Polymer Adhesion Affected by Contaminants

A simple test of polymer adhesion is to take a table tennis bat and cover it with a smooth layer of rubber, as shown in Fig. 6.4. A ping-pong ball then adheres to the smooth surface sufficiently to support its own weight. The rubber surface is so smooth that it looks shiny and liquid-like, seeming to wet the ball as it makes molecular contact, but in reality there is no fluid present. When the rubber is immersed in water, alcohol or other liquid, the ball drops off.  

An apparatus for studying this effect in more detail was described by Roberts and Tabor in 1971, as shown in Fig. 6.5. They had been particularly interested in the effect of lubricant films under car tires or beneath windshield wipers. Previously, Blok had tried to study these thick contaminant layers by placing a smooth plastic sheet on the tire rubber surface. But Roberts showed that this was unnecessary. If the rubber was molded against an optical glass surface, then the rubber itself acted as a mirror and enabled Newton s rings to be 

In an experiment on equal rubber surfaces under dry conditions, the black spot diameter was 1.37 mm, equivalent to a work of adhesion of71mJm , which is twice the surface energy of each individual surface. Thus the rubber surface energy was 35 mJ tn . But when the surfaces were immersed in water, the black spot fell to 0.63 mm in diameter, suggesting a work of adhesion reduced to corresponding to a wet surface energy of 3.4mJm . Thus the 

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