Polyurethane (PU), short for poly(urethane), is a versatile polymer synthesized through the polycondensation reaction of polyols and diisocyanates. It is known for its excellent mechanical properties and is widely used in various industrial and consumer applications, including films, coatings, adhesives, fibers, foams, and elastomers. Among the different forms of polyurethane, thermoplastic polyurethane (TPU) stands out due to its unique combination of elasticity and processability.
TPU is a class of linear segmented block copolymers, typically represented by the formula (AB)ₙ. In this structure, the "A" segment is a high molecular weight polyester or polyether (with a molecular weight range of approximately 1000 to 6000), while the "B" segment is a short-chain diol containing 2 to 12 carbon atoms. These soft and hard segments are connected by diisocyanate units, forming a structure that gives TPU both flexibility and strength. Unlike crosslinked rubber materials, TPU has a primarily linear molecular structure with minimal chemical crosslinking.
One of TPU's key characteristics lies in the presence of numerous hydrogen bonds between its molecular chains, forming what is known as physical crosslinking. These hydrogen bonds play a critical role in reinforcing the material's structure, contributing to its high modulus, tensile strength, and resilience. As a result, TPU exhibits excellent abrasion resistance, oil and chemical resistance, hydrolysis resistance, and remains stable across a wide temperature range. It also shows good resistance to microbial attack and environmental aging.
Another advantage of TPU is its thermoplastic behavior. The physical crosslinking through hydrogen bonds is reversible-under heating or in solution, the intermolecular forces weaken, allowing the material to flow or be reshaped. Upon cooling or solvent evaporation, the hydrogen bonds reform, enabling the material to recover its solid-state performance without significant loss of properties. This reprocessability makes TPU suitable for injection molding, extrusion, and other thermoplastic processing techniques.
A well-known example of TPU in daily life is spandex (also called elastane), a highly elastic fiber used in textiles. The versatility, durability, and processability of TPU make it a material of choice in industries ranging from footwear and electronics to automotive and medical devices.