thermoplastic polyurethane elastomer, also known as thermoplastic polyurethane rubber, is a kind of (AB)n-type block linear polymer, in which A is polyester or polyether with high molecular weight (1~6), B is diol with 2~12 linear carbon atoms, and the chemical structure between AB segments is diisocyanate. Thermoplastic polyurethane rubber is crosslinked by intermolecular hydrogen bonds or slightly crosslinked by macromolecular chains. With the increase or decrease of temperature, these two crosslinking structures are reversible. In the molten state or solution state, the intermolecular force is weakened, but after cooling or solvent volatilization, there is a strong intermolecular force to connect together, restoring the original solid properties. Typical TPU such as spandex. Chinese name: thermoplastic polyurethane elastomer color:: transparent, translucent, transparent yellow background, natural color. Brief introduction, classification, characteristics, synthesis method, TPU molecular structure, segment structure, hard segment content, isocyanate index, molecular weight and molecular weight distribution, raw material purity, new technology and new application. Thermoplastic polyurethane elastomer (TPU) is a kind of elastomer which can be plasticized by heating and dissolved in solvent. It has excellent comprehensive properties such as high strength, high toughness, wear resistance and oil resistance, and good processability, and is widely used in national defense. Thermoplastic polyurethane elastomer, with its excellent performance and wide application, has become one of the important thermoplastic elastomer materials, and its molecules are basically linear, with little or no chemical crosslinking. There are many physical cross-links composed of hydrogen bonds between linear polyurethane molecular chains, and hydrogen bonds strengthen its morphology, thus giving it many excellent properties, such as high modulus, high strength, excellent wear resistance, chemical resistance, hydrolysis resistance, high temperature resistance and mildew resistance. These good properties make thermoplastic polyurethane widely used in many fields such as shoes, cables, clothing, automobiles, medicine and health, pipes, films and sheets. Generally, the final product does not need vulcanization and crosslinking, which can shorten the reaction period and reduce energy consumption. Because it is basically a polymer with linear structure, it can be processed by the same technology and equipment as thermoplastic plastics, such as injection molding, extrusion, blow molding, calendering, etc., and is especially suitable for large-scale production of medium and small-sized parts. Waste materials can be recovered and reused, and different additives or fillers can be used to improve some physical properties and reduce costs during production or processing. TPU particles and powder: particle classification Polyurethane thermoplastic elastomer has two types: polyester type and polyether type, which are white irregular spherical or columnar particles with a relative density of 1.1~1.25, and the relative density of polyether type is smaller than that of polyester type. The glass transition temperature of polyether type is 1.6~16.1℃, and that of polyester type is 18.9~122.8℃. The brittleness temperature of polyether and polyester is lower than -62℃, and the low temperature resistance of polyether is better than that of polyester. Characteristics Polyurethane thermoplastic elastomer has the outstanding characteristics of excellent wear resistance, excellent ozone resistance, high hardness, high strength, good elasticity, low temperature resistance, good oil resistance, chemical resistance and environmental resistance, and the hydrolysis stability of polyether ester is far higher than that of polyester ester in humid environment. Synthesis methods The synthesis methods of TPU can be divided into two types according to whether there is solvent or not: solvent-free bulk polymerization and solvent-based solution polymerization. Bulk polymerization can be divided into one-step method and prepolymer method according to reaction steps. One-step method is to mix oligomer diol, diisocyanate and chain extender at the same time. One-step process is simple and easy to operate, but its reaction heat is difficult to eliminate and side reactions are easy to occur. Fan Li et al. synthesized polyester thermoplastic polyurethane elastomer by one-step method. Firstly, the polyester polyol and chain extender, butanediol, were weighed in a reactor, heated to 12℃ and dehydrated in vacuum. Quickly add the preheated polyurethane, stir it evenly, pour it into a preheated container, bake it in vacuum at 12℃, and then cool it to 1℃ to obtain a light yellow translucent polyurethane product, and then press it into a test piece on a flat press. The prepared TPU has high mechanical properties and damping properties. Chemical equation of TPU synthesis TPU synthesis process prepolymer method is to react oligomer diol and diisocyanate first, and then synthesize it with dry chain extender under the condition of a small amount of catalyst. The process of prepolymer method in production is complicated, energy consumption is high, and the viscosity of prepolymer is high, which increases the difficulty of process operation. However, the side reaction of prepolymer is less, and the performance of the product is better than that of one-step method. According to the continuity of the reaction process, it can be divided into batch method and continuous method. The production equipment commonly used in batch process includes automatic pouring equipment, curing oven, breaking hammer, extruder, etc., which has low production efficiency and uneven product quality, and is not suitable for large-scale production. Therefore, continuous production technology and equipment have been studied at home and abroad. The continuous process equipment is a reactive extrusion production line, and its main equipment includes raw material storage tank, pouring machine, parallel twin-screw extruder, underwater granulator, separation and drying equipment and packaging equipment. Twin-screw continuous reactive extrusion is the mainstream technology in production at present, with high production efficiency and stable product quality, which is suitable for large-scale production. Its products can be used in coatings, elastomers and adhesives. TPU molecular structure TPU is a kind of (ab) n-type block linear polymer, which consists of flexible soft segment and rigid hard segment. TPU with different segment structure has different properties, and the type of segment structure is mainly determined by the type of raw materials. The introduction of side groups in the molecular structure will reduce the orientation crystallinity between macromolecules, which will lead to the decline of mechanical properties and the deterioration of swelling properties. However, certain chemical crosslinking can improve the tensile stress and solvent resistance of elastomer and reduce permanent deformation. Hard segment content Hard segment content refers to the mass percentage of hard segment in the formula, which is an important parameter in formula design. The hydrogen bond, microphase separation degree and crystallinity directly affected by hard segment content are the main factors that determine its morphology. Generally speaking, with the increase of hard segment content, the hardness, modulus and tear strength of TPU increase, while the elongation at break decreases. Isocyanate index Because the synthesis mechanism of TPU is a step-by-step addition polymerization reaction between functional groups, the isocyanate index r (the molar ratio of diisocyanate to oligomer diol) directly affects the molecular weight. When r ≤1, the molecular weight of TPU increases with the increase of r . When r =1, the molecular weight reaches the maximum, and then the molecular weight begins to decrease again with the increase of r value. When r is between .95 and 1, the modulus, tensile strength and tear strength of TPU increase with the increase of r . Molecular weight and molecular weight distribution The molecular weight of TPU has obvious influence on its mechanical properties. With the increase of TPU molecular weight, the tensile strength, modulus and wear resistance all increase, and these properties tend to be stable when the molecular weight reaches a certain level. The tear strength and flex resistance of TPU decrease with the increase of molecular weight. On the one hand, the free volume of TPU decreases due to physical crosslinking; On the other hand, the high entanglement of TPU molecular chains and the increase of physical cross-linking reduce their internal fluidity. When subjected to external force, molecular chain rearrangement is difficult to achieve and the applied stress cannot be effectively reduced. When the proportion of low molecular weight components is large, it is extremely harmful to the heat resistance and mechanical properties of elastomers, while when the proportion of high molecular weight components is too large, it will bring inconvenience to processing and molding. Therefore, the appropriate molecular weight and molecular weight distribution should be adjusted according to the specific processing requirements of TPU for different purposes. Raw material purity 1,4- butanediol (MDI), a chain extender commonly used in TPU, is very easy to absorb water, and its purity and moisture content directly affect the actual production value and have a great influence on the molecular weight of the final product. MDI is easy to self-aggregate, and it is easy to generate dimer if it is not well preserved. The moisture content, acid value and hydroxyl value of polymeric polyols vary from batch to batch, which greatly affects the stability of TPU performance. The moisture and free carboxyl contained in raw materials, on the one hand, react with MDI and consume part of MDI, resulting in inaccurate formula design; On the other hand, the bubbles generated by the reaction play a plasticizing role, which ultimately reduces the performance of the product. Therefore, the raw materials used to synthesize TPU need to be strictly dehydrated before use. New technology and new use TPU is a rapidly developing industry, and related new technologies, new products and new uses are constantly emerging. The use of TPU extends to almost all industries, and has been widely used in many fields such as shoes, clothing, pipes, films and sheets, cables, automobiles, architecture, medicine and health, national defense, sports and leisure. TPU is recognized as a new polymer material with environmental protection and excellent performance. At present, low-end consumption is the main consumption of TPU, and its high-end consumption field is basically dominated by some multinational companies, including Bayer in Germany, BASF, Lubrizol in the United States, Huntsman, etc., all of which are increasing the research and development of new products. TPU products with high added value have been continuously developed and put into the market, and TPU materials have become one of the fastest growing thermoplastic materials. A. Footwear: sports shoes Logo, sports shoes air cushion, hiking shoes, snowshoes, golf shoes, roller skates, fabrics and inner bonding materials. B. Clothing category: snow coat, raincoat, windbreaker, cold jacket, field suit, diapers, pants, and other fabric composites (waterproof and moisture permeable). C. Medicines: surgical gowns, hats, shoes, hospital mattresses, ice packs, bandages, plasma bags, surgical dressing strips, masks, and other fabrics and lining materials, operating bed airbags. D defense supplies: fabrics and lining fabrics and airbags for aircraft fuel tanks, weapons sealing films, tent windows, military water bags, life jackets, inflatable boats, etc. E. Sporting goods: football surface and liner, inflatable bed, drinking bag, ski gloves (waterproof bag), diving suit, snow suit, swimsuit, snowboard, trademark, airbag, sweatshirt, slimming suit and other fabrics and lining materials. F industrial supplies: rubber edges of trumpet drum paper, waterproof strips, fabrics such as sound insulation materials, fireproof materials, fireproof clothing, fireproof clothing, fireproof cloth, inner composite materials and outer sheath materials of wires and cables. G. Other uses: mobile phone keys, plastic inflatable toys, bed sheets, table towels, shower curtains, furniture cloths, aprons, pianos, computer keyboards, film-covered fabrics and lining materials.