Non-isothermal Calorimetry Research Articles

Construction of epoxy composites with double alternating “rigid-flexible” structures: Curing kinetics and thermal/mechanical properties

The curing rate, post-curing modulus, and strength of epoxy resin are key performance indicators for epoxy resin-based composites. Optimizing these factors is essential for improving production processes and expanding practical applications. Functionalized rigid-flexible bilayer transition interfaces between nano-ZnO and epoxy resins were developed using chitosan (CS) and 1,2,3-Tris(glycidyloxy)propane (GTE). Their impact on the curing characteristics, rheology, and mechanical properties of the composites was investigated. The activation energies of pure EP and systems containing ZnO, ZnO-CS, and ZnO-CS-GTE were studied using non-isothermal calorimetry, obtaining 53.604 KJ/mol, 52.548 KJ/mol, 56.66 KJ/mol, and 51.482 KJ/mol, respectively. ZnO-CS-GTE showed a notably stronger promotional effect. Nano-ZnO inhibited gelation due to filler aggregation at 80 °C for 4 min, while ZnO-CS-GTE promoted gelation in just 2.6 min owing to its functional groups, achieving a modulus near 1 GPa after curing. Mechanical studies revealed that the tensile, flexural, and compressive strengths of the ZnO-CS-GTE system increased by 48.52 %, 32.67 %, and 45.01 %, respectively, with flexural and compressive moduli rising by 15.14 % and 15.76 %. The rigid-flexible interface enhanced the dispersion of epoxy resin and strengthened interfacial adhesion.

‘Cure Index’ for thermoset composites

Almost all properties of thermoset composites are more or less dependent on the situation of network formation in a system in which fillers/additives may function as cure facilitator or inversely hinder curing reactions as barriers between cure moieties or reactants by restricting the intermolecular collisions. Nonisothermal calorimetry provides useful data to quantify curing phenomenon in thermoset composites, but there is a need for a universal criterion by which one can simply give a unique signature to a thermoset composite cured at a given heating rate. This short communication proposes a dimensionless criterion, hereafter known as ‘Cure Index’, for typical epoxy-based systems containing 0D nanoparticles. The proposed universal criterion is calculated and compared for well-defined thermoset composites in order to establish the worth and powerfulness of the defined 'Cure Index'.

Dsc studies on the decomposition of chemical blowing agents based on citric acid and sodium bicarbonate

This paper investigates the decomposition kinetics of citric acid, sodium bicarbonate and their combinations in masterbatches with low density polyethylene in view of their use in injection molding with simultaneous polymer foaming. The thermal decomposition was studied by isothermal and non-isothermal calorimetry to separate the various chemical events. A multi-step autocatalytic model was used to determine the kinetic parameters. Results show that for sodium bicarbonate compounds, the kinetics can be modeled by first-order equations while for citric acid compounds it shows two steps of decomposition due to reaction intermediates. The parameters obtained for the mono-component masterbatches can be used to model the kinetics of compounds containing a mix of sodium bicarbonate and citric acid with the same multi-step coupled equations. Experiments and modelisation show that the reaction in the masterbatch proceeds in well separated steps corresponding to each species. In particular, at high heating rates such as those encountered during melting in the injection molding machine, the citric acid decomposition occurs in a single step making the analysis of the kinetics and the modelling of the overall foaming process more simple.

Analyses of the thermal decomposition of urea nitrate at high temperature

The decomposition of urea nitrate (UN) was studied using adiabatic and non-isothermal calorimetry techniques. Gas species released were identified and quantified in situ using TG-infrared spectroscopy/mass spectrometry and molar proportions of these gases were evaluated. A decomposition mechanism at high temperature was proposed based on the nature and sequences of gaseous species observed combined with literature data on decomposition of intermediate products formed. Non-isothermal decomposition kinetics of urea nitrate were measured using variable heating rates to give activation energies E/kJ mol −1 = 206 and 113 with preexponential factors Ln Z/min −1 = 47 and 21, in a closed and open system, respectively. In these systems the major UN decomposition step is strongly coupled to an endothermic dissociation reaction. Species remaining after this exothermic decomposition showed only minor exothermicity at higher temperatures. This is contrasted with the onset to adiabatic decomposition which occurred ∼30 °C below the apparent melting point (155–156 °C), and where solid (condensed) species are available.

On the isothermal kinetics analysis of transformations in metastable systems: combined use of isothermal and non-isothermal calorimetry

A procedure to optimize the isothermal calorimetric data of very slow transformation processes of metastable systems is proposed. The method uses an experimental baseline to identify the transitory effects due to the equipment. Moreover, the combined use of isothermal and non-isothermal results is shown to be effective in overcoming the intrinsic problems of low signal and signal drift for such processes. The procedure has been applied to the analysis of the nanocrystallization kinetics of the Fe60Co18Nb6B16 alloy at different devitrification stages. Based on microstructural observations, an instantaneous growth approach was assumed and a phenomenological expression of the dependence of the nucleation frequency with both the transformed fraction and the temperature was obtained.

The State of Aggregation of Casein Affects the Storage Stability of Amorphous Sucrose, Lactose, and Their Mixtures

The effect of the state of aggregation of casein (micellar or non-micellar, as milk protein concentrate [MPC] or sodium caseinate [Na-caseinate], respectively) on water sorption, plasticization, and crystallization of freeze-dried matrices containing sucrose, lactose or their blends were studied. The Guggenheim–Anderson–de Boer (GAB) equation satisfactorily fitted to the water sorption data. In most cases, sugar crystallization—studied by water sorption behavior, x-ray diffraction, and non-isothermal calorimetry—occurred significantly slower in systems containing Na-caseinate compared to MPC. The type of casein did not affect the temperature range where the glass transition (Tg) was observed. Sugar/Na-caseinate mixtures showed higher instant crystallization temperatures (up to 20°C) than sugar/MPC mixtures. X-ray diffraction showed that: (a) crystallinity increased with increasing relative vapor pressure (RVP) > 44%; (b) lactose crystallized mainly as α-lactose monohydrate regardless of casein type; and (c) that sucrose crystals predominated the patterns of the sucrose/lactose mixtures. Results suggested that the way proteins organize in space (i.e., aggregation state) affected their interactions with neighboring sugar and water molecules, which led to differences in sugar crystallization behavior.

Effect of substitution of rare earth by mischmetal on the devitrification process of Al–X–Ni–Co (X = Y, Ce, Mm) alloys

The effect of substitution of Ce or Y by mischmetal (Mm) on the glass transition and the kinetics of crystallization of quaternary Al–Ni–Co–(Ce, Y, Mm) alloys was studied using isothermal and non-isothermal calorimetry and X-ray experiments. The alloy containing Mm shows more similarity to Ce containing alloy in all the studied properties than to Y containing alloy. The glass transition is less evident for Ce and Mm containing alloys than for the alloy containing Y. For the two former alloys, the first and the second crystallization stages are strongly overlapped and yield a fully crystallized alloy. Whereas for Y alloy, the first and the second stages are well resolved and the primary crystallization yields nanosized α-Al particles embedded in a residual amorphous matrix.

A Thermodynamic Study of the AC-Di-SOL-WATER and CLD-2-WATER Surface Interaction

AbstractAdsorption-desorption studies and Immersional calorimetry were used conjointly to characterize the surface properties of two disintegrants - Ac-Di-Sol and CLD-2. Adsorption-desorption Isotherms were utilized to calculate the Integral and different Gibb's free energies. Non-Isothermal calorimetry was employed to quantify the energies of Interaction between the adsorbate and the adsorbent and obtain the differential enthalpy. These Interactions were determined to be -7.5 and -7.8 kcal per mole for Ac-Di-Sol and CLD-2, respectively. The premature decrease in the differential enthalpies and entropies and the observed immersional and adsorption-desorption hystereses were hypothesized to be the result of an Irreversible swelling process. Differences were observed between CLD-2 and Ac-Di-Sol which were believed to be due to the higher degree of cross-linking In Ac-Di-Sol.