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In-Process Compound Characterization
Assessing the quality of filled polymer compounds is of considerable importance to ensure acceptable end performance from these materials. Of particular interest is the consistency of filler loading, the uniformity of filler distribution, and the extent of dispersion determined by the presence of agglomerates. While it can be relatively straightforward to determine these measures of quality by batch sampling followed by off-line analysis (some common procedures are listed in Table 1), ideally this should be undertaken continuously during, or immediately following, compound preparation. Although this is very challenging to achieve, a number of approaches can be considered to characterize the mixing effectiveness of compounding machinery and in-line assessment of the compound produced.
These can be based on measurements of pressure and power consumption which are readily monitored during extrusion, and in some instances batch compounding, and can be indirectly related to the state of mixture quality. To this end, the specific energy consumption may be expressed as the ratio of the required power input to the effective material throughput and is expressed in kWh.kg-1. For a particular machine design and operating conditions, it is closely related to the melt viscosity of the polymer and hence the amount of filler present. Since mixture uniformity is also sensitive to specific energy input, by knowing what value is required to achieve acceptable product quality, continuous measurement of this parameter can give an overall indication of compound consistency.
Similarly, rheological measurements can provide useful information about the influence of filler type, content, and surface treatment on the overall melt viscosity of polymer containing particulate additives. At low shear rates, for example, using a parallel plate rheometer oscillating at low angular frequencies, information about the state of structure formation can also be inferred. Although various capillary and low shear rheometers are available to continuously monitor changes in viscosity during compounding, in practice, melt from compounding extruders tends to generate pressure fluctuations; hence, to provide meaningful information, consistent pressures must be first obtained, for example, by incorporating a gear pump in the line, or more acceptably, by directing a side stream of melt from the compounder for rheological analysis.
A direct and very practical approach for the assessment of additive dispersion is to analyze pressure development in polymer melt flowing through a screen pack of defined restriction. This so-called pressure filter test has been specifically applied to pigment-containing compositions but may have some relevance to compositions containing low concentrations of other fillers. The test involves using a premix of pigment masterbatch and virgin polymer (with total pigment content of 4% by weight) and metering molten material from an extruder through a screen pack, with defined mesh size, ideally using a gear pump. Pressure is developed in front of the screen which is monitored as a function of time. The test is completed either when a pressure of 120 bar is reached or after 90 min.
A promising approach for the continuous evaluation of compositional changes and mixture uniformity in filled thermoplastics is to use ultrasonic measurements. The method involves determination of the elastic behavior of solids by measurement of ultrasonic wave velocity and can be undertaken off-line in solid material and, more usefully, directly during melt compounding. By this means, transit times passing through the material can be correlated with compositional changes and, more speculatively, dispersion, in filled compounds. Absolute correlations are complicated by fluctuations in melt temperature and pressure, however.
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