Shunning the Water-Break Test for Composites

The water "breaks" on the LH side of this CFRP panel and wets on the RH side

It is amazing to me that in the year 2026 we are still having the debate about water-break testing and whether it is useful as an inspection method for validating a freshly prepared surface on a composite laminate for “cleanliness” and “wetting” prior to adhesive bonding. While water-break testing on aluminum substrates has been successfully used over the decades (and still is), it is usually done as a precleaning process prior to chemical etching or anodization—but is not recommended for use on composites, and here is why:

Science tells us that for a liquid to fully wet out a surface, the surface free energy (SFE) value of the surface must be higher (preferably much higher) than the surface tension energy (STE) value of the liquid. At 20°C (68°F), raw aluminum has a SFE of ~840mJ/m² and water ~ 82mJ/m². This relationship allows water to fully wet (sheet) on a clean raw aluminum surface. Compare this to a fiber reinforced epoxy laminate that, depending on the surface preparation, has a SFE value somewhere between 35-45 mJ/m² (plasma treated FRP surfaces can have an SFE as high as 65 mJ/m²), therefore, water at 82mJ/m² will not fully sheet the surface—instead, it will always crown and pull away from the laminate edges in a large area, low angle, beaded sheet formation. (Note: 1 mJ/m² = 1 dyne/cm for readers that are familiar with the CGS system.)

While most specifications call out deionized or distilled water for this process, either can bring contaminates to the surface from the surrounding area or from the plastic spray bottle used to disperse it. Another issue is that the water (H₂O) itself can alter the hydroxide groups that have recently been created on the freshly prepared surface. (You would rather make this exchange with the adhesive.)  Also, since composites are somewhat porous, trace moisture can remain at the subsurface of the laminate and affect the adhesive properties or promote steam at elevated temperatures during cure. At this point, many specifications call out a drying procedure that at worst presents another level of risk for contamination, and at best takes time to do—time that allows the surface to normalize—potentially back to a state of lower SFE, losing essential polar groups on the surface, thus affecting the work of adhesion.

The alternative to using the water-break test is to use a portable contact angle goniometer, preferably one that uses a ballistic microdrop of certified HPLC grade water or other liquid such as dimethyl sulfoxide (DMSO) solvent (which has a surface tension value similar to liquid epoxy ~43.5 mJ/m²) that is deposited on the (prepared) faying surfaces at select locations in the bondline or in surrogate locations just adjacent to the actual bondline. The quantifiable contact angle measurements are recorded as part of the quality verification documentation for the process.

More on Equipment:

In our facility, we use the Surface Analyst^TM from Brighton Science (Cincinnati, Ohio). This device is hand-held or it can be automated with the use of robotics.  This unit is easy to use and can be deployed on any surface (horizontal, vertical, under-wing, etc.).

Another unit of choice is the Kruss Mobile Surface Analyzer.  This unit comes with a software package that allows the user to not only quantify the contact angle measurements of up to two liquids on the surface, but to determine the SFE of the surface and break it down into polar and dispersive components.

Regardless of which tool you decide to use, this technology is a step up from water-break testing and provides the user with scientifically verifiable measurements for QA.    

For more on surface preparation and inspection, please consider taking our Adhesive Bonding of Composites and Metals course.