Liquid crystalline vinyl ester resins for structural adhesives

Abstract

Rigid-rod vinyl ester resins represent an interesting family of materials. They can be synthesized starting from the corresponding epoxy resin. Due to their composition, they exhibit outstanding properties, such as heat and chemical resistance, high impact, and corrosion resistance. These features, combined with the ease of curing, make vinyl ester resins good candidates for several industrial applications, such as glass fiber composites, adhesives, corrosion-resistant coatings, electrical encapsulation, and radiation-curable inks. In the first part of this study, a new liquid crystalline methacrylated monomer was prepared and tested as an adhesive. The synthesis was carried out by reacting methacrylic acid with a rigid-rod epoxy monomer derived from the reaction between p-(2,3-epoxypropoxy)-α-methylstilbene (DOMS, diglycidyloxy α-methylstilbene) and aniline in a molar ratio of 3 to 2. The monomer was named md3a2 (methacrylated derivative of DOMS-aniline epoxy compound). Md3a2 is liquid crystalline in a wide range of temperatures, as shown by differential scanning calorimetry (DSC) and optical microscopy. In the second part of the work, the adhesion properties of the novel liquid crystalline resin were tested and compared with those exhibited by a conventional (non-liquid crystalline) adhesive, the methacrylated derivative of an epoxy compound synthesized from a 3 to 2 molar ratio of Epon 825® (which is a commercial epoxy) and aniline. This monomer was named mep3a2 (methacrylated derivative of Epon 825-aniline epoxy compound). The adhesion of the two resins on a pretreated aluminum substrate was evaluated using mechanical tests (single-lap joint test, according to ASTM standard D1002-72) and scanning electron microscopy (SEM).