Y. Leterrier

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.

Life Cycle Assessment of Biofibres Replacing Glass Fibres as Reinforcement in Plastics

This article aims to determine the environmental performance of China reed fibre used as a substitute for glass fibre as reinforcement in plastics and to identify key environmental parameters. A life cycle assessment (LCA) is performed on these two materials for an application to plastic transport pallets. Transport pallets reinforced with China reed fibre prove to be ecologically advantageous if they have a minimal lifetime of 3 years compared with the 5-year lifetime of the conventional pallet. The energy consumption and other environmental impacts are strongly reduced by the use of raw renewable fibres, due to three important factors: (a) the substitution of glass fibre production by the natural fibre production; (b) the indirect reduction in the use of polypropylene linked to the higher proportion of China reed fibre used and (c) the reduced pallet weight, which reduces fuel consumption during transport. Considering the whole life cycle, the polypropylene production process and the transport cause the strongest environmental impacts during the use phase of the life cycle. Since thermoplastic composites are hardly biodegradable, incineration has to be preferred to discharge on landfills at the end of its useful life cycle. The potential advantages of the renewable fibres will be effective only if a purer fibre extraction is obtained to ensure an optimal material stiffness, a topic for further research. China reed biofibres are finally compared with other usages of biomass, biomaterials, in general, can enable a three to ten times more efficient valorisation of biomass than mere heat production or biofuels for transport.

Adhesion of silicon oxide layers on poly(ethylene terephthalate). I: Effect of substrate properties on coating's fragmentation process

Reference LTC-ARTICLE-1997-008View record in Web of Science URL: http://www3.interscience.wiley.com/cgi-bin/jhome/36698 Record created on 2006-06-26, modified on 2016-08-08

Adhesion of silicon oxide layers on poly(ethylene terephthalate). II: effect of coating thickness on adhesive and cohesive strengths

Reference LTC-ARTICLE-1997-007View record in Web of Science URL: http://www3.interscience.wiley.com/cgi-bin/jhome/36698 Record created on 2006-06-26, modified on 2016-08-08

Reactive processing of poly(ethylene terephthalate) modified with multifunctional epoxy-based additives

Keywords: poly(ethylene terephthalate) ; epoxy-additives ; reactive processing Reference LTC-ARTICLE-2000-006doi:10.1016/S0032-3861(99)00768-5View record in Web of Science URL: http://www.sciencedirect.com/science/journal/00323861 Record created on 2006-06-26, modified on 2016-08-08

Novel pulp fibre reinforced thermoplastic composites

The reinforcement potential of pulp fibres is presently not fully explored in thermoplastic composites. One of the reasons is that currently used processing methods comprise several severe thermomechanical steps inducing premature degradation of the fibres. Three pre-forming techniques were developed to prepare pulp fibre reinforced cellulose diacetate (CDA) pre-forms, namely filtration-forming, solvent impregnation, and commingling with polymer fibres. These techniques eliminate all thermomechanical steps, prior to final processing. The CDA polymer was nevertheless found to be very sensitive to the specific process histories relevant to each technique, contrary to the pulp fibres, whose size, shape, and mechanical properties were not affected by neither of the pre-forming processes. The tensile properties of composites compression moulded from solvent impregnated pre-forms were compared to those of ground china reed reinforced CDA. Whereas ground china reed particles were found to act merely as fillers increasing composite stiffness, a remarkable reinforcement effect was observed for the pulp fibre reinforced impregnated pre-forms. A combination of a stiffness increase by a factor 5.2 and a strength increase by a factor of 2.3 relative to the pure polymer was achieved, whereas in typical pulp fibre reinforced thermoplastics, the stiffness increase is frequently obtained at the expense of loss in strength. This work highlights the key factors which control the mechanical performance of pulp fibre reinforcements previously neglected in literature, and demonstrates the remarkable reinforcement potential of such renewable material. Furthermore, the properties achieved by optimising the extraction and processing steps indicate that pulp fibre reinforced thermoplastics composites are appropriate materials for load bearing applications.

Frequency dependent dielectric and mechanical behavior of elastomers for actuator applications

The low frequency mechanical and dielectric behavior of three different elastomers has been investigated by dynamic mechanical analysis and dielectric spectroscopy, with the aim of accounting for the frequency dependence of the characteristics of the corresponding dielectric elastomer actuators. Satisfactory agreement was obtained between the dynamic response of the actuators and a simple model based on the experimental data for the elastomers, assuming that the relatively large prestrains employed in the actuators to have little influence on the frequency dependence of their effective moduli. It was thus demonstrated that the frequency dependence of the actuator strain is dominated by that of the mechanical response of the elastomer, and that the frequency dependence of the dielectric properties has a relatively minor influence on the actuator performance.

Mechanical analysis of ultrathin oxide coatings on polymer substrates in-situ in a scanning electron microscope

Uniaxial fragmentation tests were carried out in situ in a scanning electron microscope (SEM) on 10-nm-thick silicon oxide coatings deposited by plasma enhanced chemical vapor deposition on poly(ethylene terephthalate). In order to prevent charging effects due to the isolating nature of the oxide surface, an additional conductive gold layer was sputtered onto the coating prior to its tensile loading in the SEM chamber. The gold layer was shown not to affect initiation of tensile failure of the oxide coating, and was used to achieve optimal resolution by eliminating charging effects in the low-strain range. In contrast, in the high strain range, the failure behavior of the oxide coating was found to be modified by the gold layer. It was nevertheless possible to analyze the damage mechanisms of the thin coating without a gold layer due to sufficient crack opening. The coating cohesive strength was found to be equal to 5.1 GPa, and the coating/polymer interfacial strength was found to be equal to 84 MPa using a Weibull size-dependent probability of failure for the oxide, and assuming a perfectly plastic stress transfer between the different layers.

A Method to Measure the Adhesion of Thin Glass Coatings on Polymer Films

Abstract A simple and reliable method to measure the adhesion of thin, hard coatings on polymer substrates is presented, based on the rupture mechanics of brittle films on ductile substrates. The regular fragmentation pattern of the coating obtained after straining specimens under uniaxial tension is analyzed through a classical shear-lag analysis at the coating/substrate interface. The model links the mean crack spacing measured on strained specimens to the interfacial shear strength and the reversible adhesion energy. Fragmentation tests were carried out on a PET film coated on both sides by SiO2 layers (24 nm on the thick side, and 6nm on the thin side). The interfacial shear strength was found to be close to 100 MPa for both coatings and the adhesion energy of SiO2 on PET was found to be of the order of 230 mJ m2, both values being slightly higher for the thin coating side.

Biaxial fragmentation of thin silicon oxide coatings on poly(ethylene terephthalate)

Crack patterns of 53 nm and 103 nm thick silicon oxide coatings on poly(ethylene terephthalate) films are analyzed under equibiaxial stress loading, by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The biaxial stress and strain are modeled from classical elastic membrane equations, and an excellent agreement is obtained with a finite element method. In the large pressure range, the derivation of the biaxial strain from force equilibrium considerations are found to reproduce accurately the measured data up to 25% strain. The examination of the fragmentation process of the coating under increasing pressure levels reveals that the crack onset strain of the oxide coating is similar to that measured under uniaxial tension. The fragmentation of the coating under biaxial tension is also characterized by complex dynamic phenomena which image the peculiarities of the stress field, resulting in considerable broadening of the fragment size distribution. The evolution of the average fragment area as a function of biaxial stress in the early stages of the fragmentation process is analyzed using Weibull statistics to describe the coating strength.