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While an air spray system has been used on the materials shown here, they can also be incorporated into brush, roller or powder coatings that are solvent based, water based, low VOC or high solids. The inclusion of the CRCI into the coating had no effect on the adhesion of the coating to the substrate.
Testing of the material with crosshatch scribe according to ASTM D gave no indication of any reduction in adhesion to the substrate surface due the presence of the matrix-encapsulated materials. The gloss of the coatings for the CRCI material was measured and compared against the coating without the CRCI as well as a coating containing the traditional inhibitor for steel, zinc phosphate ZnPO 4.
The difference between the two materials and the minimal impact of the CRCI material in the coating is apparent on visual inspection. Gloss measurements of the coats were performed, with the results shown in Figure 3. The control coating and the CRCI-containing coating are very similar. They are within the margin of error of the measurements across the film.
The ZnPO 4 coating has a significantly low gloss value and could be classed as a matte finish. While the materials are capable of being used as a corrosion-control agent in a primer substituting for ZnPO 4 , the advantage of the matrix-controlled release material is the potential for incorporation into a direct-to-metal DTM topcoat.
This would add corrosion inhibition functionality to the topcoat. It would potentially reduce the need for two coats in a system by removing the primer and maintaining the gloss. Through careful manipulation of the chemistry of the pigment and the size of the pores in the particle, a tailored release profile can be achieved. Figure 4 shows the difference in release rates of an active based on the pore size of the particle matrix.
This allows the active to be released quickly at the site of corrosion to stop it in its tracks, or released slowly to extend the life of the coating by giving corrosion prevention over a longer lifetime.
The active in the matrix is released into the scribe during testing by diffusion from the matrix and it preserves the shininess of the scribe as is seen with traditional chromate inhibitors. This shininess was preserved for just under hrs in NSST. The coatings themselves demonstrated similar performance to other nonchromate inhibitors shown to have an inhibitory effect on corrosion out to hrs.
No blisters or corrosion products are seen in the scribe. The shiny scribe can be seen in comparison with the controls in Figure 5. For steel, the controlled release of the actives can be seen in comparison with the traditional inhibitor. The reduction in material allows for formulation with organic inhibitors at levels low enough to remove hazard labelling. The scraped steel plates can be seen in Figure 6. Using a profilometer the depth of the scribe for the steel plates was measured after scrapping, once hrs in NSST was completed.
The scribe depth measurement was used as a method of comparison between the traditional corrosion inhibitor ZnPO 4 and the CRCI material. The results can be seen in Figure 7 and it is obvious the CRCI material has significantly slowed or stopped the corrosion process in the scribe while both the control and the ZnPO 4 sample have undergone significant corrosion in the scribe, as seen with an overall increase in scribe depth when compared to the CRCI sample.
The plots in Figure 8 show the self-healing effect demonstrated by the controlled release of the active from the matrix. The coatings without the CRCI show significant corrosion in the scribe over time - with more corrosion seen at hrs immersion compared to the sample at 24 hrs. However, in the system with the CRCI, the admittance in the scratch is hardly seen at 24 hrs and only slightly more visible at hrs.
The admittance values for the CRCI sample are much lower than without by comparison. In addition to this, no corrosion was observed in the scribe of the sample with the matrix-encapsulated material. This result demonstrates the controlled release of the corrosion-inhibiting active from the matrix.
Once in the scratch, the corrosion process is halted or significantly retarded. It is possible to remove traditional inhibitors and incorporate smart pigment materials to give the same, if not a better, level of corrosion control in coatings. Moreover, these can also remove environmental, health and safety concerns due to heavy metals. Inhibispheres are shown to be a versatile and robust replacement for traditional corrosion inhibitors. They can be easily incorporated into water, solvent or powder coatings.
They can be tailored to whatever conditions are required with control over particle size and active release rate.
They have no significant impact on the final coating properties optical and mechanical. The controlled release mechanism allows for the extension of coating life and the inhibition of corrosion directly in the scribe. While the current application of this technology is in the area of corrosion inhibition for coatings, the matrix encapsulation technology can be applied to controlled release in antifouling, fire retardants, UV absorbers, etc.