Abstract
Epoxy structural adhesives have strong adhesion, minimal shrinkage and high thermal and chemical resistance. However, despite these excellent properties, their high-energy impact resistance should be improved to satisfy the increasing demands of the automotive industry. For this reason, we used four types of silica nanoparticles with different surface groups, such as polydimethylsiloxane (PDMS), hydroxyl, epoxy and amine groups, as toughening agents and examined their effect on the glass transition temperature (Tg), crosslinking density and phase separation of epoxy structural adhesives. High-energy impact resistance, mode I fracture toughness and lap shear strength were also measured to explain the effect of surface functional groups. Silica nanoparticles with reactive functional groups increased the mode I fracture toughness of epoxy structural adhesives without sacrificing the crosslinking density. Although the mode I fracture toughness of epoxy structural adhesives could not clearly show the effect of surface functional groups, the dynamic resistance to cleavage obtained by impact wedge-peel tests showed quite different values. At a 0.3 vol% content, epoxy-functionalized silica nanoparticles induced the highest value (40.2 N/mm) compared to PDMS (34.1 N/m), hydroxyl (34.9 N/mm), and amine (36.1 N/m). All of these values were significantly higher than those of pristine epoxy structural adhesive (27.7 N/mm).
Highlights
Structural adhesives are high-strength permanent adhesives for structural materials such as metals and plastics, and they have high impact strength, as well as high peel, bending, and fatigue strength [1]
The organosilane-treated silica nanoparticles were analysed by X-ray photoelectron spectroscopy (XPS) to confirm their surface functional groups
FS-NH shows a new peak at 400 eV, which is assigned to N1s, due to the amine group of aminopropyl trimethoxysilane (APS)
Summary
Structural adhesives are high-strength permanent adhesives for structural materials such as metals and plastics, and they have high impact strength, as well as high peel, bending, and fatigue strength [1]. The following were tried: butyronitrile-based liquid rubbers, such as amine-terminated butyronitrile (ATBN), carboxylic acid-terminated butyronitrile (CTBN), and epoxy-terminated butyronitrile (ETBN) [5,6,7,8,9,10,11,12,13,14] Thermoplastic elastomers, such as olefin, urethane, and amide, are commonly used as toughening agents [15,16,17,18]. In both cases, the tlfexcess number of toughening agents decreases the glass transition temperature (Tg ) and other mechanical properties. These inorganic fillers can lessen Tg reduction, their toughening effects are inferior to those of liquid rubbers and thermoplastic elastomers
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