Less metal, more protection

Transcription

1 Less metal, more protection Performance of zinc-rich primers is enhanced by use of zinc flakes Ljudmila Kruba, Pierre Stucker, Tom Schuster The use of a mixture of lamellar zinc flakes and zinc dust in an ethyl-silicate based zinc primer has been found to maintain efficient cathodic protection at much reduced zinc contents The new formulation also avoids the problems of porous structure, blistering, pinholing and white rust commonly found with this type of primer Traditional zinc dust/ethyl silicate primers contain about 70-80% by weight of zinc in the formulation This provides a very high cathodic protection, which remains effective even after damage to the coating However, the high zinc dust content produces characteristic defects: a porous structure, white rust, pinholing, blistering and mud-cracking [1, 2, 4-7] These issues can cause problems when overcoating The main market requirement for zinc-based ethyl silicate primers today is any possible reduction of the zinc content in the formulation The primers should still satisfy a list of requirements that can be summarised as shown in Table 1 Mixing flakes with dust allows zinc content to be reduced By combining zinc pigments with different particle shapes (lamellar flakes and spherical dust) it has been found possible to significantly reduce the heavy metal content in primers (to 25% by mass in the formulation, equivalent to 117% by volume in the dry film) and to obtain coatings with excellent corrosion protection and mechanical properties The two zinc pigments work together in giving cathodic protection and in addition the zinc flakes create a very good barrier effect, providing coatings without a porous structure, blistering, pinholing or white rust [8] Solvent content is changed to optimise properties Tetraethyl silicate ("TES 40"), which forms polyethoxysiloxane after hydrolysis with water in acidic medium, was used as a reactive binder to produce self-curing zinc silicate coatings The optimised formulation of this hydrolysate is shown in Table 2 It may be observed that a mixture of solvents is used and the amount of solvent present in the hydrolysate itself is reduced, but that this is offset by the much higher content of hydrolysate in the complete formulation The properties of the pigments are summarised in Table 3 It should be noted that the zinc flake has a small particle size and a specific surface area four times higher than the zinc dust used, and a very high anodic activity On the other hand, the zinc dust has a low specific surface and surface activity The filler used was a china clay which is chemically inert and has a lamellar structure Extended pot life The formulation and basic properties of the optimised zinc flake/zinc dust primer are presented in Table 2 and 4 and compared with those of a standard zinc dust primer [3] The formulation of both hydrolysates and primers are shown A very important market demand was that the composition should have a pot life of more than eight hours after mixing of the hydrolysate with the pigments (2K system) The development of a new formulation of hydrolysate (Table 2) and the use of two zinc pigments with different particle shape and surface activity achieved a pot life of about hours (Table 4) This pot life was determined as ideal for coatings with sufficient hardening, good solvent resistance and mechanical properties after 24 hours (Table 4) Full hardening of coatings to achieve salt spray resistance took seven days Improved appearance after corrosion tests The corrosion protection properties of the primers were visually determined after the salt spray test (DIN ) and are shown in Figure 1 Figure 1a shows a typical picture of zinc dust coatings after the salt spray test: the surface shows blistering and white rust Using the zinc flake/zinc dust formulation changes this picture fundamentally: smooth and even surfaces without blistering or white rust can be seen in Figure 1b These visual differences are confirmed by the microscopic surface comparison (Figure 2) Cross-sections of the zinc flake/zinc dust and zinc dust primers after the salt spray test are shown in Figure 3 The cross-section clearly shows the difference between the conventional and new primer systems The zinc dust primer (Figure 3a) shows contacts only between dust particles The open and porous structure of these coatings explains the appearance of white rust at the surface (Figure 2a) For coatings with the zinc flake/zinc dust combination (Figure 3b) the results are different: multiple contacts between dust/dust, flake/flake and flake/dust lead to high and lasting corrosion protection by such coatings [1,8] In addition, the zinc flakes create a very good barrier effect both by retarding the access of corrosive media to the substrate and by inhibiting the diffusion of zinc white rust towards the surface The result is a smooth and even surface on the coating that practically excludes white rust formation on the surface (Figure 2b) Furthermore, this combination of different shaped zinc pigments gives an improved packing density in the film which provides very good mechanical properties The comparison of primer surfaces after a weathering test of more than 21 months shows no corrosion at the surface of either coating (Figures 4a and b) This test is continuing Environmental, application and performance benefits The performance obtained with these new primers exceeds the requirements of customers for such coatings Their advantages can be summarised as follows: Environment - Zinc content in the formulation is reduced to approximately 25% by mass; - Zinc content of the dry film at 117% by volume is 45 times lower than in standard zinc dust primer formulations Technology - Hydrolysate is stable (for more than 6 months) yet gives fast hardening; - Very simple formulation; - Good dispersion without additives; - Low settlement (no additive needed); - Good spraying properties; - Pot life more than 8 hours Coating properties - Dry film thickness: 70-80µm - Very good cathodic protection with reduced levels of zinc; - Salt spray resistance =1300 hours (=55 days) without rust; - Natural weathering: = 21 months without rust (test continuing) - Good solvent resistance after 24 hours to isopropanol and

2 methyl ethyl ketone; - Good adhesion after 24 hours and 7days - Gt0 to DIN EN ISO 2409; - Very low tendency to white rust formation; - Very hard and non-porous structure; - High barrier effect; - No blistering or pinholing; - Good mechanical properties Particle shapes are the key to adequate protection The use of two zinc pigments with different particle shapes (flakes and spherical) has allowed an ethyl-silicate based primer formulation to be developed with a significantly reduced zinc content This mixture of particle shapes increases the number of possible electrical contacts between particles, and in this way compensates for the low zinc content in the system and allows coatings to be obtained which offer high corrosion protection In addition, the zinc flakes create a very good barrier effect The final result is a coating with high packing density and mechanical properties without the porous structure, blistering, pinholing or white rust often associated with this type of primer The results achieved exceeded the market requirements for these coatings at a film thickness of 70-80µm in Lausanne, Switzerland He joined Eckart Switzerland in 1982 He is currently sales manager for zinc products and responsible for zinc activities development at Eckart Switzerland -> Dr Tom Schuster obtained his PhD in chemistry at Aachen, Germany, in 1987 After working abroad for several years, he returned to Germany in 2001 and became head of R&D at Eckart He is also responsible for marketing of protective coatings REFERENCES [1] E V Schmid, Wetter und Korrosionsschutz, Vincentz Verlag, 1983, p 392 [2] K Barton, O Kusnierik, J Lapka, Koroze a Ochrana Materialu, 1985, 29, No 3, pp [3] Wacker-Chemie GmbH, 2000, Bindemittel für anorganische Zinkstaubfarben [4] Andreas Tomanek, Silicone & Technik, Wacker Chemie GmbH, 1990, München [5] M Mitchell, M Summers, Protective Coatings Europe, vol 8, No 7, pp12-15 [6] Clive H Hare, Stephen J Wright, Journal of Coatings Technology, vol 54, No 693, October 1982, pp [7] M Taylar, G Terzakis, Protective Coatings Europe, vol 8, No 1, January 2003, pp 4-8 [8] L Kruba, P Stucker, T Schuster, Pitture e Vernici, N0 8, vol 81, 2005, pp Results at a glance - An ethyl-silicate based zinc primer formulation has been developed which offers very good corrosion protection with a much lower metal content than normal - The combination of zinc particles with two different shapes (flakes and spherical) increases the electrical contact between the particles and so maintains cathodic protection at lower zinc contents - In addition, the zinc flakes provide a barrier effect and a high packing density - The new formulation has also been shown to avoid the problems of porous structure, blistering, pinholing and white rust which are commonly found with this type of primer The authors: -> Dr Ljudmila Kruba graduated from the Chemistry-Technological Mendeleev Institute in Moscow and then worked at the Moscow Coatings Research Institute, receiving her PhD in 1987 Since 2000 she has worked for Doral and Eckart Switzerland in research and development on zinc-based coatings -> Dr Pierre Stucker obtained his PhD in chemistry in 1976

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4 Figure 1: Visual comparison of the primer surfaces after salt spray testing: a) standard zinc dust (900 h), b) zinc flake/zinc dust primer (1300 h)

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6 Figure 2 a-b: Comparison of the primer surfaces under microscope after salt spray testing

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8 Figure 3: Cross-section of standard zinc dust (a) and zinc flake/zinc dust (b) primers after salt spray testing

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10 Figure 4: Comparison of the surfaces of the standard zinc dust (a) and zinc flake/zinc dust (b) primers after 21 months' weathering exposure

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