VOLUME 15 ISSUE 7
WHAT’S HAPPENING AT MISSOURI S&T (FORMERLY UMR):
SHORT COURSE DATES
We will be offering "Introduction to Paint Formulation" Oct 21-25 (Fall 2019). 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 us at mstformulation@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
Anyone wanting to have job opening listed, please contact us at (573) 341-4419 or e-mail: mstformulation@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
Watching Paint Dry or How I Spent my Life
Michael Van De Mark, Director Missouri S&T Coatings Institute
The observation of paint drying has been a standard joke for many decades. Watching paint dry is boring and non-productive. However, I do not find it boring or a low IQ activity. The drying of water borne or solvent borne paints are a very complex event. Let’s take a look at the processes. We will discuss it in two parts water borne and solvent borne.
Water borne coatings typically dry to touch in 1-3 hr. and through dry in 2-8 hr. However, dry is not dry, since the film often requires coalescence and in some cases oxidative or reactive curing to reach its final state. The evaporation of the water is the actual drying process with glycol or a coalescent aid being additional issues. Water requires more energy to evaporate than most solvents since it hydrogen bonds to itself and to other ingredients in the paint. At time zero after application the rate of water evaporation will be very close to that of pure water. As the water evaporates the temperature of the surface will drop due to the loss of the heat of vaporization. This drop in temperature will decrease the vapor pressure of water and thus slow the loss of water.
As water is lost the concentration of resin, pigments and additives become higher at the surface than inside the coating. This sets up an osmotic flow of water to the surface from inside the coating. When water is lost the surface viscosity increases and reduces the local diffusion rate. If there is a large flow of air across the surface or the air is hot and moving it will increase the rate of water evaporation. If this is high enough the issue of differential pigment mobility will result in flooding or floating and possibly the formation of Benard Cells due to the increased driving force of the osmotic flow. The differential pigment mobility issue is why when paints are dried in the store after tinting they do not match the desired color. Differential mobility of pigments is caused by low density or fluffy pigments interacting with the flowing solvent and being moved more than small high density pigments. If they were dried at ambient the color would match. By the sales person re-tinting the paint and drying a smear of the coating again, the paint will match, but when the customer applies the paint at home and it dries at ambient it will not match. This creates a big problem. The flow of the solvent can move additives, resin or pigments causing your coating to perform differently than at ambient and slow air flow.
Recently I went to 7 stores offering paint and tinting services. All 7 used a hair dryer to dry the paint color test smears. The paint surface temperature varied from 50-75oC (measured with a non-contact laser) and all had very high velocity air movement. Thus the drying condition in the store was very different from that of the application. Also of note is that the color matching system had its data base established drying the samples at ambient.
As time passes the water loss increases the concentration of the paint components even at the substrate. As the water is lost, the rate of water diffusion and even with the extra driving force of osmotic pressure, the water movement is slowed thus reducing the evaporation rate significantly. The presence of the higher boiling glycol if used will help prevent film formation and allow the water to reach the air interface. However, if there is no glycol the coalescence of the film at the surface may block the water from reaching the air interface and evaporation. Zero VOC coatings often suffer from this issue.
Water borne coatings are affected by the relative humidity. As the RH increases the dry time increases. Higher humidity will allow a more uniform film with less osmotic flow issues. However above 90% the coating may dry so slow as to pick up other debris from the environment such as insects. At low humidity the water will evaporate faster and may exhibit more differential pigment mobility issues.
The evaporation of the water can change many coating surface properties. Color, coefficient of friction, gloss, surface hardness, mar and many other properties may be significantly altered. This issue can often be seen if there are two different resins being used. A good example is a polyurethane dispersion which is typically about 30 nanometers in diameter. If a latex resin is added to alter the hardness or other property, it typically will have a diameter of 100 nanometers. If there is osmotic flow the polyurethane dispersion will move much more than the latex changing the surface resin composition and properties relative to the interior.
Solvent borne coatings will exhibit the exact same phenomenon of osmotic flow. However, many organic solvents used in paint have a much higher evaporation rate and require less heat to evaporate. Therefore, solvent borne coatings usually exhibit a much higher temperature drop due to evaporation and a higher osmotic flow rate. For solvents such as MEK or toluene they may cause the surface temperature to drop below the dew point. At that temperature the surface will condense water on it. When this occurs the water mixes with the paint and may kick out the resin, flocculate the pigment or cause some phase separation. The overall result is a major paint defect. Most manufacturers list an application temperature range and relative humidity that the coating must have to perform. When formulating a paint we must establish the temperature and humidity range that will allow the coating to perform to specification. Thus we must watch paint dry.
Just watching paint dry is not sufficient to know if the paint will perform. We must think scientifically as to how the drying condition might change the properties of the coating. Then we must watch and test the coating as a function of drying conditions. Remember the temperature, relative humidity and air flow will all have an effect. Also the wet film thickness when applied can change things. The substrate will also have an effect. Metals conduct heat and will allow the loss of heat due to evaporation to be made up by heat transfer from both sides of the panel. In the case of wood, plastic and non-conductive substrates the surface temperature drop will be greater since heat can only come from one face. If the substrate is metal and there are supports behind the sheet metal being painted, the structure can bring more heat transfer to these areas. All these things must be considered.
The drying of paint is not a boring subject but a highly scientific study into how the coating performs with respect to its environment as it dries.