Research Highlights of Polymers Division

A substantial effort to understand and overcome sharkskin has been underway since it was first reported over 40 years ago. Sharkskin is a surface roughness that occurs in the extrusion of certain polymers such as linear low density polyethylene. (See photograph below). In the 1960s, the accidental discovery that sharkskin could be reduced or eliminated by the incorporation of a fluoropolymer polymer processing additive (PPA) allowed processors to increase throughput, reduce energy consumption, and enhance processing quality. Since then, the use of fluoropolymer PPAs has become widespread in polyolefins; resin manufacturers often add it to their polymer resins as part of an additive package. It is conjectured that fluoropolymer additives migrate to the die surface where they lower the surface energy, allowing the polyolefin to slip at the wall.

However, the study of polymer process additives is made difficult for two reasons. First, there are no available in-situ measurement tools. Thus, it is difficult to know whether a given additive migrates to the surface, and if so, does it induce slippage? Second, at a fundamental level the cause of sharkskin and the precise reason that fluoropolymer additives reduce it are still under debate. This lack of understanding of the mechanism makes it conceptually difficult to rationally design new materials. These difficulties then spill over into other areas such as manufacturing efficiency, development of new metallocene based polyolefins and expanding the usability of PPAs into new markets.

Using the NIST extrusion visualization facility, we have made substantial progress in addressing the shortcomings of the traditional techniques. We utilize stroboscopic optical microscopy to visualize the polyethylene-fluoropolymer system as they are extruded through a transparent circular capillary die. In the close-up photograph of the capillary apparatus (below), the bright cube in the center surrounds a capillary tube that is held in place by the steel gland fittings behind it. This capillary apparatus is situated at the exit of a Haake torque rheometer with a co-rotating twin screw extruder attachment. The extruder melts, mixes, and then pumps the polymer through the capillary die. The molten extrudate strand is seen exiting the sapphire tube and traveling to the bottom right of the photograph. The microscope objective lens is below the bright cube.

The extrusion visualization facility provided the critical tool to determine the cause of sharkskin in polyethylene.

Using the NIST extrusion visualization facility, we have made the following measurements that are relevant to the sharkskin and PPA problem:

•Local shear rate

•Extensional shear rate

•Coating kinetics

•Wall slip and polymer-polymer slippage

We were the first to directly visualize the fluoropolymer as it coats the capillary wall upon addition of the PPA to the host polymer. In the following sequence, we first extrude the pure host polymer. Next, we extrude the host polymer plus the PPA at a mass fraction of 0.1%. The fluoropolymer coats the surface in a stripe pattern which is difficult to discern at first but becomes sharper with time (see video micro-graph below). The formation of this structure at the surface coincides with the disappearance of sharkskin from the extrudate.

Understanding the structure of the fluoropolymer as it coats the internal die surface may lead to new additive formulations and processing strategies.

A second contribution that the extrusion visualization facility makes to the study of sharkskin and PPA is in measurement of the change in velocity profile of the polymer as it travels through the tube. The presence of the PPA has a dramatic effect on this velocity profile. Before the PPA was added, the polymer obeyed what is known as the “stick boundary condition.” The figure at the bottom-right shows the velocity of the polymer from the center of a cylindrical tube “depth = 0” to the outer wall “depth = 0.8 mm.” The velocity of the polymer goes to zero at the capillary wall. In the absence of any PPA (red curve) the velocity is a maximum at the center and decreases to zero at the wall.

In the presence of the PPA, the polymer velocity remains finite at the wall. Note the existence of two discontinuous blue points at the wall. The upper one corresponds to the LLDPE and the lower one corresponds to the PPA. This is the first direct evidence of polymer-polymer slippage. Flow velocimetry in the presence of the PPA then shows that slip occurs for all throughputs. This slip is observed to occur at the interface between the two polymers. The magnitude of the slip extrapolation length (˜200 µm) indicates that these two polymers are fully disentangled. The capillary rheo-optics methodology is shown to be an important complement to traditional capillary rheology. By providing both flow velocimetry and high speed imaging, a coherent picture of the suppression of sharkskin by use of fluoropolymer PPAs emerges. In the absence of the PPA, we confirm that slip in the die is not observed and thus is not a necessary ingredient for sharkskin. Upon addition of PPA, we observe the coating process through direct imaging. We see that the PPA forms elongated structures in the flow direction. The formation of the PPA layer coincides with the disappearance of the sharkskin in the extrudate. These results are impacting industry by providing quantitative measurement tools and in-situ screening methods for understanding how PPAs reduce sharkskin. Future work will concern the mechanism by which sharkskin occurs, measurements of slippage, and an attempt to understand why different polymers behave so differently with respect to sharkskin formation.