Thermal weight loss of silica-poly(vinyl acetate) (PVAc) sol-gel composites

Thermogravimetric analyses of sol-gel derived silica and silica-poly(vinyl acetate) (PVAc) materials show that the loss in weight between 35 and 900°C can be attributed to three distinct reactions. Samples were prepared by dissolving the reactants tetraethyl orthosilicate (TEOS), poly(vinyl acetate) (PVAc), and water in mixtures of ethanol and formamide. The lowest temperature weight loss is due to the decomposition/removal of the solvents, while the intermediate weight loss corresponds to decomposition of the PVAc. The highest temperature weight loss is related to the dehydroxylation of the silica surface. The relative amounts of ethanol and formamide have a considerable effect on processing time, drying behavior, and the resulting thermal behavior of the gels.The financial support of the Center for Ceramic Research, a New Jersey Commission on Science and Technology Center, is greatly appreciated.     

Thermogravimetric analysis of organoclays intercalated with the surfactant octadecyltrimethylammonium bromide

Summary High resolution thermogravimetric analysis has been used to study the thermal decomposition of montmorillonite modified with octadecyltrimethylammonium bromide. Thermal decomposition occurs in 4 steps.The first step of mass loss is observed from ambient to 100°C temperature range and is attributed to dehydration of adsorbed water. The second step of mass loss occurs between 87.9 to 135.5°C temperature range and is also attributed to dehydration of water hydrating metal cations such as Na⁺. The third mass loss occurs between 179.0 and 384.5°C; it is assigned to the loss of surfactant. The fourth step is ascribed to the loss of OH units due to dehydroxylation of the montmorillonite and takes place between 556.0 and 636.3°C temperature range. These TG steps are related to the arrangement of the surfactant molecules intercalating the montmorillonite. Changes in the basal spacing of the clay with surfactant are followed by X-ray diffraction. Thermal analysis provides an indication of the stability of the organo-clay.     

Thermal decomposition of natural and synthetic plumbojarosites: Importance in ‘archeochemistry’

Plumbojarosite and argentoplumbojarosite were sources of lead and silver in ancient and medieval times. The understanding of the chemistry of the thermal decomposition of these minerals is of vital importance in ‘archeochemistry’. The thermal decomposition of plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. Three mass loss steps are observed at 376, 420 and 502 °C. These are attributed to dehydroxylation, loss of sulphate occurs at 599 °C, and loss of oxygen and formation of lead occurs at 844 and 953 °C. The temperatures of the thermal decomposition of the natural jarosite were found to be less than that for the synthetic jarosite. This is attributed to a depression of freezing point effect induced by impurities in the natural jarosite. Raman spectroscopy was used to study the structure of plumbojarosite. Plumbojarosites are characterised by stretching bands at 1176, 1108, 1019 and 1003 cm⁻¹ and bending modes at 623 and 582 cm⁻¹. Changes in the molecular structure during thermal decomposition were followed by infrared emission spectroscopy. The technique shows the loss of intensity in the hydroxyl stretching region attributed to dehydroxylation. Loss of sulphate only occurs after dehydroxylation. Lead is formed at higher temperatures through oxygen evolution.     

Thermal degradation behaviour of aromatic poly(ester-amide) with pendant phosphorus groups investigated by pyrolysis-GC/MS

The thermal stability of a novel phosphorus-containing aromatic poly(ester-amide) ODOP-PEA was investigated by thermogravimetric analysis (TGA). The weight of ODOP-PEA fell slightly at the temperature range of 300-400 °C in the TGA analysis, and the major weight loss occurred at 500 °C. The structural identification of the volatile products resulted from the ODOP-PEA pyrolysis at different temperatures was performed by pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS). The P-C bond linked between the pendant DOPO group and the polymer chain disconnected first at approximately 275 °C, indicating that it is the weakest bond in the ODOP-PEA. The P-O bond in the pendant DOPO group was stable up to 300 °C. The cleavage of the ester linkage within the polymer main chain initiated at 400 °C, and the amide bond scission occurred at greater than 400 °C. The structures of the decomposition products were used to propose the degradation processes happening during the pyrolysis of the polymer.     

Thermal stability of crandallite CaAl<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>5</Subscript>·(H<Subscript>2</Subscript>O)

Thermogravimetry combined with evolved gas mass spectrometry has been used to characterise the mineral crandallite CaAl₃(PO₄)₂(OH)₅·(H₂O) and to ascertain the thermal stability of this ‘cave’ mineral. X-ray diffraction proves the presence of the mineral and identifies the products of the thermal decomposition. The mineral crandallite is formed through the reaction of calcite with bat guano. Thermal analysis shows that the mineral starts to decompose through dehydration at low temperatures at around 139 °C and the dehydroxylation occurs over the temperature range 200–700 °C with loss of the OH units. The critical temperature for OH loss is around 416 °C and above this temperature the mineral structure is altered. Some minor loss of carbonate impurity occurs at 788 °C. This study shows the mineral is unstable above 139 °C. This temperature is well above the temperature in the caves of 15 °C maximum. A chemical reaction for the synthesis of crandallite is offered and the mechanism for the thermal decomposition is given.     

Thermochemical Properties of Chemically Modified Zeolite

Zeolites chemically modified with 1, 4 or 6 M aqueous solutions of NaOH were studied by DTA, TG and ETA (emanation thermal analysis) in the temperature range 201–200°C. The structural changes in the modified zeolites at room temperature and in the modified zeolites annealed at 1000°C were studied by XRD analysis. Thermal analysis demonstrated dehydration, dehydroxylation, structural changes and a glass transition. A gradual loss in crystallinity of the chemically modified zeolites was also observed. XRD analysis revealed structural changes caused by chemical treatment and also by annealing.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal decomposition of vesuvianite

The thermal decomposition of vesuvianite was studied by means of thermal, FTIR and X-ray methods. It was found that two structural forms of vesuvianite, a high-temperature (disordered) and a low-temperature (ordered) one, differ distinctly in the mechanism of their decomposition (dehydroxylation). Dehydroxylation of low vesuvianites begins at lower temperatures (ca. 900°C), and the strong endothermic peak with maximum at ca. 1020°C is usually followed by an exothermic peak. Dehydroxylation of high vesuvianites begins at ca. 1000°C, and the OTA curve usually displays two endothermic peaks not followed by an exothermic effect. The crystallization products of vesuvianite are grossular, gehlenite and anorthite. Vesuvianite melts in the temperature range 1100–1200°C.The authors thank Mr. A. Gawel for kindly recording the X-ray powder diffractograms. This study was supported by KBN grant No 6 P201 018 04.     

Thermal decomposition of stichtite

The mineral stichtite was synthesised and its thermal decomposition measured using thermogravimetry coupled to an evolved gas mass spectrometer. Mass loss steps were observed at 52, 294, 550 and 670°C attributed to dehydration, dehydroxylation and loss of carbonate. The loss of carbonate occurred at higher temperatures than dehydroxylation.     

Thermal decomposition of the vivianite arsenates—implications for soil remediation

The presence of arsenate compounds in soils and mineral dump leachates is common. One potential method for the removal of the arsenates from soils is through thermal treatment. High-resolution thermogravimetric analysis has been used to follow this thermal decomposition of selected vivianite arsenates. This decomposition occurs as a series of steps. The first two steps involve dehydration with 6 mol of water lost in the first step and two in the second. The third major weight loss step occurs in the 750-800 °C temperature range with de-arsenation. The application of infrared emission spectroscopy confirms the loss of water by around 250 °C and the loss of arsenic as arsenic pentoxide is observed by the loss of AsO stretching bands at around 826 cm⁻¹. Thermal activation of arsenic contaminated soils may provide a method of decontamination.     

A thermogravimetric study of the alunites of sodium, potassium and ammonium

Thermogravimetry in tandem with mass spectrometry has been used to characterise the thermal decomposition of synthetic alunites of potassium, sodium and ammonium. Three mechanisms of decomposition are observed (a) dehydration, (b) dehydroxylation and (c) desulphation. The thermal decomposition of the three alunites is different. For NH₄-alunite, an additional process of de-ammoniation is observed which occurs simultaneously with dehydration. Dehydroxylation takes place in a series of four steps. De-sulphation occurs for K-alunite at 680 °C in a single step in comparison with Na and NH₄ alunites where de-sulphation is observed in a series of four steps. The temperature of desulphation is cation dependent. The thermal decomposition is not completed until around 800 °C.     

Reducing the formation of six-membered ring ester during thermal degradation of biodegradable PHBV to enhance its thermal stability

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) has attracted the attention of academia and industry because of its biodegradability, biocompatibility, thermoplasticity and plastic-like properties. However, PHBV is unstable above 160 °C during melt processing at a temperature above the melting temperature, which restricts practical applications as a commodity material. It is widely believed that thermal degradation of PHBV occurs almost exclusively via a random chain scission mechanism involving a six-membered ring transition state. Here, 2,2′-bis(2-oxazoline) (BOX) was selected to modify PHBV to control the formation of six-membered ring ester during thermal degradation. The resulting hydroxyl-terminated PHBVs (HT-PHBVs) had improved thermal stability due to a decrease in the negative inductive effect of the neighboring group of methylene groups at the β-position to the ester oxygen, and a decrease in the electron-denoting effect of substituent group of carbon atoms at α-position to the ester oxygen. The optimal reaction temperature and time were determined to be 95 °C and 6 h, respectively. Compared with those of original PHBV, the temperature determined at 5% weight loss (T_5%), the initial decomposition temperature (T₀), the maximum decomposition temperature (T_max), the complete decomposition temperature (T_f) of HT-PHBV prepared under the optimal conditions increased by 31, 24, 19 and 19.1 °C, respectively.     

Thermal decomposition of syngenite, K2Ca(SO4)2·H2O

The thermal decomposition of syngenite, K₂Ca(SO₄)₂·H₂O, formed during the treatment of liquid manure has been studied by thermal gravimetric analysis, differential scanning calorimetry, high temperature X-ray diffraction (XRD) and infrared emission spectroscopy (IES). Gypsum was found as a minor impurity resulting in a minor weight loss due to dehydration around 100 °C. The main endothermic dehydration and decomposition stage of syngenite to crystalline K₂Ca₂(SO₄)₃ and amorphous K₂SO₄ is observed around 200 °C. The reaction involves a solid-state re-crystallisation, while water and the K₂SO₄ diffuse out of the existing lattice. The additional weight loss steps around 250 and 350 °C are probably due to presence of larger syngenite particles, which exhibit slower decomposition due to the slower diffusion of water and K₂SO₄ out of the crystal lattice. A minor endothermic sulphate loss around 450 °C is not due to the decomposition of syngenite or its products or of the gypsum impurity. The origin of this sulphate is not clear.     

Thermal evolution of a slate

The thermal evolution of a slate rock sample (Berja, Almería, Spain) has been studied. The phase minerals identified in this sample were mica (illite), chlorite (clinochlore) and quartz as major components, with minor microcline, iron oxide and a mixed-layer or interstratified phase (montmorillonite-chlorite). This slate is highly silico-aluminous (48.33 mass% silica, 22.04 mass% alumina), and ca. 20 mass% of other elements, mainly Fe₂O₃ (8.35 mass%), alkaline-earths and alkaline oxides. Two main endothermic DTA effects, centered at 640 and 730°C, were observed. The more important contribution of total mass loss (7.15 mass%) was found between 500–900°C, with two DTG peaks detected at 630 and 725°C. All these effects were associated to the dehydroxylation of structural OH groups of 2:1 layered silicates mixed in the slate. The dehydroxylation of the layered silicates evidenced by dilatometry, produced a rapid increase of expansion between 600–800°C. The thermal evolution of the slate upper 800°C indicated the first sintering effects associated to shrinkage, which is also favoured by its low particle size (average 23 μm) and the presence of a liquid or vitreous phase as increasing the heating temperature. The application of thermal diffractometry to the slate sample allowed to study the formation of dehydroxylated crystalline phases from the layered silicates after heating. At 1000°C, β-quartz, dehydroxylated illite, iron oxide, relicts of microcline and the vitreous phase were present in the sample. All these results are interesting to know the thermal behaviour of a complex mineral mixture as identified in the slate.     

Controlled rate thermal analysis of sepiolite

Controlled rate thermal analysis (CRTA) technology offers better resolution and a more detailed interpretation of the decomposition processes of a clay mineral such as sepiolite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal changes in the sepiolite as the sepiolite is converted to an anhydride. In the dynamic experiment two dehydration steps are observed over the ~20–170 and 170–350 °C temperature range. In the dynamic experiment three dehydroxylation steps are observed over the temperature ranges 201–337, 337–638 and 638–982 °C. The CRTA technology enables the separation of the thermal decomposition steps.     

Thermal analysis of beaverite in comparisonwith plumbojarosite

The thermal decomposition of beaverite and plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. The mineral beaverite Pb(Fe,Cu)₃(SO₄)₂(OH)₆ decomposes in three stages attributed to dehydroxylation, loss of sulphate and loss of oxygen, which take place at 376 and 420, 539 and 844°C. In comparison three thermal decomposition steps are observed for plumbojarosite PbFe₆(SO₄)₄(OH)₁₂ at 376, 420 and 502°C attributed to dehydroxylation; loss of sulphate occurs at 599°C; and loss of oxygen and formation of lead occurs at 844 and 953°C. The temperatures of the thermal decomposition of the natural plumbojarosite were found to be less than that for the synthetic jarosite. A comparison of the thermal decomposition of plumbojarosite with argentojarosite is made. The understanding of the chemistry of the thermal decomposition of minerals such as beaverite, argentojarosite and plumbojarosite and related minerals is of vital importance in the study known as ‘archeochemistry’.     

Thermal and mechanical behaviour of flexible poly(vinyl chloride) mixed with some saturated polyesters

Four saturated polyesters poly(hexamethylene adipate), poly(ethylene adipate), poly(hexamethylene terephthalate) and poly(ethylene terephthalate) were prepared. The resulting materials were characterized by IR and ¹H NMR, end group analysis and gel permeation chromatography. The effect of blending these polyesters (5 and 10%) with poly(vinyl chloride) (PVC) in the melt was investigated in terms of changes in the thermal behaviour of PVC by studying the weight loss after 50 min at 180 °C, colour changes of the blend before and after aging for one week at 90 °C, the variation in glass transition temperature and the initial decomposition temperature. The results gave proof for the stabilizing role played by the investigated polyesters against the thermal degradation of PVC. The best results are obtained when PVC is mixed with 5% aliphatic polyesters rather than with aromatic ones. This is well illustrated not only from the increase in the initial decomposition temperature (IDT), but also from the decrease of % weight loss and from the lower extent of discolouration of PVC, which is a demand for the application of the polymer. It was also found that blending PVC with 5% of the four investigated polyesters before and after aging for one week at 90 °C gave better mechanical properties even than that of the unaged PVC blank.     

Synthesis of Porous Magnesium Oxide by Thermal Decomposition of Basic Magnesium Carbonate

On calcination of basic magnesium carbonate at 20-800°C, an induction period was observed in the initial stage of thermal decomposition. At the decomposition degree higher than 20% of the weight loss, the specific surface area varied in proportion with the decomposition degree, which points to equal rates of gas liberation and recrystallization of intermediate products into an active oxide. The individual phase of magnesium oxide alone is identified at the minimal temperature of 500°C. As the calcination temperature and time increase, the MgO phase is structured further, which decreases in its specific surface area and basicity.     

Detection of four different OH-groups in ground kaolinite with controlled-rate thermal analysis

The thermal behaviour of mechanochemically treated kaolinite has been investigated under dynamic and controlled rate thermal analysis (CRTA) conditions. Ten hours of grinding of kaolinite results in the loss of the d(001) spacing and the replacement of some 60% of the kaolinite hydroxyls with water. Kaolinite normally dehydroxylates in a single mass loss stage between 400 and 600°C. CRTA technology enables the dehydroxylation of the ground mineral to be observed in four overlapping stages at 385, 404, 420 and 433°C under quasi-isobaric condition in a self-generated atmosphere. It is proposed that mechanochemical treatment of the kaolinite causes the localization of the protons when the long range ordering is lost.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal treatment of a modified alkoxide gel precursor for the preparation of the YBa2Cu4O8 superconductor

A precursor of Y-Ba-Cu oxides was prepared by a modified alkoxide sol-gel method and its thermal decomposition in air was studied by on-line coupled TG-FTIR and High Resolution Thermogravimetric measurements. A continuous more or less stepwise weight loss was observed between room temperature and 600°C at which all organic compounds had evolved and were progressively oxidized as the temperature increased leaving only Y and Cu oxides and bariumcarbonate. Between 700 and 800°C a final weight loss was observed due to the decomposition of bariumcarbonate into oxide.The authors wish to thank Mrs. Martine Vanhamel for her technical assistance. In particular, thanks are expressed to TA Instruments for giving the possibility to use the High Resolution thermal analyser. M. K. Van Bael is an aspirant of the Belgian National Fund for Scientific Research N.F.W.O.     

Kinetics of thermal dehydroxylation of aluminous goethite

The kinetics of dehydroxylation of synthetic aluminous goethite was studied using isothermal and non-isothermal thermogravimetry. The complete isothermal dehydroxylation can be described by the Johnson-Mehl equation with up to three linear regions in plots of lnln [1/(1–y)]vs. Int Kinetics for the initial stage of dehydroxylation changed from diffusion to first-order through the temperature range 190 to 260°C. The rate of dehydroxylation was reduced by Al-substitution and increased with temperature. Activation energy for dehydroxylation, calculated from the time to achieve a given dehydroxylation extent, varied depending on the extent of dehydroxylation and Al-substitution. Non-stoichiometric OH existed in goethite and some remained in hematite after the complete crystallographic transition.