Melt viscosity also temperature, effect

As their name implies, plastics are easily deformable and once molten they may be heated not only externally, but also internally, by dissipating friction or dielectric heat. Once the pyrolysis temperature is attained, the melt viscosity starts decreasing rapidly and rising pyrolysis vapour bubbles agitate the mix. Many properties of plastics can be derived using methods used and developed by Van Krevelen [13]. The pyrolyzing plastic s rheology is poorly-documented, however, since the evolution of molecular size and strucmre with time and the effects of extraneous matter are difficult to predict. 

In spite of this evidence of possibly poor mixing, some noticeable changes were found in the rheology and thermal stability of polystyrene. In comparing two polystyrene blends, one with 5% of polymer 3 and the other with 0.1 % (control), the melt viscosity of the 5% blend was about 50% at 180 °C and 80% at 120 °C, respectively. The effect was more drastic at higher temperatures and higher shear rates. The addition of polymer 3 also seemed to improve the thermal stability of polystyrene. When the molten blend polymer was kept at 180 °C for some time, the melt viscosity of the 0.1% blend increased, whereas that of the 5% blend remained constant. 

Typically, thermoplastics, because of their high melt viscosity, require higher temperatures and injection molding pressures, or expensive mold fabrication costs to design multiple gate systems and therefore are not cost effective materials for large parts. In addition to cost, the use of multiple gates may also lead to a less structurally sound part as a result of knit lines. 

Published information on urethane polymerization detail largely concerns thermoset urethane elastomers systems.4 13 In particular, the work of Macosko et. al. is called to attention. The present paper supplements this literature with information on the full course of linear thermoplastic urethane elastomer formation conducted under random melt polymerization conditions in a slightly modified Brabender PlastiCorder reactor. Viscosity and temperature variations with time were continuously recorded and the effects of several relevant polymerization variables - temperature, composition, catalyst, stabilizer, macroglycol acid number, shortstop - are reported. The paper will also be seen to provide additional insight into the nature and behavior of thermoplastic polyurethane elastomers. 

Anszver by Author The degree of fiber bundle wetting can be controlled by the melt viscosity of the fluorocarbon polymer, which is a function of laminating temperature. Laminating pressure has also been demonstrated to have the expected effect on fiber bundle wetting. 

Rheology is the science of deformation and flow of matter. Essentially, all thermoplastic resins (and many thermosetting resins) are required to undergo flow in the molten state during the course of product manufacture. Important fabrication processes such as injection, extrusion, and calendering all involve the flow of molten polymers. In plastics fabrication, it is important to understand the effect, on melt viscosity, of such factors as temperature, pressure, rate of shear, molecular weight, and structure. It is also equally important to have reliable means of measuring viscous properties of materials. 

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