Scalable Synthesis of Thymol-Based Bisphenol E Epoxy Monomer and Related Thermosets with Low Dielectric Constant and Density and Improved Resistance to Hygrothermal Aging

Epoxy resins coming from biobased bisphenols have received significant attention; however, thymol-derived bisphenol epoxies are still essentially unknown so far. Here, a biobased bisphenol E (DHPF) intermediate was obtained from thymol and acetaldehyde via a highly efficient approach, followed by O-glycidylation to afford a bisphenol E epoxy monomer (DHEP) with a viscosity of only 3.5 Pa·s at 60 °C identical to that of a bisphenol A epoxy (DGEBA). Using 4,4′-diaminodiphenyl sulfone (44DDS) and methyltetrahydrophthalic anhydride (MTHPA) as curatives, cured DHEP thermosets were prepared through cast molding, and their properties are systematically studied and compared with DGEBA thermosets. DHEP delivers a lower dielectric constant to the resulting thermosets, especially for DHEP/MTHPA (1.9, 10 MHz). The DHEP-based thermoset also exhibits decreased water absorption, density, and thermal diffusivity and improved resistance to hygrothermal aging as indicated by storage modulus and T_g retention, and good bulk mechanical properties. Moreover, when formulated into an epoxy adhesive for bonding stainless steel sheets, DHEP endows the glue joint with a rather high lap shear strength up to 12.7 MPa. Overall, DHPF can be readily prepared in large quantities, and DHEP is potentially 100% biobased with good processability; the finalized DHEP thermosets have advantages in many properties of high interest, thereby illustrating its good prospects for real applications.