Melt flow properties

Only a limited amount of information is available concerning the effects of molecular weight distribution. There is, however, evidence that the narrower the distribution the more Newtonian are the melt flow properties. It has been noted... 

Studies of melt flow properties of polypropylene indicate that it is more non-Newtonian than polyethylene in that the apparent viscosity declines more rapidly with increase in shear rate. The melt viscosity is also more sensitive to temperature. Van der Wegt has shown that if the log (apparent viscosity) is plotted against log (shear stress) for a number of polypropylene grades differing in molecular weight, molecular weight distribution and measured at different temperatures the curves obtained have practically the same shape and differ only in position. 

When a molten plastic is forced through a die it is found that under certain conditions there will be defects in the extrudate. In the worst case this will take the form of gross distortion of the extrudate but it can be as slight as a dullness of the surface. In most cases flow defects are to be avoided since they affect the quality of the output and the efficiency of the processing operation. However, in some cases if the flow anomaly can be controlled and reproduced, it can be used to advantage - for example, in the production of sheets with matt surface finish. Flow defects result from a combination of melt flow properties, die design and processing conditions but the exact causes and mechanisms are not completely understood. The two most common defects are... 

Blind hole In regard to molding products that include holes, it is important to ensure that sufficient material surrounds the holes and melt flows property. A core pin forming blind holes is subjected to the bending forces that exist in the cavity due to the high melt pressures. Calculations can be made for each case by establishing the core pin diameter, its length, and the anticipated pressure conditions in the cavity (3). 

Saeda,S., Yotsuyanagi.J., Yamaguchi.K. Relation between melt flow properties and molecular weight in polyethylene. J. Appl. Polymer Sci. 15,277-297 (1971). 

Commodity high pressure PE-LD grades have a broad polydispersity and they contain small amounts of LCB, which results in excellent melt flow properties. Due to the broad polydispersity (PD), the mechanical properties are inferior, even compared with conventional Ziegler-Natta resins. However, narrow PD metallocene resins have poor processing properties. Recent advances in catalyst technology, have resulted in metallocene polyethenes that contain small amounts of LCB to ease the processability. 

Polymer melt flow properties determine to a large extent the characteristics of the forming process. The detailed discussion on rheology is provided in Chapter 7 Rheology of Polymer Alloys and Blends. Rheological behavior will only be summarized here in order to provide sufficient information to understand and discuss polymer forming. 

By combination of TGA, DSC, and TMA data, polymers that have the potential to melt can be obtained. These methods provide qualitative data on the thermal properties of the polymer, together with their melt flow character. These studies should be complemented by rheological measurements. It should be mentioned that the melt flow properties of these materials as a function of temperature or time have not been extensively studied. 

To explain the effect of the MWD on melt flow properties, we first consider the shear flow of a monodisperse polyethylene of molecular weight M. Figure 5.7 shows, on logarithmic scales, how the shear stress t varies with the shear strain rate y. At low y values, t is proportional to y and the... 

What is the condition for entanglements between two polymer chains to arise in a polymer melt, and what consequences such entanglements have on the melt flow properties ... 

Poly(biphenyl ether sulfone)s with improved polydispersity, a lower level of undesirable low-molecular-weight oligomeric components and improved melt flow properties have 4,4 -biphenylene, p-phenylene, 4,4 -di-phenyl sulfone and 2,2-diphenyl propane groups in the backbone. ... 

In combination with primary AOs in POs, phosphites help retain the melt-flow properties and color stability through repeated processing passes better than each AO can do alone This helps limit the amount of primary AO that is consumed in processing dramatically. Moreover, phosphites and hindered-phenol AOs can be combined as dry blends to simplify handling and feeding [3-5]. 

There have been many attempts to change the melt flow properties of a polymer by incorporating a small amount of an anisotropic or an immiscible polymer. For example, Brody [220,221] demonstrated a windup speed up to 5000 m/min by adding a small amount of copoly(chloro-l,4-phenylene ethylene dioxy-4,4 -dibenzoate/terephthalate) (CLOTH) or copoly(4-hydroxybenzoic/6-hydroxy-2-naphthoic acid) into nylon-6,6. More recently, Vassi-latos [222] disclosed the melt spinning of nylon-6,6 at speeds up to 6000 m/min with the addition of a minor amount of liquid crystalline polymers such as CLOTH. This technique clearly offsets some of the cost advantages of high-speed spinning. 

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