Thermosetting powders

The volume of thermosetting powders sold exceeds that of thermoplastics by a wide margin. Thermoplastic resins are almost synonymous with fluidized-bed appHed thick-film functional coatings whereas thermosetting powders are used almost exclusively in electrostatic spray processes and appHed as thin-film decorative coatings. 

Table 3. Physical and Coating Properties of Thermosetting Powder...

The relative reactivity of thermosetting powders can be easily deterrnined by the gel time or stroke-cure test. A small amount of powder is placed on a hot plate, usually at 200°C, and the time until the coating composition gels, or no longer forms fibers, is deterrnined. Powders are characterized by relative gel times (cure rate) as shown in Figure 1. 

Dry-Blending. Most plasticized PVC powders are prepared by a dry-blend process in which the plasticizers, stabilizers, pigments, and additives are absorbed on the porous PVC particles at elevated temperatures while they are being agitated in a high speed mixer. Thermosetting powders are almost never prepared by this process. 

Fairbanks [100] has also used ultrasound (75 W) to compression mould both thermoplastic powders (acrylics and vinyls) and thermoset powders (phenolics and allylics). The optimum conditions to produce maximum fusion are given it Tab. 5.26. 

The fundamental benefit of UV powder coatings is that they can be processed at considerably lower temperatures than traditional thermoset powders. While ordinary thermoset powders require temperatures in the range of 350 to 400°F (177 to 204°C), commercial UV curable powders require temperatures that are not higher than 220 to 250°F (104 to 121°C). A comparison of the two types is in Table 7.7. 

Thermosetting Powders Heating/Curing Temperature, c (op) Total Curing Time, min Substrate... 

Binders for thermosetting powder coatings are often called a hardener. The hardeners are a mixture of a primary resin and a cross-linker. The major types of binders can be limited to polyester, epoxy, hybrid epoxypolyester, acrylic, and UV cure types. Polyester binders are used for good exterior durability, retention of gloss, and resistance to chalking. 

In thermoset powder coatings in order to obtain the desired melt viscosity profile, within the limits of the required 7 g. low molecular weight and narrow PDi are required. 

Wissbrun, K. etal. Model for correlation of rheological measurement with leveling and curing of thermosetting powder coating. J. Coat. Technol. 1976, 48, 42. 

This process is frequently used for more intricate mouldings. The thermoset powder is placed in a heated chamber, where it is liquefied and then transferred by a piston through a channel into a second mould, where the material is kept heated under pressure while the curing cycle is completed. 

Plasti-Grit Fkmr. [Composition Materials] Crosslinked thermoset powder reinforcing agent, filler, extender for plastics. 

Vedoc. [FerreVPOwd. Coatings] Thermosetting powder coatings. 

Table 3.4 summarizes the consumption of thermosetting binders used in various regions. The epoxy-polyester systems predominate over polyester-trisglycidyl iso-cyanurate and epoxy systems. The consumption of thermoplastic coating powders, particularly those used in the fluidized-bed and flocking processes, is substantially lower than that of thermosetting powders (Table 3.5). Their production does not show a comparably significant growth rate.

In powder coatings the coating material is applied to the workpiece in the form of dry (i.e., solvent-free) thermoplastic or thermosetting powder. The powder particles are heated and melt to form a film. The thermoplastic powders melt and fuse on heating whereas the thermosetting powders also become chemically cross-linked. Two main application processes are used electrostatic spraying and fluidized-bed coating. 

DeMULS SMS GP-219 Silicone Polyol Copolymer Histar BS dispersant, pigment thermoset powder coatings... 

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