Cure monitoring of aerospace epoxy resins and prepregs by Fourier transform infrared emission spectroscopy

Spectral analysis of the infrared radiation emitted from thin films of resin transferred from the surface of high performance aerospace carbon fibreepoxy composite prepregs and heated to the cure temperature allows the cure chemistry and kinetics to be monitored in real time. Quantitative spectra with excellent signal-to-noise ratio are obtained by heating a thin resin film on a platinum hotplate fitted to the external optics of a Fourier transform infrared (FTIR) spectrometer and referencing the resulting emission (with the platinum emission subtracted) to a graphite black body at the same temperature. The resulting spectra are identical to absorption spectra and the quantitative features of the analysis are demonstrated by the appearance of isosbestic points during the curing reactions, so indicating that concentration profiles of the reacting species may be obtained. From the initial rate of amine and epoxy consumption, activation energies of 75kJ mol⁻¹ were obtained for both functional groups in the uncatalysed resin 4,4′-tetraglycidyl diamino diphenyl methane (TGDDM) with 27% 4,4′-diaminodiphenylsulfone (DDS), while values of 74 and 89kJ mol⁻¹ were obtained for amine and epoxy consumption from the TGDDM/DDS prepreg catalysed with boron trifluoride monoethylamine (Hercules 3501–6), consistent with homopolymerization occurring in the prepreg as well as amine–epoxy addition. Analysis of the FTIR emission at 177°C of resin from prepreg aged up to 90h at 23°C and 55% relative humidity shows a lowering of epoxy and amine concentration and a higher rate of cure, consistent with the formation of catalytic species. This technique may be used to monitor changes in surface properties such as tack and resin transfer, in addition to changes in the cure profile of the aged epoxy propreg.     

Cure of epoxy resins containing epoxidized polyols

A commercial epoxy resin, consisting of a mixture of diepoxides based on diglycidyl ether of bisphenol-A (DGEBA) and containing an epoxidized polypropylene glycol as reactive diluent, was characterized by ¹HNMR, FTIR, SEC, and chemical analysis. The kinetics of the cure with ethylenediamine (EDA) was catalyzed by the (OH) groups present in a large amount in the commercial formulation. A second order kinetic behavior gave an accurate fitting of results obtained by different experimental techniques (DSC in dynamic and isothermal modes and SEC in the pregel stage). The activation energy was E = 59.1 kJ/mol (14.1 kcal/mol), in very close agreement with values reported for the catalytic mechanism of the DGEBA-EDA polymerization. From the gel conversion and the critical stoichiometric ratio for samples containing an epoxy excess it was found that the average functionality of epoxidized species was f = 1.58.     

Cure kinetics of epoxy resins by differential scanning calorimetry technique

The curing reactions of epoxy resins with aliphatic amine are investigated using the differential scanning calorimetry technique with a single dynamic scan. The rate of the reaction was followed over the temperature range 30–250°C, and the activation energy and the order of the reaction are determined using four different computational methods. The activation energy for the various epoxy systems is observed in the range 40–76 kJ mol^?1 and the order of the reaction is observed to be ? 1·0.     

Cure kinetics of epoxy resins and aromatic diamines

An analysis of the cure kinetics of several different formulations composed of bifunctional epoxy resins and aromatic diamines was performed. A series of isothermal differential scanning calorimetry (DSC) runs (at higher temperature) and Fourier transform infrared spectroscopy (FT-IR) runs (at lower temperature) provided information about the kinetics of cure in the temperature range 18–160°C. All kinetic parameters of the curing reaction, including the reaction rate order, activation energy, and frequency factor were calculated and reported. Dynamic and isothermal DSC yielded different results. An explanation was offered in terms of different curing mechanisms which prevail under different curing conditions. A mechanism scheme was proposed to account for various possible reactions during cure.     

Cure monitoring of photosensitive resins by photo-dynamic mechanical analysis

A new technique, photo-dynamic mechanical analysis, was developed to monitor the cure of photosensitive resins. In this technique, the resin is dispensed on a low modulus substrate and the force needed to maintain a prescribed elongation on the sample is measured while the sample is exposed to an ultraviolet source. The force increases with the extent of cure and reaches a plateau when the resin is fully cured. Photo-dynamic mechanical analysis was found to be a rapid, continuous, and reliable technique to monitor the extent of cure of photopolymerizable resins. A finite element analysis was developed to calculate the resin modulus from the experimental force data. Thus, it was possible to monitor the resin modulus from the liquid state to the solid state.     

Cure kinetics of ternary blends of epoxy resins studied by nonisothermal DSC data

The curing kinetics of blends of diglycidyl ether of bisphenol A (DGEBA), cycloaliphatic epoxy resins, and carboxyl‐terminated butadiene‐acrylonitrile random copolymer (CTBN) in presence of 4,4′‐diamino diphenyl sulfone (DDS) as the curing agent was studied by nonisothermal differential scanning calorimetry (DSC) technique at different heating rates. The kinetic parameters of the curing process were determined by isoconversional method given by Malek for the kinetic analysis of the data obtained by the thermal treatment. A two‐parameter (m, n) autocatalytic model (Sestak‐Berggren equation) was found to be the most adequate selected to describe the cure kinetics of the studied epoxy resins. The values of E_a were found to be 88.6 kJ mol^?1 and 61.6 kJ mol^?1, respectively, for the studied two sample series. Nonisothermal DSC curves obtained using the experimental data show a good agreement with that theoretically calculated. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Cure monitoring of epoxy matrices for composites by ultrasonic wave propagation

Although ultrasonic wave propagation is a well-known technique for nondestructive analysis, it can be also applied for dynamic mechanical characterization (DMA) of polymers and composites. Most of DMA characterizations at ultrasonic frequencies are performed on thermoplastics and only a few articles are available on the characterization of the reactive properties of thermosetting resins. Therefore, in this work a complete characterization of the cure of a model epoxy system is presented, by comparing isothermal and nonisothermal data. The propagation of ultrasonic waves acting as a dynamic mechanical deformation at high frequencies can be used for the calculation of complex longitudinal bulk moduli during the cure of the epoxy resin. The evolution of attenuation and velocity during reaction is related to the strong physical changes occurring during the cure process. Furthermore, a comparison between the degree of reaction measured by Differential Scanning Calorimetry and ultrasonic data is proposed. The ultrasonic velocity (or the bulk longitudinal modulus) can be considered the most interesting parameter for cure monitoring because it follows the growth and evolution of the mechanical stiffness of the resin during cure. In particular, the obtained results suggest that the measurement of longitudinal velocity or L′ could be exploited for on-line measurements of post-gel properties. Finally, an immediate correlation is also proposed between the gel time and the end of cure and the ultrasonic data. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1969–1977, 1999     

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.     

Cure and physical properties of thermoplastic modified epoxy resins based on polyethersulfone

Dielectric, mechanical, thermal, rheological, and electron microscopy measurements are reported for five series of thermoplastic modified amine cured epoxy resin systems. The epoxy and thermoplastic components have been systematically changed to investigate the factors affecting the phase separation process. Data reported cover both changes in the physical properties occurring during cure and also of the final cured matrix. Dielectric data obtained from fully cured materials exhibits a Maxwell–Wagner–Sillers relaxation process characteristic of a phase separated morphology. Correlation of dielectric relaxation and electron microscopy data indicates that the phase structure changes with the thermoplastic content and composition of the epoxy phase. The mechanical properties change significantly at about 20–25% (w/w) of incorporated thermoplastic, coincident with the formation of a cocontinuous phase morphology. © 1995 John Wiley & Sons, Inc.     

Reversible isomerization and cure monitoring of epoxy azobenzene resins

The photoresponsive behavior of the glycidyloxyazobenzene (GOAB) monomer, synthesized using an improved method, is examined by UV/Vis spectroscopy. The monomer is cured with diethylenetriamine (DETA), forming a new epoxy resin. Proton NMR spectroscopy is used to monitor the completion of the curing reactions. Kinetics for reversible trans and cis isomerization in the cured system and also in the epoxy monomer are identified by UV/Vis spectroscopy during in situ irradiation with appropriate wavelengths (290–320 nm for UV and 400–500 nm for visible). The rates of recovery of the monomer from cis to trans forms are also obtained by heating and storing in the dark. Furthermore, the reactivity of the monofunctional GOAB monomer with a common amine, DETA, as a curing agent, is investigated using isothermal and dynamic heating scans in a DSC pan and by simultaneously monitoring the near‐FTIR spectra. The modified epoxy azobenzene proved to be reactive enough with DETA to form a network that can sustain temperatures of up to 200°C. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40770.     

Curing of epoxy resins by dicyandiamide

Summary The reaction course, the final structure and the thermal behaviour of dicyandiamide cured epoxy resins were studied by HPLC, Carbon-13 NMR (liquid and solid state), IR, gel time determination, DSC and evolved gas analysis. N, N-dimethyl-benzylamine, imidazole and Monuron were applied as three different accelerator types. Dimethylformamide was used as solvent to obtain homogeneous reaction mixtures. Two reaction pathways depending on tautomeric dicyandiamide structures are suggested explaining the reaction mechanism.     

Cure kinetics modeling of epoxy resins using a non‐parametric numerical procedure

The paper presents the development of a novel non‐parametric procedure for modeling of the chemical cure kinetics of a commercial resin‐transfer‐molding epoxy resin, RTM6. The procedure is entirely numerical and involves interpolation between experimentally determined values of cure reaction rates. The base data are obtained by Differential Scanning Calorimetry (DSC). The newly developed procedure is set against a background overview of other cure kinetics modeling approaches. It is shown that the numerical procedure can achieve a satisfactory level of accuracy in describing the progress of cure in this resin system. The important advantage, compared with other techniques, is that no information on the chemical naturr of the process is required.     

In situ cure monitoring of epoxy resins using fiber-optic Raman spectroscopy

Fiber-optic Raman spectroscopy was used to monitor the curing of epoxy resins in situ for eventual application to polymer composite processing. A 200-μm diameter quartz fiberoptic sensor immersed in liquid resin was used to obtain Raman spectra for a concentration series of diglicidyl ether of bisphenol-A in its own reaction product with diethylamine using an 820 nm continuous-wave diode laser excitation. A Raman peak at 1240 cm^?1 was assigned to a vibrational mode of the oxirane (epoxide) ring and its normalized intensity was found to be linearly related to the concentration of epoxide groups in the resin mixtures. Raman peaks at 1112 and 1186 cm^?1 associated with phenyl and gem-dimethyl resin backbone vibrations, respectively, did not change in intensity due to the curing reaction and were used as internal references to correct the Raman spectra for intensity changes due to density fluctuations and instrumental variations during the experiments. Fiber-optic Raman spectroscopy was used to monitor the extent of reaction in situ for the room-temperature cure of phenyl glicidyl ether with diethylamine. The extent of reaction of the epoxide groups calculated from the Raman spectra were in excellent agreement with kinetic data from Fourier transform near-infrared absorbance measurements made under the same conditions. © 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.

     

Cure of secondary carbamate groups by melamine-formaldehyde resins

Melamine-formaldehyde (MF) resins have been used as crosslinkers for hydroxyl-functional coreactants in thermoset coatings for about 60 years. Crosslink densities of films prepared from oligomeric urethane polyols suggested that the methoxymethyl groups of MF resins could react with urethane groups (i.e., secondary carbamate groups) as well as reacting with hydroxyl groups. Co-reactants that contain secondary carbamate groups and no hydroxyl groups have been prepared with several types of backbone structures. Cure of such co-reactants by MF resins has been studied using a gradient oven with determination of impact resistance, solvent resistance, and hardness. Several formulations from these cure profile sets have been selected for crosslink density determinations by dynamic mechanical analysis (DMA). Crosslink densities of cured films are consistent with complete conversion of secondary carbamate groups at temperatures only slightly higher than those used for cure of hydroxyl groups. The -OH groups on certain acrylic polyols were converted to secondary carbamate groups. The original acrylic and the converted acrylic were both cured with MF resins. Acid resistance was much better for films prepared from the acrylic that contained secondary carbamate groups. Presented at the 25th International Waterborne, High-Solids, and Powder Coating Symposium, New Orleans, LA, Feb. 18–20, 1998. 730 Worcester Street, Springfield, MA 01151.     

Cure behavior of epoxy resins with different kinds of onium salts as latent thermal catalysts

Two latent thermal catalysts, Dimethyl phenacylsulfonium hexafluoroantimonate, (Benzyl‐S), and triphenyl benzyl phosphonuim hexafluoroantimonate, (Benzyl‐P), were synthesized. Both these synthesized catalysts fulfill requirements for a rapid cure at a moderately elevated temperature in curing the epoxy resins of neat 3,4‐epoxycyclohexylmethyl‐3,4‐epoxycyclohexane‐carboxylate, (EPC), neat 2,2‐bis(4‐glycidyloxyphenyl)propane, (EPA), and their hybrid resin. The cure behavior of these resins cured individually with the synthesized catalysts was studied through correlation of the in situ FTIR (Fourier transform infrared) spectroscopy and DSC (differential scanning calorimetry) dynamic scanning results. Catalyst Benzyl‐S is more effective than Benzyl‐P. Resin EPC is significantly more sensitive to the latent thermal catalysts than the EPA resin. Activated chain‐end (ACE) and activated monomer (AM) mechanisms are basically adopted. Isomerization occurs in the neat EPA cure system, and transesterification takes place in all systems containing EPC species in the latter curing stage. Most importantly, ester linkages C?O in the hybrid systems have been destroyed at some time during the curing process, but some reformed at the latter curing stage. It is most likely that the C?O linkage in the EPC segments was attacked by the activated chain‐end of the epoxide group of EPA species to form a five‐membered cyclic acetal. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3539–3551, 2001     

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.