Biobased phenols and amines are attractive feedstock in rapid emerging polybenzoxazine thermosets. Renewable benzoxazines are mainly monofunctional due to structure limitation of raw materials, especially amines. Here, we report fully naturally sourced bifunctional benzoxazines by a simple one-step solventless microwave synthesis using cardanol/guaiacol- and carbohydrate-based isomannide diamine (ima). Chirality of ima led to complicated 1H NMR spectra of monomers. Heteronuclear single-quantum coherence (HSQC) and two-dimensional correlated spectroscopy (COSY) NMR and single-crystal X-ray diffraction confirmed the structure and formation of monomers. Polymerization and thermal stability of monomers and polymers were found to be either better than or comparable to widely studied fully and partly petro-based polybenzoxazines. Incorporation of rigid core ima in fully biobased polybenzoxazines showed a glass-transition temperature comparable to their petro-based aromatic diamine counterparts. Exploration of adhesive behavior revealed that the designed polybenzoxazine framework showed 2-fold higher lap shear strength than bisphenol-A (BPA) aniline polybenzoxazine. The current work highlights the utility of higher functionality renewable amines as promising candidates to design future high-performance fully biobased BPA-free polybenzoxazines. Copyright © 2019 American Chemical Society.

Bimlesh Lochab

Chemistry

(SoNS) School of Natural Sciences

Amarnath N.

Shukla S.

<p>Shiv Nadar University is a comprehensive, multidisciplinary, research-focused, and student-centric University offering a full range of academic programs at the undergraduate, postgraduate and doctoral level. With state-of-the-art infrastructure, the University comprises of academic wings, sophisticated labs, international standard sports facilities, amphitheaters, auditorium, conference rooms and smart classrooms. The University&rsquo;s goal is to become internationally recognized for the quality of its research and creative endeavors and their applicability to improving quality of life, generating new insights and expanding the boundaries of human knowledge creativity. Committed to excellence in teaching, research and service, the University aims to serve the higher education needs of India and the world beyond.</p>

Shiv Nadar University

ACS Sustainable Chemistry and Engineering

Olefin bonds participate in co-reaction with the benzoxazine functionality of the monomer and are one of the strategies used to affect the crosslink density of a polybenzoxazine network. In general, the double bond incorporation in starting material is usually catalyzed by expensive, rare earth metals affecting the sustainability of the reaction. The natural abundance of feedstocks with inherent double bonds may be a powerful platform for the development of novel greener structures, with potential applications in polymers. Here, we report the design, synthesis, and characterization of a biobased non-halogen flame retardant, consisting of naturally occurring phenols, eugenol (E), and cardanol (C). The presence of a covalently linked phosphazene (P) core allowed the synthesis of hexa-functional flame retardant molecules, abbreviated as EP and CP. The chemical structures of the synthesized EP and CP were confirmed by Fourier transform infrared (FTIR), nuclear magnetic resonance (1H, 13C, 31P NMR), and single crystal XRD (only in the case of EP). Their polymerization with cardanol sourced tri-oxazine benzoxazine monomer, C-trisapm, was followed by FTIR, NMR, and DSC studies. The thermal stability and flame retardant properties of the hybrid phosphazene-benzoxazine copolymers was determined by thermogravimetry analysis (TGA), limiting oxygen index (LOI), vertical burning, and smoke density analyses. SEM images of the char residues of the polymers with or without the addition of reactive phosphazene molecules confirmed the intumescent flame retarding mechanism. Current work highlights the utility of sustainable origin non-halogen flame retardant (FR) molecules and their utility in polybenzoxazine chemistry. © Copyright © 2020 Appavoo, Amarnath and Lochab.

Fulltext

Frontiers in Chemistry

Cardanol and Eugenol Sourced Sustainable Non-halogen Flame Retardants for Enhanced Stability of Renewable Polybenzoxazines

Polybenzoxazines (PBzs) are emerging as a highly promising and superior class of thermoset polymers for a variety of applications. However, it remains a significant challenge to substantially lower the ring-opening polymerization (ROP) temperature with an ease in processability. On the other hand, biomacromolecule chitosan (CS) is explored extensively, but its practical applications have been precluded by poor thermal and mechanical properties. Here, we developed a fully biobased copolymer of vanillin benzoxazine (V-fa) monomer with CS, which is effective in providing mutual benefits, effective lowering in ROP temperature of benzoxazine (70 °C), and an enhanced thermal stability of CS (by 85 °C, and with char yield of â32\%). To understand this unusual lowering in ROP temperature, we investigated the structural interaction mechanism between solvated CS and V-fa using in situ NMR studies. The analysis of fully intercalated co-structure demonstrated that there is a strong preference for ROP over Schiff base reaction. It is anticipated that benzoxazine molecules move within the interplanar distance of CS as supported by powder X-ray diffraction studies. An increase in V-fa content in feed ratio led to a placement of V-fa units from random to a systematic and hierarchical arrangement within the CS framework followed by its subsequent polymerization. The synergistic interactions were further supported by Fourier transform infrared, differential scanning calorimetry, scanning electron micrsocopy, thermogravimetric analysis, and tensile studies. Current work represents preparation of CS benzoxazine copolymers using a low-cost, efficient, and sustainable approach to assist metal-free ROP reaction of Bz to afford low curable temperature processable films. A new strategy is devised for the utility of CS-PBzs copolymers, enabling their extension to innovative applications in cross-domains. © 2019 American Chemical Society.

Sustainable Framework of Chitosan-Benzoxazine with Mutual Benefits: Low Curing Temperature and Improved Thermal and Mechanical Properties

Polybenzoxazines are evolving as a new class of phenolic resins with a tremendous amount of applications. The ease of structural designing, synthetic procedure, and polymerization reactions are attractive to affect their properties ensuing as an effective substitute for many existing polymers. The nonrenewable origin, toxicity, and increasing cost of petro-based raw materials are alarming issues. Therefore, the emergence of sustainable benzoxazines, especially based on naturally abundant, agro-waste origin cardanol is encouraging as it allows both solventless synthesis of monomers and processing to form highly thermally stable thermoset resins. The inherent functionalities in cardanol benzoxazine provide a platform for further structural modifications to advance their applications as adhesives, composites, and energy storage devices. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Macromolecular Chemistry and Physics

Cardanol Benzoxazines: A Versatile Monomer with Advancing Applications

Isomerization of double bonds from an allylic to propenyl position is generally mediated by expensive metal catalysts, demanding an additional synthetic step, thereby reducing sustainability of the reaction. However, such functionalities are inherent in naturally occurring compounds, enabling a versatile protocol for their industrial utility. Herein, we report the synthesis of benzoxazine monomers based on biosourced isomeric phenols, eugenol (E) and isoeugenol (IE), and biobased amine, furfurylamine (fa) to form E-fa and IE-fa monomer, respectively. The structural variation in the phenols revealed a differential chemical reactivity, during both the synthesis of the monomer and the polymerization reaction, confirming a significant influence of isomerism. The monomers only differ in the position of the double bond in the para-substituted propylene unit forming nonconjugated vs conjugated alkylene chain with the benzene ring containing benzoxazine in E-fa and IE-fa, respectively. The structure of the monomers was confirmed by 1H NMR, 13C NMR, FTIR, XRD, and mass spectrometry. The high purity of monomer was further affirmed by HPLC and DSC to demonstrate the effect of isomerization on the polymerization behavior. The extended conjugation of the double bond in IE-fa with the proximal benzoxazine ring showed a higher reactivity toward ring-opening polymerization, polymer conversion, and cross-linking reactions as supported by FTIR, NMR, and DSC-based kinetic studies. Thermal stability, mechanical properties, and adhesive analysis by TGA, DMTA, and lap shear strength measurements further supported the effect of structural isomerism of monomers with a higher potential of PIE-fa over the PE-fa network. Current work illustrates an economic, one-step, microwave-assisted, and VOC's- and catalyst-free synthesis with a simultaneous solventless processing of synthesized monomers using renewable materials as feedstocks for high-performance polymers. © 2018 American Chemical Society.

Harvesting the Benefits of Inherent Reactive Functionalities in Fully Biosourced Isomeric Benzoxazines

We report on the preparation of hexa-functional cardanol (renewable phenolic compound) benzoxazine with a phosphazene core (CPN) for use as a greener eco-friendly halogen-free flame retardant reactive additive for the formation of sustainable polyphosphazene polybenzoxazine networks for flame resistant applications. The structure and purity of the monomer was confirmed by Fourier transform infrared (FTIR), nuclear magnetic resonance (1H-, 13C-, 31P NMR) and gel permeation chromatography studies. The CPN monomer showed good compatibility with benzoxazine monomer (CPN0) as suggested by the cocuring studies. The thermal properties of the copolymer can be directly tuned by altering the composition of the monomer blend. The occurrence of phosphazene-phosphazane thermal rearrangement is also suggested for the thermal behavior (thermogravimetry analysis) at higher loading of CPN in the monomer feed ratio. An improvement in mechanical properties of the copolymer with increase in glass transition temperature was confirmed by enhancement in cross-link density as compared to neat polybenzoxazine. The reactive nature and presence of phosphazene core improved both the smoke density rating, vertical burning rating and led to higher limiting oxygen index. The FTIR and scanning electron microscopy studies of residual char supported the formation of functionalities and morphologies favorable to support the flame resistance behavior of polymer by incorporation of reactive benzoxazine with phosphazene core. Finally, we demonstrate that incorporation of cardanol phosphazene network has good compatibility with the polybenzoxazine phenolic thermosets with improvement in flame retardancy. The higher cardanol (65.7\%) and phosphorus content (3.4\%) and reactive nature of synthesized compound is attractive as a sustainable additive with the scope of their utilization with other polymeric resins. © 2017 American Chemical Society.

Eco-Friendly Halogen-Free Flame Retardant Cardanol Polyphosphazene Polybenzoxazine Networks

Mono-benzoxazine monomers based polybenzoxazines suffer from a lower crosslink density and char yield that especially is further diluted by the presence of longer alkylene chain in cardanol (C). In order to improve the crosslink density, char yield and to understand the role of higher aromatic content vs functionality, a series of cardanol-based benzoxazine monomers were synthesised. The amine condensed with cardanol and paraformaldehyde were aniline (a), 4-triphenylmethylaniline (ta), and tetra-aminophenyl methane (tapm) via solventless methodology. The C-a and C-ta are mono-benzoxazines while C-tapm is a tetra-benzoxazine monomer. The monomers were structurally characterised using FT-IR, 1H and 13C NMR spectroscopy and mass spectrometry. The ring opening polymerisation temperature (ROP) of monomers and thermal stability of resins was determined using DSC and TGA while mechanical properties were determined using rheometry. The curing kinetic study using FT-IR and DSC showed C-ta has intermediate curing behaviour between the C-a and C-tapm. The Ea for curing reaction C-a, C-ta and C-tapm was found to be 143, 126 and 70 kJ/mol. The lowest Ea for curing reaction of C-tapm (tetra-benzoxazine) is due to its highest functionality but C-ta (mono-benzoxazine) has lower Ea than C-a which can be accounted to the presence of more reactive sites provided by the additional aromatic rings in the former. It was found that the incorporation of higher aromatic ring in benzoxazine monomers is another route in enhancing the crosslink density besides higher functionality to modulate their properties. © 2016 Elsevier Ltd