Technical lignins

Table 5. Phenolic and Aliphatic Hydroxyl Contents of Milled Wood and Technical Lignins ...

The significance of phenoxy anions is well recognized in the isolation of kraft and other water-insoluble technical lignins by acid precipitation. The ioniza tion of phenoHc hydroxyl groups coupled with the reduction of molecular size renders native lignin soluble in the aqueous pulping solution, thus enabling its separation from the polysaccharide components of wood. 

Note The content of functional groups depends on the origin of the lignin in the cell wall. Great variations in molar masses and Mw/Mn-values are reported for technical lignins. 

In alkaline media, phenolic units may react with formaldehyde, forming methylol derivatives that condense with themselves or with another phenol (J, Fig. 1.4). This formaldehyde condensation reaction forms the basis for using technical lignins in the production of adhesives. 

Very little information is available on lignin spectroscopy in this range. The spectra of beech and bamboo MWLs (Fig. 4.1.18) again show only diffuse bands originating from aromatic ring deformation vibrations. Sulfur-containing technical lignins display more pronounced features because of C—S vibrations. 

Fingerprinting and identification of technical lignins from various pulping processes... 

Fengel D, Wegener G, Feckl J (1981) Contribution to characterization of analytical and technical lignins (2) Physical-chemical and electron microscopical studies Holzforschung 35 111-118... 

Native, as well as technical lignins, whether present in pulp (wood) or in the form of isolated samples, usually contain trace amounts of catechol structures These can be identified and quantified if the alkylation step in the oxidative degradation procedure is performed with diethyl sulfate since, in this case, various types of catechol units in the form of 1,2-diethoxybenzene derivatives are distinguished from guaiacyl and synngyl structures (Gellerstedt and Lindfors... 

Table 7.1.1. Total, phenolic and aliphatic hydroxyl contents of some representative milled wood and bamboo lignins (MWL and MBL) and technical lignins based on C9 unit... 

Note 8. The value C=0/C9 is meaningful only if the lignin preparations under study are sufficiently pure and have C9 structure (some technical lignins may not have a true C9 structure). In such a case, results for the total carbonyl content of lignin should be expressed as C=0/0CH3. See Chapter 7.6.3 for the significance of the methoxyl content of lignin. 

Hill M, Fricke A (1984) Ultrafiltration studies on a kraft black liquor. Tappi J 67(6) 100-103 Klinkowski PR (1978) An emerging unit operation. Chem Eng 8 165-173 Lin SY, Detroit WJ 1981 Heterogeneity of technical lignins - its significance in lignin utilization. Proc 1981 Int Symp Wood Pulp Chem. Ekman-Days 1981 (Stockholm). SPCI Rep 38,4 44-52... 

Note 2. Some technical lignins will precipitate at this pH. It is important to prevent their deposition on the fritted disc at the base of the inner tube to insure that extraction will not cease. 

There have been many attempts to replace these resins with lignin derivatives for wood composite adhesives suitable for plywood, particleboard and waferboard. Most of these studies have been empirical in nature, and few have achieved further consideration for industrial application. As wood binders, technical lignins are variable in quality and poorly reactive in comparison to conventional resin systems such as phenol-formaldehyde (PF) resins. Consequently, they are not utilized on their own. Indeed, if they were, this would adversely affect production quality and times, and necessitate equipment changes. In the wood composite industry, resins having such deleterious effects are not likely to be used even if savings could be made in terms of material costs. 

Technical lignins Pine kraft lignin O4H7 4202(H20)o 2hSo.08(OCH3)o 77 1.35 0.58 0.77... 

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