Volume 10 issue 4

What’s Happening at Missouri S&T:
(formerly UMR)

Short Course Dates

We will be offering "Basic Composition of Coatings"  September 23-27, 2013 (Fall 2013). The Basic Composition course is intended for new personnel in the coatings profession. It targets the components of coatings (resin, pigments, extenders, solvents and additives), testing and specifications, general formulation and manufacturing methods. Basic Composition is primarily a lecture course with several laboratory demonstrations.

We will be offering "Introduction to Paint Formulation"  October 21-25, 2013 (Fall 2013) . This course is intended to give the person a fundamental knowledge of how to approach a starting formulation and troubleshoot it. This course involves both lecture and laboratory work.

For more information see our web site at http://coatings.mst.edu and to register contact Catherine Hancock at cemv26@mst.edu or coatings@mst.edu or call 573-341-4419. **These courses are held on the Rolla Campus**

Employment Tab

     We have started an employment section for our students and companies. We have a full time job section, an intern / co-op section and a graduating and alumni students section . Please explore our section on employment on our web site. Anyone wanting to have job opening listed, please contact us at (573) 341-4419 or e-mail: svgwcc@mst.edu . You can also write to us at Missouri S&T Coatings Institute, BOM #2, 651 W. 13th St., Rolla, MO 65409-1020. Our web site is http://coatings.mst.edu

Technical Insights on coatings Science

Particle Size Measurement  by Dynamic Light Scattering in Coatings

Dr. Ming Hang Chen, Missouri S&T Coatings Institute

     In the coatings industry, the particle size and distribution (PSD) of the resin,  pigment and extender in the formulation is an important parameter of a coating. The PSD of the pigment particles can determine the gloss of the coating film between flat, eggshell, semi-gloss, and gloss. The hiding power of coating can also be affected by PSD of pigments. The rheological characteristics such as viscosity, flow or thixotropic behavior are also affected by the size distribution. To the formulation chemist, it is also necessary to consider the particle size of pigments when estimating the amount of dispersants. Therefore, accurate monitoring of the particle size is an important parameter to providing good quality paints.

     Manufactures of pigments and extenders normally offer the data of particle size of their products including the average particle size and its range. These data are measured when the products are powders. For most liquid coatings, we care more about the PSD of pigments when they are immersed in dispersing media.

     Dynamic light scattering (DLS) offers a convenient way to measure PSD of pigment in media. The technique measures the time-dependent fluctuations in the intensity of scattered light from a suspension of particles undergoing random, Brownian motion. Diffusion coefficient is determined from analysis of these intensity fluctuations. Subsequently, particle size is determined from Stokes-Einstein Equation. [1]

k_B- Boltzmann constant, T-Temperature in k, η-viscosity of solvent, r-radius of particle

The technique has its natural advantages: non-destructive, fast, low cost. With the proper instrumentation, it is also possible to realize onsite monitoring PSD when preparing wet slurry of pigments.

     The fundamental assumption for using DLS to measure the PSD of suspended particles is that the particles are undergoing Brownian motion. In other words, the interaction between particles should be able to be neglected compared with Brownian force from solvent molecules. Therefore, in practice, the sample must be dilute or diluted before measurement. Sometimes the dilution can alter the morphology or stability of particles. For example, some pigment may aggregate in high concentration, and break apart in low concentration. The PSD measured in low concentration may not reflect the status of pigment in high concentration.

     Another important assumption is that there is negligible multiple scattering.  Multiple scattering happens where light scattered by one particle will itself be scattered by another. This phenomenon increases with concentration. The consequence of multiple scattering is to reduce the apparent particle size and the intercept value (the signal to noise ratio). Conventional DLS, where the detecting angle is 90 degree, require the concentration of sample to be very low to prevent multiple light scattering. Today, the DLS systems use back scattering where the detecting angle is close to 180 degree and has successfully solved the issue. Back scattering  allows for much higher concentrations to be measured compared with conventional DLS.

     While DLS is a powerful technique to measure the particle size of submicron particles, it still faces some challenging from some special case, for example, if the particle size is less than 20 nano meters. Due to the small particle size, (if the light scattering index of particle is comparable to that of solvent), the light scattering intensity is low. Due to the limitation of the sensitivity of the probe of detector, the signal to noise ratio is low. In order to increase intensity, the concentration should be increased. Although the multiple light scattering is not a problem for nano scale particles, the interaction between small particles in high concentration is not out of the question. Recall that the fundamental assumption of DLS is that the particle-particle interactions should be small compared with Brownian motion. It seems that DLS cannot solve this dilemma. The good news is that an improved DLS model can break this shackle. According to the Generalized Stokes-Einstein equation, [2] the relationship between the diffusion coefficient and the viscosity of suspension can be expressed as equation 2 and 3

     At higher concentration, the viscosity of the suspension must be used instead of solvent to calculate the particle size. In this way, the regular DLS instrument with back scattering can still be used.[3] Just it is necessary to measure the viscosity of the concentrated suspension where the intensity of light scattering is high enough.

     In resins latex particle size can be measured at 0.01 to about 1% concentration.  Latex resins in coatings are generally between 0.2 to 0.05 micron in diameter with most being close to 0.1 nicron.  Another example is water borne urethanes, these are typically 0.05 to 0.02 microns in diameter and can be measured at 0.1 to 5% solids concentration.  Recent work by our group at Missouri S&T coatings Institute on Colloidal Unimolecular Polymer (CUP) particles produced 3-9 nanometer diameter particles. [3-5] These ultra small particles require 6 to 12% solids to obtain valid results.  In the CUP system the viscosity of the solution is required but for the typical latex and waterborn urethane the viscosity of water produces the correct data.

References.

1. P.R. Hiemenz, R., Principles of colloid and surface chemistry. 3th ed., Marcel Dekker, New York, 1997.
2. P.N. Segre, S.P. Meeker, P.N. Pusey, W.C. Poon, Phys Rev Lett75 (1995) 958.
3. M. Chen, C.J. Riddles, Michael Van De Mark, Colloid & Polymer Sci.
4. C.J. Riddles, W. Zhao, H.-j. Hu, M. Van De Mark, Polymer Preprints52 (2011) 232.
5. M.R. Van De Mark, S.V. Gade, A.M. Natu, The waterborne symposium; DEStech Publications, INC., New Orleans, Louisiana, 2013, p 332.