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%.

Keywords: acrylate syrup; adhesion; bulk photopolymerization; photoinitiators; polymer synthesis; structural adhesives.

Figure 1

Chemical structures of monomers and synthesis of acrylic syrups.

Figure 2

Preparation steps of the self-adhesive structural tapes (SATs).

Figure 3

Stacked plot of the proton nuclear magnetic resonance (¹H NMR) spectra of acrylic syrups. Peaks monitored for each monomer and the internal standard (naphthalene) are indicated.

Figure 4

Differential scanning calorimetry (DSC) thermographs for the uncured SATs (a) and cured SATs (b) based on ASs.

Figure 5

Adhesion to steel and tack for thermally uncured SATs based on ASs.

Figure 6

Shear strength for thermally cured aluminum–SAT–aluminum overlap joints.

Figure 7

Influence of cross-linking degree (α, DSC measurements) of thermally cured SATs on overlap shear strength for aluminum–SAT–aluminum overlap joints.

Figure 8

Influence of the monomers’ conversion of ASs (C, NMR data) on cross-linking degree (calculated using DSC data) for thermally cured SATs.

Figure 9

Storage modulus for thermally cured SATs.

Figure 10

Tangens delta for thermally cured SATs.