Volume 11 issue 1

What’s Happening at Missouri S&T:

(formerly UMR)

Short Course Dates

We will be offering "Basic Composition of Coatings"  March 24-28,2014 (Spring 2014). 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"  May 19-23, 2014 (Spring 2014). 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.

We will be offering "Introduction to Coatings Composition and Specifications" July 21-23, 2014 (Summer 2014), 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.

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**

Online Short Course

We are offering "Introduction to the Coating Systems" online short course. This course is targeted for automotive and aviation type OEM companies. This self-paced seminar will cover the painting system from the composition of paints to the evaluation of the dry film.  The pigments, resin, solvents and additives will be discussed including their influence on the coatings performance.  Color measurement, surface profile, and other evaluation criteria will be related to composition.  The importance of surface preparation and other manufacturing criteria will show the system complexity and each step's importance.

We are offering "Surface Defects: Elimination from Human and Process Contaminants" online short course. This course addresses many of the issues in prevention and minimization of defects. The course covers the defects caused by the coatings process, as well as human issues, including personal care product causes. Several of the surface defects are discussed – from basic principles and real world automotive and aircraft examples. The highly practical approach of this course will greatly aid the personnel involved in the painting operation to reduce and systematically approach issues.

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

Factors affecting diffusion of polymer chains during latex film formation

Ameya Natu, Graduate Student

Missouri S&T Coatings Institute

         Latexes are aqueous dispersions of solid polymeric particles synthesized using an emulsion polymerization technique. Due to the environmental restrictions on solvent emissions, an extensive amount of research has been conducted on water-borne resins over the last 60 years. Since the latex resins are composed of particles, development of the mechanical strength of the final latex film depends on how well the latex particles coalesce and interdiffuse. Three stages are involved during the drying process:

        Stage I: water evaporates from the film as the particles come in close contact. Stage II is characterized by deformation of particles into a closed packed structure. In Stage III, interdiffusion of chain segments between neighboring particles gives the film its final mechanical properties. This last stage is referred to as autohesion or gradual coalescence.^1,2

        The methods used to investigate the process of diffusion of polymer chains include small angle neutron scattering (SANS) and direct non-radiative energy transfer (DET). The SANS technique measures the increase in particle size during diffusion of deuterated species. The DET technique involves labelling an equal mixture of latex particles with either: a donor fluorescent chromophore and an acceptor fluorescent chromophore or b) two different fluorescent chromophores. During interdiffusion, energy transfer occurs which allows a decay profile to be analyzed in terms of time resolved fluorescence decay measurements to provide data on the extent of mixing and diffusion coefficient.³

Factors affecting the diffusion process include:

1. Molecular weight of polymer
2. Temperature 
3. Crosslinking of polymer chains
4. Latex Particle size
5. Presence of surface hydrophilic groups
6. Neutralization of surface acid groups
7. Coalescing aids

         Different models exist to determine contributions from various factors affecting the diffusion coefficient. One such model which takes into consideration the contribution of molecular weight, particle size and time is the spherical diffusion model given as:

‌M_t is the amount of substance that has diffused across the boundary at time t, R is the radius of spherical particle C_o is the initial concentration of particles. For linear polymers it has been observed that D ~ 1/M².

        The temperature has to be kept above the glass transition temperature of the polymer chains, otherwise, diffusion is restricted. Temperature dependence of polymer diffusion is described using the Williams-Landel-Ferry equation:

Where, D_o is the diffusion coefficient at reference temperature T_o, and T_∞ is the Vogel temperature corresponding to infinite viscosity or relaxation time.⁷ The diffusion coefficient can be simply expressed in terms of an Arrhenious equation ^4,5,6 as:

        ‌

        The presence of surface hydrophilic groups such as carboxylic acid and sulfonic acids retards the diffusion process since these are composed of polymers with a different Tg than the core polymer. So, interdiffusion occurs only when the annealing temperature is above the Tg of the membrane polymers. Conversion of the acid groups to its salt form also slows down the diffusion process. An ionomeric membrane is formed within the latex film, which is immiscible with the rest of the polymer and probably has a higher Tg than the polymer with acid groups. Ionic repulsion arising from the ionic groups on the polymer also hinders the diffusion process. Ammonia causes a very small reduction in diffusion while lithium < sodium <calcium hydroxide have a pronounced effect in the increasing order. This is because ammonia is easily evaporated while sodium hydroxide stays in the film. In case of calcium, diffusion ceases because the ionomer interaction is very high possibly through formation of stable ionomerically crosslinked network.^8,9

 Figure 1: latex film formation process⁹

         The crosslinking reactions and the diffusion process are competitive. During the initial stages of crosslinking, diffusion rate is also high as the temperature of the reaction is above the Tg of the polymer species. But above a certain degree of crosslinking, specific to the system involved, diffusion ceases.¹⁰ Coalescing aids enhance the rate of polymer interdiffusion since they decrease the MFFT and Tg of the polymer. A consideration of all these parameters during latex formulation is necessary to obtain final latex film with optimized properties.

 References:

1. J. W. Vanderhoff, E. B. Bradford and W. K. Carrington, J. Polym. Sci.: Symp. No. 41, (1973) 155.
2. Soren Kiil, Progress in Organic Coatings, (2006) 57 236-250.
3. P. A. Steward, J. Hearn, M. C. Wilkinson, Advances in Colloid and Interface Science, (2000) 86 195-267.
4. H. B. Kim, M. A. Winnik, Macromolecules, (1995) 28 2033-2041.
5. H. B. Kim, M. A. Winnik, Macromolecules, (1994) 27 1007-1012.
6. C. S. Kan, Journal of Coatings Technology, (1999) 71 89-97.
7. C. L. Zhao, Y. Wang, Z. Hruska, M. A. Winnik, Macromolecules, (1993) 23 4082-4087.
8. S. T. Eckersley, B. J. Helmer, Journal of Coatings Technology, (1997) 69 97-107.
9. K. Hahn, G. Ley, R. Oberthur, Colloid and Polymer Science, (1988) 266 631-639.
10. M. A. Winnink, P. Pinenq, C. Kruger, J. Zhang, P. V. Yaneff, Journal of Coatings Technology, (1999) 71, 47-60.