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Synthesis, processing, performance and application of TPU materials

2023-05-10 Page view : 20 views

TPU (thermoplastic polyurethane) is a type of polyurethane that can be plasticized by heating and can be dissolved by solvents. Compared with mixing type and casting type polyurethane, thermoplastic polyurethane has no or little chemical crosslinking in chemical structure, and its molecules are basically linear, but there is a certain amount of physical exchange.
The concept of so-called physical exchange, first proposed by Schollenberger C.S. in 1958, refers to the existence of reversible “connection points” between linear polyurethane molecular chains when exposed to heat or solvents. It is not actually a chemical crosslink. But it acts as a chemical crosslink. As a result of this physical cross-linking, the polyurethane forms a heterogeneous morphological structure theory, and the hydrogen bonding of the polyurethane strengthens its morphology and makes it resistant to higher humidity.

What is the classification of TPU

Now that we know what a TPU is, what are its classifications? According to different classification standards, TPU can have many different classifications.

For example, according to the structure of the soft segment, it can be divided into polyester type, polyether type and butadiene type , which contain ester group, ether group or butene group respectively. According to the hard segment structure, it can be divided into urethane type and urethane urea type , which are respectively obtained from diol chain extenders or diamine chain extenders. The common division is divided into polyester type and polyether type.
According to the presence or absence of crosslinking, it can be divided into pure thermoplastic and semi-thermoplastic. The former is a pure linear structure without cross-linking bonds; the latter contains a small amount of allophanate and other cross-linking bonds. According to the use of finished products, it can be divided into special-shaped parts (various mechanical parts), pipes (sheaths, rod profiles), films (sheets, sheets), adhesives, coatings, and fibers.

How is TPU synthesized

Although there are many classifications of TPU, they all belong to polyurethane in terms of molecular structure. So how did it come together? According to different synthesis processes, it is mainly divided into bulk polymerization and solution polymerization.

In bulk polymerization, it can be divided into pre-polymerization method and one-step method according to whether there is pre-reaction: pre-polymerization method is to react diisocyanate with macromolecular diol for a certain period of time, and then add chain extension to produce TPU; Molecular diols, diisoesters and chain extenders are simultaneously mixed and reacted to form TPU.
Solution polymerization is to dissolve diisocyanate in solvent first, then add macromolecular diol to make it react for a certain period of time, and finally add chain extender to generate TPU.
The type of soft segment, molecular weight, hard segment or soft segment content and TPU aggregation state of TPU will affect the density of TPU. The density is between 1.10 and 1.25, which is not significantly different from other rubbers and plastics. At the same hardness, the density of polyether TPU is lower than that of polyester TPU.


How is TPU processed

TPU resin needs to be processed to make finished products, and TPU is mainly processed by melting method and solution method. Its molecular structure is a rigid block obtained by reacting diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) with a chain extender and diisocyanate such as diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) Molecules and macromolecular polyols react to form flexible segments alternately.

Melt processing is a process commonly used in the plastics industry: such as mixing, calendering, extrusion, blow molding and molding (including injection, compression, transfer and centrifugation etc. solution processing is the dissolution of pellets in solvents or direct polymerization in solvents After making a solution, coating, spinning, etc. are performed.
TPU is made into the final product, which generally does not require vulcanization and cross-linking reaction, which can shorten the production cycle, and waste materials can be recycled and reused.


TPU performance

Like cast polyurethane (liquid) and mixing polyurethane (solid), TPU has high modulus, high strength, high elongation and high elasticity, excellent wear resistance, oil resistance, low temperature resistance and aging resistance. Among them, high tensile strength, high elongation, and low long-term compression set are all significant advantages of TPU.

1. High tensile strength and high elongation
TPU has excellent tensile strength and elongation. From the data in the figure below, we can see that the tensile strength and elongation of polyether TPU are much better than that of polyvinyl chloride plastic and rubber.

In addition, TPU can meet the requirements of the food industry without adding or adding few additives in the processing process, which is difficult for other materials such as PVC and rubber.

2. Resilience is very sensitive to temperature
The resilience of TPU refers to the degree of rapid recovery to its original shape after the deformation stress is released, which is expressed by recovery energy, that is, the ratio of the deformation retraction work to the work required for deformation. It is a function of the dynamic modulus and internal friction of the elastomer and is very sensitive to temperature.

The springback decreases with the decrease of temperature, until a certain temperature, the elasticity increases rapidly again. This temperature is the crystallization temperature of the soft segment, which depends on the structure of the macromolecular diol, and the polyether TPU is lower than the polyester TPU. At temperatures below the crystallization temperature, the elastomer becomes very hard and loses its elasticity, therefore, rebound is similar to rebounding off a hard metal surface.

3. The hardness range is Shore A60~D80
Hardness is an indicator of a material’s ability to resist deformation, nicks and scratches.
The hardness of TPU can be adjusted by adjusting the ratio of soft and hard chain segments . Therefore, TPU has a relatively wide hardness range. Its hardness ranges from Shore A60 to D80, spanning the hardness of rubber and plastic, and has a wide range of hardness in the entire hardness range. High elasticity.
With the change of hardness, some properties of TPU will change. For example, if the hardness of TPU is increased, there will be performance changes such as increased tensile modulus and tear strength, increased rigidity and compressive stress (load capacity), decreased elongation, increased density and dynamic heat generation, and increased environmental resistance.

Application field of TPU

TPU was first successfully commercialized by the American Goodrich Chemical Company (now Lubrizol) in 1958 , and has since been researched and expanded around the world. At present, manufacturers of TPU raw materials mainly include BASF, Covestro, Lubrizol, Huntsman, Wanhua Chemical, Shanghai Hengan, Ruihua, Asahikawa Chemical and so on.
As an elastomer with excellent performance, TPU has a wide range of downstream products and is widely used in daily necessities, sporting goods, toys, decorative materials and other fields. Here are a few examples.
1. Shoe material
TPU is used in shoe materials mainly due to its excellent elasticity and wear resistance. TPU-containing footwear products are much superior in wearing comfort than ordinary footwear products, so they are widely used in high-end footwear products, especially some sports shoes and casual shoes.

2. Hose
Because TPU hose has softness, good tensile strength, impact strength and high and low temperature resistance, TPU hose is widely used in China for gas and oil transportation of aircraft, tanks, automobiles, motorcycles, machine tools and other mechanical equipment hose.

3. Cable
TPU provides tear resistance, wear resistance and bending characteristics, and high and low temperature resistance is the key to cable performance. Therefore, in the Chinese market, TPU for high-end cables, such as control cables and power cables, is used to protect the coating materials of cables with complex designs, and its uses are becoming more and more extensive.

4. Medical equipment
TPU is a safe, stable, and high-quality alternative to PVC. It will not contain harmful chemical substances such as phthalate, and will migrate to blood or other liquids in medical catheters or medical bags to cause side effects. Extrusion grade and injection grade TPU can be easily used with a little adjustment in existing PVC equipment.

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