Nanomaterials have a relatively large surface area. As particles decrease in size, a greater number may be found at the surface, creating a more reactive surface than achieved with larger-size particles. Since nanoparticles provide a larger surface area and quantum effects, it is expected that replacing existing larger-size extenders with nanosize extenders could provide better coating properties. Part 1 of this article (PCI, January 2013 issue, page 44) revealed that incorporating nanoclay into polyurethane coatings significantly improved dry adhesion as well as high-pressure hot-water-spray test properties. This article discusses the same systems tested for water, humidity and corrosion resistance.

Experimental

Synthesis of Nanoclay-Based Coatings

Nanoclay, which is organically modified nanodispersable layered silicate with an average particle size of 25 nm (Al₂O₃, 2SiO₂, 2H₂O), was used for the study. The synthesis of nanoclay coatings was carried out in two steps.

1. Nanoclay Dispersion
   Generally, nanoparticle dispersion is extremely difficult due to the large surface area. Polyurethane coatings were selected because of their excellent durability. Acrylic polyol (80% on solids) and polyester polyol (20% on solids) were used as dispersing resins. The following samples were dispersed until a fineness of gauge less than 20 microns was achieved: 0% nanoclay; 2% nanoclay; and 5% nanoclay.
2. Coating Preparation
   The above dispersions were mixed with trimer isocyanate separately; the OH-NCO ratio was maintained around 1:1.5 for all experimental samples. Then all the samples were sprayed directly over mild steel and aluminum substrates, and finished with a solventborne topcoat.

Test Methods and Results

The following properties were evaluated after seven days of curing.

Water Resistance

The nanoclay-based coatings applied over mild steel and aluminum substrates were immersed in DM water at a temperature of 25±1 °C for a period of 10 days. The panels were then removed and evaluated for blisters, shrinkage and adhesion. The panels were evaluated again after 24 h. Both the immediate and 24 h results are reported. The blistering results are shown in Table 1. Blistering was determined according to ASTM D 714; blister ranking: D-Dense; MD-Medium Dense, F-Few; W-Wrinkling. Wet adhesion results are shown in Table 2. The incorporation of nanoclay at 2% and 5% concentration levels exhibited a similar level as that of the 0% level of nanoclay. This means that the addition of nanoclay does not affect the coating film properties in terms of blistering or adhesion properties.

Humidity Resistance

The panels were subjected to a Cleveland condensing humidity tester where the temperature was 38-40 °C and the relative humidity was about 95%. The panels were removed after 10 days of continuous exposure and evaluated for blisters, shrinkage and adhesion. The panels were evaluated again after 24 h. Both the immediate and 24 h results are reported. Blistering was determined according to ASTM D 714; blister ranking: D-Dense; MD-Medium Dense, F-Few; W-Wrinkling. Table 3 shows the blistering results. The results show improvement with the addition of nanoclay, especially over the mild steel substrate. The panels with 2% nanoclay had no blistering tendency, whereas slight microblisters were observed with 5% nanoclay. No significant influence was observed over the aluminum substrate.

Table 4 shows the wet adhesion results. The wet adhesion properties showed tremendous improvement with nanoclay, especially over the mild steel substrate. As with the blistering, the 2% nanoclay sample was completely intact on the substrate, whereas negligible failure was noticed with the 5% sample. However, both concentration levels of nanoclay exhibited better adhesion properties than the 0% level. There was no significant influence noticed with the aluminum panels.

Salt Spray Corrosion Resistance (ASTM B 117)

All experimental samples applied over mild steel substrate were subjected to testing in a salt spray test chamber. The salt spray was executed at an air pressure at the nozzle of 1 to 1.3 bar. Five parts of sodium chloride were dissolved in 95 parts of demineralized water, and the quality was maintained as per ISO 3696. The temperature in the cabinet was kept at 35 ± 2 °C. After 800 h, all the panels were removed and evaluated for blistering and corrosion. Blistering was determined according to ASTM D 714. The distance of corrosion/delamination along the scribe (one side) is reported in mm’s. The results are shown in Table 5. All experimental samples showed a similar level with respect to blistering and corrosion propagation. Nanoclay addition did not have any adverse effect in the salt spray test.

VDA Cyclic Corrosion Resistance

All samples applied over mild steel substrate were subjected to testing in a VDA test chamber. It is useful to test vehicle components using cycles of changing corrosive effects in accordance with VDA 621-415. The equipment is similar to that of salt spray, which is provided with a program to achieve the prescribed cycle of condensation as shown below:

• Salt spray (5% NaCl solution): 24 h
• Condensation at 38 °C: 8 h
• 23±2 °C and relative humidity: 16 h

After five cycles, all the panels were removed and evaluated for blistering and corrosion propagation. The results are shown in Table 6. No blistering was observed, even after five cycles (850 h). The corrosion propagation results showed improvement with nanoclay. However, a significant improvement is noticed at 5% concentration level. This result clearly ensures that closely packed platy nanoclay has a tendency to protect metal from corrosion.

Acetic Acid Spray Corrosion Resistance
(ISO 9227, ASTM G85)

Samples applied over aluminum substrate were subjected to testing in an acetic acid spray test chamber. The salt spray was executed at an air pressure at the nozzle of 1 to 1.3 bars. Five parts of NaCl were dissolved in 95 parts of demineralized water, and the quality was maintained as per ISO 3696. Glacial acetic acid was used to adjust pH from 3.1 to 3.3; cabinet temperature was kept at 35 ± 2 °C.  After 800 h, all the panels were removed and evaluated for blistering and corrosion propagation; blistering was determined according to ASTM D 714. The results are shown in Table 7.  The results show that the blistering tendency is almost similar in all the samples. The corrosion resistance properties over the aluminum substrate improved significantly with nanoclay. The corrosion level without nanoclay went up to 20 mm, whereas with nanoclay it reached a maximum level of 2 mm.

Conclusion

The study reveals that incorporating nanoclay improves humidity resistance, VDA cyclic corrosion over mild steel substrate and acetic acid spray corrosion over aluminum substrate. Water resistance and neutral salt spray corrosion results showed that the nanoclay had no significant effect. Overall, it can be concluded that incorporating nanoclay is an option for coating formulators to enhance durability performance – especially corrosion resistance properties.n

Acknowledgement

The authors would like to acknowledge Dr. Sudhakar Dantiki, Managing Director AkzoNobel Automotive & Aerospace coatings India Pvt Ltd., and management who allowed us to use instrumentation, which is highly appreciated.

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