Use of wood elemental composition to predict heat treatment intensity and decay resistance of different softwood and hardwood species

Heat treatment is an attractive alternative to improve decay resistance of low natural durability wood species. Decay resistance is strongly correlated to thermal degradations of wood cell wall components. Some recent studies proposed the use of wood elemental composition as a valuable marker to predict final properties of the material. These results, initially obtained with pine, have been extended to different softwood and hardwood species to check validity of the method using equipment specially designed to measure mass losses during thermal treatment. Heat treatment was performed on two softwood species (pine and silver fir) and three hardwood species (poplar, beech and ash) at 230 °C under nitrogen for different times to reach mass losses of 5, 10 and 15%. Heat-treated specimens were exposed to fungal decay using the brown rot fungus Poria placenta and the weight losses due to fungal degradation determined as well as initial wood elemental composition. Correlations between weight losses recorded after fungal exposure and elemental composition indicated that carbon content and O/C ratio can be used to predict wood durability conferred by heat treatment. Moreover, it was observed that for given curing conditions thermo-degradation patterns differed considerably according to the wood species. The sole analysis of wood physical properties like its density, thermal conductivity and diffusivity cannot allow explaining the observed differences, which should also depend on thermally activated chemical processes depending on wood chemical composition.     

Utilization of thermodesorption coupled to GC-MS to study stability of different wood species to thermodegradation

Thermodesorption coupled to gas chromatography coupled to mass spectroscopy (TD-GC-MS) has been investigated to identify volatile degradation products generated during wood heat treatment by mild pyrolysis. For this purpose, wood samples of different softwood and hardwood species have been heat treated under nitrogen for different temperatures comprised between 180 and 230 °C during 15 min in the glass thermal desorption tube of the thermodesorber and the volatile wood degradation products trapped. The trapped products were then thermodesorbed and analysed by GC-MS. Chromatograms of the different samples indicated the formation of different products resulting from degradation of lignin and hemicelluloses. Hardwoods were shown to be more sensitive to thermodegradation than softwoods, for which degradation products appear at slightly higher temperature. The important formation of acetic acid is concomitant with the formation of most of degradation products and at the origin of the difference of reactivity observed between softwoods and hardwoods.     

Prediction of the decay resistance of heat treated wood on the basis of its elemental composition

The effect of heat treatment temperature on the elemental composition of Scots pine sapwood (Pinus sylvestris) has been investigated in the range of temperatures between 220 and 250 °C. The results revealed an important increase of carbon content, while oxygen content significantly decreases. Independently of the heat treatment temperature, elemental composition is strongly correlated with the mass losses due to thermal degradations. Carbon content as well as O/C ratio seem to represent valuable markers to estimate wood degradation after heat treatment. Heat treated specimens were exposed to fungal decay using the brown rot fungus Poria placenta and the weight losses due to fungal degradation were determined. Correlations between weight losses recorded after fungal exposure and elemental composition indicated that carbon content or O/C ratio can be used to predict wood durability conferred by heat treatment allowing us to envisage the development of a proper method to evaluate the quality of heat treated wood and predict its durability. These results also confirm that chemical modifications of wood cell wall polymers are the main factors responsible for wood durability improvement against fungal decay after heat treatment.     

Investigations of the reasons for fungal durability of heat-treated beech wood

It is generally accepted that thermal treatment of wood by mild pyrolysis (retification or torrefaction) improves its durability to fungal degradation. However, this property has recently been questioned in the literature and definitely needs further investigation. The increase in durability conferred by thermal treatment is generally explained by four hypotheses: the low affinity of heat-treated wood to water; the generation of toxic compounds during heating; the chemical modification of the main wood polymers and the degradation of hemicelluloses. This study was undertaken to understand the reasons for durability of heat-treated beech wood. In order to confirm or not the above mentioned hypotheses, the durability of heat-treated beech wood towards Coriolus versicolor was evaluated according to different parameters like mass loss, wettability or chemical composition. The heat treatment was carried out in a temperature range of 20-280 °C under inert atmosphere for 10 different temperatures. The results show clearly an important correlation between the temperature of treatment and the fungal durability. At the same time, there was insufficient evidence to support the hypothesis of improved decay resistance due to generation of fungicidal compounds or due to the hydrophobic character of wood. Finally, the most plausible hypothesis to explain improvement of wood durability concerns its chemical modifications. Indeed, degradation of hemicellulose associated with other chemical modifications appearing during treatment could be the origin of improved durability. There is a good correlation between decay resistance and mass loss measurements which are directly correlated to hemicellulose degradation.     

Investigation of the chemical modifications of beech wood lignin during heat treatment

Holocellulose, Klason lignin and milled wood lignin (MWL) of beech wood were extracted before and after heat treatment and analysed using CP MAS ¹³C NMR, ¹³C NMR, ³¹P NMR and size exclusion chromatography (SEC). Experimental results showed that the thermal treatment degrades hemicelluloses and affects lignin polymer through depolymerisation due mainly to cleavage of β-aryl-ether linkages and recondensation reactions. The spectroscopic analysis of MWL demonstrated that these recondensation reactions involved mainly guaiacyl units through formation of 5,5′-biphenolic and diarylmethane structures.Analysis of molecular weight distribution of MWL by SEC indicated that average molecular weights of heat treated milled wood lignin were lower than those of native milled wood lignin.     

Thermal degradation of chlorinated rubber: Evidence for an alternative cyclic structure for chlorinated rubber

The thermal degradation under vacuum or in nitrogen of commercial chlorinated rubber (ICI Alloprene, 64.5 wt.-% chlorine) was studied by isothermal thermogravimetry and by simultaneous TG/TVA with programmed heating by using a Cahn RG thermobalance built into a thermal volatilization analysis (TVA) system. Analysis of volatile products was performed by titration and by spectroscopic methods. The only major degradation product is hydrogen chloride; five-sevenths of the total available hydrogen chloride is lost with great ease, and complete dehydrochlorination is very much easier than in poly(vinylidene chloride). Conjugation develops early in the degradation, but the minor products methane, ethylene, and hydrogen are observed in the later stages of reaction. These features cannot be reconciled with the previously proposed cyclic structure for chlorinated rubber, and an alternative structure which accounts well for the degradation behavior is suggested.     

Experimental and numerical analysis of wood thermodegradation

Torrefaction is a thermal treatment step in a temperature range of 210?C240?°C, which aims to improve the dimensional stability and durability of wood. The mass loss kinetics for torrefaction of wood samples was studied using equipment specially conceived to measure mass losses during thermal treatment. Laboratory experiments were performed under nitrogen for heating rates of 0.1, 0.25, 1, and 2?°C?min^?1. A mathematical model for the kinetics of the thermodegradation process was used and validated. Measurements of temperature distribution and anhydrous mass loss were performed on dry sample of poplar wood during torrefaction in an inert atmosphere for different temperatures. The mathematical formulation describing the simultaneous heat and mass transfers requires coupled nonlinear partial differential equations. These unsteady-state mathematical model equations were solved numerically by the commercial package FEMLAB for the temperature under different treatment conditions. A detailed discussion of the computational model and the solution algorithm is given below. Once the validity of different assumptions of the model had been analyzed, the experimental results were compared with those calculated by the model. Acceptable agreement was achieved.     

Application of analytical pyrolysis for wood fire protection control

The effect of two flame-retardant compositions (A-76% potassium carbonate, B-67% sulphate ammonium) on the process of thermal degradation of wood and the composition of volatile products of pyrolysis has been investigated by the thermogravimetry and analytical pyrolysis methods. It has been shown that the effect of flame retardants manifests itself in the low-temperature region with the formation of more thermally stable intermediate carbonized products. Upon pyrolysis of wood under the action of the composition A, the mechanism of degradation of the lignin component changes, which manifests itself in a more than two-fold increase in the guaiacol and vinylguaiacol contents in the composition of volatile products and the inhibition of the formation of carbohydrates destruction products. It has been found that the composition B has a catalytic action mainly on the process of thermal depolymerization of cellulose, favouring the increase in the formation of levoglucosan and practically does not change the yield of lignin degradation products.     

Thermal and burning properties of wood flour-poly(vinyl chloride) composite

The present study deals with the effects of wood flour on thermal and burning properties of wood flour-poly(vinyl chloride) composites (WF-PVC) using thermogravimetric (TG), cone calorimetry (CONE), and pyrolysis?Cgas chromatography/mass spectrometry (Py?CGC/MS). TG tests show that an interaction occurred between wood flour and PVC during the thermal degradation of WF-PVC. Wood flour decreased the temperature of onset of decomposition of PVC. However, the char formation could be increased by adding wood flour to PVC. CONE test indicates that wood flour had positive effects on heat release and smoke emission of PVC. Comparing with PVC, WF-PVC reduced average heat release rate and the peak HRR by about 14 and 28%, respectively; smoke production rate was also decreased. The degradation mechanism was studied by Py?CGC/MS. The results show that the volatile pyrolysis products of WF-PVC are very different from PVC. The yields of HCl and aromatic compounds decreased dramatically, and the aliphatic compounds increased by the incorporation of WF.     

Effect of chemical modifications caused by heat treatment on mechanical properties of Grevillea robusta wood

Grevillea robusta, a Kenyan wood species of low durability was heat treated under inert atmosphere in laboratory conditions at temperatures between 220 and 250 °C. Modulus of rupture (MOR) and modulus of elasticity (MOE) were determined for different heat treatment conditions. MOR and MOE reduced with increase in heat treatment weight loss. MOE reduced insignificantly for weight loss less than 16% while reduction of MOR was more significant. For a fixed heat treatment temperature by varying the treatment duration, sugar content was analysed by HPLC after acidic hydrolysis and Klason lignin was determined. The amount of sugars other than glucose decreased with treatment time and was near zero after 7 h, while lignin quantity increased gradually. Wood acidity determined by titration decreased after heat treatment indicating degradation of uronic acids present in hemicelluloses. Chemical modifications of wood components were determined by CP/MAS ¹³C NMR analysis. Spectra indicated significant degradation of hemicelluloses. Increase of treatment duration resulted in the appearance of new signals, particularly obvious on spectra of samples treated for 15 h, attributed to carbonaceous materials involved in char formation.     

Preparation and degradation of copolymers of methyl methacrylate with alkali metal methacrylates—II: Thermal analysis of the copolymers,nature of the degradation products and general characteristics of the degradation reaction

Three series of copolymers, each spanning the composition range from alkali metal methacrylate homopolymer to methyl methacrylate homopolymer, have been prepared; their degradations have been studied under programmed heating conditions, by means of simultaneous thermogravimetry and thermal volatilization analysis (TVA) in a vacuum system and by differential thermal analysis in dynamic nitrogen. Total volatile products have been characterised by infrared spectroscopy, subambient TVA and GLC. The thermal analysis data suggest that the two types of monomer unit tend to participate in degradation processes in different temperature ranges. However, in addition to those products characteristic of the degradation of each homopolymer, the copolymers give substantial amounts of methanol; this product must arise from a reaction specific to the copolymer structure.     

Effect of branching on the degradation behaviour and caloric properties of PVC

Nine different PVC samples with defined chlorine content and degree of branching have been investigated. The aim of the work was the influence of these parameters on some caloric properties as well as the degradation behaviour and the degradation products. As expected, the heat of combustion decreases with increasing chlorine content. As determined by simultaneous thermal analysis/mass spectrometry, the volatile degradation products of the pyrolysis in nitrogen atmosphere vary. The higher the chlorine content, the higher the amount of chlorinated degradation products and the lower the amount of polycyclic aromatic hydrocarbons (PAH). A higher number of branching promotes the formation of alcylated aromatic hydrocarbons.     

Thermal degradation of phenyl methacrylate-methyl methacrylate copolymers

The degradation behaviours of poly(phenyl methacrylate), four phenyl methacrylate-methyl methacrylate copolymers which span the composition range, and poly(methyl methacrylate) have been compared by using thermogravimetry in dynamic nitrogen and thermal volatilisation analysis (TVA) under vacuum, with programmed heating at 10°C/min. Volatile products have been separated by subambient TVA and identified and the cold ring fraction and partially degraded polymer have been examined by ir spectroscopy. Poly(phenyl methacrylate) resembles poly(methyl methacrylate) in degrading completely to monomer. Copolymers of phenyl methacrylate and methyl methacrylate are more stable than the homopolymers. On degradation, the major products are the two monomers. Minor products from all the copolymers include carbon dioxide, dimethylketene, isobutene and formaldehyde. Copolymers with low and moderate phenyl methacrylate contents show the formation of anhydride ring structures in the cold ring fraction and partially degraded copolymer, together with small amounts of methanol in the volatile products. Carbon dioxide is a more significant product at lower phenyl methacrylate contents.The mechanism of degradation is discussed.     

Radiation degradation of poly(arylene ether ketone)s

The resistance of five poly(arylene ether ketone)s with related chemical structures to degradation by ionizing radiation has been studied by ESR spectroscopy and yields of volatile products. All of the polymers showed high resistance to radiation with low yields of radicals after irradiation in vacuum at 77 K (when up to 84% of the radicals were identified as radical anions) and much lower yields at 300 K. The yields of volatile products were much less than reported for poly(arylene sulfone)s [1, 2]. Methyl substitution on a main-chain aromatic ring decreased the radiation resistance, but methane only comprised 10% of the volatile products from the methyl-substituted polymers. A polymer containing an isopropylidene group in the main chain and a substituent aromatic carbonyl showed significantly decreased radiation resistance. Extremely low radical yields were obtained after irradiation in air at 300 K, contrary to many polymers. XPS analysis showed an increase in C–O bonds on the surface after irradiation in air.     

The structure and stability of oxidised polybutadiene

Polybutadiene has been oxidised at 0–20°C for periods from one day up to several months and also at elevated temperatures. Microstructural changes in the polymer due to oxidation have been followed by ir and uv spectroscopy. Oxidised samples of the polymer have been degraded in nitrogen or in vacuum under programmed heating conditions by TG, DSC and thermal volatilisation analysis (TVA). In TVA degradations, non-condensable gaseous products have been studied by adsorption TVA, condensable volatile products have been separated by subambient TVA for identification, cold ring fraction materials have been examined spectroscopically and the ir spectrum of the polymer residue, after degradation to 440°C, has also been obtained. From this considerable amount of data it has been possible to propose structures present in oxidised polybutadienes and to suggest some degradative processes involved in the subsequent thermal degradation. Oxidation leads to a considerable lowering of the threshold temperature for the main decomposition process; in samples oxidised at low temperatures, an additional early stage of degradation, commencing near 100°C, is found, due to peroxide decomposition.     

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.     

Elemental composition of wood as a potential marker to evaluate heat treatment intensity

Microanalyses of pine and beech blocks treated under nitrogen at 240 °C for different times have been investigated to evaluate correlation between mass loss due to treatment and elemental composition. Oxygen content decreases with treatment intensity and is directly proportional to treatment time. In the same time, carbon content increases significantly indicating formation of carbonaceous materials within the wood structure. Acetylation has been investigated to evaluate the effect of heat treatment on the quantity of free hydroxyl groups present in the wood. The results indicate a significant decrease of reactivity of heat-treated samples compared to untreated ones. Although this decrease depends on the treatment intensity, no obvious correlation was observed between weight gain due to acetylation and mass loss due to treatment. All these results suggest that elemental composition of heat-treated wood could be a valuable marker to evaluate mass losses due to thermal degradation and consequently treatment intensity.     

Thermal degradation of wood treated with flame retardants

Wood, one of the most flammable materials, was treated with various compounds containing nitrogen, phosphorus, halogens, and boron. For a study of flame retardance from the standpoint of thermal degradation, the samples were subjected to thermogravimetry (TG), differential thermal analysis (DTA) and differential thermogravimetry (DTG) in nitrogen to determine if there were any characteristic correlations between thermal degradation behaviors and the level of flame retardance. From the resulting data, kinetic parameters for different stages of thermal degradation are obtained using the method of Broido. The energies of activation for the decomposition of samples are found to be from 72 to 109 kJ mol^–1. For wood and modified wood, the char yields are found to increase from 10.2 to 30.2%, LOI from 18 to 36.5, which indicates that the flame retardance of wood treated with compounds is improved. The flame retardant mechanism of different compounds has also been proposed.     

Degradation mechanisms and environmental effects on perfluoropolyether, self-assembled monolayers, and diamondlike carbon films

The degradation mechanisms and durability of selected lubricants and environmental effects on the lubricants which could be used for microelectromechanical/nanoelectromechanical systems (MEMS/NEMS) applications were studied in this paper. The degradation of perfluoropolyether (Z-DOL), four self-assembled monolayers (SAMs)-hexadecane thiol, perfluoroalkylsilane, and alkylsilane (C8 and C18)-and diamondlike carbon (DLC) films was investigated in high vacuum. Gaseous products and friction force were detected using a quadrupole mass spectrometer and strain gauges. It is believed that triboelectrical reaction and mechanical scission cause the degradation of Z-DOL. SAMs are believed to degrade by cleavage at an interfacial bond accompanied with triboelectrical reactions. DLC is believed to degrade by mechanical shear and thermal oxidation. Environmental effects on lubricant films were studied in high vacuum, argon, and air at various humidity levels. It was found that the environment has a significant influence on the lubricant performance. The lubricant films exhibit high friction and low durability in high vacuum. Oxygen in the air can cause the thermal oxidation of SAMs and DLC films. Water molecules can act as a lubricant for Z-DOL films at a moderate humidity level, while they can penetrate the Z-DOL films at a high humidity level. Water molecules can detach the SAM molecules from the substrate, whereas, for DLC films, water molecules can act as a lubricant.     

The thermal degradation and oxidation of polybutadiene

Detailed studies have been made of the successive stages in the thermal degradation in air and nitrogen of carboxy-terminated polybutadiene (CTPB). During oxidation at high temperatures, a protective surface film is first formed; this film ruptures at temperatures where pyrolysis leads to the formation of volatile products in the bulk of the polymer. Thermogravimetric curves for the degradation of CTPB in nitrogen are complex in shape; it appears that the free-radical crosslinking and cyclisation reactions cause an increase in the thermal stability of the polymer during degradation.