TPU Introduction

Thermoplastic polyurethane elastomer, abbreviated as TPU , also known as PU Thermoplastic is a material composed of oligomeric polyol soft segments and diisocyanates. - A linear block copolymer composed of hard segments derived from chain extenders.

TPU The molecule contains -NH-COO- The resulting polymer’s properties depend largely on the type of long-chain diol used; its hardness is adjusted by varying the hard-segment content, and its photostability can be enhanced with light stabilizers. Additionally, these properties are influenced by whether the isocyanate is aromatic or aliphatic.

The difference between aliphatic and aromatic compounds

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

In contrast, aliphatic isocyanates are primarily used to manufacture light-stabilized coatings. These products are required in applications that demand resistance to ultraviolet or sunlight, such as automotive clearcoats and many waterborne formulations.

Aliphatic isocyanate

Aliphatic polyisocyanates give polyurethane coatings   Excellent chemical resistance   and   Good aging resistance. Because it contains no phenyl groups, the use of aliphatic isocyanates ensures durable adhesion even under harsh conditions.

Aromatic isocyanates

Compared with aliphatic derivatives, the derivatives of these two product classes exhibit poor aging resistance (yellowing) and poor chemical resistance in coatings (particularly poor alkali resistance).   Therefore, aromatic polyisocyanates are primarily used indoors (e.g., in floor coatings, tank linings) or as primers. Even as primers, their use in the automotive sector is declining, as yellowing can affect the color of topcoats and lead to intercoat delamination.

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

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

TPU The soft segment can use various polyols, which are broadly classified into two types: polyether-based and polyester-based.

Polyether type (Ether): High strength, hydrolysis resistance, high resilience, and good 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 have the following effects on the material properties:  :
Tensile strength        Polyester-based      >      Polyether-based
Tear strength        Polyester-based      >      Polyether-based
Wear resistance        Polyester-based      >       Polyether-based
Chemical resistance        Polyester-based      >      Polyether-based
Evaporative moisture      Polyester-based      <      Polyether-based
Low-temperature impact toughness      Polyester-based      <      Polyether-based
Transparency          Polyester-based      >      Polyether-based
Bacterial resistance          Polyester-based      <      Polyether-based

Seven major differences

Differences in raw materials and formulations

( 1 ) Polyether type TPU The main raw materials for production are 4-4’— Diphenylmethane diisocyanate ( MDI ), polytetrahydrofuran ( PTMEG .), 1、 4— Butanediol ( BDO ), among which MDI The dosage is approximately 40% left and right, PTMEG accounts for approximately 40%， BDO accounts for approximately 20%。

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

Molecular Weight Distribution and Its Effects

The relative molecular mass distribution of polyethers follows Poisson The probability equation indicates a relatively narrow molecular weight distribution; in contrast, the molecular weight distribution of polyester diols follows Flory The probability distribution and the molecular weight distribution are relatively broad.

Comparison of hydrolytic stability

The hydrolytic stability of polyester-based thermoplastic polyurethanes is improved when protected with dicyanodiamide. Polyether ester-based and polyether-based thermoplastic polyurethanes exhibit good hydrolytic stability at elevated temperatures.

Polyesters are susceptible to hydrolytic cleavage by water molecules, and the acids produced during hydrolysis can further catalyze polyester degradation. The type of polyester used has a certain influence on the physical properties and water resistance of the elastomer. As the number of methylene groups in the polyester diol raw material increases, the water resistance of the resulting polyester-based polyurethane elastomer improves. A lower ester group content also contributes to better water resistance. Similarly, polyurethane elastomers derived from polyesters synthesized with long-chain dicarboxylic acids exhibit superior water resistance compared to those based on polyesters from short-chain dicarboxylic acids.

Comparison of microbial resistance

Polyester-based flexible thermoplastic polyurethane, when in prolonged contact with moist soil, can be degraded by microorganisms, whereas ether-based flexible or rigid thermoplastic polyurethanes, as well as ether-based thermoplastic polyurethanes or rigid thermoplastic polyurethanes, are generally resistant to microbial degradation.

Price Comparison

Polyether-based polyurethane elastomers are significantly more expensive than polyester-based polyurethane elastomers, primarily because:
① Polyether-based polyurethane elastomers exhibit excellent hydrolytic stability, low-temperature resistance, and flexural durability.
② composition TPU Compared with polyester polyols, polyether polyols used as soft segments have higher raw material costs.
③ The production process for polyether polyols is considerably more complex than that for polyester polyols.
④ Polyether polyols are difficult to control under various process conditions during the reaction.
⑤ The production of polyether polyols places high demands on production equipment, and appropriate protective measures must also be implemented during the manufacturing process.

Comparison of Variability in Processing Steps

1 , dry

As we know, polyurethane is a polar polymer that slowly absorbs moisture when exposed to air. Using moisture-absorbed TPU The material particles are melted and molded; water vaporizes at processing temperatures, resulting in a rough product surface, internal bubbles, and reduced physical properties. Therefore, to ensure product performance and prevent bubble formation caused by moisture vaporization during melt processing, TPU Before processing, the material pellets generally need to be dried.

We are ahead. TPU A comparison of the hydrolytic stability of esters and ethers has already been analyzed; since polyesters are susceptible to cleavage upon attack by water molecules, and the acids produced during hydrolysis can further catalyze polyester hydrolysis, under comparable conditions, polyesters generally… TPU Than polyether-based TPU Its moisture content is much higher; therefore, during the drying process, polyester-based materials require… TPU Pay particular attention to thoroughly drying it and strictly controlling the drying conditions.

2 , Holding Phase

During injection molding, whether in the plasticizing or injection phase, the polymer melt is subjected to the combined action of internal static pressure and external dynamic pressure. During the holding phase, the polymer melt experiences high pressure; under this pressure, the free volume between molecular chain segments is compressed. As the free volume between chains decreases, closer proximity of macromolecular segments strengthens intermolecular forces, manifesting as increased viscosity. Additionally, due to polyether-based… TPU Its ether link has low cohesive energy and a small rotational barrier, which reduces the interactions between tightly packed chain segments. Consequently, during compression, the molecular chains can shift relative to one another more readily, allowing the viscosity to vary over a wide range. Furthermore, due to the polyether nature… TPU Its molecular chain is shorter than that of polyesters. TPU It is much more flexible, so permanent deformation is less likely to occur; therefore, in the case of polyether-based… TPU During processing, when holding pressure, compared with polyester types TPU In comparison, polyether-based TPU A longer holding pressure time needs to be controlled.

3 Processing time

Because, under normal conditions, an increase in molecular weight leads to longer polymer chains, causing the center of mass of the chain to move more slowly. This results in greater opportunities for compensating relative displacements among chain segments, enhancing the flexibility of long polymer chains and increasing entanglement points. Consequently, chain reptation and slippage become more difficult, increasing flow resistance and requiring more time and energy, which manifests as shear‑thinning behavior. Typically, polyester polymers… TPU Comparison with polyether types TPU Its molecular mass is larger, so the time required for processing and molding will also be longer.

4 , processing temperature

Since, under normal circumstances, polyester-based TPU Comparison with polyether types TPU Its molecular weight distribution is relatively broad, so it requires higher processing temperatures. Due to the polyether type TPU The nitrogen–oxygen bond is relatively easy to cleave, so processing can be achieved at comparatively low temperatures.

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 difficult to break; therefore, processing it—i.e., breaking its molecular bonds—requires elevated temperatures and pressures.

6 , cooling

Due to polyester-based TPU Internal friction is relatively high, and the intramolecular cohesive energy is also substantial; consequently, cooling it to restore its original state is more difficult, necessitating a longer cooling period.

7 , liquidity

Due to polyether-based TPU Ether linkages have lower cohesive energy and smaller rotational barriers. As the relative molecular mass of polyethers increases, the chains become more flexible, exhibiting high chain flexibility and thus excellent fluidity, whereas polyester-based polymers… TPU Then it is slightly inferior.

(This article is reposted from the WeChat official account: Jingwei Polymer, for reference and study only.)