Starting from the our previous work, this article reports the successful green and innovative synthesis of flexible polyurethane foams starting from Protobind lignin and oxypropylated Protobind lignin, by following an environmentally-friendly process based on the use of microwave irradiation in order to liquefy the lignin, and on the use of water as a blowing agent. Glycerol and PEG 400 were used as liquefaction solvents, while polypropylene glycol triol or castor oil were used as chain extenders in order to produce flexible foams in controlled expansion, thus tailoring the properties of the final foams. All samples were prepared using a one-shot approach maintaining the NCO/OH ratio less than one hundred, thus reducing significantly the degree of crosslinking and giving higher flexibility to the material.
Considering the characterizations carried out, foams based on PPG triol showed a lower Tg of the soft phases, clearly separated from the hard ones, while samples based on castor oil as a chain extender presented both phases mixed. The amount of lignin in the formulation ranged from 6% to 13% and considering castor oil as renewable source, the amount of green raw materials can be more than 45%.
The kind of lignin did not greatly influence the structure and properties of the produced foams, although oxypropylated lignin was dissolved more easily in liquefaction solvents than commercial soda lignin, and, in addition, the miscibility between liquefied lignin and chain extenders (in particular with castor oil).
All the polyurethane foams showed a high content of open cells and their thermo-mechanical properties were suitable for fillings and packaging applications, such as filling for the interior parts of car seats and packaging of furniture. In this sense, foams produced by using PPG triol as a chain extender had an apparent density ranging from about 70 to 150 kg/m3 and a compression force deflection value from about 2 × 10−2 to 7 × 10−2 MPa, while foams based on castor oil had an apparent density ranging from about 130 to 210 kg/m3, and a compression strength ranging from about 7 × 10−3 to 3.5 × 10−2 MPa.
Starting from soda lignin or soda oxypropylated lignin could be considered a good alternative way to produce flexible polyurethane foams based on renewable sources, in agreement with previous work based on Kraft lignin.