Volume 6 Issue 1
What’s Happening at Missouri S&T:
Spring Short Course Dates
This spring we will be offering ?Basic Composition of Coatings" March 23-27, 2009. 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. For more information see our web site at http://coatings.mst.edu/index.html and to register contact Michael Van De Mark at firstname.lastname@example.org or call 573-341-4419. **This course is held on the Rolla campus**
This spring we will be offering ?Introduction to Paint Formulation" May 18-22, 2009. 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 Michael Van De Mark at email@example.com or call 573-341-4419. **This course is held on the Rolla campus**
Technical Insights on Coatings Science
What is causing a gloss reduction? We are using an 8 micron extender pigment at a 10% PVC and 18% PVC as titanium dioxide for a 28% PVC total.
The gloss of a coating is dependent upon much more than just the PVC, the pigment volume concentration. The most obvious other factor is the pigment diameter. Generally, the larger the pigment size the more gloss reduction is noted. Simply put, the probability of the pigment breaking the surface of the coating is higher the larger the particle size. Typically pigments larger than 10 microns can stack and reduce the gloss of a 1 mil (25 micron) dry film. This is why as the PVC increases the gloss decreases due to the stacking of the pigments. The titanium dioxide particle size should not be a problem since the typical size is between 0.2 and 0.5 microns.
However, the 8 micron extender pigment size does not mean that this is the size of all the extender pigment. Typically the pigment size is a distribution which may be narrow or broad. Much smaller pigment particles are generally not desirable since these have higher resin demands, but larger particles have the potential to reduce the gloss.
Extender pigments are generally made by the mechanical grinding of rocks. The particles are then sized through an air classification mechanism. This process does not always eliminate all the larger particles, and some processes do a better job of classification than others.
One judgment of the quality of classification is the percent retention on sieving. If no retention is seen with a 15 micron sieve, all the particles are less than 15 microns. However, if a 30 micron sieve is used and 1.5% is retained, the extender will reduce the gloss even if only a small amount of extender is used. This latter scenario implies that 1.5% is larger than 30 microns. It should be noted that there may be 50 or 75 micron particles in that 1.5%.
Another possible issue is that the pigments may not be properly dispersed. The milling process is done to de-agglomerate the pigments. Failure to do this will increase the effective particle size and result in gloss reduction. If the titanium dioxide aggregates the hide may also decrease. If the dispersion was successful but the dispersion is not stable, the system may flocculate which also results in an increased effective particle size and a loss of gloss. In this instance, the dispersant level may not be correct or a different dispersant will need to be employed. Generally, a simple grind gauge check evaluates dispersion.
If gloss decreases with time, flocculation is usually the cause. Flocculation can be evaluated by draw-downs made over a period of time with subsequent measure of gloss and hide. If hide remains constant, the titanium dioxide probably is not flocculating significantly. It should be noted that the gloss will change more for the 80o than the 60o than the 20o.
Why does the pH of the batch of water borne coatings drift with time?
Most water borne coatings are formulated to have an alkaline pH, usually between 7.5 and 9.0. The pH modifier is usually ammonium hydroxide or an amine. Because the ammonium hydroxide is in equilibrium with ammonia and water, ammonia can evaporate in open tanks which will decrease the pH. The alkaline water will also absorb carbon dioxide and thus use up some of the base to form salts. The third cause is the components in the paint such as the latex resin. These components may contain acids which will slowly react with the base since they are in the hydrophobic resin core and take time to reach the outside of the latex.
Typically a coating will drift down in pH for up to 48 hours after mixing. Most of the drift happens rapidly and while the tank is open. Once canned the drift is usually minimal.
How is the elongation of an elastomer measured?
The elongation of a coating to break can be measured or evaluated in several ways. The indirect methods are mandrel bend and GE impact tests which utilize the radius of the impact or bend to evaluate the coatings ability to stretch without breaking. These methods only give an indication and are dependent upon adhesion and other variables.
A more direct method is the use of direct elongation measurements on a free film. The film can be prepared on release paper which is a non-stick paper. Once the film is dry it can be removed and cut into “dog bones” for testing using an Instron or similar mechanical tensile tester. This method can have problems due to the surface tension of the coating and the paper. If the coating cannot be applied successfully to this substrate, a thin aluminum foil would be the next best choice. Once dried on the foil, the foil backing is removed by exposure to mercury or mercuric chloride in water. The mercury activates the aluminum and it rapidly corrodes off leaving the free film of the coating. We prefer the mercuric chloride in water approach since it produces less waste. The free film can then be cut and tested.