Modification of lignin for the production of new compounded materials

The cell walls of woody plants are compound ed materials made by in situ po lymerization of a polyphenolic matrix (lignin) into a web of fibers (cellul ose), a process that is catalysed by polyphenoloxidases (laccases) or perox idases. The first attempt to transform the basic strategy of this natural p rocess for use in human craftsmanship was the ancient lacquer method. The s ap of the lacquer tree (Rhus verniciflua) contains large amounts of a pheno l (urushiol), a polysaccharide and the enzyme laccase. This oil-in-water em ulsion solidifies in the presence of oxygen. The Chinese began using this p henomenon for the production of highly creative artwork more than 6,000 yea rs ago. It was the first example of an isolated enzyme being used as a cata lyst to create an artificial plastic compound. In order to apply this proce ss to the production of products on an industrial scale, an inexpensive phe nol must be used, which is transferred by an enzyme to active radicals that react with different components to form a compounded material. At present, the following approaches have been studied: (1) In situ polymerization of lignin for the production of particle boards. Adhesive cure is based on the oxidative polymerization of lignin using phenoloxidases (laccase) as radic al donors. This lignin-based bio-adhesive can be applied under conventional pressing conditions. The resulting particle boards meet German performance standards. By this process, 80% of the petrochemical binders in the wood-c omposite industry can be replaced by materials from renewable resources. (2 ) Enzymatic copolymerization of lignin and alkenes. In the presence of orga nic hydroperoxides, laccase catalyses the reaction between lignin and olefi ns. Derailed studies on the reaction between lignin and acrylate monomers s howed that chemo-enzymatic copolymerization offers the possibility to produ ce defined lignin-acrylate copolymers. The system allows control of the mol ecular weights of the products in a way that has not been possible with che mical catalysts. This is a novel attempt to enzymatically induce grafting o f polymeric side chains onto the lignin backbone, and it enables the utiliz ation of lignin as part of new engineering materials. (3) Enzymatic activat ion of the middle-lamella lignin of wood fibers for the production of wood composites. The incubation of wood fibers with a phenol oxidizing enzyme re sults in oxidative activation of the lignin crust on the fiber surface. Whe n such fibers are pressed together, boards are obtained which meet the Germ an standards for medium-density fiber boards (MDF). The fibers are bound to gether in a way that comes close to that by which wood fibers are bound tog ether in naturally grown wood. This process will, for the first time, yield wood composites that are produced solely from naturally grown products wit hout any addition of resins.