Physical and Chemical Properties of Lignin
The source from which lignin is obtained and the method of extraction has a strong bearing on its physical and chemical properties. Additionally, choosing the lignin with appropriate properties is an important factor in utilization of lignin for various applications [19]. As lignin is a highly crosslinked material with widely varying functionality, this natural polymer may not readily be characterized to give meaningful molecular weight data, but other parameters more directly relevant to end-use properties may be assessed. In spite of this, the molecular weight data does provide some useful guides. Major functional group compositions and molecular weights of selected lignins are shown in Table 10.3. The reactivities of these
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Table 10.3 Molecular weights and major functional group contents of some selected lignin samples [14].
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functional groups and compositions will impact on the attributes of the end products. For example, Muller and coworkers have found that Kraft lignin-based phenol formaldehyde resins have superior hardness properties to steam-exploded lignin-based phenol formaldehyde [26].
Glass transition temperature (Tg) is another important parameter in lignin, which is an indirect measure of crystallinity and degree of crosslinking of the polymer, and directly indicates the rubbery region of the material [27]. The Tg values of some common forms of lignin are shown in Table 10.4. Glass transition temperature of lignin will depend on molecular weight and chemical functionalization, as well as the amount of water and polysaccharides in the sample. In general, Tg will indicate the mobility of the lignin molecules, where a lower Tg points towards a greater mobility of molecules. While Tg generally increases with increasing molecular weight, the impact of structural variation based on the degree of polymerization has only recently been established. In these experiments, Baumberger and coworkers used a series of lignin samples isolated from transgenic poplars. They proved that the variations in Tg are closely related to the degree of polymerization of lignin as determined by thioacidolysis [28].
In addition to the thermal properties, the miscibility of lignin with other polymeric materials is an important physical property in many applications. A number of researchers have shown that potential applications of lignin can be expanded further if the miscibility
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Table 10.4 The Tg values of different lignin types [27].
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of lignin with other polymeric materials can be improved [14]. This may be done through chemical modification of lignin with appropriate hydrophobic groups such as butyrate, hydroxypropyl, and ethyl group. An alternative approach to improve the miscibility may be the use of lignin copolymers [14].