When polymers are subjected to shear in processing operations, the long chain of molecules present in them become distorted. The molecules tend to get straightened out (oriented) because of such shearing. The molecules, if they are still molten, will tend to coil up again when the shearing process ends.
The re-coiling (sometimes referred to as relaxation) may not be complete when there is a rapid cooling post-shearing. The processes of this uncoiling/re-coiling lead to multiple phenomena that are usually called elastic effects. Extrudate swell, sharkskin, melt fracture, draw down, and frozen-in orientation are some of the important elastic effects.
Extrudate (“Die”) Swell
The extrudate may swell when polymer melt emerges from a die so that the extrudate’s cross-section is more than that of the die orifice after it leaves the die.
For a capillary die, the die swell ratio, swelling ratio, extrudate swell, or the puff-up ratio is the ratio of the extrudate diameter to the die diameter. For a slit die, the appropriate ratio is of the extrudate thickness to the slit height.
The swelling takes place because the molecules become extended when the melt passes through the die; the greatest extension occurs near the die wall. After emerging from the die, the molecules tend to coil up (re-coil), contract in the flow direction, and expand in the directions transverse to the flow.
The greatest contraction in the flow direction is close to the wall where the shear is greatest when an extrudate is cut at the die face and the leading edge of the extrudate is convex.
Factors Affecting Extrudate Swell
Experimental work has demonstrated that:
- There is an increase in swelling with an increase in extrusion rate (shear rate) up to a critical shear rate
- There is a decrease in swelling with an increase in temperature at a given extrusion rate or shear rate
- Die swell is little affected by temperature at a given shear stress
- There is a decrease in die swell with an increasing length of the die for a given shear rate
- Extrudate swell through a capillary die is somewhat less than through a slit die; extrudate swell through the slit die also increases more rapidly with an increase in shear rate
- Extrudate swell will increase with the increase in the ratio reservoir diameter/capillary diameter (although little affected at ratios above 10:1).
The section entitled “Measurement of Extrudate Swell” provides the methods for the measurement of extrudate swell.
The Source of Extrudate Swell
Compensation for Die Swell by Drawing Down
Stretching, or drawing down the extrudate are common techniques to compensate solid extrudates for die swell, so that the extrudate can just pass through a sizing die. Although in this technique it is unnecessary for the two to be exactly balanced, it should be noted that drawing down causes molecular orientation.
This leads to strength enhancement in the flow direction and strength reduction in directions perpendicular to the flow, which may or may not be necessary. If the solid extrudate has varying thicknesses, drawing down will have somewhat limited effects. This is because shear rates, and therefore, die swell are higher at the thinner sections. To ensure that extrusion rates do not change throughout the cross-section, the thinner sections may also have a shorter die length, to increase the extrudate swell even further.
Compensation for Extrudate Swell with Pipe and Tubing
The situation is more complicated during the extrusion of pipe and tubing, since the extrudate is often inflated according to the dimensions of a sizing die. In this case, the assumption is that that the wall thickness will expand to the die swell appropriate to the shear rated employed, after the emergence from the extruder die.
However, there will be a reduction in the wall thickness proportional to the inflation, where the inflation is the ratio of the sizing die diameter to the external diameter of the extrusion die.
It is usually observed that distortion of the extrudate takes place when extrusion is carried out at high rates.
The distortion that is observed may be caused by the phenomenon referred to as melt fracture or elastic turbulence (and in some cases, bambooing) or an effect called sharkskin formation. These phenomena are not fully understood, and appear to have different origins.
Critical Shear Rate
When the shear rate exceeds the critical shear rate for a given polymer melt at a defined temperature, melt fracture occurs. There is a corresponding critical shear stress. Critical point is the point where these define on the shear rate-shear stress diagram (flow curve).
It is believed that melt fracture originates in the die entry region when the material is being funneled into the capillary from the die reservoir. In an extruder, this corresponds to the point where melt moves into the die parallel portion of the die. Other complicating effects also take place at the wall of the die.
Form of Distortion
Distortion is usually helical in nature, although its form may vary between polymer types. A screw thread-type distortion may appear in materials such as polypropylene and polyethylene. The extrudate may form a spiral in polystyrene, while ripples or bamboo-like repetitive kinks may be formed with other melts. The helical nature is obscured by severe and random distortion with all melts, at rates well above the critical point.
When small diameter extrudates are extruded at high rates, melt fracture is most likely to take place, with the most notable instance occurring during wire coating.
Factors Affecting Melt Fracture
Melt fracture has been widely studied, as it can be easily observed in a laboratory. Experiments have demonstrated that:
- There is an increase in the critical shear rate for a melt fracture with a rise in temperature.
- The product τcMw is a constant (τc is the critical shear stress, and Mw is the weight average molecular weight). Melt fracture starts at lower shear stresses, and rates, and takes place in high molecular weight resins than with low molecular weight polymers.
- Two polymers will have similar critical points when they have similar melt viscosities but different levels of branching.
- By tapering the die entry, there will be a significant improvement in the extrudate quality. This allows obtaining externally undistorted extrudates at rates much above the critical point. (However, there may be some internal distortion.) The critical point may significantly increase when the so-called die parallel is tapered (by up to 10º).
- The critical shear rate may also rise with an increase in the L/D ratio of the die.
All of these factors influencing melt fracture are well known and have been applied for several years, as a result of which, many operations such as high speed wire coating and operations involving high shear rates, are carried out with not much trouble from melt fracture effects.
Compared to melt fracture, the phenomenon of sharkskin has not been extensively studied, but it may represent a greater problem in industrial extrusion.
Here, instead of helical distortions, the distortion comprises of transverse ridges. When the polymer exudes from the die, distortion is caused by the melt tearing. Within the die, the melt close to the wall moves very slowly (in the layer adjacent to the wall, there is no movement). When the melt emerges, the extrudate moves away from the die face at a constant speed, resulting in the outer layers being suddenly stretched and may tear.
Incidence of Sharkskin
Experiments have shown that the critical shear rate for the onset of sharkskin (γc) is inversely proportional to the die radius (R) (i.e. the product, γcR is a constant). In the case of larger diameter dies, the critical shear rate is much lower. Although with small dies (similar to the ones used in typical laboratory rheometers) melt fracture takes place at shear rates below those for the onset of sharkskin, the reverse is likely to be observed with full-scale industrial dies.
Critical Linear Extrusion Rate
It can be shown that regardless of the die size, sharkskin can take place above a critical linear extrusion rate. In other words, for a given polymer melt, sharkskin may take place at a specific extrusion rate, with the size of the die having no effect.
Conditions Favoring Sharkskin
When the melt is partially elastic, sharkskin appears to be most severe, and has the consistency of a friable cheese. If the melt temperatures are reduced, results may improve, and the melt can become more strongly elastic when it emerges from the die.
Improved results can also be obtained by heating the die at the exit point in order to ensure that the surface layers of melt are more fluid with less tearing effect. The severity of sharkskin may vary massively.
In some cases, the distance between ridge and adjacent trough may be one-third of the extrudate cross-section. In others, the effect will not be easily visible to the naked eye, but may present itself as a matt finish or may be felt when a fingernail is run over the surface. In blow molding, sharkskin is indicated by a rough surface on the inside of a bottle as the outside is often flattened against the wall of the blow mold.
The molecular weight distribution (MWD) is the only molecular property, within a polymer type, that has an effect on sharkskin. A wide MMD usually reduces the tendency to create sharkskin effects.
When polymer molecules are in molten state, they have a tendency to coil up in the absence of external stresses; they often exist in a random coil configuration.
External Stress Application
The molecules are distorted from their randomly coiled state and are oriented during the application of external stresses (as occurring during molding, extrusion, and other shaping methods). In most processing operations, it is better to freeze (or “set”) the polymer soon after it has been shaped, such as cooling in a water bath after extruding through a die.
In such situations, there is no sufficient time for polymer molecules to re-coil (relax) fully before the melt freezes. This leads to frozen-in orientation.
Due to frozen-in orientation, plastic products display anisotropic behavior, their properties vary when measured in different directions. For instance, the tensile strength is more in the direction of orientation than in the perpendicular direction. Frozen-in orientation also affects impact strength.
For an Izod impact test sample that is injection molded with the gate at one end, the molecules are roughly aligned along the sample axis. Thus, in a standard Izod test, breaking the sample requires a fracture across the elongated molecules. This results in higher impact strength than what would be measured on unoriented samples.
Conversely, if a weight is dropped onto a flat plate to measure impact strength, the value will be lower with a more oriented molding. This is because fracture occurs more easily parallel to the direction or orientation, rather than across the molecular orientation.
Types of Orientation
Stretching the polymer melt just before its freezing produces increased orientation in a product. An important aspect in the manufacture of fibers is the uniaxial orientation, i.e. stretching in one direction. Similarly, in film manufacture, biaxial orientation involving stretching in two directions at the same time is important. To improve the hoop strength and fracture resistance of products such as piping, bottles, and other hollow containers, biaxial orientation is built up.
Desirability or Undesirability
According to circumstances, frozen-in orientation may be desirable or undesirable. Frozen-in orientation is at it's greatest when high stress is applied on the melt and there is a reduction in the time interval between shearing and setting of the melt. These conditions are common when low cooling temperatures and low melt temperatures (such as low extrusion cooling bath temperatures or low injection mold temperatures) exist.
In several extrusion processes, including the manufacture of film, the extrudate can be subjected to widespread stretching after it leaves the die. In processes such as chill-roll casting of film, it is critical to avoid tearing effects when the extruded web is stretched.
In processes where stretching the melt takes place, viscous behavior rather than elastic behavior is the more important factor. Here, the main requirement is the mobility of the melt molecules to flow past each other, although the melt should have some strength and elasticity.
The “neck-in” phenomenon is connected with chill roll casting where the edge of the extruded web has a tendency to shrink inwards towards the center of the web. This can cause the edge to become thicker than the bulk of the film. More elastic melts are less liable to neck-in, as they are able to maintain a tension in the extrusion direction.
Elastic effects can also affect parison sag, which takes place during blow molding. The thinning of the parison due to its own weight when it leaves the die is known as parison sag. A portion of the sag may be caused due to an elastic effect (chain uncoiling); viscous flow also contributes to the sag when the molecules slide past each other. It can be reasonably assumed that the elastic component resisting the sag rises as a proportion of the total as the:
- Molecular weight, and hence viscosity, is increased
- Melt temperature is reduced (when viscosity is increased)
- The length of parison per unit weight is increased, because an elastic deformation under a standard load depends on the length of the part being stretched, while the viscous flow does not (as long as the weight of the parison does not change).
This information has been sourced, reviewed and adapted from materials provided by Dynisco.
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