Zinc-free and aqueous

Transcription

1 Zinc-free and aqueous Anticorrosive primers, free of (zinc containing) anticorrosive pigments can be formulated using aqueous, internally flexibilized 2K-epoxy dispersions Salt-spray and humidity chamber test results are equivalent to those of other aqueous or solvent borne anticorrosive primers containing anticorrosive pigments Modern, aqueous epoxy dispersions and amine hardeners produce high performance primers An opinion still commonly regarding aqueous epoxy primers is that their anticorrosive properties are inferior to solvent borne systems and that the anticorrosive properties need to be improved by the use of anticorrosive pigments It is also the opinion that aqueous binders require high concentrations of these anticorrosive pigments Modern aqueous epoxy dispersions and new aqueous amine hardeners might change this opinion, however Binders, pigments and fillers must be precisely attuned to each other for this purpose The binder assumes a decisive role here It determines adhesion to the substrate and the diffusion of water and ions through the coating The pigment inhibits directly (active protection) and indirectly (barrier effect) the corrosive reactions of the substrate [1] More flexible, but water-resistant This new aqueous, internally flexibilized resin technology enables pigment additions to be carried out directly in the resin This makes more binder available into which pigments and fillers can be dispersed This also allows pigment to binder ratios to approach those of solvent borne primersthe flexibility of the epoxy resin also has a beneficial effect on the adhesive properties of a coating In general, the flexibility of the coating declines with age (Figure 1) Cupping test values of more than 3 mm were still obtained after 7 days of ageing at 50 C with the internally flexibilized, aqueous epoxy dispersion The solvent borne primer based on a non-flexibilized epoxy resin, on the other hand, declined to a value of less than 1 mm Epoxy resins can either be rendered flexible externally by means of high-boiling solvent or a coalescing agent or internally by means of flexible segments in the epoxy resin backbone However, solvents and coalescing agents usually do not permanently remain in the cured primer and contribute to subsequent embrittlement and loss of adhesion as a result of elution and exudation quite apart from their contribution to the VOC content Internally flexibilized, water-soluble epoxy resins and aqueous epoxy dispersions have chiefly been made by incorporating polyalkylene glycols [2] to date On the one hand, this enables emulsification in the aqueous medium and on the other hand, a certain degree of flexibility is introduced into the epoxy resin backbone Due to the high hydrophilicity of the polyalkylene glycols, their proportion in the epoxy resin may only be small, in order to meet the high demands for water-resistance of anticorrosive primers The new technology forming the basis of the studies described here adds other flexible segments with low hydrophilicity to the polyalkylene glycols used to date, which introduce additional functional groups into the epoxy resin selectively This makes dispersions more flexible and elastic but not more hydrophilic, and the water resistance of the anticorrosive coating made with the dispersion is retained Furthermore, the additional functional groups improve the pigment and substrate wetting as well as the stability of the dispersion under shear load Experimental The effect of active anticorrosive pigments is non-transferable from one binder system to another It is possible that a pigment which is particularly effective in one binder system only slightly improves another binder system It is, therefore, important to test the widest possible range of anticorrosive pigments in order to find the optimum pigment [3] In the new aqueous 2K-epoxy binder system, nine anticorrosive pigments were tested, as well as in addition a comparative formulation in which the anticorrosive pigment was replaced by titanium dioxide The formulation is described in Table 1, the tested anticorrosive pigments in Table 2 The dry film thickness of the coatings applied to untreated steel ("Gardobond OC") was approx 50 µm After drying for 7 days at 23 C and 50 % humidity, the following test was performed: humidity chamber test as per VLN 147 based on DIN EN ISO and salt-spray resistance as per DIN EN ISO 9227 The creep from scribe was assessed weekly In the process, as much coating as possible was removed mechanically starting from the scribe, the cleared area covered with adhesive tape and the test continued Figure 2 and Figure 3 show the results of the test Best performance without active anticorrosive pigment Regardless of the type of anticorrosive pigment, all the formulations achieved a humidity chamber test resistance of at least 672 hours Without anticorrosive pigment, it was 840 hours Other aqueous 2K-epoxy binder systems show different results: Only a few anticorrosive pigments produce good humidity chamber test resistance [3] The good humidity chamber resistance documented in Figure 2 regardless of the anticorrosive pigment used and also without such a pigment is evidently a special property of the aqueous epoxy resin dispersion used These results differ significantly from the test results obtained with other epoxy dispersions Salt-spray resistances (Figure 3) fluctuate according to the anticorrosive pigment used The best result was achieved with the formulation which contained titanium dioxide instead of the anticorrosive pigment Figure 4 shows the test results of the primer without anticorrosive pigment, the primer with the pigment ZPO and with the zinc-free pigment CHP after 1000 hours of salt-spray testing The primer without anticorrosive pigment also displays a higher initial elasticity than those with anticorrosive pigments Barium sulphate instead of zinc phosphate Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ 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2 Another test with the same aqueous binder system, but with altered pigment and filler composition compared with the formulation mentioned in Table 1, showed that the replacement of anticorrosive pigment by more cost-effective barium sulphate also produces better salt-spray test results than a formulation with anticorrosive pigment The results of both test series (replacement of ZPO by titanium dioxide or barium sulphate) were confirmed several times There are publications which already point in the same direction Thus, in [4] electrochemical impedance spectroscopy studies are described, according to which, in aqueous coatings the influence of an anticorrosive pigment and a barrier pigment on the barrier properties is relatively slight, does not last long or is non-existent The anticorrosive pigment was noted to even have a negative influence on an aqueous 2K-epoxy system In outdoor exposure tests on sandblasted steel, a significant increase in anticorrosive capacity was only partially ascertained when using anticorrosive pigments The replacement of zinc phosphate with silane modified wollastonite and flaky muscovite mica in an aqueous 2K-Epoxy system is described in [5] At a layer thickness of 120 µm, with certain filler combinations on sandblasted steel, good salt-spray results could also be achieved without zinc phosphate The effect of active anticorrosive pig-ments is therefore also achievable by means of an alternative pigmentation and the adaptation of other formulation parameters Pigment-binder interaction decisive The interaction between binder and anticorrosive pigment is important This also includes wettability of the pigment and adhesion of the binder to the pigment surface Weak adhesion of the binder can result in increased permeability of the primer [4] The test results described in this article with a new aqueous 2K-epoxy binder system indicate that the positive effect of anticorrosive pigments can be compensated by other influences Better anticorrosive properties are produced without anticorrosive pigment, regardless of whether titanium dioxide or barium sulphate is substituted or the pigment composition is varied in addition This is a special property of this new 2K-epoxy binder system Aqueous system outmatches solvent borne system The comparison with a solvent borne 2K-epoxy binder system (Table 3) shows that with a layer thickness of approximately 50 µm, the aqueous primer without anticorrosive pigment protects untreated steel equal to or better than a solvent borne primer On the one hand, the solvent borne primer formulation was adapted to the aqueous primer in order to enable a comparison with the same anticorrosive pigment concentration This solvent borne formulation with 5 % anticorrosive pigment failed after a short test time On the other hand, an optimized standard formulation (Table 4) of a solvent borne system with 11% anticorrosive pigment was tested The wet adhesion was evaluated after 1000 hours of salt-spray testing or humidity chamber testing and two hours of regeneration with cross cut test and adhesive tape pull-off (DIN EN ISO 2409) The aqueous system adhered well; the solvent based system pro-duced moderate adhesion characteristics or displayed a total loss of adhesion (Table 3) Similar results were also obtained in a two-layer system with an aqueous 2K-PUR topcoat (approximately 50 µm primer and approximately 50 µm topcoat) on untreated steel (Figure 5) The aqueous primer without anticorrosive pigment displayed the least creep from scribe Effective protection on various substrates In a further test series, the aqueous 2K-epoxy primers were tested with and without anticorrosive pigment on various substrates with a dry film thickness of approximately 50 µm The tests were performed over a period of up to 2000 hours Figure 6 shows some test panels On almost all substrates the anticorrosive pigment-free primer yielded at least as good results as the primer containing anticorrosive pigment (Table 5) The results on large-bloom, galvanized steel were the exception Poorer results were achieved without anticorrosive pigment on this substrate Advantages of zinc-free corrosion protection Some anticorrosive pigments result in a further increase in the viscosity of lacquers based on aqueous epoxy dispersions during storage Replacing the anticorrosive pigment by titanium dioxide or barium sulphate reduces this effect In Europe zinc phosphate is classified as environmentally hazardous and marked R50/53 Preparations containing between 25 % and 25 % zinc phosphate are also to be classified as environmentally hazardous in accordance with Preparation Directive 1999/45/ EC and marked R51/53; preparations with 025 % to 25 % zinc phosphate are to be marked R52/53 Efforts are therefore being made to use anticorrosive pigments with as little zinc as possible The results of this study show that active (zinc containing) anticorrosive pigments in an aqueous 2K-epoxy system based on a new, internally flexibilized epoxy dispersion can be completely dispensed with Thus, zinc-free, high performance, aqueous and VOC-compliant anticorrosive primers can be formulated? Acknowledgement The authors are grateful to Elfriede Prucher for performing the tests, as well as to the entire Epoxy Team at Cytec Surface Specialties which made a significant contribution to the development of the new epoxy dispersion References [1] W M Liu, Materials and Corrosion, 49 (1998) 8, p 576 [2] P Oldring, Waterborne & Solvent Based Epoxies and their End User Applications, Volume II, John Wiley & Sons, Chichester (1997) [3] M A Jackson, Polymers Paint Colour Journal, 180 (1990) 4270, p 608 [4] T Schauer, L Dulog, W M Liu, Farbe und Lack, 103 (1997) 1, p 28 [5 M Klawa, Farbe und Lack, 112, (2006), 8, p 34 - Nine active anticorrosive pigments were tested in an aqueous 2K-epoxy primer based on an internally flexibilized epoxy dispersion - The comparative formulation free of anticorrosive pigment produced the best results in the salt spray test It also achieved good results in the humidity chamber test - Different substrates produced similar test results - The corrosion resistance of the aqueous primer without anticorrosive pigments exceeds that of a typical solvent borne primer - It is possible to formulate zinc-free, high performance, VOC-compliant, aqueous primers THE AUTHORS» Dr Rosemaria Grasböck, studied Technical Chemistry at the Technical University of Graz and the University of Liverpool From she was at Joanneum Research, since 2003 she has been the TS&D Technical Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

3 Leader for 2K-Epoxy Systems at Cytec Surface Specialties in Graz, Austria» Dr Martin Geisberger studied Chemistry at the Technical University of Munich From he was the TS&D Technical Leader for 2K-PUR Systems, since 2005 he has been the R&D Technical Leader for 2K-Epoxy Systems at Cytec Surface Specialties in Graz, Austria * Corresponding Author Contact: Dr Rosemaria Grasböck Cytec Surface Specialties Austria GmbH Tel , Fax rosemariagrasboeck@cyteccom Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

4 Figure 1: Cupping test (DIN EN ISO 1520) after ageing for various aqueous and solvent borne primers (layer thickness approx 50 µm) Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

5 Figure 2: Humidity chamber test results of an aqueous 2K-Epoxy primer with various anticorrosive pigments (substrate: "Gardobond OC", 50 µm layer thickness, criterion: m1g1) Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

6 Figure 3: Salt-spray test results with various anticorrosive pigments (substrate: "Gardobond OC", 50 µm layer thickness, criterion: 10 mm creep from scribe and/or m1g1) Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

7 Figure 4: Salt-spray test results after 1000 h with and without anticorrosive pigment Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

8 Figure 5: Salt-spray test results for aqueous and solvent borne primer with aqueous 2K PUR topcoat Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

9 Figure 6: Salt-spray test results with and without anticorrosive pigment on various substrates Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

10 Bild zu Zinc-free and aqueous Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

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