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Keywords of this article:  extruder 
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Figure 1: With its ratio of screw of Do/Di = 1.55 and specific torque of 18 Nm/cm³, the ZSK Mc<sup>18</sup> provides very good processing conditions for producing masterbatches (figures: Coperion)
Figure 1: With its ratio of screw of Do/Di = 1.55 and specific torque of 18 Nm/cm³, the ZSK Mc18 provides very good processing conditions for producing masterbatches (figures: Coperion)
Photo: 123

A carbon black masterbatch used for coloring polyester fibers has to fulfill high standards, especially if the intended results are filmy spinning fibers for very soft and comfy textiles.

Polyester fibers are utilized in many different applications, such as in automobile construction, clothing branch or home textiles. Depending on the application, a number of different types of fibers are available. The differences lie in their fineness, cutting lengths, as well as in the various physical parameters of textiles, such as fiber strength, elongation and hot-air shrinkage.

Polyester fibers made from PET (polyethylene terephthelate) are made by melt spinning, whereby the PET melt compounded in a Coperion extruder is subsequently squeezed through spinnerets. The properties desired are obtained by adding functional additives and pigments, generally in the form of masterbatches. These masterbatches are subject to strict requirements, particularly when spinning polyester fibers that are just a few µm thick to produce especially soft and comfy textiles.

High Demands on Carbon Black Masterbatches

Black polyester fibers are generally colored with carbon black dosed as masterbatches when PET is compounded. Only a few types of carbon black that are extremely pure and have very fine particles are suitable for this. An industrially manufactured, hi-tech carbon black is used which is physically and chemically specified and produced under controlled processing conditions: industrial carbon black consists of more than 96% carbon and contains only small amounts of oxygen, hydrogen, nitrogen and sulfur.

Twin-Screw with High Dispersing Power

Co-rotating twin-screw extruders are especially suitable for compounding carbon black color batches. Parameters relevant to the compounding process include the screw diameter ratio Do/Di and the specific torque Md/a³, calculated as the ratio of the torque transmitted to the screw shafts, and the centerline distance to the third power. The ZSK Mc18 twin screw extruder (Fig 1) from Coperion with a screw diameter ratio of Do/Di = 1.55 has proven capable of compounding PET base carbon black masterbatches. Its specific torque of 18 Nm/cm³ is tuned to the free screw volume and has high dispersing power.

The process section is modularly set up from individual barrels. Therein several processing zones are set with task-specifically configured screw elements: from conveying, plasticizing, dispersing and homogenizing to degassing and pressure build-up on the compounded melt ahead of the die. The tightly intermeshing twin screw prevents stagnant areas along the entire length of the process section. The result is continuously high conveying power together with optimum self-wiping of the screws. The extruder treats the product gently and exhibits very good mixing properties.

Two Different Compounding Processes

The two processes used for compounding PET based carbon black batch are the Premix method and the Split-feed method.

In the Premix process, all components – matrix polymer, any additives and carbon black – are pre-mixed and fed together into the main intake of the twin screw extruder (Fig. 2). In order to avoid de-mixing effects and the formation of carbon black agglomerates in the intake zone, the PET, too, must be present in powder form. Since PET is available on the market only as pellets or as flakes, it has to be pulverized by courtesy of a time-consuming and cost-intensive process. The mixture is made in a container mixer, or also a quick-mixer, and then volumetrically fed to the twin screw extruder, to get the polymer molten, the carbon black dispersed, and the melt homogenized and degassed. Subsequently, the masterbatch melt is usually shaped into strands and cut up as pellets by a strand pelletizer.

By contrast, the Split-feed process (Fig. 3) offers economic advantages, since it eliminates time and cost intensive grinding, as well as the production of the pre-mixture from PET powder and carbon black. Here PET in any form – be it pellets, flakes or powder – is fed into the first barrel of the twin screw extruder and melted down. Downstream the carbon black is added separately directly into the polymer melt using a twin screw side feeder (ZS-B). There the pigment is gently worked in and dispersed. If PET pellets and carbon black powder were dosed together in the extruder intake, this would tend to compact the carbon black. Given the especially high radial forces occurring here, the still insufficiently wetted carbon black can form agglomerates that subsequently could not be completely broken up.

Tests conducted by Coperion using the ZSK Mc18 for the production of carbon black colorant batch have shown that good results can be achieved using both the Premix, as well as the Split-feed method. This applies both to dispersion quality and to the IV value of the masterbatch. The Split-feed method requires somewhat more expertise to operate the equipment; however, it offers the great advantage that it makes automation simpler to implement.

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