The Evolution of Silica Nanoparticle-polyester Coatings on Surfaces Exposed to Sunlight

The overall goal of this methodology is to examine the changes in the surface of steel coatings prepared from polyester and silica nanoparticle coated-polyester after being exposed to sunlight. This method can help answer key questions in the industrial fields and areas such as nono coatings and anticorrosion. The main advantage of this technique is it provides a comprehensive understanding of the behavior of nano coatings exposed to weathering conditions.

Demonstrating the procedure will be Jitraporn Vongsvivut and Vi Khanh Truong from our laboratories. First, rinse steel samples previously exposed to sunlight with double distilled water and then dry using nitrogen gas. Keep the samples in an airtight container to prevent any air contaminants absorbing onto the surface.

Analyze the sample surface chemistry using attenuated total reflection infrared microspectroscopy, or ATRIR. First, open the cantilever of the ATR unit and load the sample on an aluminum disk into the ATR unit located on the microscope stage. Lift the sample stage of the ATR unit upwards and use the objective to focus on the sample surface.

Press single image and add an annotation at the cross hair as center. Then make a larger overview image to cover the desired area on the sample surface by pressing image of large area. Move back to the center position.

Next, lower the ATR sample stage, then close and tighten the cantilever of the ATR unit. On the background page, select measure background once and click next. On the next measurement page, select current position and click next.

Rotate the manual pinhole wheel from open to 0.3, equivalent to a 3 micron diameter beam size and check the IR signal. Then measure the background by pressing measure background. Slowly lift the sample on the ATR sample stage upwards to make contact with the ATR crystal.

Once observing bands on the live spectrum, draw a grid map on the area of interest in the visible image. Click next. After that, fill out the parameters including sample name and select the appropriate scan time and start the measurement by clicking measure sample.

Once the scan is finished, click repeat. After saving the data, open the master file using the spectroscopy software. Choose the peak of interest on the IR spectra and right click on it.

Under the integration tab, choose integration to create two-dimensional false color maps. To perform surface wetability measurements, place one of the samples on the stage of a contact angle goniometer equipped with a nanodispenser. Adjust the position of the micro syringe assembly so that the bottom of the needle appears about a fourth of the way down in the live video window screen.

Now, raise the sample using the Z-axis until the distance between the sample and surface is about 5 milimeters. Lower the syringe until a droplet of double distilled water touches the surface. Then return the syringe to its original position.

Press the run command to record the water droplet impacting on the surface for a 20 second period using a monochrome charged couple device, or CCD camera which is integrated with the hardware. Then press the stop command to acquire a series of images. Following this, press the contact angle command to measure the contact angles from the acquired images.

To perform an optical profiling measurement, press the samples on the stage of the microscope. Focus on the surface using the 5x objective by controlling the Z-axis until the fringes appear on the screen. Press the auto command to optimize the intensity and then press the measurement command to initiate the scanning.

Save the master files. Prior to statistical roughness analysis, press the remove tilt option to remove the surface waviness. Press the contour option to analyze the roughness parameters.

Then click on the 3Di option to generate three-dimensional images of optical profiling files using compatible software. At this point, place the samples on steel disks and insert the steel disks into the magnetic holder of an atomic force microscope. Perform atomic force microscopy, or AFM scans, in tapping mode.

Mechanically load a phosphorous doped silicon probe with a spring constant of 0.9 newtons per meter, a tip curvature with a radius of 8 nanometers, and a resonance frequency of approximately 20 kilohertz. Next, manually adjust the laser reflection on the cantilever. Then, choose the auto tune command and press the tune command to tune the AFM cantilever to reach the optimum resonance frequency reported by the manufacturer.

Focus on the sample surface and move the AFM tips close to the surface. Then, click on the engage command to engage the AFM tips on the surface. Type 1 hertz into the scanning speed box.

Choose the scanning areas and press the run command to perform the scan. Following this, choose the leveling option to process the resulting topographical data. Then, save the master files.

Open the compatible AFM software and load one of the AFM master files. Press the leveling command to remove the tilting of surfaces and press the smoothing command to remove the background. Finally, press statistical parameters analysis to generate the statistical roughness.

Water contact angle measurements demonstrated that wetability of the polyester coated substrata had not changed as a result of exposure to sunlight. After one year of exposure, the silica nanopartical polyester coated samples were 1.3 times greater in hydrophobicity than the unexposed samples. The XPS spectra indicated that iron was detected on the polyester coated substrata after one and five years of exposure and that there had been a slight decrease in the carbon content of the polyester coated samples after five years of exposure.

No significant change was found in the silicon, iron and carbon levels in the silica nanoparticle polyester coated substrates. ATRIR showed that the number of carbonyl groups decreased on both the polyester and silica nanopartical polyester coated samples after five years of exposure. The microscale topographic evolution of the polyester and silica nanopartical polyester coated samples showed that the surfaces of both coatings became rougher than the original substrata after one year of exposure.

The original silica nanopartical polyester coatings were smooth on a nanometer scale, however, after ultraviolet light exposure, both coatings were found to have formed globular structures and to exhibit a significantly higher average roughness than the original substrata. Once mastered, this set of techniques can be completed in the same day if its performed properly. While attempting these procedures its important to remember to keep the sample surfaces free from dust.

Following this procedure, other methods like ATRIR can be used to answer other questions like what’s the chemical distribution across the surface of the sample. After its development, this technique enabled researchers in the field of nanocoatings to explore the extent of polymer degradation. After watching this video, you should have a good understanding of how to operate instruments that are used in the characterization of a surface.

Don’t forget, that working with the instrumentation described in this protocol can be extremely dangerous. So always take precautions to protect yourself from hazards such as laser radiation or liquid nitrogen while performing this procedure.