Flame-retarded thermoplastic polyurethane elastomer: From organic materials to nanocomposites and new prospects

Thermoplastics polyurethane elastomers (TPUs) have been widely used in auto parts, electronic appliances, wires and cables, et al. However, they are flammable, thus seriously threating the safety of human beings and property. To achieve the flame retardancy of TPUs and reduce the hazards from smoke and toxic gases as well as severe droplet, much work was carried out in the past two decades, especially in the recent decade. In this review, we focus on the fire safety of TPU and deeply review the past and current status concerning the flame retardation of TPU, covering organic materials to nanocomposites and new prospects. Starting from the structural characteristic, several basic aspects of TPU are overviewed, including thermal properties, combustion performance, and flame-retardant method. Then, on the basis of published literatures, the development course, current research hot spot, and difficulty in achieving the flame retardancy of TPU are analyzed by detailed statistic data. Further, existing inherently flame-retarded TPU and already used flame retardants in TPU are introduced thoroughly. For the most important additive-type flame-retardant mode currently, the structural design and flame-retarding effect of vital flame retardants on the fire safety of TPU are discussed in detail, which includes mono-component organic and inorganic flame retardants, modified and hybridized flame retardants, compound flame retardants, and nano flame retardants. For inherently flame-retarded TPU, this review focuses on the structural design and flame-retardant mechanism. In addition, special processes to the flame retardation of TPU are also discussed. On the basis of the past research, the flame retardation of TPU without the incorporation of traditional flame retardant elements and the bio-based flame retardation of TPU are prospected in the final part.