Using the Si-O strength Epoxy-siloxane and acrylic-siloxane systems improve the performance of corrosion protection coatings. Hybrid systems based on polysiloxanes can provide higher anticorrosive performance than traditional organic binders because the silicon-oxygen bond strength is higher than that of carbon-carbon bonds, and the silicon is already oxidised. Experiments are presented which confirm that use of a polysiloxane component increases the anticorrosive efficiency and UV resistance of epoxy and acrylic coatings. Andrea Kukacková* * Contact: Dr. Andrea Kukacková, Synpo a.s., S.K. Neumanna 1316, 532 07 Pardubice, Czech Republic, Tel. +420 466 067 256,, Fax +420 466 067 260, andrea.kukackova@synpo.cz Protective coatings technology is primarily based on organic binders. Unfortunately, organic coatings degrade as a result of thermal oxidation, photo-initiated oxidation or chemical attack. Silicon-based inorganic coatings are much more resistant to these degradation mechanisms, and organically modified polysiloxanes are generally recognised as the latest generic class of high performance protective coatings. The reason for the rapid development of polysiloxane-based coatings is clear. They offer improved performance properties and cost-effectiveness, lower VOC contents and improved health and safety features compared to traditional organic coatings [1, 2, 3, 4]. Why polysiloxanes have high resistance properties The term 'hybrid coating' is rightly used in connection with many different systems in which two (or more) binder systems with distinct properties and curing mechanisms are present [5]. As already stated, organic coatings degrade via thermal and photo-induced oxidation and are subject to chemical attack. This results in the deterioration of properties such as colour and gloss retention, flexibility, adhesion and corrosion resistance, which in turn reduces the overall coating durability and service life. Polysiloxane coatings are largely inorganic in nature and are inherently more resistant to these degradation mechanisms. The first reason for this is the presence of repeating Si-O groups in the polymer backbone. The Si-O bond strength is 108 kcal/mole. Organic coatings contain C-C bonds, with a bond strength of 83 kcal/mole. The higher energy required to cleave the Si-O bonds provides greater stability and therefore superior resistance to weathering and thermal degradation. In addition, the silicon in the polysiloxanes cannot be subject to the oxidative degradation that affects C-C bonds in organic polymers because it is already oxidised. This improves resistance to attack by atmospheric oxygen and oxidising chemicals. The versatility of polysiloxane chemistry allows a variety of functional organic groups to be attached to the inorganic siloxane backbone. Organic modification is necessary to achieve a balance of film properties, such as adhesion, flexibility and cost. The most widely used polysiloxane coatings are epoxy-siloxane and acrylic-siloxane hybrids [3, 4, 8]. Epoxy-siloxanes have excellent weathering performance The chemical combination of epoxies with polysiloxanes results in a group of epoxy-siloxane hybrids whose unique physical characteristics allow their use as durable binders for the protective coatings industry. The advantages of epoxy are combined with the strength of the polysiloxane, providing two-component ambient curing thermoset coatings. Since they have better weathering resistance than polyurethanes, excellent corrosion resistance and inherent compatibility, they offer the potential to reduce the number of coats in protective coating systems. This has lead to the elimination of one coat in zinc silicate and zinc epoxy based coating systems in cases where both appearance and the chemical protection of the zinc primer are required. The traditional high performance three-coat zinc primer/epoxy/polyurethane coating system can be replaced by the two-coat zinc primer/epoxy-siloxane system with significant savings in applied cost [4, 6, 7]. Weatherable, corrosion resistant epoxy-siloxane hybrid binders are usually formulated with aliphatic epoxy resins, silicone intermediates, oxysilanes and aminosilanes. The amine group on the aminosilane cures the epoxy resin in the usual manner and at the same time participates in hydrolytic polycondensation reactions with silicone and oxysilane components. Figures 1 and 2 show the hydrolytic polycondensation and epoxy-siloxane curing reactions [7]. Acrylic-siloxanes have two distinct curing mechanisms By combining acrylic resins with siloxane, formulators have developed high solids, low VOC, highly weatherable topcoats. Several products are commercially available and include self-curing, one-component and two-component types. One-pack types are formulated from combinations of alkoxysilane or silanol functional acrylic resins, hydroxy functional acrylic resins and silicone resin intermediates. These hybrids cure via hydrolysis of the alkoxysilane functionalities. Two-pack acrylic-siloxane hybrids have also gained market acceptance. One component is formulated from acrylate or acetoacetate functional acrylic oligomers, acrylic resins and alkoxy or silanol functional silicone resin intermediates. The other component is typically an aminosilane. These two-pack hybrids cure via a Michael addition reaction of amine and acrylate functional groups and hydrolytic condensation of the silicone resin intermediate. The curing mechanism of acrylic-siloxane hybrids is shown in Figure 3 [7]. Experimental Two groups of commercially available polysiloxane hybrid systems, the epoxy-amine-siloxanes and acrylic-amine-siloxanes, were compared with traditional organic binders (see Table 1). From the group of epoxy-amine-siloxanes, the sample EAS1 based on epoxy resin with an amine-functional polysiloxane used as the hardener and two samples containing the epoxy-siloxane resin combination with different hardeners, aminosilane (EAS2) and polyamine adduct (EAS3), were chosen. In addition, an acrylate-amine-siloxane AAS based on the combination of glycidoxy-functional acrylate with amine-functional polysiloxane hardener was chosen. Epoxy and aliphatic polyurethane organic binders were also tested for comparison. Anticorrosive coatings were formulated at PVC = 37.5 % and PVC/CPVC = 0.7 on the basis of these systems. An organic modified zinc aluminium molybdenum orthophosphate hydrate was used as the anticorrosive pigment. The prepared coatings were applied to degreased and
abraded carbon steel panels of 100 x 150 x 1 mm size (dry film thickness 70-100 µm). Two types of laboratory accelerated tests were carried out; a salt spray test according to ISO 9227 and one in a humidity chamber with SO 2 content according to ISO 3231. In all cases the panels were prepared in duplicate and stored for four weeks for hardening at 23 ± 2 C and 50 ± 5% relative humidity prior to the tests. Immediately before the accelerated tests, the samples were scribed through the coating to the bare steel substrate. The exposure time in the chambers was 1440 hours. After taking the samples from the chambers, the blistering on the paint surface was evaluated in accordance with ASTM D 714. In addition, the panels were inspected after removing the coatings from the metal plates. The degree of rusting was evaluated according to ASTM D 610 and failure at the scribe was reported according to ASTM D 1654. Using these evaluations and a Heubach rating system (100 = best, 0 = worst) the overall 'anticorrosive efficiency' of the coating systems was determined. The anticorrosive efficiency AE was calculated according to the following equations: Salt spray chamber: AE = (A + B + 2C)/4 Humidity chamber with SO 2 content: AE = (A + 2C)/3 where A: degree of blistering B: average measurement of failure at the scribe C: degree of rusting The tests revealed that hybrid coatings based on polysiloxanes had a high anticorrosive efficiency (see Figure 4). The samples EAS2 and EAS3 containing the same type of epoxy-siloxane resin but different amine hardeners showed the highest anticorrosive resistance in both tests. Sample AAS also gave highly effective corrosion protection in both tests. These anticorrosive efficiency values show that the hybrid polysiloxane systems have a higher resistance than traditional epoxy and polyurethane coatings. If the hybrid epoxy-amine-siloxane sample EAS1 is compared with the reference epoxy coating O1 (both containing the same type of epoxy resin, differing only in the type of hardener) the strong influence of the polysiloxane component contained in EAS1 in the form of amine-functional polysiloxane can be observed. Its ability to increase the anticorrosive efficiency was found in both types of accelerated laboratory tests. The same conclusion can be reached by comparing the epoxy-amine-siloxane EAS3 and the epoxy coating O1 (both containing the polyamine adduct hardener). In EAS3 the presence of polysiloxane again increased the anticorrosive efficiency. In other words, the results confirmed the assumption that the polysiloxane component should have positive influence on the corrosion protection properties of coatings. Photographs of the steel panels after 1440 hours' exposure in the salt spray chamber are shown in Figure 5. Polysiloxane component also improves UV resistance The UV resistance of the coatings was also investigated. An accelerated weathering test according to ASTM D 4587 was carried out using a QUV tester. The colour difference E* and yellowness index measurement were observed after 1000 hours of exposure. From the results obtained (see Figure 6) a clear improvement of UV resistance due to the polysiloxane content can be seen. The highest UV resistance was found with the EAS2 sample based on epoxy-siloxane resin and aminosilane hardener and with AAS, containing a polysiloxane component in the form of an amine-functional polysiloxane hardener. Both samples were slightly more UV resistant than the aliphatic polyurethane O2. EAS2 with the aminosilane hardener showed higher UV resistance than the EAS3 hardened by the polyamine adduct. A significant influence of the polysiloxane content on the UV resistance was also found when comparing the epoxy O1 and the hybrid epoxy-amine-siloxane EAS1. Similarly, the beneficial effect of polysiloxane can be observed by comparing the epoxy-amine-siloxane EAS3 with the epoxy O1. On the other hand, the reference epoxy coating showed the highest tendency to yellowing due to its high content of aromatic rings in the polymer backbone. This is also the reason why EAS1 has a lower UV resistance compared to the other two epoxy-siloxanes and the acrylic-siloxane AAS. ACKNOWLEDGEMENTS Financial support for the work reported here by Ministry of Industry and Trade of the Czech Republic is gratefully acknowledged. REFERENCES [1] N. Wood, U. Schiemann, M. Hallack, Degussa, Tego Coatings & Inks: New resin hybrid technology for the coatings formulator, Paint Coatings Industry, April 2005. [2] T. Laubender, J. Greene, Adding value to coatings by using unique organo functional silicone hybrids, Wacker-Chemie GmbH, 8th Nürnberg congress, Creative Advances in Coatings Technology, 2005. [3] A. F. Andrews, International Protective Coatings, Akzo Nobel, England: Polysiloxane topcoats - product choice for optimum performance, SSPC 2002 Technical Presentations, Tampa, Florida, November 2002. [4] N. R. Mowrer, Polysiloxane coatings innovations, Paint and Coatings Expo, Las Vegas, Nevada, 2005. [5] S. Morrison, Technology review: organic-inorganic hybrid coatings, September 2005; available at http://www.specialchem4coating.com. [6] J. M. Keijman, The evolution of siloxane epoxy coatings in the protective coatings industry, Euromat. Conference, Lisbon, Portugal, 1998. [7] N. Mowrer, Polysiloxanes, November 2003, available at http://www.ameronpsx.com/docs/presentation_polysiloxanes. pdf. [8] J. M. Keijman, Properties and use of inorganic polysiloxane hybrid coatings for the protective coatings industry, 2nd Protective Coatings Conference, Lisbon, Portugal, 2000. Results at a glance - Hybrid systems based on polysiloxanes are recently developed high performance coatings for the anticorrosive protection of metals. - Polysiloxane systems are able to provide higher performance than traditional organic binders used in the heavy-duty coatings industry (eg, epoxies or polyurethanes) because the Si-O bond strength is higher than that of C-C bonds, and the silicon is already oxidised. - Experiments are presented which confirm that the presence of the polysiloxane component has a positive influence on the anticorrosive efficiency and UV resistance of coatings based on epoxy-amine-siloxanes and acrylic-amine-siloxanes (compared with an epoxy/polyamine adduct and a 2K aliphatic polyurethane). The author: > Dr. Andrea Kukacková obtained her Ph.D. in chemistry from the University of Pardubice in the Czech Republic in 2005. She has worked since 2004 in the research institute
Synpo a.s. in Pardubice and specialises in the formulation of anticorrosive coatings.
Figure 1: Hydrolytic polycondensation reactions of polysiloxanes. Figure 2: Mechanism of epoxy-siloxane hybrid curing (together with hydrolytic polycondensation of silane/siloxane). Figure 3: Mechanism of acrylic-siloxane hybrid curing (together with hydrolytic polycondensation of silane/siloxane).
Figure 4: Results of the anticorrosive efficiency evaluation, which takes into account blistering, rusting and failure at scribe mark, after 1440 h of exposure.
Figure 5: Steel panels after 1440 hours' exposure in the salt spray chamber.
Figure 6: Results of the colour difference E* and yellowness index measurements after 1000 hours' exposure in a "QUV" panel tester.
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