Water-Based Technologies in 1K Corrosion Protection

Water-Based Technologies in Corrosion Protection Introduction Recent case studies have highlighted the importance of corrosion protection and its impact on the global economy.1 The annual cost of corrosion to the European economy is estimated to approach approximately EUR 600 billion, and best practices in corrosion protection are estimated to result in annual cost savings of 15–35%. The use of one or two-component coatings remains the preferred solution for corrosion prevention. Although the paint industry has seen a transition to water-based technologies, solvent-based systems are still preferred for corrosion-preventive metal protection for well-known reasons. These reasons: – The effect of water during application can lead to rust formation on the surface (flash rusting). – Water-based technologies require the use of surfactants or water-soluble compounds that can contribute to water transmission through the paint film. – Corrosion-inhibiting pigments or compounds are not always compatible with water or soluble in water. However, owing to the global trend towards VOC reduction in general and the increasing weight of ecological or "green" labels for paint products and changing consumer preferences, there is growing demand for high-performance, water-based Direct-To-Metal (DTM) products that can replace solvent-based technologies (such as alkyds). In paint formulations developed for application to metal surfaces, anticorrosive pigments are used, particularly to provide protection at weak points in the coating in the event of film damage. The selection of anticorrosive pigments available to the paint formulator has been significantly reduced in recent years due to health and safety concerns (such as toxic pigments like chromates or soluble barium) and, more recently, environmental concerns and the development of numerous ecological labels (water toxicity of zinc phosphate, heavy metal impurities that may be present in some pigments). Although the concept of high-gloss DTM paint without anticorrosive pigment has been successful in the architectural market, when it comes to industrial or heavy-duty coatings, it appears necessary to consider the use of anticorrosive pigments or additives in the paint formulation. This article describes the effect of anticorrosive additives and pigments with our new binders that provide the benefits of multiphase particle dispersions without their drawbacks using a proprietary "Hydrophobic Dispersion Technology" (HDT).

Polymer Design Considerations

Particle Morphology Today's polymer chemist not only has access to a wide variety of basic building blocks (monomers) and polymerization additives (surfactants, etc.) to work with, but also a continuously growing "toolbox" of processes that allow the design of a specific morphology in the polymer to achieve certain end-use properties 2,3,4. The use of a low-temperature film-forming latex (low Tg) as the sole binder in paint formulations generally results in soft coatings with unacceptably high tackiness. Various approaches have been reported to achieve the combination of low minimum film formation temperature (MFFT) with acceptable mechanical properties in coatings. Soft and hard polymer latex blending has been extensively studied as one of the possible routes to this objective 5,6. In such blends, the soft polymer provides film formation at low ambient temperature, while the hard polymer imparts block resistance and hardness. Although this technique has been successful in reducing the overall binder level required to produce a good film, it still has limitations when it comes to providing an optimal solution for balancing film formation, VOC and film hardness. Another strategy involves the use of multiphase particles synthesized by a multistage emulsion polymerization process. A wide variety of different particle morphologies are known, such as core-shell, reverse core-shell, multilayered, gradient, raspberry, confetti, etc., each with corresponding advantages and disadvantages. They can provide the formulation of solvent-free paints with excellent film-forming ability along with high block resistance, hardness and gloss. The physical and application properties of these dispersions can be related to their morphology 7. However, latex particles containing both soft and hard phases, such as core-shell morphology, are likely to suffer from similar film-formation problems as soft/hard latex blends because the hard phase can be considered a transparent filler. This would effectively reduce the critical pigment volume concentration of a paint formulation to a potentially very low level depending on the concentration of the hard phase in the polymer. A gradient morphology is employed in our HDT platform to best meet the requirements of DTM applications. This ensures that the polymer and consequently the resulting dry coating has consistent hardness/softness throughout the film. Hardness and block resistance are consequently balanced with excellent flexibility. The use of gradient morphology also avoids having heterogeneous hard and soft regions in the dry film, which could have serious consequences on the durability potential of the coating. Adhesion Adhesion is generally the most important of all technological properties of organic coatings, particularly for water-based corrosion-protective coatings. In particular, the ability of a coating to provide good adhesion to the substrate when exposed to high humidity or water is considered critical for the long-term protection provided by the coating. This is nowadays commonly achieved by including monomers in the polymer backbone that promote adhesion. For the HDT platform, monomers containing polar groups with high affinity for oxide films present on metal surfaces have been used, which not only provide excellent adhesion under dry conditions but also preserve this adhesion when the film is wet.

Water Resistance

The dried film of a standard latex contains surfactant remaining from the polymerization process. This surfactant is concentrated in areas where particle interfaces exist prior to particle coalescence. These residues provide a suitable path for water to penetrate the film. The effect can be clearly seen because the film takes on a milky appearance as water is absorbed by it. Dispersions called "surfactant-free" typically rely on the use of reactive or polymerizable surfactants. These contain a reactive group on the hydrophobic segment that can participate in free radical emulsion polymerization reactions such that they can be covalently bonded to the latex surface. With less free surfactant in the coating, the adhesion of the coating, water repellency and water resistance are particularly improved. However, the effective use and incorporation of polymerizable surfactants to prevent the surfactant itself from being embedded inside the polymer particles and to use the optimal amount of expensive raw materials is not easy. In the HDT platform, latex particles are stabilized with anionic groups bound to the polymer. A specific multistage process preferentially distributes these functional monomers on the surface of the particles, thereby optimizing their efficiency and minimizing the need for free surfactant. Additionally, the monomeric composition has been fine-tuned to achieve high polymer hydrophobicity while maintaining excellent yellowing resistance, particularly in low pigment volume concentration formulations. Although the new latexes yield a free film that is not completely impermeable due to their high hydrophobicity, the tendency to allow water permeation has been significantly reduced. Furthermore, there is no water retention or whitening during drying as seen in standard latexes. [caption id="attachment_139029" align="aligncenter"] Figure 2: Barrier properties of HDT binders[/caption]

Effect of Anticorrosive Pigment/Additive on Corrosion Resistance

ISO 12944 8 is the reference for corrosion protection of steel structures by paint systems. While systems in the previous ISO 12944-5:1998 standard consisted predominantly of solvent-based products for C3 or C4 categories, the new version published in 2017 now covers water-based paints (alkyds or acrylics) and systems. In fact, paints based on acrylic resins described in Tables A3 and A4 of ISO 12944-5:1998 were solvent-based because they were grouped with chlorinated rubber or polyvinyl chloride resins, which were only available in solvent-based form. Additionally, a new durability class ("very high") was introduced compared to the previous version and durability of more than 25 years is expected. The purpose of this study is to evaluate the effect of three different anticorrosive pigment chemistries and one liquid anti-corrosion additive on paint performance based on HDT resins and to meet the C3 or C4 anticorrosion categories defined in the ISO 12944 standard. Formulations based on HDT binders were evaluated with changes in nature and corrosion protection level according to the following table. Three levels of corrosion-inhibiting pigment were evaluated in the experimental formulation: 2.5%, 5.0% and 10.0%. Anticorrosive additive #1 was evaluated at only 2.5% of the total formulation.

Test Methods

Cross-Hatch Adhesion Test According to ISO 2409:2013, the purpose is to evaluate the adhesion of the coating film by applying and removing a pressure-sensitive tape over cuts made in the film. Paints were applied at 2 mm thickness on abrasive grit-blasted steel with Sa3 preparation and on Q-Panel at 100 µm Dry Film Thickness (DFT). They were cured for 1 day at 23°C and 50% relative humidity.

Salt Spray

Abrasive grit-blasted steel was coated with two coats of paint on R36 Q-Panel and S36 Q-Panel to reach 200 µm DFT and left to cure for 7 days. Panels were exposed to salt spray for 504 hours according to ISO 9227:2012. Salt spray resistance on Q-panels was evaluated visually while rust grade on abrasive grit-blasted steel was rated according to ISO 4628-3, blister grade was rated according to ISO 4628-2, and tape adhesion on the scribe was performed after 1 hour and 24 hour recovery and rated according to ISO 4628-8.

Results and Discussion Formulation

Paints were prepared according to the procedure performed according to the "HDT experimental formulation" procedure. Anticorrosive paints containing 10% pigment #2 and #3 were unstable and caused the formation of gritty particles. Despite various attempts (changes in the amount or structure of the dispersant), good paint stability could not be achieved and therefore the paints were not tested. Due to the partial water solubility, oil absorption and chemical structure of the anticorrosive pigment, it is quite common to encounter destabilization of the binder's surfactant system, which leads to paint stability problems. As expected, the initial gloss of the paint (Table 3) is affected by the amount of anticorrosive pigment, resulting from a shift in Critical PVC due to the higher oil absorption of anticorrosive pigments compared to titanium dioxide. A ratio of 2.5% for pigments #1 and #2 allows the preservation of high-gloss formulations, while pigment #3 significantly reduces the gloss level.

Performance

All formulations show excellent adhesion to three different steel substrates after only one day of drying. Impact resistance (according to ASTM D2794) and mandrel bend test (according to ASTM D522) were performed on all formulations, and flexibility and adhesion are provided by the binder itself and anticorrosive pigments or additives have limited impact, so all results were good despite changes in CPVC. Corrosion resistance was performed on Q panels and abrasive grit-blasted steel. All paints showed excellent resistance to corrosion. However, some blistering can be observed around the scribe with Q-Panel substrates having formulations containing anticorrosive pigments. Blister density is also related to Q-Panel surface profiles and the S-type appears somewhat more difficult than the R-type. The paint based on anticorrosive additive #1 shows excellent blister resistance regardless of the substrate. Results with abrasive grit-blasted steel are more homogeneous as shown in the table below. Anticorrosive additive #1 provided the best performance in both rust and blister resistance, while pigments #1 and #2 also showed excellent corrosion resistance even at 2.5% concentration, allowing the paints to maintain high gloss levels. At this level, blistering remains acceptable to meet C3 or C4 categories in terms of salt spray testing. To complete the evaluation according to ISO 12944, tests in a condensation chamber according to ISO 6270-1 are being conducted with different formulations. Additionally, salt spray resistance will also be exposed for up to 720 hours. The HDT binder has been proven to achieve excellent corrosion protection on metal substrates. Of course, the formulation of DTM paints remains very sensitive to content selection. Transfer of formulations from one binder to another is sometimes difficult due to possible competition between the functionalities brought by the binder (stabilization system, specific monomers, silanes, adhesion promoters...) and the other components of the formulation (dispersants and wetting agents, anti-gloss rust additives, adhesion promoters...). The selection of corrosion-inhibiting pigment or additive can also have a high impact on paint performance. As we have seen, beyond corrosion resistance, anticorrosive pigments can also affect paint stability, the gloss level of the formulation, the tendency to blister or water leakage from the paint film. Therefore, attention should be paid to the amount of anticorrosive pigment, its chemical composition and compatibility with the binder system.

Conclusion

Although the trend for environmentally friendly paint for metal started with do-it-yourself users, one of the main reasons for the transition from solvent-based to water-based products for industrial applications is the increase in VOC regulations. Binder manufacturers have managed to propose water-based resins (both one and two-component) that can compete with single-component solvent-based resins to meet the requirements of the ISO 12944 standard. With the development of the "Hydrophobic Dispersion Technology" platform, it is now possible to replace one-component solvent-based alkyd paint, reduce VOC and achieve high performance levels. Additionally, the glass transition variation of HDT latexes allows paint manufacturers to select an appropriate binder to meet other requirements that may be needed in their particular application. References 1. NACE International, "International Measures of Prevention, Application, and Economics of Corrosion Technologies Study", (2016) 2. G. Apitz, M. Dimmers, "Novel, multi-phase acrylic emulsions for environmental friendly corrosion protection", Farbe und Lack, (6-2010) 16, (2010) 3. J. Hartig, K. Ragunathan, A.Smith, A.Tuchbreiter, "New Strategies for Improving Film Properties of Zero VOC Coatings", ACS Conference, Indianapolis, 2012 4. A. Overbeek, "Polymer heterogeneity in waterborne coatings", J. Coat. Technol. Res. 23 September, 2009 5. S.T. Eckersley & B.J. Helmer, "Mechanistic considerations of particle size effects on film properties of hard / soft latex blends", J.C.T., 69, No. 864, 97 (1997). 6. A. Fream and S. Magnet, Low VOC, high performance coating formulation using surfactant free latex blends, Paper presented at the 77th Annual Meeting Technical Program of the FSCT, Dallas, Texas., October 20-22, 1999. 7. C.I. Tyre, "Novel multi-phase acrylics for High performance coatings", ACS Conference, Indianapolis, 2012 8. International Organization for Standardization: "ISO 12944, "Paints and Varnishes: Corrosion protection of steel structures by protective paint systems", 2017-2018 Maurille Sécher Global Technical Manager - Coatings Synthomer Christophe Baude EMEA Technical Service Manager - Coatings Synthomer