Durability of Nanosized Oxygen-Barrier Coatings on Polymers

Research on silicon oxide thin films developed as gas-barrier protection for polymer-based components is reviewed, with attention paid to the relations between (i) coating defects, cohesive strength and internal stress state, and (ii) interfacial interactions and related adhesion to the substrate. The deposition process of the oxide from a vapor or a plasma phase leads in both cases to the formation of covalent bonds between the two materials, with high adhesion levels. The oxide coating contains nanoscopic defects and microscopic flaws, and their respective effect on the barrier performance and mechanical resistance of the coating is analyzed. Potential improvements are discussed, including the control of internal stresses in the coating during deposition. Controlled levels of compressive internal stresses in the coating are beneficial to both the barrier performance and the mechanical reliability of the coated polymer. An optimal coating thickness, with low oxygen permeation and high cohesive strength, is determined from experimental and theoretical analyses of the failure mechanisms of the coating under mechanical load. These investigations are found relevant to tailor the interactions and stress state in the interfacial region, in order to improve the reliability of the coating/substrate assembly.

Stress controlled gas-barrier oxide coatings on semi-crystalline polymers

Yves Leterrier, Jan-Anders Månson, Gil Rochat

Thin silicon oxide (SiOx) barrier coatings formed by plasma enhanced chemical vapor deposition on poly(ethylene terephthalate) (PET) substrates were subjected to post-deposition annealing treatments in the temperature range for orientation relaxation of the polymer. The resulting change in coating internal stress state was measured by means of thermo-mechanical analyses, and its effect on the coating cohesive properties and coating/polymer adhesion was determined from the analysis of uniaxial fragmentation tests in situ in a scanning electron microscope, assuming a Weibull-type probability of failure and a perfectly plastic stress transfer at the SiOx/PET interface. The strain to failure and intrinsic fracture toughness of the ultrathin oxide coating were found to be as high as 5.7% and 10 J/m2, respectively, and its interfacial shear strength with PET was found to be close to 100 MPa. Annealing for 10 min at 150 -C did not modify the oxygen permeation properties of the SiOx/PET film, which suggests that the defect population of the oxide was not affected by the thermal treatment. In contrast, the coating internal compressive stress resulting from annealing was shown to increase by 40% the apparent coating cohesive properties and adhesion to the polymer.

2005

UV-curable epoxy primer for adhesion in steel reinforced polymer composites

Basile Golaz, Jan-Anders Månson, Véronique Michaud

In continuous composite production, for example the continuous impregnation of steel wires or cords with a thermoplastic polyurethane matrix, an ideal and cost-effective solution to improve interfacial adhesion and durability would be to deposit and cure a primer, on-line, at high processing speeds. A UV-curable epoxy primer was investigated for this purpose; it is reticulated by cationic photo-polymerisation, possibly followed by dark-curing and/or post-cure during further processing. The primer base formulation was tailored using additives including a hyperbranched monomer for tailoring cross-linking and flexibility and trimethylolpropane oxetane to increase curing speed and reduce viscosity. Curing kinetics, coating mechanical properties, and adhesion and durability at the steel-TPU interface were assessed for several combinations of coating formulation and processing parameters (time, temperature, UV intensity). Pull-out tests were analysed by finite element methods to refine interpretation of the experimental results for process-induced stresses, interfacial shear strength and energy release rate. The results demonstrated the fast-curing capability together with adhesion and durability improvements between galvanized or stainless steel reinforcements and a thermoplastic polyurethane elastomer by forming physico-chemical bonds with both the polymer and the substrate, while maintaining low internal stresses at the interface. The proposed UV-curable primer outperformed conventional surface treatments and thermally cured aminosilane primers used as benchmark in terms of processing time, adhesion and durability.

2012

Internal Stresses and Adhesion of Thin Silicon Oxide Coatings on Poly(ethylene terephthalate)

Yves Leterrier, Jan-Anders Månson

The effect of internal stresses on the cohesion and adhesion of a thin silicon oxide (SiOx) oxygen-barrier coating, evaporated on a poly(ethylene terephthalate) (PET) film substrate was studied. Internal stresses were generated during annealing in the temperature range for recrystallization of the PET,during calendering in a multilayer structure where two SiOx/PET films were laminated together with a polypropylene film, and during long-term thermal aging below the glass transition temperature of the polymer. The cohesion of the coating and its adhesion to the polymer substrate were derived from fragmentation tests, in which the failure of the oxide coating was analyzed as a function of the applied stress during uniaxial tensile loading of the substrate. The intrinsic coating strength at critical length and the interfacial shear strength were found to be equal to 1350 MPa and 73 MPa, respectively. It was found that none of the thermal treatments investigated altered the interfacial interactions. Rather, these treatments induced shrinkage of the PET substrate, which increased the coating internal compressive stress and the SiOx/PET interfacial shear strength. A linear relationship between the SiOx/PET interfacial shear strength and the coating internal stress was determined from a stress transfer analysis. The coefficient of this linear relationship, equal to-1.34 · h c/l c, where h cis coating thickness and l cis the critical stress transfer length, reproduces the experimental data with good accuracy.

2001