Polymer lignin modifiers

Macromolecules may be classified according to different criteria. One criterion is whether the material is natural or synthetic in origin. Cellulose, lignin, starch, silk, wool, chitin, natural rubber, polypeptides (proteins), polyesters (polyhydroxybutyrate), and nucleic acids (DNA, RNA) are examples of naturally occurring polymers while polyethylene, polystyrene, polyurethanes, or polyamides are representatives of their synthetic counterparts. When natural polymers are modified by chemical conversions (cellulose —> cellulose acetate, for example), the products are called modified natural polymers. 

The complex nature and interconnectivity of plant cell wall polymers preclude straightforward enzymatic digestion. There are dozens of enzyme families involved in plant cell wall hydrolysis, including cellulases, hemicellu-lases, pectinases, and lignin-modifying enzymes. The Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (IUBMB) has classified cellulases and hemicellulases, like all enzymes, into different classes based on activity. Table 33.2 and Table 33.3, compiled from the IUBMB enzyme nomenclature database (http //www.chem.qmul.ac.uk/iubmb/ enzyme/), list the IUBMB enzyme classifications for cellulases and hemicellulases.153... 

The natural polymer lignin is extracted from biomass, mainly wood by various technologies as described above. It accumulates in masses up to more than 50 x 10 tons at chemical pulp mills every year, worldwide, as a by-product of the pulp and paper industry [48]. When using it for material development, the most abundantly available types of lignin are modified by chemicals on extraction depending on the type of process and these might be used for their identification [53-55]. 

The qiplication of natural available lignin is of increased importance because of the lower cost of lignosulfonates in comparison to the cost for synthetic phenol. A substitution for 15 % of phenol with hgnin leads to a cost reduction of aboirt 9.5% [44] and a lignin modified phenolic. Table 2 lists the properties of an extracted lignin polymer [47]. 

Sarkar S, Adhikari B (2001) Jute felt composites from lignin modified phenolic resin. Polym Compos 22 518-527... 

S. Sarkar, and B. Adhikari, Jute felt composite from lignin modified phenolic resin. Polym. Compos. 22, 518-527 (2001). 

The utilization of TPS for the production of biodegradable plastics has increased and has been the object of several studies in the last decade. However, TPS has two main drawbacks namely its water affinity and its poor mechanical properties. To overcome these problems, the addition of other materials to TPS is necessary. In order to increase its water resistance, TPS has been blended with synthetic polymers and modified by cross-linking agents such as Ca and Zr salts. Substances such as waxes and lignin have also been tested to decrease the water uptake of starch-based materials. TPS s mechanical properties have usually been improved by addition of synthetic polymers, such as ethylene-acryhc acid and ethylene-vinyl alcohol copolymers. Another approach requires the use of natural fibers and mineral fillers. The inclusion of reinforcing fillers such as fibers could, however, enhance the degradation of thermoplastic starch because of the increase in the melt viscosity [79-81]. 

Vascular plant cell walls contain a wide variety of phenylpropanoids, such as monomers, dimers and polymers. Of these, the polymers (i.e., lignins and suberins) are the most abundant. According to our current knowledge, all cell-wall phenylpropanoids are derived from monomers synthesized in the cytoplasm. Following their excretion into the plant cell wall, these monomers can then be either photochemically or biochemically modified within the cell wall. 

Evidence supporting the original paradigm of lignin in wood as a random, three-dimensional network polymer is reviewed. More recent results which do not fit this simple picture are discussed. A modified paradigm is proposed in which lignin in wood is comprised of several types of network which differ from each other both ultrastructurally and chemically. When wood is deligni-fied, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

Recent systematic studies on the relation between network structure and substituents in kraft lignin, steam exploded, have shown that the lignin containing networks can be modified in new ways, cf. e.g. (80). Also the toughening of glassy, structural thermosets can be achieved by incorporating a variety of polyether and rubber-type soft segment components in the polymer network structure. 

To conclude the above methods of incorporation of modified lignin in polymer networks and blends opens new promising possibilities for the technical use of lignins and makes it competitive with other raw materials for engineering plastics. 

Thus, a small concentration of ortho- or parabenzoquinone species in an environment of phenolic functions could explain the radical enhancement upon basification. The residual spin content of the neutral or acid form of lignin is almost nil in whole wood, very small in native lignins, but significant in kraft and other chemically modified lignins. Such a stable free radical could be attributed to (a) the small equilibrium concentration of I in Equation 1, (b) a semiquinone polymer patterned after synthetic models (4y 25) containing donor and acceptor groups, or (c) radicals entrapped and stabilized in a polymeric matrix (5,15). 

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