Development of Bio-Sourced Epoxies for Bio-Composites

Xiao-Su Yi 1,2,*, Xvfeng Zhang 2,†, Fangbo Ding 3,† and Jianfeng Tong 2,† 1 Advanced Materials and Composites Department, Faculty of Science & Engineering, University of Nottingham (UNNC), Ningbo 315100, China 2 AVIC Composite Corporation Ltd. (ACC), Beijing 101300, China; 010xufeng@sina.com (X.Z.); broadtjf@sina.com (J.T.) 3 AVIC XAC, Commercial Aircraft Co., Ltd., Xi’an 710000, China; dingfangbo313@163.com * Correspondence: xiaosu.yi@nottingham.edu.cn; Tel.: +86-574-8818-0000 (ext. 8746) † These authors contributed equally to this work.


Introduction
With the present state of composite technological development, biocomposites are understood as composites that consist of biopolymer matrices, i.e., bio-sourced resins and/or natural fiber reinforcements, e.g., plant fibers (PF) [1,2].Besides existing applications in automobiles [3], this new member of the material family may provide an economical and environmentally-friendly alternative to glass-fiber-reinforced composites for quasi-structural applications in aircraft.The present paper provides an overview on the current development of biocomposite materials by an international joint project, ECO-COMPASS (Ecological and Multifunctional Composites for Application in Aircraft Interior and Secondary Structures, 2016-2019) [4], which is co-funded by the Chinese Ministry of Industry and Information Technology (MIIT) and the European Union, but with a special emphasis on bio-sourced epoxies and biocomposites.Quasi-structural composite parts were finally trial-manufactured and demonstrated using the epoxies as matrix resins.

Rosin-Sourced Epoxy as a Matrix Resin
Rosin is an abundantly available natural product, it is nontoxic and odorless, and contains various isomerized acids (>90%) and some neutral substances [5].Reactive double bonds and carboxyl groups of rosin acids render them suitable for the Diels-Alder reaction, esterification and condensation reaction usage.Therefore, rosin acid has received increasing attention as a bio-sourced form of renewable feedstock in polymer science.
Anhydride is one of the curing agents for epoxies (EPs).Owing to their characteristic bulky hydrogenated phenanthrene ring structure, rosin acids are analogous to many aromatic compounds in rigidity.However, their curing temperature is generally high, about 200-250 • C, and the curing time is long, which limits the use of the anhydride in some products and apparatus.The complex reactions of epoxy/anhydride curing include the carboxyl group reaction with an epoxy ring in the catalyzed action of accelerator, alternate ring-opening copolymerization of epoxy, and anhydride catalyzed by amine and polyetherification at high temperatures, initiated by amine and catalyzed by hydroxyl groups [6].These processes occur when an amine is added as the catalyst.An excellent reference [6] on this issue can be found in the literature.
In this study [7,8], an anhydride-type epoxy curing agent, maleopimaric (Scheme 1), was synthesized as a hardener from rosin acid (Scheme 1).This was supplied by the Ningbo Institute of Materials and Technology Engineering.A resin mixed with an E51-type epoxy and a solid phenolic epoxy was then prepared as the main component, together with an accelerator mixed of two amino imidazole salts as the thermally latent curing agent and also as the catalyst for the anhydride.The experimental details can also be find in reference [7]. Figure 1 exhibits the glass transition behavior of the formulated resin system.As shown, the curing degree increased with the curing temperature for a constant curing time of 3 h, as well as the glass transition temperature.The formulated matrix resin was finally designated as AGMP3600, with a bio-content of about 30%.reactions of epoxy/anhydride curing include the carboxyl group reaction with an epoxy ring in the catalyzed action of accelerator, alternate ring-opening copolymerization of epoxy, and anhydride catalyzed by amine and polyetherification at high temperatures, initiated by amine and catalyzed by hydroxyl groups [6].These processes occur when an amine is added as the catalyst.An excellent reference [6] on this issue can be found in the literature.
In this study [7,8], an anhydride-type epoxy curing agent, maleopimaric (Scheme 1), was synthesized as a hardener from rosin acid (Scheme 1).This was supplied by the Ningbo Institute of Materials and Technology Engineering.A resin mixed with an E51-type epoxy and a solid phenolic epoxy was then prepared as the main component, together with an accelerator mixed of two amino imidazole salts as the thermally latent curing agent and also as the catalyst for the anhydride.The experimental details can also be find in reference [7]. Figure 1 exhibits the glass transition behavior of the formulated resin system.As shown, the curing degree increased with the curing temperature for a constant curing time of 3 h, as well as the glass transition temperature.The formulated matrix resin was finally designated as AGMP3600, with a bio-content of about 30%.  Figure 2 shows the typical viscosity behavior of the trial product AGMP3600.It behaved well in the film manufacturing and subsequent prepreg production.The process condition for the prepreg using AGMP3600 as the matrix in the autoclave is shown in Figure 3.
Aerospace 2018, 5, x FOR PEER REVIEW 2 of 9 reactions of epoxy/anhydride curing include the carboxyl group reaction with an epoxy ring in the catalyzed action of accelerator, alternate ring-opening copolymerization of epoxy, and anhydride catalyzed by amine and polyetherification at high temperatures, initiated by amine and catalyzed by hydroxyl groups [6].These processes occur when an amine is added as the catalyst.An excellent reference [6] on this issue can be found in the literature.
In this study [7,8], an anhydride-type epoxy curing agent, maleopimaric (Scheme 1), was synthesized as a hardener from rosin acid (Scheme 1).This was supplied by the Ningbo Institute of Materials and Technology Engineering.A resin mixed with an E51-type epoxy and a solid phenolic epoxy was then prepared as the main component, together with an accelerator mixed of two amino imidazole salts as the thermally latent curing agent and also as the catalyst for the anhydride.The experimental details can also be find in reference [7]. Figure 1 exhibits the glass transition behavior of the formulated resin system.As shown, the curing degree increased with the curing temperature for a constant curing time of 3 h, as well as the glass transition temperature.The formulated matrix resin was finally designated as AGMP3600, with a bio-content of about 30%.  Figure 2 shows the typical viscosity behavior of the trial product AGMP3600.It behaved well in the film manufacturing and subsequent prepreg production.The process condition for the prepreg using AGMP3600 as the matrix in the autoclave is shown in Figure 3.

HOOC
The mechanical properties of AGMP 3600 laminates reinforced with different kinds of fibers and weaves were determined and are listed in Tables 1-4 in some cases compared with the state-of-theart counterparts as reference.T/℃ DSC/(mW/mg) prepreg 120℃@3h 130℃@3h 140℃@3h 150℃@3h 130℃@3h for hot press Figure 2 shows the typical viscosity behavior of the trial product AGMP3600.It behaved well in the film manufacturing and subsequent prepreg production.The process condition for the prepreg using AGMP3600 as the matrix in the autoclave is shown in Figure 3.
The mechanical properties of AGMP 3600 laminates reinforced with different kinds of fibers and weaves were determined and are listed in Tables 1-4 in some cases compared with the state-of-the-art counterparts as reference.In parallel, a 180 °C/2 h cured-rosin epoxy was also developed, designated as AGMP5600, with a higher bio-content of about 40%.It is particularly interesting to note that the glass transition temperature of AGMP5600 matrix composite reinforced with EW250F glass weave was about 220 °C.Table 4 lists the mechanical properties of the laminated composite.The temperature-dependent mechanical properties were also studied, as well as those after the hot/wet exposure (1000 h/70 °C/85% r.H.)In parallel, a 180 °C/2 h cured-rosin epoxy was also developed, designated as AGMP5600, with a higher bio-content of about 40%.It is particularly interesting to note that the glass transition temperature of AGMP5600 matrix composite reinforced with EW250F glass weave was about 220 °C.Table 4 lists the mechanical properties of the laminated composite.The temperature-dependent mechanical properties were also studied, as well as those after the hot/wet exposure (1000 h/70 °C/85% r.H.)In parallel, a 180 • C/2 h cured-rosin epoxy was also developed, designated as AGMP5600, with a higher bio-content of about 40%.It is particularly interesting to note that the glass transition temperature of AGMP5600 matrix composite reinforced with EW250F glass weave was about 220 • C. Table 4 lists the mechanical properties of the laminated composite.The temperaturedependent mechanical properties were also studied, as well as those after the hot/wet exposure (1000 h/70 • C/85% r.H.)

Epoxy Resins Based on Itaconic Acid
Itaconic acid, which is also referred to as methylenesuccinic acid, is typically produced through the fermentation of carbohydrates such as glucose or starch using Aspergillus terreus.Given its strong capacity to replace petrochemicals in the chemical industry, it has been selected as one of the top 12 potential bio-based platform chemicals by the U.S. Department of Energy [9].To the best of our knowledge, it has been widely used in the production of styrene-butadiene-acrylonitrile and acrylate latex in the paper and coating sectors.
Epoxy (EP) resin derived from itaconic acid, designated in the paper as EIA, can be synthesized following the synthetic route shown in Scheme 2. To evaluate its properties, EIA and commercial DGEBA (diglycidyl ether of bisphenol A, epoxide equivalent weight of 182-192 g/eq.) were cured with methyl hexahydrophthalic anhydride (MHHPA), respectively.The results show that EIA presented higher epoxide (0.625) and higher reactivity values than DGEBA.Relative to DGEBA, the cured EIA showed comparable or higher tensile strength (87.5 MPa), elongation at break (7.1%), flexural strength (152.4MPa) and modulus (3430.8MPa), and glass transition temperature (Tg = 130 • C).In addition, after co-monomers such as divinyl benzene (DVB) and acrylated epoxidized soybean oil (AESO) were introduced into the EIA/MHHPA system, dual-curing systems were formed, and the rigidity and toughness could be manipulated further via the various contents of rigid DVB or soft AESO, as shown in Figure 4, compared with data from reference published [10].
strong capacity to replace petrochemicals in the chemical industry, it has been selected as one of the top 12 potential bio-based platform chemicals by the U.S. Department of Energy [9].To the best of our knowledge, it has been widely used in the production of styrene-butadiene-acrylonitrile and acrylate latex in the paper and coating sectors.
Epoxy (EP) resin derived from itaconic acid, designated in the paper as EIA, can be synthesized following the synthetic route shown in Scheme 2. To evaluate its properties, EIA and commercial DGEBA (diglycidyl ether of bisphenol A, epoxide equivalent weight of 182-192 g/eq.) were cured with methyl hexahydrophthalic anhydride (MHHPA), respectively.The results show that EIA presented higher epoxide (0.625) and higher reactivity values than DGEBA.Relative to DGEBA, the cured EIA showed comparable or higher tensile strength (87.5 MPa), elongation at break (7.1%), flexural strength (152.4MPa) and modulus (3430.8MPa), and glass transition temperature (Tg = 130 °C).In addition, after co-monomers such as divinyl benzene (DVB) and acrylated epoxidized soybean oil (AESO) were introduced into the EIA/MHHPA system, dual-curing systems were formed, and the rigidity and toughness could be manipulated further via the various contents of rigid DVB or soft AESO, as shown in Figure 4, compared with data from reference published [10].As shown in Scheme 3, direct reactions between itaconic acid and epichlorohydrin generate resin EIA, which is a mixture of different oligomers of varying molecular weights.To make the best use of the carboxyl groups and itaconic acid double bond, a trifunctional EP monomer (TEIA) was designed strong capacity to replace petrochemicals in the chemical industry, it has been selected as one of the top 12 potential bio-based platform chemicals by the U.S. Department of Energy [9].To the best of our knowledge, it has been widely used in the production of styrene-butadiene-acrylonitrile and acrylate latex in the paper and coating sectors.Epoxy (EP) resin derived from itaconic acid, designated in the paper as EIA, can be synthesized following the synthetic route shown in Scheme 2. To evaluate its properties, EIA and commercial DGEBA (diglycidyl ether of bisphenol A, epoxide equivalent weight of 182-192 g/eq.) were cured with methyl hexahydrophthalic anhydride (MHHPA), respectively.The results show that EIA presented higher epoxide (0.625) and higher reactivity values than DGEBA.Relative to DGEBA, the cured EIA showed comparable or higher tensile strength (87.5 MPa), elongation at break (7.1%), flexural strength (152.4MPa) and modulus (3430.8MPa), and glass transition temperature (Tg = 130 °C).In addition, after co-monomers such as divinyl benzene (DVB) and acrylated epoxidized soybean oil (AESO) were introduced into the EIA/MHHPA system, dual-curing systems were formed, and the rigidity and toughness could be manipulated further via the various contents of rigid DVB or soft AESO, as shown in Figure 4, compared with data from reference published [10].As shown in Scheme 3, direct reactions between itaconic acid and epichlorohydrin generate resin EIA, which is a mixture of different oligomers of varying molecular weights.To make the best use of the carboxyl groups and itaconic acid double bond, a trifunctional EP monomer (TEIA) was designed As shown in Scheme 3, direct reactions between itaconic acid and epichlorohydrin generate resin EIA, which is a mixture of different oligomers of varying molecular weights.To make the best use of the carboxyl groups and itaconic acid double bond, a trifunctional EP monomer (TEIA) was designed and synthesized (Figure 4), and it generated an extremely high epoxide value of 1.16 and a low viscosity of 0.92 Pa s at 25 • C. It is well known that low resin viscosities are beneficial for manufacturing.The lower viscosities in TEIA render it easier to process than DGEBA.In Table 5, the flexural properties of TEIA cured by different curing agents are listed.When flexible poly(propylene glycol) bis(2-aminopropyl ether) (D230) is used as the curing agent, the TEIA/D230 system shows higher flexural modulus, higher strain at break and comparable flexural strength to DGEBA/D230.When rigid MHHPA was employed as the curing agent, the flexural strength, modulus and elongation at break of the TEIA/MHHPA system exceeded those of the DGEBA/MHHPA system.These results denote that TEIA may be used as a high-performance epoxy resin [11].
Aerospace 2018, 5, x FOR PEER REVIEW 7 of 9 and synthesized (Figure 4), and it generated an extremely high epoxide value of 1.16 and a low viscosity of 0.92 Pa s at 25 °C.It is well known that low resin viscosities are beneficial for manufacturing.The lower viscosities in TEIA render it easier to process than DGEBA.In Table 5, the flexural properties of TEIA cured by different curing agents are listed.When flexible poly(propylene glycol) bis(2-aminopropyl ether) (D230) is used as the curing agent, the TEIA/D230 system shows higher flexural modulus, higher strain at break and comparable flexural strength to DGEBA/D230.When rigid MHHPA was employed as the curing agent, the flexural strength, modulus and elongation at break of the TEIA/MHHPA system exceeded those of the DGEBA/MHHPA system.These results denote that TEIA may be used as a high-performance epoxy resin [11].
Scheme 3. Synthesis of the trifunctional epoxy resin of itaconic acid (TEIA).Given the presence of double bonds in itaconic acid and the low flame retardancy of EP resins, a flame-resistant DOPO (6H-dibenz(C,E)(1,2)oxaphosphorin-6-oxide) was chemically incorporated into the itaconic-acid-based EP resin, and a phosphorus-containing EP resin (EADI) was obtained (Scheme 4).The property study showed that the cured EADI network presents a comparable glass transition temperature and mechanical properties to those of the DGEBA system.In addition, excellent flame retardancy capacities with the UL94 V-0 grade used during vertical burning tests were observed for the EADI system.EADI may be used as a bio-based candidate for EP flame retardancy [12].Scheme 4. Synthetic route of phosphorus-containing itaconic-acid-based epoxy resin.

Trial Manufacturing and Demonstration
With the promising properties, as one of the project targets, biocomposites were trialmanufactured to produce interior and quasi-structural parts for potential application in airplane and Scheme 3. Synthesis of the trifunctional epoxy resin of itaconic acid (TEIA).Given the presence of double bonds in itaconic acid and the low flame retardancy of EP resins, a flame-resistant DOPO (6H-dibenz(C,E)(1,2)oxaphosphorin-6-oxide) was chemically incorporated into the itaconic-acid-based EP resin, and a phosphorus-containing EP resin (EADI) was obtained (Scheme 4).The property study showed that the cured EADI network presents a comparable glass transition temperature and mechanical properties to those of the DGEBA system.In addition, excellent flame retardancy capacities with the UL94 V-0 grade used during vertical burning tests were observed for the EADI system.EADI may be used as a bio-based candidate for EP flame retardancy [12].and synthesized (Figure 4), and it generated an extremely high epoxide value of 1.16 and a low viscosity of 0.92 Pa s at 25 °C.It is well known that low resin viscosities are beneficial for manufacturing.The lower viscosities in TEIA render it easier to process than DGEBA.In Table 5, the flexural properties of TEIA cured by different curing agents are listed.When flexible poly(propylene glycol) bis(2-aminopropyl ether) (D230) is used as the curing agent, the TEIA/D230 system shows higher flexural modulus, higher strain at break and comparable flexural strength to DGEBA/D230.When rigid MHHPA was employed as the curing agent, the flexural strength, modulus and elongation at break of the TEIA/MHHPA system exceeded those of the DGEBA/MHHPA system.These results denote that TEIA may be used as a high-performance epoxy resin [11].
Scheme 3. Synthesis of the trifunctional epoxy resin of itaconic acid (TEIA).Given the presence of double bonds in itaconic acid and the low flame retardancy of EP resins, a flame-resistant DOPO (6H-dibenz(C,E)(1,2)oxaphosphorin-6-oxide) was chemically incorporated into the itaconic-acid-based EP resin, and a phosphorus-containing EP resin (EADI) was obtained (Scheme 4).The property study showed that the cured EADI network presents a comparable glass transition temperature and mechanical properties to those of the DGEBA system.In addition, excellent flame retardancy capacities with the UL94 V-0 grade used during vertical burning tests were observed for the EADI system.EADI may be used as a bio-based candidate for EP flame retardancy [12].

Trial Manufacturing and Demonstration
With the promising properties, as one of the project targets, biocomposites were trialmanufactured to produce interior and quasi-structural parts for potential application in airplane and Scheme 4. Synthetic route of phosphorus-containing itaconic-acid-based epoxy resin.

Trial Manufacturing and Demonstration
With the promising properties, as one of the project targets, biocomposites were trial-manufactured to produce interior and quasi-structural parts for potential application in airplane and ground transportation vehicles.Figure 5 is an illustration of an interior side panel made of AGMP3600, the rosin-sourced EP, with a honeycomb sandwich core for a MA600 airplane.The side panel was manufactured using AGMP3600 prepregs in an autoclave.The composite panels are strong, lightweight, fire resistant, decorative, and impervious to mold and insects.Structure-decoration integration methods can clearly support to the production of identical or even more complex composite parts while simultaneously presenting mechanical and structural damping advantages in various applications.
Aerospace 2018, 5, x FOR PEER REVIEW 8 of 9 ground transportation vehicles.Figure 5 is an illustration of an interior side panel made of AGMP3600, the rosin-sourced EP, with a honeycomb sandwich core for a MA600 airplane.The side panel was manufactured using AGMP3600 prepregs in an autoclave.The composite panels are strong, lightweight, fire resistant, decorative, and impervious to mold and insects.Structure-decoration integration methods can clearly support to the production of identical or even more complex composite parts while simultaneously presenting mechanical and structural damping advantages in various applications.Figure 6 shows an electric race car in cooperation with Tsinghua University, China.In this case, the rosin-sourced epoxy composite was used to produce the carbon composite body with a honeycomb core.The process also used AGMP3600 prepreg.The strong conforming capacities of the materials along the curved contour were well-demonstrated.A manufacturing benefit of the biocomposites cured in an autoclave pertains to their full compatibility with the standard industrial production processes.

Conclusions
1. Rosin-sourced anhydride was developed and used as a hardener for epoxy to formulate a matrix resin with an imidazole-type latent catalyst for biocomposites.The mechanical properties of the biocomposites with the rosin-epoxy as matrix resins were tested, also under hydrothermal conditions.It was shown that the mechanical properties were generally comparable to the stateof-the-art, petroleum-sourced counterpart materials, but yielded a higher glass transition temperature.2. Epoxy resin derived from itaconic acid was also synthesized.It showed comparable or higher mechanical properties and glass transition temperatures compared to a common counterpart.A Figure 6 shows an electric race car in cooperation with Tsinghua University, China.In this case, the rosin-sourced epoxy composite was used to produce the carbon composite body with a honeycomb core.The process also used AGMP3600 prepreg.The strong conforming capacities of the materials along the curved contour were well-demonstrated.A manufacturing benefit of the biocomposites cured in an autoclave pertains to their full compatibility with the standard industrial production processes.ground transportation vehicles.Figure 5 is an illustration of an interior side panel made of AGMP3600, the rosin-sourced EP, with a honeycomb sandwich core for a MA600 airplane.The side panel was manufactured using AGMP3600 prepregs in an autoclave.The composite panels are strong, lightweight, fire resistant, decorative, and impervious to mold and insects.Structure-decoration integration methods can clearly support to the production of identical or even more complex composite parts while simultaneously presenting mechanical and structural damping advantages in various applications.Figure 6 shows an electric race car in cooperation with Tsinghua University, China.In this case, the rosin-sourced epoxy composite was used to produce the carbon composite body with a honeycomb core.The process also used AGMP3600 prepreg.The strong conforming capacities of the materials along the curved contour were well-demonstrated.A manufacturing benefit of the biocomposites cured in an autoclave pertains to their full compatibility with the standard industrial production processes.

Conclusions
1. Rosin-sourced anhydride was developed and used as a hardener for epoxy to formulate a matrix resin with an imidazole-type latent catalyst for biocomposites.The mechanical properties of the biocomposites with the rosin-epoxy as matrix resins were tested, also under hydrothermal conditions.It was shown that the mechanical properties were generally comparable to the stateof-the-art, petroleum-sourced counterpart materials, but yielded a higher glass transition temperature.2. Epoxy resin derived from itaconic acid was also synthesized.It showed comparable or higher mechanical properties and glass transition temperatures compared to a common counterpart.A Figure 6.Electric race car, its body was manufactured using AGMP3600/honeycomb sandwich composites.

1.
Rosin-sourced anhydride was developed and used as a hardener for epoxy to formulate a matrix resin with an imidazole-type latent catalyst for biocomposites.The mechanical properties of the biocomposites with the rosin-epoxy as matrix resins were tested, also under hydrothermal conditions.It was shown that the mechanical properties were generally comparable to the state-of-the-art, petroleum-sourced counterpart materials, but yielded a higher glass transition temperature.

2.
Epoxy resin derived from itaconic acid was also synthesized.It showed comparable or higher mechanical properties and glass transition temperatures compared to a common counterpart.A phosphorus-containing epoxy was also developed by incorporating DOPO into the itaconic acid EP to formulate a flame-retardant resin system.

3.
Using the rosin epoxy system, which is technologically more mature than the itaconic system, quasi-structural plant fiber reinforced components were manufactured and demonstrated for aircraft and ground transportation vehicles.The process condition was found to be fully compatible with standard industrial processes.

Figure 1 .
Figure 1.Differential Scanning Calorimetry (DSC) curves of the rosin-sourced epoxy resin system designated as AGMP3600 at different temperature conditions.

Figure 1 .
Figure 1.Differential Scanning Calorimetry (DSC) curves of the rosin-sourced epoxy resin system designated as AGMP3600 at different temperature conditions.

Figure 1 .
Figure 1.Differential Scanning Calorimetry (DSC) curves of the rosin-sourced epoxy resin system designated as AGMP3600 at different temperature conditions.

Figure 3 .
Figure 3. Process parameters for the curing of AGMP3600 prepreg in an autoclave.

Figure 3 .
Figure 3. Process parameters for the curing of AGMP3600 prepreg in an autoclave.

Figure 3 .
Figure 3. Process parameters for the curing of AGMP3600 prepreg in an autoclave.

Figure 4 .
Figure 4. Mechanical properties of the cured EP resins.EIA0 and DGEBA refer to the cured samples without comonomers; D and A denote the DVB (divinyl benzene) and AESO (acrylated epoxidized soybean oil) co-monomers, respectively.

Figure 4 .
Figure 4. Mechanical properties of the cured EP resins.EIA0 and DGEBA refer to the cured samples without comonomers; D and A denote the DVB (divinyl benzene) and AESO (acrylated epoxidized soybean oil) co-monomers, respectively.

Figure 4 .
Figure 4. Mechanical properties of the cured EP resins.EIA0 and DGEBA refer to the cured samples without comonomers; D and A denote the DVB (divinyl benzene) and AESO (acrylated epoxidized soybean oil) co-monomers, respectively.

Figure 6 .
Figure 6.Electric race car, its body was manufactured using AGMP3600/honeycomb sandwich composites.

Figure 6 .
Figure 6.Electric race car, its body was manufactured using AGMP3600/honeycomb sandwich composites.
1 A commercial product.

Table 5 .
Flexural properties of cured EP resins with D230 curing agents.

Table 5 .
Flexural properties of cured EP resins with D230 curing agents.

Table 5 .
Flexural properties of cured EP resins with D230 curing agents.