Photoinitiated cyanoacrylates for fast underwater bonding

Abstract

Strong underwater bonds between polycarbonate sheet and various other substrates were achieved using ethyl 2-cyanoacrylate gel cured with an anionic photoinitiator (dibenzoylferrocene). The gel reacts quickly with water on contact and forms a protective polyethyl cyanoacrylate (PECA) skin. This provides a diffusion barrier that protects the inner monomer from further reaction with or polymerisation by water (hydroxide ions). The PECA skin is, however, permeable to water and the inner monomer slowly reacts as water diffuses through the PECA skin. The application procedure for this adhesive is critical because of the fast reaction with water. The adhesive should be applied at the centre of the bond area in such a way as to cover the smallest exposed surface area and that it should not be spread out over the entire surface area. The PECA skin will rupture when the substrates are pressed together, exposing unreacted monomer to the fresh new surface between the two substrates. The adhesive displaces water as it propagates towards the joint edges. In this way it is able to form a strong underwater bond. Without a photoinitiator, ethyl cyanoacrylate adhesives will form strong bonds to smooth underwater surfaces. The addition of a photoinitiator (dibenzoylferrocene), however, increases the rate at which the bond strength develops. Photo differential scanning calorimetric (photo-DSC) analysis was used to determine the optimum photoinitiator concentration. A concentration of between 0.3% (m/m) and 0.1% (m/m) was found to be optimum for adhesive films between 0.5 and 1.2 mm thick. Epoxy-based underwater adhesives have the disadvantage that the cure rate is very slow at low water temperatures. In contrast, the current system features a negative temperature dependence. Therefore, the curing rate is faster at lower water temperatures, making it highly suitable for very cold environmental conditions. The underwater bonding properties of cyanoacrylate adhesive were measured on a wide range of metallic and non-metallic materials. Bond strengths ranged from 1 to 5 MPa respectively. Bond strength (0.2 mm film thickness) develops in a very short time (less than a minute) using a 5 mW/cm2 blue LED light source as trigger for the photo initiator. Water temperatures between 1.5 and 40 °C did not influence the ultimate bond strength achieved following 50 seconds of illumination. The reduction in bond strength due to the ethyl cyanoacrylates reacting with water was measured in artificial sea water and potable water. A 20% reduction in bond strength was found after 10 minutes of water exposure. The underwater bond should therefore be made as soon as possible following exposure to water. Bond ageing studies on both steel and polycarbonate showed an initial decrease in bond strength (during the first 50 hours); thereafter the bond strength stabilises.