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, taki