A STUDY ON THE DEGRADATION OF PRE-APPLIED THREADLOCKERS UNDER EXTENDED EXPOSURE TO SEVERE HYGRO-THERMAL CONDITIONS

Pre-applied threadlockers are characterized by several favorable features in terms of handling of the fastener and simplification of the assembly process. Very often, fasteners with pre-applied adhesive undergo storage and shipping in harsh environmental conditions: these may encompass both exposure to elevated temperatures and to high levels of ambient humidity. Previous studies by the authors highlighted how the performance of the adhesive, both in terms of tribological response and of breakaway torque may be impaired, when specific exposure conditions apply. This study aims at better understanding the phenomenon of hygro-thermal degradation of the adhesive, more specifically, when longer exposure periods take place. The study is based on two sets of identical specimens (zinc coated steel screws): one set has been exposed to hygro-thermal degradation for 1 month at T=90°C and rh=95% and the other set has been kept at RT and rh=50%. The screws have then been tightened (underhead and thread c.o.f. values have been recorded on a sample for each population), then cured and finally released. All measurements have been performed by means of a tightening test bench that allows sampling the separate frictional contributions at the underhead and at the thread. The outcomes of the study reveal a different behaviour of the two populations, which is in line with the literature.