Hemicelluloses thermal stability

In this chapter, polymer properties related to thermal stability and molecular motion of lignin in the solid state are discussed. Several papers on the molecular motion of in situ lignin have been published [39-42]. However, it is difficult to measure a single component in a biocomposite such as wood, since it contains three macromolecular species cellulose, hemicellulose, and lignin. In the past, the sensitivity of available experimental instruments was insufficient to detect the molecular... 

Thermal stability of hemicelluloses was performed using thermogravimetric analysis (TGA) on a thermal analyzer (TA Instruments High-Resolution Model Q-500). The apparatus was continually flushed with nitrogen. The samples weighed between 10 and 13 mg and were run fiom room temperature to 600 °C at a rate of 10 °C/min. 

Chemical and thermal stability of hemicellulose is generally lower than other celluloses, due to the lack of crystallinity and the lower degree of polymerization. This compound is very easily hydrolyzed, especially in the presence of hydroxyl groups on each unit of the chain. The results of hydrolysis include glucose, xylose, mannose, galactose, arabinose, glucuronic acid, and mannuronic acid in different amounts [9]. 

The thermogravimetric analysis of WHF/HDPE composites after various treatments is depicted in Figure 3.8. It can be seen that 5% alkali treatment caused the decrease of thermal stability of the WHF/HDPE composite. However, when the concentration of alkali increased up to 20%, the thermal stability increased. This can be explained by the fact that alkali treatment removes some particles at the outer part of the fiber skin. The particles are believed to have higher thermal stability. Hence, when the particles are removed, the overall thermal stability of the WHF decreased. However, at the higher concentration, the alkali removes hemicelluloses, which are of lower thermal stability. As a result, the overall thermal stability of the fiber and the composite increased. Figure 3.9 shows the comparison of the thermal stability values. 

Figure 15.11 shows a thermogravimetric analysis of pine flour and wood cellulose. Due to its low thermal stability, wood flour is usually used as filler only in plastics that are processed at low temperatures, lower than about 200 °C. Above these temperatures the cell wall polymers begin to decompose. High-purity cellulose pulps, where nearly all of the less thermally stable lignin and hemicelluloses have been removed, have recently been investigated for use in plastic matrices such as nylon that are processed at higher temperatures than most commodity thermoplastics [27]. 

The conversion of water-soluble wheat straw hemicellulose with succinic anhydride in aqueous alkaline solutions yields carboxyl groups containing products (6). The DS values of the products are rather low (<0.26) due to side-reactions in the aqueous medium. Despite the low functionalization, the thermal stability is increased and applications as thickening agents and metal ion binders are proposed. 

The different thermal stabilities of hemicellulose, cellulose, and lignin provide an opportunity to use pyrolysis for the thermal fractionation of biomass. The bar graph in Figure 8.1 presents a schematic overview of the different thermal stabilities of each of the main biomass fractions.The height of the bars corresponds to the approximate temperature level at which the thermal degradation rate of the biomass constituent under isothermal conditions and in an inert atmosphere reaches a maximum as can be measured by thermogravimetric analysis. ... 

Figure 8.1 Thermal stability regimes for hemicellulose, lignin, and cellulose. ...

The mechanical and physical properties of the fibers are mainly influenced by its composition, mainly cellulose, hemicellulose and lignin [3,8], As an example, higher tensile strength and higher thermal stability are obtained for fibers that contain more... 

Similarly wheat straw, another annually replenishable biomass resource available in many countries, contains about 35-38% hemicellulose (Sun and Tomkinson, 2002). Extraction of this hemicellulose, containing about 70% xylose, 13% arabinose and 14% gluclose by an ultrasound irradiation technique, improved the molecular weight as well as the thermal stability of the hemicellulose, and is a promising technique if used in conjunction with other techniques such as steam explosion . This will enable the extraction of hemicellulose in such a form that the chemical structure will be as little changed as possible from the native state. 

Figure 8.1 indicates the potential for thermal fractionation of the biomass. The order of thermochemical stability of the individual biomass constituents ranges from hemicellulose (fast degassing/decomposition from 200 to 300 °C) as the least stable polymer to... 

---