Curing Kinetics Research Articles

Cure kinetics of epoxy/graphene oxide (GO) nanocomposites: Effect of starch functionalization of GO nanosheets

Graphene oxide (GO) nanoflakes are reactive towards both epoxy resin and amine curing agent, but their inevitable agglomeration worsens resin/hardener/nanoflake interfacial interactions. In this work, starch was used to functionalize GO nanoflakes. Isoconversional cure kinetics of low concentration epoxy/GO and epoxy/starch-functionalized GO (GO-St) nanocomposites was then studied based on nonisothermal differential scanning calorimetry (DSC) analyses to evaluate the effect of starch-aided epoxy/amine/GO curing reactions. Qualitative analysis of epoxy network formation governed by GO and GO-St was done in terms of Cure Index, where Good and Excellent cure were interestingly observed as a result of starch functionalization. Quantitative cure analyses based on integral and differential isoconversional methods were also allowed for understanding conversion-dependent cure kinetics in terms of activation energy (Eα) alteration. The activation energy increased for neat epoxy system at later stage of curing reaction. However, an almost constant trend was seen for the activation energy of GO incorporated epoxy nanocomposites. Observation of a decline in Eα values for epoxy/GO-St system with respect to epoxy/GO was due to the autocatalytic curing reaction, which was attributed to facile epoxide ring opening by OH groups of starch. A good confidence was observed between calculated and observed calorimetric data.

Cure Kinetics of an Optical Polythiourethane with Amine Catalyst by IR Analysis

An optical polythiourethane based on m-xylylene diisocyanate (XDI) and 4-mercaptomethyl-3,6-dithia-1,8-octanedithiol (BES) has been studied. Triethylamine was adopted as a catalyst, and the solid-state isothermal cure reaction was carried out using FTIR spectroscopy, in the temperature range of 75°C-105°C. The -NCO absorption band of XDI was used to monitor the conversion of diisocyanate into polythiourethane. The reaction rate enhanced with an increase in the content of the catalyst, and the gel time determined by swelling test was shorter for the system with higher catalyst content. Kinetic parameters were calculated from the infrared spectrum data, and the results showed that the curing reaction of polythiourethane accords with first-order kinetic characteristics. The activation parameters obtained from the evaluation of kinetic data were △H∗=97.22 kJ mol−1, △S∗=−6.77 J K−1 mol−1, and Ea=100.23 kJ mol−1. The observed negative entropy of activation value supported the formation of a transition state in the cure reaction.

Curing and thermomechanical properties of off-stoichiometric anhydride–epoxy thermosets

In the present work, we report the preparation and characterization of a new family of thermosets based on off-stoichiometric anhydride–epoxy formulations in the presence of an anionic initiator. Diglycidyl ether of bisphenol A (DGEBA) and hexahydro-4-methylphthalic anhydride (HHMPA) have been used as epoxy and anhydride comonomers, respectively, and 1-methylimidazole (1MI) has been used as anionic initiator. The isothermal curing kinetics and the thermal properties of the stoichiometric and the off-stoichiometric systems have been compared. The kinetics of the isothermal curing has been analyzed by differential scanning calorimetry (DSC) using an isoconversional method and the Šesták–Berggren equation to determine the activation energy, the frequency factor and the reaction orders. The materials obtained were characterized by DSC and dynamic mechanical analysis. Gelation during epoxy–anhydride condensation was determined by thermomechanical analysis. At the same curing temperature, the reaction is faster in the system with excess of epoxy groups. However, the glass transition temperatures of the partially cured stoichiometric system are greater. The gelation time of the off-stoichiometric system is shorter than that of the stoichiometric one. The results indicate that the dual-curing character of off-stoichiometric DGEBA/HHMPA thermosets with 1MI as anionic initiator makes them suitable for multistage curing processes with easy control of degree of cure and material properties in the intermediate stage and enhanced final material properties.

Accelerated curing of glued-in threaded rods by means of inductive heating – part II: modelling

ABSTRACT The curing behaviour of inductively heated glued-in rods (GiR) experimentally tested in Part I of this work was further investigated with regard to the kinetics of adhesive curing, and evolution of strength with time. For that, dynamic DSC measurements were performed, and appropriate kinetic models were derived. These investigations were complemented with rheological experiments in which the adhesive were subjected to the same thermal regime as in the inductive heating of the GiR. Curing kinetics of the GiR were then compared to the increase of strength with time. The comparison showed that curing progress, defined by conversion degree, does not result in proportional strength increase. It was then attempted to estimate the influence of potential residual stresses from the induction curing on joint capacity. A relatively simple FE model was thus derived, which considered two different loading cases: a tensile force acting on the rod, and a shrinkage resulting from the induction process. Their superposition allowed for a general description of the fact that joint capacities increased with time elapsed since induction heating. Depending upon the adhesive considered, fair to very good quality of prediction were achieved.

Cure kinetics, morphology development, and rheology of a high-performance carbon-fiber-reinforced epoxy composite

Carbon-fiber-reinforced polymer (CFRP) is a thermosetting high-performance epoxy composite used in the manufacturing of advanced aircraft components. The curing kinetics and rheological characteristics can have a great effect on the macroscopic performance properties of the composite materials. Thus, understanding the curing reactions of CFRP is important to the development and optimization of composite manufacturing processes. This paper presents a comprehensive evaluation of the cure characteristics of a commercially available epoxy prepeg (pre-impregnated carbon fiber) using differential scanning calorimetry (DSC), rheology, and Fourier-transform infrared (FTIR) spectroscopy. The Ozawa and Kissinger methods are used to measure the activation energies of curing reactions. The DSC results show that the degree of curing is strongly affected by time, whereas rheological characteristics of the composite show that the gel temperature values and obtained gel point increase with the heating rate. On the other hand, the morphological study using IR microscopy reveals the phase homogeneity and resin infiltration in a three-dimensional crosslinked network of the cured epoxy prepreg when a lower heating rate is used. A lower heating rate (or slow curing process) allows the optimum intermixing of epoxy, thus promotes maximum epoxy infiltration inside the fiber reinforcement, which can improve the mechanical performance of the composite by reducing the deformation and residual stress during the curing process.

In Situ Vibrational Probes of Epoxy Gelation.

This paper presents an efficient way to measure the curing kinetics and gel point, αgel, in epoxy resins from one single experiment. The epoxy curing reaction is herein monitored using in situ and time-resolved near-infrared absorption spectroscopy (NIR). The curing profiles over different isothermal conditions are in good agreement with DSC. Furthermore, the increase of the NIR absorption bands of aromatic rings (unreactive throughout curing) probe the cure shrinkage, as more and more aromatic rings are localized within the fixed sample volume. Therefore, the gel point is determined using the onset of the aromatic absorption increase. The results are in good agreement with the theoretical gel point, as well as DMA results. This innovative approach enables gelation measurements on epoxy neat resins and film composites with an easy-to-perform, accurate, robust, and versatile method.

Effect of polyhedral oligomeric silsesquioxane on thiol-ene UV curing kinetics of waterborne polyester

The research on UV curing kinetics of waterborne polyester is the basis for regulation of film formation rate. In this paper, a kind of reinforcing agent, namely polyhedral oligomeric silsesquioxanes (POSS), is adopted as modifiers to enhance hydrophobicity and adhesion of polyester coatings, and the effect of POSS on the thiol-ene UV curing kinetics including with the aspects of activation energy, reaction order and control steps are investigated by differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR). The results show that POSS is not found to significantly affect the reaction order, but change the activation energy obviously. A small amount of POSS can affect the click reaction rate of thiol-ene. The UV curing reaction rate of thiol-ene modified by POSS is mainly controlled by propagation growth rate. There is a resistance parameter in the reaction process, which can effectively control the reaction rate of thiol-ene UV curing of waterborne polyester.

Effect of moisture and temperature on the thermal and mechanical properties of a ductile epoxy adhesive for use in steel structures reinforced with CFRP

CFRP has proven to be a material capable of being used in many engineering applications, highlighting its good properties for reinforcement of steel structures. In this study, a new CFRP-steel adhesive joint is evaluated by using a structural and ductile epoxy adhesive to join the CFRP reinforcements to the steel structure. On the one hand, the epoxy adhesive has been thermally characterized by means of its kinetics of curing (Kissinger and Model Free Kinetics methods), finding behavior similar to other epoxy adhesives. On the other hand, different surface treatments are studied in order to achieve the best mechanical properties of the joint. In addition, the suitability of this kind of joints is assessed under durability conditions (60 °C and 99% relative humidity) and under thermal fatigue. Shear tests are realized on specimens subjected to different exposure times, studying the mechanisms that affect the degradation of the adhesive joint in each environment. Breaking surfaces were analyzed by Scanning Electron Microscopy (SEM) and by Energy-dispersive X-ray Spectroscopy. A simplified Weibull model is used to evaluate the reliability of the joints during both durability and thermal fatigue processes. The thermal and mechanical properties of the adhesive are determined, showing better behavior against thermal fatigue because moisture in the durability process accelerates degradation of the adhesive joint.

Isothermal Curing Kinetics of Epoxidized Fatty Acid Methyl Esters and Triacylglycerols

AbstractPlant oil triacylglycerols are attractive renewable resources for biobased epoxy resins. We investigated the curing kinetics of three model epoxidized fatty acid methyl esters and representative epoxidized triacylglycerols with varied epoxide functionalities and distributions in the presence of a latent cationic initiator. Isothermal differential scanning calorimetry (DSC) was used to analyze the curing kinetics of the epoxy systems, and kinetic parameters (i.e., rate constants, reaction orders) were determined. Both epoxidized fatty esters and triacylglycerols followed the autocatalytic curing mechanism, and the DSC data were analyzed according to the Kamal autocatalytic model. Epoxidized methyl linoleate (EMLO) had the highest maximum curing rate, followed by epoxidized methyl linolenate (EMLON), and epoxidized methyl oleate (EMO) had the lowest maximum curing rate. We conclude that EMLO with two epoxide groups has the highest reactivity in this curing system, while the EMO with one epoxide group has the lowest reactivity. For epoxidized triacylglycerols, epoxidized camelina oil had the highest curing reactivity at higher temperatures, followed by epoxidized linseed oil and soybean oil.

Curing Kinetics and Mechanism Research of E44/T31 Insulation Paint

Epoxy resin insulation paint was prepared with epoxy resin (E44) as binder and with proper inorganic fillers and curing agent (T31) as additives. The isothermal curing reaction process of paint was studied by the differential scanning calorimetry method (DSC), and the curves of curing reaction rate versus time of paint were obtained. The curing reaction kinetics was investigated by using the phenomenological method, and the corresponding parameters of the n-order model, autocatalytic model and Kamal model were determined by fitting the experimental data, respectively. According to the values of R2 and the sum of square due to error (SSE), a suitable curing reaction kinetic model was determined. The curing reaction mechanism of paint was ascertained by the dynamic temperature DSC method and IR spectroscopy (FTIR) method. The results show that the Kamal model can be used to describe the curing kinetics of epoxy resin paint, and the total reaction orders increase from 1.30 to 2.14. The two rate constants increase with the increase of the curing temperature. The activation energy is 90.5832 kJ/mol and 68.3733 kJ/mol respectively, and the pre-exponential factors are 6.521 × 1015 s-1 and 6.3807 × 109 s-1. The curing reaction of paint consists of two steps: the first step is the addition reaction of epoxy group and primary amine or secondary amine; the second step is the etherification reaction of epoxy group and phenolic hydroxyl or alcoholic hydroxyl.
 Epoxy resin insulation paint was prepared with epoxy resin (E44) as binders and with proper inorganic fillers and curing agent (T31) as additives. The isothermal curing reaction process of paint was studied by differential scanning calorimetry method (DSC), and the curves of curing reaction rate versus time of paint were obtained. The curing reaction kinetics was studied by using the phenomenological method, the corresponding parameters of the n-order model, autocatalytic model and Kamal model were determined by fitting the experimental data, respectively. According to the values of R2 and the sum of square due to error (SSE), a suitable curing reaction kinetic model was determind. The curing reaction mechanism of paint was ascertained by dynamic temperature DSC method and IR spectrogram (FTIR) method. The results show that the kamal model can be used to describe the curing kinetics of epoxy resin paint, the total reaction orders increase from 1.30 to 2.14. The results also show that the two rate constants increase with increasing curing temperature, The activation energies are 90.5832 kJ/mol and 68.3733 kJ/mol, and the pre-exponential factor are 6.521×1015 s-1 and 6.3807×109 s-1. The curing reaction of paint in two steps, the first step is the addition reaction of epoxy group and primary amine or secondary amine. The second step is the etherification reaction of epoxy group and phenolic hydroxyl or alcoholic hydroxyl.

Tailored polyurethane acrylate blend for large-scale and high-performance micropatterned dry adhesives

Continuous roll-to-roll fabrication is essential for transferring the idea of bio-inspired, fibrillar dry adhesives into large-scale, synthetic, high-performance adhesive tapes. Toward this aim, we investigated process parameters that allow us to control the morphology and the resulting adhesion of mushroom-shaped micropatterned surfaces. Flexible silicone templates enabled the replication process of the polyurethane acrylate pre-polymer involving UV-light-induced cross-linking. For this paper, we particularly tailored the polyurethane acrylate pre-polymer by adding chemical components to tune UV curing kinetics and to reduce oxygen inhibition of radicals. We found that higher intensities of the UV light and faster reaction kinetics improved the quality of the microstructures, i.e., a larger cap diameter of the mushroom tips was achieved. The polymer blend U6E4 exhibited the fastest curing kinetics, which resulted in a micromorphology similar to that of the Ni-shim master structures. Best adhesion results were obtained for adhesive tapes made from U6E4 with 116 kPa pull-off stress, 1.4 N cm−1 peel strength and 71 kPa shear strength. In addition, repeated attachment–detachment tests over 100,000 cycles demonstrated strong robustness and reusability.

Reconstruction of the Microstructure of Cyanate Ester Resin by Using Prepared Cyanate Ester Resin Nanoparticles and Analysis of the Curing Kinetics Using the Avrami Equation of Phase Change

Bisphenol A dicyanate (BADCy) resin nanoparticles were synthesized by precipitation polymerization and used to modulate the microstructure of the BADCy resin matrix. A microscopic mechanism model was used to characterize the curing process of BADCy resin systems with different contents of the prepared nanoparticles. Due to the curing process of the thermosetting resin being analogous to the crystallization process of the polymer, the Avrami equation was used to analyze the microscopic mechanism of the curing process. The reactive functional groups, structure, and size of the prepared BADCy resin nanoparticles were characterized by FT-IR, SEM, and TEM, respectively. The kinetic parameters of different systems were then obtained using the Avrami equation, and they adequately explained the microscopic mechanism of the curing process. The results showed that the Avrami equation effectively described the formation and growth of gel particles during the curing process of the BADCy resins. The addition of nanoparticles can affect the curing behavior and curing rate. Since the reaction between the BADCy resin nanoparticles and the matrix is dominant, the formation process of the gel particles was neglected. This phenomenon can be understood as the added BADCy resin nanoparticles replacing the formation of gel particles. The reasons for accelerated curing were analyzed from the perspective of thermodynamics and kinetics. Besides this, the Arrhenius equation for non-isothermal conditions correctly accounted for the change in the cross-linked mechanism in the late-stage curing process. A comparison of the theoretical prediction with the experimental data shows that the Avrami theory of phase change can simulate the curing kinetics of different BADCy resin systems well and explain the effects of BADCy resin nanoparticles on the formation of the microstructure.

Zero-Support 3D Printing of Thermoset Silicone Via Simultaneous Control of Both Reaction Kinetics and Transient Rheology

Abstract This article presents a framework for using isothermal curing kinetics and transient rheological data to 3D print a curing thermoset silicone without support. These data are used to determ...

Effect of an Anhydride Excess on the Curing Kinetics and Dynamic Mechanical Properties of Synthetic and Biogenic Epoxy Resins

This work analyzes the effect of the anhydride excess on the nonisothermal curing kinetics and on the final properties of synthetic and biobased epoxy resins. Diglycidyl ether of bisphenol A (DGEBA) and epoxidized soybean oil (ESO) were crosslinked using methyltetrahydrophthalic anhydride (MTHPA) as a curing agent and 1-methylimidazole (1MI) as an initiator. It was shown that the ESO/MTHPA/1MI system reacts slower than the DGEBA/MTHPA/1MI system, giving place to a more significant evaporation of the curing agent during the reaction. As a result, an excess of anhydride improves the final thermal properties of the ESO/MTHPA/1MI network, contrary to the behavior observed for DGEBA/MTHPA/1MI. The knowledge of the kinetics of the curing process and the optimal amount of the curing agent for each system is of critical importance for a more efficient processing of these materials.

Effect of surface area of nanosilica particles on the cure kinetics parameters of an epoxy resin system

ABSTRACTKnowing of thermoset curing kinetics is essential for process development, quality control, and achieving desirable products. Hence, in this article, cure kinetics of an EPON 828 epoxy resin/dicyandiamide curing agent/diuron accelerator system is investigated. This resin system is usually used for the production of epoxy/glass fiber prepregs used in wind turbine blades. For this, differential scanning calorimetry analysis is used and the effect of temperature, weight percentage, and size of nanosilica is studied by conducting isothermal tests at several temperatures for samples with and without nanoparticles. An autocatalytic curing model is applied to describe the cure kinetic of system and then the variations in model parameters calculated by curve fitting using the MATLAB software. The results show that the increase in temperature, weight percentage of nanosilica from 0 to 6%, and surface area of nanosilica particles lead to the increase in curing rate, whereas the increase in the percentage and surface area of nanosilica particles significantly decreases total heat of reaction. At the end, the relation between each of model parameters and the total surface area of nanosilica particles, calculated by mathematical equations, is obtained. The allowable maximum surface area of nanosilica used in the mathematical equations is 12 m2 g−1. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47958.

Cure Kinetics and Properties of High Performance Cycloaliphatic Epoxy Resins Cured with Anhydride

In this work, a cycloaliphatic epoxy resin hexahydrophthalic acid bis(3,4-epoxycyclohexylmethyl) ester (BE) was prepared and cured with anhydride. The curing kinetics and properties of the cycloaliphatic epoxy resin were characterized and compared with (3′,4′-epoxycyclohexane) methyl-3,4-epoxycyclohexyl-carboxylate (CE) and bis(3,4-epoxycyclohexylmethyl) adipate (BA). The result shows that BE/4-methyl hexahydrophthalic anhydride (MHHPA) exhibits a higher reaction activation energy and a slower curing rate than CE/MHHPA and BA/MHHPA. The loss modulus of BE/MHHPA increased 53 and 24% compared to CE/MHHPA and BA/MHHPA, respectively. The Young’s modulus and initial degradation temperature of BE/MHHPA improved 31 and 6.4% compared with those of BA/MHHPA. Furthermore, superior moisture resistance and dielectric properties are expressed for BE/MHHPA among these epoxy resins. Those results demonstrated the potential usefulness of BE/MHHPA in the electronic packaging materials field.

Synergistic Effect of Graphene Oxide and OH-MWCNTs on the Cure Kinetics of an Epoxy-Anhydride System

Synergistic Effect of Graphene Oxide and OH-MWCNTs on the Cure Kinetics of an Epoxy-Anhydride System

Nonisothermal curing kinetics of solid resole by differential scanning calorimetry

In this work, solid resoles with different formaldehyde (F) to phenol (P) molar ratios and various contents of hexamethylenetetramine (HMTA, accelerator) and sodium hydroxide (NaOH, catalyst) were designed and characterized. Curing behavior was investigated by differential scanning calorimetry to provide useful insights for applications. Curing kinetics were analyzed by the Vyazovkin isoconversional method. The results demonstrate that the activation energy of the higher F/P molar ratio systems is lower in the initial stage but higher in the latter stage. The activation energy of the higher HMTA and NaOH system is higher in the initial stage but lower in the latter stage. The thermal stability of cured resoles was evaluated by thermogravimetric analysis. The higher F/P molar ratios or higher contents of HMTA and NaOH provide better thermal stability. This is because these synthetic conditions give higher crosslinking densities. This makes them better potential candidates for heat-resistant materials.

Kinetic Analysis of the Curing of a Partially Biobased Epoxy Resin Using Dynamic Differential Scanning Calorimetry.

This research presents a cure kinetics study of an epoxy system consisting of a partially bio-sourced resin based on diglycidyl ether of bisphenol A (DGEBA) with amine hardener and a biobased reactive diluent from plants representing 31 wt %. The kinetic study has been carried out using differential scanning calorimetry (DSC) under non-isothermal conditions at different heating rates. Integral and derivative isoconversional methods or model free kinetics (MFK) have been applied to the experimental data in order to evaluate the apparent activation energy, Ea, followed by the application of the appropriate reaction model. The bio-sourced system showed activation energy that is independent of the extent of conversion, with Ea values between 57 and 62 kJ·mol−1, corresponding to typical activation energies of conventional epoxy resins. The reaction model was studied by comparing the calculated y(α) and z(α) functions with standard master plot curves. A two-parameter autocatalytic kinetic model of Šesták–Berggren [SB(m,n)] was assessed as the most suitable reaction model to describe the curing kinetics of the epoxy resins studied since it showed an excellent agreement with the experimental data.

Shelf-life of polyfurfuryl alcohol resin: an accelerated rheokinetics study

Thermosetting resins like polyfurfuryl alcohol (PFA) tend to get prematurely cured and viscous during storage. Understanding a resin shelf-life is very helpful but no definite and validated method could be found in both scientific literature and standard methods. Cure kinetics of PFA was determined through tracing the complex viscosity changes as a function of time at elevated temperatures of 160, 170, 180, 190 and 200 °C. Adopting an empirical statistical approach, shelf-life of PFA resin at ambient temperature was estimated using the accelerated cure profiles. To this end, two functions were fitted on experimental results and then the empirical cure reaction rate constants at different temperatures were extrapolated to 25 °C. Subsequently, the viscosity versus time profiles were built for curing the resin at ambient temperature. The time interval corresponding to a viscosity range of 10–12.5 Pa.s was considered as the practical shelf-life of the resin at ambient conditions. The findings were validated by comparing the model results to the real-time, experimentally-obtained values.