ADD MONDO MINERALS TO YOUR IDEAS for Low VOC, Heavy Duty Protective Coatings Technical Bulletin 1208 Technical Bulletin 1208 Heavy Duty Protective Coatings 1
CONTENTS INTRODUCTION 2 EXPERIMENTAL 3 MATERIALS 3 FORMULATIONS 4 CORROSION TEST METHODS 6 RESULTS 6 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) 6 SALT SPRAY 8 CONCLUSIONS 10 BIBLIOGRAPHY 12 INTRODUCTION Talc is widely used as an extender in protective coatings due to its excellent barrier properties against corrosive challenge. Talc is a soft mineral with a moderate oil absorption (15 50 g /100 g) and density (2.7 2.8 kg /dm 3 ), while being inert and of low cost. Talcs of different crystal structure affect the film strength of coatings diversely. The most common talcs are in platelet form (macro-crystalline talc) giving good barrier properties and reinforcement of the coating film, but talc particles can also be blocky (micro-crystalline talc). The platy talcs are preferred for anti-corrosion performance where they presumably act in a manner similar to Mica or MIO. The extender effect comes by means of the flat, platy particles, which overlap and complicate the path water must travel to get through the coating. Based on this knowledge, talc is expected positively to influence water absorption as well as the formation of conductive pathways and therefore is expected to improve the barrier properties of various coatings. There is considerable variation in the composition of commercial talc grades owing to their diverse sources. The talc can be pure, but more often includes other minerals such as chlorite (Mg-Al-Fe-silicate) and / or carbonate based minerals. In this study the barrier and anti-corrosion performance of commercial talc grades with different composition and particle form are evaluated. The barrier properties were determined by EIS (Electrochemical Impedance Spectroscopy) measurements. Coating resistance by EIS was compared with the results of salt spray tests. An explanation of the coating formulations and the results of these accelerated tests are also discussed in this article. 2 Technical Bulletin 1208 Heavy Duty Protective Coatings
EXPERIMENTAL MATERIALS The anti-corrosion performance of 4 different talcs with different composition and morphology were tested in epoxy coatings. The barrier property of the talcs was tested by EIS (Electrochemical Impedance Spectroscopy) and via a salt spray test. The talc products differed in composition, particle form and production technique used. The macro-crystalline talcs have very platy particles, while the micro-crystalline talc (small crystal sized talc) has a blocky particle form. Magnesite (MgCO 3 ) is a very common by-mineral in talc and one talc tested contained about 15 % magnesite. High oil absorption (OA) is one problem of talc which limits its high loading into the paint formulations. To lower the OA the fines content is normally reduced by extracting the finest particles from the talc. This process is commonly called de-dusting. The properties of talc products involved in this study are shown in table 1. PROPERTY UNIT M40 M65 TALC- MAGNESITE MICRO- CRYSTALLINE TALC LOSS ON IGNITION w% 6.0 6.1 12.4 5.4 HCI SOLUBLES w% 3.8 3.4 14.7 5.9 ISO BRIGHTNESS % 77.6 76.3 88.8 72.8 DIN WHITENESS % 79.2 78.3 90.7 76.4 CIE VALUES, L* 91.32 90.88 96.27 89.95 a* - 0.40-0.37-0.05-0.45 b* 1.57 1.80 1.40 3.10 HEGMAN FINENESS µm 120 160 50 100 OIL ABSORPTION g/100 g 28 22 26 17 SPECIFIC SURFACE AREA (BET) m 2 / g 1.9 2.2 6.4 7.3 PSD BY SEDIGRAPH < 50 µm w% 99 98 93 < 40 µm w% 96 92 100 87 < 30 µm w% 86 82 99 67 < 20 µm w% 65 64 89 34 < 10 µm w% 25 35 63 17 < 5 µm w% 10 16 41 14 < 2 µm w% 9 7 22 9 < 1 µm w% 6 3 17 6 D98 µm 44 50 29 58 D50 µm 15.6 14.6 6.7 24.7 Table 1: Properties of talc products tested As polymer binder, a solution of semi-solid epoxy resin in xylene (Araldite GZ 290x90) cured with a polyamidoamine adduct (Aradur 450), both supplied by Hunstaman Materials, were used for obtaining films on steel panels. Disperbyk 180 from BYK Chemie was used as dispersing agent. Titanium dioxide (Kronos 2315) was supplied by Kronos International, Inc and micronised iron oxide red (Bayferrox 318M) was supplied by Lanxess. Technical Bulletin 1208 Heavy Duty Protective Coatings 3
FORMULATIONS Pigment Volume Concentration (PVC) is one key parameter for describing paints. The critical pigment volume concentration (CPVC) above which many coating properties change abruptly, has to be taken into account. CPVC can be calculated from oil absorption [1 2]. A relation between the PVC and the CPVC is defined as reduced pigment volume concentration (Λ). At Λ < 1 (below the CPVC), a dry coating film is a continuous coating, a composite consisting of pigment particles randomly embedded in a continuously connected matrix of polymer, whereas above the CPVC (Λ 1), there are void structures in the film due to insufficient polymer, but the pigment particles can still be thought of as being continuously connected. Above the CPVC there is not enough resin to cover the pigment surface and an important amount of the polymer is absorbed in the pigment [3]. When using different oil absorption talcs, the CPVC changes. If the PVC is kept constant, then by using higher oil absorption talcs the coating is brought nearer to the CPVC and thus gives different barrier properties. As shown in Table 1, different talc products used in this research show a difference in oil absorption which influences the coating properties when the PVC remains the same. In order to check the contribution of each talc properly, a simple coating formulation was set up and the same reduced PVC (Λ) of each coating was maintained at 0,6. Also, some coatings were formulated at the same PVC level for comparison. Tables 2.1 and 2.2 contain specific information about each coating studied. M40 M65 TALC- MAGNESITE MICRO- CRYSTALLINE TALC INGREDIENTS wt% wt% wt% wt% ARALDITE GZ290X90 31.33 28.99 30.58 26.72 DISPERBYK 180 0.64 0.63 0.63 0.64 XYLENE 9.94 9.82 9.91 9.69 KRONOS 2315 6.92 6.95 6.95 6.99 BAYFERROX 318 M 0.87 0.87 0.87 0.87 PURE, PLATY TALC WITH DE-DUSTED PSD 40.20 PURE, PLATY TALC WITH COARSE, STANDARD PSD 43.40 MAGNESITE RICH TALC NORMAL 41.22 PURE, BLOCKY TALC WITH DEDUSTED PSD ARADUR 450 10.09 9.34 9.85 8.61 46.47 TOTAL 100.0 100.0 100.0 100.0 PROPERTIES PVC 32.48 35.75 33.51 39.07 CPVC 54.11 59.55 55.80 65.06 Λ 0.60 0.60 0.60 0.60 % SOLIDS (wt) 86.55 86.91 86.67 87.26 % SOLIDS (vol) 75.00 75.00 75.00 75.00 MIX VISCOSITY AT SHEAR RATE 50 RPM, mpas 1950 1700 1680 1015 Table 2.1: Coating formulations 4 Technical Bulletin 1208 Heavy Duty Protective Coatings
M40 M65 MICRO- CRYSTALLINE TALC INGREDIENTS wt% wt% wt% ARALDITE GZ290X90 26.06 22.96 23.1 DISPERBYK 180 0.64 0.65 0.65 XYLENE 9.65 9.48 9.48 KRONOS 2315 7.01 7.03 7.02 BAYFERROX 318 M 0.88 0.88 0.88 PURE, PLATY TALC WITH DE-DUSTED PSD 47.37 PURE, PLATY TALC WITH COARSE, STANDARD PSD MAGNESITE RICH TALC NORMAL PURE, BLOCKY TALC WITH DEDUSTED PSD 51.61 ARADUR 450 8.39 7.39 7.44 51.42 TOTAL 100.0 100.0 100.0 PROPERTIES PVC 40.10 45.05 44.81 CPVC 54.12 59.63 65.14 Λ 0.74 0.76 0.69 % SOLIDS (wt) 87.37 87.85 87.83 % SOLIDS (vol) 75.00 75.00 75.00 MIX VISCOSITY AT SHEAR RATE 50 RPM, mpas 3730 4880 1650 Table 2.2: Coating formulations In order better to see the effect of the talcs, the coating formulations were kept simple in this case and no other anti-corrosion pigment was added to the formulation. The coating formulation step in this project was performed by using a high throughput system, which automatically prepares formulations in parallel using reactors equipped with dissolver blades. Raw materials can be automatically added while processing, so the normal coating processing can be applied. When the optimal formulation was obtained, these were up scaled and sprayed for further testing. The coatings were sprayed on Sa 2,5 grit blasted steel panels with a dry film thickness of around 150 µm. The coatings were allowed to cure for 1 week at room temperature, before testing. Technical Bulletin 1208 Heavy Duty Protective Coatings 5
CORROSION TEST METHODS Accelerated testing of coatings is known to formulators to speed up the performance testing before introducing the coating into the market but it is always under discussion which accelerated test is the best to represent real life coating performance. For this study, the accelerated tests Electrochemical Impendance Spectroscopy (EIS) and salt spray were used to determine the anti-corrosion performance of the coatings. EIS is an electrochemical measuring method that quantifies the protective behaviour of coatings. According to many authors [4 7] even a relatively short period of testing with EIS provides reliable data for predicting long-term behaviour. The salt spray cabinet test were performed according to ISO 9227. In this cabinet, a 5 % NaCl solution is sprayed over the panels at a temperature of 35 ± 1 C. To classify the protective coatings, salt spray testing was done for C5-M climate, part of ISO 12944, which means 1440 h of testing. RESULTS ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) EIS is an electrochemical measuring method that quantifies the protective behaviour of coatings. In this study EIS results were fitted to the most probable impedance equivalent circuit (MPEC) [9]. The test panels were immersed in standard electrolyte and EIS-parameters were measured at different moment after start of test: initially, after 24 hours, after 7 days and after 21 days. The parameters measured were the following: R c,t = Resistance of coating to ion transportation trough coating at time moment t Y 0 = Capacitance at start of measurement; lower the number better the performance Ф t = Water uptake at the time moment t n = number (0 1) that describes the deviation from ideal coating, lower the value worse is the coating (n = 1 is ideal) For the evaluation of EIS results the following interpretation rules are commonly used [12 14]: GOOD STANDARD BAD R c > 108 Ω cm 2 Y 0 < 1 10 10 s n Ω n > 0.95 Ф 24 h < 0.1 Table 3: Interpretation of EIS-measurements R c = 10 7 10 8 Ω cm 2 Y 0 = 1 3 10 10 s n Ω n = 0.90 0.95 Ф 24 h = 0.1 0.2 R c < 10 7 Ω cm 2 Y 0 > 3 10 10 s n Ω n < 0.9 Ф 24 h > 0.2 The EIS-results for different coatings are presented in Tables 4.1 and 4.2. 6 Technical Bulletin 1208 Heavy Duty Protective Coatings
UNIT M65 M40 TALC- MAGNESITE MICRO- CRYSTALLINE TALC Λ, Reduced PVC from CPVC 0.60 0.60 0.60 0.60 Actual PVC v% 32.5 32.5 33.5 39.1 R e, Electorolyte resistance (Ω cm 2 ) 1 10 2 1 10 2 1 10 2 1 10 2 R c, 504 h, Resistance of coating to ion transportation after 504 h testing time Y 0, Capasitance at the starts of measurement n, number that discribes the deviation from ideal coating, lower the value worse is the coating, n = 1 is ideal. (Ω cm 2 ) 1.08 10 8 1.13 10 8 4.57 10 7 4.51 10 7 (s n Ω 1 ) 1.49 10 10 1.61 10 10 2.99 10 10 3.89 10 10 0.940 0.929 0.895 0.888 Ф 24 h, Volume fraction of water absorbed by coating after 24 h testing time. 24 h 0.13 0.14 0.19 0.26 Summary of EIS + + + / Table 4.1: Electrical parameters for different talc coatings immersed in S solution obtained through fitting EIS data UNIT M40 M65 MICRO- CRYSTALLINE TALC Λ, Reduced PVC from CPVC 0.74 0.76 0.69 Actual PVC v% 40.1 45.1 44.8 R e, Electorolyte resistance (Ω cm 2 ) 1 10 2 1 10 2 1 10 2 R c, 504 h, Resistance of coating to ion transportation after 504 h testing time (Ω cm 2 ) 1.09 10 8 1.24 10 8 5.24 10 7 Y 0, Capasitance at the starts of measurement (s n Ω 1 ) 1.34 10 10 1.28 10 10 3.09 10 10 n, number that discribes the deviation from ideal coating, lower the value worse is the coating, n = 1 is ideal. 0.939 0.932 0.877 Ф 24 h, Volume fraction of water absorbed by coating after 24 h testing time. 24 h 0.11 0.09 0.24 Summary of EIS + + + + Table 4.2: Electrical parameters for different talc coatings immersed in S solution obtained through fitting EIS data Good Standard Bad Looking at the absolute values of the EIS measurements, the following applies: Overall an increase of talc concentration (PVC below CPVC) improves the corrosion protective performance due to a resistance increase. The talc with the best overall performance in this EIS test is M40 (pure, platy talc with de-dusted PSD) followed by M65 (pure, platy talc with coarse, standard PSD). Talc-Magnesite and Micro-crystalline talc performed with a standard resistance but with a high water absorption, high Y 0 and low n-value. It is shown with this EIS test that there are significant differences between the coatings. A change in PVC often directly influences corrosion protective performance (barrier properties). EIS is a good way to detect differences in corrosion performance even with similar coating ingredients. The effect of PVC on corrosion performance can be detected using EIS long before visual signs show. The optimal PVC from a corrosion protection point of view can further be determined by using EIS. The coatings using pure, platy talcs, especially the de-dusted versions, are suitable for the more severe conditions like immersion. Micro-crystalline (blocky) or magnesiterich talcs are limited in their applications for immersion applications, because of the higher water uptake. Based upon this study, it is seen that blocky and magnesite-rich talcs are of lower quality from a corrosion protection point of view. Technical Bulletin 1208 Heavy Duty Protective Coatings 7
SALT SPRAY In the salt spray test, one scratch down to the steel substrate was made through the coating and the sample was placed for 1440 hours in the salt spray. Classification of the salt spray test panels for C5-M climate, part of ISO 12944, focussed on creep and blisters. TALC Λ PVC MEAN COATING THICKNESS (µm) BLISTERS (ISO 4628-2) CREEP (mm) (ISO 4628-3) M40 0.60 32.5 108 ± 8 0 (S0) 2.4 M65 0.60 35.8 127 ± 5 0 (S0) 2.4 TALC-MAGNESITE 0.60 33.5 94 ± 14 0 (S0) 2.3 MICRO-CRYSTALLINE TALC 0.60 39.1 131 ± 26 0 (S0) 2.6 M40 0.74 40.1 118 ± 4 0 (S0) 2.3 TALC-MAGNESITE 0.76 45.1 159 ± 15 0 (S0) 3.3 MICRO-CRYSTALLINE TALC 0.69 44.8 176 ± 20 0 (S0) 3.1 Table 5: Salt spray results Good Standard The first number indicated under Blisters is the density of the blisters, ranging from low 1 to very high 5 and the number between brackets is the size of the blisters, ranging from small S1 to large S5. Creep is measured as the mm of rust from the 1 mm scratch that had been made in the coating panel. No blisters and a creep value of < 1 mm is excellent. Normally, epoxy coatings are applied in at least 2 layers of 75 100 µm and can achieve these requirements after 1000 hours. In this case, to see the differences between the coatings much better, only 1 layer of around 150 µm was applied. The ranking is thus now based on the relative performance of the talcs. Between the tested talcs, there was no difference observed in blistering via this salt spray test. The talcs do show a slight difference in creep results, but only the blocky and one pure platy talc at PVC 45 show a slightly higher value. The obtained higher creep values are with pure platy talc 3 only at Λ 0.76 (PVC 45) and blocky de-dusted talc only at Λ 0.69. Via this salt spray test, it is thus difficult to see the differences between the talcs, while EIS is more obvious. 8 Technical Bulletin 1208 Heavy Duty Protective Coatings
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CONCLUSIONS EIS measurements give faster and more reliable results than salt spray regarding corrosion protection. Coating developments can significantly be improved and accelerated by combining high throughput screening and EIS testing. This gives at least three times faster results compared to the manual coating preparation and testing performance via salt spray. In this study, the talc grades with different composition, particle shape and particle size distribution were ranked by their anti-corrosion performance. Table 6 shows that pure, platy talcs ( M40 and M65) give better barrier properties (mainly EIS results) compared to impure or blocky talcs (Talc-Magnesite & Micro-crystalline talc). The corrosion performance with only pure, platy talcs without any anti-corrosion pigments already give acceptable results in corrosion protection for the C5-M climate. The blocky or micro-crystalline talc (Talc 5) is worse in that respect. Pure, platy, de-dusted talc ( M40) is overall a very suitable extender for corrosion protective coatings under severe conditions like C5-M. The optimum PVC-level with this talc is 40 v% in this formulation. TALC Λ / PVC EIS SALTSPRAY SUMMARY OF RESULTS RANKING M40 0.74 / 40.0 + + + + + 1 M65 0.60 / 35.8 + + + 2 M40 0.60 / 32.5 + + + 3 M65 0.76 / 45.0 + + + / + 4 TALC-MAGNESITE 0.60 / 33.5 + / + + / 5 MICRO-CRYSTALLINE TALC 0.60 / 39.1 + 6 MICRO-CRYSTALLINE TALC 0.69 / 44.8 + / 7 Table 6: Overall results Good Standard Bad 10 Technical Bulletin 1208 Heavy Duty Protective Coatings
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BIBLIOGRAPHY [1] Tiarks et al. Formulation effects on the distribution of pigment particles in paints. Progress in Organic Coatings, 2003, 48, 140 152. DOI:10.1016 / S0300-9440(03)00095-X [2] K. W. Asbeck and M. Van Loo. Critical pigment volume relationships. Industrial and Engineering Chemical Research. 1949, 41, 1470 1475. DOI: 10.1021 / ie50475a042 [3] Rodriguez et al. The influence of pigment volume concentration (PVC) on the properties of an epoxy coating Part I. Progress in Organic Coatings, 2004, 50, 62 67. DOI: 10.1016 / j.porgcoat. 2003.10.013 [4] W. M. Bos, Prediction of coating durability, Early detection using electrochemical methods, PhD Thesis, TU Delft, The Netherlands, 2008. [5] M. Kendig, F. Mansfeld and S. Tsai, Determination of the long term corrosion behaviour of coated steel with AC impedance measurements, Corrosion Science, 1983, 23, 317. [6] J. N,. Murray, H. P. Hack, Testing organic architectural coatings in ASTM synthetic seawater immersion condition using EIS, Corrosion, 1992, 48, 671 685. [7] A. Amirudin, D. Thierry, Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals, Progress in organic coatings, 1995, 26, 1 28. [8] M. Kendig, J. Scully, Basic Aspect of electrochemical impedance application for the life prediction of organic coatings on metals, Corrosion, Nace, January 1990 [9] W. M. Bos, Prediction of coating durability, Early detection using electrochemical methods, PhD Thesis TU Delft, The Netherlands, 2008. [10] T. Nguyen, J. B. Hubbard, J. M. Pommersheim, Unified model for degradation of organic coatings on steel in neutral electrolyte, Nace (1996). [12] G. P. Bierwagen, L. He., J. Li, L. Ellingson, D. E. Tallman. Studies of new accelerated evaluation method for coating corrosion resistance thermal cyclic testing. Progress in organic Coatings 39 (2000) 67 78. [13] R. C. Bacon, J. J. Smith, F. M. Rugg, Electrolytic resistance in evaluating protective merit of coatings on metals, Industrial and engineering chemistry P 161 167, 1948. [14] G. D. Davis, L. A. Krebs, C. M. Dacres, Coating evaluation and validation of accelerated test conditions using an in-situ corrosion sensor, Journal of coating Technology Vol 74 P. 19 74, 2002. [15] D. M. Brasher, A. H. Kingsbury, Electrical measurements in the study of immersed paint coatings on metal. I. Comparison between capacitance and gravimetric methods of estimating water-uptake. Journal of Applied Chemistry, 1954, 5, 62. [16] Nguyen, VN, Perrin, FX, Vernet, JL, Water Permeability of Organic / Inorganic Hybrid Coatings Prepared by Sol-Gel Method: A Comparison Between Gravimetric and Capacitance Measurements and Evaluation of Non-Fickian Sorption Models. Corros. Sci., 47 (2) 397 412 (2005) [17] Stafford, OA, Hinderliter, BR, Croll, SQ, Electrochemical Impedance Spectroscopy Response of Water Uptake in Organic Coating by Finite Element Methods. Electrochim. Acta, 52 1339 1348 (2006) [18] J. M. Hu, J. Q. Zhang, C. N. Cao. Determination of water uptake and diffusion of Cl ion in epoxy primer on aluminum alloys in NaCl solution by electrochemical impedance Spectroscopy. Progress in Organic Coatings 46 (2003) 273 279 [19] F. Rezaei, F. Sharif, A. A. Sarabi, S. M. Kasiriha, Evaluating water transport through high solid polyurethane coating using the EIS method. J. Coat. Technol. Res., 7 (2) 209 217, 2010 DOI 10.1007 / s11998-009-9173-5 [11] J. E. O. Mayne, The mechanism of the inhibition of the corrosion of iron and steel by means of paint, Official digest, 1952, 24, 127 136. MONDO MINERALS B.V. Kajuitweg 8 1041 AR Amsterdam The Netherlands E-mail: info@mondominerals.com www.mondominerals.com The information contained in this Technical Bulletin relates only to the specific tests designated herein and does not relate to the use of our products in combination with any other material or in any process. The information provided herein is based on technical data that Mondo Minerals believes to be reliable, however Mondo Minerals makes no representation or warranty as to the completeness or accuracy thereof and Mondo Minerals assumes no liability resulting from its use for any claims, losses, or damages of any third party. Recipients using this information must exercise their own judgement as to the appropriateness of its use, and it is the user s responsibility to assess the materials suitability (including safety) for a particular purpose prior to such use. V1-032017