Thermal degradation of a polyphenylimide-quinoxaline in air and under vacuum

Three samples of poly{2,2′-[N,N′-bis(1,4-phenylene)benzophenone-3,3′,4,4′-tetracarboxylimide-6,6′-bis(3-phenyl-quinoxaline)]} (PPIQ), were prepared, differing in molecular weights and polymer chain endings. Their thermal degradation in vacuo and in air was determined by isothermal weight loss measurements. As in the case of poly-[2,2′-(1,4-phenylene)-6,6′-bis(3-phenylquinoxaline)] (PPQ), the temperature coefficients of thermal degradation in air were independent of molecular weight. However, in contrast, the temperature coefficients were independent of the type of polymer endgroups. It is, therefore, concluded that, contrary to amino-terminated PPQ's, polymer chain-end unzipping of PPIQ is of minor importance during thermal-oxidative degradation.     

Thermal degradation of poly-p-oxybenzoate under vacuum

The thermal degradation of poly-p-oxybenzoate in vacuo as function of temperature has been studied. The energy of activation of the process up to about 30% volatile formation is 59.6 kcal/mole. The degradation is preceded by induction periods, which have been shown to be due to poor heat conductivity of the polymer powder. CO, CO₂, phenol, and an unknown compound of molecular weight larger than 200 are the main degradation products found by chromatographic analysis. Infrared spectra of the original polymer, the polymer residue after degradation and of a degradation product solid at room temperature are presented. The possible reactions taking place have been indicated. The heat stability of the polymer in vacuo lies between those of polytetra-fluoroethylene and polyethylene.     

Thermal degradation of cereal straws in air and nitrogen

Multigram quantities (2.5-10 g) of highly purified IgG were obtained within 4 h from serum by using Avid AL packed in a radial-flow column. Avid AL is an affinity gel containing a synthetic, low-mol-wt ligand capable of selectively binding IgG from serum of all animal species tested. By packing the gel in a radial-flow column up to 500 mL, a high flow rate of 50 mL/min can be achieved without adversely affecting the performance of the gel and the purity of the isolated antibody.     

Thermomechanical behavior of a polyphenylquinoxaline

Crosslinking of linear poly[2,2′-(1,4-phenylene)-6,6′-bis(3-phenylquinoxaline)] (PPQ) by isothermal heat exposure in the temperature range between 425 and 490°C was investigated by means of torsional braid analysis. The change in glass transition temperature due to isothermal exposure was used as a kinetic parameter. In order to determine the effect of molecular weight and type of polymer chain ends, three PPQ samples were prepared that differed only in molecular weight and polymer chain endgroups. The apparent activation energy of isothermal crosslinking was independent of molecular weight and chain endings. Its value of 60 kcal/mole is the same as that for the thermal degradation of PPQ (determined by isothermal weight loss measurements). The rates of change of T_g at a particular temperature, however, are a function of both molecular weight (at least for these polymers that do not have a sufficiently high molecular weight) and the type of polymer chain ends. It was observed that isothermally crosslinked PPQ gave a higher break point in the TGA curve and also an increased char yield at 800°C than the linear precursor.     

Thermal degradation of natural and treated hemp hurds under air and nitrogen atmosphere

Sustainability goals are essential driving principles for the development of innovative materials in the construction industry. Natural fibers represent an attractive alternative as reinforcing material due to good mechanical properties and sustainability prerequisites. The study has been focused on the comparative investigation of chemical and physical treatments of hemp hurds and their influence on the thermal behavior of main hemp constituents in air and nitrogen atmosphere. Thermal decomposition of hemp hurds involves several parallel reactions related to heat and mass transfer processes. A comparison of DSC and TG/DTG results of hemp hurds samples before and after treatments demonstrates a better thermal stability for treated samples. It is caused by changes in chemical composition due to a partial removal of non-cellulosic components from hemp hurds structure, an increase in cellulose content and decrease in its degree of polymerization. The results show different thermal behavior of the hurds samples heated under nitrogen and air atmosphere. Based on DTG records, several-stage process of mass loss has been found for the samples under air, whereas only two-stage process under nitrogen.

     

Thermal degradation of wood treated with guanidine compounds in air: Flammability study

Wood has been treated with guanidine phosphate, guanidine nitrate, guanidine carbonate and guanidine chloride to impart flame retardancy. The samples were subjected to differential thermal analysis (DTA) and thermogravimetry (TG) from ambient temperature to 800°C in air to study their thermal behaviors. From the resulting data, kinetic parameters for different stages of thermal degradation were obtained following the method of Broido. For the decomposition of wood and flame retardant wood, the activation energy was found to decrease from 116 to 54 kJ mol^–1; the char yield was found to increase from 5.6 to 34.9%, LOI from 18 to 41.5, which indicated that the flame retardancy of treated wood was improved. Effects of the different compounds on the degradation and flammability of wood have also been proposed.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal degradation of bisphenol a polycarbonate

Bisphenol A polycarbonate (Makrolon 3000) has been purified and fractionated. Thin films of these samples have been degraded by heating at 200° under continuous evacuation (pressure less than 0- 1 Pa) for periods of several hours. The molecular weight is observed (by gel permeation chromatography) to increase with time at 200°. Eventually gel is formed. The variation of the number- and weight-average molecular weights with heating time and the effect of the initial molecular weights of the polycarbonate samples are consistent, in the main, with the Davis-Golden mechanism of thermal degradation of bisphenol A polycarbonate; i.e. the principal reactions are condensation and trifunctional branching. Chain scission due to hydrolysis of the carbonate linkage is absent under our conditions.     

Thermal analysis of polysiloxanes. II. Thermal vacuum degradation of polysiloxanes with different substituents on silicon and in the main siloxane chain

Thermogravimetric (TG) investigations of various substituted polysiloxanes of the type \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} ({\rm R}_1 {\rm R}_2 {\rm SiO\rlap{--} )}_n $\end{document} have been carried out in vacuo and the activation energies for the depolymerization processes calculated from the resulting thermograms. (R₁ and R₂ are methyl, ethyl, n-propyl, trifluoropropyl, or phenyl.) It is postulated that the activation energy is mainly a function of the inductive effect of the substituent group and that electron-withdrawing groups attached to silicon increase the activation energy, whereas electron-donating groups decrease it. A linear relation is found between the Taft constant σ^* for the substituent on silicon and the calculated activation energy for depolymerization. Product analysis results from isothermal degradations indicate that the degradation mechanism in a silmethylene siloxane polymer and a silethylene-siloxane polymer is very similar to that in polydimethylsiloxanes (PDMS). For the \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} ({\rm R}_1 {\rm R}_2 {\rm SiO\rlap{--} )}_n $\end{document} polymers, the amount of cyclotrisiloxane in the degradation products increases with the size of the substituent on silicon, and it is postulated that the rate of depolymerization is mainly influenced by short-range steric interactions between the substituents on the silicon atoms of the siloxane chain.     

Thermal cracking of vacuum distillates

The hydrotreatment of vacuum distillates under suitable operating conditions, mainly the conversion and partial pressure of hydrogen, can be used to feed a steam cracker. This hydrotreatment substantially increases the production of the products sought while preventing the formation of resins and asphaltenes.The thermal decomposition of vacuum gas-oil (VGO) and hydrotreated vacuum distillates (HVD) in the presence of steam (steam cracking) was studied in a plug flow reactor around 1053 K. The tubular reactor was a wound Incology steel tube (4 mm I.D.) heated by high-frequency induction. The total pressure in the reactor was slightly above atmospheric. By analysis (by gas and liquid chromatography, mass spectrometry, distillation and simulated distillation and asphaltene extraction) of the feedstocks and the steam cracking products, we observed that hydrotreatment of VGO transforms the initial polyaromatics into less condensed and more hydrogenated products that do not produce resins and asphaltenes during steam cracking. The cracking gasoline and the light pyrolysis products, essentially made up of α-olefins, are primary products.Considering the extreme complexity of middle and heavy steam cracking fractions, the grouping of products by families (polyaromatics, resins, asphaltenes) enabled a kinetic-semiquantitative analysis of the data to be made, and from this secondary nature of the asphaltenes could be defined together with the primary nature of the polyaromatics and resin.     

Thermal degradation of a series of polyester polyurethanes

A series of polyester polyurethanes with a range of polyester contents was prepared from an ethylene glycol/propylene glycol/adipic acid polyster, butane diol, and methylene bis (4-phenyl isocyanate) (MBPI). They were thermally degraded under vacuum and the products of degradation were identified. The urethane linkages decompose first as the temperature is increased and the stability increases with polyester content. Reaction mechanisms were proposed which account for the principal features of the reaction and the products of degradation.     

Thermal degradation of Levan

The thermal degradation of the polysaccharide Levan, of various molecular weights, was studied by means of isothermal and dynamic TG, X-ray diffraction, and IR measurements. From dynamic TG traces, two main degradation stages were observed for HL-Levans (hydrolyzed) whereas, three such stages were observed for UL-Levans (unhydrolyzed). These stages were correlated with branching effects, an interpenetrating network, and ring-fragmentation and other reactions in the late stages of the thermal degradation which result in the formation of infusible material that chars on further heating. Isothermal curves obtained for the first stage indicated the possibility of a random type of thermal degradation. Anticipated linear plots were obtained, using expressions from random degradation theory, which added further support for this type of degradation for Levan (stage 1). Various parameters were obtained for the thermal degradation of Levan, e.g., activation energies, and various experimental observations have been correlated in terms of postulated microstructures for Levan and in terms of parameters such as entropy of activation.     

Thermal degradation of UHMWPE

The thermal behaviour of ultra-high molecular weight polyethylene (UHMWPE) of different molecular weights was examined by thermal analysis methods. The melting temperatureT _m and the heat of melting H were measured by the DSC method. The thermooxidative degradation process was investigated by using a MOM Q-1500 D derivatograph at various heating rates in air atmosphere. The initial decomposition temperatureT _i was determined from the TG curves, and other characteristic temperatures of decomposition were calculated. It was found thatT _m and H are higher for UHMWPE than those for HDPE, i.e. 146C and 195 J g^–1 for UHMWPE as compared with 133C and 166 J g^–1 for HDPE. The thermal behaviour of the investigated UHMWPE samples is not significantly influenced by molecular weight.     

Thermal degradation of polystyrene

Molecular weight change studies have shown that the thermal degradation of random copolymers of styrene — namely HIPS, SAN, and ABS-at low temperatures and in air involves random chain scission. The dominant process in the degradation of HIPS is random chain scission due to weak links, whereas in SAN it is intermolecular chain transfer. In ABS, the degradation is initially random scission due to weak links and then mainly intermolecular chain transfer. The infrared spectra show that during degradation the labile weak links are attacked by oxygen and peroxidic free radicals are produced. Via hydrogen abstraction or autoxidation of olefinic links, these free radicals are responsible for the formation of aliphatic ketonic or peroxyester structures, and for isomerization and cyclization. The activation energies of overall degradation of HIPS, SAN, and ABS are 134, 142, and 92 kJ.mol^–1 respectively.Part of the PhD dissertation of Mrs. Jaya Nambiar, University of Gorakhpur, Gorakhpur-273001, 1980.     

Thermal degradation of polyethylene

Thermal degradation of low-density polyethylene (LDPE) in the temperature range from 450 to 525°C has been studied using a sieve-bottom reactor with inert gas as heat-transferring agent bubbled through the PE melt. Temperature dependence of the degradation rate was determined. Full degradation of LDPE into gaseous and wax-like hydrocarbons (alkanes and 1-alkenes) was achieved. Temperature rise and prolonging of the contact time increased the yield of the gaseous hydrocarbons and decreased the molecular weight of the wax-like product.     

Thermal degradation of polyquinoxalines

A systematic investigation of the thermal stability of nine structurally related polyquinoxalines has been conducted. The relative oxidation resistance of these polymers is controlled by two opposing structural effects. Phenyl sidegroup substitution in the heterocycle greatly improves oxidative stability, while the introduction of oxygen into the main polymer chain, in the form of ether groups, produces a negative effect of equal magnitude. These results are discussed from a mechanistic point of view. Simultaneous, dynamic thermal analysis in vacuum up to 1400°C and analysis of volatile and nonvolatile products indicates three major decomposition regions. Between 500 and 640°C, main polymer degradation takes place involving the heterocycle. This event is followed by dehydrogenation of a stable degradation product between 640 and 690°C. Above 1360°C an exothermic reaction takes place to yield highly condensed aromatic residues.     

Thermal degradation of polybenzyl

Polybenzyl has been synthesized from the benzyl compounds, chloride, methyl ether, and isopropyl ether by using stannic chloride as the Friedel-Crafts catalyst. The detailed structures of these polybenzyls have been determined by use of methods including elemental analysis, molecular weight determination, and infrared, ultraviolet/visible, and NMR spectra. The structure of these polybenzyls is globular with a high degree of branching. Thermal degradation was studied by using DTA and stepwise and isothermal degradation with analysis of both the volatile fraction and the residue. The activation energy for scissions was determined from changes in the molecular weight of the sample with time at a range of degradation temperatures of 340–440°C. The composition of the volatile fraction changed with both temperature and time of degradation. Thermal scission is directed to internal units and depends on the presence of weak links such as substituted anthracene units.     

Thermal degradation of a styrenated unsatured polyester resin

The isothermal and non-isothermal degradation of a typical styrenated phthalic acid-maleic acid-propylene glycol polyester were measured. Non-isothermal and isothermal kinetic analyses were performed on the various degradation steps observed. The values of the non-isothermal and the isothermal kinetic parameters are in good agreement.