Stress relaxation behavior of weldable crosslinked polymers based on methacrylated oleic and lauric acids

Abstract

In this work, bio-based “malleable” crosslinked polymers that present suitable stress relaxation at relatively high temperatures and thermal welding (without the addition of any extrinsic catalyst) were evaluated. Two series of polymers were prepared: one coming from the polymerization of methacrylated oleic and lauric acids with styrene and the other obtained by replacing the styrene by commercial acrylated epoxidized soybean oil (AESO). All obtained polymers present β-hydroxyester groups that can lead to associative exchange reactions when they are submitted to thermal treatments. We demonstrate that even the polymers obtained by the simplest way, that is, the radical polymerization of methacrylated oleic or lauric acids (30 and 50 wt%) with styrene, experience interesting stress relaxation that manifests itself as a decrease of about 60–70% with respect to the initial modulus after approximately 2.5 h heat treatment at 190 °C. Furthermore, the system with 70 wt% methacrylated oleic acid and 30 wt% AESO can relax stresses up to 80% at 170 °C in the same period of time and exhibits almost complete stress relaxation at 200 °C in less than 20 min. The results are directly correlated with the chemical structure of the precursors, cross-linking density and amount of hydroxyl and esters groups in the cured polymers. Moreover, the obtained polymers are insoluble in most common solvents; exhibit composition dependent glass transition temperatures (Tgs) ranging from 10 to 75 °C and after welding still retain an acceptable mechanical behavior.