Practical Applications of Wetting

The relationship between work of adhesion and practical adhesion has been reviewed by Packham [78]. 

Whereas actual measurements of contact angles are usually made in science laboratories, the principles are exploited in the water-break test and in the use of liquids of different surface tensions to assess the printability of polyolefins. 

The water-break test is a simple method to check that a metal surface is clean. A few drops of distilled water are placed on the surface, or alternatively the sample can be drawn from a bath of water. If the water does not break into droplets then 

A standard test (ASTM 1982) involves wiping a polyolefin surface with a series of liquids, starting with one of low surface tension and noting the time needed for the film to break into droplets. Liquids with increasing surface tension are used until one is found which will wet the surface for just 2 s. The surface tension of the plastic then equals that of the liquid. Twenty-two mixtures of formamide and 2-ethoxyethanol are used with surface tensions in the range 30-56 mNm .  

Wetting is not a reciprocal property [79] in that if A spreads on B, B does not necessarily spread on A. An example of this is that a liquid epoxide resin will not spread on polyethylene, but if the resin is cured it will then be wetted by molten polyethylene. Surface tensions of epoxides are about 44 mNm and that of polyethylene is about 30 mN m . A solid can force liquids of lower, but not higher surface tension to wet it. 

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However, as this book is concerned with the practical application of wet-end chemicals, these should be covered in other articles, but it is sufficient to say that good wet-end size application is required to allow these surface applied chemicals... 

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