Unmodified lignins

Figure 4. Variation in the polymer-polymer interaction parameter, B with hydroxy content of ethyl lignin (X), lignin acetate (O), unmodified lignin ( ), and hydroxypropyl lignin (—).

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... 

These benzaldehydes could then be directly used as a feedstock for various polymeric products or reduced to form phenolic benzylic alcohol derivatives (i.e., p-methylol groups). The p-methylol groups would thus be active sites, whereas in unmodified lignins, the C-l site is blocked and unreactive. In addition, the oxidative-cleavage step will hydrolyze a portion of the lignin interunit ether bonds, and thus increase the total fraction of free phenolic units to further enhance the reactivity. Other possible benefits are that the lignin would be extensively depolymerized and would form a more uniform feedstock material both conditions would give a product that is easier to handle. 

Two laboratories have been involved more recently in the synthesis and characterization of PUs based on unmodified lignins. In both approaches, the trick aimed at enhancing the lignins reactivity consisted in using a diol as third monomer, preferably capable of dissolving the lignin. 

The only published report on the use of unmodified lignins for the preparation of epoxy resins was announced by IBM in the late 1990s and dealt with the solution interaction between Kraft or organosolv lignins and various epoxy monomers, followed by a crosslinking operation induced by aromatic diamines [51]. Although details wctc scanty, it was announced that these materials were suitable for application in the electronic industry because of their excellent adhesion on copper, very good dielectric properties and temperature stability up to more than 175°C. 

The preparation method of the modified lignin-reinforced CB-BR vulcanizate is schematically shown in Fig.3. The compounding recipes illustrated in Table 2 were used for preparing five sorts of CB-BR Vu1canizates, i.e., the modified or unmodified lignin (ML or L)-... 

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