The twin-screw compounding extruder is a single piece of equipment operated in different units. Treating each unit operation as part of the toolbox can help determine what can be added or improved to handle the best quality masterbatch. #color #Best Practice

The co-rotating intermeshing twin-screw extruder (TSE) is especially suitable for processing color pigment masterbatches. It has the advantages of excellent dispersibility under high output, high pigment loading level, long service life of screw and barrel parts, and simple machine operation.

In other words, the processing of color masterbatch has its own series of challenges, such as the processing of raw materials, the dispersion of pigments, and the cleanliness between color changes. This article will explore the tools that can be used to produce high-quality masterbatches, as well as techniques to overcome processing challenges while maximizing yield and product quality.

TSE can be regarded as a "toolbox" that provides operators or process engineers with a variety of highly flexible parts that can be modified and/or moved to suit specific composite tasks. The production of high-quality pigment dispersions is widely proven on TSE, but not every pigment behaves the same and is processed in the same way. The way the pigments are handled and how they are handled in the TSE is critical to the performance of the final product.

First, even though it is considered a piece of equipment, TSE includes the dimensions of individual unit operations. To understand how to achieve the best-performing final product, it is important to carefully study each unit operation of TSE.

This article will focus on the split feed method of masterbatch mixing rather than the premix method, although both methods are acceptable methods for producing color pigment masterbatches.  

First of all, it is important to understand the raw materials of the raw materials. The choice of feeder—for example, twin screw feeder, single screw feeder, etc.—depends on the behavior of the raw material. Once the feeder is appropriately selected for each raw material, the feed intake of that material into the TSE may be the next challenge. Depending on the raw material, the material may fluidize and aerate before actually entering the machine.

The feeder can provide various twin-screw and single-screw configurations, with a variety of screw designs to suit various materials.

Placing the feeder above the TSE plays a very important role in minimizing the fluidization and aeration of the raw materials. A large amount of air is entrained with the powder (not so many particles), which requires a place to escape. If there is no filter sock or central suction system connected to the feed system, the gas escaping from the extruder will hinder the feed of raw materials.

According to experience, usually no more than 15% of the pigment powder should enter the main feed of TSE.

Regarding raw material aeration, the positioning of the height of the feeder above the extruder is also critical. Once the raw material enters the feeder, the powder has a chance to compact a little, thereby increasing its bulk density. When the feeder starts feeding, depending on the height above the extruder, the raw material will aerate, which in turn will reduce the bulk density and cause feeding problems.

According to experience, usually no more than 15% of the color pigment powder should enter the main feed of TSE to prevent feed intake problems. However, if there is already a granular masterbatch form of color pigments, no feeding problem will be encountered.

For color pigment powder loadings above 20% and up to 80%, it is strongly recommended to feed separately between the main feeder and the side feeder, where multiple side feeders can be placed downstream to add color pigments.  

The molten polymer in the TSE is affected by many factors, including external heating, internal friction, and/or heat transfer from the polymer melt. TSE is provided with a different number of zones, including heating and cooling of internal processes, in which energy is either absorbed into the system or removed from the system. Of course, as the size of the extruder increases, the influence of external heating will also decrease due to the decrease in the surface area to volume ratio.

Another effect is the internal friction that occurs between the polymer particles or powder particles inside the TSE or between the material and the screw element. For example, friction and shear stress appear in the narrow gap of the kneading block. This is completely affected by the type of screw profile installed as well as the throughput and extruder screw speed. The heat transfer in the polymer melt helps to encapsulate any unmelted polymer to help form a complete melt.

Segmented screw elements provide multiple functions and can be assembled on the shaft to suit the unit operation.

The design of the kneading and mixing zone is completely determined by the type of polymer being processed, but can include various kneading elements. These may include three-threaded and two-threaded 45° (conveying) kneading elements, two-threaded 90° (neutral) kneading elements, and/or left-handed (holding) kneading or conveying elements.

The combination of these elements provides the necessary mechanical energy and residence time to effectively melt and homogenize the polymer before it moves further downstream. The left-hand (holding) element pressurizes the melt in the TSE. It is important to design this part to prevent excessive shearing and degradation of the polymer. It is also important to note that once the material leaves the melting part, it should be completely melted in order to provide proper wetting for the color pigments introduced downstream.

To understand how to achieve the best performance of the final product, it is very important to carefully study each unit operation of TSE.

With the advancement of TSE technology, there has been room for screw element design to deviate from the classic Erdmenger profile of screw elements that have been widely used in the industry. The geometry of these so-called "involute screw elements" has changed while maintaining the self-wiping profile in the co-rotating TSE. The use of involute elements in the melting section and downstream mixing section reduces the filler incorporation restrictions observed in standard designs. Involute elements enable users to add fillers or colored pigments to the main feed of TSE while reducing the risk of insufficient melting.

Regarding downstream mixing, the use of involute elements can also reduce the limitation of insufficient filler incorporation. In general, the involute element is a tool that users can use to overcome the limitations encountered when mixing large amounts of fillers or pigments.  

As mentioned earlier, depending on the loading of color pigments in the formula, it is possible to add some color pigments to the main feed of TSE, up to about 15%, depending on its particle shape and size. However, when considering the high load of color pigments (up to 80%), it is possible to split the feed between the main feeder of the TSE and the downstream side feeder (sometimes multiple side feeders). "Material. The ratio of divided feeding depends on the type of material and the load; but in the end, finding the most effective method is a balance.

One or more side feeders can be added downstream to "split" the operation of the material feeding unit.

The melting part is a very important tool for producing high-quality color pigment dispersions on TSE. The same goes for the downstream mixing section. Once the colored pigments are effectively introduced into the TSE and the particles are properly wetted by the polymer melt, the design of the downstream mixing section is critical to dispersing and dispersing the colored pigments. As mentioned earlier, when air enters the downstream side feeder, it does entrain the pigment powder, some of which is discharged backwards through the upstream vent on the side feeder barrel itself.

However, most of the entrained air actually enters the downstream mixing section. If the design is not correct, it may cause the entrained air to move backward and return to the side feeder, obstructing the flow of material. The purpose of the downstream mixing part is to wet the pigment with the melt and transport the entrained air downstream, where atmospheric vents can be set to allow the air to escape.  

Now the product has completed most of the upstream unit operations, and the material is very close to leaving the TSE and entering the downstream equipment. One area that is sometimes overlooked is the devolatilization (or degassing) part of the machine. This is an important unit operation because when the material is vacuum applied, bubbles and/or volatiles are stripped from the polymer melt, providing non-porous and dense particles.

If there are pores in the final particle, it will affect the strength and durability of the final molded part. TSE provides a fairly simple method of applying vacuum. It consists of an open (or vented) barrel with a vacuum dome connected to a separation tank (or condenser) and a vacuum pump.

In some cases, no matter what level of vacuum is used or what extruder screw speed is used, the material will still escape from the vacuum exhaust port, blocking the exhaust port and preventing full devolatilization. In this case, a device sometimes called a side degasser (ZS-EG) will be used to help keep the material inside the TSE while allowing the gas to escape for proper devolatilization.  

Devolatilization is a downstream unit operation in which a vacuum is drawn to remove bubbles and/or volatiles from the polymer to provide dense and non-porous pellets.

It usually doesn't take much time to consider the design of the booster part of the TSE. The polymer flow in this section is a combination of the drag flow towards the screw tip due to the rotation of the screw and the upstream pressure flow away from the TSE discharge. Please pay attention to the position of the vacuum port, because excessive pressure buildup can cause devolatilization problems.

Generally, the granulation of masterbatch will involve strand or underwater granulation. A high loading of color pigments close to 80% may cause the strands to be very brittle. In this case, underwater pelletization will be a better choice, but it all depends on the type of pigment used.  

About the author: Justyn Pyz is a process engineer in Coperion's compounding and extrusion department. He is responsible for the twin-screw extrusion test at the Sewell Process Laboratory in New Jersey, as well as process development, new machine sales and technical support for existing customers. Pyz holds a master's degree in chemical engineering from the New Jersey Institute of Technology (NJIT). Contact: 856-589-0500; Justyn.pyz@coperion.com; coperion.com.

Improved transparency and cost competitiveness, coupled with its inherent heat resistance, are revitalizing OPP's prospects in hot-filled barrier containers. But hot-filled PET containers are raising the bar with higher productivity and "panelless" bottle designs.

New additives can improve transparency, stiffness, HDT and processing speed, which brings new packaging opportunities for PP.

First evaluate the current situation and define future needs. Develop a five-year plan for materials and required capabilities. Short-term solutions usually prove to be more expensive and less satisfactory after a period of time. Most importantly, understand your options.

© 2021 Gardner Business Media, Inc. Privacy Policy [Login]