Molecular weight distribution extrusion

In addition to elastic turbulence (characterised by helical deformation) another phenomenon known as sharkskin may be observed. This consists of a number of ridges transverse to the extrusion direction which are often just barely discernible to the naked eye. These often appear at lower shear rates than the critical shear rate for elastic turbulence and seem more related to the linear extrudate output rate, suggesting that the phenomenon may be due to some form of slip-stick at the die exit. It appears to be temperature dependent (in a complex manner) and is worse with polymers of narrow molecular weight distribution. 

Narrow molecular weight distribution Difficult extrusion except where NMWD resins are used. Low melt strength, good melt drawability Fiber/nonwovens Fiber/nonwovens... 

Reactive extrusion is the chemical modification of polymer while it is being transported in an extruder. In this work, polypropylene is intentionally degraded by the addition of a free radical initiator (a peroxide) during extrusion. The product has improved flow properties because of the removal of the high molecular weight tail and the narrowing of the molecular weight distribution. 

Products A wide range of bimodal and unimodal products, with a full control of comonomer distribution, can be produced, with densities ranging from 918 to 970 kg/m3 and melt flowrate from less than 0.1 to over 100. The molecular weight distribution can be controlled from narrow to broad. Advanced properties are tailor-made applications such as pipe strength, film bubble stability as well as high ESCR and stiffness in blow molding. Other special applications include extrusion coating and wire cable. 

Products Polymer density is easily controlled from 0.915 to 0.970 g/cm. Depending on catalyst type, molecular weight distribution is either narrow or broad. Melt index maybe varied from less than 0.1 to greater than 200. Grades suitable for film, blow-molding, pipe, roto-molding and extrusion applications are produced. 

Products Homopolymers can be produced with melt flows from less than 0.1 to 3,000 and isotactic content up to 99%. Random copolymers can be produced with up to 12 wt% ethylene or up to 21 wt% butene over a wide melt flow range (<0.1 to >100). A full range of impact copolymers can be polymerized with a good stiffness to impact balance. Products from narrow to broad molecular-weight distribution can be manufactured in grades suitable for film injection, molding, blow molding, extrusion and textile applications. 

Interchange reactions do not occur readily under such conditions, and the molecular weight distribution of the product may differ significantly from the random one derived above. When the polymer is melted subsequently for spinning or extrusion, the molecular weight distribution will tend to equilibrate toward the predicted most probable type, however. The rate at which this equilibrium distribution is approached will depend on the time and temperature conditions during melt process ng and will be enhanced by the presence of residual polymerization catalyst... 

R.K. Krishnaswamy, D.C. Rohlfing, A.M. Sukhadia, and K.R. Slusarz. Extrusion of broad-molecular-weight-distribution polyethylenes, Polymer. Eng. Sci. 2004, 44(12). 

Description A variety of LDPE homopolymers and copolymers can be produced on these large reactors for various applications including films, molding and extrusion coating. The melt index, polymer density and molecular weight distribution (MWD) are controlled with temperature profile, pressure, initiator and comonomer concentration. Autoclave reactors can give narrow or broad MWD, depending on the selected reactor conditions, whereas tubular reactors are typically used to produce narrow MWD polymers. 

A full range of impact copolymers can be polymerized with excellent stiffness for even the most demanding applications. Products from narrow to broad molecular-weight distribution can be manufactured in grades with proven advantage for film injection, molding, blow molding, extrusion and textile applications. 

PP granules plus the final product additive package are finished in a twin screw extrusion system (4) including any post-reactor modification of the polymer molecular weight and molecular weight distribution required by the specific product application, typically referred to as controlled rheology (CR) technology. 

The use of MFR as a measure of consistency is acceptable, but as a measure of processability it can be very misleading. This is because the shear rates of the test are much lower than typical shear rates in extrusion and injection molding. Consequently, as a result of differences in molecular weight distribution, branching, etc., two polymers with similar MFRs can have widely differing viscosities at high shear rates, and consequently their proces.sing behaviors may be quite different. The apparatus is shown in Fig. 13. 

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