The molecular structure of TPU granules consists of block copolymers formed by alternating hard and soft segments, a unique structure that endows them with excellent physical and mechanical properties. Hard segments are typically formed by the reaction of diisocyanates with small-molecule chain extenders, exhibiting rigid segment characteristics; soft segments are composed of oligomeric polyols such as polyesters, polyethers, or polyolefins, exhibiting flexible segment characteristics. The ratio and interaction between these two types directly determine key properties of TPU granules, such as hardness, elasticity, abrasion resistance, and temperature resistance.
Hard segments act as a skeletal support in TPU granules, their rigid segments forming physical cross-linking points through intermolecular forces, giving the material high strength and high modulus. As the hard segment content increases, the hardness of TPU granules significantly improves, while mechanical properties such as tensile strength, tear strength, and compressive stress also increase. For example, aromatic TPU, due to the presence of rigid benzene ring structures in its hard segments, has better heat resistance and rigidity than aliphatic TPU, but its low-temperature flexibility is relatively poor. The polar groups in the hard segments (such as urethane and urea groups) form a physical cross-linked network through hydrogen bonding, further enhancing the structural stability of the material and making it less prone to plastic deformation under stress.
The soft segments, as the elastic basis of TPU granules, directly affect the material's resilience and low-temperature performance due to the flexibility of their molecular chains and intermolecular forces. Polyester-type soft segments, containing polar ester groups, exhibit strong intermolecular forces, resulting in TPU granules with high strength and hardness, but poor hydrolysis resistance. Polyether-type soft segments, due to the non-polar nature of ether bonds, impart better flexibility and low-temperature resistance, while significantly improving hydrolysis resistance. Polyolefin-type soft segments, due to their non-polar molecular chains and poor compatibility with hard segments, have lower strength but excellent low-temperature resistance, making them suitable for extreme environment applications. By adjusting the type and molecular weight of the soft segments, the elastic range and low-temperature embrittlement temperature of TPU granules can be precisely controlled.
The compatibility between hard and soft segments is crucial to the microstructure and performance of TPU granules. If the two components have good compatibility and low microphase separation, the material exhibits a homogeneous structure with balanced mechanical properties but lacking outstanding characteristics. If the compatibility is poor, a distinct microphase separation structure forms, with the hard segment phase distributed as a dispersed phase within the continuous soft segment phase. This structure allows TPU granules to absorb energy through the slippage of the soft segment chains and transmit stress through the hard segment phase under stress, thus possessing both high elasticity and high strength. For example, aliphatic TPU, due to its moderate compatibility between hard and soft segments and high microphase separation, maintains excellent flexibility even at low temperatures.
The crosslinking method of TPU granules also significantly affects their physical and mechanical properties. Physical crosslinking forms reversible crosslinking points through hydrogen bonding between hard segments, giving the material thermoplastic processing characteristics and facilitating recycling. Chemical crosslinking, on the other hand, introduces multifunctional raw materials or forms an irreversible crosslinking network through high-temperature reactions, significantly improving the material's hardness, heat resistance, and solvent resistance, but sacrificing some elasticity and processing performance. For example, cross-linked TPU granules exhibit reduced compression set due to increased cross-linking density, but their tensile strength and elongation may decrease. A balance must be struck based on the application scenario to determine the optimal cross-linking degree.
Temperature directly impacts the molecular structure of TPU granules, affecting their performance. At low temperatures, the movement of soft segment molecular chains is hindered, increasing material brittleness. Optimizing the type of soft segments (e.g., using polyether-based segments) or adjusting the content of hard segments is necessary to lower the low-temperature embrittlement temperature. At high temperatures, the hard segment phase may soften, reducing physical cross-linking points and decreasing material strength. Introducing rigid segments or increasing cross-linking density is required to improve heat resistance. For instance, aromatic TPUs, due to the presence of rigid hard segments, maintain structural stability at high temperatures, making them suitable for high-temperature applications such as engine peripheral components.
The molecular structure design of TPU granules is crucial for balancing performance and cost. By adjusting the ratio of hard to soft segments, selecting different types of soft segments, optimizing cross-linking methods, and controlling molecular weight distribution, customized TPU granules meeting specific application requirements can be developed. For example, high-hardness TPU granules require increased hard segment content and the use of polyester-type soft segments; high-elasticity TPU granules require reduced hard segment content and the use of polyether-type soft segments; and low-temperature resistant TPU granules require optimized soft segment molecular structure to reduce phase separation. This precise molecular-level design enables TPU granules to demonstrate broad application potential in footwear, automotive, electronics, and medical fields.
