Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Patent
1995-12-19
1998-02-24
Vargot, Mathieu D.
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
264179, 264237, 425 71, C08J 518, C08L 2312, B29C 4788
Patent
active
057209146
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method of producing a polyolefinic film by a water quench process.
It is well known that, in the manufacture of polyolefinic film, particularly polypropylene film, the hot molten polymer is extruded into a water quench bath where the film solidifies. The solidified polypropylene film is taken from the bath and passed over a system of Godet rollers before being fed through a hot air oven in which the film is drawn to induce molecular orientation. Commonly the polypropylene film is slit into a plurality of tapes before being passed into the drawing oven.
Polypropylene is a hydrophobic material and therefore, in general, a polypropylene film removed from the water quench bath carries little or no water on its surfaces. However, in order to improve the properties of the polypropylene film for subsequent use it is customary for various additives to be mixed or blended with the polypropylene. The introduction of additives results in a tendency for the polypropylene film to carry over water from the water quench bath with the result that it is necessary to remove this water from the surfaces of the polypropylene film.
Water removal is conventionally effected by providing a period of vertical travel for the polypropylene film in which the water may fall from the film under gravity, and by providing suction slots to suck moisture from the surfaces of the film. It is also known to use scraper bars to assist in the removal of the water from the surfaces of the polypropylene film.
Hindered amines are additives commonly included in polypropylene in order to improve the resistance of the polypropylene to ultraviolet degradation. This is particularly important when polypropylene film is to be used in conditions where it is exposed to strong or continuous sunlight and it is important that the polypropylene does not degrade too quickly. A particular example is where the polypropylene film is to be used in the manufacture of intermediate bulk containers.
It has been found that hindered amines and other UV stabilisers are worse than other additives in promoting water carryover on the surfaces of the polypropylene film removed from a water quench bath. Indeed the water may remain on the surface of the polypropylene film through the Godet rollers, the slitting device, and into the drawing oven. This is most undesirable because the water is present in patches across the width of the polypropylene film and this leads to irregular and inconsistent drawing of the polypropylene film.
In order to ensure that water is removed from the surfaces of the polypropylene film which includes hindered amine UV stabilisers it has been necessary substantially to reduce the rate of advance of the polypropylene film. Of necessity this results in increased manufacturing costs and it is therefore important to reduce, so far as possible, the water carryover effect.
SUMMARY OF THE INVENTION
In accordance with the present invention it has been found that water carryover can be reduced by introducing an active halogen into the water of the water quench bath.
An "active halogen" is a halogen which is present in water at a pH value above 4 as a hypochlorite ion or a hypobromite ion. Such an active halogen is in equilibrium in the water with the elemental halogen, according, for example, to the equations
According to the present invention there is provided a method of producing, by a water quench process, a film of a polyolefinic material having a tendency to carry water from the water quench process on the surfaces of the film, which includes the step of introducing an active halogen into the quench water and thereby reducing the quantity of water carried on the surfaces of the film leaving the water quench process.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
More specifically in accordance with the present invention there is provided a method of producing a film of a polypropylene material which includes ultraviolet stabilisers, the method comprising the steps of extruding a film of the molten
REFERENCES:
patent: 2801446 (1957-08-01), Wolinski
patent: 4075147 (1978-02-01), Thompson
patent: 4190624 (1980-02-01), Alard et al.
patent: 5006587 (1991-04-01), Fielding
patent: 5049223 (1991-09-01), Dais et al.
Database WPI Week 8732, Derwent Publications Ltd., London GB AN 87-226509 & JP A,62,152 716 (Idemitsu Petrochem.) 7 Jul., 1987.
Bremner David Henry
Miller Joseph William Simpson
Scott & Fyfe Limited
Vargot Mathieu D.
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