Extended non-destructive testing for inline quality control of cleaning and pre-treatment processes in adhesive bonding of polymer blends

This investigation aims to demonstrate relevant pitfalls of exemplary surface contaminations on polymer blends with respect to quality assurance (QA) of adhesive joints, and highlights approaches of Extended Non-Destructive Testing (ENDT) for tackling the challenges inline during manufacture or repair processes. Greasy fingerprints were intentionally applied on surfaces of different acrylonitrile butadiene styrene (ABS) substrates before joining them with a two component (2C) polyurethane (PU) adhesive. Even when a required cleaning process was omitted before performing a pre-treatment with atmospheric-pressure plasma (APP), measured initial strengths of unaged joints did not reveal significant differences, if compared to joints produced following the qualified bonding process, which comprises a wipe-off wet cleaning before the pre-treatment step. However, after accelerated ageing mimicking the application conditions for joints, resulting strengths were more than 30% lower than for joints produced with the qualified bonding process. Thus, applying post-process random inspection of as-manufactured joints based on destructive testing, and merely evaluating initial joint strengths is not sufficient for assuring the reliability of such manufactured joints. Rather, it is recommended to encompass the intended or specified use within the formulation of the quality requirements. Introducing in-process ENDT procedures, and exemplarily monitoring the substrate surfaces with an Aerosol Wetting Test (AWT), is shown to identify latent fingerprints and to ensure a rapid and documented way to validate adherend surface quality before the application of the adhesive.