Volume 8 Issue 3
Volume 8 Issue 3
What’s Happening at Missouri S&T:
Short Course Dates
We will be offering "Basic Composition of Coatings" September 19-23, 2011 (Fall 11). 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 10-14, 2011 (Fall 11). 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 Catherine Hancock at email@example.com or firstname.lastname@example.org or call 573-341-4419. **These courses are held on the Rolla Campus**
We have expanded our web operations, and we need your help in spreading the word. We are inviting people from industry and academia to join the Missouri S&T Coatings Institute Listserv. By joining our listserve, you'll be able to receive newsletter from us which has Missouri S&T coatings short courses dates, employment advertisements and short articles related to polymers and coatings. Please reply us at email@example.com with your name, company name and e-mail address to get added to our Listserv.
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
Technical Insights on Coatings Science
Powder slurry coatings
Sagar Gade, Graduate Research Student, Missouri S&T
Being eco-friendly, powder coating, waterborne coatings and high solid coatings are the most preferred OEM coatings today. Of which, powder coatings have attracted the most attention, since the coatings are solvent free. Powder coatings are also durable and resistant to acid rain, UV as well as chip and scratch resistant, films of 100-500mm can be applied in single operation with relatively low energy consumption. Along with the preliminary advantages of powder coating, they have some major disadvantages: dust explosion hazards, inability to coat large and heat sensitive substrate, appearance limitation, material limitation, limited production flexibility, sintering during storage, contamination, color change. Wet paints can often be adjusted or tinted on-site, but this is not possible with powders. 1
Aqueous dispersion (slurries) of powder overcome some of the limitation of powder coating. 1 As the powder particles are in aqueous slurry form instead of a dry powder, the slurry avoids the dusting problem and fine airborne particulates related problems.2 This eliminates the powder explosion problem, broaden the range of application methods and reduces storage stability problems. The Tg of the powder no longer needs to be high to avoid sintering and therefore more flexible coatings can be formulated and lower baking temperature can be utilized. Settling problems can be minimized and appearance improved by using small particle size less than 10mm in slurries. The film build is reduced, conventional automotive application equipment can be used, higher flexibility in paint formulation, slurries are more homogeneous, filtration is more efficient, and the leveling is influenced by many factors. 3 However, new challenges are added since the aqueous dispersion must be stabilized without undue scarifice of properties due to presence of surfactant, and the spray rheology must be controlled.
Process for preparation an aqueous dispersion coating compositions
Powder slurry coatings are made by dispersing the powder in aqueous media (water), which gives synergistic advantage of both powder coatings and waterborne coatings. Aqueous powder slurries utilize powders which are produced by the conventional powder coating process using a twin or single screw extruder and milled to a particle size of 30 micron. The powder is then dispersed in to water at a solids content of 30-40% and wet milled to an average particle size of less than 10microns. Additives are added during the slurry production to stabilize the suspension, which introduce approximately 2% solvent. 4,5
Mechanism of drying
When powder slurry coatings are applied by air spray, electrostatic spray, dip coat or brush coat, it is not dry; the water in the interior of the coating film is free to move among the coating particles to the surface. This diffusion speed is sufficiently fast even towards the end of drying process that it resembles the rate of free water evaporation. This is not the case with the water-soluble resin coating or even an emulsion coating reaches the surface by means of diffusion. The curing of the film can be divided in three parts. 2, 4
1. Evaporation of water: here the water inside the coating film gets evaporated by means of raising the temperature up to 600-700C for 10minutes.
2. Melting and flow of the resin: then the temperature is raised up to 1200C where the resin melts and the applied coating flows into a smooth coating layer.
3. Curing of the resin: here the curing of the coating take place by raising the temperature up to 1500C-1800C for 10 minutes to get high gloss.
Powder slurry coatings are used in packaging containers 6 , automotive industry 6,7 , coil coatings 8 , can coatings, other industrial coatings, appliance coatings and many applications.
1. Wicks Z.W., Jones F.N., Pappas S.P., Organic Coatings: Science and Technology, 2nd ed.; Wiley-Interscience: USA, 1999; pp 504-506.
2. Sacharski, L.; Gessner, M., Powder slurry coating composition and its production, PCT Int. Appl. (2000), WO 2000069979
3. Woltering J., Powder Slurries for Automotive OEM Finishin, August 28, 2000, PCI Magazine
4. Sacharski L., Gessner, M., Powder slurry coating composition, US Patent 6360974 (Mar. 26,2002)
5. Williams, et al., Process for producing a powder coating composition, US Patent 5379947, (January 10, 1995)
6. Ekkehard S., Joachim W., Aqueous powder paint dispersions, US Patent 6177487, (January 23, 2001)
7. Koenraadt, et al., Process for preparing an aqueous powder coating dispersion, WO 9637561, (November 13, 2002)
8. Koch, et al., Method for coating metal substrates with a radically polymerizable coating agent and coated substrates, WO 2001091926, (November 18, 2008)