Polyethylene melt fracture

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

Narrow molecular weight distribution, which is characteristic of metallocene-based polyethylene (Fig. 7), causes processing difficulty in certain applications due to increased melt pressure, reduced melt strength, and melt fracture [14,15]. This problem can be overcome by blending the metallocene polymer with other prod-... 

Fig. 12.20 Flow velocimetry as a function of the radial position of the polyethylene inside the capillary die for a series of flow rates. Measurements were taken at x = —0.2 mm (no PPA). [Reprinted by permission from K. B. Migler, Extensional Deformation, Cohesive Failure, and Boundary Conditions during Sharkskin Melt Fracture, J. Rheol., 46, 383-400 (2002).]...

Corporation film-grade Ziegler-Natta linear low density polyethylenes will be presented. They are Resin E, Nova FP-015-A, MFI = 0.55, p = 0.9175 g/cc, and Resin C, Nova PF-120-F, MFI = 1.00, p = 0.9170 g/cc. Their capillary-flow behavior in terms of apparent shear stress vs. apparent shear rate are shown on Fig. 12.24. The melt fracture onset is also noted in Figure 12.24 and the data presented in the table below, indicate that resin E undergoes both sharkskin and gross melt fracture at lower apparent shear rates and stresses. 

Fig. 12.24 Flow curves of LLDPE resins E and C, indicating the onset of sharkskin and gross melt fracture for each resin. T — 170°C, capillary D = 1 mm, L/D = 16, with entrance angle 2a = 180°. [Reprinted by permission from E. G. Muliawan, S. G. Hatzikiriakos, and M. Sentmanat, Melt Fracture of Linear Polyethylene, hit. Polym. Process., 20, 60 (2005).]...

M. Sentmanat and S. G. Hatzikiriakos, Mechanism of Gross Melt Fracture Elimination in the Extrusion of Polyethylenes in the Presence of Boron Nitride, Rheol. Acta, 43,624—633 (2004). 

Sharkskin occurs at a lower shear rate than melt fracture, but the term melt fracture is often applied to all regular flow defects including sharkskin. Sharkskin occurs at a critical linear extrusion speed that can be raised by increasing melt temperature. Melt fracture starts at a critical shear stress and can be reduced significantly by reducing the die inlet angle, but, like sharkskin, it is also reduced by raising melt temperature. Linear low-density polyethylene is particularly prone to these defects, but they are minimized by the addition of special additives or blends with other polymers. 

Melt fracture has been a very perplexing but fascinating problem ever since it was discovered. Another problem that seems to have the same degree of perplexity and fascination is draw resonance. Both are instabilities in polymer flows. (Draw resonance may also occur in Newtonian fluids.) Draw resonance is a periodic variation in the diameter of a spinning thread line above a critical drawdown ratio. Polypropylene and high-density polyethylene are both particularly susceptible to draw resonance. Petrie and Denn have presented a comprehensive review of the numerous theoretical and experimental studies of draw resonance conducted prior to 1976 [99]. 

Initial die gaps are set to about 20 percent greater than the final film thickness, and then adjusted to accommodate changes in polymer flow which are resin and rate sensitive. Higher screw speeds increase extruder output, overall film thickness, the tendency toward melt fracture, and may alter the flow pattern. Thus, extruder speed is not a recommended control. In contrast, increased chill-roll speeds decrease film thickness, reduce film width due to increased neck in, increase uniaxial orientation, and alter the optimum air gap or drawdown distance. The optimum air gap, which produces the best orientation, crystallization, and surface properties, depends on the material and chill-roll speed. At 23 to 30 m/min (75 to 100 ft/min), the air gap for low-density polyethylene is about 100 mm (4 in), but when the line speed increases, the air gap is found by trial and error, Since the chill-roll speed controls film stretching, the take-off speed has little effect on the film dimensions. 

Melt fracture in polyethylene products can be avoided by adding a fluoropolymer, traditionally a copolymer of hexafluoropropylene and vinylidene fluoride, with trifluoromethyl side groups. The copolymer may be accompanied by other polymers or additives. There can be interaction between hexafluoropropylene and certain fimctional groups, such as amines, causing discoloration. 

DuPont Dow Elastomers earlier FreeFlow grades incorporated an ethylene copolymer and a small quantity of polyethylene glycol. These alloys were said to reduce melt fracture and to give improved high temperature resistance at long residence times. The additives were also cheaper than their competitors and were awarded Einopean and FDA clearance for food contact. The ethylene copolymer reduced the interactions with HALS that lead to lengthy initial conditioning times. 

Modern polyethylene resins and several other thermoplastic materials including poly(vinylchloride), acrylic polymers are characterized by a narrow molecular weight distribution and advantageous mechanical properties, but are subjected to surface deterioration or "melt-fracture" during processing by extrusion [10], which contains the understanding and control of the "melt-fracture" phenomenon. 

Figure 2.11 Difference between the phenomenon of matte and melt fracture (on distorted extrudates of different polymers) (1) rigid polyvinyl chloride, (2) polyethylene, (3) polypropylene. (4 5) polypropylene viewed from two angles, (6) polymethy-methacrylate, (7) polytetrafluoroethylene. (Reprinted from Ref. 66 with kind permission from Society of Plastics Engineers. Inc., Connecticut, USA.)...

Fluoropolymer-based processing additives provide a range of benefits beyond melt fracture elimination. Among these benefits are improved production capacities, and a better control of molecular orientation and final physical properties. Owing to their design and low volumetric contributions, no adverse effects have been detected which can be attributed to the PPA s presence. Careful selection of other additives used with the PPA can minimize the PPA requirement. Finally, the use of newer products, such as PPA-2, can overcome additive interferences to further enhance polyethylene properties and productivity. 

Hence, the polyethylene manufactured with Dow s catalyst system possesses sufficient levels of long-chain branching to impart higher levels of shear-thinning in the polymer melt, low melt fracture and high melt strength during the polyethylene fabrication process. 

Fluoropolymer additive was used in blown film made out of metallocene linear low density polyethylene. This additive improves processability and eliminates melt fracture. The fluoropolymer additive presence was associated with a substantial increase of dart impact strength (84%), and it had no effect on tear strength. Some slip agent of undisclosed composition was also used. With 3 wt% slip additive, the dart impact strength was reduced to the same level it had without fluoropolymer additive. The tear resistance was not affected by the addition of a slip agent. ... 

F igure 11.8. Effect of various antiblocking additives on melt fracture during extrusion of polyethylene tape. SS - synthetic silica, DE -diatomaceous earth, SCT - sUoxane coated talc. [Data from United States Patent 6,593,400, July 15,2003.]... 

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