Lignin Chemical modifications

The aromatic ring of a phenoxy anion is the site of electrophilic addition, eg, in methylolation with formaldehyde (qv). The phenoxy anion is highly reactive to many oxidants such as oxygen, hydrogen peroxide, ozone, and peroxyacetic acid. Many of the chemical modification reactions of lignin utilizing its aromatic and phenoHc nature have been reviewed elsewhere (53). 

Cellophane is, after chemical modification, obtained from the cellulose in wood, just as paper (from cellulose and lignin), cellulose fibres ( rayon ), and cellulose plastics. Leather is made from animal hides in a tanning process. 

In order to overcome the solubility limitation typical of lignin fractions, chemical modifications have been envisaged. Obviously only those methods giving nearly quantitative recovery are adequate for the purpose of measuring Mn- Table V shows results related to the acetylation technique where only a slight increase in M is observed as expected. 

Functionality Measurement of Phenolated Lignin. It is important to have knowledge of the functionality of the phenolated lignin from the point of view of further chemical modification. The amount of bound phenol in the phenolysis reaction has been measured by titrating the phenol extracted from the reaction mixture (15). This indirect method measures the unreacted phenol and determines bound phenol as the difference between the initial charge and the titrated phenol. This is sometimes misleading. 1H NMR spectroscopy is another candidate for the determination of the amount of bound phenol. However, this calculation is difficult since the number of protons before and after the phenolysis reaction is unknown. 

Ozonization of lignin forms derivatives of muconic acid that have the unique chemical structure of conjugated double bonds with two carboxyl groups. These derivatives have great potential for chemical modification. The ozonized lignin of white birch was soluble in epoxy resin at 120°C, and the free carboxyl groups were found to react with epoxide. This paper discusses developmental work on the preparation of pre-reacted ozonized lignin/epoxy resins the dynamic mechanical properties of cured resins and preliminary results of the application of these resins as wood adhesives. 

Effect of Chemical Modification with Polyethylene Glycol on the Brightness Stability of Lignin Model Compounds and High-Yield Pulp... 

H. Chemical Modification. The chemical modification of wood involves a chemical reaction between some reactive part of a wood component and a simple single chemical reagent, with or without catalyst, to form a covalent bond between the two. The wood component may be cellulose, hemicellulose, or lignin. The objective of the reaction is to render the wood decay resistant. The mechanism of the effectiveness is not known, but some possible explanations were given earlier. 

Due to its high stability and poor degradability, Kraft lignin is mainly burnt to provide the energy for paper mills [12]. Without chemical modification, it can be used as a component in construction materials [14]. 

Meshitsuka, G, and Isogai, A. 1996. Chemical structure of cellulose, hemicellulose, and lignin. In Hon, D. N.-S. (Ed.), Chemical modification of lignocellulosic materials (pp. 11-34). New York Marcel Dekker. 

Extraction of lignocellulosic materials with dioxane has been used for quite some time as a method for lignin isolation at atmospheric pressure (1) or, as recently reported, at high pressure and with supercritical C02 mixtures (2-5). In this case, it was possible to extract from wood lignin oligomers with a low degree of chemical modification, hemicelluloses were also depolymerized and extracted, but cellulose remained without significant mass losses. 

The methods listed above have the advantage of being applicable to the determination of lignin in the solid state and, being nondestructive, they obviate the risk of concurrent chemical modification. In addition, these analyses required very small amounts of sample. 

Of the above methods, the acetyl bromide method appears to have earned the most widespread acceptance. This recognition can be credited to the following advantages of the procedure it is rapid and simple, adaptable to small (milligram size) samples, does not require a correction for acid-soluble lignin, provides precise absorbance values from which the lignin content can be calculated, involves minimal chemical modification of the sample, and is accompanied by diminished interference from nonlignin products. 

Chemical Modifications. Unmodified lignin is well known for its poor solubility characteristics and its high glass transition temperature. Methods for improving the solubility (and/or reactivity) of lignin prior to crosslinking in specific network forming systems are summarized in Table I. Such systems may be based on aqueous solutions at pH below or above neutral or on solutions in polar or nonpolar solvents. Typical modifications that enhance the solubility of... 

The formation of uniform network polymers from lignin requires that phase separation prior to gel formation remain limited. The process of component demixing is controlled by both an enthalpic and an entropic contribution to Gibbs free energy. Whereas, the entropic factor requires the use of low molecular weight fractions for uniform gel formation, the enthalpic parameter necessitates chemical modification. The type of modification needed depends on the medium in which gel formation is performed. Examples of improving the solubility and compatibility with various reaction media are cited. 

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