Solventless Dual‐Cure Liquid Resins Via Circular Use of Phthalic Anhydride for Recyclable Composite Applications

Abstract

Fiber‐reinforced composites (FRCs) possess a remarkable strength‐to‐weight ratio, making them ideal light‐weighing alternative materials of metals used in automotive, aerospace, and outdoor equipment applications, but their recycling is challenging. Chemically recyclable thermoset polymers can enable fiber recovery and reuse; however, challenges remain in the separation and purification of depolymerized small molecules for efficient polymer recycling. To this end, a series of liquid resins for chemically recyclable polymer networks is designed based on phthalic anhydride, a widely produced and inexpensive chemical. The straightforward sublimation of phthalic anhydride is leveraged to enable a simple and efficient separation process for polymer recycling. To liquefy phthalic anhydride, five mono‐acryloyl‐phthalates are synthesized to obtain stable liquid resins together with phthalic diglycidyl ester. These liquid resins undergo dual‐cure reactions that comprise photopolymerization of acrylate and, subsequently, heat‐mediated epoxy‐acid polymerization reactions. These liquid resins exhibit moderate viscosities (2600–6400 cP @ 22 °C), fast curing, and robust thermomechanical properties (T_gs from 71 to 116 °C). It is demonstrated that hydrolysis of the dual‐cured polymers completes within 2 h at 80 °C, and direct sublimation produces phthalic anhydride with 82% yield. This resin system is expected to provide a cost‐competitive, highly efficient platform for recyclable FRCs.

Solventless dual‐cure epoxy‐acrylate resins are developed based on phthalic anhydride, enabling efficient chemical recycling for fiber‐reinforced composites. These liquid resins exhibit rapid photopolymerization, high glass transition temperatures, and complete degradation in alkaline conditions. Recycled phthalic anhydride is easily purified via sublimation, offering a sustainable pathway for composite manufacturing and recycling.

1. Introduction

Fiber‐reinforced composites (FRCs) with continuous fiber reinforcement exhibit exceptionally high strength‐to‐weight ratios by combining the high strength of fibers and the low density of polymer matrices. Besides a few exceptions, such as aerospace applications, FRCs are fabricated by employing liquid resin in a molding process, such as wet layup, vacuum infusion, resin transfer molding, prepreg, pultrusion, and filament winding. No solvent should be involved in the resin polymerization to maximize the strength of the resulting polymer. The liquid resin must possess optimized properties, including resin viscosity, curing temperature and duration, and thermomechanical properties of the polymer. Common liquid resin systems include unsaturated polyester resin and epoxy‐amine resin, both resulting in thermoset polymers. Due to the covalently crosslinked chemical structures (network), thermoset polymers do not exhibit viscous flow and do not dissolve in any solvent, and thus, they cannot be remolded or reclaimed through physical means. As a result, when FRC materials reach their end‐of‐life, they are disposed of in landfills, contributing to waste accumulation. To enable the recycling of the thermoset polymers and recovery of the fiber, chemical depolymerization is necessary to break down the covalent networks into soluble small molecules (monomers and/or oligomers). Given that the production of fiber reinforcement, such as carbon fibers, is costly and highly energy‐consuming, enabling fiber reclamation and reuse will significantly reduce the environmental footprint associated with FRC materials.

In the design of chemically recyclable thermosets, covalent adaptable networks (CANs) have been a suitable choice by leveraging dynamic covalent chemistry to enable depolymerization.^{[ 1} , ² , ^{3 ]} Helms et al. developed dynamic network polymers using diketoenamine bonds.^{[ 4 ]} The poly(diketoenamine)s (PDKs) are synthesized via solventless condensation reactions between ß‐triketones and an amine. The PDK depolymerization occurs in strong aqueous acid that is selective to the PDK in mixed plastic waste streams. The triketone molecules are purified by base extraction followed by precipitation in acidic conditions. Zhang et al. developed alkyl‐polycyanurates via reversible nucleophilic aromatic substitution that can be recycled using dormant covalent linkages.^{[ 5 ]} The thermosetting polymer was developed without solvent and is depolymerized by a transesterification reaction with ethanol and potassium carbonate. The reclaimed crude monomers are separated using liquid‐liquid extraction and purified via distillation. Zhang et al. leveraged epoxy‐anhydride networks consisting of tertiary amines for FRCs in neat conditions.^{[ 6 ]} The network is depolymerized in water at 200 °C in a pressure reactor without additional catalysts as the pre‐existing tertiary amines catalyze the hydrolysis reactions. The recycled monomer must be mixed with fresh epoxy to generate new resin. Other work has been done using different dynamic covalent bonds that enable depolymerization or reprocessing, such as disulfide,^{[ 7} , ⁸ , ^{9 ]} carboxylic ester,^{[ 10} , ^{11 ]} boronic ester,^{[ 12} , ^{13 ]} and thiourea.^{[ 14} , ^{15 ]} While depolymerization of the network polymers is straightforward, separating and purifying the resulting products are often tedious. It remains a challenge to design CANs from solventless liquid resins with straightforward depolymerization and separation processes.

Previously, we demonstrated the depolymerization of an epoxy‐anhydride network polyester based on phthalate monomers via transesterification with methanol (methanolysis) under ambient conditions.^{[ 16 ]} The straightforward depolymerization by methanolysis yields one major product (dimethyl phthalate) since both monomers — phthalic anhydride and diglycidyl phthalate —are derived from the same ortho‐phthalic acid. With high production (>5.8 million metric tons in 2018) and low cost ($1.57 per kg in 2024), phthalic anhydride is readily available for industrial applications.^{[ 17} , ^{18 ]} However, incorporating phthalic anhydride in solventless liquid resin systems was difficult because phthalic anhydride has a high melting temperature of 130 °C and has extremely poor solubility in organic molecules. We hypothesize that asymmetric phthalic molecules that contain polymerizable reaction handles will enable accessing phthalate‐based liquid resins since the asymmetry will disrupt the planar structure of phthalic anhydride and intermolecular π‐π stacking.

In this work, we choose to synthesize a series of mono‐acryloyl‐phthalates (MAP) that features two distinct functional groups: a (meth)acrylate and a carboxylic acid. MAP can be synthesized from the alcohol‐anhydride reaction between a hydroxyl‐containing (meth)acrylate and phthalic anhydride. A “dual‐cure” system with orthogonal polymerization mechanisms is formed by combining MAPs with an epoxide monomer. The (meth)acrylate component undergoes free radical‐mediated polymerization; the carboxylic acid components undergo epoxy‐acid polymerization that results in polyesters. In addition, the free radical polymerization can potentially obtain ultrafast reaction kinetics by leveraging photopolymerization, that is using a photoinitiator to generate free radicals under ultraviolet (UV) or visible light. We further hypothesize that since the epoxy‐acid reaction requires elevated temperatures, the intermediate polymer after the free radical polymerization will remain stable at ambient conditions, which will enable the fabrication of prepreg composite materials. The final dual‐cure polymer is a phthalate‐based polyester network, which, after hydrolysis, will result in phthalic small molecules. We study the efficacy of direct sublimation of phthalic anhydride from the depolymerized small molecules at 210 °C to demonstrate a straightforward recycling process.

2. Results and Discussion

2.1. Synthesis of Mono‐Acryloyl‐Phthalate

Five monofunctional hydroxy acrylate/(meth)acrylate monomers were used to undergo ring‐opening reaction with the phthalic anhydride (PHA), including 2‐hydroxyethyl acrylate (HEA), 2‐hydroxypropyl acrylate (HPA), 4‐hydroxybutyl acrylate (HBA), 2‐hydroxyethyl methacrylate (HEMA), and 2‐hydroxypropyl methacrylate (HPMA) (Figure  1A). We name these synthesized mono‐acryloyl‐phthalate (MAP) monomers as the (meth)acrylate monomer preceded by “Ph,” such as PhHEA. The synthesis of the MAPs was monitored using ¹H NMR (Figures S1–S5, Supporting Information). The aromatic protons of PhHEA had a distinct shift upfield to 7.95 to 7.45 ppm (up from 8.11 and 8.01 ppm for PHA) (Figure 1B). However, residual peaks of PHA protons were still present in the product spectra. A similar trend was observed in the incomplete upfield shift of the HEA acrylate peaks. The HEA acrylate peaks, ranging from 5.75 to 6.50 ppm, were observed to shift ≈0.2 ppm upfield for the PhHEA product. The conversion to PhHEA was ≈85% by comparing the integration value of the peak located at 6.44 to 6.46 ppm. Conversions for the five acrylate/(meth)acrylates range from 80–90%. Batch‐to‐batch inconsistencies were not observed, as consistent conversions of the reactants were achieved in repeat syntheses (Figure S6, Supporting Information). Yatchyshyn et al. reported similar results that the synthesis of PhHEMA does not go to completion and reaches equilibrium at 84.3% conversion.^{[ 19} , ^{30 ]} Thus, further purification of the monomer was not pursued since both unreacted hydroxy acrylates and phthalic anhydride can participate in free radical polymerization and nucleophilic ring‐opening of epoxide, respectively.

Figure 1.

A) Synthesis of mono‐acryloyl‐phthalates (MAP) using phthalic anhydride and five different hydroxy (meth)acrylates. For acrylate (R′ = H), three different R groups were used: ethyl, isopropyl, and butyl. For methacrylate (R′ = CH₃), two different R groups were used: ethyl and isopropyl. B) As an example of these syntheses, ¹H NMR spectra of PHA and HEA and the resulting MAP monomer (PhHEA) showed 85% conversion. Peaks are labeled to emphasize trace reactants in the product spectra.

2.2. Photopolymerization of Resin System

The carboxylic acid on the MAP monomer is reacted with a multifunctional epoxide to form a hydrophobic and crosslinked network. Inspired by our previous work, diglycidyl phthalate (DGPH) was used as the epoxide, as depolymerization of both DGPH and MAP will yield phthalic acid. The MAP liquid monomers were mixed with DGPH with an epoxide:acid ratio of 1:1 (Figure  2A, left and middle). For clarity, we herein refer to the resulting “dual‐cure” epoxy‐acrylate resins of PhHEA, PhHPA, PhHBA, PhHEMA, and PhHPMA as A1, A2, A3, M1, and M2, respectively (Table  1 ). To this mixture, 1 wt% of ethyl (2,4,6‐trimethylbenzoyl)phenylphosphinate (TPO‐L) photoinitiator and 1 wt% 2‐ethyl‐4‐methylimidazole (24EMI) curing agent were added. The resins were cured using 405 nm light at an intensity of 5 mW cm⁻². The orthogonality of free radical photopolymerization and heat‐mediated epoxy‐acid reaction forms an intermediate material comprising a linear poly(acrylate) and unreacted epoxide (Figure 2B, left and middle). The intermediate, photo‐cured polymer was then subsequently heated for 4 h at 130 °C, initiating the epoxy‐acid reaction (Figure 2C, left and middle).

Figure 2.

A) Reaction scheme (left) and cartoon depiction (middle) of the liquid resin comprising mono‐acryloyl‐phthalate monomers and epoxide, both with ortho‐phthalate backbones. Viscosities of the five dual‐cure resins at 22 °C (right). B) Reaction scheme (left) and cartoon depiction (middle) of the free radical photopolymerization of the resin, resulting in a linear polymer and unreacted epoxide. Polymerization reaction kinetics of the free radical polymerization using 405 nm light at an intensity of 5 mW cm^{− 2}, monitored by real‐time FTIR (right). The conversion was calculated by monitoring the peak area of the acrylate (C=C) peak at 1620 cm⁻¹ and normalized to the carbonyl peak at 1720 cm⁻¹. C) Reaction scheme (left) and cartoon depiction (middle) of the heat‐mediated epoxy‐acid reaction, resulting in a crosslinked thermoset. DMA curves of the five dual‐cured polymers showing rubbery moduli and glassy transitions (right).

Table 1.

Mono‐acryloyl‐phthalate dual‐cure resin formulations.

Resin a)	Photopolymerization handle	Mono‐acryloyl‐phthalate	wt% Phthalate b)	Viscosity [cP @ 22 °C]
A1	Acrylate	PhHEA	61	5590
A2	Acrylate	PhHPA	59	6340
A3	Acrylate	PhHBA	57	2660
M1	Methacrylate	PhHEMA	59	4460
M2	Methacrylate	PhHPMA	57	6360

^a) DGPH is added to resin using a functional group epoxy:acid ratio of 1:1

^b) wt% phthalate is calculated as the amount of phthalate groups (C₈H₄O₄) present in the resin

The MAP‐DGPH resins showed viscosities ranging from 2600 to 6400 cP at 22 °C (Figure 2A, right). The acrylate resins A1, A2, and A3 had viscosities of 5590, 6340, and 2660 cP, respectively. The methacrylate resins M1 and M2 had viscosities of 4460 and 6360 cP, respectively. We observed that the larger pendant group for A2 and M2 had an increased viscosity compared to the smaller A1 and M1. While the A3 resin has the largest pendant group, the viscosity is the lowest observed. We conclude that the increased conversion of PHA and HBA, as calculated by ¹H NMR, results in the observed lower viscosity for A3 (Figure S3, Supporting Information).

Multifunctional acrylates are commonly used in photopolymerizations because their ability to crosslink leads to the autoacceleration effect.^{[ 20} , ^{21 ]} In contrast, monofunctional acrylates, which cannot crosslink, often exhibit slow photopolymerization kinetics, making them undesirable as the primary component in most resins. Hoyle et al. reported that intermolecular interactions between monofunctional monomers can enhance polymerization kinetics by allowing these acrylates to coordinate and act as pseudo‐difunctional systems.^{[ 22 ]} Pojman et al. reported similar accelerated polymerization kinetics of monoacrylates that were coordinated via ionic interactions.^{[ 23 ]} Previously, our group reported that coordination between difunctional epoxide and methacrylic acid had a comparable effect.^{[ 24 ]} To investigate the phenomenon of intermolecular interactions of the phthalic monomers, real‐time FTIR was used to monitor the photopolymerization kinetics of the dual‐cure resin using 405 nm light at an intensity of 5 mW cm^{− 2} (Figure 2B, right). The real‐time (meth)acrylate conversion was calculated by dividing the alkene peak area at 1620 cm⁻¹ at a given time by its initial peak area. The carbonyl (C═O) stretch at 1720 cm⁻¹ was used to correct for changes in sample volume.

Resins A1, A2, and A3 reached 54, 25, and 83% conversion after 30 s of irradiation, respectively. With the absence of DGPH, the neat acrylates PhHEA, PhHPA, and PhHBA reached lower conversions of 53, 19, and 73%, respectively (Figure S7, Supporting Information). Resins M1 and M2 reached 60 and 48% conversion after 30 s of irradiation, respectively, while PhHEMA and PhHPMA reached 39 and 19% conversion, respectively. This acceleration is attributed to the hydrogen bonding coordination between the epoxide group in DGPH and the carboxylic acid in MAPs, which forms a pseudo‐dimer. We conclude that the accelerated polymerization kinetics enables more energy‐efficient polymerization, which can enable faster fabrication of composite materials.

The addition of DGPH not only increased conversion after 30 s of irradiation for the MAP monomers, but conversion after 4 min was also increased (Figure 2B, right). Resins A1, A2, and A3 reached 95, 87, and 98% conversion after 4 min of irradiation, respectively, while PhHEA, PhHPA, and PhHBA reached 77, 69, and 92% conversion, respectively. Resins M1 and M2 reached 95 and 97% conversion after 4 min of irradiation, respectively, while PhHEMA and PhHPMA reached 64 and 53% conversion, respectively. The conversion in the presence of stoichiometric epoxide was higher than that of the neat monomer, as the unreacted epoxy acted as a plasticizer, avoiding vitrification.

After completing the first‐stage photopolymerization, the polymer was heated at 130 °C for 4 h to initiate the second‐stage epoxy‐acid crosslinking. FTIR analysis confirmed the completion of this reaction (Figure  3 ). Initially, the carbonyl (C═O) peak at 1720 cm⁻¹ appeared broad due to the varied chemical environments of the carbonyl groups present in the resin. Following photopolymerization and thermal crosslinking, the carbonyl peak shoulder at 1700 cm⁻¹ disappeared, indicating the formation of a homogeneous polyester network. Additionally, the formation of a broad peak at 3500 cm⁻¹ is also observed, attributed to the formation of hydroxyl groups from the epoxy ring‐opening reaction.

Figure 3.

FTIR spectra of A1 resin. After exposure to 405 nm light at 5 mW cm⁻² light intensity, the near complete disappearance of the (C=C) peak at 1620 cm⁻¹ is observed as a result of free radical photopolymerization. The polymer is then subsequently heated at 130 °C for 4 h to initiate epoxy‐acid crosslinking, which results in a homogenous polyester network, as evidenced by the transition to a sharp carbonyl (C=O) peak at 1720 cm⁻¹.

Dynamic mechanical analysis (DMA) of the fully cured thermosets further confirmed the formation of a glassy, homogeneous thermoset network (Figure 2C, right). The glass transition temperature (T_g) was taken as the peak of the tan(𝛿) curve. The T_g values of A1, A2, A3, M1, and M2 were 73, 83, 71, 111, and 116 °C, respectively (Table  2 ). The methacrylate‐based resins had higher T_gs than their acrylate counterparts, which is consistent with the literature that the methyl group increases steric hindrance. A rubbery plateau was observed for all five dual‐cured polymers, confirming the presence of crosslinking. The rubbery modulus for A1, A2, A3, M1, and M2 are 6.9, 10.2, 11.5, 16.3, and 12.3 MPa, respectively. The rubbery modulus was calculated as the storage modulus of the polymer at a temperature of T_g + 40 °C.

Table 2.

Numerical data of the five resins, including: T_g, rubbery modulus, % C═C conversion, and fluidity. The T_g and rubbery modulus were analyzed using DMA, % C═C conversion was analyzed via FTIR, and fluidity was calculated using a parallel plate rheometer.

Resin	A1	A2	A3	M1	M2
Tg (DMA) (°C)	73	83	71	111	116
Rubbery Modulus (MPa)	6.9	10.2	11.5	16.3	12.3
% C═C Conversion	95	87	98	95	97
Fluidity (kP−1)	17.9	15.8	37.6	22.4	15.7

Polymers for composite materials benefit from increasing T_gs, rubbery moduli, and conversion of the monomer. As such, a larger area on the spider chart indicates a better material, as it reflects the optimization of key material properties; the material properties of each formulation are illustrated in Figure  4 and summarized in Table 2. The wt% of phthalate should also be maximized as it increases the theoretical yield of phthalic anhydride; for every mole of phthalate in the thermoset, one mole of phthalic anhydride is theoretically recoverable. The A1 resin has the largest recyclable content, as the resin is 61 wt% phthalate. The wt% phthalate is lowest for A3 and M2 at 57% phthalate (Table 1). The fluidity, calculated as the reciprocal of viscosity (Equation S1, Supporting Information), indicates how easily a resin can be handled. Increasing the fluidity of the resin corresponds to an improved ease of processing. To maximize the potential of these materials as composite matrices, we need to evaluate their degradability.

Figure 4.

Spider charts of the five resins showing T_g, rubbery modulus, wt% phthalate, % C=C conversion, and fluidity. The T_g and rubbery modulus were analyzed using DMA, % C=C conversion was analyzed via FTIR, and fluidity was calculated using a parallel plate rheometer. Numerical data is provided in Table 2.

2.3. Degradability of Dual‐Cure Thermosets

Three different conditions for degradation of the polymers were investigated, including methanolysis, acid‐based hydrolysis, and base‐catalyzed hydrolysis. Small discs with a thickness of 2 mm and a diameter of 10 mm were cast. The discs were then submerged in 3 m aqueous NaOH at 80 °C, 3 m aqueous HCl at 80 °C, and a saturated solution of potassium carbonate (K₂CO₃) in methanol (MeOH) at 40 °C (Figure  5A,row (i.)). The high T_g and aromatic backbone of the polymers make the polymers hydrophobic, so an elevated temperature was used to increase diffusion and degradation reaction kinetics. After 2 h at 80 °C in the 3 m NaOH solution, A1, A2, and A3 were completely degraded (Figure 5A,row (ii.)). After three weeks in the same conditions, M1 and M2 did not fully degrade and did not break apart (Figure 5A,row (iii.)). After three weeks in 3 m HCl at 80 °C, none of the five polymers showed signs of degradation (Figure 5A,row (iv.)). After 24 h in the K₂CO₃/MeOH solution, all three acrylate‐based polymers were completely degraded, while both methacrylate‐based polymers were only partially degraded (Figure 5A,row (v.)). After three more weeks under the same conditions, no additional degradation was observed for M1 and M2.

Figure 5.

A) Degradation of dual‐cured polymers in various conditions. Disk samples with a diameter of 10 mm and thickness of 2 mm were submerged in aqueous 3 m NaOH at 80 °C for (i.) 0 h, (ii.) 2 h, and (iii.) 24 h. The polymers were also subjected to (iv.) aqueous 3 m HCl at 80 °C for 24 h and (v.) saturated K₂CO₃/MeOH at 40 °C for 24 h. B) Hydrolysis scheme of the crosslinked thermoset, producing polyols and carboxylic acids.

We hypothesize that steric hindrance at the tertiary carbons on the polymethacrylate backbone prohibits hydrolysis and methanolysis reactions, leading to incomplete degradation of M1 and M2 in all tested degradation conditions. Terashima et al. subjected homopolymers of methyl acrylate (PMA) and methyl methacrylate (PMMA) to transesterification conditions using 1‐dodecanol and a titanium catalyst.^{[ 25 ]} They report transesterification yields of 96% for PMA compared to 5% for PMMA, as the α‐methyl groups on the methacrylate sterically hinder the pendant esters from transesterification. Our results were in good agreement with their findings, and we confirmed the effect of steric hindrance in crosslinked polymethacrylates.

Cho et al. utilized K₂CO₃ in MeOH, water, and dichloromethane (DCM) to produce dimethyl terephthalate from poly(ethylene terephthalate) (PET) via methanolysis under ambient conditions.^{[ 26 ]} Inspired by work done by Cho et al., we used K₂CO₃ and MeOH for the transesterification of thermoset networks in lieu of linear PET plastic.^{[ 16} , ²⁷ , ^{28 ]} Our results show that methanolysis of the MAP networks requires water as a cosolvent but not DCM. When an anhydrous K₂CO₃/MeOH solution was used, the A1 polymer did not degrade completely (Table S1, Supporting Information). This finding aligns well with Cho et al., who reported 4.5% degradation of PET in anhydrous K₂CO₃/MeOH compared to 100% PET degradation when both DCM and water were added to the reaction mixture. While methanolysis of the MAP polyester into dimethyl phthalate is a viable depolymerization route, dimethyl phthalate must be further hydrolyzed into phthalic acid so that phthalic anhydride can be recovered via thermal dehydration and sublimation. The A1 resin was selected for further characterization and monomer recovery as it has the highest recyclable content of the five MAP resins.

2.4. Reclamation of Phthalic Anhydride Via Sublimation

Hydrolysis of the acrylate‐based polyester networks via NaOH yields disodium phthalate, sodium polyacrylate, and polyols (Figure 5B). The resulting solution is acidified with HCl to produce phthalic acid, poly(acrylic acid), and NaCl. The solution was concentrated via rotary evaporation followed by drying under airflow. The crude products were subjected to the sublimation process without any additional separation of byproducts. Phthalic acid is unique in that it decomposes into its anhydride form when it decomposes. The resulting phthalic anhydride readily sublimates, making purification and isolation of the product straightforward. Phthalic anhydride from degradation of the A1 polymer was collected with a yield of 82% (Figure  6 ). ¹H NMR shows the product to be phthalic anhydride with trace amounts of water (Figure S8, Supporting Information).

Figure 6.

Reclamation of phthalic anhydride via direct sublimation. Hydrolysis of the polymer is carried out for 16 h at 80 °C. The mixture is acidified, and the liquids are removed. The resulting solids are thermally dehydrated, and the phthalic anhydride is purified and isolated to 82% yield.

2.5. Flexible and Moldable Precursor for Fiber‐Reinforced Composite

Glassy thermoset polymers are the traditional matrix material for fiber‐reinforced composites as the final material is lightweight, strong, and highly resistant to solvents and heat. Pre‐impregnated fibers or “prepreg” are ubiquitous in composite manufacturing of sports equipment and aviation as they allow simpler manufacturing of complex shapes.^{[ 29 ]} The prepreg material is partially cured, so the material must be stored at low temperatures (i.e., in a freezer) to avoid additional curing prematurely. The benefit of using a dual‐cure material is that after the initial cure, the resulting linear acrylate is plasticized by the unreacted epoxide monomer (Figure  7A). Therefore, the resulting composite material is moldable — analogous to prepreg — but is thermally stable and does not require storing in a freezer.

Figure 7.

A) Photopolymerization scheme of A1 resin system. B) First‐stage A1 polymer showed no flow behavior after being inverted for 24 h and 3 weeks. C) Rheology of the first‐stage, photopolymerized A1 polymer using 20 mm parallel plates. Strain sweep with angular frequency of ω = 2 rad s⁻¹ at 22 °C. D) Frequency sweep at a strain of 3% on the same first‐stage A1 polymer.

We observed that the first‐stage A1 resin system was a soft gel after free radical photopolymerization. To confirm that the first‐stage polymer is a soft solid and not a viscous liquid, A1 resin was photopolymerized in a 20 mL scintillation vial. The vial was inverted and monitored for 3 weeks. After 3 weeks, no flow was observed for the sample, showing long‐term stability of the gel (Figure 7B).

We used oscillatory rheology to investigate the observed viscoelasticity of the photo‐cured material. Using a parallel plate rheometer equipped with a UV curing accessory, A1 resin was photopolymerized between 20 mm parallel plates without undergoing the 2^nd‐stage heat cure. An oscillatory strain sweep was taken at an angular frequency of ω = 2 rad s⁻¹ at 22 °C to determine the linear viscoelastic region (LVR) (Figure 7C); the LVR is defined as the region where the viscoelastic parameters are independent of strain. As expected, the first‐stage A1 polymer was a soft gel, with a storage modulus of 185 to 194 Pa at a strain of 0.1 to 7.9%. A frequency sweep at a strain of 3% and 22 °C showed a sol‐gel transition at ω = 2.4 and 76 rad s⁻¹ (Figure 7D).

Flexural bars of glass‐fiber reinforced polymer were made using a wet layup process with A1 resin and 4 layers of bid fiberglass RA5277 (Figure S9A, Supporting Information). The panel was exposed to a 30 W, 405 nm LED floodlight with a measured intensity of 3.65 mW cm^{− 2} on the backside of the panel after the light passed through the whole part. To ensure complete curing of the acrylate resin, 1 wt% 2,2′‐azobis(isobutyronitrile) (AIBN) was added. The panel was then cured at 60 °C for 8 h, followed by 130 °C for 4 h. The neat A1 polymer had a flexural modulus of 3860 ± 420 MPa (Figure S9B, Supporting Information). The glass‐fiber reinforced polymer using A1 resin had a flexural modulus of 26.4 ± 1.1 GPa (Figure  8A).

Figure 8.

A) Average stress–strain curve from flexural testing of glass‐fiber reinforced composite using A1 resin (5 replicates). B) Wet layup and vacuum bagging setup for fabrication of glass‐fiber reinforced panels. The panel was then subjected to a 30 W, 405 nm LED for 10 min, flipping after 5 min. C) The moldable composite was then wrapped around a spindle, wrapped in shrink tape, and subjected to 130 °C for 4 h. The resulting rod was a stiff and lightweight composite.

To show the moldability of the first‐stage polymer, another glass‐fiber reinforced panel was made using the same wet‐layup method and photopolymerization (Figure 8B). After photopolymerization, the panel was de‐molded and wrapped around a steel spindle (Figure 8C). The composite was then wrapped with shrink tape and heated to 130 °C for 4 h. After thermal curing, the composite was de‐molded, and the result was a stiff, lightweight rod. This approach demonstrates the potential of this material as an alternative to traditional prepreg. The first‐stage of the resin‐infused fiber was easily molded and post‐cured without significant additional processing.

3. Conclusions

We demonstrated dual‐cure solventless resins for a thermoset polymer network that demonstrates promising recyclability and purification through the sublimation of phthalic anhydride. The synthesized mono‐acryloyl‐phthalate monomers, when combined with diglycidyl phthalate, produced resins suitable for composite fabrication by wet layups. Our photopolymerization results revealed increased polymerization rates facilitated by hydrogen bonding interactions between the epoxy and the carboxylic acid groups, while also delaying vitrification and increasing the conversion of (meth)acrylate groups. Dynamic mechanical analysis confirmed the formation of homogeneous glassy thermosets, exhibiting glass transition temperatures consistent with the structural characteristics of the monomers. Chemical recycling of the epoxy‐acrylate polymers was demonstrated by efficiently degrading the cured materials under alkaline conditions, yielding phthalic anhydride by direct sublimation from the depolymerization mixtures. Methacrylate‐based networks, however, exhibited resistance to degradation due to steric hindrance. This work paves the way for more sustainable thermoset polymers with simplified recycling pathways, promoting the circular use of materials in high‐performance applications. Further study on optimizing resin viscosity to enable a wide range of composite processing techniques while maintaining recyclability is currently being investigated and will be reported elsewhere.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

G.M.M. and C.W. performed in conceptualization, writing – original Draft and writing – review and edited the manuscript. G.M.M., C.J.R, T.A.K., and C.W. performed in Investigation. C.W. performed in Funding Acquisition. C.W. performed in Supervision.

Supporting information

Supporting Information

Musgrave G. M., Reese C. J., Kirk T. A., Wang C., Solventless Dual‐Cure Liquid Resins Via Circular Use of Phthalic Anhydride for Recyclable Composite Applications. Macromol. Rapid Commun. 2025, 46, 2400909. 10.1002/marc.202400909

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.