Adhesion of thermoplastic polyurethane elastomer to galvanized steel

Abstract When a thermoplastic polyurethane elastomer (TPU) is co-molded with galvanized steel, the subsequent adhesion and its durability are very sensitive to the chemical bonds created at the interface. Silane coupling primers are known for enhancing adhesion between inorganic substrates and several polymers and this study addresses their efficiency at the TPU–metal interface. γ-APS aminosilane was deposited with different parameters, including pH and solvents. Several other surface treatments were studied for comparison: degreasing, corona discharge, acidic etching, gas flame, and a zinc phosphate conversion coating. The adhesion was characterized with a single wire pull-out test derived from the microbond test geometry, after different aging conditions. Results confirmed that γ-APS worked as a coupling agent in between TPU and the inorganic substrate, outperforming all other surface treatments tested in term of strength and durability, but the silane required the appropriate deposition parameters to be really efficient. In parallel, a finite element approach was proposed for the modeling of fiber reinforced thermoplastic elastomer composites accounting for the non-linear viscoelastic material properties and its results compared to an analytical elastic shear-lag interpretation of the single filament pull-out test. The analytic interpretation was found to be sufficiently accurate for stress analysis but not for fracture mechanics. The FEM analysis offered an evaluation of the internal stresses developed within the entire system, which helped the interpretation of results. The model, compared to experimental results, also revealed that it is important to account for internal stresses, mostly for treatments leading to low intrinsic interfacial shear strength values.