Air knife coater

ASSI Frovifors Bruk, Board mill, Frovifors, Sweden In 1991, two CR filters were installed at ASSl Frovifors Bruk board mUl in Frovifors (Sweden) to concentrate and recover the waste latex coating from an air-knife coater. The 50 kg/mol cutoff polysulfone membrane produces a flux of 200 L/(m h). The mUl uses Ti02 pigment, which makes the recovery process extremely economical. The UF operates at a constant concentrate sohds concentration of 20% continuously for 10 days between cleanings [1,130]. When concentrating a standard coating color effluent steady flux of 120 L/(m h) at a coating color concentration of 10%-15% (50 L/(m h) at 40%) is achieved [36]. 

During the development of microencapsulation. Green, Schleicher, and others at NCR went looking for paper companies capable of coating the pressure-sensitive microcapsules onto paper. Appleton s coating capabilities drew their attention, because the company s air knife coater did not require the application of pressure for coating, which would damage the capsules. 

Information is given on methods of applying coatings (air-knife coaters, lock coalers, roll coalers and blade coalers) and the development of synthetic latices from SB, NB and AN for coating paper. 24 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

Air knife coater works in a way similar to a normal knife coater except that the excess of coating is blown off by air under pressure (Fig. 42). This technique works only with very fluid adhesives (less than 500 mPas) with low solids. Also the solvents should dry slowly or else blowing air could cause a skinning over the adhesive surface. When working with a water-based adhesive, the blown air should be saturated with water vapour so that the excess of adhesive could be recycled. Advantages ... 

Smoothness Smooth base paper gives a uniform and closed coated surface. The various coating processes have slightly different smoothness requirements. Air knife coaters require a relatively smooth surface because, in that method, coating... 

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

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. 

In the extrusion die coater, the coating layer is formed and calibrated in the slot and laid down on the substrate. The thickness is independent of the web thickness and can be reduced by stretching. There is no penetration through open substrates like fribrics, and there is no influence of particles on the coating. Good contact to the substrates can be provided by an air knife or an electrostatic field. 

Table 6 shows typical paper coating for-mulations with a 50 50 SBR-to-casein ratio, for use on two types of coaters. For a multiple roll coater, water would be added to adjust to a percent total solids of 50-55, giving a coating weight of 3-4 lb per 1000 sq ft. For an air knife... 

Drying Lines. Oven drying lines represent large capital investments, of the order of several million dollars, and the same line is frequently used to handle both solutions and latices. The metering equipment may be either reverse roll coaters, which offer maximum flexibility, or knife-over-roll coaters, which are less expensive. For thin films a wire-wound rod to remove excess material can be used. Because of coating difficulties encountered with latex vehicles such devices as an air knife or flexible blade are sometimes used with these products. 

Solution adhesives are applied at 10-25% total solids content by spray, doctor knife, reverse roll coater, roller, or by spatula or trowel. The adhesive is dried at room temperature or in air ovens, care being taken to eliminate the fire and health hazards associated with some solvents. 

In some modem curtain coaters, the top tank is a small volume chamber equipped with a rotating double knife. It may apply a thin film of adhesive, with reduced entrapment of air and an electronic control of thickness. With this closed application system there is no evaporation of solvents, it may work with both solvent based or water based adhesives and even the moisture curing polyurethanes. 

Standard equipment for solution based PSAs are knife over roll and reverse roll coaters (Fig. 15) [255, p. 13 ff]. For high-speed application, a closed reservoir or a slot die is preferred for better control of air bubbles [260]. Especially the knife over roll coater allows for an easy change of the coating material. 

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