Curing Kinetics Research Articles

Lithium catalyst concentration influence on bio-polyols structure and polyurethane adhesives properties

Polyol structure can be affected by the catalyst used for its synthesis and may affect the curing kinetics and mechanical strength of the final polyurethane (PU). This work aimed to study the influence of catalyst concentration on the structure of bio-based polyols and on the final properties of the polyurethane adhesive obtained from these polyols. Polyols were investigated by Fourier transform infrared (FTIR), gel permeation chromatography (GPC) and hydroxyl index analysis, with the purpose of investigating the polyols chemical state after the synthesis. Results suggest that higher catalyst concentration led to polyols with higher hydroxyl index and other differences in the polyols structure. The influence of the catalysts concentration on the cure kinetics of the polyurethanes was verified using the Fourier transform infrared (FTIR). In addition, peel strength of PU adhesives was obtained by increasing catalyst concentration on polyol synthesis. Keywords: Bio-polyol, catalyst, adhesives, polyurethane.

Preparation of a Novel Phosphorus-Nitrogen Containing Novolac Curing Agent for Epoxy Resin and flame-retardancy of its cured epoxy resin

ABSTRACTIn this study, DICY (dicyandiamide)-containing novolac (NN) was first prepared, and then DOPO (9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) was introduced to react with the unsaturated bonds from DICY to obtain a novel phosphorus-nitrogen-containing novolac resin (PNN), which was used as a curing agent for epoxy resins. The curing condition was confirmed by a non-isothermal curing kinetics study. The thermal stability and flame retardancy of the cured epoxy resin system were studied by thermogravimetric analysis (TGA), limited oxygen index (LOI) measurement, UL-94 test and cone calorimeter. The morphology of the burned chard residues was observed by Scanning electron microscopy (SEM). The results show that epoxy resin cured by the prepared PNN curing system presents excellent flame retardancy. The cured resin system, which contains only 1.31wt% phosphorus and 2.48wt% nitrogen, can achieve UL 94 V-0 rating.

Optimizing curing process of graphene oxide/waterborne epoxy blends by curing kinetics simulation considering the coupling of heat conduction and curing reaction

Curing kinetics of the waterborne epoxy emulsion (EPWE)/Graphene oxide (GO) system was investigated by non-isothermal differential scanning calorimetry (DSC), which shows an autocatalytic behavior and can be well predicted by model fitting kinetic method. By combining with the kinetics simulation and the conventional extrapolation method, an novel three-stage curing process with a high initial cure temperature and a cooling stage was proposed to inhibit the thermal shock inside materials and the uneven curing reaction during the curing process of the materials. The experimental results indicate that compared to the conventional two-stage curing process, the new curing procedure containing a cooling section significantly increased the bending strength by 14.9%, elastic modulus by 7.3% and Tg by 8 K of the cured GO/EPWE material, and the total curing time clearly decreased.

Hydroxyl‐terminated Polybutadiene Curing by 1,3‐dipolar Cycloaddition of Energetic Nitrile N‐Oxides: Room Temperature Curing Property, Kinetics, Thermodynamics, and Propellant Combustion Characteristics

Abstract5‐nitro‐1,3‐benzenedichloroaldehyde oxime was designed and synthesized to achieve room temperature curing of HTPB binders, and its structures were characterized. A new kind of 1,3‐dipolar cycloaddition reaction system was formed by HTPB as main resin, 5‐nitro‐1,3‐benzenedichloroaldehyde oxime and triethylamine as curing agent. The results revealed that the formation of isoxazoline ring in cured HTPB elastomers mainly derived form the reaction of Nitrile N‐Oxide with cis alkene of HTPB, and the cured HTPB elastomers underwent three stages decomposition. The kinetic parameters and thermodynamic parameters for the 14 % curing agent's sample were obtained from the DSC data. The activation energy was computed to be 204.75 kJ mol−1 and pre‐exponential factor was 1.38×1023 s−1.The burning rate of the propellant showed that the propellant with HTPB cured by containing −NO2 Nitrile N‐Oxides (100 % HTPB, 14 % Nitrile N‐Oxides, 2 % Aluminum and 77 % Ammonium perchlorate, Curing at 25 °C for 5 days ) had a higher burning rate and pressure exponent compared to conventional HTPB propellant (100 % HTPB, 14 % toluene diisocyanate, Nitrile N‐Oxides, 2 % Aluminum and 77 % Ammonium perchlorate, Curing at 60 °C for 5 days). Therefore, the containing −NO2 Nitrile N‐Oxides curing system could be used as a novel promising curing system for HTPB‐based solid composite propellants.

Cure kinetics of bismaleimides as basis for polyimide‐like inks for PolyJet™‐3D‐printing

ABSTRACTSince polyimides are well known for their excellent chemical and thermal stability and outstanding mechanical properties there is increasing interest in developing polyimide‐based inks to produce additively manufactured parts with properties superior to those of currently available materials. Usage of bismaleimides (BMI) as precursors allows polyimides to be fabricated via PolyJet™ printing (Stratasys Ltd., Rehovot, Israel). Characterization of the curing kinetics is a central part of process development, as fast curing initiated by UV light is desired. Here, a comprehensive study of thermal and UV curing of BMI oligomers with various molecular weights and chemical structures is presented. Fourier transform infrared spectroscopy serves as a tool for determining the curing degree. Furthermore, an estimation of the activation energy for thermal curing is performed. UV curing of the selected BMIs leads to highly crosslinked, thermoset polymers with excellent chemical resistance and thermal stability which are of great interest for PolyJet™ 3D printing. © 2018 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci.2019,136, 47244.

Curing kinetics of liquid crystalline 4,4′-Diglycidyloxybiphenyl epoxy cured with 4,4′-Diaminodiphenylsulfone

Liquid crystalline epoxy resins (LCERs) have been reported as a promising polymeric material with a high thermal conductivity. Among them, 4,4′-diglycidyloxybiphenyl (BP) epoxy with the simplest molecular structure for the formation of a liquid crystal (LC) phase appears most suitable for practical applications. In particular, it is known that BP epoxy cured by 4,4′-diaminodiphenylsulfone (DDS) exhibits a high thermal conductivity of 0.34 W/m K; however, their detailed curing reaction mechanism and kinetics have not been reported yet. Herein, the detailed mechanism and kinetics of the curing reaction between BP and DDS are reported. The curing reactions in the temperature range of 150–230 °C are investigated with the mixed model of the nth order and autocatalytic mechanism. The reaction order (m and n), reaction rate (dα/dt), rate constant (k1 and k2), and degree of conversion (α) are analyzed at several temperatures, where subscripts 1 and 2 represent the nth order and autocatalytic reaction, respectively. The kinetic parameters, activation energy, and pre-exponential factor, were determined as Ea1 = 60.76 kJ/mol, Ea2 = 57.20 kJ/mol, A1 = 4.08 × 104 min−1, and A2 = 101.22 × 104 min−1, which are in good agreement with the experimental data. Furthermore, the kinetic data could be utilized to predict and control the curing behaviors and the properties of the LCER for further application.

Thiolactone chemistry for the synthesis of functional silicone‐based amphiphilic co‐networks

ABSTRACTA series of amphiphilic co‐networks (ACNs) is prepared in a straightforward way via thiolactone chemistry by crosslinking a multivalent thiolactone‐functional poly(dimethylsiloxane) building block with poly(ethylene glycol) diacrylates. Formation of the networks is triggered by the addition of an amine, of which the nucleophilicity and steric bulk control the curing kinetics. Furthermore, some of the crosslinks can be sacrificed to introduce a fluorescent group or dye via a thia‐Michael addition, without affecting the bulk mechanical properties and swelling capabilities. The obtained ACNs exhibit a unique set of properties because of their nanophase separation, resulting in hydrophilic PEG and hydrophobic PDMS phases. Hence, swelling in both water and organic solvents is observed, of which the extent can be tuned by varying the overall PEG content. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 322–333

The effect of brominated epoxy on epoxy/phenolic reactive blends

ABSTRACTInnovative reactive blends containing epoxy and brominated epoxy (BE) incorporated with resole‐type phenolic were studied with the aim to elucidate the curing kinetics and the final thermomechanical characteristics of this unique system. Curing kinetics was investigated by means of the activation energy determined using differential scanning calorimetry (DSC ) at various heating rates analyzed by the Arrhenius equation. Both DSC and Fourier transform infrared revealed that bromine elimination at elevated temperatures (above 220 °C) had lowered the activation energy in the case of BE containing phenolic blends. The thermomechanical properties showed that the addition of conventional epoxy to resole decreased its thermal properties and modulus compared to neat resole. Distinctively, BE/resole blends exhibited increased glass‐transition temperature, compared to diglycidyl ether of bisphenol A/resole blends in combination with higher elongation and toughness compared to neat resole. It was concluded that BE/epoxy resin/phenolic reactive systems offer high T g, mechanical properties and toughness and hence are applicable for structural adhesives and for matrices of polymer‐fiber composites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47172.

Effect of Diamine Nature on Curing Kinetics of N,N'-Hexamethylene Bis(maleimide) and Properties of Glass-Reinforced Plastics

Effect of Diamine Nature on Curing Kinetics of N,N'-Hexamethylene Bis(maleimide) and Properties of Glass-Reinforced Plastics

Cure kinetics and viscosity modeling for the optimization of cure cycles in a vacuum-bag-only prepreg process

This study focuses on the development of a process-based simulation model coupled with differential scanning calorimetry (DSC) and dynamic mechanical analyzer (DMA) experiments. The cure kinetics and rheology of an epoxy-amine resin were characterized in order to predict the degree of cure and viscosity behavior in an out-of-autoclave (OOA) process condition. Both phenomenological reactions and chemo-rheological models were applied to effectively predict the degree of cure and resin viscosity. Using these results, it is possible to predict the minimum viscosity, gelation, and vitrification, which are the main process factors of the multi-stepped cure cycles, and reduce the number of process trials. It was found that there was a good correlation between the experimental results and model predictions under isothermal and non-isothermal temperature profiles. Furthermore, this study demonstrates that there is an additional scope for optimization in the conventional cure cycles recommended by prepreg manufacturers, especially when a low viscous state is required. The optimized cure cycle led to a substantially high fiber fraction (58.96 vol%) and a low void content (0.15 vol%), as compared to the conventional cure cycles.

Modeling and simulation of the curing process of epoxy resins using finite elements

This article discusses several aspects of the curing process in polymers. First, we collect experimental data for an Araldite epoxy resin and calibrate the classical model of Kamal–Sourour. It is shown that there are strong correlations between the parameters in the material parameter identification process. Thus, a curing kinetics model with reduced number of parameters is proposed, which is calibrated to the experimental test data. Second, the model is implemented into a finite element program. Here, high-order, time-adaptive time integration schemes are chosen to treat the inherent instability resulting from the curing kinetics model. Since the curing variable determines the heat source in the heat equation, a particular finite element approach is applied. After the spatial discretization, we arrive at a large system of ordinary differential equations, where the diagonally implicit Runge–Kutta method in combination with the Multilevel-Newton method is chosen, which can be seen as an analogy to internal variable theories in nonlinear quasi-static finite element approaches. Temperature and curing-dependent heat capacity and heat conductivity are considered as well. Numerical examples and a new validation experiment conclude the investigations.

Kinetics of Curing of Epoxy Oligomer by Diaminodiphenyl Sulfone: Rheology and Calorimetry

Diaminodiphenyl sulfone is used as a curing agent to obtain an epoxy-resin-based binder with improved thermal stability. The kinetics of curing of this composition is studied by rheokinetic and calorimetric methods. It is shown that complete conversion is attained at a temperature of 200°С or higher. The rate of viscosity increase in the course of time in curing under flow conditions does not depend on the shear rate; the viscosity changes monotonically. The curing reaction occurs in the homogeneous mode without separation of a crosslinked polymer; it is described by a second-order equation with self-deceleration at low temperatures. The rheokinetic characteristics and temperature dependences of kinetic parameters of the process are determined.

On anisotropy, strain rate and size effects in vat photopolymerization based specimens

The influence of build orientation, layer thickness, strain rate and size effect on the Young’s modulus, ultimate tensile strength and fracture strains in vat photopolymerization based additively manufactured specimens is investigated. Mechanical testing and subsequent scanning electron microscopy tests on additively manufactured specimens are conducted. Anisotropy in mechanical behavior is only observed in specimens fabricated in different planes. An increase in layer thickness and decrease in strain rate resulted in lower strength, stiffness and higher fracture strains. No significant size effect on strength and failure strains is observed. Cure kinetics is found to have significant influence on mechanical properties of additively manufactured specimens.

Curing kinetics and mechanical properties of epoxy based coatings: The influence of added solvent

Commercial coatings based on epoxy and amine hardener and organic solvent were prepared to investigate the effect of organic solvent on coating properties, their mechanical property and curing kinetics were discussed in terms of different solvent content. The curing degree was characterized by FT-IR, the mechanical properties were studied by tensile and adhesion tests, the non-isothermal curing kinetics and glass transition temperature of epoxy coating were also investigated by differential scanning calorimetry (DSC). The results indicated that the presence of solvent could lower the curing degree thus affecting the cross-linked structure, the tensile strength and modulus of elasticity, while the flexibility was highly improved by adding more solvent into the system. Different adhesive failure modes were observed with the increasing amount of solvent in comparison with that of pure epoxy, it transformed from cohesive to adhesive failure. Furthermore the two parameter autocatalytic kinetic model Šesták- Berggren (m,n) was introduced to describe the curing process and it shows a good agreement with the experimental rates. The results also demonstrated that activation energy of curing reaction increased with rising conversion which revealed that curing process was considered to be diffusion controlled.

Cure kinetics of poly (5-ethylidene-2-norbornene) with 2nd generation Hoveyda-Grubbs’ catalyst for self-healing applications

Self-healing polymer composites based on ring-opening metathesis polymerization (ROMP) incorporating second generation Hoveyda-Grubbs’ (HG2) catalyst as polymerization initiator and 5-ethylidene-2-norbornene (5E2N) as healing monomer are promising for self-healing applications. This is particularly true at low temperatures that may persist in the internal environment of an operating space vehicle. By using a systematic micro-Raman spectroscopic analysis and a detailed methodology specifically implemented for the investigation of 5E2N and HG2 reactants, we studied the progression of the ROMP reaction over time from room temperature down to −30 °C. The analysis is based on measuring the changes in peak intensity and integrated area of Raman phonons related to the characteristic (CC) functional groups indicating the progression of the reaction with time. The cure kinetic reaction of the 5E2N/HG2 system at low temperatures is found to follow an Arrhenius behavior showing the temperature dependence of the reaction rates. On this basis, the activation energy of the reaction is measured to be 64 (±3) kJ mol−1 (0.663 eV), a value that is about 40% higher than the data reported for similar materials.

Comparative curing kinetics study of high char yield formaldehyde- and terephthalaldehyde-phenolic thermosets

Phenol-Formaldehyde (PF) resoles exhibit excellent thermal properties, high temperature degradation and high char yield. The formaldehyde can be replaced by terephthalaldehyde (TPA), a non-toxic aromatic dialdehyde. The thermal performances of phenol-TPA (PTPA) resole are very interesting for further development and industrialization. The present investigation presents for the first time a thermo-kinetics study of curing of PTPA, in comparison with a commercial PF resole. Non-isothermal, at constant heating rates DSC experiments are performed on both resoles. PF shows one single exothermic peak, whereas PTPA exhibits two DSC peaks, suggesting a two-step curing mechanism which appears to be conversion-dependent. In addition, isoconversional analysis is used to elucidate the activation energies as a function of the degree of curing. Differential Friedman and integral Vyazovkin methods are considered for computations, giving equivalent results. Furthermore, the whole kinetic triplets (E, A, f) is elucidated for both resoles with Sestak-Berggren model fitting. Finally, isothermal isoconversional predictions are compared to experimental data.

Cure kinetics and rheology modelling of boehmite (AlOOH) nanoparticle modified epoxy resin systems

Nanoparticles can effectively increase the resin matrix properties, which is especially attractive for fabrication of high-performance fiber composites by liquid composite molding (LCM) processes. However, the cure kinetics and rheological behavior of the matrix, which are the most critical aspects in LCM processes, could be affected by the introduction of nanoparticles, depending on the surface functionalization and concentration. In this paper, the cure kinetics and rheology of a boehmite nanoparticle modified epoxy matrix is characterized and modelled. The influence of boehmite nanoparticles on cure and rheological behaviors is analyzed. By optimization of the regression method combined with model parameter analysis, a good correlation between the model parameters and nanoparticle concentration is determined. Finally, a generalized model is put forward to describe and predict the cure kinetics and rheology depending on time, temperature, and concentration of nanoparticles.

Polymers and Composites Produced from Epoxy Resins Cured with 3,5-Diethyltoluenediamine: Synthesis and Properties

The kinetics of curing of epoxy resins characterized by different functionality with 3,5-diethyltoluenediamine was studied. The kinetic constants and temperature ranges of reactions were determined. Polymers and composites were synthesized according to the established regimes. The rheological properties of composites and physicomechanical, thermomechanical, and impact properties of polymers and high-strength organoplastics were determined. Polymers based on trifunctional epoxy-novolak resin UP-643 are characterized by the highest thermal stability.

Determination of the Kinetic Parameters of the Curing Reaction of Epoxy-Imide Binder and Prepregs Based on It from Differential Scanning Calorimetry Data

Curing reactions of epoxy-imide binder and two prepregs based on it were studied by the method of differential scanning calorimetry (DSC). The effective activation energies were found, and predictions of storage time and curing time at given temperatures to different degrees of curing were made. The influence of fillers on prepreg curing kinetics was found.

Study on the demethylation of enzymatic hydrolysis lignin and the properties of lignin–epoxy resin blends

To increase hydroxyl content in lignin, demethylation of enzymatic hydrolysis lignin (DML) was conducted using 1-dodecanethiol as nucleophilic reagent and sodium methoxide as catalyst in this work. Under alkaline conditions, the demethylated lignin–epoxy resin (DML-EP) was synthesized by demethylated lignin reacted with epichlorohydrin (ECH). The optimum conditions for the synthesis of DML-EP were: 80 °C, 2.5 h, molar ratio of ECH and NaOH to phenolic hydroxyl of 7.5 and 1.2, respectively. FTIR spectrum showed the epoxy group appeared in the product. Then, the lignin-based epoxy composites were produced by cast molding method. Tensile strength test reflects that DML-EP-reinforced epoxy composites have an obvious improvement in tensile strength and elongation of about 27 and 34% compared to the pure epoxy resin with 5% content. Scanning electron microscopy images showed the fractured surface of DML-EP/epoxy composites was rough. Rheological analysis showed that the cured DML-EP/E-51 epoxy resin composites had better elasticity and bonding effect. TGA experiment showed the thermal stability of DML-EP/epoxy composites was also improved. Non-isothermal method was used to analyze the curing kinetics of E-51 resin and DML-EP/E-51 composites, and the curing apparent activation energy was calculated using the Kissinger and Ozawa methods.