Synthesis of epoxy resins derivatives of naphthalene-2,7-diol and their cross-linked products
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The aim of this study was the synthesis of three different epoxy compounds based on naphthalene-2,7-diol (2,7-NAF.EP, 2,7-NAF.WEP, 2,7-NAF.P.EP) and then their cross-linking by triethylenetetramine (TETA). All epoxides were prepared by the reaction of naphthalene-2,7-diol with epichlorohydrin but under different conditions and with other catalysts. The structures of the obtained compounds before and after the cross-linking reactions were confirmed by the attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FT-IR). The ATR/FT-IR spectra of cross-linked compounds show disappearance of the C–O–C bands (about 915 cm−1) derived from the epoxy groups. DSC and TG/DTG measurements indicated that the obtained materials possess good thermal resistance; they are stable up to about 250 °C. The hardness of the cross-linked products was determined using the Shore D method. The highest value of hardness was obtained for the 2,7-NAF.EP-POL. Additionally, the UV–Vis absorption spectra of the obtained polymers were registered and evaluated.
KeywordsEpoxy resins Naphthalene-2,7-diol Thermal properties Luminescent properties Hardness test
Epoxy resins are very important class of thermosetting polymers which were synthesized by Prishajew in 1909 . Chemical structures of the most important epoxy resins contain aliphatic, aromatic or cycloaliphatic groups and more than one epoxy group [2, 3]. These compounds found a wide range of applications in protective coatings, electronic-packaging materials, adhesives and high-performance composites, etc., due to good mechanical strength, strong adhesion, high moisture and solvent resistances, outstanding chemical resistance, good thermal and dimensional stabilities, superior electrical properties and wide formulation diversity [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. However, the inherent brittle nature and poor crack resistance limited their applications significantly. Recently, much research has been carried out to toughen epoxy resins through the addition of flexible curing agents [16, 17, 18, 19]. Wang et al.  prepared novel amines with different lengths of flexible polyoxypropylene side chains (AFPE). They used the obtained compound as a curing agent for diglycidyl ether of bisphenol A (DGEBA). DSC showed that the activation energy (Ea) value of DGEBA/AFPE increased from 45.2 to 52.4 kJ mol−1 with the increasing molecular weight of AFPE. Furthermore, when the AFPE was added into the epoxy networks, the impact strength and elongation at break improved significantly, which was mainly due to flexible polyoxypropylene side chains in the networks, decreasing cross-link density and increasing size of the cavities. Their study showed that AFPE was a novel and effective toughening agent for epoxy resins.
In general, uncured epoxy resins have only poor mechanical, chemical and heat resistance properties. However, good properties are obtained by reacting the linear epoxy resin with suitable hardeners to form three-dimensional cross-linked thermoset structures. Among commonly used curing agents for epoxy resins, amine-based ones, especially aromatic and aliphatic amines, are of prime significance in practical applications. Nowadays, most related scientific work is focused on aromatic-amine curing agents. Yet little attention is paid to developing aliphatic amine ones. Although aromatic-amine curing agents can endow their cured epoxy resins with improved thermomechanical properties and fire resistance, they still suffer from their low reactivity and high melting temperatures. For this reason, high-temperature cure must be applied to improve the compatibility of aromatic-amine curing agents with epoxy resins and to accelerate curing reactions. On the other hand, aliphatic-amine curing agents can cure epoxy resins efficiently at room temperature and at even somewhat lower temperatures without heating because they are characterized by high reactivity and low melting temperatures, accounting for their principal applications in room-temperature-cure epoxy coatings and adhesives [20, 21, 22, 23, 24, 25, 26].
Numerous studies on improvement of heat resistance of epoxy resins by increasing the cross-linking density of cured epoxy resin  or introducing bulky structures such as biphenyl or naphthalene [28, 29] were reported. The addition of naphthalene moiety is expected to improve greatly thermal property and moisture resistance [30, 31, 32]. Pan et al.  synthesized a series of novel novolac epoxy resins containing naphthalene moiety with different molecular weights via condensation of bisphenol A and 1-naphthaldehyde, followed by epoxidation with epichlorohydrin. They demonstrated that the cured naphthalene epoxy resin exhibited remarkably higher glass transition temperatures (Tg), enhanced thermal stability and better moisture resistance than the diglycidyl ether of bisphenol A (DGEBA). These pronounced good properties make it an attractive candidate for electronic encapsulation applications and composite materials. Xu et al.  conducted the synthesis of novel epoxy resin bearing naphthyl and limonene moieties. They showed that the obtained compounds have remarkably higher Tg, lower coefficient of thermal expansion, higher thermal stability, better moisture resistance and dielectric property.
This paper presents the synthesis of epoxy resins derivatives of naphthalene-2,7-diol: 2,7-NAF.EP, 2,7-NAF.P.EP and 2,7-NAF.WEP. These compounds were cross-linked using triethylenetetramine (TETA). The chemical structures of all new compounds were confirmed by spectroscopic methods. Thermal and luminescent properties of the obtained epoxy resins were also studied.
Naphthalene-2,7-diol was purchased from Sigma-Aldrich (Germany). Epichlorohydrin was from Fluka (Switzerland). 2-Propanol, sodium hydroxide, toluene, propyl carbonate, potassium carbonate, potassium acetate, triethylenetetramine (TETA), acetic acid, butanol and xylene were obtained from Avantor Performance Materials Poland S.A. All above-mentioned chemicals were used as received.
Characterization of the products
The epoxide value (EV) was determined according to the following procedure. About 0.5 g of the tested resin was weighed on an analytical balance with an accuracy of 0.0001 g by introducing it into an Erlenmayer flask (200 mL). The resin was mixed with 13 mL of the hydrochloric acid solution in dioxane. Then, five drops of cresol red solution were introduced into this mixture and were titrated with 0.2 M alcohol solution of NaOH. During neutralization, the color of the solution changed from red to purple via yellow.
Attenuated total reflection-Fourier transform infrared (ATR/FT-IR) spectra were recorded on a Bruker FTIR spectrophotometer TENSOR 27 (Bruker, Germany) using thin films of epoxy compounds. Spectra were gathered from 4000 to 600 cm−1 averaging 32 scans with a resolution of 4 cm−1.
Differential scanning calorimetry (DSC) curves were obtained on a DSC Netzsch 204 calorimeter (Netzsch, Günzbung, Germany). DSC measurements were taken in the aluminium pans with a pierced lid of the sample weight of ~ 5 to 15 mg in the nitrogen atmosphere (30 mL min−1). The empty aluminium crucible was applied as a reference. Dynamic scans were obtained at a heating rate of 10 K min−1 in the temperature range 20–550 °C. The parameters such as: decomposition temperatures (Tonset, Toffset), final decomposition temperature (Td) and enthalpy of decomposition (ΔHd) were determined.
Thermogravimetric analysis (TG/DTG) was made with the use of a thermal analyzer STA 449 F1 Jupiter (Netzsch, Selb, Germany) with the sample mass of ~ 5 to 10 mg in the helium atmosphere (20 mL min−1). Dynamic scans were made at the heating rate of 10 °C min−1 in the temperature range 0–600 °C. The main parameters of the thermal degradation were as follows: TIDT—the initial decomposition temperature of the sample, T20—the temperature at 20% mass loss, T50—the temperature for 50% mass loss, peak maximum decomposition temperature (Tmax), mk—the final mass loss at 600 °C, Tf—the final decomposition temperature. All measurements were taken in the Al2O3 crucible. As a reference, the empty Al2O3 crucible was used.
The hardness of the epoxy compounds was measured by the Shore D method on a Zwick 7206/H04 hardness tester (Germany) at 298 K. The values were taken after 15 s.
Room temperature UV–Vis reflectance spectra were registered using a horizontal sampling integrating sphere (Model PIV-756) connected to a V-660 JASCO spectrophotometer.
Synthesis of epoxy compounds
Synthesis of epoxy resin of 2,7-NAF.EP
Synthesis of epoxy resin with an average molecular weight of derivative naphthalene-2,7-diol (2,7-NAF.WEP)
Synthesis of 2,7-di[2-(2,3-epoxypropoxy)propoxy]naphthalene (2,7-NAF.P.EP)
In the first stage, 43 g of naphthalene-2,7-diol, 55.1 g of propyl carbonate and 0.13 g of potassium carbonate as a catalyst were added to a round-bottom flask of 250 mL equipped with a thermometer, mechanical stirrer, reflux condenser and a pipe for introducing liquid nitrogen. The reaction was carried out at high temperature in the range of 210–220 °C. During the 2.5 h reaction, the release of CO2 was evident. When the gas ceased to be released, the reaction was continued for 0.5 h. Then, 200 mL of chloroform and 150 mL of distilled water were added to the flask. The contents of the flask were transferred to a separating funnel. After careful separation of the phases, the solvents were distilled from the organic layer. As a result, 2,7-di(2-hydroxypropoxy)naphthalene was obtained.
In the second stage, the epoxidation reaction was performed using epichlorohydrin. The molar ratio of epichlorohydrin to the resulting diol was 10. 51 g of 2,7-di[2-hydroxypropoxy]naphthalene; 150 mL of epichlorohydrin and 0.4 g of anhydrous potassium acetate were placed in the 500-mL round-bottom flask which was equipped with a thermometer, mechanical stirrer and the Dean–Stark trap for azeotropic water separation. The reaction mixture was heated to 90 °C until all reactants were dissolved. Next, 18.5 g of NaOH suspended in 70 mL of xylene was added to the flask. During this reaction, NaCl precipitated and the resulting water was azeotropically distilled using the Dean–Stark trap. After the reaction, 400 mL of toluene was added to the flask. The whole content was heated for about 15 min. NaCl was separated, and the solvents were distilled from the filtrate. The epoxy resin (2,7-NAF.P-EP) was obtained in the form of a slightly yellow, viscous liquid with the epoxide number 0.32 .
Curing of epoxy compounds using triethylenetetramine (TETA)
Amount of epoxide/g
Results and discussion
In turn, in the spectrum of 2,7-NAF.EP-POL, the stretching vibrations of the hydroxyl group at 3332 cm−1 were observed. The group C=C gives a signal at 1626 cm−1. The signals derived from the aromatic C–H group are present at the wavelength of about 830 cm−1. The visible absorption band at the wavelength of about 1120 cm−1 indicates the vibration of C–O–C bonds. The spectrum also shows disappearance of the C–O bands derived from the epoxy group which indicates the proper course of curing reactions. Additionally, the stretching vibration of C–N group occurs at 1170 cm−1 and also the N–H bending signal is visible at 1490 cm−1.
The results of DSC analysis of the obtained resins
Name of sample
The DSC analysis showed the differences in thermal behaviour of the obtained epoxy resins (2,7-NAF.EP; 2,7-NAF.P.EP, and 2,7-NAF.WEP) and their cross-linked polymers. The DSC curves of 2,7-NAF.EP and its polymer (Fig. 10) show one distinct endothermic peak. The endothermic peaks at 333–345 °C with the ΔHd values from 85 to 256 J g−1 correspond with the total thermal degradation of these compounds.
The results of TG/DTG analysis of the obtained polymers in helium conditions
18 (600 °C)
14 (600 °C)
5 (600 °C)
The results of TG/DTG analysis of the obtained polymers in air conditions
26 (600 °C)
19 (600 °C)
5 (600 °C)
The type of atmosphere in which thermal decomposition was carried out also affects the speed of this decomposition. For example, material 2,7-NAF.EP-POL decomposes in helium with the rate of − 11.07% min−1 but in air atmosphere the maximum rate of the mass loss was − 8.18% min−1. In helium, the rate of the mass loss for 2,7-NAF.WEP-POL for first maximum peak is − 6.75% min−1, and for second maximum is − 8.85% min−1 while in case distribution in air the first maximum of decomposition is − 5.49% min−1 and the second is − 4.85% min−1. In the case of 2,7-NAF.P.EP-POL, the maximum rate of thermal decomposition in helium atmosphere is − 11.1% min−1 while in air conditions this value is − 4.73% min−1.
The shape of TG and DTG curves received in air conditions is close of the shape these curves obtained in helium atmosphere. The synthesized epoxy resins generally are characterized by good thermal resistance and stability.
The study of epoxy compounds hardness
The results of test hardness according to the Shore method
Hardness in scale D/°Sh
Absorbance measurements of the obtained epoxy resins
The synthesis and characterization of new cross-linked polymeric naphthalene derivatives with unique luminescent properties are presented. As a result of the reactions of naphthalene-2,7-diol with epichlorohydrin, three epoxide compounds were obtained under various conditions and using other catalysts. All obtained epoxy derivatives undergo cross-linking under the influence of triethylenetetramine giving solid products with different hardness.
These materials are characterized by good thermal resistance. The final decomposition temperatures for all compositions are from 424 to 430 °C. After excitation of UV radiation, the copolymers (and epoxides) emitted yellow-green radiation. This property makes the naphthalene derivatives very useful precursors for preparation of e.g.: fluorophores of luminescent materials, polymer light-emitting diodes and organic semiconductors. Easily processable fluorescent polymers containing naphthol chromophores could have potential applications in the production of high laser-resistant materials, laser dyes or fiber optic sensors. Due to suitable thermal resistance, the obtained epoxy resins can be used in the production of thermally resistant coatings.
The authors would like to thank Dr. Andrzej Bartnicki (UMCS Lublin) for his help in the synthesis of epoxy resins.
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