All-aromatic liquid crystalline thermosets as high temperatures adhesives

In this paper we will describe the synthesis and characterization of a novel series all-aromatic liquid crystal thermosets (LCTs) for high temperature (>100 °C) adhesive applications. Our ester-based oligomers, end-capped with reactive phenylethynyl functionalities, were synthesized using a standard one-pot melt condensation technique. The obtained reactive oligomers exhibit low melting temperatures and low melt viscosities, which resulted in improved flow and surface wetting. The adhesive properties of our LCTs based on 4-hydroxybenzoic acid, hydroquinone and isophthalic acid were investigated using standard (ASTM) lap-shear experiments conducted at room temperature (25 °C) and at elevated temperatures, i.e. 150, 200 and 250 °C. Our LCTs were used on Al2024, MS and Ti6Al4V substrates and we found lap-shear values of 11, 12 and 16 MPa for Al, MS and Ti, respectively. For the Al2024 substrate the lap-shear strength remained constant up to 150 °C and dropped to 4 MPa at 200 °C. A post-mortem inspection of the fracture surface, using high-resolution scanning electron microscopy (HRSEM), showed that there was no adhesive failure at the metal substrate at room temperature or at elevated temperatures. The lap-shear values of our polymers are significantly higher as reported for other commercial available LCPs such as Vectra™.