Missouri S&T FALL COATINGS SHORT COURSES

On September 13-17, the Missouri S&T Coatings Institute will offer “Basic Composition of Coatings¿?. This lecture course discusses the basic ingredients of coatings and why they are used including solvents, resins, pigments, and the different classes of additives. Testing and specification of wet and dry coatings will also be covered as well as the basics of color theory, dispersion and the equipment, and the basics of the calculations needed to understand a formulation. The course includes several demonstrations. Participants in the past have included new formulators, QC/QA lab people, purchasing agents, managers, sales, raw materials personnel, end users, crime lab technical personnel, as well as many others. Check out the course on our web site.

On September 27- October 1 the Missouri S&T Coatings Institute will offer an “Introduction to Formulation¿? short course. This hands-on, laboratory-oriented course introduces the participant to the science of formulating coatings. The course includes about 21 hours of lecture, an evening session to formulate a paint, about 14 hours of laboratory work, and a summary presentation and discussion session on Friday morning. The course is intensive and very informative. If your interest is in learning how to formulate a coating or learn more about formulation then this is an excellent course for you.

Technical Insights on Coatings Science

What do we do with QC/QA data on raw materials?
By Michael R. Van De Mark

Most coatings manufacturers do some quality assurance of incoming raw materials. Solvents are checked by gas chromatography, infrared spectroscopy, water content checked and solids content checked by evaporating a small sample. Pigments may be tested for tint strength, water content, and color match. Additives are not checked as often but solids level or IR can be very useful. For resins, common QC events include percent solids by evaporation, molecular weight by GPC, identity fingerprint by IR, solvents by GC, and functional group titration for equivalent weight. These are a few of the general tests performed in most receiving laboratories. Now the important question, what do we do with the data?

Generally the test data is compared to a specification for the product being tested. Is the water content within the specifications, is the solids content within the proper range, etc. If the product is outside of the acceptable range, the lab contacts the appropriate decision maker to either reject the product or try to use it as-is. Generally a call to the supplier is made to express concern of the quality and potential problems this may cause. If the out-of-specification material cannot be used, the material often must be replaced rapidly as to not impede production.

The QC data should also be used in control charting of performance for each vendor. The performance of each component in the coating can then be monitored and evaluated across production batches. If it is found that the molecular weight must be between 80 and 100K Daltons to meet both performance and viscosity specifications, the resin supplier can be contacted and informed of the specification requirement and that all others will be rejected. Control charting helps improve a process as more information becomes available and reduces bad or inferior coatings form lot to lot. Control charting can be used to qualify or disqualify a particular supplier, as part of a quality assurance program. In general, working off a batch costs significant manpower, decreases efficiency, and can lead to inferior products and loss of customers.

Much of the data gathered in qualifying raw materials is not used. One piece of information we have found should be used is the percent solids of the resin. If a resin is 50% solids and has a specification of plus or minus 2%, the amount of resin solids could be 48% one time and 52% another and still be within specification. If the batch uses 400 pounds of resin per 100 gallons of paint, one batch will have 192 pounds of resin solids while the other will have 208 pounds, while water is the carrier then one will have 208 pounds of water and the other only 192 pounds. This seemingly small change in percent solids represents a large effect in resin and solvent content of the paint and will have a significant effect on most coatings properties, for example, viscosity, gloss, and abrasion resistance. For most paints the addition or elimination of a gallon of solvent per 100 gallons will cause major rheological changes. The fact that this one value is so important and that resin is one of the largest components in a coating, this variable should be checked and adjusted for every batch ticket.

With modern computers and the integration of production data, the batch ticket could be compensated to the correct solids content. If the correct solids content were used, the solvent amount could be easily adjusted up or down to accommodate the correct resin solids content. The results would be a much more consistent paint.

Use of the analytical data can not only spot problems but allow a more consistent product to be produced, which is the goal of any QC/QA program. Once the tests are run, the data is there for use. By control charting and using the data, areas of the coating formulation that may need to be improved or other tests on raw materials which we must be done may be more easily diagnosed to avoid problems.

Adhesion, cohesion, how does it fail?
By Michael R. Van De Mark

Many substrates are primed and top coated and can fail in many ways during adhesion measurements. If we use the Patti tester or a torque method, the results may differ for several reasons. The Patti tester uses a metal “puck¿? glued onto a painted surface with the adhesion force for removal of the puck measured. Thus the test is basically a tensile strength-type pull test. The torque method uses the same puck glued on the paint with a nut on the puck to allow a torque wrench to remove the puck by rotation. In the torque method the force is perpendicular to the surface and is a shearing action. Both methods require heavy gauge substrates, such as 0.25 inch thick steel of yield similar to the adhesive energy. However, if 0.05 inch thick steel is used or a thinner or softer substrate, the paint will fail prematurely due to the panel buckling under the force of the Patti tester. The torque method is much less sensitive to the metal thickness since the force is in the plane of the surface. The torque method yields similar data for thin or thick substrates up to a point. For very thin stock materials which may flex during torque, the panel should be glued to a ceramic tile with a 2 ton type epoxy. This yields a ridged substrate which will not flex. The same approach is needed with the Patti test.

The force involved in causing failure is not the only important piece of information in this test. The mode of failure is perhaps more important than the force and should be recorded for each test specimen. Generally a coating system will fail at its weakest link. Failure can occur to either leave a clean interface or by failure within a matrix, which are termed adhesive and cohesive failures, respectively. In addition, failure can occur at or near any one of the interfaces formed by a coating system. For instance, failure at the substrate/primer interface is said to be an adhesive failure. The failure could occur by ripping out a piece of the substrate, which is cohesive failure of the substrate. A primer coating could fail within that layer and is called cohesive failure of the primer. If the primer delaminates from the topcoat leaving a clean interface we have again an adhesive failure between those layers. The top coat can likewise fail in a cohesive manner. In another case, the puck glue could delaminate from the paint, this is again an adhesive failure but a test failure, not a coating system failure. The glue could likewise fail in a cohesive manner or the glue could fail adhesively from puck. And lastly, the puck could conceivably fail cohesively.

It is critical that the strength of the puck and glue exceed the specification being targeted for adhesive strength of the coatings. If you are measuring the adhesion to wood, a 1500 lb adhesive should cover the issue but if you are testing epoxy primers to metal, the adhesive should exceed 4500 pounds tensile strength. The glue requires time to cure and this time needs to be evaluated prior to test protocol implementation. Also many of these adhesives are two component systems. They generally have a decreasing strength with storage time or improper mixing ratio. Date the adhesive when received and monitor the strength with time to avoid glue problems.

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