Qualitative Description of Polyurethane Films

Films were subjected to a rapid raising and lowering of the bulk liquid level. These motions strongly stimulated the surface flows previously mentioned. 

The pattern of horizontal fringes observed on the face of the films suggested a vertical wedge shape, with the film thickness increasing as one descends the film. The wedge shape of the vertical PU films has a number of implications for operational PU foam. Any cell window in PU foam does not have uniform thickness. This non-uniformity has been reported in at least two studies [24-26, 32]. In addition, it would be expected that the rupture of the cell window would occur at its thinnest point [24-26, 32]. 

As reported in the literature, interference fringes in many vertical aqueous films are horizontal with little curvature. These films also displayed mobile surfaces, rapid and extensive surface flows and rapid rates of drainage [22, 28-30]. These features were all linked to the presence of a low surface viscosity. Based on our experimental observations, the PU films stabilised by DABCO DC 198 have low surface viscosities. This conclusion is supported by reports of measurements of low surface viscosities in polyol solutions containing silicone polyether surfactants [12, 15, 38]. 

The surface tension gradients necessary to drive this type of flow can arise from three different physical phenomena surface tension gradients are necessary to support the weight of the vertical film, flows in the film or the channel of the Plateau border impose stresses on the surface monolayer which are balanced by a surface tension gradient, and extension or compression of the surface layer can create transient surface tension gradients. 

Regarding the cell windows of operational PU foam, it is apparent that surface tension gradients are constantly present, due to all three mechanisms discussed previously. In particular, the constant stretching of the bubbles during the process of foam growth acts as a potent stimuli for these flows. 

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