Article pubs.acs.org/IECR
Branched 1,6-Diaminohexane-Derived Aliphatic Polyamine as Curing Agent for Epoxy: Isothermal Cure, Network Structure, and Mechanical Properties Jintao Wan,†,‡ Cheng Li,† Hong Fan,*,† and Bo-Geng Li† †
State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ‡ IMDEA Materials Institute, C/Eric Kandel, 2, 28906 Getafe, Madrid, Spain ABSTRACT: Aliphatic diamines and polyamines are long used as important curing agents for epoxy resins, especially in room-temperature-cure epoxy coatings and adhesives due to their high reactivity and low cost. Herein we systematically evaluate our newly developed liquid branched aliphatic polyamine, N,N,N′,N′-tetra(3aminopropyl)-1,6-diaminohexane (TADH), as the curing agent for bisphenol A epoxy (DGEBA), emphasizing the isothermal cure reaction, network structure, and mechanical properties. The isothermal curing reaction of DGEBA/TADH at 40, 50 60, and 70 °C is autocatalytic, and we adequately simulate the curing kinetic rate with the extended autocatalytic Kamal model. Further isoconversional kinetic analysis reveals that the effective activation energy changes dramatically as the reaction proceeds, especially for the diffusion-controlled stage due to the chemical vitrification (Tg∞(DSC) = 119 °C). Compared to DGEBA/1,6-diaminohexane (a starting material of TADH), a dynamic mechanical analysis illustrates that the cured DGEBA/TADH network features three relaxations and shows the increased storage modulus (up to 54 °C), α and β transition temperatures, and activation energy of the α relaxation. Also, DGEBA/TADH shows enhanced mechanical properties: flexural strength (∼95 MPa), flexural modulus (∼2440 MPa), and shear strength (∼8 MPa), with good processing ability (gel time of 130−150 min at 25 °C). Due to these merits, TADH may be suitable to be used in epoxy systems, highlighting its good promise in adhesive applications.
1. INTRODUCTION Epoxy resins are mainly used in coatings, adhesives, structural composites, electronic encapsulation, printed circuit broads, sealants, and so on, owing to their excellent mechanical, thermal, anticorrosive, and electric properties, as well as good processing ability. Bisphenol A epoxy resins (DGEBAs), produced from condensation of bisphenol A and epichlorohydrin, are most commonly used to meet huge global demand (estimated roughly U.S. $18.6 billion in 20131). Curing is an essential step for processing and applications of thermosetting epoxy resins to achieve desired ultimate properties. Particularly, curing agents determine fundamental cross-linking reaction mechanisms and greatly affect the processing and ultimate properties of the shaped epoxy products. Curing agents can be divided into several main catagories according to their chemical attributes: amines, organic anhydrides, phenolic resins, polymercaptans, imidazole, complexes, etc. Very recently, renewed attention is paid to some especial epoxy curing agents with unique molecular structures2−7 and, in particular, biobased sustainable curing agents.8−12 This tendency reflects two important directions in development of epoxy curing agents: engineering ultimate properties through more sophisticated molecular design and by fostering sustainability by making use of abundant renewable starting materials. © 2017 American Chemical Society
With a shift of our attention to highly versatile epoxy curing agents commercially available, amine-based curing agents, bearing more than two reactive amino hydrogen atoms, are of primary importance because of their excellent properties such as high and tailorable reactivity, excellent corrosion resistance, easy access, and low cost. Among them, aliphatic amine curing agents are characterized by their high reactivity toward opening epoxy rings and low cost, and they can well cure epoxy resins even at room temperature, accounting for their major applications in room-temperature coatings, adhesives, potting materials, and sealants.13 However, conventional aliphatic amine curing agents always suffer from strong irritation with high vapor pressure at room temperature, fast absorption of carbon dioxide, high toxicity, and strict dosage respective to epoxy monomer or oligomers. Moreover, some aliphatic amine curing agents with long aliphatic chains are a crystal solid at room temperature, thus restricting their application in room-temperature epoxy coatings and adhesives where heating is inapplicable or infeasible. Received: Revised: Accepted: Published: 4938
February 12, 2017 April 4, 2017 April 10, 2017 April 10, 2017 DOI: 10.1021/acs.iecr.7b00610 Ind. Eng. Chem. Res. 2017, 56, 4938−4948
Article
Industrial & Engineering Chemistry Research
Scheme 1. Molecular Structures of N,N,N′,N′-Tetra(3-aminopropyl)-1,6-diaminohexane (TADH), 1,6-Diaminohexane (DAH), and Epoxy Resin (DGEBA)
Due to as-said merits of TADH, it will be interesting to further examine the applicability of TADH in epoxy system in a more comprehensive way. To better understand the TADHcured DGEBA system, herein we report an original study of the isothermal curing reaction kinetics of DGEBA/TADH using model-fitting and isoconversional methods, cured network characteristics from multiple frequency dynamic mechanical analysis, and mechanical properties of the cured epoxy. We also try to use TADH in a room-temperature epoxy adhesive formation. Our data will demonstrate that compared with 1,6diaminohexane, TADH endows the resultant DGEBA system with high reactivity, enhanced thermomechanical properties, and flexural strength and modulus and shear strength, as well as good processing ability.
To resolve these issues, simultaneously increasing molecular weights and reducing crystallization ability of aliphatic amine curing agents are very promising.14 Increasing molecular weights will notably reduce vapor pressure and toxicity and increase equivalent weight of curing agent; lowering crystallization ability will result in aliphatic amines at a liquid state. To achieve these goals, our previous work has demonstrated that introducing the branched structure into aliphatic amino molecules is very effective.15−19 To illustrate, in the previous publications,18,19 we reported the nonisothermal, isothermal curing kinetics and dynamic mechanical properties of a four-directional benzene-centered aliphatic polyamine curing agent for epoxy resins. Metaxylenediamine and acrylonitrile (CAN) were used as the starting materials for the curing agent synthesis. Metaxylenediamine is a liquid at room temperature with the melting point of 14 °C, whereas hexamethylenediamine (HAD) is much cheaper and is wildly used in huge volume (>3 billion pounds per year, representing a global market of >$4 billion20) especially for PA-66 synthesis. However, HAD is a crystal solid with the melting point of 41−42 °C, leading to their limited applications in curing agent for epoxy resin due mainly to poor processing ability without heating. In this case, modifying HAD via a chemical means to result in liquid curing agent to expand their application for epoxy curing agent for room-temperature purposes is of great interest. In another paper,21 we reported a new aliphatic polyamine curing agent (TADH) from HAD and acrylonitrile (CAN), with emphasis on the nonisothermal curing kinetics of the resulting epoxy systems. TADH is a liquid at the room temperature, thus facilitating its good mixing with epoxy at a low temperature range. In addition, the raw materials of TADH, i.e., 1,6-diaminohexane, acrylonitrile, and H2, are easy to access and inexpensive. Moreover, compared to 1,6diaminohexane (Mw = 116 Da), the much increased molecular weight of TADH (Mw = 345 Da) will reduce its volatility and toxicity and increase equivalent weight with respect to epoxy monomers or oligomers simultaneously. On the other hand, with the updated conception of sustainability in mind, HAD can be derived from biomass as glucose via a renewable process. In particular, in 2013 Rennovia Inc.20,22 announced that 100% biobased nylon-6,6 can be made by using renewable adipic acid and 1,6-diaminohexane with relatively low cost. CAN, widely used in production of carbon fiber, can be derived from a biomass as glutamic acid via a two-step method involving an its oxidative decarboxylation in water to 3-cyanopropanoic acid followed by decarbonylation elimination.23 Therefore, our newly developed aliphatic polyamine (TADH) seems more attractive based on some important industrial criteria, because the two key starting materials at least ideally come out of renewable resources, which will promote its sustainability.
2. EXPERIMENTAL SECTION 2.1. Materials. 1,6-Diaminohexane (DAH) and acrylonitrile were purchased from Shanghai Reagent Co., Ltd., China, and purified by distillation under reduced pressure prior to use. Diglycidyl ether of bisphenol A (DGEBA) was obtained from Heli Resin Co., Ltd., China, with the epoxide equivalent weight (EEW) of 196 g/equiv epoxy. The nonlinear aliphatic amine curing agent, N,N,N′,N′-tetra(3-aminopropyl)-1,6-diaminohexane (TADH), shown in Scheme 1, was synthesized by us, and the related spectrum data can be found in our previous publication.21 Due to the importance of TADH as discussed in the Introduction, herein we give our reproducible embodiment of TADH synthesis which is not yet reported in detail before. To a three-necked flask equipped with a dropping funnel, a condensation column, a thermometer, and a magnetic stirrer, 60 g of 1,6-diaminohexane (at 50−60 °C) and distilled water (120 mL) were mixed together to form a transparent solution. At 25 °C, to this solution acrylonitrile was added dropwise with stirring in 2 h, and the mixture was heated to 40 °C for 1 h. The mixture was slowly heated to reflux temperature (about 80 °C) for 20 h. After that, the water and unreacted acrylonitrile were removed with a rotatory evaporator under reduced pressure. The crude produce was washed with warmed distilled water several times and subsequently dried in a vacuum oven at 80 °C for 5 h. Finally, the yellowish viscous liquid product, N,N,N′,N′tetra(3-nitrilepropyl)-1,6-diaminohexane (compound 1), was obtained in a very good yield (∼93%). To a high-pressure autoclave (2000 mL), compound 1 (100 g), ethanol (95%, 1200 mL), Raney nickel (100 g), and NaOH (48 g) were loaded and mixed together. After replacement of air in the autoclave, H2 was charged to 20 bar with vigorous agitation (800 rpm) at room temperature. The hydrogenation process would take about 10 h, as indicated by no decrease in the pressure. After that, the catalyst was filtered off and then the filtrate was concentrated, leading to two layers. The upper layer 4939
DOI: 10.1021/acs.iecr.7b00610 Ind. Eng. Chem. Res. 2017, 56, 4938−4948
Article
Industrial & Engineering Chemistry Research
The gel time of the epoxy−amine mixture was estimated. A stoichiometric amount of DGEBA and TADH (∼5 g) was mixed well with stirring in a test tube immersed in a water bath (25 °C). The reaction time for the mixture to lose its flow ability was recorded and taken as the gel time. 2.4. Theoretical Aspect of Curing Kinetics. Curing reactions of the epoxy resins were studied using DSC. In this case, for kinetic analysis the heat flux rate was assumed to be directly proportional to the reaction rate; thus, the kinetic rate (conversion rate) can be represented as the following expression:
was collected and extracted with toluene combined with a very small fraction of water several times. Then extract was concentrated and further purified through a flash silica gel column with menthol as the eluent. Completely drying the collected fraction yielded the target polyamine N,N,N′,N′tetra(3-aminopropyl)-1,6-diaminohexane (TADH) in a good yield (∼75%). 2.2. Preparation of Cured Epoxy Sample. Stoichiometric DGEBA and DAH (epoxy/N−H = 1:1 by mole) were mixed well at 45 °C to form a transparent solution, and immediately the mixture was poured into a preheated steel module. Then the module was transferred into an oven under reduced pressure for 5 min to drive off entrapped bubbles. The following schedules were applied to cure the epoxy resin in an air-blast oven: 70 °C/1.5 h + 150 °C/2.5 h. The obtained rectangular epoxy specimens were used for the mechanical tests. Similarly, the cured DGEBA/TADH was prepared. Because of the liquid feature of TADH, DGEBA and TADH could be mixed well with stirring at room temperature. 2.3. Instrumentation and Characterization. The curing reaction of DGEBA/TADH was studied using a differential scanning calorimeter (Perkin Elemer DSC-7) in an isothermal mode. The DSC instrument was calibrated with an indium standard before any measurement. The reaction exotherm as a function of time was recorded for the different temperatures of 40, 50, 60, and 70 °C, followed by a second run from 25 to 250 °C at 10 °C/min to measure heat residual. The glass transition temperature of completely cured epoxy resin is determined with heating rate of 10 °C/min. Stoichiometric DGEBA and TADH were mixed well as quickly as possible at
