Robustly Curing Epoxy Novolacs by Linear and Cyclic Siloxane-Functionalized Eugenol: Impact of Topologies of Cross-Linkers on Properties of Thermosets

Abstract

Eugenol-based epoxy resins have been extensively studied, but eugenol-derived curing agents have been less addressed. A curing agent is decisive to fundamental curing mechanisms and greatly affects intrinsic properties of finalized epoxy thermosets. Herein, we synthesize linear (2SiEU) and cyclic (4SiEU) siloxane-functionalized eugenol hardeners to cross-link multifunctional epoxy novolacs of bisphenol A (BNE200) and ocresol (CNE202). How the linear and cyclic siloxane topologies of 2SiEU and 4SiEU affect the thermal, mechanical, fracture mechanical, and thermomechanical properties, water absorption, and wettability of resultant thermosets is highlighted. 4SiEU endows 4SiEU/BNE200 and 4SiEU/CNE202 thermosets with an elevated glass transition temperature from 52.9 and 53.9 °C to 97.3 and 89.4 °C (DSC), an increased residue (750 °C/N 2 ) from 18.9 and 20.3% to 32.6 and 43.5%, an enhanced tensile strength from 48.1 and 46.8 MPa to 61.8 and 63.9 MPa, and an impact strength from 1.56 and 1.57 kJ•m −2 to 2.70 and 2.22 kJ•m −2 , whereas the critical stress intensity factor (K IC ) decreases from 1.56 and 1.67 MPa•m 1/2 to 1.26 and 1.29 MPa•m 1/2 , respectively, compared to 2SiEU/BNE200 and 2SiEU/CNE202. All of the thermosets display the high tensile elongation at break, flexural strength, and flexural modulus above 7.3%, 72 MPa, and 2.1 GPa, respectively, with low water absorption (<∼1.8%). In general, the 4SiEU-incorporated cyclic siloxane moieties endow the superior overall performances to the obtained thermosets compared to 2SiEU because they function as additional cross-linkers and increase the content of thermally stable Si−O bonds, and they cause an additional secondary relaxation at a subroom temperature range. Nevertheless, 2SiEU delivers better fracture mechanical properties, which are likely more sensitive to the increased plastic deformation of the resin matrix. Altogether, 2SiEU and 4SiEU are capable of effectively cross-linking the standard epoxy novolacs exhibiting multiple properties of interest, expressing a good promise as robust hardeners for biobased epoxy thermosets.

Siloxane-Functionalized Eugenol-Based Novolacs: from Facile Synthesis to Their Cured Bisphenol A Epoxy Thermosets with Superb Toughness and Thermal Stability

“…Impact and fracture mechanical properties of 2SiEUPF- X /E54 thermosets. (a) V-notched Charpy impact strength; (b) critical stress intensity factor ( K IC ) and critical strain energy release rate ( G IC ); (c) comparison of K IC values with some literature reports of BPA epoxy thermosets including MDI/DGEBA, MTHPA/DGEBA, DDM/DGEBA, and 33DDS/DGEBA and biobased thermosets such as RE/HDMA, IPDA/DGEDP-Me, IPDA/NC-514, CTBN/MAESO, PFA/DGEBA, RE/DA-LIM, RE/DA-AE, 33DDS/DEUEP, and 2SiEU/CNE202 . The meanings for the abbreviations of the previously reported systems are summarized in Figure S4.…”

“…Siloxane-functionalized eugenol-derived bisphenol (2SiEU) was synthesized according to our previous work . As shown in Scheme S1, in a typical experiment, 2SiEU and paraformaldehyde (F) were dissolved in acetic acid at different molar ratios of F to 2SiEU ( X = 0.5, 0.6, 0.7, or 0.8), and a concentrated hydrochloric acid catalyst (10 wt %) was added.…”

“…In our previous work, a linear (2SiEU) and a cyclic (4SiEU) siloxane-functionalized eugenol-based curatives were synthesized and used to cross-link commodity multifunctional epoxy novolacs with considerable success. In particular, the cured epoxies exhibited superior fracture toughness and acceptable glass transition temperatures.…”

“…Noticeably, the aforementioned 2SiEU is the hydrosilylation product of 1,1,3,3-tetramethyldisiloxane and eugenol. 2SiEU can be readily synthesized from inexpensive raw materials, and the yield and selectivity are almost quantitative without separation needed . Therefore, making better use of 2SiEU to synthesize curatives for BPA epoxies will be attractive.…”

Thymol-Derived Trifunctional Epoxy Resin with Outstanding Thermomechanical Properties and Low Dielectric Constant: From High-Throughput Synthesis to High-Performance Thermoset

“…The molecular structures of some phenolic monomers frequently used in the synthesis of biobased epoxies are enumerated in Scheme S1. In addition, we have reported some case studies on eugenol-based epoxy thermosets in recent years. − Thymol has also attracted our attention as a sustainable feedstock for biobased epoxy thermosets. Thymol is a natural phenolic compound derived from plants such as thyme and oregano essential oils, and exhibits bacterial properties, antioxidant activity, and low toxicity, which is widely used in medicine, food, cosmetics, and other fields. − Different from eugenol and other methoxylated biobased phenols, thymol, without methoxyl groups on the aromatic rings, will enhance certain performances of derived polymer materials as such increased oxidization stability to result in a lighter color, improved water resistance, and enhanced electrical insulation properties.…”