Search Results

You are looking at 41 - 50 of 1,449 items for :

  • "thermal behaviors" x
  • Refine by Access: All Content x
Clear All

Abstract  

The thermal behavior of synthetic schoenite (K2SO4·MgSO4·6H2)) during heating has been studied by thermal methods. The temperatures of dehydration and decomposition of schoenite have also been determined by DTA, TG and DSC. The thermal reaction equations and the X-ray power diffraction results of the products have been given and the corresponding kinetic parameters have also been obtained.

Restricted access

Abstract  

This paper describes the thermal behaviour of blends of bisitaconimide (I) and bisnadimide (N) resins of similar structures. Bisitaconimides/bisnadimides based on 4,4'-diaminodiphenyl ether (E);2,2'-bis[4-(4-aminophenoxy)phenyl]propane (B); 1,3-bis(4-aminophenyl)benzene(R) and 1,4-bis(4-aminophenyl)benzene (H) were prepared and were designated as E-I/E-N; B-I/B-N; R-I/R-N and H-I/H-N respectively. Itaconimides had lower melting points and curing temperatures than that of corresponding nadimides. The blends of bisitaconimides and bisnadimides were prepared in the ratios of1:3, 1:1, 3:1 by solution mixing (chloroform/acetone). A decrease in the melting point and characteristic curing temperatures was observed in the blends. Thermal stability of cured resin blends was only marginally affected by the blend composition.

Restricted access

Abstract  

The thermal behaviour of H4SiW12O4024.8H2O (SiW12) was investigated by using DTA, TG and FTIR. Endothermic effects were observed at 40, 98 and 217C, corresponding to the fusion of SiW12 in its own crystallization water, boiling of the solution and decomposition of the remaining tetrahydrate into anhydrous SiW12, respectively. The mass of the sample remained constant on heating from about 250 to 400C. Subsequently, it slowly decreased and reached a constant value at about 500C. At 526C a DTA peak appeared. There was an abrupt change in the FTIR spectrum of the sample heated to 550C. The typical spectrum of the Keggin unit vanished and new bands at 807.5 and 1030 cm−1 indicated the presence of free WO3 and SiO2, respectively.

Restricted access

The urea complex of copper was synthesized and its structure was established by Fourier transform infrared (FTIR), electron spin resonance (ESR) and atomic absorption spectroscopy and elemental analysis to be Cu(urea)4Cl2. The thermal behaviour of this complex has been studied by thermogravimetry and differential thermal analysis and FTIR and ESR. Thermal analysis shows that the decomposition of the complex occurs in four stages of weight loss of different intermediates followed by three endothermal effects. The complex is thermally stable up to 428 K. The ESR and FTIR behaviour of the Cu(II)-urea complex during thermolysis was studied between 428 and 633 K. The experimental results suggest that in this temperature range the complex decomposition occurred forming thermodynamically stable regions of Cu(II) which are ferromagnetically coupled.

Restricted access

Abstract  

The topic of the present work is to study the thermal behavior of phenitoine and pharmaceuticals by means of kinetic parameters determined in non-isothermal conditions. The TG/DTG data were obtained at four heating rates. These data were processed by the following methods: Friedman (FR), Budrugeac-Segal (BS) and the modified non-parametric kinetics (Sempere-Nomen). The main conclusions of the kinetic study are The FR method is versatile, but the values of the kinetic parameters are not certain, especially by multistep processes. The BS method offer a non-variant part of the activation energy, but the kinetic description is only formal. The NPK method is able to discriminate between two or more steps of a complex process. In our case, there are a preponderant process (more than 70% of the explained variance). By the NPK method there is a non-speculative separation of the temperature, respective conversion degree dependence of the reaction rate.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: Fillipe V. Rocha, Carolina V. Barra, Silmar J. S. Franchi, Adelino V. G. Netto, Antonio E. Mauro, and Regina C. G. Frem

-thiocarbamoyl-3,5-dimethylpyrazole). However, systematic studies on the thermal decomposition of this class of compounds have not been described yet in the literature. In pursing our interest on thermal behavior [ 11 – 16 ], structural aspects [ 17 – 19

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: F. Giordano, A. Rossi, R. Bettini, A. Savioli, A. Gazzaniga, and Cs. Novák

Abstract  

The thermal behavior of binary mixtures of paracetamol and a polymeric excipient (microcrystalline cellulose, hydroxypropylmethylcellulose and cross-linked poly(vinylpyrrolidone)) was investigated. The physical mixtures, ranging from 50 to 90% by mass of drug, were submitted to a heating-cooling-heating program in the 35–180C temperature range. Solid-state analysis was performed by means of differential scanning calorimetry (DSC), hot stage microscopy (HSM), micro-Fourier transformed infrared spectroscopy (MFTIR), and scanning electron microscopy (SEM). The polymeric excipients were found to address in a reproducible manner the recrystallization of molten paracetamol within the binary mixture into Form II or Form III. The degree of crystallinity of paracetamol in the binary mixtures, evaluated from fusion enthalpies during the first and second heating scans, was influenced by the composition of the mixture, the nature of the excipient and the thermal history. In particular, DSC on mixtures with cross-linked poly(vinylpyrrolidone) and hydroxypropylmethylcellulose with drug contents below 65 and75%, respectively, evidenced the presence only of amorphous paracetamol after the cooling phase. Microcrystalline cellulose was very effective in directing the recrystallization of molten paracetamol as Form II.

Restricted access

Abstract  

The complexes of the type SnCl4(HL)·EtOH and SnCl2L2 (HL 1 : the Schiff base resulted in 1:1 condensation of isatin and aniline; HL 2 : the Schiff base resulted in 1:1 condensation of isatin and p-toluidine) have been synthesized and characterized. The thermal analysis of the new ligands and complexes has evidenced the thermal intervals of stability and also the thermal effects that accompany them. The Schiff bases thermal transformations consist in phase transitions, Carom–N bond cleavage and thermolysis processes. The different nature of the complexes generates their different thermal behaviour. The complexes lead in three steps to SnO2 and in all cases the Schiff bases degradation generates a pyrrolidone-coordinated derivative. As for the SnCl4(HL)·EtOH complexes, the SnCl4 formed during the last step is involved in two competitive processes, one consists in their volatilisation while the other one leads to SnO2. As result the SnO2 residue is smaller than the theoretically expected.

Restricted access

Thermal behaviour of rigid polyurethane foams

I. A study of rigid polyurethane foam containing additive antipyrenes

Journal of Thermal Analysis and Calorimetry
Authors: K. Gjurova, Chr Bechev, K. Troev, and G. Borisov

By means of a combined thermal analysis technique, the thermal behaviour of rigid polyurethane foams containing additive antipyrenes was studied. The presence of phosphorus- and phosphorus/chlorine-containing antipyrenes based on phosphate and phosphonate and a combination of them led to decreases in the rates of heat and weight loss during heating. More steps of decomposition were formed and the interval of decomposition was widened and shifted to higher temperatures. Some thermal characteristics make it possible to predict the optimum antipyrene compositions and concentrations with minimum time, labour and material consumption under laboratory conditions.

Restricted access

. Magaraphan et al. [ 6 ] studied the preparation, structure, properties, and thermal behavior of rigid-rod polyimide/montmorillonite nanocomposites. Castelein et al. [ 7 ] focused on the influence of heating rate on the thermal behavior and mullite formation

Restricted access