High energy surfaces wetting

To inspect for contaminants, a water break test is frequently employed. Water, being a polar molecule, will wet a high-energy surface (contact angle near 180 ), such as a clean metal oxide, but will bead-up on a low-energy surface characteristic of most organic materials. If the water flows uniformly over the entire surface, the surface can be assumed to clean, but if it beads-up or does not wet an area, that area probably has an organic contaminant that will require the part be re-processed. 

Fox HW, Hare EF, Zisman WA (1955) Wetting properties of organic liquids on high energy surfaces. J Phys Chem 59 1097-1106... 

Since slower-curing epoxy adhesives systems flow over and wet high-energy surfaces very well, there is little chance for air to become trapped at the interface. As a result, mechanical abrasion is often recommended as a substrate surface treatment prior to application of the epoxy adhesive. The added surface area and the mechanical bonding provided by the additional peaks and valleys on the surface will enhance adhesive strength. If the adhesive does not wet the substrate surface well, such as in the case of epoxy resin on polyethylene, mechanical abrasion is not recommended since it will only encourage the probability of gas voids being trapped at the interface. 

Critical surface tension Yc equals the surface tension of a hquid that exhibits zero contact angle on the solid. Any liquid (melt) with a surface tension less than that of the solid s critical surface tension will wet the surface. Uncoated inorganic fillers may have very high surface tension y > 200 mj/m, whereas polymers such as polystyrene and polyethylene have lower surface tension y < 50 mj/m. Thus, polymer melts will spread on the high-energy surfaces of fillers, unless the y value of the filler is reduced by absorbed water layers (y = 21.8mJ/m ), by contamination with low surface tension impurities, or by surface irregularities. This will result in incomplete wetting and void formation at the interface. 

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