Volume 10 issue 3
What’s Happening at Missouri S&T:
Short Course Dates
We will be offering "Introduction to Coatings Composition and Specifications" July 17-19, 2013 (Summer 2013), 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.
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 firstname.lastname@example.org or email@example.com or call 573-341-4419. **These courses are held on the Rolla Campus**
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: firstname.lastname@example.org . 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
Factors influencing coalescence of latex resins
Ameya Natu, Graduate Research Assistant
Missouri S&T Coatings Institute, Dept. of Chemistry.
Latex resins are colloidal dispersions of discrete polymer particles in a continuous aqueous phase and coalescence is the process of formation of a continuous polymeric film from these latex resins. The process of film formation involves three stages viz:
Stage 1: concentrating the solid polymer particles through evaporation of water and formation of a close packed layer of latex particles
Stage 2: Deformation of particles from their spherical shape into polyhedrons and beginning of coalescence by interfacial and capillary forces
Stage 3: Formation of continuous film by inter-diffusion of polymer chains across particle boundaries which is known as coalescence.
Initially, as the water evaporates, a white opaque film is formed. Once sufficient water has evaporated the latex particles come in contact and begin to deform. This deformation process is driven by a combination of surface and capillary forces. The diffusion of polymer chains during coalescence provides mechanical strength to the film. The quality of coalescence of the latex has a dramatic effect on the properties of final completely cured coatings. Of all the factors we will discuss how the particle size of latex resin, the molecular weight of the latex and presence of coalescent solvent affects the coalescence and film formation process in general.1,2,3,4
Eckersley and Rudin5 found that the MFFT was proportional to the number average particle diameter of latex particle. Jensen and Morgan6 found, that as the latex particle size decreased by a factor of seven, the MFFT was reduced by approximately 100 K. Larger particles create larger voids when they come in contact with each other. This leads to longer time for the voids to be filled by particle deformation reducing their efficiency of coalescing. In comparison, smaller particles provide better packing with minimum void volume and hence better and faster coalescence.7, 8 Also, Eckersley and Rudin found that larger diameter particles have smaller contact radii and so the degree of coalescence is reduced.9
Though not important in the initial stages, the molecular weight of the latex resins plays an important role during the coalescence stage of film formation. Transmission electron microscopy (TEM) studies done by various authors indicate the disappearance of latex particle contours as the coalescence proceeds. The coalescence process is diffusion controlled as the polymer chains diffuse into each other. The diffusion rate is inversely related to the molecular weight of the polymer chains. The reptation time which is the time required for a polymer chain to move a distance equal to its radius of gyration also increases as the molecular weight of the latex increases. So, higher molecular weight polymers coalesce slowly due to their slow diffusion.10, 11
Coalescing solvents are volatile plasticizers although they accelerate deformation as well as coalescence. They should be soluble in the polymer and have a low but appreciable rate of evaporation or have a mechanism to become part of the resin or lose their solvency as a function of time. The coalescing solvents reduce the minimum film forming temperature (MFFT) by lowering the effective modulus of the latex particles promoting the polymer chains to flow and diffuse. Once the film has formed, the coalescing agents generally diffuse to the surface of the film and evaporate. Recently, because of environmental restrictions, research has been directed towards finding eco-friendly substitutes for the coalescing solvents or developing resins which do not need them at all.12,13,14
- Z. W. Wicks Jr., F. N. Jones, S. P. Pappas and D. A. Wicks, Organic Coatings: Science and Technology, 3rd edition, Wiley 2007.
- K. Kendall and J. C. Padget, International Journal of Adhesion and Adhesives, 149-154, 1982.
- K. Hahn, G. Ley, H. Schuller and R. Oberthur, Colloid & Polymer Sci., 264, 1092-1096, 1986.
- M. Canpolat and O. Pekcan, Journal of Applied Poly. Sci., 59, 1699-1707, 1998.
- S. T. Eckersley and A. Rudin, Journal of Coatings Technology, 62, 89-100, 1990.
- D. P. Jensen and L. W. Morgan, Journal of Applied Poly. Sci., 42, 2845-2849, 1991.
- P. A. Steward, J. Hearn and M. C. Wilkinson, Advances in Colloid & Interface Science, 86, 195-267, 2000.
- C. S. Kan, Journal of Coatings Technology, 71, 89-97, 1999.
- S. T. Eckersley and A. Rudin, Film formation in Waterborne Coatings, Chapter 1, ACS Symposium Series, 1996.
- K. Hahn, G. Ley, H. Schuller and R. Oberthur, Colloid & Polymer Sci., 266, 631-639, 1988.
- Y. Wang and M. A. Winnink, Macromolecules, 23, 4731-4732, 1990.
- C. L. Zhao, Y. Wang, Z. Hruska and M. A. Winnik, Macromolecules, 23, 4082-4087, 1990.
- N. Jiratumnukul and M. R. Van De Mark, Journal of the American Oil Chemists’ Society, 77, 691-697, 2000.
- M. R. Van de Mark, et. al., “Waterborne Film Forming Compositions.” U.S. Patent 7,705,084 B2, issued April 27, 2010.