The water break test a common pass/fail method used to evaluate surfaces for the presence of hydrophobic contaminants, which can be detrimental to adhesion of paint or an adhesive. It is a qualitative means of evaluating surface energy, which is directly related to surface cleanliness.
In this test, a stream of water is visually evaluated as it flows over a surface:
• If it spreads out into a continuous, unbroken sheet, it indicates that the surface is substantially free of hydrophobic contaminants.
• If the surface is contaminated with low surface energy substances, the flowing water will not sheet uniformly over the surface but rather it will break into rivulets and tend to bead up (termed “water break”).
The water break test is not ideal as a quality control tool. They are messy: a relatively large amount of water is used which has to be removed and the component must be dried before coating or bonding. Cases of water break tests contaminating sensitive surfaces because of impure water or transfer of contaminants during the drying process are not uncommon. Because the result is only a binary ‘water break free/not water break free,’ it is unknown whether it is too sensitive for some applications or not sensitive enough.
Manufacturers are familiar with adhesion challenges when attempting to bond, paint, etch, or coat a substrate. Poorly prepared surfaces can undoubtedly produce problematic adhesion issues.
A common standard for measuring the effectiveness of paint adhesion relies on an archaic method: The Tape Test. While the Tape Test can provide some insight on adhesive ability, its analysis is subjective, the process is time consuming, and it is a destructive test. There are also many conditions that are difficult to control and maintain consistency: the force at which the tape is peeled, the speed at which the tape is peeled from the surface, as well as the pressure at which the crosshatch grid is applied all contribute to the tests’ downfalls.
Furthermore, it can only be done post treatment and application or in the lab for research and development. So, this test is not practical or possible to use as a surface readiness indicator prior to bonding on the manufacturing floor.
The Surface Analyst™ rapidly obtains contact angles from surfaces via Ballistic Deposition, whereby a small drop of liquid (usually water) is constructed in situ on the surface via a pulsed stream of nanoliter-sized droplets.
The contact angles established in this manner are a sensitive function of surface chemical composition.
In this work, injection molded polypropylene panels were oxidized to various levels via exposure to an atmospheric pressure plasma treatment process. Surface chemical composition was determined via X-ray Photoelectron Spectroscopy (XPS), and the chemical composition was related to the water contact angles determined using a Surface Analyst™.
There are fundamental differences between an NRL-style goniometer and the Surface Analyst, most of which contribute to the value of the measurement for development and control of surface sensitive manufacturing processes.
These include the method of liquid deposition and the method of contact angle calculation once the liquid drop is deposited.
The original motivation for these differences was to allow for a more compact and convenient instrument that could be easily handheld. However, they also significantly improve the speed and accuracy of the measurement as well as the flexibility of the types of surfaces that can be measured.
Solvent wiping and sanding procedures can greatly affect the surface energy of a substrate. To investigate the surface energy differences following different preparation procedures of an epoxy/composite laminate, several different surface conditions were created utilizing different cleaning techniques.
Measurements were obtained using a BTG Labs Surface Analyst™. The Surface Analyst is a fast, easy, accurate and nondestructive instrument that measures the contact angle of water that is applied to the surface in a precise, controlled manner.
This contact angle is determined by how strongly the surface energy of the substrate and the liquid interact with one another. The relationship between this contact angle and surface energy is complex but well understood. More importantly, this relationship correlates with the strength of adhesion of a paint, coating, print or adhesive to the substrate.
- While suitable in some cases for estimating surface energy (and therefore cleanliness or treatment level), the imprecision and subjectivity of wetting tension measurements makes them a poor choice for quality assurance and process control of surface cleaning, surface treatment, bonding, coating, and printing operations.Dyne inks are also destructive to the surface being measured. An alternative method for gauging surface condition and consistency is the Surface Analyst™, which provides a rapid, automated measurement of the water contact angle in a precise, controlled manner.This contact angle is determined by the surface energy of the substrate and the liquid and how strongly they interact with each other. This water contact angle correlates very well with the cleanliness and consistency of a surface.