Extrusion instabilities sharkskin

In blends, fluoropolymers are used in small quantities to enhance throughput, reduce the frictional properties, and increase the wear resistance. Blends comprising 0.3-50 wt% of a low molecular weight PTFE (T , < 350 °C) with engineering resin showed improved antifriction properties (Asai et al. 1991). LLDPE generally exhibits sharkskin melt fracture, but the use of fluoropolymer additives, such as the copolymer of vinyUdene fluoride and hexafluoropropylene, can help to eliminate the extrusion instability (Hatzikiriakos and Migler 2005). 

Since in industrial operations sharkskin is the first extrusion instability which limits the production rates, it is desirable to eliminate or at least postpone its onset. The effect of blending PLA with AAC on the onset of sharkskin and gross melt fracture is presmted bellow. 

Blend P10E90 (i.e. 10 wt% PEA) also exhibited the two extrusion instabilities. In this case, the onset of sharkskin was observed at shear rate 6750 s and shear stresses of 640 kPa, which is 60 kPa higher than the onset of gross melt fracture of neat AAC. The gross melt fracture of blend P10E90 appeared at 670 kPa and shear rate 7600 s. Extmdates of blend P10E90 at 160°C are presented on Fignre 7. 

During extrusion of polymer melts with high throughputs, the elastic melt properties can also lead to elastic instabilities which can result in surface distortions of the extrudate. One example are wavy distortions also described as sharkskin. Depending on the polymer, this can also lead to helical extrudate structures (stick-slip effect) or to very irregular extrudate structures (melt fracture) at even higher throughput rates [10]. 

Polymer chains anchored on solid surfaces play a key role on the flow behavior of polymer melts. An important practical example is that of constant speed extrusion processes where various flow instabilities (called sharkskin , periodic deformation or melt fracture) have been observed to develop above given shear stress thresholds. The origin of these anomalies has long remained poorly understood [123-138]. It is now well admitted that these anomalies are related to the appearance of flow with slip at the wall. It is reasonable to think that the onset of wall slip is related to the strength of the interactions between the solid surface and the melt, and thus should be sensitive to the presence of polymer chains attached to the surface. 

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...

During the extrusion of polymers different defects and flow instabilities occur at very low Reynolds numbers. The commonly known ones are sharkskin, melt fracture, slip at the wall and cork flow. These defects are of commercial importance, since they often limit the production rate in polymer processing. Many researchers have been interested in the subject, and thorough reviews on flow stability and melt fracture have been written in the last 30 years [1-4]. More recently, two review papers deahng with viscoelastic fluid mechanics and flow stability, were published by Denn [5] and Larson [6]. However, although much work has been done in the field of extrusion distortions, controversy still exists regarding the site of initiation and physical mechanisms of the instabilities. 

K.B. Migler. Sharkskin instability in extrusion. In S.G. Hatzikiriakos and K.B. Migler (Eds.), Polymer Processing Instabilities—Control and Understanding, Dekker, New York, 2005. 

Melt extrusion is characterized by at least two distinct instabilities that cause surface defects, generally known as melt fracture, or gross melt fracture, and sharkskin, or sharkskin melt fractnre (15). Sharkskin, a high frequency. 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include 1) non-uniform plastics flow in the hopper 2) troublesome bridging, with excessive barrel heat that melts the solidified plastic in the hopper and feed section and stops the plastic flow 3) variations in barrel heat, screw heat, screw speed, the screw power drive, die heat, die head pressure, and the take-up device 4) insufficient melting or mixing capacity 5) insufficient pressure-generating capacity 6) wear or damage of the screw or barrel 7) melt fracture/sharkskin (see Chapter 2), and so on. 

Commercially available thermoplastic elastomers based on block copolymers of diisocyanates and polyols were used to delay sharkskin and stick-slip instabilities in the extrusion of linear low density polyethylene. When elastomer is added in a small mass fraction to LLDPE, it deposits at the die surface during extrusion and may postpone the onset of sharkskin instability to a 12-20 times higher rate of extrusion. ... 

Migler, K., Sharkskin Instability in Extrusion Chapter 5 in Hatzikiriakos, S. and K. Migler (Eds.), Polymer Processing Instabilities Control and Understanding, Marcel Dekker, Monticello, NY, USA, pp.121-160 (2005)... 

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