Polybenzoxazine Network Research Articles

Benzoxazine/amine-based polymer networks featuring stress-relaxation and reprocessability

Amines as additives in benzoxazines are known to beneficially affect the polymerization temperature and furthermore to allow for partially reversible reaction steps yielding however a non-dynamic polybenzoxazine network. This contribution proves that the polymerization behavior of a two-component polymer of the polyetheramine Jeffamine® ED-600 and a bisphenol-A-based benzoxazine features stress-relaxation and reprocessability usually known from vitrimers. With the aim to gain a deeper understanding of the material properties of this system and the corresponding polymer structure, the reaction mechanism of a monofunctional benzoxazine and monoamine model system was studied revealing at first primary, and then secondary amine induced opening of oxazine rings, leading at first to linear polymer chains and then to covalently crosslinked networks. Both consist of repeated phenolic benzoxazine/amine motifs with permanently incorporated polyetheramine chains that do not impact the mechanical properties, compared to pure polybenzoxazine. Thermal, spectroscopic, and extraction analyses show that the addition of Jeffamine® reduces the polymerization temperature and introduces material properties such as reprocessability at the same time. Stress-relaxation measurements support the assumption that the reprocessability point to vitrimer-like molecular processes. The material shows rapid stress-relaxation of up to 11 s, a corresponding bond-exchange activation energy of 146 kJ/mol, and a topology freezing temperature of 97°C.

Greening Biobased Polybenzoxazine Network: Three Benefits in One Go

Eco-friendly initiators and environmentally safe flame-retardant (FR) adhesives are highly desired to empower sustainability in polymers. However, incorporating all of the properties simultaneously in a single step is challenging to achieve high-performance biomass sourced polymers. Herein, biobased benzoxazine monomers sourced from agro-waste phenol (cardanol, C) were synthesized using a facile solventless catalyst-free approach. Owing to their requirement of high ROP temperature (∼ 250 °C), prolonged polymerization cycle, issues of high flammability, and poor adhesive strength of the resultant polybenzoxazines limit their applications. To alleviate these issues, blends of naturally occurring phytic acid (PA) with the monomers resulted in fast polymerization kinetics (∼90% monomer conversion in 1 h) with the formation of hybrid polybenzoxazine networks at a mild temperature (∼100 °C). PA enacted as a green initiator and reactive halogen-free biobased FR and revealed favorable interactions across the polymer framework. Benefitting from the design, physical, ionic, and covalent cross-linking network of polymers with PA accounted for the extra-strong adhesive strength (71.1 kg/cm2, nearly 3.7 times higher than that of the pristine polybenzoxazine). Upon burning, the hybrid polybenzoxazines exhibited extremely low flammability, superior smoke suppression and formed intumescent char as supported by the limiting oxygen index, vertical burning, and cone calorimetry measurements. The present study indicated that the benign by design approach is a way forward to green thermosets offering excellent combination properties and potential applications in the future.

Bio‐based benzoxazine monomers and polymers based on difuran diamine

AbstractBenzoxazine monomers with high renewable content made from difuran diamine (DFDA) are presented. The benzoxazine monomers were synthesized by reacting DFDA with several bio‐based phenolic compounds and formaldehyde. These systems were purified by precipitation, and depending on composition, the resulting solid powders melt at temperatures ranging between 50 and 150°C to form low viscosity liquids that can be used to impregnate fiber reinforcements. Onset cure temperature varies depending on composition in the range of 150–240°C. The resulting polybenzoxazines have Tg's ranging from 240 to 300°C. It was found that the phenolic structure selected affects curing mechanisms of the DFDA‐based benzoxazines, Benzoxazines that have available ortho or para positions on the phenolic structures result in cure without mass loss. It was also found that adding furan rings into the backbone of the polybenzoxazine network provided the added benefit of char yield as high as 60%, as measured by thermalgravimetric analysis temperature ramps in an inert environment to 800°C. Compared with benzene‐ring‐based benzoxazine counterparts, furan‐based benzoxazines showed better thermal stability. In addition to promising processing and thermal characteristics useful for applications requiring high temperature and fire resistance, these materials contain high renewable content.

Ambient-Dried, Ultra-high Strength, Low Thermal Conductivity, High Char Residual Rate F-type Polybenzoxazine Aerogel.

The effect of the curing temperature (Tc) on the properties of PBO aerogel was investigated in this paper. The compressive strength of PBO aerogel prepared was much higher than that of PBO aerogel of the same density in other kinds of literature. With the robust F-type polybenzoxazine (PBO) aerogels with ultra-high Young’s modulus (733.7 MPa at 0.48 g/cm3 and 1070 MPa at 0.57 g/cm3), excellent properties were obtained through a facile and scalable room-temperature HCl-catalyzed sol–gel method, followed by the ambient pressure drying technique. It is found that Tc plays a vital role in the polymerization process and the evolution of the microstructure of the 3D porous PBO network, where the necks between the nanoparticles become thick and strong when Tc is up to 150 °C, resulting in a pearl necklace-to-worm transformation in the micro-structure and significant growth in mechanical properties, but if Tc is higher than 180 °C, the pore volume and specific surface area will decrease sharply. Moreover, all synthetic PBO aerogels here possessed inherent flame retardancy and a high residual char rate in the volume density (0.32–0.57 g/cm3). These properties make the F-type PBO aerogels a candidate material in aerospace applications or other fields.

Renewable benzoxazine-based thermosets from cashew nut: Investigating the self-healing, shape memory, recyclability and antibacterial activity

The preparation of sustainable bio-based polybenzoxazines with multifuctional applications is highly desirable but still challenging. Here, we propose a facile strategy to address this dilemma by introducing dioxaborolane bonds into the benzoxazine network. Renewable dynamic cardanol-based polybenzoxazine networks were prepared using the renewable waste material, cardanol (a byproduct of cashew nut processing), as the starting material and fabricated by incorporating dioxaborolane bonds. The dioxaborolane-containing cardanol-based benzoxazine thermosets (PCBOz-DBs) showed excellent reprocessability, self-healing property, shape memory behavior, and antibacterial activity. PCBOz-DBs have a controllable mechanical strength in a wide range between stretchability (elongation at break: over 587%) and stiffness (tensile strength: 5.88 MPa), as well as an excellent shape memory performance, by tailoring dioxaborolane bond content without catalyst. The polymer has a strong tissue-adhesive property and can firmly adhere to a variety of substrates, such as metal, plastic, and glass. PCBOz-DBs showed inhibition rates of 95.61% and 99.98% against Escherichia coli and Staphylococcus aureus, respectively. Moreover, the distinct network makes PCBOz-DB recyclable over multiple times without compromising its property, showing significant and self-healing properties under relatively low temperature (100 °C) and short healing time (5 min). The prepared PCBOz-DB, which is reusable, recyclable, and renewable, can achieve the goal of cleaner production by using bio-renewable biomass and reducing petrochemical resources, production, and energy emissions.

Exploiting the reversible covalent bonding of boronic acids for self-healing/recycling of main-chain polybenzoxazines

In this work, a new strategy for the synthesis of self-healable/recyclable polybenzoxazine networks under mild conditions by exploiting dynamic B–O bond exchanges is presented.

Expanding the library of nitrogen enriched polybenzoxazine thermosets prepared from side-chain type benzoxazines functionalized with polyethylenimine

We present a simple methodology to develop nitrogen enriched polybenzoxazine thermosets by integrating polybenzoxazine network with polyethylenimine (PEI). Thermosets have been developed by polymerizing side-chain type resins, which have been prepared by introducing benzoxazine moieties on the amine terminated branches of nitrogen enriched PEI, using cardanol and eugenol as bio-based phenols. The aim of this study is to investigate the potential of benoxazines synthesized using different bio-based phenols and PEI, on the performance of nitrogen enriched thermosets, as well as to contribute in the array of side-chain type benzoxazine resins. The present work is a next leg of our previously reported article on the synthesis of side-chain type benzoxazine resins using guiacol and PEI (G-pei), where the reaction parameters such as reaction medium, time, and stoichiometry were optimized. It is to be noted that, due to relatively better performance of G-pei resins prepared at 8:1:16 and 10:1:20 (phenol: amine: paraformaldehyde) molar ratio, the same stoichiometry has been specifically adopted for the preparation of cardanol and eugenol-based analogous resins in the present work. The functionalization of PEI with benzoxazine as side-chains, has been confirmed using FTIR, 1H, 13C NMR spectroscopy techniques. Polar amine groups as well as alkyl chains of phenols, are expected to alter the polymerization behaviour of oxazine moieties and the same has been investigated using DSC studies, where the curing profiles display exotherms in the temperature range of 208–222 °C. Further, rheological studies suggest solvent-less processing for cardanol based resins. Polybenzoxazine network bonded with polyethyleneimine has been confirmed from FT-IR spectra of cured thermosets, and the nitrogen content has been evidenced from elemental analysis. Furthermore, the thermal stability of the crosslinked materials has been investigated using thermogravimetric analysis (TGA), while the glass transition temperature for the cured specimens has been obtained by dynamic mechanical analysis (DMA). Moreover, polarity associated with the polybenzoxazine network as well as nitrogen enriched amine, is expected to affect the surface property of the end materials, which has been investigated by performing contact angle measurements on the surface of cured thermosets. Lap shear adhesion experiments have been additionally performed using ASTM D1002 standard, and the results indicates relatively good wettability of cardanol based resins.

Polybenzoxazines: a sustainable platform for the design of fast responsive and catalyst-free vitrimers based on trans-esterification exchanges

This work explores a new strategy, aiming for the synthesis of catalyst-free vitrimers by taking advantage of the abundant number of tertiary amines covalently bound into a polybenzoxazine network.

Sulfonic acid-containing benzoxazine surfactant and its waterborne benzoxaizne resins

A sulfonic acid-containing benzoxazine surfactant, sodium 3-dodecyl-3,4-dihydro- 2H-1,3-benzoxazine-6-sulfonate (SBZ), was synthesized via Mannich reaction using sodium 4-hydroxybenzenesulfonate and n-dodecylamine as raw materials. The chemical structure of SBZ was characterized by 1H NMR and FT-IR. Using SBZ as the surfactant, a series of waterborne benzoxazine resin (WBR) emulsions were prepared. The effects of SBZ concentration and oil–water ratio on the behavior of WBR emulsions were studied. Furthermore, polybenzoxazine film was prepared via curing a WBR emulsion with 50 vol% oil phase. The results showed that SBZ had a strong emulsifying ability. Only using low SBZ concentration of 2.0%, the high solid contents of 50–80 vol% and stable WBR emulsions were obtained. After polymerizing the WBR emulsion, a transparent and flexible polybenzoxazine film was fabricated. The SBZ surfactant was incorporated into the cross-linked polybenzoxazine network after polymerization. The film had a moderate mechanical property and a high glass transition temperature of 288 °C. This work provides a new strategy for the development of high heat-resistant and environmentally friendly waterborne benzoxazine coatings.

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

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.

Hybrid polymers based on bio-based benzoxazines with inorganic siloxane linkage to confer impressive thermal performance

Thermal performance of bio-based materials continues to be a vital concern in its path to commercialization and can be alleviated by performing hybridization of organic polymer with inorganic functionalities. The objective of the present work is to hybridize bio-based benzoxazine monomers with inorganic siloxane (Si-O-Si) linkage with an aim to develop hybrid thermosets possessing an impressive thermal performance. Hybrid bio-based benzoxazine monomers have been synthesized via Mannich like condensation of 1,3-bis(3-aminopropyl)tetramethyldisiloxane (APTMDS) with phenols of natural origin (cardanol, eugenol, guaiacol and vanillin) and paraformaldehyde using ethanol as environmentally preferable reaction medium. The structural characterization of the monomers has been performed using Fourier transform infrared (FT-IR) and nuclear magnetic resonance (1H, 13C NMR) spectroscopy. The polymerization behavior of the monomer has been evidenced using differential scanning calorimetry (DSC). In addition, rheological measurements were performed to ensure solvent-less processing of the benzoxazine resins. The contact angle measurements has been performed and surface free energy values have been calculated to study the effect of siloxane linkages on the hydrophobicity of hybrid polybenzoxazine. The influence of siloxane linkages on the thermal performance of bio-based polybenzoxazine network has been investigated by thermogravimetric analysis (TGA) which reveals Tmax to be in the range of 490–570 °C and 49–70% char yield for the thermosets. Furthermore, char yield obtained has been utilized to calculate Limiting Oxygen Index (LOI) value, which is an indicative of flame retardancy and was obtained in the range of 37–45 for thermosets. DSC investigation has been performed in order to study the glass transition temperature of the synthesized materials. Moreover, hybrid resins has been explored for adhesive application using standard procedures.

Phloretic acid: a smart choice to develop low-temperature polymerizable bio-based benzoxazine thermosets

The present work demonstrates the synthesis of new bio-based benzoxazine monomers via Mannich-like condensation of naturally occurring raw materials, for example, phloretic acid (PA) and furfurylamine (fa)/stearylamine (sa). The structure of the benzoxazine monomers has been established using proton nuclear magnetic resonance, carbon nuclear magnetic resonance and Fourier transform infrared spectroscopies. The monomers undergoes thermally activated ring opening polymerization to form polybenzoxazine networks, as revealed by non-isothermal differential scanning calorimetry, and the curing temperature for both was observed to be less than 473 K. Curing parameters of the developed monomers have also been compared with the reported bio-based monomers. The rheological behaviour of PA-fa monomer shows that the monomer has narrow processing window with liquefaction temperature at 426 K and gelation temperature at 437 K. Thermal degradation behaviour of polybenzoxazines was studied using thermogravimetric analysis which reveals that polybenzoxazine based on furfurylamine shows relatively high thermal stability and char yield which is credited to the additional cross-linking sites provided by the furan ring.

Laser-triggered shape memory based on thermoplastic and thermoset matrices with silver nanoparticles

Stimuli responsive shape memory polymers (SMPs) are challenging theme as the materials not only shows the shape changing under the traditional thermal treatment but also under other external stimuli. The present work proposes an in-situ ring opening polymerization (ROP) to combine the thermoset hard segment, i.e. polybenzoxazine, and thermoplastic soft segment, i.e. polycaprolactone and silver nanoparticles (AgNPs) as the laser-trigger. The melamine-based benzoxazine (mBz) plays its important role in allowing the ROP to create the thermoset polybenzoxazine network (PmBz) as well as providing many hydroxyl groups to further conjugation with thermoplastic polycaprolactone matrices (PCL). The AgNPs are the good alternative to select based on their function to be expressed by the very low energy (violet laser at 408 nm, 780 mW) and the surface modifiable with disulfide polycaprolactone (SS-PCL) hybridization for the highly miscible and readily dispersible in PCL matrices. The present work, for the first time, proposes light-triggered SMPs via simple molecular design and synthesis based on an in-situ ROP to combine PmBz thermoset networks and PCL thermoplastic matrices with the AgNPs-PCL to provide the light-thermal responsive function.

A mechanistic approach on the curing kinetics of benzoxazine-filled oxygen plasma treated graphene nanosheets

The present study aims to improve the understanding of the role of plasma treated graphene nanosheets on the curing characteristics of benzoxazine nanocomposites. As-synthesized benzoxazine monomers were well characterized by Nuclear Magnetic Resonance (NMR) as well as Fourier-transform infrared (FT-IR) spectroscopy. Also, the plasma treatment was utilized to alter the surface chemistry of graphene nanosheets (GNSs). Differential Scanning Calorimetry (DSC) revealed that the incorporation of GNSs into benzoxazine resin drastically reduces the curing peak temperature (Tc) and hence the enthalpy of curing reaction (∆Hc) to about 43 °C and 91 J g−1, respectively. It was found that GNSs act differently below and above 1 wt% due to their percolation threshold. The obtained activation energies (Ea) by Kissinger and Ozawa model were lower than those calculated by other researchers yet, not significantly different with each other. Then the iso-conversional model of Freidman showed that Ea of the nanocomposite varies during benzoxazine polymerization and these changes directly attributed to the evolutional rearrangement of polybenzoxazine network in the presence of GNSs and were differentiated by three regions through which the mechanistic interpretations of various phenomena can be highlighted.

Structural effects of highly π-conjugated mesogenic Schiff-base moiety on the cationic polymerization of benzoxazine and formation of ordered morphologies

A Schiff-base moiety was introduced into the chemical structure of benzoxazine through the Mannich reaction. With alternative imine groups (CN) and benzene rings in the chain segment, these highly π-conjugated systems favored the formation of strong intermolecular hydrogen bonds to the hydroxyl groups generated during polymerization, leading to a thermally initiated cationic polymerization mechanism. Compared with the bisphenol-A-based benzoxazine, the benzoxazine with the Schiff-base moiety needs higher energy to facilitate the realization of polymerization and to increase polymerization degree. In addition to the influences on the polymerization, the highly π-conjugated Schiff-base acting as a mesogenic moiety is found to form liquid crystal (LC) phases, like the nematic phase and smectic phase. Interestingly, some LC-rich domains can be stabilized by the polybenzoxazine network, which will contribute to form nitrogen-rich ordered porous carbon at the carbonization stage. For N2 and CO2 adsorption–desorption process, the carbon exhibits an enhanced CO2 adsorption performance, suggesting their capture and separation capacity for CO2.

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

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 c...

An investigation of the relationship between the performance of polybenzoxazine and backbone structure of hyperbranched epoxy modifiers

AbstractNovel hyperbranched polyether epoxies (HBPEEs) with various backbone structures were synthesized using a one‐pot A2 + B3 approach. These highly effective HBPEEs were incorporated into polybenzoxazine (PBOZ) network at various concentrations without cure‐induced phase separation. Effects of backbone structure and loading amount on the curing behavior and mechanical and thermal properties of PBOZ/HBPEE hybrid were investigated. The general trend shows that the impact strength, flexural strength and storage modulus all first increase and then decrease with filler content. For the backbone structure study, HBPEE‐3, which has the highest proportion of benzene rings, can simultaneously improve impact strength, flexural strength and storage modulus. In contrast, HBPEE‐1 and HBPEE‐2, which have fewer benzene rings and smaller intramolecular cavities, perform worse in terms of mechanical and thermal properties. Both backbone structure and loading of HBPEEs play important roles in determining the crosslink density and structures of non‐phase‐separated network. In addition, the simultaneous improvement can be explained by enhanced crosslinking density, structure of modifier, reduced hydrogen bond network and higher fractional free volume according to dynamic mechanical and thermomechanical analyses. © 2017 Society of Chemical Industry

Pegylated Polybenzoxazine Networks with Increased Thermal Stability from Miscible Blends of Tosylated Poly(ethylene glycol) and a Benzoxazine Monomer

The solubility, polymerization, and the macromolecular structure of cured blends of BPA-based benzoxazine (BA-a) in end-group tosylated poly(ethylene glycol) (mPEGOTs) are studied and compared to analogues from hydroxyl-terminated poly(ethylene glycol). BA-a can be homogeneously dispersed in both polymers where a wide loading range of ∼100–40 wt % BA-a is possible. The cure temperature for blended BA-a/mPEGOTs was up to 50 °C less than that of pure BA-a or mPEGOH blends. It is proposed that nucleophilic attack of BA-a on the end-group of mPEGOTs produces free tosylate and cationic BA-a-based initiators that act as cure catalysts—where the former was detected by TGA-FTIR and TGA-MS. H-NMR and FTIR revealed the polybenzoxazine (PBA-a) synthesized in mPEGOTs consists of a phenolic rich molecular structure with H-bonded hydroxyl residues and grafted PEG-chains. SEM confirmed that this material, P(BA-a)-graft-mPEGOTs, has a homogeneous microstructure. The glass transition and the thermal stability of the P(BA-...

Synthesis of high performance polybenzoxazine networks from bio-based furfurylamine: Furan vs benzene ring

A high performance polybenzoxazine derived from bio-based furfurylamine was synthesized in an efficient and straightforward way. For comparison, its petroleum-based analogue was also prepared from 4, 4-diaminodiphenylmethane (DDM). The chemical structures of the precursors were confirmed by Nuclear Magnetic Resonance Spectroscopy (1H-NMR and 13C-NMR) and Fourier Transform Infrared Spectroscopy (FT-IR). After the curing reaction was investigated by Differential Scanning Calorimetry (DSC) and FT-IR in detail, the thermal and mechanical properties of the cured resins were studied by Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). Results demonstrated that the polybenzoxazine based on furfurylamine (BZ-F) possessed higher glass transition temperature (270 °C) and better thermal stability associated with higher charring ability when compared with the one (BZ-DDM) derived from DDM, which might be attributed to the stronger hydrogen bonding interaction in the BZ-F system and the post-curing reaction on furan ring above 280 °C. In this paper, the hydrogen bonding effect on the properties of polybenzoxazine has been especially studied, which might give us a guidance to design and synthesize the polybenzoxazine with high performance.

Combining Elemental Sulfur with Polybenzoxazines via Inverse Vulcanization

A novel strategy to obtain sulfur-rich polybenzoxazine copolymers by reacting allyl functional benzoxazine (BA-ala) and elemental sulfur was described. Simultaneous inverse vulcanization and ring-opening reactions of benzoxazine generated soluble copolymers in specific feed ratios. Parameters such as monomer structure and feed ratios on the polymerization were studied. The thermal stability of the copolymers was investigated and compared to that of polybenzoxazines derived from neat BA-ala by using thermogravimetric analysis and differential scanning calorimetry. The surface properties of the materials as examined by scanning electron microscopy confirmed that elemental sulfur and benzoxazine copolymers can be produced without a phase separation at the micrometer level. Moreover, a sponge-like insoluble macroporous polybenzoxazine networks was obtained at the 20 wt % feed ratio of sulfur.