What is particle size and why is it important? What is a particle? How is particle size measured and described? What effects can the particle size of coatings ingredients have on the formula properties? Let’s explore these questions. Particles are defined as separate discrete entities in a continuous media. Examples of particles include powders, granules, suspensions, emulsions, and even aerosols. The most important physical properties of particulate matter are particle size and particle size distribution. In this article, I will discuss particle size and its effect on coatings properties starting with an overview of measuring and describing particle size. Then we will discuss how the particle size attributes of latexes and pigments affect coating properties.
Measuring and Characterizing Particle Size
In order to understand the effects of particle size on coatings properties we need to understand how particle size and particle size distribution are measured and described. Physical and optical Methods are used to measure particle size and distribution of many materials. Sieve analysis is the most common physical method used to measure the particle size of materials in the range from 20 microns to as large as 125 millimeters. While sieve analysis is used in many fields from agriculture to astronomy, in coatings it is most useful to characterize particle size in powder coatings and raw materials such as pigments and extenders. Sieve analysis can also be used as a preparatory method, for example as a means to isolate powder coatings of a desired particle size range in the laboratory.
A variety of optical instrumental methods are suitable to measure different particle size ranges from as small as 0.3 nanometers up to 135 millimeters. Some of the more commonly used of these are shown below with the practical range of particle sizes they can be used to measure.
- Laser Diffraction Analysis – 10 nanometers to 1000 microns
- Dynamic Light Scattering Analysis – 0.1 nanometer to 10 microns
Dynamic Light Scattering and Laser Diffraction methods are widely used for determining particle size and distribution in the coatings industry. These two methods are applicable to the measurement of particle sizes of latexes, dry pigments/extenders and pigment dispersions, three of the most important types of particulate materials used in coatings.
Very few particulate materials are a single particle size, most are a distribution of particle sizes. There are various ways to express particle size and particle size distribution. We will discuss some which are used most commonly when describing coatings or coatings raw materials. Whenever we have a distribution of something the first descriptor used is often the mean. Depending on the method we use, the mean particle size might be expressed in mass or number of particles. Another common descriptor of a distribution is the Median particle size, this is the point at which half the particles are smaller and half larger.
When referring to particle size distributions the median can also be called the D50, meaning that fifty percent of the particles are smaller than this diameter. Similarly, the D90 is the value under which ninety percent of the particles measured fall. The final common descriptor of particle size distribution we will discuss is the D10. This is the value that ten percent of the particles size distribution falls below. Taken together, these three values allow us to visualize the particle size distribution of our sample. We can determine if the distribution is normal (distributed evenly around the median) or skewed. The graphic below shows an example of these three particle size descriptors (Graphic from MICROTRAC website).
What effect does particle size have on coatings properties? The particle size of coatings ingredients can influence properties such as viscosity, rheology, film formation, gloss, and opacity. Two common coatings raw materials, latex polymers and pigments are particulate materials which have outsized effects on the properties of coatings made from them.
At first glance, latexes seem like homogeneous liquids but they actually are solid polymer particles distributed in a liquid medium (water). Latex particles are very small, typically 0.05 to 0.5 microns in diameter. Among coatings properties affected by the particle size and particle size distribution of the latexes used in them are.
- Viscosity and rheology
- Solids Content
- Film Formation
- Flow and Leveling
- Gloss
The effects of latex particle size and particle size distribution on coating viscosity and rheological properties is well documented. The relative closeness of the packing between the particles is determined by particle size and distribution and is a major determinant of viscosity and rheology. The solids content for a formulated coating is also a function of particle packing.
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Like viscosity, rheology, and solids content, latex particle size and distribution also affects the process of film formation in a latex based coating. How well the particles pack together during coalescence to form a film depends on the particle size and distribution of the latex as well as other factors like minimum film forming temperature and coalescing aids. The film formation process also affects flow, leveling and gloss of the applied paint film. Tailoring the particle size of the latex in your formulation can help you achieve the rheological properties, solids content, film formation, leveling and gloss desired in your formulation.
Pigments are solid particles suspended in a liquid or solid continuous phase. Whether they are incorporated as powders or dispersions pigments have the attributes of particles which include particle size and particle size distribution. For many discussions of pigments, we separate them into three groups, white pigments, color pigments and extender pigments. The particle size attributes of these different pigment types can affect different coating properties. Some examples of this are shown below. Like latexes, how well the pigment particles pack together is a major factor in how particle size affects properties.
- White Pigments – opacity, tint strength, viscosity
- Color Pigments – opacity, tint strength, color shade, viscosity
- Extender Pigments – gloss, volume solids, DFT, viscosity
Pigment particle size influence on the optical properties of a coating can be complex and hard to predict. Generally, larger particle size pigments provide higher opacity but with the caution that there is a maximum after which hiding can fall off. Smaller particle size pigments usually increase tint or color strength but reducing particle size too much can change the color shade of the pigment. Viscosity is influenced by particle size, with smaller average size pigments generally increasing viscosity due to more efficient packing, and higher surface area interactions between pigment particles. Careful selection of extender pigments for particle size and size distribution can help attain the desired gloss, viscosity, and volume solids.
Most of the time we paint formulators use the chemical composition of our formulas to achieve the desired coatings properties. Manipulation of the particle size and particle size distribution of ingredients can also affect these properties. Try utilizing the power of the particle size to achieve the coatings performance you and your customers are looking for.
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