Viscosity, coating colors

Dextrins are produced by dry heating (roasting) starch in the presence of an acid catalyst. They are produced in a range of viscosity and color specifications. Dextrins are primarily used as adhesives in paper conversion, such as laminating and envelope production. A low-viscosity dextrin is used in Europe as a total chlorine free (TCF) coating binder for application on TCF 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... 

Microbiological action in starch dispersions results in a drop in pH, loss of viscosity and the development of odor. Retrogradation may be accelerated by the drop in pH or especially if butanol, which complexes with amylose, is generated via starch fermentation. Sulfate-reducing bacteria will cause black deposits due to reaction with iron in the process water. For quality control, preservatives are added to starch slurry, cooked starch, surface size and coating color. 

Coating color can be heavily affected by microbial activity with a drop in pH and more problematic, changes in viscosity, which means problems in coating ... 

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. 

Natural products, like casein, starch, and soy protein used as main binders in the past, also impart the necessary viscosity and water retention to coating colors. However, they were unable to satisfy the increasing demands that were placed on runnability and coating quality, and they have gradually been superseded by syn-... 

The interaction between the polymer and water has a pronounced effect on the viscosity of the aqueous phase as weU as on the water retention of the coating color. Like aU hydrocoUoids, synthetic thickeners bind a large number of water molecules along their polymer chains, with the result that their diameter and volume increase and they occupy a greater space in the aqueous phase (Fig. 3.14). Apart from... 

The solids content greatly influences the runnability of the coating color in the coating machine. The solids content basically influences the viscosity of the color and its flow behavior at varying shear stresses in the coating machine. 

The mixing, storage and circulation tanks for coating colors are made of stainless steel, since the pH range of raw materials and coating colors tends to corrode standard steels. Most vessels are equipped with an agitator, and some also have a shell/jacket that may be heated or cooled. Agitators are mostly indispensable because, in low-viscosity colors especially, soHd color components are prone to sedimentation unless the suspension is stirred continuously. 

Apart from the increase in the viscosity of the aqueous phase due to the dissolved polymer molecules, which are present as stiff chains at the pH of the coating color... 

The most important natural binder still in use today is starch, though it is now frequently used in combination with synthetic binders. Com starch is more common in the USA, whereas potato starch is more prevalent in Europe. Native starch containing two fractions of amylose (linear chain) and amylopectin (branched chain) is not suitable for coating paper and board because the amylose fraction tends to undergo retrogradation and the viscosity of coating colors made with native starch is too high [5]. For these reasons, only treated (i.e., depolymerized) or chemically modified starches are used. Most paper mills carry out their own starch preparations in-house. 

Does not affect coating color viscosity, high levels of solid content possible Freeze-sensitive... 

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