Coated Paper Analysis Using Atomic Force Microscopy – Supplier Data By Pacific Nanotechnology
Background
Several types of microscopes are available for studying and analyzing the surfaces of coated papers. The most common microscopes are optical microscopes which can give a 2-dimensional resolution of approximately 1 micron. The primary advantages of the optical microscope are its ease of use and low cost. For higher resolution imaging, less than 1 micron, a scanning electron microscope (SEM) is typically used. The SEM can resolve surface features as small as a few nanometers. However, the SEM does not give high contrast images on flat samples, requires a vacuum, and often requires substantial sample preparation. The most common type of sample preparation is sputter coating a metal on the surface, which can cause the surface topography of the sample that will be imaged to change. Most recently the Atomic Force Microscope (AFM) is being applied for imaging the surfaces of coated papers. Advantages of the AFM are that it directly gives 3- Dimensional magnification, works in ambient air, requires no sample preparation, and the AFM has a resolution at the nanometer scale.
Atomic Force Microscope Sensor
In an AFM a very fine stylus is scanned over a surface in a raster pattern. By monitoring the motion of the stylus, a 3-Dimensional image of the surface can be created. Images may be displayed in 2-Dimensional or 3-Dimensional representations. Further, line profiles can be extracted from the image. The height and width of surface features are measured from line profiles.
AZoM - The A to Z of Materials - Left - 2-Dimensional image of the surface of a commercially available photo quality inkjet paper. The scan size is 30 microns and the scale indicates the height of the features on the paper’s surface. Right: Line profile showing the width of one of the pores in the paper
Figure 1. Left - 2-Dimensional image of the surface of a commercially available photo quality inkjet paper. The scan size is 30 microns and the scale indicates the height of the features on the paper’s surface. Right: Line profile showing the width of one of the pores in the paper.
Central to the AFM is a force sensor that measures the force between the stylus and the surface. Typically, the force sensor design is based on a light lever. In the light lever a laser beam is reflected off the backside of a cantilever into a two-section photo-detector. (Figure 2) On the bottom side of the cantilever is a stylus. Interactions between the stylus and the surface cause the cantilever to bend and the reflected light to move across the photo-detector. In this type of AFM force sensor, the cantilever is typically 40 microns wide, 100 microns long, and less than a micron thick. With such a small cantilever, forces as low as a nano-newton (10-9 Newtons) are measurable with the light lever force sensor. Thus, very small probes, less than 50 nm, can be used in the AFM and not be broken by interactions with a surface.
AZoM - The A to Z of Materials - Illustration of a light lever force sensor used in atomic force microscope. The laser is reflected from the cantilever into a two-section photo-detector.
Figure 2. Illustration of a light lever force sensor used in atomic force microscope. The laser is reflected from the cantilever into a two-section photo-detector.
The horizontal resolution in an AFM depends on the geometry of the probe and the type of sample being analyzed. On very smooth surfaces only the very end of the probe interacts with the surface and the resolution of the image depends only on the probe diameter. If the specimen has large features, more of the probe than the very end interacts with the surface and the image resolution depends on the macroscopic geometry of the probe. (Figure 3)
AZoM - The A to Z of Materials - Left - On a smooth surface only the very end of the probe interacts with the surface. Right - On a rough surface more of the probe interacts with the surface and the resolution depends on more than the probe diameter.
Figure 3. Left - On a smooth surface only the very end of the probe interacts with the surface. Right - On a rough surface more of the probe interacts with the surface and the resolution depends on more than the probe diameter.
Paper Coating Applications
Because the AFM directly shows the micron and submicron structure of paper coatings, the AFM is useful for helping to develop new paper coating processes. It is also possible to understand process failure by comparing the surface structure of paper from a good process and the structure from a failed process.
Gloss
The length scale of AFM measurements is the same length scale that controls the gloss properties of paper coatings. Thus the AFM is useful for helping to understand the relationship between surface topography, gloss and delta gloss.
Engineered Papers
A second application is the use of the AFM for engineered papers. The AFM readily measures the sizes and shape of surface particles and structure with dimensions ranging from a few nanometers to a few microns. By understanding the relationship between surface structure and coating performance, engineered papers can be improved.
The AFM is useful for visualizing surface structure, for making quantitative surface measurements, and for studying the dispersion of particles in the coating.
Surface Structure Visualization
The properties of a paper coating can depend critically on the micron and submicron structure of the paper coating The AFM is ideal for visualizing the sub micron structure of paper coatings. Such images easily show the particle size and shape of the components of the paper coating. Figure 4 illustrates the AFM images of an Opti-Gloss PCC aragonite coating before and after printing. The change in surface texture is readily visualized.
AZoM - The A to Z of Materials - Left - AFM image of the rod-like pigment particles in an Opti-Gloss PCC aragonite coating. Right - AFM image of the same coating after printing with UV ink.
Figure 4. Left - AFM image of the rod-like pigment particles in an Opti-Gloss PCC aragonite coating. Right - AFM image of the same coating after printing with UV ink.
Quantitative AFM Measurements
For many years mechanical and optical surface profilers could be used for analyzing paper surface texture. However, such profilers are limited in their horizontal resolution to approximately one micron. Thus, they can readily measure at a length scale of a few millimeters. Traditional quantitative surface roughness parameters such as Sa, Sq, Sr and Sm can be directly calculated from AFM images.
AZoM - The A to Z of Materials - Equations for surface texture that are readily used with the AFM images.
Figure 5. Equations for surface texture that are readily used with the AFM images.
As an example, it has been shown that the AFM could be used to explain anomalies in paper gloss. In a study, sheet A had a higher TAPPI (Trade Association of Paper and Pulp Institute) gloss at 75 degrees and sheet B had a higher TAPPI gloss at 20 degrees. By directly measuring the surface texture with an AFM, it was established that although the two sheets of paper had similar surface roughness, the skew and kurtosis were very different. Sheet A was significantly non-Gaussian and Sheet B was approximately Gaussian.
Particle Dispersion
In the case of clays and polycarbonates, the dispersion of particles in paper coatings can be readily established from AFM images. Figure 6 is of an Astr-Plus/Carinal 95 coating. In this image the positions of kaolin hexagonal platelets are clearly visible.
Besides measuring surface topography, images of other surface physical properties such as hardness and adhesion are measurable with the AFM. As an example, in the AFM it is possible to vibrate the stylus as it is scanned over a surface. Then by measuring the change in phase between the modulating signal and the signal coming from the photo-detector, images of surface hardness are obtained. In this technique, both the surface topography and surface hardness image are acquired simultaneously.
AZoM - The A to Z of Materials - AFM image of Astr-Plus/Carbinal 95 coating. This “engineered” coating is comprised of kaolin in a narrow particle size distribution and an ultra fine ground calcium carbonate with a latex binder.
Figure 6. AFM image of Astr-Plus/Carbinal 95 coating. This “engineered” coating is comprised of kaolin in a narrow particle size distribution and an ultra fine ground calcium carbonate with a latex binder.
This technique is ideal for visualizing the location of polymer material used in paper coatings. Figure 7 shows the topography image and hardness image of a commercially available paper coating. The dark areas in the image show the locations of the polymer material in the coating.
AZoM - The A to Z of Materials - Left - Topography . Right - Hardness image of a comercially available paper coating. These images are measured simultaneously. The gray scale in the hardness image represents the surface hardness.
Figure 7. Left - Topography . Right - Hardness image of a comercially available paper coating. These images are measured simultaneously. The gray scale in the hardness image represents the surface hardness.
Conclusion
The AFM is emerging as an important tool for characterizing and studying the micron and submicron topography and properties of paper coatings. Specific applications include the direct visualization of paper coatings, quantitative analysis of paper coatings, and the measurement of particle dispersion. Advantages of the AFM over the SEM are:
·        The AFM provides direct three-dimensional images
·        The AFM does not require sample preparation
·        The AFM can work in ambient conditions