Autocatalytic Model Research Articles

In situ cure monitoring and In-service impact localization of FRPs using Pre-implanted nanocomposite sensors

From manufacturing onset through service, cure progress and structural integrity of fibre-reinforced polymer (FRP) composites are monitored continuously using a single type of pre-implanted nanocomposite sensors. The sensors precisely respond to broadband dynamic strains up to half a megahertz, yet without imposing intrusion to host composites and downgrading the original structural integrity. In conjunction with differential scanning calorimetry and a Sesták–Berggren autocatalytic kinetic model, cure behaviors of FRPs during fabrication are evaluated accurately, in terms of the matrix polymerization degree that is correlated with subtle changes in propagation characteristics of guided ultrasonic waves captured by the pre-implanted sensors. Use of the sensors is subsequently extended to structural integrity monitoring of FRPs that are in service. Experimental validation demonstrates that a transient impact to composites can be localized and imaged with the pre-implanted sensors. This study illustrates an in situ life cycle monitoring approach for FRPs using a single type of permanently implanted sensors with neglectable intrusion to composites.

The Curing Kinetics of Multiscale [Ni(EDTA)]-2 Intercalated Zn-Al Layered Double Hydroxides: Glass Fiber–Epoxy Composite Prepreg

In the present research, the effect of Zn2Al layered double hydroxides (LDH) and nickel (II)-EDTA complex intercalated LDH (LDH-[Ni(EDTA)]-2) on the cure kinetics of glass fiber/epoxy prepreg (GEP) was explored using nonisothermal differential scanning calorimetry (DSC). The results showed that LDH caused a shift in the cure temperature toward lower temperatures while accelerating the curing of epoxy prepregs. The use of LDH-[Ni(EDTA)]-2 more profoundly influenced the acceleration of the curing process. The curing kinetics of prepregs was assessed through the differential isoconversional Friedman (FR) technique and the integration method of Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS). A decrease was detected in the E α value of glass fiber/LDH-[Ni(EDTA)]-2/epoxy (GELP) and glass fiber/LDH-[Ni(EDTA)]-2/epoxy (GELNiP) prepregs at small cure degrees relative to GEP, suggesting the catalytic effect of LDH or LDH-[Ni(EDTA)]-2 on the initial epoxy/amine reaction. Furthermore, LDH-[Ni(EDTA)]-2 performed better due to the catalyst role of nickel (II). Moreover, the activation energy exhibited lower reliance on the degree of conversion in the cases of GELP and GELNiP rather than pure epoxy prepregs. An autocatalytic model was used to evaluate the curing behavior of the system. Based on the results, the curing reaction of the epoxy prepreg can be described by the autocatalytic Šesták-Berggren model even after the incorporation of LDH or LDH-[Ni(EDTA)]-2. The kinetic parameters of the autocatalytic model (such as E α , A , m , n ) and the equations explaining the curing behavior of prepregs were introduced as well whose predictions were in line with the experimental findings.

Thermokinetic behavior of the Al2O3-PbO-B2O3 glasses

Thermokinetic behavior of the Al2O3-PbO-B2O3 glassy system was investigated by means of differential scanning calorimetry, X-ray diffraction analysis and Raman spectroscopy. The glass transition kinetics was described in terms of the Tool–Narayanaswamy–Moynihan model. The compositional evolution of the relaxation parameters was explained in terms of the structural changes and movements of the characteristic structural units detected by Raman spectroscopy. Crystal formation in the studied glassy matrices was investigated in dependence on composition and particle size. The crystal growth was suppressed by increasing particle size as well as by increasing Al2O3 content; formation of crystalline boron oxides was replaced by formation of mixed Al2O3/PbO oxides and Pb4B2O7 phase. The nucleation-growth Johnson-Mehl-Avrami model and the empirical autocatalytic model of Šesták and Berggren were used to describe the complex crystallization kinetics. The Avramov-Šesták concept of temperature dependent activation energy was successfully applied and tested on the real-life experimental data. The corresponding methodology was critically reviewed.

Photo-polymerization kinetics of a dental resin at a high temporal resolution

ObjectivesThis study: 1) aims to measure with high temporal resolution the intrinsic rate of the degree of conversion (DC) of a dental resin-based composite (RBC) photo-cured at two irradiances; 2) aims to determine the transition time at which the DC rate is maximum; 3) used two different irradiances to measure the shift in transition time; 4) aims to compare transition times measured using DC and shrinkage strain. MethodsSamples (n = 20) 1 mm thick by 10 mm diameter of Filtek One bulk-fill restorative A2 shade (3M Oral Care) were photocured for 20 s with a single emission peak (wavelength centered at 455 nm) light-emitting-diode-based light-curing unit at irradiance levels of 890 mW/cm2 and 209 mW/cm2, and initial sample temperature of T = 23 °C. The DC was measured in real-time using Attenuated Total Reflection (ATR) FTIR spectroscopy with a sampling rate of 13 DC data points per second. The data were analyzed within a phenomenological autocatalytic model. In addition, the axial shrinkage strain was measured using 3 samples of the RBC with the same outer dimensions and under similar experimental conditions using the bonded disk method and an interferometric technique. ResultsFor the 890 mW/cm2 and 209 mW/cm2 irradiance levels, the DC with time was found to agree with the model enabling the determination of transition times of 0.66 ± 0.05 s and 2.3 ± 0.2 s, and the DC at these times of 5.5 ± 0.2% and 6.4 ± 0.2%. The maximum linear strain rate at 0.76 ± 0.01 s and 1.98 ± 0.02 s for the 890 mW/cm2 and 209 mW/cm2 irradiance levels, respectively, are within two standard deviations of the corresponding transition times. SignificanceAt an irradiance level much greater than 1000 mW/cm2, the photo-polymerization kinetics of a dental RBC may be too fast to be measured accurately using ATR-FTIR spectroscopy. A viable alternative to monitor the kinetics is through the measurements of the axial shrinkage strain employing the bonded disk method and an interferometric technique.

Curing kinetics, mechanical properties and thermomechanical analysis of carbon fiber/epoxy resin laminates with different ply orientations

A carbon fiber/epoxy resin prepreg was tested by DSC, and the phenomenological nth-order and autocatalytic reaction curing kinetic models of the prepreg were established. On this basis, the optimum curing process of the prepreg was obtained. In addition, in order to explore the effect of ply parameters on the mechanical properties of the laminates, the thermomechanical and mechanical properties of the laminates of [0]10, [+ 45/ − 45]5s and [0/90]5s, prepared by compression molding, were investigated. The results show that the autocatalytic reaction model agrees well with the DSC curves obtained from the experimental data. The curing process of the carbon fiber/epoxy resin prepreg is 130 °C /60 + 160 °C/30 min. The flexural and impact strength of [0]10 laminates are higher than that of [+ 45/ − 45]5s and [0/90]5s laminates. The glass transition temperature (Tg) of the laminates is 142–146 °C, and the loss factor of [0]10 laminate is significantly higher than that of [+ 45/ − 45]5s and [0/90]5 s laminates. The great differences in the mechanical properties of the laminates are mainly due to the different amounts of 0° fibers, which lead to the different bearing and deformation capacity of the laminates under external force.Graphic abstract

Thermal hazards of benzaldehyde oxime: Based on decomposition products and kinetics analysis by adiabatic calorimeter

As a self-reactive substance, benzaldehyde oxime (BO) is prone to a highly exothermic runaway reaction and thermal hazard analysis and the reaction kinetics calculation of BO have great significance. In this work, the decomposition products of BO in nitrogen atmosphere were identified by GC–MS and HPLC techniques. The impact of the decomposition products on the decomposition behaviors of BO were analyzed by comparison of the ARC test results of pure BO and mixture of BO and decomposition products. It was found that N-benzylidene benzylamine was the intermediate decomposition product and benzoic acid, benzamide, N-benzyl benzamide, and 2,4,5-triphenylimidazole were the final products of BO. A two-step continuous autocatalytic reaction model was established to depict the decomposition process of BO. The kinetic parameters of the model were calculated by applying the nonlinear optimization method. Finally, thermal behaviors under different process temperature were predicted based on the kinetic model, and the time to maximum rate (TMRad) was predicted as 112.04 °C under 24 h, and 122.19 °C of 8 h, which offer crucial safety information to optimize the safety conditions of BO during usage, storage and transportation, which minimizes the industrial disasters.

Polydimethylsiloxane crosslinking kinetics: A systematic study on Sylgard184 comparing rheological and thermal approaches

AbstractIn this work a systematic investigation of crosslinking kinetics of Sylgard184 polydimethylsiloxane is performed in both isothermal and dynamic conditions. The results are discussed in terms of two conversions, αC and αR determined by thermal and rheological analysis, respectively. Thermal analysis can well detect the first stage of the reaction, while rheological analysis starts being sensitive only at longer time. However, once the rheological response is observable, it changes with time faster than the calorimetric one. From rheology experiments it comes out that the gel point occurs at αR = 0.53, independently of the applied thermal history. At gel point, αC is around 0.30 indicating that about 30% of the bonds involved in the crosslinking process is enough to create an infinite network. A modified version of the Kamal's autocatalytic model allows to fit and predict the experimental findings from both the techniques; however, two distinct sets of parameters have been used. The results of this work may be a useful tool to design appropriate curing cycles for the preparation of Sylgard184.

Effect of cellulosic fillers addition on curing and adhesion strength of moisture-curing one-component polyurethane adhesives

Abstract In this work, microcrystalline cellulose (MCC) and sawdust (SD) green fillers have been demonstrated as reactive fillers and moisture reservoirs for enhanced curing and adhesion strength of one-component moisture-curing polyurethane (PU) adhesives. The chemical reaction between the MCC or SD fillers and isocyanates present in the PU adhesives was tracked using Fourier-transform infrared spectroscopy (FTIR), while the joint strength was determined using lap-shear tests. The results showed that the addition of MCC or SD leads to an accelerated curing rate through the early consumption of isocyanates. The lap-shear strength of unreinforced PU adhesive joints was achieved in 12 hours. In turn, the curing time in MCC and SD reinforced joints was achieved in 5 hours. The addition of MCC and SD fillers also enhanced the lap-shear strength of the joints under full cure conditions. An autocatalytic growth model was used to describe the lap-shear strength variation with respect to curing time.

The effect of nanostructured silica synthesis temperature on the characteristics of silica filled natural rubber composite

The processing, curing, mechanical, and morphological characteristics of natural rubber (NR) filled with nanostructured silica (NS) were discussed in this paper. NS was synthesized at 60-90°C using bagasse ash as raw material. NR was mixed with NS using Haake Rheomix equipped with roller rotors. Curing kinetics of NR compounds was studied using the data obtained from a moving die rheometer. Lower compounding temperature and lower compound viscosity were observed with increasing silica synthesis temperature. Highest bound rubber content and modulus at break were found at natural rubber filled with silica synthesized at 70°C. The state-of-mix of the filler was found to increase with increasing silica synthesis temperature. The curing curve obtained showed that the vulcanization kinetics follow autocatalytic model.

Vulcanization kinetics of styrene butadiene rubber reinforced by graphenic particles

AbstractThe present study discusses the effects of graphenic particles on the kinetics of sulfur vulcanization in styrene butadiene rubber composites. Using data obtained from a cure rheometer and fitted by an autocatalytic model, it was verified that graphenic particles follow our recently established catalytic‐networking model for the effect of particles on the sulfur vulcanization of rubber, regardless of the type of particles. The magnitude of the catalytic and networking effects depends on surface chemistry and interfacial interactions of particles with rubber that can be tailored by the chemical reduction of graphene oxide. Accordingly, the reduction process decreased the catalytic effect due to the elimination of surface functional groups and increased the networking effect due to the enhancement of filler–rubber interactions and immobilization of rubber. The latter was verified by differential scanning calorimetry and bound rubber measurements.

Thermal Stability Analysis of Lithium-Ion Battery Electrolytes Based on Lithium Bis(trifluoromethanesulfonyl)imide-Lithium Difluoro(oxalato)Borate Dual-Salt.

Lithium-ion batteries with conventional LiPF6 carbonate electrolytes are prone to failure at high temperature. In this work, the thermal stability of a dual-salt electrolyte of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium difluoro(oxalato)borate (LiODFB) in carbonate solvents was analyzed by accelerated rate calorimetry (ARC) and differential scanning calorimetry (DSC). LiTFSI-LiODFB dual-salt carbonate electrolyte decomposed when the temperature exceeded 138.5 °C in the DSC test and decomposed at 271.0 °C in the ARC test. The former is the onset decomposition temperature of the solvents in the electrolyte, and the latter is the LiTFSI-LiODFB dual salts. Flynn-Wall-Ozawa, Starink, and autocatalytic models were applied to determine pyrolysis kinetic parameters. The average apparent activation energy of the dual-salt electrolyte was 53.25 kJ/mol. According to the various model fitting, the thermal decomposition process of the dual-salt electrolyte followed the autocatalytic model. The results showed that the LiTFSI-LiODFB dual-salt electrolyte is significantly better than the LiPF6 electrolyte in terms of thermal stability.

Research on Thermal Decomposition Kinetics and Thermal Safety for a New Epoxiconazole Crystal.

To clarify the thermal safety inherent in a new epoxiconazole crystal, differential scanning calorimetry (DSC) and adiabatic accelerating rate calorimetry (ARC) were used for testing and research. The Friedman method and model method were used to analyze thermal decomposition kinetics based on the DSC data, and the N-order and autocatalytic decomposition reaction kinetic models were established. The double scan method was utilized to verify the autocatalytic effect during the decomposition process. The Friedman method, N-order, and autocatalytic model methods were used to study the substance’s thermal decomposition characteristics. ARC data are utilized to verify the aforementioned prediction results and the kinetic parameters that were obtained based on ARC data from N-order and autocatalytic model methods that concur with the simulation results. This paper applies the N-order and autocatalytic model to the kinetic model to further predict thermal safety parameter time to maximum rate under adiabatic conditions.

Kinetic analysis of the curing of branched phthalonitrile resin based on dynamic differential scanning calorimetry

The aim of this work was to systematically investigate the kinetics of the curing reaction of branched phthalonitrile resin containing both flexible moiety and rigid aromatic structure in backbones with 4, 4′-diaminodiphenyl sulfone (DDS) as hardener. Differential scanning calorimetric (DSC) was mainly utilized with non-isothermal mode at different heating rates. The activation energy and the dependence of the curing activation energy with conversion were calculated and discussed based on Starink methods. Results showed that the curing reaction between branched phthalonitrile resin and 5 wt% DDS was chemistry and diffusion controlled. The predicted curves of autocatalytic kinetic model fit well with the non-isothermal DSC data that was important for predicting curing behavior of other complex phthalonitrile system.

Modeling of runaway inhibition in batch reactors using encapsulated phase change materials

Thermal runaway is a common hazard leading to process safety-related accidents. Uncontrolled release of chemical energy poses extreme risks for batch reactors. In this study, we fabricated encapsulated phase change materials (PCMs) with silica shells as inhibitors to improve the thermal management of reactors and mitigate reaction thermal runaway. The prepared encapsulated PCMs had core-shell microstructures and spherical morphologies, with an average particle diameter of 980 nm and a silica shell thickness of 100 nm. A series of inhibition experiments were conducted with 0.5 and 1 g of encapsulated PCMs. Three stages were identified from the inhibition experiments. Kinetic parameters of esterification of propionic anhydride with 2-butanol, catalyzed by sulfuric acid, were estimated based on an autocatalytic parallel reaction model. An inhibition effect-kinetic model was proposed to simulate the inhibition process of the thermal runaway reaction. The results revealed that thermal storage and heat transfer intensification of encapsulated PCMs play a crucial role in the inhibition process. The effect of stirring rate, dispersity of encapsulated PCMs, and warning temperature of thermal runaway while optimizing the injection strategy of inhibitors was assessed. The inhibition of thermal runaway under adiabatic conditions was relatively low.

Modeling Bainitic Transformations during Press Hardening.

We revisit recent findings on experimental and modeling investigations of bainitic transformations under the influence of external stresses and pre-strain during the press hardening process. Experimentally, the transformation kinetics in 22MnB5 under various tensile stresses are studied both on the macroscopic and microstructural level. In the bainitic microstructure, the variant selection effect is analyzed with an optimized prior-austenite grain reconstruction technique. The resulting observations are expressed phenomenologically using a autocatalytic transformation model, which serves for further scale bridging descriptions of the underlying thermo-chemo-mechanical coupling processes during the bainitic transformation. Using analyses of orientation relationships, thermodynamically consistent and nondiagonal phase field models are developed, which are supported by ab initio generated mechanical parameters. Applications are related to the microstructure evolution on the sheaf, subunit, precipitate and grain boundary level.

Chemorheology and Kinetics of High‐Performance Polyurethane Binders Based on HMDI

AbstractAliphatic diisocyanates, such as 1,6‐hexamethylene diisocyanate (HMDI), are preferred curing agents for the formation of polyurethanes (PUs) in applications where resistance to abrasion or degradation by ultraviolet light takes precedence. Aside from the final properties, the curing agent plays a key role in the bulk manufacturing of such materials, and it mainly affects the polymerization kinetics and their rheology. The copolymerization of HMDI and a metallo‐prepolymer derivative from hydroxyl‐terminated polybutadiene (HTPB) is studied under isothermal conditions (50–80 °C). This study is carried out by means of an indirect method, using both rotational viscometry and dynamic rheometry. At the beginning of the process, the viscosity growth fit well to a first‐order kinetic model. Afterward, the reactive system passes through gelation, from which only rheology is allowed for the investigation of the entire polymerization process. This transition is analyzed in depth together with predictions from percolation theory. The conversion degree is determined from rheological measurements, and then an autocatalytic kinetic model is applied to describe the overall process. Finally, an isoconversional method allows the evolution of activation energy to be studied. This analysis merits attention for the development of high‐performance binders that are of great interest in aerospace propulsion technology.

Model-free cure kinetics of tetra-functional epoxy reinforced with GO and p-Phenylenediamine modified GO

Tetra-functional epoxy was reinforced with graphene oxide (GO) and p-Phenylenediamine functionalized graphene oxide (fGO). FTIR and XPS analysis of fGO confirmed the attachment of amine groups on its surface. Data obtained from nonisothermal DSC scans of neat epoxy and epoxy modified with 0.1 and 0.5 wt.% GO and fGO was studied to evaluate the effect of filler content and chemistry on the curing behavior of epoxy. Cure analysis was performed quantitatively using the integral isoconversional method to identify the dependence of activation energy (Eα) on conversion. Eα for neat epoxy increased with increase in conversion, possibly because at higher conversions, reactions such as etherification and homo-polymerization controlled the reaction kinetics. The addition of GO and fGO both displayed a lower activation energy in the conversion range α = 0.5−0.9 as compared to neat epoxy. The activation energy of fGO reinforced epoxy remained almost constant with change in conversion. This behavior was attributable to the amine groups attached to the surface of the filler, which acted as a secondary hardener and triggered the epoxide ring opening reaction. This fact was supported by the comparative XPS analysis of fGO and curing agent which clearly indicated the compositional similarities between the two. Further the autocatalytic model was used to determine reaction parameters. The overall order of the curing reaction was more than unity throughout, indicative of a complex reaction mechanism.

Role of Cobalt(III) Cationic Complexes in the Self-Assembling Process of a Water Soluble Porphyrin.

Under moderate acidic conditions, the cationic (+3) complexes ions tris(1,10-phenanthroline)cobalt(III), [Co(phen)3]3+, and hexamminecobalt(III), [Co(NH3)6]3+, efficiently promote the self-assembling process of the diacid 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4) into J-aggregates. The growth kinetics have been analyzed according to a well-established autocatalytic model, in which the rate determining step is the initial formation of a nucleus containing m porphyrin units (in the range 2–3), followed by a stage whose rate constant kc evolves as a power of time. The observed catalytic rate constants and the extent of J-aggregation increase on increasing the metal complex concentration, with the phen complex being the less active. The UV/Vis extinction spectra display quite broad envelops at the J-band, especially for the amino-complex, suggesting that electronic dipolar coupling between chromophores is operative in these species. The occurrence of spontaneous symmetry breaking has been revealed by circular dichroism and the measured dissymmetry g-factor decreases on increasing the aggregation rates. The role of these metal complexes on the growth and stabilization of porphyrin nano-assemblies is discussed in terms of the different degree of hydrophilicity and hydrogen bonding ability of the ligands present in the coordination sphere around the metal center.

Progress curve analysis of microtitre plate plasma clotting assays. Assessment of tissue factor levels

MTP plasma clotting assays monitor the time course of fibrin formation in re-calcified plasma by absorbance measurements and are increasingly used as alternatives to traditional one-point clot time assays employed in clinical laboratories to detect thrombotic disorders. The parameters derived from these analyses are analogous to thromboelastography viz. time, rate and maximum extent of clot formation. The derived parameters, based on the whole course of the clotting reaction are more robust, informative and quantitative than single-point clot time assays. However, the parameters themselves are usually obtained arbitrarily by crude graphical analysis of subjectively selected points of progress curves. The current work aimed to investigate the sensitivity and reproducibility of an MTP clotting assay and examine its suitability for measuring tissue factor (TF) levels in cell culture medium and patient urine. The results demonstrate that progress curves can be analysed by fitting a logistic equation, derived from a simplified autocatalytic clot formation model. The parameters, maximum amplitude (Fm), rate constant (k), time to half-maximum amplitude (tm) and maximum rate of clot formation (vm), fit a power curve showing limiting effects with increasing TF concentration. Log/log plots of tm and k against TF concentration provide standard curves for assessment of unknowns.

The effects of carbon nanoparticles on curing kinetics of epoxy modified with triblock copolymer

This work studies the effects of combining triblock copolymers with carbon nanoparticles (carbon nanotubes, graphene and carbon black) on curing kinetics. The chosen triblock copolymer was poly (propylene glycol)-block-poly (ethylene glycol)-block-poly (propylene glycol) (PPG-b-PEG-b-PPG), which has different contents of PEG in its structure. The main objective is to investigate the influence of the miscibility of the PPG-b-PEG-b-PPG copolymer in epoxy nanocomposites. The PEG fraction in the copolymer structure was determinant for miscibility. The copolymer with the highest PEG fraction (50%) in its structure showed miscibility. The copolymer miscibility was critical for nanoparticle–matrix synergy, and the PPG-b-PEG-b-PPG copolymer nanocomposite with 50 wt% PEG showed a greater increase in E’ in relation to the nanocomposite with PPG-b-PEG-b-PPG with 10% by weight of PEG. Cure kinetics results showed that the incorporation of carbon nanoparticles delays the kinetics as the temperature increases. Additionally, the miscible block copolymer delayed the curing reaction, whereas the immiscible one accelerated it, even with the addition of nanoparticles. Finally, DSC analysis allowed to verify that the cure kinetics of all studied copolymer nanocomposite systems satisfy Kamal’s autocatalytic model.