Determination of the diffusivity of a hydrophilic migratory additive in iPP films

This article describes the measurement of the diffusivity of a commercial hydrophilic additive (CHA), Irgasurf HL560, at concentrations of 5 and 10 wt% in polypropylene films at different temperatures. The model used was a standard 1-D diffusion model, in which the film weight was expressed as a function of time as the additive concentration at the film surface was kept at zero at all times. Experimentally, two cases were compared: (1) Washing additive off the film surface with limited contact with the solvent, while PP films were stored in air and tested at 25, 40, 55, and 70 degrees C up to 20 days. (2) Immersing the film in solvent with continuous stirring at 25 degrees C for 10 days, 44 degrees C for 2 days, and 65 degrees C for 10 h. Diffusivity measured by the surface-washing method was 10(-14) to 10(-12) cm(2)/s, and 10(-12) to 10(-10) by the immersion method. TGA results showed no effect of solvent enhancement to additive diffusion at 25 degrees C but slightly enhanced at 44 and 65 degrees C. Results for both the surface-washing and immersion methods are shown for instructive purposes. It is clear that the diffusivity calculated from surface-washing is much lower than from continuous immersion, largely because the boundary condition of zero additive concentration of the film surfaces was invalid between washings as additive bloomed to the surfaces and accumulated there. The continuous immersion method more closely matched the zero boundary condition used in the model and thus gave more accurate diffusivity values than did the surface-washing method.