Abstract
This review article aims to summarize the potential of using renewable natural resources, such as lignin and tannin, in the preparation of NIPUs for wood adhesives. Polyurethanes (PUs) are extremely versatile polymeric materials, which have been widely used in numerous applications, e.g., packaging, footwear, construction, the automotive industry, the lighting industry, insulation panels, bedding, furniture, metallurgy, sealants, coatings, foams, and wood adhesives. The isocyanate-based PUs exhibit strong adhesion properties, excellent flexibility, and durability, but they lack renewability. Therefore, this study focused on the development of non-isocyanate polyurethane lignin and tannin resins for wood adhesives. PUs are commercially synthesized using polyols and polyisocyanates. Isocyanates are toxic, costly, and not renewable; thus, a search of suitable alternatives in the synthesis of polyurethane resins is needed. The reaction with diamine compounds could result in NIPUs based on lignin and tannin. The research on bio-based components for PU synthesis confirmed that they have good characteristics as an alternative for the petroleum-based adhesives. The advantages of improved strength, low curing temperatures, shorter pressing times, and isocyanate-free properties were demonstrated by lignin- and tannin-based NIPUs. The elimination of isocyanate, associated with environmental and human health hazards, NIPU synthesis, and its properties and applications, including wood adhesives, are reported comprehensively in this paper. The future perspectives of NIPUs’ production and application were also outlined.
Highlights
The aim of the present review is to summarize the current state of research and recent developments in the field of lignin- and tannin-based Non-Isocyanate PUs (NIPUs) for wood adhesives
The polycondensation pathway synthesis of NIPUs is linked to the reaction between di-or polycarbamates and diols [80], di-or polycarbonates and amino alcohols [145]
Lignin and tannin have the potential to be used as an alternative material for polyols in the manufacture of polyurethane as wood adhesives
Summary
PMDI adhesives are characterized by quick curing, formaldehyde-free emission characteristics, low loading amounts, and excellent weather resistance; they are mainly used in the production of OSB and other similar particulate wood-based panels [40,50]. The relatively greater price of pMDI compared to the other common wood adhesives is the main limiting factor for their wider industrial application Manufacturing of these resins has relied on non-renewable petroleum-based resources [51]. The production of bio-based adhesives raises additional challenges; adhesives made from renewable natural materials often have low water resistance or are too costly to compete effectively with fossil-derived polymers. The aim of the present review is to summarize the current state of research and recent developments in the field of lignin- and tannin-based NIPUs for wood adhesives
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