Coating knife

Knife coating is one popular technique for the coating of textile materials. Because the layers of the applied coating are usually very thin, the rheology of the coating is of primary importance. Rheological properties are controlled mostly by the choice of polymer. 

In PVC coating formulation fillers play a role. Filler choice mostly depends on the way the filler affect viscosity. The filler should not absorb the plasticizers nor interfere with the pseudoplastic behavior of the paste which is determined by the resin properties and by the choice of plasticizers. Fillers must be completely dispersed, since the gaps between the coated substrate and the knife are very small. There must be no lumps. Fillers should not interfere with deaeration which is 

Silica fillers play a prominent role in the paste rheology of polyurethane and rubber coating mixtures. Silica fillers are also used for surface matting. In polyurethanes, the layer thickness is even thinner than is used with PVC. Dispersion is enhanced by dispersion aids. Fillers are used at relatively low concentrations in PU formulation. But in rubber coatings, large quantities of calcium carbonate are used to decrease cost. Rheology and reinforcement are adjusted with carbon black. 

Although different rubber composition requires different equipment and processing, several general principles apply. In many mixers, especially those of older construction, fillers and oils are added in increments to facilitate dispersion. Frequently, in two stage mixing, the first stage is upside down mixing to achieve 

Two-roll mixing is another traditional method of mixing which relies on somewhat primitive equipment and on the skills of the operator. It has never been possible to establish standard procedure for this process and batch-to-batch results vary widely. 

This method employs an adjustable knife blade, bar, or rod to control the deposition of adhesive flowing onto a sheet moving under the blade. The distance between the blade edge and the adherend surface determines the adhesive coating thickness. 

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Extrusion coating is ideal for rods, tube and wire. The article is passed through a paint reservoir and then out via a die, which leaves only the correct thickness of paint in place. There are further techniques suitable for flat articles in sheet or web form. Knife coating is ideal for very thin coats, especially on continuous paper or plastic webs. The knife is either a metal doctor blade or a curtain of high velocity air (an air knife) directed onto the surface and it removes surplus material applied previously. 

Airjet spinning, cotton yarn, <5 17 Airjet textured yarn process, 20 19 Airjet texturing, 19 753 Air-knife chemical finishing, 17 513 Air knife coating, 7 10-11 method summarized, 7 5t Air-laid processes, 11 179-180 Air legislation (EU), 23 121 Airless atomization, in spray coating, 7 69-70... 

Knedel micelles, 20 489 Knelson concentrator, 16 632 Knife coatings, 7 7-8, 10-11 method summarized, 7 5t Knife-over-roll chemical finishing,... 

Poly(vinyl acetate) dry resins and ethylene-vinyl acetate (EVA) copolymers are used in solvent adhesives, which can be applied by total industrial techniques, e.g., brushing, knife-coating, roller-coating, spraying, or dipping... 

Fig. 6.17 Schematic representation of knife coating, based on building block 3.

The flow configuration of building block 3, of two non-parallel plates in relative motion, shown in Fig. 6.19, was analyzed in detail in Example 2.8 using the lubrication approximation and the Reynolds equation. This flow configuration is not only relevant to knife coating and calendering, but to SSEs as well, because the screw channel normally has constant-tapered sections. As shown in Fig. 6.19, the gap between the plates of length L is Ho and II at the entrance and exit, respectively, and the upper plate moves at constant velocity Vo-... 

Example 6.8 Knife Coating Consider a knife at angle to a substrate, which moves at velocity Vo and being covered by a thin film coating of a viscous Newtonian liquid of thickness <5, as shown schematically in Fig. E6.8. Derive an expression for the film thickness <5. 

Since the inception of the era of modem ceramics, usually defined as starting in the late 1940s, there have been many advances in ceramics processing technology. One of the most prolific advances in terms of commercial use has been the development and implementation of tape casting, also referred to as knife coating... 

The volumetric add-on of foam can also be used to control the dry pick-up on the fabric when the application method Is knife coating, But for horizontal pad application, dry pickup Is best controlled by the blow ratio,... 

PVPyrH Poly-4-vinylpyridine (PVPyr) prepared by conventional persulfate catalyzed polymerization was dissolved in ethyl alcohol or methyl Cellosolve (2 to 3% solution) for application to polyester film substances. These films adsorb heparin from solutions of sodium heparin to produce anticoagulant surfaces (PVPyrH). Films were painted on, or spread by knife coating. 

Figure 4 shows the relatively even adsorption on polyvinylpyridine surfaces spread by knife coating and dipped into radioactive heparin solution (500 fig./ml.) for 6 minutes. Six discs each 1 inch in diameter were die cut from each of two duplicate preparations which were similar in visual evenness to the ones shown. Planchet counts gave the results shown below ... 

The film is continuously coated from the front or backside by rolls or knife coating. For small-volume items frequently gravure or a roller coater is used. All these applications result in an even orientation of the metallic flakes parallel to surface. The frequently used spraying technique has a disadvantage in this case because, due to its high viscosity, the metal pigment shows only little orientation. 

Emulsifiers are added to toffees to help disperse the fat (see also Chapter 4), although it is perfectly possible to make toffees that do not contain added emulsifiers if sufficient skim milk solids are present. The emulsifying effect of a considerable quantity of skim milk solids can be replaced by a very small quantity of an emulsifier, e.g. lecithin or distilled monoglycerides, and the price of skim milk in the EU makes this an attractive proposition. Curiously, fat that is too well dispersed can cause problems. If some fat coats the surface of the piece then the toffee will cut easily as the fat lubricates the cutting knife if there is no surface fat then the toffee can stick to the knife. One solution to this is to have a cutting knife coated with PTFE. The PTFE has a very low energy surface that the toffee will not stick to. 

Application of compounds of this nature may be made in several ways. Typically, a water dilution is made and the material padded on. Another procedure might be by knife coating. Although foaming techniques have not been used widely with this product, it is possible through proper formulation to apply even lighter applications in this manner. 

Doctor blading, also known as tape casting or knife coating, uses a scraping blade, known as the doctor for the removal of excess substances from a moving surface being coated. 

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