Polyurethane is short for polyurethane, which is made from polyisocyanates reacting with polyols and contains many repeated carbamate groups (-- NH -- CO -- O --) in the molecular chain. In the actual synthesis of polyurethane resin, in addition to carbamate groups, there are groups such as urea, biuret. Polyols are long-chain molecules with hydroxyl ends, which are called "soft chain segments", and polyisocyanates are called "hard chain segments".
In the polyurethane resin produced by soft and hard chain segment, carbamate is only a minority, so it may not be appropriate to call polyurethane. In a broad sense, polyurethane is the adpolymer of isocyanic acid. Different types of isocyanates react with polyhydroxyl compounds to produce polyurethanes of various structures, thereby obtaining polymer materials with different properties, such as plastics, rubber, coatings, fibers, adhesives, etc. Polyurethane rubber polyurethane rubber was first developed in Germany in 1940, and began to be put into industrial production after 1952. In China, it began to be developed and put into production from the mid-1960s.
Polyurethane rubber is a kind of special rubber, which is made by reaction of polyether or polyester with isocyanate. There are many kinds of polyurethane rubber due to different kinds of raw materials, reaction conditions and cross-linking methods. In terms of chemical structure, there are polyester type and polyether type, and in terms of processing method, there are three kinds of mixed type, pouring type and thermoplastic type. Synthetic polyurethane rubber, generally by linear polyester or polyether reaction with diisocyanate, made of low molecular weight prepolymer, chain extension reaction, the formation of polymer, and then add the appropriate crosslinking agent, heat curing, vulcanized rubber, this method is called prepolymer or two-step method. A one-step process can also be used -- linear polyesters or polyethers are directly mixed with diisocyanates, chain extenders, and crosslinking agents to allow the reaction to occur and produce polyurethane rubber.
Thermoplastic Polyurethane rubber (TPU)
Thermoplastic polyurethane rubber is A kind of (AB) N-block linear polymer, A is A high molecular weight polyester or polyether (molecular weight 1000 ~ 6000), called long chain, B is A short chain containing 2 ~ 12 straight carbon atom diol, the chemical binding between the AB chain segments is diisocyanate.
The relationship between the structure and physical properties of TPU
1. Chain structure The A-chain segment of TPU molecule makes the macromolecular chain easy to rotate, gives polyurethane rubber good elasticity, and reduces the softening point and second-order transition point of the polymer, as well as the hardness and mechanical strength. The B-chain segment will bind the rotation of the macromolecular chain, which will increase the softening point and secondary transition point of the polymer, increase the hardness and mechanical strength, and reduce the elasticity. TPU with different mechanical properties can be prepared by adjusting the molar ratio between A and B.
In addition to the first-order crosslinking, the second-order crosslinking formed by inter-molecular hydrogen bonds must also be considered in the cross-linking structure of TPU. The primary crosslinking bond of polyurethane is different from the vulcanization structure of hydroxyl rubber, and its carbamate group, diuretic acid group and carbamate group are arranged into rigid chain segments regularly and at intervals, so the rubber produced has regular network structure, so it has excellent wear resistance and other excellent properties. Secondly, because polyurethane rubber contains many groups with high cohesive energy, such as urea-group or carbamate group, the hydrogen bond formed between molecular chains has a high strength, and the secondary cross-linking health formed by hydrogen bond also has an important influence on the performance of polyurethane rubber. Secondary crosslinking makes polyurethane rubber have the characteristics of thermosetting elastomer on the one hand, and on the other hand, this kind of crosslinking does not really cross link up, which is a virtual crosslinking, the condition of which depends on the temperature. With the increase of temperature, this crosslinking gradually weakens and even disappears. The polymer has a certain fluidity and can be processed thermoplastic. As the temperature drops, the cross-links gradually return and form again. With the addition of a small amount of filler, the distance between molecules increases, and the ability to form hydrogen bonds between molecules weakens, resulting in a sharp decline in strength.
3, groups of stability study showed that polyurethane rubber group stability in order from high to low is: ester, ether, urea, carbamate, biuret, in the process of polyurethane rubber aging, the first is the biuret and urea base formic acid ester crosslinking bond rupture, followed by carbamate and urea bond rupture, namely main chain rupture. Properties Of polyurethane rubber The elastic modulus of TPU is between rubber and plastics. Its biggest characteristic is that it has both hardness and elasticity, which is not found in other rubber and plastics. TPU can be divided into polyester type and polyether type. According to the comparison of physical properties, polyester type is better for rubber with low hardness, while polyether type is better for rubber with high hardness. Polyester rubber has good oil resistance, heat resistance and adhesion to metal, while polyether rubber has good hydrolysis resistance, cold resistance and antibacterial property.
1. Environmental characteristics TPU generally has good temperature resistance. The temperature for continuous long-term use is 80 ~ 90℃, and it can reach about 120℃ in a short time. Polyurethane also has good low temperature resistance. Polyester polyurethane has a brittle temperature of -40℃, while polyether polyurethane has a temperature of -70 ~ -80℃, but it will harden at low temperature. TPU has good oil resistance, but water resistance varies according to its structure. The most serious TPU degradation was caused by the reversibility of ester formation reaction. When esters come into contact with water, the reformation of acids is the cause of the autocatalytic reaction that leads to molecular disintegration. Polyester polyurethanes disintegrate to a greater extent when exposed to air and humidity than when immersed completely in water. This is because the acid formed by being immersed in water is constantly washed away. Polyether polyurethanes, on the other hand, are 3 ~ 5 times more hydrolytic than polyester polyurethanes, because the ether groups do not react with water. There are two reasons for the deterioration of polyurethane performance caused by water intrusion: first, the intrusive water forms hydrogen bonds with the polar groups in polyurethane, which weakens the hydrogen bonds between polymer molecules. This process is reversible, and the physical properties are restored after drying. The second is that the water intruded causes the hydrolysis of polyurethane, which is irreversible. Polyurethane darkens under long time of sunlight, and its physical properties gradually decrease. Enzyme bacteria can also lead to the degradation of polyurethane, so the polyurethane rubber used in industrial production has been added with anti-aging agent, ultraviolet absorber, enzyme inhibitor, etc.
2. Mechanical properties: The tensile strength of polyurethane rubber is high, generally up to 28 ~ 42 mpa, and TPU is in the middle, about 35 Mpa. Elongation: generally up to 400 ~ 600, the maximum is 1000%. Elasticity: Polyurethane has high elasticity, but it also has high hysteresis loss, so it has high heat generation. It is easily damaged under load conditions of multiple bending and high speed rolling. Hardness: Polyurethane has a wider range of hardness than other rubbers, with a minimum of 10 Shao's hardness, and most of the products have a hardness of 45 ~ 95. When the hardness is higher than 70 degrees, the tensile strength and constant elongation strength are higher than that of natural rubber. When the hardness is between 80 and 90 degrees, the tensile strength, constant elongation strength and tear strength are quite high. Tear strength: The tear strength of polyurethane is relatively high. When the test temperature rises to 100 ~ 110℃, the tear strength is comparable to that of styrene butadiene rubber. Abrasion resistance: Polyurethane has excellent abrasion resistance, which is 9 times higher than natural rubber and 1 ~ 3 times higher than styrene butadiene rubber. TPU has the dual characteristics of plastic and rubber. It is precisely this unique physical and chemical characteristics that require special treatment in mold design and injection molding.
1. Flow passage design: since the main flow passage is the place with the greatest pressure, when the injection pressure is relieved, the condensate of the main flow passage will increase the resistance due to elastic expansion, resulting in the water mouth sticking to the front die. Therefore, in the mold design, the release slope of the main flow passage should be increased as much as possible. And the small end size of the main channel can not be less than the injection molding machine nozzle aperture, the increase of the large end size needs to increase the cooling time, extend the injection cycle, so the increase of the release slope is mainly achieved by shortening the length of the main channel. Generally, the diameter of the small end of the main channel is about 2.5-3.0mm, the diameter of the large end is below 6.0mm, and the length should not exceed 40mm. A cold well with the diameter equal to or slightly larger than the big end shall be set at the end of the main flow channel to collect gellan and lock the outlet. The diameter of the shunt duct shall depend on the structure of the product and the length of the runner. Generally speaking, it shall not be less than 4.0mm. Better cooling effect can be obtained by using circular shunt duct.
2, the design of the gate: due to the bad liquidity TPU, in order to avoid the colloid through gate in injection and molecular orientation leads to the transverse and longitudinal shrinkage of inconsistencies, the depth and width of gate size should be the gate size larger than other thermoplastic materials, while length size is smaller than normal, so by colloid. Too long gate will cause the spray of gel during filling, which will affect the appearance of the product. The needle point gate which can cause excessive shearing heat should be avoided as far as possible.
3, exhaust groove design: the mold exhaust must be sufficient to prevent the product from burning, especially in the filling direction of the rubber material has a sharp change and the final filling part of the product should pay special attention to the exhaust setting. The depth of the exhaust groove should be distinguished according to the model of TPU. Sometimes the depth of the exhaust groove is only 0.01mm, and it will also produce a flap in the exhaust groove, which has an important relationship with the special material properties of TPU.
4. Design of cooling system: the cooling effect of the mold is good. For other thermoplastic materials, as long as the surface of the product has enough strength of the frozen layer, the product at a high temperature can be carried out ejection. For TPU, when the temperature is high, inter-molecular hydrogen bond does not recover and the tensile strength of the product is low. Forced ejecting and demoulding will only lead to product deformation. Only when the product is at a low temperature and hydrogen bond recovery is sufficient, can TPU demoulding be achieved with sufficient strength, which requires a good cooling effect of the mold.
5. Determination of shrinkage rate: The shrinkage rate of TPU varies greatly with the TPU brand used, the thickness and structure of the product, and the temperature and pressure during injection molding, and the range is between 0.1% and 2.0%. In the mold design, not only to refer to the shrinkage of raw materials data, but also according to the structure of the products, the thickness of the estimated injection molding temperature, pressure to be used for appropriate modification. For products with thicker local glue position, the pressure required by injection molding is larger, and the shrinkage rate of molded products is smaller, so the shrinkage rate of TPU needs to be reduced. And for the glue position is more uniform, thick products, it is appropriate to increase the value of shrinkage.
Injection molding processing
1. The drying of raw materials can degrade TPU due to the invasion of water. When the moisture content of TPU exceeds 0.2%, not only the appearance of the product will be affected, but also the mechanical properties will be obviously deteriorated, resulting in poor elasticity and low strength of the injected product. Therefore, it should be dried at 80℃ ~ 110℃ for 2 ~ 3 hours before injection.
2, the cylinder cleaning injection molding machine cylinder cleaning to clean, very little other raw materials will lead to the reduction of the mechanical strength of the product. The cylinder cleaned with ABS, PMMA and PE should be cleaned again with TPU nozzle before injection molding, and the residual materials in the cylinder should be removed with TPU nozzle.
3. Control of processing Temperature The processing temperature of TPU has a crucial influence on the final size, appearance and deformation of products. The temperature depends on the TPU brand and the mold design. The general trend is to increase the processing temperature in order to achieve a small shrinkage. To obtain a large shrinkage need to reduce the processing temperature. Even within the normal processing temperature range of TPU, too long stay time of raw materials in the cylinder will lead to thermal degradation of TPU, and the residual materials in the cylinder should be shot out before injection. It is also very important to control the nozzle temperature, which should be about 5℃ higher than the front end of the cartridge under normal circumstances.
4. Control of injection molding speed and pressure Low injection molding speed and longer pressure holding time will enhance molecular orientation. Although it is possible to obtain a smaller product size, the product deformation is large and the difference between transverse and longitudinal shrinkage is large. Large pressure can also lead to the gel in the mold overcompression, when the product demoulding after the size of the cavity is larger than the phenomenon.
TPU material is sensitive to shear. When the shear heat caused by too high melt speed and back pressure is too large, TPU will be thermal degraded. Therefore, TPU melt adhesive generally adopts low or medium speed. If the injection cycle is long, the delayed glue function should be used to start the mold opening after the completion of the glue, so as to prevent the raw material from being degraded after staying in the barrel for too long.