Free-Radical Bulk-Photopolymerization Process as a Method of Obtaining Thermally Curable Structural Self-Adhesive Tapes and Effect of Used Type I Photoinitiators

Abstract

A new fabrication method for thin (120 µm) thermally curable structural self-adhesive tapes (SATs) was demonstrated by utilizing a series of acrylic syrups (ASs) modified using Bisphenol A-based liquid epoxy resin. The acrylic syrups containing poly(butyl acrylate-co-butyl methacrylate-co-glycidyl methacrylate-co-2-hydroxyetyl acrylate-co-4-acryloyloxy benzophenone) were synthesized via free-radical bulk-photopolymerization (FRBP) process. Influence of different type I radical photoinitiators (PIs), i.e., α-hydroxyalkylphenones (HPs), acylphosphine oxides (APOs) and its mixtures (HPs/APOs and APO/APO) on selected physico-chemical features of obtained ASs was studied. It turned out that APO-type PIs are more effective in the FRBP process (NMR studies). Self-adhesive tests of SATs revealed that the monomers’ conversion in ASs have a significant influence on adhesion and tack. Moreover, the polymer structures formed at the UV cross-linking stage of SATs significantly affect the cross-linking degree of SATs during thermal curing (differential scanning calorimetry method). The highest values of overlap shear strength were achieved by SATs based on ASs with monomers’ conversion on the level 50–60%.

Epoxy-Based Structural Self-Adhesive Tapes Modified with Acrylic Syrups Prepared via a Free Radical Photopolymerization Process

“…In this case, ASs prepared using Omnirad 819 are characterized by a higher viscosity, solids content, M w , and M n values than the AS-TPO-type syrups. It was previously proved by the authors (considering NMR studies) that a solids content value directly correlates with monomers conversion in ASs [ 20 ]. The syrups based on TPO and prepared at the lowest agitation speed values (200 rpm or 400 rpm) exhibited the highest monomers conversion (24% or 21%, respectively).…”

“…Finally, the intact epoxy resin is trapped in the acrylic matrix consisting of the epoxyacrylate copolymers and created epoxyacrylate network. In the authors’ opinion, the self-adhesive properties mainly depend on the content of linear acrylate copolymers (formed in the free radical bulk polymerization (FRBP) process) in the applied syrup and especially on their molecular weights (as it was presented in [ 20 ]); it is known that end groups of (co)polymers act as dipoles and can interact with surface groups of steel (during the adhesion or tack tests). Thus, all the SAT samples based on the AS-TPO syrups ( M n in the range of 19,840–14,120 g/mol) reached higher values of these self-adhesive features than the SAT-819-type samples (28,670–22,700 g/mol).…”

“…In the case of SATs-TPO-based joints, the τ values were quite similar for all the samples (16.5–17.1 MPa). It is known that the shear strength of structural adhesives depends on their crosslinking degree and an optimal range of the latter parameter value is often observed [ 20 ]. Figure 6 a shows the relationship between the shear strength of Al/SAT/Al joints and the crosslinking degree of the applied SAT sample.…”

“…A UV/heat dual-curable epoxy–acrylate adhesive containing the mentioned core-shell particles exhibits outstanding water resistance [ 15 ]. Applications of epoxyacrylate copolymers (based on BA, GMA, and 2-hydroxyethyl acrylate) prepared by a conventional radical polymerization process in an organic solvent [ 16 , 17 , 18 , 19 ] or by bulk photopolymerization [ 20 ] are also known. In the latter case, the acrylate-type modifier was used in the form of a prepolymer also called acrylic syrup (i.e., a solution of the copolymer in the unreacted monomers).…”

“…This type of adhesive is commonly used in the aviation industry [ 21 , 22 , 23 , 24 , 25 ]. Preparation of the structural adhesives as thin films (based on epoxyacrylate copolymers obtained by a relatively new method, i.e., free radical bulk photopolymerization process (FRBP)) was first described in the literature by the authors of this manuscript [ 20 ].…”