UV-cured Epoxy for Adhesion in Steel Reinforced Thermoplastic Composites

This work addresses the development of composite materials that contain metallic load-bearing cords with the aim of combining outstanding mechanical performance with added functionality, including electrical conductivity, data transmission, and temperature and strain sensing. The corresponding elements are impregnated with a thermoplastic elastomer for protection, and to provide mechanical integrity and surface traction. However, in order to render the resulting smart multifunctional systems sufficiently robust and cost effective, it is necessary to find innovative solutions for the rapid establishment of a strong and durable interface between the metallic reinforcements and the surrounding elastomer. In order to ensure effective transfer of mechanical loads, the interface between metal oxides and polymers is typically modified using organosilane primers. These provide strong and durable bonds, but imply a sol-gel condensation at the interface characterized by relatively slow, temperature dependent reaction kinetics, which often precludes their application on-line. Additional processing and storage steps increase costs and may result in surface contamination or premature reaction of functional groups. It is to some extent possible to resolve these latter problems by surface activation with heat or solvent treatments prior to bonding, but such treatments may be unacceptable in terms of energy consumption and/or pollution. One of the main goals of the present work has therefore been to identify a suitable primer that can be deposited and cured on-line at high processing speeds. The first phase of the study focused on the adhesion phenomena associated with the interface of a thermoplastic polyurethane elastomer (TPU) co-moulded with galvanized steel wire, and their sensitivity to hydrothermal aging. Various surface treatments were investigated for the purposes of benchmarking and to assess their efficiency at the TPU-metal interface. γ-aminopropyltriethoxysilane (γ-APS) was deposited on the steel using water (pH5 or pH10), isopropanol or ethanol as a solvent and other surface treatments included degreasing, corona discharge, acid etching, gas flame and use of a zinc phosphate conversion coating. Adhesion was characterized using a single wire pull-out test based on the microbond test geometry on unaged specimens, after 2 months of dry aging or after 24h of hydrothermal aging in water at 50°C. γ-APS was found to increase adhesion between the TPU and the substrate, outperforming the other surface treatments in terms of strength and durability. The shear strength increased from 5-11 MPa to more than 20 MPa for the unaged specimens, and from ~5 MPa to more than 15 MPa after hydrothermal aging. However, effective treatment with γ-APS required careful optimization of the deposition parameters and a 1 hour thermal cure. At the same time, a finite element (FEM) approach was used to simulate an idealized wire-reinforced thermoplastic elastomer composite, taking into account the non-linear viscoelastic response of the matrix, and the results were compared with results from an analytical elastic shear-lag model for the single filament pull-out test. The analytical model was found to be sufficiently accurate for stress analysis, but was unsuitable for fracture properties. FEM analysis allowed evaluation of the build-up in internal stresses during processing, showing that these may reach levels of 3.3 MPa, with important consequences, particularly for treatments leading to low intrinsic interfacial shear strength. The next phase of the study involved the development of an innovative fast-curing coupling method using a cationic UV-curable primer with an optimized formulation based on an epoxy monomer. The approach was first to determine the curing kinetics and the final degree of cure of the epoxy resin by photo-DSC in order to optimize the processing conditions. The processing parameters studied were the curing time, temperature, UV intensity and thermal post-curing. The influence of the primer on the interfacial mechanical properties was then investigated using the single wire pull-out test in order to tune the formulation with respect to interfacial stress transfer through the primer before and after hydrothermal aging. The formulations comprised a hyperbranched monomer for cross-linking and flexibility, an oxetane monomer to increase curing speed and reduce viscosity, and an epoxysilane to improve the adhesion to the substrate. The proposed UV-cured epoxy primer outperformed the conventional methods in terms of processing time, adhesion and durability. The processing time was reduced from 1h for the silane primer to 5s for the UV-cured epoxy primer, and the interfacial shear strength reached more than 25 MPa before aging and more than 20 MPa after hydrothermal aging. In the final phase of the work, the fast-curing adhesion promotion system was applied to systems based on other metallic substrates and thermoplastic matrices (flame retardant TPU (FR-TPU), polyamide 12, high density polyethylene). The new primer increased adhesion to stainless steel and was also effective with FR-TPU and polyamide 12. Primer application was then successfully scaled up to the cord geometry and the interfacial adhesion tested by cord pull-out tests, as opposed to single wire pull-out tests, again before and after hydrothermal aging. In this configuration, the UV-cured primer increased the maximum pull-out force by 40% before aging and 20% after aging, even with non-optimized formulations. The primer was also found to reduce the fretting wear of steel cords, which is also a considerable advantage for long term composite properties. Finally, optical fibres sensors were successfully integrated in the TPU with sufficient interfacial stress transfer to provide the multifunctional coated transmission elements with accurate temperature and strain sensing capabilities. Use of the UV-cured epoxy primer was again shown to be highly beneficial in this case. Sensing accuracies of ±1 μstrain and ±0.2°C were achieved, with a spatial resolution of 0.10 mm.