TPU Introduction

Thermoplastic polyurethane elastomer, abbreviated as: TPU , also known as PU Thermoplastic is a material composed of low-molecular-weight polyol soft segments and diisocyanates. - Linear block copolymer composed of hard segments from chain extenders.

TPU contains within its molecule -NH-COO- The group’s many 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 it also depends on whether the isocyanate is aromatic or aliphatic.

The difference between aliphatic and aromatic compounds

Aromatic isocyanates are used in applications where discoloration due to oxidation from UV exposure is not a concern. Polyurethane coatings prepared using aromatic polyisocyanates are prone to oxidation and, therefore, tend to degrade more readily under direct sunlight.

In contrast, aliphatic isocyanates are primarily used in the manufacture of light-stabilized coatings. These products are particularly suitable for applications that require resistance to ultraviolet light or sunlight, such as automotive clearcoats and many water-based formulations.

Aliphatic isocyanate

Aliphatic polyisocyanates enable polyurethane coatings to have   Excellent chemical resistance   And   Excellent resistance to aging. Since they do not contain phenyl groups, aliphatic isocyanates ensure durable adhesion even under harsh conditions.

Aromatic isocyanate

Compared to aliphatic derivatives, the derivatives of these two types of products exhibit poorer aging resistance (yellowing) and poorer chemical resistance (particularly weaker alkali resistance).   Therefore, aromatic polyisocyanates are primarily used indoors (such as in floor coatings and tank coatings) or as primers. Even as primers, their use in the automotive sector is declining steadily, because yellowing of the primer can affect the color of the topcoat and lead to delamination between layers.

Generally speaking, aromatic polyisocyanates are not primarily used in coatings. For example, 80% of the TDI The product is used to manufacture soft foam, and... 65%MDI The product is used for manufacturing rigid foam.

Polyether type TPU With polyester type TPU The differences that exist between them

TPU The soft segment can utilize a variety of polyols, which can be broadly categorized into two types: polyether-based and polyester-based.

Polyether type (Ether): High strength, hydrolysis resistance, high resilience, and excellent low-temperature performance.

Polyester type (Ester): Better tensile properties, flexural properties, abrasion resistance, solvent resistance, and resistance to higher temperatures.

The differences in the soft segments and their impact on material properties are as follows:  :
Tensile strength        Polyester-based      >      Polyether series
Tear strength        Polyester-based      >      Polyether series
Wear resistance        Polyester-based      >       Polyether series
Drug resistance        Polyester-based      >      Polyether series
Moisture evaporation rate      Polyester-based      <      Polyether series
Low-temperature impact resistance      Polyester-based      <      Polyether series
Transparency          Polyester-based      >      Polyether series
Antibacterial resistance          Polyester-based      <      Polyether series

Seven Key Differences

Differences in raw materials and formulations

( 1 Polyether type TPU The main production raw materials are: 4-4’— Diphenylmethane diisocyanate ( MDI ) Polytetrahydrofuran ( PTMEG ), 1、 4— Butanediol ( BDO ), among which MDI The dosage is approximately 40% Left and right, PTMEG Approximately约占 40%， BDO Approximately约占 20%。

( 2 Polyester-based TPU The main production raw materials are: 4-4’— Diphenylmethane diisocyanate ( MDI ), 1、 4— Butanediol ( BDO )、Adipic acid ( AA ), among which MDI The dosage is approximately 40%， AA Approximately约占 35%， BDO Approximately约占 25%。

Molecular Weight Distribution and Its Influencing Factors

The relative molecular mass distribution of polyethers follows... Poisson The probability distribution exhibits a relatively narrow molecular weight distribution; whereas the molecular weight distribution of polyester diols follows... Flory The probability distribution and relative molecular mass distribution are relatively broad.

Comparison of Hydrolytic Stability

After being protected with carbodiimide, polyester-based thermoplastic polyurethanes exhibit improved resistance to hydrolysis. Ether-ester-based and ether-based thermoplastic polyurethanes demonstrate good hydrolysis resistance at high temperatures.

Polyesters are susceptible to hydrolytic degradation upon exposure to water molecules, and the acids generated during hydrolysis can further catalyze the hydrolysis of the polyester itself. The type of polyester used has a certain impact on the physical properties and water resistance of elastomers. As the number of methylene groups in the polyester diol raw material increases, the water resistance of the resulting polyester-based polyurethane elastomers improves. Elastomers made from polyesters with lower ester group content also exhibit better water resistance. Similarly, polyurethane elastomers synthesized using long-chain dicarboxylic acids demonstrate superior water resistance compared to those made from short-chain dicarboxylic acids.

Comparison of Microbial Resistance

Polyester-based flexible thermoplastic polyurethanes, when in prolonged contact with moist soil, can be eroded by microorganisms, whereas ether-based flexible or rigid thermoplastic polyurethanes, as well as ether-based thermoplastic polyurethanes or rigid thermoplastic polyurethanes, generally are not subject to microbial erosion.

Price comparison

Polyether-based polyurethane elastomers are significantly more expensive than polyester-based polyurethane elastomers, primarily because:
① Polyether-based polyurethane elastomers exhibit excellent resistance to hydrolysis, low-temperature performance, and flexural resistance.
② constitute TPU Compared to polyester polyols, polyether polyols used as soft segments have higher raw material costs.
③ The production process for polyether polyols is significantly more complex than that for polyester polyols.
④ It is relatively difficult to control the various process conditions during the reaction of polyether polyols.
⑤ When producing polyether polyols, the equipment used in the production process must meet high standards. Additionally, during the production process, it is important to take appropriate protective measures.

Comparison of Differences in the Processing Procedure

1 , dry

As we know, polyurethane is a polar polymer that gradually absorbs moisture when exposed to air. When it becomes hygroscopic... TPU The material particles are melted and molded; at the processing temperature, water vaporizes, resulting in an uneven surface on the finished product and the formation of internal bubbles, which degrade the material’s properties. Therefore, to ensure the performance of the finished product and prevent bubble formation caused by the vaporization of moisture during melt processing, TPU Before processing, it is generally necessary to dry the material particles.

We're in front. TPU A comparison of the hydrolytic stability between esters and ethers has also been previously analyzed. Since polyesters are easily attacked by water molecules, leading to chain scission, and the acids generated during hydrolysis can further catalyze the hydrolysis of the polyester itself, under identical conditions, polyesters generally... TPU Compared to polyether types TPU Its moisture content is significantly higher; therefore, during the drying process, polyester-based materials require... TPU Pay special attention to ensuring that it is thoroughly dried, and strictly control the drying conditions.

2、保压阶段

During injection molding, whether in the pre-plasticization stage or the injection stage, the polymer melt is subjected to the combined effects of internal hydrostatic pressure and external dynamic pressure. During the holding-pressure phase, the polymer melt is exposed to high pressure. Under this pressure, the free volume between molecular chain segments is compressed. As the free volume between molecular chains decreases, the proximity of large molecular chain segments intensifies intermolecular forces, thereby increasing the melt's viscosity. Additionally, due to the presence of polyether-based polymers... TPU Its ether bonds have lower cohesive energy and smaller rotational barriers, resulting in weaker interactions between the closely packed segments of the molecular chains. Consequently, during compression, the molecular chains undergo relatively large displacements, which allows the viscosity to vary over a wider range. Furthermore, due to the polyether-based... TPU Its molecular chain is longer than that of polyester. TPU It is much more flexible and therefore less likely to undergo permanent deformation. Consequently, in polyether-based materials... TPU During the processing, when pressure is maintained, it is compatible with polyester-based materials. TPU In comparison, polyether-based... TPU To control a longer holding time.

3 Processing time

In general, as the molecular weight increases, the molecular chain segments become longer, the center of gravity of the molecular chains moves more slowly, and there are more opportunities for the relative displacements among chain segments to cancel each other out. Consequently, the flexibility of the long molecular chains increases, the number of entanglement points rises, and it becomes more difficult for the chains to disentangle and slide past one another. This leads to increased resistance during the flow process, requiring more time and energy, and thus manifesting a heightened sensitivity of viscosity to shear. Typically, polyester-based polymers... TPU Compared to polyether types TPU Its molecular weight is high, so the time required for processing and molding will also be longer.

4 Processing temperature

Since, under normal circumstances, polyester-based... TPU Compared to polyether types TPU Its molecular weight distribution is relatively broad, so the processing temperature required during its manufacture is relatively high. Due to its polyether nature... TPU The nitrogen-oxygen bond is relatively easy to break, so processing it can be achieved at a relatively low temperature.

5 , pressure

Due to polyester-based TPU It has a relatively high intramolecular cohesive energy, and the nitrogen-oxygen bonds in its molecular structure are also more difficult to break. Therefore, processing it—i.e., breaking its molecular bonds—requires higher temperatures and pressures.

6 Cooling

Due to polyester-based TPU Internal friction is significant, and the cohesive energy within molecules is high; therefore, cooling it to restore its normal state is relatively difficult, necessitating a longer cooling period.

7 Liquidity

Due to polyether-based TPU Ether bonds have lower cohesive energy and smaller rotational barriers. As the molecular weight of polyethers increases, the chains become more flexible, giving the polymer chains a high degree of flexibility and thus exhibiting excellent fluidity. In contrast, polyester-based polymers... TPU Is slightly inferior.

(This article is reproduced from the WeChat official account: Jingwei Polymer, for reference and learning purposes only.)