There are times when trying to discern if adhesion in manufacturing will succeed seems like a deeply mysterious and opaque puzzle. The water break test is a simple and fairly straight-forward method of detecting hydrophobic (water-repellant) contaminants on surfaces, most commonly on flat sheets of metal.
With quantitative, production material surface evaluation methods in short supply, water break tests are a relatively easy pass/fail metric, albeit one with several limitations.
Diving deeper into this aquatic test gives insight into what factors need to be considered when testing for surface quality.
The water break test is conducted by putting a significant amount of water onto a material surface; either by dipping the part in water, spraying the surface with water or streaming water over the surface. The person implementing the test visually inspects the behavior of the water to detect the presence of hydrophobic contaminants, which can be detrimental to adhesion. If the water spreads out evenly on the surface (commonly referred to as sheeting off due to the uniform appearance of the water sliding down the surface) then it indicates the surface is likely free of contaminants that cause water to bead up. If the water does bead up or “break” into uneven rivulets then this indicates contamination.
The water break test doesn’t take much training or prior knowledge to understand. The test is based on the principle that water acts differently on a clean surface as opposed to one with oils or a film on it. However, being able to distinguish between water that is breaking and water that is evenly flowing over a surface takes experience. This is where the test’s subjectivity comes into play; typically, the results can be quite different depending on what is noticed by the tester.
There are many cases of water break tests contaminating a surface because of impure water or the transfer of contaminates during the drying process. Every surface that is tested upon needs to be thoroughly dried before it can be used or it will just be disposed of. This drying is typically done by wiping or baking the part, which can chemically alter the surface of the material after the test has been conducted which, essentially, negates the results of the test.
At first glance this test seems to be a resourceful answer to an age-old problem surrounding detecting contamination that leads to adhesion failure. Water is already available in most manufacturing facilities, so putting that water to work in this way seems to make a lot of sense. If this test was more reliable, exact and totally non-destructive then the low cost would be a bigger benefit to continue using it.
A critical limitation to the water break test is the fact that the understanding of the state of a surface offered by the test results is next to none. The behavior of the water across the whole surface may vary but since this is a go/no-go test it is impossible to know, with any precision, what next steps to take to better prepare the surface for adhesion. The test also doesn’t give clear and repeatable indications about where the contamination on the part begins and ends. All that can be said is that the water reacted a certain way in this general area and reacted a different way in this other area. All nuance is lost with the water break test.
Additionally, the water break test is only useful on flat surfaces. As manufacturing branches more into adhesive bonding and parts molding technology becomes more sophisticated, the shapes of parts being used in adhesion processes are able to have more complex geometries. Water break tests have highly limited effectiveness if the surfaces are not uniform and flat so the water can clearly be seen running off the surface.
Dyne ink testing is similar to water break in a number of ways. It is a visual test that uses an ink solution applied to a surface, which is visually evaluated to see if the ink beads up or wets out – much like the water used in the water break test. Dyne ink tests do incorporate a numerical component but are still subjective, qualitative examinations that give little more information than a water break test would. The only major difference is that a number is associated with the behavior of the liquid.
Benchtop goniometers are a much better alternative as they are highly precise instruments that measure contact angle which correlates to surface energy. They do this by depositing a single, tiny drop of water on a surface and by looking at the droplet from the side, the height of the droplet can be determined. The angle between the top of the drop and the surface of the material is the contact angle. The height of this angle gives an excellent indication of what is called surface energy, which is a way of describing the readiness of a surface for bonding with another substance or surface. A surface with a high surface energy is typically very amenable to adhesion.
Goniometers associate a number to surface energy by measuring the contact angle. This makes it a quantitative approach that offers abundantly more data than either a water break or dyne ink test. It is also repeatable, reliable and (for the most part) non-destructive.
The biggest hurdle to using a goniometer in production settings is that they are a laboratory instrument and cannot be used without expertise in chemistry and surface science. They are not portable so samples have to be brought into the lab to be tested which can slow down production. Having the experience and expertise required to use the instrument and track the data means adding personnel and expensive analysis equipment.
What is needed is a tool that can measure contact angle on a surface quickly and easily right on the production line. This is the only viable alternative for most manufacturers. This tool needs to conduct repeatable, quantitative, surface energy inspections that can be done with high resolution across the entire surface of a part on the production line. It must not leave any detrimental chemicals or water behind. These validation test characteristics are critical for assurance that an adhesion process will be successful.
Contact BTG Labs’ Process Experts about how to locate and validate the Critical Control Points in your manufacturing process. A free Production Line Process Walk is available to start gaining insight into where adhesion failure is originating. You don’t need to tackle the problem alone. To learn more about eliminating adhesion issues, please download the free eBook titled "Manufacturer's Roadmap to Eliminating Adhesion Issues in Production."