What is Polyurethane?
The so-called polyurethane is the abbreviation of polyurethane, which is formed by the reaction of polyisocyanate and polyol, and contains many repeated urethane groups (-NH-CO-O-) in the molecular chain. In the actual synthetic polyurethane resin, in addition to the urethane group, there are also groups such as urea and biuret. Polyols are long-chain molecules with hydroxyl groups at the end, which are called "soft segments", and polyisocyanates are called "hard segments".
In the polyurethane resin generated by the soft and hard segments, urethane is only a minority, so it is not necessarily appropriate to call it polyurethane. In a broad sense, polyurethane is an addition polymer of isocyanate.
Different types of isocyanates react with polyhydroxy compounds to form polyurethanes of various structures, thereby obtaining polymer materials with different properties, such as plastics, rubbers, coatings, fibers, adhesives, etc. Polyurethane rubber
Polyurethane rubber was first successfully developed in Germany in 1940, and it was put into industrial production after 1952, while my country was developed and put into production in the mid-1960s. Polyurethane rubber belongs to a kind of special rubber, which is prepared by the reaction of polyether or polyester with isocyanate. There are many varieties due to different types 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 types: mixing type, casting type and thermoplastic type.
Synthetic polyurethane rubber is generally made by reacting linear polyester or polyether with diisocyanate to make a low molecular weight prepolymer. After chain extension reaction, a high molecular polymer is formed, and then an appropriate cross-linking agent is added to heat it. Cured to become vulcanized rubber, this method is called prepolymerization or two-step method.
It is also possible to use a one-step method—the linear polyester or polyether is directly mixed with diisocyanate, chain extender, and crosslinking agent, so that the reaction occurs to generate polyurethane rubber.
Thermoplastic Polyurethane Rubber (TPU)
Thermoplastic polyurethane rubber is a (AB) n-type block linear polymer, A represents high molecular weight polyester or polyether (molecular weight 1000-6000), called long chain, B represents 2-12 linear carbons The atomic diol is a short chain, and the chemical bonding between the AB segments is diisocyanate.
The relationship between the structure and physical properties of TPU
1. Segment structure
The A segment in the TPU molecule makes the macromolecular chain easy to rotate, giving the polyurethane rubber good elasticity, reducing the softening point and secondary transition point of the polymer, and reducing the hardness and mechanical strength. The B segment will bind the rotation of the macromolecular chain, so that the softening point and the secondary transition point of the polymer are increased, the hardness and mechanical strength are increased, and the elasticity is decreased. By adjusting the molar ratio between A and B, TPUs with different mechanical properties can be prepared.
2. Cross-linked structure
In addition to primary cross-linking, the cross-linking structure of TPU must also consider secondary cross-linking formed by intermolecular hydrogen bonds. The primary cross-linking bond of polyurethane is different from the vulcanization structure of hydroxy rubber, and its urethane group, biuret, allophanate group and other groups are regularly and spaced into rigid segments, so the obtained The rubber has a regular network structure, so it has excellent wear resistance and other excellent properties.
Secondly, because polyurethane rubber contains many groups such as urea groups or urethane groups with large cohesive energy, the hydrogen bonds formed between molecular chains have high strength, and the secondary cross-linking formed by hydrogen bonds Health also has an important influence on the properties of polyurethane rubber. The secondary cross-linking makes the polyurethane rubber have the characteristics of thermosetting elastomer on the one hand, and on the other hand, the cross-linking is not really cross-linking, it is a virtual cross-linking, and the cross-linking state depends on the temperature.
As the temperature increases, this crosslinking gradually weakens and disappears, and the polymer has a certain fluidity and can be thermoplastically processed. When the temperature is lowered, this crosslink is gradually restored and formed again. The addition of a small amount of filler increases the distance between molecules, the ability to form hydrogen bonds between molecules is weakened, and the strength will drop sharply.
3. The stability of the group
The research shows that the order of stability of each group in polyurethane rubber from high to low is: ester, ether, urea, urethane, biuret. In the aging process of polyurethane rubber, the first is biuret and urea group The formate crosslinks are cleaved, followed by the urethane and urea linkages, ie the main chain is cleaved.
Properties of Polyurethane Rubber
The elastic modulus of TPU is between rubber and plastic. Its biggest feature is that it has both hardness and elasticity, which is not found in other rubbers and plastics.
TPU is divided into two types: polyester type and polyether type. Compared with physical properties, polyester type has better performance for low-hardness rubber, while polyether type is better for high-hardness rubber. Polyester rubber has better oil resistance, heat resistance and adhesion to metal, while polyether type is better for hydrolysis resistance, cold resistance and antibacterial properties.
1. Environmental characteristics
TPU generally has good temperature resistance, the temperature for continuous long-term use is 80 to 90 °C, and it can reach about 120 °C in a short time. The low temperature resistance of polyurethane is also good. The brittleness temperature of polyester polyurethane is -40 ° C, while the polyether polyurethane is -70 ~ -80 ° C, but it will become hard at low temperature.
The oil resistance of TPU is relatively good, but the water resistance varies depending on the structure. The most serious degradation of TPU is caused by the reversibility of the ester formation reaction. When the ester is contacted with water, the reformation of the acid is responsible for the autocatalytic reaction leading to the disintegration of the molecule. Polyester urethanes disintegrate more when exposed to moisture in air than when fully immersed in water. This is because when immersed in water, the acid formed is continuously washed away.
The hydrolysis resistance of polyether polyurethane is 3 to 5 times that of polyester polyurethane, because the ether group will not react with water.
There are two reasons why the intrusion of water leads to the decline of the performance of polyurethane: one is that the intruded water forms hydrogen bonds with polar groups in the polyurethane, which weakens the hydrogen bonds between polymer molecules. This process is reversible. After the physical properties are restored.
The second is that the invading water hydrolyzes the polyurethane, which is irreversible.
Polyurethane will discolor and darken under prolonged sunlight exposure, and its physical properties will gradually decrease. Enzyme bacteria can also lead to the degradation of polyurethane, so antioxidants, ultraviolet absorbers, anti-enzyme agents, etc. are added to the polyurethane rubber used in industrial production.
2. Mechanical properties
Tensile strength: The tensile strength of polyurethane rubber is relatively high, generally reaching 28 to 42 MPa, and TPU is in the middle, about 35 MPa.
Elongation: generally up to 400 to 600, the maximum is 1000%.
Elasticity: The elasticity of polyurethane is relatively high, but its hysteresis loss is also relatively large, so the heat generation is high. It is easily damaged under the load conditions of multiple bending and high-speed rolling.
Hardness: The hardness range of polyurethane is wider than that of other rubbers, the lowest is Shore hardness 10, and most products have a hardness of 45 to 95. When the hardness is higher than 70 degrees, the tensile strength and fixed elongation strength are higher than those of natural rubber. When the hardness is 80 to 90 degrees, the tensile strength, fixed 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°C, the tear strength is equivalent to that of styrene-butadiene rubber.
Wear resistance: The wear resistance of polyurethane is very good, 9 times higher than that of natural rubber, and 1 to 3 times higher than that of styrene-butadiene rubber
Processing requirements
TPU has the dual characteristics of plastic and rubber. It is this unique physical and chemical characteristics that requires us to be specially treated in mold design and injection molding.
Mold Design:
1. The design of the runner:
Because the sprue is the place with the highest pressure, when the injection pressure is released, the condensate in the sprue will increase the resistance due to elastic expansion, which will cause the nozzle to stick to the front mold. Therefore, the demolding slope of the sprue should be increased as much as possible when designing the mold. . The size of the small end of the sprue cannot be smaller than the diameter of the nozzle of the injection molding machine. The increase of the size of the large end requires additional cooling time and prolongs the injection cycle. Therefore, the increase of the demolding slope is mainly realized by shortening the length of the sprue.
Under normal circumstances, the diameter of the small end of the main channel is about 2.5 to 3.0 mm, the diameter of the large end is less than 6.0 mm, and the length should not exceed 40 mm. At the end of the main channel, a cold well with the same or slightly larger diameter as the big end should be set to collect cold glue and buckle the water outlet.
The diameter of the runner should depend on the structure of the product and the length of the runner. Generally speaking, it should not be less than 4.0mm. The shunt channel adopts a circular shape to obtain better cooling effect.
2. Gate design:
Due to the poor fluidity of TPU, the depth and width of the gate should be larger than those of other thermoplastic materials in order to avoid the inconsistency between the lateral and longitudinal shrinkage caused by the jetting and molecular orientation of the colloid passing through the gate, while the length dimension It is smaller than ordinary ones to facilitate the passage of colloids. A gate that is too long will cause the colloid to be ejected during filling, which will affect the appearance of the product. Pin gates that can cause excessive shearing and heat generation of the material should be avoided as much as possible.
3. Design of exhaust groove:
The exhaust of the mold must be sufficient to prevent the product from scorching, especially when the filling direction of the rubber material changes sharply and the part where the product is finally filled, pay special attention to the setting of the exhaust. The depth of the exhaust groove should be distinguished according to the type of TPU. Sometimes the depth of the exhaust groove is only 0.01mm, and a drape will be generated at 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 better. For other thermoplastic materials, as long as the frozen layer on the surface of the product has sufficient strength during injection molding, the product can be ejected and demolded at a higher temperature. For TPU, when the temperature is high, the hydrogen bonds between molecules are not restored, and the tensile strength of the product is low. Forcible ejection and demolding will only lead to deformation of the product. The key is fully recovered, and the TPU can be demolded only when the TPU has sufficient strength, which requires the cooling effect of the mold to be better.
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 its range is between 0.1% and 2.0%. When designing the mold, not only should refer to the data of the shrinkage rate of the raw material, but also according to the structure and thickness of the product to estimate the injection temperature and pressure to be used in injection molding and make appropriate corrections. For products with thicker local adhesive positions, the pressure required for injection molding is larger, and the shrinkage rate of the molded product is smaller, so it is necessary to reduce the shrinkage rate of TPU. For products with a relatively uniform glue position and a thick product, the value of the shrinkage rate should be appropriately increased.
Injection processing
1. Drying of raw materials Because the intrusion of moisture can degrade TPU
When the moisture content of TPU exceeds 0.2%, not only the appearance of the product is affected, but also the mechanical properties are obviously deteriorated, and the injection molded product has poor elasticity and low strength. Therefore, it should be dried at a temperature of 80°C to 110°C for 2 to 3 hours before injection molding.
2. Cleaning of the barrel
The barrel of the injection molding machine should be cleaned, and the mixing of very few other raw materials will reduce the mechanical strength of the product. The barrels cleaned with ABS, PMMA and PE should be cleaned again with TPU nozzle material before injection molding, and the residual material in the barrel should be removed with TPU nozzle material.
3. Control of processing temperature
The processing temperature of TPU has a crucial impact on the final size, appearance and deformation of the product. The temperature depends on the grade of TPU used and the specific conditions of the mold design. The general trend is that in order to obtain a small shrinkage rate, the processing temperature needs to be increased; to obtain a large shrinkage rate, the processing temperature needs to be lowered. Even within the normal processing temperature range of TPU, if the raw material stays in the barrel for too long, it will lead to thermal degradation of TPU, and the residual material in the barrel should be emptied before injection molding. The control of the nozzle temperature is also very important. Under normal circumstances, it should be about 5°C higher than the temperature of the front end of the barrel.
4. Control of injection speed and pressure
Lower injection speed and longer dwell time will enhance molecular orientation, and although smaller product size may be obtained, product deformation will be larger, and the difference between transverse and longitudinal shrinkage will be large. Large holding pressure will also cause the colloid to be over-compressed in the mold, and the size of the product after demolding is larger than the size of the mold cavity.
5. Control of melt speed and back pressure
TPU material is more sensitive to shearing. When the shearing heat generated by high melting speed and back pressure is too high, it will lead to thermal degradation of TPU. Therefore, low or medium speed is generally used for the melting of TPU. If the injection molding cycle is long, the delayed melting function should be used, and the mold opening will start after the melting is completed, so as to prevent the raw materials from staying in the barrel for too long and degrading.
