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MELT FRACTURE IN POLYETHYLENE FILM

IP.com Disclosure Number: IPCOM000028353D
Publication Date: 2004-May-11
Document File: 7 page(s) / 28K

Publishing Venue

The IP.com Prior Art Database

Abstract

Hydrotalcite is conventionally added to polyethylene resins to neutralize the acidic by-products from polymerization catalyst residues. Certain grades of polyethylene resins are used to prepared plastic films. These "film grades" of polyethylene typically contain a fluoroelastomer-based polymer process aid (or "PPA"). It is known that hydrotalcite is antagonistic towards the performance of the PPA. The use of a specific grade of hydrotalcite, namely HYSAFE 539, reduces this antagonistic interaction in comparison to the use of other commercially available hydrotalcites.

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ABSTRACT

                    Hydrotalcite is conventionally added to polyethylene resins to neutralize the acidic by-products from polymerization catalyst residues.  Certain grades of polyethylene resins are used to prepared plastic films.  These "film grades" of polyethylene typically contain a fluoroelastomer-based polymer process aid (or "PPA").  It is known that hydrotalcite is antagonistic towards the performance of the PPA.  The use of a specific grade of hydrotalcite, namely HYSAFE 539, reduces this antagonistic interaction in comparison to the use of other commercially available hydrotalcites.

MELT FRACTURE IN POLYETHYLENE FILM

                    The "blown film" process is well known for the preparation of polyethylene film.  The process employs an extruder which heats, melts and conveys the molten plastic and forces it through an annular die.

                    The polyethylene film is drawn from the die and formed into a tube shape and eventually passed through a pair of draw or nip rollers.  Internal compressed air is introduced from the mandrel causing the tube to increase in diameter forming a "bubble" of the desired size.  Thus, the blown film is stretched in two directions, namely in the axial direction (by the use of forced air which "blows out" the diameter of the bubble) and in the lengthwise direction of the bubble (by the action of a winding element which pulls the bubble through the machinery).  External air is also introduced around the bubble circumference to cool the melt as it exists the die.  Film width is varied by introducing more or less internal air into the bubble thus increasing or decreasing the bubble size.  Film thickness is controlled primarily by increasing or decreasing the speed of the draw roll or nip roll to control the draw-down rate.

                    The bubble is then collapsed into two doubled layers of film immediately after passing through the draw or nip rolls.  The cooled film can then be processed further by cutting or sealing to produce a variety of consumer goods.  There are a number of defects which may occur in the surface of polyethylene film produced by the blown film process.  These surface defects generally occur at high production rates (which cause high shear rates in the polyethylene which is extruded across the die).  A severe form of these surface defects in the film is referred to as "melt fracture" (or "MF").

                    This "melt fracture" phenomenon is generally observed during the blown film extrusion of linear low density polyethylene ("lldpe") - although it may also be observed during the extrusion of conventional low density polyethylene (which is typically prepared with a peroxide initiator) and Phillips high density polyethylene (which is typically produced with a supported chromium oxide catalyst).

                    The lldpe may be produced with a conventional Ziegler-Natta catalyst or with a single site catalyst (such a...