Low temperature curing polyurethane powder coatings

Low temperature curing polyurethane powder coatings

F O C U S obtainable under irradiation with UV or visible light, and powder coatings can be produced having very good storage stability and showing ve...

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F O C U S obtainable under irradiation with UV or visible light, and powder coatings can be produced having very good storage stability and showing very good flow properties. Article entitled “Lightly Cured – UV Powder Coatings based on Unsaturated Allylic Polyesters” by Dominique Burget et al, published in European Coatings Journal, Dec 2003, (12) 22-24,26

Low temperature curing polyurethane powder coatings It is well known in the coatings industry that polyurethane powder coatings have desirable performance characteristics. However, a major obstacle to their use on non-metallic substrates is the requirement of high temperature curing. A recent paper by researchers at Bayer Polymers describes remarkable improvements that have been made recently to lower polyurethane powder cure temperatures by using isocyanates with different blocking agents. The criticism of high temperature curing not only applies to conventional blocked isocyanates but also applies to excessive heat required for generation of isocyanate groups from uretdione rings. Uretdione technology is predominant in Europe, while the rest of the world employs blocked isocyanate technology with the favoured system based on E-caprolactam that deblocks around 180°C. Research in recent years has concentrated on lowering the cure temperature by the use of different aliphatic isocyanates and blocking agents. Work carried out in liquid systems illustrates the reductions in unblocking temperature achieved by alternative blocking agents: phenols reduce to 170°C but give severe discolouration; methyl ethyl ketoxime unblocks at 150°C with some yellowing; triazole also unblocks at 150°C without yellowing; dimethyl pyrazole

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(DMP) unblocks at 125°C; and diethyl malonate (DEM) unblocks at 115°C. The research and development in the field of liquid coatings offers a good starting point for the design of urethane crosslinkers suitable for powder coatings but adaptation of the technology is not easy because of fundamental differences between liquid and powder coatings, such as the differences in mobility of the reactive groups and the flow and levelling stages. The paper describes recent improvements made to lower the cure temperature of polyurethane powder coatings. Typical products available in the market include various trimethyloyl propane and aliphatic isocyanate trimers based on isophorone diisocyanate or similar products in which the IPDI is replaced by hydrogenated MDI, which offer a slightly lower cure window of 10 to 15°C with improved impact and flexibility. Triazole blocked isocyanates are also available that are capable of cure at 160°C for 30 minutes. DMP is a relatively new blocking agent that is being evaluated in powder coatings. The pyrazole moiety unblocks at lower temperature than caprolactam and triazole. An experimental crosslinker based on H12MDI and DMP is a unique product combining the flexible and tough H12MDI with good flow characteristics. If the functionality of the hydroxylated polyester is increased, or if a catalyst is added it is possible to lower the unblocking temperature even further. Good MEK resistance can be obtained at 145°C for 30 minutes but slightly higher temperatures are needed to obtain good mechanicals. The most recent low temperature cure development is the use of a DEM crosslinker. DEM has been used for years in the liquid coatings area but the absence of solid DEM blocked materials has prevented its use in powder coatings. A recent

C OAT I N G S breakthrough is the development of a solid DEM based crosslinker by Bayer designed for the powder coatings market. The interesting aspect of this crosslinker is the transesterification of the hydroxyl functional resins with the pendant ester groups present in the DEM crosslinker. Preliminary results have shown that the crosslinker is effective at 130°C. This offers new possibilities for use in low temperature curing and as a fortifying crosslinker in epoxy and hybrid formulations. Paper entitled “Recent Advances in Low Temperature Cure Crosslinkers for Polyurethane Powder Coatings” by Ramesh Subramanian and co-workers at Bayer Polymers LLC presented at the 81st Annual Meeting of the FSCT in Philadelphia, 13-14 Nov 2003. Full conference proceedings are available on CD-ROM from the Federation of Societies for Coatings Technology, 492 Norristown Road, Blue Bell, PA 19422-2350, USA. Tel: +1 610 940 0777. Fax: +1 610 940 0292

Degradation of FBE coated steel Fusion bonded epoxy (FBE) is the dominant coating used today for protecting steel reinforcing bar from corrosion. This coating has very low permeability to chloride ions, high alkaline resistance, good flexibility, and good adhesion to steel. Fabrication of fusion bonded epoxy-coated reinforcing bars (referred to as epoxy-coated rebar or ECR) is a fast efficient process. Mechanically blasted steel bars of any length or diameter are continuously fed to a chamber preheated to 230°C followed by electrostatic spraying with a thermosetting powder epoxy, which fuses into a continuous coating, is cured by residual heat in the bar, and finally cooled by quenching with water at ambient temperature. It takes only a few minutes for a 20 metre ECR to be ready for shipment. Since its introduction in the mid 70s, thousands of ECR reinforced structures have been constructed in the United States. The corrosion resistance of ECR has

JANUARY 2004