Black polyester fibers are usually colored by adding carbon black masterbatches to the PET compound. Only a few carbon blacks are suitable for this application, they are extremely pure and have very fine particle size, and generally require the following conditions: Contain at least 96 % Of C elements, and few O, H, N and S elements.
Co-rotating twin-screw extruders are especially suitable for the compounding of carbon blacks. The relevant parameters include internal / external diameter ratio Do / Di and specific torque Md / a³. Coperion's latest ZSKMc18 (see Figure 1) Black masterbatch, which has a Do / Di = 1.55, a specific torque of 18 Nm / cm³, a large free volume and a high dispersing power.
Figure 1. ZSK Mc18 provides good processing conditions for processing masterbatches (Photo: Coperion)
The process section contains several process zones: from the conveying zone, the plasticizing zone, the dispersing zone and the homogenizing zone to the exhaust zone and the build pressure zone.The intermeshing twin screws ensure that there is no dead zone in the entire process because they all have a Good continuous conveyor effect and optimized self-cleaning threaded elements allow gentle processing of materials for good mixing performance.
There are two processes for compounding PET-based carbon black masterbatches: premix and step addition.
In the premix method, all the components, including resin, additives and carbon black are mixed together and the twin screw is added together (see Figure 2). To avoid delamination and carbon black agglomerates in the feed zone, PET is used. It must be a powder. Due to the fact that PET is available on the market only with particles and flakes, it has to be pulverized with time and energy and then fed into the extruder after mixing, after melting, carbon black dispersion, The gas is devolatilized, extruded through a bar, and cut into particles.
Figure 2. Premixed process carbon black masterbatch
In contrast, the stepwise feed process (see Figure 3) provides a significant economic advantage because it lacks the time and cost of premixing the PET powder and carbon black without milling, In this process, any form of PET, it can be fed into the twin screw and then melted.The carbon black will be added separately to the melt downstream.At this point, the toner will be gently pushed into the melt and dispersed.If both at the same time The extruder will result in compression of the carbon black, which, given the strong radial forces in this zone, will form a reunion of carbon black which is not yet wet enough and will not be completely destroyed at the back.
Figure 3. Step-by-step addition process carbon black masterbatch
Coperion has done a series of carbon black masterbatch experiments using the ZSKMc 18. The results show that both of these methods can be used to achieve very satisfactory results, including dispersion and IV viscosity reduction, of which the step-by-step method has a huge Advantages - Automation of the entire process.