Coatings R&D Notebook
Specification ApproachesIs it better to specify a coating based on performance or composition?
This is a frequently asked question, and it has a complex answer. Both composition and performance specifications can be critical and applied together. Composition specifications have been successfully used where production of an end-user-developed coating is desired, as in some military applications. However, over time, technology may make the coating specification obsolete. Performance specifications have been useful for companies that know what they need the coating to do but don't know which technology will work.
Compositional specifications can, as one part of the specifications list, be important for almost all coatings. For example: the amount of resin to pigment, and the total solids, can be very important. If there is not enough resin, the paint may fail cohesively or adhesively. Setting a minimum level for the binder can aid in obtaining quality coatings, and a minimum level of hiding pigment can aid in getting paint with adequate hide. However, pigment content is usually covered by performance specifications such as hide and color.
Performance specifications are simply a set of test values the coating must attain for qualification. These test criteria are based on ASTM, MIL Specs or similar standards sometimes established by your company with defined methods and reporting. Use tests that are relatively independent of human interpretation or lab-to-lab variation.
The first step in establishing the requisite specifications is to know your needs and develop a list. Remember, there are needs beyond the simple end use. Examples include application viscosity, dry time, VOCs, hazardous volatile components, ozone depleting substances, as well as storage stability and flash point. The list of specifications should begin with the end use requirements, adding the application needs to complete the picture.
Once the general needs have been outlined, try to establish numerical values and tests to define each. A range of performance criteria are often enough. Then the list must match standard test methods to each need. Any modification from the accepted test method should be defined. These tests and values should be reviewed by everyone who has input.
Next, review these with current suppliers to ensure that you have not omitted any tests or information. Your current suppliers have a vested interest, so always interpret their input carefully to eliminate any bias toward their products in the specifications.
If a component of a paint formulation can be identified as the cause of a failure, you may want to specify the composition to exclude that product. For instance, surfactants that migrate selectively to your substrate will likely cause adhesion loss. Be careful to avoid specifications that limit technology to the current supplier, and do not allow a better or newer technology that could yield better performance with lower cost to compete. Compositional specifications are especially susceptible to this pitfall.
If your internal corporate knowledge of coatings technology is limited, do not hesitate to ask an independent consultant or have your personnel participate in continuing education programs. A consultant can review the specification list and help determine whether it is complete or needs modifying. The consultant's goal is to make the specification list meet your needs at the least cost.
Once the specification list is complete, you will still need to evaluate prospective coatings based on this list. There are three options: Trust the paint supplier to adequately test and honestly report the results; test the coatings in-house; or have an external testing lab do a comparative evaluation. Extended weathering or long-term tests are expensive and time consuming, so it may be necessary to rely on your supplier for those types of tests. But still evaluate the coating while you are using it. For example, if 20 Prohesion cycles are going to be used to evaluate a coating, the test will take five months. If your needs are urgent, you may need to look at the company's data and make a decision before you can complete your independent evaluation.
In most cases a combination of performance and composition is the best approach to specifying coatings. There is no substitute for extensive testing and field evaluations.