Abstract
Advances in stimuli-responsive materials have led to increasing popularity due to their ability to adapt intelligently and be capable of “remembering” their original shape after adopting temporary deformed shapes in various applications. At the same time, the environmental and sustainability challenges of end-of-life (EOL) disposal for these materials are particularly concerning. This review synthesizes current knowledge on how sustainable chemistry and functional material design can be bridged by integrating waste, as an effort to reach a closed-loop circular economy, into high-performance shape memory benzoxazine-based thermosets. Both agricultural and industrial waste streams, including lignin, vanillin, eugenol, diphenolic acid, and cardanol, were systematically discussed, exploiting each unique functional characteristic, such as phenol, aldehyde, allylic, carboxyl, and alkyl groups, to be utilized as valuable target sites for chemical modifications from phenolation, esterification, amination, imination, hydrothiolation and thiol functionalization, inverse vulcanization, and direct condensation to enable the shape memory effect with a controllable end-of-life (EOL) scenario. Thermoset materials initially designed with degradable linkages can be reshaped, healed, or degraded under specific triggers and conditions, retaining their properties for several reprocessing cycles until they reach their performance limits. In contrast, networks lacking dynamic functionality usually display long-term stability during service, but they require more advanced technologies for EOL management. In such cases, methods such as catalytic oxidation can be used to recover valuable fiber and resin fragments, which typically necessitates more complex processing procedures. The choice between these recycling methods should be based on the intended use, the service environment, and the desired EOL option. Implementing the waste-to-value concept with these advanced strategies undoubtedly offers environmental benefits, but it also entails “hidden costs,” such as increased processing complexity, catalyst requirements, or purification procedures, which must be weighed carefully to avoid outweighing the advantages. We conclude by outlining key areas of future advancement, with a particular emphasis on the need for thorough life-cycle and techno-economic evaluation of these waste-derived benzoxazine-based systems to transition them from laboratory proof-of-concept to operational, predictive, and industrial-scale circular systems.
Introduction
The global demand for smart or stimuli-responsive material, systems designed with the ability to adapt to environmental changes, is forecasted to increase at an annual rate of 8%, with an anticipated value of USD 133.1 billion by 2030.1–3 One such class of these smart materials, known as shape memory materials, has the capability of returning to their original shapes under certain stimuli. These can be deformed temporarily and left in such a state following the application of a suitable stimulus. This change, referred to as the shape memory effect (SME), is mainly triggered by external stimuli, most commonly heat.4–6 Their unique properties, which include easy deformability, adjustable glass transition temperature (Tg), lightweight, and high shape recovery rate, make them applicable in various fields, from aerospace engineering and biomedical devices.7–9 The SME also enables site-selective and sequential actuation behaviors,
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which can be expanded conceptually to applications driven by motion, as demonstrated in Figure 1 (right), illustrating this principle through a walking-type example that showcases the directional deformation and recovery cycles achievable in benzoxazine-derived SMPs.
Benzoxazine resins are typically synthesized via the reaction of phenol or its derivatives with aldehydes (e.g., formaldehyde, acid aldehyde, or pyromucic aldehyde) and amine groups. 13 During the polymerization process, the monomers offer several benefits such as straightforward processability, by-product-free curing, minimal shrinkage, molecular flexibility, exceptional thermal stability, enhanced chemical resistance, favorable mechanical properties, an excellent hydrophobic characteristic, and cost competitiveness.10,14–20 Thermosets produced from benzoxazine resins are selected for their advantages: their covalent network outperform the physical crosslinking of thermoplastics in mechanical strength and thermal stability, while their molecular flexibility allows for the creation of unconventional smart materials, including self-healing systems up to shape-memory applications.21–24 The combination of molecular flexibility in polybenzoxazine and the presence of free phenolic hydroxyl groups, which facilitate strong hydrogen bonding, makes them suitable for self-healing systems capable of repairing structural damage either independently or in response to stimuli.25,26 Additionally, the numerous benzene ring structures in polybenzoxazine create an inherently rigid and often brittle network, requiring the incorporation of flexible chain segments to achieve the deformability needed for shape memory performance. 27 This design flexibility is further supported by the availability of various commercially available phenols and amines, which significantly enhances the potential of benzoxazine-based systems.
Thermosetting resins, such as benzoxazine-based ones, have a great challenge associated with their environmental friendliness and sustainability with respect to end-of-life (EOL) disposal. 28 Disposal of such waste is mostly confined to landfill or incineration because they possess permanently crosslinked molecular structures, which makes recycling and reuse challenging.29–31 Therefore, there is an urgent need for more sustainable alternatives because global regulations on waste management are becoming more stringent. 32 The non-degradable nature of these materials has led to a growing awareness of the need for eco-friendly solutions, which include incorporating bio-based and recyclable components during synthesis to mitigate their environmental impact.
In response to these difficulties, recent studies have begun to investigate the substitution of traditional, petroleum-based constituents in the synthesis of benzoxazine resins with bio-based and/or waste-based compounds. For example, a renewable waste material called cardanol—produced as a by-product of cashew nut processing, one of the largest sources of industrial liquid waste—is used as a phenolic source to synthesize benzoxazine.33,34 In one such report from Krishnan et al., 35 a mono-functional bio-based benzoxazine synthesized using cardanol and Jeffamine D230 was copolymerized with a bis-functional benzoxazine prepared with guaiacol and furfurylamine to address the flexibility-rigidity trade-off. The optimized copolybenzoxazine formulations achieved balanced shape memory performance, with shape fixity ratios (Rf) exceeding 95% and shape recovery (Rr)exceeding 99% within an average of 79 s. Noting the 80/20 cardanol/guaiacol system is significant, it exhibits a much higher Tg of 139°C in comparison to 100°C seen in the 40/60 system. The combination of the mono-functional cardanol-based benzoxazine, which introduces long flexible aliphatic side chains that enhance faster mobility and recovery rates, and the bis-functional guaiacol-based benzoxazine, which offers increased crosslink density and aromaticity that provides improved shape fixity and thermal stability, is attributed to this phenomenon. This synergy at the molecular level makes it possible to tailor the actuation performance to meet the diverse thermal and mechanical needs of aerospace and automotive composites. 35 In a similar manner, a green, solvent-free synthetic route was developed by Sha et al. 36 for benzoxazine from vanillin as a phenolic source, which is primarily synthesized from lignosulfonates—a by-product of the sulfite pulping process in the paper industry. This approach effectively valorizes the large amounts of lignin generated as a by-product of current industrial paper and pulp production processes.37,38 The benzoxazine was initially synthesized via a Mannich reaction of vanillin, furfurylamine, and paraformaldehyde; and the resultant product (VFA) was subsequently used as a raw material to produce VFA-atpe by means of an aldimine condensation reaction between VFA and amine-terminated polyether (atpe). This reaction results in the formation of dynamic Schiff base bonds and incorporates flexible ether linkages into the polybenzoxazine network. The cured VFA-atpe sample exhibits strong potential as a shape memory polymer (SMP). Notably, its storage modulus (E′) significantly varies—by 100 to 1000 times—around the Tg, which is pivotal for SMEs. This material exhibits notable shape memory properties of 98% shape recovery ratio within 48 s accompanied by a relatively high Tg of 280°C. Such properties are possible due to the dynamic Schiff base bonds and the permanently crosslinked network derived from the benzoxazine structure. These remarkable properties, such as a high operating temperature, quick and efficient shape recovery, and inherent ability to be reprocessed, make these materials very promising candidates for advanced smart composite applications in demanding sectors like aerospace and automotive, particularly for functionalities like self-healing structures, adaptive components, or recyclable matrices. 36
Over the past few years, a rising number of research studies have investigated the incorporation of reactants and fillers derived from waste for the environmentally friendly production of benzoxazine-based materials. This review investigates waste sources throughout the value chain, covering forestry and wood processing as well as agriculture and agro-industry, and highlights their potential as renewable resources for producing sustainable benzoxazine-based shape memory composites. This review also aims to summarize how sustainable chemistry and functional material design can be bridged by integrating these waste streams into high-performance benzoxazine-based systems, in line with the principles of the Bio-Circular-Green (BCG) Economy.
General concept of shape memory materials
SMPs are a class of smart polymeric materials where they exhibit an intrinsic thermal response to stimuli, capable of “remembering” an original shape or adopting temporary deformed shapes.12,39,40 This unique phenomenon, referred to as SME shown in Figure 2, is brought about by the interaction of two fundamental components and is influenced by appropriate programming methods such as mechanical deformation at elevated temperatures.43,44 First, the net-point (hard segment), obtained via chemical or physical crosslinking or interpenetrating networks, provides entropic elasticity that stabilizes the network during thermomechanical processes and is responsible for shape recovery. Second, the switch (soft segment), responsible for the shape fixity, exhibits a thermal transition temperature (Ttrans), such as melting temperature (Tm) or Tg,45,46 and reversible sensitivity toward external stimuli, such as heat.
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When a polymer undergoes mechanical deformation at a temperature above its transition point with the help of an appropriate stimulus, internal stress is created within the network. At this stage, the switching domains become highly flexible, enabling the material to undergo significant deformation. Once cooled below the Ttrans, the deformed shape can be locked into a temporary shape. This happens because the switching domains lose their flexibility, causing energy to be stored in the crosslinked structure. When the material is later exposed to the same stimulus, the stored energy is released, allowing it to return to its original shape, driven by the elastic recovery properties of its structural net-point.44,47
Quantitative methods of shape memory performance
The shape memory effect is typically quantified using parameters such as the shape fixity ratio (Rf), shape recovery ratio (Rr), recovery time, and the number of recoverable cycles. Two widely used methods to determine these parameters are the bending technique and the thermomechanical cyclic tensile technique. The bending test is a simple and cost-effective way to evaluate, as shown in Figure 2. In this approach, a rectangular thin film sample is often bent to a specified angle of θ0, usually with the aid of a support fixture. The sample is first heated above its Tg for a specified duration. The deformed sample (θ1) is subsequently cooled to a temperature below Tg and kept at this point to hold its temporary shape. The sample is then reheated to the previously elevated temperature, at which point it recovers its original shape (θ2), allowing the recovery time to be recorded as the time required to reach maximum recovery. The process is repeated multiple times to determine cyclability, with testing stopping once cracking is noticed. The Rf and Rr are determined by the following equations:48–50
Another more accurate method to measure the shape memory effect is the thermomechanical cyclic tensile method, which uses equipment such as a dynamic mechanical analyzer (DMA), which is generally used in tensile configuration. The process of shape memory evaluation at the DMA incorporates four separate stages as loading, cooling, unloading, and recovery, and is further represented as shown in Figure 3. Here, the sample is stretched to the particular maximum strain (εm) after heating a rectangular thin film sample above its Tg. The stretched sample is then cooled below Tg to fix the temporary shape under either stress or strain control. Once the shape is fixed, the load is removed, allowing partial relaxation under stress-free conditions, which results in a residual strain (εu). The sample is next reheated to a temperature above Tg to recover its original shape, either in a controlled manner at constant strain (constrained recovery) or in a stress-free state (unconstrained recovery), after which a small residual strain (εp) may remain. These two different conditions of shape recovery result in different shape memory behavior. In constrained recovery, the stress formed during recovery is recorded as a temperature-time dependence with the maximum recovery stress can be determined. Such a stress response allows SMP to act as actuators. But in case no constraints are applied (unconstrained recovery) to recover the sample, the sample is free to move as a temperature-dependent function and a change in strain is measured with temperature and time. This difference is particularly important to biomedical applications where the shape memory materials might swell in relation to the surrounding tissues or other constrained spaces and therefore would not be able to recover freely as would originate tests in the lab. The procedure is repeated several times until the sample cracks to establish cyclability. Based on the stress-strain curve of the thermomechanical cyclic tensile technique, Rr and Rf can be determined using the following equations:52,53 Thermomechanical technique for assessing shape-memory properties in polymers. Adapted and redrawn with modifications from
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, with permission. Copyright 2008 Royal Society of Chemistry.
Stimuli-responsive triggering modes for SME activation
While SME can be triggered by various methods, as schematically summarized in Figure 4, the most common one is thermally induced.59,60 As an example, heat was applied to activate the SME of a renewable benzoxazine resin made from diphenolic acid (DPA), which is a condensation product of levulinic acid (a biorefinery building block of economically exploited biomass, including paper, wood, and wheat straw),
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above the Tg of the resin.
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The resin incorporates two different amine sources: stearylamine, which imparts rigidity to the molecule; and Jeffamine, which enhances the system’s mobility. The shape fixity of the resin increased to over 90% with 50 wt% or more stearylamine, while shape recovery reached 80% within 8 to 16 min, attributed to a higher E′ for better shape retention, balanced against lower molecular mobility and higher crosslinking density, which lengthened recovery times. The current system has demonstrated shape-memory-assisted self-healing, as illustrated in Figure 4(a), through its response to heat by a pre-cracked sample. In 5 min, the material regained its original shape entirely after being heated to Tg + 10°C, and the pre-existing crack was fully healed within additional an hour.
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This thermally induced SMP can also be activated by Joule heating,62,63 using electricity or light to indirectly generate heat and accelerate shape recovery. A recent study by Gholami et al.
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utilized Joule heating to establish programmable conditions for triggering shape recovery in laser-induced graphene (LIG) and polylactide (PLA) composites. The composites demonstrated the ability to respond to both electrical and infrared (IR) light stimuli, with Joule heating playing a key role in determining the required activation conditions for shape recovery. The heat in the LIG pattern is uniformly generated via electrical actuation, utilizing resistive Joule heating activated by the applied voltage shown in Figure 4(b). The high electrical conductivity and three-dimensional porous structure of LIG enable efficient and rapid internal heat generation when stimulated electrically, causing the underlying PLA matrix to warm above its Tg and recover its shape.
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Beyond Joule heating, other indirect methods for thermally triggering SMPs include magnetic actuation, photo actuation, and microwave actuation. Functional fillers can be added to SMPs to enable shape-memory activation without relying on conventional heating methods, using magnetism, electricity, or light instead. Ruenpanya et al.
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used magnetite (Fe3O4) as a superparamagnetic filler in a vanillin–furfurylamine (VFA) benzoxazine, which was alloyed with 40 wt% of polyurethane (PU) to create a triple shape memory effect. The composite containing 7 wt% magnetite displayed a broad tan delta range that facilitated effective triple-shape programming, as depicted in Figure 4(c): the initial three temporary shapes were created through thermal bending, and the recovery was initiated remotely via magnetic heating, during which Fe3O4 nanoparticles effectively absorbed alternating magnetic energy and converted it into heat.
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As reported by Leungpuangkaew et al.,
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magnetite (Fe3O4) nanoparticles not only enable magnetic activation for remote thermally induced SMPs but also facilitate shape recovery through light activation in bio-based VFA copolymerized with vegetable oil-based epoxy from castor oil (ECO) shown in Figure 4(d). The π-conjugation present in the benzene rings of VFA/ECO promotes the absorption of near-infrared (NIR) light due to π–π interactions in the polymer chains, which contributes to the photothermal effect. The interaction is further improved with the addition of the Fe3O4 nanoparticles, as they enhance light absorbance and NIR photothermal conversion for shape memory activation. The experimental data revealed high shape fixity ratios of 90 % to 95 % for VFA/ECO copolymers, with recovery times shortening from 78 s to 32 s as Fe3O4 nanoparticles content increased. These results are consistent with those of direct heating at Tg + 20°C, implying a similar shape memory effect through light activation. The copolymers were heated using an 808 nm NIR laser at 2.5 W/m2. The exposure of Fe3O4 nanoparticles to NIR light led to the generation of heat, which increased the temperature of the copolymer above its Tg, thus increasing the mobility of the polymer chains. Thereafter, the copolymers were bent toward a magnet owing to the magnetic force between the Fe3O4 nanoparticles and the external magnet, resulting in shape recovery.
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Materials can also be designed to exhibit shape memory properties in response to multiple stimuli, as demonstrated by a recent study by Bo et al.,
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which created a cardanol-based benzoxazine system that can be activated through a combination of microwave and water. The multi-responsive behavior was achieved by incorporating dynamic covalent bonds from dioxaborolane into the polybenzoxazine network. This was done by first synthesizing the benzoxazine monomers through a reaction between thiol-functionalized cardanol-derived polyol and amine, and then further modifying the resulting polymer with a bis(dioxaborolane) linker in a post-polymerization step. Actuation using an 800 W microwave generated heat of 98°C via dielectric heating shown in Figure 4(e), which was sufficient to soften the switching segments, particularly the alkyl chains and siloxane units, and initiate entropic shape recovery within 57 s. The slowdown of shape recovery (∼700 s) resulting from light actuation with infrared is attributed to localized photothermal heating on the surface, reaching a maximum temperature of up to 74°C due to light energy conversion into heat. In contrast, water-supported actuation, as shown in Figure 4(f), was achievable due to reversible plasticization of the vitrimeric network occurring when moisture interacted with dioxaborolane bonds. Water immersion caused temporary softening of the material, resulting in increased ductility and reduced breaking network interactions, allowing shapes to be modified under ambient conditions. After being dried, the material retained its crosslink density, fixing the new shape into a stable form that can last for a long time, and enabling complete reversible reprogramming through a solvent-free hydrosetting process over 10 cycles.
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These methods can be categorized as indirect thermal actuation, as they ultimately lead to thermal effects that enable shape memory, despite not directly relying on the application of heat. Various external stimuli used to trigger the shape-memory effect (SME), including (a) heat,
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(b) electrical,
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(c) magnetic,
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(d) light,
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(e) microwave, and (f) water.
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Adapted and redrawn with modifications from54–58, with permission. Copyright 2020 Wiley, and 2024 Royal Society of Chemistry.
Conversion routes from waste to resin feedstocks for targeted SME activation
The conversion of biowaste to valuable biochemicals and bioproducts is essential in promoting waste management, recycling, and the circular economy. 64 This transformation mitigates the adverse impacts of biowaste while facilitating the generation of renewable resources like bio resins and bio-based polymers. Recovery and incorporation of energy recycling together with resource recycling is an important feature toward closing the consumption and production cycle. This builds a pathway for creating renewable, waste-derived materials and contributes to a sustainable future. Biowaste typically comes from municipal waste, agricultural by-products, and food processing residues, which contain complex compounds such as cellulose, hemicellulose, lipids, proteins, and aromatic polymers. 65 Made up of numerous biomolecular components, biowaste is difficult to process and, in most cases, requires targeted chemical modification steps to convert it into a usable feedstock.
Lignin-derived benzoxazine feedstocks
Lignin and its derivatives, primarily sourced from the pulping process in the paper and pulp industry, are significant by-products of the delignification process, in which wood biomass is treated to separate cellulose fibers from lignin. Annually, up to 50 million tons of lignin are produced, which is predominantly used as low-value fuel. However, lignin has also found utility in synthesizing benzoxazines and as fillers in benzoxazine composites.28,66–68 Phenolated lignin, resulting from acid-catalyzed phenolation, has proven to be a valuable phenolic precursor for the synthesis of benzoxazine, as demonstrated by Abarro et al. 67 During the reaction, substitutions take place in the lignin side chains, converting aliphatic hydroxyl groups into aromatic hydroxyls, which results in up to ∼7 times more accessible ortho/para crosslinking sites than those found in raw lignin, corresponding to about 4.5 mmol g-1 of aromatic hydroxyl groups. An optimal phenol-to-lignin ratio of 2 is necessary to prevent partial cleavage and degradation of the backbone under acid-catalyzed conditions, which can lead to a reduction in molecular weight and dispersity. This increased density of OH groups enhance the number of reactive sites available as phenolic feedstock for benzoxazine synthesis. Two different amine sources, aniline and propargylamine, were used to further process the phenolated lignin feedstock for the synthesis of benzoxazine resin. Phenolation-induced structural modification of lignin significantly increases its reactivity toward benzoxazine formation, thereby substantially enhancing the properties of the resulting thermosets. As a result, fast polymerization and reduced curing temperatures between 225 and 230°C were observed, which can save energy in processing compared to those from systems based on bisphenol A, which has curing temperatures of approximately 260°C. It was also observed that stronger crosslinked networks formed when using propargylamine due to the secondary cyclotrimerization of its propargyl groups. This led to a higher polymerization enthalpy and enhanced overall thermal stability (Td max ≈419°C, char yield ≈52%), resulting in improved flame retardancy as indicated by the limiting oxygen index (LOI) exceeding 28—characterizing the thermoset as self-extinguishing. 67
Adjaoud et al. 69 showed a greener way to modify lignin by using phloretic acid for esterification instead of the harsher strong acid catalysts required in phenolation. A solvent-less Fischer esterification process added ester links and reactive hydroxyl groups to the lignin structure under mild conditions with a 5:1 phloretic acid to lignin ratio and 2.5 wt% of para-toluene sulfonic acid as a catalyst at 140°C for 48 h. As a result, the number of ortho-unsubstituted phenolics increased with the increased aromatic hydroxyl to 4.63 mmol g-1 and leaving the aliphatic hydroxyl content to 0.14 mmol g-1. Esterified lignin functioned as a phenolic feedstock to synthesize benzoxazine resin, utilizing three distinct amines: stearylamine (ste), furfurylamine (fa), and monoethanolamine (mea), to adjust the network rigidity and thermal response. Analyses via differential scanning calorimetry (DSC) and DMA revealed a consistent rise in curing temperature and Tg, corresponding to 188, 200, 219°C and 136, 182, 197°C, which is associated with enhanced backbone rigidity. The highest thermal stability of thermosets that used furfurylamine as the amine source (Td5% ≈ 275°C) is attributed to their aromatic furan structure. The esterified lignin-based benzoxazine, made with monoethanolamine as the amine source, displayed vitrimeric properties via internal transesterification between ester and hydroxyl groups. It had a relaxation time of under 4 min at 200°C. Without a catalyst, this topology rearrangement implies that it can be easily processed, and it may also be possible to program it to alter its shape. 69
In addition to being a phenolic precursor, lignin can also be converted into a renewable amine feedstock for the synthesis of benzoxazine. Wu et al.
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developed a catalytic funneling approach for converting lignin-derived aldehyde into 4,4′-methylenebiscyclohexanamine (MBCA), a fully bio-based diamine used in high-performance polybenzoxazine. The conversion of crude lignin aldehydes to benzyl alcohols, coupled with phenol and nickel-catalyzed demethoxylation/hydrogenation to 4,4′-methylenebiscyclohexanol (MBC), followed by amination to MBCA, can produce a 99% yield using Raney-Ni catalyst. The resulting diamine was then converted to a benzoxazine through a solventless microwave-assisted synthesis, using renewable phenols such as sesamol, guaiacol, eugenol, and propyl guaiacol as the phenolic source. Thermosets made from phenolic sources containing sesamol exhibited exceptional thermal stability properties (Td10% ≈ 401°C; Tg ≈ 315°C), surpassing commercial bisphenol-A-based polybenzoxazine (Td10% ≈ 350°C, Tg ≈ 173°C). A high Tg and char stability suggest a rigid, highly crosslinked structure, which highlights its suitability for shape-memory functions with high programming temperature (Figure 5).
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Vanillin-derived benzoxazine feedstocks
In addition to the direct valorization of lignin, its depolymerized product, vanillin, has also been identified as a versatile bio-based precursor for benzoxazine synthesis. Typically, vanillin is obtained as a by-product of the pulp and paper industry through lignin depolymerization and can be efficiently recovered from industrial waste streams such as sulfite pulping liquor.
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Vanillin stands out because it maintains both phenolic and aldehyde functionalities, with the aldehyde group remaining unchanged during oxazine ring formation, thereby enabling subsequent post-functionalization.
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This bifunctional feature facilitates various chemical modifications, such as through imine-forming condensation of the free aldehyde group with amines. Sha et al.
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demonstrated this chemical versatility by condensing an aldehyde-functionalized benzoxazine (VFA) with amine-terminated polyether (ATPE) via aldimine chemistry, successfully integrating flexible ether linkages and furan pendants to enable SME. The incorporation of atpe enhanced chain mobility and formed reversible C = N bonds, enabling the creation of a dynamic thermally reversible crosslinked network for rapid stress relaxation and shape recovery. DMA showed a sharp Tg at 280°C, with E′ decreasing from 4200 MPa at room temperature to 30 MPa at 350°C, confirming the stiff net-point and soft switching segment characteristics of SME. The thermoset is able to regain its original form within 40 s, achieving a Rr of approximately 98% at a temperature of Tg + 20°C, demonstrating the high efficacy of the aldimine-based flexible network in rapidly triggered thermal actuation shown in Figure 6(a).
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Schematic representation of the chemical modification strategies using the unreacted aldehyde group in vanillin for benzoxazine-based resin feedstocks and their corresponding shape-memory behavior. Adapted and redrawn with modifications from36,72, with permission. Copyright 2020 Elsevier, and 2024 American Chemical Society.
Similarly, Qu et al. 72 developed a reprocessable, degradable system by synthesizing a vanillin-stearylamine benzoxazine (VSA) and subsequently crosslinking its preserved aldehyde groups with 1,12-dodecanediamine via Schiff-base condensation. This modification resulted in a new type of dynamic C = N linkages without the loss of the inherent benzoxazine framework. The dynamic exchange of imine linkages enables a network rearrangement through reversible imine metathesis. These dual functions—stress relaxation and reconfigurability—collectively form the molecular foundation for the reprocessing and SME in the resulting thermoset. Figure 6(b) illustrates the thermoresponsive behavior of imine linkages. In crosslinked networks, these linkages can be exchanged easily when heated through imine bonds. This allows the thermoset to be restored to its original shape at 100°C within 5 min, while also enabling the rapid reprocessing of the thermoset without mechanical loss. Following three cycles, the crushed fragments could be reshaped into dense and defect-free samples under gentle conditions (140°C/2 MPa) with excellent thermal deformability. FTIR showed that the C = N stretch at 1643 cm-1 remained present and a slight reappearance of the aldehyde band (1680 cm-1) was observed indicating partial reversible cleavage occurred during hot-pressing. The tensile strength (σtensile) remained almost constant after the third reprocessing cycle, slightly decreasing from 16 to 13.6 MPa, while elongation at break increased from 8.1 % to 12 %, indicating the potential of the dynamic imine bond-exchange reaction for efficiently restore the crosslinked structures. 72
Unlike systems that utilize the free aldehyde feature of vanillin, Wang et al.73,74 employed a different strategy by first modifying the amine counterpart through the condensation of furfurylamine into a difuran diamine, which then reacted with vanillin to produce a bis-functional benzoxazine precursor. The incorporation of difuran diamine into the structure adds pendant benzene rings, decreases dipolar polarization, and lowers the dielectric constant, making it a suitable choice for heat-resistant insulation. Simultaneously, the unreacted aldehyde groups from vanillin form hydrogen bonds with phenolic hydroxyls, reducing the activation energy needed for the oxazine ring to open and also enhancing the molecular backbone due to increased crosslink density.
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This tunable molecular architecture was subsequently utilized to produce a shape-memory polymer via the creation of dynamic imine bonds in the oxazine network through Schiff-base chemistry. The hybrid network composition formed combines permanent oxazine crosslinks and reversible imine linkages, resulting in thermal actuation and network rearrangement with about a 3-fold increase in mechanical robustness compared to pure benzoxazines. A more complex curling-uncurling test yielded further evidence of its quick, reversible response with nearly complete unrolling within 19 s and full recovery in 46 s shown in Figure 7. The superior performance is due to the cooperative interaction between the rigid oxazine crosslinks acting as fixed points and the flexible imine bonds allowing reversible network reorganization. This system exhibits superior tensile mechanical strength and shape recovery capabilities compared to other benzoxazine-based SMPs documented in the literature.
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Schematic representation of vanillin-based benzoxazine hybrid resin synthesized via difuran diamine, showing network formation and shape-memory response. Adapted and redrawn with modifications from
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, with permission. Copyright 2024 Elsevier.
Eugenol-derived benzoxazine feedstocks
Eugenol has also been reported as a value-added product from waste stream in forest chemical industry, such as lignin black liquor produced by papermaking
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and pine wood waste from rosin-processing plants.
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The presence of phenolic and allylic sites in eugenol allows for targeted functionalities to be achieved through chemical modification techniques such as thiol–ene coupling, as demonstrated by Sriharshitha et al.
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in the development of low-toxic and sustainable benzoxazine-based vitrimers. A mono-functional benzoxazine was synthesized from eugenol and 3-amino-1-propanol, and it contained both free hydroxyl and allyl groups. These groups were then crosslinked with a tri-thiol compound via thiol–ene coupling. Bio-silica from rice husk ash was added after being modified with 3-mercaptopropyltrimethoxysilane to chemically bond with the thiol–ene vitrimer matrix, thus increasing interfacial adhesion and enhancing both mechanical robustness and thermal stability. The vitrimer network exhibited dynamic responsiveness due to hydrogen bonding of phenolic –OH and thiol –SH functionalities, along with reversible S–S and C–S covalent linkages. The sulfur functionalities, introduced via thiol–ene reaction involving functionalized bio-silica and allylic side chains of eugenol, can repeatedly experience bond deformation and reforming under mild conditions, recovering the original shape of the material within 10 s at room temperature without external heating, a rare occurrence in benzoxazine-based systems. This vitrimer network displayed both self-healing and reprocessing capabilities, alongside its shape-memory property, due to a synergistic dynamic bond exchange mechanism that incorporated reversible S–S/C–S metathesis and hydrogen-bond-assisted rearrangement. Fractured specimens reassembled within 10–12 h at room temperature without the need for catalysts or external heating, consistent with the complete surface restoration observed after approximately 10 h of self-healing in the damaged regions and 14 h longer for the BS system (Figure 8).
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Molecular design and shape-memory behavior of the eugenol-derived benzoxazine vitrimer reinforced with bio-silica from rice husk ash. Adapted and redrawn with modifications from
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, with permission. Copyright 2022 Royal Society of Chemistry. Dynamic adaptability and reconfigurability of diphenolic acid-based benzoxazine vitrimer based on reversible TER. Adapted and redrawn with modifications from
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, with permission. Copyright 2021 Royal Society of Chemistry. Thermally triggered shape recovery of the cardanol-derived polybenzoxazine vitrimer containing reversible dioxaborolane bonds. Adapted and redrawn with modifications from
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, with permission. Copyright 2022 Elsevier. Multi-stimuli-triggered shape-memory behavior of the cardanol-based polybenzoxazine vitrimer incorporating dynamic dioxaborolane linkages. Adapted and redrawn with modifications from
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, with permission. Copyright 2022 Elsevier.
Diphenolic acid-derived benzoxazine feedstocks
Accessible from waste via acid-catalyzed condensation of levulinic acid from sugarcane by-product in cellulose-rich biomass is diphenolic acid (DPA), a sustainable phenolic precursor that resembles bisphenol A structurally, which is widely used. Due to its two phenolic OH functionalities and an additional reactive carboxyl group, DPA can readily react with a range of monomers or polymers to introduce specific properties in polymer networks, offering multiple reaction sites during crosslinking and network development.79,80 The reactivity of this additional –COOH functionality has been exploited by Adjaoud et al. 81 to construct dynamic polymer networks through a molecular design involving esterification with polyethylene glycol (PEG) to produce a flexible diol-terminated macro-monomer (PEG–DPA), as shown in Figure 9. An intermediate was subsequently altered through a Mannich condensation, in the presence of monoethanolamine, to create a tetra-functional benzoxazine with pendant hydroxyl groups. During ring-opening polymerization, the in situ production of tertiary amines (NR3) acted as an internal catalyst for the transesterification (TER) reaction between proximal ester and hydroxyl groups—no external catalyst was used in this process. The resulting polybenzoxazine network displayed characteristic vitrimer properties, such as reprocessability, reshaping, and self-healing, which occurred in conjunction with dynamic transesterification exchange. Surface scratch tests showed a healing efficiency of up to 74% at 150°C within 2 h, and reshaping was possible by twisting and bending above 100°C in 2 min due to rapid stress relaxation. 82
Cardanol-derived benzoxazine feedstocks
A significant by-product of the agricultural sector is cashew nutshell liquid (CNSL), which is generated from the cashew industry and manufactured on a large scale at approximately 0.5 × 106 tons each year. The compound mainly comprises its meta-alkylphenols, with anacardic acid being the major component and smaller quantities of cardol and cardanol, the latter serving as a promising phenolic feedstock. This cardanol consists of a mixture of phenolic lipids with C15 alkyl chains that possess varying degrees of unsaturation, and can be produced in greater quantities through thermal decarboxylation and double vacuum distillation of CNSL.83,84 These trifunctional molecular structures present multiple reactive sites for further use, yet their long alkyl side chains frequently result in decreased rigidity and mechanical toughness. Bo et al. 85 fabricated a cardanol-based benzoxazine vitrimer via thiol-assisted ring opening and subsequent boronic ester metathesis, using dioxaborolane as a dynamic linkage due to its reversibility. The oxazine ring was opened by the COLBERT mechanism, where thiols catalytically cleaved the ring into C–S linkages and pendant hydroxyl groups, which can facilitate additional crosslinking. The activation process, which involved thiols, lowered the curing temperature to 188°C for the cationic polymerization reaction. The subsequent boronic ester metathesis reaction between the benzoxazine and 1,4-phenylenebisboronic acid led to the introduction of dioxaborolane linkages, which have exchangeable bonds and exhibit thermal network mobility, allowing stress to be quickly dissipated at high temperatures due to dynamic covalent exchange. The resultant vitrimer with an optimal boronic acid/OH ratio of 0.17, shown in Figure 10, displayed highly efficient shape-memory properties, including a Rf of 99% and a Rr of 93%, alongside fast thermal recovery, excellent mechanical robustness (elongation of 587% and tensile strength of 5.9 MPa), and thermal stability (Td5% > 265°C). A corresponding dynamic exchange mechanism has enabled self-healing autonomy, where reversible dioxaborolane bonds facilitate defect closure and mechanical restoration without an external catalyst, achieving visible scratch repair within 60 min at 100°C and full recovery after 24 h. 85
As most of the SMPs function under a single stimulus, Bo et al. 58 created a multi-responsive cardanol-derived benzoxazine system utilizing the unsaturated long alkyl chains of cardanol through thiol–ene click chemistry, allowing SMPs to respond to electric, microwave, and infrared stimuli, and further extending the application scope (Figure 11). The C = C bonds in the side chains served as reactive sites for thiol–ene coupling with thioglycerol to yield hydroxyl-rich polyols featuring C–S bonds. The 1,4-phenylenebisboronic acid was subsequently reacted with these polyols, forming dioxaborolane linkages via transesterification between the 1,2-diol groups on the benzoxazine backbone and the boronic ester. Reversible crosslinks were formed under mild curing conditions of 120–160°C, and exchangeable dioxaborolane bonds allowed for topological rearrangement and dynamic network mobility at temperatures around 80°C. The vitrimer demonstrated outstanding shape-memory properties under multi-stimulus conditions, wherein dynamic dioxaborolane exchange could be triggered by both direct and remote stimuli. Shape recovery was achieved at 80°C within approximately 15 s under direct heating, demonstrating a remarkably low activation temperature for any polybenzoxazine-based system and illustrating excellent exchange efficiency in the dynamic boronic network. Upon IR exposure, the surface temperature rose to approximately 74°C in 700 s, and localized photothermal conversion facilitated remote recovery without direct contact; nonetheless, microwave irradiation at a power of 800 W for just 57 s resulted in rapid, uniform heating for consistent actuation, with higher power leading to a faster shape recovery. The vitrimer network was also found to be capable of reconfiguration via a process called “hydrosetting,” in addition to thermal and remote actuation. When immersed in water, the network became temporarily softened and ductile, enabling facile reshaping at room temperature without external heat. After drying, the material regained its stiffness and retained the newly programmed geometry, which remained stable for over 6 months and could be reprogrammed for more than 10 cycles. This simple, solvent-free approach demonstrated an environmentally friendly route for low-energy reprocessing and shape reconfiguration in cardanol-based polybenzoxazines. 58
Summary of the conversion routes from waste-derived feedstocks into benzoxazine-based resins and their functional composites for targeted shape-memory applications.
Summary of comparative analysis of conversion pathways, process conditions, economic viability, and environmental aspect of various types of waste precursors as benzoxazine feedstock.
Recyclability of benzoxazine-based thermoset systems
Thermoset shape memory materials are valued for their excellent chemical and mechanical stability, yet they also pose a significant challenge: after polymerization occurs, the material cannot be reprocessed due to the rigid 3-dimensional crosslinked molecular structure. Their intrinsic rigidity makes it relatively difficult to recycle or reuse them, especially when they are reinforced with fibers. As a result, their application in sustainable material solutions is significantly limited.94,95 Introducing dynamic covalent bonds or degradable linkages into the thermoset network is a practical approach, allowing reversible bond exchange and topology rearrangement to take place in response to external stimuli such as heat, light, or chemical reagents. These reversible interactions transform a typical non-reversible crosslinked structure into a covalent adaptable network (CAN), which maintains the thermosets’ properties in terms of dimensional and mechanical stability, and exhibits reprocessability like thermoplastics.96–98 These malleable networks have been fabricated using various dynamic chemistries such as transesterification, disulfide metathesis, imine exchange, and boronic ester reconfiguration, which offer practical routes to produce recyclable, reshaping and self-healing materials.
Summary of recyclability and reprocessability characteristics of benzoxazine-based thermosets with dynamic covalent adaptability.
Chemical structure of the benzoxazine–sulfur copolymer and plausible pathways for exchange reactions within the sulfide polymer network. Adapted and redrawn with modifications from 87 , with permission. Copyright 2024 American Chemical Society.
Bo et al.
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also reported a solvent-assisted method for recycling cardanol-based polybenzoxazine vitrimers with dynamic dioxaborolane linkages. The polymer network was recyclable due to its responsive boron–oxygen coordination bonds, which were formed by thiol-assisted oxazine ring opening and subsequent boronic ester metathesis. After 96 h in tetrahydrofuran (THF) at 35°C, the dioxaborolane crosslinks in the network were partially broken, and they eventually broke down into oligomeric fragments when their crosslink density dropped below the percolation threshold. After the solvent is evaporated, the recovered liquid precursor can be easily reshaped and re-cured to reform the same polybenzoxazine network through reversible condensation between diol units and phenylboronic acid, without experiencing a significant decrease in mechanical or thermal performance even after repeated recycling (Figure 13).
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Schematic mechanism of the recycling process. Adapted and redrawn with modifications from
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, with permission. Copyright 2022 Elsevier.
Selected designs also allow for selective matrix deconstruction at the EOL for reinforcement or resin recovery. For instance, the incorporation of rigid diacetal “trigger” units into polybenzoxazine networks allows the chemical “unplugging” of the crosslinked structure into soluble low-molecular-weight fragments, which also makes it possible to recover the fiber efficiently. These vitrimers contain diacetal linkages that remain stable under normal conditions but undergo selective cleavage in acidic environments, resulting in a combination of high thermal stability and degradation resistance at a later stage. In this design, both carbon fiber and glass fiber can be recovered under acidic conditions using a DMSO/H2O solution within 8 h, where the resin is completely dissolved, resulting in clean fibers in suspended solution with no surface damage observed. 99 These degradable linkages can also lower the degradation temperature, processing time, and reduce the use of highly corrosive reagents, thus reducing the energy needs and emissions with more selectively recovering high-value components.100,101 However, trade-offs can exist between service performance, reprocessability, and degradation efficiency. Networks designed for dynamic exchange bonds are prone to decreased thermal/mechanical stability under extended service or aggressive conditions, while systems optimized for high Tg and mechanical stability frequently need higher activation temperatures, prolonged processing times, or specific solvents for exchange of bonds, 102 as can be seen for various processing conditions from Table 3. Moreover, thermosets with degradable linkages are typically synthesized through a multi-step process using specific catalysts and/or non-standard curing agents, which can increase complexity and cost, making it difficult to upscale production for commercialization in the industry. Though catalysts can further lower the amount of energy needed to reverse the bond, they also introduce additional limitations, as some catalysts are costly, not commercially available, or difficult to recover and reutilize. 103 Recent reports show that this intrinsic ability to be reprocessed is especially well-suited for use in the electronics industry. Dynamic disulfide bonds were incorporated into benzoxazine systems to demonstrate the potential for intrinsic reprocessability, along with flexible film-forming properties, making them suitable for use in applications requiring flexible electronic substrates. 87 In parallel, networks that incorporate dynamic ester bonds have been found to display ultralow dielectric properties while maintaining both reprocess- and reshaping-ability, which suggests substantial potential for use in advanced electronic packaging applications. 104
In contrast to systems utilizing reversible dynamic covalent chemistry, traditional benzoxazine thermosets cannot be recycled by topological rearrangement, but only through a chemical degradation route, as degradation involve pyrolysis or mechanical grinding are outlawed in some jurisdictions and can reduce the thermomechanical properties of the recycled products. Conventional techniques like pyrolysis and mechanical grinding are frequently hindered by environmental regulations and generally lead to the loss of valuable reinforcement and a decrease in the thermomechanical performance of the recovered materials. Catalytic oxidation has been reported to be an efficient method for breaking down benzoxazine-epoxy composites without compromising the integrity of the carbon fibers, as demonstrated by Lo et al.
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Aerospace- and automotive-grade composites made from commercial benzoxazine/epoxy resin has been effectively broken down in a mild and oxidative environment. The catalytic system used for depolymerization of the composite consisted of RuCl3 and ceric ammonium nitrate in trifluoroacetic acid, which targets benzylic methylene sites that are formed when the oxazine ring opens during polymerization and later become susceptible to hydride abstraction under oxidative conditions during depolymerization. As illustrated in Figure 14, the RuCl3–Ce(IV) catalytic system undergoes oxidation at these sites, forming transient iminium intermediates that undergo hydrolysis to yield bisphenol-F tri- or tetra-carboxylates, with each carboxylate group derived from a former methylene bridge, together with small molecules such as aniline. The reaction proceeds ply-by-ply through each layer of the laminate, separating the carbon fibers from the surrounding matrix as oxidation occurs. This was validated by EDX of the cross-sectional area in Figure 14, indicating that the highest cerium concentration occurred at the degradation front and decreased as one progressed toward the interior plies. Using this mechanism, the polymer matrix is selectively broken down while the reinforcing carbon fibers stay intact, illustrating a proof-of-concept pathway to chemical recycling of high-performance structural carbon fiber-reinforced benzoxazine/epoxy.
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Typically, such pathways that chemically degrade involve either harsh cleavage conditions or strong reagents to achieve effective matrix deconstruction. Conditions with milder characteristics tend to be associated with slower reaction kinetics and narrower processing windows, whereas more aggressive conditions may increase requirements for energy and the needs of the separation step, and in certain cases, could compromise the quality of the recovered fiber.
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Schematic representation of the ply-by-ply depolymerization of the benzoxazine-based system. Adapted and redrawn with modifications from
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, with permission. Copyright 2018 American Chemical Society.
Guidance for the selection between intrinsic reprocessability and chemical degradation should be based on the intended application and EOL scenario of the two recycling routes previously discussed. Because benzoxazine-based thermoset systems are employed across a wide range of service environments with diverse performance requirements, there is no universal recycling strategy that is optimal for all cases. The first approach is to introduce the dynamic covalent networks or degradable linkages into the polymer architecture, allowing for an intrinsic reprocessing ability and self-healing properties without a loss of structural performance. The second approach is based on a chemical degradation route designed for conventional thermosets without dynamic functionalities, in which the fully crosslinked three-dimensional network can be selectively cleaved (e.g., catalytic oxidation) to retrieve valuable fibers and resin components. For this purpose, the recycling strategy must be decided in line with the specific application demand, with consideration given to the service lifespan, acceptable processing conditions, and recovery goals, and therefore should be viewed as a design variable rather than a post-use consideration, as illustrated in Figure 15. Schematic overview of circularity strategies for benzoxazine-based thermosets via chemical degradation and intrinsic reprocessability.
Perspective and future work
An attractive closed-loop cycle to eliminate any residual waste throughout the entire material life cycle is highlighted by the functionalization of industrial and agricultural waste streams as sustainable feedstocks for fabricating robust benzoxazine-based thermoset networks with shape-memory properties. The significance of molecular design strategies of waste-derived compounds, including lignin, vanillin, eugenol, diphenolic acid, and cardanol, through strategic chemical modification has been emphasized in this review. The incorporation of these functionalities further allows EOL reprocessing and recyclability to be controlled through network mobility or dynamic covalent exchange mechanisms. These systems convert low-value by-products, present in agricultural and industrial waste streams, into high-performance renewable resins that form various value-added products. Their solvent-free and low-solvent preparation processes, low-energy production steps, and inherent recyclability through dynamic covalent chemistry are well-aligned with the closed-loop circularity concept outlined by the bio-circular-green (BCG) economy. This BCG economy represents a strategic route to sustainable development, merging economic expansion with environmental preservation. In this holistic system of sustainable production and consumption, bio-based resources in closed-loop systems facilitate reduction, reuse, and recycling.106,107 As a post-pandemic growth strategy recommended during Thailand’s APEC host year, BCG is a science-driven approach that leads to greater resource efficiency, healthier ecosystems, and reduced waste – the principles of a “sustainable new normal” based on innovation and resilience.106,108 Recommendations were made to emphasize localized resource utilization and comprehensive environmental assessment to further broaden the applicability of waste-derived benzoxazine systems as promising resins within the BCG framework, while also enabling the identification of environmental hotspots. Successful implementation of the BCG model could be achieved by establishing a pilot-scale conversion plant in Thailand that utilizes agricultural residues, such as palm oil waste, sugarcane bagasse, and cashew nut shells. This would facilitate the rapid implementation of sustainable polymer production methods. Such an initiative would accelerate the translation of lab-scale innovations into scalable, sustainable polymer production pathways.
Benzoxazine systems offer versatility and the potential for utilizing waste-derived feedstock, making them suitable choices for next-generation matrices of high-performance composites used in aerospace and other demanding applications, as the molecular design that retains the desirable properties of phenolics, including aromaticity and char formation can be adjusted.
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The majority of structural materials used in aircraft components are carbon fiber-reinforced polymer (CFRP) composites, with carbon fibers remaining the preferred reinforcement option due to their high stiffness-to-weight ratio and heat resistance.110–113 Among the matrix resins being considered for aerospace applications, the benzoxazine resin has emerged as a promising candidate to replace traditional phenolic resins, which are hindered by concerns over their toxicity.114,115 In the aerospace field, SMP and/or its composites are particularly appealing due to their lightweight characteristics and substantial recoverable deformation capabilities for self-deploying structures such as sun sails and satellite antennas. These materials can be folded up compactly before launching and then deployed into their operational configuration in orbit with minimal onboard power, allowing a lighter system with more efficient use of space.113,116–119 Recent study by Jayalath et al.
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has led to the development of an SMP system intended for deployable thin-wall structures in aerospace applications based on a cyanate ester matrix. Two composite systems were examined, a neat glass fiber-reinforced system and the same with graphene nanoplatelets. The two samples showed very similar Tg (168–169°C) and a 100% shape fixity ratio with the sample containing graphene nanoplatelets recovered faster due to improved thermal conductivity facilitating rapid heat release throughout the composite material. The addition of graphene as a supplementary reinforcement agent improved the thermal stability and mechanical properties at high temperatures up to 140°C, as demonstrated by higher modulus retention and lower stress degradation compared to a neat glass fiber-reinforced system. The stability is due to the uniform dispersion of graphene and improved load transfer, which can be advantageous for aerospace design applications that demand structural integrity under cyclic thermal exposure.
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The space environment presents challenges, and particular factors must be considered when selecting materials for aerospace applications, primarily for exposure to atomic oxygen (AO) and vacuum thermal aging (VTA), both of which are frequently employed to simulate the conditions found in low-Earth-orbit that impact their shape-memory behavior. This type of simulation is particularly relevant to the shape-memory effect, where recoverable deformation hinges on the stability and mobility of molecular switches. Subjected to an AO attack, surface erosion took place on both composite systems, and their FTIR chemical structures remained largely unaltered; although, the inclusion of graphene resulted in shallower grooves and reduced erosion levels, yet this graphene system showed less degradation in thermomechanical, mechanical, and shape recovery ratio properties compared to the neat glass fiber system. Performance degradation in the long term was found to be primarily caused by the deterioration of flexible domains during 140°C thermal aging in a vacuum, which increased the tan δ peak from 169 to 178°C, thereby slightly impeding shape recovery in the graphene system, whereas the storage modulus remained relatively stable at the design temperature (Figure 16).
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Atomic oxygen exposure simulating the low-earth-orbit environment to evaluate the durability of cyanate ester-based SMPC. Adapted and redrawn with modifications from
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. Licensed under Creative Commons CC-BY. Retention of mechanical strength over 100 recovery cycles of benzoxazine-based SMP. Adapted and redrawn with modifications from
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. Licensed under Creative Commons CC-BY.
These findings highlight the importance of cycle life in shape memory materials used in aerospace, as repeated exposure to thermal and mechanical cycling in extreme conditions, such as vacuum or thermal aging, can lead to decreased recovery efficiency over time. It is worth noting that, as of now, no benzoxazine-based SMP has been studied under aerospace-simulated conditions, although Rimdusit et al.13,121 found that the long-term durability of SMPs can be substantially enhanced by incorporating benzoxazine resin into an epoxy network, as shown in Figure 17. The resulting BA-a benzoxazine-epoxy copolymer maintained its shape fixity above 98% and recovery ratios nearly 100% following 100 consecutive thermomechanical cycles. The incorporation of benzoxazine segments creates additional secondary crosslinking sites and aromatic regions that facilitate the storage of elastic strain energy during deformation. The reduction in flexural strength intervals after a number of recovery cycles is primarily caused by fatigue rather than structural degradation, thereby highlighting the chemical resistance of the modified system. 121 A recent report by Ebrahimi et al. 122 found that the addition of carbon fiber stabilized the shape memory properties of the material in a 70/30 benzoxazine/epoxy copolymer, with recovery ratios remaining at more than 95% within 20 cycles triggered in magnetic induction. 122
The chemically modified benzoxazine systems derived from waste, as described here, already exhibit many of the characteristics expected for sustainable thermosets with high performance. Thorough reviews of these chemical modifications have revealed systems that possess: (i) glass transition temperatures significantly higher than service thresholds, (ii) retained stiffness and strength following multiple loading cycles, (iii) inherent resistance to flames and heat, and (iv) controllable network dynamics, thereby making them competitive systems for aerospace applications. A significant disadvantage of feedstocks sourced from waste streams is their variability. Further investigation is required to establish the refining and fractionation methods, and to correlate the conversion pathways with feedstock chemistry to consistently produce high Tg and robust networks. More importantly, for deployment in harsh aerospace environments, such as extreme temperatures, ultraviolet radiation, vacuum conditions, and thermal cycling, the dynamic chemistries involved along with the retention of Rf and Rr for benzoxazine-based system, remains largely unexplored. Systematic evaluation for stability under these conditions is therefore a critical research gap that must be addressed in future studies.
Conclusion
This review provides an in-depth analysis of the structure-property relationships influencing the shape memory performance of benzoxazine-based systems, including copolymers and composite architectures, as guided by molecular design strategies. Industrial and agricultural waste streams are highlighted as sustainable precursors, where each feedstock offers a distinct functional pathway for enabling the shape memory effect and adaptive functionality. These waste-derived compounds, instead of being just low-cost additives, are converted in the tailoring network architecture along with controlled thermal, mechanical, and switching behavior in benzoxazine-based systems. Lignin, accessible as a pulping by-product from the delignification process, can be converted into reactive feedstocks either as phenolic or amine sources via phenolation, esterification, or amination, respectively, resulting in a rigid benzoxazine network with a high glass transition temperature and the ability to rearrange its topology for easy reprocessing and potentially be programmed to adjust its shape. The depolymerization of lignin further enables the recovery of vanillin, whose aldehyde functionality provides an additional reactive site for introducing dynamic linkages, thereby enabling network rearrangement, reprocessing, and shape memory activation under thermally triggered conditions. Eugenol, derived from lignin-rich papermaking waste streams, exploits its allyl functionality for thiol-based modification and hybridization with bio-derived fillers, resulting in dynamic S–S/C–S networks that exhibit rapid shape recovery and self-healing at low or even ambient temperatures. Sugarcane-derived diphenolic acid serves as a renewable alternative to bisphenol A, where its pendant carboxyl group enables vitrimer-like behavior through esterification or hydrogen-bond-assisted interactions, balancing rigidity and reprocessability. Cardanol, obtained from cashew nutshell liquid, provides a versatile phenolic platform with a long aliphatic side chain that facilitates thiol functionalization, inverse vulcanization, or urethane integration, enabling multi-stimulus actuation, self-healing, and dynamic reprocessing through either covalent or supramolecular interactions. For all these systems, the shape memory performance is highly dependent on molecular design: rigid aromatic segments contribute to shape fixity and thermal stability, while flexible aliphatic chains, dynamic covalent bonds, or supramolecular interactions govern switching behavior, recovery speed, and reprocessability. These observations demonstrate that functionalities can be selectively leveraged to modulate shape fixity, recovery ratio, and programming conditions for waste-derived functionalities rather than performance sacrifice. Recyclability in benzoxazine-based thermosets can be addressed in two different strategies: intrinsic reprocessability and chemical degradation. Networks incorporating dynamic covalent bonds or degradable linkages allow reshaping, healing, and controlled end-of-life recovery due to network mobility, and many such systems can retain their functional and mechanical properties over several reprocessing cycles before reaching their performance limit. In contrast, networks without these dynamic functionalities generally exhibit long-term stability during service, but their end-of-life option relies on more technologically demanding approaches, such as through selective chemical degradation via catalytic oxidation, to recover high-value fiber and resin fragments. The choice between these strategies requires consideration of the application and service environment to be addressed, as well as the final end-of-life option, highlighting that recyclability should be considered at the molecular design level and not as a post-use solution. This review collectively shows that systems derived from waste benzoxazine can meet the principles of a closed-loop circular economy under a BCG framework, integrating efficiency, performance, and recyclability. Upcycling on a large scale comes with unforeseen expenses, since intensified processing intricacies, catalyst necessities, or purification procedures could counterbalance environmental advantages if not meticulously supervised. Significant gaps still exist in the field. Current experimental research on the long-term stability of shape memory thermosets made from waste-derived benzoxazine under extreme service conditions is limited, especially when it comes to aerospace-related situations. Further research should thus focus on (i) assessing the stability of shape memory in simulated extreme conditions, (ii) comparative studies of trade-offs between durability and reusability, (iii) techno-economic assessments for large-scale manufacturing, and (iv) life-cycle assessments to quantify actual sustainability benefits. Transferring these challenges will enable the transition of waste-derived benzoxazine systems from laboratory demonstrations to predictive, application-ready platforms for next-generation circular thermosets.
Footnotes
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work received support from the Second Century Fund (C2F) at Chulalongkorn University. The work has also been supported by the National Research Council of Thailand (NRCT) and Chulalongkorn University (N42A660910), and Thailand Science Research and Innovation Fund Chulalongkorn University (IND_FF_69_138_2100_020).
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Data Availability Statement
Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.