Driving Force of Film Formation

The film formation process is extremely complex, and there are a number of theories — or more accurately, schools of theories — to describe it. A major point of difference among them is the driving force for particle deformation surface tension of the polymer particles. Van der Waals attraction, polymer-water interfacial tension, capillary pressure at the air-water interface, or combinations of the above. These models of the mechanism of latex film formation are necessary in order to improve existing waterborne paints and to design the next generation. To improve the rate of film fonnation, for example, it is important to know if the main driving force for coalescence is located at the interface between polymer and water, between water and air, or between polymer particles. This location determines which surface tension or surface energies should be optimized. 

In recent years, a consensus seems to be growing that the surface tension of water, either at the air-water or the polymer-water interface — or both — is the driving force. Atomic force miaoscopy (AFM) studies seem to indicate that capillary pressure at the air-water interface is most important [7]. Working from another approach, Visschers and 

Estimates of Forces Operating During Particle Deformation 

Reprinted from Visschers, M., Laven, J., and Vander Linde, R., Prog. Org. CoaL, 31, 311, 1997. With permission from Elsevier. 

Gauthier and colleagues have pointed out that polymer-water interfacial tension and capillary pressure at the air-water interface are expressions of the same physical phenomenon and can be described by the Young and Laplace laws for surface energy [5]. The fact that there are two minimum film formation temperatures, one wet and one dry, may be an indication that the receding polymer-water interface and evaporating interstitial water are both driving the film formation (see Section 3.4). 

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