Injection Moulded Thermoplastic Elastomer - Metal Hybrids

Abstract

Thermoplastic – metal hybrids are a recent addition to material space. They are macroscopic composites of thermoplastic and metal with a defined structure formed during manufacturing and possess properties from both material groups. This study examines manufacturing, properties and structure of new kind of insert injection moulded thermoplastic elastomer (TPE) – metal hybrids. The bonding of TPE to planar metal surfaces was reached either through micromechanical anchoring or via use of a chemical coupling agent. The first approach was studied with poly(styrene-block-ethylene-co-butylene-block-styrene) (SEBS) – aluminium hybrids; the latter one with thermoplastic urethane (TPU) – N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane – aluminium/stainless steel/copper hybrids. The main injection moulding parameters related to adhesive strength were first studied with micromechanically bonded hybrids. Increasing the ratio of cavity depth to insert thickness increased the bond strength until saturation was reached. The most important injection moulding machine setting for maximizing the bond strength was the mould temperature. Both these effects are based on the polymer melt penetration into the microcavities at the metal surface. Compared to micromechanical bonding, the best hybrids bonded with chemical coupling agent had an order of magnitude higher bond strength. The focus with hybrids bonded with chemical coupling agent was set on comparisons between different metals and their surface modifications combined with the effects of silanization parameters on silane layer formation and bonding to TPU. The silane layers on as-received stainless steel or copper were uneven. Similar behaviour was also observed for silane on electrolytically polished substrates with thin Fe/Cr mixed oxide layer. A homogenous, thick silane layer was formed after oxidation treatment independent on the oxide structure, and on stainless steel the thickness of the silane layer was dependent on the silane concentration. The coating evenness and thickness were linked to the bond strength in corresponding TPU – metal hybrids for both copper and stainless steel; the bond strength was more sensitive to the surface modification in the case of copper. With thick silane layers the failure was mainly cohesive in the silane layer and with optimal thin layers cohesive in TPU. Contrary to stainless steel and copper, in case of aluminium an interpenetrating oxide /siloxane layer is suggested to form. Also the long-term environmental resistance of hybrids of TPU and selected metal substrates was studied. High humidity exposure led to depolymerization of the polysiloxane and deterioration of the bond in all occassions. After high temperature exposure the bond strength increased, except for hybrids of oxidized stainless steel. Based on the analysis of the exposed samples, it was deduced that the bond mechanism at the metal/silane interface is dependent on the metal oxide and its isoelectric point, except for aluminium, where Al oxide continues to grow during silanization.