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Zinc oxide tetrapod whiskers (T-ZnO) with uniform morphology were processed by thermal evaporation and condensation technique. This was used as filler in epoxy matrix resins to formulate room temperature curable, thermally conducting and high-strength composite adhesive systems. Adhesive compositions with varying extend of T-ZnOs were processed and cured using an ambient temperature curable amine hardener. Adhesive formulations which contain the same extent of conventional irregularly shaped zinc oxide particles were also processed for comparison. Various formulations were evaluated for lap shear strength (LSS), thermal conductivity and morphological examinations. Epoxy-TZnO-based adhesive formulations exhibited higher lap shear strength and thermal conductivity compared to their conventional zinc oxide-based counterparts. Thus, an optimized epoxy-TZnO formulation exhibited 42% improvement in LSS and 77% in thermal conductivity than their zinc oxide counterparts. Fracture morphology revealed a rough texture and uniform distribution of T-ZnO in the epoxy resin system.

Isocyanate terminated poly (ether ether ketone)–polymers (PEEK-NCO) were synthesized from phenolic hydroxyl functionalized poly ether ether ketone (PEEKTOH) and tolylene diisocyanate (TDI) and characterized. The PEEK-NCO polymers were then reacted with diglycidyl ether of bisphenol A (DGEBA) to form copolymers containing varying proportions of thermoplastic PEEK segments linked to DGEBA through urethane bridges. Room temperature curable, tough adhesive formulations were processed using these copolymers along with triethylenetetramine (TETA) as the curing agent. Complex viscosity, gelation time, storage modulus and loss modulus of the adhesive formulations were evaluated under isothermal conditions. Adhesive strength of the formulations was evaluated on aluminium substrates at 25 °C, 100 °C and −196 °C and the results were correlated to the composition. Cured adhesive samples were cryogenically fractured and the fracture morphologies were investigated using scanning electron microscopy. Dynamic mechanical thermal analysis was performed to understand the visco-elastic properties of the cured adhesive formulations. The PEEK- modified epoxy adhesive systems exhibited enhanced glass transition temperatures in the range of 88–90 °C vis a vis the unmodified epoxy (80 °C) and offered a broad service temperature capability from −196 °C to 100 °C.

Epoxy resins are inherently brittle due to their high crosslink density. Hybridization of epoxies with a thermoplastic phase is an effective solution for reducing the brittleness and improving the toughness without impairing the thermal and mechanical properties. The present study focuses on the development of a novel room temperature curable epoxy resin by blending with different proportions of hydroxyl terminated poly (etheretherketone)s (PEEKTOH) as thermoplastic toughening agent. The systems were cured using triethylenetetramine (TETA). Isothermal rheological studies showed that, incorporation of PEEKTOH phases reduced the gelation time of the blend from 175 min to 30 min. The glass transition temperature of the blends were improved marginally (from 74 to 80 °C) as evidenced by dynamic mechanical thermal analysis studies. The toughened and the unmodified systems were examined for their adhesive properties on aluminium substrates. It was noted that, about 19% improvement in lap shear strength (LSS) was achieved for the toughened adhesive. Higher proportions of PEEKTOH in the epoxy matrix led to increased viscosity which consequently reduced the wetting characteristics, as proven by contact angle measurements. The fracture toughness of the cured blends improved up to 86%. Fracture morphology evaluation by scanning electron microscopy (SEM) showed a brittle fracture for the unmodified epoxy and a phase separated morphology for the toughened blends. Transparency was reduced in the toughened formulations due to phase separation of PEEKTOH.