Powder-binder systems

A powder coating is the result of the application of a powder binder system, which generally consists of a resin, a crosslinker, flow additives, degassing agents, pigments or dyes, charge control agents, etc. Such a binder system is ap-pHed to the substrate as a fine, extruded powder (Fig. 20, step A), then melted at... 

Fig. 20a-d. Schematic representation of the different phases of the application of a powder binder system left - the ideal case right - the unbalanced flow/cure of a system, resulting in an orange peel surface of the coating... 

This is well covered in a review article by Iveson et al. The agglomeration regime map in Figure 19.11 shows the agglomeration regime as a function of the deformability of the powder binder system and the degree of saturation. 

Table 3. Critical Binder/Powder Ratios for Some Selected Powder-Liquid Systems...

Since the binder systems are solid at room temperature, they can be produced by the existing methods used for powder coafingsd Solid resins, pigments, photoinitiators, and other additives are premixed, then melted and dispersed in an extruder at 100 to 130°C (212 to 266°F). The molten blend is then squeezed into a thin ribbon between chilled rolls. This ribbon is further cooled to near room temperature on a water-cooled cooling belt. The cooled ribbon is broken first into flake and then ground into a fine powder ready for use. The process is illustrated in Figure 7.15. 

Because the binder systems are solid at room temperature, they can be produced by the existing methods used for powder coatings.32 Solid resins, pigments, photoinitiators and other additives are premixed, then melted and dispersed in an extruder... 

In practice, binder system may consist of three or four additives that differ in their volatility and chemical decomposition. Furthermore, interactions between the binder and the particle surfaces may alter the decomposition kinetics of the pure polymer. In view of its complex nature, a detailed analysis of thermal debinding is not useful. Instead, we consider the basic features of the process for a simplified system consisting of a powder compact with a single binder [e.g., a high molecular weight thermoplastic polymer such as poly(methyl methacrylate), poly(propylene), or poly(vinyl butyral)]. 

Dry colloidal silicas such as pyrogenic amorphous silica do not mix well with the sand and in addition they tend to absorb water from the sand-binder system. Therefore, dry colloidal silica powders should be added to the sand in the form of a paste made with water or water should be added to the sand to help mix the dry silica powder. The amount of water made to use the peste should be enough to assure good mixing of the silica powder and yet not too much to affect the strength of the core or mold when it is hardened. Generally the amount... 

Hybrid Systems. Combinations of water-soluble and water-dispersed binders may be used to achieve synergistic effects (e.g., to control the amount of organic solvents or the application behavior). Polymer dispersions or powdered binders may be used as water-dispersible binders [3.43]. The use of polymer dispersions allows the solids content of water-soluble binders to be increased, the level of organic solvents to be reduced, and the physical drying time to be shortened. The use of water-soluble binders provides a broad application spectrum and yields paint films with well-balanced properties. 

One of the main advantages of thermoplastic systems for ceramic extrusion is the lower abrasivity of the feedstock material relative to other binder systems. The lower abrasivity of a extrusion feedstock prepared with a thermoplastic hinder system compared to one with a solvent-based binder for a given ceramic powder, in this case AI2O3, is shown Fig. 1. In this study a feedstock with 58 vol.-% coarse AI2O3 powder was extruded through a steel die. Looking at the surfaces of the tubes, discoloration of the material stemming from abrasion of the die can he clearly... 

As the volume fraction of powder in a feedstock is increased, the viscosity of the feedstock rises exponentially until a critical volume fraction of powder is reached. Figure 2 shows the mixer torque (which is directly proportional to the viscosity of the feedstock) as a function of silicon carbide powder concentration in a polyethylene/wax binder system [Cle 05]. It is assume that by extrapolating the data to zero reciprocal torque, the maximum powder loading for the feedstock formulations can be determined. 

Wegmann et. al. described the extrusion of polycrystalline ceramic fibers using different thermoplastic and water-based binder systems [Weg 98]. Invariably the highest powder loadings in the study were reached using the thermoplastic binder systems with all the ceramic powders considered. Furthermore, only a thermoplastic polyethylene/wax binder sys-... 

Co-extrusion processing using thermoplastic binder systems for ceramic bodies has been described by Van Hoy [Hoy 98]. The major binder component used in this study was ethylene vinyl acetate, and methoxypoly-ethylene glycol was used as a plasticizer. Feedstocks of alumina, a piezoelectric ceramic powder and carbon black were repeatedly co-extruded to form an array of fine M -shaped structures. With this method the size of the alumina M shapes could be reduced by a factor of 915. In this example the use of solvent-based binders would have been problematical because of drying of the feedstocks and extrudates during the co-extrusion process and during handling. 

A co-extrusion technique was also applied by Beeaff and Ifilmas to produce a BaTiOs multilayer capacitor with nickel electrodes [Bee 02], Polyethylene co-butyl acrylate was used as the major binder component. The minor binder, plasticizer and surfactants used were polyethylene ethyl acetate, microcrystalline wax, mineral oil and PEG. With this binder system it was possible to produce multilayer capacitors with 10 im thick Ni electrode layers by co-extrusion. The limitation was given by the grain size of the ceramic powder used and not by the processing technique. 

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