Synthesis of isocyanurate-based imidazole carboxylate as thermal latent curing accelerator for thermosetting epoxy resins

A novel thermal latent curing accelerator, 1-(2-cyanoethyl)-2-methylimidazole/tris (2-carboxyethyl) isocyanurate adduct (2MICN-T), was successfully synthesized through an acid–base neutralization of tris(2-carboxyethyl)isocyanurate (TCEIC) and 1-(2-cyanoethyl)-2-methylimidazole (2MICN). It was further added into diglycidylether of bisphenol A based epoxy resin/methylhexahydrophthalic anhydride mixture to form one-component curing systems. With the addition of 2 wt% of 2MICN-T, the one-component system could be steadily stored for more than 1 month at room temperature, while the shelf life of 2MICN curing system was only 2 days. Nonisothermal differential scanning calorimeter also demonstrated the excellent thermal latency of 2MICN-T in low-temperature region and rapid initiation of the curing reaction when raising temperature. Compared to the cured resins with original 2MICN as accelerator, the resulted thermosets exhibited enhanced glassy storage modulus, glass transition temperature, and thermal stability when 2 wt% of 2MICN-T was applied. It was attributed to the chemical incorporation of the isocyanurate moieties with multi carboxyl groups and nitrogen-contained heterocyclic ring, effectively increasing the crosslinking density, chain rigidity, and heat resistance of the cured resin. Thus, it is suggested that 2MICN-T can play both roles as latent curing accelerator and modifier for one-component epoxy compounds, and is particularly recommended for application in electronic packaging fields.     

Synthesis of a novel lignin-based epoxy resin curing agent and study of cure kinetics,thermal, and mechanical properties

In this contribution, first of all, the methoxy groups of organic solvent lignin (OSL) was converted to phenolic hydroxyl groups through demethylation reaction for the purpose of fabricating demethylated organic solvent lignin (DOSL). In addition, the resulting DOSL was utilized as a renewable material to synthesize a novel esterified lignin (EDOSL) by reacting with isobutyryl chloride for curing of epoxy resin. Finally, commercial liquid diglycidyl ether of bisphenol A was cured by EDOSL in the presence of 4-dimethylaminopyridine (DMAP) used as a catalyst based on dual-curing mechanism. Dual curing is a processing methodology based upon the alliance of two diverse and compatible polymerization reactions occurring sequentially or simultaneously. According to the FTIR spectra and ¹H-NMR analyses, the demethylation of OSL, esterification of DOSL, and the curing reaction of epoxy resin with EDOSL were successfully conducted. The value of the phenolic hydrogen in the DOSL was approximately 4.89 mmol/g, which increased by 12.64% after demethylation. The thermal and mechanical performances of these cured epoxy samples were measured by DSC, DMA, TGA, and tensile testing. The epoxy system cured by 10%wt esterified lignin with 1%wt DMAP possessed the tensile strength of 71.54 ± 7.50 MPa and the initial degradation temperature (T_5%) of 370°C, which can compete fairly with commercial aromatic curing agents or other lignin-based agents studied currently for the curing of epoxy systems.     

Preparation of imidazole embedded polyurea microcapsule for latent curing agent

The IZ-PU microcapsule-type curing agent was prepared by interfacial polymerization method. The shell-core structure of the IZ-PU microcapsules was demonstrated by IR and SEM. The core content and coating efficiency of the IZ-PU microcapsules were analyzed by DSC test of the one-component adhesive consisting of IZ-PU microcapsules and epoxy resin. The results showed that the IZ-PU microcapsules is a proper latent curing agent to epoxy resin, its latency at room temperature is longer than 30 days.     

Preparation of 2‐ethyl‐4‐methylimidazole derivatives as latent curing agents and their application in curing epoxy resin

Three kinds of 2‐ethyl‐4‐methylimidazole (EMI) derivatives (N‐acetyl EMI, N‐benzoyl EMI, and N‐benzenesulfonyl EMI) were synthesized through the reaction of EMI with acetyl chloride, benzoyl chloride, and benzenesulfonyl chloride, respectively. And the structure was confirmed by Fourier transform infrared spectroscopy (FTIR) and ¹H‐nuclear magnetic resonance spectroscopy (¹H NMR) spectra. Furthermore, the synthesized EMI derivatives were applied in diglycidyl ether of bisphenol A epoxy resin (DGEBA) as latent curing agent. Differential scanning calorimeter (DSC) was used to analyze the curing behavior of DGEBA/EMI derivative systems, indicating DGEBA could be efficiently cured by the EMI derivatives at 110～160°C, and the corresponding curing activation energy ranged from 71 to 86 kJ/mol. Viscosity data proves that the storage life of DGEBA with N‐acetyl EMI (NAEMI), N‐benzoyl EMI (NBEMI), and N‐benzenesulfonyl EMI (NBSEMI) at room temperature was 38 d, 50 d, and 80 d, and that at 10°C was 90 d, 115 d, and 170 d, respectively. Besides, thermogravimetry (TG), izod impact strength (IIS), and tensile shear strength (TSS) were tested to characterize the thermal stability and mechanical properties of DGEBA cured by EMI derivatives. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42563.     

Deeper insights into the thermal properties of epoxy thermoset resins using torsion measurements

The glass transition temperature (T_g) of epoxy thermosets is a critical material property that depends on the component chemistry, the final cross-link density, and processing conditions. This study incorporates dynamic mechanical analysis (DMA) testing with a torsion clamp geometry on a TA Instruments DHR-2 and differential scanning calorimetry (DSC) to characterize five different two-component epoxy-amine systems. Investigation of the T_g dependence on DMA frequency and heating shows that lowering the frequency from 1 to 0.01 Hz results in a T_g very similar to that measured using DSC, while a heating rate of 0.3°C/min using DMA gives a T_g comparable to the DSC measured value at 30°C/min. The DMA technique reveals secondary relaxation transitions and peak broadening in the tan(δ) plots of poorly mixed epoxy blends, quantified using full width at half maximum (FWHM) of tan(δ) peaks, and are indicative of a non-homogeneous cross-linked network and off-ratio blending, respectively. The increase in the FWHM due to poor mixing ranges from 8% to 96%. These parameters are easily measurable and quantifiable in DMA, but are not observed in DSC. The additional DMA insights are valuable for process development and failure analysis, and can improve the understanding of epoxies.     

Structural,thermal, and mechanical properties of silanized boron carbide doped epoxy nanocomposites

In the presented study, the structural, thermal, and mechanical properties of the nanocomposites were investigated by doping silanized hexagonal boron carbide (h-B₄C) nanoparticles in varying proportions (0.5%, 1%, 2%, 3%, 4%, and 5%) into the epoxy resin by weight. For this purpose, the surfaces of h-B₄C nanoparticles were silanized by using 3-(glycidyloxypropyl) trimethoxysilane (GPS) to improve adhesion between h-B₄C nanoparticles and epoxy matrix. Then, the silanized nanoparticles were added to the resin by ultrasonication and mechanical stirring techniques to produce nanocomposites. The bond structure differences of silanized B₄C nanoparticles (s-B₄C) and nanoparticle doped composites were investigated by using Fourier transform infrared spectroscopy. Scanning electron microscopy and energy dispersion X-ray spectroscopy (SEM-EDS) technique was used to examine the distribution of nanoparticles in the modified nanocomposites. Differential scanning calorimetry and thermogravimetric analysis techniques were used to determine the thermal properties of the neat and s-B₄C doped nanocomposites. The tensile test and dynamic mechanical analysis were performed to determine the mechanical properties. When the experimental results were examined, changes in the bonding structure of the s-B₄C nanoparticles doped nanocomposites and significant improvements in the mechanical and thermal properties were observed. The optimum doping ratio was determined as 2% by weight. At this doping ratio, the T_g, tensile strength and storage modulus increased approximately 18%, 35%, and 44% compared to the neat composite, respectively.     

Model free kinetics coupled with finite element method for curing simulation of thermosetting epoxy resins

A finite element method algorithm for epoxy curing degree simulation was developed in Abaqus by integrating the discretized analytical solution of the model free kinetics into its user subroutines. This method was verified by nonisothermal and isothermal DSC experiments of an epoxy resin. By means of this method, the real manufacturing press cycle could be simulated regarding temperature distribution and curing degree with advanced curing degree‐dependent material properties. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46408.     

Effects of physical confinement (< 125 nm) on the curing behavior of phenolic resin

How the physical confinement of phenolic resin in nano porous silica (8 nm ≤ pore diameter (D_p) ≤ 125 nm) affected the polymer's curing behavior was examined by conducting differential scanning calorimetry experiments at 320 K ≤ T ≤500 K. Our results suggested that upon incorporating the phenolic resin into the silica, its curing temperature was lowered. However, what was interesting was that there was an inverse linear dependence between the pore size and the curing temperature, i.e., the smaller the pore diameter the higher the curing temperature. There was evidence that phenolic resin was unable to penetrate into 8 nm‐sized pores. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Studies on the curing behavior,thermal, and mechanical properties of epoxy resin-co-amine-functionalized lead phthalocyanine

A high performance copolymer was prepared by using epoxy (EP) resin as matrix and 3,10,17,24-tetra-aminoethoxy lead phthalocyanine (APbPc) as additive with dicyandiamide as curing agent. Fourier-transform infrared spectroscopy, dynamic mechanical analysis (DMA), differential scanning calorimetric analysis (DSC), and thermogravimetric analysis (TGA) were used to study the curing behavior, curing kinetics, dynamic mechanical properties, impact and tensile strength, and thermal stability of EP/APbPc blends. The experimental results show that APbPc, as a synergistic curing agent, can effectively reduce the curing temperature of epoxy resin. The curing kinetics of the copolymer was investigated by non-isothermal DSC to determine kinetic data and measurement of the activation energy. DMA, impact, and tensile strength tests proved that phthalocyanine can significantly improve the toughness and stiffness of epoxy resin. Highest values were seen on the 20 wt% loading of APbPc in the copolymers, energy storage modulus, and impact strength increased respectively 388.46 MPa and 3.6 kJ/m², T_g decreased 19.46°C. TGA curves indicated that the cured copolymers also exhibit excellent thermal properties.     

An evaluation of the curing characteristics of thermosetting epoxy carbon fiber sheet molding compounds and validation through computer simulations and molding trials

Sheet molding compounds (SMC) are ready-to-mold thermoset composite materials reinforced with discontinuous fibers, usually compression molded. Finite element (FE) based compression molding tools can be employed to optimize this process; FE tools require to define material models using raw material data measured through different characterization techniques. In this study, the cure kinetics of an epoxy-based carbon fiber SMC has been characterized by means of differential scanning calorimetry (DSC) and moving die rheometer (MDR) techniques. Based on these datasets, Claxton-Liska and Kamal-Souror models have been set and the compression molding of a validation plate was performed, both experimentally and virtually. The results indicate that, even if both characterization techniques are valid for SMC curing characterization, MDR technique enables the characterization of the material at real molding temperatures and the model based on MDR leads to more accurate results.     

Differential scanning calorimetry of urea–formaldehyde adhesive resins,synthesized under different pH conditions

The purpose of this study was to investigate the effects of reaction pH conditions on thermal behavior of urea–formaldehyde (UF) resins, for the possible reduction of formaldehyde emission of particleboard bonded with them. Thermal curing properties of UF resins, synthesized at three different reaction pH conditions, such as alkaline (pH 7.5), weak acid (pH 4.5), and strong acid (pH 1.0), were characterized with multiheating rate method of differential scanning calorimetry. As heating rate increased, the onset and peak temperatures increased for all three UF resins. By contrast, the heat of reaction (ΔH) was not much changed with increasing heating rates. The activation energy (E_a) increased as the reaction pH decreased from alkaline to strong acid condition. The formaldehyde emission of particleboard was the lowest for the UF resins prepared under strong acid, whereas it showed the poorest bond strength. These results indicated that thermal curing behavior was related to chemical species, affecting the formaldehyde emission, while the poor bond strength was believed to be related to the molecular mobility of the resin used. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 422–427, 2006     

Low temperature curable epoxy siloxane hybrid materials for LED encapsulant

A thermally cured epoxy‐siloxane hybrid material that is curable at low temperature (L‐expoxy hybrimer) was investigated for use as an LED encapsulant. This new hybrimer was fabricated using thermally initiated, cationic polymerization of cycloaliphatic epoxy oligosiloxane (CAEO) resin, derived from non‐hydrolytic sol – gel, mixed with oxetane hardener in the presence of a hexafluoroantimonate‐type thermo‐cationic initiator. The L‐epoxy hybrimer was cured at a lower temperature (below 120 ° C) than previously reported for an epoxy hybrimer with anhydride hardener (above 180 ° C). The L‐epoxy hybrimer showed high thermal resistance to yellowing under long‐term high temperature condition, and maintained good optical transmittance. Also, it had a high refractive index (up to 1.57), as well as the hardness (Shore D 80), and low water‐vapor permeability, w hen the new hybrimer was used to encapsulate an LED, it showed good adhesion without cracks or delamination and maintained their initial performance after the long‐term aging tests (120 and 85 ° C at 85% humidity). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39968.     

Investigation on curing reaction of phthalonitrile resin with nanosilica and the properties of their glass fiber-reinforced composites

In this work, Phthalonitrile containing benzoxazine (BA-ph) and Bisphenol A based cyanate ester (CE) were chosen as the matrix resin. Various amount of nano-SiO₂ was incorporated into BA-ph/CE and their glass fiber-reinforced composite laminates were fabricated. Curing reaction and processability of BA-ph/CE/SiO₂ blends were studied by differential scanning calorimetry and dynamic rheological analysis. Results showed that BA-ph and CE exhibited good processability and curing reaction of BA-ph/CE was not obviously affected by SiO₂. Scanning electron microscope images of the composites showed that SiO₂ particles were well dispersed in BA-ph/CE matrix. Moreover, SiO₂ could act as physical crosslinking points and diluent in matrix as well as between the glass fibers to improve the mechanical properties of composite laminates. As the results of dynamic mechanical analysis and thermogravimetry analysis, composite laminates possessed satisfactory T_g and good thermal stability. With incorporation SiO₂ particles into matrix resin, dielectric constant and dielectric loss of BA-ph/CE/SiO₂/GF composites were increased and showed frequency dependence.     

Developing toughened aromatic polybenzoxazines using thermoplastic oligomers and telechelics,part 1: Preparation and characterization of the functionalized oligomers

The preparation and characterization of three families of thermoplastic oligomers (M_n = 2918–13263 g mol^?1) based on polyarylsulfone (PSU) differing in both molecular weight and terminal functionality and one series of polyarylethersulfone (PES) of different molecular weights is reported. Infrared and nuclear magnetic resonance spectroscopy data support the formation of both the hydroxyl terminated oligomers and conversion (67–89% depending on molecular weight) to the telechelic PSU oligomer bearing reactive benzoxazine groups. Differential scanning calorimetry reveals that the onset of homopolymerization in the telechelic PSU oligomer occurs at around 100°C (peak maximum 125°C at 10 K/min) and rescans show values of the glass transition (for the homopolymers) ranging from 124 to 167°C depending on molecular weight. The influence on the oligomer backbone and terminal functionality is examined using thermal analysis. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40875.     

Dynamics and thermal properties of epoxy resin cured by new diamino disiloxanes

Two disiloxane compounds, 3,3′‐(1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxanediyl)bis(benzenamine) ( C1 ) and 4,4′‐(1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disiloxanediyl)bis(benzenamine) ( C2 ) were synthesized and used as new curing agents of DGEBA epoxy resin with an epoxy value of 0.51 ( E‐51 ). The curing kinetics of E‐51/C1 and E‐51/C2 systems was investigated by non‐isothermal differential scanning calorimetry (DSC) analyses. The activation energy (ΔE) and the characteristic cure temperatures of the two systems were determined. The two systems have the similar activation energy. The reactivity of E‐51/C1 is higher than that of E‐51/C2 . The reaction orders of E‐51/C1 and E‐51/C2 are 0.88 and 0.87, respectively, illustrating that curing reaction between the epoxy resin and curing agent ( C1 or C2 ) is complicated. The DSC result shows that E51 cured by C2 has higher Tg; whereas thermogravimetric analysis results indicate that E51 cured by C1 has higher thermal stability. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42385.     

A study on the curing kinetics of epoxy molding compounds with various latent catalysts using differential scanning calorimetry

We performed kinetic analysis in an epoxy curing system with differential scanning calorimetry (DSC) using a dynamic approach to investigate the reaction behavior of epoxy molding compounds (EMCs) according to three types of catalysts with varying latencies: triphenyl phosphine (TPP), triphenylphosphine‐1,4‐benzoquinone (TPP‐BQ), and tetraphenylphosphonium tetraphenylborate (TPP‐TPB). In dynamic approach, the well‐known model free kinetic (MFK) method was applied first to find out the variation of activation energy by the extent of conversion. By applying the MFK method, it was found that activation energy as a function of reaction conversion was nearly constant for TPP‐BQ and TPP‐TPB, but significantly reduced for TPP. By the dependence of activation energy, the model fitting method with single step reaction could be applied for TPP‐BQ and TPP‐TPB, and the nth order model was in good agreement with the MFK results. By contrast, in TPP, the reaction curve derived from MFK did not match with plot from nth order model. Isothermal curing experiments were also carried out to determine whether the assumption on kinetic predictions for the three catalysts was correct or not. As a result, TPP‐TPB and TPP‐BQ followed both MFK and nth order model. Nevertheless, TPP was more likely to follow MFK rather than nth order model. In addition, TPP‐TPB showed definitely lower conversion rate and degree of conversion compared with TPP, TPP‐BQ as expected from the catalyst structure and basicity. This study indicates the curing kinetic reaction of EMC depends on the latency of catalysts, and the MFK method can be used to describe the kinetics of curing reaction more accurately. These results help engineers in relevant fields to improve the reliability of EMCs by understanding the curing kinetic reaction of EMC with various latent catalysts. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45252.     

Curing kinetics and chemorheology of epoxy/anhydride system

The curing kinetics and chemorheology of a low‐viscosity laminating system, based on a bisphenol A epoxy resin, an anhydride curing agent, and a heterocyclic amine accelerator, are investigated. The curing kinetics are studied in both dynamic and isothermal conditions by means of differential scanning calorimetry. The steady shear and dynamic viscosity are measured throughout the epoxy/anhydride cure. The curing kinetics of the thermoset system is described by a modified Kamal kinetic model, accounting for the diffusion‐control effect. A chemorheological model that describes the system viscosity as a function of temperature and conversion is proposed. This model is a combination of the Williams–Landel–Ferry equation and a conversion term originally used by Castro and Macosko. A good agreement between the predicted and experimental results is obtained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3012–3019, 2003     

Aromatic amino acids as latent,bio-based curing agents and toughening agents for epoxy resins

In the realm of bio-based curing agents, recent investigations have focused on amino acids owing to their distinctive attributes. Nevertheless, the suitability of thermosets cured with aromatic amino acids as latent matrix materials for fiber-reinforced composites remains to be empirically established. Consequently, this study is oriented toward assessing the mechanical properties of diglycidyl ether of bisphenol A when cured with either L-tryptophan or L-tyrosine, in the presence of a latent, urea-based accelerator. The investigated properties include glass transition temperatures, tensile, flexural, compression, and fracture toughness properties. The predominant variations in the mechanical characteristics of these thermosets are confined to their Young's moduli and fracture toughness properties. This divergence is attributed to the greater presence of crystals in the L-tyrosine-cured thermoset, resulting in enhanced reinforcement and toughening effects compared to the L-tryptophan-cured thermoset.     

Differential scanning calorimetric study of curing of acrylated epoxy resin with benzoyl peroxide

The curing reaction of the acrylated diglycidyl ether of bisphenol-A (DGEBA) with benzoyl peroxide has been investigated by differential scanning calorimetry at three different heating rates. The overall cure kinetics were found to be first-order, with Arrhenius parameters E=83 kJ mol^?1 and In A = 16.5 min^?1, independent of the scan rate, up to at least 90% conversion.     

Evaluation of the complex hygrothermal behaviors of epoxy–amine systems

In this study, the complex hygrothermal behavior of two epoxy systems used for strengthening applications was studied. In these systems, property loss by plasticization simultaneously occurred with property gain during additional curing. A comparison of the changes in the glass-transition temperature (T_g) and crosslink density with water immersion at different temperatures clearly showed that the two effects of additional curing by a postcuring reaction and plasticization by water absorption were in competition with each other during the exposure. The changes in the conversion with different exposure conditions suggested that water accelerated the postcuring reaction, even at low temperatures; this resulted in a significant difference in the postcuring reaction between unexposed and exposed epoxies. The construction of the plot of T_g versus conversion for the unexposed system and the placement of the T_g for exposed systems onto this master plot provided a method for evaluating the plasticization effect while excluding the influence of additional curing. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012