Masahiro Hosoi

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°C. Their initial weight increase was proportional to the square root of time after exposure to moisture, obeying Ficks law; the hygroscopic expansion with time showed the same relationship. The equilibrium moisture content was 1.0×10-5/% 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 16×10-9 cm2/s at 30°C in comparison with 48×10-8 for PET, although the activation energies were 45 kJ/mol for PET and 41 for PEN.