What’s Happening at Missouri S&T:

Coatings Industry Summer Interns
By Michael R. Van De Mark

The Missouri S&T Coatings Institute would like to invite any companies who have interest in coatings to consider sponsoring a Missouri S&T student for a summer intern position. Most students have established their choices for this summer but we are now planning for next year, yes next year. We would like to promote the opportunities to students at Missouri S&T and to students considering a career in chemistry or science. Most students need to find summer support to be able to afford college. Working at a burger joint or in a department store may help pay the bills but will not give the student experience in their field. If you are interested in having a student, please send your contact information to Michael Van De Mark and we will include it in mailings to our students and on promotional material used to attract students from high schools to enter this exciting field.
It is important for students to know that there are companies interested in their future employment. We have found that students want to plan early for their summer employment and having a contact list early helps them and the company select the best match. We are not going to select the student for the company but will help match them. The company selects the student from those who have interest. Active recruitment of summer interns usually means a higher probability of finding an excellent employee.
We all need to visit high schools and promote the coatings industry to the students as a career choice. Whether their goal is technical sales, marketing, engineering or chemistry, the coatings industry has a place for them.

Technical Insights on Coatings Science

How is Mica typically used in coatings?
By Michael R. Van De Mark

Mica is often used in exterior architectural coatings as well as many industrial maintenance paints. Mica is considered a barrier pigment which is used to improve the weather resistance of a coating. The pigment has a large aspect ratio, like a sheet of paper, so it lays flat in a coating parallel to the surface. Oxygen and moisture must travel around the pigment to get to the substrate. The amount of mica used in a coating is typically 25-35 lbs/100 gallons. If this amount is exceeded, the pigment will not lay flat. The result is an increase in permeability rather than a decrease. The mica sheets can have a flattening effect on gloss due to the fact that other pigments may sit on top of the mica particle and stick out of the coating. Mica, in general, is not a pure white pigment and therefore it will add a dirtying of the color of a paint. A close relative to mica is micatious iron oxide. This pigment is extensively used in Europe for coatings of bridges and other steel structures. Its advantage is analogous in that it acts as a barrier pigment. This pigment is gaining popularity here in the US as well. Many of the micatious iron oxide containing systems for bridges are either epoxy or moisture cure urethane coatings.

What factors control the drying of a coating?
By Michael Van De Mark

The drying of a paint is very complicated in that several things must happen at the same time. The major factor is the evaporation of the solvent or volatile components. During this process, the surface is cooled due to the removal of heat caused by the vaporization of the solvent, i.e. heat of vaporization. The evaporation process creates concentration gradients within the coating which can result in coatings defects such as Bernard Cells, floating or flooding. As the temperature drops, the coating can pick up water from the atmosphere especially if the coating temperature drops below the dew point. Most resins are not water soluble and thus if the coating picks up water it may cause precipitation of the resin or aggregation of the pigment.
While the solvent is leaving, the coating is expected to undergo flow and leveling. Thus brush marks or orange peal disappear. As the solvent leaves the viscosity increases and the composition of the remaining solvent within the coating may change. If one solvent is more volatile than the other, the slower solvent will remain in the coating while the more volatile solvent will leave. One of the formulator’s goals is to select solvent blends which will result in the slower solvent being a good solvent to dissolve the resin. The goal is to have this solvent produce a large Mark Howink exponent while the solvent blend produces a small exponent,a. In the Mark Howink equation, viscosity is proportional to a constant, k, times the molecular weight, M, to an exponent, a. The lower the exponent, the lower the viscosity will be and thus it should be easily atomized and undergo rapid flow and leveling. If the exponent rises relatively rapidly after flow and leveling, it will prevent sag which is considered a defect. Air movement, substrate composition, and film thickness may also affect the drying.
If the coating is reaction cure, the chemical reaction will also change the rheological characteristics of the coating. As the solvent leaves the coating, the remaining components increase in concentration and since the rate of reaction is proportional to the product of the two reactant concentrations, the rate will increase. The rate is also affected by the temperature of the coating and thus the rate will be reduced if the loss of solvent causes a reduction of the temperature. Therefore, the early increase in rate of reaction by concentration increase will be offset by the decrease in temperature. If the curing process is exothermic, the coating will produce heat which may offset the heat of vaporization and thus the rate will increase with loss of solvent. If the exotherm is significant and the VOC level is small, the reaction may accelerate and rapidly cure. Baking systems can be more complex. The oven temperature is critical as well as the activation of the catalyst or loss of protecting groups. For example, in baking polyesters the acid catalyst is usually a sulfonic acid which has been blocked by reaction with an amine. The amine must be volatilized before the catalyst can function. If the release of the amine happens too slowly, the coating may not cure. If it occurs too rapidly, the coating will skin over trapping the solvent below the skin. Also, as the temperature rises the solvent will turn into a gas and will blow bubbles in the coating. The object is to slow the catalyst until most of the solvent has escaped which will reduce the chance for bubble formation.

Is there a topic you would like discussed? Contact us by e-mail at coatings@mst.edu.