If Vycel U does not have an equivalent weight and therefore does not react into the polymer, what keeps it from bleeding out of my system?
Just as rubber processing oils stay tightly bound in tire rubber, Vycel U strongly associates with the aromatic, hard block phase of the polymer’s matrix. Vycel U’s solubility parameter keeps it strongly locked within the matrix when incorporated below its compatibility threshold. When incorporated above this threshold, it will spill out into the soft block phase of the urethane where it can then bleed to the surface. The bleed or exudate characteristics of Vycel U in a particular system are dependent on three variables: the free volume of the hard block itself, the amount of Vycel U incorporated, and the application’s temperature. We recommend the Vycel U compatibility threshold be specifically determined for each individual formulation or system.
