1. The function and principle of mixing
The mixing of U-PVC formulations usually goes through two stages of hot mixing and cold mixing. The mixed ingredients are called dry mixtures. The so-called hot mixing refers to the mixing process of heating the mixture to the softening temperature or above, while the cold mixing means that the hot mixture is stirred at a low speed by the blades in the cold mixing pot, and the heat emitted is passed through the cold mixing pot clamp. The cooling water in the jacket is taken away until the raw material is lowered below the set temperature. The dry mix that has been hot mixed and cold mixed can be used after standing for a period of time to "cook". There are four aspects to the effect of hot mixing and cold mixing: the first is to make the components of the raw materials evenly distributed in space to achieve a certain degree of uniformity [2], which is obviously the basis for the full effect of each component; Mixing, so that the PVC particles have a process from the original state to crushing and micro-melting to re-aggregation, so that the dry mixture can achieve a certain pre-plasticizing effect. From the description method of the mixing state, a certain degree of dispersion can be obtained [ 3]; The third is to eliminate the components with too small particle size after the dry mix goes through the mixing process, so that the overall particle size distribution of the dry mix is relatively large and concentrated, and the apparent density and flow of the dry mix are improved. This is not only conducive to the stable transportation of dry mixes, but also can improve the output; the fourth is to eliminate the water and low volatile components in the raw materials as much as possible through hot mixing, and eliminate the influence of these components on product quality. The purpose of standing the dry mix is to eliminate the static electricity generated during the mixing process and further improve the apparent density and fluidity of the dry mix.
Taking common profiled materials as an example, PVC and CPE in the formula composition are both large particle components (average particle size Dav is 150-200μm), and the composite stabilizer will be broken into small particles after being added in the form of particles or flakes. Under the mixing conditions, it will not be as small as calcium carbonate and titanium dioxide (below 10 μm). Although the particle size of the processing aid ACR is also small, the content in the formula is very small, and the melting point is lower than the hot mixing temperature. It is melted in the mixing, so the small particle size components to be considered in the formula are calcium carbonate and titanium dioxide. Figure 1 is the particle size distribution diagram of each raw material of a certain profile formula (D10 in the figure refers to the part with 10% (volume ratio) in 100 parts of powder, the particle size value below which the particle size is lower than And so on. Dav refers to the average particle size). It is obvious from Figure 1 that the particle size of the formula material can be simply divided into two orders of magnitude, and the small particle size component is expected to be eliminated by hot mixing.
In Figure 1, D10 refers to the particle size value below which the cumulative 10% of the components are lower, D25 refers to the particle size value below which the cumulative 25% of the components are below, and so on. Dav is the average particle size. The heat source in the hot mixing process is usually the friction heat generated by the high-speed shearing of the stirring blade. The specific change process of the raw material state in the hot mixing process is analyzed: under the action of the high-speed mixer, the large PVC particles accounting for an absolute proportion are broken by the high-speed shearing action, and the specific surface area of the PVC particles increases to form a strong electrostatic adsorption; another On the other hand, the low melting point lubricants, processing aids, etc. in the formula will gradually melt during the temperature rise, adhere to the PVC particles, and play the role of adhesives between PVC and inorganic small particles; in addition, PVC After the initial crushing, the particles gradually expand as the temperature rises. When the temperature exceeds its glass transition temperature (Tg) of 87°C, the PVC particles gradually soften from the outside to the inside, and reach a certain degree of pre-plasticization, becoming soft and even soft. Micro-melted PVC particles are also conducive to the adhesion of small-sized inorganic particles. The result of all these factors is that as the mixing process progresses, the small-sized components of calcium carbonate and titanium dioxide particles in the formula will gradually Adhesive and inlaid around the PVC particles, so that the particle size of the PVC particles changes from broken to smaller and then gradually increases, and the state of the large particles of CPE in the formula changes similar to that of PVC. The blend of particle diameter eventually becomes a powder with larger particle size and uniformity. As shown in Figure 2, the left picture is the result of mixing various raw materials according to the formula and then simply stirring and mixing with a small spoon. The right picture is the dry mixture after normal heat and cold mixing. Both have the same magnification ratio.
The particle size distribution of the mixture at different mixing temperatures was analyzed by a laser particle size analyzer, and the results are shown in Figure 3.
Picture a in Figure 3 is the case where all the raw materials have not been mixed. It can be seen that there are many particles with a particle size around 10 μm. This is a small particle size component composed of ACR, calcium carbonate and titanium dioxide in the formula. With the improvement of the effect, the small particles gradually agglomerated around the large PVC particles, and the content of the small particle size components became less and less. By the time of Figure d, the proportion of particles near 10 μm has almost disappeared; on the other hand, in Figure a, The peak value of the particle size around 150μm is very high, and the frequency of occurrence is 21% during detection. This is the case that the particle size of the original PVC particles is not affected. However, in Figure b, the peak value is significantly reduced, and the left measurement is about 100μm. The proportion of particle size particles increased significantly, indicating that the PVC particles were partially broken by high-speed shearing force. With the strengthening of the mixing effect, the small particles gradually agglomerated around the PVC particles, and the particle size of the PVC particles gradually increased, as shown in the figure. The situation shown in c and d. The process of changing the particle size of the powder reacted in Figure 3 also illustrates the above-mentioned mixing principle [4].
2. The influence of the mixing process on the mixing effect
1 Material feeding sequence and its influence on mixing effect
According to the role of various raw materials and the synergistic effect between each other, the addition of various raw materials in the formula should be added in stages. A reasonable feeding sequence is to first add PVC under low-speed stirring; after high-speed stirring is started, add stabilizer and soap stabilizer when the temperature rises to 60 °C (for composite granular stabilizer, it is suitable to add PVC at the same time); when the temperature is about 80 °C , add processing aids, internal lubricants, pigments and impact modifiers (if the lubricant is compounded in the stabilizer, add it together with the stabilizer); when the temperature rises to about 100 ℃, add external lubricants such as waxes ; 110 ℃, adding calcium carbonate and titanium dioxide and other inorganic substances. First, adding PVC and stabilizer can give full play to the stabilization effect of stabilizer on PVC; early addition of internal lubrication is beneficial to the initial gelation of raw materials; then processing aids, impact modifiers, etc. are added to facilitate its mixing with PVC ; The external slip agent is added later to avoid the effect of the external slip agent on the friction and dispersion between the raw materials; the inorganic particles such as calcium carbonate and titanium dioxide are added last, on the one hand to avoid the absorption of the lubricant by the inorganic filler, on the other hand It also reduces the wear of these harder particles on the mixing equipment. Such an order of addition will make the whole mixing more scientific. However, such a feeding method has complicated process, higher requirements on equipment and control, and significantly prolongs the mixing cycle, which reduces the mixing efficiency. Therefore, in actual production, many manufacturers implement the process of mixing all raw materials at the same time. According to experience, feeding and mixing at the same time, as long as there is a suitable mixing process, the dry mixing can also well meet the production needs.
2 The effect of hot mixing and cold mixing temperature on the mixing effect
As mentioned above, the hot mixing temperature needs to be higher than the glass transition temperature of PVC. The specific setting value should be determined according to whether the purpose of hot mixing is achieved. If the hot mixing temperature is low, firstly, there are still a lot of small particle size components in the dry mixture (as shown in the hot mixing temperature and particle size distribution diagram in Figure 3). The phenomenon of raw material layering is prone to occur during the conveying process, which seriously affects the quality of the product; secondly, the pre-plasticization degree of the dry mixture is not enough, which is not conducive to obtaining a better plasticization level in the later production, and the material properties of the product are affected; the third is The water and other volatiles in the raw materials are not completely removed, which increases the precipitation in the later processing, and the products are prone to bubbles or even degradation, which seriously affects the appearance and material properties of the product. If the hot mixing temperature is set too high, the degree of pre-plasticization of the dry mixture will be too high, and the stabilizer will be consumed too much during mixing, which will affect the process control in the later production and the material and appearance properties of the product, and even affect the weather resistance of the product. sex. The setting of the hot mixing temperature should not only be determined according to the state of the dry mixture itself, but also need to be properly adjusted according to the formula and the plasticizing capacity of the extruder in the later stage. Usually the proportion of small particle size components in the formula is high, or the plasticizing ability of the extruder in the later stage is not enough, the hot mixing temperature should be appropriately higher. vice versa. Usually, the hot mixing temperature of the u-PVC profile formulation is set at around 120°C. The setting of cold mixing temperature is usually set at 40~45℃ according to experience. If the cold mixing temperature is too high, the dry mix will be stored without sufficient cooling, which will cause the raw materials in the middle part to be affected by excess heat, consume too much stabilizer, and even degrade the dry mix such as pink. The cold mixing temperature is too low, which is not conducive to improving the mixing efficiency.
3 Influence of feeding amount on mixing amount
When mixing materials, the feeding amount of each pot is usually 55-75% of the net space of the hot mixing pot. If the mixing amount is too small, the friction heat will be insufficient during hot mixing, and the temperature of dry mixing will rise slowly, which not only prolongs the hot mixing time, but also may consume too much stabilizer; It is not easy to exhaust the water vapor and so on, which will ultimately affect the quality of the dry mix.
4. Influence of dry mixing standing time on mixing effect
The hot-mixed and cold-mixed dry mix needs to stand for a period of time, generally for at least 12 hours in the storage tank. If the dry mixing time is too short, the profile is prone to wheezing (especially the parallel twin-screw reaction) during extrusion, and the phenomenon of "bridging" is likely to occur on the equipment with self-weight feeding. Tests show that after standing for 24 hours, the apparent density of the mixture increases significantly (as shown in Table 1) [5]. In addition, if a large-capacity storage tank can be used to place the dry mix, a homogenization tube is added to the storage tank, and the large batch of dry mix will be homogenized again when the material is reclaimed by weight, which is conducive to processing and Stable product quality.
5 The effect of cooling rate on dry blends In addition to the influence of the above conditions on the quality of the blends, experience has shown that the cooling rate also has a greater impact on the apparent density of dry blends (as shown in Table 2). The faster the cooling rate, the greater the apparent density of the product, which is more conducive to high-speed extrusion. The temperature of the cooling water for cold mixing is usually controlled at 13°C to 15°C.
3. Evaluation of mixing effect
As mentioned earlier, blending serves four purposes. Apparent density, fluidity, and volatile content can be directly detected. In addition, the mixture can also be judged by dry blend whiteness, rheological plasticizing properties, sieve residue, thermal stability time and particle size distribution. material quality. These eight indicators can be quantified, but the standard is not fixed. Different manufacturers can determine the test items and determine the standard indicators of the best dry mix suitable for them according to their actual conditions to analyze and control the quality and stability of the mixing process. The following describes how to use these indicators for dry mix analysis.
1 Apparent Density
As mentioned earlier, one of the goals of hot mixing is to achieve a higher apparent density of the dry mix. The specific test method can be carried out with reference to the PVC apparent density test standard GB/T 20022-2005. At the current level of technology, the filler content of high-quality profiled material formulations does not exceed 10 parts. Taking lead formulations as an example, the apparent density of unheated formulations is about 0.55g/ml, which can be increased to 0.62 after hot mixing. g/ml. There is no linear relationship between dry mix apparent density and hot mix temperature. When the hot mixing temperature reaches a certain level (small particle size components basically disappear), and then increase the hot mixing temperature, the apparent density of the dry mix does not change significantly. It is not enough to observe the apparent density only, it must be analyzed in combination with indicators such as whiteness, sieve residue and fluidity. In addition, the apparent density of formulas with a high proportion of fillers will be significantly higher than that of normal formulas.
2 Whiteness
The whiteness of the dry mix can be detected with a whiteness meter. It can reflect whether the measurement of ingredients is accurate or not, and more importantly, it can reflect the initial gelation degree of raw materials. In the case of accurate feeding, if the whiteness is lower than the normal value, it means that the hot mixing is too high, the gelation of the raw materials is too strong, and the heating time is too long. It is necessary to check whether the control of the hot mixing temperature is accurate, such as thermocouples. Whether the poor contact causes the actual hot mixing temperature to be too high, etc. Conversely, if the whiteness is higher, the degree of hot mixing may not be enough.
3 Thermal stabilization time
Thermal stability time is divided into static and dynamic thermal stability time. The static thermal stability time is usually determined by the Congo red test (GB/T 2917.1 Determination of the release of hydrogen chloride and any other acidic products from blends and products based on vinyl chloride homopolymers and copolymers at high temperature Congo red method). The dynamic thermal stability time can be analyzed by two methods: two-roll mixing tableting and torque rheological test. Under the same test conditions, the deeper the discoloration of the two-roll mixing tablet, the worse the dynamic thermal stability, and vice versa. The degradation time of the dry blend can be read directly on the rheogram by means of the torque rheology time. Dynamic thermal stability can more accurately determine the thermal stability of dry blends. The thermal stabilization time is abnormal. On the one hand, it may be due to the inaccurate addition of raw materials, resulting in insufficient or excessive actual proportion of thermal stabilizer; it may also be a problem with the temperature control system of the mixing material. When the filling amount of the raw materials in the hot mixing pot is not enough, or the stirring blade is seriously worn, the temperature rises more slowly. In the case of controlling the hot mixing time by temperature, the result is that the material undergoes a long time of hot mixing, and the waste is consumed. more heat stabilizer and ultimately lead to shorter heat stabilization time of dry mix.
4 Liquidity
The fluidity of dry mixes is commonly known as dry fluidity. The method is to perform a flow time test on a standard volume of dry mix using a standard funnel. The length of the dry flow time reflects the number of small particle size components in the dry mix. A good dry mix has a larger and uniform particle size and is easy to flow, reflecting a better mixing effect. In Fig. 3, the particle size distributions of the mixtures under the four mixing processes are very different. The dry-flow properties of these four mixtures were tested under the same conditions, and it was found that their flow times were also very different. The results are shown in Table 3. The fluidity of the dry mix is also affected by the humidity of the raw material. Therefore, the humidity factor needs to be excluded first. In addition, it is impossible to completely eliminate the small particle size components in the dry mix only through the improvement of the hot mixing process for the formulation of high loading content. Due to the presence of a large number of small particle size components, the fluidity of dry blends will be seriously affected, and even lead to stratification of raw materials, affecting processing and product quality. The fluidity of the dry mix can also be analyzed by measuring the angle of repose of the dry mix. However, this method is not easy to quantify, and the operation is relatively more difficult.
5 Volatile content
The detection of volatile matter can be carried out according to GB/T 2914 Determination method of volatile matter (including water) of plastic vinyl chloride resin and copolymer resin. The volatile content exceeds the standard, reflecting the poor removal of volatiles during hot mixing. On the one hand, it is necessary to control the volatile content of the raw materials and shorten the time for the raw materials to be stacked in the open air.
6 Rheological properties
The rheological test uses a torque rheometer, mainly to investigate the processing performance of the dry blend. In the case of accurate feeding, when the rheological curve of the dry blend to be checked is too different from the standard curve, it means that the pregelatinization degree of the dry blend has changed significantly, and there is a problem in the blending process. Equipment inspection and process correction are required. . The results of the rheological test are too different, and when reflected in the extrusion production, there will be a large process change, and the extrusion process must be greatly adjusted to be able to produce normally, or even the dry mix cannot be produced normally and the dry blend is discarded. As shown in Figure 4, the plasticization curves of sample 1 and the standard sample have good repeatability, and it can be considered that the formula measurement and mixing process of sample 1 are good. The plasticization of sample 2 is slower than the standard, and there may be problems in the process of adding or mixing raw materials. When using torque rheology test to control and analyze dry blends in normal production, two indicators, plasticizing time and plasticizing torque, are usually controlled.
7 Sifter residue
The sieve residue detection uses a vibrating sieve with two mesh sizes, one large and one small, and the effect of the mixing is reflected by analyzing the ratio of the components with too small particle size and too large particle size in the dry mixture. The detection method can refer to the detection method of PVC sieve residue (GB/T 2916 or Appendix B). If the content of small particle size components in the dry mixture is too much, it indicates that the mixing degree is not enough; if the content of large particles exceeds the standard, the actual hot mixing may be overdone, or there may be poor ventilation in the mixing pot, scaling on the pot wall, and mixing. Insufficient discharge of the material in the pot, etc., the over-heated mixture that has been bonded into hard lumps falls into the pot and cannot be sufficiently broken.
8 Particle Size Distribution
If the sieve residue is judged by two scales to judge the degree of mixing, then the particle size distribution is judged by a complete analysis of the particle size of the dry mix. As mentioned earlier, the particle size distribution of a well-mixed dry blend exhibits a high and narrow particle size distribution peak (Figure 3, panel d). Bimodal distribution, or short and wide distribution peaks, all indicate that the particles of the dry mixture are not uniform and the mixing effect is not good. According to the test, the average particle size Dav of the dry mix with good mixing effect is 160-190 μm.
Through the monitoring of the above eight indicators, not only the mixing process can be controlled, but also the accuracy of the feeding amount. If ash detection and elemental analysis are added, the feeding and mixing process can even be completely analyzed and controlled.
4. Conclusion
1. The production of u-PVC products needs to go through the process of hot mixing and cold mixing of raw materials. Through mixing, the dry mix can achieve good uniform dispersion, appropriate apparent density and fluidity, and moderate pre-plasticization, and fully exclude water and low volatile content in the raw materials. 2. The preparation effect of the dry mix of u-PVC products is mainly affected by the heating and cold mixing temperature, the feeding amount of the mixing pot, the cooling speed of the cold mixing pot and the standing time of the dry mixing;
3. The methods for evaluating the mixing effect include apparent density, whiteness, thermal stability time, dry flow, volatile content, sieve residue content, torque rheology test and particle size distribution. Judge the accuracy of feeding.
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