 g m . Art papers are produced with matt or glossy finish. The coat weight varies from 20 to > 40 g per side. Art papers are almost exclusively available in sheets. They are triple coated, usually first with a precoater and thereafter with a double blade coater. 

Coating application processes have changed significantly over the years. Three major coating processes are used today roll coaters, air knife coaters, and blade coaters. 

Blade coaters were first introduced in the 1940s. Blade coaters have several configurations, but most are of the trailing blade type. In this process, the eoating color is applied to the paper web and the excess removed by a doctor blade. 

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