Plunger speeds

The next step was the electronically compensated pump. All pumps speed the motor as resistance increases to maintain a constant solvent slow. These pumps also add a major plunger speed-up during refill and repressurization. With this modification, a pump with a single pump head and a pulse dampener could give 90% of the performance of a two-headed pump for 50% of the cost. An overall dramatic price reduction for the dual-pump HPLC system resulted. 

The density of the polymer at 25°C was obtained by using a molded disk, 0.125 in thick and 2 in in diameter. The melt density at processing temperature was obtained with an Instron Capillary Rheometer with plugged exit. The isothermal compaction at meltprocessing temperature was conducted at a plunger speed of 0.05 in/min with attainable pressures up to 25,000 psi. 

The temperature was specified as 180°C. Although IW C is the most widely used temperature, the Pebax concentrates have such a marked tendency to stick, that a lower temperature seemed more appropriate. On the other hand, an even lower temperature causes the viscosity, especially of the PS concentrates, to rise so far that high concentrations can no longer be measured at higher plunger speeds. The chosen temperature therefore represents a compromise. 

The plunger speeds were selected such that the corrected shear rates were subsequently as close as is possible to the shear rates used for interpolation (see paragraph headed Final analysis later in this section). 

In the sequence of measurements, all speeds were run in succession starting with the slowest plunger speed. As soon as the force was registered as constant, the speed was increased. In other words it was necessary to... 

Decrease injection pressure Slow injection speed Decrease booster time Decrease plunger speed... 

CONSTANT PLUNGER SPEED CIRCULAR ORIHCE CAPILLARY RHEOMETER... 

FIgur 3.3 Schematic diagram of a constant plunger speed circuiar orifice capiliary rheometer. 

This rheometer is also similar to the one described in section 3.2.1 except for two differences. Firstly, the capillary used is of very short length and secondly, the polymer is extruded by the use of dead weights (i.e. constant pressure) rather than constant plunger speed. This instrument, popularly known as the Melt Flow Indexer, is very popular in the thermoplastics industry due to its ease of operation and low cost, which more than compensates for ite lack of sophistication. The parameter measured through the melt flow indexer contains mixed information of the elastic and viscous effects of ttie pol)nner. Further, no end loss corrections have been developed for this capillary equipment nor can the melt flow index be easily related to the Weissenberg-Rabinowitsch shear rate expression. 

Constant Plunger Speed Circular Orifice Capiiiary Rheometer... 

Constant Plunger Speed Slit Orifice Capillary Rheometer... 

The capillary rheometer (Fig. 3.20) is commonly used to obtain r] at high shear rates. Basically the device consists of a barrel for melting the polymer and a plunger that pushes the melt through the capillary. The data obtained from this device consist of the pressure required to push the melt through the capillary and the volumetric flow rate (plunger speed and cross-sectional area). Two corrections are applied to this data. 

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