Thermal decomposition and stability of fatty acid methyl esters in supercritical methanol

In recent years, non-catalytic supercritical processes for biodiesel production have been proposed as alternative environmentally friendly technologies. However, conditions of high temperature and pressure that occur while biodiesel is in supercritical fluid can cause fuel degradation, resulting in low yield. In this study, we performed the thermal decomposition of fatty acid methyl esters (FAMEs) in supercritical methanol at temperatures ranging from 325 °C to 420 °C and pressure of 23 MPa to investigate the degradation characteristics and thermal stability of biodiesel. The primary reactions we observed were isomerization, hydrogenation, and pyrolysis of FAMEs. The main pathway of degradation was deduced by analyzing the contents of degradation products. We found that if FAME has shorter chain length or is more saturated, it has higher thermal stability in supercritical methanol. All FAMEs remained stable at 325 °C or below. Based on these results, we recommend that transesterification reactions in supercritical methanol should be carried out below 325 °C (at 23 MPa) and 20 min, the temperature at which thermal decomposition of FAMEs begins to occur, to optimize high-yield biodiesel production.     

Study on depolymerization of polycarbonate in supercritical ethanol

The characteristics of depolymerization of PC in supercritical ethanol were investigated in the range of 483-563 K by using a high-pressure batch autoclave reactor. Based on the qualitative and quantitative analyses of the products, a depolymerization-reaction model was proposed to explain the reaction mechanism, i.e. random scission and ester exchange reaction occurred simultaneously during the process of depolymerizaition of PC. It was suggested that the process of depolymerization consisted of subcritical region, transitional region and supercritical region. It was indicated that PC degraded with slow decrease of molecular weight determined by GPC and with the conversion of 7.5% at 513 K in subcritical region. While in the supercritical region, the molecular weight of PC decreased quickly and degraded completely in 30 min at 563 K. Continuous-distribution kinetics could be used to describe the mechanism of polymer degradation and the energy of activation for the random scission of PC in the supercritical region was 97.2 kJ/mol. Moreover, PC could be degraded completely into diethyl carbonate (DEC) and bisphenol A (BPA) with the yields of 89% and 90%, respectively, in supercritical region.     

Phenanthrene degradation in subcritical water

Subcritical water (<374 °C and <221 bar) has unique characteristics such as dramatically decreased dielectric constant, surface tension, and viscosity with increasing temperature, allowing for dissolution and reaction of organics in high-temperature water to occur. Additionally, the dissociation constant of water at temperatures of 200-300 °C is three orders of magnitude greater than that of ambient water, which may also contribute to the reactivity of subcritical water with certain organic compounds. In this study, the degradation and oxidation of phenanthrene in subcritical water were investigated. Both deionized water and water with 3% hydrogen peroxide were used in the degradation and oxidation studies. The effect of temperature on degradation efficiency has been determined with a temperature range of 100-350 °C. When the temperature was increased from 150 to 350 °C, the amount of phenanthrene degraded varied from 6 to 243 μg in each milliliter of deionized water. However, these quantities were increased to 195 μg at 150 °C and 3680 μg at 350 °C in each milliliter of water with 3% hydrogen peroxide. Several degradation products including phenol, benzoic acid, and ketones were identified by using gas chromatography/mass spectrometry (GC/MS).     

Effect of the pre-treatment of the samples on the natural substances extraction from Helianthus annuus L. using supercritical carbon dioxide

The extraction of bioactive compounds from sunflowers (Helianthus annuus L.) with supercritical carbon dioxide has been studied. The samples were treated in four different ways and the effects of two factors (pressure and temperature) were investigated at 100, 500 bar and 35, 50 °C. The best yields were obtained using a high temperature and a high pressure (50 °C and 500 bar). The dry samples produced better extraction yields than the moist samples. The bioactivities of the extracts were compared for the samples treated in different ways. The best activity profiles were obtained for the moist samples extracted at 35 °C and 500 bar.     

Degradation of benzoic acid and its derivatives in subcritical water

In this research, the stability of benzoic acid and three of its derivatives (anthranilic acid, salicylic acid, and syringic acid) under subcritical water conditions was investigated. The stability studies were carried out at temperatures ranging from 50 to 350 °C with heating times of 10–630 min. The degradation of the benzoic acid derivatives increased with rising temperature and the acids became less stable with longer heating time. The three benzoic acid derivatives showed very mild degradation at 150 °C. Severe degradation of benzoic acid derivatives was observed at 200 °C while their complete degradation occurred at 250 °C. However, benzoic acid remained stable at temperatures up to 300 °C. The degradation products of benzoic acid and the three derivatives were identified and quantified by HPLC and confirmed by GC/MS. Anthranilic acid, salicylic acid, syringic acid, and benzoic acid in high-temperature water underwent decarboxylation to form aniline, phenol, syringol, and benzene, respectively.     

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.     

Extrinsic and intrinsic structural change during heat treatment of polyaniline

HCl doped polyaniline (PAni) was synthesized electrochemically and heat treated at 150 °C, 200 °C and 250 °C for 30 min in vacuum. Different intrinsic and extrinsic structural changes due to heat treatment were determined from XRD, TGA, FT-IR, conductivity and solubility measurement. When HCl doped PAni is subjected to heat treatment, different changes are taking place in the system like doping, dedoping (extrinsic), oxidation, chain scission, cross-linking and changes in crystal structure (intrinsic). Mechanism for doping, dedoping, oxidation, chain scission and cross-linking is proposed.     

Oxidative and enzymatic degradations of l-tyrosine based polyurethanes

Oxidative and enzymatic degradations of l-tyrosine based polyurethanes were studied for biomaterial applications. Oxidative degradation was performed with 0.1 M cobalt chloride (CoCl₂) in hydrogen peroxide solutions at 37 °C and the degradation was assessed by ATR-FTIR. Results indicate that polyurethane with polyethylene glycol (PEG) shows soft segment degradation while polyurethane based on polycaprolactone (PCL) shows hard segment degradation. Enzymatic degradation of the polyurethanes was studied using proteolytic enzyme α-chymotrypsin in phosphate buffer solution (pH 7.4) at 37 °C. The enhanced degradability of l-tyrosine based polyurethanes is due to both the presence of amino acid based chain extender and the action of enzyme. The changes in the morphology of polyurethanes were analyzed by SEM. The results of the degradation study were correlated to the structure of the polyurethanes.     

The thermal degradation of poly(vinyl acetate) and poly(ethylene-co-vinyl acetate), Part I: Experimental study of the degradation mechanism

The thermal degradation mechanism of poly(vinyl acetate) (PVAc) and poly(ethylene-co-vinyl acetate) (EVA) copolymers was investigated with solid-state NMR, thermogravimetry coupled with mass spectrometry and differential thermal analysis. Between 300 and 400 °C acetic acid is eliminated (deacetylation), leaving a highly unsaturated residue or polyene. The deacetylation of PVAc is autocatalytic. Upon incorporation of ethylene entities into the polymer backbone, autocatalysis disappears. Between 400 and 500 °C, the polyene will degrade further by chain scission reactions in inert conditions or aromatise in an oxidative environment into a char, and oxidised eventually into CO₂ beyond 500 °C.In inert conditions, the deacetylation step as well as the chain scission reaction shows endothermic effects. In an oxidative environment, large exothermal effects are found for each degradation step. This indicates the occurrence of additional oxidation reactions during deacetylation, an important reorganisation of the polyene during char formation and oxidation of the latter into CO₂.     

Using subcritical/supercritical fluid chromatography to separate acidic,basic, and neutral compounds over an ionic liquid-functionalized stationary phase

We packed an ionic liquid (IL)-functionalized stationary phase – based on 1-octyl-3-propylimidazolium chloride covalently bounded to silica gel – into a 3.2 mm × 250 mm column for the simultaneous separation of acidic, basic, and neutral compounds using carbon dioxide subcritical/supercritical fluid chromatography (SFC), and examined the effects of the pressure, temperature, co-solvents, and additives on the retention behavior of the analytes. The model compounds tested for SFC separation are acetaminophen, metoprolol, fenoprofen, ibuprofen, naphthalene, and testosterone. The data indicate that hydrogen-bonding and hydrophobic interactions between the analytes and the IL-modified stationary phase seem to involve in the separation process. Simultaneous separation of acidic, basic, and neutral compounds via SFC was successful at a co-solvent content of 20% MeOH, a pressure of 110 bar, and a column temperature of 35 °C. The relative standard deviations of the retention times and peak areas at 50 ppm were all less than 4 and 8% (n = 6), respectively.     

Thermal properties and stability of cassava starch films cross-linked with tetraethylene glycol diacrylate

The thermal stability of starch cross-linked with tetraethylene glycol diacrylate was studied under nitrogen atmosphere by thermogravimetry (TG) and infrared spectroscopy (FTIR). The cross-linking reaction was confirmed by the increase in intensity of the absorption band at ca. 3330 cm⁻¹ indicating the reinforcement of hydrogen bonds and the appearance of a new band at 1726 cm⁻¹ associated with the carbonyl group of the cross-linking agent. After cross-linking the solubility of starch in water decreased to the range 9%-16%. The thermogravimetric curves of pure and cross-linked starches showed an initial stage of degradation (up to ca. 150 °C) associated with the loss of water. The main stage of degradation occurred in the range 250-400 °C corresponding to ca. 60%-70% mass loss. The activation energy (E) for the degradation process increased from 145 kJ mol⁻¹ (pure starch) to 195 kJ mol⁻¹ and 198 kJ mol⁻¹ for starch treated for 60 min by UV (30 °C) and at 90 °C, suggesting high stability after cross-linking. A higher value (240 kJ mol⁻¹) was obtained for starch treated by UV for 120 min. The main volatile products determined by FTIR which correspond to hydrocarbons and carbonyl groups are apparently associated with the scission of weak bonds in the chain (probably branched groups) and the scission of stronger bonds (glycosidic linkages), respectively.     

Thermal degradation of high density polyethylene in a reactive extruder

The thermal degradation of high density polyethylene was conducted in a reactive extruder at various screw speeds with reaction temperatures of 400 °C and 425 °C. The residence time of the extruder was estimated and the molecular weight distribution of the fed plastic and reaction products was analysed using gel permeation chromatography. A continuous kinetic model was used to describe the degradation of the high density polyethylene in the reactive extruder. The breakage kernel and the scission rate model parameters were estimated from the experimental data for a variety of cases. It was found that purely random breakage and a scission rate which had a power law dependence on molecular size of 0.474 best described the experimental data.     

Mechanism of the temperature-dependent degradation of polyamide 66 films exposed to water

Experiments were performed to elucidate the degradation mechanism of hot-pressed polyamide 66 upon exposure to water. For films exposed to water over the temperature range 25 °C-90 °C, degradation was monitored using FTIR and solid-state ¹³C NMR spectroscopies. The data are consistent with a mechanism in which (1) a radical is formed on the methylene carbon adjacent to the amide nitrogen, (2) this radical reacts with oxygen to form a hydroperoxide, and (3) the hydroperoxide decomposes to form an imide or a hydroxylated amide, both of which may cleave leading to chain scission. Water appears to facilitate degradation by increasing the flexibility of the polymer matrix through swelling rather than acting as a reactive species, at least at the early stages of the process. An apparent activation energy of 15 ± 2 kJ/mol is observed for the early stages of degradation, suggesting that segmental motions in the polymer associated with water and oxygen sorption or inter-chain radical reactions are indeed key components of the degradation process.     

A new easily accessible chiral phosphite-phosphoramidite ligand based on 2-anilinoethanol and R-BINOL moieties for Rh-catalyzed asymmetric olefin hydrogenation

A novel chiral phosphite-phosphoramidite ligand based on 2-anilinoethanol and R-BINOL moieties has been synthesized in one-pot. The ligand was evaluated in the rhodium-catalyzed enantioselective hydrogenation of α- and β-dehydroamino acid derivatives and dimethyl itaconate in three different solvents at 25 °C, at 1 or 50 bar of hydrogen pressure. The solvent and the pressure effect are discussed.     

Investigation of polypropylene degradation during melt processing using a profluorescent nitroxide probe: A laboratory-scale study

Degradation of polypropylene (PP) during melt processing was studied using a novel profluorescence technique. The profluorescent nitroxide probe, 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO) was added to PP during melt processing to act as a sensor for carbon-centred radicals. Trapping of carbon-centred radicals, formed during degradation of PP, led to an increase in fluorescence emission from TMDBIO adducts. Through analysis of viscosity changes during processing cumulative chain scission degradation was estimated. At processing temperatures of 210 °C or below, fluorescence emission from TMDBIO adducts could be correlated with cumulative chain scissions when the number of chain scissions was small. At higher temperatures, a correlation was not observed most probably due to radical-trap instability rather than decomposition of the TMDBIO. Thus, TMDBIO may be used as a profluorescent sensor for degradation of PP during melt processing when the processing temperature is 210 °C or below.     

Evaluation of Philips and Ziegler-Natta high-density polyethylene degradation during processing in an internal mixer using the chain scission and branching distribution function analysis

The oxidative and thermo-mechanical degradation of HDPE was studied during processing in an internal mixer under two conditions: totally and partially filled chambers, which provides lower and higher concentrations of oxygen, respectively. Two types of HDPEs, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturations were analyzed. Materials were processed at 160, 200, and 240 °C. Standard rheograms using a partially filled chamber showed that the torque is much more unstable in comparison to a totally filled chamber which provides an environment depleted of oxygen. Carbonyl and transvinylene group concentrations increased, whereas vinyl group concentration decreased with temperature and oxygen availability. Average number of chain scission and branching (n_s) was calculated from MWD curves and its plotting versus functional groups' concentration showed that chain scission or branching takes place depending upon oxygen content and vinyl groups' consumption. Chain scission and branching distribution function (CSBDF) values showed that longer chains undergo chain scission easier than shorter ones due to their higher probability of entanglements. This yields macroradicals that react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the oxygen concentration, temperature, and vinyl end groups' content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than the Ziegler-Natta's type at the same processing condition.     

Phase separation and phase dissolution in poly(ε-caprolactone)/poly(styrene-co-acrylonitrile) blend

A blend of poly(ε-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) containing 27.5 wt% of acrylonitrile having the critical composition (80/20 PCL/SAN) was studied. This PCL/SAN blend having a lower critical solution temperature (LCST) phase boundary at 122 °C offered an excellent opportunity to investigate, firstly the kinetics of phase separation above LCST (125-180 °C), and secondly the kinetics of phase dissolution below LCST (50-115 °C). The blend underwent a temperature-jump above LCST where spinodal decomposition (SD) proceeded, yielding a regularly phase-separated structure (SD structure). Then, it was quenched to the temperatures below LCST when the phase dissolution proceeded. Optical microscopy was used to observe the spinodal decomposition qualitatively while light scattering was used to characterize the phase separation and phase dissolution quantitatively. It was found that during phase dissolution the peak maximum moved towards a smaller angle (wavelength of concentration fluctuations increased) while the peak intensity decreased. This behavior was explained by a model. Also it was found that the fastest phase dissolution kinetics at 80 °C, which was characterized by an apparent diffusion coefficient, was about 10 times slower than the kinetics of phase separation at 180 °C.     

Investigation of thermal degradation mechanism of an aliphatic polyester using pyrolysis-gas chromatography-mass spectrometry and a kinetic study of the effect of the amount of polymerisation catalyst

The thermal degradation mechanism of the aliphatic biodegradable polyester poly(propylene succinate) (PPSu) and the effect of the polymerisation catalyst (tetrabutyl titanate, TBT) were studied using pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and TGA analysis. It is found from mass ions detection, that the decomposition takes place, mainly, through β-hydrogen bond scission and secondarily by α-hydrogen bond scission. At low pyrolysis temperatures (360 and 385 °C) gases as well as succinic anhydride, succinic acid and propanoic acid are mainly produced while allyl and diallyl succinates are formed in smaller quantities. At high temperatures (450 °C) the behaviour is inverted. Using the isoconversional methods of Ozawa and Friedman it is founded that PPSu degrades by two consecutive mechanisms. According to this analysis the first mechanism that takes place at low temperatures is autocatalysis with an activation energy of about E = 110-120 kJ/mol. The second mechanism is a first-order reaction with E of 220 kJ/mol, and corresponds to the extended β- and α-hydrogen bond scissions. These activation energies are slightly dependent on the catalyst amount and are shifted towards lower values with an increase of TBT content from 3 × 10⁻⁴ to 3 × 10⁻¹ mol TBT/mol succinic acid (SA).     

Mechanism of degradation induced embrittlement in polyethylene

The thermal oxidation of polyethylene films in air at 80 °C and 90 °C has been studied by tensile testing, IR spectrophotometry and molar mass determination from rheometric measurements. In the conditions under study, the polymer predominantly undergoes chain scission and embrittles suddenly when the weight average molar mass reaches a critical value (90 kg mol⁻¹), far before significant damage of the entanglement network (M_e = 1.9 kg mol⁻¹) in the amorphous phase.The following embrittlement mechanism is proposed: chain scission in the amorphous phase induces chemicrystallization. The thickness of the interlamellar amorphous layer (l_a) decreases until a critical value of the order of 6-7 nm, below which plasticity cannot be activated and the polymer behaves in a brittle manner, as previously shown for virgin polyethylene. Using (l_a, M_W) maps, it is possible to explain the differences observed in the embrittlement behaviour of semi-crystalline polymers predominantly undergoing chain scission.     

Understanding the role of nanosilica particle surfaces in the thermal degradation of nanosilica-poly(methyl methacrylate) solution-blended nanocomposites: From low to high silica concentration

Nanocomposites of poly(methyl methacrylate) and 12 nm silica particles have been prepared by a casting procedure which allows the homogeneous dispersion of up to 35 wt% of silica. Twelve nanocomposites with compositions ranging from 1 to 35 wt% have been prepared and studied by Scanning Electron Microscopy, Fourier Transform Infrared Spectroscopy, Differential Scanning Calorimetry, Thermogravimetric Analysis and Isothermal Chemiluminescence. Thermal stability increases outstandingly as soon as 1 wt% of silica is added to PMMA. This effect is well-known but had not been explained up to now. Of the two main processes which initiate degradation in PMMA, radical formation at labile chain ends and random chain scission, it is the former which disappears in composites with 3 wt% of silica. The origin of the thermal stabilization in these polymer composites is thus the blocking of the PMMA chain end by the silica particle. At the same time, viscous flow decreases progressively, and composites with 20 wt% of silica or over are dimensionally stable even if heated at 300 °C for several hours. The concomitant decrease of viscous flow and increase of low temperature thermal stability shift the temperature range of application of these composites strongly, and the understanding of the silica surface role allows envisaging the control of the behaviour.