Surface corrosion leads to localized failures due to the formation of pits. Anodes and cathodes develop on the surface of the metal as corrosion takes place, leading to a change in pH over the cathode and the anode. Using pH as stimulus, microcapsules containing cerium nitrate were designed which break open at basic pH. The incorporation of the inhibitor was evaluated using EDX mapping, inductively coupled plasma. The formation of the disulfide bonds was monitored using Raman Spectroscopy. The topography and the shape of the synthesized microcapsules were analyzed using Scanning Electron Microscopy. It was observed that cerium nitrate was incorporated into the polymer shell.
Encapsulation techniques have been extensively used in many areas such as pharmaceuticals1-4, food preservation5-7, pesticides 8, 9etc. The idea of encapsulation has been employed in several of these applications because of the ability of this technique to store and release the contents of the microcapsules on a need basis. Several authors have published their research with different stimuli as triggers for the on demand release. One of the less explored stimuli for the release is pH.
Corrosion is an electrochemical process, with anodes and cathodes statistically distributed throughout the surface of the metal undergoing corrosion. Corrosion inhibitors containing chromates have traditionally been used to protect the substrate from undergoing corrosion. However, owing to their carcinogenic effects10-12, the use and the application of such coatings poses significant risks to humans. The need for less aggressive and environmentally friendly inhibitors opens up opportunities for water soluble rare earth metals as inhibitors. It has been shown that the use of cerium (IV) as an inhibitor provides similar protection as that of chromates or better 13-15. Corrosion on a substrate leads to different pHs on the surface of the metal. If this pH change can be linked to some kind of polymer degradation kinetics then a pH sensitive system can be designed. It is known that disulfide bonds break when they come in close proximity to hydroxyl ions. The inclusion of such bonds in the polymer backbone would result in a structure that would degrade in basic environments. In this paper we report, the synthesis of pH sensitive microcapsules containing cerium nitrate as a corrosion inhibitor.
EXPERIMENTAL PROCEDURE
Materials. Highly butylated urea formaldehyde resin (Cymel ® U80) was used as received from CYTEC Corporation. Pentaerythritol tetra - (3 - mercaptopropionate) was purchased from TCI chemicals. Cerium nitrate hexahydrate (>99% pure) and Span® 85 was purchased from Sigma Aldrich®. Hydrogen peroxide (30% solution, ACS grade) and Tween ®20 was purchased from VWR. Acetone was used as the volatile solvent. 18 MΩ Millipore ® water was used as the non-volatile solvent in the reaction. Mineral Oil was purchased from VWR.
Procedure. The reactants were divided into two phases as shown in Figure 1. The phase 1 consisted of
FIGURE 1: Preparation of microcapsules
1:1 ratio of pentaerythritol tetra - (3 - mercaptopropionate), Cymel® U80; acetone, hydrogen peroxide, water and cerium nitrate. The phase 2, dispersion medium, consisted of mineral oil, Span® 85 and Tween® 20.
In the synthesis of these microcapsules, we use phase separated emulsion polymerization as reported by Vincent et al16.The phase 1 was added to phase 2 slowly and subjected to high shear at 2000 RPM for 30 minutes. The emulsion formed was transferred to a beaker and sonicated for 5 minutes. The formed emulsion was transferred to a rotavap. The water bath of the rotavap was set at 50 °C and a pressure of 750 mbars was applied and the revolution of the flask was set at 100 RPM. The reaction was allowed to proceed in the flask for 3 hours. After 3 hours, the formed microcapsules were vacuum filtered and centrifuged.
Characterization. Energy dispersive X- ray Spectra (EDS) of the microcapsules were analyzed for cerium nitrate. EDS mapping was used to visualize the distribution of cerium nitrate within the microcapsules. Inductively Coupled Plasma (ICP-OES) was used to measure the amount of cerium nitrate in the microcapsules. Raman Spectroscopy was used to monitor the formation of disulfide bonds. Scanning Electron Microscopy(SEM) images were taken to analyze the structure and the morphology of the microcapsules.
RESULTS
Scanning Electron Microscopy was used to study the formed structure and topography of the microcapsules. Figure 2 shows the images obtained from the SEM. It was seen that the microcapsules were spherical and more or less uniformly sized. It was seen that the size of the microcapsules could be varied with change in the stirring rate17, 18.
FIGURE 2: Scanning Electron Microscopy images of cerium nitrate containing microcapsules.
It was seen that microcapsules were agglomerated with smooth surface with tiny peaks of the polymer. However, the microcapsules were completely dryable and re-dispersible in water.
In order to see if cerium was present in the microcapsules, an EDX spectrum was used to detect cerium in the microcapsules. It was seen that cerium was detected on the surface (or just below the surface) of the microcapsules.
FIGURE 3: EDX spectra of 0.25 g cerium nitrate containing microcapsules soaked in NaOH for 10 minutes.
Four different points were chosen as shown on the SEM image on the top left corner and the composition was measured. It was seen from the spectra that various concentrations of cerium were detected at different locations. A peak for sodium also appears as the microcapsules were soaked in a 0.25 M sodium hydroxide solution. The concentration of cerium was higher at the surface (under the surface) at the point 2.
To better visualize the distribution of the cerium inside the microcapsules, localized EDX mapping was done on the surface of the microcapsules (Figure 4).
(a) (b) (c)
FIGURE 4: EDX images of 0.25 g cerium nitrate containing microcapsules. (a) SEM images of the microtomed microcapsules (b) cerium map (c) sulfur map.
(Data Type: Net Counts Mag: 650 Acc. Voltage: 15.0 kV)
It can be seen that cerium nitrate is more or less uniformly distributed within the microcapsules. It can be seen that there is a slight bleeding of the cerium in the epoxy matrix used to hold the microcapsules in place, which can be attributed due to the grinding motion of the microtome. It can also been seen that the sulfur is uniformly distributed throughout the sample and hence can be said that the disulfide bonds are present almost everywhere on the microcapsule surface.
Inductively Coupled Plasma results conclusively show that cerium nitrate had been incorporated into the microcapsules. From the preliminary ICP data we can see that low concentrations of cerium were incorporated into the microcapsules.
Table 1: Cerium nitrate concentration in microcapsules formulated with 0.25 g cerium nitrate in the formulation
Trails
Ce 413.380
Ce 413.765
Ce 418.660
Ce 448.691
Detection limit
0.014 mg/l
0.011 mg/l
0.044 mg/l
0.013 mg/l
1
9.039
9.117
9.117
9.099
2
9.34
9.435
9.419
9.396
3
9.442
9.511
9.509
9.52
mean
9.064
9.148
9.145
9.128
Disulfide bonds being the key to the pH sensitivity, was monitored using Raman spectra. They disulfide bonds presence can be detected at 500 cm-1. It can be seen from the Raman spectra, the disulfide bonds form at the end of the reaction.
FIGURE 5: Raman spectra of monomers before the reaction.
FIGURE 6: Raman spectra of cerium nitrate containing microcapsules at the end of the reaction
Peaks at 500 cm-1 indicate the formation of the disulfide bonds as the reaction proceeds due to the oxidation of the thiol in the reactant mixture. Thus, the pH sensitivity required is also incorporated by the formation of the disulfide bonds.
CONCLUSIONS
Cerium nitrate containing microcapsules were successfully synthesized. It was seen from the SEM images that uniform, spherical microcapsules were synthesized. EDS confirmed the presence of cerium on or below the surface of the microcapsule walls. It was also showed that cerium was uniformly distributed within the microcapsules with little leakage into the surrounding epoxy matrix. ICP was used to measure the concentration of cerium nitrate inside the microcapsules. It was seen that low amounts of cerium nitrate was incorporated into the microcapsules. The disulfide bond formation was tracked using Raman spectra, and formation of a peak at 500 cm-1 was detected.
