Cross-linking, lignins

Acid or alkaline treatments of particulate feeds have been shown to improve digester gas yields (17.22-25). Acid hydrolysis was not used in our work because severe reaction conditions are required, and there is considerable decomposition of the hydrolytic products under these conditions ( ). Dilute alkaline pretreatment was evaluated because alkali was shown to be more effective in promoting hydrolysis of cellulosic biomass than acid (17,26). It is postulated, for example, that sodium hydroxide breaks down the cross-linked lignin macro-molecules surrounding the cellulose fibers into alkali-soluble lower-molecular-weight units. In this way, the cellulose fibers are exposed for... 

Wood is a naturally occurring composite material consisting of lignin, cellulose, hemicelluloses (= pentoses), and water a composite consisting of oriented cellulose fibers in a continuous matrix of cross-linked lignin plasticized by water. Freshly felled (green) wood contains about 40%-60% water and air-dried wood contains about 10%-20%. The composition of wood varies according to type of tree (Table 24-14). 

The fluid loss and thickening time characteristics of the cement slurry is altered, either by increasing the molecular weight of the lignin by cross-linking with formaldehyde or epichlorohydrin or by adding agents such as sodium sulfite, sodium metasilicate, sodium phosphate, and sodium naphthalene sulfonate. 

The inability to solubilize aromatic components of suberin-enriched preparations by the methods used for lignin suggests that suberin structure is distinctly different from that of lignin, probably due to the aliphatic cross-linking and the higher degree of condensation present in suberin. 

The whole of a multi-cellular organism is contained by outer cell layers, which are described in biology texts, and maintained by connective tissue. Connective tissue is a novel, external biopolymer structure of multi-cellular organisms found within their new extracellular, circulating fluid compartments (see Section 8.9). As mentioned there, the main connective tissues, covalently cross-linked structures, are (1) those of plants, celluloses (polysaccharides), often cross-linked by lignin (2) those of lower animals and insects, mixed cross-linked polysaccharides and... 

Felby C, Hassingboe J and Lund M. 2002. Pilot-scale production of fiberboards made by laccase oxidized wood fibers board properties and evidence for cross-linking of lignin. Enzyme Microb Technol 31(6) 736—741. 

Curing The wood is thermally treated at a temperature ranging from 150 °C to 190°C under dry conditions, for 12-16 hours. During this stage, reactions occur between the hemicellulose breakdown products and the activated lignin to form cross-links. The wood leaves this part of the process at less than 1 % moisture content. 

Fig. 7. Mechanism of the cross-linking of lignin macromolecules or of the grafting of lignin onto polysaccharides by single Rb-type coniferyl radicals...

Another potential lignin modification method concerns the reaction with difunctional acid chlorides, especially terephthaloyl chloride, to form cross-linked polymeric materials (1,10,14,20). Several studies have dealt with... 

It is recommended that a reslurry of crude OSL in an organic solvent or 10% aqueous salt (e.g., NaHCOa) solution be performed to remove low-molecular-weight (mono-functional) species, waxes, and carbohydrates. This purification leads to an improvement in OSL reactivity and contributes to the usefulness of OSL as a PF resin extender or PF copolymer raw material. It is presumed that extraneous removed materials in the crude lignin react with formaldehyde but do not lead to productive cross-linking polymer formation. 

T ignin is one of the most abundant natural products constituting about one-fourth of the woody tissue in plants. Nature has chosen a unique synthetic technique to prepare this cross-linked polymeric material from coniferyl alcohol and related substances. The mechanism of lignin formation is not completely known yet, and the structural characterization of lignin has been only partially successful despite considerable research. 

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