Cure of epoxy resins with cyanoacetamides

Cyanoacetamides are a novel class of curing agents for epoxy resins. Since reaction products of epoxy compounds with cyanoacetamides have not yet been described, we investigated the reaction of phenyl-glycidylether (PGE) and N-isobutylcyanoacetamide (NICA) under the conditions of the epoxy cure (120–150°C). Twenty-two fractions of the reaction product have been separated by preparative TLC and characterized by FD and MS mass spectroscopy. The structures of 10 reaction product have been elucidated by MS, NMR, and IR techniques. They belong to the classes of cyclic urethanes, spiro-dilactones, cyclo-oxa-1-hepten-4-one-2, pyrimidones, aminocrotononitrile, and tertiary amine. This complex model reaction mixture does not enable us to propose a curing mechanism. However practical cure of Bisphenol A diglycidylether (BADGE) yields clear and tough solids with a glass transition temperature up to 200°C, good mechanical strength, and high adhesion to metal surface. Cyanoacetamides are latent hardeners requiring a curing initiator. Since N-4-chlorophenyl-N′-dimethylurea is a latent initiator, liquid, homogeneous, storage stable “one shot” systems can be formulated which harden quickly above 120°C. Heat aging properties of cured specimens are reported. A series of novel liquid, resinous, and crystalline cyanoacetamides and their potential as curing agent are described.     

Novel benzyl sulfonium salt having an aromatic group on sulfur atom as a latent thermal initiator

Three benzyl p-hydroxyphenyl methylsulfonium salts with different counter anions were synthesized as novel latent thermal initiators. Syntheses of the sulfonium salts ( 2 ) were performed by the reaction of p-hydroxyphenyl methyl sulfide with benzyl chloride followed by exchange of the counter anion (Cl^?) with SbF^?₆ ( 2a ), PF^?₆ ( 2b ), or BF^?₄ ( 2c ). In the bulk polymerization of glycidyl phenyl ether (PGE) with 2 , initiator activity of the sulfonium salts was evaluated by comparison with that of benzyl tetramethylenesulfonium hexafluoroantimonate ( 1 , R = H). Among the initiators, 2a showed the highest activity, and was much more active than 1 (R = H). Since the polymerization of PGE with 2a proceeded efficiently at more than 80°C but not at all at less than 60°C, 2a was suggested to be a good latent thermal initiator.     

Isothermal cure kinetics of epoxy/phenol‐novolac resin blend system initiated by cationic latent thermal catalyst

The investigation of the cure kinetics of a diglycidyl ether of bisphenol A (DGEBA)/phenol‐novolac blend system with different phenolic contents initiated by a cationic latent thermal catalyst [N‐benzylpyrazinium hexafluoroantimonate (BPH)] was performed by means of the analysis of isothermal experiments using a differential scanning calorimetry (DSC). Latent properties were investigated by measuring the conversion as a function of curing temperature using a dynamic DSC method. The results indicated that the BPH in this system for cure is a significant thermal latent initiator and has good latent thermal properties. The cure reaction of the blend system using BPH as a curing agent was strongly dependent on the cure temperature and proceeded through an autocatalytic kinetic mechanism that was accelerated by the hydroxyl group produced through the reaction between DGEBA and BPH. At a specific conversion region, once vitrification took place, the cure reaction of the epoxy/phenol‐novolac/BPH blend system was controlled by a diffusion‐control cure reaction rather than by an autocatalytic reaction. The kinetic constants k₁ and k₂ and the cure activation energies E₁ and E₂ obtained by the Arrhenius temperature dependence equation of the epoxy/phenol‐novolac/BPH blend system were mainly discussed as increasing the content of the phenol‐novolac resin to the epoxy neat resin. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2945–2956, 2000     

Cationic cure of epoxy resin initiated by methylanilinium salts as a latent thermal initiator

The effect of the novel N‐crotyl‐N,N‐dimethyl‐4‐methylanilinium hexafluroantimonate (CMH) initiator on cure kinetics and rheological properties of diglycidylether of bisphenol A (DGEBA) epoxy cationic system was investigated. From DSC measurements of the DGEBA/CMH system, it was found that this system exhibited excellent thermal latent characteristics at a given temperature and revealed complex cure behavior as indicated by multiple exotherms. The conversion and conversion rate of the DGEBA/CMH system increased with increasing the concentration of initiator, attributed to the high activity of CMH. Viscoelastic properties during gel formation of DGEBA initiated by CMH were investigated by rheological techniques under isothermal conditions. The gel time obtained from the modulus crossover point t(G′) = G″ was affected by a high curing temperature and the concentration of CMH, resulting in a high degree of network formation in cationic polymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2397–2406, 2001     

Near‐infrared spectroscopic studies on the cure behaviors of the CAE/DGEBA blend system initiated by a thermal latent catalyst

The latent properties and cure behaviors of an epoxy blend system based on cycloaliphatic epoxy (CAE) and diglycidyl ether of bisphenol A (DGEBA) epoxy containing N‐benzylpyrazinium hexafluoroantimonate (BPH) as a thermal latent initiator were investigated with near‐infrared (N‐IR) spectroscopy. The assignments of the latent properties and cure kinetics were performed by the measurements of the N‐IR reflectance for epoxide and hydroxyl functional groups at different temperatures and compositions. As a result, this system showed more than one type of reaction, and BPH was an excellent thermal latent catalyst without any coinitiator. The cure behaviors were identified by the changes in the absorption intensity of the hydroxyl groups at 7100 cm⁻¹ with different composition ratios. Moreover, characteristic N‐IR band assignments were used to evaluate the reactive kinetics and were shown to be an appropriate method for studying the cure behaviors of the CAE/DGEBA blend system containing a thermal latent catalyst. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 326–331, 2001     

Biaspartimide-diamines as Curing Agents for Epoxy Resins

Various 4,4′ -bis(N²-[4-(4-aminophenoxy)phenyl]aspartimido)diphenylmethane-type bisaspartimide-diamines have been used as solventless curing agents for epoxy resins. The thermal curing was performed at 170, 190, and 230°C to give a tough brown polymer. Thermogravimetric analysis of the polymer obtained showed thermal stability up to 330°C and char yields of 45% in N₂ at 800°C and 12% in air at 700°C. The thermal curing reaction was monitored using FT-IR. The synthesized polymers are useful for making composites, laminates, and adhesives.     

Reaction epoxy-amine: Suivi du mecanisme reactionnel et de la cinetique par RMN 13C, 15N et HPLC

The curing reaction of diglycidyl ether of bisphenol A (DGEBA) with metaphenylene diamine (mPDA) was investigated with high performance liquid chromatography (HPLC) and Fourier transform nuclear magnetic resonance spectroscopy (FT NMR ¹³C and ¹⁵N). With the results obtained from a series of models a mechanism was proposed and the cure kinetics were obtained at 100°C. The major conclusion is that the cure proceeded mainly by chain extension, whereas crosslinking occurred in the reaction of hydroxyl groups with epoxides and resulted in the formation of ether linkages.     

Imidazole‐type thermal latent curing agents with high miscibility for one‐component epoxy thermosetting resins

Epoxy resins are important thermosetting resins widely employed in industrial fields. Although the epoxy–imidazole curing system has attracted attention because of its reactivity, solidification of a liquid epoxy resin containing imidazoles proceeds gradually even at room temperature. This makes it difficult to use them for one‐component epoxy resin materials. Though powder‐type latent curing agents have been used for one‐component epoxy resin materials, they are difficult to apply for fabrication of fine industrial products due to their poor miscibility. To overcome this situation and to improve the shelf life of epoxy–imidazole compositions, we have developed a liquid‐type thermal latent curing agent 1 , generating an imidazole with a thermal trigger via a retro‐Michael addition reaction. The latent curing agent 1 has superior miscibility toward epoxy resins; in addition, it was confirmed that the epoxy resin composition has both high reactivity at 150 °C, and long‐term storage stability at room temperature. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2680–2688     

Cure behavior of diglycidylether of bisphenol A/trimethylolpropane triglycidylether epoxy blends initiated by thermal latent catalyst

Cure behaviors of diglycidylether of bisphenol A (DGEBA)/trimethylolpropane triglycidylether (TMP) epoxy blends initiated by 1 wt % N‐benzylpyrazinium hexafluoroantimonate (BPH) as a cationic latent catalyst were investigated using DSC and rheometer. This system showed more than one type of reaction and BPH could be excellent thermal latent catalyst without any co‐initiator. The cure activation energy (E_a) obtained from Kissinger method using dynamic DSC data was higher in DGEBA/TMP mixtures than in pure DGEBA. Rheological properties of the blend system were investigated under isothermal condition using a rheometer. The gel time was obtained from the analysis of storage modulus (G′), loss modulus (G″) and damping factor (tanδ). The crosslinking activation energy (E_c) was also determined from the Arrhenius equation based on the gel time and curing temperature. As a result, the crosslinking activation energy showed a similar behavior with that obtained from Kissinger method. And the gel time decreased with increasing TMP content, which could be resulted from increasing the activated sites by trifunctional epoxide groups and decreasing the viscosity of DGEBA/TMP epoxy blend in the presence of TMP. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2114–2123, 2000     

Mechanism of imidazole catalysis in the curing of epoxy resins

The mechanism of imidazole catalysis in the curing of epoxy resins was studied using the PGE/1-methylimidazole, 2-methylimidazole, and 1,2-dimethylimidazole model systems and another model system based on trichloromethylethylene oxide. It was demonstrated that imidazolium systems, generated in the curing reaction, show an inherent instability leading to cleavage of an N? C bond or the 2-C? H bond (2-unsubstituted imidazoles). Fourier-transform infrared spectroscopy was used to follow specific changes in the IR spectrum of the curing mixture during polymerization. The identification of carbonyl absorptions occurring during the polymerization led to the conclusion that ketone formation is a general occurrence in the cure of epoxides with nitrogen compounds. We have also shown that imidazoles are regenerated during the curing process by at least two routes. One pathway for the regeneration of the catalyst involves N-dealkylation of the imidazole via a substitution process. Another route, β-elimination, afforded carbonyl compounds, which account for the previously unexplained appearence of infrared bands in the 1650–1770 cm^?1 region during the curing process. These investigations demonstrated the true catalytic function of the imidazole. Possible mechanisms for the regeneration of the catalyst are also suggested.     

Poly-N,N-diethylacrylamide prepared by group transfer polymerization: Synthesis,characterization, and solution properties

The conditions for preparing poly-N,N-diethylacrylamide by group transfer polymerization (GTP) were investigated. While electrophiles did not catalyze the reaction, various nucleophilic substances could be used for that purpose. By using an appropriate initiator, either an ester or a carboxylic acid end group could be formed. The highest yields in the first case were obtained using tetrabutylammonium acetate and dimethylketene methyl trimethylsilyl acetal as catalyst and initiator, respectively, while the use of the corresponding bistrimethylsilyl compound as initiator gave polymers, albeit at lower yields, which carried the acidic end group. The ¹H-NMR, ¹³C-NMR, and IR spectra of the polymers were taken and used together with information obtained with soft-ionization mass spectrometric methods (MALDI-MS, ESI-MS, and FD-MS) to elucidate molecule structure, apparent molecular weight distribution, polydispersity, and possible mechanisms of the termination reaction. The poly-N,N-diethylacrylamide prepared, showed an inverse temperature dependency in its water-solubility, with a lower critical solution temperature between 29.8°C and 30°C. © 1994 John Wiley & Sons, Inc.     

The effects of glass beads and silane treatments on the curing behavior of a brominated epoxy resin: DSC analyses

The curing characteristics of a brominated epoxy resin/dicyandiamide (DICY) system filled with silane-treated glass beads are studied using isothermal differential scanning calorimetry (DSC). Three different silane coupling agents, N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane, N-[2-(vinylbenzylamino)-ethyl]-3-aminopropyl-trimethoxysilane, and 3-glycidoxypropyl-trimethoxysilane, are applied. It is found that the reaction heats of the epoxy system are little affected by the curing temperature and the untreated glass fillers, but changed with the addition of silane-coated glass beads. The effect of glass beads on the curing reaction is more significant at the low curing temperature and conversion. The silane treatment results in changes in T_g, activation energy, reaction heat, reaction rate, and reaction order. Three silanes respond differently because of their differences in the activated reaction with the matrix system. Regardless of the various curing mechanisms involved, a simple kinetic expression can describe the curing extent at 170 and 180°C with a good accuracy for all systems studied. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2063–2071, 1997     

Synthesis of novel crosslinkable polymers by atom transfer radical polymerization of cardanyl acrylate

In this work, the successful application of atom transfer radical polymerization (ATRP) to cardanyl acrylate, a polymerizable monomer derived from a renewable resource cardanol, is reported. Polycardanyl acrylate and poly(methylmethacrylate)‐cardanyl acrylate copolymers were prepared in bulk ATRP, using Copper(I) bromide/N, N, N′, N′, N″‐pentamethyl diethylene triamine (PMDETA) catalyst system at 95 °C in combination with ethyl‐2‐bromo isobutyrate initiator. The copolymers had mol. wt. (M_n) in the range 8300–2400 g/mol and polydispersity index (PDI) 1.27–2.00, depending upon the [M]₀/[I]₀ ratio. ¹H NMR analysis of the copolymer showed that unsaturation in the side chain of cardanyl acrylate is unaffected under the conditions of ATRP. This was further confirmed by studying the curing reaction of polycardanyl acrylate by supported dynamic mechanical thermal analysis (DMTA) in dual cantilever mode. The thermogravimetric analysis shows that the copolymers have improved thermal stability, by about 35 °C, in comparison with pure PMMA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5953–5961, 2005     

Effect of nanosilica on the kinetics of cure reaction and thermal degradation of epoxy resin

Nanocomposites from nanoscale silica particles(NS),diglycidylether of bisphenol-A based epoxy(DGEBA),and 3,5-diamino-N-(4-(quinolin-8-yloxy) phenyl) benzamide(DQPB) as curing agent were obtained from direct blending of these materials.The effect of nanosilica(NS) particles as catalyst on the cure reaction of DGEBA/DQPB system was studied by using non-isothermal DSC technique.The activation energy(E_a) was obtained by using Kissinger and Ozawa equations. The E_a value of curing of DGEBA/DQPB/10%NS system showed a decrease of about 10 kJ/mol indicating the catalytic effect of NS particles on the cure reaction.The E_a values of thermal degradation of the cured samples of both systems were 148 kJ/mol and 160 kJ/mol,respectively.The addition of 10%of NS to the curing mixture did not have much effect on the initial decomposition temperature(T_i) but increased the char residues from 20%to 28%at 650℃.     

Acceleration of amine/epoxy reactions with N‐methyl secondary amines

Kinetic studies established that the monomethylation of a primary amine leads to significantly higher reaction rates with glycidyl ethers. The relative rates for approximately 25 amines were determined in an alcohol solvent under pseudo‐first‐order conditions (excess epoxy). The rates were referenced to aniline. For the aliphatic amines, reactivity consistently increased upon going from a primary amine to the corresponding N‐methyl secondary amine. This acceleration effect was not seen for aniline. The enhanced reactivity was also seen in curing systems, both with pure methylated amine curing agents and with complex mixtures obtained from the partial methylation of polyamines. Economically viable partially methylated amine curing agents were obtained by the reductive alkylation of commercial polyamines with formaldehyde and by the reaction of monomethylamine with 3‐(N‐methylamino)propionitrile in the presence of hydrogen and a hydrogenation catalyst. Although actual cure performance is based on a complex combination of several factors, the acceleration due to monomethylation could be a useful tool for enhancing amine/epoxy curing reactions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 921–930, 2000     

Lanthanide–imidazole complexes as latent curing agents for epoxy resins

Imidazoles have for some time been recognized as curing agents for epoxy resins. Once the resin and the imidazole compound are mixed there is a relatively short time in which the mixture can be used, since the polymerization (curing) reaction occurs to some extent even at room temperature causing the reaction mixture to thicken. In order to circumvent this problem we have found that imidazoles can be complexed with organo-lanthanide compounds thereby tying up the imidazole and retarding its rate of reaction in the cure of epoxy materials at ambient temperatures. When it is desired to enhance the rate of cure the temperature of the mixture is simply raised. This paper concerns studies of the epoxy cure reaction with the M(THD)₃–IM series. M represents the lanthanide metals Eu, Ho, Pr, Dy, Yb, and Gd, and THD is 2,2,6,6-tetramethyl-3,5-heptanedione. Cure reactions were followed by differential scanning calorimetry and in some cases by infrared spectroscopy. We have demonstrated that these organo-lanthanide–imidazole complexes are effective thermally latent curing agents for epoxy resins. At a temperature of 150°C cure is quite rapid. In the course of these studies it has also been determined that there is an inverse correlation between the lanthanide ionic radius in the complex and the temperature at which the cure reaction occurs. Thus the Yb compound, where the imidazole is most strongly bound, cures at the highest temperature and Pr, where imidazole is bound most weakly, at the lowest. Consistent with these facts is the observation that the Yb compound also gives the longest latency period when mixed with epoxy resin.     

UV photo curing of N,N-bismaleimido-4, 4-diphenylmethane

The photo curing of a formulation consisting of N,N^′-bismaleimido-4, 4^′-diphenylmethane (BMI), which is most widely used in commercial thermal curing formulations, was performed. Parameters, such as initiator, co-initiator and temperature, which affect the curing rate and enthalpy, were studied using differential photocalorimeter. BMI undergoes copolymerization with 4-hydroxybutylvinylether (HBVE), when exposed to UV radiation, in presence of the photo initiator, Triphenylphosphine oxide (TPO). Diallylbisphenol A has been observed to be an efficient co-initiator, which improves the reaction rate coefficient and enthalpy of the photo curing process for the system BMI/HBVE/TPO.     

Formation of a highly ordered poly (N,N'-bis-phenoxyphenylpyromellitimide) powder using a high pressure curing technique

A high-pressure curing technique was developed to help determine the effects of solvent presence during the thermal curing of the polyimide poly (N,N'-bis-phenoxyphenylpyro-mellitimide) (PMDA-ODA). A powder form of this aromatic polyimide was produced from a polyamic acid solution using the high-pressure thermal curing technique. Preliminary characterization of the powder indicates a high degree of crystallinity with a measured density of 1.46 ± 0.01 g/cm³ and a distinct melting point of 594°C. The addition of chemical curing agents to the polyamic acid solution prior to thermal treatment reduced the amount of crystallinity observed in the cured material. Molecular weight measurements of the polyamic acid precursor and powder suggest that the high degree of order observed in the powder is a result of degradation during cure. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America

     

Storage stability and curing behavior of epoxy‐dicyandiamide systems with carbonyldiimidazole‐Cu (II) complexes as the accelerator

The reaction between carbonyldiimidazole (CDI) and copper (II) nitrate produces a new Cu (II) complex with nitrate as the counter anion. TGA, UV, and FTIR analyses confirmed that the coordination number of CDI in this complex is two. The acceleration effect of the complex in epoxy‐dicyandiamide (DICY) curing systems has been evaluated by DSC and TMA, and the increasing viscosity of the mixture was monitored during the storage. The results revealed that the complex is not only very effective for the acceleration of epoxy‐DICY systems, leading to a rapid gelation within 21 min at 120 °C, but it is also chemically stable at ambient temperature. This is reflected by the slow increase of viscosity of the accelerated curing systems stored at 35 °C, even over 56 days. In addition, the effects of the [Cu(CDI)₂]²⁺ complex with different counter anions, that is, bromide, chloride, nitrate, sulfate, phthalate, and acetate, were compared using a series of tests. By comparing with N,N‐dimethyl‐N′‐phenylurea (fenuron), the widely used latent accelerator for DICY‐epoxy curing systems, the complexes with bromide and nitride were found to be better, both regarding storage stability and for their acceleration effect. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3470–3476     

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.