New Crosslinkers for Polyurethane Powder Coating

August 1, 2002
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Figure 1 / Idealized Structure of Crelan VP LS 2347
Thermoset powder coating technologies have remained relatively unchanged in product availability during the last two decades. The powder coatings market needs new technology to continue to maximize growth and utility to the coatings industry. Demands in the powder coatings market for new technologies have focused on the following areas.

  • Low temperature curing technology. Advances in this area will enable the market to expand into new, temperature-sensitive substrates that are out of reach of the current technologies. In addition, low-temperature curing will make further energy savings and increased productivity possible.

  • Improved economics. This is usually achieved by low crosslinker usage in the formulation.

  • Improved performance. For example, better flexibility, improved smoothness and enhanced weatherability.

  • More robust matte systems.

    This article focuses on Bayer's newest polyisocyanate based powder crosslinker developments in an attempt to address these issues. Two of these crosslinkers are fully commercialized and the other three are in the process of being commercialized.

    Figure 2 / Idealized Structure of Crelan VP LS 2386

    Chemistry of Powder Crosslinkers

    Crelan VP LS 2347 powder crosslinker was designed to achieve the highest NCO content of any non-emissive crosslinker on the market today. With an NCO% of 19.5 (216 g/eq), a functionality of 2, and a glass-transition temperature (Tg) of 70 deg C, it is an isophorone diisocyanate (IPDI) based self-blocked uretdione type crosslinker. An idealized structure is shown in Figure 1. Uncatalyzed curing with conventional hydroxyl polyesters is achieved in 15 minutes at 180 deg C.

    Crelan VP LS 2386 powder crosslinker was designed to provide the maximal level of isocyanate content for crosslinking while minimizing the level of blocking agent. Structurally, it is based on the combination of uretdione adduct of IPDI and partially e-caprolactam blocked IPDI. This crosslinker provides 25.5% NCO, 165 g/eq, an average functionality of 2.0 and a Tg of 52-58 deg C. The idealized structure is shown in Figure 2.

    Figure 3 / Idealized Structure of Crelan VP LS 2181/1
    Curing is achieved at 180 deg C in 15 minutes; similar to conventional uretdione based crosslinkers, such as Crelan VP LS 2147. When blended with a 30 OH polyester, the binder to crosslinker ratio is 92/8, providing the desired attributes of polyurethane's with minimal crosslinker demand. Compared to the conventional e-cap blocked crosslinker, the theoretical maximum release of e-cap in this product is reduced by 20 wt%.

    Crelan VP LS 2181/1 powder crosslinker was designed to provide controlled gloss reduction through the incorporation of both blocked isocyanate and carboxylic acid functional groups on the same molecule. The structural basis for the crosslinker is the combination of isocyanate adducts blocked by e-caprolactam and a carboxylic acid functional group (see Figure 3). The acid value is 70 mg KOH (801 g/eg), and the % NCO is 10.5 (400 g/eq). The overall functionality is 3, where the molecular average functionality is 2 blocked isocyanates and a single acid functional group per molecule. The Tg is approximately 70 deg C and the appropriate curing schedule is 200 deg C for 15 minutes.

    Crelan NW 5 powder crosslinker is based on the adducts of Desmodur W diisocyanate blocked by caprolactam. This crosslinker was designed to capitalize upon the positive attributes of Desmodur W. These include higher reactivity in comparison with other caprolactam-blocked isocyanate crosslinkers, excellent weatherability, high flexibility and impact resistance in the final formulations while maintaining high hardness and solvent resistance. Caprolactam blocked Desmodur W diisocyanate, has been shown in uncatalyzed formulations to reduce the necessary curing temperatures by up to 20 deg C in comparison with IPDI or HDI based crosslinkers. NW 5 has a Tg of 54-58 deg C, a melt viscosity of 4500-7500 cPs and develops excellent properties when cured at 160 deg C for 20 minutes (see Figure 4).

    Figure 4 / Idealized Structure of Crelan NW 5
    Crelan XP 7180 powder crosslinker was designed to possess a low melt viscosity and high reactivity. These attributes were rendered possible by combining the excellent performance of crosslinkers based on Desmodur W diisocyanate with the low curing capabilities of 3,5-dimethyl pyrazole blocked isocyanates. A % NCO of 15.5 (272 g/eq) and a Tg of 42-44 deg C provide excellent flow and leveling and reduced curing times or temperatures. Uncatalyzed fully cured coatings are realized in 20 minutes at 150 deg C. Catalysis has been shown to drastically reduce the curing temperature requirements of XP 7180 to as low as 125 deg C/15 minutes. The film forming capabilities are however hindered by these low curing temperatures with conventional polyesters. High flow polyesters are currently being designed to meet the lower curing temperatures of XP 7180.

    Figure 5 / Idealized Structure of Crelan XP 7180

    Coatings Formulations, Processing and Performance
    VP LS 2347

    The formulation shown in Table 1 shows typical loading levels with this high NCO crosslinker. The 90/10 ratio of binder to crosslinker ratio capitalizes on the higher NCO content and provides excellent performance. This formulation was premixed using a Prism Pilot III Premixer that was maintained at 2500 rpm for 2 minutes. The premix was melt processed through a Buss PLK 46 twin screw extruder set at 100 rpm and 100 deg C and 120 deg C for zones 1 and 2, respectively. The extrudate was pulverized by way of a Hosokawa Micropul ACM II providing a 35-micron average particle size.

    Figure 6 / Crelan VP LS 2181/1 Gloss Reduction vs. Blending Ratio with Conventional Blocked Powder Crosslinkers
    The final powder coating was applied to aluminum panels and cured for 20 minutes at 180 deg C. The final powder coatings provided 80 inch-lbs. reverse impact, 9 mm Erichsen indentation and passed 50 acetone double rubs with minimal affect on the coating.

    The performance of the crosslinker is also compared to a conventional uretdione-based powder crosslinker in Table 2. Each of these formulations provides similar mechanical performance and solvent resistance at the appropriate loading levels.

    Figure 7 / The Effect of Filler Type on the Gloss Reduction Capability of Crelan VP LS 2181/1

    VP LS 2386

    Two example formulations containing VP LS 2386 are shown in Table 3, one with a 30-hydroxyl polyester and the other with a 50-hydroxyl polyester. These formulations were processed using a Prism Pilot III premixer set at 2500 rpm for 3 minutes. The premix was melt processed by a Buss PLK 46 twin screw extruder at 100 rpm with zones 1 and 2 set to 100 deg C and 120 deg C, respectively. The extrudate was pulverized using a Hosokawa Micropul ACM and resulted in an average particle size of 35 microns. The final powder coatings were applied to aluminum panels and cured at 180 deg C for 15 minutes and gradient oven panels were cured for 15 minutes ranging from 160-190 deg C. These white high gloss powder coatings provided 60 deg gloss values of greater than 90 while developing excellent mechanical properties. The higher hydroxyl value polyester supplied higher solvent resistance as expected.

    Figure 8 / The Effect of Filler Content on the Gloss Reduction Capabilities of Crelan VP LS 2181/1
    Additionally, VP LS 2386 is compared with the industries standard crosslinker VP LS 2256 (IPDI polyisocyanate blocked with caprolactam) combined with a 40 OH polyester in a high-gloss white formulation (see Table 4). The crosslinker demand using VP LS 2386 is approximately 60% of that for the standard crosslinker and provides equivalent performance with reduced emissions. This high NCO crosslinker does require either higher temperature or longer time in order to provide equivalent performance in an uncatalyzed formulation.

    Table 1

    VP LS 2181/1

    The bi-functionality of VP LS 2181/1 provides the formulator the ability to adjust gloss to any desired level by using a blend of conventional powder crosslinkers. The formulations shown in Table 5 are examples of the crosslinker used alone. The formulations were processed using a Prism Pilot III premixer at 2500 rpm for 2 minutes. This premix was then melt processed using a Buss PLK 46 twin screw extruder set at 100 rpm and zones 1 and 2 were set to 100 and 120 deg C, respectively.

    Table 2
    The Hosokawa Micropul ACM pulverized the final powder coating to an average particle size of 40 microns. The matte hardener used singularly produced a dead matte finish with good mechanical performance and reproducibility. Solvent resistance is diminished when low hydroxyl value polyesters are used in the formulation. VP LS 2181/1 provides the advantage maintaining a more consistent gloss development vs. processing variability with good mechanical performance. Furthermore, the crosslinker can be blended with other curing agents, as shown in Figure 6, to develop powder coatings of any desired gloss levels. Although the exact gloss level will be affected by variables such as acid coreactant type, blending crosslinkers, polyester functionality, pigment, extender, and overall PVC, the final coatings (shown in Figures 6, 7 and 8) can be manipulated to provide consistent and precise gloss reduction in industrial processes.

    Table 3
    Figure 6 reveals that a combination of 30-40 wt% VP LS 2147 (non-emissive uretdione type) and VP LS 2181/1 provides a 60 deg gloss level of 30-40. Blending with 35-40 wt% 2256 (IPDI polyisocyanate blocked by caprolactam) results in 60 deg gloss levels from 35-50. Figure 7 provides a look at the effect of extender type on the degree of gloss reduction. The blanc fix and barium sulfate extender (Schwerspat EWO)-based formulations exhibit a difference of 30 gloss points using 100% VP LS 2181/1 and varies by only 14 gloss points when the crosslinker is used in combination with 30 wt% of VP LS 2147.

    Consistent with the majority of powder coatings, powder coatings based on VP LS 2181/1 also exhibit a reduction in gloss with an increase in pigment, extender or filler concentration. Figure 8 displays how rapidly gloss levels drop with higher filler contents.

    Table 4

    NW 5

    A high-gloss white powder coating formulation based on Crelan NW 5 is shown in Table 6. This powder coating was premixed using a Prism Pilot III premixer set to 2500 rpm for 2 minutes. The premix was melt blended using a Buss PLK 46 twin screw extruder set at 200 rpm and zone 1 and 2 temperatures of 90 and 100 deg C, respectively. The extrudate was pulverized to an average particle size of 51 microns. The final powder coating was applied to Bonderite 1000 panels and cured for 20 minutes at 180 deg C. The powder coating exhibits excellent film appearance, mechanical properties, solvent resistance, and overbake stability.

    Table 5
    Tables 7-8 show performance data for NW 5-based formulations in combination with a variety of Rucote polyesters. These powder coatings are gray and were processed in the same manner as the white formulation in Table 6. However, Tables 7-8 results are derived from powder coatings, which were applied to e-coat panels and cured at 180 deg C for 20 minutes. NW 5 imparts higher levels of impact resistance than conventional IPDI based crosslinkers while maintaining good hardness and chemical resistance as shown in Tables 7-8, respectively. Additionally high levels of solvent resistance can be achieved at low curing temperatures as shown in Table 8 where curing at 160 deg C for 20 minutes in combination with Rucote GXP 1004 polyester provides 100 MEK double rubs.

    Table 6

    XP 7180

    The XP 7180 based formulation shown in Table 9 was premixed for 3 minutes at 2500 rpm. The premix was then processed by way of a Buss 26 mm twin screw extruder with Zone 1 set to 45 C, Zone 2 set to 65 at 250 rpm and maintained at 80% torque. These conditions produced a final extrudate melt temperature of 105 deg C. The extrudate was pulverized to an average particle size of 50 microns. The final powder coating formulation gelled in 162 seconds at 190 deg C and flowed 1000 mm by PCI test method for Inclined Plate flow. The results shown in Table 10 are derived from gradient oven panels. The performance data in Table 10 provides a look at the low temperature curability of XP 7180. This uncatalyzed formulation provides excellent impact resistance when cured as low at 150 deg C for 20 minutes. The low solvent resistance indicates that the formulation has not developed its ultimate properties with this curing profile, however at 165 deg C 20 minutes the powder coating is fully cured and maintains excellent mechanical properties.

    Table 9

    Summary

    Advances in polyurethane powder coatings crosslinker were presented in a context of meeting the powder industries needs. Higher NCO content crosslinkers, such as VP LS 2347 and 2386, translate to lower equivalent weights and reduced crosslinker demands; yet the performance is maintained when these crosslinkers are used as demonstrated by the example formulations. Additionally, VP LS 2181/1 clearly provides controlled gloss reduction for polyurethane powder coatings by blending with conventional crosslinkers and combining an acid co-reactant such as hydroxy alkyl amide, TGIC or GMA acrylics. Reduced curing temperatures are possible by incorporating NW 5 as a replacement for conventional IPDI-based caprolactam blocked crosslinkers. Desmodur W-based crosslinkers have proven to provide lower curing temperatures, higher impact resistance, excellent weatherability, solvent resistance while simultaneously maintain high hardness levels. XP 7180 offers even more drastic curing temperature reduction by utilizing 3,5-dimethyl pyrazole as the blocking agent.

    Acknowledgements

    The authors would like to express thanks to the following people for their input to the body of research and development discussed above: Michael Grahl, Frake Chmielewski, Tom Stadermann, Larry Smedley, Eric Vidra, Katherine Ratliff, Mike Jeffries, Lanny Venham, Reinhard Halpaap, and Hans Laas.

    This article was presented at the International Waterborne, High-Solids, and Powder Coatings Symposium February 6-8 in New Orleans. The symposium is sponsored by The University of Southern Mississippi Department of Polymer Science.

    For more information contact Bayer Corp., 100 Bayer Road, Pittsburgh, PA 15108; phone 800/662.2927; fax 412/778.4451; visit bayerpowder.com; e-mail carl.sullivan.b.@bayer.com; or Circle Number 123.

    References

    1 P. Thometzek, H Basten, K. Kohler, H.-U. Meier-Westhues, Fabe & Lack 104, 22 (1998).
    2 H.-U. Meier-Westhues, P. Thometzek, H.-J. Laas, Farbe & Lack 103, 140 (1997).
    3 U. Freudenberg, U. Meier-Westhues, H.-J. Laas, European Coatings Journal 9, 804 (1997).
    4 Freudenberg, Ulrich and Thometzek, Peter, Weather Stable, Low-Gloss Powder Coatings, Proc. Int. Waterborne, High-Solids, Powder Coatings Symp., (1999), 26th 216-231.
    5 Thometzek, Peter, Many promising powder coating perspectives, Coating (2001), 34(7) 243-246.

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