Cellulosic fibers coated in mineral sheaths for flame-retardant nonwovens

While plant-based materials composed of cellulosic fibers have a favorable sustainability profile in several respects compared to synthetic alternatives, they are flammable and often require treatment for safe use. Many traditional flame-retardant finishes are toxic, while others are difficult to integrate with fibers or fabrics. In this work, cellulosic fibers (cotton, lyocell, jute, and hemp) were coated with crystalline calcite (calcium carbonate) or gypsum (calcium sulfate dihydrate) using a “polymer-induced liquid precursor” strategy. During this process, supersaturated solutions of insoluble minerals are induced by additives to form metastable intermediate droplets before crystallizing. These droplets wet the fibers via capillary action before nucleating to form crystalline sheaths that surround the fibers; calcite coatings were notably smooth and conformal, whereas gypsum coatings were faceted and less durable. Following mineralization, fibers were assembled into nonwoven sheets using carding/needlepunching or wetlaid techniques, then subjected to flame testing. While unmineralized cellulosic nonwovens burned readily, most nonwovens derived some degree of flame-retardancy from the mineral coatings, which serve as a heat/mass transfer barrier hindering flame propagation; further, gypsum endothermically loses its waters of hydration at low temperatures. Most mineralized nonwovens either self-extinguished or were consumed very slowly by the propagation of a flameless smoldering front.