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.     

Another look at epoxy thermosets correlating structure with mechanical properties

All glassy epoxy polymers develop macroscopic properties at some degree of conversion. Our selected example, diglycidyl ether bisphenol‐A epoxy pre‐polymer, was cured with 3,3′‐diaminodiphenylsulfone at stoichiometric equivalents using a series of cure profiles to produce distinct variation in the degree of epoxy conversion and result in varying network and network connectivity. Activation energy of epoxy‐amine reaction in this selected system was ～61 kJ/mol. The calculated reaction energy barrier was found to vary with the extent of epoxy conversion and is attributed to multimechanistic reactions. Epoxy‐amine conversion was tracked in situ via near infrared spectroscopic analysis. A single cure condition (90°C) was selected for experiments focused on preferential linear chain growth and minimal branching and/or crosslinking. The physical properties for matrix materials from samples prepared to varying degrees of conversion were characterized and tested for fracture toughness, tensile, flexural, compression properties, molecular weight between crosslinks/crosslink density, and glass transition temperature(s). An empirical equation was also designed to predict molecular weight between crosslinks based on chemical connectivity and extent of reaction. POLYM. ENG. SCI., 54:1990–2004, 2014. © 2013 Society of Plastics Engineers     

Comparison of microwave and thermal cure of epoxy–anhydride resins: Mechanical properties and dynamic characteristics

The objective of this work was to compare the mechanical properties of epoxy resins cured by thermal heating and microwave heating. Epoxy–anhydride (100:80) resins were cured in a domestic microwave oven and in a thermal oven. The hardening agents included methyl tetrahydrophthalic anhydride and methyl hexahydrophthalic anhydride. Three types of accelerators were employed. Thermal curing was performed at 150°C for 20 and 14 min for resins containing 1 and 4% accelerator, respectively. Microwave curing was carried out at a low power (207 or 276 W) for 10, 14, and 20 min. All cured resins were investigated with respect to their tensile properties, notched Izod impact resistance, and flexural properties (three‐point bending) according to ASTM standards. The tan δ and activation energy values were investigated with dynamic mechanical thermal analysis, and the extent of conversion was determined with differential scanning calorimetry. The differences in the mechanical properties of the thermally cured and microwave‐cured samples depended on the resin formulation and properties. Equivalent or better mechanical properties were obtained by microwave curing, in comparison with those obtained by thermal curing. Microwave curing also provided a shorter cure time and an equivalent degree of conversion. The glass‐transition temperatures (tan δ) of the thermally and microwave‐cured resins were comparable, and their activation energies were in the range of 327–521 kJ/mol. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1442–1461, 2005     

Effect of the anhydride structure and the epoxy fortifier on the reactivity in curing of epoxy resins

The curing reactions of the epoxy resin triglycidyl-p-aminophenol with acid anhydrides using tertiary amine as catalyst were studied kinetically by differential scanning calorimetry. The dynamic scans in the temperature range 30–300°C were analyzed to estimate the activation energy and the order of the reaction for the curing process using some empirical relations. The curing process was found to follow a simple Arrhenius type kinetics having activation energies in the range 75 – 142 kJ/mol and the order of the reaction is about 1. The effect of the variation in acid anhydride structure and of the incorporation of the epoxy fortifier on the cure characteristics is investigated.     

Cure kinetics of a glass/epoxy prepreg by dynamic differential scanning calorimetry

Dicyandiamide (DICY)‐cured epoxy resins are important materials for structural adhesives and matrix resins for fiber‐reinforced prepregs. Dynamic differential scanning calorimetry (DSC) with heating rates of 2.5, 5, 10, and 15°C/min was used to study the curing behavior of the epoxy prepreg Hexply 1454 system, which consisted of diglycidyl ether of bisphenol A, DICY, and Urone reinforced by glass fibers. The curing kinetic parameters were determined with three different methods and compared. These were the Kissinger, Ozawa, and Borchardt–Daniels kinetic approaches. The lowest activation energy (76.8 kJ/mol) was obtained with the Kissinger method, whereas the highest value (87.9 kJ/mol) was obtained with the Borchardt–Daniels approach. The average pre‐exponential factor varied from 0.0947 × 10⁹ to 2.60 × 10⁹ s⁻¹. The orders of the cure reaction changed little with the heating rate, so the effect of the heating rate on the reaction order was not significant. It was interesting that the overall reaction order obtained from all three methods was nearly constant (≅2.4). There was good agreement between all of the methods with the experimental data. However, the best agreement with the experimental data was seen with the Ozawa kinetic parameters, and the most deviation was seen with the Borchardt kinetic parameters. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Cure kinetics of several epoxy–amine systems at ambient and high temperatures

Epoxy-crosslinker curing reactions and the extent of the reactions are critical parameters that influence the performance of each epoxy system. The curing of an epoxy prepolymer with an amine functional group may be accompanied by side reactions such as etherification. Commercial epoxy prepolymers were cured with different commercial amines at ambient as well as at elevated temperatures. Singularly, only epoxy–amine reactions were observed with diglycidyl ether of bisphenol-A (DGEBA)-based epoxides in our research even upon post-curing at 200°C. Etherification side reaction was found to occur at a cure temperature of 200°C in epoxides possessing a tertiary amine moiety. A combined goal of our research was to understand the effect of tougheners on the cure of epoxy–amine blend. To discern the effect of tougheners on the cure, core–shell rubber (CSR) particles were incorporated into the epoxy–amine blend. It was observed that CSR particles did not restrict the system from proceeding to complete reaction of epoxy moieties. Besides, CSR particles were found to accelerate the epoxy-amine reaction at a lower level of epoxy conversion. The lower activation energy of epoxy–amine reaction of CSR incorporated system compared to control supported the catalytic effect of CSR particles on the epoxy-amine reaction of epoxy prepolymer and amine blends.     

The effect of curing temperature on thermal,physical and mechanical characteristics of two types of adhesives for aerospace structures

The effects of cure temperatures on the thermal, physical and mechanical characteristics of two types of thermosetting structural epoxy film adhesives were determined in detail. The aim of this paper is to assess the effect of cure temperatures (82–121 °C) on the degree of cure of the two adhesives and the relevant void formations that need to be addressed in bonded part production and repair. Two thermal parameters were used to characterize the advancement of the reaction, such as degree of cure and glass transition temperature. The joint properties with respect to the cure temperatures were characterized by void content and bond-line thickness measurements and lap shear strength tests. Experimental results presented that all lap shear strengths were well within minimum shear strength (29 MPa) required by the specification of the film-type adhesive. However, the lap shear strength testing after aging at 82 °C and 95%R.H for 1000 h showed that the improved durability when the adhesive is cured at 121 °C did not occur for the 82 °C cure. Low curing conversion (75–77% degree of cure) combined with high voids (over 2 areal%) has a catastrophic effect on the bonding qualities at the metal-adhesive interface and due to lack of cohesion in the adhesive. The changes in the interface caused by the low temperature curing may contribute to an increased susceptibility of the bonded joint to moisture and consequent bond-line degradation.     

Equivalent processing time analysis of glass transition development in epoxy/carbon fiber composite systems

The cure kinetics and glass transition development of a commercially available epoxy/carbon fiber prepreg system, DMS 2224 (Hexel F584), was investigated by isothermal and dynamic‐heating experiments. The curing kinetics of the model prepreg system exhibited a limited degree of cure as a function of isothermal curing temperatures seemingly due to the rate‐determining diffusion of growing polymer chains. Incorporating the obtained maximum degree of cure to the kinetic model development, the developed kinetic equation accurately described both isothermal and dynamic‐heating behavior of the model prepreg system. The glass transition temperature was also described by a modified DiBeneditto equation as a function of degree of cure. Finally, the equivalent processing time (EPT) was used to investigate the development of glass transition temperature for various curing conditions envisioning the internal stress buildup during curing and cooling stages of epoxy‐based composite processing. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 144–154, 2002; DOI 10.1002/app.10282     

Blends of poly(ethylene terephthalate) and epoxy as matrix material for continuous fibre reinforced composites

Abstract

The morphology and mechanical properties of poly(ethylene terephthalate) (PET)–epoxy blends and the application of these blends in continuous glass fibre reinforced composites have been investigated. Epoxy resin was applied as a reactive solvent for PET to obtain homogeneous solutions with a substantially decreased melt viscosity. The epoxy resin in these solutions was cured using an amine hardener according to two different schedules. In the first, high temperature curing at 260°C preceded low temperature crystallisation of the PET at 180°C. In the second, the PET was allowed to crystallise prior to low temperature curing at 180°C. After cure, all blends revealed a phase separated morphology of dispersed epoxy in a continuous PET matrix. The flexural strength and failure strain of all cured blends showed an increase with increasing epoxy content, whereas the high temperature cured blends exhibited overall lower flexural properties than those cured at the lower temperature. Microstructural analysis and flexural properties of continuous glass fibre reinforced PET–epoxy laminates showed that the composites obtained had a low void content. These PET–epoxy laminates had increased inplane shear strength in comparison with unmodified PET based laminates, indicating considerably increased fibre–matrix adhesion.     

Investigation of curing characteristics of carbon fiber/epoxy composites cured with low‐energy electron beam

To solve the penetration depth of carbon fiber/epoxy prepreg and irradiation dose uniformity by low‐energy E‐Beam under 125 keV, the both‐side irradiation curing of prepreg was investigated. The results show that there is little thermal effect during the low‐energy electron beam irradiation curing process, even though the irradiation dosage reached 300 kGy, only 46.2°C can be tested on the prepreg surface. Due to the low curing temperature, the degree of cure of prepreg was only 61.8% at 300 kGy level of irradiation, and the glass‐transition temperature (T_g) was only 48.6°C. The degree of cure and T_g can be increased sharply by thermal postcure. After being postcured at 160°C for 30 min, the degree of cure and the T_g of prepreg reached 98.5% and 170.4°C, respectively. Interlaminar shear strength testing result indicate that the fabrication process of the composite layer by layer curing by the low‐ energy E‐Beam is a promising cure approach. POLYM. COMPOS., 36:1731–1737, 2015. © 2014 Society of Plastics Engineers     

Comparison of the curing kinetics of a DGEBA/acid anhydride epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of an epoxy resin system, based upon a diglycidyl ether of bisphenol‐A (DGEBA) with HY917 (an acid anhydride hardener) and DY073 (an amine–phenol complex that acted as an accelerator), was investigated using a conventional differential scanning calorimeter and a microwave‐heated power‐compensated calorimeter. Dynamic cure of the epoxy resin using four different heating rates and isothermal cure using four different temperatures were carried out and the degree of cure and reaction rates were compared. The cure kinetics were analyzed using several kinetics models. The results showed different activation energies for conventional and microwave curing and suggested different reaction mechanisms were responsible for curing using the two heating methods. Resins cured using conventional heating showed higher glass transition temperatures than did those cured using microwave heating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2054–2063, 2007     

Curing of epoxy resins with in situ-generated substituted ureas

An epoxy resin based on bisphenol-A diglycidylether (DGEBA) was cured with a substituted urea generated in situ from the reaction of piperidine with an equivalent amount of toluenediisocyanate (TDI). Curing at 100°C or higher temperatures, during 24 h, led to a complete conversion of epoxy groups and the appearance of aliphatic ethers and oxazolidone rings as revealed by IR spectra. The epoxy conversion was proportional to the oxazolidone concentration. The reaction heat was (?ΔH) = 61 kJ/eq. The maximum T_g was 102°C, e.g., the same value as the one obtained with piperidine alone. Thus, the cure with the substituted urea leads to a similar network but has the following advantages: increase in the latency of the initial formulation, absence of secondary amine volatilization (reproducible curing schedule), and decrease in the reaction heat per epoxy equivalent.     

A thermal and rheological investigation during the complex cure of a two‐component thermoset polyurethane

The complex cure kinetics of the reaction between oligomeric diphenylmethane diisocyanate (PMDI) and glycerol was characterized through thermal and rheological techniques. Isoconversional analysis of Differential scanning calorimetry (DSC) data resulted in the activation energy varying with conversion. Isothermal analysis gave activation energies ranging from 5 kJ/mol to 33.7 kJ/ mol, whereas nonisothermal data gave values for the activation energy ranging from 49.5 to 55 kJ/mol. Incomplete cure was evident in isothermal DSC, becoming diffusion controlled in the final stages of cure. DMA analysis on the cured material gave a glass transition temperature of 104 ± 3°C, which was evidence for vitrification of the curing system. The primary and secondary hydroxyl group reactivity was dependant on the isothermal cure temperature. Rheological studies of viscosity increase and tan δ changes with time revealed a complex cure process, with primary and secondary hydroxyl reactivity also showing dependence on isothermal cure temperatures, reflecting similar results obtained from isothermal DSC studies. The independence of tan δ on frequency was used to determine the point where the polymer formed an infinite network and was no longer able to flow, providing an overall activation energy attained at the gel point of 77.4 ± 4.4 kJ/mol. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

An infrared spectroscopic investigation of the curing reactions of the EPON 828/meta-phenylenediamine system

Mid- and near-infrared (IR) spectroscopy has been used to study the curing of a bisphenol-A based epoxy resin (EPON-828) with a tetrafunctional curing agent, viz., meta-phenylenediamine (MPDA). Three different cure cycles were used in the study. Primary amine functionality was observed to react relatively rapidly; none remained after curing for 2 h at 75°C. Secondary amine functionality was exhausted in epoxy rich samples subjected to the standard cure cycle (2 h at 75°C followed by 2 h at 125°C). In samples with stoichiometric amount or higher MPDA, complete reaction of secondary amine or epoxy groups was not observed. In amine-rich samples subjected to post curing (6 h at 175°C), evidence was seen for the reaction of hydroxyl and epoxy groups, resulting in a considerable increase in the crosslink density of these samples.     

Structural Carbon/Epoxy Prepregs Properties Comparison by Thermal and Rheological Analyses

Two different carbon/epoxy prepreg materials were characterized and compared using thermal (DSC, TGA, and DMA) and rheological analyses. A prepreg system (carbon fiber preimpregnated with epoxy resin F584) that is currently used in the commercial airplane industry was compared with a prepreg system that is a prospective candidate for the same applications (carbon fiber prepreg/epoxy resin 8552). The differences in the curing kinetics mechanisms of both prepreg systems were identified through the DSC, TGA, DMA, and rheological analyses. Based on these thermal analysis techniques, it was verified that the curing of both epoxy resin systems follow a cure kinetic of n order. Even though their reaction heats were found to be slightly different, the kinetics of these systems were nevertheless very similar. The activation energies for both prepreg systems were determined by DSC analysis, using Arrhenius's method, and were found to be quite similar. DMA measurements of the cured prepregs demonstrated that they exhibited similar degrees of cure and different glass transition temperatures. Furthermore, the use of the rheological analysis revealed small differences in the gel temperatures of the two prepreg systems that were examined.     

Isothermal Cure of an Epoxy System Containing Tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM), a Multifunctional Novolac Glycidyl Ether and 4,4′-Diaminodiphenylsulphone (DDS): Vitrification and Gelation

Times to gelation and vitrification have been determined at different isothermal curing temperatures between 200 and 240°C for an epoxy/amine system containing both tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and a multifunctional Novolac glycidyl ether with 4,4′-diaminodiphenylsulphone (DDS). The mixture was rich in epoxy, with an amine/epoxide ratio of 0·64. Gelation occurred around 44% conversion. Vitrification was determined from data curves of glass transition temperature, T_g, versus curing time obtained from differential scanning calorimetry experiments. The minimum and maximum values T_g determined for this epoxy system were T_g0=12°C and T_gmax=242°C. Values of activation energy for the cure reaction were obtained from T_g versus time shift factors, a_T, and gel time measurements. These values were, respectively, 76·2kJmol^-1 and 61·0kJmol^-1. The isothermal time–temperature–transformation (TTT) diagram for this system has been established. Vitrification and gelation curves cross at a cure temperature of 102°C, which corresponds to glass transition temperature of the gel. © of SCI.     

A new approach for determination of gel time of a glass/epoxy prepreg

Gel time of a glass/epoxy prepreg, HexPly®1454, was investigated by a parallel plate rheometer. The prepreg is based on dicyandiamide (DICY)‐cured diglycidyl ether of bisphenol A epoxy resin system. It is found that the application of the G′‐G″ crossover method for gel time determination is not suitable for this system. A new approach was proposed in which the maximum tan δ is regarded as the gel point. This can accurately define the gel point at various temperatures. The results proved that this point is independent of the applied frequency. The activation energy for the cure reaction of the system was determined via gel time determination of the prepreg at different isothermal temperatures and found to be 75.0 ± 10.2 kJ/mol. This is in good agreement with the activation energy obtained from the dynamic DSC studies. The steady‐shear rheology experiment was used to study the viscosity profile and subsequently to determine the gel point and verify the new approach for gel time determination. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Toughening of trifunctional epoxy system. V. Structure–property relationships of neat resin

This work presents an investigation into the structure–property relationships of a cured highly crosslinked epoxy/amine resin system. The mechanical, physical, and thermal properties of the cured and postcured networks were measured and compared to the chemical structures. Crosslink density was shown to be dependent upon secondary amine conversion and it determined the glass transition temperatures, water uptake, density, toughness, and compressive strength. Other properties such as compressive modulus and yield stress were determined by more short‐range molecular motions. Curing at a temperature of 150°C was shown to be the minimum temperature required to “completely” cure the network and achieve optimum mechanical, physical, and thermal properties. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 237–248, 2000     

Curing of epoxy systems at sub‐glass transition temperature

Three epoxy‐amine thermoset systems were cured at a low ambient temperature. Evolution of the reaction kinetics and molecular structure during cure at the sub‐glass transition temperature was followed by DSC and chemorheology experiments. The effect of vitrification and the reaction exotherm on curing and final mechanical properties of the epoxy thermosets was determined. Thermomechanical properties of the low‐temperature cured systems depend on the reaction kinetics and volume of the reaction mixture. Curing of the fast‐reacting system in a large volume (12‐mm thick layer) resulted in the material with T_g exceeding the cure temperature by 70–80°C because of an exothermal temperature rise. However, the reaction in a too large volume (50‐mm layer) led to thermal degradation of the network. In contrast, thin layers (1.5 mm) were severely undercured. Well‐cured epoxy thermosets could be prepared at sub‐T_g temperatures by optimizing reaction conditions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3669–3676, 2006     

Synthesis,cure kinetics,and physical properties of a new tricyanate ester with enhanced molecular flexibility

1,2,3-Tris(4-cyanatophenyl)propane, a new tricyanate ester monomer that was designed to incorporate more flexible chemical linkages at junction points in the cured macromolecular network, was synthesized in nine steps with an overall yield of 26%. The highly purified monomer exhibited an activation energy of 110 kJ/mol for auto-catalytic cure at temperatures of 210 °C–290 °C, modestly lower than the comparably measured activation energy of a commercial cyanated novolac. The overall extent of cure achievable at these temperatures was also higher for the new monomer. Many physical properties of the cured monomer, including density, thermochemical stability, moisture uptake, and the impact of hydrolytic degradation on glass transition temperature were similar to those of commercial tricyanates, with a dry glass transition temperature at full conversion of at least 340 °C. These results illustrate how careful control of the local chemical structure in the vicinity of network junction points may be utilized to improve the properties of thermosetting polymer networks.