PU Powder Coatings
While PU coatings represent about 13% of the total industrial coatings market, PU powder coatings represent about 5% of the total PU volume applied to coatings. This value is consistent with powder coatings being about 5% of all coating volumes independent of chemistry. Although the overall size of the PU powder coatings application is relatively small, it is a relevant technology
TABLE 10.12 Properties of UV curable polyurethane coating tested before and after curing
FIGURE 10.22 Operations of powder coating. The components of the powder coating must be blended (extruder operation), applied to the surface, and then fully integrated and cured in an oven.
due to its lack of need for a solvent, and good coating properties. Like all PU s, PU powder coatings are valued for their durability, abrasion resistance, low-temperature flexibility, and good aesthetics. They find common use in decorative metal coatings including automotive, lawn furniture, and home appliances for instance.
The preparation of a powder coating is unlike any of the other technologies discussed earlier. The common requirement for PU and all other powder chemistries is that all of the components must be solids at room temperature . They are subsequently blended into a homogeneous blend of materials in an extruder, pulverized into a powder—typically of particle size 10s of microns. The powder is then electrostatically sprayed at a substrate, or the preheated substrate is submerged into a fluidized bed of the powder. At this point, the coating is often not yet finished. The coated part is then heated in an oven at a process-dependent temperature for a proscribed amount of time. During this oven heating, the particles will melt and coalesce into a fully integrated coating. It is usually during this heating step that PU powder coatings will fully react to form a thermoset coating (Fig. 10.22).
To prevent the PU reaction going to completion at any preliminary step such as extrusion, pulverization, or initial coating, the reactants must not be available to react. This reservation is achieved by protecting (or "blocking") isocyanate groups from reacting until they are made available at the oven temperature. Continued innovation of PU powder coatings is generally driven to optimize the coating, minimize VOCs, and minimize the temperature at which the isocyanate deblocks, and therefore reduce energy cost by reducing oven temperatures.
There are two primary methods of blocking isocyanates: (1) complexation of the isocyanate groups with a reversible leaving group (see Chapter 2) and (2) internal blocking of the isocyanate linkage by reversible isocyanate dimer formation to the uretidione (see Chapter 3). Most formulators currently employ leaving group blocking technology since it is less complex and deblocking temperature can in principle be controlled by the choice of deblocking agent. The blocking agent method suffers from the unavoidable loss of the blocking agent as a VOC in the oven cure step. To prevent unmitigated volatilization, it is often required that the volatized blocking agent be trapped as it evolves. Several isocyanate blocking agents have developed utility for powder coatings (Table 10.13). The temperature of deblocking can be influenced by the use of catalysts, which is an area of innovation and patent activity [59-61]. While caprolactam is one of the most cited blocking agents for making PU
TABLE 10.13 Structures of chemical blocking agents and their deblocking temperatures
FIGURE 10.23 Dimenzation of a polyisocyanate to a blocked uretidione structure. Reversion back to monomers begins at about 100°C, but it is not fast enough for industrial implementation until higher temperatures. Catalysts can lower the temperature of deblocking.
powder coatings, it has been reported that color formation is a common unintended consequence of this adduct, and other agents have become much more commonly encountered. Blocked isocyanate trimers are also available to increase cross-linking of the coating. Numerous metal catalysts have proven effective in lowering deblocking temperatures and the lower end of the temperature ranges in Table 10.13 represent catalyzed values. Patented organometallic catalysts have included dibutyltin dilaurate, bismuth tris(2-ethylhexanoate), cobalt bis(2-ethylhexanoate), zirconium bis(2-ethylhexanoate), zinc bis(2-ethylhexanoate), chromium tris(2-ethylhexanoate), and titanium tetrafethyl AcAc), for instance.
A second method of blocking isocyanate reactions has been by reacting polyisocyanates to form uretidiones (Fig. 10.23). This chemistry is achieved efficiently using nitrogen- or phosphorous-based catalysts.
While reaction to uretidione occupies two of the isocyanates, there are obviously two isocyanates still available to react and that require blocking. It is possible to oligomerize the blocking agent making a chain of uretidione structures, and these do form in uretidione formation. More commonly, the unblocked isocyanate groups left from uretidione formation are reacted with the soft segment component of the powder coating [62,63]. The soft segment is usually either a polyester or polyacrylic polyol— the choice of which depending on the requirements of the final product. A polyester-based PU powder coating formulation as might be encountered is detailed in Table 10.14.
A typical preparation of this simplified powder coating (Fig. 10.22) involves physical dispersion of the solid materials followed by extrusion on a low-shear extruder (~150rpm) with mixing zone temperature at about 90 °C and an exit temperature of about. 110°C. The extrudate is chipped and then finely ground (average particle size 10s of microns) with subsequent sieving to remove larger powder sizes. The resulting powder is then electrostatically sprayed on a prepared steel substrate to a film thickness of 0.002°in and baked at 180 °C for lOmin. Properties for this coating are given in Table 10.15.
TABLE 10.14 Formulation of an illustrative polyurethane powder coating
TABLE 10.15 Properties of the polyurethane powder coating of Table 10.14