Volume 4 Issue 3

  What’s Happening at Missouri S&T:
(Missouri S&T effective January 2008)

Summer 2007 Short Course - "Introduction to Coatings Composition and Specifications"
** Register Today - Some Spaces Still Available!!!**

This summer we will be offering "Introduction to Coatings Composition and Specifications" July 16-18, 2007, in St. Louis Missouri, a course designed for the new coatings person in areas such as sales, marketing or production. The course was initiated by a number of raw material companies and distributors requesting a course with this format. This course is not as heavily technical as is our “Basic Composition of Coatings¿? and “Introduction to Paint Formulation¿? courses. The ?Introduction to Coatings Composition and Specifications?? course is a two and a half day course which will discuss the types of coatings, the basic composition of coatings and the tests and specifications used by the industry. This course will allow the participant to gain the fundamentals needed to work in this industry and to communicate more clearly. More information can be found at the above links, on our website at http://coatings.mst.edu/index.html, or by emailing coatings@mst.edu or calling 573-341-4419.

Rooms at the Drury are filling up fast!! If you have registered for this short course and need to make hotel reservations go to: http://www.druryhotels.com and "Book it Fast". Simply enter the group #162434 to get the discounted short course rate. You can also call 800-325-0720 to reserve a room at the group block rate. Time and rooms are running out, so book today! If you cannot get a room at the Drury, there are other hotels nearby, but no group rates at these.

Fall Short Course Dates

This fall we will be offering ?Basic Composition of Coatings?? September 10-14, 2007 and. 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. For more information see our web site at http://coatings.mst.edu/index.html and to register contact Michael Van De Mark at coatings@mst.edu or call 573-341-4419. **This course is held on the Rolla campus**

This fall we will be offering ?Introduction to Paint Formulation?? October 22-26, 2007. 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/index.html and to register contact Michael Van De Mark at coatings@mst.edu or call 573-341-4419. **This course is held on the Rolla campus**

Technical Insights on Coatings Science

What is the Meaning of the Solids Content of Paint?
By MIchael Van De Mark, Director, Missouri S&T (Missouri S&T) Coatings Institute

The solids content of paint is considered the sum of any component that is non-volatile and thus contributes to the total film contents. There are two methods commonly used for the determination of the solids in paint. The first is percent solids by weight. The value is calculated by taking the weights of the non-volatile components in a paint and dividing it by the total weight of the paint. We can experimentally determine this by taking an amount of paint x in grams and drying it. The residue weighs y grams. The solids % by weight is calculated as follows:

This value is often used to verify that all the ingredients have been added to the correct amount. Most raw materials used in coatings are described in terms of weight percent solids.

The second is based upon volume. The percent volume solids is used to define many factors including film build. The volume solids is difficult to measure with precision but can be calculated from the formulation relatively accurately. Experimentally it can be determined by determining the density of the paint, the density of the dry film, and the percent solids by weight. An excellent device for determining the density of the dry film is with the Micromeritics Accupyc 1330 helium displacement densitometer.

The % volume solids is also used in many paint formulation calculations including, PVC, pigment volume concentration.

Particle Size Analysis by Light Scattering
By Cynthia Riddles, Graduate Student, Missouri S&T (Missouri S&T) Coatings Institute

Many physical properties of a coating can be affected by the particle size distribution of the polymer resin or the pigment. Polymer synthesis and characterization is an essential in formulating resins for any coatings application, and requires an accurate method for determining the polymer particle size distribution. Light scattering is one technique used for this purpose.

There are two basic methods of light scattering that can be used to determine particle size; they are Static Light Scattering (SLS), and Dynamic Light Scattering (DLS). Both are reliable and cover a particle range from a few nanometers up to a few microns.1

Scattering occurs when a polymer solution that is composed of particles of the polymer suspended in some medium cause a change in the trajectory of an incident beam of radiation. Scattering can be classified into two types; elastic and inelastic. Elastic scattering arises when the energy of the incident radiation is equal to the scattered radiation; energy is conserved; only the propagation of the radiation is changed. Inelastic scattering occurs when the energy of the scattered radiation is not the same as the energy of the incident radiation; energy is not conserved.

Static Light Scattering (SLS) is a method which measures inelastic scattering. Measurement is made of the amount of light that is scattered by a solution at an angle relative to the incident light.1 SLS operates of the principle that when a very small particle interacts with a beam of light a dipole is created in the particle; this is due to the fact that the light is an electromagnetic wave. The dipole that is induced has the same oscillating frequency as the incident light. This oscillating dipole undergoes acceleration and energy is emitted in all directions perpendicular to the incident light; this emitted energy is the scattered light.2

The scattering intensity is at a maximum in the plane perpendicular to the particle that is scattering and is zero along the axis of propagation. Measurements are taken of the total scattered light intensity as a function of the scattering angle and the information is sent to a computer where it is plotted in a Zimm plot. The following is an example of the equation used:

Here c is the concentration, R(q,c) is the excess Rayleigh ratio, K is the magnitude of the scattering vector, and H is an optical constant. Scattered light intensities are measured at several angles for each solution concentration and the pure solvent. It is then possible to determine the molecular weight, Mw, the radius of gyration, rg, and the second virial coefficient, A2, for the species under investigation.3

The radius of gyration for the particle is in essence the data used for determining the particle size.

Dynamic Light Scattering (DLS) is also called Photon Correlation Spectroscopy (PCS) is a measure of the time-dependent fluctuations in the intensity of the scattered light. These fluctuations arise due to the particles undergoing what is known as Brownian Motion.4

Brownian Motion is the random movement of particles in a solution. This random movement is due to the solvent or fluid molecules hitting the particles. When a laser light that is coherent strikes the particles, scattering is observed, but is shifted in intensity due to the moving particles. This shifting of the intensity can be thought of as Doppler shifted intensities, or frequencies. The frequencies are measured at one angle and analysis is made as to the velocity of these frequency shifts.

From the shifted intensity information, a computer calculates an auto-correlation function; this describes how a given measurement relates to itself in a time dependent manner.5 This auto correlation function is then used to determine the translational diffusion coefficient, D.

After the value of D is obtained, the Stokes-Einstein equation is used to calculate the hydrodynamic radius, this is also the radius of gyration.

D = KB T
      6¿?€ η d

The earliest Dynamic Light Scattering methods measured the intensity of the scattered light at a 90° angle from the incident light. This is termed forward scattering but is sometimes referred to as classical focal angle, or side angle scattering. Today some instruments couple the 90° forward scattering with a secondary forward scattering angle being less than 90°. With this type of measurement, particle concentration becomes limited. If the concentration of particles is too high, not enough scattered light can reach the detector for accurate analysis, and the opposite is true if the particle concentration is too low.

The method of backscatter measuring has somewhat eliminated the limitations on the concentration of the particles being measured. Solutions of up to 40 % by weight can be accurately measured with particle size ranging from 0.5 nm up to 3µm.6 Backscatter is typically measured at an angle of 173° up to 180° and has the benefit of avoiding multiple scattering effects.7

Both SLS and DLS methods are used in determining the particle size distribution of polymer solutions. Instruments vary in the exact range of particle size that can be measured, but generally all are nanoscale instruments. Laser sources for the instruments are varied, a couple of examples being: He-Ne with a wavelength of 633nm, also used is Argon ion source which emits various wavelengths from the UV through Visible regions.

Detectors are also varied, and depend of the type of scattering that is being measured. In dynamic light scattering the signal values are very small and require a photon counter detector, such as an avalanche photo diode. For a Static light scattering device, a solid state detector array may be used.

Analysis of particle size distributions can be carried out by both DLS and SLS, both methods provide size information and are used widely in industry and academia. Instrumentation is benchtop and somewhat user-friendly. When both methods are used particle size analysis can be carried out on solutions and powders that cover a very broad size range, and is an effective tool for determining particle size distributions.

References:

1. Alliance Protein Laboratories Inc. Classical Light Scattering. Accessed March 2, 2007 at: http://www.ap-lab.com/light_scattering.htm#Classical_light_scattering
2. Malvern. Static Light Scattering-Overview. Accessed March 1, 2007 at: http://www.malvern.co.uk
3. Laser Light Scattering and Materials Science Group. Static Light Scattering. Accessed March 4, 2007 at: http://www.unisa.edu.au/laser/Research/SLS.asp
4. Malvern. Characterization of Polymers Using Light Scattering Techniques. Accessed March 1, 2007 at: http://www.malvern.co.uk
5. Laser Light Scattering and Materials Science Group. Dynamic Light Scattering. Accessed March 4, 2007 at: http://www.unisa.edu.au/laser/Research/DLS.asp
6. Malvern. Zetasizer Nano. Accessed March 1, 2007 at: http://www.malvern.co.uk
7. Nanotrac. Dynamic Light Scattering, Laser Light Diffraction. Accessed March 1, 2007 at: http://www.microtrac.com/productsNanotrac.cfm
   

Is there a topic you would like discussed? Contact us by e-mail at coatings@mst.edu.