Development of Lactide-based Macromonomers for Copolymerization with Acrylates to produce Adhesives and Coatings of High Renewable Contents

Abstract

A new approach was introduced for incorporating renewable biomass into existing commercial pressure-sensitive adhesive (PSA) polymers in the form of acryloyl macromonomers (MM). MMs were prepared with L-lactide and ε caprolactone via a bulk ring-opening polymerization initiated by N-hydroxyethyl acrylamide (HEAA). Acrylic adhesive copolymers were synthesized by free-radical solution polymerization in presence of 2-ethylhexyl acrylate (EHA), acrylamide and MMs. A series of MMs, synthesized using catalyzed ring-opening polymerizations, were produced containing a broad range of lactic acid and caprolactone repeat units. Results indicate that the properties and performance of adhesive polymers are strongly dependent on lactide composition. In general, increasing lactide content increases polymer hardness enhancing cohesive strength, while reducing it (i.e., increasing caprolactone content) softens the polymer. Optimal adhesion is found to require a balance between these tendencies as indicated by the existence of a clear maximum in both tack and peel data. The results demonstrate that a broad range of properties is achievable through relatively minor modifications to MM composition. It is expected that these hybrid materials can be optimized for a variety of self-adhesive applications. With the new MM approach, the relation between the dynamic wetting behavior on a soft viscoelastic surface and the rheological properties of materials can be studied. The mechanical properties of polymers are tailored through changing MM composition to provide a broad range of viscoelastic responses. It was found the wetting of these polymers supports the existence of two distinct wetting regions as opposed to the several, one in which the wetting line and ridge propagate smoothly together, and a second in which the ridge slows propagation and is eventually dropped leaving behind a residual deformation ridge. The focus is on ridge formation and properties controlling its propagation prior in the neglected former region. Although most past experimental studies emphasize the rate dependency of this process, results presented here indicate that ridge propagation is governed to a similar extent by film thickness and the vertical surface tension force. The data is used to develop a semi-empirical model consistent with the contribution of both viscous and elastic responses to the process. The ideas presented provide a new and more comprehensive view of the wetting of soft substrates.