Polymer melt performance

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

Another characteristic of this solution is its proneness to crystallization and polymerization. When parts of the exhaust system are constantly welted by Adblue on the same spot, undesired urea crystals or polymers may form if the exhaust line temperature is lower than 300°C. This phenomenon will result in uncontrolled ammonia production when the crystals or polymers melt or sublimate after being heated at significantly higher temperatures (T > 350°C). This may result in ammonia release. Furthermore, the crystals or polymers can also have an impact on the SCR catalyst cells by reducing the catalyst surface and thus reducing the catalyst performances. 

This paper reviews NSE measurements on polymer melts, networks and solutions, published since the first successful NSE study on polymers [16] was performed in 1978. The experimental observations are discussed in the framework of related microscopic models, scaling predictions or other theoretical approaches. 

Stevenson [99] conducted the first polycondensation experiments with thin polymer melt films of 0.07-5 mm thickness. The experiments were performed on metal surfaces at temperatures between 265 and 285 °C under vacuum. He varied the kind of metal and observed that the behaviour of the polycondensation rate with decreasing film thickness depended on the metal being used. He concluded that the reaction rate increased only on metals soluble in the polymerizing melts... 

Compression rates typically vary between 0.0025 and 0.0055 for pellet feedstocks. If the compression rate is too low, then the compaction rate of the solid feedstock may not be high enough to force the entrained gas out through the hopper. If the compression rate is too high, poor melting performance and solid bed breakup can occur. For example, LDPE resins can be melted very easily using a screw with a compression rate of 0.0055, while LLDPE resins perform best with a compression rate near 0.0030 [3, 4]. If an LLDPE is extruded using a screw with a compression rate of 0.0055, then solid bed breakup and solid polymer particles in the extrudate are likely to occur. 

Klein, 1., The Melting Factor in Extruder Performance, SPEL, 28, 47 (1972) Altinkaynak, A., Three-Dimensional Finite Element Simulation of Polymer Melting and Flow in a Single-Screw Extruder Optimization of Screw Channel Geometry, Ph. D. Thesis, Michigan Technological University, Houghton, MI (2010)... 

Elbirli, B., Lindt, J.T., Gottgetreu, S. R., and Baba, S.M., Mathematical Modeling of Melting of Polymers in a Single-Screw Extruder, Polym. Eng. ScL, 24, 988 (1984) Lindt, J.T. and Elbirli, B., Effect of the Cross-Channel Flow on the Melting Performance of a Single-Screw Extruder, Polym. Eng. ScL, 25, 412 (1985)... 

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