Die-swell

The phenomenon associated with the increase of the diameter of an extrudate as a polymer leaves a capillary, known as die swell or extrudate swell, has been introduced briefly back in Section 3.2. The implication at this point is that die swell is related to unconstrained elastic recovery ( co) following shear flow. Soo is related to the ratio of the primary normal stress difference to the shear stress through the equation 

Tanner s theory for die swell (see Section 3.2) for flow through a capillary leads to 

In this section we show that die swell is more complicated than indicated by Eq. 7.4 and depends on a number of factors. We then discuss how to deal with die swell in die design. 

FIGURE 7.12 Capillary extrudate swell versus j5co, the ultimate elastic recovery. ( , ) two HDPEs ( ) PS (o) LDPE (A) PP. (Data from White and Roman, 1976.) 

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Figure 5.10 (a,b) Comparison of the simulated die swell in a Couette flow for the power-... 

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... 

Non-Newtonian Fluids Die Swell and Melt Fracture. Eor many fluids the Newtonian constitutive relation involving only a single, constant viscosity is inappHcable. Either stress depends in a more complex way on strain, or variables other than the instantaneous rate of strain must be taken into account. Such fluids are known coUectively as non-Newtonian and are usually subdivided further on the basis of behavior in simple shear flow. 

The low elastic swell, unique to PVC, results from the presence of the biUion molecule flow units. Other plastics have higher die swell, which... 

High stmcture blacks ia unvulcanized mbber give higher Mooney viscosities, lower die swell, faster extmsion rates, and better and more rapid dispersion after iacorporation. In vulcanized mbber higher modulus is obtained. High stmcture blacks give lower bulk densities and high vehicle demand ia paint systems. 

Third Monomers. In order to achieve certain property improvements, nitrile mbber producers add a third monomer to the emulsion polymerization process. When methacrylic acid is added to the polymer stmcture, a carboxylated nitrile mbber with greatly enhanced abrasion properties is achieved (9). Carboxylated nitrile mbber carries the ASTM designation of XNBR. Cross-linking monomers, eg, divinylbenzene or ethylene glycol dimethacrylate, produce precross-linked mbbers with low nerve and die swell. To avoid extraction losses of antioxidant as a result of contact with fluids duriag service, grades of NBR are available that have utilized a special third monomer that contains an antioxidant moiety (10). FiaaHy, terpolymers prepared from 1,3-butadiene, acrylonitrile, and isoprene are also commercially available. 

Particulate fillers are divided into two types, inert fillers and reinforcing fillers. The term inert filler is something of a misnomer as many properties may be affected by incorporation of such a filler. For example, in a plasticised PVC compound the addition of an inert filler will reduce die swell on extrusion, increase modulus and hardness, may provide a white base for colouring, improve electrical insulation properties and reduce tackiness. Inert fillers will also usually substantially reduce the cost of the compound. Amongst the fillers used are calcium carbonates, china clay, talc, and barium sulphate. For normal uses such fillers should be quite insoluble in any liquids with which the polymer compound is liable to come into contact. 

Such elastic effects are of great importance in polymer processing. They are dominant in determining die swell and calender swell via the phenomenon often... 

Thus it is found that extrusion die swell, when compared at equivalent shear rates, tends to go down with an increase in temperature and go up with molecular weight. As shear rates are increased, polymer uncoiling is increased and die swell increases Figure 8.9). A point is reached, however, where the die swell no longer increases and in fact it can be argued that it goes down. At this point it is... 

Die swelling effects may be ignored and the melt viscosity can be assumed to be constant. 

This example illustrates the simplified approach to film blowing. Unfortunately in practice the situation is more complex in that the film thickness is influenced by draw-down, relaxation of induced stresses/strains and melt flow phenomena such as die swell. In fact the situation is similar to that described for blow moulding (see below) and the type of analysis outlined in that section could be used to allow for the effects of die swell. However, since the most practical problems in film blowing require iterative type solutions involving melt flow characteristics, volume flow rates, swell ratios, etc the study of these is delayed until Chapter 5 where a more rigorous approach to polymer flow has been adopted. 

Now it makes the solution simpler to assume that the blow up ratio is given by Dft/Di (ie rather than Db/D ). Also this seems practical because the change from D to D is caused solely by inflation whereas the change from D to Db includes die swell effects. 

So having obtained the dimensions of the tube to be produced, the procedure is to start by assuming that there is no die swell. This means that the die dimensions will be the same as those of the tube. 

Representative plots of extrudate swelling ratio as a function of NBR content are shown in Fig. 3. Shear rate increases the die-swell in all blends. The change of die-swell with NBR content exhibits a decreasing trend up to 60% of NBR, and beyond this level it shows a saturation in die-swell. Preheating of blends exhibits the minimum at 50 50 ratio irrespective of shear rates. We... 

Figure 3 Variation of die-swell with the percent NBR content in NBR-CSPE blend.

The plot of the extrudate swelling ratios with AU content are shown in Fig. 27 as a function of shear rate for both types of blends. The die-swell gradually decreases with the addition of AU in the blend up to the... 

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