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When Making Black Masterbatch, How To Solve The Problem Of Uneven Dispersion Of Carbon Black?

1. Case study

When a manufacturer makes black masterbatch, carbon black 40%, using EVA wax as a dispersant, some are very brittle when pulling strips, and some are even unbreakable, the judgment is that the carbon black is dispersed unevenly. Using twin screw extruder, after high mixer pre-mixing and added to the hopper, the temperature is 160℃, 165℃, 170℃, 180℃, and the head is 175℃.

Problems:

  1. The material in the hopper does not go down, often broken strips. At first thought that there was no material, when feeding the material, found that there were still many in the hopper, need to push manually. This problem does not occur when carbon black is 20% or 30%.
  2. When there is material, the strip is broken continuously, the speed of the discharge port is obviously uneven, and the middle two ports are fast (a total of 4 ports).
  3. There is no detection method on site, it is directly used for production, and do not understand how to easily judge the quality of masterbatch.

The solutions are as follows:

  1. Increase the amount of EVA wax
  2. Reduce the temperature by 10℃
  3. Reduce or increase the rotational speed

2. Bridge formation

The hopper is a component that stores plastic raw materials, and some add heating and blowing devices to the hopper to make a drying hopper. The shape of the hopper is generally the lower conical and the upper cylindrical. The conical cone slope has different optimal values for different particle sizes, different particle shapes, friction coefficients between particles and different bonding coefficients, otherwise either the storage capacity of the hopper is wasted, or there is a «bridge» or «funnel pipe» phenomenon of poor feeding or not unloading at all.

Definition of bridging phenomenon:

The diameter of the hopper near the feed port of the extruder gradually decreases and changes too quickly, causing the material to form a compacted solid, which will prevent the raw material from entering the extruder.

Causes of bridging phenomenon:

Since the plastic particles form an open bridge at the small mouth of the cone to support the material above it, it is more likely to occur for recycled material with large particles and irregular shape.

«Funnel into tubes» because the particles flowing down are not enough to pull their adjacent particles together, which tends to occur when the plastic particle size is small.

Especially in the case of the packing itself is very light or the proportion of filler is large, to eliminate the bridging phenomenon or minimize the possibility of bridging phenomenon, it is necessary to separate the mixed air before the powder enters the twin screw extruder, reduce the vertical height between the feeding port and the screw as much as possible, and the powder should enter the bottom of the screw, not directly added to the top of the screw. In addition, the screw structure should ensure that the air moves along the screw groove and is discharged in the exhaust section, while the gas should not flow back to the feed port for discharge.

Manufacturers generally regardless of low density and high filling, as long as the product allows, add 1-2% white oil to mix at high speed for 2-3 minutes (but pay attention to the time and method of mixing, basically powder into agglomerates is better). This approach actually removes the air in the filler, and the filler is «compacted» in some form to remove the undesirable air. Once it becomes an agglomerate, the bulk density is also increased, and the distance between the screw groove and the feeding and the direction of the feeding are not a big problem.

3. Solutions to the bridging phenomenon

The general solution is to install a vibrating device on the hopper or reduce the cone slope. If the heat on the barrel is transferred to the hopper and the hopper temperature is too high, the surface of the plastic pellets softens or bonds into blocks, and it is easier to form bridges or blockages.

Generally, there will be difficulty in unloading when calcium powder is added, which is mainly due to two reasons, one is that calcium powder itself is difficult to flow, and the other is that the external lubricant added for processing will also hinder the flow of dry powder.

The solution is to pay attention to the mixing of light calcium and heavy calcium, generally heavy calcium is better than light calcium; At the same time, the method of adding a vibrator to the lower hopper can also be adopted; From the formulation point of view, it is necessary to add more internal lubricant. Of these methods, adding a vibrator is the most effective.

Since most of the conveying of the extruder is transported by friction dragging, the temperature setting of the feeding area is generally controlled between 120-160 °C. If the temperature in this area is too low, the resin cannot adhere to the inner wall of the barrel, but only idles with the rotation of the screw, resulting in a shortage of material in the molten zone; If the temperature in this area is too high, the material will melt and plasticize in advance, forming a ring-shaped molten film on the inner wall of the barrel, which plays a lubricating role, which will also cause material shortage.

Another function of the feeding section is that the compression ratio of the solid bed is large enough to ensure that the material is sufficiently dense to achieve the state required for melting. Under normal circumstances, particles or broken sheet materials are easy to deform on the surface after compression and fit together with the inner wall of the barrel, so they are also relatively easy to compress; Due to the large sliding properties of powder materials, compression melting will lag behind particles or sheet materials in time and position. At the same time, due to the large void area of the powder material, the material density is relatively small, and a large amount of air will remain in the material, isolating the solid bed. This will further slow down the melting process and exhaust work later in the increase phase.

Due to the large internal friction and relatively high viscosity between thermoplastic elastomer particles, it is easy to produce material shortages during processing, which will cause fluctuations in production yield and degradation of materials. Generally, it is solved by improving the aspect ratio of the feeding section, forced cooling and vibration feeding.

There are also some granulation raw materials that have been processed once or more elastomers, and there are more or less filler oils, fillers, resins and other components in these raw materials. Although the internal friction and viscosity are not as exaggerated as the raw materials, due to the presence of oil that makes it difficult to convey the material feeding, it is generally not recommended to crush such materials too small. At the same time, it is necessary to increase the friction coefficient of the screw edge of the feeding section screw.

After the pigment is mixed with the resin, melt and granulate the pre-dispersion («rubbing powder») of the pigment powder needs to be paid attention to. Whether a pigment is dispersed in post-processing depends largely on the dispersant and pre-dispersion in the process of configuring this pigment. If a toner is not predispersed, then in the process of melt mixing, several base toners configured with this toner will cause different colors («decors») in various areas of the material due to not mixing evenly. Therefore, if the blending manufacturer finds that the pigment you use has a decor in the production process, then it means that the toner supply steel frame may not be too professional.

Masterbatches are pre-dispersed and mixed with the carrier, and theoretically there is no such thing as poor dispersion. However, it should be noted that the carrier of the same color will have different performance in processing.

If the bulk MFR of the masterbatch is too different from the MFR of the blended resin (regardless of size), it is entirely possible to cause color mixing. At the same time, if the natural color flexibility of the masterbatch is too good to be crushed into fine masterbatch debris as soon as possible during the mixing process, then even if the MFR matching between the masterbatch and the resin is good, it may lead to different shades of color in the final product («color mixing»).

Summary of experience:

  1. Dry the material, if it is moist, it will melt at the mouth first, but it is not completely melted, so it will be squeezed there and not go down, and the sun-dried material is easier to enter than the wet material.
  2. Reduce the temperature of the material opening, or even drop it very low, such as 100 °C, so that the material is not melted before entering, but is directly squeezed in, relying on the power of the screw, not the heat. The disadvantage of this method is that it is made for a long time, and the damage to the screw is relatively large.
  3. Use the broken screw to do it, and replace the diameter of the front section slightly smaller than the back, the gap is larger, and it is easy to feed.
  4. Enlarge the inlet, if you are doing this for a long time, you can cut the inlet larger. Of course, large cutting does not mean that you can feed more, you can install mixing equipment, you can refer to the production equipment of PS foam powder.
  5. Replace the disc of the motor, change the diameter of the rotating disc to large, so that the screw speed is accelerated, and the disguised phase stirs it in when the material has not yet dissolved. The disadvantage is that the screw is easy to stir, you need to be more cautious, and after the speed becomes faster, the temperature should be adjusted up in the heating zone behind, otherwise the carrying force of the screw is too large.

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