Isothermal Differential Scanning Calorimetry Measurements Research Articles

Crystallization Kinetics and Mechanical Properties of Nougat Creme Model Fats

Fat crystal networks result from a crystallization process, forming interlinked crystal aggregates of viscoelastic character. Palm oil-based fat crystal networks, such as chocolate and nougat spreads, often show liquid oil separation during storage because the fat crystal network is too weak to retain the liquid oils trapped within its structure. To explore the relationship between crystallization kinetics and subsequent mechanical properties, i) palm oil from three different geographical origins and with diverging crystallization properties, ii) mixtures of Ghanaian palm oil with gradually increasing additions of hazelnut oil, and iii) blends of Ecuadorian palm oil with palm stearin as a tripalmitin (PPP)-rich fraction were investigated. Kinetic parameters were acquired from an extended Avrami model by isothermal differential scanning calorimetry measurements, and the results combined with the elastic properties measured by oscillation rheology since studying crystallization kinetics alone insufficiently informs about the mechanical/structural properties needed to overcome liquid oil separation. Rate constants of all investigated fats followed bell-shaped curves, with curve progression strongly dependent on the lipid composition. Ameliorating crystallization properties entailed enhanced elastic properties. The higher the maximum rate constants, the higher the elastic modulus and the gel rigidity of the respective fats. However, two different linear regions of elastic modulus versus PPP or solid fat content resulted, depending on whether palm oil was diluted with hazelnut oil or blended with a PPP-rich fraction. Hazelnut oil strongly diluted crystallizable portions of the structuring fat, thereby decreasing the mechanical properties in a power-law fashion, because the fat crystal network became less connected between fat crystal aggregates.

Reaction sequence and formation kinetics of perovskite by calcium ferrite–titania reaction

Controlling the generation of perovskite (CaO·TiO2, CT) during sintering with vanadium–titanium magnetite (VTM) is the key to improve the strength and yield of VTM sinters. This study determined whether adding pre–formed calcium ferrite (CaO·Fe2O3, CF) effectively prevents the formation of CT during sintering with VTM. The formation behavior of CT in the CF–T system was investigated using X–ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Phase composition analysis by XRD indicates that CT is formed significant quantities in the CF–T system when the temperature is higher than the melting temperature of CF. DSC analysis shows that the CF–T samples undergo two reaction stages in continuous heating. Namely, CF first melts and forms its liquid phase, then TiO2 substitutes Fe2O3 in liquid CF to produce CT. SEM images confirm that CT could be generated at regions with a high concentration of CF. Non–isothermal DSC measurements on the formation kinetics of CT indicate that its activation energy reaches 252.00 kJ/mol and that the model function is f(α)=(1–α)2, revealing a second–order reaction based on Malek analysis. The kinetics equation of CT formation can be expressed by ln(dαdt)=2ln(1−α)−216805+70382αRT+16.71+10.33α. Quantitative calculation on enthalpy change by DSC curves shows that raising heating rates enhance the consolidation ability of CF–T samples as the melting of CF increases and the formation of CT decreases.

Temperature-Dependent Thermal and Chemical Stabilities as well as Mechanical Properties of Electrodeposited Nanocrystalline Ni

Nanocrystalline (NC) Ni electrodeposits (EDs) with a mean grain size of 34 ± 12 nm has been investigated, from room temperature to 800 °C under a purge gas of argon, by both non-isothermal and isothermal differential scanning calorimetry measurements, in combination with characterization of temperature-dependent microstructural evolution. A significant exothermic peak resulting from superimposition of recrystallization and surface oxidation occurs between 340 and 745 °C at a heating rate of 10 °C/min for the NC Ni EDs. The temperatures for recrystallization and oxidation increase with increasing the heating rate. In addition, recrystallization leads to a profound brittle–ductile transition of the Ni EDs in a narrow range around the peak temperature for the recrystallization.

Determination of the heat of pyrolysis of HDPE via isothermal differential scanning calorimetry

Due to the complexity of solid state reactions such as cracking of polymers, it is very difficult to find the required heat of the involved reactions. In this study, using a differential scanning calorimetry (DSC) instrument, a new method is proposed to determine the heat of decomposition of polymers at constant temperatures. The integration of an isothermal DSC curve with respect to time gives the overall consumed heat by the process, which consists of two components: net heat of reaction and heat loss. Finding an effective solution to measure heat loss from DSC instrument is an important part of this new approach. The heat loss is determined as function of sample mass and operating temperature. The heat loss is employed to correct the DSC results and produce the net heat of decomposition reactions. This approach is new since, firstly, it uses high-certainty isothermal DSC measurements, and secondly, the heat loss values are calculated in the decomposition range. The procedure is employed to determine the required heat for pyrolysis of high-density polyethylene at constant temperatures of 400, 410, 420 and 430 °C, and the average value was calculated to be 1375 ± 233 kJ kg−1.

Graphene-Decorated Nanocomposites for Printable Electrodes in Thin Wafer Devices

Printable electrodes that induce less stress and require lower curing temperatures compared to traditional screen-printed metal pastes are needed in thin wafer devices such as future solar cells, and in flexible electronics. The synthesis of nanocomposites by incorporating graphene nanopowders as well as silver nanowires into epoxy-based electrically conductive adhesives (ECA) is examined to improve electrical conductivity and to develop alternate printable electrode materials that induce less stress on the wafer. For the synthesized graphene and Ag nanowire-decorated ECA nanocomposites, the curing kinetics were studied by dynamic and isothermal differential scanning calorimetry measurements. Thermogravimetric analysis on ECA, ECA-AG and ECA/graphene nanopowder nanocomposites showed that the temperatures for onset of decomposition are higher than their corresponding glass transition temperature (T g) indicating an excellent thermal resistance. Printed ECA/Ag nanowire nanocomposites showed 90% higher electrical conductivity than ECA films, whereas the ECA/graphene nanocomposites increased the conductivity by over two orders of magnitude. Scanning electron microscopy results also revealed the effect of fillers morphology on the conductivity improvement and current transfer mechanisms in nanocomposites. Residual stress analysis performed on Si wafers showed that the ECA and nanocomposite printed wafers are subjected to much lower stress compared to those printed with metallic pastes. The observed parameters of low curing temperature, good thermal resistance, reasonably high conductivity, and low residual stress in the ECA/graphene nanocomposite makes this material a promising alternative in screen-printed electrode formation in thin substrates.

Accurate Cure Modeling for Isothermal Processing of Fast Curing Epoxy Resins.

In this work a holistic approach for the characterization and mathematical modeling of the reaction kinetics of a fast epoxy resin is shown. Major composite manufacturing processes like resin transfer molding involve isothermal curing at temperatures far below the ultimate glass transition temperature. Hence, premature vitrification occurs during curing and consequently has to be taken into account by the kinetic model. In order to show the benefit of using a complex kinetic model, the Kamal-Malkin kinetic model is compared to the Grindling kinetic model in terms of prediction quality for isothermal processing. From the selected models, only the Grindling kinetic is capable of taking into account vitrification. Non-isothermal, isothermal and combined differential scanning calorimetry (DSC) measurements are conducted and processed for subsequent use for model parametrization. In order to demonstrate which DSC measurements are vital for proper cure modeling, both models are fitted to varying sets of measurements. Special attention is given to the evaluation of isothermal DSC measurements which are subject to deviations arising from unrecorded cross-linking prior to the beginning of the measurement as well as from physical aging effects. It is found that isothermal measurements are vital for accurate modeling of isothermal cure and cannot be neglected. Accurate cure predictions are achieved using the Grindling kinetic model.

Degradation of glycidyl esters in RBD palm oil as a function of storage conditions

Storage of oil samples in a refrigerator results in the degradation of glycidyl fatty acid esters (G‐FE). In the present work, the degradation rate for G‐FE in RBD palm oil was determined at different temperatures with 0.4 mg/kg per month (5°C), 0.3 mg/kg per month (10°C), 0.2 mg/kg per month (15°C), 0.1 mg/kg per month (−20°C), and 0.0 mg/kg per month (20°C). The crystallization process was identified for RBD palm oil as reason for this phenomenon. Isothermal differential scanning calorimetry measurements show that this process delivers enough energy to cleave the epoxy bonding of the G‐FE (33.7 J/g [5 and 15°C] and 41.5 J/g [10°C]).Practical applications: The occurrence of glycidyl fatty acid esters is a big issue in oil processing industry especially for RBD palm oil. Short after the first announcement about the appearance of the esters in edible oils, a degradation of G‐FE was observed when oil samples were stored in a refrigerator. This observation was of great interest for analytics since oil samples usually are stored in the refrigerator until use to avoid deterioration of the oil quality. Thus, the investigation of the rate and the reason for the degradation of G‐FE during storage of RBD palm oil at lower temperatures is important to explain this phenomenon.The amount of glycidyl fatty acid ester significantly decreases during storage of palm oil samples in a refrigerator with 0.4 mg/kg per month (5°C), 0.3 mg/kg per month (10°C), and 0.2 mg/kg per month (15°C). The crystallization process was identified for RBD palm oil as possible reason for this phenomenon. Isothermal differential scanning calorimetry measurements show that this process delivers enough energy to cleave the epoxy bonding of glycidyl esters.

Cu-alloying effect on crystallization kinetics of Ti41Zr25Be28Fe6 bulk metallic glass

Compared with Ti41Zr25Be28Fe6 bulk metallic glass, (Ti41Zr25Be28Fe6)93Cu7 glassy alloy possesses a much narrower supercooled liquid region but the glass-forming ability is dramatically improved. Isochronal and isothermal differential scanning calorimetry measurements were adopted to investigate Cu-alloying effect on the crystallization transformation kinetics of Ti41Zr25Be28Fe6 glassy alloy. It is found that Cu alloying increases the activation energies of Ti41Zr25Be28Fe6 glassy alloy for glass transition and crystallization in continuous heating. Moreover, the isothermal activation energy of (Ti41Zr25Be28Fe6)93Cu7 glassy alloy increases as process of crystallization transformation, while Cu-free alloy exhibits a contrary tendency. The addition of Cu also decreases the Avrami exponent of the base alloy, resulting in the suppression of crystal nucleation and growth.

Rheological behaviour and cure kinetic studies of a trifunctional phenylethynyl-terminated imide oligomer

A phenylethynyl-terminated imide oligomer (PETI)was prepared from a triamine and 4-phenylethynylphthalic anhydride. The oligomer can be used to prepare high performance resin-based composite material via resin transfer moulding due to its low melt viscosity. The cured resin also exhibited excellent thermal stability with 5% weight loss at temperature above 531°C and higher glass transition temperature Tg (410°C) than PETI series as a result of the introduction of star-branched units. The rheological properties of the oligomer were measured and numerically fit with the dual Arrhenius model. The curing of the resin was also studied by dynamic and isothermal differential scanning calorimetry measurements. Based on the experimental data, both the curing reaction rate and the curing degree were calculated in the dynamic mode and tested under isothermal conditions.

Influence of an imidazolium salt on the curing behaviour of an epoxy-based hot-melt prepreg system for non-structural aircraft applications

This study investigates the influence of an imidazolium salt as initiator for the curing of an epoxy novolac-based hot-melt prepreg resin system for production of non-structural aircraft components. The salt decomposes at a specific temperature during curing, yielding an imidazole. This molecule initiates a fast anionic homopolymerization. The effect of the imidazolium salt on the curing kinetics is studied extensively by differential scanning calorimetry (DSC) as a function of its concentration. Onset and peak temperatures of the curing reactions are determined from dynamic DSC experiments at various heating rates. The curing behaviour at 140 °C is analysed in more detail by isothermal DSC measurements. Calculated isothermal conversion curves prove that the flame retarded epoxy novolac formulation can be cured within less than 60 min at this specific temperature by introducing six parts of the imidzolium salt per hundred parts of preformulated resin. At the same time glass transition temperatures above 130 °C evaluated by DSC or app. 155 °C determined by DMA can be reached.

Gel time and polymerization kinetics of unsaturated polyester resin/clay montmorillonite nanocomposites

This article presents the variation of viscosity, gel point, and curing kinetic process of nanocomposites based on unsaturated polyester resin (UPR) and natural nano‐clay CloisiteNa+® and organomodified Cloisite15A® montmorillonites (MMT). Different amounts (1–10 wt%) of nanoclays were added homogeneously before the cross‐linking reaction of the thermoset polymer. The complex viscosity before curing was especially dependent on the CloisiteNa+® content more than Cloisite15A®. The G' and G'' slope also as the complex viscosity criterion were used to determine the nanocomposites gel time variation. Isothermal differential scanning calorimetry measurements were performed to calculate the reaction order and the rate constant using different empirical equations. These results demonstrated a catalytic effect of the clays on the UPR cure, with diffusion controlled phenomena predominating at high conversions. The X‐ray diffraction patterns were useful for identifying the nanocomposites formation, and gave information about the clays dispersion, which served to support the rheological and polymerization kinetic results. Finally, natural MMT CloisiteNa+®decreased gel time and was more compatible with UPR than the organomodified Cloisite15A®. POLYM. COMPOS., 36:1931–1940, 2015. © 2014 Society of Plastics Engineers

Study of the Cure Reaction of Epoxy Resin Diglycidyl Ether of Bisphenol-A (DGEBA) with Meta- Phenylene Diamine

The cure process of epoxy resin diglycidyl ether of bisphenol-A (DGEBA) with aromatic amine (m-PDA) as curing agent was studied by means of differential scanning calorimetry (DSC) Perkin Elmer Pyris 6, at ratio (15 phr). Isothermal DSC measurements were conducted between 80 and 110 oC, at 10 oC intervals. The maximum degree of cure at isothermal cure temperature 110 oC was 0.9. The isothermal cure process was simulated with Kamal modifier with diffusion model, the model agrees well with the experimental data. For dynamic cure process, the activation energy was determined by two methods. One was based on Kissinger and Ozawa approach, given only one activation energy for the whole curing process. There was a slightly difference between the obtained activation energy and pre-exponential factor, 63.6 and 70.7 kJ mol-1 respectively. Another method was based on isoconversional, given activation energy at any conversion, and observed the Ea decrease with increment conversion (67-63) KJ/mol.

In-situ Cure and Cure Kinetic Analysis of a Liquid Rubber Modified Epoxy Resin

The in-situ cure and cure kinetics of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) polymerized with an anhydride hardener and its mixtures with a liquid polybutadiene rubber having hydroxyl functionality (HTPB) were studied using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) in an isothermal mode. The cure reaction was monitored in-situ by FTIR spectroscopy by observing variation in intensity of epoxy, anhydride, and ester bands. The cure reaction mechanisms by which the network structure of epoxy was developed were discussed. Isothermal mode DSC measurements were performed at selected temperatures. The reaction followed an autocatalytic mechanism, and kinetic analysis was done by a phenomenological model developed by Kamal. Good fits were obtained between the autocatalytic model and the experimental data up to the vitrification state. Afterward, the reaction became diffusion controlled. The reaction during the later stages of cure was explained by introducing a diffusion factor, which agreed well with the kinetic data. The nature of the developing morphology of modified epoxies was analyzed by optical microscopy (OM) and small angle laser light scattering (SALLS) technique. The ultimate morphology of the cured blends was analyzed using scanning electron microscopy (SEM). The cure kinetics has been correlated with the developed morphology to get insight into the mechanism of reaction-induced microphase separation.

Curing Kinetics of Cyano Functionalized Benzoxazine

A multifunctional benzoxazine monomer (BZCN) was synthesized, which has several outstanding properties, such as high thermal stability and high glass transition. To better understand the curing kinetics of BZCN, isothermal differential scanning calorimetry measurements were used to determine the kinetic parameters and the kinetic models of the curing processes of benzoxazine monomer with cyano functionality. The result shows the mechanism of the curing reaction of BZCN exhibits autocatalytic model, but doesn’t meet nth-order model. Owing to the effects of catalysis of cyano functionality, the activation energy is 89.65KJ•mol-1and the total order of reaction is 1.84, which is quite different from that of normal benzoxazine. The theoretical calculations matched reasonably well with the experimental results.

Thermal cure kinetics of epoxidized linseed oil with anhydride hardener

The thermal cure kinetics of an epoxidized linseed oil with methyl nadic anhydride as curing agent and 1-methyl imidazole as catalyst was studied by differential scanning calorimetry (DSC). The curing process was evaluated by non-isothermal DSC measurements; three iso-conversional methods for kinetic analysis of the original thermo-chemical data were applied to calculate the changes in apparent activation energy in dependence of conversion during the cross-linking reaction. All three iso-conversional methods provided consistent activation energy versus time profiles for the complex curing process. The accuracy and predictive power of the kinetic methods were evaluated by isothermal DSC measurements performed at temperatures above the glass transition temperature of the completely cured mixture (T g∞ ). It was found that the predictions obtained from the iso-conversional method by Vyazovkin yielded the best agreement with the experimental values. The corresponding activation energy (E a) regime showed an increase in E a at the beginning of the curing which was followed by a continuous decrease as the cross-linking proceeded. This decrease in E a is explained by a diffusion controlled reaction kinetics which is caused by two phenomena, gelation and vitrification. Gelation during curing of the epoxidized linseed/methyl nadic anhydride system was characterized by rheological measurements using a plate/plate rheometer and vitrification of the system was confirmed experimentally by detecting a significant decrease in complex heat capacity using alternating differential scanning calorimetry (ADSC) measurements.

Monitoring cure in epoxies containing carbon nanotubes with an optical‐fiber Fresnel refractometer

AbstractFresnel refractometry was used to follow the changes in the refractive index during the isothermal curing of a simple difunctional epoxy resin, a commercial formulated multifunctional epoxy blend, and the same two resin systems modified by the addition of multiwalled carbon nanotubes. The refractive index change, monitored via an embedded optical fiber, was correlated with the degree of cure established from isothermal differential scanning calorimetry measurements. A good correlation was established in all cases, indicating the potential of the technique to be used for online cure monitoring in thermosetting resin carbon nanocomposites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Phase stability and consolidation of glassy/nanostructured Al85Ni9Nd4Co2alloys

Al85Ni9Nd4Co2metallic glass/nanostructured ribbons and powders were used as starting materials for producing bulk amorphous/nanostructured Al-based alloys. Glassy ribbons were obtained by melt spinning at wheel surface velocities ranging from 5 to 37 m/s. The amorphous ribbons exhibited a supercooled liquid region of ∼20 K, a reduced glass transition temperature of ∼0.47 and γ ∼ 0.328. Mechanical alloying of the elemental powder mixture did not lead to amorphization. However, amorphous powders obtained by milling the glassy ribbons for 9 h exhibited a thermal stability similar to the initial ribbons. Isothermal differential scanning calorimetry measurements were used to determine the consolidation parameters of the glassy powders. Consolidation at 513 K by uniaxial hot pressing and hot extrusion indicated that the former method leads to bulk glassy samples, whereas the latter one yields nanostructured α-Al/glassy matrix composites.

Kinetic parameters of unsaturated polyester resin modified with poly(ε -caprolactone)

The curing of the neat unsaturated polyester resin (UP) with benzoyl peroxide (initiator) as well as the curing of UP modified with two poly(e-caprolactone) PCL samples (PCL2 and PCL50) of different molecular masses (M n=2⋅103 and M n=5⋅104, respectively), were investigated by non-isothermal differential scanning calorimetry (DSC), at different heating rates. The activation energy was determined from the variation of the peak exotherm temperature, T peak, upon heating rate. Besides, the degree of conversion (α) was obtained from isothermal DSC measurements at 80°C at different curing times for neat UP, UP+ PCL2 and UP+PCL50. Kinetic parameters were deduced assuming the n th order reaction kinetic model for neat UP, UP+PCL2 and UP+PCL50 systems.

Thermal stability and crystallization kinetics of Mg–Cu–Y–B quaternary alloys

The thermal stability and crystallization kinetics of the Mg 65Cu 25Y 10 and Mg 65Cu 25− x Y 10B x (numbers indicate at.%, x = 1–10) amorphous alloys are examined and compared in terms of non-isothermal and isothermal differential scanning calorimetry measurements. It is demonstrated that, with the minor addition of 1–5 at.% B, the incubation time for crystallization is prolonged and the activation energy is increased, suggesting an improvement of the thermal stability. Nevertheless, the overall nucleation and growth characteristics of the parent and B-additive alloys are similar. The minor B element appears to impose resistance to the crystallization of the major Mg 2Cu phase, possibly by blocking the path of Mg and Cu diffusion. With further addition of B upto 10 at.%, secondary crystallization phases such as MgB 4 or YB would be induced and thus lower the thermal stability.

Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS2 in Ge0.38S0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS2 and low temperature β-GeS2 polymorphs has been observed over a relatively broad range of temperatures, i.e. 420<T<494°C that correspond to viscosity of supercooled melt: 3×109>η>8×105 Pas. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS2 under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS2. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500°C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n≅4. The value of activation energy of nucleation estimated from these experiments EN=434 kJmol−1 is comparable with the activation energy of viscous flow in supercooled Ge0.38S0.62 melt (Eη=478 kJmol−1). A more complex eutectic crystallization involving both GeS2 and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.