Volume 8 Issue 2
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
What factors are important in panel preparation for corrosion testing?
Dr. Michael Van De Mark
Director, Missouri S&T Coatings Institute
There are many factors that can affect the test. These include surface cleaning, roughness, pre-treatment, coating thickness, uniformity, flaws, entrained air, and the cure of the coating. The cure can be affected by the coating thickness, temperature, humidity and time. Control over these variables should give a constant performance in the test. The substrate material that is being evaluated must be identical to that of the material desired by the customer. It should be of the same thickness, alloy, surface finish, and if possible directly from the customer. This approach will insure that at least the substrate will not be an issue.
One major error made by lab testing groups has been inadequate aging of the panels. The coating must be fully cured before initiating testing. Our laboratory usually dries coatings for 24 hrs at ambient followed by 7 days at 50oC. This is accelerated aging and more closely models a paint in service. The Air Force and many other agencies utilize this aging approach. The protocol for drying and aging of the coating must be agreed upon between the paint manufacturer and the end user to be valid.
The paint thickness and its uniformity are critical variables, which must be controlled. The thickness can change the cure time, tensile strength and rate of water and oxygen permeation through the coating. Thus the thickness will play an important role. If there are thin spots, as in the use of a wire wound draw-down rod, the rate of corrosion in the thin valleys will usually differ from that under the hill areas. If the panels are sprayed, any thin areas, poor overlap regions or simply dry fall, over-spray, which is relatively dry before it impacts the substrate, will usually result in a more rapid corrosion rate. Many of these defects or variables are not easily detected in advance or even after exposure. Careful inspection of all panels must be made prior to their test exposure. Often as many as ten panels must be coated to get five panels clean and uniform enough for testing.
The method of application must be identical to that of the end user. If a coatings manufacturer uses an air spray system but the customer uses an airless gun, the results can be significantly different. The only time a general method can be used is in the development of a coating, which is to be sold to a broad market. Here, the method used should most closely represent the majority of the intended market and not one of expedience only. The relative humidity and temperature during application and drying must be considered. The humidity can radically change adhesion and corrosion since higher humidity can increase the water content on the surface and initiate corrosion under the coating. Obviously for isocyanate systems this is a very important issue. For solvent borne paints the humidity can cause the solvents to pick up water as the film dries and decrease the properties of the paint.
Care should be taken to accurately record changes in the panels during the corrosion test i.e. after every prohesion cycle or after each 100 hours of salt spray or once a month for exterior exposure. A digital photographic record can help if the resolution of the image is high enough. Today with digital images the data can be stored on CDs and later viewed for changes.
An example of a real world issue was with a coating being used for aircraft refinishing by the Air Force. Major manufacturers developed coatings for an aluminum substrate, but the actual performance on the aircraft was not up to par. It was most notably a much lower adhesion than was reported and subsequent failures were higher. Looking at the protocol of the actual application of the paint, the surface was not being cleaned sufficiently in the field and thus creating issues. On laboratory prepared test panels the problem was not noted. If a surface preparation method is used in the field, the paint testing laboratory must emulate it as close as possible noting any potential issues. The surface is where corrosion occurs and thus any issues at the surface due to preparation will result in performance problems such as corrosion, adhesion, or delamination.