I. Introduction to TPU
Thermoplastic polyurethane elastomer (TPU), also known as PU thermoplastic, is a linear block copolymer composed of oligomer polyol soft segment and diisocyanate-chain extender hard segment. TPU molecules contain -NH-COO- groups, many of its properties depend on the type of long chain dialcohols, its hardness is adjusted by the proportion of the hard segment, its photoaging properties can be improved by adding light stabilizers, but also depends on the isocyanate is aromatic or aliphatic.
II. Classification of TPU (Thermoplastic Polyurethane)
According to different standards for classification, there are different classification methods, generally mainly the following 5 categories, the most common. 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 and butenyl group. According to the hard segment structure, they can be divided into aminoester type and aminoester urea type, which are obtained by diol chain extension or diamine chain extension, respectively. It can be divided into pure thermoplastic and semi-thermoplastic according to whether there is crosslinking. The former is a purely linear structure with no crosslinking. The latter contains a small amount of urea-formate and other cross-linked bonds. According to the synthesis process, it can be divided into bulk polymerization and solution polymerization. In bulk polymerization, prepolymerization method and one-step method can be divided according to the presence or absence of prereaction. The pre-polymerization method is to react diisocyanate with macromolecule diol for a certain time, and then add chain extender to form TPU. One-step reaction of diisocyanate mixed with macromolecule diol and chain extender to produce TPU. 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 form TPU. According to the use of products can be divided into special-shaped parts (various mechanical parts), pipe (sheath, bar profiles) and film (thin, thin), as well as adhesives, coatings and fibers.
The differences between polyether type TPU and polyester type TPU
A variety of polyalcohols can be used in the soft segment of TPU, which can be roughly divided into polyether series and polyester series. Polyether type (Ether) has high strength, hydrolysis resistance, high resilience, and good low temperature performance. Polyester type (Ester) has good tensile properties, flexural properties, friction resistance, solvent resistance and high temperature resistance. Specific differences are as follows: tensile strength: polyester > polyether tear strength: polyester > polyether is abrasion resistance: polyester > polyether is drug resistance: polyester > polyether is transparency: polyester > polyether of bacteria resistance: polyester series < polyether moisture evaporation sex: polyester series < polyether low-temperature impact: polyester is < polyether
(1) The main raw materials for the production of polyether type TPU are 4-4 '-diphenylmethane diisocyanate (MDI), polytetrahydrofuran (PTMEG), 1, 4- butanediol (BDO), of which the dosage of MDI is about 40%, PTMEG is about 40%, BDO is about 20%. (2) Polyester TPU production raw materials mainly include 4-4 '- diphenylmethane diisocyanate (MDI), 1, 4- butanediol (BDO), adipic acid (AA), in which the dosage of MDI is about 40%, AA is about 35%, BDO is about 25%
2. The molecular mass distribution and the relative molecular mass distribution of polyether follow Poisson probability equation, and the relative molecular mass distribution is narrow; However, the molecular weight distribution of polyester diol obeys the Flory probability distribution and has a wide molecular weight distribution. The molecular weight of the soft segment has an effect on the mechanical properties of polyurethane. In general, it is assumed that the molecular weight of the polyurethane is the same. If the soft segment is polyester, the strength of polyurethane increases with the increase of the molecular weight of polyester diol. If the soft segment is polyether, the strength of polyurethane decreases with the increase of the molecular weight of polyether diol, but the elongation increases. This is because the polarity of polyester soft segment itself is strong, the large molecular weight is high structure regularity, which is beneficial to improve the strength, and polyether soft segment is weak polarity, if the molecular weight increases, the relative content of the hard segment in polyurethane decreases, and the strength decreases.
3, mechanical properties comparison: polyether, polyester and other oligomer polyols composed of soft segment. Soft segment in the majority of polyurethane, different oligomer polyols and diisocyanate prepared polyurethane properties are different. The mechanical properties of polyurethane elastomer and foam obtained by using polarity polyester as soft segment are better. Because polyurethane made of polyester contains large polar ester group, this kind of polyurethane internal not only hard segment can form hydrogen bonds, and the polar groups on the soft segment can also form hydrogen bonds with the polar groups on the hard segment, so that the hard phase can be more evenly distributed in the soft phase, play the role of elastic crosslinking point. At room temperature, some polyesters can form soft segment crystallization, which affects the properties of polyurethane. Polyester polyurethane has higher strength, oil resistance and thermal oxidation stability than PPG polyether type, but poorer hydrolysis resistance than polyether type.
4, hydrolysis stability comparison: polyester thermoplastic polyurethane with carbonized diimine protection, hydrolysis resistance has been improved. Polyether ester thermoplastic polyurethane and polyether thermoplastic polyurethane have the best hydrolysis resistance at high temperature. Polyester is susceptible to the invasion of water molecules and breaks, and the acid generated by hydrolysis can catalyze the further hydrolysis of polyester. The type of polyester has certain influence on the physical properties and water resistance of elastomer. The water resistance of polyester-type polyurethane elastomer was improved with the increase of the number of methylene in polyester diol. The content of ester group is small and its water resistance is good. Similarly, the water resistance of polyurethane elastomers synthesized with long chain diacid is better than that of short-chain diacid polyester-type polyurethane.
5, microbial resistance comparison: polyester soft thermoplastic polyurethane and wet soil contact for a long time, will be eroded by microorganisms, and polyether soft or hard thermoplastic polyurethane and polyether type thermoplastic polyurethane or hard thermoplastic polyurethane is usually not eroded by microorganisms.
IV. Analysis of the reasons for the difference
1, polyether polyols: polyether polyols are alcohol polymers or oligomers containing ether bonds and hydroxyl groups on the molecular main chain structure. Because of the low cohesion energy of the ether bond in its structure, and easy to rotate. Therefore, the polyurethane material prepared by it has good low temperature flexibility, excellent hydrolysis resistance, although the mechanical properties are not as good as polyester polyol based polyurethane, but the hand feeling is good. The viscosity of the system is low, easy to be intersoluble with isocyanate, additives and other components, and the processing performance is good.
2, polyester polyols: polyester polyols are mainly produced by the condensation reaction of binary carboxylic acid and more than binary alcohol compounds. The junction is characterized by large molecular alcohols containing ester groups on the molecular main chain and hydroxyl groups on the end group. The molecular weight is generally 500~3000. Polyurethane materials based on polyester polyols usually have good mechanical properties, excellent oil resistance, wear resistance and other characteristics, but their hydrolysis resistance is poor, low temperature compliance is poor, the hand feel of its products, especially at low temperature as soft polyether polyols based polyurethane. Polyester polyols have high cohesive energy, are mostly waxy solid at room temperature, and have high viscosity after heating and melting. Their intersolubility with other raw material components used in the synthesis of polyurethane is far inferior to that of polyether polyols.
3. Under the condition that the chemical ratio of raw materials is constant, the influence of changing the length of flexible chain segment on the performance of elastomer of different soft segment types is different. The increase of soft segment molecular weight also decreases the proportion of hard segment. As the cohesion energy of the ether bond is low and the rotational barrier of the bond is small, the chain is more pliable and the proportion of soft segments increases with the increase of the molecular weight of the polyether, so the strength decreases, the elasticity increases and the permanent deformation increases. For polyester glycol, the effect of soft segment length on strength is not obvious. This is because of the existence of polar ester groups in the molecule, and the increase of the molecular weight of the soft segment and the increase of the ester groups of the polyester offset the negative effects of the increase of the soft segment and the decrease of the hard segment on the strength. In addition, the hydrolysis resistance of polyester polyurethanes decreases with the increase of polyester chain length, which is due to the increase of ester groups. The hydrolysis resistance of polyether polyurethane increases with the increase of polyether chain length.
Polyether polyurethane elastomers are much higher in price than polyester polyurethane elastomers, the main reasons are as follows: (1) polyether polyurethane elastomers have good hydrolysis resistance, low temperature resistance, bending resistance. ② Compared with polyester polyols, polyether polyols that make up the soft segment of TPU have higher raw material price. ③ The production process of polyether polyols is much more complicated than that of polyester polyols. (4) The technological conditions of polyether polyols are difficult to control in the reaction process. (5) In the production of polyether polyols, the production equipment requirements are higher, at the same time, the production process should pay attention to take certain protective measures.
Six, the conclusion
In summary, polyether type TPU has the advantages of high strength, hydrolysis resistance, high resilience and good low temperature performance. Usually used for soft foam, hard foam, hard plastics and surface coatings, high rebound soft foam processing production. Polyester type TPU has good tensile properties, flexure properties, friction resistance and solvent resistance, not easy oxidation and high temperature resistance, etc. It is mainly used in the production of soft foam, hard foam, low density semi-hard foam, soft coatings, elastomers and adhesives, solid core and microporous elastomers. The main reason for the difference between polyether type TPU and polyester type TPU is that its soft segment components are polyether type oligomer polyols and polyester type oligomer polyols respectively, and the soft segment components of TPU mainly affect the low softenness and long-term aging resistance of thermoplastic polyurethane. As far as we can see, polyester TPU is used more in the selection of raw materials by Ever Tech, while most of polyether TPU is still in the sample material testing stage. Many commercial thermoplastic polyurethane are polyester type, this thermoplastic polyurethane wear resistance, tear resistance, tensile and tear strength are superior to polyether type thermoplastic polyurethane, polyester type thermoplastic polyurethane in oil, fat and water swelling is less. However, it does not have the advantages of polyether polyurethane elastomers in terms of hydrolysis resistance, microbial degradation resistance, low temperature resistance and flexibility. Therefore, polyether thermoplastic polyurethane is recommended to be used when the above properties are required.
Seven, the difference of processing process comparison
1. Drying As we all know, polyurethane is a polar polymer that slowly absorbs moisture when exposed to air. With hygroscopic TPU granule melt processing molding, water in the processing temperature gasification, the surface of the product is not smooth, the internal bubble, physical property is reduced, so in order to ensure the performance of the product and prevent the melt processing caused by water gasification bubble, before TPU processing, generally need to dry the granule. We in front of TPU esters and ethers hydrolysis stability comparison has also made analysis, due to the invasion of polyester vulnerable to water molecules break, hydrolysis and generate acid and catalyze polyester further hydrolysis, under normal circumstances, under the same conditions, polyester TPU is much higher than water content of polyether type TPU, so in the drying process for polyester TPU is especially attention, attention should be paid to the thorough drying, strictly to control the drying conditions.
2. Pressure holding stage The polymer melt must undergo the combined action of internal static pressure and external dynamic pressure during injection molding, whether in the pre-molding stage or in the injection stage. In the pressure-holding stage, the polymer melt will be subjected to high pressure, under which the free volume between molecular chain segments will be compressed. As the free volume between molecular chains decreases, the proximity of macromolecular chain segments will strengthen the intermolecular force, indicating an increase in viscosity. In addition, due to the low cohesion energy of the ether bond of polyether TPU, the rotational barrier of the bond is small, which leads to the small interaction between the tight chain segments that strengthen the molecular chain. Therefore, the relative displacement of the molecular chain is large during compression, so the viscosity can change in a large range. In addition, because the molecular chain of polyether TPU is much more compliant than polyester TPU, it is difficult to form permanent deformation, so in the process of holding pressure of polyether TPU, compared with polyester TPU, polyether TPU has to control a longer holding time.
3, processing time because in general, the molecular weight increase the molecular chain segments, longer molecular chain center of gravity move slower, segment the relative displacement between the offset the opportunity, the more the flexibility of the molecular long chain increased, around nodes increases, the chain of relief and slip is difficult, make flow resistance increases, the need of time and energy is also increasing, the sensitivity of viscosity to shear. In general, the molecular weight of polyester TPU is larger than that of polyether TPU, so the processing and molding time will be longer.
4. Processing temperature Because the molecular weight distribution of polyester TPU is wider than that of polyether TPU in general, the required temperature in the processing process is higher. Because the nitrogen-oxygen bond of polyether TPUs is easy to break, the processing of polyether TPUs can be realized at relatively low temperature.
5. Pressure Because polyester TPU has a higher molecular cohesion energy and the nitrogen and oxygen bonds in its molecular structure are more difficult to break, higher temperature and pressure are required for its processing, namely, the destruction of its molecular bonds.
6. Cooling Due to the large internal friction and molecular cohesion energy of polyester TPU, it is difficult to cool it even if it returns to the normal state, so it needs a longer cooling time.
Because polyether TPU ether bond cohesion energy is low, the bond rotation barrier is small, with the increase of polyether relative molecular weight, the chain is more pliable, the molecular chain has a high degree of flexibility, so it shows good mobility, while polyester TPU is a little.
VIII. Analysis of various post-blending processing phenomena
Whether or not two or more polymers can be blended and the performance of the blended system is related to many factors, the most important factor is the compatibility between the various polymers. The compatibility of the blend system is related to their respective solubility parameters, polarity, surface tension, crystallization ability, viscosity and other factors. Now the following analyses are carried out:
1, esters and ethers in blending with polyether type TPU ether ester base within the group and polyester type TPU polarity is different, as well as the differences between the molecular structure, and ether base which generally in the ester resin compatibility is poor, so a mix of both will appear statified phenomenon, also with ether key molecular inter-atomic forces have a close relationship. In addition, the crystallinity of polyester is generally much stronger than that of polyether, so its compatibility is poor. But this is not true of all ethers. Because of PTMG (polyethylene tetrahydrofuran) crystallization of crystalline and polyester is similar, therefore with PTMG synthesis of polyether TPU and polyester TPU compatibility is a bit better, can be synthesized in the synthesis process, only after the processing of various physical properties will be dramatically reduced, do more harm than good, so there is no need for the blend. Ethers and esters, therefore, is to cannot be mixed together to carry on the processing, this is due to the molecular structure, molecular cohesive energy differences, molecular inter-atomic forces, crystalline differences and decided by incompatibility of the two molecules, when the both using the blending process, will appear obvious grain in the surface of specimen, there will be cloudy. Even if it can be mixed together for processing, the physical properties of the finished products after processing will still be greatly reduced, especially can not be used for processing special transparent accessories, in large quantities of production will also be very difficult, in the production process should pay special attention to do not confuse the two.
2. Blend of polyether TPU and Pebax Because Pebax itself is a block copolymer of polyether and polyamide, the physical and chemical properties of the ether group also have a certain compatibility, which is due to the role of the ether group in Pebax. Therefore, it also has a good compatibility with TPU-Ether, and it is also possible to blend the two, and adding an appropriate amount of TPU in Pebax can also improve its toughness at low temperature and room temperature.
3, esters and blending of PEBAX front we have mentioned, PEBAX itself is the polyether block copolymer with polyamide, at the same time because of ether and Ester base to a variety of reasons such as the incompatibility of determines the base containing ether PEBAX with TPU - Ester, which contains the Ester base incompatible, leading to the two can't using the blending process, will lead to the apparent effect after blending is bad and physical phenomenon such as performance degradation.
4, TPU and PVC blend PVC and TPU-Ester blend than TPU-Ether blend effect is better. Modification of TPU with polyvinyl chloride (PVC) can reduce the cost of TPU, improve the processing property of TPU, improve the weatherability of materials, improve the flame retardant property of TPU, improve the heat resistance of TPU and other properties; Modified PVC with TPU can improve the wear resistance, impact resistance, oil and chemical resistance, low temperature performance and adhesion performance of the latter.
In addition to POM, ABS, PVC and PC, there are also polystyrene (PS), polyacrylate, polyester resin, SBS, EVA, PP, CEP, polyamide and other polymers that can be blended or copolymerized with TPU.