Barus effect

The phenomenon of post-extrusion swelling or "ballooning" has been discussed in Chap. 15. It is related to the so-called Barus effect, according to which the diameter of polymer extrudates is larger than the capillary diameter when the melt is forced through an orifice. All materials with any degree of melt elasticity display this effect. The origin of the effect is related to the elastico-viscous nature of polymer melts, thus to Ni=Tu — T22. 

The negative first normal stress difference under a medium shear rate, characterized by liquid crystalline polymers, makes the material avoid the Barus effect—a typical property of conventional polymer melt or concentrated solution, i.e., when a polymer spins out from a hole, or capillary, or slit, their diameter or thickness will be greater than the mold size. The liquid crystalline polymers with the spin expansion effect have an advantage in material processing. This phenomenon is verified by the Ericksen-Leslie theory. On the contrary, the first normal stress difference for the normal polymers is always positive. 

The Barus effect can be studied with the aid of the Bagley plot. Solving... 

Barus effect (die-swell or extrudate-swell effect)... 

Fig. 7.15 Illustration of Barus effect for the transverse swell due to the slow-down of the flow at the exit of extrusion die...

In extrusion, the preheated material is forced out of the extruder with a screw (Figure 12-2) or double screw and allowed to cool in a bath or in the air. Thermoplasts, elastomers, and thermosets are extruded. As a rule, thermosets are processed in torpedo-containing extruders. With thermosets, most of the curing reaction must occur in a heated pressure chamber. Pressures can approach several hundred bars. The rate of extrusion is lowest with thick-walled bodies. Tubes are extruded at rates of up to 10 m/min, films up to 150 m/min, and telephone-cable insulating material or fibers at up to 1000 m/min. In extrusion, the Barus effect (Section 11.3.1) and melt fracture (Section 7.6.1) may be observed. Tubes, films, ducts, cable insulation, and knot-free nets are produced by extrusion. 

From the relation L=f(vij) it is obvious that the spinnability is governed by two processes, namely the cohesive break (or the swell effect) and the melt break (capillary break, melt fracture). According to Section 11.3.1, a certain amount of elastic energy can be stored in all viscoelastic fluids. This phenomenon leads, among others, to the Barus effect. 

The length of time for which the liquid remains in the spinneret orifices is 0.1-100 ms. The relaxation times for this process (see Section 11.4.2), on the other hand, lie between 100 and 1000 ms. Relaxation processes are therefore important in spinning. They are particularly evident in the Barus effect (Section 11.3.1) and in elastic turbulence (Section 7.6.1). 

Die swell is a complex rheological phenomenon [1], It can be observed as an extrudate with a cross-section (D which is greater than the die cross-section DJ. This effect, also known as extrudate swell, Barus effect, or % memory, is defined as the ratio D /Dq = B and is a feature of polymer melt flow. Die swell is associated with the viscoelastic nature of polymer melts as it exceeds the swelling of constant viscosity (Newtonian) fluids. Accordingly, for laminar flow situations, the swelling due to velocity profile rearrangements or mass balance considerations accounts for only 10-20% and cannot explain the 50-300% increase in extrudate cross-section of the polymer emerging out of a die. 

When a viscoelastic fluid flows through an orifice or a capillary, the diameter of the fluid at the die exit is considerably higher than the diameter of the orifice. This happens because, at the die exit, the viscoelastic fluid partially recovers the deformation it underwent when it was squeezed through the capillary. This type of phenomenon is known variously as extrudate swell, die swell, jet swell, Barus effect or Merrington effect. Metzner [38] discusses the history of extrudate swell... 

Die swell or extrudate swell or Barus effect is the increase in diameter of the polymeric melt extrudate upon emergence from the die. 

Staustelle, Stauscheibe, Kanalverengung (an Extruderdtlse) die ring Dilsenring die swell (Barus effect) Spritzquellung dielectric... 

Extrudate Swelling n (Barus effect) The increase in thickness or diameter, due mainly to the release of stored elastic energy, as a hot melt emerges freely from an extrusion die. In many commercial operations, because the extrudate is drawn away at speeds higher than the mean flow velocity in the die, swelling is more than offset by draw down and is not actually observed. Swelling tends to increase with extrusion rate, abruptness of the approach to the die land, and inversely... 

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