Hygroscopic Expansion of Polyester Films

The hygroscopic expansion of polyethylene terephthalate (PET) and polyethylene 2,6-naphthalate (PEN) films was measured by use of a thermomechanical analyzer at 30, 40 or 50-degrees-C. Their initial weight increase was proportional to the square root of time after exposure to moisture, obeying Fick's law; the hygroscopic expansion with time showed the same relationship. The equilibrium moisture content was 1.0 x 10(-5)/% RH for both films; the hygroscopic expansion at equilibrium was also about the same for these films. However, the diffusion constant for moisture in PEN films was 1.(6) x 10(-9) cm2/s at 30-degrees-C in comparison with 4.(8) x 10(-8)for PET, although the activation energies were 45 kJ/mol for PET and 41 for PEN.