Caseins are natural phosphorus-rich proteins, making up around 80% of milk protein. Casein was hydrolysed using acid, to give free amino groups, then reacted with phosphonic acid, formaldehyde and urea, to give a phosphorus – nitrogen bio-based PIN FR. Cotton fabric was then treated by reacting with this PIN FR in aqueous solution with dicyandiamide catalyst, curing at 180°C, then washing and drying, to achieve up to 16% w/w loading. At this loading, LOI of the cotton fabric was increased by nearly 100%, remaining increased by c. 40% after 40 wash cycles. Tensile strength and whiteness of the cotton were substantially retained. Analysis suggests that the casein-based PIN FR grafts onto the cellulose in cotton by P(=O)-O-C and C(=O)-O-C bonds and acts mainly by increasing char generation, with phosphorus preventing formation of levoglucosan by cellulose and by catalysis of dehydration of cellulose.
“Novel High-Efficiency Casein-Based P−N-Containing Flame Retardants with Multiple Reactive Groups for Cotton Fabrics”, F. Xu et al., ACS Sustainable Chem. Eng. 2019, 7, 13999−14008 http://dx.doi.org/10.1021/acssuschemeng.9b02474