TPU is a type of (AB)n segmented linear polymer. A refers to a polyester or polyether with a high molecular weight (1000-6000), and B refers to a diol containing 2-12 straight chain carbon atoms. The chemical structure between the ab segments is a diisocyanate. Thermoplastic polyurethane rubber relies on hydrogen bond cross-linking between molecules or slight cross-linking between polymer chains. As temperature rises or falls, these two types of cross-linking structures are reversible. How to know the molecular structure of TPU materials?
TPU is a (AB)n segmented linear polymer composed of a flexible soft segment and a rigid hard segment. TPU with different segment structures has different properties, and the type of segment structure mainly depends on the type of raw materials. The introduction of side chains in the molecular structure will reduce the orientation crystallinity between large molecules, leading to a decrease in mechanical properties and poor swelling properties. A certain amount of chemical cross-linking can improve the tensile stress and solvent resistance of the elastomer and reduce permanent deformation.
The hard segment content refers to the mass percentage of the hard segment and is an important parameter in formula design. The hard segment content directly affects hydrogen bonds, microphase separation and crystallinity, and is the main factor determining its morphology. Generally speaking, as the hard segment content increases, the hardness, modulus and tear strength of TPU increase, while the fracture elongation decreases.
Because TPU is synthesized by step-by-step polymerization reaction between functional groups, the isocyanate index R0 (molar ratio of diisocyanate to ethylene glycol) directly affects the molecular weight.
When R0 is ≤ TPU, the molecular weight increases with the increase of R0. When R0=1, the molecular weight reaches a large value, and then continues to increase R0, and the molecular weight begins to decrease again. When R0 is between 0.95 and 1, the modulus, tensile strength, and tear strength of TPU increase with the increase of R0.
The molecular weight of TPU has an important influence on its mechanical properties. With the increase of TPU molecular weight, the tensile strength, modulus and wear resistance increase. When the molecular weight reaches a certain level, these properties tend to be stable.
The tear strength and bending strength of TPU increase with the increase of molecular weight. On the one hand, the physical cross-linking of TPU reduces its free volume; on the other hand, the high degree of entanglement of TPU molecular chains and the increase of physical cross-linking reduce its internal flowability. When subjected to external force, the rearrangement of molecular chains is not easy to realize, and the applied stress cannot be effectively reduced.
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 processing and forming. Therefore, based on the specific processing requirements of TPU materials, the appropriate molecular weight and molecular weight distribution of TPU for different purposes should be adjusted.
1,4-butanediol (MDI) is a commonly used chain extender in TPU and is prone to moisture absorption. Its purity and water content directly affect the actual production value and have a great influence on the molecular weight of the product. MDI is prone to self-polymerization. If not stored properly, it is easy to form dimers. The water content, acid value, and hydroxyl value of polyol vary with different batches, which greatly affects the stability of TPU material sproperties.
On the one hand, the water and free carboxyl in the raw materials react with MDI, consuming part of MDI and causing inaccurate formula design; on the other hand, the bubbles generated by the reaction have a plasticization effect and reduce product performance. Therefore, the raw materials used to synthesize TPU need to be strictly dehydrated before use.