Effects of Curie particle induced accelerated curing on thermo-mechanical performance of 2K structural adhesives – Part II: Lap shear properties

Abstract

ABSTRACT Long curing times of adhesives represent a decisive disadvantage in contrast to most mechanical joining techniques. In order to compensate for this disadvantage, current research concentrates on various methods for accelerating adhesive polymerisation e.g. UV, microwave or oven curing. In addition to aforementioned methods, heating by electromagnetic induction represents a promising solution to speed up the curing process of adhesively bonded components. When components are exposed to high-frequency (HF) electromagnetic fields (EMF), heat is generated by eddy current or hysteresis losses in materials sensitive to EMF. If non-metallic materials like glass or wood are to be joined, different types of inductively heatable susceptors, e.g. meshes, fibres or particles, are added to the adhesives in order to achieve heating and consequently faster monomer cross-linking. Particles made of Curie material (Curie particles, CP) represent a relatively new type of susceptors. Even if exposed to unregulated induction power, CP can only be heated up through induction up to their specific Curie temperature (T c) above which no further CP-induced heating occurs. To guarantee an efficient and safe curing process, T c may be adjusted to the curing temperature optimal compatible for the adhesive to be cured. Thus, an accelerated curing process is designed which eliminates the need to control arising adhesive temperatures by external temperate measurements and – at the same time – prevents the adhesive from overheating. The present paper represents the second part of a study investigating the influence resulting from CP-induced accelerated curing on mechanical and thermo-mechanical properties of a 2 K epoxy resin (2 K-EPX) and a 2 K polyurethane (2 K-PUR). While investigations in the first part concentrated on adhesive bulk properties, compound characteristics of the very same adhesives bonded on different substrate materials (aluminium, G-FRP and wood) were in the focus of the present paper. The authors showed with an experimental program of 256 single lap shear (SLS) specimens that compound characteristics of the two considered adhesives are fundamentally changed by application of the introduced curing process. In most cases, inductively cured SLS joints achieved higher or equivalent lap shear strengths compared to cold cured reference sets, with a reduction in curing time from 1 or 10 days down to 10 min.