Thermal degradation of cellulose and its phosphorylated products in air and nitrogen

The thermal degradation of cellulose and its phosphorylated products (phosphates, diethylphosphate, and diphenylphosphate) were studied in air and nitrogen by differential thermal analysis and dynamic thermogravimetry from ambient temperature to 750°C. From the resulting data various thermodynamic parameters were obtained following the methods of Broido and Freeman and Carroll. The values of E_a for decomposition for phosphorylated cellulose were found to be in the range 55–138 kJ mol^?1 in air and 85–152 kJ mol^?1 in nitrogen and depended upon the percent of phosphorus contents in the samples. The mass spectrum of cellobiose phosphate indicated the absence of the molecular ion, indicating that the compound was thermally unstable. The IR spectra of the pyrolysis residues of cellulose phosphate gave indication of formation of a compound having C?O and P?O groups. A fire retardancy mechanism for the thermal degradation of cellulose phosphate has been proposed.     

Thermal and spectroscopic studies on flame-retardant cotton cellulose modified with THPC-urea-ADP and its transition metal complexes

Cotton cellulose has been treated with tetrakis(hydroxymethyl)phosphonium chloride (THPC), urea and small amounts of ammonium dihydrogen orthophosphate (ADP) to impart flame retardancy. Complexes of cell-THPC-urea-ADP with transition metals such as chromium, manganese, iron, cobalt, nickel, copper and zinc have been characterized by reflectance UV-visible spectra. The samples were subjected to differential thermal analysis and thermogravimetry from ambient temperature to 700°C in air to study their thermal behaviour. From the resulting data, various kinetic parameters for different stages of thermal degradation were obtained following the method of Broido. For the decomposition of cellulose and flame-retardant celluloses, the activation energy was found to increase from 242 to 322kJ mol^?1, the entropy of activation from 140 to 307 JK^?1 mol^?1 and the char yield from 2.5 to 31%. The free energy of activation for decomposition of cellulose and its derivatives was almost the same, viz. 148–162 kJ mol^?1, indicating that the basic steps in the decomposition of cellulose and its derivatives are the same. The IR spectra of the thermally degraded residues of cell-THPC-urea-ADP and its metal complexes indicate that dehydration takes place and a compound containing the carbonyl group is formed. The electron paramagnetic resonance signals indicate the formation of trapped and stable free radicals in the thermal degradation of cellulose and its derivatives.     

Thermal,morphological and spectroscopic studies on cellulose modified with phosphorus,nitrogen, sulphur and halogens

The kinetics of the thermal degradation of cellulose and modified cellulose, namely, cellulose phosphate, cellulose carbanilate, cellulose tosylate, chlorodeoxycellulose, bromodeoxycellulose, and iododeoxycellulose in air were studied by thermogravimetry and differential thermal analysis from ambient temperature to 700°C. The various thermodynamic functions for different stages of thermal degradation had been obtained following the procedure of Broido. The activation energies for the oxidative decomposition of cellulose and modified celluloses were found to be in the range 30–399 kJ mol^?1. The infrared spectra of the residues of modified celluloses gave indication of formation of a compound containing P?O, P? O? P (only in the case of cellulose phosphate), C?C, and C?O groups in the final residual char. The EPR signals indicated the formation of trapped and stable free radicals in the thermal degradation of all the compounds, particularly halodeoxycelluloses showed generation of large amounts of trapped free radicals during the oxidative decomposition. Scanning electron micrographs of the thermally degraded cellulose derivatives show changes in the fibrillar structure, evolution of gasesous products, and film formation depending upon the nature of the substituent in the cellulose matrix. The mechanism of thermal degradation of these compounds has been proposed.     

Thermal decomposition of cellulose ethers

The thermostability and thermal decomposition kinetics of methyl cellulose (MC), ethyl cellulose (EC), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), and hydroxypropyl–methyl cellulose (HPMC) were characterized in nitrogen and air by thermogravimetry (TG). Various methods of kinetic analysis were compared in case of thermal degradation of the five cellulose ethers. The initial decomposition temperature (T_d), temperature at the maximum decomposition rate (T_dm), activation energy (E), decomposition reaction order (n), and pre-exponential factor (Z) of the five cellulose ethers were evaluated from common TG curves and high-resolution TG curves obtained experimentally. The decomposition reactions in nitrogen were found to be of first order for MC, EC, and HPMC with the average E and ln Z values of 135 kJ/mol and 25 min⁻¹, although there were slight differences depending on the analytical methods used. The thermostability of cellulose ethers in air is substantially lower than in nitrogen, and the decomposition mechanism is more complex. The respective average E, n, ln Z values for HEC in nitrogen/air were found to be 105/50 kJ/mol, 2.7/0.5, and 22/8.3 min⁻¹, from constant heating rate TG method. The respective average E, n, and ln Z values for three cellulose ethers (EC/MC/HPMC) in air are 123/144/147 kJ/mol, 2.0/1.8/2.2, 24/28/28 min⁻¹ by using high-resolution TG technique. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2927–2936, 1999     

Thermal degradation of cellulose and cellulose esters

Cellulose, cellulose diacetate (CDA), cellulose triacetate (CTA), cellulose nitrate (CN), and cellulose phosphate (CP) were subjected to dynamic thermogravimetry in nitrogen and air. The thermostability of the cellulose and its esters was estimated, taking into account the values of initial thermal degradation temperature T_d, the temperature at the maximum degradation rate T_dm, and char yield at 400°C. The results show that these polymers may be arranged in the following order of increasing thermostability: CN < CP < regenerated cellulose < filter cotton < CDA < CTA. The activation energy (E), order (n), and frequency factor (Z) of their degradation reactions were obtained following the Friedman, Chang, Coats–Redfern, Freeman–Carroll, and Kissinger methods. The dependence of T_d, T_dm, E, n, Ln Z, and char yield at 400°C on molecular weight and test atmosphere is also discussed. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:293–304, 1998     

Flame-retardant properties of cellulose phenylthiophosphonate

The effects of sulfur atoms on the thermal degradation and flammability of cellulose phenylthiophosphonate were investigated using thermogravimetry, IR spectrometry, and limiting oxygen index flammability tester. Introduction of sulfur atoms instead of oxygen atoms in the phosphonyl groups had little influence on the thermal degradation of cellulose. The thermal reactions were altered by ion exchange with sodium ions, and degradation of the cellulose chains was retarded. Cellulose phenylthiophosphonate was self-extinguishing above 4.64% phosphorus content. The flame-retardant properties remained when the cellulose phosphorus ester was ion exchanged with sodium ions. It was concluded that introduction of sulfur atoms into phosphorus esters of cellulose was effective in preventing the decrease in flame-retardant properties by ion exchange in laundering.     

Thermal degradation and morphological studies on cotton cellulose modified with various arylphosphorodichloridites

Cellulose arylphosphonate compounds have been synthesized and investigated by the kinetics of thermal degradation by thermogravimetry (TG) and differential scanning calorimetry (DSC) from ambient temperature to 600°C. Various kinetic and thermodynamic parameters such as energy, entropy and free energy of activation have been obtained from TG curves using the Broido method and transition state theory. The high values of enthalpy change (1016 and 1025 J g^?1) of decomposition and oxidation reactions corresponding to the last two exotherms of the DSC curves of cellulose are decreased to a greater extent in the case of cellulose arylphosphonate compounds. The values of activation energy for the decomposition stage of cellulose arylphosphonate compounds lie in the range 25–49 kJ mol^?1 and are found to be lower than that of pure cellulose, namely 165 kJ mol^?1 in air atmosphere. Scanning electron micrographs of phosphorylated cotton cellulose and chars show furrowed and fractured surfaces although the morphology of the original fibres remains largely unchanged. Furthermore, higher char yields of cellulose derivatives leads to the conclusion that such derivitisation may give rise to flame‐retardant treatments for cellulosic materials. Copyright © 2004 Society of Chemical Industry     

High‐resolution thermogravimetry of cellulose esters

Cellulose dinitrate (CDN), cellulose diacetate (CDA), and cellulose triacetate (CTA) were subjected to high‐resolution thermogravimetry (TG) at a variable heating rate in air. The TG curves, the derivative TG curves, the second derivative TG curves, and heating rate curves are discussed. The thermal degradation temperature and kinetic parameters are presented and compared to those obtained with traditional TG at a constant heating rate. The degradation process of the cellulose esters is speculated. Among the three cellulose esters, CDN exhibits the lowest degradation temperature of (213°C) but the largest degradation activation energy of (237–269 kJ/mol). © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 573–578, 1999     

Thermal,spectral and morphological studies on cellulose,cellulose ethylthiophosphate and its metal complexes in air

Metal complexes of thiophosphorylated cellulose, when heated, give rise to high char yields. These and related observations suggest that such derivatisation may give rise to novel flame retardant treatments for cellulosic materials. The kinetics of thermal degradation of cellulose, cellulose ethylthiophosphate (CESP) and metal complexes of the CESP have been studied by thermogravimetry (TG) and differential thermal analysis (DTA) from ambient temperature to 700°C in dynamic air to investigate the potential flame retardance of the CESP and its metal complexes. Various parameters such as energy, entropy, enthalpy and free energy of activation have been calculated using the Broido method and transition state theory. For the decomposition stage of thermal degradation, the activation energies of the CESP samples lie in the range 53-133 kJmol^?1 and of the metal complexes, 108-177kJmol^?1, which are found to be lower than that of cellulose (187 kJmol^?1). Scanning electron micrographs of the CESP show that the fibrillar structure of cotton has become more evident and chars retain the general morphology of the original fibre although severe, localised zones of damage reflect the gross chemical and physical changes occurring during pyrolysis. The IR spectra of chars of modified samples indicate formation of compounds containing C=O, C=C and P=O groups. The mechanisms of thermal degradation of the CESP and its metal complexes have been proposed.     

Thermal studies on homogeneously synthesized cellulose p-toluenesulfonates

The thermal behavior in argon of homogeneously synthesized cellulose p-toluenesulfonates (tosylates) with a degree of substitution (DS) ranging from 0.4 to 2.3 was studied by means of thermogravimetry and derivative thermogravimetry from ambient temperature up to 500°C. For comparison, the thermal behavior of the starting celluloses used (pulps, linters, bacterial cellulose) was also examined. The thermal degradation of cellulose tosylates was initiated at lower temperature than cellulose itself and proceeds in two main stages. The temperature of the first one (169 196°C) increases with increasing DS and is independent of the molecular weight. Activation energies calculated following the method of Broido, FTIR, and ultimate analysis as well as mass spectroscopy show that the first stage of degradation is closely associated not only with the scission of tosyl ester groups but also with a partially degradation of the polymer backbone. Further, the temperature-concen-tration diagram for the system cellulose tosylate 20 /o-dichlorobenzene was studied by optical observations and calorimetric investigations. A liquid-liquid demixing interferes with the glass transition of the cellulose tosylate-solvent system. It results in the solidification of the solution. © 1996 John Wiley & Sons, Inc.     

Thermal decomposition of cellulose/synthetic polymer blends containing grafted products. II. Cellulose/polyacrylonitrile blends

The homogeneous grafting of acrylonitrile onto cellulose was carried out in a dimethyl sulfoxide/paraformaldehyde solvent system. The grafted products were added to cellulose/polyacrylonitrile (PAN) blends as compatibilizers. The thermal decomposition behavior of the blends was investigated by thermogravimetry. The thermal stability of the blends with higher grafted product content was lower by more than 100°C than that of the blends without grafted product. The accessibility values of the former blends were larger than those of the latter. The microphase-separated structures of the grafted product blends were finer than those without the product. Dynamic mechanical measurements and differential scanning calorimetry were performed to estimate the glass transition temperatures, T_g, of the blends. The variation in T_g was smaller than that in characteristic temperatures determined by thermogravimetry. The difference in thermal decomposition behavior was correlated to that in compatibility. Thermogravimetry was found to be effective for estimating the compatibility in cellulose/PAN blends containing grafted products. © 1996 John Wiley & Sons, Inc.     

Pyrolysis and combustion of cellulose. VII. Thermal analysis of the phosphorylation of cellulose and model carbohydrates during pyrolsis in the presence of aromatic phosphates and phosphoramides

Thermal gravimetric analysis, differential scanning calorimetry, and derivative thermal gravimetric analysis were utilized to characterize the thermal interactions between cellulose, 1-6, anhydro β-D-glucopyranoside, and D-glucose and model phosphate and phosphoramide flame retardants. The phosphoramides induced higher char yields than the phosphates during the pyrolysis of the mixtures of carbohydrates and organophosphorus compounds. Exothermic reactions attributed to phosphorylation and char formation were observed with each of the phosphoramide/carbohydrate mixtures and were absent with the phosphates. The individual phosphorus compounds studied showed similar thermal behavior with each of the carbohydrates indicating that the mode of interaction for these mixtures was similar. Isothermal gravimetric analysis of the organophosphorus/carbohydrate mixtures was used to measure the rate of decomposition weight loss from isothermal conditions. This weight loss was used as an indication of rate of fuel formation. The kinetics observed for these measurements indicated that the phosphoramide mixtures underwent a rapid weight loss to a final char with an effective E_act of about 55 kcal/mol while the phosphate mixtures exhibited effective E_act′s for decomposition lower than those observed for the pure carbohydrates. Mixtures of glucose with selcted arylphosphoramide esters were pyrolysed in order to determine the effect of lability of the leaving group on char formation. Gas chromatographic analysis of the pyrolysis products indicated that phenol was the favored leaving group in comparison with aniline units, but char promotion appeared to be dependent on the number of P-N bonds present in the original phosphoramide. Electron spectroscopy for chemical analysis indicated that chemically similar chars were obtained from the different organophosphorus/carbohydrate combinations.     

Studies on thermal degradation of cellulose and cellulose phosphoramides

Reaction of cellulose with hexamethylphosphoric acid triamide has been investigated under various physical conditions. Dimethylamine hydrochloride was found to be an efficient catalyst for the system. The thermal degradation of cellulose and its phosphoramide products in air was studied by DTA, TG, and DTG techniques from ambient temperature to 500°C. The data were processed for the various thermodynamic parameters following the methods of Freeman and Carroll, of Broido, and of Dave and Chopra. The energies of activation, E_a, for the degradation for various cellulose phosphoramide samples were found to be in the range of 92–136 kJ mol^?1. These values were found to decrease with increase in the degree of substitution. A mechanism for the thermal degradation of cellulose phosphoramide has been proposed. The IR spectra of char residues of cellulose phosphoramide gave an indication of the formation of compounds containing C?O and P?O groups.     

Kinetics of isothermal and non‐isothermal degradation of cellulose: model‐based and model‐free methods

BACKGROUND: The kinetics of the thermal decomposition of cellulosic materials is of interest from the viewpoint of flame retardancy for safety, optimization of incineration processes and reducing energy production from fossil sources and associated pollution. One essential step in these processes is the thermal degradation through mass and energy transport, which determines the rate of evolution of various types of products from cellulosic materials. RESULTS: Kinetic parameters have been determined using various model‐based and model‐free methods in the thermal degradation of cellulose up to 700 °C in helium atmosphere. The values of the activation energy obtained in isothermal processes and non‐isothermal processes have been found to be not far from each other. From the integral method, the random nucleation (F₁)‐type mechanism has been found most probable for cellulose degradation having an activation energy, E_a, in the range 156.5–166.5 kJ mol^?1, lnA = 20–23 min^?1, for first‐order reaction during its decomposition process at heating rates of 2, 5 and 10 °C min^?1. Based on the high correlation coefficient, many types of mechanisms seem equally good for non‐isothermal degradation of cellulose. CONCLUSION: The linear correlation coefficient has a limitation for verifying the correctness of a reaction mechanism in the study of degradation kinetics. Therefore, the correctness of a mechanism should be considered on the basis of comparing the kinetic parameters obtained from isothermal as well as non‐isothermal methods. Copyright © 2008 Society of Chemical Industry     

Pyrolysis and combustion of cellulose. II. Thermal analysis of mixtures of methyl α-D-glucopyranoside and levoglucosan with model phosphate flame retardants

The thermal degradation of methyl α-D -glucopyranoside, a cellulose model of intermediate complexity, was investigated in an attempt to gain insight into the pyrolytic reactions of analogous cellulose systems. The pure glucoside pyrolysis proceeds through formation of an intermediate of higher thermal stability. Nitrogenous bases bring about decomposition of the glucoside at lower temperatures and without formation of a detectable intermediate. Phenyl phosphates and phosphoramides induce thermal degradation of methyl α-D -glucopyranoside at lower temperatures than observed for the pure glucoside. The postulated degradation mechanism involves esterification of the glucoside followed by dehydration and skeletal rearrangements. Nitrogenous bases assist the dehydration process but reduce the yield of residue and bound phosphorus. Levoglucosan, the cellulose degradation product responsible for flaming combustion, was pyrolyzed in the presence of model flame retardants. Nitrogenous bases were found to inhibit thermal polymerization of levoglucosan and to induce its decomposition at lower temperatures. Zinc chloride exerted its effects in two stages: acid-catalyzed polymerization at lower temperatures and dehydration at higher temperatures. Phenyl phosphates and phosphoramides alter levoglucosan pyrolysis by action as Lewis acids in a manner similar to zinc chloride.     

Thermal properties of tritylated and tosylated cellulose

Triphenylchloromethane and p-toluenesulfonyl chloride were reacted with chopped or powdered cellulose, with and without premercerization, to form trityl–cellulose ethers or tosyl–cellulose esters. Powdered and premercerized cellulose samples were more readily derivatized. Differential scanning calorimetric (DSC) and thermogravimetric (TG) analyses were performed in nitrogen on these derivatives. DSC and TG thermograms were affected by the particular derivative and the degree of substitution. The decomposition temperatures for both derivatives were lower than for the unmodified cellulose. Trityl cleavage may have been detected by DSC as a broad endothermic area showing no weight loss that preceded the major endothermic decomposition peak. Decomposition temperatures were lowered, but not sufficiently to prevent decomposition products from being combustible. No increase in residue was effected. Thermal decomposition of tosyl–cellulose was substantially different from that of the trityl derivative. As the degree of tosyl substitution increased, decomposition occurred at lower temperatures as an increasing exotherm. Tosyl derivatives all produced high residues. These changes in thermal characteristics were indicative of increased flame resistance. Oxygen index (OI) values relate to flame resistance and show that tritylation was detrimental to the cellulose and that tosylation imparted some degree of flame resistance.     

A study on the mechanism research on cellulose pyrolysis under catalysis of metallic salts

Experimental research on cellulose pyrolysis under catalysis of metallic salts was done on a thermobalance and a rapid pyrolysis system. Thermogravimetric analysis showed that K⁺ catalyzed the formation of active cellulose strongly and decreased the activation energy of cellulose pyrolysis. Experimental results indicated that K⁺ would promote the formation of char and restrain the production of bio-oil largely. Fe²⁺ had a similar catalysis effect on cellulose pyrolysis with K⁺. Fe²⁺ particularly catalyzed the formation of small molecule gaseous product while K⁺ the formation of char. The addition of K⁺ or Fe²⁺ resulted in a reduction of levoglucosan formation, and enhanced the production of hydroxyacetaldehyde and other small molecule components. Levoglucosan and hydroxyacetaldehyde were formed by the decomposition of active cellulose in a parallel mode. The secondary cracking of levoglucosan would also produce hydroxyacetaldehyde. A modified cellulose pyrolysis mechanism model was proposed based on the B-S model.     

Kinetics of acid catalysed degradation of cellulose triacetate in chloroalkane solvents: 1. Dichloromethane

The kinetics of degradation of cellulose triacetate in a dichloromethane-acetic anhydride mixture with perchloric or sulphuric acid catalyst was studied. Decrease in molecular weight with time was followed viscometrically using limiting viscosity numbers, calculated by a single-point method, and Mark-Houwink constants for the solvent mixture. Dependence of first-order rate constants on catalyst concentration and temperature was investigated and Arrhenius parameters, activation entropies and catalytic coefficients were obtained. A suggested reaction mechanism for degradation based on H⁺ and Ac⁺ (acetylium ion) acting as catalytic species is proposed.     

The electrical conductivity of cellulose HYPHAN® and its complexes

The electrical conductivity, σ, of cellulose HYPHAN® and its complexes with Cr³⁺, Mn²⁺, Mo⁵⁺ or Hg²⁺ was measured from room temperature to 510°C. The activation energy, ΔE, of the samples was calculated from log σ against 1/T curves. The results show that the electrical conductivity increases by temperature with two maximum peaks. The first peak is attributed to the moisture content, while the second one is attributed to the thermal degradation of the cellulose chain. The values of the activation energy indicate that the samples change from low semiconductor to high semiconductor property with heating.     

Periodate oxidation of cellulose by homogeneous reaction

Homogeneous periodate oxidation of cellulose was achieved through methylol cellulose. The dissolution of methylol cellulose into aqueous periodate solution was followed by the gradual decomposition of methylol groups at random sites along the methylol cellulose chain. The recovery of glycol hydroxyl groups at the C₂ and C₃ positions on the glucopyranose ring during the above decomposition process caused uniform cleavage of C₂? C₃ bonds by the periodate ion. The oxidation level reached nearly 100% in 10 h. The reduced product of the resulting dialdehyde cellulose, i.e., dialcohol cellulose, resulted in mechanical properties quite different from those of conventional dialcohol cellulose. Examination of the thermal deformation and tensile properties revealed that no notable cellulose degradation occurred during the reaction. Our dialcohol cellulose gave a clear and transparent film with a flexible nature.