Atomic Force Microscopy Brings New Possibilities to Waterborne Wood Coatings
Currently, at 5 million dry pounds, waterborne coatings comprise only 10% of the wood coatings market, which is estimated at 50 million dry pounds. Total market size for this category is $400 million. The use of waterborne wood coatings is expected to grow at a rate of 7–10% annually at the expense of solventborne systems. If and when performance enhancements are brought to the market, and regulatory changes implemented at the manufacturing level, this growth in waterborne systems could very likely accelerate to occupy a higher proportion of the overall market.
The key to delivering high-performance, waterborne wood coatings could very well lie in atomic force microscopy (AFM), an advanced technology that is used for the visualization of structures in order to understand, in minute detail, the properties of a substrate and the effect of coatings on it. With AFM, visualization can be taken to atomic and molecular detail through scanning probe techniques.
Recent work in the high-resolution imaging of polymers through more conventional methods, such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), has been limited since it is very difficult for the probes to successfully image a surface as complex as wood, and the sample preparation techniques are very tedious. With AFM, sample preparation is much easier, but most of the work to date has been done on polymer films applied to glass or mica substrates. In these cases, AFM has been shown to be an excellent tool for assessing polymer film formation.
Although high-resolution imaging has been around a while, there is the added challenge of having access to the advanced level of interpretation skills that are required to use it effectively. According to a study by Professor M. Cynthia Goh at the department of Chemistry, University of Toronto, with the abundance of image enhancement procedures, it is easy for AFM to either aid or mislead the researcher. However, it is a necessary technology to get to the heart of how polymers work on different substrates.
According to Goh, understanding the evolution of macroscopic properties from the molecular structure is a prevailing theme in condensed matter studies. Nowhere is this goal more important than in the case of polymers. At the fundamental level there is the challenge of understanding why and how certain structures form, and how to control them. This information has major implications in technology since properties of polymeric materials depend not only on their atomic constitution, but also on their morphology.
Nacan has been working with Goh in applying her skills to the study of polymer physics in wood coatings. This article discusses the tests that were conducted, the overall results and the implications for future product development. These tests took place over a one-year period and provided conclusive results that could significantly impact the development of latex formulations in wood coatings and other product categories.
Atomic Force MicroscopyIt is crucial to understand what AFM is and how it works. Tracking the surface by a probe tip as a measure of topography has been around for some time. In fact, a record player works on a similar principle. The stylus profilometer, which is a common instrument in studies of surface hardness, has been used for examining surface roughness as well as topography, but has a tendency to scratch samples. SEM has limitations, in that three-dimensional information is not available, and SEM resolution and contrast are very poor. Other techniques, such as TEM, also provide only two-dimensional data.
AFM, however, can be applied to any material, offers a much higher resolution (i.e., to the atomic level), and causes far less surface damage due to the reduction of size and forces involved. The probe tip, which interacts with the sample, is mounted onto a cantilever that translates the forces acting upon the tip in a measurable quantity. AFM can provide the following four main advantages in visualization.
- Precision. The AFM can see structures at various length scales with ease and minimal sample preparation, from subnanometer to hundreds of microns.
- High Resolution. It can see the surface with very high vertical resolution and provide detailed three-dimensional visualization.
- Reduced Damage. The AFM can image samples in a relatively nondestructive manner.
- Real-Time Monitoring. Changes in morphology as conditions are varied can be achieved in real-time.
The Testing ProcessIn examining the macrostructure of wood, the ultimate goal was to design a polyurethane-acrylic latex that completely covers the wood, and delivers higher performance and durability than available products.
AFM was used to examine the surface properties of wood and measure the vertical distance of the wood’s roughness. Tests were performed on red oak and maple, the most common woods used for floor coverings.
Taking images to the micron level, the wood was examined at three stages: naked wood, after the sealer coat application, and after top coat application. Two polyurethane dispersion acrylic hybrid samples were prepared, one with a latex particle size of less than 100 nanometers, and one with a particle size greater than 200 nanometers. The Physical Properties chart (see Chart 1) indicates that all other properties of the latex formulations were kept at a constant except particle size.
The surface plot of the AFM image on red oak wood is shown in Figure 1. This surface plot was with the Z (height) scale of 400 nanometers, which is invisible to the naked eye. Vertical distance, as shown in the sectional analysis (see Figure 2), shows a maximum range of 200 nanometers. This indicates a very rough or uneven surface.
As a finer grain wood, maple recorded a vertical range of 130 nanometers (see Figures 3 and 4).
A sealer coat of the acrylic latex, with particle size less than 100 nm, was applied to red oak and maple, respectively. The surface plot for both wood surfaces indicates a substantial reduction in roughness (to approximately 20 nanometers) (see Figures 5–8, p. 16). The application of a top coat, which was applied after the sealer coat, made relatively little change to the surface roughness.
The process was repeated on maple wood using a sample formulation of the polyurethane-acrylic latex that is greater than 200 nm particle size. After application of the sealer coat, the surface was far less uniform, and the sectional analysis indicated that the vertical distance was reduced to only 50 nanometers (see Figures 9 and 10, p. 118). To eliminate other possible mitigating factors, different coalescing solvents (i.e., 20% texanol) were used to ensure that the uniformity of the film was not impacted. In changing the solvents and repeating the experiment, it was found that the results were virtually the same.
The overall coating performance results are shown in Chart 2.
The two latex formulations (i.e. smaller and larger particle size) were tested for additional performance properties including gloss, and adhesion and abrasion resistance. In all cases, the latex with larger particle size exhibited poorer chemical resistance than the latex with small particle size. It is evident that the small particle size formulation provided a better seal, superior chemical resistance and a higher gloss finish.
ConclusionFollowing these extensive studies, it was determined that latex design plays a major role in the final performance of wood coatings. To provide maximum performance, the particle size must be less than the wood surface size in order to obtain a defect-free film.
Results show that AFM can now be used to work more extensively with other substrates and polymers. Similar research is being conducted in the area of ethylene vinyl acetate latex for low-odor coatings in an effort to improve block resistance and examine in microscopic detail the polymer film formation process and its effect on the physical properties of the formulation.
For more information on microscopy, contact Nacan at 60 West Dr., Brampton, Ontario, Canada L6T 4W7; phone 905/454.4466; fax 905/454.3401.