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Improvement of curing reaction activity of one-component room temperature-curable epoxy adhesive by the addition of functionalized graphene oxide
International Journal of Adhesion & Adhesives 98 (2020) 102537
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: http://www.elsevier.com/locate/ijadhadh
Improvement of curing reaction activity of one-component room temperature-curable epoxy adhesive by the addition of functionalized graphene oxide Baoming Li a, *, Xiaoli Wang a, Mengdi Bai a, Yu Shen b a b
College of Material Science and Engineering, Fuzhou University, 2 Xue Yuan Road, Fuzhou, 350108, PR China Nantong Boyuan Automotive Parts Co., Ltd., 21 Groups of Deng Yuan Community, Chengbei Street, RuGao, 226500, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Epoxides Nanofillers Mechanical properties of adhesives Durability
Functionalized graphene oxide (GO) nanocomposites including GO-Fe and GO-Fe-P were prepared and used as a curing additive for a one-component room temperature-curable epoxy adhesive (OCRTCEA). The results showed that the functionalized GO nanocomposites were conducive to accelerating the curing reaction of OCRTCEA with a moisture-activated ketimine as a latent curing agent, and when OCRTCEA containing 1 wt% GO-Fe-P was exposed to air at room temperature for 24 h, its lap shear strength reached 11.2 MPa, which was 144% higher than that of pure OCRTCEA under the same test conditions. Furthermore, the storage stability of OCRTCEA containing 1 wt% GO-Fe-P was similar with that of pure OCRTCEA over 60 days of storage. The enhanced curing reaction activity of OCRTCEA might be due to the role of curing accelerator of GO nanocomposites, the enhanced polarity of OCRTCEA because of the addition of GO nanocomposites, and the special morphology of GO nanocomposites.
1. Introduction Room temperature-curable epoxy adhesives (RTCEAs) are a major class of adhesive, which can be applied widely, without the need for heat application in fields, such as civil construction, electrical and electronics engineering, and so on. So far, RTCEAs have been available in two forms, namely two-component RTCEA (TCRTCEA) and one-component RTCEA (OCRTCEA). For TCRTCEA, each component must be accu rately weighted and properly mixed prior to use. Thus, the various components must be separately stored until required, with the produc tion workers charged with the added responsibility of preparing epoxy adhesives with uniform properties [1]. As a result, many efforts have been made to develop OCRTCEA based systems. For OCRTCEA systems, the curing agent can be mixed with the epoxy resin but must remain inactive during storage with the need for activation of the curing agent at a later time. To achieve this capability, typical OCRTCEA systems make use of latent curing agents, for instance ketimine compounds, in an attempt to alleviate the problem of reduced shelf life [2]. The curing mechanism of an OCRTCEA system typically comprises a two-step reaction involving reaction of the ketimine compound with moisture followed by reaction of the resulting amine compound with the * Corresponding author. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.ijadhadh.2019.102537 Available online 16 December 2019 0143-7496/© 2019 Elsevier Ltd. All rights reserved.
epoxy resin [3]. It was found that the key step that affected the curing reaction activity was the hydrolysis of the ketimine compound, and the quicker this reaction, the higher the curing reaction activity of OCRT CEA was [4]. Unfortunately, the curing reaction activity of OCRTCEA with the ketimine compound was still low because of the poor hydro philicity of the epoxy resin and the large steric hindrance of the ketimine compound; the former preventing water molecules from entering the interior of the epoxy resin, and later reducing the hydrolysis reaction activity of the ketimine compound [5,6]. Ferric, as the most stable ion of iron, has a strong polarity because of its large ionic radius and trivalence state, which makes it possess coordinative affinity for groups containing oxygen, such as hydroxyl, carboxyl and epoxy groups. In this paper, the functionalized GO nanocomposites including graphene oxide – ferric (GO-Fe) and graphene oxide – ferric dihydrogen phosphate (GO-Fe-P) were prepared and used as a curing additive for OCRTCEA for the first time, and their effects on the curing reaction activity of OCRTCEA were investigated.
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
2. Experimental
Epoxy resin, diglycidyl ether of bisphenol-A (commercial name: E 51), was purchased from Sinopec Baling Petrochemical Company (China), and the other chemical reagents were obtained from Sinopharm (Shanghai) Chemical Reagent Co. Ltd. and used as received.
respectively, and then 1 g ketimine as a latent curing agent was added and the mixture was ultrasonicated at 60 � C for 8 h. After ultrasonic treatment, 2 g epoxy resin E51 was added and the mixture stirred at room temperature for 24 h. For the convenience of discussion, the resulting OCRTCEAs containing GO, GO-Fe and GO-Fe-P were desig nated as GO/EA, GO-Fe/EA and GO-Fe-P/EA, respectively, and the pure OCRTCEA containing only 1 g ketimine and 2 g E51 was designated as EA.
2.2. Characterization
3. Results and discussion
SEM and EDS analyses were carried out using a Zeiss SUPRA 55 scanning electron microscope at an accelerating voltage of 5 kV. XRD measurements were conducted on an X’Pert Pro MPD diffractometer using Cu Kα1 irradiation at a rate of 2� /min from 5� to 40� . Lap shear strength was measured on an Instron Model 3367 universal testing machine according to ISO 4587–2003. The substrate was the stainless steel and its surface treatment prior to bonding according to Ref. [7]. Briefly, the stainless steel substrate was degreased in acetone and trichloroethylene, and then immersed in an etching solution, which consisted of 35 mL saturated sodium dichromate solution dissolved in 1 L concentrated sulfuric acid at 50 � C for 15 min, and finally rinsed in running tap water before drying at 80 � C in a forced air oven for 30 min. Storage stability was evaluated with a Brookfield DV2TLV rotational viscometer according to ASTM D1337-2010 and each provided viscosity value was an average of five measurements.
3.1. Characterization of functionalized GO nanocomposites
2.1. Reagents
SEM images of GO, GO-Fe and GO-Fe-P are shown in Fig. 1. It was found that GO had a smooth surface and a curled lamellar structure. Compared with GO, GO-Fe had a large-sized planar lamellar structure with a wrinkled surface and GO-Fe-P had a leaf-like lamellar structure with a much rougher surface than GO-Fe. The special morphologies of GO-Fe and GO-Fe-P were favorable to the enhancements of their compatibility and bonding strength with epoxy resin. XRD patterns of graphite, GO, GO-Fe and GO-Fe-P are presented in Fig. 2. It was found that the diffraction peak of graphite was located at 26.5� corresponding to its (002) reflection and its interlayer distance was calculated to be 0.336 nm [9]. In contrast with graphite, the char acteristic diffraction peak of GO was located at 10.2� , which corre sponded to its (001) reflection, and its interlayer distance was calculated to be 0.866 nm. The expanded interlayer spacing can be attributed to the presence of abundant oxygen-containing functional groups on both sides of the GO sheet [11]. The XRD pattern of GO-Fe indicated that the diffraction peak of GO moved to 9.3� and its intensity decreased obvi ously, which was probably caused by the extension of the GO sheet as shown in Fig. 1 and the oxygen donor coordination of GO to ferric ions, leading to further expansion of the interlayer spacing [12]. In the XRD pattern of GO-Fe-P, the diffraction peak of GO moved to 10.0� and its intensity increased compared with GO-Fe, which might be due to a distortion of the crystal structure and morphology change after the formation of the Fe–P moieties as shown in Fig. 1.
2.3. Preparation of GO Graphene oxide was synthesized by the improved Hummers’ method according to Ref. [8]. Briefly, 2 g of graphite powder and 14 g KMnO4 were successively added to a mixture of 10 mL H3PO4 and 90 mL H2SO4. The reaction mixture was stirred at 50 � C for 12 h. After cooling to room temperature, 1.5 L deionized water was added and the reaction mixture continued to be stirred for 0.5 h. Then, 30% H2O2 was added slowly until a bright yellow reaction mixture was observed. The remaining solid material was obtained by centrifugation and then washed in succession with deionized water, 30% HCl and ethanol. The obtained GO was freeze-dried at 50 � C for 24 h.
3.2. Curing reaction activity evaluation The curing reaction of epoxy resin with the moisture-activated ketimine latent curing agent in air at room temperature was a slow process [4]. Therefore, GO nanocomposites were added into OCRTCEA in this paper and the effect of GO nanocomposites on the curing reaction activity of OCRTCEA in air at room temperature was investigated and shown in Fig. 3(a). The results showed that the lap shear strength of EA was 0.33 MPa when exposed for 5 h, and showed a slow linear increase with increase of exposure time. The lap shear strength of EA increased to 4.57 MPa when the exposure time was 24 h. Compared with EA, the lap shear strengths of GO/EA, GO-Fe/EA and GO-Fe-P/EA were 1.67 MPa, 2.02 MPa and 3.46 MPa, respectively, when exposed for 5 h, and enhanced rapidly with the increase of exposure time. The lap shear strengths of GO/EA, GO-Fe/EA and GO-Fe-P/EA were 8.01 MPa, 9.22 MPa and 11.2 MPa when the exposure time was 24 h, which were 75.3%, 102% and 144% higher than that of EA, respectively.
2.4. Preparation of functionalized GO nanocomposites GO-Fe and GO-Fe-P were prepared according to Ref. [9]. Briefly, 0.1 g GO was added to 100 mL of 0.1 moL/L HCl aqueous solution and a uniformly dispersed GO solution was obtained after ultrasonic treatment at room temperature for 2 h. 200 mL of 0.1 moL/L FeCl3 aqueous so lution was then slowly added to the above GO solution and the mixture was ultrasonicated at room temperature for 4 h. The dark brown GO-Fe powder was centrifuged, and washed with ethanol several times, and dried at 60 � C in a vacuum oven. The preparation process of GO-Fe-P was the same as that of GO-Fe except that GO-Fe and NaH2PO4 as re actants were substituted for GO and FeCl3, respectively. 2.5. Synthesis of ketimine The ketimine was synthesized according to Ref. [10]. Briefly, 0.1 moL m-phenylenediamine was reacted with 0.4 mol methyl isobutyl ketone under stirring at 170 � C for 3 h, and the water produced during the reaction was removed with a Dean–Stark trap. The reaction was regarded complete after the production of a theoretical amount of water. The residual methyl isobutyl ketone was distilled off from the mixture under reduced pressure to obtain the flaxen ketimine.
3.3. SEM observation SEM images of lap shear fracture surfaces of OCRTCEAs exposed in air at room temperature for 24 h are shown in Fig. 4. It was found that the surface of EA was smooth due to the inherent brittle characteristic of the cured epoxy resin [13]. Compared with EA, the surface of OCRTCEA containing GO or GO nanocomposite became rough and the obvious texture as indicated by the arrows was also observed. This behavior could be ascribed to good interfacial adhesion between GO or GO nanocomposite and epoxy resin as well as the deflection of propagating
2.6. Preparation of OCRTCEA 0.03 g GO, GO-Fe or GO-Fe-P was added into 5 mL sample bottle, 2
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
Fig. 1. SEM images of GO, GO-Fe and GO-Fe-P.
Fig. 2. XRD patterns of graphite, GO, GO-Fe and GO-Fe-P.
Fig. 3. Effects of GO nanocomposites on lap shear strength (a) and storage stability (b) of OCRTCEA.
crack fronts induced by the GO sheets under lap shear test [14]. Among them, GO-Fe-P/EA possessed the most ordered surface texture that was possibly related to a combination of its high curing degree and the special leaf-like lamellar structure with the rough surface of GO-Fe-P.
The dispersion performance of GO or GO nanocomposite in OCRT CEA was further analyzed by EDS and the corresponding EDS images are shown in Fig. 5, where a was the dispersion state of oxygen in GO/EA, b was the dispersion state of ferrum in GO-Fe/EA and c was the dispersion 3
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
Fig. 4. SEM images of lap shear fracture surfaces of OCRTCEAs.
Fig. 5. Optical microscopy photographs of OCRTCEAs.
state of phosphorus in GO-Fe-P/EA. It was clear that the three elements were dispersed uniformly in OCRTCEAs, respectively, which certified the excellent dispersion performance of GO and its nanocomposite.
special leaf-like lamellar structure with the rough surface of GO-Fe-P and its high curing reaction activity as a result of the strong polarity of GO-Fe-P.
3.4. Mechanism analysis
3.5. Storage stability evaluation
The possible mechanism for the improvement of curing reaction activity of OCRTCEA by the addition of functionalized GO nano composite is shown in Fig. 6. The curing reaction of OCRTCEA without GO nanocomposite was a slow process because it was not easy to absorb the moisture in the air and the decomposition of ketimine to produce mphenylenediamine that would continue to initiate the curing reaction of epoxy resin had a low reaction rate [5,6]. When the functionalized GO nanocomposite was added into OCRTCEA, it did not only produce chemical bonding with the epoxy resin and the curing agent [13] but also could absorb moisture in the air and promote water molecules to migrate from the surface to the interior of the OCRTCEA because of its strong polarity, which would accelerate the decomposition rate of ketimine. Therefore, the curing reaction activity of OCRTCEA was improved in the presence of functionalized GO nanocomposite. More over, the largest lap shear strength of GO-Fe-P/EA was ascribed to the
Storage stability was an important index to evaluate the shelf life of OCRTCEA, because it was easy to lose the flowability once it made contact with moisture in the air. In this paper, the storage stability of OCRTCEA was evaluated by investigating the viscosity change during storage and the viscosity change over time for OCRTCEA is shown in Fig. 3(b). It was found that the viscosity of EA increased slowly with the increase of time when the storage time was less than 60 days and showed a rapid increase when the storage time was more than 60 days. OCRT CEAs prepared in this paper were stored in a closed container filled with nitrogen, so the increased viscosity of EA might be due to a minute amount of water in the ketimine and epoxy resin that could lead to the decomposition of ketimene to produce m-phenylenediamine which would initiate cure of the epoxy resin at a slow rate. The variation trend of viscosity of GO/EA, GO-Fe/EA or GO-Fe-P/EA with time was similar to that of EA with a more rapid increase of viscosity for GO/EA, GO-Fe/
Fig. 6. Possible mechanism for the improvement of curing reaction activity of OCRTCEA by the addition of functionalized GO nanocomposite. 4
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
EA or GO-Fe-P/EA than for EA because of the curing acceleration of GO or GO nanocomposite.
[4] Okuhira H, Kii T, Ochi M, Takeyama H. Characterization of epoxy resin hardening with ketimine latent hardeners. J Adhes Sci Technol 2004;18:205–11. [5] Chen L, Suh BI. Effect of hydrophilicity on the compatibility between a dual-curing resin cement and one-bottle simplified adhesives. J Adhesive Dent 2013;15:325. [6] Culbertson JB. Factors affecting the rate of hydrolysis of ketimines. J Am Chem Soc 1951;73:4818–23. [7] Andrews EH, King NE. Adhesion of epoxy resins to metals. J Mater Sci 1976;11: 2004–14. [8] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun Z, Slesarev A, Alemany LB, Lu W, Tour JM. Improved synthesis of graphene oxide. ACS Nano 2010;4:4806–14. [9] Pourbeyram S. Effective removal of heavy metals from aqueous solutions by graphene oxide zirconium phosphate (GO Zr-P) nanocomposite. Ind Eng Chem Res 2016;55:5608–17. [10] Okuhira H, Kii T, Ochi M, Takeyama H. Novel moisture-curable epoxy resins and their characterization. J Appl Polym Sci 2003;89:91–5. [11] Fei P, Wang Q, Zhong M, Su BT. Preparation and adsorption properties of enhanced magnetic zinc ferrite-reduced graphene oxide nanocomposites via a facile one-pot solvothermal method. J Alloy Comp 2016;685:411–7. [12] Dong Y, Li J, Shi L, Xu J, Wang XB, Guo ZG, Liu WM. Graphene oxide–iron complex: synthesis, characterization and visible-light-driven photocatalysis. J Mater Chem 2013;1:644–50. [13] Aradhana R, Mohanty S, Nayak SK. Comparison of mechanical, electrical and thermal properties in graphene oxide and reduced graphene oxide filled epoxy nanocomposite adhesives. Polymer 2018;141:109–23. [14] Bortz DR, Heras EG, Martin-Gullon I. Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites. Macromolecules 2012;45: 238–45.
4. Conclusions The functionalized GO nanocomposites were prepared and added in OCRTCEA for the first time. The results showed that the functionalized GO nanocomposites were conducive to accelerating the curing reaction of OCRTCEA with the moisture-activated ketimine as a latent curing agent, and the storage stability of OCRTCEAs containing GO nano composites was similar with that of pure OCRTCEA within 60 days. OCRTCEA prepared in this paper with the enhanced curing reaction activity and great storage stability was convenient to be used and stored, which would be propitious to its wide applications in fields without ready access to heating, such as in civil construction, electrical and electronics engineering, and so on. References [1] Michael J. Low odor, fast cure, toughened epoxy adhesive. 2010-6-29. US7745006. [2] Browning JD, Mcginniss VD, Vajayendran BR. Single component room temperature curable low VOC epoxy coatings. 2003-11-18. US6649673B2. [3] Endo T, Sanda F, Suzuki K, Horii H, Matsuura N. One-pack moisture-curable epoxy resin composition. 2006-3-8. EP1362876B1.
5
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: http://www.elsevier.com/locate/ijadhadh
Improvement of curing reaction activity of one-component room temperature-curable epoxy adhesive by the addition of functionalized graphene oxide Baoming Li a, *, Xiaoli Wang a, Mengdi Bai a, Yu Shen b a b
College of Material Science and Engineering, Fuzhou University, 2 Xue Yuan Road, Fuzhou, 350108, PR China Nantong Boyuan Automotive Parts Co., Ltd., 21 Groups of Deng Yuan Community, Chengbei Street, RuGao, 226500, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Epoxides Nanofillers Mechanical properties of adhesives Durability
Functionalized graphene oxide (GO) nanocomposites including GO-Fe and GO-Fe-P were prepared and used as a curing additive for a one-component room temperature-curable epoxy adhesive (OCRTCEA). The results showed that the functionalized GO nanocomposites were conducive to accelerating the curing reaction of OCRTCEA with a moisture-activated ketimine as a latent curing agent, and when OCRTCEA containing 1 wt% GO-Fe-P was exposed to air at room temperature for 24 h, its lap shear strength reached 11.2 MPa, which was 144% higher than that of pure OCRTCEA under the same test conditions. Furthermore, the storage stability of OCRTCEA containing 1 wt% GO-Fe-P was similar with that of pure OCRTCEA over 60 days of storage. The enhanced curing reaction activity of OCRTCEA might be due to the role of curing accelerator of GO nanocomposites, the enhanced polarity of OCRTCEA because of the addition of GO nanocomposites, and the special morphology of GO nanocomposites.
1. Introduction Room temperature-curable epoxy adhesives (RTCEAs) are a major class of adhesive, which can be applied widely, without the need for heat application in fields, such as civil construction, electrical and electronics engineering, and so on. So far, RTCEAs have been available in two forms, namely two-component RTCEA (TCRTCEA) and one-component RTCEA (OCRTCEA). For TCRTCEA, each component must be accu rately weighted and properly mixed prior to use. Thus, the various components must be separately stored until required, with the produc tion workers charged with the added responsibility of preparing epoxy adhesives with uniform properties [1]. As a result, many efforts have been made to develop OCRTCEA based systems. For OCRTCEA systems, the curing agent can be mixed with the epoxy resin but must remain inactive during storage with the need for activation of the curing agent at a later time. To achieve this capability, typical OCRTCEA systems make use of latent curing agents, for instance ketimine compounds, in an attempt to alleviate the problem of reduced shelf life [2]. The curing mechanism of an OCRTCEA system typically comprises a two-step reaction involving reaction of the ketimine compound with moisture followed by reaction of the resulting amine compound with the * Corresponding author. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.ijadhadh.2019.102537 Available online 16 December 2019 0143-7496/© 2019 Elsevier Ltd. All rights reserved.
epoxy resin [3]. It was found that the key step that affected the curing reaction activity was the hydrolysis of the ketimine compound, and the quicker this reaction, the higher the curing reaction activity of OCRT CEA was [4]. Unfortunately, the curing reaction activity of OCRTCEA with the ketimine compound was still low because of the poor hydro philicity of the epoxy resin and the large steric hindrance of the ketimine compound; the former preventing water molecules from entering the interior of the epoxy resin, and later reducing the hydrolysis reaction activity of the ketimine compound [5,6]. Ferric, as the most stable ion of iron, has a strong polarity because of its large ionic radius and trivalence state, which makes it possess coordinative affinity for groups containing oxygen, such as hydroxyl, carboxyl and epoxy groups. In this paper, the functionalized GO nanocomposites including graphene oxide – ferric (GO-Fe) and graphene oxide – ferric dihydrogen phosphate (GO-Fe-P) were prepared and used as a curing additive for OCRTCEA for the first time, and their effects on the curing reaction activity of OCRTCEA were investigated.
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
2. Experimental
Epoxy resin, diglycidyl ether of bisphenol-A (commercial name: E 51), was purchased from Sinopec Baling Petrochemical Company (China), and the other chemical reagents were obtained from Sinopharm (Shanghai) Chemical Reagent Co. Ltd. and used as received.
respectively, and then 1 g ketimine as a latent curing agent was added and the mixture was ultrasonicated at 60 � C for 8 h. After ultrasonic treatment, 2 g epoxy resin E51 was added and the mixture stirred at room temperature for 24 h. For the convenience of discussion, the resulting OCRTCEAs containing GO, GO-Fe and GO-Fe-P were desig nated as GO/EA, GO-Fe/EA and GO-Fe-P/EA, respectively, and the pure OCRTCEA containing only 1 g ketimine and 2 g E51 was designated as EA.
2.2. Characterization
3. Results and discussion
SEM and EDS analyses were carried out using a Zeiss SUPRA 55 scanning electron microscope at an accelerating voltage of 5 kV. XRD measurements were conducted on an X’Pert Pro MPD diffractometer using Cu Kα1 irradiation at a rate of 2� /min from 5� to 40� . Lap shear strength was measured on an Instron Model 3367 universal testing machine according to ISO 4587–2003. The substrate was the stainless steel and its surface treatment prior to bonding according to Ref. [7]. Briefly, the stainless steel substrate was degreased in acetone and trichloroethylene, and then immersed in an etching solution, which consisted of 35 mL saturated sodium dichromate solution dissolved in 1 L concentrated sulfuric acid at 50 � C for 15 min, and finally rinsed in running tap water before drying at 80 � C in a forced air oven for 30 min. Storage stability was evaluated with a Brookfield DV2TLV rotational viscometer according to ASTM D1337-2010 and each provided viscosity value was an average of five measurements.
3.1. Characterization of functionalized GO nanocomposites
2.1. Reagents
SEM images of GO, GO-Fe and GO-Fe-P are shown in Fig. 1. It was found that GO had a smooth surface and a curled lamellar structure. Compared with GO, GO-Fe had a large-sized planar lamellar structure with a wrinkled surface and GO-Fe-P had a leaf-like lamellar structure with a much rougher surface than GO-Fe. The special morphologies of GO-Fe and GO-Fe-P were favorable to the enhancements of their compatibility and bonding strength with epoxy resin. XRD patterns of graphite, GO, GO-Fe and GO-Fe-P are presented in Fig. 2. It was found that the diffraction peak of graphite was located at 26.5� corresponding to its (002) reflection and its interlayer distance was calculated to be 0.336 nm [9]. In contrast with graphite, the char acteristic diffraction peak of GO was located at 10.2� , which corre sponded to its (001) reflection, and its interlayer distance was calculated to be 0.866 nm. The expanded interlayer spacing can be attributed to the presence of abundant oxygen-containing functional groups on both sides of the GO sheet [11]. The XRD pattern of GO-Fe indicated that the diffraction peak of GO moved to 9.3� and its intensity decreased obvi ously, which was probably caused by the extension of the GO sheet as shown in Fig. 1 and the oxygen donor coordination of GO to ferric ions, leading to further expansion of the interlayer spacing [12]. In the XRD pattern of GO-Fe-P, the diffraction peak of GO moved to 10.0� and its intensity increased compared with GO-Fe, which might be due to a distortion of the crystal structure and morphology change after the formation of the Fe–P moieties as shown in Fig. 1.
2.3. Preparation of GO Graphene oxide was synthesized by the improved Hummers’ method according to Ref. [8]. Briefly, 2 g of graphite powder and 14 g KMnO4 were successively added to a mixture of 10 mL H3PO4 and 90 mL H2SO4. The reaction mixture was stirred at 50 � C for 12 h. After cooling to room temperature, 1.5 L deionized water was added and the reaction mixture continued to be stirred for 0.5 h. Then, 30% H2O2 was added slowly until a bright yellow reaction mixture was observed. The remaining solid material was obtained by centrifugation and then washed in succession with deionized water, 30% HCl and ethanol. The obtained GO was freeze-dried at 50 � C for 24 h.
3.2. Curing reaction activity evaluation The curing reaction of epoxy resin with the moisture-activated ketimine latent curing agent in air at room temperature was a slow process [4]. Therefore, GO nanocomposites were added into OCRTCEA in this paper and the effect of GO nanocomposites on the curing reaction activity of OCRTCEA in air at room temperature was investigated and shown in Fig. 3(a). The results showed that the lap shear strength of EA was 0.33 MPa when exposed for 5 h, and showed a slow linear increase with increase of exposure time. The lap shear strength of EA increased to 4.57 MPa when the exposure time was 24 h. Compared with EA, the lap shear strengths of GO/EA, GO-Fe/EA and GO-Fe-P/EA were 1.67 MPa, 2.02 MPa and 3.46 MPa, respectively, when exposed for 5 h, and enhanced rapidly with the increase of exposure time. The lap shear strengths of GO/EA, GO-Fe/EA and GO-Fe-P/EA were 8.01 MPa, 9.22 MPa and 11.2 MPa when the exposure time was 24 h, which were 75.3%, 102% and 144% higher than that of EA, respectively.
2.4. Preparation of functionalized GO nanocomposites GO-Fe and GO-Fe-P were prepared according to Ref. [9]. Briefly, 0.1 g GO was added to 100 mL of 0.1 moL/L HCl aqueous solution and a uniformly dispersed GO solution was obtained after ultrasonic treatment at room temperature for 2 h. 200 mL of 0.1 moL/L FeCl3 aqueous so lution was then slowly added to the above GO solution and the mixture was ultrasonicated at room temperature for 4 h. The dark brown GO-Fe powder was centrifuged, and washed with ethanol several times, and dried at 60 � C in a vacuum oven. The preparation process of GO-Fe-P was the same as that of GO-Fe except that GO-Fe and NaH2PO4 as re actants were substituted for GO and FeCl3, respectively. 2.5. Synthesis of ketimine The ketimine was synthesized according to Ref. [10]. Briefly, 0.1 moL m-phenylenediamine was reacted with 0.4 mol methyl isobutyl ketone under stirring at 170 � C for 3 h, and the water produced during the reaction was removed with a Dean–Stark trap. The reaction was regarded complete after the production of a theoretical amount of water. The residual methyl isobutyl ketone was distilled off from the mixture under reduced pressure to obtain the flaxen ketimine.
3.3. SEM observation SEM images of lap shear fracture surfaces of OCRTCEAs exposed in air at room temperature for 24 h are shown in Fig. 4. It was found that the surface of EA was smooth due to the inherent brittle characteristic of the cured epoxy resin [13]. Compared with EA, the surface of OCRTCEA containing GO or GO nanocomposite became rough and the obvious texture as indicated by the arrows was also observed. This behavior could be ascribed to good interfacial adhesion between GO or GO nanocomposite and epoxy resin as well as the deflection of propagating
2.6. Preparation of OCRTCEA 0.03 g GO, GO-Fe or GO-Fe-P was added into 5 mL sample bottle, 2
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
Fig. 1. SEM images of GO, GO-Fe and GO-Fe-P.
Fig. 2. XRD patterns of graphite, GO, GO-Fe and GO-Fe-P.
Fig. 3. Effects of GO nanocomposites on lap shear strength (a) and storage stability (b) of OCRTCEA.
crack fronts induced by the GO sheets under lap shear test [14]. Among them, GO-Fe-P/EA possessed the most ordered surface texture that was possibly related to a combination of its high curing degree and the special leaf-like lamellar structure with the rough surface of GO-Fe-P.
The dispersion performance of GO or GO nanocomposite in OCRT CEA was further analyzed by EDS and the corresponding EDS images are shown in Fig. 5, where a was the dispersion state of oxygen in GO/EA, b was the dispersion state of ferrum in GO-Fe/EA and c was the dispersion 3
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
Fig. 4. SEM images of lap shear fracture surfaces of OCRTCEAs.
Fig. 5. Optical microscopy photographs of OCRTCEAs.
state of phosphorus in GO-Fe-P/EA. It was clear that the three elements were dispersed uniformly in OCRTCEAs, respectively, which certified the excellent dispersion performance of GO and its nanocomposite.
special leaf-like lamellar structure with the rough surface of GO-Fe-P and its high curing reaction activity as a result of the strong polarity of GO-Fe-P.
3.4. Mechanism analysis
3.5. Storage stability evaluation
The possible mechanism for the improvement of curing reaction activity of OCRTCEA by the addition of functionalized GO nano composite is shown in Fig. 6. The curing reaction of OCRTCEA without GO nanocomposite was a slow process because it was not easy to absorb the moisture in the air and the decomposition of ketimine to produce mphenylenediamine that would continue to initiate the curing reaction of epoxy resin had a low reaction rate [5,6]. When the functionalized GO nanocomposite was added into OCRTCEA, it did not only produce chemical bonding with the epoxy resin and the curing agent [13] but also could absorb moisture in the air and promote water molecules to migrate from the surface to the interior of the OCRTCEA because of its strong polarity, which would accelerate the decomposition rate of ketimine. Therefore, the curing reaction activity of OCRTCEA was improved in the presence of functionalized GO nanocomposite. More over, the largest lap shear strength of GO-Fe-P/EA was ascribed to the
Storage stability was an important index to evaluate the shelf life of OCRTCEA, because it was easy to lose the flowability once it made contact with moisture in the air. In this paper, the storage stability of OCRTCEA was evaluated by investigating the viscosity change during storage and the viscosity change over time for OCRTCEA is shown in Fig. 3(b). It was found that the viscosity of EA increased slowly with the increase of time when the storage time was less than 60 days and showed a rapid increase when the storage time was more than 60 days. OCRT CEAs prepared in this paper were stored in a closed container filled with nitrogen, so the increased viscosity of EA might be due to a minute amount of water in the ketimine and epoxy resin that could lead to the decomposition of ketimene to produce m-phenylenediamine which would initiate cure of the epoxy resin at a slow rate. The variation trend of viscosity of GO/EA, GO-Fe/EA or GO-Fe-P/EA with time was similar to that of EA with a more rapid increase of viscosity for GO/EA, GO-Fe/
Fig. 6. Possible mechanism for the improvement of curing reaction activity of OCRTCEA by the addition of functionalized GO nanocomposite. 4
B. Li et al.
International Journal of Adhesion and Adhesives 98 (2020) 102537
EA or GO-Fe-P/EA than for EA because of the curing acceleration of GO or GO nanocomposite.
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4. Conclusions The functionalized GO nanocomposites were prepared and added in OCRTCEA for the first time. The results showed that the functionalized GO nanocomposites were conducive to accelerating the curing reaction of OCRTCEA with the moisture-activated ketimine as a latent curing agent, and the storage stability of OCRTCEAs containing GO nano composites was similar with that of pure OCRTCEA within 60 days. OCRTCEA prepared in this paper with the enhanced curing reaction activity and great storage stability was convenient to be used and stored, which would be propitious to its wide applications in fields without ready access to heating, such as in civil construction, electrical and electronics engineering, and so on. References [1] Michael J. Low odor, fast cure, toughened epoxy adhesive. 2010-6-29. US7745006. [2] Browning JD, Mcginniss VD, Vajayendran BR. Single component room temperature curable low VOC epoxy coatings. 2003-11-18. US6649673B2. [3] Endo T, Sanda F, Suzuki K, Horii H, Matsuura N. One-pack moisture-curable epoxy resin composition. 2006-3-8. EP1362876B1.
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