Flame-Retardant Polyurethane Dielectric Elastomers for Fire-Safety Electroactuation

Dielectric elastomer (DE) materials exhibit large actuation strain and high energy density when stimulated by an external electric field, positioning them as promising artificial muscles for flexible actuators and soft robotics. However, the electrical breakdown of most DE materials under high electric fields can trigger combustion, thereby posing serious safety risks in human-robot interactions. Here, we have fabricated a type of flame-retardant DE material by copolymerizing flame retardants, 4-(DOPO-((4-hydroxyphenyl)imino)methyl)-2,6-dimethoxyphenol (DHIP), into the hard segments of polyurethane (PU). Incorporating the DHIP into the hard segments of PU not only endows the materials with flame-retardant properties but also alleviates the stiffening problem triggered by the rigid structure of DHIP. The modified materials exhibit superior flame-retardant properties when more than 50% of the hard segments in the prepolymer contain DHIP, and they achieve a V-0 rating according to the UL-94 flammability standards. In addition, when the DHIP accounts for 50% of the hard segments in the prepolymer, the modified DE materials retain a relatively low elastic modulus of 0.18 MPa. Under a prestretched area strain of 500% and an applied electric field of 104 MV/m, the modified DE film generates 109% area strain. These flame-retardant PU-DE materials hold promise for providing fire-safety assurance in the large-scale application of DE materials.