A Nanoparticle-Based Additive for the Improvement of Water-Based Metal Coatings
The film formation process of 1K waterborne formulations does not involve a chemical crosslinking step. The application of a dense film therefore, sometimes demands more attention compared to solventborne coatings. This might be complicated by the generally shorter open time of waterborne metal coatings. Due to the hydrophilic components in the formulation – like surfactants, salts and pigments – they are more vulnerable against water and humidity.
We have shown earlier,1,2 that a nanoparticle-based additive called Oxylink™ increases the crosslink density of waterborne coatings. In this paper we investigate and discuss the effect of Oxylink on two different metal protection systems. The additive is a formulated dispersion containing nanoparticles in water. It has a low viscosity at a solid content of ca. 40 wt.% and a shelf life of at least one year when stored between 5 °C and 40 °C.
|Table 1 Click to enlarge|
Oxylink Additive in Metal Coatings
To investigate the effect of Oxylink we chose a primer/topcoat and a direct-to-metal coating as model systems. Primer/topcoat systems comprise the more traditional setup to provide effective corrosion protection. However, recently developed DTM systems are also used in medium-duty metal-protection applications. We tested the resistance against corrosion, methyl ethyl ketone (MEK) and 2-propanol (IPA) double rubs, humidity and weathering of these two systems with and without additive. The formulations are given in Tables 1-3. Furthermore, we examined the effect of Oxylink on gloss, film-to-substrate adhesion and drying time.
The first setup (C1) was a primer/topcoat system with both primer and topcoat based on BASF Acronal S 760 (Tables 1-2). This binder comprises a self-crosslinking styrene-acrylic resin with a MFT of 22 °C (72 °F). It is advertised to be well suited for decorative metal coatings for medium-duty corrosion protection.
|Table 2 Click to enlarge|
The second system (C2) was a direct-to-metal coating based on DSM Neocryl XK-86 (Table 3). This binder is also a styrene-acrylic copolymer emulsion for use in high-performance coatings for steel protection and various general metal applications. The MFT of this binder is relatively high at 31 °C (88 °F).
In general, 1% (solid on solid) of the additive was introduced into the respective coating formulation while stirring (Tables 1-3). Due to the low viscosity of the additive, it was readily dispersed into the formulation after a few minutes. The resulting formulations were stable, and no precipitation was observed. The in-can stability was not altered compared to the formulation without Oxylink so it could still be used as a 1K system. The coatings were applied to cold rolled steel plates and dried at room temperature for two weeks. The appearance, including gloss and surface structure, of C1 and C2 was not changed by Oxylink. The coating performance and appearance were examined by various standardized tests.
|Table 3 Click to enlarge|
- The degree of crosslinking was determined by a crock meter double rub test with 2-butanone (MEK) – double rub resistance test (crosslinking) according to ASTM D 5402. The resulting parameter is the number of rubs. In a parallel test we evaluated the double rub resistance against IPA, as well.
- We tested the humidity resistance for 300 h under a condensation atmosphere with constant humidity (98% - 99%) according to DIN EN ISO 6270-2, and evaluation was done using DIN EN ISO 4628. The effect of humidity on gloss, formation of blisters and rust was rated on a scale from 0 (best, no change) to 5 (worst, complete change).
- A weathering resistance test was carried out for 1000 h using Type II UV-A bulbs and humidity (DIN EN ISO 11507, process “A”). The resulting metal plates were evaluated by comparison to the sample pictures given in DIN EN ISO 4628. The resistance was rated from 0 (best) to 5 (worst).
- The salt spray test was performed with a neutral NaCl solution spray for 300 h according to DIN EN ISO 9227. The resulting metal plates were evaluated by comparison to the sample pictures given in DIN EN ISO 4628-2 to 5. Quantitative evaluation was carried out similarly for all tests that were evaluated using DIN EN ISO 4628. The corrosion resistance was rated from 0 (best) to 5 (worst).
- Gloss was determined by measuring the reflectivity at 60° and 85° according to DIN EN ISO 2813.
- The drying time for drying level four was measured according to DIN 53150. Therefore, we coated glass slides with a 200 µm wet film and determined the duration until drying level 1 (glass pearls do not stick on coating) and 4 (a paper which is pressed by a 2 kg weight to the coating does not stick anymore) were reached.
- Film-to-substrate adhesion was determined by using the cross cut method with 1 mm distance between the cuts according to DIN EN ISO 2409. The adhesion was evaluated by ranking the results from 0 (best) to 5 (worst).
- The chemical resistance of the coating was tested against gasoline; water/butyl glycol 90/10; ethyl acetate/butyl acetate 1:1; sodium hydroxide solution (5%); hydrochloric acid (10%); acetic acid (10%); hand lotion “Atrix”; aqueous solution of a detergent (Marlon A 350, 50%); simulated bird feces; and distilled water. The test was carried out with increasing exposure times from 10 sec. to 16 h according to DIN EN ISO 2812-3 and DIN EN ISO 4628-2. The chemical resistance was evaluated by rating the results from 0 (best) to 5 (worst).
|Figure 1 Click to enlarge|
Oxylink strongly enhances the MEK and IPA double rub resistance, as shown in Figure 1. In detail, the MEK rub resistance of both coatings C1 and C2 was enhanced by a factor of 3 to 4. The improvement against IPA double rubs is in the order of factor 1.6.
Table 4 details the results for the humidity resistance test. For C1, Oxylink caused a strong improvement from a rating of 3 to a rating of 0 in gloss retention. Also, the degree of blistering could be clearly diminished from a rating of 5 to 1 (Figure 2). For C2, the gloss could not be determined because of too many large blisters, which led to a mark of 5 in blistering. By adding Oxylink, the degree of blistering could be alleviated to level 3. The gloss rating for C2 with Oxylink was 0.
|Figure 2 Click to enlarge|
The weathering resistance of the coatings was significantly improved (Table 5). Without Oxylink, C1 lost its gloss (rating 5) and showed a degree of rusting between 1 and 2. The loss of gloss was not changed by the additive. However, adding Oxylink to C1 reduced the degree of rusting to 0.
Without Oxylink, C2 also lost its gloss completely, and the gloss was rated as 5. Adding Oxylink, gloss loss for C2 was rated as 3 (medium loss of gloss) after weathering. The excellent ratings in rust inhibition and blister formation were unchanged by the additive (rating 0).
Of major importance is the fact that further performance parameters were unchanged by the additive: corrosion resistance in the salt spray, gloss, drying time and adhesion were not affected by adding Oxylink to the formulations.
The chemical resistance of the coatings with Oxylink was not altered against gasoline; ethyl acetate/butyl acetate 1:1; sodium hydroxide solution (5 %); hydrochloric acid (10 %); acetic acid (10 %); aqueous solution of a detergent (Marlon A 350, 50 %); simulated bird feces; and distilled water. Oxylink increased the resistance against hand lotion from a rating of 4 to a rating of 2 (0 = best, 5 = worst) in coating C2. Also the resistance of C2 against a mixture of water/butyl glycol was rated 4 without and rated 2 by adding Oxylink. Hence the Oxylink caused a significant improvement in chemical resistance.
|Table 4 Click to enlarge|
Oxylink yielded a significant improvement in the coating properties of both C1 and C2 metal protective coating systems. The improvements caused by Oxylink in MEK and humidity resistance are attributed to a higher degree of crosslinking in the polymeric binder.3 The binder in both coating systems is a styrene-acrylic copolymer. We propose that the inorganic additive crosslinks the acrylic groups of these copolymers. Due to the nanoscopic nature of the particles, only small amounts (0.9 % calculated on dry solids) of the additive are needed to achieve the effect.
|Figure 3 Click to enlarge|
Compared to the strong improvement in the MEK rub resistance test by Oxylink, the additive only evoked a slight reinforcement in the IPA double rub experiment. We attribute this difference to the different swelling behavior of styrene-acrylates in MEK and IPA, respectively, where the latter is more polar and a protic solvent in contrast to the ketone. We propose that the difference is an indication of the non-covalent nature of the crosslinking by the inorganic particles. If covalent crosslinking occurred, one would expect a largely solvent-independent strengthening of the polymer film. We conclude that presumably, a complexation reaction takes place between the carboxylic groups of the polymer and the surface of the particles.
The higher crosslinking, as demonstrated by MEK rub resistance, is an indicator for generally tougher coating performance. This general toughness should translate into a longer serviceable lifetime of the coating. Our assumption was supported by the fact that humidity resistance of both coatings was also increased by the additive. The higher crosslinking due to the non-covalently crosslinked polymer results in reduced swelling behavior, which causes better protection of the steel substrate against water. The higher crosslinking has also a beneficial effect on weathering resistance. Generally, both coating systems, with and without Oxylink, fared very well in this test. However, in coating C1 Oxylink reduced the degree of rusting drastically. Also, strong gloss loss was observed without the additive for both coating systems. After 1,000 h weathering, the films were dull. Even if Oxylink did not change the gloss loss of the C1 formulation, the gloss of the DTM coating C2 was much improved by Oxylink.
The undisturbed performance of the formulations containing Oxylink in the salt spray test may be surprising at first glance. Usually inorganic pigments may render a coating more hydrophilic and thus less resistant against corrosion. However, Oxylink is effective already at a very low dosage so that the crosslinking caused by the particles more than compensates the potentially more hydrophilic nature of the coatings.
|Figure 4 Click to enlarge|
One of the possible downsides of higher crosslinking of a coating might be the reduced adhesion and/or increased brittleness. Earlier work with this additive has shown that reduced adhesion might occur when using too high concentrations of the additive (more than 3 wt.%). However, in the present study, using low amounts of Oxylink, we did not find reduced adhesion or increased brittleness. We attribute this behavior to the non-covalent nature of the crosslinking that leaves the mechanical properties largely unchanged.
The effects of the altered and non-altered chemical resistance need to be discussed for the different groups of chemicals. Liquid water resistance is different from resistance against humidity. Interestingly, liquid water resistance is unchanged for C1, even if it is already very good for the non-modified coating. For C2, the liquid water resistance is somewhat improved. Also the performance of the coatings against non-polar solvents, like higher alkanes in gasoline, is excellent without Oxylink and unchanged by the additive. The additive also does not improve resistance against highly alkaline agents (bird feces and NaOH). The effect of these agents is that they hydrolyze and dissolute the polymer. As Oxylink does not affect the polymer structure itself, it cannot prevent degradation of the polymer. In contrast, Oxylink increases the resistance against hand lotion and a mixture of water/butyl glycol in C2. This is again attributed to a higher crosslink density of the coating, which hinders diffusion of substances of medium polarity into the film.
In contrast to previously reported results on clear wood coatings,2 the drying behavior of neither C1 nor C2 was improved here. Maybe the difference between the drying behavior of the wood coatings and the presented metal coatings is due to high pigment content in these formulations and subject to more detailed ongoing investigation.
We evaluated the effect of a formulated nanoparticle dispersion additive called Oxylink in two waterborne metal-protection coating systems. Significant improvements of many properties by Oxylink for metal protection coatings were found. We relate these improvements for the most part to a higher crosslinking density in the polymer, which we characterize as being non-covalent in nature.
For an overview of the quantitative results we calculated the average from the single performance values and set the best result to 100 %. The resulting comparisons of the test results are summarized in the radar graphs Figures 3 and 4. As shown in the figures, the rub resistance against MEK and IPA was drastically increased. It clearly can be seen that the evaluated metal coatings show an improved humidity and weathering resistance with Oxylink. Also a long-lasting gloss and reduced degree of rusting can be obtained by using Oxylink. In addition, resistance against aggressive chemicals like hand lotion and water/butyl glycol were improved. With these effects Oxylink is a versatile additive to strengthen the performance of waterborne acrylics.
After obtaining good results with Oxylink in wood and metal coatings we will focus further work on testing Oxylink in other applications like coatings for plastics, where there is also a steady need for faster curing times at lower temperatures. Here, Oxylink also has the potential to contribute to better processability and better performance of waterborne coatings.
This paper was presented at The Waterborne Symposium, Advances in Sustainable Coatings Technology, 2010, New Orleans, LA. Symposium Sponsored by The University of Southern Mississippi School of Polymers and High-Performance Materials.