The Dispersant Effect, Corrosion Control with Aqueous DTM Coatings

Transcription

1 The Dispersant Effect, Corrosion Control with Aqueous DTM Coatings David Tarjan, Simonida Grubjesic, Anthony Toussaint, Anthony Gichuhi, ICL Advanced Additives INTRODUCTION The coatings industry has long realized the toxic effects of inhibitive pigments based on red lead. The industry abandoned the use of lead pigments in the early 1970 s in most geographic regions around the world. Formulators have overcome the use of toxic materials with alternatives such as zinc phosphate, calcium, aluminum, strontium, phosphates, polyphosphates, borates and molybdates. In the absence of lead, the industry continued to use chromates: zinc yellow, zinc chrome, basic zinc chromate, and strontium chromate. Chromate salts provide the best anodic and cathodic passivation of all active known inhibitors. Their toxicity and environmental impact has however put them on a growing list of banned materials. Their high efficiency and low cost, remains a challenge when replacing chromates in certain industry segments such as aerospace and coil coatings, where the demand for high performance at any cost and exposures outweigh green initiatives. The economics behind tackling corrosion is an ever increasing concern. Battelle Laboratories (Columbus) and the National Bureau of Standards (NBS) in 1975 showed corrosion to equal approximately 4.2% of the gross national product (GNP) for the United States (1,2). A study conducted in 1998 by Federal Highway Administration (FHWA) and led by CC Technologies in collaboration with NACE International showed that corrosion accounted for 3.1% of the GNP. The statistics present a need for the development of new products and formulation optimization for the mitigation of corrosion products. Anticorrosive pigments come in three major categories: inhibitive, sacrificial, and barrier type. Coatings utilizing inhibitive pigments release a soluble species, such as molybdate or phosphates, from the pigment into any water that penetrates the coating. (3) These species migrate to the metal surface, where they inhibit corrosion by facilitating the growth of protective surface layers. The schematic shown in figure 1 represents the most common inhibitive mechanism of these pigments. H 2O PASSIVATION O2 NaCl aq O 2 O2 O2 PAINT FILM O 2 PASSIVE OXIDE LAYER O O O O O O O O O O O Figure 1: Passivation Mechanism for Anticorrosive Pigments SUBSTRATE

2 EU REACH regulation which took effect June 1, 2007 added zinc phosphate to the list of heavy metal based inhibitors now deemed as aquatic toxins which must carry the dead fish and dead tree warning label. See Table 1. Table 1: Toxicity Profile of Key Anticorrosive Pigments CMR (Carcinogenic, Mutagenic, Reprotoxic) Environmental Toxicity (Risk Phrases) Red Lead Zinc Chromate Strontium Chromate Cat. 1 Cat. 1 Cat.1A, 1B No N, R 5053 Very Toxic to aquatic organism N, R 5053 Very Toxic to aquatic organism N, R 5053 Very Toxic to aquatic organism Zinc Phosphate N, R 5053 Very Toxic to aquatic organism Green initiatives have created an entry point for aqueous based coatings in the general industrial marketplace traditionally dominated by solvent based coatings. The demand for high performance remains and aqueous films are subjected to the same specifications as their solvent based counterparts, regardless of their hydrophilic nature. The concept of matching performance expectations with aqueous coatings requires the optimization of the entire formulation. Managing the solubility of the corrosion inhibitor, selection of inert pigments, resin selection and additive selection play a critical role in the performance of the coating. The pigmentation of the film including the corrosion inhibitor requires a dispersant chemistry that has no negative impact on the film performance characteristics. The role of the dispersant is explored in an aqueous acrylic formulation. AQUEOUS STYRENATED ACRYLIC & CORROSION CONTROL Formulators have long since developed aqueous based coatings for corrosion control, utilizing standard dispersant technologies with limited success on controlling corrosion. The hydrophilic nature of the dispersant although efficient at dispersing inorganic pigments such as corrosion inhibitors can have a negative impact on the formulations performance when subjected to accelerated testing in accordance with ASTM B117 Static Salt Fog. Typically the mode of failure is osmotic blistering within the first 2448 hours of exposure. In this critical time period the resin and other additives are responsible for providing enough barrier properties to allow the corrosion inhibitor to begin to solubilize and passivate the metal substrate. If the integrity of the film is compromised to the extent osmotic blistering occurs, the corrosion inhibitor cannot repair the damaged sites and offers little protection. Three dispersants were used for the experiment. Their chemistries and pigment affinic groups are outlined in Table 2. Table 2: Dispersant Chemistries and Pigment Affinic Groups Dispersant ID Composition Affinic Groups Mechanism of Dispersing Loading on Inorganics Recommended Actual Loading for Testing Loading on total Formula Weight Product A Polyacrylate Anionic Deflocculating 2030% 19.4% 5.2% Product B Polyalkoxylate Neutralized Deflocculating 2030% 19.4% 5.2% Product C Carboxylate Neutralized Strongly Controlled Flocculating 510% 19.4% 5.2%

3 Product A represents the incumbent composition used in a variety of general and light industrial aqueous primers and direct to metal coating formulations. Polyacrylate chemistry lends itself as a highly effective dispersant with a large range of pigment options independent of surface treatments. Product B represents a newer technology combining acrylate based dispersant technology with alkoxylate chemistry and neutralized affinic groups, however it offers the same mechanism of deflocculating that Product A relies upon. Although Products A and B have proven to be effective at dispersing inorganic pigments their compositions allow for a detraction in overall film performance. Product C utilizes Carboxylate chemistry for controlled flocculation with neutralized affinic groups at a lower recommended loading level. The loading levels were identical to maintain the integrity of the side by side comparison. Although Product C was in excess of the recommended loading level it showcased improved performance over Products A and B exhibiting less blistering at the scribe. AntiCorrosive properties of any formulation are system dependent thus changes to the formulation will often lead to differences in the finished product. The formulation used for testing is identified in Table 3. The formulation does not contain a rheology modifier. It simply utilizes the viscosity and molecular weight of the styrenated acrylic resin and the antisettling behavior of the organic clay to control the overall viscosity. Table 3: Styrenated Acrylic Formulation RAW MATERIALS WT% Total Pigment 26.8% Water 5.1 TiO Organo Clay 0.1 Ca Carbonate 11.0 Dispersant 5.2 Sr/Zn Phosphosilicate 4.9 Defoamer 1.1 TiO Dispersants Products A,B,C Ca Carbonate 11.0 % on IO Pigment 19.4% Strontium Zinc 4.9 % on TFW 5.2 Phosphosilicate Styrenated Acrylic Styrenated Acrylic 37.5% Vol Styrenated Acrylic 50.5 Solids (Premix & Add) Tg 40 C / MFFT 48 C Plasticizer 2.7 ph Water 1.2 DPnB 4.9 DPnB Solubility %@25 C Solvent in Water 4.0% (Slowly Add) Water in Solvent 10.4% Amino Carboxylate 1.9 Sodium Nitrite 0.4 Plasticizer Solubility%@25 C Total Weight% 100 Neg. / Boiling@760mHg 347 C Critical aspects of the raw materials in the formulation listed to the right are the Tg of the resin at 40 C and the coalescence DPnB (Dipropylene glycol nbutyl ether) having a lower solubility compared to commonly formulated Butyl Cellosolve (Ethylene glycol monobutyl ether). Butyl Cellosolve is 100% soluble in water;

4 DPnB is less soluble. The combination of the resin and coalescence provides barrier properties against electrolytic attack during exposure in accelerated testing. Aqueous formulations are hydrophilic in liquid form, but require hydrophobic characteristics once cured. Selection of raw materials to deliver a properly coalesced film with hydrophobic barrier properties is crucial in providing early moisture protection during the initial 2448 hours allowing longer term inorganic corrosion inhibitors to solubilize and passivate for extended protection. Products A, B and C play a key role in providing hydrophobic barriers based on their composition and loading levels. Carboxylate chemistry exhibits a strong affinity for metal based cations and thus offers enhanced barrier properties over the Polyalkoxylate and Polyacrylate based products. While the impact of the counter ion to these dispersants cannot be understated, the focus of this study was on the polymeric portion of the dispersant. Products A, B and C were formulated using and applied using a draw down technique over Cold Roll Steel Matte Finish and subjected to ASTM B117 Static Salt Fog Testing. The results in Figure 2 explain how there is a difference in performance by simply changing the composition of the dispersant. The Cold Roll Steel substrate was prepared by solvent wipe using standard Methyl Ethyl Ketone solution. Figure 2. ASTM B117 Results 216Hrs. of Exposure / Applied to CRS at 22.5mils dft Below each of the test coupons the amine and acid values of Products A, B and C are listed. As the amine and acid values rise from left to right the level of corrosion performance also increases. The field areas of the coupons showcase the lack of resistance against ion penetration /electrolytic attack resulting in osmotic blistering. Reproducibility of the results derived from ASTM B117 is challenging. The test method consists of several variables that are not easily controlled through good lab practices. Corrosion being an

5 electrochemical mechanism allows for analytical analysis utilizing Electrochemical Impedance Spectroscopy (EIS). EIS techniques measure the impedance of a given film applied over a metal substrate over a specified frequency sweep. Graphical analysis compares the impedance values of all three formulations to determine the difference in electrochemical behavior. Figures 3 and 4 are Bode plots of the three formulations at timed intervals. Figure 3 is at 6.5 hours of immersion followed by 24 hour immersion data found in Figure 4. Figure 3. EIS Bode Plot 6.5 Hrs. Immersion The top pink line represents Product C followed by the red line for Product B and black line for Product A. The test coupons did not undergo polarization prior to testing thus the impedance values of the bare metal are included and taken as part of the impedance values of the films. The impedance values are taken from the left side of the plotted lines at lower frequencies. The slope of the line is indicative of the electrochemical behavior of the tested film. As early as 6.5 hours of immersion Product C is exhibiting a capacitance behavior with a lower slope compared to Products B and C exhibiting a traditional resistance behavior. There is no significant difference in impedance values between Products A and B. At higher frequencies the responses will converge. The difference in impedance values are attributed to the composition of the dispersants.

6 Figure 4. EIS Bode Plot 24 Hrs. Immersion Figure 4 is a Bode plot after 24 hours of immersion. At this point the difference in impedance values grows by a factor of ten. Product C is maintaining a capacitance behavior while holding steady at the 10kohm range compared to both Product A and B dropping close to the 1.0kohm range. The level of ion penetration in the form of electrolytic attack can be calculated using electrochemical techniques. By modeling the capacitance at 10 KHz, the gravimetric water uptake can be measured as a function of time and the critical period which for waterbased coatings corresponds to 2448 hours salt spray, can be pinpointed using the Brasher and Kingsbury method. The values for Product A were not tabulated considering the overwhelming lack of performance when exposed in accordance with ASTM B 117. Although Product B out performed Product A in accelerated testing the electrochemical behavior is similar and the increased acid value aids in the protection of blistering as shown in Figure 2 above.

7 Log(Cp/Co)/log(80) 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 Water Uptake of WB Acrylic Product B Critical Period Product C Time (hours) Figure 5. Water Uptake at 10 khz mil dft Figure 5 examines the difference in water uptake at 10 khz for the coatings formulated with Products B and C. The red line is Product C and the blue Product B with the arrow indicating the critical time period where the barrier properties and hydrophobicity of the raw materials in the formulation play a crucial role in allowing the corrosion inhibitor time to provide the long term protection of the formulation. The large gap between Product B and C is shown in the form of osmotic blistering in the test coupons subjected to ASTM B117 in Figure 2 above. Log(Cp/Co)/log(80) 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 Water Uptake of WB Acrylic Time (hours) Figure 6. Water Uptake at 10 khz mil dft Figure 6 is an extended evaluation, and shows that after 100 hours Product C approaches a similar amount of water penetration but still has not matched that of Product B.

8 The viscosity was evaluated using a rheometer and evaluating the differences with regards to sheer stress vs. sheer rate. The static viscosity measurements taken by Krebs Units were in line with one another. All three formulations utilizing Products A, B and C dispersants void of a rheology modifier measured between 6872 Ku s. Static viscosity measurements do not allow for an evaluation of the behavior of the dispersant chemistries, thus dynamic values using a cone plate with oscillation were conducted. Product C Product B Product A Figure 7. Sheer Stress vs. Sheer Rate 050 1/s The graph shows the impact of dispersant selection on the rheological flow curves of the formulations. Formulations dispersed with Product A and Product B appear to be more thixotropic than that dispersed with Product C. This suggests that Product C is able to give a better overall dispersion. The small graph in Figure 7 is sheer stress vs. sheer rate taken over a larger area at higher sheer less effect are noticed. To further analyze the impact of the dispersant on rheology, analyzing the low shear data, we can see that the coating dispersed with Product A has the highest yield point, followed by that containing Product B then Product C. A higher yield point is indicative of more structure indicating less effective pigment dispersion.

9 Product C Product B Product A Figure 8. Viscosity vs. Sheer Rate /s The curve shows the impact of the dispersant on the viscosity profile of the coating formulation. Product C is more efficient at dispersing the pigments resulting in a lower viscosity at low shear rates. Product B does not disperse the pigments as well resulting in a higher viscosity at low shear. The impact of dispersant is less pronounced at higher shear rates. CONCLUSION An account of all the data indicates selection of dispersants based on composition is critical in the performance of aqueous direct to metal coatings. As with all organic coatings, formulation adjustments and order of addition manipulation will have an impact on the overall performace. Carboxylate chemistry has proven to have a positive influence on corrosion resistance while maintaining dispersing efficiency compared to the Polyacrylate and Polyalkoxylate chemistries. Styrenated Acrylics are a proven resin system for direct to metal aqueous coatings and the Strontium Zinc Phosphosilicate a proven anticorrosive inorganic pigment. However, the data exhibits that their effectiveness may not be optimized in the absence of the right dispersant composition. Future studies will evaluate the impact rheological modification plays on corrosion resistance with the goal of fully optimizing aqueous direct to metal formulations. REFERENCES 1. Economic Effects of Metallic Corrosion in the United States, NBS Special Publication 5111, SD Stock No. SN , (1978).

10 2. Economic Effects of Metallic Corrosion in the United States, Appendix B, NBS Special Publication 5112, SD Stock No. SN , (1978). 3. Forsgren, A. Corrosion Control Through Organic Coatings, pp. 27. (2006)