Recent Progress of High Performance Thermosets Based on Norbornene Functional Benzoxazine Resins

A Novel Fused SiO2 and h-BN Modified Quartz Fiber/Benzoxazine Resin Ceramizable Composite with Excellent Flexural Strength and Ablation Resistance

Hypersonic vehicles encounter hostile service environments of thermal/mechanical/chemical coupling, so thermal protection materials are crucial and essential. Ceramizable composites have recently attracted intensive interest due to their ability to provide large-area thermal protection for hypersonic vehicles. In this work, a novel ceramizable composite of quartz fiber/benzoxazine resin modified with fused SiO2 and h-BN was fabricated using a prepreg compression molding technique. The effects of the fused SiO2 and h-BN contents on the thermal, mechanical, and ablative properties of the ceramizable composite were systematically investigated. The ceramizable composite with an optimized amount of fused SiO2 and h-BN exhibited superb thermal stability, with a peak degradation temperature and residue yield at 1400 °C of 533.2 °C and 71.5%, respectively. Moreover, the modified ceramizable composite exhibited excellent load-bearing capacity with a flexural strength of 402.2 MPa and superior ablation resistance with a linear ablation rate of 0.0147 mm/s at a heat flux of 4.2 MW/m2, which was significantly better than the pristine quartz fiber/benzoxazine resin composite. In addition, possible ablation mechanisms were revealed based on the microstructure analysis, phase transformation, chemical bonding states, and the degree of graphitization of the ceramized products. The readily oxidized pyrolytic carbon (PyC) and the SiO2 with a relatively low melting point were converted in situ into refractory carbide. Thus, a robust thermal protective barrier with SiC as the skeleton and borosilicate glass as the matrix protected the composite from severe thermochemical erosion and thermomechanical denudation.

Low-Temperature Terpolymerizable Benzoxazine Monomer Bearing Norbornene and Furan Groups: Synthesis, Characterization, Polymerization, and Properties of Its Polymer

There is an urgency to produce novel high-performance resins to support the rapid development of the aerospace field and the electronic industry. In the present work, we designed and consequently synthesized a benzoxazine monomer (oHPNI-fa) bearing both norbornene and furan groups through the flexible benzoxazine structural design capability. The molecular structure of oHPNI-fa was verified by the combination characterization of nuclear magnetic resonance spectrum, FT-IR technology, and high-resolution mass spectrum. The thermally activated terpolymerization was monitored by in situ FT-IR as well as differential scanning calorimetry (DSC). Moreover, the low-temperature-curing characteristics of oHPNI-fa have also been revealed and discussed in the current study. Furthermore, the curing kinetics of the oHPNI-fa were investigated by the Kissinger and Ozawa methods. The resulting highly cross-linked thermoset based on oHPNI-fa showed excellent thermal stability as well as flame retardancy (T_d10 of 425 °C, THR of 4.9 KJg^-1). The strategy for molecular design utilized in the current work gives a guide to the development of high-performance resins which can potentially be applied in the aerospace and electronics industries.

Double-Decker-Shaped Polyhedral Silsesquioxanes Reinforced Epoxy/Bismaleimide Hybrids Featuring High Thermal Stability

In this study, we synthesized bismaleimide into a functionalized double-decker silsesquioxane (DDSQ) cage. This was achieved by hydrosilylation of DDSQ with nadic anhydride (ND), reacting it with excess p-phenylenediamine to obtain DDSQ-ND-NH₂, and treating with maleic anhydride (MA), which finally created a DDSQ-BMI cage structure. We observed that the thermal decomposition temperature (T_d) and char yield were both increased upon increasing the thermal polymerization temperature, and that these two values were both significantly higher than pure BMI without the DDSQ cage structure since the inorganic DDSQ nanoparticle could strongly enhance the thermal stability based on the nano-reinforcement effect. Based on FTIR, TGA, and DMA analyses, it was found that blending epoxy resin with the DDSQ-BMI cage to form epoxy/DDSQ-BMI hybrids could also enhance the thermal and mechanical properties of epoxy resin due to the organic/inorganic network formation created by the ring-opening polymerization of the epoxy group and the addition polymerization of the BMI group due to the combination of the inorganic DDSQ cage structure and hydrogen bonding effect. The epoxy/DDSQ-BMI = 1/1 hybrid system displayed high T_g value (188 °C), T_d value (397 °C), and char yield (40.4 wt%), which was much higher than that of the typical DGEBA type epoxy resin with various organic curing agents.

Reaction-Induced Phase Separation and Morphology Evolution of Benzoxazine/Epoxy/Imidazole Ternary Blends

Introducing multiphase structures into benzoxazine (BOZ)/epoxy resins (ER) blends via reaction-induced phase separation has proved to be promising strategy for improving their toughness. However, due to the limited contrast between two phases, little information is known about the phase morphological evolutions, a fundamental but vital issue to rational design and preparation of blends with different phase morphologies in a controllable manner. Here we addressed this problem by amplifying the difference of polymerization activity (PA) between BOZ and ER by synthesizing a low reactive phenol-3,3-diethyl-4,4′-diaminodiphenyl methane based benzoxazine (MOEA-BOZ) monomer. Results indicated that the PA of ER was higher than that of BOZ. The use of less reactive MOEA-BOZs significantly enlarged their PA difference with ER, and thus increased the extent of phase separation and improved the phase contrast. Phase morphologies varied with the content of ER. As for the phase morphological evolution, a rapid phase separation could occur in the initial homogeneous blends with the polymerization of ER, and the phase morphology gradually evolved with the increase in ER conversion until the ER was used up. The polymerization of ER is not only the driving-force for the phase separation, but also the main factor influencing the phase morphologies.

Elucidating the role of acetylene in ortho-phthalimide functional benzoxazines: design, synthesis, and structure–property investigations

Novel ortho-phthalimide-benzoxazines containing acetylene have been designed and their corresponding thermosets exhibit excellent thermal stability although the expected benzoxazole cyclization at a much higher temperature did not take place.