Polypropylene temperature rise

From this relatively simple test, therefore, it is possible to obtain complete flow data on the material as shown in Fig. 5.3. Note that shear rates similar to those experienced in processing equipment can be achieved. Variations in melt temperature and hypostatic pressure also have an effect on the shear and tensile viscosities of the melt. An increase in temperature causes a decrease in viscosity and an increase in hydrostatic pressure causes an increase in viscosity. Topically, for low density polyethlyene an increase in temperature of 40°C causes a vertical shift of the viscosity curve by a factor of about 3. Since the plastic will be subjected to a temperature rise when it is forced through the die, it is usually worthwhile to check (by means of Equation 5.64) whether or not this is signiflcant. Fig. 5.2 shows the effect of temperature on the viscosity of polypropylene. 

The glass transition temperature rises with the polarity of the polymer chain. It is assumed that the decrease in the mobility of the chain in this case is due to an increase in intermolecular forces. Table 2.4 shows how the presence of polar groups such as —Cl, —OH, or —CN tends to increase Tg more than do nonpolar groups of equivalent size. Polar interactions considerably restrict rotation hence poly(vinyl chloride), (—CH2 CHC1—) , has a Tg. higher than that of polypropylene, (—CH2 — CH(CH3)—) . 

B. Monrabal, P. del Hierro, Characterization of polypropylene-polyethylene blends by temperature rising elution and crystallization analysis fractionation. Anal. Bioanal. Chem. 399, 1557-1561 (2011)... 

Fig.4 Elution step of temperature rising elution fractionation (Tref) analysis and typical Tref profiles of different polymers [5]. LLDPE linear low-density polyethylene, PP polypropylene...

The chemical composition distribution of polyolefins is measured (indirectly) by either temperature rising elution fractionation (Tref) or crystallization analysis fractionation (Crystaf). These two techniques provide similar information on the chemical composition distribution of polyolefins and can be used interchangeably in the vast majority of cases. Both methods are based on the fact that the crys-tallizability of HOPE and LLDPE depends strongly on the fraction of a-olefin comonomer incorporated into the polymer chains, that is, chains with an increased a-olefin fraction have a decreased ciystallizability. A similar statement can be made for polypropylene and other polyolefin resins that are made with prochiral monomers resins with high stereoregularity and regioregularity have higher crystalliz-abilities than atactic resins. 

Differential Scanning Calorimeter n (DSC) An instrument that measures the rate of heat evolution of absorption of a specimen while it is undergoing a programmed temperature rise (Gooch, 1997). A recorder displays the data as a trace of increase in heat per increase in temperature (d /dT) versus temperature. An example of a DSC thermogram of polypropylene is shown. The DSC has been used to study curing reactions and related properties of thermosetting resins, and heats of decomposition of resins. (An example of a DSC instrument is the PerkinElmer Diamond DSC. 

Slichter, (1959) found that a narrowing of the proton resonance line width occurred 10—20° C above Tg in the case of polyisobutylene and natural rubber but closely at Tg in the case of atactic polypropylene. The two room temperature transitions in polytetrafluoroethylene, which are so clearly visible in the specific heat-temperature curve, Fig. 12, were found by Slichter (1958a) in a fluorine nucleus magnetic resonance study to cause a drop in the second moment with rise of temperature, but the whole effect occurred over the wide temperature range of 225 to 320° K. The slight bulge" in the specific heat-temperature curve of polymethyl methacrylate from 130 to 180° K, Fig. 13, might possibly be correlated with the drop in the NMR second moment between 150° and 200° K found by Powles (1956). 

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