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. 2025 Mar 5;17(5):695.
doi: 10.3390/polym17050695.

Self-Toughened Epoxy Resin via Hybridization of Structural Isomeric Curing Agents

Woong Kwon  1 , Jiyeon Cheon  1 , Hei Je Jeong  2 , Jong Sung Won  3 , Byeong-Joo Kim  3 , Man Young Lee  3 , Seung Geol Lee  2 , Euigyung Jeong  1
Affiliations

Self-Toughened Epoxy Resin via Hybridization of Structural Isomeric Curing Agents

Woong Kwon et al. Polymers (Basel). .

Abstract

Fracture toughness is a key property of epoxy resins with a high glass transition temperature (Tg), used in carbon fiber/epoxy composites for aerospace applications. Conventional toughening methods rely on adding toughening agents, often compromising the processibility and thermal stability. This study introduces a simple self-toughening approach that enhances the fracture toughness without sacrificing other properties by controlling the cured epoxy network structure. Tetraglycidyl 4,4'-diaminodiphenylmethane (TGDDM) epoxy resin was cured using mixtures of structural isomeric curing agents, 3,3'- and 4,4'-diaminodiphenyl sulfone (3,3'- and 4,4'-DDS), at ratios of 7:3, 5:5, and 3:7. The optimal 7:3 ratio produced a resin with 30% higher fracture toughness compared to TGDDM/3,3'-DDS and 100% higher than the TGDDM/4,4'-DDS system. The Tg of the self-toughened resin ranged from 241 to 266 °C, which was intermediate between the Tg values of the TGDDM/3,3'-DDS and TGDDM/4,4'-DDS systems. This improvement is attributed to the higher crosslink density and reduced free volume of the epoxy network. These findings demonstrate that simply mixing isomeric curing agents enables self-toughening, providing a practical and efficient strategy to enhance the performance of high-Tg epoxy resins in advanced composite applications.

Keywords: curing agent; diaminodiphenyl sulfone; epoxy resin; fracture toughness; structural isomers.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
Heat flow from isothermal DSC of (a) m-TGDDM, (b) 7m,3p-TGDDM, (c) 5m,5p-TGDDM, (d) 3m,7p-TGDDM, and (e) p-TGDDM.
Figure 2
Figure 2
Degree of conversion as a function of time for the prepared epoxy samples at (a) 160, (b) 180, (c) 200, and (d) 220 °C.
Figure 3
Figure 3
Fitted activation energy values obtained using the (a) Kissinger method and (b) Flynn–Wall–Ozawa method (α = 0.9).
Figure 4
Figure 4
Mechanical properties of the prepared epoxy samples: (a) tensile strength, (b) tensile modulus, and (c) fracture toughness.
Figure 5
Figure 5
(a) Storage modulus and (b) tan δ curves of the prepared epoxy samples.
Figure 6
Figure 6
Fracture surfaces of the prepared epoxy samples: (a) m-TGDDM, (b) 7m,3p-TGDDM, (c) 5m,5p-TGDDM, (d) 3m,7p-TGDDM, and (e) p-TGDDM.
Figure 7
Figure 7
(a) Methanol uptake curves and (b) uptake data of various solvents for the prepared epoxy samples.
Figure 8
Figure 8
Schematic representation of crosslink structure growth in epoxy resins based on different types of curing agents: (a) m-TGDDM, (b) m,p-TGDDM, and (c) p-TGDDM. The blue dot with a blue line and the red dot with a red line represents m-TGDDM and p-TGDDM, respectively.
See this image and copyright information in PMC

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