Lignin helps waterborne PU foam achieve UL 94 V-0 rating
Researchers in Hong Kong have developed a halogen-free, flame-retardant flexible polyurethane foam using lignin, waterborne polyurethane and a phosphorus-based additive.
The material achieved a UL 94 V-0 rating and a limiting oxygen index of 26.5%, according to a paper published in the American Chemical Society journal Langmuir.
The researchers used an aqueous Pickering foam templating process, in which solid particles stabilize air bubbles during formation of the foam structure. Lignin acted as both a biobased stabilizer and a gelation agent, while waterborne polyurethane formed the polymer matrix.
Tests showed that lignin concentrations above 4 wt% created a physical network with the waterborne polyurethane that stabilized the foam and prevented liquid drainage.
The team combined the lignin with the halogen-free flame retardant 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthrene 10-oxide, commonly known as DOPO-HQ. The additive also acted as a reinforcing component.
Cone calorimeter testing showed that the combination reduced peak heat release from 468 kW/m² for the reference material to 280 kW/m². The foam produced a char yield of 19.41%, which the researchers said indicated improved thermal stability and flame resistance.
The resulting open-cell foam had a compressive strength of up to 231 kPa and recovered more than 99% of its original volume following compression.
Epoxy crosslinkers TGE and GPTMS were used in place of aziridine, which the researchers described as toxic. They said the substitution made the water-based manufacturing process safer and more environmentally sustainable.
The work was conducted by researchers from Hong Kong Metropolitan University’s School of Science and Technology and the Chinese University of Hong Kong’s Department of Chemistry.
The researchers said the process could provide a route to high-performance, flame-retardant flexible PU foams for insulation, transportation and furniture applications.
The paper, Sustainable Halogen-Free Flame-Retardant Flexible Polyurethane Foams via Lignin-Waterborne Polyurethane Pickering Foam Templates, was written by Zhenghao Shi, Qin Wang, Ernest Kin Wai Tsang, Jianlin Chen, Man-Hin Kwok, Jie Han and To Ngai.
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