Introduction to TPU

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Thermoplastic polyurethane elastomer, abbreviated as TPU, also known as PU thermoplastics, is a linear block copolymer composed of oligomeric polyol soft segments and diisocyanate-chain extender hard segments. TPU molecules contain -NH-COO- groups, and many of its properties depend on the type of long-chain diol used; its hardness can be adjusted by varying the proportion of hard segments. Its resistance to photoaging can be improved by adding light stabilizers, and this property also depends on whether the isocyanate is aromatic or aliphatic. The difference between aliphatic and aromatic: Aromatic isocyanates are used in applications where oxidation and discoloration due to ultraviolet light exposure are not a concern. Polyurethane coatings prepared from aromatic polyisocyanates are prone to oxidation; therefore, they degrade more easily under direct sunlight. In contrast, aliphatic isocyanates are mainly used in the production of light-stable coatings. In situations where stability against ultraviolet light or sunlight is necessary, such as in automotive lacquers and many water-based formulations, such products must be used. Aliphatic isocyanates and aliphatic polyisocyanates endow polyurethane coatings with excellent chemical resistance and good aging resistance. Due to the absence of phenyl groups, the use of aliphatic isocyanates ensures durable adhesion under harsh conditions. Compared to aliphatic derivatives, derivatives of aromatic isocyanates result in coatings with poor aging resistance (yellowing) and poor chemical resistance (especially weak alkali resistance). Therefore, aromatic polyisocyanates are mainly used indoors (for floor coatings, tank coatings, etc.) or as primers. Even primers are being used less and less in the automotive industry, as the yellowing of primers can affect the color of the topcoat and cause delamination between layers. Generally speaking, aromatic polyisocyanates are not primarily used in coatings. For example, 80% of TDI products are used in the production of soft foam, while 65% of MDI products are used in the production of hard foam. The difference between polyether-type TPU and polyester-type TPU is that various polyols can be used for the soft segments of TPU, which can generally be divided into polyether-based and polyester-based types. Polyether type (Ether): High strength, hydrolysis resistance, and high resilience, with good performance at low temperatures. Polyester type (Ester): Good tensile properties, flexural properties, wear resistance, as well as solvent resistance and resistance to high temperatures. The differences in the soft segments have the following effects on the material properties: Tensile strength – Polyester type > Polyether type; Tear strength – Polyester type > Polyether type; Wear resistance – Polyester type > Polyether type; Chemical resistance – Polyester type > Polyether type; Moisture evaporation – Polyester type < Polyether type; Low-temperature impact resistance – Polyester type < Polyether type; Transparency – Polyester type > Polyether type; Bacterial resistance – Polyester type < Polyether type. Seven key differences arise from variations in raw materials and formulations. (1) The main raw materials for producing polyether-type TPU are 4,4’-diphenylmethane diisocyanate (MDI), poly**furane (PTMEG), and 1,4-butanediol (BDO). Of these, MDI accounts for approximately 40%, PTMEG about 40%, and BDO about 20%. (2) The main raw materials for producing polyester-type TPU are 4,4’-diphenylmethane diisocyanate (MDI), 1,4-butanediol (BDO), and adipic acid (AA); among these, MDI accounts for about 40%, AA accounts for about 35%, and BDO accounts for about 25%. The molecular weight distribution and the factors affecting it follow the Poisson probability equation; the relative molecular weight distribution is relatively narrow ； The relative molecular weight distribution of polyester diols follows the Flory probability distribution, resulting in a wide relative molecular weight distribution. The molecular weight of the soft segment affects the mechanical properties of polyurethanes. Generally, assuming that the molecular weight of the polyurethane is constant, if the soft segment is a polyester, then the strength of the polyurethane increases as the molecular weight of the polyester diol increases ； If the soft segment is a polyether, the strength of the polyurethane decreases as the molecular weight of the polyether diol increases, whereas the elongation rate increases. This is because polyester soft segments are inherently highly polar; a higher molecular weight leads to greater structural regularity, which is beneficial for improving strength. In contrast, polyether soft segments have lower polarity, and as the molecular weight increases, the relative content of the hard segments in the polyurethane decreases, resulting in reduced strength. Mechanical properties compare the soft segments composed of oligomer polyols such as polyethers and polyesters. The soft segment constitutes the majority in polyurethanes, and polyurethanes prepared from different oligomer polyols and diisocyanates exhibit varying properties. Polyurethane elastomers and foams obtained using polar polyesters as soft segments exhibit good mechanical properties. This is because polyurethanes made from polyesters contain polar ester groups; in such polyurethanes, hydrogen bonds can form not only between the hard segments but also partially between the polar groups on the soft segments and those on the hard segments. This allows the hard phases to be distributed more evenly within the soft phases, serving as sites for elastic cross-linking. At room temperature, certain polyesters can form soft segment crystals, affecting the properties of polyurethanes. Polyester-type polyurethanes have higher strength, oil resistance, and thermal oxidative stability than PPG polyether-type ones, but lower hydrolysis resistance. Hydrolytic stability: After protection with carbodiimide, the hydrolysis resistance of polyester-based thermoplastic polyurethanes is improved. Polyetherester thermoplastic polyurethanes and polyether thermoplastic polyurethanes have the best hydrolysis resistance at high temperatures. Polyesters are susceptible to degradation by water molecules, and the acid produced by hydrolysis can further catalyze the hydrolysis of the polyester. The type of polyester has a certain influence on the physical properties and water resistance of the elastomer. As the number of methylene groups in the polyester diol precursor increases, the water resistance of the resulting polyester-based polyurethane elastomers improves. It has a low ester content, which also gives it good water resistance. Similarly, polyurethane elastomers prepared from polyesters synthesized using long-chain dicarboxylic acids exhibit better water resistance than those made from polyester-based polyurethanes using short-chain dicarboxylic acids. Microbial resistance: When in prolonged contact with moist soil, polyester-based soft thermoplastic polyurethanes are subject to microbial degradation, whereas ether-based soft or rigid thermoplastic polyurethanes, as well as ether-based thermoplastic polyurethanes or rigid thermoplastic polyurethanes, generally are not affected by microbes. In terms of price, polyether-based polyurethane elastomers are much more expensive than polyester-based polyurethane elastomers. The main reasons for this are: ① Polyether-based polyurethane elastomers possess excellent hydrolysis resistance, low-temperature resistance, and bending resistance. ②Compared to polyester polyols, the polyether polyols that make up the TPU soft segments have higher prices for their raw materials. ③The production process of polyether polyols is much more complex compared to that of polyester polyols. ④The various process conditions during the reaction of polyether polyols are difficult to control. ⑤When producing polyether polyols, high requirements are placed on the production equipment, and certain protective measures must also be taken during the production process. Comparison of differences in the processing process 1. Drying: As we know, polyurethane is a polar polymer that gradually absorbs moisture when exposed to air. It is molded by melt processing using hygroscopic TPU pellets; water vaporizes at the processing temperature, resulting in an uneven surface on the product and the formation of bubbles inside, which lowers its physical properties. Therefore, in order to ensure the performance of the product and prevent bubbles caused by the vaporization of water during melt processing, it is generally necessary to dry the pellets before TPU processing. As we also analyzed in the previous comparison of the hydrolytic stability of TPU esters and ethers, since polyesters are prone to being attacked by water molecules and broken down, and the acids produced by hydrolysis can further catalyze the hydrolysis of these polymers, under identical conditions, polyester-based TPUs generally have a much higher water content than ether-based TPUs. Therefore, special attention must be paid to polyester-based TPUs during the drying process; it is essential to ensure that they are thoroughly dried, with strict control over the drying conditions. 2. During the holding phase of injection molding, the polymer melt is subjected to the combined effect of internal static pressure and external dynamic pressure, whether in the pre-plasticization stage or the injection stage. During the holding pressure stage, the polymer melt is subjected to high pressure; under this pressure, the free volume between molecular chains is compressed. As the free volume between molecular chains decreases, the proximity of these chains leads to an increase in intermolecular forces, which results in an increase in viscosity. Additionally, since polyether TPU has a lower cohesive energy within its ether bonds and a smaller rotational barrier for these bonds, the forces acting between the closely packed segments of the molecular chains are weaker. As a result, during compression, the molecular chains shift relative to each other by more, which allows viscosity to vary over a wider range. Furthermore, since the molecular chains of polyether TPU are much more flexible than those of polyester TPU, permanent deformation is less likely to occur. Therefore, during the holding pressure stage in the processing of polyether TPU, a longer holding time is required compared to polyester TPU. 3. Processing time: Under normal conditions, an increase in molecular weight leads to longer molecular chain segments. The center of gravity of the molecular chains moves more slowly, resulting in more opportunities for the relative displacement between the chain segments to cancel each other out. This increases the flexibility of the long molecular chains and leads to an increase in entanglement points, making it difficult for the chains to separate and slide past one another. As a result, the resistance during the flow process increases, and both more time and energy are required, which reflects the sensitivity of viscosity to shear forces. Generally, polyester-based TPU has a higher molecular weight compared to ether-based TPU, which is why it takes longer to process and shape. 4. Processing temperature: Since, under normal conditions, polyester-based TPU has a wider molecular weight distribution compared to ether-based TPU, a higher temperature is required during its processing. Since the N–O bonds in polyether TPU are relatively easy to break, a relatively low temperature is sufficient for processing it. 5. Pressure: Due to the high internal binding energy of polyester-based TPU, as well as the difficulty in breaking the nitrogen-oxygen bonds in its molecular structure, higher temperatures and pressures are required to process it and break its molecular bonds. 6. Cooling: Due to the high friction within polyester-based TPU and the large amount of energy stored within its molecules, it is difficult for such materials to return to their normal state, which requires a longer cooling time. 7. Fluidity: Due to the lower cohesive energy of the ether bonds in polyether TPU and the smaller rotational barrier of these bonds, as the relative molecular mass of the polyether increases, the chains become more flexible. The molecular chains exhibit high flexibility, which results in good fluidity; polyester TPU, on the other hand, performs slightly less well in this regard.