Kneading Importance

Recent advances in the application of ultrafine talc for enhanced mechanical and thermal properties have been studied [12]. A particularly important use is of finely divided filler in TPO as a flame-retardant additive. In a representative formulation, 37 parts of E-plastomer, Ml 2.0, density 0.92, 60 parts of amorphous EPR, and 4 parts of fine carbon black were dry blended, kneaded at 180°C, pelletized, and press molded into test pieces, which showed oxygen index 32 versus 31 in the absence of a filler. The oxygen index is a measure of flame retardancy. 

In polymer processing, compaction is an important and necessary step in order to reduce the interparticle, unoccupied spaces and thus eliminate air. It is essential for melting in both single-screw extruders as well as for twin-rotor processors, as we shall see in Chapters 5 and 10. In twin-rotor devices, such as Co-TSEs, for example, the large and repeated deformation of compacted particulates by the kneading elements, which induces large plastic deformation of particulates, is the dominant melting mechanism. 

It is important to note that similar beneficial mixing flows and solid polymer particulate charge deformations to those discussed earlier also occur in the multilobal variants of the counterrotating, intermeshing TSEs, as well as in CMs. We used the co-rotating, fully intermeshing twin screw kneading element pairs, because the relentless expansion/ contraction cycles can best be demonstrated with them. 

If a twin-screw extruder is stopped and opened, several zones can be clearly distinguished [9]. Depending on whether the extruder is fed with a solid or a liquid material, two different situations occur. In the case of a solid feed (a polymer or solid monomer), the channels near the feed hopper are more or less filled with solids. This material melts, resulting in a zone where the channels are only partly filled. At the end of the screw, close to the die, a zone exists where the channels are completely filled with polymer. If the extruder is fed with a liquid monomer, the first part does not necessarily need to be partly empty but, as will be explained later, for reasons of stability it is advisable to create a zone where the channels are not fully filled. In particular, the fully filled zone is very important for correct functioning of the extruder. In this zone the pressure is built up, the mixing and kneading mainly... 

The extruder can be fed with a paste or a dry powder. In the case of a dry powder the extruder will have a kneading section where a liquid is add to the powder and the powder is kneaded to a paste. In the other case the paste is made in a separate kneader before feeding it into the extruder. The rheological properties of the paste are very important, so the formulation of the paste is the most... 

In the dry process it is said that the order of mixing the ingredients is very important to obtain good sparks. At first potassium nitrate and pine charcoal are mixed intimately, then pine soot is added to the mixture, kneading it together, and at last sulphur is added to this. 

A similar situation exists in the case of the term kneading. There are screw machines whose primarily task are the mixing or conveying of paste-like compositions. In such cases the kneading itself is of secondary importance, although it cannot be ignored. 

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