In this series of five blog posts, we’re examining the analytical approaches that you can use to get a better understanding of paints and coatings at a nanostructural level. Number two is an approach that I think no coatings scientist can do without – crystal phase analysis using X-ray diffraction.
Since their discovery by Röntgen in 1895, X-rays have found many uses in physics, chemistry and materials science. In fact, I’ve already covered the use of X-ray fluorescence (XRF) to determine elemental composition in my previous blog post. But despite being based on the same process of hitting a sample with X-rays, XRF’s analytical cousin X-ray diffraction (XRD) works in a rather different way and provides completely different information.
Using XRD for crystal phase analysis
XRD operates on the principle of irradiating a crystalline material with monochromatic X-rays and then measuring the angular intensities of the same wavelength X-rays as they scatter from the material. In contrast to X-ray fluorescence for elemental analysis, (in which the individual elements in a material respond with a characteristic X-ray spectrum), in XRD, X-rays are scattered by the regular arrangement of molecules in crystalline lattices. In large enough crystals, the scattered waves interfere to produce a diffraction pattern.
Diffraction patterns give information about both the molecules in a material and the crystal structure they have in solid form – a combination called a ‘phase’. For larger crystals the diffraction patterns are strong and can tell you lots about the internal structure of the crystals, whereas for smaller crystals (such as nanocrystals) the interference is weak, and instead only a scattering pattern is analyzed. Nevertheless, a scattering pattern can still give you information about the size and shape of nanocrystals and nano-layers.
In days gone by, you’d have had to interpret these scattering and diffraction patterns manually, which required more maths than most people would be comfortable with, but the good news now is that all the analysis is automated.
This means that you queue your sample for automated acquisition and data analysis, and find out a large amount of information about your sample – including the materials present, the distribution of phases, the characteristics of crystallites, thin films and nanoparticles, as well as details of strain, preferred orientation, layer thickness, and much more.
In terms of the amount of information that can be gained using a single technique, that’s pretty impressive. But what is XRD actually useful for in the field of paints and coatings?
What is XRD used for in the paint industry?
Checking chemical purity
You’ll hardly need me to tell you that the performance of paints and coatings depends crucially on the quality of the components that go into the formulation. So it’s little wonder that one of the most widespread uses of XRD in the paints and coatings industry is to check the purity of incoming material.
Top of the list for inspection is titanium dioxide or TiO2, which is used as a pigment in many coatings because of its opacity and white color. However, it comes in two polymorphs, anatase and rutile, with the latter being preferred because of its higher refractive index. Fortunately, determining the ratio of these two is an easy task for XRD, allowing manufacturers to make sure that their supplies don’t deviate from what’s needed (the use of XRD for this task is even the subject of a standard method, ASTM D3720).
Fine-tuning color performance
In addition to simply checking supplies, XRD has a role to play in more advanced formulation science, by looking more closely at the color performance of a particular component.
For example, the various phases of iron oxide used in formulations provide gradations of color that are important for achieving the right balance of tinting strength and hue. Built upon many years or artistic endeavor, there are plenty of other pigments where understanding the exact nature and purity of an inorganic or organic compound – or mixtures of them – is critical to achieving a reproducible and enduring color specification.
There is also a large new world of extenders, which can be mineral or organic particles that are more cost-effective than the main color pigments and which can contribute positively to the final color performance. But their mineral purity is important, and phase analysis is frequently used to identify and screen any mineral inconsistencies.
(By the way, color will be a big focus of the next blog post in this series, when we’ll be talking about the effect of particle size and shape).
Investigating smart coatings
Last but certainly not least in this short review of the applications of XRD are so-called ‘smart’ coatings. These are coatings that respond in some way to an external influence – such as varying their refractive index in response to temperature or electric field. In many of them the ‘smart’ function is contained within crystalline pigment particles, which require the same analytical considerations as mentioned above for ‘color’ pigments.
But whatever exciting properties they may have, smart coatings are still fundamentally coatings, and so can be investigated using the same techniques. Once again, XRD is of particular value, because it is unsurpassed in its ability to provide nano-scale information on surfaces and polycrystalline thin films. For example, XRD experiments dedicated to surface thin films can tell you about residual stresses, preferred orientation, film thickness and other aspects of surface microstructure – all of which are essential in order to know how your final coating is performing and whether it’s going to remain ‘smart’.
XRD – In-depth crystal phase analysis
All the above examples show the growing importance of XRD in the world of paint formulation and application. This is especially true given the growing pressure on quality of supplies, and as the accelerating need for higher-performing coatings piles pressure on manufacturers to understand what’s happening in coatings at the nanoscale.
In conclusion, as someone who’s spent much of my career using XRD, it might come as no surprise to hear me say this – but if you allowed me just one analytical technique to use in the paints and coatings sector, it would have to be XRD!
Written by: Patricia Kidd, Posted by: Matvern Panalytical (www.materials-talks.com)