The kinetics of model reactions of curing epoxy resins with amines

The isothermal course of the reaction of phenylglycidyl ether and N,N-methylglycidylaniline with dibutyl amine at various temperatures was investigated DSC. The data were treated on the basis of a reaction scheme with two processes in parallel, one of them auto-catalyzed. A good fit with the experiment was reached only when the order of both processes with respect to amine had been reduced to half its original value. An assumption that the kinetics of the amine-epoxy resin rection are considerably affected by the formation of various complexes through hydrogen bonds may be an explanation. A simple mathematical model has been suggested to estimate this influence. Because of the relatively high heats of interaction for the formation of complexes, the dependence of the measured heat on the degree of conversion is not linear. The magnitude of the error caused by neglecting this fact in investigation of the kinetics by DSC has been estimated.     

Curing epoxy resins with anhydrides. Model reactions and reaction mechanism

The mechanism of acid curing of epoxy resins catalyzed with tertiary amines was investigated by using model systems composed of phenylglycidyl ether and benzoic acid or acetic acid anhydrides in the presence of benzyldimethylamine. The reaction was studied by NMR spectrometry, liquid chromatography, and ozone absorption. The main findings are that (1) the tert-amine is bound chemically and irreversibly during the reaction under the formation of a quaternary ammonium salt and (2) 1-phenyloxypropanediol-2,3-dibenzoate or diacetate is the main reaction product. The suggested reaction mechanism involves initiation in which the tertiary amine reacts with the epoxy group, giving rise to a zwitterion that contains a quaternary nitrogen atom and an alkoxide anion; the latter immediately reacts with the anhydride and quaternary salt is formed. In a later stage the carboxy anion of the quaternary salt reacts first with the epoxy group, then with the anhydride. By this reaction diester is formed and the carboxy anion is regenerated.     

Model reactions of amine curing of glycidylamine epoxy resins: Homopolymerization of N-methylglycidylaniline

The kinetics of homopolymerization of the monofunctional epoxide N-methylglycidylaniline in the presence of a tertiary amine or an amino alcohol has been followed by reversed phase high performance liquid chromatography and size exclusion chromatography. The reaction products were identified by mass spectrometry using potassium ionization of desorbed species (K⁺IDS). 1,3-Di-N-methylanilino-2-propanol (P) was the main reaction product and low molecular weight oligomers with M_n > 600 were also formed. The molecular weight and fraction of oligomers decrease with increasing concentration of the initiator. The suggested complex reaction mechanism involves formation of four stable oligomeric series initiated by reaction of the epoxide with either an OH group of (a) the amino alcohol, (b) product P, (c) traces of water, or (d) the tertiary amine to form ionic species resulting in the ionic propagation. Regeneration of the initiator and formation of new initiating centers during the polymerization are the causes of low molecular weights of oligomers. © 1992 John Wiley & Sons, Inc.     

Effects of tertiary amine accelerators on curing of epoxide resins

The effects of seven tertiary amine accelerators on curing of bisphenol-type epoxide resins using azelaic acid as a curing agent have been investigated. The structure of the cured resins is characterized and reaction and structure schemes are proposed. The reaction mechanism and the resulting structure of the resin depend on the basicity of the accelerator. With increasing accelerator basicity crosslinking in the cured resin increases. Characterization results indicate that the network structure consists of ether bonds or a mixture of ether and ester bonds; the linear structure consists of only ester bonds. The structure and, therefore, the properties of the cured epoxide resin may thus be regulated by selection of the amine basicity.     

Curing studies of epoxy resins with phosphorus‐containing amines

The curing system of diglycidyl ether of bisphenol A (DGEBA) with two phosphorus‐containing amine compounds—bis(3‐aminophenyl)methyl phosphine oxide and bis(4‐aminophenyl)‐bis(9,10‐dihydro‐9‐oxa‐10‐oxide‐10‐phosphaphenanthrene‐10‐yl)methane—was studied with differential scanning calorimetry under isothermal and nonisothermal conditions and compared with the DGEBA/diamino diphenyl methane system. The isoconversional method was used to evaluate the dependence of the effective activation energy on the extent of conversion. Modulated differential scanning calorimetry and dynamic mechanical thermal analysis were used to study the phenomena of vitrification and gelation. The thermal and flame‐retardant properties were evaluated, and the limiting oxygen index values of the phosphorylated resins, above 30, confirmed that phosphorus‐containing epoxy resins are effective flame retardants. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1676–1685, 2006     

Curing behavior and kinetics of epoxy resins cured with liquid crystalline curing agent

This article describes the synthesis of a liquid crystalline curing agent 4,4′-bis-(4-amine-butyloxy)-biphenyl (BABB), and its application as a curing agent for the epoxy resin (DGEBA) in comparison with normal curing agent, 4,4′-diaminobiphenyl (DABP). BABB was investigated with polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scatting, and the results showed that BABB displayed smectic liquid crystalline phase. The curing behaviors of DGEBA cured with BABB and DABP were studied by using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and dynamic mechanical analysis (DMA). The results indicated that BABB showed a higher chemical reactivity than DABP. The kinetics was studied under isothermal conditions using an isoconversional method, and the isothermal DSC data can be fitted reasonably by an autocatalytic curing model. The nematic droplet texture was observed for the resulting polymer network of DGEBA/BABB system, while the DGEBA/DABP system showed an isotropic state. The storage modulus of DGEBA/BABB system was enhanced in comparison with DGEBA/DABP system because of the formation of LC phase, whereas the glass transition temperatures decreased because of the introduction of flexible spacer group.     

Kinetics of curing reactions for epoxy-amine/polyethersulphone resins

Differential scanning calorimetry (DSC) was used to indicate the relative extents of the different cure reactions of the 4-glycityloxyl-N, N-diglycidylaniline (MY0510), polyglycidyl ether of phenol formaldehyde novolac (DEN431) and 3,3 diamino diphenylsulphone (3,3 DDS) resin systems and how these were affected by the presence of polyethersulphone (PES). The extent of reactions at any given time decreased with increasing PES concentration and the reaction rate maximum shifted to longer times. The cured resin systems were examined using dynamic mechanical analysis (DMA). Broader β-transitions of lower intensities were observed in specimens containing PES, suggesting an increased range of relaxations within the transition.     

Calorimetric investigation of polymerization reactions. III. Curing reaction of epoxides with amines

The curing reactions of epoxy resin with aliphatic diamines and the reaction of phenyl glycidyl ether with butylamine as a model for the curing reactions were investigated with a differential scanning calorimeter (DSC) operated isothermally. The heat of reaction of phenyl glycidyl ether with butylamine is equal to 24.5 ± 0.6 kcal/mole. The rate of reaction was followed over the whole range of conversion for both model and curing reactions. The reactions are accelerated by the hydrogen-bond donor produced in the system. The rate constants based on the third-order kinetics were determined and discussed for the model reaction and for the chemically controlled region of curing reactions. The activation energies for these rate constants are 13-14 kcal/mole. At a later stage of conversion, the curing reactions become controlled by diffusion of functional groups. The final extent of conversion is short of completion for most isothermally cured and even for postcured samples because of crosslinking. It was quantitatively indicated that the final conversion of isothermal cure corresponds to the transition of the system from a viscous liquid to a glass on the basis of the theory of glass transition temperature of crosslinked polymer systems.     

Comparative studies on the curing kinetics and thermal stability of tetrafunctional epoxy resins using various amines as curing agents

The curing reactions of the epoxy resins tetraglycidyl diaminodiphenyl methane (TGDDM) and tetraglycidyl methylenebis (o-toluidine) (TGMBT) using diaminodiphenyl sulfone (DDS), diaminodiphenyl methane (DDM) and diethylenetriamine (DETA) as curing agents were studied kinetically by differential scanning calorimetry. The dynamic scans in the temperature range 20°–300°C were analyzed to estimate the activation energy and the order of reaction for the curing process using some empirical relations. The activation energy for the various epoxy systems is observed in the range 71.9–110.2 kJ·mol^–1. The cured epoxy resins were studied for kinetics of thermal degradation by thermogravimetry in a static air atmosphere at a heating rate of 10 deg·min^–1. The thermal degradation reactions were found to proceed in a single step having an activation energy in the range 27.6–51.4 kJ·mol^–1.

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Zusammenfassung Die Vernetzungsreaktionen der Epoxidharze Tetraglycidyl-diamino-diphenyl-methan (TGDDM) und Tetraglycidyl-methylen-bis(o-toluidin) (TGMBT) unter Verwendung von Diaminodiphenylsulfon (DDS), Diaminodiphenylmethan (DDM) und Diethylentriamin (DETA) als Vernetzungsmittel wurden kinetisch mittels DSC untersucht. Die dynamischen Scans im Temperaturbereich 20°–300°C wurden analysiert, um unter Anwendung einiger empirischer Gleichungen die Aktivierungsenergie und die Reaktionsordnung des Vernetzungsprozesses zu ermitteln. Die Aktivierungsenergie der einzelnen Epoxy-Systeme liegt im Bereich 71.9–110.2 kJ·mol^–1. An der ausgehärteten Harze wurde mittels TG in einer statischen Luftatmosphäre un deiner Aufheizgeschwindigkeit von 10 Grad/min die Kinetik des termischen Abbaues untersucht. Man fand, daß die thermiscehn Abbaureaktionen in einem Schritt ablaufen und ihre Aktivierungsenergie im Intervall 27.6–51.4 kJ·mol^–1 liegt.

     

The curing of epoxy resins with aminophenoxycyclotriphosphazenes

Aminophenoxycyclotriphosphazenes have been used as curing agents for epoxy resins. The thermal curing was performed in stages at 120–125 and 175–180°C followed by postcuring at 225°C to give tough brown polymers. The thermal curing reaction was monitored using FTIR and differential scanning calorimetry. Thermogravimetric analysis of the cured resins has shown thermal stability up to 350–340°C. The char yield obtained in nitrogen at 800°C was about 55–42% and in air at 700°C was about 40–32%. Graphite cloth laminates were prepared. The mechanical properties evaluated were found superior to those of commonly used epoxy resin systems. These resins are useful for making fire- and heat-resistant composites, laminates, molded parts, and adhesives.     

Microwave-enhanced bismuth triflate-catalyzed epoxide opening with aliphatic amines

In the presence of a catalytic amount of Bi(OTf)₃·4H₂O and under microwave irradiation, neat mixtures of epoxides and amines afforded smoothly the corresponding 2-amino alcohols. A wide variety of aliphatic amines were reacted with cycloalkene oxide, styrene oxide, and stilbene oxide. The reaction proceeded rapidly and afforded the 2-amino alcohols in high up to quantitative yields. All products could be obtained without aqueous work-up by simple filtration.     

Cross-linking epoxide resins with hydrolysates of chrome-tanned leather waste

Differential scanning calorimetry was employed to investigate the reaction of diglycidyl ethers of bisphenol A (DGEBA) of mean molecular mass 348–480 Da, with collagen hydrolysate of chrome-tanned leather waste in a solvent-free environment. The reaction leads to biodegradable polymers that might facilitate recycling of plastic parts in products of the automotive and/or aeronautics industry provided with protective films on this basis. The reaction proceeds in a temperature interval of 205–220°C, at temperatures approx. 30–40°C below temperature of thermal degradation of collagen hydrolysate. The found value of reaction enthalpy, 519.19 J g⁻¹ (= 101.24 kJ mol⁻¹ of epoxide groups) corresponds with currently found enthalpy values of the reaction of oxirane ring with amino groups. Reaction heat depends on the composition of reaction mixture (or on mass fraction of diglycidyl ethers in the reaction mixture); proving the dependence of kinetic parameters of the reaction (Arrhenius pre-exponential factor A (min⁻¹) and activation energy E _a (kJ mol⁻¹)) did not succeed. Obtained values of kinetic parameters are on a level corresponding to the assumption that reaction kinetics is determined by diffusion.     

Curing of bismaleimides—I. Effect of amines on the curing behaviour and thermal stability

This paper describes the curing behaviour of 4,4′-(bismaleimido)diphenyl ether (BE) in the presence of diamines containing amide groups in the back-bone. The effects of the concentration of diamine on the melting point (T_m), exothermic peak position (T_exo) and heat of polymerization were studied by differential scanning calorimetry. Increase in the concentration of amines in bismaleimide-amine adducts reduced T_m and T_exo. Thermal stabilities of resins, cured at 200° C for 30 min were evaluated by dynamic thermogravimetry in nitrogen. Char yields and the decomposition temperatures of BE increased in the presence of diamines.     

Photogeneration of polyfunctional amines and novel thermal curing reactions of epoxy resin and polyurethane oligomer using these amines

Photochemical reactions of various blocked polyfunctional amines such as bis(4-formylaminophenyl)methane (FAPM), bis(4-acetylaminophenyl)methane (AAPM), bis(4-benzoylaminophenyl)methane (BAPM), 2,4-diformylaminotoluene (DFAT), m-xylene diformamide, and bis[[(2-nitrobenzyl)oxy]carbonyl]hexane-1.6-diamine were carried out to give the corresponding free amines in THF solution, in epoxy resin, or in polyurethane oligomer with terminal isocyanate groups. Photolysis of FAPM and DFAT to produce the corresponding polyfunctional amines such as 4,4'-methylenedianiline and 2,4-diaminotoluene proceeded with 80 and 75% conversions, respectively, in THF solution under UV irradiation at 5 h. AAPM and BAPM also produced the corresponding photo-Fries rearrangement products with 32 and 38% conversions, respectively, under the same irradiation conditions. The photolysis of those compounds also occurred with similar conversions in the epoxy resin and in the polyurethane oligomer under UV irradiation; and the thermal curing reactions of the epoxy resin and the polyurethane oligomer with photogenerated polyfunctional amines proceeded smoothly after heating at 140°C for 2 hrs. © 1993 John Wiley & Sons, Inc.     

Investigation of the curing reactions of some multifunctional epoxy resins using differential scanning calorimetry

Curing reaction of three tetrafunctional epoxy resins in the presence of tetraethylene tetramine was examined by differential scanning calorimetry at different heating rates. The kinetic parameters of the curing reaction were determined using various computational methods (Barrett, Borchardt–Daniels and Kissinger). The heating rate shows a great influence on the curing process. The activation energy varied in the range 43–80 kJ/mol, and the order of the curing reaction is observed to be ≈1.0 with slight variations.     

Mechanical relaxation mechanism of epoxide resins cured with aliphatic diamines

The mechanism of low-temperature relaxations in bisphenol-A-type epoxide resins cured with aliphatic diamines, with aliphatic diamines in the presence of salicylic acid as an accelerator, and with tertiary amines was investigated to compare the dynamic mechanical properties and the chemical structure of these networks. Mechanical relaxations are observed at about ?140 and ?60°C. The former relaxation is denoted the γ relaxation and the latter the β relaxation. The β relaxation of the cured epoxide resins containing hydroxyether groups is a sum of contributions from the relaxation of these groups and of other parts of the network structure. A new relaxation due only to the motion of the hydroxyether group can be estimated from the difference of tanδ curves between the aminecrosslinked and ether-crosslinked systems. The γ relaxation is attributed to the motion of a polymethylene sequence consisting of at least four carbon atoms.     

Mechanical relaxation mechanism of epoxide resins cured with acid anhydrides

The mechanism of low-temperature mechanical relaxation in epoxide resins cured with various acid anhydrides has been investigated by comparing dynamic mechanical properties and chemical structures of these networks. One mechanical relaxation, denoted as the β relaxation, is observed at about ?70°C. The β relaxation is affected by the chemical structure of the curing agents, but not by that of the epoxide resins. In addition, the strength of the relaxation increases linearly with increasing concentration of diester linkage introduced in the network by reaction with acid anhydrides. From these results, it is concluded that the β relaxation mechanism of the anhydride-cured systems involves the motion of diester segments included in the network structures.     

Stereochemistry of reaction products of 1,3-dimethylthymine epoxide with amines

Reaction of 1,3-dimethylthymine epoxide with amines afforded cis and trans adducts, stereostructures of which were elucidated and treatment of trans adduct with boron trifluoride etherate afforded cis adduct; mechanism for the isomerization is presented.