TPU Molecular Structure

Chain segment structure
TPU is a (AB) n-type block linear polymer composed of flexible soft segments and rigid hard segments. TPUs with different segment structures have different performance, and the type of segment structure is mainly determined by the type of raw material. Introducing side groups into the molecular structure can reduce the orientation crystallinity between macromolecules, leading to a decrease in mechanical properties and a decrease in swelling performance; A certain amount of chemical crosslinking can improve the elongation stress and solvent resistance of the elastomer, and reduce permanent deformation.
Hard segment content
Hard segment content refers to the mass percentage of hard segments in the medium, and is an important parameter in formula design. The hydrogen bonding, microphase separation degree, and crystallization performance directly influenced by the content of hard segments are the main factors determining their morphology. Generally speaking, with the increase of hard segment content, the hardness, modulus, and tear strength of TPU increase, while the elongation at break decreases.
isocyanate index
Due to the synthesis mechanism of TPU being a gradual addition polymerization reaction between functional groups, the isocyanate index r0 (molar ratio of diisocyanate to oligomer glycol) directly affects the molecular weight. When r0 ≤ 1, the molecular weight of TPU increases with the increase of r0. When r0=1, the molecular weight reaches its maximum, and then continues to increase the r0 value, and the molecular weight begins to decrease again. When r0 is between 0.95-1, TPU modulus, tensile strength, tear strength, etc. increase with the increase of r0.
Molecular weight and molecular weight distribution
The molecular weight of TPU has a significant impact on its mechanical properties. As the molecular weight of TPU increases, the tensile strength, modulus, and wear resistance all increase. When the molecular weight reaches a certain level, these properties tend to stabilize. The tear strength and flexural strength of TPU decrease with the increase of molecular weight. On the one hand, physical crosslinking of TPU reduces its free volume; On the other hand, the high entanglement of TPU molecular chains and the increase in physical cross-linking reduce their internal fluidity. When subjected to external forces, molecular chain rearrangement is difficult to achieve and cannot effectively reduce the applied stress. When the proportion of low molecular weight components is large, it is extremely harmful to the heat resistance and mechanical properties of the elastomer, while when the proportion of high molecular weight components is too large, it will bring inconvenience to the processing and molding. Therefore, suitable molecular weight and molecular weight distribution should be adjusted according to the specific processing requirements of TPUs for different purposes.
Raw material purity
The commonly used chain extender 1,4-butanediol (BDO) in TPU is highly absorbent, and its purity and moisture content directly affect the actual production value, greatly affecting the molecular weight of the final product. MDI is prone to self aggregation, and if stored poorly, it can easily generate dimers. The moisture content, acid value, and hydroxyl value of polymeric polyols vary depending on the batch, greatly affecting the stability of TPU performance. The moisture and free carboxyl groups contained in the raw materials, on the one hand, react with MDI and consume some MDI, resulting in inaccurate formula design; On the other hand, the bubbles generated by the reaction play a plasticizing role, ultimately reducing the performance of the product. Therefore, the raw materials used for synthesizing TPU need to be strictly dehydrated before use.