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Thermoplastic polyurethane (TPU) is a multifunctional thermoplastic polymer with elastomeric properties. The raw material is colorless square or round particles, which are composed of hard segment chains and soft segment chains. The carbamates in the hard segment can form intermolecular hydrogen bonds, so that the hard segment aggregates to form physical crosslinking points, providing thermoplastic polyurethane with physical properties such as tensile resistance, wear resistance and heat resistance; its soft segment is composed of polyether, polyester or their mixture, which provides thermoplastic polyurethane with rubber-like elasticity. The intermolecular hydrogen bonds of thermoplastic polyurethane can be dissociated by increasing the temperature, and the intermolecular hydrogen bonds and physical crosslinking can be re-formed by lowering the temperature, so that thermoplastic polyurethane can be processed by thermoplastic material processing technology, such as injection molding, extrusion, blow molding, calendering and slush molding.
Thermoplastic polyurethane has the advantages of high transparency, good tensile strength and high toughness, and has excellent tear resistance, good dimensional stability, excellent electrical insulation and good biocompatibility. Thermoplastic polyurethane has been fully recognized and utilized in the fields of electronics, automobiles, electric tools, household appliances, sanitary products, oil-water separators, medical treatment, etc. Thermoplastic polyurethane also has both the processing performance of plastics and the physical and mechanical properties of rubber. Its wear resistance, oil resistance, low temperature resistance, and radiation resistance are particularly excellent. It is increasingly widely used in the interior and exterior decoration of the automotive industry and as a functional material.
In 1958, Bayer of Germany prepared thermoplastic polyurethane (TPU) for the first time. In 1963 and 1965, Phillips and Shell of the United States developed thermoplastic styrene-butadiene-styrene (SBS) block polymer elastomers. Since the United States, Japan and European countries began to mass-produce olefin thermoplastic elastomers in the 1970s, the technology has been constantly innovating.
The molecular chain of thermoplastic polyurethane is linear, and there are many physical crosslinks formed by hydrogen bonds between the molecular chains. The intermolecular hydrogen bonds of thermoplastic polyurethane can be dissociated by increasing the temperature, and the intermolecular hydrogen bonds and physical crosslinks can be re-formed by lowering the temperature.
Thermoplastic polyurethane is mainly composed of hard segment chains and soft segment chains. At room temperature, the soft segment is in a highly elastic state, and the hard segment is in a glassy or crystalline state. The soft segment is generally composed of soft oligomer diols, which provide elasticity for thermoplastic polyurethane; the hard segment of thermoplastic polyurethane is composed of diisocyanates and small molecule chain extenders, which provide rigidity for thermoplastic polyurethane, as shown in the figure below.
The soft and hard segments of thermoplastic polyurethane are thermodynamically incompatible systems, which will produce microphase separation in the system.
Thermoplastic polyurethane can be classified according to different standards.
(1) Thermoplastic polyurethanes can be divided into polyester type, polyether type and polybutadiene type according to the soft segment structure, which contain ester groups, ether groups or butene groups respectively.
(2) Thermoplastic polyurethanes can be divided into yellowing type (MDI, TDI, NDI, etc.) and non-yellowing type (HDI, IPDI, etc.) according to the isocyanate structure used.
(3) Thermoplastic polyurethanes can be divided into urethane type and urethane urea type according to the hard segment structure, which are prepared by diol chain extension or diamine chain extension respectively.
(4) Thermoplastic polyurethanes can be divided into full thermoplastic type and semi-thermoplastic type according to whether they are cross-linked or not. There are no chemical crosslinks between the molecules of the full thermoplastic type, only physical crosslinks mainly based on hydrogen bonds, which are soluble in solvents such as dimethylformamide and are pure linear structures; there are a small amount of chemical crosslinks such as allophanate between the molecules of the semi-thermoplastic type. These chemical crosslinks are thermodynamically unstable and will break at processing temperatures above 150¡ãC, and will regenerate again after molding and cooling. The presence of a small amount of chemical crosslinks plays an important role in improving the compression permanent deformation and tearing permanent deformation of the product.
Thermoplastic polyurethane has high toughness, high strength, high elasticity, insulation and good strain capacity. Under long-term load, the strain curve decreases less, and it is suitable for working in harsh environments with long-term loads, and has good mineral oil resistance and excellent wear resistance. Thermoplastic polyurethane has outstanding cold resistance, can be recycled, has good biocompatibility, and is non-toxic, but the heat resistance of thermoplastic polyurethane is not outstanding, the long-term working temperature cannot be higher than 80¡ãC, and the acid, alkali and water resistance are poor, especially polyester thermoplastic polyurethane. Thermoplastic polyurethane is a polar block copolymer that is resistant to non-polar solvents (such as fuels, oils, lubricants, etc.), but sensitive to polar organic solvents and non-polar solvents. Thermoplastic polyurethane is also addable like plastics and can be manufactured through thermoplastic processes such as extrusion and injection molding.
The synthesis process of thermoplastic polyurethane is divided into bulk polymerization and solution polymerization.
The experimental principle of bulk polymerization is that the main chain of polyurethane contains carbamate, and thermoplastic polyurethane is generated by the gradual addition reaction of isocyanate group and hydroxyl group of diol. If polyether diol or polyester diol is used to participate in the synthesis of polyurethane, the reaction formula is as follows:
Thermoplastic polyurethane can be divided into prepolymerization and one-step method in bulk polymerization: the prepolymerization method is to react diisocyanate with macromolecular diol for a period of time, and then add chain extender to generate thermoplastic polyurethane; the physical and mechanical properties of the product obtained by prepolymerization are better than those of one-step method, but the process control is more difficult. The one-step method is to mix and react diisocyanate, macromolecular diol and chain extender at the same time to generate thermoplastic polyurethane.
Solution polymerization generally uses polar solvents. The advantages of the solution method are stable, slow, easy to control, and good uniformity. The disadvantages are that the solvent purity is high, and the solvent is volatile, which can cause environmental pollution. The strength of the produced film is not as high as that of the bulk polymerization method. ?The solution polymerization method also has a one-step method and a prepolymerization method. All raw materials can be added to the reactor together and reacted at a certain temperature. Polyester diol, chain extender and MDI can also be added to the reactor first, and then the solvent is added after the viscosity increases. Finally, the material is cooled and filtered.
The injection molding process is to add the dry and preheated pellets into the barrel of the injection molding machine. After heating and flowing, they are pressed to the nozzle by the plunger or screw and injected into the preheated mold. The hot melt fills the mold cavity and is cooled and molded. The product is obtained after demolding. The injection molding process can adopt single-station annular continuous injection or multi-station disc injection with a high level of automation.
The process of extrusion molding is to add dry thermoplastic polyurethane pellets from the hopper to the barrel of the extruder, heat them and transfer them to the rotating screw for heat plasticization, extrude them from the die, cool them to shape them, and then finish them. Thermoplastic polyurethane with a Shore hardness of less than 92 is suitable for this method, such as: special-shaped parts, hoses, cable jackets, films, etc.
The compression molding process is relatively simple. The pellets and compound materials are sent to the internal mixer for internal mixing. After being pressed into tablets, they are sent to the preheated mold and molded for 1~5min at a pressure of 1~7MPa. After demolding and cooling, the product is obtained.
The calendering process is to send the thermoplastic polyurethane melt and ingredients to a preheated internal mixer for plasticization, and then calender it into a continuous sheet through the gap between two or more identically rotating rollers, cool it and form it, and then pull it out for coiling. The calendering process includes two stages: the front stage includes ingredients, plasticization and feeding to the calender, and the back stage includes calendering, pulling, cooling, coiling, etc. The temperature of thermoplastic polyurethane during plasticization is generally 140-170¡æ, and the temperature during calendering is generally 130-160¡æ. Due to the high viscosity of thermoplastic polyurethane melt, it is generally necessary to add a small amount of lubricants and other additives.
Thermoplastic polyurethane is mainly used in wear-resistant products, high-strength oil-resistant products and high-strength and high-modulus products.
In the automotive industry, thermoplastic polyurethane can be used to produce guide sleeves, shaft seals, bearings, gear lever connecting sleeves, washers, gaskets, sealing pads, door and window seals, hydraulic pipes, chair back handles, etc. Thermoplastic polyurethane reinforced with glass fiber can improve the rigidity and impact of reinforced polyurethane elastomers, which are used to make large parts such as automobile bumpers. Thermoplastic polyurethane and its alloys can also be used to produce automotive products that make it easier for vehicles to drive on ice and snow in cold areas. They are better than anti-skid chains and snow tires, and their performance is very ideal. For example, a car tire made of thermoplastic polyurethane has been used in zero cold areas.
Thermoplastic polyurethane is widely used in travel shoes, sports shoes and sports products, such as the outer layer of ski boots and the soles of sports shoes. Among them, thermoplastic polyurethane is most commonly used in football shoes. Thermoplastic polyurethane shoe materials have the characteristics of lightness, comfort, good elasticity, variety, easy processing, and recyclability.
In the pipe industry, thermoplastic polyurethane can be made into different types of pipes through extrusion molding, especially in the manufacture of pneumatic pipes and fire hoses.
Thermoplastic polyurethane can also be used as an adhesive, which is a solution adhesive or hot melt adhesive. Thermoplastic polyurethane can also replace rubber on escalator handrails to become a polyurethane handrail formed by extrusion. Compared with traditional rubber handrails, the new polyurethane handrail is environmentally friendly, beautiful, low energy consumption and high efficiency. Thermoplastic polyurethane can also be used to manufacture cable sheaths, military and marine facilities and 3D printing technology.
Thermoplastic polyurethane particles are not toxic; thermoplastic polyurethane solutions contain organic solvents and are handled as hazardous materials.
Thermoplastic polyurethane should be stored in a dry room temperature environment. To prevent condensation, it is recommended that the material be allowed to reach room temperature before opening the package for materials stored at low temperatures. After opening the bag, if there is still material in the bag, the bag should be sealed as tightly as possible. Granular materials should only be exposed to the ambient air when absolutely necessary.
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