Note: Descriptions are shown in the official language in which they were submitted.
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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,
CA 02072599 2002-10-25
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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
3
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,
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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,
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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
0
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. ,
