Imidazole and chromium acetylacetonate as additives for the cure and fracture toughness of epoxy resins and their composites

The effects of additives such as 2-undecyl-imidazole (C₁₁Z) and chromium acetylacetonate (Cr(acac)₃) were examined on the curing behavior and fracture toughness of tetraglycidyldiaminodiphenyl methane/diaminodiphenyl sulphone (TGDDM/DDS) epoxy resins and their composites. The C₁₁Z additive alone reacted with TGDDM epoxy resins at about 127°C and increased the resin viscosity, resulting in an acceptable resin content for composite processing. Further addition of Cr(acac)₃ to TGDDM/DDS/C₁₁Z formulation increased the fracture toughness 5.7 times compared to the typical TGDDM/DDS/BF₃MEA epoxy formulation used for the preparation of laminates. The interlaminar fracture toughness of the laminates prepared by TGDDM/DDS/C₁₁Z/Cr(acac)₃ formulation was only twice as much as that prepared by typical TGDDM/DDS/BF₃MEA. This was due to the fiber bridging contribution to the interlaminar fracture toughness. Based on the experiment, this fiber bridging contribution was only dependent on the fiber content. Thus, the interlaminar fracture toughness is approximated by the sum of the fracture toughness of epoxy matrix and the estimated fiber bridging contribution.     

Synthesis,characterization, and properties of low viscosity tetra‐functional epoxy resin N,N,N′,N′‐ tetraglycidyl‐3,3′‐diethyl‐4,4′‐diaminodiphenylmethane

Tetra‐functional epoxy resin N,N,N′,N′‐tetraglycidyl‐3,3′‐diethyl‐4,4′‐diaminodiphenylmethane (TGDEDDM) was synthesized and characterized. The viscosity of TGDEDDM at 25°C was 7.2 Pa·s, much lower than that of N,N,N′,N′‐tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM). DSC analysis revealed that the reactivity of TGDEDDM with curing agent 4,4′‐diamino diphenylsulfone (DDS) was significantly lower than that of TGDDM. Owing to its lower viscosity and reactivity, TGDEDDM/DDS exhibited a much wider processing temperature window compared to TGDDM/DDS. Trifluoroborane ethylamine complex (BF₃‐MEA) was used to promote the curing of TGDEDDM/DDS to achieve a full cure, and the thermal and mechanical properties of the cured TGDEDDM were investigated and compared with those of the cured TGDDM. It transpired that, due to the introduction of ethyl groups, the heat resistance and flexural strength were reduced, while the modulus was enhanced. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40009.     

Differential scanning calorimetry studies of the cure of carbon fiber–epoxy composite prepregs

Diaminodiphenyl sulfone (DDS) cured tetraglycidyl-4,4'-diaminodiphenyl methane (TGDDM) epoxies, whose cure reactions are accelerated by BF₃:amine catalysts, are the most common composite matrices utilized in aerospace high performance, fibrous composites. To process reproducible composites requires an understanding of the cure reactions and how these reactions are modified by the BF₃:amine catalysts. In this article we report systematic differential scanning calorimetry (DSC) studies of (i) the constituents of BF₃:NH₂C₂H₅-catalyzed TGDDM–DDS epoxies and their mixtures, (ii) the effect of BF₃:NH₂C₂H₅ concentration on the cure reactions, (iii) the nature of the catalyzed cure reactions, and (iv) the environmental sensitivity of the catalyst. DSC studies are also reported on the cure reaction characteristics and environmental sensitivity of commercial C fiber–TGDDM–DDS epoxy prepregs.     

The effects of additives on curing properties,resin contents,and mechanical properties of graphite/epoxy composites

The effects of additives such as boron trifluride-monoethylene amine (BF₃MEA) and fumed silica in the TGDDM/DDS epoxy formulations on the curing properties, resin contents, and mechanical properties of their graphite/epoxy (Gr/Ep) composites were investigated. The addition of BF₃MEA increased the viscosity of resin as well as the resin contents of cured laminates because of its catalytic effect. Although the fumed silica was considered a thickening agent, it also acted like a co-catalyst with BF₃MEA. As the resin content of cured laminates was increased, the excess resin was likely to accumulate in the interlaminar region, which increased the interlaminar shear strength but decreased the flexure strength as well as the interlaminar fracture toughness value, G_IC.     

Epoxy resin cure. II. FTIR analysis

Fourier transform infrared (FTIR) spectroscopy is used to determine the cure rate of an epoxy resin consisting of Tetraglycidyl-4,4′-diaminodiphenyl methane (TGDDM) and diaminodiphenylsulfone (DDS). Cure rates at 120 and 160°C are shown to increase noticeably when 1% BF₃–MEA is added to either TGDDM to TGDDM plus DDS. Fluoroboric acid is shown to increase the cure rates even more than the BF₃–MEA. These Results combined with the NMR results in the accompanying article indicate that BF₃–MEA is not a catalyst for epoxy resin cure. Instead it is rapidly hydrolyzed to fluoroboric acid which acts as the catalyst.     

Fibre reinforced composites of multifunctional epoxy resins

Glass and carbon fibre reinforced epoxy composites were fabricated for N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenyl methane (TGDDM) and its formulated systems with tri- and di-functional reactive epoxy diluents using 30% diaminodiphenyl sulphone (DDS) as a curing agent. The epoxy laminates were evaluated for their physical, chemical and mechanical properties [at room (26°C) and high (100°C) temperatures]. A marginal increase (<20%) in the mechanical properties of CFRP was found compared with GFRP laminates. Incorporation of epoxy diluents altered the mechanical properties of the composites significantly. The incorporation of triglycidyl-4-aminophenol diluent to TGDDM systems resulted in an improvement in mechanical properties of about 2–6%.     

Tertiary‐amine‐free,non‐planar,sulfone‐containing,tetrafunctional epoxy and its application as a high temperature matrix

Non‐amine‐derived tetrafunctional epoxies have several advantages over the amine‐derived N,N,N′,N′‐tetraglycidyl‐4,4′‐diaminodiphenyl methane (TGDDM) in high temperature applications. Although two non‐amine‐derived tetrafunctional epoxies were developed in our laboratory, further improvements in toughness using less loading amount is still desirable. Thus, a tertiary‐amine‐free, non‐planar and triphenylmethane‐containing tetrafunctional epoxy (STFE) with a sulfone spacer was synthesized. When it was mixed with diglycidyl ether of bisphenol A (DGEBA) and cured with 4,4′‐diaminodiphenylsulfone (DDS), both thermal and mechanical performances outperformed TGDDM. Moreover, STFE modified system shows the highest toughness (35.7 kJ m^–2) among three amine‐free and triphenylmethane‐containing epoxies at merely 5 wt% loading. Molecular simulation and thermomechanical analysis results suggest that the improved mechanical properties could be related to the geometry of the molecule and larger free volume. Despite a marginal drop in T_g, the thermal degradation temperature is better than that of TGDDM/DDS. In addition, the moisture resistance of STFE/DGEBA/DDS is much better than that of TGDDM/DDS. Thus, STFE modified DGEBA could be a potential replacement for TGDDM in some high temperature applications. © 2020 Society of Chemical Industry     

Glass fiber-reinforced epoxy composites

Glass fiber-reinforced epoxy composites were prepared from the matrix resins tetraglycidyl diaminodiphenylmethane

1 Systematic name: N,N,N′,N′-Tetrakis(2,3-epoxypropyl)-4,4′-diaminodiphenylmethane.

(TGDDM) and tetraglycidyl bis(o-toluidino)-methane

2 Systematic name: N,N,N′,N′-Tetrakis(2,3-epoxypropyl)-4,4′-bis(o-toluidino)methane.

(TGMBT) using various amines like 4,4′-diaminodiphenylmethane (DDM), 4,4′-diaminodiphenylsulfone (DDS) and diethylene triamine (DETA) as curing agents. The fabricated laminates were evaluated for their mechanical and dielectrical properties and chemical resistance. The composites prepared using an epoxy fortifier (20 phr) showed significant improvement in the mechanical properties.     

New Epoxy Resins Containing Hard-soft Segments: Synthesis, Characterization and Modification Studies for High Performance Applications

We have attempted two methods to improve the properties of the epoxy materials for high performance applications. Solventless reactions of epoxy backbones tailored with hard-soft segments were adopted to improve the toughness. This was followed by curing of epoxy groups with cyanate to enhance the properties of epoxy formulations. We have reported here the synthesis of new epoxy resins having hard-soft segments based on aromatic and aliphatic backbones. An attempt was made at the modification and characterization of TGDDM/DDS (Tetraglycidyl 4,4^′′-diaminodiphenyl methane/diaminodiphenyl sulphone) system with new epoxides and 4-dicyanato diphenyl-2,2^′-propane (DCDPP). Thus, new epoxides of 1,4-, 1,5- and 1,6-methylene groups with terephthalate/isophthalate backbones were synthesized and the intermediates were characterized by FT-IR, ¹H/¹³C-NMR spectroscopic methods. The synthesized epoxides were used to modify the TGDDM/DDS/DCDPP. The neat cast laminates were made and characterized for their physical and mechanical properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isothermal curing by dynamic mechanical analysis of three epoxy resin systems: Gelation and vitrification

Times to gelation (t_gel) and times to vitrification (t_vit) during isothermal curing for the epoxy systems diglycidyl ether of bisphenol A (DGEBA)/1,3‐bisaminomethylcyclohexane (1,3‐BAC), tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM)/4‐4′‐diaminodiphenylsulfone (DDS), and TGDDM/epoxy novolac (EPN)/DDS were measured at different curing temperatures. This article reports on a method to determine t_gel and t_vit by dynamic mechanical analysis (DMA). Gelation was determined at the onset of the storage modulus or by the peak of the loss factor. Vitrification was defined as the curve of the storage modulus as the curve reached a constant level (endset) in DMA tests. The experimental values obtained for t_gel and t_vit were compared with values obtained by other experimental methods and with theoretical values (t_gel's) or indirect determinations (t_vit's). From kinetic analysis by differential scanning calorimetry, conversions corresponding to gelation were obtained for the three systems; this yielded a constant value for each system that was higher than theoretical value. Values of the apparent activation energies of the DGEBA/1,3‐BAC, TGDDM/DDS, and TGDDM/EPN/DDS epoxy systems were obtained from plots of t_gel's against reciprocal temperatures. They were 53.2, 58.2, and 46.5 kJ/mol, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 78–85, 2002     

Fluid uptake behavior of multifunctional epoxy blends

Profound changes in network architecture from blending trifunctional (m-triglycidylaminophenol, mTGAP) or tetrafunctional (tetraglycidyldiaminodiphenylmethane, TGDDM) epoxides with diglycidyl ether of bisphenol-A (DGEBA) and a curative amine (3,3′-diaminodiphenylsulfone, 3,3′-DDS) were observed using PVT, DMA, and PALS analyses. Increasing multifunctional content, which increased the crosslink density (with the expected increase in T_g), produced a decrease in the average free volume hole size (V_h) accompanied by a counterintuitive increase in fractional free volume (FFV). This unusual inverse relationship between FFV and V_h allowed clear resolution of their respective roles in equilibrium moisture uptake vs. the rate of uptake (diffusivity). Equilibrium water uptake increased with increasing multifunctional content, concomitant with the increase in FFV. Water diffusivity, on the other hand, decreased with increasing multifunctional content, concomitant with the decrease in V_h. The decreasing V_h in the epoxy blends also had interesting consequences for organic solvent sensitivity. MEK ingress was substantial in the binary DGEBA/DDS epoxy and completely inhibited for most of the blends, implying hole size selectivity was responsible for the MEK uptake inhibition. MEK uptake was precluded in epoxies whose V_h was below a critical threshold value of ～68 Å³. A small amount of mTGAP or TGDDM was sufficient to reduce the V_h of DGEBA/DDS epoxy below the threshold and prevent MEK uptake.     

The effect of hygrothermal fatigue on physical/mechanical properties and morphology of neat epoxy resin and graphite/epoxy composite

The effect of moisture absorption, desorption, and thermal spiking on the physical/mechanical properties of TGDDM/DDS epoxy resin was investigated and compared to the Gr/Ep composite. The mechanism of moisture diffusion in the neat resin was described on the morphological level. The diffusion rate of moisture in epoxy resin was found to depend on the mobility of molecular chains within an inhomogeneous epoxy network. Two well-known concepts of plasticization of amorphous polymers, the lubricity theory and the gel theory, were invoked to describe the interactions between the absorbed moisture and the resin network. Slight permanent changes in properties of the neat resin were observed after one absorption-desorption cycle. In the thermal spiking experiment, only the spiking temperature above the glass transition of the moisture saturated epoxy resins changed their internal structure and produced very small (thin) microcracks. By comparison with the neat epoxy resin, the Gr/Ep composites contain the reinforcement-matrix boundary region, characterized by the highest restrictions to molecular mobility. The absorbed moisture during the static hygrothermal fatigue cannot effectively plasticize this region. But during thermal spiking, the formation of microcracks is observed within the reinforcement-matrix boundary region as well as an increase in the moisture content.     

Cure kinetics of multifunctional epoxies with 2,2′‐dichloro‐4,4′‐diaminodiphenylmethane as hardener

The curing behavior of the epoxy resin N,N,N′,N′‐tetraglycidyldiaminodiphenyl methane (TGDDM) with triglycidyl p‐aminophenol as a reactive diluent was investigated using 2,2′‐dichloro‐4,4′‐diaminodiphenylmethane (DCDDM) as the curing agent. The effect of the curing agent on the kinetics of curing, shelf‐life, and thermal stability in comparison with a TGDDM‐diaminodiphenylsulfone (DDS) system was studied. The results showed a lesser activation energy at the lower level of conversion with a broader cure exotherm for the epoxy‐DCDDM system in comparison with the epoxy‐DDS system, although the overall activation energy for the two systems was comparable. TGA studies showed more stability in the epoxy‐DCDDM system than in the epoxy‐DDS system. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2097–2103, 2000     

High performance epoxy resins cured in the presence of BF3 catalyst

Dynamic mechanical experiments have been conducted on an epoxy system made with tetraglycidyl 4,4′-diaminodiphenyl methane (TGDDM) and polyglycidyl ether of Bisphenol A Novalac that were cured with 4,4′-diaminodiphenyl sulfone (DDS) in the presence of Boron trifluoride monoethylamine catalyst (BF₃:EtNH₂). As the concentration of BF₃:EtNH₂ increased, the low temperature β-transition magnitude increased slightly. The α₁-transition observed in the uncatalyzed system decreased significantly with the addition of BF₃:EtNH₂ catalyst. The α₂ or glass transition temperature of this system increased with increasing catalyst concentration. Both the catalyzed and uncatalyzed epoxy formulations studied in this work are important due to their similarity to systems used commercially in epoxy matrix composites.     

Characterization of the cure of TGDDM/DDS epoxy resins by chemiluminescence I. Spectral and thermal analysis

The emission of weak visible chemiluminescence (CL) during the cure of a tetraglycidyl 4,4′-diaminodiphenyl methane (TGDDM)-based epoxy resin, with three different concentrations of 4,4′-diaminodiphenylsulfone (DDS) has been studied at 135°C. Spectral analysis indicates that the CL originates from trace oxidation of the TGDDM resin and the emission intensity is sensitive to the viscosity changes during cure. From thermal analysis data, sharp discontinuities in CL intensity are shown to occur at the gel point. The temperature dependence of CL from a cured resin also shows a sharp discontinuity at T_g. These results indicate that CL provides a sensitive monitor of both the kinetics of gelation and the network formation in this epoxy resin.     

Two-Stage Moisture Absorption Behavior and Hydrolysis of Cured Dicyanate Ester Resins

A two-stage moisture absorption behavior for the cured 4,4-dicyanato 1,1-diphenolethane (DCDPE) resins was found when they were exposed to 60C/100%RH environment. The first stage followed the Fickian diffusion, whereas the secondary stage was a swelling-controlled diffusion in that the diffusion coefficient was decreased with time. With increasing the sample thickness, the two-stage absorption behavior became less discernible. After long term exposure to moisture, DCDPE resins were prone to hydrolysis to form voids in the networks. When chromium acetylacetonate (Cr(acac)₃) was incorporated in the resin formulations as a curing accelerator, it also accelerated the hydrolysis, facilitating the formation of voids.     

Multifunctional epoxy resins

The curing behavior of epoxy resins prepared by reacting epichlorohydrin with 4,4′-diaminodiphenyl methane (DADPM)/4,4′-diaminodiphenyl ether (DADPE) or 4,4′-diaminodiphenyl sulfone (DDS) was investigated using DDS and tris-(m-aminophenyl)phosphine oxide (TAP) as curing agents. A broad exothermic transition with two maxima were observed in the temperature range of 100–315°C when TAP was used as the curing agent. The effect of varying DDS concentration on curing behavior of epoxy resin was also investigated. Peak exotherm temperature (T_exo) decreased with increasing concentration of DDS, whereas heat of curing (ΔH) increased with an increase in amine concentration up to an optimum value and then decreased. Thermal stability of the resins, cured isothermally at 200°C for 3 h, was investigated using thermogravimetric analysis in a nitrogen atmosphere. Glass fiber-reinforced multifunctional epoxy resin laminates were fabricated and the mechanical properties were evaluated. © 1993 John Wiley & Sons, Inc.     

Dynamic mechanical experiments for probing process-structure-property relations in amine-cured epoxies

Dynamic mechanical experiments were performed on an amine-cured high performance epoxy system containing two types of epoxies and one amine. The system approximates a commercial resin system used widely as matrix material for graphite composites and whose main ingredients and composition are 88.5 percent of tetraglycidyl 4,4′-diamino diphenyl methane (TGDDM) and 11.5 percent of polyglycidyl ether of Bisphenol A Novalac epoxy with varying compositions of 4,4′-diaminodiphenyl sulfone (DDS), ranging from 19 to 40 PHR. Specifically, the effects of DDS and Novalac content as well as processing conditions including the particle size of solid components, the mixing speed, mixing temperature, and the mixing duration of the components on the dynamic mechanical properties of the epoxy polymer were investigated. A viscoelastic transition in the dynamic spectrum which was observed to be quite sensitive to the sample's T_g in conjunction with incomplete crosslinking reactions allowed for interrelating the composition and processing variables to the structure and bulk properties of this epoxy system.     

Epoxy resins based on aromatic glycidylamines. V. Shelf life of TGDDM-based epoxy resins

The effect of aging on resin composition was investigated as a part of a study concerned with the evaluation of epoxies containing N, N, N′, N′-tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM). Long-term stability of three different epoxy resins based on TGDDM and their mixtures with 4,4′-diaminodiphenylsulfone (DDS) was followed at 23 ± 2°C at a relative humidity ranging from 45% to 55%, by means of GPC and HPLC; short-term stability of the resins was evaluated at 125°C.     

Moisture effects during cure of high-performance epoxy matrices

The effects of absorbed moisture on the cure reactions and subsequent solid-state properties of a high-performance epoxy system were investigated in this study. The resin systems investigated were a model system, TGDDM–Novalac–DDS–BF₃:MEA (TNDB), and its commercial analog, Hercules 3501–6. The samples were exposed to three environments: a liquid water environment at 50°C; an 85% relative humidity vapor environment at 50°C; and an evacuated, desiccated atmosphere at 22°C. Differential scanning calorimetry (DSC) thermograms were obtained for the Hercules 3501-6 formulation as well as various compositions of the TNDB system for samples exposed to each of the three environments. Moisture was found to accelerate the cure especially for formulations exposed to the vapor environment. Dynamic mechanical experiments were also performed on cured, thin film samples. The “wet” samples were produced by adding water to the resin mixture prior to B-staging. The moisture was shown to increase the extent of reaction, but produce a slightly lower degree of crosslinking as related to the sample's glass transition. This has been attributed to chain-extension reactions, such as the hydroxyl–epoxide etherification reaction, which are favored in the presence of hydroxyl groups from water.