Melt elasticity melts

Melt homogeneity Melt strength Melt extensibility Melt elasticity Melt viscosity... 

Polyolefin melts have a high degree of viscoelastic memory or elasticity. First normal stress differences of polyolefins, a rheological measure of melt elasticity, are shown in Figure 9 (30). At a fixed molecular weight and shear rate, the first normal stress difference increases as MJM increases. The high shear rate obtained in fine capillaries, typically on the order of 10 , coupled with the viscoelastic memory, causes the filament to swell (die swell or... 

The Weissenbetg Rheogoniometer is well suited to research on homogeneous viscoelastic fluids and elastic melts. For oscillatory shear a second motor-drive mechanism is added. This allows the use of 60 frequencies in the range of 7.6 x 10 to 40 Hz at ampHtudes between 2 x 10 and 3 X 10 rad. An electronic circuit improves the precision of oscillatory measurements, particularly at frequencies neat the natural resonance frequency of the instmment itself (298). 

Melt elasticity is of considerable importance in understanding much of the behaviour of polyethylene when processing by film extrusion techniques and when blow moulding. The complex relationships observed experimentally here have been summarised by the author elsewhere. ... 

PEs, as other polymers, exhibit nonlinear behavior in their viscous and elastic properties under practical processing conditions, i.e., at high-shear stresses. The MFI value is, therefore, of little importance in polymer processing as it is determined at a fixed low-shear rate and does not provide information on melt elasticity [38,39]. In order to understand the processing behavior of polymers, studies on melt viscosity are done in the high-shear rate range viz. 100-1000 s . Additionally, it is important to measure the elastic property of a polymer under similar conditions to achieve consistent product quality in terms of residual stress and/or dimensional accuracy of the processed product. 

Because of the melt-elasticity effects of the material, it does not draw down in a simple proportional manner thus, the draw-down process is a source of errors in the profile. Errors are significantly reduced in a balanced situation such as circular extrudate. These errors must be corrected by modifying the die and takeoff equipment. 

Many of the new plastics, blends, and material systems require special, enhanced processing features or techniques to be successfully injection molded. The associated materials evolution has resulted in new plastics or grades, many of which are more viscoelastic. That is, they exhibit greater melt elasticity. The advanced molding technology has started to address the coupling of viscoelastic material responses with the process parameters. This requires an understanding of plastics as viscoelastic fluids, rather than as purely viscous liquids, as is commonly held... 

When he interviewed with Meyer in 1926, Mark outlined a typically thorough program. He proposed a team of organic and physical chemists, and physicists who would evaluate the influence of structure on properties such as rigidity, elasticity, melting point, and water absorption. Work, he proposed, would shift toward new material development and into the manufacturing facilities to evaluate the effects of processing... 

However, melt elasticity is more closely related to the third moment known as the z-average molecular weight M. The values of are most often determined using either light-scattering photometry or ultracentrifugation. It is shown mathematically as ... 

The mechanical synthesis of block and graft copolymer is a method of sizable versatility. It can be performed (as already stated) during polymer processing and in standard equipment. The reaction, also, can be carried out by subjecting a mixture of two or more polymers to mechanical degradation in either the solid, elastic-melt, or solution states. It is, also, possible to induce reaction mechanically between polymers and monomers. 

Rheological properties of filled polymers can be characterised by the same parameters as any fluid medium, including shear viscosity and its interdependence with applied shear stress and shear rate elongational viscosity under conditions of uniaxial extension and real and imaginary components of a complex dynamic modulus which depend on applied frequency [1]. The presence of fillers in viscoelastic polymers is generally considered to reduce melt elasticity and hence influence dependent phenomena such as die swell [2]. 

Mendelson (169) studied the effect of LCB on the flow properties of polyethylene melts, using two LDPE samples of closely similar M and Mw plus two blends of these. Both zero-shear viscosity and melt elasticity (elastic storage modulus and recoverable shear strain) decreased with increasing LCB, in this series. Non-Newtonian behaviour was studied and the shear rate at which the viscosity falls to 95% of the zero shear-rate value is given this increases with LCB from 0.3 sec"1 for the least branched to 20 sec"1 for the most branched (the text says that shear sensitivity increases with branching, but the numerical data show that it is this shear rate that increases). This comparison, unlike that made by Guillet, is at constant Mw, not at constant low shear-rate viscosity. 

Mendelson and Finger (172) have reported in a later paper the effects of LCB, at constant MW and MWD, on melt elasticity (from an analysis of data on die swelling), and confirm the conclusion that it decreases the melt elasticity as... 

Ham and co-workers (173) compared a LDPE with three HDPE samples, the MJMn ratios of which bracketted that of the LDPE their non-Newtonian behaviour, as measured by the exponent in a power-law relationship between stress and shear rate, also bracketted that of the LDPE. However, the LDPE had considerably higher melt elasticity than the HDPE, which was ascribed to LCB. 

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