Thermoplastic polyurethane (TPU) is a class of polyurethane elastomers synthesized through a step-growth polymerization reaction between isocyanates and hydroxyl-containing compounds. The fundamental chemical reaction can be represented as:
R-N=C=O + R'-OH ⇌ R-NH-CO-OR'
In this reaction, the isocyanate group reacts with a hydroxyl group to form a urethane linkage.
Typically, TPU is produced using three main components:
Long-chain polyols with average molecular weights ranging from 600 to 4000 (which form the soft segments);
Chain extenders, which are low molecular weight diols (MW ~61–400); and
Diisocyanates, which act as the hard segment precursors.
The final properties of TPU-ranging from soft and flexible to rigid and high-modulus-depend largely on the selection and ratio of these raw materials.
Soft Segment Materials
The soft segment contributes flexibility, elasticity, and controls TPU's performance at low temperatures, as well as its resistance to solvents and weathering. These segments are generally made from hydroxyl-terminated polyesters or polyethers.
Polyesters are more commonly used and include materials such as polyadipate, polycaprolactone, and aliphatic polycarbonate diols.
Typical polyethers include polypropylene glycol (PPG) and polytetramethylene ether glycol (PTMEG). Sometimes, a blend of polyester and polyether polyols is used to optimize performance by combining hydrolytic resistance, mechanical strength, and elasticity.
Hard Segment Materials
The hard segment is formed from diisocyanates and short-chain diols. The most widely used diisocyanate in TPU production is 4,4'-diphenylmethane diisocyanate (MDI), known for its high reactivity and rigidity. Other diisocyanates used include hexamethylene diisocyanate (HDI) and 3,3'-dimethyl-4,4'-biphenyl diisocyanate (TODI), selected for specific performance or processing needs.
Common chain extenders are low molecular weight diols such as ethylene glycol, 1,4-butanediol, 1,6-hexanediol, and hydroquinone bis(2-hydroxyethyl) ether, which help build the hard segments and contribute to mechanical reinforcement.
Other Additives
In addition to the primary components, TPUs often include various additives to enhance processing and long-term stability:
Mold release agents, typically derivatives of fatty acids, silicones, or fluoropolymers, are added in small amounts (0.1%–0.2%) to aid demolding.
Stabilizers: Aromatic carbodiimides are used particularly in polyester-based TPUs to reduce hydrolytic degradation (1%–2% by mass). Hindered phenols and amines help combat thermal oxidation.
UV stabilizers, such as benzotriazole or benzophenone derivatives, are often combined with HALS (hindered amine light stabilizers) to prevent yellowing and degradation from sunlight.
Fillers: Mineral fillers like calcium carbonate, talc, and silica improve rigidity or reduce cost. Reinforcing fillers include mica, glass fibers, and organic fibers.
Lubricants, including graphite, molybdenum disulfide, PTFE powders, and silicone oils, improve wear resistance and surface characteristics.
Plasticizers may also be incorporated to fine-tune flexibility and processing behavior.
This flexible formulation system is one of TPU's greatest advantages. By adjusting the chemistry and formulation, manufacturers can create TPU grades tailored for specific performance criteria-ranging from soft and elastic films to rigid structural parts-all while maintaining thermoplastic reprocessability.