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Melt Fracture (melt + fracture)

Distribution by Scientific Domains
Polymers and Materials Science100%

Selected Abstracts

Effect of diatomite/polyethylene glycol binary processing aid on the melt fracture and the rheology of polyethylenes

POLYMER ENGINEERING & SCIENCE, Issue 7 2005
Xiaolong Liu
The influence of polyethylene (PE) glycol (PEG), diatomite, and diatomite/PEG binary processing aid (BPA) on the rheological properties and the sharkskin melt fracture of three PEs was studied using a capillary rheometer. When diatomite or PEG is added to the PE matrix, they have little effect on the viscosity reduction of PEs, while the diatomite/PEG BPA shows a synergetic effect on the viscosity reduction of PEs. The incorporation of small amount of BPA was found to increase the shear-thinning behavior and decrease the melt viscosity significantly. Meanwhile, the critical apparent shear rate for the onset of sharkskin melt fracture of PEs is increased. The mechanism for BPA to improve the rheological properties and the melt flow instability of PEs was discussed. POLYM. ENG. SCI., 45:898,903, 2005. © 2005 Society of Plastics Engineers [source]

The effect of surface energy of boron nitride on polymer processability

POLYMER ENGINEERING & SCIENCE, Issue 8 2004
Nimish Rathod
Flow instabilities manifest themselves as distortions on the extrudate surface (melt fracture). They are usually observed at high production rates in many polymer processing operations. Certain fluoropolymers/fluoroelastomers have long been used as processing aids for surface melt fracture elimination. Recent developments have shown that a small amount of boron nitride (BN) powder may successfully eliminate surface melt fracture and also delay the onset of gross melt fracture to higher rates. It has also been reported that a combination of BN and fluoropolymer/fluoroelastomer enhances the effectiveness of the polymer processing even further. The main objective of the present work was to measure the surface properties of a number BN powders, mainly surface energy, in order to gain a better understanding of its performance as a processing aid. Based on this study, it can be concluded that surface energy plays an important role in deciding the possible interactions between the processing aid, polymer melt and the extruding surface. It is observed that the lubricious nature of BN along with an optimum balance of its polar (non-dispersive) and non-polar (dispersive) components of surface energy renders BN a successful processing aid in eliminating both sharkskin and gross melt fracture phenomena. Polym. Eng. Sci. 44:1543,1550, 2004. © 2004 Society of Plastics Engineers. [source]

Gross melt fracture of polyethylene.

POLYMER ENGINEERING & SCIENCE, Issue 3 2002
I: A criterion based on tensile stress
We propose the use of a critical tensile stress as a criterion for the onset of gross melt fracture (OGMF). This stress is estimated from the entrance pressure drop using the method of Cogswell. Carbon black tracer was used to verify that what we deemed to be GMF, based on the inspection of extrudate, was indeed the result of a brittle rupture. An orifice die was used to eliminate the complications arising from the presence of a capillary. Polyethylenes having a variety of molecular structures were used to evaluate the usefulness of the critical tensile stress for characterizing polymers. The critical stress was found to be independent of temperature and contraction ratio. It is also independent of entrance angle, as long as this is equal to or greater than 90 degrees. The critical stress is thus a material property and can be used to rate the tendency of polymers to exhibit gross melt fracture. [source]

Gross melt fracture of polyethylene.

POLYMER ENGINEERING & SCIENCE, Issue 3 2002
II: Effects of molecular structure
The effect of molecular structure (MW, MWD and LCB) on the critical tensile stress (,c) for the onset of gross melt fracture (OGMF), proposed in Part I (1) as a material-dependent criterion for fracture, was determined for a group of polyethylenes varying in structure. These included linear low and high-density polyethylenes and several materials produced using metallocene and constrained geometry catalysts. It was found that the critical stress is independent of MW, for constant polydispersity but increases with increasing long chain branching and polydispersity. The addition of boron nitride particles had no effect on the ,c up to a level of 0.5% by weight. [source]

Ultrasonic improvement of rheological and processing behaviour of LLDPE during extrusion

POLYMER INTERNATIONAL, Issue 1 2003
Shaoyun Guo
Abstract The effects of ultrasonic oscillations on die pressure, productivity of extrusion, melt viscosity and melt fracture of linear low density polyethylene (LLDPE) as well as their mechanism of action were studied in a special ultrasonic oscillation extrusion system developed in our Laboratory. The experimental results showed that, in the presence of ultrasonic oscillations, the melt fracture or surface distortion of LLDPE extrudate is inhibited or disappears. The surface appearance of the LLDPE extrudate was greatly improved. The productivity of LLDPE extrudate was increased in the presence of ultrasonic oscillations. The die pressure, melt viscosity and flow activation energy of LLDPE decreased with the rise in ultrasonic intensity. The shear sensitivity of LLDPE melt viscosity decreased due to the increase of its power law index in the presence of ultrasonic oscillations. Inducing ultrasonic oscillations into LLDPE melt greatly improved its processability. A possible mechanism for the improved processibility is proposed. © 2003 Society of Chemical Industry [source]