Solventborne, 2-pack (2K) epoxy-amine coating systems have been commonly used to formulate high-performance metal primers. Crosslinkable at ambient temperatures, they provide excellent adhesion as well as corrosion protection to many metallic substrates. However, due to new, lowered VOC limits enacted in certain areas as well as other proposed regulation changes in North America and Europe, there is a need to reduce VOC by shifting to waterborne 2K epoxy systems.
Unfortunately, existing waterborne 2K epoxy primers are generally regarded as inferior in terms of corrosion and humidity resistance, when compared to their solventborne counterparts. In addition, it is believed that many waterborne systems require the use of inorganic corrosion inhibitive pigments, such as zinc phosphates, in order to provide adequate corrosion protection. The possible drawbacks of using anti-corrosion pigments include higher formulation cost, difficulties in pigment dispersion, poor formula stability, and potential environmental concerns. For example, primers containing between 2.5% to 25% of zinc phosphate are classified as environmentally hazardous in Europe in accordance with Preparation Directive 1999/45/EC.
It was found that a novel, internally emulsified and flexibilized waterborne epoxy resin dispersion, Epoxy 386, could be used to formulate low-VOC (~100 g/L and less) 2K primers, which are free of corrosion-inhibitive pigments, but with anti-corrosion performance comparable to a commercially available solventborne 2K epoxy system, while out-performing another commercially available waterborne 2K epoxy system by a significant margin.
Furthermore, this epoxy resin is shear stable, which enables formulators the flexibility of grinding pigments, such as titanium dioxide and barium sulfate, directly in the epoxy portion using high-shear dispersers or bead mills. Since the hardener resin generally represents the smaller component, direct grinding inside the epoxy portion means more liquid binder volume is available for processing. This leads to other formulation possibilities including improved ease of manufacturing and higher pigment-to-binder (P/B) ratio for cost reduction.
Moreover, the flexibility of this epoxy resin provides an added benefit of enhancing the adhesive properties of a coating. It was previously shown1 that while the flexibilities of both waterborne and solventborne epoxy primers could decline upon aging for 7 days at elevated temperature (50 °C), only waterborne primers based on Epoxy 386 maintained adequate flexibility after aging compared to two other primers based on non-flexibilized epoxy resins.
ExperimentalMaterial and Formulation
Two commercially available 2K epoxy-amine anti-corrosion primers were obtained in the United States for this study. The first primer, WB-X, was a waterborne coating with VOC reported at <250 g/L. The second primer, SB-Y, was a solventborne semi-gloss system with VOC reported at <300 g/L after solvent reduction as suggested by its manufacturer.
Three slightly different test formulations, with calculated VOC levels of 102 g/L, 92 g/L and 80 g/L, were evaluated in this study (Table 1). Pigments were ground directly in the epoxy portion.
Characteristics of the waterborne epoxy resin and hardener used in these formulas were:
i) Epoxy 386 – a 52% active epoxy resin dispersion with epoxy equivalent weight (EEW) at ~1,000 g/mol as supplied. Viscosity ranges from 300 – 1,500 mPa.s.
ii) Hardener 2188 – a 55% active, hydrophobically modified aliphatic amine hardener with H-equivalent weight (HEW) at ~380 g/mol as supplied. Viscosity ranges from 6,000 – 14,000 mPa.s. This hardener is commercially available for 2K waterborne anti-corrosion primers, and it was found to work well together with Epoxy 386.
A stoichiometry ratio HEW:EEW of 0.75:1.00 was chosen since this ratio was previously determined to provide a very good balance in corrosion (salt spray) and humidity resistance.2
The difference in calculated VOC among these formulas is due to (i) the usage of different delivery forms of a polymeric type pigment dispersant (6208/60 is supplied in a blend of organic solvents, while 6208 is supplied in mostly water and contains only 1% of organic solvent); and (ii) the usage of a high-boiling ester alcohol solvent, EA. It is believed that at least some EA remained in the coating films even after curing at room temperature for more than 10 days. However, we took a conservative approach in our VOC calculation and treated EA as 100% volatile for this study.
Primer and Coated Panel Preparation
Commercial primers WB-X and SB-Y were prepared according to their respective manufacturers’ recommendation before application. Both systems required an induction period of 30 minutes after mixing Parts A and B (activation), and were applied to panels shortly after the induction was finished. Formulation details such as stoichiometry and P/B ratio of either primer are unknown.
Test formulas #1, #2 and #3 were prepared by mixing Parts A and B together, with an additional 6% of water to lower viscosity for ease of spraying. An induction period of 30 minutes was allowed before application. All test panels were coated shortly after the induction was finished, except those prepared specifically for pot-life study.
All metal panels were coated by conventional pressure-feed air spray. Three types of metal panels were used in salt spray and crosshatch adhesion tests: sand-blasted steel (SB, Custom Lab Specialties brand, 11 gauge cold rolled), cold rolled steel (CRS, Q-Panel brand Type S, ground finish) and aluminum (AL, Q-Panel brand Type A, bare mill finish). All other tests were performed on CRS panels only. All test panels were single coated. Dry film thickness (DFT) on these panels was targeted as shown in Table 2. A slightly higher film thickness of SB-Y was targeted on CRS and AL panels based on recommendation from SB-Y’s manufacturer.
All coated panels were dried and cured at room temperatures (72 ± 2) °F for 10 days before subjected to corrosion-related testing, including salt spray (fog), humidity resistance and water immersion tests. All other testing was done after the primers were allowed to dry and cure at room temperatures for 7 days.
Performance TestingCorrosion Testing
Salt spray testing was done according to ASTM B 117-03. Coated panels were scribed according to ASTM D 1654-05 before exposure. Ratings for maximum creepage at scribes were given according to the same method. A rating of “10” represents zero creepage, while a rating of “0” represents a maximum creepage of >16 mm.
Humidity resistance testing was done according to ASTM D 4585-99 using a Cleveland condensing type humidity cabinet with temperature set at (38 - 40) °C.
Water immersion testing was done by immersing the coated panels in de-ionized (DI) water with temperature maintained at (40 - 42) °C. At each evaluation interval, panels were removed from the water, wiped dry, rated immediately and then returned back to water immersion.
For all three corrosion tests mentioned above:
(i) Panel appearance was evaluated and the degree of blistering was rated according to ASTM D 714 - 02. A blister size rating of “10” represents no blistering, while a size rating of “2” represents large blisters. Frequency of blisters was also rated according to ASTM D 714- 02, with “F” = few, “M” = medium, “MD” = medium dense, and “D” = dense.
(ii) Degree of surface rusting was rated according to ASTM D 610 – 01. A “10” rating represents ≤ 0.01% of surface rusted, while a “0” rating represents > 50% surface rusted. Rust distribution type was described as either “S” (spot rusting), “G” (general rusting), “P” (pinpoint rusting), or “H” (hybrid rusting).
Crosshatch adhesion was performed according to ASTM 3359-07, with 2 mm spacing between cuts. 3M’s Scotch brand tape type 898 was used. A rating of “5B” represents 0% of coating detached, while “0B” represents > 65% of coating detached. For adhesion ratings less than 5B, numbers in parenthesis following the adhesion rating represent the estimated percentage of coated area with coating detached.
Pencil hardness was checked on CRS panels per our internal method SOP ST-LC-26, which is similar to ASTM D 3363-05.
Gloss (60°) was measured using a handheld gloss meter on CRS panels according to our internal method SOP ST-LC-28, which is similar to ASTM D 523-89.
Chemical resistance was evaluated using 24 hour spot test. Testing was performed on CRS panels according to internal method SOP ST-LC-46, which is similar to ASTM D 1308-02. Spots were evaluated immediately after reagents were removed.
MEK resistance (double rub) was checked on CRS panels per our internal method SOP ST-LC-23, which is similar to ASTM D 5402-93.
Results and DiscussionCorrosion Testing
Evaluation results of scribed panels after 168 hrs (1 week), 504 hrs (3 weeks) and 1,008 hrs (6 weeks) of salt spray exposure are summarized in Table 3.
Formula #1 Compared to WB-X and SB-Y
Epoxy 386-based formula #1 performed slightly better than SB-Y after 1,008 hrs of exposure, even though SB-Y had a small advantage of slightly higher dry film thickness on CRS and AL panels. The creepage of formula #1 on CRS was better than that of SB-Y after 1,008 hrs (Figure 1). In addition, while both AL and SB panels of formula #1 showed no blistering or rusting, an AL and a SB panel of SB-Y showed small signs of corrosion at the unscribed area.
On the other hand, even though formula #1 is more or less equal in corrosion protection to WB-X on AL panels, formula #1 out-performed WB-X significantly on both SB and CRS panels. WB-X started showing face rust and blisters after only 168 hrs of exposure, while formula #1 remained free of rust or blisters at the unscribed area even after 1,008 hrs of exposure. Moreover, creepage rating of WB-X on CRS was much worse than that of formula #1 (Figure 1).
Comparing Formulas #1, #2 and #3
In general, formulas #1 and #2 performed similarly on all three types of metal panels. This indicates the slight reduction in overall formula VOC (by substituting the polymeric dispersant from its high VOC to low VOC version) had no significant negative impact in salt fog resistance. Furthermore, coated CRS panels of both formulas #1 and #2 have similar gloss levels after activation (Figure 5). This suggested that the small amount of solvents coming from the dispersant did not make a difference in the film formation, and therefore, probably did not have any impact in the barrier property of either primer.
On the other hand, CRS panels of formula #3 performed somewhat better than those of both #1 and #2 in salt spray resistance (Figure 2). This is contradictory to what was originally expected – that the addition of high-boiling solvent EA in a low-VOC waterborne formula would enhance film formation and, therefore, improve the resulting barrier property against corrosion.
It was thought that at least some EA solvent molecules remained in the primer films of formulas #1 and #2, even after curing at room temperature for 10 days. If this was indeed the case, these EA molecules might become the ‘weak links’ within those films, resulting in poorer barrier properties of #1 and #2. In addition, since all three Epoxy 386-based formulas performed similarly in humidity resistance and water immersion tests (see results shown in Tables 4 and 5), these ‘weak links’ created by leftover EA molecules might be especially sensitive to the presence of salt used in salt fog. Additional work is needed in order to investigate this further.
Humidity resistance results, after exposure periods of 168, 504 and 1,008 hrs, are summarized in Table 4.
Formula #1 performed very similarly to SB-Y up to 504 hrs of exposure. However, #1 showed a few small blisters after 1,008 hrs and was therefore slightly worse than SB-Y, which remained free of blisters. In contrast, WB-X showed signs of blistering at 504 hrs and got worse (in terms of size and frequency of blisters) after 1,008 hrs of exposure (Figure 3).
On the other hand, formulas #1, #2 and #3 performed very similarly after exposure to condensing water, indicating the formula differences among them had no visible effect on performance.
Water immersion test results after 168, 504 and 1,008 hrs are summarized in Table 5.
Results from the water immersion test showed a similar trend to that of the humidity resistance test – formulas based on Epoxy 386 performed very comparable to SB-Y, and in turn both systems performed significantly better than WB-X (Figure 4). Note that WB-X showed more severe blistering after water immersion than exposure in a Cleveland type humidity cabinet.
Table 6 lists a summary of results. All Epoxy 386-based primers showed very good adhesion to all three types of metal panels (no more than 1% of coating loss) and they were comparable to both WB-X and SB-Y. SB-Y showed a slightly inferior adhesion on SB panels.
After 7 days of curing at room temperature, the pencil hardness of all Epoxy 386-based primers was slightly lower than that of their waterborne counterpart WB-X, which in turn was softer than SB-Y. The relative softness of primers based on formulas #1, #2 and #3 was somewhat expected since Epoxy 386 is internally flexibilized by design. Conversely, after conditioning at room temperature for 2 months, all primers became harder. The pencil hardness of formulas #1 through #3 was about equal to that of WB-X, and SB-Y remained the hardest. In this study, pencil hardness of different primers did not show any direct correlation to their corresponding anti-corrosion performance. Additionally, test data reported in this paper confirmed that the chemical modification utilized to make Epoxy 386 flexible did not seem to impart any negative influence in corrosion resistance.
MEK double-rub and 24-hour spot test results indicated Epoxy 386-based primers have better resistance to sulfuric acid and sodium hydroxide solutions, equal resistance to MEK, but slightly poorer resistance to xylene, when compared to WB-X. In contrast, SB-Y has better resistance to sulfuric acid, sodium hydroxide and MEK, but poorer resistance to xylene, when compared to Epoxy 386-based primers.
Overall, primers made with Epoxy 386 have chemical resistance ranked somewhere in between WB-X and SB-Y. This might still be acceptable in applications where (i) the primers are protected by their topcoats; or (ii) the primers are not expected to be exposed to harsh chemicals for extended period of time. Additionally, chemical resistance of formulas #1 - #3 might be further improved by adjusting the stoichiometry ratio and other formulation parameters.
Lastly, we checked the pot-life of formulas #1 and #2 by monitoring their gloss (Figure 5) and salt spray resistance on CRS panels at different time intervals after activation.
Even though 60° gloss profiles of both formulas #1 and #2 indicated that their end of pot-life is approximately 3 hrs after activation, salt spray test results showed only a slight drop-off in corrosion resistance even when these two primers were applied 5 hrs after activation. Additional tests, including humidity resistance and water immersion, are needed to confirm if overall properties at 5 hrs after activation are indeed acceptable. Moreover, future work includes the pot-life evaluation of formula #3.
ConclusionsBy utilizing the novel epoxy resin evaluated in this study, together with the correct selection of a matching hardener as well as the employment of proper formulation techniques, formulating high-performance, waterborne 2K epoxy-amine primers with properties equal to or better than commercially available primers is quite possible. At the same time, the following requirements can be fulfilled:
- low VOC (< 100 g/L);
- free of anti-corrosion pigments, including zinc phosphate;
- excellent corrosion resistance;
- very good adhesion;
- pot-life of 3 hrs or more; and
- more formulation flexibility in terms of pigment grinding.
Future WorkAdditional R&D work is on-going since there are still a few questions to be answered. For example, it was previously found that Epoxy 386-based primers containing no corrosion-inhibitive pigments actually had better salt fog resistance than similar primers containing zinc phosphate or other anti-corrosion pigments.1,2 This seems contradictory to conventional wisdom. There might be drawback of using these active pigments, which is more than enough to negate any benefits provided by them.
AcknowledgementThe authors want to give special thanks to our colleague, Mr. Karl Sundberg, for his contribution in performing most of the application testing necessary for this paper. We also want to express our gratitude to the entire Epoxy Team, with members spreading across Europe and Americas, for their support in the development of this new 2K epoxy-amine primer system.
This paper was presented at the American Coatings Conference, Charlotte, NC, 2008.