Non-isothermal Curing Kinetics Research Articles

Preparation and Properties of Epoxy Adhesives with Fast Curing at Room Temperature and Low-Temperature Resistance.

Developing a highly efficient multifunctional epoxy adhesive is still an enormous challenge, which can rapidly cure at room temperature and has excellent low-temperature resistance performance and is crucial for the epoxy adhesive and electrical sealing fields during severe cold seasons. Herein, diglycidyl phthalate (DP) was synthesized with phthalic anhydride (PA) and epichlorohydrin (ECH) to enhance the curing rate and low-temperature resistance of bisphenol A diglycidyl ether (DGEBA) adhesive. The modified DP/DGEBA adhesives were systematically analyzed by gel time, mechanical properties, and aging resistance (time, temperature, and dry/wet treatment). The results showed that DP with highly active ester groups significantly accelerates the curing speed of DP/DGEBA. DP's rigid aromatic ring-benzene ring and flexible group-ester group gave the adhesive better low-temperature resistance. When the addition of DP was 10 wt % (based on the mass of DGEBA), the gel time of DP/DGEBA epoxy adhesives was reduced by 58 min compared to unmodified DGEBA epoxy adhesive, and after aging at low temperature (-20 °C) for 7 days, the tensile shear strengths of polyvinyl chloride (PVC) and aluminum plate increased by 76.2 and 80.6%, respectively. The results of non-isothermal curing kinetics and dynamic mechanical analysis suggested that when the amount of DP was 10 wt %, the reaction activation energy of DP/DGEBA epoxy adhesive decreased by 4.0%, and the cross-linking density increased by 8.9%. Moreover, the toughness of the modified adhesive was also improved. This study opens up a feasible way for the development of a low temperature-resistant epoxy adhesive cured rapidly at room temperature in practical application.

Modelling the non-isothermal curing kinetics of peroxide crosslinking polyolefin copolymers for photovoltaic module lamination

This study presents a model for determining the non-isothermal curing kinetics of peroxide crosslinking polyolefin copolymers used for encapsulation of silicon solar cells in photovoltaic modules. Therefore, rheological data of ethylene vinyl acetate copolymer (EVA) and polyolefin elastomer (POE) were analyzed. The curing kinetics were described assuming a temperature dependent Arrhenius rate coefficient and implementing a reaction rate model. Validation was performed under isothermal and non-isothermal conditions. An excellent agreement of experimental and model data was ascertained. The model was used to determine the non-isothermal crosslinking conversion during lamination of EVA or POE based photovoltaic mini-modules. Temperatures were tracked by positioning seven sensors within the mini-modules. Moreover, a thermal simulation model was implemented. Also on module level, the experimental results corroborated the simulated data. A lamination temperature of 150 °C led to insufficient curing of POE based modules. Similar conversion rates were achieved for EVA and POE encapsulants at 150 and 160 °C, respectively.

Toughness modification of SBS/CRMA on epoxy asphalt: curing behaviour and low-temperature cracking characteristic analysis

ABSTRACT The limited utilisation of epoxy asphalt primarily stems from its inherent brittle cracking behaviour. This investigation aims to mitigate this issue by introducing styrene–butadiene-styrene/crumb rubber modified asphalt (SBS/CRMA). Furthermore, this research assesses the influence of SBS/CRMA on the curing behaviour of epoxy asphalt (EA) and the low-temperature fracture characteristics of epoxy asphalt concrete (EAC). The curing behaviour of EA was systematically examined using diverse analytical techniques, including attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR), rotational viscosity, and non-isothermal curing kinetics analysis. Notably, SBS/CRMA expedites the curing process of epoxy asphalt, possibly attributed to the amine constituents in the crumb rubber, which acts as catalyst. Subsequently, the study scrutinised the fracture characteristics of EAC through the application of the semicircle bending (SCB) test and acoustic emission (AE) methodology. The results substantiate that SBS/CRMEAC imparts a proficient toughening effect, underpinned by an augmented stress relaxation capacity due to the reinforcing influence of elastomers such as crumb rubber and SBS. The fracture process was delineated into three distinct stage and the AE signal in matrix epoxy asphalt (MEA) exhibited concentration during the macroscopic crack extension and final fracture stages. In stark contrast, SBS/CRMEAC exhibited a uniform distribution and showcased ductile fracture characteristics.

Understanding the curing kinetics of boron-based hyperbranched polysiloxane reinforced and toughened epoxy resin by rheology

The curing kinetics of epoxy resin (EP) play an important role in optimizing the final properties of the resin, such as thermal stability, flame retardancy and mechanical properties, etc. However, full understanding of the curing kinetic remains a formidable challenge due to the progressively crosslinked structure and the multi-stage exothermic curing reactions. In this study, a hyperbranched polysiloxane (PBPS) with a Si-O-B backbone, containing reactive hydroxyl and epoxy end groups, is synthesized to facilitate the amine curing of EP. For the first time, the contribution of boron to the curing kinetics of epoxy resin is systematically revealed. The Kamal-Sourour model and the Arrhenius four-parameter model, having good fittings to rheology data, successfully describe the isothermal and non-isothermal curing kinetics of the epoxy-amine system loaded with PBPS, respectively. Acting as a Lewis acid, the boron in PBPS coordinates with epoxy groups, efficiently reducing the induction time of curing of the epoxy-amine system. The reaction rate of the non-catalytic stage is increased with reduced activation energy. Meanwhile, a gentle autocatalytic curing stage is achieved with PBPS. The effect of PBPS on the initial viscosity of EP leads to an increase in the activation energy of the viscous flow stage of the non-isothermal process. The curing behavior determines the final properties of EP. The combined effect of improved curing conditions, which endow EP with high crosslink density, and the flexible segments of PBPS exhibit excellent toughening effects. PBPS cavities can in situ form filament structures, thereby reinforcing and toughening the EP. This systematic study fills the knowledge gap regarding the mechanism of hyperbranched polysiloxane participating in the curing of epoxy resin and its influence on the final properties.

Fully bio-based epoxy resins derived from magnolol and varying furan amines: Cure kinetics, superior mechanical and thermal properties

With rapidly shrinking petroleum resources and the deteriorating environment, the use of renewable green bio-based resources to replace petroleum-based raw materials and the development of new bio-based epoxy resins are hot topics of research today. In this study, a new bio-functional epoxy monomer based on magnolol (EMDBP) and two different curing agents based on furfuryl amine with various side groups (PDFA and BDFA) were synthesized and utilized to produce two sets of fully bio-based epoxy thermosets. The non-isothermal and isothermal curing kinetics of EMDBP/PDFA and EMDBP/BDFA were systematically investigated using different scanning calorimetry (DSC) according to the Friedman isoconversional differential method and model-fitting approach, respectively. The results indicate that the autocatalytic curing mechanism dominated the curing reaction of the fully bio-based curing system at a low conversion stage whereas diffusion controlled the curing reaction at a high conversion stage. With an attached benzene ring, BDFA has a more reactive amine group than PDFA. Thus, the curing reaction rate of the BDFA curing system was much faster than that of the PDFA curing system, which also increased significantly with temperature. Additionally, the glass transition temperature of the EMDBP systems (Tg, 151.3 °C and 168.4 °C) was increased by 71.3% to 95.7% compared to the diglycidyl ether of bisphenol A (E51) systems (Tg, 77.3 °C and 98.3 °C), which was also higher than those of most previously reported fully bio-based epoxy resins. Analysis by nanoindentation and tensile testing showed that the Young's modulus, hardness, tensile strength, and modulus of EMDBP systems were also greatly improved. Furthermore, thermogravimetric investigations demonstrated that EMDBP systems possessed excellent thermal stability at high temperatures, with a char yield of about 47.5 wt% at 800 °C. Consequently, these novel fully bio-based epoxy thermosets with superior heat resistance, mechanical properties, and thermal stability have the potential to replace commercial epoxy resins and be utilized under high temperature environments.

Novel epoxy-ended hyperbranched polyether derived from xylitol as sustainable tougheners for epoxy resin

Families of bio-based hyperbranched epoxy (HBPEEs) were successfully prepared using renewable xylitol, bisphenol A and epichlorohydrin as feedstocks. The structure and physical performances of HBPEEs were characterized by FTIR, NMR, GPC and viscosity evaluation. Then, they were used as high efficient tougheners for diglycidyl of bisphenol A/diethylenetriamine (DGEBA/DETA) networks. The results indicated that the incorporation of HBPEEs into the DGEBA system can significantly enhance the mechanical properties and solvent resistance properties of the hybrid (HBPEEs/DGEBA). In particular, toughened hybrids with 15 wt% HBPEE-2 exhibited the outstanding impact strength than that of neat DGEBA (37.4 kJ/m2 vs. 11.8 kJ/m2) due to its excellent compatibility of with DGEBA. Simultaneously, the ductile fracture morphology of the modified hybrids was also confirmed by SEM. In addition, the activation energies of HBPEE-2/DGEBA hybrids calculated from the non-isothermal curing kinetics were lower than that of neat DGEBA. All of the results suggested that the bio-based HBPEEs could be used as efficient tougheners applied in epoxy resins industry.

Synthesis of antibacterial polyether biguanide curing agent and its cured antibacterial epoxy resin

ABSTRACT At present, bacteria continue to threaten human health, and the resistance of bacteria to antibiotics continues to increase, so the development of new antibacterial agents and antibacterial materials is increasingly important to ensure human health. In this paper, three polyether biguanide compounds with high antibacterial properties were synthesized by reacting polyetheramine T403 with o-tolylbiguanide, m-tolylbiguanide and p-tolylbiguanide (o-TTB, m-TTB and p-TTB), respectively. The antimicrobial performance of polyether biguanide against E. coli and S. aureus was evaluated using a minimum inhibitory concentration method, and the results showed that the synthesized polyether biguanide exhibited efficient and broad-spectrum antimicrobial effects. Among them, o-tolyl biguanide derivative o-TTB showed the best antimicrobial performance, with minimum inhibitory concentrations of 20 and 15 μg/mL against E. coli and S. aureus, respectively. Then, epoxy resin E51 was cured using the obtained TTB as a curing agent to prepare an epoxy resin with antibacterial properties. The inhibition of the growth of S. aureus by the cured o-TTB/E51 resin was investigated by incubating the cured epoxy resin with bacteria, and the results showed that the cured resin had a significant inhibitory effect on the growth of bacteria. The non-isothermal curing kinetics of the o-TTB/E51 system were investigated by differential scanning calorimetry (DSC) to determine the optimized curing reaction temperature, curing kinetic parameters and curing kinetics equation.

Finite element modelling of non-isothermal curing kinetics for resin transfer moulding process

The numerical modelling and simulation of empirical cure rate models presented for the non-isothermal resin transfer moulding curing process cycle. The nth order model, Kamal and Sourour’s (K & S) model and their modified forms used to study the efficacy of Comsol multi-physics simulator without adding any user defined program. The appropriate value of mould temperature selected from the isothermal curing simulation of nth order model. The transient variation of degree of cure and temperature within the saturated preform has been presented. It has been observed that the models having two rate constant were not converging for the initial temperature lower than 315 K. Therefore, for the sake of uniformity we used initial temperature as 315 K for all the models and mould temperature as 380 K. Although, process cure time was less for K & S model, we observed the large temperature gradient inside the composite plate for the two rate constant models. To add on the complexity in the simulation three dimensional curved plate is simulated for 10 mm, 20 mm and 30 mm thickness. Definitely, with increase in thickness of the composite plate, the time to complete the cure reaction is increased.

Approaches on the non-isothermal curing kinetics of epoxy/PCL blends

The non-isothermal curing kinetics of DGEBA/PCL/MTHPA/DEH 35 blends at 10, 20 and 30 phr of PCL were investigated applying Ozawa, Kissinger, Friedman autocatalytic, Friedman isoconversional and Málek models. From verified discrepancies between theoretical and experimental fits measured based on the models parameters such as activation energy (Ea) and molecular collisions frequency (ln A), Ozawa and Kissinger presented higher deviation most likely due to the considered invariable energy along with the curing, while in Friedman and Málek's perspectives it is changeable, hence providing better data adjustments. PCL led to lower collision frequency among molecules, suggesting lower density microstructure related to epoxy, which corroborates to the system composed by physical (hydrogen bonds) and chemical (covalent bonds) crosslinking, altogether might provide swollen coils and occasionally, an interconnected structure that percolates the system resulting in (macro) gelation instead reticulation affording the workable character on epoxy/PCL blends.

Non-isothermal curing kinetics of biocomposite from poly(furfuryl alcohol) and graphene oxide or reduced graphene oxide with different contents of oxygen-groups by DSC

The non-isothermal curing kinetics of furfuryl alcohol (FA) with p-toluenesulfonic acid (p-TSA-2 wt%), two graphene oxides with different content of oxygen and a high thermally reduced graphene oxide at different concentrations (2, 4, 6 wt%) were studied by DSC using four heating rates (5, 7.5, 10 and 15 K min−1). The graphene particles were characterized by infrared spectroscopy, Raman spectroscopy, 13C solid-state NMR, and X-ray photoelectron spectroscopy. The formulations highlighted the importance of the amount of graphene as well as the effect of the oxygen groups on the curing process. The results showed that graphene had a catalytic effect on the polymerization reaction of FA depending on the oxidation degree. FA curing presented two well-defined stages: polycondensation at relatively low temperature, and crosslinking at high temperature. The kinetic parameters of Kamal and Kamal-Chern diffusion models were applied successfully for working with experimental data. The double fitting method was used to determine the kinetic parameters and to establish the kinetics equation.

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.

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.

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.

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.

Comparison of Hydrogenated Bisphenol A and Bisphenol A Epoxies: Curing Behavior, Thermal and Mechanical Properties, Shape Memory Properties

Hydrogenated bisphenol A epoxy resin was cured using different kind of curing agents, resulting in epoxy networks with better shape memory properties than bisphenol A epoxy networks. The non-isothermal curing kinetics investigated by differential scanning calorimetry (DSC) demonstrated that hydrogenated bisphenol A epoxy showed lower curing reactivity than bisphenol A epoxy, while it still could be cured well. The thermal and mechanical properties as well as shape memory properties were studied by dynamic mechanical analysis (DMA), DSC, thermogravimetric analysis (TGA), three-point bending test and U-type shape memory test and cyclic stretch test using DMA. Results manifested that hydrogenated bisphenol A epoxy systems exhibited lower shape transition temperature (lower T g ), slightly higher modulus, better toughness, much faster shape recovery rate, and better elongating ability at temperature above T g than bisphenol A epoxy systems, which was due to the rigidity of cyclohexane ring from its steric hindrance and favorable segmental mobility when absorbing external energy such as heating or bending. Moreover, the shape fixity and shape recovery ratio of all the samples were as high as 96.3~98.5% and 100% and their cycling stability during shape memory test was excellent. Although lower than bisphenol A epoxy networks, hydrogenated bisphenol A epoxy networks possessed high thermal stability with initial degradation temperature (Td5%) of >305 °C.

Non-Isothermal Curing Kinetics of a Modified Benzoxazine Resin System Suitable for Hot-Melt Prepreg Preparation

Non-isothermal DSC was used to study the cure kinetics of a flame-retardant benzoxazine resin system (BZ501-1) suitable for hot-melt prepreg preparation. The variation of activation energy with conversion for BZ501-1 system was investigated by using Flynn-Wall-Ozawa method, and the average value of activation energy obtained from this method was 100.5 kJ/mol. After that, the model function that depends on the reaction mechanism for the non-isothermal cure of BZ501-1 system was determined by Málek method, and the result showed that Šesták-Berggren model can generally simulate well the reaction rate. Finally, the isothermal curing behavior of BZ501-1 at 200 °C was predicted by using the obtained rate equation, and the result showed that there was no significant difference between the experiment conversions and the predicted ones. The predicted results at different temperatures indicated that the curing rate increased gradually with increasing the curing temperature.

Non-isothermal kinetics of epoxy resin curing reaction under compressed CO2

The non-isothermal curing kinetics of epoxy resin system consisting of diglycidyl ether of bisphenol A (DGEBA) and m-xylylenediamine (MXDA) were investigated by high-pressure differential scanning calorimetry at different heating rates of 5, 10, 15 and 20 K min−1 under compressed carbon dioxide (CO2) of 0.1–8 MPa. The activation energies were determined by isoconversional Kissinger–Akahira–Sunose method and Vyazovkin numerical method, and the average values decreased from 53.5 to 46.8 and 54.1 to 44.5 kJ mol−1, respectively, when the CO2 pressure increased from 0.1 to 8 MPa. It is indicated that the curing reaction of epoxy resin occurs more easily under higher-pressure CO2 due to the promoted molecular movement, especially in the supercritical state. The non-isothermal curing kinetics were investigated by nth-order model and two-parameter autocatalytic model (Sestak–Berggren model), and the latter one analyzed by Malek method exhibited good agreement with the experimental data except the initial and final stages of the curing reaction.

Study of curing kinetics of anhydride cured petroleum-based (DGEBA) epoxy resin and renewable resource based epoxidized soybean oil (ESO) systems catalyzed by 2-methylimidazole

The non-isothermal curing kinetics of diglycidyl ether of bisphenol-A (DGEBA) and bio-based epoxy resin (ESO) both cured with methyl hexahydrophthalic anhydride (MHHPA) curing agent in the presence of 2-methylimidazole as catalyst (2-MI) was investigated using differential scanning calorimetry (DSC) at three different heating rates 5, 10, 15°C/min. The peak temperature was found to be dependent on the heating rates and the structure of anhydride as well as molar ratio of epoxy groups to MHHPA and catalyst. The activation energy (Ea) was determined by Kissinger and Ozawa methods. The two kinetic parameter m and n were obtained by Sestak–Berggren autocatalytic model. The curve obtained by the predicted values of the non-isothermal Malek method matched perfectly with the experimental data for DGEBA, ESO and their blend.

Rheokinetics in curing process of polyfurfuryl alcohol: effect of homologous acid catalysts

Curing reaction of polyfurfuryl alcohol (PFA) resin was investigated using rheological measurements in the presence of different acid catalysts. A homologous series of dicarboxylic acid catalysts with even number of carbons of 2, 4 and 6 were chosen, i.e., oxalic acid (OX, 2 carbon), succinic acid (SU, 4 carbon) and adipic acid (AD, 6 carbon). Acidity of these catalysts in terms of pK a was in the range of 1.25–4.43. Firstly, non-isothermal curing kinetics were investigated, and then, isothermal studies were performed at three different temperatures using 3 and 5 wt% of each acid catalyst. Isothermal cure behavior was best described by an empirical Arrhenius model of viscosity. An isoconversional method was applied to compute the changes in the effective activation energy as a function of degree of conversion. The change in activation energy at 50–10 kJ mol−1 was observed for PFA/3 wt% OX system due to the vitrification and diffusion-controlled processes. Gel time, t gel, was determined as a function of curing temperature, acid catalyst type and its concentration. The gelation time was consistent for all the formulations with an extent of conversion of about 0.7. Significant differences were observed in the curing behavior of resins in the presence of different acid catalysts. Increasing of the curing temperature significantly accelerated the curing process and increased curing rate constant of the PFA resin.

Green Epoxy Resin System Based on Lignin and Tung Oil and Its Application in Epoxy Asphalt

In this work, lignin-based epoxy was synthesized by reacting epichlorohydrin (ECH) and partially depolymerized lignin (PDL) and then used for modification of asphalt. The Diels–Alder adduct of methyl esters of eleostearic acid and maleic anhydride (ME-MA) was synthesized and used as a biobased curing agent. The structure of PDL-epoxy was characterized by FTIR and 31P NMR. Nonisothermal curing kinetics and thermal properties of the cured epoxy resins were studied by differential scanning analysis, dynamic mechanical analysis, and thermogravimetric analysis, respectively. Curing behaviors of PDL-epoxy and a commercial epoxy DER332 were compared. Effects of epoxy content on rheological properties of the modified asphalt binder were studied using a parallel plate rheometer. Results show that the elastic behavior of asphalt binder at elevated temperatures was improved with increase in epoxy content.