Abstract
UV-cross-linkable and thermally curable self-adhesive structural tapes (SATs) were compounded using solid commercial benzoxazine resins (Araldite MT 35700 and Araldite MT 35910) and a photoreactive epoxyacrylate copolymer (EAC). As initiators of benzoxazine resin polymerization and epoxy component cationic polymerization, two kinds of latent curing agents (LCAs) were tested, i.e., amine type and ionic liquid type. The influence of the benzoxazine resin and the LCA type on the UV-cross-linking process, the self-adhesive features and thermal curing behavior of UV-cross-linked tapes, as well as the shear strength of cured aluminum/SAT/aluminum joints and thermal stability of adhesives were investigated. It was found that the amine additive and the benzoxazine resin take part in the UV-cross-linking process of the EAC as hydrogen donors, which is confirmed by an increase in cohesion (+86%) and a decrease in adhesion (−25%) of SATs. The highest results of adhesion to steel (47 N/25 mm) and overlap shear strength (11.1 MPa) values were registered for SATs based on Araldite MT 35910 and contained 7.5 wt. parts of the amine-type hardener. The formation of a polyacrylate-benzoxazine network has a significant impact on the course of the thermal curing process and the thermomechanical properties of adhesive joints, which was also confirmed by the Cure Index calculation.
Highlights
Benzoxazine resins (BRs) are a novel class of thermosetting phenolic resins that have a characteristic oxazine ring [1]
It has been reported that some acids, bases, and Lewis acid can effectively decrease the induction for benzoxazine polymerization [7–9]
The following components were used for the preparation of epoxyacrylate copolymers (EAC): n-butyl acrylate (BA); glycidyl methacrylate (GMA); 4-hydroxybutyl acrylate (HBA)
Summary
Benzoxazine resins (BRs) are a novel class of thermosetting phenolic resins that have a characteristic oxazine ring (a heterocyclic six-membered ring with oxygen and nitrogen atoms) [1]. They can by synthesized via the Mannich condensation reaction of a phenolic derivative, formaldehyde, and a primary amine [2]. The polymerization of BRs typically takes place through thermally accelerated ring-opening polymerization (ROP), which occurs with or without initiators, and no by-products are produced. The polymerization of BRs takes place by heating (between 160 and 220 ◦ C) because BRs typically contain a small amount of phenolic derivatives (raw material) as impurity [6].
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