Search Results
Abstract
The thermal degradation of a sort of polyvinyl chloride was investigated. Complex processes for polyvinyl chloride degradation were evidenced. The kinetic analysis of dehydrochlorination and of subsequent processes was carried out. A change of mechanism was detected when dehydrochlorination goes to completion. The values of non-isothermal kinetic parameters determined by various methods are in a satisfactory agreement. The obtained results allowed some clarifications concerning the thermal degradation steps.
Abstract
The thermal degradation of sodium hyaluronate, xanthan and methylcellulose was evaluated by thermogravimetric and infrared analysis. Kinetic parameters such as activation energy and pre-exponential factor were determined considering the Ozawa and Freeman–Carroll methods. The results suggest changes in the degradation mechanism with the fraction of mass loss for both the studied polysaccharides. The activation energy values determined by the Freeman–Carroll method are higher than those obtained by the Ozawa method under the same conditions, probably because in the first method a first order reaction was assumed and the thermal history effects were eliminated since only one TG curve was used to determine the kinetic parameters. Low thermal stability was observed for polyanions e.g. sodium hyaluronate (Na-Hy) and xanthan(XT) in comparison with methylcellulose (MC) which is a neutral polysaccharide. By infrared spectroscopy, it was observed that at low temperatures there occured only the scission of the exocyclic groups for both polysaccharides and that the scission of strong links in the backbone occurred at high temperatures, in agreement with the kinetic parameters determined for the degradation reaction.
Kinetics of thermal degradation of polymers
Complementary use of isoconversional and model-fitting methods
Abstract
The thermal degradation of polymers has been studied quite extensively using thermogravimetric measurements. For the kinetic description, most of the times single rate heating data and model-fitting methods have been used. Since the thermal degradation of the polymers is a very complex reaction, the choice of a reliable model or a combination of kinetic models is very important. The advantages or the disadvantages of using a single heating rate or multiple heating rates data for the determination of the kinetic triplet have been investigated. Also, the activation energy has been calculated with the isoconversional and model-fitting methods. The reaction model was determined with the model-fitting method. The limits of all these procedures were investigated with experimental data of the thermal degradation of the poly(ethylene adipate) (PEAd).
Abstract
The lignin preparations isolated from pine and beech wood were subjected to a thermogravimetric analysis (TG). The lignin preparations were also used to obtain samples of different degrees of thermal degradation characterised by mass-losses in the interval from 10 to 60% of their initial mass. These samples were subjected to elementary analysis and the content of methoxy groups. It was observed that the content of these functional groups declined in products in which the degree of thermal degradation exceeded 30%, which corresponds to temperatures over 450°C.
The kinetic study of thermal degradation takes into account the validity of the Arrhenius equation. From TG data, the activation energy,E a and pre-exponential factor,A, are evaluated. These results are interpreted by using the ‘kinetic compensation effect’ as basis. A linear correlation between In(A) andE a is obtained in all cases studied. However, in a plot of the logarithm of the rate constant as a function of reciprocal temperature for the same series of reactions, the thermal oxidative degradations of Nylon-6 and PVC display a point of concurrence and one isokinetic temperature, whereas those of HIPS and PC do not. Therefore, in the thermal oxidative degradations of Nylon-6 and PVC a ‘true’ compensation effect occurs, which could be related to the bulk properties of metal oxides, such as different valence states, whereas for other polymers it displays only an ‘apparent’ compensation effect. This means that degradation is largely independent of the bulk properties of oxides, but may be related to the distribution of different kinds of active links in the polymer surface having different activation energies.
Abstract
The thermal degradation of copolymers based on butyl acrylate-methyl acrylate-acrylic acid used as acrylic pressure-sensitive adhesives, especially for bonding of plasticizer containing materials, has been investigated using thermogravimetry and pyrolysis-gas chromatography at 250°C. It was observed that during the pyrolysis of butyl acrylate-methyl acrylate-acrylic acid copolymers unsaturated monomers as methyl acrylate, methyl methacrylate, butyl acrylate and butyl methacrylate were formed. During the side-chain butyl acrylate-methyl-acrylate-acrylic acid-copolymer degradation the presence of methyl alcohol and butyl alcohol was observed.
Abstract
Thermal degradation of copolymers, prepared from glycidyl methacrylate and acrylonitrile in varying molar ratios using 2,2′-azobisisobutyronitrile as an initiator, was studied by thermogravimetry, derivative thermogravimetry, differential thermal analysis and mass spectrometry. The fragmentation patterns in the mass-spectra were interpretable by comparison with the known degradation patterns of the related materials. Thermal kinetic parameters, including activation energies and order of reaction of the degradation of the prepared copolymers, were calculated from their thermoanalytical data. These parameters suggest an overall increase in thermal stability with increasing content of acrylonitrile in the copolymers.
Abstract
The effect of molar mass and, in the case of dextran, of the degree of branching on the thermal degradation kinetics of dextran and pullulan was studied in the presence and absence of oxygen. Although the initial mass loss of the dextran samples occurred at higher temperatures than that of the pullulan samples, the overall thermal degradation activation energies were lower for dextran than for pullulan. In the case of dextran the thermal stability was found to decrease with molar mass and degree of branching. The molar mass of pullulan, in the range of 104 to 105 g/mol, appeared to have no significant influence on the thermal characteristics of the samples.
Influence of some minerals on the cellulose thermal degradation mechanisms
Thermogravimetic and pyrolysis-mass spectrometry studies
Abstract
The influence of different inorganic salts (MgCl2, ZnCl2, NiCl2 and H2PtCl6) on the primary mechanisms of cellulose thermal degradation has been conducted by using thermogravimetric (TG-DTG) and pyrolysis-mass spectrometry (Py-MS) analysis at low heating rate (10°C min-1) from ambient temperature to 500°C. The results clearly demonstrate that the used salts influence the primary degradation mechanisms. Furthermore, we can assume that some inorganic salts could be considered as specific catalysts and some others as inhibitors. MgCl2 promotes selectively initial low temperature dehydration as observed both by TG and Py-MS. ZnCl2 strongly changes the thermal behaviour of impregnated sample. The maximum mass loss rate temperature is shifted to lower temperature and on the basis of our results we can conclude that ZnCl2 acts as catalyst in all primary degradation mechanisms. NiCl2 and H2PtCl6 do not modify significantly the cellulose thermal behaviour but change the composition of both produced gases and liquids suggesting that these minerals catalyse some secondary reactions.
Abstract
The thermal degradation of poly(vinyl acetate) (PVA), poly(vinyl alcohol) (PVAL), vinyl acetate-vinyl alcohol (VAVAL), vinyl acetate-vinyl-3,5-dinitrobenzoate (VAVDNB) and vinyl alcohol-3,5-dinitrobenzoate (VALVDNB) copolymers have been studied using differential thermal analysis (DTA) and thermogravimetry (TG) under isothermal and dynamic conditions in nitrogen. Thermal analysis indicates that PVA and PVAL are thermally more stable than VAVAL copolymers, being PVAL the most stable polymer. The presence of small amounts of vinyl-3,5-dinitrobenzoate (VDNB) in PVA or PVAL produces a marked decrease in the thermal stability of both homopolymers, being VALVDNB copolymers the less stable materials. The apparent activation energy of the degradative process was determined by the Kissinger and Flynn-Wall methods which agree well.
