CA2072599C

CA2072599C - Stretch wrapping of a horizontal beam

Info

Publication number: CA2072599C
Application number: CA002072599A
Authority: CA
Inventors: David Charles Climenhage
Original assignee: DuPont Canada Inc
Current assignee: Coveris Flexibles Canada Inc
Priority date: 1990-06-26
Filing date: 1991-06-21
Publication date: 2003-10-07
Anticipated expiration: 2011-06-21
Also published as: HUT65769A; AU641253B2; CA2072599A1; PL297165A1; EP0502132A1; GB2245572A; WO1992000344A1; TW205016B; AU8005291A; NZ238679A; GB9014181D0

Abstract

A process for wrapping articles, particularly large or heavy articles, with a stretch wrap film is disclosed. The film is at least 45 µm thick and is made from a polymer blend, such blend comprising (i) from 1 to 20 wt.-% of at least one polybutene having a number average molecular weight of from 500 to 10,000 and (ii) a polyethylene selected from the group consisting of at least one linear ethylene/C4-C10 .alpha.-olefin copolymer having a density of from 0.915 to 0.940 g/cm3 and blends of such copolymer with a second polymer, selected from a homopolymer of ethylene and a copolymer of ethylene and vinyl acetate, said second po-lymer having a density of from 0.910 to 0.940 g/cm3, said polyethylene having up to 70 wt.-% of said second polymer, and wherein said film has a low crystallinity.

Description

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,~: '~ay~ t/003~1 1Pt?/~CA91/00226 STRETCH WRAPPING OF A HORIZONTAL BEAD
Technical Field The present invention relates to the wrapping of an article with a stretch wrap film. In particular it relates to wrapping of a rolled and other products, e.g.
of paper, carpet, non-woven~fabric, fibre spools, with a stretch wrap film.
Background Art Stretch wrap films are known. For example, U.S.
Patent 4 657 982 to Breck and Mollison, which issued 1927 April 14, discloses stretch wrap films r,ade fro- o0-9G
percent by weight of a polyethylene, 0.5 tc 10.0 pence.~..
by . weight of a polybutene having a number a~,~er age molecular weight of from 500 to 1500 and 0.5 to 10.0 percent by weight of a polybutene having a number average molecular weight of from 1700 to 10 000. -West German Patent 2 821 733 A49, to British Cellophane Limited,.
published 1978 November 30, discloses a method of manufacturing a wrapping film auitable for use in spin wrapping applications, comprising extruding a blend of polyethylene and 0.5 to 10 percent by weight of polyisobutylene to form a film, and treating one surface of the film with corana discharge. Loa: density polyethylenes are exemplified. U.S. Patent 4 337 lEe to Climenhage and Eadie, which issued 1982 June 29 also describes cling films made from polyolefins, an elastomer and one or more of an N,N-bis(2-hydroxyethyl) all:ylamine, a mineral oil, a liquid polyolefin, and organic phosphate ester, polyethylene glycol, glycerol oleate and N-(3 alkoxy-2-hydroxypropyl) ethanolamine. .
It is also known to wrap articles in a spin wrap operation whereby the articles are rotated about a vertical axis. The film used to wrap such articles is usually made from low density polyethylene or linear log:
density polyethylene blended with at least one polyrutene in a conventianal blown film process. film thicknesses -are generally from 20 ~cm to 30 Vim. Unsuccessful,attempts have been made to wrap large roll 'stock, e.g. paper, carpets and the like, on a horizontal beam, c_- heav?
articles, e.g. bricks on a pallet, with linear lo:: density polyethylene film, where such film has been.made using a conventional blown film process and has a thickness of about 51 Vim. Such attempts showed non-uniform stretching of the film, especially if it is prestretched to at least about 100%. Prestretching at higher ratios tends to cause a higher frequency of breaks in the film. Such processes are wasteful. The present invention is intended to alleviate the aforementioned difficulties:
Qisclosure of Invention Accordingly the present invention proviaes i::
process for wrapping an article with a stretch crag filr, an improvement wherein the film is at least 45 um thick:
and is made from a polymer blend, such blend comprising i) from 1 to 20 wt.o of one polybutene having a number average molecular weight of from 500 to 10 000 and ii) a polyethylene selected from the group consisting of at least one linear ethylene/octene-1 copolymer having a density of from 0.905 to 0.940 g/cm3 with a homopolymer of ethylene, said homopolymer having a density of from 0.910 to 0.940 g/cm3, said po:Lyethylene having up to 70 wt.o of said second polymer, and wherein said film has a low crystallinity.
Preferably the article is cylindrical, and circular in cross-section.
In one embodiment the article has a ratio of length to height of at least about 1.5.
In another embodiment of the process the filr is prestretched at least 200%, preferably at least 250'---_, an~
more preferably at least 400%, immediately pric: tc wrapping the article.
In yet another embodiment the ethylene/C_-":
a-olefin copolymer is an ethylene/octene-1 copolymer or an -ethylene/hexene-1 copolymer having a density of from C.910 W~.. .1,00344 z o ~ ~ ~ ~ ~~ PCT/CA91/00226 to 0.930 g/cm3.
In a further embodiment the polybutene has a number average molecular weight of from 700 to 5000, especially from 900 to 3000.
In yet another embodiment the polymer blend contains from 2 to 12 wt.% of polybutene, especially from 3 to 10 wt.%.
In a further embodiment the film is from 50 um to 110 ~m in thickness.
In a further embodiment the gloss of the f ilm is at least about 90%, measured using ASTM Proce3ure D-2~5-at a 20° reflectance angle.
In another embodiment the gloss of the fil- is from 90 to 120%.
In another embodiment the haze of~the film is from about 4% to 6%, as measured using ASTM Procedure D-1003.
The present invention is particulary suited to wrapping large or heavy articles. In one embodiment the article is a beam of paper, fabric or carpet. The article is more usually wrapped on a horizontal wrapping machine because it is more convenient 1.o support the weight of the article or roll horizontally, although a vertical wrapping machine may be used, if~convenient.
Molecular weights of the polybutenes referred to herein, unless otherwise specified, are number average molecular weights determined using ASTI~i Procedure D-2503-f7.
The term "polybutene°' as used herein in relation to the present invention, refers~to polymers having a backbone predominantly based on n-butane or isobutylene.
Polybutenes known in the trade as polyisobutylenes may be made by catalytically polymerizing an isobutylene-rich miactures, with more 1- and 2-butanes being incorporated in the lower weight polyisobutylenes than in the higher molecular weight polyisobutylenes. Polybutenes may also -be synthesized, by a low temperature catalytic process, fVG ./003~t ~ fCT/CA; ~022b ,...

from a refinery gas stream containing polymerizable olefins, particularly isobutylene.
The constituents of the composition of the film may be blended by methods known in the art, e.g. the constituents may be preblended, the polybutene may be incorporated in a masterbatch concentrate and then °'let down'° by admixing with polyethylene, or the polybutene may be directly injected into the film extruder. In any event, the polybutene should be well mixed with the s polyethylene in order to provide as uniforrz a filr; as possible. The blend may be directly extruded into file form in a flat film or blown film process which: wauses the film to have low crystallinity, as described more fulll-hereinafter. ' Crystallinity is not easily measured in.films of the present invention but an indication of crystallinity may be given by the clarity of the film. Indicators of.
crystallinity are gloss and haze levels of the film.
Gloss is measured by ASTM Procedure D-2457, measured at a ZO 20' reflectance angle and haze is measured by ASTr:
Procedure D-1003, both with 51 ~m.thick films. In the context of the present invention, gloss levels of at least 90% are desirable, with preferred levels being from 90 4 to 120%. Haze levels of from about 4% to about 6% are preferred, and especially from 4% to 5.5%. For comparison, films made by conventional processes have gloss levels of from about 35% to 70% and haze levels of from 7% to 15%.
The blown film process is the preferred process for forming~the film useful in the present invention. In the blown film process the polymer blend is extruded through a circular die. The resulting film is pulled fro the die by cooperating nip rollers which collapse the tubular film at the nip. The tubular film, between the circular die and the nip is sometimes referred to as a bubble. While in the molten or plastic formative 'state the tubular film is expanded by air or inert gas admitted ,w:. ., i~~ , X2/00344 PCT/CA91/00226 into the bubble through an orifice in the centre of the circular die. The film may be cooled by directing air onto the exterior of the film while in the plastic formative state. The film is cooled until it is in the 5 solid state. Cooling to the solid state is often referred to as quenching. The faster the film is cooled the lower the level of crystallinity. Fast quenching, in the blown film process, may be accomplished by passing the expanding film about a cooled mandrel which is situated within the bubble. One such process, using a cooled mandrel, is disclosed in Canadian 893 216 which issued 197a Ferruary to M. Bunga and C.V. Thomas. Fast head transfer fror..
the film to the mandrel can be improved by using a gas inside the bubble which has excellent heat transfer 15 properties, such as~helium. Such a process would provide the level of crystallinity required in the present invention. Conventional blown film processes do not have rapid quenching and do not permit development of the required level of crystallinity.
It is to be understood that the films used in the present invention may contain ultra-violet light stabilizers, pigments, antioxidants and other modifiers known in the art.
The invention may also be better unaerstood by reference to the following examples:
Example I:
On a horizontal stretch wrapping apparatus, a large roll of non-woven fabric was attempted to be stretch wrapped with a 51 ~cm thick stretch wrap film ( Contr of ) .
Such film was made from a blend of polybutene and linear low density polyethylene, which had been made into film using a conventional blown film process. The apparatus was capable of prestretching film at ratios from 50 to 650%. The film yielded in a non-uniform manner, and tended to lack high prestretch capability ana have insufficient cling, at prestretch~ ratios of 50 to' 100%.
The film tended to stretch and then fail to stretch, we Iioo~ ~ ~ ~ ~ ~ c~ ~crica~ )0226 forming transverse lanes 25 'to 50 mm wide, 51 um thick, followed by a length 1 ~cm thick. The unstretched portions were hazy and the stretched portions were clear. Above stretch ratios of 100% the film tended to brew: more frequently, which caused a loss of productivity. Similar performance was noted with stretch wrap films having a thickness of 51 ~cm, made from non-linear polyethylene in a conventional blown film process.
A similar film composition to the Control was blown into film form, using a blown fil,:. process in o:hich there was a cooled metal mandrel and heliu-: gas ins~~--._ the film bubble. The film (Film A) was substantiall:: less crystalline than the Control film, as evidenced by the clarity of Film A as opposed to the lack of clarity in the Control film and the lack of haze of Film A compared to the Control film. While gloss and haze was not measured for Film A as used in this example, films with the same composition, made under similar conditions, had a gloss of 100.2% and haze of 5.1% (the averages of ten measurements). Film A was u~>ed to wrap large rolls of paper at prestretch ratios of .>0, 100; 150, 200, 250, 300, 350, 400, 450, 500, 550, 600 and 650%. No film breaks were experienced over tPxe entire prestretch ratio range.
Additionally the film stretchE:d evenly at all prestretch ratios and cling properties were retained. Furthermore the film was crystal clear after stretching. This example clearly demonstrates the surprising improvement in perforiaance of films having low crystallinity.
example II
eighteen spools of synthetic fibre were wrapped into a three-layer unit on a vertical, stretch wrap . machine. Two three-layer units were then wrapped together using a second vertical stretch wrap machine. The film used to stretch wrap the spools and units was a 30.5 ~xm thick co~nercially available conventional stretch wrap polyethylene film. The prestretch ratio was about~.120%.
The film, after being wrapped around the spools and units Vd.. X2/00344 ~ ~ '~ ~ ~ ~ ~ PCT/CA91/00226 was hazy and exhibited non-uniform stretch, sometimes called zebra stripes in the art. Additionally, although the process generally ran well, there was one period of film breakage. About 200 g of film was required for each package.
The film was then replaced with Film A of, Example I, of the present invention, and operated at prestretch ratios of from 350 to 450%. While a low breakage frequency was experienced at a prestretch retie of 450%, there were no breaks at 350, 375 and 425%. The film stretched uniformly and was clear. About 125 g of film was required for each package. This example clearly demonstrates the improved performance of the present invention. ,

Claims

CLAIMS:

1. In a process for wrapping an article with a stretch wrap film, an improvement wherein:
the film is at least 45 µm thick and is made from a polymer blend, such blend comprising i) from 1 to 20 wt.%
of one polybutene having a number average molecular weight of from 500 to 10 000 and ii) a polyethylene selected from the group consisting of at least one linear ethylene/octene-1 copolymer having a density of from 0.915 to 0.940 g/cm3 and a homopolymer of ethylene having a density of from 0.910 to 0.940 g/cm3, said polyethylene having up to 70 wt.% of said homopolymer of ethylene, and wherein said film has a low crystallinity.

2. A process according to Claim 1 wherein the gloss of the film is at least about 90%.

3. A process according to Claim 1 wherein the haze of the film is from about 4% to about 6%.

4. A process according to Claim 1 or Claim 2 or Claim 3 wherein the article is cylindrical, and circular in cross-section.

5. A process according to Claim 1 or Claim 2 or Claim 3 wherein the article has a ratio of length to height of at least about 1.5.

6. A process according to Claim 1 or Claim 2 or Claim 3 wherein the film is prestretched at least 200%
immediately prior to wrapping the article.

7. A process according to Claim 1 or Claim 2 or Claim 3 wherein the film is prestretched at least 400%
immediately prior to wrapping the article.

8. A process according to Claim 1 or Claim 2 or Claim 3 wherein the polybutene has a number average molecular weight of from 700 to 5000.

9. A process according to Claim 1 or Claim 2 or Claim 3 wherein the polymer blend contains from 2 to 12 wt.% of polybutene.

10. A process according to Claim 1 or Claim 2 or Claim 3 wherein the film is from 50 µm to 110 µm in thickness.