Nanjing, China – While the sustainability attributes of cellulose-based materials are appealing, using them in place of plastics is often unsuccessful because of their poor wet mechanical and barrier properties. Polymer coatings are commonly used to improve their performance, particularly polyurethanes, but there are solvent concerns.
Now, a group at Nanjing Forestry University has been looking to create an alternative material based on lignin and a water-based PU emulsion that might be more sustainable than petroleum-based plastic films, and without solvent hazards.
They took what they described as an environmentally blocked Kraft lignin-based waterborne IPDI-based PU emulsion and used it to make a hydrophobic, translucent, biodegradable recycled fibre composite film via a blocked terminal isocyanate group. Lignin is preferable to polyols from vegetable oils because it does not compete with the food chain.
Furthermore, the PU emulsion was stable on storage, in contrast to most lignin-based PUs that are not stable on long-term storage as they require excess isocyanate and forms gels. Blocking the free isocyanate group meant they were water stable, but the isocyanate group was freed by hot pressing, and thus able to bond with hydroxyl groups on the surface of the fibre network.
The tensile strength, Young’s modulus and toughness of the composite film were all higher than that of a control film. It also had good water stability, with water contact angle and wet strength both better than the control
Additional benefits of the composite film include UV-blocking properties and good thermal stability. The group anticipates that it could have applications as an environmentally friendly packaging and construction material, including for food and as an agricultural covering.
The work has been published in the journal Industrial Crops and Products.