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Highly enhanced performance of epoxy composites via novel phthalazinone-bearing hybrid system as matrix
Composites Part A 131 (2020) 105772
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Composites Part A journal homepage: www.elsevier.com/locate/compositesa
Highly enhanced performance of epoxy composites via novel phthalazinonebearing hybrid system as matrix
T
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Fengfeng Zhanga, Lishuai Zonga,b, Zhihuan Wenga,b, Feng Baoa, Nan Lia,b, Jinyan Wanga,b, , Xigao Jiana,b a b
Department of Polymer Science & Materials, Dalian University of Technology, 116024, China Liaoning Province Engineering Technology Center of High Performance Resins, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phthalazinone Composites Fibers Adhesion
A novel phthalazinone-bearing epoxy resin, namely TEPZ, was designed and synthesized as functional modifier and prepreg agent for carbon fiber TGDDM epoxy composites by a “one pot two steps” method. Three-dimensional TEPZ/TGDDM/DDS system was constructed and optimized by modulating curing procedure and concentration of DDS to give thermosets with excellent thermal resistance and mechanical strength. The storage modulus and glass transition temperature (Tg) increased significantly with composition of 100 phr TGDDM, 30 phr TEPZ and 40 phr DDS as compared to those of TGDDM/DDS. The tensile strength, impact strength and flexural strength increased simultaneously. Afterwards, continuous unidirectional carbon fiber reinforced laminates were fabricated, followed by systematical investigation of their mechanical and interfacial properties. The interfacial study indicated that the polarity and wettability of TEPZ played an important role during the modification.
1. Introduction Rapid development of aerospace high-tech fields offers additional challenges to thermosetting matrix composites, which should be qualified for processing adequate thermal-resistance and modulus to meet the requirements of high performance engineering. Epoxy resin is a versatile class of thermosets with series of fascinating properties, such as high stiffness, chemical resistances, thermal and dimensional stability, reasonable manufacturability and low shrinkage during process [1]. However, the comprehensive properties above are closely related with the structure of the resins as well as their corresponding thermosets. For example, higher crosslink density of the thermoset will promote their modulus and thermal resistance without sacrificing other fundamental properties [2]. As a result, the exploration of multifunctional (trifunctional or tetra functional) epoxides, which contributes effectively for increasing crosslink density of thermosets, evolves as one of the most attractive strategies towards the target of high-tech aerospace applications [3]. As typical representative of multifunctional epoxides, N, N, N′, N′tetraglycidyl-4, 4′-diaminodiphenyl methane, namely TGDDM, has revealed superior performances as the aerospace matrix for fiber reinforced composites since 1970s. Considerable efforts have been
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dedicated on investigation of matrix modification and interface properties of continuous carbon fiber reinforced composites. (i): Matrix modification could bring about improvements including heat resistance, flame retardant, and mechanical strength. Jena et al. [4] blended tetra-functional epoxy resin (TGDDM) with diallyl bisphenol A (DBA) and bismaleimide (BMI) for rigid risers with serving temperature more than 280 °C. Zhou et al. [5] displayed several multifunctional epoxy-anhydride systems in terms of their glass transition temperatures and thermal stability. Besides physical blend and chemical modification methods, introduction of novel curing agent for TGDDM resins are also efficient strategy to improve their thermal resistance and strength, as well as their flame retardant, transparency [6–9]. However, previous works usually concentrate attention of optimization inherent properties of TGDDM/DDS systems via shifting the curing procedure and the amount of amine. Few architecture diversities of epoxy monomers have been directly explored with comparable properties as TGDDM from a perspective of molecular structure, as shown in Fig. 1 [18–21]. Their Tgs are always ranging from 218 to 280 °C and storage modulus generally varies between 2000 and 4000 MPa. Unfortunately, such networks exhibit elevated storage modulus while sacrificing Tg, and vice versa [9–17], which is worthy for thorough study. (ii): Study of the interfacial behaviors of continuous fiber reinforced composites, such as
Corresponding author. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.compositesa.2020.105772 Received 27 June 2019; Received in revised form 25 December 2019; Accepted 12 January 2020 Available online 13 January 2020 1359-835X/ © 2020 Published by Elsevier Ltd.
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composites that is seldom explored. 2. Experimental details 2.1. Materials 4, 4′-tetradiglycidyl diaminodi-phenyl methane (TGDDM, 75%) and 4, 4′-diaminodiphenyl sulfone (DDS, 99.7%) are all obtained from Aladdin Chemical Reagent Co. Ltd, and dried in vacuum prior to use. 1, 2-dihydro-2-(4-aminophenyl)-4-[4-(4-aminophenoxy) phenyl] (2H) phthalazin-1-one (DA-DHPZ, 99.8%) is supplied by Dalian Polymer New Material Company (China). The commercially available, presoaked carbon fibers T700SC-12K-50C are purchased from Toray Industries Inc. Epichlorohydrin (ECl, 99.5%), anhydrous sodium sulfate (Na2SO4, 99.8%), toluene (99.7%) and acetone (99.8%) are purchased from Tianjin Fuyu Fine Chemical Factory (China). Ethanol (99.9%), chloroform (99.7%) and other organic solvents are all purchased from Tianjin Damao Chemical Reagent Factory (China). 2.2. Synthesis of the TEPZ The details of the TEPZ synthesis process were depicted in the Fig. 2. DA-DHPZ (6.3 g, 0.015 mol) and ECl (41.63 g 0.45 mol) were mixed in a 500 mL flask equipped with magnetic stirring under nitrogen atmosphere at 85 °C. After refluxing for 6 h, 10 wt% aqueous sodium hydroxide solution (NaOH aq.) was added dropwise within 30 min then maintained the temperature for 5 h. After cooling, the mixture was extracted with toluene and washed with deionized water for several times until the pH of the solution reached neutral. Then the solvent of organic phase was removed under reduced pressure to get yellow solid powder (Yield: 74–85%; HPLC Purity: 83.5%). The 1H NMR, FTIR, HPLC, LC-MS and HRMS spectra details for TEPZ and TGDDM are in the Supplementary Data. Fig. 1. The structure reported by other researchers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. Preparation of composites The composition of matrix was optimized via following procedure: the mixture of TGDDM and 0–100 phr TEPZ was continuously stirred at 110 °C until homogeneous. Then, the curing agent DDS was added with weight amount relative to the mixture, and the equivalent weight ratios of mixture to DDS were 10:3, 10:4, 10:5 (10:5 was approximately stoichiometric equivalent ratio). The bubbles were removed under vacuum for 30 min by oil pump. After that, the mixture was poured into a preheated mold, then was immediately transferred into the air-circulating oven, followed by step curing process including 130 °C for 1 h, 150 °C for 1 h, 180 °C for 4 h. The carbon fiber composites were prepared by following process: the obtained optimized TEPZ/TGDDM/DDS system was dissolved in DMF for presoaking T700 carbon fiber, then the presoaking T700 carbon fiber was winded continuously to the frame to afford unidirectional prepregs. The prepregs were dried at 80 °C for 2 h, 100 °C for 1 h, 130 °C for 1.5 h in the vacuum oven to accelerate the possible reaction between the original carbon fiber and TEPZ. Fourteen tailored prepregs were compacted into a steel mold with a pressure of 1.5 MPa at 180 °C for 1 h and 2.5 MPa at 200 °C for 0.5 h. After the steel mold was removed, the post-curing procedure of laminate composite was 180 °C for 1 h, 200 °C for 1 h, 230 °C for 1 h. The fiber volume contents in the solidified laminates were controlled to be 60–67% after limited resin bleeding, which were calculated by ASTM D3171-15 standard. The obtained laminate composites should be sawed and sanded into 80 × 10 × 2 mm3 size and 20 × 10 × 2 mm3 size for flexural strength and inter-laminar shear strength tests, respectively.
physical interaction, chemical bonding, and morphological changes occurring in fiber could result in increased performance while served as structural materials. Conventional modifications like diamine or acid treatment, polymer grafting, surfactants treatment and physical mixture of carbon fibers have been most commonly utilized [22] for enhancing their interfacial adhesion. Such modifications could be realized by those emerging technologies, including electrophoretic deposition [23], atmospheric pressure plasma [22] and 3D printing [24], resulting in significant promotion of their comprehensive properties. However, some of the strategies mentioned above involve tedious process and higher cost, leading to lag of practical industrial application rather than laboratory research. Therefore, we describe a particularly simple, convenient and effective approach for enhancing interaction between fiber and matrix. Notably, there are only few attempts focused on evaluating the effects of polarity and wettability between reinforcement and matrix on the mechanical properties of carbon fiber reinforced epoxy composites. This paper will integrate both of matrix modification and interfacial property study into the system to realize synergetic effect of chemical and physical interaction between matrix and fibers, finally achieving wettability and polarity matching. Inspired with the conception, we incorporate polar structure, unsymmetrical and kink non-coplanar phthalazinone, into epoxy resin architecture as functional modifiers for TGDDM/DDS system, and also as a prepreg agent for carbon fiber to improve its wettability for epoxy matrix. The resin and composite system possess excellent thermal resistance along with mechanical properties after thermal cure. This work may provide a new perspective for the development of the manufacture for multi-functional epoxy
2.4. Characterization methods The chemical structures of synthesized epoxy monomer were 2
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Fig. 2. Synthesis routes of TEPZ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
determined by 1H NMR in CDCl3 on a Bruker AVANCE III 500 spectrometer, and the FT-IR spectrum that was collected by a Thermo Nicolet Nexus 470 Fourier transform infrared (FT-IR) spectrometer. Our product and the TGDDM were measured by High performance liquid chromatography (HPLC) and Liquid Chromatography-Mass Spectrometry (LC-MS) for purity analysis, which were performed on an Alliance 2695–2696 instrument and Agilent 1100 instrument, respectively. High resolution mass spectrometry (HRMS) was performed on Thermo Scientific LTQ-Orbitrap XL. Thermo gravimetric analysis (TGA) that were performed on the METTLER TGA SDTA851 analyzer were employed to evaluate the thermal stability of the cured products from 30 to 800 °C with the heating rate of 20 °C/min under continuous nitrogen flow (50 mL min−1). The step curing procedure was confirmed by Differential Scanning Calorimetry (DSC) using a Mettler DSC STAR System instrument over a temperature range of 25 to 300 °C under nitrogen atmosphere. Dynamic mechanical analysis (DMA) measurements were examined using a METTLER DMA SDTA8610e instrument at 1 Hz and a heating rate of 5 °C/min from 25 °C to 300 °C under air atmosphere with the sample dimension of 25 × 6 × 3 mm3. Rheological behavior was investigated by a TA AR2000 instrument at a frequency of under a strain of 0.02 N, 1 Hz. Elemental composition of the presoaking fibers was analyzed by Xray photoelectron spectroscopy (XPS, ESCALAB 220i-XL, VG, UK) equipped with a monochromated Al Kα source (1486.6 eV) at a base pressure of 2 × 10−9 mbar. The resin content of cured materials based on the ASTM D3171-15 standard. Annex A7 procedure was used to digest the matrices of sample containing carbon fiber at 450 °C for 6 h under ambient
conditions, and a paralleled pure epoxy sample was heated simultaneously for calculating the original mass of the carbon fiber in the composite. The tensile strength measurement set (AI-7000M-2) was used to measure the tensile properties of the composite sample according to the ASTM D638-96 standard. The sample was dumbbell-shaped with the dimensions of 115 mm (length) × 4 mm (thickness). A Charpy impact strength measurement set (XCJ-4) was performed to measure the impact property of the cured samples according to the Chinese standard GB/T 1843-2008. The specimen dimensions were 80 × 10 × 4 mm3 without notch. The flexural strength and interlaminar shear strength were determined using an Instron-5869 machine with a capacity of 500 N on T700 composite samples according to ASTM D790-10 and ISO 14130 standards, respectively. The brittle fracture surfaces morphology of composites were observed by SEM (FEI QUANTA 450). The samples were coated with a thin conductive gold layer before the SEM observation in order to capture a stable and clear image.
3. Results and discussion 3.1. Rheology analysis The complex viscosity (η*) of epoxy/DDS systems as a function of temperature was a vital parameter in evaluating their processability. As shown in the Fig. 3(a), (b) and Table 1, the TEPZ/TGDDM/DDS systems exhibited excellent processability with processing window (η* < 10 Pa.s) ranging from 67 to 140 °C. It became gradually narrower as the increasing loading of TEPZ in system mainly due to the high 3
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Fig. 3. The rheological behavior of TEPZ/TGDDM/ DDS hybrids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 The process temperature windows (η* < 10 Pa·s) of epoxy/DDS systems.
Table 2 The data of thermo mechanical properties for all the composite samples.
Samples
Temperature range/ °C (η* < 10 Pa·s)
Samples
Er/MPa
Ve/mol·m−3
E0/MPa
Tr/K
0 phr TEPZ/TGDDM/40 phr DDS 10 phr TEPZ/TGDDM/40 phr DDS 20 phr TEPZ/TGDDM/40 phr DDS 30 phr TEPZ/TGDDM/40 phr DDS 50 phr TEPZ/TGDDM/40 phr DDS 100 phr TEPZ/TGDDM/40 phr DDS TEPZ/40 phr DDS
58–198 61–200 62–200 71–198 78–202 97–200 138–205
TGDDM/30 phrDDS TGDDM/10 phrTEPZ/30 phrDDS TGDDM/20 phrTEPZ/30 phrDDS TGDDM/30 phrTEPZ/30 phrDDS TGDDM/50 phrTEPZ/30 phrDDS TGDDM/100 phrTEPZ/30 phrDDS TGDDM/40 phrDDS TGDDM/10 phrTEPZ/40 phrDDS TGDDM/20 phrTEPZ/40 phrDDS TGDDM/30 phrTEPZ/40 phrDDS TGDDM/50 phrTEPZ/40 phrDDS TGDDM/100 phrTEPZ/40 phrDDS TGDDM/50 phrDDS TGDDM/10 phrTEPZ/50 phrDDS TGDDM/20 phrTEPZ/50 phrDDS TGDDM/30 phrTEPZ/50 phrDDS TGDDM/50 phrTEPZ/50 phrDDS TGDDM/100 phrTEPZ/50 phrDDS
19.78 21.88 23.02 18.88 19.73 22.38 20.90 22.55 21.43 24.76 16.94 23.32 21.03 19.08 14.89 15.40 13.82 17.07
1522 1560 1706 1321 1357 1566 1602 1742 1615 1700 1170 1609 1653 1520 1170 1083 957 1174
4797 4494 4275 4064 4026 3588 4052 4170 3820 4600 3854 3677 4343 3965 3600 3652 4022 3060
521 562 541 573 583 573 523 519 532 584 580 581 510 503 512 570 579 583
stiffness of phthalazinone units. The viscosity for TEPZ/DDS system was significantly higher than that of TGDDM/DDS system in same temperature range, also ascribed to the kink non-coplanar rigid aromatic heterocyclic structure in TEPZ. When the shearing force was applied on TEPZ/DDS system, the interpenetration and entanglement imposed by the rigid aromatic heterocyclic structure significantly hindered the molecular motion between crosslinked network, resulting in higher viscosity of TEPZ/DDS than that of TGDDM/DDS. That meant it was difficult to obtain void-free TEPZ/DDS sample only with powder compaction technology.
Er: the storage modulus at (Tg + 30 °C); E0: the storage modulus at room temperature; Tr = (Tg + 30)/K.
directly related to a more rigid and dense crosslinking network formed during the cure procedure. So we calculated the crosslinking density by the rubber elasticity theory with the Eq.: Ve = Er/3RTr. The results were shown in the Table 2. In details, when the samples blended with 30 or 40 phr DDS, the Ve was increased with the incorporation of TEPZ then declined compared with those of TGDDM/DDS systems. However, when the samples with 50 phr DDS, the Ve values decreased with the
3.2. Mechanical parameters of TEPZ/TGDDM/DDS systems DMA had been extensively employed for characterizing the dynamic behaviors of the polymer chain segments which greatly related to the modulus and internal friction of the composites. As shown in Fig. 4(a), the storage modulus dropped down for certain degree when with 30 phr or 50 phr DDS loading. The higher storage modulus for samples usually
Fig. 4. DMA results of TEPZ/TGDDM/DDS hybrids. (a) The storage modulus of the TEPZ/TGDDM/DDS hybrids. (b) The Tg of the TEPZ/TGDDM/DDS hybrids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 4
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Table 3 The tensile behaviors of the composite samples. Samples
Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strain %
TGDDM/40 phr DDS TGDDM/10 phr TEPZ/40 phr DDS TGDDM/20 phr TEPZ/40 phr DDS TGDDM/30 phr TEPZ/40 phr DDS TGDDM/50 phr TEPZ/40 phr DDS TGDDM/100 phr TEPZ/40 phr DDS
37.60 39.41 58.97 54.32 42.11 49.47
3.21 3.37 3.50 3.64 3.71 3.48
1.23 1.33 1.85 1.62 1.05 1.55
± ± ± ± ± ±
5.1 3.9 6.5 5.0 5.9 2.1
± ± ± ± ± ±
0.19 0.17 0.10 0.06 0.33 0.14
± ± ± ± ± ±
0.32 0.24 0.24 0.17 0.18 0.11
Table 4 The mechanical parameters of TEPZ/TGDDM/DDS systems. Samples
Storage modulus GPa
Tg (tanδ)°C
Impact strength KJ·m−2
Flexural Strength MPa
30 phr TEPZ/TGDDM/40 phr DDS TGDDM/0 phr TEPZ/40 phr DDS
4.6 ± 0.1 3.6 ± 0.2
281 ± 6 215 ± 7
21.2 ± 2.2 14.5 ± 2.6
115 ± 4.1 81 ± 5.5
Fig. 5. The dielectric properties and SEM images of cured hybrids. (a) The dielectric loss of samples at 30 and 120 °C. (b) The dielectric constant of samples at 30 and 120 °C. (c) The SEM images of cured hybrids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The XPS spectra of unpresoaked carbon fiber and the survey spectrum. (a) The C1s spectra of unpresoaked carbon fiber. (b) The O1s spectra of unpresoaked carbon fiber. (c) The survey spectra of the presoaked and unpresoaked carbon fiber. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
adding of TEPZ. At first, all the results revealed that the loading amount of the TEPZ within appropriate range could effectively promote crosslinking density of the forming networks, owing to the comparatively high curing efficiency with a better covalent crosslinking network. Secondly, in the system with the least amount of curing agent (30 phr), the crosslinking density firstly increased which was due to the chemical bonds between the composite epoxy and DDS, and then decreased due to the incomplete curing with the least amount of DDS. Finally, with the dosage of 50 phr, the curing reaction between the epoxy group and amidogen was restricted by the steric hindrance of phthalazinone group, leading to decrease in mobility of the polymer chains, and even some curing agent still left intact as plasticizing agent, thus the cured
resultant obtained a relatively low crosslinking density. Obviously, the increase of Ve corresponding to the denser structure would stem from the higher storage modulus. In addition, it was notable that when the samples contained 100 phr TEPZ, the Ve values always increased in a certain extent, which may be related to the stronger van der Waals force existed in these systems. So we speculated that the physical crosslinking (originated from the van der Waals force) and covalent crosslinking were the two main factors affecting the storage modulus. As shown in Fig. 4(b), the Tg of the networks showed an overall increasing trend as the increase loading of TEPZ. This phenomenon could be ascribed to the higher crosslinking density and the rigid phthalazinone structure in TEPZ. Generally, the decreased crosslinking 5
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Fig. 7. The XPS spectra of presoaked carbon fiber. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Schematic of composite interface between carbon fiber and epoxy.
What's more, the rigidity of polymer chains was enhanced by the incorporation of phthalazinone group. The tight and rigid crosslinking networks was the key to achieving higher storage modulus and Tg simultaneously. The effect of the TEPZ on the tensile properties of the composites sample was studied by tensile test. The tensile strength, tensile modulus and tensile strain were listed in Table 3. It could be observed most of the composite samples showed higher tensile strength and tensile
density should result in the decline of Tg. But as shown in Table 2, the Tg of samples shown the increasing trend as containing more TEPZ. Essentially, the TEPZ epoxy monomer had the special twisted non coplanar structure, which was different from the comparatively linear structure of TGDDM. Hence, under the condition of appropriate amount of DDS, the TEPZ bearing phthalazinone group with steric hindrance and increased functionality shortened the average chain length between neighboring crosslinking points leading to tight crosslinking networks. 6
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volumes to the system. Hence, the observed reduction of dielectric loss was consistent with DMA study. Additionally, the SEM image also gave evidence for tight polymer networks in Fig. 5(c). No separated phase could be monitored. That meant polymer networks that was originated from high compatibility of TGDDM and TEPZ epoxy contributing efficiently to energy dissipation, which was benefit for promotion of their mechanical properties [26]. Eventually, TEPZ (30 phr)/TGDDM (100 phr)/DDS (40 phr) system was selected as a more promising matrix for further investigation in this work. 3.3. Surface chemical compositions of carbon fiber XPS analysis was conducted to help quantitative comparison for determining the surface chemical composition of the presoaked and unpresoaked carbon fiber. The signal assignment and their corresponding proportion were listed in Fig. 7. The results exhibited clearly that the unpresoaked carbon fiber surface contained mainly carbon and oxygen elements and the presence of hydroxyl, carbonyl, and carboxyl groups were related to hydrophilic surface of unpresoaked carbon fiber, as shown in Fig. 6(a and b). Correspondingly, due to functionalization of the unpresoaked carbon fiber with epoxy resin (TGDDM or TEPZ), an obvious increase on O/C ratios of presoaked specimens could be found in Fig. 7(a, b and h). On the other hand, Fig. 6(c) also displayed that more oxygen-containing functional groups on the presoaked fiber was indicative of successful modification of carbon fiber. When with both TEPZ and TGDDM as the prepreg agent, the resultant synergism derived from the wettability of polar groups in TEPZ made the resin bond with carbon fiber firmly, enabling for more effective inter-diffusion between fiber and prepreg agent which was primarily responsible for interfacial modification. In addition, little amount of NeC]O bonding presented in Fig. 7 (e, f, j and k) that possibly rooted from the epoxy or the reaction between fiber and matrix was found in all presoaked specimens after presoaking. The possible interaction mechanism was depicted in Fig. 8.
Fig. 9. The Flexural strength and ILSS strength of the fiber composites.
modulus than pure TGDDM due to the introduction of TEPZ. When 20 phr TEPZ were added into the TGDDM/DDS system, tensile strength and elongation at failure of TGDDM/20 phrTEPZ/40 phrDDS increased to 58.97 MPa and 1.85%, respectively. Obviously, the formation of homogeneous crosslinking network simultaneously exhibited positive effects on the tensile properties of the composite samples, which was mainly benefited from chemical crosslinks between epoxy and cure agent. Besides, the special structure of TEPZ could offer more physical crosslinking, such as van der Waals force and polymer chain entanglement, which played a crucial role for the improvement. In other words, the physical interactions could compensate the deterioration of the tensile properties caused by lower crosslinking density (Table 2). Finally, the tensile modulus of composite samples was enhanced evidently after the incorporation of 50 phr TEPZ. In addition, the impact strength and flexural strength shown in Table 4 was simultaneously increased by 31.6% and 41.9%, respectively. Such enhancement may be ascribed to the interpenetrating networks imposed by TEPZ and TGDDM that formed during thermal cure procedure. In order to further verify the speculation, dielectric properties and morphological study of cured products were carried out. In Fig. 5(a), the presence of polar moieties in TEPZ resulted in the slightly increase of dielectric constants. The dielectric loss properties of the networks were shown in Fig. 5(b). TEPZ-containing system exhibited a lower dielectric loss. Based on the published work [25], dielectric loss could be effectively lowered down by incorporation of low polarizability elements and improvement of the free volume of polymer networks. In TEPZ/TGDDM/DDS system, bulky phthalazinone structure disrupted the close packing of polymer chains and endowed high free
3.4. Interfacial enhancement mechanism Fig. 8 depicted a possible correlation between fiber and epoxy prepreg agents. Pisanova [27] proposed that the polymer carrying polar groups would provide an intensive adhesion strength by acid–base interactions. In other words, the polymer or prepreg agents had ability to act as an electron donor or electron acceptor to interact with epoxy or carboxyl group on carbon fibers. In CF-TEPZ/TGDDM/DDS systems, the acid-base interactions happened after wetting contact between fibers and prepreg agents was displayed in Fig. 8 (middle image). Moreover, other types of interaction contributed to the interface adhesion as well. For instance, epoxy rings of the resins opened at the interface and
Fig.10. The thermo mechanical properties of the fiber composites. (a): the storage modulus of the fiber composites. (b): the glass transition temperature of the fiber composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 7
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Fig. 11. SEM images for the interface of carbon fiber reinforced composites. (a): the cross sectional image of CF-TGDDM/DDS sample (2000×). (b): the cross sectional image of CF-TGDDM/TEPZ/DDS sample (2000×). (c): the side view of CF-TGDDM/DDS sample (1000×). (d): the side view of CF-TGDDM/ TEPZ/DDS sample (1000×). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
formed chemical (covalent) bonds with the fibers to enhance the strength, notably, which were irreversible processes. Therefore, the incorporation of TEPZ containing polar phthalazinone group exhibited the capability for varies types of interactions, including multiple covalent bonds, hydrogen bonds, and van der waals force, at the interface. The excellent mechanical properties of composites were the manifestation for the forming of intensive interfacial adhesion, interlocking, and entanglement among the matrix after the thermal sinter.
interior. After undergoing continuously boiling for 72 h, the water absorption of CF-TEPZ/TGDDM/DDS and CF-TGDDM/DDS was 0.42% and 1.16%, respectively, which was caused by failures in the interfaces of the composites. The decrease of water uptake rate of TEPZ-containing composites could be associated with the shielding function of polar TEPZ-fiber binding. The matrix with optimized curing networks would benefit for hindering the diffusion of water through the composites. Meanwhile, it could be seen from Fig. 10(a) that the CF-TEPZ/ TGDDM/DDS composites maintained higher storage modulus than that of CF-TGDDM/DDS composites after treating, which was consistent with previous analysis. In Fig. 10(b), an unconspicuous sub-transition of tan δ could be identified in the range of 200 °C-300 °C, possibly caused by the existence of certain incomplete curing fragments in the composites.
3.5. Interfacial property testing of composites In order to evaluate the interfacial interaction effect, the flexural strength and interlaminar shear strength of the fiber composites were tested. As shown in Fig. 9, they all displayed same trend of changes. The interlaminar shear strength of TGDDM/T700 composites was 57.8 MPa with the increase of 59.9%, which were comparable to the values reported by Li [28] and Wang [29]. The composites fabricated from TEPZ/TGDDM hybrids and presoaked carbon fibers exhibited slightly increases in flexural strength. Two factors mainly contributed to the improvement. For one thing, TEPZ epoxy could bring about excellent interfacial wettability for carbon fibers stemmed from its strong polarity that resulted in the formation of robust adhesion between fiber and matrix. For another, the high stiffness of phthalazinone could endow the composites a higher modulus and thermal resistance, which was consistent with the analysis of DMA. Afterwards, the water dipping treatment was carried out to assess the bonding toughness of composite
3.6. Surface morphology of composites The SEM images directly showed evidences for the interfacial information between carbon fiber and epoxy. The phenomenon of fiber debonding and fiber pull out occurred with some fragmented resin covered on the fiber surface as shown in Fig. 11(a), and it could also be observed from the Fig. 11(c) that fiber was peeled off the interface of the composites with disordered array. However, the introduction of TEPZ utilized as prepreg agent for the carbon fiber surface enabled composite to improve the interfacial strength between fiber and epoxy as shown in Fig. 11(b and d). After detailed observation of Fig. 11(b), 8
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the cross sectional view of the TEPZ/TGDDM carbon fiber composites was flat and fibers were packed tightly without cracks. Therefore, the TEPZ was believed as a qualified prepreg agent, which could be confirmed by the embedded fiber. These results were also consistent with the mechanical properties of epoxy/T700 composites.
[6]
4. Conclusions
[7]
[5]
Novel tetra-functional TEPZ was successfully synthesized and characterized using FT-IR, 1H NMR, LC-MS, HPLC and HRMS. TEPZ/ TGDDM blending system were prepared and investigated. The thermal analysis and mechanical tests showed that the TGDDM/DDS (100 phr/ 40 phr) system combined 30 phr TEPZ possessing better behaviors in terms of their comprehensive properties than those of neat TGDDM/ DDS. The mechanical properties test showed that the tensile strength, flexural strength and impact strength of the combined system increased severally for 44.5%, 41.9% and 31.6% than those of TGDDM/DDS. After reinforced by carbon fibers, the resulting laminates exhibited excellent mechanical properties and hydrothermal stability, which would definitely benefit for its application on aerospace structural materials.
[8]
[9]
[10] [11] [12]
[13]
[14]
CRediT authorship contribution statement
[15]
Fengfeng Zhang: Writing - original draft, Writing - review & editing, Investigation, Methodology. Lishuai Zong: Writing - review & editing, Supervision. Zhihuan Weng: Supervision. Feng Bao: Software, Investigation, Methodology. Nan Li: Software, Investigation, Methodology. Jinyan Wang: Project administration, Supervision. Xigao Jian: Funding acquisition.
[16]
[17]
[18]
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[19] [20] [21]
Acknowledgements
[22]
This work was supported by the National Nature Science Foundation of China (nos. U1663226, 51673033, and 51873027).
[23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compositesa.2020.105772.
[25]
[26]
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Contents lists available at ScienceDirect
Composites Part A journal homepage: www.elsevier.com/locate/compositesa
Highly enhanced performance of epoxy composites via novel phthalazinonebearing hybrid system as matrix
T
⁎
Fengfeng Zhanga, Lishuai Zonga,b, Zhihuan Wenga,b, Feng Baoa, Nan Lia,b, Jinyan Wanga,b, , Xigao Jiana,b a b
Department of Polymer Science & Materials, Dalian University of Technology, 116024, China Liaoning Province Engineering Technology Center of High Performance Resins, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phthalazinone Composites Fibers Adhesion
A novel phthalazinone-bearing epoxy resin, namely TEPZ, was designed and synthesized as functional modifier and prepreg agent for carbon fiber TGDDM epoxy composites by a “one pot two steps” method. Three-dimensional TEPZ/TGDDM/DDS system was constructed and optimized by modulating curing procedure and concentration of DDS to give thermosets with excellent thermal resistance and mechanical strength. The storage modulus and glass transition temperature (Tg) increased significantly with composition of 100 phr TGDDM, 30 phr TEPZ and 40 phr DDS as compared to those of TGDDM/DDS. The tensile strength, impact strength and flexural strength increased simultaneously. Afterwards, continuous unidirectional carbon fiber reinforced laminates were fabricated, followed by systematical investigation of their mechanical and interfacial properties. The interfacial study indicated that the polarity and wettability of TEPZ played an important role during the modification.
1. Introduction Rapid development of aerospace high-tech fields offers additional challenges to thermosetting matrix composites, which should be qualified for processing adequate thermal-resistance and modulus to meet the requirements of high performance engineering. Epoxy resin is a versatile class of thermosets with series of fascinating properties, such as high stiffness, chemical resistances, thermal and dimensional stability, reasonable manufacturability and low shrinkage during process [1]. However, the comprehensive properties above are closely related with the structure of the resins as well as their corresponding thermosets. For example, higher crosslink density of the thermoset will promote their modulus and thermal resistance without sacrificing other fundamental properties [2]. As a result, the exploration of multifunctional (trifunctional or tetra functional) epoxides, which contributes effectively for increasing crosslink density of thermosets, evolves as one of the most attractive strategies towards the target of high-tech aerospace applications [3]. As typical representative of multifunctional epoxides, N, N, N′, N′tetraglycidyl-4, 4′-diaminodiphenyl methane, namely TGDDM, has revealed superior performances as the aerospace matrix for fiber reinforced composites since 1970s. Considerable efforts have been
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dedicated on investigation of matrix modification and interface properties of continuous carbon fiber reinforced composites. (i): Matrix modification could bring about improvements including heat resistance, flame retardant, and mechanical strength. Jena et al. [4] blended tetra-functional epoxy resin (TGDDM) with diallyl bisphenol A (DBA) and bismaleimide (BMI) for rigid risers with serving temperature more than 280 °C. Zhou et al. [5] displayed several multifunctional epoxy-anhydride systems in terms of their glass transition temperatures and thermal stability. Besides physical blend and chemical modification methods, introduction of novel curing agent for TGDDM resins are also efficient strategy to improve their thermal resistance and strength, as well as their flame retardant, transparency [6–9]. However, previous works usually concentrate attention of optimization inherent properties of TGDDM/DDS systems via shifting the curing procedure and the amount of amine. Few architecture diversities of epoxy monomers have been directly explored with comparable properties as TGDDM from a perspective of molecular structure, as shown in Fig. 1 [18–21]. Their Tgs are always ranging from 218 to 280 °C and storage modulus generally varies between 2000 and 4000 MPa. Unfortunately, such networks exhibit elevated storage modulus while sacrificing Tg, and vice versa [9–17], which is worthy for thorough study. (ii): Study of the interfacial behaviors of continuous fiber reinforced composites, such as
Corresponding author. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.compositesa.2020.105772 Received 27 June 2019; Received in revised form 25 December 2019; Accepted 12 January 2020 Available online 13 January 2020 1359-835X/ © 2020 Published by Elsevier Ltd.
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composites that is seldom explored. 2. Experimental details 2.1. Materials 4, 4′-tetradiglycidyl diaminodi-phenyl methane (TGDDM, 75%) and 4, 4′-diaminodiphenyl sulfone (DDS, 99.7%) are all obtained from Aladdin Chemical Reagent Co. Ltd, and dried in vacuum prior to use. 1, 2-dihydro-2-(4-aminophenyl)-4-[4-(4-aminophenoxy) phenyl] (2H) phthalazin-1-one (DA-DHPZ, 99.8%) is supplied by Dalian Polymer New Material Company (China). The commercially available, presoaked carbon fibers T700SC-12K-50C are purchased from Toray Industries Inc. Epichlorohydrin (ECl, 99.5%), anhydrous sodium sulfate (Na2SO4, 99.8%), toluene (99.7%) and acetone (99.8%) are purchased from Tianjin Fuyu Fine Chemical Factory (China). Ethanol (99.9%), chloroform (99.7%) and other organic solvents are all purchased from Tianjin Damao Chemical Reagent Factory (China). 2.2. Synthesis of the TEPZ The details of the TEPZ synthesis process were depicted in the Fig. 2. DA-DHPZ (6.3 g, 0.015 mol) and ECl (41.63 g 0.45 mol) were mixed in a 500 mL flask equipped with magnetic stirring under nitrogen atmosphere at 85 °C. After refluxing for 6 h, 10 wt% aqueous sodium hydroxide solution (NaOH aq.) was added dropwise within 30 min then maintained the temperature for 5 h. After cooling, the mixture was extracted with toluene and washed with deionized water for several times until the pH of the solution reached neutral. Then the solvent of organic phase was removed under reduced pressure to get yellow solid powder (Yield: 74–85%; HPLC Purity: 83.5%). The 1H NMR, FTIR, HPLC, LC-MS and HRMS spectra details for TEPZ and TGDDM are in the Supplementary Data. Fig. 1. The structure reported by other researchers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. Preparation of composites The composition of matrix was optimized via following procedure: the mixture of TGDDM and 0–100 phr TEPZ was continuously stirred at 110 °C until homogeneous. Then, the curing agent DDS was added with weight amount relative to the mixture, and the equivalent weight ratios of mixture to DDS were 10:3, 10:4, 10:5 (10:5 was approximately stoichiometric equivalent ratio). The bubbles were removed under vacuum for 30 min by oil pump. After that, the mixture was poured into a preheated mold, then was immediately transferred into the air-circulating oven, followed by step curing process including 130 °C for 1 h, 150 °C for 1 h, 180 °C for 4 h. The carbon fiber composites were prepared by following process: the obtained optimized TEPZ/TGDDM/DDS system was dissolved in DMF for presoaking T700 carbon fiber, then the presoaking T700 carbon fiber was winded continuously to the frame to afford unidirectional prepregs. The prepregs were dried at 80 °C for 2 h, 100 °C for 1 h, 130 °C for 1.5 h in the vacuum oven to accelerate the possible reaction between the original carbon fiber and TEPZ. Fourteen tailored prepregs were compacted into a steel mold with a pressure of 1.5 MPa at 180 °C for 1 h and 2.5 MPa at 200 °C for 0.5 h. After the steel mold was removed, the post-curing procedure of laminate composite was 180 °C for 1 h, 200 °C for 1 h, 230 °C for 1 h. The fiber volume contents in the solidified laminates were controlled to be 60–67% after limited resin bleeding, which were calculated by ASTM D3171-15 standard. The obtained laminate composites should be sawed and sanded into 80 × 10 × 2 mm3 size and 20 × 10 × 2 mm3 size for flexural strength and inter-laminar shear strength tests, respectively.
physical interaction, chemical bonding, and morphological changes occurring in fiber could result in increased performance while served as structural materials. Conventional modifications like diamine or acid treatment, polymer grafting, surfactants treatment and physical mixture of carbon fibers have been most commonly utilized [22] for enhancing their interfacial adhesion. Such modifications could be realized by those emerging technologies, including electrophoretic deposition [23], atmospheric pressure plasma [22] and 3D printing [24], resulting in significant promotion of their comprehensive properties. However, some of the strategies mentioned above involve tedious process and higher cost, leading to lag of practical industrial application rather than laboratory research. Therefore, we describe a particularly simple, convenient and effective approach for enhancing interaction between fiber and matrix. Notably, there are only few attempts focused on evaluating the effects of polarity and wettability between reinforcement and matrix on the mechanical properties of carbon fiber reinforced epoxy composites. This paper will integrate both of matrix modification and interfacial property study into the system to realize synergetic effect of chemical and physical interaction between matrix and fibers, finally achieving wettability and polarity matching. Inspired with the conception, we incorporate polar structure, unsymmetrical and kink non-coplanar phthalazinone, into epoxy resin architecture as functional modifiers for TGDDM/DDS system, and also as a prepreg agent for carbon fiber to improve its wettability for epoxy matrix. The resin and composite system possess excellent thermal resistance along with mechanical properties after thermal cure. This work may provide a new perspective for the development of the manufacture for multi-functional epoxy
2.4. Characterization methods The chemical structures of synthesized epoxy monomer were 2
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Fig. 2. Synthesis routes of TEPZ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
determined by 1H NMR in CDCl3 on a Bruker AVANCE III 500 spectrometer, and the FT-IR spectrum that was collected by a Thermo Nicolet Nexus 470 Fourier transform infrared (FT-IR) spectrometer. Our product and the TGDDM were measured by High performance liquid chromatography (HPLC) and Liquid Chromatography-Mass Spectrometry (LC-MS) for purity analysis, which were performed on an Alliance 2695–2696 instrument and Agilent 1100 instrument, respectively. High resolution mass spectrometry (HRMS) was performed on Thermo Scientific LTQ-Orbitrap XL. Thermo gravimetric analysis (TGA) that were performed on the METTLER TGA SDTA851 analyzer were employed to evaluate the thermal stability of the cured products from 30 to 800 °C with the heating rate of 20 °C/min under continuous nitrogen flow (50 mL min−1). The step curing procedure was confirmed by Differential Scanning Calorimetry (DSC) using a Mettler DSC STAR System instrument over a temperature range of 25 to 300 °C under nitrogen atmosphere. Dynamic mechanical analysis (DMA) measurements were examined using a METTLER DMA SDTA8610e instrument at 1 Hz and a heating rate of 5 °C/min from 25 °C to 300 °C under air atmosphere with the sample dimension of 25 × 6 × 3 mm3. Rheological behavior was investigated by a TA AR2000 instrument at a frequency of under a strain of 0.02 N, 1 Hz. Elemental composition of the presoaking fibers was analyzed by Xray photoelectron spectroscopy (XPS, ESCALAB 220i-XL, VG, UK) equipped with a monochromated Al Kα source (1486.6 eV) at a base pressure of 2 × 10−9 mbar. The resin content of cured materials based on the ASTM D3171-15 standard. Annex A7 procedure was used to digest the matrices of sample containing carbon fiber at 450 °C for 6 h under ambient
conditions, and a paralleled pure epoxy sample was heated simultaneously for calculating the original mass of the carbon fiber in the composite. The tensile strength measurement set (AI-7000M-2) was used to measure the tensile properties of the composite sample according to the ASTM D638-96 standard. The sample was dumbbell-shaped with the dimensions of 115 mm (length) × 4 mm (thickness). A Charpy impact strength measurement set (XCJ-4) was performed to measure the impact property of the cured samples according to the Chinese standard GB/T 1843-2008. The specimen dimensions were 80 × 10 × 4 mm3 without notch. The flexural strength and interlaminar shear strength were determined using an Instron-5869 machine with a capacity of 500 N on T700 composite samples according to ASTM D790-10 and ISO 14130 standards, respectively. The brittle fracture surfaces morphology of composites were observed by SEM (FEI QUANTA 450). The samples were coated with a thin conductive gold layer before the SEM observation in order to capture a stable and clear image.
3. Results and discussion 3.1. Rheology analysis The complex viscosity (η*) of epoxy/DDS systems as a function of temperature was a vital parameter in evaluating their processability. As shown in the Fig. 3(a), (b) and Table 1, the TEPZ/TGDDM/DDS systems exhibited excellent processability with processing window (η* < 10 Pa.s) ranging from 67 to 140 °C. It became gradually narrower as the increasing loading of TEPZ in system mainly due to the high 3
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Fig. 3. The rheological behavior of TEPZ/TGDDM/ DDS hybrids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 The process temperature windows (η* < 10 Pa·s) of epoxy/DDS systems.
Table 2 The data of thermo mechanical properties for all the composite samples.
Samples
Temperature range/ °C (η* < 10 Pa·s)
Samples
Er/MPa
Ve/mol·m−3
E0/MPa
Tr/K
0 phr TEPZ/TGDDM/40 phr DDS 10 phr TEPZ/TGDDM/40 phr DDS 20 phr TEPZ/TGDDM/40 phr DDS 30 phr TEPZ/TGDDM/40 phr DDS 50 phr TEPZ/TGDDM/40 phr DDS 100 phr TEPZ/TGDDM/40 phr DDS TEPZ/40 phr DDS
58–198 61–200 62–200 71–198 78–202 97–200 138–205
TGDDM/30 phrDDS TGDDM/10 phrTEPZ/30 phrDDS TGDDM/20 phrTEPZ/30 phrDDS TGDDM/30 phrTEPZ/30 phrDDS TGDDM/50 phrTEPZ/30 phrDDS TGDDM/100 phrTEPZ/30 phrDDS TGDDM/40 phrDDS TGDDM/10 phrTEPZ/40 phrDDS TGDDM/20 phrTEPZ/40 phrDDS TGDDM/30 phrTEPZ/40 phrDDS TGDDM/50 phrTEPZ/40 phrDDS TGDDM/100 phrTEPZ/40 phrDDS TGDDM/50 phrDDS TGDDM/10 phrTEPZ/50 phrDDS TGDDM/20 phrTEPZ/50 phrDDS TGDDM/30 phrTEPZ/50 phrDDS TGDDM/50 phrTEPZ/50 phrDDS TGDDM/100 phrTEPZ/50 phrDDS
19.78 21.88 23.02 18.88 19.73 22.38 20.90 22.55 21.43 24.76 16.94 23.32 21.03 19.08 14.89 15.40 13.82 17.07
1522 1560 1706 1321 1357 1566 1602 1742 1615 1700 1170 1609 1653 1520 1170 1083 957 1174
4797 4494 4275 4064 4026 3588 4052 4170 3820 4600 3854 3677 4343 3965 3600 3652 4022 3060
521 562 541 573 583 573 523 519 532 584 580 581 510 503 512 570 579 583
stiffness of phthalazinone units. The viscosity for TEPZ/DDS system was significantly higher than that of TGDDM/DDS system in same temperature range, also ascribed to the kink non-coplanar rigid aromatic heterocyclic structure in TEPZ. When the shearing force was applied on TEPZ/DDS system, the interpenetration and entanglement imposed by the rigid aromatic heterocyclic structure significantly hindered the molecular motion between crosslinked network, resulting in higher viscosity of TEPZ/DDS than that of TGDDM/DDS. That meant it was difficult to obtain void-free TEPZ/DDS sample only with powder compaction technology.
Er: the storage modulus at (Tg + 30 °C); E0: the storage modulus at room temperature; Tr = (Tg + 30)/K.
directly related to a more rigid and dense crosslinking network formed during the cure procedure. So we calculated the crosslinking density by the rubber elasticity theory with the Eq.: Ve = Er/3RTr. The results were shown in the Table 2. In details, when the samples blended with 30 or 40 phr DDS, the Ve was increased with the incorporation of TEPZ then declined compared with those of TGDDM/DDS systems. However, when the samples with 50 phr DDS, the Ve values decreased with the
3.2. Mechanical parameters of TEPZ/TGDDM/DDS systems DMA had been extensively employed for characterizing the dynamic behaviors of the polymer chain segments which greatly related to the modulus and internal friction of the composites. As shown in Fig. 4(a), the storage modulus dropped down for certain degree when with 30 phr or 50 phr DDS loading. The higher storage modulus for samples usually
Fig. 4. DMA results of TEPZ/TGDDM/DDS hybrids. (a) The storage modulus of the TEPZ/TGDDM/DDS hybrids. (b) The Tg of the TEPZ/TGDDM/DDS hybrids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 4
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Table 3 The tensile behaviors of the composite samples. Samples
Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strain %
TGDDM/40 phr DDS TGDDM/10 phr TEPZ/40 phr DDS TGDDM/20 phr TEPZ/40 phr DDS TGDDM/30 phr TEPZ/40 phr DDS TGDDM/50 phr TEPZ/40 phr DDS TGDDM/100 phr TEPZ/40 phr DDS
37.60 39.41 58.97 54.32 42.11 49.47
3.21 3.37 3.50 3.64 3.71 3.48
1.23 1.33 1.85 1.62 1.05 1.55
± ± ± ± ± ±
5.1 3.9 6.5 5.0 5.9 2.1
± ± ± ± ± ±
0.19 0.17 0.10 0.06 0.33 0.14
± ± ± ± ± ±
0.32 0.24 0.24 0.17 0.18 0.11
Table 4 The mechanical parameters of TEPZ/TGDDM/DDS systems. Samples
Storage modulus GPa
Tg (tanδ)°C
Impact strength KJ·m−2
Flexural Strength MPa
30 phr TEPZ/TGDDM/40 phr DDS TGDDM/0 phr TEPZ/40 phr DDS
4.6 ± 0.1 3.6 ± 0.2
281 ± 6 215 ± 7
21.2 ± 2.2 14.5 ± 2.6
115 ± 4.1 81 ± 5.5
Fig. 5. The dielectric properties and SEM images of cured hybrids. (a) The dielectric loss of samples at 30 and 120 °C. (b) The dielectric constant of samples at 30 and 120 °C. (c) The SEM images of cured hybrids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. The XPS spectra of unpresoaked carbon fiber and the survey spectrum. (a) The C1s spectra of unpresoaked carbon fiber. (b) The O1s spectra of unpresoaked carbon fiber. (c) The survey spectra of the presoaked and unpresoaked carbon fiber. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
adding of TEPZ. At first, all the results revealed that the loading amount of the TEPZ within appropriate range could effectively promote crosslinking density of the forming networks, owing to the comparatively high curing efficiency with a better covalent crosslinking network. Secondly, in the system with the least amount of curing agent (30 phr), the crosslinking density firstly increased which was due to the chemical bonds between the composite epoxy and DDS, and then decreased due to the incomplete curing with the least amount of DDS. Finally, with the dosage of 50 phr, the curing reaction between the epoxy group and amidogen was restricted by the steric hindrance of phthalazinone group, leading to decrease in mobility of the polymer chains, and even some curing agent still left intact as plasticizing agent, thus the cured
resultant obtained a relatively low crosslinking density. Obviously, the increase of Ve corresponding to the denser structure would stem from the higher storage modulus. In addition, it was notable that when the samples contained 100 phr TEPZ, the Ve values always increased in a certain extent, which may be related to the stronger van der Waals force existed in these systems. So we speculated that the physical crosslinking (originated from the van der Waals force) and covalent crosslinking were the two main factors affecting the storage modulus. As shown in Fig. 4(b), the Tg of the networks showed an overall increasing trend as the increase loading of TEPZ. This phenomenon could be ascribed to the higher crosslinking density and the rigid phthalazinone structure in TEPZ. Generally, the decreased crosslinking 5
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Fig. 7. The XPS spectra of presoaked carbon fiber. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Schematic of composite interface between carbon fiber and epoxy.
What's more, the rigidity of polymer chains was enhanced by the incorporation of phthalazinone group. The tight and rigid crosslinking networks was the key to achieving higher storage modulus and Tg simultaneously. The effect of the TEPZ on the tensile properties of the composites sample was studied by tensile test. The tensile strength, tensile modulus and tensile strain were listed in Table 3. It could be observed most of the composite samples showed higher tensile strength and tensile
density should result in the decline of Tg. But as shown in Table 2, the Tg of samples shown the increasing trend as containing more TEPZ. Essentially, the TEPZ epoxy monomer had the special twisted non coplanar structure, which was different from the comparatively linear structure of TGDDM. Hence, under the condition of appropriate amount of DDS, the TEPZ bearing phthalazinone group with steric hindrance and increased functionality shortened the average chain length between neighboring crosslinking points leading to tight crosslinking networks. 6
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volumes to the system. Hence, the observed reduction of dielectric loss was consistent with DMA study. Additionally, the SEM image also gave evidence for tight polymer networks in Fig. 5(c). No separated phase could be monitored. That meant polymer networks that was originated from high compatibility of TGDDM and TEPZ epoxy contributing efficiently to energy dissipation, which was benefit for promotion of their mechanical properties [26]. Eventually, TEPZ (30 phr)/TGDDM (100 phr)/DDS (40 phr) system was selected as a more promising matrix for further investigation in this work. 3.3. Surface chemical compositions of carbon fiber XPS analysis was conducted to help quantitative comparison for determining the surface chemical composition of the presoaked and unpresoaked carbon fiber. The signal assignment and their corresponding proportion were listed in Fig. 7. The results exhibited clearly that the unpresoaked carbon fiber surface contained mainly carbon and oxygen elements and the presence of hydroxyl, carbonyl, and carboxyl groups were related to hydrophilic surface of unpresoaked carbon fiber, as shown in Fig. 6(a and b). Correspondingly, due to functionalization of the unpresoaked carbon fiber with epoxy resin (TGDDM or TEPZ), an obvious increase on O/C ratios of presoaked specimens could be found in Fig. 7(a, b and h). On the other hand, Fig. 6(c) also displayed that more oxygen-containing functional groups on the presoaked fiber was indicative of successful modification of carbon fiber. When with both TEPZ and TGDDM as the prepreg agent, the resultant synergism derived from the wettability of polar groups in TEPZ made the resin bond with carbon fiber firmly, enabling for more effective inter-diffusion between fiber and prepreg agent which was primarily responsible for interfacial modification. In addition, little amount of NeC]O bonding presented in Fig. 7 (e, f, j and k) that possibly rooted from the epoxy or the reaction between fiber and matrix was found in all presoaked specimens after presoaking. The possible interaction mechanism was depicted in Fig. 8.
Fig. 9. The Flexural strength and ILSS strength of the fiber composites.
modulus than pure TGDDM due to the introduction of TEPZ. When 20 phr TEPZ were added into the TGDDM/DDS system, tensile strength and elongation at failure of TGDDM/20 phrTEPZ/40 phrDDS increased to 58.97 MPa and 1.85%, respectively. Obviously, the formation of homogeneous crosslinking network simultaneously exhibited positive effects on the tensile properties of the composite samples, which was mainly benefited from chemical crosslinks between epoxy and cure agent. Besides, the special structure of TEPZ could offer more physical crosslinking, such as van der Waals force and polymer chain entanglement, which played a crucial role for the improvement. In other words, the physical interactions could compensate the deterioration of the tensile properties caused by lower crosslinking density (Table 2). Finally, the tensile modulus of composite samples was enhanced evidently after the incorporation of 50 phr TEPZ. In addition, the impact strength and flexural strength shown in Table 4 was simultaneously increased by 31.6% and 41.9%, respectively. Such enhancement may be ascribed to the interpenetrating networks imposed by TEPZ and TGDDM that formed during thermal cure procedure. In order to further verify the speculation, dielectric properties and morphological study of cured products were carried out. In Fig. 5(a), the presence of polar moieties in TEPZ resulted in the slightly increase of dielectric constants. The dielectric loss properties of the networks were shown in Fig. 5(b). TEPZ-containing system exhibited a lower dielectric loss. Based on the published work [25], dielectric loss could be effectively lowered down by incorporation of low polarizability elements and improvement of the free volume of polymer networks. In TEPZ/TGDDM/DDS system, bulky phthalazinone structure disrupted the close packing of polymer chains and endowed high free
3.4. Interfacial enhancement mechanism Fig. 8 depicted a possible correlation between fiber and epoxy prepreg agents. Pisanova [27] proposed that the polymer carrying polar groups would provide an intensive adhesion strength by acid–base interactions. In other words, the polymer or prepreg agents had ability to act as an electron donor or electron acceptor to interact with epoxy or carboxyl group on carbon fibers. In CF-TEPZ/TGDDM/DDS systems, the acid-base interactions happened after wetting contact between fibers and prepreg agents was displayed in Fig. 8 (middle image). Moreover, other types of interaction contributed to the interface adhesion as well. For instance, epoxy rings of the resins opened at the interface and
Fig.10. The thermo mechanical properties of the fiber composites. (a): the storage modulus of the fiber composites. (b): the glass transition temperature of the fiber composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 7
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Fig. 11. SEM images for the interface of carbon fiber reinforced composites. (a): the cross sectional image of CF-TGDDM/DDS sample (2000×). (b): the cross sectional image of CF-TGDDM/TEPZ/DDS sample (2000×). (c): the side view of CF-TGDDM/DDS sample (1000×). (d): the side view of CF-TGDDM/ TEPZ/DDS sample (1000×). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
formed chemical (covalent) bonds with the fibers to enhance the strength, notably, which were irreversible processes. Therefore, the incorporation of TEPZ containing polar phthalazinone group exhibited the capability for varies types of interactions, including multiple covalent bonds, hydrogen bonds, and van der waals force, at the interface. The excellent mechanical properties of composites were the manifestation for the forming of intensive interfacial adhesion, interlocking, and entanglement among the matrix after the thermal sinter.
interior. After undergoing continuously boiling for 72 h, the water absorption of CF-TEPZ/TGDDM/DDS and CF-TGDDM/DDS was 0.42% and 1.16%, respectively, which was caused by failures in the interfaces of the composites. The decrease of water uptake rate of TEPZ-containing composites could be associated with the shielding function of polar TEPZ-fiber binding. The matrix with optimized curing networks would benefit for hindering the diffusion of water through the composites. Meanwhile, it could be seen from Fig. 10(a) that the CF-TEPZ/ TGDDM/DDS composites maintained higher storage modulus than that of CF-TGDDM/DDS composites after treating, which was consistent with previous analysis. In Fig. 10(b), an unconspicuous sub-transition of tan δ could be identified in the range of 200 °C-300 °C, possibly caused by the existence of certain incomplete curing fragments in the composites.
3.5. Interfacial property testing of composites In order to evaluate the interfacial interaction effect, the flexural strength and interlaminar shear strength of the fiber composites were tested. As shown in Fig. 9, they all displayed same trend of changes. The interlaminar shear strength of TGDDM/T700 composites was 57.8 MPa with the increase of 59.9%, which were comparable to the values reported by Li [28] and Wang [29]. The composites fabricated from TEPZ/TGDDM hybrids and presoaked carbon fibers exhibited slightly increases in flexural strength. Two factors mainly contributed to the improvement. For one thing, TEPZ epoxy could bring about excellent interfacial wettability for carbon fibers stemmed from its strong polarity that resulted in the formation of robust adhesion between fiber and matrix. For another, the high stiffness of phthalazinone could endow the composites a higher modulus and thermal resistance, which was consistent with the analysis of DMA. Afterwards, the water dipping treatment was carried out to assess the bonding toughness of composite
3.6. Surface morphology of composites The SEM images directly showed evidences for the interfacial information between carbon fiber and epoxy. The phenomenon of fiber debonding and fiber pull out occurred with some fragmented resin covered on the fiber surface as shown in Fig. 11(a), and it could also be observed from the Fig. 11(c) that fiber was peeled off the interface of the composites with disordered array. However, the introduction of TEPZ utilized as prepreg agent for the carbon fiber surface enabled composite to improve the interfacial strength between fiber and epoxy as shown in Fig. 11(b and d). After detailed observation of Fig. 11(b), 8
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the cross sectional view of the TEPZ/TGDDM carbon fiber composites was flat and fibers were packed tightly without cracks. Therefore, the TEPZ was believed as a qualified prepreg agent, which could be confirmed by the embedded fiber. These results were also consistent with the mechanical properties of epoxy/T700 composites.
[6]
4. Conclusions
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[5]
Novel tetra-functional TEPZ was successfully synthesized and characterized using FT-IR, 1H NMR, LC-MS, HPLC and HRMS. TEPZ/ TGDDM blending system were prepared and investigated. The thermal analysis and mechanical tests showed that the TGDDM/DDS (100 phr/ 40 phr) system combined 30 phr TEPZ possessing better behaviors in terms of their comprehensive properties than those of neat TGDDM/ DDS. The mechanical properties test showed that the tensile strength, flexural strength and impact strength of the combined system increased severally for 44.5%, 41.9% and 31.6% than those of TGDDM/DDS. After reinforced by carbon fibers, the resulting laminates exhibited excellent mechanical properties and hydrothermal stability, which would definitely benefit for its application on aerospace structural materials.
[8]
[9]
[10] [11] [12]
[13]
[14]
CRediT authorship contribution statement
[15]
Fengfeng Zhang: Writing - original draft, Writing - review & editing, Investigation, Methodology. Lishuai Zong: Writing - review & editing, Supervision. Zhihuan Weng: Supervision. Feng Bao: Software, Investigation, Methodology. Nan Li: Software, Investigation, Methodology. Jinyan Wang: Project administration, Supervision. Xigao Jian: Funding acquisition.
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[17]
[18]
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[19] [20] [21]
Acknowledgements
[22]
This work was supported by the National Nature Science Foundation of China (nos. U1663226, 51673033, and 51873027).
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compositesa.2020.105772.
[25]
[26]
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