Internal stresses and adhesion of thin silicon oxide coatings on poly(ethylene terephthalate)

The effect of internal stresses on the cohesion and adhesion of a thin sili con oxide (SiOx) oxygen-barrier coating, evaporated on a poly(ethylene tere phthalate) (PET) film substrate was studied. Internal stresses were generat ed during annealing in the temperature range for recrystallization of the P ET, during calendering in a multilayer structure where two SiOx/PET films w ere laminated together with a polypropylene film, and during long-term ther mal aging below the glass transition temperature of the polymer. The cohesi on of the coating and its adhesion to the polymer substrate were derived fr om fragmentation tests, in which the failure of the oxide coating was analy zed as a function of the applied stress during uniaxial tensile loading of the substrate. The intrinsic coating strength at critical length and the in terfacial shear strength were found to be equal to 1350 MPa and 73 MPa, res pectively. It was found that none of the thermal treatments investigated al tered the interfacial interactions. Rather, these treatments induced shrink age of the PET substrate, which increased the coating internal compressive stress and the SiOx/PET interfacial shear strength. A linear relationship b etween the SiOx/PET interfacial shear strength and the coating internal str ess was determined from a stress transfer analysis. The coefficient of this linear relationship, equal to -1.34 (.) h(c)/l(c), where h(c) is coating t hickness and l(c) is the critical stress transfer length, reproduces the ex perimental data with good accuracy.