ISSN:
0032-3888
Keywords:
Chemistry
;
Chemical Engineering
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
Notes:
In poly(butylene terephthalate) based compositions, thermal degradation of the polymer matrix during processing has to be minimized to achieve quality products. Experience has shown a temperature-residence time relationship, indicating that for mixing systems with high product temperatures, the residence time of the product has to be reduced to avoid excessive thermal degradation. The power density of the mixing system is related to the specific energy input and the residence time of the product. From rheology, the power density is also known for a simple shear deformation which can thus be used to characterize the shear intensity of a particular mixing process. Comparing two different mixing systems by their power density provides us with a qualitative better understanding why higher shear is permitted with lower residence time. From theoretical considerations it was found that for a temperature-sensitive product, like PBT, the power density in the mixing operation can be further raised, taking into account that with shorter residence time a higher product temperature is permitted. Production-scale test work was carried out on a 200 mm screw-diameter continuous kneader to investigate the effect of running conditions and screw design on the thermal degradation of two different types of PBT. Results have shown that for the high-viscosity PBT a linear relationship exists between product temperature and the viscosity retained upon compounding. In a two-stage kneader only minor thermal degradation is encountered in the melting section, but conditions become critical in the mixing stage due to the viscosity increase after introducing the glass fibers to the melt. A new feature in compounding thermodegradable products is the addition of unmolten polymer into the mixing stage of the kneader since this leads to a reduction in the product end temperature and, consequently, thermal degradation of the matrix material. The limited results obtained so far indicate that an optimum exists as to the amount of pellets added. At a 15 percent level the product temperature was reduced by 20°C as compared to 10°C at 20 percent. An energy balance carried out on the continuous kneader indicates that because of the low melt viscosity approximately 30 percent of the energy put into the product in the melting section of the kneader originates from external heating. A rough comparison shows that the power density of a continuous kneader is twice that of a single-screw extruder designed for compounding PBT, but, can be tolerated because of the considerably lower residence time in the former mixing system.
Additional Material:
9 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pen.760180208
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