Pellet boundary reformation

PELLET BOUNDARY REFORMATION IN REHEATED POLYSTYRENE MOLDINGS... 

Two narrow molecular weight distribution pelletized polystyrene samples and one of somewhat broader distribution were observed for pellet boundary reformation following molding in the range of 100-200°C and subsequent reheat at 130-150 C. Results show that samples molded below a certain temperature (molecular weight dependent) demonstrate definite signs of pellet boundary reformation upon reheat while those molded above this temperature do not. In most cases, this cutoff temperature is consistent with the Tn transition temperature as indicated by DSC. 

Pellet boundary reformation was tested by placing the samples in a preheated brass box (used for even heat distribution) at 130-150°C. The samples were checked every five minutes for signs of distortion (pellet boundary formation). Heating was discontinued after two consecutive checks yielded no change, typically after 25-30 minutes. The samples were then compared and photographed as a function of molding temperature. 

Fig. 2 shows the pellet boundary phenomena of narrow MWD PS-118. At the lower molding temperatures, such as shown in Fig. 2a, gross distortions and whole pellets can be seen. At one point (Fig. 2b), an abrupt change in sample distortion is observed for the narrow temperature range between 155.7 and 157.1°C. All samples molded below 157.UC showed some, if not extensive, pellet boundary reformation on reheat while those molded above 155.7 C did not. 

Fig. 2. Narrow MWD PS-118 samples (center section of insens) following molding at the temperatures shown (°C) and reheated at 150 C. (a) Lower temperature moldings showing gross pellet boundary reformation, (b) Medium temperature moldings encompassing the transition" temperature, (c) Higher temperature moldings showing no distortion.

Several years ago, industry discovered that both minimum and maximum temperature and pressure limits exist for optimum injection molding of polystyrene. Samples injected below these minimums would not fill the molds. Those injected above the maximum limits would stick to the mold. Boyer ascribed the lower temperature limit to the liquid-liquid (T/i) transition in polystyrene. In the present experiments, the molding temperature at which the pellet boundaries no longer reform on reheat seem to fall in the neighborhood of this liquid-liquid transition. 

Photographs of the reheated moldings of PS-100 are shown in Figure 5. Note that in the same molding temperature range that pellet boundaries disappeared in PS-118 (Fig. 5a vs. Fig. 2b), PS-100 continues to show distinctive pellet reformation. At higher temperatures (Fig. 5b) pellet boundaries are still evident. Indeed, the final disappearance does not occur until 168.0-175.5 C (Fig. 5c). This constimtes a very real difference in behavior from the narrow molecular weight distribution samples shown previously and is plotted as point C in Fig. 4. The reasons for this... 

The final model hence includes one ordinary second-order differential equation (3.32) with an integral boundary condition at the surface (3.41) for each of the reactions. The numerical solution of the catalyst effectiveness factor can be carried out using the orthogonal collocation method by Villadsen and Michelsen [512]. The steam reforming reaction takes place mainly in the outer shell of the catalyst particle, since large particles are used to limit pressure drop. In this case it is advantageous to divide the catalyst pellet into two sections, an inner section and an outer section, divided by a spline point and with an appropriate coupling between the two sections. The spline collocation method has been used by [525] and [181]. A description of the method, spline collocation, can be found in [512]. 

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