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Chemical Changes in Polyetherimide (PEI) Joined by Friction-based Injection Clinching Joining (F-ICJ) Technique

Abstract

The study of new joining technologies for multimaterial structures is motivated by the environmental friendly policy of weight reduction in the automotive industry. Friction-based Injection Clinching Joining (F-ICJ) (European Patent Application EP 14182938.2) is a new staking-based joining technique, which uses frictional heating to produce joints in an energy-efficient and fast manner. In F-ICJ a non-consumable tool applies friction and force to soften or melt and deform a polymeric stud fitted in a through hole of a joining partner. After consolidation, the deformed stud (stake) mechanically joins the partners by creating a staked joint. The thermomechanical process exposes the polymer to high temperatures (250 - 290°C) and deformation rates resulting from the combination of tool rotational speeds of 8000-12000 rpm and short joining cycles (5-10 seconds). Resulting changes in material properties can affect the global performance of the joints. This study aims to investigate the chemical changes occurred in the polymeric partner of hybrid joints on polyetherimide (PEI) and aluminum 6082-T6. Molecular weight distribution (Size-Exclusion Chromatography - SEC) and chemical structure (Fourier transform infrared spectroscopy – FTIR) analyses were performed on the base material and PEI extracted from a joint to evidence any possible chemical changes caused by frictional heating. The polymeric sample from the joint showed number-average molecular weight (Mn) 23% smaller and weight-average molecular weight (Mw) 15% smaller than the PEI base material. FTIR spectra show reduction in the intensity of phthalimide bands owing to the F-ICJ process. Therefore, these analyses identified the extension of chain scission in the PEI caused by shear and heat imposed by F-ICJ process, which are fundamental for process optimization.
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