Dielectric modeling of the curing process

Dielectric data from an epoxy resin system were used as the foundation for dielectric modeling of the curing process. This resin system (DGEBA-polyamide) was chosen as an easily processible model system. Dielectric average relaxation times, defined as the reciprocal of the angular frequency at which the loss component of the dielectric constant reaches a maximum, were determined for a 40°C isothermal cure. The changes in the average relaxation time through the cure exhibited similar behavior to those for viscosity, which inspired the correlation of the two properties. The dielectric relaxation time was modeled using a six-parameter model analogous to that used for viscosity. The model parameters were in turn associated with both intrinsic properties of the system and reaction kinetics describing the cure. The possibility of extending the relaxation time model for use with single-frequency data by means of a time-frequency correlation was also investigated. Combined, these two modeling methodologies provide a powerful constitutive approach for describing dielectric properties of thermosetting polymers during cure.     

Phthalonitrile‐epoxy blends: Cure behavior and copolymer properties

Binary blends composed of 4,4′‐bis(3,4‐dicyanophenoxy)biphenyl (biphenyl PN) and diglycidyl ether of bisphenol A (epoxy resin) and oligomeric n = 4 phthalonitrile (n = 4 PN) and epoxy resin were prepared. The cure behavior of the blends was studied under dynamic and isothermal curing conditions using differential scanning calorimetry, simultaneous thermogravimetric/differential thermal analysis, infrared spectroscopy, and rheological analysis. The studies revealed that phthalonitrile‐epoxy blends exhibited good processability and that they copolymerized with or without the addition of curing additive. In the absence of curing additive, the blends required higher temperatures and longer cure times. The thermal and dynamic viscoelastic properties of amine‐cured phthalonitrile‐epoxy copolymers were examined and compared with those of the neat epoxy resin. The properties of the epoxy resin improved with increasing biphenyl PN content and with n = 4 PN addition. Specifically, the copolymers exhibited higher glass transition temperatures, increased thermal and thermo‐oxidative stabililty, and enhanced dynamic mechanical properties relative to the commercially available epoxy resin. The results showed that the phthalonitrile‐epoxy blends and copolymers have an attractive combination of processability and high temperature properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Curing behaviour of syndiotactic polystyrene–epoxy blends: 1. Kinetics of curing and phase separation process

The modification of the curing behaviour and the phase separation process for an epoxy resin blended with a crystalline thermoplastic was investigated in the case of the diglycidylether of bisphenol‐A (DGEBA)/4,4′‐methylene bis(3‐chloro‐2,6‐diethylaniline) (MCDEA) blended with syndiotactic polystyrene (sPS) and cured at 220 °C. Phase separation taking place during curing of the blend was investigated by differential scanning calorimetry (DSC) and optical microscopy in order to get a better understanding of the complex interactions between cure kinetics of epoxy matrix and crystallisation of sPS, both influenced by blend composition. Results suggested that phase separation and crystallisation of sPS occurred at almost similar times, with phase separation just being ahead of crystallisation. DSC and near‐infrared measurements were used for the determination of the cure kinetics. Slow delays on the cure reactions were observed during the first minutes for the sPS‐containing blends compared with the neat DGEBA/MCDEA system but, after some time, the reaction rate became faster for the blends than for the neat matrix. Phase separation occurring in the mixtures may explain this particular phenomenon. Copyright © 2004 Society of Chemical Industry     

Cure kinetics and thermal stability of micro and nanostructured thermosetting blends of epoxy resin and epoxidized styrene‐block‐butadiene‐block‐styrene triblock copolymer systems

The compatibility of styrene‐block‐butadiene‐block‐styrene (SBS) triblockcopolymer in epoxy resin is increased by the epoxidation of butadiene segment, using hydrogen peroxide in the presence of an in situ prepared catalyst in water/dichloroethane biphasic system. Highly epoxidized SBS (epoxy content SBS >26 mol%) give rise to nanostructured blends with epoxy resin. The cure kinetics of micro and nanostructured blends of epoxy resin [diglycidyl ether of bisphenol A; (DGEBA)]/amine curing agent [4,4′‐diaminodiphenylmethane (DDM)] with epoxidized styrene‐block‐butadiene‐block‐styrene (eSBS 47 mol%) triblock copolymer has been studied for the first time using differential scanning calorimetry under isothermal conditions to determine the reaction kinetic parameters such as kinetic constants and activation energy. The cure reaction rate is decreased with increasing the concentration of eSBS in the blends and also with the lowering of cure temperature. The compatibility of eSBS in epoxy resin is investigated in detailed by Fourier transform infrared spectroscopy, optical and transmition electron microscopic analysis. The experimental data of the cure behavior for the systems, epoxy/DDM and epoxy/eSBS(47 mol%)/DDM show an autocatalytic behavior regardless of the presence of eSBS in agreement with Kamal's model. The thermal stability of cured resins is also evaluated using thermogravimetry in nitrogen atmosphere. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Dynamically cured PP/MAH-g-SEBS/epoxy blends

Abstract

A new method concerning the simultaneous reinforcing and toughening of polypropylene (PP) is reported. Dynamic cure of the epoxy resin with 2-ethylene-4-methane-imidazole was successfully applied in the PP/maleic anhydride grafted styrene–ethylene–butylene–styrene (MAH-g-SEBS) triblock co-polymer, and the obtained blends were named as dynamically cured PP/MAH-g-SEBS/epoxy blends. The stiffness and toughness of the blends are in a good balance, and the smaller size of the epoxy particle in the PP/MAH-g-SEBS/epoxy blends shows that MAH-g-SEBS was also used as a compatibiliser. The structure of the dynamically cured PP/MAH-g-SEBS/epoxy blends is the embedding of the epoxy particles by MAH-g-SEBS. The cured epoxy particles as organic filler increase the stiffness of the PP/MAH-g-SEBS blends, and the improvement in the toughness is attributed to the embedded structure. The tensile strength and flexural modulus of the blends increase with increasing epoxy resin content, and the impact strength reaches a maximum of 342 J m^?1 at the epoxy resin content of 10wt-%. Differential scanning calorimetry analysis shows that the epoxy particles in the dynamically cured PP/MAH-g-SEBS/epoxy blends could have contained embedded MAH-g-SEBS, decreasing the nucleating effect of the epoxy resin. Wide angle X-ray diffraction analysis shows that the dynamical cure and compatibilisation do not disturb the crystalline structure of PP in the blends.     

Microdielectric analysis and curing kinetics of an epoxy resin system

Microdielectric analysis (DEA) was carried out to investigate the cure behavior of a bisphenol F epoxy/aromatic amine resin system using an online dielectric cure monitoring technique. Ionic conductivity measured by a microdielectric sensor under isothermal conditions was correlated to the degree of cure and glass‐transition temperature, which are determined by differential scanning calorimetry (DSC). Results obtained by isothermal DSC measurement were used to establish a cure kinetic model for the epoxy resin. Experimental results show that the ratio of the ion conductivity to the initial ion conductivity, Logσ/Logσ₀, has a linear relation with the glass‐transition temperature. Furthermore, correlations between ion conductivity and degree of cure and cure rate are established using the best fit of the measured data. Cure behavior of the epoxy resin obtained by DEA is compared with that predicted by the cure kinetics model. Good agreement was observed. POLYM. ENG. SCI., 47:150–158, 2007. © 2007 Society of Plastics Engineers     

Dynamically cured polypropylene/epoxy blends

The dynamic vulcanization process, usually used for the preparation of thermoplastic elastomers, was used to prepare polypropylene (PP)/epoxy blends. The blends had crosslinked epoxy resin particles finely dispersed in the PP matrix, and they were called dynamically cured PP/epoxy blends. Maleic anhydride grafted polypropylene (MAH‐g‐PP) was used as a compatibilizer. The effects of the reactive compatibilization and dynamic cure were studied with rheometry, capillary rheometry, and scanning electron microscopy (SEM). The crystallization behavior and mechanical properties of PP/epoxy, PP/MAH‐g‐PP/epoxy, and dynamically cured PP/epoxy blends were also investigated. The increase in the torque at equilibrium for the PP/MAH‐g‐PP/epoxy blends indicated the reaction between maleic anhydride groups of MAH‐g‐PP and the epoxy resin. The torque at equilibrium of the dynamically cured PP/epoxy blends increased with increasing epoxy resin content. Capillary rheological measurements also showed that the addition of MAH‐g‐PP or an increasing epoxy resin content increased the viscosity of PP/epoxy blends. SEM micrographs indicated that the PP/epoxy blends compatibilized with PP/MAH‐g‐PP had finer domains and more obscure boundaries than the PP/epoxy blends. A shift of the crystallization peak to a higher temperature for all the PP/epoxy blends indicated that uncured and cured epoxy resin particles in the blends could act as effective nucleating agents. The spherulites of pure PP were larger than those of PP in the PP/epoxy, PP/MAH‐g‐PP/epoxy, and dynamically cured PP/epoxy blends, as measured by polarized optical microscopy. The dynamically cured PP/epoxy blends had better mechanical properties than the PP/epoxy and PP/MAH‐g‐PP/epoxy blends. With increasing epoxy resin content, the flexural modulus of all the blends increased significantly, and the impact strength and tensile strength increased slightly, whereas the elongation at break decreased dramatically. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1437–1448, 2004     

In-process control of epoxy composite by microdielectrometric analysis

The curing of an epoxy prepolymer based on the diglycidyl ether of bisphenol A (DGEBA) with dicyandiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry, differential scanning calorimetry, viscosity measurements, and insolubles in THF for gel-point detection. Interpreting dielectric data with respect to chemorheology continues to be the subject of scientific discussion. The focus of this issue is to give an industrial point of view on the collected on-line dielectric measurements during an epoxy/fiber glass composite cure. Hence, isothermal polymerizations of DGEBA/DDA/imidazole resin were examined and dielectric properties such as ionic conductivity were related to the cure kinetics by conversion through an experimentally established equation. This mathematical model was used to predict reaction advancement of epoxy processing under nonisothermal cure conditions. This model is shown to be able to forecast both isothermal and nonisothermal cure data of unaged resin. According to these results, cure monitoring was carried out on prepregs. Whereas some deviations of the law were observed at the time of the last stage of the cure, good correlation was obtained for the reaction rate during the in-mold process curing time.     

Properties of cyanate ester-cured epoxy/polyphenylene oxide blends as a matrix material for Kevlar fiber composites

Epoxy/polyphenylene oxide (PPO) blends were cured with multifunctional cyanate ester resin. The effects of the PPO content on the cure behavior in the cyanate ester-cured epoxy were investigated with Fourier transform infrared spectroscopy (FTIR). The cure reaction in the epoxy/PPO blends was faster than that of the neat epoxy system. FTIR analysis revealed that the cyanate functional group reactions were accelerated by adding PPO and that several co-reactions had occurred, such as cyanate-hydroxyl addition and epoxy-cyanate addition. This was caused by the reaction of cyanate ester with the PPO phenolic end-group and water yielding imidocarbonate and carbamate intermediate which can react with cyanate ester to form cyanurate. Then the cyanurate can react further with the epoxy resin. Thermal mechanical analysis showed that the thermal stability of the epoxy/PPO blends is improved by adding PPO. The morphology of the fiber-rich areas in the composite is different from that of the epoxy/PPO blend without Kevlar fiber. In the pure polymer blends with high PPO content (30 and 50 phr), phase separation and phase inversion were observed. In the composites, the majority of the epoxy resin migrates to the polar fiber surface, resulting in epoxy-coated fibers. So the interfacial shear strength (IFSS) between Kevlar fiber and the epoxy/PPO blends is almost the same as that between Kevlar fiber and neat epoxy. The presence of PPO does not affect the interfacial property in the epoxy/PPO/fiber composite. So the interlaminar shear strength (ILSS) increase with the PPO content is due to an increase in the composite's ductility or toughness.     

Cure behavior of epoxy resin containing castor oil and cashew nut shell liquid and its derivative

The cure behavior of epoxy resin with a conventional amide‐type hardener (HD) was investigated in the presence of castor oil (CO), cashew nut shell liquid (CNSL), and cashew nut shell liquid–formaldehyde resin (CFR) with dynamic differential scanning calorimetry (DSC). The activation energy of the curing reaction was also calculated on the basis of nonisothermal DSC thermograms at various heating rates. A one‐stage curing was noted in the case of epoxy resin filled with CO, whereas the epoxy resin with CNSL and CFR showed a two‐stage curing process. A competitive cure reaction was noted for the epoxy resin/CNSL(or CFR)/HD blends. In the absence of HD, CFR showed lower values of curing enthalpy than that of CNSL. The activation energy of epoxy resin curing increased with increasing CNSL and CFR loading. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Exploring equivalence of information from dielectric and calorimetric measurements of thermoset cure—a model for the relationship between curing temperature, degree of cure and electrical impedance

The paper presents a phenomenological model, which accounts for the observed similarity between change in the imaginary electrical impedance maximum (IIM) during the cure of a commercial epoxy system, RTM6, and the cure reaction rate data collected by differential scanning calorimetry (DSC) experiments on the same resin. The model is developed by analysing the information content of dielectric cure monitoring signals and calorimetric cure monitoring signals under both isothermal and non-isothermal cure conditions. The absolute values of the coefficients in the model equation are shown to be indicative of the relative effects of the polymerisation reaction and of the temperature on the dielectric signal. The information presented here strengthens the suggestion that on-line dielectric measurements may be used to quantify the degree of cure in thermosetting resins in real time.     

Elastomer‐modified epoxy/amine systems in a resin transfer moulding process

The effect of the elastomer structure on toughening highly crosslinked epoxy systems in a resin transfer moulding process (RTM) was investigated. Two kinds of elastomers containing carboxyl functionalized groups were used: (1) a reactive liquid elastomer based on carboxyl‐terminated butadiene‐acrylonitrile copolymer (CTBN), (2) a preformed core‐shell rubber (CSR). The introduction of CTBN rubber caused the modification in the glass transition temperature due to the miscibility in the epoxy matrix, whereas CSR particles did not. During cure, these elastomers affected the morphological, rheological and dielectric behaviour of epoxy/amine systems. A blend of 5% CTBN and 5% CSR exhibited a bimodal distribution of rubber particle sizes (analyzed by transmission electron microscopy) whereas scanning electron microscopy showed the glass fibre‐matrix cohesion in fracture surfaces. A semi‐empirical model was used (developed by Castro‐Macosko for describing chemorheological behaviour of epoxy/amine systems for the RTM process). The increase in viscosity and the reduction in ion conductivity were the two key parameters to monitor the cure process. The presence of rubber affected the rheological behaviour involving initial viscosity and gel point. The investigation of temperatures, pressures and ionic conductivities in various glass fibre layers was conducted to control the front flow filling and the cure reaction. The introduction of rubber modified the inflexion area of the cured rubber–epoxy blends by changing the cure rate. Copyright © 2004 Society of Chemical Industry     

Effect of epoxy resin on thermal,dynamic, and mechanical properties of rigid poly(vinyl chloride)

This work is concerned with the effect of an epoxy resin on the properties of rigid poly(vinyl chloride) (PVC). The epoxy resin concentrations of 0, 1, 2, 4, and 6 phr were used to prepare PVC/epoxy polymer blends and the viscoelastic behavior of the blends was investigated by dynamic mechanical thermal analysis and rheometry test. The results revealed that the low molecular weight epoxy resin did not greatly affect the viscoelastic properties of PVC. From the morphological point of view, the smallest droplet size of epoxy dispersed in the polymer blends was found in the sample with 1 phr epoxy resin, and the largest one was for the sample with 6 phr epoxy. The thermal properties of PVC/epoxy blends were investigated using differential scanning calorimetry and thermogravimetric analysis, as well. According to our research, the initial decomposition temperature of PVC was increased about 6°C by the incorporation of epoxy resin. The results of tensile test showed that the addition of epoxy resin decreased the elongation‐at‐break of PVC about 50% in the samples without calcium carbonate and about 25% in the samples containing calcium carbonate. Moreover, the failure mode of PVC was changed from a ductile fracture mode to a brittle fracture mode with the addition of epoxy resin. J. VINYL ADDIT. TECHNOL., 25:E72–E79, 2019. © 2018 Society of Plastics Engineers     

Cure behavior,morphology, and mechanical properties of the melt blends of epoxy with polyphenylene oxide

Cure behavior, miscibility, and phase separation have been studied in blends of polyphenylene oxide (PPO) with diglycidyl ether of bisphenol A (DGEBA) resin and cyanate ester hardener. An autocatalytic mechanism was observed for the epoxy/PPO blends and the neat epoxy. It was also found that the epoxy/PPO blends react faster than the neat epoxy. During cure, the epoxy resin is polymerized, and the reaction‐induced phase separation is accompanied by phase inversion upon the concentration of PPO greater than 50 phr. The dynamic mechanical measurements indicate that the two‐phase character and partial mixing existed in all the mixtures. However, the two‐phase particulate morphology was not uniform especially at a low PPO content. In order to improve the uniformity and miscibility, triallylisocyanurate (TAIC) was evaluated as an in situ compatibilizer for epoxy/PPO blends. TAIC is miscible in epoxy, and the PPO chains are bound to TAIC network. SEM observations show that adding TAIC improves the miscibility and solvent resistance of the epoxy/PPO blends. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 26–34, 2000     

Settling behavior of fillers in thermosetting epoxy casting resins during cure

Using a theory based on a modified Stokes' law equation, the settling behavior of fillers was investigated (glass beads and silica powder) in thermosetting epoxy casting resins during cure. In this study a suspension (a thermosetting epoxy casting resin) containing a large amount of filler particles is assumed to be a homogeneous fluid, and the settling phenomena are treated such that one or more particles of the filler settle into the homogeneous fluid under gravity without interference from other particles. The viscosity increase of the fluid during cure is taken into consideration. The experimental results agree well with the theoretical predictions when the settling distances are small and the particle size distribution is narrow. When these conditions are not satisfied, various effects are observed such as convection, filtering, combination, bottom effect, and a broad particle size distribution. In addition, the formation of a compression zone and of a compaction zone are clearly observed.     

Cure behavior of epoxy/MWCNT nanocomposites: The effect of nanotube surface modification

The effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing behavior of diglycidyl ether of bisphenol A (DGEBA) epoxy resin was studied using differential scanning calorimetry (DSC), rheology and infrared spectroscopy (IR). Activation energy (E_a) and rate constants (k) obtained from isothermal DSC were the same for the neat resin and fluorinated MWCNT system (47.7 and 47.5 kJ/mol, respectively) whereas samples containing carboxylated MWCNTs exhibited a higher activation energy (61.7 kJ/mol) and lower rate constant. Comparison of the activation energies, rate constants, gelation behavior and vitrification times for all of the samples suggests that the cure mechanisms of the neat resin and fluorinated sample are similar but different from the carboxylated sample. This can be explained by the difference in how the fluorinated nanotubes react with the epoxy resin compared to the carboxylated nanotubes. Although the two systems have different reaction mechanisms, both systems have similar degrees of conversion as calculated from the infrared spectroscopic data, glass transition temperature (T_g), and predictions based on DSC data. This difference in reaction mechanism may be attributed to differences in nanotube dispersion; the fluorinated MWCNT system is more uniformly dispersed in the matrix whereas the more heterogeneously dispersed carboxylated MWCNTs can hinder mobility of the reactive species and disrupt the reaction stoichiometry on the local scale.     

Characterization of cure reactions of anhydride/epoxy/polyetherimide blends

The cure kinetics of blends of epoxy (diglycidyl ether of bisphenol A)/anhydride (nadic methyl anhydride) resin with polyetherimide (PEI) were studied using differential scanning calorimetry under isothermal conditions to determine the reaction parameters such as activation energy and reaction constants. By increasing the amount of PEI in the blends, the final cure conversion was decreased. Lower values of final cure conversions in the epoxy/PEI blends indicate that PEI hinders the cure reaction between the epoxy and the curing agent. The value of the reaction order, m, for the initial autocatalytic reaction was not affected by blending PEI with epoxy resin, and the value was approximately 1.0. The value of n for the nth order component in the autocatalytic analysis was increased by increasing the amount of PEI in the blends, and the value increased from 1.6 to 4.0. A diffusion‐controlled reaction was observed as the cure conversion increased and the rate equation was successfully analyzed by incorporating the diffusion control term for the epoxy/anhydride/PEI blends. Complete miscibility was observed in the uncured blends of epoxy/PEI at elevated temperatures up to 120 °C, but phase separations occurred in the early stages of the curing process. © 2002 Society of Chemical Industry     

Preparation and properties of dynamically cured PP/MAH‐g‐EVA/epoxy blends

A method concerning with the simultaneous reinforcing and toughening of polypropylene (PP) was reported. Dynamical cure of the epoxy resin with 2‐ethylene‐4‐methane‐imidazole (EMI‐2,4) was successfully applied in the PP/maleic anhydride‐grafted ethylene‐vinyl acetate copolymer (MAH‐g‐EVA), and the obtained blends named as dynamically cured PP/MAH‐g‐EVA/epoxy blends. The stiffness and toughness of the blends are in a good balance, and the smaller size of epoxy particle in the PP/MAH‐g‐EVA/epoxy blends shows that MAH‐g‐EVA was also used as a compatibilizer. The structure of the dynamically cured PP/MAH‐g‐EVA/epoxy blends is the embedding of the epoxy particles by the MAH‐g‐EVA. The cured epoxy particles as organic filler increases the stiffness of the PP/MAH‐g‐EVA blends, and the improvement in the toughness is attributed to the embedded structure. The tensile strength and flexural modulus of the blends increase with increasing the epoxy resin content, and the impact strength reaches a maximum of 258 J/m at the epoxy resin content of 10 wt %. DSC analysis shows that the epoxy particles in the dynamically cured PP/MAH‐g‐EVA/epoxy blends could have contained embedded MAH‐g‐EVA, decreasing the nucleating effect of the epoxy resin. Thermogravimetric results show the addition of epoxy resin could improve the thermal stability of PP, the dynamically cured PP/MAH‐g‐EVA/epoxy stability compared with the pure PP. Wide‐angle x‐ray diffraction analysis shows that the dynamical cure and compatibilization do not disturb the crystalline structure of PP in the blends. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

环氧基苯基硅油改性环氧树脂绝缘胶的研究

采用环氧基苯基硅油(PEPDMS)作为环氧树脂绝缘胶的增韧增柔改性剂。研究了PEPDMS对环氧树脂相容性、机械性能、耐热性和电性能的影响。实验结果表明,PEPDMS与环氧树脂具有良好的相容性,对环氧树脂具有显著的增强增韧的效果,且对环氧树脂的耐热性能和电性能无不良影响。但PEPDMS的加入使环氧树脂的玻璃化转变温度(T_g)略有下降。     

Chemical interpretation of the relaxed permittivity during epoxy resin cure

The isothermal cure of a diglycidyl ether of Bisphenol-A (DGEBA) epoxy resin with diaminodiphenylsulfone has been characterized by microdielectrometry and differential scanning calorimetry. The cure temperatures ranged from 410 to 460K. The behavior of the relaxed (or static) dielectric permittivity vs. cure time and temperature was determined from the microdielectrometry data. The DSC data was fit to an autocatalyzed reaction kinetics model, which was then used to predict reactive group concentrations as a function of cure time and temperature. The temperature dependence of the relaxed permittivity at constant chemical conversion was examined in the context of the Onsager theory for the relaxed permittivity of a system of independent dipoles. This analysis indicated that the dipoles in the resin are not independent, as assumed by the Onsager theory, and that the behavior is similar to that observed in polyethers. An empirical modification to the Onsager theory was used in conjunction with the kinetic model to estimate dipole moments for the epoxide, primary amine, and reacted (secondary and tertiary) amine groups. The relative and absolute values of the dipole moments were in good agreement with estimates based on the structures, leading to the conclusion that the observed decrease of the relaxed permittivity during cure of this epoxy/amine system is due to the changing concentrations of polar reactive groups.