Critical wetting tension

An adhesive should possess a Hquid surface tension that is less than the critical wetting tension of the adherend s surface. 

Fogging is formation of small water droplets (visible condensation) on the surface of a polymer film. Undesirable effects may result from fog formation, such as reduction of clarity and dripping. Incorporation of antifogging agents eliminates the reduction of transparency by migration to the surface and increases the polymer surface critical wetting tension. This results in... 

TABLE 27.1. Critical wetting tension of some common polymeric materials [1]. 

Table III, Values of Critical Wetting Tension (7c) for Several Polymers...

Critical Wetting Tensions of Some Chemical Groups at 20°C... 

In metal bonding the issue of wetting is easily settled. Clean metal surfaces have extremely high critical wetting tensions, in the order of several hundred dynes/cm. As a consequence, adhesives with a typical surface tension of 35 dynes/cm have little trouble wetting metal surfaces. If wetting problems occur, it is almost always the result of contamination on the substrate surface, a condition that can be quickly checked with the water break test. If water with a surface tension of 73 dynes/cm wets the surface, it is a safe assumption that no problem will be encountered with lower surface tension adhesives. 

Unfortunately, wetting problems on plastics are not as easily resolved. Since adhesives and plastics are both polymers, they may have similar surface tensions and critical wetting tensions. Under the most favorable of conditions, adhesives will still wet the surface. However, plastics such as polytetrafluoroethylene, polyethylene, or polypropylene have such low critical wetting tension that bonding is not practical without surface treatments designed to raise its value. 

In the presence of salt it was assumed that electrical double layer repulsion is suppressed and the action of the SDS/NaCl solutions was simply to enhance wetting. In a parallel study with isopropanol /water mixtures 5 we found that there exists a linear relationship between wjj and yl where Wq = 0 when the liquid surface tension is equal to the critical wetting tension yc. The value of Yc in these alcohol/water mixtures was ca 22 mNm i which is lower than Ys PET because of the well known adsorption effect 20 and hence the complete inhibition of adhesion due to wetting occurs when yc YS Similar adsorption effects have been observed for SDS solutions 2 and this is reflected by the value of yc oi ca lOmNm determined in this work from contact angle measurements on Mylar film. The measured Wq (= 0.95 yN) in water gives the fallowing expression for as a function of yl assuming that a linear interpolation can be made between YL YC 10 Nm l and yl Ywater 72.2 mNm"l. 

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