Sensitizer melt index

The Gyna-ir Correlation can be sharpened by the two-phase argument. With the crosslinking agents confined to their own phase, the quantity of gel should be determined by the total surface area of that phase and, hence, the dispersion of the E/ABP in the polyethylene matrix. If we assume that better blending is achieved by matching the component melt viscosities, then Qninv should be dependent on the sensitizer melt index (MI), the relative volumes of the components -blend ratio, BR - and the total sensitizer concentration (Co). From least squares analyses of our results, we find ... 

Mechanical properties, such as elastic modulus and yield point, that depend on crystallinity per se are not seriously affected by low to moderate doses of ionizing radiation. On the other hand, those mechanical properties that are sensitive to interlamellar activity are most dramatically affected by the low to moderate radiation doses. This is seen in the ultimate tensile strength and elongation at failure of the polyolefins. It is also reflected in the large change in melt index between 0 and 18 Mrad, which indicates formation of cross-links that increase with increasing... 

Ultrathene [Quantum], TM for a series of ethylenevinyl acetate copolymer resins for adhesives, conversion coatings, and thermoplastic modifiers. Wide range of melt indexes. Improves specific adhesion of hot-melt, solvent-based, and pressure-sensitive adhesives. 

FIGURE 242 In comparison with Ziegler catalysts, a typical Cr/silica Phillips catalyst is usually much less sensitive to H2 as a means of controlling the polymer melt index. 

ASTM Method 1238, Standard Test Method for Flow Rates of Thermoplastics by Extrusion Plastometer [6], completely defines the melt index, both the instrument and the experimental procedure. Temperatures and instrument configurations for common materials are given along with recommended calibration procedures and round-robin results. This method must be strictly followed if consistent melt index results are to be obtained. A melt index measurement is performed at low stress levels compared to capillary rheometers measurements (typical weights are in the range of 1.2-21.6 kg) and is sensitive to variations in operating procedure, instrument cleanliness (between tests), and operator bias (items such as packing and sample preparation). 

Figures 18.30(b) and (c) show similar data for temperatures of 0 and -20 °C, respectively. With the exception of the low melt index PP copolymer, the notch sensitivities are similar to those at room temperature but the marked drop in overall level brought about by a decrease in temperature is again clearly demonstrated.

Figures 18.31 and 18.32 show the combined data for room temperature and -20 "C for HDPE, and for ABS terpolymer and toughened PS, respectively. Of particular note is the very small effect of temperature on both the impact level and notch sensitivity of HDPE, although the effect of MW (or melt index) is again clearly demonstrated. This indicates the suitability of HDPE for low-temperature applications.

Figure 2.14 shows the relative notch sensitivities of some polyolefins. Among the polyolefins, the low-melt-index polypropylene copolymer (GMT 61) has the lowest notch sensitivity, whereas the higher-melt-index polyethylene and polypropylene homopolymers have the highest. 

Figure 2.15 shows the variation in standard Izod impact strength (1 mm notch tip radius) with temperature for the same range of materials. By far the most temperature sensitive is the low-melt-index polypropylene (PP) copolymer. It is interesting to compare the sharp fall-off in notched impact strength with this material between 23°C and 0°C with the steady values obtained for falling weight impact strength for the same temperature range (Figure 2.16). The removal of points of stress concentration is thus the most vital factor governing the impact performance at tanperatures of 90°C and below.

FIGURE 2.15 Influence of temperature on Izod impact strength of various polymers of low and high melt indexes. GMT 61 is a low-melt-index and low-notch-sensitivity polypropylene copolymer. KMT 61 is a high-melt-index and low-notch-sensitivity polypropylene polymer. HDPE 65045 is a high-melt-index and high-notch-sensitivity high-density polyethylene. 

The determination of the melt index is very sensitive to a number of factors that reduce its precision. These faetors include temperature control wear on the die, barrel, and piston and operator inconsistencies. Data from ASTM D 1238 indicate a eoeffieient of variation within the same laboratory of 1.7-5.6%, with an interlaboratoiy eoeffieient of variation of 5-16%. In general, the coefficients of variation increase toward low and high values of melt index. 

A erude measure of the shear sensitivity of a polyethylene resin can be obtained if it is extruded from a melt indexer under two different loads. It is conventional to make this determination using the standard load of 2.16 kg and one of 10.0 kg. The ratio of the mass of extrudate in 10 min at high load relative to that at low load is a dimensionless number known as the melt index ratio (MIR) or fiow rate ratio (FRR). The higher the number, the greater is the sample s propensity to undergo shear thinning. 

Too little has been published about the flow properties of PET as a criterion for processing. The results of melt flow index (MFI) testing conditions do not correlate with the processing behavior in the case of PET. This may be caused by the discrepancy between the shear rates in testing and processing. MFI is defined as the amount of polymer melt (in g) extruded within 10 min through an orifice of specified diameter at a standard load and temperature. In the case of PET, this method was not very popular until recently due to the sensitivity of this material to hydrolytic degradation. 

Criteria for solvent purity include careful measurement of physical constants such as melting point, refractive index, or conductance, but even these techniques may not detect some trace impurities. In many instances gas-liquid or other sensitive chromatographic methods may be used. 

Some properties, such as heat capacity, refractive index, and density, are not particularly sensitive to molecular weight but many important properties are related to chain length. Figure 3.2 lists three of these. The melt viscosity is typically proportional to the 3.4 power of the average chain length so 17 is proportional to Thus, the melt viscosity increases rapidly... 

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