Cross-linked lignin adhesives

Recently, adhesives that include lignin in their formulation have shown several properties similar to phenol-formaldehyde resins [11, 12] and are extensively utilized in the plywood, particleboard, fiberboard and laminated wood industries. The chemistry of the lignin-based adhesives has been studied from an empirical point of view [13] however, the molecular mechanism and the interactions of the structural cross-linked 4-hydroxyphenylpropanoids of lignin that drive the adhesion phenomenon remain unclear. 

Compared to previous gluten based adhesives the present resins are applicable in liquid form, thus without any need for modifications of the application systems in particleboard factories. Relative to other protein adhesives such as soy-based adhesives based on reaction with formaldehyde some of the resins presented here have several advantages (i) they cannot and do not produce any aldehyde emission as neither formaldehyde nor any other volatile aldehyde was used in some of the formulations (ii) the percentage of natural materials was increased up to 70% for one type of formulation and up to 95% for others. Furthermore, in relation to resin formulations based on different cross-linking reactions other than those with formaldehyde the resins presented here have other advantages they are competitive with alternate natural resin systems such as those based exclusively on tannins and/or lignins. 

Lignin is an amorphous, cross-linked polymer network consisting of an irregular array of variously bonded hydroxy- and methoxy-substituted phenylpropane units [13]. The chemical structure varies depending on its source. Lignin is less polar than cellulose and acts as a chemical adhesive within and between fibers. 

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