Development of low temperature curing, 120[degrees]C, durable, corrosion protection powder coatings for temperature sensitive substrates.

Commercial low temperature cure powder coatings, including candidates representative of all the major coating chemistries, were evaluated. Nearly all failed to adequately react at a cure schedule of 120[degrees]C for 30 min, and none, even when prepared at their manufacturer's lowest recommended cure conditions, met the stringent performance needs for temperature-sensitive military applications. Initial research is presented toward developing low temperature cure powder coatings that simultaneously meet all performance requirements at this target cure schedule. Using commercial resins, corrosion inhibitors, and catalysts, this research effort has closed gaps in low temperature cure coating performance and has helped to identify critical deficiencies. This study establishes direction for future developments in new resin and catalyst technologies.

Keywords: Differential scanning calorimetry, catalysis, corrosion, corrosion protection, powder, aluminum, low temperature cure

**********

Significant effort is expended and cost is incurred each year to procure, use, and dispose of toxic and hazardous materials associated with the use of solvent-borne corrosion protection coatings. Powder coatings have the potential to eliminate more than 95% of the volatile organic compounds and hazardous air pollutants released during the application of such coatings. Over the past years, powder coatings have increasingly gained popularity as a result of their ecological advantages as well as their economical and performance benefits. (1-7) There are numerous military and civilian applications that require protective coatings, but they involve substrates that are made from materials such as low-tempered metal alloys, composites, plastic, or wood that would be structurally compromised by thermal treatments required to cure conventional powder coatings. The need for powder coatings that cure at ever lower temperatures has been presented extensively and much work on their development has been reported in the literature. (8-28)

This article highlights research to develop a weatherable powder coating that cures at or below 120[degrees]C within 30 min. For specific military applications, this cure schedule is dictated by the temperature sensitive nature of 2024-grade aluminum alloy that is used in several types of aircraft parts, weapon systems, and support equipment. Due to the T3 heat treatment, prolonged exposure to temperatures above 120[degrees]C can compromise the structural integrity of the alloy. In addition to the low temperature cure target, the final coating must also meet functional requirements for corrosion and chemical resistance, adhesion, impact strength, and exterior durability. With the help of research and qualification partners representing several military and government agencies, a comprehensive list of coating performance specifications and tests was compiled from military specification reports and joint test protocols. An abbreviated summary of the performance criteria is provided in Table 1. Only surface quality lacks a quantified specification target at this time.

This article presents the results from a benchmark evaluation of several state-of-the-art low temperature cure powder coatings. Additionally, progress from early efforts to develop powder coatings specifically for a 120[degrees]C/30 min cure schedule is evaluated against performance targets.

EXPERIMENTAL

Materials and Test Substrates

All the materials used in this study, including finished powders and raw materials, were used as received from their manufacturers. In formulation studies, four grades of acid functional polyester resins, denoted A, B, C, and D with corresponding equivalent molecular weights of 1700, 1600, 1630, and 1650, were combined 93:7 with triglycidylisocyanurate (TGIC) (Araldite PT 810, Huntsman Chemical). Three different catalysts were used in this work: choline chloride (Actiron CC6, Synthron Inc.), and two benzyltrimethylammonium halides (chloride and bromide salts, both from Aldrich Chemical). Two types of corrosion inhibitors were used: zinc phosphate (Halox, Rockwood Pigments), and barium metaborate (Butrol 23, Buckman Laboratories).

Chromated aluminum, 2024T3, was the primary substrate for this research. For testing coating flexibility, a softer T0 annealed version of the same alloy was used with an anodized surface treatment; this substrate type is specified in MIL-PRF-85285. All aluminum test substrates were obtained from Q-Panel Lab Products. For standardized impact testing, untreated 1008 steel panels (R-46, Q-Panel Lab Products) were used.

Melt Compounding and Powder Grinding

Raw materials were dry-blended by either hand shaking in a bag (one minute) or in a Henschel mechanical mixer (60 sec at 2000 rpm). Each formulation, 4.5 kg in size, was then melt-mixed on a 50-mm twin-screw extruder (lab model Baker Perkins) at 500 rpm with a max barrel temperature of 88[degrees]C. Extrudate was passed through water-cooled pinch-rolls and collected onto a stainless steel belt; from exit of the extruder, approximately 60 sec was required to reach ambient temperature. Powder grinding was performed using an air classifying mill, ACM-5, followed by sieving through a 140 mesh screen. This process produced powders with a mean size of 40-50 microns (95% < 105 microns) as measured using a Malvern Series 2600 laser analyzer.

Thermal Analysis 

Samples, 15-20 mg in size, were analyzed using a Perkin Elmer DSC 7. The testing protocol utilized an isothermal hold at 120[degrees]C for 30 min followed by a rapid quench and then a temperature scan from 25-300[degrees]C at 10[degrees]C/min. Heat of reaction and the corresponding cured powder percent conversion curve were obtained from the isothermal portion of the test while the cured network [T.sub.g] and residual heat of reaction were assessed from the follow-up scan. Both heats of reaction were used to calculate actual percent conversion at 120[degrees]C for 30 min.

Coating Preparation

Prior to coating, test substrates were cleaned with an MEK wipe. All powders were applied in an ETI Flexicoat[R] manual powder coating booth using a Nordson SureCoat[R] cup gun with an applied voltage of 70 kV, application pressure of 30 psi, and rinse rate setting of 20 psi. Curing was performed in a Blue-M convection oven. For each panel, mean coating thickness and standard deviation were monitored based on six measurements using an ElektroPhysik Minitest 4100. After curing, panels were held at ambient conditions for a minimum of 24 hr before testing.

Property Evaluation

Adhesion testing was performed using a Gardner crosshatch knife and Permacel[R] tape in accordance with ASTM D 3359. Flexibility testing followed ASTM D 522 and was performed with a Gardner mandrel bend tester. Direct impact strength was tested in accordance with ASTM D 5420 using a Gardner impact tester. Pencil hardness was assessed following ASTM D 3363. Solvent resistance was determined using the MEK double rub test, ASTM D 5402, with failure report at substrate read-through.

S[O.sub.2] and salt fog corrosion tests were performed using standard ASTM test methods G 85 and B 117, respectively, on both steel and aluminum substrates. For salt fog testing, time to failure, assessed as greater than 1/8 in. undercutting from edge of scribe on coating, up to the test duration of 2000 hr, was reported. For S[O.sub.2] testing, performance was reported as creep after 500 hr of exposure rated on an ASTM scale. A failure rating of 6 corresponds to undercutting from edge of scribe by more than 1/16 in.

For accelerated weathering, an Atlas Ci35a Xenon Weather-Ometer[R] was used. To assess performance, color coordinates were measured as a function of exposure using a Macbeth Colorimeter (Color-Eye 7000A) following ASTM D 2244. The [DELTA]E color change is reported after 2000 hr of exposure.

Gloss was measured at 60[degrees] using a BYK Gardner Tri-Gloss Meter. Coating surface quality was determined using a BYK Gardner Wavescan[TM] distinctness of image (DOI) instrument calibrated relative to the Powder Coating Institute's (PCI) surface quality standards. Results are reported in PCI units ranging from 1-10, with 10 corresponding to the best quality. In samples where gloss was inadequate to allow for use of the Wavescan, visual assessments were made by side-by-side comparison to the surface quality standards.

RESULTS AND DISCUSSION

Commercial Low Temperature Cure Performance

To assess current state-of-the-art technology in low temperature cure powder coatings, product literature, websites, and technical support from many commercial manufacturers were consulted. Several best-in-class low temperature curing powders were sampled from each of the major conventional chemistries including acrylate, epoxy, urethane, and polyester crosslinked using either triglycidylisocyanurate (TGIC) or hydroxyalkylamide (HAA). For each of the five chemistry families, differential scanning calorimetry (DSC) scans at 10[degrees]C/min were used to down-select the fastest low temperature reaction kinetics based on onset and peak exotherm temperatures. The down-selected powders were then applied and cured as nominally 3-mil thick powder coatings according to their respective manufacturer's lowest recommended cure schedule and evaluated.

[FIGURE 1 OMITTED]

Key performance attributes for each coating are summarized in Table 2. These were evaluated using the corresponding test methodologies indicated in Table 1. The cure schedule used for each coating is also provided in Table 2. Responses that fail to meet Table 1 performance criteria have been shaded dark. Overall, there are tradeoffs among the chemistries, and clearly no solution meets all cure and performance expectations. When selected powders were prepared closer to their manufacturer's standard cure conditions, coating performance improved, most notably chemical resistance. For example, the resistance to MEK double rubs increased to the test limit of 200 for the PE/TGIC and PE/HAA coatings, and to 175 for the acrylate. These results underscore the challenge of designing a powder coating that can effectively crosslink at 120[degrees]C within 30 min and simultaneously meet the performance goals of this research effort. This is further emphasized by the plot in Figure 1, which simultaneously compares each of the five candidate coatings relative to select specification targets.

Many low temperature curing limitations identified in Table 2 can be linked to the nature of the crosslinking mechanism or the base resin chemistry. (29) The acid/hydroxyl reaction is the least reactive mechanism represented. Its curing temperature is limited by the nature of the esterification reaction and the need to drive off water to obtain high conversions. HAA crosslinkers rely on this scheme. Slightly lower temperatures can be used to cure hydroxyl functional resins with protected or dimerized isocyanates to yield urethane coatings. However, the deblocking temperatures of the protective group or the ring-opening kinetics of the uretidione limit the minimum cure temperature to approximately 140[degrees]C. (30,31)

[FIGURE 2 OMITTED]

The other three coatings chemistries, PE/TGIC, epoxy, and acrylate, are all based on epoxy type reactions. These have the greatest potential for low temperature cure. Epoxies can crosslink with a variety of different chemical functionalities, such as acids, aromatic hydroxyls, amines, or even through catalyzed homopolymerization. (32) Use of bisphenol-A (BPA) and novolacmodified epoxies is effectively restricted to interior applications because of their poor weathering attributes. Poor exterior durability is evident in the epoxy coating in Table 3. (29) Cycloaliphatic epoxies do not suffer from the ultraviolet light instability of the aromatics, but resins suitable for powder coatings are unavailable.

For exterior applications, acrylate resins functionalized with epoxy moieties such as glycidyl methacrylate are increasingly finding use. They offer excellent exterior durability and scratch resistance, but many times this comes at the price of poor chemical resistance and brittleness. Acid functional polyesters are often combined with multifunctional epoxy crosslinkers, most commonly TGIC, to deliver both low temperature cure kinetics and exterior durability. Despite concerns about the potential toxicity of TGIC, there are no commercial alternatives that offer similar performance. Of the commercially available resins, PE/TGIC chemistry offers the best opportunity to develop 120[degrees]C cure, exterior powder coatings with the properties outlined in Table 1. The next section presents development efforts toward achieving the low temperature cure and coating performance goals building on PE/TGIC chemistries.

Resin Screening Study

Four different commercial acid functional polyester resins were sampled from their manufacturers as candidates for low temperature curing powder coatings. These were incorporated into the general factorial screening study illustrated in Figure 2 wherein each of the four resins, combined 93/7 with TGIC crosslinker, were formulated with two different corrosion inhibitors, either zinc phosphate or barium metaborate, and a choline chloride catalyst at levels of 0, 0.3, or 0.5 percent of total formulation. Details on the powder formulations are given in Table 3. Sample panels of each of the 24 powder coatings produced in this design were prepared by curing at 120[degrees]C for 30 min.

For the sake of rapid screening, the complete performance specification list was pared down to eight key performance attributes that include: adhesion, flexibility, toughness, hardness, chemical resistance, gloss, and surface quality. The corresponding tests for these are indicated in Table 1 and the screening results are summarized in Table 4. All test coatings met the 2.3-3.2 mil thickness specification.

At a high level, the results in Table 4 showed a significant effect of resin type on coating performance with resin D systems, and in particular formulations D-2 and D-3, meeting the greatest number of the key performance goals. Compared to resin systems A and B, systems C and D offered simultaneous improvements in flexibility and chemical resistance, but, depending on corrosion inhibitor type, suffered slight to moderate reductions in gloss. Irrespective of resin system, use of barium metaborate appears to adversely affect gloss, especially at higher catalyst loadings. Within the resin D formulations, those with zinc phosphate and added catalyst outperformed analogous formulations with barium metaborate, particularly in chemical resistance. Across all formulations investigated, including the best overall performers, the most severe deficiency was seen in direct impact toughness with values well below the 150 in.-lb goal. These results emphasize the strong effect of component interactions and the potential for competing tradeoffs between performance attributes.

[FIGURE 3 OMITTED]

Many of the important factors that likely differentiate these four resins such as chemical structure, molecular weight, functionality, polydispersity, as well as type and level of precatalysis were not provided by their manufacturers. Without this information it is difficult to draw correlations between chemistry and performance. It is possible, however, to compare the reaction kinetics of the four resin systems to learn more about their differences and how these might affect coating performance.

Figure 3 is a compilation of plots for DSC-measured heat evolution as a function of time at 120[degrees]C for the four resin systems. Without catalyst addition (formulations 1 and 4 in each resin series), both the reaction rate and total evolved heat were strongly dependent on the base resin. This was not unexpected as manufacturers commonly precatalyze their resins. The results in Figure 3 suggest resins A and B were less precatalyzed than resins C and D. In fact, without additional catalyst, resin A did not show any appreciable reaction at 120[degrees]C, whereas added catalyst had little effect on the total heat of reaction for resins C and D with average values of approximately 22 and 15 J/g, respectively. The lesser precatalyzed resins, A and B, showed the greatest increase in reaction rate and total exotherm with catalyst addition. In these same systems, corrosion inhibitor selection had a notable effect on reaction kinetics; relative to formulations with zinc phosphate, use of barium metaborate is associated with a faster reaction rate and higher total heat of reaction. This is consistent with the overall reduction in gloss observed with barium metaborate. Moreover, barium metaborate with the highly precatalyzed resin D may have over-accelerated the reaction producing network heterogeneity and caused a drop in chemical resistance.

[FIGURE 4 OMITTED]

Using follow-up DSC scans, reaction conversion not realized after 30 min at 120[degrees]C was measured for each of the 24 resin screening formulations. It should be noted that the crosslinked coatings made in this work have glass transition temperatures between 50 and 60[degrees]C, and thus, at the 120[degrees]C cure condition vitrification should not limit reaction conversion. The recovered residual heat of reaction was added to the 120[degrees]C exotherm to calculate a total potential heat of reaction. Figure 4 shows the results of this analysis. Inspection of the data shows that for resins A and B, only certain combinations of catalyst and corrosion inhibitor yielded complete reaction conversion; whereas for resins C and D, complete reaction conversion was attained in all formulations. For the most part, the total potential heat of reaction was unaffected by formulation changes. Exceptions of note occurred in resins B and D. Barium metaborate alone or the combination of zinc phosphate and choline chloride both acted to lower the total potential heat of reaction in resin B by about 35%. In resin D, a choline chloride loading of 0.5% caused a 20% reduction in the total potential heat of reaction. This observation may be indicative of undesirable prereaction during processing that could have the effect of lowering the measurable exotherm.

[FIGURE 5 OMITTED]

Increased Catalyst

Higher catalyst loadings of 0.6 and 0.75% were investigated in resins C and D using only the zinc phosphate corrosion inhibitor. Adhesion and flexibility were unaffected by the increased catalyst levels while hardness, gloss, and surface quality showed slight formulation specific changes. The lone exception was in resin D with 0.75% catalyst where both pencil hardness and gloss dropped significantly to values of F and 55, respectively.

Combined with earlier results, direct impact and chemical resistance are plotted as a function of catalyst level in Figure 5. Resin system C was relatively unaffected by catalyst loading, suggesting that this commercial resin was already highly catalyzed. A more pronounced effect was evident in resin D where chemical resistance was improved to the test limit of 200 double rubs with 0.3 and 0.5% catalyst but dropped off at 0.6%, while impact values simultaneously increased to the test limit of 160 in.-lb. A trade-off in chemical resistance and impact is not unusual, but it is surprising that this occurred at higher catalyst loadings. Normally, increasing crosslink density improves chemical resistance and penalizes impact resistance. Partial reaction during processing could lead to gel particles and heterogeneous crosslinking of the coating film. This might explain the reduced solvent resistance and improved impact results at the highest catalyst loadings.

Catalyst Type

To this point, only choline chloride has been considered as the added catalyst in coating formulations. Many other candidates exist for the acid/epoxy reaction and several of these have been screened in our laboratory specifically for rapid cure kinetics at 120[degrees]C, latency at 93[degrees]C melt processing, and minimal yellowing under 160[degrees]C over bake conditions. (33) In addition to the choline chloride, our work has identified promise in two benzyltrimethylammonium salts based on either bromide (BTMA-Br) or chloride (BTMA-Cl) counter ions.

As an extension to the present study, BTMA-Br and BTMA-Cl, along with choline chloride, were screened in formulations based on resin D with either zinc phosphate or barium metaborate corrosion inhibitors. Two catalyst levels were considered, 0.5 and 0.75%. These levels were chosen to capture the low and high extremes of the potential formulation space. In a first pass at the formulation design, a higher catalyst level of 1.0% was selected, but screening tests found gel formation during extrusion, so the level was lowered to 0.75%. The performance of coatings prepared in this study is summarized in Table 5, with shading again used to denote below target performance.

Relative to noncatalyzed controls (blends D-1 and D-4 in Table 4), good flexibility and hardness were generally realized across the formulations summarized in Table 5, and in several formulations considerable improvements in toughness were made. At the same time, gloss, surface quality, and chemical resistance were compromised; the effect was more significant in the most highly catalyzed formulations. The limited exceptions were the combinations of zinc phosphate and BTMA-Cl, which showed slightly enhanced chemical resistance. In four of the six formulations, the higher catalyst level actually caused a reduction in DSC measured heat of reaction at 120[degrees]C in 30 min. Prereaction during melt compounding is the most probable cause.

The Table 5 formulations based on low-level (0.5%) choline chloride with either corrosion inhibitor represent formulation replicates of chemistries studied in the resin screening study, specifically formulations D-3 and D-6 in Table 4. Similarly, an analogue to the high-level (0.75%) choline chloride with zinc phosphate formulation in Table 5 was previously examined in the increased catalyst study. Even though the noted formulations are the same, processing changes were made between the earlier and later studies. Specifically, hand mixing of components was used in the resin screening and increased catalyst work, whereas more intense machine (Henschel) mixing was used in the catalyst screening study. Comparing 0.5% analogue formulations, the more aggressively mixed versions showed significant improvements in impact resistance and simultaneous reductions in chemical resistance. This is similar to the effect observed in Figure 4 with an increase in the catalyst loading to the highest levels. It is reasonable to suspect that improved reagent mixing increased reagent homogeneity and, thereby, catalyst efficiency. Further building in this direction, a comparison of 0.75% analogues reveals a precipitous drop in both toughness and chemical resistance in the more intensely mixed catalyst screening formulation. This suggests an upper limit where over-catalysis is systemically detrimental. Even below this limit, the lack of processing robustness of highly catalyzed formulations may challenge their viability.

CONCLUSIONS

As a benchmark for this research, several commercial low temperature cure powder coatings, including candidates representative of all the major coating chemistries, were sampled and tested. The results highlight tradeoffs that challenge the ability to achieve desired coating performance in a low temperature cure powder coating. With these materials as reference, experimental low temperature cure powder coatings built on commercial acid functional polyester resins with TGIC crosslinker were formulated and studied. Despite similar functional densities, base resin selection had a significant influence on coating performance, but with corrosion inhibitor type, catalyst type, and catalyst loading all having strong and sometimes confounded interactions. For instance, the effect of corrosion inhibitor type was found to be dependent on both the base resin and added catalyst type as well as catalyst level. The corrosion inhibitors themselves appeared to dually act as catalysts for the epoxy/acid reaction. Their effect was difficult to predict, however, complicated in part by the localized surface effects of heterogeneous catalysis.

The best overall coating performance was realized in formulations with heavily precatalyzed resins. Even with these resins, additional catalysis was required to attain adequate network formation for good coating flexibility, hardness, and chemical resistance in a 120[degrees]C cure. Further increases in catalyst level or more aggressive reagent mixing produced a limited window of dramatically improved impact resistance but with a concomitant drop in chemical resistance. These coatings may have been effectively impact-toughened by heterogeneous network formation or, possibly, the higher levels of catalysis may have supported epoxy/epoxy reaction of crosslinker moieties and thereby produced a more chain extended network. At the highest extreme of catalyst loading investigated, gelation during melt extrusion and/or systemic degradation of coating performance became prohibitive.

Overall, progress was made toward closing identified performance gaps for 120[degrees]C/30 min cure powder coatings, but further improvements are needed. This study sets a foundation for subsequent developments in new low temperature cure resins, catalysts, and formulation technologies. Future work will also more closely consider the processing window of highly catalyzed systems and will utilize more rapid extrudate cooling methodologies representative of commercial powder coating production.

ACKNOWLEDGMENTS

The authors thank Douglas S. Richart for his consultation on low temperature powder coating developments. This research was supported in part by the U.S. Department of Defense Contract DACA72-02-C-0025, through the Strategic Environmental Research and Development Program (SERDP), Pollution Prevention Program Manager Chuck Pellerin. This work was presented at the 31st International Waterborne, High-Solids, and Powder Coatings Symposium, and is published here with the permission of The University of Southern Mississippi's Polymer Science Press.

References

(1) Miller, E.P. and Taft, D.D., Fundamentals of Powder Coating, Society of Manufacturing Engineers, Dearborn, MI, 1974.

(2) Misev, T.A., Powder Coatings: Chemistry and Technology, John Wiley & Sons, New York, 1991.

(3) Richart, D.S., Coating Processes (Powder Technology), Howe-Grant, M. (Ed.), John Wiley & Sons, Inc., NY, Vol. 6, pp. 635-661, 1993.

(4) Misev, T.A. and van der Linde, R., Prog. Org. Coat., 34, 160-168 (1998).

(5) Farrel, R., Metal Finishing, 98, 135-141 (2000).

(6) Papasavva, S., Kia, S., Claya, J., and Gunther, R., Eur. Coat. J., 1-2, 92-95 (2001).

(7) Harsch, M., Eyerer, P., Finkbeiner, M., and Saur, K., "Environmental Assessment of Paint Technologies," Proc. International Waterborne, High-Solids, and Powder Coatings Symposium, New Orleans, LA, pp. 469-481, 1997.

(8) Armstrong, B., Powder Coating, Sept., 60-61 (2002).

(9) Bergmans, A., DeCock, C., Bertens, J., and Kooijmans, P., Shell International Research, "Acid Functional Polyester Resins and Lower Temperature Curable Powder Coating Compositions Comprising Them," European EP 1,006,163 (2000).

(10) Bergmans, A., Walravens, M., DeCock, C., and Riviere, J., Polymers Paint Colour Journal, Oct., 17-19 (2001).

(11) Danick, C., "Low Temperature Crosslinking for Powder Coatings," Proc. of 76th Annual Meeting of the Federation of Societies for Coatings Technology, October 11-14, New Orleans, LA, 1998.

(12) Johansson, M., Malmstrom, E., Jansson, A., and Hult, A., "Novel Concept for Low Temperature Curing Powder Coatings Based on Hyperbranched Polyesters," JOURNAL OF COATINGS TECHNOLOGY, 72, No. 905, 49 (2000).

(13) Laas, H.J., Meier-Westhues, H.U., Halpaap, R., Freudenberg, U., and Klee, H.P., "Polyurethane Powder Coating Compositions Which Have a Low Stoving Temperature," Bayer, U.S. Patent 5,847,044, 1998.

(14) Malmstrom, E., Johansson, M., Jansson, A., and Hult, A., "Novel Concept for Low Temperature Curing Powder Coatings Based on Hyperbranched Polymers," Proc. International Waterbome, High-Solids, and Powder Coatings Symposium, New Orleans, LA, pp. 373-382, 1999.

(15) McBride, P.J., J. Oil. Col. Chem. Assoc., 65, 257-262 (1982).

(16) Pappas, S.P. and Hill, L.W., "Kinetic Parameter Considerations for Maximizing Stability and Minimizing Cure Temperature of Thermosetting Coatings. Sulfonium Salts as Latent Thermal Initiators for Cationic Polymerization," JOURNAL OF COATINGS TECHNOLOGY, 53, No. 675, 43 (1981).

(17) Pont, J.D. "Further Advances in Low Temperature Cure Polyurethane Powder Coatings," Proc. International Waterborne, High-Solids, and Powder Coatings Symposium, New Orleans, LA, pp. 232-245 (1999).

(18) Rijkse, K., Mod. Paint Coat., April, 36-39 (2001).

(19) Roescher, A. and DeRuiter, B., "New Isocyante-Based Low-Temperature Hardeners for Powder Coatings." Proc. XXV FATIPEC Congress, Torino, Italy, pp. 175-190, 2000.

(20) Ruth, W.G., Greth, S.L., Seelig, C.A., and Schreffler, D.A., "Powder Coating of Non-Crystalline and Crystalline Epoxy Resins," Morton International Inc., U.S. Patent 6,140,430, 2000.

(21) Sohoni, M. and Figlioti, P., "Developments in Low Temperature Cure: Urethane Powder Curing Agents," Proc, International Waterborne, High-Solids, and Powder Coatings Symposium, New Orleans, LA, pp. 267-280, 1998.

(22) Subramanian, R. and Sullivan, C.J., "Unsaturated Polyesters in Low Temperature Cure Powder Coatings," Proc. International Waterborne, High-Solids, and Powder Coatings Symposium, New Orleans, LA, pp. 381-389, 2001.

(23) Wenning, A., Franzmann, G., and Spyrou, E., "Powder Coating Compositions Comprising Crystalline Urethane Acrylates and Use Thereof," Degussa AG, U.S. Patent 0175433 A1, 2003.

(24) Witzeman, J.S., Prog. Org. Coat., 27, 269-276 (1996).

(25) Christ, U., Macromolecular Symposia, 187, 759-770 (2002).

(26) Harris, S., Paintindia, 51, 51-56 (2001).

(27) Kreis, W., Polymers Paint Colour Journal, 188, 12-13, 16-17 (1998).

(28) Bell, G., Paint & Ink International, 11, 2,4,8,10 (1998).

(29) Wicks, Z.W., Jones, F.N., and Pappas, S.P., Organic Coatings: Science and Technology, 2nd Ed., Wiley-Interscience, NY, 1999.

(30) Wicks, D.A. and Wicks, Z.W., Prog. Org. Coat., 36, 148-172 (1999).

(31) Wicks, D.A. and Wicks, Z.W., Prog, Org. Coat., 41, 1-83 (2001).

(32) Epoxy Resins: Chemistry and Technology, May, C.A. (Ed.), Marcel Dekker Inc., NY, 1988.

(33) Merfeld; G., Molaison, C., Koeniger, R., Acar, A.E., Mordhorst, S., Suriano, J., Irwin, P., Warner, R., Gray, K., Smith, M., Kovaleski, K., Finley, S., Meredith, D., Naguy, T., and Spicer, M., "Acid/Epoxy Catalyst Screening for Low Temperature (120[degrees]C) Powder Coatings," Proc. International Waterborne, High-Solids, and Powder Coatings Symposium, New Orleans., LA, 2004.

Glen Merfeld,**** Steve Mordhorst, Rainer Koeniger, A. Ersin Acar, Chris Molaison, Joe Suriano, Pat Irwin, and Ron Singh Warner -- GE Global Research*

Ken Gray -- Crosslink Powder Coatings, Inc. ([dagger])

Mark Smith -- Honeywell-Department of Energy**

Kevin Kovaleski and Greg Garrett -- NavAir ([double dagger])

Steve Finley and Debora Meredith -- AFMC***

Mike Spicer and Tom Naguy -- AFRL ([dagger][dagger][dagger])

Presented at the 31st Annual International Waterborne, High-Solids, and Powder Coatings Symposium, February 18-20, 2004, in New Orleans, LA.

* One Research Cir., Building K1, Room 4B37, Niskayuna, NY.

([dagger]) Clearwater, FL.

** Kansas City, MO.

([double dagger]) Patuxent River, MD.

*** WPAFB, OH.

([dagger][dagger][dagger]) WPAFB, OH.

**** Author to whom correspondence should be addressed.

Table 1 -- Target Performance Specifications for 120[degrees]C/30 min
Cure Powder Coatings

                                                          Specification*
Property              Test               Units             LSL      USL

Thickness             Gauge (eddy        mils              2.3      3.2
                      current, magnetic
                        induction)
Adhesion              Crosshatch ASTM    ASTM scale        4B       --
                      D 3359-97
Flexibility           Mandrel bend       failure dia. in.  --       0.25
                      ASTM D 522-93
Toughness             Direct impact      in.-lb            150      --
                      ASTM D 5420
Hardness              Pencil hardness    pencil #          2H       --
                      ASTM D 3363
Exterior Durability   Xenon arc, 2000    delta color       --       2
                      hr ASTM G 26-96
Chemical Resistance   Skydrol fluid      delta pencil #    --       2
                      immersion, 7
                      days
                      MEK double rubs    double rubs       200      --
                      ASTM D 5402
Color Match           Colorimetry ASTM   delta color       --       2
                      D 2244
Gloss                 60[degrees]        gloss units       90       --
                      gloss ASTM D 523
Surface               DOI wavescan,      standard #        --       --
  Quality ([dagger])  calibrated to
                      PCI standards
Corrosion Resistance  Salt fog, ASTM B   hr to failure     2000     --
                      117
                      SO2 500 hr, ASTM   scribe undercut   7        --
                      G 85               rating
                      Cyclic on          cylces to         80       --
                      scribed steel GM   failure
                      954P
                      Filiform ASTM      in.               --       0.25
                      2803-93

*LSL=Lower Spec Limit; USL=Upper Spec Limit.
([dagger]) Specification not yet defined.

Table 2 -- Commercial Low Temperature Cure Powder Coating Performance

Property              Test                PE/TGIC        PE/HAA

Cure Schedule*        --                  148[degrees]C  154[degrees]C
                                          20 min         30 min
Adhesion              Crosshatch          5B             4B-5B
Flexibility           Mandrel bend        0.125          1
Toughness             Direct impact       20             20
Hardness              Pencil hardness     H              HB
Chem. Resist.         MEK double rub      184            80
Durability            Xenon arc           0.5            5.6
Surface               PCI standard        4              5
  Quality ([dagger])
Corrosion Resistance  Salt fog (AL)       >2000          >2000
                      Salt fog (Steel)    <1576          <1081
                      S[O.sub.2] (AL)     8              8
                      S[O.sub.2] (Steel)  9              10

Property              Test                Urethane       Epoxy

Cure Schedule*        --                  177[degrees]C  121[degrees]C
                                          20 min         15 min
Adhesion              Crosshatch          5B             5B
Flexibility           Mandrel bend        0.125          0.125
Toughness             Direct impact       100            20
Hardness              Pencil hardness     HB             HB
Chem. Resist.         MEK double rub      62             200
Durability            Xenon arc           0.6            2.8
Surface               PCI standard        4              4
  Quality ([dagger])
Corrosion Resistance  Salt fog (AL)       >2000          >2000
                      Salt fog (Steel)    <1576          <2085
                      S[O.sub.2] (AL)     5              5
                      S[O.sub.2] (Steel)  9              5

Property              Test                Acrylate

Cure Schedule*        --                  148[degrees]C
                                          15 min
Adhesion              Crosshatch          5B
Flexibility           Mandrel bend        0.125
Toughness             Direct impact       20
Hardness              Pencil hardness     H
Chem. Resist.         MEK double rub      67
Durability            Xenon arc           --
Surface               PCI standard        8
  Quality ([dagger])
Corrosion Resistance  Salt fog (AL)       >2000
                      Salt fog (Steel)    <674
                      S[O.sub.2] (AL)     5
                      S[O.sub.2] (Steel)  6

*Manufacturer recommended lowest temperature cure schedule.
Shaded responses indicate failure relative to performance requirements
provided in Table 1.
([dagger]) Specification not defined.

Table 3 -- Powder Coating Formulations Used in Resin Screening
Experiments

                                   Formulation Number
Component             1        2        3        4        5        6

Acid polyester         65.0     65.0     65.0     65.0     65.0     65.0
TGIC                    4.9      4.9      4.9      4.9      4.9      4.9
Curing catalyst         0.0      0.3      0.5      0.0      0.3      0.5
Flow promoter           1.5      1.5      1.5      1.5      1.5      1.5
Degassing agent         0.5      0.5      0.5      0.5      0.5      0.5
Antioxiant              1.0      1.0      1.0      1.0      1.0      1.0
Zinc phosphate          5.0      4.7      4.5      0.0      0.0      0.0
Barium metaborate       0.0      0.0      0.0      5.0      4.7      4.5
Filler & pigment       22.1     22.1     22.1     22.1     22.1     22.1
Totals                100      100      100      100      100      100

Table 4 -- Resin Screening Summary

                                          Resin A
Test                    A-1     A-2     A-3     A-4    A-5    A-6

Adhesion                0B      1B      2B      0B     3B     4B
Flexibility.            0.75    NF      1       0.25   1      1
Toughness               0       20      20      0      0      0
Hardness                H       H       F       F      F      H
Chem. resist.           4       4       7       3      21     73
60[degrees] Gloss       89      91      80      90     70     63
Surface
Quality                 4       4       4       4      4      5

                                         Resin B
Test                    B-1     B-2     B-3     B-4    B-5    B-6

Adhesion                0B      2B      4B      2B     5B     5B
Flexibility.            0.66    0.75    0.75    0.7    0.5    0.135
Toughness               0       0       20      20     20     20
Hardness                2H      2H      2H      2H     2H     3H
Chem. resist.           8       10      30      10     110    85
60[degrees] Gloss       88      90      85      86     75     65
Surface
Quality                 4       4       4       4      4      5

                                          Resin C
Test                    C-1     C-2     C-3     C-4    C-5    C-6

Adhesion                4B      4B      3B      4B     4B     4B
Flexibility.            NF      0.5     NF      NF     NF     0.13
Toughness               20      20      20      20     20     40
Hardness                2H      2H      2H      3H     H      H
Chem. resist.           122     116     106     68     136    178
60[degrees] Gloss       84      76      78      75     70     67
Surface
Quality                 3       5       3       3      4      4

                                          Resin D
Test                    D-1     D-2     D-3     D-4    D-5    D-6

Adhesion                4B      4B      5B      5B     4B     4B
Flexibility.            NF      0.13    NF      NF     NF     NF
Toughness               20      20      20      60     40     40
Hardness                H       2H      2H      2H     H      2H
Chem. resist.           77      199     200     125    67     96
60[degrees] Gloss       78      63      69      59     54     49
Surface
Quality                 4       5       4       5      5      5

NF = No Failure
Shaded responses indicate failure relative to performance requirements.

Table 5 -- Catalyst Screening Study

                                       Resin D
                                    Zinc Phosphate
                      BTMA-Cl (a)     BTMA-Br (a)    C. Chloride
Test                  Low     High    Low  High      Low  High

Adhesion              4B      4B      4B   4B        4B   4B
Flexibility           NF (b)  NF      NF   NF        NF   NF
Toughness             <20     120     60   160       160  40
Hardness              2H      2H      2H   3H        2H   2H
Chem. resist.         107     94      54   54        24   31
60[degrees] gloss     46      27      36   29        40   50
Surface quality       2       3       2    3         3    4

                                     Resin D
                                 Barium Metaborate
                     BTMA-Cl (a)    BTMA-Br (a)   C. Chloride
Test                 Low   High     Low   High    Low  High

Adhesion             4B    4B       4B    4B      4B   4B
Flexibility          NF    NF       NF    NF      NF   NF
Toughness            60    140      160   40      80   130
Hardness             2H    2H       3H    3H      2H   H
Chem. resist.        64    53       61    27      34   20
60[degrees] gloss    27    16       26    26      61   37
Surface quality       3    2        2     3       4    3

(a) Catalysts: benzltrimethylammonium chloride (BMTA-Cl) and bromide
(BTMA-Br), Choline Chloride Catalyst Levels; Low -0.5, High -0.75 pph.
(b) NF = No Failure.
Shaded responses indicate failure relative to performance requirements.