Lignin possesses a lot of hydroxyl groups and is easily accessible as a by-product of the pulp and paper industries,it is a perfect candidate as a polyol precursor for polyurethane. Lignin-based polyurethanes are typically created by reacting diisocyanate with either modified or unmodified lignin.
However, lignin has drawbacks as well, not the least of which is that it renders the material brittle; as a result, it is frequently combined with other polyols to enhance performance. Even still, only approximately 30% of lignin is actually practicable. Another drawback is the lignin hydroxyl group's limited reactivity with isocyanates, particularly aromatic isocyanates, as a result of steric hindrance. To solve this issue, modification techniques like hydroxypropylization are frequently used.
The end-sealing isocyanate approach as an alternative way for producing lignin-based polyurethanes which is widely used in the field of adhesives has been created by a research team at the ABC Federal University in Brazil.The initial interaction with the polyol hydroxyl group is prevented by the isocyanate group's first reaction with the terminal agent, which results in the formation of a functional group that is chemically stable. Only above the temperature at which the protecting group dissociates do polyurethane bonds develop.
Lignin from eucalyptus hardwood was extracted and then reacted with MDI using castor oil as the second polyol and diisopropylamine as the terminal agent. They claim that the resultant polyurethane is appropriate for use in single-component adhesives or coatings that activate using heat. Single-lap shear tests on steel substrates, in particular, demonstrate the superior performance of hydroxypropyl lignin-based polyurethanes.
The advantages of this process including lower sensitivity to moisture, lower toxicity and better storage stability due to the reduced concentration of cyanate esters. It can also be used to control the molar mass and molecular structure of polyurethanes.