Introduction and application of thermoplastic polyurethane elastomer (TPU)
What is thermoplastic polyurethane elastomer?
Polyurethane elastomer is a variety of polyurethane synthetic materials (other varieties refer to polyurethane foam, polyurethane adhesives, polyurethane coatings and polyurethane fibers), and thermoplastic polyurethane elastomer is one of the three major types of polyurethane elastomers. , people are used to calling it TPU (the other two major types of polyurethane elastomers are cast polyurethane elastomers, referred to as CPU, and mixed polyurethane elastomers, referred to as MPU).
TPU is a type of polyurethane elastomer that can be plasticized by heating and dissolved by solvents. Compared with CPU and MPU, TPU has no or very few chemical cross-links in its chemical structure. Its molecular chain is basically linear, but there are A certain amount of physical cross-linking is the structurally distinctive thermoplastic polyurethane elastomer to be introduced below.
Structure and classification of TPU
Thermoplastic polyurethane elastomer is an (AB) type block linear polymer. A represents a high molecular weight polymer polyol poly(ester or polyether, with a molecular weight of 1000 to 6000), which is called a long chain; B represents a polymer containing 2 to Diols with 12 straight chain carbon atoms are called short chain.
In the structure of thermoplastic polyurethane elastomer, the A segment is called the soft segment, which has flexible and soft characteristics, making TPU extensible; the urethane chain generated by the reaction of the B segment and isocyanate is called the hard segment. It has rigid and hard properties. People can make TPU products with different physical and mechanical properties by adjusting the ratio of A and B segments.
According to the soft segment structure, it can be divided into polyester type, polyether type and butadiene type, which contain ester group, ether group or butenyl group respectively. According to the hard segment structure, it is divided into urethane type and carbamate urea type, which are obtained by glycol chain extender or diamine chain extender respectively. The most common classification is polyester type and polyether type.
What are the synthetic raw materials of TPU?
(1) Polymer diol
Macromolecular diols with molecular weights in the range of 500 to 4000 and bifunctional groups, whose content in TPU elastomers is 50% to 80%, play a decisive role in the physical and chemical properties of TPU.
Polymer glycols suitable for TPU elastomers include polyester and polyether: polyesters include polytetramethylene adipate glycol (PBA), polyε-caprolactone glycol (PCL), Polyhexamethylene carbonate glycol (PHC); polyethers include polyoxypropylene ether glycol (PPG), tetrahydrofuran homopolyether glycol (PTMG), etc.
(2) Diisocyanate
The molecular weight is small but the function is outstanding. It not only plays the role of connecting soft segments and hard segments, but also gives TPU various good physical and mechanical properties. The diisocyanates suitable for TPU include: diphenylmethane diisocyanate (MDI), methylene Bis(-4-cyclohexyl isocyanate) (HMDI), p-phenyl diisocyanate (PPDI), 1,5-naphthalene diisocyanate (NDI), p-phenyl dimethyl diisocyanate (PXDI), etc.
(3) Chain extender
The molecular weight is 100 to 350, which is a small molecular glycol. The chain extender has a small molecular weight, an open-chain structure and no substituted groups, which is helpful for TPU to obtain high hardness and high scalar weight. Chain extenders suitable for TPU include: 1,4-butanediol (BDO), 1,4-bis(2-hydroxyethoxy)benzene (HQEE), 1,4-cyclohexanedimethanol (CHDM), Monophenyldimethylglycol (PXG), etc.
Modification application of TPU as toughening agent
In order to reduce product costs and obtain additional properties, polyurethane thermoplastic elastomers, as common toughening agents, can be used to toughen a variety of thermoplastics and modified rubber materials.
Since polyurethane is a highly polar polymer, it is compatible with polar resins or rubbers: for example, it can be used with chlorinated polyethylene (CPE) to make medical products; blended with ABS, it can be used instead of engineering thermoplastics; Used together with polycarbonate (PC), it has oil resistance, fuel resistance, impact resistance and other properties, and can be used to make automobile bodies; used together with polyester to improve its toughness; in addition, it can be used with polyvinyl chloride, polyoxymethylene (POM) or polyvinylidene Good compatibility with vinyl chloride; polyester polyurethane can be well compatible with 15% nitrile rubber or 40% nitrile rubber/polyvinyl chloride blend; polyether polyurethane can also be used with 40% nitrile rubber/polyvinyl chloride blend It has good compatibility with the mixed rubber; it can also be co-compatible with acrylonitrile-styrene (SAN) copolymer; it can form an interpenetrating network (IPN) structure with reactive polysiloxane. Most of the above blended rubbers have been officially launched as products.
In recent years, domestic research on the toughening of POM by TPU has been increasing. On the one hand, the blending of TPU and POM improves the high temperature resistance and mechanical properties of TPU. On the other hand, TPU plays a significant toughening effect on POM. Researchers have proven that compared to the POM matrix in the tensile fracture test, the POM alloy added with TPU changes from brittle fracture to ductile fracture. The addition of TPU also gives POM shape memory properties. The crystalline region of POM serves as the stationary phase of the shape memory alloy, and the amorphous TPU and POM amorphous regions serve as reversible phases. When the recovery response temperature is 165°C, the recovery time is 120 s. When, the recovery rate of this alloy reaches more than 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 composite materials with good performance cannot be produced. Therefore, surface treatment methods such as plasma, corona, wet chemistry, primer, flame or reactive gas are often used for the latter. For example, American Air Products and Chemicals Company conducts F2/O2 active gas surface treatment on ultra-high molecular weight polyethylene fine powder with a molecular weight of 3 to 5 million, and adds it to the polyurethane elastomer at a ratio of 10%, which can significantly improve its performance. Flexural modulus, tensile strength and wear resistance. In addition, the above-mentioned directional elongation short fibers with a length of 6 to 35 mm are also surface treated with F2/O2 active gas, which can improve the stiffness and tear toughness of the composite material.
What are the application fields of TPU?
In 1958, the American Goodrich Chemical Company (now renamed Lubrizol) first registered the TPU brand name Estane. In the following 40 years, more than 20 brand names have been launched around the world, and each brand has several series of products.
As an elastomer with excellent performance, TPU has a wide range of downstream product directions and is widely used in daily necessities, sporting goods, toys, decorative materials and other fields. Here are a few examples.
①Shoe materials
TPU is used in shoe materials mainly due to its excellent elasticity and wear resistance. Footwear products containing TPU are much superior to ordinary footwear products in terms of wearing comfort. Therefore, they are widely used in high-end footwear products, especially some sports shoes and casual shoes.
②Hose
Because TPU hoses are soft and have good tensile strength, impact strength and high and low temperature resistance, TPU hoses are widely used in China for gas and oil transportation in aircraft, tanks, automobiles, motorcycles, machine tools and other mechanical equipment. hose.
③Cable
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 is used for high-end cables such as control cables and power cables. It is used to protect the coating materials of complexly designed cables, and its uses are becoming increasingly widespread.
④Medical equipment
TPU is a safe, stable and high-quality alternative material to PVC. It does not contain harmful chemical substances such as phthalates, which can migrate into the blood or other liquids in medical catheters or medical bags to cause side effects. It is also specially developed for extrusion. Extrusion grade and injection molding grade TPU can be easily used with just a little debugging on existing PVC equipment.
⑤Cars and other means of transportation
Using nylon fabric extrusion-coated on both sides with polyurethane thermoplastic elastomer, inflatable combat attack rafts and reconnaissance rafts carrying 3 to 15 people can be made. Its performance is far superior to that of vulcanized rubber inflatable rafts; polyurethane thermoplastic elastomer reinforced with glass fiber can be used To make molded parts on both sides of the car itself, door skins, bumpers, scuff strips and grilles and other body parts.