Impact of Cold Plasma Treatment on the Bulk Properties of Polypropylene Films

Extruded polypropylene (PP) films were exposed to cold air plasma treatment. Plasma treatment of PP films resulted in essential changes in their bulk properties. Maximal elongation, ultimate tensile strength (UTS), and toughness of the films were increased. The toughness of the films was increased from U_T0=(3323±400) MPa to U_(T_PT)=(4434±400) MPa. This increase is due to the growth of both the maximal elongation and the UTS of the plasma-treated samples. We relate improvement of mechanical properties of PP to the morphological transformations revealed in the plasma-treated PP films. Plasma treatment of PP samples was also followed by the modification of their surface properties. Plasma treatment resulted in hydrophilization of PP films followed by hydrophobic recovery. Bulk and surface properties of the plasma-treated PP films evolve with time. The following hierarchy of the temporal scales related to the studied relaxation processes is established: τ_HR>τ_ε=τ_T=τ_UTS>τ_E, where τ_HR,τ_ε,τ_T,τ_UTS and τ_E are the time scales of the change in: apparent contact angle (hydrophobic recovery), elongation, toughness, ultimate tensile strength, and Young modulus, respectively. The longest of the relaxation times is related to the surface processes, i.e., hydrophobic recovery. The stress-strain curves of the virgin and plasma-treated PP are well described with the twin-slope linear dependencies. The post-plasma-treatment recovery of the tangent modulus is reported.