What is Thermoplastic polyurethane elastomer?

What is Thermoplastic polyurethane elastomer?

Polyurethane elastomer is a variety of polyurethane synthetic materials (other varieties refer to polyurethane foam, polyurethane adhesive, polyurethane coating and polyurethane fiber), and Thermoplastic polyurethane elastomer is one of the three types of polyurethane elastomer, People commonly refer to it as TPU (the other two major types of polyurethane elastomers are cast polyurethane elastomers, abbreviated as CPU, and mixed polyurethane elastomers, abbreviated as MPU).

TPU is a kind of polyurethane elastomer that can be plasticized by heating and dissolved by solvent. Compared with CPU and MPU, TPU has little or no chemical cross-linking in its chemical structure. Its molecular chain is basically linear, but there is a certain amount of physical cross-linking. This is the Thermoplastic polyurethane elastomer that is very characteristic in structure.

Structure and classification of TPU

Thermoplastic polyurethane elastomer is a (AB) block linear polymer. A represents a polymer polyol (ester or polyether, molecular weight of 1000~6000) with high molecular weight, called long chain; B represents a diol containing 2-12 straight chain carbon atoms, called a short chain.

In the structure of Thermoplastic polyurethane elastomer, segment A is called soft segment, which has the characteristics of flexibility and softness, making TPU have extensibility; The urethane chain generated by the reaction between the B segment and isocyanate is called a hard segment, which has both rigid and hard properties. By adjusting the ratio of A and B segments, TPU products with different physical and mechanical properties are made.

According to the soft segment structure, it can be divided into polyester type, polyether type, and butadiene type, which respectively contain ester group, ether group, or butene group. According to the hard segment structure, it can be divided into urethane type and urethane urea type, which are respectively obtained from ethylene glycol chain extenders or diamine chain extenders. The common classification is divided into polyester type and polyether type.

What are the raw materials for TPU synthesis?

(1) Polymer Diol

Macromolecular diol with a molecular weight ranging from 500 to 4000 and bifunctional groups, with a content of 50% to 80% in TPU elastomer, plays a decisive role in the physical and chemical properties of TPU.

The polymer Diol suitable for TPU elastomer can be divided into polyester and polyether: polyester includes polytetramethylene Adipic acid glycol (PBA) ε PCL, PHC; Polyethers include polyoxypropylene ether glycol (PPG), tetrahydrofuran polyether glycol (PTMG), etc.

(2) Diisocyanate

The molecular weight is small but the function is outstanding, which not only plays the role of connecting the soft segment and the hard segment, but also endows the TPU with various good physical and mechanical properties. The diisocyanates applicable to the TPU are: Methylene diphenyl diisocyanate (MDI), methylene bis (-4-cyclohexyl isocyanate) (HMDI), p-phenyldiisocyanate (PPDI), 1,5-naphthalene diisocyanate (NDI), p-phenyldimethyl diisocyanate (PXDI), etc.

(3) Chain extender

The chain extender with a molecular weight of 100~350, belonging to small molecular Diol, small molecular weight, open chain structure and no substituent group is conducive to obtaining high hardness and high scalar weight of TPU. The chain extenders suitable for TPU include 1,4-butanediol (BDO), 1,4-bis (2-hydroxyethoxy) benzene (HQEE), 1,4-cyclohexanedimethanol (CHDM), p-phenyldimethylglycol (PXG), etc.

Modification Application of TPU as a Toughening Agent

In order to reduce product costs and obtain additional performance, polyurethane thermoplastic elastomers can be used as commonly used toughening agents to toughen various thermoplastic and modified rubber materials.

Due to its high polarity, polyurethane can be compatible with polar resins or rubbers, such as chlorinated polyethylene (CPE), which can be used to make medical products; Blending with ABS can replace engineering thermoplastics for use; When used in combination with polycarbonate (PC), it has properties such as oil resistance, fuel resistance, and impact resistance, and can be used to make car bodies; When combined with polyester, its toughness can be improved; In addition, it can be well compatible with PVC, Polyoxymethylene or PVDC; Polyester polyurethane can be well compatible with 15% Nitrile rubber or 40% nitrile rubber/PVC blend; Polyether polyurethane can also be well compatible with 40% nitrile rubber/polyvinyl chloride blend adhesive; It can also be co compatible with acrylonitrile styrene (SAN) copolymers; It can form interpenetrating network (IPN) structures with reactive polysiloxanes. The vast majority of the above-mentioned blended adhesives have already been officially produced.

In recent years, there has been an increasing research on the toughening of POM by TPU in China. The blending of TPU and POM not only improves the high-temperature resistance and mechanical properties of TPU, but also significantly toughens POM. Some researchers have shown that in tensile fracture tests, compared to POM matrix, the POM alloy with TPU has transitioned from brittle fracture to ductile fracture. The addition of TPU also endows POM with shape memory performance. The crystalline region of POM serves as the fixed phase of the shape memory alloy, while the amorphous region of amorphous TPU and POM serves as the reversible phase. When the recovery response temperature is 165 ℃ and the recovery time is 120 seconds, the recovery rate of the alloy reaches over 95%, and the recovery effect is the best.

TPU is difficult to be compatible with non-polar polymer materials such as polyethylene, polypropylene, Ethylene propylene rubber, butadiene rubber, isoprene rubber or waste rubber powder, and cannot be used to produce composites with good performance. Therefore, surface treatment methods such as plasma, corona, Wet chemistry, primer, flame or reactive gas are often used for the latter. For example, the American Air Products and Chemicals Company has carried out F2/O2 active gas surface treatment on ultra-high molecular weight polyethylene fine powder with a molecular weight of 3-5 million, and added it to polyurethane elastomer at a ratio of 10%, which can significantly improve its Flexural modulus, tensile strength and wear resistance. And the F2/O2 active gas surface treatment can also be applied to the directionally elongated short fibers with a length of 6-35mm, which can improve the stiffness and tear toughness of the composite material.

What are the application areas of TPU?

In 1958, Goodrich Chemical Company (now renamed Lubrizol) registered the TPU brand Estane for the first time. Over the past 40 years, there have been more than 20 brand names around the world, and each brand has several series of products. At present, the main TPU raw material manufacturers in the world are: BASF, Covestro, Lubrizol, Huntsman Corporation, McKinsey, Golding, etc.

As an excellent elastomer, TPU has a wide range of downstream products, which are widely used in daily necessities, sports goods, toys, decorative materials, and other fields. Below are a few examples.

① Shoe materials

TPU is mainly used for shoe materials due to its excellent elasticity and wear resistance. Footwear products containing TPU are much more comfortable to wear than regular footwear products, so they are more widely used in high-end footwear products, especially some sports shoes and casual shoes.

② Hoses

Due to its softness, good tensile strength, impact strength, and resistance to high and low temperatures, TPU hoses are widely used in China as gas and oil hoses for mechanical equipment such as aircraft, tanks, automobiles, motorcycles, and machine tools.

③ Cable

TPU provides tear resistance, wear resistance, and bending characteristics, with high and low temperature resistance being the key to cable performance. So in the Chinese market, advanced cables such as control cables and power cables use TPUs to protect the coating materials of complex cable designs, and their applications are becoming increasingly widespread.

④ Medical devices

TPU is a safe, stable and high-quality PVC substitute material, which will not contain Phthalate and other chemical harmful substances, and will migrate to the blood or other liquids in the medical catheter or medical bag to cause side effects. Moreover, the specially developed extrusion grade and injection grade TPU can be easily used with a little debugging in the existing PVC equipment.

⑤ Vehicles and other means of transportation

By extruding and coating both sides of nylon fabric with polyurethane thermoplastic elastomer, inflatable combat attack rafts and reconnaissance rafts carrying 3-15 people can be made, with much better performance than vulcanized rubber inflatable rafts; Polyurethane thermoplastic elastomer reinforced with glass fiber can be used to make body components such as molded parts on both sides of the car itself, door skins, bumpers, anti friction strips, and grilles.