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

Abstract

New modifiers (i.e., acrylic syrups; ASs) of epoxy-resin-based thermally curable structural self-adhesive tapes (SATs) were prepared via a free radical bulk polymerization (FRBP) of n-butyl acrylate, butyl methacrylate, glycidyl methacrylate, and hydroxybutyl acrylate. In the process, two kinds of UV-photoinitiators (i.e., monoacylphosphine oxide/Omnirad TPO and bisacylphosphine oxide/Omnirad 819) and various mixing speed of the monomers mixture (200-1000 rpm) were applied. The TPO-based syrups exhibited a lower copolymers content (10-24 wt%), dynamic viscosity (<0.1 Pa·s), molecular weights (M_n and M_w, and polydispersity (1.9-2.5) than these with Omnirad 819. Additionally, the higher mixing speed significantly reduced monomers conversion and viscosity of ASs as well as molecular weights of the acrylate copolymers. These parameters influenced the properties of thermally uncured (e.g., adhesion) and thermally cured SATs (shear strength of aluminum/SAT/aluminum overlap joints). Better self-adhesive features were observed for SATs-TPO (based on ASs with lower monomers conversion, M_n and M_w); however, a slightly higher shear strength was noted for the thermally cured SAT-819 (ASs with higher monomers conversion, M_n and M_w). An impact of polydispersity of the acrylate copolymers as well as crosslinking degree of thermally cured SATs on the mechanical strength was also revealed.

Keywords: aerospace; epoxyacrylate compositions; photopolymerization; structural adhesives; thermoset polymers.

Figure 1

Schematic chemical structures of the tested monomers, photoinitiators, and the prepared acrylic syrups (BA: n-butyl acrylate, BMA: butyl methacrylate, GMA: glicydyl methacrylate, HEA: hydroxyethyl acrylate, O.819: Omnirad 819, O.TPO: Omnirad TPO).

Figure 2

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

Figure 3

Differential scanning calorimetry (DSC) thermographs for the uncured (a) SATs-TPO and (b) SATs-819 tapes.

Figure 4

Self-adhesive features of the selected UV-crosslinked (thermally uncured) SATs: (a) adhesion to steel, (b) tack, and (c) cohesion at 20 °C.

Figure 5

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

Figure 6

The influence of (a) the crosslinking degree of thermally cured SATs on overlap shear strength of Al/SAT/Al joints and (b) mixing speed on the crosslinking degree.

Figure 7

Photos of (a) Al/SAT-819-600/Al (slight adhesive outflow) and (b) Al/SAT-TPO-800/Al overlap joints (significant adhesive outflow).