Kraft Lignin Bio-Polymer for Polyurethane Foams Synthesis
With the world moving towards bio-based alternatives; Kraft Lignin Bio-Polymer for Polyurethane Foams is gaining a lot of attention as an attractive bio-polymer for a probable partial replacement of both the Polyol and diisocyanate. Lot of investigation is also going on in the field of PU foams as well to replace the conventional raw materials with bio-based alternative.
Polyurethane foam is one of the four basic types of products that can be made from raw, liquid polyurethane. They are comprised of two chemicals which, when mixed and heated, form liquid polyurethane prior to being further processed. These chemicals are polyol, a type of complex alcohol, and diisocyanate, a petroleum byproduct that reacts strongly with alcohol. By combining the two, a stable long-chain molecule is formed. It’s a polymer, or plastic, known as urethane. Flexible polyurethane foam (‘PU foam’) is a thermosetting plastic.
Lignin is the second most abundant renewable biopolymer found in nature. The impressive properties of lignin, such as its high abundance, low weight, environmentally friendliness and its antioxidant, antimicrobial, and biodegradable nature, along with its CO2 neutrality and reinforcing capability, make it an ideal candidate for the development of novel polymer composite materials. Therefore, lignin has gained the attention of researchers as a raw material for the synthesis of value-added products of commercial importance.
Lignin has also been exploited to synthesize various chemicals like esters, ethers, bio-adhesives, biolubricants, foams, nano-particles and nano-composites. There are two main types of lignin based on extraction processes, sulfur lignin and sulfur-free lignin. Sulfur-containing lignin is produced by commercial chemical pulping processes (paper and pulp industries) including kraft lignin and lignosulfonates. Kraft lignin is a highly condensed CC structure, containing higher amount of phenolic hydroxyl groups.
Utilisation of kraft lignin for making bio-based PU foams in different ways
- Kraft lignin as a reactive additive to bio-polyol:
- Kraft lignin as a crosslinking agent to polyol:
- Chemically modified kraft lignin as a bio-polyol:
- Kraft lignin as a pre-polymer to a isocyanate:
Kraft lignin as a prepolymer to isocyanate is the most promising option for bio-based PU foams as the hydroxyl groups from lignin can be directly reacted with isocyanate to form urethane groups, various strategies have been devised to incorporate lignin into the PU products not only as a blend component, but also as a macromonomer with or without chemical modification to replace petroleum-derived polyols. The volatile raw material prices of MDI & TDI is a major concern in the PU foam industry today. So Kraft Lignin, being a most economical product, when used as a pre-polymer to isocyanate also results in an eco-friendly product. Considering the merits of excellent biodegradation, low cost, and environmental friendliness, lignin-based materials exhibit potential application for sustainable materials.