Cure of epoxy resins with aromatic amines: High heat-distortion studies

Castings having unexpectedly high heat-distortion temperatures result when certain treated resins of the EPON

1 EPON is a registered trademark of the Shell Oil Company.

828 type and about 75% of the stoichiometric amount of m-phenylenediamine, are postcured for 10–20 hr. at 175–200°C. The improvement in heat-distortion temperature is about 100°C., to values as high as 250°C. A recrystallized resin has given the highest values. Other glycidyl ethers of polyphenols have shown this phenomenon to a lesser degree, but other amine curing agents, including isomers and substitution products of m-phenylenediamine, have not. Some evidence of a new curing reaction has been developed, by NMR and pyrolysis studies of model compounds, which supports the postulate that the m-phenylenediamine is alkylated with a fifth epoxy group during the postcure, presumably at a ring carbon, resulting in greater crosslinking.     

Effect of aromatic and aliphatic amines as curing agents in sulfone epoxy monomer curing process

The kinetics of the curing of sulfone epoxy (SEP) monomers using aromatic and aliphatic amine curing agents was studied via differential scanning calorimetry (DSC). SEP curing is a two-stage process involving SEP/electron donation and electron donation to either aliphatic or aromatic curing agents. The SEP/electron donation curing process occurred readily since semi-electron-withdrawing curing agents are induced by nucleophilic substitution in the first stage. In the second stage, SEP is cured by the semi-electron-withdrawing curing agents. The kinetic parameters of the curing process were determined using a conversional method derived from Ozawa’s and Kissinger’s methods, which are typically used for kinetic analysis of data for thermal treatments. The higher melting points and steric bulk of the aromatic curing agents resulted in higher curing activation energies than for the aliphatic curing agents. The aliphatic curing agents also increased the activation energy of the curing process due to their electron-withdrawing and cross-linking properties as well as the viscosity of the epoxy/amine curing system. Cured SEP/aromatic curing agent materials possessed higher glass transition temperatures than cured SEP/aliphatic curing agent materials.     

Curing of epoxy resins with amines

Summary Using determination of the critical molar ratio of functional groups necessary for gel formation with amino groups in excess, network formation was investigated in the reaction of diglycidyl ether of Bisphenol A (DGEBA) and nitrogen-containing epoxy resins — diglycidylaniline and N,N,N,N-tetraglycidyl-4,4-diaminodiphenylmethane with aromatic or aliphatic amines. It was found that in the case of nitrogen-containing epoxides the crosslinking is accompanied by pronounced cyclization, unlike in systems with DGEBA. Gelation depends on the relative reactivity of the primary and secondary hydrogen atom of the amino group. In the case of nitrogen-containing polyepoxides, the dependence of reactivities of adjacent glycidyl groups is also operative.     

Polymeric reagents: Part 1. Synthesis of polymer-anchored amines useful for curing epoxy resins

A range of polymers carrying amino-functions [NHEt, NHBuⁿ, NEt₂, NPr, N(CH₂CH₂CH₂NH₂)₂, NMeCH₂CH₂CH₂NH₂ or NEtCH₂CH₂CH₂ NH₂] has been synthesised via amination of styrene-vinylbenzyl chloride copolymers, poly(epichlorohydrin) and poly(2-chloroethyl vinyl ether), and via homopolymerisation and copolymerisation (with styrene) of (n-butyl)(vinylbenzyl)-ammonium chloride. Some of the polymers showed promise as curing agents for epoxy resins.     

Effect of structure of aromatic diamines on curing characteristics,thermal stability,and mechanical properties of epoxy resins. I

The curing behavior of diglycidyl ether of bisphenol-A (DGEBA) with aromatic diamines having aryl–ether, aryl–ether–carbonyl, and aryl–ether–sulfone linkages was studied using differential scanning calorimetry (DSC). Aromatic diaminessuch as 1,3-bis(aminophenoxy)benzene (R), 1,4-bis(aminophenoxy)benzene (H),2,2′-bis[4-(4-aminophenoxy)phenyl]propane (B), 4,4′-bis(4-aminophenoxy)benzo-phenone (P), and bis[4-(4-aminophenoxy)phenyl]sulfone (S) were synthesized and characterized in the laboratory. Curing of DGEBA was done using both stoichiometric and nonstoichiometric amounts of diamines and the reaction was monitored using DSC. The reactivity of the diamines depended on the structure. The presence of electron withdrawing groups, even though significantly apart from the reaction site, reduced the nucleophilicity. No significant change was observed in the activation energy for curing, which was around 56 ± 2 kJ/mol. The glass transition temperature of the epoxy network depended on the structure and was higher when diamines P and S were used in comparison to diamines R, H, and B. The cured resins were stable up to 300°C, and maximum char yield (i.e., 32% at 600°C) was obtained in DGEBA cured with diamine P. The room temperature mechanical properties only changed marginally with the structure of the diamines. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1759–1766, 1998     

Electrochemical curing of epoxy resins

An electrochemical method has been developed for bonding electrically-conductive adherends. The procedure is based on the electrochemical generation of a curing agent from an otherwise chemically-unreactive precursor mixed with an epoxy resin sandwiched between the bonding members. The one-part epoxy resin is storage stable and cured rapidly on passage of current.     

Toughening of epoxy resins with aromatic polyesters

Polyesters, prepared by direct polycondensation from bisphenol A and aliphatic dicarboxylic acids [adipic acid (AD), suberic acid, sebacic acid (SE), and dodecanedioic acid], were used to improve the toughness of the diglycidyl ether of the bisphenol A/diaminodiphenyl methane epoxy system. Polyesters had the number average molecular weight (M_n) ranging from 4300 to 19,200 g/mole. The epoxy systems modified with the AD system (M_n = 6400 g/mole) and the SE system (M_n = 10,200 g/mole) showed phase separated structures with discrete domains of 0.2 μm, but other systems showed smooth fracture surfaces when observed by scanning electron microscopy. The modified epoxy systems except for the AD system and SE system showed two tan δ peaks corresponding to the α and β transitions of the epoxy resin. The modified epoxy systems showed maximum values of K_1c at around 10 wt % of polyester and maximum flexural properties at 5 wt % of polyester. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2464–2473, 2000     

Synthesis and properties of cationic resins derived from aniline-modified epoxy resins and various amines

Aniline-modified epoxy resin which contains tertiary amine in the middle of the polymer chain was synthesized by the reaction of aniline and epoxy resin. The resulting aniline-modified epoxy resin and two commercial epoxy resins with different epoxy equivalent weights were reacted with 2-ethylhexanol-blocked toluene diisocyanate (2-EH-blocked TDI) to obtain thermally crosslinkable epoxy resins. These epoxy resins were subsequently reacted with various secondary amines and partially neutralized with acetic acid to give thermally crosslinkable cationic resins. The resulting cationic resins were dissolved in suitable solvents and mixed with deionized water to form emulsions. The crosslinking properties, emulsion, and electrodeposition properties of these resins were studied in some detail. The electro-deposition yields of the emulsions prepared from aniline-modified epoxy resins were higher than those of other emulsions. The crosslinked films prepared from aniline-modified epoxy resins were also glossier than those prepared from commercial epoxy resins. High deposition yield and high glossiness were the characteristic properties of the aniline-modified epoxy resins. Thermal properties were not affected by aniline-modified epoxy resins.     

Effect of structure of aromatic imide–amines on curing behavior and thermal stability of diglycidyl ether of bisphenol‐A

The curing behavior of diglycidyl ether of bisphenol‐A (DGEBA) with aromatic imide–amines having aryl ether, sulfone, and methylene linkages was studied using differential scanning calorimetry (DSC). Six imide–amines of varying structure were synthesized by reacting 1 mol of naphthalene 1,4,5,8‐tetracarboxylic dianhydride (N) or 4,4′‐oxodiphthalic anhydride (O) with excess (>2 mol) of 4,4′‐diaminodiphenylether [E] or 4,4′‐diaminodiphenyl methane [M] or 4,4′‐diaminodiphenyl sulfone [S]. The imide–amines prepared by reacting O or N with S, M, and E have been designated as OS/NS; OM/NM, and OE/NE, respectively. Structural characterization of imide–amines was done using FTIR, ¹H NMR, ¹³C NMR, and elemental analysis. The curing behavior of DGEBA in the presence of stoichiometric amount of imide–amines was investigated by recording DSC scans. A broad exothermic transition was observed and the peak exotherm temperature was found to be dependent on the structure of imide–amines. The peak exotherm temperature (T_p) was lowest in case of imide–amines OE and highest in case of imide–amines NS/OS. Thermal stability of isothermally cured DGEBA in the presence of imide–amines was evaluated by dynamic thermogravimetry. The char yield was highest for resin cured with imide–amines NE. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Gelation in the curing of epoxy resins with anhydrides

Summary The statistical theory of branching processes has been used in an analysis of gelation in the system diepoxide-cyclic anhydride-tert.-amine in step and initiated reaction mechanisms. Experimental data indicate that critical conversion depends on the concentration of tert.-amine, which suggests that a mechanism with initiation is operative.     

Radiation curing of hyperbranched polyester resins

A research area that has obtained increasing interest during the last decade concerns improvement of macromolecular properties by changes in the macromolecular architecture. One group of these materials is dendritic polymers, which are highly branched structures exhibiting very different properties compared with linear polymers. One potential application for these polymers is as radiation curable thermoset resins. This article describes a study where the use of an aliphatic hyperbranched polyester as a base for new radiation curable thermoset resins. The hyperbranched polyesters have been characterized with respect to cure rate and final mechanical properties compared with conventional resins. It is shown that hyperbranched polyesters can be used as versatile scaffolds for various radiation curable resin structures. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 612–618, 2000     

Studies of the curing kinetics and thermal stability of epoxy resins using a mixture of amines and anhydrides

This article describes the curing and thermal behavior of diglycidyl ether of bisphenol A with phthalic anhydride (PA)/pyromellitic anhydride/diaminodiphenyl sulfone (DDS) or a mixture of anhydrides and amines in varying ratios as curing agents. The kinetics of the curing behavior was investigated with a multiple‐rate method. The activation energy of the curing reaction as determined in accordance with Ozawa's method was found dependent on the structure of the anhydride and on the ratio of amines to anhydrides. The activation energy was highest with sample DP3 (0.25 : 0.75) and DM3 (0.25 : 0.75). We evaluated the thermal stability of epoxy resin, cured isothermally, by recording thermogravimetric traces in a nitrogen atmosphere. The char yield was highest for resins cured with a mixture of DDS and PA (0.5 : 0.5) and a mixture of DDS and pyromellitic dianhydride (0.25 : 0.75). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3919–3925, 2006     

Pyridinyl-containing benzoxazine: Unusual curing behaviors with epoxy resins

A pyridinyl-containing benzoxazine (6) was synthesized from the Mannich condensation of 4-phenyl-2,6-bis(4-aminophenyl) pyridine (2), phenol, and paraformaldehyde. For the purpose of properties comparison, a benzoxazine (7), which is structurally similar to (6) except for the pyridinyl group, was prepared. The solvent effect on the synthesis of (6) was discussed, and toluene/ethanol was found to provide (6) with the best purity and yield. The pyridinyl group provides solubility and acts as a catalyst for the ring opening of benzoxazine, as supported by the forward curing in the DSC thermograms. When curing with epoxy resins, a carbonyl absorption at 1670 cm⁻¹ and 192 ppm was observed in the IR and ¹³C NMR spectra. It is proposed that the formation a cyclic amide structure is responsible for the absorption. A reaction mechanism including nucleophilic addition, Diels–Alder reaction, and rearrangement was proposed. The pyridinyl group acts as a crosslinking site, and results in thermosets with good thermal properties.     

The cure of epoxy novolacs with aromatic diamines. II. Statistical parameters of the resulting networks

The buildup of the network arising from the reaction of an epoxy novolac (derived from a phenol—formaldehyde novolac resin), with 4,4'diaminodiphenylsulfone (DDS), was modeled using a kinetic-statistical procedure that takes into account (i) the complex structure of the starting novolac, (ii) the actual polymerization kinetics including the different reactivity of primary and secondary amine hydrogens, and (iii) the formation of intramolecular cycles. Predictions of gel conversion and sol fraction were in excellent agreement with experimental results. Glass transition temperatures T_g's of networks prepared at different stoichiometries were predicted using Nielsen's equation (J. Macromol. Sci.-Rev. Macromol. Chem., C3 , 69, 1969) and the concentration of elastic chains arising from the model. Theoretical predictions explained the presence of a maximum in the experimental T_g for formulations containing a 20% amine excess. Experimental values of T_g as a function of conversion, for stoichiometric samples, were correlated using a model proposed in the literature. © 1993 John Wiley & Sons, Inc.     

Dynamic thermomechanometry of networks from acrylated epoxy resins

Dynamic thermomechanical analysis of networks from acrylated DGEBA type epoxy resins, crosslinked through u.v. induced polymerization, was performed to elucidate the effect of the functionality and functionality distribution of the oligomers on the network properties. Partially acrylated and propionated DGEBA resins were employed in which the acrylic functionality was changed in the range 2-1 double bonds per mole of the resin. The dynamic storage modulus, measured with a Rheovibron instrument, was found, in the high temperature region (>150°C), to follow the classical laws of rubber elasticity when the proper number of elastic effective chains was taken into account. The glass-rubber transition temperature was found to decrease linearly as a function of the fraction of free chain ends. The influence of the presence of CBr₄ as a chain transfer agent on the network properties was also preliminarily investigated.     

Investigations on the curing of epoxy resins with hexahydrophthalic anhydride

The bifunctional epoxides bisphenol A diglycidyl ether (BADGE) and hexahydrophthalic diglycidyl ester (HHDGE) as well as the monoepoxides phenyl glycidyl ether (PGE) and cyclohexane carboxylic acid glycidyl ester (CHGE) were cured with hexahydrophthalic anhydride (HHPA) in the presence of benzyldimethylamine (BDMA) or 1-methylimidazole (1-MI) as catalysts at 100–140°C. Investigations of the curing kinetics gave sigmoidal-shaped curves with marked induction periods. IR analysis of the cured products revealed that the propagation proceeds not only by the esterification reaction of epoxide with anhydride but also by chain anhydride formation by the reaction of carboxylate with anhydride groups. ¹³C-NMR investigations of the soluble polymers showed that most of the peaks resulting from double bonds could not be assigned to structures formed by initiation reactions that had previously been proposed for the anhydride curing of epoxides. In analogy to a postulated mechanism for the decarboxylation condensation of HHPA alone in the presence of tertiary amines, it is proposed that an isomerization product of HHPA is one of the molecules that initiate the curing reaction.     

Esters from castor oil functionalized with aromatic amines as a potential lubricant

Vegetable oils are very promising alternatives to fossil lubricants due to their abundance, low cost, excellent performance, and environmental friendliness. Due to its multifunctional structure, castor oil is an excellent precursor in the synthesis of new biolubricants. However, it showed poor thermal-oxidative stability and a higher pour point. This study used castor oil fatty acids prepared by transesterification (EHRO), epoxidation (TEPO), and oxirane ring opening with the aromatic amines aniline (ANIL) and p-anisidine (ANIS). The chemical structure of these oils was verified by ¹H and ¹³C NMR analysis, and mass spectrometry. Measurements show that the presence of an aromatic amine increases the viscosity resulting in 172 (ANIL) and 199 (ANIS) cSt at 40°C, but reduces viscosity index to 16 and 1, respectively. In addition, the amine groups can scavenge radicals increasing their thermal and oxidative stability. These products do not oxidize copper, and tribological analysis reveals that ANIS has the lowest torque with wear equivalent to commercial mineral lubricant NH-140.     

Interdependence between the curing,structure, and the mechanical properties of phenolic resins

The interdependence between the curing conditions, structure, and the mechanical properties of tow neat phenolic resin systems was investigated. Changes of the distribution of the void diameters were characterized by light‐ an scanning electron microscope analyses. Tensile tests and dynamic mechanical thermo analysis were performed to determine the influence of the hardener concentration and the curing temperature on the mechanical and the thermomechanical properties. The study reveals that the hardener concentration predominately influenced the microscopic structure, and thus the mechanical properties of the phenolic resin systems. By varying the postcuring times, it can be shown that independent from the microstructure of the phenolic resin system, the degree of cure has a strong influence on the mechanical properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3173–3185, 1999     

A kinetic model for curing reactions of epoxides with amines

Curing reactions of epoxy resins are accelerated by added hydrogen-bond donor solvent and hydroxyl groups produced during the course of polymerization. A kinetic model comprising several different polycondensation and polyaddition reactions that occur simultaneously is developed. The concept of diffusion controlled reactions is employed to describe the change of reaction rate constants with conversion after the formation of an infinite crosslinking network. Good agreement is obtained between the model predictions and experimental data available in the literature.