The idea of using natural products in adhesives is not new. Animal blood and casein adhesives are one of the oldest materials used as wood adhesives. They were displaced by higher performance synthetic polymers in the 1950s. However, due to cost and environmental concerns, the natural products are now regaining a prominent position at least with regard to development. A primary objective is to develop new synthetic materials using natural resources as a starting point which will provide similar or enhanced properties to their petroleum-based counterparts. One of the naturally occurring polymers that is achieving significant attention is lignin - a waste product of the pulp and paper industry.
Lignins intuitively should be considered as good wood and paper adhesives since they are the substances that hold plant fibers together. They are one of the most abundant organic polymers on earth, constituting 25 to 33% of the dry mass of wood. Lignins are recovered mainly as byproducts from wood pulping operations with about 75 million tons produced annually worldwide.
Over the years, lignin has been one of the most intensely researched raw materials for wood adhesives, mainly because of its very low cost. In fact, paper mills and pulp producers treat lignin as a waste and pay for its removal. The annual sales of lignin as specialty chemicals in 1998 amounted to only 1% of the total lignin production. The remaining 99% is burned in an energy recovery step or disposed of in waste streams. Lignin production is expected to rise significantly with the current shift toward biorefineries, in which lignin is also a byproduct.
Lignin provides biomass based resins with a phenolic-like molecular structure. Lignin is unusual as a biopolymer and more difficult to adapt to adhesives than starch, cellulose or soy because of its heterogeneity and imprecise composition. Native lignin is a crosslinked polymer, but the polymers have to be partially degraded to allow them to be separated from the cellulosics. For adhesive purposes, these degraded lignin resins need to be further polymerized to obtain useful properties.
Lignin from the pulping process does not lead to a useful product because of the cost of separating the lignin from the pulping chemicals and the variability of the product. However, lignin sulfonates, contained in the spent sulfate liquids from sulfite pulping of wood have been found to be a more useful feedstock for the production of reactive lignin.
Lignin based wood adhesives have been prepared with formaldehyde or other aldehydes. Although they have been claimed to provide properties competitive with conventional phenol formaldehyde binders, the quality of lignin-based resins varies significantly depending on wood source, the pulping operation, and various other factors.
The major disadvantages of lignin are that they have lower reactivity towards formaldehyde or other aldehydes than other resins, long curing times at elevated temperatures, and they are highly corrosive to processing equipment. The lignin based resins also have a high degree of variability and are dark in color. For reactive lignin to be used in the production of adhesives, they need to be modified in some manner (e.g., by hydroxymethylation, epoxidation, conversion to an isocyanate).
Over the last decade there has been an enormous effort to develop lignin based adhesive. These have been explored in many large volume applications such as floor coverings and binders for composite wood products. This has met with little significant commercial success principally due to the disadvantages noted above. However, companies are taking a new look at lignin based adhesives as a result of the drive toward protecting the environment and reducing our dependency on petroleum based raw materials.
For example, Huntsman International LLC has made significant commitments to the development of new materials based on natural raw materials. They have recently signed a Memorandum of Understanding with Lignol regarding the development of applications based on Lignol’s lignin HP-L product. Huntsman is assessing the performance of lignin in a range of applications with the intention of exploiting these attributes in polyurethane systems including adhesives, foams, and coatings. 1
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Anon, "New Biorefineries Could Provide Raw Material for Coatings", JCT Coatings Tech, March 1, 2008.