The so-called elastic body refers to the glass transition temperature is lower than room temperature, elongation at break >50%, after the withdrawal of external force better recovery of polymer materials. And vitrification temperature higher than room temperature polymer materials called plastics. In the elastomer, the elongation at break is larger (>200%), the 100% fixed elongation stress is smaller, and the better elasticity can be called rubber. Therefore, elastomer is a more extensive kind of polymer material than rubber.

Polyurethane, or PU for short, is a kind of polymer material containing more repeated carbamate groups (-NHCOO-) in the molecular chain. Polyurethane elastomer is a special kind of elastomer. From the perspective of molecular structure, polyurethane elastomer is a kind of block polymer. Generally, it is composed of oligomer polyol flexible long chain to form a soft segment, and diisocyanate and chain extender to form a hard segment, and the hard segment and soft segment are arranged alternately to form repeating structural units. In addition to polyurethane containing ammonia ester groups, hydrogen bonds can be formed within and between molecules, and the soft and hard segments can form microphase regions and produce microscopic phase separation. These structural characteristics make polyurethane elastomers with excellent wear resistance and toughness, known as "wear-resistant rubber". Polyurethane elastomers according to the molding process of the products can be roughly divided into three categories: one is the pouring polyurethane elastomer (CPU), two is the thermoplastic polyurethane elastomer (TPU), three is the mixing type polyurethane elastomer (MPU).

Thermoplastic PU (TPU for short), also known as PU thermoplastic, is a linear block copolymer composed of oligomer polyol soft segment and diisocyanate-chain extender hard segment. It is a kind of polyurethane elastomer that can be plasticized by heating and dissolved by solvent.

Compared with general plastics and rubber materials, TPU has a wide range of hard layer, outstanding mechanical properties, excellent high/low temperature resistance, good processing performance, excellent environmental protection performance, strong plasticity, strong designability, good transparency and other excellent characteristics. It has both the high elasticity of rubber materials and the high strength of engineering plastics. Because of its excellent performance and wide application, it has become one of the important thermoplastic elastomer materials. Compared to CPU and MPU, the molecular chain of TPU is basically linear, with little or no chemical cross-linking in the chemical structure. There are many physical crosslinks formed by hydrogen bonds between linear polyurethane molecular chains, which strengthen the morphology of TPU, thus giving it many excellent properties, such as high modulus, high strength, excellent wear resistance, chemical resistance, hydrolysis resistance, high and low temperature and mold resistance.

TPU is A (AB) type block linear polymer, A stands for high molecular weight polymer polyol poly (ester or polyether, molecular weight 1000 ~ 6000), known as long chain; B stands for 2-12 straight chain carbon diols, called short chains. In the structure of thermoplastic polyurethane elastomer, A chain segment is called soft chain segment, with flexible and soft characteristics, so that TPU has extensibility; The ammonia ester chain generated by the reaction of B chain segment and isocyanate is called hard chain segment, which has rigid and hard properties. People make TPU products with different physical and mechanical properties by adjusting the proportion of A and B chain segments.

According to the soft segment structure can be divided into polyester type, polyether type and butadiene type, they contain ester group, ether group or butadiene group respectively. According to the hard segment structure, it can be divided into ammonia ester type and ammonia ester urea type, which are obtained by diol chain extender or diamine chain extender respectively. The common division is divided into polyester type and polyether type.

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Polyether type (Ether) : high strength, hydrolysis resistance and high resilience, good low temperature performance. Polyester (Ester) : good tensile properties, flexural properties, friction resistance, solvent resistance and high temperature resistance.

(2) performance comparison tensile strength, tear strength, wear resistance, drug resistance, transparency: polyester > polyether bacterial resistance, low temperature impact: polyester < polyether system

(1) Polymer diol is a large molecule diol with double functional groups in the molecular weight range of 500 ~ 4000. Its content in TPU elastomer is 50% ~ 80%, which plays a decisive role in the physical and chemical properties of TPU. Suitable for TPU elastomer polymer diols are polyester and polyether two categories: polyester has poly tetramethylene adipate diol (PBA), poly ε hexanolide diol (PCL), poly hexamethylene carbonate diol (PHC); Polyether category of poly oxide propylene ether diol (PPG), tetrahydrofuran all polyether diol (PTMG).

(2) The diisocyanate has a small molecular weight but outstanding function, which not only connects the soft segment and the hard segment, but also endodes TPU with various good physical and mechanical properties. The diisocyanate suitable for TPU includes: Diphenylmethane diisocyanate (MDI), 4,4' -dicyclohexylmethane diisocyanate (HMDI), p-phenyldiisocyanate (PPDI), 1, 5-naphthalene diisocyanate (NDI), p-phenyldimethyl diisocyanate (PXDI), etc.

(3) The chain extender has a molecular weight of 100 ~ 350, which is a small molecule dialcohol. The chain extender with a small molecular weight and an open chain structure without substituent group is conducive to TPU obtaining high hardness and high scalar quantity. The chain extenders suitable for TPU are: 1,4 butanediol (BDO), 1,4 pairs (2 hydroxyethoxy) benzene (HQEE), 1,4 cyclohexane dimethanol (CHDM), p-phenyldimethyl diol (PXG), etc.

TPU can be divided into bulk polymerization and solution polymerization according to different synthetic processes.

图 生产氨酯基的反应

In bulk polymerization, it can be divided into pre-polymerization and one-step polymerization according to whether there is pre-reaction. The pre-polymerization method is to react diisocyanate and macromolecule diol for a certain time, and then add chain extension to produce TPU. TPU is synthesized by mixing macromolecules of diol, diiso-acid ester and chain extender in one step. Solution polymerization is to dissolve diisocyanate in solvent first, then add macromolecule diol to make it react for a certain time, and finally add chain extender to generate TPU. Soft segment type, molecular weight, hard segment or soft segment content and TPU aggregation state affect the density of TPU, and the density is about 1.10~1.25, no significant difference from other rubber and plastics. The density of polyether TYPE TPU is lower than that of polyester type TPU at the same hardness.

Synthetic TPU particles need to be processed in a variety of ways to form the final product, mainly using melting method and solution method for TPU processing. Melt processing is used in the plastic industry commonly used processes: such as mixing, calendering, extrusion, blow molding and molding (including injection, compression, transfer and centrifugal, etc.); Solution processing is a solution made of granules dissolved in solvent or directly polymerized in solvent and then coated and spun. TPU final products generally do not need to be vulcanized crosslinking reaction, can shorten the production cycle, waste materials can be recycled and reused.