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Several factors affecting viscosity (full version)
Several factors affecting viscosity
Viscosity is one of the most important basic concepts of polyethylene processability and a quantitative expression of fluidity. The factors affecting viscosity are melt temperature, pressure, shear rate and relative molecular weight. , the following are introduced respectively.
(1) temperature
From the previous analysis, we can see that the viscosity of polyethylene is a function of shear rate, but the viscosity of polyethylene is also affected by temperature. Therefore, it is of practical significance to study the effect of temperature on viscosity only when the shear rate is constant.
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Justice. Generally speaking, the viscosity of polyethylene melt is more sensitive than shear. The results show that the viscosity of polyethylene melt decreases exponentially with the increase of temperature. This is because, with the increase of temperature, the movement between molecules is accelerated, which reduces the entanglement between polyethylene molecular chains and increases the distance between molecules, resulting in the decrease of viscosity. It is easy to form, but the shrinkage of the product is large, which will also lead to decomposition, too low temperature, high melt viscosity, difficult flow, poor formability and high elasticity, which will also make the shape stability of the product poor.
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However, different polyethylene viscosities have different degrees of temperature. Polyformaldehyde is the least sensitive to temperature change, followed by polyethylene, polypropylene and polystyrene, and cellulose acetate is the most sensitive. Table 1 lists the temperature sensitivity of some commonly used polyethylene. For very sensitive polyethylene, temperature control is very important, otherwise the viscosity changes greatly, which makes the operation unstable and affects the product quality.
In practice, for the melt with good temperature sensitivity, we can consider increasing the molding temperature of polyethylene during the molding process to improve the fluidity of polyethylene.
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Dynamic characteristics, such as PMMA, PC, CA, PA. However, for polyethylene with poor sensitivity, the improvement of flow performance is not obvious by increasing temperature, so in general, increasing temperature can not improve flow performance. Such as POM, PE, PP and other nonpolar polyethylene, even if the temperature rises greatly, the viscosity decreases very little. In addition, raising the temperature must be limited by certain conditions, that is, the molding temperature must be within the allowable molding temperature range of polyethylene, otherwise polyethylene will degrade. The loss of molding equipment is large, and the working conditions deteriorate, which is not worth the loss. The magnitude of activation energy is used to represent matter.
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The relationship between viscosity and temperature is of quantitative significance. Table 2 shows the activation energy of some polyethylene at low shear rate.
(2) the influence of pressure
There is a tiny space between molecules and molecular chains in polyethylene melt, which is called free volume. So polyethylene is compressible. In the process of injection, the maximum external pressure of polyethylene can reach tens or even hundreds of MPa. Under this pressure, the distance between macromolecules decreases, the activity range of chain segments decreases, the distance between molecules decreases and the interaction between molecules decreases.
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With the increase of force, the dislocation between chains becomes more difficult, which shows that the overall viscosity increases.
However, under the same pressure, the viscosity of different polyethylenes increases differently. Polystyrene (PS) is the most sensitive to pressure, that is, the viscosity increases rapidly with the increase of pressure. Compared with low-density polyethylene, high-density polyethylene has less influence on viscosity, and polypropylene is affected by pressure equivalent to moderate polyethylene.
The fact that increasing pressure leads to increasing viscosity shows that poly can be improved simply by increasing pressure.
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The flow rate of ethylene melt is not suitable. Too high pressure not only can't obviously improve the filling performance of the fluid, but also the filling performance may sometimes decrease due to the increase of viscosity, which not only causes excessive power loss and excessive equipment wear, but also causes flash and increases the internal stress of the product. In addition, if the pressure is too high, there will be injection defects such as product deformation, resulting in excessive power consumption. But if the pressure is too low, it will cause a shortage of materials.
Combined with the influence of temperature on viscosity, it can be found that the increase of pressure affects the viscosity of polyethylene melt within the normal processing parameters of polyethylene.
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The effect of lowering the temperature on the viscosity of polyethylene is similar. For example, for many polyethylenes, when the pressure increases to 100 MPa, the change of viscosity is equivalent to a temperature decrease of 30 ~ 50℃.
(3) the influence of shear rate
With the increase of shear rate, the viscosity of polyethylene generally decreases. However, when the shear rate is low and high, the viscosity hardly changes with the change of shear rate.
Under the premise of constant temperature and pressure, the viscosity reduction degree of different polyethylene is different. In other words,
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Although the viscosity of most polyethylene melts decreases with the increase of shear rate, the sensitivity of different polyethylene to shear rate (shear stress) is different.
The sensitivity of viscosity of several commonly used polyethylenes to shear rate is shown in Table 4.
The enlightenment of this point is that increasing the shear rate will significantly reduce the viscosity of polyethylene and improve its fluidity within a certain range of shear rate. However, it is best to adjust the process within the range where the melt viscosity is insensitive to the shear rate, otherwise, due to the shear rate,
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The fluctuation will cause unstable processing and quality defects of polyethylene products.
(4) the influence of polyethylene structure
For polyethylene, the viscosity of polyethylene increases with the increase of relative average molecular weight at a given temperature. The greater the relative molecular mass, the stronger the intermolecular force and the higher the viscosity.
The smaller the relative molecular weight of polyethylene, the less the dependence of viscosity on shear rate; The greater the molecular weight, the greater the dependence of viscosity on shear rate. Wide molecular weight distribution resin
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And has low melt viscosity and excellent processability. Because that low molecular weight chain part is beneficial to improve the fluidity of the resin melt.
(5) Influence of low molecular weight additives
Low molecules can reduce the force between macromolecular chains and play the role of "lubrication", thus reducing the viscosity of the melt and the viscosity fluidization temperature. Such as adding plasticizer and solvent, so that the resin can be easily filled and molded.
Common methods to improve the fluidity of polyethylene
Increase screw speed
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Choose an amorphous brand.
raise the temperature
The dissolution of HSHMPE fiber includes two processes: pre-swelling and dissolution. The optimum swelling temperature is related to the nature of the solvent, and it takes some time to completely swell. The solution viscosity increases with the increase of polyethylene mass fraction or the decrease of solution temperature, and the solution viscous flow activation energy increases with the increase of polyethylene mass fraction. Mineral oil as solvent has better solubility to HSHMPE fiber than kerosene. The extraction mechanism of polyethylene jelly fiber is the double diffusion of fiber solvent and extractant.
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At present, the main solvents used in HSHMPE fiber manufacturing are paraffin oil and decalin, which constitute wet spinning and dry spinning process routes respectively. The above two solvents have their own advantages, but the technology of using paraffin oil as solvent in China is mature, and the equipment and operation are very reliable. At the same time, the international patent of the process route with paraffin oil as solvent has expired, while the process route with decalin as solvent still involves international patent infringement. The process route with paraffin oil as solvent is also divided into different process implementations and different equipment configurations, and the final equipment investment gap is also higher than this.
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